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Mogale Gold

www.isa.org    7/20/2010 9:44:00 AM    ReadCount:594

Control system created for mine tailings reprocessing

Mine tailings are the crushed and milled rock residue that remains after mineral extraction. These tailings must be deposited for storage in a cost effective way that also meets environmental guidelines and mandates. Dams, dumps, and other types of surface piling are some of the more common tailings deposition methods used today. However, these all pose serious environmental concerns, as tailings often contain trace and sometimes substantial quantities of the metals found in the host ore, as well as certain amounts of chemicals and compounds used in the mining processes. As a result, establishing and maintaining tailings dumps represent a significant cost for mining companies.
Historically, mineral separation processes have been somewhat inefficient because after the required crushing, grinding, and metallurgical treatment, some amount of valuable metal is left behind in the residue. But recently, improved mining technologies and the ever-increasing price of gold in the marketplace have opened the opportunity for mining companies to purchase old mining dumps and metallurgically treat the tailings in order to recover residual gold.


Carbon-in-leach (CIL) and elution


Leaching, sometimes referred to as metallurgical extraction, is a chemical process for removing minerals by dissolving the mineral and moving it from a solid to liquid state. At Mogale Gold, this is accomplished by absorbing the dissolved gold onto activated carbon. This form of carbon is extremely porous (a single gram can have the same surface area as a small parking lot), making it well suited for adsorbing gold.
During elution, gold is stripped off the carbon by a hot chemical mix of caustic and cyanide. At an operator’s command, the Opto Controls control system opens the necessary valves, starts a series of feed pumps, and begins transferring water, caustic (sodium hydroxide), and sodium cyanide into the elution make-up tank. A large diesel burner is started, and the cyanide solution is circulated through heat exchangers to bring the solution to a temperature of 80-90°C, after which, the solution is fed into the elution make-up tank.
There are several devices and many types of instrumentation used in this process, and control system connects to and monitors and/or regulates nearly all of them, including motors on the caustic and cyanide feed pumps, pressure indicators, temperature probes on storage tanks, air valves, and flowmeters. Of particular importance are the variable speed drives (VSDs) that regulate the pump motors. To ensure this equipment operates optimally and proper flow rates are maintained, several proportional integral derivative (PID) loops were created.