Axial pressure distribution, flow behaviour and breakage within a HPGR investigation using DEM

Abstract

The pressure distribution between the rolls in a HPGR (High Pressure Grinding Rolls) is usually expected to vary significantly in the axial direction due to the end effects at the end of the rolls. This is based on the idea that particles near the ends of the rolls are less well confined which makes them more mobile with reduced ability to be trapped and crushed. This can reduce the performance efficiency of a HPGR quite significantly. Experimental measurement of this axial pressure distribution and its consequences for the breakage efficiency, particularly at industrial scale, is very difficult. So little is known about its nature or how this may change with choice of confining structures (such as cheek plates). This problem is tractable using DEM to predict the coupled flow and breakage of particles within the HPGR. A replacement strategy is used where particles are broken when the applied force exceeds a size dependent limit. This allows the DEM model to capture the fracture dynamics sufficiently to allow flow and load prediction. The model is calibrated against an experimentally measured product size distribution and predicts very close agreement across the entire resolved range of particle sizes. This model is used to explore the pressure distribution and how this, and the coarseness of the particles, changes along the rolls. Two mechanisms are identified for producing breakage in the HPGR. For a HPGR with cheek plates the discharge mass flow rate is found to be nearly independent of axial position with the product size distribution being axially invariant. The frictional effects of the cheek plates lead to a moderate axial flow towards each end of the rolls that balances the reduced vertical flow to give axially very uniform discharge behaviour. Finally, it is shown that only small changes in the confining cheek plate locations are needed to allow significant axial bypass, strong axial variation of discharge mass flow rate and a coarser product.