JP3994988B2 - Method of recovering and using metal components contained in slag slag containing chromium - Google Patents

Description

  The present invention relates to a method for recovering and utilizing a metal component in slag generated when refining chromium-containing steel in a refining furnace such as a converter, an AOD furnace, or a VOD furnace.

  Conventionally, chromium-containing steel, for example, stainless steel, is made by melting chromium-containing molten metal as a main raw material in an electric furnace, and then using an AOD furnace or a VOD furnace to perform oxidative refining to minimize chromium oxidation loss. It is common to manufacture. Also, in recent years, technology to perform oxidative refining in the top-bottom converter using hot metal and alloy iron from the blast furnace, or directly add chromium ore into the top-bottom converter without using alloy iron. Various types of stainless steel manufacturing techniques have been proposed, such as a technique in which a crude molten metal obtained by smelting reduction is charged into another top-bottom blowing converter and subjected to oxidative refining.

  In these stainless steel manufacturing processes, it is a common problem that expensive chromium migrates into the slag due to the oxidation of chromium that proceeds simultaneously with the decarburization reaction in the smelting furnace. The extent to which the amount of chromium oxide, represented by) can be reduced, is extremely important economically. Here, chromium oxide means a state in which chromium is oxidized to a divalent, trivalent or hexavalent number. In the analysis of normal slag, the total of these is expressed in terms of chromium, and the total chromium (T・ Cr). Accordingly, in this invention as well, the (T · Cr) concentration represents the total concentration of chromium in the oxidized state as described above.

  Therefore, in order to recover the generated (T · Cr), reduction recovery treatment with FeSi or the like is performed after oxidative refining. However, as the chromium oxide is reduced and recovered, the chromium concentration in the slag decreases and the reduction efficiency also decreases, so the amount of reducing materials such as FeSi and Al used for the reduction inevitably increases. It will mimic the increase. Accordingly, reduction is generally performed until the (T · Cr) concentration becomes about 0.7 to 3 wt%, and then slag is discharged.

  However, in this treatment method, the reduction treatment varies, or depending on the state after cooling, chromium in the slag may be reoxidized to generate harmful chromium peroxide, so-called hexavalent chromium, and this slag is reclaimed. It becomes a big obstacle when it is used for such applications. This is because even when most of the slag is not problematic, if slag containing chromium peroxide is mixed, it is impossible to reuse all of the slag.

With respect to this problem, Patent Document 1 proposes a method of adding FeS in order to suppress generation of chromium peroxide in slag.
JP-A-8-104553

  However, when FeS is added to the slag, a large amount of S-containing gas is generated and adversely affects the environment. Therefore, it is necessary to recover this exhaust gas. The actual adoption is difficult because there are many harmful effects due to the addition of FeS, such as the fact that it is generated and that S in the exhaust gas is high and it is difficult to reuse it as a combustion gas.

  In addition, the magnetic iron content in the slag can be recovered by magnetic separation, but the stainless steel refining slag contains a large amount of metal that is difficult to separate from the slag, and it is difficult to recover this metal component. It is rare.

  An object of the present invention is to propose an effective method for recovering and using metal components in slag slag containing chromium.

  The inventors have made various studies on the recycling of slag slag containing chromium that meets the above-mentioned purpose, and paid attention to the fact that the smelting furnace is located in the steelworks, and in the steelmaking upstream process of the smelting furnace. It has been found that it is advantageous to recycle slag for a blast furnace. That is, by returning the slag to the blast furnace, the metal component in the slag can be recovered into molten iron, and the slag modified here can be reused for civil engineering and architectural purposes.

This invention is derived from the above findings.
That is, in the present invention, the chromium-containing molten metal charged into the smelting furnace is subjected to a reduction treatment on the slag generated by the oxidative refining performed by supplying oxygen to reduce the (T · Cr) concentration in the slag to 0.3. Adjust to ~ 3.0wt%, then add boron oxide to the slag to solidify the slag, then add the slag directly or after sintering to the blast furnace, and in the blast furnace, add the metal components in the slag to the molten iron Before the decarburization refining, the molten iron recovered and recovered from the blast furnace is subjected to hot metal pretreatment that supplies oxygen at an oxygen unit of 2 Nm ³ / t or more, and reduced from the slag into the molten iron in the blast furnace. This is a method for recovering and using a metal component contained in a chromium-containing steel refining slag, wherein the recovered B is removed, and then the molten iron is charged into a refining furnace to perform decarburization refining.

Here, subjecting added to blast furnace slag produced during decarburization refining directly or after sintering the recovery and reuse of the metal components in the slag is compatible advantageously Upon implementation.

  According to the present invention, recycling of stainless steel refining slag, which has been difficult in the past, can be realized almost completely, so that both cost reduction and productivity improvement can be achieved in refining and further in the ironmaking process. .

Now, when the slag generated during refining of stainless steel is added to the blast furnace, the metal contained in the slag and the metal oxides such as FeO, Cr ₂ O ₃ and MnO are melted or reduced, and are put into the hot metal in the blast furnace. Collected. On the other hand, since the slag itself is completely reduced in the blast furnace and reformed into a so-called blast furnace slag that does not contain oxides, the modified slag is converted into civil engineering such as roadbed material, cement, or raw concrete aggregate, Can be reused for construction purposes.

  Here, if the (T · Cr) concentration in the stainless steel refining slag is too high, when this slag is added to the blast furnace, the amount of chromium reduced to the molten iron increases, and the chromium concentration in the molten iron increases. The increase in the chromium concentration in the molten iron is not particularly a problem if all of the blast furnace molten iron is used as a refining raw material for chromium-containing steel, but it is usually used as a refining raw material for ordinary steel. It is necessary to stop the chromium content to such an extent that it does not cause problems. In other words, it is necessary to reduce the (T · Cr) concentration in the slag to some extent. On the other hand, as described above, in order to reduce the (T ・ Cr) in the slag to an extremely low concentration range, the amount of reducing material used must be increased along with the reduction in reduction efficiency. The cost for reduction increases in inverse proportion to the Cr) concentration.

  According to the knowledge of the inventors, when the reduction treatment is completely performed until the (T · Cr) concentration is about 0.3 wt% or less, the problem of chromium peroxide is solved, but the variation (T · In order to reduce the Cr) concentration to 0.3 wt% or less, as shown in FIG. 1, a great reduction cost is required. On the other hand, in order to reuse slag, the recycling cost for reuse in the blast furnace, specifically, the cost of colemanite and handling, or the use of inexpensive raw materials such as Ni slag to limit the Cr concentration is restricted. The cost increases due to the increase in raw material costs accompanying the increase in the amount of raw materials and the increase in auxiliary materials accompanying dechromation in the steelmaking process. As shown in Fig. 1, if the (T · Cr) concentration exceeds 3 wt%, the recycling and reduction costs, and the refining cost that combines both, make the (T · Cr) concentration less than 0.3 wt%. It turns out that the cost equal to or more than the case is necessary. That is, if the (T · Cr) concentration is 3 wt% or less and slag is not reused, it will be disadvantageous in terms of cost. Another problem is that when the (T · Cr) concentration exceeds 3 wt%, the mass tends to be pulverized after crushing. The reason for this is not clear, but it is presumed that when the (T · Cr) concentration exceeds 3 wt%, the properties of the slag change and the addition yield of the collimite added in the furnace decreases. Therefore, in order to reuse slag without problems such as cost increase and powdering, it is necessary to make the (T · Cr) concentration 3 wt% or less.

  Moreover, it is common to use Si containing substances, such as FeSi, for the reducing material of chromium oxide in the slag, but when reducing with FeSi, the Si concentration in the molten steel increases. That is, FIG. 2 shows the relationship between the (T · Cr) concentration in the slag and the Si concentration in the molten steel when it is reduced with FeSi. From the figure, it can be seen that the (T · Cr) concentration in the slag can be suppressed to 3.0 wt% or less by making the Si concentration in the molten steel 0.1 wt% or more.

On the other hand, when the Si concentration in the molten steel increases, Si is oxidized during the decarburization process in the secondary refining, and the slag volume increases, resulting in various problems such as a decrease in alloy addition yield and formation of inclusions. May cause. In addition, when the Si content is high, the amount of nitrogen adsorbed in the steel output increases, and it becomes difficult to melt extremely low nitrogen steel. According to the investigations by the inventors, these phenomena frequently occur when the Si content exceeds 0.3 wt%, so it is desirable to limit the Si content to 0.3 wt% or less.
From the above knowledge, it can be seen that the reduction treatment is preferably performed using FeSi and under the condition that the amount of Si in the molten steel is 0.10 to 0.30 wt%.

Furthermore, the second reason for performing reduction using a Si-containing substance such as FeSi is to suppress an increase in the Al ₂ O ₃ concentration in the slag. In other words, when the Al ₂ O ₃ concentration in the slag is high, when the slag is put into the blast furnace, the viscosity of the blast furnace slag becomes high and hinders the discharge and unloading work. Here, the Al ₂ O ₃ concentration in the blast furnace slag usually has to be adjusted to 14 or 15 wt% or less, but since Al ₂ O ₃ is mixed from other raw materials, the Al ₂ O ₃ in the slag is The lower the better. Considering the level at which slag is practically reused (5 to 50 kg / t per blast furnace discharge), Al ₂ O ₃ needs to be 10 wt% or less, preferably 5 wt% or less. If, for example, FeSi is used as the reducing material after the actual refining of stainless steel, the amount of Al ₂ O ₃ can be maintained at about 2 wt% or less.

Next, when adding the stainless steel refining slag to the blast furnace, the stainless steel refining slag was easily pulverized, but the powdered stainless steel slag was found to be difficult to use in the blast furnace. That is, stainless steel refining slag is likely to be pulverized after cooling due to volume expansion that occurs during the phase transformation of so-called dicalcium silicate {(CaO) ₂ SiO ₂ }. Compared to slag, this powdering is remarkable because the P content is low and the phase is unstable. This naturally pulverized slag is extremely fine and has the problem of flying when transported, etc. In addition, when added to the blast furnace, it impedes the air permeability in the blast furnace, so it is difficult to use in the blast furnace. I understood it.

  Therefore, prior to cooling the slag, the inventors add boron oxide to the slag after the reduction treatment to prevent slag from being pulverized, cool and solidify, and then pulverize and adjust the particle size again. I tried to do that. That is, as described above, most of the slag that is naturally cooled and solidified is pulverized, and the particle size of the slag is extremely small as illustrated in Table 1. Will interfere with sex. Therefore, boron oxide is added to the slag after the reduction treatment to prevent slag from becoming pulverized and solidified. After the solidified slag is crushed and then passed through a sieve, the slab is separated from the 10mm diameter. By sorting, a larger amount of massive slag can be obtained, while a slag having a diameter of 10 mm or less can also have a relatively large diameter, and can be advantageously introduced into a blast furnace or a sintering furnace. Incidentally, the particle size distribution of the slag obtained by crushing the slag, which is solidified by adding boron oxide, is as shown in Tables 2 and 3.

Moreover, although the ratio of massive slag and powdered slag depends on the selection of a crusher or the like, it is preferably about 40 to 60 wt%, that is, about half. In order to solidify the slag, it is necessary to add boron oxide (B ₂ O ₃ ) concentration in the slag to be about 0.1 to 0.5 wt%. In this case, a method is used in which boron oxide is added to the furnace after the steel is discharged and discharged, while stirring with gas from the bottom blowing tuyere. On the other hand, in the method of putting in the slag discharge container in advance, the agitation is not sufficiently performed and the yield of boron addition is lowered.

  Here, the boron oxide used to solidify the slag is directly brought into the blast furnace by the slag, and is reduced in the blast furnace and contained in the molten iron. The resulting molten iron has a high B concentration. If this B remains in the product stage, depending on the steel type, there will be a problem in the material, so it is necessary to remove it in the steel making process. Although this B can be removed by oxidation refining in the converter of the next process, it has been newly found that if this B is removed by refining, there is a problem in terms of the subsequent recycling of slag.

That is, for the converter slag as well as the above-described stainless steel refining slag, it is effective to add to the blast furnace and recover CaO and iron content in the converter slag, but if B is removed by the converter, It was found that B transferred to the converter slag was reduced and recovered again to the molten iron of the blast furnace, and B was circulated and concentrated by repeating the above cycle. Then, it tried to remove B by hot metal preliminary treatment. In the hot metal pretreatment, since the treatment temperature was low, there was a possibility that B could not be removed. However, as shown in FIG. 3, if oxygen was increased to 2 Nm ³ / t molten iron or more, It was found that can be removed. In addition, FIG. 3 investigated the relationship between supply oxygen amount and B density | concentration at the time of supplying oxygen in various concentrations of B containing molten iron in a topped car.

  Here, in addition to normal oxygen gas, oxygen may be solid oxygen that reacts in so-called molten iron such as iron ore or scale to cause an oxidation reaction. Also, the method of adding oxygen may be selected as appropriate, such as a method of injecting together with lime or the like, or a method of performing only oxygen from the top blowing lance with the purpose of supplying heat.

  As described above, in the refining of stainless steel, after performing oxidation refining first, for example, by adding FeSi with a Si concentration in the molten steel of 0.10 to 0.30 wt%, (T · Cr) in the generated slag After adjusting the concentration to 0.3 to 3.0 wt% and then discharging the molten steel, in the smelting furnace, boron oxide is added to the slag and then discharged, cooled and solidified, and then the solidified slag is crushed and sieved In addition, lump slag with a diameter exceeding 10 mm is added directly to the blast furnace, and only powdered slag with a diameter of 10 mm or less is added to the sintering furnace and added to the blast furnace. The slag recovered and reformed into blast furnace slag is used for the same applications as conventional blast furnace slag such as roadbed materials.

In addition, the molten iron discharged from the blast furnace is subjected to oxidative refining such as hot metal pretreatment, and preferably oxygen of 2 Nm ³ / t molten iron or more is supplied and oxygen in the molten iron is substantially 1 ppm or less. Remove until about. Then, the molten iron is charged into the converter, and normal decarburization refining is performed.For the converter slag discharged here, by adding it to the blast furnace and recycling it as in the above stainless steel refining slag, Reduce the amount of lime used in the blast furnace.

  The method according to the present invention is effective not only for the operation of melting stainless steel by the so-called converter method but also for the operation of melting stainless steel by the AOD method or VOD method. In particular, it is effective in operations that increase the basicity of slag to 1.8 or more for the purpose of desulfurization during reduction. This is because slag powdering becomes severe when the basicity is 1.8 or more. When desulfurization is performed at the time of reduction, the upper limit of slag basicity is not particularly limited in the present invention, but at 3.0 or higher, hatching deteriorates and desulfurization efficiency in the converter does not increase, and desulfurization refining is not performed. As the slag volume increases, there are many economic adverse effects such as an increase in chrome loss, so a practical upper limit is around 3.0.

  In addition, the refining furnace used for the above-described stainless steel refining is not particularly limited, and there is no problem whether the refining gas is blown from the bottom or the side of the bath from below the bath surface. Is preferred. The reaction vessel used as the hot metal preliminary treatment may be any type of reaction vessel that injects or top blows oxygen, iron oxide, or the like, in addition to a topped car, a pan or a converter. This also applies to the refining of ordinary steel, which is subsequently performed, and there is no particular limitation on the reaction vessel such as the converter.

Stainless steel was refined using an upper-bottom converter with a capacity of 175 t. Here, an example will be described in which a so-called Cr ore is smelted and reduced using two upper bottom blowing converters and then decarburized in another upper bottom blowing converter. That is, in a converter charged with a chromium-containing molten metal (C: 5.5 wt%, Cr: 9-13 wt%), oxidative refining is applied for 60-70 minutes to decarburize, and C at the end of refining is 0.07-0.20. Adjusted to the wt% range. For CaO during blowing, the Si in FeCr was calculated so that the slag basicity was 2.0. Subsequent reduction refining was performed by changing the basicity of the slag to about 1.8 to 2.5 based on the analysis result of the sampling performed before the reduction. Specifically, the oxidized chromium was estimated and FeSi for reduction was determined, and CaO was added to match the SiO ₂ produced by FeSi. In order to improve fluidity, fluorite was added at 5 to 10 wt% of the lime input, and MgO was adjusted to remain several wt% in the slag to protect the refractory in the converter.

Next, after the molten stainless steel was produced at a temperature of 1680 to 1720 ° C., the slag remaining in the furnace was colemanite (B ₂ O ₃ : 39.7 wt%, FeO ₃ : 1 to ₂ wt%, SiO ₂ : 11 wt%, CuO: 21 wt%, MgO: 2.9 wt%). Here, the B ₂ O ₃ content in the colemanite was about 40 wt% and the crystal water was 7 to 8 wt%. The (T · Cr) concentration in the slag was 0.5 to 3 wt%. B ₂ O ₃ in the slag was 0.3 to 0.5 wt%. When adding the colemanite, N ₂ was blown into the furnace from the bottom blowing tuyere and stirred at 110 Nm ³ / min.

  After the slag was cooled and solidified, it was crushed and sieved, and massive slag with a diameter exceeding 10 mm was directly put into the blast furnace from the hopper hopper. On the other hand, the powder slag having a diameter of 10 mm or less was subjected to bedding and mixed with other sintering raw materials, and then made into sintered ore in a sintering furnace. The obtained sintered ore was added into the blast furnace together with the lump slag.

  According to the above procedure, the refining slag was repeatedly added to the blast furnace for every refining charge. The composition of the refining slag added to the blast furnace is as shown in Table 4. Moreover, in the blast furnace operation performed in parallel with the said refining operation, the result of having investigated about the molten iron component and blast furnace slag component for every tapping is shown in Table 5 and 6.

  From Table 5, the blast furnace slag discharged from the blast furnace was completely different from the normal blast furnace slag, and could be used for civil engineering and construction such as hearth materials, cement raw materials, and ready-mixed aggregates. Furthermore, it is clear from the comparison between the composition of the smelting slag in Table 4 and the composition of the blast furnace slag in Table 5 that the metal components in the smelting slag were recovered in the molten iron in the blast furnace. Incidentally, the amount of refining slag recycled in the blast furnace was 2400 to 3000 t / month, and the amount of molten iron produced during that time was 300,000 t.

Next, after the molten iron melted in the blast furnace was received by the topped car, the hot metal preliminary treatment was performed by the topped car. As shown in Table 6, the concentration of B in the molten iron was 3 to 8 ppm.
That is, by receiving the molten iron of 250t to Topidoka, with blown 15 Nm ³ / min of oxygen oxygen from the lance from 20 Nm ³ / min and an injection lance relative to the molten iron, the 350 kg / min and CaO iron oxide from the injection lance 50 to 200 kg / min The blowing process was performed for approximately 10 to 25 minutes. The effective oxygen intensity in this treatment was 3.3 to 8.2 Nm ³ / t molten iron.

Here, the effective oxygen basic unit is a basic unit used per molten iron of oxygen from which oxygen used for secondary combustion of top blown oxygen is removed, as defined by the following equation.
[Equation 1]
Effective oxygen intensity = (injection oxygen + oxygen in injection iron oxide + top-blown oxygen /
(1 + PCR)) / molten metal weight Here, PCR: secondary combustion rate = CO ₂ / (CO + CO ₂ )
CO, CO ₂ : CO, CO ₂ concentration in exhaust gas

As shown in FIG. 3, B in the molten iron was suppressed to 1 ppm or less by the hot metal preliminary treatment. Thereafter, molten iron was charged into the converter and decarburized and refined. The amount of blown-off C was 0.03 to 0.20%, but in all cases, B remained at 1 ppm or less. Here, when B ₂ O ₃ in the slag was investigated after the steel was discharged from the converter, it was not recognized in the analytical detection range in any charge. Subsequently, all the converter slag discharged from the converter was added to the blast furnace, and it was used for recycling in the same manner as the above-described refining slag.

It is a graph which shows the relationship between (T * Cr) density | concentration in slag and refining cost. It is a graph which shows the relationship between Si content in molten steel, and (T * Cr) density | concentration in slag. It is a graph which shows the relationship between the oxygen basic unit supplied to molten iron, and B density | concentration in molten iron.

Claims (2)

The slag generated by the oxidative refining performed by supplying oxygen to the chromium-containing molten metal charged in the smelting furnace is reduced to adjust the (T · Cr) concentration in the slag to 0.3 to 3.0 wt%. Then, boron oxide is added to the slag to solidify the slag, and then the slag is added directly or after sintering to the blast furnace, and the metal components in the slag are reduced and recovered in the molten iron in the blast furnace, and then Prior to decarburization and refining, the molten iron discharged from the blast furnace is subjected to a hot metal pretreatment that supplies oxygen at an oxygen unit of 2 Nm ³ / t or more to remove B that is reduced and recovered from the slag into the molten iron in the blast furnace. And then, the molten iron is charged into a smelting furnace and decarburized for refining, and a method for recovering and using a metal component contained in a chromium-containing steel smelting slag. The slag generated during decarburization refining is added to a blast furnace directly or after sintering and used for recovering and using metal components in the slag according to claim 1. Method.

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