Microbial extraction of nickel from Sukinda chromite overburden by Acidithiobacillus ferrooxidans and Aspergillus strains

In this study, the recovery of nickel from a low-grade chromite overburden was attempted by employing two fungal strains, Aspergillus niger and Aspergillus fumigatus, and a mixed culture of mesophilic acidophiles (predominantly Acidithiobacillus ferrooxidans). Various factors were studied for bioleaching of chromite overburden such as, temperature, pH and pulp density. It was found that the At. ferrooxidans culture solubilized nickel effectively at temperatures ranging from 30 °C to 37 °C, whereas the organism was not able to solubilize nickel at higher temperatures, such as 45 °C. The use of higher pulp density resulted in a decrease of the percent nickel recovery whereas lower pulp density resulted in higher recovery values. Besides, increased supplemental ferrous iron increased the leaching efficiency of the At. ferrooxidans culture. The maximum nickel solubilization was 40%, at 2% pulp density, and 24%, at 10% pulp density, at 30 °C after 28 days leaching at 150 rpm.In the case of fungal strains, a comparison of leach ability of chromite overburden and roasted overburden was made. The factors studied were pulp density and reaction time. The adapted fungal strain showed better leaching results as compared to the unadapted strains. The in situ nickel leaching efficiency of a laboratory stock culture of A. niger showed maximum recovery of 34% nickel with roasted chromite overburden, at 2% pulp density, while 32% nickel was solubilized by A. fumigatus, under the same conditions at 30 °C and 150 rpm, after 28 days incubation.