TWI604060B - Hot metal desulphurization method and desulfurizer - Google Patents

Description

Hot metal desulfurization method and desulfurizer

The invention relates to a method for desulfurizing molten iron and a desulfurizing agent.

The molten iron from the blast furnace contains high concentration of sulfur (S) due to coke or the like used in the blast furnace. Sulfur is a component that basically affects the quality of steel. Therefore, in the steel making step, hot metal desulfurization and molten steel desulfurization are performed corresponding to the quality of the required steel. Therefore, in the case of desulfurization of molten iron, a method of adding a desulfurizing agent mainly composed of inexpensive quicklime (CaO) and stirring and mixing is used.

In such a desulfurization treatment, a slag-forming agent is used in order to promote slag formation of the added desulfurizing agent. A fluorite (CaF ₂ )-based slagging agent is known as a slag-forming agent excellent in slag-forming of a desulfurizing agent. However, fluorine (F) contained in fluorite has an adverse effect on the environment and the human body, and is an element for setting a discharge standard. Therefore, when the slag after the desulfurization treatment using fluorite is recovered, the elution of fluorine from the recovered product becomes a problem, and therefore a desulfurizing agent having high desulfurization energy and no fluorine component is required.

For example, in Patent Document 1, as a desulfurizing agent which does not contain a fluorine component used in desulfurization, it is revealed that CaO having a low melting point is contained. Iron steel additive for Al ₂ O ₃ minerals (calcium aluminate).

Further, for example, in Patent Document 2, as a desulfurizing agent which does not contain fluorine, a calcium aluminate having a chemical composition of CaO: 40 to 60%, Al ₂ O ₃ : 60 to 40% by mass ratio, and the like are disclosed. A desulfurizing agent mixed with calcium aluminate and quicklime (CaO).

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2003-129122

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2007-46083

However, the iron steel additive disclosed in Patent Document 1 has a high mixing ratio of calcium aluminate to the desulfurizing agent as high as 100%, which results in an increase in the cost of the desulfurization treatment.

Further, the iron steel additive disclosed in Patent Document 2 has a high mixing ratio of calcium aluminate to the desulfurizing agent, such as 77% to 100%, so that the cost of the desulfurization treatment is increased as in Patent Document 1. .

Accordingly, the present invention has been made in view of the above problems, and an object thereof is to provide a desulfurization method and a desulfurizing agent for molten iron which can obtain a specific desulfurization energy without using fluorine and can reduce the cost of the desulfurization treatment.

In order to achieve the above object, according to an embodiment of the present invention, a desulfurization method is provided, in which a desulfurizing agent is added to molten iron when the molten iron is stirred for desulfurization, and the desulfurizing agent is a mass concentration ratio of Al ₂ O ₃ / (Al ₂ O ₃ + CaO) is obtained by mixing the calcium aluminate and quicklime so that the mixing ratio of the calcium aluminate is more than 0% by mass and not more than 75% by mass.

Further, according to an embodiment of the present invention, a desulfurizing agent is provided which has a mass concentration ratio of Al ₂ O ₃ /(Al ₂ O ₃ +CaO) of more than 0 and 0.20 or less, and a mixing ratio of calcium aluminate. A mixture of calcium aluminate and quicklime is mixed in an amount of more than 0% by mass and not more than 75% by mass.

According to the method for desulfurizing molten iron and the desulfurizing agent of the present invention, obtaining specific desulfurization energy without using fluorine can also reduce the cost of the desulfurization treatment.

12‧‧‧Trolley

14‧‧‧Iron water pot

16‧‧‧ molten iron

18‧‧‧desulfurizer

20a, 20b‧‧‧ mechanical agitated desulfurization unit

21‧‧‧ Impeller

211‧‧‧ Impeller shaft

24‧‧‧Desulfurizer Addition Department (added above)

241‧‧‧ hopper

242‧‧‧cutting device

243‧‧‧Input slot

25‧‧‧Desulfurizer Addition Department (top blow addition)

251‧‧‧Distributor

252‧‧‧cutting device

253‧‧‧Powder supply tube

254‧‧‧ top blow gun

G‧‧‧Transporting gas

Figure 1 is a graph showing the relationship between the alumina ratio in the slag and the S concentration in the slag.

Fig. 2 is a schematic view showing a mechanically agitated desulfurization apparatus used in the first embodiment of the present invention.

Fig. 3 is a schematic view showing a mechanically agitated desulfurization apparatus used in a second embodiment of the present invention.

The form for carrying out the invention (hereinafter referred to as an embodiment) will be described in detail below. The detailed descriptions set forth below are intended to provide a comprehensive description of the specific embodiments of the invention. However, it should be understood that one or more embodiments may be implemented without the specific details. In addition, in order to clarify the drawings, well-known structures and devices are shown in simplified form.

Prior to the present invention, the present inventors have noticed that the ratio of alumina contained in the calcium aluminate in the desulfurizing agent affects the desulfurization energy, and the lime water (CaO) and calcium aluminate are put into various kinds in the molten iron. The desulfurizing agent prepared by mixing is mixed, and the ratio of alumina in the slag and the S concentration (% by mass) in the slag are investigated for the generated slag. Further, the alumina ratio is a ratio calculated based on the mass % of CaO and Al ₂ O ₃ in the slag represented by the formula (1). In the formula (1), (%Al ₂ O ₃ ) represents the mass % of Al ₂ O ₃ in the slag, and (%CaO) represents the mass % of CaO in the slag.

In addition, in the present investigation, calcium aluminate produced by preliminarily mixing the mixed quicklime and alumina by mixing the quicklime and the alumina in a mass concentration ratio of CaO/Al ₂ O ₃ = 1/2 is used. . Further, the calcium aluminate produced by the above method is pulverized, and the pulverized calcium aluminate and quicklime have a specific concentration in a mass concentration ratio of Al ₂ O ₃ /(Al ₂ O ₃ +CaO) exceeding 0 and 0.46 or less. A desulfurizing agent produced by mixing the concentration. Further, the desulfurizing agent produced by this method does not contain fluorine.

Fig. 1 shows the results of investigations on the ratio of alumina in the slag produced by the above-mentioned desulfurizing agent and the S concentration in the slag. As a result of the investigation, it is understood that the S concentration in the slag is increased by more than 2% in the region where the alumina ratio in the slag exceeds 0% and is less than 20%. This result is presumed to be due to the fact that when the alumina ratio is as low as more than 0% and less than 20%, the calcium aluminate is prevented from being biased by uniformly mixing the calcium aluminate and the quicklime in advance, and the calcium aluminate is not biased around the lime particles. The ground exists and the effect of improving the desulfurization energy of lime is obtained efficiently. Here, the region in which the alumina ratio in the desulfurizing agent of the present investigation exceeds 0% and is 20% or less corresponds to a region in which the ratio of calcium aluminate in the desulfurizing agent exceeds 0% and 30% or less. In other words, the inventors of the present invention have thought of the effect of obtaining a low melting point of a desulfurizing agent by alumina and a melting of calcium aluminate by a role of calcium aluminate in a region where the ratio of calcium aluminate in the desulfurizing agent is low. The effect of increasing the dissolution rate of the desulfurizing agent by the generated liquid phase is effective to promote the melting of the lime of the desulfurizing agent.

On the other hand, it can be understood that in the region where the alumina ratio in the slag exceeds 20%, the S concentration in the slag is drastically lowered, so that the S concentration in the slag becomes less than 2%. This is presumed to be due to the effect of increasing the liquid phase rate due to the lower melting point of the alumina-added desulfurizing agent, the ratio of alumina to the S in the slag (the S concentration in the slag relative to the S concentration in the molten iron) The effect of the reduction) has a greater impact.

From the results of the above investigation, the present inventors have learned that among the desulfurizing agents having a lower ratio of alumina to calcium aluminate, the desulfurizing agent having a higher qualitative ratio than the alumina has a lower liquid phase ratio, but Since the S distribution ratio in the slag becomes high, the alumina is uniformly dispersed to effectively promote the melting of the lime. Therefore, the inventors of the present invention thought that by setting the mass concentration ratio of the desulfurizing agent to Al ₂ O ₃ /(Al ₂ O ₃ +CaO) to more than 0 and 0.20 or less, even if the mixing of the calcium aluminate is relatively low, the liquid phase Among the desulfurizing agents having a lower rate of improvement, specific desulfurization energy can still be obtained, and thus the present invention has been completed. Further, the specific desulfurization energy is at least equivalent to the desulfurization energy of a desulfurizing agent having a high mixing ratio of a fluorite desulfurizing agent or calcium aluminate.

<First embodiment> [Composition of mechanical stirring desulfurization device]

Next, a method of desulfurizing the molten iron 16 according to the first embodiment of the present invention will be described with reference to Fig. 2 . The desulfurization method of the molten iron 16 of the first embodiment is a mechanically agitated desulfurization method using a mechanical agitation type desulfurization apparatus 20a. First, the configuration of the mechanically agitated desulfurization apparatus 20a used in the desulfurization method of the molten iron 16 of the first embodiment will be described.

The mechanically agitated desulfurization apparatus 20a includes an impeller 21 made of a refractory material provided at one end of the impeller shaft 211, a desulfurizing agent addition unit 24 that stores the desulfurizing agent 18 and is added to the molten iron 16, and a dust collecting hood 22 and Exhaust pipe 23.

Impeller 21 is impregnated. It is buried in the molten iron 16 accommodated in the molten iron pot 14 loaded by the trolley 12, and the stirring blade of the molten iron 16 is stirred by rotation. Further, the impeller 21 is configured to be provided on the other end side of the impeller shaft 211. The lifting device and the rotating device are not vertically movable in the vertical direction, and are rotatable with the impeller shaft 211 as a rotating shaft.

The desulfurizing agent addition unit 24 has a hopper 241 that adds a desulfurizing agent 18 to the molten iron 16 and stores the desulfurizing agent 18; a cutting device 242 that cuts the desulfurizing agent 18 from the hopper 241; and an input tank 243, It puts the cut desulfurizing agent 18 into the molten iron 16.

The desulfurizing agent 18 is made to have a mass concentration ratio of Al ₂ O ₃ /(Al ₂ O ₃ +CaO) in the desulfurizing agent 18 of more than 0 and 0.20 or less, and the aluminum in the desulfurizing agent 18 Calcium aluminate and quicklime are uniformly mixed and produced by mixing the calcium acid in an amount of more than 0% by mass and not more than 75% by mass. In the above investigation, the inventors of the present invention have learned that the alumina ratio of the desulfurizing agent 18 is 20% or less and the mixing ratio of the calcium aluminate in the desulfurizing agent 18 is more than 0% by mass and 30. Below the mass%, the S distribution ratio in the slag can be increased. Furthermore, the present inventors have confirmed that the mixing ratio of the calcium aluminate in the desulfurizing agent 18 is more than 30% by mass and 75 by setting the alumina ratio of the desulfurizing agent 18 to 20% or less as described in the following examples. Specific desulfurization energy can also be obtained in the range of mass % or less.

Calcium aluminate is produced by mixing quicklime and alumina at a specific mass ratio, pre-melting and pulverizing. Here, the specific mass ratio of the quicklime to the alumina of the calcium aluminate is a mass ratio calculated by the method in which the ratio of the alumina in the desulfurizing agent 18 and the mixing ratio of the calcium aluminate satisfy the above conditions, and the mass ratio is CaO. /Al ₂ O _{3 is} indicated. For example, when the ratio of the alumina in the desulfurizing agent 18 is 20% and the mixing ratio of the calcium aluminate is 50%, the mass ratio of calcium aluminate is calculated to be 3/2 of CaO/Al ₂ O ₃ .

The dust collecting hood 22 is placed to cover the upper portion of the hot pot 14. The exhaust pipe 23 is mounted on the dust collecting hood 22. The mechanically agitated desulfurization apparatus 20a sucks the exhaust gas or dust generated during the process to the dust collector (not shown) connected to the exhaust pipe 23 through the exhaust pipe 23.

Further, any molten iron can be used for the molten iron 16 of the first embodiment. In other words, the molten iron 16 is made of molten iron in a state of being smelted from a blast furnace or molten iron which has been subjected to at least one treatment such as de-purging or dephosphorization in advance.

[Desulfurization method of molten iron]

Next, the desulfurization method of the molten iron 16 of the first embodiment will be described. First, the trolley 12 that supports the hot metal pan 14 is moved such that the position of the impeller 21 becomes substantially the center of the hot metal pot 14. Next, the impeller 21 is lowered and immersed in the molten iron 16 to rotate the impeller 21. Further, the desulfurizing agent 18 accommodated in the hopper 241 is cut out by a cutting device 242 by a necessary amount, and is added to the molten iron 16 via the input tank 243. The amount of the desulfurizing agent 18 is determined by the desulfurization energy of the desulfurizing agent 18 and the composition of the molten iron 16. The temperature, the target S concentration after the treatment, and the processing time are determined. Subsequently, after stirring for a specific period of time, the rotation of the impeller 21 is stopped, the impeller 21 is raised, and the desulfurization treatment is ended.

As described above, the desulfurization method of the molten iron 16 of the first embodiment is a desulfurization method using a mechanically agitated desulfurization apparatus 20a to which the desulfurizing agent 18 is added, wherein a ratio of mass ratio of calcium aluminate to quicklime is used. ₂ O ₃ /(Al ₂ O ₃ +CaO) is a mixture of more than 0 and 0.20 or less, and the mixing ratio of calcium aluminate is more than 0% by mass and not more than 75% by mass as the desulfurizing agent 18. Therefore, the effect of improving the dissolution rate of the desulfurizing agent 18 due to the low melting point of the desulfurizing agent 18 due to alumina and the liquid phase formed by the melting of the calcium aluminate can be effectively obtained. Thereby, at least a specific desulfurization energy equivalent to that of a desulfurizing agent or a fluorine-containing desulfurizing agent having a high mixing ratio of calcium aluminate of, for example, more than 75% can be obtained. Therefore, compared with the case where the mixing ratio of calcium aluminate is as high as 75% of the desulfurizing agent, the amount of expensive calcium aluminate can be reduced, and the cost of the desulfurization treatment can be reduced. Moreover, the amount of fluorine in the treated slag can be reduced as compared with when a fluorine-containing desulfurizing agent is used.

Further, when the mass concentration ratio of Al ₂ O ₃ /(Al ₂ O ₃ +CaO) is 0, since the effect of improving the liquid phase ratio due to alumina cannot be obtained, high desulfurization energy cannot be obtained.

<Second embodiment> [Composition of mechanical stirring desulfurization device]

Next, a method of desulfurizing the molten iron 16 according to the second embodiment of the present invention will be described with reference to Fig. 3 . First, the configuration of the mechanically agitated desulfurization apparatus 20b used in the desulfurization method of the molten iron 16 of the second embodiment will be described.

The mechanically agitated desulfurization apparatus 20b of the second embodiment differs from the mechanical agitating desulfurization apparatus 20a of the first embodiment in the configuration of the desulfurizing agent addition unit 25, and the other configuration is the same as that of the first embodiment. In other words, the mechanically agitated desulfurization apparatus 20b includes the impeller 21, the dust collecting hood 22, and the exhaust pipe 23, as in the first embodiment. Further, the mechanical agitation type desulfurization device 20b is provided with a storage desulfurizing agent 18 and is top-blown and added Desulfurizer addition unit 25. The desulfurizing agent addition unit 25 has a distributor 251 that stores the desulfurizing agent 18, a cutting device 252 that cuts out the desulfurizing agent 18 from the distributor 251, and a powder supply pipe 253 that supplies the cut desulfurizing agent. 18, the transfer gas G, and the top blow gun 254 are connected to the front end of the powder supply pipe 253, and the desulfurizing agent 18 is sprayed together with the transfer gas G to the molten iron 16.

The carrier gas G may be any one of an inert gas, a non-oxidizing gas, and a reducing gas, and may be, for example, nitrogen gas or argon gas.

The desulfurizing agent 18 is the same as the desulfurizing agent 18 of the first embodiment.

[Desulfurization method of molten iron]

Next, the desulfurization method of the molten iron 16 of the second embodiment will be described. First, the trolley 12 that supports the hot metal pan 14 is moved such that the position of the impeller 21 becomes substantially the center of the hot metal pot 14. Next, the impeller 21 is lowered and immersed in the molten iron 16 to rotate the impeller 21. Further, the desulfurizing agent 18 accommodated in the distributor 251 is cut out by a necessary amount by the cutting device 252, and the cut desulfurizing agent 18 is sprayed together with the conveying gas G through the powder supply pipe 253 and the top blowing lance 254. Top blowing is carried out in the molten iron 16 . The amount of the desulfurizing agent 18 is determined by the desulfurization energy of the desulfurizing agent 18 and the composition of the molten iron 16. The temperature, the target S concentration after the treatment, and the processing time are determined. Subsequently, after stirring for a specific period of time, the rotation of the impeller 21 is stopped, the impeller 21 is raised, and the desulfurization treatment is ended.

As described above, the desulfurization method of the molten iron 16 of the second embodiment is a desulfurization method using a mechanically agitated desulfurization apparatus 20b in which the desulfurizing agent 18 is top-blown, wherein a ratio of mass ratio of calcium aluminate to quicklime is used. Al ₂ O ₃ /(Al ₂ O ₃ +CaO) is more than 0 and 0.20 or less, and the mixing ratio of the calcium aluminate in the desulfurizing agent 18 is uniformly mixed in a manner of more than 0% by mass and not more than 75% by mass. Desulfurizer 18. Therefore, similarly to the first embodiment, the effect of improving the dissolution rate of the desulfurizing agent due to the low melting point of the desulfurizing agent by alumina and the liquid phase formed by the melting of the calcium aluminate can be effectively obtained.

Further, in the desulfurization method of the molten iron 16 of the second embodiment, the desulfurizing agent 18 is added by top blowing, and the desulfurizing agent 18 having a small particle size can be efficiently added to the case where the desulfurizing agent 18 is added. Therefore, in the desulfurization method of the molten iron 16 of the second embodiment, the desulfurization agent 18 having a small particle size can be used to increase the reaction boundary area, and high desulfurization energy can be obtained.

<Modification>

The present invention has been described above with reference to the specific embodiments, but is not intended to limit the invention. Various modifications of the disclosed embodiments and other embodiments of the invention will be apparent to those skilled in the art. Therefore, the scope of the claims should be construed as including the modifications or embodiments of the invention.

For example, in the method for desulfurizing molten iron according to the present invention, in addition to the desulfurizing agent 18, an oxidizing agent such as metal aluminum, aluminum ash or cerium may be added to the desulfurization treatment. At this time, the oxidizing agent may be stored in a container different from the hopper 241 or the distributor 251 provided in the mechanically agitated desulfurization apparatus 20a, 20b. In the same manner as in the case of the desulfurizing agent 18, it is added in a specific amount.

Further, for example, the mechanically agitated desulfurization apparatus of the present invention may be provided with two desulfurizing agent addition units 24 and 25. At this time, the desulfurizing agent 18 is added to the two desulfurizing agent addition portions 24 and 25 so that the total amount of addition is a specific addition amount.

In the second embodiment, the desulfurizing agent 18 obtained by mixing the quicklime and the calcium aluminate is stored in the distributor 251, and is added to the molten iron 16, but the present invention is not limited thereto. For example, at least two dispensers 251 may be provided, and different limes and calcium aluminate are stored in different dispensers 251, and quicklime and calcium aluminate are cut out by a specific amount, and are simultaneously blown with the conveying gas G to the top. Hot metal 16. At this time, the added quicklime and calcium aluminate are cut out so that the ratio of the alumina contained in the calcium aluminate to the total addition amount is more than 0% and less than 20%, and the calcium aluminate is mixed with the total addition amount. The rate is more than 0% by mass and not more than 75% by mass. Further, the quicklime and the calcium aluminate are simultaneously cut out and uniformly mixed by the transfer gas G when being transported by the powder supply pipe 253 and the top blow gun 254. The desulfurizing agent 18 is mixed before being sprayed, and the alumina of the desulfurizing agent 18 can be more uniformly dispersed than the desulfurizing agent addition unit 25 of the second embodiment.

Further, in the above embodiment, the molten iron 16 in the hot metal pot 14 is stirred by the impeller 21. The mechanical agitated desulfurization devices 20a, 20b are mixed, but the invention is not limited thereto. For example, the stirring of the molten iron 16 may not use a mechanically agitated desulfurizing device such as the impeller 21, but may be a gas sprayed from the top blowing spray gun or a spray gun immersed in the molten iron 16 or self-set in the hot water pot. The gas injected from the wing of the bottom of the 14 is stirred. Body agitating desulfurization device. At this time, the desulfurizing agent 18 may be added as in the first embodiment, or may be sprayed together with a stirring gas from a spray gun that sprays a stirring gas. Further, in such a gas agitation type desulfurization apparatus, a hot water tank 14 may be replaced with another container such as a tortoise or a torpedo that can accommodate the molten iron 16.

Further, in the above embodiment, the quicklime is mixed with alumina and then pre-melted to produce calcium aluminate. However, the present invention is not limited thereto. For example, calcium aluminate may also use a plurality of minerals composed of CaO and Al ₂ O ₃ (3CaO.Al ₂ O ₃ , 12CaO.7Al ₂ O ₃ , CaO.Al ₂ O ₃ , CaO.2Al ₂ O ₃ or the like, quicklime and alumina, and a plurality of kinds of raw materials are selected and mixed in such a manner that CaO/Al ₂ O _{3 has} a specific mass ratio similar to that of the above embodiment, and further premelted.

[Examples]

Next, experiments conducted by the inventors and results thereof will be described by way of examples.

In the present embodiment, as shown in Fig. 1, experiments were carried out on the comparative examples corresponding to the two levels of the first and second embodiments and the three levels. In the experiment, the 200 t of molten iron 16 was subjected to desulfurization treatment under various conditions shown in Table 1, and the S concentration in the molten iron before and after the treatment was investigated.

In Examples 1, 3, and 4, desulfurization was carried out using a mass concentration ratio of Al ₂ O ₃ /(Al ₂ O ₃ +CaO) of 0.20 and a mixing ratio of calcium aluminate of 30% by mass, 75% by mass, and 50% by mass. The agents were treated in the desulfurization method of the first embodiment. Further, in Example 2, a desulfurizing agent having a mass concentration ratio of Al ₂ O ₃ /(Al ₂ O ₃ +CaO) of 0.20 and a calcium aluminate mixing ratio of 30% by mass was used, and the desulfurization of the second embodiment was used. The method is processed. Further, in Comparative Example 1, as the desulfurizing agent, quicklime and calcium aluminate having a mass concentration ratio of Al ₂ O ₃ /(Al ₂ O ₃ +CaO) of 0.50 and a calcium aluminate ratio of 75 mass% were used. The mixture was added in the same manner as in the first embodiment. Furthermore, in Comparative Example 2, a mixture of 97% by mass of quicklime and 3% by mass of fluorite was used as a desulfurizing agent using fluorite, and was added in the same manner as in the first embodiment. Further, in Comparative Example 3, only the quicklime was used as the desulfurizing agent, and the addition was carried out in the same manner as in the first embodiment. Further, other conditions such as the number of rotations of the impeller 21 or the processing time are the same at all levels.

As a result of the experiment, in Examples 1 to 4, it was confirmed that the desulfurization rate was improved as compared with the case where only the quicklime was used as the desulfurizing agent in Comparative Example 3. Further, in Examples 1 to 4, the desulfurization ratios equivalent to those of Comparative Examples 1 and 2 were obtained, and it was confirmed that the desulfurization energy equivalent to Comparative Examples 1 and 2 was obtained. Thereby, according to the desulfurization method and the desulfurizing agent of the present embodiment, it can be confirmed that the specific desulfurization energy can be obtained without using fluorine, and the cost of the desulfurization treatment can be reduced.

Claims (6)

A method for desulfurizing molten iron characterized by stirring a molten iron for desulfurization treatment, adding a desulfurizing agent to the molten iron, the desulfurizing agent having a mass concentration ratio of Al ₂ O ₃ /(Al ₂ O ₃ + Calcium aluminate and quicklime are mixed so that the mixing ratio of calcium aluminate is more than 0 and not more than 0.20, and the mixing ratio of calcium aluminate is more than 0 mass% and 75 mass% or less. The method for desulfurizing molten iron according to claim 1, wherein the mechanical stirring type desulfurization device is used when the molten iron is desulfurized. The method for desulfurizing molten iron according to claim 1 or 2, wherein the desulfurizing agent is added to the molten iron. The method for desulfurizing molten iron according to claim 1 or 2, wherein, when the desulfurizing agent is added to the molten iron, the desulfurizing agent is passed through a top-blown lance and transported with the desulfurizing agent. Add with a gas top blow. The method for desulfurizing molten iron according to claim 4, wherein the calcium aluminate and the raw lime are mixed in the transport to the top blown lance. A desulfurizing agent characterized in that the mass concentration ratio of Al ₂ O ₃ /(Al ₂ O ₃ +CaO) is more than 0 and 0.20 or less, and the mixing ratio of the calcium aluminate is more than 0% by mass and not more than 75% by mass. A mixture of calcium aluminate and quicklime is mixed.