Milling, mechanical

Mechanical Mills with Mir Classifiers. To improve the end fineness and achieve a sharper topsize cutoff point, many mechanical impact mills are fitted with integral air classifiers (Fig. 13). These can be driven separately from the mill rotor or share a common drive. The material to be ground is introduced into the mill section of the machine, where impact size reduction takes place. The airflow through the machine carries the partially ground product to the air classifier, which is usually some form of rotating turbine. The speed of rotation determines which particle size is internally recycled for further grinding and which is allowed to exit the machine with the airflow. Machines are available up to 375 kW and can achieve products with essentially all material <20 fim. 

Figure 7. Particle size distribution of traditionally processed cowpea paste and mechanically milled cowpea flour. Reproduced with permission from Ref. 9. Copyright 1983, American Association of Cereal Chemists.

These non-existent allotropes, which are impurity-stabilized phases, are fee Sc, fee Y-Ce, the bcc Ho, Er, Tm and Lu and fee phases of Nd, Sm, Gd and Dy, some of which have been described as formed at room temperature during mechanical milling. A number of fee high-pressure polymorphs, for instance, are actually compounds, with a structure related to the NaCl-type, formed by reaction with O, N and/or H during mechanical milling (see also Alonso et al. 1992). 

Figure 6. Typical comparison on the dehydriding (DH)/rehydriding (RH) profiles between the samples that were prepared by mechanically milling NaH/AI with (a) 4 mol % TiH2, (b) 4 mol % metallic Ti under Ar atmosphere for 10 h.

Wang, P, Orimo, S., Tanabe, K., Fujii, H. 2003. Hydrogen in mechanically milled amorphous boron. J Alloys Comp 350 218-221. 

A.O.C. magnet-controlled mechanical model mill (Uni-Ball magnetomill) A.O.C. electric discharge-assisted mechanical mill... 

Milling atmosphere, which is a neutral protective gas (helium or argon) during mechanical milling or hydrogen under pressure up to 0.9 MPa under reactive mechanical alloying processes. 

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... 

In the first procedure, called a two-step method, either premilled pure metallic elements or a mechanically milled (MM) precast intermetallic ingot are hydrogenated under gas hydrogen pressure to form a desired metallic or intermetallic-... 

Mechanical milling (MM) of cast ingots followed by hydrogenation... 

R.A. Vatin, T. Czujko, Z.S. Wronski, Particle size, grain size and g-MgH effects on the desorption properties of nanocrystalline commercial magnesium hydride processes by controlled mechanical milling, Nanotechnology 17 (2006) 3856-3865. 

G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, Hydrogen storage properties of the mechanically milled MgH -V nanocomposite, J. Alloys Compd. 291 (1999) 295-299. 

Table 2.15 Composition of powders and processing parameters for reactive mechanical milling of Mg-H powders [63]... 

It is also clearly seen in Fig. 2.34 that the relative frequency distribution of powder particles remains log-normal starting from Mg through any powder regardless of the duration of reactive milling time. This is exactly the same behavior as already described for mechanically milled commercial MgH powders (Fig. 2.20). The experimental coefficient of variation, CV(ECD) = 5D(ECD)/M(ECD) (where SD is the standard deviation... 

For the sake of clarity, it must be mentioned that Huot et al. [24, 35] reported that at 350°C the absorption/desorption PCT plateau pressure hysteresis of the unmilled MgH was quite substantial, while the hysteresis of the same material milled for 20 h was very small. They argued that the plateau pressure difference observed for the uiimilled material was due to a very slow desorption kinetics which did not allow reaching equilibrium. In turn, ball milling increased desorption kinetics, which allowed reaching equilibrium and eliminated hysteresis. However, it must be noticed that the situation reported by Huot et al. is completely opposite to what is observed in Fig. 2.43, in which MgH synthesized by reactive mechanical milling shows a pressure hysteresis. 

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