Ferredox: A biohydrometallurgical processing concept for limonitic nickel laterites

The Ferredox process is a biohydrometallurgical concept targeting tropical limonitic laterites, focusing on simplified and low-intensity processing while being amenable to modularization. This approach aims to reduce both technical implementation risks and capital costs for tropical limonite projects. Key to the Ferredox process is the reductive dissolution of goethite, the predominant nickel limonite host mineral, using elemental sulphur as a reductant at ambient conditions. This dissolution yields a ferrous iron-based pregnant leach solution that allows for the recovery of nickel and cobalt prior to iron removal. Iron is removed through ferrous iron oxidation, resulting in precipitation as jarosite or schwertmannite, without the addition of a neutralizing agent. The soluble ferric iron produced during oxidation is repurposed to generate acid for the oxidation of elemental sulphur, which is then reused in the reductive leaching step. Unlike other hydrometallurgical methods for limonite, Ferredox operates at ambient pressure and temperature with mild acid concentrations (pH 1.2–2). While described specifically for nickel limonite ore, the Ferredox concept is intended for broader applications in the biohydrometallurgical processing of iron oxide ores containing base metals, including copper.