Planetary mills

A serious limitation of the ball or tube mill is that it operates effectively only below its critical speed, as given by equation 2.10. In the planetary mill, described by Bradley et alS2y>, this constraint is obviated by rotating the mill simultaneously about its own axis and about an axis of gyration, as shown in Figure 2.32. In practice, several cylinders are incorporated in the machine, all rotating about the same axis of gyration. 

It is possible to achieve accelerations up to about 15g in practical operation. For further details of the operation of the planetary mill, reference may be made to Bradley et al.<23> and Kitschen et al.<24) The planetary mill is used for the preparation of stabilised slurries of coal in both oil and water, and it can also handle paste-like materials. 

Lyophilized copolymer was ground down to colloidal dimensions (<1 iJim) using a laboratory planetary mill (Pulverisette, Fritsch GmbH). A 110-mg sample of copolymer powder, 20 mg of bovine insulin, and 20 mg of GOD were mixed, and the mixture was compressed into a disk-shaped matrix of 5-mm thickness and 15-mm diameter. 

There are other advantages of employing magnetic ball mills besides the control of mechanical milling modes. Since the centrifugal force becomes a secondary factor in milling, and the reactor shell rotates at low RPM, contamination from balls and shell wear is lower than in a vibrational or a planetary mill there is less ball wear involved and contaminations with Fe from steel become less of the problem. Also lower rotations and uniaxial movement of reactors paced on horizontal axle allow... 

Elansari et al. [201] developed a novel method of synthesizing alkali metal hydrides Na, KH, RbH, and CsH by reactive mechanical milling of pure alkaline metals under hydrogen pressure up to 30 bars in a planetary mill (Retsch PM 400). The reaction proceeds in 16 h and gives 3-15 g of very pure alkali metal hydride with FCC crystal structure (space group Fm3m). 

Figure 7.11 Some basic types of roll arrangements (a) two-high, (b) three-high, (c) four-high, (d) cluster, (e) tandem rolling with three stands, and (f) planetary milling. McGraw-Hill Encyclopedia of Science and Technology, Vol. 11, 8th ed., p. 54. Copyright 1997 by McGraw-HiU.

Figure 8.24 (a) Shaker mill and (b) planetary mill (reproduced by permission of The Royal Society... 

Fig. 1.4 Planetary mill vial with antenna mounted on cover equipped with temperature and pressure sensors for GTP wireless monitoring gas conditions during milling...

A typical task is the milling of minerals in the pm-range with power consumption of approximately 50 Wh kg-1. A tumbling ball mill would need 1 hour of milling time at a specific power input of 50 W kg-1 for the comminution, a stirred media mill would need 2 min at 1400 W kg-1, a vibration mill 40 s at 4500 W kg-1, and a planetary mill 5 s at 36,000 W kg-1 (Fig. 14.4). 

Planetary mills operate at higher stress intensities than tumbling mills. Grinding drums are placed on a rotating bracket and rotate at about 400 rpm such that accelerations of 60 g and over are possible. These mills are used for nanofine milling in batch mode (Fig. 14.8). 

K-absorption spectrum of yttrium in YH3, which was mechanically treated in a ball planetary mill during 20 min. with angular rate of 1630 rot./min. is shown in Fig. 6 (c). This absorption spectrum, as seen from the picture, is shifted relatively to the spectrum of untreated YH3 hydride to the side of lower energies, which in accordance with the found out correlation has to testify to thermal stability of reduction of treated yttrium hydride. Actually, we determined by the method of hydrogen thermal desorption, that as a result of mechanic dispersing of this hydride its temperature of decomposition decreased more than on 300 °C (Fig. 7), i.e. the... 

Powders of MgH2, TiHi s and YH3 hydrides, obtained from the gas phase, were mechanically grinded. Grinding was carried out in a ball planetary mill in argon medium at rotation speed of 1630 rot/min. Grinding balls and hydride bulk were in ratio of 20 1. 

It has been established that under intense milling in a planetary mill in a nitrogen atmosphere, titanium hydride may decompose with formation of defected structure characterized by partly non-occupied tetrahedron voids in a face-centered cubic sublattice of titanium atoms. This effect is accompanied by decrease in the crystal lattice parameter of titanium hydride subjected to intense milling for 60 min. 

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