ing pulp. Any falling off in the quantity of sand entering causes a lowering of the sand level within the cone and finally a "breaking through" or discharge of muddy water through the spigot, while any sudden increase in the pulp supply raises the sand level in the cone to a point where sand is carried over in the slime product. This fluctuation is not so important when working with large units which allow of a fair margin of variation, but with small units constant attention is needed in order to obtain satisfactory work.

The mechanical classifier, on the other hand, provided that it is not overloaded and that the ratio of liquid to solid is sufficiently

large to allow of the disentanglement of the sand from the slime by the action of gravity, will perform its function efficiently regardless of wide variations in pulp flow, and with practically no attention.

A very satisfactory system of sand leaching by direct treatment in use in at least one Mexican mill consists in separating the sand in a mechanical classifier, adding clear solution to the product and passing it to a second similar classifier, and then transferring from this by means of a stream of clear solution to a Butters distributor tank with peripheral overflow. When the tank is full of sand the leaching treatment is proceeded with and completed in the same tank, and the residue sluiced out of the dump.

A useful form of classifier for thoroughly washing the sand free from slime before discharging it into the Butters distributor for this system of collection would seem to be the Dorr washing classifier which combines three machines in one, the first compartment separating the sand from the slimey water or solution and discharging it successively into a second and third compartment containing clear solution which completes the removal of the entangled slime.

The Leaching Treatment.-Leaching is usually carried out in circular tanks of wood or steel with a false bottom covered with some filtering medium. A grid is first made on the bottom of the tank by laying wooden strips about 2 in. by 3 in. on edge, a foot apart, and nailing across and on top of these, smaller strips having a square section of 1 to 11⁄2 in., and leaving 1-in. spaces between them. This grid is preferably made in sections sufficiently small to be handled by two men, as it has to be lifted periodically for the purpose of cleaning out accumulated sand and slime that may have worked through the filter. On top of the grid is laid a circular mat usually composed of ordinary cocoa fibre matting, and on this a cloth of a texture fine enough to prevent the passage through it of slime and fine sand. The best material to use for this purpose is closely woven jute cloth such as is used in Mexico and Central America for sacking coffee for export. The material commonly known as "burlap" is not suitable as a top covering because the texture is usually too coarse. Six or eight-ounce duck is often placed above the cocoa mat instead of jute and answers the purpose well except that it tends to reduce the rate of percolation considerably.

The top covering, whatever it may be, is securely fastened in contact with the side of the tank, either by extending it a few inches up and tacking a flexible strip of wood over it, or in the case of a steel tank by driving a thick rope with a wooden caulking tool down between the grid and the side of the tank. For this purpose the grid is made 2 in. smaller in diameter than the size of the tank, and is enclosed by a continuous rim made

from a flexible strip of wood, so as to give a solid backing for the rope caulking.

A pipe, 11⁄2 to 2 in. in diameter, enters the tank through the bottom for drawing off the solution. The system of connecting the outflow pipes from the whole system of leaching tanks into a single header for conveyance to the precipitating house is not recommended, as it makes control of individual charges difficult both as regards sampling the effluent solution and also adjusting its rate of flow. If the precipitation house is at a distance from the leaching tanks, the individual pipes may discharge through regulating valves into a small tank or box near by, from which the solution may flow in a large common pipe line to the extractor house. If it is desired to circulate two different grades of solution, the small receiving tank may have two compartments, and the solution from any tank may be readily diverted to either compartment.

When the sand tank is filled and ready for treatment the solution may be run onto the surface of the charge and allowed to percolate downward, or it may be introduced under pressure beneath the filter and caused to percolate upward. The latter method is chiefly valuable where the charge contains a large admixture of fine material and slime, such as results when a dam of accumulated tailings is dried and coarse and fine mixed together for treatment of the whole by percolation. Such a charge might pack so densely under the influence of gravity percolation, especially if drained dry after a wash, that no subsequent washes would pass through, whereas by continuous upward percolation and removal of the effluent from the surface a rapid rate of flow may be maintained throughout the treatment. The usual practice, however, is to apply the solution on top and let it gravitate downward through the charge. It is often found a good plan to fix a stand pipe of about two inches in diameter against the side of the tank, the upper end terminating at the top and the lower end extending down through the filter mat which is tightly caulked around it. This acts as a vent for the air driven down in front of the mass of descending solution, which would

otherwise be forced to escape upward through the sand in the form of bubbles, thus forming channels and spoiling the uniformity of the subsequent percolation. The leaching cock is better kept closed until the whole charge is saturated and the solution stands permanently above the level of the sand. It may then be opened and regulated to any desired rate of flow. When the cyanide process was first introduced a great point was made of letting the solution remain in contact for many hours before drawing off. This soaking procedure does not seem to have anything to recommend it, since diffusion is almost impossible during this stagnation period, and consequently the film of solution in contact with the particles would soon become saturated, and also denuded of its oxygen, with the result of delaying rather than hastening the dissolution of the precious metals. The idea underlying this practice was probably the desire to effect the dissolving with as small a bulk of strong solution as possible, both as a means of saving cyanide and also of obtaining a solution richer in the precious metals than would otherwise be the case. A method of accomplishing this result without retarding the dissolving action has been used with good results in some mills, notably at Kalgurli, and El Oro, Mexico.

This consists in attaching a small air lift to the outside of the tank, arranged so that as the solution percolates and reaches the space beneath the filter bottom it is at once returned to the surface of the sand. In this way a small bulk of strong solution is kept circulating through the charge for as long as may be deemed advisable, and in its passage is given an opportunity of taking up oxygen to replace that which has been absorbed by the ore.

There are two methods in use for applying the solution. The first is to pump it on as fast as it percolates, thus keeping the charge always saturated or even covered, and the second is to pump on enough to saturate the charge and cover it to a depth several inches and then to drain it off and allow the sand to stand dry for several hours before the next application of solution. The second method is almost always preferable, because, except in the case of highly oxidized gold ores, the circulating solution does not

carry sufficient dissolved oxygen to satisfy the reducing action of the charge and needs to be supplemented by other opportunities for oxygen absorption. By draining dry between washes the air is drawn down into the interstices of the charge as the solution recedes, and finds ideal conditions for absorption. In the case of very fine sand it is useful to apply a vacuum under the filter to remove the excess moisture due to the increased action of capillarity. When dealing with exceptionally reducing ores compressed air may then be introduced beneath the filter, if necessary, as is done at the Homestake. The system of draining dry between washes as compared with continuous leaching has been known to result in an increased extraction of 25% under actual working conditions on combined gold and silver ore.

The rate of percolation is usually measured by the number of inches per hour that the level of the solution on top of the sand falls as the wash percolates downward and flows out through the discharge pipe. Generally speaking the finer the sand the slower is the rate of percolation, but Julian and Smart1 state that "if an ore be crushed to pass through an ordinary assayer's sieve of 90 mesh and washed free of slime, a good leachable product may be obtained, but if this fine ore be mixed with coarser particles, percolation becomes slower than the average of the two sizes treated separately." The presence of slime, even when uniformly distributed, has a serious retarding action on the rate of percolation, especially when leaching fine grades of sand. The depth of the charge is also a factor to be considered, there being a slight retardation in percolation for every additional foot in depth. As regards standard rates of percolation 3 inches and over per hour is considered good, 11⁄2 inches fair, and 3/4 inch bad.

Retained Moisture. The finer the sand the more moisture is retained in the pores of the charge, but even with comparatively coarse sand there is a zone at the bottom that will never drain even approximately dry. This zone or layer is usually about 6 inches in depth regardless of the depth of the tank, and would therefore suggest the superiority of deep tanks over shallow ones, 1 Cyaniding Gold and Silver Ores, page 54 (Second Edition).