Investigating the formation of zeolitic compounds from dissolution of clays through vibrational spectroscopy

Abstract

The dissolution of clays in alkaline media is one of the important pathways in the hydrothermal formation of zeolites; despite this it is not fully understood. In this work we investigate the dissolution of kaolinite, halloysite and metakaolin and the subsequent re-precipitation of zeolites via infrared spectroscopy. Specifically, the behaviour in Bayer synthetic liquor was focused on. It was found that the appearance of a broad H-bonding absorbance suggests dissolution for all these clays begins by ingress of OH− into the interlayer region. In addition, the calcined kaolinite (metakaolin) appears to reform surface or accessible Al-OH species as shown by the sharp peaks observed at ∼3620 and ∼3680 cm−1. All clay dissolution leads to observable silicate species in the solution at early times and water dominates at later times. The rate at which the clays dissolve was found to be in the order kaolinite < halloysite = calcined kaolinite. The dissolution of halloysite and kaolinite showed no subsequent zeolite Linde-type A (LTA) formation but dissolution of calcined kaolinite showed that desilication product formation followed an LTA > sodalite transformation pathway in caustic while this was not observed in Bayer liquor. Finally, infrared can be used to monitor the silicate in solution (provided samples have not aged and an appropriate control is used) as well as monitoring the solids formed through a peak ratio method. Having said this, LTA is harder to monitor than sodalite where other supporting evidence was required.