Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
`` 1069313
~ his invention relates to a process for the direct
reduction of iron oxide-containing materials, preferably iron
ores, to produce sponge iron by a treatr~lent with solid
carbonaceous reducing agents in a rotary kiln at a temperature
below the softening and ~nelting point of the charge, in which
the kiln at~l~osphere flows opposite to the direction of movement
of the charge, at least part of the solid carbonaceous reducing
agents is charged in agglomerated form into the rotary kiln at '
the charging end thereof, and' oxygen-containing gases are fed
1 n into the rotary kiln.
The use of highly reactive solid carbonaceous reducing
agents having a high content of volatile constituents has the
advantage that the reaction rate i8 increased, the volatile
constituents can be used for the reduction and heat supply
in the rotary kiln, and inexpensive reducing agents can be used.
On the other hand, the problem arises to enable a utilization
in the rotary kiln of those volatile constituents which are
' released near the charging end and to minimize the quantity of
fine-grained reducing agent entrained by the exhaust gas from
20 the rotary kiln.
For this reason, in processes in which such reducing
agents are used, at least a considerable part of these reducing
agents is blo~m or thrown into the rotary kiln at the discharge
end thereof and is distributed along the rotary kiln over a
certain distance (British Patent Specification 1,222,1 23; British
Patent Specification 1,307,869; "Stahl und Eisen", 85, (1965),
page 1375; Australian Printed Application 423,616). As a
result, a large part of the volatile constituents is released
in the rear part of the rotary kiln and can be utilized in the
30 rotary kiln. In that process, however, a highly exact, defined
and uniform distribution of the reducing agent to the several zones
of the rotary kiln is required in order to avoid trouble due to
~ !
"` lO~g31~
fluctuations of temperature, pressure and gas composition. ~o
ensure the required distribution of the reducting agent, its
particle size distribution must be constant. If part of the
reducing agent having a high content of volatile constituents
is fed into the rotary kiln at the charging end thereof, only
part or none of the volatile constituents released in the first
part of the rotary kiln can be utilized in the rotary kiln.
It is also known to pelletize the surplus carbonaceous
material separated from the material discharged from the kiln
and to recycle the pelletized surplus carbonaceous material to
the charging end of the rotary kiln. Due to its pelletization,
the surplus carbonaceous material resists disintegration in
at least part of the length of the kiln and the losses due to
solids entrained by the exhaust gases are minimized (British
Patent Specification 1,264,452). Whereas this practice results
in a reduction of the losses caused by the entrainment of
recycled surplus carbon which has been devolatilized, it does
not constitute a suggestion how the problems described herein-
before can be solved.
It is known from the U.S. Patent SpeciPication
3.097.090 to charge the rotary kiln with fine-grained coal and
with surplus coal in the form of lump coal or coal pellets.
The lump coal should form a protective layer covering the
surface of the charge and is recycled when it has been screened
from the matter discharged from the kiln. A use of highly
reactive coal having a high content of volatile constituents
is not mentioned and would result in an excessively high
content of volatile constituents in the exhaust gas.
It is known from the British Patent Specification
1,110,667 to impregnate coked coal pellets with liquid hydro-
carbons and then to charge them into the rotary kilns in zones
in which the charge is already at a temperature of at least 600C.
-- 2 --
1069~13
~or this reason, the pellets rnust be blown or thrown into the
rotary kiln or fed through the ~qhell thereof.
It is an obàect of the invention to provide a
technically simple and economic process which enables the use
of highly reactive coal having a high content of volatile
; constituents whereas the above-mentioned disadvantages of the
known processes are avoided. Specifically, the volatile
constituents should be utilized to a high degree in the rotary
kiln, the losses of solid reducing agent due to entrainment
by the exhaust gases should be minimized, and a uniform
operation of`the kiln and high throughput rates are to be
achieved.
~his object is accomplished according to the invention
in that highly reactive, solid carbonaceous reducing agents which
contain 20-50% volatile constituents are charged in agglomerated
form into the rotary kiln at the charging end thereof and are
heated up at such a rate that they disintegrate into fine
particles substantially in and before the end of the heating-up
zone of the rotary kiln.
~he oxygen-containing gases - generally air - may be
blown in by means of shell tubes, which are spaced along the
kiln, and1or in a jet from the discharge end of the rotary kiln.
Preferred highly reactive solid carbonaceous reducing agents
which contain 20-50% volatile constituents are brown coal and
subbituminous coal. The reducing agent may be agglo~erated by
being pelletized or briquetted. ~he agglomerates have
preferably a particle size in the range of about 1-7 cm.
~rag~ents of agglomerates may also be used. ~he disintegration
of the agglomerates in and before the end of the heating-up
zone of the rotary kiln is controlled in such a manner that
the disintegration takes place throughout the length of the
heating-up zone whereas a complete disintegration of all
~0693~3 r
agglomerates in the initial portion of the heating-up zone is
avoided ~he reducing zone begins approximately at that point
of the rotary kiln where the temperature of the charge has
reached the reducing temperature and remains virtually constant.
It will be understood that a reduction resulting in lower iron
oxides takes place even before the reducing zone thus defined.
~he disintegration in the heating-up zone should result in a
particle size which is desirable for the reduction, i.e., which
is as small as possible, below 5 mm and in any case below 8 mm.
A small disintegration or continued disintegration in the reducing
zone is not disturbing.
According to a preferred feature of the invention, !
the highly reactive solid carbonaceous reducing agents are
charged in the form of briquettes into the rotary kiln at the
charging end thereof. Briquettes are highly suitable and can
be made more economically than pellets because the reducing
~;agents which are used can be briquetted without binders when
they have the particle size which results from drying and they
need not be disintegrated to pelletizable particles and be
pelletized with addition o~ binders. The briquetting may be
effected at the temperature at which the coal is discharged
from the dryer or to which the coal cools down before entering
the presses. lhe briquettes leaving the presses are at a
temperature of about 40-60C. Cylindrical briquettes are
preferred.
According to another preferred feature of the invention,
part of the solid carbonaceous reducing agent is blown into the
rotary kiln at the discharge end thereof in the direction of
flow of the kiln atmosphere. In this way, the sponge iron
product can be protected in the final part of the reducing zone
even if only a very small amount of residual carbon is left
in the charge. Only a small required surplus of carbon is
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69313
required in the discharged material in this case.
According -to a further preferred feature, the
agglomerated solid carbonaceous reducing agent has a particle
size of 1-7 cm. ~his particle size range results in very good
operating conditions.
- According to a further preferred feature, the
agglomerated reducing agent is heated up in the heating-up
zone of the rotary kiln at a rate of 7-15C/min, preferably
9-12C/min. Atthis heating-up~ate,the agglomer~tes ~the reducing agentare
devolatilized at a lower rate than the same reducing agent when
it is not agglomerated. This results in a more uniform devola- ;
tilization throughout the heating-up zone and in a smaller
heat consumption. A heating-up rate in this range results
also in a particularly desirable disintegration of the
.
agglomerated reducing agent and in lower losses due to a
formation of dust and burning.
According to yet another preferred feature, the
charge is at a temperature of 900-950C in the reducing zone.
Most of the volatile constituents of the agglomerates of the
reducing agent are released up to this ternperature range so
that the H2 content of the degasified reducing agent reaches
its maximum when the reducing zone is reached. In the reducing
zone, the reducing agent is gasified and the gasification
products are optimally utilized for the reduction in the
reducing zone. ~hese temperatures result also in a low-tempera-
ture coke having a sufficiently high strength for its continued
movement in the reducing zone of the kiln.
According to another preferred feature, the carbon
contained in the matter discharged from the rotary kiln amounts
to less than 2~ by weight, preferably less than 1% by weight,
of the quantity of iron oxide-containing material which has
been charged. ~his simplifies the separation of surplus carbon
`` ~0693~3
from the material which has been discharged or eliminates the
need for such separation. Any separated carbonaceous material
can be recycled into the rotary kiln.
According to a ~urther preferred feature, a solid
carbonaceous reducing agent of low reactivity i9 charged into
the rotary kiln in a quantity of up to about 0 1% by weight
carbon per 1% by weight Fe in the iron oxide-containing material
which is charged. ~he reducing agent of low reactivity may be
charged into the rotary kiln at the charging end thereof or
may be blown in at the discharge end. The reducing agent of ~--
low reactivity is circulated almost without losses due to
burning and formation of dust! ~his practice avoids a depletion
of carbon in the charge in the final part of the rotary kiln
and permits of the use of only a small surplus of carbon re~uired
for the sake of safety.
According to a further preferred feature, a reducing
flame is maintained in the final portion of the reducing zone.
This permits also of the use of only a small surplus of carbon
required for the sake of safety. The reducing flame may be
produced by a central burner which is fed with oil or gas and
with a deficienty of air.
According to a further preferred feature, at least
80% of highly reactive solid carbonaceous reducing agent are
.
charged in agglomerated form into the rotary kiln at the
charging end thereof. Particularly good operating conditions
result from that practice.
~he advantages of the invention reside in that highly
reactive solid carbonaceous reducing agents are used in an ~
economical and technically simple manner and an optimum utiliza- -
tion of the volatile constituents in the rotary kiln is enabled,
the losses due to dust entrained by the exhaust gases are small,
and optimum operating conditions in conjunction with a high
throughput rate can be obtained.
