Mining’s hardest workers are too small to see

Aerial view of an arid, craggy landscape surrounding tailing ponds.

…and we need to know what they’re doing

The image that is conjured up when thinking about mining is a vast underground network of tunnels, big open pits, larger than life machinery or grease-covered workers with headlamps on. But the largest workforce out in any mining operation is the microbes that are working 24/7, constantly influencing the environment.

“Bacteria are present in every aspect of mining, but we don’t fully understand the impacts they can have on water quality,” says Professor Lesley Warren, an aqueous and microbial geochemist with the Lassonde Institute of Mining and University of Toronto’s Department of Civil Engineering.

Warren works to determine the identities and roles of microbes in order to gain an understanding of the bacteria’s beneficial—and detrimental—processes. Effective biological tools, based on Warren’s research, are being developed for industry especially related to water quality management.

Water is a necessary resource for municipalities, agriculture, manufacturing, power generation—and mining. According to a 2009 report by Statistics Canada, mining accounts for two per cent of water use in Canada. Mine operations need water for mineral processing and metal recovery, but the discharge of water used in these activities can have adverse effects on surrounding surface and groundwater systems.

Water availability is a major concern for the mining industry. Fresh, clean water is a scarce resource and industry competes with many stakeholders for access. The mining industry has made great strides to minimize its effect on water quality and waste. The industry uses environmental management strategies, monitors discharges, and recycles used water, but water pollution remains a significant concern. So minimizing water requirements, while increasing the yield of recoverable minerals, is environmentally, socially and economically responsible.

Aerial view of an arid, craggy landscape surrounding tailing ponds.

Aerial view of an arid, craggy landscape surrounding tailing ponds.

“Acid mine drainage is the number one priority pollution issue for the mining industry on a global scale,” according to Warren. “It refers to the creation of acidic water. When sulphide minerals are exposed to water and air, sulphuric acid is generated through a natural, microbial-driven chemical reaction.”

Sulphuric acid lowers the pH of water, which lowers the quality of downstream water systems and increases their dissolved metal content. It is most commonly formed through the oxidation of pyrite (iron sulphide), which produces ferric sulphate and sulphuric acid. Because the majority of metal deposits are high in sulphide minerals and mining activities increase the amount of rock surface exposed to air and water, acid mine drainage remains of great concern.

Several researchers in the Lassonde Institute work to tackle water issues in the mining industry. In addition to Warren, Professors Elizabeth Edwards and Vladimiros Papangelakis explore the industrial applications for microbial and technological processes to solve mining’s pressing water challenges.

Professor Elizabeth Edwards (Department of Chemical Engineering & Applied Chemistry) works in subsurface bioremediation and remediation. For more than 20 years, she has studied microbes in contaminated sites, looking at how micro-organisms have adapted to both aerobic and anaerobic environments. The field applications of Edwards’ research involves harnessing the power of microbial metabolic processes for detoxification. One of her first commercial projects involved using bacteria to stimulate de-chlorination of solvents in industrial sites. Now, the same strategy is being applied to other contaminants.

Edwards has worked with Professor Vladimiros Papangelakis (Department of Chemical Engineering & Applied Chemistry) on a biological wastewater treatment project for acid mine drainage. Using bioleaching—the process of using bacteria to dissolve minerals instead of chemical solutions—of pyrrhotite tailings to recover nickel and elemental sulphur, making the wastewater more benign.

The oxidation of iron and of sulphur is achieved biologically. The project uses two bioleaching approaches: one for high nickel in acidic conditions at high solids concentration and one for ultramafic concentrates (high nickel and magnesium) at a neutral pH with oxygen and nitrate. The bacterium Thiobacillus denitrificans facilitates the nickel and magnesium dissolution, sulphate production and nitrate usage. In the nascent stages, the results were a two-to-four per cent nickel extraction at pH 6, but this may be more effective at a lower pH.

Papangelakis is also working on a project using forward osmosis to desalinate water for industrial usage, which could be more efficient and cost effective than existing technologies. Brackish water—a mixture of fresh and salt water—is generally unusable by industry because the salt corrodes equipment. As a water purification technique, forward osmosis is more energy efficient, and cheaper, than reverse osmosis.