UBC Theses and Dissertations
Rheological effects on ultra-fine grinding in stirred mills Yue, Jian
A pilot scale study was conducted on ultra-fine grinding in a horizontal stirred mill. The main objectives were to investigate grinding kinetics, the influences of rheology and effects of bead size on performance criteria. The main criteria were breakage rate, particle size reduction, product size distribution and power consumption. The results confirm the "first-order" breakage rates in stirred mills. The product size distributions are best represented by the Rosin-Rammler equation. A method of determining breakage rates versus particle size is demonstrated which shows that the 10% top size fraction is most appropriate for the determination of specific breakage rates. The slurries ground exhibit Newtonian to pseudoplastic to yield pseudoplastic behavior over the range of conditions studied, corresponding to coarse low percentage solids to fine high percentage solids. The flow curves of grinding products were modeled using the Casson equation. It was shown that yield stress is the dominant rheological parameter which influences the grinding performance. A Low yield stress favors high breakage rates, narrow size distributions, and increased production of -10 μm fines. The bead sizes and its composition play a very important role in stirred mills. There exists an optimum ratio of bead size to feed size with respect to breakage rate, size reduction ratio and size distribution. The ratio was confirmed to be about 20/1. The existence of fine beads in grinding media has negative effects on grinding performance.