Cu_{2}S + 2Cu_{2}O ➡ 6Cu + SO_{2}.
This stage of the blow also occupies about one hour or more, according
to circumstances. It commences with a vivid red flame accompanied by
smoke, but this soon dies out and a thin purple, almost colourless,
flame results, which continues practically unchanged for the remainder
of the blow (D). The temperature of the white metal is to some extent
judged by the appearance of the flame, a red-brown colour indicating
the correct temperature. If the colour be too red, the metal is too
cool, and coal is thrown in; if the tint be too orange, the temperature
is too high, and dope is added. Constant punching of the tuyeres by
long steel chisels is required during this stage of the blow, owing
to the lessened heat production due to diminution of iron, and also
to the marked tendency for the liberated copper to chill round the
tuyeres. The end of the blow is most difficult to judge, and although
the size and colour of the flame offer some criterion, the usual and
most important guide is the emission of small shots of copper which no
longer stick to the hood situated above the converter throat, but which
rebound from it. This is the stage where the skill and judgment of the
skimmer are most tried.
When the blow is considered satisfactory, the character of the metal is
further tested by pouring a small quantity on to the floor—a rugged and
uneven surface indicating satisfactory metal. If poured too soon, the
copper is coarse and impure; if poured too late, heavy losses in the
slag result, owing to excessive oxidation of the metal.
The copper is then poured into a ladle, and conveyed to the refining
and casting furnaces.
The whole operation for a straight run occupies about two hours, but
the time required in general naturally depends upon the rapidity of
working, and particularly on the grade of matte, and the volume and
pressure of the blast.
The slags during the early part of the blow generally carry about 2
per cent. of copper, after the white metal stage is passed, they are
usually much richer, on account of the intensely oxidising atmosphere
which prevails, and the decreasing quantity of protecting sulphur.
These later slags often contain upwards of 20 per cent. of copper, and
in consequence as much slag as possible is poured off during the early
stages of the blow, and the quantity towards the close is kept at a
minimum.
The subsequent treatment of the converter slag depends very much upon
the conditions of work at the smelter; at Anaconda, the iron contents
of this slag are very useful in the blast-furnace charge, as there
is a shortage of suitable basic flux for the silica of the rather
siliceous charges. The slag is poured from the converters into ladles,
and conveyed to a slag-casting machine, consisting of a conveyor belt
carrying cast-iron moulds which are sprayed with cold water, the slag
being thus cast into cakes suitable for the blast-furnace charge.
At Tennessee, the pyritic-smelting slags are already too ferruginous
for any addition of irony converter-slags in the blast-furnace charge
to be desirable, and the only metallurgical treatment for which these
are suited is that of recovering from them the large amount of copper
which they carry. The molten converter slag is, therefore, poured
directly into the blast-furnace settlers, and by this means, the slags
are cleaned and the values recovered.
At the new Tooele Smelter, under Mathewson’s organisation, the molten
converter slags are poured directly into the reverberatory furnaces,
there being no blast-furnace or settler plant, and the cleaning and
settling are thus very satisfactorily conducted.
=Systems of Working=: _Acid-lined Converter_.—The “normal” system of
working—_i.e._, blowing a matte-charge first to white metal, then to
blister copper—is not always practicable nor economically the best
practice, and the system of operating the charges depends largely
upon the working conditions, which are subject to much variation at
different smelters. Even at the same plant, the procedure has to be
varied according to the attendant circumstances.