The ores, having been broken and ground, generally in tube mills, until they pass a 150 to 200-mesh sieve, are transferred to the leaching vats, which are constructed of wood, iron or masonry; steel vats, coated inside and out with pitch, of circular section and holding up to woo tons, have come into use.
The ore, supposed to have been salt-roasted, is charged loosely into the leaching vat and treated with water (to which sulphuric acid or copper sulphate may have been added), to remove soluble salts, which might later on be precipitated with the silver (base-metal chlorides), or overcharge the solution (sodium chloride and sulphate), or interfere with the solvent power (sodium sulphate).
Sodium chloride, characteristic of the Augustin process in which the ores, after a chloridizing roast, were extracted with brine, and the silver precipitated by copper, has almost wholly fallen into disuse; and potassium cyanide, which has become a very important solvent for finely divided gold, is rarely used in leaching silver ores.
This method of extraction contrasts favourably in time with the leaching process, which is so slow that over io,000,000 tons of ore are always under treatment on the immense leaching floors of the company's works in Spain.
The result is a great destruction of the humus of the soil, and great leaching and washing, especially in the light loams of the hill country of the United States.
The leaching is generally carried out with a strong, medium, and with a weak liquor, in the order given; sometimes there is a preliminary leaching with a weak liquor.
If the burningup of humus and the leaching of the soil could be prevented, there is no reason whyia cotton soil should not produce good crops continuously for an indefinite time.