For the production of cobalt, a pure sulphate of cobalt, mixed with ammonium sulphate, was prepared by heating purpureocobaltic chloride with sulphuric acid. The aqueous solution of this contained 11.64 grm. of cobalt per litre; and the electrolyte was made up as follows: 100 c.c. cobalt sulphate solution. 30 grm. ammonium sulphate. 30 grm. ammonia (of 0·905 sp. gr.). 500 c.c. water. The cathode was a platinum plate 94 cm. long by 5.9 cm. wide, and a similar platinum plate served as anode. The strength of current was 0.7 ampere at 3 volts, and the current density D100 = 0.6 ampere. The deposited cobalt weighed 8.133 grm., of which 7.319 grm. separated from the cathode in the form of a coherent and fairly strong plate. The metal was brightly lustrous on the side next to the platinum, but on the other side it was dull and grey; it was not tarnished, however, and showed but little oxide. On heating in pure oxygen it lost 0.23 per cent. in weight, so that it must have contained 0.55 per cent. of cobaltic oxide (Co,O3 + 2H2O); in other words, 0.32 per cent. of the total weight of cobalt had been deposited as oxide. In a second experiment, the electrolyte consisted of:250 c. c. cobalt solution. = 30 grm. ammonium sulphate. 50 grm. ammonia (sp. gr. = 0.905). 250 c.c. water. 0.6 ampere. A polished nickel plate, 9 cm. long by 7.6 cm. wide, was used as cathode, and a platinum plate as anode. The current strength was 0.8 ampere at a pressure of 3.2 volts, and the current density was D100 The action was stopped after 30 hours, and afforded 2.9 grm. of metal, of which 2-2 grm. were easily separated from the cathode in the form of thin and curled fragments of plate. The metal so obtained was in parts perfectly lustrous, but in many places was flecked with brilliant tarnished spots, or tinged with brown. On heating in hydrogen, it lost 0.15 per cent. in weight, which corresponds to 0.36 per cent. of cobaltic oxide (Co2O3 + 2H2O). Hence 0.21 per cent. of the whole of the cobalt had been deposited as oxide. The determinations of oxide made in this way, however, are likely to be a little too high, because the deposited metal retained traces of ammonium salts, even after very thorough washing; and these were volatilised on heating in hydrogen, affording a slight brownish ring of deposit in the cooler part of the tube. After heating in hydrogen, the cobalt had a uniformly metallic appearance, and in parts formed plates with a beautiful lustre. Its colour, as compared with nickel, was distinctly bluish-white, like that of zinc. Summary. If one might judge solely from patent specifications, of which the best known have been more or less fully quoted here, the problem of extracting nickel by electrolysis on the commercial scale might be considered as solved. The methods that have been worked out for electrolytic analysis* show that nickel may be separated quantitatively in the form of a dense and lustrous deposit from quite a number of salts. Again, the processes are not less numerous by which a bright and adhesive coating of nickel may be deposited on the more important metals. The analyst and the electro-plater attain their respective objects with certainty, but no process of depositing nickel continuously for the industrial extraction of the metal has yet been made public. Since the metallurgy of copper † has been taken as typical of that of nickel, it might at first seem possible that the crude nickel could be refined without difficulty after the manner of the electrolytic process for copper. It is, however, only necessary to call to mind the nature of the impurities in nickel to recognise that this process has no prospect of success. The objections to the direct electrolytic treatment of nickel ore and matte will be understood on reference to the account of analogous processes in connection with copper extraction. There remain only the wet processes for the treatment of the nickel ores, which permit the electrolytic precipitation of the nickel after the bulk of the impurities has been removed from the solution by precipitation. But the conditions necessary for such are: The best electrolyte is the sulphate, and although on the one hand it may with advantage exhibit an acid reaction, yet on the other hand it must not contain any of the mineral acids in the free condition. Boric and phosphoric acids, however, appear to be harmless. Of basic solutions only those which are ammoniacal can be used. The unsatisfactory character and the cost of working with insoluble, but yet not indestructible, anodes have been sufficiently explained already. Under these circumstances an E.M.F. of 3 volts must be applied to obtain a current density of 60 to 100 amperes per sq. metre [0-04 to 0.65 amp. per sq. in.]; and it is only when depolarising agents, such as organic acids, are present that the difference of potential may be reduced to 2 to 2.5 volts. Among the organic acids that are good conductors, and of which the price is not prohibitive, the author has already recommended the use of the sulphonic acids of the less valuable distillation products of tar. *A. Classen, Quantitative Analyse durch Elektrolyse, 3rd. Ed., 1892. +G. Langbein, Galvanische Metallniederschläge, 3rd. Ed., 1895. See Chapters on Lead and Zinc. Although there is no longer any difficulty in producing thin deposits of nickel which leave nothing to be desired in respect of soundness, lustre, and colour, it has not yet been found possible even with the greatest care, to obtain thicker plates that could be subjected direct to mechanical working. Even in nickelplating, if an extra thick coating be required, it is necessary to stop the process after a certain thickness of nickel has been deposited, and to coat this with a thin film of copper before proceeding further. Only in this way will the nickel coating be adhesive, for electro-deposited nickel shows a tendency to break away, and to flake off as the thickness is increased. It may be that the cause of this is to be ascribed to the formation of an almost imperceptible film of oxide, as would seem possible from A. Winkler's experiments. This, however, is only a conjecture, and no sufficiently inexpensive method of obviating the difficulty has yet been found. Thus there is at present no publicly described electrolytic process which, at a sufficiently reasonable cost, can produce a metal that is capable of being worked mechanically. And so long as the electro - deposited nickel has yet to be fused and refined subsequently, the commercial practicability of electrolytic methods must be held in doubt. The Applications of Nickel.-Pure nickel is used in considerable quantities for cooking and table utensils. Nickeled. iron sheet, made by welding together nickel and iron plates, is employed for similar purposes. Many objects in common use consist of a cheaper metal electro-plated with nickel. Among the alloys may be included-cupro-nickel for coinage, &c., nickelcopper-zinc as German silver, &c., and nickel iron, such as nickel steel for armour plates. Cobalt.-Excepting the method described above for the production of pure cobalt, it will not be necessary to enter into details of the treatment of this metal, inasmuch as nearly all that has been written concerning nickel is equally applicable to cobalt. CHAPTER XIII. METALS OF THE PLATINUM GROUP. THERE is but little need to refer to this group of metals, since there is not much prospect of electrolytic methods of extraction being applied to them, owing to the fact that most of their compounds may be readily decomposed by a slight expenditure of heat. Several methods of depositing and separating the metals by electrolysis have been published during the last few years. It should be stated, howewer, that these methods have been mainly devised for purposes of analysis or electro-plating, and may therefore be found described in such works as those of Classen and Langbein, to which reference has been made so frequently.* In A process for the separation of gold and platinum has already deen described in the Chapter relating to the former metal. a similar way by using a platinic chloride solution as electrolyte, platinum may be separated from certain other metals (iridium and rhodium) of this group. The use of the Siemens electric furnace (see p. 131) for melting platinum has not been successful owing to the readiness with which the carbon of the electrode passes over into the platinum; and platinum containing carbon is inapplicable to most purposes. If platinum is to be melted by electrical means, it will be either by placing the metal as a resistance in a powerful electric circuit, or by causing the electric arc to pass to the metal from an electrode of the same material, as Slavianoff has proposed for use in the treatment of iron (see Chapter on Iron, p. 389). * See also Translator's Treatise on Electro-Metallurgy. 404 ADDENDA. TABLE I.-SHOWING The Value of EQUAL CURRENT VOLUMES AS EXPRESSED IN AMPERES PER SQUARE DECIMETRE, PER SQUARE FOOT, AND PER SQUARE INCH OF ELECTRODE SURFACE. By this table the current density may be expressed in amperes per square decimetre, square foot, or square inch, any one of them being given. Thus a current of 1 ampere per square decimetre has the same electrolytic value as one of 9-29 amperes per square foot, or 0-0645 per square inch. To find the value of intermediate numbers not shown above, add together the various numbers representing the hundreds, tens, units, and decimals of the given quantity. Thus 27-5 amperes per square decimetre (20+7+05) is equivalent to 185-8 +65 +464 = 255-44 amperes per square foot, or 1-2903 + 0·4516 + 0·0323 = 1.7742 amperes per square inch. Note. This table is re-printed from the Translator's Treatise on Electro-Metallurgy. = Amperes per Square Inch. |