Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CONTINUOUS REDUCTION OF IRON-CONTAINING MATER AL
The present invention relates to a process for the continuous
reduction of iron-containing material, in particular of ~ine-
particled iron ore.
Fine-particled iron ore is available in large quantities and
at relatively low prices. This is because the cheap transportation
of ores through pipelines requires that the ores exist in a
finely ground state with an average particle size of about
50 microns. ~his, in turn, means that the ores transported
inexpensively have to be reconverted into a shape permitting
further metallurgical processing, which means that the ores
must be pelletized prior to the further processing. Pelletization
is a relatively expensive matter, as the pellets have to be
fired, so the benefits obtained by cheap transportation are
lost again owing to the inevitably necessary conversion of
the ores into pellets.
There has been no lack of attempts to reduce ores continuously,
attention being paid mainly to coarse-particled to lumpy ores,
as the continuous reduction of very fine-particled iron ores
is problematic, regardless of whether the reduction is to be
carried out in the solid phase or into the liquid phase.
Direct reduction processes which are carried out in the solid
f~
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phase and~ for example, in rotary kilns have been known for
a long time. Thus, British Patent No. 1,35~,7~0 describes
a process in which iron ore is reduced by blowing in natural
gas beneath the ore charge. The exhaust gdses formed in the
S process are mixed with air and burned over the ore hed. The
iron carriers are lump ore, ore pellets, and optionally also
sintered pellets. The nature of the direct reduction processes
carried out in rotary kilns requires that both the iron carriers
and the reducing agents, if not reducing gases, be present
in lumped form.
In fact, it is worth preventing the materials used (or materials
which have already been reduced or pre-reduced) from being
blown out of the rotary tube by the blasting action of flames
and settling on the filters. For this reason, attention should
be paid to the properties of stability at high temperatures,
when selecting lumped ore for the charge.
Now lumped ore and coal having the necessary stability to high
temperatures are rare and are consequently correspondingly
expensive. The conversion of fine-particled ores into ore
^-20 pellets is also an expensive procedure, as pelletizing apparatuses
can be operated only at considerable expense.
With regard to reduction into the liquid phase, we have described
in our Luxembourg Patent No. 71 ~35 a process for the production
of liquid pig iron permitting simultaneous recovery of a gaseous
o~
mixture which is used for the pre-reduction oF the ore ch~rge.
~ carbon carrier is introduced by means of a carrier gas into
a first zone of an iron bath, deep in it, carbon being b~nded
to the iron and the bath being intentionally carburized. In
an adjacent zone of the bath, subjected to continuous thorouyh
mixing, the excess carbon is oxidised off by means o~ an incoming
oxygen blast and is converted into carbon monoxide, while the
ores at least partially pre-reduced by the resultant reducing
gases in an adjacent reaction chamber are char3ed into the
immediate vicinity of the exothermic reaction zone and are
caused to melt.
When carrying out this process, a carbon injection lance is
obviously needed, entailing high expenditure. Moreover, it
should be noted that in the course of the process very hot
exhaust gases are formed, whose temperature is about 1450C.
If these exhaust gases are to be utilised for the pre-reduction
of ore, a cooling stage must be provided. It is also advisable
to remove the dust from the exhaust gases.
What is desired, therefore, is a process which permits inexpensive
~ 20 fine-particled iron carriers to be reduced with the aid of
inexpensive solid reducing agents and which allows work to
be carried out both within the scope of reduction in the solid
phase and into the liquid phase and, particularly in the latter
case, allows the problems arising from the high temperature
of the exhaust gases to be transformed into an advantage.
. .
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The present invention provides a process in which fine-particled
ores as well as ground hard (mineral) coal in a ratio of 2:1
to 5:1 are heated separately in a flow reactor with the aid
of hot gas, the hot components are mixed, the temperature of
the mixture lying between 470 and 530C, and the components
are then briquetted while hot, and the hot briquettes are exposed
to sufficiently high temperatures, on the one hand, to cause
the iron oxide to be reduced and, on the other hand, to cause
C0 gas to be formed, which is burnt by a deliberate supply
of oxygen.
Fine ores can be mixed with softening, caking, hard coals acting
as a binder and also with non-softening, carbon-containing
substances such as low-volatile hard coals, coke breeze, fine-particled
low-temperature coke, and/or petroleum coke. However, it is
possible to use non-caking hard coal and to provide an addition
of pitch as a binder.
In a preferred process, fine ores (as well as optionally non-
softening carbon-containing substances), prior to mixing with
softening caking hard coal and prior to briquetting, are brought
to a temperature above the briquetting temperature, while the
softening caking hard coal is brought to a temperature below
its softening point. The temperatures of the components are
¦ adjusted prior to mixing in such a way that the desired briquetting
¦ temperature turns out to be the temperature o~ the mixture.
~3!301~9
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Brique~ting preferably takes place on a roller press operating
at pressures of between 1 and 5 Kg/cm roller width.
After leaving the roller press, the ready briquettes can be
kept for a prolonged period, at least 30 minutes, in the region
of the press temperature and thus additionally strengthened.
In another embodiment of the process according to the invention,
the pressed bodies are charged hot. This saves the otherwise
I necessary cooling process, and the sensible heat of the pressed
¦ bodies is not wasted.
j 10 According to a first embodiment of the process according to
¦ the invention, the hot briquettes are charged into a conventional! rotary kiln at the delivery end of which there is located at
least one burner which ensures the necessary starting temperature
at the beginning of the process and compensates for any temperature
` 15 variations in the course of the process.
Since carbon monoxide which is to be used to heat the furnace
is formed during the reduction process, the rotary kiln is
provided with tuyeres which permit the aimed supply of fresh
air. The reduction process itself is caused predominantly
by the reaction with the solid carbon. The products formed
when carrying out the process are pure scrap metal and coke
breeze. The pure scrap metal obtained can be processed metallurgically
without a need for expensive sampling, analysis, and grading,
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as is the case when using conventional pure scrap metal.
Although the temperatures prevailing in the rotary kiln during
the metallurgical processing of the brique~tes lead to a certain
loss of strength, the binder coal is simultaneously transformed
into a stable coke framework which counteracts this loss of
strength.
In the rotary kiln direct reduction process carried out here,
the carbon originating from the binder is additionally used
as a heat carrier and reducing agent.
Another embodiment of the process according to the invention
involves charging the hot briquettes into a converter containing
an iron bath whose carbon content is as high as possible and
which is decarburised continuously by a blast of oxygen, the
bath simultaneously being flushed by blowing in inert gas through
the converter bottom.
Owing to the high carbon content of the iron bath,the briquettes
immediately enter a strongly reducing medium as they impinge
~ upon the bath surface in general, regardless of whether and
how much slag is located above the bath. In fact, it can be
assumed that a proportion, if not the majority, of the metallurgical
reactions take place inside the slag during the refining operation,
as is revealed by the fact that metallic iron granules are
invariably found in converter slags. This explains why the
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oxygen jet with which the bath is supplied does not cause a
disturbance in the present instance, as the briquette carbon
would be oxidised before the briquette entered the bath, but
the bath is caused to bubble so intensively by the oxygen jet
that the briquettes enter the turbulent layer consisting of
slags and of iron, where reduction also takes place.
It can be assumed that Fe203 contained in the briquettes is
pre-reduced with the briquette carbon according to Fe203 ~ C
-~ FeO ~ CO, while the FeO in the bath passes into the metallic
phase under the effect of the bath carbon according to FeO
t C -~Fe ~ CO.
It is immaterial whether this actually applies or not, providing
that sufficient carbon always enters the bath through the addition
of briquettes to guarantee that the bath remains as saturated
as possible with carbon, since, in the final analysis, the
energy is supplied by the combustion of carbon.
I
The process according to the invention is suitable for the
continuous production of liquid iron as, owing to the continuous
~ charging of briquettes, carbon is continuously introduced into
the bath and therefore saturates it. Fine ore is simultaneously
reduced continuously into iron and supplies energy to the bath,
since carbon is burnt by the blowing of oxygen. Blowing can
obviously take place intermittently, in which case the intensity
and duration of the addition of oxygen depe d on the temperature
gL1 8~D~
prevailing in the bath and on ~he carbon conten~ of the bath.
These can be estimated by analysis of the exhaust gases or
can be determined by taking samples.
With regard to the flushing of the bath with inert gas through
the converter bottom, this measure is an important aid, as
the thickness and the consistency of ~he slag layer can be
influenced by varying the intensity of flushing, as described
in our Luxembourg Patent No. 81 207. In fact, intensive flushing
with inert gas causes the slag foam to collapse, whereas a
thick, foamy slag layer can be formed by curbing the intensity
of flushing. Thin-layered, non-foaming slag is formed during
the flushing operation; decarburisation, as well as after-combustion
of the CO over the surface of the bath, is promoted. This
is of particular importance in the process according to the
invention, as the zone located above the bath should not consist
predominantly of oxygen, on the one hand, and high temperatures
should prevail in that very place, on the other hand.
Owing to the flushing with inert gas, the conditions favourable
for the addition of briquettes can consequently be created
according to the invention by heating the briquettes sufficiently
above the bath and not subjecting the~, either above the bath
or inside the slag layer, to excessive contact with oxygen,
so that reduction can take place without a sig~ificant interruption.
A possible embodiment of the process according to the invention
g
involves introducing ;nto the apparatus, in which the heating
of the ore end of the coal as well as briquetting take place,
the hot exhaus~ gases formed during decarburisation of ~he
bath in order to utilise their sensible heat as well as their
energy of combustion.
Another possible embodiment involves using the exhaust gases
as heating or as reducing gases. In -fact, about 75% of the
exhaust gases consist of C0, the remainder being C02 and N2.
It is important during the decarburisation process to take
into consideration the fact that more or less significant quantities
of sulphur can be present in the carbon-containing substances
used. For th;s reason, slag forming agents are added, as known,
in order to obtain a basic, free-flowing slag having a predominant
desulphurisation effect. The slag forming agents can be introduced
1~ into the bath either individually or as constituents of the
briquette.
It should also be emphasised that the process according to
the invention is not only suitable for the reduction of oxidic
iron carriers but that it ;s quite possible to use iron containing
dust or roller sinter instead of fine ore, in the interests
of recycling and of environmental protection.
