Precipitation Of Silver

129. Precipitation Of Silver. The silver is precipitated by means of a strong solution of calcium poly-sulphide, called, in the works, simply "calcium," or "sulphide," which is led into the silver tub, by a hose from the elevated vat in which it is kept. It throws down the silver, and other metals, as sulphides, in the form of a dark brown mud, which soon turns black. At the same time it restores the hypo, which was altered in dissolving metals, so that it can be used again.

It is imperative that not more of the calcium sulphide be used than the quantity required to precipitate the metals. If a little silver remains in the solution, it is not lost, as the hypo is used again; therefore, it is best to add a little less of the calcium sulphide than would be required to precipitate the whole of the metal. If too much is used, the excess remains unchanged, and mixed with the hypo, in the re-use of which it converts some of the silver chloride in the ore into sulphide, which cannot be leached out.

In practice it is not difficult to precipitate aright. A circular motion is given to the solution in the silver tub, the stream of calcium sulphide is turned in, and allowed to run as long as it is seen to cause a distinct precipitation. The liquor is then stirred, thoroughly and vigorously, for a couple of minutes. The circular motion is checked by a reversed movement of the stirrer; a few minutes are allowed for partial settling, a sample is taken in a glass, and a little calcium sulphide added. If a considerable precipitate is produced, a few more gallons of the precipitant are run into the tub, and the stirring and testing are repeated. When the calcium sulphide produces only a very slight precipitation, the contents of the silver tub are left undisturbed for a few hours.

130. If, in the test, calcium sulphide gives no precipitate, it may be that too much has been used in the tub; therefore another sample is taken, and tested by adding a few drops of solution of iron sulphate, which instantly gives a black precipitate if there is the least excess of calcium sulphide. If neither of the tests gives a precipitate, all is right; but if it is found that too much calcium sulphide has been employed, it must be counteracted by an addition of silver solution from another vat, or, if there is none to be had, some iron sulphate may be used instead. There is, however, no trouble if care be used. The experienced workman knows almost the exact moment when enough calcium sulphide has been added, by a white cloudiness which appears in the liquor.

The precipitate settles in a few hours, and the renovated hypo is drawn off, by means of an arrangement similar to that used in the gold tub, and pumped into the elevated tank, whence it is again led to the leaching vats as required.

131. If, when the leaching of a vat is finished, a dark scum is seen on the surface of the ore, it is taken off and returned to the roasting. It is caused by a small quantity of metal sulphide, which remained suspended in the hypo when it was drawn from the silver tub, and is thus filtered out by the ore. It contains more or less silver.

132. The metal contained in the weak solution in the wash-tubs, is also precipitated by means of calcium sulphide, but, after the settling, the liquid, which is little more than water, is allowed to run to waste. The metal should be completely precipitated, because, as the liquid is not used again, any which might remain in solution would be lost. An excess of the precipitant can do no harm in this case.