Study on the treatment of cyanide gold extraction wastewater by persulfate-electrochemical oxidation

The cyanide gold extraction wastewater from a gold smelter was treated using a persulfate-electrochemical oxidation (PS-EO) coupling process. The optimal process conditions and significance level of each factor were determined using orthogonal experiments, and the precipitates and cathodic precipitates in the system were analyzed and characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope-energy dispersive spectroscopy (SEM-EDS), and the oxidation removal reaction mechanism of cyanide was further explored. The results showed that voltage, time and PS concentration were the main influencing factors of the cyanide removal extent. When the voltage was 5 V, PS concentration 0.5 g·L–1, pH 7, plate spacing 2.5 cm and reaction time 240 min, the removal extents of total cyanide (CNT), free cyanide (CN–), Cu and Zn ions in wastewater reached 98.33%, 100%, 99.53% and 98.86%, respectively. The removal of cyanide was mainly attributed to the indirect oxidation by SO4•–, •OH and O2•– active radicals produced by activated PS and the direct oxidation of S2O82–. The contribution rate of S2O82– direct oxidation was 42.38%. The oxidation contribution rates of SO4•–, •OH and O2•– active free radicals were 15.75%, 29.33% and 12.54%, respectively. Compared with simple PS and EO treatment processes, the PS-EO combined process reduced the amount of oxidants and helped to improve the current density of the system, as well as the generation of active free radicals such as SO4•–, •OH and O2•–. Additionally, after PS-EO treatment, there were almost no precipitates such as Zn(CN)2, CuCN, Zn(OH)2 and Cu(OH)2 produced in the wastewater. The cyanide ions in complex ions such as Cu(CN)32– and Zn(CN)42– were completely oxidized to N2 and CO2, and then under the influence of an electric field, the released Cu2+ and Zn2+ ions were directed to migrate to the cathode and precipitate.Finally, the treated wastewater could be directly returned to the grinding or leaching the ore process, thus achieving the goals of being harmless and utilizing cyanide wastewater resources.