Disc Attrition Mills

Disc-type attrition mills are employed in the food industry for grinding cereals and grains as well as in the pharmaceutical industry. Other applications include the grinding of paint pigments and inks. Buhrstone mills are an ancient example of a disc attrition mill. 

Disc mills crush material between two grinding plates mounted on rotating discs (also termed disc attrition mills). 

Intermediate Grinders Roller mflls Hammer mills Tumbling mills Disc attrition mills... 

Disc attrition mills (a) single-disc mill, (b) double-disc mill, and (c) Buhr mill. 

Disc attrition mill 12 0.07 <1 - 103 3 Cellulose, asbestos, rubber... 

FIGURE 8.2 Equipment commonly used in wet-milling operations, (a) Disc-attrition mill, (b) Hydrocyclone (courtesy of American Association of Cereal Chemists). 

Disc-type attrition mills have surfaces that rotate past each other at high speeds with close tolerances. One or both discs may be rotated usually in opposite directions but also may rotate in the same direction. These mills are the modern version of the early Buhrstone mills. Clearances between the discs may be adjusted with springs. The grinding plates may be an abrasive or steel. Feed material... 

Disc-type attrition mills have surfaces that rotate past each other with close clearance and high speed, usually in opposite directions but sometimes in the same direction at different speeds. Clearances are adjustable with spring loading in increments of 0.001 in. Maximum feed sizes are 10-25 mm, diameters 12-48in., and speeds 1200-7000rpm. Table 12.5(b) is a list of materials that have been ground in disc mills. Some data are in Table 12.7(b). 

FIGURE 56.1 Attrition mill used for the determination of attrition, (a) and (b) are views of a disc held in place at the bottom of the mill to retain the catalyst. 

These types of mills, as those illustrated in Figure 4.5, make use of shear forces for size reduction, mainly in the fine size range of particles. There are several basic designs of attrition mills. The single disc mill (Figure 4.5a) has... 

The steeped stock is fed to disc or attrition mills, and the starch suspension is allowed to rest for 10-24 h. The fiber is removed by screening, and the starch is recovered by centrifugation. The starch is washed with water to remove excess alkali and then dried (Juliano 1984). Protein could be recovered from the steeping and centrifugation effluents after first adjusting the pH to 6.4. The protein is recovered via the use of filter presses or centrifuges (Figure 8.3). 

Forces commonly used in food processes for particle size reduction are compressive, impact, attrition (or shear), and cutting forces. More than one force usually participates in the comminution operation in industrial size-reduction equipment. In particular, crushing rolls use mainly compressive forces, hammer mills are based on impact, disc mills cause particle attrition through shear force application, and rotary knife cutters use cutting forces. 

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