Relationship between thermal dehydroxylation and aluminium extraction from a low-grade kaolinite : Role of clay chemistry and crystallinity

The presented research investigates the impact of clay crystallinity and iron oxide content on thermal dehydroxylation of kaolinite and thus the impact on aluminium dissolution. Two clay samples with different crystallinity and purity levels (designated low-grade for the impurity rich and disordered kaolin, and high grade for the highly crystalline and relatively pure kaolin) underwent thermal treatment from ambient temperature to 850 °C in 50 °C increments, followed by acid digestion to recover aluminium. Infrared spectroscopy and X-ray diffraction were used to characterise the clays after thermal treatment and following acid digestion to gain insights into the chemistry of the process. Lower temperatures were required to achieve complete thermal dehydroxylation for the low-grade kaolin at 450 °C, in comparison to high-grade kaolin, which was complete at 650 °C. Solid-state 27Al Nuclear magnetic resonance (NMR) highlighted the emergence of Al(IV) at 600 °C and Al(V) at 650 °C, the latter of which corresponded to the complete thermal dehydroxylation of the kaolinite and was the main aluminium species responsible for aluminium dissolution. The iron oxide phases present within the low-grade kaolin resulted in phase transformation to insoluble aluminium spinels at lower temperatures (750 °C), which caused a reduction in the acid solubility of aluminium and iron. However, there were benefits associated with aluminium recovery from the low-grade kaolin, namely, less energy being required to achieve complete thermal dehydroxylation for aluminium dissolution. This finding holds significance for the utilisation of low-grade kaolin by industry, such as for production of new commodities such as high purity alumina.