going tests should not be carried beyond the critical point or the end of the phase of free settling.

Determination of D (the ratio of solution to solids in the discharge required from the thickener). There is a point of maximum density beyond which any given slime will not settle and it is of course useless to require a discharge of greater density than this. In the Dorr Company's instructions already quoted this point is determined as follows:

"The dilution of discharge "D" that can be expected may be determined by allowing a pulp of consistency about 3:1 to stand about 48 hours in a graduate stirring gently with a glass rod every four or five hours." For instance,

Note. After each reading decant as much clear solution as possible.

Dry weight of solids in graduate

Number of cc corresponding to 1 foot of length of graduate

Then if the slime needs 36 hours to attain its maximum density or the density required, the cubic capacity of the thickener must be sufficiently large to retain any given portion of the pulp in that tank for the period ascertained as necessary by the test.

The following example is given by Coe and Clevenger on a pulp that required 4.31 sq. ft. of area per ton of solids per 24 hours for free settlement. A compression test was made in a cylinder 12 in. deep with a 3 to 1 pulp, giving these readings:

=

"Since an area of 4.31 sq. ft. is required per ton of solids per 24 hours the total solids per square foot retained in the thickening zone must be 19 X 2000 367 lb. or 19 hours supply per square foot. There is 24 X 4.31 required a 5-hour supply of each of the pulp consistencies, 1.16 to 1, 1.275 to 1, 1.47 to 1, and a 4-hour supply of a 1.7 to 1 pulp.

"The solids per cubic foot in the above pulps are 43.2 lb., 37.6 lb., 33.7 lb., and 30 lb., respectively. The depth of each class of pulp would therefore be 2.23 ft., 2.57 ft., 2.87 ft., and 2.58 ft., or a total depth of 10.25 ft. To this depth must be added a foot for the loss due to the pitch of the drags in the thickener and 1.5 ft. for the depth of the feed, since the feed is thick and the volume will be proportionately low. The total calculated depth of the tank would be 12.75 ft. If proper allowance were made for storage capacity. the tank might be inconveniently deep. In this case it would be better practice to make the tank 12 ft. deep and make various allowances for additional capacity by increasing the diameter of the tank to give a 30 per cent. increase in area.”

In regard to the practical operation of Dorr thickeners the' writer has observed in some instances a tendency on the part of the operator to extend the vertical intake cylinder down from 2% to 34 of the depth of the tank, under the impression that the settlement would be helped thereby, the idea apparently being that if the slime is fed to the lower part of the tank it will stay there. This practice, however, defeats its own object and changes the apparatus from a settler into a classifier. The reason of this

will be seen from the diagrammatic sketches, Figs. 12 and 13. Fig. 12 illustrating the incorrect arrangement shows how the deep cylinder destroys the tranquility of a large part of the tank

INCORRECT METHOD

CORRECT METHOD

FIGS. 12 and 13.-Sketch of Dorr Thickener.

charge causing upward currents which tend to raise and carry off the more flocculent material, and reducing the available settling capacity of the tank. The action may be compared with that of a spitzlutte, wherein the further the baffle-plate is lowered the greater is the velocity and lifting power of the current in the

lower part of the box. Fig. 13 shows the more approved arrangement in which the intake cylinder extends down only from 12 in. to 18 in. below the surface or just sufficient to prevent the surface disturbance which would be caused by the inflowing pulp, such disturbance being further minimized by allowing the stream to fall onto a deflecting board, either floating on the surface or fixed close to the surface. This deflector is made of smaller diameter than the cylinder so as to allow an annular space for the pulp to pass downward into the charge. It may also be made in the form of a shallow tray with holes bored all around the vertical sides, and this plan works well when a screen is used to intercept waste and other matter which would tend to block the holes.

Slime Extraction Treatment.-There are two different principles applied to the treatment of slime, (1) agitation, (2) leaching in thin cakes by pressure. Of these two the first is in almost universal use. One of the earliest attempts to cyanide slime was that of Charles Butters who devised a combination treatment whereby the pulp was agitated by stirring arms in a tank fitted with a filter bottom. When the dissolution of the gold was complete, the charge was allowed to settle, and while the clear solution was being decanted a vacuum was applied under the filter to reduce the moisture in the mass of settled slime. The latter feature was, however, soon abandoned and simple agitation, settlement, and decantation resorted to.

John R. Williams in his system of agitation dispensed with the stirring gear and substituted a method of transfer from tank to tank in contact with cyanide solution by means of centrifugal pumps. His tanks were made with a moderately conical bottom connected at the centre with a centrifugal pump. When a charge of thickened slime had been collected and the surplus water decanted the pump was started and at the same time a stream of weak solution was played on top of the charge diluting the pulp and washing it into the pump which delivered into a second similar tank. If this treatment was insufficient to dissolve all the gold, the operation could be repeated as often as necessary.

Both the Butters and Williams methods gave good results on the Rand and were developed simultaneously.

Aeration. In the treatment of the accumulated slime from the storage dams on the Rand it was soon found that a very poor extraction was obtained by the methods that were perfectly satisfactory for the fresh or current product. This was traced by W. A. Caldecott1 to the formation in the dams of products of partial oxidation, among which were ferrous sulphate, ferrous sulphide, and sulphuretted hydrogen: these being strong reducing agents abstracted all the free oxygen from the solution and checked the dissolution of the gold. When, however, they were thoroughly oxidized by blowing air through the pulp before it came in contact with cyanide, a normal extraction was obtained in the subsequent cyanidation.

To introduce the necessary air into the pulp H. T. Durant inserted a small automatic valve into the suction of the centrifugal pump so that by slightly choking the flow of pulp in the main suction line the little "snifter valve" would admit a continuous current of atmospheric air. This air in passing through the rotor of the pump was broken up and atomized in such a way as to render it very effective for the purpose in view.

In practice, the snifter value needed constant attention to secure a steady supply of air and for mechanically stirred tanks a widely adopted substitute, in cases where an additional amount of aeration is needed, consists in fitting a small air lift against the inner side of the tank. This gives excellent aeration, rarely gets out of order, and consumes very little power. One length of sixinch pipe discharging through a tee or ell just above the surface of the pulp is usually sufficient to aerate a charge of 80 to 100 tons of slime with a three to one dilution.

Agitating Devices. Although the old form of mechanical stirring ag tator is (1919) rarely installed in a modern plant it is not to be despised being a simple and efficient device for agitating slime pulp. The main points to be observed in the construction are (1) to see that the gears and clutches are sufficiently heavy, 1 Proceedings Chem., Met. and Min. Soc. of S. A., July, 1897.