Granulation(redirected from arachnoid g)
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granulation(gran-yŭ-lay -shŏn) A network of convective cells in the solar photosphere. It consists of bright irregularly shaped (often polygonal) granules, separated by dark narrow intergranular lanes. The individual granules are about 1000 km in diameter, have an upward velocity of about 0.5 km s–1, and exhibit a horizontal flow of material – outward from the center – with a velocity of about 0.25 km s–1; their average lifetime is about 8 minutes. They represent the changing tops of currents from the convective zone, bringing hot gases to the photosphere, where the gases cool and then return via the intergranular lanes.
Granulation is readily visible under favorable conditions, when it gives the photosphere a mottled appearance. It may be best seen near the center of the Sun's disk, where foreshortening is not significant. See also supergranulation.
the process of reducing a substance to very small pieces (granules). Granulation is required to impart improved technical properties to a substance, to prevent sintering (adhesion) and increase friability, to make it possible to use small portions of a material, and to facilitate loading and transporting. Depending on the method of granulation, the same substance can vary in hardness, hydraulic properties, and so on. Granulation is used chiefly in certain areas of the chemical industry, in metallurgy, in power system management, and in agriculture.
In the chemical industry granulation is used in making fertilizers (superphosphate, ammonium nitrate, and so on) and plastics. Ammonium nitrate is granulated by spraying a melt into hollow towers 30–35 m high, where the spray solidifies into grain as it falls. Granulation of superphosphate powder in rotating drums shapes it into spherical grains. The granulation of fertilizers (urea, ammonium nitrate, and others) in an air-fluidized bed and by other methods holds promise. Polymers are granulated either by packing low-bulk powdered materials into dense, regular shapes (grains or tablets) or by pulverizing large blocks, all possible wastes, and scrap articles made from polymers. The methods of granulating thermoplastic polymers from melts is the most widely used. In this case granulation is also used to mix the polymer with plasticizers, to remove moisture and volatiles, and to introduce various ingredients, such as antioxidants and dyes. The thermoplastic polymer melt is pressed through an extruder head in the form of roving or ribbon, which is cut into granules either immediately upon emerging from the head or after cooling in air or water. Heat-setting polymers are granulated by pulverizing the cold material in a grinding unit (for example, between corrugated rolls), as well as by mechanical packing of the powdered materials at room or high temperature.
In metallurgy, the liquid products of a melt—slags (for subsequent use as ballast in road construction, as a filler for concrete, and as an active additive in making cement and slag brick), mattes (for treatment in a pulverized state and removal of sulfur particles), and certain metals (to facilitate their use in small portions)—are granulated. Three methods of granulation are used in metallurgy: (1) wet granulation, in which a stream of water is fed into a stream of melt that is flowing in a trough, and then both streams pass into a basin containing water, where the melt breaks up into particles, which then solidify as fine grains or pieces; (2) semidry granulation, in which the molten material, mixed with a stream of water, drops onto a rotating drum with blades and is thrown off as drops, which solidify upon contact with the air; and (3) dry granulation, in which the molten material is split into granules by a stream of compressed air, nitrogen, or steam.
In power system management, boiler slags are granulated to accelerate their setting in order to stop coating or clogging of the boiler’s ash hopper and convection surface.
In agriculture granulation is used to make granulated feeds. Granulators that operate on the stamping principle are most often used for this purpose; farinaceous feeds treated with steam or mixed with water or molasses fall on to a rotating disk or annular matrix, are squeezed out through holes 3–16 mm in diameter by roller pressure, and are cut into granules by knives.
REFERENCESPozin, M. E. Tekhnologiia mineral’nykh solei, 3rd ed., parts 1–2. Leningrad, 1970.
Matusevich, L. N. Kristallizatsiia iz rastvorov v khimicheskoi pro-myshlennosti. Moscow, 1968.
Gel’perin, N. I., V. G. Ainshtein. and V. B. Kvasha. Osnovy tekhniki psevdoozhizheniia. Moscow, 1967.
V. L. PEBALK
a technique in jewelry-making whereby small gold or silver balls (with a diameter as small as 0.4 mm) are fused to filigree ornaments. Granulation creates an interplay of texture and chiaroscuro and enriches the ornamental rhythm of the article. The technique has been known since ancient times, notably in Mesopotamia, ancient Greece, and the Caucasus and was widely practiced in the Middle Ages, especially in ancient Rus’; granulation is still used today.