Roller mills

The class of roller mills includes those pulverizers in which the stressing occurs between two surfaces that roll off each other, i.e. spherical or cylindrical rollers and a smooth conical or bowl-shaped grinding face. The milling implements are enclosed in a housing. The grains form a layer of material and are stressed by pressure and shear. The contact forces are either gravitational or centrifugal, or are extraneous hydraulic or spring forces. The numerous designs of roller mills can be classified according to the following operating conditions or mechanical arrangements: the use of centrifugal or extraneous force; the shape of the rolling body (roller or ball); the orientation of the axis of the rotating rolling surface as well as the direction of pressing (radial pressing with a vertical shaft, radial pressing with a horizontal shaft, axial pressing with a vertical shaft); the drive of the rolling bodies or of the grinding face.

Roller mills are suitable for pulverizing soft to moderately hard materials down to a fineness of less than 200 /xm. Because the grinding chamber is enclosed and the residence time is longer, drying can be combined with pulverizing without difficulty. Designs that can tolerate temperatures up to about 700 °C are available. The field of application of roller mills is very large and includes the dressing of ores, cement feedstock and ceramic masses and the pulverizing of coal, phosphate, gypsum, bauxite and dyestuffs. Large mills achieve throughputs of up to 150 x 103 kg/h. Roller mills are displacing ball mills for the materials listed above. Basically mills using extraneous force promise more than those using centrifugal force because with the former the pressing force and the rolling speed can be independently adjusted, whereas with the latter they are coupled together.

Of the numerous types of roller mill in existence only two will be discussed here as examples. Pendulum mills (Fig. 4.28) are centrifugal roller mills in which one or more rollers are suspended free to swing from a crosshead c. On rotation the centrifugal force presses these onto the layer of material on the grinding face. A stop prevents direct contact

Principle of a pedulum mill

Figure 4.28 Principle of a pedulum mill: a, roller; b, grinding face; c, crosshead.

Principle of a two-roller disc mill

Figure 4.29 Principle of a two-roller disc mill: a, rollers; b, grinding face; c, pressing springs.

between roller and ring. Ploughshare-like vanes in front of the rollers guide the material onto the grinding face. Figure 4.29 shows the principle of an extraneous-force roller mill. The grinding face is a flat plate with an outer rim. Two or more fixed rollers are pressed against it either hydraulically or by the force of springs, and the plate is driven around.

 
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