Milling, Smelting and the Environment

After the ores has been brought to surface, the process of getting the metal out can create harmful substances. Apart from the ores they use and the wastes they produce, milling and smelting run on fuels and use chemicals to extract the metals. These substances also can present a hazard to the environment.

The milling process uses plenty of water, and large mills may use several hundred liters a minute. This water contains small concentrations of various organic and inorganic reagents used in the milling process. Many companies recycle all or part of the water back into their mills instead of discharging it into the environment. If it has to be discharged, then it also must be treated by capturing or destroying the chemicals.

One particularly important example is gold extraction. Most mills extract gold using weak solutions of sodium cyanide under slightly alkaline conditions. Traces of cyanide are left in gold mill effluents. Cyanide is not a single chemical element, so it can be broken down into the elements that make it up, namely carbon and nitrogen. The waste waters from the mill can be held in a pond where sunlight and contact with air break the cyanide down. Another choice is to incorporate a cyanide destruction process into the mill circuit, in which waste waters are aerated or chemicals are added that react with the cyanide to form less hazardous substances.

The use of cyanide in mining sometimes raises public concern. It is a dangerous poison and can be fatal if ingested. But since cyanide is also a compound, and an unstable one at that, it is relatively easy to destroy any excess in waste waters, leaving them suitable for discharge to the environment.