Hydrothermal dewatering of lignite water slurries: Part 2 surface properties and stability

In this paper, the mechanism of the stability of lignite lignite water slurry after hydrothermal dewatering/treatment (HTD) was studied by the measurement of the yield stress and the fractal dimension of the slurry. The effects of HTD on the surface properties of low rank coal were characterized by the contact angle, zeta potential, and Fourier transform infrared (FTIR) spectroscopy. After HTD, the hydrophobic attraction between coal particles was found to increase, as shown by the increase of the contact angle, and the electrical double layer repulsion was found to decrease, due to the decrease of zeta potential. These effects lead to an increase on the inter-particle attractions, which is also proven by the theoretical calculations on the interaction energy using the extended DLVO theory. The stability measurements show that the low rank coal water slurry (CWS) becomes more stable after HTD, which is probably due to the formation of a network structure (soft sediment) by coal aggregating across the whole volume of the slurry to prevent coal particles from settling down. The network structure (soft sediment) in coal water slurry after HTD was verified and quantified by the yield stress and fractal dimension. Our findings indicated that the static stability of coal water slurry may be acquired by the network structure of the aggregated coal particles.