205

C.—3

In considering the treatment for any particular ore or tailings three points are to be taken into account (a) the condition of the gold, whether fine, or coarse ; (b) the material, whether acid or neutral; (c) the metals contained in the material having a strong affinity for cyanogen. The condition of the gold'—that is, its coarseness or fineness —determines the length of time required in contact with the cyanide solution to effect its dissolution. On the Witwatersrand, generally speaking, the gold is very fine, and a period of twelve hours standing under a strong solution is usually sufficient with tailings to dissolve as much of the gold as it is commercially profitable to extract. Further treatment with weak solution is chiefly in the nature of washing to remove the already dissolved gold. In the treatment of pyritic concentrates a very much longer contact with the solvent is required. By an acid ore is understood the presence of the products of the partial decomposition of pyrites, usually the sulphuric acid, soluble salts such as protosulphate of iron, or insoluble basic iron-salts. All these substances are destructive to cyanide, forming with it compounds useless in the extraction of gold. As to other metals found in the ore certain copper compounds are most troublesome. It has been found, however, that in many cases this difficulty may be overcome by the use of weak solutions. The usual course of treatment of fine milling-tailings on the Witwatersrand is as follows : The vats are filled within an inch or two of the top with the tailings. Sufficient solution of cyanide of potassium to thoroughly saturate and cover the material is then run on, generally about one third of the dry weight of the ore. The first solution thus applied, the strong solution may be 0-3 per cent, strength. It should always be borne in mind that the weaker the solution employed the more particularly selective action of the chemical comes into play. In other words, as long as a solution just strong enough to dissolve the gold in a reasonable time is used any additional strength is a waste, in that compounds of iron and other substances, or even the atmosphere, will destroy a proportion of the chemical in a strong solution when they would not act on weak solutions. After standing for say twelve hours, during which time small quantities of the solution may be drained off occasionally to effect artificial diffusion, all this solution is drained through the zinc-boxes until the tailings are dry. A weaker solution —say, 0-15 per cent—about one half in quantity of the first solution is then run on, and may be started draining within an hour or two. A further weak wash, or if the quantity of stock solution will allow, a water-wash is then applied and drained off. After the tank is thoroughly drained dry it is ready to discharge. The solutions that have been drained through the fine boxes into the storage-vats, or have been pumped to the upper reservoirs, are ready for making up to strength for further use on the next charge, practically all the gold having been extracted from the solutions in their passage through the zinc-shavings. This treatment applies to a class of ore which may be considered as rather an exceptionally simple one, of a kind which, on the Band, is getting rarer as old accumulations of fine milling-tailings are getting worked up, and more deep lead-ore is being crushed. An average extraction of 80 per cent, out of ore containing 6dwts. per ton is nothing uncommon in dealing with this class of ore. The treatment of acid ore, or rather tailings rendered acid by the partial oxidation of the crushed pyritous ore, offers more difficulty than in the case of the free milling-ore. In order to overcome the cyanide destroying qualities of the acid or iron-salts present we are obliged to have recourse to neutralisation by means of an alkali or alkaline earth—caustic soda or lime—with or without a preliminary wash-water to remove such soluble cyanides as may be present. If the quantity of cyanide present is large, and by testing a considerable proportion it is found to be soluble in water, a preliminary water-wash is generally applied. It has hitherto frequently been the custom to water-wash in the same tank in which the subsequent treatment with cyanide is effected, the wash-water being run to waste. Mr W. E. Peldtmann is of opinion that this is a practice which cannot be too strongly condemned, as accounting largely for the so-called mysterious discrepancy between the expected yield of gold, as estimated from assays made before and after the treatment, and the actual return. When the wash-water, charged with acid out of the ore, comes in contact with the residual quantities of cyanide solution lying in the bottom and adhering to the sides of the tank a certain quantity of hydro-cyanic gas is liberated, which, diffusing through the whole tank, is capable of dissolving a not inconsiderable amount of gold from the ore. The worst feature of this is that it is not precipitated even if passed through zinc, and is consequently run to waste with the wash-water. If, in order to economize, caustic soda and lime, when water-washing, is adopted, it should be done in tanks which are reserved for the special purpose, and the ore should then be transferred to the cyanide-treatment vats. This means a little extra cost for handling, but will, in most cases, amount to less than the loss of gold incurred by the other method. In order to neutralise the remaining acid a quantity of a solution of caustic soda, of which the amount necessary must be approximately determined by experimental test, is run on, allowed to stand for an hour or so, and then drained off into an alkali sump, there to be made up to strength with the next lot. When practicable it is a good plan to mix powdered lime with the ore, as this not only saves caustic soda, but also keeps the cyanide solutions freer from suspended ferric hydrate, which, in the case of using caustic soda alone, makes them very turbid, and fouls the zinc. The preliminary treatment having been finished, cyanide solutions are applied. The strength of the first solution may vary from 025 to 0-5 per cent. The number and strength of subsequent solutions and washes, and the time of contact required, depend entirely on the quality and nature of the ore. It may be noted that, in the case of ores requiring a long treatment, it is preferable, rather than allowing the charge to stand under one solution for two or three days, to draw off the strong solution completely and apply a fresh lot every twenty-four hours. In the case of concentrates requiring two or three weeks or longer under a strong solution it is found beneficial to draw off every two or three days, and even to turn the ore over once or twice in course of treatment.