Drying kinetics of nickeliferous limonitic laterite ores

Abstract

The drying kinetics of a nickeliferous limonitic laterite ore were investigated in the temperature range of 44–228 °C, using a thermogravimetric technique, and the results were characteristic of a non-hygroscopic porous solid. For drying temperatures below 100 °C, the drying rate curves exhibited three stages: a relatively short incubation or set-up period, a constant rate drying period and a decelerating rate drying period. At temperatures above the boiling point of water, there was no constant rate drying period and only the incubation and the decelerating rate stages were observed. The graphical critical moisture fraction, X Cr , which is the moisture content at which the transition from the constant rate period to the decelerating rate period occurs, was found to be 0.064 and was independent of the drying temperature and thus the drying rate. Using the concepts of the relative drying rate ( f) and the characteristic moisture content ( Φ), a characteristic drying curve was determined for the ore. The drying kinetics, when the sample had attained the equilibrium temperature, exhibited non-Arrhenian behavior and the drying process was approximated using two linear relationships. The activation energies over the two temperature ranges were determined to be as follows: 25.5 kJ/ mole 44 ° C⩽T⩽100 ° C 12.9 kJ/ mole 100 ° C⩽T⩽228 ° C The activation energies were both relatively low and this indicates that the process was diffusion controlled in both temperature regimes. It is postulated that at temperatures below 100 °C, the rate-determining step was attributed to boundary layer control while at higher temperatures the rate-determining step was internal diffusion control. Effective diffusion coefficients ( D e) were calculated from the drying rate data and they varied with both concentration and temperature. At low moisture contents, a maximum was observed in the effective diffusion coefficient and this was explained in terms of a maximum in the vapour phase diffusivity at low moisture contents. At temperatures below the boiling point of water, the plateau at the maximum was extended and the diffusion coefficient was relatively constant over the concentration range X=0.02 –0.055. The maximum in the diffusion coefficient was found to vary with temperature as T n , where n=1.8.