Iron and sulphur management options during Ni recovery from (bio)leaching of pyrrhotite tailings Part 2: Strategies for sulphur fixation during biomass-induced magnetization of goethite and jarosite

Abstract

In the context of recovering Ni from pyrrhotite tailings though a (bio)leaching process, Part 1 of this investigation explored a strategy to concentrate iron from the residual jarosite-goethite assemblage using controlled release of reducing gases from biomass to promote transformation at temperatures below 500 °C to an inverse spinel phase amenable to magnetic separation. While most of the remaining sulphur is associated with K in the form of arcanite (K2SO4), a water-soluble potassium sulfate that can easily be removed through rinsing, the present study focuses on implementing a complementary fixation strategy to minimize the significant loss of sulphur to the gas phase and to access the residual amount trapped in magnetic spinel to obtain a cleaner more valuable iron product. Through the use of potassium additives (K2CO3, KOH) during biomass-induced magnetization experiments, more than 90% of the original sulphur could be stabilized in the form of arcanite. Moreover, after a further thermal oxidation treatment of the rinsed magnetite product in the presence of KOH, the residual sulphur content could be lowered from 1.8 down to 0.08 wt%. Although the major advantage of fixing the sulphur in the form of a readily-extractable arcanite phase is to easily purify the magnetite product, subsequent exchange of the potassium sulfate solution with calcium chloride to promote the precipitation of high-purity gyspum (CaSO4·2H2O) represents a viable option to contain sulphur in a highly stable form, which can also represent a by-product suitable for use in the construction industry. On the other hand, the potassium remaining in solution in the chloride form can be precipitated as sylvite (KCl) for use as fertilizer.