Improvement of aluminium extraction from low-grade kaolinite by iron oxide impurities: Role of clay chemistry and morphology

Abstract

This study explored the influences of iron impurities present in a low-grade kaolinite (17.46% Fe2O3) on the aluminium dissolution kinetics in comparison to a high-grade kaolinite (0.93% Fe2O3) in hydrochloric acid with common industrial clay to acid ratios. The kinetic data was the first kaolinite dissolution data to be modelled to the Brouers, Weron and Sotolongo (BWS) fractal kinetic equation, revealing the low-grade kaolinite had an activation energy of 22.21 kJ mol−1 in contrast to a high-grade kaolinite with an activation energy of 91.37 kJ mol−1. The high-grade kaolinite’s activation energy was consistent with literature results; however, the low-grade kaolin’s activation energy was 3.5 times lower than that reported in literature. In order to determine the impact of iron impurities and disorder in the kaolinite on the reaction kinetics, the low-grade kaolin had the iron phases selectively extracted whereas the high-grade kaolin was spiked iron prior to the kinetic study. The results found that the activation energy increased with the decreasing iron content of the clay. The iron present within the low-grade kaolinite aided in reducing the activation energy by two mechanisms: 1) chemically, by the formation of ferric chloride, a known leaching agent, and 2) physically, by the dissolution of iron oxide within the clay structure, which increased the number of active sites for acid attack.