Biooxidation of thiocyanate-containing refractory gold tailings from Minacalpa, Peru

Refractory, sulfidic tailings from gold mining operations may be biooxidized to increase the amount of gold recoverable by cyanide extraction. Tailings containing thiocyanate must be pretreated to remove thiocyanate prior to biooxidation due to its toxicity to microorganisms that oxidize sulfide minerals. Thiocyanate in tailings at Minacalpa, Peru, was only partially extractable with water or with dilute sulfuric acid. The thiocyanate also was not completely biodegradable at neutral pH. Consequently, tailings remained inhibitory toward biooxidation after these treatments, releasing thiocyanate into bioleach solutions. The non-extractable form of thiocyanate in Minacalpa tailings was probably copper thiocyanate. A novel approach was developed to remove this thiocyanate using ferric sulfate solution. Tailings were detoxified by washing with either reagent ferric sulfate or solutions derived from biooxidation of pyrite. Detoxified tailings, i.e., those thoroughly washed with ferric sulfate solution, were biooxidized, exhibiting a fairly linear relationship between sulfide-S oxidation and cyanide gold recovery. A process for biooxidation of Minacalpa tailings would involve an initial washing step with biooxidation solutions followed by stirred tank biooxidation or agglomeration of detoxified tailings onto support rock and biooxidation in heaps.