Refining method in electric furnace

Abstract

To provide a refining method in an electric furnace capable of stabilizing, at a high level, slag-foaming height when using biomass charcoal as a carbonaceous material to be supplied by being blown into the furnace.SOLUTION: When refining molten iron at an electric furnace 1, a refining method in an electric furnace blows a carbonaceous material into the electric furnace from a coal feed lance 4, using biomass charcoal as the carbonaceous material, with a water content in the carbonaceous material being 7.8 to 15 mass%, or preferably 8.2 to 11 mass%. An oxygen gas is blown into an inside of the electric furnace from an oxygen feed lance 5, and a distance from a carbonaceous material addition location 12 at which a central axis of the coal feed lance 5 intersects with a molten iron surface 14 to an ignition point center location 13 at which a central axis of the oxygen feed lance 5 intersects with the molten iron surface 14 (the distance between the carbonaceous material addition location - the ignition point center location) is within 1 m. However, the biomass charcoal means carbonized biomass.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a refining method in an electric furnace when refining molten iron in an electric furnace. [Background technology]

[0002] When refining molten iron in an electric furnace, an arc is formed between a graphite electrode and the iron source or molten iron, and the molten iron temperature is raised by heating with the arc. Flux is added to the electric furnace, and a slag layer made of molten flux is formed on the surface of the molten iron. At this time, the slag is thickened by foaming to form foamed slag. By enveloping the arc between the graphite electrode and the molten iron with foamed slag, nitrogen gas from the atmosphere is prevented from being entrapped in the arc, nitrogen absorption into the molten iron is suppressed, and the final nitrogen concentration in the molten iron can be reduced. To achieve this, a technology is needed to stabilize the foaming height of the foamed slag at a high level.

[0003] Conventionally, as shown in Figure 1, a coal supply lance 4 is used in an electric furnace 1 to spray powdered carbonaceous material together with a carrier gas onto molten iron 10, and another oxygen supply lance 5 is used to spray a gas mainly composed of oxygen onto the molten iron 10. This causes the carbonaceous material to react with oxygen, generating CO gas, which is then suspended in the slag, causing the slag to foam.

[0004] In electric furnace steelmaking, coal and the like have traditionally been used as a carbonaceous material. Regarding these carbonaceous materials, the use of biomass charcoal produced by carbonizing plants has been considered from the perspective of reducing fossil fuel use to prevent global warming and achieving carbon neutrality with the aim of zero emissions. For example, Patent Document 1 discloses a technology in which biomass charcoal is used as an auxiliary fuel or recarburizer in an electric arc furnace steelmaking process.

[0005] Here, when biomass charcoal is used as the carbonaceous material, there is a problem that the carbon concentration in the carbonaceous material is low and the apparent density of the carbonaceous material is low, resulting in poor formability. As a method for suppressing carbonaceous material scattering, Patent Document 1 discloses a method of using biomass charcoal with high bulk density by using coconut shell as a raw material, but this method limits the raw material and manufacturing method of the carbonaceous material and lacks versatility. On the other hand, increasing the injection rate of the carbonaceous material is thought to be effective, but increasing the supply rate of the carbonaceous material may require modifying or enlarging the equipment, which can be difficult. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-46726 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a refining method in an electric furnace that can stabilize the foaming height of slag at a high level when biomass charcoal is used as the carbonaceous material to be injected into the furnace. [Means for solving the problem]

[0008] That is, the gist of the present invention is as follows. [1] A refining method in an electric furnace, characterized in that when refining molten iron in an electric furnace, a carbonaceous material is injected into the electric furnace from a coal feed lance, biomass charcoal is used as the carbonaceous material, and the moisture content of the carbonaceous material is 7.8 to 15 mass%. However, the biomass charcoal means carbonized biomass. [2] The refining method in an electric furnace according to [1], characterized in that the content of moisture contained in the carbonaceous material is 8.2 to 11 mass %. [3] A refining method in an electric furnace according to [1] or [2], characterized in that oxygen gas is blown into the electric furnace from an oxygen supply lance, the point where the central axis of the oxygen supply lance intersects with the molten iron surface is set as the carbonaceous material addition position, the point where the central axis of the oxygen supply lance intersects with the molten iron surface is set as the ignition point center position, and the distance between the carbonaceous material addition position and the ignition point center position is set to within 1 m. [Effects of the Invention]

[0009] In the present invention, when biomass charcoal is used as the carbonaceous material to be injected into an electric furnace from a coal feed lance, the moisture content of the carbonaceous material is set to 7.8 to 15 mass%, thereby stabilizing the slag foaming height at a high level and enabling sufficient coverage of the arc formed between the graphite electrode and the surface of molten iron. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 2 is a front cross-sectional view showing an outline of an electric furnace. [Figure 2] FIG. 2 is a plan view showing the supply of carbonaceous material by a coal supply lance and the blowing of oxygen gas by an oxygen supply lance. DETAILED DESCRIPTION OF THE INVENTION

[0011] In this invention, biomass is used as the main raw material for the charcoal that is injected into the electric furnace. There are various definitions of biomass, but in this invention, it is defined as "renewable, biologically derived organic resources excluding fossil resources." This is the definition in the "Biomass Japan Comprehensive Strategy," which was approved by the Cabinet in December 2002 and supervised by the Ministry of Agriculture, Forestry and Fisheries. Specific examples of biomass include wood (especially thinned wood), palm oil, rice straw, and rice husks.

[0012] In the present invention, biomass is used as a raw material and carbonized to produce a carbon material. Biomass carbonization is a process in which biomass is heated in a non-oxidizing atmosphere to decompose carbon compounds. Biomass that has been carbonized using biomass as a raw material and carbonized is referred to as "biomass charcoal."

[0013] As mentioned above, the use of biomass charcoal as a carbonaceous material poses the problem of poor foaming properties. Therefore, in this invention, we conceived the idea of ​​foaming slag using not only CO gas generated by the reaction between the injected carbonaceous material and oxygen gas, but also other gases. Specifically, slag foaming is achieved using HO gas in addition to CO gas. To achieve this, the moisture content of the carbonaceous material is controlled within an appropriate range. When the carbonaceous material is sprayed onto molten iron, HO gas is also generated. When the moisture is converted into HO gas, the surrounding area is cooled due to an endothermic reaction. In particular, electric furnace slag is heated to a high temperature by the arc, resulting in low viscosity and high tendency for bubbles to rise and fall. Therefore, the aforementioned cooling increases the slag viscosity, facilitating the suspension of HO gas in the slag. This results in a high slag foaming effect.

[0014] Conventionally, when biomass charcoal is used as the carbonaceous material to be injected into an electric furnace, the moisture content initially decreases during carbonization, and then moisture accumulates during transportation. The moisture content (on arrival basis) of the carbonaceous material upon arrival at the site of use was 6.5 to 7.7 mass%, as shown in Table 1 of Patent Document 1. In contrast, the present invention has discovered that by actively adding moisture to biomass charcoal and adjusting the moisture content of the carbonaceous material to 7.8 to 15 mass%, slag foaming can be successfully achieved. It is more preferable to set the moisture content of the carbonaceous material to 8.2 to 11 mass%. The moisture content of the carbonaceous material can be evaluated as the weight loss when 5 g of a sample crushed to a particle size of 250 μm or less is dried at 107 ± 2°C to constant weight.

[0015] As described above, as shown in Figure 1, powdered carbonaceous material and a carrier gas are sprayed onto molten iron 10 using a coal feed lance 4, and a gas mainly composed of oxygen is sprayed onto molten iron 10 from a separate oxygen feed lance 5. This causes the carbonaceous material and oxygen to react with each other, generating CO gas, which then suspends CO gas bubbles in the slag, causing the slag to foam. Here, as shown in Figures 1 and 2, the point where the central axis of the coal feed lance 4 intersects with the molten iron surface 14 is defined as the carbonaceous material addition position 12, and the point where the central axis of the oxygen feed lance 5 intersects with the molten iron surface 14 is defined as the fire center position 13.

[0016] In conventional electric furnaces, the maximum flow rate of the carrier gas that can be injected when injecting carbonaceous material with the coal feeding lance 4 is 3 Nm per lance due to equipment restrictions. 3 / (hr·ton of molten steel), the carrier gas is not able to sufficiently agitate the slag surface 15 or the molten iron surface 14. As a result, there is a high possibility that the injected carbonaceous material will accumulate on the slag surface or the molten iron surface, and then scatter without being incorporated into the slag 11 or the molten iron 10. Meanwhile, the maximum flow rate of oxygen gas from the oxygen supply lance 5 has traditionally been 30 Nm3 per lance. 3 / (hr·ton of molten steel), which is greater than the carrier gas flow rate from the coal delivery lance 4, it is possible to expect increased agitation of the slag surface 15 and molten iron surface 14 by the injected oxygen gas. Therefore, electric furnace refining was performed while varying the distance between the carbonaceous material addition position 12 defined above and the ignition point center position 13 defined above (distance 16 between the carbonaceous material addition position and the ignition point center position) on the molten iron surface 14. As a result, it was found that better foaming of the slag 11 could be achieved by setting the distance 16 between the carbonaceous material addition position and the ignition point center position within 1 m. Regarding this phenomenon, it can be inferred that if the distance (distance 16 between the carbonaceous material addition position and the carbonaceous material center position) between the center of the oxygen jet supplied from the oxygen supply lance 5 (ignition point center position 13) and the center of the carbonaceous material landing position (carbonaceous material addition position 12) on the molten iron surface 14 is too far, the stirring effect of the oxygen jet supplied with oxygen at the carbonaceous material landing position on the molten iron surface 14 becomes smaller, and although the carbonaceous material accumulates on the slag and molten iron surface, the amount that scatters without being incorporated into the slag 11 and molten iron 10 increases. [Example]

[0017] The present invention was carried out using an AC electric furnace as the electric furnace 1 shown in Figure 1. The electric furnace 1 had 70 t of molten iron from the previous charge and 5 to 8 t of slag remaining in the furnace. In this charge, 110 t of scrap was continuously supplied to the furnace, and raw materials (referred to as auxiliary raw materials) serving as sources of CaO, SiO2, and MgO were also added to form molten slag, and an arc was applied from the graphite electrode 2. Additionally, two lances (coal feed lance 4 and oxygen feed lance 5) installed on the furnace wall 6 were used, with biomass coal as a carbonaceous material being sprayed from the coal feed lance 4 and oxygen gas being sprayed from the oxygen feed lance 5 onto the molten iron 10. The flow rate of the carrier gas from the coal feed lance 4 was 1.5 to 2.5 Nm 3 / (hr·ton of molten steel), the oxygen gas flow rate from the oxygen supply lance 5 is 15 to 25 Nm 3 / (hr·ton of molten steel)

[0018] Biomass charcoal used had a fixed carbon concentration of 70-75% when dried, with other components being volatile matter and ash. This biomass charcoal was allowed to absorb a predetermined amount of moisture to adjust the moisture content. In addition, the distance between the average position of the biomass charcoal landing point (carbonaceous material addition position 12) on the molten iron surface 14 and the center position 13 of the oxygen supply fire point was changed within a range of 0-1.5 m. The moisture content was analyzed using the method described above.

[0019] In this example, scrap and auxiliary materials were added over 38 to 42 minutes, and then the molten iron was heated to 1640 to 1650°C over 7 to 10 minutes. Four to five minutes after the completion of scrap and auxiliary material addition, the arc irradiation from the graphite electrode was temporarily stopped, and the height of the slag being formed was measured. The amount of slag was 100 to 110 kg per ton of molten iron, and the distance between the tip of the graphite electrode 2 and the molten iron surface 14 was 440 to 460 mm.

[0020] The production conditions and results are shown in Table 1. Nos. 1 to 11 in Table 1 are examples of the present invention, and Nos. 12 to 17 are comparative examples. Numerical values ​​outside the range of the present invention and the preferred range of the present invention are underlined.

[0021] [Table 1]

[0022] Table 1 shows the relationship between the moisture content of the biomass charcoal and the distance 16 between the carbonaceous material addition position and the center position of the hot spot on the resulting foamed slag height. In Tests 1 to 7, when the moisture content of the biomass charcoal was in the range of 7.8 to 15 mass% and the distance 16 between the carbonaceous material addition position and the center position of the hot spot was 1.1 to 1.5 m, the foamed slag height was 460 to 510 mm, and the foamed slag was able to cover the arc generated between the graphite electrode 2 and the molten iron surface 14. In particular, in Tests 1 to 4, where the moisture content of the carbonaceous material was 7.8 to 8.0 mass% or 11.5 to 15 mass%, the foamed slag height was 460 to 480 mm, whereas in Tests 5 to 7, where the moisture content of the carbonaceous material was 8.2 to 11 mass%, the foamed slag height was 490 to 510 mm, indicating that the foamed slag height was even higher.

[0023] Furthermore, when the moisture content was in the range of 7.8% by mass to 12% by mass and the distance 16 between the carbonaceous material addition position on the molten iron surface and the center position of the hot spot was 1.0 m or less, as in Tests 8 and 9, the foamed slag height was 490 to 510 mm, which was even higher than Tests 1 to 4. Furthermore, when the moisture content was in the range of 8.2 to 11% by mass and the distance 16 between the carbonaceous material addition position on the molten iron surface and the center position of the hot spot was 1.0 m or less, as in Tests 10 and 11, the foamed slag height was 520 to 530 mm, which was even higher than Tests 5 to 7. This is presumably because, when the distance 16 between the carbonaceous material addition position and the center position of the hot spot was close to 1.0 m or less, the oxygen jet from the oxygen supply lance 5 had a significant agitation effect on the molten iron surface 14 at the carbonaceous material addition position 12, improving the proportion of the carbonaceous material that was incorporated into the slag and molten iron without scattering.

[0024] On the other hand, in Tests Nos. 12, 13, and 16, the foamed slag height was low at 370 to 430 mm, and the arc was exposed from the slag. This was because the moisture content was low at less than 7.8 mass%, and the HO gas generation rate was insufficient. In Tests Nos. 14, 15, and 17, the foamed slag height was low at 380 to 420 mm, and the arc was exposed from the slag. This is presumably because the moisture content exceeded the upper limit specified in the present invention, and the HO gas generation rate was high, causing the HO gas bubbles in the slag to coarsen, making them more likely to float and detach from the slag. [Explanation of symbols]

[0025] 1 electric furnace 2. Graphite electrodes 4 Coal feeding lance 5 Oxygen supply lance 6 Furnace wall 10 Molten Iron 11 Slag 12 Carbon material addition position 13 Fire point center position 14 Molten iron surface 15 Slag surface 16 Distance between carbonaceous material addition position and center of fire point

Claims

1. A refining method in an electric furnace, characterized in that, when refining molten iron in an electric furnace, a carbonaceous material is injected into the electric furnace from a coal feeding lance, biomass charcoal is used as the carbonaceous material, and the moisture content of the carbonaceous material is set to 7.8 to 15 mass%. However, the biomass charcoal means carbonized biomass.

2. 2. The refining method in an electric furnace according to claim 1, wherein the content of moisture contained in the carbonaceous material is 8.2 to 11 mass %.

3. A refining method in an electric furnace as described in claim 1 or claim 2, characterized in that oxygen gas is blown into the electric furnace from an oxygen supply lance, the point where the central axis of the carbon supply lance intersects with the molten iron surface is the carbon material addition position, the point where the central axis of the oxygen supply lance intersects with the molten iron surface is the ignition point center position, and the distance between the carbon material addition position and the ignition point center position is within 1 m.

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