JP2007327120A - Method for refining molten iron - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently refine molten iron such as molten pig iron or molten steel by efficiently adding/dispersing flux for refinement such as a desulfuring agent into the molten iron while using a mechanically stirring device having a rotary stirring blade. <P>SOLUTION: When refining the molten iron by using the mechanically stirring device having the stirring blade 4, and stirring and mixing the molten iron 3 and the flux 7 for refinement by rotating the stirring blade, this refining method includes controlling the rotation speed of the stirring blade so that the relationship between the rotation speed and a diameter of the stirring blade can satisfy the expression of F×R>200, wherein (F) represents the rotation speed (rpm) of the stirring blade; and (R) represents the diameter (m) of the stirring blade. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for refining molten iron by using a mechanical stirrer equipped with a stirring blade and rotating the stirring blade to stir and mix molten iron and a refining flux.

  The hot metal discharged from the blast furnace usually contains a high concentration of sulfur (S) which adversely affects the quality of the steel, and depending on the required quality level, The desulfurization treatment is performed by various methods such as a combination of the desulfurization treatment and the desulfurization treatment with molten steel. Among these, for desulfurization of hot metal, an inexpensive desulfurization agent mainly composed of CaO (lime) is widely used as a refining flux, and the desulfurization reaction in this case is a reaction represented by “CaO + S → CaS + O”. Proceed based on the formula.

In this desulfurization treatment, a fluorite (CaF ₂ ) type fossilizing agent and an alumina (Al ₂ O ₃ ) type fossilizing agent are used as an accelerator for CaO hatching. For example, quick lime which is a source of CaO is used. CaO—CaF ₂ -based desulfurization agents mixed with about 5% by mass of fluorite are widely used. However, these hatching accelerators are generally expensive, and increasing the blending ratio of such hatching accelerators leads to an increase in the cost of the desulfurization agent. Furthermore, when the blending ratio of the hatching accelerator is increased, the CaO concentration in the desulfurizing agent is lowered, and there is a concern that the reaction efficiency is lowered. In addition, in order to improve the reaction efficiency of desulfurization, there are a method in which quick lime and a calcium carbide desulfurizing agent or a soda desulfurizing agent are used in combination, and a method in which limestone (CaCO ₃ ) is mixed in quick lime. Both are desulfurization agents based on the premise that a fluorite (CaF ₂ ) hatching accelerator is added. Under circumstances where there is a concern about the environmental impact of fluorine in recent years, fluorine-based soot It is desired to efficiently desulfurize without using a crystallization accelerator.

As an example of a desulfurization agent that does not use CaO or meteorite, a calcium carbide desulfurization agent and a soda desulfurization agent have been put into practical use, both of which have advantages and disadvantages. Calcium carbide-based desulfurization agents have a strong desulfurization ability, but there are safety problems such as the generation of acetylene gas in the post-treatment of desulfurization slag after the desulfurization treatment. Moreover, since it is expensive and dangerous, it is extremely difficult to handle. A soda-based desulfurization agent is relatively inexpensive, but has a high alkalinity, and thus has a great influence on the refractories of the processing furnace and the processing vessel. Moreover, since Na is contained in exhaust gas, the removal process is required. Furthermore, since the content of Na ₂ O in the slag is high, there is a restriction on the reuse to cement and the like, which is undesirable from the influence on the environment.

  As another desulfurizing agent, metal Mg is also well known. Metal Mg easily reacts with sulfur in the hot metal to produce MgS. However, since the boiling point of metal Mg is as low as 1100 ° C., there is a risk that the hot metal vaporizes and scatters in the hot metal at 1250 to 1500 ° C. In addition, since the generated Mg vapor is diffused into the atmosphere without sufficiently contributing to the desulfurization reaction, the desulfurization efficiency is poor. Moreover, there is a problem that the metal Mg itself is very expensive.

  Generally, the hot metal desulfurization treatment is performed by dispersing a desulfurizing agent in the hot metal. Therefore, in order to improve the efficiency of the desulfurization reaction, it is important to study the desulfurization agent as described above, but it is more effective to increase the reaction interfacial area by improving the dispersion of the desulfurization agent. As a method for dispersing the desulfurizing agent, there is a desulfurization method using a mechanical stirring device that mechanically stirs the molten iron with a rotating stirring blade (also referred to as “rotary blade” or “impeller”). In this method, the desulfurization agent is cut out from a hopper and the like, and the desulfurization agent is introduced from the inlet installed in the upper part of the processing vessel such as a hot metal ladle, and the hot metal vortex generated by the stirring blade rotating in the processing vessel is used. A desulfurizing agent is dispersed in the hot metal.

  Such a dispersion method by mechanical stirring has a great deal of knowledge in the field of chemical engineering, and dispersion in liquid with a high density solid or liquid, or a solid or liquid having a density equivalent to the liquid is well known. By the way. However, in the steel process such as hot metal desulfurization reaction, the object to be entrained (desulfurizing agent) is extremely low density compared to the bath (molten iron), and it is difficult to entrain, and the bath is handled at a high temperature. Because it is special, it is difficult to apply conventional knowledge as it is, and its own research and development is being conducted.

  In addition, in the actual process, in addition to the difficulty of dispersing the low density particles, there is also a phenomenon in which sintering and agglomeration occur at a high temperature when adding or dispersing the desulfurizing agent or in the hot metal. The reaction efficiency can be significantly reduced. To further improve the desulfurization efficiency, it is possible to further reduce the particle size of the powdered desulfurization agent used in the past, but when adding the desulfurization agent, the amount of desulfurization agent splashed increases. As a result, the amount of the desulfurizing agent reaching the hot metal surface decreases. In addition, since the agglomeration phenomenon described above is likely to occur, a significant improvement in the desulfurization reaction cannot be expected. In order to increase the desulfurization efficiency, it is important how to efficiently disperse the desulfurizing agent while suppressing aggregation.

  As a method for promoting the dispersion of the desulfurizing agent in the hot metal, Patent Document 1 discloses that in a hot metal desulfurization process using a stirring blade, a hot metal colliding with the current plate is formed by immersing the current plate in the hot metal bath. A method has been proposed in which a desulfurizing agent is entrained in hot metal by a downward flow. Further, in Patent Document 2, in the desulfurization treatment of hot metal using a stirring blade, a protruding rectifier is provided on the side wall portion of the processing vessel, and a desulfurizing agent is added using the turbulent flow of the hot metal that collides with the rectifier. There has been proposed a method of involving the inside.

However, when using obstacles such as rectifier plates and rectifiers under high temperature and strong agitation such as mechanical stirring type desulfurization method, it is necessary to use very strong obstacles, and it is expensive to make and maintain them. There is a problem that it requires a lot of cost and labor. In addition, since the obstacle is immersed in the hot metal, there is a problem that the amount of hot metal that can be desulfurized is reduced.
JP 49-44927 A Japanese Patent Laid-Open No. 51-112416

  The present invention has been made in view of such circumstances, and the object of the present invention is to use a mechanical stirring device having a rotating stirring blade, such as a mechanical stirring type desulfurization method, so as to use hot metal, molten steel, and the like. When refining molten iron, it is possible to efficiently add and disperse refining fluxes such as desulfurizing agents in molten iron. For this reason, for example, in the desulfurization treatment of hot metal, a fluorine-based hatching accelerator is used. The object is to provide a method for refining molten iron that enables efficient refining such that high desulfurization efficiency can be ensured without the need.

The method for refining molten iron according to the first invention for solving the above-mentioned problem uses a mechanical stirring device equipped with a stirring blade, rotates the stirring blade and stirs and mixes the molten iron and the refining flux. In refining molten iron, the number of revolutions of the stirring blade and the diameter of the stirring blade satisfy the relationship of the following formula (1).
F × R> 200 (1)
However, in Formula (1), F is the rotation speed (rpm) of a stirring blade, R is the diameter (m) of a stirring blade.

  The method for refining molten iron according to the second invention is characterized in that, in the first invention, the refining flux is a refining flux mainly composed of CaO.

  The method for refining molten iron according to the third invention is characterized in that, in the second invention, the refining flux has a fluorine content of 1% by mass or less.

  According to a fourth aspect of the present invention, there is provided a method for refining molten iron according to any one of the first to third aspects, wherein, during refining, the number of revolutions of the stirring blade is 50% or more of the period during which the stirring blade is rotating. The diameter of the stirring blade satisfies the relationship of the formula (1).

  The molten iron refining method according to the fifth invention is the method of refining molten iron according to any one of the first to fourth inventions, wherein at least 5 minutes have elapsed from the start of rotation of the stirring blade at the start of refining. The diameter of the stirring blade satisfies the relationship of the formula (1).

  A molten iron refining method according to a sixth invention is characterized in that, in any one of the first to fifth inventions, the molten iron is hot metal, and the refining is desulfurization treatment.

  According to the present invention, when refining molten iron using a mechanical stirring device having a stirring blade, the product of the rotation speed of the stirring blade and the diameter of the stirring blade, that is, the peripheral speed of the stirring blade is maintained at a predetermined value or more. Since the molten iron is stirred, the refining flux such as a desulfurizing agent can be efficiently entrained in the molten iron. As a result, the reaction interfacial area increases, and the amount of refining flux used is small. However, it is possible to refine the molten iron efficiently.

  The present invention will be described below.

Conventionally, the utilization efficiency of the desulfurization agent in the hot metal desulfurization process using the mechanical stirrer is at most about 10% even when a high utilization efficiency CaO-CaF ₂ desulfurization agent is used, and the remaining 90%. Remained unreacted and was very inefficient. As a result of various analyzes on the cause of the low utilization efficiency, the present inventors have found that there are the following problems.

  That is, in the hot metal desulfurization process using a mechanical stirring device, the desulfurizing agent is continuously or intermittently added from above the processing vessel onto the hot metal that has been vigorously stirred by the rotating blades. In order to accelerate the desulfurization reaction, the higher the reaction interface area, the more advantageous.For this reason, a granular or powdery desulfurization agent is used. Gets worse. On the other hand, if a desulfurization agent having a large size is used in consideration of scattering at the time of addition, the reaction interface area cannot be ensured, and the desulfurization reaction is stagnated. The CaO-based desulfurizing agent mainly used as a desulfurizing agent has poor wettability with hot metal and is difficult to be caught in the hot metal, and the desulfurizing agent added to the hot metal is strongly stirred. Sintering and agglomeration in the bath, the reaction interface area decreases. In particular, CaO-based desulfurization agents that do not contain fluorine have a great influence on them. For this reason, it becomes a subject how to suppress agglomeration in hot metal under strong stirring and to entrain the desulfurizing agent, that is, to secure an interfacial area.

  For this reason, the present situation is that a desulfurization agent having a particle size that is difficult to scatter is used, and the addition amount of the desulfurization agent is increased to secure a reaction interface area and obtain desulfurization ability. In addition, when a hatching accelerator containing fluorine, such as fluorite, is not used, the desulfurization ability that is reduced is compensated by increasing the amount of CaO added or extending the treatment time. It is. However, an increase in the amount of desulfurizing agent used leads to an increase in cost and an increase in the amount of generated slag, and an extension of the treatment time is not preferable because it causes a decrease in productivity.

  Based on these, in order to increase the utilization of desulfurization agents, especially CaO-based desulfurization agents that do not contain fluorine, in order to increase the reaction interfacial area without increasing the amount of desulfurization agent used, the rotating behavior of the stirring blades The relationship between the desulfurization agent and the reaction interfacial area was investigated.

  In the hot metal desulfurization treatment using the mechanical stirring device, when the rotation speed of the stirring blade is increased, the desulfurization agent is caught in the hot metal from a certain rotation speed, and the desulfurization capacity is increased. Therefore, conventionally, desulfurization treatment is performed in the vicinity of the rotational speed that becomes the boundary, but there are many unknown portions with respect to details of the rotational conditions.

  Therefore, the relationship between the rotation speed of the stirring blade and the desulfurization capacity was studied. As a result, it was found that the desulfurization capacity is influenced by the rotational speed of the stirring blade, but is not simply correlated with the rotational speed of the stirring blade, but is correlated with the peripheral speed of the stirring blade.

  That is, it has been found that in order to increase the entrainment of the desulfurizing agent in the bath, it is only necessary to increase the peripheral speed of the stirring blade, that is, to increase the kinetic energy of the desulfurizing agent. The peripheral speed of the stirring blade is expressed by the product (r × ω) of the radius r (m) of the stirring blade and the rotational angular velocity ω (rad / sec). Therefore, when the rotation speed F (rpm) of the stirring blade is used, The product of the rotation speed F (rpm) of the stirring blade and the diameter R (m) of the stirring blade (F × R = F × 2r: unit = “rpm · m”) is a parameter of the peripheral speed of the stirring blade. However, how much the product (F × R) becomes involved in the bath of the desulfurizing agent is not well understood with respect to dispersion of low density particles such as refining of molten metal. There is no information at all in the system where the aggregation of the underlying particles occurs. Furthermore, there are no examples of application to hot metal refining processes.

  Therefore, first, the dispersion behavior of low density particles in the bath was verified by a water model experiment. About 60 kg of water was placed in an acrylic cylindrical container having a diameter of 0.5 m, and stirring was performed with a four-blade stirring blade. As the low density particles, hollow plastic particles having a diameter of 1 mm or less were used and added to the water bath before stirring. As a result of observing the behavior of the low density particles at that time by rotating the stirring blades at various rotational speeds and observing the behavior of the low density particles, the low density particles are reached when the lower end of the vortex formed in the container reaches the stirring blades. It was confirmed that dispersion in water started. Furthermore, using a stirring blade having a different diameter, the state of entrainment of low density particles at various rotational speeds was investigated. As a result, if the conditions under which the entrainment state (uniform dispersibility) of the low-density particles is remarkably improved are summarized, the occurrence of entrainment of the low-density particles correlates with the peripheral speed, that is, the product (F × R) of the stirring blade at that time. I found.

  Next, based on these verification results, in the hot metal desulfurization process using an actual mechanical stirring device, the rotation speed of the stirring blade and the diameter of the stirring blade were changed to investigate the entrainment of the desulfurizing agent. As the desulfurization agent, a desulfurization agent mainly composed of CaO not containing fluorine was used. As a result, a significant improvement in desulfurization efficiency is recognized as the product (F × R) increases, and the effect is particularly large when the product (F × R) is 200 rpm · m or more. A further increase in the effect was confirmed at 250 rpm · m or more.

  Based on these results, in the present invention, using a mechanical stirring device equipped with a stirring blade, rotating the stirring blade, molten iron such as hot metal or molten steel, and a refining flux such as a desulfurizing agent or a dephosphorizing agent, When the molten iron is refined by stirring and mixing, the product (F × R) of the rotation speed F (rpm) of the stirring blade and the diameter R (m) of the stirring blade is 200 rpm · m or more. In setting the product (F × R) to 200 rpm · m or more, either the rotation speed F of the stirring blade or the diameter R of the stirring blade may be adjusted, or both may be adjusted.

  A specific implementation method of the present invention will be described below with reference to the accompanying drawings, taking as an example the case where the present invention is applied to a hot metal desulfurization process performed by a mechanical stirring device. FIG. 1 is a schematic sectional view showing an example of a mechanical stirring desulfurization apparatus to which the present invention is applied. In addition, the mechanical stirring apparatus for performing a desulfurization process is called the mechanical stirring type desulfurization apparatus.

  As shown in FIG. 1, the mechanical stirring desulfurization apparatus includes a refractory stirring blade 4 that is immersed and buried in a hot metal 3 accommodated in a hot metal ladle 2 and swirled to stir the hot metal 3. The agitating blade 4 is moved up and down in a substantially vertical direction by an elevating device (not shown) and swiveled around a shaft 4a by a rotating device (not shown). Further, in the mechanical stirring type desulfurization apparatus, the granular or powdery desulfurizing agent 7 and the granular or powdery deoxidation source 8 are placed on the bath surface of the hot metal 3 accommodated in the hot metal ladle 2 as a refining flux. Add the inlet 6 for adding on top and the powdered desulfurizing agent 7A and the powdered deoxidation source 8A by blowing up toward the hot metal 3 accommodated in the hot metal ladle 2 as a refining flux. An upper blowing lance 5 is installed. Further, an exhaust duct port (not shown) connected to a dust collector (not shown) is provided at an upper position of the hot metal ladle 2 so that gas and dust generated during the desulfurization process are discharged. .

  The input port 6 is provided with a supply device including a hopper 9 containing a granular or powdered desulfurizing agent 7, a rotary feeder 10 for quantitatively cutting out from the hopper 9, and a granular or powdered deoxidation source 8. It is connected to a supply device comprising a hopper 11 to be accommodated and a rotary feeder 12 for quantitatively cutting out from the hopper 11, and a granular or powdery desulfurizing agent 7 and a granular or powdery deoxidation are supplied from an inlet 6. The power source 8 can be adjusted and supplied independently at an arbitrary timing. Further, the top blowing lance 5 includes a supply device including a hopper 13 for storing the powdery desulfurizing agent 7A, a cutting device 14 for quantitatively cutting out from the hopper 13, and a powdery deoxidation source 8A. It is connected to a supply device composed of a hopper 15 to be accommodated and a cutting device 16 for quantitatively cutting out from the hopper 15, and from the top blowing lance 5 together with a carrier gas such as nitrogen gas or Ar gas, The desulfurization agent 7A and the powdery deoxidation source 8A can be independently adjusted and supplied at arbitrary timings. In addition, what is necessary is just to select suitably from the magnitude | size of the desulfurization agent to be used, etc. whether it blows in from the top blowing lance 5 or adds from the insertion port 6. FIG. The deoxidation sources 8 and 8A are for reducing the oxygen concentration of the hot metal 3 and promoting the desulfurization reaction, and are not necessarily required, but are preferably used to promote the desulfurization reaction.

  In the mechanical stirring-type desulfurization apparatus configured as described above, first, the position of the carriage 1 on which the hot metal ladle 2 is mounted is adjusted so that the position of the stirring blade 4 is substantially the center of the hot metal ladle 2, and then the stirring is performed. The blade 4 is lowered and immersed in the hot metal 3. If the stirring blade 4 is immersed in the hot metal 3, the stirring blade 4 starts to rotate, and the product (F × R) of the rotation speed F (rpm) of the stirring blade 4 and the diameter R (m) of the stirring blade 4 is The rotation speed F of the stirring blade 4 is increased according to the diameter R (m) of the stirring blade 4 to be used until it becomes 200 rpm · m or more, preferably 250 rpm · m or more. When the rotational speed F of the stirring blade 4 reaches a predetermined rotational speed, the desulfurizing agent 7 or the desulfurizing agent 7A is added continuously or intermittently. In this case, in order to accelerate the desulfurization reaction in parallel with the addition of the desulfurizing agent 7, 7A or before and after the addition of the desulfurizing agent 7, 7A, the deoxidation source 8, 8A is supplied to the hot metal ladle 2. Is preferred. The diameter R of the stirring blade 4 is the rotational diameter at the upper end surface of the stirring blade 4 regardless of the shape of the stirring blade 4.

  Even after the supply of the predetermined amount of the desulfurizing agent 7, 7A is completed, the molten iron 3 is further stirred by the stirring blade 4, and when stirring for a predetermined time is performed, the number of rotations of the stirring blade 4 is decreased and stopped. When the swirling of the stirring blade 4 is stopped, the stirring blade 4 is lifted and waited above the hot metal pan 2. The generated slag floats to cover the hot metal surface, and the desulfurization process of the hot metal 3 is completed in a stationary state. The slag generated after the desulfurization treatment is discharged from the hot metal ladle 2 and conveyed to the next refining process.

  In this way, the desulfurization treatment of the hot metal 3 is performed. In order to promote the desulfurization reaction, it is preferable to maintain the product (F × R) of the rotation speed F of the stirring blade 4 and the diameter R of the stirring blade 4 over 200 rpm · m over the entire period of the desulfurization treatment. The desulfurization reaction can be promoted within a range of 50% or more of the entire period of the desulfurization treatment. In particular, the desulfurization reaction can be efficiently promoted by applying the present invention for 5 minutes from the start of the treatment in which the desulfurization agent 7 or 7A is entrained in the hot metal 3.

As the desulfurizing agent 7, 7A to be used, various desulfurizing agents such as calcium carbide-based desulfurizing agent, soda-based desulfurizing agent, and metallic Mg can be used, but they are inexpensive. Therefore, it is preferable to use a CaO-based desulfurization agent. Moreover, it is preferable to use only a CaO-based desulfurization agent without using a fluorine source such as fluorite in combination because environmental measures and the reuse of generated slag are easy. That is, a CaO-based desulfurization agent having a fluorine content of 1% by mass or less is preferable. As the CaO-based desulfurization agent, quick lime (CaO), dolomite (MgCO ₃ · CaCO ₃ ), slaked lime (Ca (OH) ₂ ), limestone (CaCO ₃ ) and the like can be used. In the present invention, the desulfurizing agent 7, 7A can be forcibly engulfed in the molten iron 3, so that it can be sufficiently desulfurized without using a fluorine source.

  Moreover, as the deoxidation sources 8, 8A, metal Al or aluminum dross powder is desirable because it can be obtained at a low cost as an aluminum source. Further, other Al sources such as atomized powder obtained by atomizing an aluminum melt with gas and cutting powder generated when an aluminum alloy is polished and cut may be used. Moreover, Si alloy like ferrosilicon, Mg alloy, etc. can also be used. These are preferably in the form of a powder when added to the surface of the hot metal 3 together with the carrier gas, and fine powder that normally scatters is fine when added as an upper spray. It is possible to use.

  As described above, according to the present invention, since the desulfurizing agent 7 and 7A can be efficiently involved in the hot metal 3, the interface area of the desulfurization reaction is increased, and the amount of the desulfurizing agent 7 and 7A used is small. In addition, the hot metal 3 can be efficiently desulfurized without using a fluorine-based hatching accelerator.

  In addition, although the said description was performed about the desulfurization process of hot metal, this invention is applicable not only to the desulfurization process of hot metal, but also to the desiliconization process, dephosphorization process, and decarburization process of hot metal. In short, as long as it is a method of refining molten metal using a mechanical stirring device, the present invention can be carried out according to the above.

  About 150 tons of hot metal in the hot metal ladle was desulfurized using the mechanical stirring desulfurization apparatus shown in FIG.

  The hot metal discharged from the blast furnace is subjected to two desiliconization treatments between the blast furnace casting floor and the hot metal ladle after receiving, and then transported to the mechanical stirring type desulfurization apparatus shown in FIG. Carried out. The composition of the hot metal used was carbon: 4.0 to 4.6% by mass, silicon: 0.05 to 0.15% by mass, Mn: 0.10 to 0.15% by mass, phosphorus: 0.10 to 0 The sulfur concentration before the desulfurization treatment was 0.02 to 0.04 mass%. The hot metal temperature before desulfurization treatment is 1320 to 1450 ° C., a desulfurization agent mainly composed of CaO is used as a desulfurization agent, and a deoxidation source is aluminum dross containing about 50% by mass of metal Al. Powder was used.

  Various refractory stirring blades having different diameters were used and immersed in the hot metal, and then rotated to form a vortex in the hot metal. Thereafter, 5 to 10 kg / t of a desulfurizing agent was added according to the sulfur concentration before the desulfurization treatment, and the desulfurization treatment was performed for 10 minutes. At that time, the product (F × R) of the rotation speed F of the stirring blade and the diameter R of the stirring blade was changed in the range of 130 to 260 rpm · m.

  As a result of the desulfurization treatment, when the product (F × R) is 130, 160, 190, 210, 240, and 260 rpm · m, the desulfurization rates are 8%, 9%, 11%, 15%, 16%, and 21%, respectively. Thus, it was confirmed that the desulfurization efficiency was improved as the product (F × R) increased. In particular, desulfurization efficiency was high when the product (F × R) was 200 rpm · m or more, and further improvement of desulfurization efficiency was confirmed when the product (F × R) was 250 rpm · m or more.

It is a schematic sectional drawing which shows one example of the mechanical stirring type desulfurization apparatus with which this invention is applied.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Bogie 2 Hot metal ladle 3 Hot metal 4 Stir blade 5 Top blowing lance 6 Input port 7 Desulfurization agent 8 Deoxidation source 9 Hopper 10 Rotary feeder 11 Hopper 12 Rotary feeder 13 Hopper 14 Cutting device 15 Hopper 16 Cutting device

Claims (6)

When refining molten iron by stirring and mixing the molten iron and the refining flux using a mechanical stirring device equipped with a stirring blade, the rotation speed of the stirring blade and the diameter of the stirring blade are A method for refining molten iron, characterized by satisfying the relationship of the following formula (1).
F × R> 200 (1)
However, in Formula (1), F is the rotation speed (rpm) of a stirring blade, R is the diameter (m) of a stirring blade.   The method for refining molten iron according to claim 1, wherein the refining flux is a refining flux mainly composed of CaO.   The method for refining molten iron according to claim 2, wherein the refining flux has a fluorine content of 1% by mass or less.   2. The refining, wherein the number of rotations of the stirring blade and the diameter of the stirring blade satisfy the relationship of the formula (1) for a period of 50% or more of the period during which the stirring blade is rotating. Item 4. The method for refining molten iron according to any one of Items 3.   The rotation speed of the stirring blade and the diameter of the stirring blade satisfy the relationship of the expression (1) until at least 5 minutes have elapsed from the start of rotation of the stirring blade at the start of refining. Item 5. The method for refining molten iron according to any one of Items 4 to 5.   The method for refining molten iron according to any one of claims 1 to 5, wherein the molten iron is hot metal, and the refining is a desulfurization treatment.

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