Field of the invention

The present invention relates to methods of purifying precious metals from precious metal-bearing materials.

Background of the invention

Methods of extracting precious metals from precious metal bearing materials are known in the art. Generally, these methods involve extracting the precious metal from the precious metal bearing material (usually referred to as the “leaching” of precious metal from the precious metal bearing material) and then capturing the leached precious metal from the solution.

Various means to achieve the leaching of the precious metal from the precious metal bearing material exist in the art, each with their own specific chemistry. For example, some use highly poisonous inorganic cyanides to extract the precious metal from the precious metal bearing material. Such processes are associated with considerable environmental concerns (where accidental leakages can result in environmental contamination) as well as considerable health and safety concerns (where inadvertent cyanide exposure can cause notable human health concerns). Further, although such processes have utility for gold and silver recovery, they are of limited utility when extracting other precious metals, such as platinum or palladium. Cyanide processes are generally carried out at an alkaline pH, typically around a pH of 11.5. The alkaline pH brings with it a specific chemical environment that is different to that associated with extraction processes that utilise acidic pHs.

Also in the art is WO 2017/158561 , which uses a different means to achieve the leaching of the precious metal from the precious metal bearing material, this method having its own specific chemistry. In WO 2017/158561 , precious metals are leached from the ore using an aqueous acidic oxidant mixture, and so associated with such methods is an acidic pH. Extracting the precious metal from the precious metal bearing material in WO 2017/158561 achieves good isolation of the precious metal whilst avoiding the concerns associated with using cyanide. In WO 2017/158561 , the capturing of the leached precious metal from the pregnant solution is preferentially achieved by using cyclodextrin. Although capturing the leached precious metal using cyclodextrin has its benefits, such as the ability of cyclodextrin to capture the precious metal at acidic pHs, the inventors of the present application made the realisation that this method generally requires the precious metal bearing material to contain above a certain level of precious metal, generally above 100ppm. This therefore limits the applicability of cyclodextrin as the means for capturing the metal from solution in extraction processes, as the levels of precious metals in precious metal-bearing material can vary. For example, typical levels of gold in ore are 1 ppm. Higher levels of gold in ore do exist - such as up to 100ppm, and possibly also 5000ppm - but such very high levels typically require a pre-concentration step.

Accordingly, there remains a need for new methods of purifying precious metals from precious metal-bearing materials.

Summary of the invention

In a first aspect there is a method of purifying precious metal from a precious metal-bearing material comprising the steps of: a) forming an aqueous acidic oxidant mixture from an oxidant, an acid, and a source of bromide ion, said aqueous acidic oxidant mixture having a pH in the range of 0 to 6; wherein the oxidant is one or more of hydrogen peroxide, ozone, chlorine, hypochlorous acid, hypobromous acid, a hypochlorite salt, and a permanganate salt; b) contacting the aqueous acidic oxidant mixture with a resin and the precious metal bearing-material, to oxidise the metal and form a metal resin complex; and c) recovering the metal from the metal resin complex; wherein the resin is an ion exchange resin comprised of an organic polymer backbone to which a series of functional groups are attached, said functional groups containing at least one heteroatom.

It has been surprisingly found that, when using the specific aqueous acidic oxidant mixture of step a) as the leaching means, the use of an ion exchange resin with functional groups containing at least one heteroatom provides improved purification of precious metal from a precious metal-bearing material. The method is applicable to precious metal-bearing materials that contain a range of precious metal contents, and is effective even when the level of precious metal in the metal-bearing material is low. The method also provides improved metal capture compared with capturing using activated carbon.

Without wishing to be bound by theory, it is thought that the precious metal that has been leached using the aqueous acidic oxidant mixture can be effectively captured from solution by the ion exchange resin by virtue of the at least one heteroatom within the series of functional groups. It is thought that the at least one heteroatom coordinates with the leached metal, thereby capturing the metal onto the resin, allowing the metal to be recovered. The capture by the ion exchange resin is effective in the acidic pH environment associated with the use of the aqueous acidic oxidant mixture.

Detailed description

Used throughout, the language “up to” means “up to an including”.

As used herein, the term “precious metal” refers to metals such as gold, silver, platinum, palladium, rhodium, copper, titanium, and rare earth metals. The skilled person will understand that the term “rare earth metal” refers to the metals yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and scandium.

Preferably, the precious metal is one or more of gold, silver, platinum, palladium, rhodium, copper, titanium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and scandium.

More preferably, the precious metal is one or more of gold, silver, platinum, palladium, rhodium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and scandium.

More preferably, the precious metal is one or more of gold, silver and copper.

More preferably, the precious metal is gold and/or silver. Most preferably, the precious metal is gold.

As used herein, the term “precious metal-bearing material” refers to the material that contains the precious metal. The precious metal-bearing material may comprise one or more precious metals. The precious metal-bearing material may contain several precious metals from which one or more than one such metal is desired to be isolated. The method of the invention may recover two or more metals, either in admixture, or, preferably separately. The material may also comprise additional metals further to the one or more precious metals, including impurities such as arsenic, mercury and lead. The invention has particular utility in selectively extracting one or more desired precious metals from the precious-metal bearing material thereby separating the precious metal from such impurities.

The precious metal-bearing material can take various forms, as will be appreciated by the skilled person. The precious metal-bearing material can be selected from an ore (a naturally occurring rock or sediment), including a concentrate of such ore, sea water, waste material, a metal mixture, a human body component, a medical device, or a consumer product. Examples of an ore include mineral deposits (such as mineral veins) and the like obtained from waterways, causeways, mines, and other Earth-bound sources known in the art. Examples of a human body component include teeth, bones, heart, muscle, joints, legs, arms, hands, fingers, knees, feet, among others. Examples of medical devices include life support systems and devices, such as a diagnostic machine, a dialysis machine, a medical implant (for example, a pacemaker), a tooth filling, tooth enamel, tooth inlay, dentures, an artificial joint, an artificial limb or other artificial appendage, or materials removed after diagnostic, radiodiagnostic or therapeutic administration that comprise e.g. metal-containing nanoparticles. Examples of consumer products include a jewellery item, an electronics item, and other metal products such as an ingot, bar or currency coin. Examples of a jewellery item include a ring, a bracelet and a necklace, among others. Examples of an electronics item include a computer, a monitor, a power supply, an amplifier, a preamplifier, a digital to analog converter, an analog to digital converter, and a phone, among others. Examples of waste material includes tailings from previous mining efforts, bio-waste, and waste derived from sewer plants.

The skilled person will appreciate that the precious metal-bearing material may contain various levels of precious metal, depending on the type of precious metal-bearing material, and the type of precious metal(s) that it contains. The methods disclosed herein are effective at a variety of levels, and are effective at high levels of precious metal, but are also effective even when there are low levels of precious metal in the precious metal-bearing material. For example, the precious metal-bearing material may comprise the precious metal in an amount ranging from 0.1 ppm to 10000 ppm. The precious metalbearing material may comprise the precious metal in levels of at least 0.1 ppm, at least 1 ppm, or at least 5 ppm. The precious metal-bearing material may comprise the precious metal in levels of up to 10 ppm, up to 30 ppm, up to 50 ppm, up to 100 pm, up to 500 ppm, up to 800 ppm, or up to 10000 ppm.

Preferably, the precious metal-bearing material is an ore that contains gold and/or silver. More preferably, the precious metal-bearing material is an ore containing gold at a level of 0.1 to 5000 ppm and optionally also containing silver at a level of 1 to 500 ppm.

As will be appreciated by the skilled person, an ore may be subjected to preliminary steps such as reducing the size of its particles and/or agglomerating particles to provide controlled size agglomerates. The ore may be reduced in size so as to be processable as fluid slurry, and brought into contact with the oxidant mixture in vats.

Disclosed herein, there is the step of forming an aqueous acidic oxidant mixture from an oxidant, an acid, and a source of bromide ion, said acidic oxidant mixture having a pH in the range 0 to 6. This is denoted herein as step a). Formation of an aqueous acidic oxidant mixture is taught in WO 2017/158561 . In step a), without wishing to be bound by theory, it is thought that the oxidant reacts with the bromide ion to form bromine species including BrOH.

Disclosed herein, the “aqueous acidic oxidant mixture” refers to the mixture formed when combining the oxidant, acid, and a source of bromide ion, in an aqueous medium. The water present within the aqueous medium may be tap water, well water distilled water, or sea water. The oxidant is one or more of hydrogen peroxide, ozone, chlorine, hypochlorous acid, hypobromous acid, a hypochlorite salt, and a permanganate salt. The use of these oxidants results in a more environmentally friendly process with fewer health and safety concerns.

Preferably, the oxidant is one or more of hydrogen peroxide, ozone, chlorine, hypochlorous acid, a hypochlorite salt, and a permanganate salt.

Various amounts of oxidant may be used to form the aqueous acidic oxidant mixture. Preferably, the amount of oxidant added to form the aqueous acidic oxidant mixture relative to the amount of the source of the bromide ion added to form the aqueous acidic oxidant mixture is in the range of 0.1 : 1 to 10: 1 by weight, more preferably in the range of 1 :1 to 10:1 by weight.

The skilled person will appreciate that “ozone” refers to O3. Ozone can be introduced as a gas by bubbling through the remaining components to form the aqueous acidic oxidant mixture. Ozone can be generated in situ by a variety of possible methods, as will be appreciated by the skilled person.

The skilled person will appreciate that “chlorine” refers to CI2. Chlorine can be introduced as a gas by bubbling through the remaining components to form the aqueous acidic oxidant mixture. Chlorine can be generated in situ by a variety of possible methods, as will be appreciated by the skilled person.

The skilled person will appreciate that “hypochlorous acid” refers to HOCI. This may be provided for use in the process as part of an aqueous solution which can vary in concentration, possible concentrations being 5-100 wt.%, 5-70 wt.%, 20-70 wt.%, 30-70 wt.%, or 30-60 wt.%.

The skilled person will appreciate that “hypobromous acid” refers to HOBr. This may be provided for use in the process as part of an aqueous solution which can vary in concentration, possible concentrations being 5-100 wt.%, 5-70 wt.%, 20-70 wt.%, 30-70 wt.%, or 30-60 wt.%.

The skilled person will appreciate that “hypochlorite salt” refers to any salt capable of generating the hypochlorite anion (CIO-) in solution. Preferably, the hypochlorite salt is an alkali metal hypochlorite salt e.g. lithium hypochlorite, potassium hypochlorite, and/or sodium hypochlorite. Preferably, the hypochlorite salt is sodium hypochlorite. The skilled person will appreciate that “permanganate salt” refers to any salt capable of generating the permanganate anion (MnC ) in solution. Preferably, the permanganate salt is an alkali metal permanganate salt e.g. lithium permanganate, potassium permanganate, and/or sodium permanganate. Preferably, the permanganate salt is potassium permanganate.

Preferably, the oxidant is hydrogen peroxide and/or ozone.

More preferably, the process disclosed herein comprises the step a) of forming an aqueous acidic oxidant mixture from hydrogen peroxide, an acid, and a source of bromide ion, said aqueous acidic oxidant mixture having a pH in the range of 0 to 6.

The skilled person will appreciate that “hydrogen peroxide” refers to H2O2. The use of hydrogen peroxide provides an economic, commercial and environmental preference to other oxidants. More specifically, hydrogen peroxide is available commercially at very large scale and the breakdown products are water and oxygen, resulting in environmentally friendly credentials.

Various amounts of hydrogen peroxide may be used to form the aqueous acidic oxidant mixture. Preferably, the amount of hydrogen peroxide added to form the aqueous acidic oxidant mixture relative to the amount of the source of the bromide ion added to form the aqueous acidic oxidant mixture is in the range of 0.1 :1 to 10:1 by weight, more preferably in the range of 1 :1 to 10:1 by weight.

Hydrogen peroxide may be provided for use in the process as part of an aqueous solution which can vary in concentration, possible concentrations being 5-100 wt.%, 5-70 wt.%, 20-70 wt.%, 30-70 wt.%, or 30-60 wt.%.

Hydrogen peroxide is available from many companies on a commercial basis and can be supplied at large scale by road or rail. These companies include but are not limited to PeroxyChem, Solvay GmbH, Kemira, Arkema. Some companies offer the concept of on site generation, which may be compatible with the method disclosed herein.

The acid referred to in step a) of the method disclosed herein is preferably sulphuric acid, nitric acid, hydrochloric acid, acetic acid, carbonic acid, formic acid, or combinations thereof. Preferably, the acid referred to in step a) of the method disclosed herein is sulphuric acid and/or nitric acid. The acid may be added as a concentrated acid or as a dilute solution. The skilled person will appreciate the amount of acid required to be added in step a) in order to achieve the stated pH range.

The source of the bromide ion may be supplied in an aqueous solution or dispersion, or in solid form.

The source of bromide ion in step a) of the method disclosed herein can be a bromide salt. Preferably, the source of bromide ion is an alkali metal bromide, more preferably lithium bromide, potassium bromide, sodium bromide, or combinations thereof. Most preferably, the source of bromide ion is sodium bromide.

Various amounts of the source of bromide ion may be used to form the aqueous acidic oxidant mixture. The amount of the source of bromide ion added to form the aqueous acidic oxidant mixture can for example range from 1 to 20 wt.% based on the total weight of the aqueous acidic oxidant mixture. The amount of the source of bromide ion added to form the aqueous acidic oxidant mixture can be at least 1 wt.%, preferably at least 5 wt.% based on the total weight of the aqueous acidic oxidant mixture. The amount of the source of bromide ion added to form the aqueous acidic oxidant mixture can be up to 20 wt.%, preferably up to 15 wt.% based on the total weight of the aqueous acidic oxidant mixture.

The aqueous acidic oxidant mixture of step a) has a pH in the range of 0 to 6. The aqueous acidic oxidant mixture can have a pH of at least 0.1 , at least 0.8, or at least 1 . The aqueous acidic oxidant mixture can have a pH of up to 5, preferably up to 3, more preferably up to 2. The pH is measured by standard methods known in the art, for example using a standard electronic pH meter or colour coded test strip, across temperatures ranging from 5°C to 75°C.

The skilled person will appreciate that the oxidant and the source of bromide ion are not necessarily mutually exclusive terms. For example, the oxidant and the source of the bromide ion may both be hypobromous acid (thereby providing a direct source of BrOH to the aqueous acidic oxidant mixture). However preferably, the oxidant and the source of bromide ion are different chemical species. The skilled person will appreciate that the acid and the source of bromide ion are not necessarily mutually exclusive terms. For example, the source of the bromide ion and the acid may both be hypobromous acid. However preferably, the source of bromide ion and the acid are different chemical species.

The skilled person will appreciate that the acid and the oxidant are not necessarily mutually exclusive terms. For example, the acid and the oxidant may both be hypobromous acid. However preferably, the acid and the oxidant are different chemical species.

More preferably, all three of the oxidant, the source of bromide ion, and the acid, are different chemical species.

Disclosed herein, there is the step of contacting the aqueous acidic oxidant mixture with a resin and the precious metal bearing-material, to oxidise the metal and form a metal resin complex. This is referred to as step b).

In step b), the resin and the precious metal bearing-material can be added to the aqueous acidic oxidant mixture simultaneously, or sequentially. “Sequentially” meaning one after the other. For example, when the resin and the precious metal bearing-material are added to the aqueous acidic oxidant mixture sequentially, the resin can be added before, or after, the addition of the precious metal bearing-material. The timing of these steps can be tailored at the convenience of the processing facilities.

Step b may for example comprise the steps of: contacting the aqueous acidic oxidant mixture with the precious metal bearingmaterial to oxidise the metal and form a metal bromide salt solution; and contacting the metal bromide salt solution with the resin to form a metal resin complex.

It will be understood that the term “metal bromide salt solution” refers to the solution formed by contacting the aqueous acidic oxidant mixture with the precious metal-bearing material.

Step b may for example comprise the steps of: contacting the aqueous acidic oxidant mixture with the resin to form an oxidant resin mixture; and contacting the oxidant resin mixture with the precious metal bearing-material, to oxidise the metal and form a metal resin complex.

It will be understood that the term “oxidant resin mixture” refers to the mixture formed by contacting the aqueous acidic oxidant mixture with the resin.

In step b, the metal in the precious metal bearing-material is oxidised by the aqueous acidic oxidant mixture, and the resin allows the formation of a metal resin complex. As used herein, the oxidation of the metal in the precious metal bearing-material can be referred to as the extracting, or leaching, of the metal from the precious metal bearing-material. It will be understood that the term “metal resin complex” refers to the species formed from the interaction of the oxidised metal (i.e. once extracted from the precious metal-bearing material) with the resin. It will be understood from this that the metal resin complex refers to the resin with the metal complexed thereon or therein.

The precious metal-bearing material contains the precious metal (or metals) M in a variety of possible states, typically ranging from 0 (M°) to 4 (M ⁴⁺ ). Without wishing to be bound by theory, it is thought that in step b) the precious metal of the precious metal bearing-material is oxidised by the species BrOH resulting from step a). It is thought that the precious metal is oxidised to form metal bromide species including MBr _n and/or MBr _n +r where n is the valency of the oxidised metal ion M ⁿ⁺ .

The amount of metal bromide MBr _n and/or MBr _n +r contained in the resulting solution can vary, for example between 1 mg/L to 100 mg/L. The amount of metal bromide MBr _n and/or MBr _n +r contained in the resulting solution can be at least 0.1 mg/L, at least 1 mg/L, at least 2 mg/L, or at least 3 mg/L. The amount of metal bromide MBr _n and/or MBr _n +r contained in the resulting solution can be up to 10000 mg/L, up to 100 mg/L, up to 80 mg/L, or up to 50 mg/L. It will be understood that the term “resulting solution” refers to the solution formed from the combination of the aqueous acidic oxidant mixture and the precious metal bearing-material (including for example the “metal bromide salt solution” disclosed herein). It will also be appreciated that, when the resin and precious metal-bearing material are added either simultaneously or in immediate succession, the metal bromide species MBr _n and/or MBr _n +r may form and then immediately react to result in the formation of the metal resin complex. This scenario is referred to as a “Resin in leach” process.

The amount of metal-bearing material that contacts the aqueous acidic oxidant mixture can be tailored depending on e.g. the type of precious metalbearing material, but can for example be added in amounts of 1 g to 1000 g, preferably 10 g to 800 g, more preferably 30 g to 600 g. The ratio, by weight, of precious metal-bearing material : aqueous acidic oxidant mixture can be tailored depending on e.g. the type of precious metal-bearing material, and the scale at which the method is being conducted, and can for example range from 1 :0.2 to 1 :10000. The ratio, by weight, of precious metal-bearing material : aqueous acidic oxidant mixture can be at least 1 :0.2, preferably at least 1 :1 , more preferably at least 1 :2. The ratio, by weight, of precious metal-bearing material : aqueous acidic oxidant mixture can be up to 1 :10000, up to 1 :1000, up to 1 :10, preferably up to 1 :8, more preferably up to 1 :7.

The resin is an ion exchange resin. The use of the ion exchange resin, following the specific aqueous acidic oxidant mixture of step a) as the leaching means, provides improved capture of the precious metal. The present inventors surprisingly found particular applicability of the ion exchange resin with the functional groups disclosed herein to the particular chemistry resulting from use of the aqueous acidic oxidant mixture of step a).

The term “ion exchange resin” takes its usual definition in the art, and so refers to a material that acts as a medium for ion exchange that is generally insoluble in aqueous mediums. It will be understood from this that the ion exchange medium is substantially insoluble in the aqueous solutions and mixtures disclosed herein, e.g. the aqueous acidic oxidant mixture, and the metal bromide salt solution. By “substantially insoluble”, it is meant that less than 0.1 mg/ml of the resin dissolves in the aqueous solutions and mixtures disclosed herein (e.g. the aqueous acidic oxidant mixture, and the metal bromide salt solution) at 25 °C.

As used herein, definitions relating to the ion exchange resin generally refer to the features of the ion exchange resin per se, i.e. prior to its addition to the aqueous solutions and mixtures disclosed herein. After its addition, it will be understood that, due to the acidic nature of the leaching means of step a), it is possible that protonation of certain groups of the ion exchange resin may occur.

The ion exchange resin is preferably a porous material. The porosity of the ion exchange resin increases the surface area available for ion exchange.

Consistent with what will be understood from the term “ion exchange resin”, the ion exchange resin used in the method disclosed herein is comprised of a polymer backbone (sometimes referred to as a polymer matrix), to which a series of functional groups are attached.

Specifically, in the method disclosed herein, the ion exchange resin is comprised of an organic polymer backbone to which a series of functional groups are attached. The ion exchange resin may essentially consist of an organic polymer backbone to which a series of functional groups are attached.

The ion exchange resins used in the method disclosed herein are commercially available from a variety of sources, with commercially available resins including but not limited to the following, which are all ion exchange resins with polystyrene backbones functionalised with the following groups:

- SEPLITE® LSC660: functionalised with guanidine groups

- SEPLITE® LSC740: functionalised with thiol groups

- SEPLITE® LSC710: functionalised with iminodiacetic acid groups

- AMBERSEP® 21 K XLT Mesh Anion Exchange Resin (CI-): functionalised with quaternary ammonium groups

- Purogold™ MTA5015SO4: functionalised with quaternary ammonium groups

- LEWATIT® MonoPlus TP 214: functionalised with thiourea groups

- Puromet™ MTS9140: functionalised with thiourea groups

- LEWATIT MP 62 WS: functionalised with tertiary amine groups

- LEWATIT TP 106: functionalised with quaternary ammonium groups

As used herein, the term “polymer” takes its usual definition the art and so refers to a homopolymer or copolymer formed from the polymerisation of one or more monomers. As such, this term covers e.g. linear polymers, branched polymers, and cyclic polymers.

As used herein, the term “homopolymer” takes its usual definition in the art, and so refers to a polymer whose polymer chains comprise one type of monomer. As used herein, the term “co-polymer” takes its usual definition in the art, and so refers to a polymer whose polymer chains comprise two or more different types of monomers. The skilled person will appreciate therefore that the term “co-polymer” encompasses polymers that include three different types of monomers (which can at times be referred to in the art specifically as “terpolymers”). The term “block co-polymer” takes its usual definition in the art and so refers to a copolymer whose polymer chains include two or more blocks of monomers. Each block is comprised of a particular monomer type, where at least two of the blocks present comprise a different monomer type to one another. A di-block co-polymer, a tri-block copolymer, and a tetra-block copolymer, each refer to copolymers with two, three, and four monomer blocks respectively.

As used herein, the term “monomer” takes its usual definition in the art and so refers to a molecular compound that may chemically bind to another monomer to form a polymer. Unless expressly stated to the contrary, any monomer referred to herein should be understood to include all enantiomers, diastereomers, racemates and mixtures thereof of the monomers in question.

It will be understood that the term “polymer backbone” refers to the series of covalently bonded atoms that create a continuous molecular chain which acts as a scaffold to which the functional groups are attached. In line with the usual definition in the art, the polymer backbone is generally the longest continuous molecular chain, to which other chains and functional groups may be regarded as being pendant.

It will be understood that the term “organic polymer backbone” refers to a polymer backbone that includes carbon-carbon covalent bonds.

The organic polymer backbone may be crosslinked or uncrosslinked. Preferably, the organic polymer backbone is crosslinked with a crosslinking agent such as divinylbenzene, hexamethylenetetramine, a functionalized silane, isocyanate, peroxide, or combinations thereof. More preferably, the organic polymer backbone is crosslinked with divinylbenzene. The amount of crosslinker can vary, but can be for example 1 % to 50% by weight, based on the total weight of the polymer backbone and crosslinker. The organic polymer backbone can for example be polystyrene, polyvinyl toluene, poly(vinylbenzyl chloride), polyvinyl acetate, polyvinyl butyral, polyvinyl ether, polyethylene, polyurethane, or acrylonitrile butadiene styrene.

Preferably, the organic polymer backbone is a vinyl polymer backbone, which will be understood as referring to a polymer backbone formed from vinyl monomers (i.e. those monomers including in their structure the formula -CH=CH ₂ ). For example, the organic polymer backbone can be polystyrene, polyvinyl toluene, poly(vinylbenzyl chloride), polyvinyl acetate, polyvinyl butyral and polyvinyl ether. More preferably, the organic polymer backbone is polystyrene. In a particularly preferred embodiment, the organic polymer backbone is polystyrene crosslinked with divinylbenzene.

Disclosed herein, a series of functional groups are attached to the organic polymer backbone. It will be understood that this attachment is generally by covalent bonding to the organic polymer backbone. This attachment may be achieved by standard procedures known in the art. By “series” it is meant that there are a plurality of functional groups attached to the polymer backbone. For a given ion exchange resin, the functional groups may be the same or different.

The functional groups contain at least one heteroatom. The term “heteroatom” takes its usual definition in the art, and so refers to an atom that is not carbon or hydrogen. Preferably, the heteroatom is one or more of N (nitrogen), S (sulphur), O (oxygen), and P (phosphorus). More preferably, the heteroatom is one or more of N, S, and O, more preferably one or more of N and S. It will be appreciated that, when the heteroatom is one or more of the listed options, additional heteroatoms other than those recited in this list may also be present in the functional groups. Unless expressly stated to the contrary, the atomic species given for the heteroatom are to be understood as encompassing that species irrespective of whether or not it is in a neutral state. In particular, when the series of functional groups contain N, this encompasses scenarios where the N is positively charged, for example as part of a quaternary ammonium group.

Without wishing to be bound by theory, it is thought that the at least one heteroatom coordinates with the oxidised metal (i.e. the metal extracted from the precious metal-bearing material), thereby capturing the precious metal and forming the metal resin complex. Therefore, the ion exchange resins used in the method disclosed herein may be referred to as chelating resins.

Particularly preferred is when the series of functional groups includes one or more of an iminodiacetic acid group, a thiourea group, a quaternary ammonium group, a guanidine group, an amine group, and a thiol group. The skilled person will be familiar with the molecular structure implied by these groups. Particularly good metal extraction is achieved when the ion exchange resin is functionalised with these groups. Within this embodiment, the functional groups of the series can be iminodiacetic acid groups, thiourea groups, quaternary ammonium groups, guanidine groups, amine groups, or thiol groups.

Even more preferably, the series of functional groups includes one or more of a thiourea group, a quaternary ammonium group, and a guanidine group. Particularly good metal extraction is achieved when the ion exchange resin is functionalised with these groups. Within this embodiment, the functional groups of the series can be thiourea groups, quaternary ammonium groups or guanidine groups.

The skilled person will be familiar with appropriate techniques to form the resulting structure of the organic polymer backbone to which the series of functional groups are attached. Indeed, the polymer backbone with the series of functional groups attached can be formed by standard reaction procedures known in the art, with the attachment between the functional group and the backbone being located at an appropriate point of the molecular framework of the functional group, as will be appreciated by the skilled person. For example, the resulting structure may be achieved directly by polymerisation of one or more monomers. Or, the resulting structure may be achieved by first forming the polymer backbone, and then subsequently introducing the functional groups to the polymer backbone.

The term “iminodiacetic acid” refers to the formula HN(CH ₂ CO2H) ₂ . When the series of functional groups includes an iminodiacetic acid group, the functional groups comprise one or more of the following moieties:

The term “thiourea” refers to the formula S=C(NR ¹ R ² )(NR ³ R ⁴ ), where

R ¹ , R ² , R ³ and R ⁴ may be the same or different and are each independently selected from H or an alkyl group. Preferably, R ¹ , R ² , R ³ and R ⁴ are the same or different and are each independently selected from H or a Ci-Ce alkyl group. Within this embodiment, R ¹ , R ² , R ³ and R ⁴ are the same or different and can each be independently selected from H or a C1-C3 alkyl group. More preferably, R ³ and R ⁴ are both H.

When the series of functional groups includes a thiourea group, the functional groups comprise one or more of the following moieties, with R ¹ , R ² , R ³ and R ⁴ taking the same meaning and preferences as those stated above: The term “quaternary ammonium” refers to the formula [NR ⁵ R ⁶ R ⁷ R ⁸ ] ⁺ , where R ⁵ , R ⁶ , R ⁷ and R ⁸ may be the same or different and are each independently selected from H or an alkyl group. Preferably, R ⁵ , R ⁶ , R ⁷ and R ⁸ are the same or different and are each independently selected from H or a Ci-Ce alkyl group. More preferably, R ⁵ , R ⁶ , R ⁷ and R ⁸ are each an alkyl group, preferably a Ci-Ce alkyl group, and may be the same or different. Preferably, R ⁵ , R ⁶ , R ⁷ and R ⁸ are each a C1-C3 alkyl group, and may be the same or different.

When the series of functional groups includes a quaternary ammonium group, the functional groups comprise one or more of the following moieties, with R ⁵ , R ⁶ and R ⁷ taking the same meaning and preferences as those stated above:

The term “guanidine” refers to the formula (R ⁹ R ¹⁰ N)(R ¹¹ R ¹² N)C=N-R ¹³ where R ⁹ , R ¹⁰ , R ¹¹ , R ¹² and R ¹³ may be the same or different and are each independently selected from H or an alkyl group. Preferably, the alkyl group is a Ci-Ce alkyl group, more preferably a C1-C3 alkyl group. More preferably, the term “guanidine” refers to the formula HN=C(NH ₂ )2, which the skilled person will appreciate is non-derivatised guanidine, where R ⁹ , R ¹⁰ , R ¹¹ , R ¹² and R ¹³ are each H.

When the series of functional groups includes a guanidine group, the functional groups comprise one or more of the following moieties, with R ⁹ , R ¹⁰ , R ¹¹ , R ¹² and R ¹³ taking the same meaning and preferences as those stated above:

The term “thiol” refers to the formula R ¹⁴ -SH where R ¹⁴ is an alkyl group, preferably a Ci-Ce alkyl group, more preferably a C1-C3 alkyl group.

When the series of functional groups includes a thiol group, the functional groups comprise one or more of the following moieties:

The term “amine” refers to the formula NR ¹⁵ R ¹⁶ R ¹⁷ , where R ¹⁵ , R ¹⁶ and R ¹⁷ may be the same or different and are each independently selected from H or an alkyl group. Preferably, R ¹⁵ , R ¹⁶ and R ¹⁷ are the same or different and are each independently selected from H or a Ci-Ce alkyl group. More preferably, R ¹⁵ , R ¹⁶ and R ¹⁷ are the same or different and are each an alkyl group, preferably a Ci-Ce alkyl group. Preferably, R ¹⁵ , R ¹⁶ and R ¹⁷ are each a C1-C3 alkyl group, and may be the same or different.

When the series of functional groups includes an amine group, the functional groups comprise one or more of the following moieties, with R ¹⁵ and R ¹⁶ taking the same meaning and preferences as those stated above: As used throughout, it will be understood that the symbol “ denotes the end of the molecular fragment and so refers to the point at which the moieties are attached to the polymer backbone. The moieties may be attached directly to the polymer backbone e.g. by way of a direct bond, or, they may be attached via an alkyl group, such as a C1-C10 alkyl group, preferably a Ci-Ce alkyl group, more preferably a C1-C3 alkyl group.

As used herein, the term “alkyl” refers to a straight or branched saturated or unsaturated alkyl group. Preferably, the alkyl group is a saturated alkyl group. More preferably, the alkyl group is a straight alkyl group. As used herein, the term "(Ca-Cb)alkyl" wherein a and b are integers refers to a straight or branched chain alkyl having from a to b carbon atoms. Thus, by way of example, a C1-C10 alkyl group refers to a group having from 1 to 10 carbon atoms, and so includes methyl, ethyl, n-propyl, isopropyl, n- butyl, isobutyl, sec-butyl, t-butyl, n- pentyl, n-hexyl, heptyl, octyl, nonyl and decyl. Meanwhile, a Ci-Ce alkyl group refers to a group having from 1 to 6 carbon atoms, and so includes methyl, ethyl, n-propyl, isopropyl, n- butyl, isobutyl, sec-butyl, t-butyl, n-pentyl, n-hexyl.

Each of the functional groups disclosed herein has selectivity for different precious metals. Therefore, the functional group can be tailored depending on which precious metal is desired to be extracted. For example, an ion exchange resin functionalised with an iminodiacetic acid group has good metal selectivity for copper. An ion exchange resin functionalised with a thiourea group has good metal selectivity for silver. An ion exchange resin functionalised with a quaternary ammonium group has good metal selectivity for gold and silver, particularly gold. An ion exchange resin functionalised with a guanidine group has good metal selectivity for gold and silver, particularly gold. An ion exchange resin functionalised with a thiol group has good metal selectivity for copper. As a result, a resin with a particular type of functional group can be selected so as to extract a particular precious metal. Alternatively, a series of the resins with a series of different types of functional groups can be used in sequence so as to extract a series of different precious metals.

The process disclosed herein may comprise the additional step of subjecting the material from which the precious metal has been extracted to a decontamination step, said decontamination step comprising the step of contacting the material with one or more of the resins disclosed herein. It has been found that certain functional groups, such as thiol groups, have selectivity for impurities such as arsenic, mercury, and lead. Therefore, not only can certain resins disclosed herein be used to selectively extract precious metals in preference to impurities, but certain resins disclosed herein can then be used to decontaminate the material leftover from the process (sometimes referred to as “tailings”), providing a “clean up” operation for the tailings leftover from the process.

Step b) may further comprise a series of sub steps, where the aqueous acidic oxidant mixture is contacted with a first resin, and then the aqueous acidic oxidant mixture is contacted with a second resin. It will be appreciated that such scenarios will form a first metal resin complex and a second metal resin complex (corresponding to the first and second resins). It will also be understood that the first resin and the second resin are subject to the mandatory, optional and preferred resin definitions disclosed herein, and accordingly are each an ion exchange resin comprised of an organic polymer backbone to which a series of functional groups are attached, said functional groups containing at least one heteroatom. When step b) further comprises a series of sub steps, where the aqueous acidic oxidant mixture is contacted with a first resin, and then the aqueous acidic oxidant mixture is contacted with a second resin, this is preferentially in combination with one of the following orders of addition: (i) the first resin is added to the aqueous acidic oxidant mixture, followed by the addition of the precious metal-bearing material, followed by the addition of the second resin; (ii) the first resin is added simultaneously with the precious metalbearing material to the aqueous acidic oxidant mixture, followed by the addition of the second resin; (iii) the precious metal-bearing material is added to the aqueous acidic oxidant mixture, followed by the addition of the first resin, and then the second resin. More preferably, the addition of the first and second resin is in combination with order of addition (iii). The first resin and the second resin can be the same or different, but are preferably different, where the first resin has a first series of functional groups, and the second resin has a second series of functional groups, where the first and second series are different. This allows for the first resin to selectively extract a first precious metal, and then the second resin to selectively extract a second (and different) precious metal. Preferably, the functional groups of the first series are iminodiacetic acid groups, thiourea groups, quaternary ammonium groups, guanidine groups, amine groups, or thiol groups, and the functional groups of the second series are iminodiacetic acid groups, thiourea groups, quaternary ammonium groups, guanidine groups, amine groups, or thiol groups, wherein the first and second series are different. More preferably, the functional groups of the first series are thiourea groups, quaternary ammonium groups, or guanidine groups, and the functional groups of the second series are thiourea groups, quaternary ammonium groups, or guanidine groups, wherein the first and second series are different. Step b) may include additional sub-steps using third and further different resins with third and further different series of functional groups, with the preferable functional groups being in accordance with those disclosed above.

Although not essentially required, the method disclosed herein is compatible with being following by an additional metal capture step using activated carbon.

The resin may be provided as a plurality of beads with a particle size distribution such that more than 95% of the particles have a diameter of 0.1 to 10mm, 0.2 to 5mm, 0.1 to 2.5mm or 0.2 to 1.5mm. The bulk density can vary, for example from 100 g/l to 2000 g/l, preferably from 200 to 900 g/l, more preferably from 500 g/l to 900 g/l, or from 600 g/l to 850 g/l. The absolute density can vary, for example from 100 to 2000 g/l, preferably from 200 to 1500, more preferably from 500 to 1200 g/l.

The amount of resin used in step b) can vary depending on the application in question, but can for example range from 0.1 -100g per 100ml of aqueous acidic oxidant mixture or metal bromide salt solution. The amount of resin used in step b) can be at least 0.1g, at least 1g, at least 2g, preferably at least 5g, per 100ml of aqueous acidic oxidant mixture or metal bromide salt solution. The amount of resin used in step b) can be up to 100g, up to 80g, up to 50g, preferably up to 20g, per 100ml of aqueous acidic oxidant mixture or metal bromide salt solution.

Disclosed herein, there is the step of recovering the metal from the metal resin complex. This is referred to as step c), and the skilled person will appreciate that this can be carried out by a variety of possible means. For example, the resin, complete with the complexed metal (the metal resin complex) can be removed from the remaining components of the process disclosed herein by appropriate solid-liquid separation techniques (such as sieving, for example), before subjecting the resin to a suitable process to separate the metal from the resin. For example, the metal can be recovered from the metal resin complex by stripping, incineration, ashing or burning of the resin. In one embodiment, step b) is carried out by passing the mixture of the precious metal-bearing material and aqueous acidic oxidant mixture through a column including the resin, and step c) is carried out by stripping the resin on the column.

The method may be monitored by continuous measurement of the pH and the oxidation reduction potential (ORP) of the mixture of the precious metalbearing material and aqueous acidic oxidant mixture. The oxidation-reduction potential is a measure of electrical potential which represents the tendency of one chemical species to oxidize or reduce another chemical species. Solutions of chemical species with a high (positive) potential tend to gain electrons thereby oxidising the other species, i.e., cause the other species to lose electrons. Conversely, a solution with a low (negative) potential will have a tendency to lose electrons thereby reducing the other species, i.e., cause the other species to gain electrons. In an aqueous solution the ORP is commonly measured and expressed in millivolts. The ORP is measured as a relative value by detecting the difference between the electrical potential of a measurement electrode (typically a platinum or gold electrode) and the electrical potential of a reference electrode (typically a silver-silver chloride electrode) in contact with the solution. The typical accuracy for an ORP meter is +5 mV. The ORP measurements provide an indication of the presence of oxidising species, such as BrOH, which is thought to result from step a). The ORP of the mixture of the precious metalbearing material and aqueous acidic oxidant mixture is preferably at least +500 mV, or ranges from +700 to +900 mV, as measured with in-situ measurements by an Milwaukee model MW 500 ORP meter.

For the process disclosed herein, the preferred order of addition to form the aqueous acidic oxidant mixture is to add the acid after the remaining components. In other words, the preferred order of addition is to first combine the oxidant and the source of the bromide ion to form an oxidant-bromide mixture, and then add the acid to the oxidant-bromide mixture to form the aqueous acidic oxidant mixture. The term “oxidant-bromide mixture” refers to the mixture formed from combining the oxidant and the bromide.

Preferably, the aqueous acidic oxidant mixture is stirred for at least 5 minutes before the aqueous acidic oxidant mixture is contacted with the precious metal bearing-material and resin.

The following non-limiting examples illustrate the invention.

Example 1

Formation of the aqueous acidic oxidant mixture and extraction of precious metal from precious metal-bearing material

To a stirred 750 ml solution of water, 70 g of sodium bromide was added, followed by 30 ml of hydrogen peroxide with a concentration of 30-55 wt.%. The solution was stirred and the pH reduced to between 0 and 6 by adding acid. The reaction was stirred for a minimum of 5 min. To this, 400 g of crushed ore containing gold was added and stirred for 5mins-24hr (depending on the release profile of the ore), the ore granules varying in size from 10-500 microns. If necessary, pH adjustments are made to maintain an ORP (oxidation reduction potential) above 600 vs the silver electrode and the pH in the above range.

The ore was filtered, washed with 100 ml of fresh aqueous acidic oxidant mixture and the resulting gold bromide salt solution collected for gold extraction. The tailings (i.e. spent ore material left over after the precious metal has been removed) are washed on a separate circuit with either water or a basic water formed by the addition of lime, sodium hydroxide, sodium bicarbonate or sodium carbonate.

Example 2 Capture of extracted precious metal using resin

The following demonstrates the improvements afforded by using resins, as opposed to traditional adsorption onto carbon, to capture the metal from the metal bromide salt solution resulting from steps a) and b).

Using a similar methodology to experiment 1 , a metal bromide salt solution was obtained, containing 4mg/L of soluble gold bromide species. 100 ml of this solution was stirred with 10 g of the ion exchange resin Purogold™ MTA5015SO4 for 1 hr. This ion exchange resin has a polystyrene backbone crosslinked with divinylbenzene, and is functionalised with quaternary ammonium groups. The reaction mixture was filtered and the residual gold species remaining in solution was measured by atomic adsorption spectrometry.

Using the same metal bromide salt solution again, which again contained 4mg/L of soluble gold bromide species, 100 ml was stirred with 10g of activated carbon for 1 h. The reaction mixture was filtered and the residual gold species remaining in solution was measured by atomic adsorption spectrometry.

The amount of residual gold remaining in solution indicates the amount of gold that was not captured by the resin or carbon. Therefore the lower the amount of residual gold remaining in solution, the better the uptake by the resin or carbon.

When using Purogold™ MTA5015SO4, the amount of residual gold in solution was measured as 0.2 mg/l, giving a 95% yield.

When using carbon, the amount of residual gold in solution was measured as 8 mg/l of gold, representing a 55% yield.

This demonstrates the improvement associated with using an ion exchange resin vs carbon in conjunction with the leaching step a) disclosed herein.

Example 3

“Resin in leach” process

To a stirred 750 ml solution of water, 70 g of sodium bromide was added, followed by 30 ml of hydrogen peroxide with a concentration of 30-55 wt.%. The solution was stirred and the pH reduced to between 0 and 6 by adding acid. The reaction was stirred for a minimum of 5 min. To this, 400 g of crushed ore containing gold was added together with 20 g of resin and stirred for 5mins-24hr (depending on the release profile of the ore), the ore granules varying in size from 10-500 microns. If necessary, pH adjustments are made to maintain an ORP (oxidation reduction potential) above 600 vs the silver electrode and the pH in the above range. The mixture was passed through a filter to leave the resin beads in the filter and the tailings for further treatment.

Example 4

Results using a series of ion exchange resins

The following tests are based on a methodology involving providing 100ml of a metal bromide salt solution and adding 1-20 g of resin and shaking/stirring for 30 min. The resins were obtained from commercially available sources. The precious metals to be extracted are listed on the table below, together with the results, and the resin functionalisation in question.

Example 5 Metal purification on a large scale

A series of columns are set up each containing an ion exchange resin.

An aqueous acidic oxidant mixture is prepared and contacted with a metalcontaining material, and the result is circulated through the columns, optionally using multiple cycles, until the precious metal has been absorbed by the resin in each column.