Impact attrition of spray-dried burkeite particles

Particle attrition in manufacturing plants handling particulate solids could cause processing as well as environmental problems, and lead to the degradation of product quality. Spray-dried powders are particularly prone to attrition because of their porous and often weak structure. Spray-dried burkeite particles are a good example and have been used as a model porous powder to investigate the effect of structure on their breakage propensity. The particles are subjected to well-defined stresses due to impact, and the change in the particle size distribution is determined by particle size analysis based on sieving. It is found that impact breakage of burkeite is affected by the structure, and some unexpected trends for breakage are observed; for a given impact velocity some smaller particle sizes break to a greater extent as compared with larger particles. This is attributed to uncontrollable variations of porosity (and hence particle envelope density) as a function of size. To study the effect of porosity on attrition of the spray-dried powders, structure visualisation and analysis have been carried out by Scanning Electron Microscope (SEM) and X-ray microtomography (XRT). SEM images show three levels of structure in a single particle of spray-dried burkeite. Based on the XRT results, the particle envelope density increases as particle size increases, and the variation of envelope density influences the impact breakage. Once the relevant values of the envelope density are taken into account, then the trend of impact breakage data becomes as expected, and the material mechanical properties of the particles can be inferred from the breakage results.