Method for dephosphorizing molten iron

Abstract

The present invention provides a method for dephosphorizing molten iron, which can effectively increase the phosphorus distribution ratio and promote the dephosphorizing reaction of the molten iron by applying electric energy by using an electrochemical method. A method for dephosphorizing molten iron, in which an electrode in contact with the molten iron is used as an anode and an electrode only in contact with molten slag is used as a cathode, and an electric current is applied between the molten slag and the molten iron through the two electrodes. The applied current density I of the current satisfies the following relational expression (1) among the molten iron temperature T of the molten iron, the phosphorus distribution ratio Lp of the molten slag, and the necessary phosphorus distribution ratio Lp'of the molten slag. In relational expression (1), I represents the applied current density (A / m2), alpha represents a constant, Lp represents the phosphorus distribution ratio (-) of the molten slag, Lp'represents the necessary phosphorus distribution ratio (-) of the molten slag, and T represents the molten iron temperature (K) of the molten iron.

Description

Technical Field

[0001] This invention relates to a method for dephosphorizing molten iron. In particular, this invention relates to a method for dephosphorizing molten iron that can promote the dephosphorization reaction. Background Technology

[0002] Phosphorus in molten steel tends to segregate at grain boundaries in materials formed from molten steel, reducing the strength and toughness of the steel. Therefore, the phosphorus content in molten steel is usually controlled to be low during the steelmaking process.

[0003] In recent years, the demand for high-grade steel manufacturing has been increasing, requiring further reductions in the phosphorus content of molten steel. However, due to factors such as the declining quality of iron ore, a raw material for steelmaking, the burden of dephosphorization treatment in molten steel has become increasingly severe. From this technical perspective, the development of technologies to reduce the phosphorus concentration in molten steel after treatment has become essential.

[0004] In existing steel dephosphorization technologies, because the phosphorus oxides in molten steel are acidic, lime-based fluxes such as CaO are added to the molten steel during production to increase the basicity of the resulting molten slag. By adding lime-based fluxes such as CaO to the molten steel, the dephosphorization capacity of the molten slag can be improved, and the equilibrium phosphorus concentration of the molten steel can be reduced.

[0005] However, molten slag with high basicity has a high melting temperature. During the pretreatment of molten pig iron, as the steel temperature decreases, the viscosity of the molten slag increases. Therefore, the slag formation of CaO from the lime-based flux absorbed by the molten steel is insufficient, leading to a decrease in dephosphorization efficiency. To improve this lower dephosphorization efficiency and compensate for the reaction efficiency of the dephosphorization reaction, an excessive amount of refining agent needs to be added to the molten steel. Using an excessive amount of refining agent increases the cost of refining the molten steel. Furthermore, to reduce the phosphorus concentration in the molten steel to extremely low levels, the amount of molten slag becomes excessive. This results in problems such as insufficient capacity in the steel refining equipment or slag discharge equipment.

[0006] Therefore, in the dephosphorization treatment of molten steel with the goal of extremely low phosphorus, in order to promote the slag formation of CaO contained in lime-based flux and reduce the amount of flux added, the following method is adopted: adding halides such as fluorite (CaF2) to balance dephosphorization capacity and reduce the amount of molten slag, thereby maintaining a high dephosphorization rate in the extremely low phosphorus region.

[0007] However, adding halides such as fluorite (CaF2) to molten steel increases the content of fluorine (F) and other compounds in the resulting molten slag. In recent years, with increasing public concern about environmental issues, the use of fluorine (F) leaching, which may cause problems, has been restricted. From this perspective, it is difficult to use halides such as fluorite (CaF2) in molten steel. Therefore, a technology is needed to efficiently dephosphorize molten steel without increasing slag basicity.

[0008] In view of the above, research has been conducted on technologies for efficiently dephosphorizing molten steel without increasing the basicity of the molten slag, focusing on electrical energy. For example, Non-Patent Literature 1 discloses a method for promoting the dephosphorization reaction (hereinafter referred to as the slag-metal reaction) that occurs at the slag-molten iron interface based on an electrochemical concept. According to Non-Patent Literature 1, the concept of promoting the reaction by electrical energy is demonstrated by polarizing the potential of the molten iron to the positive potential side to oxidize and remove iron in the slag, and conversely, polarizing it to the negative potential side to reduce iron ions in the slag and return them to the molten iron. Subsequently, various studies related to the slag-metal reaction have been reported based on Non-Patent Literature 1.

[0009] Patent document 1 discloses a method for reducing the content of granular iron in molten slag and the fluctuation of metallic iron content in slag per furnace by energizing an electrode in contact with molten slag and an electrode in contact with an iron bath.

[0010] Patent Document 2 discloses a method for transferring impurities such as phosphorus and sulfur from steelmaking slag to molten scrap iron deposited at the bottom of the furnace, where they are absorbed and regenerated into steelmaking slag. The steelmaking slag regeneration method described in Patent Document 2 applies an electric current by using the slag-side electrode as the anode and the molten scrap iron side as the cathode, thereby reducing phosphorus and sulfur in the slag and transferring them to the molten scrap iron.

[0011] Existing technical documents

[0012] Patent documents

[0013] Patent Document 1: Japanese Patent No. 7158570 Specification

[0014] Patent Document 2: Japanese Patent Application Publication No. 11-302719

[0015] Non-patent literature

[0016] Non-patent document 1: Masanori Tokuda, “Coupling phenomenon in slag-metal reaction”, Proceedings of the Japan Institute of Metals, Vol. 15, No. 6 (1976), accepted on February 17, 1976. Summary of the Invention

[0017] The problem that the invention aims to solve

[0018] However, these existing technologies still have the following problems that need to be addressed. Specifically, Non-Patent Document 1 discloses a method for promoting the dephosphorization reaction based on the electrochemical treatment of the slag-metal reaction. However, the methods for promoting the dephosphorization reaction of molten iron described in Non-Patent Document 1 all involve altering the mixing potential or equilibrium potential of the dephosphorization reaction system of molten iron through additives, etc., rather than promoting the dephosphorization reaction by applying electrical energy externally. On the other hand, the technology disclosed in Patent Document 1 involves applying an electric current between the slag and metal. Moreover, in the molten slag treatment method described in Patent Document 1, the purpose of applying an electric current between the slag and metal is to reduce the content and fluctuation of granular iron present in the slag. In other words, in the molten slag treatment method described in Patent Document 1, the effect of applying electricity between the slag and metal is to promote the agglomeration and coarsening of granular iron; therefore, the impact on the electrochemical reaction is not considered, nor is it to promote the dephosphorization reaction of molten iron.

[0019] In the technology disclosed in Patent Document 2, electrical energy is used to heat the molten slag and scrap iron to melt them, without contributing to the electrochemical reaction. Furthermore, the technology disclosed in Patent Document 2 promotes the removal of phosphorus from the molten slag and its transfer to the molten iron, a process known as phosphorus reversion, rather than promoting the dephosphorization reaction in the molten iron.

[0020] The present invention was made in view of the above circumstances, and its purpose is to provide a method for dephosphorizing molten iron, which can apply electrical energy by electrochemical means to effectively increase the phosphorus distribution ratio and promote the dephosphorization reaction of molten iron.

[0021] Methods for solving problems

[0022] Therefore, in order to solve the above-mentioned problems, the inventors conducted various experiments and found that in the method of dephosphorizing molten iron by applying an electric current between molten slag and molten iron, controlling the applied current based on the relationship between the phosphorus distribution ratio of the molten slag, the necessary phosphorus distribution ratio of the molten slag, and the temperature of the molten iron can effectively increase the phosphorus distribution ratio and promote the dephosphorization reaction of the molten iron. This invention is based on the above insights, and its main points are as follows.

[0023] That is, the dephosphorization method of molten iron of the present invention, which can advantageously solve the above-mentioned problems, is a dephosphorization method of molten iron in which an electrode in contact with the molten iron is used as an anode and an electrode in contact only with the molten slag is used as a cathode, and a current is applied between the molten slag and the molten iron through the two electrodes. The method is characterized in that the applied current density I satisfies the following relationship (1) between the molten iron temperature T, the phosphorus distribution ratio LP of the molten slag and the necessary phosphorus distribution ratio LP' of the molten slag.

[0024]

[0025] In equation (1), I is the applied current density (A / m). 2 ), where α is a constant, LP is the phosphorus distribution ratio of the molten slag (-), LP' is the necessary phosphorus distribution ratio of the molten slag (-), and T is the molten iron temperature (K).

[0026] Furthermore, in the dephosphorization method for molten iron of the present invention, the following can be considered as more preferred solutions: (a) The carbon concentration [C] in the molten iron is 4.0% by mass or less, and the applied current density I (A / m) is... 2 ) satisfies the following relation (2),

[0027] In equation (2), I is the applied current density (A / m). 2 ), LP is the phosphorus distribution ratio of the molten slag (-), LP' is the required phosphorus distribution ratio of the molten slag (-), and T is the molten steel temperature (K) of the above-mentioned molten iron; (b) No electric arc discharge occurs when the current is applied between the molten slag and the molten iron; (c) The current value of the above current is 5000 (A) or less; (d) The liquid phase fraction of the above-mentioned molten slag is 60% or more by volume; etc.

[0028] Invention Effects

[0029] According to the present invention, by applying electrical energy to molten iron using an electrochemical method, the phosphorus distribution ratio can be effectively increased without modifying the slag, thereby promoting the dephosphorization reaction of the molten iron. Attached Figure Description

[0030] Figure 1 This is a schematic diagram showing the outline of a molten iron dephosphorization apparatus for implementing the dephosphorization method of this embodiment.

[0031] Figure 2 This is a graph showing the relationship between the dephosphorization treatment time (minutes) and phosphorus concentration (mass%) of molten iron when implementing the dephosphorization method of this embodiment.

[0032] Figure 3 This is a graph showing the relationship between the applied current density and the phosphorus distribution ratio of phosphorus in the molten iron when applying current to the electrodes using a dephosphorization device for molten iron. Detailed Implementation

[0033] [First Implementation]

[0034] The dephosphorization method for molten iron according to the first embodiment will be described. The dephosphorization method for molten iron in this embodiment is a method in which an electrode in contact with the molten iron is used as the anode, and an electrode in contact only with the molten slag is used as the cathode, and an electric current is applied between the molten slag and the molten iron through these two electrodes. The dephosphorization apparatus for molten iron used to implement the dephosphorization method of this embodiment will be described.

[0035] <Overview of the molten iron dephosphorization unit>

[0036] Figure 1 This is a schematic diagram showing an outline of a molten iron dephosphorization apparatus used in the molten iron dephosphorization method for implementing this embodiment. Figure 1 As shown, the molten iron dephosphorization apparatus 100 includes an MgO crucible 101, a ramming mix 102, and an induction melting furnace 103. Molten iron, formed from the melting of scrap steel, molten pig iron, etc., is loaded into the MgO crucible 101. The MgO crucible 101 is preferably made of a material with low solubility in molten iron and thermodynamic stability.

[0037] In addition to MgO crucible 101, CaO crucibles and Al2O3 crucibles can also be used as crucibles. The shape of MgO crucible 101 is not particularly limited and can be cylindrical. When MgO crucible 101 is cylindrical, its cross-sectional area can be 0.005–0.030 m². 2 For example, it can be 0.018m 2 .

[0038] The ramming mix 102 is a refractory material. The ramming mix 102 covers the sidewalls and bottom surface of the MgO crucible 101 and is attached to the inner wall of the induction melting furnace 103. From the viewpoint of heat resistance and durability, the thickness of the ramming mix 102 is preferably 100-150 mm. The induction melting furnace 103 is a crucible-type low-frequency induction furnace. The induction melting furnace 103 is capable of maintaining molten iron at a high temperature. Induction heaters can be installed on the sidewalls of the induction melting furnace 103.

[0039] Furthermore, the hot metal dephosphorization apparatus 100 includes a cathode 104 and an anode 105 for applying current between the molten slag 200 and the molten iron 300 contained in the MgO crucible 101. The cathode 104 is in contact only with the molten slag 200 and not with the molten iron 300 present below it. Since the temperature of the molten slag 200 in the MgO crucible 101 is extremely high, the cathode 104 is preferably formed of a heat-resistant material. Graphite or artificial graphite is preferred as the material for forming the cathode 104. The cathode 104 can be rod-shaped or flat.

[0040] The anode 105 is in contact with the molten iron 300. The anode 105 is also in contact with both the molten slag 200 and the molten iron 300. Since the molten slag 200 and the molten iron 300 loaded into the MgO crucible 101 are at extremely high temperatures, the anode 105 is preferably made of a heat-resistant material. The material used to form the anode 105 can be a composite material such as carbon or C-MgO. The anode 105 can be, for example, the metal core of a stirring lance immersed in the molten iron (molten metal) to agitate it by blowing in inert gases such as argon or nitrogen, or a graphite-containing refractory brick installed below the surface of the molten iron (molten metal).

[0041] The cathode 104, installed in the induction melting furnace 103, is connected to the negative terminal of the DC power supply 106 located outside the induction melting furnace 103 via a wire 107. The anode 105, installed in the induction melting furnace 103, is connected to the positive terminal of the DC power supply 106 located outside the induction melting furnace 103 via a wire 107. In this way, an electrical circuit is formed by connecting the two electrodes, consisting of the cathode 104 and the anode 105, to the DC power supply 106 located outside the induction melting furnace 103.

[0042] The upper surfaces of the MgO crucible 101, ramming mix 102, and induction melting furnace 103 in the molten iron dephosphorization apparatus 100 are covered by heat insulation plates 108. The heat insulation plates 108 block the upper surfaces of the MgO crucible 101, ramming mix 102, and induction melting furnace 103, maintaining the temperature of the molten iron 300 contained in the MgO crucible 101. The heat insulation plates 108 can be made of any heat-insulating material; there are no particular restrictions.

[0043] <Dephosphorization of Molten Iron Based on Electric Current>

[0044] In the dephosphorization method for molten iron in this embodiment, the electrode in contact with the molten iron is used as the anode, and the electrode in contact only with the molten slag is used as the cathode. Current is applied between the molten slag and the molten iron through these two electrodes.

[0045] The following describes the dephosphorization of molten iron based on the application of electric current in the dephosphorization method of this embodiment.

[0046] Industrial pure iron is loaded into an MgO crucible 101. Ramming material 102 is embedded in the outer wall of the MgO crucible 101, and the industrial pure iron is melted using an induction melting furnace 103 to produce molten iron 300. The phosphorus concentration in the molten iron 300 is adjusted to reach a specified range, thereby producing molten iron 300. Here, the phosphorus concentration in the molten iron 300 can be 0.01 to 0.20% by mass, preferably 0.05 to 0.15% by mass, more preferably 0.08% by mass. Furthermore, the total amount of molten iron 300 produced by melting industrial pure iron can be set to 5 to 30 kg, preferably 10 to 20 kg, more preferably 15 kg.

[0047] Furthermore, in order to form molten slag 200 on the upper surface of the molten iron 300 within the system of the molten iron dephosphorization device 100, flux is added to the upper surface of the molten iron 300. The amount of flux added can be appropriately determined based on the internal volume of the MgO crucible 101, the total amount of molten iron 300, etc. For example, the amount of flux added can be 10 to 30 kg / molten iron-t, preferably 15 to 25 kg / molten iron-t, and more preferably 20 kg / molten iron-t.

[0048] There are no particular restrictions on the composition of the flux, as long as it contains components capable of forming molten slag 200. For example, the flux may contain CaO, SiO2, FeO, MgO, etc. When the flux contains CaO, SiO2, FeO, and MgO as its components, their content, based on mass, can be 22.5% (CaO), 28.0% (SiO2), 42.5% (FeO), and 7.0% (MgO).

[0049] After flux is added to the upper surface of the molten iron 300, the temperature of the molten iron 300 inside the MgO crucible 101 is maintained within the range of 1300–1700°C, preferably 1585–1615°C. By adding flux to the molten iron 300 and maintaining its temperature, molten slag 200 and molten iron 300 are formed within the system of the molten iron dephosphorization device 100. Molten slag 200 forms on the surface of the molten iron 300. A molten slag 200 (slag) – molten iron 300 (metal) interface is formed between the molten slag 200 and the molten iron 300.

[0050] The molten slag 200 thus formed has a molten slag composition in which a cathode 104 and an anode 105 for applying current between the molten slag 200 and the molten iron are inserted.

[0051] The cathode 104 is immersed in the molten slag 200 formed within the system of the hot metal dephosphorization apparatus 100. The cathode 104 is immersed only in the molten slag 200. On the other hand, the anode 105 is immersed in both the molten slag 200 and the molten iron 300 formed within the system of the hot metal dephosphorization apparatus 100. That is, the anode 105 can be immersed in both the molten slag 200 and the molten iron 300, which are formed from carbonaceous refractory C-MgO bricks. Thus, a direct current is applied to the molten slag 200 and the molten iron 300 using a direct current power supply 106 between the two electrodes, the cathode 104 and the anode 105, provided on the hot metal dephosphorization apparatus 100.

[0052] The applied current density between the electrodes is preferably determined after comprehensively considering the dephosphorization treatment time, the necessary phosphorus distribution ratio based on the target phosphorus concentration, and the power cost. Specifically, the achievable phosphorus concentration can be reduced and the dephosphorization rate increased by setting the applied current density between the electrodes to increase the applied current density.

[0053] Therefore, although applying a large current between the two electrodes can shorten the processing time required for dephosphorization of molten iron and obtain molten iron (molten metal) with a phosphorus concentration below the target level, the power cost would be too high. From this technical point of view, the applied current density can be set in a manner that satisfies the following relationship (1) to apply current between the electrodes.

[0054]

[0055] In equation (1), I is the applied current density (A / m). 2 ), where α is a constant, LP is the phosphorus distribution ratio of the molten slag (-), LP' is the necessary phosphorus distribution ratio of the molten slag (-), and T is the molten iron temperature (K).

[0056] In the dephosphorization method for molten iron in this embodiment, the applied current density I calculated using formula (1) is the current density that can achieve the dephosphorization effect of molten iron to the minimum. Here, when a current density of more than or equal to the applied current density I calculated using formula (1) is applied between the molten slag 200 and the molten iron 300, there are two main effects.

[0057] First, as shown in equation (5) below, the production efficiency of molten steel can be improved by increasing the reaction rate of the dephosphorization reaction of molten iron.

[0058] Second, the electricity cost required for dephosphorizing molten iron will increase. In other words, if the applied current density used for dephosphorizing molten iron increases excessively, both productivity and production costs will rise. Therefore, the upper limit of the applied current density used for dephosphorizing molten iron should be determined by comprehensively considering factors such as the allowable time, cost, and allowable current of the power supply for the dephosphorizing operation.

[0059] From this technical point of view, the value of the applied current density I can be calculated using the relation (1), which not only ensures the dephosphorization effect of molten iron, but also provides the preferred conditions for the dephosphorization method of molten iron that take into account the allowable time, cost and allowable current of the power supply for the operation of dephosphorizing molten iron. Therefore, it is of great significance.

[0060] Specifically, in the dephosphorization method for molten iron in this embodiment, the applied current density I can be set to a value of 150 to 600 (A / m²) using the relationship (1). 2 The preferred value is 200–500 (A / m). 2 If the applied current density I is 150 (A / m). 2 This reduces the phosphorus concentration in the molten iron (300 μL) and increases the dephosphorization rate, making it a preferred method. If the applied current density I is 600 A / m³, it is preferable. 2 The following parameters can suppress the electricity cost during the dephosphorization treatment of molten iron 300, making it a preferred option.

[0061] Then, in the dephosphorization method of molten iron in this embodiment, the phosphorus concentration of phosphorus contained in molten iron 300 can be measured after a certain period of time has elapsed since the time when current is first applied between the two electrodes.

[0062] Figure 2 This is a graph showing the time-varying phosphorus concentration in molten iron when an electric current is applied between the electrodes to perform dephosphorization using a molten iron dephosphorization device. That is, Figure 2 The relationship between dephosphorization treatment time (minutes) and phosphorus concentration (mass %) in molten iron is shown. Figure 2 As shown, the higher the applied current value, the higher the dephosphorization rate and the lower the phosphorus concentration reached.

[0063] Figure 3 This is a graph showing the relationship between the applied current density and the phosphorus distribution ratio of phosphorus in the molten slag when using a hot metal dephosphorization device to apply current to the electrodes for dephosphorization treatment of molten iron. Figure 3As shown, it can be understood that the logarithm of the phosphorus distribution ratio Lp in the molten slag 200 increases linearly with respect to the applied current density I applied to the two electrodes. This trend remains the same even when the composition of the molten slag 200 is changed. Furthermore, this trend is maintained even when dephosphorizing the molten iron 300 by combining it with agitation using an air-blowing lance. Moreover, the inventors have confirmed that even when the steel temperature T is changed when the dephosphorization method of the molten iron 300 is applied, the logarithm of the phosphorus distribution ratio in the molten slag 200 increases linearly with respect to the applied current density I.

[0064] The principle behind the dephosphorization method for molten iron in this embodiment, which promotes the dephosphorization reaction, is as follows: By applying an electric current to the molten iron 300 and the molten slag 200, the potential on the molten iron 300 side polarizes towards the positive potential side, while the potential on the molten slag 200 side polarizes towards the negative potential side. This potential change is called overvoltage. Here, due to the change in Gibbs free energy equivalent to the overvoltage, the equilibrium reaction equation for the dephosphorization reaction of phosphorus contained in the molten iron 300 and the equilibrium constant K for the phosphorus dephosphorization reaction are shown below.

[0065]

[0066] By applying an electric current to two electrodes using the molten iron dephosphorization device 100, the phosphorus contained in the molten iron 300 undergoes a dephosphorization reaction, and the phosphorus (P) contained in the molten iron 300 is converted into phosphorus ions (P2+). 5+ Phosphate ions (P) 5+ The concentration of ) [P] 5+ The equilibrium constant K of the dephosphorization reaction of phosphorus contained in molten iron 300 increases. As a result, it can be considered that the concentration [P] of phosphorus (P) contained in molten iron 300 will decrease. At this time, if the reaction order of the dephosphorization reaction of phosphorus contained in molten iron 300 is set as first order, and the reaction rate v of the dephosphorization reaction is expressed as a first-order function of phosphorus concentration [P], then the following relationship (5) is obtained. In relationship (5), t is the dephosphorization treatment time t (s) of phosphorus contained in molten iron, k is the apparent reaction rate constant of the dephosphorization reaction of phosphorus contained in molten iron, and [P]e is the equilibrium phosphorus concentration when the dephosphorization reaction of phosphorus contained in molten iron reaches dissolution equilibrium.

[0067]

[0068] According to equation (5), the equilibrium phosphorus concentration [P]e when the dephosphorization reaction of phosphorus contained in molten iron 300 reaches dissolution equilibrium decreases, which will increase the reaction rate v of the dephosphorization reaction of phosphorus contained in molten iron 300. In this case, let the temperature of molten steel when the dephosphorization reaction of phosphorus contained in molten iron 300 reaches dissolution equilibrium be T (K), let the phosphorus distribution ratio of phosphorus contained in molten slag 200 when no current is applied to molten iron 300 and molten slag 200 be Lp, and let the necessary phosphorus distribution ratio of phosphorus contained in molten slag 200 after applying current to molten iron 300 and molten slag 200 be Lp'. Then, the current application density I required to obtain the necessary phosphorus distribution ratio Lp' of phosphorus is expressed by the following equation (1).

[0069]

[0070] In equation (1), I is the applied current density (A / m). 2 ), where α is a constant, Lp is the phosphorus distribution ratio of the molten slag (-), Lp' is the necessary phosphorus distribution ratio of the molten slag (-), and T is the molten iron temperature (K).

[0071] That is, in equation (1), the phosphorus concentration C in the molten slag at a steel temperature T (K) after a specified time is determined. 炉渣 The concentration of phosphorus in molten iron, C 铁 The phosphorus distribution ratio Lp(-) of the molten slag 200 is calculated. Next, the necessary phosphorus distribution ratio Lp'(-) of the molten slag 200 is set when the current is applied to the two electrodes by the hot iron dephosphorization device 100, so that the dephosphorization reaction of the phosphorus contained in the hot iron 300 proceeds and reaches dissolution equilibrium.

[0072] Then, the phosphorus distribution ratio Lp(-) of the molten slag 200 at the steel temperature T(K) is calculated, and the necessary phosphorus distribution ratio Lp'(-) of the molten slag 200 is set. Using the relationship (1), the applied current density I(A / m) corresponding to the necessary phosphorus distribution ratio Lp'(-) of the molten slag 200 can be calculated. 2 ).

[0073] Here, the constant α in relation (1) is calculated as follows. First, the applied current density I (A / m²) is obtained. 2 The relationship between the applied current density I (A / m³) and the overvoltage η generated between the electrodes formed by the cathode 104 and the anode 105 when current is applied to the molten iron 300 and the molten slag 200. 2 The relationship between the applied current density I (A / m) and the overvoltage η can be expressed by the following equation (6). Therefore, the applied current density I (A / m) can be expressed as... 2 The slope α is calculated from the graph showing the relationship between the voltage η and the overvoltage η.

[0074]

[0075] On the other hand, when an electric current is applied to molten iron 300 and molten slag 200, the change in Gibbs free energy ΔG when the current is applied is... 初始 And the change in Gibbs free energy ΔG when phosphorus in molten iron reaches dissolution equilibrium after the application of current. 平衡 , respectively represented by the following relation (7) and relation (8).

[0076]

[0077] In equation (7), R represents the gas constant, T represents the molten steel temperature (K), and the phosphorus distribution ratio of the molten slag is Lp(-). 炉渣 a represents the activity of phosphorus in molten slag. 铁 This indicates the activity of phosphorus in molten iron.

[0078]

[0079] Furthermore, according to equations (7) to (8), the overvoltage η generated between the electrodes formed by the cathode 104 and the anode 105 when current is applied to the molten iron 300 and the molten slag 200 is represented by the following equation (9). Moreover, by comparing equations (6) and (9), the constant α in equation (1) can be calculated.

[0080] It should be noted that in relation (9), η represents overvoltage, R represents gas constant, F represents Faraday constant, Z represents valence number, Lp represents phosphorus distribution ratio of molten slag at steel temperature T (K), and Lp' represents necessary phosphorus distribution ratio.

[0081]

[0082] Thus, in the dephosphorization method for molten iron in this embodiment, by setting the phosphorus distribution ratio Lp(-) of the molten slag at the molten steel temperature T(K) and the necessary phosphorus distribution ratio Lp'(-) of the molten slag, and using the relationship (1) with the constant α determined, the current application density I of the current required to obtain the necessary phosphorus distribution ratio Lp' of phosphorus can be determined.

[0083] As described above, according to the invention of the first embodiment, based on the relationship between the phosphorus distribution ratio of the molten slag, the necessary phosphorus distribution ratio of the molten slag, and the molten iron temperature, the phosphorus distribution ratio can be effectively increased by controlling the applied current density, thereby promoting the dephosphorization reaction of the molten iron.

[0084] [Second Implementation]

[0085] The dephosphorization method for molten iron according to the second embodiment will be described. The dephosphorization method for molten iron in this embodiment is characterized in that, in the dephosphorization method for molten iron described in the above embodiment, the carbon concentration [C] in the molten iron is 4.0% by mass or less, and the applied current density I (A / m²) is... 2 ) satisfies the following relation (2).

[0086]

[0087] In equation (2), I is the applied current density (A / m). 2 Lp is the phosphorus distribution ratio (-) of the molten slag, Lp' is the required phosphorus distribution ratio (-) of the molten slag, and T is the temperature of the molten iron (K). The technical features of the dephosphorization method for molten iron in this embodiment will be described below.

[0088] The dephosphorization method for molten iron in this embodiment uses the constant α calculated from the relationship (1) used in the dephosphorization method of molten iron in the above embodiment, and determines α = 5.264 × 1 ² Relationship (2). That is to say, in the steel dephosphorization method of this embodiment, by adopting relationship (2) and measuring the phosphorus concentration after applying a current with applied current density I to molten slag 200 and molten iron 300, the molten slag distribution ratio Lp' can be obtained.

[0089] Furthermore, in the dephosphorization method for molten iron according to this embodiment, the current applied to the molten slag and molten iron is controlled based on the current value. Therefore, in the dephosphorization method for molten iron according to this embodiment, even if the electrical properties of the molten iron change, the impact on the aforementioned overvoltage change can be reduced.

[0090] Furthermore, by applying a current with an application current density that satisfies the relationship (2) used in the dephosphorization method of molten steel in this embodiment to the molten slag and molten iron, the phosphorus distribution ratio can be effectively increased without modifying the slag, and the dephosphorization method of molten iron in this embodiment can be applied to molten iron of various composition systems.

[0091] In the dephosphorization method for molten iron according to this embodiment, the carbon concentration [C] of the carbon contained in the molten iron to which the current is applied is preferably 4.0% by mass or less. If the carbon concentration [C] of the carbon contained in the molten iron to which the current is applied is 4.0% by mass or less, the carbon content that may be present in the molten iron can be ensured, and therefore this is preferred. It should be noted that the carbon concentration [C] of the carbon contained in the molten iron to which the current is applied can be 0.1% by mass or more.

[0092] Thus, the dephosphorization method for molten iron in this embodiment can be applied without particularly limiting the carbon concentration within the range of carbon concentration that may be contained in the molten iron, which is excellent.

[0093] As described above, according to the invention of the second embodiment, the phosphorus distribution ratio can be effectively improved and the dephosphorization reaction of the molten iron can be promoted without being affected by overvoltage changes caused by changes in the electrical properties of the molten iron, or by the concentration of components such as carbon [C] and phosphorus [P] contained in the molten iron.

[0094] [Third Implementation]

[0095] The dephosphorization method for molten iron according to the third embodiment will be described. The characteristic of the dephosphorization method for molten iron in this embodiment is that, in the dephosphorization method for molten iron of the above embodiment, arc discharge does not occur due to the application of the current between the molten slag and the molten iron.

[0096] The following describes the technical features included in the dephosphorization method for molten iron according to this embodiment.

[0097] The dephosphorization method for molten iron in this embodiment is a method of dephosphorizing molten iron without generating an electric arc discharge due to the current applied between the molten slag and the molten iron. That is, in the dephosphorization method for molten iron in this embodiment, no electric arc discharge is generated due to the current applied between the molten slag and the molten iron, thereby promoting the reaction between phosphorus and iron oxides contained in the molten iron and effectively carrying out dephosphorization of the molten iron.

[0098] From the viewpoint of promoting the dephosphorization reaction of molten iron, it is undesirable for an electric arc to occur within the system of the molten iron dephosphorization apparatus due to the current applied between the molten slag and the molten iron. Hereinafter, the reasons for employing conditions in the molten iron dephosphorization method of this embodiment that prevent electric arc discharge from occurring due to the current applied between the molten slag and the molten iron will be explained.

[0099] In the dephosphorization method of molten iron in this embodiment, as shown in the following equilibrium reaction formula (10), phosphorus (P) contained in the molten iron before dephosphorization reacts with iron oxide (FeO) contained in the molten iron to generate phosphorus pentoxide, and the iron oxide (FeO) contained in the molten iron before dephosphorization is reduced to iron (Fe). The equilibrium constant Kp in the chemical equilibrium reaction formula (10) is shown in the following relationship formula (11).

[0100]

[0101] Here, in equation (11) representing the equilibrium constant Kp, a P2O5 a represents the activity of phosphorus pentoxide (P₂O₅) in molten iron. Fe a represents the activity of iron (Fe) contained in molten iron. P a represents the activity of phosphorus (P) in molten iron. FeO It indicates the activity of iron oxide (FeO) contained in molten iron.

[0102] It should be noted that the activity a of phosphorus (P) in molten iron... P The phosphorus concentration [P] and its activity coefficient f in molten iron can be used as a reference. p , is represented by the following relation (12).

[0103]

[0104] Furthermore, in the equation (11) representing the equilibrium constant Kp, by taking the logarithm of both sides and rearranging the relationships between the activities of phosphorus pentoxide (P2O5), iron (Fe), phosphorus (P), and iron oxide (FeO) contained in the molten iron, the following equation (13) is obtained.

[0105]

[0106] Therefore, it can be clearly seen from equation (13) that the dephosphorization reaction of molten iron represented by chemical equilibrium reaction equation (10) is promoted by lowering its reaction temperature T. In general, in the dephosphorization method of molten iron, the current applied between the molten slag and the molten iron causes an electric arc discharge, thereby increasing the reaction temperature T of the dephosphorization reaction of molten iron.

[0107] From this technical point of view, in the dephosphorization method of molten iron in this embodiment, the electric arc discharge is not generated due to the current applied between the molten slag and the molten iron, thereby reducing the reaction temperature T and promoting the dephosphorization reaction of the molten iron.

[0108] Furthermore, from the chemical equilibrium reaction equation (10) and the relationship equation (11) representing the equilibrium constant Kp, it can be seen that by increasing the activity of iron oxides (FetO) contained in molten iron, the reaction between iron oxides contained in molten slag and phosphorus can be promoted, thereby promoting the dephosphorization reaction of molten iron. In addition, by reducing the activity of phosphorus pentoxide (P2O5), the reverse reaction of the dephosphorization reaction of molten iron caused by the decomposition reaction of phosphorus pentoxide (P2O5) contained in molten slag can be suppressed.

[0109] On the other hand, according to the activity a of phosphorus (P) contained in molten iron... P Relationship (12), activity a of phosphorus (P) P The phosphorus concentration [P] and its activity coefficient f in molten iron. P The product of the carbon concentration [%C] in the molten steel and the activity coefficient f. P The larger the carbon concentration [%C] in the molten steel, the greater the activity coefficient f. P This can increase the activity of phosphorus (P) in molten iron. P .

[0110] In the dephosphorization method for molten iron in this embodiment, in order to prevent arc discharge from occurring due to the application of current between the molten slag and the molten iron, the required current value is preferably 5000 (A) or less.

[0111] In the dephosphorization method for molten iron according to this embodiment, if the current applied between the molten slag and the molten iron is 500 to 5000 A or less, the energy balance in the dephosphorization reaction of the molten iron can be maintained without generating an electric arc discharge, thus ensuring the thermal efficiency of the dephosphorization treatment. From this technical viewpoint, a direct current electric arc furnace is preferred as the molten iron dephosphorization apparatus to which the dephosphorization method of this embodiment can be applied.

[0112] DC electric arc furnaces have low power consumption and low unit consumption of electrodes and refractory materials, as well as less noise and flicker. In addition, by installing preheating and continuous charging equipment for scrap steel on the DC electric arc furnace, high-temperature exhaust gas can be used for preheating, and heat loss caused by opening the furnace lid during scrap steel charging can be prevented, thereby reducing energy consumption.

[0113] In addition, DC electric arc furnaces are increasingly incorporating scrap preheating, continuous charging equipment, and eccentric bottom tapping methods. If an eccentric bottom tapping method is used in a DC electric arc furnace, steel can be tapped quickly without tilting the furnace, resulting in good efficiency. Furthermore, slag is less likely to flow into the ladle during tapping, making it a preferred method for maintaining the cleanliness of the molten steel.

[0114] As explained above, according to the invention of the third embodiment, by ensuring that no electric arc discharge occurs to reduce the reaction temperature T, and by setting the current value applied between the molten slag and the molten iron to 5000 (A) or less, the phosphorus distribution ratio can be effectively increased, thereby promoting the dephosphorization reaction of the molten iron. Furthermore, the dephosphorization method for molten iron of the third embodiment can be implemented using a DC electric arc furnace.

[0115] [Fourth Implementation]

[0116] The dephosphorization method for molten iron according to the fourth embodiment will be described. The characteristic of the dephosphorization method for molten iron in this embodiment is that, in the dephosphorization method for molten iron of the above embodiments, the liquid phase fraction of the molten slag is 60% by volume or more. Hereinafter, the technical features included in the dephosphorization method for molten iron of this embodiment will be described.

[0117] In the dephosphorization method for molten iron according to this embodiment, molten slag 200 is formed in a dephosphorization apparatus 100, such as a refining reaction vessel containing molten iron 300 (molten metal). At this time, the molten slag 200 is added to the upper surface of the molten iron 300 in such a thickness that the cathode 104 can only be immersed in it. An electrode made of a conductive material is configured to be immersed only in the molten slag 200, serving as the cathode 104.

[0118] In the dephosphorization method for molten iron in this embodiment, the composition of the molten slag 200 used for dephosphorizing the phosphorus contained in the molten iron 300 is preferably that of the molten slag 200 containing CaO, SiO2, FeO, MgO, etc., commonly used in dephosphorization refining. According to the dephosphorization method for molten iron in this embodiment, as can be clearly seen from the relations (1) and (2), the composition of the molten slag 200 is not particularly limited, and the necessary phosphorus distribution ratio of the molten slag 200 can be increased.

[0119] In the dephosphorization method for molten iron according to this embodiment, in addition to requiring that the cathode 104 of the molten iron dephosphorization apparatus 100 be immersed only in the molten slag 200, the liquid phase fraction of the molten slag 200 is preferably 60% by volume or more in order to improve the reaction efficiency of the dephosphorization reaction. The liquid phase fraction of the molten slag 200 is only required to allow the cathode 104 and anode 105, which are used to apply current between the molten slag 200 and the molten iron, to be inserted into the molten slag 200. If the liquid phase fraction of the molten slag 200 is 60% by volume or more, the cathode 104 can be sufficiently immersed in the molten slag 200, which promotes the dephosphorization reaction of the molten iron, and is therefore preferred. If the liquid phase fraction of the molten slag 200 is 95% by volume or less, the operation of the molten iron dephosphorization apparatus 100 becomes easier, and is therefore preferred. It should be noted that the liquid phase fraction of the molten slag 200 refers to the proportion of liquid phase in the molten slag 200.

[0120] As described above, according to the invention of the fourth embodiment, by setting the liquid phase fraction of the molten slag to 60% by volume or more, the cathode of the hot metal dephosphorization device can be fully immersed in the molten slag, thereby effectively increasing the phosphorus distribution ratio and promoting the dephosphorization reaction of the hot metal.

[0121] [Other Implementation Methods]

[0122] The present invention has been described above with reference to the embodiments described herein, but the present invention is not limited to the embodiments described above. Within the scope of the present invention, various modifications to the structure and details of the present invention can be made that are understandable to those skilled in the art. In addition, programs, systems, or apparatuses constructed by combining the features included in the various embodiments in any manner are also included within the scope of the present invention.

[0123] Example

[0124] The effects of the present invention will be specifically described below based on the embodiments, but the present invention is not limited to these embodiments.

[0125] (Example 1 of the invention)

[0126] The dephosphorization method for molten iron according to this embodiment was implemented using an electric arc furnace. Scrap steel, iron phosphate (FeP), carbon materials, and CaO-SiO2-FeO-MgO slag were charged into the electric arc furnace, and these steelmaking raw materials were melted using an alternating electric arc. Inside the electric arc furnace, 300 t of molten steel and 30 kg / t of molten slag were obtained.

[0127] Next, the furnace-mounted graphite electrode for the alternating current arc is immersed in the molten slag as the cathode. The metal core of the stirring lance is immersed in the molten steel as the anode. Further, while using the stirring lance at 2.0 Nm... 3 Argon gas (Ar) is blown in per minute while an average current density of 300 A / m is applied to the molten slag-steel space. 2 Apply a DC current of 2100A for 30 minutes to perform dephosphorization treatment on the molten steel (Level 1).

[0128] Then, during the dephosphorization treatment of molten steel, samples of molten steel were taken before dephosphorization treatment (0 minutes), 10 minutes after the start of dephosphorization treatment, 20 minutes after the start of dephosphorization treatment, and 30 minutes after the start of dephosphorization treatment (after dephosphorization treatment was completed) to determine the phosphorus concentration in the molten steel and obtain the phosphorus distribution ratio (actual phosphorus distribution ratio). Table 1 shows the phosphorus distribution ratio and slag composition ratio of molten steel under the dephosphorization treatment time. At the same time, the slag liquid phase fraction and whether the relationship shown in the above formula (2) are shown are also shown. It should be noted that in Example 1 of the invention, the necessary phosphorus distribution ratio of the molten slag is set to 100.

[0129] Meanwhile, in the dephosphorization method for molten iron in Invention Example 1, the presence or absence of an electric arc was confirmed, and the current value of the current applied between the molten slag and the molten steel was set to a predetermined current value.

[0130] (Examples 2-6 of the invention)

[0131] In Invention Examples 2 to 5, the average current density of the direct current applied to the molten slag-molten steel space is 300 to 350 (A / m²). 2 Except for variations in the applied current value below 5000 (A), the dephosphorization treatment of molten steel is performed in the same manner as in Invention Example 1 (levels 2 to 5).

[0132] On the other hand, in Example 6 of the invention, the average current density of the DC current applied to the molten slag-molten steel space is set to 3000 (A / m). 2The applied current value is set to 21000 (A) to generate an arc discharge (level 9).

[0133] Table 1 shows the phosphorus distribution ratio (actual phosphorus distribution ratio) and slag composition ratio under the dephosphorization treatment time of molten steel. It also shows the slag liquid phase ratio and whether the relationship shown in equation (2) above is satisfied. Furthermore, in the dephosphorization methods for molten iron in Examples 2-6 of the Invention, the presence or absence of an electric arc was confirmed, and the current value applied between the molten slag and molten steel was set to a predetermined current value.

[0134] (Comparative Examples 1-2)

[0135] Without applying direct current between the molten slag and molten steel, the dephosphorization treatment of the molten steel is performed in the same manner as in Example 1 (Level 6). Furthermore, the average current density of the direct current applied between the molten slag and molten steel is set to 200 (A / m²). 2 Under the condition that the above relationship (2) is not satisfied, dephosphorization treatment of molten steel is carried out (level 7). Table 1 shows the phosphorus distribution ratio (actual phosphorus distribution ratio) and slag composition ratio of molten steel under the dephosphorization treatment time. At the same time, the slag liquid phase ratio and whether the relationship shown in the above relationship (2) are shown are also shown.

[0136] (Comparative Example 3)

[0137] Without applying direct current between the molten slag and the molten steel, the dephosphorization treatment of the molten steel was performed in the same manner as in Example 1 of the Invention (Level 8). Table 1 shows the phosphorus distribution ratio (actual phosphorus distribution ratio) and slag composition ratio of the molten steel under the dephosphorization treatment time. At the same time, the slag liquid phase ratio and whether the relationship shown in the above-mentioned relationship (2) are shown are also shown.

[0138]

[0139] Table 1 shows the experimental conditions and results at each level used in Invention Examples 1-6 and Comparative Examples 1-3. It should be noted that Table 1 shows the necessary phosphorus distribution ratio and evaluates whether the calculated actual phosphorus distribution ratio reaches this value. It can be clearly seen from Table 1 that when the slag liquid phase fraction is 100% (Invention Examples 1-2, 4-6), the calculated actual phosphorus distribution ratio (the phosphorus distribution ratio obtained from the experimental results) does not reach the necessary phosphorus distribution ratio when the above relationship (2) is not satisfied (Comparative Example 2).

[0140] On the other hand, it can be seen that when a DC current with a current density that satisfies the above relationship (2) is applied under the condition that the slag liquid phase fraction is 100% (Examples 1-2, 4-6), the necessary phosphorus distribution ratio is achieved (Examples 1-6).

[0141] Furthermore, as shown in Examples 1 to 5, even when the applied current is set to 5000 (A) or less and no arc discharge occurs, the necessary phosphorus distribution ratio can be achieved very well. From this technical point of view, it is clear that the dephosphorization method for molten iron in this embodiment can be suitably implemented using a DC electric arc furnace that can avoid the generation of arc discharge.

[0142] Furthermore, it is known that the higher the average current density of the DC current applied to the molten steel, the earlier the necessary phosphorus distribution ratio is reached, and the shorter the dephosphorization treatment time can be. In addition, under the condition that the slag liquid phase fraction is 30%, the solidified slag becomes an obstacle and the electrode cannot be impregnated, so the dephosphorization method of the molten iron of the present invention cannot be applied (Comparative Example 3).

[0143] However, when the slag liquid phase fraction is above 60% by volume, although the electrode can be immersed in the molten slag, considering the reaction efficiency of the dephosphorization reaction, the slag liquid phase fraction is preferably above 60% by volume. It should be noted that these trends are independent of the carbon concentration [C] and phosphorus concentration [P] of the molten iron.

[0144] Thus, in the dephosphorization method for molten iron of the present invention, under the conditions of setting the slag liquid phase fraction to 60% or more and applying a direct current with a current density satisfying the above-mentioned relationship (2), the necessary phosphorus distribution ratio can be achieved. That is, it is clear that by using the dephosphorization method for molten iron of the present invention and setting the prescribed conditions, the phosphorus distribution ratio can be effectively improved and the dephosphorization reaction of molten iron can be promoted.

[0145] Industrial availability

[0146] The dephosphorization method for molten iron according to the present invention can effectively increase the phosphorus distribution ratio and promote the dephosphorization reaction of molten iron without modifying the slag, thus contributing to the development of the steel industry and related industries, and is extremely useful in industry.

[0147] Symbol Explanation

[0148] 100 Iron Dephosphorization Unit

[0149] 101 MgO crucible

[0150] 102 Ramming material

[0151] 103 Induction melting furnace

[0152] 104 Cathode (Graphite Electrode)

[0153] 105 Anode (MgO-C electrode)

[0154] 106 DC power supply

[0155] 107 conductor

[0156] 108 heat insulation board

[0157] 200 Molten Slag

[0158] 300 molten iron

Claims

1. A method for dephosphorizing molten iron, wherein, The method for dephosphorizing molten iron is characterized by using an electrode in contact with the molten iron as the anode and an electrode in contact only with the molten slag as the cathode, and applying an electric current between the molten slag and the molten iron through these two electrodes. The applied current density I satisfies the following relationship (1) between the molten iron temperature T, the phosphorus distribution ratio LP of the molten slag, and the necessary phosphorus distribution ratio LP' of the molten slag. In equation (1), I is the applied current density (A / m). 2 ), where α is a constant, LP is the phosphorus distribution ratio of the molten slag (-), LP' is the necessary phosphorus distribution ratio of the molten slag (-), and T is the molten iron temperature T (K).

2. The dephosphorization method for molten iron according to claim 1, characterized in that, The carbon concentration [C] in the molten iron is less than 4.0% by mass. The applied current density I (A / m 2 ) satisfies the following relation (2), In equation (2), I is the applied current density (A / m). 2 ), LP is the phosphorus distribution ratio of the molten slag (-), LP' is the required phosphorus distribution ratio of the molten slag (-), and T is the molten iron temperature (K).

3. The method for dephosphorizing molten iron according to claim 1 or 2, characterized in that, An electric arc discharge does not occur when the current is applied between the molten slag and the molten iron.

4. The dephosphorization method for molten iron according to claim 3, characterized in that, The current value is 5000 (A) or less.

5. The method for dephosphorizing molten iron according to claim 1 or 2, characterized in that, The liquid phase fraction of the molten slag is 60% or more by volume.

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