The effect of mill speed on the breakage rates is illustrated in
Fig. 8, which also shows its effect on the frequency of collisions
and the median collision energy loss. It shows that breakage rates
increase significantly with fraction of critical speed, especially
above 20%, dropping above the critical speed. This is associated
with the increase in median specific collision energies, and the
reduction in their specific frequency (number of collisions divided
by ball load) for mill speeds above, about, 50%. It is important to
state, however, that the assumption of perfect mixing (iii) of balls
and powder charge becomes questionable as mill frequencies approach
and surpass the critical speed, since, in these conditions,
particles are more amenable to centrifuge than the balls, given
their finer sizes.