This section is from the book "Leaching Gold and Silver Ores. The Plattner And Kiss Processes: A Practical Treatise", by Charles Howard Aaron. Also available from Amazon: Leaching Gold And Silver Ores.
1. Leaching, or lixiviation, originally meant the extraction of alkaline salts from ashes, by pouring water on them. The resulting liquid was called a leach, lixivium, or lye. In metallurgy, at the present day, leaching means the extraction of metal salts from ores, by means of a watery solvent. The solution so obtained is called the leach or lixivium. Leaching may be done in several ways - by filtration, by decantation, or by flowing.
2. Filtration may be upward or downward; the solvent is passed through the prepared ore and through a filter which retains all solid matter.
3. Decantation is drawing or pouring off the solution obtained by mixing the solvent with the ore, after allowing the solid matters to subside.
4. Flowing is allowing a stream of the solvent to flow continuously into the lower part, and out of the upper part of a vessel containing the ore. The latter is usually kept suspended in the liquid by gentle stirring, and the vessel is so deep that only clear, or nearly clear liquid rises to the outlet.
6. It is necessary that the metals be combined with some substance which renders them soluble in the liquid used. For gold and silver, chlorine is the most suitable substance for this purpose. In order, then, to extract gold and silver from ores by leaching, these metals are combined with chlorine; the resulting compounds, or metal salts, are dissolved, and the leach is separated from the undissolved matter by filtration. The metals are then separated from the leach by precipitation.
7. Chemical combination differs from mere mixture, or mechanical combination. When two or more substances are merely mixed, each remains the same as it was before, and may be separated from the others without having undergone any alteration of its properties; but when they are chemically combined, they unite to form a substance which usually differs from any one, and from all of the original components - a substance which is the same throughout, or homogeneous, and from which the components can only be recovered by chemical action. Thus gold and chlorine might, under certain conditions, be mixed, and again separated unchanged, by a current of air; in the interval the gold would remain a metal, and the chlorine a gas. If combined, gold chloride would be formed, which is neither a metal nor a gas. It bears no resemblance, in appearance or properties, to either gold or chlorine; nor can either of those substances be obtained from it by any merely mechanical process of separation.
8. Solution also differs from mixture; the difference is best explained by an example. Sand, clay, or gold powder can be mixed with water, but not dissolved in it. They may remain a long time suspended, but will ultimately settle, or may be at once separated from the water by filtering; while sugar, salt, or gold chloride will dissolve in water, will not settle, and will pass with the water through a filter, having in the act of dissolving become a liquid, which the other substances, not dissolving, did not.
9. Precipitation means throwing down. It is effected by adding to a solution a substance, either solid or liquid, which acts chemically, and causes the dissolved substance, or some of its components, to become insoluble in the liquid; or, so changes the liquid as to render it incapable of dissolving the substance, which is therefore thrown down, or precipitated. An example of the first kind of precipitation is seen when a drop of muriatic acid is added to a solution of silver nitrate. Silver chloride is formed, which, being insoluble in the liquid, separates in the solid state. If this silver chloride be placed in hot brine, it will be dissolved, and will be again thrown down on the addition of sulphuric acid, which so changes the brine as to render it incapable of dissolving silver chloride. This is the second kind of precipitation.
Precipitation may also be caused by a change of temperature. Thus, if the hot brine in which the silver chloride was dissolved were allowed to become cool, the silver chloride would, unless in very small quantity, be precipitated, because cold brine cannot dissolve so much as when hot. Again, as any solvent can only dissolve a certain quantity of a substance, it follows that if a saturated solution be exposed to evaporation, the dissolved substance, not evaporating, must be thrown down in proportion to the diminution in quantity of the solvent. In the latter two cases the effect is generally called crystallization, or deposition, rather than precipitation. A substance which causes precipitation, when added to a solution, is called a precipitant.
10. Chlorine is a greenish gas, and is usually produced from common salt. When metals are combined with chlorine, the resulting compounds are metal chlorides, and are distinguished by the names of the respective metals, as gold chloride, silver chloride, and further, by the prefixes sub, or di, proto, bi, ter, tetra, penta, as copper chloride, copper protochloride, mercury proto-chloride, and bichloride, etc., representing different proportions of chlorine combined with the respective metals. The terms sesqui and per, are also used to designate certain ratios of combination.
11. Gold terchloride, for lixiviation, usually called simply gold chloride, is made by exposing the pulverized ore, containing the metal in small particles, to the action of chlorine and moisture. It is extracted by leaching with cold water, in which it dissolves readily, and the gold is precipitated in the metallic state, as a brown powder, by a solution of iron sulphate, known in commerce as copperas, or green vitriol, which takes to itself the chlorine, and leaves the gold insoluble. The metal is collected, washed, dried, melted, and cast as a bar or ingot.
12. If the rock, or ore containing the gold, is free from opposing substances, it may be chlorinated without being roasted; but in general, ores which are treated by lixiviation contain the gold so combined, or mixed with other substances, thai a preliminary roasting is necessary.
13. Silver chloride is made, for lixiviation, by means of heat, in a roasting furnace, is extracted by leaching the ore with a solution of calcium hyposulphite, being insoluble in simple water, and the silver is precipitated as silver sulphide, by a solution of calcium polysulphide. The precipitated sulphide, in the form of a black mud, is collected, washed, dried, roasted, and melted with an addition of scrap-iron, which takes the sulphur remaining after the roasting, and sets the silver free.
14. Silver sometimes exists naturally combined with chlorine, in ore, and is then soluble without roasting; but in general it is combined with antimony, sulphur or arsenic, or with base metal sulphides, oxides, etc., and then roasting is necessary.