Carburizing method of molten iron

Abstract

The present invention provides a method for carburizing molten iron in an electric furnace, which allows for the use of biomass char as a carburizing material without imposing restrictions on the raw materials or carbonization methods of the biomass char. [Solution] The charcoal material is biomass charcoal, containing a total of 12-41% by mass of volatile components and water. A method for carburizing molten iron, characterized in that the angle θ between the central axis of the lance and the stationary molten iron surface is in the range of 15 to 45°, and the carbon material is blown in such a way that the central flow velocity v (m / s) of the carrier gas at the tip of the lance satisfies the following equation (1). [Math 1] TIFF2026047794000014.tif16146 However, d p :Average particle diameter of the carbon material by mass (mm), h:Height measured vertically from the stationary molten iron surface to the center of the lance tip (mm), d:Inner diameter of the nozzle at the tip of the lance (mm).

Description

[Technical Field]

[0001] The present invention relates to a method for carburizing molten iron, and more particularly to a method for carburizing molten iron using biomass char as a carburizing material in an electric furnace. [Background technology]

[0002] In the steel industry, coal and other carbon materials are used as reducing agents and auxiliary fuels. From the perspective of reducing the use of fossil fuels to prevent global warming, these carbon materials are being considered as replacements with carbonized plant biomass materials (hereinafter referred to as biomass charcoal). Here, biomass is defined in the "Biomass Japan Comprehensive Strategy," which is under the jurisdiction of the Ministry of Agriculture, Forestry and Fisheries and was approved by the Cabinet in December 2002, as "renewable, biologically derived organic resources excluding fossil resources." Specifically, it refers to plants themselves, waste and unused materials generated from processing, and waste and unused materials from food and sewage sludge. Among these, plant biomass is suitable for use in the steel industry because its properties such as composition, moisture content, and shape are stable during carbonization, and it is also desirable in terms of carbon dioxide recycling. Specifically, plant biomass refers to coffee grounds, seaweed, wood, coconut shells, and rice husks. Examples of wood include construction waste, sawmill residues, and forest residues. Examples of tree species include evergreen trees such as Japanese cedar, cypress, pine, and eucalyptus. The above are examples of plants that can be used as raw materials for biomass charcoal, but there are no restrictions on the raw materials as long as they do not deviate from the definition mentioned above.

[0003] Regarding the technology for using biomass coal in electric furnaces, for example, Patent Document 1 discloses a technology for using biomass coal as an auxiliary fuel or carburizing agent in arc furnace steelmaking.

[0004] Biomass coal has properties that differ from conventionally used coal, mainly in its microstructure. More specifically, because biomass coal has a porous structure derived from plant cell walls, it is characterized by a lower apparent density compared to coal, and has problems such as scattering due to the influence of air currents in electric furnaces, which reduces utilization efficiency. Patent Document 1 aims to solve this problem by developing a material with a bulk density of 0.50 t / m³ on an anhydrous basis. 3 To utilize the biomass charcoal described above, a technology has been published that uses coconut or oil palm shells as a raw material for biomass charcoal. However, biomass charcoal that can be used in steelmaking processes has challenges in that it is limited in terms of raw materials and carbonization methods, and there are concerns about procurement risks, etc. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2009-046726 [Non-patent literature]

[0006] [Non-Patent Document 1] Ozawa et al., "Penetration Behavior of Solid Particles into Liquid Metals," Iron and Steel, 69 (1983), pp. 753-759. [Overview of the project] [Problems that the invention aims to solve]

[0007] The present invention aims to provide a method for carburizing molten iron in an electric furnace, which allows for the use of biomass char as a carburizing material without imposing any restrictions on the raw materials or carbonization methods of the biomass char. [Means for solving the problem]

[0008] In other words, the gist of this invention is as follows: [1] A method for carburizing molten iron in an electric furnace having a coal-feeding lance for blowing coal material into the furnace, The carbon material is biomass charcoal produced by carbonizing plant-based biomass, and is a carbon material containing a total of 12 to 41% by mass of volatile components and moisture.

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Advantages of the Invention

[0009] The present invention provides a method for adding carbon to molten iron that can efficiently use biomass charcoal as a reducing agent and auxiliary fuel in an electric furnace without being restricted by the density of the carbon material when using biomass charcoal as a carbon addition material in the electric furnace.

Brief Description of the Drawings

[0010] [Figure 1] It is a schematic cross-sectional view showing an electric furnace, (A) is a side view, and (B) is a front view. [Figure 2] It is a view showing a lance arranged obliquely with respect to the static molten iron surface.

Embodiment for Carrying Out the Invention

[0011] This invention was made in view of the above background and provides a technology for the highly efficient use of biomass charcoal as a reducing agent or auxiliary fuel, regardless of its apparent density. The inventors conducted tests to investigate in detail the behavior of biomass charcoal when it comes into contact with molten iron. Specifically, biomass charcoal was dropped onto molten iron using a small furnace and photographed using a high-speed camera. As a result of the tests, it was observed that when biomass charcoal comes into contact with molten iron, it bursts accompanied by the ejection of gas. This is caused by the rapid increase in volume when volatile components and moisture contained in the biomass charcoal vaporize due to the temperature rise caused by contact with molten iron, and the porous structure is destroyed from the inside. This is a phenomenon not seen in conventionally used charcoal materials such as coal. After bursting, the pulverized biomass charcoal becomes even more easily scattered than before contact with molten iron, but because its surface area is larger, it can be expected to dissolve into the molten iron with high efficiency if contact with the molten iron can be maintained. Therefore, the inventors considered ways to maintain contact between the ruptured biomass char and molten iron, and conceived of achieving this by allowing the char to penetrate deeply into the molten iron and rupture it inside the molten iron.

[0012] Next, the inventors investigated the supply conditions for the char material that would enable biomass char to penetrate sufficiently deep into the molten iron. Non-patent document 1 describes the critical conditions for solid particles to penetrate molten metal using the dimensionless Weber number (hereinafter referred to as We), where We is shown in equation (2) c When it is greater than this, solid particles can penetrate the molten metal. Equation (2) below is obtained by substituting equation (9) into equation (15) in Non-Patent Literature 1. Equations (3) and (4) below are obtained from equations (6) and (14) in Non-Patent Literature 1.

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Equation

Equation

[0013] In the process of blowing carbonaceous particles from the lance, the velocity of the particles is determined by the velocity of the carrier gas. The flow velocity of the carrier gas discharged from the lance decreases as it moves away from the lance. The following Equation (7) is derived from the law of conservation of momentum of the carrier gas.

Equation

[0014] In an electric furnace, generally as shown in Figure 2, noting that the lance 15 is arranged obliquely with respect to the stationary molten iron surface 14, taking θ: Angle (rad) formed by the central axis 16 of the lance and the stationary molten iron surface 14, h: Height (mm) measured vertically from the stationary molten iron surface 14 to the tip center 17 of the lance 15, since L = h / sinθ, the v in Equation (7) L is the v in Equation (6) pBy substituting this, the discharge rate of the carrier gas required to penetrate the powder into the molten iron can be expressed by equation (8).

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[0015] In this invention, it is necessary for the biomass charcoal particles to be pulverized after entering the molten iron and before floating up, and to remain in contact with the molten iron. Therefore, the required carrier gas flow velocity is greater than the velocity determined by the right-hand side of equation (8). Furthermore, the presence of slag on the surface of the molten iron in the electric furnace inhibits the movement of the carrier gas, and the gas blowing itself deforms the molten iron surface. Both of these factors work unfavorably for particle penetration, requiring a blowing velocity greater than that on the right-hand side of equation (8). The inventors conducted tests to determine how much the carrier gas flow velocity needs to exceed the right-hand side of equation (8). Specifically, they first substituted 2tanφ = 2tan12.08° = 0.428 into equation (8), and then √(2σ) in equation (8). gl W ec / ρ l Assuming that ) is a constant, the value of this constant was determined based on the test results, and it was found to be √(2σ gl W ec / ρ l The result obtained was ) = 11.06. Substituting this result into equation (8), equation (1) was obtained. That is, it was found that under conditions that satisfy equation (1), the scattering of pulverized biomass charcoal can be suppressed and the utilization efficiency can be increased.

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[0016] Next, the inventors investigated the applicability range of formula (1). Firstly, regarding θ, under conditions where θ is small, the decay of powder particle velocity becomes significant, and the utilization efficiency of the carbon material decreases. Also, in the range where θ is large, although the powder penetrates the molten iron, its horizontal velocity is small, causing it to accumulate near the landing point, which hinders the penetration of carbon material blown in later, thus reducing the utilization efficiency. As a result of tests conducted by the inventors, the utilization efficiency of the carbon material was high in the range of θ from 15 to 45°, and particularly high in the range of 16 to 40°, and even more preferably in the range of 16 to 25°.

[0017] Furthermore, for coal material to rupture, it needs to contain a certain amount of volatile matter and moisture to generate gas. Also, if the volatile matter and moisture content is high, the amount of fixed carbon, which is effective as a reducing agent and auxiliary heat source, decreases, and the utilization efficiency of the coal material actually decreases. As a result of tests conducted by the inventors, it was found that the total amount of volatile matter and moisture that maximizes the utilization efficiency of the coal material is in the range of 12 to 41 mass%, and in particular, the utilization efficiency is further increased in the range of 15 to 35%. Note that the moisture content in the coal material referred to here means the moisture contained in the coal material particles. The moisture content should be determined based on the arrival basis, i.e., the condition of the lot upon delivery (i.e., the total moisture content), as described in the "Dictionary of Coal Utilization Technology Terms," ​​edited and published by the Combustion Association, December 26, 1983.

[0018] The following describes preferred embodiments of the present invention from the viewpoint of carbon particle size. When the particle size is small, the movement of the particles is easily affected by disturbances. On the other hand, if the particle size is too large, the reaction area after rupture is relatively small, making it difficult to enjoy the effect of improving utilization efficiency. From the results of tests, the inventors found that the utilization efficiency of the carbon material can be improved when the particle size of the carbon material used is 1.0 to 20.0 mm, and more preferably 3.0 to 12.0 mm. [Examples]

[0019] In implementing the present invention, six types of biomass charcoal shown in Table 1 were used. The volatile matter and moisture content of the biomass charcoal were altered by changing the heat treatment temperature and time during the carbonization process. In addition, the fixed carbon content, volatile matter content, and moisture content of one type of anthracite and the six types of biomass charcoal used as comparative examples are also shown in Table 1. The apparent density of the anthracite and the various biomass charcoals was measured using the underwater weighing method described in JIS Z 8807-2012, and the fixed carbon content and volatile matter content were measured in accordance with JIS M 8812. The moisture content was measured in accordance with JIS M 8820. However, the moisture content measured according to JIS M 8820 includes moisture adhering to the surface of the charcoal material. However, because biomass charcoal is porous, moisture easily penetrates into the pores, and the moisture adhering to the surface of the charcoal material is less than the moisture inside the charcoal material. Therefore, the moisture measured according to JIS M 8820 can be considered as the moisture inside the charcoal material. Furthermore, the biomass charcoal was supplied into the furnace as a powder, and the average particle size was calculated by measuring the mass-based particle size distribution in advance using a sieving method.

[0020] [Table 1]

[0021] Table 2 shows an example of implementing the present invention in an electric furnace with an internal diameter of 6.5 m, capable of holding 175 tons of molten iron, a tapping capacity of 110 tons, and leaving 65 tons of seed molten iron in the furnace after tapping, along with a comparative example.

[0022] [Table 2]

[0023] The electric furnace will be described below with reference to Figure 1. Both Figure 1s are vertical cross-sectional views of the electric furnace 1. Figure 1(A) is a side view corresponding to a cross-section perpendicular to the slag outlet 4, which will be described later, and Figure 1(B) is a front view corresponding to a cross-section perpendicular to the side view. The electric furnace 1 has a furnace bottom 10 at the bottom of the furnace body and a furnace lid 9 at the top of the furnace body. Three graphite electrodes 2 are inserted into the furnace from the furnace lid 9 as upper electrodes, and power is supplied to the furnace. Scrap is used as the iron source, and raw materials are continuously charged by a horizontal conveyor 3. It should be noted that the embodiments of the present invention are not limited to the number of upper electrodes and the power supply method, and can be implemented without departing from the spirit of the present invention, regardless of whether it is a single-phase AC type, a DC type, or the number of electrodes. Similarly, the raw material charging method can also be changed without departing from the spirit of the present invention, such as batch type or shaft type continuous feeding.

[0024] As shown in the side view of Figure 1(A), the electric furnace 1 has a slag outlet 4 on its side wall that is 1000 mm wide and 995 mm high. The insertion of the door lance 6 and sampling of the molten iron 13, which will be described later, were performed through the slag outlet 4. The electric furnace 1 also has a tapping port 5.

[0025] The electric furnace 1 has a movable door lance 6 with an outlet diameter of 28 mm, as shown in the side view of Figure 1(A), which serves as the coal supply lance 11. It can supply a maximum of 60 kg / min of coal material into the furnace via the slag port 4. The height and angle of the door lance 6 can be controlled individually during its movement. In implementing the present invention, the height h of the tip center 17 of the lance relative to the stationary molten iron surface 14 and the angle θ of the lance central axis 16 were changed within the movable range of the door lance 6 (see Figure 2). Carbon dioxide with a purity of 99.5% by volume or higher was used as the carrier gas for supplying biomass coal. The maximum flow rate of the carrier gas was 200 Nm³. 3 At 1 / h, the maximum flow velocity v at the center of the lance tip is 361 m / s. Here, it is known that the flow velocity v at the center of the lance tip is four times the average linear flow velocity obtained by dividing the carrier gas flow rate by the nozzle outlet cross-sectional area, and was calculated from the carrier gas flow rate during carbon injection and the nozzle inner diameter d at the lance tip.

[0026] The electric furnace 1 has a coal supply lance 11 used for supplying coal material, as well as oxygen lances 12 for supplying oxygen gas into the furnace for the purpose of supplying a heat source by oxidation and refining. These include one door lance 6 shown in the side view and two fixed lances 7 fixed to the furnace wall 8 shown in the front view. A maximum of 4000 Nm³ of oxygen gas can be supplied from each oxygen lance 12. 3 / h, total 10000Nm 3 A supply of oxygen gas at a rate of / h can be placed into the furnace. In the present invention example and comparative example, oxygen gas with a purity of 99.5% by volume or higher was used.

[0027] In the present invention example and comparative example, the raw material was continuously fed in while an arc was applied using the graphite electrode 2, oxygen gas was supplied by the oxygen lance 12, and carbon material was supplied by the carbon supply lance 11 to melt the iron raw material. Furthermore, when 30% of the raw material had been charged, 50 kg / t-steel of quicklime was added. After the entire amount of raw material had been charged, the arc was further applied, oxygen gas and carbon material were supplied, and the furnace temperature was raised and the molten iron was oxidized and refined to produce molten steel. After that, the molten steel was sampled and analyzed. The concentrations of carbon and phosphorus in the collected sample were measured using spark emission spectrometry, and after confirming that the predetermined molten steel components had been obtained, the tap opening 5 provided at the bottom of the furnace was opened and the molten steel was poured into a ladle (not shown). Here, the predetermined components in the present invention example and comparative example were 0.04 to 0.05 mass% of carbon and 0.015 to 0.018 mass% of phosphorus.

[0028] During the supply of charcoal, the angle θ of the lance's central axis relative to the stationary molten iron surface was measured from the insertion angle of the door lance 6. Furthermore, the height of the lance tip center measured from the lowest point of the furnace bottom was calculated from the insertion angle and length of the door lance 6. The height h of the lance tip center measured from the stationary molten iron surface was calculated by subtracting the depth of molten iron calculated from the amount of iron source charged, and this height was controlled to remain constant.

[0029] The effect of the present invention on improving the utilization efficiency of charcoal materials was evaluated from the perspective of the unit consumption of charcoal materials. Here, since the proportion of carbon content differs depending on the type of charcoal material, we focused on the fixed carbon content of the charcoal material and multiplied the unit consumption of the charcoal material used by the fixed carbon content (mass%) and divided by 100, and the value "of which fixed carbon content" is also listed in Table 2.

[0030] Furthermore, for Comparative Examples 5-10 and Examples 1-19, we evaluated them by marking ◎ for particularly good fixed carbon content per unit, ○ for equivalent to or reduced to anthracite, and × for increased carbon content per unit.

[0031] Compared to the anthracite shown in Comparative Examples 1-4, the unit consumption worsened in Comparative Example 5 (higher volatile matter and moisture content) and Comparative Example 6 (lower volatile matter and moisture content) of the biomass coal, in Comparative Example 7 (smaller biomass coal injection angle) and Comparative Example 9 (larger biomass coal injection angle), and in Comparative Examples 8 and 10 (lower injection speed).

[0032] On the other hand, in Examples 1 to 17, biomass charcoal was used while achieving a unit consumption rate equivalent to that of anthracite. In particular, in Examples 11 to 16, 18, and 19, where the particle size of the charcoal material was 1.0 to 20.0 mm, the unit consumption rate was further reduced compared to Example 10, where the particle size was smaller than the preferred range, and Example 17, where it was larger, using the same biomass charcoal. [Explanation of Symbols]

[0033] 1 Electric furnace 2 Graphite electrodes 3. Horizontal conveyor 4. Sludge outlet 5 Steel tapping port 6-door lance 7 Fixed Lance 8 Wall surfaces 9 Hearth cover 10 Hearth bottom 11 Coal transport lance 12 Oxygen Lance 13 Molten iron 14. Static molten iron surface 15 Lance 16 Center axis 17 Tip center

Claims

1. A method for carburizing molten iron in an electric furnace having a coal-feeding lance for blowing coal material into the furnace, The aforementioned charcoal material is biomass charcoal produced by carbonizing plant-based biomass, and is a charcoal material containing a total of 12 to 41% by mass of volatile components and moisture. A method for carburizing molten iron, characterized in that the angle θ between the central axis of the lance and the stationary molten iron surface is in the range of 15 to 45°, and the carbon material is blown in such a way that the central flow velocity v (m / s) of the carrier gas at the tip of the lance satisfies the following equation (1). [Math 1] However, d ｐ :Average particle diameter of the carbon material by mass (mm), h:Height measured vertically from the stationary molten iron surface to the center of the lance tip (mm), d:Inner diameter of the nozzle at the tip of the lance (mm).

2. The average particle diameter d of the carbon material based on mass ｐ The method for carburizing molten iron according to claim 1, characterized in that the thickness is 1.0 to 20.0 mm.

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