Technical Implications of Organic Syntheses in Ball Mills - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 267

Although this parameter significantly influences chemical kinetics and
therefore the reaction rate, commercial ball mills for laboratory-scale ap-
plications (PBM, MBM) do not allow temperature regulation. Individual
solutions are available; either the instalment of a heating mantel for a
milling beaker of a MBM 47,71 or construction of a PBM allowing temperature
regulation of the complete milling chamber. 51 Switching from laboratory to
pilot or industrial scale the type of ball mill often changes to stirred media
mills. These types of ball mills have a static milling chamber, thus easily
providing the opportunity for heating or cooling the mill feed. 8,47
Owing to these technological hurdles, most organic reactions carried out
in ball mills are operated at room temperature, while during processing the
temperature of the milling beaker, and as a result that of the mill feed,
increases. The final temperature strongly depends on r MB .Typicalequi-
librium temperatures after prolonged milling for Z1hare40-801C. 29,54
Utilization of high-density materials like steel or WC can lead to a T of
120 1C, measured at the surface of the milling beaker and the mill
charge. Calculation of the reaction conditions in the impact zone results
in maximal temperatures of up to 3000 K. 51,96,97 Efforts to determine the
actual temperature of steel milling balls by opening the milling
beakers and transferring the balls into a calorimeter result in values of
200 1C. 98 However, these experiments were conducted by operating the
ball mill at a low F, which influences the final temperature. Owing to
continuous release of frictional heat, the bulk temperature increases
with increasing t, n, r MB ,andF 1 . For several applications, especially for
temperature-sensitive materials, the employment of a milling auxiliary
may be desirable for homogeneous heat distribution and reduction of local
hot spots in the impact zone of the milling beakers. 63,99 Alternatively, a
cycle-mode is often used: ball milling for a discrete time interval alternates
with pauses, allowing the milling beaker and feed material to cool
down. 81,82
Since liquefaction of the mill charge during ball milling may disturb the
eciency of energy transfer and the chemical reaction itself, 100 conditions
have to be set-up keeping all reagents in the solid or solidified state. This
can either be realized through the application of high-melting inorganic
milling auxiliaries or by (continuous) cooling of the milling beakers. One
possibility is cryogenic milling, whereby the feed material is frozen with
liquid nitrogen and afterwards comminuted for a short time to avoid un-
freezing. However, the energy distributed by frictional and impact forces is
so high that the milling time is reduced to several minutes only before room
temperature is reached. Special equipment for MBMs allows continuous
cooling with liquid nitrogen. Performing reactions under such conditions
reduces molecular motion almost completely and thus most chemical re-
actions afforded significantly lower yields. 61,63,101 Unfreezing of the finely
ground reaction mixtures can lead to post reactions, which may be very fast
and result in high conversions, 63 since the surface energy of the particles is
very high.