Abstract
This paper describes the effect of chemical composition on the crystallization rate of minerals in an Al2O3-SiO2-H2O system. Eleven Al2O3-SiO2 gels different in Al/(Al+Si) ratio were reacted with water at 473 K for 2, 10, 49 and 280 days. The gels and the products were analyzed with X-ray diffractometry (XRD) and thermal gravimetry (TG). The weight ratios of minerals were determined from X-ray diffraction intensities with an internal standard method. Boehmite crystallized from a pure alumina gel in 2 days, and kaolinite began to crystallize in 10 days from the gels with the Al/(Al+Si) ratios of 0.3 to 0.6. Pseudoboehmite, which has the structure close to boehmite but has poor crystallinity, was formed from the gels with the Al/(Al+Si) ratios of 0.7 to 0.9. Kaolinite and boehmite crystallized fast when the Al/(Al+Si) ratios of the gels were similar to those of kaolinite and boehmite, respectively. On the other hand, kaolinite and boehmite formed extremely slowly from the gels with intermediate compositions between kaolinite and boehmite. The crystallization rates of minerals in an Al2O3-SiO2-H2O system depend on the composition of starting materials.
