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

¹ Commonwealth Scientific and Industrial Research Organization, PO Box 90, Bentley, W.A. 6982, Australia
² Department of Mineralogy, South Australian Museum, North Terrace, Adelaide, S.A. 5000, Australia
³ School of Earth and Environmental Science, University of Adelaide, Adelaide, S.A. 5005, Australia
⁴ School of Chemical Engineering, University of Adelaide, Adelaide, S.A. 5005, Australia

Correspondence: ^* E-mail: haipeng.wang{at}csiro.au

The oxidation behavior of synthetic -NiS in air has been investigated over the temperature range 670–700 °C. The -NiS was ground and sieved to give a particle size ranging from 53 to 90 µm. Three oxidation paths were observed:

No Ni₃S₂ (heazlewoodite) was observed over the course of -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 -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 -NiS structure with the release of SO₂ rather than by an intrinsic thermal decomposition of -NiS to -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:

The approximate values of k₁ are k₂ are 3 x 10^–4s^–1 and 5 x 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 x 10^–5s^–1 to 1.2 x 10^–5s^–1). The reaction mechanism in the temperature range 670 to 680 °C is constant with E_a = 868.2 kJ/mol.

Key Words: NiS • oxidation • chain reaction • kinetics