Mining: Waste reduction

When we consider the incredible advances of technology the 20th century witnessed, it is hard to believe just how little the mining industry has changed. Most of the precious gems mined in the world, such as emeralds, diamonds and rubies, are still collected by troops of people chipping away at exposed rock or coal faces.

Even in so-called developed countries, mine site injuries and fatalities are still a sad reality, due to mine collapses and machine accidents. Most of the advances in mining techniques have been aimed at improving worker safety rather than minimising environmental impact. This aim has led to much more automation of mining processes, which has meant that electrical and electronic engineers are now just as necessary at mine sites as mechanical, mining and structural engineers.

Anyone who has experienced a visit to a bauxite mine will understand the immensity of scales involved at such sites. Gargantuan tractors, trucks and shovels dot the landscape, and these huge machines are now equipped with high-tech electronic controls. Automation may in fact help the environmental cause because high-level technologies have the potential to reduce energy consumption with fuzzy logic, much as washing machines can now identify the amount of water and energy is needed for the amount of laundry sitting in the tub.

Processing methods often involve the use of toxic chemicals, or substances with toxic by-products, and this is a major cause for concern. Extracting copper from its ore requires sulfuric acid, a common acid type found in acid rain. Gold processing plants emit arsenic, mercury and sewage, and cyanide is used to separate gold from the effluent. Cyanide is one of the few substances that will cause the extremely stable, meaning non-reactive, gold metal to ionise and form a compound. The solid gold element is “dissolved” in the cyanide solution, the solid effluent is filtered out, and then the gold solid is precipitated in pure form. However, the cyanide also returns to its original, toxic, form.

A recent copper mine proposed for Arizona was to employ a new technique that would recycle the sulfuric acid used in processing the copper, and a great deal of money was spent for a gold mine planned in California’s Mojave Desert to have closed tanks to contain the cyanide-laced effluent, to prevent wildlife from drinking it. These methods are, however, short-term solutions.

Biochemist Jim Whitlock in South Dakota has identified a bacteria that feeds on carbon and nitrogen, the ingredients in cyanide. Cyanide-bearing waste can be passed through a series of tanks in a bioreactor filled with bacteria, which renders the effluent virtually free of cyanide. This kind of process, which uses a natural organism to treat waste, is called bioremediation.

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