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Page 40
The construction of larger furnaces and the employment of higher
temperatures led to the production of a highly carbonized, fusible
metal, without any special design on the part of the manufacturers in
producing it. This pig iron, however, could be used only for a few
purposes for which metallic iron was needed; but it was produced
cheaply and with little loss of metal, and the attempt to decarbonize
this product and bring it into a state in which it could be hammered
and welded was soon successfully made. This process of decarbonization,
or some modification of it, has successfully held the field against
all so-called, direct processes up to the present time. Why? Because
the old fashioned bloomeries and Catalan forges could produce blooms
only at a high cost, and because the new processes introduced failed
to turn out good blooms. Those produced were invariably "red short,"
that is, they contained unreduced oxide of iron, which prevented the
contact of the metallic particles, and rendered the welding together
of these particles to form a solid bloom impossible.
The process of puddling cast iron, and transforming it by
decarbonization into wrought iron, has, as everybody knows, been in
successful practical operation for many years, and the direct process
referred to so closely resembles this, that a short description of the
theory of puddling is not out of place here.
The material operated on in puddling is iron containing from 2� to 4
per cent. of carbon. During the first stage of the process this iron
is melted down to a fluid bath in the bottom of a reverberatory
furnace. Then the oxidation of the carbon contained in the iron
commences, and at the same time a fluid, basic cinder, or slag, is
produced, which covers a portion of the surface of the metal bath, and
prevents too hasty oxidation. This slag results from the union of
oxides of iron with the sand adhering to the pigs, and the silica
resulting from the oxidation of the silicon contained in the iron.
This cinder now plays a very important part in the process. It takes
up the oxides of iron formed by the contact of the oxidizing flame
with the exposed portion of the metal bath, and at the same time the
carbon of the iron, coming in contact with the under surface of the
cinder covering, where it is protected from oxidizing influences,
reduces these oxides from the cinder and restores them to the bath in
metallic form. This alternate oxidation of exposed metal, and its
reduction by the carbon of the cast iron, continues till the carbon is
nearly exhausted, when the iron assumes a pasty condition, or "comes
to nature," as the puddlers call this change. The charge is then
worked up into balls, and removed for treatment in the squeezer, and
then hammered or rolled. In the Wilson process the conditions which we
have noted in the puddling operation are very closely approximated.
Iron ore reduced to a coarse sand is mixed with the proper proportion
of charcoal or coke dust, and the mixture fed into upright retorts
placed in the chimney of the puddling furnace. By exposure for 24
hours to the heat of the waste gases from the furnace, in the presence
of solid carbon, a considerable portion of the oxygen of the ore is
removed, but little or no metallic iron is formed. The ore is then
drawn from the deoxidizer into the rear or second hearth of the
puddling furnace, situated below it, where it is exposed for 20
minutes to a much higher temperature than that of the deoxidizer. Here
the presence of the solid carbon, mixed with the ore, prevents any
oxidizing action, and the temperature of the mass is raised to a point
at which the cinder begins to form. Then the charge is carried forward
by the workmen to the front hearth, in which the temperature of a
puddling furnace prevails. Here the cinder melts, and at the same
time the solid carbon reacts on the oxygen remaining combined with the
ore, and forms metallic iron; but by this time the molten cinder is
present to prevent undue oxidation of the metal formed, and solid
carbon is still present in the mixture to play the same role, of
reducing protoxide of iron from the cinder, as the carbon of the cast
iron does in the ordinary puddling process. I have said that the cast
iron used as the material for puddling contains about 3 per cent. of
carbon; but in this process sufficient carbon is added to effect the
reduction of the ore to a metallic state, and leave enough in the mass
to play the part of the carbon of the cast iron when the metallic
stage has been reached.
It would be interesting to compare the Wilson with the numerous other
direct processes to which allusion has already been made, but there
have been so many of them, and the data concerning them are so
incomplete, that this is impossible. Two processes, however, the Blair
and the Siemens, have attracted sufficient attention, and are
sufficiently modern to deserve notice. In the Blair process a metallic
iron sponge was made from the ore in a closed retort, this sponge
cooled down in receptacles from which the air was excluded, to the
temperature of the atmosphere, then charged into a puddling furnace
and heated for working. In this way (and the same plan essentially has
been followed by other inventors), the metallic iron, in the finest
possible state of subdivision, is subjected to the more or less
oxidizing influences of the flame, without liquid slag to save it from
oxidation, and with no carbon present to again reduce the iron oxides
from the cinder after it is formed. The loss of metal is consequently
very large, but oxides of iron being left in the metal the blooms are
invariably "red short."
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