electrolytic refining of lead haa never been as widely applied as in the case of copper, due to the fact that the operation is more expensive in comparison with the low price of the metal, and that the ordinary fur nace-refined lead of commerce is 99.98 per cent pure. In some cases, however, the lead carries valuable impurities that are not readily re covered by furnace methods, and the process of electrolytic refining is resorted to. The prin ciples involved are the same as for copper, the impure lead being used as anode in a solution of lead fluosilicate as electrolyte. The cathode is a rolled sheet of pure lead. The electrolyte ordinarily carries 60 to 70 grams of lead per litre, as fluosilicate, and 80 grams of free hydro fluosilicic acid. Lead normally tends to give a fine crystalline deposit, but by the addition of 0.1 per cent of gelatin to the electrolyte, this is changed to a smooth, coherent deposit. The temperature has no effect on the deposit, but the current used is sufficient to maintain the bath at about 30° C. (86° F.). The current density used is 12-16 amperes per square foot, and the voltage per tank is 0.30-0.38. Tanks are arranged in series with the electrodes in each tank in multiple. The purity of the re fined lead is about 99.995 per cent Silver.= The parting of the gold and silver when the silver is in excess, or the refining of auriferous .silver, is also carried on by an electrochemical process. In this process the electrodes are arranged horizontally, the anodes above and separated from the cathode by a porous diaphragm. The cathode is a thin sheet of silver formed into an endless belt which travels horizontally below the series of anodes. The upper surface of the belt is smeared with graphite to prevent a close adherence of the crystals of deposited silver. These crystals am brushed off at the end of the tank upon a con veyor-belt and removed at once from the electrolyte. Another modification has a horizon tal graphite plate for cathode, from which the silver crystals are removed by hand with a scraper. The electrolyte carries 1 to 3 per cent of silver, 4 to 6 per cent of copper, and one-tenth of 1 per cent of free nitric acid. A certain amount of the acid is consumed in dis solving the copper present in the silver —about one and one-half pounds to each 1,000 ounces of silver treated.
Gold.— The electrolytic process has been used in the recovery of gold from its solution in potassium cyanide, after cyanide extraction. The cyanide liquor is electrolyzed between iron anodes and sheet lead cathodcs, using low cur rent density. Chemical precipitation of the gold, using zinc or aluminum is usually pre ferred, however.. In addition to this recovery process, electrolytic refining is practised to a considerable extent. The crude gold is used as the anode, in a solution of gold chloride with hydrochloric acid as the electrolyte. The cathode is a thin sheet of pure gold. A cur rent density of 90 amperes per square foot at a low voltage (say 1 volt) is employed. The gold is deposited in crystalline form, leaving the impurities m the anode as a sludge, or dissolved in the electrolyte.
Antimony.—Antimony has been produced by an electrochemical process, but never on any extended scale. One process consists in leach ing the sulphide ore with sodium sulphide, and extracting the dissolved antimony from the solu tion by electrolysis, using iron cathodes, from which the deposited metal is broken by ham mering when it reaches a thicicness of about one-tenth of an inch. The electrolyzing cell is separated into two compartments by. a porous diaphragm, the anode being carbon in a solu tion of sodium chloride. The chlorine from the anode compartment is used in the manufacture of bleaching powder, and the exhausted sodium sulphide from the cathode compartment is used to leach more ore. One method of working up
the slimes from the electrolytic lead-refining process gives a sodium sulphide solution carry ing antimony, which is treated in a similar man ner. Attempts have also been made to refine antimony in both sulphide and fluoride solu tions.
Nicket—While the electrolytic processes have not proved available for the commercial winning of metallic nickel from its ores, its electrolytic refining is successfully accomplished, though the details of the process employed are guarded as a trade secret. As is well known, electroplating with nickel is simple and easy. When, however, a thicker deposition is at tempted, the metal scales off of the cathode in thin flakes which cannot be collected and melted into ingots' at a commercial profit The tend ency of any iron and cobalt present in the crude nkkel to be deposited on the cathode along with the nickel is a serious drawback — and incidentally compels attention to the fact that electrolytically deposited metal is not neces sarily pure. It has been proved by experiment that nickel may be thus deposited in thick plates if the operation is conducted with a hot electro lyte— m the neighborhood of 65° C. (150° F.) — and with a comparatively high current den sity. Difficulty is experienced under these con ditions with the evolution of hydrogen from the cathode, causing pitting of its surface. An indirect method of refining nickel by electro lysis consists in the deposition of its principal impurity (say copper), the release of other im purities (say silver and platinum) in the anode sludge, leaving the pure nickel in solution in the electrolyte, from which it is then de posited.
Calcium.— The production of metallic cal cium by electrolysis may be accomplished economically by using fused calcium chloride as the electrolyte. The principal process used in this country is that of Seward and von Kugelgen. The cell consists of a circular iron box through the bottom of which projects a conical iron cathode, insulated from the box. The anode is a carbon lining, also insulated from the box. Above the cathode at the level of the bath is a water-cooled collecting ring within which the metal collects, it being lighter than the bath. By the time the collecting ring is full of metal the top layer is solidified, and the solid metal is gradually lifted up through the ring by a hook, the freshly collected metal building on underneath as it solidifies, thus making a stick or rod of metal Magnesium.— Being, like aluminum, reduc ible with difficulty by ordinary furnace methods, magnesium is prepared almost solely by electro lysis. The raw material used is gcamallite,' the double chlorice of magnesium and potas sium. The operation is carried on in a cylin drical steel box, which is made the cathode by i suitable electrical connections. The anode• is of carbon, and it is enclosed in a porcelain cylinder open at the bottom and with slotted sides, and having tube at the top for the escape of the chlorine gas set free at the anode. The charge of carnallite is kept in a fused condition by heat applied externally to the steel box. All oxygen is excluded from the appara tus by the introduction of some other gas (usually nitrogen) into the space above the electrolyte. This is necessary in order to pre vent the oxidizing of the metallic magnesium, which rises and floats on the surface of the electrolyte. A tendency of the globules of magnesium to gather a film of oxide sufficient to prevent coalescence is overcome by the addi tion of fluorspar (calcium fluoride) to the molten mass. While the metal thus obtained is not strictly. pure, it :s sufficiently so to be available for all commercial purposes.