Leaching The Silver

123. Leaching The Silver. At this point some operators remove the ore to other leaching vats, from an idea that the small quantity of the leaching liquid used for silver which remains in the filter, is injurious to the chlorination of the next lot of ore. I do not think it worth while to take this trouble. I even found it unnecessary to take any special pains to wash the filters before recharging, any further than is done in washing the hypo out of the ore, which is necessary in order to avoid waste. The traces of hypo remaining, if not oxidized to harmless sulphate by the action of air, must be so, instantly, on the admission of the chlorine. It is true that in the latter case hydrochloric acid would be formed, but in such minute quantity as to be of no importance. At the worst the only inconvenience would be a slightly increased consumption of chlorine.

When the gold leaching has been carried as far as is desired, the water in the vat is allowed to subside below the surface of the ore. The solution of calcium hyposulphite is then admitted, and the discharge is directed into one of the extra silver or wash tubs.

It is best to have a separate trough to convey the silver solution to the tubs, not only because it is convenient to have the gold and silver tubs at opposite ends of the set of leaching vats, but also because it is often necessary, when working several vats, to run both gold and silver solution at the same time. I therefore have two troughs, extending in front of and below the leaching vats, side by side, but inclined in opposite directions, so as to lead, the one to the gold, the other to the silver tubs.

The hypo should never be allowed to flow into the gold tubs, because it forms a combination from which the gold cannot be precipitated by iron sulphate. As, however, the precipitant used for silver throws down gold also, whether dissolved in water or in hypo, there is not the least danger of losing any gold which may find its way to the silver tubs.

124. As the water runs out of the ore mass in the vat, the hypo follows it, and as soon as a sweet taste, indicating the presence of silver, is perceptible, the stream is turned into the main silver tub, because the wash water, after precipitation of any metal it may contain, is thrown away, but the hypo is preserved for future use. The flow is not allowed to be too rapid, but is checked by "kinking" the pipe, as directed in the gold leaching. There is no guide for this but experience, and the strength of the silver solution coming from the ore, which is known by its more or less sweet taste.

125. When a sweet taste is no longer perceived in the solution coming from the vat, a test is made by taking some of it in a glass, and adding some solution of calcium sulphide. If a precipitate is produced, it is certain that metal of some sort is still being extracted from the ore. The question then is, what metal, or rather whether there is any, or much, silver, for it is an unfortunate circumstance that the hypo dissolves lead sulphate, and other base metal compounds, especially after the extraction of silver is finished, or when the hypo is too strong.

Some idea of the nature of the metal which is being extracted by the hypo, may be obtained from the color of the precipitate. Silver gives a dark brown, copper a reddish brown, lead and antimony light, sometimes yellowish shades. It is, however, often impossible to say, from the appearance of the precipitate, whether it contains silver or not, and the sweet taste, when it is no longer strong, may be masked by other and less agreeable flavors. I have not been able to find a very simple test for the presence of silver in this case, but I will give the one I use, which can be made in ten minutes.

126. The precipitate in the glass is stirred, or shaken, to make it curdle, allowed to settle for a few seconds, and a portion of the liquid is poured off; the remainder, with the precipitate, is heated in a small porcelain dish. The precipitate blackens, shrinks, and settles, so that nearly all of the liquor can be poured off without much loss of precipitate. A little nitric acid, and a fraction of a grain of salt, are then added, and heat is again applied. The action is very rapid, and in a few seconds nothing remains of the black precipitate but a yellow mass, principally sulphur, but possibly in part silver chloride. To prove this, ammonia is added, very cautiously unless after cooling and diluting, until the contents of the dish smell strongly of it.

If copper is present, it gives a blue color; silver chloride is dissolved in the ammonia, which is then poured upon a filter, and to the liquid which passes through, and is received in a test tube, nitric acid is added, drop by drop, while the test tube is inclined away from the operator's eyes, until the smell of ammonia is no longer perceptible. If there was any silver in the precipitate, it now appears as a white cloud of chloride, which curdles on being shaken, or, if in very small quantity, as a slight milkiness in the liquid. A little practice will enable the operator to judge as to whether it is worth while to continue the leaching, or not.

127. When the quantity of silver in the leach is inconsiderable, the hypo is turned off, but, as it is not to be wasted, water is again led into the vat as soon as the ore is uncovered, to displace the hypo which would otherwise be retained. As soon as it is found, by the taste, that the hypo is almost washed out, and nearly pure water begins to come, the stream is turned into one of the wash-tubs, and the influx of water to the vat is stopped. The ore is allowed to drain, to facilitate which it may be dug over with a shovel. It is then removed.

128. The leaching of a charge occupies from eight to forty-eight hours, according to the richness and character of the ore, and the skill displayed in the roasting.