C

136. C. Concentrations containing Silver, but little or no Gold. - These concentrations generally contain less iron pyrites, and more lead, zinc, copper, antimony, and arsenic, which makes them more difficult to roast. They require from 5 to 20 per cent of salt, according to their richness. The salt is usually charged with the ore, but as some ores are more liable to melt if it is added at the commencement of the roasting, than if the addition is made at a later period, it is sometimes necessary to complete the oxidation, under a moderate heat before the salting.

137. The roasting is commenced with only the heat required for the ignition of the sulphur. Stirring is kept up almost continuously, in order to prevent sintering, or partial melting. If the salt has not been mixed with the ore at the time of charging, it is added as soon as the smell of burning sulphur has become faint, under a dull red heat. The heat is then raised cautiously, the ore swells, becomes flaky, and somewhat sticky, appearing as though it were moist, which is caused by the fusing together of the metal sulphates and the salt.

At this period the ore must not be stirred too frequently, or more than is required to ensure an equal heat, as it facilitates the escape of chlorine, which should be retained as long as possible within the mass. The furnace doors may be closed while the stirring is intermitted, because what is now required is heat rather than air.

It is a good plan to heap the ore near to the fire-wall, and let it remain for a time, then spread it, and again gather it into a heap. The roaster should be careful to avoid pushing the somewhat adhesive ore against the wall of the furnace with the hoe. The charge on the finishing hearth must be changed from end to end at least twice; once before the heat is raised, and again after. It is drawn out while still giving off an abundance of chlorine and volatile chlorides, and is allowed to remain an hour before being spread on the cooling floor.

138. The degree of heat which may be used depends on the quality of the ore. Zincblende requires a high temperature for its decomposition. Iron, copper and antimony sulphides, especially the latter, require a low heat at first, but are not very liable to cause sintering after oxidation. Lead is troublesome, as all its compounds are very fusible.

Some ores do not bear a high heat, after the ad-dition of salt, without a great loss of silver; yet if zincblende is present a strong heat is requisite for its oxidation. Such an ore is roasted without salt until a sample, ground in a mortar, and carefully horned, or panned, shows no sulphides. It is then allowed to cool to a moderate red heat, salted and mixed. If a strong odor of chlorine is not thus developed, some dried copperas, in the proportion of about sixty pounds to the ton of ore, must be added and mixed. When the proper smell is obtained, the charge is withdrawn.

Washing The Ore

139. Washing The Ore. The roasted and cooled ore, previously sifted if lumpy, is placed in the leaching vat, and cold water is passed slowly upward through the mass, the discharge pipe having been connected with the water supply for that purpose. When the vat is full, the water is admitted above the ore, and allowed to flow out through the discharge pipe. The washings are allowed to run to waste, unless they contain copper, in which case they are conducted to vats which contain scrap iron, by which the copper is precipitated.

The reason for introducing the water at first in the manner described is, that a certain quantity of silver chloride is dissolved by the strong solution of base chlorides and residual salt, which is formed on the first introduction of water. By operating as described, this solution is brought above the ore, and on being diluted with a further quantity of water, deposits the silver chloride in and upon the ore mass, where it remains until the hypo is admitted, while the major part of the base chlorides, remaining dissolved, is removed by the washing.

A portion of base metal chlorides, however, is also deposited with the silver chloride, and, on this account, some operators prefer to admit the water at once above the ore, and to save the silver which is thus carried out, by allowing the washings to flow through a series of launders, together with an additional stream of water. In the launders is placed a quantity of wood shavings, or some similar material, on and among which the silver chloride, together with base metal chlorides, is deposited. The shavings, with the metal chlorides, are then gathered up, and the silver is extracted, either by leaching, or by a smelting operation in a crucible. The flow of water through the ore is continued, until a test with calcium sulphide gives no precipitate.

140. If the ore contains lead chloride, it is washed with cold water until the greater part of the copper and iron chlorides, and salt, are removed. It is then treated with hot water, as long as any metal can be extracted, after which it may be necessary to again apply cold water, to cool the ore before admitting the hypo, so that too much base metal may not be extracted with the silver. Hot water, if applied at first, increases the solubility of silver chloride in the solution of base chlorides and salt; but after these are in the main removed, by means of cold water, it may be used with advantage. If, however, it were immediately followed by the hypo, more base metal would be extracted than would be the case if the ore were cooled; for, as before remarked, there are base metal compounds in the roasted ore which are insoluble in both hot and cold water, but which are soluble in the hypo, especially if it is warm.

Lead chloride is almost insoluble in cold water, but dissolves readily by the aid of heat. The sulphate is not dissolved by water, hot or cold. Hence it is better that lead should be chloridized in the roasting, because it can then be removed by washing. If lead is extracted in the silver leaching, it cannot, like copper, be retained in the matte when melting, but inevitably goes into the bullion, because its sulphide is easily reduced by iron at a red heat, while the copper sulphide is not.

141. From this point the leaching and precipitation are carried on exactly as directed, after the gold leaching, for concentrations containing gold and silver. If the ore contains a little gold, a portion of it may be obtained with the silver, owing to the formation, during the roasting, of the peculiar gold chloride described (30).