Of these, the second was used directly for the production of the anodes, the first was concentrated by roasting and fusing with silicious materials, and the third was employed for the preparation of the solutions. The composition of two separate samples of the matte, as used for the anodes, is given in the following table, and as the composition was variable, a third column is appended, showing the average of a large number of analyses made at a later period than the others :

The anodes were 80 cm. high by 80 cm. wide by 4 mm. thick [2 ft. 7 in. x 2 ft. 7 in. x in.]. They were made by allowing the melted matte to flow out of the furnace into a large iron tank, from which it was removed by iron ladles and cast in iron moulds. The moulds were sunk in the ground so that the sulphides might cool very slowly, as otherwise the anode plates developed cracks, and were easily fractured. A long copper strip 2 cm. wide by 3 mm. thick [ in. x in.] was laid in the mould before pouring, so that it might be cast into the anode, into which it penetrated to about the centre. The free end of the strip, outside the bath, was bent over and fastened by a screw clamp to the positive conductor, which consisted of a copper rod of about 3 cm. [1-2 in.] diameter, and so served to make electrical connections with the matte. To avoid rupture of the anodes under their own weight, for each weighed about 24 cwts., they were supported upon two strips of wood placed beneath them in the bath.

The cathodes consisted of copper plates 80 cm. by 80 cm. by 1 mm. [2 ft. 7 in. x 2 ft. 7 in. × 0.04 in.], with four strips of copper 2 cm. [0.8 in.] wide riveted to each, in order that they might be attached to cross strips which were laid upon transverse wooden slats placed above the vat. The connections of copper strips with the negative conductor were like those adopted for the anodes.

The electrolyte was made by extracting the richest mattes

(those carrying about 50 per cent. of copper) with dilute sulphuric acid, and when ready for use contained about 27 to 28 grms. of copper and 15 grms. of iron per litre [about 4 oz. of copper and about 2 oz. of iron per gallon]. In order to facilitate the constant circulation of the solution during the course of electrolysis, the vats were arranged one above the other in terrace form, and a pipe of 2 in. internal diameter led from the bottom of one vessel to the rim of that next below.

Two Siemens and Halske dynamos of the CF-type, of a size sufficient to deposit 5 cwts. of copper in 24 hours, were used to produce the necessary current. These dynamos, running at 700 and 800 revolutions, gave a current of 430 amperes × 35 volts. The current density in each bath was about 30 amperes per sq. metre [2.8 amperes per sq. ft.], and the electromotive force required was at first I volt per bath.

The profits to be earned by the process were estimated by Marchese in the following way:-Starting with a matte containing 15 to 20 per cent. of copper, 14 per cent. of lead, and 0.05 per cent. of silver, and expecting to recover all these metals (the copper by direct electrolysis, and the other two from the insoluble anode residue by a subsequent treatment) he arrived at the following results. In the ton of matte he reckoned :

Value of 1 ton of matte = Fr. 320

But he was able to purchase the matte at Fr. 112.5 per ton, because only the copper contents were taken into account, and he thus reckoned upon obtaining at once a clear gain of Fr. 207.5 per ton of matte, or Fr. 1383.33 per ton of copper. The interest upon the capital locked up in the shape of copper in the baths was thus estimated: there were 20 anodes of 125 kg. each in every bath, and as there were 58 baths, the total weight of anode material was 145 tons; and this represented (in round numbers, at Fr. 100 per ton) Fr. 14,500. But the anodes gradually gave up their copper during the electrolysis, and he therefore took the half of this sum as the average value of the material in the baths throughout the operation, and so arrived at the sum of Fr. 8000.

Then 580 kg. of copper were deposited daily in the 58 baths. But since three months were required to produce copper plates of the usual marketable thickness, there remained in the baths 90 × 580 = 26,100 kg. of copper (because here also the half of the total copper is taken as an average); and the value of this is Fr. 32,000. The percentage of the copper in the solution itself was so small that it was regarded as negligible. The result of this calculation is that an annual production of 210

tons necessitates the sinking of a capital of Fr. 40,000 in the form of copper in the baths. But since the electrolytic copper recovered is chemically pure, it is estimated as being worth Fr. 125 to 140 more per ton than is the ordinary copper of commerce. And this, calculated on the annual outturn of 210 tons, amounts to nearly Fr. 30,000, which is equivalent to an interest of 75 per cent. on the capital of Fr. 40,000.

The Stolberg plant when at first set in operation, fulfilled all expectations; the baths worked satisfactorily, and the separated copper was pure. After a few days, however, the resistance of the baths began to increase, and required, in some instances, an electromotive force of 5 volts. The primary cause of this increase was a dense deposit of separated sulphur at the anode, which hindered the access of the electrolyte to the undecomposed sulphide within. But there was a still greater difficulty to be encountered. The copper and iron being dissolved out of the anodes, they lost all cohesion, so that large fragments crumbled away and filled up the space between the anodes and cathodes at the bottom of the vat. There they formed a shortcircuit for the current, because their conductivity was higher than that of the electrolyte, and the bath remained undecomposed. Finally, polarisation must have added considerably to the apparent resistance of the bath. The fact that such polarisation existed is proved by the observation that a lower E. M.F. was required for the operation of baths that had been cut out of the circuit for a few days. A modification of the process suggested by the last-mentioned observation was tried, but was soon given up again. Then, as the formation of PbO, was considered to be the cause of the polarisation, the percentage of copper in the anodes was increased, and that of lead diminished, but with no better result; a change in the proportion of iron also failed to produce any improvement. Further, the deposited copper was found to contain antimony, bismuth, lead, iron, zinc, and sulphur. Hence it was determined to abandon the use of matte for anodes, in favour of lead, which would be insoluble. A small-scale experiment was therefore tried, and as the results were satisfactory, a larger bath was erected. The electrolyte employed was the same as had been used in the Marchese baths, only it had the advantage of remaining serviceable for a longer time, because the proportion of iron that it contained was not being constantly increased through the action of the solution upon the anodes. The E.M.F. required was 17 volts; and the results were at first good. But in a short time the quantity of copper deposited fell to 60 per cent. of that which should theoretically have been obtained, and the potential rose to 2.15 volts.

2

Use of Depolarisers.-The cause of this defect also lay in the polarisation of the anodes, which became coated with PbO2

under the oxidising influence of the current, and so yielded a back-electromotive force that opposed and weakened the electrolysing current. It was then hoped that the introduction of a reducing agent would eliminate this source of trouble. Sulphurous acid was selected for the purpose, and was led into contact with the anodes that it might combine with the oxygen separated there, and so form sulphuric acid. A small experiment was therefore made with a bath containing four lead anodes of 0.37 sq. metre [4 sq. ft.] surface area and four coppercoated lead cathodes of like dimensions. The electrolyte contained per litre 39 grms. of copper, 144 grms. of iron in the ferrous state, 3-9 grms. of iron in the form of ferric salts, and 96 grms. of free sulphuric acid.* The sulphurous acid was obtained by burning sulphur, and, mixed with air, was injected into the bath. The E. M.F. required was not diminished through the introduction of the reducing agent, but the yield of copper was increased, and the metal was purer, containing 99.984 per cent. of copper. A large quantity of sulphuric acid was, of course, formed, and this extracted so much soluble material from the copper mattes that trouble was caused by the crystallisation of salts in the baths. It should here be added that a patent was granted for the use of sulphurous acid as a depolarising agent in 1885. A larger bath was then arranged on the lines of the experimental apparatus. At first the gases from matte-calcining furnaces were led into the baths to provide the sulphurous acid required; but the gases were found to be too dilute, and were, therefore, replaced by those from the muffles used in the calcining of zinc ores. But various circumstances (among others, the offer of a new process by Siemens & Halske) interfered with the prosecution of the work in this direction, which would have involved the transfer of the whole plant. The use of insoluble anodes, and of the depolarising agent, was a great departure from the original Marchese process, and it is to be regretted that the experiments which were set on foot could not be carried through; experiments, however, with other depolarisers, both at Stolberg and in other installations, have not as yet led to satisfactory results.

Body's Process.-In working the Marchese process, it was remarked that a part of the current was used up in converting the ferrous sulphate present in the liquors into the corresponding ferric compound, and that this salt again attacked the anode material. It was, therefore, only natural that more attention should be given to the action produced by iron salts, with the object of utilising it in some convenient way. The first step in this direction, at least, in connection with electrolytic processes,

[This is equivalent to about 6 ozs. of copper, 23 ozs. of ferrous iron, oz. of ferric iron, and 1 oz. of sulphuric acid per gallon of liquid.] + German Patent 32,866, March 13, 1885.

is to be found in a patent granted to Body; but in purely metallurgical works the utilisation of iron salts had, of course, long been known. Although Body's apparatus and process are not concerned with copper extraction alone, but were intended for the electrolytic extraction of metals from ores in general, they may be here described as being the forerunner of the well-known processes of Siemens & Halske and Hoepfner.

The vessel, A (Figs. 123 and 124), is made of Portland cement, and is painted within and without with an impermeable material. The partition walls, S, which are also of Portland cement, do not reach quite to the bottom of the bath; and in the space thus left beneath them are placed plates of some material (like felt) that is pervious to water. The raised floor is covered with a carbon plate, C, in connection with the positive pole of an electrical generator; and the inner surfaces of the

outer walls of the vessel are also lined with carbon, D, which, however, is practically unnecessary. The metal plates, K, which form the cathodes, are suspended in the space outside the partitions, S. A solution of ferric salts with sodium chloride is used as electrolyte. The ore, which has been previously moistened with a similar solution, and is still saturated with it, is placed in the inner space, J, and is here kept in constant motion during electrolysis by means of the stirrer, R. The solution enters through the opening, O, in the raised floor, follows the course of the arrow marked in the figure over the partitions, S, and, finally, after traversing the cathode compartment, escapes through the aperture, O, in these outer chambers. While the solution is thus slowly circulating the following reactions take place :

*U.S.A. Patent 33,815, Jan. 5, 1886.