[Footnote 1: Paper read before the Iron and Steel Institute.]
By J. MASSENEZ, Hoerde.
If in the acid and the basic Bessemer processes the molten pig iron is
taken direct to the converter from the blast furnace, there is the
disadvantage that the running of the individual blast furnaces can
hardly ever be kept so uniform as it is desirable should be the case
in order to secure regularity in the converter charges. In the
manufacture of Bessemer steel the variable proportions of silicon and
of carbon here come chiefly under consideration, while in the basic
process it is chiefly the varying proportions of silicon and of
sulphur; and in cases where either ores containing variable
percentages of phosphorus, or puddle slags, are treated, the varying
proportion of phosphorus has also to be considered. This disadvantage
of the irregular composition of the individual blast furnace charges
is obviated in a simple and effective manner by W.R. Jones's mixing
process. In this as much pig iron from the various blast furnaces of a
works as is sufficient for a large number of Bessemer charges, say
from seven to twelve charges, or, in other words, from 70 to 120 tons
of pig iron, is placed in a mixing vessel. Only a portion of pig iron
placed in the mixer is taken for further treatment for steel, while
new supplies of pig iron are brought from the blast furnace. In this
way homogeneity sufficient for practical purposes is obtained.
In the treatment of phosphoric pig iron, which is employed in the
production of basic steel, it is, however, not sufficient merely to
conduct the molten pig iron in large quantities to the converter in a
mixed condition, but the problem here is to render the proportion of
sulphur also independent of the blast furnace process to such an
extent that the proportion of sulphur in the finished steel is so low
that the quality of the steel is in no way influenced by it. The
question of desulphurization has, especially of late years, become of
the utmost importance, at any rate for the iron industry of the
Continent. By the great strike of 1889, the German colliers have
succeeded in greatly improving their wages; and with this increase in
wages not only is there a distinct diminution in the amount of coal
wrought, but, unfortunately, the coal produced since then is raised in
a much less pure condition than was formerly the case. Consequently
the proportion of sulphur in the coke has considerably increased.
Whereas formerly this proportion did not exceed one per cent., it has
now in many cases risen to 18 per cent.; so that an unpleasant ratio
exists between the wages of the workmen and the amount of sulphur in
the coal raised. It is therefore not remarkable that, even when ores
fairly free from sulphur are treated, it easily happens that a
sulphureted pig iron is obtained.
In order to effect satisfactory desulphurization, attention has been
bestowed on the fact that iron sulphide is converted by manganese into
manganese sulphide and iron. If sulphureted pig iron, poor in
manganese, is added in a fluid condition to manganiferous molten pig
iron, poor in sulphur, the metal is desulphurized, and a manganese
sulphide slag is formed. It may be urged that it does not seem
necessary to effect the desulphurization by means of the reaction of
the manganese and iron sulphide outside of the blast furnace, as it is
possible, by suitably directing the blast furnace, by the employment
of manganiferous ores or highly basic slag, so to desulphurize the
iron in the blast furnace itself that it would be unnecessary further
to lower the percentage of sulphur. Every blast furnace manager,
however, will have observed that, even with every precaution in the
blast furnace practice, pig iron will often be obtained with so high a
percentage of sulphur as to render it useless for the Bessemer acid or
basic processes. If the desulphurization in the blast furnace is
carried sufficiently far, it is always necessary to work the furnace
hot, and thus to obtain hotter iron than is desirable for further
treatment in the converter. On the other hand, the method of further
desulphurization outside the blast furnace, described in this paper,
presents the double advantage that part of the blast furnace can be
kept cooler, and thus lime and coke be saved, and that there is a
certainty that no red-short charges are obtained in the treatment in
the converter, while the pig iron passes to the converter at a
suitable temperature.
[Illustration: FIGS. 1 through 5]
A further advantage presented by the direct process described in this
paper is that the Bessemer works is independent of the time at which
the individual blast furnaces are tapped, as the pig iron required for
the Bessemer process can be taken at any moment from the
desulphurizing plant. In Hoerde, where the mixing and desulphurizing
process has for a considerable time been regularly in use, it has been
found that all the chief difficulties formerly encountered in the
method of taking the fluid pig iron direct from the various blast
furnaces to the converter have been obviated. At Hoerde the mixing and
desulphurizing plant shown in the accompanying engravings is employed.
This apparatus holds 70 tons of pig iron. It is, however, advisable to
have an apparatus of greater capacity, say 120 tons. The apparatus has
the shape of a converter, and the hydraulic machinery by which it is
moved is simple and effective. An hydraulic pressure of eight
atmospheres is sufficient to set it in motion. The vessel is provided
with a double lining of firebricks of the same quality as those used
for the lining of blast furnaces. This lining is gradually attacked
only along the slag line, and does not require repair until it has
been in use for some six weeks. Further repairs are then necessary
every three weeks. Only the few courses of spoilt bricks are renewed,
and for the repairs, including the cooling of the vessel, a period of
two or three days is required. At the end of the week the vessel is
kept filled, so that its contents suffice for the last charge to be
blown on Saturday. On Sunday night the vessel is again filled. The
consumption of manganese is very low; theoretically, it is the
quantity required for the formation of manganese sulphide, and in
practice it has been found that this amounts to about 0.2 per cent.
The proportion of manganese which the desulphurized pig iron coming
from the vessel should contain is best kept at about 1.5 per cent. in
order to render the desulphurization as complete as possible. Thus, a
mean proportion of 1.7 per cent. of manganese in the pig iron passing
into the vessel is more than sufficient to effect a thorough
desulphurization. Indeed, 1 to 1.2 per cent. of manganese is
sufficient to effect a satisfactory desulphurization. For the extent
of the removal of the sulphur, the temperature and the duration of the
reaction are of importance. It has been found that if highly
sulphureted pig iron is poured from the blast furnace into the
desulphurizing vessel, fifteen to twenty minutes are sufficient to
effect the desulphurization requisite for the steel process. The part
played by the duration of the process is seen from the results
obtained with the last charges, if the vessel is emptied at the end of
the week without fresh pig iron being added from the blast furnace.
If, for example, 60 tons of pig iron with 0.065 per cent. of sulphur
remain in the vessel, the proportion of sulphur with the last charges
falls to 0.03 per cent. The iron in the vessel remains sufficiently
fluid for several hours. When necessary, a little wood is thrown in.
It has been found quite unnecessary to obtain heat by passing and
burning a current of gas above the bath of metal.
A number of results, showing the separation of sulphur at the Hoerde
Works, was published a few months ago[2] by Professor P. Tunner, one
of our honorary members.
[Footnote 2: "Oesterreichische Zeitschrift fur Berg und
Huttenwesen," 1891, No. 19.]
The totals represent, respectively, 138,500 kilogrammes of pig iron
and 98,654 kilogrammes of sulphur.
Thus, from 138,500 kilogrammes of pig iron there has been eliminated
179,577-98,654 = 80,923 kilogrammes of sulphur, or, in other words,
45.063 per cent.
The proportion of sulphur in the slags rises with that in the iron
from the blast furnace to 17 per cent., an inappreciable portion of
the sulphur of the slag being oxidized to sulphurous anhydride by
access of air. An analysis of the slag yielded the following results:
Per cent.
Sulphur 17.07
Manganese 30.31
Phosphoric anhydride 0.61
Iron 7.13
Bases 35.04
An analysis of an average sample gave:
Per cent.
Manganese sulphide 28.01
Manganous oxide 20.23
Ferrous oxide 25.46
Silica 18.90
Alumina 5.00
Lime 3.53
Magnesia 0.43
The great convenience and certainty presented by the method described
in this paper will in all probability lead to its general adoption. As
a matter of fact, several works are now occupied with the installation
of this mixing and desulphurizing plant.
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