The mineralogy and reactivity of pyrrhotite from selected nickel ore deposits and its effect on flotation performance

The non-stoichiometric sulfide mineral pyrrhotite (Fe(1-x)S), common to many nickel ore deposits, occurs in a variety of crystallographic forms, each with subtly differing physical and chemical properties. Since there appears to be little agreement in the literature as to how these varying properties influence the reactivity and flotation performance of pyrrhotite, the manipulation of pyrrhotite flotation performance is not necessarily simple, especially since there is no uniformity as to whether pyrrhotite is recovered or rejected during flotation. Using nickeliferous pyrrhotite samples derived from the Nkomati ore in South Africa, Phoenix ore in Botswana and Sudbury ore in Canada, the mineralogy and reactivity of magnetic (Fe7S8), non-magnetic (Fe9S10) and inter grown magnetic and non-magnetic pyrrhotite was characterised and the relationship to flotation performance developed. Rest potential, cyclic voltammetry and oxygen uptake measurements were used to quantify the reactivity of pyrrhotite and demonstrated that magnetic pyrrhotite was the most reactive, whereas non-magnetic pyrrhotite was relatively unreactive. In some scenarios, the magnetic pyrrhotite was so reactive it was already passivated and appeared to be relatively unreactive. Subsequent microflotation tests showed that the relatively unreactive non-magnetic pyrrhotite had the best collectorless flotation recovery and that only with the addition of reagents could differences in the flotation performance of magnetic and mixed pyrrhotite samples be identified. These differences in pyrrhotite flotation performance were primarily attributed to the propensity of the different pyrrhotite samples for oxidation and formation of hydrophilic ferric hydroxides. In turn, this was evaluated according to the effect that pyrrhotite crystallography, mineral chemistry and mineral association have in controlling the surface characteristics of pyrrhotite and its subsequent flotation performance. This study has clearly demonstrated the importance of mineralogy as a tool to understand pyrrhotite flotation performance which may be utilised for plant optimisation.