but against that is to be set the decreased rate of percolation as already noted and also the superior opportunities for aeration in a shallow charge. This wet layer at the bottom can be dried and aerated by the application of a vacuum under the filter after the solution has ceased to percolate by gravity. A vacuum used in this way is especially advantageous in the leaching of clean fine sand, which although it admits of rapid percolation by gravity yet retains a disproportionate amount of moisture which can only be displaced by pressure.

An important point in securing good leaching is to see that the mats and filter bottom are kept clean and thoroughly pervious to the solution. Sometimes the fine upper cloth may take on a deposit of lime carbonate, especially if aeration by compressed air is made use of, and in that case it will be necessary to clean it with dilute muriatic acid. Even under ordinary conditions the mats and the spaces underneath them tend in time to get blocked up with fine slimy material and require periodical cleansing.

The quantity of solution used during treatment varies from 34 to 11⁄2 times the weight of ore, according to circumstances, and has to be determined by the metallurgist from economical tests.

The solution strength is on the whole usually higher than is used in agitation processes, and various strengths are used on the same charge. When treating accumulated tailings, the common practice is to begin with one or two weak washes so that the cyanicides may expend themselves before the stronger solution is applied. With freshly crushed ore, especially if milling or collecting be done in cyanide solution, this is usually unnecessary and strong solution may be added at once, with the advantage of saving time and beginning the dissolving action as early as possible. The strength of this solution varies, but a concentration of 0.3% KCN or 6 pounds per ton of solution is probably used more often than any other.

The cyanide necessary to make up the stock solution to the desired strength is best added before the wash is applied to the

sand tank. It is a common practice to hang cans containing lumps of cyanide in the stream of solution as it flows onto the sand, but this method is not recommended as it cannot produce a wash of uniform strength.

The most approved plan is to keep a solution tank expressly for strong washes. The solution therein is tested once or twice a day and the necessary quantity of cyanide placed in a basket hanging below the surface. When the lumps are all dissolved the contents of the tank are well mixed either by blowing compressed air into it for a short time or by circulating with a centrifugal pump. The resulting strong solution can then be pumped or run by gravity onto the sand charges needing it.

Draining and Washing. When all the soluble precious metal has been dissolved the strong rich solution in the charge is displaced by repeated washes of weak barren solution and finally by a water wash. Julian and Smart's experiments1 go to show that given a certain volume of wash solution available its displacing efficiency is considerably increased by dividing it up into a number of small washes rather than applying the whole of it at once.

The quantity of water given at the finish is determined by the amount of moisture held in the charge when drained and ready to be sent to the dump, because although it would be desirable to give sufficient to displace the whole of the cyanide solution held as moisture, this usually could not be done without increasing the total quantity of solution in stock, with the result that the surplus would have to be run to waste, consequently only such an amount of water is given as is just equal to the quantity of moisture sent out of the plant in the residue.

It is advisable to assay the effluent solutions frequently throughout the treatment, especially during the washing-out stage of the process so as to observe the progress of displacement of the value-bearing liquor and avoid losing dissolved gold in the residues.

1 Cyaniding Gold and Silver Ores, page 58 (Second Edition.).

Discharging the sand is done in various ways. Where water is plentiful sluicing is a cheap and effective method. If this is not available, the tanks are so constructed that cars may be run underneath and the residue shovelled into them through doors in the tank bottom. Where labor is expensive, the Blaisdell excavating machinery in conjunction with a conveyor belt and stacker is probably the most satisfactory way of disposing of the residue.

CHAPTER IV

SLIME TREATMENT

Many definitions of the word "slime" have been attempted but the one that best expresses its meaning from a practical point of view is that given by Park1 that it is that part of the pulverized ore that is not percolable on a commercial scale without the use of pressure. The consequences of this definition are that slime cannot be treated commercially by the same methods as sand and requires entirely different handling. The fundamental constituent of slime is the amorphous or clayey matter present in all ores to a greater or less extent and this is supplemented by varying amounts of exceedingly fine granular material produced in the process of crushing or pulverizing.

For some years after the leaching process of cyaniding was well established no feasible method was discovered for treating the slime which had been separated from the percolable part of the pulp: consequently the object of the metallurgist was to avoid as much as possible the formation of slime in milling, and in so doing there was a tendency to produce a pulp whose maximum sized particles were too coarse to liberate the precious metals and expose them to the action of the cyanide solution. With the development, however, of successful methods of treating the slime, the practice in milling was aimed more and more at the production of a pulp fine enough to yield the maximum extraction by cyanide.

This principle developed rapidly until the process of "allsliming" came to be considered as the panacea for all the ills of the cyanide process and in discussing the erection of a new mill it was considered almost heresy to suggest the possibility that "all-sliming" might not be the most commercially profitable method of treating the particular ore in question. The expres1 The Cyanide Process, page 129 (Fifth Edition).

sion "all-sliming" does not usually mean grinding everything to a "slime" in the sense of the definition already laid down, but merely grinding sufficiently fine to avoid mechanical difficulties in the treatment of the whole of it by slime methods. In the majority of instances the product of "all-sliming" contains from 5% to 35% of sand that will not pass a 200 sieve; moreover, a good proportion of the sand -200 sieve is perfectly leachable if properly freed from slime, as has been already explained, so it is fairly safe to say that of the usual "all-slime" pulp from 30% to 50% does not come under the terms of the definition of slime, that is, it would be perfectly practicable to leach it by gravity on a commercial scale.

In many cases it may be just as effective and more convenient to consider the ground product all as "slime" and deal with it as such, but the writer has shown instances where it was more profitable to make a separation and treat the percolable part by leaching. For example, in one "all-sliming" mill the plant capacity restricted the agitation period of each charge to 24 hours; this was sufficient to give a good extraction on the true slime, but the fine granular material required considerably longer. Here the general extraction was improved, the tonnage increased, and the costs lowered by erecting a subsidiary leaching plant and removing as a percolable product about 40% of the finished pulp.

The question of "all-sliming" any given ore should be determined solely by commercial considerations. Extraction tests will, of course, be made but it will not be sufficient for these to show an increased extraction; the amount of such increase must be weighed against the increased cost of treatment and an estimate made of the probable net gain, if any. Even when allsliming is indicated it usually will not be profitable in practice to reduce the entire ore to accord with the definition of slime. In pulverizing an ore the first thing to separate as a finished product is the natural slime, next the most friable portions will yield to disintegration, and so on, until there is left a residuum of the toughest and most refractory material. This latter will