Kinetics of chloridization of nickel-bearing lateritic iron ore by hydrogen chloride gas

The selective chloridization of nickel in a lateritic iron ore by gaseous HCl is based on the principle of relative thermal stability of iron and nickel chlorides. This aspect has been discussed with differential thermal analysis (DTA) and thermogravimetric (TG) data of the hydrated chlorides of iron and nickel. The kinetics of chloridization of nickel in a lateritic nickel ore from Orissa, India, have been studied by using both pure HCl (g) and the HCl (g) + N₂ mixture. The sharp decrease in the rate of chloridization of nickel at temperatures above 250 °C is attributed to the rapid decomposition of molten ferric chloride hydrate (FeCl₃ · 3H₂O), which blocks the pores of the reactant solid. Therefore, kinetics of chloridization follow both the pore-blocking model (logarithmic rate law) and diffusion-controlled mechanisms. Very low values of apparent activation energy and effective diffusivity derived from the rate constants of the diffusion-controlled process suggest that diffusion of HCl (g) takes place either in a dissolved state in the molten ferric chloride (at 100 °C to 150 °C) or through cracks and fissures formed on the surface due to rapid decomposition of ferric chloride at 200 °C to 250 °C. Because of the complexity of the reaction system, the rate of chloridization of nickel is almost independent of grain size.