JPH0452206A - Method for dephosphorizing chromium-contained molten iron - Google Patents

Abstract

PURPOSE:To efficiently execute dephosphorization in Cr-contained molten iron by pulverizing natural fluorite containing the specific ratios of CaF2 and SiO2, mixing the specific quantity of lime powder and blowing to below the molten iron surface containing the specific quantity of Cr together with oxygen source. CONSTITUTION:The natural fluorite containing >=70wt.% CaF2 and >=5wt.% SiO2 is pulverized and in this fluorite powder, the lime powder is mixed so that wt. ratio of CaO/CaF becomes >=4/6, and the mixed powder together with the oxygen source composed of gaseous oxygen or iron oxide are blown into the molten iron containing >=3wt.% chromium from below the molten iron surface so that SIGMAO2/(CaO+CaF2) becomes 20-120/Nlkg. Wherein, SIGMAO2 is the total quantity (Nl) of the gaseous oxygen quantity in the carrier gas and O2 quantity when the iron oxide decomposes into Fe and O2.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for dephosphorizing hot metal containing chromium (hereinafter referred to as chromium-containing hot metal).

[Conventional technology]

In recent years, methods of melting stainless steel raw materials without using an electric furnace have been studied from the viewpoint of power saving. In other words, by melting scrap using carbonaceous material such as coke, or by
This method involves directly melting and reducing chromium ore. In this case, since the oxygen potential during reduction or dissolution is low, almost 100% of the phosphorus contained in the raw material migrates into the metal. Therefore, in order to industrialize the melting method of these stainless steel raw materials using carbonaceous materials, the development of dephosphorization technology is an essential issue.

However, dephosphorization of chromium-containing hot metal is generally very difficult because it reduces the activity of chromium phosphorus. In addition, in the case of oxidative dephosphorization, chromium is preferentially oxidized, so Cr
The fluidity of the slag deteriorates due to the generation of , Os, and as a result, the membrane phosphorization reaction stagnates, and the basicity decreases, which adversely affects dephosphorization. These problems arise, and furthermore, it becomes difficult to dephosphorize this chromium-containing hot metal.

Therefore, the dephosphorization methods that have been adopted so far have been to use an expensive strongly basic flux and also to use a fluxing agent (slag forming agent), which is also expensive. Examples of methods using these strong basic fluxes include methods using carbonates of alkali metals such as lithium carbonate (Li-COl) (for example, Japanese Patent Publication No. 57-32688) and methods using BaO-BaClt-based fluxes ( For example, Japanese Patent Publication No. 61-403). Also, as a CaO system.

Cab-CaF, a method using system flux (for example, Japanese Patent Publication No. 63-481) and Cab-NaF system in which NaF is more effective than Ca F z (for example, Japanese Patent Publication No. 61-198)
49422), but these CaO-based ones are also available.

This method uses a large amount of fluxing agents such as CaF2 and NaF.

[Problem that the invention seeks to solve]

The problem with the above method is that one strong basic substance (L i 2 C○
.

(BaO, etc.) itself is high, and a large amount of flux (CaFz, NaF, BaC12) is used, which increases the cost and shortens the life of the refractory.

In addition, in the case of Ca0-CaFt system,
As described in Publication No. 1, there is a restriction that the calcium fluoride used must be of high purity. In other words, it is necessary to use natural fluorite as a source after refining it, or use industrially produced C as a source.
It was necessary to use a F t.

The present invention aims to solve these problems and provide an inexpensive and efficient method for dephosphorizing chromium-containing hot metal.

[Means to solve problems]

In the present invention, quicklime powder is added to fluorite powder obtained by crushing naturally produced fluorspar containing 70% by weight or more of CaFz and 51% by weight or more of SiOz, and the weight ratio of CaO/CaFz is 4z6 or more. This mixed powder is mixed with an oxygen source consisting of gaseous oxygen or iron oxide at a concentration of 3% by weight.
The present invention provides a method for dephosphorizing chromium-containing hot metal, which is characterized by blowing into the chromium-containing hot metal from below the surface of the hot metal.

In implementing the present invention, the mixed powder and an oxygen source.

ΣOx/(CaO+CaF2) is 20~12ONl'
kg into the hot metal, where Σ0□ is the total amount (Nf) of the amount of gaseous oxygen in the carrier gas and the amount of 0□ when iron oxide is decomposed into Fe and 0-rich, and
Actually, the mixed powder has a total amount of CaO and CaFz of 7
It contains 0% by weight or more, and the remainder may contain 5iO1, limestone, iron oxide, or the like.

The oxygen source may be gaseous oxygen only in the carrier gas without containing iron oxide.

[Effect]

According to the present invention, the dephosphorization reaction proceeds even if naturally produced fluorspar containing a relatively large amount of Stow as an impurity is used as a refining agent. Due to the infusion of Sing,
When the hot metal to be treated contains Si, SiO□
A slag containing a large amount of

Although the slag basicity will decrease, according to the method of the present invention, the dephosphorization reaction will proceed even under conditions where such low basicity slag is produced, and the chromium oxidation reaction will be reduced to ΣOt /
It can be suppressed by making the ratio of (CaO+CaFz) appropriate.

At the same time, the desulfurization reaction proceeds, and desulfurization treatment can be performed together with dephosphorization.

〔Example〕

The experiment was conducted by using 5 tons of molten metal and injecting the refining agent of the present invention together with O3-enriched gas directly into the molten metal. The carbon concentration of the hot metal was about 5.6%, and the chromium concentration was about 12%.

Table 1 shows the analytical values of the fluorite used. CaFt:80
%, , S to! 713.6%, and this naturally occurring fluorite was crushed and used as it was.

The composition of the refining agent is shown in Table 1. Refining 151 is 45% quicklime - 45% fluorspar - 10% mill scale, and refining agent ■ is 40% quicklime - 45% fluorspar - 15.
% limestone. Ca○/CaF in these refining agents
z is 1.2 to 1.3.

Σ○z/(CaO+CaF*) was varied in the range of 50 to 9 ON1/kg by varying the amount of scale mixed in the powder or the flow rate of 02 in the carrier gas. The basic unit of powder was 40 to 60 kg/lon. Processing temperatures ranged from 1470 to 1310°C, with an average of about 1400°C.

Table 3 shows the blowing conditions. Table 4 shows the slag components after treatment.
Table 5 also shows the metal components before and after treatment.

As seen in Table 4, SiO□ in fluorite is 13.
6%, and the silicon concentration at the start of the dephosphorization treatment is about 0.15%, so the SiOz concentration in the slag after the second treatment is about 10%, and the basicity (C
ab/Sing) was also observed to be less than 3. However, in all cases, dephosphorization progressed efficiently, and as shown in Table 1, a dephosphorization rate of 42 to 49% was obtained despite the low unit consumption of refining agent at 40 to 60 kg/ton. Ta. This is inconceivable using conventional common sense. That is, as shown in Japanese Patent Publication No. 63-481, in order to dephosphorize efficiently, fluorite must be of high purity and the amount of CaF* should be larger than CaO. Two conditions are required: F t < 1) to lower the melting point of the slag and make it a slag with good fluidity.The present invention breaks this common sense and achieves an unexpectedly efficient escape. The phosphorus rate was obtained. In other words, in the conventional method, the amount of fluxing agent is increased mainly from the viewpoint of ensuring the fluidity of the slag.
In the Ft system, the CaO/CaFz ratio is
In the aC1z system, BaO/BaC1, Cab-NaF
In the system, the CaO/NaF ratio is set to less than 1, respectively.
Efficient dephosphorization could not be achieved unless the mixing ratio of the flux was substantially 60% or more. Therefore, as already mentioned, a large amount of expensive fluxing agent is used, which not only increases costs, but also promotes melting and loss of refractories and reduces workability.
This is a disadvantageous condition in terms of both cost.

Moreover, these refining agents were required to be expensive and contain few impurities such as SiOz. In contrast, the present invention uses inexpensive naturally occurring fluorite, reduces the amount of fluorite used, and efficiently dephosphorizes with semi-molten slag. As a result, almost no erosion of the refractories is observed, making it an extremely superior method in terms of cost, workability, and productivity.

In addition, in the case of oxidative dephosphorization, oxidation loss of chromium is usually involved, but in the present invention, ΣOx/(CaO+CaF
2) By setting the ratio to an appropriate value, this chromium loss could be significantly reduced. As shown in Example 2 and Example 4 of Table 5, ΣOx/(CaO+CaF2)=52-56NI
/kg, the chromium concentration before and after treatment is 11.9
6/11.92 (%) (Example 2), 12.25/
It was 12.29 (%) (Example 4), which was a variation within 5 analytical errors, and no chromium loss was observed. However, as shown in Example 1.3.5, when this ratio was increased further to increase the oxidizing power, there was no change in the dephosphorization efficiency, but the oxidation loss of chromium increased. In other words, appropriate oxidation conditions exist, and under the present experimental conditions, the value is Σ0
2/(CaO+CaF2) was approximately 5081/kg. Note that the appropriate oxidation conditions may vary depending on the flow state of the bath or the blowing method, but Σ
Oz/(CaO+CaF2) is 20-12ONl/kg
Within this range, the object of the present invention can be achieved.

Also, gaseous oxygen in the carrier gas is the oxygen source for the phosphorous oxide film, and a solid acid such as mill scale or iron ore may be used as the oxygen source.

However, the use of these solid acids causes a large drop in bath temperature, which is disadvantageous from the standpoint of thermal compensation.

The inventors have also experienced that when these solid acids are used, the oxidation loss of chromium is greater than when using gas acids.
It is better to use air acid.

〔effect〕

As stated above, the present invention uses naturally occurring fluorite as it is, and is an inexpensive method that uses only a small amount of fluorite, while still being able to efficiently dephosphorize, which is different from conventional dephosphorization of chromium-containing hot metal. It has achieved what was previously impossible.

Furthermore, in the present invention, the slag can be made into a semi-molten state, and the amount of erosion of the refractory can be reduced. That is, the present invention provides an extremely inexpensive and efficient method for dephosphorizing chromium-containing hot metal.

Claims (4)

[Claims] (1) Quicklime powder is added to fluorite powder obtained by crushing naturally produced fluorite containing 70% by weight or more of CaF_2 and 5% by weight or more of SiO_2, so that the weight ratio of CaO/CaF_2 is 4/6 or more. A method for dephosphorizing chromium-containing hot metal, which comprises mixing the powder and blowing this mixed powder together with an oxygen source consisting of gaseous oxygen or iron oxide into hot metal containing 3% by weight or more of chromium from below the surface of the hot metal. (2) Mixed powder and oxygen source are ΣO_2/(CaO+CaF
The method for dephosphorizing chromium-containing hot metal according to claim 1, wherein _2) is blown into the hot metal at a concentration of 20 to 120 Nl/kg. However, ΣO_2 is the total amount (Nl) of the amount of gaseous oxygen in the carrier gas and the amount of O_2 when iron oxide is decomposed into Fe and O_2. (3) The method for dephosphorizing chromium-containing hot metal according to claim 1 or 2, wherein the mixed powder contains 70% by weight or more of a total amount of CaO and CaF_2, and the remainder contains SiO_2 and limestone or iron oxide. (4) The method for dephosphorizing chromium-containing hot metal according to claim 1 or 2, wherein the oxygen source consists essentially of gaseous oxygen in the carrier gas.