JP3348828B2 - Method for reforming slag containing chromium oxide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reforming chromium oxide-containing slag generated during smelting of chromium-containing steel such as stainless steel, and more particularly to a method for preventing hexavalent Cr from being eluted from the slag. It is intended to make effective use.

[0002]

2. Description of the Related Art The manufacturing process of stainless steel includes:
Conventionally, a process combining an electric furnace with AOD or VOD, a process using a converter, and the like have been put to practical use. Recently, Fe-Cr alloys, pre-reduced Cr
A technique has been developed in which inexpensive Cr ore is directly melted and reduced without using an expensive Cr source such as pellets.

[0003] Since the refining of such chromium-containing steel is an oxidizing refining centering on decarburization using a gas mainly composed of oxygen, the slag generated during the refining includes chromium oxide which is inevitably generated thermodynamically. Things will be included. Stainless steel slag after decarburization refining contains 10 wt% or more of Cr in the form of chromium oxide, and discarding as it is wastes expensive Cr. For this reason, after decarburization refining, Fe-Si and
A reducing treatment such as adding a reducing agent such as Al to recover the Cr in the slag into the molten steel is performed. However, in order to make Cr in the slag as close as possible to zero, it is necessary to add a large amount of an expensive reducing agent, which causes an increase in cost. Normally, due to the cost balance between Cr loss due to slag disposal and the addition of a reducing agent, some Cr oxide remains in the slag after the reduction treatment.

In such a slag containing Cr oxide, there is a concern that hexavalent Cr may be eluted from the slag depending on conditions, and there is a problem in using it as a roadbed material or a civil engineering landfill material as it is. To solve such a problem, for example, JP-A-63-140044
In the publication, the slag containing chromium oxide is discharged to another container in a molten state, and the slag to be melted in the container is agitated, and a reducing agent in an amount sufficient to reduce chromium oxide is added. A method for treating slag has been proposed. JP-A-6-171993 proposes a method for modifying chromium oxide-containing slag by adding a predetermined amount of aluminum ash and magnesia-based industrial waste to chromium oxide-containing slag in a molten state. . Also, JP-A-8-104553 discloses that
There has been proposed a method for modifying a stainless steel slag in which a divalent S compound or further boron oxide is added to a chromium oxide-containing slag in a molten state while blowing and stirring an inert gas.

[0005]

However, the above-mentioned prior art is a slag treatment that requires an extra treatment of adding various additives to the slag, which complicates the process and completely eliminates elution of Cr ions. From the standpoint of prevention. In response to the above-mentioned problem, the present inventors have previously disclosed in Japanese Patent Application No. 9-7558, an aqueous solution containing sulfur and / or a sulfur compound having an oxidation number of +5 or less to a chromium oxide-containing substance. A method for modifying the chromium oxide-containing material, such as mixing the chromium oxide-containing material with an unaged blast furnace slow cooling slag and allowing it to stand at the dew point, was proposed. According to this method, hexavalent in slag
Cr can be completely reduced and slag can be reused. In particular, even a chromium oxide-containing slag containing as high as 50 mg / l of eluted hexavalent Cr can be completely reduced and rendered harmless.

However, even when this method is used, extra treatment is required for slag reforming, and particularly when the chromium oxide content in the slag is high, the required amount of the reducing agent is large. In addition, the processing time for reduction becomes longer, and the problem that slag processing becomes economically expensive remains. The present invention advantageously solves the above-mentioned problems of the prior art, and reforms chromium oxide-containing slag generated in the smelting process of chromium-containing steel in which oxidizing smelting is performed using a gas mainly composed of oxygen to form hexavalent. It is an object of the present invention to propose an economically inexpensive method for reforming slag containing chromium oxide, which prevents the elution of Cr and makes it available for effective use.

[0007]

Means for Solving the Problems In order to achieve the above object, the present inventors have intensively studied an economically inexpensive slag reforming method. As a result, in order to make the slag reforming process inexpensive, the slag is inspected and separated, the reduction process is limited to slag containing hexavalent Cr, and the amount of slag to be reduced is minimized. 6 during the slag
I realized that it is important not to generate valence Cr or to minimize the generation of chromium.

Therefore, the present inventors first studied the mechanism of generating hexavalent Cr. As a result, it has been found that hexavalent Cr is generated in an intermediate stage of a cooling process in which the molten state is solidified and then cooled, and furthermore, the amount of the generated hexavalent Cr varies greatly depending on the state of the slag. FIG. 2 schematically shows a process of generating hexavalent Cr. The chromium oxide (CaCr ₂ O _{4 in} FIG. 2) in the slag is an intermediate stage of the cooling process.
Oxygen is oxidized from the surface due to the surrounding oxygen potential in the range of 1000 ° C, and only the vicinity of the surface is hexavalent Cr.
O ₄ ). As described above, the generation amount of hexavalent Cr largely depends on the surface area, that is, is greatly affected by the solidification state of the slag. In order to reduce the generation amount of hexavalent Cr, it is important that the solidified state of the slag is not powdery but is as massive as possible.

In order to reduce the amount of chromium oxide in the slag to a predetermined amount or less, the basicity of the final slag after the reduction is adjusted to 2.5 or less.
It was found that adding Si-containing metal and reducing Cr in slag is an effective means. The present invention has been made by further study based on the above findings.

That is, the present invention recovers chromium oxide-containing slag generated in a refining process of chromium-containing steel in which oxidizing refining is performed using a gas mainly composed of oxygen, and the hexavalent Cr concentration of the recovered slag is recovered. And then subjecting the slag containing only hexavalent Cr to a slag reduction treatment in which a substance containing a sulfur compound having an oxidation number of +5 or less is brought into contact with the slag only under humid conditions. In the method, the determination is preferably made up of a primary determination using a simple reagent and a secondary determination using a chemical analysis performed on the slag determined to be detected in the primary determination.

Further, the present invention recovers chromium oxide-containing slag generated in a refining process of chromium-containing steel in which oxidizing refining is performed using a gas mainly composed of oxygen, and transfers the recovered slag in a molten state to B. After adding the contained materials and cooling and solidifying, the hexavalent Cr concentration of the slag is determined, and only the slag containing hexavalent Cr is brought into contact with a substance containing a sulfur compound having an oxidation number of +5 or less under humid conditions under humid conditions. A method for modifying a chromium oxide-containing slag characterized by performing a treatment, and in the present invention, the B-containing substance has a B ₂ O ₃ content of 0.
It is preferable to add the slag in an amount of 3 wt% or more, and in the present invention, the final slag basicity is 2.5 or less, preferably
2.3 It is preferable that the content is not more than, and in the present invention, after the oxidizing and refining, a Si-containing metal is added into the refining furnace, and the Si in the molten steel is added.
It is preferable to carry out a reduction treatment to make the content 0.10 wt% or more. In the present invention, the determination is made using a simple reagent.
It is preferable to comprise a secondary determination using a chemical analysis performed on the slag determined to be detected in the primary determination and a secondary determination.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, chromium oxide-containing slag generated in a refining process of chromium-containing steel in which oxidizing refining is performed using a gas mainly composed of oxygen is recovered, cooled, solidified, and recovered. A sample is taken from the slag to determine the hexavalent Cr concentration in the slag. Judgment is made using a simple reagent 1
It is composed of a secondary determination and a secondary determination using chemical analysis performed on the slag determined to be detected in the primary determination. FIG. 1 shows a processing flow of a slag containing chromium oxide in the present invention.
Shown in

The primary judgment uses a simple reagent which is extremely sensitive to chromium oxide. This reagent is not particularly limited as long as it reacts when the concentration of hexavalent chromium, which has conventionally been a problem, exceeds 0.05 mg / l.
In the primary determination, the slag was immersed in 10 times the volume of the slag, allowed to permeate for 1 h or more, and then, as a simple reagent, a pH adjuster and diphenylcarbazide were added to the aqueous solution, and the color of the solution was measured. The viewing method is most preferred. If the liquid develops color, it indicates that the hexavalent chromium concentration is 0.05 mg / l or more. The simple reagent introduced into the aqueous solution by this method reacts when the hexavalent chromium concentration exceeds 0.05 mg / l, and does not particularly limit the kind of the reagent.

According to the above-mentioned method, the detection and non-detection of hexavalent chromium ion can be determined by the presence or absence of coloring of the aqueous solution, so that the analysis can be performed more quickly than the conventional chemical analysis method, and the cost required for the analysis can be greatly reduced. There are advantages such as possible.
However, in this method, the absolute value of the hexavalent chromium concentration is unknown, and there is a possibility that the color may be formed by ions other than hexavalent chromium. Therefore, if the detection is not performed by this rapid analysis method, it can be determined that there is no problem, and the slug of the charge that is not detected can be used as it is.

On the other hand, the slag detected in the primary determination is
A sample is taken again and sent to the secondary determination. In the secondary determination, the collected sample is immersed in water (immersion time:
6hr), measure the amount of hexavalent Cr eluted by chemical analysis,
Cr elution is recognized as “detection” and hexavalent Cr
If no elution is observed, it is regarded as "not detected". Here, “not detected” means a case where the analysis value is 0.05 mg / l or less, which is the lower limit of analysis.

Next, the slug of the charge, which is not detected in the secondary determination, is directly reused. On the other hand, the slag of the charge detected in the secondary determination has an oxidation number of +5.
A slag reduction treatment is performed in which a substance containing a sulfur compound having a valency of less than or equal to that of the slag is brought into contact with the substance under wet conditions. In the slag reduction treatment in the present invention, the chromium oxide-containing slag detected in the secondary determination is added to a substance containing a sulfur compound having an oxidation number of +5 or less contained in a blast furnace slow-cooled slag, substantially an aqueous solution Is contacted under wet conditions to reduce hexavalent Cr and render it harmless.

As the sulfur compound having an oxidation number of +5 or less,
Unaged blast furnace slow cooling slag and blast furnace slag elution water generated during sprinkling cooling of the blast furnace slag are preferred. S ions in slowly cooled blast furnace slag SO ₄ ^- believed to reduce Cr ⁶⁺ in the oxidation. The valence of S ions in the slag varies from -2 to 10/3 valence, and all S ions before +6 valence are considered to be effective.

Further, the slag reduction treatment for bringing a substance containing a sulfur compound having an oxidation number of +5 or less into contact is specifically carried out by:
Method of adding unaged blast furnace slow-cooled slag to chromium oxide-containing slag and mixing and then leaving it in the open or steam aging, or blast furnace slag elution water generated when chromium oxide-containing slag is sprinkled and cooled in blast furnace slag It is preferable to use a method of immersion in water, a method of combining these methods, or the like.

The important point here is that when a pentavalent or less sulfur compound is mixed with the slag, it is added and mixed through an aqueous solution. That is, the mechanism by which the S ions in water reduce hexavalent chromium in the slag is important, and a treatment such as simply adding FeS to the molten slag is not effective. In the present invention, before the limited slag reduction treatment as described above, the B-containing substance is preferably added to the recovered slag in a molten state, and the slag is cooled and solidified. Thereby, the slag cooled and solidified becomes a lump, and the generation of a large amount of hexavalent Cr can be suppressed. The B-containing substance to be added is not particularly limited, but is preferably B-containing ore (for example, colemanite) or anhydrous borax (Na ₂ B ₄ O ₇ ). The B-containing substance may be added so that B ₂ O ₃ is contained in the slag in an amount of 0.3 wt% or more. The amount of B ₂ O ₃ in the slag
If the content is less than 0.3 wt%, slag agglomeration is insufficient, and the formation of hexavalent Cr is promoted. Further, even if B ₂ O ₃ is contained in the slag in excess of 1.0 t%, the above-mentioned effect is saturated and the effect corresponding to the addition cannot be expected, so the amount of B ₂ O ₃ in the slag is 1.0 t% or less. It is desirable to do.

The addition of the B-containing substance to the slag
When oxidizing and refining is performed in an OD furnace or a top-bottom blow converter, it is preferable to reduce the slag with FeSi or the like and then to perform the slag after tapping when the slag is in the refining furnace. On the other hand, in VOD refining, etc., B is reduced and the amount of B in the molten steel increases, so the B is not added to the slag in the pot, but the slag is discharged into a container (slag pot, pallet, etc.) in which the B-containing substance is placed beforehand, It is preferable to add a B-containing substance therein.

In the present invention, the basicity of the final slag after oxidative refining, which is decarburizing refining, and after reduction treatment with FeSi or the like,
2.5 or less, preferably 2.3 or less. This changes the structure of the chromium oxide in the slag, and reduces the generation of hexavalent Cr. If the basicity of the slag exceeds 2.5, CaCr ₂ O ₄
It changes to CaCrO ₄ and the generation of hexavalent Cr becomes remarkable. The slag basicity at the time of oxidative refining is desirably set to 2.0 or more from the viewpoint of reducing refractory erosion.

In the present invention, it is desirable to adjust the amount of chromium oxide in the slag to 2% by weight or less after oxidizing refining, which is decarburizing refining. As an adjustment method, after the oxidation refining, which is decarburization refining, a reduction treatment is performed in which a metal containing Si is added into the refining furnace in order to reduce Cr in the slag and recover it in molten steel. Metals containing Si include Fe-Si, Si-M
n is preferred.

A metal containing Si may be added so that the Si content in the molten steel is 0.10 wt% or more, and reduced until the chromium oxide in the slag becomes 2 wt% or less. It is also possible to directly confirm whether chromium oxide in the slag is 2 wt% or less by sampling from the slag, but since slag is not usually analyzed, Si
It is preferable to directly sample and confirm that the content is 0.10 wt% or more. Alternatively, a method may be employed in which sampling is performed during blowing immediately before the end of oxidizing refining, the amount of chromium oxidized at that time is obtained, and the amount of reduction is estimated therefrom.

If the amount of the Si-containing metal added is small and the Si content [Si] in the molten steel is less than 0.10 wt%, the chromium oxide in the slag exceeds 2% and, as shown in FIG. The elution appearance rate of Cr increases. If the Si content [Si] in the molten steel is 0.10 wt% or more, the chromium oxide in the slag becomes 2% or less and hexavalent Cr
The elution appearance rate of is drastically reduced. The dissolution appearance rate of hexavalent Cr is the appearance rate of slag in which hexavalent Cr is detected at 0.05 mg / l or more in all slags. The number of slags where Cr was detected at 0.05 mg / l or more) / the total number of slags x 100%.

[0025]

EXAMPLE Hot metal from a blast furnace was subjected to a dephosphorization treatment, and then charged into a first converter, to which chromium ore was added and melt-reduced to obtain chromium-containing hot metal. Next, the chromium-containing hot metal was charged into a second converter (upper and lower blown), and subjected to decarburization refining, which is oxidation refining using a gas mainly composed of oxygen. After the reduction, the basicity of the slag was adjusted to 2.0 to 2.5. Note that the refining slag in the first converter is considered to be substantially no problem because it is reduction refining, but a primary determination was made using a simple reagent. As a result, out of 300 charges, the slag detected by the primary judgment was only 4 charges, and the "detection" by the secondary judgment was 0 charges.

Therefore, the slag in the oxidation refining of the second converter will be described below. A sample was taken using a sublance during oxidative refining, and endpoint control was performed. The oxygen blowing was terminated when it was determined that the carbon content had been predicted by the end point control. Subsequently, Fe-Si was added into the furnace to reduce Cr in the slag. During the reduction treatment, the content of Si in the molten steel was adjusted as shown in Table 1. After the reduction treatment was completed and molten steel was tapped from the converter, colemanite was added as a B-containing substance to the molten slag in the furnace to adjust the amount of B ₂ O ₃ in the slag shown in Table 1. Thereafter, the slag was discharged into a slag pot and cooled and solidified. Under the conditions shown in Table 1, 150 charges were carried out for each example, and this was an example of the present invention.

Further, the molten steel is subjected to secondary refining,
Cast through a tundish. The slag during the secondary refining or casting is discharged into a tundish filter pan in which colemanite, the amount of which is adjusted so as to be the amount of B ₂ O ₃ in the slag shown in Table 1, is placed beforehand. Cooled and solidified in a pan. In addition, the case where the B-containing substance was not added was also performed as a comparative example.

A sample was collected from the slag cooled and solidified for each charge, and subjected to a primary judgment using a simple reagent. The primary determination method was as described above, and a pH adjuster and diphenylcarbazite were used as simple reagents. Next, the slug of the charge determined as "detected" in the primary determination is
A sample was taken again, crushed and pretreated, immersed in water for 6 hours, and analyzed for the amount of hexavalent Cr eluted by chemical analysis. The case where the elution amount of hexavalent Cr was 0.05 mg / l or more was defined as "detection". The reason for setting it to be 0.05 mg / l or more is that the analysis limit of hexavalent Cr is 0.05 mg / l, and if it is less than that, it cannot actually be detected.

An unaged blast furnace slowly cooled slag was added and mixed only to the slag determined to be "detected" in the secondary determination, and a slag reduction treatment of blowing steam into the mixture was performed. In addition,
In each of the examples of the present invention, the slag in which hexavalent Cr was detected in the primary determination was moved to a temporary storage place and temporarily stored until the chemical analysis for the secondary determination was completed. At that time, the slag was classified for each charge and temporarily placed. Therefore, a space of about 30 mx 30 m was required for temporarily storing 30 charges.

Further, Comparative Example 1 shows that the converter slag which did not carry out Si adjustment in the molten steel stage did not add B, and
Processed without the next judgment. As a result, the amount of processing is enormous and it is practically difficult to separate the slag for each charge and temporarily place it until the analysis for the secondary determination is completed. Was done.

In Comparative Example 2, the secondary refining slag and the T / D slag in which the Si was not adjusted in the molten steel stage were processed without adding B without performing the primary determination. Since the amount of slag was small, the slag was separated for each charge at the temporary storage and temporarily stored. However, a temporary storage space of about 30m x 60m was required. Thereafter, the result of the secondary slag analysis was waited, and thereafter, slag reduction treatment was performed only on the slag for 10 charges in which elution was detected.

However, for this purpose, not only did all slag need to be retained for two weeks, but also all slag had to be transported to a temporary storage site once, and a covered temporary storage site that did not allow rainwater to enter was required. And other problems. Table 1 shows the results.

[0033]

[Table 1]

The slag determined as "not detected" in the primary determination, 2
Regarding the slag that was determined as “not detected” in the next determination, and the slag that was determined as “detected” in the secondary determination and subjected to the slag reduction treatment, a sample was taken and the elution amount of hexavalent Cr was investigated.
In each case, the elution amount of hexavalent Cr was less than the analysis limit of 0.05 mg / l, and it was confirmed that there was no problem when used as a roadbed material or a civil engineering landfill material.

As described above, according to the present invention, the slag containing chromium oxide can be reformed by an inexpensive method by eliminating the necessity of performing the entire slag reduction treatment. It became possible to do.

[0036]

According to the present invention, chromium oxide-containing slag can be reformed and hexavalent without significant cost increase.
Cr elution can be prevented, and it can be effectively used as a roadbed material, a civil engineering landfill material, etc., and a remarkable industrial effect is achieved. In addition, slag processing can be speeded up, and there is no need for a large slag processing plant.
Further, effects such as reduction of chemical analysis cost can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a flow of a slag reforming method of the present invention.

FIG. 2 is a schematic explanatory view schematically illustrating a process of generating hexavalent Cr in slag.

FIG. 3 is a graph showing the Si content [Si] in molten steel and the appearance rate of hexavalent Cr elution.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-10-324547 (JP, A) JP-A-11-106243 (JP, A) JP-A-11-100242 (JP, A) 279817 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C04B 5/00-5/06

Claims (5)

(57) [Claims] Claims 1. A chromium oxide-containing slag generated in a smelting process of a chromium-containing steel in which oxidizing refining is performed using a gas mainly composed of oxygen is recovered, and a hexavalent Cr concentration of the recovered slag is determined. And a slag reduction treatment in which only a slag containing hexavalent Cr is brought into contact with a substance containing a sulfur compound having an oxidation number of +5 or less in a wet state. 2. A chromium oxide-containing slag generated in a chromium-containing steel refining process in which oxidizing refining is performed using a gas mainly containing oxygen, and a B-containing substance is added to the recovered slag in a molten state. After adding and solidifying by cooling, 6
The chromium oxide-containing slag is characterized in that the slag containing hexavalent Cr is determined and subjected to a slag reduction treatment in which only a slag containing hexavalent Cr is brought into contact with a substance containing a sulfur compound having an oxidation number of +5 or less under humid conditions. Reforming method. 3. The method according to claim 1, wherein the B-containing substance is a slag containing B ₂ O _{3 of} 0.3%.
The chromium oxide-containing slag reforming method according to claim 2, wherein the slag is added so as to be 3 wt% or more. 4. The method according to claim 1, wherein after the oxidizing refining, a reduction treatment is performed by adding a Si-containing metal into the refining furnace to reduce the Si content in the molten steel to 0.10 wt% or more. A method for modifying a chromium oxide-containing slag according to any one of the above. 5. The method according to claim 1, wherein the determination comprises a primary determination using a simple reagent and a secondary determination using chemical analysis performed on the slag determined to be detected in the primary determination. 5. The method for reforming a chromium oxide-containing slag according to any one of items 4 to 4.