189

C.—3

The ferrocyanide of potassium formed according to the last reaction is reacted upon, if sufficient acid be present, by an additional quantity of ferrous sulphate, with production of Prussian blue, according to the reaction :— 3 (K 4 FeCy c ) + 6 FeSO 4 + 30 = FeA + 6 K,SO 4 + Fe 7 Cy 18 . This production of Prussian blue gives a blue colour to the surface of the tailings, or to the solution, and indicates at once that the washing and neutralising operations have not been carried on properly, and that a great loss of cyanide is taking place. Ferric salts, when present unmixed with ferrous salts, decompose the cyanide solution with the formation of hydrocyanic acid and precipitation of ferric hydrate, according to the reactions, — Fe 2 (50 4 ) 3 + 4KCy = F a Cy 6 + 3 K 3 SO 4 . With further decomposition: Fe a Cy c + 6 H. 2 O = Fe 3 (OH) 6 + 6 HCy. This means that, other things being equal, one molecule of ferric sulphate decomposes six molecules of cyanide. If 1 per cent., or 201b., of ferric sulphate existed in the ore, very nearly the same weight of cyanide, costing £2 55., would be destroyed. If a mixture of ferric and ferrous sulphate, as is probable, exists in partially oxidized ores, it causes the production, when ferrous sulphate is in excess, of ferrous ferrocyanide, according to the reaction, — 12 KCy + 3 FeSO 4 + 4 Fe 2 (50 4 ) 3 = Fe 3 (3 FeCy 6 ) 2 + 6 K 2 SO 4 . When ferric sulphate is in excess, the production of ferric ferrocyanide (Prussian blue), according to the reaction, —■ 18 KCy + 3 FeSO 4 +4 Fe (50 4 ) 3 = Fe 4 (FeCy 6 ) 3 + 9 KSO,. These reactions show clearly that washing by water and neutralising by a caustic alkali must be employed to arrive at satisfactory and economical results. It is more than probable that many of the failures already recorded are due to the lack of these precautions. In addition to these reactions, there are many with unknown compounds, the composition of which cannot be expressed, even where the greatest precautions are used and the operations supervised with the greatest ability and knowledge. Precipitation of the Gold.— Zinc precipitates the dissolved gold, as the cyanide has more affinity for it than for the gold. The theoretical reaction is— 2 KAuCy 2 + Zn = 2 Au + K 2 ZnCy 2 but more zinc goes into solution than this reaction calls for. The consumption in South Africa, as a matter of fact, is lib. of zinc to loz. of gold, instead of lib. of zinc to 61b. of gold, as called for by the reaction. This excessive consumption of zinc must be ascribed to other action than the mere replacement of zinc for gold in the double cyanide of gold and potassium. There is comparatively little exact knowledge of the reactions taking place in the zinc-precipi-tation boxes. One fact is known positively, and that is that hydrogen is evolved. This does not occur, however, when zinc alone is exposed to a cyanide solution, but after gold is deposited on the zinc, or when zinc is placed in contact with iron. In other words, a galvanic couple is formed, the water is decomposed, and hydrate of zinc is formed, which is attacked by the cyanide, forming a double cyanide of zinc and caustic potash. The probable reactions may be expressed as follows : — Zn + 2 H0. 2 = 2 H + Zn 2 (HO). Zn 2 (HO) + ± KCy = ZnK 2 Cy 4 + 2 KHO. This production of caustic alkali explains the increased alkalinity of the solution after passing the zinc-precipitation boxes. It may be considered advantageous to a certain extent, however, as carbonic acid, which decomposes the solution, is absorbed by the caustic potash, with formation of a carbonate of the alkalies. Ammonia is formed also, as is indicated by the strong odour of that gas about the boxes. The precipitate contains, besides the precious metals, many of the base metals, which may be dissolved by the solution. The principal of these are copper, arsenic, and antimony, and, in the condition in which it is refined, large quantities of zinc and the impurities of the zinc. An incomplete analysis of the precipitate formed at the Mercier Works, Fairfield, Utah, is here given :— Zinc ... ... ... ... ... ... ... ... 391 Calcium carbonate ... ... '... ... ... ... 36-7 Gold ... ... ... ... ... ... ... ... 44 Cyanogen ... ... ... ... ... ... ... 3-5 Sulphur... ... ... ... ... ... ... ... 2-6 Iron ... ... ... ... ... ... ... ... 2-4 Undetermined residue ... ... ... ... ~. ... 6 - 0 The gangue of the ore treated here is a silicious limestone, and the lime in the precipitate can thus be accounted for; but in what manner it has become reunited with carbonic acid is a mystery. It would seem improbable that calcium cyanide, formed in the first place by the reaction of cyanide on the calcite, was decomposed by the hydrate of zinc formed in precipitation with formation of caustic lime, which in turn was changed to carbonate by atmospheric action, for the consumption of cyanide would in this case be much higher than it is in reality (1'271b. per ton); yet this sequence of reactions, hovvever improbable, is the only way in which we can account for it, unless we consider the carbonate of lime to be dissolved by the alkaline solution without decomposition. In this case it might be reprecipitated by several of the reactions which occur in the zinc-boxes. Copper was found by C. A. Aaron to be precipitated by zinc only in contact with .iron, in a non-metallic form, from a solution of commercial cyanide of potassium. The precipitate dissolved with an evolution of gas, which, as it had no odour, was certainly not cyanogen gas.