(_a_) To allow of the formation, from the mixture of sulphides
and oxides in the roasted materials from the calciners,
of a copper matte and a slag.
(_b_) To maintain such a high temperature as to render these
products perfectly fluid, and thus to allow the matte and
slag to settle and separate thoroughly.
In spite of the neutral atmosphere, however, the smelting of the
roasted materials usually results in a higher concentration than would
be expected from the calculation of the sulphur, copper, and iron in
the charge. The reason of this is that the smelting operation results
in some further elimination of the sulphur, which causes the production
of a higher grade matte. This additional elimination of sulphur in the
reverberatory furnace smelting of the roasted charge is due to the
reactions which take place on melting, between the oxides, sulphates,
and sulphides of copper, all of which exist in the products from the
roasters. These reactions are expressed by the equations—
Cu_{2}S + 2Cu_{2}O ➡ 6Cu + SO_{2}
Cu_{2}S + CuSO_{4} ➡ 3Cu + 2SO_{2},
which indicate a further addition of copper to the matte, and a
corresponding loss of sulphur. Thus a typical reverberatory charge of
the following composition:—
Silica, 27·2 per cent.
Iron, 31·0 "
Lime, 2·3 "
Sulphur, 8·4 "
Copper, 8·3 "
should theoretically yield, on melting down, a matte running—
─┐ ┌─ ─┐ ┌─
[8]Cu (8·3) -> Cu_{2}S 10·4 │ │ Cu 8·3 │ │ Cu 30 %
├─ = ─┤ S 8·4 ├─ or ─┤ S 30 %
S (8·4−2·1) -> FeS 17·6 │ │ Fe 11·3 │ │ Fe 40 %
─┘ └─ ─┘ └─
In actual practice however, the matte resulting from the reverberatory
smelting of the charge had the composition—
┌─
│ Cu 45 per cent.
─┤ S 27 "
│ Fe 28 "
└─
the 3 per cent. loss of sulphur causing a 15 per cent. increase in the
copper contents of the matte.
Experience in the working of the plant enables the management to
determine this important factor with fair accuracy, and thus from a
knowledge of the composition of the roaster product, to regulate and
control the grade of the matte produced at the reverberatories. In
modern reverberatory practice, therefore, the control of the furnace
products is carried out at the roasting plant, and the reverberatory
furnace has simply to melt the charge and ensure good settling.
_Anaconda Practice_ affords a good illustration. The foreman of the
reverberatory furnaces simply charges what is sent him from the
roasters, and practically nothing else is put in,[9] his duty being to
smelt this mixture and to obtain from it a clean slag and fluid matte.
He is not responsible for the grade of the matte, and if this is not
satisfactory, some change is made in the working at the roasters. The
reverberatory foreman does not learn the composition of the materials
passing into his furnace until he is furnished with the daily assay
reports on the following day.
Reverberatory smelting is essentially a British process, developed
in Wales, as already explained, owing to a plentiful supply of good
furnace coal yielding a long flame, and also of good refractory
material. Many Swansea workmen were, in the early days of American
development, and are still, employed in charge of such copper furnaces,
and it is largely due to British technical skill and to American genius
for organisation and development that reverberatory smelting in the
large furnaces at modern works has become so very successful.
=The Principles of Modern Practice.=—Success in modern reverberatory
work has been due to the recognition of the fact, that with the
maintenance of constant high temperature on large masses of material,
thorough fusion and separation of the products can be very efficiently
conducted.
=The Requirements for Successful Reverberatory Work.=—Since the action
in the furnace is performed mainly by the effects of heat, it is
necessary that—
A. The melting should be as rapid as possible.
B. The losses of heat during melting should be reduced
to a minimum.
The temperature required for the formation of slag and for obtaining
a thorough fluidity of the materials is from 1,400° to 1,600° C., and
the methods of achieving the proper conditions can best be stated as
the avoiding of all circumstances likely to cool the furnace or to
interfere with the melting down of the charge.
A. To ensure rapidity of melting, it is essential that a very large
quantity of coal shall be burned as rapidly as possible. This requires—
i. A large grate area.
ii. A good draft.
iii. The firing and grating to be conducted so as to
interfere as little as possible with the regularity
and degree of heating.
In localities where a supply of suitable coal is not available, other
methods of heating, such as the use of oil or gaseous fuel, are
necessary.