Investigation of comminution in a Wiley Mill: Experiments and DEM Simulations

摘要

Particle size reduction of dry granular material by mechanical means, also known as milling or comminution, is undoubtedly a very important unit operation in pharmaceutical, agricultural, food, mineral and paper industries. Particle size reduction rate was studied by conducting parametric studies experimentally and computationally using Discrete Element Method (DEM). Studies were performed with lactose non-pareils (spheres) to understand the effect of blade speed (rotational), feed rate, and blade-wall tolerance in a Wiley mill. The size and shape of the resulting progeny of particles were analyzed by sieve analysis and microscope/image analysis techniques respectively. The feed rate determines the hold up of material in sizing chamber and hence energy required for size reduction. Greater size reduction was observed at higher speeds and low feed rates owing to the greater centrifugal force experienced by the particles and longer mean free path lengths respectively. Particle shape analysis revealed fragmentation to be the dominant mechanism of size reduction at higher speeds. Increase in blade-wall tolerance resulted in accumulation of powder bed which was found to be significant at low impeller speeds. Single particle impact studies were performed using Dynamic Mechanical Analyzer to determine the force required to break the granule. In this method, the granules were subjected to a quasi-static compression process and the corresponding force was continuously recorded. The flow and fragmentation of non-pareils were simulated using DEM, an explicit numerical technique scheme, which calculates interaction forces between grains for each grain-grain contact, and the resulting motion of each grain. The fragmentation of each spherical particle was simulated using the Grady's Algorithm. The diameter of the resultant progeny particle was evaluated based on the fracture toughness of material, propagation velocity of longitudinal elastic waves in the material, and the induced strain rate.