Leaching The Gold Ore

96. Leaching The Gold Ore. After the ore is supposed to have remained long enough in contact with chlorine - that is, from 12 to 40 hours - the plug hole in the cover is opened, and tried with ammonia as before. This must still produce a dense fume; if not, the chances are that a poor result will be obtained in leaching, unless chlorination is repeated. If appearances are satisfactory, a little ore is taken out through the hole with a sampler, finely ground, and carefully washed in a saucer, or a horn. Neither gold nor sulphides should be visible with a lens, but, as before remarked, some ores do not show the gold well, even before chlo-rination, and for such this test is not conclusive.

97. At this point a difficulty arises, from the fact that a large quantity of chlorine remains in the vat, the escape of which into the leaching room is not desirable. If the cover of the vat be raised, the gas is visible like a green sea above the ore; a pestilential sea whose waves, surging forth upon the slightest disturbance of the air, threaten to envelope and suffocate the workman, unless he hold his breath while hoisting the cover and fastening the tackle, and then retreat in haste till the storm is over; and even then the same trouble recurs while water enters, and displaces the chlorine still remaining beneath the filter, and permeating the loose mass of damp ore.

To overcome this difficulty, some operators fill the vat with water before lifting the cover, and, by means of a rubber hose, convey the expelled gas, either out of doors, or into another vat charged with ore. There are two objections to this plan - firstly, a great deal of chlorine is absorbed by the water, which has an unfavorable effect in the precipitation of gold, making it more difficult to settle, as well as wasting the precipitant; and, secondly, the water, entering the tub in one large stream, descends through the bed of ore at one point, and rises through it in other parts, giving a solution of nearly uniform strength throughout the mass, and requiring a large quantity of water to complete the leaching, taking more time, and giving finally a weaker solution than is necessary, for the precipitation takes place better when the solution is strong.

The best way to apply the water is by spraying it all over the surface of the ore in the vat, so that, percolating downward through the mass, it carries the greater part of the gold chloride in the first portion of the water, and a rich solution is obtained at once, while less water is required to remove the whole of the gold, and the stronger solution thus finally obtained gives a better precipitation on addition of iron sulphate. But if the spraying is undertaken by hand with a sprinkler, the loose ore, settling as it becomes wet, expels chlorine in such volumes as to render it impossible for a man to stand near the vat.

The course I adopted was to place a coil of lead or rubber pipe, in which were numerous small holes, around the vat under the cover. After raising the cover, the water supply pipe was connected with the perforated coil, and the water, under moderate pressure, issued in a number of fine jets, and showered all over the ore, through which it descended gradually, dissolving the gold chloride as it passed, so that the first solution which passed through the filter, and filled the space under it, was very rich, while that which finally remained on the top was very weak. The discharge was then started, and the water thenceforth admitted in the usual way, keeping the surface of the ore covered until the leaching was finished. This plan, however, is still open to the objection that it gives a solution containing much free chlorine, while the chlorine which is expelled is not removed from the room; hence it is very desirable that means should be used to abstract the surplus from the vat before raising the cover, or admitting water. The appliance I am about to describe will not only do this, but will also save the chlorine, for use in another charge of ore, with little waste.

98. Before lifting the cover, the plug-hole being open, one of the lower openings, either the nipple through which the gas was admitted, or the discharging pipe, is connected by a rubber hose with the interior of an inverted bell of sufficient size, which is immersed in a tank of water, precisely like the "gasometer" of gasworks, and like it, counterpoised and suspended by ropes or chains passing over rollers, so as to allow the bell to rise or fall in its water tank. By adding weights to the counterpoise, the bell is gradually raised, and the chlorine is withdrawn from the vat and enters the bell, while air, entering the vat through the open plug-hole in the cover, replaces the abstracted chlorine. The filling of the vat with water is then proceeded with in any way that may be convenient, without annoyance from escaping chlorine. The chlorine in the bell may be again expelled when wanted, and caused to enter the same, or another vat, by simply removing a part of the counterpoise, and allowing the bell to descend by its own weight.

When the ore in the vat is covered by the water, the discharge pipe is let down to the trough which leads to the gold tub, and the plug is removed from it. To prevent disturbance of the ore by the water flowing into the vat, the stream is received on a perforated board, or on a sack laid upon the ore. The inflow and the outflow must be equal, so that the ore remains submerged. If the outflow is too rapid, it is lessened by simply "kinking" the pipe, which, for this reason, is of pure rubber, and long enough not only to reach the trough, but a foot or so more; it can then be "kinked" so as to entirely stop the flow if desired.

99. A few ounces of the strong solution of gold chloride first obtained, are set aside for a purpose which will shortly appear. If the ore contains no copper, and has been properly roasted, the lixivium will be of an amber color. A green hue indicates the presence of copper, and a very dark, almost black, appearance, produced by iron perchloride, shows that the ore has been improperly roasted.