Technical Implications of Organic Syntheses in Ball Mills - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 253

Table 10.1 Overview of materials applicable for mechanochemical syntheses in ball
mills.

Vickers
hardness

Density (r)
(g cm 3 )

Energy
entry

Possible impurities
due to wear Material

Stainless steel 550 7.8 High Fe, Cr

Hardened
steel

750 7.9 High Fe, Cr, C

Tungsten
carbide

1200 14.8 Very high WC, Co

Agate 1000 2.7 Very low SiO 2

Al 2 O 3 c Corundum 1750 3.9 Low

Zirconia a ZrO 2 , MgO/Y 2 O 3 b 1200 5.9 Moderate

Very low c PTFE Elastic 2.1 F, C

a Magnesia-stabilized zirconia ¼ MSZ. Yttrium-stabilized zirconia ¼ YSZ.
b The deterioration of milling balls made from those materials is generally low compared to the
other materials listed.
c In the case of polymer-coated steel balls the energy entry is high.

the abrasive behaviour and generate catalytic species in situ. Thus, substi-
tution of Cu (co)-catalyst(s) in several reactions by milling vessel and milling
balls made from Cu has been reported. 69
Deterioration of the milling tools is generally a problem since the milling
balls continuously lose weight, 69 which leads to a decrease of the moving
masses with consequences for the kinetic energy. Since the abrasive poten-
tial not only depends on the material properties of the milling balls or
containment surface but also on the hardness and particle size distribution
of the mill feed, 62,74 it is important to monitor the weight loss. If the de-
terioration exceeds a specific value or if control experiments reveal that
parameters like yield or selectivity tend to change significantly, the grinding
media have to be replaced.

10.3.3 Number of Milling Balls
As indicated with Eq. (10.2) the mass of the grinding bodies influences E kin
of the moving bodies in a ball mill and thus the energy that can be trans-
formed into chemical or thermal energy. Changing the mass of the grinding
bodies by use of different materials is one possibility to influence the energy
density. On the other hand, the number of the milling balls n MB and their
size d MB are important parameters (cf. Eq. 10.1). 65,75,76
Generally, one has to differentiate between mechanochemical syntheses in
ball mills carried out in single-ball mode (n MB ¼ 1) or multi-ball mode (n MB Z2).
The possibility to run a ball mill with less than two milling balls depends on
the type of mill used for the experiments. Whereas it is possible to operate a
MBM or a VBM using only one milling ball, similar experiments would fail in
planetary ball mills, since the wear stress would increase significantly. Ac-
cording to Figure 10.4 multiple milling balls are necessary to give the required
trajectories inside the vessel, which are responsible for the energy transfer and