Passive treatment for the removal of residual cyanide in drainage from closed gold mine tailing ponds

摘要

In the gold mining industry, tailings from the cyanide leaching process contain variable quantities of a number of cyanide compounds; as tailings are transferred to a pond for sedimentation and consolidation of the solid fraction, the supernatant water contains residual cyanide. A natural degradation of cyanide in tailings ponds is reported in the specialised bibliography, but in most cases, this is not sufficient to reduce the residual cyanide concentration to levels which would allow these waters to be directly discharged into the existing receiving water courses. In order to develop low cost and environmental friendly long-term methods for treating leachates from tailing ponds, wetland-based passive systems have been tested at the site of a gold mine in Spain. In this case study, most cyanide compounds are in the 'weak acid dissociable' (WAD) form and, almost all of them as copper-cyanide. In order to obtain relevant information concerning the performance, effectiveness and suitability of engineered wetland systems for the treatment of wastewaters containing cyanides (and metals), several experiments at laboratory and field scale were undertaken. At the laboratory scale, tests included static aerobic cells, anaerobic dynamic columns, oxidation cascades and a laboratory wetland. An average CNwad removal rate of 90.36 percent was found in the lab wetland, achieving residual CNwad concentrations below 2 mg/l. Removal rates of Cu and other metals reached values in the range of 90-99 percent (from 50 mg/l to 1 mg/l). Taking into consideration the experimental data, a pilot scale passive system, including aeration cascades and aerobic and anaerobic cells, has been constructed at the site of a local gold mine and it is currently being monitored.