JP2872586B2 - Reforming method of stainless steel slag - 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 stainless steel slag, and more particularly to a method for effectively preventing elution of harmful substances which may be concerned when recycling the slag.
It is intended to make effective use of the slag.

[0002]

2. Description of the Related Art Effective use of slag, which is industrial waste,
Considering the cost reduction and the dedicated storage area in the factory site, this is an important issue for steelworks. Effective use of steelmaking slag includes recycling to sintering plants,
It is mainly used for roadbed materials and landfill materials for civil engineering work. For example, regarding application to landfill materials for civil engineering, see the Kawatetsu Technical Report (19
86, vol.18, No.1, p.20-24), a method for modifying and solidifying slag using a powdering inhibitor has been proposed.

[0003] Incidentally, stainless steel slag is greatly different from other steelmaking slag in that slag contains several percent of Cr oxide. In other words, stainless steel slag after decarburization refining usually contains more than 10 wt% of Cr in the form of chromium oxide.
Cr is wasted. Therefore, after the decarburization refining, a so-called reduction treatment is performed in which a reducing agent such as FeSi or Al is added to recover Cr in the slag in the molten steel. However, if the Cr concentration in the slag after the reduction is made to be zero as much as possible, the reduction efficiency becomes poor at a low Cr concentration, so that a large amount of expensive FeSi or Al is required, resulting in a significant increase in cost. Therefore, the Cr concentration in the slag after the reduction treatment is usually 0.5 ~, due to the cost balance between Cr loss due to slag disposal and addition of FeSi and Al.
It is about 3.0 wt%.

[0004] However, even with this slag Cr concentration of about 0.5 to 3.0 wt%, hexavalent Cr may be eluted. When stainless steel slag is used as roadbed material and landfill material for civil engineering, it is an absolute condition that environmental pollution does not occur due to elution of Cr ions from the slag, especially Cr peroxide (hexavalent C
It is an absolute condition not to elute r).

As a method for preventing the elution of hexavalent Cr from stainless steel slag, for example, in Japanese Patent Application Laid-Open No. 63-140044,
A method has been proposed in which a chromium oxide-containing slag discharged in a molten state is transferred to another container, and a sufficient amount of a reducing agent for reducing Cr oxide is added while stirring is provided. However, the above method is usually performed in a decarburization smelting furnace.
This is a technology relating to a so-called reduction treatment in which FeSi reduction is simply transferred to another container, and according to the embodiment, 2% still remains in the slag even after such reduction treatment is performed.
Since chromium oxide before and after remains, elution of hexavalent Cr cannot be completely prevented.

[0006]

The object of the present invention is to completely prevent the elution of Cr ⁶⁺ ions from stainless steel slag, and to be effective as a roadbed material or a landfill material for civil engineering without fear of environmental pollution. The aim is to propose a method for reforming stainless steel slag that can be used.

[0007]

As a process for producing stainless steel, various methods such as an electric furnace-AOD method, a converter-VOD method, a chromium ore smelting reduction method and a converter decarburization method have been put to practical use. The composition of slag generated during decarburization refining differs depending on the process, but in each case, several percent
Cr remains as described above. By the way, the present inventors have investigated the elution of hexavalent Cr from slag to water for the various stainless steel slags described above. When the S concentration in the slag is low, the elution of hexavalent Cr from the slag Has been found to easily occur. That is, even when the Cr concentration in the slag is as low as 1 wt% or less, if the S concentration in the slag is low, elution of hexavalent Cr may occur.

[0008] The inventors of the present invention have thought that it would be possible to prevent the elution of hexavalent Cr by increasing the S concentration in the slag, and added an S compound to the molten slag after the reduction treatment. Apparently, unexpected results were achieved in achieving the intended purpose. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. After the decarburizing refining, the molten chromium oxide-containing slag that has undergone the reduction treatment is blown with an inert gas and stirred.
A method for reforming stainless steel slag (1st invention), characterized by adding a valent S compound to increase the S concentration in the slag to 0.20 wt% or more.

[0010] 2. After the decarburization refining, a divalent S compound and boron oxide are added to the chromium oxide-containing slag in a molten state that has undergone a reduction treatment to reduce the S concentration in the slag to 0.20 wt% or more and the B ₂ O ₃ concentration. A method for reforming stainless steel slag characterized by being at least 0.1 wt% (second invention).

3. After the decarburization refining, the divalent S compound and boron oxide are added to the molten chromium oxide-containing slag that has undergone the reduction treatment while blowing and stirring an inert gas to reduce the S concentration in the slag to 0.20 wt. % And B ₂ O ₃ concentration
A method for reforming stainless steel slag characterized by being at least 0.1 wt% (third invention).

In each of the above-mentioned inventions, as the divalent S compound, FeS, CaS and the like are suitable, and among them, FeS is particularly suitable.

[0013]

FIG. 1 shows the results of a study on the relationship between the S concentration in the decarburizing smelting furnace slag after the reduction treatment during the melting of stainless steel and the elution amount of hexavalent Cr. The dissolution test of hexavalent Cr was performed by a method according to the Environment Agency Notification No. 13. As shown in the figure, when the S concentration in the slag was less than 0.2 wt%, it was found that elution of hexavalent Cr might occur.

Therefore, on a laboratory scale, the slag having an S concentration of 0.2 wt% or less in slag with elution of hexavalent Cr is redissolved, and an S compound is added to increase the S concentration to increase the hexavalent Cr.
It was investigated whether or not elution could be prevented. In addition,
FeS and CaS were used as the S compound. The addition of the S compound was performed while blowing the N ₂ gas into the molten slag and stirring the slag. FIG. 2 shows the obtained results. As is clear from the figure, FeS and CaS were added, and S in the slag was added.
If the concentration is increased to 0.2wt% or more, hexavalent Cr from slag
It has been found that the elution of can be completely prevented.
In addition, in compounds of sulfur oxides such as gypsum (CaSO ₄ ),
Although the S concentration in the slag increased, elution of hexavalent Cr could not be completely prevented.

As described above, if the divalent S compound such as FeS or CaS is added so that the S concentration in the slag becomes 0.2 wt% or more, elution of hexavalent Cr from the stainless steel slag can be completely prevented. Can be. However, S in the slag
If the concentration is too high, slag partially remaining in the furnace may increase the S concentration in the molten steel during the next refining, so the S concentration in the slag is preferably about 2 wt% or less.

Here, when the S concentration in the slag decreases, 6
Although the reason why the valent Cr is easily eluted has not yet been clearly elucidated, it is considered as follows from the electrochemical reaction. For Cr, the following equilibrium equation

(Equation 1) ^Holds , and the amount of Cr ⁶⁺ decreases under the condition that the electron emission is small.

As for reduction of cations in slag, Fe ions and S are reported in the following documents. [ИЗВЕСТИЯ ВЫСШИХ УЧЕБНЫХ
{1967}. Н. Боронен
ков et al. 10-14] According to this, Fe ²⁺ ← Fe ³⁺ + e ⁻ is reacted by applying a potential difference, and the Fe ²⁺ / Fe ³⁺ ratio is measured as a current value, but the CaS concentration in the slag is measured. It is shown that the reduction reaction of Fe ³⁺ ion progresses as the value increases. Here, it is considered that the phenomenon in which the electrons existing as S ²⁻ in the molten slag contribute to the reduction may exist for Cr as well.

The addition time of the divalent S compound according to the present invention is not desirable because during the decarburization blowing, the yield of S due to vaporization and desulfurization decreases and the S concentration in stainless steel increases due to an increase in input S. When the slag solidifies or loses fluidity, the added divalent S compound does not mix sufficiently uniformly in the slag, and the effect of preventing elution is reduced. Therefore, it is necessary to add the divalent S compound at a time when the slag is in a molten state before the slag is solidified after the completion of the reduction treatment. At this time, the slag is stirred simultaneously by blowing an inert gas, or It is necessary to add a predetermined amount of boron oxide in a complex manner or to perform at least one of the treatments. This is because simply adding a divalent S compound to a molten slag does not necessarily result in 2 slag.
This is because it cannot be guaranteed whether the valence S compound is uniformly mixed in the slag.

As described above, when stainless steel slag is used as a landfill material for civil engineering, elution of Cr ⁶⁺ ions must be completely prevented. This is because uniform mixing is premised, but slag agitation or compound addition of boron oxide by blowing in an inert gas extremely effectively acts on such uniform mixing.

First, the inert gas blowing and stirring will be described. The inert gas is used as a stirring gas because, in an oxidizing gas such as oxygen or air, Cr present in slag in trivalent is further oxidized. It is a condition that is likely to become hexavalent Cr in order to prevent it, and in addition, FeS is oxidized,
This is to prevent S from being released as SOx gas outside the furnace and not yielding in the slag. Here, N ₂ gas or Ar gas is advantageously adapted as the inert gas. The flow rate of this inert gas is desirably about 0.5 to 5.0 Nm ³ / min · t-slag, and sufficient stirring and homogenization can be achieved with this amount.
At the same time, the inside of the furnace can be maintained in a reducing atmosphere. Further, the method of blowing the inert gas may be any of a bottom-blow tuyere, a side-blow tuyere, and a method in which a lance is immersed and blown from above, and may be a blow method suitable for a decarburizing refining device.

Next, the composite addition of boron oxide will be described. The addition of a boron oxide such as colemanite has the effect of lowering the viscosity of the slag. Therefore, the divalent S added to the slag can be prepared without special slag stirring or the like.
The compounds are mixed well. Here, at least 0.1 wt% of B ₂ O ₃ is required in the slag for uniform mixing by the decrease in viscosity of the slag. Therefore, in the present invention, the B ₂ O ₃ concentration in the slag is Limited to 0.1 wt% or more. In addition, boron oxide also has an effect of preventing dusting during cooling of the slag, and this is also useful as a slag reforming method in this respect. However, if the concentration of B ₂ O ₃ in the slag is too high, the refractory wears significantly, so the B ₂ O ₃ concentration is preferably about 0.4 wt% or less.

Furthermore, if both slag stirring by blowing inert gas and viscosity reduction of slag by combined addition of boron oxide are used in combination, uniform mixing of S in slag can be achieved in a shorter time, and operation efficiency can be reduced. It is more advantageous in that respect. It should be noted that swinging the furnace body has the effect of more quickly uniforming the S in the slab, and therefore, it is advantageous to use this furnace body swing as needed.

[0023]

EXAMPLES The experiments were performed using a 100 ton stainless steel decarburization smelting furnace. After tapping the molten stainless steel (tap tapping temperature: 1660-1700 ° C), the residual Cr concentration in the furnace
For slag of 0.5 to 2.5 wt%, FeS (S content: 36.4 wt%), CaS (S content: 44.4 wt%), and colemanite (B ₂ O ₃ ) were obtained under the conditions shown in Tables 1 and 2. Content:
39.7wt%). After the treatment, the results of investigation on the S concentration of the slag discharged into the slag pot and the elution state of Cr ⁶⁺ are also shown in Tables 1 and 2. Here, the detection rate of Cr ⁶⁺ ion, when performing the dissolution test, the total number of tests under each condition
Cr ⁶⁺ concentration> 0.05 mg / l refers to the ratio of the test.
In addition, the detection rate of Cr ⁶⁺ was investigated by performing 10 or more for each charge.

[0024]

[Table 1]

[0025]

[Table 2]

As is clear from Tables 1 and 2, no leaching of Cr ⁶⁺ was observed from the slag treated according to the present invention. On the other hand, No.30 which did not perform any processing
In this case, the S concentration in the slag was extremely low, and therefore, elution of Cr ⁶⁺ was detected at a rate of 4.5%. Similarly, No. 34 to which only colemanite was added had a low S concentration in the slag,
% Eluted Cr ⁶⁺ . In addition, FeS and CaS were added, but were not actively stirred.
1, 33, 38, 39), the effect of the agitation treatment was not sufficient, and the S concentration in the slag could not be increased to 0.20 wt% or more (Nos. 35, 36, 37). In any case where the S concentration in the slag could not be increased to 0.20 wt% or more (No. 32), the elution of Cr ⁶⁺ from the slag could not be completely prevented.

[0027]

As described above, according to the present invention, elution of hexavalent Cr from stainless steel slag containing chromium oxide can be completely prevented, and therefore, the stainless steel slag can be removed from the roadbed without fear of environmental pollution. It can be used effectively for materials and landfills for civil engineering.

[Brief description of the drawings]

FIG. 1 S concentration in slag and hexavalent before slag reforming
4 is a graph showing a relationship with a Cr elution amount.

FIG. 2 is a graph showing the relationship between the S concentration in slag and the amount of hexavalent Cr eluted when an S compound is added to hexavalent Cr eluted slag.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Sumio Yamada 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Works Co., Ltd. Chiba Works (72) Inventor Ryuichi Asaho 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Inside the Chiba Works, Steel Works Ltd. Co., Ltd. (72) Inventor Hiroyuki Tobo 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Works, Ltd.Steel Research Laboratories (72) Inventor Shin Okubo 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawatetsu Mining Co., Ltd. Inside the factory (72) Inventor Yukio Sato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Prefecture Kawatetsu Mining Co., Ltd.Chiba (56) References JP-A-8-49007 (JP, A) JP-A-8-26791 (JP, A) JP-A-6-171993 (JP, A) (58) Fields investigated (Int. . ⁶ , DB name) C04B 5/06

Claims (4)

(57) [Claims] 1. A devalent S compound is added to a chromium oxide-containing slag in a molten state that has undergone a reduction treatment after decarburizing and refining of stainless steel, while blowing an inert gas and stirring. The method for modifying stainless steel slag, wherein the slag is 0.20 wt% or more. 2. After decarburizing and refining stainless steel, a divalent S compound and boron oxide are added to a chromium oxide-containing slag in a molten state that has undergone a reduction treatment to reduce the S concentration in the slag to 0.20 wt%. A method for reforming stainless steel slag characterized by the above and a B ₂ O ₃ concentration of 0.1 wt% or more. 3. After the decarburizing refining of stainless steel, a divalent S compound and boron oxide are added to the molten chromium oxide-containing slag subjected to the reduction treatment while blowing and stirring an inert gas. S concentration in slag is more than 0.20wt% and
A method for reforming stainless steel slag, wherein the concentration of B ₂ O ₃ is 0.1 wt% or more. 4. The divalent S according to claim 1, 2 or 3,
A method for modifying a stainless steel slag in which the compound is FeS.