June 2, 1898] 



NATURE 



1 1 



cent, of crystallised copper sulphate and 5 to 6 per cent, of sul- 

 phuric acid, run in. The impure copper plates, of course, form 

 the soluble anodes, the thin sheets receiving the deposit of pure 

 copper A current of from 100 to 200 amperes per square 

 metre is usually employed, the E. M. F. being o"2 to 0*4 volt. 

 The electrical energy needed is, therefore, 0"i to 0-2 electrical 

 horsepower hour per pound of copper deposited. Considerable 

 variations in the details of working are found in different works, 

 owing to the varying local conditions. When a larger current 

 density is employed the amount of electric energy required to 

 deposit a pound of copper is greater, but, on the other hand, the 

 copper is deposited more quickly, and therefore, for a given 

 output, less copper is locked up in the baths, less labour is re- 

 quired, and a smaller plant is sufficient. In order to obtain a 

 homogeneous deposit of copper the solution must be kept in 

 circulation, otherwise a deficiency of copper near the kathode 

 surface would arise, which would lead to the formation of a non- 

 coherent impure deposit. Of the impurities in the anodes, gold, 

 silver, and part of the arsenic and antimony remain undissolved, 

 whilst iron, nickel, and the remainder of the arsenic and anti- 

 mony pass into solution. The two latter metals are deposited 

 along with the copper if they are allowed to accumulate too 

 largely in the solution, especially if the amount of free acid 

 present is small. The solutions must therefore be purified 

 from time to time, and this forms the main difficulty of the pro- 

 cess. By blowing air through the solution, after neutralising it 

 with cupric oxide, ferric arsenate and basic antimony sulphate 

 are deposited ; but large quantities of copper sulphate are thus 

 accumulated, which are difficult to dispose of. Where cheap 

 power is available, the impure solutions may be electrolysed 

 with insoluble anodes of lead and the copper, arsenic and anti- 

 mony deposited ; otherwise evaporation and recrystallisation 

 must be resorted to. 



The anode slimes which contain Au, Ag, Se, Te, Bi, Sb, and 

 As, are worked up to recover the precious metals. In 1896, 

 137,000 tons of electrolytic copper were obtained, of which the 

 United States produced more than all other countries together. 

 The greater part of this pure copper is employed for electrical 

 purposes, where its high conductivity is of paramount import- 

 ance. The electrolytic copper is obtained in a coarsely crystal- 

 line condition, and is fused before use. Mr. Elmore aims at 

 depositing the copper directly in the form in which it is to be 

 employed ; copper tubes, for example, are made by depositing the 

 metal upon a rotating cylinder, the surface of the deposit being 

 constantly polished by a prismatic piece of agate which moves 

 backwards and forwards parallel to the axis of the cylinder. 

 This produces a very dense and tough deposit, and at the same 

 time permits of the employment of a current density as high as 

 600 amperes per square metre. The removal of the cylinder 

 from the tube is very simple when it is made of some easily 

 fusible alloy. 



The electrolytic process for making aluminium has entirely 

 superseded the chemical process, the superiority of the former 

 (from a commercial point of view) being demonstrated by the 

 diminution in the price of aluminium from over 20s. per lb. in 

 1888 to about \s. ^(i. to-day. The electrolyte employed is a 

 solution of alumina in a fused mixture of the fluorides of 

 aluminium and of the alkali or earth-alkali metals. Minet has 

 used a mixture of common salt and aluminium fluoride, but it 

 would appear that the solvent usually employed is cryolite from 

 which iron and silicon have been removed by a preliminary 

 electrolysis. The baths consist of large iron, carbon-lined 

 boxes, the lining forming the kathode. The anode consists of 

 massive blocks of carbon suspended above the bath, and dipping 

 under the fused electrolyte almost to the bottom of the bath. The 

 electrolyte is maintained in the fused state by the heat generated 

 by the passage of the current, and the aluminium collecting on 

 the bottom of the bath is run off from time to time. The 

 alumina alone undergoes decomposition, the oxygen combining 

 with the carbon anode and escaping as carbonic anhydride. 

 Anhydrous alumina is shovelled on to the surface of the bath as 

 required, and serves to protect the fused mass below from loss 

 of heat by radiation. Although attempts have been made to 

 refine aluminium containing iron and silicon, they do not appear 

 to have met with success, and it is therefore necessary to exclude 

 these impurities from the materials used. The pure alumina 

 used in the process is prepared from bauxite. A current of 7000 

 amperes is passed through each bath (the current density being 

 probably about 25 amperes per sq. cm. of kathode), an 

 E.M.F. of about 5 volts being required. The current efficiency 



NO 1492, VOL. 58] 



is considerably less than the theoretical amount, owing to some 

 secondary action, so that from 14 to 18 electrical horse-power 

 hours are required to produce a pound of metal. The annual 

 production of aluminium is rapidly increasing, and is at present 

 considerably over 2000 tons. Notwithstanding the very large 

 consumption of electrical energy in this manufacture, it is 

 interesting to note that the cost of the pure alumina is the 

 largest individual item in the total cost of production. 



The problem of utilising aluminium presents as great diffi- 

 culties as that of its economical production. Mr. A. E. Hunt, 

 of the Pittsburg Reduction Company, has recently given an 

 interesting account of the applications of aluminium, from which 

 it appears that these difficulties are being overcome. 



The energetic reducing action of aluminium is utilised in 

 many ways, the most important being the production of steel 

 castings ; two to five ounces of aluminium per ton suffices to 

 remove oxygen from the steel, and so to obviate to a great 

 extent the formation of blow-holes in the castings. A little 

 aluminium added from time to time to the baths of molten zinc 

 used in galvanising, removes the oxide and keeps the baths fluid. 

 The addition of a little aluminium in making brass castings 

 increases their soundness and strength in a similar way. 



Aluminium is also used instead of brass for a multitude of 

 small cast and stamped objects which do not require to be 

 soldered ; there would still appear to be no trustworthy method of 

 permanently soldering aluminium. Aluminium may possibly be 

 used as a conductor of electricity, though at present the ad- 

 vantage in price lies with copper ; the specific conductivity of 

 aluminium is 63 to 64 per cent, of that of copper, whilst copper 

 is 3*3 times as heavy. 



The history of the electro-metallurgy of zinc is mainly a 

 record of failures. Zinc is readily deposited from neutral or 

 slightly acid aqueous solutions or from the fused chloride, but, 

 from the former, is very prone to separate in a spongy form. 

 Mylius and Fromm show that this is probably due to the form- 

 ation of traces of oxide, and is prevented by the presence of 

 reducing agents. Vigorous circulation of the solution is also 

 advantageous. The presence of metals more electro-negative 

 than zinc, which deposit on it and promote its oxidation, also 

 produces the spongy deposit. The difficulty of insuring the 

 absence of such metals from solutions obtained from zinc ores, 

 as well as the low price of the metal, which precludes any 

 elaborate purification, probably account for the slow progress of 

 this industry. Progress is, however, being made. Dieffenbach's 

 process is in successful operation at Duisberg in Germany. In 

 this a solution of zinc chloride, obtained by leaching a zinciferous 

 iron pyrites after submitting it to chlorinating roasting, is 

 electrolysed ; but further details are wanting. 



The Ashcroft process obtains coherent zinc by employing a 

 somewhat basic solution of zinc sulphate or chloride in the 

 kathode compartments of the electrolytic cells, whilst the 

 Siemens and Halske process employs somewhat acid zinc 

 sulphate solution. Both these processes are at work on the 

 large scale, but their ultimate success does not seem to be yet 

 quite assured ; so that a more lengthy description may be dis- 

 pensed with. 



At Tarnowitz an alloy of zinc and silver with a little lead and 

 copper, obtained by desilverising lead with zinc containing about 

 0-5 per cent, of aluminium, was refined electrolytically, using a 

 slightly basic concentrated solution of zinc and magnesium 

 chlorides as electrolyte, and rotating zinc plates as kathode. 

 The insoluble anode mud thus obtained contained about 75 per 

 cent, of silver, and the zinc deposited was almost chemically pure. 



Electro-galvanising is also now somewhat largely employed, 

 the electrolyte being a solution of zinc sulphate. Here again 

 close attention to the current density and composition of the 

 solutions is required to secure a smooth and adherent deposit. 



Nickel. — Whilst it is perfectly easy to deposit a very thin film 

 of nickel by electrolysis, the metal peels off if a thicker deposit 

 is attempted. According to Foerster, however, tough, homo- 

 geneous plates of nickel of any thickness may be deposited from 

 aqueous solutions of the sulphate or chloride if they are heated 

 to from 50° to 90° C. The nickel obtained is, however, not so 

 pure as is the case with copper, cobalt and iron being found in 

 the refined metal in about the same quantities present in the 

 unrefined. Electrolytic nickel is now a commercial article, part 

 of it being obtained from alloys of copper and nickel contain- 

 ing a considerable amount of sulphur, which are used as anodes, 

 the copper being first deposited, whilst the nickel goes into 

 solution, from which it is subsequently deposited. 



