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mechanical grinding of wheat or other grains to produce flour. Milling separates the fine, mealy parts of grain from the fibrous bran covering. In prehistoric times grain was crushed between two flat stones.
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a machine for grinding various materials. Mills are distinguished from crushers in that they grind the material more finely (down to a particle size of less than 5 mm). Mills are divided into five types according to the shape and type of the working member and its speed of motion (see Table 1).
Drum mills are widely used in mineral dressing, in the production of cement and coal dust, and in the chemical and metallurgical industries. In such mills a cylindrical or conical drum is
|Table 1. Classification of mills|
|Group||Type of working member||Speed of working member|
|I..........||Drum, including ball, rod, pebble, and self-grinding||Low|
|II..........||Roller, roll, ring, friction-ball, and crusher-roll||Medium|
|IV..........||Vibrating, with rocking housing||High|
|V..........||Air-pressure and aerodynamic, without grinding bodies||High|
half-filled with grinding bodies and rotates about its horizontal axis. Raw material is loaded into one end of the drum, and the milled product is removed at the other end, usually through hollow journals in the end plates. As the drum rotates, the freely moving grinding bodies pulverize the material by impact, abrasion, and crushing. The grinding bodies are cast iron or steel balls 30–150 mm in diameter, cylindrical cast iron or steel rollers 16–25 mm in diameter and 30–40 mm long, round steel rods up to 130 mm in diameter and equal in length to the length of the drum, flint or mineral pebbles up to 200 mm in size, and large pieces of the ore being crushed. Accordingly, a distinction is made among ball,rod, pebble, ore-pebble, and self-grinding mills.
The drum of a mill rotates at 60–95 percent of the “critical speed of rotation” (ncr = 42.3/ rpm, where D is the inside diameter of the drum in meters). When the critical speed of rotation is significantly exceeded, the grinding bodies are pressed to the side of the drum by centrifugal force, and grinding ceases. Operation above the critical speed of rotation requires smooth lining plates within the drum. The inside surface of the drum is covered by steel casting or rubber lining plates to provide protection from wear.
Drum mills are designed for dry or wet grinding. Modern ball or rod mills have drums 0.9 to 5 m in diameter and 0.9 to 8 m long (in cement production, they are 4 m in diameter and up to 15 m long). The drums of self-grinding mills are up to 10.5 m in diameter and 3.8 m long; the drive power of such mills is up to 7,000 kilowatts (kW). As of 1974, mills 12.2 m in diameter and with a power of up to 20,000 kW were planned.
The efficiency of a mill is directly proportional to power consumption (for equal coarseness of the initial material and the product). Material with a particle size of up to 30 mm is handled by ball and rod mills; self-grinding mills are suitable for particle sizes up to 300 mm. The particle size of the product may be less than 0.04 mm. The wear of the steel balls in grinding is 1–3 kg per ton of ore. Power consumption per ton of ore is 10–20 kW-hr.
To obtain a product of a given size, drum mills are usually used in conjunction with classifiers, liquid cyclones or air separators, which sort the material emerging from the mill into fine (finished) and large particles. The latter are returned to the mill for further grinding (called the closed cycle).
The mechanism of the ball mill has been known for more than 150 years. Drum mills have been used since the 1880’s and have been in wide use since 1910. Large-diameter self-grinding mills were developed in the 1930’s but have been used in industry only since the 1950’s.
Mills in which the mechanism operates at moderate speeds (medium-speed mills) are used for dry grinding of soft and moderately hard materials (coals, cement raw material, phosphorites, graphite, sulfur, talc, and mineral paints). Medium-speed mills may be of the roller, roll, ring, friction-and-ball, or crusher-roll types. The main patents for medium-speed mills of various types date to the 1860’s to 1890’s. The roller mill was invented by Schranz in Germany in 1870.
A medium-speed roller mill consists of a hermetically sealed body and a rotating horizontal milling ring to which two rollers up to 1,200 mm in diameter are pressed by springs. The raw material is introduced into the milling ring and is crushed by the rollers when the ring rotates. The mill operates in a closed cycle, with an air classifier located directly above it; air circulation is maintained by a blower. The particle size of the initial material in roller mills is usually up to 20 mm, but it may be as high as 50 mm in certain cases. The particle size of the finished product is characterized by the residue of 10–20 percent of the material when screened through a sieve with 0.088-mm openings.
Crusher-roll mills are used for milling feldspar and dolomite in the production of ceramics and refractories. In such mills, material is crushed and ground between the cylindrical surfaces of the rollers and the flat bottom of the pan. The rolls are up to 1.8 m in diameter and 0.8 m long. Crusher-roll mills (Chile mills) have their origin in drag-stone mills used in gold-mining operations in ancient Mexico (heavy boulders were dragged by horses around the bottom of a basin strewn with rocks). The main patents for modern crusher-roll mills were granted in the 1850’s.
Hammer (shaft) mills are used for making powdered fuel from soft coals, shale, and peat. A rotor with rigidly attached or hinged hammers rotates in a housing. The raw material is fed to the rotor and crushed by blows from the hammers. Hot air is fed into the mill, and drying of the fuel takes place simultaneously with the milling. The milled and dried material is carried into the shaft, from which fine particles of the product are carried by the air stream into the furnace and the large particles, requiring further milling, are returned to the rotor. Shaft mills are high-speed machines; the linear velocity at the hammer ends is up to 65 m/sec. The diameter of the rotor is up to 1.6 m, and the length, up to 2 m. The fuel introduced into shaft mills has previously been reduced to a particle size of less than 15 mm. The product is a coarse grist; the residue on a screen with 0.088-mm holes is 30–60 percent. Shaft mills have been in use since 1925, although the patent for a beater cross mill with hinged hammers was granted to H. Currier in Great Britain in 1875.
Pin beater mills, or disintegrators, are used for milling soft materials such as coal and dry clay.
Vibrating mills are used for low-output milling of materials of moderate hardness with particle sizes from 2 to 0.06 mm and finer. The drum of the mill is filled with balls to 80 percent of its volume and is mounted on springs. The action of a mechanical vibrator (a rotating unbalanced load) causes the drum to perform circular vibrations of high speed (up to 3,000 per min) and small radius (3–5 mm). The material loaded into the drum is ground by its frequent collisions with the balls in the vibrating body. The volume of the drum of vibration mills does not exceed 1,000 liters, and the efficiency is low. The first vibrating mills were introduced in the 1930’s.
Air-pressure mills are used for very fine milling (to a grain size of 0.001–0.05 mm). Ground material is introduced into ejectors facing one another. Compressed air under a pressure of 0.4—0.8 meganewton per sq m (40–80 kilograms-force per sq cm), super-heated steam, or hot gaseous combustion products are introduced into the ejectors. The material enters the grinding chamber at very high speed (up to 500 m/sec) through acceleration tubes. The particles of the material collide with each other in flight and disintegrate; the milled material is drawn off from the chamber to a classifier. The large fraction is returned to the ejectors. The idea of using a stream of compressed gas for im-parting velocity to a particle in pulverization was patented in 1880, but the development of air-pressure mills began only in 1925.
New electrophysical milling methods using high-frequency currents, electric-pulse techniques, and electrohydraulic impact are under study. However, massive drum mills, including self-grinding mills, will apparently continue to be used for large-scale milling.
REFERENCESRomadin, V. P. Pyleprigotovlenie. Moscow-Leningrad, 1953.
Andreev, S. E., V. V. Zverevich, and V. A. Perov. Droblenie, izmel’chenie i grokhochenie poleznykh iskopaemykh. Moscow, 1966.
Akunov, V. I. Struinye mel’misy, 2nd ed. Moscow, 1967.
Il’evich, A. P. Mashiny i oborudovanie dlia zavodov po proizvodstvu keramiki i ogneuporov. Moscow, 1968.
Schubert, H.Aufbereitung fester mineralischer Rohstoffe, vol. 1. Leipzig, 1968.
V. A. PEROV
What does it mean when you dream about a mill?
Something that is being ground to flour or meal can indicate the disintegration of the ego and false beliefs in self. It is the coming to grips with the “nitty gritty” of life, and behaving according to one’s true values and beliefs.