Note: Descriptions are shown in the official language in which they were submitted.
s
~ CA 02534863 2006-02-06 ~PCTIEP200 4 / 0 0 7 9 Q 4
WO 2005/014866 PCT/EP2004/007904
'" _1_
PROCESS AND PLANT FOR REDUCING SOLIDS CONTAINING IRON OXIDE
The present invention relates to a process and a plant for reducing solids
containing
iron oxide, in particular iron ore, in which fine-grained solids are heated
and at least
partly calcined in a preheating and/or calcining stage, are prereduced in a
first fluidized-
bed reactor downstream of the preheating and/or calcining stage, and are
reduced in a
second fluidized-bed reactor and briquetted in a briquetting stage downstream
of the
second reactor at a temperature above 500°C.
From DE 44 10 093 C1 there is known such a process for the direct reduction of
iron
ores to obtain sponge iron (DRI), in which in a first reactor with circulating
fluidized bed
a prereduction is effected at temperatures between 550 and 650°C. In a
downstream
second reactor with classical fluidized bed, into which heated gas containing
hydrogen
is introduced as reducing agent for fluidizing purposes, the solids are
further reduced,
so that the product for instance has a degree of metallization of more than
90%.
During the transport of sponge iron (DRI), the iron is usually briquetted for
safety rea-
sons, for instance because of the risk of fire and because of the better
handling (forma-
tion of dust). Such briquetting is effected subsequent to the reduction of the
iron, the
still hot sponge iron mostly being cooled while being supplied to the
briquetting plant.
For increasing the strength of the briquet it is, however, desirable that
briquetting takes
place at rather high temperatures of e.g. about 700°C. At this
temperature, however,
the fine-grained sponge iron has a very poor flow behavior, which makes
briquetting
more difficult. To improve the flow behavior of spong iron and ensure a good
proc-
essability, about 0.5 wt-% magnesium oxide (Mg0) is added to the sponge iron
prior to
briquetting through a pressure feeder upstream of the briquetting plant.
Magnesium
oxide has no measurable negative influence on the strength or stability of the
sponge
iron briquet, but it is expensive due to costly processing steps, so that the
manufactur-
ing costs for sponge iron briquets are also rising. In addition, magnesium
oxide is
hygroscopic and very fine-grained, usually with a grain size below 100 Nm, so
that it is
difficult to store and to use.
13 July 2004 O 1 P 96 WO
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A process of producing sponge iron from oxidic iron ores is also known from DE-
OS 1
458 756, in which the reduction should be effected at rather high
temperatures. To
avoid an effect known as bogging or fouling, which occurs during the reduction
at such
high temperatures and in which the solids stick or weld together to form
agglomerates,
so that the fluidized bed sinks down in the reduction reactor, the addition of
about 0.05
wt-% of very finely ground oxides or carbonates of magnesium is proposed.
These
additives should be rather fine-grained and preferably have a grain size of
distinctly
less than 297 pm, in particular less than 44 pm. However, this also leads to
the above-
described problems during the storage or use of the additives. In addition, in
this proc-
ess the additives must be added to the iron ore e.g. via a pressure feeder
before the
reduction stage. This leads to a rise in the investment costs for a plant for
performing
the process. Due to the lower temperatures in the reduction stage, a
comparatively
poor calcination is achieved in this known process, when carbonates of
magnesium are
added to the reduction stage. This can only be compensated by longer retention
times,
which are, however, likewise undesirable. On the other hand, these problems do
not
occur when magnesium oxide is added to the reduction stage instead of
carbonates of
magnesium. However, this involves the above-mentioned disadvantages of the
high
costs and the poor handling properties of magnesium oxide.
Therefore, it is the object of the present invention to provide a process and
a plant for
reducing solids containing iron oxide, which is characterized by an improved
flow be-
havior of the product and a lower consumption of energy.
In accordance with the invention, this object is solved by a process as
mentioned
above, in which magnesite (MgC03) together with the solids containing iron
oxide is
added to the preheating andlor calcining stage, which magnesite is at least
partly
calcined in the preheating and/or calcining stage to obtain magnesium oxide.
As com-
pared to magnesium oxide, magnesite is available at a distinctly lower price,
so that the
costs for producing briquets from sponge iron can be decreased. Since the
magnesite
is at least partly calcined to obtain magnesium oxide, the flowability of the
sponge iron
prior to briquetting is improved. Briquetting therefore can also take place at
high tem-
peratures, at which the flow behavior of the sponge iron usually is
deteriorated. By
means of this hot briquetting, the strength of the briquet is increased as
compared to
cold briquetting at lower temperatures.
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Since the magnesite is heated in the preheating and/or calcining stage
together with
the solids containing iron oxide, the supply of heat to the two reactors for
reducing the
solids containing iron oxide need not be ensured by a strong heating of the
reducing
agent, e.g. hydrogen. The energy efficiency of the endothermal reduction
process
hence can be increased in that the solids containing iron oxide and the
magnesite are
heated already in the preheating andlor calcining stage to the temperature
required for
reduction.
Due to the common supply of the solids containing iron oxide and the magnesite
into
the preheating and/or calcining stage it is not necessary to add additional
magnesium
oxide to the solids before a pressure feeder upstream of the reactors. In this
way, the
investment costs for the commonly used separate supply of magnesium oxide are
also
eliminated. The magnesite frequently contains impurities such as iron oxide
andlor
limestone, which do not disturb the further processing steps, but are in part
even de-
sired for the further processing of the iron.
The energy efficiency of the process of the invention can further be increased
in that
the magnesite together with the solids containing iron oxide is calcined in
the preheat-
ing and/or calcining stage at temperatures of 400 to 1250°C, in
particular at 540 to
1000°C. In accordance with the invention, the temperature range for
calcining can also
lie between 1000 and about 1250°C. Due to the particularly high
temperatures in the
preheating and/or calcining stage, as compared to the known processes, the
supply of
heat for the endothermal reduction of iron oxide by means of hydrogen need not
be
effected by a strong heating of the hydrogen commonly used as reducing agent.
In accordance with a preferred embodiment of the invention, more than 50 %,
prefera-
bly about 90 %, of the magnesite added to the preheating and/or calcining
stage to-
gether with the solids containing iron oxide have a grain size between 300 pm
and 3
mm, in particular between 400 pm and 1 mm. For the process in accordance with
the
invention there can also be used magnesite with a grain size between 1.25 and
3 mm.
The storage and the handling properties of the magnesite are improved thereby
without
deteriorating the flowability of the sponge iron. In the course of the
process, the rela-
tively coarse-grained magnesite or the magnesium oxide is ground in the
preheating
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and/or calcining stage or in the reactors provided downstream of the same. The
appli-
cability of the magnesium oxide in briquetting thereby is increased without
deteriorating
the handling properties of the additives.
An improved flow behavior and a good processability of sponge iron, in
particular in the
briquetting stage, is achieved in accordance with the invention when between
0.1 and 5
wt-%, in particular about 0.5 wt-% magnesite is added to the solids containing
iron
oxide before and/or during the supply into the preheating and/or calcining
stage. The
solids supplied to the briquetting stage from the second reactor contain for
instance
between 0.1 and 5 wt-°I°, in particular about 0.5 wt-°lo
magnesium oxide, which was
obtained by calcining the magnesite in the preheating and/or calcining stage.
To further
improve the processability of the solids reduced in the second reactor in the
briquetting
stage, the same can be heated together with the magnesium oxide in a heating
stage
upstream of the briquetting stage to a temperature above 600°C, in
particular about
700°C, and can be introduced into the briquetting stage in the hot
condition. This pro-
vides for a further reduction of the energy required for forming in the
briquetting stage.
To largely avoid the formation of agglomerates in the reactors during the
reduction, the
solids containing iron oxide are reduced in the first and second reactors,
preferably at
temperatures below 700°C, in particular at about 630°C. At these
temperatures, the
bogging effect known from the prior art does not occur. As a result, the
magnesite
supplied to the preheating and/or calcining stage is not required already in
the reduc-
tion stage for forming magnesium oxide, but ensures the flowability of the
sponge iron
during the supply to a briquetting plant. The degree of fluidization of the
solids contain-
ing iron oxide in the first and second reactors hence is particularly high
during the
reduction, so that there can occur a good transfer of heat and a good reaction
with the
reducing agent. The solids containing iron oxide are reduced in the first and
second
reactors to obtain metallic iron with a degree of metallization of more than
75 %, in
particular more than 90 %.
The object underlying the invention is further solved with a plant for
reducing solids
containing iron oxide, comprising a preheating and/or calcining stage, a first
and a
second reactor each constituting a fluidized-bed reactor, and a briquetting
stage, in that
the preheating stage includes means for the simultaneous continuous or
discontinuous
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introduction of iron-oxide-containing solids and magnesite, and that upstream
of the
briquetting stage a heating stage is provided. Since iron-oxide-containing
solids and
magnesite are introduced together, the same are heated in the preheating
and/or
calcining stage, so that the heat required for the subsequent endothermal
reduction of
5 the iron oxide need not be ensured by a strong heating of the reducing
agent. The
heating stage upstream of the briquetting stage also makes it possible that
the sponge
iron withdrawn from the reduction reactors together with magnesium oxide
obtained
from the magnesite can be heated to a temperature of for instance about
700°C, which
is optimal for briquetting. The reduction can be effected at comparatively low
tempera-
tares, so that the inclination of the iron oxide to form agglomerates is
largely sup-
pressed.
The two series-connected reactors, in which the reduction takes place, can for
instance
be fluidized-bed reactors with a stationary fluidized bed. To provide for
improved mass
15' and heat transfer conditions during the reduction, at least one of the two
reactors
should, however, preferably be a fluidized-bed reactor with a circulating
fluidized bed or
an annular fluidized bed.
In accordance with a preferred embodiment of the invention, the first and/or
the second
reactor has a plurality of nozzles or inlet openings for supplying a heated,
gaseous
reducing agent, such as hydrogen. The reducing agent can also be used for
fluidizing
the solids reduced in the reactors.
The energy efficiency of the plant in accordance with the invention can be
improved in
that the preheating and/or calcining stage includes a first preheater, for
instance a
Venturi preheater, with a downstream first cyclone and a second preheater
(calcining
stage) with a downstream second cyclone, the first and/or the second cyclone
being
connected with the first Venturi preheater via a conduit for recirculating
dust separated
from waste gas. The dust heated in the preheating and/or calcining stage thus
is util-
ized for preheating the solids containing iron oxide and the magnesite.
Further developments, advantages and possible applications of the invention
can also
be taken from the subsequent description of an embodiment and the drawing. All
fea-
tures described and/or illustrated in the drawing form the subject-matter of
the invention
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per se or in any combination, independent of their inclusion in the claims or
their back-
reference.
The only Figure shows a process diagram of a process and a plant in accordance
with
an embodiment of the present invention. In the process for reducing solids
containing
iron oxide as shown in the Figure, for instance moist iron ore together with
magnesite
(MgC03) is introduced via a supply conduit 1 into a Venturi preheater 2, in
which the
solids containing iron oxide and the magnesite are dried and heated. Via
conduit 3, the
solids containing iron oxide together with the magnesite are introduced into a
cyclone
4, in which the dust-laden waste gases are separated from solids.
Via conduit 5, the dust-laden waste gases are supplied to a filter 6, for
instance an
electrostatic precipitator or a scrubber, from which the dust is recirculated
to the pre-
heating stage via conduit 7.
The solids separated from the waste gas in the cyclone 4 are supplied via
conduit 8 to
a calcining stage 9 or a second preheater to which a burner 9a is associated,
by means
of which a major part of the energy is supplied to the process. In the
calcining stage 9,
the solids and the magnesite are preheated to a temperature of for instance
about
850°C. Due to this high temperature in the calcining stage 9, the
magnesite is calcined
to obtain magnesium oxide, which together with the solids containing icon
oxide is
supplied via conduit 10 to a second cyclone 11. Therein, the solids are
separated from .
dust-laden waste gas, which is supplied to the first Venturi preheater 2 via
conduit 12.
As a result, the solids containing iron oxide and the magnesite are heated and
dried in
the Venturi preheater 2 by the waste gases of the second cyclone 11.
The solids separated in the second cyclone 11 are supplied via a conduit 13
with pres-
sure feeder to a first reactor 14, which for instance includes a circulating
fluidized bed.
By supplying hydrogen, the heated ore containing iron oxide is prereduced in
the first
reactor 14 and via conduit 15 introduced into a second reactor 16, which can
be a
stationary fluidized-bed reactor. Heated hydrogen as reducing agent is also
introduced
into the second reactor 16, so that the iron oxide is reduced in the second
reactor 16.
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From the second reactor 16, sponge iron with a high degree of metallization is
with-
drawn together with magnesium oxide and via conduit 17 introduced into a
heating
stage 18, in which the solids are heated to a temperature of about
700°C and via con-
duit 19 are introduced into a briquetting stage 20 in the hot condition.
Downstream of the reactors 14 and 16 recirculating cyclones can be provided,
in which
dust-like solids are separated from the gases leaving the reactors. In a waste
gas
treatment stage 21, these waste gases can be cleaned and be heated in a heater
22,
before they are recirculated to the reactors 14, 16.
Example (Reduction of Iron Ore)
In a plant corresponding to the Figure, 61.2 t/h of moist iron ore with 7.8 %
moisture
and 300 kg/h of magnesite with a grain size of less than 1 mm were supplied to
the
Venturi preheater 2 via conduit 1. Together with the magnesite, the iron ore
was dried
and heated in the Venturi preheater 2 and introduced via the cyclone 4 into
the calcin-
ing stage 9, in which the iron ore and the magnesite were heated to a
temperature of
850°C.
From the dust-laden waste gases separated in the cyclone 4, 2.6 t/h of dust
were
separated in the filter 6, which dust contained 25 kg/h of magnesium oxide.
This dust
was recirculated to the preheating stage via conduit 7.
54.2 t/h of the iron ore heated to 850°C in the preheater 9 together
with 150 kg/h of
magnesite calcined to magnesium oxide were introduced via conduit 13 with
pressure
feeder into the reactor 14 and the reactor 16 provided downstream of the same.
The
reduction in the reactors 14 and 16 at a temperature of about 630°C
provided 37 t/h of
a product with a degree of metallization of 91 %. The product which contained
about 34
t/h of metallic iron and 150 kg/h of magnesium oxide was introduced via
conduit 17 into
the further heating stage 18. In the same, the metallic iron and the magnesium
oxide
were heated to 700°C and via conduit 19 were introduced into the
briquetting stage 20
in the hot condition.
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List of reference numerals:
1 conduit
2 (first) Venturi preheater
3 conduit
4 (first) cyclone
5 conduit
6 filter
7 conduit
8 conduit
9 calcining stage (second
preheater)
9a burner
10 conduit
11 (second) cyclone
12 conduit
13 conduit with pressure feeder
14 (first) reactor
15 conduit
16 (second) reactor
17 conduit
18 heating stage
19 conduit
20 briquetting stage
21 waste gas treatment stage
22 heater
