other (where it had been in contact with the cathode plate). The fracture is crystalline, and the metal is said to contain 99.7 to 99.9 per cent. of antimony.—TRANSLATOR.] The electrolytic extraction of antimony has scarcely passed out of the experimental stage; but it may be assumed that, if the problem cannot be solved by the electrolysis of sulphides, it can scarcely be hoped that a satisfactory solution will be found for it. Treatment of the Crude or Electro-deposited Antimony. None of the methods of obtaining antimony-whether metallurgical or electrolytic-is capable of yielding a directly marketable product. Either the metal is too impure or it is obtained in a loose and pulverulent form. A process of fusion either for refining or for running the metal together is, therefore, unavoidable in every case. The impurities may consist of sulphur, arsenic, iron, copper, and other metals. In order, as far as possible, to prevent the contact of the metal with iron, the impure antimony is melted in crucibles or reverberatory furnaces, in the first place with antimony sulphide (Sb,S,, that has been liquated from antimony glance), by which means iron and copper are principally removed, and afterwards with basic substances, such as potash or soda, together with a small proportion of oxide or sulphide of antimony. In this way the metal becomes desulphurised, with the formation of a slag containing thio-salts of antimony. The antimony obtained electrolytically and brushed off the cathodes must be washed and dried, and is then fused together under a slag of antimony oxysulphide or of thio-antimonites, with basic additions, such as soda or potash. The Applications of Antimony have already been described in referring to its properties, they include alloying, deposition, pigments, thermo-electric couples, antimony chloride. CHAPTER X. CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, MANGANESE. CHROMIUM. = Properties of Chromium.-Chromium (Cr; atomic weight 52; specific gravity=6 to 7) is a bright grey, very lustrous, and very hard crystalline metal. Its fusing point has not yet been determined with accuracy, but it is a little higher than that of platinum, or approximately 2000° C. Chromium containing carbon melts at a temperature between 1600° and 1800° C. The alloying properties of chromium have not been so fully examined as those of other metals on account of the infusibility of the metal; and only the alloys with iron (ferro-chrome, and chrome steel) are of any practical importance. Since a metal containing carbon is yielded by most of the processes employed in extracting chromium, many of the properties ascribed in the books to pure chromium in reality belong to its carbon or carbide alloy. Pure chromium oxidises very slowly in cold air; but, on heating, the surface becomes readily covered with a thin film of oxide, which may possess any colour from yellow to blue. Even in oxygen it does not burn as readily as iron. Chromium combines with sulphur and with the halogens, evolving a very considerable amount of heat. It is especially prone to combine with carbon and silicon, and it has already been stated that it is almost impossible to obtain by simple reduction processes a metal that is free from carbon. Carbides have not yet been isolated as such; but a metal very rich in carbon is easily obtained. Carburised chromium is much harder, and resists chemical influences far more than does the pure metal. The oxide, Cr2O, and the so-called chromic acid, CrO3, are the chief compounds with oxygen that are of technical importance; the first-named forms a series of (chromic) salts with acids, the latter a series of chromates or chromic acid salts with bases. Occurrence in Nature. In nature chromium is mainly found as oxide in chrome iron ore, FeO. Cr,O,, which rarely, if ever, corresponds accurately to this formula, the FeO being in part replaced by MgO, and the Cr2O in part by Al,O, or Fe2O3. Crocoisite is a chromate of lead, PbCrÒ. Chrome iron ore alone is practically used for the extraction of chromium. The Reduction Process.-Chromium, which is to be used for the manufacture of iron alloys, may be reduced direct from the chrome iron ore in admixture with iron ore, either in blast furnaces or in crucibles, the product being an alloy of iron and chromium known as ferro-chrome. If crucibles are employed, and the alloy is to be obtained in the fused condition, it will be necessary to employ regenerative gas-firing. The The The metal can only be obtained unalloyed by treating the pure oxide, but even then it will not be free from carbon. method of obtaining the oxide from chrome iron ore is somewhat tedious. A chromate is first obtained by calcining the finely-divided ore with potash or soda in the presence of air, and from this the oxide is produced by roasting with sulphur. oxide formed in this way is washed free from the alkaline sulphates produced during the treatment with sulphur, and it is then reduced by heating it with carbon in plumbago crucibles. In this manner a grey metallic powder is obtained, which shows no sign of fritting even when regenerative gas-firing has been employed. The metal can only be fused with the aid of electricity, for which purpose an apparatus heated either by means of an electrical resistance or by the arc may be used. With the aid of the former apparatus (see pp. 88, 110, 112, 113) the author was first successful in obtaining fused chromium. The simplest form of this apparatus may again be figured at this place for convenience in illustrating the principle used in heating (Fig. 172). Within the space left between the fire-bricks, ABCDFS, a mixture of chromium oxide and carbon is packed around the carbon rod, k, which forms a bridge between the thick carbon rods, K, and thus lies in the midst of the mixture. The passage of a powerful current of 8 to 10 amperes per sq. mm. of cross-sectional area of the small rod [5000 to 6500 amps. per sq. in.] is sufficient to raise the temperature of the mass to such an extent that the oxide is reduced, and the resulting chromium fuses together. The use of this furnace was explained in an earlier chapter, and it is, therefore, unnecessary to dwell further upon it here. Moissan's Furnace.-Moissan,* who has described the best method of obtaining non-carburised chromium, at first recommended the use of the apparatus shown in Fig. 173. The hearth of this miniature furnace consisted of a limestone block (pierre de Courson), in which a rectangular cavity had been prepared. The walls of the cavity were lined with magnesia plates 10 mm. [0.4 in.] thick, and then again with carbon plates of the same thickness. Two stout carbon rods were introduced, one through each opposite side wall, the carbon lining being, of course, cut away so that it did not touch the rod at any point and so make contact with it or start an electric arc. At right angles to the electrodes, but at a somewhat lower level, a carbon tube is passed through a third wall. The tube may be set level, or at any angle of less than 30° with the horizon, and serves to contain the * Comptes Rendus, 1893, vol. cxvii., p. 679. substance to be heated, which either rests in or is passed through it. The end of the tube should be some 10 mm. [0.4 in.] below the electric arc. The cover of the melting chamber, like the body of the furnace, consists, first, of a carbon plate, then of a slab of magnesia, and, finally, of a limestone block. Chaplet's Furnace.--Chaplet seeks to distribute the heat. more evenly in the furnace by submitting the tube containing The the charge to the influence of a larger number of arcs. Fig. 175.-Chaplet's modified electric furnace. The upper portion of the furnace contains the muffle tubes and the movable electrodes. The muffle is sufficiently inclined downwards, and may consist of a simple tube having an opening, o, at its lowest point, through which the molten material may flow into the collecting well. It may also be doubled, and possess a V-form with an opening at the narrow part of the V, to allow of the escape of the molten charge (Fig. 175). The electrodes, c, in sufficient number, are so disposed that the whole mass in the tube, d, may be heated. Additional electrodes, c', are arranged near the mouth of the tube containing the charge, so that the arcs playing between them may prevent the solidification of the mass at this point. * German Patent 77,896. Refining of Crude Chromium.-Both the containing tube (whether it be made of a mixture of clay and graphite or of any other refractory material) and the carbon of the charge itself tend to prevent the formation of a pure chromium free from carbon, silicon, &c., by the smelting of a mixture of chromic oxide and carbon in an apparatus of this description. The resulting metal must therefore be refined. Moissan has ingeniously solved this problem by adapting to it the principles of the basic open-hearth steel process. By heating lime with chromic oxide he obtains a basic substance which serves for the oxidation as well as the fluxing of the impurities. If the hearth of an electric furnace be lined with this calcium chromite mixture the carbon and silicon present in the metal that is heated upon it will be oxidised and removed by the chromic oxide, in the same way that they are eliminated from iron by means of oxide of iron in the open hearth process. Details as to the cost of this process have not yet been published. It is scarcely to be doubted that the method is capable of application on a large scale; but the author would recommend that the crude carburised chromium should be obtained in pulverulent form by reduction in crucibles or in the electric resistance furnace, in order to save the expense of fusion in the first instance. The Precipitation Process.-The separation of chromium from its fused salts by means of zinc, and especially from chromium chloride or the double chloride of chromium, and an alkali metal was first effected by Wöhler.* The yield is very small, but the metal is of great purity. The alkali metals have also been used for the decomposition of chloride of chromium.† ELECTROLYSIS OF CHROMIUM COMPOUNDS. Electrolysis of Dissolved or Fused Chromium Compounds with Insoluble Anodes.-If any one work deserve to be foremost in the field of electro-metallurgy, it is that of Bunsen on the production of chromium by electrolysis. It merits especial recognition not only on account of the observation itself, and of the arrangements devised for the experiment, but because at the time at which it was accomplished, electrical energy, measuring instruments, and other requirements were not nearly so readily available as they are now. It should be pointed out to modern experimenters, to whom time or opportunity has failed for the study of the classics of electro-chemistry, that, more than forty years ago, Bunsen showed the result of electrolysis to be dependent in many instances on the density of the current. His account of his observations ran as follows: * Liebig's Annalen der Chemie und Pharmacie, vol. cxi., p. 230. |