Understanding Iron Impurities in Australian Kaolin and Their Effect on Acid and Heat Activation Processes of Clay

Iron impurities present in the crystal structure of kaolin minerals or in accessory species are frequently encountered in clay deposits. As knowledge of the location and states of the iron is crucial when modifying the properties of clays by activation, it is important that new deposits are well characterized in terms of the amount and location of this metal. The Western Australia Noombenberry deposit has been identified as a large resource of kaolin composed largely of halloysite and kaolinite. We sampled six from one hundred drill holes and grouped them according to major mineral and iron impurities. First, we characterized them to understand the source of iron impurities. Then, we performed three physicochemical activation processes of samples involving acid treatment (by 3 M HCl), heating at 600 °C, and a combination of both. State-of-the-art tools, including X-ray diffraction, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, and nuclear magnetic resonance, revealed the properties of kaolin, iron impurities, and the changes incurred after activation. The iron impurities were found to be linked to non-kaolin minerals, i.e., in mica or illite. Once the iron was removed mainly by acid activation, the surface area, pore volume, and negative surface charges increased, and that was significant for halloysite-rich samples. These properties helped adsorb N₂ gas compared to the raw kaolin. Therefore, knowing the iron's location and states in associated mineral species and their dissolution/retention may expand the scope of material development for gas adsorption. They are also useful in other applications like clay purification and adsorbent or additive formulations.