In the previous report4), we suggested that the grindability or the rate of grinding of particles was closely connected with the size distribution pattern of the ground material, which ranged from the bulk crushing to the surface grinding. We assumed, also, that the size distribution of the ground material was affected by the strength of grinding force.
This paper is intended to verify the above-mentioned assumption by means of the ball-mill grinding, which is the most general operation pattern comprizing factors we can easily vary.
Silica balls with four different diameters (and naturally of different weights) and sintercorund balls with one diameter were respectively employed at various rotational speeds of the mill. To investigate the results of the experiment, we assumed the characteristic number of β, which indicates a characteristic of the size distribution relative to the ratio of cumulative weight percent of the coarse part (Y_(r)to that of the fine part (Y_(f)). (Cf. Fig. 4).
When the ball weight or the speed of rotation increased, (or the grinding force increased), β also increased as shown in Eq. 4-2 and Figs. 8 and 9. The values of k_β and k_x were closely related to the particle diameters, viz., when the fed particles were small, the effect of grinding force was small.
From this we may conclude that in fine grinding, the weight of a ball does not count much.
It was observed that in general the grinding rate of particle (X) was determined by the grindability of particle (Γ) and the mechanical efficiency of grinding (η_m), which is the probability of ball impact or that of other grinding forces. Therefore, in fine grinding, Γ becomes negligible, and η_m essential.
Upon this, we assumed that η_m (or impact number per one rotation of the mill) was constant irrespective of the speed of rotation of the mill, and obtained by experiment the relative values of η_m as shown in Fig. 10.
Furthermore, according to H.E. Rose's suggestion9), from the supposed contact area of a couple of balls and probability of impact of balls in the mill., we derived relative efficiency of ball impact, as shown in Eq. 5-13, and ascertained the experimental results and Eq. 5-13, as shown in Fig. 17.