It's as good as the metals

Biological process to extract minerals from ocean mineral lumps

 
By T V Jayan
Last Updated: Saturday 04 July 2015

Black gold: the polymetallic n indian metallurgists have devised an ecofriendly process to extract valuable metals from polymetallic nodules (mineral lumps) found on deep ocean floors. Scientists from the department of metallurgy at Bangalore's Indian Institute of Science (iisc) used the marine bacterium Bacillus M1 and commonly used starch to biologically leach the nodules to extract metals of economic value such as cobalt, nickel, copper and manganese. The bacterium was isolated from nodules mined from the Indian Ocean.

As of yet, no country worldwide is harvesting these nodules at a commercial scale, because land-based mineral sources are still considered cheap. Polymetallic nodules are found in very deep waters, implying that considerable mechanical resources are required to haul them up. This is the factor hindering their commercialisation. However, they have been acknowledged as a potential mineral source once the land-based reserves either run out or become too expensive to mine. Their existence has been known for almost a century. But most research on the process of utilising them has been carried out in the West, that too on nodules from the Atlantic and Pacific Oceans. "Far too little studies have been conducted on the Indian Ocean nodules," says Amitava Mukherjee, one of the iisc scientists.

Nodules from one ocean can vary widely in characteristics from that of another, as several physical and chemical factors influence their formation and composition. Mineral ores present in the nodules are broadly divided into oxides of manganese and iron. Other metals, which are present in smaller quantities, are part of either of these concentrations.

The iisc scientists let the microbes feed on the nodules, made available in powdered form. After four hours, the leach pulp was taken out and centrifuged to obtain what is called leach liquor. Once dried, the iisc scientists found that 25-45 per cent of the metals present in the nodules had been leached in the liquor. In order to harvest higher quantities of the metals, the scientists tried out several methods. In one, a series of four leaching processes were undertaken, with each lasting for four hours. In the second, a two-step leaching process was tried out using the bacterium acidithiobacillus thiooxidant. In the third, again involving two stages, the researchers added diluted starch to the residue separated after the first cycle and it proved to be a much better medium for the Bacillus M1 to operate. This process yielded more than 80 per cent of all the metals present in the nodules when the starch concentration was three per cent. In the first two processes, the recovery was relatively less.

Currently, the public sector company, Hindustan Zinc Limited, is operating a pilot-scale plant for processing polymetallic nodules on the behalf of the Union government's department of ocean development. However, the process used is ammoniacal leaching, which is highly-energy intensive. The iisc process has no such drawback. It is also much simpler and less expensive. Moreover, the bacteria operate at near neutral ph and room temperature.

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