DESCRIPTION
Title of Invention: Method of Production of Reduced Iron
and Facility for Production of the Same
Technical Field
[OOOl] The present invention relates to a method of
production and production facility o,f reduced iron. In
particular, it relates to a method of production and
production facility of reduced iron able to efficiently
obtain reduced iron with a stable high metallization
ratio without being affected by variations in the
properties of the feed material.
Background Art
COO021 In recent years, the method of producing
reduced iron utilizing the dust containing iron oxide
produced in the ironmaking and steelmaking processes
(blast furnace dust, rotary furnace dust, etc. Below,
sometimes called "good quality dust") and iron ore powder
or other iron oxide powder (below, sometimescalled "good
quality iron oxide powder") has been broadly used.
[0003] As such,a method of production of reduced iron,
the technique of mixing ironmaking dust or iron ore or
other iron oxide powder and a carbonaceous reducing agent
to form agglomerates and obtain a pellet- or briquetteshaped
agglomerate material and heating and reducing this
agglomerate material in a rotary hearth furnace or other
reducing furnace with a continuously moving hearth (for
example, PLT 1) and the technique of mixing an iron oxide
powder and a carbonaceous reducing agent, then heating
and reducing it in a rotary kiln- (for example, PLT 2)
have been known.
[0004] Due to the recent promotion of recycling at
ironmaking plants and the orientation toward zero
emissions, not only the good quality dust produced as a
byproduct in the ironmaking and steelmaking processes,
but also the melting furnace dust or electric furnace
dust with high slag content produced as a byproduct of
melting furnaces or electric furnaces, the rolling scale
produced as a byproduct of the rolling processes, the
sludge from the pickling or neutralization process
produced as a byproduct of the pickling-neutralization
processes, and other poor quality iron oxide powder have
also been used as material for producing reduced iron.
[OOOS] In general, such poor guality iron oxide powder
is higher in material oxidation degree than good quality
dust, so if used as a feed material for producing reduced
iron, to maintain the metallization ratio of'the final
product reduced iron (final product metallization ratio)
at the same level, the load in the heating and reduction
process increases. Here, the material oxiaation degree
(sometimes simply referred to as the "oxidation degree")
is an indicator defined by the mass percentage of the
amount of oxygen currently actually bonded with the iron
to the theoretical amount of oxygen able to be bonded
when all of the iron-containing components in the
material (metal iron, wustite, magnetite, hematite)
become hematite. Further, the final product metallization
ratio is an indicator defined by the mass percentage of
the metal iron to the total iron content in the final
product reduced iron. The metallization ratio of the
final product is preferably high because the load of
finishing reduction of the reduced iron in a melter after
the reduction process is decreased.
[0006~ Furthermore, such poor quality iron oxide
powder is larger in variation of the components compared
with good quality dust, so the the extent of variation of
the load in the heating and reduction step also becomes
larger.
[0007] That is, up to now, if trying to use the above
poor quality iron oxide powder in existing processes for
producing reduced iron from good quality dust or iron ore
powder or other good quality iron oxide powder, it is
necessary to deal with the increase in load in the
heating and reduction process and, further, devise
measures to strengthen the capability for dealing with
variations of the material properties
Citation List
5 Patent Literature
[0008] PLT 1. Japanese Patent Publication No. 2003-
293020A ,,
PLT 2. Japanese Patent Publication No. 2001-280849A
PLT 3. Japanese Patent Publication No. 2000-160219A
10 PLT 4. Japanese Patent Publication No. 2012-126963A
PLT 5. Japanese Patent Publication No. 2010-222667~
PLT 6. Japanese Patent Publication No. 59-25910A
Summary of Invention
Technical Problem
15 [0009] When using a rotary hearth type reducing
furnace to produce reduced iron, the production capacity
of reduced iron, that is, the amount of the iron oxide in
the feed material reduced in the furnace per unit time,
is mainly determined by the hearth area and furnace
2 0 temperature of the rotary hearth. In actual operation, to
maximize the production efficiency, it is common to raise
the furnace temperature as high as possible within the
range of the restrictions of the facility.
[OOlO] As explained above, recently, use of poor
2 5 quality iron oxide powder is being promoted for producing
reduced iron. It is becoming necessary to reduce material
with a higher oxidation degree than the past using
existing rotary hearth furnaces
[OOll] When using such poor quality iron oxide powder,
3 0 in order to produce the reduced iron by the same
)metallization ratio as up to before, the amount of
reduction required in the reducing furnace becomes
greater. However, raising the furnace temperature more
than now is difficult in many respects. Further, it is
3 5 not easy to increase the hearth area of a rotary furnace.
Therefore, the retention time in the reduction furnace is
extended to produce the reduced iron from such feed
material.
[0012] As a result, the increase in the amount of use
of the poor quality iron oxide powder is accompanied with
a drop in the productivity of the reduced iron.
Furthermore, poor quality iron oxide powder is not only
high in oxidation degree, but ,is also large in variation
of properties of the material even when of the same
origin. On the other hand, the time the agglomerate
material is kept in a rotary furnace hearth at the time
of production of reduced iron is generally a short one
such as 15 to 25 minutes or so. Changing the reduction
time to deal with variations in the properties of the
material is also extremely difficult in actual operation.
[0013] Accordingly, when poor quality iron oxide
powder is used to produce reduced iron while satisfying
the required value of the final product metallization
ratio, operation with an extra margin of safety
considering variations in the properties of the material
becomes essential. As a result, the facility has to be
operated according to the upper limit of range of the
oxidation degree of the material and the overall
productivity falls.
COO141 The reduction reaction of an iron oxide-based
agglomerate material powder containing a carbonaceous
material as a reducing agent proceeds as the carbon
monoxide present in the pores of the agglomerate material
reduces the hematite, magnetite, wustite, and other iron
oxide.
[0015] In this reaction system, the carbon monoxide
functioning as the reducing agent is produced by reaction
of the carbon in the carbonaceous material contained in
the agglomerate material and the carbon dioxide present
in the pores. This carbon monoxide generating reaction
(Boudouard reaction, C+C02+AQ=2CO) is an endothermic
reaction, so to make the reduction reaction proceed, it
is necessary to continue to supply an amount of heat for
the heat of reaction from outside the agglomerate
material.
[0016] Here, the agglomerate material of the reduced
iron-producing material is in general a porous material
with a porosity of 30% or more. The apparent heat
conduction rate becomes smaller than the value predicted
from the average of the components, so the heat
conduction in the agglomerate material is extremely slow.
[0017] That is, in a reduction reaction of a
agglomerate material in a rotary hearth furnace, the heat
conduction process where the heat which is supplied by
radiant heat from the furnace atmosphere to the surface
of the agglomerate material is conducted to the inside of
the agglomerate material is the rate-controlling step of
the overall operation (for example, see PLTs 3 and 4).
[0018] PLT 3 discloses a technique of improving the
internal heat conduction rate of a agglomerate material
of iron ore containing a carbonaceous material by
stipulating specific properties of the carbonaceous
material and the ore. Further, PLT 4 discloses the
technique of mixing a wire-like additive for promoting
heat conduction. However, neither technique inherently
suppresses the rise of porosity due to the oxygen
component which had been bonded with the iron oxide being
separated along with the reduction reaction.
[0019] As this reduction reaction proceeds, the
porosity of the agglomerate material increases. As a
result, the speed of heat conduction at the inside falls.
Unless breaking this relationship, to promote a reduction
reaction using a reducing furnace mainly employing
radiant heating such as a rotary hearth furnace, it is
necessary to continue raising the furnace temperature,
but in continuous operation at ahigh furnace
temperature, the load on the refractories becomes larger.
This leads to the drop in the operating rate due to
repair.
[0020] Here, consider the case of making up for the
insufficient capacity of a rotary hearth furnace
accompanying use of poor quality iron oxide powder with a
high oxidation degree by means of extension of the length
of the rotary hearth furnace or additional installation.
[0021] Even if using poor quality iron oxide powder
with a high oxidation degree, at the final stage of the
rotary hearth furnace, the metallization ratio of the
reduced iron reaches at least 60% or so. Consequently,
the porosity also becomes larger. That is, at the time of
the final stage at the rotary hearth furnace, the heat
conduction rate of the reduced iron has already become
sufficiently small. If trying to extend the length of a
rotary hearth furnace where the only mechanism for
supplying heat to the agglomerate material is radiant
heating so as to treat the reduced iron by finishing
reduction, it can easily be predicted that the facility
would become extremely large.
100221 Further, the technique of not expanding a
single reducing furnace but using a two-stage step for
proceeding with the reduction reaction has been described
in PLTs 5 and 6. PLT 5 describes the method of using a
fluidized bed reaction system to reduce hematite or
goethite in advance by a reducing gas to magnetite, then
adding a carbonaceous reducing agent to the obtained
semireduced productreducing agent, and radiating the
mixture with microwaves to heat it for further reduction
to obtain the metal iron.
100231 In this case, the size of the equipment of the
fluidized bed reaction system for the advance reduction
is kept down, but the finishing reduction is performed by
irradiation by microwaves. If considering the energy
required for reduction and.the efficiency of irradiation,
it can be easily predicted that the required power
facilities would become excessive.
100241 PLT 6 proposes using the exhaust gas from a
reducing furnace comprised of a rotary kiln for
preheating and preliminary reduction of the material and
increasing the content of carbon monoxide in the exhaust
gas to speed up the preliminary reduction. The aim of
this is to improve the reduction efficiency at the rotary
kiln. However, this technique is predicated on reduction
in a rotary kiln. Further, the exhaust gas temperature of
a rotary kiln is lower than the temperature inside the
furnace, so it can be easily guessed that the effect of
the preliminary reduction able to be obtained using this
exhaust gas would be smaller compared with when making
the reducing furnace itself a two-stage one. Further, at
the level of the art at that time, use of poor quality
iron oxide powder was not considered as a precondition,
so this art cannot be applied as is to poor quality iron
oxide powder. To use poor quality iron oxide powder, use
of an efficient rotary hearth type reducing furnace would
be better in heat transfer and reduction reaction, but as
explained above, the problem would remain of
deterioration of the conduction of heat to the inside of
the feed agglomerate material.
LOO251 The present invention was made in consideration
of the above state of the art in production of reduced
iron and the problems in the same. It has as its problem
to stably produce reduced iron of a high metallization
ratio, even when using material with a large variation of
properties or poor quality iron oxide powder with a high
oxidation degree, without large scale remodeling of an
existing rotary hearth furnace and by just increasing
inexpensive and easy finishing reduction facilities, and
has as its object the provision of a method of production
and facility for production of reduced iron solving the
above problem.
Solution to Problem
COO261 The inventors studied in depth the methods for
solving the above problem. As a result, they obtained the
following discoveries:
100271 (a) First, they discovered that when mixing
powder iron oxide-containing material and a carbonaceous
reducing agent to form agglomerates and reducing the
agglomerate material (below, in this Description,
referred to as the "feed agglomerate material"), the
porosity increases along with the progress in the
reduction reaction and the conduction of heat from the
surface of the agglomerate material to the inside becomes
the rate-controlling step of the reduction reaction. This
problem can be solved by dividing the reduction process
into two stages. In the first stage of reduction (below,
referred to as the "first reduction" and the reducing
furnace at this time referred to as the "first reducing
furnace"), the majority of the iron oxide in'the feed
agglomerate material is reduced. In this case, the
obtained reduction product (the reduction product
obtained by the first reduction being referred to as the
"first reduction product") is raised in porosity, so left
alone, the heat conductivity is poor. For this reason,
the inventors discovered that if performing the second
stage reaction by a method enabling the increase in
porosity to be used to promote the reduction reaction, it
would be possible to produce reduced iron with a high
metallization ratio stably and with a high efficiency.
COO281 (b) By using a rotary hearth type reducing
furnace (RHF) for the first reduction since the feed
agglomerate material is low in porosity and is good in
heat conduction and by using a rotary kiln or sliaft
furnace for the second stage of reduction (below,
referred to as the "second reduction" and the reducing
furnace at this time referred to as the "second reducing
furnace") since the first reduction product becomes high
in porosity and therefore the reducing gas in the furnace
easily passes through the pores to diffuse to the inside
and raises the reducing ability, the inventors discovered
it is possible to produce high metallization ratio
reduced iron with a high thermal efficiency. Further, in
the second reduction, the reducing gas penetrates from
the pores of the first reduction product to the inside
and reduces the iron oxide. For this reason, in the
second reduction, a certain concentration of reducing gas
becomes necessary. Therefore, the inventors discovered
that by making the concentration of CO gas in the
reducing atmosphere in the second reduction 10 vol% to 85
vol%, it is possible to efficiently reduce the first
reduction product.
[0029] (c) The first reduction product also contains a
portion of reduced iron with low metallizationreduced
iron with low metallization. Usually, reduced iron with
low metallization is low in strength and is powderized at
the time of discharge from the first reducing furnace.
Even if a high metallization ratio can be obtained in the
second reducing furnace, as a result of the powdering,
the reduced iron becomes smaller in particle size, so at
the time of handling in the later processes, there is a
concern over generation of dust and reoxidation. On the
other hand, reduced iron with a high metallization ratio
is high in strength, so is not easily rendered a powder.
Therefore, the inventors discovered that if reducing just
the powdered part in the first reduction product in the
second reducing furnace using a rotary kiln, it is
possible to promote the reduction reaction while
producing granulated reduced iron. However, if the amount
of the powdered part is too great as well, the progress
of the second reduction is inhibited. The inveniors
confirmed that when using a second reducing furnace
comprised of a rotary kiln type reducing furnace, if a
reduction,product containing particles of the reduction
product of a size of less than 3 mm in no more than 75%,
the second reaction sufficiently proceeds in the rotary
kiln type reducing furnace. . .
[0030] (d) Furt.hermore, when using a second reducing
furnace comprised of a rotary kiln, if raising the
furnace temperature so as to promote the reduction of
reduced iron with a small particle size and low
metallization ratio, a deposit called a "kiln ring" forms
inside the furnace. It was learned that if such a kiln
ring is formed, continuation of operation sometimes
becomes difficult. Therefore, the inventors discovered
that if controlling the furnace temperature of the rotary
kiln to between llOO°C and 1200°C, it is possible to
promote a reduction reaction while preventing the
formation of a kiln ring and form the reduced iron into
granules.
[0031] The present invention was made based on the
above discoveries and has as its gist the following:
(1) A method of production of reduced iron comprising
mixing an iron oxide-containing material and a reducing
agent to form agglomerates and treating the agglomerate
material by two consecutive stages of reduction, the
method of production of reduced iron further comprising
(i) applying a rotary hearth type reducing furnace to a
first reduction then
(ii) applying a rotary kiln type or shaft type reducing
furnace to a second reduction,
in the second reduction, a concentration of CO gas in the
reducing atmosphere being 10 vol% to 85 ~01%.
(2) The method of production of reduced iron according to
(I), wherein a metallization ratio of a first reduction
product of the reduction product produced by the first
reduction is 65 mass% to 90 mass%.
(3) The method of production of reduced iron according to
(1) or ( Z ) , wherein when applying a rotary kiln type
reducing furnace to the second reduction to treat the
first reduction product, an atmospheric temperature
inside the rotary kiln type reducing furnace is over
110O0C to 1200°C.
(4) The method of production of reduced iron according to
any one of (1) to ( 3 ) , further comprising classifying a
first reduction product produced by the first reduction
so that the classified fine size part of the reduction
product contains the reduction product of a size of less
than 3 mm in 75 mass% or less, applying a rotary kiln
type reducing furnace to reduce the classified fine size
part of the reduction product by the second reduction to
obtain a second reduction product, and mixing the second
reduction product with the classified coarse size part of
the first reduction product.
(5) The method of production of reduced iron according to
any one of (1) to (4), wherein, the iron oxide-containing
material includes at least one of melting furnace dust,
electric furnace dust, rolling scale, and sludge from the
pickling or neutralization process.
(6) A facility for production of reduced iron mixing an
iron oxide-containing material and a reducing agent to
form agglomerates and reducing the feed agglomerate
material using two consecutive reducing furnaces,
the facility for production of reduced irbn provided
with:
(a) a rotary hearth type reducing furnace as a first
reducing furnace for reducing the feed agglomerate
material and
(b) a rotary kiln type or shaft type reducing furnace as
a second reducing furnace further reducing a first
reduction product of the reduction product produced at
the first reducing furnace.
(7) The facility for production of reduced iron according
to (6), further comprising a classifying facility for
classifying the first reduction product and comprising a
facility for charging the fine size part of the first
reduction product at the classifying facility into the
second reducing furnace comprising of a rotary kiln type
reducing furnace and mixing a second reduction product
produced by the second reducing furnace and the coarse
part ofthe first reduction product obtained at the
classifying facility.
Advantageous Effects of Invention
100321 According to the present invention, it is
possible to easily produce reduced iron with a high
metallization ratio from poor quality iron oxide powder
without using a large scale rotary hearth type reducing
furnace and without raising the furnace temperature to
raise the reaction speed. As a result, it is possible to
avoid the capital investment for a large sized rotary
hearth type reducing furnace.
Brief Description of the Drawings
[0033] F I G . 1 is a flow chart showing one aspect of
the present invention.
FIG. 2 is a graph showing the test results of Example 1.
F I G . 3 is a graph showing the test results of Example 2.
FIG. 4 is a flow chart showing one aspect in the case of
classifying a first reduction product.
F I G . 5 a graph showing the test results of Example 5.
F I G . 6 is a graph showing a relationship of a fine size
classification rate (mass%) and the final'product
metallization ratio ( % ) .
Description of Embodiments
[0034] The method of production of reduced iron of the
present invention (below, sometimes referred to as the
"method of production of the present invention")
comprises mixing an iron oxide-containing material and a
reducing agent to form agglomerates and treating the
agglomerate material by two consecutive stages of
reducing furnaces, further comprising (i) using a first
reducing furnace comprised of a rotary hearth type
reducing furnace, then (ii) using a second reducing
furnace comprised of a rotary kiln type or shaft type
reducing furnace, in the second reduction, a
concentration of CO gas in the reducing atmosphere being
10 vol% to 85 ~01%.
100351 The facility for production of reduced iron of
the present invention (below, sometimes referred to as
"the facility for production of the presentinvention")
is a facility for production of reduced iron mixing an
iron oxide-containing material and a reducing agent to
form agglomerates and reducing the agglomerate material
using two stages of reducing furnaces, provided with (a)
a first stage rocary hearth type reducing furnace for
receiving and reducing the agglomerate material and (b) a
second stage rotary kiln type or shaft type reducing
furnace installed after the rotary hearth type reducing
furnace, receiving reduced iron discharged from the
rotary hearth type reducing furnace, and further reducing
the reduced iron.
[00361 Below, the present invention (the method of
production of the present invention and the facility for
production of the present invention will sometimes be
referred to together as "the present invention") will be
explained.
COO371 The reduction product discharged from the first
reducing furnace is defined as "the first reduction
product", while the reduction product discharged from the
second reducing furnace is defined as "the second
reduction product". Further, the reaction inside the
second reducing furnace for obtaining the second
reduction product from the first reduction product will
be referred to as "the finishing reduction reaction".
[0038] FIG. 1 shows one aspect of the present
invention using two stages of reducing furnaces to
produce reduced iron. The iron oxide-containing material
and reducing agent (carbonaceous reducing agent) are
crushed and mixed to form agglomerates to obtain the feed
agglomerate material. The feed agglomerate material is
dried, then charged into the first stage rotary hearth
type reducing furnace and heated and reduced to produce
the first.reduction product. Next, the first reduction
product is charged into the second stage rotary kiln type
or shaft type reducing furnace and heatedand reduced to
,.produce the second reduction product. .,,
[0039] In the present invention, to raise the
metallization ratio of the first reduction product, which
dropped due to the high oxidation degree of the material
before reduction resulting from use of poor quality iron
oxide powder, to a predetermined value, the first
reduction product reduced in the rotary hearth type
reducing furnace is additionally heated and reduced. A
rotary kiln type or shaft type reducing furnace with a
relatively low furnace temperature is added. Furthermore,
to promote the finishing reduction reaction of the
reduced iron inside the rotary kiln type or shaft type
reducing furnace, the carbon monoxide or other reducing
gas in the furnace atmosphere is held at a high
concentration.
[0040] Normally, in a reduction reaction of an iron
oxide agglomerate material containing a carbonaceous
material in a rotary hearth type reducing furnace,
reducing gas derived from the contained carbonaceous
material is supplied from the inside of the agglomerate
material, so the effect of the atmosphere' outside of the
agglomerate material on the reduction reaction is
negligibly small. That is, the pressure of the gas
produced from inside the agglomerate material is higher
than the atmospheric pressure, so there is little entry
of gas from the atmospheric side to the inside of the
agglomerate material.
[0041] However, the first reduction product discharged
from the rotary hearth type reducing furnace of the first
reducing furnace already reaches a metallization ratio of
at least 60% or so. It is guessed that the porosity also
becomes correspondingly high. Therefore, the inventors
thought that if the first reduction product obtained by a
rotary hearth type reducing furnace undergoes finishing
reduction, in the second reducing furnace, the furnace
atmosphere gas could easily diffuse through the pores to
the inside of the agglomerate material and therefore the
reduction reaction could be made to proceed more
efficiently. That is, they thought that if the
concentration of reducing gas inside the second reducing
furnace performing the finishing reduction of the first
reduction product is raised, the reducing gas could
diffuse to the inside of the first reduction product and
reduce the iron oxide.
roo421 In the present invention, a typical example of
a reducing gas effective for reduction of an iron oxide
agglomerate material containing a carbonaceous material
is CO (carbon monoxide) gas. The concentration of CO gas
in a rotary hearth type reducing furnace at the time of
normal operation is less than 10 vol%, so the
concentration of CO gas inside the second reducing
furnace is preferably made 10 vol% or more. Further, the
inventors studied this and, as a result, learned that
when the CO gas concentration is raised, the effect of
promotion of the reduction reaction becomes satlirat.ed, so
set an upper limit of the CO gas concentration of 85%
(FIG. 3).
100431 The method of raising the concentration of
reducing gas inside the second reducing furnace may be
the method of injecting reducing gas from the outside or
the method of setting the combustion air ratio of the
burners, operated inside the furnace for maintaining the
furnace temperature, to less than 1 to thereby cause
incomplete combustion. Further, it may also be the method
of feeding a coal-based carbonaceous material in addition
to the first reduction product and causing it to
decompose inside the furnace to generate a reducing gas.
[0044] According to the present invention, the
reducing gas required for the reduction reaction of the
iron oxide inside the second reducing furnace is supplied
from outside the agglomerate material directly to the
inside of the first reduction product, so it is not
necessarily required to cause a Boudouard reaction inside
the agglomerate material and it is not necessary to
supply the amount of heat required for a Boudouard
reaction to the agglomerate material. That is, in the
finishing reduction of the first reduction product with
the high porosity, the limitation of the speed due to
heat conduction can be avoided and the reduction reaction
can made to proceed even at a relatively low furnace
temperature.
[0045] Furthermore, by using a second reducing furnace
comprised of a rotary kiln type or shaft type reducing
furnace, a strong interaction between the atmospheric gas
and the first reduction product which cannot be expected
with a rotary hearth type reducing furnace where the
agglomerate material is reduced while standing still on
the hearth can be obtained. That is, in the case of a
rotary kiln type reducing furnace, the agitation of the
first reduction product by rotation of the drum and,
further, in the case of a shaft type reducing furnace,
the flow of gas due to the gas being pushed inside the
furnace and the contact with the first reduction product,
enable the diffusion and penetration of the reducing gas
into the pores of the first reduction proauct to be
promoted. As explained above, in the present invention,
the fact of the first reduction product being a high
porosity is effectively utilized to promote the reduction
reaction in the second reducing furnace.
COO461 If the reduction reaction does not proceed to a
certain extent in the first reduction, the porosity of
the first reduction product will not become sufficiently
large. Since the pressure of generation of gas from
inside the pores is greater than the atmospheric
pressure, in the second reduction, the reducing gas will
not be able to diffuse into and penetrate the pores and
the reduction reaction will not proceed.
COO471 The inventors studied this and as a result set
the desirable lower limit of the metallization ratio of
the first reduction product to 65%. They confirmed that
if a metallization ratio of 65% or more is secured, the
porosity becomes sufficiently high and the reduction
reaction proceeds in the second reduction (FIG. 2).
COO481 Further, if the reduction time in the first
reducing furnace is sufficiently long, the metallization
ratio of the first reduction product approaches the
target metallization ratio of the second reduction
product. In general, the metallization ratio of reduced
iron available on the market is 90% or so. That is, if
the metallization ratio of the reduced iron is 90% at the
time of completion of reduction at the first reducing
furnace, there is no need for finishing reduction at the
second reducing furnace. From the above, the
metallization ratio of the first reduction product after
the first reduction ends is preferably 65% to 90%.
100491 Further, when using a second reducing furnace
comprised of a rotary kiln type reducing furnace, the
first reduction product can be mixed and stirred inside
of the rotary kiln reducing furnace while forming
granules (increasing the particle size).
[0050] In the finishing reduction of the first
reduction product with the high porosity,' limitation of
the speed by the heat conduction can be avoided. Even at
a relatively low temperature, the reduction reaction can
be promoted. However, when the furnace temperature of the
second stage rotary kiln type reducing furnace is llOO°C
or less, the finishing reduction reaction speed falls, so
the furnace temperature is preferably over llOO°C. On the
other hand, if the furnace temperature of the second
stage rotary kiln type reducing furnace exceeds 1200°C,
there is significant formation of a kiln ring, so the
furnace temperature is preferably 1200°C or less.
100511 For the iron oxide-containing material, for
example, melting furnace dust, electric furnace dust,
rolling scale, sludge from the pickling or neutralization
process, and other poor quality iron oxide powder can be
used.
100521 Next, the method of classifying the first
reduction product and charging the fine size part to the
second reducing furnace will be explained. In the first
reduction product, a reduction reaction proceeds inside
the agglomerate material, whereby the metallization ratio
reaches at least 60% or so. Along with this, the porosity
of the first reduction product increases.
LOO531 The inventors discovered, based 43 this, that
if proactively charging the part of the first reduction
product with a small metallization ratio and high
porosity into the second reducing furnace and further
reducing it there, reduced iron with a high metallization
ratio can be efficiently obtained. Further, the inventors
discovered that if the second reducing furnace is a
rotary kiln type reducing furnace, finishing reduction
where the atmospheric gas easily penetrates from the
pores to the inside of the first reduction product will
proceed. Furthermore, they discovered that in t.he process
of the reduction, the powder first reduction product will
form granules (increase in particle size) and finally
reduced iron of a high metallization ratio and large
particle size will be obtained.
LO0541 As the properties of the first reduction
product (composition of ingredients, particle size
distribution, etc.), the properties obtained under normal
operating conditions are sufficient. They are not limited
to specific properties. However, if powder of the
reduction product of a particle size less than 3 mrn
exceeds 75 mass%, the heat conduction will limit the
speed of reaction in the second stage rotary kiln type
reducing furnace and the benefit of control of the
atmosphere for promoting the finishing reduction will not
be sufficiently enjoyed. For this reason, the first
reduction product preferably contains powder of a
particle size of less than 3 mm in no more than 75 mass%.
LOO551 Furthermore, it was learned that the fine size
part of the first reduction product is not reduced much
by the reduction reaction, is low in metallization ratio,
and contains much residual carbon. Conversely, it was
learned that the coarse particle size part of the first
reduction product is sufficiently reduced by the
reduction reaction, is high in metallization ratio, and
has less residual carbon.
100561 This is believed to be because when charging a
feed agglomerate material into the first stage rotary
hearth type reducing furnace, if impact or other action
causes the feed agglomerate material to powderize, this
powderized material blocks heat t.o the surrounding
agglomerate material and prevents the reduction reaction
from proceeding.
LO0571 , Therefore, the inventors came up with the idea
of classifying the first reduction product and treating
only the size part (powder like) with low metallization
ratio in the second stage rotary kiln type reducing
furnace for finishing reduction and granulat.ion.
[0058] FIG. 4 shows one example of production of
reduced iron using two stages of reducing furnaces. Up
until production of the first reduction product at the
first stage rotary hearth type reducing furnace, the
method of production is the same as that shown in FIG. 1.
However, in the method of production shown in FIG. 4, the
first reduction product is classified and only the fine
size part (low metallization ratio powder part) is
charged into the second stage rotary kiln type reducing
furnace and heated and reduced to obtain the second
reduction product.
100591 The production facility for production of
reduced iron shown in FIG. 4 comprises (a) a first stage
rotary hearth type reducing furnace to heat and reduce a
feed agglomerate material which consists of an iron oxide
powder and a carbonaceous reducing agent, (b) a
classifying facility installed after the rotary hearth
type reducing furnace to classify the reduction product
discharged from the first stage rotary hearth type
reducing furnace (first reduction product), and (c) a:
second stage rotary kiln type reducing furnace to heat
and reduce only the fine size part of the reduction
product.
[0060] Since the classifying facility is a facility
classifying the reduction product of temperature between
about 700°C and about llOO°C discharged from the rotary
hearth type reducing furnace, therefore the classifying
facility requires heat resistance. For example, a fixed
type screen (grizzly) or a water-cooled type of roller
screen is preferable. A roller screen can be adjusted in
the classifying size by changing the distance between
axes, therefore is preferable as a classifying facility.
[0061] The coarse size part (coarse particles) has a
high metallization ratio, so is made to bypass the
finishing reduction process and is merged with the second
reduction product obtained by finishing reduction of the
fine size part to obtain the final product reduced iron.
According to the method of production shown in FIG. 4,
the amount of treatment at the second stage rotary kiln
type reducing furnace can be reduced, the'body of the
rotary kiln type reducing furnace can be reduced in size,
and the exhaust gas treatment facility and heating use
burners and other ancillary equipment of the reducing
furnace can also be reduced in size. This leads to
reduction of the capital investment.
[0062] If setting the reference size of classification
when classifying the first reduction product to a finer
size, the amount of treatment at the second stage rotary
kiln type reducing furnace decreases, so the reducing
furnace can be made smaller in size. However, on the
other hand, the mass ratio of the fine size park (powder)
of 3 mm or less size treated at the rotary kiln type
reducing furnace (below, referred to as the "3 mm or less
ratio",) increases and the reduction reaction speed in the
rotary kiln type reducing furnace falls. As shown in FIG.
5, if the 3 mm or less ratio is over 75%, the finishing
reduction does not proceed.
COO631 FIG. 6 shows the relationship between' the fine
size classification rate (ratio (mass%) of the product
forming the fine size part in the product subjected to
classification) and the metallization ratio ( % ) of the
final product. From FIG. 6, it will be understood that if
the fine size classification rate is 50% or more, a
metallization ratio of the final product of 80% or more
can be secured.
Examples
[0064] Next, examples of the present invention will be
explained, but the conditions in the examples are only
illustrations employed for confirming the workability and
effects of the present invention. The present invention
is not limited to these illustrations of conditions. The
present invention can employ various conditions so long
as not departing from the gist of the present invention
and so long as achieving the object of the ptesent
invention.
[0065] Example 1
To confirm the optimal division of load ak the first
reducing furnace and the second reducing furnace, poor
quality iron oxide powder of melting furnace dust mixed
with electric furnace dust was mixed as the iron oxidecontaining
material with coal to form a test-use
agglomerate material (below, referred to as "tablets")
which was then subjected to reduction tests.
[0066] The prepared tablets all contained, by mass
percentage, metal Fe=l%, Fe0=35%, Fe203=34%, C=15%.
[0067] The inventors prepared two test-use reducing
furnaces (furnace A and furnace B ) . The furnace A was a
test-use el.ectric furnace with a furnace gas colr~position
of N2: 100% and a furnace temperature held at 1250°C,
while the furnace B was a test-use electric furnace with
a furnace gas composition of (CO: 40%+N2:60%) and a
furnace temperature held at 1150°C. The furnace B had a
furnace atmosphere of a reducing atmosphere stronger than
the furnace A, but had a furnace temperature 100°C lower.
The test conditions are shown in Table 1.
100681 At each run, first, the tablets were reacted
for exactly a predetermined time at the inside of the
furnace A while stationary. Inside the furnace A, the
furnace temperature was a sufficiently high 1250°C. The
tablets also contained a carbonaceous material.
Therefore, the heat conducted from the furnace atmosphere
by radiant heating to the surface of the tablets was
conducted to the inside of the tablets whereby the
carbonaceous material was gasified to generate CO gas
causing the reduction reaction to proceed. The reduction
reaction up to here will be called "the first stage",
while the reduction reaction from here on will be called
"the second stage.
[0069] At the second stage, part of the tablets
finished being subjected to the reduct.ion reaction at the
first stage continued to be treated by a reduction
reaction in the furnace A at the same furnace temperature
and the same atmosphere (runs 1, 3, and 5). The remaining
tablets were quickly moved to the inside of the furnace B
after the end of the first reduction reaction and
continued to be treated to a reduction reaction in the
furnace B (runs 2, 4, and 6) .
[0070] The atmosphere inside the furnace B was held at
a (CO: 40%+N2:60%) strong reducing atmosphere, so the
reduction reaction of the tablets could be expected to
proceed faster than inside the furnace A. Therefore, in
furnace B, the furnace temperature was set lower or the
reaction time was set shorter so that the total reaction
time became shorter than that for the tablets continued
to be reduced in the furnace A.
[0071] Table 2 shows the results of analysis of the
metal.lization ratios of the tablets after the end of the
first stage and the second reduction reaction shown in
Table 1.
[0072] First, take note of run 3 and run 4. Run 3
&~
caused reaction in an N2: loo%, 1250°C furnace A for 15
minutes. As opposed to this, run 4 first caused a
reaction in furnace A for 10 minutes, then caused a
reaction at a furnace temperature 1150°C, strong reducing
atmosphere (CO: 40%) furnace B for 2.5 minutes for a
total of 12.5 mihutes.
100731 At run 4, the total reaction time was 2.5
minutes shorter than run 3 and the furnace temperature of
the second reducing furnace was 100°C lower. Despite this,
the final metallization ratio after the end of the second
reduction could be maintained at the same value as run 3.
That is, this shows that switching the inside of a
furnace to a strong reducing atmosphere in the middle of
a reduction reaction can greatly improve the reaction
speed of the tablets.
100741 As opposed to this, at run 5 and run 6, the
first reduction time was extended 2.5 minutes from run 3
and run 4 to conduct slmilar tests. In this case, run 6,
where the total reaction time was shortened and the
furnace temperature of the second reduclng furnace was
lowered to 1150°C, was higher in metallization ratio
[0075] This is believed to be because the
metallization ratio at the first stage end point was
higher compared with run 3 and run 4 and the porosity of
the tablets became large and that therefore at the time
of the second reduction reaction, the atmospheric gas
easily diffused into the pores of the tablets and the
reduction reaction proceeded more easily.
100761 At run 1 and run 2, the first reduction time
was made 5 minutes. After that, the reduction continued
to be performed in the furnace A for 5 minutes (run 1)
and, at the second stage, the strong reducing atmosphere
furnace B was used for reduction at 1150°C for 2.5 minutes
(run.21. The metallization ratios of the products were
compared.
r.00771 In this case, unlike runs 3 to 6, the
"'-.metallizationr atio of the second reduction product was
higher at run 1 of continued reduction at the furnace A
than at run 1 of second reduction in the furnace B. This
is believed to be because the metallization ratio at the
first reduction end point was a low 40% so sufficient
pores were not formed for enabling the reducing gas in
the furnace to diffuse into the first reduction product
at the second stage.
[00781 At run 7, the reduction time at the first stage
furnace A was extended by 20 minutes. In this case, when
the first reduction ended, the metallization ratio of the
first reduction product reached 90%. Therefore, in this
case, there was no need to go to the trouble of adding
facilities to perform the second reduction. Just the
first stage reduction was sufficient.
roo791 FIG. 2 shows the relationship between the
metallization ratio ( % ) of the second reduction product
after the end of the reduction at the second stage at the
furnace A or furnace B and the metallization ratio ( % ) of
the first reduction product after the end'of the first
reduction at the furnace A in the case of treating the
first reduction product obtained by the first reduction
in the furnace A by further (1) continued reduction for 5
minutes in the furnace A (runs 1, 3, 5, and 7, circle
marks in figure) and (2) reduction for 2.5 minutes in the
furnace B (runs 2, 4, and 6, square marks in figure).
[0080] From the above test results, it will be
understood that to improve the final metallization ratio,
it is necessary that the metallization ratio of the first
reduction product at the first stage end point has to be
a certain value or more. Further, reduced iron with a
metallization rate at the first stage end point reaching
90% can be used as is as a final product.
[008lJ . Example 2
Next, the inventors conducted a test for evaluating the
effects of the furnace atmosphere on the metallization
ratio in the second reduclng furnace. In this test as
well, they used two test-use reducing furnaces (furnace A
and furnace B). The furnace A was a test-use electric
furnace with a furnace gas composition of N2: 100% and a
furnace temperature held at 1250°C, while the furnace B
was a test-use electric furnace with an inside able to
controlled to a reducing atmosphere and with a furnace
temperature held at 1150°C. The tablets use3 were the same
as in Example 1.
[0082] In this test, at all runs, the tablets reacted
In the furnace A for 10 minutes at the first stage were
quickly moved to the inside of the furnace B. Inside the
furnace B, the second reduction reaction was performed.
At the respective runs, the CO concentrations ln the
furnace B were changed to 40%, 70%, 85%, and 90% and the
effects on the metallization ratios of the tablets were
evaluated. The different test conditions are shown in
Table 3. Among these, run 4 with a CO concentration in
the furnace B at the second stage of 40% was the same as
run 4 of Example 1.
[0083] Table 4 shows the test results. By comparing
runs 4a, 4b and run 4, it will be understood that if
raising the CO concentration in the furnace B, the
metallization ratio after the end of the second stage is
also improved. However, the metallization ratio of run 4c
where the CO concentration was further raised to 90%
became the same value as the run 4b where CO: 85%.
100841 FIG. 3 shows the relationship between the
metallization ratio ( % ) of the second reduction product
after the end of the second reduction at the runs 4, 4a,
4b, 4c and the CO concentration ( % ) in the second
reducing furnace (furnace B). From FIG. 3, it will be
understood that by increasing the CO concentration in the
second reducing furnace, the metallization ratio also
increase.s.,.,..ebvuetn if the CO concentration exceeds 85%
and a strong reducing atmosphere is set, there is almost
no effect on the metallization ratio.
[0085] Example 3
Two test-use reducing furnaces (furnace A and furnace B)
were prepared. The furnace A here was a rotary hearth
type reducing furnace with a furnace gas composition of
N2: 100% and a furnace temperature held at 1250°C, while
the furnace B was a ~12Ox1200mmH small sized test-use
shaft furnace wlth a furnace gas composition made (CO:
30%+N2:70%) and a furnace temperature held at i150°C. The
furnac? B had a furnace atmosphere of a reduclng
atmosphere stronger than the furnace A, but had a furnace
temperature 100°C lower.
[0086] Meltlng furnace dust (poor quality lron oxide
powder) and coal were mixed to form agglomerates. The
feed agglomerate material (by mass%, metal Fe: 1%, FeO:
38%, FezO3: 31%, C: 14%) was flrst reduced at the furnace
A (first stage) to produce a first red~ction~product.
This first reduction product was charged in the furnace B
where the first reduction product was treated for
finishing reduction (second reduction).
[0087] The finishing reduction conditions were as
follows :
Shaft furnace: inside diameter 3120xheight 1200 rnm
Furnace temperature: 1150°C
Furnace atmosphere: CO 30%+N2 70%
The test conditions are shown in Table 5.
COO881 At each run, first, as the first stage
reduction, the tablets were reacted for exactly a
predetermined time in the furnace A.
[0089] Next, at the second stage, part of the tablets
finished in reduction reaction at the first stage
continued to be treated by a reduction reaction in the
furnace A at the same furnace temperature and the same
atmosphere (runs 1, 3, and 5). The remaining tablets were
quickly &ved to the inside of the furnace B after the
end of the first reduction reaction and continued to be
treated to a reduct~on reactlon at the furnace B (runs 2,
4, and . 6 ) .
[00901 The atmosphere insxde the furnace B was held at
a (CO: 30%+N2:70%) strong reducing atmosphere. The
reduction reaction of the tablets could be expected to
proceed faster than inside the furnace A, so in the
furnace B, the furnace temperature was set lower or the
reaction time was set shorter so that the total reaction
time became shorter than the tablets next reduced in the
furnace A.
[0091] Table 6 shows the results of analysis of the
metallization ratio of tablets after the first stage and
after the end of the second reduction reaction shown in
Table 5 .,
[0092] First, take note of run 3 and run 4. Run 3
causes reaction at N2: 100% in a 1250°C furnace A for 15
minutes. As opposed to this, run 4 causes reaction at the
furnace A first for 10 minutes, then reaction at a
furnace temperature 1150°C in the strong reducing
atmosphere (CO: 30%) furnace B for 2.5 minutes for a
total 12.5 minutes of reaction.
[00931 At run 4, the total reaction time was made 2.5
minutes shorter than run 3 and the furnace temperature of
the second reducing furnace was 10O0C lower. Despite this,
the .fi..nal...me.tallization ratio' after the end of the second
reduction could be maintained at the same value as the
run 3. That is, this shows that by switching the inside
of the furnace to a strong reducing atmosphere in the
middle of the reduction reaction, the reaction speed of
the tablets is greatly improved.
[0094] As opposed to this, at run 5 and run 6, the
first reduction time was extended 2.5 minutes from the
run 3 and run 4 and similar tests were performed. In this
case, at run 6, where the total reaction time was
shortene'd and the furnace temperature of the second
reducing furnace was lowered to 1150°C, the metallization
ratio became higher.
[0036] , This is believed to be because the
metallization ratio at the first stage end point was
higher compared with run 3 and run 4, the porosity of the
tablets became greater, and, at the time of the second
reduction reaction, the atmospheric gas easily diffused
inside the pores of the tablets and thereby the reduction
reaction easily proceeded.
[0096] At run 1 and run 2, the first reduction time
was made 5 minutes. After that, the material continued to
be reduced as is in the furnace A for 5 minutes (run 1).
At the second stage, the strong reducing atmosphere
furnace B was used for reduction at 1l5O0C for 2.5 minutes
(run 2). The metallization ratios of the products were
compared.
COO971 In this case, unlike runs 3 to 6, the
metallization ratio of the second reduction product was
higher at the run 1 of continued reduction at the furnace
A than at the run 2 of the second reduction at the
furnace B. This is believed to be because the
metallization ratio at the first reduction end point was
a low 45% and at the second stage, sufficient pores
enabling the reducing gas in the furnace to diffuse into
the first reduction product were not formed.
[0098] At run 7, the reduction time at the first stage
furnace A was extended by 20 minutes. In this case, when
the first reduction ended, the metallization ratio of the
first reduction product reached 91%. Therefore, in this
case, there was no need to go to the trouble of adding
facilities for performing the second reduction. Just
reduction by the first stage rotary hearth type reducing
furnace was sufficient.
[0099] Example 4
Melting furnace dust (poor quality iron oxide powder) and
coal ,were mixed to form agglomerates. The feed
agglomerate material (by mass%, metal Fe: 1%, FeO: 38%,
Fe203: 31%, C: 14%) was reduced at a furnace temperature
1250°C rotary hearth type reducing furnace (first stage)
to produce a first reduction product. This first
reduction product was charged into a 430Ox3400mrnL small
sized rotary kiln where the first reduction product was
treated for finishing reduction (second reduction).
[OlOO] The finishing reduction conditions were as
follows :
Rotary kiln: inside diameter 300xlength 3400 mm
Speed of rotary kiln: 3.2 rpm
Amount of first reduction product charged: 120 kg/h
Standing time of first reduction product: 20 minutes
Burner fuel: LPG
Combustion air ratio: 0.7 (CO concentration in furnace:
20 ~01%)
The furnace temperature and results of finishing
reduction are shown in Table 7.
[OlOl] At run 5-1, the furnace temperature of the
rotary kiln (second stage) was made 1200°C for the
finishing reduction. As a result, the metallization ratio
was +7%, the ratio of particle size 28 mm reduced iron
was +24%, and the ratio of particle size < 3 mm reduced
iron was -18%. It was learned that in reduction at the
second stage rotary kiln, both the reduction reaction and
granulation proceed. Note that, along with continuation
of the second stage reduction reaction, a kiln ring was
formed inside the rotary kiln furnace, but its growth was
slow. Operation could be continued by periodically
removing it during operation. The effect was light.
[0102] At run 5-2, the furnace temperature of the
rotary kiln (second stage) was made 1250°C for the
finishing reduction, but a kiln ring was formed inside
the furnace and quickly grew closing the inside of the
kiln furnace, so continued operation became difficult.
[01031 At run 5-3, the furnace temperature of the
rotary kiln (second stage) was made llOO°C for the
finishing reduction, but it was learned that the
metallization ratio was -3%, the ratio of reduced iron of
a particle size of 8 mm or more was +9%, the ratio of
reduced iron of a particle size of less than 3 mm was
+I%, and both the reduction reaction and granulation
failed to proceed.
[0104] At run 5-4, the furnace temperature of the
rotary kiln (second stage) was made 1130°C lor the
finishing reduction. The metallization ratio was +5%, the
ratio of particle size28 mm reduced iron was +19%, and
the ratio of particle slze <3 mm reduced iron was -13%.
It was learned that in the reduction at the second stage
rotary kiln, both the reduction reaction and granulation
proceeded. Further, formation of a kiln ring inside the
rotary kiln furnace also could not be observed.
[0105] At run 5-5, the furnace temperature of the
rotary kiln (second stage) was made 1170°C for the
finishing reduction. The metallization ratio was +6%, the
ratio of particle size28 mm reduced iron was +22%, and
the ratio of particle size <3 mm reduced iron was -15%.
It was learned that in the reduction at the second stage
rotary kiln, both the reduction reaction and granulation
proceeded. Further, formation of a kiln ring inside the
rotary kiln furnace also could not be observed.
[QlQ6] Example 5
Melting furnace dust (poor quality iron oxide powder) and
coal were mixed to form agglomerates. The agglomerate
material (by mass%, metal Fe: 1%, FeO: 38%, Fe203: 31%, C:
14%) was reduced at a furnace temperature 1250°C rotary
hearth type reducing furnace (first stage), then suitably
classified to produce first reduction products of ratios
of particle sizes less than 3 mm of 29 mass%, 38 mass%,
65 mass%, and 90 mass%. These first reduction products
were charged into small size rotary kilns and treated for
finishing rgduction (second stage) . The furnace
temperature of the klln was made 1150°C.
[0107] The finishing reduction conditions were as
follows :
Rotary kiln: inside diameter 300xlength 3400 mm
Speed of rotary kiln: 3.2 rpm
Amount of first reduction product charged: 120 kg/h
Standing time of first reduction product: 20 minutes
Burner fuel: LPG
Combustion air ratio: 0.7 (CO concentration in furnace:
20 ~01%)
The test conditions and results are shown in Table 8 and
FIG. 5.
[OlOS] FIG. 5 shows the results of Table 8 with the
less than 3 mm ratio of the first reduction product
(mass%) on the abscissa and the amount of change (A%) of
the metallization ratio in the heating and reduction at
the rotary kiln (second reducing furnace) on the
ordinate. From FIG. 5, it will be understood that if the
less than 3 mm ratio of the first reduction product
exceeds 75 mass%, the reduction reaction inside the
rotary kiln furnace will no longer proceed.
[0109] Example 6
Melting furnace dust (poor quality iron oxide powder) and
coal were mixed to form agglomerates. The agglomerate
material (by mass%, metal Fe: 1%, FeO: 38%, Fe203: 31%, C:
14%) wa. ,s .,. .. r... e duced in a furnace temperature 1250°C rotary
hearth type reducing furnace (first stage), then
classified by a predetermined reference size. Only the
fine size part was charged into a small size rotary kiln
(second stage) and treated by finishing reduction (second
stage) .
[OllO] The finishing reduction conditions were as
follows :
Rotary kiln: inside diameter 300xlength 3400 mm
Speed of rotary kiln: 3.2 rpm
Furnace temperature of rotary kiln: 1150°C
Burner fuel: LPG
Combustion air ratio: 0.7 (CO concentration-in furnace:
20 vol%,)
The test conditions and results are shown in Table 9 and
FIG. 6.
[Olll] At run 7-1, the first reduction product was not
classified in advance. The entire amount was reduced in
the rotary kiln. In this case, the total amount of
processing at the first stage rotary hearth type reducing
furnace (loo%, 20.0t/h) was reduced by finishing
reduction at the rotary kiln. As a result, the difference
between the metallization ratio of the second reduction
product after treatment at the rotary kiln (at run 7-1,
this becomes the final product' reduced iron) and the
metallization rate of the first reduction product (below,
called the "A metallization ratio") was 7.9%.
[0112] At run 7-2, the first reduction product
discharged from the rotary hearth type reduciny furnace
was classified by a reference size of 16 mm. Only the
fine size part was treated in the rotary kiln for
finishing reduction. The coarse size part bypassed this.
Finally, the two were mixed to obtain the final product
reduced iron.
[0113] The mass ratio of the fine size part to the
total treated amount of the first reduction product was
63%. This shows that compared with treating the entire
amount, the specification of the second stage rotary kiln
type reducing furnace can be reduced to about six-tenths.
The final A metallization ratio became 7.6% for a rate of
rise of reduction rate no different from the A
metallization ratio 7.9% of run 7-1.
[0114] At run 7-3, the classification reference size
was made 12 mm. In the same way as run 7-2, the fine size
part was treated in the rotary kiln for finishing
reduction and then was mixed with the bypassed coarse
size.part to obtain the final product reduced iron. In
this case, the mass ratio of the fine size part with
respect to the total treated amount of the first
reduction product was 48%. It will be learned that the
specification of the rotary kiln can be reduced to about
half. The final A metallization ratio was also 7.3%.
Compared with run 7-1 and run 7-2, the same level of
finishing reduction could be maintained.
[0115] At run 7-4, the classification reference size
was lowered to 4 mm. At this time, the mas: ratio of the
fine size part became just 16% of the total treatment
amount of the first reduction product. Compared with
treatment of the entire amount, it is believed possible
to greatly reduce the scale of the rotary kiln.
[0116] However, in this case, the A metallization
ratio became 2.2%. Compared with run 7-1 where no advance
classification was performed, the final amount of rise of
the metallization ratio was extremely small. This is
believed because the classification reference size was a
small 4 mm, so the less than 3 mrn mass ratio of the fine
size part reached as high as 73.0% and progress in the
finishing reduction in the rotary kiln was obstructed.
[01171 FIG. 6 shows the correlation between the fine
size classification ratio (mass%) of the result of
classification of the first reduction product at the
different runs of the examples and the metallization
ratio ( % ) of the final product reduced iron. Note that,
the metallization ratio 81.4 ( % ) of the product reduced
iron when not classifying the material, but treating the
entire amount of the first reduction product by finishing
reduction in the rotary kiln type reducing furnace is
shown at the fine size classifying ratio 100%.
[01181 From FIG. 6, it will be learned that it is
sufficient to set the classification reference size so
that the mass percentage of the fine size part obtained
by classification to the entire amount of the first
reduction product becomes 50% or more.

[0121] Table 3
101221 Table 4
Run Metallization ratio ( % 1
After end of I After end of
flrst reduction second reducclon
4b 6 5 84
nc 84
I I I I I I I L I L , Y O
3 1250 1i0. o Furnace 4 IN^, 100% 11250 15.0 F~rnac6 A IN?, 100%
4 1250 /10.0 l~urnace A IN?, 100% 11150 12.5 /Furnace B CO, 30%
I01231 Table 5
Run Metallization ratio 1 % )
After end of / After end of
Run Flrst stage
5 1250 12.5
6 1250 12.5
7 1250 20.0
flrst stage second stage
1 145 ( 62
I 57
Second stage
Furnace
atmosphere
Nz, 100%
N2, 100%
Furnace temp.
(OC)
1250
1150
Furnace temp. Reduction
( OCi tlme (mln)
1 1250 5.0
2 1250 5.0
101241 Table 6
Furnace A
Furnace A
Furnace A
Furnace used
Furnace A
Furnace A
Reduction
tlme (mlni
5.0
2.5
N,, 100%
N,, 100%
N,, 100%
Furnace used
Furnace A
Furnace B
1250
1150
-
Furnace
atmosphere
NZr 100%
CO, 30%
hi 7 n 9
5.0
2.5
-
Furnace A
Furnace B
-
NZ, 70%
N2, 100%
CO, 30%
N2, 70% -

101271 Table 9
semireduced iron I
Classification )(mi No / 6
Run 7-1 7-2
Total treatment amount (t/h) 20 20
of semlreduced iron
P 73.5 73.5
7-3
20
----
73.5
reference size classification
Treatment after classification Total anount Fine size Coarse size
kilned part part
. . kilned bypassed
Kilning or bypass amount (t/h) 20 12.6 7.4
-100% -63% -37%
Metallization ratio after ( % ) 73.5 69.5 82.2
classification
<3 mm ratio (mass%) 11.5 18.3 0
A 7.9 10.9 - A metallization ratio
due to kilning
Metallization ratio after ( % ) 81.4 80.4 82.2
7-4
2 0
73.5
kilning or after bypass 1
Merging After merging
Metallization ratio ( % ) 81.4 81.1
of final product
A metallization ratio (A%) 7.9 7.6
(final productsemireduced
iron)
Fine sized
part
kilned
9.5
-48%
65.8
24.3
13.8
79.6
After merging
80.7
7.3
Coarse size
part,
bypassed
10.5
-52%
81.8
0 -
81.8
Fine sized
part
kilned
3.2
-16%
51.3
7 3
9.4
60.8
Coarse size
part
bypassed
16.8
-84%
78.5
0 -
78.5
After merging
75:7
2.2
W
LC
I
Industrial Applicability
101281 The present invention can be utilized in the
ironmaking industry. The iron oxide contained in the
ironmaking dust discharged from the ironmaking process
5 can be reduced to metal iron by the present invention and
thereby be reutilized as a material for ironmaking.
CLAIMS
Clalm 1.
A method of production of reduced iron comprising
mixing an iron oxide-containing material and a reducing
agent to form agglomerates and treating the agglomerate
material by two consecutive stages of reduction, the
method of production of reduced iron further comprising
(i) applying a rotary hearth type reducing
furnace to a first reduction then
(ii) applying a rotary kiln type or shaft type
reducing furnace to a second reduction,
in the second reduction, a concentration of CO
gas in the reducing atmosphere being 10 vol% to 85 ~01%.
Claim 2.
The method of production of reduced iron according
to claim 1, wherein a metallization ratio of a first
reduction product of the reduction product produced by
said first reduction is 65 mass% to 90 mass%.
Clalm 3.
The method of production of reduced iron according
to claim 1 or 2, wherein when applying a rotary kiln type
reducing furnace to said second reduction to treat said
first reduction product, an atmospheric temperature
inside said rotary kiln type reducing furnace is over
llOO°C to 1200°C.
Claim 4.
The method of production of reduced iron according
to.aray..,.aonfe claims 1 to 3, further comprising
classifying a first reduction product produced by said
first reduction so that the classified fine size part of
the reduction product contains the reduction product of a
size of less than 3 mm in 75 mass% or less, applying a
rotary kiln type reducing furnace to reduce said
classified fine size part of the reduction product by
said second reduction to obtain a second reduction
product, and mixing the second reduction product with the
classified coarse size part of the first reduction
product. - .
. "
Claim 5. -. .
' The method of production of reduced iron according
to any one of claims 1 to 4, wherein said iron oxidecontaining
material includes at least one of melting
furnace dust, electric furnace dust, rolling scale, and
sludge from the pickling or neutralization process.
* Claim 6."
A facility for production of reduced iron mixing an
10 iron oxide-containing material and a reducing agent to
form agglomerates and reducing the feed aggl'qmerate
material using two reducing furnaces,
said facility for production of reduced iron
provided with:
15 (a) a rotary hearth type reducing furnace as a
first reducing furnace for reducing said feed aggl-omerate
material and
(b) a rotary kiln type or shaft type reducing
furnace as a second reducing furnace further reducing a
2 0 first reduction product of the reduction product produced
at said first reducing furnace.
Claim 7.
The facility for production of reduced iron
according to claim 6, further comprising a classifying
2 5 facility for classifying said first reduction product and
comprising a facility for charging the fine size part of
the first reduction product at said classifying facility
into saicG,s.e,conrde ducing furnace comprising of a rotary
kiln type reducing furnace and mixing a secondreduction
30 product produced by-said second reducing furnace and the
.coarse part of the first reduction product 'obtained at
said classifying facility.