Process for recovering metal from feedstocks

Abstract

The invention relates to methods for recovering precious metals from feedstocks including solid feedstocks including urban feedstocks, such as electronic waste, and leachates thereof, and / or from liquid feedstocks. In certain examples, methods for recovery of target metals, such as gold, using a support material are provided, as are target metal-laden materials so produced.

Description

[0001] PROCESS FOR RECOVERING METAL FROM FEEDSTOCKS FIELD OF INVENTION

[0002] The invention relates to methods for recovering precious metals from feedstocks including urban feedstocks, such as electronic waste and leachates thereof. In particular examples, selective methods for target metal reduction, binding, and / or recovery are utilised in the methods disclosed herein. BACKGROUND

[0003] The following includes information that may be useful in understanding the present inventions. It is not an admission that any of the information provided herein is prior art, or relevant, to the presently described or claimed inventions, or that any publication or document that is specifically or implicitly referenced is prior art. Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0004] There is an abundance of materials containing trace metals throughout the world, including aqueous solutions and solid materials. However, due to the relative scarcity of the metal component relative to the non-metal matrix, recovering these metals in efficient, environmentally safe ways is extremely challenging. For example, the removal of toxic metal ions from aqueous liquid waste streams is a significant challenge for a wide range of industries.

[0005] Similarly, as ore grades for the mining and refining of virgin metals decrease, increased interest is being shown in obtaining metals from sources such as low-grade mining ores, smelter tailings, electronic waste, and automotive catalytic converters. Recovering metals from these feedstocks, however, is often economically prohibitive. Factors that influence the viability of any recovery process include the metal concentration of a feedstock (and hence the amount of feedstock required for processing); the presence of impurities such as other metals or refractory materials; and the volume of effluent generated. There is therefore a place for alternative solutions that aim to mitigate at least some of these problems, thereby improving the economics for the recovery of metals from low-grade or recalcitrant feedstocks.

[0006] Traditional techniques for refining metals include pyrometallurgy and hydrometallurgy. In pyrometallurgy, a feedstock is smelted at high temperature (typically in the presence of a suitable reductant and / or catalyst). This requires a non-trivial energy input (and associated emissions), and therefore there is a practical minimum metal concentration required in a feedstock. In hydrometallurgy, the feedstock is treated with a lixiviant solution that leaches the desired metal (specifically or otherwise) into an ionic or complexed soluble form. Subsequent steps are required to recover the target metal(s) from solution (e.g. electrowinning). Due to practical considerations related to efficient mixing of a solid in a liquid, particularly at commercially relevant scale, a set solid / liquid ratio is generally used for hydrometallurgical processes. This means that a lower concentration of target metal in the feedstock results in more dilute target metal solutions, and that the concentration of target metal in the leachate is less than that in the solid feedstock. Depending on the temperature and pressure requirements for leaching, this approach may allow for lower grade feedstocks to be processed in comparison to pyrometallurgy. Consideration needs to be made for the possible use of corrosive (e.g. acidic) or toxic (e.g. cyanide) solutions; any consumption of solution components during feedstock treatment; and dealing suitably with waste effluent. Pyrometallurgy and hydrometallurgy techniques are not mutually exclusive, and may be used sequentially over multiple steps to refine specific metals.

[0007] Recovery of gold from gold containing ores is a typical example of a hydrometallurgical approach that has a number of issues. The amount of gold in gold bearing ores has been declining for over a hundred years as easier to recover resources with higher gold content have been depleted through extensive mining. As such, hydrometallurgical techniques have been used to recover traces of gold from large volumes of rock. Cyanide-based lixiviants have been successfully employed for many years, but suffer from toxicity issues and challenges with processing certain ore types.

[0008] Waste electrical and electronic equipment (also referred to herein as "e-waste"), such as printed circuit boards from computers, cell phones, notebooks and LCD displays, represent a large target for metal recovery systems. Waste electrical and electronic equipment are an example of an urban precious metal feedstock. Approximately 53.6 million tonnes of electronic waste were generated worldwide in 2019, with global generation of e-waste projected to grow to 74.7 Mt by 2030 (Forti et al, The Global E-waste Monitor 2020, United Nations University, UN-ITR, ITU, ISWA, (2020)). On a per weight basis, this e-waste contains high amounts of valuable base and precious metals (including gold) in comparison to virgin ore. Recovering metals from this feedstock is attractive because the mining and refining processes used to obtain virgin metals consumes energy and water resources (and generates related carbon emissions); recycling means that these costs need not be borne again. As an example, recycling copper reportedly requires 85% less energy than producing virgin material from ore (Khaliq et al, Resources 3, pp 152-179 (2014)).

[0009] While much endeavour has been applied to recovering precious metals such as gold from e-waste using pyrometallurgy and hydro metallurgy approaches, sustainable success has yet to be achieved. Pyrometallurgy approaches commonly involve incinerating circuit boards to liberate and fractionate metals - this is energy and capital intensive, and produces dangerous gases such as dioxins.

[0010] Hydrometallurgy approaches commonly involve using strong acids or cyanide-based solutions to leach metals - this is highly toxic, expensive and can be non-recyclable. The heterogeneity of e-waste as a feedstock also makes it difficult to economically apply these existing technologies.

[0011] Automotive catalytic converters are a second example of a precious metal feedstock, containing significant quantities of precious metals such as palladium, platinum and rhodium, in greater concentration than in virgin ores (Xu et al, Environ. Sci. Technol. 53, pp 733-742 (2019)). Energy intensive pyrometallurgical methods are currently employed to recycle these materials.

[0012] Accordingly, there remains a need for cost effective and energy efficient methods of recovering metals from these and other feedstocks, and particularly such methods having minimised environmental impact.

[0013] The present invention relates to methods of recovering precious metals from feedstocks, including hydrometallurgical leachates derived from solid feedstocks such as electronic waste, ore and / or catalytic converters using selective recovery techniques that complement or replace traditional pyrometallurgy and hydrometallurgy approaches, or to at least provide the public with a useful choice in this regard. It is anticipated that this will lead to the capture of value from low-grade or waste streams of metal that are currently neglected, and / or provide for lower cost, energy efficient and / or environmentally sound alternatives to existing approaches. SUMMARY OF THE INVENTION

[0014] The present invention responds to a need in the art. The present invention relates to methods for recovering precious metals (referred to herein as "target metals") from precious metal feedstocks, such as but not limited solid feedstocks including virgin and reworked ores including tailings, electronic waste, and automotive catalytic converters.

[0015] The invention generally relates to methods of recovering one or more target metals from a feedstock, such as a leachate containing a target metal or target metals (usually referred to herein as a target metal-pregnant solution) selectively over other metals that may be or are present in solution. The invention thus relates to methods of preparing such target metal-pregnant solutions from a feedstock, such as a solid feedstock, in addition to methods of recovering one or more target metals from such target metal-pregnant solutions. This leaves a target metal-barren solution containing lower, and in particularly contemplated examples essentially no target metal(s), or indeed no target metal(s). In certain examples, the method involves the recovery of one or more target metals from the target metal-pregnant solution. For example, one or more target metals are concentrated on a biomass-derived support material which is then recovered, for example by physically separating it from the target metal-barren solution by conventional solid-liquid separation techniques. The target metalladen support material removed from solution is also referred to herein as a target metal concentrate. In various examples, the methods disclosed herein are capable of concentrating one or more target metals to a substantial degree, such that the one or more target metals are concentrated to the support material by a concentration factor of about 10 to 1000 or more, relative to its concentration in solution, in addition to being physically separated from other metals.

[0016] In such examples contemplated herein, the methods result in the production of a target metal-laden support material that is amenable to further processing, for example repeated use in the methods discussed here, or still further concentration for example by pyrometallurgical manipulation.

[0017] Accordingly, the overall concentration factor achievable using the methods discussed herein coupled with established refinement techniques, for example concentration and recovery methods such as pyrometallurgical processing, in certain examples will be about 10,000 to about 100,000 or more. In various examples, the methods disclosed herein broadly comprise an approach where a base metal leach is performed to remove at least some non-target metal(s) present in the solid feedstock, the target metal(s) are dissolved into solution and the solid residue is then removed from solution by solid-liquid separation techniques known to those familiar with the art, yielding a target metalpregnant solution. It will be appreciated by those skilled in the art that a number of methods of recovering one or more target metals from a target metal-pregnant solution exist. Particularly contemplated examples of methods to recover one or more target metals from such solutions are disclosed and exemplified herein. In certain examples, the target metal-pregnant solution is contacted with a support material, such as a support material capable of binding one or more target metals or metal ions. In certain examples, the target metal-pregnant solution may be subjected to hydrometallurgical recovery employing one or more of ion-exchange, chelation, solvent extraction, precipitation, electrowinning, or reduction techniques, including any pertinent combination of two or more of such techniques.

[0018] In various examples, the methods disclosed herein broadly comprise an approach where a base metal leach is performed to remove at least some non-target metal(s) present in the solid feedstock, the target metal(s) are dissolved into solution and the solid residue is then removed from solution by solid-liquid separation techniques known to those familiar with the art, and the target metal-pregnant solution so formed is then contacted with a support material under conditions in which the target metal is adsorbed to the support material.

[0019] Accordingly, in a first aspect, the invention relates to a method of recovering one or more target metals from solid feedstock, the method comprising:

[0020] a) providing solid feedstock, said solid feedstock comprising at least one target metal and one or more non-target metals;

[0021] b) contacting the solid feedstock with a base metal lixiviant in a base metal leach to provide a base metal leach solution, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids, and one or more oxidants;

[0022] c) removing the base metal leach solution to yield a target metal leach (TML) feedstock;

[0023] d) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof;

[0024] e) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution;

[0025] f) recovering at least some of the one or more target metals from the target metal-pregnant solution.

[0026] In one example, the recovery of at least some of the one or more target metals from the target metalpregnant solution comprises contacting the target metal-pregnant solution with a support material. In one example, the recovery of at least some of the one or more target metals from the target metalpregnant solution comprises adding at least one target metal recovery agent to the target metalpregnant solution.

[0027] In a further aspect, the invention relates to a method of recovering one or more target metals from solid feedstock, the method comprising:

[0028] a) providing solid feedstock, said solid feedstock comprising at least one target metal and one or more non-target metals;

[0029] b) contacting the solid feedstock with a base metal lixiviant in a base metal leach to provide a base metal leach solution, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids, and one or more oxidants;

[0030] c) removing the base metal leach solution to yield a target metal leach (TML) feedstock;

[0031] d) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof; e) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution;

[0032] f) adding at least one target metal recovery agent (for example, hydrogen peroxide) to the target metal-pregnant solution;

[0033] g) contacting the target metal-pregnant solution with a support material;

[0034] h) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and I) separating the metal-laden support material from the target metal-barren solution; and j) recovering the at least one target metal from the target metal-laden support material.

[0035] In one example, the method of recovering one or more target metals from solid feedstock comprises: a) providing solid feedstock, said solid feedstock comprising at least one target metal and one or more non-target metals;

[0036] b) contacting the solid feedstock with a base metal lixiviant in a base metal leach to provide a base metal leach solution, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids, and one or more oxidants (e.g. hydrogen peroxide);

[0037] c) removing the base metal leach solution to yield a target metal leach (TML) feedstock;

[0038] d) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; and any combination of any two or more thereof; e) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution;

[0039] f) adding at least one target metal recovery agent (for example, hydrogen peroxide) to the target metal-pregnant solution;

[0040] g) contacting the target metal-pregnant solution with a support material;

[0041] h) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and I) separating the metal-laden support material from the target metal-barren solution; and j) recovering the at least one target metal from the target metal-laden support material.

[0042] In one example, the method of recovering one or more target metals from solid feedstock comprises: a) providing solid feedstock, said solid feedstock comprising at least one target metal and one or more non-target metals;

[0043] b) contacting the solid feedstock with a base metal lixiviant in a base metal leach, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids; one or more oxidants; c) contacting at least a portion of the resulting base metal leach solution with an oxidant in a second base metal leach to provide a second base metal leach solution, wherein the second base metal leach solution comprises dissolved metal ions of at least one non-target metal; d) removing the second base metal leach solution to yield a target metal leach (TML) feedstock; e) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; and any combination of any two or more thereof; f) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution; g) adding at least one target metal recovery agent (for example, hydrogen peroxide) to the target metal-pregnant solution;

[0044] h) contacting the target metal-pregnant solution with a support material;

[0045] i) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and j) separating the metal-laden support material from the target metal-barren solution; and k) recovering the at least one target metal from the target metal-laden support material.

[0046] In a further aspect, the invention relates to a method of recovering one or more target metals from a solution, wherein the solution comprises dissolved metal ions of at least one target metal, the method comprising:

[0047] a) providing a solution comprising dissolved metal ions of at least one target metal, wherein the solution has been prepared in a method comprising:

[0048] I. contacting a solid feedstock comprising one or more target metals and one or more non-target metals with a base metal lixiviant, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids; one or more oxidants; II. contacting at least a portion of the resulting base metal leach solution with hydrogen peroxide in a second base metal leach to provide said solution comprising dissolved metal ions of at least one non-target metals;

[0049] ill. removing the base metal leach solution to yield a target metal leach (TML) feedstock; iv. adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof;

[0050] v. maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metalpregnant solution;

[0051] b) adding at least one target metal recovery agent to the target metal-pregnant solution; c) contacting the target metal-pregnant solution with a support material; d) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and e) separating the metal-laden support material from the target metal-barren solution; and f) recovering the at least one target metal from the target metal-laden support material.

[0052] In one example, the solution comprises dissolved metal ions of at least one target metal and dissolved metal ions of one or more non-target metals.

[0053] In one example, the method of recovering one or more target metals from a solution comprises: a) providing a solution comprising dissolved metal ions of at least one target metal, wherein the solution has been prepared in a base metal leach, said base metal leach comprising:

[0054] i. contacting a solid feedstock comprising one or more target metals and one or more non-target metals with a base metal lixiviant, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids; one or more oxidants; II. contacting at least a portion of the resulting base metal leach solution with hydrogen peroxide in a second base metal leach to provide said solution comprising dissolved metal ions of at least one non-target metals;

[0055] ill. removing the second base metal leach solution to yield a target metal leach (TML) feedstock;

[0056] iv. adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; and any combination of any two or more thereof;

[0057] v. maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metalpregnant solution;

[0058] b) adding at least one target metal recovery agent to the target metal-pregnant solution; c) contacting the target metal-pregnant solution with a support material;

[0059] d) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and e) separating the metal-laden support material from the target metal-barren solution; and f) recovering the at least one target metal from the target metal-laden support material.

[0060] In one example, the method of recovering one or more target metals from a solution comprises: a) providing a solution comprising dissolved metal ions of at least one target metal, wherein the solution has been prepared in a base metal leach, said base metal leach comprising:

[0061] I. contacting a solid feedstock comprising one or more target metals and one or more non-target metals with a base metal lixiviant, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids; one or more oxidants; ii. contacting at least a portion of the resulting base metal leach solution with hydrogen peroxide in a second base metal leach to provide a said solution comprising dissolved metal ions of at least one target metal and dissolved metal ions of at least one nontarget metals;

[0062] ill. removing the base metal leach solution to yield a target metal leach (TML) feedstock; iv. adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds, and cyanide;

[0063] v. maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metalpregnant solution;

[0064] b) adding hydrogen peroxide to the target metal-pregnant solution;

[0065] c) contacting the target metal-pregnant solution with a support material;

[0066] d) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and e) separating the metal-laden support material from the target metal-barren solution; and f) recovering the at least one target metal from the target metal-laden support material.

[0067] In one example, the solution is formed by dissolution of a solid feedstock.

[0068] In a further aspect, the invention relates to a method of recovering one or more target metals from a solution, wherein the solution comprises dissolved metal ions of at least one target metal, the method comprising:

[0069] a) providing a solution comprising dissolved metal ions of at least one target metal, wherein the solution has been prepared in a method comprising:

[0070] i. contacting a solid feedstock comprising one or more target metals and one or more non-target metals with a base metal lixiviant, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids; one or more oxidants; II. contacting at least a portion of the resulting base metal leach solution with hydrogen peroxide in a second base metal leach to provide said solution comprising dissolved metal ions of at least one non-target metals;

[0071] ill. removing the base metal leach solution to yield a target metal leach (TML) feedstock; iv. adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof; v. maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metalpregnant solution;

[0072] b) recovering at least some of the one or more target metals from the target metal-pregnant solution.

[0073] In a further aspect, the invention relates to a method of recovering one or more target metals from a solution, wherein the solution comprises dissolved metal ions of at least one target metal, the method comprising:

[0074] a) providing a solution comprising dissolved metal ions of at least one target metal,

[0075] b) adding to the solution a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof;

[0076] c) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution; d) recovering at least some of the one or more target metals from the target metal-pregnant solution.

[0077] In one example, the invention relates to a method of recovering one or more target metals from a solution, wherein the solution comprises dissolved metal ions of at least one target metal, the method comprising:

[0078] a) providing a solution comprising dissolved metal ions of at least one target metal,

[0079] b) contacting at least a portion of the solution with hydrogen peroxide,

[0080] c) adding to the solution a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof;

[0081] d) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution; e) recovering at least some of the one or more target metals from the target metal-pregnant solution.

[0082] In a further aspect, the invention relates to a method of recovering one or more target metals from a solution, wherein the solution comprises dissolved metal ions of at least one target metal, the method comprising:

[0083] a) providing a solution comprising dissolved metal ions of at least one target metal,

[0084] b) adding to the solution a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof; c) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution; d) contacting the target metal-pregnant solution with a support material;

[0085] e) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and f) separating the metal-laden support material from the target metal-barren solution; and g) recovering the at least one target metal from the target metal-laden support material.

[0086] In one example, the invention relates to a method of recovering one or more target metals from a solution, wherein the solution comprises dissolved metal ions of at least one target metal, the method comprising:

[0087] a) providing a solution comprising dissolved metal ions of at least one target metal,

[0088] b) contacting at least a portion of the solution with hydrogen peroxide,

[0089] c) adding to the solution a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof;

[0090] d) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution; e) adding hydrogen peroxide to the target metal-pregnant solution;

[0091] f) contacting the target metal-pregnant solution with a support material;

[0092] g) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and h) separating the metal-laden support material from the target metal-barren solution; and I) recovering the at least one target metal from the target metal-laden support material.

[0093] In another aspect, the invention relates to a method of preparing a target metal-laden cellulose material, the method comprising

[0094] a) adding to a solution comprising dissolved metal ions of at least one target metal and dissolved metal ions of at least one non-target metal prepared in a method as described herein a filterable cellulose material and a reducing agent, wherein the reducing agent has a reducing potential effective to reduce at least one species of target metal ions to target metal but ineffective to reduce at least one species of non-target metal ions; and

[0095] b) maintaining the solution for a time and under conditions suitable to precipitate the at least one target metal to the cellulose material to form a target metal-laden cellulose material; and c) separating the metal-laden cellulose material from the solution by filtration; and

[0096] d) recovering the target metal-laden cellulose material.

[0097] In another aspect, the invention relates to a metal-laden cellulose material prepared by a method as contemplated herein. In one example, the metal-laden cellulose material comprises gold, for example, gold nanoparticles. In another aspect, the invention relates to a method of providing a target metal-pregnant solution comprising one or more target metals, the method comprising:

[0098] a) providing solid feedstock, said solid feedstock comprising at least one target metal and one or more non-target metals;

[0099] b) contacting the solid feedstock with a base metal lixiviant in a base metal leach to provide a base metal leach solution, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids, and one or more oxidants;

[0100] c) removing the base metal leach solution to yield a target metal leach (TML) feedstock;

[0101] d) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof;

[0102] e) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution comprising one or more target metals.

[0103] In another aspect, the invention relates to a method of providing a target metal-pregnant solution comprising one or more target metals, the method comprising:

[0104] a) providing a solution comprising one or more target metals;

[0105] b) adding to the solution comprising one or more target metals a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof;

[0106] c) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution comprising one or more target metals.

[0107] In one example, the solution comprising one or more target metals is prepared in a method disclosed herein, such as a method employing a base metal leach solution formed using a base metal lixiviant comprising, consisting essentially of, or consisting of one or more of the group consisting of: one or more organic acids, one or more mineral acids, and one or more oxidants.

[0108] Any of the examples described herein can relate to any of the aspects presented herein.

[0109] In one example, the solid feedstock is particulate solid feedstock.

[0110] In one example, the solid feedstock material is selected from the group consisting of e-waste, precious metal bearing ore, precious metal bearing sand, precious metal bearing clay, and a combination of any two or more thereof. In one example, one of the target metals is selected from the group consisting of gold, silver, palladium, platinum, and rhodium. In one example, one of the target metals is selected from the group consisting of gold, palladium, platinum, and rhodium.

[0111] In one example, the target metal is gold.

[0112] In one example, the solid feedstock material is selected from the group consisting of e-waste, gold bearing ore, gold bearing sand, gold bearing clay and a combination of any two or more thereof. In one example, the solid feedstock material is selected from the group consisting of e-waste.

[0113] In one example, the base metal lixiviant comprises, consists essentially of, or consists of sulfuric acid, optionally together with one or more oxidants.

[0114] In one example, the base metal lixiviant comprises, consists essentially of, or consists of sulfuric acid and hydrogen peroxide.

[0115] In one example, the target metal leach (TML) feedstock comprises less than 10% by weight base metals.

[0116] In one example, the TML feedstock comprises less than 5% by weight base metals.

[0117] In another example, the TML feedstock comprises less than 2% by weight base metals.

[0118] In one example, the target metal lixiviant comprises one or more chlorine-containing compounds. In one example, the target metal lixiviant comprises an aqueous solution comprising one or more chlorine-containing compounds.

[0119] In one example, the target metal lixiviant comprises an aqueous solution comprising one or more chlorine-containing compounds and hydrogen peroxide.

[0120] In one example, one or more of the one or more chlorine-containing compounds is selected from the group consisting of: trichloroisocyanuric acid (TCCA), dichloroisocyanuric acid (DCCA), sodium trichloroisocyanurate, sodium dichloroisocyanurate, chlorine.

[0121] In certain examples, for instance when the TML feedstock comprises less than about 10% base metals, for example when the TML feedstock comprises less than about 5% base metals, or when the TML feedstock comprises less than about 2% base metals, the target metal lixiviant comprises, consists essentially of, or consists of TCCA.

[0122] In one example, the pH of the TML solution (which comprises at least the TML lixiviant and the TML feedstock) is maintained within a range of from pH 3 to pH 7 over a reaction period.

[0123] In one example, the method comprises a TML step comprising addition of a pH increasing agent. The pH increasing agent may be selected from suitable agents to increase pH including magnesium hydroxide, calcium hydroxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide, sodium carbonate, sodium bicarbonate, magnesium oxide, and calcium hypochlorite or precursors of any one thereof.

[0124] In one example, the target metal lixiviant comprises one or more iodine-containing compounds and / or one or more bromine-containing compounds.

[0125] In one example the temperature of the TML solution is maintained in a range from 30-45°C. In one example, the target metal recovery agent is a reducing agent.

[0126] In one example, the target metal recovery agent is hydrogen peroxide. For example, when the target metal lixiviant comprises, consists essentially of, or consists of TCCA, the target metal recovery agent is hydrogen peroxide.

[0127] In one example, the target metal recovery agent is an agent capable of destabilising target metalligand binding.

[0128] In one example, the conditions suitable for binding of the at least one target metal to the support material include the presence of one or more recovery agents.

[0129] In one example, the conditions suitable for binding of the at least one target metal to the support material include the presence of one or more organic acids, or one or more mineral acids.

[0130] In one example, the organic acid is ascorbic acid.

[0131] In one example, the support material comprises, consists essentially of, or consists of cellulose or a cellulosic material, and / or the support material comprises, consists essentially of, or consists of non-cellular biomass, and / or the support material comprises, consists essentially of, or consists of non-microbial biomass.

[0132] In one example, the target metal-laden support material is separated from the solution by filtration. In one example, the at least one target metal is recovered from the target metal-laden support material by ashing.

[0133] In one example, when the solid material is electronic waste, the base metal leach is preceded by one or more pre-processing steps selected from the group consisting of: chip removal; grinding, milling, or comminuting electronic waste, for example to a preselected size, for example to provide particulate solid material; removal of certain density fractions; removal of certain size particles or fractions; removal of one or more magnetic materials; removal of at least a portion of non-target material; and any combination of two or more thereof.

[0134] In one example, the conditions suitable for binding of the at least one target metal to the support material include the presence of one or more reducing agents having a reducing potential effective to reduce only one species of metal ions present in the solution to metal.

[0135] In one example, the conditions suitable for binding of the at least one target metal to the support material include the presence of one or more reducing agents having a reducing potential effective to reduce only one species of the target metal ions present in the solution to target metal.

[0136] In one example, the conditions suitable for binding of the at least one target metal to the support material include the presence of one or more reducing agents having a reducing potential effective to reduce only one species of the target metal ions present in the solution to target metal and ineffective in reducing all species of non-target metal ions present in the solution.

[0137] In one example, the conditions suitable for binding of the at least one target metal to the support material include the presence of one or more reducing agents having a reducing potential effective to reduce at least one species of target metal ions present in the solution to target metal, but ineffective to reduce all species of non-target metal ions. In one example, the reducing agent is selected from the group consisting of organic acids, hydrazines, hydrides, borohydrides, and inorganic acids.

[0138] In one example, the reducing agent comprises, consists essentially of, or consists of one or more inorganic acids such as one or more mineral acids, or one or more organic acids.

[0139] In one example, the reducing agent is selected from the group consisting of ascorbic acid or a salt thereof, citric acid or a salt thereof, formic acid or a salt thereof, lactic acid or a salt thereof, malic acid or a salt thereof, oxalic acid or a salt thereof, tartaric acid or a salt thereof, and uric acid or a salt thereof.

[0140] In one example, the reducing agent comprises, consists essentially of, or consists of ascorbic acid. In one example, the reducing agent comprises, consists essentially of, or consists of formic acid. In one example, when one of the species of target metal ions is gold, the reducing agent is ascorbic acid or a salt thereof.

[0141] In one example, the target metal-pregnant solution is an aqueous solution containing more than lOppm of the target metal.

[0142] In one example, at least about 90% of the target metal is bound to the support material and / or recovered.

[0143] In one example, at least about 95%, or at least about 99%, of the target metal is bound to the support material and / or recovered.

[0144] In one example, the concentration factor of the target metal from the target metal-pregnant solution to the support material is greater than 100.

[0145] In one example, the concentration factor of the target metal from the target metal-pregnant solution to the support material is greater than 1000.

[0146] In one example, in the maintaining step the support material is in contact with the target metalpregnant solution for between about 0.5 and 48 hours.

[0147] In one example, the maintenance of the solution for a time and under conditions suitable to bind the at least one target metal to the support material is for a time and under conditions suitable to form a target metal-barren solution.

[0148] In one example, the recovery step comprises burning of the metal laden support material or chemical dissolution of the target metal / support material complex to release the target metal.

[0149] In one example, the target metal-pregnant solution comprises at least one non-target metal and the reducing agent preferentially reduces the target metal over the non-target metal, and the non-target metal(s) remains in the aqueous solution in the separating step.

[0150] In one example, the target metal is bound to the support material over the non-target metal in the binding step such that the mass ratio of target metal to non-target metal on the support material increases by a factor of at least 2 when compared to the mass ratio in the target meta I -pregnant solution.

[0151] In one example, the non-target metal is a base metal selected from one or more of lead, copper, and / or nickel. In one example, the non-target metal is a base metal selected from a first row transition metal including Fe, Ni, Mn, Cu, Zn, Cr, V, Co, and / or Al, and / or Sn.

[0152] In one example, the solid feedstock comprises a solid material comprising less than 5% of target metal.

[0153] In one example, the target metal lixiviant comprises a thiourea-based solution, or a thiosulphate-based solution, or a thiocyanate-based solution, or a halogen-based solution.

[0154] In one example, the target metal lixiviant does not comprise cyanide.

[0155] In one example, the target metal lixiviant does not comprise aqua regia.

[0156] In one example, the pH of the solution prior to the recovery step is maintained within the range of from about 3 to about 10.

[0157] In one example, at least a portion of the support material and / or the target metal-barren solution is reused in a further repeat of the method.

[0158] In one example, one or more additional components are added to the target metal-barren solution such that it can act as a lixiviant.

[0159] In one example, the one or more additional components are selected from one or more of thiourea, thiosulphate, thiocyanate, a halogen, nitric acid, hydrochloric acid.

[0160] In one example, the target metal-barren solution is treated with chlorine gas.

[0161] In one example, at least 25% of the target metal-barren solution is reused.

[0162] In one example, the non-target material includes one or more base metal(s).

[0163] In one example, the oxidant in the second base metal leach is hydrogen peroxide.

[0164] In one example, the solid feedstock comprises at least one target metal and two or more non-target metals.

[0165] In various examples, the addition of support material to the solution precedes the addition of reducing agent by a time sufficient to allow for dispersal of the support material in the solution.

[0166] In one example, the addition of reducing agent to the solution precedes the addition of support material.

[0167] In various examples, the reducing agent has a reducing potential effective to reduce only one species of metal ions present in the solution to metal.

[0168] In various examples, the reducing agent has a reducing potential effective to reduce only one species of the target metal ions present in the solution to target metal.

[0169] In various examples, the reducing agent has a reducing potential effective to reduce only one species of the target metal ions present in the solution to target metal and ineffective in reducing all species of non-target metal ions present in the solution.

[0170] In various examples, the reducing agent has a reducing potential effective to reduce at least one species of target metal ions present in the solution to target metal, but ineffective to reduce all species of non-target metal ions. In various examples, the method comprises the additional step of adding to the target metal-pregnant solution another reducing agent having a reducing potential effective to reduce at least one species of target metal ions still present in the solution to target metal.

[0171] In various examples, the method comprises the additional step of adding to the target metal-pregnant solution another reducing agent having a reducing potential effective to reduce at least one species of target metal ions still present in solution to target metal, but ineffective to reduce at least one species of non-target metal ions.

[0172] In one example, the reducing agent has a reducing potential effective to reduce at least one species of non-target metal ions present in the solution, and is added in an amount insufficient to reduce all of the non-target metal ions present.

[0173] In one example, the reducing agent has a reducing potential effective to reduce at least one species of non-target metal ions present in the solution, but ineffective to reduce all species of non-target metal ions.

[0174] In one example, the reducing agent has a reducing potential effective to reduce all species of non-target metal ions present in the solution to target metal, and is added in an amount insufficient to reduce all of the non-target metal ions present.

[0175] In various examples, the method comprises the additional step of separating the target metal-laden support material from the target metal-pregnant solution and adding the support material to a target metal-pregnant solution and reducing agent.

[0176] In one example, the target metal-laden support material is separated from the target metal-barren solution by filtration.

[0177] In one example, the support material comprises, consists essentially of, or consists of cellulose or a cellulosic material.

[0178] In one example, the reducing agent comprises an inorganic acid.

[0179] In various examples, the method comprises the preliminary step of pre-processing the solid feedstock. In various examples, the method comprises a preliminary pre-processing step selected from the group consisting of: chip removal; grinding, milling, or comminuting electronic waste, for example to a preselected size, for example to provide particulate solid material; removal of certain density fractions; removal of certain size particles or fractions; removal of one or more magnetic materials; removal of at least a portion of non-target material; and any combination of two or more thereof. In various examples, the pre-processing step removes at least a portion of non-target material from the feedstock.

[0180] In various examples, the non-target material includes one or more base metal(s).

[0181] In a particularly contemplated example, the target metal is a noble metal. For example, the target metal is a noble metal selected from the group consisting of gold, silver, mercury, rhenium, ruthenium, rhodium, palladium, osmium, iridium, and platinum. In one example, the target metal is selected from the group consisting of gold, silver, ruthenium, rhodium, palladium, osmium, iridium, and platinum. In another particularly contemplated example, the target metal is a platinum group metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum.

[0182] In a particularly contemplated example, the target metal is selected from the group consisting of gold, silver, platinum and palladium.

[0183] In various examples, the non-target metal is a metal present in the feedstock or leachate other than a target metal. For example, non-target metals commonly present in e-waste include Al, Cr, Cu, Fe, Mn, N I, Pb, Sn, Zn. For example, non-target metals commonly present in catalytic converters include Al, Cr, Fe, Mn, Ni, and rare earth elements. Accordingly, in various examples the non-target metal is selected from the group consisting of Al, Cr, Cu, Fe, Mn, Ni, Pb, Sn, Zn, rare earth elements, and any combination of two or more thereof.

[0184] In one example the target metal-pregnant solution contains between about O.lppm to 1500ppm, or between about O.lppm to lOOOppm, or between about O.lppm to 500ppm, or between about O.lppm to 200ppm, or between about O.lppm to lOOppm, or between about O.lppm to 50ppm, or between about O.lppm to 20ppm of the target metal. In one example the target metal-pregnant solution contains between about 0.5ppm to 1500ppm, or between about 0.5ppm to lOOOppm, or between about 0.5ppm to 500ppm, or between about 0.5ppm to 200ppm, or between about 0.5ppm to lOOppm, or between about 0.5ppm to 50ppm, or between about 0.5ppm to 20ppm of the target metal(s). In one example the target metal-pregnant solution contains between about lppm to 1500ppm, or between about lppm to lOOOppm, or between about lppm to 500ppm, or between about lppm to 200ppm, or between about lppm to lOOppm, or between about lppm to 50ppm, or between about lppm to 20ppm of the target metal(s).

[0185] In one example the target metal-barren solution contains less than O.lppm, or less than lppm, or less than 2ppm, or less than 5ppm, or less than lOppm, or less than 20ppm, or less than 50ppm, or less than lOOppm of the target metal. In one example the target metal-barren solution contains between about 0.001 and lOOppm, or between about 0.001 and 50ppm, or between about 0.001 and 50ppm, or between about 0.01 and 50ppm of the target metal(s).

[0186] In one example the target metal-pregnant solution contains at least 10 times more target metal than the target metal-barren solution. In one example the target metal-pregnant solution contains at least 20 times, or at least 40 times, or at least 45 times or at least 50 times more target metal than the target metal-barren solution.

[0187] In one example the metal laden support material comprises greater than lOOppm, greater than 200ppm, greater than 500ppm, or greater than lOOOppm of the target metal(s).

[0188] In one example the metal laden support material comprises greater than 2000ppm, greater than 3000ppm, greater than 4000ppm, greater than 5000ppm, greater than 6000ppm, greater than 7000ppm, greater than 8000ppm, greater than 9000ppm, greater than lOOOOppm, greater than 15000ppm, greater than 20000ppm, greater than 25000ppm, or greater than 30000ppm of the target metal(s).

[0189] In one example the metal laden support material comprises greater than 0.1% (w / w) target metal(s), for example greater than 0.2% (w / w), greater than 0.3% (w / w), greater than 0.4% (w / w), greater than 0.5% (w / w), greater than 0.6% (w / w), greater than 0.7% (w / w), greater than 0.8% (w / w), greater than 0.9% (w / w), or greater than 1% (w / w) target metal(s). In one example the metal laden support material comprises greater than 1.5% (w / w) target metal(s), for example greater than 2% (w / w), greater than 3% (w / w), greater than 4% (w / w), greater than 5% (w / w), greater than 6% (w / w), greater than 7% (w / w), greater than 8% (w / w), greater than 9% (w / w), or greater than 10% (w / w) target metal(s).

[0190] In a particular example, the target metal-pregnant solution includes at least one further metal, in addition to the one or more target metal(s).

[0191] In one example, the conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material include the presence of one or more target metal recovery agents.

[0192] In one example, the conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material include the presence of one or more reducing agents and / or the presence of a target metal reducing environment.

[0193] In one example the reducing agent preferentially reduces the one or more target metal(s) over the further metal, facilitating the binding of target metal(s) to the support material, such that the further metal remains in the target metal-barren solution at the separating step.

[0194] In one example the binding of the target metal(s) over the further metal is such that the mass ratio of target metal(s) to further metal on the support material increases by a factor of at least 2 when compared to the mass ratio of the metals in the target metal-pregnant solution. In one example the mass ratio increased by a factor of at least 3, or at least 5, or at least 8, or at least 10, or at least 20, or at least 50, or at least 100, or at least 200. In one example the target metal is gold, palladium, platinum, or rhodium. In one example the target metal is gold, silver, palladium, platinum, or rhodium. In one example the further metal is selected from one or more of copper and nickel.

[0195] In one example the concentration factor of the target metal from the target metal-pregnant solution to the support material is greater than 5 or greater than 10, or greater than 20, or greater than 50, or greater than 100, greater than 200, greater than 300, greater than 400, greater than 500, greater than 600, greater than 700, greater than 800, greater than 900, or greater than 1000.

[0196] In one example the concentration factor of the target metal from the target metal-pregnant solution to the support material is greater than 1500, greater than 2000, greater than 3000, greater than 4000, greater than 5000, greater than 6000, greater than 7000, greater than 8000, greater than 9000, greater than 10000, greater than 15000, greater than 20000, greater than 25000, or greater than 30000.

[0197] In one example, in the binding step the support material is in contact with the target metal-pregnant solution for between about 0.5 and 48 hours. In one example between about 0.5 and 24 hours, or between about 0.5 and 12 hours, or between about 0.5 and 4 hours, or between about 1 and 3 hours. In one example the binding step is carried out at ambient temperature, for example between about 15 and 35 °C. In other examples, the binding step is carried out at temperatures above ambient, for example above about 35 °C.

[0198] In one example where the target metal is gold the support material is selected from the group consisting of cellulose, modified cellulose, and cellulosic materials.

[0199] In one example where the target metal is gold the support material comprises cellulose. In various examples, the cellulosic material is selected from the group consisting of wood, wood chips, sawdust, wood pulp, kraft pulp, and paper.

[0200] In certain examples, the separation step includes at least one of: gravity separation of the metal laden support material from the target metal-barren solution and removal of the target metal-barren solution; centrifugation and removal of the target metal-barren solution; and filtration of the metal laden support material from the target metal-barren solution.

[0201] In certain examples, the separating step comprises separating the metal laden support material by filtration, wherein during the filtration at least 50% of the target metal-barren solution is removed from the metal laden support material. In one example at least 60%, or at least 70%, or at least of 80%, or at least 90%, or at least 95% of the target metal-barren solution is removed during filtration. In certain examples, particularly in examples where filters with a larger pore size are used, yield will be improved by recirculation or re-filtering the filtrate, and particularly the early filtrate such as the first 5% or 10% of the filtrate, to capture smaller particles of the support material. As those skilled in the art will recognise, a balance between speed and a high proportion of collection will usually need to be reached to maximise process efficiency, though this may depend on various factors including the targeted yield, the feedstock, the target metals, and the like.

[0202] In certain examples, the separating step comprises gravity separation of the metal laden support material from the target metal-barren solution, wherein at least 50% of the target metal-barren solution is removed. In one example at least 60%, or at least 70%, or at least of 80%, or at least 90%, or at least 95% of the target metal-barren solution is removed.

[0203] In certain examples, the separating step comprises separating the metal laden support material by centrifugation, wherein during the centrifugation at least 50% of the target metal-barren solution is removed from the metal laden support material. In one example at least 60%, or at least 70%, or at least of 80%, or at least 90%, or at least 95% of the target metal-barren solution is removed during centrifugation.

[0204] In certain examples, the separation steps includes filtration and at least one of gravity separation and centrifugation.

[0205] In certain examples the separating step includes drying the support material.

[0206] In certain examples the target metal lixiviant dissolves at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90% of the target metal(s) from the precious metal feedstock. In particular examples, the target metal lixiviant dissolves 95-99.5% of the target metal(s).

[0207] In certain examples the target metal lixiviant solution is a thiourea-based aqueous solution, or a thiosulphate-based aqueous solution, or a thiocyanate-based aqueous solution, or a halogen-based aqueous solution, or an aqua regia-based solution. In particular examples, the target metal lixiviant comprises a chloride source and an oxidant. In particular examples, the target metal lixiviant comprises a water miscible solvent, an oxidant and an acid - as specified by Foley et al in WO2016 / 168933 (incorporated herein by reference).

[0208] In particularly contemplated examples, the target metal lixiviant comprises a halogen solution, or an acetic acid and chlorine, or hydrochloric acid and peroxide. In particularly contemplated examples, for instance when the target metal lixiviant is a thiol-containing compound, the target metal undergoes a ligand exchange step between extraction and binding to the support material.

[0209] In certain examples, the method comprises one or more steps prior to the base metal leach, such as a pre-processing step. Accordingly, in one example the method of recovering one or more target metals comprises prior to the base metal leach a pre-processing step comprising one or more operations selected from the group consisting of: chip removal; grinding, milling, or comminuting electronic waste, for example to a preselected size, for example to provide particulate solid material; removal of certain density fractions; removal of certain size particles or fractions; removal of one or more magnetic materials; removal of at least a portion of non-target material; and any combination of two or more thereof.

[0210] In certain examples, selectivity for the target metal(s) is improved by removing at least a portion of non-target materials prior to the dissolving steps. In particular examples, the method includes a preprocessing step wherein at least a portion of non-target materials are removed prior to the base metal leach.

[0211] In particular examples the pre-processing step comprising one or more operations including but not limited to: chip removal; grinding, milling, or comminuting electronic waste, for example to a preselected size, for example to provide particulate solid material; removal of certain density fractions; removal of certain size particles or fractions; removal of one or more magnetic materials; removal of at least a portion of non-target material; and any combination of two or more thereof. In particular examples, the pre-processing step includes at least one of the above operations. In another example, the pre-processing step includes at least two of the above unit operations. In another example the pre-processing step includes at least three of the above unit operations. In another example, the pre-processing step includes at least four of the above unit operations.

[0212] In particular examples the pre-processing step removes at least 50% of the non-target materials prior to the precious metal recovery, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95%. In particular contemplated examples, the pre-processing step removes less than 20%, or less than 10%, or less than 5%, or less than 2%, or less than 1% of the target metal(s).

[0213] In one example the feedstock is electronic waste comprising, consisting essentially of, of consisting of printed circuit boards (PCBs). In one example the PCBs comprise at least lOppm, or at least 20ppm, or at least 50ppm, or at least lOOppm, or at least 200ppm, or at least 300ppm, or at least 400ppm, or at least 500ppm target metal. Since PCBs contain a plurality of metals including base metals, precious metals and target metals, it is recognised there is substantial benefit in using a lixiviant that dissolves the target metal selectively over other non-target metals. However, since precious metals are typically less reactive than base metals it is recognised that selectivity of dissolution may be low. Hence, the methods disclosed herein enable the binding and recovery of desired target metals through the use of selected processing steps and reagents.

[0214] In one example, the target metal(s) and non-target materials are substantially selectively reduced by the reducing agent in a ratio of greater than 1:1000 (target metal : non-target material), or greater than 1:500, or greater than 1:200, or greater than 1:100, or greater than 1:50, or greater than 1:20, or greater than 1:10, or greater than 1:5, or greater than 1:2, or greater than 1:1. In one example, the target metal(s) and non-target materials are substantially selectively dissolved by the lixiviant in a ratio of greater than 1:1000 (target metal : non-target material), or greater than 1:500, or greater than 1:200, or greater than 1:100, or greater than 1:50, or greater than 1:20, or greater than 1:10, or greater than 1:5, or greater than 1:2, or greater than 1:1.

[0215] In particular examples, the invention comprises at least a partially selective reducing step and an at least partially selective dissolving step. In other examples, the method includes a pre-processing step to remove at least a portion of the non-target materials and at least a partially selective dissolving step.

[0216] In particularly contemplated examples the target metal is gold.

[0217] In various examples, the reducing agent is selected from the group comprising ascorbic acid, hydrazine sulfate, citric acid, and sodium borohydride, and any combination of two or more thereof. In particular examples, the reducing agent comprises, consists essentially of, or consists of ascorbic acid. In particular examples, the lixiviant comprises, consists essentially of, or consists of acetic acid. In another example, for example where target metal(s) and non-target materials are dissolved non-selectively (i.e. , all metals are dissolved), the support materials may selectively bind the target metal(s). In particular examples the target metal(s) and non-target materials are bound by the support materials in a ratio of at least 1:1 (target metal : non-target metal), or at least 5:1, or at least 10:1, or at least 20:1, or at least 50:1 or at least 100:1, or at least 200:1, or at least 500:1, or at least 1000:1, or at least 5000:1, or at least 10000:1. In particular examples, the support materials bind target metal and substantially no non-target metal.

[0218] In certain examples, the steps of the method prior to the recovery step are each performed in the same vessel. In one example, the addition and maintaining steps are performed in the same vessel. An additional separation step may be required to separate the metal laden support material from the target metal-barren solution and remaining solid feedstock material.

[0219] In various examples the method further includes a recycling step wherein at least a portion of the support material is reused, for example, reused in a further binding step.

[0220] In one example the method further includes a recycling step wherein at least a portion oftarget metalbarren solution is reused as lixiviant or partially used as lixiviant in a dissolving step.

[0221] It is appreciated that in certain examples the target metal-barren solution contains metal ions and in some instances may even include limited target metal ions. Accordingly, in some examples the target metal-barren solution is treated to remove excess metal ions or other compounds prior to reuse, such as prior to returning it for use in a leach step. In some examples, at least a portion of the target metal-barren solution is mixed with makeup water prior to returning to a leach step.

[0222] It is further recognised that, where lixiviant is to be reused, one or more additional components may need to be added to the target metal-barren solution such that it can act as a target metal lixiviant and dissolve target metal(s). For example, in certain examples where the target metal lixiviant is thiosulfate or thiourea or chlorine, the active lixiviant agent(s) is at least partially recharged to enable further dissolution of the target metal(s). In certain examples, these additional components are added to the target metal-barren solution prior to returning to a leach step. By way of non-limiting example, additional oxidant and / or additional acid or base and / or additional counter ion is added. Additionally or alternatively, the target metal-barren solution is treated to adjust the pH, the oxygen reduction potential (ORP), the temperature or any other physical properties that might be known to those skilled in the art in order to make it a suitable lixiviant.

[0223] In particular examples, at least 25% of the target metal-barren solution is reused, for example is returned to a dissolving step. In other examples, at least 35%, or at least 45%, or at least 55%, or at least 65%, or at least 75%, or at least 85%, or at least 95% of the target metal-barren solutions is returned to a dissolving step.

[0224] In another example, the invention further includes the step of recovering the target metal(s) from the metal laden support material. In certain examples, the recovery step includes contacting the metal laden support material with a condition which triggers the release of the target metal(s), such as the release of substantially all of the target metal(s), from the support material.

[0225] In one example, the condition is a solution containing a compound that triggers release of the target metal. In one example, the release solution contains one or more of a compound that releases the target metal(s) from the support material. In one example, the release solution contains any one or more of cysteine, thiosulphate, thiourea, hydrochloric acid, oxidant, halide source, and / or oxidant and halide source, or aqua regia.

[0226] In certain examples, the support material is re-used in whole, in significant part or in part with fresh support material. Additionally or alternatively, the condition triggers release of the target metal. By way of example, the release solution comprises conditions that trigger the release of the target metal(s) or metal ions. By way of example, the conditions may be of pH less than 5, or pH less than 4, or pH less than 3, or pH less than 2. Alternatively, the conditions may be between pH 1 and 5, or between pH 2 and 5, or between 2 and 4. By way of further example, the conditions may be pH greater than 8, or pH greater than 9, or pH greater than 10, or pH greater than 11, or pH greater than 12. Alternatively, the pH may be between pH 8 and 13, or between pH 9 and 13, or between 10 and 13. Additionally or alternatively, the conditions may be at an oxidation-reduction potential suitable for release of the target metal(s).

[0227] Alternatively, the recovery step includes burning or chemical dissolution of the metal laden support material to release the target metal(s).

[0228] In certain examples the feedstock is an urban precious metal feedstock comprising a solid material comprising about 10%, less than about 10%, about 5%, less than about 5%, about 1%, less than about 1%, or less than 0.1%, or less than 0.01%, or less than 0.001%, or less than 0.0001% of target metal.

[0229] In certain examples where the target metal is gold, preferably the solid feedstock material is e-waste, or gold bearing ore, or gold bearing sand, or gold bearing clay.

[0230] In certain examples where the target metal is palladium, preferably the solid feedstock material is e-waste, or automotive catalytic converters, or industrial catalysts, or fine chemical catalysts, or palladium bearing ore, or palladium bearing sand, or palladium bearing clay.

[0231] In certain examples where the target metal is platinum, preferably the solid feedstock material is e-waste, or automotive catalytic converters, or industrial catalysts, or fine chemical catalysts, or platinum bearing ore, or platinum bearing sand, or platinum bearing clay. In certain examples where the target metal is rhodium, preferably the solid feedstock material is e-waste, or automotive catalytic converters, or rhodium bearing ore, or rhodium bearing sand, or rhodium bearing clay.

[0232] In certain examples where the target metal is silver, preferably the solid feedstock material is e-waste, or automotive catalytic converters, or industrial catalysts, or fine chemical catalysts, or silver bearing ore, or silver bearing sand, or silver bearing clay.

[0233] In another aspect, there is provided a system for the recovery of target metal from electronic waste, the system comprising:

[0234] (a) optionally a vessel configured for contacting electronic waste with a base metal lixiviant such that at least a portion of one or more non-target metal(s) dissolve to produce a base metal leach solution;

[0235] (b) optionally, a vessel configured for contacting the solid feedstock once the base metal leach solution has been removed with a target metal lixiviant to provide a target metal-pregnant solution;

[0236] (c) optionally, a vessel configured for contacting the target metal-pregnant solution with a ligand exchange solution to converting at least some of the dissolved target metal complex into a target metal complex that is more suitable for one or more of the subsequent steps;

[0237] (d) a vessel configured for contacting a support material with the target metal-pregnant solution under conditions suitable for binding of at least a portion of the target metal(s) to the support material, wherein the support material becomes metal laden, and the target metal-pregnant solution becomes barren (that is, becomes a target metal-barren solution);

[0238] (e) a separator configured for substantially separating the metal laden support material from the target metal-barren solution; and

[0239] (f) optionally, a recovery module configured for recovery of the target metal(s) from the metal laden support material.

[0240] In certain examples, the system includes one or more conduits for removing the base metal leach solution from the vessel in (a), and / or introducing into the vessel in (b) the target metal lixiviant once the base metal leach solution has been removed, and / or one or more conduits for moving the solid feedstock to the vessel in (b), and / or for passing the target metal-pregnant solution from the vessel in (b) to the vessel in (c) and / or (d). In certain examples, the vessel in (a) is the same as or part of the vessel in ( b) , and / or is the same as or part of the vessel in (c), and / or is the same as or part of the vessel in (d). In another example, the system includes one or more conduits for passing the target metal-barren solution containing metal laden support material from the vessel in (e) to the separator (e). In certain examples, the system includes one or more conduits for passing the separated metal laden support material in (e) to the recovery module in (f).

[0241] In certain examples, the method comprises an optional ligand exchange step. In certain examples, the optional ligand exchange step is performed in the same vessel as step (b), for example by introducing a chloride source and optionally an oxidant into the vessel.

[0242] The separator is configured for separating the metal laden support materials by filtration, wherein at least a portion of the target metal-barren solution is removed from the metal laden support material. In certain examples, in addition to at least some filtration, the separator is configured for gravity separating the metal laden support material from the target metal-barren solution wherein at least a portion of the target metal-barren solution is removed from the metal laden support material.

[0243] In certain examples, in addition to at least some filtration, the separator is configured for separating the metal laden support materials by centrifugation, wherein at least a portion of the target metalbarren solution is removed from the metal laden support material;

[0244] In certain examples, the recovery module includes an element for contacting the metal laden support material with a solution.

[0245] In certain examples, the recovery module includes an element for burning the metal laden support material to release the target metal.

[0246] In particular examples, the system includes pre-processing configured to remove at least a portion of the non-target material prior to passing to the dissolution vessel.

[0247] In one example the pre-processing comprises one or more operations including but not limited to: chip removal; grinding, milling, or comminuting electronic waste, for example to a preselected size, for example to provide particulate solid material; removal of certain density fractions; removal of certain size particles or fractions; removal of one or more magnetic materials; removal of at least a portion of non-target material; and any combination of two or more thereof.

[0248] In particular examples, the pre-processing (also referred to herein as the pre-processing step) includes at least one of the above operations. In another example, the pre-processing step includes at least two of the above operations. In another example the pre-processing step includes at least three of the above operations. In another example, the pre-processing step includes at least four of the above operations.

[0249] In particular examples the pre-processing step removes at least 50% of the non-target materials prior to the precious metal recovery, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95%. In particular contemplated examples, the pre-processing step removes less than 20%, or less than 10%, or less than 5%, or less than 2%, or less than 1% of the target metal(s).

[0250] In particular examples of various aspects disclosed herein, the support materials comprises, consists essentially of, or consists of cellulose.

[0251] Additionally or alternatively, a mixture of support material species may be used. In various examples, where a mixture of support materials is used, at least one of the support materials is cellulose.

[0252] In one example the target metal lixiviant comprises a combination of water, acetic acid, hydrochloric acid, hydrogen peroxide (H2O-C2H4O2-HCI-H2O2).

[0253] The invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, in any or all combinations of two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

[0254] To those skilled in the art to which the invention relates, many changes in construction and widely differing examples and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.

[0255] Other aims, aspects, features and advantages of the present invention will become apparent from the following description. It should be understood, however, that the detailed description and the specific examples, while indicating certain particularly contemplated examples of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

[0256] BRIEF DESCRIPTION OF THE FIGURES

[0257] These and other aspects of the present invention, which should be considered in all its novel aspects, will become apparent from the following description and non limiting examples, which is given by way of example only, with reference to the accompanying figures, in which:

[0258] Figure 1 presents five photographs of filter pads comprising different amounts of support material recovered following binding of target metals, as described herein in Example 4, in which: B is sample 25-B; C is sample 25-C; D is sample 25-D; 24-C is sample 24-C; and E is sample 25-E.

[0259] Figure 2 presents a diagram depicting the reuse of support material and reiterative recovery of target metal from solution, as described in Example 6 herein. The concentration of gold in solution is shown in the various samples as follows: 0. Fresh Au solution (27.1 ppm Au); 1. After CFA and ascorbic acid added and sample centrifuged; 2. Filtrate of 1; 3. Fresh Au solution (27.1 ppm Au) added to CFA (lx used) recovered from 1; 4. After ascorbic acid added and sample centrifuged; 5. Filtrate of 4; 6. Fresh Au solution (27.1 ppm Au) added to CFA (2x used) recovered from 4; 7. After ascorbic acid added and sample centrifuged; 8. Filtrate of 7.

[0260] Figure 3 presents a diagram showing characterisation of gold laden cellulose biomass. A) SEM image of sole cellulose support material. B) SEM image of AuNPs loaded on cellulose. C) SEM-EDS mapping confirming nanoparticles on cellulose are gold and highlighting their purity. D) XPS showing gold on biomass exists in its metallic state (oxidation state of 0).

[0261] Figure 4 presents three graphs depicting the results of recovery of gold using TCCA in a method as herein contemplated, as described in Example 4 herein.

[0262] Figure 5 presents three graphs depicting the results of recovery of gold using calcium hydroxide in a method as herein contemplated, as described in Example 5 herein.

[0263] Figure 6 presents a graph depicting a time course of gold recovery in a representative method contemplated herein utilising magnesium hydroxide and calcium hypochlorite as , as described in Example herein.

[0264] Figure 7 presents three graphs depicting the results of recovery of gold using magnesium hydroxide in a method as herein contemplated, as described in Example 7 herein.

[0265] Figure 8 presents a graph depicting the effect of temperature on gold recovery in a representative recovery method as described herein in Example 8.

[0266] DETAILED DESCRIPTION

[0267] The invention relates to methods relating to the recovery of precious and / or desirable metals from feedstocks, including from solution, such as solution formed by the dissolution of solid metal containing feedstock, such as electronic waste, ores, tailings, catalytic converters, and the like. In certain examples the methods comprise the dissolution of one or more species of target metal ions and binding to a support material, followed by separation of the support material and recovery of the target metal. In certain examples, the methods comprise the provision of a solution comprising one or more target metals, in some examples with one or more non-target metals present, and recovery of at least some of the one or more target metals. In other examples, the methods relate to the preparation of a target metal-pregnant solution comprising one or more target metals.

[0268] A wide range of solid feedstocks, of metal-containing solutions, including solutions having a wide range of concentrations of target and non-target metals, and of support materials are amenable to use in the methods disclosed herein. Examples of commercially relevant representative examples, such as those providing loadings of 2% Au on cellulose support material, are exemplified herein. Broadly, in certain examples of the recovery methods contemplated herein, a support material (such as cellulose) is added to a target metal-pregnant solution comprising target metal (gold or other precious metal) ions and fully suspended, typically by mechanical agitation. At commercially relevant scale, the target metal-pregnant solution will generally have from 1 -10 ppm gold to 1000s of ppm target metal and 10s or 100s to 1000s or 10000s or greater ppm of non-target metal. The target metal-pregnant solution and support material is maintained under conditions suitable to allow binding of the target metal to the support material. On the completion of binding, the precious metal-laden support material is conveniently filtered from the solution, which from the perspective of target metal is now barren. In certain examples, the filtrate is tested to determine the concentration of residual precious metal, and optionally the presence or absence of one or more non-target metals or other contaminants, and the target metal is recovered from the target metal-laden support material, for example by burning. However, as disclosed herein, methods to allow for the recovery of target metal and the reuse of support material are disclosed herein, and will in certain circumstances be advantageously employed.

[0269] Selected definitions

[0270] It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7). These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

[0271] Those skilled in the art will appreciate the meaning of various terms of degree used herein. For example, as used herein in the context of referring to an amount (e.g., "about 9%"), the term "about" represents an amount close to and including the stated amount that still performs a desired function or achieves a desired result, e.g. "about 9%" can include 9% and amounts close to 9% that still perform a desired function or achieve a desired result. For example, the term "about" can refer to an amount that is within less than 10% of, within less than 5% of, within less than 1% of, within less than 0.1% of, or within less than 0.01% of the stated amount. It is also intended that where the term "about" is used, for example with reference to a figure, concentration, amount, integer or value, the exact figure, concentration, amount, integer or value is also specifically contemplated.

[0272] The term "and / or" can mean "and" or "or". The term "comprising" as used in this specification means "consisting at least in part of". When interpreting each statement in this specification that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises", and the terms "including", "include" and "includes" are to be interpreted in the same manner.

[0273] The term "consisting essentially of" when used in this specification refers to the features stated and allows for the presence of other features that do not materially alter the basic characteristics of the features specified.

[0274] The term "consisting of" as used herein means the specified materials or steps of the claimed invention, excluding any element, step, or ingredient not specified in the claim.

[0275] The term "contacting" refers to the bringing together, frequently by the mixing of and / or interaction between, two or more entities, such as two or more solutions or substances. One example of this is the contact between a target metal-pregnant solution and a support material. A further example of this is the contact between a lixiviant and a solid feedstock material.

[0276] The term "target metal" includes both elemental metal and ions of a target metal or a plurality of metals. It is recognised that a particular target metal may exist in different ionic states (including elemental form) or a plurality of ionic states in different steps of the methods or parts of the systems disclosed herein. In specifically contemplated examples, the target metal is dissolved or partially dissolved in the solutions employed herein, either as an ion (or ions), salts or complex or elemental form or a combination thereof. Similarly, the target metal may exist in solid form either as an ion (or ions), salts or complex or elemental form or a combination thereof as the context dictates.

[0277] The term "non-target material" refers to materials from the precious metal feedstock (such as, but not limited to electronic waste or automotive catalytic converters) that are not immediately desired for recovery. The non-target material may contain metals that are not target metal(s), and / or non-metals and may include epoxy, ceramics, glass fibre. Non-target material accordingly includes, but is not limited to, base metals. It will be appreciated that, in certain examples disclosed herein relating to iterative recovery of target metals, a first iteration of the method (i.e., the addition, maintaining, and recovery steps of the method) in which a first reducing agent is employed will target the recovery of a first target metal or a combination of target metals, during which one or more other metals present will be considered a "non-target metal", and where a second or subsequent iteration of the method (i.e., a second or subsequent round of addition, maintaining and recovery steps) is performed during which one or more of the metals previously considered a non-target metal becomes the or a target metal. Such examples contemplate the advantageous selectivity of recovery afforded by certain methods disclosed herein.

[0278] The term "base metal" is used interchangeably with "non-target metal" and refers to metal(s) in similar states of matter as per the term "target metal" above, but that are not precious (i.e., target) metals. A non-exhaustive list of examples of base metals include copper, tin, nickel, lead, iron, and zinc, and other first row transition metals including Mn, Cr, V, Co, together with Al and Sn.

[0279] The term "pregnant solution" refers to a solution, such as an aqueous solution, containing one or more species of dissolved target metal. For example, a target metal-pregnant solution contains dissolved target metal(s). In some instances a target metal-pregnant solution also contains at least some undissolved target metal(s) and / or non-target metal.

[0280] The term "target metal-barren solution" refers to a solution, including an aqueous solution, containing a depleted amount of dissolved target metal compared with the target meta I -pregnant solution. It is recognised that in specifically contemplated examples, one or more target metal(s) is substantially or completely absent from the target metal-barren solution.

[0281] The term "target metal recovery agent" as used herein contemplates one or more agents that supports, facilitates or enhances the recovery of target metal from the target metal-pregnant solution, including in combination with one or more other agents, such as a reducing agent, organic acid, or the like. In certain examples, the target metal recovery agent is capable of decomposing an or the active oxidant, or is capable of reducing effectiveness of coordinating ligands, as required.

[0282] In certain examples herein, the target metal recovery agent is capable of reducing effectiveness of coordinating ligands (e.g., hydrogen peroxide to oxidise thiosulfate) or otherwise capable of disrupting target metal ligand binding, for example, binding of one or more target metals such as gold by thiol-containing compounds.

[0283] In certain examples, the target metal recovery agent is a reducing agent when used in conjunction with a target metal lixiviant comprising a chlorine-containing compound (e.g., is a reductant like hydrogen peroxide for chlorine based systems), or a hydroxide source for iodine based systems. For example, the target metal recovery agent may be a reducing agent when present in a target metalpregnant solution comprising chloride ions. One such example of a target metal recovery agent for use with chloride lixiviants is hydrogen peroxide. The terms "reducing agent" and "reductant" and grammatical equivalents thereof refer to an agent or combination of agents capable of reducing a target species, for example, capable of reducing a species of metal ion to elemental metal.

[0284] The terms "bind", "bound", and "binding" and the like, when used in relation to the methods and systems disclosed herein, refers to the association of target metal with a support material(s), including as the context dictates the use of the support material to bind target metal, or the process of metal associating with and / or binding to the support material(s).

[0285] The terms "support" and "support material" refers to a material, typically a biomass-derived material, such as cellulose, to which target metal can be bound. As contemplated herein, when the support material is present in a target metal-pregnant solution comprising target metal in its atomic form (e.g., that resulting from the reduction of target metal ions by a reducing agent), the atomic target metal associates with and / or is bound to the support material. Without wishing to be bound by any theory, the applicants understand that the support material facilitates the formation of nano-particles of target metal, such that atomic target metal is associated with the support material from which it is recoverable at commercially-relevant scale. It may be used in the plural sense for a mixture of support materials.

[0286] The terms "filterable support" and "filterable support material" refer to a support material which can be recovered from solution by filtering, particularly at commercially-relevant scale. It may be used in the plural sense for a mixture of support materials.

[0287] The term "metal laden support material" means a support material that has bound or is associated with one or more target metals. The term "ppm" refers to parts per million and relates to the concentration of an entity (such as a metal or metal ion, a compound, moiety, support material, or the like) in comparison to another entity - that is, the weight:weight ratio of the respective entities. In specific examples contemplated herein, ppm is used in reference to a target or non-target metal in comparison to a solution in which it occurs. The term "decanted" or "decant" or the like refers to the removal of the upper portion of solution from a liquid, or a solid / liquid mixture, for example a solid / liquid mixture in which the solid fraction has been allowed to settle.

[0288] The terms "leachate" or "leach solution" when used herein refers to an aqueous solution in which one or more metals, such as one or more target metal(s), are dissolved. In certain examples of the methods contemplated herein, a leachate or leach solution is formed by addition of a lixiviant to a feedstock. In certain examples, a leachate or leach solution will be a target metal-pregnant solution comprising one or more target metals or target metal ions. In certain examples, a leachate or leach solution will be a base metal leachate or base metal leach solution comprising one or more non-target metals or non-target metal ions.

[0289] The term "ligand" as used herein refers to a moiety, molecule, compound or macromolecule capable of binding to a target. In the context of this disclosure, the term ligand will typically mean a moiety, molecule, compound or macromolecule capable of binding to a target metal, for example to form a target metal complex, such as a target metal complex suitable for binding.

[0290] The term "lixiviant" refers to an aqueous solution that is capable of dissolving metal(s) into an aqueous form. The term "base metal lixiviant" accordingly refers to an aqueous solution that is capable of dissolving one or more base metal(s). The term "target metal lixiviant" accordingly refers to an aqueous solution that is capable of dissolving one or more target metal(s).

[0291] The terms "e-waste" and "electronic waste" refers to electronic waste or waste electrical and electronic equipment (commonly referred to as WEEE).

[0292] The term "PCB" means printed circuit board, a form of electronic waste.

[0293] The terms "selectivity", "selectively", and grammatical equivalents when used in reference to an agent, such as a lixiviant, a leachate, a target metal recovery agent, a reducing agent, or a support material, refers to the ability of the agent to preferentially interact, such as to interact with a specified metal ion or species thereof. For example, when used in reference to a reducing agent, selectivity or selectively refers to the ability of the reducing agent to favourably reduce the one or more specified metal ion species over one or more other metal ion species present (for example one or more non-target metal ion species in a sample and / or solution). In another example, when used in reference to a support material, selectivity or selectively refers to the ability of the support material to favourably bind the one or more specified metal ion species over one or more other metal ion species present (for example one or more non-target metal ion species in a sample and / or solution).

[0294] A "system" as used herein comprises one or more vessels, containers, pipework and other features that would be typically employed to enable the extraction of metals from a feedstock. By way of example, the "system" may include vessels, conduits, pumps, pressure valves, heat exchangers, filters, instrumentation (pressure sensors, flow sensors, pH sensors) and mixing tees (static mixers). The term "urban precious metal feedstock" and grammatical equivalents refers to precious metal-rich materials produced and used in urban environments, including feedstocks such as but not limited to electronic waste and automotive catalytic converters.

[0295] While the following description focuses on particular examples of the invention, namely the recovery of gold and other precious metals from solid feedstock material and / or target metal-pregnant solutions, it should be appreciated that the invention may be applicable to production of alternative target metals as will be known by persons of ordinary skill in the art to which the invention relates. As described herein, in a first aspect, the invention relates to a method of recovering one or more target metals from solid feedstock, the method comprising:

[0296] a) providing solid feedstock, said solid feedstock comprising at least one target metal and one or more non-target metals;

[0297] b) contacting the solid feedstock with a base metal lixiviant in a base metal leach to provide a base metal leach solution, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids, and one or more oxidants;

[0298] c) removing the base metal leach solution to yield a target metal leach (TML) feedstock;

[0299] d) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof;

[0300] e) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution;

[0301] f) optionally adding at least one target metal recovery agent to the target metal-pregnant solution;

[0302] g) recovering at least some of the one or more target metals from the target metal-pregnant solution.

[0303] In another aspect, the invention relates to a method of recovering one or more target metals from solid feedstock, the method comprising:

[0304] a) providing solid feedstock, said solid feedstock comprising at least one target metal and one or more non-target metals;

[0305] b) contacting the solid feedstock with a base metal lixiviant in a base metal leach to provide a base metal leach solution, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids, and one or more oxidants;

[0306] c) removing the base metal leach solution to yield a target metal leach (TML) feedstock;

[0307] d) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof; e) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution;

[0308] f) optionally adding at least one target metal recovery agent to the target metal-pregnant solution;

[0309] g) contacting the target metal-pregnant solution with a support material;

[0310] h) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and I) separating the metal-laden support material from the target metal-barren solution; and j) recovering the at least one target metal from the target metal-laden support material.

[0311] In another aspect, the invention relates to a method of recovering one or more target metals from solid feedstock, the method comprising:

[0312] a) providing solid feedstock, said solid feedstock comprising at least one target metal and one or more non-target metals;

[0313] b) contacting the solid feedstock with a base metal lixiviant in a base metal leach to provide a base metal leach solution, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids, and one or more oxidants;

[0314] c) removing the base metal leach solution to yield a target metal leach (TML) feedstock;

[0315] d) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof;

[0316] e) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution;

[0317] f) adding at least one target metal recovery agent to the target metal-pregnant solution; g) contacting the target metal-pregnant solution with a support material;

[0318] h) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and I) separating the metal-laden support material from the target metal-barren solution; and j) recovering the at least one target metal from the target metal-laden support material.

[0319] Base metal leach

[0320] The present invention in certain examples provides a method of removing target metals from a solution that has undergone a base metal leach (BML), ensuring that the feedstock for the target metal leach (TML) is substantially free of interfering base metals. The inventors have found that the TML process described is particularly effective when the composition of the solid feedstock for TML comprises less than 10% base metals, for instance, less than 5% base metals, as this significantly improves the selectivity and efficiency of the TML steps for target metal recovery. Having a BML prior to the TML is crucial for removing a substantial portion of base metals, such as copper, nickel, iron, and lead, which would otherwise interfere with the TML process. These base metals tend to compete with target metals for leaching reagents and may lead to undesirable side reactions or complex formation, reducing the efficiency of the target metal extraction. By pre-treating the solid feedstock with a BML, the remaining TML feedstock has reduced levels of base metals, ideally under 5%, which enables a more selective dissolution and recovery of target metals.

[0321] In certain examples, such as when the TML feedstock comprises less than 2% base metals, the TML is even further optimised. With base metal content below this threshold, side reactions are minimised to an even greater extent, leading to reduced reagent consumption, increased economic viability, and lower contamination in the final extracted target metals. Additionally, maintaining a low base metal concentration helps achieve a cleaner leach solution in the TML, facilitating downstream processing steps such as biosorption. This is particularly advantageous in high-purity applications, where trace amounts of base metals can compromise product quality.

[0322] This is particularly the case when TCCA is used as a lixiviant. As described herein, the present disclosure recognises that a target metal lixiviant comprising, consisting essentially of, or consisting of TCCA is effective in recovering one or more target metal when the TML feedstock to which it is added comprises less than 10% base metal, for example, comprises less than about 5% base metal, or less than about 2% base metal.

[0323] Since examples of feedstocks contemplated for use herein, such as printed circuit boards and other e-waste, typically comprise a plurality of different metals and alloys, particularly contemplated examples of the present methods provide for separating out some metals prior to precious metal recovery. Those skilled in the art will recognise that most metals have solubility properties based on their electronic configuration, placement on the periodic table, size, hardness / softness as well as other factors. As such, it is possible to substantially dissolve certain metals while leaving others substantially undissolved based on their solubility properties. For example, base metals such as copper, zinc, aluminium, iron, and tin tend to be at least partially soluble in sulfuric acid while precious metals such as gold, palladium, platinum and silver are substantially less soluble. As such, selective dissolution may be used to separate some metals from the feedstock, such as a printed circuit board material.

[0324] By way of non-limiting example, certain base metals may be selectively dissolved and separated from precious metal bearing printed circuit board material using techniques including but not limited to:

[0325] • Kell process (oxidative pyrolysis)

[0326] • Sulphuric acid leach

[0327] • Hydrochloric acid leach

[0328] • Nitric acid leach

[0329] • Ammonia leach

[0330] The Kell process comprises a hydrometallurgical alternative to smelting of concentrates containing base metals and precious metals. The process involves three main steps (commonly on mining ores): 1 - Aqueous pressure oxidation in an acidic sulphate medium to dissolve the base metals.

[0331] 2 - Roasting of the previous materials to improve leaching conditions in the final stage.

[0332] 3 - Leaching of precious metals in chloride media to dissolve the remaining metal contents. The key feature of this process is the separation of the base metal and precious metal chemistries (www.saimm.co.za / Conferences / Pt2010 / 181-186_Liddell.pdf).

[0333] A sulphuric acid leach comprising 2M sulphuric acid and 35% hydrogen peroxide in a 4:1 ratio can be used in a two-step process to selectively dissolve the base metals, including copper, iron, nickel, tin, zinc and aluminium as the most common metals. A solid to liquid ratio of 1:10 was found to be most optimal (Behnamfard et al. Waste Management, 33 (2013) 2345-2363). Furthermore, Kaya (Waste Management, Vol57 (2016) 64-90) incorporated herein by reference, provides a detailed table of base metal lixiviants: mainly sulphuric and oxidant, also a 6M HNO3 system.

[0334] It is also reported that 1 - 6M nitric acid can be used for the dissolution of base metal, particularly copper, lead and tin (plus other common base metals in shredded e-waste). When concentrations of nitric acid of over 4M are used, tin precipitates as metastannic oxide. Mecucci et al (Journal of Chemical Technology and Biotechnology, Vol 77 (2002) 449-457 claim that nitric acid has advantages over sulphuric acid as it forms less precipitates. Furthermore, nitric acid itself is oxidising so requires less or no extra oxidant and has the potential to be regenerated and / or recycled ore easily.

[0335] Fazhul et al (dspace. unimap. edu.my / dspace / bitstream / 123456789 / 7476 / l / Selective%20leaching %20for%20the%20recovery%20of%20Copper.pdf) provide a lixiviant comprising ammonia that selectively leaches certain base metals (particularly copper) in the presence of other common base metals. Additionally or alternatively Sun et al. (Environmental Science and Technology Letters, 49 (2015) 7981-7988) provide a process comprising ammonium (7.55 wt %) with 196 g / L ammonia carbonate with air at room temperature and a solid to liquid ratio of lg / 5 ml can be used to leach to copper, while removing low amounts of other base metals, predominantly zinc (<5% total).

[0336] Following the base metal leach, a target metal leach is performed, resulting in the formation of a target metal-pregnant solution.

[0337] In one example, the base metal lixiviant comprises sulphuric acid. In one example, the base metal lixiviant is sulphuric acid and hydrogen peroxide.

[0338] In one example, the target metal lixiviant comprises a chlorine-containing compound. For example, the target metal lixiviant is selected from the group consisting of trichloroisocyanuric acid (TCCA), dichloroisocyanuric acid (DCCA), sodium trichloroisocyanurate, sodium dichloroisocyanurate, chlorine. In a particularly contemplated example, the target metal lixiviant comprises, consists essentially of, of consists of TCCA.

[0339] In another example, the base-metal lixiviant comprises, consists essentially of, or consists of hydrochloric acid and hydrogen peroxide (HCI-H2O2). This mineral-acid / oxidant combination provides a simple, low-cost, and readily recyclable lixiviant suitable for feedstocks containing a high proportion of copper, nickel, or iron. The system generates soluble metal chlorides efficiently while producing a low-organic effluent. Hydrogen peroxide acts as an in-situ oxidant to form and maintain metal ions in solution while regenerating active chloride. The leachate produced may be readily separated, yielding a solid residue amenable to a target-metal leach with halogen-containing or sulphur-containing lixiviants such as trichloroisocyanuric acid, thiourea, or thiosulphate, as described herein. The absence of organic acids in this formulation also reduces foaming and facilitates closed-loop recycling of the acid solution in continuous operations. In another example, the hydrochloric acid concentration may range from about 1 M to 6 M, for example 2 M to 4 M, and the hydrogen-peroxide concentration may range from about 0.5 M to 3 M, for example 1 M to 2 M. The lixiviant may be prepared using water or recycled acid liquor. The leach may be conducted at temperatures of 25 °C to 70 °C for 0.5 hours to 5 hours, with vigorous stirring to maintain solids in suspension. In some cases, the addition of hydrogen peroxide may be gradual or metered to control exothermicity and maintain oxidation potential between approximately 500 mV and 700 mV versus Ag / AgCl. This system is particularly suitable where rapid removal of copper or nickel is required prior to a halogen-based target-metal leach.

[0340] In a further example, the base-metal lixiviant comprises, consists essentially of, or consists of acetic acid and hydrogen peroxide (CH3COOH-H2O2). This organic-acid oxidising system is milder and more environmentally benign than mineral-acid systems, and can be used for pre-treatment or surface de-metalisation of feedstocks prior to a more aggressive leach. It is particularly effective for dissolving tin and lead oxides while leaving copper and noble metals largely intact. This selectivity makes it useful where a chloride-free downstream process is desirable or where effluent minimisation is a design priority. Following removal of the base-metal leachate, the remaining solid may be subjected to a target-metal leach comprising one or more halogen-containing compounds such as trichloroisocyanuric acid or sodium trichloroisocyanurate, or sulphur-containing compounds such as thiourea or thiosulphate, thereby achieving sequential dissolution of non-target and target metals.

[0341] In examples employing the organic-acid oxidant system, the acetic acid concentration may be between about 1 M and 6 M, for example 2 M to 4 M, and the hydrogen-peroxide concentration may be between about 0.1 M and 3 M, for example 0.5 M to 1.5 M. The temperature may range from 15 °C to 60 °C, for example between 25 °C and 40 °C, with contact times of 2 hours to 12 hours depending on the reactivity of the feedstock. Mild agitation is sufficient; vigorous mixing is typically unnecessary. These conditions enable controlled dissolution of tin, lead, and surface oxides while avoiding substantial attack on copper or noble-metal phases.

[0342] In another example, the base-metal lixiviant comprises, consists essentially of, or consists of hydrochloric acid and acetic acid without an added oxidant. This acid-digestion system provides a controllable dissolution pathway useful for feedstocks where partial de-metalisation is required prior to oxidation or where over-dissolution of the target metal is to be avoided. The acetic acid moderates solution acidity, enhances wetting of polymeric or resinous feedstock components such as circuitboard laminates, and can facilitate dispersion of particulate solids. This leach composition is advantageous for use in counter-current or staged leach circuits, where it may function as an initial digestion step prior to an oxidising leach or a halogen-based target-metal leach.

[0343] In another example, the hydrochloric acid concentration may range from about 1 M to 5 M, and the acetic acid concentration may range from about 1 M to 4 M. The temperature of the leach may be maintained between about 20 °C and 80 °C, for example 40 °C to 60 °C, for a period of 1 hour to 8 hours. Air sparging or agitation may be used to provide limited oxidation through dissolved oxygen. The relatively lower oxidation potential of this system makes it useful for gradual digestion of surface oxides or partially corroded metallic phases, producing a residue suitable for subsequent oxidising leaches or direct target-metal leaching.

[0344] In still other examples, the acid mixture described above is combined with an alternative oxidant selected from nitric acid, ferric chloride, or sodium hypochlorite. Each oxidant may be chosen according to reagent availability, process temperature, or recycling strategy. For instance, ferric chloride provides a self-regenerating oxidant couple that can be cycled repeatedly within a closed leach circuit; nitric acid offers a high oxidation potential beneficial for refractory oxides; and sodium hypochlorite can be generated in situ from brine to reduce reagent cost. These oxidised chloride systems are compatible with subsequent target-metal leaches employing halogen- or sulphur-containing lixiviants, since both leaches operate within a chloride-rich aqueous medium that stabilises target-metal complexes and promotes efficient dissolution.

[0345] Where an alternative oxidant is employed, representative conditions are as follows:

[0346] • Ferric chloride (FeCIs) concentration between 0.1 M and 2 M, for example 0.3 M to 1 M;

[0347] temperature between 30 °C and 70 °C.

[0348] • Nitric acid (HNOs) concentration between 0.1 M and 3 M, for example 0.5 M to 2 M, with total acidity adjusted to a pH of about 0 to 1; temperature between 25 °C and 60 °C.

[0349] • Sodium hypochlorite (NaOCI) concentration between 0.05 M and 1 M, for example 0.1 M to 0.5 M; temperature maintained below 50 °C to limit chlorine evolution.

[0350] The hydrochloric-acetic medium may be retained within the ranges described above. Contact times may be 1 hour to 6 hours. Each of these oxidant systems may be integrated into a closed or semi-closed leach circuit, with redox potential typically maintained between 400 mV and 800 mV versus Ag / AgCI to sustain metal dissolution while minimising formation of volatile chlorine.

[0351] Each of the foregoing lixiviant systems can be integrated into the broader recovery methods described herein. In particular, these acid-oxidant leaches produce a target-metal-leach feedstock containing less than about 10% by weight of base metals, which can subsequently be contacted with a targetmetal lixiviant selected from one or more halogen- or sulphur-containing compounds as described elsewhere in this specification. Residual chloride or organic acid species from the base-metal leach may in some examples enhance subsequent dissolution of noble metals by stabilising chloro- or thiometal complexes. Conversely, where a chloride-free target-metal leach is preferred (for example, thiosulphate or thiourea systems), the milder acetic-acid / hydrogen-peroxide base-metal leach may be selected to minimise chloride carry-over. The ability to tailor the leach sequence provides flexibility to accommodate a wide range of feedstock compositions while optimising reagent economy, selectivity, and environmental performance.

[0352] In alternative examples, the base-metal leach (BML) may be carried out using a hot sulphuric-acid system configured to dissolve transition-metal sulphides, oxides, or intermediate compounds present in the solid feedstock. In these examples, the base-metal lixiviant comprises, consists essentially of, or consists of sulphuric acid and water, optionally containing one or more oxidising agents such as oxygen, air, hydrogen peroxide, ferric ions, manganese dioxide, or nitrate ions. The acid leach provides selective dissolution of copper, nickel, cobalt, iron, and zinc, yielding a base-metal-rich sulphate liquor while leaving a solid residue enriched in gold, silver, and platinum-group metals suitable for subsequent target-metal leaching.

[0353] In representative examples, the sulphuric-acid concentration may range from 50 g L1to 500 g L1, for example 100-300 g L1, and the leach may be performed at temperatures between 50 °C and 260 °C, for example 80 °C to 180 °C, under either atmospheric or autoclave conditions. The oxidant (for example, air, O2, or H2O2) may be supplied to maintain an oxidation-reduction potential (ORP) of 400-700 mV versus Ag / AgCI, depending on the mineralogy of the feedstock. The solid-to-liquid ratio may be adjusted between 5 wt % and 30 wt %, such as, for example, 10-20 wt %, to ensure good slurry suspension and reaction kinetics. Leach durations may range from 0.5 hours to 8 hours for atmospheric leaching and up to 2 hours for high-temperature pressure leaching. Where heat treatment or conditioning is required, the residue may be subjected to a thermal stage at 300-700 °C in air or inert atmosphere to decompose jarosites, remove carbonaceous materials, or oxidise persistent sulphides.

[0354] The sulphate-based BML offers several advantages when with a halogen or solvometallurgical targetmetal leach. The process effectively removes reactive base metals that can consume halide oxidants or interfere with subsequent precious-metal dissolution, while converting sulphide minerals to stable, non-acid-generating sulphates. The moderate acidity and high selectivity minimise the risk of leaching noble metals at this stage, thereby improving downstream recovery efficiency. Sulphate liquors produced in this step can be readily neutralised, treated for metal recovery, or recycled within a closed loop to reduce reagent consumption.

[0355] In certain examples, the sulphuric-acid leach may be followed by a controlled conditioning step in either acid or alkali medium. For example, the slurry may be conditioned in sulphuric acid at a concentration of 25-300 g L1to maintain an acidic environment, or alternatively in an alkaline medium adjusted to a pH of 10-14 using calcium or sodium hydroxide. The alkaline conditioning stage neutralises excess acidity, decomposes jarosites, and improves the permeability and porosity of the leach residue prior to the subsequent target-metal leach. Alternatively, a mild oxidative pre-roast or reductive calcine may be applied to enhance liberation of precious-metal inclusions before the targetmetal leach.

[0356] The ability to operate this sulphate base-metal leach across a wide range of temperatures and acid strengths enables flexible integration with the previously described halogen-based target-metal leaches. In particular, residues derived from this BML exhibit reduced concentrations of copper, nickel, and iron and are therefore highly compatible with chloride, bromide, or non-aqueous halogen lixiviants used in the target-metal leach (TML) steps disclosed herein. This combination enhances selectivity, reduces reagent consumption in the TML, and improves overall meta I -recovery efficiency while maintaining a closed, low-emission hydrometallurgical circuit.

[0357] Target metal leach

[0358] In various examples, following the base metal leach, the target metal leach comprises the addition to the feedstock of the target metal lixiviant and the maintenance of the target metal leach for a time of from about 30 minutes to about 24 hours. For example, the target metal leach is maintained for a time of from about 30 minutes to about 18 hours, from about 30 minutes to about 15 hours, from about 30 minutes to about 12 hours, from about 30 minutes to about 9 hours, or from about 30 minutes to about 6 hours.

[0359] In one particularly contemplated example, following the base metal leach, the target metal leach comprises the addition to the feedstock of a target metal lixiviant comprising TCCA and the maintenance of the target metal leach for a time of from about 30 minutes to about 24 hours. For example, the target metal leach is maintained for a time of from about 30 minutes to about 18 hours, from about 30 minutes to about 15 hours, from about 30 minutes to about 12 hours, from about 30 minutes to about 9 hours, or from about 30 minutes to about 6 hours.

[0360] In various examples, such as following the base metal leach, the target metal leach comprises the addition to the feedstock of the target metal lixiviant wherein the conditions under which the target metal leach is maintained comprise an acidic pH. For example, the target metal leach is maintained at a pH of from about 3 to about 7.

[0361] Maintaining the pH within the range of 3 to 7 for the target metal leach (TML) is critical for controlling the form and activity of chlorine species during the reaction, which significantly impacts both the efficiency and safety of the leach process. Specifically, a pH within this range stabilises chlorine as hypochlorous acid (HOCI), which is the active oxidant that promotes efficient target metal leaching. If the pH drops below 3, chlorine tends to convert to CI2 gas, which is both hazardous and less effective in solution for the intended reactions. Conversely, at a pH above 7, the chlorine species would primarily shift to hypochlorite ions (OCI ), which do not have the desired oxidation potential for target metal leaching and could reduce extraction efficiency.

[0362] The examples illustrate the optimal pH range of 3 to 7, specifically validating the efficacy of trichloroisocyanuric acid (TCCA) in the TML at this pH range. Within this range, the oxidation-reduction potential (ORP) typically stabilises between approximately 850 and 1200 mV, creating a strong oxidising environment necessary for target metal dissolution while also retaining chlorine equivalents in solution as hypochlorous acid. This optimal ORP range, facilitated by the pH, supports the selective dissolution of target metals without excessive side reactions or loss of chlorine gas, which can reduce efficiency.

[0363] The timing of base additions during the TML is mainly to maintain this pH window, keeping the reaction conditions in an optimised state. For instance, at the one-hour mark, the pH may drop to approximately 2.0 due to ongoing reactions. At this point, a controlled addition of base raises the pH slightly, maintaining it within a narrower range of 1.5 to 3.0. As the reaction progresses to around 2.5 hours, a further base addition helps to adjust the pH into the broader optimal range of 4 to 8, balancing the reaction speed and safety while maintaining effective hypochlorous acid concentration. Additionally, the ORP, which is typically around 1100-1200 mV at one hour, tends to stabilise between 850 and 950 mV at 2.5 hours. This gradual decrease in ORP reflects the ongoing consumption of oxidants in the reaction while ensuring effective leaching conditions are sustained.

[0364] By managing the pH and ORP in this way, the TML process maintains all chlorine species in solution as effective oxidants, avoids gas-off of chlorine, and creates an environment optimised for the dissolution of target metals. This approach not only maximises the efficiency of the TML but also ensures safe and controlled reaction conditions, even with varying base additions over time.

[0365] It will be appreciated that maintenance of a desired pH encompasses the addition of one or more pH-modifying agents, such as a pH increasing agent, such as a base or an agent able to generate hydroxide ions in solution, and / or a pH decreasing agent, such as an acid or an agent able to generate H_ions in solution.

[0366] In one example, the method comprises a TML step comprising addition of a pH increasing agent. The pH increasing agent may be selected from suitable agents to increase pH including magnesium hydroxide, calcium hydroxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide, sodium carbonate, sodium bicarbonate, magnesium oxide, and calcium hypochlorite or precursors of any one thereof.

[0367] In one particularly contemplated example, following the base metal leach, the target metal leach comprises the addition to the feedstock of a target metal lixiviant comprising TCCA and the maintenance of the target metal leach at a pH of from about 3 to about 7. For example, the target metal leach comprises the addition to the feedstock of a target metal lixiviant comprising TCCA and the maintenance of the target metal leach at a pH of from about 3 to about 7 by one or more additions of a pH increasing agent. In one example, the pH increasing agent is magnesium oxide. In the context of using a halogen such as trichloroisocyanuric acid (TCCA) in the TML process, the inventors have found that temperature control between approximately 30°C and 45°C plays an important role in optimising both yield and reaction efficiency. The examples within this specification demonstrate that maintaining the TML at moderate temperatures between about 30-35°C maximises gold recovery, with yields peaking at approximately 92% over a reaction time of two days. This temperature range ensures that TCCA remains in its most reactive and stable form as hypochlorous acid, effectively maintaining the required oxidative potential without excessive off-gassing or degradation of the oxidant.

[0368] At elevated temperatures between about 40-45°C, while the reaction time is significantly reduced (3 to 6 hours), the yield is observed to moderate around 70-73%. However, the faster reaction rate at these higher temperatures is advantageous for scenarios where shorter process times are preferred, though it may require additional pH control to prevent rapid shifts that could otherwise lead to increased chlorine off-gassing and reduced efficiency.

[0369] The examples provided in this specification support the efficacy of these temperature-optimised ranges for TCCA, highlighting the balance that can be achieved between reaction time and yield. This balance allows the process to be adapted according to specific operational needs, either prioritising maximum recovery in a moderate temperature range or faster processing at elevated temperatures. Overall, the data indicate that the selected temperature range (30-45°C) provides a controlled environment for the TCCA-driven TML, sustaining an ideal oxidative potential and pH stability, thus enhancing the overall efficiency of the metal extraction process.

[0370] Accordingly, in one example, the method of recovering one or more target metals from solid feedstock comprises:

[0371] a) providing solid feedstock, said solid feedstock comprising at least one target metal and one or more non-target metals;

[0372] b) contacting the solid feedstock with a base metal lixiviant in a base metal leach to provide a base metal leach solution, wherein the base metal lixiviant comprises, consists essentially of, or consists of sulphuric acid and hydrogen peroxide;

[0373] c) removing the base metal leach solution to yield a target metal leach (TML) feedstock;

[0374] d) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant comprising, consisting essentially of, or consisting of TCCA;

[0375] e) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution; f) adding at least one target metal recovery agent to the target metal-pregnant solution, wherein the target metal recovery agent is hydrogen peroxide;

[0376] g) contacting the target metal-pregnant solution with a support material;

[0377] h) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and I) separating the metal-laden support material from the target metal-barren solution; and j) recovering the at least one target metal from the target metal-laden support material.

[0378] In another example, the method of recovering one or more target metals from solid feedstock comprises:

[0379] a) providing solid feedstock, said solid feedstock comprising at least one target metal and one or more non-target metals;

[0380] b) contacting the solid feedstock with a base metal lixiviant in a base metal leach, wherein the base metal lixiviant comprises, consists essentially of, or consists of sulphuric acid and hydrogen peroxide;

[0381] c) contacting at least a portion of the resulting base metal leach solution with hydrogen peroxide in a second base metal leach to provide a second base metal leach solution, wherein the second base metal leach solution comprises dissolved metal ions of at least one non-target metal;

[0382] d) removing the second base metal leach solution to yield a target metal leach (TML) feedstock; e) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant comprising, consisting essentially of, or consisting of TCCA;

[0383] f) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution; g) adding at least one target metal recovery agent to the target metal-pregnant solution, wherein the target metal recovery agent is hydrogen peroxide;

[0384] h) contacting the target metal-pregnant solution with a support material;

[0385] i) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and j) separating the metal-laden support material from the target metal-barren solution; and k) recovering the at least one target metal from the target metal-laden support material.

[0386] In particularly contemplated examples, such as examples above, the target metal leach is maintained: a) a pH of from about 3 to 7; or

[0387] b) for a time of about 1 to 6 hours; or

[0388] c) a temperature of from about 30 °C to about 45 °C; or

[0389] d) any two or more of a) to c); or

[0390] e) each of a) to c). In a further particularly contemplated example, the pH is maintained by addition, including periodic addition, of a pH increasing agent.

[0391] In another aspect, the invention relates to a method of recovering one or more target metals from a feedstock from which one or more non-target metals have been removed, for example in a base metal leach as herein described.

[0392] In one example, the feedstock is a target metal-pregnant solution, such as a target metal-pregnant solution prepared in a method as herein described.

[0393] In one example, the invention relates to a method of recovering one or more target metals from a solid feedstock from which one or more non-target metals have been removed, for example in a base metal leach as herein described.

[0394] In certain examples, the solid feedstock is a TML feedstock as herein contemplated.

[0395] In one example, the target metal leach (TML) feedstock comprises less than 10% by weight base metals.

[0396] In one example, the TML feedstock comprises less than 5% by weight base metals.

[0397] In another example, the TML feedstock comprises less than 2% by weight base metals.

[0398] In one example, the target metal leach (TML) feedstock comprises less than 10% by weight of the base metals originally present in the solid feedstock.

[0399] In one example, the TML feedstock comprises less than 5% by weight of the base metals originally present in the solid feedstock.

[0400] In another example, the TML feedstock comprises less than 2% by weight of the base metals originally present in the solid feedstock.

[0401] Accordingly, in one example the invention relates to a method of recovering one or more target metals from a TML feedstock, the method comprising:

[0402] a) providing a TML feedstock, said TML feedstock comprising at least one target metal and from which one or more non-target metals has been removed;

[0403] b) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof;

[0404] c) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution; d) optionally adding at least one target metal recovery agent to the target metal-pregnant solution; and

[0405] e) recovering at least some of the one or more target metals from the target metal-pregnant solution.

[0406] Accordingly, in one example the invention relates to a method of recovering one or more target metals from a TML feedstock, the method comprising: a) providing a TML feedstock, said TML feedstock comprising at least one target metal and from which one or more non-target metals has been removed;

[0407] b) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof;

[0408] c) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution; d) adding at least one target metal recovery agent to the target metal-pregnant solution; e) contacting the target metal-pregnant solution with a support material;

[0409] f) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and g) separating the metal-laden support material from the target metal-barren solution; and h) recovering the at least one target metal from the target metal-laden support material.

[0410] As discussed herein, the methods contemplated here are suitable for recovering metals from aqueous solutions containing metal ions as well as from solid feedstock materials. In particular, the present invention provides methods for recovering metals from aqueous solutions in a manner that has a number of cost and environmental advantages over existing methods.

[0411] In a particular aspect there is provided method of recovering one or more target metals from a solution, wherein the solution comprises dissolved metal ions of at least one target metal, the method comprising:

[0412] a) providing a solution comprising dissolved metal ions of at least one target metal, wherein the solution has been prepared in a method comprising:

[0413] i. contacting a solid feedstock comprising one or more target metals and one or more non-target metals with a base metal lixiviant, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids, one or more oxidants; II. contacting at least a portion of the resulting base metal leach solution with hydrogen peroxide in a second base metal leach to provide said solution comprising dissolved metal ions of at least one target metal and dissolved metal ions of at least one non- target metals;

[0414] ill. removing the second base metal leach solution to yield a target metal leach (TML) feedstock;

[0415] iv. adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; cyanide; and any combination of any two or more thereof; v. maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metalpregnant solution;

[0416] b) adding at least one target metal recovery agent to the target metal-pregnant solution; c) contacting the target metal pregnant solution with a support material;

[0417] d) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and e) separating the metal-laden support material from the target metal-barren solution; and f) recovering the at least one target metal from the target metal-laden support material.

[0418] In various examples, the base metal leach solution, or the second base metal leach solution when produced, may be at least in part reused or recycled, for example, in a further iteration of one of the methods described herein.

[0419] In accordance with the methods disclosed herein, a target metal leach is done in which dissolution of target metal using a lixiviant solution occurs, thereby yielding a target metal-pregnant solution. By way of non-limiting example, in one example when gold is the target metal, the target meta I -pregnant solution is produced by dissolving the target metal(s) in a lixiviant comprising a thiourea-based solution, or a thiosulphate-based solution, or a thiocyanate-based solution, or a halogen-based solution, and examples of suitable conditions can be found in Aylmore, Developments in Mineral Processing 15, pp 501-539 (2005) and references therein.

[0420] Lixiviant systems suitable for dissolving one or more target metals (referred to herein as target metal lixiviants) act at different rates, pH's, temperatures, and ORPs, and those skilled in the art will appreciate how the conditions may be optimised to ensure the target metal(s) are dissolved efficiently. Examples of suitable lixiviant systems include:

[0421] Water and Chlorine

[0422] A lixiviant system of water as the solvent plus chlorine gas can be used to dissolve gold. This system is not selective but will dissolve most metals. (A Straightforward Route to Tetrachloroauric Acid from Gold Metal and Molecular Chlorine for Nanoparticle Synthesis. doi:10.3390 / met5031454)

[0423] Water miscible solvent, oxidant and chloride

[0424] According to WO 2016 / 168933, herein incorporated by reference, a lixiviant system comprising glacial acetic acid as the solvent, an oxidant and a chloride source can be used to dissolve gold selectively. For example, acetic acid, hydrochloric acid and chlorine gas was shown to selectively dissolve gold from printed circuit boards. Similarly, a lixiviant system of glacial acetic acid as the solvent plus hydrochloric acid plus calcium hypochlorite can be used to dissolve gold selectively. Additionally, a lixiviant system of glacial acetic acid as the solvent plus hydrogen peroxide plus hydrochloric acid plus calcium chloride can be used to dissolve gold selectively.

[0425] Potassium iodide and iodine

[0426] A lixiviant system of water as the solvent plus iodine plus potassium iodide can be used to dissolve gold in accordance with US 3957505. Thiourea

[0427] A lixiviant system of thiourea plus ferric ions in a water solvent with a pH between 1-3 can be used to dissolve gold. (Alternative Lixiviants to Cyanide for Leaching Gold Ores, DOI: 10.1016 / S0167-4528(05)15021-2)

[0428] Thiosulfate

[0429] A lixiviant system of thiosulfate plus ammonia plus copper (II) can be used to dissolve gold.

[0430] (Alternative Lixiviants to Cyanide for Leaching Gold Ores, DOI: 10.1016 / 50167-4528(05)15021-2) Aqua regia

[0431] A lixiviant system of nitric acid plus hydrochloric acid volume ratio of approximately 1:4 can be used to dissolve gold. (Cyanide and Other Lixiviant Leaching Systems for Gold with Some Practical Applications, DOI: 10.1080 / 08827509508914125)

[0432] Cyanide

[0433] A lixiviant system of sodium cyanide (0.02-0.1%) in a water solvent with a pH between 10-11 that is saturated with air can be used to dissolve gold. (Cyanide and Other Lixiviant Leaching Systems for Gold with Some Practical Applications, DOI: 10.1080 / 08827509508914125).

[0434] In particular examples, the target metal lixiviant system may be used to selectively dissolve target metal while leaving non-target metal and / or non-target material substantially undissolved or dissolved to a smaller degree.

[0435] In particular examples, the target metal(s) and non-target metals are substantially selectively dissolved by the lixiviant in a ratio of greater than 1:1000 (target metal : non-target metal), or greater than 1:500, or greater than 1:200, or greater than 1:100, or greater than 1:50, or greater than 1:20, or greater than 1:10, or greater than 1:5, or greater than 1:2, or greater than 1:1.

[0436] According to WO2016 / 108933, systems that use acetic acid show greater selectivity towards gold than systems that use other solvents such as water. One such system is the glacial acetic acid, hydrochloric acid and calcium hypochlorite system. This lixiviant showed a molar ratio of 1 part gold to 1.32 parts copper and 0.87 parts nickel at 4 minutes where all the gold is dissolved, starting with an input of 1 part gold to 133.6 parts copper and 38 parts nickel. Another system is glacial acetic acid plus hydrochloric acid plus chlorine, this system showed a molar ratio of 1 part gold to 2.4 parts copper and 0.6 parts nickel at 1 minute where all the gold is dissolved starting with an input of 1 part gold to 138.7 parts copper and 46.6 parts nickel.

[0437] In particular examples, the feedstock material / lixiviant mixture may need to be gently heated to over 30 °C, or over 40 °C or over 50 °C to assist with dissolution of the target metals. Similarly, the mixture may be agitated, sonicated, vibrated or otherwise treated to assist with dissolution.

[0438] In alternative examples, the target-metal leach may be conducted using an acid-oxidant system comprising a mixture of organic and mineral acids together with hydrogen peroxide. In one example, the target-metal lixiviant may comprise, consist essentially of, or consist of water, acetic acid, hydrochloric acid, and hydrogen peroxide (H2O-CH3COOH-HCI-H2O2). Such compositions promote dissolution of target metals including gold, silver, palladium, and platinum from complex solid feedstocks such as printed circuit boards or automotive catalytic converter powders, while also enabling partial removal of residual base metals.

[0439] In use, the solid feedstock may be contacted with the acid-oxidant lixiviant at a temperature between about 20 °C and 70 °C for a period of 1-6 hours, optionally under agitation sufficient to maintain suspension of solid material and promote oxidation of exposed metallic phases. Hydrogen peroxide acts as an oxidising agent to regenerate chloride ions present in the solution, maintaining leaching activity and redox potential. The resulting target-metal leach solution typically contains ionic species of Au3+, Ag+, Pd2+, and Pt2+and may be separated from the solid residue, which is substantially depleted in these target metals and enriched in inert components.

[0440] In certain examples, this acid-oxidant target-metal lixiviant can be used in place of, or in combination with, other halogen- or sulphur-containing lixiviant systems described herein. In still further examples, the H2O-CH3COOH-HCI-H2O2 lixiviant may serve as an initial target-metal leach in a multi-stage process, wherein a subsequent halogen-, sulphur-, or solvometallurgical leach is performed according to the methods disclosed herein.

[0441] In certain examples, the hydrochloric acid concentration in the lixiviant may be between about 2 M and 5 M, for example between 3 M and 4.5 M. The acetic acid concentration may be between about 2 M and 6 M, for example between 2 M and 3 M, while the hydrogen-peroxide concentration may be between about 1 M and 4 M, for example between 1 M and 2 M. The temperature of the leach may be maintained between about 20 °C and 70 °C, for example between 40 °C and 60 °C, and the leaching time may range from 1 hour to 6 hours depending on the particle size of the solid feedstock and the degree of metal dissolution desired.

[0442] In alternative examples, the target-metal leach (TML) may be performed using a non-aqueous or water-miscible solvometallurgical lixiviant comprising one or more halide salts and one or more oxidising agents dissolved in an organic solvent such as 3-methoxy-3-methyl-l-butanol (MMB). In particular examples, the lixiviant may comprise, consist essentially of, or consist of MMB, lithium bromide, and lithium bromate, optionally together with elemental bromine or hydrogen peroxide as an auxiliary oxidant. This system advantageously dissolves gold, silver, and platinum-group metals via formation of halide-complex ions under strongly oxidising conditions while operating in a recyclable, closed-loop solvent medium.

[0443] In other examples, the solvometallurgical target-metal leach may employ organic solvents and copper-halide oxidants, for example dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile, methanol, or ethanol containing cupric bromide, cupric chloride, or bromine as the halogen source and oxidising agent. Such organic halide systems operate under similar principles to the MMB-halide-oxidant leaches described above but rely on the redox couple Cu2+ / Cu+to maintain dissolution potential. These solvents are water-miscible and can form biphasic systems with aqueous process streams, facilitating integration with downstream biosorption or solvent-exchange recovery steps. The use of copper-halide oxidants provides rapid and selective dissolution of precious metals as AuBr4 or AuCl4 complexes under moderate conditions (typically 20 °C - 90 °C) while allowing regeneration of the oxidant in situ by aeration or hydrogen-peroxide addition.

[0444] In representative examples, the solvometallurgical target-metal lixiviant may comprise, consist essentially of, or consist of one or more halide salts and one or more oxidising agents dissolved in an organic solvent, for example 3-methoxy-3-methyl-l-butanol (MMB) or an organic solvent selected from dimethylformamide, dimethyl sulfoxide, acetonitrile, methanol, or ethanol. The halide component may be selected from lithium bromide, sodium bromide, potassium bromide, lithium chloride, sodium chloride, cupric bromide, cupric chloride, or mixtures thereof, while the oxidant may be selected from lithium bromate, lithium chlorate, bromine, chlorine, hydrogen peroxide, sodium chlorite, sodium perchlorate, lithium perbromate, or the in-situ oxidation of cuprous halides to cupric halides. The use of paired halide-oxidant reagents provides a high oxidation-reduction potential while maintaining halogen species in a stable solvated form, enabling efficient dissolution of gold, silver, and platinumgroup metals in a closed-loop, regenerable solvent system.

[0445] The solvometallurgical leach is advantageously conducted after completion of a base-metal leach (BML) carried out according to any of the aqueous acid-oxidant systems described herein, for example a sulphuric-acid / hydrogen-peroxide leach or an H2O-CH3COOH-HCI-H2O2 leach. Performing the solvometallurgical TML on the residue from the aqueous BML provides enhanced selectivity, as the majority of copper, nickel, tin, and lead are removed prior to exposure to the halogen-based solvent. The precious-metal residue can therefore be treated under milder halogen conditions without excessive consumption of oxidant or halide and without co-extraction of base-metal contaminants. In representative examples, the solvometallurgical target-metal leach solution may be prepared to contain one or more halide salts, for example lithium bromide at 1 - 3 mol L1or more generally 0.5 -4 mol L1, together with one or more oxidising agents such as lithium bromate at 0.1 - 0.5 mol L1or more broadly 0.05 - 1 mol L L In other examples, the concentration of cupric halide may range from 0.05 mol L1to 1 mol L1, for example 0.1 - 0.5 mol L L The oxidation-reduction potential (ORP) of the system is maintained above 600 mV versus Ag / AgCI, typically stabilising between 650 mV and 900 mV, and the pH of the reaction medium is controlled between 1 and 4, for example 1 to 3. The leach may be performed at temperatures of 20 °C to 90 °C, for example 40 °C to 70 °C, for a period of 0.5 to 48 hours, most commonly 8 to 24 hours, with agitation sufficient to maintain a uniform slurry and ensure contact between the solid residue and solvent. The pulp density is maintained below 25 wt %, for example about 10 wt %, to promote mass transfer and minimise viscosity. After leaching, the mixture may be filtered to separate the target-metal-pregnant solvent, which may be recycled or regenerated for subsequent leaching cycles, while the solid residue is washed and either discarded or reused as a carrier medium.

[0446] Following the solvometallurgical target-metal leach, the target-metal-pregnant solvent may be contacted with a biosorbent or solid support material as described elsewhere in this specification. In certain examples, the solvent phase may be mixed with an aqueous or biphasic system containing a cellulose-based support, chitosan, lignocellulose, or other biopolymer, optionally in the presence of a mild reducing agent such as ascorbic acid. Under these conditions, dissolved metal complexes can transfer from the organic phase to the support surface, where reduction or ion-exchange binding occurs. This approach provides a selective, low-energy recovery step compatible with both the MMB-halide-oxidant and copper-halide-solvent systems, minimising chemical waste while concentrating the target metals onto a recoverable solid phase. In certain exemplary examples, the biosorption step may be carried out by contacting the metal-laden solvometallurgical liquor with the support material at a support-to-metal mass ratio of 100:1 to 400:1, for example about 200:1, at ambient to 50 °C for 1 - 3 hours under gentle agitation. The resulting metal-laden support material may then be separated by filtration or gravity settling, washed, and processed in accordance with the recovery modules described elsewhere herein. These concentration and temperature ranges balance dissolution efficiency and solvent stability. They are expected to provide high extraction yields (typically > 95 % for gold and palladium) while limiting halogen volatilisation. At lower halide concentrations (< 1 mol Lx) or below 40 °C, reaction rates decrease but selectivity improves, which may be desirable when treating mixed feedstocks.

[0447] Conversely, higher oxidant concentrations or temperatures above 70 °C accelerate leaching kinetics and may be favoured where short residence times are advantageous. The ability to tune these parameters allows either the MMB-halide-oxidant or copper-halide-organic-solvent leach to be integrated flexibly with the preceding aqueous base-metal leach and subsequent recovery steps described elsewhere in this specification.

[0448] In alternative examples, the target-metal leach (TML) may be carried out using an aqueous chloride lixiviant configured to dissolve precious and platinum-group metals from the solid residue obtained following a base-metal leach. In such examples, the target-metal lixiviant comprises, consists essentially of, or consists of hydrochloric acid or a chloride brine together with an oxidising agent selected from chlorine gas, hypochlorous acid, sodium hypochlorite, hydrogen peroxide, or nitrate ions. The solid feedstock may include residues from the previously described sulphuric-acid or acidoxidant base-metal leach, typically enriched in gold, silver, and platinum-group metals. Under these conditions, the chloride medium forms stable chloro-complexes such as [AuCk] , [PdCle]2, [PtCle]2, and [AgCI?] , enabling efficient dissolution of precious metals in a cyanide-free process.

[0449] In representative examples, the chloride concentration of the lixiviant may range from 1 mol L-1to 10 mol L-1, for example 2 - 6 mol L-1, corresponding to approximately 35 - 200 g L-1Cl depending on the chloride source. The oxidant may be supplied at concentrations sufficient to maintain an oxidation-reduction potential (ORP) of 600 - 950 mV versus Ag / AgCI, with oxygen, chlorine, or hydrogen peroxide continuously sparged or dosed during the leach. The temperature may be maintained between 50 °C and 150 °C, for example 80 °C - 120 °C, and the pH controlled between 0 and 7.5, for example 0 - 4 for maximum dissolution efficiency. The pulp density may be adjusted to 5 - 25 wt %, most commonly 10 - 15 wt %, with agitation sufficient to keep solids in suspension. Leach durations may range from 0.5 hours to 24 hours, depending on feed composition and particle size. Following leaching, the mixture is filtered to yield a target-metal-pregnant chloride solution and a leach residue depleted in precious metals.

[0450] This chloride-based TML offers several advantages when used after a preceding sulphuric-acid or acidoxidant BML. The removal of copper, nickel, and iron in the earlier stage prevents excessive reagent consumption during the chloride leach and avoids co-precipitation of base-metal chlorides. The aqueous chloride system operates under relatively mild conditions and provides high leach selectivity (> 95 % Au and Ag extraction) while permitting straightforward reagent regeneration. The use of hydrogen peroxide or sodium hypochlorite as oxidants eliminates the need for gaseous chlorine handling, simplifying plant safety and environmental management. The chloride solution can be regenerated via air sparging, electrolysis, or oxidation of residual ferrous ions to ferric chloride, enabling closed-loop reagent recycling.

[0451] In certain examples, the chloride leach may be followed by a biosorption or ion-exchange recovery step as described elsewhere herein. The target-metal-pregnant chloride liquor may be contacted with a biosorbent or functionalised support material such as cellulose, lignocellulose, chitosan, or ionexchange resin under mild agitation to recover precious metals via adsorption, ion exchange, or reduction. The metal-laden support material may then be separated, washed, and processed for metal recovery, while the chloride solution is recycled for further leaching. This integration of the chloride leach with biosorption recovery enables a low-emission, cyanide-free hydrometallurgical circuit combining high extraction yield with sustainability.

[0452] At lower chloride concentrations (< 2 mol Lx) or temperatures below 60 °C, reaction kinetics slow but selectivity increases, which may be desirable when treating mixed-metal residues. Conversely, higher chloride strengths or temperatures above 100 °C accelerate leaching rates and are advantageous when short residence times or compact reactor designs are preferred. The ability to adjust chloride strength, oxidant potential, and temperature allows this TML to be readily combined with any of the previously described aqueous or solvometallurgical leaches, providing operational flexibility across diverse feedstocks.

[0453] It will be appreciated by those skilled in the art on reading this disclosure that a range of agents and / or manipulations are suitable for providing the conditions of paragraph f) above. In various examples, the conditions comprise the presence of one or more reducing agents for reducing one or more species of various target metals. Examples of such reducing agents are exemplified herein in the Examples. Advantageously, the methods disclosed herein employ a reducing agent capable of providing selectivity in terms of the metal or metals it reduces. This, typically coupled with the use of selected support material, enables the efficient recovery of a desired target metal or metals from solution, typically from a solution including a solution comprising at least one other metal species that remains in solution. The methods contemplated herein thus enable the efficient recovery of highly pure target metal from complex solutions.

[0454] In particular examples, the conditions (for example, the presence of one or more reducing agents) selectively reduce the target metal ions to their elemental and / or colloidal forms while leaving any non-target metals present dissolved in their ionic form.

[0455] In certain examples, the target metal(s) and non-target materials are substantially selectively reduced by the conditions (for example, the presence of one or more reducing agents) in a ratio of greater than 1:1000 (target metal : non-target material), or greater than 1:500, or greater than 1:200, or greater than 1:100, or greater than 1:50, or greater than 1:20, or greater than 1:10, or greater than 1:5, or greater than 1:2, or greater than 1:1.

[0456] In certain examples, the reducing agent has a reducing potential effective to reduce at least one species of non-target metal, such as at least one species of non-target metal ions that are more reactive than one or more target metal ion species, and is added in an amount insufficient to reduce all of said at least one species of non-target metal ions. Maintaining the solution for a period leads to the reduction of one or more target metal ions to elemental target metal, which in the presence of the support material binds to said support material.

[0457] In particularly contemplated examples, the reducing agent is and is added in an amount sufficient to lead to the reduction of substantially all of at least one species of target metal ions to target metal. In particularly contemplated examples, the reducing agent is and is added in an amount sufficient to provide, at the maintaining step, a solution in which substantially all of at least one target metal is in elemental form. In particularly contemplated examples, the reducing agent is and is added in an amount sufficient to provide, at the maintaining step, a solution in which substantially all of at least one target metal is in elemental form, and in which at least some of at least one non-target metal is present as an ion. For example, the reducing agent is and is added in an amount sufficient to provide, at the maintaining step, a solution in which substantially all of at least one target metal is in elemental form, and in which substantially all non-target metals are present as ion species.

[0458] The invention has particular utility in efficiently recovering target metal ions, including in certain examples efficiently recovering substantially all or all of the target metal ions present in a solution, so in some examples the target metal-pregnant solution contains more than lOOOppm, or more than 500ppm, or more than 200ppm, or more than lOOppm, or more than 50ppm target metal.

[0459] In one example the target metal-pregnant solution contains between about O.lppm to 1500ppm, or between about O.lppm to lOOOppm, or between about O.lppm to 500ppm, or between about O.lppm to 200ppm, or between about O.lppm to lOOppm, or between about O.lppm to 50ppm, or between about O.lppm to 20ppm of the target metal. In one example the target metal-pregnant solution contains between about 0.5ppm to 1500ppm, or between about 0.5ppm to lOOOppm, or between about 0.5ppm to 500ppm, or between about 0.5ppm to 200ppm, or between about 0.5ppm to lOOppm, or between about 0.5ppm to 50ppm, or between about 0.5ppm to 20ppm of the target metal. In one example the target metal-pregnant solution contains between about lppm to 1500ppm, or between about lppm to lOOOppm, or between about lppm to 500ppm, or between about lppm to 200ppm, or between about lppm to lOOppm, or between about lppm to 50ppm, or between about lppm to 20ppm of the target metal.

[0460] Those skilled in the art will recognise that the methods disclosed herein have application to metalcontaining solutions prepared via a variety of approaches from a variety of feedstocks. The methods disclosed herein will thus in some examples involve one or more pre-processing steps, such as dissolution of solid feedstock to provide a solution comprising target metal.

[0461] In certain examples the method is performed in a vessel or system configured for first dissolving non-target metal from a solid feedstock, such as electronic waste. In particular examples, electronic waste such as ground, milled, or particulate printed circuit boards are added to the vessel, wherein a suitable base metal lixiviant is applied leading to at least partial dissolution of one or more non-target metal(s) to produce a base metal leach solution containing one or more species of non-target metal ions.

[0462] Generally, once the base metal leach (or second base metal leach as the case may be) is completed, the base metal leach solution is removed from the feedstock, and the feedstock is then contacted with a target metal lixiviant, by for example introducing the target metal lixiviant into the vessel in which the feedstock remains after removal of the base metal leach solution (or second base metal leach solution as applicable) has been done. However, in certain examples, this is by introducing the feedstock into a different vessel into which the target metal lixiviant is or has been introduced.

[0463] In one configuration of a system contemplated herein, following optional pre-processing as described herein, the target metal-pregnant solution is contacted in a vessel with one or more support materials under conditions suitable for binding to the support material. Upon contact, at least a portion of the target metal ions are bound to the support material such that the support materials become metal laden and the target metal-pregnant solution becomes barren, that is, a target metal-barren solution. In accordance with certain examples, the support material is contacted with the target metal-pregnant solution for at least 2 minutes, or at least 5 minutes, or at least 10 minutes, or at least 30 minutes, or at least 60 minutes, or at least 120 minutes, or for a period of time necessary to bind at least 50% of the precious target metal(s), or at least 60% of the precious target metal(s), or at least 70% of the precious target metal(s), or at least 80% of the precious target metal(s), or at least 90% of the precious target metal(s), or at least 95% of the target metal(s). The time period is in certain examples between about 0.5 and 48 hours, or between about 0.5 and 24 hours, or between about 0.5 and 12 hours, or between about 0.5 and 4 hours, or between about 1 and 3 hours.

[0464] In a particularly contemplated example, the support material preferentially binds the target metal over one or more further metal or metals (such as a non-target metal) that may be present in the target metal-pregnant solution. The further metal(s) is then separated from the target metal in the separation step while the further metal remains in the target metal-barren solution. Examples herein show the preferential nature of the target metal binding step. The factor of binding and recovery will in part depend on the ratio of the metals in the target metal-pregnant solution, for example if they are already in similar quantities the mass ratio may not change as much as if there is a large excess of the further metal. However, in certain examples the target metal-pregnant solution and the support material are maintained under conditions in which the target metal binds to the support material preferentially over any further metal (e.g., any non-target metal(s)) in the binding step such that the mass ratio of target metal to further metal in the target metal-pregnant solution prior to addition of the support material compared to the ratio of the target metal of the further metal bound to the support material increases by a factor of at least 2, or at least 3, or at least 5, or at least 8, or at least 10, or at least 20, or at least 50, or at least 100, or at least 200. The upper limit of the increase in ratio will in part be dependent on the starting ratio, but may be 1,000 or higher. In one example the target metal is gold. In one example the further metal is selected from one or more of lead, copper, and / or nickel.

[0465] The conditions necessary to bind the target metal(s) will depend on a number of factors including one or more of the target metal, the oxidant, the recovery agent, the reducing agent, pH, the support material, and the concentration of the various components. However, in accordance with a particular example disclosed herein, the support material is added to the target meta I -pregnant solution at acidic pH.

[0466] Support materials suitable for use in the methods discussed herein are frequently prepared for use in industrial processes, for example by washing in a buffer solution, for example phosphate, Tris, saline, acetate and / or perchlorate, or in an acid wash, prior to use. While such washing can be utilised in conjunction with the methods contemplated herein, it is not generally required.

[0467] A number of support materials, including filterable support materials, are capable of binding target metal once reduced to its atomic form. In certain examples, the support material, such as the filterable support material is advantageously selected from the group consisting of cellulose, modified cellulose, and cellulosic materials. In certain examples, the support material comprises, consists essentially of, or consists of cellulose. In certain examples, the support material comprises, consists essentially of, or consists of a lignocellulosic material, such as a lignocellulosic material selected from the group consisting of wood, wood chips, sawdust, wood pulp, and paper. In various examples, the support material comprises, consists essentially of, or consists of non-cellular biomass. For example, the support material comprises, consists essentially of, or consists of non-microbial biomass. Selected examples include but are not limited to cellulose, starch, chitosan, chitin, and lecithin.

[0468] In one example, the support material does not comprise microbes or microbial material. In one example, the support material does not comprise viable microorganisms, such as viable bacteria. In particular examples wherein the target metal(s) ion is gold, filterable support materials such as cellulose or modified cellulose are used.

[0469] In certain examples, the target metal-pregnant solution contains relatively high amounts of target metal, for example greater than 1000 ppm. It is therefore surprising the support material still has the capacity to bind high levels of target metal, and in various examples do so in relatively short time periods, for example in 3 hours or less, even where the target metal is at lower or higher concentrations.

[0470] Upon recovery of at least some of the target metal(s), such as at least partial binding of the target metal, the solution becomes a target metal-barren solution, wherein the target metal-barren solution contains less of the target metal than the target metal-pregnant solution. In particular examples, the target metal-barren solution contains less than O.lppm or less than lppm, or less than 2ppm, or less than 5ppm, or less than lOppm, or less than 20ppm, or less than 50ppm, or less than lOOppm of the target metal. In one example the target metal-barren solution contains between about 0.001 and lOOppm, or between about 0.001 and 50ppm, or between about 0.001 and 50ppm, or between about 0.01 and 50ppm of the target metal(s). In particular examples, the target metal-pregnant solution contains at least 10 times more target metal(s) than the target metal-barren solution. In one example the target metal-pregnant solution contains at least 20 times, or at least 40 times, or at least 45 times, or at least 50 times more target metal(s) than the target metal-barren solution.

[0471] Those skilled in the art will, on reading this disclosure, appreciate that a number of scalable techniques for separating solids, such as the metal laden support material, from liquids, exist and are suitable for use in the methods disclosed herein. In particularly contemplated examples, separation of the metal-laden support material from the solution is conveniently performed in a separation step that comprises filtration.

[0472] It is anticipated that the initial part of the separation step will in certain examples occur in the same vessel as the binding step, wherein the metal laden support material is simply allowed to concentrate via gravity separation. In other examples, the metal laden support material and target metal-barren solution are filtered directly, or are passed to a separation module. Examples of processes and / or equipment to separate a support material from a target metal-barren solution, and thus being suitable to comprise a separation module in the systems disclosed herein, will be familiar to those skilled in the art. However, by way of example, the metal laden support material may be separated by gravity separation, centrifugation, filtration or a combination thereof such that in each case the target metalbarren solution is at least partially removed from the metal laden support material.

[0473] Reference to substantially separating should be taken to mean physically separating at least a portion of the target metal-barren solution from the metal laden support material. Physically separating refers to having them in separate non-touching locations, for example separate containers rather than touching layers within the same container.

[0474] In specifically contemplated examples, the metal laden support material is separated from the target metal-barren solution by filtration. Those familiar with the art will recognise the appropriate conditions and equipment necessary for separating the target metal-barren solution from the metal laden support material, which following separation can be recovered, for example by being passed to a recovery module in a system contemplated herein.

[0475] In certain examples, the separating step comprises separating the metal laden support material by filtration, wherein during the filtration at least 50% of the target metal-barren solution is removed from the metal laden support material. In one example at least 60%, or at least 70%, or at least of 80%, or at least 90%, or at least 95% of the target metal-barren solution is removed during filtration. As an example, solutions containing the metal laden support material may be filtered under vacuum through filters with pore size of approximately 1 - 10 pm or larger to separate the support material from the target metal-barren solution. As another example, a cross flow filtration device or membrane bioreactor device may be used to remove the target metal-barren solution.

[0476] In various examples, the metal laden support material gravity separates from the target metal-barren solution over a time period, for example prior to filtration. Following gravity separation, at least a portion of the target metal-barren solution can be filtered, decanted, syphoned or otherwise removed leaving the concentrated metal laden support material from which the target metal is then recovered. In certain examples, the separating step comprises gravity separation of the metal laden support material from the target metal-barren solution, wherein at least 50% of the target metal-barren solution is removed. In one example at least 60%, or at least 70%, or at least of 80%, or at least 90%, or at least 95% of the target metal-barren solution is removed. By way of example, a solution of the support material may be left to sediment by gravity for up to 2 hours, or up to 6 hours, or up to 12 hours, or up to 24 hours, or up to 48 hours, or up to 72 hours before removing the target metalbarren solution.

[0477] In an alternative example, the metal laden support material can be separated from the target metalbarren solution by centrifugation and removal the target metal-barren solution. Those familiar with the art will recognise the appropriate conditions and equipment necessary for separating the target metal-barren solution from the metal laden support material, which following separation can be recovered, for example by being passed to a recovery module in a system contemplated herein.

[0478] In certain examples, the separating step comprises separating the metal laden support material by centrifugation, wherein during the centrifugation at least 50% of the target metal-barren solution is removed from the metal laden support material. In one example at least 60%, or at least 70%, or at least of 80%, or at least 90%, or at least 95% of the target metal-barren solution is removed during centrifugation.

[0479] Those skilled in the art will recognise operation of a centrifuge will be dependent on the volumes of liquid addressed and the rate of separation required. There are also a number of centrifuge systems that may be employed with the methods and systems disclosed herein including suitable continuous flow centrifugation or decanter centrifuge device. The separating step is important for a number of reasons. The separating step removes the metal laden support material, and therefore the target metal, from the other components originally present in the target metal-pregnant solution (and thus usually still present in the target metal-barren solution). The other components can be toxic or corrosive, such as cyanide or acids. The separation step also allows for concentration of the target metal. Following the separation step, the metal laden support material will in certain examples comprise greater than lOOppm, greater than 200ppm, greater than 500ppm, greater than lOOOppm, greater than 2000ppm, greater than 3000ppm, or more of the target metal.

[0480] In certain examples, the metal laden support material comprises greater than greater than 4000ppm, greater than 5000ppm, greater than 6000ppm, greater than 7000ppm, greater than 8000ppm, greater than 9000ppm, greater than lOOOOppm, greater than 15000ppm, greater than 20000ppm, greater than 25000ppm, or greater than 30000ppm of the target metal(s).

[0481] In still further examples, the support material comprises about 3.5% (w / w) target metal, about 4% (w / w), about 4.5% (w / w), about 5% (w / w), about 5.5% (w / w), about 6% (w / w), about 6.5% (w / w), about 7% (w / w), about 7.5% (w / w), about 8% (w / w), about 8.5% (w / w), about 9% (w / w), about 9.5% (w / w), about 10% (w / w), about 11% (w / w), about 12% (w / w), about 13% (w / w), about 14% (w / w), about 15% (w / w), or more than 15% (w / w), target metal.

[0482] In various examples, the support material comprises less than about lOOppm non-target metal. For example, the support material comprises less than about 90ppm non-target metal, less than about 80ppm, less than about 70ppm, less than about 60ppm, less than about 50ppm, less than about 40ppm, less than about 30ppm, less than about 20ppm, less than about lOppm, less than about 5ppm, less than about 2ppm, about lppm, or less than about lppm non-target metal.

[0483] In various examples, the mass ratio of target metal to non-target metal present on and / or bound to the support material is at least about 100, at least about 500, at least about 1,000, or higher.

[0484] Further the inventors have shown significant concentration factors of the target metal from the target metal-pregnant solution to the separated support material. The concentration factor of the target metal from the target metal-pregnant solution to the support material ( I . e. , the number of times more concentrated the target metal is in the support material compared to solution) is greater than 5, greater than about 10, greater than about 20, greater than about 50, greater than about 100, greater than about 200, greater than about 300, greater than about 400, greater than about 500, greater than about 600, greater than about 700, greater than about 800, greater than about 900, or greater than about 1000.

[0485] In various examples the concentration factor of the target metal from the target metal-pregnant solution to the support material is greater than about 1500, greater than about 2000, greater than about 3000, greater than about 4000, greater than about 5000, greater than about 6000, greater than about 7000, greater than about 8000, greater than about 9000, greater than about 10000, greater than about 15000, greater than about 20000, greater than about 25000, or greater than about 30000.

[0486] This concentration is important as, for example, although lixiviants are widely used in hydrometallurgy to extract metal, the metal must still be recovered from the solution. The methods described herein, including but not limited to the binding and / or separation steps, allow in certain examples for selective separation and / or concentration of metals. For example, certain of the Examples presented herein demonstrate preferential recovery of gold from a solution comprising a number of other metals, including zinc, nickel and copper. The reduced gold binds to the support material, so that in the separating step the gold-laden support material is separated from the other metals including copper still present in the target metal-barren solution.

[0487] Those familiar with the art will on reading this disclosure recognise suitable recovery processes and / or equipment for recovering target metal(s) from the target metal-pregnant solution. Similarly, those familiar with the art will on reading this disclosure recognise suitable recovery processes and / or equipment for recovering target metal(s) from the metal laden support material.

[0488] In a particularly contemplated example, the separated metal laden support material is dried and burnt (also referred to herein as "ashing") and / or smelted to recover the target metal(s), which may be separated from the ash using conventional pyrometallurgy or hydro metallurgy techniques known to those skilled in the art (Hennebel et al., New Biotechnology 32, pp 121-127 (2015)). In certain particularly contemplated examples, the support material (such as cellulose) produces very little ash itself, meaning the predominant component of the burnt metal laden support material is the target metal. The state of the target metal after pyrometallurgical treatment will depend on the metal, for example, noble metals such as gold will typically be in metallic form, while other target metals such as Pd, Pt, Rh will typically be recovered as an oxide or as a mixture of metallic and oxide forms.

[0489] However, by way of further non-limiting examples, the metal may be released from the metal laden support material by resuspension in, or washing with, a solution suitable for target metal dissolution. For instance, in certain examples the metal may be released from the metal laden support material by any conditions suitable for dissolving the target metal from other feedstocks, in addition to pH adjustment, such as pH adjustment to a pH less than 3, or pH adjustment to a pH greater than 10. In certain examples the target metal can be resuspended in its colloidal form by complete dissolution of the support material.

[0490] In an alternative example, the metal laden support material may be contacted with a liquid containing a compound to elicit release of the target metal(s) into the liquid. By way of example, aqueous cysteine may be used in certain examples to elicit the release of the target metal(s). In certain examples wherein the target metal(s) is gold, approximately 0.3 mM, or approximately 1 mM, or approximately 10 mM, or approximately 30 mM, or approximately 60 mM cysteine solutions may be contacted with the metal laden support material (Kenney et al, Geochimica et Cosmochimica Acta 82, pp 51-60 (2012)). In a related example, aqueous thiosulphate, thiourea, thiocyanate, cyanide or other thiol ligands may be used to elicit release of gold from the support material. Additionally or alternatively, other conditions such as a change in oxidation-reduction potential or temperature may be used to promote release of the target metal(s).

[0491] The concentrated solutions may then be subjected to separation and purification procedures such as precipitation of impurities, solvent extraction, binding and ion- exchange to isolate and / or further concentrate the target metal(s). Subsequently, the solutions can be treated by electrorefining process, chemical reduction, or crystallization for target metal(s) recovery or other methods that those skilled in the art will be aware of. Release of the target metal will in certain examples be done in a smaller volume than the initial leachate, thus concentrating the solution and making it more amenable to conventional refining techniques such as those familiar to persons skilled in the art.

[0492] It will be apparent that the recovery step can recover the target metal in metallic or ion form.

[0493] Reference to recovering the target metal should therefore be taken to include recovery of metallic metal or metal ions.

[0494] In particular examples of the methods and systems disclosed herein, the target metal is gold. In such examples, the separated gold laden support material may be dried at ambient temperature or 30°C or 50°C to minimise water content and then incinerated, for example by muffle furnace or gas torch gently so as to minimise the loss of ash generated. This ash may then be treated with nitric acid to solubilise base metals, filtered, and the gold-containing residue treated with aqua regia (1 part nitric acid to 3 parts hydrochloric acid) to generate a solution of chloroauric acid. In a related example, the gold laden support materials may directly undergo the aforementioned acid treatment without requiring prior incineration. Gold may be precipitated and smelted from chloroauric acid using methods known to those with ordinary knowledge of the art.

[0495] In particular examples of the methods and systems disclosed herein, the target metal is gold. In such examples, the separated gold laden support materials may be dried at ambient temperature or 30 °C or 50 °C to minimise water content and then incinerated by gas torch gently so as to minimise the loss of ash generated or by furnace at above 400 °C, or above 500 °C, or above 600 °C, or above 1000 °C. Borax flux may be used to help bind the gold and minimise any loss, the flux also cleans away any oxide impurities from the gold. This ash may then be treated with nitric acid to solubilise base metals, filtered, and the gold-containing residue treated with aqua regia (1 part nitric acid to 3 parts hydrochloric acid) to generate a concentrated solution of chloroauric acid. In a related example, the gold laden support material may directly undergo the aforementioned acid treatment without requiring prior incineration. Gold may be precipitated and smelted from chloroauric acid using methods known to those with ordinary knowledge of the art.

[0496] Those skilled in the art will appreciate there may be benefit in treating the feedstock, such as electronic waste, prior to dissolving the target metal(s) with a lixiviant. It is well known that certain feedstocks, particularly electronic waste, contain a wide variety of non-target materials including elements and compounds. Some or all of those non-target materials may have a deleterious effect on the recovery of the target metal(s). In many examples there will be significant benefit to preprocessing the feedstock, such as the electronic waste, prior to dissolving the target metal(s) in the lixiviant to remove certain non-target materials such as certain components, elements, metals and / or compounds.

[0497] As such, according to another aspect, there is provided a method of recovering one or more target metal(s) from a solid feedstock such as electronic waste, the method comprising:

[0498] a) an optional pre-processing step comprising removing at least a portion of non-target material(s) from the solid feedstock, such as the electronic waste;

[0499] b) contacting the solid feedstock with a base metal lixiviant in a base metal leach to provide a base metal leach solution, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids, and one or more oxidants; c) removing the base metal leach solution to yield a target metal leach (TML) feedstock;

[0500] d) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; and any combination of any two or more thereof; e) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution;

[0501] f) adding target metal recovery agent to the target metal-pregnant solution;

[0502] g) contacting the target metal-pregnant solution with a support material;

[0503] h) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and i) separating the metal-laden support material from the target metal-barren solution; and j) recovering the at least one target metal from the target metal-laden support material.

[0504] In various examples the pre-processing step comprises one or more operations selected from the group consisting of: chip removal; grinding, milling, or comminuting electronic waste, for example to a preselected size, for example to provide particulate solid material; removal of certain density fractions; removal of certain size particles or fractions; removal of one or more magnetic materials; removal of at least a portion of non-target material; and any combination of two or more thereof. For certain feedstocks, such as electronic waste, there are a number of benefits to performing one of more of these pre-processing steps prior to precious metal recovery, including reducing the volume of the material to be processed in the target metal recovery step, removal of certain fractions that may interfere with precious metal recovery or reduce the efficiency of the binding steps, and enabling selective recovery of certain valuable fractions.

[0505] In particular examples the pre-processing step removes at least 50% of the non-target material, such as the non-target or base metal(s), prior to the precious metal recovery, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the non-target materials. In particularly contemplated examples, the pre-processing step removes less than 20%, or less than 10%, or less than 5%, or less than 2%, or less than 1% of the target metal(s).

[0506] For example, in one example, there is provided a pre-processing step including multiple operations including chip removal, grinding, milling, or comminuting electronic waste, for example to a preselected size, for example to provide particulate solid material; removal of certain density fractions; removal of certain size particles or fractions; removal of one or more magnetic materials; removal of at least a portion of non-target material; and any combination of two or more thereof. Systems suitable for carrying out such pre-processing steps in conjunction with the remainder of the method steps are also contemplated herein.

[0507] In certain examples, the leach solution obtained from the base-metal leach or from the subsequent target-metal leach may be cooled or otherwise conditioned to precipitate silver as silver chloride (AgCI). This recovery step may be integrated directly with the base-metal leach liquor or applied to a separated target- meta I -pregnant solution. Cooling induces supersaturation of AgCI and provides a simple, selective recovery of silver without the need for additional reagents. The process advantageously minimises reagent cost, yields a stable and easily filterable solid, and enables regeneration of the remaining chloride solution for reuse in the base-metal or target-metal leach stages. In representative examples, the pregnant solution may be cooled from ambient temperature (« 25 °C) to between about 0 °C and 10 °C, for example about 4 °C, and held for a period of 0.5 to 4 hours to complete precipitation. The resulting AgCI may be separated by filtration or decantation and washed with a dilute chloride solution to remove soluble impurities. Silver recovery efficiencies above 95 % may be achieved under these conditions. Where integration with a target-metal leach is desired, the cooled filtrate may be reheated or neutralised and then treated with halogen- or sulphur-based lixiviants as described elsewhere herein.

[0508] In another example, the pregnant solution from the target-metal leach may be subjected to solvent extraction using an organic extractant such as methyl isobutyl ketone (MIBK) to selectively recover gold from chloride-containing solutions. This recovery technique can optionally follow the base-metal leach, or be applied after a halogen-based target-metal leach step. Solvent extraction provides excellent selectivity for AuCl4 complexes, and the organic phase can be stripped using a mild reducing agent to deposit metallic gold directly. The method allows continuous operation, re-use of the solvent, and high-purity metal recovery with minimal waste generation.

[0509] In certain exemplary examples, the organic-to-aqueous (O:A) ratio may be between 1:5 and 1:1, for example about 1:3. The extraction may be performed at 20 °C to 50 °C with a contact time of 5 to 30 minutes, followed by phase separation. Stripping or reduction of the loaded organic may be carried out using ascorbic acid, oxalic acid, or sodium bisulphite at concentrations between 0.1 M and 1 M, for example 0.2 to 0.5 M, at temperatures between 20 °C and 60 °C. These ranges provide efficient transfer of gold to the organic phase and complete reduction to metallic gold with recovery efficiencies exceeding 98 %. The raffinate from extraction may be returned to the leach circuit or used as feed to subsequent recovery steps.

[0510] In other examples, the pregnant leach solution obtained after the base-metal leach may be treated with an oxalate-containing reagent, such as oxalic acid or an alkali-metal oxalate, to precipitate tin, nickel, and lead as their respective oxalate salts. This approach enables selective removal of multiple base-metal contaminants from the leach liquor while maintaining dissolved precious-metal species in solution. The resulting oxalate precipitates are chemically stable and can be decomposed thermally to recover the metals as oxides. The step can be integrated with either the base-metal leach effluent or the target-metal leach raffinate to refine or recycle the solution.

[0511] In representative examples, oxalic acid may be added to achieve a concentration between 0.05 M and 1 M, for example about 0.2 to 0.5 M, at a pH adjusted to between 1 and 3. The solution may be maintained at 20 °C to 60 °C for 0.5 to 4 hours to allow full precipitation. The stoichiometric ratio of oxalate to dissolved metal may range from 1.0 to 3.0 moles per mole of total metal ions. These conditions may yield greater than 95 % precipitation of tin and lead oxalates and about 80-90 % removal of nickel. The filtrate, now purified of base metals, may be directed to a subsequent halogen-or sulphur-based target-metal leach for precious-metal recovery.

[0512] In still other examples, platinum-group metals (PGMs) such as platinum or palladium present in the leach liquor may be recovered by cementation using iron powder or iron scrap. This process may be optionally integrated with the target-metal leach when PGMs are present or carried out as a separate post-treatment of the leachate. Cementation with iron offers a simple, inexpensive route to recover noble metals by exploiting the electrochemical potential difference between Fe and the PGM ions. The resulting metal-laden solids may be filtered and refined to produce high-purity PGM concentrates. In representative examples, iron may be introduced at a stoichiometric excess of 10 % to 200 % relative to the total dissolved noble metals. The leach liquor may be maintained at temperatures of 25 °C to 70 °C and pH 0.5 to 2.5, for example pH 1 to 2, for 0.5 to 5 hours under mild agitation.

[0513] Recovery efficiencies above 90 % are typically achieved under these conditions. The spent iron can be regenerated by acid leaching and recycled to the process, reducing reagent cost and waste generation.

[0514] Each of the above recovery operations may be optionally integrated with the base-metal leach and / or the target-metal leach steps described elsewhere in this specification. For example, silver precipitation may follow directly after the base-metal leach to refine the liquor before target-metal leaching, while gold solvent extraction or PGM cementation may be performed on the target-metal-pregnant solution. The specific sequence of integration may be selected according to the metal composition of the feedstock, the selectivity required between base and target metals, and overall process efficiency. These optional recovery steps provide flexibility to tailor the process for various industrial wastes or mineral concentrates, enabling improved selectivity, reagent recycling, and environmental performance. In some examples, the base metal leach and target metal leach systems may be used in combination with the adsorption methods onto a solid support as described herein.

[0515] In alternative examples, the target-metal-pregnant solution obtained after any of the previously described base metal or target metal leaches, for example aqueous, halogen-based, or solvometallurgical leaches may be subjected to a hydrometallurgical recovery stage employing one or more of ion-exchange, chelation, solvent extraction, precipitation, electrowinning, or reduction techniques. In such examples, the target-metal-pregnant solution may be contacted with a sorbent or ion-exchange medium comprising one or more functional groups selected from thiol, thiouronium, polyamine, amide, carboxylate, or sulphonate groups. The sorbent may be in the form of a resin, fibre, carbon, biological material, or other particulate solid capable of selectively binding preciousmetal species such as Au, Ag, Pt, Pd, Rh, Ir, or Ru from the leachate while allowing non-target basemetal ions to remain in solution.

[0516] In representative examples, the sorption step may be conducted at temperatures between 20 °C and 80 °C, at an acid strength of 0.1 - 3 mol L1HCI or equivalent ionic strength, with a solid-to-liquid ratio of 1 : 50 to 1 : 500 (w / v). Contact times may range from 0.5 to 6 hours, with gentle agitation to maintain suspension and ensure complete interaction between the sorbent and leach solution. The resulting loaded sorbent or resin may then be subjected to an elution step using a solution containing one or more eluants selected from acidic thiourea, sulphite, hydrosulphite, or chloride salts, to release the adsorbed metals into an enriched eluate. Alternatively, the loaded sorbent may be directly incinerated or thermally decomposed to produce a high-grade metal concentrate or mixed preciousmetal sponge suitable for further refining.

[0517] In certain examples, the eluate obtained from the desorption stage may be treated by precipitation using hydroxide, carbonate, or sulphide reagents to recover target metals as insoluble compounds, or by electrowinning or chemical reduction to produce metallic powders or cathode deposits. The recovered metals may include gold, silver, platinum-group metals, or rare metals present in minor quantities in the feedstock. The barren leachate or residual acid from this process may be recycled to earlier stages of the base-metal or target-metal leach circuits, thereby conserving reagents and minimising waste generation. These recovery techniques provide a flexible and scalable approach that can be tailored to the composition of the pregnant solution. When integrated with the previously described base-metal and target-metal leaches, they enable selective recovery of high-value metals while maintaining a closed-loop hydrometallurgical process. The combination of sorption, elution, and regeneration steps ensures minimal reagent loss, high metal recovery efficiency, and compatibility with the biosorption-based recovery systems described elsewhere herein. Together, these modules provide an environmentally responsible, cyanide-free recovery route that achieves high purity of target metals and effective separation from base-metal and gangue components.

[0518] Chip removal

[0519] In certain examples relating to e-waste, the method includes one or more steps configured to remove chips and other surface mounted components from printed circuit boards prior to precious metal recovery. Those skilled in the art will recognise that chips and other components are typically fixed to printed circuit boards with solder. Soldering on printed circuit boards typically comprises tin and lead, tin and silver or a combination thereof. As such, a chip removal process typically comprises the removal of solder through elevating the temperature of the solder above the melting point of the solder or by dissolving the solder in a suitable solvent. Chips and other components can then be simply removed by shaking, vibrating, scraping or knocking the PCBs such that the chips and other components fall off.

[0520] Those skilled in the art will appreciate suitable processes and / or equipment for heating and removing chips and other components from printed circuit boards. However, by way of non-limiting example, the chips and other components may be heated in a trommel device configured for tumbling a plurality of printed circuit boards at elevated temperatures. For example Wang et al (Waste Management, Vol 53, July 2016, 218-224) provide an automated system for dissembling PCBs with heated air at 265°C can remove solder in 8 mins.

[0521] Additionally or alternatively, the chips and other surface mounted components may be removed or recovered by immersing the printed circuit boards in a solvent system suitable for selectively dissolving the solder. Those skilled in the art will recognise there are a number of solvent systems suitable for selectively dissolving solder. For example Yang Jian-guang et al (Journal of Hazardous Material, Vol 304, March 2016, 409-416) provide a 1:1 mixture of SnCI4 and HCI which can remove up to 99% of Tin at 60-90°C. Additionally or alternatively, Manis Kuma Jha et al (Hydrometallurgy, Vol 121-124, June 2012, 28-34) provide a nitric acid solution for dissolution of Lead and Tin solder comprising 0.2M HNO3 with solid: liquid ratio of lg / lOOml at 90 °C recovers 99.99% of lead in 120 minutes.

[0522] During a chip removal process it may be beneficial to recover solder, solder components, chips and other surface mounted components once removed from the printed circuit boards. Those skilled in the art will appreciate there are a number of methods for recovering such materials, for example chips and other surface mounted components may be recovered in a tray and sorted through size exclusion screens.

[0523] Grinding

[0524] The dissolution of solid feedstocks will typically be greatly facilitated by reducing the size of the feedstock units, for example by grinding tailings to reduce average particle size. In another example, due to the heterogeneous nature of electronic waste, particularly PCBs, it will usually be desirable to grind the PCB material to a particular size prior to precious metal recovery. In addition, it is recognised that significant quantities of precious metals are embedded within chips or other components and can only be accessed if the PCB material is first ground to a particular size fraction. Those skilled in the art will appreciate that grinding any material typically results in a distribution of sizes and the resulting distributed sizes may be separated through well known size exclusion or size separating technologies such as size exclusion screens. Those skilled in the art will recognise PCBs can be ground to a particular size though various well know crushers or grinders, such as hammer mills, ball mills, ring rings and shredders or a combination of two or more such mills. By way of example, the grinding step might include a cutting stage followed by a two-step grinding and crushing stage. In the first stage to reduce the size to an average 3 mm, followed by sieving of particles and fed into a second stage to reduce the average size below 1 mm, ideally in the range of 0.1 - 0.9 mm. Large particles are sieved and returned to the earlier grinding stage for further clarifying (Silvas et al. Waste Management, Vol 46, December 2015, 503-510).

[0525] By way of example, grinding to a particle size of less than 1mm improves the rate of copper and other base metal extractions, however below 0.5mm no significant improvements are observed. When grinding to particle sizes below 0.075 mm the copper and lead extraction rates are adversely affected (Chen et al. Journal of Cleaner Production, Vol 95, May 2015, 142-147). Since shredding, grinding and milling are energy intensive processes, there is a desire to maximise extraction efficiency while minimising energy consumption.

[0526] In particular examples, the pre-processing step includes grinding the printed circuit boards to less than 3cm, or less than 1cm, or less than 5000 microns, or less than 2000 microns, or less than 1000 microns, or less than 500microns, or less than 200microns, or less than lOOmicrons, or less than 50microns, or less than 20 microns. In a preferred example, the pre-processing step includes grinding the PCBs to an average size of 100-900microns.

[0527] Density fraction removal

[0528] By way of non-limiting example, particles from a given feedstock containing significant amounts of metal will usually have a higher density than particles that are substantially free of metals. In examples relating to e-waste, once the printed circuit board material has been ground to a smaller particle size, the particles within the ground material will usually differ in weight and density. As such it may be beneficial to separate particles that are substantially free of metals from particles comprising metals. Those skilled in the art will recognise there are a number of well know technologies for separating out ground materials based on density. However, by way of non-limiting example, such particles may be separated using floatation, shaker tables and / or electrostatic separation.

[0529] In a particular example relating to e-waste, wherein the printed circuit boards are ground to <200 microns, the method includes removal of substantially metal free particles by electrostatic separation. For example according to Kaya (Waste Management, Vol 57, Nov 2016, 64-90) are three types of electrostatic separation:

[0530] 1 -Corona electrostatic separation (most useful in producing a metallic and non-metallic mixture with little to no cross-contamination, works best with a 0.6 - 1.2 mm PCB size for industrial applications); 2 - Eddy current separation (useful for recovering Al);

[0531] 3 - triboelectric separation.

[0532] Any one or more of such methods are amenable to use in the pre-processing step(s) contemplated herein.

[0533] Removal of magnetic material

[0534] Precious metals and many base metals are non-magnetic so it may be beneficial to separate magnetic material prior to further treatment. For example, in the context of e-waste, once a printed circuit board material has been ground to a smaller particle size, it may be beneficial to separate magnetic material from non-magnetic material. Magnetic material may be removed using a magnetic field to physically remove magnetic material from non-magnetic materials such as ground printed circuit board substrate. For example, metal and non-metal components are completely dissociated from one another with a crush size below 0.6 mm (Guo Chao Wang Hui et al. Waste Management Vol 31, Sep-Oct 2011, 2161-2166). The authors disclose a two step crushing process, which uses sieve sizes (anything above 1.25 mm is recycled into the feed loop), followed by electrostatic separation and magnetic separation.

[0535] Recycling Solutions

[0536] The inventors recognise the efficiency of the process may be improved by recycling certain solutions and / or the support material used in the methods disclosed herein. For example, certain examples comprise the recycling of at least a portion of the target metal-barren solution separated from the metal laden support materials back into the dissolution step. In other examples, the metal-laden support material is recovered and reused, for example as exemplified herein in Example 6.

[0537] It is anticipated that in certain examples, a recycling step reduces the amount of one or more reagents, such as water, needed by the overall system thus making it increasingly cost and / or environmentally efficient. In certain examples, the efficiency of the process is facilitated by increasing the amount of target metal bound to the support material, by iteratively binding target metal to reused support material thereby increasing the concentration of target metal with respect to support material.

[0538] In examples where the target metal-barren solution is reused, it is appreciated that the target metalbarren solution may contain metal ions and in some instances may even include some residual target metal ions. In some examples the target metal-barren solution is treated to remove excess metal ions or other compounds prior to recycling. In some examples, at least a portion of the target metalbarren solution is mixed with makeup water or other suitable liquid prior to recycling.

[0539] It is further recognised that additional components may need to be added to the target metal-barren solution such that it can act as a lixiviant. For example, in certain examples where the lixiviant is thiosulfate or thiourea or chlorine, the active lixiviant agent(s) may need to be at least partially recharged to enable further dissolution of the target metal(s). In certain examples, these additional components are added to the target metal-barren solution prior to recycling. Additionally or alternatively, the target metal-barren solution may need to be treated to adjust the pH, the ORP, the temperature or any other physical properties that might be known to those skilled in the art in order to make it a suitable lixiviant. In certain examples of the systems contemplated herein for carrying out the methods disclosed, there is provided a passage for returning at least a portion of the target metal-barren solution from the or a separation vessel to the dissolution vessel.

[0540] In particular examples at least 25% of the target metal-barren solution is recycled. In other examples, at least 35%, or at least 45%, or at least 55%, or at least 65%, or at least 75%, or at least 85%, or at least 95% of the target metal-barren solutions is recycled.

[0541] Unless indicated otherwise, the order of steps described in the methods described herein has been optimised by trials carried out by the inventors to ensure that the process provides an efficient yield and an economically viable recovery method.

[0542] The invention is further described with reference to the following examples. It will be appreciated that the invention as claimed is not intended to be limited in any way by these examples.

[0543] EXAMPLES

[0544] Example 1A: Gold Extraction via Biosorption under Varied Au-to-Biomass Ratios Materials and Methods

[0545] A total of 200 g of e-waste material was used as the starting mass for the extraction process. This material underwent sequential leaching steps designed to separate and concentrate base and precious metals.

[0546] Base Metal Leach (BML) Process

[0547] Reagents and Setup:

[0548] The base metal leaching process was conducted using a IL solution with a 20% solid loading of the 3mm grind e-waste material. The leachant consisted of 5 M hydrochloric acid (HCI), and air was sparged through the solution at a rate of 14 L / min to enhance leaching efficiency and maintain adequate mixing.

[0549] Procedure:

[0550] The mixture was stirred continuously during the leaching process to selectively dissolve base metals. All of the resulting base metal-leached powder was carried through into the subsequent target metal leach.

[0551] Target Metal Leach (TML) Process

[0552] Reagents and Setup:

[0553] Following the base metal leach, a target metal leach was performed. The reaction vessel contained 1.1L of solution with a 10% solid loading (116 g) of the pre-treated e-waste powder. The solution was acidified with 3% acetic acid to maintain the desired pH, and 67.5 g of calcium hypochlorite (Ca(OCI)2) was added as an oxidizing agent.

[0554] Procedure:

[0555] A sparging process was applied for 1 hour to achieve an oxidation-reduction potential (ORP) of 987 mV. The ORP level was monitored to ensure stability, aiding in the selective dissolution of target metals, primarily gold, from the material.

[0556] Biosorption Trials (Treatments A-C) Following the target metal leach, biosorption trials were conducted using a lixiviant solution containing approximately 20.4 ppm of gold (Au). The Au-to-biomass (Au) ratio was varied across the treatments as follows:

[0557] • Treatment A: Au ratio of 1:200

[0558] • Treatment B: Au ratio of 1:400

[0559] • Treatment C: Au ratio of 1:600

[0560] Each treatment was incubated for 2 hours to allow for optimal biosorption of gold onto the biomass under controlled conditions.

[0561] Results

[0562] The total yield of recovered metals was determined by summing the metal content obtained from each leachate. This included the base metal leach (BML) lixiviant, the target metal leach (PML) lixiviant, and the content from aqua regia (AR) applied to the remaining TML-treated powder. This cumulative yield was calculated to represent 100% recovery for the material processed.

[0563] Base Metal Leach (BML) and Target Metal Leach Output

[0564] The base metal leach (BML) process was conducted on the initial 200 g of e-waste material. Following the leaching procedure, the composition and recovery percentages of various metals in the lixiviant were as shown in the table below:

[0565] Table 1.

[0566]

[0567] In the BML, the high recovery rates for copper, nickel, lead, and iron demonstrate the effectiveness of the BML process for selectively extracting base metals from the e-waste material. No gold was detected in the lixiviant, indicating that precious metals were retained in the residue for subsequent leaching and biosorption processes. After the BML, the residue (i.e. the TML feedstock) contains <5% base metal content.

[0568] Following the base metal leach, the precious metal leach (PML) was conducted on the BML residue to target the extraction of gold and remaining metals. The results from the residue and the lixiviant are detailed in the table above.

[0569] The PML process successfully extracted a significant portion of gold, with 68.6% recovered in the lixiviant and 31.4% retained in the treated powder. Minimal amounts of copper, nickel, lead, and iron were also detected in the lixiviant, indicating selective leaching of target metal. Biosorption Trials

[0570] Following the PML process, biosorption was conducted to recover gold and other metals from the TML lixiviant. The data for the input TML lixiviant, supernatant after biosorption, and biosorbed mass across all treatments (Au ratios of 1:200, 1:400, and 1:600) are as follows:

[0571] Table 2.

[0572]

[0573] The biosorption process varied across treatments, with optimal gold recovery observed at an Au ratio of 1:400 (Treatment B), where 57% of the initial gold content was biosorbed onto the biomass. In contrast, copper showed minimal biosorption across all treatments, primarily remaining in the supernatant.

[0574] Summary

[0575] The combined BML and TML processes demonstrate effective separation of base and target metals from e-waste. The majority of base metals were recovered during the BML process, while precious metals, particularly gold, were targeted in the PML step. The biosorption trials further isolated gold, with the best recovery observed at an Au-to-biomass ratio of 1:400 in Treatment B.

[0576] Example IB: Base metal leach from e-waste using H2O2 and H2SO4

[0577] The objective of this experiment was to determine a method to extract >90% of the base metals from two different samples of types of e-waste.

[0578] Materials and Methods:

[0579] Table 3: E-waste type 1 composition:

[0580]

[0581] Table 4: PCB type 2 composition:

[0582]

[0583] On each e-waste composition 1 and 2 two sequential BML leaches were performed

[0584] BML #1 Reaction - 1.5L of 2M H2SO4 was added to 300 g of e-waste. 60% H2O2 was added to the slurry at a rate of 1 mL / min / kg of e-waste for 40 mins then the rate of addition of 60% H2O2 was increased to 1.8 mL / min / kg of e-waste for 110 mins, after which the e-waste was filtered from solution and prepared for BML #2

[0585] BML #2 Reaction - after BML #1, 1.5L of 1.5M H2SO4 was added to the e-waste. Then 60% H2O2 was added to the slurry at a rate of 3.1 mL / min / kg of e-waste for 270 mins after which the e-waste was filtered from solution.

[0586] Results

[0587] For the type 1 composition of e-waste 92.5% of the base metals were leached and for the type 2 composition of e-waste 92.1% of the base metals were leached.

[0588] Conclusion

[0589] The dual base metal leaches using H2O2 and H2SO4 on e-waste were effective at removing >90% of the base metals present in two types of e-waste with different base metal compositions.

[0590] Example 1C - High vs Low Acid BML Comparison

[0591] This experiment compares two base metal leach (BML) methods on e-waste using different acid concentrations. Specifically, the comparison involves a BML using 1.5M sulfuric acid (H2SO4) versus a BML with a combination of 0.5M H2SO4 and 0.75M magnesium sulphate (MgSCM) under identical conditions for e-waste mass, solution volume, and reaction parameters. The analysis evaluates mass balance, and the effects on BML solution treatments.

[0592] Materials and Methods

[0593] The base metal leach comprised two BML treatments:

[0594] 1. Treatment A: 1.5 M H2SO4 solution.

[0595] 2. Treatment B: 0.5 M H2SO4 with 0.75 M MgSCU solution.

[0596] Each treatment used a 1.5 L reaction volume with 300 g of e-waste. Each e-waste sample was treated in two BML leaches using the same concentration and peroxide addition rate for both experimental conditions.

[0597] Results

[0598] Base Metal Removal - The BML with 1.5M H2SO4 removed more base metals compared to the BML with 0.5M H2SO4 and 0.75M MgSCU, suggesting higher effectiveness in base metal leaching for the higher acid concentration.

[0599] Mass Balance - Overall, more metals were leached with the 1.5M H2SO4 treatment Conclusion

[0600] The 1.5M H2SO4 solution demonstrated greater efficacy in removing base metals from e-waste compared to the lower acid concentration with MgSCM. The findings indicate that using higher concentration sulphuric acid (i.e. >1M) in combination with peroxide is desirable to ensure effective base metal leach conditions.

[0601] Example 2: Gold Extraction via Biosorption under Serial Dilution of Au-to-Biomass Ratios Materials and Methods

[0602] A total of 200 g of e-waste material was processed through a sequential leaching and biosorption procedure aimed at optimizing gold recovery by varying the Au-to-biomass ratio.

[0603] The base Metal Leach (BML) Process was as described in example 1.

[0604] Target Metal Leach (TML) Process

[0605] Reagents and Setup:

[0606] The TML process used a 1.1L solution with a 10% solid loading (116 g) of pre-leached e-waste powder. The solution was acidified with 3% acetic acid to control pH, and 67.5 g of calcium hypochlorite (Ca(OCI)2) was added as an oxidizing agent.

[0607] Procedure:

[0608] The TML solution underwent sparging for 15 hours, reaching an oxidation-reduction potential (ORP) of 245 mV, which facilitated the selective leaching of target metals, including gold. The final gold concentration in the lixiviant was 20.0 ppm.

[0609] Biosorption Trials:

[0610] Following the target metal leach, biosorption trials were conducted using the lixiviant containing approximately 20.0 ppm of gold (Au). The Au-to-biomass ratio was adjusted across treatments as follows:

[0611] Treatment RO-1-16A: Au ratio of 1:200

[0612] Treatment RO-1-16B: Au ratio of 1:400

[0613] Treatment RO-1-16C: Au ratio of 1:600

[0614] Each treatment was incubated for 2 hours to enable biosorption under controlled conditions.

[0615] Results

[0616] The total yield of recovered metals was calculated as described in example 1. The base metal leach and target metal leach processes yielded the metal compositions described in example 1.

[0617] Following the TML, biosorption trials were conducted to capture gold and other metals from the lixiviant. The results for each Au-to-biomass ratio treatment (1:200, 1:400, and 1:600) are as follows:

[0618] Table 5.

[0619]

[0620]

[0621] The 1:400 Au-to-biomass ratio provided the highest biosorption efficiency for gold, capturing 82% of the available gold in the biosorbed mass. This suggests that the 1:400 ratio is the optimal concentration for gold recovery under the conditions tested, as it maximized biosorption while minimizing residual gold in the supernatant.

[0622] Further biosorption experiments were carried out using an ORP of 200mV, and Au to biomass ratio of 1:250 and 300ml of lixiviant:

[0623] Table 6

[0624]

[0625] This experiment investigated the biosorption efficiency for gold recovery under conditions of 200 mV ORP, an Au-to-biomass ratio of 1:250, and a 300 mL lixiviant volume. In the initial TML lixiviant, 5.96 mg of gold was present. Post-biosorption analysis showed that 1.30 mg of gold remained in the supernatant, accounting for 21% of the initial gold, while 4.15 mg, or 70% of the initial gold, was captured in the biosorbed mass. The biosorption process demonstrated strong selectivity for gold, as other metals like copper, nickel, and lead remained predominantly in the supernatant, showing negligible biosorption. This selectivity suggests that the process is well-suited for isolating gold while leaving other metals in solution. Iron, however, exhibited unusual behaviour, with 111% of the initial iron amount found in the biosorbed mass, likely due to contamination from external sources, such as ferromagnetic stirring equipment.

[0626] Example 3 - Iodine-Based Target Metal Leach (PML) for Gold Recovery Materials and Methods

[0627] An iodine-based TML process was tested which utilizes a circular setup, allowing for repeated iodine recycling. This provides advantages in recycling the iodine (lower cost) and reduced water consumption.

[0628] The lixiviant solution consisted of iodide (I ) and triiodide (Is-) at a neutral pH of ~7 and room temperature, h-is generated from action of hydrogen peroxide on a KI solution and results in an oxidising potential providing high selectivity for gold.

[0629] The process started with 660 kg of dry, base metal leached e-waste. Additional reagents include 11.3 L of 60% hydrogen peroxide (H2O2), and an optional sulfuric acid (H2SO4) component, if required for pH adjustments. Any water necessary to maintain the lixiviant concentration is also added.

[0630] TML Step (lodine / Iodide Leach) The primary leach process uses a solution of iodide / triiodide (I / I3 ) at a neutral pH of approximately 7. This solution, prepared in a total volume of 6600 L with a concentration of 150 mM potassium iodide (KI), facilitates the dissolution of gold from the e-waste. A 792 L wash of 50 mM KI is subsequently used to rinse the material, yielding 1000 kg of wet e-waste residue, which includes remaining solids and undissolved materials.

[0631] Basification Step Following the TML leach, the iodine solution is subjected to a basification step, where the pH is raised to 12 using 3.6 kg of calcium hydroxide (Ca(OH)2). This converts triiodide (I3 ) to iodate (IO3 ), and converts any dissolved residual base metals to their insoluble hydroxide forms. An additional 4 kg of filteraid (CFA250) was introduced to enhance filtration. A water wash of 80L completes the step, yielding approximately 20 kg of wet base metal waste for removal.

[0632] Gold Reduction Step - The solution is now ready for the selective reduction of gold. Using 3.7 kg of cellulose as a substrate and 3.6 kg of hydrazine sulfate (N2H6SO4) as a reducing agent, gold is deposited onto the cellulose. This is filtered and washed with an additional 50 L water. The final product of this step is 8 kg of wet gold-cellulose composite.

[0633] Lixiviant Regeneration and Recycling - The lixiviant, now at approximately 6450 L, undergoes regeneration to restore the iodide concentration for reuse. Any remaining wet e-waste residue and a portion of the liquid waste (~1000 L) are separated, resulting in a total of 1000 kg of solid waste. The regenerated lixiviant is then circulated back to the PML step, maintaining a continuous, circular process that minimizes waste and maximizes reagent reuse.

[0634] This circular PML process exemplifies an efficient, low-waste method for gold recovery from e-waste, with all major reagents regenerated and recirculated to the initial leaching step.

[0635] Hydrogen peroxide was added to convert iodide into triiodide, the active oxidant that facilitates gold dissolution. Basification is achieved with calcium hydroxide (Ca(OH)2), raising the pH to 12 and precipitating dissolved base metals. After gold reduction onto a cellulose support using hydrazine or hydroxylamine, the lixiviant is regenerated by concentrating the post-reduction solution through reverse osmosis (RO).

[0636] Results

[0637] Testing of the iodine-based PML process at an 800L scale demonstrated efficient gold recovery and recycling capabilities. When taken into the lab for additional treatment, the process achieved 100% iodine recovery, while in-press washing yielded an 85% recovery rate, indicating some loss during larger-scale processing.

[0638] Conclusions

[0639] The iodine-based PML process provides a promising recovery method for gold from e-waste while recycling reduces cost and water consumption.

[0640] Example 4 - Optimising Gold Recovery Using TCCA with Sequential Addition of Bases Materials and Methods:

[0641] Experiments were carried out to examine the effect of varying TCCA concentrations on gold extraction from e-waste using a sequential addition of bases. Each test was performed with 20 g of e-waste, 200 mL of warm water, and an initial addition of Mg(OH)2 solution. Subsequent additions of Ca(OH)2 and Ca(CIO)2 aimed to maintain optimal pH and ORP for maximum gold leaching.

[0642] The experiment combined 20 g of e-waste with 200 mL of warm water and 0.33 mL of 60% Mg(OH)2 solution (equivalent to 1.67 mL / L). The mixture was then heated to 42°C.

[0643] Treatment 1 comprised 6 g of TCCA (30 g / L) Treatment 2 and 3 comprised TCCA quantities adjusted to 5.2 g (26 g / L) and 4 g (20 g / L), respectively. The solution was stirred for 1.5 hours, with samples taken every 30 minutes, including two samples at the 150-minute mark.

[0644] Following the initial stirring, 0.52 g of Ca(OH)2 (2.6 g / L) was introduced to each mixture, and stirring was continued for another hour. Samples were collected at 10, 30, and 60 minutes, with additional sampling at the 60-minute mark.

[0645] In the final step, 0.6 g of Ca(CIO)2 (3 g / L) was added, and the solution was stirred for an additional 2.5 hours. Samples were collected at 10, 30, 60, and 360 minutes to monitor ORP, pH, and gold concentration, providing a comprehensive analysis of the process dynamics and extraction efficiency.

[0646] Results

[0647] Gold Recovery

[0648] The results for recovery of gold are shown in Figure 4A, 4B and 4C. Here, gold extraction is optimised at higher pH i.e. 3-6 and ORP 1000-1200.

[0649] The varying concentrations of TCCA (30 g / L, 26 g / L, and 20 g / L) impacted the gold leaching rate, with the highest concentration showing faster initial extraction. Optimal extraction was achieved with the 30 g / L TCCA concentration in treatment 1, demonstrating the highest gold recovery.

[0650] ORP and pH Stability: The sequential addition of Ca(OH)2 and Ca(CIO)2 helped maintain consistent ORP and pH levels, which was important for sustaining the leaching environment. The Ca(CIO)2 addition contributed to ORP stability during extended leaching.

[0651] Conclusion

[0652] The experiments demonstrate that a higher TCCA concentration (30 g / L) in conjunction with Mg(OH)2, Ca(OH)2, and Ca(CIO)2 additions effectively optimises gold recovery from e-waste. The sequential base addition protocol maintains ORP and pH levels, enhancing the efficiency of the leaching process.

[0653] Example 5 - Gold Extraction Using Calcium Hydroxide as Base

[0654] Materials and Methods A process was developed to test calcium hydroxide (Ca(OH)2) for leaching gold from e-waste. The following procedure was followed in three treatments with varied TCCA (trichloroisocyanuric acid) concentrations.

[0655] Treatment 1- 20 g of e-waste was combined with 200 mL of warm water and 0.3 g of Ca(OH)2 (1.5 g / L). Then, 6 g of TCCA (30 g / L) was added to the mixture, and it was stirred for 2.5 hours.

[0656] Afterward, an additional 0.4 g of Ca(OH)2 (2.5 g / L) was introduced to determine if more gold could be extracted.

[0657] Treatment 2 - Similar to Treatment 1, but with 5.2 g of TCCA (26 g / L).

[0658] Treatment 3 - Similar to Treatment 1, but with 4 g of TCCA (20 g / L).

[0659] Results

[0660] The gold concentration was observed to stabilize at just under 20 ppm for both the 30 g / L and 26 g / L TCCA treatments, indicating effective extraction - see Figure 5A.

[0661] Optimal gold extraction occurred from pH2-6 and ORP 1100-1250, as shown in Figures 5B and 5C. Conclusion

[0662] This experiment demonstrated that Ca(OH)2 is a viable base for gold leaching. A concentration of 3.5 g / L Ca(OH)2 was found to provide good gold extraction efficiency.

[0663] Example 6 - Using Calcium Hypochlorite (Ca(CIO)2) for Additional Base in Gold Extraction In this experimental series, calcium hypochlorite (Ca(CIO)2) was used to evaluate the effectiveness of Ca(CIO)2 in facilitating gold leaching and its impact on the overall process.

[0664] 20g of e-waste was combined with 200 mL of water at 42°C and 0.33 mL of 60% Mg(OH)2 solution (1.67 mL / L). Trichloroisocyanuric acid (TCCA) was added as the oxidizing agent, with the amount varying across treatments: Treatment A: 6 g of TCCA (30 g / L), Treatment B: 5.4 g of TCCA (27 g / L), Treatment C: 4.8 g of TCCA (24 g / L). After 2.5 hours of stirring, Ca(CIO)2 was added to each treatment, equivalent to the mole amount of 3.3 mlVL of 60% Mg(OH)2 solution. Treatment A: 2.1 g (10.4 g / L), Treatment B: similar to A with adjusted TCCA concentration, Treatment C: similar to A with adjusted TCCA concentration. Five hours into the process, additional Ca(CIO)2 was introduced to examine its effect on further gold leaching: Treatment A: 1.5 g (7.5 g / L), Treatment B: 1 g (5 g / L), Treatment C: 0.3 g (1.5 g / L).

[0665] Results

[0666] The use of Ca(CIO)2 as an additional base achieved good gold extraction levels, with a maximum observed gold concentration of 18 ppm, which was similar to the levels achieved using Mg(OH)2 or Ca(OH)2. See Figure 6.

[0667] Conclusion: Calcium hypochlorite (Ca(CIO)2) exhibited good gold extraction efficiency.

[0668] Example 7 - Using Magnesium Hydroxide (Mg(OH)2) as Base in TCCA Gold Extraction

[0669] In this experiment, Magnesium hydroxide (Mg(OH)2) was used to examine whether it could achieve gold extraction when used to control the pH in combination with TCCA as an oxidising agent. 20g of e-waste was combined with 200 mL of water at 42°C and 0.33 mL of 60% Mg(OH)2 solution (1.67 mL / L). Trichloroisocyanuric acid (TCCA) was added as the oxidizing agent. After 2.5 hours of stirring, a second dose of 3.3 mL / L of 60% Mg(OH)2 solution was added.

[0670] Results

[0671] The use of (Mg(OH)2) as a base exhibited good pH control keeping the pH of the leach between 1.5 and 7 affording gold extraction, with a maximum observed gold concentration of 14 ppm (Figures 7A-7C).

[0672] Conclusion: Magnesium hydroxide (Mg(OH)2) exhibited good pH control keeping the pH allowing for gold extraction from e-waste.

[0673] Example 8 - Effect of pH Control with HCI and NaOH on Gold Extraction with TCCA:

[0674] Target metal leach experiments were conducted at both room temperature (121 hours) and 40°C (5 hours) with the following treatments:

[0675] • Treatment A: E-waste TML at room temperature, 121 hours, pH controlled with HCI and NaOH to maintain a stable environment.

[0676] • Treatment B e-waste TML at 40°C, 5 hours, pH adjustment with HCI.

[0677] • Treatment C e-waste at room temperature, 121 hours, with no pH adjustment.

[0678] • Treatment D e-waste Material at 40°C for 5 hours, with NaOH added for pH adjustment. Results:

[0679] • Room temperature reactions converged to around pH 3, while reactions at 40°C settled between pH 1-2.

[0680] • Reaction B (TCCA with HCI) performed best at room temperature but poorly at 40°C, indicating temperature effects on gold liberation, especially with high residual-copper e-waste.

[0681] Conclusion: Temperature had a significant impact on pH and ORP. HCI addition at room temperature was beneficial for e-waste with high copper content.

[0682] Example 9

[0683] In this example, gold extraction from e-waste using TCCA was tested under different pH control and temperature conditions. Three treatments were designed to adjust pH using HCI or NaOH, with experiments conducted at both room temperature (121 hours) and at an elevated temperature of 40°C (5 hours).

[0684] Results showed that:

[0685] • At room temperature, pH naturally stabilized around 3 in all treatments, with Treatment A (no pH adjustment) yielding the highest gold recovery.

[0686] • At 40°C, the pH remained lower, between 1-2, and Treatment A again produced the highest gold yield, with elevated temperatures enhancing recovery across all treatments.

[0687] Adding HCI (Treatment B) slightly reduced gold extraction, likely due to adverse effects at high temperatures. Overall, TCCA was effective for gold leaching without pH control, particularly at 40°C, where extraction efficiency was maximized without additional base or acid adjustments.

[0688] Example 10 - Evaluation of NaDCCA and DCDMH as Chlorine Oxidants for Gold Extraction Materials and Methods

[0689] This experiment tested sodium dichloroisocyanurate (NaDCCA) and dichlorodimethylhydantoin (DCDMH) as alternative chlorine oxidants in gold extraction processes, to determine their viability compared to standard oxidants.

[0690] 1. NaDCCA Treatment: NaDCCA was added to e-waste leach solutions in controlled trials to assess its reactivity and ability to sustain oxidation necessary for gold dissolution.

[0691] 2. DCDMH Treatment: DCDMH was similarly tested to examine its oxidizing capabilities, focusing on its potential to release hypochlorous acid (HOCI), which is critical for effective gold leaching.

[0692] Results

[0693] • NaDCCA: Low gold yields were observed.

[0694] • DCDMH: Yields were similarly low.

[0695] Conclusion

[0696] Both NaDCCA and DCDMH yielded gold extraction but at lower levels versus TCCA, primarily due to low oxidizing efficiency and instability in maintaining necessary chlorine levels. This suggests that TCCA remains the more reliable chlorine-based oxidant for this application.

[0697] Example 11 - Optimising TCCA Performance for Gold Extraction at Various Temperatures Materials and Methods

[0698] This study investigated the effect of temperature on gold extraction efficiency using trichloroisocyanuric acid (TCCA) as the oxidant in a TML. Experiments were conducted with e-waste leach solutions at three temperature settings: ambient (~20°C), moderate (30-35°C), and elevated (40-45°C).

[0699] 1. Ambient Temperature (~2O°C):

[0700] o Reaction time: approximately 3 days.

[0701] o Objective: Assess gold yield without additional heating.

[0702] 2. Moderate Temperature (30-35°C):

[0703] o Reaction time: 2 to 2.5 days.

[0704] o Objective: Test if a slight increase in temperature improves yield within a shorter timeframe.

[0705] 3. Elevated Temperature (40-45°C):

[0706] o Reaction time: 3 to 6 hours.

[0707] o Objective: Examine the impact of higher temperatures on reaction speed and gold yield. Results

[0708] • Ambient Temperature: Gold yield reached up to 77% over the extended timeframe.

[0709] • Moderate Temperature (30-35°C): Yield was highest, peaking at 92%, with optimal reaction duration of about 2 days.

[0710] • Elevated Temperature (40-45°C): Yield was moderate, around 70-73%, achieved within 3 to 6 hours.

[0711] Conclusion

[0712] Temperature significantly impacts the extraction yield with TCCA. A moderate temperature of 30-35°C over 2 days offers a balanced approach, achieving high yields within a manageable timeframe.

[0713] Example 12 - Effect of Initial Temperature with TCCA + NaOH

[0714] Objective: To determine the impact of different initial temperatures on gold extraction efficiency using 41 g / LTCCA, followed by NaOH addition to control pH.

[0715] • Treatments:

[0716] o 1 - 25°C, followed by 13 mL / L of 50% NaOH.

[0717] o 2 - 35°C, followed by 13 mL / L of 50% NaOH.

[0718] o 3 - Room temperature, followed by 13 mL / L of 50% NaOH.

[0719] o 4 - 30°C, followed by 13 mL / L of 50% NaOH.

[0720] Results:

[0721] As can be seen in Figure 8, varying initial temperatures impacted reaction rates, with the 35 °C treatment providing more consistent extraction.

[0722] Conclusion: Higher initial temperatures, particularly 35 °C, may improve reaction stability and extraction, though NaOH pH control is critical across all temperatures.

[0723] Example 13 - Extended Reaction Times with MgO and TCCA

[0724] Objective: To evaluate extended PML reaction times at 42°C with MgO as a base.

[0725] Treatments - Various e-waste samples with 41 g / L TCCA and 4.2 g / L MgO for an 8-hour duration.

[0726] Results: Gold extraction was consistent across treatments, but longer reaction times were necessary for optimal recovery.

[0727] Conclusion: Using MgO with an 8-hour reaction period at 42°C is effective for gold leaching, providing stable yields and minimizing excess reagent use.

[0728] Example 14 - Thiourea Leaching Experiments Using FeC as Oxidant

[0729] Materials and Methods

[0730] A series of experiments was conducted to investigate gold leaching from e-waste using thiourea (Tu) and ferric chloride (FeCI 3) under acidic conditions. The experiments were performed at a pH of 1 using sulfuric acid (H2SO4) with 100 mL of water as the solvent. Four variations were tested:

[0731] • Treatment 1: 20 g e-waste, 1.2 g Tu, 1.5 g FeCIs. • Treatment 2: 20 g e-waste, 0.6 g Tu, 0.75 g FeCIs.

[0732] • Treatment 3: 10 g e-waste, 2.4 g Tu, 3.0 g FeCIs.

[0733] • Treatment 4: 20 g e-waste, 2.4 g Tu, 3.0 g FeCIs.

[0734] Results

[0735] The gold recovery varied depending on the FeCIs concentration used in each treatment, indicating that the degradation rate of FeCIs influences the extraction efficiency:

[0736] • Treatment 1 (1.5 g FeCIs) : 213 mg / kg of gold.

[0737] • Treatment 2 (0.75 g FeCIs) : 201 mg / kg of gold.

[0738] • Treatment 3 (3.0 g FeCIs with 10 g e-waste): 384 mg / kg of gold.

[0739] • Treatment 4 (3.0 g FeCIs with 20 g e-waste): 258 mg / kg of gold, similar to Treatment 1. The rate at which the FeCh degrades determines the amount of Au extracted. In 20% e-waste loading:FeCI3 @ 0.75g, 1.5g and 2.4g shows extraction of Au @ 201, 213 and 258 mg / Kg respectively.

[0740] Adding 0.75g more FeCh to Treatment 2 (total 1.5g FeCh) recovers 248 mg / kg - similar to Treatment 4 which uses twice as much oxidant (3.0g)

[0741] Conclusion

[0742] The experiments demonstrate that thiourea and iron chloride provide effective gold extraction. FeCIs concentration plays a key role in gold leaching efficiency, with higher amounts resulting in greater gold recovery. However, adding extra thiourea did not significantly enhance the extraction yield. Example 15: Biosorption with Modified pH Using TCCA and Ascorbic Acid

[0743] Materials and Methods:

[0744] This experiment aimed to enhance gold biosorption by modifying pH levels using TCCA in combination with a secondary treatment of ascorbic acid. E-waste was pre-treated with 1 g / L of cellulose-based filter aid (Becocel), stirred, and then treated with ascorbic acid to promote biosorption.

[0745] Procedure:

[0746] 1. 1 g / L of Becocel was added to the biosorption solution and stirred for 20 minutes.

[0747] 2. Ascorbic acid (0.5 g / L) was subsequently introduced, with the solution stirred for 1 hour to ensure thorough interaction.

[0748] 3. The resultant Au@becocel complex was isolated via filtration and then subjected to aqua regia digestion for yield determination.

[0749] Results:

[0750] • Gold Recovery: The biosorption yield of gold was highly effective, with results ranging from 93% to 100%.

[0751] Au@Becocel Integrity: No palladium contamination was detected, confirming the selectivity and purity of the gold recovery process. Conclusion:

[0752] The addition of ascorbic acid and the cellulose-based filter aid significantly increased the biosorption efficiency, achieving high gold recovery. The method demonstrates that it integrates well with a process that includes a base metal leach which minimises the amount of other metals in the targetmetal leach process. This provided for high-purity gold recovery with minimal contamination, making it suitable for refining applications in electronic waste processing.

[0753] Example 16 - Gold Extraction from E-Waste Using Hypochlorite and TCCA

[0754] Objective

[0755] This study compares the efficiency of hypochlorite and TCCA in extracting gold from three different samples of base metal leached e-waste (A, B and C).

[0756] Materials and Methods

[0757] In each treatment, 20 g of e-waste was leached in 200 mL of solution under the following conditions:

[0758] • Hypochlorite Leach: 54 g / L Ca(OCI)2 in 5% acetic acid, conducted at room temperature.

[0759] • TCCA Leach: 41 g / L TCCA in water at 42°C.

[0760] Three replicates (A and C) or two replicates (B) were averaged to provide data.

[0761] Results and Conclusion

[0762] Table 8

[0763]

[0764] In the given conditions, the TCCA PML performed slightly better than the hypochlorite PML with all samples of base metal leached e-waste (by 6, 2 and 14% for A, B and C, respectively). The most favourable result was achieved in both cases for the sample with lowest total metal content - B (2.2%). The TCCA type leach showed less dependence on higher residual metal content in the sample than hypochlorite.

[0765] Modest gold extraction yields were obtained for sample A containing high amounts of residual iron which is present as steel (indicated by high nickel concentration). When iron is present as steel, it plays a smaller role in reducing effectiveness of the TML due to the higher corrosion resistance relative to copper, tin and aluminium (which consume oxidant). Sample C contains the highest non-steel metal content and shows the lowest gold target metal yields.

[0766] Overall, both TCCA and hypochlorite leaching achieved stable gold recoveries across diverse e-waste types, making it more reliable for various compositions.

[0767] Example 17 - Precious-Metal Recovery Using a Calcium Hypochlorite Lixiviant and

[0768] Biosorption A series of experiments were conducted to demonstrate recovery of gold from electronic waste using a two-stage leach process followed by biosorption.

[0769] The process employed:

[0770] 1. A base-metal leach (BML) using hydrochloric acid;

[0771] 2. A target-metal lixiviant (TML) containing calcium hypochlorite and acetic acid; and

[0772] 3. A biosorption stage using microbial biomass to recover dissolved gold.

[0773] Base-Metal Leach

[0774] E-waste fragments were subjected to leaching in a 1 L glass reactor at 20 % solids loading using 5 M HCI with an air flow rate of 14 L min-1.

[0775] All solids recovered from this stage were transferred without washing into the subsequent TML step. Target-Metal Leach

[0776] The TML stage was performed using:

[0777] • 1.1 L solution containing 10 wt % solids (116 g) of the BML residue,

[0778] • 3 % (v / v) acetic acid, and

[0779] • 67.5 g Ca(OCI)2 as oxidant.

[0780] The slurry was sparged with air for either 1 hour or 15 hours, depending on experiment, to establish oxidation-reduction potentials (ORP) of 987 mV or 245 mV, respectively.

[0781] The total gold yield in the BML leach solution was 0% and in the TML leach solution ~69% indicating good precious-metal yield from this stage.

[0782] Biosorption - First Series

[0783] The lixiviant from the 1 h sparge experiment (ORP « 987 mV, [Au] = 20.4 ppm) was incubated with serial dilutions of microbial biomass in 30 mL aliquots:

[0784]

[0785] The gold yield ranged from 19-57% of biosorption input.

[0786] Biosorption - Second Series

[0787] A second series employed the lixiviant after 15 h sparging (ORP « 245 mV, [Au] = 20.0 ppm).

[0788] Incubations were conducted under identical conditions for 2 hours.

[0789]

[0790] The gold yield ranged from 68-82% of biosorption input.

[0791] Biosorption - Large-Scale A tenfold-scale experiment (300 mL PML solution, [Au] = 19.9 ppm, ORP « 200 mV) was conducted using a 1 : 250 Au:Microbe ratio and 2 h incubation.

[0792] Overall gold yield = 70% of biosorption input.

[0793] Alternative Target-Metal Lixiviants

[0794] Alternative target metal leach chemistries were tested with the biosorption step:

[0795] 1. I2 / KI System

[0796] - 10 g BML residue + 1 g I2 + 4 g KI in 100 mL H2O.

[0797] - Au « 68 ppm (pre-dilution) 20 ppm (post-dilution).

[0798] - Biosorption at 1 : 1000 (Au:biomass) under variable NaCI concentrations (0-4 g per 20 mL).

[0799] - Enhanced biosorption observed at high NaCI.

[0800] 2. FeCIs I Thiourea System

[0801] - 10 g BML residue + 2.4 g thiourea + 1.74 g FeCIs in H2SO4 (pH « 1), 100 mL H2O.

[0802] - Au « 37 ppm (pre-dilution).

[0803] - Biosorption (1 : 1000 Au:Microbe) showed effective gold recovery.

[0804] Conclusion

[0805] The experiments confirm that a Ca(OCI)2 / acetic-acid lixiviant followed by microbial biosorption effectively recovers gold from e-waste under mild conditions.

[0806] Alternative oxidant systems (I2 / KI, FeCIs / thioure) demonstrate gold dissolution and biosorption potential.

[0807] Example 18. Stoichiometry

[0808] This example presents experiments investigating the amount of support material and reductant to be added for useful recovery of one or more target metals from solution.

[0809] Materials and Methods:

[0810] Varying amounts of support material and reducing agent were added to a 20 ppm gold electronic waste leachate. Two different amounts of the support material (see Table 9 below), in this case fine cellulose, were added to samples of the leachate and fully suspended by mechanical agitation.

[0811] Reducing agent, in this example ascorbic acid, was then added in two different amounts (again, see Table 9 below) to samples of the support material / leachate mixture, and agitation was continued for several minutes to ensure adequate mixing and reduction of the target metal ions to target metal and binding to the support material. A negative control (21-A in Table 9 below), in which no cellulose or ascorbic acid were added, was included.

[0812] After the addition of the reductant, the samples were maintained for a suitable time (here, 4 hours), and the target metal-laden support material was then recovered by filtration. The filtrate was tested to determine the concentration of residual target metal, while the amount of gold recovered from the support material was determined by digesting the cellulose support material via aqua regia (AR). Results:

[0813] As shown in Table 1 below, changing the amount of cellulose from 1.67mg / ml (21-E) to 5mg / ml (21-F 8i 21-G) did not appreciably impact the amount of gold recovered on the support material or that remained present in the filtrate. Similarly, changing the amount of ascorbic acid from 0.17mg / ml (21-G) to lmg / ml (21-E & 21-F) did not appreciably impact gold recovery. The concentration of gold present in the solution of the negative control remained unchanged.

[0814] Table 9. Effect of relative amounts of support material and reducing agent

[0815]

[0816] These results show that efficient recovery of target metal is possible using the methods described herein across a large range of amounts of each of the support material (here, cellulose) and the reducing agent (here, ascorbic acid).

[0817] Example 19. Addition of support material and reducing agent

[0818] This example presents experiments investigating the order of addition of support material and reducing agent on the recovery of one or more target metals from solution.

[0819] Materials and Methods:

[0820] Support material and reducing agent were added to a 20 ppm gold electronic waste leachate. For one sample (21-D in Table 10 below), the reducing agent was added to the leachate first and mixed by mechanical agitation, followed by addition of the support material and further mechanical agitation. The order of addition was reversed for the other sample (21-E in Table 10 below), with support material added first followed by reducing agent. Again, a negative control (21-A in Table 10 below), in which no cellulose or ascorbic acid were added, was included.

[0821] As for Example 1 above, samples were maintained for 4 hours following addition of reductant, then the target metal-laden support material was recovered by filtration. The filtrate was tested to determine the concentration of residual target metal, while the amount of gold recovered from the support material was determined by digesting the cellulose support material via anaerobic respiration. Results:

[0822] As shown in Table 10 below, changing the order of addition of support material and reducing agent did not appreciably impact the amount of gold recovered on the support material or that remained present in the filtrate. The recovery of gold observed for 21-E, in which support material was added first, was comparable to that observed with 21-D, in which reducing agent was added first. Again, the concentration of gold present in the solution of the negative control remained unchanged.

[0823] Table 10. Effect of relative amounts of support material and reducing agent

[0824]

[0825]

[0826] These results show that efficient recovery of target metal is possible using the methods described herein irrespective of the order in which the support material (here, cellulose) and the reducing agent (here, ascorbic acid) were added. Accordingly, and without wishing to be bound by any theory, the inventors expect that the methods described herein are amenable to implementation across a range of practical considerations, and can accommodate a range of workflows, plant designs, and the like, without negatively impacting yield and / or efficiency.

[0827] Example 20. Support material

[0828] This example presents experiments exploring the characteristics of the support material and their effect on the recovery of one or more target metals from solution. Cellulose is a desirable support material for efficient recovery of target metal by burning as it has a low ash content. Several grades of cellulose were investigated.

[0829] Materials and Methods:

[0830] Fine, medium, and coarse examples of support material were each added to separate samples of 27.1 ppm gold electronic waste leachate together with reducing agent. Here, equivalent amounts of fine cellulose (CFA150, 24-A in Table 11 below), medium cellulose (CFA250, 24-B in Table 11 below), and coarse cellulose (CFA1200, 24-C in Table 3 below), were assessed. Equivalent amounts of ascorbic acid reducing agent was added to each sample as shown in Table 11. All samples were incubated with agitation for 1 hour to allow for reduction and binding of target metal to the support material.

[0831] The target metal-laden support material was then recovered by filtration. The filtrate was tested to determine the concentration of residual target metal, while the amount of gold recovered from the support material was determined by digesting the cellulose support material via anaerobic respiration. Results:

[0832] As shown in Table 11 below, the grade (particle size) of cellulose had no appreciable difference on the yield of gold recovered. However, as shown in Table 11, the time taken to filter the sample comprising coarse cellulose (24-C, CFA1200) was much less than that taken to filter the fine and medium cellulose-containing samples (24-A, CFA150; 24-B, CFA250; respectively).

[0833] Table 11. Effect of support material

[0834]

[0835] These results show that efficient recovery of target metal is possible using the methods described herein with support materials of quite varied coarseness. Interestingly, the use of comparatively coarse support material enables the rapid filtration and separation of metal-laden support material without negatively impacting the efficiency of recovery. Those skilled in the art will appreciate that improvements in processing speeds have the potential to have meaningful commercial impact.

[0836] Example 21. Support material

[0837] This example presents experiments exploring the relative amounts of support material required for efficient recovery of one or more target metals from solution.

[0838] Materials and Methods:

[0839] Coarse support material (cellulose CFA1200) was added in varying amounts to separate samples of 27.1 ppm gold electronic waste leachate along with reducing agent. Here, five amounts of cellulose CFA1200, ranging from 10 mass equivalents (relative to target metal amount) to 185 mass equivalents (as shown in Table 4 below) were assessed. Equivalent amounts of ascorbic acid reducing agent was added to each sample as shown in Table 4. All samples were incubated with agitation for 1 hour to allow for reduction and binding of target metal to the support material.

[0840] As above, the target metal-laden support material was recovered by filtration. The filtrate was tested to determine the concentration of residual target metal, while the amount of gold recovered from the support material was determined by digesting the cellulose support material via anaerobic respiration. Results:

[0841] As shown in Table 12 below, the amount of support material across the assessed range had not appreciable impact on the efficiency of recovery. Greater than 99% recovery was observed in all samples tested, from 25-B, in which a 1:10 mass equivalent of Au:cellulose was employed, resulting in a yield of 99.2%, through to 25-E, in which a 1:185 mass equivalent of Au:cellulose provided a yield of 99.3%.

[0842] Table 12. Effect of amount of support material

[0843]

[0844] These data show that a >99% yield can be achieved when using the methods described herein with as little as a 1:10 mass equivalent of target metal :support material.

[0845] While in certain examples, minimising the amount of support material required to enable efficient recovery will be advantageous, in certain examples (particularly those in which recovery and reuse of the support material is possible) the use of higher mass equivalents of support material can be advantageously employed. As shown in Figure 1, inspection of the filter pads resulting from each sample showed that, at least for small volumes, a greater amount of support material will enable easier recovery of support material from the filter support, thereby minimising loss of target metal by undesired loss of metal laden support material - compare Figure 1 B (25-B) to Figure 1 E (25-E). Example 22. Selectivity of metal recovery

[0846] This example presents the analysis of the experiments described above to elucidate the selectivity of metal recovery, and particularly the selectivity of target metal over base (non-target) metals present in the test solutions.

[0847] Materials and Methods:

[0848] Various samples from the preceding Examples were analysed to determine the concentration of certain base metals commonly seen in electronic waste before and after binding.

[0849] Results:

[0850] As shown in Table 13 below, the methods employed in the Examples above were highly selective for the target metal, gold. From a starting concentration of 27.1 ppm, the amount of residual gold present in filtrate after binding was minimal, with a maximum residual concentration observed in sample 24-C1 of 1.27 ppm.

[0851] In contrast, as presented in Table 13 essentially all of each non-target metal (Sn, Ni, Fe, Al, Zn, Cu, Mg) was retained in solution after gold metal binding took place.

[0852] Table 13. Selectivity of target metal over base metals

[0853]

[0854] These results show that efficient and highly selective recovery of target metal is possible using the methods described herein, even in the presence of complex solutions comprising multiple species of metal ions / multiple non-target metals. Those skilled in the art will appreciate that the methods described and exemplified herein are capable of iterative binding steps, whereby different metals are sequentially targeted for binding / removal from the leachate solution through selection of appropriate reducing agents and / or support materials.

[0855] Example 23. Recovery and reuse of support material

[0856] This example presents experiments exploring the effect of reusing the support material on the recovery of one or more target metals from solution.

[0857] Materials and Methods: Coarse support material (cellulose CFA1200) was added to separate samples of 27.1 ppm gold electronic waste leachate along with ascorbic acid reducing agent. All samples were incubated with agitation for 1 hour to allow for reduction and binding of target metal to the support material.

[0858] As above, the target metal-laden support material was recovered by filtration. The filtrate was centrifuged and tested to determine the concentration of residual target metal, while the amount of gold recovered from a sample of the support material was determined. After the first round of binding, the metal laden support material was air dried for several minutes until it was visually dry. The dried, metal-laden cellulose was then used in a second round of binding. The recovered support material was added to fresh leachate solution comprising 27.1 ppm gold together with fresh ascorbic acid, incubated with agitation, followed by filtration. This procedure was repeated for a third round of binding, again with fresh leachate and reducing agent. The amount of gold recovered was assessed as described in the Examples above.

[0859] Results:

[0860] As shown in Figure 2, substantially complete depletion of gold from the leachate and binding to the reused cellulose support material was achieved in each round of binding. In this case, a gold loading of 1:50 Au:support material was repeated 3 times leading to a theoretical 3:50 Au:cellulose support material loading.

[0861] Example 24. Recovery conditions

[0862] This example presents the analysis of the conditions affecting the binding of target metal to support material and the resulting recovery and yield.

[0863] Materials and Methods:

[0864] Electronic waste feedstock was incubated with chlorine based lixiviant (prepared by dissolving trichloroisocyanuric acid (TCCA) in water) to dissolve target and non-target metals. The reaction was quenched by addition of sodium hydroxide after completion at 3 hours to pH =~7 (to minimize CI2 gas off) and filtered. Once the e-waste had been filtered, aliquots of the resulting solution were then either acidified (with HCI, see Table 6 below), basified (with NaOH, see Table 6) or left unchanged and a standard dechlorination process was applied (0.5mL H2O2 added in a single portion). After the cyanuric acid (derived from TCCA) was filtered from solution, identical binding experiments (50mg cellulose (CFA250) support material, 40mg ascorbic acid) with 30 minutes incubation were performed. Results:

[0865] The results presented in Table 14 below show that a pH of about 4.5 or less when the reaction is dechlorinated substantially improved target metal yield compared to pH of ~7 or above - compare 35-A & 35-B to DY-35-C & 35-D.

[0866] Table 14. Effect of reaction conditions (pH) on recovery

[0867]

[0868] These data suggest that, in order to achieve acceptable binding and useful yield, an acidic pH dechlorination step is desirable.

[0869] Example 25. Target metal recovery

[0870] This example presents the analysis of the conditions affecting the binding of target metals other than gold to the support material and the resulting recovery of such target metals.

[0871] Materials and Methods:

[0872] Palladium (Pd) is also present in e-waste, albeit usually at lower concentrations than gold. A concentrated Pd solution was formed using lixiviant created by exposing spent 10% Pd / C catalyst to chlorine in water. The binding conditions described in the Examples above (40ml solution, 150mg cellulose (CFA250) support material, 40mg ascorbic acid) were used.

[0873] Results:

[0874] The results presented in Table 15 below show that palladium was able to be recovered with high yield.

[0875] Table 15. Recovery of target metals other than gold

[0876]

[0877] These data show that target metals other than gold, in this case palladium, can be readily recovered from concentrated lixiviant using the methods described herein.

[0878] Example 26. Recovery of target metals using various reducing agents

[0879] This example presents the analysis of the use of various reducing agents and their effect on the binding of target and various non-target metals to support material and the resulting selectivity of recovery and on yield.

[0880] Materials and Methods:

[0881] The representative reducing agents ascorbic acid, hydrazine sulfate, citric acid (in equimolar amounts) and sodium borohydride were assessed for gold reduction and selectivity with regard to base metals. Samples of each reductant was incubated with complex mixtures of gold and various non-target metals, one for 1 hour and an otherwise equivalent sample overnight.

[0882] Results:

[0883] As shown in Table 16 below, sodium borohydride was effective to remove all gold within 1 hour, whereas ascorbic acid removed 90% of the gold present at this time point. In contrast, hydrazine only removed ~40% of the gold within this period, and citric acid removed only a negligible amount. After stirring overnight, ascorbic acid had removed all the gold, as had the stronger reductant sodium borohydride. Despite the overnight incubation, gold recovery with hydrazine remained inefficient, with only marginally more gold removed compared to the shorter duration incubation. Overnight incubation with citric acid was ineffective at removing gold from solution.

[0884] Turning to the base (non-target) metals, some degree of removal of each base metal other than sodium was observed after stirring overnight with sodium borohydride. Likewise, some degree of removal of each base metal was observed after overnight stirring with citric acid. Neither ascorbic cid or hydrazine were effective to remove base metals with overnight stirring, showing high selectivity for gold.

[0885] able 16. Effect of reductant on recovery of target and non-target metals

[0886]

[0887] ithout wishing to be bound by any theory, the inventors believe that the base metals removed from solution overnight with sodium borohydride have been reduced, but are not attached to the support material and are as fine particulate flakes. Metals complexed by citric acid are unlikely to have been reduced and instead are electrostatically attached to the cellulose support material. These data show that ascorbic acid and hydrazine both show excellent selectivity for old over the non-target base metals commonly found in e-waste, with ascorbic acid also showing excellent efficacy in recovering gold.

[0888] xample 27: Gold Recovery Using Thiourea Leach

[0889] Materials and Methods: A 150 g e-waste sample undergoes sulfuric acid BML, followed by a thiourea TML and an ascorbic acid-assisted biosorption step. Base Metal Leach (BML) Process:

[0890] 1. E-waste is leached in IL of 3M H2SO4 at 60°C with 0.5% H2O2 for 2 hours.

[0891] 2. The residue is filtered and retained for TML. Target Metal Leach (TML) Process:

[0892] 1. The residue is introduced into a IL solution with 3 g / L thiourea and ferric chloride, maintaining pH at 1.5.

[0893] 2. The mixture is stirred at 50°C, with ORP held around 950 mV for 2 hours.

[0894] Biosorption Process:

[0895] 1. After TML, the solution is adjusted to neutral pH with ascorbic acid (3 g / L) to selectively precipitate gold.

[0896] 2. The solution is then passed through a bed of biosorbent resin, which is effective in retaining fine gold particles.

[0897] Results: The combined thiourea and ascorbic acid biosorption process recovers an expected 88% of gold, with ascorbic acid aiding in selective precipitation and biosorption.

[0898] Example 28: Gold Extraction with Hypochlorite Leach

[0899] Materials and Methods: A 200 g e-waste sample undergoes sulfuric acid BML, followed by a hypochlorite-based TML and a biosorption process using ascorbic acid-coated biomass.

[0900] Base Metal Leach (BML) Process:

[0901] 1. E-waste is leached in 2L of 5M H2SO4 at 45°C with 1% H2O2, stirred for 2 hours.

[0902] 2. The residue is filtered for the TML.

[0903] Target Metal Leach (TML) Process:

[0904] 1. The residue is treated with 40 g / L Ca(OCI)2 in 5% acetic acid, adjusted to pH 3.8, and stirred at 35°C.

[0905] 2. ORP is maintained around 1000 mV over 3 hours.

[0906] Biosorption Process:

[0907] 1. The TML solution is cooled, and ascorbic acid (4 g / L) is added to reduce gold ions to a more biosorbable form.

[0908] 2. The solution is mixed with biomass coated with ascorbic acid, optimized for gold recovery, and stirred for 2 hours.

[0909] Results: The optimized conditions allow for an expected 90% gold recovery, with the ascorbic acid-coated biomass facilitating efficient biosorption of reduced gold ions.

[0910] Example 29: Gold Extraction Using Iodine-Based Leach

[0911] Materials and Methods: A 50 g e-waste sample is processed through sulfuric acid BML, followed by an iodine TML and biosorption with ascorbic acid and resin.

[0912] Base Metal Leach (BML) Process:

[0913] 1. The e-waste is treated in IL of 4M sulfuric acid with 0.5% H2O2, maintained at 40°C for 3 hours.

[0914] 2. The residue is filtered and retained forTML. Target Metal Leach (TML) Process:

[0915] 1. The residue is leached with 1 g / L iodine and 5 g / L KI, with pH adjusted to 7 using MgO, at 30°C.

[0916] 2. ORP is maintained at approximately 950 mV for 4 hours.

[0917] Biosorption Process:

[0918] 1. Ascorbic acid (5 g / L) is added to the leach solution to reduce gold ions.

[0919] 2. The solution is then passed through a biosorption resin bed, specifically designed to capture reduced gold.

[0920] Results: The use of iodine, ascorbic acid, and biosorption resin achieves an expected 88% total gold recovery. The ascorbic acid helps in reducing gold ions, improving biosorption efficiency.

[0921] Example 30: Dual Oxidant TML

[0922] Materials and Methods: A 120 g e-waste sample undergoes sulfuric acid BML, followed by TML with TCCA and hypochlorite under optimized conditions, with biosorption enhanced by ascorbic acid.

[0923] Base Metal Leach (BML) Process:

[0924] 1. The e-waste is treated with 1.5L of 4M H2SO4 at 50°C for 2.5 hours with air sparging.

[0925] 2. The residue is filtered and prepared for TML.

[0926] Target Metal Leach (TML) Process:

[0927] 1. The residue is added to a IL solution containing 30 g / L TCCA and 5 g / L Ca(OCI)2, adjusting pH to 4.2.

[0928] 2. The TML is conducted at 42°C, with ORP set at 1150 mV for 4 hours.

[0929] Biosorption Process:

[0930] 1. After TML, cellulose support and ascorbic acid (3 g / L) are introduced to reduce dissolved gold ions.

[0931] 2. The solution is stirred with cellulose-based biosorbent that has been treated with ascorbic acid, allowing selective gold binding.

[0932] Results: This process yields an expected total gold recovery of 94%, with the ascorbic acid-treated algae enhancing gold biosorption by creating a more effective binding environment.

[0933] Example 31: H2SO4 and H2O2 BML with TCCA TML

[0934] Materials and Methods: A 100 g e-waste sample is processed with a sulfuric acid and H2O2 BML, followed by TCCA TML and ascorbic acid biosorption.

[0935] Base Metal Leach (BML) Process:

[0936] 1. E-waste is treated in IL of 5M H2SO4 with 1% H2O2 at 45°C for 3 hours.

[0937] 2. The residue is retained for TML.

[0938] Target Metal Leach (TML) Process:

[0939] 1. The residue is leached in a IL solution with 25 g / L TCCA, at pH 4.5. 2. ORP is maintained at 1050 mV, with temperature held at 40°C for 3.5 hours. Biosorption Process:

[0940] 1. Ascorbic acid (2 g / L) is added to the TML solution to reduce gold ions.

[0941] 2. The solution is mixed with cellulose-based biosorbent pre-treated with ascorbic acid, enhancing gold biosorption.

[0942] Results: An expected 91% gold recovery is achieved with this process, with ascorbic acid facilitating biosorption efficiency.

[0943] Example 32: Gold Extraction via Sequential BML, TML with TCCA, and Ascorbic Acid Biosorption

[0944] Materials and Methods: A 100 g sample of e-waste undergoes a sulfuric acid BML, followed by a TML with TCCA, and is finally subjected to biosorption using ascorbic acid.

[0945] Base Metal Leach (BML) Process:

[0946] 1. The e-waste is treated in a 2L solution of 5M sulfuric acid (H2SO4) at 50°C with air sparging for 3 hours.

[0947] 2. After filtering, the residue is retained for TML as a TML feedstock containing less than 5% base metals.

[0948] Target Metal Leach (TML) Process:

[0949] 1. The residue is added to a IL solution containing 30 g / L TCCA with 5% acetic acid, adjusted to pH 4.5, at 42°C.

[0950] 2. The mixture is stirred with pH maintained at pH 5 for 4 hours.

[0951] Biosorption Process:

[0952] 1. The TML solution containing dissolved gold is treated with ascorbic acid (2 g / L) as a reducing agent at room temperature for 1 hour.

[0953] 2. Cellulose-based biosorbent is then introduced at a 1:200 Au-to-biosorbent ratio, stirred for an additional 2 hours to capture any remaining gold.

[0954] Results: The combination of TML and ascorbic acid biosorption yields an expected total gold recovery of 92%, with ascorbic acid enhancing gold capture in the biosorption step.

[0955] Example 33: H2SO4 BML with Sequential H2O2 Addition and Thiourea TML with Ascorbic Acid Biosorption

[0956] Materials and Methods:

[0957] A 120 g e-waste sample undergoes a sulfuric acid and H2O2 BML, followed by thiourea TML and ascorbic acid biosorption.

[0958] Base Metal Leach (BML) Process:

[0959] 1. The e-waste is treated in a IL solution of 3M sulfuric acid with an initial H2O2 dose (0.5% concentration) at 60°C for 2 hours to enhance base metal dissolution. 2. An additional dose of H2O2 (0.25%) is added after the first hour to maintain an optimal ORP around 450 mV.

[0960] 3. Following the leach, the residue is filtered, washed, and prepared for the TML step.

[0961] Target Metal Leach (TML) Process:

[0962] 1. The filtered residue is placed in a solution containing 3 g / L thiourea and 1.5 g / L ferric chloride at pH 1.5, with temperature controlled at 40°C.

[0963] 2. The reaction is maintained for 2 hours with ORP controlled at approximately 950 mV to optimise gold leaching efficiency.

[0964] 3. The solution is filtered, retaining the lixiviant for the subsequent biosorption process.

[0965] Biosorption Process:

[0966] 1. Ascorbic acid (2 g / L) is introduced into the thiourea lixiviant to reduce gold ions, adjusted to pH 3 for optimal biosorption conditions.

[0967] 2. The solution is stirred with a cellulose-based biosorbent for 1 hour, allowing gold to adhere effectively to the biosorbent material.

[0968] Expected Results: This process is anticipated to yield a high recovery of gold, with ascorbic acid enabling selective reduction of gold ions and efficient biosorption onto the cellulose-based support. The combination of H2O2-enhanced sulfuric acid BML, controlled thiourea leach conditions, and ascorbic acid biosorption is projected to achieve a recovery efficiency of approximately 90%.

[0969] Example 34: Gold Extraction Using Sequential BML, TML with TCCA, and Biosorption with Hydrazine Sulfate

[0970] Materials and Methods: A 100 g sample of e-waste undergoes sulfuric acid BML, followed by TML with TCCA, and a biosorption step using hydrazine sulfate as the reducing agent.

[0971] Base Metal Leach (BML) Process:

[0972] 1. The e-waste is treated in a 2L solution of 5M sulfuric acid (H2SO4) at 50°C with air sparging for 3 hours to dissolve base metals.

[0973] 2. The residue is filtered and retained for TML.

[0974] Target Metal Leach (TML) Process:

[0975] 1. The residue is leached in a IL solution containing 30 g / L TCCA and 5% acetic acid, with pH adjusted to 4.5 and temperature held at 42°C.

[0976] 2. ORP is maintained at approximately 1100 mV for 4 hours.

[0977] Biosorption Process:

[0978] 1. The TML solution containing dissolved gold ions is adjusted with hydrazine sulfate (2 g / L) at pH 3 to reduce gold ions.

[0979] 2. The solution is then stirred with chitosan-based biosorbent for 1 hour to bind the reduced gold. Results: This combination of TML and hydrazine sulfate biosorption is expected to yield an expected 89% recovery of gold, with hydrazine sulfate effectively reducing gold ions for efficient biosorption. Example 35: Gold Recovery Using Thiourea Leach with Citric Acid Biosorption

[0980] Materials and Methods: A 150 g e-waste sample is subjected to sulfuric acid BML, followed by a thiourea TML and citric acid biosorption.

[0981] Base Metal Leach (BML) Process:

[0982] 1. E-waste is treated in a IL solution of 3M sulfuric acid (H2SO4) at 60°C for 2 hours with 0.5% H2O2 to enhance base metal dissolution.

[0983] 2. The residue is filtered and retained for TML.

[0984] Target Metal Leach (TML) Process:

[0985] 1. The residue is transferred to a IL solution containing 3 g / L thiourea and 1.5 g / L ferric chloride, with pH adjusted to 1.5 and ORP controlled at 950 mV.

[0986] 2. The mixture is stirred at 50°C for 2 hours.

[0987] Biosorption Process:

[0988] 1. The TML solution is treated with an equimolar amount of citric acid (2 g / L) to reduce gold ions at pH 4.

[0989] 2. A cellulose-based biosorbent is introduced, and the solution is stirred for 2 hours to maximize gold capture.

[0990] Results: This method achieves a total gold recovery of approximately 85%, with citric acid effectively aiding in biosorption by reducing gold ions to a biosorbable form.

[0991] Example 36: Gold Extraction Using Hypochlorite Leach and Biosorption with Sodium Borohydride

[0992] Materials and Methods: A 200 g e-waste sample undergoes sulfuric acid BML, followed by hypochlorite-based TML, and biosorption with sodium borohydride.

[0993] Base Metal Leach (BML) Process:

[0994] 1. The e-waste is treated in a 2L solution of 5M sulfuric acid with 1% H2O2 at 45°C for 2 hours.

[0995] 2. The residue is filtered and retained for TML.

[0996] Target Metal Leach (TML) Process:

[0997] 1. The residue is leached in a solution of 40 g / L calcium hypochlorite (Ca(OCI)2) in 5% acetic acid, with pH adjusted to 3.8.

[0998] 2. The TML is conducted at 35°C with ORP controlled at 1000 mV over a period of 3 hours.

[0999] Biosorption Process:

[1000] 1. Sodium borohydride (1 g / L) is added to the TML solution to reduce gold ions.

[1001] 2. The solution is then stirred with a biosorbent resin for 1 hour, allowing the gold to bind

[1002] effectively. Results: This approach yields an expected 90% recovery rate of gold, with sodium borohydride reducing gold ions effectively for biosorption.

[1003] Example 37: Gold Extraction Using Dual Oxidant TML and Biosorption with Citric Acid Materials and Methods: A 120 g e-waste sample undergoes sulfuric acid BML, followed by a TML using TCCA and hypochlorite, and biosorption with citric acid.

[1004] Base Metal Leach (BML) Process:

[1005] 1. The e-waste is treated in a 1.5L solution of 4M sulfuric acid at 50°C for 2.5 hours, with air sparging.

[1006] 2. After filtering, the residue is prepared for TML.

[1007] Target Metal Leach (TML) Process:

[1008] 1. The residue is introduced into a IL solution containing 30 g / L TCCA and 5 g / L Ca(OCI)2, with pH adjusted to 4.2.

[1009] 2. The TML is conducted at 42°C, with ORP set at 1150 mV and maintained for 4 hours.

[1010] Biosorption Process:

[1011] 1. Citric acid (equimolar at 2 g / L) is added to the TML solution at pH 5 to reduce gold ions. 2. The solution is stirred with ascorbic acid-coated biosorbent for 1 hour.

[1012] Results: The optimized conditions yield a expected 92% recovery of gold, with citric acid facilitating effective biosorption by stabilizing reduced gold ions for biosorption.

[1013] Example 38: Gold Extraction Using Sequential BML and TML with Sodium Borohydride Biosorption

[1014] Materials and Methods: A 150 g e-waste sample is processed through sulfuric acid BML, TCCA TML, and sodium borohydride biosorption.

[1015] Base Metal Leach (BML) Process:

[1016] 1. The e-waste is leached in a 1.5L solution of 4M sulfuric acid at 50°C for 3 hours.

[1017] 2. After filtering, the residue is retained for TML.

[1018] Target Metal Leach (TML) Process:

[1019] 1. The residue is added to a IL solution containing 25 g / L TCCA at pH 4.5.

[1020] 2. The solution is stirred at 40°C, with ORP held at 1050 mV for 3.5 hours.

[1021] Biosorption Process:

[1022] 1. Sodium borohydride (1.5 g / L) is added to the TML solution to reduce gold ions.

[1023] 2. The solution is then mixed with a cellulose-based biosorbent for 1 hour to capture the gold.

[1024] Results: This sequence yields an expected 88% recovery of gold, with sodium borohydride acting as an efficient reducing agent, enabling enhanced biosorption.

[1025] Example 39. - Effect of other biomass derived support materials Other non-cellular biomass derived support materials, in addition to cellulose, may be used to recover gold from solution (see table below). A solution containing 20ppm gold derived from leaching e-waste as described in previous examples was used to test this. In the presented cases, less gold is retained in the solid filtered from solution compared to cellulose- a fact that is wholly due to the solubility of the biological supports in lixiviant media or their particle size. Any soluble component of support material, or component smaller than the filter pore, still binds gold, but it is not in a filterable and therefore recoverable form.

[1026] The trend of deposition yields of cellulose>starch>chitin>chitosan>lecithin observed supports this. Cellulose is completely insoluble and has a robust polymer structure, whereas the amylose component of starch can form fine colloidal suspensions capable of passing through a standard sized filter (approx. 10pm). Although chitin is traditionally insoluble, hydrolysis of the acetyl groups can convert it into chitosan, which is fairly soluble in the acidic lixiviant conditions (pH=l). Finally, lecithin is partially soluble in water and forms emulsions rendering the filterable component very small. In our experiments, lecithin formed a thin gel which could not be filtered under standard conditions used hence the lack of deposition yield.

[1027] Without wishing to be bound by any theory, the inventors believe it is therefore important to select a biosorbent that is totally insoluble in the target metal-pregnant solution for maximum yields of recovered target metal. Without wishing to be bound by any theory, the inventors believe that certain non-cellular support materials are likely to be more suitable in some applications, for example applications not explicitly detailed in this Example.

[1028] Table 17. Effect of type of biomass on recovery of target metal

[1029]

[1030] Example 40 - Characterisation of gold on biomass

[1031] In the cases presented here, the target metal gold is reduced by the reductant to a nanoparticle form on the biomass. To prove this, gold laden biomass was analysed by SEM (scanning electron microscopy), SEM-EDS (electron dispersive spectroscopy), and XPS (X-ray photoelectron spectroscopy).

[1032] Analysis of biomass from the cellulose material generated in Example 10 above is presented in Figure 3. In contrast to solely cellulose substrate (Figure 3a), the gold laden cellulose biomass (Figure 3b) contains nanoparticulate material as seen by SEM. This was confirmed to be gold by SEM-EDS (Figure 3c), which also highlighted the purity of the particles by absence of any other signals in the spectrum. XPS spectra of these show they exist in the form of Au(0) determined by the two spin-orbit split signals at 84.2 eV (4f 7 / 2)and 87.8 eV (4f 5 / 2) (Figure 3d)

[1033] Example 41

[1034] Methodology A milled electronic waste feedstock (D80 « 500 m) is subjected to a base metal leach comprising water, acetic acid (2-3 M), hydrochloric acid (3-4.5 M), and hydrogen peroxide (1-2 M). The leach is conducted at 40-60 °C for 1-6 hours under agitation, maintaining an oxidation-reduction potential (ORP) between 500 and 700 mV versus Ag / AgCl. After removal of the base metal leachate, the solid residue contains less than 5 wt % base metals. The residue is then subjected to a target metal leach comprising trichloroisocyanuric acid (TCCA) at pH 3-7 and 30-45 °C for 1-6 hours under mild agitation. Following leaching, the target metal-pregnant solution is contacted with cellulose-based support material in the presence of ascorbic acid (0.5-2 mM) at 20-40 °C for 1-3 hours to recover dissolved metals.

[1035] Results

[1036] The base metal leach removes more than 90 % of copper, nickel, tin, and lead. The subsequent target metal leach dissolves more than 90 % of gold and a significant proportion of silver while maintaining reagent stability. The biosorption step captures more than 90 % of the dissolved target metals, and cooling of the raffinate to 4 °C precipitates more than 95 % of silver as silver chloride.

[1037] Example 42

[1038] Methodology

[1039] A comminuted waste catalyst and printed circuit board mixture is subjected to a base metal leach comprising sulphuric acid (150-250 g Lx) and hydrogen peroxide (0.5-2 % v / v) at 50-80 °C for 2-4 hours. The residue, containing less than 2 wt % base metals, is then contacted with an organic halide lixiviant comprising 3-methoxy-3-methyl-l-butanol containing lithium bromide (1-3 mol Lx) and lithium bromate (0.1-0.5 mol Lx). The leach is conducted at 40-70 °C for 8-24 hours with a pulp density of 10 wt % and an ORP maintained above 600 mV versus Ag / AgCl. The resulting pregnant solution is contacted with a biphasic aqueous phase containing chitosan and ascorbic acid to transfer and bind dissolved precious metals onto the biopolymer support.

[1040] Results

[1041] The base metal leach removes more than 95 % of copper and nickel. The organic halide leach achieves greater than 95 % gold and 90 % palladium dissolution under closed-loop solvent conditions. The biosorption step transfers more than 90 % of the dissolved metals to the solid phase, with less than 1 % organic solvent loss per cycle.

[1042] Example 43

[1043] Methodology

[1044] Comminuted printed circuit boards containing high copper content are subjected to a base metal leach comprising hydrochloric acid (2-4 M) and hydrogen peroxide (1-2 M) at 25-60 °C for 0.5-5 hours with agitation, maintaining an ORP of 500-700 mV. The residue is then leached in an organic solvent comprising dimethylformamide containing cupric bromide (0.1-0.5 mol Lx) and bromine as oxidant. The leach is conducted at 20-90 °C for 0.5-24 hours with a pulp density of less than 25 wt %. The pregnant organic phase is stripped with 0.2-0.5 M ascorbic acid at 20-60 °C, and the aqueous strip solution is contacted with a cellulose support for 1-3 hours at 25 °C. Results

[1045] The base metal leach reduces copper and nickel levels in the residue to less than 5 wt %. The organic halide leach dissolves over 90 % of gold within 2-8 hours and achieves a high selectivity for precious metals over base metals. The stripped aqueous phase yields over 90 % recovery of gold on the cellulose support, while the organic phase is successfully regenerated by oxidation of cuprous to cupric halide.

[1046] Example 44

[1047] Methodology

[1048] A mixed electronic waste concentrate containing gold, silver, and palladium is subjected to a sulphate-based base metal leach comprising sulphuric acid (100-300 g Lx) and hydrogen peroxide at 80-180 °C under air sparging for 0.5-8 hours. The resulting residue is conditioned by either maintaining acidic conditions (25-300 g L1H2SO4) or neutralising to pH 10-14 using sodium hydroxide to improve permeability. The residue is then treated with a chloride leach comprising hydrochloric acid (2-6 M) and sodium hypochlorite as oxidant, maintaining an ORP of 600-950 mV and temperature between 80-120 °C for 0.5-24 hours. The resulting pregnant solution is contacted with an ion-exchange resin containing thiol functional groups at 20-80 °C for 0.5-6 hours.

[1049] Results

[1050] The sulphate leach removes more than 90 % of base metals and yields a porous residue enriched in precious metals. The chloride leach dissolves more than 95 % of gold and silver and more than 85 % of platinum-group metals. The ion-exchange step selectively binds precious metals, achieving recovery efficiencies above 90 %. The chloride solution is regenerated and reused, maintaining a closed-loop process with reduced reagent demand.

[1051] Example 45

[1052] Methodology

[1053] Ground electronic waste containing gold and solder metals is subjected to a selective base metal leach comprising acetic acid (2-4 M) and hydrogen peroxide (0.5-1.5 M) at 25-40 °C for 2-8 hours to dissolve tin and lead oxides. A subsequent mild sulphuric-acid leach is applied to remove remaining base metals, leaving less than 5 wt % in the residue. The residue is then treated with an ammoniacal thiosulphate lixiviant comprising ammonium thiosulphate (0.1-1.0 M), ammonia (0.5-2 M), and copper(II) ions (50-500 ppm) at pH 9-10.5 and 25-50 °C for 2-12 hours under aeration. The pregnant solution is contacted with cellulose in the presence of hydrogen peroxide (0.5-2 mM) at 20-40 °C for 1-3 hours.

[1054] Results

[1055] The selective leach removes more than 90 % of tin and lead while retaining the majority of copper and noble metals. The ammoniacal thiosulphate leach dissolves more than 85 % of gold with minimal co-dissolution of base metals. Biosorption recovers more than 90 % of dissolved gold from the pregnant solution, and the thiosulphate reagent is regenerated with minimal degradation, supporting closed-loop operation. Example 46

[1056] Methodology

[1057] Comminuted printed-circuit-board waste containing gold, copper, and nickel is subjected to a basemetal leach comprising hydrochloric acid (4-6 M) and air oxidation at 25-70 °C for 0.5-6 hours, maintaining an oxidation-reduction potential (ORP) of 500-750 mV with agitation. The leached residue is separated and transferred directly to a precious-metal leach containing bromine and sodium bromide in aqueous medium.

[1058] The bromine lixiviant is prepared by dissolving 0.05-0.2 mol L1Br? in 0.3-1.0 mol L1NaBr, generating in situ tribromide species. Leaching is carried out at 20-80 °C for 0.5-12 hours with a solids loading below 25 wt %, optionally with air sparging at 0.5-2 L min1to maintain oxidising conditions (ORP = 200-500 mV).

[1059] The resulting bromide pregnant solution is contacted with a solid support, for example cellulose or another biopolymer, under conditions suitable to bind dissolved gold. Other recovery methods described elsewhere herein - such as precipitation, solvent extraction, cementation, electrowinning, or thermal recovery - may likewise be employed. The barren bromide solution may be regenerated by oxidation of bromide or bromine species with air or hypobromite before reuse. The barren bromide solution may be regenerated by oxidation of bromide or bromine species with air or hypobromite before reuse.

[1060] Results

[1061] The base-metal leach decreases copper and nickel contents of the solid residue to below 5 wt %, providing a clean substrate for precious-metal dissolution. The bromine / sodium-bromide lixiviant may be used to dissolve > 90 % of gold within 4-8 hours, demonstrating high selectivity toward precious metals. Subsequent recovery using the described methods may be used to yield > 90 % overall gold recovery, and the regenerated bromide lixiviant may retain > 90 % of its oxidising capacity for repeated use.

[1062] ***

[1063] Any method detailed herein also corresponds to a disclosure of a device and / or system configured to execute one, or more, or all, of the method actions. Likewise, any disclosure of a device and / or system detailed herein corresponds to a method of making and / or using the device and / or system, including a method of using that device according to the functionality detailed herein. And any disclosure of a device and / or system detailed herein also corresponds to a disclosure of otherwise providing that device and / or system.

[1064] It should be noted that various changes and modifications to the presently preferred examples described herein will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be included within the present invention.

[1065] The invention has been described herein, with reference to certain preferred examples, in order to enable the reader to practice the invention without undue experimentation. However, a person having ordinary skill in the art will readily recognise that many of the components and parameters may be varied or modified to a certain extent or substituted for known equivalents without departing from the scope of the invention. It should be appreciated that such modifications and equivalents are herein incorporated as if individually set forth.

[1066] Where in the foregoing description reference has been made to integers or components having known equivalents thereof, those integers are herein incorporated as if individually set forth.

[1067] Titles, headings, or the like are provided to enhance the reader's comprehension of this document, and should not be read as limiting the scope of the present invention.

[1068] The entire disclosures of all applications, patents and publications, cited above and below, if any, are hereby incorporated by reference. The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge in any country in the world.

[1069] The invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, in any or all combinations of two or more of said parts, elements or features.

[1070] In at least some exemplary examples, any feature disclosed herein can be utilized in combination with any other feature disclosed herein unless otherwise specified. While various examples have been described, they have been presented by way of example only, and not limitation. Changes in form and / or detail can be made therein without departing from the invention.

[1071] Aspects of the invention have been described by way of example only, and it should be appreciated that variations, modifications and additions may be made without departing from the scope of the invention, for example when present the invention as defined in the claims. Furthermore, where known equivalents exist to specific features, such equivalents are incorporated as if specifically referred in this specification.

Claims

CLAIMS1. A method of recovering one or more target metals from solid feedstock, the method comprising:a) providing solid feedstock, said solid feedstock comprising at least one target metal and one or more non-target metals;b) contacting the solid feedstock with a base metal lixiviant in a base metal leach to provide a base metal leach solution, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids, and one or more oxidants;c) removing the base metal leach solution to yield a target metal leach (TML) feedstock; d) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds; and any combination of any two or more thereof;e) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metal-pregnant solution; f) recovering at least some of the one or more target metals from the target metal-pregnant solution.

2. A method of recovering one or more target metals from a solution, wherein the solution comprises dissolved metal ions of at least one target metal, the method comprising:a) providing a solution comprising dissolved metal ions of at least one target metal, wherein the solution has been prepared in a method comprising:i) contacting a solid feedstock comprising one or more target metals and one or more non-target metals with a base metal lixiviant, wherein the base metal lixiviant comprises, consists essentially of, or consists of one or more of the group consisting of: one or more organic acids, one or more mineral acids; one or more oxidants; ii) contacting at least a portion of the resulting base metal leach solution with hydrogen peroxide in a second base metal leach to provide a said solution comprising dissolved metal ions of at least one target metal and dissolved metal ions of at least one non- target metals;ill) removing the base metal leach solution to yield a target metal leach (TML) feedstock; iv) adding to the TML feedstock a target metal lixiviant in a target metal leach, said target metal lixiviant selected from the group consisting of: one or more halogen-containing compounds; one or more sulphur-containing compounds;v) maintaining the target metal leach for a time and under conditions suitable for dissolution of at least some of the one or more target metals to form a target metalpregnant solution;b) optionally adding at least one target metal recovery agent to the target metal-pregnant solution;c) recovering at least some of the one or more target metals from the target metal-pregnant solution.

3. The method according to claim 1 or claim 2 wherein the recovery of at least some of the one or more target metals from the target metal-pregnant solution comprise:a) optionally adding at least one target metal recovery agent to the target metal-pregnant solution;b) contacting the target metal-pregnant solution with a support material;c) maintaining the solution for a time and under conditions suitable to bind the at least one target metal to the support material to form a target metal-laden support material; and d) separating the metal-laden support material from the target metal-barren solution.

4. The method according to any one of the preceding claims, wherein recovery of the one or more target metals from the target-metal-pregnant solution comprises one or more steps selected from the group consisting of:a) contacting the target-metal-pregnant solution with a biosorbent or solid support material comprising cellulose, lignocellulose, chitosan, chitin, starch, or other biopolymer under conditions suitable to bind the one or more target metals to form a target-metal-laden support material;b) precipitating one or more target metals by addition of a reagent selected from oxalate, hydroxide, carbonate, sulphide, or cysteine to form an insoluble target-metal compound; c) recovering one or more target metals by cementation, comprising contacting the target- metal-pregnant solution with an elemental reductant selected from iron, zinc, aluminium, or copper under acidic conditions to deposit the target metal;d) extracting one or more target metals into an organic phase using a solvent-extraction reagent selected from methyl isobutyl ketone (MIBK), tributyl phosphate, di-2-ethylhexyl phosphoric acid, or mixtures thereof;e) electrochemically recovering one or more target metals by electrowinning or electrorefining from the target-metal-pregnant solution or from an eluate derived therefrom; or f) recovering one or more target metals by thermal or pyrometallurgical treatment of a metalladen support material to yield an ash or metallic concentrate enriched in the one or more target metals.

5. The method according to any one of claims 1 to 4 wherein the solid feedstock is particulate solid feedstock.

6. The method according to any one of claims 1 to 5 wherein one of the target metals is selected from the group consisting of gold, silver, palladium, platinum, and rhodium.

7. The method according to any one of the preceding claims wherein the solid feedstock material is selected from the group consisting of e-waste, precious metal bearing ore, precious metal bearing sand, precious metal bearing clay, and a combination of any two or more thereof.

8. The method according to any one of the preceding claims wherein the target metal is gold.

9. The method according to any one of the preceding claims wherein the solid feedstock material is selected from the group consisting of e-waste, gold bearing ore, gold bearing sand, gold bearing clay and a combination of any two or more thereof.

10. The method according to claim 8 wherein the solid feedstock material is selected from the group consisting of e-waste.

11. The method according to any one of the preceding claims wherein the base-metal lixiviant comprises, consists essentially of, or consists of:a) water, acetic acid, hydrochloric acid, and hydrogen peroxide (H2O-CH3COOH-HCI-H2O2); orb) hydrochloric acid and hydrogen peroxide (HCI-H2O2); orc) acetic acid and hydrogen peroxide (CH3COOH-H2O2); ord) hydrochloric acid and acetic acid without an added oxidant; ore) hydrochloric acid and acetic acid together with an oxidant selected from nitric acid, ferric chloride, or sodium hypochlorite; orf) an acid-oxidant system comprising one or more organic acids and one or more oxidants selected from hydrogen peroxide, ferric ions, manganese dioxide, or nitrate ions; or g) a sulphate-based system comprising sulphuric acid and water optionally containing one or more oxidising agents selected from oxygen, air, hydrogen peroxide, ferric ions, manganese dioxide, or nitrate ions; orh) sulphuric acid and hydrogen peroxide; orI) any combination of two or more of the foregoing lixiviant systems.

12. The method according to any one of the preceding claims wherein the base metal lixiviant comprises, consists essentially of, or consists of sulfuric acid, optionally together with one or more oxidants.

13. The method according to any one of the preceding claims wherein the target metal lixiviant comprisesa) one or more chlorine-containing compounds selected from trichloroisocyanuric acid (TCCA), dichloroisocyanuric acid (DCCA), sodium trichloroisocyanurate, sodium dichloroisocyanurate, or chlorine; orb) one or more halogen-containing compounds selected from bromine, iodine, chlorine, or halide salts and their corresponding oxidants, including lithium bromide and lithium bromate, sodium bromide and sodium bromate, or lithium chloride and lithium chlorate; or c) a solvometallurgical system comprising one or more halide salts and one or more oxidising agents dissolved in an organic solvent selected from 3-methoxy-3-methyl-l-butanol (MMB), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile, methanol, or ethanol; ord) an organic halide system comprising an organic solvent as described in (c) and one or more cupric halides selected from cupric chloride or cupric bromide; ore) an aqueous chloride lixiviant comprising hydrochloric acid or a chloride brine together with an oxidising agent selected from chlorine gas, hypochlorous acid, sodium hypochlorite, hydrogen peroxide, or nitrate ions; orf) a sulphur-containing lixiviant comprising one or more of thiourea, thiosulphate, or thiocyanate; org) a cyanide lixiviant comprising sodium cyanide or potassium cyanide in aqueous medium; or h) an iodide lixiviant comprising iodine and potassium iodide; orI) any combination of two or more of the foregoing lixiviant systems..

14. The method according to claim 13 wherein the target metal lixiviant comprises an aqueous solution comprising one or more chlorine-containing compounds.

15. The method according to claim 14 wherein the target metal lixiviant comprises an aqueous solution comprising one or more chlorine-containing compounds and hydrogen peroxide.

16. The method according to any one of claims 13 to 15 wherein one or more of the one or more chlorine-containing compounds is selected from the group consisting of: TCCA, DCCA, and chlorine.

17. The method according to any one of the preceding claims wherein the target metal lixiviant comprises one or more iodine-containing compounds and / or one or more bromine-containing compounds.

18. The method according to any one of the preceding claims wherein one or more oxidants or one or more target metal recovery agents comprises hydrogen peroxide.

19. The method according to any one of the preceding claims wherein the conditions suitable for binding of the at least one target metal to the support material include the presence of one or more organic acids, and / or one or more mineral acids.

20. The method according to claim 19 wherein the organic acid is selected from the group consisting of: ascorbic acid, citric acid, formic acid, lactic acid, malic acid, oxalic acid, tartaric acid, and uric acid.

21. The method according to claim 20 wherein the organic acid is ascorbic acid.

22. The method according to any one of the preceding claims wherein the support material comprises, consists essentially of, or consists of cellulose or a cellulosic material, and / or the support material comprises, consists essentially of, or consists of non-cellular biomass, and / or the support material comprises, consists essentially of, or consists of non-microbial biomass.

23. The method according to any one of the preceding claims wherein the target metal-laden support material is separated from the solution by filtration.

24. The method according to any one of the preceding claims wherein the at least one target metal is recovered from the target metal-laden support material by ashing.

25. The method according to any one of the preceding claims, wherein the target metal lixiviant comprises, consists essentially of, or consists of TCCA, and the target metal leach is maintained: a) a pH of from about 3 to 7; orb) for a time of about 1 to 6 hours; orc) a temperature of from about 30 °C to about 45 °C; ord) any two or more of a) to c); ore) each of a) to c).

26. The method according to claim 25, wherein the pH is maintained by addition, including periodic addition, of a pH increasing agent.

27. The method according to claim 26 wherein the pH-increasing agent comprises one or more of magnesium hydroxide, calcium hydroxide, calcium hypochlorite, magnesium oxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide, sodium carbonate, or sodium bicarbonate.

28. The method according to any one of the preceding claims wherein when the solid material is electronic waste, the base metal leach is preceded by one or more pre-processing steps selected from the group consisting of: chip removal; grinding, milling, or comminuting electronic waste, for example to a preselected size; removal of certain density fractions; removal of certain size particles or fractions; removal of one or more magnetic materials; removal of at least a portion of non-target material; and any combination of two or more thereof.

29. The method according to any one of the preceding claims wherein the conditions suitable for binding of the at least one target metal to the support material include the presence of one or more reducing agents having a reducing potential effective to reduce only one species of metal ions present in the solution to metal.

30. The method according to any one of the preceding claims wherein the conditions suitable for binding of the at least one target metal to the support material include the presence of one or more reducing agents having a reducing potential effective to reduce only one species of the target metal ions present in the solution to target metal.

31. The method according to any one of the preceding claims wherein the conditions suitable for binding of the at least one target metal to the support material include the presence of one or more reducing agents having a reducing potential effective to reduce only one species of the target metal ions present in the solution to target metal and ineffective in reducing all species of non-target metal ions present in the solution.

32. The method according to any one of the preceding claims wherein the conditions suitable for binding of the at least one target metal to the support material include the presence of one or more reducing agents having a reducing potential effective to reduce at least one species of target metal ions present in the solution to target metal, but ineffective to reduce all species of non-target metal ions.

33. The method according to any one of claims 29 to 32 wherein the reducing agent is selected from the group consisting of organic acids, hydrazines, hydrides, borohydrides, and inorganic acids.

34. The method according to any one of claims 29 to 33 wherein the reducing agent comprises, consists essentially of, or consists of one or more inorganic acids such as one or more mineral acids, or one or more organic acids.

35. The method according to any one of claims 29 to 34 wherein the reducing agent is selected from the group consisting of ascorbic acid or a salt thereof, citric acid or a salt thereof, formic acid or a salt thereof, lactic acid or a salt thereof, malic acid or a salt thereof, oxalic acid or a salt thereof, tartaric acid or a salt thereof, and uric acid or a salt thereof.

36. The method according to any one of claims 29 to 35 wherein when one of the species of target metal ions is gold, the reducing agent is ascorbic acid or a salt thereof.

37. The method according to any one of the preceding claims wherein the target metal-pregnant solution is an aqueous solution containing more than lOppm of the target metal.

38. The method according to any one of the preceding claims wherein at least about 90% of the target metal is bound to the support material and / or recovered.

39. The method according to claim 38 wherein at least about 95%, or at least about 99%, of the target metal is bound to the support material and / or recovered.

40. The method according to any one of the preceding claims wherein the concentration factor of the target metal from the target metal-pregnant solution to the support material is greater than 100.

41. The method according to claim 40 wherein the concentration factor of the target metal from the target metal-pregnant solution to the support material is greater than 1000.

42. The method according to any one of the preceding claims wherein in the maintaining step the support material is in contact with the target metal-pregnant solution for between about 0.5 and 48 hours.

43. The method according to any one of the preceding claims wherein the recovery step comprises burning of the target metal-laden support material or chemical dissolution of the target metal / support material complex to release the target metal.

44. The method according to any one of the preceding claims wherein the target metal-pregnant solution comprises at least one non-target metal and the reducing agent preferentially reduces the target metal over the non-target metal, and the non-target metal(s) remains in the solution in the separating step.

45. The method according to any one of the preceding claims wherein the target metal is bound to the support material over the non-target metal in the binding step such that the mass ratio of target metal to non-target metal on the support material increases by a factor of at least 2 when compared to the mass ratio in the target metal-pregnant solution.

46. The method according to any one of the preceding claims wherein the non-target metal is selected from one or more of lead, copper, and / or nickel.

47. The method according to any one of the preceding claims wherein the solid feedstock comprises a solid material comprising less than 5% of target metal.

48. The method according to any one of the preceding claims wherein the target metal lixiviant comprises a thiourea-based solution, or a thiosulphate-based solution, or a thiocyanate-based solution, or a halogen-based solution.

49. The method according to any one of the preceding claims wherein the pH of the target metalpregnant solution prior to the recovery step is maintained within the range of from about 3 to about 10.

50. The method according to any one of the preceding claims wherein at least a portion of the support material and / or the target metal-barren solution is reused in a further repeat of the method.

51. The method according to any one of the preceding claims wherein one or more additional components are added to the target metal-barren solution such that it can act as a lixiviant, such as a base metal lixiviant.

52. The method according to claim 51 wherein the one or more additional components are selected from one or more of thiourea, thiosulphate, thiocyanate, a halogen, nitric acid, and hydrochloric acid.

53. The method according to any one of the preceding claims wherein the target metal-barren solution is treated with chlorine gas.

54. The method according to any one of the preceding claims wherein at least 25% of the target metal-barren solution is reused.

55. The method according to any one of the preceding claims wherein the non-target material includes one or more base metal(s).

56. A method of preparing a target metal-laden cellulose material, the method comprising:a) adding to a solution comprising dissolved metal ions of at least one target metal and dissolved metal ions of at least one non-target metal prepared in a method as claimed herein a filterable cellulose material and a reducing agent, wherein the reducing agent has a reducing potential effective to reduce at least one species of target metal ions to target metal but ineffective to reduce at least one species of non-target metal ions; and b) maintaining the solution for a time and under conditions suitable to precipitate the at least one target metal to the cellulose material to form a target metal-laden cellulose material; andc) separating the metal-laden cellulose material from the solution by filtration; and d) recovering the target metal-laden cellulose material.

57. A target metal-laden cellulose material prepared by the method as claimed in any preceding claim.

58. The method according to claim 56 or the target metal-laden cellulose of claim 57 wherein the target metal is gold.

59. The method according to claim 56 or 58 or the target metal-laden cellulose of claim 57 or 58 wherein the gold is gold nanoparticles.

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