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 is provided with a melting step for obtaining molten pig iron (P) by melting an iron source material containing reduced iron in a submerged arc furnace (1), the melting step being operated so as to satisfy the slag ratio S / P, which is the slag mass per unit mass of the molten pig iron, within the range of 0.15-2.0 t / t-p, and the slag ratio S / P being the slag mass per unit mass of the molten pig iron. The method further includes adding a slag forming material to adjust the alkalinity (CaO / SiO2) of molten slag (S) formed on the molten pig iron (P), and adding a carbon material to adjust the C content of the molten pig iron (P), where the alkalinity (CaO / SiO2) of molten slag refers to the mass fraction of CaO with respect to SiO2, and at least one or both of the following: adding a slag forming material to adjust the alkalinity (CaO / SiO2) of molten slag (S) formed on the molten pig iron (P), and adding a carbon material to adjust the C content of the molten pig iron (P).

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 Document 1 discloses a reduction raw material that suppresses reduction pulverization and ensures the strength of the reduction raw material by inhibiting the reduction of hydrogen-based iron oxide in the temperature range of 550-600°C and allowing the reduction of iron oxide to occur above 700°C. This reduction raw material is a pellet with a two-layer structure having a porous body and a coating. The porous body contains bentonite at a concentration of 0.1 parts by mass to 10.0 parts by mass relative to its Fe₂O₃ content. The coating contains bentonite at a concentration of 0.1 parts by mass to 10.0 parts by mass relative to the total amount of Ca and Fe compounds contained therein.

[0005] Patent document 2 proposes an effective utilization method for montmorillonite clay pretreated with a dispersant to improve the strength of pellets. The montmorillonite clay contains bentonite, and as an example, the amount of montmorillonite clay incorporated is approximately 0.2 to 1.0 kg, or approximately 0.4 to 0.8 kg, or approximately 0.4 to 0.7 kg per 1 MT (megaton) of pellet-forming particles.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2017-119910

[0009] Patent Document 2: Japanese Patent Publication No. 2021-507116 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] 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.

[0012] 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.

[0013] 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.

[0014] Here, since the smelting process using SAF requires a great deal of energy (electricity), it is necessary to maximize energy efficiency. However, in the past, in the series of molten pig iron manufacturing processes that use low-grade iron ore as raw material to manufacture iron ore pellets, reduce them in a solid reduction furnace to produce reduced iron, and then melt them in an SAF furnace, appropriate operating guidelines for achieving high energy efficiency have not been established.

[0015] Because calcined iron ore pellets are prone to pulverization in solid reduction furnaces, Patent Document 1 aims to suppress this reduction pulverization and ensure the strength of the calcined pellets. Patent Document 2 also aims to improve the strength of the pellets. Both only consider the properties of the pellets and do not consider achieving high energy efficiency in the aforementioned series of molten pig iron manufacturing processes.

[0016] 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.

[0017] Problem Solving Methods

[0018] To solve the above problems, the inventors conducted in-depth research and obtained the following insights: by making the ratio of slag remaining in the submerged arc furnace to molten pig iron within a given range in the melting process, the energy efficiency in the melting process is increased. That is, the molten pig iron manufacturing method of the present invention, which advantageously solves the above problems, has a melting process and is operated in such a way that the slag ratio S / P shown in 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 is the mass fraction of CaO relative to SiO2. The melting process includes melting an iron source raw material containing reduced iron in a submerged arc furnace to obtain molten pig iron, and further includes 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.

[0019] S / P={S (DRI) +S (SCR) +S (FLUX)} / {P (DRI) +P (SCR)}(1)

[0020] In the formula,

[0021] S (DRI) : The amount of slag (t) originating from the reduced iron produced in the submerged arc furnace.

[0022] S (SCR) The amount of slag (t) originating from iron source materials other than reduced iron in the submerged arc furnace.

[0023] S (FLUX) The amount of slag (t) originating from the addition of slag-forming materials and carbon materials in the submerged arc furnace.

[0024] P (DRI) : The amount of molten pig iron (t) derived from reduced iron in the submerged arc furnace.

[0025] P (SCR) : The amount of molten pig iron (t) derived from iron source raw materials other than reduced iron in the submerged arc furnace.

[0026] 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 (f):

[0027] (a) Further includes the following steps:

[0028] 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

[0029] The reduced iron manufacturing process includes any step of manufacturing the reduced iron from the aforementioned raw materials for direct reduction;

[0030] (b) Includes lump ore as the raw material for direct reduction used in the above-mentioned reduced iron manufacturing process;

[0031] (c) Intentionally setting one or more of the following: the timing and amount of adding slag-forming materials selected from the above-mentioned reduced iron, iron source raw materials other than the above-mentioned reduced iron, the timing and amount of discharging molten pig iron and slag in the submerged arc furnace, so that the above-mentioned slag ratio S / P is in the range of 0.15 to 2.0;

[0032] (d) Operate in a manner that satisfies the above-mentioned slag ratio S / P range of 0.25 to 0.9;

[0033] (e) 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;

[0034] (f) 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.

[0035] The effects of the invention

[0036] According to the method for manufacturing molten pig iron of the present invention, high energy efficiency can be obtained when iron source raw materials containing reduced iron are melted in a submerged arc furnace to obtain molten pig iron. Attached Figure Description

[0037] Figure 1 This is a schematic diagram illustrating a method for manufacturing molten pig iron according to one embodiment of the present invention.

[0038] Figure 2 This is a graph showing the relationship between the slag ratio S / P in a submerged arc furnace and the unit power consumption.

[0039] Figure 3 This is a graph showing the relationship between the slag ratio (S / P) and power efficiency in a submerged arc furnace.

[0040] Symbol Explanation

[0041] 1. Submerged Arc Furnace (SAF)

[0042] 2 Furnace wall

[0043] 3 electrodes

[0044] 4 (Input) Raw Materials

[0045] P Molten pig iron

[0046] S Molten Slag Detailed Implementation

[0047] 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.

[0048] (Melting process)

[0049] 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.

[0050] 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.

[0051] The ratio of the mass of slag S to the mass of molten pig iron P is set as the slag ratio S / P(t / tp), which is expressed by the following formula (1).

[0052] S / P={S (DRI) +S (SCR) +S (FLUX)} / {P (DRI) +P (SCR)}(1)

[0053] In the formula,

[0054] S(DRI): The amount of slag (t) originating from reduced iron in the submerged arc furnace.

[0055] S(SCR): The amount of slag (t) originating from iron source materials other than reduced iron in the submerged arc furnace.

[0056] S(FLUX): The amount of slag (t) derived from the addition of slag-forming materials and carbon materials in the submerged arc furnace.

[0057] P(DRI): The amount of molten pig iron (t) derived from reduced iron in the submerged arc furnace.

[0058] P(SCR): The amount of molten pig iron (t) derived from iron source materials other than reduced iron in the submerged arc furnace.

[0059] If the slag ratio (S / P) is in the range of 0.15~2.0 t / tp, the unit power consumption can be reduced, and the power efficiency can be improved. Here, the unit power consumption refers to the electricity required to produce a unit mass of molten pig iron, and the power efficiency refers to the percentage of energy used in the production of molten pig iron (i.e., the production of iron source materials, melting and heating of slag, and reduction of iron oxide) relative to the input electricity. The preferred slag ratio (S / P) is in the range of 0.25~0.9 t / tp. When the slag ratio (S / P) is below the lower limit, the slag thickness is too thin, and the heat of the electric arc is dissipated outside the furnace due to radiation, resulting in an increase in the unit power consumption and a decrease in power efficiency. When the slag ratio (S / P) is above the upper limit, the electricity is consumed in the sensible heat and melting heat of the slag, increasing the unit power consumption for producing molten pig iron and decreasing the power efficiency. One of the objectives of this invention is to increase the amount of low-grade iron ore used. Therefore, it can be assumed that when only low-grade iron ore is used without dilution of the composition using concentrate ore or pellet feed ore, the slag ratio S / P is greater than 0.6t / tp.

[0060] 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%.

[0061] 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 preferred for reuse as a roadbed material such as cement.

[0062] 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, besides carbon, is contained in the sulfur (S) of the carbon material. (FLUX) It remains in the molten slag S. The C content in the molten pig iron P is preferably adjusted to a range of 2-5% 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.

[0063] (Preparation process for raw materials used in direct reduction)

[0064] 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.

[0065] 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.

[0066] 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.

[0067] (Reduced iron manufacturing process)

[0068] 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.

[0069] Example

[0070] like Figure 1As shown, the power consumption and power efficiency per unit mass of molten pig iron were investigated using a 4-6 t batch submerged arc furnace 1. Table 1 shows the composition of the reduced iron used in the investigation and the composition of the blast furnace slag as an example.

[0071] [Table 1]

[0072]

[0073] A submerged arc furnace 1 was charged with 500 kg of reduced iron (based on molten pig iron) and a given amount of blast furnace slag. Electricity was applied to electrode 3 to begin heating. After the start of heating, 400 kg of reduced iron was added every 350 kWh of electricity consumed, repeated eight times. Based on the complete melting of the added 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 as 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, divided by the electricity consumption per unit mass. Table 2 shows the investigation conditions and results of the added charge composition. Adjustments were made with a slag basicity of 1.26 and a C content of 2.5–4.5% by mass in the molten pig iron. The input power was kept constant at 2.7 MW.

[0074] [Table 2]

[0075]

[0076] Figure 2 The diagram shows the relationship between the slag ratio S / P and the unit electricity consumption. Figure 3 The relationship between slag ratio (S / P) and power efficiency is shown. The results indicate that a slag ratio (S / P) of 0.15–2.0 t / tp can maintain low power consumption and high power efficiency. Furthermore, setting the slag ratio to a range of 0.25–0.9 t / tp further reduces power consumption and further increases power efficiency.

Claims

1. A method for producing molten pig iron, comprising a melting step operated in a manner satisfying a slag ratio S / P in the range of 0.15 to 2.0 t / tp as shown in the following formula (1), wherein the melting step comprises melting an iron source material containing reduced iron 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 Carbon materials are added to adjust the C content of the molten pig iron. in, The basicity (CaO / SiO2) of the molten slag is the mass fraction of CaO relative to SiO2. S / P={S (DRI) +S (SCR) +S (FLUX) } / {P (DRI) +P (SCR) }(1) In the formula, S (DRI) : The amount of slag (t) originating from the reduced iron produced in the submerged arc furnace. S (SCR) The amount of slag (t) originating from iron source materials other than reduced iron in the submerged arc furnace. S (FLUX) The amount of slag (t) originating from the addition of slag-forming materials and carbon materials in the submerged arc furnace. P (DRI) : The amount of molten pig iron (t) derived from reduced iron in the submerged arc furnace. P (SCR) : The amount of molten pig iron (t) derived from iron source raw materials other than reduced iron in the submerged arc furnace.

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 timing and amount of adding slag-forming materials selected from the reduced iron, iron source raw materials other than the reduced iron, and the timing and amount of discharging molten pig iron and slag in the submerged arc furnace are intentionally set so that the slag ratio S / P is in the range of 0.15 to 2.

0.

5. The method for manufacturing molten pig iron according to claim 4, wherein, The operation is carried out in a manner that satisfies the slag ratio S / P being in the range of 0.25 to 0.

9.

6. The method for producing molten pig iron according to any one of claims 1 to 5, 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.

7. The method for producing molten pig iron according to any one of claims 1 to 6, 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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