1911 Encyclopædia Britannica/Aluminium
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. ἄλς (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 1820 the word was changed into alumina. In 1760 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 1808 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, Preparation.so that when F. Wöhler repeated the experiment at Göttingen 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 properties, and prepared several compounds hitherto unknown. Early in 1854, H. St Claire Deville, accidentally and in ignorance of Wöhler'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 processes—the 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 £17 per ℔, 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. Héroult and C. M. Hall, by devising the electrolytic method now in use, inaugurated the present era of industrial electrolysis.
The chief natural compounds of aluminium are four in number: oxide, hydroxide (hydrated oxide), silicate and fluoride. Corundum, the only important native oxide (Al2O3), occurs in large deposits in southern India and the United States. Although it contains a higher percentage of metal Ores.(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 Héroult-Hall process as a solvent, it is sometimes made artificially. Aluminium silicate is the chemical body of which all clays are nominally composed. Kaolin or China clay is essentially a pure disilicate (Al2O3·2SiO2·2H2O), occurring in large beds almost throughout the world, and containing in its anhydrous state 24.4% of the metal, which, however, in common clays is more or less replaced by calcium, magnesium, and the alkalis, the proportion of silica sometimes reaching 70%. Kaolin thus seems to be the best ore, and it would undoubtedly be used were it not for the fatal objection that no satisfactory process has yet been discovered for preparing pure alumina from any mineral silicate. If, according to the present method of winning the metal, a bath containing silica as well as alumina is submitted to electrolysis, both oxides are dissociated, and as silicon is a very undesirable impurity, an alumina contaminated with silica is not suited for reduction. Bauxite is a hydrated oxide of aluminium of the ideal composition, Al2O3·2H2O. It is a somewhat Widely distributed mineral, being met with in Styria, Austria, Hesse, French Guiana, India and Italy; but the most important beds are in the south of France, the north of Ireland, and in Alabama, Georgia and Arkansas in North America. The chief Irish deposits are in the neighbourhood of Glen ravel, Co. Antrim, and have the advantage of being near the coast, so that the alumina can be transported by water-carriage. After being dried at 100° C., Antrim bauxite contains from 33 to 60% of alumina, from 2 to 30% of ferric oxide, and from 7 to 24% of silica, the balance being titanic acid and water of combination. The American bauxites contain from 38 to 67% of alumina, from 1 to 23% of ferric oxide, and from 1 to 32% of silica. The French bauxites are of fairly constant composition, containing usually from 58 to 70% of alumina, 3 to 15% of foreign matter, and 27% made up of silica, iron oxide and water in proportions that vary with the colour and the situation of the beds.
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 oxy-hydrogen 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 best 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 180° 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 Al3F5·2NaF 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 0.9 to decompose it, and it is easily soluble in the fused sulphides of the alkali metals. It can also be reduced metallurgic ally 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 sodium, formation of the pure double chloride of sodium and aluminium, and preparation of the metal by the interaction of the two former substances. To produce the Chemical reduction.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, 100 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 École Normale in Paris. Early in 1855 he conducted large-scale experiments at ]avel 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 Glaciére, 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. Péchiney 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 obvious—the 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 Batter sea, who also employed cryolite, made his own sodium, and was able to sell the product at 3s. 9d. per oz. This enterprise only lasted about four years. Between 1860 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 ℔ 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 Sal ind res, 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 expensive sodium hydroxide (caustic soda), and replacing carbon by a mixture of iron and carbon—the so-called carbide of iron—he invented the highly scientific method of winning the alkali metal which has remained in existence almost to the present day. In 1872 sodium prepared by Deville’s process cost about 4s: per ℔, the greater part of the expense being due to the constant failure of the retorts; in 1887 Castner’s sodium cost less than 1s. per ℔, 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 Wallsend, and by 1889 were selling the metal at 11s. to 15s. per ℔. The Aluminium Company’s price in 1888 was 20s. per ℔ and the output about 250 ℔ per day. In 1889 the price was 16s., but by 1891 the electricians commenced to offer metal at 4s. per ℔, and aluminium reduced with sodium became a thing of the past.
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 before the public. The first electrical method worthy of notice is that patented by E. H. and A. H. Cowles Electrical reduction.in 1885, which was worked both at Lockport, New York, U.S.A., and at Milton, Staffordshire. The furnace consisted of a flat, rectangular, firebrick box, packed with a layer of finely-powdered charcoal 2 in. thick. Through stuffing-boxes at the ends passed the two electrodes, made after the fashion of arc-light carbons, and capable of being approached together according to the requirements of the operation. The central space of the furnace was filled with a mixture of corundum, coarsely-powdered charcoal and copper; and an iron lid lined with firebrick was luted in its place to exclude air. The charge was reduced by means of a 50-volt current from a 300-kilowatt dynamo, which was passed through the furnace for 1½ hours till decomposition was complete. About 100 ℔ of bronze, containing from 15 to 20 ℔ of aluminium, were obtained from each run, the yield of the alloy being reported at about 1 ℔ per 18 e.h.p-hours. The composition of the alloys thus produced could not be predetermined with exactitude; each batch was therefore analysed, a number of them were bulked together or mixed with copper in the necessary proportion, and melted in crucibles to give merchantable bronzes containing between 1¼ and 10 % 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. Crätzel 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 fluoride 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 thrown down, and the raw materials had accordingly to be prepared in a pure state. After prolonged experiments in a factory owned by Messrs Bernard Frères at St Michel in Savoy, Minet’s process was given up, and at the close of the 19th century the Héroult-Hall method was alone being employed in the manufacture of aluminium throughout the world.
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 80 ℔ 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 1000° 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. Héroult 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 1888 Hall was at work on a commercial scale at Pitts burg, 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 ℔ per day, but by 1894 the turbines of the Swiss and French works ruined the enterprise About 1897 the Bernard factory at St Michel passed into the hands of Messrs Péchiney, 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 Société Métallurgique Suisse erected plant driven by a 500 h.p. turbine to carry out Héroult’s alloy process, and at the end of that year the Allgemeine Elektricitäts Gesellschaft united with the Swiss firm in organizing the Aluminium Industries Actien Gesellschaft of Neuhasen, which has factories in Switzerland, Germany and Austria. The Société Electrométallurgique Française, started under the direction of Héroult in 1888 for the production of aluminium in France, began operations on a small scale at Froges in Isère; but soon after large works were erected in Savoy at La Praz, near Modane, and in 190 51 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, remodelled to suit modern requirements. In 1905 this company began works for the utilization of another water»power at Loch Leven.
In 1907 a new company, The Aluminium Corporation, was started in England to carry out the production of the metal by the Héroult 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 Héroult 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 10 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 ampéres per sq. ft. of cathode surface, and the number of rods in the anode is such that each delivers 6 or 7 ampères per sq. in. of cross-sectional area. The working temperature lies between 750° and 850° C., and the actual yield is 1 ℔ 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 prolonged exposure to air, it has a slight violet shade. Its atomic weight is 27 (26.77, H=1, according toProperties. 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 654.5°, 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 considerable increased by rolling. It is not magnetic. It stands near the positive end of the list of elements arranged in electromotive series, being exceeded only by the alkalis and metals of the alkaline earths; it therefore combines eagerly, under suitable conditions, with oxygen and chlorine. Its coefficient of linear expansion by heat is 0.0000222 (Richards) or 0.0000231 (Roberts-Austen) per 1° C. Its mean specific heat between 0° and 100° is 0.227, and its latent heat of fusion 100 calories (Richards). Only silver, copper and gold surpass it as conductors of heat, its value being 31.33, (Ag=100, Roberts-Austen). Its electrical conductivity, determined on 99.6 % metal, is 60.5 % that of copper for equal volumes, or double that of copper for equal weights, and when chemically pure it exhibits a somewhat higher relative efficiency. The average strength of 98% metal is approximately shown by the following table:—
tons per sq. in.
| Ultimate Strength
tons per sq. in.
of Area %
|Cast . . .||3||7||15|
|Sheet . . .||5½||11||35|
|Bars . . .||6½||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 air, an almost invisible coating of oxide being produced, just as happens with zinc; but this film is very permanent and prevents further attack. Exposure to air and rain also causes slight corrosion, but to nothing like the same extent as occurs with iron, copper or brass. Commercial electrolytic aluminium of the best quality contains as the average of a large number of tests, 0.48% of silicon and 0.46 % of iron, the residue being essentially aluminium itself. The metal in mass is not affected by hot or cold water, the foil is very. slowly oxidized, while the amalgam decomposes rapidly. Sulphuretted hydrogen having no action upon it, articles made of it are not blackened in foggy weather or in rooms where crude coal gas is burnt. To inorganic acids, except hydrochloric, it is highly resistant, ranking well with tin in this respect; but alkalis dissolve it quickly. Organic acids such as vinegar, common salt, the natural ingredients of food, and the various extraneous substances used as food preservatives, alone or mixed together, dissolve traces of it if boiled for any length of time in a chemically; clean vessel; but when aluminium utensils are submitted to the ordinary routine of the kitchen, being used to heat or cook milk, coffee, vegetables, meat and even fruit, and are also cleaned frequently in the usual fashion, no appreciable quantity of metal passes into the food. Moreover, did it do so, the action upon the human system would be infinitely less harmful than similar doses of copper or of lead.
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 the finished 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, to sea-water or to corrosive influences of any kind in contact with another metal, or if it be mixed with another metal so as to form an alloy which is not a true chemical compound, the other metal being highly negative to it, powerful galvanic action will be set up and the structure will quickly deteriorate. This explains the failure of boats built of commercially pure aluminium which have been put together with iron or copper rivets, and the decay of other boats built of a light alloy, in which the alloying metal (copper) has been injudiciously chosen. It also explains why aluminium is so difficult to join with low temperature solders, for these mostly contain a large proportion of led. This disadvantage, however, is often overestimated since in most cases other means of uniting two pieces are available.
The metal produces an enormous number of useful alloys, some of which, containing only 1 or 2 % of other metals, combine the lightness of aluminium itself with far greater hardness and strength. Some with 90 to 99 % of other metals Alloys. 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 Uses.brittle 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 industry—especially where dead weight has to be moved about, and lightness is synonymous with economy—for 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 confectionery 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 vogue 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 0.100 in. in diameter, while the former weighs about 79 ℔ and the latter 162 ℔ per mile. Assuming the materials to be of equal tensile strength per unit of area-hard-drawn copper is stronger, but has a lower conductivity-the adoption of aluminium thus leads to a reduction of, 52 % in the weight, a gain of 60 % 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 ioints, 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 is no better for weighing 30 cwt. than for weighing ro cwt. A saucepan is required to have a certain diameter and a certain depth in order that it may hold a certain bulk of liquid: its weight is merely an encumbrance. Copper being 3% times as heavy as aluminium, whenever the latter costs less than 3% times as much as copper it is actually cheaper. It must be remembered, too, that electrolytic aluminium only became known during the last decade of the 10th century. Samples dating from the old sodium days are still in existence, and when they exhibit unpleasant properties the defect is often ascribed to the metal instead of to the process by which it was won. Much has yet to be learnt about the practical qualities of the electrolytic product, and although every day’s experience serves to place the metal in a firmer industrial position, a final verdict can only be passed after the lapse of time. The individual and collective influence of the several impurities which occur in the product of the Héroult cell is still to seek, and the importance of this inquiry will be seen when We consider that if cast iron, wrought iron and steel, the three totally distinct metals included in the generic name of “iron”—which are only distinguished one from another chemically by minute differences in the proportion of certain non-metallic ingredients—had only been in use for a comparatively few years, attempts might occasionally be made to forge cast iron, or to employ wrought iron in the manufacture of edge-tools. (E. J. R)
Compounds of Aluminium.
Aluminium oxide or alumina, Al2O3, 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, for they culminate in the production of artificial ruby and sapphire (see Gems, Artificial).
Aluminium Hydrates.—Several hydrated forms of aluminium oxide are known. Of these hydrargillite or gibbsite, Al(OH)3, diaspore, AlO(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 Al(OH)3⋅H2O is obtained; at 300° this yields AlO(OH), which on ignition gives alumina, Al2O3. 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, K2Al2O4, 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, Al2O3. 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, AlCl3, was first prepared by Oersted, who heated a mixture of carbon and alumina in a current of chlorine, a method subsequently improved by Wöhler, 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 (194°). Its vapour density at temperatures above 750° corresponds to the formula AlCl3; below this point the molecules are associated. It is very hygroscopic, absorbing water with the evolution of hychlochloric acid. It combines with ammonia to form AlCl3⋅3NH3; and forms double compounds with phosphorus pentachloride, phosphorus oxychloride, selenium and tellurium chlorides, as well as with many metallic chlorides; sodium aluminium chloride, AlCl3⋅NaCl, 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, AlBr3, is prepared in the same manner as the chloride. It forms colourless crystals, melting at 90°, and boiling at 265°-270°. Aluminium iodide, AlI3, results from the interaction of iodine and aluminium. It forms colourless crystals, melting at 185°, and boiling at 360°. 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⋅18H2O, 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⋅18H2O 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 (AlN) is obtained as small yellow crystals when aluminium is strongly heated in nitrogen. The nitrate, Al(NO3)3, is obtained as deliquescent crystals (with 8H2O) 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 Al8(PO4)3(OH)15·9H2O, is a naturally occurring basic phosphate, while the gem-stone turquoise (q.v.) is Al(PO4) (OH)3·H2O, 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.