C.—3

208

it is ready to run direct into the leaching-vats again. The discharge system indicated is the bottom discharge. The third design, shown in Fig. 11, is a combination of those shown in Pigs. 9 and 10, and is fitted with a pipe-service to enable the operator, if desired, to run the solution up through the sand in the leaching-vats. As shown in the sketch, the plant is designed for side discharge, but all of them could be adapted for different systems of discharge, as may be found advantageous according to local conditions. In a description of the cyanide process in reference to the treatment of gold-ores at the Wit-watersrand-Transvaal goldfields, read before the Society of Chemical Industry in April, 1894, by Henry de Mosenthal, F.1.C., F.C.S., he states that the strongest solution he had seen used was 0-6 per cent., in which case the weak solution was 0-3 per cent. In some works he had seen a 03--per-cent. strong solution, and a 01-per-cent. weak solution used. The amount of cyanide required to dissolve lOOoz. troy of gold is theoretically 4 - 531b., but in practice it is found it takes at least 3001b. to extract lOOoz. gold out of 400 tons tailings. This would mean fib. of cyanide per ton of tailings treated, but none of the mines are working so low as this. Besides, it has been found that, the more pyritic the ore, the more cyanide is necessary. In actual practice in some of the works they were using 21b. of 90-per-cent. cyanide per ton of tailings, whilst at others the quantity was considerably less. At the Eobinson Mine the average in 1893 was l'l6lb. per ton of tailings. The chief reasons why the consumption of cyanide in the tanks is so enormously in excess of that which is theoretically necessary are : — Firstly.—That the tailings contain finely-divided pyrites. The action of the air on this in the set-tling-dams causes the tailings to become acid. The action of ferric and ferrous sulphates upon cyanide —in acid tailings —is probably to form some of the Prussian blues. This acidity has been ruined by using alkali. Formerly lime-water was used; now, however, caustic soda seems to be preferred, most works using about Jib. of caustic soda to a ton of tailings. But, although less than in acid solutions, the pyrites present still consumes a good deal of cyanide with the formation of iron prussiates. Secondly.—That the carbonic acid is absorbed from the atmosphere and produces the following reaction: — 2 KCy + CO 2 + H,O = K 2 CO 3 + 2 HCy whereby volatile prussic acid is formed. Thirdly. —That a good deal of cyanide remains mixed with the closely-packed, and to some extent porous, particles of ore in the tanks. In reference to the method of precipitation, Mr. Mosenthal states he has seen the new process patented by Messrs. Siemens and Halske, whose representative, Dr. Yon Gerret, was working at the Eand Central Ore-reduction Company's Works. They precipitate the gold out of the cyanide liquor by means of electricity, using iron as a positive electrode, and thin lead foil as a negative electrode. The gold and silver is deposited on the lead, whilst the cyanide is deposited on the iron, and there forms Prussian blue. The iron is coated with a special material, which causes the Prussian blue to adhere to it, instead of floating in a flocculent condition in the liquor, as it otherwise would. This Prussian blue is, by treatment with carbonate of potash, converted into cyanide, whilst the gold and silver are separated from the lead foil by cupillation. It is claimed that the cost of this process is about the same as that now in use, with the advantage of the regained cyanide, in addition to which the liquor after treatment is purer than the liquor that has been through the zinc boxes. Dr. Yon Gerret was working with 10 volts and using about 10 amperes per ton of ore, but Mr. Mosenthal states he has since learned that any tension over six volts decomposes the cyanide to some extent. Just as he was leaving the goldfield Mr. Andre arrived at Johannesburg from the Gold and Silver Scheido Anstalt at Frankfort to try a new process, according to which the gold is precipitated from the cyanide liquor by means of aluminium. The following is the cost, per ton of tailings treated, of using the cyanide process at the Eobinson Mine in South Africa :• — s. d. Wages ... ... ... ... ... ... ... 1 2-81 Supplies and stores ... .. ... ... ... ... 0 612 Fuel, assaying, melting, &c. ... ... ~. ... ... 0 5 - 24 Cyanide, l-161b. ... ... ... ... ... ... 2 049 Zinc, 0-23 ... ... ... ... ... ... ... 0 1-13 Filling and discharging vats ... ... ... ... ... 1 6-81 Eoyalty ... ... ... ... ... ... ... 1 4-38 Total ... ... ... ... ... ... 7 6-68 The New Eecoveey Pbocess.—Electbical Pbecipitation pbom Cyanide Solution. At a meeting of the Chemical and Metallurgical Society of South Africa, recently held, Mr. A. Yon Gernet read the following paper upon the Siemens and Halske gold-recovery process, which has been tried on a large scale at the Worcester Mine for four months, after long and exhaustive preliminary experiments at the works of the Eand Central Ore-reduction Company. Mr. Yon Gernet said :— " Although the electrical precipitation of gold, extracted from ores by cyanide, has only been recently introduced into South Africa, it has been in use in Europe and Asia as far back as 1888. The author having been connected with the process from the beginning, he is thereby enabled to give some information concerning its development, based on actual working data, which may be of general interest. In 1887 Dr. Siemens found that the gold anodes used in electroplating at his