The grinding of talc was carried out under reduced pressure using a hammer screen mill which was putting into a vacuumn vessel. The effect of the pressure of the atmosphere on the grinding efficiency was investigated experimentally. The grinding conditions were from 0.08 to 93.3kPa in the vacuum vessel.
Finer products were obtained as the pressure of the atmosphere decreased. In particular, the particle size of the products was greatly influenced when the pressure of the atmosphere was less than 2.67kPa. The ratio of the incremental specific surface area produced by grinding less than 2.67kPa to that at 93.3kPa has a tendency to increase with decreasing particle size. On the other hand, the influence of the opening size of the screen on the size distribution of products at 0.08kPa was recongized. Therefore, grinding under a reduced peesure, where the friction drag of the air effect is negligible, is expected to be useful for improving grinding efficiency.

View full abstract

Based upon a simplified grinding mechanism, an analysis of the selection function is made for vibration mills. The mechanism considered is that a particle nipped by two colliding balls is to be ground only if the ball impact generates or activates at least one crack within the particle and the crack is big enough for fragmentation. The derived selection function increases with increasing size up to a certain value and then decreases with increasing particle size. This trend has been observed elsewhere by experiment. Also, the present paper examines the effects of the ball on the selection function.

View full abstract

In general, it is well known that the energy efficiency of a grinding process decreases with decrease in produced particle size or increase in input energy. In this paper, a grinding method to produce submicron particles was investigated based on the experimental results of single particle crushing and simultaneous grinding. The size reduction energy was defined as the elastic strain energy input to a particle up to the instant of fracuture. It was assumed that the kinetic energy of a grinding medium which collides against a particle was converted completely into fracture energy.
As a result, it was found that the charge of a binary mixture of media having considerable difference in the size was effective for ultrafine grinding. The experimental verification of the above considerations was performed for limestone using a laboratory scale ball mill. The grinding medium charged was a binary mixture of 20mm alumina porcelain and 1.2mm steel balls. In wet grinding, the ground product with all particles smaller than 3μm was obtained after 6 hours of grinding.

View full abstract

Contamination during fine grinding is an important problem for a raw powder of newly developed materials. Wear behaviors of grinding media, which were made of steel, alumina and zirconia, were examined during fine grinding using a vibration ball mill. Ball wear was nearly proportional to a specific surface area of ground products. It was supposed that the wear in wet grinding is more than in dry grinding, because wet grinding is more effective for fine grinding. Wear was effected by the materials of the grinding media. Ball wear, which was made of zirconia, was the least of three kinds of balls, that were used. Linear wear was not as much as ball wear.

View full abstract

The batch grinding of silica sand was carried out using a coaxial circular multivessel type beads mill. The long-term grinding caused reversed grinding where the particle size enlarged and reduced simultaneously. This reversed grinding occurred when the particle size became less than a critical value. The critical median value was larger when the peripheral speed of the agitator pin was larger. Howerer, the BET specific surface area even increased within the range of reversed grinding.
The grinding rate constant estimated from Tanaka's equation was proportional to (the peripheral speed of the pin)^1.55, and it was inversely proportional to the particle concentration. The number of pins gave a different effect on the grinding performance from that resulting from the peripheral speed of the pin.

View full abstract

Baker's yeast suspended in pure water was disintegrated by the agitator bead mill using glass beads having a diameter of 0.5 to 2.0mm. ADH (alcohol dehydrogenase) was released faster than protein and G-6-PDH(glucose-6-phosphate dehydrogenase), and a concept of the selective release of enzyme was defined. The final weight of the protein released and the activities of enzymes, the release rate constant for protein and ADH, and the release selectivity between the protein and ADH were dependent on the bead size, the weight of beads and the agitation speed. The optimum condition for release existed for each operation parameter. The electrical conductivity of the yeast slurry had good correlation with the amount of G-6-PDH and protein released. This result indicated that the degree of releaase for these substances could be estimated from the mesurement of electric conductivity.

View full abstract