Phase Equilibrium between Ni-S Melt and CaO-Al₂O₃ Based Slag in CO-CO₂-SO₂ Gas Mixtures at 1773 K

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To provide thermodynamic data for converting the nickel matte to liquid nickel, an experimental study was conducted in the phase equilibrium between the Ni–S alloy and the CaO–Al₂O₃ based slag melted in a MgO or Al₂O₃ crucible at 1773 K under controlled P_SO2 of 10.1 kPa and P_O2 in a range between 5.0×10⁻³ and 1.6 Pa by using CO–CO₂–SO₂ gas mixtures. The contents of nickel in these slags at P_O2 of 1.6 Pa, above which solid NiO is expected to precipitate in equilibrium state, were found to be 5.6 and 7.2 mass% for the slags melted in the MgO and Al₂O₃ crucibles, respectively. The contents of sulfur and oxygen in the nickel melt at P_O2 of 1.6 Pa were 2 and 0.4 mass%, respectively. The CaO–Al₂O₃ based slag with the mass% ratio (CaO/Al₂O₃) of about 1 melted in the MgO crucible was homogeneous in the investigated P_O2 range. While, that with the ratio of about 0.5 melted in the Al₂O₃ crucible formed two phases composed of a liquid and an Al₂O₃–NiO solid solution whose nickel content was increased with increasing P_O2. The dissolutions of nickel and sulfur in the slag melted in the MgO crucible were discussed on the basis of distribution ratio and sulfide capacity, respectively.