(113a) Experimental Study and DEM Simulation of Granule Breakage by Impact

During transportation and handling of granulates, breakage and attrition occur, which change the particle size distribution and deteriorate the product quality and sometimes may form harmful toxic dust. In order to optimise the existing production processes and minimize the product quality losses, the breakage behaviour of granulates must be investigated by experiments and simulations.

A single granule impact test has been used to study the breakage behaviour of granulates. The impact experiments were studied with the help of granule impact tester for three industrial spherical granulates (γ-Al₂O₃, synthetic zeolite and sodium benzoate) at different velocities (from 10 to 50 m/s). The elastic and plastic breakage behaviour was explained with the help of the SEM image analysis of fracture surface of the fragments. It has been shown that the breakage behaviour is strongly dependent on the impact energy, macroscopic and microscopic structure of the granules. Fig. 1 shows the breakage probability as function of the mass-related impact energy. The breakage probability was determined by using particle size distributions after the impacts at different velocities and described by using a modified Weibull distribution. It was shown that for the breakage of small granules the larger mass-related impact energy is needed than for larger granules.

Fig. 1.  Breakage probability P of examined granulates versus mass-related impact energy W_m.

A two dimensional discrete element simulation of the impact test of granules at different velocities has been performed. The DEM simulation shows the crack propagation inside the specimen by breaking the solid bridges of binder between the primary particles (Fig. 2). The large impact velocities lead to the complete disintegration (break up) of the granule into predefined primary particles. The contact and bond forces between primary particles (Fig. 2) during the wave propagation in the granule by impact were evaluated. After calibration, the correlation between the experiments and the simulations is obtained.

Fig. 2. Fractured specimen (left) and corresponding shear forces in unbroken solid bridge bonds (right) of impact at 50 m/s (t=0.5 ms).

The DEM simulation approach provides a suitable tool to investigate the cracking mechanism of the different structured granules and mechanical micro properties of primary particles and the bonding agents.