Effect of rice hulls additions and calcination conditions on the whiteness of kaolin

In this study, the rice hulls were used as an addition agent to whiten the soft kaolin by a calcination method. Emphasis was given on the effect of the whitening conditions, including the impurity amounts of Fe and Ti, calcination atmosphere, calcination temperature, holding time at the optimal temperature and particle size. Both kaolins before and after being whitened were characterized by chemical analysis, thermal analysis (TG–DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectrum analysis (FT-IR) and whiteness determination. Chemical analysis showed no significant change of Fe and Ti contents after being whitened. Moreover, whiteness measurement revealed that the highest whiteness was 90.60% based on optimum calcination conditions, keeping the whiteness unchanged for a long time. TG–DSC analysis indicated that a redox reaction occurred in the calcination process. The phase transition and morphology change of kaolin were confirmed by XRD, FT-IR and SEM results. In addition, the whitening mechanism was also explored.