Specification
Title of the Invention PROCESS FOR DESULFURIZATION OF MOLTEN PIG IRON
Technical Field [0001]
The present invention relates to a process for desulfurizing molten pig iron by immersing an immersing freeboard and an injection lance into the molten pig iron in a molten pig iron ladle and injecting an inert gas and CaO powder from the injection lance, and particularly to a desulfurization method of molten pig iron in which metallic Al is added in order to increase the reaction efficiency of CaO.
This application claims priority on Japanese Patent Application No. 2008-228502 filed on September 5,2008, the content of which is incorporated herein by reference.
Background Art [0002]
Since molten pig iron run off from a blast furnace includes a large amount of sulfur (S), which adversely affects the quality of steel, a desulfurization treatment of the molten pig iron is conducted as a process of a molten pig iron preliminary treatment. Meanwhile, in recent years, demand for the manufacture of high-quality steels has risen, therefore it is desirable to employ a method that can lower sulfur at a lower cost. [0003]
For some time, a desulfurization agent including cheap CaO (lime) as the main component has been widely used in the desulfurization treatment of molten pig iron, but the ratio of CaO that contributes to a desulfurization reaction is generally low. As a result, a desulfurization assisting agent or a desulfurization agent, such as fluorite (CaF), soda ash, magnesium (Mg), calcium carbide (CaC2), is used to increase the efficiency of the desulfurization reaction. However, if fluorite is used, there is a problem in that
fluorine (F) is mixed into desulfurized slag, and, if soda ash is used, there is a problem in that sodium (Na) is mixed into desulfurized slag. Therefore, when considering the recycling of slag, the use of any of the above agents is not preferable from the viewpoint of environment preservation. In addition, it is not desirable to use a large amount of Mg and CaC2 from the standpoint of suppressing production costs. As such, a technology for the desulfurization treatment of molten pig iron, which uses no fluorite, soda ash or the like and is cheap, is desirable. [0004]
As a desulfurization method of molten pig iron, which does not use fluorite, soda ash, Mg or CaC2, a desulfurization method in which Al is added has long been known (for example, refer to Patent Documents 1 and 2).
Patent Document 1 discloses a desulfurization method of molten pig iron that adds Al in advance to molten pig iron so that the concentration of Al becomes from 0.01 to 0.1 times that of Si in the molten pig iron and from 0.2 to 1.0 times that of S to be desulfuirzed, and then injects CaO, which is a desulfurization agent, to the molten pig iron together with a carrier gas.
However, if Al is added alone, splashing occurs, which is accompanied by a lot of difficulties in operation. In addition, if CaO is injected after the previous addition of Al is completed, the desulfurization treatment time is prolonged. As a result, the temperature of the molten pig iron is lowered and thus a heating treatment is required in the post process, which results in a rise in the production costs. [0005]
As a method to solve such problems, Patent Document 2 suggests a desulfurization method of molten pig iron in which an amount of Al, determined depending on the amount of Si and S in the molten pig iron, is injected to the molten pig iron together with CaO at the same time via a carrier gas before the amount of CaO which corresponds to 15% by mass (hereinafter, simply referred to as %) of the amount of CaO determined depending on the amount of S included in the molten pig iron is injected.
However, the method described in Patent Document 2 conducts a treatment in the
atmosphere without using an immersing freeboard or the like, and thus oxygen in the atmosphere and the added Al react with each other, which leads to a decrease in the amount of Al to be consumed by the reaction with CaO, therefore it is practically difficult to desulfurize the molten pig iron to a level making the concentration of residual S be 50 ppm or less (refer to FIG. 3 in Patent Document 2).
Related Art Documents Patent Documents [0006]
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. S54-037020
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. S55-110711
Disclosure of the Invention
Problems to be Solved by the Invention
[0007]
The present invention has been made in consideration of the above problems, and an object of the present invention is to provide a desulfurization method of molten pig iron capable of developing a treatment capacity that stably desulfurizes the molten pig iron so as to make the concentration of residual S be 50 ppm or less at a low cost within a short time.
Means for Solving the Problems [0008]
As a result of repeating a variety of experimental and theoretical studies to solve the above problems, the inventors of the present invention have made the technical findings below. [0009]
(A) The present invention supposes a desulfurization method of molten pig iron that desulfurizes the molten pig iron by, basically, immersing an immersing freeboard and an injection lance into the molten pig iron in a molten pig iron ladle and injecting an inert gas and CaO powder from the injection lance. Additionally, the present invention is based on a desulfurization method of molten pig iron that adds metallic Al at a predetermined timing in order to increase the reaction efficiency of CaO. [0010]
In this case, if the metallic Al is added before oxygen inside the immersing freeboard is substituted with the inert gas, since most of the added metallic Al reacts with the oxygen, thereby changing to Al2O3, the metallic Al, which is added to increase the reaction efficiency of CaO, cannot contribute to the improvement of the reaction efficiency of CaO and is needlessly wasted.
On the other hand, if the metallic Al is added after oxygen inside the immersing freeboard is substituted with the inert gas, since the loss of the metallic Al due to the above-described oxidization is reduced, the Al consumes desulfurization-discharged oxygen generated from the surface of quick lime, and CaO- Al2O3, which is a liquid phase with a high S absorption capacity, is formed and made to be slag on the surface of the quick lime, whereby desulfurization treatment capacity is improved. [0011]
For example, in a case in which 35 kg of metallic Al with an Al grade of 90% is fed and 1300 kg of CaO with a CaO grade of 98% is injected (at an injection speed of 200 kg/min) to 3601 of molten pig iron, the relationship between the variation in oxygen partial pressure and treatment efficiency from the start of treatment is as shown in (a) and (b) in FIG. 1. Further, the treatment efficiency is represented by CaO-k values (= In (the concentration of S before treatment)/ (the concentration of S after treatment)/the amount of quick lime per the unit amount of molten pig iron).
As is clear from FIG. 1, in order to make the CaO-k value which represents the treatment efficiency be 0.5 or more, the metallic Al needs to be added to the molten pig iron after the oxygen partial pressure PO2 inside the immersing freeboard becomes at least
0.1 MPa or less. Since the CaO-k value reaches the saturation state at about 0.55, when adding the metallic Al to the molten pig iron, it is desirable to feed the metallic Al into the molten pig iron after the oxygen partial pressure PO2 inside the immersing freeboard becomes 0.01 MPa or less. [0012]
As shown in FIG. 1, in a case in which the metallic Al is added after the injection ratio of CaO powder becomes about 30% by mass of the total amount from the time at which the injection of CaO powder into the molten pig iron began (treatment starting), the CaO-k value is lowered with a gentle gradient. That is, even when the metallic Al is added after 30% by mass of the total amount of CaO powder to be used has been injected, the CaO powder injected into the molten pig iron has already floated and cumulated on the surface of the molten pig iron. Therefore, the above-described reaction of 3CaO+3S+2Al→3CaS+Al203 does not proceed in the molten pig iron, thereby lowering the CaO-k value which represents the treatment efficiency. [0013]
As a result, it is necessary to feed the metallic Al into the molten pig iron before 30% by mass of the total amount of CaO to be used is injected. In addition, since the addition of the metallic Al forms CaO- Al2O3 on the surface of the CaO powder and thus lowers the melting point, the wettability of the CaO powder and the molten pig iron is improved and thus the reaction efficiency of a permanent reaction is improved. Therefore, there is another technical implication in that a reaction time for the permanent reaction after the slagmaking of CaO (formation of CaO- Al2O3) can be ensured by adding the metallic Al before 30% by mass of the total amount of CaO powder to be used is injected. [0014]
(B) From the viewpoint of increasing the above permanent reaction efficiency by maintaining a high desulfurization reaction property of the floated CaO powder, it is desirable to remove blast furnace slag present on the molten pig iron in the molten pig iron ladle before conducting the above treatment. More specifically, it is desirable to remove

the blast furnace slag inside the molten pig iron ladle before immersing the immersing freeboard and the injection lance into the molten pig iron in the molten pig iron ladle. That is, the loss of the metallic Al due to the oxidization of the added metallic Al by the blast furnace slag can be reduced, therefore the reaction efficiency of CaO powder can be increased. [0015]
(C) Regarding the feeding location of the metallic Al, it is desirable to perform
feeding at a portion on the surface of the molten pig iron which is brought into an exposed
state since the injection of the inert gas stirs the molten pig iron so as to remove blast
furnace slag in the vicinity of the stirring. Thereby, the loss of the metallic Al due to the
oxidization reaction with the slag can be reduced, and therefore the reaction efficiency of
CaO powder can also be increased.
[0016]
(D) In addition, from the viewpoint of increasing the reaction efficiency of CaO
powder, it is desirable to inject CaO powder with a pore diameter of 3 µm or more, which
is represented by, for example, salt-burnt quick lime. Since the molten pig iron intrudes
into the pores on the surface of CaO powder, the contact area between the CaO powder
and the molten pig iron is significantly enlarged, and thus the reaction efficiency of CaO
powder can be increased.
[0017]
(E) From the viewpoint of increasing the desulfurization treatment speed, it is
desirable to concurrently use CaO powder, metallic Al and metallic Mg in the starting
phase of the desulfurization treatment. That is, it is desirable to inject metallic Mg in
addition to CaO powder from the injection lance together with the feeding of metallic Al
from the phase in which the injection of CaO powder into the molten pig iron is started to
the starting phase of desulfurization when the concentration of S in the molten pig iron is
100 ppm or more. As to the desulfurization capability of the metallic Mg in a high S
concentration range of 100 ppm or more, the metallic Mg has a desulfurization efficiency
four or more times higher than that of CaO powder after the injection of metallic Al, with
respect to a unit of consumption. Thus, the treatment speed can be increased by concurrently using metallic Mg. It is particularly effective in a case in which the concentration of S included in the molten pig iron is high, and thus the treatment time is assumed to become long. [0018]
Based on the above finding, the inventors of the present invention have found a desulfurization method of molten pig iron capable of achieving a desulfurization capacity that can make the concentration of residual S be 50 ppm or less at a low cost within a short time. The summary thereof is as follows. [0019]
(1) A desulfurization method of molten pig iron according to the present
invention is a method in which an immersing freeboard and an injection lance are
immersed into the molten pig iron inside a molten pig iron ladle, and an inert gas and CaO
powder are injected from the injection lance, thereby desulfurizing the molten pig iron, the
method includes a process of adding a metallic Al on a surface of the molten pig iron in a
part of a duration after an oxygen partial pressure in the immersing freeboard reaches 0.1
MPa or less due to the inert gas injected into the immersing freeboard, and before 30% by
mass of a total amount of the CaO powder to be used is injected.
[0020]
(2) The desulfurization method of molten pig iron described in the above (1) may further include a process of removing a blast furnace slag inside the molten pig iron ladle so as to be 0.5 t or less before immersing the immersing freeboard and the injection lance into the molten pig iron inside the molten pig iron ladle.
(3) In the desulfurization method of molten pig iron described in the above (1) or (2), the injection lance may be immersed into the molten pig iron inside the molten pig iron ladle while injecting the inert gas and the CaO powder from the injection lance, and the metallic Al may be fed at an exposed portion on the surface of the molten pig iron formed by the inert gas and the CaO powder injected after immersing the injection lance. [0021]
(4) In the desulfurization method of molten pig iron described in the above (1), the pore diameter of the CaO powder may be 3 urn or more.
(5) In the desulfurization method of molten pig iron described in the above (1) or (2), a metallic Mg may be injected in addition to the CaO powder from the injection lance together with the metallic Al at the starting phase in which the inert gas and the CaO powder begin to be injected from the injection lance.
Effects of the Invention [0022]
In the desulfurization method of molten pig iron according to the present invention described in the above (1), metallic Al is fed into molten pig iron in a part of a duration after the oxygen partial pressure inside an immersing freeboard becomes 0.1 MPa or less and before 30% by mass of the total amount of CaO powder to be used is injected. Thereby, the loss of the metallic Al by oxidation reaction or the like can be reduced, and it is possible to ensure a reaction time for a permanent reaction after the making of CaO slag (formation of CaO- Al2O3). Therefore, the added metallic Al can be used effectively, and thus the reaction efficiency of CaO can be increased. [0023]
The case described in the above (2) further includes a process that previously removes blast furnace slag in the molten pig iron ladle, therefore, the loss of the metallic Al by oxidation reaction with blast furnace slag can be reduced, and thus the reaction efficiency of CaO powder can be increased. That is, the desulfurization reaction property of CaO powder can be maintained to a high degree, and the treatment efficiency can be increased. [0024]
In the case described in the above (3), the loss of the metallic Al by oxidization reaction between the metallic Al and the blast furnace slag can be further reduced by feeding the metallic Al into the molten pig iron directly through a portion on the surface of the molten pig iron in an exposed state, which is not covered with the blast furnace slag.
[0025]
In the case described in the above (4), since CaO powder with a pore diameter of 3 µm or more is injected, the contact area between the CaO powder and the molten pig iron is significantly enlarged, and thus the reaction efficiency of the CaO powder can be increased. [0026]
In the case described in the above (5), since CaO powder, metallic Al and metallic Mg are concurrently used, desulfurization treatment time can be significantly shortened. Therefore, it is particularly effective in a case in which the concentration of S included in the molten pig iron is high, and thus the treatment time is assumed to become long. [0027]
As described above, according to the desulfurization method of molten pig iron according to the present invention, the addition timing of metallic Al is optimized in terms of the relationship between the oxygen partial pressure inside the immersing freeboard and the amount of injected CaO powder. Therefore, the reaction efficiency of the CaO powder is improved, whereby it is possible to achieve a desulfurization treatment capacity that can make the concentration of residual S be 50 ppm or less at a low cost within a short time.
Brief Description of the Drawings [0028]
FIG. 1 illustrates graphs of the relationship between the variation in oxygen partial pressure and treatment efficiency from the moment of treatment starting. In the graph (a) in FIG. 1, the horizontal axis represents the injection ratio of CaO, and the vertical axis represents oxygen partial pressure PO2. In addition, in the graph (b) in FIG. 1, the horizontal axis represents the injection ratio of CaO, and the vertical axis represents CaO-k values.
FIG. 2 A is a view schematically illustrating a desulfurization method according to an embodiment of the present invention.
FIG 2B is a view schematically illustrating a subsequent process of the same desulfurization method.
FIG. 2C is a view schematically illustrating a subsequent process of the same desulfurization method.
FIG 2D is a view schematically illustrating a subsequent process of the same desulfurization method.
FIG. 2E is a view schematically illustrating a subsequent process of the same desulfurization method.
Embodiments of the Invention [0029]
FIGs. 2A to 2E are views schematically illustrating the desulfurization method of molten pig iron according to an embodiment of the present invention, and the optimal configuration for conducting the present invention will be hereinafter described with reference to the above drawings.
Firstly, blast furnace slag 2 floating on the top surface of molten pig iron 1 in a molten pig iron ladle 3 is removed before the blast furnace slag 2 in the molten pig iron ladle 3 becomes 0.5 t (thickness of 10 mm) or less. Then, the molten pig iron ladle 3 is moved toward the bottom of an immersing freeboard 4.
Additionally, an injection lance 5 formed with a refractory material is moved down together with the immersing freeboard 4 from the location shown in FIG 2A, and the moving-down operation is stopped after the bottom portion of the immersing freeboard 4 is immersed into the molten pig iron 1 inside the molten pig iron ladle 3 (FIG 2B). [0030]
After the bottom portion of the immersing freeboard 4 is immersed into the molten pig iron 1 as described above, the supply of an inert gas and CaO powder is started from the top end of the injection lance 5. Additionally, the injection lance 5 is moved further down, and the moving-down operation is stopped in a state in which the bottom portion of the injection lance 5 is immersed deeper into the molten pig iron 1 as shown in
FIG. 2C. If the injection lance 5 is immersed into the molten pig iron 1 while injecting the CaO powder together with the inert gas from the injection lance 5, the clogging at the tip of the injection lance 5 can be prevented, which is desirable. [0031]
Further, in th& above injection process, the amount of the inert gas and CaO powder injected varies depending on the conditions, such as the concentration of S included in the molten pig iron 1, the amount of molten pig iron to be treated, the amount of desulfurization or the like. However, for example, it is possible to use an injection speed of the inert gas of 11 Nm /min and an injection speed of the CaO powder of 200 kg/min. In addition, it is desirable to use nitrogen gas or argon gas as the kind of inert gas. [0032]
Air in the immersing freeboard 4 is sequentially exhausted via an exhaust pipe 4a by continuing the injection of the inert gas and the CaO powder from the bottom end of the injection lance 5 in this state. Additionally, as shown in FIG. 2D, metallic Al 6 (hereinafter, simply described as 'metallic Al') is fed into the molten pig iron 1 from a feeding opening 4b present at the top portion of the immersing freeboard 4 in a state in which the oxygen partial pressure inside the immersing freeboard 4 becomes 0.1 MPa or less, and the amount of the injected CaO powder does not reach 30% by mass of the total amount to be injected.
Here, in the actual equipment, it is also possible to previously obtain the relationship between the amount of the inert gas to be injected and the oxygen partial pressure inside the immersing freeboard 4, and then determine the oxygen partial pressure inside the immersing freeboard 4 based on the relationship. Alternatively, it is also possible to measure the oxygen partial pressure in exhaust gas by providing a meter for measuring oxygen concentration in exhaust gas at the exhaust pipe 4a and then determine the oxygen partial pressure inside the immersing freeboard 4 based on the measured values. Furthermore, it is also possible to determine the oxygen partial pressure by calculation using the following formulae (1) to (3).
PO2/PO2 (O)=exp(-VN2/V0×T1) - (l)
PO2=0.2×exp(-VN2/V0×Tl) - (2)
PO2 (O)=0.2 (initial oxygen partial pressure) - (3)
In these formulae, VN2 (the volume of the injected inert gas) = the speed of the injected inert gas Nm /min x (molten pig iron temperature (°C) + 273) / 273 x treatment time, and V0 (corrected volume) = the total volume of the immersing freeboard 4x(l/4). [0033]
Meanwhile, if the metallic Al is added in a state in which the oxygen partial pressure inside the immersing freeboard does not become 0.1 MPa or less, the metallic Al does not sink into the molten pig iron 1 due to the light weight thereof. Therefore, most of the added metallic Al reacts with oxygen in the atmosphere so as to change to Al2O3 alone. Therefore, the metallic Al added to increase the reaction efficiency of CaO is needlessly wasted, and thus cannot contribute to the improvement of the reaction efficiency of CaO. [0034]
However, if the metallic Al is added after the oxygen inside the immersing freeboard 4 has been substituted with the inert gas so that the oxygen partial pressure becomes 0.1 MPa or less, most of the loss of the metallic Al due to the above-described oxidization does not occur. As a result, in the molten pig iron 1, the CaO-k values representing the desulfurization treatment capacity are improved according to the reaction formula 3CaO+3S+2A1→3CaS+Al2O3. More desirably, since the CaO-k values are saturated at about 0.55, the feeding of the metallic Al from the feeding opening 4b into the exposed molten pig iron 1 as shown in FIG. 2D is started after the oxygen partial pressure inside the immersing freeboard 4 reaches 0.01 MPa or less, which corresponds to the above range. [0035]
The addition of the metallic Al is finished before 30% by mass of the total amount (total mass) of the CaO powder to be used has been injected. That is because,
when the metallic Al is added after 30% by mass of the total amount of the CaO powder to be used has been injected, the CaO powder injected into the molten pig iron 1 has already been floated from the inside of the molten pig iron 1 and cumulated on the surface of the molten pig iron 1, therefore the CaO cumulated on the surface of the molten pig iron 1 is not activated even when the metallic Al is added to the molten pig iron 1.
However, in a case in which the metallic Al is added before 30% by mass of the total amount of the CaO powder to be used is injected, the amount of CaO powder cumulated on the surface of the molten pig iron 1 is small, and most of the added and melted metallic Al comes into contact with the CaO powder so as to be consumed in the making of CaO slag (a state in which CaO- Al2O3 is formed on the surface of the CaO powder), thereby increasing the use efficiency of the quick lime. [0036]
As shown in FIG 2E, the addition of the metallic Al and the injection of the metallic Mg are conducted before 30% by mass of the total amount of CaO powder to be used is injected and stopped at a stage at which a predetermined amount has been reached, and then only CaO powder is injected.
It is possible to maintain a state in which the above-described molten metallic Al contributes to the desulfurization reaction during the remaining treatment time even while the CaO powder is being injected. Naturally, CaO slagmade after 30% by mass of the total amount of CaO powder to be used has been injected can also contribute to the desulfurization reaction even after being floated, but the remaining period of the treatment time is short, therefore a period during which CaO contributes to desulfurization reaction becomes short by a like amount. [0037]
In the present embodiment, in order to increase the reaction efficiency of CaO powder to the maximum extent, the metallic Al is injected into the molten pig iron 1 at the above-described optimized timing, but the metallic Al also reacts with the blast furnace slag 2 inside the molten pig iron ladle 3 so as to change to Al2O3.
Therefore, from the viewpoint of increasing treatment efficiency by maintaining
the desulfurization property of CaO powder to a high degree, as described above, it is preferable to remove the blast furnace slag 2 inside the molten pig iron ladle 3 before the immersing freeboard 4 and the injection lance 5 are immersed into the molten pig iron 1 in the molten pig iron ladle 3. Thereby, the loss of the metallic Al by the oxidization reaction with the blast furnace slag 2 can be reduced, and thus the reaction efficiency of the CaO can be increased.
Meanwhile, the less the residual amount of the blast furnace slag 2 is, the more preferable it is, and the amount is desirably 0.5 t or less. Although the residual amount depends on the internal diameter of the molten pig iron ladle 3, it is preferable to make the lower limit of the residual amount of the blast furnace slag 2 be 0.11 or more since there is a concern in that, as the amount of slag to be removed becomes larger, the operation time for removing slag becomes too long, thereby lowering the productibility. [0038]
In addition, from the viewpoint of increasing the reaction efficiency of the CaO powder, it is preferable to inject, for example, CaO powder with a pore diameter of 3 urn or more (preferably 5 urn or more) and a diameter of 30 urn or less, which is represented by salt-burnt quick lime. The molten pig iron intrudes into the pores on the surface of the CaO powder so that the contact area between the CaO powder and the molten pig iron 1 is significantly enlarged. As a result, an area that becomes CaO-Al2O3 is enlarged, therefore, the effect of the addition of Al can be further remarkably developed.
Meanwhile, the particle diameter of the CaO powder is not particularly limited, and, for example, powder with a particle diameter of 0.2 mm or less can be used. The above-described operation by the metallic Al can be remarkably developed by using powder with a particle diameter of 0.2 mm or less. [0039]
In the desulfurization method according to the present embodiment described thus far, CaO powder and metallic Al are used as flux, but, from the viewpoint of shortening the treatment time, it is desirable to concurrently use CaO powder, metallic Al and metallic Mg. More specifically, it is preferable to inject metallic Mg from the
injection lance 5 in addition to the inert gas and the CaO powder when feeding the metallic Al. Since the desulfurization treatment capacity of metallic Mg is higher than that of CaO powder, the desulfurization treatment time can be significantly shortened by concurrently using metallic Mg. Therefore, the treatment time can also be adjusted by injecting metallic Mg from the injection lance 5 in an amount that corresponds to the concentration of S included in the molten pig iron 1. This is particularly effective in a case in which the concentration of S included in the molten pig iron 1 is high and thus the treatment time is assumed to exceed the cycle time. [0040]
In addition, compared with a case in which a desulfurization treatment is conducted only with CaO powder and metallic Mg, the amount of metallic Mg to be used can be significantly reduced by concurrently using CaO powder, metallic Al and metallic Mg.
The reason is as below. That is, since metallic Mg is characterized by having a desulfurization efficiency which is extremely high in the case of a high S concentration (0.01% or more), but decreases as the S concentration decreases (0.01% or less), in the case of conducting a desulfurization treatment only with CaO powder and metallic Mg, it is necessary to use a large amount of metallic Mg. In contrast, in the case of concurrently using CaO powder, metallic Al and metallic Mg, when S concentration is high immediately after starting the desulfurization treatment, metallic Mg develops the high desulfurization treatment capability, and, when the S concentration decreases as the treatment time passes, CaO activated by the addition of metallic Al develops a stabilized desulfurization treatment capability. Therefore, it is possible to suppress the amount of metallic Mg to be used. As a result, by concurrently using CaO powder, metallic Al and metallic Mg, it is possible not only to shorten the treatment time, but also to reduce the production cost.
Examples [0041]
Next, examples of the present invention will be described with reference to Table 1. However, the conditions in the present examples are conditions employed to prove the feasibility and remarkable effects of the present invention, and the present invention is not limited only to these conditions. In the present examples, a molten pig iron ladle with a diameter of 5 m and a height of 6 m and an immersing freeboard with a diameter of 3 m and a height of 8 m were used. Furthermore, the immersion depth of an injection lance was 2.2 m, and N2 gas was used as a carrier gas with a flow rate of 11 Nm3/min. In addition, the average particle size of metallic Al added from the top portion of the immersing freeboard was 30 mm, and the particle size of metallic Mg injected from the injection lance together with CaO powder at the same time was 300 µm or less. Moreover, general quick lime with an average pore diameter of about 1 µm was used as CaO powder except Invention Examples 3 and 4. [0042]
Comparative Example 1 is an example in which the addition of metallic Al is started early (that is, metallic Al was added when the oxygen partial pressure inside the immersing freeboard was higher than 0.1 MPa). Comparative Example 2 is an example in which the addition of metallic Al is started late (that is, metallic Al was added when the injected amount of CaO powder exceeded 30% by mass of the total amount of CaO powder to be used). Comparative Example 3 is an example in which metallic Al was not added and metallic Mg was injected at the starting phase of treatment. Comparative Example 4 is an example in which metallic Al was added not on the surface of molten pig iron, but on blast furnace slag. All of Comparative Examples 1 to 4 exhibited poor desulfurization rate. [0043]
On the other hand, all of Invention Examples 1 to 6 satisfy the requisite conditions of the present invention, and can obtain good desulfurization rate compared with each of Comparative Examples 1 to 4.
In Invention Example 1, blast furnace slag inside the molten pig iron ladle was not sufficiently ejected when immersing the immersing freeboard, and thus the amount of
residual blast furnace slag was large, therefore desulfurization rate was slightly lowered compared with Invention Example 2. In addition, since Invention Example 3 used salt-burnt CaO powder with an average pore diameter of 5 µm, Invention Example 3 could obtain a good desulfurization rate. Furthermore, since Invention Example 5 concurrently used metallic Mg at the starting phase of desulfurization, Invention Example 5 could obtain a good desulfurization rate, and also shorten the desulfurization treatment time compared with Invention Example 6 in which metallic Mg was not used. [0044] [Table 1]
Industrial Applicability [0045]
According to the present invention, it is possible to provide a desulfurization method of molten pig iron capable of developing a treatment capacity that stably desulfurizes the molten pig iron so as to make the concentration of residual S be 50 ppm or less at a low cost and within a short time.
Reference Signs List [0046]
1 MOLTEN PIG IRON
2 BLAST FURNACE SLAG DESULFURIZED SLAG
3 MOLTEN PIG IRON LADLE
4 IMMERSING FREEBOARD
4a EXHAUST PIPE
4b FEEDING OPENING
5 INJECTION LANCE
6 METALLIC Al

CLAIMS
1. A desulfurization method of molten pig iron, wherein an immersing freeboard
and an injection lance are immersed into the molten pig iron inside a molten pig iron ladle,
and an inert gas and CaO powder are injected from the injection lance, thereby
desulfurizing the molten pig iron, the method comprising:
adding metallic Al on a surface of the molten pig iron in a part of a duration after oxygen partial pressure in the immersing freeboard reaches 0.1 MPa or less due to the inert gas injected into the immersing freeboard, and before 30% by mass of a total amount of the CaO powder to be used is injected.
2. The desulfurization method of the molten pig iron according to Claim 1, further
comprising
removing a blast furnace slag inside the molten pig iron ladle so as to be 0.5 t or less before immersing the immersing freeboard and the injection lance into the molten pig iron inside the molten pig iron ladle.
3. The desulfurization method of the molten pig iron according to Claim 1 or 2,
wherein
the injection lance is immersed into the molten pig iron inside the molten pig iron ladle while injecting the inert gas and the CaO powder from the injection lance, and
the metallic Al is fed at an exposed portion on the surface of the molten pig iron formed by the inert gas and the CaO powder injected after immersing the injection lance.
4. The desulfurization method of the molten pig iron according to Claim 1 or 2, wherein the CaO powder has a pore diameter of 3 urn or more.
5. The desulfurization method of the molten pig iron according to Claim 1 or 2, wherein metallic Mg is injected in addition to the CaO powder from the injection
lance together with the metallic Al at a starting phase in which the inert gas and the CaO powder begin to be injected from the injection lance.