Structural evolution of coal sludge filter cake and optimization mechanism of combined chemicals in the process of filtration pressing

The structure of coal sludge filter cake has an important influence on the dewatering effect. In this study, the characteristics of the filter cake structure and its evolution law in the process of filter press dewatering were investigated by using the homemade pressurized dewatering device and CT scanning methods. The filter press process was modified into four stages. Among them, the cake dewatering stage mainly removes the internal water of the cake, and the more developed the pore structure of the cake in this stage, the better the dewatering effect of the filter press; the cake compression stage mainly compresses the cake at last, and the higher the compression of the cake in this stage, the lower the porosity and water content of the cake will be in the end. The polymerised ferric sulphate (PFS) and α-hemihydrate gypsum (α-HH) were used to carry out filtration aid experiments on coal slurry. The filtration-assisting mechanism was analyzed from various aspects, such as the flocculation structure of coal slurry particles, the composition of the physical phase of the filter cake, and the three-dimensional structure of the filter cake. The results showed that the combination of PFS and α-HH significantly enhanced the filtration effect of coal slurry, and the optimal dosage of the two was determined to be 1711.44 g/t and 48.33 mg/g by central combinatorial design (CCD). Under the optimal conditions, the water content of the filtrate cake was reduced to 16.97% and the filtration rate was increased to 145.89 mL/(s·m²). The addition of α-HH made the absolute value of filtrate zeta potential to 1.45 mV, which reduced the electrostatic repulsive force between coal slurry particles, enhanced the flocculation effect of PFS, and increased the chord length of flocs to 71−171 μm. Meanwhile, in the PFS system, the hydration reaction of α-HH was accelerated, and a large number of gypsum dihydrate crystals were formed, which formed the intertwined skeleton in the flocs of coal slurry, and played the role of the skeleton constructor. These gypsum dihydrate crystals increase the index of compression dehydration (I_CD) of coal tailing filter press to 176.30. The pore size of coal slurry filter cake was increased from 12.14% to 14.39% during the filtration process, which improved the pore coordination number and connectivity of the coal slurry filter cake and thus improved the filtration effect.