Behavior of silicon-containing minerals during Bayer digestion

Digestion behaviors of several single silicon-containing minerals including kaolinite, illite and pyrophyllite in aluminate solution were investigated. The result shows that the digestion rates of kaolinite, illite and pyrophyllite in aluminate solution were influenced by reaction conditions such as temperature. Adopting quantum chemistry theoretical calculation, using CASTEM procedure module, the slab model of kaolinite, illite, pyrophyllite and their ordinary complete cleavage plane (001) were geometry optimized at GGA-PW91 level. The micro-essence of the different reaction activities among kaolinite, illite, pyrophyllite in aluminate solution was opened out. It is found that the reaction activities of kaolinite, illite, pyrophyllite are obviously different and the reaction activity of pyrophyllite is the highest. The Si-O bond strength of illite is the strongest and its Al-O bond strength is comparatively the weakest, so Si in it is the most difficult to be removed. The Si-O bond strength of pyrophyllite is the weakest, so Si in it is relatively easy to be removed. The micro-electron structure of complete cleavage plane and the Si-O bond strength of kaolinite, illite and pyrophyllite are evidently influenced by the existence of OH−, of which the surface character and the reduction of Si-O bond strength of kaolinite are influenced most greatly by OH−. Therefore, the desilication of kaolinite in alkaline solution at relatively low temperature is easier.