The Strength of Agglomerates and Their Breakage During Fluidisation.

The breakage behaviour of agglomerated materials is of interest to many industries which handle powders in a granulated form. Products which pass through an agglomeration process include fertilisers, pharmaceutical materials and some bulk chemicals. The current work relates the breakage rate of agglomerates to their mechanical strength in a particular piece of agglomeration equipment, namely, a fluidised bed. A study of the strength of test agglomerates composed of sand with a polymeric binder (polyvinyl pyrrolidone) was carried out. The strength of agglomerate bars was determined using a "three-point bend test" designed to elucidate fracture mechanics parameters, particularly Kc (the critical stress intensity factor). Indentation tests enabled the agglomerate yield strength to be found and therefore the size of the fracture "process zone" to be estimated. Agglomerates with a range of binder concentrations, and hence strengths, were fluidised and their breakage rates were determined for the different compositions. The breakage rate was found to be proportional to the reciprocal of the critical stress intensity factor as measured earlier. The breakage rate was also found to be proportional to the excess gas velocity in both a bubbling bed and a bed with gas jets. High-speed video recordings have shown that the main mechanism of fines generation is oblique impact of particles at relatively low velocities (of the order of a metre per second or less). This process is analogous to abrasive wear, which has. been simulated in a test outside the bed. Again, the wear rate was found to be proportional to 1/Kc. The usefulness of a fracture mechanics approach to agglomerate strength and breakage in process equipment has been confirmed using a fluidised bed