U.S. patent application number 09/836062 was filed with the patent office on 2002-01-31 for process to preheat and carburate directly reduced iron (dri) to be fed to an electric arc furnace (eaf).
Invention is credited to Burba, Gianni, Guastini, Fabio, Pavlicevic, Milorad, Primavera, Alessandra.
Application Number | 20020011132 09/836062 |
Document ID | / |
Family ID | 8168812 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020011132 |
Kind Code |
A1 |
Pavlicevic, Milorad ; et
al. |
January 31, 2002 |
Process to preheat and carburate directly reduced iron (DRI) to be
fed to an electric arc furnace (EAF)
Abstract
The present invention relates to a high energy-saving process
for preheating and carburation of directly reduced iron (DRI) to be
conveyed to a melting process, by using melting furnace off-gas,
which is characterized by a high temperature and a high
CO-content.
Inventors: |
Pavlicevic, Milorad; (Udine,
IT) ; Burba, Gianni; (Latisana, IT) ;
Primavera, Alessandra; (Udine, IT) ; Guastini,
Fabio; (Gorizia, IT) |
Correspondence
Address: |
JENKINS & WILSON, PA
3100 TOWER BLVD
SUITE 1400
DURHAM
NC
27707
US
|
Family ID: |
8168812 |
Appl. No.: |
09/836062 |
Filed: |
April 17, 2001 |
Current U.S.
Class: |
75/380 ;
266/138 |
Current CPC
Class: |
C21B 13/0086 20130101;
Y02P 10/20 20151101; Y02P 10/216 20151101; Y02P 10/136 20151101;
C21C 5/52 20130101; C21B 13/14 20130101; Y02P 10/134 20151101 |
Class at
Publication: |
75/380 ;
266/138 |
International
Class: |
C21C 005/30; C21B
009/14 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2000 |
EP |
00111036.0 |
Claims
1. A process for preheating and carburation of directly reduced
iron (DRI) to be fed to a melting furnace, including the steps of:
adjusting the temperature and the composition of melting furnace
off-gas outside the melting furnace to a value, suitable to preheat
and carburate the DRI, contacting the DRI with said adjusted gas to
preheat and carburate the DRI.
2. A process according to claim 1, characterized in that at least
the temperature of the melting furnace off-gas is measured before
adjusting the temperature and the composition of the melting
furnace off-gas.
3. A process according to claim 1, characterized in that at least
the composition of the melting furnace off-gas is measured before
adjusting the temperature and the composition of the melting
furnace off-gas.
4. A process according to one or more of the preceding claims,
characterized in that said melting furnace is an electric arc
furnace (EAF).
5. A process according to one or more of the preceding claims,
characterized in that said melting furnace off-gas is adjusted to a
temperature of .ltoreq.800.degree. C. and a CO/CO.sub.2-ratio of at
least 1.
6. A process according to one or more of the preceding claims,
characterized in that the temperature of said melting furnace
off-gas is adjusted to 450-800.degree. C.
7. A process according to one or more of the preceding claims,
characterized in that the composition of said melting furnace
off-gas is adjusted to have a CO/CO.sub.2-ratio of 1-3.
8. A process according to one or more of the preceding claims,
characterized in that the temperature of said melting furnace
off-gas is adjusted to 450-800.degree. C. and its composition to
have a CO/CO.sub.2-ratio of 1-3.
9. A process according to one or more of the preceding claims,
characterized in that the adjustment of temperature and composition
of said melting furnace off-gas is performed by burning a mixture
of air and/or O.sub.2, recycle gas and optionally CH.sub.4, with
the melting furnace off-gas.
10. A process according to claim 9, characterized in that said
recycle gas is obtained from gas, which has been contacted with the
DRI in the preheating and carburation step (preheating
off-gas).
11. A process according to claim 9 or 10, characterized in that
said recycle gas is obtained after removing fines, cooling the gas,
pressurizing and intermediate storage of the gas.
12. A process according to one or more of the preceding claims,
characterized in that said DRI is contacted with said adjusted gas
by a vertically split flow.
13. A process according to one or more of the preceding claims,
characterized in that said DRI is preheated to a temperature of
300-700.degree. C.
14. A process according to one or more of the preceding claims,
characterized in that said DRI is carburated to a C-content of 0-3
wt.-%.
15. A process according to one or more of the preceding claims,
characterized in that said adjusted gas contains 0-30% H.sub.2,
which is produced by the reaction of CH.sub.4 with O.sub.2.
16. A process according to one or more of the preceding claims,
characterized in that the temperature of said melting furnace
off-gas is about 1200.degree. C.-1600.degree. C. before adjustment
thereof.
17. A plant for performing the process according to claim 1,
comprising: a melting furnace, a means for the adjustment of the
melting furnace off-gas, and a preheater for contacting the DRI
with said adjusted melting furnace off-gas.
18. A plant according to claim 17, characterized in that said means
for the adjustment of the melting furnace off-gas is a combustion
chamber.
19. A plant according to claim 17 or 18, further comprising means
for measuring at least the temperature of the melting furnace
off-gas.
20. A plant according to claim 17, 18 or 19, further comprising
means for measuring at least the composition of the melting furnace
off-gas.
21. A plant according to one or more of the claims 17 to 20,
characterized in that said melting furnace is an electric arc
furnace (EAF).
22. A combustion chamber, comprising a controlled burner,
controlled inlets for the off-gas, CH.sub.4, air and/or O.sub.2 and
cooling recycled gas and a controlled outlet for transferring said
adjusted gas to the preheater.
23. A plant according to one or more of the claims 17 to 21,
characterized that said plant further comprises a device for
separating fines from the preheater off-gas, a cooling device for
cooling the preheater off-gas, a pressurizing device and an
intermediate storage device, from which the recycled preheater
off-gas is transferable to the combustion chamber and/or a fume
plant.
24. A plant according to one or more of the claims 17 to 21 or 23,
characterized in that said preheater comprises at least one
discharging device at the bottom and at least one gas inlet for the
gas to be contacted with the DRI.
25. A plant according to one or more of the claims 17 to 21, 23 or
24, characterized in that said preheater has essentially a
cylindrical form.
26. A plant according to one or more of the claims 17 to 21, 23, 24
or 25, characterized in that said preheater comprises at least two
conical discharging devices located at the bottom of said
cylindrical container and three gas inlets for the gas to be
contacted with the DRI, wherein one gas inlet is located centrally
at the bottom of the preheater and two gas inlets are located at
the side of said conical discharging devices.
27. Use of the process according to claim 1 in the production of
steel.
Description
[0001] The present invention relates to a high energy-saving
process for preheating and carburation of directly reduced iron
(DRI) to be conveyed to a melting process, by using melting furnace
off-gas, which is characterized by a high temperature and a high
CO-content.
FIELD OF THE INVENTION
[0002] According to the state of the art, steel can be produced
from iron ores/iron minerals by direct reduction of the solid iron
ore to produce sponge iron ("directly reduced iron", DRI). This DRI
is then fed to a melting furnace or a smelting furnace, for
example, an electric arc furnace (EAF), or the like. Due to the
need of reducing steel production costs, research directed towards
these integrated processes that relate to the production of liquid
steel from Fe mineral, has increased.
[0003] The only integrated process which has found an industrial
application is the one quoted in U.S. Pat. Nos. 5,447,550 and
5,296,015, which essentially concerns the production of DRI in a
shaft through hot discharging, pneumatic transportation and feeding
to an EAF. In order to make the continuous DRI production process
compatible with the discontinuous EAF process, a part of the DRI
product is cooled and stored in a parallel circuit. Obviously, the
presence of a reduction shaft in proximity of the melting process
zone is required.
[0004] Principally, heat can be transferred from a gas to an object
through two different ways. First, heat can be transferred via the
sensitive power/heat of a gas, which corresponds to its
temperature, flow rate and composition. Secondly, the latent
heat/power of a gas can be used, for example in an EAF off-gas,
which has a high CO-content, the heat generated by the exothermic
oxidation reaction of CO: CO+1/2O.sub.2=>CO.sub.2.
[0005] The processes disclosed in the U.S. Pat. Nos. 5,447,550 and
5,296,015 enable to recover the sensitive heat of the charged DRI,
but present some disadvantages that make it non-competitive from an
economical point of view, especially as regards high productivity.
In fact, besides the difficulties associated with the utilisation
of pneumatic transport, i.e. high percentage of fines in the
product, the necessity of a treatment gas circuit to avoid the DRI
reoxidation, the main problem is the high power (sensitive
power+latent power) dissipated by the EAF off-gases.
[0006] Another method that allows some energy saving in the melting
process consists in preheating the EAF feeding (i.e. the DRI) by
means of the sensitive heat of the gases coming out of the melting
process. There are several patents related to this method, for
example U.S. Pat. No. 4,736,383, WO 98/43032 or U.S. Pat. No.
4,002,465, which is mainly used for scrap pre-heating and which is
suitable for different plants. However, all these processes are
associated with high emissions of CO.sub.2.
[0007] A very important parameter for the DRI composition in the
production of liquid steel concerns the C content (carburation):
the optimal theoretic C-content value is the one which facilitates
or enables the reduction of the remaining FeO (wustite) during the
melting process in the EAF, according to the following
reaction:
FeO+C=>Fe+CO
[0008] Thus, it would be desirable to adjust the C-content of the
DRI before transferring the DRI to a melting furnace. Carburation
is generally carried out in the mineral reduction reactor disclosed
in U.S. Pat No. 4,897,113, No. 4,734,128, No. 4,752,329. Although
carburation can be achieved in this manner, it is associated with a
rather high consumption of CH.sub.4. In order to achieve an
increase the C-content of 1%, 18-20 Nm.sup.3 CH.sub.4 per ton DRI
have to be consumed.
[0009] Therefore, it is an object of the present invention to
provide a process for preheating and carburation of directly
reduced iron (DRI) to be conveyed to a melting furnace which does
not have the above-mentioned disadvantages and which considerably
reduces the energy consumption in steel production.
[0010] These problems are solved by a process according to claim 1
for preheating and carburation of directly reduced iron (DRI) to be
fed to a melting furnace, preferably an electric arc furnace (EAF).
The process according to the present invention has the advantage
that both the latent heat and the sensitive heat of the melting
furnace off-gas are used to preheat the DRI. Further, due to the
high CO-content of the melting furnace off-gas, natural gas for
carburation of said DRI is saved and the addition of carbon in the
melting furnace or the EAF can be avoided or at least considerably
reduced.
[0011] In a preferred embodiment of the present invention at least
the temperature of the melting furnace off-gas is measured before
adjusting the temperature and the composition of the melting
furnace off-gas.
[0012] In another preferred embodiment at least the composition of
the melting furnace off-gas is measured before adjusting the
temperature and the composition of the melting furnace off-gas.
[0013] According to the present invention it is preferred that the
melting furnace is an electrical furnace (EAF).
[0014] The values of temperature, pressure and composition of the
off-gases are measured and the temperature and CO/CO.sub.2-ratio of
said gas are adjusted in order to obtain a composition that may be
utilized for preheating and carburization of DRI.
[0015] In the process according to the present invention, it is
preferred that the temperature of the melting furnace off-gas is
adjusted to .ltoreq.800.degree. C., preferably 450-800.degree. C.,
and/or its composition to have a CO/CO.sub.2-ratio of at least 1,
preferably 1-3. The upper limit of 800.degree. C. is determined by
the fact that usually the CO/CO.sub.2-ratio in the off-gas coming
from the furnace is not over 5,5. This value corresponds to a
temperature of 800.degree. C. for the carburation zone (see FIG.
2A). The lower limits of 450.degree. C. for T and 1,0 for the
CO/CO.sub.2-ratio are determined by the fact that below these
values no remarkable deposition of C is verified due to the kinetic
of carburation.
[0016] The adjustment of temperature and composition of the melting
furnace off-gas is preferably performed by burning a mixture of air
and/or O.sub.2, recycle gas obtained from gas which has been
contacted with the DRI, and optionally CH.sub.4, with the melting
furnace off-gas.
[0017] The recycle gas is preferably obtained after removing fines,
cooling the gas, pressurizing and intermediate storage of the
gas.
[0018] The DRI is preferably preheated to a temperature of
300-700.degree. C. using the adjusted melting furnace off-gas. The
use of melting furnace off-gas has the advantage that the melting
furnace off-gas does not have to be transferred to a fume plant and
wasted.
[0019] In a preferred embodiment the DRI is contacted with said
adjusted gas by a vertically split flow.
[0020] According to the present invention the DRI is preferably
carburated to a C-content of 0-3 wt-%. Thus, the addition of carbon
in the melting furnace or EAF can be avoided or at least
considerably reduced.
[0021] It is further preferred that the adjusted gas contains 0-30%
H.sub.2 which is produced by the reaction of CH.sub.4 with O.sub.2.
The presence of H.sub.2 has the advantage that the reducing
capability of the atmosphere used to preheat the DRI is increased,
which prevents Fe in the DRI from being (re)oxidized to FeO and/or
helps to reduce residual FeO in the DRI to Fe.
[0022] The present invention also comprises a plant for performing
the process according to claim 1.
[0023] It is preferred that the plant for performing the process of
the present invention comprises a means for measuring at least the
temperature of the melting furnace off-gas and/or the composition
of the melting furnace off-gas. This means is preferably a
combustion chamber.
[0024] In said plant, the adjustment of the melting furnace off-gas
is preferably performed in a combustion chamber, comprising a
controlled burner, controlled inlets for melting furnace off-gas,
CH.sub.4, air and/or O.sub.2 and cooling recycled gas and a
controlled outlet for transferring said adjusted gas to the
preheater.
[0025] In a preferred embodiment, the plant further comprises a
device for separating fines from the preheater off-gas, a cooling
device for cooling the preheater off-gas, a pressurizing device and
an intermediate storage device, from which the recycled preheater
off-gas is transferable to the combustion chamber and/or a fume
plant. Thus, the preheater off-gas can be advantageously recycled
and used to cool the melting furnace off-gas.
[0026] The preheater comprises preferably at least one discharging
device at the bottom and at least one gas inlet for the gas to be
contacted with the DRI.
[0027] It is further preferred that the preheater has essentially a
cylindrical form.
[0028] In a preferred embodiment, the preheater comprises at least
two conical discharging devices located at the bottom of said
cylindrical container and three gas inlets for the gas to be
contacted with the DRI, wherein one gas inlet is located centrally
at the bottom of the preheater and two gas inlets are located at
the side of said conical discharging devices. This construction has
the advantage that the DRI can be efficiently contacted with the
adjusted melting furnace off-gas.
FIGURES
[0029] FIG. 1 shows the thermodynamic flow of the Boudouard
reaction as a function of the temperature for a feeding gas
characterized by a CO/CO.sub.2-ratio of 2 at a temperature of
750.degree. C. or higher.
[0030] FIG. 2A shows the thermodynamic conditions for reduction and
carburation in a Fe--C--O thermodynamic diagram
(CO+CO.sub.2=100%).
[0031] FIG. 2B shows the thermodynamic conditions for reduction and
carburation in the presence of 20% (CO+CO.sub.2).
[0032] FIG. 3 shows a plant for preheating and carburation of DRI
to be fed to an EAF.
[0033] FIG. 4 is an enlarged view of the preheater of FIG. 3.
[0034] FIG. 5 shows a side view of the preheater and sectional
views indicating the positions of gas inlets.
DETAILED DESCRIPTION
[0035] The present invention describes a procedure which permits to
preheat and carburate DRI using the off-gas of the melting process,
usually performed in an EAF. According to this process, the energy
consumption in the production of liquid steel can be considerably
reduced in different ways:
[0036] utilisation of latent heat and sensitive heat of the melting
furnace off-gas;
[0037] reduction of natural gas (CH.sub.4) consumption for the
carburation;
[0038] addition of carbon in the melting furnace or the EAF can be
avoided or at least reduced.
[0039] Carburation can be carried out in a preheating reactor
thanks to the exothermic Boudouard reaction (2COC+CO.sub.2) which
is catalysed by Fe and whose thermodynamic flow is shown in FIG. 1
for a gas having a CO/CO.sub.2-ratio of 2 at a temperature of
750.degree. C. or higher. Although the C deposit reaction is
favoured by low temperatures (exothermic reaction), it does not
show a remarkable reaction rate for T<400-500.degree. C.
[0040] It is possible to use different types of melting furnaces,
for example smelting furnaces. However, the use of an EAF is
preferred in the present invention.
[0041] The composition of the gas fed to the preheating reactor and
its temperature should be defined in order to avoid the Fe
reoxidation reaction and, if possible, to favour the reduction of
remaining FeO present in the DRI. In FIG. 2A, a thermodynamic
diagram is shown, which gives the min. value of the
CO/CO.sub.2-ratio to be present in the gas to heat the DRI without
incurring in its reoxidation. The limit conditions are also
summarized in Table 1. The components of the off-gases are in this
case only CO and CO.sub.2. In the real case, off-gases are composed
of CO, CO.sub.2 and other components, i.e. N.sub.2, H.sub.2,
H.sub.2O. In this case, the C+CO.sub.22CO equilibrium curve moves
towards left (see FIG. 2B for a (CO+CO.sub.2) content of the gas
phase of 20%), fixing new limit conditions.
1TABLE 1 Limit conditions for obtaining preheat-reduction and/or
preheat- carburation CO/CO.sub.2-ratio T/.degree. C. FeO
reduction/Formation of CO 1.6 730 Lower limit FeO
reduction/Carburation 1.6 730 Lower limit Fe oxidation/Carburation
1.6 730 Higher limit Fe oxidation/Formation of CO
[0042] Although the melting furnace off-gas composition will
usually be adjusted using recycle gas, CH.sub.4 and O.sub.2 and/or
air, it is also possible according to the present invention to use
the melting furnace off-gas without a subsequent adjustment of its
composition.
[0043] The off-gas of the melting process is preferably conveyed
into a combustion chamber where, after mixing with recycle gas and,
if necessary, with CH.sub.4 and O.sub.2 and/or air (in order to
increase the CO/CO.sub.2 ratio), a reducing gas with desired
temperature and composition is generated. Cold fed DRI can be
preheated, depending on the gas used, up to a temperature of
300-700.degree. C. In this case, the preheating is to a large
extent achieved by the heat generated by the exothermic carburation
reaction.
[0044] The upper limit of the DRI preheating temperature depends on
the characteristics of the processed material, but may be limited
by the following considerations:
[0045] a too high temperature could lead to DRI pellet
sticking;
[0046] a faster Fe re-oxidation kinetic occurs at high
temperatures, even at high values of the CO/CO.sub.2-ratio;
[0047] C, initially deposited on the DRI according to the Boudouard
reaction:
[0048] 2COC+CO.sub.2 may be (re)gasified.
[0049] Generally, the following conditions are applicable to
achieve a combined preheating and carburation:
T.sub.inlet gas.ltoreq.800.degree. C. CO/CO.sub.2.gtoreq.1 (compare
FIG. 2A)
[0050] Preferred operative ranges to obtain preheating and
carburation are:
T.sub.inlet gas=450.degree. C.-800.degree. C. with
CO/CO.sub.2=1-3.
[0051] The DRI is preheated to a temperature of 300 to 700.degree.
C.
[0052] It is possible in the process according to the present
invention to obtain a C-content in the DRI up to CDRI=0-3%
[0053] Short Description of the Plant
[0054] A suitable plant according to the present invention should
focus on the possibility of carburation and preheating DRI using
melting furnace off-gases, preferably EAF off-gases, which
otherwise would directly flow to the fume plant.
[0055] In such a plant, the sensitive heat of fumes with a rich
carbon monoxide content is used, in addition with the heat produced
by their combustion with air or an air/O.sub.2-Mixture in a chamber
capable of burning such fumes in a controlled manner. If necessary,
CH.sub.4 and O.sub.2 can be injected in the combustion chamber in
order to increase the CO/CO.sub.2-ratio and to obtain a reducing
atmosphere through the reaction of partial CH.sub.4 oxidation to
yield CO and H.sub.2.
[0056] The combustion products exchange their sensitive and latent
heat with the DRI in a preheater which is charged and operated
seperately from the EAF. Dust is removed from fumes leaving the
preheater, which are then cooled, passed through a booster and
afterwards conveyed to the above-mentioned combustion chamber.
[0057] The proportion of cold fumes, which has not been used in the
closed process cycle, is conveyed to an exhaust gas treatment
plant.
[0058] In a variant of the plant, the gases coming from the
preheater are directly conveyed to the fume plant. In this case the
temperature of the off-gases flowing to the preheater may be
controlled, for instance, using a heat exchanger. In any case the
plant is equipped with instruments for measuring the temperature
and the composition of the melting furnace off-gas.
[0059] The range of flow rates of the adjusted off-gas to be fed to
the preheater is very wide. The lower value is the theoretic value
required for the carburation and preheating of the DRI. The maximum
value depends on the pressure drop in the preheater and on the
stability of the material inside the preheater. Flow rates range
from about 300 to about 2,500 Nm.sup.3/t.
[0060] In the following example a plant and a process according to
the present invention are described. However, the example is not
intended to limit the scope of the present invention.
EXAMPLE
[0061] A plant in accordance with the present invention is shown
schematically in FIG. 3. The plant includes an EAF (1), capable of
melting DRI for the production of liquid steel. The secondary
product of the melting operation (EAF off-gas) is the off-gas of
the decarburation phase. Since it is mainly made of CO, it can bum
and produce the heat necessary to preheat the DRI.
[0062] This off-gas leaving the EAF with a temperature of about
1200.degree. C. to about 1600.degree. C. enters the combustion
chamber (2), where it is oxidised with air (air/O.sub.2) in an
atmosphere thermally controlled by the recycle gas according to a
ratio, which permits to obtain fumes at a temperature of
<800.degree. C. and a CO/CO.sub.2-ratio of 1-3. If necessary,
CH.sub.4 and .degree. 2 can be injected in the combustion chamber
in order to increase the CO/CO.sub.2-ratio and to obtain a reducing
atmosphere through the reaction of partial CH.sub.4 oxidation to
yield CO and H.sub.2. The H.sub.2-content can vary between 0 to
about 30%.
[0063] Since the combustion products in the following heating phase
enter in straight contact with the DRI, gases must be devoid of
oxidant characteristics, and in order to achieve this, the
CO/CO.sub.2-ratio has to be regulated by acting on the combustion
air flow.
[0064] The fumes that leave the combustion chamber enter the
preheater (3) through some distributors at the bottom of the
cylindrical area.
[0065] The preheater (3) can be seen as a shaft for inside
refractory preheating and is equipped with a tapered discharging
device at the bottom for the descent of the solid material once the
heating has concluded. While ascending, the adjusted melting
furnace off-gas transfers its heat to the DRI which is loaded in
solid form at ambient temperature prior to the preheating by means
of a charging system, and leaves the preheater through a collector
located at the upper side of the preheater (3). Afterwards, the
gases pass to the operations of removement of the fines and a dust
separator (4) (cyclone and scrubber) and are subsequently cooled by
means of a heat exchanger (5). The gas temperature reached at the
end of these treatments allows using a fan (6) capable of adjusting
the pressure back to the values approximating the ones present in
the combustion chamber. The fan performance does not exceed 600-800
mm H.sub.2O.
[0066] The gases thus compressed are pushed into a storage unit (7)
with the aim of filling the flow gas power, if any, during
start-ups and preheating stoppages.
[0067] The off-gas of the storage unit (7) is split by a three-line
valve, regulated by controlling the temperature of the products of
the fume partial combustion of chamber (2), which divides the
off-gas into two independent flows: the first one is directed to
the combustion chamber, the second one is destined to the exhaust
gas treatment plant.
[0068] The preheater, which is shown in FIG. 4, is not very high
and has a considerable horizontal extension. Thus, a specific gas
distribution is required to homogeneously contact all the charged
material mass. The heating gas is injected through two distinct
ways: the first one consisting in an injection between the two
discharge cones at the bottom of the cylindrical area (8), the
second one through two distribution rings placed at the top of the
two discharge cones (9). In FIG. 5, sectional views of the
preheater are shown, indicating the positions of the gas
inlets.
* * * * *