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Encyclopedia Britannica - Main :: ALM-ANC |
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ALUMINIUM (symbol Al; atomic weight 27.0) , a metallic chemical element. Although never met with in the free state, aluminium is very widely distributed in combination, principally as silicates. The word is derived from the Lat. alumen (see ALUM), and is probably akin to the Gr. Ors (the root of salt, halogen, &c.). In 1722 F. Hoffmann announced the base of alum to be an individual substance; L.B. Guyton de Morveau suggested that this base should be called alumine, after Sel alumineux, the French name for alum; and about 182o the word was changed into alumina. In 176o the French chemist, T. Baron de Henouville, unsuccessfully attempted " to reduce the base of alum " to a metal, and shortly afterwards various other investigators essayed the problem in vain. In 18o8 Sir Humphry Davy, fresh from the electrolytic isolation of potassium and sodium, attempted to decompose alumina by heating it with potash in a platinum crucible and submitting the mixture to a current of electricity; in 1809, with a more powerful battery, he raised iron wire to a red heat in contact with alumina, and obtained distinct evidence of the production of an iron-aluminium alloy. Naming the new metal in anticipation of its actual birth, he called it alumium; but for the sake of analogy he was soon persuaded to change the word to aluminum, in which form, alternately with aluminium, it occurs in chemical literature for some thirty years. In the year 1824,endeavouring to prepare it by chemical means, H. C. Oersted heated its chloride with potassium amalgam, and failed in his object simply by reason of the mercury, so that when F. Wohler repeated the experiment at t onpar. Gottingen in 1827, employing potassium alone as the reducing agent, he obtained it in the metallic state for the first time. Contaminated as it was with potassium and with platinum from the crucible, the metal formed a grey powder and was far from pure; but in 1845 he improved his process and succeeded in producing metallic globules wherewith he examined its chief Ammonium Alum. Caesium Alum. Potash Alum. Rubidium Alum. tC. too parts water tC too parts water tC. too parts water tC loo parts water dissolve. dissolve. dissolve. dissolve. 0 2.62 0 0.19 0 3.9 0 0.71 to 4'5 to 0.29 to 9.52 to I09 50 15.9 50 1.235 5o 44II 50 4.98 8o 35'2 8o 5.29 8o 134'47 8o 21.6o too 70'83 too 357.48 _ Poggiale C. Setterberg Poggiale C. Setterberg Ann. Chico. phys. Ann. 1882, 211, p. 104. [3] 8, p 467. properties, and prepared several compounds hitherto unknown. Early in 1854, H. St Claire Deville, accidentally and in ignorance of WShler's later results, imitated the 1845 experiment. At once observing the reduction of the chloride, he realized the importance of his discovery and immediately began to study the commercial production of the metal. His attention was at first divided between two processesthe chemical method of reducing the chloride with potassium, and an electrolytic method of decomposing it with a carbon anode and a platinum cathode, which was simultaneously imagined by himself and R. Bunsen. Both schemes appeared practically impossible; potassium cost about X17 per lb, gave a very small yield and was dangerous to manipulate, while on the other hand, the only source of electric current then available was the primary battery, and zinc as a store of industrial energy was utterly out of the question. Deville accordingly returned to pure chemistry and invented a practicable method of preparing sodium which, having a lower atomic weight than potassium, reduced a larger proportion. He next devised a plan for manufacturing pure alumina from the natural ores, and finally elaborated a process and plant which held the field for almost thirty years. Only the discovery of dynamo-electric machines and their application to metallurgical processes rendered it possible for E. H. and A. H. Cowles to remove the industry from the hands of chemists, till the time when P. T. L. Hermit and C. M. Hall
The chief natural compounds of aluminium are four in number: oxide, hydroxide (hydrated oxide), silicate and fluoride. Corun- a. dum, the only important native oxide (Al203), occurs in large deposits in southern India and the United States. Although it contains a higher percentage of metal (52.9 %) than any other natural compound, it is not at present employed as an ore, not only because it is so hard as to be crushed with difficulty, but also because its very hardness makes it valuable as an abrasive. Cryolite (AlF3.5NaF) is a double fluoride of aluminium and sodium, which is scarcely known except on the west coast of Greenland. Formerly it was used for the preparation of the metal, but the inaccessibility of its source, and the fact that it is not sufficiently pure to be employed without some preliminary treatment, caused it to be abandoned in favour of other salts. When required in the Heroult- Hall
Kaolin
Kaolin
Before the application of electricity, only two compounds were found. suitable for reduction to the metallic state. Alumina itself is so refractory that it cannot be melted save by the oxyhydrogen blowpipe or the electric arc, and except in the molten state it is not susceptible of decomposition by any chemical reagent. Deville first selected the chloride as his raw material, but observing it to be volatile and extremely deliquescent, he soon substituted in its place a double chloride of aluminium and sodium. Early in 1855 John Percy suggested that cryolite should be more convenient, as it was a natural mineral and might not require purification, and at the end of March in that year, Faraday exhibited before the Royal Institution samples of the metal reduced from its fluoride by Dick and Smith. H. Rose also carried out experiments on the decomposition of cryolite, and expressed an opinion that it was the beet of all compounds for reduction; but, finding the yield of metal to be low, receiving a report of the difficulties experienced in mining the ore, and fearing to cripple his new industry by basing it upon the employment of a mineral of such uncertain supply, Deville decided to keep to his chlorides. With the advent of the dynamo, the position of affairs was wholly changed. The first successful idea of using electricity depended on the enormous heating powers of the arc. The infusibility of alumina was no longer prohibitive, for the molten oxide is easily reduced by carbon. Nevertheless, it was found impracticable to smelt alumina electrically except in presence of copper, so that the Cowles furnace yielded, not the pure metal, but an alloy. So long as the metal was principally regarded as a necessary ingredient of aluminium-bronze, the Cowles process was popular, but when the advantages of aluminium itself became more apparent, there arose a fresh demand for some chief method of obtaining it unalloyed. It was soon discovered that the faculty of inducing dissociation possessed by the current might now be utilized with some hope of pecuniary success, but as electrolytic currents are of lower voltage than those required in electric furnaces, molten alumina again became impossible. Many metals, of which copper, silver and nickel are types, can be readily won or purified by the electrolysis of aqueous solutions, and theoretically it may be feasible to treat aluminium in an identical manner. In practice, however, it cannot be thrown down electrolytically with a dissimilar anode so as to win the metal, and certain difficulties are still met with in the analogous operation of plating by means of a similar anode. Of the simple compounds, only the fluoride is amenable to electrolysis in the fused state, since the chloride begins to volatilize below its melting-point, and the latter is only 5 below its boiling-point. Cryolite is not. a safe body to electrolyse, because the minimum voltage needed to break up the aluminium fluoride is 4.0, whereas the sodium fluoride requires only 4.7 volts; if, therefore; the current rises in tension, the alkali is reduced, and the final product consists of an alloy with sodium. The corresponding double chloride is a far better material; first, because it melts at about i8o C., and does not volatilize below a red heat, and second, because the voltage of aluminium chloride is 2.3 and that of sodium chloride 4-3, so that there is a. much wider margin of safety to cover irregularities in the electric pressure. It has been found, however, that molten cryolite and the analogous double fluoride represented by the formula Al2Fs.2NaF are very efficient solvents of alumina, and that these solutions can be easily electrolysed at about 800 C. by means of a current that completely decomposes the oxide but leaves the haloid salts unaffected. Molten cryolite dissolves roughly 30 % of its weight of pure alumina, so that when ready for treatment the solution contains about the same proportion of what may be termed " available " aluminium as does the fused double chloride of aluminium and sodium. The advantages lie with the oxide because of its easier preparation. Alumina dissolves readily enough in aqueous hydrochloric acid to yield a solution of the chloride, but neither this solution, nor that containing sodium chloride, can be evaporated to dryness without decomposition: To obtain the anhydrous single or double chloride, alumina must be ignited with carbon in a current of chlorine, and to exclude iron from the finished metal, either the alumina must be pure or the chloride be submitted to purification. This preparation of, a chlorine compound suited for electrolysis becomes more costly and more troublesome than that of the oxide, and in addition four times as much raw material must be handled. At different times propositions have been made.to win the metal from its sulphide. This compound possesses a heat of formation so much lower that electrically it needs but a voltage of o9 to decompose it, and it is easily soluble in the fused sulphides of the alkali metals. It can also be reduced metallurgically by the action of molten iron. Various considerations, however, tend to show that there cannot be so much advantage in employing it as would appear at first sight. As it is easier to reduce than any other compound, so it is more difficult to produce. Therefore while less energy is absorbed in its final reduction, more is needed in its initial preparation, and it is questionable whether the economy possible in the second stage would not be neutralized by the greater cost of the first stage in the whole operation of winning the metal from bauxite with the sulphide as the intermediary. The Deville process as gradually elaborated between 1855 and 1859 exhibited three distinct phases:Production of metallic Chemical sodium, formation of the pure double chloride of sodium reduction. and aluminium,and preparation of the metal by the inter- action of the two former substances. To produce the alkali metal, a calcined mixture of sodium carbonate, coal and chalk was strongly ignited in flat retorts made of boiler-plate; the sodium distilled over into condensers and was. preserved under heavy petroleum. In order to prepare pure alumina, bauxite and sodium carbonate were heated in a furnace until the reaction was complete; the product was then extracted with water to dissolve the sodium aluminate, the solution treated with carbon dioxide, and the precipitate removed and dried. This purified oxide, mixed with sodium chloride and coal tar, was carbonized at a red heat, and ignited in a current of dry chlorine as long as vapours of the double chloride were given off, these being condensed in suitable chambers. For the production of the final aluminium, Too parts of the chloride and 45 parts of cryolite to serve as a flux were powdered together and mixed with 35 parts of sodium cut into small pieces. The whole was thrown in several portions on to the hearth of a furnace previously heated to low redness and was stirred at intervals for three hours. At length when the furnace was tapped a white slag was drawn off from the top, and the liquid metal beneath was received into a ladle and poured into cast-iron moulds. The process was worked out by Deville in his laboratory at the Ecole Normale in Paris. Early in 18$5 he conducted large-scale experiments at javel in a factory lent him for the purpose, where he produced sufficient to show at the French Exhibition of 1855. In the spring of 1856 a complete plant was erected at La Glaciere, a suburb of Paris, but becoming a nuisance to the neighbours, it was removed to Nanterre in the following year. Later it was again transferred to Salindres, where the manufacture was continued by Messrs. Pechiney till the advent of the present electrolytic process rendered it no longer profitable. When Deville quitted the Javel works, two brothers C. and A. Tissier, formerly his assistants, who had devised an improved sodium furnace and had acquired a thorough knowledge of their leader's experiments, also left, and erected a factory at Amfreville, near Rouen, to work the cryolite process. It consisted simply in reducing cryolite with metallic sodium exactly as in Deville's chloride method, and it was claimed to possess various mythical advantages over its rival. Two grave disadvantages were soon obviousthe limited supply of ore, and, what was even more serious, the large proportion of silicon in the reduced metal. The Amfreville works existed some eight or ten years, but achieved no permanent prosperity. In 1858 or 1859 a small factory, the first in England, was built by F. W. Gerhard at Battersea, who also employed cryolite, made his own sodium, and was able to sell the product at 3S. gd. per oz. This enterprise 1. asonly lasted about four years. Between 186o and 1874 Messrs Bell Brothers manufactured the metal at Washington, near Newcastle, under Deville's supervision, producing nearly 2 cwt. per year. They took part in the International Exhibition of 1862, quoting a price of 40S. per lb troy. In 1881 J. Webster patented an improved process for making alumina, and the following year he organized the Aluminium Crown Metal Co. of Hollywood to exploit it in conjunction with Deville's method of reduction. Potash-alum and pitch were calcined together, and the mass was treated with hydrochloric acid; charcoal and water to form a paste were next added, and the whole was dried and ignited in a current of air and steam. The residue, consisting of alumina and potassium sulphate, was leached with water to separate the insoluble matter which was dried as usual. All the by-products, potassium sulphate, sulphur and aluminate of iron, were capable of recovery, and were claimed to reduce the cost of the oxide materially. From this alumina the double chloride was prepared in essentially the same manner as practised at Salindres, but sundry economies accrued in the process, owing to the larger scale of working and to the adoption of W. Weldon's method of regenerating the spent chlorine liquors. In 1886 H. Y. Castner's sodium patents appeared, and The Aluminium Co. of Oldbury was promoted to combine the advantages of Webster's alumina and Castner's sodium. Castner had long been interested in aluminium, and was desirous of lowering its price. Seeing that sodium was the only possible reducing agent, he set himself to cheapen its cost, and deliberately rejecting sodium carbonate for the more ex-pensive sodium hydroxide (caustic soda), and replacing carbon by a mixture of iron and carbonthe so-called carbide of ironhe invented the highly scientific method of winning the alkali metal which has remained in existence almost to the present day. In 187 2 sodium prepared by Deville's process cost about 4S: per lb, the greater part of the expense being due to the constant failure of the retorts; in 1887 Castner's sodium cost less than Is. per lb, for his cast-iron pots survived 125 distillations. In the same year L. Grabau patented a method of reducing the simple fluoride of aluminium with sodium, and his process was operated at Trotha in Germany. It was distinguished by the unusual purity of the metal obtained, some of his samples containing 99.5 to 99.8 %. In 1888 the Alliance Aluminium Co., organized to work certain patents for winning the metal from cryolite by means of sodium, erected plant in London, Hebburn and Wall
About 1879 dynamos began to be introduced into metallurgical practice, and from that date onwards numerous schemes for utilizing this cheaper source of energy were brought eeatdca before the public. The first electrical method worthy reduction. of notice is that patented by E. H. and A. H. Cowles in 1885, which was worked both at Lockport, New York
TI the necessary proportion, and melted in crucibles to give merchantable bronzes containing between r; and ro % of aluminium; Although the copper took no part in the reaction, its employment was found indispensable, as otherwise the aluminium partly volatilized, and partly combined with the carbon to form a carbide. It was also necessary to give the fine charcoal a thin coating of calcium oxide by soaking it in lime-water, for the temperature was so high that unless it was thus protected it was gradually converted into graphite, losing its insulating power and diffusing the current through the lining and walls of the furnace. That this process did not depend upon electrolysis, but was simply an instance of electrical smelting or the decomposition of an oxide by means of carbon at the temperature of the electric arc, is shown by the fact that the Cowles furnace would work with an alternating current. In 1883 R. Cratzel patented a useless electrolytic process with fused cryolite or the double chloride as the raw material, and in 1886 Dr E. Kleiner propounded a cryolite method which was worked for a time by the Aluminium Syndicate at Tyldesley near Manchester, but was abandoned in 1890. In 1887 A. Minet took out patents for electrolysing a mixture of sodium chloride with aluminium fluoride, or with natural or artificial cryolite. The operation was continuous, the metal being regularly run off from the bottom of the bath, while fresh alumina and flouride were added as required. The process exhibited several disadvantages, the electrolyte had to be kept constant in composition lest either fluorine vapours should be evolved or sodium throwh down, and the raw materials had accordingly to be prepared in a pure state. After prolonged experiments in a factory owned by Messrs Bernard
The original Deville process for obtaining pure alumina from bauxite was greatly simplified in 1889 by K. T. Bayer, whose improved process is exploited at Larne in Ireland and at Gardanne in France. New works on the same process have recently been erected near Marseilles. Crude bauxite is ground, lightly calcined to destroy organic matter, and agitated under a pressure of 70 or 8o lb per sq. in. with a solution of sodium hydroxide having the specific gravity 1.45. After two or three hours the liquid is .diluted till its density falls to 1.23, when it is passed through filter-presses to remove the insoluble ferric oxide and silica. The solution of sodium aluminate, containing aluminium oxide and sodium oxide in the molecular proportion of '6 to 1, is next agitated for thirty-six hours with a small quantity of hydrated alumina previously obtained, which causes the liquor to decompose, and some 70 % of the aluminium hydroxide to be thrown down. The filtrate, now containing roughly two molecules of alumina to one of soda, is concentrated to the Original gravity of 1.45, and employed instead of fresh caustic for the attack of more bauxite; the precipitate is then collected, washed till free from soda, dried and ignited at about r000 C. to convert it into a crystalline oxide which is less hygroscopic than the former amorphous variety. The process of manufacture which now remains to be described was patented during 1886 and 1887 in the name of C: M. Hall in America, in that of P. T. L. Heroult in England and France. It would be idle to discuss to whom the credit of first imagining the method rightfully belongs, for probably this is only one of the many occasions when new ideas have been born in several brains at the same time. By r888 Hall was at work on a commercial scale at Pittsburg, reducing German alumina; in 1891 the plant was removed to New Kensington for economy in fuel, and was gradually enlarged to 1500 h.p.; in 1894 a factory driven by water was erected at Niagara Falls, and subsequently works were established at Shawenegan in Canada and at Massena in the United States. In 1890 also the Hall process operated by steam power was installed at Patricroft, Lancashire, where the plant had a capacity of 300 lb per day, but by 1894 the turbines of the Swiss ' and French works ruined the enterprise.' About 1897 the Bernard
Messrs Pechiney, the machinery soon being increased, and there, under the control of a firm that has been concerned in the industry almost from its inception, aluminium is being manufactured by the Hall process on a large scale. In July 1888 the SocieteMetallurgique Suisse erected plant driven by a 500 h.p. turbine to carry out Heroult's alloy process, and at the end of that year the Allgemeine Elektricitats Gesellschaft united with the Swiss firm in organizing the Aluminium Industrie Actien Gesellschaft of Neuhasen, which has factories in Switzerland, Germany and Austria.. The Societe Elecirometallurgique Francaise, started under the direction of Heroult in 1888 for the production of aluminium . in France, began operations on a small scale at Froges in Isere; but soon after large works were erected in Savoy at La Praz, near Modane, and in 1905. another large factory was started in Savoy at St Michel. In 1895 the British Aluminium Company was founded to mine bauxite and manufacture alumina in Ireland, to prepare the necessary electrodes at Greenock, to reduce the aluminium by the aid of water-power at the Falls of Foyers, and to refine and work up the metal into marketable shapes at the old Milton factory of the Cowles Syndicate, re-modelled to suit modern requirements. In 1905 this company began works for the utilization of another water-power at Loch Lever}. In .1907 a new company, The Aluminium Corporation , was started in England to carry out' the production of the metal by the Heroult process, and new factories were constructed near Conway' in North Wales and at Wallsend-on-Tyne, quite close to where, twenty. years before, the Alliance Aluminium Co. had their works.The Heroult cell consists of a square iron or steel box lined with carbon rammed and baked into a solid mass; at the bottom is a' cast-iron plate connected with the negative pole of the dynamo, but the actual working cathode is undoubtedly the layer of already reduced and molten metal that lies in the bath. The anode is formed of a bundle of carbon rods suspended from overhead so as to be capable of vertical adjustment. The cell is filled up with cryolite, and the current is turned on till this is melted; then the pure powdered alumina is fed in continuously as long as the operation proceeds. The current is supplied at a tension of 3 to 5 volts per cell, passing through ro or 12 in series; and it performs two distinct functions:(1) it overcomes the chemical affinity of the aluminium oxide, (2) it overcomes the resistance of the electrolyte, heating the liquid at the same time. As a part of the voltage is consumed in the latter duty, only the residue can be converted into chemical work, and as the theoretical voltage of the aluminium fluoride in the cryolite is 4.0, provided the bath is kept properly supplied with alumina, the fluorides are not attacked. It follows, therefore, except for mechanical losses, that one charge of cryolite lasts indefinitely, that the sodium and other impurities in it are not liable to contaminate the product, and that only the alumina itself need be carefully purified. 'The operation is essentially a dissociation of alumina into aluminium, which collects at the cathode, and into oxygen, which combines with the anodes to form carbon monoxide, the latter escaping and being burnt to carbon dioxide outside. Theoretically 36 parts by weight of carbon are oxidized in the production of 54 parts of aluminium; practically the anodes waste at the same rate at which metal is deposited. The current density is about 700 amperes per sq. ft. of cathode surface, and the number of rods in the anode is such that each delivers 6 or 7 amperes per sq. in. of cross-sectional area. The working temperature lies between 750 and 85n C., and the actual yield is r lb of metal per 12 e.h.p. hours. The bath is heated internally with the current rather than by means of external fuel; because this arrangement permits the vessel itself to be kept comparatively cool; if it were fired from without, it would be hotter than the electrolyte, and no material suitable for the construction of the cell is competent to withstand the attack of nascent aluminium at high temperatures. Aluminium is so light that it is a matter requiring some ingenuity to select a convenient solvent through which it shall sink quickly, for if it does not sink, it short-circuits the electrolyte. The molten metal has a specific gravity of 2.54, that of molten cryolite saturated with alumina is 2.35, and that of the fluoride Al2F6 2NaF saturated with alumina 1.97. The latter therefore appears the better material, and was originally preferred by Hall; cryolite, however, dissolves more alumina, and has been finally adopted by both inventors. Aluminium is a white metal with a characteristic tint which most nearly resembles that of tin; when impure, or after pro- longed exposure to air, it has a slight violet shade. Its Properties. atomic weight is 27 (2 6.77, 77, 11=1, according to J. Thomsen). It is trivalent. The specific gravity of cast metal is 2.583, and of rolled 2.688 at 4 C. It melts at 626 C. (freezing-point 6 54.5, Heycock and Neville). It is the third most malleable and sixth most ductile metal, yielding sheets 0.000025 in. in thickness, and wires 0.004 in. in diameter. When quite pure it is somewhat harder than tin, and its hardness is considerably increased by rolling. It is not magnetic. It stands near the positive end of the list
1 Elastic Limit, Ultimate Reduction tons tons per sq. in. per sqstrength, of Area % sq. in. Cast . 3 7 15 Sheet . 52 11 35 Bars 6i 12 40 Wire '. 7-13 13-29 60 Weight for weight, therefore, aluminium is only exceeded in tensile strength by the best cast steel, and its own alloy, aluminium bronze. An absolutely clean surface becomes tarnished in damp
The highly electro-positive character of aluminium is most important. At elevated temperatures the metal decomposes nearly all other metallic oxides, wherefore it is most serviceable as a metallurgical reagent. In the casting of iron, steel and brass, the addition of a trifling proportion (0.005 %) removes oxide and renders the molten metal more fluid, causing thefinished products to be more homogeneous, free from blow-holes and solid all through. On the other hand, its electro-positive nature necessitates some care in its utilization. If it be exposed to damp
The metal produces an enormous number of useful alloys, some of which, containing only r or 2 % of other metals, combine the lightness of aluminium itself with far greater hardness Alloys. and strength. Some with 90 to 99 % of other metals exhibit the general properties of those metals conspicuously improved. Among the heavy alloys, the aluminium bronzes (Cu, 90-97.5 %; Al, 10-2.5 %) occupy the most important position, showing mean tensile strengths increasing from 20 to 41 tons per sq. in. as the percentage of aluminium rises, and all strongly resisting corrosion in air or sea-water. The light copper alloys, in which the proportions just given are practically reversed, are of considerably less utility, for although they are fairly strong, they lack power to resist galvanic action. This subject is far from being exhausted, and it is not improbable that the alloy-producing capacity of aluminium may eventually prove its most valuable characteristic. In the meantime, ternary light alloys appear the most satisfactory, and tungsten and copper, or tungsten and nickel, seem to be the best substances to add. The uses of aluminium are too numerous to mention. Probably the widest field is still in the purification of iron and steel. To the general public it appeals most strongly as a material for constructing cooking utensils. It is not brittle Uses. like porcelain and cast iron, not poisonous like lead-glazed earthenware and untinned copper, needs no enamel to chip off, does not rust and wear out like cheap tin-plate, and weighs but a fraction of other substances. It is largely replacing brass and copper in all departments of industryespecially where dead weight has to be moved about, and lightness is synonymous with economyfor instance, in bed-plates for torpedo-boat engines, internal fittings for ships instead of wood, complete boats for portage, motor-car parts and boiling-pans for cone fectionery and in chemical works. The British Admiralty employ it to save weight in the Navy, and the war-offices of the European powers equip their soldiers with it wherever possible. As a substitute for Solenhofen stone it is used in a modified form of lithography, which can be performed on rotary printing-machines at a high speed. With the increasing price of copper, it is coming into yogue as an electrical conductor for uncovered mains; it is found that an aluminium wire 0.126 in. in diameter will carry as much current as a copper wire o Too in. in diameter, while the former weighs about 79 lb and the latter 162 lb per mile. Assuming the materials to be of equal tensile strength per unit of areahard-drawn copper is stronger, but has a lower conductivitythe adoption of aluminium thus leads to a reduction of, 52 % in the weight, a gain of 6o% in the strength, and an increase of 26 % in the diameter of the conductor. Bare aluminium strip has recently been tried for winding-coils in electrical machines, the oxide of the metal acting as insulators between the layers. When the price of aluminium is less than double the price of copper aluminium is cheaper than copper per unit of electric current conveyed; but when insulation is necessary, the smaller size of the copper wire renders it more economical. Aluminium conductors have been employed on heavy work in many places, and for telegraphy and telephony they are in frequent demand and give perfect satisfaction. Difficulties were at first encountered in making the necessary joints, but these have been overcome by practice and experience. Two points connected with this metal are of sufficient moment to demand a few words by way of conclusion. Its extraordinary lightness forms its chief claim to general adoption, yet is apt to cause mistakes when its price is mentioned. It is the weight of a mass of metal which governs its financial value; its industrial value, in the vast majority of cases, depends on the volume of that mass. Provided it be rigid, the bed-plate of an engine
Compounds of Aluminium. Aluminium oxide or alumina, AI203, occurs in nature as the mineral corundum (q.v.), notable for its hardness and abrasive power (see EMERY), and in well-crystallized forms it constitutes, when coloured by various metallic oxides, the gem-stones, sapphire, oriental topaz, oriental amethyst and oriental emerald. Alumina is obtained as a white amorphous powder by heating aluminium hydroxide. This powder, provided that it has not been too strongly ignited, is soluble in strong acids; by ignition it becomes denser and nearly as hard as corundum; it fuses in the oxyhydrogen flame or electric arc, and on cooling it assumes a crystalline form closely resembling the mineral species. Crystallized alumina is also obtained by heating the fluoride with boron trioxide; by fusing aluminium phosphate with sodium sulphate; by heating alumina to a dull redness in hydrochloric acid gas under pressure; and by heating alumina with lead oxide to a bright red heat. These reactions are of special interest
Aluminium Hydrates.Several hydrated forms of aluminium oxide are known. Of these hydrargillite or gibbsite, Al(OH)3, diaspore, A1O(OH), and bauxite, Al2O(OH)4, occur in the mineral kingdom. Aluminium hydrate, Al(OH)3, is obtained as a gelatinous white precipitate, soluble in potassium or sodium hydrate, but insoluble in ammonium chloride, by adding ammonia to a cold solution of an aluminium salt; from boiling solutions the precipitate is opaque. By drying at ordinary temperatures, the hydrate AI(OH)3H20 is obtained; at 300 this yields AlO(OH), which on ignition gives alumina, Al203: Precipitated aluminium hydrate finds considerable application in dyeing. Soluble modifications were obtained by Walter Crum (Journ. Chem. Soc., 1854, vi. 216), and Thomas Graham (Phil. Trans., 1861, p. 163); the first named decomposing aluminium acetate (from lead acetate and aluminium sulphate) with boiling water, the latter dialysing a solution of the basic chloride (obtained by dissolving the hydroxide in a solution of the normal chloride) Both these soluble hydrates are readily coagulated by traces of a salt, acid or alkali; Crum's hydrate does not combine with dye-stuffs, neither is it soluble in excess of acid, while Graham's compound readily forms lakes, and readily dissolves when coagulated in acids. In addition to behaving as a basic oxide, aluminium oxide (or hydrate) behaves as an acid oxide towards the strong bases with the formation of aluminates. Potassium aluminate, K2Al204, is obtained in solution by dissolving aluminium hydrate in caustic potash; it is also obtained, as crystals containing three molecules of water, by fusing alumina with potash, exhausting with water, and crystallizing the solution in vacuo. Sodium aluminate is obtained in the manufacture of alumina; it is used as a mordant in dyeing, and has other commercial applications. Other aluminates (in particular, of iron and magnesium), are of frequent occurrence in the mineral kingdom, e.g. spinel, gahnite, &c. Salts of Aluminium.Aluminium forms one series of salts, derived from the trioxide, Al203. These exhibit, in certain cases, marked crystallographical and other analogies with the corresponding salts of chromium and ferric iron. Aluminium fluoride, AlF3, obtained by dissolving the metal in hydrofluoric acid, and subliming the residue in a current of hydrogen, forms transparent,. very obtuse rhombohedra, which are insoluble in water. It forms a series of double fluorides, the most important of which is cryolite (q.v.); this mineral has been applied to the commercial preparation. of the metal (see above). Aluminium chloride, AiCla, was first prepared by Oersted, who heated a mixture of carbon and alumina in a current of chlorine, a method subsequently improved by Wohler, Bunsen, Deville and others. A purer product is obtained by heating aluminium turnings in a current of dry chlorine, when the chloride distils over. So obtained, it is a white crystalline solid, which slowly sublimes just below its melting point (r94). Its vapour density at temperatures above 750 corresponds to the formula AICI3; below this point the molecules are associated. It is very hygroscopic, absorbing water with the evolution of hydrochloric acid. It combines with ammonia to form AlC13.3NH3; and forms double compounds with phosphorus peutachloride, phosphorus oxychloride, selenium and tellurium chlorides, as well as with many metallic chlorides; sodium aluminium chloride, AlC13NaCl, is used in the production of the metal. As a synthetical agent in organic chemistry, aluminium chloride has rendered possible more reactions than any other substance; here we can only mention the classic syntheses of benzene homologues. Aluminium bromide, AIBr3, is prepared in the same manner as the chloride. It. forms colourless crystals, melting at 90, and boiling at 265-270. Aluminium iodide, A113, results from the interaction of iodine and aluminium. It forms colourless crystals, melting at 185, and boiling at 36o. Aluminium sulphide, Al2S3, results from the direct union of the metal with sulphur, or when carbon disulphide vapour is passed over strongly heated alumina. It forms a yellow fusible mass, which is decomposed by water into alumina and sulphuretted hydrogen. Aluminium sulphate Al(SO4)3, occurs in the mineral kingdom as keramohalite, Al2(SO4)3.18H20, found near volcanoes and in alum-shale; aluminite or websterite is a basic salt, Al2(SO4)(OH)4.7H2O. Aluminium sulphate, known commercially as " concentrated alum" or " sulphate of alumina," is manufactured from kaolin or china clay, which, after roasting (in order to oxidize any iron present), is heated with sulphuric acid, the clear solution run off, and evaporated. Alum cake " is an impure product. Aluminium sulphate crystallizes as Al2(SO4)3.18H20 in tablets belonging to the monoclinic system. It has a sweet astringent taste, very soluble in water, but scarcely soluble in alcohol. On heating, the crystals lose water, swell up, and give the anhydrous sulphate, which, on further heating, gives alumina. It forms double salts with the sulphates of the metals of the alkalis, known as the alums (see ALUM). Aluminium nitride (AIN) is obtained as small yellow crystals when aluminium is strongly heated in nitrogen. The nitrate, Al(NO3)3, is obtained as deliquescent crystals (with 8H20) by evaporating a solution of the hydroxide in nitric acid. Aluminium phosphates may be prepared by precipitating a soluble aluminium salt with sodium phosphate. Wavellite Als(PO4)3(OH),5.oH2O, is a naturally occurring basic phosphate, while the gem-stone turquoise (q.v.) is Al (PO4) (OH)3H2O, coloured by traces of copper. Aluminium silicates are widely diffused in the mineral kingdom, being present in the commonest rock-forming minerals (felspars, &c.), and in the gem-stones, topaz, beryl, garnet, &c. It also constitutes with sodium silicate the mineral lapis-lazuli and the pigment ultramarine (q.v.). Forming the basis of all clays, aluminium silicates play a prominent part in the manufacture of pottery and porcelain. End of Article: ALUMINIUM (symbol Al; atomic weight 27.0) If you wish, you can link directly to this article.
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