JP2005187912A - Method for preliminarily treating molten pig iron - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for preliminarily treating molten pig iron, which inhibits the lowering of a temperature in the molten pig iron and the increasing of slag amount, and dephosphorizes it with a higher efficiency than ever, even when a flux is blown into the molten pig iron with an injection method. <P>SOLUTION: This preliminary treatment method is an improved version of a technology of blowing the dephosphorization flux into the molten pig iron 2 together with a carrier gas, through an immersion lance 3 immersed in the molten pig iron retained in a treatment vessel 1. Specifically, the method includes spraying oxygen gas onto the bath face F of the molten pig iron through an assistant lance 4, from which the slag has been removed by a surfacing dephosphorization agent and carrier gas that has been blown through the immersion lance. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a hot metal pretreatment method, and more particularly to a technique for efficiently dephosphorizing hot metal while effectively applying heat to the hot metal.

  In a steelmaking process using hot metal melted in a blast furnace as a raw material, before decarburizing and blowing the hot metal in a converter, most of the silicon (symbol: Si) and phosphorus (symbol: P) contained in the hot metal is contained. A so-called “hot metal preliminary treatment” is generally performed in which oxygen gas or solid iron oxide is used for oxidation removal in advance. This hot metal pretreatment method is classified according to the type of processing vessel to be used, and one is a process using a converter. While the hot metal is stirred with a bottom blowing gas, flux such as iron oxide and lime, oxygen In this method, gas is supplied from the hot metal. The other is a treatment in a hot metal ladle or a kneading car (hereinafter referred to as a topped car). A lance is immersed in the hot metal held in these treatment containers, and flux is blown with a carrier gas through the immersion lance. This is processing of a method (hereinafter referred to as an injection method).

  Comparing the two, the injection system using the topped car has the advantage that the reaction efficiency of the oxidizing agent is high and the processing cost is low. On the other hand, the free board (space above the bath surface) in the container is small, and it is necessary to keep the supply rate of flux and carrier gas low in order to avoid blowing hot metal from the container, and the processing speed is low. That is, there is a drawback that the processing time is long. Further, since a flux containing solid iron oxide (mill scale, sintered dust, etc.) is used as an oxidant, there is a drawback that the hot metal temperature is lowered by sensible heat of the solid iron oxide and heat absorption during decomposition. If the hot metal temperature decreases, the refining process (desulfurization, decarburization) in the converter of the next process is adversely affected, so that the preliminary process must be interrupted when the lower limit of the hot metal temperature is reached. For this reason, it may be difficult to continue the pretreatment up to a desired phosphorus concentration.

Therefore, a means for solving such a decrease in hot metal temperature in the injection system with such a topped car is disclosed. That is, CO gas generated by reacting the flux in the hot metal with the flux blown into the hot metal from the immersion lance immersed in the hot metal by blowing oxygen gas into the topped car through the top blowing lance during the treatment. Is a technique in which CO ₂ is subjected to secondary combustion and the combustion heat is applied to the hot metal (see Patent Documents 1 and 2). Thereby, since the fall of hot metal temperature is suppressed, the amount of flux supply can be increased from the conventional amount by the heat margin. As a result, the processing speed is improved, and it is possible to perform preliminary processing to a lower phosphorus concentration in the same time.

In addition, when degassing the hot metal by blowing CaO-based flux into the deep part of the hot metal bath with a carrier gas from the tip of the double pipe lance immersed in the hot metal in the processing vessel, it floats on the hot metal during the dephosphorization process. A hot metal pretreatment method in which oxygen gas is blown into the dephosphorization slag from the middle of the double pipe lance to burn the granular iron present in the slag and at the same time secondary combustion of the gas generated by the dephosphorization treatment Is also disclosed (see Patent Document 3). According to this method, it is possible to reduce the temperature drop of the hot metal during the dephosphorization process by heat generated by the combustion of the granular iron in the slag and the secondary combustion of the gas generated by the dephosphorization process.
JP-A-63-180345 Japanese Patent Laid-Open No. 3-24216 JP-A-9-41015

  However, in recent years, quality requirements for steel products have become very strict, and a lower level of phosphorus concentration is required than ever. In order to meet this demand, it is necessary to further increase the amount of hot metal passing through the hot metal pretreatment. Therefore, it has been necessary to increase the processing speed of the hot metal pretreatment more than before and to increase the pretreatment cycle (treatment charge within a predetermined time).

  Here, considering the behavior of the flux blown into the hot metal, the flux is generally a mixture of iron oxide (mill scale, sintered dust, etc.) and lime. The flux blown into the hot metal from the immersion lance floats while reacting with carbon and phosphorus in the hot metal bath, and in particular, phosphorus is oxidized by iron oxide and combined with lime in the flux and taken into the flux. However, in order for phosphorus to continue to exist stably in the flux, it is necessary that the oxygen potential of the flux is maintained at a certain value, in other words, a certain concentration of iron oxide is present. . The flux that floats in the bath reacts with carbon in addition to phosphorus, but iron oxide in the flux reacts a lot with carbon, so iron oxide is rapidly consumed during the float. For this reason, when the flux reaches the bath surface, the iron oxide in the flux decreases, and the phosphorus taken up more than in the hot metal may be released (so-called recovered phosphorus). This phenomenon is unavoidable in the injection method.

Therefore, in order to increase the dephosphorization rate by the injection method, the following two approaches can be considered in light of the above phenomenon.
(1) Increasing the amount of flux blown and increasing the amount of dephosphorization (2) Maintaining the oxygen potential of the flux at a required value and suppressing the release of phosphorus from the flux However, in order to implement the means of (1) As described above, the supply rate of the solid oxygen source (iron oxide) is increased, and the problem that the hot metal temperature decreases is unavoidable.

  For this problem, as described in Patent Documents 1 to 3, it is effective to supply oxygen gas into the slag on the free board or the hot metal bath surface. However, if the supply amount of oxygen gas is increased too much, there is a problem that the refractory in the vicinity of the free board of the processing container is melted, so that there is a limit to increasing the supply amount of oxygen gas. Further, when the amount of flux blown is increased, the amount of slag generated by the hot metal preliminary treatment inevitably increases, and a troublesome problem that the processing amount increases newly arises.

  In view of such circumstances, the present invention provides a hot metal pretreatment method that can suppress a decrease in the hot metal temperature and an increase in the amount of slag even if the flux is blown into the hot metal by an injection method, and can perform dephosphorization more efficiently than conventional methods. The purpose is that.

  In order to achieve the above-mentioned object, the inventors have conducted intensive studies on the effect (2), and have realized the results in the present invention.

  That is, the present invention blows a dephosphorization flux into the hot metal with a carrier gas through the immersion lance immersed in the hot metal held in the processing container, and degassed the hot metal through the immersion lance. A hot metal pretreatment method characterized in that oxygen gas is blown through an auxiliary lance onto a hot metal bath surface from which slag has been removed by floating of a phosphorus agent and a carrier gas. In this case, in order to further promote the secondary combustion of the CO gas in the exhaust gas, another upper blowing lance is suspended above the slag layer floating on the bath surface of the hot metal, and oxygen gas is passed through the upper blowing lance. You may spray. The dephosphorization flux preferably contains no calcium fluoride.

According to the present invention, oxygen gas is sprayed on the hot metal bath surface without slag, so that the oxygen potential of the floated flux can be maintained high. As a result, the phosphorus taken up by the flux is not released into the hot metal, and the dephosphorization efficiency of the hot metal is improved.
That is, it becomes possible to increase the preliminary processing speed while suppressing a decrease in the hot metal temperature without increasing the flux amount.

  Hereinafter, the best mode for carrying out the present invention will be described based on the circumstances leading to the invention.

  Conventionally, an upper blowing lance is suspended from above on the freeboard of the processing vessel, and oxygen gas is injected toward the slag layer floating on the hot metal bath surface. However, it was mainly aimed at heat compensation, that is, suppression of a decrease in hot metal temperature. In other words, when dephosphorizing the hot metal, carbon in the hot metal is oxidized and CO gas is generated by the solid oxidizing agent in the dephosphorization flux blown through the immersion lance immersed in the hot metal bath. The gas was secondarily burned in the space above the bath surface, and the heat generated was used for heat compensation. In that sense, the blowing of oxygen gas into the free board through the top blowing lance had only an indirect effect with respect to the dephosphorization reaction.

  Moreover, even if oxygen gas is supplied into the slag layer on the hot metal bath surface as in the technique described in Patent Document 3, it does not contribute to the improvement of the dephosphorization reaction rate. This is because supplying oxygen gas into the slag does not prevent a decrease in iron oxide content in the flux that is in the process of ascent and does not contribute to an increase in the surrounding oxygen potential. Moreover, since the oxidation of granular iron in the slag is accompanied by a large exotherm, local hot areas are formed in the slag, which is a disadvantageous condition for dephosphorization. Could be conducive.

  Accordingly, the inventors have made various studies on whether or not oxygen gas blown from such an upper blowing lance can directly contribute to the dephosphorization reaction to improve the dephosphorization rate. As a result, the oxygen gas from the upper blowing lance is passed through the dephosphorization flux blown from the immersion lance and the hot metal bath surface (hereinafter also referred to as the floating surface) on which the carrier gas floats, without passing through the slag layer. I found out that I should spray.

That is, the flux that floats on the air bearing surface takes in phosphorus by a reaction in the middle of floating in the hot metal bath, but conventionally iron oxide is consumed by a decarburization reaction in the middle of the floating, The potential drops to 10 ^-15 atm and releases phosphorus. Therefore, it was thought that if oxygen (gas) was directly supplied to this air bearing surface, the flux and the oxygen potential around it would rise again to about 10 ^-13 atm, and the release of phosphorus could be suppressed. The air bearing surface is in an active state because the bath surface fluctuates due to the flotation energy of the carrier gas, and the sprayed oxygen gas is easily caught in the hot metal by the bath surface wobbling. Oxygen can be supplied.

  The present invention embodies this idea. As shown in FIG. 1, the flux blown into the hot metal bath 2 through the immersion lance 3 and the hot metal bath surface F on which the carrier gas floats are supplemented. Oxygen gas is blown through the lance 4 without passing through the slag layer 6. This is because the floating surface F rises as the carrier gas rises, and the slag approaches the surroundings, so that the hot metal is easily exposed. However, if the amount of slag on the hot metal bath surface is large from the beginning of the treatment, or if the treatment is prolonged and the total amount of flux blown becomes large, the slag may cover the air bearing surface F. In this case, the molten metal surface may be exposed by pushing away the slag on the bath surface with the kinetic energy of the oxygen gas supplied from the auxiliary lance 4. This is possible by appropriately adjusting the oxygen gas supply speed and dynamic pressure.

  In the present invention as well, oxygen gas supplied through the auxiliary lance 4 that does not reach the air bearing surface F such as flux reacts with CO gas in the freeboard of the topped car. It has the same thermal compensation effect as top blown oxygen. However, in order to further enjoy the effect of heat compensation, in addition to the use of the immersion lance 3 and the auxiliary lance 4 according to the present invention, as shown in FIG. It may be supplied above the slag layer 6 to further promote the combustion (secondary combustion) of CO gas in the free board 7. Therefore, this is also added to the present invention.

  As the auxiliary lance 4, it is necessary to appropriately adjust the distance between the gas outlet (tip) and the air bearing surface, so that the auxiliary lance 4 moves independently of the immersion lance 3 as shown in FIGS. 1 and 2. It is desirable to be free. Further, the auxiliary lance 4 is a steel tubular body, but since it receives a thermal load, it is preferable to use a water cooling system. For this purpose, the same cooling means (used as in the conventional top blowing lance) For example, a triple-tube structure lance similar to the oxygen lance of an oxygen top blow converter can be used. In addition, a consumable lance may be used instead of the water cooling method. In this case, it is preferable to extend the life of the lance by subjecting the lance surface to an antioxidant process such as calorizing treatment. Further, the secondary blow upper lance 5 is preferably water-cooled or anti-oxidized in the same manner as the auxiliary lance 4.

  In the present invention, the form of the processing container is not particularly limited as long as it is generally used in the steel industry such as hot metal ladle, chaotic car (topped car) and the like.

  Further, conventionally, in order to accelerate the hatching of the flux and improve the dephosphorization rate, there has been often an operation of blowing a dephosphorization flux containing fluorite (the main component is calcium fluoride). On the other hand, in recent years, from the viewpoint of environmental protection, reduction of fluorine-containing waste has been required, and it has become necessary to reduce the amount of fluorite used as much as possible. The present condition is that the alternative material which exhibits is not found. In the present invention, the hot metal dephosphorization rate can be improved by the oxygen gas supply method, so that the hot metal pretreatment is performed at the same dephosphorization rate without using a dephosphorization flux containing fluorite. Therefore, an invention that makes this a requirement is also proposed.

(Example 1)
A dephosphorization flux was blown into the hot metal 2 held in the topped car 1 having a capacity of 300 tons via a dipping lance 3 with a carrier gas. At that time, applying the hot metal pretreatment method according to the present invention shown in FIG. 1, through the auxiliary lance 4, toward the dephosphorization flux blown through the immersion lance and the floating surface F of the carrier gas, Oxygen gas was blown separately. The distance between the tip of the auxiliary lance 4 and the uppermost end of the raised portion of the air bearing surface F was 200 mm. As the dephosphorization flux, a mixture of sintered ore powder and quicklime powder was used, and nitrogen was used as the carrier gas. The treatment was performed for 20 minutes and the results are shown in Table 1.
(Comparative Example 1)
The auxiliary lance was not used, and the hot metal pretreatment conventionally performed by blowing a dephosphorization flux from the immersion lance 3 into the hot metal with a carrier gas was performed. The results are also shown in Table 1.

As can be seen from Table 1, according to this conventional pretreatment method, the temperature drop of the hot metal is large, leading to an increase in the temperature raising material (for example, coke) used in converter steelmaking as the next step. In addition, the same amount of oxygen gas as in Example 1 was blown through the immersion lance 3, but at this time, the immersion lance 3 was quickly worn out and the dephosphorization process could not be continued after 5 minutes. It became possible. On the other hand, in the present invention of Example 1, it is clear that the temperature drop is small and the dephosphorization rate is improved.
(Comparative Example 2)
Except for supplying oxygen gas in a state in which the immersion lance is immersed in the slag without directing the tip of the auxiliary lance 4 to the dephosphorization flux blown through the immersion lance 3 and the floating surface F of the carrier gas, Dephosphorization treatment was performed in the same manner as in Example 1. The results are also shown in Table 1.

  From Table 1, it can be seen that according to the pretreatment method of Comparative Example 2 in which oxygen gas is not jetted onto the air bearing surface F, the hot metal temperature drop is reduced, but the dephosphorization rate is not improved. On the other hand, in the present invention of Example 1 in which oxygen gas is injected onto the air bearing surface F, it is clear that the temperature drop of the hot metal is small and the dephosphorization rate is improved.

(Example 2)
As shown in FIG. 2, dephosphorization similar to that in Example 1 except that oxygen gas for secondary combustion of CO gas in the exhaust gas is supplied via the top blowing lance 5 in the vicinity of the opening in the topped car 1. Processed. The results are shown in Table 2.

  Table 2 also shows a conventional operation result without using an upper blowing lance for injecting oxygen as Comparative Example 3. According to the present invention, not only the improvement of the hot metal dephosphorization rate but also the hot metal temperature It is clear that descent suppression is achieved.

  As described above, by supplying oxygen gas to an appropriate position of the hot metal via a simple auxiliary lance, it has become possible to simultaneously improve the dephosphorization rate of the hot metal and suppress the temperature drop during processing.

The figure which shows typically the pretreatment dephosphorization of the hot metal which concerns on this invention using the immersion lance which supplies the dephosphorization flux in hot metal, and the auxiliary | assistant lance which blows oxygen gas directly on floating surfaces, such as the flux of a hot metal bath surface It is. It is a figure which shows typically the pretreatment dephosphorization of the hot metal using the top blowing lance which injects the oxygen which accelerates | stimulates the secondary combustion of exhaust gas in addition to the immersion lance and auxiliary | assistant lance of FIG.

Explanation of symbols

1 Processing container (Topy car)
2 Hot metal 3 Immersion lance 4 Auxiliary lance 5 Top blowing lance 6 Slag layer 7 Free board F Floating surface of dephosphorization flux and carrier gas

Claims (3)

When dephosphorizing the hot metal by blowing a dephosphorization flux into the hot metal with a carrier gas through an immersion lance immersed in the hot metal held in the processing vessel,
A hot metal pretreatment method, characterized in that oxygen gas is blown through an auxiliary lance to the hot metal bath surface from which slag has been removed by the floating of the dephosphorizing agent and carrier gas blown through the immersion lance. Furthermore, in order to promote the secondary combustion of the CO gas in the exhaust gas, another upper blowing lance is suspended above the slag layer floating on the bath surface of the hot metal, and oxygen gas is injected through the upper blowing lance. The hot metal pretreatment method according to claim 1. The hot metal pretreatment method according to claim 1 or 2, wherein the dephosphorization flux does not contain calcium fluoride.

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