._--Wet methods are only employed for
low grade ores (under favourable circumstances ore containing from 1/4 to
1% of copper has admitted of economic treatment), and for gold and
silver bearing metallurgical products.

The fundamental principle consists in getting the ore into a solution,
from which the metal can be precipitated. The ores of any economic
importance contain the copper either as oxide, carbonate, sulphate or
sulphide. These compounds are got into solution either as chlorides or
sulphates, and from either of these salts the metal can be readily
obtained. Ores in which the copper is present as oxide or carbonate are
soluble in sulphuric or hydrochloric acids, ferrous chloride, ferric
sulphate, ammoniacal compounds and sodium thiosulphate. Of these
solvents, only the first three are of economic importance. The choice of
sulphuric or hydrochloric acid depends mainly upon the cost, both acting
with about the same rapidity; thus if a Leblanc soda factory is near at
hand, then hydrochloric acid would most certainly be employed. Ferrous
chloride is not much used; the Douglas-Hunt process uses a mixture of
salt and ferrous sulphate which involves the formation of ferrous
chloride, and the new Douglas-Hunt process employs sulphuric acid in
which ferrous chloride is added after leaching.

Sulphuric acid may be applied as such on the ores placed in lead, brick,
or stone chambers; or as a mixture of sulphur dioxide, nitrous fumes
(generated from Chile saltpetre and sulphuric acid), and steam, which
permeates the ore resting on the false bottom of a brick chamber. When
most of the copper has been converted into the sulphate, the ore is
lixiviated. Hydrochloric acid is applied in the same way as sulphuric
acid; it has certain advantages of which the most important is that it
does not admit the formation of basic salts; its chief disadvantage is
that it dissolves the oxides of iron, and accordingly must not be used
for highly ferriferous ores. The solubility of copper carbonate in
ferrous chloride solution was pointed out by Max Schaffner in 1862, and
the subsequent recognition of the solubility of the oxide in the same
solvent by James Douglas and Sterry Hunt resulted in the "Douglas-Hunt"
process for the wet extraction of copper. Ferrous chloride decomposes
the copper oxide and carbonate with the formation of cuprous and cupric
chlorides (which remain in solution), and the precipitation of ferrous
oxide, carbon dioxid