RECOVERY OF CYANIDE FROM EFFLUENTS USING CARBON DIOXIDE

Abstract

Cyanidation effluents are treated in order to reduce the cyanide level down to regularity limits. Cyanide recovery is the most desired route for treatment of effluents of high cyanide consuming ores, in particular, due to economic and environmental incentives. In this study, carbon dioxide (CO2) was utilised as an alternative for acidification of cyanide solutions prior to absorption/recovery of hydrogen cyanide (HCN(g)) in alkaline solutions. Kinetic tests have indicated that reaction time is an important parameter for the recovery of cyanide, which reached 89.1% over a period of 90 min. Effects of concentration of cyanide (0.5-1.5 g/L NaCN), flow rate of carbon dioxide (0.38-1.15 L/min. CO2) and time (30-90 min.) on the recovery of cyanide (%) were investigated in detail by a two-level full factorial design (23). The statistical evaluation of the data showed that flow rate of carbon dioxide and time were statistically significant parameters. Efficiency of the process was not affected by the concentration of cyanide. The results demonstrated that high recoveries of cyanide up to 93.1% could be achieved under suitable conditions. Introduction of air (1.15 L/min.) as a carbon dioxide source was found to be inefficient and require longer reaction periods for high recoveries i.e. no recovery at 90 min. vs. 95.4% at 24 h. A separate test performed using a real pregnant leaching solution (PLS) yielded a cyanide recovery of 49.7%. These finding demonstrated that using carbon dioxide can be used for acidification of cyanide solutions for cyanide recovery.