Process and system for removing cyanide from an aqueous fluid

Abstract

The invention provides a process for removing cyanide from a cyanide-bearing aqueous fluid comprising free cyanide and one or more metal cyanide complexes, the process comprising: (i) contacting the cyanide-bearing aqueous fluid with an anion- exchange absorbent to absorb the one or more metal cyanide complexes, thereby producing a metal-lean aqueous fluid comprising free cyanide; (ii) acidifying at least one of the cyanide-bearing aqueous fluid and the metal-lean aqueous fluid to speciate the free cyanide as HCN; and (iii) removing HCN from the metal-lean aqueous fluid to produce a cyanide-lean aqueous fluid.

Description

Process and system for removing cyanide from an aqueous fluidPriority cross-reference

[0001] The present application claims priority from Australian provisional patent application No. 2023902617 filed on 17 August 2023, the contents of which should be considered to be incorporated into this specification by this reference.Technical Field[2] The invention relates to a process for removing cyanide from an aqueous fluid comprising free cyanide and one or more metal cyanide complexes. The process comprises contacting the aqueous fluid with an anion-exchange absorbent to absorb the metal cyanide complexes, acidifying the aqueous fluid, and removing HCN from the aqueous fluid to produce a cyanide-lean aqueous fluid. The process is particularly applicable to cyanidation process streams in hydrometallurgical processes, and it will be convenient to describe the invention primarily in that context. The invention also relates to a method of purifying a cyanide-bearing aqueous fluid from a cyanidation process stream in a hydrometallurgical process, such as a gold leaching process, and to a system for removing cyanide from an aqueous fluid comprising free cyanide and metal-complexed cyanide.Background of Invention[3] Cyanidation technology is commonly used to extract gold and other precious metals from precious metal-bearing ores and concentrates. After the leaching and gold recovery processes, a tailings solution or slurry is produced which typically contains cyanide in both free and metal-complexed forms. Historically, such streams have been treated with a cyanide detoxification process in which cyanide is chemically destroyed before transfer to a tailings dam for further degradation of any residual cyanide species, but there are now strong environmental and economic incentives to avoid cyanide detoxification and instead recover and recycle cyanide within the process. Moreover, the recovery of metal values from the tailings stream, in particular from dissolved copper cyanide complexes, may also be desirable.[4] One approach to address at least some of these concerns is dry stacked tailings technology, in which a recyclable cyanidation solution is separated from the tailing solids, the solids are washed with water and then removed from the process as dry tailings. However, to maintain the water balance in the process, a bleed stream at least equivalent to the wash water input is needed. This bleed stream should ideally provide an outlet to limit build-up of detrimental species in the cyanidation process, but should also contain low levels of environmentally harmful cyanide species if recycled within the minerals processing plant or discharged into the environment.[5] Other cyanidation processes are water scarce operations where the process water needs to be largely recycled with the least possible discharge of aqueous effluent. Cyanide-free water is required for some parts of the operation e.g. the grinding circuit. In such scenarios, it may be desirable to remove substantially all cyanide species from cyanide-bearing process streams for recycling in the process, thereby reducing the external water requirements of the overall process and also providing an outlet for detrimental species which would otherwise build up.[6] Ion-exchange resin technology has previously been used to recover cyanide species from cyanide-bearing aqueous fluids, such as cyanidation process streams. Strong base ion-exchange resins have excellent absorption capacity for anionic cyanometallate species including copper, iron, nickel, zinc, gold and silver cyanide complexes, and the pregnant resin can subsequently be regenerated by elution with non-acidic and / or acidic eluants to desorb the metals. Free cyanide, however, is only weakly absorbed on ion-exchange resins.[7] One proposal to achieve satisfactory recovery of all cyanide species is to pre-load the ion-exchange resin with cyanide-reactive copper species, e.g. copper cyanide (CuCN), so that free cyanide in the solution to be processed is taken up as anionic copper cyanide complexes, e.g. Cu(CN)32' and Cu(CN)2’. The copper cyanide complexes thus formed, together with cyanometallates already present in the solution, are retained on the resin so that a good overall cyanide removal is obtained. For example, the cyanide removal process disclosed in US patent 6,919,030 involves absorbing anionic copper cyanide species on an ion-exchange resin, eluting a portion of the copper cyanide species from the resin, acid-treating the resultant resin to removecyanide while retaining the residual copper on the resin, and recycling the acid-treated resin to the absorption step. The acid treatment converts the residual copper cyanide species to cyanide-reactive copper species such as CuCN which take up free cyanide in the absorption step to re-form anionic copper cyanide species.[8] A significant difficulty with this approach is that iron cyanide complexes such as ferrocyanide, i.e. Fe(CN)e4', may also be present in the solution. Iron and copper cyanometallates have similar affinity for ion-exchange resins, so that both species will be present when a copper cyanide-loaded resin is acid treated. Nickel and zinc cyanide complexes such as Ni(CN)42’ and Zn(CN)42’ have very strong absorption affinities and will also remain on the resin. In the presence of iron cyanide complexes, copper, nickel and zinc cyanometallates precipitate under acidic conditions as insoluble mixed-metal cyanide complexes. This can block the pores of the resin, and cause irreversible buildup of metals including the nickel and zinc. A further issue is that repeated acid conditioning and alkaline resin adsorption can degrade the resin due to osmotic shock and resin breakage.[9] Another proposed approach is to add a soluble copper-containing compound to the cyanide-bearing aqueous fluid before contact with the ion-exchange resin. The free cyanide thus complexes with the added copper so that all cyanide is present in an absorbable metal-complexed form when the fluid is contacted with the ion-exchange resin. Solid copper-containing compounds such as CuCN may be used for this process. Alternatively, as disclosed in WO2022 / 087656, a copper-iron mixed-metal cyanide complex formed by precipitation in the eluate regeneration circuit may be recycled to the cyanide-bearing aqueous fluid to take up the free cyanide.

[0010] While effective to remove all cyanide from the cyanide-bearing fluid in a single absorption step, this approach has the disadvantage of requiring a large inventory of ion-exchange resin. Sufficient absorption capacity must be provided to remove not only the metals initially present in the cyanide-bearing aqueous fluid but also the metals added to complex the free cyanide. The absorption circuit, including resin absorption columns and downstream equipment for processing the eluate (metal precipitation, recovery of the free cyanide released during precipitation, and barreneluate recycling), represents a significant capital and operating cost to the process and it is desirable to minimise the size of the circuit.

[0011] It is also desirable that elution of metals from the ion-exchange resin is highly efficient since non-eluted metals represent lost absorption capacity in the subsequent absorption cycle. The need for high elution efficiencies must be balanced against the imperative to minimise the volume of eluant.

[0012] It is further desirable to extract value from the metals removed from the cyanide-bearing aqueous fluid, particularly but not necessarily only from copper. In various previously disclosed approaches, the recovered metals will be precipitated from the eluate in impure form or as mixtures of metal compounds. The recovery of one or more of the metals as comparatively pure compounds may enhance their value as saleable products.

[0013] There is therefore an ongoing need for processes to remove cyanide from a cyanide-bearing aqueous fluid, which at least partially addresses one or more of the above-mentioned short-comings, or provides a useful alternative.

[0014] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.Summary of Invention

[0015] The present invention relies on a two-step approach for removing cyanide from a cyanide-bearing aqueous fluid comprising both free and metal-complexed cyanide. In the first cyanide removal step, metal cyanide complexes are absorbed on anion-exchange absorbent (such as a strong base ion-exchange resin), with a substantial proportion of the free cyanide remaining in the treated aqueous fluid. The free cyanide, speciated as HCN, is then removed from the aqueous fluid in a second cyanide removal step, typically by a process involving transfer of HCN into a gas phase before capture in an alkaline solution. In some embodiments, the HCN is removed by transfer across a gas membrane.

[0016] Advantageously, this approach reduces the amount of anion-exchange absorbent required for effective removal of metals from the cyanide-bearing aqueous fluid compared to prior approaches where free cyanide is complexed with added metal (such as copper) before or during the absorption step. This advantage may be enhanced, in some embodiments, by acidifying the cyanide-bearing aqueous fluid before contact with the anion-exchange absorbent. The effect of this may be to release additional free cyanide from the metal cyanide complexes while reducing the average valency of the anionic metal cyanide complexes, for example by converting Cu(CN)43’ and CU(CN)32' to Cu(CN)2’. Without limitation by theory, the metal complexes thus occupy fewer absorption sites, on average, in the anion-exchange absorbent, providing a greater capacity for metal absorption. The additional free cyanide is removed together with the initially present free cyanide in the second cyanide removal step.

[0017] The effective removal of metals from the cyanide-bearing aqueous fluid prior to the second cyanide removal step also eliminates or at least mitigates the risk of metal precipitation (e.g. as CuCN) during the second cyanide removal step.

[0018] The inventors have found that very low cyanide levels, such as below 500 ppb, can be achieved by this two-step approach, allowing the aqueous phase to meet regulatory thresholds for disposal.

[0019] Given that HCN is ultimately removed from the aqueous fluid in the second cyanide removal step, there is also an opportunity to oxidise thiocyanate, if present in the cyanide-bearing aqueous fluid, to free cyanide prior to the HCN removal step. In such embodiments, the aqueous fluid product of the two-step process may thus be effectively depleted of both cyanide and thiocyanate.

[0020] The absorbed metal complexes may subsequently be extracted (eluted) from the pregnant anion-exchange absorbent with a non-acidic aqueous extractant (eluant) and precipitated as one or more metal compounds with release of free cyanide as HCN. The HCN is then recovered from the extractant (eluate), typically by a similar process to that used in the second cyanide removal step performed on the cyanide- bearing aqueous fluid.

[0021] While the process disclosed herein requires an additional cyanide removal step to be performed on the cyanide-bearing aqueous fluid, this provides the opportunity in some embodiments to exploit synergies between the similar HCN removal processes applied to (i) the cyanide-bearing aqueous fluid and the (ii) extractant (eluate).

[0022] In accordance with a first aspect the invention provides a process for removing cyanide from a cyanide-bearing aqueous fluid comprising free cyanide and one or more metal cyanide complexes, the process comprising: (i) contacting the cyanide-bearing aqueous fluid with an anion-exchange absorbent to absorb the one or more metal cyanide complexes, thereby producing a metal-lean aqueous fluid comprising free cyanide; (ii) acidifying at least one of the cyanide-bearing aqueous fluid and the metal-lean aqueous fluid to speciate the free cyanide as HCN; and (iii) removing HCN from the metal-lean aqueous fluid to produce a cyanide-lean aqueous fluid.

[0023] In some embodiments, the total cyanide concentration (i.e. cyanide present as free cyanide and metal-complexed cyanide) in the cyanide-bearing aqueous fluid is less than 500 mg L1.

[0024] In some embodiments, the cyanide-bearing aqueous fluid comprises at least one copper cyanide complex. The cyanide-bearing aqueous fluid may further comprise at least one iron cyanide complex. In some embodiments, the copper concentration in the cyanide-bearing aqueous fluid is less than 200 mg L’1, such as less than 100 mg L’1. In some embodiments, the cyanide-bearing aqueous fluid comprises gold. The gold concentration may be less than 1 mg L1, such as less than 0.2 mg L1.

[0025] In some embodiments, the process comprises acidifying the cyanide- bearing aqueous fluid before or during contact with the anion-exchange absorbent, thereby converting at least a portion of at least one metal cyanide complex from a multivalent anionic species to a monovalent anionic species and releasing free cyanide.

[0026] The cyanide-bearing aqueous fluid may be acidified to a pH of from about 4 to about 8, such as from about 5 to about 7.5, before or during contact with the anion-exchange absorbent. In some embodiments, the multivalent anionic species comprises CU(CN)43’ and / or Cu(CN)32' and the monovalent anionic species is Cu(CN)2’.

[0027] In some embodiments, the cyanide-bearing aqueous fluid comprises thiocyanate and the process comprises oxidising thiocyanate in at least one of the cyanide-bearing aqueous fluid and the metal-lean aqueous fluid to produce free cyanide. Preferably, the thiocyanate is oxidised in the metal-lean aqueous fluid.

[0028] Oxidising the thiocyanate may comprise adding an oxidising agent to the cyanide-bearing aqueous fluid or the metal-lean aqueous fluid, preferably to the metallean aqueous fluid, at a pH of less than 7, such as less than 6, for example about 5. In some embodiments, the oxidising agent comprises ozone.

[0029] In some embodiments, removing the HCN from the metal-lean aqueous fluid comprises transferring HCN into a gas phase.

[0030] In some embodiments, removing the HCN from the metal-lean aqueous fluid comprises (i) contacting the metal-lean aqueous fluid with a first side of a gas membrane, (ii) contacting a first alkaline solution with a second side of the gas membrane, and (iii) absorbing the HCN through the gas membrane into the first alkaline solution to produce a first cyanide solution.

[0031] In some embodiments, the cyanide-lean aqueous fluid has a cyanide concentration of less than 500 ppb.

[0032] In some embodiments, the anion-exchange absorbent is a strong base ionexchange resin.

[0033] In some embodiments, the process further comprises extracting the anion- exchange absorbent with at least one non-acidic aqueous extractant to produce an aqueous extract comprising one or more extracted metal cyanide complexes. The non- acidic aqueous extractant may comprises an anion selected from the group consisting of chloride, bromide, nitrate, thiocyanate, thiosulfate, sulfate and perchlorate. In some embodiments, the non-acidic aqueous extractant comprises chloride in an amount of at least 3 mol / litre. In some embodiments, the non-acidic aqueous extractant comprises free cyanide.

[0034] In some embodiments, the process comprises a plurality of absorptionextraction cycles wherein each cycle comprises: (i) extracting the anion-exchange absorbent consecutively with a first extractant fraction and a second extractant fraction of the at least one non-acidic aqueous extractant, wherein the first extractant fraction comprises a higher concentration of metal cyanide complexes than the second extractant fraction; (ii) recovering the aqueous extract consecutively in a first extract portion and a second extract portion, wherein the first extract portion comprises a higher concentration of extracted metal cyanide complexes than the second extract portion; and (iii) recycling the second extract portion to the first extractant fraction for a subsequent cycle.

[0035] The volume of the first extract portion may be equal to or greater than the combined volume of (i) the first extractant fraction and (ii) any pre-eluant contacted with the anion exchange absorbent before extracting with the first extractant fraction.

[0036] The process may further comprise: removing extracted metal cyanide complexes and free cyanide from the first extract portion, thereby forming a barren extract; and recycling most of the barren extract to the second extractant fraction.

[0037] The process may further comprise: washing the anion-exchange absorbent with a minor portion of the barren extract after extracting the anion-exchange absorbent with the second extractant fraction.

[0038] The process may further comprise: purging a minor portion of the barren extract.

[0039] In some embodiments, the anion-exchange absorbent comprises copper cyanide and iron cyanide complexes and at least 95%, such as at least 99%, of the absorbed copper and at least 98%, such as at least 99%, of the absorbed iron is extracted into the aqueous extract.

[0040] In some embodiments, the anion-exchange absorbent is recycled after extraction for contact with the cyanide-bearing aqueous fluid in step (i), wherein the recycled anion-exchange absorbent is not acid treated after extraction or is acid treated only after multiple absorption-extraction cycles with the non-acidic aqueous extractant. In some such embodiments, the process may further comprise extracting the anion-exchange absorbent with an acidic eluant, after multiple absorption-extraction cycles with the non-acidic aqueous extractant, thereby removing adsorbed zinc cyanide and / or nickel cyanide complexes. In some embodiments, the process may further comprise extracting the anion-exchange absorbent with an acidic eluant comprising thiourea, after multiple absorption-extraction cycles with the non-acidic aqueous extractant, thereby removing absorbed gold.

[0041] In some embodiments, the process comprises multiple absorption-extraction cycles, each absorption-extraction cycle comprising:(i) contacting the cyanide-bearing aqueous fluid with the anion-exchange absorbent to absorb the one or more metal cyanide complexes, thereby producing the metal-lean aqueous fluid comprising free cyanide;(ii) extracting the anion-exchange absorbent with the at least one non-acidic aqueous extractant to produce the aqueous extract comprising one or more extracted metal cyanide complexes; and(iii) providing the anion-exchange absorbent after step (ii) for step (i) of the next absorption-extraction cycle, wherein the anion-exchange absorbent is extracted with an acidic extractant between steps (ii) and (iii) in at least one but not each of the absorption-extraction cycles, thereby removing adsorbed zinc cyanide and / or nickel cyanide complexes (but preferably leaving at least 90% of any absorbed gold in the anion-exchange resin), and / or wherein the anion-exchange absorbent is extracted with an acidic extractant comprising thiourea between steps (ii) and (iii) in at least one but not each of the absorption-extraction cycles, thereby removing absorbed gold.

[0042] In some embodiments, the process comprises (i) acidifying at least a portion of the aqueous extract; (ii) precipitating one or more metal compounds; and (iii) separating the one or more metal compounds from an acidified aqueous extract.

[0043] The aqueous extract may comprise at least one copper cyanide complex, and precipitating the one or more metal compounds comprises adding a sulfide source to the aqueous extract and selectively precipitating copper sulfide.

[0044] The acidified aqueous extract may comprise at least one iron cyanide complex, and the process comprises adding a metal salt to the acidified aqueous extract, thereby precipitating a metal-iron cyanide complex; and separating the metal- iron cyanide complex from the acidified aqueous extract. The metal salt may comprise ferric cation and the metal-iron cyanide complex comprises Prussian blue.

[0045] In other embodiments, the aqueous extract comprises at least one copper cyanide complex and at least one iron cyanide complex, and precipitating the one or more metal compounds comprises precipitating a mixed-metal cyanide complex comprising copper and iron.

[0046] The process may further comprise removing HCN from the acidified aqueous extract. Removing the HCN from the acidified aqueous extract may comprise (i) contacting the acidified aqueous extract with a first side of a gas membrane, (ii) contacting a second alkaline solution with a second side of the gas membrane, and (iii) absorbing the HCN through the gas membrane into the second alkaline solution to produce a second cyanide solution.

[0047] In some such embodiments, the second alkaline solution is produced by (i) absorbing the HCN present in the metal-lean aqueous fluid into a first alkaline solution, preferably through a gas membrane, to provide a first cyanide solution; and (ii) adding base to the first cyanide solution to produce the second alkaline solution, wherein the second cyanide solution has a higher cyanide concentration than the first cyanide solution.

[0048] In some embodiments, the process further comprises recycling at least a portion of the acidified aqueous extract to form at least a portion of the non-acidic aqueous extractant.

[0049] In some embodiments, the aqueous extract comprises at least one copper cyanide complex and at least one iron cyanide complex, and the process comprises (i) acidifying and adding a sulfide source to at least a portion of the aqueous extract; (ii) selectively precipitating copper sulfide, thereby releasing free cyanide as HCN, (iii) separating the copper sulfide from an acidified aqueous extract comprising at least one iron cyanide complex and the HCN; (iv) removing at least a portion of the HCN fromthe acidified aqueous extract, preferably by absorbing the HCN through a gas membrane into a second alkaline solution to produce a second cyanide solution; (v) thereafter, adding a metal salt, e.g. a ferric salt, to the acidified aqueous extract to precipitate a metal-iron cyanide complex, e.g. Prussian blue; (vii) separating the metal- iron cyanide complex from a barren extract; and (viii) thereafter, recycling at least a portion of the barren extract to form at least a portion of the non-acidic aqueous extractant.

[0050] In some embodiments, the cyanide-bearing aqueous fluid comprises zinc cyanide and / or nickel cyanide complexes, wherein the zinc cyanide and / or nickel cyanide complexes are substantially retained in the anion-exchange absorbent after extracting the anion-exchange absorbent with the at least one non-acidic aqueous extractant.

[0051] In some embodiments, at least 50%, or at least 60%, of the free cyanide in the cyanide-bearing aqueous fluid remains present in the metal-lean aqueous fluid.

[0052] In some embodiments, at least 95%, preferably at least 99%, of the one or more metal cyanide complexes in the cyanide-bearing aqueous fluid is absorbed by the anion-exchange absorbent.

[0053] In some embodiments, the cyanide-bearing aqueous fluid comprises a cyanidation process stream of a hydrometallurgical process, preferably a gold leaching process. In some embodiments, the cyanide-bearing aqueous fluid comprises at least one selected from a tails stream and a wash stream from a gold leaching process. In some embodiments, the cyanide-bearing aqueous fluid is not a pregnant leach solution.

[0054] In accordance with a second aspect the invention provides a method of purifying a cyanide-bearing aqueous fluid from a cyanidation process stream in a hydrometallurgical process, preferably a gold leaching process, comprising (i) removing cyanide from the cyanide-bearing aqueous fluid by a process according to any embodiment of the first aspect; and (ii) discharging the cyanide-lean aqueous fluid from the hydrometallurgical process or recycling the cyanide-lean aqueous fluid for use in a cyanide-free operation of the hydrometallurgical process.

[0055] The method may comprise discharging the cyanide-lean aqueous fluid to the environment, wherein the cyanide-lean aqueous fluid has a cyanide concentration of less than 500 ppb.

[0056] In accordance with a third aspect, the invention provides a system for removing cyanide from a cyanide-bearing aqueous fluid comprising free cyanide and one or more metal cyanide complexes, the system comprising: (i) an absorption unit for contacting the cyanide-bearing aqueous fluid with an anion-exchange absorbent to absorb the one or more metal cyanide complexes, thereby producing a metal-lean aqueous fluid comprising free cyanide; (ii) acid dosing apparatus configured to acidify at least one of the cyanide-bearing aqueous fluid and the metal-lean aqueous fluid, thereby speciating the free cyanide as HCN; and (iii) a first cyanide-removal unit to remove HCN from the metal-lean aqueous fluid, thereby producing a cyanide-lean aqueous fluid.

[0057] In some embodiments, the acid dosing apparatus is configured to acidify the cyanide-bearing aqueous fluid before or during contact with the anion-exchange absorbent.

[0058] In some embodiments, the system comprises an ozone dosing apparatus configured to add ozone to at least one of the cyanide-bearing aqueous fluid and the metal-lean aqueous fluid, preferably downstream of the acid dosing apparatus. In some embodiments, the system comprises an ozone dosing apparatus configured to add ozone to the metal-lean aqueous fluid downstream of the acid dosing apparatus.

[0059] In some embodiments, the first cyanide-removal unit is configured to transfer the HCN from the metal-lean aqueous fluid into a gas phase.

[0060] In some embodiments, the first cyanide-removal unit comprises a gas membrane and is configured to (i) contact the metal-lean aqueous fluid with a first side of the gas membrane, (ii) contact a first alkaline solution with a second side of the gas membrane, and (iii) absorb the HCN through the gas membrane into the first alkaline solution to produce a first cyanide solution.

[0061] In some embodiments, the system comprises an extraction circuit configured to: (i) extract the anion-exchange absorbent with at least one non-acidic aqueousextractant to produce an aqueous extract comprising one or more extracted metal cyanide complexes; (ii) acidify at least a portion of the aqueous extract to form an acidified aqueous extract; (iii) precipitate and separate one or more metal compounds from the acidified aqueous extract; (iv) remove HCN from the acidified aqueous extract; and (vi) recycle at least a portion of the acidified aqueous extract to form at least a portion of the non-acidic aqueous extractant.

[0062] The system may comprise a second cyanide removal unit, wherein the second cyanide-removal unit comprises a gas membrane and is configured to (i) contact the acidified aqueous extract with a first side of the gas membrane, (ii) contact a second alkaline solution with a second side of the gas membrane, and (iii) absorb the HCN through the gas membrane into the second alkaline solution to produce a second cyanide solution.

[0063] In some such embodiments, the system is configured to: (i) absorb the HCN present in the metal-lean aqueous fluid through a gas membrane into a first alkaline solution to provide a first cyanide solution; and (ii) add base to the first cyanide solution to produce the second alkaline solution.

[0064] In some embodiments, the system comprises a copper precipitation unit configured to i) acidify and add a sulfide source to at least a portion of the aqueous extract; (ii) selectively precipitate copper sulfide, thereby releasing free cyanide as HCN; and (iii) separate the copper sulfide from the acidified aqueous extract.

[0065] The system may further comprise an iron precipitation unit downstream of the copper precipitation unit, the iron precipitation unit configured to (i) add a metal salt, e.g. a ferric salt, to the acidified aqueous extract; (ii) precipitate a metal-iron cyanide complex, e.g. Prussian blue; and (iii) separate the metal-iron cyanide complex from a barren extract.

[0066] In some embodiments, the system is configured to: (i) extract the anion- exchange absorbent consecutively with a first extractant fraction and a second extractant fraction of the at least one non-acidic aqueous extractant; (ii) recover the aqueous extract consecutively in a first extract portion and a second extract portion; (iii)recycle the second extract portion to the first extractant fraction; and (iv) acidify the first extract portion to form the acidified aqueous extract.

[0067] Unless the context requires otherwise, where the terms “comprise”, “comprises”, “comprised” or “comprising” are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[0068] As used herein, the terms “first”, “second”, “third” etc in relation to various features of the disclosed devices, methods, systems etc are arbitrarily assigned and are merely intended to differentiate between two or more such features that the device, methods, systems etc may incorporate in various embodiments. The terms do not of themselves indicate any particular orientation or sequence. Moreover, it is to be understood that the presence of a “first” feature does not imply that a “second” feature is present, the presence of a “second” feature does not imply that a “first” feature is present, etc.

[0069] Further aspects of the invention appear below in the detailed description of the invention.Brief Description of Drawings

[0070] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[0071] Figure 1 schematically depicts a system for removing cyanide from a cyanide-bearing aqueous fluid according to embodiments of the invention.

[0072] Figure 2 schematically depicts an elution circuit for extracting an anion- exchange absorbent with non-acidic aqueous extractant according to embodiments of the invention.

[0073] Figure 3 is a graph showing residual free cyanide concentration in a NaCN solution after gas membrane cyanide removal to NaOH stripping solution, as performed in Example 2.

[0074] Figure 4 is a graph showing the breakthrough curves for various species when a cyanide-bearing aqueous fluid was passed through a strong base ion-exchange resin in Example 5.

[0075] Figure 5 is a graph showing residual free cyanide concentration in a model solution after gas membrane cyanide removal to NaOH stripping solution, as performed in Example 6.

[0076] Figure 6 is a graph showing the effect of chloride concentration on elution efficiency from a pregnant ion-exchange resin column, using a chloride + cyanide eluant, as performed in Example 7.

[0077] Figure 7 schematically depicts a first elution configuration used for cycles 1 - 9 in the continuous test campaign of Examples 13-14.

[0078] Figure 8 schematically depicts a second elution configuration used for cycles 1 1 -15 in the continuous test campaign of Examples 13-14.

[0079] Figure 9 schematically depicts a second elution configuration used for cycles 16-20 in the continuous test campaign of Examples 13-14.

[0080] Figure 10 is a graph showing the copper, iron and thiocyanate concentrations in the resin adsorption barren solution for each cycle in the continuous test campaign of Examples 13-14.

[0081] Figure 1 1 is a graph showing the free cyanide concentrations in the resin adsorption feed (“Ads Preg”) and barren solutions (“Ads barren”) for each cycle in the continuous test campaign of Examples 13-14.

[0082] Figure 12 is a graph showing the residual concentration of copper and iron on barren resin for each cycle in the continuous test campaign of Examples 13-14.

[0083] Figure 13 is a graph showing the elution efficiency of copper and iron from resin for each cycle in the continuous test campaign of Examples 13-14.

[0084] Figure 14 is a graph showing the copper and iron concentrations in the E1 resin eluate for each cycle in the continuous test campaign of Examples 13-14.

[0085] Figure 15 is a graph showing the copper and iron concentrations in the E2 resin eluate for each cycle in the continuous test campaign of Examples 13-14.

[0086] Figure 16 is a graph showing the fraction of eluted copper and iron reporting to the E1 resin eluate for each cycle in the continuous test campaign of Examples 13- 14.

[0087] Figure 17 is a graph showing the residual concentration of zinc and nickel on barren resin for each cycle in the continuous test campaign of Examples 13-14.

[0088] Figure 18 is a graph showing the zinc and nickel concentrations in the eluate when eluting the resin with acid after 20 cycles during the continuous test campaign of Examples 13-14.

[0089] Figure 19 is a graph showing the iron concentration in the extract before sulfidization, after sulfidization, and after Prussian blue precipitation, for each cycle in the continuous test campaign of Examples 13-14.

[0090] Figure 20 is a graph showing the thiocyanate concentration in the extract before sulfidization, after sulfidization, and after Prussian blue precipitation, for each cycle in the continuous test campaign of Examples 13-14.

[0091] Figure 21 is a graph showing the residual free cyanide concentration in the resin adsorption barren solution after gas membrane cyanide removal to NaOH stripping solution, for each cycle in the continuous test campaign of Examples 13-14.

[0092] Figure 22 is a graph showing the residual free cyanide concentration in the E1 barren solution after gas membrane cyanide removal to NaOH stripping solution, for each cycle in the continuous test campaign of Examples 13-14.

[0093] Figure 23 is a graph showing the breakthrough curves for copper and iron when a cyanide-bearing aqueous fluid with pH of 11 , or acidified to pH of 7 or 5, was passed through a strong base ion-exchange resin in Example 16.

[0094] Figure 24 is a graph showing the elution efficiency of copper and iron, absorbed onto an ion-exchange resin from an acidified (pH 5) loading solution, when eluted with a chloride + cyanide eluant in Example 18.

[0095] Figure 25 is a graph showing the breakthrough curve of gold when a cyanide-bearing aqueous fluid acidified to pH of 5 was passed through a strong base ion-exchange resin in Example 19.

[0096] Figure 26 is a graph showing the elution efficiency of gold absorbed onto an ion-exchange resin from an acidified (pH 5) loading solution, when eluted with acid (BV 1 to 6) then acid + thiourea (BV 7-13) in Example 20.

[0097] Figure 27 is a graph showing the change in thiocyanate, cyanate and cyanide concentrations with time when adding ozone to a solution of 10 mM KSCN at pH 5 in Example 23.

[0098] Figure 28 is a graph showing the change in thiocyanate, cyanate and cyanide concentrations with time when adding ozone to a solution of 10 mM KSCN at pH 12 in Example 23.

[0099] Figure 29 is a graph showing the change in thiocyanate, cyanate and cyanide concentrations with time when adding ozone to a solution of 10 mM KSCN + 10 mg L’1copper at pH 5 in Example 24.

[0100] Figure 30 is a graph showing the change in thiocyanate, cyanate and cyanide concentrations with time when adding ozone to a solution of 10 mM KSCN + 0.5 M NaCI at pH 5 in Example 25.Detailed DescriptionProcess for removing cyanide from a cyanide-bearing aqueous fluid

[0101] The present invention relates to a process for removing cyanide from a cyanide-bearing aqueous fluid comprising free cyanide and one or more metal cyanide complexes. The process comprises: (i) contacting the cyanide-bearing aqueous fluid with an anion-exchange absorbent to absorb the one or more metal cyanide complexes, thereby producing a metal-lean aqueous fluid comprising free cyanide; (ii) acidifying at least one of the cyanide-bearing aqueous fluid and the metal-lean aqueous fluid to speciate the free cyanide as HCN; and (iii) removing HCN from the metal-lean aqueous fluid to produce a cyanide-lean aqueous fluid.Cyanide-bearing aqueous fluid

[0102] The process disclosed herein removes cyanide from a cyanide-bearing aqueous fluid comprising free cyanide and one or more metal cyanide complexes. The cyanide-bearing aqueous fluid may be a solution or slurry process stream in a hydrometallurgical process, for example a precious metal, e.g. gold, cyanidation process. In some embodiments, it is or comprises a tails stream from a cyanidation process, i.e. the residual solution or slurry remaining after the primary target metal(s) have been removed. In some embodiments, the cyanide-bearing aqueous fluid is taken as a purge stream from a process stream in a hydrometallurgical process to limit the build-up of water and / or metal species (such as copper, iron, zinc and / or nickel cyanide complexes) in the cyanidation leach circuit. Advantageously, the process disclosed herein may facilitate the disposal of the cyanide-bearing aqueous fluid in accordance with regulations for cyanide discharge to the environment, while also allowing the recovered cyanide to be recycled to the hydrometallurgical cyanidation process. Alternatively, the cyanide-lean aqueous fluid may be recycled within the overall hydrometallurgical process for use in a cyanide-free operation, for example ore grinding. This may reduce the water intake to the hydrometallurgical process.

[0103] While the process disclosed herein is considered particularly applicable in hydrometallurgical processing in mining operations, other cyanide-bearing aqueous fluids may also be treated, for example spent cyanide-containing plating solutions from the metal plating industry.

[0104] As used herein, free cyanide refers to cyanide which is not complexed to a metal, and includes cyanide in both the alkaline (deprotonated) form, CN’, and the acidic (protonated) form, HCN. It will be appreciated that these species exist in equilibrium in aqueous fluids with the relative abundance dependent on the pH. In most use scenarios, the initial cyanide-bearing aqueous fluid is highly alkaline so that the free cyanide is speciated primarily or entirely as CN’.

[0105] As used herein, metal cyanide complexes refer to coordination complexes of metal cations with one or more CN’ ligands. Typically, the metal cyanide complexes comprise multiple CN’ ligands, as the only ligands. Such metal cyanide complexes,when dissolved in aqueous solution, may be anionic species and thus susceptible to absorption by anion-exchange absorbents.

[0106] In some embodiments, the cyanide-bearing aqueous fluid includes one or more selected from copper, iron, nickel, zinc, gold and silver cyanide complexes. In some embodiments, the aqueous fluid includes copper cyanide complexes and / or iron cyanide complexes. In some embodiments, the aqueous fluid includes copper cyanide complexes, or both copper and iron cyanide complexes. Copper is commonly coleached in cyanidation processes, and the process disclosed herein provides the opportunity to recover copper values from the fluid.

[0107] In some embodiments, the aqueous fluid comprises gold, typically at low levels such as less than 1 mg L1. Some residual gold is commonly present in cyanidation tails solution from gold mining operations. The amount of residual gold present largely depends on the efficiency of activated carbon recovery of gold that is impacted by many factors, e.g. pH, pregnant leach solution composition and carbon adsorption operation conditions. There is typically more residual gold present if the PLS contains high copper as copper strongly competes with gold for carbon adsorption.

[0108] The process disclosed herein may find particular application for processing tailings and purge streams from hydrometallurgical processes which may be diluted and / or depleted of metal values compared to leach solutions used in the primary leach operation. The cyanide-bearing aqueous fluid may therefore have substantially lower concentrations of metal- and cyanide-species than are typically present in pregnant leach solutions produced in leaching of ores or concentrates. The pregnant leach solution of copper ores or concentrates typically contains copper concentration from 500 to 2000 mg L’1(based on the data of three large Australian copper-gold mines), but can contain up to 5000 mg L’1copper for some operations. To effectively leach gold from such ores or concentrates, the total CN:Cu molar ratio needs to be maintained at >4:1 , meaning that the total cyanide concentration (as CN’ in both free and copper complexed cyanide) of the pregnant leach solution is typically in the range of 1000 to 4000 mg L1. The gold concentration in the pregnant leach solution is typically in the range of 5 to 20 mg L’1, but can be lower or higher depending on the ore type and grade.

[0109] In some embodiments of the present process, therefore, the total cyanide concentration (as both free and metal complexed cyanide) is less than 500 mg L’1, for example less than 400 mg L’1. In some embodiments, the copper concentration is less than 200 mg L’1, such as less than 100 mg L’1. In some embodiments, the gold concentration is less than 1 mg L’1, such as less than 0.2 mg L’1.

[0110] The cyanide-bearing aqueous fluid prior to treatment typically has an alkaline pH, and the free cyanide and the metal cyanide complexes may be anionic species. For example, the cyanide-bearing aqueous fluid may have a pH over above 8, or above 9, such as above 10, or in the range of 9 to 12. The counter ions in solution may include common cations such as sodium, potassium and calcium.Absorption

[0111] The process disclosed herein involves a step of contacting the cyanide- bearing aqueous solution with an anion-exchange absorbent to absorb the one or more metal cyanide complexes. A metal-lean aqueous fluid, still containing free cyanide, is thus produced.

[0112] Absorption technology to remove metal cyanide complexes from cyanidation process streams is now well-established, and conventional anion-exchange absorbents and absorption process equipment used for such applications may be employed in the process of the present invention. In some embodiments, the anion-exchange absorbent is a basic ion-exchange resin, for example a strong base ion-exchange resin. Such resins typically include porous polymers with absorption sites provided by quaternary amino groups (-NR3+X_, where X’ is the exchangeable anion). Suitable strong base ion-exchange resins are known in the art and are commercially available. One example of a suitable resin is Purolite MTA5013SO4 (rebranded from Purolite A500 / 2788).

[0113] It is also envisaged that water-immiscible liquid anion-exchange absorbents may be used, with the absorption step conducted by liquid-liquid extraction techniques. Long-chain alkyl quaternary ammonium salts, such as Aliquat 336, dissolved in hydrocarbon solvents may be used as the liquid anion-exchange absorbent, for example as disclosed in Int. J. Miner. Metall. Mater. 2016, 23, 1258.

[0114] The process may involve contacting the aqueous solution with ion-exchange resin, for example in the form of beads. The beads may be loaded by batch contact with the solution, for example in an agitated tank or a resin-in-leach or resin-in-pulp type process. Alternatively, the ion-exchange resin is present in a column, and the aqueous fluid is passed through the column. In some embodiments, the aqueous fluid is flowed continuously through the column at a suitable flow rate to allow near- quantitative absorption of the metal cyanide complexes on the absorption sites until the resin is fully loaded.

[0115] The aqueous solution may be flowed through one, or more than one, column of ion-exchange resin. In some embodiments, the aqueous solution is flowed through two or more, preferably two, columns arranged in series, with the majority of absorption taking place in the first column and the second column scavenging any metal cyanide complexes that break through the first column. This advantageously allows a high loading of metals to be produced on the first column while mitigating the risk of metals breakthrough into the treated aqueous fluid for subsequent processing.

[0116] In a particularly preferred mode, a plurality of three or more columns of ionexchange resin are operated in carousel mode. In this mode, the aqueous solution is flowed through two or more, preferably two, columns arranged in series while a third column is regenerated by elution. In each cycle, when the first resin column in sequence reaches a target loading, for example as determined by a maximum concentration of metal, e.g. copper, in the fluid passing to the second column, the first column is taken offline for resin elution. The second resin column then proceeds as the first resin column in sequence with the third column, now regenerated, brought back online and proceeding as the second column in sequence. The first column is then regenerated by elution (as discussed below). Once the elution is complete the eluted column is ready to be brought back online as the second column in sequence in the next cycle. While this carousel mode is described for a three-column scenario, it will be appreciated that the same principle may be applied with more than two columns online in each cycle, arranged in series and / or in parallel, and that more than one column may be off-line for regeneration in each cycle.

[0117] As a result of the contact with anion-exchange absorbent, the metal content in the metal-lean aqueous fluid is reduced in comparison with the cyanide-bearing aqueous fluid. Preferably, the metal-lean aqueous fluid is substantially free of metal cyanide complexes, or sufficiently depleted of metal cyanide complexes to meet the process-specific maximum threshold for metal-complexed cyanide in the treated stream. It will be appreciated, however, that quantitative removal of metal is not required in all implementations. In some embodiments, at least 90 mol%, or at least 95 mol%, or at least 98 mol%, such as at least 99 mol% of the metals initially present as metal cyanide complexes in the cyanide-bearing aqueous fluid are removed from the metal-lean aqueous fluid. In some embodiments, at least 95 mol%, or at least 98 mol%, such as at least 99 mol% of the non-ferrous metals initially present as metal cyanide complexes in the cyanide-bearing aqueous fluid are removed from the metal-lean aqueous fluid. Iron-complexed cyanide is generally considered to be irreversibly bound and thus less harmful. Accordingly, higher concentrations of iron may be acceptable in the metal-lean aqueous fluid, provided that non-ferrous metals such as copper are near-quantitatively removed. In some embodiments, the cyanide-bearing aqueous fluid comprises copper cyanide complexes and at least 95 mol%, or at least 98 mol%, such as at least 99 mol% of the copper initially present in the cyanide-bearing aqueous fluid is removed from the metal-lean aqueous fluid.

[0118] After contact with the anion-exchange absorbent, the metal-lean aqueous fluid comprises free cyanide, which is only weakly absorbed by typical anion-exchange absorbents. It is not required that all of the free cyanide initially present in the cyanide- bearing aqueous fluid remains in the metal-lean aqueous fluid, since absorbed free cyanide can be recovered during regeneration (e.g. elution) of the anion-exchange absorbent. However, it may be preferred that a substantial portion of the free cyanide is not absorbed, since this increases the availability of absorption sites for metal- cyanide complexes. In some embodiments, the metal-lean aqueous fluid comprises at least 50% of the initially present free cyanide, such as at least 60%, or at least 70%.Acidifying the aqueous fluid

[0119] The process disclosed herein involves a step of acidifying at least one of the cyanide-bearing aqueous fluid and the metal-lean aqueous fluid to speciate the free cyanide as HCN.

[0120] The aqueous fluid is preferably acidified sufficiently, before and / or after the absorption step, such that the free cyanide speciates predominantly as HCN in the metal-lean aqueous fluid. This facilitates the removal of free cyanide in the next process step. Since the pKa of HCN is 9.2, it is preferred that the metal-lean aqueous fluid has a pH of less than 8, or less than 7.5, such as less than 7. Acid titration experiments have demonstrated that free cyanide is near-quantitatively converted to the HCN form at pH of about 7 and lower. In some embodiments, the metal-lean aqueous fluid after acidification has a pH of no less than 2, or no less than 3, for example a pH of between 2 and 5, such as between 3 and 4. Excessive acidification, beyond what is needed to speciate the free cyanide as HCN and to facilitate HCN removal in the subsequent process step may add costs and present further difficulties for disposal or re-use of the treated aqueous fluid.

[0121] The aqueous fluid may be acidified with a mineral acid such as hydrochloric acid or sulfuric acid.

[0122] In some embodiments, the metal-lean aqueous fluid is acidified. In some embodiments, only the metal-lean aqueous fluid is acidified, i.e. acidification is performed only after the absorption step. This approach may be preferred to avoid or mitigate safety concerns associated with the presence of free HCN during absorption on the anion-exchange absorbent, and potentially to avoid mitigating measures such as negative pressure operation.

[0123] In some embodiments, the cyanide-bearing aqueous fluid is acidified, i.e. acidification is performed at least in part, or entirely, before or during the contacting of the cyanide-bearing aqueous fluid with the anion-exchange absorbent. Such acidification may release additional free cyanide from one or more metal cyanide complexes present in the cyanide-bearing aqueous fluid. However, it is to be understood that the cyanide-bearing aqueous fluid comprises free cyanide prior to the acidification. The cyanide-bearing aqueous fluid may thus be acidified to a pH of lessthan 8, or less than 7.5, such as less than 7 before or during the contact with the anion- exchange absorbent.

[0124] In some embodiments, the cyanide-bearing aqueous fluid is not acidified to an extent where metals precipitate from solution. The inventors have found that metals are not precipitated at pH of 7 or higher even when iron, nickel and zinc are present together with copper in aqueous solutions representative of cyanidation tailings.

[0125] In other embodiments, acidification before or during the contacting of the cyanide-bearing aqueous fluid with the anion-exchange absorbent causes precipitation of a portion of the metals in the cyanide-bearing aqueous fluid. In such scenarios, the residual solution still containing free cyanide and unprecipitated metal cyanide complexes may be separated from the precipitate, for example by filtration, before contact with the anion-exchange absorbent. For example, acidification of aqueous fluids comprising iron cyanide complexes may cause precipitation of mixed metal iron cyanide complexes such as mixed zinc-iron cyanide complexes. Preferably, all or most any copper in solution (such as at least 90%) remains unprecipitated. The inventors have found that little copper is precipitated at pH of 5 or 6 even when iron, nickel and zinc are present together with copper in aqueous solutions representative of cyanidation tailings.

[0126] The cyanide-bearing aqueous fluid may thus be acidified to a pH of no less than 4, or no less than 5, such as no less than 6. In some embodiments, the cyanide- bearing aqueous fluid is acidified to a pH of between 4 and 8, such as between 5 and 7.5, before or during the contacting of the cyanide-bearing aqueous fluid with the anion- exchange absorbent.

[0127] Acidification before or during the absorption step may provide a number of advantages. Firstly, acidification may release additional free cyanide from the metal cyanide complexes, thereby reducing the average valency of the anionic metal cyanide complexes. Without limitation by theory, this is expected to increase the metal absorption capacity of the anion-exchange absorbent, since each anionic metal cyanide complex occupies a number of absorption sites corresponding to its valency. Thus, monovalent, divalent and trivalent metal cyanide complexes will occupy one, two and three absorption sites respectively. By reducing the average valency, e.g. byconverting a portion of the multivalent metal cyanide complexes to monovalent complexes, fewer absorption sites are required to retain each complex.

[0128] For example, copper cyanide complexes in aqueous solution may include trivalent, divalent and monovalent cyanometallate species (Cu(CN)43-, Cu(CN)32’ and CU(CN)2') which exist in equilibrium with each other and free cyanide. At reduced pH values, CU(CN)43’ and Cu(CN)32' convert to Cu(CN)2’ with concomitant release of free cyanide.

[0129] Secondly, acidification before or during the absorption step will increase the speciation of the cyanide as HCN and reduce the prevalence of CN’. This in turn will reduce the competitive absorption of anionic free cyanide to the anion-exchange absorbent, providing enhanced capacity for metal absorption.

[0130] The acidification-induced release of additional free cyanide can be accommodated in the methods disclosed herein because free cyanide, in the form of HCN, is removed in a second cyanide removal step. By contrast, the cyanide-bearing aqueous fluid cannot be acidified to enhance the metal loading capacity of the anion- exchange absorbent in single-step cyanide removal methods which rely on pre- complexation of free cyanide because acidification would release cyanide from the metal cyanide complexes as HCN that cannot be recovered by the anion-exchange absorbent.

[0131] In some embodiments, the cyanide-bearing aqueous fluid is acidified both (i) before or during; and (ii) after the contacting of the cyanide-bearing aqueous fluid with the anion-exchange absorbent. The cyanide-bearing aqueous fluid may be acidified before or during the contact to a pH sufficient to reduce the average valency of the metal cyanide complexes, but not so low that there is a risk of undesirably precipitating metal species from solution (e.g. precipitating a substantial portion of the copper content). For example, such a pH may be in the range of 4 to 8, such as about 5 to about 7.5. After the absorption of the metal cyanide complexes, the metal-lean aqueous fluid may be then further acidified without risk of metal precipitation, for example to a pH of between 2 and 5, such as between 3 and 4. This may be desirable to facilitate efficient removal of the HCN in the next process step.Removing HCN

[0132] The process disclosed herein involves a step of removing HCN from the metal-lean aqueous fluid to produce a cyanide-lean aqueous fluid. Typically, removing the HCN from the metal-lean aqueous fluid comprises transferring HCN from the metallean aqueous fluid into a gas phase.

[0133] In some embodiments, the HCN is removed from the metal-lean aqueous fluid by a gas membrane process. Such a process comprises contacting the metallean aqueous fluid with one side of a gas membrane, contacting an alkaline solution (i.e. a stripping solution) with the opposite side of the gas membrane, and absorbing the HCN through the gas membrane into the alkaline solution. The HCN dissolves into the alkaline solution and dissociates to the CN’ form, thus forming a cyanide solution.

[0134] Suitable gas membranes (also known as gas-filled membranes) are generally porous hydrophobic membranes, typically comprising hydrophobic polymers such as polypropylene (PP) or polytetrafluoroethylene (PTFE). Due to its hydrophobic character, the pores of the gas membrane remaining gas-filled in operation, with the two aqueous fluids in close proximity but separated by the gas membrane. HCN is thus able to devolatilise from the metal-lean aqueous fluid, diffuse into the gas phase in the membrane pores, and then diffuse into the alkaline solution on the opposite side of the membrane.

[0135] The use of a gas membrane approach advantageously provides fast kinetics of HCN removal from the metal-lean aqueous fluid. Moreover, this advantage may be enjoyed without the risk of metal precipitation inside or against the gas membrane that can occur when removing free cyanide from metal-containing cyanide-bearing aqueous fluids.

[0136] The gas membrane may be configured as one or more hollow gas membrane fibers, with the metal-lean aqueous fluid flowed through the lumen of the fibers and the alkaline solution in contact with the outside of the fibers. For example, a plurality of hollow gas membrane fibers is arranged in parallel in a shell. The metallean aqueous fluid may be flowed through the lumen of the hollow fibres, with thealkaline solution flowed along the fibers on the shell-side, in either counter-current or co-current direction.

[0137] The metal-lean aqueous fluid may be recirculated repeatedly for contact with the gas membrane, with additional HCN removed in each pass. In a batch process, the recirculation may continue until a target HCN concentration is reached. This may be done in multiple discrete cycles, for example by flowing the entire batch of metallean aqueous fluid from a first reservoir past the gas membrane into a second reservoir in a first cycle, then flowing the entire batch past the (or another) gas membrane in the second cycle, and continuing these discrete cycles until the target HCN concentration is reached. Alternatively, the batch may be continuously recirculated past the gas membrane, with the treated fluid that exits the gas membrane being continuously remixed into the batch, until the target HCN concentration is reached. The inventors have found that the HCN concentrations of less 500 ppb can be obtained with the later approach in less than 1 hour using commercially representative process conditions (flow rates vs membrane surface areas). Even better results are expected with the first approach.

[0138] In a continuous process, the metal-lean aqueous fluid may be flowed through a series of membrane contactors (e.g. dozens with no reservoir in-between) so that the cyanide-lean aqueous fluid leaving the final contactor has a free cyanide meeting specification, for example below 500ppb. If the stream is larger than the contactor capacity (in terms of flowrate), several trains, each containing multiple contactors and separately operated, can be used.

[0139] The alkaline solution may comprise any suitable base, preferably a mineral base such as sodium hydroxide, potassium hydroxide or calcium hydroxide (lime). In some embodiments, the alkaline solution comprises sodium hydroxide.

[0140] The alkaline solution may also be recirculated repeatedly for contact with the gas membrane, with additional HCN absorbed in each pass. The alkaline solution may be recirculated - in discrete cycles or with continuous re-mixing as described above - until a desired cyanide concentration is obtained. A sufficient concentration of base should thus be provided in the initial alkaline solution to obtain the desired cyanideconcentration, with higher cyanide concentrations typically preferred as this minimises the volume of cyanide solution to be transported or further processed.

[0141] In practice, however, the concentration of the initial alkaline solution, and thus the ultimate cyanide concentration, may be constrained by the need to avoid or mitigate osmotic distillation. Osmotic distillation, i.e. water vapour transfer from a low ionic strength solution to a high ionic strength solution, will occur across the gas membrane if there is a substantial imbalance between the salinity of the alkaline solution and the metal-lean aqueous fluid. If the alkaline solution is too concentrated, water will be absorbed together with HCN, thus diluting the cyanide solution. The salinity of the metal-lean aqueous fluid will depend on the origin of the cyanide-bearing aqueous fluid being processed. However, in some scenarios, it is expected that the alkaline solution should have a base concentration of no more than 2 M, or no more than 1 .5 M, to avoid excessive osmotic distillation.

[0142] As will be described hereafter, this constraint may be overcome in some embodiments by adding additional base to the cyanide solution and using the resultant further alkalised solution as the stripping solution in a second gas membrane process unit present in the elution circuit.

[0143] While the use of a gas membrane provides attractive kinetics and has been found effective to obtain very low levels of free cyanide in the cyanide-lean aqueous fluid, other cyanide removal processes are also envisaged and may be used. In some embodiments, for example, the HCN is removed from the metal-lean aqueous fluid by an AVR (acidification-volatilisation-reneutralisation) process. In such a process, a stream of gas such as air is contacted with the acidified metal-lean aqueous fluid under conditions suitable to volatilise the HCN into the gas. The gas is then contacted with an alkaline solution, such as a lime solution, to absorb and react the HCN. The volatilisation of HCN from the metal-lean aqueous fluid is the rate-limiting step, and various techniques may be used to enhance the rate of volatilisation. These include (i) reactor designs to enhance the gas-liquid interface area, such as packed towers, (ii) increasing the ratio of gas to liquid, (iii) increasing the gas flow rate, and (iv) reducing the pH of the metal-lean aqueous fluid.

[0144] It will be appreciated that the choice of HCN removal process, and the process design and operating conditions (e.g. membrane area and recirculation time in a gas membrane), will dependent on the maximum HCN concentration that remains acceptable in the cyanide-lean aqueous fluid. In some embodiments, the cyanide-lean aqueous fluid has a cyanide concentration (including both free and metal-complexed cyanide) of less than 10 ppm, or less than 5 ppm, or less than 1 ppm. In some embodiments, the cyanide-lean aqueous fluid has a cyanide concentration or less than 500 parts per billion (ppb), which is the standard for cyanide surface discharge according to Australian regulations (Cyanide Management, Australian Government 2008).

[0145] The cyanide-lean aqueous fluid may be used and / or disposed of in any suitable way. In some embodiments, all or part of the cyanide-lean aqueous fluid is discharged to a tailings dam or other environmentally acceptable outlet. In some embodiments, all or part of the cyanide-lean aqueous fluid may be reused internally in the process disclosed herein, for example as a wash in the elution circuit as will be described hereafter. In some embodiments, the cyanide-lean aqueous fluid is recycled elsewhere within a hydrometallurgical process, in particular for use in a cyanide-free operation, such as ore grinding, where it may be used instead of fresh water.Thiocyanate oxidation

[0146] In some embodiments, the cyanide-bearing aqueous fluid to be treated in the process comprises thiocyanate (SCN-). For example, thiocyanate may be formed when cyanide-based lixiviants are used to leach reactive sulfide ores. In such embodiments, the process may comprise a step of oxidising at least a portion of the thiocyanate to produce free cyanide. The regenerated free cyanide may be captured in the process, typically together with free cyanide that was already present in the cyanide-bearing aqueous fluid.

[0147] In some embodiments, the process comprises oxidising thiocyanate in at least one of the cyanide-bearing aqueous fluid and the metal-lean aqueous fluid, preferably in the metal-lean aqueous fluid after absorption of the metal cyanide complexes. Alternatively or additionally, thiocyanate absorbed on the anion-exchange absorbent (e.g. strong base anion-exchange resin) in the absorption step is desorbedinto an aqueous extract (eluate) and may be oxidised in the extract to form free cyanide, as will be described hereafter.

[0148] The thiocyanate may be oxidised by adding a suitable oxidising agent to the relevant solution containing thiocyanate, although it is also envisaged that thiocyanate in solution may be electrolytically oxidised. Suitable oxidising agents may include chlorine, a peroxide, or ozone. In some embodiments, the oxidising agent is ozone.

[0149] In some embodiments, the thiocyanate is oxidised in solution at an acidic pH, i.e. a pH of less than 7, or less than 6, such as less than 5. The inventors have found that over-oxidation of thiocyanate to cyanate (OCN-) is suppressed under acidic conditions, and ascribe this to an increased susceptibility of CN’ to oxidation compared to HCN.

[0150] The thiocyanate may be oxidised by adding a suitable oxidising agent such as ozone to the cyanide-bearing aqueous fluid, despite the presence of metal cyanide complexes in solution. Preferably, the cyanide-bearing aqueous fluid is acidified to a pH of below 7 before adding the oxidising agent. However, the inventors have found that the presence of copper cyanide complexes has a limited adverse effect on selectivity when oxidising thiocyanate with ozone at pH 5.

[0151] Preferably, therefore, the process comprises oxidising thiocyanate by adding a suitable oxidising agent such as ozone to the metal-lean aqueous fluid, i.e. downstream of the absorption step and thus after the metal cyanide complexes have already been removed. Preferably, the metal-lean aqueous fluid has a pH of less than 7, or less than 6, such as less than 5, when the oxidising agent is added, to suppress oxidation of the free cyanide.

[0152] The generation of additional free cyanide by thiocyanate oxidation can be accommodated because HCN is ultimately removed from the metal-lean aqueous fluid, e.g. via a gas membrane. The resultant cyanide-lean aqueous fluid may thus be effectively depleted of both cyanide and thiocyanate.Extraction

[0153] After absorbing the one or more metal cyanide complexes into the anion- exchange absorbent, the absorbent may approach or reach a maximum loading of metal species and thus require regeneration. The process may thus comprise a step of extracting the anion-exchange absorbent with at least one non-acidic aqueous extractant to produce an aqueous extract comprising one or more extracted metal cyanide complexes. Where the anion-exchange absorbent is a solid absorbent such as an ion-exchange resin, the extraction step may comprise eluting the anion-exchange absorbent with the non-acidic aqueous extractant. The aqueous extractant is thus an eluant and the aqueous extract is an eluate.

[0154] In some embodiments, the anion-exchange absorbent comprises absorbed copper cyanide and / or iron cyanide complexes, for example both copper cyanide and iron cyanide complexes. The extraction step may remove all or most of the absorbed iron cyanide complexes, and preferably also a high proportion of the absorbed copper cyanide complexes. This ensures that the extracted absorbent has a high capacity for absorption when recycled to a subsequent absorption step. Moreover, highly effective removal of the iron is particularly desirable if intermittent acid regeneration of the absorbent is required to remove strongly absorbed species such as nickel or zinc cyanide complexes. In the presence of iron cyanide complexes, absorbed copper, nickel and zinc may not be removed in an acid regeneration but may instead undesirably precipitate in the absorbent as insoluble mixed-metal cyanide complexes. Residual copper may also precipitate in the absorbent, for example as CuCN, under acidic conditions.

[0155] In some embodiments, therefore, at least 95%, or at least 98%, preferably at least 99%, of the absorbed iron is extracted into the aqueous extract. In some embodiments, at least 80%, or at least 90%, preferably at least 95%, or at least 98%, such as at least 99%, of the absorbed copper is extracted into the aqueous extract.

[0156] Any zinc cyanide and / or nickel cyanide complexes may be substantially retained in the anion-exchange absorbent after the extraction, meaning that at least 80%, or at least 90%, remain absorbed. Any gold cyanide complexes may also be substantially retained in the anion-exchange absorbent after the extraction, meaning that at least 80%, or at least 90%, remain absorbed.

[0157] The non-acidic aqueous extractant (or eluant) and the extraction (or elution) methodology may thus be chosen to provide a high extraction efficiency for the extractable metal complexes, particularly for the copper and iron cyanide complexes. As used herein, a non-acidic aqueous extractant has a pH sufficiently high to avoid decomplexation of the absorbed copper and other metal cyanide complexes and precipitation of metal species on the absorbent, and is thus distinguished from strongly acidic regeneration eluants. In some embodiments, the pH of the non-acidic aqueous extractant may be greater than about 4.5, or 5, or 6, or 7.

[0158] The non-acidic aqueous extractant may comprise a displacement anion, which may be an anion with a relatively lower affinity for the absorbent than the absorbed metal cyanide complexes. Nevertheless, when the displacement anions are provided at sufficiently high concentration in the extractant, the exchange equilibria favour desorption and extraction of at least some of the metal cyanide complexes, including copper and iron cyanide complexes. After continuous or multi-stage extraction, a high extraction efficiency can thus be obtained. After extraction the absorption sites are thus occupied primarily by the displacement anions, with relatively small amounts of residual strongly absorbed metal species such as nickel and zinc cyanide complexes. The absorbent can thus be used in a subsequent absorption cycle.

[0159] In some embodiments, the displacement anion is selected from the group consisting of chloride, bromide, nitrate, thiocyanate, thiosulfate, sulfate, perchlorate and combinations thereof. Chloride is a particularly suitable displacement because of its overall compatibility in the process. In some embodiments, the displacement anion, such as chloride, is present in an amount of at least 1 mol / litre , or at least 2 mol / litre, or at least 3 mol / litre, or at least 3.5 mol / litre, such as about 4 mol / litre or higher. Chloride concentrations of at least 3 mol / litre, such as about 4 mol / litre, are preferred to achieve highly effective elution of copper.

[0160] In some embodiments, the unloaded absorbent may then be treated with a further non-acidic aqueous extractant comprising a different displacement anion (e.g. chloride where other than chloride was used as the first displacement anion) before recycling the absorbent (e.g. in chloride form) to the absorption step. The change inanion on the adsorbent is conducted to improve the adsorption kinetics and recovery of copper and iron cyanide complexes.

[0161] In some embodiments, the non-acidic aqueous extractant comprises free cyanide. In some embodiments, the non-acidic aqueous extractant comprises both free cyanide and the displacement anion as disclosed above, for example chloride in an amount of at least 1 mol / litre, or at least 2 mol / litre , or at least 3 mol / litre , or at least 3.5 mol / litre. It is proposed that the cyanide facilitates the displacement of copper cyanide complexes from strong base ion-exchange resins by converting Cu(CN)2’ and CU(CN)32' to the more weakly bound Cu(CN)43’.

[0162] In some embodiments, a sequence of different non-acidic aqueous extractants is used, with the aim of efficiently extracting the copper and iron cyanide complexes into a small total volume of extractant. For example, a strong base ionexchange resin may be contacted with a conditioning eluant (or pre-eluant), for example under static conditions (a pre-soak eluant) or under continuous flow conditions achieved by continuously recirculating the conditioning eluant over the resin, before subsequent continuous elution with a second eluant. In one exemplary embodiment, the resin is conditioned by recirculation or pre-soaking with a high cyanide concentration conditioning pre-eluant, e.g. comprising at least 1 mol / litre CN’ (preferably together with chloride), for a time sufficient to predispose the copper cyanide complexes for elution, e.g. for about 2 hours. Such a conditioning eluant may be particularly helpful when the cyanide-bearing aqueous fluid was acidified before contact with the an anion-exchange absorbent, since copper absorbed from that solution may be present as strongly coordinating Cu(CN)2’. The resin is then continuously eluted with a low cyanide concentration eluant, as disclosed herein e.g. comprising 20 mmol / litre CN’ and at least 3 mol / litre Cl’, to near-quantitatively elute the copper and iron cyanide complexes. The resin may then be washed with water or other suitably pure aqueous fluid to remove free cyanide before recycling the resin to the absorption step.

[0163] Another approach to reduce the total volume of metal-containing extract is to internally recycle a portion of the extract. Typically, the extractable metals are desorbed at relatively high concentrations early in the extraction (elution) process, withsubsequent extraction producing lower concentrations of the desorbed metal species. Nevertheless, to achieve a high overall percentage extraction, it may be necessary to continue extracting the absorbent while producing low metals concentrations, thus producing an undesirably high volume of extract if the later fractions are combined with the high concentration initial fractions. In some embodiments, therefore, a final fraction of the extract is internally recycled to form all or part of an initial fraction of the non- acidic aqueous extractant in a subsequent extraction step, i.e. in extraction of a new batch of pregnant anion-exchange absorbent.

[0164] In some embodiments, the anion-exchange absorbent is extracted consecutively with i) optionally, a conditioning eluant (pre-eluant), (ii) a first extractant fraction of the non-acidic aqueous extractant; iii) a second extractant fraction of the non-acidic aqueous extractant; and (iv) optionally, an aqueous wash. The resultant aqueous extract is collected as a first extract portion, containing most of the extracted metal cyanide complexes, and a second extract portion containing only a minor amount of the extracted metal cyanide complexes. Only the first extract portion is sent for metal recovery, while the second extract portion is internally recycled, with reagent top-ups as needed, to form the first extractant fraction for a subsequent extraction.

[0165] In some preferred embodiments, the volume of the first extract portion is equal to or greater than the combined volume of (i) the first extractant fraction and (ii) the optional pre-eluant. The inventors have found that this minimises hold-up of metals such as copper in the internal recycle stream, so that a high overall extraction efficiency is obtained.

[0166] The first extract portion is sent for metals and free cyanide recovery, as further described hereafter, resulting in the production of a barren extract, typically with very low metals content. Most of this barren extract is then recycled, with reagent top- ups as needed, to form the second extractant fraction for a subsequent extraction. A minor portion may be purged. In some embodiments, a minor portion of the barren extract is used (without reagent to-up) as the aqueous wash. This advantageously minimises the introduction of added water into the elution circuit, and thus the required size of the purge which may represent a loss of valuable reagents such as chloride.

[0167] In one set of exemplary embodiments, a column of pregnant ion-exchange resin is eluted with i) about 0.5 bed volumes (BV) of conditioning eluant (pre-eluant), (ii) about 4.5 BV of the first extractant fraction; iii) about 4 BV of the second extractant fraction; and (iv) about 0.5 BV of aqueous wash (total 9.5 BV). The aqueous extract (also 9.5 BV) is divided into (i) about 5 BV as the first extract and (ii) about 4.5 BV as the second extract fraction.

[0168] In embodiments where the anion-exchange absorbent is a water-immiscible liquid, the absorbed metal cyanide complexes may be extracted into the non-acidic aqueous extractant with liquid-liquid extraction techniques.

[0169] After the extraction step, the regenerated anion-exchange absorbent may be recycled for contact with the cyanide-bearing aqueous fluid. The anion-exchange absorbent may thus pass through multiple cycles of absorption and regeneration. As already disclosed herein, this cycling may be done in carousel mode.

[0170] In some embodiments, the recycled anion-exchange absorbent is either not acid treated after extraction or is acid treated only after multiple absorption-extraction cycles, such as more than three, or more than five, absorption-extraction cycles. Advantageously, avoiding an acid treatment after each absorption cycle may reduce the formation of insoluble species in resin absorbents and extend the lifetime of the absorbent.

[0171] However, the anion-exchange absorbent after non-acidic extraction may contain residual metal species, for example nickel, zinc and / or gold cyanide complexes which have strong affinities for absorbents such as strong base anion-exchange resins. In typical cyanidation processes, nickel and zinc are relatively minor components of the total leached metals, so that the retention of these metals on the absorbent after extraction will not prevent the cycling of the absorbent through multiple absorptionextraction cycles. Over time, however, strongly absorbed cyanometallates will build up and an acid regeneration step may be required.

[0172] The anion-exchange absorbent may thus be subjected to an intermittent acid treatment, for example with a mineral acid such as H2SO4 which is capable of removing absorbed nickel and zinc cyanide complexes. HCI may also be effective to removenickel cyanide complexes. In some embodiments, gold may be retained on the anion- exchange absorbent after this intermittent acid treatment. The acid extractant for nickel and / or zinc removal may thus be substantially free of thiourea.

[0173] As disclosed herein, the iron content in the absorbent is preferably very low when acid treating, to avoid the precipitation of copper-iron, nickel-iron or zinc-iron mixed metal cyanide complexes. Thus, acid treatment is generally effected only after a thorough extraction with the non-acidic aqueous extractant to deplete the iron and preferably also the copper on or in the absorbent.

[0174] Nickel and zinc may be recovered from the acid extract I eluate by conventional means. In one embodiment, a solution containing iron cyanide complexes, for example a portion of the non-acidic aqueous extract, is added to precipitate the nickel and zinc as mixed-metal complexes which can then be recovered by filtration.

[0175] Any gold present in the cyanide-bearing aqueous fluid will also be absorbed on the anion-exchange absorbent, and strongly retained on the absorbent during (i) the non-acidic extraction (to remove copper and iron) and (ii) any intermittent acid treatments to remove nickel and / or zinc. The anion-exchange absorbent may thus be subjected to an intermittent gold extraction treatment, for example with an acidic extractant comprising thiourea, to extract gold. Preferably, the gold extraction treatment will be performed as a separate step immediately after acid treatment to extract nickel and zinc, thus avoiding contamination of the extracted gold with these elements.

[0176] Gold may be recovered from the thiourea-containing acid extract (eluate) by conventional means. In some embodiments, a chemical reducing agent such as sodium borohydride is added to precipitate gold metal from solution. Preferably, the pH of the thiourea-containing acid extract is adjusted to at least 1 , or at least 1 .5, such as at least 2, before adding the chemical reducing agent. The avoidance of strongly acidic conditions has been found to limit the consumption of sodium borohydride by hydrolysis.

[0177] The frequency of the intermittent treatments to remove nickel and / or zinc (typically with thiourea-free acidic extractant) and the intermittent treatments to remove gold (typically with a thiourea-containing acidic extractant) can be independently selected based on the rate at which these species build up on the absorbent. In many implementations, multiple absorption-extraction cycles (such as more than three, or more than five) can be conducted with only non-acidic extractions before it becomes necessary to use an acidic extractant to remove nickel, zinc or gold.Metal precipitation

[0178] After extracting the anion-exchange absorbent with non-acidic aqueous extractant, the resultant aqueous extract comprises one or more extracted metal cyanide complexes. It may thus be desirable to recover the metals and associated cyanide, and to recycle the barren extractant back to the extraction step.

[0179] In some embodiments, the process thus further comprises acidifying at least a portion of the aqueous extract, precipitating one or more metal compounds, and separating the one or more metal compounds from the acidified aqueous extract. For some metals, such as copper, precipitating the corresponding metal compound comprises releasing cyanide into the acidified aqueous extractant in the form of HCN. A first extract portion of the aqueous extract, as described above, may be acidified with a second extract portion internally recycled.

[0180] The aqueous extract may be acidified with a mineral acid such as hydrochloric or sulfuric acid. Hydrochloric acid has the advantage of introducing chloride, which may already be present in the extractant as the displacement anion. Thus, extraneous anions are not introduced to the extractant when the barren extract is recycled. However, the inventors have found that sulfate ions, introduced when acidifying the extract with sulfuric acid, may also be accommodated without ill effect since the sulfate concentration will remain at acceptable levels due to purging.

[0181] In some embodiments, the aqueous extract comprises copper cyanide complex, and copper sulfide (CU2S) is precipitated by adding acid and a sulfide source to the aqueous extract. One suitable sulfide source is sodium sulfide (Na2S). Other suitable sulfides may include K2S, NaHS, KHS, H2S, CaS. Preferably, the coppersulfide is selectively precipitated, so that a high purity copper sulfide product is produced. This may enhance the value of the recovered copper as a saleable product. In some common scenarios, the aqueous extract will comprise both copper cyanide and iron cyanide complexes, and there is a risk of co-precipitating iron with the copper. However, the inventors have found that iron co-precipitation can be avoided or mitigated by adding the sulfide source to the aqueous extract before the initiation of acid-induced precipitation. Copper is thus selectively precipitated as copper sulfide, with the iron cyanide species primarily remaining in solution. The precipitated copper sulfide may then be separated from the copper-lean acidified aqueous extract by conventional methods such as settling and decantation, filtration or centrifugation. Preferably, the resultant copper-lean acidified aqueous extract has a very low copper concentration, such as less than 50 mg L1, preferably less than 20 mg L1.

[0182] Optionally, the iron cyanide complexes may then be recovered by adding a suitable metal salt to the acidified aqueous extract, precipitating a metal-iron cyanide complex, and separating the metal-iron cyanide complex from the acidified aqueous extract. Dissolved iron in cyanide solutions is generally present as ferrocyanide, Fe(CN)64’, or ferricyanide, Fe(CN)e3’, species. These complexes have very high stability constants (Fe(CN)e3’: 1042 3; Fe(CN)e4’: 1035 4) so that the cyanide does not decomplex under typical hydrometallurgical process conditions and is generally considered irrecoverable. However, ferrocyanides (and ferricyanides) form waterinsoluble mixed-metal complexes with a variety of metal cations including iron, copper, nickel and zinc, particularly under acidic conditions.

[0183] In some embodiments, the metal salt comprises ferric (Fe3+) cation. The precipitated metal-iron cyanide complex may thus comprise the compound Felll[Fe"(CN)6]3, also known as Prussian blue. One advantage of using ferric cation to precipitate the iron cyanide complexes is that any excess ferric will precipitate as Fe(OH)3 when the acidified aqueous extract is subsequently alkalized. Moreover, Prussian blue is a potentially saleable product. However, it is not excluded that other metal salts are used to precipitate iron cyanide complexes. The inventors have also demonstrated effective recovery with ferrous (Fe2+) salts. Another alternative is to use the nickel and / or zinc salts which are intermittently extracted, in an acidic solution, from the anion-exchange absorbent. Stoichiometrically controlled dosing of this acidicsolution into the acidified aqueous extract will precipitate the iron content as insoluble zinc-iron and / or nickel-iron mixed metal cyanide complexes.

[0184] As an alternative to sequential precipitation of copper and iron compounds, the copper and iron may intentionally be co-precipitated by acidifying the aqueous extract without adding sulfide. The resultant contamination of the copper product with iron may be acceptable, in some scenarios, if a subsequent iron removal step can be avoided or reduced in size. The copper and iron are expected to precipitate, at least in part, as a mixed-metal cyanide complex having the formula CuxMyFe(CN)6, where M represents one or more metal cations, 0 < x < 4, 0 < y < 4, and x and y are related so that the mixed-metal cyanide complex is neutral, having regard to the oxidation state of the copper, the iron and the metal cations. The copper in the mixed-metal cyanide complex may be in the Cu+or Cu2+oxidation states, and the iron may be in the Fe2+or Fe3+oxidation states. In embodiments where the extract comprises a high concentration of sodium, such as above 1 mol / litre, the precipitate may include Cu2Na2Fe(CN)e.

[0185] As yet another alternative, part or all of the iron present in the acidified aqueous extract may not be precipitated but may instead be controlled at acceptable levels in the extractant, despite recycling of the barren extract, via a purge stream. Due to the exceptionally high stability constants, iron cyanide complexes are considered non-harmful and it may thus be acceptable to dispose of a purge stream still containing dissolved iron. This option may be preferred for example when the iron content in the initial cyanide-bearing aqueous fluid is relatively low.Thiocyanate oxidation in extractant

[0186] In some embodiments, the acidified aqueous extract comprises thiocyanate. For example, a portion of any thiocyanate present in the cyanide-bearing aqueous fluid may be weakly absorbed on the anion-exchange absorbent and then extracted into the non-acidic extractant. Optionally, the process may thus comprise a step of oxidising thiocyanate in the aqueous extract, preferably in the acidified aqueous extract after metal precipitation, to produce free cyanide. The thiocyanate may be oxidised by adding a suitable oxidising agent such as chlorine, a peroxide, or ozone. In some embodiments, the oxidising agent is ozone.Free cyanide recovery from extractant

[0187] The acidified aqueous extract may comprise free cyanide, in the form of HCN. This free cyanide may originate from two main sources: 1 ) by release of metal- complexed cyanide as free cyanide when metals (in particular copper) are precipitated from the aqueous extract, and / or 2) free cyanide intentionally added to the extractant to facilitate metal desorption from the anion-exchange absorbent. Secondary sources of free cyanide may also include 3) weakly absorbed free cyanide extracted from the anion-exchange absorbent, and 4) free cyanide produced by oxidising thiocyanate present in the acidified aqueous extract (as discussed above). Free cyanide may therefore need to be removed from the acidified aqueous extract to prevent build-up due to barren extract recycling and to allow purging of the acidified aqueous extract.

[0188] In some embodiments, therefore, the process comprises a step of removing at least a portion of the HCN from the acidified aqueous extract. Typically, removing the HCN from the acidified aqueous extract comprises transferring HCN from the acidified aqueous extract into a gas phase. In some embodiments, the HCN is removed from the acidified aqueous extract by a gas membrane process, as previously described herein.

[0189] In some embodiments, the process thus includes two gas membrane process steps: one for removing HCN from the metal-lean aqueous fluid after the absorption step; and one for removing HCN from the acidified aqueous extract after the extraction step. This provides the opportunity to obtain various synergies between the two gas membrane process steps.

[0190] For example, a single stripping solution may be used in both process steps. As previously described, the concentration of the alkaline stripping solution used in the first gas membrane process may be limited by the need to avoid or minimise osmotic distillation from the metal-lean aqueous fluid. However, the acidified aqueous extract is, in at least some embodiments, a highly saline solution since the extractant (or eluant) may comprise high chloride (or other anion) concentrations. Much more concentrated alkaline stripping solutions may thus be used to remove HCN from the acidified aqueous extract without unacceptable osmotic distillation.

[0191] Thus, in some embodiments, the process comprises (i) absorbing the HCN present in the metal-lean aqueous fluid into a first alkaline solution, preferably through a gas membrane, to provide a first cyanide solution; and (ii) adding base (e.g. NaOH) to the first cyanide solution to produce the second alkaline solution and (iii), absorbing the HCN present in the acidified aqueous extract into the second alkaline solution, preferably also through a gas membrane. In this manner, all cyanide removed from the process, as HCN, can be obtained in a single, concentrated cyanide solution, for sale or re-use in a hydrometallurgical process.

[0192] HCN may be removed from the acidified aqueous extract at any point after acidification of the aqueous extract. For example, it may be recovered after precipitating and separating all metal species from the aqueous extract. In some embodiments, however, the HCN is removed after precipitating and recovering copper (thus releasing HCN), but before precipitating out iron. Iron precipitation does not release additional free cyanide, and it may be preferable to conduct this precipitation step in the absence of free cyanide.Extractant recycling

[0193] After removal of the metal and HCN from the acidified aqueous extract, the resultant barren extract may be recycled to the aqueous extractant and / or the aqueous wash. The barren extract typically comprises a high concentration of chloride or other displacement anion, and recycling is thus necessary to minimise the losses of this component. The process may thus comprise alkalizing the barren extract and recycling it, with addition of reagent to-ups as needed (e.g. chloride and free cyanide addition) to form at least a portion of the non-acidic aqueous extractant. In some embodiments, a minor portion of the barren extract is purged to provide an outlet for impurities which would otherwise build-up in the extraction circuit.Method of purifying a cyanide-bearing aqueous fluid from a cyanidation process stream in a hydrometallurgical process

[0194] The present invention further relates to a method of purifying a cyanide- bearing aqueous fluid from a cyanidation process stream in a hydrometallurgical process, such as a gold leaching process. The method comprises (i) removing cyanidefrom the cyanide-bearing aqueous fluid by a process as previously disclosed herein; and (ii) discharging the cyanide-lean aqueous fluid from the hydrometallurgical process or recycling the cyanide-lean aqueous fluid for use in a cyanide-free operation of the hydrometallurgical process.

[0195] In some embodiments, the cyanidation process stream is a tails stream of the cyanidation process, i.e. the residual solution or slurry remaining after the primary target metal(s) have been removed. In some embodiment, the cyanidation process stream is a wash stream produced by washing tailing solids from the tails stream. In some embodiments, the cyanide-bearing aqueous fluid is not a pregnant leach solution produced by leaching an ore or concentrate.

[0196] In some embodiments, the cyanide-lean aqueous fluid is discharged to the environment, for example to a lake or river. In such cases, the cyanide lean aqueous fluid will have a cyanide concentration that meets regulatory requirements, such as less than 500 ppb in Australia.

[0197] In other embodiments, the cyanide-lean aqueous fluid is recycled for use within the overall hydrometallurgical process, potentially as a replacement for externally sourced water. Due to the low content of both free and metal-complexed cyanide, the cyanide-lean aqueous fluid may advantageously be used in a cyanide-free operation of the hydrometallurgical process, such as but not limited to ore grinding.System for removing cyanide from a cyanide-bearing aqueous fluid

[0198] The present invention further relates to a system for removing cyanide from a cyanide-bearing aqueous fluid comprising free cyanide and one or more metal cyanide complexes. The system comprises: (i) an absorption unit for contacting the cyanide-bearing aqueous fluid with an anion-exchange absorbent to absorb the one or more metal cyanide complexes, thereby producing a metal-lean aqueous fluid comprising free cyanide; (ii) acid dosing apparatus configured to acidify at least one of the cyanide-bearing aqueous fluid and the metal-lean aqueous fluid, thereby speciating the free cyanide as HCN; and (iii) a first cyanide-removal unit to remove HCN from the metal-lean aqueous fluid, thereby producing a cyanide-lean aqueous fluid.

[0199] The system is generally suitable for performing the process of the invention, and the skilled person will appreciate that system features already described in that context may be present in embodiments of the system.

[0200] The absorption unit may thus utilise a strong base ion-exchange resin as the anion-exchange absorbent. In some embodiments, the absorption unit may comprise one or more columns of ion-exchange resin, for example a plurality of columns (e.g. at least 3) configured to operate in carousel mode as described herein.

[0201] Acid dosing apparatus for acidifying process streams in hydrometallurgical processes is well-known to the skilled person, and may include dosing pumps, mixing equipment to combine acid and the stream being treated, and pH sensors to provide feedback and thus facilitate control of the acidification. In some embodiments, the acid dosing apparatus is configured to acidify the cyanide-bearing aqueous fluid before or during, preferably before, contact with the anion-exchange absorbent. As already disclosed herein, this may increase the capacity of the anion-exchange absorbent for metal removal.

[0202] The first cyanide-removal unit may be configured to transfer the HCN from the metal-lean aqueous fluid into a gas phase. In some preferred embodiments, the first cyanide-removal unit comprises a gas membrane. The unit may thus be configured to (i) contact the metal-lean aqueous fluid with a first side of the gas membrane, (ii) contact a first alkaline solution with a second side of the gas membrane, and (iii) absorb the HCN through the gas membrane into the first alkaline solution to produce a first cyanide solution.

[0203] The system may be configured to extract the absorbed metal cyanide complexes from the anion-exchange absorbent once a target metal loading has been achieved. In some preferred embodiments, the system thus comprises an extraction circuit (e.g. elution circuit when using solid absorbents) configured to: (i) extract (elute) the anion-exchange absorbent with at least one non-acidic aqueous extractant (eluant) to produce an aqueous extract (eluate) comprising one or more extracted metal cyanide complexes; (ii) acidify at least a portion of the aqueous extract to form an acidified aqueous extract; (iii) precipitate and separate one or more metal compounds from the acidified aqueous extract; (iv) remove HCN from the acidified aqueous extract; and (vi)recycle at least a portion of the acidified aqueous extract (barren eluate) to form at least a portion of the non-acidic aqueous extractant.

[0204] The extraction circuit may thus comprise a second cyanide removal unit, which preferably also includes a gas membrane. The second cyanide-removal unit may thus be configured to (i) contact the acidified aqueous extract with a first side of the gas membrane, (ii) contact a second alkaline solution with a second side of the gas membrane, and (iii) absorb the HCN through the gas membrane into the second alkaline solution to produce a second cyanide solution.

[0205] In some embodiments, the first and second cyanide removal units are integrated to allow all free cyanide in the process to be recovered in a single, preferably high concentration, cyanide product. The system may thus be configured to: (i) absorb the HCN present in the metal-lean aqueous fluid into a first alkaline solution through a gas membrane to provide a first cyanide solution; and (ii) add base to the first cyanide solution to produce the second alkaline solution. The second alkaline solution is then used as the stripping solution in the second cyanide-removal unit.

[0206] In common use scenarios, the aqueous extract will comprise copper cyanide complexes, and typically also iron cyanide complexes. In some embodiments, therefore, the system comprises a copper precipitation unit configured to i) acidify and add a sulfide source (e.g. sodium sulfide) to at least a portion of the aqueous extract;(ii) selectively precipitate copper sulfide, thereby releasing free cyanide as HCN; and(iii) separate the copper sulfide from the acidified aqueous extract. Advantageously, the copper sulfide may be produced in high purity, improving its value as a saleable product.

[0207] In some such embodiments, the system further comprises an iron precipitation unit downstream of the copper precipitation unit. The iron precipitation unit is configured to (i) add a metal salt, such as a ferric salt, to the acidified aqueous extract; (ii) precipitate a metal-iron cyanide complex, such as Prussian blue; and (iii) separate the metal-iron cyanide complex from the resultant barren extract.

[0208] The precipitates may be separated and recovered by conventional techniques in hydrometallurgical process, such as filtration, centrifugation, clarificationetc. Optionally, a flocculant may be added to facilitate separation of e.g. the precipitated metal-iron cyanide complex.

[0209] The system may be designed to extract the anion-exchange absorbent with high efficiency, while producing only a small volume of extract that must be processed to remove metals and cyanide. Thus, a portion of the extract with low metals content may be internally recycled to form part of the non-acidic aqueous extractant. In some embodiments, therefore, the system is configured to: extract the anion-exchange absorbent consecutively with a first extractant fraction and a second extractant fraction of the at least one non-acidic aqueous extractant; recover the aqueous extract consecutively in a first extract portion and a second extract portion; recycle the second extract portion to the first extractant fraction; and recover the metals and cyanide only from the first extract portion.Embodiments

[0210] The invention will now be described with reference to Figure 1 , which depicts system 100 for removing cyanide from a cyanide-bearing aqueous fluid according to some embodiments of the invention. Aqueous fluid 102 comprises both free cyanide (as CN’) and metal cyanide complexes. In some embodiments, the metal cyanide complexes include anionic copper, iron, nickel and zinc cyanide complexes, with the nickel and zinc cyanide complexes present as minor components relative to the copper and iron cyanide complexes. In some embodiments, the metal cyanide complexes include anionic gold cyanide complexes. While copper, iron, nickel and zinc cyanometallates are typical components of cyanidation process streams in minerals processing operations, it should be appreciated that the process is not limited to the treatment of aqueous fluids comprising these specific cyanometallates or combinations thereof.

[0211] Aqueous fluid 102 may be a process stream in a cyanidation process, for example derived by processing a cyanidation tails slurry which contains residual free cyanide from the cyanidation leaching step, base metal cyanide complexes leached from the ore and residual ore solids. As depicted in Figure 1 , tails slurry 104 is clarified in thickener 106 and the clarified tails solution 108 is then optionally processed via ion-exchange absorption unit 1 10 for subsequent recycling of metal-lean tails solution 1 12 in the overall cyanidation process, for example to form part of the cyanidation lixiviant.

[0212] Underflow 1 14 of thickener 106 is sent to filter unit 1 16 where the ore solids are filtered, washed with wash water 1 18 and discharged from the process as dry tailings 120. The clarified liquid effluent from the filtration unit, comprising the liquid component of underflow 1 14 and wash water 1 18, forms aqueous fluid 102. Aqueous fluid 102 thus comprises both free cyanide and metal-complexed cyanide species which were present in cyanidation tails slurry 104, and these species should be removed before fluid 102 can be discharged into the environment or used elsewhere in the minerals processing plant.

[0213] The overall process depicted in process flow diagram 100 has the advantage that only a small proportion of cyanidation tails slurry 104 is treated to remove free cyanide. Nevertheless, this may provide sufficient purge capacity to prevent unacceptable accumulation of undesirable soluble components in the cyanidation lixiviant, while also maintaining the overall water balance of the process. While process flow diagram 100 depicts aqueous fluid 102 as a wash / purge stream, as described, it will be appreciated that this is merely an exemplary embodiment and the invention is generally applicable to the treatment of cyanide-bearing aqueous fluids comprising both free and metal-complexed cyanide species.

[0214] Aqueous fluid 102 is sent to absorption unit 128 where it is contacted with an anion-exchange absorbent 130 so that the metal cyanide complexes are absorbed into the absorbent. Absorbent 130 may be a strong base ion-exchange resin, for example in resin beads. Aqueous fluid 102 may be passed continuously through a single column of the resin until the absorption capacity of the resin is near-exhausted (i.e. until shortly before breakthrough of the cyanometallates is expected). Alternatively, aqueous fluid 102 may be passed through two columns in series, operating in carousel mode as described herein.

[0215] The resultant metal-lean aqueous fluid 126 still comprises most of the free cyanide that was initially present in aqueous fluid 102, because free cyanide is only weakly absorbed on anion-exchange absorbents.

[0216] At least one of cyanide-bearing aqueous fluid 102 and metal-lean aqueous fluid 126 is acidified to speciate the free cyanide. Mineral acid 124, such as hydrochloric or sulfuric acid, is thus added (i) before absorption unit 128 by acid dosing apparatus 124a, or (ii) after absorption unit 128 by acid dosing apparatus 124b, or (iii) by both acid dosing apparatus 124a and 124b. Addition of at least a portion of mineral acid 124 before absorption unit 128 may advantageously reduce the average valency of the metal cyanide complexes, thus increasing the absorption capacity of absorbent 130 for metal removal.

[0217] Optionally, system 100 further comprises an ozone dosing apparatus 125 configured to add ozone to either cyanide-bearing aqueous fluid 102 or metal-lean aqueous fluid 126, for the purposes of oxidising any thiocyanate present in cyanide- bearing aqueous fluid 102. The ozone dosing apparatus is preferably located downstream of at least one of acid dosing apparatus 124a or 124b (e.g. at locations 125a or 125b indicated) so that the relevant stream can be acidified before adding ozone.

[0218] The acidified metal-lean aqueous fluid 126 is then sent to HCN-recovery unit 122 where HCN is removed from the aqueous fluid, typically by transfer into a gas phase. In some embodiments, HCN-recovery unit 122 includes a gas membrane. Aqueous fluid 126 and alkaline solution 127 are contacted with opposite sides of the gas membrane, by flow along the membrane typically in multiple passes as described herein, allowing the HCN to diffuse through the gas membrane into the alkaline solution and forming cyanide solution 129.

[0219] The resultant cyanide-lean aqueous fluid 132 is then discharged from the process, or recycled to the cyanidation process, for example as wash water 1 18. Cyanide-lean aqueous fluid 132 may advantageously have a low content of both free and metal-complexed cyanide, such as less than 500 ppb, potentially allowing its discharge to the environment.

[0220] Metal-loaded anion-exchange absorbent 130a, as produced in absorption unit 128, is then subjected to extraction in extraction unit 134 with non-acidic aqueous extractant 136, optionally comprising a solution of free cyanide (CN’) and chloride (Cl’). The absorbed copper cyanide and iron cyanide complexes are thus extracted from theloaded absorbent into aqueous extract 138. It is preferred that a high proportion of the copper cyanide and iron cyanide complexes are extracted from the absorbent, and a cyanide-chloride extractant solution with chloride concentration over 3 mol / litre has been found particularly effective to achieve this goal. However, other non-acidic aqueous extractant compositions may be used provided that they are capable of desorbing and extracting the copper and iron cyanide complexes.

[0221] Prior to the extraction step, loaded absorbent 130a may be transferred to a separate extraction vessel, as represented by dotted arrow 140 in Figure 1 , where the extraction takes place. Alternatively, however, the absorbent is maintained in the same vessel, e.g. a resin column, through both the absorption and extraction steps. Absorption unit 128 and extraction unit 134 may thus comprise the same process equipment, with the absorbent cycled in situ between absorption and extraction cycles (as represented in this case by dotted arrows 140 and 142), optionally in carousel mode as described herein.

[0222] In embodiments where anion-exchange absorbent 130 is a strong base ionexchange resin, eluant 136 may be passed continuously through a column of the loaded resin to elute the copper cyanide and iron cyanide complexes, thereby producing aqueous eluate 138.

[0223] Once the copper cyanide and iron cyanide complexes have been extracted from anion-exchange absorbent 130a in extraction unit 134, the barren absorbent is cycled back to absorption unit 128 where it is again used to absorb metal cyanide complexes from aqueous solution 102. In this manner, anion-exchange absorbent 130 may be recycled repeatedly between absorption and extraction steps (i.e. multiple absorption-extraction cycles).

[0224] Nickel, zinc and gold cyanide complexes have a higher affinity for certain anion-exchange absorbents than copper cyanide and ferrocyanide complexes, and thus in at least some embodiments the nickel, zinc cyanide and gold complexes initially present in cyanide-bearing aqueous fluid 102 are predominantly or entirely retained on absorbent 130a after the non-acidic extraction step. Aqueous extract 138 thus contains little or no nickel, zinc or gold. However, as the nickel, zinc and gold species were relatively minor components in aqueous fluid 102, the retention of these species doesnot substantially impair the absorption capacity of absorbent 130 until it has passed through multiple absorption-extraction cycles.

[0225] If the level of absorbed nickel and zinc cyanometallate species does exceed an undesirable level after multiple cycles, anion-exchange absorbent 130 may be subjected to an acid extraction in acid regeneration unit 144, using acid 146, for example sulfuric or hydrochloric acid. Acid 146 is preferably substantially free of thiourea, to avoid gold elution. Once again, the absorbent may be physically transferred to a separate vessel for the acid regeneration step, or retained in the same vessel for intermittent in situ acid treatment. In either case, because the acid extraction step is performed after the non-acidic extraction with aqueous extractant 136, the absorbent contains little or no residual copper cyanide and iron cyanide complexes when acidified. Under these conditions, the decomplexed nickel and zinc cations desorb and are extracted into the acidic extract 148. Free cyanide 150, released as HCN under the acidic conditions, may be recovered from the acid regeneration unit.

[0226] The acid regenerated absorbent 130b, now containing very low levels of absorbed metal species (except, in some embodiments, small amounts of gold), is then returned to absorption unit 128 and extraction unit 134 for further absorption and extraction cycles. Depending on the amount of nickel and zinc in aqueous fluid 102, it is expected that many cycles of absorption and extraction can be conducted before an acid regeneration step is required. Advantageously, the lifetime of the absorbent may thus be extended as it is not subjected to regular switching between neutral / alkaline and highly acidic conditions.

[0227] In embodiments where small amounts of gold are present in cyanide-bearing aqueous fluid 102, the gold will accumulate on the absorbent over multiple cycles, even when intermittently exposed to acidic elution to remove nickel and / or zinc. Once the level of absorbed gold cyanometallate species reaches a threshold level where recovery is desired, anion-exchange absorbent 130 may be subjected to a further intermittent acidic extraction in gold recovery unit 184, using acidic extractant 186 which comprises thiourea. Once again, the absorbent may be physically transferred to a separate vessel for the acid regeneration step, or retained in the same vessel for intermittent in situ treatment. In either case, because the gold recovery step isperformed after the non-acidic extraction with aqueous extractant 136 and typically also directly after an acidic extraction with acid 146, the absorbent contains little or no residual copper-, iron-, nickel- or zinc cyanide complexes when extracted with extractant 186. Under these conditions, the gold desorbs into the gold-bearing extract 188 with little or no contamination by previously extracted metals. Gold values can be recovered from extract 188 by conventional means such as chemical reduction. The regenerated absorbent 130c, now containing very low levels of absorbed metal species, is then returned to absorption unit 128 and extraction unit 134 for further absorption and extraction cycles.

[0228] After non-acidic extraction in extraction unit 134, at least a portion of aqueous extract 138, comprising the extracted copper cyanide and iron cyanide complexes, is sent to copper precipitation unit 160. There, sulfide 162 (e.g. sodium sulfide) and acid 163 are added to selectively precipitate the copper as copper sulfide and release the associated cyanide as HCN. Copper sulfide product 164 is separated and recovered, and can be sold to recover the copper value of aqueous fluid 102.

[0229] The resultant copper-lean acidified extract 166 is then sent to HCN-recovery unit 168 where HCN is removed from the fluid, typically by transfer into a gas phase. In some embodiments, HCN-recovery unit 168 includes a gas membrane. Acidified extract 166 and alkaline solution 131 are contacted with opposite sides of the gas membrane, by flow along the membrane typically in multiple passes as described herein, allowing the HCN to diffuse through the gas membrane into the alkaline solution and forming cyanide solution 135. Cyanide solution 135 may be sold or recycled to the cyanidation process.

[0230] In some embodiments (as depicted), alkaline solution 131 comprises cyanide solution 129, the product of HCN-recovery unit 122. Thus, all free cyanide recovered from aqueous fluid 102 (either initially present as free cyanide, or produced by decomplexation in the process) is recovered in a single cyanide solution 135. Optionally, additional base 137, such as NaOH, is added to cyanide solution 129, thus increasing the concentration of alkaline solution 131. This advantageously facilitates both (i) the recovery of HCN in HCN-recovery unit 122, where a low concentration alkaline solution may be required to avoid osmotic distillation, and (ii) the subsequentrecovery of HCN in HCN-recovery unit 168, where a high concentration alkaline solution may be accommodated due to the highly salinity of acidified extract 166. Thus, all HCN may be recovered to a single, high concentration cyanide solution 135.

[0231] After HCN removal, cyanide-lean acidified extract 169 is sent to iron precipitation unit 171 . There, metal salt 173 is added to selectively precipitate the iron cyanide complexes as a metal-iron cyanide complex. In some embodiments, the metal salt is a ferric salt, thus precipitating Prussian blue. In other embodiments, the metal salt may be nickel and / or zinc salts present in acidic extract 148. The metal-iron cyanide complex 175 is separated and recovered, and can optionally be sold.

[0232] In a variant, iron precipitation unit 171 is located upstream of HCN-recovery unit 168. This is possible because iron precipitation is not expected to release further free cyanide. However, it may be preferable from a safety perspective to remove all free cyanide before precipitating and recovering the iron.

[0233] In another variant, the iron precipitation unit is located upstream of, or replaces, copper precipitation unit 160. Acidification of aqueous extract 138, without addition of sulfide, will precipitate a copper-ferrocyanide mixed-metal cyanide complex, such as Cu2Na2Fe(CN)6, and release free cyanide as HCN. If copper is in excess, all iron will thus be removed. The iron-containing precipitate can then be separated from the iron-lean acidified extract, which is sent downstream for recovery of any residual copper in copper precipitation unit 160 and removal of HCN in HCN-recovery unit 168. While effective, one disadvantage of this approach is that at least a portion of the copper is recovered as a mixture with iron.

[0234] After removal of copper, iron and free cyanide, most of the acidified barren extract 177 is recycled, after re-alkalization with reagent top-up stream 179 (e.g. NaCN + NaCI), to non-acidic extractant 136. A minor portion of acidified barren extract 177 is removed as outlet stream 181. A portion of this stream may be purged, providing an outlet for impurity species which would otherwise build-up in the extractant circuit, and another portion may be used (instead of water) to wash the anion-exchange absorbent as the final stage of the extraction.

[0235] The elution step of an embodiment of the invention will now be described with reference to Figure 2, which depicts elution circuit 234. Elution circuit 234 may be part of an overall process such as system 100 described above. Elution circuit 234 includes elution column 235 containing strong base ion-exchange absorbent 230a, which has been loaded with copper, iron, nickel and zinc cyanometallates via an absorption step as disclosed herein. To initiate the elution step, absorbent 230a is subjected to a conditioning step in the form of a pre-elution soak with pre-eluant 280 (a conditioning eluant). Pre-eluant 280 contains free cyanide (CN_) and chloride (Cl’). The free cyanide and chloride concentrations may be relatively high, for example the cyanide concentration may be above 0.5 mol / litre, or above 1 mol / litre, such as about 1.2 mol / litre, and the chloride concentration may be above 3 mol / litre, or above 4 mol / litre, such as about 4.5 mol / litre. Pre-eluant 280 is loaded in an amount of less than one bed volume (BV), for example 0.5 BV, such that all the ion exchange resin is in contact with pre-eluant 280, and the pre-eluant is allowed to contact the absorbent for a suitable soak time, for example between 1 and 3 hours. The high concentrations of cyanide and chloride in the soak assist to desorb the copper and iron cyanometallates, thus allowing effective elution of these species into a smaller total volume of eluate. As an alternative, the conditioning step can be conducted by continuously recirculating pre-eluant 280 through absorbent 230a for a time sufficient to condition the copper and iron cyanometallates for desorption in the subsequent main elution step.

[0236] After the static or continuous conditioning step, absorbent 230a is continuously eluted by passing eluant 236 through column 234 as a suitable flow rate, for example about 2 BV / hour. The eluant may comprise chloride with a concentration of at least 3 mol / litre, such as about 4.0 mol / litre, and optionally free cyanide, for example with a concentration of 10-100 mmol / litre, such as 20 mmol / litre. The continuous elution is performed with two sequential fractions: eluant faction 236a, an internally recycled elution stream containing a low copper content, and then eluant fraction 236b which is preferably essentially free of copper.

[0237] Eluate portion 238a, a first portion of the eluate 238 containing most of the extracted copper and iron cyanide complexes, is recovered from column 235 and sent to metals and cyanide removal units 290 (corresponding to units 160, 168 and 171 inFigure 1 ), thus producing barren extract 277 of which a small portion 281 is purged. Eluate portion 238b, a subsequent portion with only low copper concentration, is internally recycled within elution unit 234 to form, together with top up reagents 292 (chloride and cyanide as needed), eluant fraction 236a. Barren extract 277, which is substantially free of metal cyanide complexes, is recycled to form, together with top up reagents 292 (chloride and cyanide as needed), eluant fraction 236b.

[0238] The volume of eluate portion 238a is selected to ensure that this fraction contains most of the total copper and iron cyanide complexes desorbed from the ion- exchange resin, while minimising the total volume of eluate sent for further processing. Because the copper elution profile from ion-exchange resins may exhibit a long tail of low concentration copper after the primary copper elution peak, a small proportion of the total copper cyanide complexes may report to eluate portion 238b. However, it may be more efficient to internally recycle this fraction as eluant 236b than to dilute the eluate sent for further processing via the precipitation unit.

[0239] Preferably, the volume of the eluate portion 238a is equal to or greater than the combined volume of (i) pre-eluant 280 and (ii) 236a. Thus, eluate 238 is only directed to second eluate portion 238b when, or after, elution with copper-containing eluant fraction 236a ceases. This minimises hold-up of metals such as copper in the internal recycle stream, so that a high overall extraction efficiency is obtained.

[0240] After the elution with eluant fraction 236b, absorbent 230a may be washed with wash solution 284, for example to remove free cyanide. The wash solution may advantageously be a portion of barren extract 277. The unloaded ion-exchange resin 230 can then be returned to an absorption step, or an intermittent acid regeneration step, as described.

[0241] It will be appreciated that the optimised absolute and relative volumes of pre- eluant 280, eluant fractions 236a and 236b, wash solution 284, eluate portions 238a and 238b, and purge 281 , will be implementation specific. In one exemplary embodiment, pre-eluant 280, eluant fractions 236a and 236b, and wash solution 284 comprise 0.5 BV, 4.5 BV, 4 BV and 0.5 BV respectively; eluate portions 238a and 238b comprise 5 BV and 4.5 BV respectively, and purge 281 comprises 0.5 BV.EXAMPLES

[0242] The present invention is described with reference to the following examples. It is to be understood that the examples are illustrative of and not limiting to the invention described herein.Example 1. General procedures for free cyanide recovery by gas membrane contactor

[0243] Investigations of free cyanide recovery were conducted using with a 3M Liqui-Cel™ MiniModule 1 x 5.5 membrane contactor with an active surface area of 0.18 m2. To start the test, 1 L free cyanide containing solution was acidified to a target pH using 1 M H2SO4. The pH was adjusted and maintained at the target pH using a computer controlled Metrohm 906 Titrando unit. It was observed that the pH gradually decreased slowly with acid addition from the initial pH of c.a. 10.5 to c.a. 7 during which CN’ was converted to HCN (pKa of HCN: 9.2). The subsequent sharp drop in pH with further acid addition indicated that essentially all CN’ had already been converted to HCN by about pH 7.

[0244] 0.2 L of NaOH solution (0.5 M - 1.5 M) was used as the strip solution, recirculating through the membrane contactor (in the same direction to the strip solution unless otherwise specified) at the same flowrate as the cyanide containing solution, being 300 - 500 mL min1. The cyanide solution was flowed on the lumenside and the strip solution on the shellside. A solution sample was taken from the feed and from the final strip solution for free cyanide measurement by potentiometric titration. Periodic samples were taken from the acidified stream (with pH adjusted to above pH 1 1 ) for free cyanide measurement by O.l. CN analyser (using pH 6 buffer as a carrier solution) (detection limit: 0.5 ppb).

[0245] For scale-up studies, a 3M Extra-Flow 2.5 x 8 membrane contactor with an active surface area of 1 .5 m2was used. To start the test, 10 L free cyanide containing solution (simulating cyanidation tails solution following ion-exchange resin removal of metal cyanides) was agitated and acidified to a target pH using 2 M H2SO4. The pH was maintained at the target pH (pH 3 or pH 6) during the test using a computer controlled Metrohm 906 Titrando unit. 1 L of 1 M NaOH solution was used as the stripsolution. The acidified cyanide solution and the NaOH strip solution were recirculated through the gas membrane contactor counter-current to each other at 10 L min-1.Example 2. Initial investigations of flow directions and flow rates

[0246] An initial study was conducted using the method of Example 1 with cyanide solution having initial pH of 3 and a NaOH strip solution of 0.5 M. Both solutions were flowed through the contactor at 0.5 L min-1. Experiments were conducted with the flows in opposite direction (counter-current flow) and the same direction (same direction flow). The results are shown in Figure 3. The cyanide concentration was reduced to below 500 ppb (0.5 mg / L) after a period of between 30 and 40 minutes in both cases. The slightly inferior results for the counter-current flow experiment may have been caused by one of the flows being directed downwards, in contrast to the same direction flow experiment where both flows were directed upwards and thus expected to better fill the flow channels.

[0247] A further study was conducted using cyanide solution with initial pH of 4 and a NaOH strip solution of 0.5 M to investigate the effect of recirculation flow rate. Flow rates of 0.3, 0.4 and 0.5 L min-1were used. All flow rates were effective to remove HCN to below 500 ppb (by 50 minutes), but with the best results obtained at the highest flow rate (240 ppb after 40 minutes).Example 3. Effect of cyanide solution pH

[0248] A study of the effect of cyanide solution pH was conducted using the method of Example 1 with using cyanide solutions having different initial pH, a NaOH strip solution of 0.5 M, and solution flows of 0.5 L min1. Cyanide solution pH values of 2, 3, 4 and 5 were investigated. The results are shown in Table 1. The cyanide uptake curves for all experiments were similar to Figure 3, with cyanide concentration reduced to below 500 ppb (0.5 mg / L) by 40 minutes. Cyanide recoveries of 89-95% were estimated based on the strip solution analysis, with the losses attributed to volatilization to atmosphere in the small-scale experiment. Evidently, the pH of the cyanide solution in the investigated range has little effect since free cyanide is fully speciated to HCN below pH of about 7.Table 1.Example 4. Effect of strip solution NaOH concentration

[0249] A study of the effect of strip solution concentration was conducted using the method of Example 1 with pH 4 cyanide solution, different concentration NaOH strip solutions, and solution flows of 0.5 L min1. NaOH concentrations of 0.5, 1 and 1 .5 M were investigated. The results are shown in Table 2. No effect of NaOH concentration on HCN removal was evident. However, the use of a higher NaOH concentration means that the same strip solution can be used for more cyanide recovery cycles to produce a smaller volume of more concentrated cyanide product. The upper limit of the NaOH concentration is governed by potential osmotic distillation, meaning that water vapour can transfer from the acidified cyanide stream to the strip solution when NaOH concentration is overly high due to large imbalance of ionic strength on both sides of the gas membrane. This osmotic distillation effect can be undesirable as it increases the volume of the strip solution and reduces the recovered cyanide concentration. No volume change in the strip solution was observed when 0.5 and 1 M NaOH were used; whilst the strip solution volume increased by 1 mL within one hour when 1.5 M NaOH were used.Table 2.Example 5. Absorption of solutions comprising free and metal-complexed cyanide on ion-exchange resin

[0250] Purolite MTA5013SO4 ion-exchange resin was used for the resin column adsorption tests. The Purolite MTA5013SO4 was pre-conditioned to convert from sulfate to chloride form prior to resin adsorption using the following method provided by Purolite: (i) in a resin column, pump 8 bed volumes (BVs) of 1 M NaCI at a flowrate of 2 BV hr1; (ii) displace entrained liquor with 6 BVs deionised (DI) water at 5 BV hr1.

[0251] The column adsorption was conducted using a 7.5 mL resin column coupled with a fraction collector. A synthetic feed solution representative of a cyanidation tails solution from a gold mining operation, comprising free cyanide, Cu(CN)32’, Fe(CN)63’, Zn(CN)42', Ni(CN)42' and thiocyanate (SCN-) with composition shown in Table 3, was pumped through the column as the required flow rate.

[0252] The following experiments were conducted.• Exp #1 : 350 BVs (2.6 L) at 5 BV hr1. The first 187 BVs was collected in three fractions (92 BVs, 31 BVs and 64 BVs), and the last 120 BVs was collected using a fraction collector.• Exp #2: 350 BVs (2.6 L) at 10 BV hr1. The first 230 BVs was collected in two fractions (70 BVs and 160 BVs), and the last 120 BVs was collected using a fraction collector.• Exp #3: 350 BVs (2.6 L) at 15 BV hr1. The first 230 BVs was collected in one fraction, and the last 120 BVs was collected using a fraction collector.• Exp #4: Same as Exp #1 but with a higher thiocyanate concentration (0.603 mM instead of 0.166 mM).

[0253] A sample was taken from each of the large fractions and one from every 5thsmall fractions. These samples were subject to metal analysis by ICP, SCN analysis by HPLC and free cyanide analysis by potentiometric titration with silver nitrate. Uponcompletion of the column adsorption, a resin sample (~1 mL) was taken for resin stripping to determine the exact loading of various species on the resin.Exp #1

[0254] Exp #1 was conducted at 5 BV hr1. The composition of the feed solution, and the first three fractions of barren solution, are shown in Table 3.Table 3.

[0255] No break-through of metals was seen for the first three large fractions of adsorption barren (187 BVs in total), indicating compete recovery of the metal cyanides by resin adsorption. The subsequent fractions (7.5 mL each) were collected by auto fraction collector and the adsorption profiles of various species are shown in Figure 4. Copper started to break through (concentration > 1 mg L’1) from the 253rdBV and its concentration increased slowly to > 10 mg L’1from the 318thBV. Iron started to break through (concentration > 1 mg L-1) from the 238thBV; the iron concentration increased with BVs to above the feed concentration (10.4 mg L’1) from the 298thBV, indicating a desorption process occurring due to the competitive adsorption of other more strongly adsorbed metal cyanide species such as Cu / Ni / Zn. There was no break-through of Ni and Zn throughout the 350 BVs (concentrations below detection limit of ICP: <0.02 mg L’1).

[0256] Thiocyanate started to break through in the second large fraction of adsorption barren and its concentration slowly increased with BVs to above the feed concentration (0.166 mM) due to competitive adsorption of other species. Free cyanide started to break through in the first large fraction of adsorption barren due to its weakaffinity towards resin adsorption. Fluctuations and losses of free cyanide were attributed to loss of volatile cyanide during sample collection.

[0257] Upon completion of passing 350 BVs through the resin adsorption column, a sample of the pregnant resin (mixed before sampling to homogenise) was stripped to determine the loadings of various species. The loading capacity of MTA5013SO4 resin is 1 150 mM equivalent and Table 4 shows that more than half of its capacity was loaded with copper cyanide, followed by iron cyanide and free cyanide. The reported free cyanide loading is likely to be less than its actual loading due to potential cyanide loss as HCN during resin stripping at -pH 7.Table 4.achloride loading calculated based on the difference of the loading capacity of resin (1150 mM equivalent) and the measured loading of other speciesbequivalent loading was calculated taking into account charge of the species (e.g. as each Fe(CN)64-occupies four resin adsorption sites, its equivalent loading = 4 x actual loading in mM).Exp #2

[0258] Exp #2 was conducted at 10 BV hr1. Similar results, with only slightly reduced absorption efficiency, were observed compared to 5 BV hr1. Copper and iron broke through (concentration > 1 mg L’1) from the 226thBV and 220thBV respectively (vs. 253 BVs and 238 BVs for 5 BV hr1). There was no break-through of Ni and Zn until the 326thBV; 0.1 mg L’1Zn was detected afterwards. Free cyanide broke through in the first large fraction of adsorption barren solution due to its weak affinity towards resin adsorption.Exp #3

[0259] Exp #3 was conducted at 15 BV hr1. The composition of the feed solution, and the major fraction of barren solution, are shown in Table 5.Table 5.

[0260] At the 230thBV (the first 7.5 mL fraction collected by fraction collector), the copper concentration was already > 3 mg L’1and iron concentration > 2 mg L’1. The results indicate that copper and iron started to break through (concentration > 1 mg L’1) within the first 229 BVs, i.e. much earlier than at 5 or 10 BV hr1. As a result, much lower volumes of solution may be treated at 15 BV hr1to ensure complete recovery of copper cyanides.

[0261] Exp #4 was conducted at 5 BV hr1but with elevated thiocyanate concentration in the feed (0.603 mM vs 0.166 mM in Exp #1 ). Copper broke through (concentration > 1 mg L’1) from the 241stBV (vs. 253rdBV for Exp #1 ), and the break through concentrations increased more rapidly (> 10 mg L’1at 291stBV vs. 318thBV for Exp #1 ). There was no break-through of Ni and Zn until the 326thBV; 0.1 mg L’1Zn was detected afterwards. Free cyanide broke through in the first large fraction of adsorption barren solution due to its weak affinity towards resin adsorption. For solutions containing higher thiocyanate concentrations, the bed volumes of solution treated for each adsorption cycle will be slightly reduced to accommodate competitive adsorption of thiocyanate and to ensure complete recovery of Cu / Zn / Ni cyanides.

[0262] The results of Exp #1 - Exp #4 indicate that, although copper cyanide started to break through at 200 - 250 BVs, the initial breakthrough concentrations were low. To enable the treatment of more bed volumes of solution for each adsorption cycle while ensuring complete recovery of Cu / Ni / Zn cyanides, two resin columns in series can be used for resin adsorption, operating in a carousel mode. In this approach, the 1stresin column will recover the majority of the metal cyanides whilst the 2ndresin column scavenges the residual metal cyanides that break through the 1stcolumn. When the 1stresin column reaches a target break-through copper concentration it will be taken offline for resin elution and the 2ndresin column will move forward as the 1stresin column;once elution of the 1stresin column is complete the eluted column will be brought back as the 2ndcolumn in the series.Example 5. Comparative absorption of different solutions on ion-exchange resin

[0263] Aqueous solutions were prepared having the compositions shown in Table 6 below.Table 6.

[0264] Absorption tests using solutions 1 and 2 were performed using a strong base ion-exchange resin (Purolite A500 / 2788) loaded into an 8 mL resin column coupled with a fraction collector. 100 bed volumes (BVs) of the test solution were pumped through the resin column at 5 BV hr1, with 1 BV fractions collected by the fraction collector. One sample out of every five fractions was subjected to metal analysis by ICP.

[0265] For sample 1 , free cyanide broke through at 20 BV, but copper and iron only broke through at 64 BV. By contrast, for sample 2, copper and iron broke through at 50 BV. This is attributed to the increased copper concentration in Sample 2. By complexing the free cyanide with copper, larger ion-exchange resin beds must be provided to remove the metal cyanide complexes without risk of breakthrough.Example 6. Free cyanide removal from cyanidation tails solutions after metal removal

[0266] Gas membrane removal of free cyanide was conducted in a larger scale gas membrane (1.5 m2active surface area) by the method of Example 1. The syntheticfeed solution, having the composition shown in Table 7, modelled a cyanidation tails stream after removal of the metal cyanide complexes on a strong base ion-exchange column. Tests were conducted after acidification to pH 6 and pH 3 respectively using a flowrate of 10 L min1.Table 7.

[0267] The cyanide recovery results are shown in Figure 5 and Table 8. For both pH 3 and pH 6 complete recovery of cyanide was achieved within one hour. The slightly faster recovery kinetics observed at pH 3 vs pH 6 were ascribed to a stronger driving force for CN’ to HCN equilibrium at the lower pH. At pH 3, 45 minutes were required to reduce the cyanide concentration from 200 ppm to less than 500 ppb (which is Australian cyanide surface discharge standard according to Cyanide Management, Australian Government 2008) while 50 minutes was required for pH 6. The slightly longer time required for cyanide removal, compared to Examples 2-4, is because of the higher membrane-surface-area : solution-volume ratio when using the smaller-scale membrane contactor (0.18 m2L’1compared to larger-scale contactor 0.15 m2L’1). Further reduction in pH below pH 3 would increase the acid and subsequent reneutralisation caustic demands which could offset any benefit of additional increase in kinetics (which is not expected to be significant).Table 8.Example 7. Resin elution experiments

[0268] A large-scale resin adsorption was conducted to generate pregnant resin for resin elution investigations, using a 500 mL ion exchange resin column containing preconditioned Purolite MTA5013SO4 resin. 65 BVs (33 L) of a synthetic cyanide-bearing solution, containing Cu(CN)32', Fe(CN)e2', Ni(CN)42’ and Zn(CN)42’ and having the composition shown in Table 9, was pumped through the resin column at 5 BVs hr1. Solution samples were taken from the pregnant and barren adsorption solutions for ICP analysis of metals. A resin sample was taken after the adsorption and subject to resin strip to determine the loadings of various species on the pregnant resin.Table 9.

[0269] The loaded resin was split into 100 mL portions for resin elution investigations using a column elution setup. The following elution protocol was employed:1 . Presoak with 0.5 BV of 4.5 M NaCI + 1 .2 M NaCN for 2 hours, circulating at 10 BV hr1.2. Elution at 2 BV hr1with varying elution conditions as shown in Table 10.

[0270] 1 BV fractions were collected and a sample from each fraction was analysed for copper and iron by ICP. A resin sample was taken after elution and subject to resin strip to determine the residual loadings of various species on the barren resin.Table 10.Change in chloride concentration

[0271] It is economically preferable to minimise the chloride concentration during elution while still maintaining acceptable elution efficiencies. The effect of chloride concentration on elution efficiency, based on Elution #1 -3, is seen in Figure 6 and Table 1 1.Table 11.

[0272] As seen in Figure 6, copper elutes with a peak below 2 BV but with a long subsequent tail which continues beyond 10 BV. While iron elution was quantitative in all three experiments, the copper elution efficiency decreased with decreasing chloride concentration.Change in cyanide concentration

[0273] In an effort to improve the copper elution efficiency with 2 M NaCI, the NaCN concentration was increased from 20 mM to 40 mM in Elution #4 and to 60 mM inElution #5. As seen in Table 12, increasing cyanide concentration had little effect on the efficiency of copper elution at 2 M NaCLTable 12.Effect of sulfate

[0274] The use of H2SO4 to precipitate metals is economically preferable compared to HCI but will introduce sulfate into the elution circuit, with the sulfate building up to a level determined by the rate of purging. The effect of sulfate in eluant on elution efficiencies was thus investigated in Elution#6 and Elution#7. No adverse effect was observed; the elution efficiencies for Elution#5, #6 and #7 were 94%, 95% and 95% respectively.Example 8. Precipitation of copper-iron mixed metal complex from eluant

[0275] A strong base ion-exchange resin (Purolite A500 / 2788) was loaded by contacting it with a synthetic cyanide-bearing solution (composition: 5 mM SCN’ + 3.5 mM copper [Cu(CN)32’] + 1 mM iron [Fe(CN)e3’] + 1 mM zinc [Zn(CN)42’] + 1 mM nickel [Ni(CN)42-]) in a batch process overnight. The loaded resin was rinsed and eluted as follows: Resin presoak with 0.5 BV of 3 M NaCI + 1 M NaCN solution for 2 hours, (2)elution with 8 BVs of 4 M NaCI + 10 mM NaCN solution at 2 BV hr1, (3) wash with 1 BV water at 2 BV hr1, (4) elution with 8 BVs of 1 M H2SO4 solution. The first four bed volumes of the alkaline elution were collected and used in the subsequent precipitation tests, since most of the copper and iron was eluted by this time.

[0276] Copper precipitation experiments were conducted using a 100 ml reactor coupled to a gas scrubber containing 0.5 M NaOH solution. The reactor and scrubber were sealed and operated under a slight negative pressure so that the hydrogen cyanide gases were captured in the scrubber. The alkaline eluate solution was transferred to the reactor (100 mL) and acidified by adding 1 M sulfuric acid with a Dosimat to reduce the pH of the solution to desired pH values in stages. At each pH stage, the precipitation reaction was allowed to occur for 10 minutes before a sample was taken for the analysis of various species. Upon completion of the test, the pH was adjusted to above 1 1 by NaOH addition before waste disposal. The results are shown in Table 13 and discussed below.Table 13.

[0277] Copper and iron started to precipitate below pH 4.5 but accelerated at pH of 3. At pH 3, about 70% copper was precipitated and iron precipitation was close to complete. The precipitation of copper and iron appeared to be correlated. A Cu:Fe molar ratio in the precipitate was calculated based on the change in solution concentrations. At pH 3 where the majority of copper and nearly all iron were precipitated, the Cu:Fe molar ratio was 1 .8. This ratio suggests the formation of either the Cu(ll) complex Cu2Fe(CN)e or the Cu(l) complex Cu2Na2Fe(CN)e according to Reaction (1 ) or Reaction (2). Although no oxygen was added on purpose, there wasoxygen present in the solution and in the headspace that could have oxidised copper to Cu(ll).4CU(CN)32- + 2Fe(CN)64' + 16H++ O2 2Cu2Fe(CN)6+ 12HCN + 2H2O (1 )4CU(CN)32- + 2Fe(CN)64' + 12H++ 4Na+2Cu2Na2Fe(CN)6+ 12HCN (2)

[0278] The continuing but slow precipitation of copper below pH 3, after depletion of iron, was potentially due to CuCN and I or CuSCN precipitation. However, due to the high chloride concentration, precipitation of single-metal copper cyanide complexes was inhibited and copper precipitation was incomplete even at pH 2.

[0279] Another precipitation experiment was conducted on the same alkaline eluate solution, similar to that described above but conducted under a nitrogen atmosphere to eliminate oxygen. Almost identical results were obtained, with a Cu:Fe ratio of 1.9 in the precipitate formed at pH 3. This suggests that Cu2Na2Fe(CN)6, rather than Cu4Fe(CN)6, is precipitated in the absence of oxygen. The presence of high sodium concentrations (NaCI introduced to provide a 4 M chloride concentration in the eluant) may have contributed to the formation of Cu2Na2Fe(CN)6 rather than Cu4Fe(CN)e.Example 9. Copper precipitation by sulfidization after iron precipitation

[0280] After the low-oxygen precipitation reaction in Example 8, the precipitate was recovered by filtration and the filtered supernatant solution (containing 360 mg / L Cu but very low iron, zinc and nickel, at pH 2) was subjected to sulfidization with sodium sulfide (Na2S). The resultant precipitation of copper sulfide caused the copper concentration in solution to decrease to 10 mg / L in 10 minutes.Example 10. Single stage metal precipitation

[0281] Several attempts were made to precipitate copper and iron in a single stage from an alkaline eluate solution, so as to minimise the unit operations in the process. A synthetic eluate solution was prepared with the following composition: free cyanide: 168mM; thiocyanate: 0.5 mM; Cu(CN)32’: 4000 mg L’1(Cu), Fe(CN)62: 1000 mg L’1(Fe); NaCI: 4 M. The solution was acidified by 2 M H2SC to a target pH and the pH was maintained during the test using a computer controlled Metrohm 906 Titrando unit.

[0282] In a first attempt, sulfidization was attempted in the presence of calcium with the aim of simultaneously precipitating cooper as CU2S and iron as CaNa2Fe(CN)e. Acidification was accompanied by addition of two times stoichiometric addition of sodium sulfide (based on CU2S formation) and calcium chloride. Solution samples were taken I filtered I pH adjusted at various time intervals for ICP analysis of metals. However, copper recovery was inhibited by the presence of calcium: the added calcium competed with copper for sulfide to precipitate CaS rather than forming CaNa2Fe(CN)e.

[0283] In a second attempt, the pH of the alkaline solution was reduced to 2.7, causing near-quantitative precipitation of iron (<74 mg L’1remaining in solution), probably as Cu2Na2Fe(CN)e. Copper precipitation was incomplete at this stage due to the exhaustion of iron. Without filtering the slurry, two times stoichiometric sulfide (based on CU2S formation) was added. This immediately decreased the copper concentration to 89 mg L’1, giving 98% overall copper precipitation rate. However, the iron concentration increased significantly after sulfide addition (to 660 mg L’1), indicating a re-dissolution of the iron-containing precipitate. The redissolution could not be attributed to a temporary increase in pH following sulfide addition, as a repeat experiment with acidified pH of 1.3 prior to sulfide addition, maintaining pH below 3 throughout, gave a similar result. It is proposed that the Kspof CU2S (2.5 x 10’48) could be significantly lower than that of Na2Cu2Fe(CN)e which via the equilibrium between the various species drives the re-dissolution of Na2Cu2Fe(CN)e precipitate and formation of CU2S regardless of pH.Example 11. Selective and sequential recovery of copper and iron from eluate

[0284] A synthetic eluate solution was prepared with the following composition: free cyanide: 168mM; thiocyanate: 0.5 mM; Cu(CN)32': 4000 mg L’1(Cu), Fe(CN)e2: 1000 mg L’1(Fe); NaCI: 3.9 M. 0.45 L of this solution was acidified by 4 M H2SO4 to a target pH and the pH was maintained during the test using a computer controlled Metrohm 906 Titrando unit. Three times stoichiometric addition of sodium sulfide (based on CU2S formation) was then added to precipitate the copper; excess sulfide was added to compensate the significant loss of sulfide as H2S due to the experimental setup used where vacuum was applied to maintain negative pressure for safety reasons. Upon addition of Na2S, the pH of the solution increased to > pH 6 but was reduced to andcontrolled at -pH 4.5 by auto-controlled H2SO4 addition for optimal copper sulfide precipitation. Sulfide precipitation was allowed to occur for 10 minutes before the precipitate (mainly CU2S) was recovered by filtering and a solution sample was taken.

[0285] A stoichiometric addition of ferric ion (as 60% w / v ferric chloride solution) was then added to the filtrate. Upon the addition of ferric chloride solution, the pH decreased (e.g. to pH 2.2) and was adjusted back to pH 4.5 with the addition of 50% NaOH. Iron precipitation was allowed to occur for 10 minutes before a solution sample was taken. Solution samples were taken, filtered and pH adjusted at various time intervals for ICP analysis of metals.

[0286] Calcium chloride and ferrous chloride was also tested as alternatives to ferric chloride for iron cyanide precipitation. Initial scoping experiments showed that calcium was ineffective to precipitate iron. The results for ferric and ferrous precipitation reagents are shown in Table 14.Table 14.

[0287] Sulfidization was highly effective for copper recovery as CU2S, achieving 97- 99% copper recovery for all the tests, but ineffective for iron. The effective precipitation of copper is due to two factors: 1 ) very low solubility product of CU2S (2.5 x 10’48) and 2) copper cyanide being a moderately strong complex (log( n) = 27.0 for Cu(CN)32’). In comparison, ferrous cyanide is a very strong complex (log( n) = 35.4) while FeS has amuch higher solubility product of 8 x 10’19. This selective precipitation of copper has the advantage that a high purity CU2S product is obtained.

[0288] Stoichiometric addition of FeCh to the filtrate resulted in complete precipitation of iron cyanide as Prussian Blue in 10 minutes (Fe in solution from 1 154 to 6 mg L’1), according to Reaction 1 .4FeCl2+ 3Na4Fe(CN)6= Fe4[Fe( V)6]3+ 12NaCl (1 )

[0289] This reaction was very fast due to a very low solubility product of Prussian Blue (3.0 x 10’41). The majority of the residual copper post sulfidization was also coprecipitated during this process. In case of accidental overdosing of ferric ion, the excess ferric will either precipitate with the residual sulfide or precipitate as Fe(OH)3 when pH is increased above 3.

[0290] Stoichiometric addition of FeCh (as a fine powder) resulted in partial precipitation of iron cyanide, probably according to Reaction 2.FeCl2+ Na4Fe(CN)6= FeNa2Fe(CN)6+ 2NaCl (2)

[0291] Another stoichiometric addition of FeCk increased the iron precipitation to 95%. This suggests that the slower precipitation kinetics could have been a result of the time needed to dissolve FeCh powder. A third stoichiometric addition of FeCk increased the solution iron concentration, indicating that any excess ferrous ion (in case of overdosing) would remain soluble in solution (as opposed to ferric ion that would precipitate out as Fe(OH)3.Example 12. Filtration and flocculation of Prussian blue precipitate

[0292] The Prussian blue precipitate of Example 1 1 was fine, passing through a 1 .2 pm glass fibre filter paper but retained on a 0.45 pm filter paper. After precipitation as described in Example 1 1 , the precipitate settled out of the supernatant given sufficient time. Organic cationic flocculants such as Zetag92 (available from Solensis) and Magnafloc 800HP (available from BASF) were found to improve the rate of settling.Example 13. Continuous test campaign - methods

[0293] A bench scale proof-of-concept continuous testing of the integrated cyanide recovery process was conducted for 20 cycles of 1 day each (4 weeks excluding weekends).Resin absorption

[0294] Purolite MTA5013SO4 resin was converted from a sulfate form to a chloride form using the following method: pumping 8 BVs of 1 M NaCI through the resin column at 2 BV hr1followed by 5 BVs of DI water wash at 5 BV hr1. Three 100 mL resin columns were operated in a carousel mode in the resin circuit. Two columns in series were used for resin adsorption while the third column was undergoing resin elution. The use of two columns for adsorption was to ensure complete removal of metal cyanides (Cu / Fe / Zn / Ni) from the cyanidation tails solution. To fit in 24-hour adsorption cycles, 290 BVs (29L) of synthetic cyanidation tails solution, containing Cu(CN)32', Fe(CN)e2', Ni(CN)42’ and Zn(CN)42’ and having the composition shown in Table 15, was pumped through the two adsorption columns at 12 BV hr1. Upon completion of each adsorption cycle in 24 hours, the first resin column was taken offline for resin elution. The second resin column was moved forward as the first column for next adsorption cycle and the eluted resin column was returned to adsorption as the second adsorption column.Table 15.Resin elution

[0295] Upon completion of each resin adsorption, the first resin column was subjected to resin elution for copper and iron. A split elution configuration (Elution Configuration #1 ; depicted in Figure 7) was used for cycles 1 -9, allowing the recycling of two eluant streams back to the next elution. This elution procedure is detailed below:1 . Presoak with 0.5 BV of 4.5 M NaCI + 1 .2 M NaCN for 2 hours, re-circulating at 10 BV hr1(0.5 BV pre-eluant; “PE”);2. Elution at 2 BV hr1with 8 BVs of 3 M NaCI + 20 mM NaCN (5 BV of Eluant 1 followed by 3 BV of Eluant 2);3. 0.5 BV of water wash;4. The first 4 BVs of resin eluate (“E1”), high in copper and iron, subjected to metal precipitation and cyanide recovery by gas membrane; 3 BVs of the barren solution (containing high chloride) recycled back to the next elution to form (with NaCI and NaCN reagent top-ups) Eluant 2 and the remaining 1 BV purged;5. The last 5 BVs of resin eluate (“E2”), low in copper and iron, recycled to the next elution to form (with NaCI and NaCN reagent top-ups) Eluant 1 .

[0296] Upon completion of elution, a resin sample (1 mL; replaced with fresh resin) was taken for resin strip to determine the barren resin loading of metals.

[0297] To improve the resin elution efficiency and reduce Cu and Fe lockup in the elution circuit, a revised elution configuration was used for cycles 1 1 -15: Elution Configuration #2 depicted in Figure 8. Elution was conducted at 2 BV hr1with 8.5 BVs of 4 M NaCI + 20 mM NaCN, and 5 BV of eluate was taken as E1 .

[0298] To also increase the recycling of reagents, the purge rate was decreased and the 0.5 BV wash water was replaced with recycled barren solution for cycles 16- 20: Elution Configuration #3 depicted in Figure 9. Only 0.5 BV of the barren solution (E1 after metal precipitation and cyanide recovery) was purged with the remainder recycled to Eluant 2 (4BV) and wash (0.5 BV).

[0299] Six solution samples (Recycled Eluant 1 & 2, E1 & E2, barren resin strip sample 1 & 2) were analysed for SCN’ and Cl’ by HPLC, metals (Cu, Fe, Ni and Zn) by ICP and cyanide by O.l. CN Analyser or potentiometric silver nitrate titration.

[0300] After the last elution cycle, an acid elution was conducted following the Cu / Fe alkaline elution to remove the Zn / Ni adsorbed and accumulated on resin over nearly 7 cycles (three columns rotated for 20 cycles). The acid elution method used is shown below:1 . Elution with 6 BVs of 1 M H2SO4 at 2 BV hr1;2. 1 BV of water wash.

[0301] 1 BV fractions were taken and a sample from each fraction was analysed by ICP for Zn and Ni.Metal precipitation

[0302] The E1 fraction from the above resin elution was subject to metal precipitation. Two approaches were tested in this study: 1 ) copper sulfide precipitation (sulfidization) + Prussian Blue precipitation + gas membrane recovery of cyanide (cycles 1 - 7); 2) sulfidization + gas membrane recovery of cyanide + Prussian Blue precipitation (cycles 8 - 20). The procedure for cycles 8-20 is detailed below:1 . Sulfidization. The E1 solution was acidified by 4 M H2SO4 to a target pH 4.5 and the pH was maintained during the test using a computer controlled Metrohm 906 Titrando unit. Three times stoichiometric sodium sulfide (based on CU2S formation) was then added to precipitate the copper; excess sulfide was added to compensate the significant loss of sulfide as H2S due to the experimental setup used where vacuum was applied to maintain negative pressure for safety reasons. Sodium sulfide was added when the pH decreased below pH 5. Upon the addition of sulfide, the pH increased to -pH 7.5 and then decreased to pH 4.5 with further H2SO4 addition. Copper sulfide precipitation was allowed to occur for 10 minutes before the precipitate was recovered by filtering.2. Gas membrane recovery of HCN: described below.3. Prussian Blue precipitation: A stoichiometric amount of ferric ion (as 60% w / v ferric chloride solution) was added to the gas membrane-treated filtrate from Step 1 that still contained ferrous cyanide. The addition of ferric chloride decreased the pH but was adjusted back to pH 5 by addition of 50% NaOH for Prussian Blue precipitation so that any excess ferric would precipitate as Fe(OH)3. Precipitation was allowed to occur for 10 minutes before the precipitate was centrifuged and filtered. The majority of the barren solution was recycled as Eluant 2 in the next elution with the remaining 1 or 0.5 BV purged.

[0303] Four solution samples (sulfidization barren, gas membrane feed & barren and Prussian Blue precipitation barren) were analysed for SON’ and Cl’ by HPLC,metals (Cu, Fe, Ni and Zn) by ICP and cyanide by 0.1. CN Analyser or potentiometric silver nitrate titration.Cyanide recovery by gas membrane

[0304] A 3M Liqui-Cel™ MiniModule 1 x 5.5 membrane contactor was used for cycles 1 -4, and a larger 3M Liqui-Cel™ MiniModule 1.7 x 8.75 membrane contactor was used for cycles 5-20. An inline filter (Pall AcroPak 1000 Capsules 0.8 / 0.2 pm) was used in front of the gas membrane contactor to filter any particulates, if present.

[0305] Gas absorption from resin adsorption barren solution. As each resin adsorption cycle produced 29 L (290 BVs) of solution for free cyanide recovery, only a portion of the barren solution (1 L) was treated to demonstrate the technical feasibility. The gas membrane cyanide recovery was conducted at pH 3 using a solution recirculating in batch mode (see Example 1 ). 200 mL of 1 .5 M NaOH was used as the strip solution; the strip solution was re-used for many cycles to allow the accumulation of recovered cyanide and thus production of a concentrated cyanide solution.

[0306] Gas absorption from E1 eluate. Gas membrane recovery of HCN from E1 resin eluate was conducted either before (cycles 1 -6) or after (cycles 7-20) Prussian Blue precipitation at pH 5. 400 mL of 3 or 4 M NaOH recirculating in batch mode was used as the strip solution; concentrated NaOH was used to balance the ionic strength in the filtrate containing 3 or 4 M NaCI and therefore to avoid osmotic distillation. The strip solution was re-used for as many cycles as possible to allow the accumulation of recovered cyanide.

[0307] Four solution samples for each operation above (gas membrane feed & barren, NaOH strip and HCN scrubber) were analysed for cyanide by O.l. CN Analyser or potentiometric silver nitrate titration.Example 14. Continuous test campaign - results

[0308] The results for the continuous test campaign described in Example 13 are presented in this Example.Resin absorption

[0309] The copper, iron and thiocyanate concentrations in the resin adsorption barren solution are shown in Figure 10. The copper concentrations in the adsorption barren were close to the ICP detection limit (0.1 mg L’1) for the first 12 cycles, followed by a slight increase but largely below 0.5 mg L1for the rest of the cycles (50 mg L1in feed). The iron concentration showed similar trend with its concentration increased to -2 mg L’1towards end of the campaign (10 mg L’1in feed). The small amount of Cu / Fe breakthrough in the last few cycles was ascribed to: 1 ) carry-over effect of poor resin elution efficiencies for the first three cycles and; 2) the increased loadings of Zn / Ni due to their accumulation over cycles. From cycle 14 onwards, thiocyanate largely passed through the adsorption columns (10 mg L’1in feed and barren). The typical loadings of copper and iron on the pregnant resin were -45,000 g t’1and -9,000 g t’1respectively.

[0310] Nickel concentration in the adsorption barren was essentially zero for all cycles while zinc concentration was below or close to the ICP detection limit (0.1 mg L’1). Zinc and nickel remained on the resin during the NaCI + NaCN alkaline elution for Cu / Fe and were allowed to accumulate over cycles; the average loadings of zinc and nickel on three resin columns over 20 cycles (6-7 loading and elution cycles for each resin column) were -12,000 g t’1and -2,300 g t’1respectively.

[0311] Figure 1 1 shows the free cyanide concentrations in the resin adsorption feed (“Ads Preg”) and barren solutions (“Ads barren”) measured by silver nitrate potentiometric titration. The feed concentrations were around the target concentration of 200 mg L’1whilst the barren concentrations were around 150 mg L’1. This indicates that some free cyanide was adsorbed onto the resin which was recovered during resin elution.Resin elution

[0312] The elution results for copper and iron are shown in Figure 12 (loading on barren resin) and Figure 13 (elution efficiency). Elution of iron was very effective, with quantitative elution (typically <50 g t’1remaining on resin vs -9000 g t’1for pregnant resin) achieved regardless of the elution configurations and NaCI concentrations used. More than 97% of eluted iron reported to the E1 resin eluate.

[0313] Elution of copper according to Elution Configuration #1 (see Figure 7) left high copper loading on the barren resin which increased over the first three cycles, with correspondingly poor resin elution efficiencies (from 87% down to 79% and then 74%). The poor elution efficiency in the first cycle was ascribed in part to unrepresentative resin sampling from the top of the resin column where elution appeared to be worse than other sections of the column. The resin sampling method was hence rectified from cycle 4 whereby a representative resin sample was taken after a good mixing of all resin from the column. The decline in elution efficiency over cycle 2 and 3 was attributed to loss of NaCI in the recycled Eluant 2 caused by an unintended, setup-specific dilution by wash water.

[0314] From cycle 4, the NaCI concentration in eluant was increased to 4 M. This resulted in an immediate drop in copper loading on the barren resin and an increase in resin elution efficiency to 91 % for cycle 4. The resin elution efficiency remained high at 95%-96% over cycles 5-8. From cycle 9 to cycle 12, the Cl’ concentration in the recycled Eluants 1 & 2 decreased to 3 M or below, again due to unintended dilution. As a result, the resin elution efficiencies for cycles 9-12 were lower at 92%-94%. The close correlation between chloride concentration and resin elution efficiency highlights the importance of monitoring and maintaining chloride concentration to achieve a high elution efficiency. From cycle 13, the chloride top-ups to Recycled Eluants 1 & 2 were calculated based on HPLC measured results. With chloride in eluant properly controlled at about 4M, near-quantitative copper elution was obtained for cycles 13-20.

[0315] Figure 14 and Figure 15 show the copper and iron concentrations in E1 and E2 resin eluates respectively, while the deportment of the eluted copper and iron to E1 is shown in Figure 16. For all cycles >97% of eluted iron reported to E1 . For cycles 1 - 10 when Elution Configuration #1 was used (see Figure 7), 87 - 93% of eluted copper reported to E1 and 7 - 13% to E2. Because E2 was recycled as Eluant 1 to the next elution, this resulted in a significant portion of eluted copper being held up in the Recycled Eluant 1 . High concentrations of copper in the recycled eluant 1 can reduce the overall resin elution efficiency of copper due to the equilibrium formed between solution and resin phases during elution. The holdup of eluted copper in resin circuit was mainly caused by only the first 4 BVs of eluate being taken as fraction E1 for metalsprecipitation, with the following 1 .5 BVs of relatively high copper fraction eluant recycled as part of E2 in the resin circuit.

[0316] To minimise this copper holdup, Elution Configuration #2 (see Figure 8) was proposed where a larger fraction of high copper containing eluate (5 BVs) was taken as E1 for metals precipitation. Elution Configuration #2 was tested from cycle 11 and resulted in immediate decrease in copper concentration in the E2 fraction which was recycled to Recycled Eluant 1 . The deportment of eluted copper was also significantly improved with typically 96 - 98% eluted copper reporting to E1. Most importantly, the reduced copper concentration in the Recycled Eluant 1 increased the overall resin elution efficiency to >99% for the last 8 cycles (Figure 13).

[0317] To minimise losses of NaCI (a significant process cost), the purge stream should be minimised. Elution Configuration #3 (see Figure 9) was thus tested for cycles 16 - 20, with 4.5 BVs of E1 barren recycled with contained NaCI and only 0.5 BVs purged. The E1 barren after metal removal and cyanide recovery had negligible Cu, Zn and Ni, ~ 10 mg L’1Fe and -30 mg L’1thiocyanate (see below), and it was thus proposed to use 0.5 BV of this barren solution to replace the wash water at end of the elution. The use of E1 barren as wash water further reduced the copper loading on the barren resin to 200 - 400 g t’1(likely due to the contained NaCI) and further increased the elution efficiency from -99% to >99.4% (Figure 13).

[0318] Elution of zinc and nickel was ineffective using the alkaline eluant (NaCI + NaCN). Zinc and nickel in the E1 and E2 fractions were below or close to the ICP detection limit throughout. The loadings of zinc and nickel on the barren resin, shown in Figure 17, increased with cycles indicating the accumulation of these metals. After 20 cycles, the zinc and nickel loadings on the resin were -10000 g t’1and -2000 g t’1respectively.

[0319] The 20 alkaline elution cycles were followed by an acid elution (6 BVs of 1 M H2SO4 at 2 BV hr1) to elute nickel and zinc and regenerate the resin. Figure 18 shows a sharp elution peak for zinc which was essentially complete in 4 BVs. The elution of nickel was kinetically slower but also largely completed in 6 BVs. A barren resin sample was taken and subject to resin stripping. Both zinc and nickelconcentrations in the resin stripping solution were below ICP detection limit, indicating complete elution of zinc and nickel by the acid elution.

[0320] Sulfuric acid was used to adjust and maintain the pH for metal precipitation and gas membrane recovery of cyanide from E1. As a result, sulfate was introduced into the resin circuit via the Recycled Eluant 2. Sulfate was removed from the resin circuit by the purge of E1 barren following metal precipitation and gas membrane recovery of cyanide. Therefore, sulfate accumulates in the resin elution circuit until a steady state is reached. Under Elution Configuration #1 and #2, with 1 BV purge, sulfate accumulated to about 0.2 M in Recycled eluant 1 and 2. Under Elution Configuration #3, with 0.5 BV purge, sulfate concentration increased to over 0.4 M. No adverse effect of sulfate on resin elution efficiency were observed.Metal precipitation

[0321] Two approaches were tested in this study: copper sulfide precipitation (sulfidization) + Prussian Blue precipitation + gas membrane recovery of cyanide (cycles 1 - 7); 2) sulfidization + gas membrane recovery of cyanide + Prussian Blue precipitation (cycles 8 - 20). The second approach offers potential advantages including: 1 ) absence of free cyanide during Prussian Blue precipitation and filtering and; 2) absence of sulfide during Prussian Blue precipitation (any residual sulfide from sulfidization removed by gas membrane), avoiding contamination of the Prussian Blue product, and 3) reduced contact time between liberated free cyanide and excess sulfide, thus reducing formation of thiocyanate by-product.

[0322] During the sulfidization step, the recovery of copper from the E1 feed was near-quantitative, with the precipitate containing copper as CU2S. The copper concentration remaining in the barren solution after precipitation was <20 mg L’1throughout.

[0323] Figure 19 shows the change in iron concentrations before and after sulfidization (“SART feed” and “SART barren”) and after Prussian Blue precipitation, and the accumulative iron removal efficiencies. Some iron was removed during sulfidization with the remainder of iron effectively removed by Prussian Blue precipitation. Prussian Blue precipitation only requires stoichiometric addition of ferricions. Slight overdosing of ferric is not expected to be an issue as the overdosed ferric irons will precipitate as ferric hydroxide. It was noticed, however, that significant overdosing of ferric could result in some soluble iron remaining in the barren solution (e.g. cycles 3 - 9 where ferric was overdosed). The residual iron was suspected to result from the following chemistry: the overdosed ferric caused the formation of some ferric-cyanide (higher stability than ferrous-cyanide) releasing some ferrous ions into the barren solution that remained soluble at pH ~5. From cycle 10, the iron concentration in the Prussian Blue precipitation feed was measured by atomic absorption spectroscopy (AAS) and the ferric addition was calculated based on the measured iron values. This accurate addition of ferric resulted in very low iron concentrations in the barren solution (typically <10 mg L’1) with >97% overall iron removal efficiencies. As the barren solution is recycled to resin elution as Recycled Eluant 2, low iron concentration is desirable otherwise it would consume significant amount of cyanide in the eluant.

[0324] Figure 20 shows the change in thiocyanate concentrations before and after sulfidization (“SART feed” and “SART barren”) and after Prussian Blue precipitation. There was generally a slight increase in thiocyanate following sulfidization. The large increase of thiocyanate in the Prussian Blue precipitation barren solution for cycles 1 - 7 was due to the longer period of cyanide and sulfide co-existence. Once free cyanide was removed before Prussian Blue precipitation in cycles 8 - 20, little additional thiocyanate was generated and the thiocyanate concentration in the Prussian Blue precipitation barren was typically ~30 mg L’1.

[0325] The Prussian Blue precipitation barren solution during Cycles 8-10 thus contained low levels of thiocyanate (~30 mg L’1) and metals (Cu < 20 mg L’1, Fe < 10 mg L’1and close-to-zero Ni and Zn), allowing this solution to be recycled back to resin elution as Recycled Eluant 2 and as wash solution for recovering I reusing the chloride contained within.Cyanide recovery by gas membrane

[0326] HCN absorption from resin adsorption barren solution. The CN’ concentration in the resin adsorption barren solution (feed to gas membrane contactor) was in the range of 150 - 200 mg L’1(Figure 1 1 ). Figure 21 shows the CN’ concentrationafter gas membrane recovery for a given time. For cycles 1 -4, a longer recovery time (60 to 70 minutes) was required to reduce the cyanide concentration below 0.5 mg L’1. In the later cycles 5-20 using a larger membrane surface area contactor, the CN’ concentration was reduced to <0.5 mg L’1typically within 20 minutes (the exception for cycle 16 and 17 was due to unintended dilution of the NaOH in the strip solution; a fresh batch of NaOH strip solution was used from cycle 18).

[0327] HCN absorption from E1 eluate. The CN’ concentration in E1 resin eluate was around 6000 mg L’1(or 231 mM) for cycles 1 1 - 20 when the resin Elution Configuration #2 and #3 were used. The cyanide introduced through pre-elution (0.5 BVs of 1 .2 M) and Eluant 1 (4.5 BVs of 20 mM) was calculated to be 138 mM in 5 BVs of E1. The difference (-90 mM) is due to the free cyanide adsorbed during resin adsorption (see Figure 1 1 ).

[0328] Figure 22 shows the CN’ concentration in E1 barren after gas membrane recovery. In the later cycles 5-20 using a larger membrane surface area contactor, 30 minutes was initially sufficient to reduce the cyanide concentration from -6000 to <0.5 mg L’1(cycles 8 - 1 1 ). When the NaOH in the strip solution started to diminish at cycle 12, 50 minutes was required to reduce the cyanide concentration to 0.8 mg L’1. A fresh batch of NaOH strip solution was used from cycle 13 and 40 minutes was used for the rest of the cycles. The CN’ concentration was reduced to <0.4 mg L’1for cycles 13 - 17 but slowly increased to 0.8 mg L’1over cycles 18 - 20 as the free NaOH in the strip solution decreased.

[0329] To balance the ionic strength in the E1 resin eluate containing 4 M NaCI and to avoid osmotic distillation, 4 M NaOH was used in the strip solution. The NaCN concentration in the strip solution at cycle 20 was 5.2 g L’1, equivalent to -10% NaCN (or 2 M). However, it is expected that the cyanide concentration can increase to 4 M or 20% NaCN.Example 15. Acidification of cyanidation tails solutions before metal absorption - calculations

[0330] The preceding Examples have demonstrated the approach of using resin adsorption to remove metal cyanides followed by whole stream acidification for cyaniderecovery by gas membrane. An alternative is to neutralise I weakly acidify (e.g. pH 5 - 7) the cyanidation tails solution before resin adsorption. A calculation was conducted to estimate the copper cyanide speciation with pH in a cyanidation tails solution and the results are shown in Table 16. At pH 1 1 , copper speciates mainly as Cu(CN)32’ and to a lesser extent Cu(CN)43’, with each adsorbed copper occupying two or three resin adsorption sites. By acidifying the tails stream to pH 5 before resin adsorption, the majority of the copper (-90%) speciates to Cu(CN)2’ which only occupies one resin adsorption site. An intermediate outcome is expected at pH 7. The resultant release of free cyanide can be accommodated since free cyanide is already present and removed downstream by gas membrane. By moving acidification forward before resin adsorption, it is expected that the copper adsorption capacity of the resin can be increased, potentially doubled, reducing the size and cost of the absorption section.Table 16.Example 16. Acidification of cyanidation tails solutions before metal absorption

[0331] Purolite MTA5013SO4 ion-exchange resin was used for the resin column adsorption tests. The Purolite MTA5013SO4 was pre-conditioned to convert from sulfate to chloride form prior to resin adsorption using the following method: (i) in a resin column, pump 8 bed volumes (BVs) of 1 M NaCI at a flowrate of 2 BV hr1; (ii) displace entrained liquor with 6 BVs deionised (DI) water at 5 BV hr1.

[0332] The column adsorption was conducted using a 7.5 mL resin column coupled with a fraction collector. A synthetic feed solution representative of a cyanidation tails solution from a gold mining operation, comprising free cyanide, Cu(CN)32', Fe(CN)e3', Zn(CN)42', Ni(CN)42' and thiocyanate (SCN-) with composition shown in Table 15, was used.

[0333] The following experiments were conducted:• Exp #1 : The solution with a natural pH of -pH 1 1 was used without pretreatment. 350 BVs of the solution was pumped through the column at a flow rate of 10 BV hr1. The first 230 BVs was collected in two fractions (1stfraction: 70 BVs and 2ndfraction: 160 BVs), and the last 120 BVs was collected using fraction collector.• Exp #2: The solution was pH adjusted to and maintained at pH 7. 430 BVs of the solution was pumped through the column. The first 230 BVs was collected in two fractions (1stfraction: 43 BVs and 2ndfraction: 187 BVs), and the last 200 BVs was collected using fraction collector.• Exp #3: Gold (1 mg L1) was added to the solution. The solution was then adjusted to a pH of 5. A precipitate formed and was recovered by filtration after 10 minutes. 436 BVs of the filtered solution was then pumped through the column. The first 204 BVs was collected in two fractions (1stfraction: 60 BVs and 2ndfraction: 144 BVs), and the last 232 BVs was collected using fraction collector.

[0334] A sample was taken from each of the large fractions and one from every 5thsmall fractions. These samples were subject to metal analysis by ICP. Upon completion of the column adsorption, a resin sample (~1 mL) was taken for resin stripping to determine the exact loading of various species on the resin.

[0335] Figure 23 shows the comparison of copper and iron resin adsorption profiles at pH 11 , pH 7 and pH 5. At pH 11 , the copper and iron started to break through (concentration > 1 mg L’1) from the 226thBV and the 221stBV respectively. At pH 7, both copper and iron started to break through from the 270thBV. The results suggested that by adjusting the solution pH to pH 7, the same size of resin column could treat approximately 50 BVs more solution compared to pH 1 1 . At pH 5, the adsorption of copper and iron were much stronger with breakthrough (concentration >1 mg L-1) only observed from the 396thBV.

[0336] Table 17 shows that at the breakthrough point, acidification to pH 7 enabled the recovery of approximately 6500 g t’1more copper and 1400 g t1more iron onto theresin. After acidification to pH 5, the resin loadings of copper and iron at the breakthrough point were nearly doubled compared with those at pH 1 1 .Table 17.

[0337] The increased loading of copper (and some iron) achieved by acidifying the solution is believed to be due to the change in copper cyanide speciation with pH. Table 16 shows that at pH 1 1 , copper is adsorbed mainly as Cu(CN)32' and to a lesser extent CU(CN)43’, with one copper occupying two or three resin adsorption sites. At pH 7, most of copper is adsorbed as Cu(CN)32’ and some as CuCN2’, the latter only occupies one resin adsorption site. At pH 5 the majority of the copper (-90%) is adsorbed as Cu(CN)2’ resulting in the observed high loading capacity for copper at this pH.Example 17. Precipitation during acidification of cyanidation tails solutions before metal absorption

[0338] Further experiments were conducted to investigate the precipitation observed in Example 16 when the solution was acidified to pH 5. The synthetic feed solution (with composition shown in Table 15) was acidified and maintained at different pH values for a time sufficient to allow any precipitation to be completed. The precipitate formed from 500 mL solution after acidification to pH 5 was recovered by filtration and re-dissolved in 50 mL of 20 mM cyanide solution. The various solutions were then subjected to metal analysis by ICP and the results are shown in Table 18.Table 18.

[0339] All metals were stable in solution at pH of 7 and above, but co-precipitation of iron, zinc and a small amount of copper was observed at pH of 6 and below. The precipitate is expected to be mainly a zinc-iron mixed metal cyanide complex. No precipitation from a copper cyanide solution was observed when acidified to pH 5.Example 18. Elution of metals absorbed from acidified cyanidation tails solution

[0340] Purolite MTA5013SO4 ion-exchange resin was pre-conditioned to chloride form as described in Example 16. Column adsorption was conducted using a 210 mL resin column. A synthetic feed solution simulating acidification of a cyanidation tails solution after acidification to pH 5, with the composition shown in Table 19, was used to load the resin. The CN:Cu ratio in the feed was 2:1 so that copper is present as CU(CN)2'. LOW zinc concentration was used to simulate the loss of zinc from the feed solution as zinc iron cyanide precipitate during acidification (see Example 17). 2 mg L’1gold was added to investigate absorption and elution of gold if any remains in the tails solution being processed. Au, Cu, Fe, Ni and Zn were present in the form of their respective cyanide complexes, i.e. Au(CN)2’, Cu(CN)2’, Fe(CN)62', Ni(CN)42’ and Zn(CN)42’.Table 19.

[0341] 100 BVs (21 L) of solution was pumped through the column at 6 BVs per hour. Solution samples were taken from the pregnant and barren adsorption solutionsfor ICP analysis of metals. A resin sample was taken after the adsorption and subject to resin strip to determine the loadings of various species on the pregnant resin. The results are shown in Table 20.Table 20.

[0342] Elution of this loaded resin to elute copper and iron was conducted in a 100 mL resin column with the following alkaline elution procedure:1 . Presoak with 0.5 BV of 4.5 M NaCI + 2 M NaCN for 2 hours, re-circulating at 10 BV hr1;2. Elution at 2 BV hr1with 8 BVs of 4 M NaCI + 20 mM NaCN;3. 0.5 BV of 3 M NaCI wash.

[0343] A high concentration of cyanide (2M NaCN) was used in the presoak to facilitate the conversion of Cu(CN)2’ to Cu(CN)43’ for elution. Figure 24 shows the elution profiles of copper and iron, while Table 21 shows the loading of metals on the resin. A large copper peak is present in the first 2 bed volumes of the copper elution profile, followed by a gradual but continuous elution tail. 98% copper was eluted in 9 BVs, despite the initial loading of predominantly Cu(CN)2’ which is strongly retained. The elution of iron was essentially complete in the first 2 bed volumes. Little nickel or zinc was eluted during the alkaline resin elution.Table 21.

[0344] Following the alkaline elution, the resin was subject to an acid elution using 6 BVs of 1 M H2SO4 at 2 BV hr1. The majority of the nickel was eluted in the first 2 BV’s, with 97% elution efficiency after 6 BV’s. Zinc elution was essentially complete after 3 BV’s.Example 19. Gold absorption during resin loading

[0345] In Exp#3 of Example 16, 1 mg / L of gold was present in the synthetic tailings solution loaded onto the ion-exchange resin at pH 5. ICP analysis demonstrated that no gold was lost from the solution upon acidification. Figure 25 shows the gold adsorption profile during resin adsorption. The gold concentration in the solution flowing from the column was below 0.01 mg L’1for the first 285 BVs, indicating a very strong adsorption of gold with >99% gold recovery. The gold concentration slowly increased thereafter. At the 396thBV where copper and iron started to break through (concentration > 1 mg L’1; c.f. Figure 23), the gold concentration was 0.07 mg L’1. In a resin circuit with two adsorption columns operating in a carousel mode, the tiny amount of gold that breaks through the first column is expected to be recovered by the second column, enabling a complete recovery of the residual gold from the tails. The total gold loading on the resin was 1253 g t’1(based on resin stripping results) upon completion of the 436 BVs of feed.Example 20. Gold elution

[0346] Elution of gold was also monitored in the elution study of Example 18. Less than 1 % of the gold loaded on the resin was eluted during the alkaline elution procedure for copper and iron elution (total 9 BV’s eluate). Due to the low amounts and the nearcomplete retention, gold can thus be allowed to accumulate on the resin for many cycles of resin adsorption and alkaline resin elution before a gold-selective elution treatment is required (similar to the approach proposed for nickel and zinc).

[0347] Figure 26 shows that during the subsequent acidic elution of nickel and zinc using 6 BVs of 1 M H2SO4, no gold was eluted (first 6 BV’s of the Figure; see Example 18). An acid I thiourea elution was then conducted to elute the gold. 6 BVs of eluant containing 1 M H2SO4 + 0.5 M thiourea, followed by 1 BV water wash, were pumpedthrough the column at 60°C and 2 BV hr1. As seen in Figure 26, gold was immediately eluted from the 7thBV with 89% gold eluted in 7 BVs of thiourea-containing eluate. Complete elution of gold would be expected with additional eluant flow and / or increased thiourea concentrations. The rate of gold elution is expected to increase with both acid and thiourea concentrations up to at least 1 M H2SO4 and 1 M thiourea.Example 21. Gold recovery from acid thiourea resin eluate

[0348] Reductive precipitation using sodium borohydride (SBH) was investigated for gold recovery from the acidic thiourea eluate produced in Example 20. SBH has the advantage of not reacting with thiourea, but is known to hydrolyse at low pH. Table 22 shows gold recovery results using different experimental conditions: initial pH adjustment with NaOH, amount of SBH added, and reaction time.

[0349] At unadjusted low pH values (1 M H2SO4), significant excesses of SBH were required to obtain significant gold recoveries. This was ascribed to SBH consumption by hydrolysis. When the pH was adjusted to less acidic values, excellent gold recoveries were obtained, with 100% gold recovered at pH 2 in 3 minutes and the gold precipitate remaining stable for at least 30 minutes.Table 22.Example 22. Regeneration of cyanide from thiocyanate by ozone

[0350] Tailings solutions from cyanidation processes commonly contain significant levels of thiocyanate (SCN ), particularly when reactive sulfide ores are processed. Experiments were thus conducted to investigate whether thiocyanate could be regenerated to cyanide in the process for capture together with the cyanide already present in the tailings solution.

[0351] A Triogen LAB2B™ ozone generator was used to produce ozone for the tests. The ozone was fed to a 4L reactor fitted with a gas sparger with the off-gases sent to two ozone scrubbers in series (containing 50 mM potassium iodide, 2L and 0.5 L respectively, fitted with gas spargers). Calibration studies showed that pure oxygen at 2 L min1fed to the ozone generator produced approximately 7.4 g hr1ozone when the variable output control knob was set at 10.

[0352] Tests were conducted by bubbling ozone through thiocyanate solutions of various compositions in the reactor. Solution samples were taken at various time intervals with pH adjusted to > pH 10.5 for thiocyanate and cyanate analysis by HPLC and cyanide analysis by potentiometric silver nitrate titration. Ozone sent to the scrubbers was calculated based on thiosulfate titration of the iodine (I2) produced when ozone reacts with iodide.

[0353] A scoping test was first conducted using 4 L of 10 mM KSCN at its natural pH (pH 7.7). The reaction between ozone and thiocyanate occurs via Reaction 3.O3+ SCN~ + H2O - SO42~ + HCN + H+(3)

[0354] Due to a high solubility of ozone in water (109 mg L’1at 25 °C) and fast reaction kinetics of Reaction 3, no change in solution colour of the ozone scrubber containing KI was observed initially, indicating that all ozone was retained and / or reacted in the reactor. At 24 minutes, the KI solution in the first ozone scrubber started to change colour (yellow), indicating that ozone had begun escaping from the reactor, and the test was immediately stopped for safety precaution. A solution sample was taken with pH adjusted to 1 1 for cyanide analysis by potentiometric silver nitratetitration. The cyanide concentration was measured to be 8 mM. The pH of the solution had decreased to pH 2.7 due to the production of acid in Reaction 3.

[0355] Based on this scoping test, a standard test procedure was derived and applied to the rest of the tests to facilitate the comparison of results. Tests were conducted on 4 L of 10 mM thiocyanate solutions for 24 minutes using 2 L min-1pure oxygen as feed gas and variable output control at 10; samples were withdrawn at 0, 5, 10, 15, 20 and 24 minutes for thiocyanate and cyanate analysis by HPLC and cyanide analysis by potentiometric silver nitrate titration. The ozone produced over 24 minutes was estimated to be 2.96 g, equivalent to 15.4 mM in 4 L solution and sufficient to oxidise all the thiocyanate to cyanide.Example 23. Impact of pH on regeneration of cyanide from thiocyanate by ozone

[0356] The speciation of cyanide (as HCN and / or CN’) is influenced by pH. This impact of pH on cyanide speciation could affect the kinetics of cyanide regeneration from thiocyanate and the further oxidation of cyanide to cyanate via Reaction 4. Cyanate can be further oxidised to carbonate and nitrogen via Reaction 5.O3+ 3CN~ -► 30CN~ (4)O3+ 2OCN~ + H2O ^ 2HCO2+ N2(5)

[0357] Two tests were conducted by the method of Example 22 to investigate the impact of adjusting the initial pH of the 10 mM KSCN solution to pH 5 or pH 12 by acid or alkali addition. This was to ensure that the cyanide produced existed entirely as either HCN or CN’ and therefore to allow differentiation of this impact.

[0358] Figure 27 shows the change in thiocyanate, cyanate and cyanide concentrations with time for the test at pH 5. Thiocyanate concentration decreased almost linearly to 0.6 mM (94% conversion) with cyanide increasing to 7.7 mM in 24 minutes, indicating fast reaction kinetics of thiocyanate oxidation by ozone. Only a small amount of cyanate concentration, up to 0.2 mM, was formed by further oxidation of the cyanide product. The pH of the solution after 24 minutes had decreased to pH 2.5 due to the acid production in Reaction 3.

[0359] Figure 28 shows the change in thiocyanate, cyanate and cyanide concentrations with time for the test at pH 12. A reduced thiocyanate oxidation rate was observed at pH 12 compared to pH 5, with 1.8 mM thiocyanate remaining (82% conversion) at 24 minutes. Oxidation of the cyanide product to cyanate was significantly enhanced at pH 12, indicating that cyanide in the form of CN’ is more susceptible to oxidation to cyanate than HCN. The pH of the solution after 24 minutes had decreased to 11.2.Example 24. Impact of copper on regeneration of cyanide from thiocyanate by ozone

[0360] The influence on copper on thiocyanate oxidation was investigated to determine whether thiocyanate can be regenerated in a solution containing metal cyanide, e.g. in a tailings solution prior to resin absorption. An experiment was thus conducted by the method of Example 22 using 4 L of a solution of 10 mM KSCN and 10 mg L’1copper (as CuCNa2’) at pH 5. Figure 29 shows the change in thiocyanate, cyanate and cyanide concentrations with time. The results show an enhanced thiocyanate oxidation rate in the presence of copper (c.f. Figure 27), reaching 100% conversion rate in 24 minutes and suggesting a catalysing effect of copper. The oxidation of the cyanide product to cyanate was also significantly enhanced by copper. The cyanate concentration increased to 1.42 mM at 15 minutes, seven times the concentration in the absence of copper. The cyanate concentration subsequently decreased which may be due to further oxidation of cyanate via Reaction 5. As a result of these effects, cyanide production in the presence of 10 mg L1copper was slightly lower than in the absence of copper (c.f. Figure 27). The pH of the solution after 24 minutes had decreased to 2.5.Example 25. Impact of chloride on regeneration of cyanide from thiocyanate by ozone

[0361] The influence of chloride on thiocyanate oxidation was also investigated due to the presence of this species in relevant streams in a cyanidation process. An experiment was thus conducted by the method of Example 22 using 4 L of a solution of 10 mM KSCN and 0.5 M NaCI at pH 5. Figure 29 shows the change in thiocyanate, cyanate and cyanide concentrations with time. The results show an enhancedthiocyanate oxidation rate in the presence of chloride (c.f. Figure 27), reaching 100% conversion in 20 minutes. Without limitation by theory, ozone may be oxidising chloride to hypochlorite or chlorine which in turn reacts with thiocyanate. The oxidation of the cyanide product to cyanate was also significantly enhanced by the presence of chloride. The cyanate concentration increased initially and then started to decrease from 15 minutes likely due to further oxidation of cyanate. Due to a combination of these two opposing effects, the cyanide production rate was similar in the presence or absence of 0.5 M NaCI. The pH of the solution after 24 minutes had decreased to 2.2.

[0362] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Claims

Claims1 . A process for removing cyanide from a cyanide-bearing aqueous fluid comprising free cyanide and one or more metal cyanide complexes, the process comprising:(i) contacting the cyanide-bearing aqueous fluid with an anion-exchange absorbent to absorb the one or more metal cyanide complexes, thereby producing a metal-lean aqueous fluid comprising free cyanide;(ii) acidifying at least one of the cyanide-bearing aqueous fluid and the metallean aqueous fluid to speciate the free cyanide as HCN; and(iii) removing HCN from the metal-lean aqueous fluid to produce a cyanide- lean aqueous fluid.

2. The process according to claim 1 , wherein the cyanide-bearing aqueous fluid comprises at least one copper cyanide complex.

3. The process according to claim 2, wherein the cyanide-bearing aqueous fluid further comprises at least one iron cyanide complex.

4. The process according to any one of claims 1 to 3, comprising acidifying the cyanide-bearing aqueous fluid before or during contact with the anion-exchange absorbent, thereby converting at least a portion of at least one metal cyanide complex from a multivalent anionic species to a monovalent anionic species and releasing free cyanide.

5. The process according to claim 4, wherein the cyanide-bearing aqueous fluid is acidified to a pH of from about 5 to about 7.5, before or during contact with the anion-exchange absorbent.

6. The process according to claim 4 or claim 5, wherein the multivalent anionic species comprises Cu(CN)43’ and / or Cu(CN)32' and the monovalent anionic species is Cu(CN)2’.

7. The process according to any one of claims 1 to 6, wherein the cyanide-bearing aqueous fluid comprises thiocyanate and wherein the process comprises oxidising thiocyanate in at least one of the cyanide-bearing aqueous fluid and the metallean aqueous fluid to produce free cyanide.

8. The process according to any one of claims 1 to 7, wherein removing the HCN from the metal-lean aqueous fluid comprises transferring HCN into a gas phase.

9. The process according to any one of claims 1 to 8, wherein removing the HCN from the metal-lean aqueous fluid comprises (i) contacting the metal-lean aqueous fluid with a first side of a gas membrane, (ii) contacting a first alkaline solution with a second side of the gas membrane, and (iii) absorbing the HCN through the gas membrane into the first alkaline solution to produce a first cyanide solution.

10. The process according to any one of claims 1 to 9, wherein the cyanide-lean aqueous fluid has a cyanide concentration of less than 500 ppb.11 .The process according to any one of claims 1 to 10, further comprising extracting the anion-exchange absorbent with at least one non-acidic aqueous extractant to produce an aqueous extract comprising one or more extracted metal cyanide complexes.

12. The process according to claim 11 , wherein the anion-exchange absorbent comprises copper cyanide and iron cyanide complexes and wherein at least 95% of the absorbed copper and at least 98% of the absorbed iron is extracted into the aqueous extract.

13. The process according to claim 11 or claim 12, comprising a plurality of absorption-extraction cycles wherein each cycle comprises: extracting the anion-exchange absorbent consecutively with a first extractant fraction and a second extractant fraction of the at least one non-acidic aqueous extractant, wherein the first extractant fraction comprises a higher concentration of metal cyanide complexes than the second extractant fraction;recovering the aqueous extract consecutively in a first extract portion and a second extract portion, wherein the first extract portion comprises a higher concentration of extracted metal cyanide complexes than the second extract portion; and recycling the second extract portion to the first extractant fraction for a subsequent cycle.

14. The process according to any one of claims 11 to 13, wherein the anion-exchange absorbent is recycled after extraction for contact with the cyanide-bearing aqueous fluid in step (i), wherein the recycled anion-exchange absorbent is not acid treated after extraction or is acid treated only after multiple absorption-extraction cycles with the non-acidic aqueous extractant.

15. The process according to claim 14, further comprising extracting the anion- exchange absorbent with an acidic eluant, after multiple absorption-extraction cycles with the non-acidic aqueous extractant, thereby removing adsorbed zinc cyanide and / or nickel cyanide complexes.

16. The process according to claim 14 or claim 15, further comprising extracting the anion-exchange absorbent with an acidic eluant comprising thiourea, after multiple absorption-extraction cycles with the non-acidic aqueous extractant, thereby removing absorbed gold.

17. The process according to any one of claims 11 to 16, comprising (i) acidifying at least a portion of the aqueous extract; (ii) precipitating one or more metal compounds; and (iii) separating the one or more metal compounds from an acidified aqueous extract.

18. The process according to claim 17, wherein the aqueous extract comprises at least one copper cyanide complex and wherein precipitating the one or more metal compounds comprises adding a sulfide source to the aqueous extract and selectively precipitating copper sulfide.

19. The process according to claim 17 or claim 18, wherein the acidified aqueous extract comprises at least one iron cyanide complex and wherein the process comprises adding a metal salt to the acidified aqueous extract, thereby precipitating a metal-iron cyanide complex; and separating the metal-iron cyanide complex from the acidified aqueous extract.

20. The process according to claim 19, wherein the metal salt comprises ferric cation and wherein the metal-iron cyanide complex comprises Prussian blue.21 . The process according to any one of claims 17 to 20, further comprising removing HCN from the acidified aqueous extract.

22. The process according to claim 21 , wherein removing the HCN from the acidified aqueous extract comprises (i) contacting the acidified aqueous extract with a first side of a gas membrane, (ii) contacting a second alkaline solution with a second side of the gas membrane, and (iii) absorbing the HCN through the gas membrane into the second alkaline solution to produce a second cyanide solution.

23. The process according to claim 22, wherein the second alkaline solution is produced by (i) absorbing the HCN present in the metal-lean aqueous fluid into a first alkaline solution, preferably through a gas membrane, to provide a first cyanide solution; and (ii) adding base to the first cyanide solution to produce the second alkaline solution, wherein the second cyanide solution has a higher cyanide concentration than the first cyanide solution.

24. The process according to any one of claims 1 to 23, wherein the cyanide-bearing aqueous fluid comprises at least one selected from a tails stream and a wash stream from a gold leaching process.

25. A method of purifying a cyanide-bearing aqueous fluid from a cyanidation process stream in a hydrometallurgical process, comprising (i) removing cyanide from the cyanide-bearing aqueous fluid by a process according to any one of claims 1 to 24; and (ii) discharging the cyanide-lean aqueous fluid from the hydrometallurgicalprocess or recycling the cyanide-lean aqueous fluid for use in a cyanide-free operation of the hydrometallurgical process.

26. A system for removing cyanide from a cyanide-bearing aqueous fluid comprising free cyanide and one or more metal cyanide complexes, the system comprising:(i) an absorption unit for contacting the cyanide-bearing aqueous fluid with an anion-exchange absorbent to absorb the one or more metal cyanide complexes, thereby producing a metal-lean aqueous fluid comprising free cyanide;(ii) acid dosing apparatus configured to acidify at least one of the cyanide- bearing aqueous fluid and the metal-lean aqueous fluid, thereby speciating the free cyanide as HCN; and(iii) a first cyanide-removal unit to remove HCN from the metal-lean aqueous fluid, thereby producing a cyanide-lean aqueous fluid.

27. The system according to claim 26, wherein the acid dosing apparatus is configured to acidify the cyanide-bearing aqueous fluid before or during contact with the anion-exchange absorbent.

28. The system according to claim 26 or claim 27, wherein the first cyanide-removal unit comprises a gas membrane and is configured to (i) contact the metal-lean aqueous fluid with a first side of the gas membrane, (ii) contact a first alkaline solution with a second side of the gas membrane, and (iii) absorb the HCN through the gas membrane into the first alkaline solution to produce a first cyanide solution.

29. The system according to any one of claim 26 to 28, comprising an extraction circuit configured to:(i) extract the anion-exchange absorbent with at least one non-acidic aqueous extractant to produce an aqueous extract comprising one or more extracted metal cyanide complexes;(ii) acidify at least a portion of the aqueous extract to form an acidified aqueous extract;(iii) precipitate and separate one or more metal compounds from the acidified aqueous extract;(iv) remove HCN from the acidified aqueous extract; and(vi) recycle at least a portion of the acidified aqueous extract to form at least a portion of the non-acidic aqueous extractant.

30. The system according to claim 29, comprising a second cyanide removal unit, wherein the second cyanide-removal unit comprises a gas membrane and is configured to (i) contact the acidified aqueous extract with a first side of the gas membrane, (ii) contact a second alkaline solution with a second side of the gas membrane, and (iii) absorb the HCN through the gas membrane into the second alkaline solution to produce a second cyanide solution.

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