Experimental investigation made previously on microbiological leaching of nickel and cobalt from the laterite nickel ore of Sukinda Valley reveals that the recovery was not very much promising under any favorable conditions. Therefore, in order to improve the efficiency for bioleaching, the homogenized lateritic ore in palletized form is thermally pretreated by roasting at different temperatures. The parameters studied for the bioleaching experiments were the four types of pretreated ore which were roasted at different temperatures, i.e., 300 o C, 400 o C, 600 o C and 800 o C, in shake flask by using a mixed mesophilic acidophilic bacterial consortium consisting predominantly of the Acidithiobacillus ferrooxidans strain. It was observed that the pretreated ore at 600 o C with 10% (w/v) pulp density showed maximum recovery of nickel and cobalt, i.e., 59.18% (4.556 ppm) and 65.09% (0.546 ppm), using 10% (v/v) (2.5×10 8 cells/ ml) consortium concentration at 1.5 pH, 30 o C, and 150 rpm after an incubation period of 31 days.

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In the present study lateritic nickel ore was used for bacterial leaching using a mixed consortium of mesophilic acidophiles. The microorganisms were adapted to 1 gram nickel/L prior to leaching. For the experiments, lateritic ore in different forms such as raw, roasted, roasted ore presoaked in dilute sulphuric acid and palletized pretreated roasted (400 o C and 600 o C) ore were taken. The leaching experiments were conducted in 9 K + with 40 L capacity bioreactor using 10% (v/v) inoculum concentration at 10% (w/v) pulp density. The aeration was maintained at 2-3 L/min and the speed of agitator and temperature at 400-500 rpm and 35 o C. The maximum extraction of nickel and cobalt was observed with pretreated ore (600 o C) at 10% pulp density (77.23% and 73.22%) respectively within 31 days at pH 1.5 and least extraction in case of raw ore i.e., 9.47% nickel and 41.12% cobalt respectively.