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.
201. However carefully the ore may be treated, a certain portion of gold remains in the tailings, varying, other things being equal, according to the character of the ore. In case this results, in part, from the presence of particles too large to be entirely dissolved, that portion can be extracted by re-treatment, either with gas, by chlorine water, or by amalgamation. But a portion, which is sometimes quite considerable, still remains, and resists every mode of chlorination or amalgamation, and can only be extracted by smelting with lead.
In general, it may be said that gold which can be seen by the aid of a lens, after finely grinding, and carefully washing, a sample of the tailings, can be extracted by chlorination, or amalgamation; but that which cannot thus be rendered visible can neither be chlorinated nor amalgamated, even if the ore be re-roasted, with or without re-grinding. (If re-ground it forms a pasty mass, which cannot be leached, being almost impervious to water.) Of course the ore is supposed to be properly roasted in the first instance.
Concentrated sulphides containing gold, and free from lead, will frequently yield as much as 98 per cent of the fire assay, if moderately rich, but the extraction of 95 per cent is considered a good result, from ore containing $100 worth of gold in a ton. As the material loses about 24 per cent of its weight in the roasting and leaching, it will readily be perceived that, if there has been no sensible loss of gold in the roasting, the tailings from such ore will assay about $6 per ton, and for every hundred tons of ore treated there will be 76 tons of tailings, containing $456 worth of gold.
Tailings of this character, consisting chiefly of iron peroxide, make an excellent flux for the smelting of galena. All the precious metal they contain is extracted, together with the lead set free from the galena by the action of the iron oxide, and may be considered as clear profit to the smelter, who is obliged to use some kind of iron flux, and can find nothing, unless it be metallic iron, better adapted to his purpose than these tailings. Chlorination tailings can also be utilized in the manufacture of red paint.