The

in Fig. 152. It was thus an easy matter to regulate the current strength in the subsidiary circuit in such a way that the acid should be generated at exactly the required rate, and that the acidity of the electrolyte should thus be maintained constant. Thus, for example, a current of 0.01 ampere would produce 0.43 grm. [6.6 grains] of sulphuric acid in 24 hours, and this would suffice for the neutralisation of 0.36 grm. [5.5 grains] of zinc oxide. Or, to avoid all possibility of rendering the cathode zinc impure by platinum, the use of the second circuit may be omitted, and the necessary quantity of acid may be added to the bath from time to time (at about 12-hour intervals). quantity thus required for a total electrode surface of about 600 sq. cm. [93 sq. in.] would amount to about 0-2 grm. [308 grains] of sulphuric acid 24 hours; and this quantity would be distributed through about 15 litre [2.5 pints] of solution. The anodes, from which bubbles of hydrogen were seen occasionally to rise, were removed from the bath once every day in order that the impurities clinging to them might be brushed away. After they had been replaced the electrolysis proceeded as before. Any particles that might become detached were retained in the anode compartments by the diaphragms. No visible evolution of hydrogen was permitted at the cathode, and if any isolated bubbles of gas were observed they were removed by raising the electrode plate. The distance between the electrodes in our experiments amounted to several centimetres, the current density was at least 1 ampere per sq. dm. [0-065 per sq. in.] of cathode surface, and the slope of potential between the anodes and cathodes amounted to from 0·3 to 0.6 volt.

Fig. 152.-Connections with subsidiary electrodes in zinc deposition.

"It is advisable to deposit zinc on both sides of the cathode, because, otherwise, the separation of grey zinc may easily commence on the back. This arrangement has also the additional advantage that the number of the electrodes that may be introduced into a bath can be increased at will.

"The current density of 1 ampere used corresponded to a separation of about 29 grms. of zinc per sq. dm. [28.9 grains per sq. in.] in 24 hours, so that a layer about 0.4 mm. [0·016 in.] in thickness was deposited in the day. The electrolysis could be continued ordinarily for from two to three days without any formation of porous zinc. The cathodes became coated with a solid white coat of zinc which never exceeded 2 mm. [0·08 in.] in thickness. At the edges of the plates, however, the deposit was thicker, and it was here that, as the operation proceeded, rough patches and growths were formed, in which gradually small nests

of spongy zinc were found, and this was the sign that the cathode must be replaced."

No further remarks upon the work of Kiliani, and Mylius and Fromm need be made here, since it refers to the use of soluble (metallic zinc) anodes, and is, therefore, chiefly of interest in connection with electro-plating and electro-typing; and it will suffice to point out that in these two branches of electrochemistry the current density usually ranges from 200 to 300, and in exceptional cases rises to 700 amperes per sq. metre [0·13 to 0.2, or 045 amp. per sq. in.], and often the electrolytes used have a reducing action. Reference will be made to some of the points observed in practical experience with these processes after treating of the third method of extracting zinc.

3. Electrolysis of Dissolved or Fused Zinc Salts with Insoluble Anodes.-Luckow's Process.-Luckow, in his patent,* to which attention has already been drawn, made several proposals in connection with the use of insoluble anodes, among which the introduction of a current of sulphurous acid into the electrolyte vat may be especially noted-a method of procedure that has been frequently re-discovered at a later date. Luckow, however, was not very fortunate in this use of sulphurous acid, inasmuch as he proposed to employ it as a means of converting into a harmless substance the chlorine that is evolved at the anodes when solutions of zinc chloride are being electrolysed. It must not be forgotten that the use of Luckow's deoxidising agent increases the acidity of the electrolyte at an enormous rate. Thus, whether the solution used be the chloride or the sulphate, whether, therefore, the group Cl2 or SO4 (= SO, + O) be liberated from the deposited zinc, the use of SO, to render these anions harmless will always be attended by the formation of a quantity of free acid, which is double that corresponding theoretically to the zinc precipitated, thus:

3

2HCl + H2SO4.

If, therefore, the liberated acid is to be neutralised by means of zinc ore, as specified in Luckow's patent, it is evident that after the first neutralisation two molecules of zinc salt will be contained in the liquid for every one that had previously been decomposed. Hence in a very short time it would become necessary to find some other means of neutralising the acid (the proportion of which is doubled at each operation) other than that of allowing it to dissolve zinc compounds. But, at the same time, the direct use of solutions which have been neutralised by means of the ores is not permissible in practice, owing to the quantity of impurities that are thus introduced into them. Although Luckow's process, as patented, has not been adopted on the large

*German Patent 14,256 (see p. 285).

scale, the specification contained some valuable suggestions to which reference will be made later. At present a short account will be given of a few of the best known among the numerous processes that have been patented.

The Létrange Process. Létrange, in numerous patent specifications, describes in general terms all the methods and conditions of work of a process for the treatment of zinc-bearing substances, excepting those which must be observed for the production of coherent zinc plates. The patent specification prescribes many details of work that are quite impracticable; and the process might have been left unnoticed here had not the accounts published in various technical journals and text books led to the belief that it was in actual use in zinc works. It cannot be disputed that Létrange has made practical experiments in the electrolysis of zinc on a large scale at his works in St. Dénis, but it is certain that if he obtained any practical result, it was not by following accurately the details of his patented process. According to this patent, "Zinc blende is roasted at so low a temperature, that very little sulphur is removed, most of the sulphide being converted into sulphate. The sulphate is dissolved in water, and decomposed by the electric current (a process which appears to be very simple on paper!), and the sulphuric acid thus set free serves to dissolve calamine and zinc oxide. For this purpose it is caused to flow through a system of masonry basins, which are connected with one another by pipes, and are filled with the zinc-bearing material. As soon as the first vessel is deprived of its zinc oxide, it is disconnected from the series, is re-charged, and is then again introduced as the last of the series. The solution saturated with zinc is conducted to a reservoir, from which it is usually led to the electrolysis vessel, which is placed at a somewhat lower level. In this reservoir the zinc solution may be purified (!) from foreign metals according to known methods. The silver and lead contained in the zinc mineral collects in the extraction-vats, and is therefore not lost. The electrolytic tanks are constructed of wood lined with lead, or of glass, cement, &c. and contain zinc (or copper or brass) plates as cathodes, together with carbon anode plates. The solution of zinc sulphate is introduced at the bottom of the vat, whilst the liberated sulphuric acid escapes freely at the same rate from the top on account of its lower specific gravity. The zinc separates at the cathode. If the zinc oxide to be treated should be very pure, the process is simplified because the solution may be effected in the electrolyte tank itself. In that case, either two vessels placed in communication are used, or else one vessel is divided into two compartments by a porous division. The one vessel

*German Patent 21,775. Austrian Priv. of Nov. 12, 1881, &c. [English Patent, 3211, July 22, 1881.]

(or compartment) serves to receive the anode and the zinc oxide or calamine, as the case may be, and the other contains the cathode on which the deposited zinc is separated. Since zinc oxide is a bad conductor of electricity, it is mixed with a certain proportion of carbon, in order to bring it into direct conducting communication with the anode, and so to facilitate solution."

In the whole of this description, discussion of the difficulties is simply evaded. They are lightly passed over, while the simpler matters are treated as broadly as possible.

Other Patented Processes. In the same way there is a whole series of inventors who for the most part specify some method, usually well known already, for effecting the solution of the zinc contained in ores, metallurgical products or waste material; they then combine this method with the electrolytic treatment of the resulting solution, and the invention is complete. It will suffice to give the references in the accompanying footnote* to some patents of this description.

Squire and Currie + deposit the zinc as an amalgam by employing a cathode of mercury in an alkaline solution, and then distil off the mercury. This also is impracticable from the technical point of view.

Some other patentees utilise the anode energy. Thus, Siemens & Halsket propose to use the same process for zinc blende that they introduced for the treatment of copper matte and sulphide of copper ores. But it has been found that when iron salts are present in the electrolyte, the deposition of good solid zinc at the cathode is impossible.

Currie suggests the recovery of certain other metals in the form of insoluble chlorides at the anode, during the electrolysis of chloride of zinc solutions.

In C. Hoepfner's process || poor oxidised ores and metallurgical products are treated with solutions of caustic alkali, and the resulting solutions, after purification with zinc dust, are introduced into the electrolysis tanks. During electrolysis chlorates will be formed in the anode compartments where haloid salts of the alkalies and alkaline earths are present. The anode and cathode cells are separated by suitable (!) diaphragms. Such diaphragms are unquestionably necessary for the success of the process, but, unfortunately, no material that would be suitable for use with the proposed solutions has yet been found.

* Lea and Hammond, English Patent 10,868, Aug. 25, 1886; Croselmire, English Patent, 4,286, March 20, 1888; Burghardt, German Patent 49,682; Heinzerling, German Patent t4,435; Choate, U.S. A. Patents 512,361, 518,711, 518,732, &c.

+ English Patent 12,249, Sept. 27, 1886.

German Patent 42,243 (see Chapter on Copper).

§U.S. A. Patent 466,720, Jan. 5, 1892. [English Patent 212, Jan. 5, 1892.]

German Patent 62,946, 1891.

Lange and Kosmann * propose to obtain both zinc and sulphuric acid by the electrolysis of zinc sulphate solutions. For this purpose zinc blende is roasted; the roaster gases are led into water containing roasted ore in suspension, and thus the electrolyte is produced. No details are given concerning the electrolysis itself.

Use of Deoxidising Agents.-At this point reference may be made to an investigation, to which allusion was made in the first edition of the book, but which the author has been unable as yet to begin, owing to the want of time and opportunity. Experience, alike in electrolytic analysis and in electro-plating, have shown that certain organic compounds are especially adapted for use as depolarising agents. Classen, in his work on Chemical Analysis by Electrolysis, recommends the use of oxalic acid in combination with the oxides of the metals to be deposited, and accompanied by alkaline oxalates. In electro-plating, tartaric and oxalic acids, cyanides, and other organic substances have been employed with special advantage. The organic matter contained in these substances is usually sacrificed, being in most cases. oxidised to carbon dioxide, &c. But it is always possible to select an organic compound, with intermediate products between the initial and the final stages of oxidisation, and to interrupt the process at a moment when the oxidisation has not proceeded so far as to render the material worthless. Among the substances most suitable for this purpose are the distillation products of coal-tar. The author has generally used the so-called liquid carbolic acid, or mixture of cresols, which remains after the removal of the more volatile carbolic acid. Nevertheless, other organic compounds may also be available. The cresol cannot be used as such, inasmuch as it is insoluble, or only very slightly soluble, in water, and is, moreover, a very bad conductor of electricity. But its conversion into a soluble compound is a problem which presents no difficulty. When an alkaline solution of the substance may not be used (it dissolves readily in caustic soda or potash) the cresol is digested with strong sulphuric acid, and is thus readily converted into cresol-sulphonic acids, which, either as such or in the form of salts, fulfil the two conditions necessary for good electrolytes, high conductance and easy solubility in water. After sufficiently long continued electrolysis these sulphonic acids are oxidised completely at the anode into carbon dioxide, water, and sulphuric acid, but by interrupting the electrolysis at suitable moments the whole series of intermediate products that are theoretically possible may be produced. There can be no doubt that a number of oxidation processes for organic compounds may be combined with electrolytic methods for the extraction of metal with great advantage to both. It may, indeed, be assumed that such a * German Patent 57,761. [English Patent 8716, June 5, 1890.]