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AND See also:STEEL . See also:Hearth furnaces are employed in certain metallurgical operations, e.g. in the See also:air-reduction See also:process for smelting See also:lead ores. The principle is essentially that of the Catalan forge. Such furnaces are very wasteful, and have little to recommend them (see Schnabel, Metallurgy, 1905, vol. i. p. 409). Reverberatory Furnaces.—Blast furnaces are, from the intimate contact between the See also:burden to be smelted and the See also:fuel, the least wasteful of See also:heat; but their use supposes the possibility of obtaining fuel of See also:good quality and See also:free from See also:sulphur or other substances likely to deteriorate the See also:metal produced. In all cases, therefore, where it is desired to do the See also:work out of contact with the solid fuel, the operation of burning or heat-producing must be per-formed in a See also:special See also:fire-See also:place or See also:combustion chamber, the See also:body of See also:flame and heated See also:gas being afterwards made to See also:act upon the See also:surface of the material exposed in a broad thin layer in the working See also:bed or laboratory of the See also:furnace by reverberation from the See also:low vaulted roof covering the bed. Such furnacesareknown by the See also:general name of reverberatory or reverbatory furnaces, also as air or See also:wind furnaces, to distinguish them from those worked with compressed air or blast. Originally the See also:term See also:cupola was used for the reverberatory furnace, but in the course of See also:time it has changed its meaning, e and is now given to a small blast furnace such as that used by FIG. 4.-Reverberatory Furnace (See also:elevation at flue end). See also:iron-founders--reverberatory smelting furnaces in the same furnaces are connected with the same See also:chimney stack, the damper See also:trade being called air furnaces. takes the See also:form of a sliding See also:plate in the mouth of the connecting flue, See also:Figs. 2, 3 and 4 represent a reverberatory furnace such as is used so that the See also:draught in one may be modified without affecting the for the See also:fusion of See also:copper ores for See also:regulus, and may be taken as gener- others. The fire See also:bridge is partially protected against the intense MNNftb;v.,/lam P/ damper G, which may be raised or lowered by a See also:chain reaching to the ground, and serves for regulating the See also:speed of the exhaust gases, and thereby the draught of air through the fire. Where several heat of the body of flame issuing through the fire See also:arch by a passage to which the air has free See also:access. The material to be melted is introduced into the furnace from the hoppers HH through the charging holes in the roof. When melted the products See also:separate on the bed (which is made of closely packed See also:sand or other infusible substances), according to their See also:density; the lighter earthy matters forming an upper layer of slag are See also:drawn out by the slag hole K at the flue end into an iron See also:wagon or See also:bogie, while the metal subsides to the bottom of the bed, and at the termination of the operation is run out by the tap hole L into moulds or granulated into See also:water. The opposite opening M is the working See also:door, through which the See also:tool for stirring the See also:charge is introduced. It is covered by a plate suspended to a See also:lever, similar to that seen in the end elevation (fig. 4) in front of the slag hole. According to the purposes to which they are applied, reverberatory furnaces may be classed into two See also:groups, namely, fusion or melting furnaces, and calcining or wasting furnaces, also called calciners. The former have a very extended application in many branches of See also:industry, being used by both founders and smelters in the fusion of metals; in the concentration of poor metallic compounds by fusion into regulus; in the reduction of lead and See also:tin ores; for refining copper and See also:silver; and for making malleable iron by the puddling processes and See also:welding. Calcining furnaces have a less extended application, being chiefly employed in the See also:conversion of metallic sulphides into oxides by continued exposure to the See also:action of air at a temperature far below that of fusion, or into chlorides by roasting with See also:common See also:salt. As some of these substances (for example, lead sulphide and copper See also:pyrites) are readily fusible when first heated, but become more refractory as See also:part of the sulphur is dissipated and See also:oxygen takes its place, it is important that the heat should be very carefully regulated at first, otherwise the See also:mass may become clotted or fritted together, and the oxidizing effect of the air soon ceases unless the fritted masses be broken small again. This is generally done by making the bed of the furnace very See also:long in proportion to its breadth and to the fire-See also:grate See also:area, which may be the more easily done as a not inconsiderable amount of heat is given out during the oxidation of the ore—such increased length being often obtained by placing two or even three working beds one above the other, and allowing the flame to pass over them in See also:order from below upwards. Such calciners are used especially in roasting See also:zinc See also:blende into zinc See also:oxide, and in the conversion of copper sulphides into chlorides in the wet extraction process. In some processes of lead-smelting, where the minerals treated contain sand, the long calciner is provided with a melting bottom See also:close to the fire-place, so that the desulphurized ore leaves the furnace as a glassy slag or silicate, which is subsequently reduced to the metallic See also:state by fusion with fluxes in blast furnaces. Reverberatory furnaces See also:play an important part in the manufacture of See also:sodium carbonate; descriptions and illustrations are given in the See also:article See also:ALKALI MANUFACTURE.
Muffle, Crucible and See also:Retort Furnaces.—A third class of furnaces is so arranged that the work is done by indirect See also:heating; that is, the material under treatment, whether subjected to calcination, fusion or any other process, is not brought in contact either with fuel or flame, but is raised to the proper temperature by exposure in a chamber heated externally by the products of combustion. These are known as muffle or chamber furnaces; and by supposing the crucibles or retorts to represent similar See also:chambers of only temporary duration, the See also:ordinary pot melting air furnaces, and those for the reduction of zinc ores or the manufacture of See also:coal gas, may be included in the same See also:category. These are almost invariably air furnaces, though sometimes air under pressure is used, as, for example, in the combustion of small anthracitic coal, where a current of air from a See also:fan-blower is sometimes blown under the grate to promote combustion. Types of muffle furnaces are figured in the article See also:ANNEALING, HARDENING AND TEMPERING.
Furnace Materials.—The materials used in the construction of furnaces are divisible into two classes, namely, ordinary and refractory or fire-resisting. The former are used principally as casing, walls, pillars or other supporting parts of the structure, and includes ordinary red or yellow bricks, See also:clay-See also:slate, See also:granite and most See also:building stones; the latter are reserved for the partsimmediately in contact with the fuel and flame, such as the lining of the fire-place, the See also:arches, roof and flues, the See also:lower part if not the whole of the chimney lining in reverberatory furnaces, and the whole of the See also:internal walls of blast furnaces. Among such substances are fireclay and firebricks, certain sandstones, See also:silica in the form of ganister, and Dinas See also: In most cases, however, the latter tendency is guarded against, in making up the See also:paste for moulding, by adding to the fresh clay a certain proportion of burnt material of the same See also:kind, such as old bricks or potsherds, ground to a coarse See also:powder. Coke dust or graphite is used for the same purpose in crucible making (see See also:FIREBRICK). The most highly valued fireclays are derived from the Coal See also:Measures. Among the See also:chief localities are the neighbourhood of See also:Stourbridge in See also:Worcestershire and Stannington near See also:Sheffield, which See also:supply most of the materials for crucibles used in steel and See also:brass melting, and the pots for See also:glass houses; See also:Newcastle-on-See also:Tyne and Glenboig near See also:Glasgow, where heavy blast furnace and other firebricks, gas retorts, &c., are made in large quantities. Coarse-grained but very strong firebricks are also made of the See also:waste of See also:china clay See also:works. In See also:Belgium the clay raised at Andenne is very largely used for making retorts for zinc furnaces. The See also:principal See also:French fireclays are derived from the See also:Tertiary strata in the See also:south, and more nearly resemble See also:porcelain clays than those of the Coal Measures. They give wares of remarkably See also:fine texture and surface, combined with high refractory See also:character. In See also:Germany, Ips and See also:Passau on the See also:Danube, and See also:Gross Almerode in See also:Hesse, are the best known localities producing fireclay goods, the crucibles from the last-mentioned place, known as See also:Hessian crucibles, going all over the See also:world. These, though not showing a See also:great resistance to extreme heat, are very slightly affected by sudden alternations in heating, as they may be plunged See also:cold into a strongly heated furnace without cracking, a treatment to which French and See also:Stour-bridge pots cannot be subjected with safety. See also:Plumbago or graphite is largely used in the See also:production of crucibles, not in the pure state but in admixture with fireclay; the proportion of the former varies with the quality from 25 to nearly 5o %. These are the most enduring of all crucibles, the best lasting out 70 or 8o meltings in brass foundries, about 5o with See also:bronze, and 8 to ro in steel-melting. Silica is used in furnace-building in the forms of sand, ganister, a finely ground See also:sandstone from the Coal Measures of See also:Yorkshire, and the analogous substance known as Dinas clay, which is really nearly pure silica, containing at most about 22 % of bases. Dinas clay is found at various places in the Vale of See also:Neath in South See also:Wales, in the form of a loose disintegrated sandstone, which is crushed between rollers, mixed with about r % of lime, and moulded into bricks that are fired in kilns at a very high temperature. These bricks are specially used for the roof, fire arches, and other parts subjected to intense heat in reverberatory steel-melting furnaces, and, although infusible under ordinary conditions, are often fairly melted by the heat without fluxing or corrosion after a certain amount of exposure. Ganister, a slightly plastic siliceous sand, is similarly used for the lining of See also:Bessemer steel converters; it is found in the neighbourhood of Sheffield. Alumina as a refractory material is chiefly used in the form of See also:bauxite, but its applications are somewhat special. It has been found to stand well for the linings of rotatory puddling furnaces, where, under long-continued heating, it changes into a substance as hard and infusible as natural See also:emery. In the See also:angle of repose of the charge, which is introduced at the upper end, and is pushed down the slope by fresh material, when necessary, in the contrary direction to the flame which enters at the lower end. Gerstenhofer's pyrites burner is a furnace of this class. It has a tall See also:vertical chamber heated from below, and traversed by numerous narrow See also:horizontal See also:cross bars at different heights. The ore in fine powder is fed in at the See also:top, through a hopper, in a See also:regular thin stream, by a pair of rollers, and in falling lodges on the fiats of the bars, forming a See also:talus upon each of the height corresponding to the angle of See also:rest of the material, which is, however, at See also:short intervals removed to lower levels by the arrival of fresh ore from above. In this way a very large surface is exposed to the heat, and the ore, if containing sufficient sulphur to maintain the combustion, is perfectly burned when it arrives at the bottom; if, however, it is imperfectly sized or See also:damp, or if it contains much earthy See also:matter, the result is not very satisfactory. There are many other furnaces in which the same principle is utilized. 2. See also:Mechanical stirrers constitute a second See also:division of mechanical furnaces, in which the labour of rabbling or stirring the charges is performed by combinations of levers and See also:wheel-work taking See also:motion from a rotating See also:shaft, and more or less perfectly imitating the action of See also:hand labour. They are almost entirely confined to puddling furnaces. 3. Revolving furnaces, the third and most important division of mechanical furnaces, are of two kinds. The first of these resemble an ordinary reverberatory furnace by having a See also:flat bed which, however, has the form of a circular disk mounted on a central shaft, and receives a slow See also:movement of rotation from a water-wheel or other motor, so that every part of the surface is brought successively under the action of the fire, the charge being stirred and ultimately removed by passing under a See also:series of fixed scraper arms placed above the surface at various points. See also:Brunton's calciner, used in the " burning " of the, pyritic minerals associated with tin ore, is a See also:familiar example of this type. The hearth may either rotate on an inclined See also:axis, so that the path of its surface is oblique to that of the flame, or the working part may be a hollow See also:cylinder, between the fireplace and flue, with its axis horizontal or nearly so, whose inner surface represents the working bed, mounted upon See also:friction rollers, and receiving motion from a special See also:steam-See also:engine by means of a central See also:belt of See also:spur gearing. Furnaces of the second kind were first used in alkali works for the conversion of sulphate into carbonate of sodium in the process known as See also:black ash fusion, but have since been applied to other processes. As calciners they are used in tin mines and for the chlorination of silver ores. Mechanical furnaces are figured in the article ALKALI MANUFACTURE. Use of Heated Air.—The calorific intensity of fuel is found to be very considerably enhanced, if the combustion be effected with air previously heated to any temperature between that of boiling water and a dull red heat, the same effect being observed both with solid and gaseous fuel. The latter, especially when brought to the burning point at a high temperature, produces a heat that can be resisted by the most refractory substances only, such as silica, alumina and magnesia. This is attained in the regenerative furnace of See also:Siemens, detailed See also:consideration of which belongs more properly to the subject of iron.
See also:Economy of Waste Heat.—In every See also:system of artificial heating, the amount of heat usefully applied is but a small proportion of that See also:developed by combustion. Even under the rnost advantageous application, that of evaporation of water in a steam See also:boiler where the gases of the fire have to travel through a great length of flues bounded by thin iron surfaces of great heat-absorbing capacity, the temperature of the current at the chimney is generally much above that required to maintain an active draught in the fireplace; and other tubes containing water, often in considerable See also:numbers, forming the so-called fuel economizers, may often be interposed between the boiler and the chimney with marked See also:advantage as regards saving of fuel. In reverberatory and air furnaces used in the different operations of iron manufacture, where an extremely high temperature has to be maintained in spaces of comparatively small extent, such as the beds of puddling, welding and steel-melting furnaces, the temperature of the exhaust gases is exceedingly high, and if allowed to pass directly into the chimney they appear as a great body of flame at the top. It is now general to See also:save a portion of this heat by passing the flame through flues of steam boilers, air-heating apparatus, or both—so that the steam required for the necessary operations of the forge and heated blast for the furnace itself may be obtained without further See also:expenditure of fuel. The most perfect method of utilizing the waste heat hitherto applied is that of the Siemens re-; generator, in which the spent gases are made to travel through chambers, known as regenerators or recuperators of heat, containing a quantity of thin firebricks piled into a cellular mass so as to offer a very large heat-absorbing surface, whereby their temperature is very considerably reduced, and they arrive tat the chimney at a heat not exceeding 300 or 400 degrees. As soon as the bricks have become red hot, the current is diverted to an adjacent chamber or pair of chambers, and the acquired heat is removed by a current of cool
of which can be classified under three heads of gravitating furnaces, gas or air passing towards the furnace, where it arrives at a See also:tern-mechanical stirrers and revolving furnaces. perature sufficiently high to ensure the greatest possible heating r. to gravitating. furnaces the bed is laid at a slope just within the ( effect in combustion.
See also:Paris See also:Exhibition of 1878 bricks very hard and dense in character, said to be of pure alumina, were exhibited by See also: The former may be used as a bed for melting platinum in the same way as lime or magnesia, without affecting the quality of the metal. Ferric oxide, though not strictly infusible, is largely used as a protecting lining for furnaces in which malleable iron is made, a portion of the ore being reduced and recovered in the process. In an oxidizing See also:atmosphere it is indifferent to silica, and therefore siliceous bricks containing a considerable proportion of ferric oxide, when used in flues of boilers, brewers' coppers, &c. and similar situations, are perfectly fire-resisting so long as the heated gas contains a large proportion of unconsumed air. The red firebricks known as See also:Windsor bricks, which are practically similar in See also:composition to soft red sandstone, are of this character. The electric furnace has led to the See also:discovery of several important materials, which have been employed as furnace linings. See also:Carborundum (q.v.) was applied by Engels in 1899, firebricks being washed with,carborundum paste and then baked. Siloxicon, a See also:compound of carbon, See also:silicon and oxygen, formed from carbon and silica in the electric furnace, was patented by E. G. Acheson in 1903. It is very refractory, and is applied by mixing with water and some See also:bond, such as sodium silicate or gas-See also:tar. An amorphous, soft silicon See also:carbide, also formed in the electric furnace, was patented by B. See also:Talbot in 1899. For basic linings, magnesia crystallized in the electric furnace is being extensively used, replacing See also:dolomite to some extent (see E. Kilburn See also:Scott, " Refractory Materials for Furnace Linings," See also:Faraday See also:Soc., Igo6, p. 289). Furnace Construction.—In the construction of furnaces See also:provision has to be made for the unequal expansion of the different parts under the effect of heat. This is especially necessary in the See also:case of reverberatory furnaces, which are essentially weak structures, and therefore require to be See also:bound together by complicated systems of tie rods and uprights or See also:buck staves. The latter are very commonly made of old flat bottom rails, laid with the flat of the flange against the See also:wall. Puddling furnaces are usually entirely cased with iron plates, and blast furnaces with hoops See also:round each course of the stack, or in those of thinner constructions the firebrick work is entirely enclosed in a wrought iron casing or jacket. Such parts as may be subjected to extreme heat and the fretting action of molten material, as the tuyere and slag breasts of blast furnaces, and the fire See also:bridges and bed plates of reverberatory furnaces, are often made in See also:cast iron with See also:double walls, a current of water or air being kept circulating through the intermediate space. In this way the metal, owing to its high conductivity and low specific heat as compared to that of water, is kept at a temperature far below its melting point if the water is renewed quickly enough. It is of course necessary in such cases that the circulation shall be perfectly free, in order to prevent the See also:accumulation of steam under pressure in the interior of the casting. This method has received considerable See also:extension, notably in furnace-smelting of iron ores containing See also:manganese, where the entire hearth is often completely water-cased, and in some lead furnaces where no firebrick lining is used, the lower part of the furnace stack being a See also:mere double iron See also:box cooled by water sufficiently to keep a coating of slag adhering to the inner See also:shell which prevents the metal from being acted upon. Mechanical Furnaces.—The introduction and withdrawal of the charges in fusion furnaces is effected by See also:gravitation, the solid masses of raw ore, fuel and See also:flux being thrown in at the top, and flowing out of the furnace at the taphole or slag run at the bottom. Vertical kilns, such as those used for burning See also:limestone, are worked in a similar manner—the raw stone going in at the top, and the burnt product falling through holes in the bottom when allowed to do so. With reverberatory calciners, however, where the work is done upon a horizontal bed, a considerable amount of hand labour is expended in raking out the charge when finished, and in See also:drawing slags from fusion furnaces; and more particularly in the puddling process of refining iron the amount of See also:manual exertion required is very much greater. To diminish the See also:item of expenditure on this See also:head, various kinds of mechanical furnaces have been adopted, all In iron-smelting blast furnaces the waste gases are of considerable fuel value, and may render important services if properly applied. Owing to the conditions of the work, which require the See also:maintenance of a sensibly reducing atmosphere, they contain a very notable proportion of carbonic oxide, and are drawn off by large wrought iron tubes near the top of the furnace and conveyed by See also:branch pipes to the different boilers and air-heating apparatus, which are now entirely heated by the combustion of such gases, or mixed with air and exploded in gas engines. Formerly they were allowed to See also:burn to waste at the mouth of a short chimney place above the furnace top, forming a huge body of flame, which was one of the most striking features of the Black See also:Country landscape at See also:night. Laboratory and Portable Furnaces.—Small air-furnaces with hot plates or sand See also:bath flues were formerly much employed in chemical laboratories, as well as small blast furnaces for crucibles heated with See also:charcoal or coke. The use of such furnaces has very considerably diminished, owing to the general introduction of coal-gas for heating purposes in laboratories, which has been rendered possible by the invention of the See also:Bunsen burner, in which the mixture of air and gas giving the least luminous but most powerfully heating flame is effected automatically by the effluent gas. These burners, or modifications of them, have also been applied to muffle furnaces, which are convenient when only a few assays have to be made—the furnace being a mere clay shell and soon brought to a working temperature; but the fuel is toe expensive to allow of their being used habitually or on a large See also:scale. See also:Petroleum, or rather the heavy See also:oils obtained in tar refineries, having an equal or See also:superior heating power to coal-gas, may also be used in laboratories for producing high temperatures. The oil is introduced in a thin stream upon a series of inclined and channelled bars, where it is almost immediately volatilized and burnt by air flowing in through parallel orifices. Furnaces of this kind may be used for melting cast iron or bronze in small quantities, and were employed by H. Sainte Claire Devine in experiments in the metallurgy of the platinum See also:group of metals. Sefstrom's blast furnace, used in See also:Sweden for the assay of iron ores, is a convenient form of portable furnace applied to melting in crucibles. It consists of a See also:sheet-iron cylinder about 8 or 9 in. in See also:diameter, within which is fixed one of smaller See also:size lined with fire-clay. The space between the two cylinders serves as a heater and distributor for the blast, which is introduced through the nozzle at the bottom, and enters the furnace through a series of several small tuyeres arranged round the inner lining. Charcoal is the fuel used, and the crucibles stand upon the bottom of the clay lining. When a large body of fuel is required, the cylinder can be lengthened by an iron hoop which fits over the top See also:ring. Deville's portable blast furnace is very similar in principle to the above, but the body of the furnace is formed of a single cast iron cylinder lined with fireclay, closed below by a cast iron plate perforated by a ring of small holes—a hemispherical See also:basin below forming the air-heating chamber. Additional information and CommentsThere are no comments yet for this article.
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