A comparison of acid treatment in the dewatering of Chinese and Australian lignites by mechanical thermal expression at high temperatures

Mechanical thermal expression (MTE) is an effective and efficient method of dewatering high moisture content lignite. However the high total organic carbon (TOC) of the wastewater requires further chemical treatment, before discharge to the environment. In this work, changes in the characteristics of the solid products and the wastewater are reported, when sulphuric acid is used as part of the MTE process. Two lignites were tested: Loy Yang (LY) from Australia and ShengLi (SL) from China. The characteristics of the solid products and wastewater were analysed for both. The effect of expression temperature (200°C and 240°C) on moisture content reduction is evident for LY, but it is only marginal for SL. The final moisture content of the solid product in either case is not impacted by the addition of sulphuric acid, but the mercury porosimetry intrusion analysis showed a greater reduction in pore volume for LY. It is postulated that acid treatment suppresses the solubilization of phenols and carboxylic groups in the case of LY. For SL with a different maceral structure, as the concentration of the solubilized phenolic compounds is much lower when no acid is used, the impact of acid addition is much less significant. It appears that the addition of acid inhibits the dissociation of organics from the lignite macerals and also reduces the rate of dissolution, leading to lower TOC in the wastewater. Thus, the wastewater becomes easier to treat after sulphuric acid addition during MTE. The MTE process also removes dissolved salt and organically-bound minerals which aids in reducing the fouling components in the lignite and possibly the PM₁₀ concentrations in the flue gas following combustion.