Aluminium is an important metal of modern times. With applications ranging from transportation to packaging, construction to paints, the need for aluminium is ever-increasing. Bauxite is the main ore of aluminium. India, one of the largest producers of bauxite in the world, is estimated to have produced close to 20 million tonnes of bauxite during 2013-14 alone.
Before obtaining aluminium from bauxite, the ore needs to be processed for removal of impurities such as iron, calcium and silica. Conventional processes, involving the use of toxic chemicals, are expensive, energy intensive, and not eco-friendly. An unexpected discovery in a different field of science has led to a new unconventional solution to these problems.
Micro-organisms modify their chemical environments through their internal metabolic processes. Specifically, this is through redox reactions, i.e. electron addition and removal. These micro-organisms could present a simple yet invaluable method to process ores. Dr. K. A. Natarajan, an honorary professor at the Department of Materials Engineering, Indian Institute of Science, has elaborated in his new paper on how this can be used for removal of impurities in bauxite.
“Various microorganisms relevant to bauxite beneficiation are present in the water bodies as well as soils associated with bauxite deposits,” says Dr. Natarajan.
Of the 8 species of bacteria and 4 species of fungi found in the mine water samples from Western Indian bauxite, Dr. Natarajan and his team show that Paenibacillus polymyxa, a bacterium, can effectively remove calcium and iron from low grade bauxites i.e. ore with a lower percentage of alumina content; and conditioned silicate bacteria can remove silica in the bauxite.
If implemented, taking care of these micro-organisms is relatively simple. The main energy source for bacteria being sucrose, this could be easily supplemented with cheaper molasses or brewery wastes. However, as the bacteria produce organic acids, this could hamper their growth. Fortunately, the presence of some other minerals in the bauxite can somewhat help neutralise this effect. However, scaling up, testing and optimization can be done only with collaboration of a mining industry and sufficient funding.
Interestingly, there have also been previous similar unconventional attempts elsewhere in the world to remove impurities in ores. Dr. Natarajan suggests combining the use of micro-organisms with planting suitable plant species at the ore and red mud disposal sites may help reduce wasting resources and pollution control in ore-processing. Though trees are usually perceived as hurdles for mining operations, ironically, they might help us mine better.
“These scientific results and the developed process open up potentials for development of biotechnological routes for the beneficiation of Indian bauxites,” says a hopeful Dr. Natarajan. The use of microorganisms for mining has also been established to be beneficial to Indian iron ores, copper, lead-zinc and other low grade mineral deposits.
We have already outsourced medicine-making to micro-organisms. Perhaps it is time to strike another deal with our microscopic partners!
About the author
K A Natarajan is a Professor at the Department of Materials Engineering, Indian Institute of Science, Bangalore.
About the paper
The paper appeared in the journal Transactions of the Indian Institute of Metals earlier this month. DOI 10.1007/s12666-015-0597-6