Patent Application
20250179594
The present invention relates to a method for producing molten pig iron in the steel industry.
Reducing carbon dioxide emissions is required across all industries, as mandated by the SDGs and the Paris Agreement, for example. Among all manufacturing industries, the steel industry emitting large amounts of CO2 is particularly required to reduce CO2 emissions. In the steel industry, the molten pig iron production process accounts for 60% or more of the CO2 emissions of all processes. The molten pig iron production process involves reducing and melting iron ores using a carbon source to produce molten pig iron. The reason that the molten pig iron production process involves very high CO2 emissions is considered to be the use of coke or coal for the reduction and melting. There has been demand for the development of a molten pig iron production process that does not discharge CO2 to the outside of the system.
An iron-making process intended for net-zero carbon emissions has been devised.
Non Patent Literature 1 describes a review of the technological prospects for achieving the long-term goal of carbon dioxide reduction in the steel industry. For example, Non Patent Literature 1 introduces a method (CCS: Caron dioxide Capture and Storage) of separating CO2 from an exhaust gas generated through a reduction reaction, and storing CO2 in an isolated manner so as to reduce CO2 emissions to the outside. Another known method is a technology (CCU: Carbon dioxide Capture and Utilization) for separating CO2 from an exhaust gas to reuse it. The technology involves synthesizing CH4 using CO2 in an exhaust gas and blowing the synthesized CH4 into a blast furnace through its tuyere so as to use the synthesized CH4 for a reduction reaction again.
However, the existing technologies have the following problems. That is, although applying the CCS method can contribute to the reduction of CO2 emissions, the process would consume lots of energy. The method also has problems in that the C source should be moved to the outside of the system and the storage capacity is limited. Meanwhile, when CH4 is synthesized from CO2 to blow the synthesized CH4 into a blast furnace through its tuyere, a line for blowing CH4 into the tuyere would be needed, which is problematic.
The present invention has been made in view of the above circumstances and aims to provide a method for producing molten pig iron by circulating a C source through a process and feeding the recovered C source into a shaft furnace through its throat as a carbon-material-containing agglomerated ore to produce molten pig iron.
A method for producing molten pig iron according to the present invention that advantageously solves the above problems includes a first step of producing a carbon-material-containing agglomerated ore from an iron-containing raw material and a carbon-containing raw material; a second step of blowing an oxygen-containing gas into the carbon-material-containing agglomerated ore to reduce and melt the carbon-material-containing agglomerated ore, thereby producing molten pig iron; and a third step of bringing a carbon-containing gas containing carbon monoxide and carbon dioxide produced as a by-product of the reduction into contact with a porous material, thereby recovering carbon, in which in the first step, the carbon recovered in the third step is used for a part of or an entirety of the carbon-containing raw material.
The method for producing molten pig iron according to the present invention may have more preferable solutions as follows.
According to the present invention, molten pig iron can be produced by charging a carbon-material-containing agglomerated ore using the recovered carbon into a shaft furnace through its throat. Thus, the recovered carbon can be circulated through a process without the need to modify the shaft furnace.
Hereinafter, a method for producing molten pig iron according to the present embodiment will be described with reference to the drawings. The following embodiment only illustrates examples of an apparatus and a method for embodying the technical idea of the present invention. Thus, the configuration of the present invention is not limited thereto. That is, the technical idea of the present invention can be modified in various ways within the technical scope recited in the claims.
In a first step, an iron-containing raw material 4 and a carbon-containing raw material 6 are mixed to produce the carbon-material-containing agglomerated ore 26. The iron-containing raw material 4 is composed mainly of crushed iron ores, and may also contain dust generated in a steel mill, for example. In a second step, the obtained carbon-material-containing agglomerated ore 26 is charged into the blast furnace 32, and a blow gas 34 is blown into the furnace to allow a reduction reaction to proceed and thus produce the molten pig iron 36. In the third step, an exhaust gas 38, which has been produced as a by-product of the reduction reaction in the second step, is recovered, and the carbon monoxide contained in the exhaust gas 38 is brought into contact with a porous material so that a process of precipitating solid-state carbon and thus recovering it is performed. The exhaust gas 38 treated herein preferably contains an exhaust gas 40 produced as a by-product of a refining process of molten iron.
A part of or all of the carbon-containing raw material 6 used in the first step is the solid-state carbon recovered in the third step. If the carbon content of the recovered solid-state carbon alone is insufficient, a carbon source such as biomass may be used to supplement the deficiency. In such a case, the carbon-containing raw material 6 contains a carbon source such as biomass. Further, the particle size of the carbon-containing raw material 6 is preferably 100 μm or less. If the iron-containing raw material 4 used in the first step is crushed iron ores and contains a large amount of crystal water, it is preferable to apply a heat treatment step of subjecting the iron-containing raw material 4 to heat treatment in the temperature range of 300° C. to 1000° C. before the first step. Hereinafter, each step will be described in detail.
The first step is a step of mixing an iron-containing raw material and a carbon-containing raw material to produce a carbon-material-containing agglomerated ore. In the example shown in
In the first step, if the carbon content of the obtained carbon-material-containing agglomerated ore 26 is less than a target carbon mass percent, it is preferable to include biomass, for example, in the carbon-containing raw material 6 to achieve the target carbon mass percent. In addition, the carbon-material-containing agglomerated ore 26 is preferably obtained by cold forming because it is necessary to avoid combustion and gasification of carbon. Examples of the cold forming method include a method of adding a cement-based solidification agent, for example, and then granulating the mixture with a pelletizer or a drum mixer, and a method of performing compression forming using a briquette machine, for example. In addition, it is preferable to set the carbon mass percent of each particle of the carbon-material-containing agglomerated ore to 15 mass % or less to maintain the strength after a reduction process. Herein, if the carbon mass percent of each particle of the carbon-material-containing agglomerated ore exceeds 15 mass %, the crushing strength of the agglomerated material as measured with an autograph (1 mm/min) will be less than the threshold (2.5 MPa), which is not preferable.
The second step is a process of producing molten pig iron by reducing and melting the iron-containing raw material 4 contained in the carbon-material-containing agglomerated ore 26, which has been produced in the first step, using a countercurrent reduction layer. In the example shown in
The third step is a step of precipitating solid-state carbon from an exhaust gas or the like, which has been produced as a by-product of the reduction reaction in the second step, for recovery purposes. The exhaust gas 38 produced as a by-product of the reduction reaction and the exhaust gas 40 produced as a by-product of a refining step of molten iron contain carbon monoxide, carbon dioxide, hydrogen, and water. In the method for producing molten pig iron according to the present embodiment, it is acceptable as long as each of the exhaust gases 38 and 40 contains at least carbon monoxide and carbon dioxide. In the third step of the present embodiment, as shown in
Each of the exhaust gases 38 and 40 used in the present embodiment contains carbon monoxide, carbon dioxide, hydrogen, and water. The exhaust gases 38 and 40 are caused to fill the gas reforming furnace 110. Next, a hydrogen gas supplied from a hydrogen supply unit is added to the gas reforming furnace 110 to form a mixed gas. An internal space of the gas reforming furnace 110 filled with the mixed gas is then heated to 800 to 1200° C. When the internal space of the gas reforming furnace is heated to a temperature in such a range, carbon dioxide in the mixed gas that is present within the gas reforming furnace undergoes a water-gas shift reaction with hydrogen in accordance with the following chemical equation (I), thereby generating carbon monoxide and water.
A water-gas shift reaction between carbon dioxide and hydrogen is reversible. In a region where the reaction temperature is above 500° C., the chemical equilibrium of the water gas shift reaction shifts in the direction in which carbon monoxide is generated. Thus, in the method for producing molten pig iron of the present embodiment, setting the temperature in the gas reforming furnace 110 to 800 to 1200° C. can efficiently convert carbon dioxide into carbon monoxide, and thus generate carbon monoxide as a raw material of solid-state carbon. That is, in the gas reforming step, setting the temperature in the gas reforming furnace 110 to 800 to 1200° C. can efficiently reform the mixed gas.
The water-gas shift reaction that proceeds in the gas reforming furnace 110 is a reaction between carbon dioxide contained in the mixed gas, hydrogen contained in the mixed gas, and hydrogen in the hydrogen gas supplied from the hydrogen supply unit. Herein, it is preferable to supply hydrogen from the hydrogen supply unit so that the hydrogen concentration in the mixed gas (with reference to the gas composition excluding a nitrogen gas and an inert gas) becomes 58 volume % or more. If the hydrogen concentration in the mixed gas is 58 volume % or more, it is possible to increase the proportion of carbon monoxide in a water-gas shift reaction, and also recover solid-state carbon in the following carbon separation step, which is preferable. A hydrogen concentration of 58 volume % or more in the mixed gas has been determined by taking into consideration the conditions under which a carbon monoxide gas would increase through a water-gas shift reaction in the temperature range of 800 to 1200° C. The conditions under which a carbon monoxide gas would increase can be determined based on the relationship between the temperature of the water-gas shift reaction and an equilibrium constant. Note that the upper limit of the hydrogen concentration in the mixed gas may be less than 100 volume %, and the upper limit of the hydrogen concentration in the mixed gas may be determined in accordance with the carbon dioxide concentration.
As the hydrogen gas supplied from the hydrogen supply unit disposed outside of the gas reforming furnace 110, a hydrogen gas derived from renewable energy is preferably used. This can further reduce carbon dioxide emissions, which is preferable. Note that the amount of the hydrogen gas supplied from the hydrogen supply unit can be set by taking into consideration the amount of hydrogen contained in the exhaust gases 38 and 40.
<Step of Removing Water from Carbon-Containing Gas>
The reformed gas after the water-gas shift reaction of the mixed gas is supplied to the water removal device 120. The reformed gas contains water (water vapor) generated through the water-gas shift reaction. The water contained in the reformed gas is removed while passing through the water removal device 120. The removal of the water herein can be effected by a method of passing the reformed gas containing the water through a layer filled with an adsorbent in the water removal device 120, or a method of passing the reformed gas through a separating film, for example. The reformed gas preferably has the water removed therefrom so as to satisfy the relationship of the following expression (1). By removing water from the reformed gas so as to satisfy the relationship of the expression (1), carbon recovery efficiency in the carbon recovery step is increased. In the expression (1) below, [H2O] represents the water concentration (volume %) in the reformed gas, and [H2] represents the hydrogen concentration (volume %) in the reformed gas.
An adsorbent or a separating film can be used to remove the water from the reformed gas, as described above. As the adsorbent, oxide, such as silica, zeolite, or alumina; calcium chloride; magnesium sulfate; and so on can be used. Examples of the separating film for water in the gas include a carbon film, a resin film, and an inorganic film. Such films have pores with a diameter on the order of subnanometers. The water in the gas is recovered as liquid water at room temperature by the water removal device 120.
The dehumidified gas with the water removed therefrom becomes a raw material of solid-state carbon. The solid-state carbon is separated from the carbon monoxide in the carbonization apparatus 100 by a bimolecular decomposition reaction (II) that occurs when two carbon monoxide molecules are decomposed as represented by the following chemical equation (II), or by a monomolecular decomposition reaction (III) that occurs when one carbon monoxide molecule reacts with hydrogen, as represented by the following chemical equation (III).
The carbon monoxide contained in the dehumidified gas preferably satisfies CO/(CO+CO2) of 0.5 or more, more preferably 0.7 or more. Thus, solid-state carbon can be efficiently recovered from the carbon monoxide.
The dehumidified gas is supplied to a carbon separating unit in the carbonization apparatus 100, which includes a porous material, at a predetermined supply rate.
<Step of Bringing Carbon-Containing Gas into Contact with Porous Material to Separate Solid-State Carbon>
A method for recovering carbon from the dehumidified gas includes a step of bringing the dehumidified gas into contact with the porous material to separate solid-state carbon. Bringing the dehumidified gas into contact with the porous material and separating carbon from the dehumidified gas are performed within the carbon separating unit in the carbonization apparatus 100. A filled layer filled with a porous material 102 is provided in the carbon separating unit in the carbonization apparatus 100 as shown in an example of
As shown in
In the step of bringing the dehumidified gas into contact with the porous material 102 to separate solid-state carbon, the contact between the dehumidified gas and the porous material 102 is preferably effected under an atmosphere in the temperature range of 500 to 800° C. The temperature at which the reformed gas is brought into contact with the porous material 102 is preferably 500° C. or higher because the decomposition reaction of the carbon monoxide is promoted, while the temperature is preferably 800° C. or lower because thermal energy generated by the decomposition reaction of the carbon monoxide can be effectively utilized. The temperature at which the reformed gas is brought into contact with the porous material 102 is within the range of 500 to 800° C. which corresponds to the temperature condition adopted for a direct-reduction iron-making reaction. Note that the contact between the dehumidified gas and the porous material may be effected by passing the dehumidified gas through the layer filled with the porous material 102 provided within the carbon separating unit. Accordingly, a decomposition reaction of carbon monoxide represented by the above chemical equation proceeds. As the decomposition reaction of carbon monoxide proceeds, the solid-state carbon that constitutes the carbon monoxide is precipitated on the surface of the porous material 102. When a porous iron material is used, solid-state carbon precipitated on its surface partially or entirely causes carburizing to form iron carbide.
In the step of bringing the dehumidified gas into contact with the porous material 102 and separating solid-state carbon, the contact between the dehumidified gas and the porous material 102 is preferably effected under an atmosphere where the pressure of the dehumidified gas is 1.0 to 10 atm. The pressure of 1.0 atm or more at which the dehumidified gas is brought into contact with the porous material 102 corresponds to the pressurization condition, and allows the equilibrium of a decomposition reaction of the carbon monoxide to shift rightward, thus promoting the decomposition reaction of the carbon monoxide, which is preferable. The pressure of 10 atm or less can ensure the safety of the carbon separating unit of the carbonization apparatus 100 with respect to the legal restraints, which is preferable.
The open porosity, as measured by the Archimedes' method, of the porous material 102 with which the dehumidified gas is brought into contact is preferably 50 to 99%, more preferably 80 to 95%. When the open porosity of the porous material is 50% or more, the dehumidified gas passes through the pores of the porous material to promote the decomposition reaction of the carbon monoxide and also absorb the decomposed carbon, which is preferable. Meanwhile, when the open porosity of the porous material 102 is 99% or less, the thermal shock resistance to the dehumidified gas supplied to the porous material 102 can be maintained, which is preferable. The pore diameter of the porous material 102 is preferably 10 μm or more. Note that the open porosity of the porous material 102 is measured by the Archimedes' method. Specifically, a value measured by a measurement method defined by the Japanese Industrial Standards (JIS R2205; 1992) can be used as the open porosity of the porous material.
The porous material 102 with which the dehumidified gas is brought into contact preferably contains at least one element selected from the group consisting of iron, platinum, nickel, cobalt, rhodium, and palladium. That is, the porous material may contain one metal element selected from the group consisting of iron, platinum, nickel, cobalt, rhodium, and palladium, or a combination of two or more of such metal elements. Further, the porous material may be a metal compound containing one metal element selected from the group consisting of iron, platinum, nickel, cobalt, rhodium, and palladium as carbide, oxide, carbonate, sulfate, or the like.
Among such metals adopted as the porous material 102 with which the dehumidified gas is brought into contact, iron, platinum, and nickel are preferable. When platinum or nickel is used as the porous material, each element functions as a decomposition reaction catalyst for the carbon monoxide, and the catalyst would undergo little deterioration, which is preferable. In addition, when platinum or nickel is used as the porous material 102, solid-state carbon generated by a decomposition reaction of the carbon monoxide is precipitated as graphite alone, which is preferable.
The porous material 102 with which the dehumidified gas is brought into contact is more preferably iron. The use of iron as the porous material 102 causes the porous material 102 to be carburized by carbon generated by the decomposition reaction of the carbon monoxide to thus obtain austenite (i.e., Y iron) with the carbon being dissolved as a solid-solution. The use of iron as the porous material 102 also causes the porous material 102 to be carburized by carbon generated by the decomposition reaction of the carbon monoxide to obtain cementite (i.e., iron carbide (Fe3C)) through a reaction between the carbon and the iron, which is preferable. Austenite and cementite resulting from the carburizing of the porous material 102 by carbon generated by a decomposition reaction of carbon monoxide have almost the same hardness as quenching steel and thus can be directly used as iron-making raw materials. Further, the porous material 102 with which the dehumidified gas is brought into contact is preferably one or more types of iron selected from iron oxide and reduced iron.
The porous material 102 with which the dehumidified gas is brought into contact is particularly preferably an iron whisker. An iron whisker is a crystal that grows on the surface of a crystal in an outward direction like a beard and is formed such that, when compression stress is generated around the surface of the crystal, a new crystal grows on the original crystal in an outward direction in an attempt to relax the stress. Since such an iron whisker has a small start point of crystal growth and tends to keep growing continuously, it is formed as a single crystal in the shape of an extremely long and thin beard and has a length of 1 mm or more relative to a diameter of about 1 μm. When an iron whisker is adopted as the porous material with which the dehumidified gas is brought into contact, it is possible to allow the iron whisker to be carburized by the carbon generated by a decomposition reaction of carbon monoxide, thereby obtaining a whisker formed of austenite or formed of cementite.
The iron whisker as the porous material 102 with which the dehumidified gas is brought into contact can be produced by a method of producing an iron whisker described in Non Patent Literature 2, for example. According to the method of producing an iron whisker, it is possible to obtain an iron whisker as a porous material with an open porosity of 90% or more and a pore diameter of 10 μm or more. The iron whisker obtained by the method of producing an iron whisker also has a high open porosity and thus can be preferably used as the porous material 102 used for a carbon recovery method. It should be noted that even when a metal other than iron is used as the metal for forming the porous material 102 with which the dehumidified gas is brought into contact, the porous material 102 can be produced in the same manner.
The carbon adsorbed on the porous material 102 is recovered within a carbon recovery unit 130. Herein, “recovering the carbon adsorbed on the porous material” includes recovering the solid-state carbon precipitated on the surface of the porous material 102 or recovering the carbon causing carburizing inside of the porous material as a solid solution or a metal carbide compound of the carbon and the metal element contained in the porous material.
The carbon obtained by the decomposition reaction of the carbon monoxide contained in the dehumidified gas is precipitated on the surface of the porous material 102. The carbon obtained by the decomposition reaction of the carbon monoxide contained in the dehumidified gas also causes carburizing inside of the porous material 102 and reacts with the metal element forming the porous material, thereby forming a carbon solid solution or a metal carbide compound. Further, the carbon obtained by a decomposition reaction of the carbon monoxide contained in the dehumidified gas is precipitated on the surface of the carbon solid solution or the metal carbide compound.
The carbon precipitated on the surface of the porous material 102 can be recovered by subjecting the porous material 102 containing the carbon to an operation of separating powder and granular materials using a sieve mesh, for example. In addition, to recover the carbon that has become a carbon solid solution or a metal carbide compound by reacting with the metal element forming the porous material after carburizing the inside of the porous material 102, it is possible to directly recover the carbon solid solution or the metal carbide compound. It should be noted that when the recovered carbon is to be used as an iron-making raw material, using iron as the porous material 102 can collectively use them as the iron-making raw material, which is preferable, without performing an operation of separating the recovered carbon from the porous material 102.
As described above, in the third step according to the present embodiment, bringing the carbon monoxide contained in the dehumidified gas into contact with the porous material 102 can promote the decomposition reaction of the carbon monoxide and separate solid-state carbon, recovering the carbon as the solid-state carbon, or as a carbon solid solution or a carbon metal compound containing the carbon. In addition, since molten pig iron can be produced by using, as a raw material, the carbon-material-containing agglomerated ore obtained with the recovered carbon, the recovered carbon can be circulated through a process, reducing CO2 emissions to the outside of the system. The exhaust gases 38 and 40, the mixed gas, and the dehumidified gas of the present embodiment are examples of a carbon-containing gas containing carbon monoxide and carbon dioxide.
Although the present embodiment describes an example in which the second step is performed with the blast furnace 32, the present invention is not limited thereto. For example, molten pig iron may be produced by performing a reduction step of heating the carbon-material-containing agglomerated ore to 1160 to 1450° C. using a rotary hearth furnace, instead of the blast furnace 32, to reduce and melt them, and then cooling them to obtain reduced iron, and a melting step of melting the reduced iron using an electric furnace, for example. Even when a rotary hearth furnace is used, it is possible to use the existing rotary hearth furnace as is as long as the carbon-material-containing agglomerated ore obtained with the recovered carbon is used.
The influence of the carbon recovered by the device shown in
The particle size of iron ore A and the particle size of the recovered carbon were each set to −105 μm. “−105 μm” herein represents the particle size of the iron ore A that has passed through a sieve with a mesh size of 105 μm. A sample was obtained by mixing the iron ore A and the weighed powder of the recovered carbon in a mortar without pressing a pestle against each other for 3 minutes. Thus, uniform mixed powder was formed without changing the particle size during mixing. To the sample, carbon was added in an amount of 0.8 times the oxygen molar content in the iron oxide, and further added in an amount of 0.2 times the iron molar content in the iron oxide or the iron carbide, and then, the sample was mixed. By adding carbon in an amount of 0.8 times the oxygen molar content in the iron oxide, the carbon was used as a reducing material, and by adding carbon in an amount of 0.2 times the iron molar content in the iron oxide or the iron carbide, the effect of carburizing the metallic iron was attempted to be achieved. That is, carbon was provided with two functions that are a reducing material and a carburizing material.
The uniformly mixed sample was press-formed with an applied pressure of 98 MPa for 30 seconds into a cylindrical shape with a diameter of 10 mm and a height of 10 mm. The formed sample was heated to 1300° C. at a heating rate of 10° C./min in an atmosphere where 5 volume % of N2—Ar mixed gas was supplied at a flow rate of 0.5 NL/min. The generated gas was analyzed with an infrared spectrophotometer to calculate the reduction rate of the iron ores.
Although the reason for the above is not clear, the inventors consider it due to the difference in the particle size of the carbon-containing materials. Specifically, the recovered carbon is in the form of fibers of about several nm as shown in
The method for producing molten pig iron according to the present invention has the effect of reducing CO2 emissions to the outside of the system. In addition, the method can, by recovering CO2 from an exhaust gas as solid-state carbon, and producing a carbon-material-containing agglomerated ore by combining the recovered solid-state carbon with an iron-containing raw material, increase the reducibility of the agglomerated ores. This can reduce the unit consumption of carbon required for reduction and contribute to reducing environmental burdens in the steel industry. Thus, the present invention is industrially quite advantageous.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-034616 | Mar 2022 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2023/007389 | 2/28/2023 | WO |
