EA048166B1 - METHOD OF CARBON-CATALYSED THIOSULPHATE LEACHING OF GOLD-BEARING MATERIALS - Google Patents

Abstract

The invention relates to a thiosulfate leaching method for the extraction of precious metals. This method leaches the precious metal from a precious metal-containing material by contacting the material with a powdered carbon material, thiosulfate, an anion exchange resin, and an oxidizing agent, forming a pulp containing the leached precious metal. The powdered carbon materials serve as a catalyst for the rate of dissolution and extraction of the precious metal during leaching.

Description

Cross-reference to related applications

The invention claims priority to the filing date of U.S. Provisional Application Nos. 62/794,887, filed January 21, 2019, and 62/819,005, filed March 15, 2019, entitled Method for Thiosulfate Leaching of Gold-Bearing Materials, the contents of which are incorporated herein by reference in their entirety.

Field of technology

The invention generally relates to leaching of metal-containing materials and, in particular, to thiosulfate leaching of materials containing precious metals.

State of the art

The traditional cyanide leaching/carbon-in-pulp process has been the primary method for gold recovery for decades. The lixiviant most commonly used in gold leaching is cyanide. Cyanide is relatively low cost, its chemistry is simple and well understood, and it forms strong complexes with gold that are stable in aqueous solutions. Although cyanide leaching is effective for leaching gold from some carbonaceous or mixed ores, there are significant environmental concerns associated with the use of cyanide in gold leaching processes, and for some gold-bearing ores, such as carbonaceous ores, cyanide consumption can be high.

Thiosulfate is one of the most successful alternative leaching agents for efficient gold leaching. An example of a thiosulfate leaching process for precious metal-containing materials is shown in U.S. Patent 7,544,232. The ammonia-thiosulfate system typically uses copper ions as the oxidizing agent. This is because oxygen has a low solubility and its slow rate of reduction on the gold surface results in a slow dissolution rate. Although copper ions are a better oxidizing agent for gold than oxygen, oxygen is still required to regenerate the oxidizing agent. Unfortunately, the combination of oxygen and copper in the ammonia-thiosulfate system has been reported to increase the rate of thiosulfate decomposition to form tetrathionate and other polythionates. Ammonia-free thiosulfate systems may be more attractive from an environmental standpoint.

In response to the need for an environmentally acceptable replacement for cyanide in gold leaching, considerable work has been done to improve the kinetics of thiosulfate leaching. Experiments with thiosulfate leaching at temperatures in the range 40-80°C and excess oxygen pressures of 10-100 psig, respectively, showed that gold recovery was dependent on temperature and thiosulfate concentration, but was less sensitive to excess oxygen pressure. Increasing temperature, Cu(II) concentrations, and dissolved oxygen were found to have a positive effect on the rate of gold dissolution. The rate of gold dissolution could be enhanced in the presence of sulfide minerals by galvanic interactions, with the rate of oxygen reduction being higher at the surface of the sulfide minerals than at the surface of the gold. Finally, U.S. Patent 1,016,1,016 discloses a process for pretreating thiosulfate-refractory oxide ores using activated carbon. In the pre-treatment process, prior to thiosulfate leaching, the ore pulp is contacted with activated carbon in the presence of oxygen. The pre-treatment process has been found to increase gold recovery from 23-26% to 71-86%.

Unfortunately, the effectiveness of thiosulfate in gold leaching can be very sensitive to the ore. Some oxide ores can be inherently refractory. They do not provide sufficient gold leaching in a thiosulfate leaching system and do not leach as efficiently as cyanide. Thiosulfate recovery of gold from some oxide ores can be minimal. Since oxide ores do not contain sulfides (or have very low sulfide levels), refractory properties cannot be reduced in the same way as sulfide ores (e.g., roasting, biooxidation, or pressure oxidation).

There is a need for a thiosulfate leaching process to overcome the problem associated with the recalcitrance of some oxide ores in a thiosulfate leaching system.

The essence of the invention

These and other problems are solved by various embodiments and configurations of the present invention. The invention is generally directed to leaching materials containing precious metals with thiosulfate in the presence of carbon.

In some embodiments, the leaching process may include the steps of:

(a) providing a powdered carbon material comprising one or more of activated carbon, activated charcoal, coke, solid carbon obtained from coconut shell and/or elemental carbon, calcined resin and mixtures thereof;

(b) providing a precious metal containing material having a first thiosulfate leaching value of the precious metal in the absence of contact with a powdered carbon material;

(c) contacting a material containing a precious metal with a powdered carbon material

- 1 048166 material, thiosulfate, an anion exchange resin and an oxidizing agent to form a pulp; and (d) leaching a precious metal from the suspended precious metal-containing material, wherein the anion exchange resin has a greater affinity for the leachable precious metal than the powdered carbon material, and the suspended precious metal-containing material has a second thiosulfate leaching value of the precious metal that is greater than the first thiosulfate leaching value of the precious metal.

In some embodiments, the leaching process may include the steps of:

(a) contacting a gold-containing material with a carbonaceous material and a thiosulfate lixiviant and/or an anion exchange resin to form a pulp, wherein the carbonaceous material comprises one or more of activated carbon, activated charcoal, coke, solid carbon obtained from coconut shell and/or elemental carbon, calcined resin, and mixtures thereof;

(b) subsequently contacting the pulp with an oxidizing agent to leach gold from the gold-bearing material and precipitate the leached gold on an anion exchange resin; and (c) contacting the gold-bearing anion exchange resin with an eluent to produce a gold-bearing eluent and a de-goldened ion exchange resin sent for reuse to step (a).

In some embodiments, the leaching process may include the steps of:

(a) contacting a precious metal containing material with a powdered carbon material, thiosulfate and an anion exchange resin to form a pulp, wherein the precious metal containing material has a first thiosulfate leaching value of the precious metal in the absence of contact with the powdered carbon material, and the powdered carbon material comprises one or more of activated carbon, activated charcoal, coke, solid carbon obtained from coconut shell and/or elemental carbon, calcined resin and mixtures thereof; and (b) contacting the pulp with an oxidizing agent to leach precious metal from the suspended precious metal-containing material, wherein the anion exchange resin has a greater affinity for the leachable precious metal than the powdered carbon material, and the suspended precious metal-containing material has a second thiosulfate leaching value of the precious metal that is greater than the first thiosulfate leaching value of the precious metal.

A precious metal could be, for example, gold.

Leaching can be carried out in the presence of molecular oxygen, but in the absence or near absence of a surrogate oxidizer such as copper.

The addition of a carbonaceous material such as finely divided carbon may result in increased dissolution rates and increased gold recovery. Without being bound by theory, it is believed that the carbonaceous material and the precious metal such as gold form a galvanic pair and that the rate of oxygen reduction is increased as a result of the increased surface area available on the finely divided carbonaceous material. In particular, the use of activated carbon as a catalyst in thiosulfate leaching of gold is an alternative process that may not require the addition of surrogate oxidants (such as copper), can operate at temperatures as low as 30°C and may have low levels of thiosulfate degradation, making it an attractive alternative for gold leaching.

Whether or not to remove the carbon after leaching depends on the particle size of the carbon used. When coarse coal is used, it is usually removed. When fine coal is used, it is usually not removed.

Finely dispersed carbon may be brought into contact with the precious metal-containing material either separately after the material has been ground, or before and/or during the grinding. In the latter case, the coal particles may be large in size but are ground to a fine size similar to that of the precious metal-containing material.

Prior to leaching, the suspended precious metal-containing material may have little or no contact with the oxidizing agent.

The various components of the pulp may be added in any order. In some embodiments, the anion exchange resin and/or thiosulfate lixiviant are contacted with the precious metal-containing material prior to the carbon material.

A precious metal-containing material may be cyanide leachable (and therefore not cyanide refractory) but not thiosulfate leachable (i.e., the material is a thiosulfate refractory precious metal-containing material). In other words, cyanide leaching of precious metals from a precious metal-containing material may be more efficient than thiosulfate leaching of precious metals. Even when leaching of a precious metal-containing material has similar precious metal recovery using either cyanide or thiosulfate as the leaching agent, the cyanide leachable material may be more efficient than thiosulfate leaching of precious metals from a precious metal-containing material.

- 2 048166 leaching agent, the leaching method of the present invention can further increase the extraction of precious metal by thiosulfate. The material containing the precious metal can be enriched or unenriched. Typically, the precious metal is in a matrix that is primarily one or more oxides. For example, the material containing the precious metal can contain more oxides than sulfides.

The gold recovery process can be carried out at ambient conditions (room temperature and atmospheric pressure) in less than 24 h. Increasing the process temperature can further enhance gold recovery and/or pretreatment kinetics.

The carbonaceous material may be removed from the pretreated pulp by screening, typically requiring that about 90% or more, and even more typically about 98% or more, of the carbon be retained on the screen, while about 90% or more, and more typically about 95% or more, of the precious metal-containing material pass through the screen. The relative means, medians, modes, and P80 particle sizes of the carbon and precious metal-containing material are selected to achieve at least these levels of separation.

After separation of the coal, the leached pulp can be discharged to tailings and the recovered carbon material can be sent to the leaching stage for reuse.

The present invention can provide a number of advantages depending on the specific configurations. Thiosulfate leaching of oxide ores in oxygenated water and in the presence of activated carbon or other carbonaceous material can significantly increase the gold recovery achieved by other thiosulfate leaching processes. The process can have low operating costs and provide a simple method for thiosulfate leaching of gold from oxides. Attrition from pulp agitation is usually the only cause of carbon loss and can be minimized by proper design of mixers and reactors. Carbonaceous material can be returned to the process and reused, which reduces operating costs. Only inexpensive air (or more expensive oxygen gas) is the reactant consumed, making the economics of the process very attractive.

These and other advantages will be apparent from the description of aspects, variations and configurations of the present invention.

In the context of the present description, the expressions at least one, one or more and and/or are open-ended expressions that are both conjunctive and disjunctive when used. For example, each of the expressions at least one of A, B and C, at least one of A, B or C, one or more of A, B and C, one or more of A, B or C, A, B and/or C and A, B or C, means only A, only B, only C, A and B together, A and C together, B and C together, or A, B and C together. If each of A, B and C in the above expressions refers to an element such as X, Y and Z, or a class of elements such as _X1 - _Xn , Yi-Ym and _Z1 - _Z0 , the phrase will refer to a single element selected from X, Y and Z, a combination of elements selected from a single class (e.g. X1 and _X2 ), and also to a combination of elements selected from two or more classes (e.g. Y1 and _Z0 ).

In this regard, the terms one or more and at least one may be used interchangeably in this description. It should also be noted that the terms comprising, including and having may be used interchangeably.

Activated carbon is a form of carbon that has been processed to create small, low-volume pores that increase the surface area available for adsorption or chemical reaction. Activated carbon may be granulated, extruded, beaded, impregnated, and/or polymer-coated.

The term carbon includes carbonaceous organic material such as one or more of the following: activated carbon (activated charcoal or activated bituminous carbon), hard coal (e.g., peat, lignite, sub-bituminous coal, bituminous coal, steam coal, anthracite, and graphite), brown coal, coke, hard carbon derived from coconut shell or elemental carbon, calcined pitch, and mixtures thereof.

The term means, as used in this specification, is to be given the broadest interpretation possible pursuant to 35 U.S.C. § 112(f) and/or 112, paragraph 6. Accordingly, a claim including the term means includes all structures, materials, or acts set forth in this specification and all equivalents thereof. In addition, the structures, materials, or acts and their equivalents include all those described in the summary section, the list of drawings, the detailed description of the invention, the abstract, and the claims themselves.

The term precious metal refers to gold and silver.

A thiosulfate-resistant precious metal-containing material is a material in which at least a portion of the precious metal-containing material is inherently resistant to recovery by thiosulfate leaching.

Unless otherwise stated, all ingredient or composition levels refer to the active portion of this product.

- 3 048166 component or composition and do not include impurities, such as residual solvents or by-products, that may be present in commercially available sources of such components or compositions.

All percentages and ratios are based on the total weight of the composition unless otherwise stated.

It should be understood that each maximum numerical limitation recited in the present invention is considered to include each lesser numerical limitation as an alternative, as if such lesser numerical limitation were expressly recited in the present invention. Each minimum numerical limitation recited in the present invention is considered to include each greater numerical limitation as an alternative, as if such greater numerical limitation were expressly recited in the present invention. Each numerical range recited in the present invention is considered to include each narrower numerical range that falls within such broader numerical range, as if such narrower numerical ranges were all expressly recited in the present invention. For example, the phrase from about 2 to about 4 includes the integer and/or ranges of integers from about 2 to about 3, from about 3 to about 4, and every possible range based on real (e.g., irrational and/or rational) numbers, such as from about 2.1 to about 4.9, from about 2.1 to about 3.4, and so on.

The foregoing is a simplified summary of the invention in order to provide an understanding of some aspects of the invention. This summary is neither a comprehensive nor an exhaustive overview of the invention and its various embodiments. It is not intended to identify key or critical elements of the invention or to define the scope of the invention, but rather to present certain concepts of the invention in a simplified form as a prelude to the more detailed description presented below. It should be understood that other embodiments of the invention are possible using, alone or in combination, one or more of the features set forth above or described in detail below.

List of drawings

The accompanying drawings are included in the description and form a part thereof to illustrate several examples of the present invention. These drawings, together with the description, explain the principles of the invention. The drawings merely illustrate preferred and alternative examples of how the invention may be realized and used and should not be construed as limiting the invention to the examples illustrated and described. Additional features and advantages will become apparent from the following, more detailed description of various aspects, variations and configurations of the invention as shown in the drawings referred to below.

Fig. 1 is a schematic representation of a process flow according to an embodiment of the invention.

Fig. 2 is a graph of gold recovery (vertical axis) versus time (horizontal axis) illustrating the effect of carbon pretreatment on gold recovery from sample S1 by CaTS-RIL.

Fig. 3 is a graph of gold recovery (vertical axis) versus time (horizontal axis) illustrating the effect of carbon pretreatment and fine carbon on recovery via thiosulfate leaching from sample S1.

Fig. 4 is a graph of gold recovery (vertical axis) versus time (horizontal axis) illustrating the effect of fine coal on gold recovery from coal flotation tailings.

Fig. 5 is a graph of gold recovery (vertical axis) versus time (horizontal axis) illustrating the effect of fine carbon on gold recovery using thiosulfate from sample S3.

Fig. 6 is a graph of gold recovery (vertical axis) versus time (horizontal axis) illustrating the effect of carbon pretreatment on gold recovery from sample S1.

Fig. 7 is a graph of gold recovery (vertical axis) versus time (horizontal axis) illustrating the effect of fine coal on gold recovery from coal flotation tailings.

Fig. 8 - shows galvanic interactions between gold and carbon.

Fig. 9 is a graph of gold recovery (vertical axis) versus carbon concentration (horizontal axis) illustrating the effect of activated carbon on gold recovery using thiosulfate.

Fig. 10 is a graph of gold recovery (vertical axis) versus time (horizontal axis) illustrating the effect of copper, resin and temperature on the thiosulfate leaching of gold from S1 in the presence of carbon (10 g/kg).

Fig. 11 is a graph of gold recovery (vertical axis) versus time (horizontal axis) illustrating the effect of thiosulfate concentration on gold recovery from sample S2 in the presence or absence of 10 g/kg carbon.

Fig. 12 - graph of gold extraction (vertical axis) versus time (horizontal axis)

- 4 048166 axis), which illustrates the effect of coarse activated carbon on gold recovery by thiosulfate leaching.

Fig. 13 is a graph of gold recovery (vertical axis) versus time (horizontal axis) illustrating the effect of carbon-solid contact on thiosulfate leaching of gold.

Fig. 14 is a graph of gold recovery (vertical axis) versus time (horizontal axis) illustrating the effect of fine activated carbon on the thiosulfate leaching of gold from sample S2.

Detailed description of the invention General description

The present invention provides a method for leaching materials containing a precious metal. The method can be carried out before or during thiosulfate leaching and increases the overall recovery of the precious metal from thiosulfate-resistant materials containing the precious metal. Leaching is carried out by means of substantially simultaneous mixing of a pulp comprising a material containing the precious metal, water, thiosulfate, carbon material and dissolved molecular oxygen (as an oxidizer) and leaching in the presence of various components for a predetermined holding time. The carbon material remains in the pulp during leaching to ensure an increase in gold recovery compared to the prior art.

In some embodiments, the thiosulfate lixiviant contacts the suspended precious metal-containing material prior to the carbonaceous material. Typically, at least about 50 mol%, more typically at least about 75 mol%, and even more typically at least about 95 mol% of the thiosulfate lixiviant has been contacted with the suspended precious metal-containing material prior to any or most of the carbonaceous material being added to the pulp. In other words, the suspended precious metal-containing material typically contains greater than about 0.001, more typically greater than about 0.0025, and even more typically greater than about 0.005 M thiosulfate prior to contact with any or most of the carbonaceous material.

In some embodiments, the ion exchange resin is contacted with the suspended precious metal-containing material prior to the carbon material. Typically, at least about 50 wt.%, at least about 75 wt.%, and more typically at least about 95 wt.% of the ion exchange resin is contacted with the suspended precious metal-containing material prior to any or most of the carbon material being added to the slurry.

Surprisingly and unexpectedly, the amount of carbon material required to obtain a relatively high gold recovery (e.g., at least about 80%, more typically at least about 85%, more typically at least about 90%, and most typically at least about 95%) is relatively low compared to the pretreatment process of U.S. Patent No. 10,161,016. Without being bound by theory, it is believed that the carbon material and the precious metal form a galvanic couple and that the rate of oxygen reduction is increased due to the increased surface area available on the carbon material. The use of activated carbon, in particular, as a catalyst in thiosulfate leaching of precious metal may not require the addition of a surrogate oxidizer (such as copper), and it can operate at lower temperatures, making the process more attractive for gold leaching when the use of cyanide is eliminated. Reduced coal requirements allow the carbon material to be disposed of as waste along with tailings without significantly reducing the economic efficiency of the process.

The precious metal containing material may be an oxide ore, concentrate, tailings, leach residue, roasted material, and other oxide materials containing precious metal. Typical oxide ores and concentrates containing precious metal may contain silicates, phosphates, iron oxides, and hydroxides, as well as relatively low levels of residual sulphides.

The optimum concentration of thiosulfate in the solution to be maintained during leaching, and hence the optimum concentration of thiosulfate in the thiosulfate lixiviant solution, will depend upon the nature of the material being leached, but will typically be from about 0.005 to about 2 moles (M), more typically from about 0.02 to about 0.5 M, and still more typically from about 0.05 to about 0.2 M. The source of fresh thiosulfate 148 may be any available thiosulfate-containing compound such as sodium thiosulfate, potassium thiosulfate, calcium thiosulfate, ammonium thiosulfate, or some other thiosulfate-containing material or thiosulfate precursor. In some embodiments, the thiosulfate lixiviant contains substantially no ammonia, with no more than about 25%, more commonly no more than about 10%, and even more commonly no more than about 5% of the thiosulfate combined with ammonia.

While leaching can be carried out at atmospheric, subatmospheric or superatmospheric pressures and over a wide range of pulp temperatures, the catalytic effect of the carbon material unexpectedly makes it possible to carry out leaching rapidly at temperatures as low as

- 5 048166 only 30 degrees Celsius.

The leaching option will be discussed with reference to Fig. 1.

In a typical leaching process, a precious metal-containing material 100 is mixed in a mixing tank, vessel or other suitable reactor with an oxidizer 108, a thiosulfate lixiviant 110 and a carbonaceous material 118, such as activated carbon, thiosulfate and water 104 to form a leachable slurry 116. The oxidizer 108, typically molecular oxygen, is typically contacted by bubbling the slurry. The molecular oxygen may be supplied from a suitable source, such as air, oxygen-enriched air or technically pure oxygen, preferably atmospheric air. Any water, such as raw water or relatively pure process water, may be used in the process. Other suitable reactors, such as a pulse column, may be any reactor capable of properly mixing the carbon, the suspended precious metal-containing material and the gas.

However, it has been unexpectedly and surprisingly discovered that relatively high leaching kinetics can be realized without the pretreatment step disclosed in U.S. Patent 10,161,016. Therefore, a typical embodiment of the leaching process of FIG. 1 eliminates (or is carried out without) the pretreatment step. Typically, the various components, namely, the precious metal-containing material 100, the water 104, the thiosulfate lixiviant 110, and the carbonaceous material 118, are added substantially simultaneously (i.e., before the leaching of the precious metal begins or before the pulp is contacted with the ion exchange resin in a resin-on-leaching or resin-in-pulp leaching process, or before solvent extraction in a solvent extraction leaching process). In some embodiments, the various components are added before the oxidizing agent 108 is added, although in other embodiments, the oxidizing agent 108 may be added substantially simultaneously with the other components. In some embodiments, the precious metal containing material 100, water 104, and carbon material 118 are mixed before, during, or immediately after milling, but before adding the oxidizing agent 108. The thiosulfate lixiviant 110 may or may not be added before or during milling.

As shown in Fig. 1, the thiosulfate lixiviant 110 in the reactor leaches one or more precious metals from the precious metal-containing material 100. Due to the presence of the carbonaceous material 118 during leaching, other surrogate oxidants 108 (in addition to molecular oxygen) are generally not required during the extraction 120 of the precious metal.

The absence or substantial absence of (added and/or total) copper in the leachate during precious metal recovery 120 can significantly simplify the process. The elimination or near elimination of (added and/or total) copper from the leachate during precious metal recovery 120 can provide the advantage of being able to achieve consistently high and reproducible precious metal recoveries over a wider pH range than has previously been possible with other thiosulfate leaching processes. The oxidative decomposition of thiosulfate to polythionates and sulfates is markedly accelerated in the presence of copper ions. The oxidative decomposition reactions are significantly slowed at elevated oxygen partial pressures in the absence or near absence of copper. Typically the copper concentration (added and/or total in solution) is no more than about 20 parts per million (ppm), more commonly no more than about 15 ppm, more commonly no more than about 10 ppm, more commonly no more than about 5 ppm, and even more commonly no more than about 2.5 ppm. In this process, leaching may be carried out at a pH of about 7-12, more commonly about 8-11, more commonly about 8-10, and even more commonly at a pH of less than 9. The oxidation-reduction potential (ORP) during precious metal extraction is typically in the range of about 10 to about 350 mV, and more commonly about 150 to about 300 mV (relative to a standard hydrogen electrode (SHE)).

The weight ratio of the carbon material 116 (either added or total, including added and originally present carbon material in the precious metal-containing material) to the precious metal-containing material 100 during the extraction 120 of the precious metal may vary depending on the requirements of the particular ore, the properties of the carbon material itself, and the desired level of precious metal extraction. In some embodiments, for the coarse carbon material 118, the weight ratio of the precious metal-containing material 100 to the carbon material 118 is typically greater than about 50:1, more often about 100:1, even more often at least 125:1, even more often at least 150:1, even more often at least 175:1, and even more often at least 200:1. In some embodiments, the weight ratio of the precious metal-containing material 100 to the carbon material 118 is no more than 1000:1, more often no more than about 850:1, still more often no more than about 750:1, still more often no more than about 650:1, and still more often no more than about 500:1. In other embodiments, lower weight ratios of the precious metal-containing material 100 to the carbon material 118 are used.

In some embodiments, the precious metal-containing material is substantially free of organic carbon. In other words, the precious metal-containing material typically contains no more than about 1 wt.%, more commonly no more than about 0.5 wt.%, and even more commonly no more than

- 6 048166 than about 0.25 wt.%, even more often contains no more than about 0.1 wt.%, even more often contains no more than about 0.05 wt.%, and even more often contains no more than about 0.01 wt.% organic carbon.

Fine particles of carbon in the carbon material 118 are typically not separated from particles of the leached material 100 that does not contain precious metal. After the precious metal is extracted, fine particles of carbon are typically sent to tailings or waste along with the leached material 100 that does not contain precious metal. However, as shown by the optional step 140, the particles of carbon can be separated from the leached material that does not contain precious metal, for example, by screening (for particles of carbon larger than particles of the leached material that does not contain precious metal) or flotation and are discarded separately or reused in the pulp 116 for leaching.

Fine particles of carbon in the carbon material 118 can be obtained by adding coarse coal to the grinding step for grinding the material containing the precious metal together with the coal to obtain a combined material containing the precious metal and the carbon material, fed in the mixing step. The particle size of the completely ground carbon material 118 and the material 100 containing the precious metal is usually at least smaller than the size of 80% of the particles passing through openings of about 48 mesh (300 microns), more often 80% of the particles passing through openings of about 100 mesh (150 microns) and even more often 80% of the particles passing through openings of about 200 mesh (75 microns).

Leaching in the precious metal recovery process is typically accomplished by resin-in-pulp (RIP) processing. Resin-in-pulp processing may be performed in any suitable vessel. The preferred vessel is a pachuca, which is an air-agitated, conical-bottomed vessel with air injected into the bottom of the cone. The advantage of a pachuca is that it reduces the breakage of the resin beads and improves the distribution of the resin beads in the pulp compared to mechanically agitated systems. RIP recovery is preferably performed in four or more vessels connected in series, more preferably the number of pachucas is between four and eight.

The ion exchange resin must have a greater affinity for the precious metal selected for recovery (e.g. gold or silver) than for the carbon material. Accordingly, strongly basic anion exchange (SBA) resins are used, although some medium basic resins can also be used. These resins primarily contain quaternary ammonium functional groups with fixed positive charges. Examples of commercial strongly basic ion exchange resins are Purolite™ A194™ and A500™. The gold uptake efficiency of these resins is independent of pH and they can operate effectively over the entire pH range. Medium basic resins contain a mixture of tertiary, secondary and primary amine groups. Commercial examples are Purolite™ S992™ and BASF™ Aurix™. Examples of other chelating resins include Purolite™ MTS8140™, MTS9200™ and MTS9240™, and DuPont™ AmberSep 91419™, AmberSep 21KXLT™ and AmberLyst A21™. The sorbed precious metal can be eluted with any eluent suitable for the particular resin, such as a thiourea/sulfuric acid eluent, an isothiouronium eluent, a thiol polymer, a polythionate eluent, an alkali such as sodium hydroxide, etc.

Other processes for extracting precious metals may be used, such as cementation (e.g. zinc or aluminium precipitation) and electrowinning.

Experimental part.

The following examples are presented to illustrate some embodiments of the invention and should not be construed as limitations of the invention set forth in the appended claims. All parts and percentages are by weight unless otherwise indicated.

The catalyzed thiosulfate leaching of gold from oxide ores was investigated. The pretreatment process described in U.S. Patent 10,161,016 was tested, but the results were sometimes difficult to reproduce. Since a process that could enhance gold recovery from thiosulfate leaching to such an extent was promising, it was decided to investigate it further. In this regard, the objectives were to understand the effect of activated carbon during pretreatment and during thiosulfate leaching of gold, the reasons for the lack of reproducibility, and the mechanism by which carbon pretreatment could provide high gold recovery from an ore that would otherwise be considered thiosulfate refractory. Experimental work has shown that it is possible to increase gold recovery from some ores without the need for ammonia or copper at low temperatures and atmospheric pressures by adding small amounts of finely dispersed carbon during thiosulfate leaching of gold.

The addition of finely divided activated carbon results in increased dissolution rates and increased gold recovery. It has been suggested that activated carbon and gold form a galvanic couple and that the rate of oxygen reduction is increased due to the increased surface area available on the activated carbon. The use of activated carbon in

- 7 048166 as a catalyst in thiosulfate leaching of gold is an alternative process that may not require the addition of surrogate oxidants (e.g. copper) and which can operate at low temperatures, making it a more attractive option for gold recovery when the use of cyanide is not possible.

In the first set of experiments, the three samples used were ground in a rod mill using tap water at a pulp density of 60 wt% to a _P80 of 75 µm. The ground material was then homogenized and divided into 200 g portions for the leaching experiments. The 200 g portion was then divided into 25 g portions for the main analyses. The solid fraction was analyzed for gold content by smelting followed by dissolution in acid and analyzed by atomic absorption spectrometry (AAS). Some samples were also analyzed for natural carbon content; total carbon was determined using a LECO analyzer, organic carbon was also determined using a LECO analyzer after removal of inorganic carbon by dissolution in HCI. Inorganic carbon was determined as the difference between total carbon and organic carbon. The solutions were analyzed for gold by AAS, for copper and other metals by inductively coupled plasma (ICP) spectrometry, and for thiosulfates and polythionates by ion chromatography.

Resin in leach (RIL) experiments using calcium thiosulfate were carried out in a 1 L stainless steel stirred tank. Unless otherwise stated, the experimental conditions were: 50 °C, pH 8 (adjusted and maintained with 20 wt % lime slurry), 0.1 M calcium thiosulfate (CaTS), 20 ppm Cu(II) (as _{CuSO4 ^5H2O )} , 0.5 L/min O2, 35 wt % solids, 20 ml/L resin (Purolite A500™ (which is a macroporous polystyrene cross-linked with divinylbenzene)) and a hold time of 24 h; stirring was conducted at 400 rpm. The activated carbon added in the leaching experiments was ground using a mortar and pestle and its particle size was not measured.

300 g of the solid phase was suspended in deionized water to the desired pulp density and heated to the desired temperature in a water bath. Once the reaction temperature was reached, the resin was added to the reaction vessel and then the calculated amounts of calcium thiosulfate and copper sulfate were added to the reactor. Finally, oxygen sparging was started and this was considered the start of the test. The pulp samples were typically aged for 1, 3, 6 and 24 h. The resin in the samples was first screened and returned to the leach tank and the remaining pulp was filtered. The solid phase was dried in an oven at 50 degrees Celsius overnight and then analyzed for gold and, in some cases, natural carbon content. The solutions were analyzed for gold and natural polythionate content. For activated carbon leaching tests, the carbon was ground using a mortar and pestle and its particle size was not measured.

The ore pretreatment conditions were 50 degrees Celsius, pH 8, 0.5 L/min _O2 , 35 wt% solids, 90 g activated carbon/kg ore and 400 rpm. The pretreatment duration was 6 h. After pretreatment, the carbon was screened and the solids were re-slurried to the desired pulp density for the RIL experiment.

The gold grades of samples S1, S2 and S3 were 1.41, 2.54 and 3.63 g/t, respectively. Organic carbon was found only in sample S3 (1.56 wt%). The mineralogical composition of the three samples is shown in Table 1. Although samples S1 and S2 contained 1.6% pyrite, leaching results indicate that most of the gold was liberated.

Table 1

Mineralogical composition of samples

Mineral Rational chemical formula Sample SI Sample S2 Sample S3 Quartz SiO2 96.4 98.3 59.7 Jarosite _KFe3 ( _SO4 )2(OH) ₆ 1,9 Pyrite _FeS2 1,9 1,7 Calcite CaCQ ₃ 6.3 Dolomite CaMg( _CO3 ) ₂ 20.8 Gypsum CaSO4« _2H2O 4.9 Kaolinite _Al2Si2O5 (OH) ₄ 4.2 Mica Kal ₂ (AlSi ₃ Oio)(OH) ₂ 4.1

Figure 2 shows the effect of carbon pre-treatment on gold leaching from sample S1. For comparison, a cyanide leach test was also run and the gold recovery was 81.3%. The gold recovery in the baseline thiosulfate RIL experiment was 26.2% over 24 h. The cyanide and thiosulfate leaching recoveries for sample S1 were 81.3 and

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26.2%, indicating that the ore was refractory to thiosulfate leaching; possible reasons for this refractory are believed to be related to the rate of oxygen reduction at the gold surface. The results of two thiosulfate leaching tests in which the ore was pretreated according to the method of U.S. Patent 10,161,016 are also shown in Fig. 2. Both pretreatments were carried out under identical conditions; both experiments increased the rate of gold dissolution as well as its final recovery to 80.1-85.8%, but the experiment was not reproducible. Nevertheless, the positive effect of the pretreatment on thiosulfate leaching of gold was evident. Based on these and previous observations, the aim of further experiments was to address two questions: first, how does carbon pre-treatment affect the ore such that gold recovery via thiosulfate leaching is enhanced, and second, why was reproducibility not always achieved in these tests?

An important observation in these tests was that some trithionate (20-40 ppm) and tetrathionate (15-25 ppm) were observed throughout the thiosulfate leach tests where the ore was pretreated with carbon, but were not observed in the baseline experiment. This observation led to the hypothesis that some fine activated carbon particles were carried over into the thiosulfate leach and that this carbon was responsible for the increased gold recovery. In fact, the residues from the leach tests where the feed was pretreated were analysed for organic carbon and found to contain 0.08 wt% (no carbon was present in the samples initially).

To investigate this hypothesis, additional tests were conducted in which fine activated carbon (1-10 g/kg ore) was added during the RIL leaching. The effect of adding fine activated carbon (1-10 g/kg) to the CaTS-RIL tests is shown in Figure 3. First, increasing the carbon loading from 1 to 5 g/kg resulted in increased gold dissolution kinetics and an increase in gold recovery from 72.3 to 83.0%; a further increase in the carbon loading to 10 g/kg also resulted in an increased recovery (84.4%) and a slightly higher rate. Second, with 1 g/kg carbon loading, the resulting dissolution curve was similar to that obtained in the first carbon pretreatment experiment. Finally, duplicate experiments at 5 g/kg carbon loading gave reproducible results. These results support the hypothesis that the fine activated carbon carried over into the RIL was responsible for the increased recovery observed during RIL following standard carbon pretreatment as described in U.S. Patent 10,161,016.

Figure 4 shows the gold dissolution curves for sample S2 in the cyanide and thiosulfate leaching experiments. The NaCN concentration was increased from 0.2 to 2.0 g/L to increase the gold recovery from 56.1 to 74.6%; although not shown in Figure 4, the recovery in the 2 g/L NaCN test after 48 hours of leaching was 82.0%. Gold dissolution from sample S2 was slower than from sample S1, regardless of the lixiviant used. Gold recovery by thiosulfate leaching in the absence of carbon was 15.4%, but it is interesting to note that recovery increased during the first six hours, when recovery was 26.4%, and then decreased towards the end of the experiment. This experiment was repeated but the same pattern of change was observed. The results indicate that gold was released from the resin and then precipitated back onto the solid phase. It should also be mentioned that no gold was detected in solution during this experiment. However, the addition of fine carbon in the thiosulfate leaching experiments with sample S2 also increased gold recovery to 61.4 and 76.4% when fine carbon was added at 10 and 20 g/kg, respectively.

Finally, Figure 5 shows the effect of fine carbon on the gold recovery by thiosulfate from sample S3. Sample S3 was not as refractory to thiosulfate as samples S1 and S2. The addition of fine carbon did not significantly affect gold recovery in this test; in fact, after adding carbon to the thiosulfate leach, gold recovery decreased from 69.7 to 68.6%. In the context of the present work, the main difference between sample S3 and samples S1 and S2 may be the carbon content of each ore (in addition to differences in mineralogical composition). Sample S3 had an organic carbon content of 1.56 wt%, while no carbon was detected in either sample S1 or S2. Thus, it is suggested that the carbon originally present in sample S3 played a similar role to the addition of activated carbon. Since carbon was present in the baseline experiment, the addition of fine carbon was not expected to have a significant effect on gold recovery from sample S3.

The leaching results obtained with the three samples support the hypothesis that fine activated carbon included in the thiosulfate leach test was responsible for the increase in recovery observed after the standard carbon pretreatment disclosed in U.S. Patent 10,161,016. The patent discloses an increase in gold recovery from 60.7 to 71.1% when the pretreatment duration was increased from 6 to 24 hours; however, the patent does not disclose a significant benefit from increasing the temperature or using oxygen instead of air during the pretreatment. These results are consistent with the present results and can also be explained by the hypothesis that fine carbon particles during

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CaTS-RIL are responsible for the increase in gold recovery after pretreatment. That is, increasing the duration of pretreatment probably increases carbon attrition, while the type of gas used or the pretreatment temperature does not affect it. The difference in the amount of carbon introduced into the CaTS-RIL tests as a result of the breakdown during pretreatment may also explain why reproducibility is difficult to achieve in these experiments.

So far, it can only be concluded that the presence of activated carbon during thiosulfate leaching increases gold recovery and that reproducibility of leaching tests can be achieved by controlling the carbon content in these experiments. RIL experiments in the presence of activated carbon have shown that carbon has a positive effect on gold recovery, but they do not allow conclusions to be drawn about the effect of carbon pre-treatment on gold recovery by thiosulfate leaching. Although it is difficult (if not impossible) to prove that carbon pre-treatment has no effect on gold recovery in the RIL process other than by the fact that carbon fines are carried over into the thiosulfate RIL process, it was decided to conduct three different pre-treatment experiments.

Preventing carbon attrition during pre-treatment was difficult, but it was decided to conduct two experiments using sample S1 to try to distinguish the effect of pre-treatment from the effect of carbon during thiosulfate leaching. The first experiment involved pre-attrition of the activated carbon to minimise the amount of carbon fines carried over into the RIL tests; the second experiment involved the use of a special experimental setup. This setup consisted of a mesh pocket with activated carbon in it, which was immersed in a mixing tank containing the slurry containing the sample to be pre-treated; the slurry was also pumped through the mesh pocket to increase contact between the carbon and the solids while minimising carbon attrition.

Figure 6 shows the results of the pre-crushed carbon and the pre-crushed carbon pre-treatments. The recovery in the test with the pre-crushed carbon was the same as in the baseline experiment; that is, the recovery in this test was similar to that observed without the pre-treatment. The recovery in the test with the pre-crushed carbon pre-treatment was greater than that in the test with the first carbon pre-treatment. However, the RIL tailings for the latter experiment were also analyzed for carbon and were found to contain 0.07 wt% organic carbon. The differences in recovery between the virgin and pre-crushed carbon pre-treatments are likely due to the increased attrition of the carbon during the pre-treatment and the amount of fines entering the RIL test. Unfortunately, the tailings for the first carbon pre-treatment test were not analyzed for native carbon content.

After 24 h, 5 g/t fine carbon was added to the thiosulfate leach test, resulting in an increased gold dissolution rate and increased recovery.

The second experiment involved carbon pretreatment of sample S1 followed by removal of the carbon from the solids by flotation prior to thiosulfate leaching. After flotation, the metal content of the flotation tailings from the carbon pretreatment sample was 0.55 g/t; the organic carbon content of the flotation tailings was 0.06 wt%. Except for the difference in the method of contacting the ore with the carbon, the other pretreatment conditions were the same: 50 degrees Celsius, pH 8, 35 wt% solids, 0.5 L/min _O2 , and six hours.

Two RIL tests were conducted on the flotation tailings: a baseline experiment without carbon addition and a RIL test in the presence of 10 g/kg fine carbon (see Figure 7). The 24-hour recovery in the baseline experiment was 3.6%, while the addition of fine carbon resulted in an increase in gold recovery to 63.6% after 6 hours of leaching. The results of these experiments indicate that the carbon pretreatment disclosed in U.S. Patent 10,161,016 does not directly affect the thiosulfate leach and that the fine carbon formed during the pretreatment is responsible for the increased recovery observed during the thiosulfate leach.

To explain the effect of carbon on the thiosulfate recovery of gold, it is currently hypothesized that carbon and gold form a galvanic couple in which carbon acts as a cathode, thus providing an increased area for the reduction of copper or oxygen, and gold acts as an anode and dissolves. Gold is the most noble metal in the absence of lixiviant reagents. However, the presence of lixiviant reagents affects the reduction potential of gold, making gold dissolution in aqueous solution possible, at least from a thermodynamic standpoint. Standard potentials for gold in water and in the presence of thiosulfate are given in Table 2, all values relative to the standard hydrogen electrode (SHE). Table 2 also includes the reduction potentials for gold and carbon under the thiosulfate leaching conditions used in this work (50 °C, pH 8, 0.1 M thiosulfate, 1.24x10 ^-5 M CO2, 5.08x10 ^-8 M Au(S2O3)2 ^3- ).

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Table 2

Standard reduction potentials for gold and carbon

Reaction Standard potential Potential under leaching conditions. Au’+ e ·+ Au E°= 1.83 V Not calculated 4H ⁺ + CO2 +4e -> C + 2H2O E° = -0.21 V E = 0.38 V Au(8gO3)2 ³ ' + e Au+ 2S2O3 ² · E° = -0.15 V E = 0.19 V

In the thiosulfate system, the reduction potential of gold is 192 mV less than that of carbon, meaning that if carbon and gold were to form galvanic couples, the carbon would be cathodically protected by the gold, and gold dissolution would probably be accelerated by the carbon acting as an increased surface for the electrochemical reduction of oxygen (see Fig. 8).

It is known that the rate of dissolution of the gold electrode increases when it is connected to a pyrite electrode. It is also known that the rate of gold dissolution increases in those leaching experiments in which gold was in the presence of pyrite. This has been attributed to galvanic interactions between gold and various sulfides (pyrite, chalcopyrite, chalcocite, marcasite). It has been reported that the rate of dissolution of the gold electrode increases linearly with increasing area of the pyrite electrode, which is equivalent to what is observed in Fig. 9, which shows the net recoveries at various carbon concentrations for the results presented so far.

The influence of various parameters on the stability of carbon-catalyzed thiosulfate leaching of gold was assessed. The aim of the following experiments was not to optimize the process, but to analyze the sensitivity of the recovery to Cu(II) and thiosulfate concentrations, resin addition, and temperature.

Fig. 10 further shows the effect of Cu(II) on gold recovery with thiosulfate in the presence of 10 g/kg carbon. The dissolution curves with and without Cu(II) showed that copper ions have no effect on the net gold recovery or the kinetics of its dissolution. Various authors have reported slow oxygen reduction at the gold surface in ammonia-thiosulfate systems and more recently in ammonia-free systems. To overcome this problem, copper ions have commonly been added to the ammonia-thiosulfate system to enhance gold dissolution; this is possible because ammonia stabilizes the copper ions as Cu(NH3)i+, thus allowing Cu(II) to oxidize gold. In the absence of ammonia, Cu(II) is rapidly reduced to Cu(I), resulting in a lower solution redox potential (Eh) which may be insufficient to achieve acceptable gold dissolution rates; This may help explain why the addition of copper ions to the calcium thiosulfate system does not significantly affect either the dissolution kinetics or the resulting gold recovery.

Oxygen is commonly added to the thiosulfate leach system to regenerate copper ions, but it has also been reported that adding oxygen accelerates the breakdown of thiosulfate in the presence of Cu(II) to trithionate and tetrathionate, which is why oxygen addition and the Eh of the system should be carefully controlled. It has also been reported that adding 50 ppm Cu(II) to a sodium thiosulfate leach system does not affect gold recovery, but increases trithionate and tetrathionate formation.

Figure 10 also shows that the removal of resin from the dough had a negative effect on both the dissolution kinetics and the final gold recovery. The recovery in the absence of resin was 78.7%, whereas in the presence of resin the recovery was 82.3-85.1%. This could be due to the stability of the gold-thiosulfate complex or to the reduction of the concentration gradient at the surface-solution interface, which could affect the rate of gold dissolution.

Reducing the temperature to 30 degrees Celsius (Fig. 10) resulted in slower dissolution kinetics, but with a final recovery of 82.3%, which was close to that obtained at 50 degrees Celsius. Therefore, adding carbon to the thiosulfate leach system may also reduce energy requirements. Unfortunately, due to limitations in the amount of S1 sample available for this work, it was not possible to conduct tests at higher carbon doses and lower temperatures.

Figure 11 shows the effect of thiosulfate concentration on gold recovery from sample S1 in the presence and absence of 10 g/kg carbon. In the absence of carbon, the initial rate of gold dissolution increases with increasing thiosulfate concentration; however, in all tests in the absence of carbon, gold recovery decreases over time after 3-6 h of leaching. The reasons for this behavior were not understood, and it was not possible to investigate further. The addition of carbon helped to solve the problem, and higher recoveries (52-70%) were observed in these experiments. However, it was interesting to note that the final recovery decreased with increasing thiosulfate concentration. It is suggested that

- 11 048166 that such behavior was specific to this sample, and the conclusions cannot be extrapolated to other samples tested in this work.

The above results (except for the carbon treatment tests) are consistent with the tests carried out in the presence of ground activated carbon. However, from a process point of view, this may mean that the added carbon will be lost in the leach tailings. It was therefore decided to carry out a test in which coarse carbon was added to the leach test so that it could be recovered later. Figure 12 shows a comparison of the experiment carried out with coarse activated carbon (70 g/kg) with the tests carried out with fine carbon at two different dosages (1 and 10 g/kg). The initial dissolution rate in the presence of coarse carbon was lower than that observed in the experiments with fine carbon; this was to be expected since the frequency of contact between the coarse carbon and the gold surface would be lower than if fine carbon were used instead. However, the final recovery was 79.4%, which was higher than the recovery in the presence of 1 g/kg fine coal (72.3%), but lower than in the presence of 10 g/kg (84.4%). The amount of coal in the tailings was 0.11 wt%, corresponding to a loss of 1.1 g/kg (~5% loss of coal). To decide whether the use of fine or coarse coal is a better alternative, it is necessary to consider the economics of the process.

Thiosulfate destruction is an important factor from an economic and chemical point of view. The formation of tetrathionate, in particular, was associated with gold losses from gold precipitation reactions. The main thiosulfate destruction products observed in this work were trithionate and tetrathionate, but in some cases pentathionate and hexathionate were also observed, although they were not quantified. Thiosulfate consumption was less than 2.11 g thiosulfate/kg ore, but there was no clear correlation between thiosulfate destruction and operating conditions.

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Table 3

Summary of gold recovery and thiosulfate consumption

Test No. Ore Temp. °C [ _{S2O32 ]} ^M [Cu(II)] mg/l [Resin] ^cm3 /l [C] g/kg Extraction % s ₂ o ₃ ² - consumption g/kg CaTS (1) SI 50 0.10 20 20 0 26.2 0,00 CaTS (2) SI 50 0.10 20 20 Pre-processing 80.1 0,06 CaTS (I) SI 50 0.10 20 20 Pre-processing 85.8 0,00 CaTS (3) SI 50 0.10 20 20 1 72.3 0,00 CaTS (4) SI 50 0.10 20 20 5 83,0 0.61 2 CaTS (13) SI 50 0.10 20 20 5 85.1 0.36 CaTS (8) SI 50 0.10 0 20 5 85.1 0,60 CaTS (9) SI 50 0.10 20 0 5 78.7 0.91 CaTS (10) SI 50 0.10 0 0 5 78.7 2,P CaTS (5) SI 50 0.10 20 20 10 84.4 1,10 CaTS (12) SI 30 0.10 20 20 10 82.3 0.56 CaTS (17) S2 50 0.10 20 20 0 15.4 0.14 CaTS (18) S2 50 0.10 20 20 10 61.4 1.44 CaTS (24) S2 50 0.10 20 20 0 15.4 0.09 CaTS (27) S2 50 0.10 20 20 20 76.4 1.28 CaTS (28) S2 50 0.20 20 20 0 8.3 0,00 CaTS (29) S2 50 0.05 20 20 10 69.7 0.09 CaTS (30) S2 50 0.05 20 20 0 10.2 0,00 CaTS (31) S2 50 0.20 20 20 10 52.4 1,16 CaTS (32) S3 50 0.10 0 20 0 69.7 1.36 CaTS (33) S3 50 0.10 0 20 10 68.6 0.71

Having confirmed that the presence of fine carbon particles was necessary during the thiosulfate leaching of gold to catalyze the reaction, an experiment was conducted to determine whether carbon-solid contact was necessary to achieve this effect. The experiment involved placing coarse carbon in a mesh pocket within the RIL tank to minimize contact between the carbon and the solid phase. Figure 13 shows that the gold recovery for this C-in-mesh experiment was very close to that observed in the baseline experiment. Figure 13 also shows that high dissolution rates and high gold recoveries were obtained when the temperature was reduced to 30°C and when neither copper sulfate nor resin were added to the leaching test.

Finally, Fig. 14 shows the effect of fine carbon on the thiosulfate leach of sample S2. The kinetics of gold dissolution in sample S2 were slower than those observed with sample S1 in the presence of both cyanide and thiosulfate. Compared to sample S1, sample S2 required a longer residence time and a higher NaCN concentration to achieve complete recovery of the exfoliated gold. The baseline RIL-thiosulfate run showed an increase in gold recovery during the first six hours, however, at the end of the run, gold recovery decreased from 42.1 to 15.4%. The recovery of 15.4% calculated from the tailings assay is in good agreement with the recovery calculated from the gold in resin assay (16.1%). It is generally assumed that oxygen reduction is slow and that a catalyst is needed to obtain acceptable kinetics. The results above show that in the presence of activated carbon,

- 13 048166 for gold leaching at an acceptable leaching rate, copper is not necessary.

The effect of activated carbon on the thiosulfate leaching of gold is investigated. Fine activated carbon is shown to enhance gold recovery and its dissolution kinetics in a calcium thiosulfate leaching system. Carbon and gold are assumed to form a galvanic couple, with the additional area provided by the activated carbon increasing oxygen recovery, thus enhancing the electrochemical oxidation of gold. In the presence of fine carbon, the addition of copper ions does not affect either gold recovery or its dissolution rate; it is also possible to lower the leaching temperature from 50 to 30 degrees Celsius and still obtain high gold recovery. Coarse carbon can be used instead of fine carbon to reduce carbon loss in tailings. Thiosulfate destruction was in the range of 2.11 kg/t, with trithionate and tetrathionate being the main destruction products.

The present results make the use of activated carbon as a catalyst in thiosulfate leaching of gold an attractive process alternative in situations where cyanide cannot be used. This process, however, will be limited to stirred/fluidized bed reactors and cannot be applied to heap leaching.

Many variations and modifications of the invention may be used. It is possible to implement some features of the invention without implementing others.

For example, in one alternative embodiment, powdered activated carbon is added to a slurry containing thiosulfate, a precious metal containing material, and an oxidizing agent during precious metal leaching.

In another alternative embodiment, the precious metal-containing material is first pretreated in accordance with US 10,161,016, after which the carbon is removed and thiosulfate and finely divided carbonaceous material are added. In an optional pretreatment step (not shown in Fig. 1), carried out prior to leaching, the precious metal-containing material is mixed with the carbonaceous material and water to form a slurry and, after the pretreatment, the slurry is contacted with a thiosulfate lixiviant. Accordingly, the pretreatment is usually carried out in the substantial absence of a thiosulfate lixiviant. Typically, the pretreatment slurry contains no more than about 0.02 M, more typically no more than 0.01 M, and still more typically no more than 0.005 M thiosulfate. Suitable reaction conditions during the optional pretreatment can provide relatively high leaching kinetics. Typically, the pretreatment process is conducted at atmospheric pressure and temperature, although the use of a higher operating temperature (e.g., typically about 25 or higher, more typically about 35°C or higher, and more typically about 50°C or higher, and in some embodiments typically in the range of about 25 to about 65°C) may provide increased reaction kinetics. The pH of the pulp is typically about pH 7 or higher, more typically about pH 8 or higher, and more typically pH 9 or higher. The oxidation reduction potential (ORP) of the pulp is typically greater than about 100 mV, more typically greater than about 200 mV, and typically less than about 750 mV, and more typically less than about 500 mV (Ag/AgCl electrode). The rate of bubbling of the oxidizing agent (e.g., molecular oxygen) through the pulp during precious metal recovery is typically in the range of about 0.05 to about 5, and more typically about 0.10 to about 2.5 L O ₂ /L pulp/min. The residence time of the pulp in the mixing tank is typically in the range of from more than about 1 hour to about 24 hours, depending on the temperature, the concentration of dissolved oxygen in the solution, and the type of ore. Typically, the pulp density is in the range of from about 20 to about 50 wt.% solids, but may be as low as about 1% or as high as about 60%. Typically, the solids content of the pulp is in the range of from about 20 to about 30 wt.%. As such, the processing conditions, in particular the time and temperature of the leaching process, the amount of carbon material, and the rate of oxygen addition, are adjusted to optimize the recovery of the precious metal. This option may require lower fine carbon consumption to achieve relatively high precious metal recovery.

In another embodiment, during the thiosulfate leaching, in addition to the ion exchange resin (which has a greater affinity for the dissolved precious metal than for the carbon material), a coarse carbon material is added, then after the leaching step and before the sediment is removed to the tailings, the carbon material is removed, for example, by screening. The coarse carbon material has a P ₈₀ size of at least about 80 mesh (0.177 mm), more often at least about 50 mesh (0.297 mm). Typically, the coarse carbon material includes, for example, granular activated carbon, extruded activated carbon and activated carbon in the form of beads.

The present invention in its various embodiments, configurations, or aspects includes the components, methods, processes, systems, and/or devices generally depicted and described herein, including various embodiments, configurations, aspects, subcombinations, and subgroups of the present invention. Upon review of the present invention, those skilled in the art will understand how to make and use the present invention. The present invention in its various variations

- 14 048166 embodiments, configurations and aspects include the provision of devices and processes that lack elements not shown and/or described in the present invention or in various embodiments, configurations or aspects, including the absence of such elements that could be used in previous devices or processes, for example, to improve performance, achieve simplicity and/or reduce the cost of implementation.

The foregoing discussion of the invention is presented for purposes of illustration and description. The foregoing does not limit the invention to the embodiment or embodiments disclosed herein. In the foregoing detailed description, for example, various features of the invention are grouped together in one or more embodiments, configurations, or aspects in order to facilitate disclosure of the invention. Features of embodiments, configurations, or aspects of the invention may be combined into alternative embodiments, configurations, or aspects other than those discussed above. This manner of disclosure should not be construed as reflecting an idea that the claimed invention requires more features than are expressly recited in each claim. Rather, as reflected in the following claims, the inventive aspects include fewer features than all the features of a single embodiment, configuration, or aspect disclosed above. Accordingly, in the claims below, incorporated into this detailed description, each independent claim is an independent preferred embodiment of the invention.

Furthermore, although the description of one or more variants, configurations or aspects and some variations and modifications are included in the description of the invention, other variations, combinations and modifications are within the scope of the invention, for example, as may be understood by those skilled in the art after reading the present invention. It is intended to obtain rights that include alternative variants, configurations or aspects, to the permitted extent, including alternative, interchangeable and/or equivalent structures, functions, ranges or steps to the claimed ones, regardless of whether such alternative, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed in the present description, and without the intention to publicly disclose any patentable subject matter of the invention.

Claims (5)

1. A thiosulfate leaching method for extracting gold and/or silver from gold- and/or silver-containing material, comprising: a) contacting a gold- and/or silver-containing material with a carbon-based material, a thiosulfate lixiviant, and an anion exchange resin to form a pulp, wherein the weight ratio of the gold- and/or silver-containing material to the carbon-based material is greater than 125:1, and the carbon-based material comprises one or more of activated carbon, activated charcoal, coke, solid carbon obtained from coconut shell and/or elemental carbon, calcined resin, and mixtures thereof; b) subsequently contacting the pulp with an oxidizing agent to leach gold and/or silver from the gold- and/or silver-containing material and precipitating the leached gold and/or silver on an anion exchange resin; and c) contacting the gold- and/or silver-containing anion exchange resin with the eluent to form a gold- and/or silver-containing eluent and a de-metalized anion exchange resin sent for reuse to step (a). 2. The method according to claim 1, characterized in that the gold- and/or silver-containing material is contacted with most or all of the anion exchange resin before the gold- and/or silver-containing material is contacted with most or all of the carbon-based material, contains no more than 0.01 wt.% organic carbon before contacting the carbon-based material, and is not pre-treated by pre-contacting with the carbon-based material and an oxidizer, and the average particle size of the carbon in the carbon-based material is greater than the average particle size of the gold- and/or silver-containing material, wherein the method after step (c) further comprises removing the carbon-based material from the leached gold- and/or silver-containing material to form a carbon-depleted pulp, and the carbon-based material is removed by screening. 3. The method of claim 1, further comprising removing from the pulp after step (c) at least 95% of the carbon from the carbon-based material, after step a) the pulp contains more than 0.005 M thiosulfate, the gold- or silver-containing material is in contact with most or all of the thiosulfate lixiviant prior to contact with the oxidizing agent, the gold- or silver-containing material contains more oxides than sulfides and contains no more than 1 wt.% organic carbon prior to contact with the carbon-based material, and the degree of gold and/or silver recovery by thiosulfate leaching of the gold- and/or silver-containing material in the absence of prior contact with carbon is less than the degree of gold and/or silver recovery by thiosulfate leaching of the gold- and/or silver-containing material after prior - 15 048166 contact with coal. 4. The method of claim 1, wherein the gold and/or silver-containing material is contacted with most or all of the thiosulfate prior to contacting the gold and/or silver-containing material with most or all of the carbon-based material, the anion exchange resin comprises a strongly basic or moderately basic resin, the oxidizing agent is molecular oxygen, and the pulp during the subsequent contacting step has a pH of from pH 7 to pH 12, a thiosulfate concentration of from 0.005 to 2 M, a total copper concentration of no more than 20 ppm, and an oxidation-reduction potential in the range of from 100 to 350 mV vs. standard hydrogen electrode (SHE), and the contact rate of molecular oxygen with the pulp during the subsequent contacting step (b) is at least 0.10 L O ₂ /L pulp/min, and the weight ratio of gold and/or silver-containing material to carbon-based material is more than 200:1, wherein the gold and/or silver-containing material contacts most or all of the anion exchange resin prior to contact with the oxidizing agent, the gold and/or silver-containing material is an oxide ore that is amenable to gold and/or silver recovery by cyanidation, and at least a portion of the gold and/or silver-containing material is refractory to gold and/or silver recovery by thiosulfate leaching. 5. The method according to claim 1, characterized in that the gold- and/or silver-containing material contains a combination of gold and silver, and the gold and silver are extracted from the gold- and/or silver-containing material by a thiosulfate leaching process.