TWI570244B - Preliminary treatment method for molten iron - Google Patents

Description

Preparation method of molten iron

The present invention relates to a method for preparing a molten iron which is continuously subjected to a desulfurization treatment and a dephosphorization treatment of a molten iron by using a converter type refining furnace through a slag discharging step in the middle.

In recent years, there has been a strong demand to reduce the amount of greenhouse gas emissions. In the steel industry, when decontamination treatment or decarburization refining is carried out using a processing vessel such as a converter or a molten iron pan, iron filings (iron) are placed in the molten iron in the furnace. Scrap) and other cold iron sources reduce the energy needed to make steel. The reason is that the cold iron source is different from the iron oxide such as iron ore charged in the blast furnace, and it is not necessary to carry out the reduction. Therefore, compared with the method of refining the pig iron from the blast furnace to produce molten steel, a small amount of energy can be used. The consumption and the amount of greenhouse gas emissions are small to produce molten steel.

In addition, in recent years, it is advantageous in terms of cost and quality, and therefore, a refining method for preliminarily removing phosphorus from molten iron before performing decarburization refining by a converter to perform pre-dephosphorization (also referred to as "pre-dephosphorization" deal with"). Usually, the dephosphorization treatment is carried out by adding an oxidizing agent (oxygen source such as oxygen) and a dephosphorization refining agent (CaO-based flux) to the molten iron, and oxidizing the phosphorus in the molten iron to form phosphorus oxidation. And absorb it into the slag-removing dephosphorization refining agent In theory, the dephosphorization reaction is advantageous as the refining temperature is lower. That is, the dephosphorization reaction is easily carried out at a temperature lower than the molten iron phase of the molten steel stage, and a dephosphorization treatment can be carried out using a small amount of an oxidizing agent and a dephosphorization refining agent. Therefore, by performing the above-described preliminary dephosphorization treatment, although the number of processing steps is increased, the amount of slag generated in all the steel refining steps can be reduced.

In addition, the molten iron from the blast furnace is contained in a range of 0.3% by mass to 0.6% by mass. If the molten iron containing cerium is dephosphorized, the ruthenium is first removed by oxidation and the molten iron is removed. After the cerium concentration is lowered to a certain extent, the phosphorus in the molten iron is oxidized and removed. The slag containing SiO ₂ as a main component is generated by the oxidation of the above ruthenium, and the slag hinders the dephosphorization reaction. This is because slag having a basicity ([CaO (% by mass)) / [SiO ₂ (% by mass))) of 1.2 or more is necessary for the dephosphorization reaction, but it is produced by oxidation of ruthenium. SiO ₂ has a function of lowering the alkalinity of the slag.

In the steel refining step including the combination of blast furnace and converter, cold iron such as iron filings The heat source for melting of the source is mainly composed of the sensible heat of the molten iron and the heat of combustion of carbon and helium in the molten iron, and it is basically impossible to melt a large amount of cold iron source. Further, as described above, in the case where the untwisting treatment and the dephosphorization treatment are performed as the preliminary treatment for the molten iron, carbon and bismuth in the molten iron which becomes the combustion heat source are removed in addition to the decrease in the temperature of the additional molten iron in the processing step. It is also reduced by the above-described deodorization treatment and dephosphorization treatment, so that the melting of the cold iron source in the converter is more disadvantageous.

Therefore, in the case of performing the molten iron preparation process, in order to melt more In the cold iron source, for example, Patent Document 1 proposes a method for preparing a molten iron in which the alkalinity of the slag at the end of the deodorization treatment is first performed when the molten iron is removed and dephosphorized using a converter type refining furnace. Adjusting CaO in a way that is in the range of 0.3~1.3 The desulfurization treatment is performed by supplying the flux, and thereafter, the slag generated in the furnace is discharged from the furnace mouth by the tilting refining furnace, and then the CaO-based flux is newly added to perform dephosphorization treatment. Further, Patent Document 2 proposes a method for preparing a molten iron in which a desulfurization treatment is performed using a converter type refining furnace, and after the dephosphorization treatment is completed, the molten iron is discharged, and the slag is not discharged. In the state where it remains in the furnace, the molten iron of the next charge is charged into the refining furnace, and oxygen is supplied to perform the deodorization treatment, and after the deodorization treatment, the blowing is temporarily interrupted and set. The intermediate slag discharging step of the slag is discharged, and thereafter, the dephosphorization treatment is continued.

The technique of the above Patent Document 1 can be carried out by using a converter type refining furnace In addition to the effect, the technique of the patent document 2 removes the slag generated by the dephosphorization treatment (hereinafter also referred to as "dephosphorization", in addition to the effect, in addition to the effect of the de-phosphorization treatment and the dephosphorization treatment. The slag ") is reused in the deodorization treatment, so that the temperature drop due to the addition of the slagging agent in the deodorization treatment step can be reduced.

That is, by employing the technique of Patent Document 1 or Patent Document 2, the heat loss in the preliminary processing step of the molten iron can be reduced, so that the blending ratio of the cold iron source can be increased as compared with the prior, and the greenhouse effect gas can be realized. Reduction in discharge amount or reduction in manufacturing cost.

[Prior Art Literature]

[Patent Literature]

[Patent Document 1]: Japanese Patent Laid-Open No. Hei 10-152714

[Patent Document 2]: Japanese Patent Laid-Open No. Hei 11-323420

However, the above problems exist in the above prior art.

In the case where the desulfurization treatment, the intermediate slag removal, and the dephosphorization treatment are continuously performed by a converter type refining furnace as in the technique disclosed in Patent Document 1 or Patent Document 2, in order to set the alkalinity of the slag to dephosphorization When the CaO-based flux used in the dephosphorization treatment is reduced, the slag containing a large amount of SiO ₂ (hereinafter also referred to as "de-slurry slag") generated in the decarburization treatment must be discharged from the converter-type refining furnace. More than the specified amount.

From this point of view, with regard to the above prior art, regarding Patent Document 1 In the technique, it is considered that when the alkalinity of the slag at the end of the deodorization treatment is controlled to 0.3 to 1.3, the slag sufficiently exhibits fluidity, and the slag discharge of the slag is sufficiently performed. However, if only the alkalinity of the slag is controlled to 0.3 to 1.3, the slag is insufficiently formed and the fluidity is poor, and it is difficult to discharge the slag in a short time without flowing the molten iron, and vice versa. Excessive molding, in which the slag overflows from the furnace mouth during the untwisting process, and the operation or the like is hindered, so that it is difficult to sufficiently control the slag discharge.

Further, the technique of Patent Document 2 proposes to perform intermediate slag discharge by setting the alkalinity of the slag slag to 1.0 to 3.0 and the cerium concentration in the molten iron to 0.20% by mass or less. However, the reason is that when the concentration of cerium in the molten iron at the end of the decanting treatment is higher than 0.20% by mass, the content necessary for adjusting the alkalinity of the slag at the time of dephosphorization treatment in the next step to 2.0 is included. The substance of CaO becomes excessive and becomes disadvantageous in terms of cost, but there is no consideration for the slagging property of the slag.

In other words, the techniques disclosed in Patent Document 1 and Patent Document 2 have a problem in that the slag can not be sufficiently discharged, and the amount of the CaO-based flux must be increased in the dephosphorization treatment in the next step, or dephosphorization is required. The concentration of phosphorus in the molten iron after the treatment becomes high.

The present invention has been made in view of the above problems in the prior art, and its object is The present invention proposes a method for preparing a molten iron, which is a method for preparing a molten iron which is continuously subjected to a desulfurization treatment and a dephosphorization treatment of a molten iron by using a converter type refining furnace through a slag discharging step in the middle. The slag discharge property of the slag slag generated in the mashing treatment is improved, and the temperature reduction of the molten iron can be suppressed and the dephosphorization treatment in the next step can be performed at low cost.

The first embodiment of the present invention developed to solve the above problems is as A method for preparing a molten iron after the molten iron is supplied to the converter-type refining furnace to be subjected to a desulfurization treatment, and a part of the slag present in the furnace is self-rotating furnace-type refining furnace in a state in which molten iron remains in the furnace. After the discharge, the CaO-based flux and the oxygen source are supplied to the converter type refining furnace for dephosphorization treatment, and the molten iron subjected to the dephosphorization treatment is discharged from the converter type refining furnace, thereby melting the furnace using a converter type refining furnace. The iron is subjected to a degassing treatment and a dephosphorization treatment, and the method for preparing the molten iron is characterized in that the suction gas sucked by the exhaust gas processing equipment of the converter type refining furnace is used in the above-described deodorization treatment (suction gas) The concentration of at least one or more gaseous species containing carbon atoms is analyzed, and the end time of the dislocation treatment is determined based on the analysis value.

A second embodiment of the present invention is a melting method according to the first embodiment of the present invention. The iron preparation processing method is characterized in that the exhaust gas discharged from the converter type refining furnace in the deodorization treatment is calculated based on an analysis value of a concentration of a gaseous substance containing carbon atoms in the intake gas and a flow rate of the suction gas. The discharge rate of the carbon in the above-described calculated exhaust gas is a maximum value after the discharge rate of the carbon in the exhaust gas is a maximum value, and the pattern of the end of the dislocation process is determined.

According to a third aspect of the present invention, in a method for preparing a molten iron according to the first embodiment of the present invention, the CO gas concentration, the CO ₂ gas concentration, and the CO gas and the CO ₂ gas are based on the inhaled gas. The analysis value of any of the total concentrations is a maximum value, and the fluctuation pattern which is increased again after the minimum value is determined, and the end time of the above-described dislocation processing is determined.

According to a fourth aspect of the present invention, in a method of preparing a molten iron according to the first embodiment of the present invention, the CO gas concentration, the CO ₂ gas concentration, and the CO gas and the CO _{2 are} based on the inhaled gas. The fluctuation pattern calculated by the product of the analysis value of any of the total concentration of the gas and the flow rate of the suction gas determines the end time of the deodorization process, and the fluctuation pattern is the CO gas flow rate and CO _{2 in} the intake gas. Any one of the gas flow rate and the total flow rate of the CO gas and the CO ₂ gas is a maximum value, and the fluctuation pattern is increased again after the minimum value.

According to a fifth embodiment of the present invention, the second embodiment to the present invention According to a fourth aspect of the present invention, in the method of preparing a molten iron according to any of the above-described embodiments, in the variation pattern which becomes a maximum value and then increases again, the value which is increased again becomes 90 with respect to the maximum value. The time point above the value of the predetermined ratio of % or more and 150% or less is used as a reference, and the end time of the above-described dislocation processing is set to a predetermined elapsed time range.

According to a sixth embodiment of the present invention, the second embodiment to the present invention According to a fifth aspect of the present invention, in the method of preparing a molten iron according to any one of the embodiments, the maximum value is a maximum value, and a difference between a maximum value and a minimum value in a variation pattern that increases again after a minimum value is 10% of a maximum value. the above.

According to a seventh aspect of the present invention, in a method of preparing a molten iron according to the first embodiment of the present invention, the CO gas concentration, the CO ₂ gas concentration, and the CO gas and the CO ₂ gas in the suction gas are used. The end time of the above-described dislocation processing is set to a predetermined elapsed time range based on the time point when the analysis value of any of the total concentrations is equal to or greater than the predetermined threshold value.

An eighth embodiment of the present invention is a melting method according to a seventh embodiment of the present invention. In the method for preparing an iron, the exhaust gas treatment device has a function of recovering the exhaust gas of the converter-type refining furnace that is sucked in as a fuel gas, and sucks the exhaust gas of the converter-type refining furnace together with the exhaust gas treatment device. In the atmosphere, at least a part of the CO gas in the exhaust gas is burned, and the time when the CO gas concentration in the inhaled gas after the combustion is equal to or greater than a predetermined threshold of 2.0 vol% or more and 18.0 vol% or less is used as a reference. The end time point of the above-described dislocation processing is set within a predetermined elapsed time range.

According to a ninth aspect of the present invention, in a method for preparing a molten iron according to the first embodiment of the present invention, the CO gas concentration, the CO ₂ gas concentration, and the CO gas and the CO _{2 in} the suction gas are used. Any one of the CO gas flow rate, the CO ₂ gas flow rate, and the total flow rate of the CO gas and the CO ₂ gas in the intake gas calculated by multiplying the analysis value of any of the total concentration of the gas and the flow rate of the intake gas. The time point of the above-described dislocation processing is set to be within a predetermined elapsed time range based on the time point of the predetermined threshold or more.

A tenth embodiment of the present invention is the first embodiment of the present invention. A method for preparing a molten iron, which is characterized in that the discharge of the spin-type refining furnace in the above-described deodorization treatment is calculated based on an analysis value of a concentration of a gaseous substance containing carbon atoms in the inhaled gas and a flow rate of the inhaled gas. The discharge speed of the carbon in the gas is based on the time point at which the discharge speed is equal to or greater than a predetermined threshold value. The end time of the dislocation process is set within a predetermined elapsed time range.

An eleventh embodiment of the present invention is the first embodiment of the present invention to A method for preparing a molten iron according to any one of the tenth embodiments, wherein the slag generated in the dephosphorization treatment of the previous heat is 30% by mass in the furnace, and the next furnace is used. The secondary molten iron is charged into a converter type refining furnace and subjected to deodorization treatment.

According to a twelfth aspect of the present invention, in the method of preparing a molten iron according to any one of the first to eleventh embodiments of the present invention, the converter type is The alkalinity ([CaO (mass%)) / [SiO ₂ (% by mass))) of the slag present in the refining furnace is controlled to be in the range of 0.80 to 1.50.

According to the present invention, in the case of using a converter type refining furnace, the desulfurization treatment and the dephosphorization treatment of the molten iron are continuously performed through the intermediate slag, and the contents of the suction gas sucked by the exhaust gas treatment equipment of the converter type refining furnace are contained. When the analysis value of the gaseous substance of the carbon atom determines the end time of the dislocation treatment, the deviation of the determination of the completion of the dislocation treatment is greatly reduced, and the slag can be continuously formed in the state where the slag is sufficiently formed and the fluidity is high. On the other hand, the degreasing furnace slag can be sufficiently discharged in a short time without flowing out the molten iron, and further, the cost of the dephosphorization treatment after the deodorization treatment and the variation in the phosphorus concentration in the molten iron after the treatment can be reduced.

1‧‧‧ Converter type refining furnace

2‧‧‧ top blow gun

3‧‧‧ bottom air outlet

4‧‧‧iron outlet

5‧‧‧ molten iron

5a‧‧‧Fused iron after dislocation treatment

5b‧‧‧Fused iron after dephosphorization

6‧‧‧Desulfurization slag

7‧‧‧Dephosphorization furnace slag

8‧‧‧cold iron source

9‧‧‧Oxygen

10‧‧‧ bottom blowing gas

11‧‧‧ skirt

12‧‧‧ flue

13‧‧‧ gas probe

14‧‧‧Gas analysis device

15‧‧‧Loading the pot

Figure 1 is a graph showing changes in the discharge rate of carbon in the exhaust gas during the deodorization treatment. The chart of the example.

2(a) to 2(c) are diagrams for explaining changes in the discharge speed of carbon in the exhaust gas.

Fig. 3 is a schematic cross-sectional view showing a converter type refining furnace used in a method for preparing a molten iron according to the present invention.

4 is a view showing the relationship between the concentration of CO gas in the intake gas and the slagging property of the slag of the slag at the end of the devolatilization treatment.

5(a) to 5(f) are schematic views for explaining a preliminary processing method of the present invention in accordance with the steps.

Fig. 6 is a graph showing the relationship between the intermediate slag discharge time and the Si concentration in the molten iron in the method of the present invention as compared with the conventional method.

First, the basic technical idea of the present invention will be described.

The inventor of the present invention uses a converter-type refining furnace to continuously perform deodorization treatment and dephosphorization treatment on the molten iron through the discharge of the degreasing furnace slag after deodorization treatment (hereinafter also referred to as "intermediate slag discharge"). In order to improve the slagging property of the above-mentioned slag slag, various factors affecting the slagging property of the slag slag are repeatedly studied.

As a result, it is clear that the slag discharge property of the slag slag in the intermediate slag is in addition to the fluidity of the slag slag itself, and the molding condition of the slag slag is greatly affected, in order to remove the slag. The slag discharge property is good, and it is important that the slag is sufficiently slag at the time of slag discharge to exhibit good fluidity, and the slag is sufficiently molded to reduce the volume specific gravity.

The slag slag generated in the mashing treatment is formed by slag-forming SiO ₂ produced by combustion of ruthenium in the molten iron and slag-forming material added or left in the furnace. In the initial stage of the deodorization treatment in which the molten iron temperature is low and the rhodium concentration in the molten iron is high, the decoupling reaction is preferentially performed, and the amount of SiO ₂ in the furnace is gradually increased. In order to ensure fluidity during slag discharge, it is important to add or A slag-forming material containing CaO such as quicklime or steel slag is preliminarily charged, and the composition of the slag is adjusted to an appropriate range.

On the other hand, the rate of CO gas generation is at the initial stage of the dislocation treatment. In the low position, as the desorption reaction proceeds, the concentration of ruthenium in the molten iron is reduced to less than 0.20% by mass, the temperature of the molten iron rises, the decarburization reaction becomes active, and the rate of generation of CO gas gradually increases. Further, the concentration of iron oxide in the slag is relatively low at the initial stage of the deodorization treatment in which the concentration of antimony in the molten iron is low, but the concentration of antimony in the molten iron is lowered as the deodorization reaction proceeds. The amount of slag is increased, and the concentration of iron oxide in the slag is gradually increased.

Further, if the concentration of iron oxide in the slag increases by more than 10% by mass, Then, the low melting point of the slag or the increase in the liquid phase ratio becomes remarkable, and the increase in the temperature in the furnace increases the fluidity of the slag. Further, the generation of CO gas by the reaction between the iron oxide in the slag and the molten iron bath or the molten iron droplets involved in the slag is also active, and the slag contains a large amount of CO gas bubbles, which becomes a so-called "made" The state of slag forming. When the slag is started to be formed, the amount of oxygen supplied to the slag layer by the oxygen supplied from the top-blowing lance is also increased, and the oxidation of iron or the like is promoted. Therefore, depending on the oxygen supply conditions such as the height of the lance, there is also The molding height is accelerated to increase, and the so-called "slopping" is achieved.

If it can be controlled and maintained in the slagging state when the slag is removed from the slag, then The slag has a very small specific gravity, and even if the same slag quality is about 10 times as large, it can be quickly discharged when the slag flows out from the furnace mouth. The slag does not cause the molten iron to flow out. However, if the slag formation in the untwisting treatment is excessively performed, there is a possibility that the slag overflows from the furnace mouth and hinders the operation, so care is required.

However, in the prior art, it is not established to control and maintain the above-mentioned slagging. The method, or the molding state is appropriately evaluated, and the technique of shifting from the dislocation treatment to the timing of the intermediate slag discharge is appropriately determined. Therefore, it is difficult to stably increase the slag discharge rate of the slag in the intermediate slag.

The inventors and the like have appropriately evaluated the molding state of the slag in the dislocation treatment. In the state, the method of determining the timing of ending the dislocation treatment and starting the intermediate slagging is repeatedly studied. As a result, it has been found that in the degassing treatment using the converter type refining furnace, the discharge speed of the carbon in the exhaust gas shows a specific fluctuation pattern, and the molding state becomes the most suitable for slag discharge within a specific range of the variation pattern, and ends. The timing of the slag discharge in the middle of the slag removal process can be based on the concentration of the gaseous substance containing carbon atoms contained in the suction gas (hereinafter also referred to as "inhalation gas") sucked by the exhaust gas treatment equipment attached to the converter type refining furnace. The analysis is performed to determine the development of the present invention.

1 is a transition (change over time) of a discharge rate of carbon contained in an exhaust gas (amount of carbon contained in an exhaust gas discharged per unit time), a flow rate of a suction gas, an oxygen supply rate, and CO in an intake gas. The graph showing the sum of the gas concentration and the CO ₂ gas concentration together shows that the discharge speed of the carbon contained in the exhaust gas is based on the concentration of ruthenium in the molten iron under the condition that the slag is easily molded. The molten iron is subjected to a CO gas concentration, a CO ₂ gas concentration, and a suction gas flow rate in the suction gas sucked into the exhaust gas treatment equipment attached to the converter type refining furnace at the time of the deuterium concentration reduction of 0.10% by mass or less. Calculated in the standard state).

As can be seen from the figure, in the case of the formation of the slag, the discharge rate of the carbon contained in the exhaust gas shows a specific fluctuation pattern as follows: after the start of the deodorization treatment, the discharge gas is discharged as the degassing reaction proceeds. The discharge rate of the contained carbon gradually rises (stage I), and decreases after temporarily displaying the maximum value, and shows the minimum value (stage II), and then increases again (stage III). Further, regarding the discharge rate of the carbon in the exhaust gas, strictly speaking, the exhaust gas processing device also includes a sucked air contained in the CO _2, CO ₂ in the air, but a trace amount, it can ignore the influence of the resulting .

The discharge speed of carbon in the exhaust gas shows the specificity as described above The reason for the change pattern, the inventors and the like are considered as follows.

First, the stage I is a stage in which the amount of CO gas is gradually increased as the temperature of the molten iron rises due to the deodorization treatment and the concentration of rhodium in the molten iron is lowered, and the generation of the decarburization slag at the initial stage of the decarburization reaction is generated. The amount is still small, and the temperature is low and unformed, so as shown in Fig. 2(a), the CO gas can be easily discharged to the outside of the furnace through the slag layer. However, when the deuteration reaction is carried out and the phase II is obtained, the amount of CO gas generated is increased, and the temperature of the desulfurization slag is increased, and the viscosity is reduced. Therefore, as shown in Fig. 2(b), the generated CO gas is taken. The slag is formed in the slag, and the discharge rate of carbon in the exhaust gas is reduced by an apparent one-time. When the phase III is further removed by the deoximation reaction, the CO gas above the slag can not be taken in, and the molding is saturated. Therefore, as shown in Fig. 2(c), the CO gas is discharged to the outside of the furnace. The discharge rate of carbon in the exhaust gas starts to rise again.

In addition, it can be considered that the variation pattern of the discharge speed of carbon in the exhaust gas is also It is related to the following phenomenon.

In the stage I of the deodorization treatment, the oxygen supplied from the top blowing lance is mainly consumed in the deodorization reaction and the decarburization reaction, but if it is in the stage II, the amount of generated slag increases, the thickness of the slag layer increases, and the slag The concentration of iron oxide in the rise increases, and the oxygen supplied Since the oxidation reaction of iron is consumed, the oxygen used for decarburization is correspondingly reduced, and the discharge rate of carbon in the exhaust gas is lowered. In particular, when the slag is started to be formed and the thickness of the slag layer is increased, the amount of oxygen supplied to the slag layer obtained by oxygen supply from the top blowing lance is accelerated, and the concentration of iron oxide is increased. Next, in the case of the stage III, CO gas is generated by the reaction of iron oxide and molten iron in the slag, and the iron oxide in the slag is correspondingly reduced, and the iron oxide in the slag is in a state of balance, so CO gas The rate of generation increases again, and the rate of carbon discharge in the exhaust gas begins to rise.

According to the above discussion, the discharge rate of carbon in the exhaust gas is extremely large. The value becomes a minimum value and increases again from the minimum value. If the deodorization treatment is completed at the stage of the above-mentioned stage III in which the slag is in the molding state, and the intermediate slag is started, the slag discharge rate of the slag can be surely increased. . In addition, it is confirmed that the cerium concentration in the molten iron in this stage is steadily lowered to 0.10 mass% or less which can be efficiently dephosphorized by reducing the amount of the dephosphorization refining agent used in the subsequent dephosphorization treatment.

Further, the inventors found that in the specific region of the above stage III, Specifically, when the maximum value of the discharge speed of carbon in the exhaust gas is 90% or more and 150% or less, the slag removal rate of the slag can be further increased.

The reason for this is considered to be as follows. As described above, by performing the decarburization reaction, the temperature of the slag is increased, the viscosity is lowered, and the fluidity is improved, and the specific gravity of the slag is apparently formed by molding. As the size becomes smaller, the height (thickness) of the slag layer on the molten iron bath increases and it becomes easy to flow out from the furnace mouth. When the discharge rate of the carbon in the exhaust gas is less than 90% with respect to the maximum value, the slag is insufficiently formed, and the slag discharge rate of the slag is also insufficient. On the other hand, the case where the dislocation treatment is ended with respect to the above maximum value exceeding 150% At the time, there is a problem that the slag overflows from the furnace mouth before the end of the dislocation treatment and hinders the operation.

The present invention has been developed based on the novel findings described above, The method is: after the desulfurization treatment is performed on the molten iron supply source in the converter type refining furnace, at least a part of the slag present in the furnace is discharged from the converter type refining furnace (intermediate slag discharge), and thereafter, to the converter The molten iron in the refining furnace is supplied with a CaO-based flux and an oxygen source to perform dephosphorization treatment, and the molten iron is discharged, and the inhalation is taken in the exhaust gas treatment equipment of the converter type refining furnace in the above-described deodorization treatment. The concentration of the gaseous substance containing carbon atoms in the gas is analyzed, and the end time of the dislocation treatment is determined based on the analysis value.

The method for preparing the molten iron of the present invention described above is as shown in FIG. A converter type refining furnace 1 for top blowing and bottom blowing. The top blowing is performed by supplying the oxygen-containing gas 9 as an oxygen source from the tip end of the top blowing lance 2 to the molten iron 5 via the top blowing lance 2 that can be raised and lowered inside the converter type refining furnace 1. As the oxygen-containing gas 9, oxygen (industrial pure oxygen), oxygen-enriched air, air, a mixed gas of oxygen and an inert gas, or the like can be used.

On the other hand, the bottom blowing is performed by passing through the bottom of the converter type refining furnace 1 The bottom-blowing tuyere 3 is blown into the molten iron by the bottom blowing gas 10. The bottom blowing gas 10 may be a gas containing oxygen or an inert gas such as argon or nitrogen. The bottom blowing gas 10 has a function of promoting the stirring of the molten iron 5 by blowing into the molten iron to promote melting of the cold iron source, and may also have a function of blowing the slag forming agent into the molten iron from the bottom blowing port 3. The function of the gas for transportation.

Further, a skirt 11 for lifting and covering the mouth of the converter type refining furnace 1 and a flue 12 connected thereto are provided above the converter type refining furnace 1. The exhaust gas discharged from the converter type refining furnace 1 is sucked through the flue 12 by an exhaust gas treatment device (not shown), and is sprinkled with water to remove the dust, and then the suction speed is measured. Exhaust When the gas processing apparatus has a function of recovering the sucked inhaled gas as a fuel gas, the inhaled gas can be recovered as a fuel gas or discharged according to the composition or the flow rate, but in the dislocation treatment, since the CO gas is generated at a low speed, Usually not recycled as fuel gas. Here, in addition to the exhaust gas discharged from the converter type refining furnace 1, the intake gas sucked by the exhaust gas treatment equipment (not shown) includes a differential pressure from the atmospheric pressure in the skirt 11 and a converter type refining. Air (atmosphere) that is inhaled by the size of the gap between the furnace 1 and the skirt 11 (hereinafter also referred to as "skirt height").

The oxygen in the air sucked into the skirt together with the high-temperature exhaust gas reacts with the CO in the exhaust gas to generate CO ₂ until substantially any of oxygen and CO is exhausted. The flue 12 is provided with a gas take-up probe 13 for taking in the analysis inhalation gas, and is connected to the gas-taking probe 13 to provide CO, CO ₂ , O _{2 ,} etc. in the intake gas taken by the probe 13 by the gas. The gas composition is analyzed by a gas analysis device 14. That is, it is configured such that the gas composition of the suction gas taken by the gas taking probe 13 is continuously or intermittently measured by the gas analyzer 14 . Further, there is a case where a steam boiler is provided in a portion through which a high temperature gas such as the flue 12 passes.

In the case where the exhaust gas treatment apparatus recovers the high-temperature exhaust gas as the fuel gas, the operation of the exhaust gas treatment apparatus is generally to minimize the height of the skirt and to minimize the air taken in. However, since the rate of generation of CO gas during the deodorization treatment is small (the amount of generation is small), recovery of the fuel gas is not normally performed, and thus the height of the skirt can be arbitrarily set. Therefore, for example, when the flue 12 has the function of a steam boiler, it is desirable to raise the skirt 11 to actively suck in air, thereby burning the CO gas in the exhaust gas, and recovering the heat of combustion (thermal energy) as a high pressure. Vapor. On the other hand, the flue 12 does not have the function of a steam boiler. In order to suppress the suction of air, the position of the skirt 11 can be lowered to operate the exhaust gas processing apparatus, whereby the heat load of the flue 12 or the like can be reduced.

The composition of the intake gas taken in by the exhaust gas treatment device is completely different depending on the characteristics of the exhaust gas treatment device or its operating conditions. In other words, when the position of the skirt 11 is lowered and the amount of intake of air is suppressed, the concentration of CO and CO ₂ is relatively high, and when the skirt 11 is raised and the air is actively taken in, it is from a converter type refining furnace. The rate of generation of the CO gas of 1 is not more than a certain ratio (specifically, about 30 vol% or more) with respect to the gas suction rate of the exhaust gas treatment equipment, and only CO ₂ is detected in the analysis of the intake gas, and CO is not detected. The reason for this is that CO in the exhaust gas is burned by O ₂ in the air taken in. Further, the concentration of CO and CO ₂ in the inhaled gas is relatively low due to dilution with N ₂ . In addition, FIG. 1 shows an example of an operation of reducing the skirt 11 and suppressing the inhalation of air after confirming the ignition of the exhaust gas after the start of the dislocation process using the exhaust gas treatment device having the gas recovery function but not including the steam boiler. .

Further, the discharge speed of carbon in the exhaust gas shown in FIG. 1 described above is based on the intake gas measured by the gas take-up probe 13 disposed in the flue of the exhaust gas treatment device shown in FIG. The CO gas concentration, the CO ₂ gas concentration, and the gas suction speed of the exhaust gas treatment equipment (the intake gas flow rate in the standard state) were calculated. In the measurement of the gas composition of CO and CO ₂ , an infrared absorption type analyzer is usually used. However, compared with the measurement of the gas flow rate, the measurement method is slow in response speed, and a delay time of about 10 seconds to several tens of seconds is generated. Time is applied to the correction. Further, in order to prevent leakage of the exhaust gas, the amount of gas suction in the exhaust gas treatment device is larger than the amount of gas generated in the spinner type refining furnace in the decarburization treatment or the decarburization treatment, so that air is taken in from the furnace mouth and the skirt. At least a portion of the produced CO is oxidized to become CO ₂ .

Therefore, in order to determine the end time of the dislocation treatment, the following method is effective: the exhaust gas is obtained by measuring the CO concentration and the CO ₂ concentration in the inhaled gas, and the suction speed of the exhaust gas treatment equipment (flow rate in the standard state). The rate of carbon discharge. However, according to the experimental results of the inventors and the like, as long as the operating conditions (skirt height, mouth pressure, etc.) of the converter exhaust gas treatment apparatus are fixed, the discharge speed of carbon in the exhaust gas can be replaced instead of the inhaled gas. Any one of the CO gas concentration, the CO ₂ gas concentration, and the total concentration of the CO gas and the CO ₂ gas becomes a maximum value, and the fluctuation pattern which is increased again after the minimum value is determined, and the time point at which the dislocation process is completed is determined. In this case, if the amount of air to be inhaled is relatively small and the operating conditions of the CO remaining in the inhaled gas are determined, the time point at which the dislocation process is completed can be determined based on the variation pattern of the total concentration of the CO gas and the CO ₂ gas. When the amount of air to be sucked hardly changes and the CO ₂ gas concentration is stable, the time point at which the dislocation process is completed may be determined based on the fluctuation pattern of only the CO gas concentration. Further, if the amount of air to be sucked is sufficiently large, and the operating conditions in which the CO in the exhaust gas is completely combusted, the time point at which the dislocation process is completed may be determined based on the fluctuation pattern of only the CO ₂ gas concentration.

Further, instead of the discharge speed of carbon in the exhaust gas, the flicker is determined based on the fluctuation pattern calculated from the product of the CO gas concentration and/or the CO ₂ gas concentration in the inhaled gas and the flow rate of the inhaled gas. The point in time at which the processing ends. The fluctuation pattern is a fluctuation pattern in which the flow rate of the CO gas in the intake gas, the flow rate of the CO ₂ gas, and the total flow rate of the CO gas and the CO ₂ gas become maximum values, and becomes a minimum value and then increases again. In this case, if the amount of air to be sucked is relatively small and the operating conditions of the CO remaining in the intake gas are determined, the time point at which the dislocation process is completed can be determined based on the fluctuation pattern of the total flow rate of the CO gas and the CO ₂ gas. When the amount of air to be sucked hardly changes and the flow rate of the CO ₂ gas is stable, the time point at which the dislocation process is completed may be determined based on the fluctuation pattern of the flow rate of only the CO gas. Further, if the amount of air to be sucked is sufficiently large, and the operating conditions in which the CO in the exhaust gas is completely combusted, the time point at which the dislocation process is completed may be determined based on the fluctuation pattern of the flow rate of only the CO ₂ gas.

In addition, the suction gas flow rate is the height of the skirt before and after the dislocation treatment As a result of the adjustment and the large change (see Fig. 1), as described above, in the measurement of the flow rate of the suction gas and the analysis of the gas composition, since the response speed is different, if the flow rate of the suction gas greatly changes, the exhaust gas is measured. The change in the carbon discharge rate becomes a cause of error. Therefore, when determining the maximum value and the minimum value, it is preferable to operate so as not to greatly vary the flow rate of the intake gas before and after the maximum value and the minimum value. The height of the skirt is usually controlled in such a manner that the suction pressure of the gas is kept constant, or is controlled in such a manner that the height of the skirt is kept constant. In these cases, the fluctuation of the flow rate of the suction gas becomes a maximum value and a minimum value. The level is not a problem.

As described above, it may be based on the discharge speed of carbon in the exhaust gas or the CO gas concentration in the suction gas, the CO ₂ gas concentration, the total concentration of the CO gas and the CO ₂ gas, the CO gas flow rate, the CO ₂ gas flow rate, and the CO gas. When any of the measured values of the total flow rate of the CO ₂ gas is a maximum value and becomes a minimum value and then increases again, the end time of the dislocation process is determined, and the difference between the maximum value and the minimum value is a maximum value. In the case of 10% or more, it is preferable to determine the maximum value and the minimum value in the above-described variation pattern. The reason is that if the difference is less than 10%, a slight variation caused by fluctuations in the flow rate of the intake gas or the like is mistaken for the variation pattern related to the molding phenomenon of the slag, and therefore, slight fluctuations are ignored to reliably prevent erroneous detection. . Further, when the difference between the maximum value and the minimum value of the measured value is less than 10% of the maximum value, it is preferable to ignore the maximum value and the minimum value in the fluctuation pattern, and to continue monitoring the fluctuation of the measured value. The variation pattern until the maximum value and the minimum value become 10% of the maximum value appears.

In addition, the end time of the dislocation treatment is desirably such that it becomes extremely small as described above. The fluctuation value that is increased again after the value is within a predetermined elapsed time range based on the time point at which the maximum value is equal to or greater than the value of the predetermined ratio of 90% or more and 150% or less. The reason why the value of the predetermined ratio with respect to the maximum value is the threshold value is that the region which is increased again after the minimum value of the fluctuation pattern is formed by the formation of the slag, and the slag height in the furnace is sharply increased. In the region where the maximum value is less than 90%, the slag discharge rate is insufficient due to insufficient slag formation, and on the other hand, more than 150% of the area is before the end of the dislocation treatment. The slag overflows from the furnace mouth and hinders the operation. In addition, the time point of the completion of the dislocation process may be a time point itself in which the fluctuation value is equal to or greater than the threshold value, and it may be several tens of seconds required for the operation or the operation of the device to be performed from the time point. After that, there will actually be no obstacles. The above elapsed time is preferably set in the range of 0 second to 50 seconds, and more preferably in the range of 0 second to 30 seconds. If it is in the range of the above-mentioned time, the deodorization treatment can be completed, and the intermediate slag discharge can be sufficiently performed without excessively molding and hindering the operation.

In addition, the end time point of the dislocation process determined in the above manner can be set to The time point at which any of the above-described measured values is equal to or greater than the value at which the maximum value is a predetermined ratio may be a time point at which a predetermined processing time elapses from the time point. However, in the latter case, it is desirable to set it to any of the above measured values relative to the maximum value. The dislocation treatment was terminated at a time point of more than 150%.

Further, in addition to the method determined based on the fluctuation pattern as described above, the method of determining the end time of the dislocation treatment is also effective as follows: the discharge rate of carbon in the exhaust gas or the concentration of CO gas in the inhaled gas, CO _{2, based on the time point at which the} gas concentration, the total concentration of the CO gas and the CO ₂ gas, the CO gas flow rate, the CO ₂ gas flow rate, and the total flow rate of the CO gas and the CO ₂ gas are equal to or greater than a predetermined threshold value, For the specified elapsed time range. Here, the elapsed time from the time when the measured value is equal to or greater than the threshold value to the end time of the dislocation processing is preferably in the range of 0 second to 50 seconds, and more preferably set to 0 second. Within the range of ~30 seconds.

Among them, in the case of these methods, it must be in the middle of the dislocation process. The portion of the segment, for example, the maximum value observed on the fluctuation pattern of the carbon discharge rate shown in Fig. 1, is not recognized as the above-described measured value being equal to or greater than the threshold value. For this reason, it is desirable to end the supply of 1.2 times the amount of stoichiometrically necessary oxygen calculated from the Si concentration in the molten iron before the deodorization treatment and the Si concentration in the molten iron after the target deodorization treatment. Preferably, after the time point of 1.5 times of oxygen, the time when the measured value is equal to or greater than a predetermined threshold value is set. Further, the threshold value may be a value obtained empirically, and may be a value calculated by using a variable such as a molten iron temperature or a Si concentration in the molten iron, and is preferably a slagging condition in consideration of intermediate slag discharge or It is determined by the actual results of the subsequent dephosphorization treatment.

Here, the exhaust gas treatment equipment has a row of a recovery converter type refining furnace When the gas is used as the fuel gas and the flue gas is included in the exhaust gas, and the exhaust gas processing equipment is operated such that at least a part of the CO gas in the exhaust gas is actively burned in the degassing process. For survey based When the threshold value described above determines the end time of the dislocation process, specifically, the slag discharge property of the slag is determined based on the CO gas concentration in the intake gas at the end time of the dislocation process.

In this example, the slagging property of the slag slag is melted before the dislocation treatment The concentration of bismuth in the iron and the mass of the slag slag generated are substantially fixed, and the amount of the slag slag discharged from the converter type refining furnace is evaluated under the condition that the tilting angle of the converter type refining furnace at the time of slag removal is fixed. . Specifically, the mass of the slag slag which is discharged to the slag receiving container which is directly under the converter-type refining furnace is 50% or more of the mass of the slag slag which is present in the furnace, and is evaluated as "slag discharge". In the case where the above value is 30% or more and less than 50%, the case where the above value is less than 30% is evaluated as "poor slag discharge".

Figure 4 shows the results of the above investigation. According to the figure, the dislocation process ends. The higher the concentration of the CO gas in the inhaled gas, the higher the slag discharge property of the slag, and the slag discharge does not occur when the CO gas concentration in the suction gas at the end of the devolatilization treatment is 6.0 vol% or more. bad". In other words, it is preferable that the end of the dislocation treatment can be determined based on the CO gas concentration in the inhalation gas in the deodorization treatment, and in this case, the value of the CO gas concentration in the inhaled gas is set to 6.0 vol% or more. Determine the threshold for the end time of the dislocation process.

In addition, the refining furnace such as the converter type refining furnace is a batch type of molten iron in the furnace. Or the refining of the molten steel, the refining of one unit is referred to as "heating", and the number of furnaces in Fig. 4 indicates the number of times. Further, in the untwisting treatment, since the amount of generated exhaust gas is small, the concentration of CO gas in the suction gas is changed by the air taken in between the furnace mouth of the converter type refining furnace and the skirt, and the suction gas is changed. The threshold value of "CO gas concentration is 6.0 vol% or more" is to make the operating conditions (oxygen supply amount, skirt) Height, mouth pressure, etc.) Obtained under fixed operating conditions. Therefore, as long as it is under this condition, it can be fully used as a threshold.

Further, a value suitable as the threshold value is also set according to the exhaust gas treatment The operating conditions such as the inhalation capacity and the operating conditions such as the oxygen supply rate differ. The exhaust gas treatment device has a function of recovering the exhaust gas of the converter type refining furnace as a fuel gas, and when the air is actively sucked in the deodorization treatment to burn the CO in the exhaust gas, the end time of the dislocation process is determined. The threshold value is preferably such that the concentration of the CO gas in the inhaled gas is in the range of 2.0 vol% or more and 18.0 vol% or less, and the appropriate value is empirically selected based on the actual results of the slagging rate or the slag discharge time during the intermediate slag discharge. And use. When the threshold value of the CO gas concentration is 2.0 vol% or more, even if only the measured value of the CO gas concentration is used instead of the fluctuation pattern of the carbon discharge rate in the exhaust gas, it is possible to reduce misidentification when the molding of the slag is sufficiently promoted. On the other hand, when the threshold value of the CO gas concentration is 18.0 vol% or less, it is possible to prevent the slag from being excessively molded and to prevent the operation, and to accurately determine the molding state for efficiently performing the slag discharge.

As described above, the molding condition of the slag in the deodorization treatment and the end of the dislocation The appropriate timing for starting the intermediate slag discharge can be determined only based on the information obtained by the inhalation gas, and the necessary oxygen amount, the exhaust gas temperature, and the slag in the deodorization blowing obtained by calculating the operating conditions obtained in advance can also be determined. Information such as the discharge state from the furnace mouth, the outflow condition of the slag from the tapping port, the change in the vibration of the spray gun or the sublance, the change in the sound during the oxygen blowing, or the estimation technique of the slag grade obtained by the known method. The combination is performed such that the above determination conditions are more accurate.

Next, the method of preparing the molten iron of the present invention will be specifically described by taking the case of using the converter type refining furnace shown in Fig. 3 as an example.

First, as shown in Fig. 5 (a), the molten iron preparation method of the present invention is carried out in a converter type refining furnace 1 in which a cold iron source 8 such as iron filings is loaded, via a loading pot 15, and is not subjected to untwisting. The treatment and dephosphorization treatment, that is, the molten iron 5 before the preliminary treatment (the molten iron charging step).

Next, as shown in FIG. 5(b), the molten iron 5 in the converter type refining furnace 1 is supplied with an oxygen-containing gas or an oxygen-containing gas and iron oxide as an oxygen source, and is subjected to a deodorization treatment (deintercalation treatment step). At this time, the ruthenium contained in the molten iron 5 reacts with the supplied oxygen to carry out a depurination reaction (Si + O ₂ → SiO ₂ ). The molten iron temperature rises due to the heat of combustion of the crucible obtained by the decoupling reaction, and the melting of the cold iron source 8 in the molten iron is promoted.

Then, as the above-described decarburization treatment progresses, the concentration of ruthenium in the molten iron is gradually lowered to generate CO gas, and if the deodorization treatment is further performed, the amount of generation of the slag slag is increased, and the temperature of the molten iron is increased, and the slag is slag. The composition and physical properties also change, and the slag is formed by the generated CO gas.

Here, the degreasing slag 6 produced by the above-described decarburization treatment prevents the phosphorus transition from the decomposition of the phosphorus oxide (P ₂ O ₅ ) contained in the dephosphorization slag 7 described below during the deodorization treatment. To the molten iron 5 (this phenomenon is referred to as "rephosphorization"), it is preferable to set the alkalinity ([CaO (% by mass)) / [SiO ₂ (% by mass))) after the completion of the deuteration treatment. It is 0.80 or more. Further, the upper limit of the alkalinity of the slag slag 6 is not required to be specified in terms of the detachment reaction. However, a high alkalinity means that the ratio of CaO to SiO ₂ produced is high, and since the amount of the slag slag 6 is increased, the upper limit is preferably set to about 1.50. More preferably, it is less than 1.30, and further preferably less than 1.20.

Further, adjusting the alkalinity of the slag slag 6 to the above range can be carried out by adding a flux such as a CaO-based flux to the furnace before the mashing treatment and the mashing treatment. In the case where the CaO-based flux is added in the initial stage of the deuterium treatment, when the alkalinity of the de-slagging slag 6 is the lowest, the time at which the amount of SiO ₂ generated is the highest, the deodorization treatment is completed. When the alkalinity at the end is 0.80 or more, the alkalinity in the previous deodorization treatment must be 0.80 or more. In addition, when the alkalinity at the end of the deodorization treatment is 0.80 or more by CaO contained in the dephosphorization slag 7 of the previous heat remaining in the furnace, it is basically unnecessary to add CaO-based assistance. Flux such as flux.

Further, the method of adding the CaO-based flux may be a method of introducing a granular or bulk flux from a hopper on a furnace or a method of feeding a powdery flux through the top-blowing lance 2, and is not particularly limited.

Further, the alkalinity of the slag slag 6 can be calculated based on the following formula (1): alkalinity = [(the amount of residual CaO in the furnace (kg / molten iron - t)) + (the amount of added CaO in the deodorization treatment (kg /Fused iron-t))]/[(Amount of residual SiO ₂ in the furnace (kg/melt-t)) + (Amount of SiO ₂ produced during the deodorization treatment (kg/melt-t))]... ).

Further, the amount of SiO ₂ generated in the deodorization treatment in the above formula is calculated from the change in the concentration of rhodium in the molten iron before and after the deodorization treatment.

Further, the oxygen source used for the delining treatment may be only the oxygen gas 9 supplied from the top blowing lance 2, and in addition to the above oxygen gas 9, iron oxide (not shown) may be used in combination. However, in view of the fact that one of the objects of the present invention is to melt the cold iron source 8 in a large amount, the iron oxide which absorbs heat during temperature rise and decomposition is not preferable. Therefore, it is preferred to avoid the use of iron oxide as an oxygen source.

In addition, one embodiment of the present invention is a variable pattern which is obtained by measuring the discharge rate of carbon contained in the exhaust gas discharged from the converter-type refining furnace in the degassing process, and becomes a maximum value and becomes a minimum value and then increases again. , decided to dislocate Since the time point is ended, it is preferable to perform an operation of keeping the supply rate of the oxygen source as constant as possible while the discharge speed of the carbon is a maximum value and a minimum value. Further, the usual operation is to complete the dislocation treatment in a short time, so that the operation of increasing the oxygen supply rate or the introduction of the iron oxide is performed at the initial stage of the deodorization treatment, and thereafter, the oxygen supply rate is kept constant and stably stabilized. The maximum value and the minimum value of the carbon discharge rate were measured.

After the molten iron is subjected to the dislocation treatment in the above-described manner and the determination is completed, the top-blowing lance 2 is immediately raised to complete the dislocation treatment, but the end time of the dislocation treatment is usually necessary after the above-mentioned determined time point. After several tens of seconds required for the operation or operation of the device or the like. Immediately thereafter, as shown in Fig. 5(c), the converter type refining furnace 1 is tilted to the side opposite to the side on which the tap hole 4 is provided, so that the low alkalinity containing a large amount of SiO ₂ generated during the decanting treatment is removed. The crucible slag 6 is discharged from the mouth of the converter type refining furnace 1 (intermediate slagging step). Further, in Fig. 5(c), the molten iron after the disintegration treatment is indicated as 5a in order to distinguish it from the molten iron 5 before the deodorization treatment.

The slag discharge rate of the slag slag 6 in the intermediate slag removal step is preferably set to be efficiently dephosphorization in a dephosphorization treatment step using a small amount of the CaO-based flux. 30% by mass or more. Further, in order to make the total amount of the CaO-based flux used between the self-melting iron preparation treatment and the decarburization refining step in the next step smaller than the conventional method in which the intermediate slag removal step is not performed, it is more preferable to stably ensure the quality of 50%. Slag discharge rate above %.

However, when the untwisting slag 6 is discharged by more than 80% by mass, the slag of the newly added CaO-based flux in the dephosphorization treatment step in the next step is impaired, and the dephosphorization reaction is hindered. Therefore, in the present invention, it is preferable to control the slag discharge rate of the slag slag 6 to be in the range of 50% by mass to 80% by mass.

Further, the above slag discharge rate is defined by the following formula (2).

Slag discharge rate (% by mass) = (discharge slag quality) × 100 / [(quality of slag produced in the de-slipping treatment step) + (residual slag quality of the previous heat)] (2)

After the middle slag is discharged, as shown in FIG. 5(d), a CaO-based flux or an oxygen source is supplied to the molten iron 5a after the deodorization treatment remaining in the converter type refining furnace to perform dephosphorization treatment (dephosphorization treatment step). . Here, in the dephosphorization treatment step, the alkalinity of the dephosphorization slag 7 generated in the furnace is preferably adjusted to a range of 1.2 to 3.0. The reason for this is that if the alkalinity is 1.2 or more, the dephosphorization ability of the slag is in an appropriate range, and the phosphorus concentration in the molten iron can be lowered by a small amount of slag. On the other hand, when it is 3.0 or less, the dephosphorization reaction can be carried out without impairing the slag of the CaO-based flux and making the fluidity of the slag an appropriate range.

Here, the oxygen source used in the dephosphorization treatment step may be the same as the deodorization treatment, and the oxygen gas 9 from the top-blowing lance 2 is mainly used, and part of the iron oxide is used. However, one of the objects of the present invention is to expand the use of the cold iron source 8. Therefore, it is preferable to use a small amount of iron oxide which absorbs heat when heating or decomposing as much as possible. For this reason, it is preferable to adjust the T.Fe concentration of the dephosphorization furnace slag 7 by appropriately controlling the oxygen supply conditions, the height of the lance, and the like, and to promote the slag formation of the CaO-based flux without using the iron oxide.

Further, as the CaO-based flux used in the dephosphorization treatment, quicklime (CaO), limestone (CaCO ₃ ), hydrated lime (Ca(OH) ₂ ), or the like can be used. However, it is not limited to these. For example, a CaO-based flux containing 40% by mass or more of CaO and optionally containing other components such as fluorine or alumina may be used. Further, the method of adding the above-mentioned CaO-based flux is the same as the case of adding a CaO-based flux in the deodorization treatment, and it is possible to introduce the granular and bulk flux from the hopper on the furnace or to pass the powdery flux through the top-blowing lance. There is no particular limitation on the method of 2 inputs.

In the dephosphorization treatment step, phosphorus in the molten iron is oxidized by oxygen in the supplied oxygen source to become phosphorus oxide (P ₂ O ₅ ), and is taken into slag by the CaO-based flux. and as a dephosphorizing agent functioning dephosphorization slag formation 7 become stable form of the compound (3CaO.P ₂ O _5), whereby the dephosphorization of molten iron 5a. In addition, the end of the dephosphorization treatment may be performed when the dephosphorization treatment time has elapsed for a predetermined period of time or when the dephosphorization reaction is performed to lower the phosphorus concentration of the molten iron 5a to a predetermined value or less.

After the dephosphorization treatment is completed, as shown in FIG. 5(e), the converter type refining furnace 1 is tilted in a direction opposite to the direction in which the slag is discharged in the middle, and the molten iron 5b in the converter type refining furnace 1 is passed through the tap hole 4 It flows out to a molten iron holding container (not shown) (ironing step). Further, in Fig. 5(e), the molten iron after the dephosphorization treatment is shown as 5b in order to distinguish it from the molten iron 5a after the deodorization treatment. After the iron discharge of the molten iron 5b is completed, as shown in Fig. 5 (f), the converter type refining furnace 1 is reversed so that the furnace opening faces upward, and the iron discharge is completed.

Further, in the above-described tapping step, the dephosphorization slag 7 is mixed in the molten iron 5b flowing out of the tap hole 4 and flows out, and the outflow of the dephosphorization furnace slag 7 is unavoidable, but is small, and is intended The discharge of the following dephosphorization slag discharged is clearly distinguished. Therefore, a small amount of molten iron 5b (not shown) and almost all of the dephosphorization slag 7 which have not flowed out remain in the converter type refining furnace 1 after the completion of the tapping.

Further, since the dephosphorization furnace slag 7 has a high concentration of phosphoric acid, if it is directly used as the slag of the next heat, the phosphorus concentration in the molten iron is increased. Therefore, all of the dephosphorization furnace slag was previously discharged after the above-described tapping step was completed. However, when all of the dephosphorization furnace slag is discharged, the amount of the slag-forming material used to generate the slag slag required for the deodorization treatment of the next heat is increased, resulting in an increase in the cost of the auxiliary material. Therefore, in the present invention, it is preferred to adjust the refining conditions in the deodorization treatment to prevent the re-phosphorization derived from the dephosphorization furnace slag, and to perform the dephosphorization treatment after the intermediate slag discharge. After the iron flows out, 30% by mass or more of the dephosphorization slag remaining in the converter type refining furnace remains in the furnace, and is used as a part of the raw material of the slag slag of the next heat. The residual dephosphorization slag is more preferably 50% by mass or more.

Further, the slag discharging method in which the dephosphorization slag remains as described above may be the same as the slag slag discharging method which is generally performed, and when the furnace body after the iron tapping is tilted toward the opposite side of the tap hole and the slag is discharged from the furnace port, A method of adjusting the tilting angle so that a part of the slag remains in the furnace. Further, in order to make the fluidity of the dephosphorization slag suitable for slag discharge, it is preferable to adjust the alkalinity of the slag after the dephosphorization treatment to a range of 1.2 to 1.8, and to adjust the concentration of the iron oxide to 10% by mass or more.

Then, when all of the dephosphorization slag 7 remains in the furnace of the converter type refining furnace 1, and when a part of the dephosphorization slag 7 is discharged from the converter type refining furnace 1 after the above-described tapping step, As shown in Fig. 5 (f), after the converter type refining furnace 1 is in an upright state, a small-sized cold iron source 8 is charged into the converter type refining furnace 1 from the hopper on the furnace, or the converter is tilted several times forward and backward. In the refining furnace 1, the dephosphorization slag 7 remaining in the furnace is solidified (the dephosphorization slag solidification step), whereby the molten iron 5b remaining in the furnace is solidified. The dephosphorization slag solidification step is for preventing the clogging of the bottom air outlet 3 due to the dephosphorization slag 7 and the molten iron 5b remaining in the furnace flowing into the bottom of the bottom air outlet 3, preferably at least the dephosphorization slag 7 and the molten iron 5b is solidified and solidified before being solidified, and the bottom blowing gas 10 is ejected from the bottom blowing port 3. Here, in the case where the bottom blowing gas is continuously supplied, this step can also be omitted.

After the dephosphorization slag solidification step, the molten iron charging step shown in Fig. 5 (a) is returned again, and the next heat treatment and dephosphorization treatment are carried out in accordance with the above steps.

According to the invention described above, a converter-type refining furnace can be used, and the de-slipping treatment and the dephosphorization treatment are continuously performed through the intermediate slag, so that the ginseng can be replaced by The heat loss caused by the refining vessel is effectively used as a heat source for melting the cold iron source. Further, according to the present invention, the concentration of the gaseous substance containing carbon atoms in the intake gas (exhaust gas) sucked into the exhaust gas treatment equipment of the converter type refining furnace determines the end time of the dislocation treatment, so that it is always possible to take off When the slag has a small specific gravity and a high fluidity, the slag is discharged in the middle, so that the slag discharge rate of the slag can be stably increased.

Further, in the above description of the present invention, the following description has been made on the use of a converter type refining furnace in which the discharge of the degreasing furnace slag after the deodorization treatment (hereinafter also referred to as "intermediate slag discharge") is applied to the molten iron. In this case, it is preferable to use two or more converter type refining furnaces for the refining of the molten iron 5, and at least one converter type refining furnace 1 is used in the present invention. The molten iron is pretreated, and the molten iron of the molten iron preparation process of the present invention is subjected to decarburization refining by at least one of the remaining converter type refining furnaces.

[Example 1]

The following molten iron preparation process is carried out: using a converter-type refining furnace having a capacity of 300 tons as shown in FIG. 3, the refining oxygen is blown to the 300-ton molten iron accommodated in the furnace from the top-blowing lance, and is set at the bottom of the furnace. The bottom air outlet blows nitrogen for stirring into the molten iron, and the molten iron is subjected to a de-corrosion treatment and a dephosphorization treatment. The molten iron preparation process specifically includes the following steps: after the iron filings are placed in the converter type refining furnace shown in FIG. 3, the molten iron is charged, and the quicklime as a CaO-based flux is added as needed, and is supplied from the top blowing lance. After the oxygen is removed, a part of the slag is discharged, oxygen is supplied from the top blowing lance, and quicklime is added as a CaO-based flux to perform dephosphorization treatment, and then the molten iron is discharged, and thereafter, the dephosphorization slag is completely discharge.

In addition, the molten iron for performing the above molten iron preparation treatment is used at a temperature of 1250 ° C~ The molten iron at 1320 ° C, the cerium concentration is 0.20% by mass to 0.55% by mass, and the phosphorus concentration is about 0.12% by mass. In addition, the target cerium concentration in the molten iron after the above-described deodorization treatment is set to 0.03 mass%, and the target alkalinity of the slag slag after the completion of the devolatilization treatment is set to a range of 0.6 to 0.9. Further, the target molten iron temperature at the end of the above-described deodorization treatment is set to 1300 ° C to 1340 ° C, and the control is performed by fixing the iron filing amount to 80 kg/t, depending on the temperature of the molten iron to be charged. At the initial stage of the deodorization treatment, a cold iron source and/or iron ore which is a cooling material or a carbon material and/or ferroniobium which are heat sources are added, and the amount of addition and the amount of oxygen supplied are adjusted.

In addition, in the dephosphorization treatment to be carried out, the amount of quicklime used is adjusted so that the alkalinity of the dephosphorization slag is in the range of 1.6 to 2.0 based on the amount of the slag remaining in the furnace and the estimated value of the composition.

The above molten iron preparation process determines the end time point of the dislocation treatment by the following two methods.

. In the first method, the oxygen required for the enthalpy concentration in the molten iron to be 0.03 mass% calculated from the enthalpy concentration in the molten iron before the preliminary treatment and the deuterium oxygen efficiency (empirical value) corresponding to the concentration is calculated. The current method of time points (comparative example).

. In the variation pattern of the discharge rate of the carbon in the exhaust gas, as shown in FIG. 1, the time at which the discharge rate of the carbon which rises again after the maximum value and the minimum value is equal to or greater than the maximum value is determined as a reference. When the carbon discharge rate is in the range of 100% or more and 150% or less with respect to the maximum value after the lapse of the lapse of the lapse of the lapse of the lapse of the lapse of the lapse of the lapse of the lapse of the lapse of the lapse of the lapse of the lapse of the lapse of the lapse of the lapse of the lapse of the above-mentioned time,

Further, in either method, the oxygen supply rate from the top blowing lance was fixed to 30,000 Nm ³ /hr, and nitrogen gas was blown from the bottom at a blowing speed of 1200 Nm ³ /hr.

In addition, the dislocation treatment will be determined by the above methods 1 and 2 The molten iron preparation process at the end time point was carried out for several tens of times in each method, and the slag discharge rate of the slag slag in the intermediate slag of each method was compared.

At this time, the slag discharge (intermediate slag discharge) of the slag slag after the completion of the devolatilization treatment is started immediately after the completion of the devolatilization treatment, and the top blasting gun is raised and then the furnace body is tilted, and the furnace is not hindered from being placed in the furnace. The slag on the moving trolley is subjected to the slag, and the tilting angle of the converter-type refining furnace is increased as much as possible within the range in which the molten iron flows out. The end of the intermediate slag is set to be usable by the weighing value. When the amount of slag is sufficient, it is difficult to cause only the slag to flow out without flowing the molten iron, and the slag discharge time becomes any one of the longest time allowed for the operation. In addition, the slag discharge rate of the slag slag in the middle slag is a mass measuring device that measures the slag of the slag pot by a weighing device provided on a moving trolley provided under the furnace, according to the following (2) Determined by the formula.

Slag discharge rate (% by mass) = (discharge slag quality) × 100 / [(quality of slag produced in the de-slipping treatment step) + (residual slag quality of the previous heat)] (2)

The above results are shown in Table 1. In the conventional method (comparative example) in which the concentration of Si in the molten iron is determined to be the end of the dislocation treatment, the slag discharge rate of the slag after the deodorization treatment is in the range of 20% by mass to 70% by mass. The deviation and the average slagging rate are 37% by mass. On the other hand, in the example of the present invention in which the release of the carbon in the exhaust gas is determined, the slag removal rate of the slag after the mashing treatment is 50. In the range of %% to 80% by mass, the average slagging rate is 67% by mass, and the slagging rate of 50% by mass or more can be stably achieved. Further, it is understood that the amount of the slag-forming material added during the dephosphorization treatment can be drastically reduced by increasing the slagging rate of the intermediate slag.

In addition, FIG. 6 shows the relationship between the time required for the intermediate slag discharge and the Si concentration in the molten iron before the preliminary treatment, and it is understood that in the example of the present invention, the operation required for the intermediate slag discharge is required. The room can be greatly shortened to less than 10 minutes. By shortening the slag discharge time, the processing pitch of the molten iron preparation process can be shortened to a level substantially equal to the processing pitch of the decarburization treatment in the next step, so that the molten iron preparation process can be performed on almost all of the molten iron.

[Embodiment 2]

The molten iron after the dephosphorization treatment is discharged, and the molten iron in the next heat is placed in the furnace, and the alkalinity of the slag in the deodorization treatment is controlled to be in the range of 0.9 to 1.2. In the same manner as the invention example in which the desorption treatment was determined by the method 2 of the first embodiment, the molten iron preparation was carried out continuously for 10 times in order to prevent the re-phosphorization derived from the dephosphorization slag. Processed experiments.

As a result, the amount of the CaO-based flux (lime) used in the deodorization treatment can be greatly reduced from 6.0 kg/molten iron t (refer to Table 1) in the case of the inventive example of Example 1 to 2.0 kg/ Fused iron t. In addition, the dephosphorization slag in which the previous heat is left in the furnace hardly adversely affects the amount of quicklime used in the dephosphorization treatment or the phosphorus concentration in the molten iron after the treatment, so that the above-mentioned state can be reduced. The advantage of the amount of quicklime used in the deodorization process.

[Example 3]

The same conditions as in Example 2 were carried out except that the end time of the deodorization treatment was determined by the same method as in the method 1 of the first embodiment, and the amount of quicklime used in the dephosphorization treatment was 7.0 kg/t. In the comparative example in which the molten iron preparation process was performed, a comparative experiment was carried out by changing the method of determining the end time of the dislocation process in the comparative example to the invention example of the following method 3.

Specifically, the molten iron preparation process is repeated as follows: as shown in FIGS. 5( a ) to 5 ( f ), the dephosphorization slag 7 generated in the molten iron preparation process of the previous heat is not discharged and remains in the entire The converter type refining furnace 1 is filled with iron scraps 8, and then the molten iron 5 is placed in the above-described converter type refining furnace 1, and then quicklime is added as needed, and oxygen gas 9 is supplied from the top blowing lance 2 to perform deodorization treatment. A part of the slag slag 6 is discharged, and thereafter, quicklime is added, and oxygen gas 9 is continuously supplied from the top-blowing lance 2 to perform dephosphorization treatment. In addition, the exhaust gas treatment device of the converter-type refining furnace 1 has a function of recovering the intake gas as a fuel gas, and has an intake capacity of about 90000 Nm ³ /hr to 100,000 Nm ³ /hr in a state where the skirt 11 is raised during the blowing. . Further, the flue 12 includes a steam boiler (exhaust heat boiler), and since the suction gas is not recovered during the dislocating process, the skirt 11 is raised and sucked into the atmosphere, and the CO gas in the exhaust gas is actively burned as a high pressure. Vapor for energy recovery.

At this time, the method of determining the end time of the above-described dislocation treatment is to use the same method as the method 1 of the first embodiment (comparative example) and the following method 3 (invention example) and compare them.

. Method 3: At the end of the supply of oxygen which is 1.2 times the stoichiometrically necessary amount of oxygen calculated from the Si concentration in the molten iron before the deodorization treatment and the Si concentration in the molten iron after the target deodorization treatment After the time point, the end of the deodorization treatment is determined based on the time point when the CO gas concentration in the inhaled gas is 6.0 vol% or more, and the CO gas concentration is 6.0 vol% or more after about 20 seconds from the above time point. Method for ending the dislocation treatment in the range of 18.0 vol% or less (Example of the invention)

According to the above method 1 and method 3, 100 furnaces of molten iron preparation were respectively carried out, and the average slagging rate of the slag slag in the intermediate slag after the completion of the mashing treatment and the molten iron after the molten iron preparation treatment were investigated. The phosphorus concentration in the medium is shown in Table 2.

According to the results, it is understood that the average slagging rate of the slag slag in the method 1 of determining the end time of the mashing treatment is 47% by mass by the same method as the above, and the detachment is determined by the method suitable for the present invention. The average slagging rate of the slag slag in the method 3 at the end of the treatment is greatly increased to 62% by mass, and the phosphorus concentration in the molten iron after the molten iron preparation is greatly reduced, and the deviation thereof The quasi-bias can also be greatly reduced.

Claims (15)

A method for preparing a molten iron, which is obtained by removing a molten iron from a molten iron in a converter-type refining furnace and then removing the molten iron in the furnace, and a part of the slag present in the furnace is self-rotating furnace type The refining furnace is discharged, and then a CaO-based flux and an oxygen source are supplied to the converter type refining furnace for dephosphorization treatment, and the molten iron subjected to the dephosphorization treatment is discharged from the converter type refining furnace, thereby using a converter type refining furnace. The molten iron is subjected to the above-described deodorization treatment and the above-described dephosphorization treatment, and the molten iron preparation processing method is characterized in that at least the inhaled gas sucked into the exhaust gas treatment device of the converter type refining furnace is used in the deodorization treatment. Analysis of a concentration of one or more gaseous substances containing carbon atoms, wherein the spin-type refining furnace in the above-described deodorization treatment is calculated based on an analysis value of a concentration of a gaseous substance containing carbon atoms in the inhaled gas and a flow rate of the inhaled gas The discharge rate of carbon in the discharged exhaust gas is a maximum value based on the calculated discharge rate of carbon in the exhaust gas, and becomes a minimum value again. Large variation pattern, the decision time of the end point of silicon removal process. A method for preparing a molten iron, which is obtained by removing a molten iron from a molten iron in a converter-type refining furnace and then removing the molten iron in the furnace, and a part of the slag present in the furnace is self-rotating furnace type The refining furnace is discharged, and then a CaO-based flux and an oxygen source are supplied to the converter type refining furnace for dephosphorization treatment, and the molten iron subjected to the dephosphorization treatment is discharged from the converter type refining furnace, thereby using a converter type refining furnace. The molten iron is subjected to the above-described deodorization treatment and the above-described dephosphorization treatment, and the molten iron preparation processing method is characterized in that at least the inhaled gas sucked into the exhaust gas treatment device of the converter type refining furnace is used in the deodorization treatment. The concentration of one or more gaseous substances containing carbon atoms is analyzed, wherein an analysis value based on any one of a CO gas concentration, a CO ₂ gas concentration, and a total concentration of CO gas and CO ₂ gas in the inhaled gas becomes a maximum value The change pattern that becomes a minimum value and then increases again determines the end time of the above-described dislocation process. A method for preparing a molten iron, which is obtained by removing a molten iron from a molten iron in a converter-type refining furnace and then removing the molten iron in the furnace, and a part of the slag present in the furnace is self-rotating furnace type The refining furnace is discharged, and then a CaO-based flux and an oxygen source are supplied to the converter type refining furnace for dephosphorization treatment, and the molten iron subjected to the dephosphorization treatment is discharged from the converter type refining furnace, thereby using a converter type refining furnace. The molten iron is subjected to the above-described deodorization treatment and the above-described dephosphorization treatment, and the molten iron preparation processing method is characterized in that at least the inhaled gas sucked into the exhaust gas treatment device of the converter type refining furnace is used in the deodorization treatment. An analysis of a concentration of one or more gaseous substances containing carbon atoms, wherein the analysis value based on the concentration of the CO gas in the inhaled gas, the concentration of the CO ₂ gas, and the total concentration of the CO gas and the CO ₂ gas is the same as the above The fluctuation pattern calculated by the product of the flow rate of the inhaled gas determines the end time of the dissociation process, and the fluctuation pattern is the CO gas flow in the inhaled gas. Either a flow rate of the CO ₂ gas flow rate and total flow rate of the CO gas in a CO ₂ gas becomes a maximum value variation pattern becomes the minimum value increases again. The method for preparing a molten iron according to any one of the items 1 to 3, wherein the maximum value is increased to a minimum value and then increased again. In the variation pattern, the value of the re-increase is set to a predetermined elapsed time range based on the time point at which the maximum value is equal to or greater than the value of the predetermined ratio of 90% or more and 150% or less. Inside. The method for preparing a molten iron according to any one of the first to third aspect, wherein the maximum value and the minimum value in the variation pattern which is increased to a minimum value and which is increased to a minimum value It is more than 10% of the maximum value. The method for preparing a molten iron according to the fourth aspect of the invention, wherein the difference between the maximum value and the minimum value in the fluctuation pattern which becomes the minimum value and becomes the minimum value is 10% or more of the maximum value. A method for preparing a molten iron, which is obtained by removing a molten iron from a molten iron in a converter-type refining furnace and then removing the molten iron in the furnace, and a part of the slag present in the furnace is self-rotating furnace type The refining furnace is discharged, and then a CaO-based flux and an oxygen source are supplied to the converter type refining furnace for dephosphorization treatment, and the molten iron subjected to the dephosphorization treatment is discharged from the converter type refining furnace, thereby using a converter type refining furnace. The molten iron is subjected to the above-described deodorization treatment and the above-described dephosphorization treatment, and the molten iron preparation processing method is characterized in that CO in the suction gas taken in by the exhaust gas treatment device of the converter type refining furnace in the above-described deodorization treatment The concentration of the gaseous substance containing at least one of the gas concentration and the CO ₂ gas concentration, wherein the concentration of the CO gas in the inhaled gas, the concentration of the CO ₂ gas, and the total concentration of the CO gas and the CO ₂ gas are analyzed. The analysis value of any concentration is based on the time point when the oxygen supply of 1.2 times the amount of oxygen required for the deuteration is completed, and the time is equal to or higher than the predetermined threshold value. By setting the end time of the above-described deodorization treatment to a predetermined elapsed time range, it is possible to discharge 30% by mass or more of the slag present in the furnace from the converter type refining furnace after the completion of the above-described deodorization treatment. In the manner, the above-described deodorization treatment is completed in a state where the slag is molded. The method for preparing a molten iron according to claim 7, wherein the exhaust gas treating device has a function of recovering exhaust gas of the inhaled converter type refining furnace as a fuel gas, and is combined with the exhaust gas treating device. At least a part of the CO gas in the exhaust gas is burned in the exhaust gas of the converter-type refining furnace, and the CO gas concentration in the exhaust gas after the combustion is 2.0 vol% or more and 18.0 vol% or less. The time point is the reference, and the end time of the above-described dislocation processing is set to a predetermined elapsed time range. A method for preparing a molten iron, which is obtained by removing a molten iron from a molten iron in a converter-type refining furnace and then removing the molten iron in the furnace, and a part of the slag present in the furnace is self-rotating furnace type The refining furnace is discharged, and then a CaO-based flux and an oxygen source are supplied to the converter type refining furnace for dephosphorization treatment, and the molten iron subjected to the dephosphorization treatment is discharged from the converter type refining furnace, thereby using a converter type refining furnace. The molten iron is subjected to the above-described deodorization treatment and the above-described dephosphorization treatment, and the molten iron preparation processing method is characterized in that, in the above-described deodorization treatment, in the intake gas sucked by the exhaust gas treatment device of the above-described converter type refining furnace The concentration of the gaseous substance containing at least one of the CO gas concentration and the CO ₂ gas concentration is analyzed, and the flow rate of the inhaled gas is measured, wherein the concentration of the CO gas in the inhaled gas, the concentration of the CO ₂ gas, and CO gas in the intake gas analyzer according to any one concentration value of the total concentration of CO gas and a CO ₂ gas in the product of the flow rate of the suction gas is calculated After the amount of either a flow rate of the CO ₂ gas flow rate and total flow rate of the CO gas with a CO ₂ gas in the off 1.2 times the amount of oxygen in the stoichiometric silicon necessary time point of oxygen at the end of the supply becomes more than a predetermined threshold value The time point is used as a reference, and the end time of the above-described dislocation treatment is set to a predetermined elapsed time range, whereby 30% by mass or more of the slag present in the furnace can be rotated after the completion of the above-described deodorization treatment. In the manner in which the refining furnace is discharged, the above-described deodorizing treatment is completed in a state where the slag is molded. A method for preparing a molten iron, which is obtained by removing a molten iron from a molten iron in a converter-type refining furnace and then removing the molten iron in the furnace, and a part of the slag present in the furnace is self-rotating furnace type The refining furnace is discharged, and then a CaO-based flux and an oxygen source are supplied to the converter type refining furnace for dephosphorization treatment, and the molten iron subjected to the dephosphorization treatment is discharged from the converter type refining furnace, thereby using a converter type refining furnace. The molten iron is subjected to the above-described deodorization treatment and the above-described dephosphorization treatment, and the molten iron preparation processing method is characterized in that, in the above-described deodorization treatment, in the intake gas sucked by the exhaust gas treatment device of the above-described converter type refining furnace The concentration of at least one or more gaseous substances containing carbon atoms is analyzed, and the flow rate of the inhaled gas is measured, wherein the off-gas is calculated based on an analysis value of a concentration of a gaseous substance containing carbon atoms in the inhaled gas and a flow rate of the inhaled gas The discharge rate of carbon in the exhaust gas discharged from the converter type refining furnace in the enthalpy treatment is required to end the supply and discharge at the discharge speed After 1.2 times the amount of oxygen in the stoichiometric point of time of the oxygen, becomes the predetermined threshold value as a reference point in time, the end time point of the silicon removal process is set within a predetermined elapsed time, In this way, after the completion of the above-described deodorization treatment, 30% by mass or more of the slag present in the furnace is discharged from the rotary furnace type refining furnace, and the above-described deodorization treatment is completed in a state where the slag is molded. The method for preparing a molten iron according to any one of the items 1 to 3, wherein the dephosphorization treatment of the previous heat is generated In the state where the slag remains in the furnace at 30% by mass or more, the molten iron of the next heat is charged into the converter type refining furnace to perform the deodorization treatment. The method for preparing a molten iron according to the sixth aspect of the invention, wherein the slag generated in the dephosphorization treatment of the previous heat is 30% by mass or more in the furnace, and the next heat is used. The molten iron is charged into a converter type refining furnace for deodorization treatment. The method for preparing a molten iron according to any one of the items 1 to 3, wherein the above-mentioned deodorization treatment is present in the converter The alkalinity ([CaO (mass%)) / [SiO ₂ (% by mass))) of the slag in the refining furnace is controlled to be in the range of 0.80 to 1.50. The method for preparing a molten iron according to claim 11, wherein the alkalinity ([CaO (% by mass)) / [SiO of the slag present in the converter type refining furnace at the end of the above-mentioned deodorization treatment ₂ (% by mass)]) The control is in the range of 0.80 to 1.50. The method for preparing a molten iron according to claim 12, wherein the alkalinity ([CaO (% by mass)) / [SiO of the slag present in the converter type refining furnace at the end of the above-described dehaling treatment ₂ (% by mass)]) The control is in the range of 0.80 to 1.50.