Discrete element modelling of ribbon milling: A comparison of approaches

Roller compaction followed by milling of the generated ribbons is a typical dry granulation route. It is desirable to be able to predict the size distribution of the granules exiting the mill based on the ribbon properties and mill operational conditions. Two DEM-PBM approaches for predicting this size distribution are compared; a direct approach where the size distribution is experimentally determined, and an indirect approach where the successive change in size distribution due to each stressing event is determined mathematically by the PBM. The experimental component of the direct approach assumes shear deformation to be the dominant breakage mechanism. This approach provides a reasonable agreement to experimental data, though the influence of mill parameters is not experimentally tested. When considering breakage to be driven by impact, the indirect approach predicts the correct magnitude of fines generation, though incorrectly predicts the fine fraction to increase with impeller speed. When abrasion is assumed to be the dominant breakage mechanism, the indirect approach suggests the same trend, though with a less pronounced effect of impeller speed and a closer agreement to experimental data. Prediction accuracy is expected to improve by considering distributions of stressing conditions and material strength, the latter being explicitly captured in the experimental component of the direct approach. Furthermore, the direct approach accounts for the variable loading conditions of the fragments in the mill.