Electrolytic oxidation of cuprocyanide electroplating waste waters under different pH conditions

The main purpose of this work was to investigate the electrolytic oxidation of cuprocyanide solution with various total cyanide to copper molar ratios ranging from 2.8 to 20 and under different pH conditions. In strong alkaline solution (pH12), cuprocyanide ions Cu(CN) _n ^/(n–1)– , wheren=2, 3 or 4, are directly electroxidized, and copper oxide precipitates on the anode. Cyanate ions, as well as nitrogen gas, were detected as the products and 0.30–0.43 g mol of total cyanide was destroyed per Faraday. For less alkaline solutions (pH<12), cuprocyanide ions first dissociated to free cyanide ions and then electroxidized. At a pH of 10.5–11.7, cyanate ion and brown azulmin polymer were produced in the anolyte. In the neutral solution (pH=7.0–8.6), carbonate and ammonium ions and azulmin were formed and 0.52–0.56 g mol of total cyanide was destroyed per Faraday. In weak acidic solution (pH=5.2–6.8), oxalate and ammonium ions and white oxamide were produced and 1.01–1.18 g mol of total cyanide were destroyed per Faraday.Nomenclature C _CN molar concentration of total cyanide (kmol m^–3) - C _Cu molar concentration of total copper (kmol m^–3) - C _d equivalent concentration of cyanide destroyed due to the formation of cupric oxide (kg m^–3) - C _f concentration of cyanide destroyed by dissociation of complex ion to free cyanide ion and then electroxidized (kg m^–3) - C _i initial concentration of total cyanide (kg m^–3) - C _t change of total cyanide concentration during electrolysis (kg m^–3) - F Faraday constant (96 487 C mol^–1) - K ₁,K ₂,K ₃ formation constant of dicyanocuprate, tricyanocuprate and tetracyanocuprate ions - R molar ratio of total cyanide concentration to total copper concentration (i.e.C _CN/C _Cu) - W weight of precipitates on electrodes or in anolyte (kg) - angle of incidence