Optimization of Flocculation–Settlement and Continuous Gravity Discharge for Unclassified Sulfide Ore Tailings in a Deep Mine Vertical Sand Tank

The continuous thickening and high-concentration discharge of fine unclassified sulfide ore tailings remain a critical challenge for deep-mine cemented paste backfill systems, yet a systematic optimization framework integrating flocculation chemistry, hydrodynamic simulation, and continuous discharge control has been lacking. This study fills this gap by combining experimental flocculation–settlement tests, response surface methodology (RSM), CFD-PBM-KTGF numerical simulations, and industrial-scale validation for a vertical sand tank operating at over 1000 m depth. A non-ionic polyacrylamide with an optimized molecular weight was selected, achieving a settling velocity approximately five times that of natural sedimentation. Response surface methodology (RSM) analysis revealed that slurry solids concentration, flocculant solution concentration, and flocculant dosage were all significant individual factors affecting the solid flux, whereas none of their pairwise interactions were statistically significant. The optimized parameters achieved a target solid flux that meets the mine’s backfill capacity requirements. CFD-PBM simulations revealed that floc growth occurs predominantly in the mid-depth zone and established a stable sand bed height of 12 m for continuous discharge. A 34 h industrial trial validated the optimized conditions, maintaining average underflow and overflow concentrations well within industrial specifications while achieving a stable dynamic mass balance. This work provides a comprehensive, experimentally validated framework for optimizing tailings thickening and continuous gravity discharge, offering both theoretical guidance and practical reference for deep-mine backfill operations.