Flocculation and filtration performance of coal slime water: role of flocculant molecular weight and coagulants

Coal slime water treatment is a critical process in coal preparation plants. However, the presence of fine particles and the limitations of traditional flocculants result in high filtration resistance and extended filtration time. This study investigated the effects of flocculants, i.e. polyacrylamide (PAM) with molecular weights of 16 million, 18 million, and 20 million, along with the addition of coagulants (CaCl2 and MgSO4), on the flocculation and filtration performance of slime water. Focused beam reflectance measurement (FBRM) and low-field nuclear magnetic resonance (LF-NMR) were employed to evaluate floc size and filter cake pore structure. Sedimentation and filtration tests were conducted to determine the settling velocity, filtration resistance, porosity, and filter cake moisture under different treatment conditions. The results revealed that increasing the molecular weight of flocculants enhanced the settling velocity and filtration resistance. However, it also increased the porosity and moisture content of the filter cake. The addition of coagulants CaCl2 and MgSO4 further improved filtration efficiency by significantly reducing filtration time and resistance, promoting the formation of larger flocs (~100 μm), and increasing the porosity of the filter cake. These findings highlighted the combined effects of flocculants and coagulants in improving the flocculation and filtration performance of slime water.