Chemistry Reference
In-Depth Information
Table 10.3 Experimental results
a
for variation of the batch size, n
initial,i
, and the
effect on the filling degrees, F
i
(
),
b
and the time to reach quantitative
yield, t
97%
, for the reaction of vanillin and barbituric acid in a PBM
(Scheme 10.4).
79
n
initial,i
(mmol) F
MB,material
F
MB,packing
F
GS,packing
F
total
t
97%
(min)
100 0.25 0.45 0.30 0.55 30
125 0.25 0.45 0.38 0.63 28
150 0.25 0.45 0.45 0.70 30
175 0.25 0.45 0.53 0.78 29
200 0.25 0.45 0.60 0.85 28
a
Reaction conditions: PBM Fritsch P6, milling vessel (V
MV
¼
0.25 L) and milling balls made from
MSZ (d
MB
¼
20 mm), equimolar ratio of substrates, rpm
¼
550 min
1
.
b
Calculation according to the equations given in parentheses: F
MB,material
(Eq. 10.10), F
MB,packing
(Eq. 10.10), F
GS,packing
(Eq. 10.11), and F
total
(Eq. 10.9).
regarding the ratio of substrate amount and milling balls. One possibility to
measure this ratio is the so-called ''ball-to-powder-ratio'', which is often
applied to ascribe experimental conditions processing inorganics in ball
mills.
9,30
This value is often expressed as a mass ratio, which makes this
parameter inapplicable to describing the general relationship due to large
differences in material or bulk densities. Thus, a volume-based measure like
F
GS,packing
(Eq. 10.11) should be preferred. Table 10.3 summarizes the in-
fluence of variations in n
initial,i
. The total filling degree, F
total
, ranges from
0.55 to 0.85. Despite the high values for F
total
the times taken to reach
quantitative yield were 30 min, with negligible variations. It seemed that the
filling degree with respect to the substrates has only a minor influence,
unless the optimal value for the milling balls has been chosen. Even a
residual free volume for milling ball movement of 15% (1
F
total
¼
1
0.85)
of V
MV
is apparently enough to achieve high conversion.
10.4 Process Parameters
Variables described within this section are parameters used for controlling
the energy entry during the process. The values whose influence on chemical
reaction will be discussed here are (i) the operating frequency (n, in some
cases also f; in the case of PBM also rpm), (ii) the reaction time (t) and (iii)
the temperature (T). Whereas the operating frequency is directly correlated
to the kinetic energy of the grinding media and, thus, to the energy that can
be dissipated as heat to the mill charge (originated from frictional forces),
the parameters t and T assign the progression of a chemical reactions.
Consequently, the chemical kinetics of a desired reaction also contribute to
the performance of a solid-state reaction carried out in a ball mill.
10.4.1 Operating Frequency
The energy entry in ball mills is facilitated by the acceleration, trajectories,
and impact of milling balls, resulting in wear phenomena that are able to
Search WWH ::
Custom Search