PLANETARY BALL MILLS THE “PLANETARY PRINCIPLE”
In the planetary ball mill, every grinding jar represents a “planet”. This planet is located on a circular platform, the so-called sun wheel. When the sun wheel turns, every grinding jar rotates around its own axis, but in the opposite direction. Thus, centrifugal and Coriolis forces are activated, leading to a rapid acceleration of the grinding balls (see fi g. 1).
The result is very high pulverization energy allowing for the production of very fi ne particles. The enormous acceleration of the grinding balls from
Fig. 1: In the planetary ball mill, centrifugal and Coriolis forces permit grindings down to the submicron range.
one wall of the jar to the other produc- es a strong impact effect on the sample material and leads to additional grinding effects through friction. For colloidal grinding and most other applications, the ratio between the speed of the sun wheel and the speed of the grinding jar is 1: -2. For applications where a high- er energy input has to be achieved, planetary ball mills with other ratios as well as high performance ball mills such as the Emax are the perfect choice.
FINAL FINENESS DOWN TO THE NANOMETER RANGE
Figure 2 shows the result of grinding of alumina (Al2
O3) at 650 min-1 in the
PM 100. After 1 hour of size reduction in water with 1 mm grinding balls, the mean value of the particle size distri- bution is 200 nm; after 4 hours it is
[%]
100 90 80 70 60 50 40 30 20 10 0
0.01
100 nm. In a further trial, the material was initially ground for 1 hour with 1 mm grinding balls and then for 3 hours with 0.1 mm grinding balls (see fi g. 3). In this case, an average value of 76 nm was achieved. The grinding results show
Colloidal grinding of alumina in the PM 100 Q3
Q3 [%] x50
1 h = 200 nm 4 h = 100 nm
100 90 80 70 60 50 40 30 20 10 0
0.1 particle size
Fig. 2: Grinding of alumina in water with 1 mm grinding balls after 1 hour (blue) and after 4 hours (green)
1 10 [μm] 0.01
that planetary ball mills can produce particle sizes in the nanometer range. The choice of the right ball size plays a crucial role in the success of nanogri- nding.
x50 H2 O = 76 nm
0.1 particle size
Fig. 3: Grinding of alumina with a 1 mm grinding ball (1 hour) and then with 0.1 mm balls (3 hours) in water
1
10 [μm]
Conclusion
Planetary Ball Mills are ideally suited for classical mixing and size reduction processes in which a high energy input is required. They are suitable for both dry and wet grinding; with the latter even grind sizes down to the nanometer range can be achieved depending on the sample material. RETSCH offers various models and extensive accessories, making the PM product range universally applicable.
www.retsch.com | the sample 39 | page 09
percent by volume
percent by volume
Direct contact: 866-473-8724
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