Cyclic degradation of thiocyanate in cyanide barren solution by manganese oxides

Abstract

This study describes the degradation of thiocyanate by manganese oxides. With NaOH and CaO as precipitating agents, two manganese oxides, i.e. MnOx/Na and MnOx/Ca, were prepared using MnSO4·H2O as Mn resource and air as oxidant. The influence of various reaction parameters like pH, manganese oxide dosage and initial thiocyanate concentration were systematically investigated with MnO2 as a reference. Based on an entire process dynamics model and the initial kinetics analysis, the rate-limiting step (formation of the precursor complexes) and reaction orders were derived independently. The reaction pathway was detected and the formation of the precursor complexes between surface positively charged manganese oxides and thiocyanate was the first step of thiocyanate degradation, after which electron transfer occurred within the precursor complexes and intermediate products were formed, which were rapidly released to produce SO42− and CN−, and some of the CN− would be further oxidized to HCO3− and NH4+. The formation of precursor complexes is related to the charge distribution on the surface of the manganese oxide and the protonation of thiocyanate in solution, which makes the lower pH favorable. The results of cyclic treatment of cyanide barren solution showed good regenerability of MnOx/Ca and gradual reduction of undesired secondary salts (especially SO42− ions) in the degraded liquid, which made it a potentially reliable technique to regulate the thiocyanate concentration during the cyanidation process of gold ores economically.