ELECTROCHEMICAL RECOVERY OF GOLD FROM CONCENTRATE BY USING SULFUR-GRAPHITE ELECTRODE AS THE LEACHING AGENT SOURCE

Transcription

1 Journal Journal of Chemical of Chemical Technology Technology and Metallurgy, and Metallurgy, 53, 3, 53, 2018, 3, ELECTROCHEMICAL RECOVERY OF GOLD FROM CONCENTRATE BY USING SULFUR-GRAPHITE ELECTRODE AS THE LEACHING AGENT SOURCE Rashid K. Nadirov, Leila I. Syzdykova, Aisulu K. Zhussupova Department of General and Inorganic Chemistry Faculty of Chemistry and Chemical Engineering al-farabi Kazakh National University, al-farabi av., 71 Almaty , Kazakhstan Received 07 March 2017 Accepted 20 December 2017 ABSTRACT In the present paper electrochemical leaching has been applied to recover gold from the gold concentrate (Vasilkovskoe deposit, Kazakhstan) into water solution. The specially prepared sulfur-graphite electrode (SGE) was used as a source of sulfurcontaining anions, acting as leaching agents. To prevent anodic oxidation of sulfur-containing anions, the cathodic compartment was separated from the anodic one with PVC separator. Cathodic polarization of SGE immersed into NaOH aqueous solution in the presence of gold concentrate leads to gold partial recover into solution. The following leaching process conditions allow to obtain maximum gold recovery (93 %): С NaOH = 2 M; i = 80 ma cm -2 ; L:S = 7; leaching duration = 210 min. The SGE consumption is about 55 g per 1 g of the gold in the solution under the above-mentioned conditions. Keywords: gold, electrochemical leaching, sulfur - graphite electrode. INTRODUCTION Despite the effectiveness of cyanides to leach gold from sulphide ores and concentrates, application of them involves significant risk due to their high toxicity [1, 2]. Therefore, considerable efforts are expended to develop new gold recovery process that excludes cyanides. Among them, some sulfuric compounds are commonly interesting and widely investigated [3, 4]. It has been found in the early 90s that when elemental sulfur is dissolved in alkaline solutions, the solution formed can be used to recover gold from the raw material [5-10]. This is due to the formation of sulfur containing anions, such as HS -, S, S n, S 2, which can form complexes with auric ions and can be considered as leaching agents. Although not so aggressive as cyanide, the sulfur containing anions offer several technological advantages including their lower toxicity and efficiency. A lot of fundamental studies have been contributed to gold leaching by using aforementioned leaching agents. The solutions of calcium hydroxide and sodium hydroxide are attracting the most attention [11]. Sulfur in such solutions is presented in polysulfide (S n ), hydrosulfide (HS - ), and thiosulfate (S 2 ) forms which are in equilibrium with each other. During the partial oxidation of the sulfide minerals, the gold is accumulated in the elemental sulfur; this circumstance is an important argument in favour of using the sulfur-containing species at leaching. The instability over time is considered as the big drawback of using sulfur-containing agents for gold leaching. Thus, thiosulfate ion can react with the formation of sulfate ions and elementary sulfur [1, 2]: S 2 + 2O 2 + H 2 O = 2SO 4 (1) 2S 2 + 2H 2 O = 2SO 4 + 4S 0 + 2OH - (2) In an aqueous solution at a light exposed polysulfide ions are oxidized to form sulfur: 2S n + 2H 2 O + O 2 = 2nS 0 + 4OH - (3) The processes mentioned lead to a considerable consumption of sulfur per unit of gold amount recovered to a solution and, consequently, to increase the amount of elemental sulfur in the pulp. This circumstance requires 556

2 Rashid K. Nadirov, Leila I. Syzdykova, Aisulu K. Zhussupova higher efforts for separating sulfur (with gold) from the ore material and for further separating gold from sulfur. The combination in one apparatus of sulfur-containing agents generation and direct leaching of gold from the raw material may allow to reduce dramatically the amount of sulfur in the final pulp. This approach can be implemented using an electrochemical method. We have implemented a process of sulfur-containing anions generation by the cathodic polarization of specially designed sulfur-graphite electrode in an alkaline solution in the presence of gold concentrate, followed by a gold leaching in the same apparatus (electrolyzer). There are many examples of successful use of electrochemical leaching of gold by the anodic generation of leaching agents [13-15]. However, this approach has not been previously proposed for obtaining sulfurcontaining ones. EXPERIMENTAL Characteristics of the original concentrate The gold concentrate (Vasilkovskoe deposit, Kazakhstan), particle size -74μm, was used for the experiments. The mineralogical composition of the concentrate determined by XRD method, is presented in Table 1. Chemical composition of the concentrate, determined by optical emission spectroscopy with inductively coupled plasma, is presented in Table 2. The concentrate was used as received. The specific surface of the ground ore, determined by the BET method, was 3.07 m 2 g -1. Sulfur-Graphite Electrode Preparing Pre-grinding the mixture of sulfur and graphite (mass ratio 70:30) was melted at a temperature of o C. The resulting homogeneous mass was applied to the stainless steel with a mesh size of cm and 0.5 mm thick rods, and then held up to cure at room temperature. Electrochemical Measurements of Sulfur-Graphite Electrode (SGE) Electrochemical measurements in three-electrode configuration were taken in a specifically pdesigned cell divided into two parts every with volume of 50 ml. The prepared sulfur-graphite (SG) plate was taken as a working electrode. A plate of stainless steel was used as counter electrode. A solution of NaOH served as an electrolyte. Chlorine-silver electrode (Ag/AgCl in saturated KCl solution) with potential V (vs. a normal hydrogen electrode (NHE)) was used as a reference electrode. All potentials values here are presented vs. NHE. Potentiostat-galvanostat «Elins P-40X» was used to maintain the potential or current (depending on which of the modes, potentiostatic or galvanostatic, is used in the experiment) of working electrode. Aliquots (5 ml) of solution were collected from the cathode compartment at predetermined time intervals and the concentration of anionic sulfur species in a solution were analyzed using well-known methods [16]. Electrochemical Leaching The kinetics of electrochemical leaching of precious metals from ore was studied on the specifically designed test unit. A schematic diagram of the experimental setup is presented in Fig. 1. The electrolytic cell was made of resin plate; a PVC separator was used to divide the cell into two compartments of 250 ml. SG plate with a geometric surface 25 cm 2 and a lead plate were used as a cathode and an anode, respectively. The experimental setup was operated using the potentiostat-galvanostat, mentioned earlier. To carry out the leaching experiments, the concentrate samples were mixed with 0,5 M NaOH solution, and the resultant slurry was introduced into the cathode compartment. The anode compartment was filled with alkaline solution. Each test on electrochemical leaching Table 1. Mineralogical composition of the concentrate. FeAsS SiO 2 Al 2 FeS 2 CaO MgO Other Content, % Table 2. Chemical composition of the concentrate. Fe As Si Al S total S Sulfate Au Ag Other Content, % (g/t) (48.7) (4.9)

3 Journal of Chemical Technology and Metallurgy, 53, 3, 2018 where m 1 and m o are the masses of gold in the residue after leaching and in the original concentrate, respectively. RESULTS AND DISCUSSION Electrochemical Measurements CV curves, obtained at SGE in 1 M NaOH, are presented in Fig. 2. The cathodic peak in the curve corresponds to the two-electron sulfur recovery to sulfide- and polysulfide ions [17]: S 0 + 2e = S (4) ns 0 + 2e = S n (5) Fig. 1. Schematic diagram of experimental setup for electrochemical leaching (1 - slurry; 2 - stirring device; 3 - Sulfur-Graphite Electrode; 4 - PVC separator; 5 - potentiostat-galvanostat; 6 - lead plate). was carried out at 25 ± 3 o C. Impeller speed was 120 min -1. Aliquots (5 ml) of slurry were collected from the cathode compartment at predetermined time intervals and, after filtration, the concentration of silver and gold in the leach liquor were analyzed using atomic adsorption spectrometer ICP OS Optima 8000 (PerkinElmer). The solid residue after leaching was analyzed by X- ray fluorescence (Spectroscan) as well as by DRON-3M model X-ray diffractometer. The gold recovery into the solution was calculated by the formula: The anodic peak, respectively, associated with the oxidation of the resulting sulfide ions to elemental sulfur. The cathodic potentiostatic curves of the sulfurgraphite electrode obtained in 1 M H 2 SO 4 are presented in Fig. 3. The constant value of the current density during the electrolysis indicates a lack of passivation of the working electrode. Over the time of electrolysis, the solution turns yellow, which confirmed the formation of the sodium polysulfide. It was of interest to determine the anionic forms of sulfur in the anolyte in the process of cathodic polarization. We expected that sulfide, polysulfide and hydrosulphide ions were formed due to the hydrolysis of S n and S : S + Н 2 О = HS - + OH - (6) S n + H 2 O = HS - n + OH- (7) In addition, thiosulfate ions may be formed in alkaline solution: Fig. 2. CV curve of sulfur-graphite electrode in 1 M NaOH solution. 558

4 Rashid K. Nadirov, Leila I. Syzdykova, Aisulu K. Zhussupova Fig. 3. Potentiostatic curves of sulfur-graphite electrode (SGE) in 1 M NaOH solution. 4S + 6OH - = 2S + S 2 + 3H 2 O (8) Table 3 presents the results of the dependence of sulfur concentration change in different form, from the duration of electrolysis. The values of the total sulfur concentration in the solution are correlated with the values obtained from the Faraday s law: where m is the mass of the substance liberated at an electrode in grams (the mass of sulfur in this case), I is the current in Amperes, τ is the duration of electrolysis, M is the molar mass of the substance in grams per mol, CE is current efficiency as a fraction of a unit, F = 26.8 is Faraday constant. In this case, the total output values of CE have been calculated for the processes of anodic dissolution of sulfur. Estimated value of the CE for the target processes of sulfur transition into solution is , which is acceptable value in the electrochemical production. Electrochemical Leaching of the Concentrate General Issues It has been found that stirring of pulp consisting of concentrate and NaOH solution under cathodic polari- Table 3. Change of sulfur concentration in the catholyte with time. Content of sulfur Duration of electrolysis, min in different forms Stotal, g/l S, g/l Sn, g/l (HS - n + HS - ), g/l S2O3, g/l

5 Journal of Chemical Technology and Metallurgy, 53, 3, 2018 Fig. 4. Effect of the current density and NaOH concentration on Au recovery (leaching duration min; L:S = 5). zation of SGE leads to partial dissolution of gold. This process is caused by the interaction of metallic gold with the anionic form of sulfur that formed under condition of electrolysis; simplistically these interactions can be represented as equations (9-11): 4Au + 8S 2 + O 2 + 2H 2 O = 4Au(S 2 ) OH (9) 2Au + 2H 2 S + 2HS - = Au 2 (HS) 2 S + H 2 (10) Au + S n = AuS - + (n-1)s + e (11) As mentioned by Hiskey and co-workers [18], the stability constant of the sulfide and thiosulfate complexes of gold are and 10 26, respectively. This fact determines the thermodynamic substantiation of gold dissolution in the presence of the abovementioned anionic forms of sulfur. At the same time to find acceptable conditions of electrochemical leaching of gold, it is important to investigate the influence of experimental factors on the process feasibility. To establish the need of usage the diaphragm during the electrochemical leaching of gold concentrate, a control experiment was carried out in a non-diaphragm electrolyzer. It has been found that the extraction of gold in the solution in this case does not exceed % for 5 hours of electrolysis. It is evident that the sulfurcontaining anions formed at the cathodic polarization of SGE are oxidized to form the particles which are inactive against the gold in the concentrate. Thus, the main series 560 of experiments was carried out in a diaphragm cell. The impact of the following factors on electrochemical leaching of gold was investigated (the range of variation of this parameter is shown in brackets): Concentration of NaOH ( mol/l); Liquid : Solid (L:S) ratio (5-10); Current density (i, ma/cm 2 ); Leaching process duration ( min). The following values were used as a response: Au recovery from ore, %; Au leaching rate. The impact of current density and the concentration of alkali on the extraction of gold are presented in Fig. 4. At a current density less than 30 ma cm -2, the catholyte yellowing occurs very slowly, indicating a low rate of generation of the polysulfide ions. When the current density is near 100 ma cm -2, destruction of SGE is visually observed. These circumstances limited the usable range of cathodic current density values. With increasing NaOH concentration growth of gold, extraction is occurs as well at the same current density until reaching the alkali concentration 2 mol L -1. Further increasing of the alkali concentration does not lead to an increase in the gold recovery. The increasing current density leads to an increase in the value of gold extraction, after reaching current density 80 ma cm -2 the dependence reaches a plateau. Further experiments were carried out with 2 M NaOH and 80 ma cm -2. It was of interest to investigate the impact of the

6 Rashid K. Nadirov, Leila I. Syzdykova, Aisulu K. Zhussupova Fig. 5. Effect of the leaching duration on Au recovery. duration of electrolysis and L: S ratio on the gold extraction (Fig. 5). Increasing L:S ratio increases the degree of gold extraction at the same electrolysis duration, and at L:S = 7 the gold recovery takes its maximum value. In turn, the degree of gold extraction reaches 90 % at the electrolysis duration of 210 minutes. Thus, the following process conditions are preferred: С NaOH = 2 M; i = 80 ma/cm 2 ; L:S = 7; Leaching Duration is 210 min. Under these conditions, the gold recovery reaches 93 %. The SGE consumption is about 55 g per 1 g of the gold in the solution. Kinetics issues Reducing the duration of the leaching process is one of the factors that reduce the cost price of the precious metal in the solution. Therefore, the influence of experimental factors on the rate of gold extraction was investigated. Theoretically, the rate of gold dissolution may be limited by the following factors: the rates of formation of anionic sulfur species, i.e. leaching agents; the rates of chemical reactions between the sulfurcontaining anions with a metallic gold (mainly according to reactions 9-11), including adsorption processes, if necessary; sulfur containing anions diffusion to the surface of the gold in the ore. In the study of the kinetics of dissolution of powdered minerals the main difficulty is keeping the value of the specific surface of minerals. For monodisperse material, it can be assumed that for short periods of time, the surface of the individual grains varies slightly. Comparison of the specific surface area of ore sample after 200 minutes of leaching (3.01 m 2 g) with a surface area of the starting material showed the validity of mentioned thesis as applied to our experiment (change of specific surface area is less than 2 %). Dependence of the change of the rate of gold dissolution from the square root and stirrer speed as well as the cathodic current density is presented in Fig. 6. The analysis of the data presented in the Fig. 6 indicates that: gold leaching rate increases linearly with the square root of the stirrer rotation speed increasing up to 110 min -1 ; after reaching this value the metal leaching rate reaches a plateau at mol L -1 min -1 ; gold leaching rate is limited by the rate of generation of leaching agents until the current density reaches 80 ma cm -2 ; upon reaching this value, the current density is no longer influences the gold leaching rate. Thus, at a current density of 80 ma cm -2 and a stirrer speed of 110 min -1 the rate of the whole leaching process is limited by the rate of chemical reactions between the anion with a metallic gold. 561

7 Journal of Chemical Technology and Metallurgy, 53, 3, 2018 Fig. 6. Effect of the square root of the stirrer rotation speed and current density on Au leaching rate. CONCLUSIONS This study demonstrates for the first time effective recovery of gold to aqueous solution from the concentrate using electrochemical leaching. Cathodic polarization of specially prepared sulfur-graphite electrode in the NaOH aqueous solution was used to obtain sulfurcontaining anions that were further utilized as leaching agent. The rate of the process of gold dissolution was found to be 33x10-6 mol L -1 min -1 at the cathodic current density of 80 ma cm -2 and stirrer rotation speed of 110 min % of gold recovery has been achieved under the conditions of С NaOH = 2 M; i = 80 ma/cm 2 ; L:S = 7; leaching duration = 210 min. Acknowledgements This research was supported by the grant of the Ministry of Education and Science of the Republic of Kazakhstan 5715/GF 4. REFERENCES 1. M.I. Jeffrey, P.L. Breuer, The cyanide leaching of gold in solutions containing sulfide, Miner. Eng., 13, 10, 2000, N. Kuyucak, A. Akcil, Cyanide and removal options from effluents in gold mining and metallurgical processes, Miner. Eng., 50, 2013, M.G. Aylmore, D.M. Muir, Thiosulfate leaching of gold-a review, Miner. Eng., 14, 2, 2001, A.C. Grosse, G.W. Dicinoski, M.J. Shaw, P.R. 562 Haddad, Leaching and recovery of gold using ammoniacal thiosulfate leach liquors (a review), Hydrometallurgy, 69, 1, 2003, D. Wei, L. Chai, R. Ichino, M. Okido, Gold leaching in an alkaline thiourea solution, J. Electrochem. Soc., 146, 2, 1999, J. Zhang, K. Lan, F. Ding, Z. Yuan, L. Yang, Leaching gold and silver by lime-sulfur-synthetic-solution (LSSS). Part I. Synthesizing the LSSS and dissolving pure gold and silver with it, Prec. Met., 16, 1992a, S. Zheng, Y.Y. Wang, L.Y. Chai, Research status and prospect of gold leaching in alkaline thiourea solution, Miner. Eng., 19, 13, 2006, J. Zhang, X. Lan, F. Ding, Z. Yuan, S. Shuai, Leaching gold and silver by lime-sulfur-synthetic-solution. Part II. Treating ores with the LSSS. Prec. Met., 1992b, 16, C.G. Anderson, S.M. Nordwick, Pretreatment using alkaline sulfide leaching and nitrogen species catalyzed pressure oxidation on a refractory gold concentrate, EPD Congress 1996, The Minerals, Metals and Materials Society, 1996, J.Y. Chen, T. Deng, G.C. Zhu, J. Zhao, Leaching and recovery of gold in thiosulfate based system - a research summary at ICM, T. Indian I. Metals, 49, 6, 1996, Z.H. Fang, B.L. Han, Leaching gold using oxidation products of elemental sulfur in Ca(OH)2 solution under oxygen pressure, Chinese J. Proc. Eng., 2, 3, 2002,

8 Rashid K. Nadirov, Leila I. Syzdykova, Aisulu K. Zhussupova 12. M.I. Jeffrey, C.G. Anderson, A fundamental study of the alkaline sulfide leaching of gold, SSb, 1, 2003, A. Azizi, C.F. Petre, C. Olsen, F. Larachi, Untangling galvanic and passivation phenomena induced by sulfide minerals on precious metal leaching using a new packed-bed electrochemical cyanidation reactor, Hydrometallurgy, 107, 3, 2011, Z.H. Liu, Y. Li, X.X. Zhou, J. Du, C.Y. Tao, Research progress of electro-oxidation intensification leaching for refractory ore, Advanced Materials Research, 236, 2011, A.L. Samusev, V.G. Minenko, Productivity of chemical-electrochemical gold leaching from rebellious ore, J. Min. Sci., 50, 1, 2014, T.D. Gornostayeva, V.A. Pronin, V.Ya. Semenov, Russian Patent SU , Y. Jung, S. Kim, B.S. Kim, D.H. Han, S.M. Park, J. Kwak, Effect of organic solvents and electrode materials on electrochemical reduction of sulfur, Int. J. Electrochem. Sc., 3, 5, 2008, J.B. Hiskey, V.P. Atluri, Dissolution chemistry of gold and silver in different lixiviants, Miner. Process. Extr. M., 4, 1986,