[Footnote 1: A paper read at the Cincinnati Meeting of the
American Institute of Mining Engineers, by Willard P. Ward, A.M.,
M.E., February, 1884.]
The numerous direct processes which have been patented and brought
before the iron masters of the world, differ materially from that now
introduced by Mr. Wilson. After a careful examination of his process,
I am convinced that Mr. Wilson has succeeded in producing good blooms
from iron ore, and I think that I am able to point out theoretically
the chief reasons of the success of his method.
Without going deeply into the history of the metal, I may mention the
well known fact that wrought iron was extensively used in almost all
quarters of the globe, before pig or cast iron was ever produced.
Without entering into the details of the processes by which this
wrought iron was made, it suffices for my present purpose to say that
they were crude, wasteful, and expensive, so that they can be employed
to-day only in a very few localities favored with good and cheap ore,
fuel, and labor.
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 2½ 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."
In the Siemens process pieces of ore of the size of beans or peas,
mixed with lime or other fluxing material, form the charge, which is
introduced into a rotating furnace; and when this charge has become
heated to a bright-red heat, small coal of uniform size is added in
sufficient quantity to effect the reduction of the ore.
The size of the pieces of the material employed prevents the intimate
mixture of the particles of iron with the particles of carbon, and
hence we would, on theoretical grounds, anticipate just what practice
has proved, viz., that the reduction is incomplete, and the resulting
metal being charged with oxides is red-short. In practice, blooms made
by this process have been so red-short that they could not be hammered
at all.
It would be impracticable in this process to employ ore and carbon in
as fine particles as Wilson does, as a very large portion of the
charge would be carried off by the draught, and a sticking of the
material to the sides of the rotating furnace could scarcely be
avoided. I do not imagine that a division of the material into
anything like the supposed size of molecules is necessary; we know
that the graphitic carbon in the pig-iron employed in puddling is not
so finely divided, but it is much smaller particles than bean or pea
size, and by approximating the size of the graphite particles in pig
iron, Wilson has succeeded in obtaining good results.
If we examine the utilization of the heat developed by the combustion
of a given quantity of coal in this process, and compare it with the
result of the combustion of an equivalent amount of fuel in a blast
furnace, we shall soon see the theoretical economy of the process. The
coal is burned on the grate of the puddling-furnace, to carbonic acid,
and the flame is more fully utilized than in an ordinary
puddling-furnace, for besides the ordinary hearth there is the second
or rear hearth, where additional heat is taken up, and then the
products of combustion are further utilized in heating the retorts in
which the ore is partly reduced. After this the heat is still further
utilized by passing it under the boilers for the generation of steam,
and the heat lost in the gases, when they finally escape, is very
small. In a blast furnace the carbon is at first burned only to
carbonic oxide, and the products of combustion issue mainly in this
form from the top of the furnace. Then a portion of the heat resulting
from the subsequent burning of these gases is pretty well utilized in
making steam to supply the power required about the works, but the
rest of the gas can only be utilized for heating the blast, and here
there is an enormous waste, the amount of heat returned to the furnace
by the heated blast being very small in proportion to the amount
generated by the burning of that portion of carbonic oxide expended in
heating it, and the gases escape from both the hot-blast and the
boilers at a high temperature.
In the direct process under consideration the fuel burned is more
completely utilized than in the puddling process, to which the cast
iron from the blast furnace is subjected to convert it into wrought
iron.
The economy claimed for this process, over the blast furnace and
puddling practice for the production of wrought iron, is that nearly
all the fuel used in the puddling operation is saved, and that with
about the same amount of fuel used in the blast furnace to produce a
ton of pig iron, a ton of wrought iron blooms can be made. I had no
opportunity of weighing the charges of ore and coal used, but I saw
the process in actual operation at Rockaway, N.J. The iron produced
was hammered up into good solid blooms, containing but little cinder.
The muck-bar made from the blooms was fibrous in fracture, and showed
every appearance of good iron. I am informed by the manager of the
Sanderson Brothers' steel works, at Syracuse, N.Y., that they
purchased blooms made by the Wilson process in 1881-1882, that _none_
of them showed red-shortness, and that they discontinued their use
only on account of the injurious action of the titanium they contained
on the melting pots. These blooms were made from magnetic sands from
the Long Island and Connecticut coasts.
[Illustration: NEW PROCESS FOR MAKING WROUGHT IRON FROM THE ORE.]
The drawing given shows the construction of the furnace employed. I
quote from the published description:
"The upper part, or deoxidizer, is supported on a strong
mantel plate resting on four cast iron columns.
"The retorts and flues are made entirely of fire-brick, from
special patterns. The outside is protected by a wrought iron
jacket made of No. 14 iron. The puddling furnace is of the
ordinary construction, except in the working bottom, which is
made longer to accommodate two charges of ore, and thus
utilize more of the waste heat in reducing the ore to metallic
iron.
"The operation of the furnace is as follows: The pulverized
ore is mixed with 20 per cent. of pulverized charcoal or coke,
and is fed into an elevator which discharges into the hopper
on the deoxidizer leading into the retorts marked C. These
retorts are proportioned so that they will hold ore enough to
run the puddling furnace 24 hours, the time required for
perfect deoxidation. After the retorts are filled, a fire is
started in the furnace, and the products of combustion pass up
through the main flue, or well, B, where they are deflected by
the arch, and pass out through suitable openings, as indicated
by arrows, into the down-takes marked E, and out through an
annular flue, where they are passed under a boiler.
"It will be noticed that the ore is exposed to the waste heat
on three sides of the retorts, and owing to the great surface
so exposed, the ore is very thoroughly deoxidized, and reduced
in the retorts before it is introduced into the puddling
furnace for final reduction. The curved cast iron pipes marked
D are provided with slides, and are for the purpose of
introducing the deoxidized ore into the second bottom of the
furnace. As before stated, the furnace is intended to
accommodate two charges of ore, and as fast as it is balled up
and taken out of the working bottom, the charge remaining in
the second bottom is worked up in the place occupied by the
first charge, and a _new_ charge is introduced. As fast as the
ore is drawn out from the retorts the elevator supplies a new
lot, so that the retorts are always filled, thus making the
process continuous."
The temperature of the charge in the deoxidizer is from 800° to 1,000°
F.--_Amer. Engineer._
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