Partitioning of transition metals during magnetization of oxidized pyrrhotite tailings

Low-temperature magnetization using reducing gas derived from biomass pyrolysis is being explored as an option to recover Fe from oxidized waste generated during Ni extraction from bio-leaching of pyrrhotite-rich tailings. The study investigates the potential sequestration of the main transition metal contaminants—Cu and Ni—into the magnetic phases. In the original oxidized pyrrhotite tailings, Ni and Cu were associated with goethite. Magnetization at 460 °C led to full conversion of goethite and jarosite into magnetite through solid-state transformation. 93% of the magnetite was recovered in the magnetic fraction by magnetic separation. Nickel and Cu accumulation was respectively 4 times and 2 times greater in the magnetic than non-magnetic fraction, indicating that the magnetic fraction acted as a sink for these transition metals. Metal fractionation of the magnetic fraction showed that a greater proportion of total Ni resided with the crystalline and refractory phases compared to Cu (47% of total Ni vs. 29% of total Cu). The greater affinity for Ni to incorporate into magnetite was further demonstrated using magnetized goethite co-precipitated with 100 mmol Cu or Ni kg−1: most Ni was associated with the amorphous and crystalline Fe whereas 60% of total Cu was extracted as a sorbed/exchangeable species. The greater Cu mobility during sequential extraction of magnetite is likely attributable to its monovalent state in this reducing system. Low-temperature magnetization coupled with biomass pyrolysis may represent a viable option to manage iron and leftover transition metals in oxidized, (bio)leached pyrrhotite wastes as a way to produce much smaller volumes of inert wastes.