Copper electrowinning from cyanide solutions

The objective of this research was to explore an efficient process to recover cyanide and copper from barren gold cyanide solution. The research work described here concerns an investigation into fundamental and practical aspects o f two options for electrowinning copper from cyanide solution. These two options are: (a) the use o f an alternative anode reaction to limit the electro-oxidation o f cyanide in concentrated cyanide solutions and (b) the use of a graphite fibre cathode to electrowin copper from dilute cyanide solution. (1) A critical literature survey was conducted to examine the stability constants of copper cyanide species. The distributions and the equilibrium redox potentials of copper cyanide species were calculated using the most reliable stability constants. They are dependent on the mole ratio of cyanide to copper, total cyanide concentration, pH and temperature. Potential measurements have confirmed the validity of the calculated results. The pH-potential diagram was drawn using the Gibbs free energy data derived by selecting the most reliable stability constants. (2) Direct copper electrowinning from dilute cyanide solutions was conducted in a membrane cell. The accumulation o f deposited copper on the graphite felt as the plating proceeds significantly improves the conductivity o f the graphite felt, increases the specific surface area and benefits copper deposition. Copper can be deposited on the graphite felt from low concentration solutions (1-2 gL-1 Cu and CN:Cu mole ratio = 3-4) with 50-80 % current efficiency, the removal of around 40% Cu and an energy consumption of 1-2 kWh/kg Cu in the superficial current density range 30 - 100 Am-2 at 40 °C. (3) Copper electrowinning from concentrated copper cyanide solution (70 g L-1 Cu) was conducted using four sacrificial species (sulphite, methanol, thiocyanate and ammonia) at 40 to 60 °C. Only sulphite can decrease the anodic current efficiency of cyanide oxidation from ~ 100 to 10-20 % over the current density range of 250-500 Am-2. With increasing CN:Cu mole ratio from 3 to 4.5, the anodic current efficiency of cyanide oxidation increased and the copper deposition current efficiency decreased. As regards the recovery of copper from barren gold cyanide solution, it has been shown that using sulphite oxidation as an alternative anode reaction, copper can be electrowon from a cyanide electrolyte containing about 70 gL-1 Cu (CN:Cu = about 3) and 0.5 M Na2SO3 at a cathode current efficiency of about 95% with a energy consumption of about 0.8 kWh/kg Cu at 250 Am-2. (4) In alkaline solutions, sulphite is oxidized to sulphate on the graphite anode in a two-electron reaction. The reaction order with respect to sulphite ions is below 1 at low potentials(< 0.4 V vs. SCE) and 1 at high potentials. The reaction order for hydroxide ions is close to zero. Two Tafel slopes were observed, 0.060 - 0.64 V decade-1 at low potentials and 0.19-0.20 V decade-1 at high potentials in the temperature range 40 - 60 °C . Sulphite oxidation in alkaline solution appears to undergo an electron-radical mechanism. (5) The anodic oxidation of copper cyanide has been studied using a graphite rotating disk with reference to cyanide concentration (0.05-4 M) , CN:Cu mole ratio (3-12), temperature (25-60 °C) and hydroxide concentration (0.01-0.25 M) . Copper had a significant catalytic effect on cyanide oxidation. In the low polarization region (< about 0.4 V vs. SCE) , cuprous cyanide is oxidized to cupric cyanide complexes which further react to form cyanate. At a CN:Cu ratio of 3 and [OH-] = 0.25 M, the Tafel slope was about 0.12 V decade-1. Cu(CN)32- was discharged on the electrode surface. With increasing CN:Cu mole ratio and decreasing pH, the dominant discharged species shifted to Cu(CN)43-. In the high polarization region (about 0.4 -0.6 V vs. SCE), cuprous cyanide complexes were oxidized to copper oxide and cyanate. When the concentration of cyanide was high and the pH low, cyanogen was formed, but no copper oxide. (6) Sulphite oxidation is enhanced by the presence of copper cyanide. The effect of sulphite on limiting the oxidation of copper cyanide decreases with increasing mole ratio of cyanide to copper. This is related to the shift in the discharged species from Cu(CN)32- to Cu(CN)43- with increasing mole ratio of cyanide to copper. Sulphite is oxidized to sulphate. At [Cu] = around 1 M, CN:Cu = 3 -3.2, [OH-] = 0.05-0.25 M, [SO32-] = 0.4-0.6 M and the temperature = 50 - 60 °C, the anode current efficiency of sulphite oxidation reached 80-90%) as the anodic current efficiency of cyanide fell to 20 to 10 %. [Scientific formulae used in this abstract could not be reproduced.]Applied Science, Faculty ofMaterials Engineering, Department ofGraduat