This attractive method of separating gold from silver has made some headway, and will in many cases replace other methods of parting. The electrolyte used is a solution of silver nitrate, or it may be started with dilute nitric acid. The cathode consisting of thin silver plate and the anode of the silver-gold alloy usually in plates about 3/8 inch in thickness. The electrolyte vats consist of rectangular wooden vessels, about 26 inches wide and 20 deep in internal cross section and 12 feet long. Each vat is divided transversely into seven compartments, and each compartment holds four cathode plates suspended vertically and transversely, each reaches to within 1½ inches from the sides of the vat, and 6 inches from the bottom. These are soldered on to a copper rod on their upper horizontal edge, the ends of the bar projects for about half an inch beyond the plates, and are attached to a supporting hook (v), which serves to suspend the cathode from the positive (P) and negative (N) conductors of the bath, the positive pole being insulated. The cathode plates (a) are about 4½ inches wide and 8½ inches deep, and are cast so as to be rectangular on three sides, but the upper side has a slightly projecting lug in the centre with an eye in it, so as to enable it to be suspended almost wholly in the bath. Five of these anode plates are so arranged to hang vertically with their edges slightly overlapping so as to form what is equivalent to a continuous plate parallel to the cathode. Each of these is suspended by a double hook (h) from a rectangular metal frame (R), which rests directly on the positive conductor (P), and is insulated from the negative (N) by an insulating sheath (I).
Since valuable material is locked up while refining operations are proceeding, it is advisable to shorten the time required for the solution and deposition of the silver to a minimum. In other words the current density must be as high as is consistent with not heating the solution, keeping impurities in solution, or leaving them at the anode. If silver alone is to be separated, the density may be 0.2 ampere per square inch. When copper is also present or accumulates in the solution, the current density should not exceed 0.13 amperes per square inch. The E.M.F. necessary will be 1.5 volts per cell. For the compartment of the size indicated a current of 150 amperes, with a cathode area of 8 square feet will serve for the deposition of 31 2/3lbs. of silver in 24 hours, or each anode plate of the dimensions given will have its silver contents removed in 36 hours.
Silver is not precipitated from its nitrate solutions under the conditions named, in a coherent form, but as fine bright crystals, which do not adhere. These would soon bridge the space between anode and cathode the short circuit the metals. In order to prevent this scrapers passing between anode and cathode remove any growths, and allow the crystals of silver to fall to the bottom of the bath. The scrapers are simply vertical wooden laths nailed on to a horizontal frame. Two pairs are provided for each cathode plate, each pair forking the plate, and so placed that as the frame (F) moves backward and forward with a reciprocating motion the laths sweep any growths of crystals off from end to end.
The gold separating from the anode plates would fall off and mix with the silver were this not provided for by enclosing them in a narrow rectangular frame (G), the sides of which are covered with filter cloth.
A tray is provided to facilitate the removal of the silver crystals, and arrangements are provided for lifting the whole of the framework with the scrapers, the anodes, and the cathodes out of the vat. The silver crystals are removed, washed, pressed and smelted, and the gold powder, if pure, can be similarly treated. As a general rule, however, the gold is contaminated with oxides of silver, lead per-oxide, bismuth, oxide, and some silver. At the St. Louis Smelting and Refining Works, where this process was in operation, the gold so obtained is melted down with the addition of more silver, if necessary, and the bullion parted in the ordinary way with nitric acid. It would thus appear as if the electrolytic process was looked upon as more suitable for refining and removing a large quantity of silver from a small amount of gold rather than a separative process per se. It is considerably cheaper than the Gutzkow, and has the advantage of being worked with dilute solutions in the cold, and without any evolution of noxious fumes.
Moebius has devised an improved apparatus for his process, the main improvements being the continuous delivery of the refined silver crystals and the facilities given for introducing and replacing the anode bars, as well as the perfect solution of the latter. It consists essentially of a shallow rectangular vat A. The cathode C consists of a broad endless belt of silver 1-32-inch in thickness, which travels on rollers in the bath B, b, in the direction of the arrows. The anode (G), consisting of gold-bearing silver, lies horizontally above the belt in a frame (E), covered with filter cloth, this cloth being paraffined or oiled, to protect it from the action of the acid solutions. The vat contains strong solution of potassium, or sodium nitrate, so acidified with nitric or sulphuric acid as to keep all the copper in solution. The silver crystals are deposited on the cathode belt, and are carried forward and dropped on to the belt D, on which they are carried upwards out of the liquid, they then pass over the pulley (d) at (O), and drop into the trough R. A scraper (S) serves to remove those carried beyond O. The rollers are driven by a chain belt (n) outside the vat. The necessary current is introduced through the copper bar (K), and flows through the stout wire (M) attached to and above K to the anode bar below. The point of the wire is provided with a platinum cap (g), and the lower portion, which might come in contact with the liquid, is protected by paraffin and sheathed in rubber. The brush F connected on to the negative conductor L serves to make connection with the cathode.
The Improved Moebius Plant
The improved Moebius plant was successfully introduced by the Guggenheim Smelting Co., N.J., in 1904. It contains 48 electrolytic tanks. The tanks are constructed of 2-inch pitch pine, coated with acid proof paint, and measure 14 feet 3 inches long, 16 inches wide and 7 inches deep. The electrolyte contains 0.1 per cent, free nitric acid, 4 to 5 per cent, copper, and 1.2 per cent, silver. The consumption of acid is about 1½lb. for every 1000oz. dore parted.
In each tank are placed six frames (18 inches square and 1 inch deep), over which muslin diaphragms are stretched. The frames are divided by strips into four sections, in each of which an anode is placed. The dore contains about 980 silver to 3 to 8 gold, and the anodes cast are 15 x 3½ x ½ inch. The cathode belt runs either within half an inch of the muslin, and is 31 feet long by 15 inches wide. At first the belts were made of silver-coated rubber, but owing to the buckling of the rubber silver belt sheets, 1-32 inch thick, were introduced. The upper surface of the belt is coated with a graphite composition, as otherwise the silver will adhere firmly in places, rendering the belt brittle and useless. This preparation is applied once a fortnight. The belt moves at the rate of three feet per minute, and about 3 horse-power is required to drive the chain and gearing which move the rolls in the vessel. A 40 horse-power drives a general electric generator capable of delivering 300 amperes at 150 volts, the actual supply for 24,000oz. of silver daily being 220 amperes at 90 volts. The silver obtained is practically pure.