Cyanide effluent treatment by electrocoagulation using airlift reactor: Modeling and optimization by response surface methodology

Abstract

The extraction processes of gold, silver, and various metals from ores consume 13% of 1.1 million metric tons of hydrogen cyanide produced worldwide annually. Therefore, developing efficient processes to treat the issuing effluents before releasing them into the environment is imperative. In this context, the present study aims to evaluate the effect of three factors: inter-electrodes distance (d), current density (J) and electrolysis time (t) on the cyanide removal efficiency by a combination between Electro-Coagulation process and Internal Loop Airlift Reactor. Employing an airlift reactor enables an effective dispersion of the coagulant without the need for additional stirring. The optimization of the process was conducted using Response Surface Methodology in conjunction with Box–Behnken Design. The statistical analysis results reveal that electrolysis time(t)and current density (J) have significant effects on reactor performance to reduce cyanide with a confidence level of 99%, and 97.5%, respectively. The multiple regression analysis method demonstrates the model's overall predictability as the coefficient of determination was 98 % for the cyanide removal efficiency. The investigation inside the RSM 2 and 3-dimensional graphs allowed us to define the optimal domains of the variables to reduce cyanide. At a current density of 68 A.m−2 and an inter-electrodes distance of 13mm during 57.5min of electrolysis time, 98.27% of cyanide removal is achieved. The knowledge obtained through this study using the airlift reactor operating in batch mode and the EC process for the treatment of cyanide effluent confirms EC's ability to successfully remove cyanide and might be useful for the transposition from batch to continuous mode.