Enabling grate discharge in large ball mills: Transient blockage and a restriction-based predictive framework

Abstract

Copyright © 2026. Published by Elsevier Ltd.Slurry transport in tumbling mills is rarely resolved at the particle scale, limiting the prediction of discharge performance in large ball mills. In particular, the application of grate discharge in mills of 20–28 ft diameter has been constrained by uncertainty in evacuation capacity and the risk of restriction under high-throughput conditions. This work presents a particle-resolved Smoothed Particle Hydrodynamics–Discrete Element Method (SPH–DEM) framework to investigate slurry–particle interactions at the discharge interface. The granular phase is resolved by DEM within a GPU-based Blaze framework, and the slurry phase is advanced by a natively integrated SPH module built on the DualSPHysics formulation. Within this implementation, particles relevant to charge dynamics and discharge interactions are explicitly resolved by DEM, while the sub-resolution fine and ultra-fine fraction is represented by the slurry-phase properties (density, viscosity, solids fraction). This combination enables direct simulation of industrial-scale ball mills, capturing free-surface and multiphase dynamics at the discharge interface without relying on homogenization assumptions (e.g., porous-medium or averaged-mixture representations) for the resolved particulate bed. The results show that discharge is not governed by geometric open area alone, but by a dynamically evolving effective discharge area reduced by transient particle–slot interactions. These finite-duration restriction events are continuously formed and resolved, producing a statistically stable deficit in discharge capacity at the mill scale. This behavior is captured through a dimensionless Withdrawal Restriction Number, which combines particle size, slot geometry, and expansion effects. The effective discharge area follows a bounded relation as a function of this parameter, defining transitions between self-cleaning and restriction-dominated regimes. At the system level, grate discharge suppresses slurry pooling, reduces total power draw in the industrial comparison reported here, and shifts energy utilization toward impact-dominated events, enabling operation at lower grinding media load without measurable product coarsening in the analyzed plant case.

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