Effect of temperature on the dewaterability of hematite suspensions

Abstract

Many industrial processes perform solid–liquid separation operations at elevated temperatures. One such example is the Bayer process, which is used to produce alumina from bauxite. During the thickening and washing of the process residue (red mud), the operational temperature decreases from around 110 to 35 °C, and thus an understanding of the effects of temperature on suspension dewaterability becomes important. Since the composition of Bayer process residues is quite complex, the effects of temperature on the flocculation and dewaterability behaviour of suspensions was investigated using hematite as a model system. Experiments were carried out on untreated and polymer flocculated hematite suspensions and the effects of temperature were measured in terms of settling rate, supernatant turbidity and equilibrium bed height. The dewaterability of settled suspensions was then determined by measuring the compressive yield stress and hindered settling function using a custom-built pressure filtration rig. To complement the compressibility tests, shear yield stress experiments were also performed as a function of temperature. An increase in temperature was found to improve settling rates and reduce supernatant turbidites. Furthermore, it was found that the equilibrium bed height, compressive yield stress and shear yield stress increase with temperature, thus indicating that the network strength becomes stronger with a rise in temperature. The hindered settling function was also observed to increase with temperature, resulting in faster rates of dewatering. Therefore, while a decrease in temperature has beneficial effects in terms of settling rate, supernatant turbidity and rate of dewatering, the final solids concentration is severely compromised.