TABLE 5.-OTHER ASSUMED CONDITIONS AND CALCULATED RESULTS. TYPE "B" FLOW SHEET

The two requisites for efficient working of the system are a sufficient volume of precipitated solution and a perfect precipitation yielding a tail solution that is for practical purposes barren. It is possible under these conditions to get a very satisfactory elimination of the dissolved precious metals from the residue.

Regarding elimination of the cyanide it is not quite so satisfactory because, as in all other washing processes, the quantity of final water wash available is limited by the daily tonnage of moisture eliminated from the plant, as otherwise the stock solutions would soon increase to an unmanageable bulk. In the case of gold ores where a weak cyanide solution can be used with effect the loss of cyanide in the residue is inconsiderable, but for silver ores where a treatment solution of 0.1% or over is employed it will often be found profitable to use a filter of some kind before discharging the residue from the last thickener to the dump.

It has often been observed, when working this process, that although the dissolving action in the treatment tanks may have apparently ceased before the pulp enters the washing thickeners, additional small amounts of precious metal seem to pass into solution during the washing out process, corroborating the theory already alluded to of the efficacy of successive washes for the dissolving process, a tentative explanation of which has already been given.

Another phenomenon, rather difficult to explain, is also sometimes in evidence in the use of this process. Even in cases where little or no dissolving action is observed in the successive thickeners in which cyanide solution is used as the diluent yet when the pulp reaches the last thickener and is diluted with water marked dissolution of the metals seems to take place. This may perhaps be due to the increased activity of the ions in a less viscous medium than is afforded by the stock solution. On the other hand it seems possible that the dissolving effect may be only apparent and that in reality the result may be due to a liberation of pregnant solution previously adsorbed by the colloidal constituents of the pulp. Such a liberation might conceivably be brought about by the changes in the properties of the electrolyte due to

excessive dilution with water. One such change might be traced to a decrease of lime in solution sufficient to cause the curds or aggregations of clayey matter to disintegrate into their original units liberating soluble matter which had been entangled with them. If it be asked why a similar action is not displayed when making washed assay samples for the determination of these various points, the answer might lie in the fact that such washed assay samples are now almost universally prepared on laboratory vacuum filters, so that they are not subjected to the same conditions as obtain in the working of the plant.

CHAPTER V

MILLING IN CYANIDE SOLUTION

Milling in Cyanide Solution.-This was first tried on the Rand in 1892 but was soon abandoned, probably on account of difficulties arising out of the fact that there was at that time no method of treating the slime. It was introduced as a practical system in New Zealand in 1897 and in South Dakota in 1899. Since then the practice has grown until at the present time it may be considered standard. Almost the only exception to its use is in the case of mills where the ore is subjected to amalgamation before the cyanide treatment.

The reasons for this exception are two: first that the presence of lime hardens the amalgam. This is not considered an objection on the Rand, where amalgamation is usually carried out in a lime water pulp. Second, the cyanide solution is said to attack the copper plates through the coating of amalgam and gradually to destroy them. Clennell,' however, states that with solutions of .03 to .07 per cent. KCN the action on the plates is not serious, and E. S. Leaver (Mining & Scientific Press of Aug. 22, 1908) says that at the Nevada Goldfield Reduction works plate amalgamation had been in use one year while milling with 0.1% cyanide solution without any particular effects on the plates having been noted. E. H. Nutter (M. & S. Press, Dec. 14, 1907) says that he found the method satisfactory at Bodie and at the Liberty Bell though it was necessary to renew the plates twice a year. A third objection has been raised, especially on the Rand, to the effect that crushing in solution renders impossible a correct sampling of the ore head for assay. The statement is made that the only way to obtain a representative assay of the ore entering the mill is to sample the pulp after it leaves the mortar boxes, and this is of course rendered difficult if not impossible if cyanide 1 The Cyanide Handbook, page 342 (Second Edition).

solution is used in place of water. It would seem, however, that if the system has advantages as a money saver (and this class of objęctor if often willing to admit the fact), it would be more profitable to use it and put up with a degree of uncertainty about the head assay, if it is really impossible to get a correct sample of the ore except at the lip of the mortar boxes.

The advantages of milling in cyanide solution are mainly two: The first is that the ore is brought into contact with the solvent at the earliest possible moment, thus reducing the time needed for the dissolving treatment proper and decreasing the size of plant necessary. It is no uncommon thing to find from 70% to 80% of the gold in an ore dissolved during milling and before the pulp reaches the treatment tanks, though silver does not respond to the same extent, the amount dissolved from a silver ore during milling being usually from 5% to 25%.

The second advantage is that the introduction of large quantities of water into the solution stock is avoided. It is obvious that, considering any cyanide plant as a whole, no more water can be taken into the system daily than is withdrawn from that system in the same period; consequently, if, with every ton of slime, a ton of water be taken into the plant, then for every ton of slime residue discharged a ton of solution must be got rid of in some way or other. Some do this in the form of moisture in the slime residue; and others, who prefer to displace part of this solution moisture by water, run to waste an equivalent quantity of solution from the tailing of their precipitation-boxes, so as to preserve the balance in the system. This difficulty is much accentuated where the slime residue is treated in filter-presses or vacuum-filters; it is to some extent mitigated where double filtering is practised, that is, where the excess water is extracted from the slime before cyanide solution is added, though even here, when the cake contains 25% moisture, for every three tons of slime a ton of water is introduced into the system and a corresponding ton of solution has to be eliminated at some point, rendering it difficult or impossible to give a displacing water-wash to the residue cake without running to waste weak solution. It is