Test data (Fig. 2) also showed that an increase in the amount of acid used from 1.1 s to 1.2 s resulted in a decrease in aluminium extraction by about 20–30% after 3 h of digestion, whilst any decrease as a result of increase in pulp density from 20 to 25% (Fig. 2 and Fig. 3) was generally not significant. Accordingly an acid concentration of 1.1 s and a pulp density of 20–25% seemed to represent the optimum conditions for aluminium extraction from the bauxite slimes. It was also noted that there would be a need to blend the high alumina content samples (such as sample S16), to reduce the overall alumina content to about 50%. The digested pulp from this sample gave lower extraction levels and the digested pulp tended to cake spontaneously after digestion because of higher degree of supersaturation.