Flocculation–Sedimentation of Coal Slurry Water with Calcium Chloride and Polyacrylamide: Experiments and Particle Aggregation Sensitivity Analysis

To address the problems of fine particle size, high ash content, strong suspension stability, and poor settling performance of coal slurry water in coal preparation plants, enhanced flocculation–sedimentation experiments using combined CaCl2 and polyacrylamide were conducted, and a particle-scale numerical sensitivity model was employed to investigate microscopic aggregation behavior under different conditions. The coal slurry water was first characterized in terms of water quality, mineral composition, and particle size distribution. Single-factor tests with CaCl2, nonionic polyacrylamide (PAM), and anionic polyacrylamide (APAM), followed by combined CaCl2 + PAM and CaCl2 + APAM tests, were then performed. The results showed that the appropriate dosages of CaCl2, PAM, and APAM in the single-factor tests were 1.4 g/L, 4 mg/L, and 0.6–1.0 mg/L, respectively. Among the combined systems, CaCl2 + APAM exhibited superior overall settling performance. At 1.2 g/L CaCl2 and 0.8 mg/L APAM, the settling rate reached 805 mm/min and the supernatant turbidity decreased to 41.7 NTU. In the numerical model, the particle composition was reconstructed according to the measured particle size distribution, with the 1–5 μm ultrafine fraction retained to improve consistency between the model and the actual coal slurry particle size characteristics. Sensitivity analysis showed that, when the effective particle adhesion parameter increased from 0.2 to 0.8, the mean floc size at 60 s increased from 10.655 to 13.065 μm, the aggregation rate increased from 0.00356 to 0.01034 s−1, and the apparent fractal dimension increased from 1.065 to 1.184, indicating that enhanced effective particle adhesion significantly promotes the formation of stable aggregates. In addition, as the CaCl2 dosage increased from 0.6 to 2.4 g/L, the corresponding ionic-strength increment increased from 16.219 to 64.877 mmol/L, accompanied by stronger electrostatic screening and increasing trends in the simulated mean floc size and aggregation rate. These results indicate that CaCl2-induced electrical double-layer screening and polyacrylamide-promoted effective particle adhesion jointly influence the aggregation of coal slurry particles, while the CaCl2 + APAM system provides favorable solid–liquid separation performance. The findings provide a reference for optimizing combined reagent addition and improving sedimentation treatment of coal slurry water in coal preparation plants.