Chemistry Reference
In-Depth Information
Figure 10.6 Correlation between operating frequency of two different types of ball
mills and yield of product as well as energy intensity E
m
(Eq. 10.16, see
Section 10.5).
29
Reproduced with permission of the Royal Society of Chemistry.
The higher centrifugal force in the case of a PBM generates g-forces that
multiply the force generated by the propulsion drive system (cf. Figure 10.5).
This effect results in the higher operating frequencies of a MBM in com-
parison to a PBM under the prerequisite that the yield of a chemical reaction
is the same.
Comparison of a MBM, a VBM, and a mortar grinder was undertaken by
Wang and Liu using the pinacol coupling of aromatic aldehydes and ketones
as their model reaction.
60
The results indicate that the selectivity of the
reaction is lower for accomplishment under the conditions of high-energy
ball milling (MBM, VBM) compared to the experiments in the mortar
grinder. Obviously, the increased energy density in the ball mills initiate
decomposition of the vicinal diols to secondary alcohols. Such ''association-
dissociation equilibrium''
60
has been verified for the grinding of C
60
with
mortar and pestle yielding the dumbbell-shaped dimer C
120
.
66
10.3.2 Grinding Material
The choice of material for milling beakers and grinding media (milling balls)
is important for the performance of organic reactions. Material properties
like density (r
MB
), Young's modulus (E
Y,MB
), hardness, or abrasion behaviour
significantly influence the outcome of a chemical reaction performed in
a ball mill as well for particle refinement (cf. Eq. 10.1).
7,51,67
Based on
Eq. (10.2) the kinetic energy E
kin
of a system is determined by the moving
mass m and the relative velocity v:
55
E
kin
ΒΌ
0.5mv
2
(10.2)
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