Removal of Thiocyanate from Gold Mining Process Water

Thiocyanate is a common contaminant in gold mining effluent. The high level of cyanide used in gold and silver extraction leads to the formation of thiocyanate (SCN^–) in the presence of sulfide minerals. It is a hydrophilic species and reacts with metal sulfides to suppress their flotation. Gold mine wastewater can contain up to 1,000 mg/L of SCN^– ions, which increase with water reuse and can adversely affect the flotation of gold-bearing sulfide minerals. Managing cyanide effluents in mineral processing is extremely important given their negative impact on process efficiency and environmental concerns. In this work, environmentally friendly electrochemical advanced oxidation process (EAOPs) was applied to remove thiocyanate from gold mining process water. An electrochemical cell was fabricated using a pair of carbon electrodes as the anode and cathode. Electrodes were prepared by casting an aqueous carbon/PVDF binder slurry onto graphite sheets. Batch recirculation experiments showed that SCN^– was oxidized to sulfate (SO₄^2–) at an applied voltage of 5 V and a constant flow rate of 1.5 mL/min. The concentration of SCN^– was reduced from 160 mg/L to 3 mg/L, while 120 mg/L of SO₄^2– was produced after 180 minutes of operation. Continuous flow tests revealed that at 3 V, 85% of the SCN^– was removed with a total thiocyanate removal of 2301 mg/m² at a flow rate of 1.5 mL/ min for 5 mg/L SCN^– solution during 180 minutes of operation. The removal efficiency decreased from 85% to 22% as the influent concentration increased from 5 mg/L to 20 mg/L at 3 V and 1.5 mL/min. In the treatment of gold mining process water, 47% of the SCN^– was removed in 5 minutes of operation, but then decreased to about 25% in 120 minutes of treatment at 5 V with a flow rate of 1.5 mL/min.