The aluminium content from chemical analysis represented the total aluminium content of the sample without reference to its distribution among gibbsite and kaolinite mineral phases. The resistance of kaolinite to dissolution in acid medium has already been mentioned. It is also known that aluminium in bauxite could be incorporated in the goethite or haematite lattice (Solymár and Ferenczi, 1979), which may not totally be available for leaching unless the goethite or haematite lattice has been completely broken down. In addition, not all the gibbsite or kaolinite contents of the material are expected to dissolve. The achievement of about 80% extraction efficiency in 3 h can therefore be considered a good result. Data not presented in this paper indicated that higher than 80% recovery were achieved when the residence times were increased in certain cases.