The mechanism and kinetics of α-NiS oxidation in the temperature range 670-700 °C

The oxidation behavior of synthetic α-NiS in air has been investigated over the temperature range 670-700 °C. The α-NiS was ground and sieved to give a particle size ranging from 53 to 90 μm. Three oxidation paths were observed: (i) α-NiS + 3/2 O₂ → NiO + SO₂ (ii) 3α-NiS + O₂ → Ni₃S₂ + SO₂ (iii) Ni₃S₂ + 7/2 O₂ → 3NiO + 2SO₂ No Ni₃S₂ (heazlewoodite) was observed over the course of α-NiS oxidation at 670 and 680°C. The dominant oxidation path at this temperature is path i. At 700 °C, however, all three oxidation paths were observed. As an intermediate oxidation product, Ni₃S₂ steadily exsolved from α-NiS, reaching a maximum quantity after about 80 min of oxidation, declining afterward, and approaching annihilation at 160 min of oxidation. Experimental results show that the exsolution of Ni₃S₂ is likely triggered by the loss of one third of S in the α-NiS structure with the release of SO₂ rather than by an intrinsic thermal decomposition of α-NiS to α-Ni_1-xS + Ni₃S₂. The eventual annihilation of Ni₃S₂ was caused by a further oxidation of Ni₃S₂ to NiO. Oxidation paths 2 and 3 form a typical single chain reaction: α-NiS → k₁ Ni₃S₂ → k₂ NiO The approximate values of k₁ are k₂ are 3 × 10^-4s^-1 and 5 × 10^-4s^-1 respectively. Oxidation temperature was found to play important roles both in the oxidation kinetics and the oxidation mechanism. By decreasing 10 °C from 680 to 670 °C, the average reaction rate (dy/dt, where y is the reaction extent) over the experiment time scale almost decreased to one third of its original rate (from 3.3 × 10^-5s^-1 to 1.2 × 10^-5s^-1). The reaction mechanism in the temperature range 670 to 680 °C is constant with E_a = 868.2 kJ/mol.