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

Abstract

Considering the potential for recovering Ni from pyrrhotite tailings though a (bio)leaching process, the fate of the large amounts of oxidized Fe and S produced in the form of residual hydroxide (goethite) and sulfate (jarosite) needs to be addressed. Low-temperature transformation of such ferric phases to an inverse spinel phase amenable to magnetic separation using biomass as a source of reducing gas can represent a clean option for iron management. In this context, biomass-induced reducing experiments at temperatures below 500 °C were performed to identify the mechanisms that influence the formation of magnetic spinel from goethite, K-jarosite and hematite decomposition. Microstructural characterization of the products from experiments on crystalline hematite at 450 °C indicates a solid-state transition to magnetic spinel initiated at surface defects, where the non-porous passivating reacted layer leads to slow kinetics. This study also confirms that efficient transformation of goethite to magnetite at low temperature involves an intermediate transition to a reactive hematite nanocrystalline phase with high microporosity inherited from the (oxy)hydroxide precursor, allowing effective percolation of the reducing gases throughout the grains. In the case of K-jarosite, the resulting phase assemblage is complex and sensitive to experimental conditions. Optimization of the release of reducing gases prevents the formation of ferrous sulfates leading to complete partitioning of Fe in oxide form associated with arcanite (K2SO4) down to 480 °C. However, although the abundance of magnetite is significant, persistence of residual hematite suggests that it has low porosity and hence limited reactivity. Nevertheless, for a jarosite-goethite assemblage, complete transformation to magnetite is observed suggesting that the early transition of goethite to spinel promotes magnetite nucleation during the subsequent decomposition of K-jarosite. As coexisting arcanite is water-soluble, rinsing of the product acts as a means of liberating the magnetic material and allows extraction of most of the residual potassium and sulphur. Consequently, low-temperature biomass-induced magnetization represents a viable option for managing Fe after Ni extraction from pyrrhotite-rich tailings where both goethite and jarosite are precipitated, with the additional potential to recover the produced biochar.