Optimization for the COD reduction and thermodynamics research of coal-series kaolin

Coal-series kaolin is the product of seam deposition in coal-accumulating basin, and may always contain reducing substances. The reducing substances such as carbon matter and pyrite could cause relatively high chemical oxygen demand (COD) of the raw kaolin, resulting in low quality kaolin products. In this study, narrow range classification and calcination were investigated to evaluate decreasing COD on coal-series kaolin from the Hubei Province. The kaolin and roasting products were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF) and thermogravimetry–differential scanning calorimetry (TG-DSC) analyses. The results showed that the COD value in kaolin ore (19,252 µg/g) was largely concentrated in the size fraction of – 150 to + 60 μm. The optimum conditions for reducing COD were identified to be – 74 to + 60 μm at a roasting temperature of 450 °C, and the COD value decreased from 27517 to 585 µg g⁻¹. Moreover, the COD value and E_a showed linear correlation (R² = 0.932) among different particle sizes, and G(α) = ${\left[1-{\left(1-\alpha \right)}^{\frac{1}{2}}\right]}^{\frac{1}{2}}$ could be regarded as the most possible mechanism function for the thermal decomposition of coal-series kaolin.