Molten iron manufacturing method

Abstract

Provided is a method for producing molten pig iron, in which a high energy efficiency is rendered possible in a melting step in which reduced iron is melted in a submerged arc furnace. The method for producing molten pig iron includes a melting step which comprises: melting an iron source material containing reduced iron having an average metallization degree of 60% or higher in a submerged arc furnace to obtain molten pig iron; and adding a slagging material in order to adjust the basicity (CaO / SiO2) of molten slag to be formed on the molten pig iron and / or adding a carbonaceous material in order to adjust the C content of the molten pig iron. The metallization degree is a percentage of the ratio of the mass of the iron metal component to the total mass of the iron components contained in the reduced iron, and the basicity (CaO / SiO2) of the molten slag is a mass proportion of CaO to SiO2.

Description

[Technical Field]

[0001] The present invention relates to a method for producing molten iron by melting an iron source material containing reduced iron in a submerged arc furnace. 3 kg. In this specification, "x to y" representing a numerical range means not less than x and not more than y, and includes the boundary value. "tp" is a unit representing the mass of molten iron. [Background technology]

[0002] In recent years, there has been a demand to reduce CO2 emissions to reduce the environmental impact. Even in the steel industry, the production of pig iron using the direct reduction (DR) method has been attracting attention as an alternative to the blast furnace method, which emits a large amount of CO2. In the DR method, for example, iron-containing agglomerates are reduced in a shaft furnace to produce direct reduced iron (DRI). This reduced iron or iron scrap, or other iron source materials, are charged into an electric arc furnace (EAF) or submerged arc furnace (SAF) and heated and melted. After separating the slag, pig iron is produced.

[0003] Pellets and lump ore are used as agglomerates for direct reduction. Pellets are made by mixing and granulating fine ore or dust with auxiliary materials and a binder, and then firing the mixture. Bentonite is often used as the binder.

[0004] Patent Documents 1 and 2 disclose techniques for obtaining molten metal by arc-heating pre-reduced metal or solid reduced iron. These techniques describe the use of reduced iron with a high metallization rate, which can reduce the thermal energy required for melting the reduced iron, and therefore use of reduced iron with a metallization rate of 60% or more, preferably 80% or more, and more preferably 90% or more.

[0005] Patent Document 3 discloses a method for reducing and melting iron raw materials containing iron oxide, in which the iron raw materials containing iron oxide are charged into a DC arc furnace and reduced and melted. Patent Document 3 describes a method using an iron raw material containing iron oxide, which has an iron metallization rate of 45% or more and 95% or less, and contains 4 to 20 mass% of oxides other than iron oxide.

[0006] Patent Document 4 discloses an electric furnace equipped with one or more upper electrodes, one or more bottom-blowing tuyeres, a mechanical agitator equipped with an impeller, and a charging device for charging iron oxide-containing raw materials. In operation using this electric furnace, iron oxide-containing raw materials are charged onto molten pig iron seed, where they are melted and reduced. This allows for the melting and reduction of iron oxide-containing raw materials with a high iron yield. This method for melting and reducing iron oxide-containing raw materials using an electric furnace allows the entire amount of CO gas generated in a DC arc furnace to be used for reduction in a pre-reduction furnace. Furthermore, this method can suppress an increase in the carbon consumption rate without reducing the overall reduction efficiency, and it can suppress an increase in the reduction heat required in the DC arc furnace, thereby preventing an increase in the electricity consumption rate. This method describes the use of iron oxide-containing raw materials with a metallization rate of 45% or more and 95% or less. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-105415 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-074120 [Patent Document 3] Japanese Patent Application Publication No. 2018-003075 [Patent Document 4] International Publication No. 2019 / 082762 Summary of the Invention [Problem to be solved by the invention]

[0008] Conventionally, electric arc furnaces (EAFs) have been used primarily as equipment for melting reduced iron in the DR process. Due to equipment limitations, EAFs require the use of high-quality reduced iron with a low slag ratio (high Fe content). Consequently, high-grade iron ore has inevitably been used as the raw material for reduced iron, i.e., the raw ore for iron ore pellets. Examples of high-grade iron ore include South American ore, concentrate ore whose grade has been improved by prior ore dressing, and pellet feed ore.

[0009] However, there are challenges in utilizing South American ore, concentrate ore, and pellet feed ore in Japan. Due to Japan's geographical conditions, South American ore inevitably increases freight rates. Concentrate ore and pellet feed ore are also derived from high-grade iron ore, as beneficiation of high-grade iron ore is more efficient than beneficiation of low-grade iron ore, and for the same reason, freight rates increase. Therefore, the direct reduction process using high-grade iron ore poses a cost challenge.

[0010] Therefore, in Japan, studies are underway to apply the DR process to low-grade iron ore produced in Australia, India, etc. However, it is difficult to melt the reduced iron produced from low-grade iron ore using the current EAF, so the use of a submerged arc furnace (SAF) is being considered.

[0011] Since the melting process using SAF requires a large amount of energy (electricity), it is necessary to improve energy efficiency as much as possible. In EAF, reduced iron with a high metallization rate is generally used to reduce the heat of melting.

[0012] Patent Documents 1 to 4 each describe the use of reduced iron with an appropriate high metallization rate. However, all of these documents are intended for heating by arc heat, and do not mention the effectiveness of heating systems that use Joule heat, such as submerged arc furnaces. Furthermore, they state that the preferred range of metallization rate is 80% or 90% or higher, aiming to improve the reduction efficiency in electric furnaces.

[0013] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing molten pig iron that can achieve high energy efficiency in the melting step when producing molten pig iron by melting reduced iron in a submerged arc furnace. [Means for solving the problem]

[0014] As a result of intensive research aimed at solving the above-mentioned problems, the inventors have discovered that, as long as the metallization rate of reduced iron charged into a submerged arc furnace is within a predetermined range, energy efficiency is improved in the processes from pre-reduction to melting. That is, a method for producing molten pig iron according to the present invention, which advantageously solves the above-mentioned problems, includes a melting step in which an iron source raw material containing reduced iron having an average metallization rate of 60% or more is melted in a submerged arc furnace to obtain molten pig iron, and the melting step includes at least one or both of adding a slag former to adjust the basicity (CaO / SiO2) of molten slag formed on the molten pig iron and adding a carbonaceous material to adjust the C content of the molten pig iron, wherein the metallization rate is a percentage of the ratio of the mass of metallic iron components to the mass of total iron components contained in the reduced iron, and the basicity (CaO / SiO2) of the molten slag refers to the mass fraction of CaO to SiO2.

[0015] The method for producing molten iron according to the present invention comprises the steps of: (a) a step of preparing raw materials for direct reduction, further including an optional step of preparing iron ore pellets produced using low-grade iron ore having a total Fe content of 63% by mass or less; a reduced iron production process comprising any step of producing the reduced iron from the raw material for direct reduction, (b) the raw material for direct reduction used in the reduced iron production process includes lump ore; (c) adding the slag former so that the basicity (CaO / SiO2) of the molten slag is in the range of 1.0 to 1.3; (d) adding the carbon material so that the C content in the molten iron is in the range of 2 to 5 mass %; This may be a more preferable solution. [Effects of the Invention]

[0016] According to the method for producing molten pig iron of the present invention, high energy efficiency can be achieved when iron source materials containing reduced iron are melted in a submerged arc furnace to produce molten pig iron. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram illustrating a method for producing molten iron according to one embodiment of the present invention. [Figure 2] 1 is a graph showing the relationship between the average metallization rate of reduced iron and the power consumption rate. [Figure 3] 1 is a graph showing the relationship between the average metallization rate of reduced iron and power efficiency. [Figure 4] 1 is a graph showing the relationship between the treatment time and the metallization rate when iron ore pellets are reduced in an electric furnace. [Figure 5] 1 is a graph showing the relationship between the metallization rate of reduced iron and the reduction energy per unit metallization rate % required to obtain the metallization rate. DETAILED DESCRIPTION OF THE INVENTION

[0018] The following is a detailed description of embodiments of the present invention. The following embodiments are intended to exemplify equipment and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope defined in the claims.

[0019] (Melting process) Figure 1 is a schematic diagram illustrating the configuration of a submerged arc furnace suitable for use in a method for producing 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 a furnace wall 2. In the submerged arc furnace 1, electrodes 3 are inserted into the molten slag S, and an arc is generated between the multiple electrodes 3 or between the electrodes 3 and the molten pig iron P, and the raw materials are heated and melted by resistance heating.

[0020] In this embodiment, an iron source raw material 4 containing reduced iron is charged into a submerged arc furnace. At the same time, at least one or both of the following is included: adding a slag former to adjust the basicity (CaO / SiO2) of molten slag S formed on molten pig iron P; and adding a carbon material 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 to SiO2 in the molten slag.

[0021] In this embodiment, the iron source raw material to be melted in the submerged arc furnace includes reduced iron. The reduced iron may be produced in advance by a direct reduction method, or commercially available reduced iron may be purchased and used. Iron scrap or scale may also be used as the iron source raw material. The mass ratio of the reduced iron to the iron source raw material is preferably 50 to 100%.

[0022] When the metallization rate of the reduced iron is 60% or higher, the overall power consumption rate is low and energy efficiency is excellent. There is no particular upper limit to the metallization rate. If the metallization rate is too high, the reduced iron, which is the raw material 4 charged from above the slag S in the submerged arc furnace 1, may conduct electricity through the slag, resulting in a decrease in resistance heat. Therefore, the upper limit of the metallization rate is preferably about 90%. More preferably, the upper limit of the metallization rate is 80%.

[0023] As for the slag formers, limestone (CaCO3) or quicklime (CaO) is preferably used as a CaO source, and silica (SiO2) is used as a SiO2 source. The basicity (CaO / SiO2) of the molten slag S is preferably in the range of 1.0 to 1.3. When the basicity of the molten slag is in this range, it is suitable for reuse as a roadbed material such as cement.

[0024] Coke or coal can be used as the carbonaceous material. It can be charged in chunks or in powder form using a gas transport. The ash content of the carbonaceous material other than carbon is added to the molten slag. FLUX The C content in the molten pig iron P is preferably adjusted to a range of 2 to 5 mass %. If the C content is within this range, the pig iron can be used as it is or as a raw material for steelmaking in the next step.

[0025] (Preparation process of raw materials for direct reduction) In this embodiment, it is preferable to have a step of preparing raw materials for direct reduction, which includes an optional step of preparing iron ore pellets produced from low-grade iron ore having a total Fe content (hereinafter also referred to as T.Fe) of 63% by mass or less. The raw materials for iron ore pellets generally consist of iron ore, binder, and auxiliary materials. In this embodiment, low-grade iron ore refers to iron ore having a T.Fe content of 63% by mass or less. In addition, it is preferable that the low-grade iron ore has a crystal water content of 4% by mass or more.

[0026] Bentonite is preferred as a binder for iron ore pellets. Any known or arbitrary binder, such as organic or inorganic binders, that can achieve the same effect may also be used. Quicklime, limestone, dolomite, and other auxiliary materials may also be mixed with the iron ore pellets. Furthermore, lump ore may also be prepared as a raw material for direct reduction in addition to the iron ore pellets. Lump ore is generally iron ore with a size of approximately 10 to 35 mm, and is used in the reduced iron production process without being crushed.

[0027] Iron ore pellets may be prepared by being manufactured through a general crushing, mixing, granulation, and firing process, or pre-manufactured iron ore pellets may be prepared. When manufacturing iron ore pellets, each process can be carried out using conventionally known equipment and conditions as described below. The crushing process can be carried out using a crusher such as a general ball mill. The mixing process can be carried out using a general high-speed stirring mixer or concrete mixer. The granulation process can be carried out using a general pelletizer or drum mixer. The firing process can be carried out using a general rotary kiln or electric furnace.

[0028] (reduced iron manufacturing process) In this embodiment, it is preferable to have a reduced iron production process that includes any step of producing the reduced iron from raw materials for direct reduction. In the reduced iron production process, reduced iron is produced from iron ore pellets or lump ore as essential raw materials. A solid reduction furnace such as a general shaft furnace may be used to produce reduced iron. There are no particular restrictions on the reducing gas, but it is preferable to use, for example, hydrogen gas produced using renewable energy. Depending on the production method used, for example, a mixed gas consisting of, by volume, 55% H2, 35% CO, and the remainder being CO2 and CH4, or a mixed gas consisting by volume of 75% H2, 20% CO, and the remainder being CO2 and N2, can be suitably used. [Example]

[0029] The specific power consumption and power efficiency per unit mass of molten iron were investigated using a 4-6 ton batch-type submerged arc furnace 1, as shown in Figure 1. Table 1 shows the composition of the reduced iron used in the investigation. Both furnaces used the same low-grade ore pellets, with only the metallization rate being changed. In Table 1, M.Fe represents metallic iron.

[0030] [Table 1]

[0031] Reduced iron equivalent to 500 kg of molten pig iron and a predetermined amount of blast furnace slag were charged into submerged arc furnace 1, and heating was initiated by applying current to electrode 3. After the start of current application, eight additional charges of 400 kg of reduced iron were added at 350 kWh power consumption intervals. The power consumption per unit mass of molten pig iron (kWh / tp) was calculated from the power consumption when the molten pig iron temperature reached approximately 1600°C after all the charged reduced iron was melted. The power efficiency was calculated by dividing the theoretical sum of the heating energy for the molten pig iron, the heating energy for the slag, and the reduction energy for iron oxide by the power consumption. Table 2 shows the investigated conditions and results. The amount of reduced iron charged was kept constant, and the average metallization ratio of the charged reduced iron was adjusted by changing the blending ratio of reduced iron DRI1 and reduced iron DRI2. The slag basicity was 1.26, and the carbon content of the molten pig iron was adjusted to 2.5–4.5 mass%. The slag ratio in the furnace, which is the mass fraction of molten slag per unit mass, was adjusted to 1.5t / tp using a slag former. The input power was kept constant at 2.7MW.

[0032] [Table 2]

[0033] Figure 2 shows the relationship between the average metallization rate and the power consumption rate in Table 2. Figure 3 shows the relationship between the average metallization rate and the power efficiency in Table 2. The results in Figure 2 indicate that the power consumption rate decreases almost linearly with the average metallization rate. This indicates that the energy required for reduction in a submerged arc furnace per unit percentage decrease in the metallization rate remains almost constant. This is consistent with the fact that the power efficiency shown in Figure 3 remains almost constant. The reason for the slight decrease in power efficiency in No. T8 is thought to be that the metallization rate of the reduced iron was so high that electricity was passed through the raw material layer deposited on top of the slag layer rather than the slag layer.

[0034] Figure 4 shows the relationship between time and metallization rate during gas reduction in an electric furnace for iron ore pellets, the raw material for reduced iron DRI1 and DRI2 listed in Table 1. Reduction was performed in a 50% CO2-50% H2 atmosphere at 900°C. As is clear from Figure 4, the reduction rate was fast in the early stages of reduction and then slowed as the metallization rate increased. The reason for this is thought to be as follows: In gas reduction, reduction begins at the periphery of the pellet. Therefore, as the pellet's metallization rate increases, the gas diffusion resistance from the surface to the unreduced central portion and the reaction interface itself become smaller. Therefore, in gas reduction to produce reduced iron, the energy required for reduction per unit percentage of metallization rate increases as the metallization rate increases. The energy required for the reduction test was converted into the amount of reduced iron charged into a submerged arc furnace. Figure 5 shows the relationship between the energy required for reduction per unit percentage of metallization rate of reduced iron and the metallization rate of reduced iron, and the energy required per unit mass of metallic iron. On the other hand, as mentioned above, the energy required per unit mass of molten iron per unit percentage of metallization of reduced iron in a submerged arc furnace was almost constant, at approximately 6 (kWh / tp) / %. This indicates that reduction using a submerged arc furnace requires less energy when the metallization rate is in the range of 60% or higher, and the difference has tended to widen. Considering operational fluctuations, it is preferable that the metallization rate of the reduced iron charged into a submerged arc furnace be between 60% and 80%. [Explanation of symbols]

[0035] 1 Submerged Arc Furnace (SAF) 2 Furnace wall 3 electrodes 4 (Input) Raw materials P hot metal S Molten slag

Claims

1. The method comprises melting an iron source material containing reduced iron having an average metallization rate of 60% to 80% in a submerged arc furnace to obtain molten iron, and adjusting the basicity (CaO / SiO ２ and / or adding a carbon material to adjust the C content of the molten pig iron; Here, the metallization rate is a percentage of the ratio of the mass of metallic iron components to the mass of the total iron components contained in the reduced iron, and the basicity of the molten slag (CaO / SiO ２ ) is SiO of CaO ２ A method for producing molten iron, which refers to the mass fraction of

2. a step of preparing raw materials for direct reduction, the step including an optional step of preparing iron ore pellets produced using low-grade iron ore having a total Fe content of 63% by mass or less; and a reduced iron production step comprising any step of producing the reduced iron from the raw materials for direct reduction.

3. The method for producing molten iron according to claim 2 , wherein the raw materials for direct reduction used in the reduced iron production step include lump ore.

4. The basicity of the molten slag (CaO / SiO ２ 4. The method for producing molten iron according to claim 1, wherein the slag former is added so that the ratio of slag ratio to slag strength 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 3, wherein the carbonaceous material is added so that the C content in the molten pig iron is in the range of 2 to 5 mass%.

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