1. The metals contained in the ore are brought into solution at the expense of the ferric salts in the electrolyte, which are thus reduced to the ferrous state.

2. The dissolved metal is deposited at the cathode.

3. The chlorine which is set free at the anode peroxidises the ferrous salts that have been produced, and any excess of chlorine that may escape absorption in this way is able to act directly upon the ore.

The Siemens-Halske Matte-Refining Process.-Siemens & Halske then went a step beyond this, and caused the reactions between the ferric salts and the copper compounds to take place entirely outside the electrolytic vessel. They stored up, so to speak, the anode energy in a part of the electrolyte in order that they might utilise it outside the baths. The process

was thus described in the first German patent taken out by the firm.*

The powdered copper pyrites is roasted at a moderate temperature, preferably in a Gerstenhöfer furnace, in such a way that the iron is almost completely oxidised, whilst the copper is contained in the roasted material, partly as copper sulphate, and partly as copper oxide, but mainly as cuprous sulphide. The finely-divided material, after calcination, is treated with the solution flowing from the electrolysing tanks. This leaching is best performed in a series of vats, through which the liquid flows successively in such a manner that it passes last through the vat that was latest charged with the ore. The solution, which is thus newly enriched with copper sulphate, and no longer contains any ferric salt, is now returned to the electrolytic cells, where it is first deprived of its copper, and is then peroxidised, so that it may be used afresh to extract the copper from another charge of ore. The process is therefore continuous, and the same solution may be used repeatedly until, owing to the absorption of foreign metals previously contained in the ore, it has become too impure for the process of electrodeposition.

This solution, for use in separate electrolyte cells, should be introduced continuously nearly at the bottom of the cells which surround the cathode plates; then rising to the top of these, and depositing a part of its copper by electrolytic action on the cathode on its way, it flows over the top rim of the membrane into the anode compartment, through which it passes to make its final escape from the bottom of this cell (Fig. 125). During the passage of the electrolyte through the anode cell, the ferrous sulphate that it contains is first converted into a basic ferric sulphate, which in turn is changed into the normal ferric sulphate by the absorption of sulphuric acid produced through *German Patent 42,243, Sept. 14, 1886. [English Patent 14,033, Nov. 1, 1886.]

the electrolysis of the copper sulphate; the higher specific gravity of the latter salt causes it to sink to the bottom

R

of the vessel. The liquid escaping from this compartment, therefore, contains less copper than was present in it when introduced into the cathode cell; and it also contains neutral ferric sulphate in solution. This solution has now the power of converting cupric and cuprous sulphides and copper oxide into copper sulphate. In thus acting upon the first-named copper compound the ferric sulphate is reconverted into ferrous sulphate, whilst the liberated oxygen serves to oxidise the sulphide of copper. The product from the roasting of the ore at a low temperature, as above explained, contains most of its copper in the form of sub-sulphide; but the iron is present as peroxide, a substance which is not attacked by ferric sulphate, and is scarcely affected by sulphuric acid. The cuprous sulphide, however, is energetically dissolved by the ferric solution. The chemical processes, which take place during the electrolysis, and the leaching process are clearly shown in the following equations:

Fig. 125. Siemens & Halske electrolytic cell for treating copper ore.

I. xH2SO4 + 2CuSO4 + 4FeSO4 = 2Cu + 2Fe2(SO4)3 + xH2SO4
II. (α) xH2SO4 + Cu¿S + 2Fe2(SO4)3

(b)

(c)

(d)

=

2CuSO4 + 4FeSO4 + S + xH2SO4 = CuSO4 + H2O

CuO + H2SO4
3CuO+ Fe2(SO4)3 = 3CuSO4 + Fe2O3

CuO+2FeSO4 + H2O = CuSO4 + (Fe2O3 + SO3) + H2

A comparison of the equations I. and II. (a), shows that if the ore hold all its copper in the form of cuprous sulphide, the electrolyte, after passing through the leaching vats, will contain exactly the same quantity of copper sulphate, ferrous sulphate, and free sulphuric acid as it did prior to electrolysis; and that it is, therefore, completely regenerated, and may be used again for the electrolytic decomposition. But if the copper be present in the ore partly as copper oxide, it is evident from equations II. (b), (c), and (d) that in this case the solution will be richer in copper, but poorer in respect of iron and sulphuric acid than it was before electrolysis.

It is not necessary to point out that the raw matte may be used in lieu of the roasted material, because the copper is present almost entirely in the form of cuprous sulphide. In this case, however, iron would also be dissolved, and a complete

uniformity of the solutions in regard to copper and iron could not be maintained. It is to be remarked that in the described electrolytic process no polarisation occurs, and that the position of the two electrode materials in the electro-chemical series gives rise to no counter electromotive force.

Whilst, with matte anodes, an E.M.F. of 1.5 volts is necessary to give the required current density, 07 volt will suffice when the above process is adopted. And, again, whilst in the former case, about one-third of the current volume is used for other reduction processes, and is therefore lost; by the alternative method there is no loss whatever of this nature.

In order to produce the rapid circulation of liquid through the vats, which is necessary for satisfactory work, the cells are placed in terrace form (Fig. 126), and all the cathode compartments, K1, K2, K ̧, are connected together by siphons, h1, h2, h, in one

Fig. 126.-Arrangement of vats in the Siemens-Halske process. group, while the anode cells, A,, A2, A3, are similarly connected in another series by the siphons, k1, ką, ką. In order to maintain the level of liquid in the vessels independent of the quantity of solution added, the ends of the siphons in the lower vats are turned upwards for the space, a, which is equal to the difference in height, B, between two consecutive vessels.

Modified Siemens-Halske Process.-From a subsidiary patent* taken out by the same firm, it must be concluded that the use of this process led to difficulties. The electrolytic cells had previously been divided into two (positive and negative) compartments by a membrane; but it is shown in the second patent specification that these membranes are liable to become torn during electrolysis. The membranes have either too high

* German Patent 48,959, Jan. 3, 1889. [English Patent 3533 Feb. 27,

an electrical resistance, or else they are not sufficiently durable, for they stretch and allow the solutions to escape.

Figs. 127, 128, and 129 show an electrolyte cell in which this

Siemens & Halske's electrolyte cell (1889).

evil is remedied. A flat vessel, G, made of wood or of other suitable material, and coated with lead, is provided with a perforated false bottom, L, on which the anode, A, is extended. The anode may consist, either of plates of retort carbon in direct

electrical connection with one another, or of perforated lead plates covered with small fragments of retort carbon, or, finally, of deeply corrugated lead plates containing perforations to allow of the passage of the electrolyte. The horizontal anode is provided with the necessary insulated electrical connections, and is covered with a layer of some filtering material that may serve to prevent the escape of the solution surrounding the anodes The filter may be of felt or any other suitable organic or inorganic material. The cathodes consist of the surfaces of the cylinders, K, which are quite covered by the electrolyte, and are constantly maintained in slow revolution by the waterproof belt, S. These cylinders may be made of a wooden core coated with wax, cement, or other material, and surrounded with a conducting material, which is electrically connected in any suitable way with the journals of the cylinders and the conductors, k.

The regenerated solution, consisting of copper and ferrous sulphate solutions, is conveyed in a continuous stream into the liquid which is already covering the cylinders. The rotation of

the latter effects the continual mixture of the solution down to the partition separating it from the anode compartment. The tube, U, conducts the solution away from the space beneath the filter at the same rate as the regenerated liquor is run into the upper compartment through C; and, there is, in consequence of this, a constant but slow transference of liquid through the filter from the cathode to the anode compartment. Here the ferrous salt is reconverted into ferric sulphate by the liberated oxygen, and the ferric salt, having a higher specific gravity, sinks to the bottom and is at once carried away through U, so that by properly regulating the inflow of liquid, the strength of the current, and the quantity of copper and iron in the solution, the result of the process should be that the electrolyte in the upper compartment loses some two-thirds of the copper contained in it, while in the anode portion the whole of the ferrous salt is peroxidised to the ferric state. The solution is uninterruptedly conveyed from the anode cell to the extraction tank, and after acting upon the ore powder, it circulates through the whole system again.

According to later accounts the anodes were afterwards made of specially prepared homogeneous round carbon rods, a (see Figs. 130 and 131), of which every 109 were bound together into one group by means of a thoroughly insulated cast-lead frame, forming a system 1600 mm. long by 405 mm. wide [5 ft. 6 in. × 1 ft. 4 in.]. The connection with the main conductors is made by means of the lead strips, V, cast on to the frames.

The electrolyte vessels are shallow wooden tanks rendered * Grusonwerk-Magdeburg. Das Siemens'che Kupfergewinnungsverfahren aus Erzen.