WO2025208183A1

WO2025208183A1 - A process for recovery of precious metals from thiosulfate solutions

Info

Publication number: WO2025208183A1
Application number: PCT/AU2025/050326
Authority: WO
Inventors: Elsayed Abdelrady ORABY; Jacobus Johannes EKSTEEN
Original assignee: Curtin University
Current assignee: Curtin University
Priority date: 2024-04-05
Filing date: 2025-04-04
Publication date: 2025-10-09
Anticipated expiration: 2026-10-05

Abstract

A process for recovering dissolved precious metal from an aqueous solution containing thiosulfate and precious metal. The process includes contacting the precious metal- thiosulfate solution with amino acid (or derivative thereof) and activated carbon to adsorb precious metal onto the activated carbon and form loaded activated carbon. The process also includes recovering precious metal from the loaded activated carbon.

Description

A PROCESS FOR RECOVERY OF PRECIOUS METALS FROM THIOSULFATE SOLUTIONS

TECHNICAL FIELD

A process is disclosed for recovering dissolved precious metal from a thiosulfate solution (such as leachate) using activated carbon. The process involves the treatment of either the leachate or the activated carbon (or both) with an amino acid or an amino acid salt (such as a metal glycinate) in order to improve precious metal extraction/recovery.

A process is also disclosed for recovering precious metal from a preg-robbing precious metal-containing material, such as a preg-robbing precious metal-containing ores.

As used herein, the term “ore” is understood to mean a naturally occurring solid material from which a metal or valuable mineral can be extracted profitably. The term “ore” as used herein includes a concentrate of an ore and materials that are regarded as “waste” materials because the concentration of the metal or the valuable mineral was thought to be too low to be regarded as an ore at a one time but is regarded as being an ore at a later time.

As used herein, the term “preg-robbing material” is typically meant a material, such as a natural ore, that contains components that can rob (i.e. re-adsorb onto the ore itself) precious metals that entered the leachate. The components that lead to the readsorption of precious metals are typically carbonaceous materials that are associated with the precious-metals-bearing material, such a mineral ore, but may also include electronic waste and other process intermediates and mineral tailings and wastes. These carbonaceous materials typically have carbon in the form of organic carbon or some graphitised form of it.

As used herein, the term “precious metal” means gold, silver and the platinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum. However, of these precious metals, the process is particularly applicable to the recovery of gold and/or silver, and discussion will therefore focus on these two precious metals.

BACKGROUND ART

Cyanide has historically been a favoured leachant of precious metals from ores. However, cyanide has fallen from favour in recent times due to its toxicity to humans and the environment. Cyanide leaching of preg-robbing ores is also problematic because the precious metal-cyanide complex adsorbs from solution onto the carbon associated with the ore itself, leading to reduced precious metal recovery. Carbonaceous materials associated with precious-metals-bearing materials with these back-adsorbing properties can be found in mineral ores, mineral tailings, process intermediates, process wastes, and electronic wastes, such as printed circuit boards and integrated circuit chips.

The thiosulfate leaching of precious metals is a more environmentally friendly process than cyanide leaching. The thiosulfate leaching system is moreover a good alternative leaching solution for preg-robbing ores because the precious metal- thiosulfate complex does not re-adsorb from solution onto the carbon (or other preg- robbing components) in the ore. Despite some of the advantages in using alkaline thiosulfate systems as a lixiviant (typically in the presence of copper ions, and with aqueous ammonia or ammonium salts and/or calcium being present in industrial lixiviant systems), they suffer from some challenges such as the formation of higher order polymerized forms of thiosulfates, such as polythionates, that complicate the chemical speciation and subsequent metal recovery processes from solution, such as ion exchange.

However, where ion-exchange resins are used to recover precious metals from the pregnant leach solution, these resins are prone to polythionate (a byproduct from thiosulfate leaching) poisoning or fouling and are highly sensitive to process variation. High concentrations of polythionates adsorb strongly onto the resin and they compete with the precious metals’ adsorption. Furthermore, ion exchange resins are ideally suited for clear solutions (without suspended solids), and poorly suited for direct adsorption from pulps, which implies expensive solid-liquid separation prior to ion exchange. Moreover, resins are very expensive and degrade physically and chemically over time.

Activated carbon is a much cheaper adsorbent than ion-exchange resin and is well-suited to be combined in leach systems where appropriate, such as carbon-in-leach and carbon-in-pulp systems. Activated carbon is a form of particulate carbon having a high surface area to volume ratio and is commonly used for adsorption applications. It is produced in bulk in many (developed and developing) countries and can have many different sources of carbon (from coconut char, to charred fruit kernels, to synthetic sources). It is thus not limited to a small number of proprietary suppliers. It is chemically robust in that it can adsorb numerous organic and carbon-bearing inorganic compounds, but cannot adsorb thiosulfates, polythionates or other oxy-sulfur anions (such as sulfate, sulfite, disulfate, disulfite, peroxodi sulfates, persulfate, etc.) of the form SxO_y ^z" where S and O are sulfur and oxygen, x and y are their atomic ratios and z being the anionic charge. It is also a robust adsorption media of a convenient particle size (generally from 0.5 to 5 mm, such as from around 1.5-4 mm) and easy to separate from the ore slurry (particles typically smaller than 0.15 mm) through interstage screens of the appropriate screen aperture. It allows easy sampling of the carbon (compared to resin) to obtain the precious metal content on carbon separately from the precious metal content remaining in ore in slurry. Many measurement, control, handling, and process modeling technologies have been developed to deal with activated carbon.

However, notwithstanding the advantages of activated carbon, precious metal recovery by activated carbon is problematic because the gold thiosulfate complex adsorbs very poorly onto activated carbon.

The above extraction process difficulties are therefore impediments to the uptake of thiosulfate leaching technology in the broader mineral processing industry.

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.

SUMMARY OF THE DISCLOSURE

In a first aspect there is disclosed a process for recovering dissolved precious metal from a solution containing thiosulfate (as herein defined) and precious metal, the process including the steps: a) contacting the precious metal- thiosulfate (as herein defined) solution with amino acid (or derivative thereof) and activated carbon to adsorb precious metal onto the activated carbon and form preci ous-metals-loaded activated carbon; and b) recovering precious metal from the loaded activated carbon.

The precious metal- thiosulfate (as herein defined) solution may comprise a leachate.

As used herein, the term “thiosulfate” is intended to include higher order polymerized forms of thiosulfates, such as polythionates, or other oxy-sulfur anions (such as sulfate, sulfite, disulfate, disulfite, peroxodi sulfates, persulfate, etc.) of the form SxO_y ^z" where S and O are sulfur and oxygen, x and y are their atomic ratios and z is the anionic charge. A certain portion of thiosulfate may self-convert to polythionate, and a portion may be oxidised or reduced to another SxO_y ^z" form, and these variations are within the scope of the definition of “thiosulfate”.

Accordingly, the precious metal- thiosulfate leachate may comprise one or more precious metal-thiosulfate aqueous complexes, metal polythionate aqueous complexes and other metal-S_xO_y ^z' aqueous complexes. The precious metal- thiosulfate leachate may have been previously formed from an earlier leaching step in which a precious metal containing material (such as an ore) was leached with a thiosulfate containing leachant. Alternatively, the leaching step may occur in situ in step (a), such that there is simultaneous formation of the precious metal- thiosulfate (as herein defined) solution and contacting the solution with amino acid (or derivative thereof) and activated carbon.

The concentration of thiosulfate in the solution may be a minimum of 0.01 M. The concentration of thiosulfate may be a minimum of 0.05 M. The concentration of thiosulfate may be a minimum of 0.1 M. The concentration of thiosulfate may be a maximum of 0.2 M. The concentration of thiosulfate may be a maximum of 0.3 M. The concentration of thiosulfate may be a maximum of 0.5 M. The concentration of thiosulfate may be a maximum of 1 M.

The concentration of precious metal in solution will vary depending on the nature of the source of the precious metal. In the case where the precious metal- thiosulfate leachate was formed from an earlier step of leaching a precious metal containing material (such as an ore), the precious metal concentration will depend on the grade of the material and the effectiveness of the leach. The concentration of precious metal in the leachate may be a minimum of 0.01 mg/L. The concentration of precious metal in the leachate may be a minimum of 0.05 mg/L. The concentration of precious metal in the leachate may be a minimum of 0.1 mg/L. The concentration of precious metal in the leachate may be a minimum of 0.2 mg/L. The concentration of precious metal in the leachate may be a minimum of 0.5 mg/L. The concentration of precious metal in the leachate may be a minimum of 1 mg/L. The concentration of precious metal in the leachate may be a minimum of 1.5 mg/L. The concentration of precious metal in the leachate may be a minimum of 2 mg/L.

The amino acid may comprise an alpha amino acid or its metals or ammonium (NH₄ ⁺) salts. The amino acid may comprise one or more of Glycine, Histidine, Valine, Alanine, Phenylalanine, Cysteine, Aspartic Acid, Glutamic Acid, Lysine Methionine, Serine, Threonine, and Tyrosine. The metal salts may be alkali metals (lithium, sodium, potassium, etc.) or alkaline earth metals (beryllium, magnesium, calcium, strontium, barium). In one embodiment, the amino acid is glycine. Glycine is a simple amino acid that is easy and cheap to produce on an industrial scale. Glycine has a number of advantages: it is a cheap reagent produced in bulk industrial quantities, it is an environmentally safe and stable reagent, yet it is enzymatically destructible at suitable pH’s and is easily metabolized in most living organisms. It has no transport or storage limitations and is produced by a diverse range of countries, thereby averting supplychain security risks.

In another embodiment, the amino acid is glutamic acid.

Derivatives of amino acids may include amino acid ions, aqueous amino acid complexes (such as base metal- amino acid complexes), dissolved amino acid salts (such as metal (or ammonium) amino acid salts or their mixtures) or other compounds derived from reaction of an amino acid with another organic molecule. Examples include calcium, sodium or base metal salts such as base metal glycinates and base metal glutamates, such as copper glycinate, sodium glycinate, mono-sodium glutamate or ammonium glycinate. These salts may also arise due to in-situ formation between the amino acid and pH modifiers (eg NaOH or Ca(OH)2) or other species already present in the leachate or leach system. Alternatively, the amino acid derivative may be a peptide derivative, such as a di- or tri- peptide.

Where hydroxides are added to the solution, they may have a minimum concentration of 0.001 g/L. They may be present at a concentration of up to 150 g/L.

In another embodiment, the leachate may be contacted with a combination of two or more amino acids. The combination of amino acids may comprise glycine and one or more other amino acids. For example, the combination may comprise glycine and histidine.

The amino acid may be produced via bacteria or through abiotic processes. It may be sourced through normal commercial channels or produced on site. If produced on site, the amino acid may be produced either singly or in combination at the ore processing site from an appropriate nutrient medium by a range of microorganisms and bacteria, such as (but not limited to):

• Achromobacter paradoxus

• Aeromonas hydrophila

• Aeromonas sp.

• Bacillus circulans

• Bacillus megaterium

• Bacillus mesentericus

• Bacillus polymyxa

• Bacillus subtillis

• Bacillus sp.

• Candida sp.

• Chromobacterium flavum

• Penicillium sp.

• Pseudomonas aerginosa

• Pseudomonas liquefaciens

• Pseudomonas putida

• Pseudomonas sp.

• Sarcina flava

The amino acid may be present at a minimum concentration of 0.00 IM. In an embodiment, the amino acid may be present at a minimum concentration of 0.002 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.003 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.005 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.007 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.01 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.02 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.03 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.05 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.07 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.1 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.2 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.3 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.5 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.7 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.8 M. In an embodiment, the amino acid may be present at a minimum concentration of 1 M.

In an embodiment, the amino acid may be present at a concentration up to 2 M. In an embodiment, the amino acid may be present at a concentration up to 1.7 M. In an embodiment, the amino acid may be present at a concentration up to 1.5 M. In an embodiment, the amino acid may be present at a concentration up to 1.2 M. In an embodiment, the amino acid may be present at a concentration up to 1 M.

Without wishing to be limited to theory, it is believed that the presence of amino acid (such as glycine) or derivative either during the leaching step or subsequently added to the precious metal- thiosulfate leachate may lead to formation of precious metal thio-amino acid complexes (for example, gold thio-glycinate complexes) which have more affinity for adsorption onto carbon than gold thiosulfate complexes only. The amino acid or derivative may instead or in addition behave as a ferrying mechanism for precious metals by stripping the precious metals from their thiosulfate (or polythionate, etc) complexes and ferrying the precious metals as their amino acid complexes to the carbon. The exact mechanism of adsorption is unknown. It is only known that, in the absence of amino acids, precious metals thiosulfate (and polythionate, etc) complexes adsorb poorly onto activated carbon (to the point that it is totally infeasible to use), whereas the addition of amino acids or their salts to the precious metals- thiosulfate leachate makes recovery of these metals through carbon adsorption possible.

The process may further include a leaching step prior to or during step (a). The leaching step may comprise leaching a precious metal containing material with a leaching solution that contains a thiosulfate lixiviant to produce the leachate containing precious metal- thiosulfate complexes. As used herein, the term “lixiviant” refers to a specific active chemical that is used to complex with and enable the extraction of a target element (in this case, precious metal) from a material (e.g., ore).

In some cases when a preg-robbing precious metal-containing material is being treated, the absence of amino acids during the leaching step minimises precious metals adsorption onto the carbonaceous materials.

In other embodiments where the precious metal-containing material is not preg- robbing, the leaching may be conducted concurrently during step (a). In this case, the precious metal-containing material is leached with a solution containing thiosulfate and amino acid in step (a) in the presence of activated carbon such that the precious metal is leached and adsorbed onto the activated carbon in a single step.

The leaching step may be followed by a solids separation step, to substantially remove solid leach residue, such as spent ore (or “ripios”), including any remaining carbonaceous material, from the leachate. The solids separation step is advantageous where the precious metal-containing material is preg-robbing.

The leachant may further include a dissolved base metal, such as aqueous copper ions. The presence of a base metal in solution can enhance adsorption of the precious metal on the activated carbon. Without wishing to be limited by theory it is believed that the base metal forms dissolved amino acid salts in solution, such as copper glycinates, which adsorb onto the activated carbon. The dissolved precious metal then engages in a metal-ion exchange with the copper/base metal, by which the copper or base metal is released from carbon to the leachate and the precious metal then adsorbed onto the carbon.

The base metal may be derived from the precious metal-containing material or separately added. The base metal may be present at a minimum concentration of 0.001 g/L. The base metal may be present at a minimum concentration of 0.005 g/L. The base metal may be present at a minimum concentration of 0.01 g/L. The base metal may be present at a minimum concentration of 0.05 g/L. The base metal may be present at a minimum concentration of 0.1 g/L. The base metal may be present at a minimum concentration of 0.5 g/L. The base metal may be present at a minimum concentration of 0.7 g/L. The base metal may be present at a minimum concentration of 1 g/L. The base metal may be present at a minimum concentration of 1.5 g/L. The base metal may be present at a minimum concentration of 2 g/L. The base metal may be present at a minimum concentration of 3 g/L. The base metal may be present at a minimum concentration of 5 g/L. The base metal may be present at a maximum concentration of 20 g/L. The base metal may be present at a maximum concentration of 15 g/L. The base metal may be present at a maximum concentration of 12 g/L. The base metal may be present at a maximum concentration of 10 g/L. In an embodiment, the base metal may be present in a concentration between 0 and 20g/L.

In one embodiment, step (a) may comprise adding amino acid to the leachate to produce a treated leachate, then contacting the treated leachate with activated carbon.

In another embodiment, step (a) may comprise contacting the leachate with activated carbon that has been pretreated with amino acid or derivative thereof (such as, amino acid ions, amino acid salts or amino acid metals complexes. The pretreatment may comprise coating the activated carbon with amino acid or derivative thereof. The activated carbon may have a minimum particle size of 1.5 mm. The activated carbon may have a maximum particle size of 4 mm. In an embodiment, the pretreatment may comprise treating activated carbon with base metal amino acid complexes having a concentration of at least 0.01 g/L. In another embodiment, the pretreatment may comprise treating activated carbon with base metal amino acid complexes having a concentration of up to 50 g/L. The pretreatment may comprise treating activated carbon with base metal amino acid complexes in a concentration range of 0.01 g/L to 50 g/L . The base metal may be copper. Without wishing to be limited to a particular mechanism, it is believed that the copper or other base metal engages in a metal-ion exchange with the precious metal, by which the copper or base metal is released from carbon to the leachate and the precious metal then adsorbed onto the carbon. The concentration of activated carbon in the solution may range from 0.1 to lOOg/L. The metals can also be adsorbed using carbon in column.

In another embodiment, the leachate may further include aqueous ammonia. The presence of aqueous ammonia (or equivalently ammonium hydroxide) may improve the metals adsorption onto carbon. The ammonia may be present at a concentration of at least 0.5 g/L, such as between 1.0 and 50 g/1.

The process may be conducted at a pH of at least 5. In an embodiment, the process may be conducted at a pH of at least 6. In another embodiment, the process may be conducted at a pH of at least 7.

In an embodiment, the process is conducted under alkaline conditions, such as at least pH 8. In another embodiment, the process may be conducted at a pH of at least 9. In another embodiment, the process may be conducted at a pH of at least 10.

The process may be conducted at a maximum pH of 12.5.

In a second aspect there is disclosed a process for recovery of precious metal from a non-preg-robbing precious metal-containing material, the process including:

(i) leaching the precious metal containing material with a leaching solution that contains a thiosulfate (as herein defined) lixiviant, amino acid and activated carbon to adsorb the leached precious metal in situ onto the activated carbon and form loaded activated carbon;

(ii) recovering precious metal from the loaded activated carbon.

In a third aspect there is disclosed a process for recovery of precious metal from a preg-robbing precious metal-containing material, the process including:

(i) leaching the precious metal containing material with a leaching solution that contains a thiosulfate (as herein defined) lixiviant to produce a precious metal and thiosulfate containing leachate and solid leaching residue; (ii) contacting the precious metal- thiosulfate (as herein defined) leachate with amino acid (or derivative thereof) and activated carbon to adsorb precious metal onto the activated carbon and form loaded activated carbon;

(iii) recovering precious metal from the loaded activated carbon.

In a fourth aspect, there is disclosed a process for treating activated carbon, including:

(i) contacting activated carbon with an amino acid or derivative thereof (as herein defined) to form a coated activated carbon comprising an at least partial coating of the amino acid or derivative thereof on the activated carbon; and

(ii) drying the coated activated carbon.

The activated carbon treatment process may be conducted using an aqueous solution of an amino acid (or derivative thereof). The derivative may comprise a glycinate, such as a sodium or calcium glycinate. The amino acid may be present at a minimum concentration of 0.001M. In an embodiment, the amino acid may be present at a minimum concentration of 0.002 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.003 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.005 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.007 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.01 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.02 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.03 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.05 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.07 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.1 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.2 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.3 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.5 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.7 M. In an embodiment, the amino acid may be present at a minimum concentration of 0.8 M. In an embodiment, the amino acid may be present at a minimum concentration of 1 M.

The concentration of activated carbon in the treatment solution may range from 0.1 to lOOg/L.

The aqueous treatment solution may further include a dissolved base metal, such as aqueous copper ions. The base metal may be present at a minimum concentration of 0.001 g/L. The base metal may be present at a minimum concentration of 0.005 g/L. The base metal may be present at a minimum concentration of 0.01 g/L. The base metal may be present at a minimum concentration of 0.05 g/L. The base metal may be present at a minimum concentration of 0.1 g/L. The base metal may be present at a minimum concentration of 0.5 g/L. The base metal may be present at a minimum concentration of 0.7 g/L. The base metal may be present at a minimum concentration of 1 g/L. The base metal may be present at a minimum concentration of 1.5 g/L. The base metal may be present at a minimum concentration of 2 g/L. The base metal may be present at a minimum concentration of 3 g/L. The base metal may be present at a minimum concentration of 5 g/L. The base metal may be present at a maximum concentration of 20 g/L. The base metal may be present at a maximum concentration of 15 g/L. The base metal may be present at a maximum concentration of 12 g/L. The base metal may be present at a maximum concentration of 10 g/L. In an embodiment, the base metal may be present in a concentration between 0 and 20g/L.

The aqueous solution may further include aqueous ammonia or ammonium hydroxide. The ammonia or ammonium hydroxide may be present at a concentration of at least 0.5 g/L, such as between 1.0 and 50 g/1. The process may be conducted at a pH of at least 5. In an embodiment, the process is conducted under alkaline conditions. The process may be conducted at a maximum pH of 12.5.

The activated carbon may have a minimum particle size of 1.5 mm. The activated carbon may have a maximum particle size of 4 mm. The treatment process may comprise treating the activated carbon with a base metal amino acid complex in a concentration range of 0.001 g/L to 50 g/L. The base metal may be copper.

The treated carbon may undergo a washing step prior to the drying step (ii).

The drying step may be conducted at ambient temperature. The drying step may be conducted at an elevated temperature, such as at least 40 C. The drying temperature may be a maximum of 150 C, such as 200 C.

In a fifth aspect, there disclosed an activated carbon product that includes an at least partial coating of an amino acid or amino acid derivative thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

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:

Figure l is a plot of gold loading on activated carbon versus time from thiosulfate-copper-ammonia and thiosulfate-copper-ammonia-glycine systems;

Figure 2 is a plot of gold concentration in solution versus time from thiosulfate- copper-ammonia and thiosulfate-copper-ammonia-glycine systems;

Figure 3 is a plot of gold loading versus time from thiosulfate and thiosulfateglycine systems using pre-treated activated carbon;

Figure 4 is a plot of gold concentration in solution versus time from thiosulfate and thiosulfate-glycine systems using pre-treated activated carbon; Figure 5 is a plot of copper concentration in solution versus time for thiosulfate and thiosulfate-glycine systems using pre-treated activated carbon;

Figure 6 is a plot of gold loading on activated carbon versus time from a thiosulfate- glycine system (8 g/L carbon);

Figure 7 is a plot of gold concentration in solution versus time for a thiosulfateglycine system adsorbed with 8 g/L fresh activated carbon.

Figure 8 is a plot of copper loading on activated carbon versus time from a thiosulfate- glycine system (8 g/L carbon);

Figure 9 is a plot of gold and silver concentration versus time in a thiosulfate- Cu-glycine system using fresh activated carbon at 6 g/L carbon

Figure 10 is a plot of gold loading on activated carbon versus time for thiosulfate-copper-ammonia and thiosulfate-copper-glycine systems (6 g/L carbon);

Figure 11 is a plot of gold loaded on carbon from thiosulfate-Cu-glycine system using fresh activated carbon at 6 g/L carbon in the presence of different cations (Ca²⁺ vs Na⁺).

Figure 12 is a plot of gold loaded on carbon from a thiosulfate-Cu-glycine system using fresh activated carbon at 6 g/L carbon in the presence of different glycine concentrations.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Examples

Non-limiting Examples of a process for extracting dissolved precious metal from a leachate will now be described.

Example 1

In Example 1, the adsorption of gold (~2.2 mg/L) from two synthetic gold bearing leachate systems (pH 10.5 at room temperature) was compared. The two systems were:

- Thiosulfate-copper-ammonia; and

- Thiosulfate-copper-ammonia-glycine.

Two tests were conducted to achieve this aim. In each test, the gold bearing leachate was contacted with (unpretreated) activated carbon in the indicated ratios.

Test 1 : Gold adsorption from thiosulfate-copper-ammonia system

Solution: 250 mL having 0.1 M sodium thiosulfate, 0.2 M ammonia, 300 ppm dissolved copper and approximately 2.2 mg/L gold.

Carbon: 1.5 g activated carbon (ie, 6 g C per litre of solution)

Test 2: Gold adsorption from thiosulfate-copper-ammonia-glycine system

Solution: 250 mL having 0.1 M sodium thiosulfate, 0.2 M ammonia, 10 g/L glycine, 300 ppm Cu and approximately 2.2 mg/L gold.

Carbon: 1.5 g activated carbon (ie, 6 g C per litre of solution)

Results:

The amount of gold adsorbed on the activated carbon (g/t) versus time for each system is illustrated in Figure 1. It can be seen that there was a significantly higher rate of gold adsorption in the thiosulfate-copper-ammonia-glycine system, as compared with the thiosulfate-copper-ammonia system, with the quantity of adsorbed gold being approximately six times higher after 24 hours.

Table 1

Table 1 and Figure 2 show the concentration of gold in solution versus time for both tests. Again, it can be seen that the rate of reduction in concentration of gold due to adsorption on carbon is significantly greater in the presence of amino acid. Example 2

In Example 2, gold-thiosulfate solutions were contacted with activated carbon that had been pretreated with an amino acid derivative, specifically copper glycinate. Two 1.5 g samples of fresh activated carbon were each loaded with solution containing 1000 ppm Cu glycinate for 24 hours at room temperature. The pretreated carbon was then washed and dried in an oven at a temperature of 70 °C.

The 1.5 g samples of the pretreated carbon were respectively contacted with solutions having the following compositions:

Solution 1 : 250 mL - 0.1 M sodium thiosulfate.

Solution 2: 250 mL - 0.1 M sodium thiosulfate and 10 g/L Glycine.

Figure 3 shows the amount of gold adsorbed on the pretreated carbon (g/t) versus time for each solution. It can be seen that there was an initial significantly higher rate of gold adsorption on carbon that had been pretreated with solution 2 (glycine containing) as compared with solution 1 (glycine free). However, the adsorption rate of gold on glycine pretreated carbon decreased after around 5 hours, Conversely, as seen in Figure 4 and Table 2, the rate of reduction in concentration of gold in solution is significantly greater when adsorbed on activated carbon pretreated with solution 2, as compared with activated carbon pretreated with solution 1.

Table 2 Ion exchange between gold in solution and copper (from Cu glycinate) adsorbed on the carbon was observed in each system. However, it was found that copper desorption from carbon pretreated with solution 2 (glycine containing) was much faster as compared with solution 1 (glycine free), as is evident from Figure 5. Without wishing to be limited to theory, it is believed that the presence of glycine/amino acids adsorbed on the carbon enhances the desorption of copper from the pre-treated carbon and the copper is stabilised as Cu(II) glycinate in the solution.

Example 3

In Example 3, a gold- thiosulfate- glycine aqueous solution having the composition:

Solution: 250 mL having 0.1 M sodium thiosulfate, 22.4 g/L glycine (0.3M); pH 11.0, room temperature, 150 ppm Cu, and 3 mg/L Au, was contacted with 2.0 g of (unpretreated) activated carbon (ie, a solids: liquid ratio of 8 g/L carbon). It can be seen from Figure 6 that the rate of gold adsorbed on carbon is essentially linear over time. Figure 7 shows the concentration of gold in solution versus time during this adsorption indicating that the rate is similarly linear.

Figure 8 shows the copper adsorption on carbon from thiosulfate-glycine system.

Example 4

Example 4 is a comparison of the adsorption of multiple elements, Au, and Ag, on to unpretreated carbon from two aqueous solutions, one glycine containing and the other glycine free:

Solution 1 : 250 ml having 0.1 M thiosulfate, 250 ppm Cu, 0.25 MNHj, 10 ppm

Ag, 3 ppm Au.

Solution 2: 250 ml having 0.1 M thiosulfate, 250 ppm Cu, 20 g/L glycine; 10 ppm Ag, 3 ppm Au.

The results are shown in Figures 9 and 10. Figure 9 shows gold and silver loaded on carbon from a thiosulfate-Cu-glycine aqueous solution using fresh (ie, unpretreated) activated carbon at 6 g/L carbon. The rate of adsorption of gold from solution is significantly higher than that for silver. This may lead to a selective adsorption of gold over silver in the presence of thiosulfate and amino acids.

Figure 10 compares gold loading on carbon from thiosulfate-Cu-ammonia and thiosulfate-Cu-glycine solutions using fresh (ie unpretreated) activated carbon at 6 g/L carbon. Similarly to Example 1, the rate of gold adsorption from the glycine containing solution was significantly higher than that from the glycine free solution. The glycine containing solution differs from that used in Example 1 mainly in that it does not include ammonia- and the amount of gold adsorbed after 24 hours is lower (260 g/t as compared with 290 g/t in Figure 1). This observation suggests that the presence of ammonia slightly improves the metals adsorption.

Example 5

Example 5 is a comparison of the adsorption of gold onto carbon over time in the presence of sodium and calcium cations (Na versus Ca). The addition of sodium or calcium hydroxide forms sodium or calcium glycinate (respectively) in situ.

Solution: 250 mL having 0.1 M sodium thiosulfate, 20 g/L glycine, pH 11.0, room temperature, 150 ppm Cu, and 3 mg/L Au.

Adding calcium hydroxide (CaOH)2 significantly improves the gold adsorption on carbon compared with sodium hydroxide as shown in Figure 11. This finding was consistent with the observed effect of added cations on the gold cyanide adsorption on carbon adsorption, according to the following order H⁺ > Ca²⁺ > Mg²⁺ > K⁺ > Na⁺

Example 6

Example 6 illustrates the effect of glycine concentration in the gold thiosulfate solutions on the amount of gold adsorbed onto the activated carbon.

Solution: 250 mL having 0.1 M sodium thiosulfate, 150 ppm Cu, and 3 mg/L Au, at pH 11.0, room temperature.

Different glycine concentrations were added and the pH was readjusted to pH 11. Figure 12 clearly shows that increasing the glycine concentration enhances the gold adsorption. Without wishing to be limited to theory, it is believed that adding glycine may lead to formation of gold thio-glycinate complexes which have more affinity for adsorption onto carbon than gold thiosulfate only.

Whilst a number of specific method embodiments have been described, it should be appreciated that the method may be embodied in many other forms.

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.

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 provisional 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 provisional claims at a later date so is to further define or re-define the invention or inventions.

Claims

1. A process for recovering dissolved precious metal from an aqueous solution containing thiosulfate (as herein defined) and precious metal, the process including: a) contacting the precious metal- thiosulfate (as herein defined)'solution with amino acid (or derivative thereof) and activated carbon to adsorb precious metal onto the activated carbon and form loaded activated carbon; b) recovering precious metal from the loaded activated carbon.

2. The process of claim 1, wherein the aqueous solution containing thiosulfate is a leachate.

3. The process of claim 1 or 2, wherein the concentration of thiosulfate in solution is a minimum of 0.01 M.

4. The process of any preceding claim, wherein the precious metal- thiosulfate solution was formed from a prior leaching step in which a precious metal containing material (such as an ore) was leached with a thiosulfate (as herein defined) lixiviant.

5. The process of any one of claims 1 to 3, wherein the precious metal- thiosulfate solution was formed in situ during step (a) in which a precious metal containing material (such as an ore) was leached with a thiosulfate (as herein defined) lixiviant.

6. The process of any preceding claim, wherein the amino acid is glycine or a glycine salt.

7. The process of any preceding claim, wherein the amino acid is present at a minimum concentration of 0.00 IM.

8. The process of claim 4, wherein step (a) comprises adding the amino acid to the leachate to produce a treated leachate, then contacting the treated leachate with the activated carbon.

9. The process of claim 4, wherein step (a) comprises contacting the leachate with activated carbon that has been pretreated with the amino acid or derivative thereof, such as a base metal glycinate.

10. The process of claim 5, wherein step (a) comprises contacting the amino acidprecious metal- thiosulfate solution with the activated carbon.

11. The process of claim 5, wherein step (a) comprises contacting the amino acidprecious metal- thiosulfate leachate with activated carbon that has been pretreated with the amino acid or derivative thereof, such as a base metal glycinate.

12. The process of any preceding claim, wherein the precious metal- thiosulfate solution further includes ammonia.

13. The process of claim 12, wherein the ammonia has a concentration of at least 0.5 g/L.

14. The process of any preceding claim, wherein the process is conducted at a pH of at least 5.

15. The process of any preceding claim, wherein the activated carbon has a particle size between 0.5 mm and 5 mm.

16. The process of any preceding claim, wherein the precious metal- thiosulfate solution further includes a dissolved base metal, such as copper.

17. The process of claim 16, wherein the base metal concentration is from 0 to 20 g/L.

18. A process for recovery of precious metal from a non-preg-robbing precious metal-containing material, the process including:

(i) leaching the precious metal containing material with a leaching solution that contains a thiosulfate (as herein defined) lixiviant, amino acid and activated carbon to form a leached precious metal in solution and to adsorb the leached precious metal in situ onto the activated carbon and form loaded activated carbon;

(ii) recovering precious metal from the loaded activated carbon.

19. The process of claim 18, wherein the leaching solution further includes a dissolved base metal, such as copper.

20. A process for recovery of precious metal from a preg-robbing precious metalcontaining material, the process including:

(i) leaching the precious metal containing material with a leaching solution that contains a thiosulfate (as herein defined) lixiviant to produce a precious metal and thiosulfate containing leachate and solid leaching residue;

(ii) contacting the precious metal- thiosulfate (as herein defined) leachate with amino acid (or derivative thereof) and activated carbon to adsorb precious metal onto the activated carbon and form loaded activated carbon;

(iii) recovering precious metal from the loaded activated carbon.

21. The process of claim 18, further including a solids separation step to substantially remove the solid leaching residue from the leachate.

22. The process of claim 20 or 21, wherein the leachate further includes a dissolved base metal, such as copper.

23. The process of claim 18 or 20, wherein the activated carbon has been pretreated with amino acid or derivative thereof.

24. A process for treating activated carbon, including:

(ii) drying the coated activated carbon.

25. An activated carbon product that includes an at least partial coating of an amino acid or amino acid derivative thereon.