Effect of biochemical reactions in enhancement of rate of leaching

Microbial dissolution of metals involves direct adhesion/attack of the bacterial cell on the sulfide surface and/or allowing chemical oxidation of liberated sulfides by ferric iron generated microbially or through oxidation by air. The prolonged recycling of Fe2+/Fe3+ couple by the bacterium is essential to keep an environment of high redox potential that is required for efficient leaching. Acidophile Thiobacillus ferrooxidans (Tf) has the unique ability to oxidize ferrous to ferric and reduced sulfur compounds to sulfate. Enhancement of rate of reactions involving oxidation of Fe2+ <----> Fe3+ and S2- <----> SO42- is essential in order to improve the overall leaching kinetics. This could be achieved by vigorous production of lixiviant (Fe3+) and augmentation of conversion step of elemental sulfur to sulfate. These reactions are important in avoiding the formation of jarosite. Otherwise it leads to a decrease in availability of ferric iron by blocking the mineral sites on ore particles. This creates a kinetic barrier due to slow diffusion of reactants and products through the precipitation zone. The aim of this work is to correlate the rate of biochemical reactions with the efficacy of leaching of metals. In that direction, experiments were conducted with copper concentrate that contains chalcopyrite and pentlandite as major phases. Operating conditions such as solid's concentration, agitation, aeration, nutrient requirements, residence time and pH regulation, etc. were optimized for selective leaching of nickel. pH plays a critical role for the maintenance of cyclic process of Fe2+ <----> Fe3+ and eliminate the ferric iron precipitation which control the whole leaching process. Increase in the rate of ferrous and reduced sulfur oxidation was achieved by Tf cultures adapted to flotation concentrates. Concurrently, amplification in the rate of Ni leachability was observed. (C) 2004 Elsevier Ltd. All rights reserved.