PRODUCING METHOD OF REDUCED IRON BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an efficient producing method of reduced iron with high operability and productivity without melting by using molded bodies or compacts in which an iron oxide-based fine raw material and a reducing material such as a coal are mixed.
Priority is claimed on Japanese Patent Application No. 2008-093344 and Japanese Patent Application No. 2008-306789, the contents of which are incorporated herein by reference.
Description of Related Art
A technique of producing reduced iron by blending and mixing a carbonaceous reducing material and moisture in a dust powder (fine raw material of iron) containing a large amount of iron oxide generated in iron and steelmaking process, molding the mixture into molded bodies or compacts in the form of a pellet or a briquette, drying the molded bodies or the compacts, and feeding and heating the molded bodies or the compacts in a reducing furnace has been known.
In addition, in recent years, means for effective utilization of fine ore which is difficult to use in a sintering process or in a blast furnace have been required because of concerns about depletion of resources, and a producing method of reduced iron using iron oxide of a fine ore as a main raw material also has been known.
As a method related to the conventional techniques, Japanese Unexamined
Patent Application, First Publication No. 2004-285399 discloses a method for
coalescence of metal iron, which is generated by heating molded bodies or compacts at a
high temperature in a reducing furnace, into grains while separating the metal iron from
slag. Japanese Unexamined Patent Application, First Publication No. 2004-285399 also
discloses a technique of reducing a concentration of sulfur contained in metal iron by
controlling slag basicity in a predetermined range.
Japanese Unexamined Patent Application, First Publication No. 2006-283136 discloses a method of adding a reforming material containing SiO2 to molded bodies or compacts in order to reduce accumulation or deposition on a hearth during the reduction of the molded bodies or the compacts in a reducing furnace.
In addition, Japanese Unexamined Patent Application, First Publication No. S55-122832 discloses a method of producing high-strength hardened pellets by forming silicate or hydrosilicate bond upon hydrothermal hardening to be used in a kiln and a method of producing metallized pellets from the produced hardened pellets by using the kiln.
However, in the method of separating the metal iron and the slag components by carburization and melting of the metal iron, which is described in the above Japanese Unexamined Patent Application, First Publication No. 2004-285399, it is necessary to heat the reducing furnace to a temperature greater than or equal to a melting point of the metal iron which is varied with the extent of carburization. Under such a high temperature, there are problems in that refractory wear in the furnace is significant, specific energy consumption necessary for the heating increases, and productivity is reduced.
In addition, there is also a problem in that the sulfur derived from a carbonaceous material is contained in the metal iron to cause the metal iron to be molten.
In order to reduce the concentration of the sulfur contained in the metal iron, a reduction
potential in the reducing furnace, that is, CO/(CO+CO2) is required to be highly
maintained, and it is necessary that an additional amount of the carbonaceous material to
be fed into the reducing furnace be more than or equal to the amount required for the
reduction, carburization, and melting. Since the excessive feed of the carbonaceous
material significantly reduces the melting point of the metal iron, there is a high
possibility of mutual melting of the granulated metal iron and there is a concern that the
metal iron flows on the hearth. In this case, a yield of the granulated metal iron is
reduced and operability is significantly reduced.
If reduced iron can be produced without melting of the metal iron, the sulfur
derived from the carbonaceous material can be prevented from being molten in the metal
iron. Accordingly, it is unnecessary to highly maintain the reduction potential of the
reducing furnace and cost for the carbonaceous material as a raw material can be saved.
Simultaneously, the flowing of the metal iron on the hearth, associated with the mutual
melting of the granulated metal iron, can be prevented and there is no concern that
operability is damaged. However, at present, these cannot be achieved by the method
described in the above Japanese Unexamined Patent Application, First Publication No.
2004-285399.
In the method described in the above Japanese Unexamined Patent Application, First Publication No. 2006-283136, a molten compound stabilized by iron oxide and SiO2 is easily generated. Accordingly, the reduction of iron oxide by a reducing material takes a considerable amount of time and thus there is a problem in that productivity is reduced.
Moreover, in the method of producing the metallized pellets by using the kiln, which is described in the above Japanese Unexamined Patent Application, First
Publication No. S55-122832, unlike a method of producing reduced iron using a
moving-hearth reducing furnace, high-strength molded bodies or compacts are required
to prevent reduced iron from being pulverized. However, it is hard that all the molded
bodies or the compacts have such an ideal strength, and actually, irregularity in strength
cannot be completely avoided practically. Accordingly, it is inevitable that partial
pulverization occurs before the reduction. As a result of the pulverization, the slag
components are easily accumulated in the furnace. The accumulated materials, as
deposited materials referred to as so-called kiln rings, grow along an inner wall of the
furnace and there is concern that the grown materials interfere with discharge of the
metallized pellets. In this case, productivity of the metallized pellets is significantly
reduced.
SUMMARY OF THE INVENTION
The invention is contrived in view of the above-described situation and an object of the invention is to provide an efficient producing method of reduced iron containing a greater concentration of metal iron with high operability and productivity, without damaging reducibility of iron oxide as a main raw material when the reduced iron formed of the metal iron and slag components is produced by heating molded bodies or compacts, in which an iron oxide-based main raw material and a carbonaceous reducing material are mixed, in a reducing furnace.
The invention employs the following means to solve the above-described problems and achieve the object.
(1) A method for producing reduced iron including a mixture of metal iron and slag
components by drying a molded body or a compact obtained by molding a raw material which contains iron oxide as a main component and to which a carbonaceous reducing material for reduction is added, and then by feeding the molded body or the compact into
a moving-helth reducing to reduce the molded body or the compact, the method including: adding at least one of an oxide-based reforming material containing CaO as a main component and an oxide-based reforming material containing MgO as a main component to the raw material when molding the molded body or the compact; setting the total content of CaO, SiO2, MgO, and Al2O3 in the molded body or the compact to be in the range of 8 to 20 mass% with respect to the total mass of the molded body or the compact from which the carbonaceous reducing material is excluded; and setting slag basicity of (CaO%+MgO%)/Si02% which is calculated using the mass% of the slag components in the molded body or the compact to be in the range of 0.9 to 3.0. (2) In the above-described method for producing the reduced iron (1), the content of Al2O3 may be in the range of 5 to 19 mass% with respect to the total content of CaO, SiO2, MgO, and Al2O3 in the molded bodies or the compacts during molding of the molded bodies or the compacts.
(3) In the above-described method for producing the reduced iron (1) or (2), the oxide-based reforming material containing CaO as a main component or the oxide-based reforming material containing MgO as a main component added to the raw material during molding of the molded bodies or the compacts may have an 80% minus-sieve particle diameter of less than or equal to 2 mm.
According to the method for producing reduced iron (1), without forming an iron oxide-based compound which reduces reducibility by the reaction between iron oxide and slag components upon reducing molded bodies or compacts, it is possible to stabilize a high content of a metal iron in the molded bodies or the compacts attained by heating for a predetermined time, reduce cost for raw materials and fuel fed in a reducing furnace, and highly efficiently produce reduced iron with high operability and productivity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing a relationship between (CaO+MgO)/SiO2 in slag components and a metallization ratio.
FIG. 2 is an optical microscope photograph of a cross-section of reduced iron obtained when slag basicity is controlled so as to satisfy conditions for a producing method of a reduced iron according to the invention.
FIG 3 is an optical microscope photograph of a cross-section of reduced iron obtained when slag basicity is controlled so as not to satisfy the conditions for the reduced iron producing method according to the invention.
FIG. 4 is a detailed electron microscope photograph of a portion of the cross-section of the reduced iron shown in FIG. 3.
FIG. 5 is a diagram showing a relationship between an 80% minus-sieve particle diameter of MgO and a metallization ratio of reduced iron.
DETAILED DESCRIPTION OF THE INVENTION Hereinafter, preferred embodiments of the invention will be described in detail with reference to the accompanying drawings.
The invention is a technique built on the basis of results of various inspections focused on an influence of contained slag components on reducibility of iron oxide as a main raw material in view of the above-described problems. By this technique, a method of efficiently producing reduced iron containing a greater concentration of metal iron with high operability and productivity, without damaging reducibility of the iron oxide as a main raw material when the reduced iron formed of the metal iron and slag components is produced by heating in a reducing furnace molded bodies or compacts in
which an iron oxide-based main raw material and a carbonaceous reducing material are
mixed is provided.
Firstly, hereinafter, the contents and results of inspections performed by the inventors of this application will be described in detail before describing the producing method of the reduced iron according to the invention.
Iron oxide as a main raw material for molded bodies or compacts to be fed into a reducing furnace in order to produce reduced iron according to the invention is dusts (for example, converter dust, electric furnace dust, melting furnace dust, blast furnace dust, and the like) or fine ores reducing an aeration property in a sintering process or in a blast furnace and interfering with production. The former dusts are generated in processes of melting, reduction, and refining, and contain oxides such as CaO, SiO2, Al2O3, MgO, and the like as main components of refining slag. The latter fine ores mainly contain oxides such as SiO2, Al2O3, and the like as gangue components.
It is preferable that these iron oxides be utilized as an iron raw material from the viewpoint of resource recycling. As a method thereof, a method of mixing a predetermined amount of carbon-based reducing material such as coal to form molded bodies or compacts such as pellets or briquettes and holding the molded bodies or compacts for a given length of time in a heating furnace to produce reduced iron has been widely known.
During the formation of the molded bodies or the compacts, a predetermined amount of binder for maintaining a strength is added. A starch powder such as cornstarch is representative as the binder of this type.
In pelletizing and molding, by adding a predetermined amount of moisture to the raw material and drying the raw material, the composition of the raw material in the obtained molded bodies or compacts becomes uniform and bonding forces between the
fine particles are increased, and thus the strength of the molded bodies or the compacts
can be maintained. As a result, the molded bodies or the compacts at the time of
transporting between the processes and feeding into the furnace can be prevented from
being mechanically broken. In addition, explosive pulverization associated with sudden
evaporation of the moisture in the molded bodies or the compacts upon feeding the
molded bodies or the compacts into the furnace also can be prevented.
In producing the reduced iron by heating the molded bodies or the compacts in the reducing furnace, a phenomenon in which iron oxide and slag components are reacted to form a phase of a low melting point and some of the molded bodies or the compacts are molten occurs depending on a heating temperature and the slag composition. By this phenomenon, a resultant phenomenon in which a reduction mechanism of the iron oxide changes from an indirect reaction between the solid oxides and a reducing gas generated from the carbon-based reducing material to a direct reaction between a molten oxide phase and the solid carbon-based reducing material and a reduction rate increases was found. An influence of the slag components during this time on the reduction of the iron oxide was also found.
Referring to the above findings, the invention is applied to the reduction of the iron oxide in the molded bodies or the compacts containing the carbonaceous reducing material therein. An example of the process is a producing method of reduced iron using a rotary hearth in which molded bodies or compacts are fed onto a rotating hearth of a heating furnace arranged in a donut shape and heated for a predetermined time to be discharged. In this process, when an amount of generated molten slag is large, accumulated materials on the hearth are significantly grown, interfere with the heating furnace, and thus become an obstacle to the rotation. In this manner, problems in facilities can occur. Accordingly, it is necessary to improve machinability of the
accumulated materials generated on the hearth. In the above-described Japanese
Unexamined Patent Application, First Publication No. 2006-283136, a method of adding
an oxide-based reforming material containing SiO2 to control an amount of liquid phase
of slag in order to improve the machinability of the accumulated materials on the hearth
which are an obstacle to production is disclosed. However, since a compound having a
low melting point, referred to as Fayalite (=2FeO • SiO2), is generated by the reaction
between the iron oxide and SiO2 and reducibility of FeO is reduced, it is difficult to
produce reduced iron with a high metallization ratio.
The inventors have considered the reduction of the iron oxide in the molded bodies or the compacts formed of the iron oxide raw material such as dust and the carbonaceous reducing material from the thermodynamic viewpoint, and as a result of repeated experiments, found that reducibility of the iron oxide in the molded bodies or the compacts has a relationship with activity of FeO in the molten slag generated by heating the molded bodies or the compacts. In other words, the activity of FeO in the molten slag is determined by the interaction between the other components and the concentration of contained FeO, and when, for example, SiO2 and the like are present, FeO becomes more stable and the reducibility is thereby reduced. When basic components such as CaO and MgO are present, FeO becomes more active and the reducibility is thereby improved.
That is, even when the concentration of FeO in the molten slag is the same, the reducibility of FeO is improved as the activity of FeO is high in the molten slag. The inventors have focused on the slag amount and the slag composition in order to efficiently reduce the iron oxide in the molded bodies or the compacts, and as a result of repeated examinations, they found a method of increasing the reducibility of the iron oxide in the molded bodies or the compacts. Regardless of the processes, this idea can
be applied in common to reduce materials formed of iron oxide such as iron ore or dust
and slag components.
Hereinafter, conditions of the invention will be described in detail.
As described above, a reduction experiment was performed under conditions for efficiently reducing FeO, and it was found that slag basicity, that is, (CaO+MgO)/SiO2 has an influence on the reduction of FeO on the basis of the result of the experiment. After converter dust, coal, and CaO powder and MgO powder having a particle diameter less than or equal to 2 mm were blended and kneaded in predetermined amounts, tablets having the outer diameter of 30 mm and the height of 17 mm were molded and held for 15 minutes in the furnace which was controlled under N2 atmosphere at 1250°C, and then the tablets were taken out to be provided for chemical analysis. In this case, the total mass of CaO, SiO2, MgO, and Al2O3 was in the range of 8 to 20% in terms of mass% based on the total mass of the molded bodies or the compacts from which the carbonaceous reducing material was excluded.
A relationship between a metallization ratio (= M.Fe%/T.Fe%) of the obtained reduced iron and (CaO%+MgO%)/SiO2% is shown in FIG. 1. As shown in FIG. 1, the metallization ratio of the reduced iron has a strong relationship with the slag basicity. The metallization ratio of the reduced iron became maximum when the slag basicity was approximately 1.4 to 1.7. Under this basicity condition (slag basicity of approximately 1.4 to 1.7), the metallization ratio (maximum value) of the reduced iron was approximately 95%.
FIGS. 2 and 3 show optical microscope photographs of typical cross-sections of the obtained reduced iron. White portions in FIGS. 2 and 3 are the metal iron and gray portions are slag. As shown in FIG. 2, when the slag basicity was controlled to be 1.28, the metallization ratio of the reduced iron was 95%. A structure of the reduced iron is
constituted by the metal iron generated in a network form and the slag present in the gaps of the network of the metal iron. As shown in FIG. 3, when the slag basicity was controlled to be 0.7, the metallization ratio of the reduced iron was 78%. A structure of the reduced iron is constituted by the metal iron scattered in grains and the slag phase largely remaining around the metal iron. The result obtained by high resolution observation of a cross-section of the reduced iron with an electron microscope is shown in FIG. 4. In addition, the result obtained by analyzing the structure with an energy dispersive X-ray spectrometer (EDX) is shown in FIG 4. It was found that Fayalite (2FeO • SiO2) is generated in the molten slag and remains together with FeO.
By arranging these results, it was found that the metallization ratio after reduction is greater than or equal to 85% by controlling (CaO+MgO)/Si02 in the range of 0.9 to 3.0. It is preferable that the metallization ratio of the obtained reduced iron get greater. This is because a portion of the residual iron oxide reduces a melt efficiency by an endothermic reaction associated with the reduction and the rest of the residual iron oxide as slag reduces a yield of molten iron in manufacturing the molten iron by melting the reduced iron. Accordingly, a condition for producing molten iron without reducing the melt efficiency is that the metallization ratio of the reduced iron is greater than or equal to 85%, that is, (CaO+MgO)/SiO2 is in the range of 0.9 to 3.0.
The condition is determined as follows. When (CaO+MgO)/SiO2 is less than or equal to 0.9, FeO becomes more stable by the influence of SiO2 as described above and thus reducibility is reduced. When (CaO+MgO)/SiO2 is greater than 3.0, CaO and MgO singly increase a melting point of the slag components and thus the amount of molten slag is reduced. Accordingly, it is thought that the reaction between the molten iron oxide and the solid reducing material, that is, the advantage of smelting reduction is damaged. Particularly, when (CaO+MgO)/SiO2 is controlled in the range of 1.4 to 2.2,
reduced iron in which the metallization ratio is greater than 90% can be obtained and a
very high melt efficiency can be expected.
In order to take and control the above-described interaction between the iron oxide and the slag, a predetermined amount of slag is required in the molded bodies or the compacts. Accordingly, the total mass of CaO, SiO2, MgO, and Al2O3 which are main components of the slag is controlled in the range of 8 to 20% in terms of mass% based on the total mass of the molded bodies or the compacts from which the carbonaceous reducing material is excluded. When the slag amount is less than 7%, the amount of slag molten by the reaction with FeO is small and thus the advantage thereof is reduced. When the slag amount is more than 20%, the amount of iron in the reduced iron is reduced and thus excess energy which is consumed for melting of slag is required in manufacturing molten iron by melting the reduced iron.
It has been known that Al2O3 as one of the slag components easily forms spinel (MgO • Al2O3), which is a hard mineral phase at a high melting point, with MgO-containing oxide. Since MgO in the slag is reduced by the formed spinel and thus the melting point of the slag is increased, an amount of the molten slag is reduced and a reduction efficiency of FeO is reduced. Accordingly, it is preferable that the content of Al2O3 be controlled in the range of 5 to 19 mass% with respect to the total content of CaO, SiO2, MgO, and A12O3.
Since better effects are obtained, it is preferable that, for example, during molding of the raw material into the molded bodies or the compacts such as briquettes or pellets, quicklime as a CaO source, light burnt magnesite or dolomite as a MgO source be blended and mixed with the iron oxide-based raw material, the carbonaceous reducing material, and if required, the binder in predetermined amounts and the mixture be uniformly dispersed in the molded bodies or the compacts to control the slag composition.
Chemical analysis about the iron ore as a main raw material for the molded bodies or the
compacts, the iron oxide-based main raw material such as dust and the carbonaceous
reducing material is performed in advance before molding to determine additional
amounts of CaO and MgO on the basis of the result of the analysis.
It is preferable that these additional materials have a smaller particle diameter from the viewpoint of uniformity and efficiency of the reaction at the time of heating, and better effects are obtained when they are added in the form of powders than in the form of clumps. Specifically, it is preferable that an 80% minus-sieve particle diameter be less than or equal to 2 mm. It is more preferable that the 80% minus-sieve particle diameter be less than or equal to 1.5 mm. The 80% minus-sieve particle diameter is a particle diameter when the mass of a powder passing through a sieve is 80% of the total mass upon sieving the powder. Herein, converter dust, coal, and MgO having a different particle diameter were blended and kneaded in predetermined amounts to mold them into tablets having the outer diameter of 30 mm and the height of 17 mm. The tablets were held for 15 minutes in a furnace which was controlled under N2 atmosphere at 1250°C, and then taken out to be provided for chemical analysis. FIG. 5 shows a relationship between a metallization ratio of reduced iron and an 80% minus-sieve particle diameter of MgO when (CaO+MgO)/SiO2 is controlled in the range of 1.56 to 1.58. As shown in FIG. 5, when the 80% minus-sieve particle diameter of MgO is less than or equal to 2 mm, the metallization ratio of the reduced iron is high.
As a method of providing the 80% minus-sieve particle diameter less than or equal to 2 mm, for example, light burnt magnesite generally having a large particle diameter greater than or equal to 2 mm is pulverized by a rod mill or a ball mill. However, the pulverization method for the additional materials of the invention is not limited to this method.
In addition, it is preferable that an operation temperature of the reducing furnace
be controlled at a melting temperature of the carburized metal iron required for the
separation into the molten slag and the metal iron generated by reduction, for example at
1400°C, or less. It is more preferable that the reduction temperature be 1385°C or less
and it is most preferable that the reduction temperature be 1350°C or less. When the
metal iron is molten, an amount of the metal iron entrapped in the accumulated materials
at the hearth increases and thus the accumulated materials form rock beds. Accordingly,
machinability of the accumulated materials is significantly reduced. As a result,
operability and productivity of the reducing furnace is significantly reduced.
In the above description, the control of the slag basicity by adding both of CaO and MgO has been described. However, the invention is not limited only to the above example. The slag basicity may be controlled by adding only one of CaO and MgO.
EXAMPLE
Next, examples of the invention will be described. Conditions for the examples are one example of the conditions employed to confirm the feasibility and advantages of the invention and the invention is not limited only to the one example of the conditions. The invention can employ various conditions as long as the object of the invention can be achieved without departing from the gist of the invention.
Coal was blended in a raw material in which various dusts including various slag components were blended, so that oxygen contained in iron oxide in the dusts and carbon contained in the coal had the same amount in terms of molar concentration (mol%0/mol%C=l). An 80% minus-sieve particle diameter of the blended raw materials was set to 120 µm. An additional amount of quicklime (CaO) or light burnt magnesite (MgO) and a particle diameter were changed for mixing and molding into briquettes having different slag amounts and different compositions was performed.
After dried, the briquettes were fed onto a rotary hearth having a diameter of 20 m and an
operation was performed. A temperature in the furnace was controlled in the range of
1000 to 1350°C by a LNG burner and the briquettes were held for 15 minutes in the
furnace. Table 1 shows values measured by chemical analysis of the compositions of
the slag in the fed briquettes and metallization ratios of obtained reduced iron. As
shown in Table 1, it is found that reduced iron having a metallization ratio greater than or
equal to 85% is obtained under the conditions of the examples.
In comparative examples not satisfying the conditions of the invention, reducibility of iron oxide is reduced, and thus a metallization ratio of obtained reduced iron is less than 85% and this result is unsatisfactory.
TABLE 1
[Tablel]
(Table Removed)
As above, the preferred embodiments of the invention have been described with reference to the accompanying drawings, but the invention is not limited only to the examples. It is obvious to those skilled in the art that various changes and
modifications may be made in a category described in claims, without departing from the
technical scope of the invention.
Industrial Applicability
A efficient producing method of reduced iron containing a greater concentration of metal iron with high operability and productivity, without damaging reducibility of iron oxide as a main raw material when the reduced iron formed of the metal iron and slag components is produced by heating molded bodies or compacts, in which an iron oxide-based main raw material and a carbonaceous reducing material are mixed, in a reducing furnace can be provided.

CLAIMS
1. (Currently Amended) A method for producing a reduced iron including a mixture of
metal iron and slag components by drying a molded body or the compact obtained by
molding a raw material which contains an iron oxide as a main component and to which
a carbonaceous reducing material for reduction is added, and then by feeding the molded
body or the compact into a moving-hearth reducing furnace to reduce
the molded body or the compact, the method comprising:
adding at least one of an oxide-based reforming material containing CaO as a main component and an oxide-based reforming material containing MgO as a main component to the raw material when molding the molded body or the compact;
setting the total content of CaO, SiO2, MgO, and Al2O3 in the molded body or the compact to be in the range of 8 to 20 mass% with respect to the total mass of the molded body or the compact from which the carbonaceous reducing material is excluded; and
setting a slag basicity of (CaO%+MgO%)/SiO2% which is calculated using the mass% of the slag components in the molded body or the compact to be in the range of 0.9 to 3.0.
2. (Original) The method for producing a reduced iron according to claim 1,
wherein the content of Al2O3 is in the range of 5 to 19 mass% with respect to the total content of CaO, SiO2, MgO, and Al2O3 in the molded body or the compact during molding of the molded body or the compact.
3. (Original) The method for producing a reduced iron according to claim 1 or 2,
wherein the oxide-based reforming material containing CaO as a main component or the oxide-based reforming material containing MgO as a main component added to the raw material during molding of the molded body or the compact has an 80% minus-sieve particle diameter of less than or equal to 2 mm.