Regeneration of cyanide by ozone oxidation of thiocyanate in cyanidation tailings

Cyanidation tailings usually contain important amounts of thiocyanate. A method has been developed to transform this waste thiocyanate into cyanide to be recycled to the cyanidation plant. Thiocyanate was oxidized to cyanide by using ozone in mildly acidic pH conditions as an oxidant. Preliminary experiments were performed with synthetic thiocyanate solutions. Results were corroborated with industrial effluents from two mines in northern Quebec. To avoid decomposition of the regenerated cyanide, it was necessary to keep copper concentration below 10 mg/l and pH below 7.0. Up to 85 % cyanide recovery was obtained with a consumption of about 1.0 Kg ozone per Kg of thiocyanate. When copper was present in the tailings, it was eliminated with hydrogen peroxide or precipitated with a cationic complexing agent before cyanide regeneration. This last alternative is particularly attractive because it may permit recovery of the metal. The thiocyanate regeneration method was also applied directly to slurries. When the ore was acidic, good cyanide conversions were obtained, but when the ore contained basic minerals that maintained a high pH level, substantial destruction of the regenerated cyanide was observed.