A process for treating leach residue

Abstract

A process is disclosed for treating a leach residue to remediate the chemistry of a leach residue in a heap to an environmentally acceptable state. The leach residue may be produced at the end of a heap leach operation. The process includes the treatment of the leach residue with an amino acid.

Description

[0001] A PROCESS FOR TREATING LEACH RESIDUE

[0002] TECHNICAL FIELD

[0003] A process is disclosed for treating a leach residue to remediate the chemistry of a heap to an environmentally acceptable state. The process is particularly applicable to the treatment of a heap of a leach residue, such as that produced at the end of a heap leach operation. The process includes the treatment of the residue with an amino acid. The process may also include the reduction of concentrations of any toxic species such as cyanides in the residue. The process may also include adjusting the pH of the residue closer to neutral.

[0004] BACKGROUND ART

[0005] Heap leaching has become an increasingly attractive mineral extraction process for metals (including base metals and precious metals) due to its low cost and suitability for processing a wide variety of ore types and grades. Heap leaching comprises the application of a leaching solution (e,g, via drip irrigation) to a heap of crushed (and typically agglomerated) ore supported on an impermeable leach pad. The leach solution percolates through the heap and dissolves metal values from the ore to form a pregnant leach solution. The pregnant leach solution is then collected (such as in a pond) and treated to recover the metal values.

[0006] As used herein, the term “ore” means a solid material containing a valuable target metal. It includes naturally occurring ores minerals, ore concentrates (such as concentrates formed by mineral processing operations such as flotation), tailings or waste (such as ripios).

[0007] As used herein "a heap" includes a heap or a dump or any other type of pile of a material which contains an ore to be processed to remove a metal from the ore.

[0008] As used herein "heap leach operation" means a process of actively operating a heap leach by applying a solution to a heap and removing a target metal(s) from the ore in the heap during the period of the heap leach operation and prior to decommissioning of the heap at the completion of the heap leach operation.

[0009] At the completion of a heap leach operation, a leach residue (often termed “ripios”) remains in the heap, and noting that the leach residue may contain at least some ore with the target metal(s). The leach residue must be treated to ensure that it complies with regulatory and safety requirements for closure / decommissioning. Depending on the condition and chemistry of the leach residue, the treatment may comprise one or more of the following actions:

[0010] 1. Removal of any residual toxic species (such as cyanides) in the leach residue, introduced during the leaching process. 2. Adjusting the heap pH of the leach residue to neutral (or close to it).

[0011] 3. Evaporation of the remediation pond solution.

[0012] 4. Revegetation of the residue heap.

[0013] The treatment of the leach residue may conventionally include volatilization of toxic species (such as cyanide) by spraying of an aqueous wash solution onto the heap and dissolving the toxic species in the wash solution as it travels through the heap. The run-off solution is then collected in a pond where the toxic species can be subjected to a first phase (Phase I) of natural sun degradation, and the treated run-off recycled to the heap for further washing. The chemistry of the wash solution may be adjusted, such as by addition of acid (e,g, hydrochloric or sulfuric acid), to convert the majority of cyanide to hydrogen cyanide which is more susceptible to volatilization. Volatilization may be accelerated by using evaporation sprayers or paddles

[0014] Once the level of toxic species in the residue falls to a predetermined threshold (such as approximately 10 ppm in the case of cyanide), the treated run-off solution may then be subjected to a second stage (Phase II) comprising chemical detoxification. During the chemical detoxification stage, one or more reagents are added such as hydrogen peroxide (H2O2), NaHS or SMBS (Na2S20s) with CuSO4 as a catalyst.

[0015] The two treatment phases should result in detoxification of the wash solution and in the case of cyanide, the substantial elimination of free cyanide and reduction of weak acid dissociable (WAD) cyanide to less than 0.2 mg / L. The detoxification results in the production of carbon dioxide and ammonia from cyanide containing phases.

[0016] H2O2 + CN-WAD CNO’ + H2O

[0017] Na2S2O5+ 2O2+ H2O + 2CN-WAD 2 CNO’ + 2 NaHSO4

[0018] 2 Cu2++ Fe(CN)64’ Cu2Fe(CN)6

[0019] CNO’ + H++ H2O CO2+ NH3

[0020] The conventional detoxification process typically requires significant volumes of wash solutions. In the case of cyanide removal from a precious metal (e.g., gold, silver and the platinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum) containing leach residue, to ensure that all cyanide is removed from the heap through washing with water, a minimum of 3 BV (bed volumes) or ideally 4-5 BV is required to be flushed through the heap. As used herein, the term “bed volume” means the amount of water that is equivalent to the volume of the heap. Typically, the wash solution is irrigated at half the usual heap leach irrigation rate to concentrate the residual cyanide which may indirectly facilitate some precious metal leaching. However, if during Phase I (natural degradation) it is necessary to reduce the pH of the water, this would greatly influence the amount of precious metal that would be recovered because the cyanide in solution would be present in a form that could not complex precious metal. For example, at pH 9.2 half of the cyanide in solution is in the form of hydrogen cyanide (which would not complex precious metal) and free cyanide (which can complex precious metal). The addition of acid to reduce the pH to say 8.0 would convert the majority of the cyanide in solution to hydrogen cyanide which does not complex precious metal.

[0021] Furthermore, the natural volatilization of hydrogen cyanide from solution also depends on the solution temperature. Hydrogen cyanide would naturally sublime or boil off at temperatures above around 25°C which may be problematic for heaps in cooler climates. This would also be a problem for the evaporation of the wash water containing the cyanide.

[0022] In view of the above difficulties, it is very likely that the only effective means to remove free cyanide and hydrogen cyanide in the wash water is through chemical detoxification (Phase II). Once this is done the detoxified water can be safely discharged into the environment. However, chemical detoxification requires the introduction of chemical species that add to the cost and complexity of the detoxification process.

[0023] It would therefore be desirable to provide an alternative process for treating leach residue that overcomes, or at least alleviates one or more difficulties of the conventional process.

[0024] The above references to the background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the apparatus and method as disclosed herein.

[0025] SUMMARY OF THE DISCLOSURE

[0026] Applicant has previously developed technology for leaching metal values using amino acid lixiviants, as described, for example, in PCT / AU2014 / 000877 and PCT / AU2016 / 050171.

[0027] Applicant has now surprisingly discovered that amino acids may be used in the treatment of leach residues, such as metal value-containing residues remaining in a heap at the completion of a heap leach operation.

[0028] This finding is not confined to leach residues from leaching metal values using amino acid lixiviants and extends to residues that have been produced by leaching metal values using other lixiviants such as cyanide or thiourea or in alkaline or acidic solutions (e.g, sulfuric acid based solution). The amino acid treatment may be used in the closure / decommissioning or remediation of a heap after completion of a heap leach operation. It may be used for leach residues of varying pH and produced by leaching processes that include a variety of lixiviants. The leach residues may be produced by lixiviants other than amino acids, such as leach residues produced by cyanide leaching (in the case of leaching precious metal ores) or acidic leaching solutions (such as those containing sulfuric acid in the case of leaching base metal ores).

[0029] The treatment may be used to remediate the chemistry of heap residues to an environmentally acceptable state. The treatment may include adjusting the pH of the leach residues closer to neutral in a situation where the residues have an acidic or alkaline pH. The treatment may also extract any remaining metal values from the leach residues.

[0030] In a first aspect there is disclosed a process for treating an acidic or alkaline leach residue containing metal values, including adding an amino-acid (or derivative thereof) to the leach residue and adjusting the pH of the leach residue towards neutral.

[0031] The process may include producing a product solution containing leached metal values.

[0032] The leach residue may have been produced by leaching a metal value-containing material, such as an ore, ore concentrate, waste material, a process by-product or e- waste, with an acidic or alkaline leach solution.

[0033] The leach residue may comprise solid and / or liquid components.

[0034] The solid components may comprise the leached metal value-containing material, such as leached ore or leached ore concentrate.

[0035] The liquid component may comprise residual process solution such as one or more of pregnant leach solution, wash solution and barren solution. The residual process solution may be recycled.

[0036] The pH of the leach residue may be determined by measuring the pH of a solution that is flushed through the heap. The pH of the solution may be conveniently measured as the solution discharges from the heap. The pH of the solution may be measured elsewhere in a solution circuit. The pH may be measured using a pH meter that measures the hydrogen ion concentration in solution. The pH may be measured at any other suitable measurement option.

[0037] In an embodiment, the leach residue may comprise a heap of leached ore. In another embodiment, the leach residue may comprise a vat of leached ore. In a further embodiment, the leach residue comprises a tailings dam. The process for treating the leach residue may advantageously comprise remediating or terminating a heap of the leach residue (ripios). It may therefore comprise part of a closure strategy for a heap after completion of a heap leach operation.

[0038] The leach residue may contain metal values that were originally present in the metal- value-containing material and were not completely extracted during a main heap leach operation. The metal values may be selected from one or more of precious metals and base metals. The precious metals may include gold, silver and the platinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum. The base metals may include copper, nickel, cobalt, zinc and lead.

[0039] In an embodiment, the metal value is one or more precious metals, such as gold and / or silver.

[0040] The leach residue may be acidic or alkaline, depending on the type of a leach solution that was used to leach the metal value-containing material. Where the metal value is a precious metal such as gold, the leach solution, and therefore the leach residue, may be alkaline. Where the metal value is a base metal, the leach solution, and therefore the leach residue, may be acidic.

[0041] The leach residue may contain a toxic species, such as cyanide.

[0042] As used herein, the term “cyanide” means any inorganic chemical species that contains a C atom triple bonded to a nitrogen atom. A cyanide may comprise a cyanide anion, such as that derived from a cyanide salt, e,g, alkali metal salt, or it may comprise hydrogen cyanide (HCN).

[0043] The leach solution may have contained one or more lixiviants.

[0044] As used herein, the term “lixiviant” refers to a specific active chemical of a leaching solution that complexes with and enables the extraction or dissolution of target elements from a feed material (e.g., ore). A “primary lixiviant” is the primary (or main) active chemical in the leaching solution that complexes with and enables the extraction or dissolution of target elements from the feed material.

[0045] In the case of leaching precious metals, in particular gold, the leach solution may have contained a cyanide-containing phase as the lixiviant. Typically, the leach solution would have been alkaline and contained a cyanide-containing lixiviant. The cyanide-containing lixiviant may have been a CN“ anion derived from HCN or a soluble salt such as NaCN.

[0046] The amino acid may be any suitable amino acid.

[0047] As used herein, the term "amino acid" means an organic compound containing both a carboxyl (-COOH) and an amino (-NH2) functional group. In many cases, the amino acid contains a -CHR or CH2 group. In most cases the amino (-NH2) group and the carboxyl (- COOH) group connects to the same -CHR or -CH2 connecting group and are referred to primary alpha- amino-acids. The "R" group in the -CHR connecting group can take on any organic structure, such as aliphatic hydrocarbon groups to complex organic structures including aromatic groups, heterocyclic groups, and poly-nuclear groups or various other organic groups. In its simplest form, the R-group is only hydrogen, in which case the molecule reverts to the simplest primary alpha-amino-acid, called glycine.

[0048] The amino acid derivative may comprise an amino acid salt.

[0049] As used herein, the term “amino acid salt” includes an alkali metal salt, for example, a sodium or potassium glycinate. Alternatively, the amino acid salt may be an alkaline earth salt (for example its calcium salt).

[0050] For ease of discussion, when the term “amino acid” is used herein, it should be understood to also encompass an amino acid derivative such as an amino acid salt.

[0051] In an embodiment, the amino acid is selected from glycine and glutamic acid and salts thereof.

[0052] In an embodiment, the amino acid salt is Na-glycinate.

[0053] In another embodiment, the amino acid is hydrogen glycinate.

[0054] The amino acid may be conveniently added to the leach residue in an aqueous solution, such as wash water, recycled process water or saline water.

[0055] The concentration of amino acid in the aqueous solution is preferably lower than the concentration of amino acid that would ordinarily be used in a leaching solution during an active heap leach operation, in order to minimize cost and complexity of the decommissioning process. For example, during a heap leach operation, the concentration of amino acid would typically be at least 0.5 g / 1. However, during the present process for treating leach residue, lower concentrations of amino acid would instead typically be used. The amount of amino acid added would be dictated by the amount of contained target metal in the leach residue.

[0056] The amino acid may have a concentration in solution of at least 0.001 g / L. In an embodiment, the concentration of amino acid may be a minimum of 0.005 g / L. In another embodiment, the concentration of amino acid may be a minimum of 0.007 g / L. In another embodiment, the concentration of amino acid may be a minimum of O.Olg / L. In another embodiment, the concentration of amino acid may be a minimum of 0.02 g / L. In an embodiment, the concentration of amino acid may be a minimum of 0.05 g / L. In another embodiment, the concentration of amino acid may be a minimum of 0.07 g / L. In another embodiment, the concentration of amino acid may be a minimum of O.lg / L.

[0057] While the concentration of amino acid may be relatively high (such as up to 15 g / L) it would generally be economically advantageous have a lower concentration. In an embodiment, the concentration of amino acid may be up to 1 g / L. In another embodiment, the concentration of amino acid may be up to 0.7 g / L. More commonly the concentration of amino acid may be less than 0.5 g / L. In another embodiment, the concentration of amino acid may be less than 0.3 g / L. In another embodiment, the concentration of amino acid may be less than 0.2 g / L.

[0058] The amino acid or derivative has a natural pH of near neutral. For example, glycine or hydrogen glycinate has a natural pH in solution of around 6.0. Accordingly, addition of the amino acid to the leach residue also advantageously results in adjustment of the pH of the residue towards neutral. In other words, the addition of the amino acid to an acidic leach residue would increase pH and addition to an alkaline leach residue would decrease pH. If necessary, additional acid or base could be added to accelerate the neutralization process. However, the majority of the neutralization process can be effected by the amino acid which is more environmentally benign than neutralization solely by addition of acid (e.g., sulfuric or hydrochloric acid) or base (e.g., lime, (Ca(OH)2) or caustic (NaOH).

[0059] The amino acid may be added to the heap by spraying as an aqueous solution (or by other suitable means like drippers).

[0060] The amino acid may be applied to the leach residue using drippers or sprayers.

[0061] Amino acid is a lixiviant and as it travels through the leach residue, it complexes with residual metal values in the residue and forms a product solution containing the leached metal values. Amino acid is a particularly effective lixiviant for precious metals and base metals, as disclosed for example in applicant’s patent specifications PCT / AU2014 / 000877 and PCT / AU2016 / 050171, the entire disclosures of which are incorporated herein by reference.

[0062] The applied amino acid may combine with residual species in the residue to form the treatment solution in situ in the residue. For example, where the leach residue contains cyanide from a previous heap leach operation, the application of an amino-acid (or derivative thereof) to the leach residue may form an aqueous amino-acid and cyanide containing solution in situ in the residue.

[0063] The aqueous treatment solution containing the amino acid may further contain other additives that cooperate synergistically with amino acid to enhance the treatment of the residue. In the case of treating an acidic leach residue, one such additive is thiourea, and the aqueous solution may further include thiourea which reacts with the amino acid to form an amino acid-thiourea compound that enhances recovery of the metal values from the residue. Applicant’s patent specification PCT / AU2018 / 051060 describes the synergistic nature of amino acid and thiourea and its entire disclosure is also incorporated herein by reference. Accordingly, the amino acid assists in moving the residue pH towards neutral. The amino acid-thiourea compound also complexes with remaining metal values in the leach residue to recover those metal values.

[0064] The metal value may be recovered from the product solution from the heap through traditional column extraction with ion exchange resins or activated carbon for base metals or precious metals.

[0065] The leach residue may contain a toxic species, such as cyanide, that was previously used in the heap leach operation to recover the metal values. In the case of leaching gold, the heap leach operation may have used a solution that contained a cyanide lixiviant. Advantageously, the addition of amino acid to the cyanide containing leach residue forms a particularly effective leaching solution in situ. Although the treatment of the leach residue with the amino acid-containing solution causes the desirable reduction of cyanide over time as it is washed from the residue, the in-situ formation of the leaching solution allows the contemporaneous extraction of residual metal values. Cyanide enhances the lixiviant action of amino acid when leaching precious metals. Accordingly, the combination of cyanide and amino acid enables greater quantities of precious metal to be recovered from the residue than would otherwise be possible. Furthermore, the combination of cyanide and amino acid can tolerate a broader pH window down to pH 8.0 which would allow gold extraction in more than a few BV washes i.e. gold in solution would be present after 3-4 BV’s which would maximize the recovery of gold during this closure plan.

[0066] The combination of cyanide and amino acid also preserves the cyanide and converts any weak acid dissociable (WAD) cyanide into free cyanide for metal value leaching.

[0067] In the case of a leach residue comprising a heap of leached ore or a tailings dam, once the pH of the residue is approaching neutral, it is possible to commence further remediation such as by revegetation. A particular advantage of amino acid treatment is that the amino acid is a good fertiliser that encourages plant growth. Accordingly, once the pH of the residue is (for example) below 8.0 when treating an alkaline residue, one could commence revegetation while still applying amino acid to the residue. If residual cyanide or any WAD cyanide remains after the present process has been completed, then it may be necessary to include a chemical detoxification step where hydrogen peroxide (H2O2) or sodium metabisulfite (SMBS) could be added to the system to degrade the residual cyanide and possibly also some of the glycine.

[0068] Once the cyanide in the leach residue is reduced to an acceptable level, the treated process water may be discharged safely into the environment.

[0069] The process may further include revegetating the heap after the pH of the heap has been adjusted to a suitable level, such as below 8.

[0070] In a second aspect, there is disclosed a method of remediating a heap of a leach residue containing metal values, including supplying an aqueous amino-acid (or derivative thereof) containing treatment solution to the heap of the leach residue and adjusting the pH of the leach residue in the heap towards neutral.

[0071] The process may include producing a product solution containing leached metal values.

[0072] The method of the second aspect may comprise a part of a closure / termination plan for the heap at the end of a heap leach operation.

[0073] In a third aspect there is provided a method of remediating a heap of a leach residue containing cyanide and a precious metal value, including: supplying an aqueous amino-acid (or derivative thereof) to the heap of the leach residue and forming an amino acid-cyanide containing treatment solution in situ in the heap; extracting at least a part of the precious metal from the leach residue with the treatment solution to form a product solution; collecting the product solution; recovering the precious metal from the product solution; and removing cyanide from the product solution.

[0074] The cyanide may be removed from the product solution (either before or after precious metal recovery) by volatilization. The volatilization process may be similar to that of the conventional process.

[0075] In a fourth aspect, there is disclosed a process for leaching and then remediating a heap of a metal- value containing material, including: leaching a heap of the metal-value containing material in a heap leach operation (as herein defined) and producing a metal value-containing leach residue; discontinuing the heap leach operation; treating the leach residue by adding an amino-acid (or derivative thereof) to the leach residue.

[0076] The above-described leach residue treatment methods may further include one or more of the following steps:

[0077] • Assessing the chemistry of the leach residue heap, such as by measuring the pH of the leach residue and / or determining the presence and / or concentration of chemical species in the residue. The chemical species may be toxic. The chemical species may be a residual lixiviant from the previous leaching process. The residual lixiviant may be a cyanide, such as where the leach residue was an acidic precious metal containing residue.

[0078] • Selecting the chemistry of a treatment solution that includes the amino acid for application to the heap. The treatment solution may include other additives depending on the chemistry of the leach residue heap. For example, in the case of remediating a heap of acidic, base-metal containing leach residue, the treatment solution may include thiourea in addition to amino acid.

[0079] • Applying the heap with the treatment solution until the pH of the heap is adjusted to a value close to neutral.

[0080] • Recovering product solution arising from reaction of the treatment solution with the leach residue.

[0081] • Extracting residual metal values from the product solution that are leached from the leached residue.

[0082] • If necessary, further adjusting the pH and other chemistry of the product solution to produce treated process water that complies with local environmental regulations. The adjustment may be by way of addition of chemicals, such as acid, base or oxidant, or by agitation and / or evaporation.

[0083] • Discharging the treated process water to the environment.

[0084] • Revegetating the remediated heap.

[0085] Advantages of the disclosed process for treating leach residue include one or more of the following:

[0086] 1. Use of a cheap and environmentally benign species (amino acid) to adjust the pH of the leach residue towards neutral.

[0087] 2. Where the leach residue contains cyanide, taking advantage of the synergistic effect of amino acid and cyanide to recover more metal values during the treatment than otherwise possible. In the case of gold recovery, the inventors have found in experimental work that the present process may recover from 30-50% more gold as compared to 10-20% using the conventional process. Additionally, any residual WAD cyanide is converted to free cyanide that can be used for metal value leaching.

[0088] 3. Where the leach residue is acidic, taking advantage of the synergistic effect of amino acid and any other additives like thiourea to recover more metal values during the treatment than otherwise possible. In the case of gold and / or base metal recovery, the inventors have found in experimental work that the present process may recover from 50-80% more gold and / or 20-30% more base metals (such as copper) as compared to 10-20% of each using the conventional process.

[0089] 4. Residual amino acid in the residue after completion of the process will assist in establishment of revegetation because glycine is a nitrogen source similar to a fertilizer.

[0090] 5. Utilizing the value of recovered metal values to pay for the cost of heap closure.

[0091] 6. The potential for reduced water requirements for residue treatment as compared to the conventional treatment process.

[0092] 7. Potentially allowing remediation of a heap in a single stage.

[0093] BRIEF DESCRIPTION OF THE DRAWINGS

[0094] Notwithstanding any other forms which may fall within the scope of the apparatus and method as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:

[0095] Figure 1 is a schematic drawing of a process for treating a heap of leach residue according to the present disclosure;

[0096] Figure 2 is a plot of gold extraction (%),and glycine concentration (g / L) at NaCN concentration of 50 mg / L for static bottle roll tests (BRTs),

[0097] Figure 3 is a plot of gold extraction (%),and glycine concentration (g / L) at NaCN concentration of 10 mg / L for static bottle roll tests (BRTs),

[0098] Figure 4 is a plot of gold extraction (%),and glycine concentration (g / L) at NaCN concentration of 5 mg / L for static bottle roll tests (BRTs),

[0099] Figure 5 is a plot of gold extraction (%) versus time for varying glycine concentrations at NaCN concentration of 50 mg / L,

[0100] Figure 6 is a plot of gold extraction (%) versus time for varying glycine concentrations at NaCN concentration of 10 mg / L, Figure 7 is a plot of gold extraction (%) versus time for varying glycine concentrations at NaCN concentration of 5 mg / L,

[0101] Figure 8 shows gold extraction for column tests at varying NaCN and glycine concentrations, and

[0102] Figure 9 is a plot of gold extraction (%) versus time for column tests at varying glycine and NaCN concentrations.

[0103] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENT

[0104] Figure 1 illustrates a heap closure strategy 10 including a heap of leach residue 20 containing residual precious metal values. The leach residue comprises ripios 30 from a heap leach operation in which a gold-containing ore was leached with an alkaline cyanide leachant. It is noted that the invention is not confined to the use of an alkaline cyanide leachant. The ripios comprises a combination of the leached gold-containing ore and residual alkaline process solution such as one or more of pregnant leach solution, wash solution and barren solution. The ripios is alkaline (around pH 9) and includes both residual gold and residual cyanide that was a lixiviant used to leach the gold in the heap leach operation.

[0105] In the heap closure strategy 10, an aqueous amino-acid solution 40 comprising hydrogen glycinate is applied to the top of heap of leach residue 20 by sprayers 50.

[0106] As the solution 40 travels through the heap, the amino acid, typically having a pH of 6, reduces the pH of the alkaline ripios towards pH 7 and therefore contributes to forming an environmentally benign heap.

[0107] The pH of the residue is determined by the use of a pH meter 120. The pH of the leach residue is determined by measuring the pH of the solution that is flushed through the heap. The pH of the solution is conveniently measured as the solution exits the heap. The pH meter measures the hydrogen ion concentration in solution. It is noted that a skilled person would appreciate that there are other options for determining the pH of the residue.

[0108] The pH meter 120 further includes a CN meter that measures the cyanide content of the solution as it leaves the heap. It therefore monitors the decreasing CN content over time.

[0109] In addition, the amino acid in the solution 40 and the residual cyanide in the heap together form a particularly effective leaching solution in situ. The leaching solution causes dissolution of residual gold in the heap and forms a product solution containing dissolved gold. Cyanide enhances the lixiviant action of amino acid when leaching precious metals.

[0110] Accordingly, the combination of cyanide and amino acid enables greater quantities of precious metal to be recovered from the residue than would otherwise be possible by simple washing of the heap with water. The product solution is collected towards the base of the heap where it drains into a PLS / BLS pond 60. Fresh hydrogen glycinate 70 is also added to the pond. Pregnant leach solution from the pond is pumped 80 to a gold elution circuit 90 where the PLS is treated by carbon in column (C-I-C) 100 and recovered by elution. The barren leach solution 110 remaining after gold recovery is recycled to the sprayers 50 for further irrigation of the heap. Spent electrowinning solution containing cyanide is recycled to the PLS / BLS pond 60 from the elution circuit 90.

[0111] The PLS / BLS pond 60 further includes a paddle wheel or sprayer 130 to agitate the PLS / BLS solution and assist in the evaporation and volatilization of the HCN in the solution.

[0112] The amount and flow rate of amino acid added to the heap is controlled (such as by a VSD pump) to a given addition rate L / m2 / hr in response to measured pH and / or CN concentration in the heap using the meter 120.

[0113] During the course of the treatment of the leach residue, the CN and WAD cyanide in solution is converted to HCN (g) or to CO2 (g) and NH3 (g) as per the above chemical reactions.

[0114] The above-described embodiment is an effective process for remediating the heap of leach residue.

[0115] As noted above, the Applicant discovered that amino acids may be used in the treatment of leach residues, such as metal value containing residues remaining in a heap at the completion of a heap leach operation, to remediate the residues.

[0116] The Applicant’s considerable knowledge of the effectiveness of amino acids, in particular glycine, as a lixiviant for gold-bearing material provides the applicant with a high level of assurance of the effectiveness of amino acids in remediation applications.

[0117] EXAMPLES

[0118] Non-limiting Examples of experimental work in relation to a process for treating alkaline heap leach residues containing precious metal values in accordance with embodiments of the invention are described below.

[0119] Samples of leach residues were subjected to static bottle roll tests (BRTs) and column test work to simulate treatment of a heap of leach residue after completion of a heap leach operation involving an alkaline cyanide leach of gold containing ore. In both the BRTs and the column tests, varying concentrations of amino acid (in the form of glycine) were added to the sample in a slurry with varying cyanide concentration (as NaCN).

[0120] Example 1 Leach residue A comprised the residue from leaching a gold containing ore with an alkaline cyanide leachant, and had the composition set out in Table 1:

[0121] Table 1: Leach residue A composition Samples of leach residue A were subjected to static bottle roll tests (BRTs) to simulate treatment of a heap of the leach residue after completion of a heap leach operation involving an alkaline cyanide leach of gold containing ore. In the BRTs, varying concentrations of amino acid (in the form of glycine) were added to the sample in a slurry with varying cyanide concentration (as NaCN) and at a pH of 10.5 or 11.5. Samples of leach residue A were subjected to twenty-four static BRTs over a 96-hour period.

[0122] The experimental conditions of the BRTs are set out in Table 2:

[0123]

[0124] Table 2: Experimental conditions of BRTs

[0125] Three BRTs (Cyanide 12, 11 and 22) served as the baseline for cyanidation, with no additions of glycine and initial sodium cyanide concentrations set at 50, 10, or 5 ppm, respectively. These cyanide concentrations were maintained throughout the BRTs following standard cyanidation BRT procedures.

[0126] The remaining twenty-one BRTs tested various combinations of cyanide with glycine at different concentrations. Specifically, sodium cyanide levels were kept at 50, 10, or 5 ppm, while varying amounts of glycine (3, 7.5, and 15 g / L) were added. The tests were performed at two pH levels, 10.5 and 11.5. The solids percentage was constant at 33%.

[0127] The intention of this Example was to simulate the decreasing cyanide concentration of a heap (such as what occurs during the decommissioning of a heap after a heap leach operation as the heap is washed with water to remove cyanide) and its effect on extraction of gold from the leach residue.

[0128] The results of the BRTs are illustrated in Figures 2 to 7.

[0129] The results showed that the addition of glycine significantly enhanced gold extraction over that achieved from cyanide alone, leading to considerable gold extraction improvement despite reducing cyanide concentrations.

[0130] Referring to Figures 2, 3 and 4, as the baseline cyanide concentrations were lowered from 50 ppm (Figure 2) to 10 ppm (Figure 3) to 5 ppm (Figure 4), the baseline gold (Au) extraction was 66.8%, 16.3%, and 9.3%, respectively. For each cyanide concentration, the optimal gold extraction was achieved with the addition of the highest concentration of glycine tested, namely 15 g / L of glycine. At this concentration, the Au extraction increased to 87.1%, 77.5%, and 70% for the respective cyanide concentrations.

[0131] Moreover, referring to Figures 5 to 7, the gold extraction process demonstrated faster kinetics as the glycine concentration was elevated, demonstrating that higher glycine levels accelerate the extraction process even in the presence of relatively low cyanide concentrations.

[0132] In order to control the pH of each sample during the BRT, alkaline reagents such as NaOH and / or lime were added to the sample. These reagents were observed to be consumed during the course of each BRT in order to maintain an alkaline pH. However, in the absence of such alkaline reagents, the pH of the sample would inevitably decrease towards neutral given the acidic nature of amino acids.

[0133] It was observed that cyanide was consumed throughout the BRTs. For example, cyanide consumption was reduced from the initial concentrations of 50 ppm, 10 ppm, and 5 ppm upon the addition of 15 g / L glycine, with corresponding reductions of 42%, 81%, and 65%, respectively, for each cyanide concentration. The cyanide consumptions assisted in removal of cyanide from the residue and facilitated heap decommissioning.

[0134] In conclusion, when the heap is decommissioned and water-washed, the cyanide concentrations naturally decrease, and for a given cyanide concentration adding glycine would extract more gold than cyanidation. As a result, the study findings suggest that adding glycine at lower cyanide concentrations can continue the leaching process, leading to effective gold recovery while adhering to environmental regulations, potentially eliminating the need for an additional detoxification step but notably recovering more gold.

[0135] Example 2

[0136] Example 2 concerns column test work using leach residue B having the composition set out in Table 3 and the experimental conditions for the columns in Table 4.

[0137] Table 3: Composition of Leach Residue B

[0138] Table 4: Experimental conditions of Columns

[0139] A heap comprising leach residue B typically includes 400 ppm cyanide at the end of the active heap leach operation. In the column testwork of Example 2, the effects of lower cyanide concentrations at varying glycine concentration (300 and 600 ppm) were investigated.

[0140] Figure 8 shows the gold extraction under varying cyanide and glycine concentrations. When 500 mg / L (ppm) of glycine was added to the baseline cyanide concentration of 400 ppm cyanide, there was an increase of gold extraction of approximately 10%. Further, at a lower cyanide concentration (such as after a decrease in cyanide due to water washing), Figure 8 indicates the amount of gold recovery can still be significant. Specifically, the amount of gold recovered at a lower cyanide concentration of 250 ppm and glycine concentration of 300 ppm is still higher than that for the baseline cyanide concentration (500 ppm). The recovery is around 6% higher for a glycine concentration of 600 ppm. Accordingly, Example 2 shows that for a given cyanide concentration, similar or higher gold extraction can be attained with glycine addition.

[0141] Figure 9 shows the extraction kinetics of the tests as compared with the baseline concentration of 400 ppm NaCN. At 250 ppm NaCN with 300 ppm glycine, Au extraction kinetics exceeded that of the baseline 400 ppm NaCN cyanidation. At 250 ppm NaCN with 600 ppm glycine, Au extraction kinetics increased more rapidly than the baseline 400 ppm NaCN. At 400 ppm NaCN (baseline) with 500 ppm glycine, Au extraction kinetics surpassed the baseline cyanidation rate.

[0142] It was also observed that at 250 ppm NaCN with 300 ppm glycine, cyanide consumption matched that for the 400 ppm NaCN baseline, reflecting similar reagent consumption kinetics.

[0143] The results demonstrated that adding glycine would benefit not only the overall extraction but also the amount of gold per given volume i.e. faster leaching kinetics. This indicates that more gold will be recovered at lower cyanide concentrations in the presence of glycine than at baseline cyanide concentrations. It was also observed that cyanide continued to be removed from the residue during the column tests which facilitates heap decommissioning.

[0144] Whilst a specific method embodiment has been described, it should be appreciated that the method may be embodied in many other forms. For example, while the illustrated embodiment relates to an alkaline heap of cyanide-containing leached gold ore, the present method may also be used for treatment of other previously leached materials, such as acidic leach residue, residue that does not contain cyanide, residue that contains other metal values (eg, base metals like copper) and residue that contains other toxic chemical species. The method may further extend to applying other chemical species (such as thiourea) in addition to amino acid to the residue.

[0145] In the claims which follow, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” and variations such as “comprises” or “comprising” are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the apparatus and method as disclosed herein.

[0146] Further patent applications may be filed in Australia or overseas on the basis of, or claiming priority from, the present application. It is to be understood that the following claims are provided by use of example only and are not intended to limit the scope of what may be claimed in any such future applications. Features may be added to or omitted from the claims at a later date so as to further define or re-define the invention or inventions.

Claims

CLAIMS1. A process for treating acidic or alkaline leach residue containing metal values, including adding an amino-acid (or derivative thereof) to the residue and adjusting the pH of the leach residue towards neutral.

2. The process of claim 1, wherein the leach residue comprises a heap of alkaline, cyanide-containing leach residue or a heap of acidic, sulphuric acid containing leach residue.

3. The process of claim 1 or 2, wherein the metal values are selected from: a. base metals such as copper, nickel, zinc, lead and cobalt; and / or b. precious metals such as gold, silver, palladium and PGMs.

4. The process of any preceding claim, wherein the amino acid is selected from glycine and glutamic acid.

5. The process of any preceding claim, wherein the amino acid derivative is a salt, such as Na-glycinate or H- glycinate.

6. The process of any preceding claim, wherein the amino acid or derivative thereof is added in an aqueous solution to the leach residue, such as in process water or saline water.

7. The process of any preceding claim, wherein the amino-acid is added to the heap through drippers or sprays.

8. The process of claim 4, wherein the concentration of amino acid in the aqueous solution is from 0.001 to 15 g / L.

9. The process of any preceding claim, wherein the leach residue is produced by leaching a metal containing material selected from an ore, an ore concentrate, tailings, waste material or e-waste.

10. The process of claim 9, wherein the leach residue is produced by leaching a precious metal containing material with an alkaline leachant containing a cyanide- containing phase.

11. The process of claim 10, wherein the residue additionally comprises cyanide.

12. The process of claim 11, wherein the amino acid and cyanide together form a leaching solution in situ.

13. The process of claim 11, wherein the residual metal values comprise precious metals, such as gold.

14. The process of claim 9, wherein the leach residue is produced by leaching a base metal containing material with an acidic leachant.

15. The process of any preceding claim, wherein the product solution is recovered from the heap using ion exchange or activated carbon.

16. The process of any preceding claim, further including revegetating the leach residue after the pH of the residue has been adjusted to near neutral.

17. A method of remediating a heap of a leach residue containing metal values, including supplying an aqueous amino-acid (or derivative thereof) containing treatment solution to the heap of the leach residue and adjusting the pH of the leach residue in the heap towards neutral.

18. A method of remediating a heap of a leach residue containing cyanide and a precious metal value, including: supplying an aqueous amino-acid (or derivative thereof) to the heap of the leach residue and forming an amino acid-cyanide containing treatment solution in situ in the heap; extracting at least a part of the precious metal from the leach residue with the treatment solution to form a product solution; collecting the product solution;recovering the precious metal from the product solution; and removing cyanide from the product solution.

19. A process for leaching and then remediating a heap of a metal-value containing material, including: leaching the heap of the metal value-containing material in a heap leach operation and producing a metal- value containing leach residue; discontinuing the heap leach operation; and treating the leach residue by adding an amino-acid (or derivative thereof) to the leach residue.

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