SELECTIVE GOLD RECOVERY METHOD BY GREEN CHEMISTRY FROM A GOLD-CONTAINING ELEMENT AND A PLATINUM RING

Abstract

A process for the selective dissolution of gold from an element also containing a platinum group metal, the process comprising the following steps: - providing a support comprising gold and a platinum group metal selected from platinum, palladium, rhodium, and ruthenium, - immersing the support in an ionic liquid solution comprising a complexing ionic liquid having a thiocyanate anion and optionally an additional solvent ionic liquid, the concentration of the complexing ionic liquid being greater than 0.1 mol / L, to selectively dissolve the gold present in the support, the precious element not being dissolved, the selective dissolution of the gold taking place either in the presence of an oxidant, for example iron, whereby the gold is dissolved in the ionic liquid solution, or by applying a potential between the support and a counter electrode immersed in the ionic liquid solution.whereby the gold is dissolved in solution and electrodeposited onto the counter electrode in a single step. Figure for the abbreviation: 6.

Description

Title of the invention: METHOD FOR SELECTIVE RECOVERY OF GOLD BY GREEN CHEMISTRY STARTING FROM AN ELEMENT CONTAINING GOLD AND A PLATINUM Technical field

[0001] The present invention relates to the general field of the recovery and valorization of gold from a support containing other elements, and in particular precious metals such as platinum.

[0002] The invention relates to a method for selective recovery of gold.

[0003] The invention is particularly interesting since it allows gold to be selectively recovered even if the waste contains platinoids.

[0004] The invention finds applications in numerous industrial fields, and in particular for the recovery of gold from waste or residue originating from waste electrical and electronic equipment (WEEE). State of the prior art

[0005] Currently, given the available resources and the growing demand for innovative technologies, the recovery and recycling of noble metals represents a strategic axis for many industrialists.

[0006] For example, among waste electrical and electronic equipment (WEEE), small electronic and telecommunications equipment has the richest metal fraction. It can contain up to 40 different metals, including precious metals. In particular, gold and palladium represent more than 80% of the economic value of this waste. For example, in electronic cards, gold and palladium are used to make stainless contacts or solders and are also present in electronic components (resistors, capacitors, etc.). The gold and palladium contents of this waste are generally much higher than those found in mines. For example, the gold content of electronic cards is between 10g / t and 1,000g / t, while the content of gold mines reaches 10g / t in the richest underground mines and only 1g / t in open-pit mines.The palladium content varies between 5 and 100 g / t in electronic cards and peaks at 7 g / t in underground mines.

[0007] It is therefore essential to be able to recycle and recover this waste. The WEEE recycling process typically includes three main stages: collection, disassembly to be able to recover the WEEE elements containing the highest content of recoverable elements and treatment using physicochemical processes to recover the metallic phases.

[0008] Typically, after being disassembled, the part of interest of the DEES is crushed to obtain particles. A separation phase makes it possible to recover the fraction of non-ferrous metals (by eddy current or magnetic separation for example). This fraction is then treated by mixed processes coupling pyrometallurgy and hydrometallurgy.

[0009] Blast furnaces intended for the exploitation of copper ores are used for the heat treatment phase. In these furnaces, a series of pyrometallurgical steps makes it possible to incinerate residual plastics but also to recover certain metals in the form of salts or oxides (Zn, Fe, etc.). Despite the significant losses of elements such as aluminum or tantalum, it makes it possible to concentrate the precious metals and copper in a phase called "black copper". This phase is then treated by electrolysis to obtain, on the one hand, high-purity copper cathodes and, on the other hand, an anodic mud loaded with gold, platinoids and silver.

[0010] The treatment of this anodic residue is essentially based on hydrometallurgical processes using cyanides to dissolve the gold (McArthur-Forest process in particular) or aqua regia to dissolve the platinoids. The dissolved metals are then separated and recovered in metallic form, as salts or as a solution.

[0011] Other lixiviant systems exist (Thiourea, Thiocyanate, aqua regia, etc.). However, their effectiveness remains lower than that of cyanidation and their environmental advantage is questionable. Many solvometallurgical solutions (hydrometallurgy in organic solvents) have also been studied. However, the sustainable aspect of the process is questionable because these organic solvents are often volatile, which leads to significant losses.

[0012] Even if conventional hydrometallurgy has advantages and in particular a much lower energy expenditure than the pyrometallurgical route, this route has several disadvantages. First, hydrometallurgical processes produce acid leachates during the recovery of particles with ions in mixtures. The selective recovery of gold and palladium from such a matrix is ​​difficult due to the chemical similarities of these elements. In addition, the hydrometallurgy of precious metals uses toxic or highly corrosive reagents and produces a significant quantity of effluents that must be treated, resulting in significant management, use and treatment costs in particular.

[0013] It is therefore important to find alternative ways to recover metals from WEEE, remedying these drawbacks.

[0014] The recovery of metals in ionic liquid or deep eutectic solvent (DES) media currently appears to be a particularly interesting green chemistry route. Ionic liquids are solvents with a wide electrochemical window, liquid at room temperature, extremely stable and allowing applications in air. They have a very low vapor pressure which reduces worker exposure and limits unwanted solvent losses. They also have complexing and dissolution properties superior to organic solvents. It has already been demonstrated that it is possible to recover certain metals in ionic liquid media.

[0015] For example, the article by Popescu et al. (“Recovery of Silver and Gold From Electronic Waste by Electrodeposition in Ethaline Ionie Liquid”, REV. CHIM. (Bucharest) (2020), 71, n°1, 122-132) describes the use of an electrochemical process for the dissolution and recovery of gold and silver. The precious metal concentrate (Ag 39.7wt and Au 18.9%) is subjected to an electrodissolution step (anodic dissolution) in a mixture of ethaline and iodine. Iodine has the ability to oxidize most elements, including precious metals such as gold. Since gold and silver have deposition potentials, it is possible to electrodeposit them separately. Cathodic deposits contain 30 to 70% gold and 99% silver Ag, respectively.

[0016] This document does not describe a waste containing both gold and a platinoid.

[0017] The publication by Abbott et al. (“Electrocatalytic recovery of elements from complex mixtures using deep eutectic solvents”, Green Chem. (2015), 17, 2172-2179) demonstrates that electrogeneration of oxidative species is an effective dissolution method in DES medium for gold and silver. Here, ethaline is used to simplify the speciation of solutes and to allow the modification of the redox potentials compared to aqueous solutions. The redox catalyst used is iodine. The effectiveness of this approach is demonstrated for three ore samples: Cu / Zn, Ga / As and Au / Ag.

[0018] This document does not describe a waste containing both gold and a platinoid.

[0019] The publication by Jayakumar et al. (“Electrochemical behavior of ruthenium (III), rhodium (III) and palladium (II) in l-butyl-3-methylimidazolium chloride ionic liquid” Electrochimica Acta (2009), 54, 6747-6755) studies the electrochemical behavior and in particular the electrodeposition of ruthenium, rhodium and palladium in ionic liquid medium ([BMIM][C1]).

[0020] In this study there is no mention of gold.

[0021] Thus, none of the works describe a process for selective recovery of gold in relation to a platinoid. Statement of the invention

[0022] An aim of the present invention is to propose a method, overcoming the drawbacks of the prior art, making it possible to selectively recover gold from a support comprising gold and at least one platinoid chosen from platinum, palladium, rhodium and ruthenium, the process having to be simple to implement, involving few steps and involving mild temperature conditions, while limiting energy and reprocessing costs, in order to be able to be transposed to an industrial scale.

[0023] For this purpose, the present invention provides a method for selectively dissolving gold from an element containing gold and a platinoid, the method comprising the following steps:

[0024] - providing an element comprising gold and a platinoid chosen from platinum, palladium, rhodium and ruthenium,

[0025] - immersing the element in an ionic liquid solution comprising a liquid complexing ionic having a thiocyanate anion and optionally an additional ionic liquid solvent, the concentration of complexing ionic liquid being greater than 0.1mol / L, to selectively dissolve the gold present in the element, the platinoid not being dissolved,

[0026] the selective dissolution of gold taking place:

[0027] - either in the presence of an oxidant, for example an iron salt, whereby the gold is dissolved in the ionic liquid solution,

[0028] - either by applying a potential between the element and a counter-electrode immersed in the ionic liquid solution, whereby the gold is electrodissolved in the ionic liquid solution and electrodeposited onto the counter electrode in a unit step.

[0029] The invention is fundamentally distinguished from the prior art by the use of a thiocyanate-based ionic liquid to selectively recover gold from a support containing gold and at least one platinoid. The (electro-)chemical system is of the ionic liquid type, with an anion capable of ensuring extraction selectivity specific to gold over the platinoid.

[0030] The support may contain one or more platinoids, possibly in the form of metal oxide.

[0031] According to a first embodiment, an oxidant is used. Advantageously, the oxidant is iron chloride.

[0032] According to this first embodiment variant, the support may be electrically conductive or electrically insulating. The support may be in particulate form. This first embodiment variant is particularly advantageous in the case where it is not possible to shape the support in a form compatible with an electrochemical process.

[0033] According to a second embodiment variant, the process is carried out electrochemically. According to this variant, the gold can be simultaneously electrodissolved at the anode and electrodeposited to recover it in metallic form at the cathode (counter-electrode).

[0034] Advantageously, the applied potential is between 0.3V and 0.8V vs Ag+ / Ag.

[0035] According to these different embodiments, the concentration of ionic liquid complexing agent is, advantageously, less than or equal to Imol / L.

[0036] Advantageously, the selective dissolution of gold takes place at a temperature between 10°C and 150°C, preferably between 20°C and 90°C, and even more preferably between 30°C and 80°C, the ionic liquid solution preferably being stirred. Such conditions improve the diffusion coefficients.

[0037] Advantageously, the cation of the complexing ionic liquid is chosen from a pyrrolidinium, an ammonium and an imidazolium, advantageously an N,N-dialkylimidazolium.

[0038] Preferably, the complexing ionic liquid is l-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]).

[0039] Advantageously, the liquid solution further comprises 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][NTf2]).

[0040] Advantageously, the platinoid is palladium.

[0041] Advantageously, the support is waste electrical and electronic equipment, for example an electronic card.

[0042] According to another advantageous embodiment, the element is a metallic sludge, for example obtained according to the following steps:

[0043] - provide waste electrical and electronic equipment,

[0044] - crushing waste electrical and electronic equipment in order to obtain particles,

[0045] - implement a separation step on the particles to obtain on the one hand a fraction of non-ferrous metals and on the other hand a fraction of ferrous metals,

[0046] - carry out a pyrometallurgical and / or hydrometallurgical treatment on the fraction non-ferrous metals, resulting in a metal sludge.

[0047] The process has many advantages:

[0048] - simultaneously meet economic and environmental objectives for the re selective recovery of gold in a matrix also containing a platinoid such as palladium,

[0049] - both ensure the selective electro-dissolution of gold, and at the same time the recover electrochemically, in a single operation (simultaneously), and thus ensure the regeneration of the bath,

[0050] - the anion of the complexing ionic liquid complexes the metal to be dissolved by reducing the complexing force of the latter, which makes it possible to overcome the problems of solubility and stability of the complexing agent,

[0051] - eliminate the cost and reprocessing of the complexing species that is regenerated during the deposition of gold and to introduce the active species in a suitable concentration promoting the kinetics of dissolution and recovery of gold,

[0052] - the deposition potential of gold avoids the deposition of another metal, avoids the degradation of the cation of the ionic liquid and minimizes the energy required for deposition,

[0053] - use the ionic liquid as an ionic conductor, avoiding the addition of a salt conductor to ensure ionic conduction during the electrolysis stage,

[0054] - having an ionic liquid solution that is liquid at room temperature, facilitating its use in an industrial environment,

[0055] - considerably reduce the constraints linked to the use of concentrated acid and to temperature, such as the use of aqua regia, cyanides, various acids for dissolving metals because ionic liquids are non-volatile, non-flammable and chemically stable at temperatures above 200°C,

[0056] - avoid classic extraction steps of platinoids compared to gold, which are complex and expensive.

[0057] Other characteristics and advantages of the invention will emerge from the additional description which follows.

[0058] It goes without saying that this additional description is given only as an illustration of the subject of the invention and must in no case be interpreted as a limitation of this subject. Brief description of the drawings

[0059] The present invention will be better understood on reading the description of exemplary embodiments given purely for informational purposes and in no way limiting, with reference to the appended drawings in which:

[0060] [Fig. 1] is a graph representing the intensity I as a function of the potential E, at a scanning speed Vb = 50 mV / s, of an anode comprising a gold wire and a palladium wire in an electrolytic solution SI, according to a particular embodiment of the invention,

[0061] [Fig.2] is a chronoamperogram obtained during electrochemical dissolution of an anode comprising a gold wire and a palladium wire in the electrolytic solution SI, according to another particular embodiment of the invention,

[0062] [Fig.3] is a chronoamperogram obtained during electrochemical dissolution of an anode comprising a gold wire and a palladium wire in the electrolytic solution SI, according to another particular embodiment of the invention,

[0063] [Fig.4] is a photograph obtained by scanning electron microscopy of a counter- platinum electrode after implementation of the selective gold recovery method, according to another particular embodiment of the invention,

[0064] [Fig.5] represents an EDX analysis carried out on the platinum counter-electrode observed in [Fig.4], in an area where there was no deposit,

[0065] [Fig.6] represents an EDX analysis carried out on the platinum counter-electrode observed in [Fig.4], at the level of an area on which there has been a deposit.

[0066] Detailed description of particular embodiments

[0067] Although this is in no way limiting, the invention particularly finds applications in the field of electrical and electronic equipment and, more particularly, for the recovery of gold present in waste or residue originating from waste electrical and electronic equipment.

[0068] The method could be applied to any other element containing gold. For example, it could be a.... or a....

[0069] The process for recovering waste electrical and electronic equipment, for example an electronic card, may comprise the following steps:

[0070] a) provide waste electrical and electronic equipment (WEEE),

[0071] b) grinding the WEEE to obtain particles,

[0072] c) implementing a separation step on the particles to recover from a on the one hand a fraction of non-ferrous metals (for example by eddy current and / or magnetic separation) and on the other hand a fraction of ferrous metals,

[0073] d) carrying out a pyrometallurgical treatment and / or hydrometallurgical treatment on the non-ferrous metal fraction to recover the transition metals (e.g. copper) and other metals (tin, lead, etc.), whereby an element containing at least gold and a platinoid chosen from platinum, palladium, rhodium and ruthenium is obtained; the element thus obtained is, for example, in the form of a metal sludge having a high concentration of precious metals (PGM),

[0074] e) carrying out the method for selective separation of gold by immersing the element in an ionic liquid solution comprising a complexing ionic liquid and, optionally, an additional ionic liquid solvent, the complexing ionic liquid having a thiocyanate anion, the concentration of complexing ionic liquid being greater than 0.1mol / L, to selectively dissolve the gold present in the element, the platinoid not being dissolved.

[0075] the selective dissolution of gold taking place:

[0076] - either in the presence of an oxidant, for example an iron salt, whereby the gold is dissolved in the ionic liquid solution,

[0077] - either by applying a potential between the element and a counter-electrode immersed in the ionic liquid solution, whereby the gold is electrodissolved in solution and electrodeposited onto the counter electrode in a unit step.

[0078] According to a first embodiment variant, to implement the method of selective dissolution of gold, the element is immersed in the ionic liquid solution in the presence of an oxidant. This embodiment is particularly advantageous when it is not desired or not possible to shape the element in the form of a electrode or even deposit it on an electrically conductive substrate.

[0079] By oxidant is meant a species capable of oxidizing the gold contained in the element in ionic form. The oxidant is in particular a metal salt. An iron salt will preferably be chosen, in particular iron chloride, for example in the form of iron (III) chloride hexahydrate (FeCl3.6H2O).

[0080] The dissolved gold can be deposited or electrodeposited onto a support in a subsequent step.

[0081] Advantageously, the ionic liquid solution contains an oxidant and a reducer so as to form a redox couple (also called an electrochemical shuttle). Simultaneously:

[0082] - the oxidant can oxidize gold from a metallic form to an ionic form (by means of which the oxidant is reduced) according to Au Au+ + e, and

[0083] - the reducer can reduce gold from an ionic form to a metallic form (whereby the reducer is oxidized) according to Au+ + e Au.

[0084] The gold can thus be deposited on a support immersed in the ionic liquid solution. For example, it can be a support made of glassy carbon or platinum.

[0085] The oxidation of the reducing agent and the reduction of the oxidant make it possible to form new oxidant / reducing species and / or to regenerate the species initially present in solution. The process is economical since the redox couple in solution simultaneously ensures the redox reactions, so that the consumption of reagent is zero; the solution can be used for a new cycle.

[0086] Advantageously, the redox species pair is a metal pair, preferably chosen from Mn2+ / Mn3+, Co2+ / Co3+, Cr2+ / Cr3+, Cr3+ / Cr6+, V2+ / V3+, V4+ / V5+, Sn2+ / Sn4+, Ag+ / Ag2+, Cu+ / Cu2+, Ru4+ / Ru8+ or Fe2+ / Fe3+, a pair of organic molecules, a pair of metallocenes such as Fc / Fc+, or a pair of halogenated molecules such as for example C12 / C1 or C1 / C13.

[0087] For example, Fe2+ / Fe3+ will be chosen. These are soluble in their two oxidation states, they are not toxic, they do not degrade the ionic liquid and they have adequate redox potentials.

[0088] According to a second variant embodiment, the element is:

[0089] - either in the form of an electrically conductive substrate which can be directly connected to a voltage (or current) generator

[0090] - either deposited on an electrically conductive substrate which can be connected to a ge voltage (or current) generator.

[0091] The element acts as a working electrode (also called anode or positive electrode). A counter electrode (also called cathode or negative electrode) and, preferably, a reference electrode are also immersed in the ionic liquid solution so as to form an electrochemical cell. A potential is applied between the element and the counter electrode, whereby the gold is electrodissolved in solution and electrodeposited in metallic form on the counter electrode in a single step. Alternatively, it would be possible to apply a current.

[0092] The application of the potential (or current) to the positive electrode (or to the negative electrode) simultaneously generates the electro-dissolution of the gold at the positive electrode and the electrodeposition / recovery of the gold in metallic form at the negative electrode. Thus, the gold extraction step simultaneously allows the recovery of the gold in metallic form and the regeneration of the ionic liquid solution, for example for use in a new treatment cycle.

[0093] The conductive substrate on which the element is deposited is, for example, made of tin-doped indium oxide, vitreous carbon, stainless steel, titanium or a noble metal.

[0094] The counter electrode may be vitreous carbon, stainless steel, carbon, titanium, noble metals, tin-doped indium oxide, platinum, a gold surface.

[0095] The reference electrode is an Ag+ / Ag type electrode, for example produced by balancing a silver wire with a solution comprising the complexing ionic liquid and silver thiocyanate (AgSCN).

[0096] According to this second embodiment variant, the ionic liquid solution could also comprise an oxidant or an electrochemical shuttle.

[0097] We will now describe in more detail the ionic liquid solution used for these different embodiments.

[0098] The ionic liquid solution may contain one or more ionic liquids. For example, the ionic liquid solution comprises two ionic liquids: a complexing ionic liquid and a solvent ionic liquid.

[0099] Ionic liquid means the association comprising at least one cation and one anion which generates a liquid with a melting temperature lower than or close to 100°C.

[0100] A complexing ionic liquid is an ionic liquid which promotes the electro-dissolution of gold and which has a complexing power on gold. The latter must also prevent the dissolution of palladium.

[0101] A solvent ionic liquid is understood to mean an ionic liquid which does not allow the dissolution of precious metals and which has no or very little complexing power.

[0102] The cation of the complexing ionic liquid and the cation of the solvent ionic liquid are independently chosen from the following families: imidazolium, pyrrolidinium, ammonium, piperidinium and phosphonium.

[0103] Advantageously, the solvent ionic liquid and the complexing ionic liquid have the same cation to increase the solubility of the complexing ionic liquid in the ionic liquid solvent.

[0104] Preferably, the cation of the complexing ionic liquid is an imidazolium cation. Particularly advantageously, the cation of the complexing ionic liquid and the cation of the solvent ionic liquid are imidazolium. This cation is stable up to a cathodic potential sufficiently high to envisage electrochemical deposition of gold when the conditions of mixing and association of the ionic liquids are respected (in terms of anion nature and concentration). An A / A-dialkylimidazolium will advantageously be chosen, and preferably l-butyl-3-methylimidazolium (BMIM). This cation has a good viscosity and a good diffusion coefficient, which promotes the movement of the species in the ionic liquid solution.

[0105] The anion of the complexing ionic liquid is the thiocyanate anion. This anion promotes the dissolution of gold, without dissolving palladium. In the case of a mixture of ionic liquids, it will preferably be between 1% mol and 50% mol, to improve the transport properties of the medium, and more efficient the electrochemical process (resistance, ionic conductivity, viscosity, diffusion of species, etc.)

[0106] The anion of the solvent ionic liquid can be organic or inorganic. The anion of the solvent ionic liquid has no or very little complexing affinity, which excludes anions having a ligand role on precious metals: halides, amines and / or sulfur. Advantageously, anions will be used which make it possible to obtain a wide electrochemical window, a low viscosity, a low melting temperature (liquid at room temperature), good solubility with the complexing ionic liquid and / or which do not lead to hydrolysis (degradation) of the ionic liquid.

[0107] For example, the TFSI anion will be chosen for the solvent ionic liquid.

[0108] As an illustration, we can choose a mixture [BMIM][SCN] and [BMIM][TFSI].

[0109] The ionic liquid solution may further comprise a drying agent and / or an agent promoting the transport of matter.

[0110] The drying agent is for example a salt which does not intervene in the reactions at the electrodes and which does not react with the solvent. The drying agent may be chosen from MgSO4, Na2SO4, CaCl2, CaSO4, K2CO3, NaOH, KOH and CaO.

[0111] The agent promoting the transport properties may be a salt, a third ionic liquid or deep eutectic solvent (DES), water or another organic solvent.

[0112] Illustrative and non-limiting examples of an embodiment:

[0113] Example 1: Study of the electrochemical dissolution of an Au / Pd- electrode

[0114] The electrolytic cell comprises three electrodes:

[0115] - a platinum cathode as a counter electrode,

[0116] - an anode made of two metal wires (one gold wire and one palladium wire) is carried in working electrode,

[0117] - an Ag+ / Ag reference electrode made by balancing a silver wire with a solution of l-butyl-3-methylimidazolium thiocyanate [BMIM][SCN], containing 10 mmol / L of silver thiocyanate AgSCN.

[0118] The electrolytic solutions tested are:

[0119] - SI formed only from the ionic liquid thiocyanate of 1-butyl-3-methylimidazolium [BMIM] [SCN].

[0120] - S2 comprising bis(trifluoromethanesulfonyl)imide of l-butyl-3-methylimidazolium [BMIM][NTf2] and l-butyl-3-methylimidazolium thiocyanate [BMIM][SCN] at a concentration of 0.1M.

[0121] Firstly, a voltammetric study of the system was carried out in the SI electrolytic solution in order to determine the oxidation potential of the anode ( [Fig.l]). The scanning speed is Vb = 50 mV / s. The curve I(E) shows the appearance of an oxidation current from 0.2 V vs Ag+ / Ag, which corresponds to the oxidation of the anode.

[0122] Electrochemical dissolution tests were then carried out in air for working temperatures of 40°C and 80°C. The temperature is kept constant by a recirculating oil bath. The solution is stirred at 600 revolutions / min (rpm) by a stirrer bar during dissolution.

[0123] The electrochemical dissolution tests of the anode were carried out under potentiostatic control for 1000s, i.e. by applying a fixed potential E, located beyond 0.2 V vs Ag+ / Ag, typically between 0.7 V and 1.1 V vs Ag+ / Ag.

[0124] [Fig.2] illustrates the chronoamperogram obtained during the electrochemical dissolution of the anode in the electrolytic solution S1 at 40°C. At the end of the chronoamperometry, the anode is extracted from the electrolytic solution thus obtained, then the two wires are weighed. The dissolution rate Vd of the gold is also determined.

[0125] The faradaic efficiency Rf of gold dissolution is calculated assuming the formation of Au(I) during dissolution. The efficiency is calculated by comparing the effective charge Q and the charge required to dissolve the amount of gold dissolved via the reaction: To To(l) + 6"

[0126] The protocol which has just been described was reproduced at 80°C for the SI solution and at 80°C for the S2 solution.

[0127] The results for each of the tests are collected in Table 1 below.

[0128] [Table 1] Solution T CO E(VVS Ag / Ag+) Q(C) Au dissolved (mg) Pd dissolved (mg) Vd (mg / cmVh) Rf SI 40 °C 0.75 1.31 1.33 0.02 48.5 98% SI SO^C 0.50 4.41 9.09 0.09 331 99% S2 80°C 0.8 0.42 0.67 0 16.4 78% These results confirm the electrochemical dissolution, or anodic dissolution, of gold selectively over palladium dissolution. A yellow surface layer is present on the surface of the palladium wire. Without being bound by theory, it appears that, initially, the palladium is dissolved very slightly, then a protective layer is formed and protects the surface from dissolution.

[0129] Example 2: Study of direct electrodissolution-electrodeposition of gold in the electrolytic solution S2-

[0130] The electrolytic cell comprises three electrodes:

[0131] - a platinum plate (Pt-Rh alloy) as a counter electrode,

[0132] - an anode formed of two metal wires (a gold wire and a pre-plated palladium wire) (labilely protected via example 1) is carried as a working electrode, and

[0133] - an Ag+ / Ag reference electrode made by balancing a silver wire with a solution of l-butyl-3-methylimidazolium thiocyanate [BMIM][SCN], containing 10 mmol / L of silver thiocyanate AgSCN.

[0134] The electrolytic solution used is SI solution (l-butyl-3-methylimidazolium thiocyanate [BMIM][SCN]).

[0135] The electrochemical dissolution test was carried out in air at a working temperature of 40°C. As in Example 1, the temperature was kept constant by a recirculating oil bath. The solution was stirred at 600 rpm by a stirrer bar during dissolution.

[0136] The electrochemical dissolution of the anode was carried out under potentiostatic control for 45 min, i.e. by applying a fixed potential, located beyond 0.2 V vs Ag+ / Ag ([Fig.3]), for example 0.7 V vs Ag+ / Ag. The chronoamperogram confirms the electrochemical dissolution of the anode in the electrolytic solution.

[0137] At the end of the dissolution step, the platinum counter-electrode is removed. It is weighed, observed by scanning electron microscopy (SEM) and its composition is analyzed by energy dispersive X-ray spectroscopy (EDX).

[0138] The plate is covered by a gold deposit which is visible to the naked eye (figures 4). The SEM-EDX analysis confirms on the one hand that the part of the platinum counter-electrode not covered by the deposit does not contain gold ([Fig.5]) and on the other hand that the deposit is gold ([Fig.6]). No trace of palladium is observable.

[0139] The dissolution rate of gold is also determined.

[0140] The same protocol is applied for a temperature of 80°C.

[0141] The results of the tests are collected in Table 2 below.

[0142] [Table 2] Solution t co E (Vvs Ag / Ag+) Q(C) Dissolution (mg) Vd (mg / cmVh) Rf Au recovered (mg) Au Pd SI 40 °C 0.75 3.09 3.85 0 31.6 61% 0.29 SI 80T 0.50 8.41 16.88 0 138.4 98% 0.39 These results confirm the dissolution and electrodeposition of gold. The selectivity of gold dissolution leads to a high-purity deposit. The same experiment can be conducted by reversing the counter electrode and the working electrode.

[0143] Example 3: Study of the chemical dissolution of gold and palladium in different electrolytic solutions

[0144] The dissolution device used in this example consists of a pillbox containing 1 ml of the following solutions:

[0145] - S0 formed from the ionic liquid l-butyl-3-methylimidazolium thiocyanate [BMIM][SCN]

[0146] - SI formed from the ionic liquid l-butyl-3-methylimidazolium thiocyanate [BMIM][SCN] in which 0.5M of iron(III) chloride hexahydrate FeCl3 ,6H2O is dissolved

[0147] - S2 formed from the ionic liquid bis(trifluoromethanesulfonyl)imide of l-butyl-3-methylimidazolium [BMIM][NTf2] to which l-butyl-3-methylimidazolium thiocyanate [BMIM][SCN] at a concentration of IM and 0.5M iron(III) chloride hexahydrate FeCl3,6H2O have been added

[0148] - S3 formed from the ionic liquid bis(trifluoromethanesulfonyl)imide of l-butyl-3-methylimidazolium [BMIM][NTf2] to which l-butyl-3-methylimidazolium thiocyanate [BMIM][SCN] at a concentration of 0.1M and 0.5M iron(III) chloride hexahydrate FeCl3,6H2O have been added.

[0149] The chemical dissolution tests were carried out in air at a working temperature of 80°C. The temperature is kept constant by heating using a heating block. The solution is stirred at 400 revolutions / min (rpm).

[0150] A metal wire (gold or palladium) is placed in the test pillbox for 24 hours. The wire is then weighed. Table 3 below lists the different mass losses.

[0151] [Table 3] Solution T CO Dissolved Au (mg) Dissolved Pd (mg) so so°c 0.01 0.02 SI SOT 8.19 0.02 82 BOT 9.28 0.02 83 SOT 0 0 As before, the results confirm the chemical dissolution of the gold wire and not the palladium wire. The thiocyanate concentration must be greater than 0.1M to observe sufficient dissolution of the gold in 24 hours. Below this concentration, dissolution takes longer. Beyond 1M of thiocyanate, increasing the concentration does not lead to an improvement in the results.

[0152] The presence of the iron salt ensures dissolution. As in Example 1, the surface of the palladium is covered with a yellow film after the test.

Claims

Claims

1. A method for selectively dissolving gold from an element containing gold and a platinoid, the method comprising the following steps: - providing an element comprising gold and a platinoid selected from platinum, palladium, rhodium and ruthenium, - immersing the element in an ionic liquid solution comprising a complexing ionic liquid having a thiocyanate anion and optionally an additional ionic liquid solvent, the concentration of complexing ionic liquid being greater than 0.1mol / L, to selectively dissolve the gold present in the element, the platinoid not being dissolved, the selective dissolution of the gold taking place: - either in the presence of an oxidant, for example an iron salt, whereby the gold is dissolved, - or by applying a potential between the element and a counter electrode immersed in the ionic liquid solution, whereby the gold is electrodissolved and electrodeposited on the counter electrode in a unit step.

2. Method according to claim 1, characterized in that the oxidant is iron chloride.

3. Method according to one of claims 1 and 2, characterized in that the support is in particulate form.

4. Method according to claim 1, characterized in that the potential is between 0.3V and 0.8 V vs Ag+ / Ag.

5. Method according to any one of the preceding claims, characterized in that the concentration of complexing ionic liquid is less than or equal to 1mol / L.

6. Method according to any one of the preceding claims, characterized in that the selective dissolution of the gold takes place at a temperature between 10°C and 150°C, preferably between 20°C and 90°C, and even more preferably between 30°C and 80°C, the ionic liquid solution preferably being stirred.

7. Method according to any one of the preceding claims, characterized in that the cation of the complexing ionic liquid is chosen from a pyrrolidinium, an ammonium and an imidazolium, advantageously an N, V-dialkylimidazolium.

8. A method according to any preceding claim, ca-

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10.

11.

12. characterized in that the complexing ionic liquid is l-butyl-3-methylimidazolium thiocyanate. Method according to the preceding claim, characterized in that the liquid solution further comprises l-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. Method according to any one of the preceding claims, characterized in that the platinoid is palladium. Method according to any one of the preceding claims, characterized in that the element is waste electrical and electronic equipment, for example an electronic card. Method according to any one of claims 1 to 10, characterized in that the element is a metallic sludge, for example obtained according to the following steps: - supply waste electrical and electronic equipment, - crush waste electrical and electronic equipment to obtain particles, - implement a separation step on the particles to obtain on the one hand a fraction of non-ferrous metals and on the other hand a fraction of ferrous metals, - carry out pyrometallurgical and / or hydrometallurgical treatment on the non-ferrous metal fraction, resulting in a metal sludge.

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