Prediction of mill performance, throughput and product size distribution in full 3D SAG mills with feed and discharge is now possible using a multi-physics, particle-scale model which combines charge and slurry behaviour, breakage and attrition of resolved coarse particulates, and grinding of unresolved fines in the slurry phase. This uses a fully two-way coupled DEM+SPH model to represent the behaviour of the coarse solids (DEM) and fine slurry (SPH) phases as well as the interactions between these phases. Size reduction of feed material is included in the DEM sub-model through four inter-related comminution mechanisms. These include body breakage and surface attrition (via chipping, rounding and abrasion) which create explicit size and shape modification of the resolved coarse particles. Body breakage makes use of a particle-replacement method and breakage characterisation data to pack super-quadric progeny into each fracturing parent particle. Fine unresolved fragments both in the feed and resulting from coarse fracture are transferred to the SPH slurry phase where the viscosity then spatially varies with fines content. Collisions and shear due to the motion and stressing load of the coarse DEM particles generate local energy dissipation in the SPH slurry which is used to calculate size-dependent grinding rates for the unresolved fines. The transient slurry size distribution is then predicted by solving a coupled set of population balance equations for each SPH particle allowing prediction of mass transfer through to the size classes from the grinding. Dispersive transport of unresolved fines is solved by a Fickian diffusion term in these equations whilst advection is automatically treated by the SPH. The ability of the particle-scale model to predict SAG mill performance is explored for an industry standard 1.8 m diameter by 0.6 m long AG/SAG pilot mill.