Vibratory Ball Mills

Vibratory Tube Mill. In the laboratory vibratory tube mill shown in Fig. 20, oscillatory motion of the balls is complicated. The path of the balls and particles is determined by many factors, including vibrational speed, amplitude, curvature of the sides of the mill chamber wall, horizontal motion of the mill, and charge contact with the upper surface of the mill chamber. The balls gyrate around the chamber wall, sometimes traveling horizontally in a spiral path. Gyration in cylindrical chambers occasionally produces deformation and/or wear grooves in the inner walls of the chamber. These grooves may extend 360° or less, depending on whether the balls make contact with the upper surface of the chamber wall. Balls also revolve at different rates and directions, causing substantial shearing action, which is desirable in mixing and blending operations.

Fig. 20 Megapact vibratory ball mill. Courtesy of Battelle Memorial Institute

Impact forces acting on powders in a vibratory mill are a function of the rate of milling, amplitude of vibration, and mass of the milling medium. High-energy milling forces can be obtained by using high vibrational frequencies and small amplitudes of vibration. The mill shown in Fig. 20 operates at 3300 rpm, with a 2 mm (0.08 in.) amplitude, reaching a maximum acceleration rate of 12.2 g, where g is the gravitational acceleration at 9.81 m/s2 (32.3 ft/s2). Large production mills operate at relatively low vibrational frequencies and high amplitude (for example, 1000 to 1500 rpm and up to 12 mm, or 0.48 in.).

The vibratory ball mill is an excellent means of producing solid-state alloyed and dispersion-strengthened metals in amounts up to 4.5 kg (10 lb) or more, depending on the apparent density of the powder. Figure 21(a) and 21(b) show the microstructure of an aluminum-iron-cerium alloy and a transmission electron micrograph of oxide dispersion in the same alloy after solid-state alloying in a laboratory vibratory mill of the type shown in Fig. 20.

Fig. 21 Homogenization of Al-Fe-Ce alloy by high-energy milling. (a) Untreated rapidly solidified power after hot pressing. (b) Hot pressed rapidly solidified power after high-energy milling in a Megapact mill

In large tube mills of the type shown in Fig. 22, vibratory motion of the medium decreases from the chamber walls to the center of the mill tube; consequently, the milling effect is less at the center of the mill than adjacent to the chamber walls. Efficient operation is obtained at a ball fill of 60 to 80% of the volume of the mill chamber for tubes about 500 mm (20 in.) in diameter, with 100% fill of the volume between the balls.

Fig. 22 Pilot- and production-size vibratory mills

In vibratory mills, the grinding medium receives rapid impulses at a rate proportional to the vibrational frequency of the mill. Impact forces acting on the powder exceed shearing and friction forces. The entire charge slowly revolves counterclockwise to the oscillatory vibrations, so that grinding and intense mixing occur simultaneously. Vibratory mills utilize smaller mediums because of higher impact forces, frequencies, and acceleration; thus, a higher specific surface is available for milling. The rate of processing in a vibratory mill is the following:

• Proportional to the density of balls (diameter constant)

• Proportional to the diameter of the balls (density constant)

• Proportional to the cube of the frequency of vibration

• Negligible for speeds less than 900 to 1000 rpm

• Proportional to the square root of ball diameter/mean particle diameter ratio

• Not significantly affected by chamber diameter

• Increased as amount of powder in mill decreases

• Greater with balls than cylinders or other shapes

Sweco vibratory mills (Fig. 23) are equipped with a grinding chamber in the form of a vertical cylinder with a solid center axis. A double-ended motor with eccentric weights rigidly attached to the bottom of the chamber generates high-frequency, three-dimensional vibrations. The chamber and motor assembly are mounted on a base and are supported by compression springs. Vibration of the grinding medium within the chamber creates the milling action. Vibrations are transmitted from the sides and base of the chamber to the grinding medium. Particles trapped within the medium are broken down by high-frequency impaction.

Vibratory Mill
Fig. 23 Sweco vibratory wet grinding mill

The medium is packed to provide near-maximum packing density. The packed mass slowly gyrates horizontally, rises near the outer wall of the chamber, and descends as it approaches the inner wall. This motion facilitates distribution of the charge in dry grinding and serves to maintain solids in suspension in wet milling. The Sweco mill is not widely used to mill metal powders. It is particularly ill suited for high-density metals, because high density causes particles to settle to the bottom and become caked.

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