Using bacteria to achieve environmentally friendly mining processes

By Belinda Bleeze

Advancements in minerals processing technology is being driven by the increase in demand for precious metals. The increase in demand is resulting in a depletion of high grade ore, resulting in the use of more complex low-grade ore. Current mining practices use froth flotation as a technique to separate valuable minerals from the surrounding material which is found to be increasingly difficult with an increase in ore complexity. The effectiveness of separation is often enhanced through the use of toxic chemicals which alter the surface chemistry of both the target mineral and the surrounding material. These chemicals, although effective, are toxic to the environment and are known contributors of acid mine drainage.

My project aims to employ the relatively new technique of bio-flotation. This method replaces the toxic chemicals with biological alternatives. Specific types of bacteria, which naturally occur at mine sites, can be used to replace the use of toxic chemicals. In particular, the bacterial metabolites, known as Extracellular Polymeric Substances (EPS) show a promising potential for effective separation. To optimise this technique it is important that we form an understanding of the bacterial – mineral system. This includes the EPS composition and how it changes in different growth conditions, mechanisms of attachment for both the cell and EPS along with the surface chemistry changes of the mineral. The use of EPS as a bio-reagent is ideal as it eliminates any negative effects that may occur due to the bacteria in the system.

The crucial factor required for successful separation is the selective attachment of bacteria/EPS to a mineral. The selective attachment of Leptospirillum ferrooxidans (L.f) has been investigated using a scanning electron microscope. The results show L.f selectively attaches to pyrite over chalcopyrite for exposures less than 7 days. The selective attachment is seen to correspond with a greater presence of hydrophilic compounds (compounds which are attracted to water) on the mineral surface. Bio-flotation, like other flotation processes, uses the attachment of particles to air bubbles. As the attachment of L.f and EPS to pyrite produces a hydrophilic surface, attachment of pyrite to the air bubbles is supressed. This allows greater separation of pyrite and chalcopyrite, without the use of toxic chemicals.

Figure 1: L.ferrooxidans and EPS attached to pyrite surface
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