Mechanical/thermal dewatering of lignite. Part 4: Physico-chemical properties and pore structure during an acid treatment within the MTE process

The Mechanical Thermal Expression (MTE) process is a dewatering process carried out at moderately elevated temperatures and pressures. In this study, the MTE dewatering behaviour of lignite/water and lignite/acid slurries of six coal samples from Australia, Germany and Greece were compared. The extent of alkali cation removal was also studied, along with the COD of the wastewater, as an indicator of the amount of organic carbon loss. Furthermore, the physical changes that the lignite experiences at different MTE process temperatures and acid concentrations were evaluated using mercury porosimetry, helium pycnometry, moisture holding capacity and CO2 adsorption methods. Total pore volume, residual water content, and sodium and potassium content were significantly reduced with increasing severity of the MTE process. Acid treatment improves the removal of inorganics but has less significant effects on the water removal and the physical properties of the dewatered lignite. Furthermore, a decrease in moisture holding capacity at high dewatering temperatures correlates with a reduction of the pore volume.