Molten iron manufacturing method

Abstract

Provided is a method for producing molten iron, in which high energy efficiency can be achieved in a melting step for melting reduced iron in a submerged arc furnace. Specifically provided is a method for producing molten iron, the method having a melting step for melting an iron-source raw material including reduced iron in a submerged arc furnace 1 to obtain molten iron P, said step including one or both of adding a slag-making material in order to adjust the basicity (CaO / SiO2)of molten slag S formed on the molten iron P and adding a carbonaceous material in order to adjust the C content of the molten iron P, wherein an operation is performed such that a slag ratio S / P, which is the mass of the slag per unit mass of the molten iron, satisfies the range of 0.15-2.0 t / t-p, where the basicity (CaO / SiO2) of the molten slag is defined as the mass fraction 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 Document 1 proposes a raw material for reduction that suppresses hydrogen-based iron oxide reduction in the temperature range of 550 to 600°C and promotes iron oxide reduction at 700°C or higher, thereby suppressing reduction disintegration and ensuring the strength of the raw material for reduction. This raw material for reduction is a pellet with a two-layer structure consisting of a porous body and a coating layer. The porous body portion contains bentonite in an amount of 0.1 to 10.0 parts by mass relative to the Fe2O3 content. The coating layer contains bentonite in an amount of 0.1 to 10.0 parts by mass relative to the total of the Ca compounds and Fe compounds contained therein.

[0005] Patent Document 2 proposes the use of smectite clay pretreated with a dispersant to improve the strength of pellets. The smectite clay contains bentonite, and examples of the amount of smectite clay blended per 1 MT (megaton) of pellet-forming particles are about 0.2 to about 1.0 kg, about 0.4 to about 0.8 kg, or about 0.4 to about 0.7 kg. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-119910 [Patent Document 2] Special Publication No. 2021-507116 Summary of the Invention [Problem to be solved by the invention]

[0007] 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.

[0008] 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.

[0009] 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.

[0010] Because the melting process using SAF requires a significant amount of energy (electricity), it is necessary to improve energy efficiency even if only slightly.However, until now, no appropriate operational guidelines have been established for achieving high energy efficiency in the molten iron production process, which involves producing iron ore pellets from low-grade iron ore, reducing these in a solid reduction furnace to produce reduced iron, and then melting this in an SAF furnace.

[0011] In Patent Document 1, iron ore pellets after sintering are prone to pulverization in a solid reduction furnace, and the aim is to suppress this reduction pulverization and ensure the strength of the sintered pellets. Patent Document 2 also aims to improve the strength of the pellets. Both of these methods only consider the properties of the pellets, and do not consider the realization of high energy efficiency in the series of molten iron production processes described above.

[0012] 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]

[0013] As a result of intensive research aimed at solving the above-mentioned problems, the inventors have discovered that energy efficiency in the melting process can be improved by keeping the ratio of the amount of slag to molten pig iron accumulated in a submerged arc furnace within a predetermined range during the melting process. Specifically, a method for producing molten pig iron according to the present invention, which advantageously solves the above-mentioned problems, comprises a melting process in which an iron-source raw material containing reduced iron is melted in a submerged arc furnace to obtain molten pig iron, and the melting process includes at least one or both of adding a slag former to adjust the basicity (CaO / SiO2) of the molten slag formed on the molten pig iron and adding a carbonaceous material to adjust the C content of the molten pig iron, and is characterized in that the slag ratio S / P, expressed by the following formula (1), is in the range of 0.15 to 2.0 t / tp, where the basicity (CaO / SiO2) of the molten slag refers to the mass fraction of CaO to SiO2. S / P={S (DRI) +S (SCR) +S (FLUX)} / {P (DRI) +P (SCR)}(1) where: S (DRI) : Amount of slag derived from reduced iron in the submerged arc furnace (t), S (SCR) : Amount of slag derived from iron source materials other than reduced iron in the submerged arc furnace (t), S (FLUX) : Amount of slag derived from added slag formers and added carbonaceous materials in the submerged arc furnace (t), P (DRI) : amount of molten iron derived from reduced iron in the submerged arc furnace (t), P (SCR) : Amount of hot metal derived from iron source materials other than reduced iron in the submerged arc furnace (t) is.

[0014] The method for producing molten iron according to the present invention comprises the steps of: (a) further comprising a step of preparing raw materials for direct reduction, the step including an optional step of preparing iron ore pellets produced from low-grade iron ore having a total Fe content of 63% by mass or less, and a step of producing reduced iron from the raw materials for direct reduction, (b) the raw material for direct reduction used in the reduced iron production process includes lump ore; (c) intentionally setting one or more of the timing and amount of addition of the reduced iron, iron source raw materials other than the reduced iron, and additive slag formers, and the timing and amount of discharge of the molten iron and slag from the submerged arc furnace so that the slag ratio S / P is in the range of 0.15 to 2.0; (d) operating the furnace so that the slag ratio S / P is in the range of 0.25 to 0.9; (e) adding the slag former so that the basicity (CaO / SiO2) of the molten slag is in the range of 1.0 to 1.3; (f) 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]

[0015] 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]

[0016] [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 slag ratio S / P in a submerged arc furnace and the power consumption rate. [Figure 3] 1 is a graph showing the relationship between the slag ratio S / P in a submerged arc furnace and power efficiency. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] (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, heating and melting the raw materials by resistance heating.

[0019] 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.

[0020] The ratio of the mass of slag S to the unit mass of molten iron P is defined as the slag ratio S / P (t / tp), which is expressed by the following equation (1). S / P={S (DRI) +S (SCR) +S (FLUX)} / {P (DRI) +P (SCR)}(1) where: S (DRI) : Amount of slag derived from reduced iron in the submerged arc furnace (t), S (SCR) : Amount of slag derived from iron source materials other than reduced iron in the submerged arc furnace (t), S (FLUX) : Amount of slag derived from added slag formers and added carbonaceous materials in the submerged arc furnace (t), P(DRI) : amount of molten iron derived from reduced iron in the submerged arc furnace (t), P (SCR) : Amount of hot metal derived from iron source materials other than reduced iron in the submerged arc furnace (t) is.

[0021] A slag ratio S / P in the range of 0.15 to 2.0 t / tp can reduce the power consumption rate and improve power efficiency. Here, power consumption rate refers to the power required to produce a unit mass of molten pig iron, and power efficiency refers to the percentage ratio of the energy used to produce molten pig iron, i.e., melting and heating the iron source materials and slag, and reducing iron oxide, to the input power. A slag ratio S / P in the range of 0.25 to 0.9 t / tp is preferred. If the slag ratio S / P is below the lower limit, the slag thickness is too thin, causing the heat from the arc to radiate outside the furnace, increasing the power consumption rate and reducing power efficiency. If the slag ratio S / P exceeds the upper limit, power is consumed for the sensible heat of heating the slag and for melting the slag, increasing the power consumption rate for molten pig iron production and reducing power efficiency. One of the objectives of the present invention is to increase the use of low-grade iron ore. Therefore, if only low-grade iron ore is used and the components are not diluted with concentrate ore or pellet feed ore, the slag ratio S / P may exceed 0.6 t / tp.

[0022] 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%.

[0023] As for the slag formers, limestone (CaCO3) or quicklime (CaO) is preferably used as a CaO source, and silica stone (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 it is in 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 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 power consumption rate and power efficiency per unit mass of molten iron were investigated using a 4- to 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 and the composition of blast furnace slag as an example.

[0030] [Table 1]

[0031] Reduced iron equivalent to 500 kg of molten pig iron and a specified amount of blast furnace slag were charged into the 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 had 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 conditions and results for the additional charge. The slag basicity was 1.26, and the carbon content of the molten pig iron was adjusted to 2.5–4.5% by mass. The input power was constant at 2.7 MW.

[0032] [Table 2]

[0033] Figure 2 shows the relationship between slag ratio S / P and power consumption. Figure 3 shows the relationship between slag ratio S / P and power efficiency. These results show that when the slag ratio S / P is in the range of 0.15 to 2.0 t / tp, the power consumption can be kept low and power efficiency can be maintained at a high level. Furthermore, when the slag ratio is in the range of 0.25 to 0.9 t / tp, the power consumption can be further reduced and power efficiency can be further increased. [Explanation of symbols]

[0034] 1 Submerged Arc Furnace (SAF) 2 Furnace wall 3 electrodes 4 (Input) Raw materials P hot metal S Molten slag

Claims

1. The iron source material containing reduced iron is melted in a submerged arc furnace to obtain molten iron, and the basicity (CaO / SiO ２ and adding a carbonaceous material to adjust the C content of the molten pig iron, and the operation is carried out so that the slag ratio S / P represented by the following formula (1) falls within the range of 0.44 to 0.90 t / t-p. Here, the basicity of the molten slag (CaO / SiO ２ ) is the SiO of CaO ２ A method for producing molten iron, which refers to the mass fraction of S / P={S （ＤＲＩ） +S （ＳＣＲ） +S （ＦＬＵＸ） } / {P （ＤＲＩ） +P （ＳＣＲ） }(1) where: S （ＤＲＩ） : Amount of slag derived from reduced iron in the submerged arc furnace (t), S （ＳＣＲ） : Amount of slag derived from iron source materials other than reduced iron in the submerged arc furnace (t), S （ＦＬＵＸ） : Amount of slag derived from added slag formers and added carbonaceous materials in the submerged arc furnace (t), P （ＤＲＩ） : amount of molten iron derived from reduced iron in the submerged arc furnace (t), P （ＳＣＲ） : amount of molten iron derived from iron source materials other than reduced iron in a submerged arc furnace (t) is.

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. 4. The method for producing molten iron according to claim 1, wherein one or more selected from the timing and amounts of addition of the reduced iron, iron source raw materials other than the reduced iron, and an additional slag former, and the timing and amounts of discharge of the molten iron and slag from a submerged arc furnace are intentionally set so that the slag ratio S / P is in a range of 0.44 to 0.

90.

5. 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.

6. 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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