Ni–Fe–Co alloy – magnesia-iron-silicate slag equilibria and the behavior of minor elements Cu and P in nickel slag cleaning

The increasing demand for metals and the tightening environmental restrictions on industries have led to a situation where improvements of new and existing process technologies for higher metal recoveries from primary and secondary resources are needed. This, in turn, requires a better understanding of process chemistry. The present study provides important fundamental information on the equilibrium reduction path and the formation of metal alloy in an electric furnace under nickel slag cleaning conditions, as well as the chemical properties of Ni, Co, Cu and P at these conditions. High-temperature equilibration experiments were conducted on magnesia-iron-silicate slags at silica saturation at 1400 ˚C (1637.15 K) and in the oxygen partial pressure range of 10⁻¹¹–10⁻⁷ atm (1.01325 × 10⁻⁶–10⁻² Pa). The samples were analyzed by electron microprobe (EPMA) for major and minor elements and laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) for trace element compositions. Initially the slag contained 2 wt% Ni and 1 wt% Cu, Co and P each, added as oxides, and at P_O2 = 10⁻⁷ atm, these metals were only present in the slag with no metal alloy formed. At lower partial pressures, the alloy formed and its composition changed from 90wt-% Ni-rich to 50 wt-% Ni – 40 wt% Fe alloy as the P_O2 decreased. Copper vaporized to a great extent at all conditions. The minor metals were shown to be present as NiO, PO_2.5, CoO and CuO_0.5 species in slag over the P_O2 = 10⁻⁸–10⁻¹⁰ atm range.