Removal of cyanide by anodic oxidation for wastewater treatment

The aim of this work was to investigate the feasibility of using a bipolar trickle tower electrochemical reactor consisting of graphite Raschig rings as the electrodes to remove cyanide from a cyanide containing effluent. Direct and indirect methods were both found to be efficient for removing the cyanide in either continuous or recirculating batch modes. The cyanide concentration in the solution was reduced from 1500 to less than 60 mg l −1 and from 1000 to less than 30 mg l −1 with an energy consumption in the order of 18.4–26.8 kW h per kg cyanide removed during continuous operation. In the case of a cyanide concentration of 300 mg l −1, full removal of cyanide was achieved with an energy consumption of 78.5 kW h kg −1. However, less than 30 mg l −1 could be achieved with an energy consumption of 8.75 kW h kg −1 for this initial concentration. During treatment the rate of removal decreased as the concentration decreased with time. More energy was used to reduce the cyanide content from 30 mg l −1 to zero (70 kW h kg −1) than to reduce the cyanide concentration from 300 to 30 mg l −1 (8.75 kW h kg −1). Moreover, the cyanide content could be reduced from 1500 to less than 300 mg l −1 with quite a low energy consumption (3 kW h per kg of CN removed, i.e. 3.6 kW h per cubic metre of effluent treated). Full removal of cyanide has also been achieved for all initial cyanide concentrations during the recirculating batch operation. The energy consumptions for these experiments were 5.15 for 300 mg l −1, 5.14 for 1000 mg l −1 and 6.3 for 1500 mg l −1 in kW h per kg cyanide removed. The mass transfer coefficient for this reactor was also determined with an average value of 4.35×10 −4 ms −1 being obtained. This is a typical value for such a cell.