Milling, Smelting and the Environment

After the ore has been taken up to surface, the process of getting the metal out can also 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 danger to the environment.

The milling process uses a large amount of water, and large mills can use several hundred litres per minute. This water contains small concentrations of many organic and inorganic reagents used in the process of milling. 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 destroying or capturing the chemicals.

Gold extraction is one particularly important example. Most mills extract gold using weak solutions of sodium cyanide under conditions which are slightly alkaline. 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 nitrogen and carbon. The waste waters from the mill can be held in a pond where sunlight and contact with air  break  the  cyanide  down. Another option is to incorporate a cyanide process of destruction 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 at times makes the public to be concerned about the situation. 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 appropriate for discharge to the environment.