A PROCESS FOR RECOVERING GOLD FROM ORES

Abstract

A process for recovering gold from a refractory gold ore, comprising the steps of: electrolyzing a mixture consisting of the ore particles and an aqueous bromide solution in an electrolytic cell having anode and cathode, wherein bromine is produced at the anode by oxidation of the bromide, thereby dissolving gold in the aqueous phase; separating the ore particles from the aqueous phase to obtain a leach liquor; adjusting the pH of the leach liquor to the alkaline range to produce a gold-containing precipitate; collecting the gold-containing precipitate and recycling a bromide-containing barren solution for reuse as an aqueous bromide feed solution.

Claims

1. A process for recovering gold from a refractory gold ore, comprising the steps of: electrolyzing a mixture consisting of the ore particles and an aqueous bromide solution in an electrolytic cell having anode and cathode, wherein bromine is produced at the anode by oxidation of the bromide, thereby dissolving gold in the aqueous phase; separating the ore particles from the aqueous phase to obtain a leach liquor; adjusting the pH of the leach liquor to the alkaline range to produce a gold-containing precipitate; collecting the gold-containing precipitate and recycling a bromide-containing barren solution for reuse as an aqueous bromide feed solution.

2. A process according to claim 1, wherein the pH of the leach liquor is adjusted to the alkaline range by electrolyzing the leach liquor in an electrolytic cell having anode and cathode, whereby hydroxide ions are produced at a cathode upon water reduction.

3. A process according to claim 1, wherein the refractory ore is sulfide-containing ore.

4. A process for recovering gold from a refractory gold ore in an electrolytic cell having anodic and cathodic compartments, comprising the steps of: feeding an anolyte to the anodic compartment, the anolyte being a mixture of the ore particles and an aqueous bromide solution; feeding a catholyte to the catholyte compartment, the catholyte being AuBr2 and/or AuBr-r ions-containing pregnant leach solution, which was obtained after the leaching of a refractory gold ore with an aqueous bromine/bromide lixiviant; applying an electrical voltage across the electrodes, thereby oxidizing bromide to bromine at the anode to dissolve gold in the anolyte and reducing water at the cathode to create an alkaline environment and precipitate a gold-containing solid in the catholyte; separating the ore particles from the anolyte stream to obtain a leach liquor; separating the gold-containing precipitate from the catholyte to collect the gold-containing precipitate and obtain bromide-containing barren solution for reuse as an aqueous bromide feed solution.

5. A process according to claim 4, wherein the feeding of the anolyte to the anodic compartment includes the circulation of an anolyte stream between the anodic compartment and a leach reactor where ore particles were mixed with the aqueous bromide solution; and the feeding of the catholyte to the catholyte compartment includes the circulation of a catholyte stream between the cathodic compartment and a reactor accommodating pregnant leach solution.

6. A process according to claim 5, comprising directing the leach liquor that is separated from the anolyte stream, to the circulation of the catholyte stream, and using the barren solution that is separated from the catholyte stream as feed solution in the circulation of the anolyte stream.

7. A process according to claim 4, wherein the refractory ore is sulfide-containing ore.

8. A process according to claim 3, wherein the sulfide content of refractory ore is from 1 to 10 wt %.

9. A process according to claim 1, where the alkaline environment at which gold-containing solid is precipitated is at pH in the range from 7.5 to 11.5.

Description

IN THE DRAWINGS

[0042] FIG. 1 is a process design according to the prior art EP 476862.

[0043] FIG. 2 is a process design according to one variant of the invention.

[0044] FIG. 3 is a process design according to another variant of the invention.

[0045] FIG. 4 is an illustration of an electrochemical cell used in the experimental work.

[0046] FIG. 5 is a photo of H-shaped cell used in the experimental work.

[0047] FIG. 6 is SEM-EDS microscopy image of a gold-containing solid recovered by the method of the invention.

[0048] FIG. 7 is SEM-EDS microscopy image of a gold-containing solid recovered by the method of the invention.

[0049] FIG. 8 is pH versus time plot measured in the anodic and cathodic sides of an electrochemical cell operated by the method of the invention.

[0050] FIG. 9 is a bar diagram showing separability of metals from PLS as function of pH.

EXAMPLES

[0051] In the refractory gold ore studies reported below, an ore obtained from a gold mining site in Kyrgyzstan was tested. The ore was analyzed to determine its metal constituents. Analysis reported herein was performed using 1) Fire Assay Gold Analysis: FAA505: 50 g fire assay, Atomic Absorption finish, Gold; and 2) Multi Elements ICPMS: IMS40B: 4 acid digestion, ICPMS, Finish, Multi Elements (49 elements). Results are set out in Table 1.

TABLE-US-00001 TABLE 1 element concentration units Al 4.46 % As 1.34 % Fe 4.15 % Ca 2.60 % S 2.6 % Ag 18 ppm Au 4.12 ppm Cu 436 ppm

[0052] Ore samples were milled; the particle size distribution set out below was obtained (measured by laser diffraction using Malvern PSD 3000): D.sub.90=720 μm, D.sub.50=169 μm, D.sub.10=4.45 μm. The crushed ore was used in the leaching studies.

Example 1 (Comparative)

Leaching Refractory Gold Ore by Addition of Aqueous Br.SUB.2./Br— Lixiviant to the Leach Reactor

[0053] Gold leaching from refractory ore was tested by mixing 120 gr bromine/bromide leach solutions with 60 gr of refractory ore samples in 0.5 L glass bottles. Leach solutions with varying bromine/bromide concentrations were used, as tabulated below, to determine the level of gold leaching with increasing loading of bromine/bromide. After 30 minutes of mixing, the content of the vessel was filtered on a glass fiber filter, and the depleted ore was analyzed by fire assay. Test conditions and results are set out in Table 2.

TABLE-US-00002 TABLE 2 Concentration of leach solution [Br.sub.2 ];[Br.sup.−] Total Br2 Total NaBr Gold leaching (wt %) (g/100 g ore) (g/100 g ore) (%) 5%;5% 10/100 10/100 0.7% 10%;10% 20/100 20/100 9.1% 20%;20% 40/100 40/100 86.1%

[0054] The results indicate that high volumes of bromine/bromide aqueous reagent are needed to reach acceptable leaching level.

Examples 2 and 3

Leaching Refractory Gold Ore with the Aid of Electrolytically Generated Aqueous Br.SUB.2./Br— Reagent

[0055] The experimental set-up used for the leaching tests included a 1 L glass reactor equipped with Ika Eurostar 60 mixer and an electrochemical cell (Electro MP Cell from ElectroCell), connected with a peristaltic pump (WATSON MARLOW323 D) to enable circulation of a slurry consisting of sodium bromide solution and the particulate ore between the reactor and the electrochemical cell. A side view of the individual elements used to assemble the electrochemical cell (30) are shown in FIG. 4. The graphite electrode plates are indicated by numerals (31), (36). A flow frame (33) made of polyvinylidene fluoride (PVDF) is positioned between the pair of electrodes, which are spaced 5 mm apart. The open area (34) of the PVDF frame provides the flow space for the electrolyzed solution, to be exposed to the potential applied across electrodes; the active area of each electrode is 204 cm.sup.2. The other (non-active face) of each electrode is stacked to PVDF plate (35, 32), interposed between the electrode and a first (37) and second (38) end plates made of stainless steel which are positioned across the respective ends of the structure. Openings to receive mounting bolts (39) are located at the corners of the end plates and the PVDF plates, to allow the entire set of plates to be held together tightly. On juxtaposing the individual plates together, a passage is created through openings (40) to direct incoming stream of slurry from the reactor into space (34). Outgoing stream is returned to the reactor via a passage created by openings (41) (direction of fluid flow is marked by arrows).

[0056] The glass reactor was charged with sodium bromide solution. The ore is added gradually with stirring (100 rpm). The peristaltic pump was then turned on (150 rpm), and the stirring velocity at the reactor was lowered to 70 rpm; the circulation flow rate was 55 ml/min. After 5 minutes, the operation of the electrochemical cell began; the cell was operated at a constant amperage of 2 A. The circulated solution acquired a red-brown hue indicating the formation of bromine. At the end of the experiments, the slurry was discharged from the reactor and filtered. Gold leaching from the depleted ore was measured by fire assay method. The exact test conditions and results are set out in Table 3.

TABLE-US-00003 TABLE 3 Ore Leaching Circulation gold leaching Example (g) solution time Current (%) 2 200 800 g of 20 wt % 17 hours 2 A 79.5 NaBr solution 3 100 900 g of 5 wt % 8.5 2 A 81.3 NaBr solution

Example 4

Separation of Gold from Pregnant Leach Solution in Electrochemically-Created Alkaline Environment

[0057] A pregnant leach solution containing 0.11 ppm of gold (measured by ICP-MS) was introduced into an undivided electrochemical cell (Electro MP Cell from ElectroCell). The area of each of the two electrodes mounted in the cell was ˜200 cm. The electrodes were spaced 5 mm apart. A voltage of 4 V was applied. During the process, the pH of the solution changed to alkaline, reaching a pH of about 8.73. Formation and settling of solids was observed in the electrochemical cell. After 6 hours, the cell content was discharged and filtered. The gold concentration measured in the filtrate (glass microfiber filter disc from Sartorius was used for the filtration) was 10 ppb; hence 91% gold recovery was determined.

Example 5

Leaching Gold from Refractory Ore Using Electrolytically Generated Br.SUB.2 .in Anodic Half-Cell with Simultaneous Separation of Gold from Pregnant Leach Solution in Cathodic Half-Cell

[0058] The experimental set-up is shown in FIG. 5. The H-shaped electrochemical cell (1) consists of an anode compartment (2) and cathode compartment (3), each in the form of an essentially cylindrical glass flask, about 100 mm high with outer diameter of 55 mm and inner diameter of 50 mm. The two vessels are connected by a passage (20), consisting of two parts (20A) and (20C) extending from the lateral surfaces of the anodic and cathodic cylindrical flasks, respectively; the parts are joined together to create a passage which is 120 mm long, with a diameter of ˜35 mm. Hence the total length of the H-shaped electrochemical cell is 230 mm. The compartments are separated by a glass microfiber disc from Sartorius (90 mm) placed transversely in the middle of the passage (21) joining the two compartments; the disc blocks transfer of solid particles from one side to another via the passage joining the flasks.

[0059] The anode (A) and the cathode (C) are made of graphite plates i.e., 150 mm long, 30 mm wide, 5 mm thick plates. In operation, the liquid level in the H-shaped electrochemical cell is 40 mm. The anode and cathode are submerged in the anolyte and catholyte, respectively, such that the lowermost end of the electrode is 15 mm above the bottom of the flask; hence the active area for each electrode is 30 mm×25 mm. The electrodes are positioned concentrically in the cylindrical flask, that is, the longitudinal axis of the electrode is coaxial with the cylindrical flasks, thus the electrodes are spaced 17 cm apart (face-to-face distance). Anode (A) and cathode (C) are electrically connected to a source of direct current (not shown).

[0060] The crushed sample (8.25 g) and 4 wt % aqueous sodium bromide solution (80 g) were added to the anodic compartment. A previously obtained pregnant leach solution (72.5 g) was loaded into the cathode side.

[0061] Electric potential of 8V was applied between the electrodes for 3.5 hours. Elemental bromine evolved at the anode side where the ore was suspended in the sodium bromide solution, indicated by the appearance of the characteristic red-brown color. On the cathode side, the solution became hazy. At the end of the experiment, pH at the anode side and cathode side was ˜2 and ˜12, respectively.

[0062] The content of each compartment was separately withdrawn from the cell and filtered to remove the solids (i.e., the metal-depleted ore was removed from the anolyte and the gold-containing precipitate is removed from the catholyte). The clear filtrate from the anodic side and the clear filtrate from the cathodic side were assayed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to detect gold (the instrument used was Nexion300XX, PerkinElmer). The results are tabulated in Table 4.

TABLE-US-00004 TABLE 4 Anode side Cathode side Initial concentration 23.8 ppb of Au in the aqueous phase (ppb) 0 final concentration 26 ppb <10 ppb of Au in the aqueous phase (ppb)

[0063] It is seen that in the anodic side, where elemental bromine evolved, leaching from refractory gold ore was achieved, with gold being solubilized in the aqueous leach solution. In parallel, in the cathodic side, under alkaline pH build-up, gold was separated from the aqueous pregnant leach solution.

Example 6

Leaching Gold from Refractory Ore Using Electrolytically Generated Br.SUB.2 .in Anodic Half-Cell with Simultaneous Separation of Gold from Pregnant Leach Solution in Cathodic Half-Cell: SEM-EDS Analysis of the Precipitate Collected in the Cathodic Side

[0064] The H-shaped electrochemical cell illustrated in FIG. 5 was used in the experiment. The procedure and operation parameters are as described in Example 5.

[0065] A crushed sample (10.6 g) and 4% wt % aqueous sodium bromide solution (100 g) were added to the anodic compartment. The cathodic side was charged with a previously obtained pregnant leach solution (101.4 g), spiked with gold dissolved in aqueous bromine/bromide, to investigate the separability of gold under water reduction in the cathodic side.

[0066] An electric potential of 8V was applied across the electrodes for 3.5 hours. At the end of the experiment, the content of each compartment was withdrawn from the cell and filtrated to remove the solids. The solid that was formed in the cathode side was gently washed with 45 ml of DD water, dried and analyzed by SEM-EDS microscopy. Images obtained are appended as FIGS. 6 and 7, with the corresponding analysis tabulated alongside the image.

[0067] In the left image of FIG. 6, there is seen a light region approximately in the middle of the image. The region and some additional points in its environment were examined to determine the chemical elemental composition (total of six points—see the right image). The results tabulated indicate that gold is separable from an aqueous pregnant leach solution under alkaline pH build-up in the cathodic side; gold settled in a particulate form within non-gold containing particles. FIG. 7 and the corresponding chemical composition also indicate the presence of gold-rich region (point 1) as opposed to gold-free region (point 2) in the solid produced at the cathodic side.

Example 7

Leaching Gold from Refractory Ore Using Electrochemically-Generated Br.SUB.2 .with Simultaneous Separation of Gold from Pregnant Leach Solution Under Electrochemically-Generated Alkaline Environment: A Process Design Based on Circulated Anolyte and Catholyte Streams

[0068] The experimental set-up used in this example is shown schematically in FIG. 2. It includes an electrochemical flow cell (1) divided into anodic and cathodic compartments (2 and 3, respectively). The electrochemical cell (1) used in the experiment is the one described in detail in Examples 2 and 3. In this experiment the cell was divided with Daramic separator into two compartments. Each compartment is equipped with a peristaltic pump (WATSON MARLOW323 D) to enable circulation of a slurry consisting of sodium bromide solution and the particulate ore between tank (5) and the anodic compartment (2), and the circulation between the PLS held in tank (7) and the cathodic compartment (3). In the experiment, tank (5) was a 1 L glass reactor equipped with Ika Eurostar 60 mixer and tank (3) a round-bottomed flask.

[0069] An ore was introduced into the 1 L glass reactor (5). The ore sample had 2.14 ppm gold content; it consisted of 50% fresh ore and 50% of depleted ore. Aqueous sodium bromide (800 g of 4 wt % solution) was added to reactor (5). The slurry was kept under stirring at 130 rpm. PLS from a previous run (856 g, gold content 75 PPB) was charged into the round-bottomed flask (7).

[0070] The anolyte and catholyte streams were pumped by their respective peristaltic pumps at 100 rpm. The cell current was set to 2 A and the experiment lasted 6 hours. pH variation was recorded periodically for the anodic and cathodic sides. The pH versus time plots in FIG. 8 show the built-up of an alkaline environment in the cathodic side.

[0071] At the end of the experiment, the depleted ore was recovered by filtration from the anodic side and analyzed by fire assay to determine the metals content remained in the ore. In the fire assay it was found that gold concentration in the ore was reduced to 0.46 ppm gold, indicating 79% of the gold was leached with the aid of the electrolytically-generated bromine.

[0072] The precipitate formed in the cathodic side was isolated by filtration. The precipitate and its mother liquor (the filtrate of the cathodic side) were submitted to ICP analysis to determine the amount of metal separated from the PLS under the alkaline conditions. The ICP analysis indicates that 63% of the gold was recovered (28 PPB gold measured in the filtrate). The gold content in the precipitate collected was ˜ 1 ppm. ICP analysis detected other metals that were recovered alongside gold, as tabulated below.

TABLE-US-00005 TABLE 5 Element concentration units Element concentration units Ag 13 ppm Na 3.75 % Al 1.32 % Ni 97 ppm As 1.35 % Pb 0.19 % Cd 31 ppm Sb 464 ppm Cu 473 ppm Sr 110 ppm Fe 7.35 % Ti 25 ppm Mg 4.78 % V 11 ppm Mn 0.92 % Zn 0.49 %

Example 8

Metals Recovery from PLS Under pH Variation

[0073] In the next set of experiments, recovery of different metals from PLS mix was measured as function of pH, to determine optimal pH for metals separation. To 100 grams samples of PLS mix was added sodium hydroxide to reach a target pH (the pH range investigated was from 4 to 12 at increments of two pH units, namely, at pH=4, 6, 8, 10 and 12; solutions with pH=4, 6, 8 and 10 were formed by addition of 10% aqueous NaOH solution to the PLS sample; the solution with pH=12 was prepared by addition of aqueous and solid NaOH). Then solids precipitated were separated by filtration from each sample and metal content was determined by ICP analysis. Results are shown in the form of a bar diagram in FIG. 9. It is seen that separability of gold reaches maximum values at around pH=10; pH increase to 12 could lead to a drop gold recovery.