Two-step ferric and biological leaching as an environmentally friendly method for improving nickel sulfide concentrate processing

Abstract

The nickel and copper extraction from a nickel sulfide concentrate during a two-step oxidation process was compared to one-step biooxidation. A new approach was applied to the processing of nickel concentrates: successive ferric leaching and bioleaching of the residue obtained after ferric leaching. The two-step process included ferric leaching by biologically generated ferric sulfate solution and subsequent biooxidation by moderately thermophilic microbial communities. For experiments, a concentrate containing 16.6 % Ni and 1.73 % Cu was used. Pentlandite, violarite, pyrite, pyrrhotite, and chalcopyrite were the main sulfide minerals in the concentrate. After several cycles (13 h in total) of ferric leaching (80 °C; pulp density, 10 % (w/v)) with a solution containing 10 g/L of Fe^3 +, 58.9 % of nickel and 35.0 % of copper were dissolved. Subsequent biooxidation at 45 or 50 °C (pulp density, 5 % (w/v)) using moderately thermophilic microbial communities allowed achieving maximum nickel recovery (∼98–100 %) four and eight days earlier than direct bioleaching, reducing bioleaching duration ∼1.5 times. A comparison of metal extraction using microbial communities growing at 45 and 50 °C in the bioleaching step was carried out; the duration of a nearly complete nickel leaching in the two-step process was shown to be five days shorter in the first case. Ferric leaching was not shown to alter the rate of dissolution of nickel sulfides during bioleaching. Patterns of changes in the chemical and mineral composition of leach residues, as well as in the microbial community, were revealed.