(579d) Recovery Options for Iron and Sulphur Produced during Nickel Extraction from (bio)Leaching of Pyrrhotite Tailings

The development and application of a circular economy model to tailings management in the mining industry has the potential to address issues of mineral resource shortages and environmental pollution, while minimizing waste and generating economic profits^[1]. For example, the valorization of the 100 million metric tons of pyrrhotite tailings contained in the Sudbury area, having an estimated average grade of ~0.8 wt% nickel^[2],may be economically viable and would eliminate remediation required to prevent acid mine drainage. While this approach is technically feasible using (bio)leaching strategies, pyrrhotite (Fe_1-xS) oxidation results in the generation of large volumes of residual iron and sulphur in the form of goethite (a-FeOOH) and jarosite (KFe₃(SO₄)₂(OH)₆). Consequently, iron and sulphur management options will have a major impact on the economics and overall environmental footprint. In this context, the objective of this study was to investigate the low-temperature (<500^oC) transformation of goethite and jarosite to an inverse spinel (e.g. magnetite, maghemite) amenable to magnetic separation using reducing gas produced from biomass, and fixation of the released sulphur in a recoverable phase. Magnetization of hematite (a-Fe₂O₃) was also considered as it represents an important product of goethite and jarosite decomposition.

Based upon the microstructural characterization of the products obtained from experiments performed on crystalline hematite, a solid-state mechanism for magnetic spinel formation, initiated at surface defects, was identified, where the low porosity of the reacted layer results in slow kinetics. On the other hand, goethite was readily converted to magnetite, as it involves an intermediate transition to a highly porous nanocrystalline reactive hematite phase allowing effective percolation of the reducing gases throughout the grains. The transformation of jarosite led to a phase assemblage consisting of magnetic spinel and arcanite (K₂SO₄), although persistence of minor residual hematite occurred. However, for a jarosite (75 wt%) / goethite (25 wt%) mixture, which represents a more realistic ferric residue of pyrrhotite (bio)leaching, complete transformation to magnetic spinel was observed, suggesting that the early transition of goethite to spinel promotes magnetite nucleation during K-jarosite decomposition.

Considering that about 55% of the total sulphur was lost to the gas phase during the transformation of the jarosite/goethite mixture, addition of potassium in the form of KOH and/or K₂CO₃ to the iron-bearing load was investigated as a means to trap the volatile sulphur and to access the minor amount locked in the magnetized product. This strategy was found to be very effective, especially in the case of KOH, where more than 90% of the sulphur could be fixed in the form of arcanite. As this phase is water soluble, it can easily be recovered through rinsing of the product. Furthermore, exchange with CaCl₂ led to stabilisation of sulphur as high-purity gypsum (CaSO₄•2H₂O), suitable for use in the construction industry, where the potassium remaining in solution could be precipitated as sylvite (KCl) for use as fertilizer.

^[1] Kinnuen P.H.-M. & Kaksonen, A.H., “Towards circular economy in mining: Opportunities and bottlenecks for tailings valorization”, J. Clean. Prod., 228, pp. 153-160 (2019).

^[2] Peek E., et al., “Nickeliferous pyrrhotite – Waste or resource?”, Miner. Eng., 24, pp. 625-637 (2011).