Cyanide removal and recovery by electrochemical crystallization process

Alkaline chlorination, an efficient but high chemical cost process, is commonly employed for cyanide (CN⁻) removal from CN-rich wastewater streams. CN⁻ removal and recovery through the precipitation of Prussian Blue (Fe₄^III [Fe^II (CN)₆ ]₃, PB) or Turnbull’s Blue (Fe₃^II [Fe^III (CN)₆ ]₂, TB) were realized using iron salts, leading to a cost-effective and sustainable process producing a valuable recovery product. However, the precipitation of PB and TB is highly affected by pH and dissolved oxygen (DO). CN⁻ removal and recovery from CN-containing water by crystallization of PB and/or TB were investigated using dissolved iron that was electrochemically generated from a sacrificial iron anode under various pH values, initial CN⁻ levels (10 to100 mg/L) and DO levels (aeration, mechanical mixing, and N₂ purging). It was shown that the complexation of CN⁻ with Fe ions prevented the vaporization of HCN under acidic pH. At pH of 7 and initial CN⁻ concentration of 10 mg/L, CN⁻ removal efficiency increases linearly with increasing Fe:CN⁻ molar ratios, reaching 80% at the Fe:CN⁻ molar ratio of 5. A clear blue precipitate was observed between the pH range of 5–7. CN⁻ removal increases with increasing initial CN⁻ concentration, resulting in residual CN⁻ concentrations of 8, 7.5 and 12 mg/L in the effluent with the Fe:CN⁻ molar ratio of 0.8 for initial concentrations of 10, 50 and 100 mg CN⁻ /L, respectively. A polishing treatment with H₂ O₂ oxidation was employed to lower the residual CN⁻ concentration to meet the discharge limit of <1 mg CN⁻ /L.