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ELECTROMETALLURGY
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ELECTROMETALLURGY . The See also:present See also:article, as explained under See also:ELECTROCHEMISTRY, treats only of those processes in which See also:electricity is applied to the See also:production of chemical re-actions or molecular changes at See also:furnace temperatures. In many of these the application of See also:heat is necessary to bring the substances used into the liquid See also:state for the purpose of See also:electrolysis, aqueous solutions being unsuitable. Among the earliest experiments in this See also:branch of the subject were those of See also:Sir H. See also:Davy, who in 1807 (Phil. Trans., i8o8, p. I), produced the See also:alkali metals by passing an intense cur-See also:rent of electricity from a See also:platinum See also:wire to a platinum dish, through a See also:mass of fused See also:caustic alkali. The See also:action was started in the See also:cold, the alkali being slightly moistened to render it a conductor; then, as the current passed, heat was produced and the alkali fused, the See also:metal being deposited in the liquid See also:condition. Later, A. Matthiessen (Quarterly Journ. Chem. See also:Soc. viii. 30) obtained See also:potassium by the electrolysis of a mixture of potassium and See also:calcium chlorides fused over a See also:lamp. There are here foreshadowed two types of electrolytic furnace-operations: (a) those in which See also:external See also:heating maintains the electrolyte in the fused condition, and (b) those in which a current-See also:density is applied sufficiently high to develop the heat necessary to effect this See also:object unaided. Much of the earlier electrometallurgical See also:work was done with furnaces of the (a) type, while nearly all the later developments have been with those of class (b). There is a third class of operations, exemplified by the manufacture of calcium See also:carbide, in which electricity is employed solely as a heating See also:agent; these are termed electrothermal, as distinguished .from electrolytic. In certain electrothermal processes (e.g. calcium carbide production) the heat from the current is employed in raising mixtures of substances to the temperature at which a desired chemical reaction will take See also:place between them, while in others (e.g. the production of See also:graphite from See also:coke or See also:gas-See also:carbon) the heat is applied solely to the production of molecular or See also:physical changes: In See also:ordinary electrolytic work only the continuous current may of course
be used, but in electrothermal work an alternating current is equally available.
Electric Furnaces.—Independently of the question of the application of external heating, the furnaces used in electrometallurgy may be broadly classified into (i.) arc furnaces, in which the intense heat of the electric arc is utilized, and (ii.) resistance and incandescence furnaces, in which the heat is generated by an electric current overcoming the resistance of an inferior conductor.
Excepting such experimental arrangements as that of C. M.
Despretz (C. R., 1849, 29) for use on a small See also:scale in the laboratory,
Pichou in See also:France and J. H. See also: In these arrangements,
which were similar if not identical, the furnace See also:charge was
crushed to a See also:fine See also:powder and passed through two or more electric
arcs in See also:succession. When used for Ore smelting, the reduced
metal and the accompanying slag were to be caught, after leaving the arc and while still liquid, in a See also:hearth fired with ordinary See also:fuel. Although this See also:primitive furnace could be made to See also:act, its efficiency was See also:low, and the use of a See also:separate See also:fire was disadvantageous. In 1878 Sir See also: At once the attractive force of the solenoid on the iron cylinder was automatically reduced, and the falling of the latter caused the negative carbon to rise, starting an arc between it and the metal in the crucible. A counterpoise was, placed on the solenoid end of the balance beam to act against the attraction of the solenoid, the position of the counterpoise determining the length of the arc in the crucible. Any See also:change in the resistance of the arc, either by lengthening, due to the sinking of the charge in the crucible, or by the burning of the carbon, affected the See also:pro-portion of current flowing in the two shunt circuits, and so altered' the position of the iron cylinder in the solenoid that the length of arc was, within limits, automatically regulated. Were it not for the use of some such device the arc would be liable to See also:constant fluctuation and to frequent extinction. The crucible was surrounded with a See also:bad conductor of heat to minimize loss by See also:radiation. The positive carbon was in some cases replaced by a See also:water-cooled metal See also:tube, or See also:ferrule, closed, of course, at the end inserted in the crucible. Several modifications were proposed, in one of which, intended for the heating of non-conducting substances, the electrodes were passed horizontally through perforations in the upper See also:part of the crucible walls, and the charge in the See also:lower part of the_crucible was heated by radiation. The furnace used by See also:Henri See also:Moissan in his experiments on reactions at high temperatures, on the See also:fusion and volatilization of refractory materials, and on the formation of carbides, silicides and borides of various metals, consisted, in its simplest form, of two superposed blocks of See also:lime or of See also:limestone with a central cavity cut in the lower See also:block, and with a corresponding but much shallower inverted cavity in the upper block, which thus formed the lid of the furnace. See also:Horizontal channels were cut on opposite walls, through which the carbon poles or electrodes were passed into the upper part of the cavity. Such a furnace, to take a current of 4 H.P. (say, of 6o amperes and 5o volts), measured externally about 6 by 6 by 7 in., and the electrodes were about o•4 in. in See also:diameter, while for a current of too H.P. (say, of 746 amperes and too volts) it measured about 14 by 12 by 14 in., and the electrodes were about 1.5 in. in diameter. In the latter See also:case the crucible, which was placed in the cavity immediately beneath the arc, was about 3 in. in diameter (internally), and about 32 in. in height. The fact that See also:energy is being used at so high a See also:rate as too H.P. on so small a charge of material sufficiently indicates that the furnace is only used for experimental work, or for the fusion of metals which, like See also:tungsten or See also:chromium, can only be melted at temperatures attainable by See also:electrical means. Moissan succeeded in fusing about lb of either of these metals in 5 or 6 minutes in a furnace similar to that last described. He also arranged an experimental tube-furnace by passing a carbon tube horizontally beneath the arc
' Cf. Siemens's See also:account of the use of this furnace for experimental purposes in See also:British Association See also:Report for 882.
Are furnaces.
in the cavity of the lime blocks. When prolonged heating is required at very high temperatures it is found necessary to See also:line the furnace-cavity with alternate layers of See also:magnesia and carbon, taking care that the lamina next to the lime is of magnesia; if this were not done the lime in contact with the carbon crucible would form calcium carbide and would slag down, but magnesia does not yield a carbide in this way. Chaplet has patented a muffle or tube furnace, similar in principle, for use on a larger scale, with a number of electrodes placed above and below the muffle-tube. The arc furnaces now widely used in the manufacture of calcium carbide on a large scale are chiefly developments of the Siemens furnace. But whereas, from its construction, the Siemens furnace was intermittent in operation, necessitating stoppage of the current while the contents of the crucible were poured out, many of the newer forms are specially designed either to minimize the See also:time required in effecting the withdrawal of one charge and the introduction of the next, or to ensure See also:absolute continuity of action, raw material being constantly charged in at the See also:top and the finished substance and by-products (slag, &c.) withdrawn either continuously or at intervals, as sufficient quantity shall have accumulated. In the See also: In the See also:United States a revolving furnace is used which is quite continuous in action. The class of furnaces heated by electrically incandescent materials has been divided by Borchers into two See also:groups: (1) those in which the substance is heated by contact fncan" with a substance offering a high resistance to the descence furnace& current passing through it, and (2) those in which the substance to be heated itself affords the resistance to the passage of the current whereby electric energy is converted into heat. Practically the first of these furnaces was that of Despretz, in which the mixture to be heated was placed in a carbon tube rendered incandescent by the passage of a current through its substance from end to end. In 188o W. Borchers introduced his resistance-furnace, which, in one sense, is the converse of the Despretz apparatus. A thin carbon See also:pencil, forming a See also:bridge between two stout carbon rods, is set in the midst of the mixture to be heated. On passing a current through the carbon the small rod is heated to incandescence, and imparts heat to the surrounding mass. On a larger scale several pencils are used to make the connexions between carbon blocks which form the end walls of the furnace, while the side walls are of fire-See also:brick laid upon one another without See also:mortar. Many of the furnaces now in constant use depend mainly on this principle, a core of granular carbon fragments stamped together in the See also:direct line between the electrodes, as in Acheson's See also:carborundum furnace, being substituted for the carbon pencils. In other cases carbon fragments are mixed throughout the charge, as in E.H. and A.H. Cowles's See also:zinc-smelting See also:retort. In practice, in these furnaces, it is possible for small See also:local arcs to be temporarily set up by the shifting of the charge, and these would contribute to the heating of the mass. In the remaining class of furnace, in which the electrical resistance of the charge itself is utilized, are the continuous-current furnaces, such as are used for the smelting of See also:aluminium, and those alternating-current furnaces, (e.g. for the production of calcium carbide) in which a portion of the charge is first actually fused, and then maintained in the molten condition by the current passing through it, while the reaction between further portions of the charge is proceeding. For ordinary metallurgical work the electric furnace, requiring as it does (excepting where waterfalls or other cheap See also:sources of See also:power are available) the intervention of the See also:boiler uses and and See also:steam-See also:engine, or of the gas or oil engine, with a advan- See also:tages. consequent loss of energy, has not usually proved so economical as an ordinary direct fired furnace. But in some cases in which the current is used for electrolysis and for the production of extremely high temperatures, for which the calorific intensity of ordinary fuel is insufficient, the electric furnace is employed with See also:advantage. The temperature of the electric furnace, whether of the arc or incandescence type, ispractically limited to that at which the least easily vaporized material available for electrodes is converted into vapour. This material is carbon, and as its vaporizing point is (estimated at) over 35000 C., and less than 4000° C., the temperature of the electric furnace cannot rise much above 35000 C. (63300 F.); but H. Moissan showed that at this temperature the most See also:stable of See also:mineral combinations are dissociated, and the most refractory elements are converted into vapour, only certain borides, silicides and metallic carbides having been found to resist the action of the heat. It is not necessary that all electric furnaces shall be run at these high temperatures; obviously, those of the incandescence or resistance type may be worked at any convenient temperature below the maximum. The electric furnace has several advantages as compared with some of the ordinary types of furnace, arising from the fact that the heat is generated from within the mass of material operated upon, and (unlike the blast-furnace, which presents the same advantage) without a large See also:volume of gaseous products of See also:combustion and atmospheric See also:nitrogen being passed through it. In ordinary reverberatory and other heating furnaces the burning fuel is without the mass, so that the See also:vessel containing the charge, and other parts of the plant, are raised to a higher temperature than would otherwise be necessary, in See also:order to compensate for losses by radiation, convection and See also:conduction. This advantage is especially observed in some cases in which the charge of the furnace is liable to attack the containing vessel at high temperatures, as it is often possible to maintain the See also:outer walls of the electric furnace relatively cool, and even to keep them lined with a protecting crust of unfused charge. Again, the construction of electric furnaces may often be exceedingly crude and See also:simple; in the carborundum furnace, for example, the outer walls are of loosely piled bricks, and in one type of furnace the charge is simply heaped on the ground around the carbon resistance used for heating, without containing-walls of any See also:kind. There is, however, one (not insuperable) See also:drawback in the use of the electric furnace for the smelting of pure metals. Ordinarily carbon is used as the electrode material, but when carbon comes in contact at high temperatures with any metal that is capable of forming a carbide a certain amount of See also:combination between them is inevitable, and the carbon thus introduced impairs the See also:mechanical properties of the ultimate metallic product. Aluminium, iron, platinum and many other metals may thus take up so much carbon as to become brittle and unforgeable. It is for this See also:reason that Siemens, Borchers and others substituted a hollow water-cooled metal block for the carbon See also:cathode upon which the melted metal rests while in the furnace. Liquid metal coming in contact with such a See also:surface forms a crust of solidified metal over it, and this crust thickens up to a certain point, namely, until the heat from within the furnace just overbalances that lost by conduction through the solidified crust and the cathode material to the flowing water. In such an arrangement, after the first instant, the melted metal in the furnace does not come in contact with the cathode material. Electrothermal Processes.—In these processes the electric current is used solely to generate heat, either to induce chemical reactions between admixed substances, or to produce a physical (allotropic) modification of a given substance. Borchers predicted that, at the high temperatures available with the electric furnace, every See also:oxide would prove to be reducible by the action of carbon, and this prediction has in most instances been justified. Alumina and lime, for example, which cannot be reduced at ordinary furnace temperatures, readily give up their See also:oxygen to carbon in the electric furnace, and then combine with an excess of carbon to form metallic carbides. In 1885 the See also:brothers Cowles patented a See also:process for the electrothermal reduction of oxidized ores by exposure to an intense current of electricity when admixed with carbon in a retort. Later in that See also:year they patented a process for the reduction of aluminium by carbon, and in 1886 an electric furnace with sliding carbon rods passed through the end walls to the centre of a rectangular furnace. The impossibility of working with just sufficient carbon to reduce the alumina, without using any excess which would be See also:free to 234 form at least so much carbide as would suffice, when diffused through the metal, to render it brittle, practically restricts the use of such processes to the production of aluminium Atumin- See also:alloys. Aluminium See also:bronze (aluminium and See also:copper) 'um alloys, and ferro-aluminium (aluminium and iron) have been made in this way; the latter is the more satisfactory product, because a certain proportion of carbon is expected in an alloy of this See also:character, as in ferromanganese and See also:cast iron, and its presence is not objectionable. The furnace is built of fire-brick, and may measure (internally) 5 ft. in length by 1 ft. 8 in. in width, and 3 ft. in height. Into each end See also:wall is built a See also:short iron tube sloping downwards towards the centre, and through this is passed a bundle of five 3-in. carbon rods, See also:bound together at the outer end by being cast into a See also:head of cast iron for use with iron alloys, or of cast copper for aluminium bronze. This head slides freely in the cast iron tubes, and is connected by a copper rod with one of the terminals of the dynamo supplying the current. The carbons can thus, by the application of suitable mechanism, be withdrawn from or plunged into the furnace at will. In starting the furnace, the bottom is prepared by ramming it with See also:charcoal-powder that has been soaked in See also:milk of lime and dried, so that each particle is coated with a film of lime, which serves to reduce the loss of current by conduction through the lining when the furnace becomes hot. A See also:sheet iron case is then placed within the furnace, and the space between it and the walls rammed with limed charcoal; the interior is filled with fragments of the iron or copper to be alloyed, mixed with alumina and coarse charcoal, broken pieces of carbon being placed in position to connect the electrodes. The iron case is then removed, the whole is covered with charcoal, and a cast iron cover with a central flue is placed above all. The current, either continuous or alternating, is then started, and continued for about 1 to Iz See also:hours, until the operation is See also:complete, the carbon rods being gradually withdrawn as the action proceeds. In such a furnace a continuous current, for example, of 3000 amperes, at 5o to 6o volts, may be used at first, increasing to 5000 amperes in about See also:half an See also:hour. The reduction is not due to electrolysis, but to the action of carbon on alumina, a part of the carbon in the charge being consumed and evolved as carbon monoxide gas, which See also:burns at the orifice in the cover so See also:long as reduction is taking place. The reduced aluminium alloys itself immediately with the fused globules of metal in its midst, and as the charge becomes reduced the globules of alloy unite until, in the end, they are run out of the tap-hole after the current has been diverted to another furnace. It was found in practice (in 1889) that the See also:expenditure of energy per See also:pound of reduced aluminium was about 23 H.P.-hours, a number considerably in excess of that required at the present time for the production of pure aluminium by the electrolytic process described in the article ALUMINIUM. Calcium carbide, graphite (q.v.), See also:phosphorus (q.v.) and carborundum (q.v.) are now extensively manufactured by the operations outlined above. 'Electrolytic Processes.—The See also:isolation of the metals See also:sodium and potassium by Sir See also:Humphry Davy in 1807 by the electrolysis of the fused hydroxides was one of the earliest applications of the electric current to the extraction of metals. This pioneering work showed little development until about the See also:middle of the 19th See also:century. In 1852 See also:magnesium was isolated electrolytically by R. See also:Bunsen, and this process subsequently received much See also:attention at the hands of Moissan and Borchers. Two years later Bunsen and H. E. Sainte Claire Deville working independently obtained aluminium (q.v.) by the electrolysis of the fused See also:double sodium aluminium chloride. Since that date other processes have been devised and the electrolytic processes have entirely replaced the older methods of reduction with sodium. Methods have also been discovered for the electrolytic manufacture of calcium (q.v.), which have had the effect of converting a laboratory curiosity into a product of commercial importance. See also:Barium and See also:strontium have also been produced by electrometallurgical methods, but the processes have only a laboratory See also:interest at present. See also:Lead, zinc and other metals have also been reduced in this manner. For further See also:information the following books, in addition to those mentioned at the end of the article LECTROCHEMISTRY, may be consulted : Borchers, Handbuch der Elektrochemie; Electric Furnaces (Eng. trans. by H. G. See also:Solomon, 1908) ; Moissan, The Electric Furnace (19o4); J. Escard, Fours electriques (1905); See also:Les See also:Industries electrochimiques (1907). (W. G. Additional information and CommentsThere are no comments yet for this article.
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