Screening bioleaching systems and operational conditions for optimal ni recovery from dry electroplating sludge and exploration of the leaching mechanisms involved

Recovery of valuable metals from electroplating sludge by bioleaching has attracted growing attention. However, extraction efficiency of metals was generally rather low even at a low pulp density of 1%–2% owing to failing to find the most suitable bioleaching system. In this work, high Ni recovery yield from electroplating nickel slag at a pulp density of 10% was attained by determining the best bioleaching process. The results showed the sulfur-A.t system exhibited the greatest extraction efficiency among the three bioleaching systems tested. The dissolved concentration of Ni and leaching efficiency reached 5400 mg/l and 90%, respectively, after 9 days of contact. Furthermore, 100% of Ni extraction was obtained at a pulp density of 10% by the sulfur-A.t system through optimizing operational conditions. Acid dissolution by biogenic H₂SO₄ was the dominating mechanism for Ni leaching in the sulfur-A.t system. However, other unknown leaching mechanisms also occurred which resulted in 19% Ni extraction. A direct contact between the sludge and cells was necessary for efficient Ni release by the sulfur-A.t system.