In cases where fine grinding is not necessary to liberate the gold values, it is often possible to classify the coarsely ground ore into a sand and slime product (at, say, about 200 mesh) treat the former by so-called percolation leaching and the latter by the agitation method that has been described above. This is also called a Heap Leach Amenability Test.
For laboratory tests a convenient apparatus for percolation leaching consists of a 3-ft. length of 1¾ in. inside diameter lucite or glass tube held by clamps in a vertical position on the laboratory bench. The upper end is left open, while the lower end is provided with a rubber stopper fitted with a ¼-in. glass outlet tube, to which is connected a length of rubber hose. Several shallow drainage slots radiating from the center are cut on the face of the stopper, a circular piece of light cotton filter cloth slightly larger in diameter than the stopper is placed over the slots, and the stopper inserted into the tube. The filter cloth will wedge between the stopper and sides of the tube and be held firmly in place. The end of the outlet tube, which is provided with a regulating clamp and small glass nozzle, is arranged to carry the effluent solution into a Winchester quart bottle. Solution is supplied to the tube by means of an inverted stoppered bottle conveniently placed on a shelf above the bench and having a large outlet hose the end of which is submerged below the solution level in the tube. As the level drops, air is admitted to the bottle and enough solution flows out to submerge the tube and re-establish the level.
In carrying out a test, the tube is filled to a depth of about 24 in. with the deslimed sand, and the feed bottle filled with about 2 liters cyanide and lime solution of known strength. The outlet clamp is closed, and the solution run into the percolation tube to a level about 6 in. above the surface of the ore. The end of the inlet hose is fixed at about this level. The outlet clamp is then adjusted to give a flow of 5 to 10 cc per min. into the effluent collecting bottle. Under the constant head, and if the slime content of the sand is low, there is little difficulty in maintaining this flow, while the supply is automatically maintained, as above described.
When all the solution has run through (at the end of 4 to 8 hr., depending on feed rate), the charge should be allowed to aerate for some time, while the effluent solution is titrated for cyanide and lime content and made up to strength by suitable reagent addition. This solution is then transferred to the feed bottle, and the cycle of operations repeated.
The leaching operations should be continued for several days at least, at the end of which time one or two water washes are applied and the sands discharged, dried, and assayed.
In connection with Heap Leach Amenability Test, E. M. Hamilton remarks as follows:
One of the principal difficulties in such percolation tests lies in the fact that the charge is so much shallower than a working charge that there is not sufficient head to overcome the capillarity of the interstices, so that, even with sand coarse enough to percolate very rapidly, the level of solution will not fall much below the level of the sand. The result of this is that the charge is not aerated (as it is in practice), by the air following the solution down into the interstices, between each wash. This may be overcome by applying a vacuum under the filtering medium after the solution has ceased to percolate by gravity. In this way the residual solution is drawn off and the air follows it down. The charge should then stand for several hours before the next wash is applied. This procedure is more important than it may seem, since a difference in extraction of 20 to 30 per cent has been in some instances observed according to whether the vacuum was applied or not.