EA047382B1 - METHOD FOR EXTRACTING GOLD FROM ORES - Google Patents

Abstract

The invention presents a method for extracting gold from refractory gold-bearing ore, which includes the steps of electrolysis of a mixture consisting of ore particles and an aqueous solution of bromide in an electrolytic cell containing an anode and a cathode, wherein bromine is formed at the anode during oxidation of bromide, under the action of which gold dissolves in the aqueous phase; separating the ore particles from the aqueous phase to obtain leaching liquor; adjusting the pH of the leaching liquor to the alkaline range to obtain a gold-containing precipitate; collecting the gold-containing precipitate and recycling the bromide-containing de-goldened solution for repeated use as the initial aqueous solution of bromide.

Description

Cyanidation is the most common method used in the gold mining industry to extract gold from gold ores. An aqueous leaching medium containing elemental bromine (Br ₂ ) together with bromide (Br ^- ), hereinafter referred to as bromide-bromide lixiviant, is a potential replacement for cyanide as it can leach gold from ores by oxidizing the precious metal and dissolving the gold in an aqueous solution with formation of AuBr2 ^- and/or AuBr ₄ ^- complexes. The ore particles are then separated from the gold-containing aqueous phase (pregnant leach solution; PLS). The productive leach solution is processed to recover gold.

For example, US 4,684,404 shows that gold metal can be dissolved in an aqueous solution in the presence of sodium bromide and elemental bromine, for example in an aqueous reagent containing 1.0 vol% Br ₂ and 1.0 vol% sodium bromide. US 4,684,404 also notes that gold/bromine complexes formed in aqueous solution can be recovered using organic solvents such as methyl isobutyl ketone (MIBK), diisobutyl ketone (DIBK). -isobutyl ketone) and ethyl ether. Other methods that can be used to recover gold from PLS, and which are described in US 4684404, are electrodeposition, activated carbon adsorption and ion exchange.

The effect of bromide lixiviant on various ores was also investigated by Melashvili et al. [Conference of Metallurgists Proceedings ISBN: 978978-1-926872-24-7, published by the Canadian Institute of Mining, Metallurgy and Petroleum].

The use of electrolytically generated bromine to leach gold by direct treatment of gold ore in a tank in which free bromine is produced by electrolysis of a bromide solution is described in US Pat. No. 4,904,358. The sodium bromide solution is supplied to a tank containing the ore. The solution flows down through the ore to be leached. Two electrodes installed in the reservoir are connected to a power source. Electrolysis occurs and elemental bromine is formed from bromide. Bromine becomes available to attack the gold, which dissolves in the aqueous phase. The productive leach solution is separated from the ore. By using zinc cementation (adding zinc metal, a metal that is a strong reducing agent), precious metals can be extracted from PLS.

However, the primary motivation for incorporating electrolytic bromine production into gold recovery technology is the efficient reuse of bromide ions that are formed after bromine oxidizes gold, namely the reoxidation of bromide ions to elemental bromine for reuse in the process. Methods based on this approach of re-oxidizing bromide to elemental bromine through electrolysis are described in publications EP 476862 and WO 2017/199254, illustrating gold recovery processes consisting of three main stages. During the first stage, the gold ore is leached with a bromide lixiviant. The suspension is separated by filtration to obtain a solid phase and an aqueous phase, which contains AuBr2 ^- /AuBr4 ^- ions, that is, a productive leach solution. During the second stage, gold is recovered from the productive leach solution by passing the solution through a column packed with an anion exchange resin, adsorption by activated carbon, zinc cementation or solvent extraction (EP 476862) or by solvent extraction using a phosphorus or amine extractant and a specific stripping process (WO 2017 /199254). The resulting solution with a low bromide content is fed with bromide. Then, during the third stage, the bromide solution suitable for electrolysis is passed through one or more electrolytic cells to oxidize the bromide to elemental bromine to produce an aqueous bromine solution, which is recycled and used in the leaching step. This provides a closed-cycle technology with efficient use of bromine.

Another technological solution based on electrolytically generated bromine is shown in FIG. 1 (Fig. 7 from publication EP 476862). The electrochemical cell (83) is divided into anodic (91) and cathodic (93) compartments by a cation exchange membrane (89). The goal is to recover the gold contained in the productive leach solution by electrodeposition of gold on the cathode and the formation of bromine on the anode side. Anolyte consists of an acidic bromide solution. Bromine is generated on the anode side, and an aqueous solution of bromine is supplied to the leaching tank (97). After the liquid and solids are separated (99), the leach liquor is fed to the cathode side where gold is deposited onto the cathode. EP 476862 states that the pH of the solution supplied to the cathode is strongly acidic. The catholyte effluent is directed to a make-up tank (101) and fed with bromide by adding alkali metal bromide to obtain an anolyte feed solution.

Gold ores that are difficult to cyanidate are called refractory ores. Refractory ores and leach residues (a type of waste generated from mining and processing plants called tailings) have always caused problems in the gold mining industry. The present invention relates to gold leaching using electrolytically generated bromine from such gold sources.

The main type of refractory gold ore that is resistant to cyanidation is

- 1 047382 sulfide-containing ore is poured. It is an ore containing sulfide minerals (such as pyrite, pyrrhotite and stibnite) that are impermeable to the leach solution. As a result, the leach solution cannot easily reach the gold locked in the sulfide minerals. Therefore, sulfide-containing refractory ores are treated by roasting to convert sulfide to oxide or other oxidative pretreatment methods before they can be subjected to leaching (eg, cyanidation). Bromine-bromide lixiviant is an alternative to cyanidation for leaching refractory gold ore due to its ability to simultaneously oxidize sulfide and gold. However, it is shown [see higher; Conference of Metallurgist Proceedings ISBN] that the bromine consumption when leaching refractory gold ore to achieve approximately 70% recovery is very high (more than 500 kg/t). The experimental results presented in this publication are more or less consistent with these data. These observations imply that in order to meet the need for large quantities of bromine to process refractory gold ore, it may be necessary to either directly add elemental bromine to the leach reactor (with the high pressure and corrosive nature of its vapors) or supply too large volumes of bromine-bromide aqueous reagent.

Other types of refractory ores contain gold bound to carbonaceous matter, or gold in the form of a solid solution along with other minerals such as arsenic, iron, copper, antimony and tellurium.

We have now found a technological solution that eliminates the above-mentioned difficulties associated with leaching of refractory ores, and additionally includes the stage of efficient gold recovery. In a variant of the method, recovery of gold from refractory ore using a bromine/bromide lixiviant and separation of the metal from the leach liquor is carried out in an electrolytically similar manner. The bromine produced at the anode by the oxidation of bromide in aqueous solution dissolves the gold from the refractory ore that is added to the anode side of the electrolytic cell or which circulates through the anode side of the electrolytic cell. After removing the ore particles, a leach liquor containing soluble AuBr complexes is obtained₂ /AuBr4^-. The alkaline environment formed by the generation of hydroxyl ions at the cathode during the electrolysis of this leach liquor provides good gold recovery by precipitating one or more insoluble forms of gold in the cathode compartment. The sediment is collected; We found that the gold concentration in the degold solution was acceptably low, indicating good gold recovery through alkaline-induced precipitation. Of course, gold was discovered after re-dissolving the sediment in aqua regia. Without wishing to be bound by a particular theory, mechanisms that may explain gold recovery from alkaline electrolyzed leach liquor include chemical precipitation of Au³⁺ from solution in the form of Au(OH)3; chemical precipitation of Au sulfide or sulfate¹⁺ and/or Au³⁺; or reductive Au deposition⁽⁰⁾ from the volume of solution, together with other hydroxides. Electrodeposition of gold at the cathode appears to play little role in the recovery of gold from alkaline electrolyzed leach liquor.

Therefore, the present invention relates to a method for extracting gold from refractory gold ore, which includes the stages of electrolysis of a mixture consisting of ore particles and an aqueous solution of bromide in an electrolytic cell containing an anode and a cathode, and bromine is formed at the anode during the oxidation of bromide, under the influence of which gold dissolves in the aqueous phase;

separating ore particles from the aqueous phase to produce leach liquor;

adjusting the pH of the leach liquor to the alkaline range to produce one or more gold-bearing precipitates;

collecting the gold-bearing precipitate (or precipitates) and recycling the bromide-containing degold solution for reuse as the initial aqueous bromide solution.

It should be noted that gold recovery from leach liquor can be accomplished downstream of the leach step by adding an alkaline agent (eg, an alkali metal hydroxide) to the leach liquor to obtain a pH value greater than 7.5, for example ranging from 8 to 12, or by electrolysis of leach liquor. That is, ore leaching using bromine generated at the anode and gold separation from the leaching liquor through alkalization are successfully carried out. Therefore, in an embodiment of the present invention, the pH of the leach liquor is shifted to the alkaline range by electrolysis of the leach liquor in an electrolytic cell containing an anode and a cathode (in a second electrolytic cell), with hydroxyl ions being produced at the cathode by reducing water to produce an alkaline environment.

However, as stated above, a more elegant approach to leaching gold from refractory ore and recovering gold from the productive leach solution involves simultaneously using anode-generated bromine to dissolve gold from the refractory ore and cathode-generated hydroxyl ions to precipitate gold-bearing solids from the producing solution.

- 2 047382 leaching. The method can be carried out either in a batch reactor consisting of an undivided or divided electrolytic cell, or in a semi-continuous mode, for example with the circulation of anolyte and catholyte streams through two separate closed flow circuits, the anolyte stream being a suspension of ore in an aqueous solution of bromide, and the catholyte stream consists of the productive leach solution obtained in the previous run. We use the term leach liquor to refer to the aqueous phase separated from the ore residue after leaching a portion of the ore with a bromide lixiviant; The term productive leach solution in the context of the present invention is a broader term, it may be leach liquor obtained from previous runs of the process using another batch of refractory ore. PLS typically contains residual Br ₂ , for example up to 1.2 wt.%.

Accordingly, the present invention provides a method for recovering gold from a refractory gold ore in an electrolytic cell containing anode and cathode compartments, which includes the steps of:

supplying anolyte to the anode compartment, wherein the anolyte is a mixture of ore particles and an aqueous solution of bromide;

supplying catholyte to the cathode compartment, wherein the catholyte is a productive leaching solution containing AuBr ₂ ^- and/or AuBr ₄ ^- ions, which was obtained after leaching of refractory gold ore with an aqueous bromine-bromide lixiviant;

applying electrical voltage to the electrodes, due to which bromide is oxidized to bromine at the anode and dissolves gold in the anolyte, and water is reduced at the cathode to create an alkaline environment for the precipitation of a solid substance containing gold in the catholyte;

separating ore particles from the anolyte to produce leach liquor;

separating the gold-containing precipitate from the catholyte to collect the gold-containing precipitate and obtain a degold-free solution containing bromide for reuse as the original aqueous bromide solution.

More preferably, supplying the anolyte to the anode compartment involves circulating an anolyte stream between the anode compartment and a leaching reactor where the ore particles are mixed with an aqueous bromide solution, and supplying the catholyte to the cathode compartment involves circulating a catholyte stream between the cathode compartment and a reactor containing a productive leach solution.

The leach liquor and the bromide-containing degold solution, which are separated from the anolyte stream and the catholyte stream, respectively, can be used during subsequent operation as feed solutions for the cathode side and anode side of the electrochemical cell, respectively.

The approach described above is depicted in FIG. 2, which illustrates a process based on the solubilization of Au ⁽⁰⁾ from an ore by electrolytically generated bromine with simultaneous electrolytically controlled precipitation of the valuable metal from the productive leach solution (obtained in the previous run) at an alkaline pH. The main component of the installation shown in FIG. 2 is an electrochemical cell (1), divided into anode and cathode compartments (2 and 3, respectively). The first circulation circuit (4) provides a flow path between the anode compartment (2) and a stirred tank (5) equipped with suitable stirring means. The reservoir (5) serves as a leaching reactor. A second circulation circuit (6) provides a flow path between the cathode compartment (3) and a stirred tank (7) containing the productive leach solution.

A slurry consisting of refractory gold ore and water is fed into the reactor through a feed line (8). The sulfide content of refractory ores that are amenable to leaching by the method of the present invention ranges from 0.5 wt% to 30 wt%, for example from 1 wt% to 10 wt%, for example from 1 wt% to 5 wt.%. The solid/water weight ratio may range from 1:10 to 1:2. Refractory gold ore can be leached in crushed form, but very fine grinding is not required, and crushing the ore to a particle size distribution (measured by laser diffraction) corresponding to D ₉₀ < 750 µm and D ₅₀ < 200 µm has been shown to provide good results on a laboratory scale.

The suspension flows through a circulation circuit (4) between the leach tank/reactor (5) and the anode compartment (2) using a pump used to transport the suspension. On a laboratory scale, the volumetric flow rate of anolyte ranges from 30 ml/min to 90 ml/min. A solid-liquid separation unit (9), for example a filtration unit or a gravity separator, is in fluid communication with the suspension flow (for example, with a circulation circuit (4)) to ensure the separation of degold ore particles (10) from the aqueous phase, while an aqueous stream is formed consisting of the productive leach solution (11), which is collected and used in the next leach run.

The productive leach solution obtained from the previous leach stage is added to the tank/reactor (7). The productive leach solution forms a catholyte stream which is circulated between the reservoir (7) and the cathode side (3) of the cell using a suitable pump

- 3 047382 (not shown in the picture). On a laboratory scale, the catholyte flow rate ranges from 30 ml/min to 90 ml/min. The reduction of water that occurs at the cathode produces hydroxyl ions in parallel with the evolution of hydrogen, which is removed from the system (the hydrogen produced can likely be used for various purposes), and with the reduction of bromine to bromide (the residual bromine contained in the PLS). The precipitate progressively accumulates in the catholyte flow in the alkaline environment generated at the cathode at a pH of at least 7.5, for example at least 8, for example in the range from 8 to 12, preferably from 8.5 to 11.5, for example from 9 to 11 or from approximately 9.5 to 10.5. The circulation circuit (6) of the flow is equipped with a block (12) for separating solids and liquids to obtain a solid precipitate (13) and a degold solution (14). The degold solution can be fed with bromide by adding a fresh bromide source (not shown in the figure) and used as a feed stream in the subsequent leach step to supply the bromide component of the bromide leachate.

Large scale processes, such as 100 t/h leaching, may typically require circulation of anolyte and catholyte streams at a flow rate ranging from about 200 m ³ /h to about 500 m ³ /h. During operation, the anolyte stream, consisting of a mixture of refractory ore and an aqueous solution of alkali metal bromide, and the catholyte stream, consisting of the productive leach solution obtained in the previous run, are continuously recirculated in the stream circuits (4 and 6, respectively) of the plant shown in FIG. . 2. The anolyte gradually acquires a characteristic red-brown color due to the bromine generated at the anode; leaching, which occurs using bromine generated at the anode, occurs until the level of solubilized gold reaches a predetermined threshold. The cell is then stopped and the anolyte is released into the separation block (9) to obtain leach liquor (11) (eg, for use as a catholyte stream in the next run). Similarly, catholyte samples are periodically collected to determine the gold concentration in the aqueous phase. The separation block (12) is activated in response to a signal indicating that substantially all of the solubilized gold has been removed from the aqueous phase. The gold-containing solids (13) are subsequently processed using methods known in metallurgy to recover the precious metal, while the gold-free solution (14) is stored until the next run.

Fig. 3 illustrates the following technological solution providing continuous operation based on the concept shown in FIG. 2. This variant of the method includes directing the leach liquor separated from the anolyte stream into the catholyte stream circulation and using the degold solution separated from the catholyte stream as a feed solution fed to the anolyte circulation stream.

After the start-up phase and after reaching stable conditions, the leach liquor (11), collected by separating the anolyte flow into solid and liquid phases in the separation block (9), is fed into the reactor (7) containing the productive leach solution. Similarly, the degold solution (14), collected when the catholyte stream is separated into solid and liquid phases in the separation block (12), is used to supply the aqueous bromide solution to the anode side of the cell.

Accordingly, another aspect of the present invention is a method for recovering gold from a refractory ore in an electrolytic cell containing anode and cathode compartments, comprising the steps of:

circulation of the anolyte flow through the anode compartment, wherein the anolyte flow consists of a mixture of ore particles and an aqueous solution of bromide, while bromine is formed at the anode during the oxidation of bromide, due to which gold dissolves in the aqueous phase;

circulation of the catholyte flow through the catholyte compartment, wherein the catholyte flow consists of a productive leaching solution containing AuBr ₂ ^- and/or AuBr ₄ ^- ions, which was obtained after leaching of refractory gold ore with an aqueous bromine-bromide lixiviant, while a precipitate containing gold;

separating the ore particles from the anolyte stream to produce leach liquor and using this leach liquor as a feed solution for the cathode compartment;

separating the gold-containing precipitate from the catholyte stream to obtain a bromide-containing degold solution and using this degold solution as a bromide-containing feed solution for the anode compartment.

Next, the design of the electrolytic cell (1) and its operation will be described in more detail regarding the materials of the electrodes, their spatial configuration, the voltage values and current densities used.

The anode (A) and cathode (C) installed in the electrolytic cell (1) are connected to a DC power supply (not shown in the figure). The cell operates at a voltage ranging from 1 V to 9 V, for example from 3 V to 8 V, and at a current density ranging from 5 mA/cm ² to 100 mA/cm ² , for example from 10 mA /cm ² to 20 mA/cm ² . The anode (A) may be made of graphite (which is generally preferred due to its low cost and good resistance to bromine) or suitable metals including transition metal oxide electrodes.

- 4 047382 coated (for example, electrodes coated with RuO2 and IrO ₂ ), for example - coated electrodes made of titanium or an alloy of titanium with other metals. The cathode (C) can be made of graphite or titanium alloy (for example, grade 7 titanium alloy). A compartmentalized cell configuration is achieved using a separator in the form of an ion exchange membrane (Nation® type) placed in the space between the electrodes to separate the cell into cathode and anode compartments, or a diaphragm (e.g., a microporous separator made from glass microfibers or sieves made from of inert materials such as polyvinylidene fluoride (PVDF) to provide a physical barrier between the electrodes and prevent movement of solids between compartments.

The electrodes are preferably flat, plate-shaped and parallel to each other. In laboratory scale installations, good results have been obtained using electrodes that have a spacing ranging from about 3 mm to about 7 mm, each electrode having an active surface area ranging from about 5 cm ² to about 300 cm ² . Large-scale production typically requires the use of an electrolytic unit with multiple individual cells, that is, the installation of a large number of anodes and cathodes that are alternately spaced across the width of the electrolytic unit and electrically connected to opposite poles of the DC source. The inlet and outlet ports of the electrolytic unit are connected to suitable piping systems to uniformly distribute the anolyte and catholyte flows into the spaces between adjacent electrodes and to collect and discharge the effluent flows.

It is also possible to use electrolytic cells with cylindrical symmetry, that is, with a housing containing anode and cathode compartments arranged coaxially and concentrically to provide annular spaces (for example, the inner compartment is the anode compartment) into which anolyte and catholyte can be loaded or through which recycle anolyte and catholyte flows. One or more rod-shaped anodes and one or more rod-shaped cathodes are installed in corresponding compartments.

As stated above, methods for recovering gold from a solid precipitate separated from a productive leach solution in an alkaline environment are known in principle. The solid can be processed at the mining site or elsewhere using standard hydrometallurgical processes such as re-leaching, concentration, separation and recovery. For the above, methods such as pH change, solvent extraction, absorption using ion exchange or activated carbon can be used. See, for example, The Chemistry of Gold Extraction for a description of suitable methods; John Marsden, lane House; SME, 2006. It should be noted that the solid precipitate may contain metals other than gold, as shown by the experimental results below, for example Mn, Ni and Sb. Accordingly, the method of the present invention is potentially applicable to the recovery of gold and other valuable metals from ores.

Brief description of graphic materials

Fig. 1 is a diagram of a method according to publication EP 476862 in accordance with the prior art.

Fig. 2 is a diagram of a method according to one embodiment of the present invention.

Fig. 3 is a diagram of a method according to another embodiment of the present invention.

Fig. 4 is an illustration of the electrochemical cell used in the experimental work.

Fig. 5 is a photograph of the H-shaped cell used in the experimental work.

Fig. 6 is a scanning electron microscope with energy dispersive system (SEM-EDS) image of a gold-bearing solid obtained by the method of the present invention.

Fig. 7 is an SEM-EDS image of a gold-bearing solid obtained by the method of the present invention.

Fig. 8 is a plot of pH measured at the anodic and cathodic sides of an electrochemical cell operating according to the method of the present invention versus time.

Fig. 9 is a histogram showing the release of metals from PLS as a function of pH.

Description of embodiments of the invention

The refractory gold ore studies described below tested ore mined from a gold deposit in Kyrgyzstan. The ore was analyzed to determine its metallic components. The analysis described in this publication was performed using 1) Flame Gold Assay: FAA505: Flame Gold Assay for 50 g of ore with atomic absorption termination; and 2) multielement inductively coupled plasma mass spectrometry (ICPMS): IMS40B: hydrolysis with a mixture of 4 acids, flame analysis with

- 5 047382

ICPMS-ending, multi-element (49 elements). The results are presented in table. 1.

Table 1

Element Concentration Units A.I. 4.46 % As 1.34 % Fe 4.15 % Ca 2.60 % S 2.6 % Hell 18 million' ¹ Au 4.12 million' ¹ Si 436 million' ¹

Ore samples were ground; the particle size distribution given below was obtained (measured by laser diffraction using a Malvern PSD 3000 instrument): _D90 = 720 µm, D5o = 169 µm, Dio = 4.45 µm. The crushed ore was used in leaching studies.

Example 1 (comparative). Leaching of refractory gold ore by adding an aqueous Br ₂ /Br ^leachant to the leaching reactor.

Leaching of gold from refractory ore was studied by mixing 120 g of bromide leaching solutions with 60 g of refractory ore samples in 0.5 L glass vials. Leaching solutions with varying bromine/bromide concentrations were used as listed in the table below to determine the level of gold leaching with increasing bromine/bromide concentration. After stirring for 30 minutes, the contents of the vessel were filtered through a glass fiber filter and the degold ore was analyzed by fire assay. The test conditions and its results are given in table. 2.

table 2

Concentration of the leaching solution [Br ₂ ]; [Br] (wt.%) Total amount of Br ₂ (g/100 g of ore) Total amount of NaBr (g/100 g ore) Gold Leaching (%) 5%; 5% 10/100 10/100 0.7% 10%; 10% 20/100 20/100 9.1% 20%;20% 40/100 40/100 86.1%

The results indicate that large volumes of aqueous bromide reagent are required to achieve acceptable leaching levels.

Examples 2 and 3. Leaching of refractory gold ore using electrolytically generated aqueous Br ₂ /Br reagent.

The experimental setup used for the leaching tests consisted of a 1 L glass reactor equipped with an Ika Eurostar 60 stirrer and an electrochemical cell (Electro MP Cell from ElectroCell) and connected to a peristaltic pump (WATSON MARLOW 323D) to circulate the suspension consisting of the solution sodium bromide and ore particles between the reactor and the electrochemical cell. A side view of the individual elements used to assemble the electrochemical cell (30) is shown in FIG. 4. The graphite electrode plates are designated by reference numbers (31), (36). A flow frame (33) made of polyvinylidene fluoride (PVDF) is located between a pair of electrodes that are spaced 5 mm apart. The free space (34) of the polyvinylidene fluoride frame provides space for the flow of the electrolyzed solution exposed to the voltage applied to the electrodes; the active area of each electrode is 204 cm ² . The other (inactive) surface of each electrode is aligned with a PVDF plate (35, 32) located between the electrode and the first (37) and second (38) end plates made of stainless steel, which are located at the respective ends of the structure. Holes for mounting bolts (39) are located in the corners of the end plates and

PVDF plates to ensure tight bonding of the entire set of plates. When individual plates are combined with each other, a passage is formed through the holes (40), directing the flow of suspension from the reactor into space (34). The effluent flow returns to the reactor through the passage formed by the holes (41) (the direction of liquid flow is indicated by arrows).

- 6 047382

A solution of sodium bromide was loaded into a glass reactor. Ore was added gradually with stirring (100 rpm). Then the peristaltic pump was turned on (150 rpm) and the stirring speed in the reactor was reduced to 70 rpm; the volumetric flow rate was 55 ml/min. After 5 minutes, operation of the electrochemical cell began; the cell operated at a constant current of 2 A. The circulating solution acquired a red-brown color, indicating the formation of bromine. At the end of the experiments, the suspension was drained from the reactor and filtered. Leaching of gold from degold ore was measured by flame analysis. The exact test conditions and its results are given in table. 3.

Table 3

Example Ore (g) Leaching solution Circulation time Current Gold Leaching(%) 2 200 800 g of NaBr solution with a concentration of 20 wt. % 17 o'clock 2 A 79.5 3 100 900 g of NaBr solution with a concentration of 5 wt. % 8.5 hours 2 A 81.3

Example 4. Extraction of gold from a productive leaching solution in an electrochemically created alkaline environment.

The productive leach solution, containing 0.11 ^ppm gold (as measured by ICP-MS), was fed into a non-compartmented electrochemical cell (Electro MP Cell from ElectroCell). The area of each of the two electrodes installed in the cell was approximately 200 cm2. The electrodes were spaced 5 mm apart from each other. A voltage of 4 V was applied. During the process, the pH of the solution changed to alkaline and reached a value of approximately 8.73. The formation and precipitation of solids in an electrochemical cell was observed. After 6 hours, the contents of the cell were drained and filtered. The gold concentration measured in the filtrate (using a glass microfiber filter disk from Sartorius) was 10 ^ppb ; thus, gold recovery was determined to be 91%.

Example 5. Leaching gold from refractory ore using Br ₂ electrolytically generated in the anode half-cell, while simultaneously extracting gold from the productive leach solution in the cathode half-cell.

The experimental setup is shown in Fig. 5. The H-shaped electrochemical cell (1) consists of an anode compartment (2) and a cathode compartment (3), each of which has the shape of a substantially cylindrical glass vial having a height of approximately 100 mm, an outer diameter of 55 mm and internal diameter equal to 50 mm. The two vials are connected by a passage (20) consisting of two parts (20A) and (20C) extending from the side surfaces of the anode and cathode cylindrical vials, respectively; the parts are connected to each other to form a passage which has a length equal to 120 mm and a diameter approximately equal to 35 mm. Accordingly, the total length of the No-shaped electrochemical cell is 230 mm. The compartments are separated by a Sartorius glass microfiber disc (90 mm) placed transversely in the middle of the passage (21) connecting the two compartments; the disc blocks the transfer of solid particles from one side to the other through the passage connecting the bottles.

The anode (A) and cathode (C) are made of graphite plates, for example plates having a length of 150 mm, a width of 30 mm and a thickness of 5 mm. During operation, the liquid level in the H-shaped electrochemical cell is 40 mm. The anode and cathode are immersed in anolyte and catholyte, respectively, in such a way that the lower end of each electrode is located 15 mm above the bottom of the bottle; Accordingly, the active area of each electrode is 30 mm x 25 mm. The electrodes in a cylindrical bottle are located concentrically, that is, the longitudinal axis of the electrode is coaxial with the cylindrical bottles, so the electrodes are located at a distance of 17 cm from each other (distance from surface to surface). The anode (A) and cathode (C) are electrically connected to a direct current source (not shown in the figure).

A crushed sample (8.25 g) and an aqueous solution of sodium bromide (80 g) with a concentration of 4 wt.% were placed in the anode compartment. The previously obtained productive leaching solution (72.5 g) was loaded into the cathode compartment.

An electrical potential of 8 V was applied between the electrodes for 3.5 hours. Elementary

- 7 047382 bromine was released at the anode side, where the ore was suspended in the sodium bromide solution, as evidenced by the appearance of a characteristic red-brown color. On the cathode side the solution became cloudy. At the end of the experiment, the pH on the anode side and cathode side was approximately 2 and approximately 12, respectively.

The contents of each compartment were individually removed from the cell and filtered to remove solids (that is, the metal-free ore was removed from the anolyte, and the gold-bearing precipitate was removed from the catholyte). The anodic side clear filtrate and the cathode side clear filtrate were analyzed using inductively coupled plasma mass spectrometry (ICP-MS) for gold detection (using a Nexion300XX from PerkinElmer). The results are shown in table. 4.

Table 4

Anode side Cathode side Initial Au concentration in aqueous phase ( ^ppb ) 0 23.8 billion' ¹ Final Au concentration in aqueous phase ( ^ppb ) 26 billion' ¹ <10 billion' ¹

It can be seen that on the anode side, where elemental bromine was released, leaching from the refractory gold ore was ensured, while the gold was solubilized in the aqueous leaching solution. In parallel, on the cathode side, with the formation of an alkaline pH, gold separated from the aqueous productive leaching solution.

Example 6 Leaching gold from refractory ore using Br ₂ electrolytically generated in the anode half-cell while recovering gold from the productive leach solution in the cathode half-cell: SEM-EDS analysis of the precipitate collected on the cathode side.

The experiment used an H-shaped cell illustrated in FIG. 5. The procedure and operating parameters were the same as those described in Example 5.

A crushed sample (10.6 g) and an aqueous solution of sodium bromide (100 g) with a concentration of 4 wt.% were placed in the anode compartment. The previously obtained productive leach solution (101.4 g), enriched with gold dissolved in an aqueous bromine/bromide solution, was loaded into the cathode compartment to study the release of gold during the reduction of water on the cathode side.

An electrical potential of 8 V was applied between the electrodes for 3.5 hours. At the end of the experiment, the contents of each compartment were removed from the cell and filtered to remove solids. The solid formed on the cathode side was carefully washed with 45 ml double-distilled water, dried and analyzed by SEM-EDS microscopy. The resulting images are attached in the form of Figs. 6 and 7, and the corresponding analysis results are presented in the form of tables next to the images.

In the left image from FIG. 6 you can see a light area located approximately in the middle of the image. This area and some additional points around it were examined to determine the chemical elemental composition (6 points in total - see right image). The results presented in the table show that gold can be recovered from an aqueous productive leach solution at an alkaline pH formed on the cathode side; gold precipitated in particle form among particles that did not contain gold. Fig. 7 and the corresponding chemical composition also indicate the presence of a high gold content region (point 1) and a gold free region (point 2) in the solid obtained at the cathode side.

Example 7. Leaching of gold from refractory ore using electrochemically generated Br ₂ with simultaneous separation of gold from the productive leaching solution in an electrochemically generated alkaline medium: scheme of a method based on circulating flows of anolyte and catholyte.

The experimental setup used in this example is shown schematically in FIG. 2. It includes a flow electrochemical cell (1), divided into anode and cathode compartments (2 and 3, respectively). The electrochemical cell (1) used in the experiment is described in detail in examples 2 and 3. In this experiment, the cell was divided into two compartments by a Daramic separator. Each compartment was equipped with a peristaltic pump (WATSON MARLOW 323D) to circulate a slurry consisting of sodium bromide solution and ore particles between the reservoir (5) and the anode compartment (2), and circulation between the PLS contained in the reservoir (7) and the cathode compartment (3). In the experiment, reservoir (5) was a 1 L glass reactor equipped with an Ika Eurostar 60 stirrer, and reservoir (7) was a round bottom flask.

The ore was placed in a glass reactor (5) with a volume of 1 liter. The ore sample had a gold content of 2.14 ^ppm ; it consisted of 50% fresh ore and 50% depleted ore. An aqueous solution of sodium bromide (800 g solution with a concentration of 4 wt.%) was added to the reactor (5). The suspension was stirred at a speed of 130 rpm. PLS from previous run (856 g, 75 ^ppb gold)

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Claims (2)

loaded into a round-bottomed flask (7). The anolyte and catholyte flows were pumped by corresponding peristaltic pumps at a speed of 100 rpm. The current in the cell was set to 2 A, and the experiment lasted for 6 hours. Changes in pH on the anode and cathode sides were periodically recorded. Graphs of pH versus time shown in Fig. 8 demonstrate the formation of an alkaline medium on the cathode side. At the end of the experiment, the depleted ore was isolated by anode side filtration and analyzed using flame analysis to determine the metal content remaining in the ore. In flame analysis, the gold concentration in the ore was found to have decreased to 0.46 ^ppb gold, indicating that 79% of the gold was leached by electrochemically generated bromine. The precipitate formed on the cathode side was isolated by filtration. The precipitate and its mother liquor (the filtrate obtained from the cathode side) were subjected to ICP analysis to determine the amount of metal recovered from the PLS under alkaline conditions. ICP analysis showed that 63% gold was recovered (28 ^ppb gold measured in leachate). The gold content of the collected precipitate was approximately ¹ ppb. ICP analysis also found other metals co-occurring with gold, as shown in the table below. Table 5 Element Concentration Units Element Concentration Units Hell 13 million' ¹ Na 3.75% AI 1.32% Ni 97 ppm ¹ As 1.35 % Pb 0.19 % Cd 31 mln' ¹ Sb 464 mln' ¹ Si 473 million' ¹ Sr 110 million' ¹ Fe 7.35% Ti 25 ppm ¹ MD 4.78% V 11 million' ¹ MP 0.92 % Zn 0.49 % Example 8 Metal recovery from PLS at different pH values. The next series of experiments examined the recovery of various metals from the PLS mixture as a function of pH to determine the optimal pH value for metal recovery. Sodium hydroxide was added to samples of the PLS mixture weighing 100 g until the target pH value was reached (the range of pH values was studied from 4 to 12 in increments of 2 pH units, namely pH 4, 6, 8, 10 and 12; solutions with pH 4 , 6, 8 and 10 were obtained by adding a 10% NaOH aqueous solution to the PLS sample; a pH 12 solution was obtained by adding an aqueous solution and solid NaOH). Precipitated solids were then isolated from each sample and metal content determined by ICP analysis. The results are presented in the form of a histogram in Fig. 9. It can be seen that the release of gold reaches maximum values at a pH value approximately equal to 10; increasing pH to 12 may result in decreased gold recovery. CLAIM 1. A method for extracting gold from sulfide-containing refractory gold ore in an electrolytic cell containing anode and cathode compartments, including stages in which an anolyte is supplied to the anode compartment, wherein the anolyte is a mixture of ore particles and an aqueous solution of bromide; the catholyte is supplied to the cathode compartment, the catholyte being a productive leaching solution containing AuBr ₂ - and/or AuBr4 ions, which was obtained after leaching of refractory gold ore with an aqueous bromine/bromide lixiviant; electrical voltage is applied to the electrodes, due to which bromide is oxidized to bromine at the anode with the dissolution of gold in the anolyte, and water is reduced at the cathode with the formation of an alkaline medium and the precipitation of a solid substance containing gold in the catholyte; separating ore particles from the anolyte stream to produce leach liquor; separating the gold-containing precipitate from the catholyte to collect the gold-containing precipitate and obtain a degold-free solution containing bromide for reuse as the original aqueous bromide solution. 2. The method according to claim 1, characterized in that the supply of anolyte to the anode compartment includes circulation of the anolyte flow between the anode compartment and the leaching reactor, where ore particles are mixed with an aqueous solution of bromide, and the supply of catholyte to the cathode compartment includes circulation of the catholyte flow -