DEM study on the surface mixing and whole mixing of granular materials in rotary drums

Image-based methods have been widely used in the measurement of granular mixing in rotary drums. However, the results are usually obtained based on particle images acquired from an exposed surface of the granular bed. To what extent the obtained data from the bed surface (surface mixing) can reflect the mixing of the whole granular bed (whole mixing), is still a question to be answered. In the present work, an experimentally validated DEM model is applied to investigate the difference between surface mixing and whole mixing of mono-sized particle groups (differently colored) in a D206 rotary drum which has a glass observing window on the drum end. Ten sets of DEM simulations are performed, with rotation speed and the drum length as varying parameter. Quantitative features (characteristic time constant obtained from a second-order process model) and qualitative features (particle pattern) of the surface mixing and the whole mixing are then compared in detail. For the short drums, no significant difference was found between these two mixing processes in the aspects of the particle pattern as well as the time constant (maximal deviation 8%); with increased drum length, however, the deviation in time constant is non-negligible (up to 33%). (C) 2017 Elsevier B.V. All rights reserved.