Method for refining molten iron

Abstract

This method for refining molten iron includes denitrifying molten iron in an electric furnace in which at least two sets of lance groups including an oxygen-feeding lance and a carbon-feeding lance are installed, wherein: in the at least two sets of lance groups, oxygen and a carbonaceous material are sprayed so as to achieve confluent blowing in which an oxygen-feeding jet from the oxygen-feeding lance and a carbonaceous-material jet from the carbon-feeding lance collide with each other above the surface of the molten iron; in the at least two sets of lance groups, the rate at which oxygen is fed from the oxygen-feeding lance is 0.083 Nm3 / min or greater per ton of molten iron; and the spraying is performed under conditions for forming an ignition point of high carbon concentration and an ignition point of high oxygen concentration.

Description

Iron smelting method

[0001] This invention relates to a method for refining molten iron using an electric furnace. This application claims priority based on Japanese Patent Application No. 2024-205678, filed in Japan on November 26, 2024, the contents of which are incorporated herein by reference.

[0002] In recent years, there has been a growing demand for high-grade steel with low nitrogen (N) concentrations. To melt such high-grade steel in an electric furnace, it is essential to reduce the N concentration in the molten iron after the melting process in the electric furnace. However, electric furnaces have many openings, allowing a large amount of air to enter, and furthermore, nitrogen absorption reactions tend to proceed rapidly through the arc from the electrodes. Therefore, to suppress nitrogen absorption in the electric furnace, it is common practice to use a large amount of slag and form it, thereby covering the arc with the formed slag and suppressing nitrogen absorption. When forming the slag, oxygen gas and carbon material are blown upward from a lance in the electric furnace, and the oxygen jet and the jet consisting of carbon material and carrier gas are combined between the nozzle outlet of the lance and the surface of the molten iron. This burns the carbon in the carbon material within the slag, and the CO gas generated by the combustion of carbon in the carbon material forms the slag.

[0003] On the other hand, in order to melt down high-grade steel with a low nitrogen concentration, it is necessary not only to prevent nitrogen absorption from the atmosphere but also to promote denitrification from the molten iron. Patent Document 1 discloses a method for denitrifying molten steel in which an oxygen-containing gas and a carbon source are simultaneously blown by defining the blowing direction.

[0004] Denitrification in molten iron proceeds by releasing nitrogen gas into CO bubbles; therefore, it is necessary to induce a decarburization reaction within the molten iron. However, if a large amount of slag is used for forming to suppress nitrogen absorption from the atmosphere, the carbon material may not reach the molten iron. The method described in Patent Document 1 does not take into account nitrogen absorption from the atmosphere, so when a large amount of slag is used for forming, the carbon source may not be sufficiently supplied to the molten steel, resulting in insufficient denitrification.

[0005] International Publication No. 2024 / 038715

[0006] In view of the aforementioned problems, the present invention aims to provide a method for refining molten iron that can reduce the nitrogen concentration in molten iron by promoting the denitrification reaction in molten iron while suppressing nitrogen absorption from the atmosphere.

[0007] The inventors have found that by arranging two or more sets of lances for acid supply and lances for coal supply in an electric furnace, and by setting the coal supply rate and acid supply rate in each lance to an appropriate range, it is possible to suppress nitrogen absorption due to slag forming and promote the decarburization reaction in molten iron, thereby reducing the nitrogen concentration in molten iron.

[0008] The present invention is as follows: [1] A method for refining molten iron using an electric furnace equipped with at least two sets of lances, each including an acid lance and a coal lance, wherein slag is present above the molten iron, and oxygen and coal are blown in the at least two sets of lances such that the acid jet from the acid lance and the coal jet from the coal lance collide above the surface of the molten iron, and for at least 50% of the total period from the start to the end of the blowing of oxygen and coal, the acid supply rate from the acid lance in the at least two sets of lances is set to 0.083 Nm per ton of molten iron. ３ A method for refining molten iron, characterized by performing spraying such that the coal supply rate and acid supply rate in the first set of lances are 1 / min or more, satisfy equation (1), and the coal supply rate and acid supply rate in the second set of lances are 1 / min or more, satisfy equation (2). {Coal supply rate (kg / min)} - {Acid supply rate (Nm ３ / min)}×(32 / 22.4)×(12 / 16)>0...(1) {Coal feeding rate (kg / min)}-{Acid feeding rate (Nm ３} / min) × (32 / 22.4) × (12 / 16) < 0 ... (2) [2] The method for refining molten iron according to [1] above, characterized in that in at least one group of lances, the acid lance and the coal lance are installed such that the length of the line segment connecting the intersection of the central axis of the acid lance and a plane perpendicular to the central axis and passing through the nozzle opening of the acid lance and the point obtained by projecting the center of the nozzle opening of the coal lance onto the said plane is 9.0 times or less the diameter of the nozzle opening of the acid lance. [3] The method for refining molten iron according to [1] or [2] above, characterized in that, in two groups of lances, the distance from the center of the smallest circle on the molten iron surface that includes all the electrodes when the electrodes in the electric furnace are viewed from above to the line segment connecting the centers of the fire points where each of the acid jets collides with the molten iron surface is 1.50 times or less the diameter of the circle.

[0009] According to the present invention, it is possible to provide a method for refining molten iron that can reduce the nitrogen concentration in molten iron by promoting the denitrification reaction in molten iron while suppressing nitrogen absorption from the atmosphere.

[0010] Figure 1 is a diagram illustrating the arrangement of lances inside an electric furnace. Figure 2A is a diagram illustrating the distance between the nozzle outlet centers of the acid lance and the coal lance when viewed from the side. Figure 2B is a diagram illustrating the distance between the nozzle outlet centers of the acid lance and the coal lance when viewed from the direction of the nozzle tip. Figure 3 is a diagram illustrating the relationship between the ignition point and the position of the electrodes. Figure 4 is a diagram illustrating the entire period from the start to the end of oxygen and coal spraying.

[0011] First, let's explain the terms used in this invention. "Oxygen-supplying lance" refers to a pipe-shaped component used for blowing a gas containing a high concentration of oxygen. "Carbon-supplying lance" refers to a pipe-shaped component used for blowing particles containing a high concentration of carbon, such as carbon material, together with a carrier gas. "Molten iron" refers to a molten iron alloy, where an iron alloy is an alloy containing 50% or more iron atoms by mass. It is particularly preferable when the iron atom content in the iron alloy is 70% or more, and more preferable when it is 85% or more. "Carbon material" refers to a substance manufactured using carbon sources such as coal or coke as raw materials. The particle size and carbon content of the carbon material can be adjusted by crushing or calcining these carbon source raw materials. "Carbon supply rate" in this invention refers to the weight of carbon supplied from the carbon material per ton of molten iron and per unit time (1 minute). "Oxygen supply rate" in this invention refers to the amount of oxygen supplied per ton of molten iron and per unit time (1 minute) converted to standard conditions (Nm³). ３ ) means. In this technical field, "blowing upwards" means blowing a predetermined substance from the top of the molten iron toward the molten iron surface below. "Acid jet" refers to the roughly conical spread of oxygen discharged from the nozzle tip of the acid lance. "Coal jet" refers to the roughly conical spread of coal material and carrier gas discharged from the nozzle tip of the coal lance.

[0012] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 is a schematic diagram of the inside of the electric furnace 1 viewed from above. The electric furnace 1 is, for example, an AC type electric furnace and is provided with three electrodes 3. On the side of the electric furnace 1, a first set of lances 10a is installed, consisting of an acid supply lance 11a and a coal supply lance 12a, and a second set of lances 10b is installed, consisting of an acid supply lance 11b and a coal supply lance 12b. In the example of Figure 1, the lances are installed as wall lances, but the acid supply lance and coal supply lance may be installed so as to be inserted from, for example, the slug door. The electric furnace may also be a DC type, and there is no particular limit to the number of electrodes, but 1 to 3 is preferred.

[0013] When performing denitrification treatment in an electric furnace 1 containing molten iron and slag, first, an arc is generated from electrode 3, and as shown in Figure 1, acid jets 13a and 13b are blown from acid lances 11a and 11b, respectively, and coal jets 14a and 14b are blown from coal lances 12a and 12b, respectively. At this time, the coal jets 14a and 14b collide with the acid jets 13a and 13b above the molten iron surface and merge. By using this merging method, the coal material is accelerated by the acid jets, and the proportion of coal material that dissipates before reaching the molten iron or burns due to reaction with the atmosphere is reduced, and the coal material is stably supplied to the fire point. The carrier gas for blowing the coal material from the coal lances is CO ２ Gas, air, Ar gas, etc., are used.

[0014] First, in the first set of lances 10a, the acid supply rate in the acid supply lance 11a is reduced compared to the coal supply rate in the coal supply lance 12a, creating a condition where the carbon material does not completely burn in the acid supply jet 13a. This condition allows the carbon material to reach the molten iron and dissolve into it. On the other hand, in the second set of lances 10b, the coal supply rate in the coal supply lance 12b is reduced compared to the acid supply rate in the acid supply lance 11b, creating a condition where the carbon material completely burns in the acid supply jet 13b. This condition allows oxygen to reach the molten iron and dissolve into it. By blowing oxygen and carbon material under these conditions, the carbon concentration becomes high near the fire point 15a and the oxygen concentration becomes high near the fire point 15b. Then, due to the flow of the molten iron, the molten iron with a high carbon concentration and the molten iron with a high oxygen concentration diffuse, and a decarburization reaction occurs between the fire points 15a and 15b. The distance between the ignition point 15a and the ignition point 15b is not particularly limited, but it is preferably 300 mm or more and 4000 mm or less.

[0015] As described above, by flowing the molten iron near the ignition point with the acid injection jet and promoting the decarburization reaction between the ignition points, CO bubbles are generated and slag is formed, and the denitrification reaction in the molten iron can be promoted while suppressing nitrogen absorption from the atmosphere. Therefore, in order to flow the molten iron near the ignition point, during 50% or more of the entire period from the start to the end of the injection of oxygen and carbon material, the acid injection rate from the acid injection lances 11a and 11b is 0.083 Nm per ton of molten iron ３ / min or more respectively. If the acid injection rate is lower than this, the molten iron with a high carbon concentration and the molten iron with a high oxygen concentration near the ignition point do not sufficiently diffuse, and the decarburization reaction does not proceed sufficiently. Preferably, the acid injection rate from the acid injection lances 11a and 11b is 0.100 Nm ３ / min or more per ton of molten iron respectively. Note that the higher the acid injection rate, the more the decarburization reaction is promoted and denitrification is also promoted, but the splashing of the molten iron increases. Therefore, the acid injection rate from the acid injection lances 11a and 11b is preferably 0.480 Nm ３ / min or less per ton of molten iron respectively.

[0016] Furthermore, in order to promote the decarburization reaction between the ignition points 15a and 15b, as described above, it is necessary to increase the carbon concentration near the ignition point 15a and increase the oxygen concentration near the ignition point 15b. Therefore, during 50% or more of the entire period from the start to the end of the injection of oxygen and carbon material, in the first lance group 10a, the acid injection rate and the carbon injection rate are adjusted so as to satisfy the following formula (1), and in the second lance group 10b, the acid injection rate and the carbon injection rate are adjusted so as to satisfy the following formula (2). {Carbon injection rate (kg / min)} - {Acid injection rate (Nm ３ / min)} × (32 / 22.4) × (12 / 16) > 0 ... (1) {Carbon injection rate (kg / min)} - {Acid injection rate (Nm ３ / min)} × (32 / 22.4) × (12 / 16) < 0 ... (2)

[0017] Here, in equations (1) and (2), the coal supply rate is given as the amount of molten iron per ton and the weight of carbon supplied from the coal material per unit time (1 minute) (kg). On the other hand, in equations (1) and (2), the acid supply rate is given as the amount of oxygen supplied per unit time (1 minute) (Nm³) per ton of molten iron and the amount of oxygen supplied per unit time (1 minute) (Nm³). ３ ) is assumed. Therefore, in equations (1) and (2), the oxygen molecule (O ２ The formula is derived by converting the molecular weight of the oxygen to its weight (22.4 liters) per mole of oxygen at standard conditions (0°C, 1 atm), and further adjusting the weight ratio in the decarburization reaction using the atomic weights of carbon and oxygen. In order to more stably form a fire point with a high carbon concentration and a fire point with a high oxygen concentration, the value on the left side of equation (1) is preferably 2.0 or higher, and the value on the left side of equation (2) is preferably -2.0 or lower.

[0018] If either equation (1) or (2) is not satisfied, the decarburization reaction will not be sufficiently promoted even if the molten iron near the fire point is allowed to flow. Alternatively, the acid supply rate and coal supply rate may be adjusted in the first lance group 10a to satisfy equation (2), and the acid supply rate and coal supply rate may be adjusted in the second lance group 10b to satisfy equation (1).

[0019] As shown in Figure 4, "the entire period from the start to the end of oxygen and carbon material spraying" refers to the period during which acid and carbon are being supplied in all lance groups. However, immediately after the start and immediately before the end of acid and carbon supply, the supply rate and supply rate may be lower than the target value. Also, there may be a difference in the start timing of acid and carbon supply between the first lance group and the second lance group. Of the entire period from the start to the end of oxygen and carbon material spraying, preferably equations (1) and (2) should be satisfied for 60% or more of the entire period.

[0020] Furthermore, while single-hole nozzle lances are used for the acid supply lances 11a and 11b, a coherent lance that surrounds the acid supply jet with the flame of a burner may also be used. When combined blowing is performed, the central axis of the coherent lance is directed toward the approximately conical shape of the acid supply jet before the acid supply jet reaches the molten iron. When using a single-hole nozzle lance, the apex angle of the cone is approximately 20 to 28° when only oxygen is injected. On the other hand, when using a coherent lance, the acid supply jet is injected so that it is surrounded by the flame of a burner. Therefore, although it depends on the ratio of the acid supply rate to the fuel gas flow rate, the apex angle of the cone is approximately 6 to 20°.

[0021] Furthermore, in order to further reduce the loss of coal material, it is preferable that in at least one lance group, the distance between the nozzle centers of the acid-supplying lance and the coal-supplying lance is 9.0 times or less the diameter of the nozzle opening of the acid-supplying lance. There is no particular lower limit to the distance between the nozzle centers of the acid-supplying lance and the coal-supplying lance, but it is more preferable that it be 3.0 times or more. If the cross-sectional shape of the nozzle opening of the acid-supplying lance is elliptical, the diameter of the nozzle opening of the acid-supplying lance is calculated as "(major axis + minor axis) / 2". The distance between the nozzle centers will be explained below.

[0022] Figure 2A is a schematic diagram of the acid supply lance 11a and coal supply lance 12a viewed from the side, and Figure 2B is a schematic diagram of the acid supply lance 11a and coal supply lance 12a viewed from the direction of the nozzle tip. Here, the distance L between the centers of the nozzles is １ As shown in Figure 2A, the distance L is defined on a line segment on plane A that is perpendicular to the central axis of the acid lance 11a and passes through the nozzle opening 21. The length of the line segment connecting the intersection of the central axis of the acid lance 11a and plane A and the point obtained by projecting the center of the nozzle opening 22 of the coal lance 12a onto plane A is defined as the distance between the nozzle centers. The high-speed acid jet 13a draws in surrounding gases and particles, so the distance L between the nozzle centers is １ The diameter D of the nozzle opening 21 of the acid supply lance 11a １If the distance between the nozzle centers is 9.0 times or less, the coal particles dispersed from the coal-feeding jet 14a will also be drawn into the acid-feeding jet 13a, further reducing coal loss. More preferably, the distance between the nozzle centers is 7.0 times or less the diameter of the nozzle opening of the acid-feeding lance.

[0023] Note that while Figures 2A and 2B illustrate the first set of lances 10a as an example, the same applies to the second set of lances 10b. Therefore, the distance L between the nozzle centers of the acid supply lance 11b and the coal supply lance 12b is also relevant. ２ The diameter D of the nozzle opening of the acid supply lance 11b ２ It is preferably 9.0 times or less, and more preferably 7.0 times or less.

[0024] Furthermore, as molten iron with high carbon and high oxygen concentrations diffuses into the surroundings, a decarburization reaction occurs even in the region between the two fire points. As a result, as mentioned above, the denitrification reaction by CO bubbles proceeds, and the slag is formed by the CO bubbles generated in the decarburization reaction, suppressing nitrogen absorption. In particular, if the region between the two fire points is near the electrodes, the nitrogen absorption suppression effect is even greater due to the arc. Specifically, as shown in Figure 3, when L is the distance from the center 32 of the circumscribed circle 31 of the three electrodes 3 on the molten iron surface to the line segment connecting the centers of the respective fire points 15a and 15b, it is preferable that the distance L is 1.50 times or less the diameter D of the circumscribed circle 31. In this way, the decarburization reaction proceeds between the fire points, and the slag thickness increases due to forming. As a result, the slag thickness increases even near the electrodes, so the arc can be sufficiently covered, and nitrogen absorption can be further suppressed. More preferably, the distance L is 1.40 times or less the diameter D of the circumscribed circle 31. The lower limit of the distance L is not particularly limited and may be zero times the diameter D of the circumscribed circle 31. In other words, the line segment connecting the centers of the respective fire points 15a and 15b may pass through the center 32.

[0025] In the example shown in Figure 3, the circumscribed circle was defined for the case with three electrodes. However, regardless of the number of electrodes, the circumscribed circle is defined as the smallest circle on the molten iron surface that includes all electrodes when viewed from above. Therefore, in the case of a DC electric furnace with one electrode, the smallest circle on the molten iron surface that includes that single electrode when viewed from above means the outer circumference of the electrode on the molten iron surface.

[0026] Furthermore, this embodiment assumes that nitrogen absorption is suppressed by forming the slag. Therefore, if the slag thickness before processing (before forming) is small, the amount of carbon material and oxygen added should be increased to generate more CO bubbles, and the slag thickness should be increased by forming. To further suppress nitrogen absorption, it is preferable that the slag thickness in the formed state be greater than or equal to the distance between the lower end surface of the electrode and the molten iron surface.

[0027] Furthermore, in this embodiment, an example was described in which a group of lances is formed with one acid lance and one coal lance. On the other hand, a group of lances may be formed that includes multiple acid lances or coal lances in order to form the same fire point. For example, in the first group of lances, a group of lances may be formed with one acid lance and two coal lances in order to form a fire point with a high carbon concentration. Then, in the second group of lances, a group of lances may be formed with two acid lances and one coal lance in order to form a fire point with a high oxygen concentration.

[0028] Furthermore, although two sets of lance groups are installed in the example in Figure 1, three or more sets of lance groups may be installed. In this case, the third and subsequent sets of lance groups do not need to satisfy the conditions of equation (1) or (2) described above, and oxygen and carbon material may be blown under any conditions. When installing the third and subsequent sets of lance groups, it is preferable to have a larger number of lance groups as the size of the electric furnace increases. On the other hand, considering the effort involved in installing the lance groups, it is preferable to have four or fewer sets of lance groups.

[0029] Next, the present invention will be further described based on examples. The conditions in the examples are one set of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one set of conditions. The present invention can adopt various conditions as long as it does not deviate from the gist of the present invention and achieves the object of the present invention.

[0030] In the experiment, an AC electric furnace equipped with three electrodes in the center of the furnace and two or three sets of lance groups consisting of one or two oxygen supply lances and one or two carbon supply lances were installed on the side surface of the furnace. This electric furnace has a capacity to process a maximum of 200 tons of molten iron weight, holds 175 t of molten iron and 65 - 146 kg of slag per ton of molten iron, and generates an arc from the three electrodes to energize in this state. The content of iron atoms in the molten iron at this time was 96 - 99% by mass, and the composition of the slag was 60% by mass of CaO, ２ 30% by mass of SiO ２ and 10% by mass of MgO. For the oxygen supply lance, a single-hole nozzle was used except for Test Nos. 17 and 18, and a coherent lance was used in Test Nos. 17 and 18. Then, in each of the two or three sets of lance groups, carbon material was blown upward from the carbon supply lance and oxygen was blown upward from the oxygen supply lance. The spraying conditions when the target value was reached in each of the two sets of lance groups are shown in Table 1 and Table 2. Also, in Test Nos. 21 and 22, since three sets of lance groups were used, the spraying conditions when the target value was reached in the third set of lance groups are shown in Table 3. Under the conditions with combined blowing, the respective jets were made to merge before reaching the molten iron. The carbon content of the carbon material used was 87% by mass, and a material containing 90% or more of the total weight with a spherical equivalent diameter of 0.1 - 4.0 mm was used. The oxygen gas used had an oxygen purity of 93% by volume ratio. Also, CO ３ / hr. In each test, oxygen and carbonaceous material were sprayed so that the total period from the start to the end of the spraying was 10 minutes. The oxygen feeding rate was measured by a flow meter. The carbon feeding rate was calculated from the weight change per unit time of the tank containing the carbonaceous material and the carbon content of the carbonaceous material.

[0031] As an evaluation method, samples of molten iron were taken before spraying the carbonaceous material and oxygen and after finishing the 10-minute spraying, and the nitrogen concentration [N] in the molten iron was quantified. When the value Δ[N] obtained by subtracting the nitrogen concentration [N] before spraying from the nitrogen concentration [N] after spraying was -0.0015 mass% or less, it was evaluated that the effect of the invention was obtained. The test results are shown in Table 4.

[0032]

[0033]

[0034]

[0035]

[0036] In Tables 1 to 3, D １ , D ２ , D ３ represents the diameter (mm) of the nozzle opening of the oxygen feeding lance in each lance group, and L １ , L ２ , L ３represents the distance (mm) between the nozzle centers of the acid lance and coal lance in each lance group. Specifically, the distance between nozzle centers represents the length (mm) of the line segment connecting the intersection of the central axis of the acid lance and the plane containing the nozzle opening of the acid lance perpendicular to the central axis, and the point obtained by projecting the center of the nozzle opening of the coal lance onto the same plane. In addition, L in Table 4 represents the distance (mm) from the center of the circumscribed circle of the three electrodes on the molten iron surface to the line segment connecting the fire points formed by the acid jets of each lance group. D in Table 4 represents the diameter (mm) of the circumscribed circle. The slag amount in Table 4 represents the amount of slag (kg / ton) per ton of molten iron before an arc is generated from the electrodes and current is applied. The period percentage in Table 4 represents the percentage (%) of the period during which spraying was performed under the conditions shown in Tables 1 and 2, when the total period is defined as the period from the start to the end of oxygen and coal spraying.

[0037] In the inventive examples, Tests No. 1 to No. 22, the spraying conditions for the first set of lances satisfied equation (1), and the spraying conditions for the second set of lances satisfied equation (2), resulting in a decrease in nitrogen concentration in the molten iron before and after spraying. In particular, the length of the line segment connecting the intersection of the central axis of the acid supply lance and a plane perpendicular to the central axis, and the point obtained by projecting the center of the nozzle opening of the coal supply lance onto the said plane, was 9.0 times or less the diameter of the nozzle opening of the acid supply lance (L １ / D １ and / or L ２ / D ２ When the value was ≤ 9.0, the nitrogen concentration tended to decrease further. Furthermore, similarly, when the distance L from the center of the circumscribed circle on the molten iron surface of the three electrodes to the line segment connecting the centers of the respective fire points formed by the blowing from the two sets of lances was 1.50 times or less the diameter D of the circumscribed circle, the nitrogen concentration also tended to decrease further.

[0038] On the other hand, in comparative examples No. 101 to No. 120, spraying was performed under conditions that did not satisfy the conditions of equation (1) or (2). As a result, the decarburization reaction did not proceed sufficiently in the region between the heating points, and the nitrogen concentration in the molten iron became high. Also, in comparative examples No. 121 and No. 123, combined spraying was not performed in one of the lance groups. As a result, there was a large loss of carbon material, and consequently, the nitrogen concentration in the molten iron became high. Also, in comparative examples No. 122 and No. 124, in one of the lance groups, the acid supply rate per ton of molten iron was too low, so the carbon material did not reach the molten iron, or molten iron with a high carbon concentration or molten iron with a high oxygen concentration did not diffuse sufficiently. As a result, the decarburization reaction did not proceed sufficiently in the region between the heating points, and the nitrogen concentration in the molten iron became high. Also, in comparative examples No. In sample 125, the period during which spraying was carried out under conditions satisfying equations (1) and (2) was short. As a result, the decarburization reaction did not proceed sufficiently, and the nitrogen concentration in the molten iron became high.

[0039] According to the present invention, the nitrogen concentration in molten iron can be reduced by promoting the denitrification reaction in molten iron while suppressing nitrogen absorption from the atmosphere, making it highly valuable for industrial applications.

Claims

1. A method for refining molten iron using an electric furnace equipped with at least two sets of lances, each including an acid lance and a coal lance, wherein slag is present above the molten iron, and oxygen and coal are sprayed in the at least two sets of lances such that the acid jet from the acid lance and the coal jet from the coal lance collide above the surface of the molten iron, and for at least 50% of the total period from the start to the end of the spraying of oxygen and coal, the acid supply rate from the acid lance in the at least two sets of lances is set to 0.083 Nm per ton of molten iron. ３ A method for refining molten iron, characterized by performing spraying such that the coal supply rate and acid supply rate in the first set of lances are 1 / min or more, satisfy equation (1), and the coal supply rate and acid supply rate in the second set of lances are 1 / min or more, satisfy equation (2). {Coal supply rate (kg / min)} - {Acid supply rate (Nm ３ / min)}×(32 / 22.4)×(12 / 16)>0...(1) {Coal feeding rate (kg / min)}-{Acid feeding rate (Nm ３ (min)} × (32 / 22.4) × (12 / 16) < 0 ... (2) 2. The method for refining molten iron according to claim 1, characterized in that, in at least one group of lances, the acid lance and the coal lance are installed such that the length of the line segment connecting the intersection of the central axis of the acid lance and a plane perpendicular to the central axis and passing through the nozzle opening of the acid lance and the point obtained by projecting the center of the nozzle opening of the coal lance onto the said plane is 9.0 times or less the diameter of the nozzle opening of the acid lance.

3. The method for refining molten iron according to claim 1 or 2, characterized in that the distance from the center of the smallest circle on the molten iron surface that includes all the electrodes when the electrodes in the electric furnace are viewed from above, to the line segment connecting the centers of the fire points where each of the two lance groups of acid jets collides with the molten iron surface, is 1.50 times or less the diameter of the circle.

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