Abstract:Illite occurs widely in bauxite ores and results in low alumina grade of the ores. Differential thermal analysis (DTA), thermal gravimetric analysis (TGA) and X-ray diffraction analysis (XRD) show the OH groups split off from the structural framework of illite between 500 ℃ and 700 ℃. With the increase in temperature up to about 1 100 ℃, the layer structure of illite breaks up and Si in the layers is transformed into the amorphous state. Meanwhile, mullite comes out at 1 100 . Quartz occurring in illite keeps unchanged in structure in the range of 500-1 200 ℃. A desilication process from illite by thermochemical activation followed by alkali leaching is therefore developed on the basis of the behavior that amorphous silica is alkali soluble. The investigation finds that the optimum parameters for desilication are activation temperature of 1 100-1 150 ℃, activation time of 90-120 min, leaching temperature of 95-110 ℃, leaching time of 90-120 min and concentration of caustic soda (Na₂O_k) 120-150 g/L. An overall desilication about 45% is attained under these conditions. XRD analysis confirms that the active amorphous SiO₂ has been dissolved in the alkali solution and removed from the samples, while quartz and mullite have not. The investigation also shows that the formation of mullite during activation and formation of sodium hydroaluminosilicates (Na₉₆Al₉₆Si₉₆O₃₈₄ and 0.95Na₂O•Al₂O₃•3.25SiO₂•4.79H₂O) during leaching lead to the relatively low desilication of illite. 

Key words: illite; thermochemical activation; desilication