Method for producing molten iron using solid reduction furnace and submerged arc furnace

Abstract

Provided is a method for producing molten iron, whereby high energy efficiency can be achieved in a melting step when producing reduced iron from iron ore in a solid reduction furnace and producing molten iron by melting the reduced iron in an SAF. This method for producing molten iron comprises: a step for producing first reduced iron from low-grade iron ore pellets; a step for producing second reduced iron from high-grade iron ore pellets; a step for producing third reduced iron from lump ore; a step for preparing fourth reduced iron; a step for adjusting the alkalinity by adding a slag-making material; and satisfies the following formula. 150.0 < = S1 * W1 + S2 * W2 + S3 < = 400.0, where S1 is the slag ratio of the first reduced iron, W1 is the blending rate of the first reduced iron, S2 is the average slag ratio of the second to fourth reduced iron, W2 is the total blending rate of the second to fourth reduced iron, and S3 is the amount of slag forming material added in the melting step.

Description

Technical Field

[0001] The present invention relates to a method for producing molten iron using a solid reduction furnace and a submerged arc furnace. Background Art

[0002] In recent years, the concept of reducing CO2 emissions has attracted much attention in the steel industry, which has a large CO2 emission, as a method for producing pig iron that does not rely on the blast furnace method in order to reduce the environmental load. In the DR method, iron ore raw materials such as iron ore pellets and lump ore are first gas-reduced in a solid state in a solid reduction furnace (vertical furnace) to produce direct reduced iron (DRI, hereinafter also referred to as "reduced iron"). The reduced iron is melted in a melting furnace such as an electric arc furnace (EAF) and a submerged arc furnace (SAF), and molten iron is obtained in the state of separating pig iron slag (molten slag).

[0003] Iron ore pellets used as an iron ore raw material in the DR method are produced by mixing powdered ore with a binder and auxiliary materials, granulating the mixture into green pellets, and then firing the green pellets. Bentonite is often used as the binder.

[0004] Patent Document 1 proposes a double-layer pellet comprising a porous body made of an iron ore raw material and a coating layer covering the porous body. The coating layer comprises a Ca compound, an Fe compound, and 0.1 to 10.0 parts by mass of bentonite based on the total of the Ca compound and the Fe compound.

[0005] Patent Document 2 proposes to use montmorillonite clay pretreated with a dispersant to improve the strength of pellets. Montmorillonite clay includes bentonite, and Patent Document 2 lists as examples the amount of montmorillonite clay blended per 1 MT (million tons) of pellet-forming particles of about 0.2 to about 1.0 kg, or about 0.4 to about 0.8 kg, or about 0.4 to about 0.7 kg.

[0006] Prior art literature

[0007] Patent Literature

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

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

[0010] However, in the past, in the DR method, an electric arc furnace (EAF) was mainly used as a device for melting reduced iron. In the EAF, due to its equipment limitations, high-quality reduced iron with a low slag ratio (high Fe content) is required. Therefore, high-grade iron ore must be used as the raw material of the reduced iron (i.e., the raw material ore of iron ore pellets). Examples of high-grade iron ore include South American ore, concentrate that has been previously subjected to beneficiation treatment to improve its grade, and pellet feed ore.

[0011] However, there are problems in using South American ores, concentrates, and pellet feed ores in Japan. For South American ores, the freight rate is bound to rise due to Japan's geographical conditions. For concentrates and pellet feed ores, since the efficiency of beneficiating high-grade iron ore is better than that of beneficiating low-grade iron ore, most of them are basically high-grade iron ore, and the freight rate rises for the same reason. Therefore, there are cost issues in the direct reduction process using high-grade iron ore.

[0012] Therefore, in Japan, the application of low-grade iron ore produced in Australia, India and other countries to the DR process is being studied. In the current EAF, it is difficult to melt the reduced iron produced using low-grade iron ore as a raw material. Therefore, the use of a submerged arc furnace (SAF) is being studied.

[0013] Here, a very large amount of energy (electricity) is required in the melting process using SAF, so it is required to improve energy efficiency as much as possible. However, in the conventional process of manufacturing molten iron, which uses low-grade iron ore as a raw material to manufacture iron ore pellets, reduces the pellets in a solid reduction furnace to manufacture reduced iron, and melts the pellets in SAF, no optimal raw material design guide for achieving high energy efficiency has been obtained.

[0014] In Patent Document 1, the fired iron ore pellets are prone to pulverization in the solid reduction furnace, so the purpose is to suppress the reduction pulverization and ensure the strength of the fired pellets. Patent Document 2 also aims to improve the strength of the pellets. Both Patent Documents 1 and 2 only consider the properties of the pellets, and do not consider the point of achieving high energy efficiency in the above series of molten iron manufacturing processes.

[0015] Therefore, in view of the above problems, the present invention aims to provide a method for producing molten iron, which can achieve high energy efficiency in the melting process when producing reduced iron from iron ore in a solid reduction furnace and melting the reduced iron in a submerged arc furnace to produce molten iron.

[0016] The present inventors have conducted intensive studies to solve the above-mentioned problems and have found that the energy efficiency in the melting step is improved by setting the total slag ratio in the melting step to be within a predetermined range.

[0017] That is, the gist of the present invention is as follows.

[0018] [1] A method for producing molten iron, characterized by comprising:

[0019] The iron ore preparation step (A) includes: a step (A-1) of preparing a first iron ore pellet produced by using a low-grade iron ore having a total Fe content of 63% by mass or less as a raw material, an arbitrary step (A-2) of preparing a second iron ore pellet produced by using a high-grade iron ore having a total Fe content of more than 63% by mass as a raw material, and an arbitrary step (A-3) of preparing lump ore;

[0020] The reduced iron preparation step (B) includes: a step (B-1) of producing the first reduced iron from the first iron ore pellets, an arbitrary step (B-2) of producing the second reduced iron from the second iron ore pellets, an arbitrary step (B-3) of producing the third reduced iron from the lump ore, and an arbitrary step (B-4) of preparing the fourth reduced iron produced in advance; and

[0021] A melting step (C) of melting the first reduced iron, and any of the second reduced iron, the third reduced iron, and the fourth reduced iron in a submerged arc furnace to obtain molten iron, and adding a slag-making material to adjust the basicity of the molten slag formed on the molten iron;

[0022] The method for producing molten iron satisfies the following formula (1).

[0023] 150.0≤S1×W1+S2×W2+S3≤400.0···(1)

[0024] in,

[0025] S1: Slag ratio of the first reduced iron (kg / t)

[0026] W1: The blending ratio of the first reduced iron (-)

[0027] S2: Average slag ratio of the second to fourth reduced iron (kg / t)

[0028] W2: Total blending ratio of the second to fourth reduced iron (-)

[0029] S3: The amount (kg / t) of the slag-forming material added in the melting step (C).

[0030] [2] The method for producing molten iron according to [1], wherein one or more selected from S1, W1, S2, W2 and S3 is intentionally set to satisfy the above formula (1).

[0031] [3] The method for producing molten iron according to [2], wherein one or both of S1 and S3 are intentionally set so as to satisfy the above formula (1).

[0032] [4] A method for producing molten iron according to [2] or [3], wherein S1 is set by setting one or more of the composition of the low-grade iron ore used in the step (A-1) and the types and proportions of the binder and auxiliary materials added in the step (A-1).

[0033] [5] The method for producing molten iron according to [1], wherein the following formula (2) is satisfied.

[0034] 250.0≤S1×W1+S2×W2+S3≤350.0···(2)

[0035] [6] The method for producing molten iron according to [5], wherein one or more selected from S1, W1, S2, W2 and S3 is intentionally set to satisfy the above formula (2).

[0036] [7] The method for producing molten iron according to [6], wherein one or both of S1 and S3 are intentionally set so as to satisfy the above formula (2).

[0037] [8] A method for producing molten iron according to [6] or [7], wherein S1 is set by setting one or more of the composition of the low-grade iron ore used in the above-mentioned step (A-1), and the types and proportions of the binder and auxiliary materials added in the above-mentioned step (A-1).

[0038] [9] The method for producing molten iron according to any one of [1] to [8], wherein the slag-making material is added in such a manner that the basicity (CaO / SiO2) of the molten slag is 1.0 to 1.3.

[0039] According to the method for producing molten iron of the present invention, when reducing iron is produced from iron ore in a solid reduction furnace and the reduced iron is melted in a submerged arc furnace to produce molten iron, high energy efficiency can be achieved in the melting step. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a graph showing the relationship between the total slag ratio in the furnace and the power efficiency in the melting step. DETAILED DESCRIPTION

[0041] Hereinafter, an embodiment of the method for producing molten iron using a solid reduction furnace and a submerged arc furnace of the present invention will be described. It should be noted that the embodiment described below is an example of the present invention being embodied, and the configuration of the present invention is not limited to this specific example.

[0042] The method for producing molten iron using a solid reduction furnace and a submerged arc furnace according to one embodiment of the present invention comprises: an iron ore preparation step (A), comprising: a step (A-1) of preparing a first iron ore pellet produced by using a low-grade iron ore having a total Fe content of 63% or less as a raw material, an arbitrary step (A-2) of preparing a second iron ore pellet produced by using a high-grade iron ore having a total Fe content of more than 63% as a raw material, and an arbitrary step (A-3) of preparing a lump ore; a reduced iron preparation step (B), comprising: preparing a first iron ore pellet from the first iron ore pellet; The invention further comprises a step (B-1) of producing a first reduced iron, an arbitrary step (B-2) of producing a second reduced iron from the second iron ore pellets, an arbitrary step (B-3) of producing a third reduced iron from the lump ore, and an arbitrary step (B-4) of preparing a fourth reduced iron produced in advance; and a melting step (C) of melting the first reduced iron, and any of the second reduced iron, the third reduced iron, and the fourth reduced iron in a submerged arc furnace to obtain molten iron, and adding a slag-making material to adjust the basicity of the molten slag formed on the molten iron. Furthermore, the invention is characterized in that the following formula (1) is satisfied.

[0043] 150.0≤S1×W1+S2×W2+S3≤400.0···(1)

[0044] in,

[0045] S1: Slag ratio of the first reduced iron (kg / t)

[0046] W1: The blending ratio of the first reduced iron (-)

[0047] S2: Average slag ratio of the second to fourth reduced iron (kg / t)

[0048] W2: Total blending ratio of the second to fourth reduced iron (-)

[0049] S3: The amount (kg / t) of the slag-forming material added in the melting step (C).

[0050] The raw materials of iron ore pellets are usually composed of iron ore, a binder and auxiliary materials. In the present invention, low-grade iron ore refers to iron ore with a total Fe content (hereinafter referred to as T.Fe) of 63% by mass or less. In addition, in the present invention, high-grade iron ore refers to iron ore with T.Fe exceeding 63% by mass. Low-grade iron ore preferably has a crystal water content of 4% or more by mass.

[0051] The iron ore pellets made from low-grade iron ore as raw material are referred to as first iron ore pellets, and the iron ore pellets made from high-grade iron ore as raw material are referred to as second iron ore pellets. The iron ore pellets used in this embodiment include at least the first iron ore pellets. When the first iron ore pellets are not included, the weight of the slag described later is insufficient, and the energy efficiency is reduced. The iron ore pellets used in this embodiment may all be the first iron ore pellets, but may also include the second iron ore pellets.

[0052] Bentonite is preferred as a binder for iron ore pellets, but any known or arbitrary binder such as an organic or inorganic binder that can achieve the same effect may be used. In addition, quicklime, limestone, dolomite, etc. may be mixed as auxiliary raw materials.

[0053] The first and second iron ore pellets can be prepared by manufacturing through general crushing, mixing, granulation, and calcining processes, or pre-manufactured iron ore pellets can be prepared. When manufacturing iron ore pellets, each process can be carried out using the following conventionally known devices and conditions. The crushing process can be carried out using a general ball mill or other crusher. The mixing process can be carried out using a general high-speed stirring mixer, concrete mixer, etc. The granulation process can be carried out using a general granulator, drum mixer, etc. The calcining process can be carried out using a general rotary kiln, electric furnace, etc.

[0054] As a raw material for reducing iron, in addition to iron ore pellets, lump ore can also be prepared. Lump ore is generally iron ore with a size of about 10 to 35 mm, and is used in the reduction step without being crushed.

[0055] In the reduced iron preparation step (B), reduced iron is manufactured from the first iron ore pellets as the required raw material, the second iron ore pellets as the optional raw materials, and the lump ore. In the manufacture of reduced iron, a general solid reduction furnace such as a vertical furnace can be used. There is no particular limitation on the reducing gas, but in accordance with the manufacturing method used, for example, a mixed gas containing H2: 55% and CO: 35% by volume, and the remainder consisting of CO2 and CH4, or a mixed gas containing H2: 75% and CO: 20% by volume, and the remainder consisting of CO2 and N2 can be used. The reduced iron obtained from the first iron ore pellets, the second iron ore pellets, and the lump ore are referred to as the first, second, and third reduced iron, respectively. In addition, in addition to the reduced iron obtained as described above, reduced iron manufactured in advance can be prepared, which is referred to as the fourth reduced iron.

[0056] In the melting process, the reduced iron prepared in the above process is melted using SAF, and separated into molten iron and molten slag formed on the molten iron. The furnace diameter of SAF is preferably 5 to 25 m. For example, SAF with a production capacity of 50 t / h belongs to the above. When the furnace diameter of SAF is 5 to 25 m, the maintenance of the furnace and the floor space of the equipment are suitable.

[0057] In addition, in the melting process, slag-forming materials are added to adjust the basicity of the molten slag formed on the molten iron. The basicity of the molten slag is calculated by the weight ratio of CaO / SiO2. The basicity of the molten slag is preferably 1.0 to 1.3. When the basicity of the molten slag is 1.0 to 1.3, it is suitable when the molten slag is solidified, crushed and reused as a roadbed material such as cement. The slag-forming materials added to adjust the basicity of the molten slag are preferably, for example, limestone (CaCO3) and quicklime (CaO) as the CaO source, and silica (SiO2) as the SiO2 source.

[0058] The total value (kg) of CaO, SiO2, Al2O3, and MgO contained in 1000kg of the total Fe content of the first reduced iron is defined as the slag ratio S1 (kg / t). It should be noted that when multiple types of low-grade iron ore are used as raw materials, the average slag ratio corresponding to the blending ratio of each type is defined. In addition, the blending ratio of the first reduced iron to the total amount of molten reduced iron is defined as W1.

[0059] The weighted average of the total value (kg) of CaO, SiO2, Al2O3, and MgO contained in 1000kg of the total Fe content of the second reduced iron, the total value (kg) of CaO, SiO2, Al2O3, and MgO contained in 1000kg of the total Fe content of the third reduced iron, and the total value (kg) of CaO, SiO2, Al2O3, and MgO contained in 1000kg of the total Fe content of the fourth reduced iron is set as the average slag ratio S2 (kg / t). In addition, the total blending ratio of the second to fourth reduced irons to the total amount of molten reduced iron is set as W2. It should be noted that,

[0060] W2=1-W1

[0061] The relationship is established.

[0062] The total amount of Fe in the molten iron in the melting process is represented by the total value (kg) of CaO, SiO2, Al2O3, and MgO contained in 1000 kg of the added slag-making material, which is S3 (kg / t).

[0063] S1 and S2 are obtained from the total value of CaO, SiO2, Al2O3, and MgO contained in the raw materials of each reduced iron. The amount of each oxide can be measured by a conventionally known method such as titration, atomic absorption spectrometry, and fluorescent X-ray method.

[0064] In this embodiment, it is important that S1, W1, S2, W2, and S3 satisfy the following formula (1). Here, "S1×W1+S2×W2+S3" is the total slag ratio per 1000 kg of the total Fe content of molten iron in the melting process, hereinafter referred to as the "total slag ratio".

[0065] 150.0≤S1×W1+S2×W2+S3≤400.0···(1)

[0066] If the above formula (1) is satisfied, high energy efficiency can be obtained in the melting process. When the above formula (1) is not satisfied and the total slag ratio exceeds 400.0 kg / t, the slag is excessive and a large amount of energy is required for electric heating, so the energy efficiency in the melting process is reduced. When the above formula (1) is not satisfied and the total slag ratio is less than 150.0 kg / t, the slag is insufficient, and there is a possibility that the electrode passes through the layer of molten slag and is immersed in the molten steel or the electrode does not reach the layer of molten slag. When the electrode is immersed in the molten steel, the power supply to the layer of molten slag is reduced, so the efficiency of heating using electricity is reduced. Furthermore, when the electrode immersed in the molten steel is a self-burning electrode, it is possible to melt and be damaged. On the other hand, when the electrode does not reach the layer of molten slag, arc discharge and the like occur, and the use of electricity becomes unstable.

[0067] It is preferred to intentionally set one or more selected from S1, W1, S2, W2 and S3 in a manner that satisfies the above formula (1). In addition, it is more preferred to intentionally set one or both of S1 and S3 in a manner that satisfies the above formula (1). This is because, for lump ore and pre-manufactured reduced iron, there is a limit on the amount of use due to the market circulation and the amount, so S2 and W2 have low freedom in actual operation, and W1 (=1-W2) ultimately has low freedom in actual operation.

[0068] S1, W1, S2, W2, and S3 preferably satisfy the following formula (2).

[0069] 250.0≤S1×W1+S2×W2+S3≤350.0···(2)

[0070] If the above formula (2) is satisfied, the energy efficiency is further improved.

[0071] It is preferred that at least one selected from S1, W1, S2, W2 and S3 is intentionally set so as to satisfy the above formula (2). Furthermore, as described above, it is more preferred that one or both of S1 and S3 is intentionally set so as to satisfy the above formula (2).

[0072] The setting of S1 is preferably carried out by setting one or more selected from the composition of the low-grade iron ore used in step (A-1), and the types and proportions of the binder and auxiliary materials added in step (A-1). The setting of S2 is preferably carried out by setting one or more selected from the composition of the high-grade iron ore used in step (A-1), and the types and proportions of the binder added in step (A-1), the composition of the lump ore, and the composition of the purchased reduced iron.

[0073] Example

[0074] As raw materials for iron ore pellets, ores A to D (low-grade iron ores) and ore H (high-grade iron ore) shown in Table 1 were prepared. Table 1 shows the composition (mass %) of each iron ore. As a binder, bentonite containing 3% CaO, 60% SiO2, 15% Al2O3, and 3% MgO by mass was prepared. As an auxiliary raw material, limestone containing 53% CaO, less than 1% SiO2, less than 1% Al2O3, and 1% MgO by mass was prepared.

[0075] [Table 1] (mass %)

[0076]

[0077] Iron ore pellets were produced from various iron ores A to D and H. First, 300 kg of various iron ores were prepared and crushed in a ball mill to obtain various iron ore powders. Bentonite was added to the various iron ore powders in an amount (mass %) as shown in Table 2 relative to the amount of iron ore powder, and limestone was further added in such a manner that the basicity was 0.2. The mixture was mixed at 20 rpm for 3 minutes using a concrete mixer. Next, the mixed raw materials were placed in a The pellets were granulated in a granulator while adding water. Pellets of 9.5 to 12 mm were collected and further rotated in the granulator for 10 minutes to obtain green pellets. The green pellets were kept in an electric furnace at 1200 to 1350° C. for 25 minutes to produce iron ore pellets.

[0078] [Table 2]

[0079]

[0080] The iron ore pellets obtained above were charged into an electric furnace in the proportions shown in Table 2, and a gas having a volume ratio of CO:H2:CO2:N2=8:40:7:45 was flowed at 400-850°C for 340 minutes to obtain reduced iron. The reduced iron obtained by reducing the iron ore pellets using low-grade ores A to D as raw materials was respectively referred to as "first reduced iron", and the reduced iron obtained by reducing the iron ore pellets using high-grade ore H as raw material was referred to as "second reduced iron". Table 2 shows the blending ratio of the iron ore pellets as the raw material of the reduced iron, the slag ratio S1 of the first reduced iron, and the slag ratio S2 of the second reduced iron.

[0081] The obtained reduced iron was charged into SAF and melted. CaO was added as slag-making material so that the basicity of the slag formed during melting, i.e., the weight ratio of CaO / SiO2, was 1.2. A 100 kg furnace for testing was used for SAF. Table 3 shows W1, W2, S1, S2, S3, and the total slag ratio.

[0082] [Table 3]

[0083]

[0084] [Current efficiency measurement]

[0085] The reduced iron was heated to 1700°C and melted in SAF, and the consumed electric energy was confirmed. The ratio of the calculated energy required for theoretical heating and melting to the actually input electric energy was calculated as the electric efficiency, and the results are shown in Table 3. In operation, the electric efficiency is preferably 50% or more, and more preferably 70% or more.

[0086] Figure 1 The relationship between the total slag ratio in the SAF and the power efficiency of each example is shown. It can be confirmed that by adjusting the total slag ratio in the SAF to within the range of the above formula (1) of the present invention, a high current efficiency of 50% or more can be obtained. It can be further known that when the range of the slag ratio is particularly within the range of the above formula (2), the current efficiency is 70% or more, which is more preferred. On the other hand, in the comparative example where the slag ratio is outside the range of the above formula (1), the power efficiency is less than 50%, indicating that the operating efficiency is low. In summary, the effect of the present invention is obvious.

[0087] Industrial Applicability

[0088] According to the method for producing molten iron of the present invention, when reducing iron is produced from iron ore in a solid reduction furnace and the reduced iron is melted in a submerged arc furnace to produce molten iron, high energy efficiency can be achieved in the melting step.

Claims

1. A method for producing molten iron, characterized in that: have: The iron ore preparation step (A) includes: a step (A-1) of preparing a first iron ore pellet produced by using a low-grade iron ore having a total Fe content of 63% by mass or less as a raw material, an arbitrary step (A-2) of preparing a second iron ore pellet produced by using a high-grade iron ore having a total Fe content of more than 63% by mass as a raw material, and an arbitrary step (A-3) of preparing lump ore; The reduced iron preparation step (B) includes: a step (B-1) of producing first reduced iron from the first iron ore pellets, an arbitrary step (B-2) of producing second reduced iron from the second iron ore pellets, an arbitrary step (B-3) of producing third reduced iron from the lump ore, and an arbitrary step (B-4) of preparing fourth reduced iron produced in advance; and A melting step (C) of melting the first reduced iron, and any of the second reduced iron, the third reduced iron, and the fourth reduced iron in a submerged arc furnace to obtain molten iron, and adding a slag-forming material to adjust the basicity of the molten slag formed on the molten iron; The method for producing molten iron satisfies the following formula (1): 150.0≤S1×W1+S2×W2+S3≤400.0···(1) in, S1: slag ratio of the first reduced iron (kg / t), W1: the first reduced iron ratio (-), S2: average slag ratio of the second to fourth reduced iron (kg / t), W2: the total blending ratio of the second to fourth reduced iron (-), S3: The amount (kg / t) of the slag-forming material added in the melting step (C).

2. The method for producing molten iron according to claim 1, wherein: One or more selected from S1, W1, S2, W2 and S3 are intentionally set so as to satisfy the above-mentioned formula (1).

3. The method for producing molten iron according to claim 2, wherein: One or both of S1 and S3 are intentionally set so as to satisfy the above-mentioned formula (1).

4. The method for producing molten iron according to claim 2 or 3, wherein: S1 is set by setting one or more selected from the composition of the low-grade iron ore used in the step (A-1), and the types and blending ratios of the binder and auxiliary materials added in the step (A-1).

5. The method for producing molten iron according to claim 1, wherein: The following formula (2) is satisfied: 250.0≤S1×W1+S2×W2+S3≤350.0···(2).

6. The method for producing molten iron according to claim 5, wherein: One or more selected from S1, W1, S2, W2 and S3 are intentionally set so as to satisfy the above-mentioned formula (2).

7. The method for producing molten iron according to claim 6, wherein: One or both of S1 and S3 are intentionally set so as to satisfy the above-mentioned formula (2).

8. The method for producing molten iron according to claim 6 or 7, wherein: S1 is set by setting one or more selected from the composition of the low-grade iron ore used in the step (A-1), and the types and blending ratios of the binder and auxiliary materials added in the step (A-1).

9. The method for producing molten iron according to any one of claims 1 to 8, wherein: The slag-forming material is added so that the basicity of the molten slag, i.e., CaO / SiO2, is 1.0 to 1.3.

Images