Electrochemical treatment of cyanide-containing wastewater

Electrochemical destruction of cyanide is not new, many years to metal electroplating industry for handling small waste volume. HSA (High Surface Area) electrochemical cell units in Canada and the United States have been located in the Pamuel Sumac mine in Ontario and the Dientelo Nylov mine in the United States.

Electrochemical treatment of metal cyanide solutions involves electrooxidation of cyanide (at the anode) and electroreduction of the metal (at the cathode). The oxidation of cyanide can be further enhanced by the addition of salt to cause electrochlorination.

Upon electroreduction, the complexed metal cyanide ions are reduced at the cathode, precipitation or deposition of the metal occurs, and a corresponding amount of cyanide ions is produced. Upon oxidation of the anode, cyanide is converted to cyanate, and any thiocyanate in the solution is oxidized to cyanate and sulfate. The addition of salt to the solution in electrochlorination produces active chloride ions which, as in conventional alkaline chlorination processes, promote the oxidation of cyanide and thiocyanate to cyanate.

The uniqueness of HSA technology lies in the design of electrochemical reactors. It uses carbon fiber filaments as the electrode material to provide a large surface area on small-sized electrodes, which can improve the flow of solution and increase the electrical efficiency. The above HSA electrochemical device test results show that this method is effective in removing cyanide and recovering cyanide and heavy metals. But it does not remove iron cyanide.

Two methods of operation were investigated in the test: (1) cyanide and metal recovery; (2) cyanide destruction and metal recovery. The first method involves the electroreduction and deposition of metal in the metal cyanide complex and the release of free cyanide into the solution, which allows the treated lean liquid to be returned to the cyanide system. The second method produces cyanate during the electrooxidation of cyanide, while simultaneously producing metal electroreduction and metal electrodeposition.

The required operating temperature is around 60 °C. The oxidation of cyanide can be promoted by the addition of a salt to provide chloride ions. In order to remove the deposited metal, a periodic metal desorption stage is also required. The metal can be recovered from the desorption solution by conventional electrolysis or by chemical precipitation.

HSA Reactor Co., Ltd. claims that its process can destroy cyanide at a cost roughly equivalent to half that of the alkali chlorination process. However, in electrochemical systems, the real value is generated by the recovery of cyanide.