Method for producing molten iron

Abstract

This invention provides a method for producing molten iron that can efficiently dissolve large quantities of solid reduced iron. [Solution] When producing molten iron by melting solid reduced iron in an electric furnace, the solid reduced iron is introduced while a slag of 100 mm or more is present on the molten iron, thereby ensuring time for the solid reduced iron to pass through the slag and allowing the slag to adhere to the surface of the solid reduced iron, making it less likely for the solid reduced iron to stick together.

Description

Technical Field

[0001] The present invention relates to a method for producing molten iron using an electric furnace.

Background Art

[0002] In recent years, in order to achieve carbon neutrality, a method of melting solid reduced iron in an electric furnace to produce molten steel has been widely adopted. On the other hand, solid reduced iron contains components other than metallic iron, such as iron oxide and gangue components, and also contains voids that reduce thermal conductivity, so it is more difficult to melt than scrap. Therefore, various techniques have been proposed to efficiently melt solid reduced iron. Patent Document 1 discloses a method of preheating a cold iron source in a preheating chamber and charging the preheated cold iron source into a melting chamber for melting. Patent Document 2 discloses a method of charging solid reduced iron so that the specific charging rate of the solid reduced iron is less than a predetermined value.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] In recent years, it has been required to further improve the productivity of molten iron, and when continuously charging solid reduced iron, the charging rate of solid reduced iron tends to increase. In the methods described in Patent Documents 1 and 2, when the charging rate of solid reduced iron is increased, the solid reduced irons tend to aggregate, and the surface area and melting rate of the solid reduced iron decrease due to the aggregation, so the productivity of molten iron cannot be sufficiently increased.

[0005] In view of the aforementioned problems, the present invention aims to provide a method for producing molten iron that can efficiently dissolve a large amount of solid reduced iron. [Means for solving the problem]

[0006] The inventors of this invention have diligently investigated methods to prevent solidification, as increasing the rate at which solid reduced iron is introduced causes the molten iron surrounding the solid reduced iron to cool and solidify, making it easier for the solid reduced iron particles to stick together. As a result, they focused on the fact that contact with molten iron while slag is attached to the surface of the solid reduced iron inhibits adhesion between the solid reduced iron and the solidified iron, thereby suppressing the aggregation of the solid reduced iron. They found that by allowing sufficient time for the solid reduced iron and slag to be in contact, a large amount of solid reduced iron can be efficiently dissolved.

[0007] The present invention is as follows: [1] A method for producing molten iron by dissolving solid reduced iron in an electric furnace, A method for producing molten iron, characterized by adding solid reduced iron while a slag of 100 mm or more is present on the molten iron. [2] A method for producing molten iron according to [1] above, characterized by adding the solid reduced iron in such a way that the conditions of the following equation (1) are met. F / S < 1.00 (1) In the formula, F represents the supply rate of solid reduced iron (t / min), and S represents the slag volume (m³) in the electric furnace. 3 ) represents. [3] The method for producing molten iron according to [1] or [2] above, characterized in that the solid reduced iron contains a total of 4.0% by mass or more of Al2O3, SiO2, CaO, and MgO as gangue components. [Effects of the Invention]

[0008] According to the present invention, a large amount of solid reduced iron can be efficiently dissolved. [Brief explanation of the drawing]

[0009] [Figure 1] This is a diagram illustrating the internal structure of an electric furnace. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 is a diagram illustrating the internal structure of an electric furnace. The electric furnace 1 comprises an electrode 2 for heating solid reduced iron and molten iron by an arc, an input section 3 for feeding in solid reduced iron, carbon material, quicklime, etc., and an exhaust section (not shown) for discharging gas and dust generated by melting solid reduced iron. Although Figure 1 shows one electrode 2, there may be two or more electrodes, and the electric furnace may be DC or AC. There may be two or more input sections 3.

[0011] When adding solid reduced iron, first, with the seed molten metal 4 and slag 5 remaining, the electrode 2 is lowered to near the molten iron surface and an arc is generated from the lower end of electrode 2. Then, the solid reduced iron is added from the input section 3 and heated to melt it. Alternatively, the arc may be generated after adding the initial amount of solid reduced iron. In addition, when initially adding solid reduced iron, auxiliary materials such as lime or charcoal may be added to adjust the basicity and improve the reduction efficiency. This causes the iron oxide (FeO) in the solid reduced iron to be reduced by the carbon in the solid reduced iron or molten iron, generating CO gas, which then agitates the molten iron and solid reduced iron. Note that reduction does not necessarily occur after the solid reduced iron has melted; reduction may also occur during the melting of the solid reduced iron. Solid reduced iron is continuously added from the input section 3 while the arc is being generated.

[0012] Then, once the target amount of solid reduced iron is added and completely dissolved, the molten iron, minus the amount of seed molten iron to be used in the next charge, is discharged from a tap (not shown) located at the bottom of the electric furnace. After that, a portion of the slag is discharged from a slag outlet (not shown) as needed.

[0013] Next, we will explain the more detailed conditions for adding solid reduced iron. As mentioned above, when solid reduced iron is added, the molten iron surrounding it cools, and if the solid reduced iron particles are in close proximity, the cooled molten iron solidifies into iron, causing the solid reduced iron particles to stick together. On the other hand, if slag is applied to the surface of the solid reduced iron particles, the adhesion between the solid reduced iron and the solidified iron is inhibited, thus suppressing the sticking of the solid reduced iron particles together.

[0014] However, generally speaking, it is difficult to ensure that the solid reduced iron introduced into the electric furnace is sufficiently coated with slag and in contact with the molten iron. Typically, the slag in the electric furnace is formed to cover the arc in order to improve heat transfer efficiency, and the large amount of gas generated by forming comes into contact with the surface of the solid reduced iron, thus hindering contact with the liquid phase of the slag. In addition, since solid reduced iron contains carbon and oxygen, CO gas is also generated from the solid reduced iron, and the bubbles of CO gas also hinder contact with the liquid phase of the slag.

[0015] Therefore, in this embodiment, in order to ensure sufficient time for slag to adhere to the surface of solid reduced iron in the electric furnace, the solid reduced iron is introduced when there is 100 mm or more of slag on the molten iron. When solid reduced iron is introduced, it descends through the slag and reaches the surface of the molten iron. If the slag thickness is less than 100 mm, there is insufficient time for the solid reduced iron and the slag to come into contact, and the slag cannot adhere sufficiently to the surface of the solid reduced iron. Preferably, the slag thickness is 200 mm or more. On the other hand, if the slag thickness is too large, the wear of electrodes and refractories due to forming will increase, so it is preferable that the slag thickness be 1000 mm or less.

[0016] When slag is forming when charging solid reduced iron, the thickness of the forming slag may be 100 mm or more. However, while continuously charging solid reduced iron, always ensure that the slag thickness is 100 mm or more. If the slag thickness is 100 mm or more, even if the gas phase generated by the forming contacts the surface of the solid reduced iron, slag can be sufficiently adhered to the surface of the solid reduced iron. Incidentally, when starting the charging of solid reduced iron, if the slag thickness is 100 mm or more in the stationary state, it is not necessarily necessary to form slag.

[0017] The components of the slag are not particularly limited. The slag is mainly generated by adding auxiliary materials such as a lime source and a carbon material for the purpose of adjusting the basicity and reduction efficiency. In this embodiment, after tapping the molten iron in the previous charge, the slag generated up to the previous charge may be left as it is so as to always ensure that the slag thickness is 100 mm or more. When the slag thickness can be sufficiently ensured, a part of the slag may be removed after tapping the molten iron in the previous charge.

[0018] Also, when solid reduced iron is charged and the solid reduced irons are crowded together, it becomes difficult for slag to adhere to the surface of the solid reduced iron. Therefore, when charging solid reduced iron, it is preferable to adjust the supply rate so as to satisfy the following formula (1). F / S < 1.00 (1)

[0019] In the formula, F represents the supply rate of solid reduced iron (t / min), and S represents the slag volume in the electric furnace (m 3) represents this. Note that the lower the supply rate of solid reduced iron, the less likely it is for solid reduced iron particles to stick together. Therefore, although the lower limit of the supply rate of solid reduced iron is not particularly defined, when the supply rate F of solid reduced iron is 0.30 t / min or more, by setting the slag thickness to 100 mm or more, the effect of preventing the sticking of solid reduced iron particles and the decrease in the dissolution rate becomes significant. On the other hand, when solid reduced iron is charged, the molten iron temperature also decreases. Therefore, in order to keep the molten iron temperature constant, it is preferable to adjust the power for generating the arc according to the supply rate of solid reduced iron.

[0020] Also, metallic iron, which is the main component in solid reduced iron, has poor wettability with slag, and from this perspective, it is also a factor that inhibits the adhesion between solid reduced iron and slag. On the other hand, the gangue components contained in solid reduced iron have good wettability with slag and contribute to the adhesion of slag to the surface of solid reduced iron. Therefore, it is preferable that the gangue components are contained in the solid reduced iron to be charged at 4.0 mass% or more.

[0021] Typical gangue components include Al2O3, SiO2, CaO, and MgO. That is, it is preferable that these oxides are contained in the solid reduced iron in a total amount of 4.0 mass% or more. Note that not all of these oxides need to be contained, and even if some of these oxides are not contained, it is acceptable. When charging solid reduced iron containing a large amount of gangue components, since the basicity of the slag tends to decrease, it is preferable to adjust the amount of the lime source charged together according to the content of the gangue components in the solid reduced iron.

[0022] Furthermore, while there are no particular limitations on the other components of solid reduced iron, a higher carbon content results in greater CO gas generation and easier slag formation. When using solid reduced iron with a high carbon content, even if the slag thickness before formation is relatively small, it is possible to increase the slag thickness to 100 mm or more through forming. However, since a high carbon content generates a large amount of CO gas originating from the carbon in the solid reduced iron, and contact with the slag is more easily hindered by bubbles, the slag thickness may be adjusted according to the carbon content of the solid reduced iron.

[0023] Furthermore, while there are no particular limitations on the metallization rate of solid reduced iron, FeO has better wettability with slag compared to metallic iron. Also, since FeO contributes to the generation of CO gas as an oxygen source, a higher FeO content makes the slag easier to foam. However, a high FeO content increases the proportion of iron that dissolves into the slag without being reduced, leading to a decrease in yield. From these perspectives, a metallization rate of 85-97% is preferable. [Examples]

[0024] Next, embodiments of the present invention will be described. The conditions in the embodiments are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0025] In an electric furnace with a molten iron surface diameter D of 6.8 m (when a specified amount of solid reduced iron is dissolved), 150 tons of molten iron with a C concentration of 0.1-0.3 mass% and a molten iron temperature of 1550-1570°C were held, and slag of different thicknesses was held for each test number. Then, an arc was generated from the electrodes to form the slag, and 10 tons of solid reduced iron were added at the input rate shown in Table 1. The measurement results of the slag thickness in the formed state when the solid reduced iron was added are shown in Table 1. The slag thickness was determined by inserting an iron rod into the slag through the slag discharge port, pulling it out, and measuring the area of ​​slag adhering to the iron rod.

[0026] Furthermore, the time from the end of adding solid reduced iron until the dissolution of the solid reduced iron was completed (dissolution completion time) was also measured. When measuring the dissolution completion time, if bubbles were generated in the molten iron or slag due to the generation of CO gas, it was assumed that undissolved solid reduced iron was hidden in the slag. If the dissolution of the solid reduced iron was not completed one minute after the end of adding solid reduced iron, the measurement of the dissolution completion time was stopped, and it was evaluated as if undissolved solid reduced iron remained.

[0027] Furthermore, all of the solid reduced iron samples used were HBI samples with a length of approximately 110 mm, a width of approximately 50 mm, and a thickness of approximately 35 mm. They contained 85-92% T.Fe, 83-87% M.Fe, and 1.2-1.5% C, with the gangue components shown in Table 1 as other components. Note that the gangue component content shown in Table 1 represents the total amount of Al2O3, SiO2, CaO, and MgO. The test results are shown in Table 1.

[0028] [Table 1]

[0029] As shown in Table 1, in Examples No. 11 to No. 23, the slag thickness was 100 mm or more in all cases, which allowed for sufficient contact time between the solid reduced iron and the slag, and enabled efficient dissolution of the solid reduced iron by suppressing the adhesion of the solid reduced iron particles to each other. Among these, in Examples No. 13 to No. 16, the supply ratio of solid reduced iron to slag volume F / S was less than 1.00, resulting in a slightly shorter dissolution completion time compared to Examples No. 11 and No. 12.

[0030] Furthermore, in Nos. 17 to 19, the dissolution completion time was slightly shorter compared to Nos. 11 and 12 because the solid reduced iron contained a large amount of gangue components. In particular, in Nos. 20 to 23, the supply ratio of solid reduced iron to slag volume F / S was less than 1.00, and the solid reduced iron contained a large amount of gangue components, resulting in a significantly shorter dissolution completion time compared to the other examples, Nos. 11 to 19.

[0031] On the other hand, in comparative examples No. 1 to No. 9, the slag thickness was less than 100 mm in all cases, so sufficient time could not be secured for the solid reduced iron to come into contact with the slag, and the solid reduced iron could not be efficiently dissolved. In particular, in No. 1 and No. 2, the supply ratio of solid reduced iron to slag volume F / S was less than 1.00, and the solid reduced iron contained a large amount of gangue components, but in all cases the slag thickness was less than 100 mm, so the solid reduced iron could not be efficiently dissolved. [Explanation of Symbols]

[0032] 1 Electric furnace 2 electrodes 3 Input section 4 types of bath 5 slags

Claims

1. A method for producing molten iron by dissolving solid reduced iron in an electric furnace, A method for producing molten iron, characterized by adding solid reduced iron while a slag of 100 mm or more is present on the molten iron.

2. The method for producing molten iron according to claim 1, characterized in that the solid reduced iron is added in such a way that the conditions of the following equation (1) are satisfied. F / S<1.00 (1) In the formula, F represents the supply rate of solid reduced iron (t / min), and S represents the volume of slag in the electric furnace (m³). 3 ) represents.

3. The aforementioned solid reduced iron has Al as a gangue component. 2 O 3 SiO 2 A method for producing molten iron according to claim 1 or 2, characterized in that it contains a total of 4.0% by mass or more of CaO and MgO.

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