High temperature calorimetry of sulfide systems. I. Thermochemistry of liquid and solid phases of Ni + S

An experimental arrangement suitable for application of high temperature calorimetry to liquid sulfide systems has been developed. Using this approach, we have measured the integral enthalpies of mixing of Ni + NiS at 1100 K to form liquid alloys with compositions from X_Nis = 0.576 to X_Nis = 0.754. Partial enthalpies of the two components also were measured. After correcting for the enthalpies of fusion of Ni and NiS at 1100 K, the results of all measurements can be represented by an analytical expression which reflects subregular behavior of the mixing enthalpies ΔH_mix^l?1 = X_Nis²X_NiA + X_NisX_NiS²B with A = ?97.712 kJ mol^?1 and B = ?4.772 kJ mol^?1.The standard enthalpies of formation of the high and low temperature forms of NiS were evaluated from the calorimetrically measured enthalpy change associated with the reaction between nickel and sulfur at 1021 K. The standard enthalpies of formation of Ni₃S₂ (heazlewoodite), Ni₇S₆ and Ni_0.958S were determined from the enthalpy changes of reactions in which the compounds were formed from NiS and Ni at 873 K and 833 K. The standard enthalpy of formation of NiS₂(vaesite) was obtained from the enthalpy change of the reaction of NiS₂ with Ni to give NiS at 873 K. The following values are reported for the standard enthalpies of formation of the phases studied (in kJ mol^?1): ΔH_f,NiS(HT)⁰ = ?88.1 ± 1.0 ΔH_{f, Ni0.958S}⁰ = ?93.2 ± 0.7ΔH_f,Ni7S6⁰ = ?582.8 ± 5.7 ΔH_f,NiS(LT)⁰ = ?91.0 ± 1.0ΔH_f,Ni3S2(LT)⁰ = ?217.2 ± 1.6 ΔH_f,NiS2⁰ = ?124.9 ± 1.0.