Browsing by Author "Maria, Matome Batsalelwang"

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    Extraction of Alumina from South African calcined clays
    (University of the Witwatersrand, Johannesburg, 2024) Maria, Matome Batsalelwang
    The growing need for high-purity alumina (HPA) is driven by its critical importance in sophisticated batteries and electronic applications. Typically produced from high- purity aluminum (Al) metal using the alkoxide method, HPA production is closely linked to the Bayer process, which relies on Al metal derived from alumina (Al2O3) extracted from bauxite ores. However, the scarcity of alumina-rich bauxite reserves has prompted interest in alternative local sources of alumina including kaolinitic clays, which are abundant and widespread. Although the Bayer process is not appropriate for treating kaolinitic clays owing to their high silica content, efforts have been made to explore alkaline and acidic processing methods for alumina extraction. In particular, the HCl acid leaching process has been extensively studied as a viable acidic processing route. South Africa possesses abundant kaolinitic clays despite the lack of bauxite reserves, making it necessary to investigate the potential for alumina extraction via HCl processing. This study investigated the dissolution of aluminum and iron species from thermally activated clay in a hydrochloric acid solution, emphasizing the selectivity of aluminum extraction over iron impurities. Furthermore, the effect of different calcination and acid leaching conditions on the dissolution of aluminum and impurity species in a hydrochloric acid solution was investigated. Three different kaolinitic clays sourced from South Africa, namely ball (BL), flint (FL), and fire (F) clays, were analyzed to ascertain their chemical and mineralogical properties. It was observed that the raw BL and FL clays utilized in this study shared similar mineralogical compositions, whereas the F clay contained additional clay minerals such as carbonates and smectite. However, mineralogical analysis revealed that BL and FL clays were predominantly rich in kaolinite (82.4% and 91.1%, respectively), whereas raw F clay exhibited a lower kaolinite content (16.5%) and a higher amount of quartz (44.7%). The calcination of kaolinitic clay was performed at temperature range of 600 to 800 °C (interval of 50 °C) for a period of 8 hours. Infrared spectroscopy and X-ray diffraction were employed to confirm the structural and mineralogical changes, iii revealing the transformation of kaolinite into amorphous metakaolinite. The research revealed that calcination temperature significantly influenced alumina extraction, with varying effects on iron content. The calcination temperatures necessary for the complete dehydroxylation of kaolinite into amorphous metakaolinite were uniform across all samples of raw BL, FL, and F clays at 600°C. At this temperature, BL clay demonstrated an Al extraction 79.4%, FL clay showed a recovery of 78.4%, and F clay exhibited an Al extraction of 36.7%. However, although the Al extraction from F clay increased with an increase in calcination temperature, it did not exceed 50 %. Conversely, an increase in the thermal decomposition temperature was accompanied by a decline in the amount of Al extracted from calcined BL clay. The incorporation of iron oxide species in calcined BL clay resulted in the formation of insoluble aluminum spinels at higher temperatures, particularly starting from 700°C until 800 °C. This process led to a decrease in the acid solubilities of both aluminum and iron. Interestingly, the extraction of aluminum from calcined FL clay exhibited a fluctuating pattern: it decreased at 650°C and 750°C but increased at 700°C (69.5%) and 800°C (71.8%). Both calcined BL and FL clays at 600°C demonstrated high aluminum extraction, although accompanied by high iron content (67.8% and 66%, respectively), whereas calcined FL clay at 800°C offered high aluminum extraction with moderate iron content (40.2%). In comparison with BL clay, the lower amount of iron oxides in FL clay did not present a clear trend for the formation of spinels. Statistical analysis showed that leaching time had a significant impact on aluminum extraction of BL clay. Meanwhile, the acid concentration (pertaining only to FL clay), leaching temperature, solid/liquid ratio, and leaching time and solid/liquid ratio interaction significantly affected aluminum extraction for FL and F clays. A higher iron content has ramifications in HPA production for subsequent processing of the leachate solution. However, the reduced energy requirement for achieving complete dehydroxylation, which is essential for aluminum dissolution, presents a significant advantage for aluminum recovery from both BL and FL clays. The choice between BL and FL clays would depend on either a lower energy requirement in the pre-treatment stage with the associated higher cost of processing leachate containing elevated iron content, or a higher energy demand in the iv pretreatment phase with a lower cost linked to processing leachate with moderate iron content. Notably, for FL clay, there was flexibility in assessing the three calcination temperatures that yielded high aluminum dissolution. The findings of this study hold considerable importance for utilizing South African kaolinitic clays as a cost-effective, locally sourced alumina resource for HPA production.