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Metallurgical Uses Fluxes for Metallurgy

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Metallurgical Uses Fluxes for Metallurgy METALLURGICAL USES FLUXES FOR METALLURGY HAROLD R. KOKAL AND MADHU G. RANADE A metallurgicalf2uxis a substance that is added to combine with some acidic components are sometimes added. In nonferrous pro- gangue (unwanted minerals) during ore smelting, with impurities in cesses slags and fluxes are acidic with silica as the primary com- a molten metal, or with other additives in metal refining processes ponent, and although the use of basic flux such as limestone or lime to form a slag that can be separated from the metal. Because slags is not extensive, some amount is used to modify slag properties and are immiscible with the metallic melt and are of lower density, a some refining slags are lime based. separation of the slag and metal occurs if the viscosity and surface The production of iron is accomplished by smelting of ores, tension are of the proper values. The chemistries of slags are pellets, and sinter in blast furnaces with subsequent refining of adjusted to provide the proper melting point, viscosity, surface molten iron and scrap in oxygen-blown processes. The electric tension, conductivity, specific heat, density, or chemical properties furnace is used to make steel, stainless steels, ferroalloys, and to effect the desired task. In addition to absorbing impurities from special alloys, whereas ferromanganese is often made in blast fur- the metal, the purposes of the slag are to thermally insulate the metal naces. Fluxes are used in all of these processes, and either limestone bath, protect the molten metal from the atmosphere, and control the or lime is the major flux component with some dolomite, dolomitic chemical potential of the system. Several excellent references on lime, silica, alumina, and fluorspar being used. In ironmaking, the slags and metallurgical fluxes are available (Boynton, 1980, Lank- flux is added either by direct charging or through sinter and fluxed ford, Jr., et al., 1985, Turkdogan, 1983, Rosenqvist, 1974, Fine and pellets. In steel refining by the basic oxygen process, flux is added Gaskell, 1984). as lime and dolomitic lime that are either charged as lumps or The function of the slag might vary at different stages within injected as fines. Some small amount of limestone is sometimes a process prior to final melting. For example, the slag composition used. Fluorspar is often added as lumps or mixed with other fine and behavior will change as materials descend in the iron blast materials in briquettes. Alumina is sometimes added to blast fur- furnace, or as they melt in the early stages of formation of steel- naces either by direct charging of sized lumps or through sinter. making slags. Selection of the chemistry for a slag might be in- Typical acid fluxes are sand, gravel, quartz rock, used silica fluenced by factors outside the primary function as when blast brick, or raw siliceous ore (Lankford, Jr., et al., 1985). Olivine, furnace slags are used to make cement, rock wool insulation, or which is a magnesium-silicate mineral, has been added to iron blast fertilizer supplements. Sometimes slags contain a sufficient amount furnaces to enhance removal of alkalis (Lankford, Jr., et al., 1985). of valuable recoverable elements to be sold as raw materials for Alumina in the form of bauxite, aluminiferous clay, and recycled other processes. alumina brick has been used as flux. Alumina can function am- Fluxes are often referred to as acid, basic, or neutral. Acid photerically as either an acid or base, for example it can form fluxes are those that generally form acids in water and bases are aluminum silicate in high silica slags or calcium aluminate in those that would generally form bases in water. Typical acid fluxes lime-bearing slags (Lankford, Jr., et al., 1985). are silica, alumina, and phosphorus, although alumina can function Limestone (calcium carbonate) and dolomite (magnesium-cal- as either an acid or base. Typical basic fluxes are lime and magnesia. cium carbonate), or fluxstones as they are sometimes called (Boyn- Fluorspar, or calcium fluoride, is a neutral substance because it can ton, 1980), and their calcined forms, lime and dolomitic lime are be viewed as the reaction product of a base and an acid. The degree the major basic fluxes. In the latter cases, the carbonates are de- of acidity (ratio of acids to bases) or basicity (ratio of bases to acids) composed in a kiln-type process to drive off carbon dioxide that is often specified to characterize the slag chemistry for a particular might otherwise interfere with the subsequent smelting or refining system. The system might also be referred to as acid or basic process or require expensive forms of energy to effect the decom- depending on the choice of slag, for example, basic steelmaking position (e.g., by combustion of coke in the iron blast furnace). uses slags with more bases (lime and magnesia) than acids (silica Limestone is considerably less expensive than lime because it is not and alumina). calcined; however, lime is far more difficult to handle and reacts Many forms of minerals and compounds have been used as readily with water. Dolomite and dolomitic lime are used for their fluxes depending on the process requirements, availability, costs, magnesia content. requirements for recycling of intermediate products, and environ- Fluorspar is a neutral flux often used as an additive in steel- mental concerns. Because slags are most often mixtures of oxides making slags to improve fluidity, but it has also been used in and silicates, fluxes are usually oxides, carbonates (which decom- combination with lime as a primary slag in electroslag refining pose to oxides) and silicates. Slags comprising phosphates, borates, (Duckworth and Hoyle, 1969). Fluorspar is available in several sulfides, carbides, or halides have also been used (Rosenqvist, 1974, grades, with the lowest grade often used for steelmaking. When the Moore, 1981, Szekely et al., 1989). In ironmaking and steelmaking, price of fluorspar has risen, substitutes including aluminum smelting slags are basic, and the primary flux added is lime-bearing, although dross, borax, manganese ore, titania, iron oxides, silica sand, and 662 INDUSTRIAL MINERALS AND ROCKS calcium chloride have been used (Peters, 1982). Briquettes con- producers owned limestone and dolomite quarries, but most have taining various combinations of low grade manganese ore fines, mill sold the operations and now recycle slags or purchase flux as needed. scale, precipitator dust, recycled slag, and fine fluorspar have been Because limestone is very readily available around the world, used as an addition to the steelmaking process to form a fluid slag. a suitable source can almost always be found. Long distance trans- Some flux materials are added to repair smelting and refining portation of stone is not common due to the low value and wide- vessel refractory linings. The selection of the flux to be added spread occurrence. However, higher valued lime might be trans- depends on the type of process and type of refractory. In the basic ported greater distances. In some special cases, due to the purity oxygen process for steel, lime from the flux combines with silica required, flux might be moved long distances. For example, low formed by oxidation of silicon from the metal to form a slag. The impurity flux has been transported from Japan to Australia to steelmaking vessel is lined with basic magnesia brick that will be supplement local sources for use in fluxed pellets (Murray, 1988, attacked by the slag, so common practice includes addition of 5 to personal communication). 10% magnesia (magnesium oxide) to help prevent attack on the For many years in the United States, limestone and dolomite refractory in the vessel (Boynton, 1980, Lankford, Jr., et al., 1985). were transported to the steel producing centers for use in blast In copper smelting, an acid slag practice is used, and silica is added furnaces, lime plants, and sinter plants. The sinter plants were often for repair of refractories in some parts of the smelting furnaces located at or near steel mills to take advantage of the availability (Anderson, 1961, Fowler, 196 1). of a variety of iron-bearing raw materials, coke fines, and slag. The slag formed in one operation can also serve as a flux in Recently, some major iron producers have incorporated flux into another. Slags formed in refining operations are often recycled to pellets, and the fluxed pellets are transported to iron producing a smelting operation to provide valuable flux materials and also to centers. recover elements that otherwise might be lost. For example, steel- The trend towards the use of fluxed pellets in blast furnaces has making slags are recycled to the blast furnace smelter to recover required a change in the transportation and distribution patterns of unreacted, pre-calcined lime, iron, and manganese oxides. The limestone and dolomite. In the United States, fluxed pellets are not limitation on the recycle of steelmaking slag to the iron blast furnace made near the steel mills, but instead are made at the mines in is typically the impurity content, particularly the sulfur, phosphorus, Minnesota and Michigan that are located farther from the steel mills and zinc. In copper smelting, converter slags can be recycled to than the limestone and dolomite quarries. Therefore, the limestone recover copper and flux. The amount recycled is affected by the and dolomite must first be transported to the mines before being minor elements such as bismuth, antimony, and arsenic. returned to the steel mills in the form of fluxed pellets. Fluxed Since about 1980, ironmakers and steelmakers have been using pellets made in Minnesota with flux from Michigan are transported synthetic slags for external treatment of hot metal from the blast as far as Alabama and Utah.
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