ALUM I N I U M method now in use, placed the business finally under the con- to treat aluminium in an identical manner. In practice, however, trol of electricians, and inaugurated the present era of industrial it cannot be thrown down electrolytically with a dissimilar anode so as to win the metal, and certain difficulties are still met with in electrolysis. The chief natural compounds of aluminium are four in number : the analogous operation of plating by means of a similar anode. oxide, hydroxide (hydrated oxide), silicate, and fluoride. Corundum, Of the simple compounds, only the fluoride is amenable to electrothe only important native oxide (A1203), occurs in large lysis in the fused state, since the chloride begins to volatilize below melting-point, and the latter is only 5° below its boiling-point. Ores. deposits in southern India and the United States. its Cryolite is not a safe body to electrolyse, because the minimum Although it contains a higher percentage of metal (52-9 per cent.) than any other natural compound, it is not at voltage needed to break up the aluminium fluoride is 4 ’0, whereas present employed as an ore, not only because it is so hard as to be the sodium fluoride requires only 4‘7 volts ; if, therefore, the crushed with difficulty, but also because its very r hardiness makes current rises in tension, the alkali is reduced, and the final product it valuable as an abrasive. Cryolite (AL^I’a. 6iS aF) is a double consists of an alloy with sodium. The corresponding double fluoride of aluminium and sodium, which is scarcely known except chloride is a far better material ; first, because it melts at about on the west coast of Greenland. Formerly it was used for the 180° C., and does not volatilize below a red heat, and second, preparation of the metal, but the inaccessibility of its source, and because the voltage of aluminium chloride is 2 -3 and that of sodium the fact that it is not sufficiently pure to be employed without some chloride 4'3, so that there is a much wider margin of safety to preliminary treatment, caused it to be abandoned in favour of other cover irregularities in the electric pressure. It has been found, howsalts. When required in the Heroult-Hall process as a solvent, it is ever, that molten cryolite and the analogous double fluoride represometimes made artificially. Aluminium silicate is the chemical sented by the formula A12F6. 2NaF are very efficient solvents of body of which all clays are nominally composed. Kaolin or China alumina, and that these solutions can be easily electrolysed at about clay is essentially a pure disilicate (AI2O3. 2Si02. 2H20), occurring 800° C. by means of a current that completely decomposes the in large beds almost throughout the world, and containing in its oxide but leaves the haloid salts unaffected. Molten cryolite disanhydrous state 24*4 per cent, of the metal, which, however, in solves roughly 30 per cent, of its weight of pure alumina, so that common clays is more or less replaced by calcium, magnesium, and when ready for treatment the solution contains about the same prothe alkalis, the proportion of silica sometimes reaching 70 per cent. portion of what may be termed “ available ” aluminium as does the Kaolin thus seems to be the best ore, and it would undoubtedly be fused double chloride of aluminium and sodium. The advantages used were it not for the fatal objection that no satisfactory process lie with the oxide because of its easier preparation. Alumina dishas yet been discovered for preparing pure alumina from any solves readily enough in aqueous hydrochloric acid to yield a solumineral that is chemically a silicate. If, according to the present tion of the chloride, but neither this solution, nor that containing method of winning the metal, a bath containing silica as well as sodium chloride, can be evaporated to dryness without decomposialumina is submitted to electrolysis, both oxides are dissociated, tion. To obtain the anhydrous single or double chloride, alumina and as silicon is a very undesirable impurity, an alumina con- must be ignited with carbon in a current of chlorine, and to exclude taminated with silica is not suited for reduction. Bauxite is a iron from the finished metal, either the alumina must be pure, or hydrated oxide of aluminium of the ideal composition, A1203.2H20. the chloride be submitted to purification. Thus preparation of a It is a somewhat widely distributed mineral, being met with in chlorine compound suited for electrolysis becomes more costly and Styria, Austria, Hesse, and French Guiana; but the most important more troublesome than that of the oxide, and in addition four beds are in the south of France, the north of Ireland, and in times as much raw material must be handled. At different times propositions have been made by Bucherer, Alabama, Georgia, and Arkansas in North America. The chief Irish deposits are in the neighbourhood of Glenravel, Co. Antrim, Blackmore, and others, to win the metal from its sulphide. This and have the advantage of being near the coast, so that the alumina compound possesses a heat of formation so much lower that eleccan be transported by water-carriage. After being dried at 100° C., trically it needs but a voltage of 0 ’9 to decompose it, and it is easily Antrim bauxite contains from 33 to 60 per cent, of alumina, from soluble in the fused sulphides of the alkali metals. It can also be 2 to 30 per cent, of ferric oxide, and from 7 to 24 per cent, of reduced metallurgically by the action of molten iron. Various silica, the balance being titanic acid and water of combination. considerations, however, tend to show that there cannot be so much The American bauxites contain from 38 to 67 per cent, of alumina, advantage in employing it as would appear at first sight. As it is from 1 to 23 per cent, of ferric oxide, and from 1 to 32 per cent, of easier to reduce than any other compound, so it is more difficult to silica. The French bauxites are of fairly constant composition, produce. Therefore while less energy is absorbed in its final containing usually from 58 to 70 per cent, of alumina, 3 to 15 per reduction, more is needed in its initial preparation, and it is cent, of foreign matter, and 27 per cent, made up of silica, iron questionable whether the economy possible in the second stage oxide, and water in proportions that vary with the colour and the would not be neutralized by the greater cost of the first stage in the situation of the beds. whole operation of winning the metal from bauxite with the sulphide Before the application of electricity, only two compounds were as the intermediary. found suitable for reduction to the metallic state. Alumina itself The Deville process as gradually elaborated between 1855 and is so refractory that it cannot be melted save by the oxy-hydrogen 1859 exhibited three distinct phases :—Production of metallic blowpipe, or the electric arc, and except in the molten state it is sodium, formation of the pure double chloride of sodium j not susceptible of decomposition by any chemical reagent. Deville and aluminium, and preparation of the metal by the r first selected the chloride as his raw material, but observing it to interaction of the two former substances. To produce be volatile and extremely deliquescent, he soon substituted in its the alkali metal, a calcined mixture of sodium carbonate, coal, and place a double chloride of aluminium and sodium. Early in 1855 chalk was strongly ignited in flat retorts made of boiler-plate ; the Percy suggested that cryolite should be more convenient, as it was sodium distilled over into condensers, and was preserved under a natural mineral and might not require purification, and at the heavy petroleum. In order to prepare pure alumina, bauxite and end of March in that year, Faraday exhibited before the Royal sodium carbonate were heated in a furnace until the reaction was Institution samples of the metal reduced from its fluoride by Dick complete ; the product was then extracted with water to dissolve and Smith. Rose also carried out experiments on the decomposi- the sodium aluminate, the solution treated with carbon dioxide, tion of cryolite, and expressed an opinion that it was the best of all and the precipitate removed and dried. This purified oxide, mixed compounds for reduction ; but, finding the yield of metal to be with sodium chloride and coal tar, was carbonized at a red heat, low, receiving a report of the difficulties experienced in mining the and ignited in a current of dry chlorine as long as vapours of the ore, and fearing to cripple bis new industry by basing it upon the double chloride were given off, these being condensed in suitable employment ot a mineral of such uncertain supply, Deville decided chambers. For the production of the final aluminium, 100 parts to keep to his chlorides. With the advent of the dynamo, the of the chloride and 45 parts of cryolite to serve as a flux were position of affairs was wholly changed. The first successful idea of powdered together, and mixed with 35 parts of sodium cut into using electricity depended on the enormous heating powers of the small pieces. The whole was thrown in several portions on to the arc. The infusibility of alumina was no longer prohibitive, for the hearth of a furnace previously heated to low redness, and was molten oxide is easily reduced by carbon. Nevertheless, it was stirred at intervals for three hours. At length when the furnace found impracticable to smelt alumina electrically except in presence was tapped a white slag was drawn off from the top, and the liquid of copper, so that the Cowles furnace yielded, not the pure metal, metal beneath was received into a ladle and poured into cast-iron but an alloy. So long as the metal was principally regarded as a moulds. The process was worked out by Deville in his laboratory necessary ingredient of aluminium-bronze, the Cowles process was at the l3cole Normale in Paris. Early in 1855 he conducted largepopular, but when the advantages of aluminium itself became more scale experiments at Javel in a factory lent him for the purpose, apparent, there arose a fresh demand for some cheap method of where he produced sufficient to show at the French Exhibition of obtaining it unalloyed. It was soon discovered that the faculty of 1855. In the spring of 1856 a complete plant was erected at La inducing dissociation possessed by the current might now be utilized Glaciere, a suburb of Paris, but becoming a nuisance to the with some hope of pecuniary success, but as electrolytic currents neighbours, it was removed to Nanterre in the following year. are of lower voltage than those required in electric furnaces, molten Later it was again transferred to Salindres, where the manufacture alumina again became impossible. Many metals, of which copper, was continued by Messrs. Pechiney till, after Deville’s death in silver, and nickel are types, can be readily won or purified by the 1881,|the advent of the present electrolytic process rendered it no electrolysis of aqueous solutions, and theoretically it may be feasible longer profitable. Deville received much encouragement and
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