For same feed size, the size distributions of products after grinding
for various determined grinding time under different ball media
are given in Fig. 3. Size distributions in each test became finer
regularly according to grinding time. The size distributions for 5
and 10 min grinding time are plotted in Fig. 4 in order to compare
the effect of different ball media in same graph. It can be seen that
coarser ball media in which 70 mm, 60 mm, 50 mm top size ball
diameter would lead to rapid disappearance of coarser particles
than finer ball media in which 40 mm, 30 mm, 20 mm top size ball
diameter.
As seen in Fig. 4, the difference in size distributions of obtained
by finer and coarser ball loads was significant for the particles larger
than 0.212 mm. Coarser ball loads were found to be efficient for
coarser particles having larger energy for breakage. However, finer
ball loads having larger surface area were efficient for particles
smaller than 0.212 mm.
Using the particle size distributions of feed and mill content the
grinding process in pilot scale ball mill was modelled and the specific
breakage rate parameters (ri/di) were calculated for the each
test using Whiten’s (1974) perfect mixing model given in Eq. (1)
(Napier-Munn et al., 1996). The variation of specific rate of breakage
with particle size all through the grinding time in each test is
shown in Fig. 5. The difference between breakage rate of particles
at 5 and 10 min under different ball media tests are given in Fig. 6 as well