Filtration of low-rank coal from the viewpoints of moisture migration and water occurrence state

Currently, research on low rank coal (LRC) filtration has focused on the variation of particle properties and cake structure, but few reports have focused on the moisture migration and water occurrence state. Herein, combining surface property tests, low-field nuclear magnetic resonance (LF NMR) technique, and Young-Laplace equation, the effects of cetyltrimethylammonium bromide (CTAB) and cationic polyacrylamide (CPAM) on LRC filtration were investigated from the viewpoints of moisture migration and water occurrence state. The results showed that the optimal dosages of CTAB and CPAM were 0.8 kg/t and 30 g/t, respectively. CTAB reduced the surface tension of the filtrate to 30.75 mN/m, while it increased the hydrophobic functional groups on the coal surface and the free water content in the filter cake 5.39 % and 7.17 %, respectively. Thus, CTAB increased the hydrophobicity of the pore walls and the flowability of the filtrate, and reduced the moisture binding of the pores. This promoted the moisture migration and caused part of the non-flowable capillary water in the filter cake to be transformed into flowable free water. However, CPAM reduced the absolute value of Zeta potential by 20.20 mV, decreasing the electrostatic repulsion between coal particles. Meanwhile, CPAM increased the floc size to 20–60 μm, which promoted floc formation and improved the pore structure, increasing the filter cake porosity by 5.19 %. This reduced the filtration resistance of the LRC and promoted the transformation of capillary water to free water, reducing the capillary water content by 12.27 %. This work provides a new perspective to reveal the filtration mechanism of LRC.