Effect of rock shape representation in DEM on flow and energy utilisation in a pilot SAG mill

Abstract

The impact of the shape representation used for rock particles in a DEM model of a SAG mill on charge dynamics and energy utilisation is investigated. A 1.8-m diameter pilot scale SAG mill which includes realistic end walls and a discharge grate is used. This size of mill is used for scale-up and design processes for industrial SAG mills so understanding the effect of approximating the particles as being spherical or non-round is important. The cataracting component of the flow is found to be insensitive to the particle shape used in the model but the cascading part of the flow is moderately sensitive with the shoulder moving slightly higher, the bulk toe moving closer into the charge and the centre of circulation moving more strongly higher. The end walls are found to have a strong effect on the flow dominating the dynamics in the outmost parts of the charge and strengthening the particle microstructure via the axial compression induced by the transfer of the weight of the charge to the end walls. The cataracting stream consists of coherent bands of predominantly finer material being thrown on ballistic trajectories. Its form is a superposition of one component formed by the flow from the belly lifters and a second from the flow along the radial end wall lifters. The net power draw is found to increase slightly, but larger changes in the pattern of energy usage within the mill are identified. Rock shape has a significantly stronger impact on the large size classes in the charge and leads to strong migration of energy usage from the finer material to the coarser and from the rock fractions to the media.

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