JP7626032B2 - Desulfurization method for molten iron - Google Patents

Description

The present invention relates to a method for desulfurizing molten iron by stirring the molten iron to which a desulfurizing agent has been added using an impeller immersed in the molten iron.

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 have been developed to improve the reaction efficiency of the desulfurizing agent with the molten pig iron.

For example, Patent Document 1 discloses a method in which the 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 lowered 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 iron to rise to the surface of the molten iron, and the impeller rotation speed is then increased again from that state. As a result, 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.

Patent Document 2 discloses a method in which a desulfurizing agent is added to molten pig iron in multiple additions, 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 then added onto the floated slag. According to this method, if 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.

JP 2008-101262 A JP 2011-42865 A

However, the methods described in Patent Document 1 and Patent Document 2 are characterized by reducing the impeller rotation speed during the process. This causes the following problems. That is, since molten iron has a high specific gravity, once a flow of molten iron is formed by stirring with the impeller, a large inertial force is generated, and the flow rate of the molten iron does not decrease immediately even if the impeller rotation speed is reduced. For this reason, it is necessary to wait 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 can react sufficiently, the processing time has to be extended. In addition, in order to avoid extending the processing 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, an investigation was conducted into suppressing the flow of molten 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.

The present invention was made in consideration of these circumstances, and its purpose is to propose a method for desulfurizing molten iron that can improve the reaction efficiency of the added desulfurizing agent.

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.

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;
(i) rotating the impeller while varying an axial 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 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;
(E) further 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 any one or more or all of the desulfurizing agent addition steps are performed;
This is thought to be a preferable solution.

According to the present invention, the impeller is lowered when the desulfurization agent is added or before or after, which creates an unsteady flow in the molten iron and makes it easier for the newly added desulfurization agent to disperse in the molten iron without agglomerating. In addition, if the impeller is rotated while varying the axial position of the impeller while it is lowering, an even more unsteady flow is created in the molten iron and the amount of desulfurization agent that is captured in the vortex 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.

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. 2A is a top view of the AA portion of FIG. 1, and FIG. 2B is a top view of the BB portion of FIG. 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. FIG. 4 is a top view of the CC portion of FIG. 3. 1 is a graph showing the change in impeller rotation speed and immersion depth over time in an example of a method for desulfurizing molten iron according to the present invention.

The following is a detailed description of the embodiments of the present invention. Note that the following embodiments are intended to exemplify devices and methods for embodying the technical ideas of the present invention, and are not intended to specify the configurations 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.

Figure 1 is a schematic front view showing an outline of an agitation type desulfurization device suitable for use in a molten pig iron desulfurization treatment method according to one embodiment of the present invention. Figure 2(a) is a top view of the A-A portion of Figure 1. Figure 2(b) is a top view of the B-B portion of Figure 1. The agitation type desulfurization device 1 is composed of a guide 2 that is erected across a molten pig iron ladle 40 filled with molten pig iron 41, an agitator 3 that looks into the molten pig iron ladle 40 and agitates the molten pig iron, and a lifting device 35 that raises and lowers the agitator 3 along the guide 2.

The lifting device 35 is composed of a wire 36 fastened to the motor Mw and the carriage 4 via a pulley. The agitator 3 is raised and lowered by winding or pulling out the wire 36 that suspends 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 process. 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 that is 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.

The agitator 3 has a carriage 4 that houses an impeller drive device 30 consisting of a drive motor 31 and a reducer 32, a drive shaft 5 that extends below the carriage 4, and an impeller 6 that is connected to the drive shaft 5 and stirs 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 according to commands from the controller 7.

The desulfurization agent 9 is added to the molten iron 41 by the desulfurization agent adding device 8. The desulfurization agent adding device 8 is capable of cutting a predetermined amount of the desulfurization agent 9 from the hopper 60 in response to a command from the controller 7 and adding it to the molten iron 41 via the chute 50.

Schedule information for the rotation pattern of the impeller 6, the lifting and lowering pattern of the impeller 6, and the addition pattern of the desulfurization agent 9 is input to the controller 7. The controller 7 can be configured as a computer or a relay sequencer, etc.

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 consists of 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 always 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.

As shown in another type of agitator type desulfurization device 1' in Figures 3 and 4, a roller with a pusher 10 may be interposed between the carriage 4 and the guide 2. For example, the roller with a pusher 10 is made up of a hydraulic cylinder 11 whose end is supported by the carriage 4 and 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 created 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 in place. This allows the agitator 3 to rise and fall along the guide 2 with less rattling, reducing the impact load acting on the agitator 3 and the guide 2.

Next, the operation of the process in the molten pig iron desulfurization process method 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 throughout the molten pig iron 41, thereby performing desulfurization.

Conventional lifting and lowering control of the impeller 6 involves lowering the impeller 6 into the molten iron ladle 40 to immerse the impeller 6 in the molten iron 41, then adding a desulfurizing agent and rotating the impeller 6 while maintaining a constant height to stir the mixture for desulfurization. Once the desulfurization process is complete, the impeller 6 is raised and removed from the molten iron ladle 40.

In contrast, the present embodiment includes a first, second, ... n-th desulfurization agent addition step in which the desulfurization agent 9 is added to the molten pig iron 41 at least once during the desulfurization process, and a first, second, ... n-th impeller lowering step in which the immersion depth of the impeller 6 in the molten pig iron 41 is made deeper than the previous immersion depth during the desulfurization process. Here, n is an integer of 1 or more. Immediately after, simultaneously with, or immediately before each desulfurization agent addition step, the impeller lowering step is executed. That is, the desulfurization agent 9 is added one or more times during the desulfurization process, and the immersion depth of the impeller 6 is deepened 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.

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 can be obtained when the impeller 6 is rotated while vibrating in the horizontal direction while it is descending. The method of rotating the impeller 6 while vibrating in the horizontal direction while it is descending and the effect thereof will be described later.

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

In this embodiment, when the desulfurization agent addition step is performed two or more times, an example of the schedule of the impeller lifting and rotation pattern and the desulfurization agent 9 addition pattern is shown in FIG. 5. FIG. 5(a) is a diagram showing the change in impeller rotation speed (vertical axis) over 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 (indicated as the depth from the stationary bath surface: vertical axis) over time (horizontal axis t) during the desulfurization process. The arrow in FIG. 5(b) indicates the timing of adding the desulfurization agent. The immersion depth position of the impeller 6 is, for example, the center of gravity position 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).

In the schedule shown in FIG. 5, the rotation speed of the impeller 6 is increased to the desired rotation speed N1 for desulfurization by 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 that can disperse the light-gravity desulfurization agent 9 in the molten iron 41. Note that the rotation speed N1 can be set appropriately to ensure the desired stirring state depending on the equipment specifications, equipment load, or usage conditions of the impeller 6.

The first addition AS1 of desulfurization agent 9 is carried out after the rotational speed of impeller 6 reaches the rotational speed N1 for the desired desulfurization process, and a predetermined amount (e.g., about 2/3 of the total) of desulfurization agent 9 is added. At this time, the addition AS1 of desulfurization agent 9 is carried out in synchronization with a change in the immersion depth of impeller 6. For example, the immersion depth of impeller 6 is deepened immediately after the addition AS1 of desulfurization agent 9.

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

After the first addition of desulfurizing agent 9 AS1, 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 added as the second addition of desulfurizing agent 9 AS2. In this embodiment, immediately after the second addition of desulfurizing agent 9 AS2, the impeller 6 is moved further downward to make the immersion deeper.

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.

By further increasing the immersion depth of the impeller 6, the flow of the molten pig iron 41 becomes unsteady again, so that the additionally added desulfurization agent 9 is more easily absorbed 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 more easily.

As with the first addition of desulfurizing agent 9 AS1, the change in 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 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 it easier for the desulfurizing agent 9 to be absorbed into the molten pig iron 41 is reduced. On the other hand, if the change in 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, reducing the dispersion effect of the desulfurizing agent.

After the second addition of the desulfurization agent 9 AS2, the stirring process is continued for a predetermined time at the position where the immersion depth of the impeller 6 is deepened 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 (processing termination step te). Note that after the second addition of the desulfurization agent 9 AS2, the stirring process is continued for a predetermined time at the position where the immersion depth of the impeller 6 is deepened, 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 had approached a steady state under the condition where the immersion depth of the impeller 6 was deepened, unsteady again. At this time, it is possible to make the flow of the molten pig iron 41 unsteady by further increasing 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 the contact area between the molten pig iron 41 and the desulfurization agent 9 may decrease. Therefore, after the immersion depth of the impeller 6 is increased to a certain degree, it is advisable to make the flow of the molten pig iron bath unsteady by decreasing the immersion depth of the impeller 6.

In addition, it is preferable to set the time interval between the first addition of desulfurization agent 9 AS1 and the second addition of desulfurization agent AS2 to 10 to 50% of the planned 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 of desulfurization agent 9 AS1 and the second addition of desulfurization agent AS2 is shorter than 10% of the planned 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 essentially equivalent to the case where the desulfurization agent is added only once. On the other hand, if the time interval between the first addition of desulfurization agent 9 AS1 and the second addition of desulfurization agent AS2 is longer than 50% of the planned time from the completion of the treatment start step to the completion of the treatment end step, the timing of the second and subsequent additions of desulfurization agent will be delayed, and the reaction time of the desulfurization agent added second and subsequent times with the molten 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 process end step be in the range of 10 to 50% of the planned time from the completion of the process start step to the completion of the process end step.

When the rotational flow rate of the molten pig iron increases due to impeller stirring, a conical depression forms on the molten pig iron surface 41a. When the apex of this cone is located in the molten pig iron below the lower end face of the impeller, a vacuum vortex (hollow vortex portion) is formed in the interior surrounded by the lower end face of the impeller and the molten pig iron. Some of the added flux is sucked into this hollow vortex portion and adheres to and solidifies on the lower end face of the impeller, missing the opportunity to react with the molten pig iron and becoming one of the factors that hinders the improvement of the desulfurization reaction efficiency of the added desulfurization agent.

In this embodiment, during the period in which the impeller 6 is lowered to deepen the immersion depth immediately after the addition of the desulfurizing agent 9, it is preferable to rotate the impeller while varying the axis of rotation 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 horizontally while the impeller is lowering. In this way, by rotating the impeller while vibrating it horizontally while the impeller is lowering, a further unsteady flow is generated in the molten iron, and the amount of desulfurizing agent that is taken into the vortex cavity formed directly below the impeller and does not substantially contribute to the reaction can be reduced. That is, when the impeller vibrates horizontally, 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 desulfurizing agent 9 will be sucked into the vortex of the cavity immediately after it is added, and if any desulfurizing agent 9 has already been sucked into the vortex of the cavity, it will also leave the vortex, increasing the contact area between the molten iron 41 and the desulfurizing agent 9 and making it easier for the desulfurization reaction to proceed.

During the descent of the impeller, the method of rotating the impeller while vibrating it in the horizontal direction 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 begins 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 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 ascending and descending, the rod of the hydraulic cylinder is advanced to eliminate the gap between the clamp roll 24 at the end of the rod and the mast 20 (return to the original state). 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 moderately in the horizontal direction.

When a roller 10 with a pusher is interposed between the carriage 4 and the guide 2, the roller 10 with a pusher may be loosened when the impeller 6 is lowered, so that an appropriate clearance is provided between the carriage 4 and the guide 2, and the carriage 4 can vibrate in a direction perpendicular to the direction of the carriage rising and lowering (a direction parallel to the ground, i.e., the horizontal direction) when the carriage 4 rises and falls. This allows the impeller 6 to vibrate appropriately in the horizontal direction via the carriage 4 while the carriage 4 is descending.

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 wobble more as it descends, making it difficult for the impeller to descend smoothly.

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. In the case where the desulfurizing agent is added multiple times, it may be performed only 0 to 1 minute before the first desulfurizing agent addition, or may be performed 0 to 1 minute before all of the desulfurizing agent additions. Adding a deoxidizer immediately before the addition of a deoxidizer not only reduces the oxygen potential near the surface of the molten iron 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 product (e.g., Al ₂ O ₃ ) produced by the immediately preceding addition of the deoxidizer to react with the subsequently added desulfurization agent (e.g., CaO) to form a low-melting-point oxide, making it easier for the slag to be incorporated into the molten iron.

Using the molten iron desulfurization method according to the above embodiment, molten iron was desulfurized under the following conditions. As a comparative example, the condition in which the impeller was immersed to a constant depth during desulfurization is shown. The amount of molten iron contained in the molten iron ladle was 300 t, and the bath depth of the molten iron ladle was 4.5 m. The chemical composition of the molten iron before desulfurization was C: 4.3 mass% and S: 0.023 mass%. The target chemical composition after desulfurization was C: 4.3 mass% and S: 0.001 mass%. Lime was used as the desulfurization agent. 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.

(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 stirring step: Add a deoxidizing agent and treat for 1 minute at an impeller speed of N1 and an immersion depth of 1.8 m.
First desulfurization agent addition step AS1: Add 2/3 of the planned amount of desulfurization agent.
First impeller lowering step: The impeller is lowered while vibrating to an immersion depth of 2.1 m.
Second stirring step: After the descent is completed, the vibration is stopped and stirring is maintained for 2 minutes.
Second desulfurization agent addition step AS2: Add 1/3 of the planned amount of desulfurization agent.
- Second impeller lowering step: The impeller is lowered while vibrating to an immersion depth of 2.3 m.
Third stirring step: After the descent is completed, the vibration is stopped and stirring is maintained for 2 minutes.
Impeller rising step: The impeller is raised to a immersion depth of 2.1 m 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.

Comparative Example
The desulfurization process is carried out in the following order:
Treatment start step ts: A deoxidizer is added, the impeller is lowered, and the impeller rotation speed is increased to N1.
First stirring step: The mixture is treated for 3 minutes at an impeller speed of N1 and an 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: Keep the immersion depth at 2.1 m 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: The immersion depth is kept at 2.1 m and stirring is maintained 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.

Desulfurization treatment was carried out 1,300 times for both the invention example and the comparative example. The average desulfurization unit was 2.65 kg/t-molten iron for the invention example, while 4.16 kg/t-molten iron was required for the comparative example. As can be seen from these results, the invention example has improved desulfurization efficiency compared to the comparative example.

The molten iron desulfurization method of the present invention contributes to reducing the desulfurization consumption rate and improves productivity, making it industrially useful.

Reference Signs List 1, 1' Agitation type desulfurization device 2 Guide 3 Agitator 4 Carriage 5 Drive shaft 6 Impeller 7 Controller 8 Desulfurization agent adding device 9 Desulfurization agent 10 Roller with pusher 11 Hydraulic cylinder 12 Rod 13 Bracket 14 Roller 15 V-shaped groove 18 Rail 20 Mast 21 Vibration suppression mechanism 22 Hydraulic cylinder 22A Rod 23 Pressing force transmission mechanism 24 Clamp roll 30 Impeller drive device 31 Drive motor 32 Reducer 35 Lifting device 36 Wire 40 Molten iron ladle 41 Molten iron 50 Chute 60 Hopper Mw Motor

Claims (7)

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. 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. 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. 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. 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. 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. 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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