Grinding Media Mills

Tumbling mills are commonly used in industry for grinding large quantities. There are three distinct types, each with different application areas  

The motion of the grinding media depends on the rotation speed, and can be cascading at low speeds (Fig. 14.7), cataracting at medium speed, and centrifugal at higher speeds. The volume fraction of the grinding media amounts to 0.3, while the mass fraction of the grinding stock is typically less than 0.2 of the total mass. 

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). 

Vibration mills are filled with 60-80 % spheres, cylpebs or rods. The grinding drum is attached to a frame on springs and is driven by an eccentric drive at a frequency of 1000-1500 rpm and amplitudes of approximately 10 mm. The tubes can be up to 4 m in length, with diameters of up to 0.65 m. The continuous operation is performed without classifier at throughputs of up to 15 tonnes per. Vibration mills are use for the microfine grinding of hard material (Fig. 14.9). 

Centrifugal mills are a special design of vibration mill with a fixed amplitude (Fig. 14.10). 

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CRUSHING AND GRINDING EQUIPMENT WET/DRY GRINDING—MEDIA MILLS... 

Mixing. The most widely used mixing method is wet ball milling, which is a slow process, but it can be left unattended for the whole procedure. A ball mill is a barrel that rotates on its axis and is partially filled with a grinding medium (usually of ceramic material) in the form of spheres, cylinders, or rods. It mixes the raw oxides, eliminates aggregates, and can reduce the particle size. 

The pebble mill is a tube mill with flint or ceramic pebbles as the grinding medium and may be lined with ceramic or other nonmetaUic liners. The rock-pebble mill is an autogenous mill in which the... 

Material and Ball Charges The load of a grinding medium can be expressed in terms of the percentage of the volume of the mill that it occupies i.e., a bulk volume of balls half filhng a mill is a 50 percent ball charge. The void space in a static bulk volume of balls is approximately 41 percent. Since the medium expands as the mill is rotated, the ac tual running volume is unknown. 

PEBBLE MILL. A jacketed steel cylinder rotating on a horizontal axis and containing flint or porcelain pebbles as the grinding medium. Its operation is similar to that of a ball mill. It is used for grinding and mixing of dry chemicals, pigments, food products, and the like. Pebble mills are usually lined with alumina, buhrsione, or similar material to protect the walls from wear. 

These facts and the scattering of results in differently sized mills made a systematic investigation of the grinding process necessary [78]. The grinding process in bead mills is determined by the frequency and the intensity of the collision between beads and grinding medium. According to this assumption, the grinding result... 

The tube mill originated in the cement industry, where an extremely fine product is required. Originally flint pebbles were employed as the grinding medium, and the mills were lined with silex or silex brick, a hard siliceous material. Recently, however, it has been found that the efficiency is greatly increased by using a hard-iron or steel liner and steel balls, the output per horsepower being far greater. 

Spiral Jet Mill The original design of the spiraljet mill, also called a pancake mill, is shown in Fig. 21-71. This design was first described by Andrews in 1936 and patented under the name Micronizer. A number of nozzles are placed in the outer wall of the mill through which the grinding medium, a gas or steam, enters the mill. 

A key to the performance of media mills is the selection of an appropriate grinding medium. Jorg Schwedes and his students have developed correlations which are effective in determining optimal media size for stirred media mills [Kwade et al.. Powder Technm., 86 (1996) and Becker et ah, Int. J. Miner. Process., 61 (2001)]. Although these correlations were developed for stirred media mills, the principles developed apply to all media mills. 

Ball and Pebble Mills. Ball mills are hardened steel shells with closed ends that use steel balls as the grinding medium (Figure 4). Pebble mills have steel shells and ends but they are lined with burrstone or synthetic stone (porcelain) and they use natural or porcelain balls as the grinding media (8) (Figure 5). 

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