Copper recovery and cyanide oxidation by electrowinning from a spent copper-cyanide electroplating electrolyte.

Copper-cyanide bleed streams arise from contaminated baths from industrial electroplating processes due to the buildup of impurities during continuous operation. These streams present an elevated concentration of carbonate, cyanide and copper, constituting a heavy hazard, which has to be treated for cyanide destruction and heavy metals removal, according to the local environmental laws. In the Brazilian Mint, bleed streams are treated with sodium hypochlorite, to destroy cyanide and precipitate copper hydroxide, a solid hazardous waste that has to be disposed properly in a landfill or treated for metal recovery. In this paper, a laboratory-scale electrolytic cell was developed to remove the copper from the bleed stream of the electroplating unit of the Brazilian Mint, permitting its reutilization in the plant and decreasing the amount of sludge to waste. Under favorable conditions copper recoveries around 99.9% were achieved, with an energy consumption of about 11 kWh/kg, after a 5-h electrolysis of a bath containing copper and total cyanide concentrations of 26 and 27 g/L, respectively. Additionally, a substantial reduction of the cyanide concentration was also achieved, decreasing the pollution load and final treatment costs.