Mineralization of cyanide originating from gold leaching effluent using electro-oxidation: multi-objective optimization and kinetic study

This study examines the electro-oxidation (EO) of cyanide originating from an industrial plant ' s gold leaching effluent. Experiments were carried out in a laboratory-scale batch cell reactor. Monopolar configuration of electrodes consisting of graphite (anode) and aluminum (cathode) was employed, operating in galvanostatic mode. Response Surface Methodology (RSM), based on a Box-Behnken experimental Design (BBD), was used to optimize the EO operational conditions. Three independent process variables were considered: initial cyanide concentration ([CN-](0) = 1000-2000 mg L-1), current density (J =7-107 mA cm(-2)), and stirring velocity (eta = 250-750 rpm). The cyanide conversion, %R-COD, and EC) were fitted to second-order polynomial models with adjusted correlation coefficients (Radj2 of ca. 98, 99 and 87%, respectively. The kinetic analysis, performed at optimal EO operational conditions, allowed determination of time required to meet Colombian permissible discharge limits. The predictive capacity of kinetic expressions was verified against experimental data obtained for gold leaching effluent. Total cyanide removal and 96% of COD reduction were obtained, requiring EC of 71.33 kWh kg(-1) and 180 min. The BOD5 (biological oxygen demand)/COD ratio increased from 4.52 x 10(-4) to 0.5573, confirming effluent biodegradability after EO treatment. Graphic <g The variation of cyanide (CN-), cyanate (CNO-) and ammonium (NH4+) ions concentrations vs. time at alkaline conditions. EO operational conditions: [CN-]0 = 1000 mg/L, J = 100 mA/cm(2) , eta = 750 rpm, [NaCl] = 0.15 M and pH 11.1.