F

146. F. Ores Containing Coarse Gold, or an Alloy of Gold and Silver. - If the gold is too coarse, it will not be entirely dissolved in a single operation by the above described process of chlorination, or if it is alloyed with a considerable proportion of silver, unless in very fine particles, the chlorination may be obstructed by the formation of a crust of silver chloride on each of the particles. Gold which is too coarse is more easily collected by amalgamation; yet it can be dissolved by leaching with chlorine water, that is, water through which chlorine has been passed until no more is absorbed. An alloy of gold and silver can be leached out with brine of common salt, saturated with chlorine. The chlorine acts on both of the metals, and the brine dissolves the chlorides as fast as they are formed. The dissolved metals may be precipitated together, by means of plates of copper.

147. The Mears process, recently introduced, seems to be well adapted to this class of ore. It is conducted as follows: The roasted ore is treated in rotating, lead lined, iron cylinders, with the addition of water and chlorine, the latter being forced into the cylinders under heavy pressure. The gold is said to be dissolved very rapidly, not more than two hours being required. The pulp is then thrown into leaching vats, and leached in the usual manner. An addition of salt to the charge in the cylinder would be useful in case of an alloy of gold and silver. The advantages of applying the chlorine, or chlorinated brine, under pressure, have long been known.

Calcium Poly Sulphide

148. Calcium Poly Sulphide. This precipitant for silver is made by boiling lime and sulphur together in water. Lime is calcium oxide, and, when boiled with sulphur, a part of it is decomposed; the calcium combines with sulphur, making calcium sulphide, while the oxygen, and some undecomposed lime, combines with another portion of sulphur, making calcium hyposulphite, which is the same as the solution used in leaching silver.

When the solution of sulphide is used for the purpose of precipitation, the water that it contains is added to the leaching solution, which would thus be gradually diluted but for the hypo which the precipitant also contains. This is the reason why the volume of the leaching solution frequently increases, notwithstanding a certain amount of waste. Not only is hypo formed in making the calcium sulphide, but the sulphide itself absorbs oxygen from the air, and changes into hypo to a certain extent.

Calcium sulphide may be made in an iron pot, over a fire, with frequent stirring. A neater way is to make it in a wooden tub, using a jet of steam for the heating.

It can thus be made in the vat in which it is kept.

The vat is two-thirds filled with water, and steam is admitted. As soon as the water is hot, freshly slaked lime is mixed with it, and flowers of sulphur added by rubbing through a sieve. When the sulphide is made in this way, two or three hours boiling suffices.

149. The proportions of lime and sulphur vary with the quality of the lime. About 1 1/2 pounds of average lime is required for one pound of sulphur; 75 pounds of lime, 50 pounds of sulphur, and 120 gallons of water will produce a solution of suitable strength, and will suffice for the precipitation of from 25 to 50 pounds of silver, according to the quantity of base metal in the lixivium. If a direct fire is used for the boiling, the water which evaporates must be replaced, but if steam is used, a little less water than the prescribed quantity should be taken, as some will be added by the condensation of steam. It is however to be observed, that with an insufficient quantity of water, the operation proceeds very slowly. The proper strength of the solution is indicated by a density of about 10° Beaume. If below 6° it will dilute the hypo too much, when used for the precipitation.

The chief points to be observed in making the sulphide are, that the lime must be slaked before use, and must not be in excess. If it is not previously slaked, time and material will be wasted. If used in too great proportion, insoluble compounds are formed, or, at least, compounds which are not sufficiently soluble to be useful. An excess of sulphur is not injurious. When the solution is properly made, with good lime, it is nearly of the color of strong coffee; it deposits, on cooling, few, if any, needle-shaped, yellow crystals, and the residue consists only of a small quantity of matter, of a dirty greenish color.