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Bacteria, our gold laying geese of the future?

Of all human peculiarities, one is unique – our love for gold. We adore the yellow metal and have labeled it ‘noble’. We have attached so much value to it that it is practically the basis for all modern currency. India consumes 800 tonnes of gold annually. There’s a wide gap between indigenous gold production and demand, making it necessary for urgent exploration, mining and extraction of all available gold resources in the country.

Gold occurs naturally in its native form and is usually in combination with quartz. These ores, called ‘free milling’ ores, have gold in its least associated form. This ore can easily be crushed, washed and milled to recover the gold contained within it. However, to increase the quantity of gold extracted, the ore is treated in a conventional process involving cyanide for solubilising the gold, followed by recovery processes.

After centuries of mining, these free milling ores are fast depleting. The time has come to tap into refractory ores, another type of ore where gold is associated with sulphides such as pyrite and arsenopyrite.

But the gold in refractory ores is so strongly stuck inside the ore’s structure, that it cannot be physically freed with just grinding. Moreover, only liberated gold particles on the surface of the ore can be extracted using cyanide solution. One possible solution before ‘cyanidation’ is bacterial oxidation. First implemented in 1986 in South Africa, this method has now spread to the Americas, Australia, China, Russia and the rest ofAfrica.

“Bacterial oxidation offers the advantage of significantly reduced capital and energy costs while producing environmentally acceptable effluents”, says Dr. K. A. Natarajan from the Department of Materials Engineering, Indian Institute of Science (IISc), Bangalore.

Set up by Prof. K. A. Natarajan 30 years ago, his lab at IISc focuses on mineral biotechnology. Taking an interdisciplinary approach, his team works at the juncture of microbiology, molecular biology and metallurgy to provide uncanny solutions for mining minerals effectively.

Chemolithotrophs, i.e. bacteria that use inorganic reduced compounds as a source of energy, such as Acidithiobacillus ferrooxidans, are well known for their ability to dissolve many sulphide minerals. They are present in the ore deposits and surrounding water. Deriving carbon from carbon dioxide and energy from iron and sulfur, At. ferrooxidans generates a good solvent for leaching a number of sulphide minerals.

Dr.Natarajan’s team isolated a strain of At. ferrooxidans, TfH6, from the Hutti Gold Mines in Karnataka. As the by-products of the gold solubilisation reaction do not support the bacterium to survive, TfH6 was made to further adapt to tolerate those extreme conditions, as well as arsenic.

After extensive experimentation, the team succeeded in oxidising more than 90% of the sulphides resulting in enhanced recovery of gold and silver after cyanidation. Dr.Natarajan’s team in collaboration with Engineers India Limited developed a process design package for a demonstration bioreactor plant at Hutti a few years ago, financially supported by the Department of Biotechnology, Govt. of India.

This demonstration plant is a first and only effort in this direction in India. This attempt opens up biotechnological solutions to problems faced by mining industries all over the country. “Our efforts at Hutti mark only a beginning and lots of ground remains to be covered,” beams a hopeful Dr.Natarajan.

About the scientist:

Dr. K. A. Natarajan is currently an honorary professor at the Department of Materials Engineering, Indian Institute of Science, Bangalore. With over 350 scientific research publications in the fields of Biometallurgy, Hydrometallurgy, Mineral Processing, Corrosion, and Environmental Control, Dr. Natarajan is a recipient of several awards including the Raja Ramanna Fellowship (DAE) and the National Metallurgist Award.

Email: kan@materials.iisc.ernet.in