Studies into the preparation of alum from slime waste from the Awaso Bauxite Washing Plant

Abstract

Bauxite mining has been going on at Awaso in the Western Region of Ghana, West Africa, since 1941. Materials from the pit are usually passed through the washing plant to remove impurities such as plant material, and clay, and fines in the bauxite material. The washing process has generated several thousands of tonnes of slime waste that has been impounded in tailings dams. Studies were carried out to determine the suitability of the slime material for the preparation of alum. Samples of slimes were taken from the slime dams and characterised chemically and mineralogically. Samples of selected slime material of various compositions from the slime dams were leached at boiling temperature with varying acid concentrations and pulp density to determine the optimum condition of treatment. Methods of reducing Fe and Cr levels as the main impurities in the solution were investigated and jar tests were carried out to determine the potency of the product solution and crystalline alums. The results showed that the slime material could be used directly without any pre-treatment and the main impurities in the solution, Fe and Cr, could be reduced by varying the initial acid concentration, pH management and the use of calcined waste clay from the bauxite mine. Both solution and crystalline alum of acceptable commercial quality and potency could be produced. At a dosage of 80 ppm in the case of crystalline alum, the pH of water treated with the locally produced and imported alum were 6.8 and 6.6 respectively, whilst the colour (Haizen), turbidity (NTU) and TDS (mg/l) were 75 and 75; 0.83 and 0.64; and 161 and 154 respectively. In the case of liquid alum the required dosage was relatively higher.