JP7548256B2 - Stirring blade and method for desulfurizing molten iron - Google Patents

Description

The present invention relates to a stirring blade and a method for desulfurizing molten iron.

In recent years, the proportion of ultra-low P and S steel types, which have low P and S concentrations, is increasing due to the growing need for higher purity and higher performance steel materials. In order to reduce impurity levels in the steelmaking process, technology is needed to produce ultra-low P and S steel without increasing costs or the amount of slag generated, making it important to improve the reaction efficiency of refining agents. In addition, productivity must be maintained or improved, and improving reaction speed is also essential.

A widely used industrial method for reducing the S concentration in steel is to desulfurize the molten pig iron to a low S concentration range by using a mechanical stirrer (impeller) to perform molten pig iron desulfurization (mechanical stirring desulfurization). This mechanical stirring molten pig iron desulfurization process allows high-speed processing to a low S concentration range. In mechanical stirring molten pig iron desulfurization, techniques such as screw-type stirring blades and installation of baffles in the reaction vessel are known to further improve the reaction efficiency of the desulfurizing agent and increase the processing speed, but this has been difficult to achieve due to the shortened lifespan of the stirring blades and the vessel refractory material.

In the mechanical stirring molten pig iron desulfurization process, a powdered or granular desulfurization agent is added to the surface of the molten pig iron, and is introduced into the cavity (vortex) of the molten pig iron formed by the rotation of the stirring blade. This increases the contact area with the molten pig iron, aiming for high desulfurization agent reaction efficiency. For example, a lime (CaO)-based desulfurization agent is used as the desulfurization agent. The molten pig iron side is stirred by the rotation of the stirring blade, and the desulfurization reaction progresses as the sulfur in the molten pig iron is supplied to the reaction interface with the desulfurization agent. The reaction vessel that holds the molten pig iron is a cylindrical pot type, and the stirring blade is inserted from above into the center of the cross section of the vessel. It is known that the stirring force can be increased by installing a baffle plate on the inner wall of the vessel and offsetting the rotation position, as opposed to rotating the stirring blade at the center of the vessel. In addition, by devising the shape of the stirring blade, it is possible to promote the entrainment of the desulfurization agent and extend the life of the stirring blade.

Agitator blades of various shapes have been used to agitate molten metal, and various shapes have been proposed in order to reduce uneven stirring of the molten metal and obtain a high desulfurization rate in a short period of time (e.g., Patent Documents 1 and 2).
Furthermore, Patent Document 3 discloses a stirring type composition adjustment device having an upper impeller and a lower impeller for the purpose of forming a turbulent flow throughout the entire pot and efficiently adjusting the composition.

JP 2012-167868 A JP 2009-114506 A JP 2003-279272 A

However, the impeller shapes disclosed in Patent Documents 1 to 3 have very complex shapes, which makes construction difficult and time-consuming and costly. In addition, impellers that improve the mixing efficiency due to their complex shapes also have the problem that the effect cannot be maintained because the refractory material wears out during use and the shape changes due to the adhesion of desulfurizing agents and slag.

Therefore, the present invention was made with a focus on the above problems, and aims to provide an agitator blade and a method for desulfurizing molten iron that can improve desulfurization efficiency without making the agitator blade have a complex shape.

According to one aspect of the present invention, there is provided an agitator blade for use in a molten pig iron desulfurization process, in which agitation is provided by rotating an agitator blade immersed in molten pig iron, and a desulfurization agent is added to the molten pig iron at least one of the timings during and before stirring by the agitator blade, thereby reducing the sulfur concentration in the molten pig iron. The agitator blade comprises: two first agitator blades protruding from a rotation axis of the agitator blade, the protruding directions of which are perpendicular to the rotation axis and in opposite directions to each other; and two second agitator blades protruding respectively from the two first agitator blades, the protruding directions of which are perpendicular to the protruding direction of the first agitator blades and a direction parallel to the rotation axis, and in opposite directions to each other, and the second agitator blades are disposed such that the widthwise center positions of the second agitator blades are moved a distance L from the center position of the rotation axis such that the widthwise center positions of the second agitator blades are moved a distance L that satisfies formula (2).
0.5≦｜L/(R-0.5T)｜≦1...(2)
however,
L: Distance from the rotation axis to the center position of the second agitating blade (m)
R: Length of the first agitating blade (m)
T: Width of second agitating blade (m)

According to one aspect of the present invention, there is provided a method for desulfurizing molten pig iron, in which stirring is performed by rotating a stirring blade immersed in the molten pig iron, and a desulfurizing agent is added to the molten pig iron at least during or before stirring by the stirring blade, thereby reducing the sulfur concentration in the molten pig iron. The stirring blades include two first stirring blades that protrude from the rotation axis of the stirring blade and have protruding directions perpendicular to the rotation axis and opposite to each other, and two second stirring blades that protrude from the two first stirring blades, respectively, and have protruding directions perpendicular to the protruding direction of the first stirring blade and the direction parallel to the rotation axis, and opposite to the radial direction of the rotation axis, and the second stirring blades are arranged at positions where the center positions of the width of each of the second stirring blades are moved a distance L from the center position of the rotation axis to satisfy formula (2).

According to one aspect of the present invention, a stirring blade and a method for desulfurizing molten iron are provided that can improve desulfurization efficiency without making the stirring blade have a complex shape.

1A and 1B are schematic diagrams showing an agitator blade used in a water model experiment, in which (A) is a plan view and (B) is a side view. 1A and 1B are schematic diagrams showing an agitator blade used in a water model experiment, in which (A) is a plan view and (B) is a side view. 1A and 1B are schematic diagrams showing an agitator blade used in a water model experiment, in which (A) is a plan view and (B) is a side view. 1 is a graph showing the results of a water model experiment. FIG. 1 is a schematic diagram showing a mechanical stirring type desulfurization device according to an embodiment of the present invention. 1A and 1B are schematic diagrams showing an agitator blade according to one embodiment of the present invention, in which (A) is a plan view and (B) is a side view. 1A is a schematic diagram showing an agitating blade having an inclination angle, (A) being a plan view, (B) being a side view, and (C) being a front view. FIG. 1A is a schematic diagram showing an agitating blade having an inclination angle, (A) being a plan view, (B) being a side view, and (C) being a front view. FIG.

In order to solve the above problems, the inventors have conducted extensive research and studies into a molten pig iron desulfurization method in which an impeller is immersed in the molten pig iron, rotated to provide agitation, and a lime-based desulfurization agent is added to the molten pig iron at least either during or before stirring with the impeller, thereby reducing the sulfur concentration in the molten pig iron. As a result, the inventors have found that a high desulfurization reaction efficiency can be obtained by using an impeller equipped with two first impellers that protrude from the rotation axis of the impeller and protrude in directions perpendicular to the rotation axis and in opposite directions, and two second impellers that protrude from each of the two first impellers and protrude in directions perpendicular to the protruding direction of the first impellers and the direction parallel to the rotation axis and in opposite directions to each other. Specifically, the inventors conducted extensive research into various impeller shapes in water model experiments and mechanically stirred molten iron desulfurization processes, and discovered an impeller shape that is simple and easy to install, and can promote the entrainment of desulfurizing agents and the stirring of molten iron.

First, the inventors conducted a water model experiment using a four-blade agitator blade 4A shown in FIG. 1 as a base to investigate the effect of changing the position of the agitator blade on the entrainment of the desulfurization agent. As shown in FIG. 1(A), the agitator blade 4A has an agitator blade 41A, and has two sets of agitator blades 41A, each set consisting of two agitator blades 41A facing each other around the rotating shaft 40A. Of these two sets, one set of agitator blades 41A is the first agitator blade 411A, and the other set of agitator blades 41A is the second agitator blade 412A. In the example shown in FIG. 1(A), the two agitator blades 41A protruding left and right from the rotating shaft 40A are the first agitator blade 411A, and the two agitator blades 41A protruding upward and downward from the rotating shaft 40A are the second agitator blade 412A. In addition, the length of one set of agitator blades 41A in the extension direction of the agitator blades 41A in a plan view is also referred to as the diameter d of the agitator blade 4A. In the water model experiment, the diameter d of the first agitator blade 411A and the second agitator blade 412A were set to the same length. In addition, the width T of the agitator blade 41A and the blade height b, which is the vertical height, were also set to the same for the first agitator blade 411A and the second agitator blade 412A.

In this study, as shown in Figures 2 and 3, the agitator blade 41A was moved a certain distance in the extension direction of the first agitator blade 411A (left and right direction in Figures 2 (A) and 3 (A)) so that the second agitator blade 412A was separated from each other, based on the agitator blade 4A in Figure 1. The concave depth of the generated vortex when rotated at the same rotation speed and the number of particles dispersed in the lower region from the agitator blade when plastic particles simulating a desulfurization agent were added were investigated by a water model experiment. In this study, the movement position of the second agitator blade 412A was defined as the movement amount ratio r as shown in equation (1) to organize the results. Here, L is the length (m) from the rotating shaft 40A to the center position of the second agitator blade 412A, that is, the distance (m) in the extension direction of the first agitator blade 411A to the center position of the second agitator blade 412A when the rotating shaft 40A in a plan view is the origin. In this study, as shown in Figs. 1(A) to 3(A), the rotation direction of the stirring blade 4A when viewed from above is set to be counterclockwise in plan view, and the positive and negative values of L are set with the rotation direction being positive. In other words, the stirring blade 4A shown in Figs. 2 and 3 has the second stirring blade 412A moved toward the tip side of the first stirring blade 411A so that the center position of the second stirring blade 412A moves in the same direction as the rotation direction, relative to the second stirring blade 412A in Fig. 1, so that L is positive. R is the length (m) of the first stirring blade 411A, that is, the length (m) of the first stirring blade 411A in the extension direction from the rotating shaft 40A to the end of the first stirring blade 411A in plan view. T is the width (m) of the second stirring blade 412A, that is, the length (m) of the second stirring blade 412A in the extension direction of the first stirring blade 411A in plan view. Since the denominator of equation (1) is a positive value, when the value of L is positive, the value of r is also positive, and when the value of L is negative, the value of r is also negative.
r=L/(R-0.5T)...(1)

In this study, water model experiments were conducted using mixing blade 4A with nine conditions of movement ratio: +1.0, +0.75, +0.5, +0.25, 0, -0.25, -0.5, -0.75, and -1.0. In the water model experiments, the rotation speed was kept constant and the depth of the depression in the center of the generated vortex was measured from outside the container.

Figure 4 shows the relationship between the movement ratio r of the stirring blade 4A and the vortex depth and the number of entrained plastic particles as results of the water model experiment. In the water model experiment, the stirring blade 4A was immersed in water contained in a ladle-shaped transparent cylindrical container simulating a molten iron container, and the vortex depth was the distance from the water surface position when the stirring blade 4A was not rotating to the bottom end of the vortex generated when it was rotated. Figure 4 shows the vortex depth as an index, with the vortex depth when the movement ratio r is 0 being set to 1. As shown in Figure 4, it was found that the greater the absolute value of the movement ratio r, the deeper the vortex depression.

In the water model experiment, plastic particles simulating a desulfurizing agent were added to the bath, and the number of particles dispersed in the lower region below the stirring blade 4A was counted. In Figure 4, the number of particles dispersed in the region below the lower end of the stirring blade is taken as the number of particles entrained, and the number of particles entrained is expressed as an index when the number of particles entrained when the movement amount ratio r is 0 is taken as 1. As shown in Figure 4, when the absolute value of the movement amount ratio r is about 0.25, the number of particles entrained hardly increases, but when the absolute value of the movement amount ratio r is 0.5 or more, the number of particles entrained increases by more than two times. In particular, it was found that when the value of the movement amount ratio r of the second stirring blade 412A is positive, that is, when the center position of the second stirring blade 412A is moved in the same direction as the rotation direction, the increase in the number of particles entrained is greater.

The reason why the vortex recess depth increases as the absolute value of the movement amount ratio r increases is thought to be because, even if the diameter d of the stirring blade 4A is the same, when the blade part is moved, the maximum outer diameter D increases and this maximum outer diameter contributes to the formation of the vortex. Note that the maximum outer diameter D is the length of the straight line that passes through the rotating shaft 40A in a plan view and connects the ends of the two second stirring blades 412A, which is the maximum length, as shown in Figures 1 (A) and 2 (A). Furthermore, the increase in the number of particles entrained, i.e., the number of dispersed particles, is thought to be influenced by the fact that the maximum outer diameter D increases as the blade part moves, deepening the depth of the vortex generated at the same rotation speed, and the change in shape increases the entrainment of particles.

On the other hand, when the particle dispersion behavior was carefully observed for the difference between the case where the second agitating blade 412A was moved to the positive side (the case where the center position of the second agitating blade 412A was moved in the same direction as the rotation direction) and the case where the second agitating blade 412A was moved to the negative side (the case where the center position of the second agitating blade 412A was moved in the opposite direction to the rotation direction), it was confirmed that the particles in the bath hit the side of the first agitating blade 411A on the side where the second agitating blade 412A does not protrude, moved from the center of the agitating blade toward the outside along the side, and then detached from the tip of the agitating blade 4A and dispersed in the bath. When the second agitating blade 412A was moved to the negative side, it was observed that the particles were retained inside the L-shaped portion formed by the side of the second agitating blade 412A on the rotating shaft 40 side and the side of the first agitating blade 411A. On the other hand, when the second agitator blade 412A is moved to the forward side, the particles that have moved along the blade side are smoothly detached from the blade tip, and it was found that the difference in the particle retention state leads to the difference in the number of dispersed particles. Therefore, it is better to move the second agitator blade 412A in the forward direction. In other words, it is better to rotate the agitator blade 4A so that the rotation direction of the first agitator blade 411A is opposite to the protruding direction of the second agitator blade 412A from the first agitator blade 411A, relative to the shape of the agitator blade 4A. In addition, the behavior of particles peeling off from the blade due to such a change in shape was significantly confirmed in the agitator blade 4A with a movement amount ratio r of +0.5 or more, and it was thought to be one of the factors that increased the number of particles entrained when the movement amount ratio r was +0.5 or more.

In addition, compared to the base four-blade agitator blade 4A shown in FIG. 1, the agitator blade 4A in which the second agitator blade 412A is moved as shown in FIG. 2 and FIG. 3 does not increase the overall volume of the agitator blade, so there is no need to reduce the amount of molten iron that can be accommodated in the treatment vessel. In addition, since there is no increase in volume, the weight of the agitator blade 4A does not increase when the same refractory material is used, and there is no need to increase the holding strength of the agitator. In other words, it was found that it is possible to promote the dispersion of the desulfurization agent and improve desulfurization efficiency without making the shape complicated and without increasing the volume or weight of the agitator blade 4A.

In the following detailed description, an embodiment of the present invention will be described with reference to the drawings. In the description of the drawings, identical or similar parts are given the same or similar reference numerals, and duplicate explanations will be omitted. The drawings are schematic and may differ from the actual product. In addition, the embodiments shown below are examples of devices and methods for embodying the technical concept of the present invention, and the technical concept of the present invention does not specify the materials, structure, arrangement, etc. of the components as described below. The technical concept of the present invention may be modified in various ways within the technical scope defined by the claims.

<Stirring blade and molten iron desulfurization method>
A method for desulfurizing molten pig iron according to an embodiment of the present invention is carried out by using, as an example, a mechanical stirring desulfurization apparatus as shown in Fig. 5. In the example shown in Fig. 5, a ladle-type molten pig iron ladle 2 is used as a treatment vessel for accommodating molten pig iron. As for the shape of the treatment vessel, a ladle-type treatment vessel as shown in Fig. 5 is optimal because the desulfurization treatment is performed by the mechanical stirring desulfurization apparatus, but a torpedo car can also be used.

The molten iron 3 tapped from the blast furnace is received by a molten iron ladle 2 or a torpedo car mounted on a cart 1, and the received molten iron 3 is transported to a mechanically stirred desulfurization device. When the molten iron is received by a torpedo car, it is desirable to transfer it to a ladle-type treatment vessel prior to the desulfurization treatment. The molten iron 3 to be subjected to the desulfurization treatment according to this embodiment is molten iron produced in a blast furnace, electric furnace, or shaft furnace, and may have any composition. For example, it may have been subjected to a desiliconization treatment or a dephosphorization treatment in advance. The desiliconization treatment is a treatment in which an oxygen source such as oxygen gas or iron ore is added to the molten iron 3 prior to the dephosphorization treatment in order to perform the dephosphorization treatment efficiently, thereby mainly removing the Si in the molten iron.

The mechanical stirring desulfurization device is equipped with a refractory stirring blade 4 that is immersed and buried in the molten pig iron 3 contained in the molten pig iron ladle 2 and rotates to stir the molten pig iron 3. The stirring blade 4 is raised and lowered in a substantially vertical direction by a lifting device (not shown), and rotates around a shaft to which the stirring blade 4 is connected at its tip by a rotating device (not shown) consisting of a drive motor and a reducer. The mechanical stirring desulfurization device is also equipped with an inlet 6 for placing and adding a desulfurization agent or a deoxidizing source on the bath surface of the molten pig iron 3 in the molten pig iron ladle 2. An up-blowing lance 5 for adding a desulfurization agent by blowing it from above toward the molten pig iron 3 in the molten pig iron ladle 2 may also be installed. Furthermore, an exhaust duct port (not shown) connected to a dust collector (not shown) is provided above the molten pig iron ladle 2, and gas and dust generated during the desulfurization process are discharged.

The inlet 6 is connected to a supply device consisting of a hopper 11 that contains the desulfurizing agent and a rotary feeder 12 for feeding a fixed amount from the hopper 11, and a supply device consisting of a hopper 13 that contains the deoxidizing source and a rotary feeder 14 for feeding a fixed amount from the hopper 13. The structure allows at least one of the desulfurizing agent and the deoxidizing source to be independently adjusted and fed from the inlet 6 at any desired timing.

Similarly, the top-blowing lance 5 is connected to a supply device consisting of a hopper 7 that contains the desulfurizing agent and a rotary feeder 8 for feeding a fixed amount from the hopper 7, and a supply device consisting of a hopper 9 that contains the deoxidizing source and a rotary feeder 10 for feeding a fixed amount from the hopper 9. The top-blowing lance 5 is structured so that it can independently adjust and feed at least one of the desulfurizing agent and the deoxidizing source at any timing together with the conveying gas. The supply of the desulfurizing agent and the deoxidizing source from the top-blowing lance 5 is also called top-blowing addition.

As the desulfurizing agent, various desulfurizing agents such as not only CaO-based desulfurizing agents but also calcium carbide-based desulfurizing agents, soda-based desulfurizing agents, metallic Mg, etc. can be used, but it is preferable to use a CaO-based desulfurizing agent whose main component is lime because it is inexpensive. Also, it is preferable to use only a CaO-based desulfurizing agent without using a fluorine source such as fluorite in order to take environmental measures and to easily reuse the slag generated. As the CaO-based desulfurizing agent, quicklime (CaO), dolomite ( _MgCO3.CaCO3 ), slaked lime (Ca(OH) ₂ ), limestone ( _CaCO3 ) _, etc. can be used.

As the deoxidizing source, it is preferable to use metallic Al or aluminum dross powder, which is inexpensive and can be obtained as an aluminum source. Other Al sources may also be used, such as atomized powder obtained by atomizing molten aluminum with gas, or cutting powder generated when polishing or cutting an aluminum alloy. These may be added from the inlet 6 separately from the CaO-based desulfurizing agent, or may be added by top blowing from the top blowing lance 5 to the surface of the molten iron 3 together with the carrier gas.

After the molten pig iron ladle 2 is transported to the mechanical stirring desulfurization device, the position of the cart 1 carrying the molten pig iron ladle 2 is adjusted so that the stirring blade 4 is positioned approximately at the center of the molten pig iron ladle 2. Next, the stirring blade 4 is lowered and immersed in the molten pig iron 3. Once the stirring blade 4 is immersed in the molten pig iron 3, the stirring blade 4 starts to rotate and increases its speed to a predetermined rotation speed. Once the rotation speed of the stirring blade 4 reaches the predetermined rotation speed, the rotary feeder 8 is started and the desulfurization agent in the hopper 7 is added to the molten pig iron from the inlet 6 or the top blowing lance 5. In this way, the desulfurization agent may be added to the molten pig iron 3 by natural drop from the inlet 6, or the desulfurization agent may be supplied to the molten pig iron 3 by spraying it onto the molten pig iron bath surface together with the carrier gas from the top blowing lance 5, or both may be used in combination.

The supply of the deoxidizing source into the molten metal ladle 2 is preferably carried out in order to promote the desulfurization reaction, and may be carried out in parallel with the addition of the desulfurizing agent, before or after the addition, or throughout the entire desulfurization treatment period.
After stirring for a predetermined time, the rotation speed of the stirring blade 4 is reduced and stopped. After the rotation of the stirring blade 4 is stopped, the stirring blade 4 is raised and made to stand by above the molten pig iron ladle 2. The generated slag floats up and covers the surface of the molten pig iron, and the desulfurization of the molten pig iron 3 is completed in the stationary state. After the desulfurization, the generated slag is discharged from the molten pig iron ladle 2, and the molten pig iron ladle 2 is transported to the next refining process.

In this embodiment, the agitator blades 4 shown in Figures 2, 3 and 6 are used in the desulfurization treatment. The agitator blades 4 are based on the knowledge obtained in the water model experiment. The agitator blades 4 include two first agitator blades 411 that protrude from the rotation shaft 40 of the agitator blade 4 and have protruding directions perpendicular to the rotation shaft 40 and opposite to each other, and two second agitator blades 412 that protrude from each of the two first agitator blades 411 and have protruding directions perpendicular to the protruding direction of the first agitator blades 411 and a direction parallel to the rotation shaft 40 and opposite to each other. The first agitator blades 411 and the second agitator blades 412 are also collectively referred to as agitator blades 41.

Further, the second agitating blades 412 are disposed at positions where the center positions of the width of each of the second agitating blades 412 are shifted from the center position of the rotating shaft 40 by a distance L that satisfies the following formula (2). L, R, and T in formula (2) are the same as those in formula (1). The positive and negative values of the movement amount ratio r are also the same as those in formula (1). In other words, the positive and negative values of the movement amount ratio r are positive when the direction of deviation (movement direction) of the center position of the second agitating blade 412 from the rotating shaft 40 is the same as the rotation direction, and are negative when it is the opposite direction to the rotation direction.
0.5≦｜L/(R-0.5T)｜≦1...(2)
however,
L: Distance from the rotating shaft 40 to the center position of the second agitating blade 412 (m)
R: Length of the first agitating blade 411 (m)
T: width of the second agitating blade 412 (m)

By satisfying formula (2), the movement ratio r can be set to a range of +0.5 or more or -0.5 or less in the graph shown in FIG. 4, improving the stirring efficiency and promoting the entrainment of the desulfurization agent added to the molten iron 3. This makes it possible to obtain high desulfurization efficiency. In addition, such an agitator blade 4 is a conventional cross-shaped agitator blade as shown in FIG. 1, in which the position of the second agitator blade 412 is simply shifted, and has a simpler shape than those of Patent Documents 1 to 3, making it excellent in terms of workability and construction costs. Furthermore, since there is less change in shape during use, high desulfurization efficiency can be maintained for a long period of time. Furthermore, the agitator blade 4 according to this embodiment can obtain the effect of improving the stirring efficiency without increasing the diameter d of the agitator blade 4 compared to the conventional shape as shown in FIG. 1, and can improve the desulfurization efficiency.

In addition, conventional knowledge has shown that it is preferable to use a four-blade agitator blade in order to maintain ease of installation and effectiveness during use, and that increasing the diameter of the agitator blade and increasing the rotation speed are effective ways to increase the agitator efficiency. However, when the diameter of the agitator blade is increased, the volume and weight of the agitator blade increase, resulting in problems such as a decrease in the amount of molten iron that can be stored in the treatment vessel and a decrease in the rotation speed due to the increased weight of the agitator blade. In contrast, in this embodiment, the increase in the volume and weight of the agitator blade 4 can be suppressed, so these problems associated with larger size do not occur.

Furthermore, in this embodiment, when stirring, it is preferable that the rotation direction of the stirring blade 4 is opposite to the protruding direction of the second stirring blade 412 from the first stirring blade 411. In such a rotation direction, the movement amount ratio r becomes positive, and as shown in Figure 4, a higher stirring efficiency can be obtained compared to when the movement amount ratio r is negative.
As shown in formula (2), the absolute value of the movement amount ratio r is preferably 1 or less. If the absolute value of the movement amount ratio r exceeds 1, the connection length between the first stirring blade 411 and the second stirring blade 412 becomes shorter, so that the durability of the stirring blade 4 decreases. In particular, it is more preferable that the movement amount ratio r of the second stirring blade 412 is +1. In this case, the stirring blade 4 has the same shape as shown in FIG. 6, and the highest stirring efficiency can be obtained as shown in FIG. 4. In other words, it is preferable that the second stirring blade 412 is disposed at a position where the center positions of the width of each of the second stirring blades 412 are moved by a distance L that satisfies formula (3) from the center position of the rotating shaft 40 so that the two second stirring blades 412 are separated from each other.
L/(R-0.5T)=1...(3)

<Modification>
Although the present invention has been described above with reference to specific embodiments, it is not intended that the invention be limited by these descriptions. By referring to the description of the present invention, other embodiments of the present invention including various modifications in addition to the disclosed embodiments will be apparent to those skilled in the art. Therefore, it should be understood that the embodiments of the invention described in the claims also include embodiments including these modifications described in this specification, either alone or in combination.

For example, in the above embodiment, as shown in Figures 2, 3, and 6, the agitator blade 41 has no inclination angle, but the present invention is not limited to such an example. An inclined agitator blade 41 is an agitator blade 41 in which the end face of the tip of the protruding direction of the agitator blade 41 is inclined with respect to the vertical direction when viewed from the protruding direction of the agitator blade 41 and a direction perpendicular to the vertical direction. For example, as shown in Figures 7 and 8, the agitator blade 41 may have an inclination angle such that the length protruding from the rotating shaft 40 becomes shorter and the diameter d becomes smaller toward the lower end side of the agitator blade 4.

In the above embodiment, the width T of the first agitating blade 411 and the width T of the second agitating blade 412 are the same, but the present invention is not limited to such an example. The width T of the first agitating blade 411 and the width T of the second agitating blade 412 may be different values. For example, the widths T may be set to different values due to the difference in the wear rate between the first agitating blade 411 and the second agitating blade 412.
Furthermore, in the above embodiment, the desulfurizing agent is added to the molten iron 3 stirred by the stirring blade 4, but the present invention is not limited to this example. The desulfurizing agent may be added to the molten iron 3 at least either during or before the molten iron 3 is stirred by the stirring blade 4.

The following describes an example carried out by the present inventors. In the example, lime was used as the desulfurization agent, and the molten pig iron 3 was desulfurized using the mechanical stirring desulfurization device shown in Figure 5. The chemical components of the molten pig iron 3 used before the desulfurization treatment were C: 3.5-5.0 mass%, Si: 0.1-0.3 mass%, S: 0.025-0.035 mass%, and P: 0.10-0.15 mass%, and the molten pig iron temperature was in the range of 1250-1350°C. The desulfurization treatment used a molten pig iron ladle 2 capable of storing 250-350 tons of molten pig iron 3 as the treatment vessel, and the amount of molten pig iron to be treated was approximately 300 tons.

In the desulfurization process, the desulfurization agent used was used at a unit rate of 5.0 to 6.0 kg/ton of molten iron, and the desulfurization stirring time was constant. In addition, metallic aluminum was added to the molten iron as a deoxidizing agent before the addition of the desulfurization agent. After the desulfurization process was completed, the desulfurization slag was removed from the molten iron ladle, and a molten iron sample was taken to analyze the sulfur concentration in the metal.

In both the example and the comparative example, the stirring blade 4 had a diameter d of 1.4 m, a blade height b of 0.8 m, and no inclination angle as shown in Figures 1, 2, 3, and 6. In the example, six types of stirring blades 4 with a movement ratio r of +0.25, +0.5, +1.0, -0.5, and -1.0 were used. The output (current value of the drive motor) relative to the number of times the stirring blade 4 was used was kept constant, and 300 desulfurization treatments were performed with each stirring blade 4. The immersion position of the stirring blade was the center position of the treatment vessel, and the immersion depth from the stationary molten iron surface when the stirring blade was immersed (in a non-rotating state) was kept constant. In addition, as a comparative example, two types of stirring blades with a movement ratio r of 0 and +0.25 were used to perform the desulfurization treatment in the same manner.

Table 1 shows the average sulfur concentration in the molten pig iron before and after 300 desulfurization treatments, as well as the desulfurization rate. The desulfurization rate was defined as {(S concentration before treatment) - (S concentration after treatment)}/(S concentration before treatment) x 100. When a four-blade stirring blade 4 with a movement amount ratio r of 0 was used (Comparative Example 1) and when a stirring blade with a movement amount ratio of 0.25 was used (Comparative Example 2), the desulfurization rate was less than 70%, but when the movement amount ratio was ±0.5 or more (Examples 1, 2, 3, and 4), the desulfurization rate was significantly improved to 85% or more, and it was confirmed that when the movement amount ratio was 1 (Example 4), the desulfurization rate was high at 95% or more.

REFERENCE SIGNS LIST 1 Carriage 2 Molten iron ladle 3 Molten iron 4, 4A Stirring blade 40, 40A Rotating shaft 41, 41A Stirring blade 411, 411A First stirring blade 412, 412A Second stirring blade 5 Top blowing lance 6 Feeding port 7, 9, 11, 13 Hopper 8, 10, 12, 14 Rotary feeder

Claims (5)

1. A stirring blade used in a molten pig iron desulfurization treatment, comprising: a stirring blade immersed in molten pig iron is rotated to stir the molten pig iron; and a desulfurization agent is added to the molten pig iron at least during or before stirring by the stirring blade, thereby reducing a sulfur concentration in the molten pig iron,
Two first agitating blades protruding from the rotation shaft of the agitating blade, the protruding directions being perpendicular to the rotation shaft and opposite to each other;
Two second agitating blades protruding from the two first agitating blades, respectively, and protruding directions perpendicular to the protruding direction of the first agitating blades and the direction parallel to the rotation shaft, and in opposite directions to each other;
Equipped with
The second agitating blade is disposed at a position where the widthwise center position of each of the second agitating blades is shifted from the center position of the rotation shaft by a distance L that satisfies formula (2).
0.5≦｜L/(R-0.5T)｜≦1...(2)
however,
L: Distance from the rotation axis to the center position of the second agitating blade (m)
R: Length of the first agitating blade (m)
T: Width of the second agitating blade (m) The agitator blade according to claim 1, wherein the widthwise center position of each of the second agitator blades is disposed at a position that is a distance L away from the center position of the rotating shaft so as to satisfy formula (3).
L/(R-0.5T)=1...(3) A method for desulfurizing molten pig iron, comprising: stirring the molten pig iron by rotating a stirring blade immersed in the molten pig iron; and adding a desulfurizing agent to the molten pig iron at least during or before stirring by the stirring blade, thereby reducing a sulfur concentration in the molten pig iron,
The method for desulfurizing molten iron comprises the following agitator blades: two first agitator blades protruding from a rotation shaft of the agitator blades, the protruding directions of which are perpendicular to the rotation shaft and in opposite directions to each other; and two second agitator blades protruding from the two first agitator blades, the protruding directions of which are perpendicular to the protruding direction of the first agitator blades and to a direction parallel to the rotation shaft, and are in opposite radial directions of the rotation shaft, wherein the second agitator blades are disposed at positions where the width center positions of the second agitator blades are moved a distance L from the center position of the rotation shaft such that equation (2) is satisfied.
0.5≦｜L/(R-0.5T)｜≦1...(2)
however,
L: Distance from the rotation axis to the center position of the second agitating blade (m)
R: Length of the first agitating blade (m)
T: Width of second agitating blade (m) The method for desulfurizing molten iron according to claim 3, wherein the rotation direction of the impeller is opposite to the direction in which the second impeller protrudes from the first impeller. 5. The method for desulfurizing molten iron according to claim 3 or 4, wherein the widthwise center positions of the second agitating blades are arranged at positions shifted from the center position of the rotating shaft by a distance L that satisfies formula (3).
L/(R-0.5T)=1...(3)

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