Method for producing molten iron

Abstract

To provide a method for producing molten iron using an electric furnace having an upper electrode, the method capable of improving carburization efficiency or heat-transmission to the molten iron when adding a solid carbon source to the molten iron in the electric furnace, even in using the solid carbon source with smaller apparent density.SOLUTION: A method for producing molten iron uses an electric furnace including a coal feeding lance 4 and one or more upper electrodes 2. In the electric furnace, a solid carbon source with apparent density 350 to 600 kg / m3 is used. The method satisfies an expression (1): 0.3≤HM / HE<1.0 (1), in which, assuming a space O to which a cross section of an upper electrode 2 is projected from a tip 20 of the upper electrode 2 to a stationary molten iron surface 6 along a central axis of the upper electrode 2, the space O intersects with a line L extending a central axis of the coal feeding lance 4, and HM is a height from the stationary molten iron surface to a point PM being a highest point from among points on a line segment L1 defined by cutting the line L by the space O, and HE is a height from the stationary molten iron surface to the tip 20 of the upper electrode 2.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing molten iron using an electric furnace having an upper electrode. [Background technology]

[0002] In the steel industry, blast furnace steelmaking uses iron ore as the main raw material and large amounts of coke as a reducing agent. Therefore, from the perspective of preventing global warming, electric arc furnace steelmaking has attracted attention as a steelmaking method that does not require a reducing agent. In electric arc furnace steelmaking, carbonaceous materials such as coal are also used as auxiliary fuel or recarburizers. For these carbonaceous materials, alternatives such as biomass charcoal produced by carbonizing plants, waste plastics, and papermaking waste are being considered from the perspective of reducing fossil fuel use to prevent global warming and recycling waste materials with the aim of achieving zero emissions. For example, Patent Document 1 discloses a technology that uses biomass charcoal as an auxiliary fuel or recarburizer in an electric arc furnace steelmaking process. Hereinafter, carbonaceous materials added in solid form and their alternatives will be collectively referred to as solid carbon sources.

[0003] The solid carbon source includes those with low apparent density. Because strong air currents exist inside electric furnaces, if solid carbon sources with low apparent density are used as is, they easily scatter, posing a problem of preventing heating and carburization of the molten iron. Patent Document 1 discloses a method for suppressing the scattering of solid carbon sources, in which coconut shells are used as a raw material to utilize biomass charcoal with high bulk density. However, this method limits the raw materials and production methods of the fixed carbon source, and lacks versatility.

[0004] As a technique for merging the arc and the solid carbon source, Patent Document 2 discloses a technique in which the supply position of the solid carbon source is positioned below the electrode. In Patent Document 2, the invention is carried out using anthracite, which has a high bulk density, as the fixed carbon source. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-046726 [Patent Document 2] Japanese Patent Application Publication No. 2023-4698 [Non-patent literature]

[0006] [Non-Patent Document 1] Toshio Nanjo, "Arc Heating Lectures: Fundamentals and Applications of Arc Heating (2)," Electroheat, Vol. 27, No. 4, pp. 73-84 Summary of the Invention [Problem to be solved by the invention]

[0007] Even when the invention described in Patent Document 2 is used, if a solid carbon source having a low apparent density as described above is used, the solid carbon source may scatter and not reach the surface of the molten steel. Therefore, when a solid carbon source having a low apparent density is used, there is a problem that the arc and the solid carbon source cannot be stably joined together.

[0008] An object of the present invention is to provide a method for producing molten iron using an electric furnace having an upper electrode, which can improve the efficiency of heating and carburizing the molten iron when a solid carbon source having a low apparent density is added to the molten iron in the electric furnace, even when a solid carbon source having a low apparent density is used. [Means for solving the problem]

[0009] That is, the gist of the present invention is as follows. [1] In an electric furnace having a coal supply lance for injecting a solid carbon source into the furnace and one or more upper electrodes, the apparent density is 350 to 600 kg / m 3 Using the solid carbon source, a space O is assumed where the cross section of the upper electrode is projected along the central axis of the upper electrode from the tip to the stationary molten iron surface, and one or more points in the space O intersect with a line L extending the central axis of the coal feeding lance, and among the points on the line L1 obtained by cutting the line L at the space O, point P is the point with the maximum vertical height from the stationary molten iron surface. MThe height measured vertically from the stationary molten iron surface to the M And H M (mm) satisfies the following formula (1): 0.3≦H M / H E <1.0 (1) However, H E : Height (mm) measured vertically from the surface of the stationary molten iron to the tip of the upper electrode [2] The method for producing molten iron according to [1], characterized in that the voltage of the arc is set in the range of 200 V to 1500 V while the solid carbon source is being supplied. [3] The method for producing molten iron according to [1] or [2], characterized in that the carrier gas for the solid carbon source is argon. [Effects of the Invention]

[0010] According to the present invention, scattering of a solid carbon source having a low apparent density is suppressed in a method for producing molten iron, and the use of the solid carbon source as a recarburizer and auxiliary fuel in an electric furnace is realized. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a front cross-sectional view of an electric furnace having an upper electrode. [Figure 2] FIG. 2 is a perspective view showing the positional relationship between the coal feeding direction from the coal feeding lance and the upper electrode. DETAILED DESCRIPTION OF THE INVENTION

[0012] First Embodiment In the present invention, the solid carbon source is a carbonaceous material having an apparent density of 350 to 600 kg / m 3 A solid carbon source with an apparent density of 600 kg / m 3 The effects of the present invention can be fully achieved with a solid carbon source having a low apparent density of 350 kg / m or less. 3 If the density is less than 350 kg / m, the present invention cannot fully exert its effect. 3 The above was targeted.

[0013] In an electric furnace 1 having an upper electrode 2 as shown in Figure 1, an arc formed between the upper electrode 2 and molten iron 5 is used as the main heat source. Here, as shown in Figure 2, consider a space O, which is a projection of a cross section of the upper electrode 2 along the central axis of the upper electrode from the tip 20 of the upper electrode 2 to the stationary molten iron surface 6. As shown in Figure 2, the space O is a space from the tip 20 of the upper electrode 2 toward the stationary molten iron surface 6 of the molten iron 5. The surface of the space O is located on an extended plane of the surface of the upper electrode 2. The tip 20 of the upper electrode 2, which is the upper end of the space O, is defined as the space upper end O. T The position where the bottom end of the space O comes into contact with the stationary molten iron surface 6 is defined as the bottom end O B Let's say.

[0014] It is known that a strong airflow is formed along the arc formed between the upper electrode 2 and the molten iron 5 from the tip 20 of the upper electrode 2 toward the stationary molten iron surface 6, and this airflow is called the arc jet 7. In FIG. 2, the arc jet 7 flows downward in the space O. It is also known that when the arc jet 7 collides with the stationary molten iron surface 6, a part of the arc jet 7 changes direction from the stationary molten iron surface 6 toward the side wall or ceiling of the furnace, creating an airflow called a reverse flow 8. The reverse flow 8 flows from the bottom O of the space O B The arc flows in a direction corresponding to the inclination of the arc with respect to the stationary molten iron surface 6.

[0015] In studying ways to prevent the scattering of a solid carbon source added toward molten iron in an electric furnace, the present inventors focused on the arc jet 7 generated along the arc, which is used as the main heat source in an electric furnace. If a solid carbon source with a low apparent density is supplied toward the stationary molten iron surface 6 near the arc, the scattering of the solid carbon source is promoted by the reversing flow 8. Therefore, the present inventors came up with the idea of controlling the supply position of the solid carbon source to avoid the influence of the reversing flow 8.

[0016] One method for supplying the solid carbon source to the stationary molten iron surface 6 without being affected by the reverse flow 8 is to increase the kinetic energy of the solid carbon source. Furthermore, for a solid carbon source of the same mass, the kinetic energy can be increased by increasing the velocity in the direction toward the stationary molten iron surface. The inventors focused on the fact that the arc jet 7 moves toward the molten iron at a high velocity before colliding with the stationary molten iron surface 6. Specifically, they came up with the idea that by merging the solid carbon source with the arc jet 7 in the space above the stationary molten iron surface 6, it would be possible to make the solid carbon source move toward the stationary molten iron surface 6 at a high velocity.

[0017] In order to solve the above problem, the inventors focused on the height at which the solid carbon source is joined to the arc jet 7 in the space above the stationary molten iron surface 6.

[0018] Here, the portion where the line L passes through the space O is designated as a line L1 as shown in FIG. 2. The point at the highest height from the stationary molten iron surface 6 on the line L1, where the line L extending from the central axis of the coal feeding lance 4 intersects with the space O, is designated as a point P M As shown in FIG. 2, when the direction of the line L, which is the direction of the carbonaceous material discharge from the coal feeding lance 4, is downward, the point P M The position of is the position where the line L intersects with the surface of the space O.

[0019] In an electric furnace in which a solid carbon source is supplied into the furnace using a coal supply lance 4, in order to make the solid carbon source supplied from the tip 21 of the coal supply lance 4 join the arc jet, as shown in Figure 2, a line L extending the central axis of the coal supply lance 4 must intersect with at least one of the spaces O formed by projecting the cross section of the upper electrode 2 along the central axis of the upper electrode from the tip 20 of the upper electrode 2 to the stationary molten iron surface 6. To achieve this, the intersection P between the line L and the surface of the space O defined above must be M is the upper limit O of the space O T The intersection point P of the line L and the surface of the space O M is the upper limit of the space O T If it is higher, the solid carbon source will collide with the upper electrode 2 and the solid oxygen source cannot be merged with the arc jet 7 .

[0020] Since the reverse flow 8 occurs near the stationary molten iron surface 6, the intersection point P of the line L and the surface of the space O M When a solid carbon source with a low apparent density is supplied in a state where the surface of the space O is located near the stationary molten iron surface 6, B The solid carbon source is scattered by the reverse flow 8 that flows radially from the vicinity of the surface 6 of the stationary molten iron. Therefore, the inventors have experimentally investigated the height of the confluence position at which the solid carbon source with a low apparent density can be stably confluent with the arc jet 7. From the results of the experiments conducted by the inventors, it was found that the point P M The vertical height from the stationary molten iron surface is H M (mm), and the height measured vertically from the stationary molten iron surface 6 to the tip 20 of the upper electrode 2 is H E (mm), it was discovered that by supplying the carbonaceous material so that these satisfy formula (1), it is possible to stably merge the carbonaceous material into the arc jet even when a solid carbon source with a low apparent density is used. It is more preferable that the left side of the following formula (1) is 0.5 or more and the right side is 0.8 or less. H M / H E When is close to 1, the powder may collide with the electrode due to diffusion, resulting in loss. M / H E If it is ≦0.8, this problem can be avoided. 0.3≦H M / H E <1.0 (1)

[0021] Second Embodiment A more preferred embodiment of the present invention is described below. The solid carbon source supplied from the tip 21 of the coal supply lance 4 is distributed radially from the central axis L of the coal supply lance 4 around the central axis L. Here, if the arc length of the arc formed from the upper electrode 2 toward the stationary molten iron surface 6 is short, the distribution of the supplied solid carbon source tends to cause a portion of the supplied solid carbon source to be affected by the reverse flow 8. If the arc length is 200 mm or longer, the supplied solid carbon source will not be affected by the reverse flow 8, resulting in good results. Furthermore, as the arc length increases, the arc jet flow velocity near the molten steel surface decreases, resulting in insufficient downward kinetic energy imparted to the solid carbon source by the arc jet. As a result, the solid carbon source has difficulty penetrating the molten slag coexisting on the molten iron surface and therefore has difficulty reaching the stationary molten iron surface. If the arc length is 1500 mm or shorter, the solid carbon source can obtain sufficient kinetic energy from the arc jet 7. From the results of experiments conducted by the present inventors, it was found that the solid carbon source can be merged with the arc jet and stably supplied to the molten iron surface when the arc length is in the range of 200 mm to 1500 mm. As disclosed in Non-Patent Document 1, the arc length (mm) is proportional to the arc voltage (V), and the above arc length corresponds to an arc voltage of 200 V to 1500 V.

[0022] Third Embodiment Furthermore, from the viewpoint of preventing the arc and arc jet from becoming unstable, the inventors came up with the idea of using a gas with low dissociation energy and large molecular weight as a carrier gas for the carbonaceous material. A known example of a gas with low dissociation energy and large molecular weight is argon, which is a monoatomic molecule with a large atomic weight and can be used from the viewpoint of economic rationality. From the results of tests, the inventors confirmed that using argon as a carrier gas stabilizes the arc and arc jet and promotes the confluence of the arc jet and solid carbon source. [Example]

[0023] In carrying out the present invention, six types of solid carbon sources (anthracite and solid carbon sources A to E) were used, as shown in Table 1. To test multiple apparent densities, solid carbon sources A to E were prepared by carbonizing plants through heat treatment. The apparent densities were varied by changing the raw material and the heat treatment temperature and time. Table 1 also lists the physical properties, fixed carbon content, and volatile component content of the anthracite and solid carbon sources A to E used as comparative examples. The apparent densities of the anthracite and various solid carbon sources were measured using the underwater weighing method described in JIS Z 8807-2012. The content of each component was measured in accordance with JIS M 8812. Note that the solid carbon source in embodiments of the present invention is not limited to plant-derived charcoal; solid carbon sources such as waste plastics and papermaking waste can also be used within the scope of the present invention. In Table 1 and Table 2 described below, items and values outside the scope of the present invention and values outside the preferred range of the present invention are underlined.

[0024] [Table 1]

[0025] Table 2 shows an example of the present invention implemented in an electric furnace 1, as shown in Figures 1 and 2, with an inner diameter of 6.5 m, a capacity to hold 175 tons of molten iron, a tapping rate of 110 tons, and 65 tons of seed metal remaining in the furnace during tapping. The electric furnace 1 is a three-phase AC electric furnace in which three graphite electrodes (upper electrodes 2) are inserted into the furnace from the furnace roof ceiling 10 above the furnace body and energized. The radius of the electrodes used as upper electrodes 2 was 14 inches, or 356 mm. The present invention is not limited to the number of upper electrodes 2 or the three-phase AC system. It can also be implemented with one, two, or four or more upper electrodes, or with DC or single-phase AC energization systems, without departing from the spirit of the present invention. The electric furnace had a slag discharge port 11, 1000 mm wide and 995 mm high, on its side wall. Inserting the lance of the manipulator 9 (described later) or sampling molten steel was performed through the slag discharge port 11.

[0026] The electric furnace 1 has one lance (hereinafter referred to as the coal feeding lance 4) connected to a movable manipulator 9 as the coal feeding lance 4, and can feed a solid carbon source into the furnace at a maximum rate of 60 kg / min. The feed position of the solid carbon source can be changed within the movable range of the manipulator 9. The carrier gas for feeding the solid carbon source can be selected from carbon dioxide and argon, and in the examples of the present invention and the comparative examples, carbon dioxide with a purity of 99.5% by volume or more and argon with a purity of 99.99% by volume or more were used as the carrier gas for the carbonaceous material.

[0027] The electric furnace 1 has one oxygen supply lance 3 connected to a manipulator 9 independently of the supply system of the solid carbon source and two oxygen supply lances 3 fixed to the inner wall of the electric furnace, and each lance can supply up to 4000 Nm 3 / h, total 10000Nm 3 / h of oxygen gas can be supplied into the furnace. In the examples of the present invention and the comparative examples, oxygen gas with a purity of 99.5% by volume or more was used.

[0028] In the present invention and comparative examples, scrap was used as the iron raw material. 6,200 kg of quicklime was charged into an electric furnace containing 65 t of seed molten metal. Scrap was then continuously fed into the furnace while an arc was applied and oxygen gas and a solid carbon source were supplied to melt the iron raw material. The furnace was then heated and the molten iron was oxidized and refined by applying an arc and supplying a solid carbon source and oxygen gas. The molten iron was then sampled and analyzed.

[0029] When supplying a solid carbon source into the furnace, the height of the stationary molten iron surface is estimated from the mass of the charged iron raw material, and the height of the electrode tip is estimated from the length of the upper electrode measured before energization and the height of the electrode gripper, and the difference is calculated. E The height (measured vertically from the stationary molten iron surface 6 to the tip 20 of the upper electrode 2) was calculated. The point P with the maximum height in the range (line L1) where the line L intersects with at least one of the spaces O was M The vertical height from the stationary molten iron surface is H M (mm), and each H E During the operation, M / HE The carbonaceous material supply position was controlled so that the temperature was constant.

[0030] The iron sample was analyzed by spark emission spectrometry to measure the concentrations of carbon and phosphorus, and after confirming that the desired molten steel composition had been obtained, a tap hole (not shown) was opened in the hearth, and the molten steel was poured into a ladle (not shown). The desired molten steel composition in the present invention and comparative examples was 0.04 to 0.06 mass% carbon and 0.015 to 0.018 mass% phosphorus.

[0031] The effect of the present invention was evaluated from the viewpoint of the unit consumption of the solid carbon source. Here, since the proportion of carbon contained varies depending on the type of solid carbon source, attention was focused on the fixed carbon content of the solid carbon source, and the unit consumption of the solid carbon source was multiplied by the fixed carbon content (mass%), and the result was divided by 100. The value obtained is shown in Table 2 as "fixed carbon content," and a comparison was made with anthracite shown in the comparative example.

[0032] [Table 2]

[0033] Compared to the anthracite shown in Comparative Examples 1 to 4, the consumption rate increased in Comparative Examples 5 and 6, in which a solid carbon source was supplied outside the range of formula (1), and in Comparative Example 7, in which the apparent density of the solid carbon source was low. On the other hand, in Inventive Examples 1 to 12, despite the use of a solid carbon source with a low apparent density, the consumption rate was equivalent to that of anthracite. Furthermore, Inventive Examples 7 and 8, in which the arc voltage was in the range of 200 V to 1500 V, had better consumption rates than Inventive Example 6, which had a low arc voltage, and Inventive Example 9, which had a high arc voltage. Furthermore, Inventive Examples 10 and 12, in which argon gas was used as the carrier gas, had better consumption rates than Inventive Examples 6 and 11, which used carbon dioxide. [Explanation of symbols]

[0034] 1 electric furnace 2 Upper electrode 3 Oxygen supply lance 4 Coal feeding lance 5. Molten Iron 6 Stationary molten iron surface 7 Arc Jet 8 Reverse flow 9 Manipulator 10 Furnace cover ceiling surface 11 Slag discharge port 20 Tip 21 Tip O space O T top of space O B lower end of space

Claims

1. In an electric furnace having a coal feeding lance for injecting a solid carbon source into the furnace and one or more upper electrodes, an apparent density of 350 to 600 kg / m ３ A solid carbon source of the above is used, and a space O is assumed as a projection of a cross section of the upper electrode along the central axis of the upper electrode from the tip of the upper electrode to the stationary molten iron surface, and one or more of the spaces O intersect with a line L extending the central axis of the coal feeding lance, and a line segment L is formed by cutting the line L at the space O. １ Point P is the point with the maximum vertical height from the stationary molten iron surface. Ｍ The height measured vertically from the stationary molten iron surface to the Ｍ Toshi, H Ｍ (mm) satisfies the following formula (1): 0.3≦H Ｍ / H Ｅ <1.0 (1) However, H Ｅ : Height (mm) measured vertically from the surface of the stationary molten iron to the tip of the upper electrode

2. 2. The method for producing molten iron according to claim 1, wherein the voltage of the arc is set to a range of 200V to 1500V while the solid carbon source is being supplied.

3. 3. The method for producing molten iron according to claim 1, wherein the carrier gas for the solid carbon source is argon.

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