Abstract
Bioleaching of olivine, a natural nickel-containing magnesium-iron-silicate, was conducted by applying chemoorganotrophic bacteria and fungi. The tested fungus, Aspergillus niger, leached substantially more nickel from olivine than the tested bacterium, Paenibacillus mucilaginosus. Aspergillus niger also outperformed two other fungal species: Humicola grisae and Penicillium chrysogenum. Contrary to traditional acid leaching, the microorganisms leached nickel preferentially over magnesium and iron. An average selectivity factor of 2.2 was achieved for nickel compared to iron. The impact of ultrasonic conditioning on bioleaching was also tested, and it was found to substantially increase nickel extraction by A. niger. This is credited to an enhancement in the fungal growth rate, to the promotion of particle degradation, and to the detachment of the stagnant biofilm around the particles. Furthermore, ultrasonic conditioning enhanced the selectivity of A. niger for nickel over iron to a value of 3.5. Pre-carbonating the olivine mineral, to enhance mineral liberation and change metal speciation, was also attempted, but did not result in improvement as a consequence of the mild pH of chemoorganotrophic bioleaching.
Highlights
The increasing demand and diminishing availability of raw materials requires us to look beyond conventional resources; the importance of low-grade ores is expected to increase in the near future [1]
It was found that the tested fungus Aspergillus niger leached substantially more nickel from olivine than the tested bacterium, Paenibacillus mucilaginosus, and outperformed the fungi Penicillium chrysogenum and Humicola grisea
A selectivity factor of 2.2 was achieved for nickel compared to iron
Summary
The increasing demand and diminishing availability of raw materials requires us to look beyond conventional resources; the importance of low-grade ores is expected to increase in the near future [1]. An example is the depletion of accessible high-grade sulfidic ores, which makes it necessary to seek more abundant but lower grade ores, often rich in siliceous mafic minerals (olivine, pyroxene, amphibole and biotite). The processing of such ores, is linked to high processing costs when using traditional extraction routes (e.g., high pressure acid leaching (HPAL) and ferro-nickel smelting). Potentially more sustainable approach is the application of biohydrometallugy, wherein microorganisms act as renewable chemical producers [1] of substances that deteriorate and dissolve minerals, thereby liberating the immobilized metals into solution (leachate). The three main mechanisms that can act in the solubilization of metals are: acidolysis, complexolysis and redoxolysis [4]
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