Dissolution kinetics and removal mechanism of kaolinite in diasporic bauxite in alkali solution at atmospheric pressure

A new chemical pre-desilication process of kaolinite in diasporic bauxite in alkali solution at atmospheric pressure was proposed. The dissolution kinetics and mechanism were studied by chemical analysis, XRD and SEM. The kinetic results of dissolution process show that the kaolinite is symbiotic with diaspore but without cladding. The dissolution ratio of kaolinite is close to 100% at 100 degrees C for 90 min. The dissolution kinetic equation is 1-(1-alpha)(1/3)=7.88x10(6)exp[-64434/(RT)]t. With the low L/S (L/S=10:1), the dissolution ratio of kaolinite decreases to 55%. This is due to the formation of lamellar hydroxyl-sodalite (OH-SOD) which is deposited on the surface of kaolinite and hinders the further dissolution of kaolinite. Under the optimum conditions, the A/S (mass ratio of Al2O3 to SiO2) of dissolved residues is increased to 8.55, while the A/S of the bauxite is only 4.97.