Three-Dimensional Predominance Volume Diagrams: The Ni-As-S-O System

The 3D predominance volume diagram (PVD) for the Ni-As-S-O system at 700 K (427 °C) was constructed using independent gases O₂, SO₂, and As₄O₆. The system has 15 four-condensed-phase invariant points. Invariant points, fixed by any combination of boundaries and condensed phases totaling four, were determined by a Gibb’s minimization routine that searched from a corner of the PVD along each undetermined line emanating from this and subsequently-identified invariants. The search direction was determined from the cross product of the two independent planes’ normal direction vectors. The 0.25 atm and 1.00 atm isobaric surfaces intersect the Ni₃As₂O₈, NiO, NiSO₄, Ni_1−xS, NiS₂, and Ni₁₁As₈ phases. The 1.00 atm surface also intersects the NiAs phase. The PVD results agree with published ternary and quaternary phase diagram Alkemade lines. If the roasting of gersdorffite (NiAsS) parallels the roasting of enargite (Cu₃AsS₄), gersdorffite is expected to initially react irreversibly with O₂ in the roasting gas to form NiS and NiSO₄ topochemical layers with arsenic being expelled as As₄O₆ gas and perhaps As₄ gas once a diffusion-controlled topochemical reaction sequence is established. Thermochemical calculations showed that the formation of NiO from NiSO₄ and Ni₃As₂O₈, if present, becomes more favorable as the temperature is increased with complete conversion to NiO above 1086 K and 1160 K (813 °C and 887 °C), respectively, at 1.00 atm.