JP2008081845A - Method for treating slag containing chrome steel - Google Patents

Abstract

A chromed steel refining slag treatment method is proposed in which chromium contained in a slag is fixed in a trivalent state with respect to slag generated in a large amount during refining of chromium-containing steel.
For slag generated when refining chromium-containing steel by AOD or VOD, the reduction of chromium is induced by adjusting the basicity of the slag to CaO / SiO ₂ > 1.2. The slag hot water surface area (Sm ²⁾ indicates that the valent chromium oxide Cr ₂ O ₃ has a concentration of 3.0 wt% or less, and then the smelted chromium-containing molten steel is exposed to the atmosphere. ) And the slag volume (Vm ³ ) ratio (S / Vm ⁻¹ ) of 4 or less, the method for treating chrome-containing steel refining slag discharged in a vessel.
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Description

  The present invention relates to a method for treating slag generated at the time of smelting chromium-containing steel outside the furnace, and in particular, proposes a technique for generating slag that can be reused as resources such as civil engineering materials and building materials. .

Chromium-containing steels, typically stainless steel, are generally produced by melting in an electric furnace and then subjected to out-of-furnace refining such as AOD method and VOD method, and generate slag at each stage. Such slag is generated during removal of impurities (C, O, S, P, and non-metallic inclusions) contained in the molten steel mainly by the addition of a deoxidizing agent, a desulfurizing material and a dephosphorizing material. In particular, in the case of chromium-containing steel, Cr ₂ O ₃ is contained in the slag. As other components, Si, Al, etc. in the molten steel are oxidized during the deoxidation process and reduction of Cr ₂ O ₃ to become oxides such as SiO ₂ , Al ₂ O _3, etc. A molten slag is formed together with CaO or MgO added for the purpose of protection.

  In particular, in the out-of-core refining of chromium-containing steel such as stainless steel, part of chromium in molten steel is oxidized during the oxygen blowing stage for decarburization to produce chromium oxide, which is transferred into the molten slag. It is known to do. For this reason, reduction treatment of chromium oxide is usually performed at the end of the refining process, but some of the chromium oxide remains in the slag, and such slag cannot be reused as roadside material as a valuable resource. was there.

  However, in recent years, attention has been focused on reusing such slag as a material for civil engineering due to problems such as reduction of waste and depletion of natural resources.

  For this reason, various studies have been made on methods for treating such chromium oxide-containing slag. For example, as a method for preventing elution of hexavalent chromium from exhaust slag, many techniques such as Patent Document 1, Patent Document 2, and Patent Document 3 are disclosed. However, both of these methods are a method of reducing slag discharged outside the furnace at a high temperature and a method of processing the slag by adding Ba salt or the like, which has a high processing cost and a large amount of waste. Since it is not the method of processing slag, it was unsuitable as a processing method of chromium-containing steel slag generated in large quantities.

Further, Patent Document 4 and Patent Document 5 propose that the recycling treatment of chromium-containing steel slag is performed in a melting-refining process. However, these methods may not be reused as resources unless the slag cooling process after evacuation is sufficiently controlled, and are still insufficient as a slag treatment method.
JP-A-48-71371 Japanese Patent Laid-Open No. 52-152651 JP-A-5-345658 JP-A-8-302418 Japanese Patent Laid-Open No. 8-29517

  Therefore, in view of the above-mentioned situation that the prior art has, the present invention is a trivalent state of chromium contained in the slag with respect to the slag generated in large quantities during the refining of chromium-containing steel. This paper proposes a method for treating smelting slag containing chromium.

  The inventors have the biggest problem in recycling chromed steel refining slag, and some of the trivalent chromium oxide contained in this slag is further oxidized to form hexavalent chromium oxide. Under the recognition that there is a change, we conducted diligent research to solve the problem. As a result, it was found that trivalent chromium oxide was oxidized by reaction with free oxygen ions contained in atmospheric oxygen and slag, and changed into hexavalent chromium oxide. It has also been determined that the largest source of is undehydrated CaO.

The present invention has been developed based on such knowledge, and the basicity of slag generated when refining chromium-containing steel by AOD or VOD is set to CaO / SiO ₂ > 1. By adjusting to 2, the reduction of chromium is led, the concentration of the trivalent chromium oxide Cr ₂ O ₃ is 3.0 wt% or less, and then the refined chromium-containing molten steel is replaced with the slag after being discharged. The slag surface area (Sm ² ) and the slag volume (Vm ³ ) ratio (S / Vm ⁻¹ ), which indicates the area exposed to the atmosphere, is discharged into a container having 4 or less. It is the processing method of chromium steel refining slag.

In the present invention, the reduction treatment of chromium is 5 wt% or less in the Cr ₂ O ₃ concentration after the reduction, the molten steel temperature at the time of exhausting is 1550 ° C. or more, and the slag is treated with an inert gas atmosphere. It is desirable to cool down.

  As described above, according to the present invention, in the refining of chromium-containing steel, most of the chromium oxide contained in the slag generated in large quantities during this refining can be fixed to trivalent, Effective use is possible.

As described above, in the out-of-furnace refining of chromium-containing steel such as stainless steel, chromium in the molten steel is oxidized and Cr ₂ O ₃ is generated in the oxygen blowing period for decarburization in the refining process. This Cr ₂ O ₃ generates refined slag containing chromium oxide together with oxides such as Si and Al added as a reducing agent and CaO and MgO added as a flux.

With respect to the chromium-containing refining slag, the inventors first studied the mechanism of formation of hexavalent chromium oxide mainly caused by free oxygen ions contained in the slag. As a result, when free CaO and Cr ₂ O ₃ are present in the chromium-containing refining slag, CaO · Cr ₂ O ₃ formed in the high temperature phase of refining is generated, and this is the undehydrated CaO in the subsequent slag cooling process. It has been found that it is oxidized by free oxygen ions generated from the above and undergoes a phase transformation represented by the following formula (1) to produce CaCrO ₄ which is a hexavalent chromium oxide.
2CaO + 2CaO · Cr ₂ O ₃ + 3O ₂ → 4CaCrO ₄ (1)

Therefore, the inventors have considered that the method of preventing the oxidation reaction of the above formula (1) is effective in preventing the formation of hexavalent chromium oxide, and the following of the above oxidation reaction. I came up with a suppression mechanism like That is, when the basicity CaO / SiO ₂ (hereinafter simply abbreviated as C / S) of the out-of-furnace slag is adjusted to 1.2 or more, Si contained in the molten steel acts as a reducing agent, and slag It was found that the trivalent chromium oxide Cr ₂ O ₃ therein was reduced and recovered as metal Cr. As a result, the Cr ₂ O ₃ concentration in the slag is reduced, and the production of hexavalent chromium oxide is suppressed. Not particularly need to be limited to, _Cr 2 _{O 3} concentration in the chromium-containing molten steel after reduction is less 3.0 wt%. More preferably, it is 1.2 wt% or less. In addition, when a reducing agent is added for the purpose of promoting the reduction of Cr ₂ O ₃ , it is preferable to add Si, Al, Mn, etc. having a stronger affinity for oxygen than Cr.

Even when the CaO becomes slag formation failure due to a temperature insufficient or stirring shortage during Haikasu, since the periphery of the non-slag formation CaO becomes locally high C / S, the CaO by SiO ₂ slag formation The effect is reduced, and hexavalent chromium oxide is easily generated. Therefore, although it is not particularly limited, it is preferable that the molten steel temperature before waste is 1550 ° C. or higher. More preferably, it is 1570 degreeC or more. Further, in order to reduce the concentration of the trivalent chromium oxide Cr ₂ O ₃ itself contained in the slag, it is possible to add a reducing agent such as Si, Al and Mn, which has a stronger affinity for oxygen than Cr. preferable.

  Next, the production mechanism of hexavalent chromium oxide mainly caused by atmospheric oxygen will be described. It has been conceived that the reaction shown in the above formula (1) can be suppressed and the production of hexavalent chromium oxide can be suppressed by reducing the area of the slag exposed to the atmosphere as much as possible after evacuation.

That is, when the ratio (S / V) of the slag hot water surface area (S) to the slag volume (V) is adjusted to 1.8 or more and 4 or less as is apparent from Table 1, the slag capacity is reduced to the atmosphere. The exposed area is sufficiently small so that the reaction according to the above formula (1) does not proceed. Even if the trivalent chromium compound Cr ₂ O ₃ is contained in the slag, the trivalent chromium compound is fixed as it is. Formation of valent chromium oxide can be prevented. Here, the reason why S / V is limited to 4 or less is that when S / V is larger than 4, the area exposed to the atmosphere is large with respect to the slag volume, and depending on conditions, hexavalent chromium oxide is generated. Because there is.

  Furthermore, in order to prevent oxidation by the atmosphere, it is preferable that the waste slag is received in a container such as a slag pot and cooled in an inert gas atmosphere. Specific examples include blowing an inert gas and cooling in a container filled with an inert gas. The type of the inert gas is not particularly limited, but argon, nitrogen, helium, carbon dioxide and the like are preferable. In order to fill and seal the container with an inert gas, it is preferable to cover the slag pot.

  Refined slag was produced by variously changing the basicity and cooling conditions of the slag produced during out-of-furnace refining of chromium-containing steel raw materials by the AOD method or VOD method. In addition, when each condition is changed extremely in operation with an actual machine, there may be a problem in operation. For some (comparative example), a small amount of slag was produced in the laboratory.

  About each produced refined slag, the elution amount of hexavalent chromium was measured based on the environmental agency notification No. 46 which is the verification method of a soil environmental standard. Furthermore, the slag composition was specified by fluorescent X-ray diffraction, and the presence or absence of undehydrated CaO was specified by micro-observation of the sampled sample. The results are shown in Table 1.

  According to the experimental results shown in Table 1, since elution of hexavalent chromium is not detected, it can be seen that chromium in the slag is fixed to trivalent. However, in the comparative example, since at least one or more is out of the range defined in the present invention, a very small amount of hexavalent chromium is eluted.

Claims (4)

For slag generated when refining chromium-containing steel with AOD or VOD, by adjusting the basicity of this slag to CaO / SiO ₂ > 1.2, the reduction of chromium was led to trivalent chromium oxidation. The slag hot water surface area (Sm ² ) and slag volume indicate the area in which the product Cr ₂ O ₃ has a concentration of 3.0 wt% or less, and the smelted chromium-containing molten steel is then exposed to the atmosphere. A method for treating chrome-containing steel refining slag, characterized in that it is discharged into a container having a ratio (S / Vm ^-1 ) to (Vm ³ ) of 4 or less. The method for treating chromium-containing steel refining slag according to claim 1, wherein the reduction treatment of chromium is to reduce the Cr ₂ O ₃ concentration after the reduction to 1.2 wt% or less. The method for treating a chrome-containing steel refining slag according to claim 1 or 2, wherein the molten steel temperature at the time of waste is set to 1550 ° C or higher. The said slag is cooled in inert gas atmosphere, The processing method of chromium-containing steel refining slag of any one of Claims 1-3 characterized by the above-mentioned.