Similar results could be obtained for tests with pre-calcined bauxite at 25% pulp density (Fig. 8). However, the degree of impurity removal obtained with the use of NaOH solution or pre-calcined bauxite could not be attained with the pre-calcined lithomargic clay (Fig. 8). It seems the reactivity of metakaolinite in the pre-calcined lithomargic clay was lower than that of dehydrated gibbsite in the pre-calcined bauxite. Fe concentration in this case could only be reduced to from 14.5 mg/L or 6.5 mg/L at 25% or 30% pulp density respectively.