but they do not show any changes in the principle of the apparatus as described in the patent specification.

The electrolyte vats consist at present of long tanks of pitch pine, tarred on the inside about 0.6 metre in width (inside measurement) and 3.75 metre in length [2 ft. x 12 ft.]. Each of these vats is divided by transverse partitions into seven compartments, and every compartment contains three rows of anodes and four cathodes. The anodes are suspended in linen or cotton

Fig. 136.-The Moebius apparatus - plan.

bags, the cathodes merely hang in the baths. Wooden scrapers are supported from frames, which are free to travel backwards and forwards on the rim of the vat; and these scrapers serve to break off the silver crystals as they grow upon the surface of the cathode, so that all possibility of the current becoming shortcircuited is avoided. Under the electrodes, and covering almost the whole floor area of each section of the vat, is a deep tray with a linen bottom, which is intended to receive the principal portion of the crystals detached by the scraper. By means of a

frame, all these arrangements may be lifted together from the vats in which they are suspended. The silver to be refined is cast into anode plates, a (Fig. 133), which are usually 6 to 10 mm. [ to in.] thick. These are suspended by the double hooks, h, shown in Fig. 135, from a metal frame, R (Figs. 133, 135, 136), which serves both as a support for the plates, and as a means of connection with the current. For the latter purpose, it is placed at one side, in direct contact with the conductor, P, whilst on the other side it is separated from the negative conductor, N, by the insulating sheath, I. In order to prevent the undissolved gold, which is left at the anodes in the form of a brown powder, from mingling with the silver brushed from the cathodes, the anodes are enclosed in bags of closely woven filter cloth, which are stretched on the wooden support, G (Figs. 133, 135); and this in turn is suspended from the frame, R.

The cathodes, k, consist of thin rolled silver plates, soldered to a copper rod, which is suspended in a horizontal position.

+

+

Fig. 137.-The Moebius apparatus-scheme of electrical connections. At each end of this rod, where it projects beyond the edges of the silver plate, is attached a supporting-hook, v, which serves to suspend the cathode from the positive (P) and negative (N) conductors of the bath; the hook on the one side is therefore placed in direct metallic connection with the negative lead, N, but that on the other side is insulated from the positive conductor, P, as shown in Fig. 134. The advantage of this arrangement is that both conductors are made available as supports, while only one is used to convey the current.

The conductors, P(+) and N (-), are thick copper, brass, or bronze rods; and these rods must not only be thick enough to carry the required current, but they must obviously be capable also of bearing the weight of the electrodes and their supports. The several sections of the vat, usually seven in number, are electrically coupled in series, as shown in the scheme of connections sketched in Fig. 137.

The arrangement of the scrapers, s, is shown in Figs. 134, 135, and 136; they are simply wooden slats, of which two pairs are provided for each cathode plate. They are forked, so as to embrace the cathodes from above, and are supported by the frame, F, shaped like a grid-iron, by which the necessary reciprocating motion is imparted to them. The frame is supported on rollers, which run over the rails, n, these being securely

fastened to stringers forming part of the framework, O, of the top of the vat. The frame is moved to and fro by means of a sliding cam-rail, which is attached to one of the short sides of the vat, and to which a reciprocating motion is imparted by an eccentric. The attachment between them is made by a hinged pin on the frame, which engages with a socket in the slide-bar. The frame, O, together with all parts of the apparatus that dip into the bath, may be raised at one operation by the lifting arrangement indicated in Fig. 134.

The flat wooden boxes, C, serve as collecting trays for the muddy deposit of silver. They are so made that they may be lowered with sufficient clearance, one into each of the compartments of the vat. Thin strips of wood are attached to the sides of the tray, and, being extended upwards, are fastened to the electrode frame, so that the whole may be lifted out of the bath together. The bottom of the tray consists of a grid of laths, fastened by pins, dowelled into the wooden sides of the tray. Before starting the electrolysis, this grid is covered with coarse filtering cloth or sacking, in order that the silver crystals may be retained in it when the tray is raised from the vat. By knocking out the connecting-pins, the bottom may be at once detached from the sides of the tray when it has received its charge of silver and after removal from the vat.

The

The baths are filled with a weak acidulated solution of silver nitrate as electrolyte, or, if desired, the work may be begun with very dilute nitric acid. As soon as the above described apparatus has been placed in the bath the electrical connections may be completed. The maintenance of a constant current density is of far less importance in this process than in most electrolytic operations. Especially at the outset, or so long as the solution contains but little copper, a very high current density is permissible; it may exceed 300 amperes per sq. metre [0.2 ampere per sq. in.]. In course of electrolysis the copper from the anode (which may frequently contain only 95 per cent. of silver) gradually accumulates in the bath, so that the proportion of copper present will, on the average, be higher than that of silver. former may amount to about 4 per cent., and the latter to 0.5 per cent., when from 0.1 to 1 per cent. of free nitric acid are present. It is essential that as the percentage of copper increases the proportion of nitric acid be also raised; the current density should be lowered to about 200 amperes per sq. metre [0.13 ampere per sq. in.]. The electromotive force required will usually vary from 14 to 1.5 volts per cell. In each of the compartments shown in Figs. 133 to 137 a current of 150 amperes (with an available cathode area of 0.75 sq. metre, or 8 sq. ft.) should suffice to transfer 21.6 kg. [473 lbs.] of silver from the anode to the cathode in 36 hours. Now, if three rows, each with five anodes of the above-named dimensions, be suspended in the baths, about 1.5

kg. [3.3 lbs.] of silver will be dissolved from every anode plate (or, in other words, almost the whole of the silver introduced will have been carried over to the cathode) in the time quoted. It is, therefore, possible to treat quite considerable quantities of silver within a small area; moreover, the metal is only locked up in the apparatus for a short time, which is a very important consideration in dealing with the precious metals. The cost of the process is not high, as may be easily calculated from the data given above; the separation of gold is practically complete; and, finally, the operation is conducted without any evolution of gas. In spite of the high current density, the heating of the solution is very slight, thanks to the high electrical conductance of the electrode and electrolyte, and especially to the constant movement of the solution caused by the motion of the scrapers.

At intervals of 24 hours the whole of the apparatus suspended in the bath, including the silver trays, should be lifted out by means of its supporting frame. After allowing the trays to drain into the bath they are separated from the electrode frames and taken to the washing plant in order that they may be emptied of silver deposit. This emptying is done, as already indicated, by removing the pins which hold the bottom grids and the filtering cloth to the sides of the trays. The silver is washed, pressed by hydraulic power, dried, and melted together. The anode cells are emptied once or twice a week, according to the proportion of gold in the silver that is being refined. The further treatment of the anode deposit will depend upon its composition. (See Gold.)

[The Pennsylvania Lead Company's Plant.-This Company have had the Moebius process in operation at their Pittsburg Works since 1886, and a valuable account of their present practice has been given by Faunce.* It is stated that there are 14 tanks, each of which is divided into seven separate electrolyte cells, but that they are not all in constant use. The details of the plant are similar in most respects to those of the Frankfurt installation. The current density used is 0.125 amp. per sq. in. of cathode surface, and with a solution of silver and copper nitrate containing to 1 per cent. of free nitric acid, and a space of about 1 in. between the electrodes, it is reckoned that each kilogramme of silver deposited requires about H.P.hour; in other words, 80 oz. of silver is the equivalent of 1 H.P.hour. The average outturn of 70 baths amounts to 33,000 oz. per diem, and the metal (which before use has been refined by cupellation until it contains less than 2 per cent. in all of lead, copper, bismuth, and other impurities) is so far purified by the electrolytic process that the crystals, when run into ingots, afford a metal 999 to 999 fine. The anode slimes, after being [Journ. Franklin Instit., 1895, and Mineral Industries, 1895, vol. iv., p. 355.] † [99.9 to 99.95 per cent. of silver.]

*

granulated and parted, yield gold that should be 996 to 998 fine. When the proportion of copper in the electrolyte rises much above 4 to 5 per cent. the solution must be run off from the vats and purified to remove the copper and lead that it contains. But little copper should be present in the anodes, as otherwise this metal would tend to be deposited with the silver. The cost of labour in the process is comparatively slight, a staff of three men by day and one by night sufficing for the whole of the work in connection with the management of the plant, as well as for the melting of the silver and the refining of the gold. In addition to these hands a carpenter is employed daily in repairing and renewing the tanks (about one-seventh of the whole number being always thrown out of use for repairs), and a mason and a machinist are also required about one day a week for general work.-TRANSLATOR.]

g

Fig. 140.-The Moebius anode connection (new form).

Later Form of Moebius Apparatus.-In a newer form of apparatus,* shown in Figs. 138, 139, and 140, Moebius has

* [English Patent 469, Jan. 8, 1895, and U.S. A. Patent 532,209, Jan. 8,