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sintering,process of forming objects from a metal powder by heating the powder at a temperature below its melting point. In the production of small metal objects it is often not practical to cast them. Through chemical or mechanical procedures a fine powder of the metal can be produced. When the powder is compacted into the desired shape and heated, i.e., sintered, for up to three hours, the particles composing the powder join together to form a single solid object.
or agglomeration, in metallurgy, the heat process for pelletizing fines (ores, ore concentrates containing waste metals, and others), as components of the metallurgical charge, by sintering these materials in order to impart the necessary shape and properties (chemical composition, structure) to them for melting. Sintering takes place either by direct adhesion of separate heated particles of the charge as their surfaces are softened or as a result of the formation of fusible compounds binding the particles when the product to be sintered cools down. The heat required for sintering is obtained either from combustion of carbon fuel added to the material to be sintered or, where sulfurous ore concentrates are to be sintered, from oxidation of sulfides. In practice, sintering is carried out most often on grate bars, with a downdraft suction of air to the bed of cinder charge lying on the grates. Fuel combustion progresses downward to successive layers of charge as the bed advances. The charge must be made as homogeneous as possible. The sinter charge must exhibit the necessary permeability to gas to promote uniform oxidation of the fuel during the sintering process and to obtain a firm and porous sinter of proper chemical composition; the major factors here are grain size and the initial moisture content.
The principal feed materials for sintering are fine untreated ores (8–10 mm) and ore concentrates, as well as fuel (coke breeze and anthracite breeze up to 3 mm), flux (limestone and dolomite up to 3 mm), and in some cases fine wastes (flue dust, scale, and others). The end product is sinter cake. Over 95 percent of the sinter is used in ferrous metallurgy; sinter is used in aluminum production, nickel production, and lead production in nonferrous metallurgy. The industrial production of sinter dates to the early part of the 20th century (USA).
The sintering process includes preparation of the charge, including proportioning or batching the individual components, mixing, moistening, and pelletizing; sintering a prepared charge on sintering machines; and processing the hot sintered cake by fragmentation, screening to remove lumps up to 5–10 mm, cooling up to 100°C and sorting. Sintering is closely coordinated with the operation of process machinery preparing raw materials for sintering. This relationship places a premium on stabilization of the principal input parameters of the process (blending and proportioning of materials, chemical composition, moisture content, and so on), which opens up avenues for comprehensive automation of the sintering process. Sintering is carried out at sintering plants, which include stockpiles for blending and storing reserves of charge materials, receiving hoppers, departments for comminution of coke and limestone (also for calcining limestone), a charge preparation department, a sintering department, and a department for processing the finished sinter cake.
The operation of receiving the raw material, proportioning and preparing the charge, placing the charge on sinter machines, and processing the finished sinter cake are completely mechanized and are even automated to a certain extent in modern sinter plants.
The ore, concentrates, flue dust, and other additives which do not require crushing are fed to the charge preparation department out of the receiving hoppers or from the stockpile on conveyors. Coke breeze and lime are sent to the size-reduction department and then to the charge preparation department. Return fines (fines screened from the sintering process) are also routed to the charge preparation department. The charge preparation department is equipped with hoppers of sufficient capacity to keep the sintering machines operating steadily for eight to ten hours. Specified quantities of each of the components of the charge are discharged from the charge hoppers by batch feeders to be placed on a collecting conveyor which transfers the charge to the primary mixing drum and then to the charge hoppers of the sintering machine in the sintering department. Before being charged on the sintering machine, the charge undergoes secondary mixing, moistening, and partial pelletizing in pelletizing drums.
Upon arrival at the discharge end of the sintering machine, the sintering cake is crushed and sorted to remove fines (returns), which are recycled to the charger. The sinter cake is then cooled and sorted. The waste gases are suctioned off through a gas duct and gas scrubber by an exhaust fan and are vented to the atmosphere via the smokestack.
Sintering machines are the basic process equipment in the sintering process. Conveyor-type sintering machines featuring an endless train of grate-bottomed sinter buggies (pallets) in motion are widely used. The buggy passes under the feeder, which lays down a bed of charge of 250–400 mm on the pallet and then passes under the ignition furnace, where the solid fuel contained in the surface zone of the sinter bed is ignited. The exhaust fan draws air downward through the bed (80–100 m3/min per square meter of sintering area); the combustion zone (15–20 mm) progresses downward through the bed at a speed of 20–40 mm/min. Much of the charge melts at temperatures of 1200–1500°C, in the combustion zone of the solid fuel. As the combustion zone progresses downward, the semimolten mass in the upper portion of the bed cools to form sinter cake. Gases emanating from the combustion zone dry out and heat the lower portions of the sinter bed, from which hygroscopic and hydrate water, carbon dioxide gas, and other volatiles are driven off, as well as sulfur, arsenic, and other harmful impurities. The world’s largest sintering machines, with 312 sq m of sintering area and a bed width of 4 m, are operated in the USSR, with an hourly output of 1–2 tons per square meter of sintering area, and a yearly output of 2–3 million tons of sinter.
The worldwide production of iron-ore sinter amounts to roughly 330 million tons (1967), of which the USSR accounts for 128 million tons (1968).
REFERENCESBazilevich, S. V., and E. F. Vegman. Aglomeratsiia. 1967.
Spravochnik aglomeratchika. Kiev, 1964.
Patkovskii, A. B. Aglomeratsionnye fabriki chernoi metallurgii. Moscow, 1954.
S. V. BAZILEVICH, E. F. VEGMAN, and A. G. MIKHALEVICH
in technology, a process for obtaining solid and porous materials and items from fine powdery or pulverized materials at high temperatures. The physicochemical properties and structure of materials are also frequently altered by sintering. Sintering is used in, for example, agglomeration, coking, the preparation of poorly caking coals for coking, and the production of ceramics and refractory items. It is one of the technological processes in powder metallurgy.
The welding together and growth of contact area between two or more initially distinct particles at temperatures below the melting point, but above one-half of the melting point in kelvins. Since the sintering rate is greater with smaller than with larger particles, the process is most important with powders, as in powder metallurgy and in firing of ceramic oxides.
Although sintering does occur in loose powders, it is greatly enhanced by compacting the powder, and most commercial sintering is done on compacts. Compacting is generally done at room temperature, and the resulting compact is subsequently sintered at elevated temperature without application of pressure. For special applications, the powders may be compacted at elevated temperatures and therefore simultaneously pressed and sintered. This is called hot pressing or sintering under pressure.
Certain compacts from a mixture of different component powders may be sintered under conditions where a limited amount of liquid, generally less than 25 vol%, is formed at the sintering temperature. This is called liquid-phase sintering, important in certain powder-metallurgy and ceramic applications. See Ceramics, Powder metallurgy