Kinetics of Lime-Enhanced Hydrogen Reduction of Solid Nickel Sulphide

Investigations have been carried out on the hydrogen reduction of solid nickel sulphide (β-NI₃S₂) in the presence of lime. The effects of the charge composition, temperature (500-700°C). hydrogen flow rate, time of reduction and particle size of the sulphide have been studied. Lime was found to tremendously enhance the reduction process and drastically stifle H₂S emission into the off-gas. Temperature as well as hydrogen flow rate were found to affect the reduction process and best results were achieved (in static bed experiments) with 200% CaO addition at 630°C in 2 hr employing a hydrogen flow rate of 0.2 l/min. Thermodynamic considerations and several experimental findings indicate that the progress of the Ni₃S₂-CaO-H₂ reaction is governed by the intrinsic kinetics of the Ni₃S₂-H₂ reaction. Kinetic analysis reveals the observance of Jander's linear rate equation indicative of phase boundary control at the sulphide/gas interface. Scanning electron microscopic studies on the reduced nickel sulphide pellets show that like in solid state transformations, discontinuous precipitation (cellular morphology) is exhibited.