xides, have not met with commercial
success. When using iron as the precipitant, it is desirable that the
solution should be as neutral as possible, and the quantity of ferric
salts present should be reduced to a minimum; otherwise, a certain
amount of iron would be used up by the free acid and in reducing the
ferric salts. Ores in which the copper is present as sulphate are
directly lixiviated and treated with iron. Mine waters generally contain
the copper in this form, and it is extracted by conducting the waters
along troughs fitted with iron gratings.

The wet extraction of metallic copper from ores in which it occurs as
the sulphide, may be considered to involve the following operations: (1)
conversion of the copper into a soluble form, (2) dissolving out the
soluble copper salt, (3) the precipitation of the copper. Copper
sulphide may be converted either into the sulphate, which is soluble in
water; the oxide, soluble in sulphuric or hydrochloric acid; cupric
chloride, soluble in water; or cuprous chloride, which is soluble in
solutions of metallic chlorides.

The conversion into sulphate is generally effected by the oxidizing
processes of weathering, calcination, heating with iron nitrate or
ferric sulphate. It may also be accomplished by calcination with ferrous
sulphate, or other easily decomposable sulphates, such as aluminium
sulphate. Weathering is a very slow, and, therefore, an expensive
process; moreover, the entire conversion is only accomplished after a
number of years. Calcination is only advisable for ores which contain
relatively much iron pyrites and little copper pyrites. Also, however
slowly the calcination may be conducted, there is always more or less
copper sulphide left unchanged, and some copper oxide formed.
Calcination with ferrous sulphate converts all the copper sulphide into
sulphate. Heap roasting has been successfully employed at Agordo, in the
Venetian Alps, and at Majdanpek in Servia. Josef Perino's process, which
consists in heating the ore with iron nitrate to 50 deg.-150 deg. C., is
said to possess several advantages, but it has not been applied
commercially. Ferric sulphate is only used as an auxiliary to the
weathering process and in an electrolytic process.

The conversion of the sulphide into oxide is adopted where the
Douglas-Hunt process is employed, or where hydrochloric or sulphuric
acids are cheap. The calcination is effected in reverberatory furnaces,
or in muffle fur