Method for producing molten pig iron

Abstract

Provided is a method for producing molten pig iron capable of achieving high energy efficiency in a melting step for melting reduced iron in a submerged arc furnace. This method for producing molten pig iron comprises a melting step for obtaining molten pig iron by melting an iron source material containing reduced iron having an average metallization ratio of 60% or more in a submerged arc furnace. And further comprising at least one or two of the following: adding a slagging material to adjust the alkalinity (CaO / SiO2) of molten slag formed on the molten pig iron, and adding a carbon material to adjust the C content of the molten pig iron. Here, the metallization ratio is the percentage of the ratio of the mass of the metallic iron component contained in the reduced iron to the mass of the total iron component, and the alkalinity (CaO / SiO2) of the molten slag is the mass fraction of CaO to SiO2.

Description

Technical Field

[0001] This invention relates to a method for producing molten pig iron by melting an iron source material containing reduced iron in a submerged arc furnace. In the following description, "t" as a unit of mass represents 10... 3 kg. In this specification, "x~y" indicates a numerical range, meaning above x and below y, including boundary values. Additionally, "tp" is the unit for expressing the mass of molten pig iron. Background Technology

[0002] In recent years, efforts have been made to reduce CO2 emissions in order to mitigate environmental burdens. In the steel industry, the use of direct reduction (DR) to produce pig iron as a substitute for the high CO2 emissions of the blast furnace method has attracted significant attention. In the DR method, iron-containing agglomerate is reduced in a shaft furnace, for example, to produce direct reduced iron (DRI). This reduced iron, along with iron scrap and other iron source materials, is then heated and melted in an electric arc furnace (EAF) or submerged arc furnace (SAF). Pig iron is then produced by separating the slag.

[0003] Agglomerated ore, used as a raw material for direct reduction, employs pellets and lump ore. Pellets are produced by mixing / granulating fine ore, dust, by-products, and a binder, followed by calcination. Bentonite is commonly used as the binder.

[0004] Patent documents 1 and 2 disclose a technique for obtaining molten metal by arc heating a pre-reduced metal or solid reduced iron. In this technique, since using reduced iron with a high metallization rate can reduce the heat energy required for melting the reduced iron, the technique of using reduced iron with a metallization rate of 60% or more, preferably 80% or more, and more preferably 90% or more is described.

[0005] Patent Document 3 discloses a reduction / melting method for reducing / melting iron raw materials containing iron oxide by feeding them into a DC electric arc furnace. Patent Document 3 describes a method using iron raw materials containing iron oxide with an iron metallization rate of 45% or more and 95% or less, and containing 4 to 20% by mass of oxides other than iron oxide.

[0006] Patent Document 4 discloses an electric furnace comprising: one or more upper electrodes, one or more bottom tuyeres, a mechanical stirrer with an impeller, and a feeding device for feeding iron raw materials containing iron oxide. Operation using this electric furnace involves feeding the iron raw materials containing iron oxide onto initially molten pig iron for melting and reduction. Therefore, the melting and reduction of iron raw materials containing iron oxide can be performed with a high iron yield. In the method for melting and reducing iron raw materials containing iron oxide using this electric furnace, all CO gas generated in the DC electric arc furnace can be used as CO gas for reduction in the pre-reduction furnace. Furthermore, this method can suppress the increase in the unit consumption of carbon materials without causing a decrease in overall reduction efficiency, suppress the increase in the reduction heat required in the DC electric arc furnace, and prevent an increase in the unit consumption of electricity. As described in this method, an iron oxide-containing raw material with a metallization rate of 45% or more and 95% or less is used.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2003-105415

[0010] Patent Document 2: Japanese Patent Application Publication No. 2009-074120

[0011] Patent Document 3: Japanese Patent Application Publication No. 2018-003075

[0012] Patent Document 4: International Publication No. 2019 / 082762 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] However, in the past, the electric arc furnace (EAF) was mainly used as the equipment for melting reduced iron in the DR process. Due to equipment limitations, the EAF required high-quality reduced iron with a low slag ratio (high Fe content). Therefore, the raw material for this reduced iron, namely the iron ore pellets, must be high-grade iron ore. Examples of high-grade iron ore include: South American ore, concentrate ore that has undergone pre-processing to improve its grade, and pellet feed ore.

[0015] However, in Japan, there are problems with the efficient utilization of South American ores, concentrates, and pellet feedstocks. Due to Japan's geographical conditions, transportation costs for South American ores are inevitably increased. For concentrates and pellet feedstocks, the efficiency of beneficiating high-grade iron ore is superior compared to low-grade iron ore; therefore, most are derived from high-grade iron ore, leading to increased transportation costs for the same reason. Consequently, cost issues exist in direct reduction processes using high-grade iron ore.

[0016] Therefore, research is underway in Japan on applying low-grade iron ore from countries such as Australia and India to the DR process. In existing EAFs (Extended Equivalent Furnaces), it is difficult to melt reduced iron produced from low-grade iron ore. Therefore, the use of a submerged arc furnace (SAF) has been investigated.

[0017] Here, because the melting process using SAF requires a great deal of energy (electricity), it is necessary to maximize energy efficiency. In EAF, reduced iron with a high metallization rate is typically used to reduce its heat of fusion.

[0018] Patent documents 1-4 describe their respective applicable reduced iron with high metallization rates. However, they all focus on heating based on electric arc heat and do not mention the effect of heating systems based on Joule heating, such as submerged arc furnaces. Furthermore, they describe a preferred range of metallization rate of 80% or 90% or higher, with the goal of improving reduction efficiency within the electric furnace.

[0019] The present invention was made in view of the above circumstances, and its object is to provide a method for manufacturing molten pig iron in which high energy efficiency can be achieved in the melting process when reducing iron is melted in a submerged arc furnace to manufacture molten pig iron.

[0020] Problem Solving Methods

[0021] To address the aforementioned issues, the inventors conducted in-depth research and arrived at the following insight: if the metallization rate of the reduced iron charged into the submerged arc furnace is within a given range, the energy efficiency in the process from pre-reduction to melting increases. That is, the method for manufacturing molten pig iron according to the present invention, which advantageously solves the above problems, includes a melting step comprising melting an iron source material containing reduced iron with an average metallization rate of 60% or more in a submerged arc furnace to obtain molten pig iron, and further comprising at least one or both of the following: adding slag-forming materials to adjust the basicity (CaO / SiO2) of the molten slag formed on the molten pig iron, and adding carbon materials to adjust the C content of the molten pig iron, wherein the metallization rate is the percentage of the mass of metallic iron component contained in the reduced iron relative to the mass of the total iron component, and the basicity (CaO / SiO2) of the molten slag refers to the mass fraction of CaO relative to SiO2.

[0022] It should be noted that the method for manufacturing molten pig iron of the present invention can be a more preferred solution, such as (a) to (d):

[0023] (a) Further includes the following steps:

[0024] The preparation process for raw materials used in direct reduction includes any step of preparing iron ore pellets from low-grade iron ore with a total Fe content of 63% by mass or less; and

[0025] The reduced iron manufacturing process includes any step of manufacturing the reduced iron from the aforementioned raw materials for direct reduction;

[0026] (b) Includes lump ore as the raw material for direct reduction used in the above-mentioned reduced iron manufacturing process;

[0027] (c) Add the above-mentioned slag-forming material so that the basicity (CaO / SiO2) of the above-mentioned molten slag is in the range of 1.0 to 1.3;

[0028] (d) Add the above-mentioned carbon material so that the C content in the above-mentioned molten pig iron is in the range of 2 to 5% by mass.

[0029] The effects of the invention

[0030] According to the method for producing molten pig iron of the present invention, high energy efficiency can be obtained when melting iron source raw materials containing reduced iron in a submerged arc furnace to obtain molten pig iron. Attached Figure Description

[0031] Figure 1 This is a schematic diagram illustrating a method for manufacturing molten pig iron according to one embodiment of the present invention.

[0032] Figure 2 It is a graph showing the relationship between the average metallization rate of reduced iron and the unit electricity consumption.

[0033] Figure 3 This is a graph showing the relationship between the average metallization rate of reduced iron and electrical efficiency.

[0034] Figure 4 This is a graph showing the relationship between processing time and metallization rate when reducing iron ore pellets in an electric furnace.

[0035] Figure 5 It is a graph showing the relationship between the metallization rate of reduced iron and the reduction energy required per unit metallization % to obtain that metallization rate.

[0036] Symbol Explanation

[0037] 1. Submerged Arc Furnace (SAF)

[0038] 2 Furnace wall

[0039] 3 electrodes

[0040] 4 (Input) Raw Materials

[0041] P Molten pig iron

[0042] S Molten Slag Detailed Implementation

[0043] The embodiments of the present invention will now be described in detail. The following embodiments illustrate apparatus and methods for embodying the technical concept of the present invention, but do not limit the structure of the present invention to the following. That is, the technical concept of the present invention can be modified in various ways within the scope of the claims.

[0044] (Melting process)

[0045] Figure 1 This is a schematic diagram illustrating the preferred configuration of a submerged arc furnace used in a method for manufacturing molten pig iron according to one embodiment of the present invention. The submerged arc furnace 1 holds molten pig iron P and molten slag S within the furnace wall 2. In the submerged arc furnace 1, electrodes 3 are inserted into the molten slag S, and an electric arc is generated between multiple electrodes 3 or between an electrode 3 and the molten pig iron P, melting the raw material through resistance heating.

[0046] In this embodiment, an iron source material 4 containing reduced iron is charged into a submerged arc furnace. Simultaneously, at least one or both of the following are included: adding slag-forming materials to adjust the basicity (CaO / SiO2) of the molten slag S formed on the molten pig iron P, and adding carbon materials to adjust the C content of the molten pig iron P. The basicity (CaO / SiO2) of the molten slag refers to the mass fraction of CaO relative to SiO2 in the molten slag.

[0047] In this embodiment, reduced iron is included as the iron source material molten in the submerged arc furnace. Reduced iron can be manufactured in advance by direct reduction or commercially available reduced iron can be purchased and used. Iron scrap and scale can also be used as the iron source material. The mass ratio of reduced iron in the iron source material is preferably 50% to 100%.

[0048] When the metallization rate of reduced iron is above 60%, the total power consumption per unit decreases, resulting in excellent energy efficiency. There is no specific upper limit on the metallization rate. However, if the metallization rate is too high, there is a risk that the reduced iron (as raw material 4) fed from above the slag S in the submerged arc furnace 1 may experience reduced resistance heat due to energization within the slag. Therefore, the upper limit of the metallization rate is preferably set at around 90%. More preferably, the upper limit of the metallization rate is set at 80%.

[0049] For the slag-forming materials, limestone (CaCO3) and quicklime (CaO) are preferably used as the CaO source, and silica (SiO2) is used as the SiO2 source. The basicity (CaO / SiO2) of the molten slag S is preferably set in the range of 1.0 to 1.3. If the basicity of the molten slag is within this range, it is preferable to reuse the slag as a roadbed material such as cement.

[0050] Carbon materials can be made from coke or coal. Besides using lumpy carbon materials, powdered carbon materials can also be conveyed via gas transport. The ash content, excluding carbon, in the composition of carbon materials is considered as sulfur (S). FLUX It is contained and remains in the molten slag S. The C content in the molten pig iron P is preferably adjusted to a range of 2 to 5% by mass. If it is within this range, it is preferred to use it directly as pig iron or as a raw material for steelmaking in subsequent processes.

[0051] (Preparation process for raw materials used in direct reduction)

[0052] In this embodiment, a preparation step for raw materials for direct reduction is preferred. This step includes any step of preparing iron ore pellets made from low-grade iron ore with a total Fe content (hereinafter also referred to as T.Fe) of 63% or less by mass. The raw materials for iron ore pellets typically include iron ore, binder, and by-products. In this embodiment, low-grade iron ore refers to iron ore with a T.Fe content of 63% or less by mass. Furthermore, the low-grade iron ore preferably has a water of crystallization content of 4% or more by mass.

[0053] Bentonite is preferred as a binder for iron ore pellets. However, any known or arbitrary binder, such as organic / inorganic binders, that achieves the same effect can also be used. Additionally, quicklime, limestone, dolomite, etc., can be mixed in as a by-product of iron ore pellet production. Furthermore, in addition to iron ore pellets, lump ore can also be prepared as a raw material for direct reduction. Lump ore is typically iron ore with a size of about 10-35 mm, and is used directly in the reduced iron manufacturing process without being crushed.

[0054] Iron ore pellets can be prepared by conventional crushing, mixing, granulation, and calcining processes, or pre-made iron ore pellets can be prepared. In the case of manufacturing iron ore pellets, each process can be carried out using existing, known apparatus and conditions, as listed below. The crushing process can be carried out using conventional ball mills or similar crushers. The mixing process can be carried out using conventional high-speed mixers, concrete mixers, etc. The granulation process can be carried out using conventional granulators, drum mixers, etc. The calcining process can be carried out using conventional rotary kilns, electric furnaces, etc.

[0055] (Reduced iron manufacturing process)

[0056] In this embodiment, a reduced iron manufacturing process is preferred, which includes any step of manufacturing the aforementioned reduced iron from raw materials for direct reduction. In the reduced iron manufacturing process, reduced iron is produced from iron ore pellets or lump ore, which are essential raw materials. In the manufacturing of reduced iron, any conventional solid reduction furnace, such as a shaft furnace, can be used. There are no particular restrictions on the reducing gas; however, hydrogen, produced using renewable energy sources, is preferred. Depending on the manufacturing method used, for example, a mixed gas with H2: 55% H2, CO: 35% CO2 by volume, and the remainder being a mixture of CO2 and CH4, or a mixed gas with H2: 75% H2, CO: 20% CO2 by volume, and the remainder being a mixture of CO2 and N2, can be suitably used.

[0057] Example

[0058] like Figure 1 As shown, the unit power consumption and power efficiency per unit mass of molten pig iron were investigated using a batch submerged arc furnace with a capacity of 4-6 t. Table 1 shows the composition of the reduced iron used in the investigation. Both used the same low-grade ore pellets, only the metallization rate was changed. M.Fe in Table 1 represents metallic iron.

[0059]

[0060] In a submerged arc furnace 1, 500 kg of reduced iron (based on molten pig iron) and a given amount of blast furnace slag were charged. Electricity was applied to electrode 3 to begin heating. After the start of energization, 400 kg of reduced iron was added for every 350 kWh of electricity consumed, repeated eight times. Based on the complete melting of the charged reduced iron, the electricity consumption per unit mass of molten pig iron (kWh / tp) was calculated according to the electricity consumption when the molten pig iron temperature reached approximately 1600°C. The power efficiency was calculated by dividing the sum of the theoretical heating energy of the molten pig iron, the heating energy of the slag, and the reduction energy of the iron oxide by the electricity consumption per unit mass. Table 2 shows the investigation conditions and results. The amount of reduced iron input was kept constant, and the average metallization rate of the input reduced iron was adjusted by changing the ratio of reduced iron DRI1 to reduced iron DRI2. Adjustments were made with a slag basicity of 1.26 and a C content in the molten pig iron of 2.5–4.5% by mass. Adjustments were made using slag-forming materials to achieve a slag ratio of 1.5 t / tp per unit mass of molten slag. The power input was kept constant at 2.7 MW.

[0061]

[0062] Figure 2 The table shows the relationship between the average metallization rate and the unit power consumption in Table 2. Figure 3 The relationship between average metallization rate and power efficiency is shown in Table 2. Figure 2 The results show that, relative to the average metallization rate, the unit power consumption decreases almost linearly. This indicates that, for each unit percentage decrease in metallization rate, the energy required for reduction in the submerged arc furnace remains essentially constant. Therefore, compared to... Figure 3 The results show that the power efficiency remains basically constant. The slight decrease in power efficiency in No. T8 can be attributed to the fact that the metallization rate of the reduced iron is too high, resulting in the energization of the raw material layer accumulated above the slag layer, rather than the energization of the slag layer itself.

[0063] Figure 4 The diagram shows the relationship between time and metallization rate during gas reduction of iron ore pellets in an electric furnace, where the iron ore pellets serve as the raw materials for reduced iron DRI1 and DRI2 as shown in Table 1. Reduction was carried out in a gaseous atmosphere of 50 volume CO - 50 volume % H2 at 900°C. According to... Figure 4 It is clear that the reduction rate is fast in the initial stage of reduction, and slows down as the metallization rate increases. The reason for this can be attributed to the following: In gas reduction, reduction occurs from the periphery of the pellet. Therefore, if the metallization rate of the pellet increases, the diffusion resistance of gas from the surface to the unreduced central portion, and the reaction interface itself, decrease. Thus, in the gas reduction of reduced iron, the energy required for reduction per unit % metallization increases with increasing metallization rate. Converting the energy required for the reduction experiment into the amount of reduced iron charged into the submerged arc furnace, the relationship between the energy required for reduction per unit % metallization rate and per unit mass of metallic iron and the metallization rate of reduced iron is shown below. Figure 5 On the other hand, as mentioned above, the energy required for reduction in the submerged arc furnace per unit percentage of metallization and per unit mass of molten pig iron is essentially constant, approximately 6 (kWh / tp) / %. Consequently, the energy required for reduction in the submerged arc furnace is small within a range of metallization rates above 60%, and this difference tends to widen. Considering operational variations, it is preferable to set the metallization rate of the reduced iron charged into the submerged arc furnace between 60% and 80%.

Claims

1. A method for producing molten pig iron, comprising a melting step, said melting step including melting an iron source raw material containing reduced iron with an average metallization rate of 60% or more in a submerged arc furnace to obtain molten pig iron, and further comprising at least one or two of the following: Adding slag-forming materials to adjust the basicity (CaO / SiO2) of the molten slag formed on the molten pig iron, and Carbon materials are added to adjust the C content of the molten pig iron. in, The metallization rate is the percentage of the mass of metallic iron in the reduced iron relative to the total mass of iron, and the basicity (CaO / SiO2) of the molten slag is the mass fraction of CaO relative to SiO2.

2. The method for manufacturing molten pig iron according to claim 1, further comprising: The preparation process for raw materials used in direct reduction includes any step of preparing iron ore pellets from low-grade iron ore with a total Fe content of 63% by mass or less; and The reduced iron manufacturing process includes any step of manufacturing the reduced iron from the raw materials for direct reduction.

3. The method for manufacturing molten pig iron according to claim 2, wherein, The material includes lump ore as the raw material for direct reduction used in the reduced iron manufacturing process.

4. The method for producing molten pig iron according to any one of claims 1 to 3, wherein, The slag-forming material is added so that the basicity (CaO / SiO2) of the molten slag is in the range of 1.0 to 1.

3.

5. The method for producing molten pig iron according to any one of claims 1 to 4, wherein, The carbon material is added so that the C content in the molten pig iron is in the range of 2 to 5% by mass.

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