Control of particle characteristics in the production of fine powder by grinding

The kinetics of fine dry grinding of a high volatile A bituminous coal in laboratory-scale mills (a Bleuler, rotary ring and puck mill and a Retsch, high-speed planetary mill) were investigated. The results indicate essentially "normal" breakage behavior in the Bleuler mill, but "abnormal" behavior in the planetary mill at relatively coarse sizes (greater than about 20 {dollar}mu{dollar}m). Both the mills show clear evidence of re-agglomeration as fine material accumulates in the mills. A simplified population balance model for simultaneous breakage and growth has been formulated to investigate the re-agglomeration of fine particles ground in the Bleuler mill. Breakage rates and breakage distributions are assumed to follow the patterns typically observed in simple grinding systems. It is recognized, however, that solid particles and agglomerates may exhibit different breakage behavior. Based on the comparisons of model simulations and experimental data, it appears that agglomerate rebreakage rates and fragment size distributions are quite different from those for solid particles.; Particle shape has been analyzed by quantitative macro-geometric shape descriptors such as elongation, angular variability (which describes the variations in the angle between adjacent edges) and radial variability (which is a measure of the deviation of a given profile from an ideal circle) for various systems of ground particles using an image analysis system. The shape descriptors have been demonstrated to be capable of differentiating among the various shapes present in powder systems. Measured distributions of the shape descriptors have been used to investigate the effects of crushing and grinding conditions on product particle shape and the role of shape in hydrocyclone classification.; Finally, methods for producing dust for medical research were investigated using three laboratory-scale grinding devices--the Bleuler mill, the high-speed planetary mill and a Sweco vibratory mill. Results in terms of the product size distribution, particle shape and contamination introduced in the grinding process have been compared with the natural airborne dust collected from underground coal mines using a passive dust collector designed for this study. A new method to produce respirable dust particles was designed. It involves rapid grinding of relatively coarse feed material in an open circuit using the Bleuler mill operated in an air-swept mode. Freshly generated respirable dust particles are removed immediately by classification using a modified 10-mm nylon cyclone personal sampler. The system is capable of producing respirable dust according to the definition given by the American Conference of Governmental Industrial Hygienists (ACGIH).