JP7614505B2 - Method for discharging slag from a smelting furnace - Google Patents

Description

This relates to a method of discharging slag from a refining furnace in which the molten slag on top of the molten iron is discharged from the furnace throat by tilting the furnace while leaving the molten iron in the furnace.

In steel refining, molten slag is formed on top of the molten iron contained in a refining furnace, and impurities contained in the molten iron are transferred to the molten slag. When a converter is used as the refining furnace, it has a furnace mouth at the top and a tapping port on the side, and the entire refining furnace is tilted. Prior to refining, molten iron and slag raw materials are charged through the furnace mouth and refining is carried out. After refining is complete, the furnace is tilted to tap only the molten steel from the tapping port, and then it is tilted to the other side to discharge the molten slag from the furnace mouth.

In recent years, in steel refining using converters, a method has been widely used in which dephosphorization and decarburization are performed separately within the same converter. Molten iron is charged into the converter, dephosphorization slag is formed and dephosphorization is performed, and after dephosphorization is completed, the converter is tilted to the side opposite the tap hole and only the dephosphorization slag is discharged from the throat, and then decarburization is performed in the same converter. After decarburization is completed, the furnace is tilted to tap only molten steel from the tap hole, and then it is tilted to the opposite side and the decarburization slag is discharged from the throat.

When the refining furnace is tilted to discharge the dephosphorization slag from the throat, if the dephosphorization slag remains in the furnace after it has been discharged, the phosphorus that migrated into the dephosphorization slag will return to the molten iron during the continuing decarburization refining, and the overall dephosphorization refining process will not be sufficient. On the other hand, if an attempt is made to reduce the amount of slag remaining in the furnace when the slag is discharged, the molten iron will also be discharged from the throat along with the slag. The discharged slag discharged from the throat is collected in a slag ladle waiting below the refining furnace. When the molten iron is discharged together with the slag, the slag and the molten iron react in the slag ladle to generate gas, which causes the slag to foam. If the outflow of molten iron is minor, the slag discharge operation can be continued by adding a slag sedative to the slag ladle, which has the effect of suppressing slag foaming. However, if the outflow of molten iron is excessive, the slag will overflow from the slag ladle, causing equipment damage and the associated deterioration in productivity. In the first place, if molten iron is discharged when slag is discharged, it will reduce the iron yield during refining.

In Patent Documents 1 and 2, when tilting a refining furnace to discharge slag from the furnace mouth, the tilting speed is changed according to the tilting angle of the refining furnace, and the tilting speed is slowed in the latter half of the slag discharge, or the time spent discharging the slag is increased in the latter half of the slag discharge compared to the first half, thereby minimizing the amount of molten iron discharged when discharging the slag, enabling rapid discharge, and increasing the slag discharge rate.

JP 2007-077481 A JP 2005-264210 A

When using a conventional slag discharge method, molten iron flows out together with the slag at the final stage of slag discharge, even though the slag discharge is still continuing. If an attempt is made to minimize the amount of slag remaining in the refining furnace, the amount of molten iron that flows out becomes excessive.
Patent Documents 1 and 2 propose methods for suppressing the outflow of molten iron entrained by the slag flow. However, even if the methods described in Patent Documents 1 and 2 are applied, there is a limit to the improvement.

The present invention aims to suppress the amount of molten iron flowing out while maintaining the amount of slag discharged from the refining furnace in a method for discharging slag from a refining furnace in which the molten slag in the upper layer of the molten iron is discharged from the furnace throat by tilting the refining furnace while leaving the molten iron in the furnace.

A numerical model experiment was conducted on the behavior of the fluid inside a refining furnace when the furnace is tilted to discharge slag from the furnace throat. As a result, it was found that surface waves are generated on the surface of the fluid inside the refining furnace during tilting, and that the molten iron can flow out regardless of the slag flow due to the influence of the waves generated in the molten iron. It was also found that the height of the surface waves on the surface of the fluid inside the refining furnace can be reduced and the amount of molten iron flowing out can be reduced by setting the tilting pattern of the refining furnace to at least one "tilt stop - tilt - tilt stop" pattern, setting the tilt angle change in this "tilt" to 1.5 degrees or less, and setting the stop time in the subsequent "tilt stop" to 2 seconds or more.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
(1) A method for discharging slag from a refining furnace in which the molten slag in the upper layer of the molten iron is discharged from the throat by tilting the refining furnace while leaving the molten iron in the furnace, the method comprising the steps of: executing two or more tilting stops with a stop time of 0.5 seconds or more during the slag discharge; and performing specific tilting control in a part or all of the section until the slag discharge is stopped, in which the change in tilt angle between one tilting stop and the next tilting stop is 1.5 degrees or less, and the tilting stop time after the tilting angle change of 1.5 degrees or less is 2 seconds or more.
(2) The method for discharging slag from a refining furnace described in (1) above, characterized in that the tilt angle θ (degrees) of the upright position of the refining furnace (the position where the furnace mouth is facing straight up) is set to 0 degrees, the maximum tilt angle when discharging slag is set to θend (degrees), and the specific tilt control is performed within a range in which the tilt angle is θa (degrees) determined by the following formula (1).
θa≦0.95θend…(1)

In this invention, in a slag discharge method in which the molten slag on top of the molten iron is discharged from the throat by tilting the refining furnace while leaving the molten iron in the furnace, at least one "tilt stop - tilt - tilt stop" pattern is set for the tilt pattern of the refining furnace during slag discharge, the change in tilt angle during the "tilt" is set to 1.5 degrees or less, and the stop time during the subsequent "tilt stop" is set to 2 seconds or more, thereby reducing the wave height of the surface waves on the fluid surface in the refining furnace and reducing the amount of molten iron flowing out when the slag is discharged.

1A and 1B are schematic cross-sectional views showing the tilting state of a refining furnace, in which FIG. 1A shows the state during refining and FIG. FIG. 1 is a conceptual cross-sectional view showing the rippling state of the slag-metal interface in the furnace during slag discharge. 7A and 7B show the relationship between time (t) and tilt angle (θ) during tilting, where (A) is the basic tilting pattern, (B) is the tilting pattern shown in FIG. 4 where Δt _S = 2 seconds and Δθ _M = 1.5 degrees, and (C) is the tilting pattern shown in the embodiment of FIG. 7. FIG. 13 is a diagram showing the state of maximum wave height amplification when the tilting stop time _ΔtS in the tilting pattern during slag discharge is changed. FIG. 13 is a diagram showing the state of the maximum wave height amplification degree when the tilt angle Δθ _M per tilting pattern during slag discharge is changed. 1A and 1B are cross-sectional conceptual diagrams showing rippling at the slag-molten iron interface in the furnace when the slag is discharged, and FIG. 1B shows the maximum tilt angle (θend) at which the slag is started to be discharged. FIG. 13 is a diagram showing the molten iron outflow index when the tilting pattern during slag discharge is changed.

Using a 350-ton converter as a model of a refining furnace for refining molten iron, the fluid behavior of molten iron and molten slag when the slag is discharged was analyzed by a numerical model experiment based on computational fluid dynamics. For the numerical model experiment, FLUENT (registered trademark), a general-purpose thermal fluid analysis software made by ANSYS Japan, Inc., was used. Since the fluid inside the refining furnace has an interface between the gas phase and the molten slag phase, and an interface between the molten slag phase and the molten iron phase, the multiphase flow model provided by FLUENT was applied to clarify the behavior of the surface waves generated at these interfaces.

The refining furnace contains 350 tons of molten iron and dephosphorization slag at the end of dephosphorization refining. The bulk density of the dephosphorization slag is smaller than its true density due to foaming.

As shown in Figure 1, the refining furnace 1 tilts around the rotation axis 3. Figure 1 (A) shows the state during refining, and (B) shows the state during tilting. Here, the tilting angle when the refining furnace is tilted in the tilting direction 6 for discharging slag is represented as θ (degrees), the tilting speed is represented as ω (degrees/second), and the time during tilting is represented as t (seconds). When the tilting speed is represented in degrees/minutes, it is represented as Ω (degrees/minute). The tilting angle θ (degrees) of the refining furnace in the upright position (during refining) (position where the furnace throat 2 faces straight up) is 0 degrees, the tilting angle at which the slag 5 begins to be discharged from the furnace throat 2 as shown in Figure 6 (A) is θini (degrees), and the maximum tilting angle at which the slag is discharged as shown in Figure 6 (B) is θend (degrees).

The tilting movement of the refining furnace is a motion pattern that repeats a stop for a predetermined time and a tilt for a predetermined time and angle. Hereinafter, stop is represented by the subscript S (Stop) and tilt is represented by the subscript M (Move). Furthermore, when expressing the average tilting speed combining stop and tilt, it is represented by the subscript A (Average). Therefore, the tilting movement of the refining furnace is a repeating pattern of a stop time _ΔtS and tilting by a tilt angle _ΔθM during a tilting time _ΔtM . The tilting speed (degrees/minute) during _ΔtM is _ωM . The average tilting speed _ωA is _ωA = _ΔθM / ( _ΔtS + _ΔtM ) = _ΔtM・_ωM / ( _ΔtS + _ΔtM )
It becomes.

In the numerical calculation, as the basic tilt pattern, a pattern in which Δt _S =0.5 seconds, Δt _M =1.0 seconds, and ω _M =1 degree/second are repeated in sequence, as shown in FIG. 3A. In this basic tilt pattern, the average tilt speed is
ω _A = 1/1.5 = 0.67 degrees/second (Ω _A = 40 degrees/minute)
It becomes.

Next, the refining furnace 1 was tilted in the slag discharge process, and a numerical model experiment was performed by changing the tilting pattern from the tilting angle θini at which the slag 5 starts to be discharged from the throat 2 as shown in FIG. 6(A) to the maximum tilting angle θend at the time of slag discharge as shown in FIG. 6(B). During the time lapse from θini to θend, attention was paid to the surface waves generated at the interface between the molten slag phase and the molten iron phase in the refining furnace 1 containing the molten iron 4 and the slag 5, and the wave height H of the surface waves was extracted (see FIG. 2). The surface waves had about 1 to 5 standing wave crests in the furnace, and the wave height at the time when the maximum wave height was formed during one period of the standing wave was taken as the wave height H. From θini to θend, the tilting pattern also involved repeated tilting stops and tilting. The number of repetitions after θini is indicated by the subscript n. The first repetition after θini is indicated by the subscript 1. The wave height of the surface waves at the nth repetition is indicated as H _n . The increase ratio of the wave height H per repetition of the tilting pattern is expressed as "H _n /H _n-1 ". The maximum value of "H _n /H _n-1 " from θini to θend is extracted and called the "maximum wave height amplification degree." The smaller this maximum wave height amplification degree is, the more the tilting pattern is evaluated as being able to suppress surface waves in the refining furnace.

In both patterns, the basic tilt pattern described above was used as the tilt pattern between θ = 0 and θini. In the numerical model experiments, θini = 55 degrees and θend = 85 degrees.

First, various tilt patterns were set during the time lapse from θini to θend, and for each tilt pattern, a constant value of _ΔtS between 0.5 and 20 seconds was adopted to evaluate the maximum wave height amplification. Note that for each pattern, _ωM was set constant at 1.0 degree/second, and the calculation was performed with two levels of tilt angle _ΔθM per time, 0.2 degrees and 1.5 degrees. Figure 3(B) shows the tilt patterns for _ΔtS = 2 seconds and _ΔθM = 1.5 degrees.

The results are shown in Figure 4. The horizontal axis is Δt _S and the vertical axis is the maximum wave height amplification. In both cases where Δθ _M = 0.2 degrees and 1.5 degrees, the maximum wave height amplification changes significantly before and after Δt _S = 2 seconds, and when Δt _S is less than 2 seconds, the maximum wave height amplification is greater than 1, indicating a tendency for surface waves to be amplified in the refining furnace, whereas when Δt _S is 2 seconds or more, the maximum wave height amplification is less than 1 in all cases, which indicates that even if surface waves are generated in the furnace at θini, the wave height of the surface waves continues to attenuate thereafter.

4, data where Δt _S =2 seconds and Δθ _M =1.5 degrees (called "high speed pattern 1") is compared with data where Δt _S =1 second and Δθ _M =0.2 degrees (called "low speed pattern 1"). The average tilt speed of high speed pattern 1 is represented as ω _A ^H1 and the average tilt speed of low speed pattern 1 is represented as ω _A ^L1 . In high speed pattern 1, Δt _M =1.5 seconds, and in low speed pattern 1, Δt _M =0.2 seconds. Therefore,
ω _A ^H1 = 1.5/(2 + 1.5) = 0.43 deg/sec ω _A ^L1 = 0.2/(1 + 0.2) = 0.17 deg/sec That is, even though the average tilt speed ω _A of low speed pattern 1 is more than twice as slow as that of high speed pattern 1, the surface waves attenuate in high speed pattern 1 (Δt _S = 2 seconds) and amplify in low speed pattern 1 (Δt _S = 1 second).

From the above numerical calculation results, it was found that in the tilting pattern of the refining furnace during the slag discharge process, when the tilting stop and tilting are repeated, the surface waves generated in the refining furnace can be attenuated by setting the tilting stop time _ΔtS to 2 seconds or more, even though the average tilting speed is fast. Note that when _ΔtS exceeds 20 seconds, the effect of attenuating the surface waves is saturated. Therefore, maintaining _ΔtS at more than 20 seconds does not have an adverse effect on preventing the outflow of molten iron, but leads to an extension of the operating time. Therefore, it is preferable that _ΔtS is 20 seconds or less.

Next, various tilt patterns were set over time from θini to θend, and for each tilt pattern, a constant value of _ΔθM between 0.1 degrees and 1.9 degrees was adopted to evaluate the maximum wave height amplification. In each pattern, Δt _S was set constant at 2 seconds. In addition, for each pattern, numerical calculations were performed for three types of _ωM : _ωM = 0.167, 0.667, and 8 degrees/second ( _ΩM = 10, 40, and 480 degrees/minute). The basic tilt pattern was used as the tilt pattern between θ=0 and θini, and θini = 55 degrees and θend = 85 degrees, which is the same as in FIG. 4.

The results are shown in Figure 5. The horizontal axis is Δθ _M and the vertical axis is the maximum wave height amplification, with the plots divided into ω _M = 0.167 degrees/second (△), 0.667 degrees/second (+), and 8 degrees/second (●). Regardless of the ω _M condition, the maximum wave height amplification changes significantly around Δθ _M = 1.5 degrees, and when Δθ _M is over 1.5 degrees, the maximum wave height amplification is greater than 1, indicating a tendency for surface waves to be amplified in the refining furnace, whereas when Δθ _M is 1.5 degrees or less, the maximum wave height amplification is less than 1 in all cases, which indicates that even if surface waves are generated in the furnace at θini, the wave height of the surface waves continues to attenuate thereafter.

5, data where Δθ _M =1.5 degrees and ω _M =8 degrees/second (called "high speed pattern 2") is compared with data where Δθ _M =1.7 degrees and ω _M =0.167 degrees/second (called "low speed pattern 2"). The average tilt speed of high speed pattern 2 is represented as ω _A ^H2 and the average tilt speed of low speed pattern 2 is represented as ω _A ^L2 . In high speed pattern 2, Δt _M =0.19 seconds, and in low speed pattern 2, Δt _M =10.2 seconds. Therefore,
ω _A ^H2 = 1.5/(2 + 0.19) = 0.68 deg/sec ω _A ^L2 = 1.7/(2 + 10.2) = 0.14 deg/sec That is, even though the average tilt speed ω _A of the low speed pattern 2 is more than four times slower than that of the high speed pattern 2, the surface waves are attenuated in the high speed pattern 2 (Δθ _M = 1.5 deg) and are amplified in the low speed pattern 2 (Δθ _M = 1.7 deg).

From the above numerical calculation results, it was found that in the tilting pattern of the refining furnace during the slag discharge process, when the tilting stop and tilting are repeated, the surface waves generated in the refining furnace can be attenuated regardless of the average tilting speed by setting the tilting angle Δθ _M per tilt to 1.5 degrees or less. Note that when Δθ _M is less than 0.2 degrees, the effect of reducing the maximum wave height amplification is saturated. Therefore, setting Δθ _M to less than 0.2 degrees does not have an adverse effect on the suppression of molten iron outflow, but increases the number of tilts and decreases the average tilting speed during slag discharge, leading to an extension of the operating time. Therefore, it is preferable that Δθ _M is 0.2 degrees or more.

Summarizing the results shown in Figure 4 and Figure 5, when tilting and stopping are repeated in the section from the start of slag discharge to the stop of slag discharge, by setting the change in tilt angle between one tilt stop and the next tilt stop to 1.5 degrees or less, and setting the tilt stop time after a tilt angle change of 1.5 degrees or less to 2 seconds or more, it is possible to suppress the surface waves generated on the slag metal surface in the refining furnace, and therefore to suppress the amount of molten iron flowing out during slag discharge.

Next, the basic tilting pattern (a pattern in which Δt _S =0.5 sec, Δt _M =1.0 sec, ω _M =1 degree/sec are repeated in sequence) was applied as the tilting pattern from the upright state of the refining furnace at θ=0 through θini to θend, and a pattern was applied in which the tilt angle was changed to 1.5 degrees or less only once between θini and θend, and the tilting stop time after the tilt angle change of 1.5 degrees or less was set to 2 seconds or more, to examine the effect on the amount of molten iron outflowing from the refining furnace. Note that the preconditions for the numerical model experiment were the same as those for the numerical model experiment, including that θini=55 degrees and θend=85 degrees.

In Comparative Example 1, the basic tilt pattern (Δt _S =0.5 sec, Δt _M =1.0 sec, ω _M =1 deg/sec repeated in sequence) was applied throughout the entire tilt period from θ=0 to θend. The average tilt speed ω _A =0.67 deg/sec (Ω _A =40 deg/min).

Next, in Examples 1 to 4, as shown in FIG. 3C, tilt control was performed in which a pattern different from the basic tilt pattern (referred to as a "specific tilt pattern") was inserted repeatedly for only one section between θini and θend, based on the tilt pattern (basic tilt pattern) of Comparative Example 1. That is, the specific tilt pattern was inserted after Δt _S =0.5 seconds at a predetermined tilt angle θa of the basic tilt pattern. A tilt of Δt _M =1 second at ω _M =1.5 degrees/second was performed to set Δθ _M =1.5 degrees, and a stop of Δt _S =2 seconds was performed immediately after that to set the specific tilt pattern. After that, a tilt of ω _M =0.5 degrees/second was performed for Δt _M =1 second, and then the tilt pattern was returned to the basic tilt pattern. In Examples 1 to 4, the timing of inserting the specific tilt pattern (tilt angle θa) was determined as follows.
Example 1: θa = 55 degrees, θa/θend = 0.647
Example 2: θa = 79 degrees, θa/θend = 0.929
Example 3: θa = 80.5 degrees, θa/θend = 0.947
Example 4: θa = 81.5 degrees, θa/θend = 0.958

At the end of the slag discharge process, when the tilting angle reached θend, tilting was stopped, and the stoppage was continued for 0.5 seconds in Comparative Example 1 and Examples 1 to 3, and for 2 seconds in Example 4, after which the refining furnace was returned to the upright position. The total amount of molten iron that flowed out from the converter throat from the start to the end of tilting was calculated, and the amount of molten iron that flowed out in Comparative Example 1 was normalized to 100, and the molten iron outflow amount index was calculated for Examples 1 to 4, which was plotted on the vertical axis of Fig. 7. As is clear from Fig. 7, the amount of molten iron outflow was reduced in all of Examples 1 to 4 compared to the Comparative Example.
From the results of Examples 1 to 4 described above, it has become clear that the amount of molten iron outflow can be reduced by performing specific tilting control in a part or all of the section until the slag discharge is stopped, in which the change in tilt angle between one tilting stop and the next tilting stop is set to 1.5 degrees or less, and the tilting stop time after the tilting angle change of 1.5 degrees or less is set to 2 seconds or more.

In other words, as specified in the present invention, in a slag discharge method in which the molten slag on top of the molten iron is discharged from the throat by tilting the refining furnace while leaving the molten iron in the furnace, the amount of molten iron that flows out during slag discharge can be reduced by performing two or more tilt stops with a stop time of 0.5 seconds or more during slag discharge, and performing specific tilt control in part or all of the section from the start of slag discharge to the stop of slag discharge, in which the change in tilt angle between one tilt stop and the next tilt stop is 1.5 degrees or less, and the tilt stop time after the tilt angle change of 1.5 degrees or less is 2 seconds or more. In all of Examples 1 to 4 and Comparative Example 1, θend is set to the same value, so the amount of slag remaining in the refining furnace at the time slag discharge was completed was the same.

The patterns of Examples 1 to 4 above are examples of the present invention in which the specific tilt control is applied to a part of the section from the start of slag discharge to the end of slag discharge, whereas the plot of Δt _S ≦2 seconds in Fig. 4 and the plot of Δθ _M ≦1.5 degrees in Fig. 5 are examples in which the specific tilt control is applied to the entire section from the start of slag discharge to the end of slag discharge.

In addition, comparing Examples 1 to 4, it is found that, compared to Example 4 in which θa/θend>0.95, the amount of molten iron flowing out during slag discharge can be further reduced by adjusting the timing of the specific tilting control to θa/θend≦0.95. That is, as specified in the present invention, it is preferable to set the tilting angle θ (degrees) of the upright position of the refining furnace (the position where the furnace mouth faces straight up) to 0 degrees, set the maximum tilting angle during slag discharge to θend (degrees), and perform the specific tilting control within a range in which the tilting angle is θa (degrees) determined by the following formula (1).
θa≦0.95θend…(1)

To calculate θa, the value of the maximum tilting angle θend when discharging the slag is required. θend can be determined by measuring the refractory profile inside the furnace in advance to determine the furnace volume at the time of tilting and the amount of molten iron in the furnace, and it is possible to predict the tilting angle at which the molten iron will flow out when tilted quasi-statically. However, in practice this is affected by rippling of the molten iron, so it is a good idea to carry out tests in advance with different tilting methods to determine θend.

1 Refining furnace 2 Throat 3 Rotating shaft 4 Molten iron 5 Slag 6 Tilting direction θ Tilting angle θini Tilting angle at which slag starts to be discharged from the throat θend Maximum tilting angle when slag is discharged Δt _S Tilting stop time Δt _M Tilting time Δθ _M Tilting angle (per time) ω Tilting speed (degrees/second)
ω _A average tilting speed Ω Tilt speed (degrees/min)

Claims (2)

In a slag discharge method in which a refining furnace is tilted while leaving molten iron in the furnace, and molten slag in an upper layer of the molten iron is discharged from a furnace throat, the method comprises: performing tilting stops with a stop time of 0.5 seconds or more two or more times during slag discharge;
In part or all of the section up to the stop of slag discharge,
The method for discharging slag from a refining furnace is characterized in that, as a specific tilting control, a tilting angle change between one tilting stop and the next tilting stop is set to 1.5 degrees or less, and the tilting stop time after the tilting angle change of 1.5 degrees or less is set to 2 seconds or more. The method for discharging slag from a refining furnace according to claim 1, characterized in that the tilt angle θ (degrees) of the refining furnace in the upright position (the position where the furnace mouth is facing straight up) is 0 degrees, the maximum tilt angle when discharging slag is θend (degrees), and the specific tilt control is performed within a range in which the tilt angle is θa (degrees) determined by the following formula (1).
θa≦0.95θend…(1)

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