Solid state and phase transformation mechanism of kaolin sintered with limestone for alumina extraction

Abstract

The present work aims to investigate the effects of the solid-state thermal reactions at different sintering temperatures on the phase transformation mechanisms and the microstructural changes, the self-disintegration mechanism as well as the efficiency of alumina percent recovery during alumina extraction from kaolin by the lime-sinter process. The sinters were produced at different temperatures within the range 800 °C - 1400 °C then subjected to leaching process (at the optimum dissociation of 1:5 solid : liquid ratio) by using sodium carbonate solutions (120 g/L) which finally were given to the alumina-pregnant solution. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used for characterizing the transformational changes resulted during the thermal reactions between the sintered mineral components in the raw kaolin and limestone samples (i.e., kaolinite and calcite). Microstructural and textural changes of the obtained sinters and sludges were characterized by means of X-ray diffractometry (XRD/XRF), scanning electron microscopy (SEM/EDX) and particle size granulometry. The results indicated that the sintering temperature at 1360 °C was the optimum for inducing of the solid-state reaction between the clay and carbonate components with enhancement of alumina productivity by the highest recovery (80.49%). At this superlative sintering condition, the kaolinite was dehydroxylated and transformed into metakaolinite while the calcite was decomposed into calcium oxide, then they reacted together forming to the calcium aluminates that are considered as the highly effective and generative precursor phases in the alumina extraction. Moreover, the obtained sludges were obviously exhibited morphological changes with size enlargements during the leaching process due to the formation of calcium carbonate particles that agglomerated at the surfaces of the insoluble sludge particles.