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in engineering, fine crushing of a solid material (to particles of less than 5 mm). Pulverization is used extensively for the concentration of mineral products in mining, as well as in metallurgy, the chemical industry, and construction.
Pulverization has been known since ancient times. The stone mortar and pestle were known earlier than 8000 B.C. Before 3500 B.C., manually operated milling stones were used in Egypt and China to pulverize grain, but only partially in mining. In the 16th century stamps (dropping pestles) began to be used to pulverize ore. Machine pulverization began to develop in the second half of the 19th century. The principle of operation of the ball mill, the basic pulverizing device, was already known 150 years earlier; the prototype of the modern mill was invented in the 1870’s.
Methods of pulverization include crushing, impact, and grinding; compressive strain and shear are very important in these processes. In essence pulverization is a process of formation of new surfaces. Under the influence of external forces, stresses arise, causing microscopic cracks that are capable of partial closing (self-healing) when the load is removed. A certain maximum concentration of microscopic cracks per unit volume may cause at least one large crack, which leads to the breakup of the piece. The surface-active molecules of the substances present in the surrounding environment are adsorbed on the walls of the crack and prevent self-healing (the “Rebinder effect”). Where the load on the piece is repeated, these cracks may give rise to a large crack, and so on. The phenomenon of concentration of the substance on the surface of the cracks explains the action of softening solutions that promote pulverization. As the size of the pieces decreases during the pulverization process, their strength increases, because the small particles have fewer structural defects. With very fine pulverization, particles of a few microns (μ) and smaller may form flakes and concretions because of the forces of molecular cohesion. In this case new small pieces occur simultaneously during pulverization and are partially consolidated as a result of aggregation. Surface-active substances, which cover the particles with a very fine film that inhibits conglomeration, are added to prevent aggregation. In many cases pulverization is associated with chemical changes on the surface of the particles. The size distribution of particles in the products of pulverization is usually regular. The specific surface may serve as a measure of coarseness, since it is inversely proportional to the average size of the particles.
For pulverization of mineral products and materials the cement and chemical industries use mainly drum mills (ball, rod, pebble, and self-pulverization mills); in the building-materials industry edge-runner mills are used to pulverize clay, quartz, and feldspar. Soft and medium-hardness nonabrasive materials such as phosphorites and coals, are pulverized in roller and ring mills. Vibration mills are used for very fine pulverization of small amounts of material with grain sizes of 1–2 mm to 0.05 mm. Superfine pulverization of materials with grain sizes of 0.1–0.2 mm to particles 2–10 μ in size is done in air-pressure mills.
The indexes of machine productivity for pulverization include not only the weight but also the coarseness of the initial material and of the product. The expenditure of energy for pulverization depends on the strength (pulverizability) of the material, the coarseness of the initial material, and the degree of mill loading. To increase the output of mills and reduce repulverization of the material, the pulverization process is often carried on in a closed cycle with a sorting device; the finished pulverized product is separated from the material unloaded from the mill, and the coarse material is returned to the mill. Mills operate efficiently only at a certain degree of pulverization; therefore, pulverization is often done twice (two stages) or, less frequently, three times, to produce a fine product. Various pulverization techniques are possible here. For example, in two-stage pulverization the first-stage mill may work in an open cycle, and the second-stage mill may have a closed cycle.
New principles of pulverization are developing based on the use of the electrohydraulic effect (electric discharge in water), high-frequency currents, and collisions of counterstreams of air carrying solid particles (so-called air-pressure mills).
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V. A. PEROV