ELECTROWINNING OF GOLD FROM CYANIDE SOLUTIONS G. Cifuentes a, C. Vargas a, J. Simpson a, R. Esteban b a Departamento de Ingeniería Metalúrgica, Facultad de Ingeniería, Universidad de Santiago de Chile, Avenida Libertador Bernardo O Higgins 3363, Casilla 1233, Santiago, Chile. E-mail: cvargas@lauca.usach.cl; gcifuent@lauca.usach.cl b División El Teniente, CODELCO-CHILE, Millán #1, Rancagua, Chile. Abstract The electrowinning of gold from solutions originating from the cyanidation of cleaner concentrates from froth flotation, was investigated at various experimental conditions in order to obtain the best conditions of the electrolysis. The experimental study consisted of designing a cilindrical electrowinning cell and to observe the behavior of different variable on the recovery and selectivity of the electrowinning of gold process. Then, with these conditions, is evaluated the corresponding variable to the type of flow with a rectangular cell to accomplish a comparison between these two types of reactors. Finally, were compared the results obtained between the laboratory tests and pilot plant tests, in order to obtain a scaling, giving as result that the experiences to pilot scale can be simulated as of laboratory level tests directly. Keywords: Electrowinning, gold, Cyanidation, concentrate, Flotation Introduction The electrolytic recovery of gold starting from cyanide solutions had its application to industrial scale in the XX century with the electrolytic method Simens-Halske (Adamson 1972). The Gold in cyanide solutions was deposited on lead cathodes that were removed periodically by fusion. When taking place two phases in the fusionated cathode the lead it was strained in form of ingots, being alone the gold which was recovered. Finally the lead ingots were laminated to produce new cathodic material. Zadra in 195 designs and carries out experimental tests in a steel wool cathode what allowed to increase the cathodic surface that meant an increase in the quantity of electroplated gold. The electrowinning of gold from cyanide solutions in comparison with the precipitation with zinc, presents two big advantages. First, after the electrolysis the solution can be recycled to the extraction process, since chemical reagents are not added that impede its new use. Second, is a highly selective process what allows to obtain high purity product. Experimental A cylindrical electrolytic cell of 253,17 cm 3 of volume was used where the flow of the electrolyte entered for the base of the cylinder and it left for it dams for three
holes in the superior part, connected to hoses that converge to the recirculation tank. A rectangular cell of same volume was also used, where the electrolyte feeding was carried out for one of its ends, for the inferior part and it dammed for the other end in the superior part. A fine iron wool cathode, 2grams per liter of volume cell (equivalent to aprox. 1.92 g of wool for the cells in study) was disposed on a stainless steel 316L cathodic mesh and a perforated sheet of great area of stainless steel 316L as anode were used as electrodes. A solution coming from cyanidation of a cleaner concentrate froth flotation was used, which was upset and heated in a recirculation tank, which was good to homogenize the electrolyte and to control variables as temperature, ph, concentration and conductivity. Results and discusions In all the experiences for the study of variables a solution starting from the cyanidation of a cleaner concentrate. Cylindrical cell of 253,17 cm 3 : Influence of the current density In Figure 1 is shown the result of the kinetic of recovery of gold and copper for different current densities. These experiences were also carried out with a feeding flow to the cell of 6,4x1-3 l/min cm 2 and a temperature of 25ºC and the means concentration of 3 ppm Au, 52 ppm Cu, 25 ppm Ag and 3 ppm of free cyanide. 2 1 2 3 4 5 6 67,7 A/Kg of Cathode (Gold) 15,7 A/Kg of Cathode (Gold) 152,1 A/Kg of Cathode (Gold) 67,7 A/Kg of Cathode (Copper) 15,7 A/Kg of Cathode (Copper) 152,7 A/Kg of Cathode (Copper) Figure 1: Effect of the current density in the recovery of gold and copper. The results show that when increasing the current density the quantities of recovered metal increase, ignoring the selectivity of the process. This implies that the quantity of recovered metal will depend on the applied current density, but the selectivity of the process is not controlled by the current density, therefore, when increasing the current density, the electric field increases implying that it increases the gold, silver and copper deposition on the surface of the cathode. This deposition was not controlled in selective form since when increasing the current density the cathodic
overpotential it also increased. It was also that as it lapses the time, the current efficiency in the gold recovery decreases. This can be an approach that allows to estimate the time of operation. Influence of the cell potential 9 7 5 3 2 1 1 2 3 4 5 6 1,5 Volts (Gold) 2,5 Volts (Gold) 3, Volts (Gold) 1,5 Volts (Copper) 2,5 Volts (Copper) 3, Volts (Copper) Figure 2: Effect of the cell potential in the recovery of gold and copper. In Figure 2 it can observe that as the tension increases in the system a bigger copper codeposition it takes place because the overpotential increases, favoring the deposition of this metal ( E o o Cu (CN ) 2,75 V / Cu NHE,E,63 V Au(CN / Au NHE ). 3 )2 A very narrow relationship it exists among the current density that controls the recovery and the generated potental in the cell that controls the selectivity of the process. Because in this process type is obtained a free of impurities product like the copper, it is that it becomes necessary to control the cell potential. Once fixed the potential the current density should be maximized, with the purpose of obtaining a product with the smallest quantity in impurities and in the possible time of electrobtención smallest. In this case, when increasing the cell tension it decreases the current efficiency of the process with regard to the gold. This is due to that increases the cell potential the cathodic overpotential increase, allowing that the reactions of other complexes are favored and increase the codeposición of other metals as the copper. The previuos analysis of the results allow conclude that the variable that should be controlled is the cell potential. For this reason to carry out the experiences of feeding flow to the cell and temperature of the solution is considered that the potential with which should be worked is 2,5 V what means a cathodic sobrepotential of 1,45 V. Influence of the feeding flow to the cell All these experiences were carried out at 25ºC, 2,5 V and a cathodic overpotential of - 1,45 V. In Figure 3 can be observed that as the feeding flow increases the gold recovery is increased. This demonstrates an improvement in the gold complex
diffusion in the limit layer, since when increasing the feeding flow, the limit later thickness decreases between the cathodic surface and the solution bulk, allowing also a renovation of the complex of gold in that region more dinamyc. 2 1 2 3 4 5 6 Time(h) 3,2E-3 l/min cm2 (Gold) 12,8E-3 l/min cm2 (Gold) 6,4E-3 l/min cm2 (Copper) 6,4E-3 l/min cm2 (Gold) 3,2E-3 l/min cm2 (Copper) 12,8E-3 l/min cm2 (Copper) Figure 3: Effec of the feeding flow in the recovery of gold and copper. The feeding flow variable can also be analyzed with the cathodic polarization curves that it is shown in Figure 4. For a given current density current when increasing the feeding flow to the cell it decreases the cathodic overpotential, implying a bigger selectivity of the process. The copper codeposition decreases product of the decrease of the cathodic overpotential. Operationally when fixing the potential, the selectivity of the process is controlled, and when increasing the flow of the solution the current density it is maximized, obtaining a product with smaller degree of impurities and in a smaller time of electrowinning like it is shown in Figure 3. Figure 4: Cathodic polarization curve for differents feeding flows. The scanner velocity was 5 mv/min. Influence of the temperature All the following experiences were carried out to 2,5 V, with a cathodic overpotential of -1,45 V and a feeding flow of 6,4x1-3 l/min cm 2.
12 2 1 2 3 4 5 6 25ºC (Gold) 3ºC (Gold) 35ºC (Gold) ºC (Gold) 25ºC (Copper) 3ºC (Copper) 35ºC (Copper) ºC (Copper) Figure 5:Effect of the temperature in the recovery of gold and copper. In Figure 5 is observed that when increasing the temperature of the solution the recovery of gold increases too. For example for a temperature of ºC the gold recovery was,62% for a time of 1 hour, and, also, for short times the copper recovery is low, of the order of 1,18%, what implies to obtain a product with a high selectivity degree at very short times. The increase of the temperature of the electrolyte favors the increase of the diffusion coefficient of the complex Au (CN ) 2, improving the diffusion in the limit layer. In Figure 6 for a given cell potential, when increasing the temperature of the solution the current density that can be applied increases, what implies a bigger recovery of gold and silver. In the copper case the recovery doesn't increase since the cathodic overpotential is maintained constant. This effect is the same that takes place when increasing the feeding flow. Then if the cell potential is fixed, the selectivity of the process is controlled, and when increasing the temperature of the solution the current density it is maximized, obtaining a product with smaller degree of impurities and in a smaller time of electrowinnig. Figure 6: Polarization curves for differents temperatures. The scanner velocity was 5 mv/min. Effect of the initial concentration of Au, Ag, Cu and CN For this variable a initial concentrations of gold of 25,7; 24,1; 13,8 and 6,8 ppm were used, 26,8; 24,8; 13,8 and 7,3 ppm of silver, 537, 57; 282 and 145 ppm of copper and free cyanide of 32; 212 and 133 ppm. All these experiences were carried out to
25 ºC, 2,5 V, a cathodic overpotencial of -1,45 V and a feeding flow of 6,4x1-3 l/min cm 2. Figure 7: Cathodic Polarization Curves for differents concentrations of Au, Cu, Ag y CN -. The scanner velocity was 5mV/min. Case 1 variation of Au, Cu, Ag and CN - concentrations In Figure 7 the cathodic polarization curves are shown for the first case. Can be observed that the curves move in parallel form toward to the left(increse the overpotential) as the silver, copper and cyanide concentrations decreases in same proportion approximately. This implies that when decrease the concentration of these species and to apply a current density constant, the cathodic overpotential increases proportionally, implying that the copper codeposition increases. Figure 8: Cathodic Polarization Curves for differents concentrations of gold. The scanner velocity was 5 mv/min. Case 2 variation only of the initial concentration of Au In Figure 8 the cathodic polarization curves are presented for different concentrations of gold in the solution, being observed an increase in the slope of the cathodic polarization curve in the measure that the initial concentration of metal increases. Finally of these experiences can be concluded that the gold and free cyanide concentratios in the solution are the main variables that it modifies the cathodic
polarization curves. If increases the complex of gold concentration, for a certain potential, the current density that you can apply increases, implying an increase of the limit current of the system. Now, if the free cyanide concentration decreases in the system the copper deposition it is favored by formation of copper complexs of smaller coordination number. Comparison between the cell types For this analysis both experiences were carried out with a cell tension of 2,5 V,a feeding flow of 6,4x1-3 l/min cm 2 and a temperature of 25ºC, for initial concentrations of gold and copper of 25 and 52 ppm respectively. In Figure 9 are presented the results of the kinetic of gold and copper recovery for the cylindrical and rectangular cells. In this case it is observed that the gold recovery in the rectangular cell increases in 31,52% and the copper recovery decreases 3,19% for a hour of operation, having a final gold recovery of 87,17% and,3% of copper. This is due to that the cathodic overpotential for the rectangular cell decreases, being the process is more selective. 9 7 5 3 2 1 1 2 3 4 5 6 Gold, Cylindrical Cell Copper, Cylindrical Cell Gold, Rectangular Cell Copper Rectangular Cell Figure 9: Influence of cell type in the recovery of gold and copper. Another factor that helps to that the process in the rectangular reactor has a high recovery is the traversal flow, that maintains a more homogeneous system in the cathodic surface compared with the longitudinal flow of the cylindrical cell.then, to level laboratory the rectangular cell presents comparative advantages with regard to cylindrical cell. Scaling to plant pilot The results to laboratory scale regarding to pilot pilot were compared for similar operation conditions, with the purpose of establishing a scaling between these experiences. Both experiences were carried out with 2,5 volts, 25ºC, with a feeding flow to the cell of 6,4x1-3 l/min cm 2 and a cathodic mass density of 2g/l. The cilyndrical cell volume to pilot scale was 6. cm 3.
9 7 5 3 2 1 1 2 3 4 5 6 Laboratory Pilot Figure 1: Comparison between experimental tests and pilot plant. As can be observed in the exposed results, the experiences accomplished to laboratory scale are quite similar to the accomplished to pilot scale. For this reason is not necessary to introduce fitting factors between these experiences. Finally it is concluded that the experiences to pilot scale can be simulated as of experiences at laboratory level. References Harrison, J.A. and Thompson, J., 1973. The electrodeposition of precious metals. Electrochemical Acta, 18: 829-834. Yannopoulos, J.C.,1991.The extractive metallurgy of gold. Van Nostrand Reinhold, New York. Marsden, J. and House, I., 1993. The Chemistry of Gold Extraction. Ellis Horwood, London.