Desulfurization method for molten iron

Abstract

To provide a method for desulfurizing a molten iron, capable of improving reaction efficiency of an added desulfurizing agent.SOLUTION: A method for desulfurizing a hot metal that is held in a hot metal ladle and to which a desulfurizing agent is added, by stirring the hot metal with an impeller immersed in the hot metal includes: a treatment-starting step of starting the desulfurization by immersing the impeller in the hot metal up to a predetermined position and rotating the impeller at a predetermined rotational frequency; a desulfurizing agent-adding step of adding the desulfurizing agent to the hot metal during the desulfurization; an impeller-lowering step of lowering the impeller to a depth deeper than a previous immersion depth of the impeller into the hot metal during the desulfurization; and a treatment-stopping step of stopping the rotation of the impeller and pulling up the impeller above a surface of the hot metal bath, wherein: the desulfurizing agent-adding step is performed two times; and the impeller-lowering step is performed immediately after, simultaneously, or immediately before each of the desulfurizing agent-adding step.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a method for desulfurizing molten iron by stirring the molten iron, to which a desulfurizing agent has been added, with an impeller immersed in the molten iron. [Background technology]

[0002] Currently, the mainstream method for desulfurizing molten pig iron is to add a desulfurizing agent to the molten pig iron contained in a molten pig iron ladle and mechanically stir the molten pig iron using an impeller. In this method, technologies for improving the reaction efficiency of the desulfurizing agent with the molten pig iron have been investigated.

[0003] For example, Patent Document 1 discloses a method in which an impeller is rotated at a predetermined rotation speed from the start of desulfurization until a predetermined time has elapsed, the impeller rotation speed is then relatively lowered and maintained for a certain period of time, and then the impeller rotation speed is relatively increased from the reduced rotation speed and maintained for a certain period of time. According to this method, the impeller rotation speed is temporarily lowered during the desulfurization treatment period to allow the desulfurizing agent that has reacted with the sulfur in the molten pig iron to rise to the surface of the molten pig iron, and the impeller rotation speed is then increased again from that state. It is described that the rising desulfurizing agent collides with the impeller, causing cracks and the appearance of a new reaction interface, which causes a new desulfurization reaction to proceed.

[0004] Patent Document 2 discloses a method in which a desulfurizing agent is added to molten pig iron in multiple batches, and for at least one of the second and subsequent additions, stirring is temporarily stopped or the stirring force is weakened to float slag to the surface of the molten pig iron, and the desulfurizing agent is added onto the floated slag. According to this method, if a new desulfurizing agent is added in a stirring state in which the already added desulfurizing agent can be dispersed in the molten pig iron, the newly added desulfurizing agent will aggregate when it comes into contact with the molten pig iron surface, so the desulfurizing agent is added with the stirring force temporarily stopped or weakened. It is then described that the stirring force is returned to a state in which the desulfurizing agent can be dispersed, and the new desulfurizing agent in the floated slag is introduced into the molten pig iron together with the slag, thereby improving the desulfurization reaction rate. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2008-101262 A [Patent Document 2] JP 2011-42865 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, the methods described in Patent Document 1 and Patent Document 2 are both characterized by reducing the impeller rotation speed during the treatment. This has caused the following problems. That is, since molten pig iron has a high specific gravity, once a flow of molten pig iron is formed by stirring with the impeller, a large inertial force is generated, and the flow rate of the molten pig iron does not decrease immediately even if the impeller rotation speed is reduced. For this reason, it is necessary to wait for a while after reducing the impeller rotation speed before increasing the impeller rotation speed or adding a desulfurization agent, and if one tries to secure a stirring time after increasing the impeller rotation speed so that the added desulfurization agent will react sufficiently, the treatment time has to be extended. In addition, in order to avoid extending the treatment time, no significant desulfurization promotion effect was observed when the impeller rotation speed was increased or the desulfurization agent was added immediately after reducing the impeller rotation speed. Furthermore, a study was conducted to suppress the flow of molten pig iron by braking or reversing the impeller rotation, but this put a load on the motor and impeller, and an increase in equipment maintenance costs was expected.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to propose a method for desulfurization of molten iron that can improve the reaction efficiency of the added desulfurization agent. [Means for solving the problem]

[0008] The present invention provides a method for desulfurizing molten pig iron, which advantageously solves the above-mentioned problems, for desulfurizing molten pig iron held in a molten pig iron ladle and to which a desulfurizing agent has been added, by stirring the molten pig iron with an impeller immersed in the molten pig iron, the method including a process initiation step of immersing the impeller in the molten pig iron to a predetermined position and rotating the impeller at a predetermined rotation speed to start the desulfurization process, a desulfurization agent addition step of adding the desulfurization agent to the molten pig iron during the desulfurization process, an impeller lowering step of increasing the immersion depth of the impeller in the molten pig iron to a depth greater than the previous immersion depth during the desulfurization process, and a process termination step of stopping the rotation of the impeller and raising the impeller above the bath surface of the molten pig iron, the impeller lowering step being performed immediately after, simultaneously with, or immediately before the desulfurization agent addition step. It is something.

[0009] The method for desulfurizing molten iron according to the present invention comprises the steps of: (A) the amount of descent of the impeller in the impeller lowering step is in the range of 1 to 10% of the bath depth of the molten iron held in the molten iron ladle; (a) rotating the impeller while varying a rotation axis position of the impeller within a range of 0.5 to 5% of a diameter of a blade portion of the impeller during the impeller lowering step; (C) performing the desulfurization agent addition step two or more times; (D) The time interval from one of the desulfurization agent addition steps to the next desulfurization agent addition step and the time interval from the last desulfurization agent addition step to the treatment end step are set to a time in the range of 10 to 50% of the scheduled time from the completion of the treatment start step to the completion of the treatment end step; (E) further comprising: performing a deoxidizer addition step of adding a deoxidizer to the molten iron during the desulfurization treatment at least once; (F) the deoxidizing agent addition step is performed immediately before one or more or all of the desulfurizing agent addition steps are performed; This is thought to be a preferable solution. Effect of the Invention

[0010] According to the present invention, the impeller is lowered when the desulfurization agent is added or before or after the addition, so that an unsteady flow is generated in the molten pig iron, and the newly added desulfurization agent is easily dispersed in the molten pig iron without agglomeration. Furthermore, if the impeller is rotated while varying the axial position of the impeller while the impeller is lowered, an even more unsteady flow is generated in the molten pig iron, and the amount of desulfurization agent that is taken into the vortex part of the cavity formed directly below the impeller and does not substantially contribute to the reaction can be reduced. As a result, it is possible to improve the desulfurization reaction efficiency of the added desulfurization agent. [Brief description of the drawings]

[0011] [Figure 1] 1 is a schematic front view showing an outline of an agitation type desulfurization apparatus suitable for use in a molten iron desulfurization method according to the present invention. [Diagram 2] 2A is a top view of a portion viewed from the line AA in FIG. 1, and FIG. 2B is a top view of a portion viewed from the line BB in FIG. [Diagram 3] FIG. 2 is a schematic front view showing an outline of another agitation type desulfurization apparatus suitable for use in the molten iron desulfurization method according to the present invention. [Figure 4] FIG. 4 is a top view of a portion viewed from CC in FIG. [Diagram 5] 1 is a graph showing the change over time in impeller rotation speed and immersion depth in an example of a method for desulfurizing molten iron according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, the embodiments of the present invention will be described in detail. Note that the following embodiments are merely examples of devices and methods for embodying the technical ideas of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical ideas of the present invention can be modified in various ways within the technical scope described in the claims.

[0013] Fig. 1 is a schematic front view showing an outline of an agitation type desulfurization apparatus suitable for use in a molten pig iron desulfurization method according to one embodiment of the present invention. Fig. 2(a) is a top view of a portion viewed from the line AA in Fig. 1. Fig. 2(b) is a top view of a portion viewed from the line BB in Fig. 1. The agitation type desulfurization apparatus 1 is composed of a guide 2 arranged vertically across a molten pig iron ladle 40 filled with molten pig iron 41, a stirrer 3 that enters the molten pig iron ladle 40 and stirs the molten pig iron, and a lifting device 35 that lifts and lowers the stirrer 3 along the guide 2.

[0014] The lifting device 35 is composed of a motor Mw and a wire 36 fastened to the carriage 4 via a pulley. The agitator 3 is raised and lowered by winding or pulling out the wire 36 suspending the carriage 4 with the motor Mw. The lifting device 35 makes it possible to control the elevation of the impeller 6 during the desulfurization treatment. As shown in FIG. 2(a), the guide 2 is provided with a rail 18. In addition, the carriage 4 is provided with a roller 14 journaled on the carriage 4 so that it can slide along the rail 18 without any gaps. This allows the agitator 3 to be raised and lowered along the guide 2 with little rattling.

[0015] The agitator 3 has a carriage 4 housing an impeller drive device 30 consisting of a drive motor 31 and a reducer 32, a drive shaft 5 extending below the carriage 4, and an impeller 6 connected to the drive shaft 5 for agitating the molten metal. The impeller 6 is, for example, a blade-shaped impeller 6. The drive shaft 5 of the impeller 6 is arranged with its axis facing up and down and can be rotated by the drive motor 31. The rotation speed of the drive motor 31 is controlled in response to commands from a controller 7.

[0016] The desulfurization agent 9 is added from a desulfurization agent adding device 8 toward the molten iron 41. The desulfurization agent adding device 8 is capable of cutting out a predetermined amount of the desulfurization agent 9 from a hopper 60 in response to a command from a controller 7, and adding it to the molten iron 41 through a chute 50.

[0017] Schedule information regarding the rotation pattern of the impeller 6, the lifting and lowering pattern of the impeller 6, and the addition pattern of the desulfurizing agent 9 is input to the controller 7. The controller 7 can be configured with a computer, a relay sequencer, or the like.

[0018] In the agitation type desulfurization device 1 used in this embodiment, the mast 20 is suspended from the lower end of the carriage 4. The mast 20 is cylindrical and extends downward from the carriage 4 to cover the drive shaft 5. A vibration suppression mechanism 21 is further provided to suppress horizontal vibration of the mast 20. The vibration suppression mechanism 21 is supported by the guide 2 and includes a hydraulic cylinder 22 that expands and contracts toward the mast 20, and a drum-shaped clamp roll 24 that is supported via a pressing force transmission mechanism 23 provided at the end of the rod 22A of the hydraulic cylinder 22. The vibration suppression mechanism 21 is provided at two or more locations in the circumferential direction of the mast 20 to suppress vibration of the mast 20. The clamp roll 24 is constantly pressed against the mast 20 with a predetermined pressure by the hydraulic cylinder 22. This suppresses vibration of the mast 20 when the carriage 4 is raised and lowered or when the impeller 6 is rotating, and reduces the impact load acting on the carriage 4, the agitator 3, and the guide 2.

[0019] As shown in another type of agitator-type desulfurization device 1' in Figs. 3 and 4, a roller 10 with a presser may be interposed between the carriage 4 and the guide 2. For example, the roller 10 with a presser is made up of a hydraulic cylinder 11, the end of which is supported by the carriage 4, which presses the guide 2 toward the guide 2, and a roller 14, which is supported by a bracket 13 provided at the end of the rod 12. The roller 14 is always pressed against the rail 18 with a predetermined pressure by the hydraulic cylinder 11, so that no gap is generated between the roller 14 and the rail 18. It is preferable that the rail 18 is convex and the V-shaped groove 15 of the roller 14 slides on the rail 18. This allows the agitator 3 to rise and fall along the guide 2 with less rattling, and the impact load acting on the agitator 3 and the guide 2 can be reduced.

[0020] Next, the operation of the desulfurization method for molten pig iron according to this embodiment will be described. The impeller 6 is inserted from above into the molten pig iron ladle 40 containing the molten pig iron 41, so that the impeller 6 is immersed in the molten pig iron 41. The impeller 6 is then rotated at a predetermined rotation speed, so that the molten pig iron 41 in the molten pig iron ladle 40 is mechanically stirred, and the desulfurization agent 9 added to the molten pig iron 41 is dispersed in the molten pig iron 41, thereby performing desulfurization.

[0021] In the conventional lifting control of the impeller 6, the impeller 6 is lowered into the molten pig iron ladle 40 to immerse the impeller 6 in the molten pig iron 41, and then a desulfurization agent is added and the impeller 6 is rotated and stirred while maintaining a constant height to perform desulfurization. Then, when the desulfurization process is completed, the impeller 6 is raised and removed from the molten pig iron ladle 40.

[0022] In contrast, the present embodiment includes first, second, ..., n-th desulfurization agent adding steps for adding the desulfurization agent 9 to the molten pig iron 41 at least once during the desulfurization process, and first, second, ..., n-th impeller lowering steps for increasing the immersion depth of the impeller 6 in the molten pig iron 41 during the desulfurization process from the previous immersion depth. Here, n is an integer of 1 or more. Immediately after, simultaneously with, or immediately before each desulfurization agent adding step, the impeller lowering step is executed. That is, the desulfurization agent 9 is added once or more during the desulfurization process, and the immersion depth of the impeller 6 is increased immediately before, simultaneously with, or immediately after the addition of the desulfurization agent 9 for each addition of the desulfurization agent 9. More specifically, the controller 7 outputs a command to the impeller lifting device 35 to lower the impeller 6 in synchronization with the addition timing of the desulfurization agent 9, that is, to change the immersion depth of the impeller 6 in the molten pig iron 41.

[0023] In this embodiment, it is more preferable to execute each impeller lowering step immediately after each desulfurization agent addition step. This is because the effect of rotating the impeller 6 while vibrating it in the horizontal direction while the impeller 6 is lowering can be obtained. The method of rotating the impeller 6 while vibrating it in the horizontal direction while the impeller 6 is lowering and the effect of the method will be described later.

[0024] In this embodiment, it is preferable to perform the desulfurization agent addition step two or more times. By adding the desulfurization agent 9 in separate steps, the added desulfurization agent 9 is more likely to be taken into the molten pig iron 41 in a dispersed state, the contact area between the molten pig iron 41 and the desulfurization agent 9 is increased, and the desulfurization reaction is more likely to proceed.

[0025] FIG. 5 shows an example of a schedule of the impeller lifting and rotation patterns and the desulfurization agent 9 addition pattern in the case where the desulfurization agent addition step is performed two or more times in this embodiment. FIG. 5(a) is a diagram showing the change in impeller rotation speed (vertical axis) with the passage of time (horizontal axis t) during the desulfurization process, and FIG. 5(b) is a diagram showing the change in the immersion depth of the impeller 6 (represented as the depth from the stationary bath surface: vertical axis) with the passage of time (horizontal axis t) during the desulfurization process. The arrow in FIG. 5(b) indicates the timing of introducing the desulfurization agent. The immersion depth position of the impeller 6 is, for example, the position of the center of gravity of the impeller 6. D0 shown on the vertical axis is the position of the surface 41a (molten iron surface) of the molten iron 41 in a non-stirred state (standing still).

[0026] 5, the rotation speed of the impeller 6 is increased to the rotation speed N1 for the desired desulfurization process upon the start of desulfurization process ts (processing start step), and the rotation speed is controlled to the target rotation speed N1. Here, the desulfurization process rotation speed N1 is a rotation speed at which the desulfurization agent 9, which has a light specific gravity, can be dispersed in the molten pig iron 41. The rotation speed N1 may be set appropriately so as to ensure a desired stirring state depending on the equipment specifications, equipment load, or usage conditions of the impeller 6.

[0027] The first addition AS1 of the desulfurization agent 9 is performed after the rotation speed of the impeller 6 reaches the rotation speed N1 for the desired desulfurization treatment, and a predetermined amount (for example, about 2 / 3 of the total) of the desulfurization agent 9 is added. At this time, the addition AS1 of the desulfurization agent 9 is performed in synchronization with the change in the immersion depth of the impeller 6. For example, the immersion depth of the impeller 6 is deepened immediately after the addition AS1 of the desulfurization agent 9.

[0028] When the molten pig iron 41 is stirred continuously for a certain period of time by the impeller 6 under the same conditions, the flow of the molten pig iron 41 approaches a stationary state of rigid body rotation. In other words, the flow of the molten pig iron 41 approaches a flow in which the entire molten pig iron 41 rotates as a unit (like the rotation of the earth). When the desulfurization agent 9 is added to the molten pig iron 41 in such a flow state, the desulfurization agent 9 is not taken into the molten pig iron 41, but simply rotates together with the molten pig iron 41 on the bath surface of the molten pig iron 41, and the desulfurization reaction does not proceed easily. On the other hand, immediately after the immersion depth is deepened while rotating the impeller 6, the flow of the molten pig iron 41 becomes unsteady. By putting the molten pig iron 41 into such a flow state immediately after adding the desulfurization agent 9, the desulfurization agent 9 is easily taken into the molten pig iron 41, the contact area between the molten pig iron 41 and the desulfurization agent 9 increases, and the desulfurization reaction proceeds easily.

[0029] After the first addition AS1 of the desulfurizing agent 9, the stirring process is continued for a predetermined time with the impeller 6 immersed to a deeper depth as described above. Then, after the predetermined time has elapsed, a predetermined amount (e.g., about 1 / 3 of the total) of desulfurizing agent 9 is additionally added as the second addition AS2 of the desulfurizing agent 9. In this embodiment, immediately after the second addition AS2 of the desulfurizing agent 9, the impeller 6 is moved further downward to make the immersion deeper.

[0030] The second addition AS2 of the desulfurizing agent 9 may be performed simultaneously with the operation of moving the impeller 6 further downward to make the immersion depth deeper, or immediately after the operation of moving the impeller 6 further downward to make the immersion depth deeper.

[0031] By further increasing the immersion depth of the impeller 6, the flow of the molten iron 41 becomes unsteady again, so that the additionally added desulfurization agent 9 is more easily taken into the molten iron 41, the contact area between the molten iron 41 and the desulfurization agent 9 increases, and the desulfurization reaction proceeds more easily.

[0032] As in the case of the first addition AS1 of the desulfurizing agent 9, the change in the immersion depth is preferably 1 to 10% of the bath depth of the molten pig iron held in the molten pig iron ladle. If the change in the immersion depth is less than 1% of the bath depth of the molten pig iron held in the molten pig iron ladle, the flow state of the molten pig iron 41 hardly changes, and the effect of making the desulfurizing agent 9 more easily taken into the molten pig iron 41 is reduced. On the other hand, if the change in the immersion depth is more than 10% of the bath depth of the molten pig iron, the added desulfurizing agent 9 is easily sucked into the vortex depressions formed on the surface of the molten pig iron 41 and aggregates, and the dispersion effect of the desulfurizing agent is reduced.

[0033] After the second addition AS2 of the desulfurization agent 9, the stirring process is continued for a predetermined time at the position where the immersion depth of the impeller 6 is increased as described above. Then, after the predetermined time has elapsed, the rotation of the impeller 6 is stopped and the impeller 6 is raised, and the process is terminated (process termination step te). Note that, after the second addition AS2 of the desulfurization agent 9, the stirring process is continued for a predetermined time at the position where the immersion depth of the impeller 6 is increased, and then the impeller 6 may be moved upward and the stirring process may be continued for a predetermined time at the shallower immersion depth, and then the process may be terminated. The reason for terminating the stirring process after the shallower immersion depth of the impeller 6 is to make the flow of the molten pig iron 41, which has approached a steady state under the condition where the immersion depth of the impeller 6 is increased, unsteady again. At this time, it is possible to make the flow of the molten pig iron 41 unsteady by further deepening the immersion depth of the impeller 6, but if the immersion depth of the impeller 6 is made too deep and the blades of the impeller 6 are completely buried in the molten pig iron, no depressions will be formed on the surface of the molten pig iron 41, and there is a risk of a decrease in the contact area between the molten pig iron 41 and the desulfurization agent 9. Therefore, after making the immersion depth of the impeller 6 deeper to a certain extent, it is advisable to make the flow of the molten pig iron bath unsteady by making the immersion depth of the impeller 6 shallower.

[0034] In addition, it is preferable that the time interval between the first addition AS1 of the desulfurizing agent 9 and the second addition AS2 of the desulfurizing agent is 10 to 50% of the scheduled time from the completion of the treatment start step to the completion of the treatment end step. If the time interval between the first addition AS1 of the desulfurizing agent 9 and the second addition AS2 of the desulfurizing agent is shorter than 10% of the scheduled time from the completion of the treatment start step to the completion of the treatment end step, the time interval is too short and is substantially equivalent to the case where the desulfurizing agent is added only once. On the other hand, if the time interval between the first addition AS1 of the desulfurizing agent 9 and the second addition AS2 of the desulfurizing agent is longer than 50% of the scheduled time from the completion of the treatment start step to the completion of the treatment end step, the timing of the addition of the second or subsequent desulfurizing agent will be delayed, and the reaction time of the desulfurizing agent added second or subsequent times with the molten pig iron may be insufficient. Therefore, it is preferable that the time interval from one desulfurization agent addition step to the immediately following desulfurization agent addition step and the time interval from the last desulfurization agent addition step to the treatment termination step be in the range of 10 to 50% of the scheduled time from the completion of the treatment start step to the completion of the treatment termination step.

[0035] When the rotational flow rate of the molten pig iron increases due to impeller stirring, a conical depression is formed on the molten pig iron surface 41a, and when the apex of this cone is located below the lower end face of the impeller in the molten pig iron, a vacuum vortex (hollow vortex portion) is formed inside the molten pig iron surrounded by the lower end face of the impeller. A part of the added flux is sucked into this hollow vortex portion and adheres to and solidifies on the lower end face of the impeller, so it misses the opportunity to react with the molten pig iron, which is one of the factors that hinders the improvement of the desulfurization reaction efficiency of the added desulfurization agent.

[0036] In this embodiment, during the period in which the impeller 6 is lowered to increase the immersion depth of the impeller 6 immediately after the addition of the desulfurizing agent 9, it is preferable to rotate the impeller while varying the axial position of the impeller in the radial direction of the impeller within a range of 0.5 to 5% of the diameter of the impeller blade. That is, it is preferable to rotate the impeller while vibrating it in the horizontal direction during the lowering of the impeller. By rotating the impeller while vibrating it in the horizontal direction during the lowering of the impeller in this way, a more unsteady flow is generated in the molten pig iron, and the amount of desulfurizing agent that is taken into the vortex cavity formed immediately below the impeller and does not substantially contribute to the reaction can be reduced. That is, when the impeller vibrates in the horizontal direction, the upper end of the vortex cavity temporarily comes off the lower end surface of the impeller, and the formation of the vortex cavity is eliminated. This reduces the probability that the desulfurization agent 9 will be sucked into the vortex cavity immediately after it is added, and if any desulfurization agent 9 has already been sucked into the vortex cavity, it will also detach from the vortex cavity, increasing the contact area between the molten iron 41 and the desulfurization agent 9 and making it easier for the desulfurization reaction to proceed.

[0037] The method of rotating the impeller while vibrating it in the horizontal direction during its descent is not particularly limited. For example, when the impeller 6 is lowered, the clamp roll 24 is loosened to provide an appropriate clearance between the mast 20 and the clamp roll 24, so that the carriage 4 can vibrate in a direction perpendicular to the direction of the carriage 4 as it ascends and descends (a direction parallel to the ground, i.e., a horizontal direction) when the carriage 4 ascends and descends. That is, when the carriage 4 starts to ascend and descend, the rod 22A of the hydraulic cylinder 22 of the vibration suppression mechanism 21 is retreated to provide a gap between the clamp roll 24 provided at the end of the rod 22A and the mast 20, and the carriage 4 is lowered in this state. Then, when the carriage 4 has completed its ascending and descending, the rod of the hydraulic cylinder is advanced to eliminate (return to the original position) the gap between the clamp roll 24 provided at the end of the rod and the mast 20. As a result, while the carriage 4 is descending, horizontal vibrations are transmitted to the carriage 4 via the mast 20, causing the impeller 6 to vibrate appropriately in the horizontal direction.

[0038] In addition, when the pressing roller 10 is interposed between the carriage 4 and the guide 2, the pressing roller 10 may be loosened when the impeller 6 is lowered to provide an appropriate clearance between the carriage 4 and the guide 2, so that the carriage 4 can vibrate in a direction perpendicular to the ascending and descending direction (a direction parallel to the ground, i.e., a horizontal direction) when the carriage 4 ascends and descends. As a result, the impeller 6 vibrates appropriately in the horizontal direction via the carriage 4 while the carriage 4 is descending.

[0039] The range of variation of the impeller's rotation axis position is preferably within 0.5 to 5% of the diameter of the impeller's blades. If it is less than 0.5% of the diameter of the impeller's blades, the above effect cannot be obtained. If it is more than 5% of the diameter of the impeller's blades, the impeller will have a large wobble during descent, making it difficult for the impeller to descend smoothly.

[0040] Furthermore, in this embodiment, it is preferable to perform at least one deoxidizing agent addition step of adding a deoxidizing agent to the molten iron during the desulfurization process. Examples of deoxidizing agents include metallic aluminum (new lump, recycled), aluminum slag, metallic silicon, and ferrosilicon. The addition of the deoxidizing agent has the effect of lowering the oxygen potential near the surface of the molten iron, which is in a high state due to exposure to the atmosphere by stirring, and making the desulfurization reaction more likely to occur thermodynamically. The effect of this deoxidizing agent addition step is further enhanced if it is performed immediately before one or more or all of the desulfurizing agent addition steps are performed. More specifically, it is preferable to add the deoxidizing agent 0 to 1 minute before the addition of the desulfurizing agent. If the desulfurizing agent is added multiple times, it may be added only 0 to 1 minute before the first desulfurizing agent addition, or 0 to 1 minute before all of the desulfurizing agent additions. Adding a deoxidizer immediately before the addition of a desulfurization agent not only reduces the oxygen potential near the molten iron surface as described above, making the desulfurization reaction more likely to occur thermodynamically, but also improves the wettability of the molten iron and slag by causing the deoxidization products (e.g., Al2O3) produced by the addition of the deoxidizer immediately before the deoxidization agent to react with the desulfurization agent (e.g., CaO) added subsequently to form low-melting-point oxides, making it easier for the slag to be incorporated into the molten iron. EXAMPLES

[0041] The molten pig iron was desulfurized under the following conditions using the molten pig iron desulfurization method according to the above embodiment. As a comparative example, the condition in which the impeller was immersed to a constant depth during the desulfurization is shown. The amount of molten pig iron contained in the molten pig iron ladle was 300 t, and the bath depth of the molten pig iron ladle was 4.5 m. The chemical composition of the molten pig iron before the desulfurization was C: 4.3 mass% and S: 0.023 mass%. The target chemical composition after the desulfurization was C: 4.3 mass% and S: 0.001 mass%. The type of desulfurization agent used was lime. The impeller rotation speed N1 was kept constant at 130 rpm during the treatment. Al slag was used as the deoxidizer, and the total treatment time was about 10 minutes. The immersion depth is expressed as the depth from the stationary bath surface.

[0042] (Example of the invention) The desulfurization process is carried out in the following order: · Processing start step ts: The impeller is lowered, and the impeller rotation speed is increased to N1. First mixing step: Add deoxidizer and process for 1 minute at impeller speed N1 and immersion depth of 1.8 m. First desulfurization agent addition step AS1: Add 2 / 3 of the planned amount of desulfurization agent. First impeller descent step: The impeller oscillates and descends to a immersion depth of 2.1m. · Second stirring step: After the descent is complete, stop the vibration and maintain stirring for 2 minutes. Second desulfurization agent addition step AS2: Add 1 / 3 of the planned amount of desulfurization agent. - Second impeller descent step: The impeller oscillates and descends to a immersion depth of 2.3m. · Third stirring step: After the descent is completed, the vibration is stopped and stirring is maintained for 2 minutes. Impeller rising step: Raises to a immersion depth of 2.1m without vibration. · Fourth stirring step: Stir for 1 minute after the rise is complete. · Processing end step te: The rotation of the impeller is stopped and the impeller is lifted out of the molten iron ladle.

[0043] (Comparative Example) The desulfurization process is carried out in the following order: · Treatment start step ts: A deoxidizing agent is added, the impeller is lowered, and the impeller rotation speed is increased to N1. First mixing step: treatment for 3 minutes at impeller speed N1 and immersion depth of 2.1 m. First desulfurization agent addition step AS1: Add 2 / 3 of the planned amount of desulfurization agent. Second stirring step: maintain immersion depth at 2.1m and keep stirring for 3 minutes. Second desulfurization agent addition step AS2: Add 1 / 3 of the planned amount of desulfurization agent. Third stirring step: Keep the immersion depth at 2.1m and keep stirring for 5 minutes. · Processing end step te: The rotation of the impeller is stopped and the impeller is lifted out of the molten iron ladle.

[0044] Desulfurization was performed 1,300 times for both the invention example and the comparative example. The average desulfurization unit was 2.65 kg / t-molten pig iron for the invention example, while 4.16 kg / t-molten pig iron was required for the comparative example. As can be seen from this result, the desulfurization efficiency of the invention example is improved compared to the comparative example. [Industrial Applicability]

[0045] INDUSTRIAL APPLICABILITY The method for desulfurization of molten pig iron of the present invention contributes to a reduction in the desulfurization unit consumption and improves productivity, and is therefore industrially useful. [Explanation of symbols]

[0046] 1, 1' Agitation type desulfurization equipment 2 Guide 3 Mixer 4 Carriage 5 Drive shaft 6 Impeller 7 Controller 8. Desulfurization agent addition device 9. Desulfurization Agent 10 Roller with pusher 11 Hydraulic cylinder 12 Rod 13 Bracket 14. Lola 15 V-shaped groove 18 Rail 20 Mast 21 Vibration suppression mechanism 22 Hydraulic cylinder 22A Rod 23 Pressure transmission mechanism 24 Clamp roll 30 Impeller drive unit 31 Drive motor 32 Reducer 35 Lifting device 36 Wire 40 Molten metal ladle 41 Molten Iron 50 Shots 60 Hopper MW motor

Claims

1. A method for desulfurizing molten iron, which is held in a molten iron ladle and to which a desulfurizing agent has been added, by stirring the molten iron with an impeller immersed in the molten iron, comprising the steps of: a process initiation step of immersing the impeller in the molten iron to a predetermined position, and then starting to rotate the impeller at a predetermined rotation speed to initiate the desulfurization process; a desulfurization agent addition step of adding a desulfurization agent to the molten iron during the desulfurization treatment; an impeller lowering step of lowering the impeller into the molten iron during the desulfurization treatment to a depth greater than the previous immersion depth; a process terminating step of starting to raise the impeller after decelerating the impeller, and raising the impeller above the bath surface of the molten iron after stopping the rotation of the impeller, The desulfurization process is performed by keeping the rotation speed of the impeller constant, The impeller lowering step is performed immediately after, simultaneously with, or immediately before the desulfurization agent addition step. Here, "immediately after and immediately before the desulfurization agent addition step" refers to the period during which the added desulfurization agent is taken into the molten iron due to the unsteady flow of the molten iron caused by the descent of the impeller. A method for desulfurizing molten iron.

2. The amount of descent of the impeller in the impeller lowering step is in the range of 1 to 10% of the bath depth of the molten iron held in the molten iron ladle. The method for desulfurizing molten iron according to claim 1.

3. During the impeller lowering step, the impeller is rotated while varying the position of the axis of rotation of the impeller within a range of 0.5 to 5% of the diameter of the blade portion of the impeller. The method for desulfurizing molten iron according to claim 1 or 2.

4. The desulfurization agent addition step is carried out two or more times. A method for desulfurizing molten iron according to any one of claims 1 to 3.

5. The time interval from one of the desulfurization agent addition steps to the immediately following desulfurization agent addition step and the time interval from the last desulfurization agent addition step to the treatment end step are set to a time in the range of 10 to 50% of the scheduled time from the completion of the treatment start step to the completion of the treatment end step. The method for desulfurizing molten iron according to claim 4.

6. Furthermore, a deoxidizer addition step of adding a deoxidizer to the molten iron during the desulfurization treatment is performed at least once. A method for desulfurizing molten iron according to any one of claims 1 to 5.

7. The deoxidizing agent addition step is performed immediately before any one or more or all of the desulfurizing agent addition steps are performed. The method for desulfurizing molten iron according to claim 6.

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