Ferredox: A biohydrometallurgical processing concept for limonitic nickel laterites

Abstract The Ferredox process is a biohydrometallurgical concept flow sheet targeting tropical limonitic laterites. The process components are designed to (a) facilitate simplified and low-intensity processing, (b) be amenable to modularization, (c) reduce technical process implementation risk, and (d) reduce capital costs for tropical limonite projects. The Ferredox process has been enabled by the recently demonstrated reductive dissolution of goethite, the most prominent nickel limonite host mineral, using elemental sulphur as a reductant at ambient conditions. The reductive dissolution process results in a ferrous iron-based pregnant leach solution, facilitating the recovery of nickel and cobalt prior to iron removal. Iron removal is achieved by ferrous iron oxidation resulting in iron precipitation as either jarosite or schwertmannite, without neutralizing agent addition. Soluble ferric iron, resulting from ferrous oxidation, is used in a reductive acid generation step for the oxidation of elemental sulphur. The resulting ferrous-based acid solution is redeployed to the reductive leaching step. A distinguishing feature, compared to other hydrometallurgical leaching options for limonite, is that all key process components occur at ambient pressure and temperature conditions with mild acid concentrations (pH 1.2–2). The Ferredox concept is described in the context of a nickel limonite ore but is intended as a generic concept for biohydrometallurgical processing of iron oxide ores containing base metals, including copper.