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 (Fe-4(III)[Fe-II(CN)(6)](3), PB) or Turnbull's Blue (Fe-3(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 N-2 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 H2O2 oxidation was employed to lower the residual CN- concentration to meet the discharge limit of <1 mg CN-/L.