Acid dissolution of mining waste—a new eco-friendly method for extracting metals

New research shows that magnesium and other metals in mine waste can be solubilized and recovered thanks to acid-producing bacteria. Researcher Nathan van Wyk believes that the method has great potential for reducing environmental pollution, extending the life of mines and providing raw materials for industry.

Each year, more than three billion tons of metal are produced for use in various products. However, the metal content of ores has decreased with exploitation, which has made it more difficult to access the metals. This has resulted in increased metal prices and more harmful wastes. In addition, there is an increased demand for so-called rare earth elements, which are often found in very low concentrations, which can also generate a lot of mine waste.

The mine waste is often disposed of in large, open piles at the mine. It is an environmental hazard, because metals in the waste can leach into groundwater, fine windblown waste can spread contaminants, and some chemicals used in metals extraction can also cause pollution. But these wastes also presents an opportunity.

The waste still contains significant quantities of several useful metals. So if we can extract the metals from the waste, we can both reduce environmental pollution and produce more raw materials, says van Wyk, researcher in ecology.

Acid dissolution

In his doctoral thesis, van Wyk investigated how the metals can be extracted from the waste using acids produced by bacteria. The wastes he examined were mainly bauxite residues, a mineral that contains aluminum and is used in aluminum production, and several wastes that are resultant of magnesium production.

The research shows that the acids extracted close to 100% of the magnesium from the magnesium-rich waste. And when it comes to the bauxite residues, the acids extracted several metals: 68% of aluminum, 80% of calcium, and 59% of titanium, as well as some rare earth elements—42% of yttrium was also solubilized.

“Acid dissolution of mine waste is a promising way to make use of valuable resources. It reduces the amount of waste, protects the environment, generates raw materials for industry and extends the life of the mines. And also advances ‘green’ technology while providing jobs. There are many advantages to the application of this method, instead of just leaving the waste behind,” says van Wyk.

The remains can be used as concrete

A positive spin-off of the method is that residual material can be combined with cheap additives to produce a type of rapid curing concrete.

“With the waste from our process, we can produce construction materials, like concrete. Our concrete doesn’t need to be calcinated (an energy intensive process) during production as the energy had already been spent.

“Over 7% of all the energy used in industry globally is used for the production of concrete and cement. So by using this waste we can achieve zero solid waste from magnesium mining, and reduce global energy consumption, to a degree,” says van Wyk.

Van Wyk believes that the acid dissolution method needs some development to achieve large-scale production, but laboratory scale work reveals an elegant and effective process, and he hopes it will be implemented by the mining industry in the future.

“We have developed a new technology that has several applications within the mining industry. Through this technology, we can turn some mining operations into a zero-waste facility,” says van Wyk.