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Cyanide Destruction
Figure 12. Gold Acres Heap at Cortez Gold Mine.
Cyanide Heap Biological Detoxification, Phase II
Primary Issue Addressed: Cyanide Destruction
Secondary Issues Addressed: Gold Recovery and Economics; Biological Treatment
Project Site: Placer Dome U.S., Cortez Gold Mine located in Crescent Valley, Nevada
Collaborating Entities: MSE, Placer Dome U.S., Cortez
Gold Mine, Whitlock
and Associates, Spearfish, South Dakota
Cost Share: In-kind services provided by Cortez Gold Mine.
Project Description
The Mine Waste Technology Program (MWTP), Project 46, Cyanide Heap Biological Detoxification, Phase II, is ongoing at the Cortez Gold Mine (CGM) in northeastern Nevada using the Gold Acres Heap shown in Figure 12. Placer Dome, Inc., owns 60% of CGM through a joint venture with Kennecott Exploration Ltd. Placer Dome Inc., is a national and international mining company. This project is the second phase of the MWTP Project 11, a large-scale column study conducted at McClelland Laboratories in Sparks, Nevada.
Many active mine sites, mines in the closure stage, and some abandoned mines are and have used cyanide to remove and recover precious metals. Discharges from these sites normally contain significant amounts of metal cyanide complexes and concentrations of thiocyanate, soluble heavy metals, nitrate, sulfate, and ammonia.
Chemical, physical and biological processes have been developed to attempt to clean up seeps and discharges. Strong oxidants such as hydrogen peroxide, chlorine dioxide, Caros Acid, ozone, and sulfur dioxide have shown effectiveness in some applications. Biological processes, alkaline chlorination, reverse osmosis, and ion exchange have been effective in removing thiocyanate, cyanides, and heavy metals. Typically, biological processes incur lower capital costs at substantially lower operational costs while producing a treated effluent that is compatible with receiving waters and the environment.
The conventional method of heap detoxification consists of rinsing the spent heap leach pad with fresh or treated water until regulatory standards for solids and solutions are achieved. Several pore volumes of water are typically required to detoxify cyanide, which lengthens the treatment process. In addition, metals continue to be present in the heap effluent after cyanide detoxification, which can prevent heap closure. Consequently, large quantities of solution are generated that require treatment and add to the overall treatment costs. Biological heap detoxification is a process that uses bacteria to detoxify a spent heap leach pad by destroying cyanide, nitrates, and sulfates and removing metals.
Status
The field demonstration was implemented in September 2004 with the biological treatment being spray applied to the Gold Acres Heap. Monitoring is scheduled to continue into early FY06. The final project report will be completed in FY06