The mechanism and kinetics of -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 O 2 → NiO + SO 2 (ii) 3α-NiS +O 2 → Ni 3 S 2 + SO 2 (iii) Ni 3 S 2 + 7/2 O 2 → 3NiO + 2SO 2 No Ni 3 S 2 (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, Ni 3 S 2 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 Ni 3 S 2 is likely triggered by the loss of one third of S in the α-NiS structure with the release of SO 2 rather than by an intrinsic thermal decomposition of α-NiS to α-Ni 1-x S + Ni 3 S 2 . The eventual annihilation of Ni 3 S 2 was caused by a further oxidation of Ni 3 S 2 to NiO. Oxidation paths 2 and 3 form a typical single chain reaction: The approximate values of k1 are k2 are 3 × 10 -4 s -1 and 5 × 10 -4 s -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 -5 s -1 to 1.2 × 10 -5 s -1 ). The reaction mechanism in the temperature range 670 to 680 °C is constant with E a = 868.2 kJ/mol.