Crushing(redirected from Pressing stone)
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in engineering, the process of breaking pieces of hard material to reduce their size. The pieces are broken by external forces that overcome the forces of cohesion among the particles of the material. Crushing does not differ in principle from grinding, but it is arbitrarily considered that crushing yields products larger than 5 mm and grinding yields products smaller than 5 mm. Crushing can be accomplished by pressure, splitting, abrasion, or striking. Firm and abrasive materials are crushed mostly by pressure; firm and viscous materials, by pressure combined with abrasion; soft and brittle materials, by splitting and striking.
The work in crushing is expended on the deformation of the piece of material and the formation of a new surface of small pieces. A large part of the energy expended is dispersed in the form of heat, and only a small part is converted into the free surface energy of the solid. The total work of crushing is equal to the sum of the work of deformation and the formation of new surfaces. This general formula was put forth by P. A. Rebinder in 1944. For approximate calculations it is assumed that the work required to crush a piece of size D at a given degree of crushing is directly proportional to D2.5.
Crushing is described by the degree of crushing—that is, by the ratio of sizes of the largest pieces of material before and after crushing. Another index is the unit expenditure of energy—the kilowatt-hours (kW-hr) per ton of crushed material. Crushing is usually combined with screening.
A distinction is made between open and closed crushing cycles. In the first case the product, which has already been sized, is screened off before entering the crusher and is recovered after crushing; in the second case, the material is screened after crushing into small pieces (the finished product) and large pieces, and the large pieces are run through the same crusher again. Several steps (stages) are used in sequence in order to produce a high degree of crushing. In ore enrichment, two, three, or four stages of crushing are used, and the unit expenditure of energy to crush pieces of material 900-1,200 mm in size into pieces 25 mm in size is 1.5-3.0 kW-hr per ton of ore.
Hand crushing and firecrushing were known 3,000 years before the Common Era. The simplest devices—falling pestles driven by a waterwheel—were used as early as the Middle Ages and were described by G. Agricola. Machine crushing began to develop in the early 19th century.
Hydroexplosive, thermal, electrothermal, and other means of crushing have been under study since the 1950’s in the USSR and other countries; however, the mechanical processes described above will be the main ones used in the next several decades.
Crushing is used in the mining, metallurgical, chemical, and food industries, in construction, and in agriculture.
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Andreev, S. E., V. V. Zverevich, and V. A. Perov. Droblenie, izmel’chenie i grokhochenie poleznykh iskopaemykh, 2nd ed. Moscow, 1966.
Trudy Evropeiskogo soveshchaniia po izmel’cheniiu. Moscow, 1966. (Translated from German.)
Arsh, E. I., G. K. Vitort, and F. B. Cherkasskii. Novye metody drobleniia krepkikh gornykh porod. Kiev, 1966.
Ponomarev, I. V. Droblenie i grokhochenie uglei. Moscow, 1970.
V. A. PEROV