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.SOLUTION: An electric furnace includes a coal feeding lance 4 for blowing a solid carbon source into the furnace and one or more or n pieces of upper electrodes 10. In the electric furnace, a solid carbon source with apparent density 350 to 600 kg / m3 is used as the solid carbon source. The solid carbon source is supplied so that an expression (1) below is satisfied by a minimum distance xi from among distances x1, x2, ..., xn (mm) from a point PL at which a line L extending a central axis of the coal feeding lance 4 intersects with a stationary molten iron surface 6, to points PE1, PE2, ..., PEn at which lines respectively extending central axes of the upper electrodes intersect with the stationary molten iron surface: xi≥rE+0.65 VA (1). However, rE is an upper electrode radius (mm) and VA is an arc voltage (V).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 furnace steelmaking has attracted attention as a steelmaking method that does not require a reducing agent. Electric furnace steelmaking also uses carbonaceous materials such as coal as an auxiliary fuel. With a view to reducing fossil fuel use and recycling waste materials with the aim of achieving zero emissions to prevent global warming, alternatives to these carbonaceous materials, such as biomass charcoal produced by carbonizing plants, waste plastics, and papermaking waste, are being considered. 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 a solid form and their alternatives will be collectively referred to as solid carbon sources.

[0003] Solid carbon sources include those with low apparent density. Because strong air currents exist inside electric furnaces, adding solid carbon sources with low apparent density directly to the furnace can easily scatter, hindering heating and carburization of the molten iron. Patent Document 1 discloses a method for suppressing the scattering of solid carbon sources, which utilizes biomass charcoal with high bulk density by using coconut shell as a raw material. However, this method limits the raw materials and production method of the fixed carbon source, making it less versatile. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-046726 [Non-patent literature]

[0005] [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]

[0006] 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 is added to the molten iron in the electric furnace. [Means for solving the problem]

[0007] That is, the gist of the present invention is as follows. [1] In an electric furnace having a lance for injecting a solid carbon source into the furnace (hereinafter referred to as a coal feeding lance) and one or more upper electrodes, The solid carbon source has an apparent density of 350 to 600 kg / m 3 The point where the line extending the central axis of the coal feeding lance intersects with the stationary molten iron surface is called point P L The point where the line extending the central axis of each upper electrode intersects with the stationary molten iron surface is defined as point P E1 , P E2 , …, P En year, The point P L and the point P E1 , P E2 , …, P En Distances x1, x2, …, x n (mm) is the smallest of x i year, The x i A method for producing molten iron, characterized in that a solid carbon source is supplied so that the following formula (1) is satisfied: x i ≧r E +0.65V A (1) where r E :Top electrode radius (mm), V A : arc voltage (V), n: number of upper electrodes.

[0008] [2] The electric furnace has one or more lances (hereinafter referred to as oxygen supply lances) for injecting oxygen gas into the furnace, and the point where the line extending the central axis of the oxygen supply lance intersects with the stationary molten iron surface is called point P. O1 , P O2 , …, P Om year, The point P O1 , P O2 , …, P Om and the aforementioned P L Distance x O1 , x O2 , …, x Om The method for producing molten iron according to [1], characterized in that the solid carbon source is supplied so that (mm) satisfies the following formula (2): x Ok ≧r Nk +0.27d k (k=1,2,…,m) (2) where m is the number of oxygen supply lances, r Nk :P Ok The outlet radius of the oxygen supply lance (mm) corresponding to k :P Ok From the tip center of the oxygen supply lance corresponding to P Ok The distance to the [Effects of the Invention]

[0009] 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]

[0010] [Figure 1] FIG. 1 is a front cross-sectional view of an electric furnace having an upper electrode. [Figure 2] 1A and 1B are diagrams showing the positional relationship between the upper electrode, arc jet, oxygen supply lance, and coal supply lance, where (A) is a plan view and (B) is a front cross-sectional view. [Figure 3] FIG. 2 is a plan view showing the positional relationship between an upper electrode, an oxygen supply lance, and a coal supply lance. DETAILED DESCRIPTION OF THE INVENTION

[0011] First Embodiment In an electric furnace 3 having an upper electrode 10 as shown in Figure 1, the arc formed between the upper electrode 10 and molten iron 5 is used as the main heat source. It is known that a strong airflow is formed from the upper electrode 10 toward the molten iron 5 along the arc formed between the upper electrode 10 and the molten iron 5, as shown in Figure 2(B), and this airflow is called an arc jet 7. 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.

[0012] In studying how to prevent the scattering of a solid carbon source added toward molten iron in an electric furnace, the present inventors investigated the effects of the arc jet 7 generated along the arc, which is used as the main heat source in an electric furnace, and the reverse flow 8 that occurs when the arc jet 7 collides with the stationary molten iron surface 6 and changes direction toward the side wall or ceiling of the furnace (see FIG. 2(B)). Here, if a solid carbon source with a low apparent density is supplied near the arc, the reverse flow 8 promotes the scattering of the solid carbon source. Therefore, the present inventors came up with the idea of controlling the supply position of the solid carbon source to avoid the effects of the reverse flow 8.

[0013] Here, the point where the line extending from the central axis of the upper electrode 10 intersects with the stationary molten iron surface 6 is called point P E In FIG. 2B, the first upper electrode 11 is located at a point P E1 , point P for the second upper electrode 12 E2 , the point P E3 is stated.

[0014] In an electric furnace 3 having an upper electrode 10, an arc is generated between the upper electrode 10 and molten iron 5, and an arc jet 7 is formed along the arc in the vicinity of the arc and flows toward the stationary molten iron surface 6. The arc jet 7 then collides with the stationary molten iron surface 6, and a flow is formed from the collision point toward the periphery along the stationary molten iron surface 6, becoming a reverse flow 8. Point P E The flow velocity of the reverse flow 8 decreases as the distance from the nozzle increases.

[0015] Therefore, even when using a solid carbon source with a low apparent density, we came up with the idea of supplying the solid carbon source at a position sufficiently far from the upper electrode 10 to avoid scattering, and we investigated the distance between the upper electrode 10 and the carbon material supply position as follows.

[0016] It is known that the arc in an electric furnace is generated at an angle of 30° with respect to the central axis of the upper electrode 10. Therefore, the arc jet 7 affects a wider area than the area obtained by projecting the electrode cross section along the central axis onto the stationary molten iron surface. Specifically, the arc jet 7 affects a wider area than the area obtained by projecting the electrode cross section along the central axis onto the stationary molten iron surface 6. E The longer the time, the more widespread the impact.

[0017] As described above, the arc has an angle of 30° at most with respect to the central axis of the upper electrode. Therefore, the distance H between the tip of the upper electrode 10 on the molten iron side (upper electrode tip 30) and the stationary molten iron surface 6 is E (mm) and arc length L A (mm) and H E =L A cos30° (3) Furthermore, as published in Non-Patent Document 1, L A is the arc voltage V A There is a correlation between (V) and equation (4). L A =V A (4) Therefore, considering the slope of the arc in equation (3), H E and V A The relationship shown in equation (5) holds between H E =V A cos30° (5)

[0018] Here, in order to estimate the range of influence of the reversed flow 8, the inventors needed to take into consideration that the radius of the arc jet 7 expands from the upper electrode 10 toward the stationary molten iron surface 6, and that the reversed flow 8 is generated from the collision point of the molten iron 5 and the arc jet 7 in a direction away from the upper electrode 10. Therefore, they considered that the range would be wider than that predicted from equations (3) and (4), and determined the range through testing.

[0019] As described above, the point where the line extending from the central axis of the upper electrode 10 intersects with the stationary molten iron surface 6 is called point P E From the results of the test by the inventor, it is found that the influence range of the reverse flow 8 on the stationary molten iron surface is from the point P E It was found that the radius r shown in equation (6) is at the center. r=r E +H E ×0.75 (6) where r E is the radius of the upper electrode 10. Since cos30°=0.867, substituting equation (5) into equation (6) gives: r=r E +V A cos30°×0.75 =r E +0.65V A (7) is derived.

[0020] As shown in Figures 2 and 3, the point where the line extending from the central axis of the coal feeding lance 4 intersects with the stationary molten iron surface 6 is called point P L In addition, the point where the line extending from the central axis of each of the n upper electrodes (11, 12, . . . 1n) intersects with the stationary molten iron surface 6 is defined as point P E1 , P E2 , …, P En Far away.

[0021] From the result of the above formula (7), in an electric furnace in which a solid carbon source is supplied into the furnace using the coal supply lance 4, in order to suppress the scattering of a solid carbon source having a low apparent density, L and the point P E1 , P E2 , …, P En The distance between x1, x2, …, xn (mm), where x1, x2, ..., x n (mm) is the smallest of x i When the above x i must satisfy the following formula (1) (see Figure 2(A)). x i ≧r E +0.65V A (1)

[0022] Second Embodiment A more preferred embodiment of the present invention will be described below. The inventors have noticed that in an electric furnace, in addition to the arc jet 7 generated by the arc of the upper electrode 10, a strong air current is generated by the oxygen gas jet used for heating and refining. That is, they thought that scattering of the solid carbon source could be further suppressed by supplying the solid carbon source so as to avoid the oxygen gas jet, as in the case of the arc jet.

[0023] In an electric furnace, oxygen gas is supplied by spraying it onto molten iron from the tip of the oxygen supply lance 20. It is generally known that the jet of oxygen gas from the tip of the oxygen supply lance spreads from the outlet of the oxygen supply lance 20 that blows oxygen gas at an angle of 15° on one side with respect to a line extending from the central axis of the oxygen supply lance. Therefore, since tan 15° = 0.27, as in the case of an arc jet, the point where the line extending from the axis of the oxygen supply lance 20 that supplies oxygen gas intersects with the stationary molten iron surface is called point P O Let P be the point O and the aforementioned P L Distance x O The solid carbon source should be supplied so that (mm) satisfies the following formula (7) (see Figure 2(A)). x O ≧r N +0.27d (8) However, r N is the outlet radius (mm) of the oxygen supply lance 20, and d is the distance from the center of the tip of the oxygen supply lance 32 to P OWhen the electric furnace has one or more oxygen supply lances (21, 22, ... 2 m), the point where the line extending from the central axis of each oxygen supply lance (21, 22, ... 2 m) intersects with the stationary molten iron surface is called point P O1 , P O2 , …, P Om Let P be the point O1 , P O2 , …, P Om and the aforementioned P L Distance x O1 , x O2 , …, x Om It is more preferable to supply the solid carbon source so that (mm) satisfies the following formula (2) (see FIG. 3). x Ok ≧r Nk +0.27d k (k=1,2,…,m) (2) where m is the number of oxygen supply lances, r Nk :P Ok The outlet radius of the oxygen supply lance (mm) corresponding to k :P Ok Point P from the center of the tip of the oxygen supply lance 32 Ok The distance to the [Example]

[0024] In carrying out the present invention, five types of solid carbon sources were used, as shown in Table 1. To test for multiple apparent densities, plants carbonized by heat treatment were used as solid carbon sources. The apparent densities were varied by changing the raw material and the heat treatment temperature and time. Table 1 also shows the fixed carbon content and volatile component content of anthracite and five types of solid carbon sources used as comparative examples. The apparent densities of anthracite and the 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 type of solid carbon source is not limited to plant-derived charcoal; solid carbon sources such as waste plastics and papermaking waste can also be used within the spirit and 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.

[0025] [Table 1]

[0026] Table 2 shows an example of the present invention implemented in an electric furnace with an inner diameter of 6.5 m, capable of holding 175 t of molten iron, with a tapping rate of 110 t, leaving 65 t of seed molten iron in the furnace after tapping. As shown in Figs. 1 to 3, this electric furnace is a three-phase AC electric furnace in which three graphite electrodes are inserted into the furnace from the furnace cover at the top of the furnace body as upper electrodes 10 (first upper electrode 11, second upper electrode 12, third upper electrode 13) and electricity is applied. The upper electrode radius r of the electrode used as the upper electrode 10 is E The diameter of the upper electrode 10 was 14 inches, or 356 mm. Note that the embodiment of the present invention is not limited to the number of upper electrodes 10 and the three-phase AC system, and can be practiced with one, two, or four or more upper electrodes, or with current systems such as DC and single-phase AC, without departing from the spirit of the present invention. The electric furnace had a slag discharge port 31 on its side wall, measuring 1000 mm wide and 995 mm high, and insertion of the lance of the manipulator 9, described below, or sampling of molten steel was performed through the slag discharge port 31.

[0027] The electric furnace had one lance (hereinafter referred to as coal feed lance 4) connected to a movable manipulator as a coal feed lance, and was capable of feeding a maximum of 60 kg / min of solid carbon source into the furnace. The outlet diameter of the coal feed lance 4 was 28 mm. The feed position of the solid carbon source could be changed within the movable range of the manipulator 9. In the examples of the present invention and the comparative examples, carbon dioxide gas with a purity of 99.5% by volume or more was used as the carrier gas for the carbonaceous material.

[0028] As shown in FIG. 3, the electric furnace has one lance (first oxygen supply lance 21) connected to a manipulator 9 independently of the supply system of the solid carbon source, and two lances (second oxygen supply lance 22 and third oxygen supply lance 23) fixed to the inner wall of the electric furnace as oxygen supply lances 20. Each lance can supply up to 4000 Nm 3 / h, total 10000Nm3 The outlet diameter of each lance is 32 mm, that is, the outlet radius r N 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.

[0029] In the present invention and comparative examples, scrap was used as the iron raw material. After 6,200 kg of quicklime was charged into the seed molten metal, scrap was continuously fed into the furnace while applying the arc voltage V shown in Table 2. A The iron raw material was melted by arc current application and the supply of oxygen gas and a solid carbon source. After that, the temperature inside the furnace was raised and the molten iron was oxidized and refined by arc current application and the supply of solid carbon source and oxygen gas, and then the molten iron was sampled and analyzed. The supply position of the solid carbon source was adjusted within the movable range of the manipulator 9 so that the formula (2) was always satisfied relative to the oxygen supply lances fixed to the inner wall (second oxygen supply lance 22, third oxygen supply lance 23). In addition, the oxygen supply lance connected to the manipulator 9 was designated as the first oxygen supply lance 21, and the lances fixed to the inner wall were designated as the second oxygen supply lance 22 and the third oxygen supply lance 23, and the corresponding dk and x Ok The results are shown in Table 2. The iron samples were analyzed by measuring the concentrations of carbon and phosphorus using spark emission spectrometry. 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.

[0030] 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.

[0031] [Table 2]

[0032] Compared to the anthracite shown in Comparative Examples 1 to 6, the consumption rate increased in Comparative Examples 7 and 8, in which the solid carbon source was supplied near the electrode, and in Comparative Examples 9 and 10, in which the apparent density of the solid carbon source was low. On the other hand, in Inventive Examples 1 to 14, in which the solid carbon source was supplied so as to satisfy formula (1), the apparent density was lower than that of anthracite, but the consumption rate was equivalent to that of anthracite. Furthermore, Inventive Examples 1 to 6, 9, and 10, in which oxygen gas was supplied so as to satisfy formula (2), had better consumption rates than Inventive Examples 7, 8, and 11 to 14, in which the solid carbon source and oxygen gas were supplied at positions close to each other, deviating from formula (2). [Explanation of symbols]

[0033] 3. Electric furnace 4 Coal feeding lance 5. Molten Iron 6 Stationary molten iron surface 7 Arc Jet 8 Reverse flow 9 Manipulator 10 Upper electrode 11 1st upper electrode 12 2nd upper electrode 13 Third upper electrode 20 Oxygen supply lance 21 First oxygen supply lance 22 Second oxygen supply lance 23 Third oxygen supply lance 30 Top electrode tip 31 Slag discharge port 32 Tip of oxygen supply lance

Claims

1. In an electric furnace having a lance for injecting a solid carbon source into the furnace (hereinafter referred to as a coal feeding lance) and one or more upper electrodes (n), The solid carbon source has an apparent density of 350 to 600 kg / m ３ The point where the line extending the central axis of the coal feeding lance intersects with the stationary molten iron surface is called point P Ｌ The point where the line extending the central axis of each upper electrode intersects with the stationary molten iron surface is defined as point P Ｅ１ , P Ｅ２ , ..., P Ｅｎ year, The point P Ｌ and the point P Ｅ１ , P Ｅ２ , ..., P Ｅｎ Distance x from １ , x ２ , …, x ｎ (mm) the smallest one is x ｉ year, The x ｉ A method for producing molten iron, comprising supplying a solid carbon source so that the following formula (1) is satisfied: x ｉ ≧r Ｅ +0.65V Ａ (1) However, Ｅ : Upper electrode radius (mm), V Ａ : arc voltage (V), n: number of upper electrodes.

2. The electric furnace has one or more lances (hereinafter referred to as oxygen supply lances) for injecting oxygen gas into the furnace, and the point where the line extending the central axis of the oxygen supply lance intersects with the stationary molten iron surface is called point P. Ｏ１ , P Ｏ２ , ..., P Ｏｍ year, The point P Ｏ１ , P Ｏ２ , ..., P Ｏｍ and the above P Ｌ Distance x from Ｏ１ , x Ｏ２ , …, x Ｏｍ 2. The method for producing molten iron according to claim 1, wherein the solid carbon source is supplied so that (mm) satisfies the following formula (2): x Ｏｋ ≧r Ｎｋ +0.27d ｋ (k=1,2,…,m) (2) where m is the number of oxygen supply lances, r Ｎｋ :P Ｏｋ The outlet radius of the oxygen supply lance (mm) corresponding to d ｋ :P Ｏｋ From the tip center of the oxygen supply lance corresponding to P Ｏｋ The distance (mm) to the

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