Iron and sulphur management options during Ni recovery from (bio)leaching of pyrrhotite tailings, Part 1: Mechanisms of low-temperature transformation from goethite and jarosite to magnetic spinel

•Biomass-induced magnetization of jarosite-goethite assemblage is achieved below 500 °C.•Transformation of hematite, an intermediate phase, to magnetite is solid-state.•The reduction of jarosite-geothite produced magnetite and water-soluble arcanite.•Arcanite, the only sulfate product, is recovered through rinsing.