JP2001234222A - Dephosphorization method for molten pig iron - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dephosphorization method for molten pig iron which is capable of making dephosphorization efficiency higher than heretofore by utilizing waste gas characteristics which may be rapidly measured. SOLUTION: In the molten pig iron treatment of blowing a flux for refining together with carrier gas into the molten pig iron held in a refining vessel and dephosphorizing the molten pig iron, the temperature and/or CO concentration and CO2 concentration of the waste gases are successively measured during the treatment and an index to judge the period when a decarburization reaction is vigorous in accordance with the measured value is selected or determined and the supply rate of the flux for refining and/or carrier gas is changed according to the value of this index.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for dephosphorizing hot metal, and more particularly to a technique for suppressing decarburization reaction and increasing dephosphorization efficiency in so-called "hot metal pretreatment".

[0002]

2. Description of the Related Art In recent years, in a steelmaking process, in order to reduce the refining load in a converter, silicon (hereinafter referred to as Si), phosphorus (hereinafter referred to as Si) contained in the hot metal before the hot metal is charged into the converter. ,
P) is removed. This is called hot metal pretreatment, in which a refining flux mainly composed of iron oxide and / or an oxidizing gas (mainly oxygen gas) is blown into the hot metal through a lance immersed in the hot metal. In addition, Si and P contained in the hot metal are transferred to slag as oxides. At that time, Si in the hot metal has a higher affinity for oxygen than P, so that it is first preferentially oxidized, and then P is oxidized. Oxidation of this P (also called dephosphorization reaction)
Occasionally, the carbon (hereinafter C) separately contained in the hot metal is also oxidized, but the oxidation of the C (also referred to as decarburization reaction) is active when the P concentration in the hot metal decreases in the latter half of the dephosphorization reaction. Tend to be. Therefore, when the decarburization reaction becomes active,
Since the ratio of oxygen used in the decarburization reaction to the total amount of the injected solid oxidizer and oxygen gas increases, the dephosphorization efficiency decreases, and the time for dephosphorization to a low P concentration region increases. Undesired problems such as an increase in processing cost and a decrease in hot metal temperature. Therefore, the appearance of a technique for more efficiently dephosphorizing in the hot metal pretreatment has been desired.

[0003] The product of the decarburization reaction is CO
Gas or CO ₂ gas. Therefore, there is an attempt to effectively utilize information such as the temperature and components of the exhaust gas during the hot metal pretreatment for the hot metal pretreatment. For example, JP-A-2
Japanese Patent Publication No. 209411 and Japanese Patent Publication No. 7-17933 measure the CO and CO ₂ concentrations in exhaust gas sequentially, calculate the oxygen efficiency during decarburization using the measured values, and then calculate the P concentration in the hot metal. A technique for estimating is disclosed. Japanese Patent Application Laid-Open No. Sho 62-7807 proposes a technique for adjusting the carrier gas flow rate and / or the blowing depth of the flux based on the P concentration of the hot metal during the treatment to increase the dephosphorization oxygen efficiency. Incidentally, the publication states that when the P concentration in the hot metal becomes 0.03 to 0.04 mass%, the flow rate of the carrier gas is increased to obtain high dephosphorization oxygen efficiency.

[0004]

The above-mentioned JP-A-62-78
The technique described in Japanese Patent Application Laid-Open No. 07-2007 is so-called "static" in which the flow rate of the carrier gas is increased when the P concentration reaches a certain value. However, the dephosphorization reaction in the actual hot metal pretreatment involves the temperature of the hot metal,
Various factors such as the components and the stirring characteristics of the reaction vessel influence, and a desired dephosphorization efficiency cannot be obtained only by fixing the P concentration to a certain value and changing the flow rate of the carry gas. Further, Japanese Patent Application Laid-Open No. 2-209411 and Japanese Patent Publication No.
According to the technique estimated from the decarbonation efficiency described in Japanese Patent No. 17933, the P concentration in the hot metal has a large variation, and it is impossible to accurately determine whether or not the P concentration has reached an appropriate P concentration for changing the carrier gas flow rate. It was difficult. Therefore, there is no technology that has been used to improve the dephosphorization efficiency in the hot metal pretreatment by applying the information obtained by measuring the exhaust gas characteristics to the technology described in Japanese Patent Application Laid-Open No. 62-7807. Is the current situation.

[0005] In view of such circumstances, an object of the present invention is to provide a method for dephosphorizing hot metal which can utilize a characteristic of exhaust gas which can be measured quickly and which can increase the dephosphorization efficiency as compared with the prior art.

[0006]

Means for Solving the Problems The inventor has conducted intensive studies to achieve the above object and has embodied the results in the present invention.

That is, according to the present invention, in a hot metal pretreatment for blowing a refining flux together with a carrier gas into a hot metal held in a refining vessel and dephosphorizing the hot metal, the temperature and / or the CO concentration of the exhaust gas during the processing are reduced. And CO ₂ concentration are sequentially measured, and an index for judging when the decarburization reaction is active is selected or obtained based on the measured value, and the supply rate of the refining flux and / or the carrier gas is determined according to the value of the index. The method is a method for dephosphorizing hot metal, characterized in that

The present invention is also a method for dephosphorizing hot metal, wherein the index is the temperature of the exhaust gas or the sum of the CO concentration and the CO ₂ concentration of the exhaust gas.

Further, the present invention is a method for dephosphorizing hot metal, wherein the index is a carbon concentration in the hot metal calculated based on a flow rate of exhaust gas, a CO concentration, a CO ₂ concentration and a hot metal amount.

[0010] In addition, the present invention is a method for dephosphorizing hot metal, wherein a time when the decarburization reaction becomes active is defined as an inflection point in the time-dependent change of the index value.

According to the present invention, the characteristics of the exhaust gas are measured successively, and the values are used immediately or processed to obtain a judgment index for determining when to change the operating factor for suppressing the decarburization reaction. Since the operation is performed based on the index, the dephosphorization efficiency can be increased as compared with the conventional case.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the circumstances leading to the invention.

First, the inventor reviewed the conventional concept regarding the dephosphorization reaction in hot metal. Oxidation of phosphorus in the hot metal occurs in competition with elements, such as C, Si, and Mn, which coexist in the hot metal. Then, when the supply rate of oxygen to the hot metal is constant, of these elements, Si, which has the strongest affinity for oxygen,
Is first oxidized according to the following formula:

[Si] +2 [O] = (SiO ₂ ) Here, [C], [Si], [P] and [O] represent the concentrations of C, Si, P and O in the hot metal by (SiO ₂ ), (P
₂ O ₅ ) represents the concentration of SiO ₂ and P ₂ O _{5 in the} slag. [O] is supplied from an oxygen source, which is produced by refining flux (mainly C
aO and iron oxide) (solid oxygen) and gaseous oxygen in the oxidizing gas.

Thereafter, as the Si concentration in the hot metal decreases, the oxidation of C and P proceeds according to the following equation.

[C] + [O] = CO (gas) 2 [P] +5 [O] = (P ₂ O ₅ ) Further, the CO gas that has escaped from the surface of the hot metal to the atmosphere above is the oxygen and The reaction is performed according to the following formula to generate CO ₂ (gas).

CO + 1 / O ₂ = CO ₂ Therefore, the inventor has determined the temperature of the exhaust gas during the dephosphorization treatment, CO,
If you focus on the CO ₂ concentration, as explained below,
We thought that these phenomena (decarburization reaction and dephosphorization reaction in hot metal) could be traced.

At the beginning of the dephosphorization reaction, P
Since the concentration is high and the amount of oxygen used for oxidizing P is large, the amount of oxygen used for decarburization is suppressed, and the amount of generated CO gas is suppressed. Further, since the generation of CO ₂ is also suppressed, the temperature of the exhaust gas rapidly rises immediately after the start of the reaction, but thereafter, the rate of the rise becomes gentle. However, when the oxidation of P proceeds and the P concentration in the hot metal decreases, the oxidation rate of P decreases, and the amount of oxygen used for the oxidation of C increases.
As a result, the amount of generated CO gas increases, and the temperature of the exhaust gas also sharply increases. Therefore, if the oxidation rate of P decreases,
If it is possible to grasp the point at which the oxidation rate of C increases, then the conditions for blowing the inert gas, oxidizing gas and / or refining flux can be changed to suppress the accompanying decarburization amount and improve the dephosphorization efficiency. it can.

Next, in order to confirm the above idea, the inventor conducted a test in an actual hot metal pretreatment operation. As shown in FIG. 1, the test apparatus is a torpedo car that is currently frequently used as a smelting vessel in hot metal pretreatment. Then, the hot metal was held in the torpedo car, and iron oxide of the flux for refining was blown through a lance by feeding nitrogen oxide as a carrier gas and CaO as an oxygen gas as a carrier gas. From the start to the end of the blowing, the temperature, the flow rate, the CO, and the CO ₂ concentration of the exhaust gas were sequentially measured by the respective sensors and devices.

An example of the measurement result is shown in FIGS. Here, FIG. 2 shows the exhaust gas temperature, and FIG. 3 shows C in the exhaust gas.
This is a change over time of the sum of the O gas and CO ₂ gas concentrations. In each of the figures, it is clear that there is a point where the curve greatly changes (referred to as an inflection point). This inflection point is shown in FIG.
And at about the same time in FIG. This means
It is suggested that the oxidation rate of C increases as the oxidation rate of P decreases at these inflection points.

Therefore, the inventor added the data of the flow rate of the exhaust gas and the weight of the hot metal to these measured values, and calculated the C concentration in the hot metal according to the following equation. That is, the reaction amount of C was investigated over time to confirm the above suggestion. as a result,
As shown in FIG. 4, the change with time of the C concentration is not affected by the change in FIG.
An inflection point appeared at the same time as. ΔC = {(Q · C _CO + Q · CO _CO2 ) · Δt} × (1 / 22.4) × 12 × 1000 × (1 / W _HM ) × 100 (1) Equation [C] = [ C] _I- ΔC where ΔC: decarburization amount (% by mass) Q: exhaust gas flow rate (Nm ³ / min) C _CO : CO concentration in exhaust gas (volume fraction) CO _CO2 : CO ₂ concentration in exhaust gas (volume Rate) Δt: Exhaust gas analysis time (min) W _HM : Hot metal mass (ton) [C]: C concentration in hot metal (% by mass) [C] _I : C concentration in initial hot metal (% by mass) From the results and confirmation, the inventor determined the exhaust gas temperature, the sum of CO gas and CO ₂ gas, or the hot metal in determining the time during which the oxidation of P was reduced during the hot metal pretreatment and the oxidation of C was prioritized. The C concentration in the sample was used as an index. Incidentally, when the P concentration in the hot metal when the inflection point was reached was estimated from the existing regression equation for dephosphorization efficiency, it was 0.040% by mass. In addition, this is a case where the exhaust gas is sucked by the IDF with a constant suction speed, and when the exhaust gas flow rate is constant at a known value, the exhaust gas flow rate is not measured sequentially, The calculation may be performed using a known value.

By the way, considering this inflection point as the point where the rate-limiting step of the oxidation rate of P shifts from the oxygen supply rate-limiting to the liquid phase-side mass transfer rate-limiting, after the inflection point, the same stirring force as before is used. If the hot metal was agitated, the dephosphorization rate after that would be further reduced. Then, the inventor increases the stirring gas power by increasing the inert gas flow rate of the carrier gas after the inflection point, or changes the supply speed of the refining flux, or changes the oxidizing gas. The aim was to suppress decarburization at the end of dephosphorization.

In the present invention, as the inert gas,
It is desirable to use argon gas or nitrogen gas. In addition, as the oxidation source, it is best to use solid pure iron oxide in addition to oxygen gas, but sinter ore, dust, and iron oxide-containing substances such as mill scale may be used instead of pure iron oxide. .
Furthermore, when the temperature of the exhaust gas during the treatment increases, the CO / CO ₂ concentration in the exhaust gas also increases. In some cases, it is not possible to specify the exact value, and it is desirable to use both of them. In addition, in the present invention, in order to facilitate identification of the inflection point during actual operation, it is preferable to differentiate the value of each index with time and determine the magnitude of the differential value. At that time, the inflection point can be found more accurately by using a so-called “second-order differential value”.
At the inflection point, the “second-order differential value” becomes almost 0, so that the determination can be made without error. In addition, in the present invention, the smelting vessel may be any smelting vessel having a bottom, but is preferably a converter or a torpedo car.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hot metal that has just been tapped from a blast furnace and has the chemical composition shown in Table 1 is supplied to a topped car 1 shown in FIG.
And transported to a hot metal pretreatment plant equipped with a blowing device for refining flux. And the lance 4 in the hot metal 2
, And a dephosphorization flux (75% sintered ore + 25% CaO) was blown through the molten iron to perform a hot metal pretreatment operation. The carrier gas used for blowing was 6 Nm ³
/ Min oxygen gas and ³ Nm ³ / min nitrogen gas. At this time, the temperature of the exhaust gas, the concentration of CO and CO _2, and the flow rate of the exhaust gas were determined by the respective sensors 5 as shown in FIG.
Measurements were made sequentially using 6, 7, or instruments 9, 10. The blowing was performed under the conditions according to the method for dephosphorizing hot metal according to the present invention (invention example) and under the conditions not following (comparative example).

Table 2 shows the blowing conditions collectively. Inventive Example 1 continuously measures the exhaust gas characteristics, determines the inflection point, which is the point of increase in the decarburization rate, using the carbon concentration in the hot metal calculated from the measured value as an index, and determines the inflection point from then on until the end of processing The period is a case where the nitrogen flow rate and the acid feed rate were changed. Inventive Examples 2 and 3 know the inflection point with the same index as in Inventive Example 1 and change the nitrogen flow rate in Inventive Example 2 and the acid supply rate in Inventive Example 3 during the subsequent increase in the decarburization reaction. It is. Inventive Examples 4 and 5 determine that the point at which the CO ₂ concentration sharply increases at an exhaust gas temperature of approximately 500 ° C. is an inflection point. In Inventive Example 4, when the flow rate of nitrogen gas is increased thereafter, Inventive Example 5 This is the case where the subsequent acid supply rate is reduced.
In contrast to these invention examples, Comparative Example 1 was a case where the acid supply rate and the flow rate of the inert gas were blown at a constant rate even after the inflection point, and Comparative Example 2 was a conventional method in which This is a case where the time when the P concentration becomes 0.04% by mass is estimated, and the nitrogen gas flow rate is increased after that time. In each case, the processing was interrupted at the time of the inflection point and the point of change, and sampling was performed.

[0026]

[Table 1]

[0027]

[Table 2]

Table 3 summarizes the results of the operation. From Table 3, Invention Example 1 in which the processing conditions were changed at the inflection point of the index calculated from the exhaust gas temperature and the CO and CO ₂ concentrations in the exhaust gas,
In Comparative Examples 1 and 2 in which the flow rate of the inert gas was constant from the start to the end of the treatment, the amount of decarburization at the end of the treatment was suppressed, and the deoxygenation efficiency at the end of the treatment was improved. It is clear that Further, as in Invention Examples 4 and 5, even when the flow rate of the inert gas is increased using the exhaust gas temperature and the CO ₂ concentration as indices, an improvement in the dephosphorization oxygen efficiency in the final stage can be similarly seen. On the other hand, in Comparative Example 2 in which the treatment conditions were changed at a certain P concentration value, the decarburization amount at the end of the treatment was small, but the dephosphorization oxygen efficiency was reduced. The dephosphorization oxygen efficiency is defined by the following equation.

Dephosphorization oxygen efficiency (%) = {(oxygen unit contributing to dephosphorization) × 100} / {oxygen unit in oxidizing agent (flux) + oxygen gas unit}

[0030]

[Table 3]

[0031]

As described above, according to the present invention, the pretreatment of hot metal to a low P concentration region can be performed with higher dephosphorization efficiency than before, thereby reducing the cost of hot metal pretreatment and reducing the processing time. Shortening is expected.

[Brief description of the drawings]

FIG. 1 is a view showing an apparatus in which a method for dephosphorizing hot metal according to the present invention is performed.

FIG. 2 is a diagram showing a change over time in exhaust gas temperature during a dephosphorization treatment.

FIG. 3 is a diagram showing the change over time of the sum of the CO concentration and the CO ₂ concentration of the exhaust gas during the dephosphorization treatment.

FIG. 4 is a diagram showing a change with time of the carbon concentration in hot metal calculated based on measured values of exhaust gas characteristics.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 Topped car 2 Hot metal 3 Hot metal 3 Exhaust gas duct 4 Lance 5 Thermocouple 6 Sampling tube 7 Pitot tube 8 Top blowing acid lance 9 CO, CO ₂ analyzer 10 Recorder 11 Manometer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shuji Takeuchi 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Chiba Works of Kawasaki Steel Corporation (72) Inventor Naoshi Ogawa 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Inside the Chiba Works, Steel Works Co., Ltd. (72) Inventor Mototatsu Sugisawa, 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Chiba Works, Chiba Works, Ltd. (72) Inventor Shigeru Ogura, 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture 4K013 BA03 CB04 CC04 CF11 FA01 FA02 FA03 4K014 AA03 AC16 AD01 AD17

Claims (5)

[Claims] In a hot metal pretreatment for blowing a refining flux together with a carrier gas into hot metal held in a refining vessel and dephosphorizing the hot metal, the temperature and / or the CO concentration and the CO ₂ concentration of the exhaust gas during the processing. Are sequentially measured, and an index for judging when the decarburization reaction becomes active is selected or obtained based on the measured value, and the supply rate of the refining flux and / or the carrier gas is changed according to the value of the index. A method for dephosphorizing hot metal, comprising: 2. The method for dephosphorizing hot metal according to claim 1, wherein the index is an exhaust gas temperature. 3. The method according to claim 1, wherein the index is the CO concentration of the exhaust gas and the CO concentration.
_{2. The} method for dephosphorizing hot metal according to claim 1, wherein the sum of the _two concentrations is used. 4. The method according to claim 1, wherein the index is a flow rate of exhaust gas, a CO concentration,
_{2. The} method for dephosphorizing hot metal according to claim 1, wherein the carbon concentration in the hot metal is calculated based on the CO2 concentration and the amount of hot metal. 5. The method for dephosphorizing hot metal according to claim 1, wherein the time when the decarburization reaction becomes active is defined as an inflection point in the time-dependent change of the index value. .