JP4488305B2 - Fluorine elution suppression method for electric furnace slag - Google Patents

Description

  The present invention relates to a method for suppressing the elution of fluorine from electric furnace slag generated in a steel melting and refining method using an electric furnace and a ladle refining furnace.

  The steel melting and refining method in the electric furnace is generally only scrap melting and oxidation refining in the electric furnace, and the reductive refining takes the ladle refining furnace (the ladle under the lid with the heating electrode, The refining furnace is made to function as a refining furnace by covering with a lid.Hereafter, the integrated refining method is called “smelting furnace”, and the unintegrated case is simply called “ladder”) Has been done. Among them, the conventional method relating to slag processing is as follows.

  (1) Make an oxidizing slag for oxidation refining in an electric furnace. When the oxidation refining is finished, the oxidizing slag is separated and transferred to the slag pan and the molten steel is transferred to the ladle. At this time, it is desirable that the oxidizing slag that has been discharged to the ladle along with the molten steel is scraped and removed from the slag pan before the ladle refining.

  (2) In the ladle refining furnace, a new slagging agent is added to the molten steel surface and heated to produce reducing slag and reductive refining.

  (3) After reductive refining, when recirculating vacuum degassing is performed, the ladle is moved under a recirculating vacuum degassing device (RH degassing device), and after continuous recirculation vacuum degassing refining, continuous casting Transfer from the bottom nozzle to the tundish or ingot mold.

  (4) After the molten steel is put out into a tundish or a steel ingot mold, the reducing slag remaining in the ladle is tilted and poured into the slag pan, and the disposal is performed. Here, the remaining hot water (residual molten steel) is separated and transferred to an electric furnace or ladle, or separated after simultaneous transfer.

  (5) When reusing reducible slag, after transferring the reducible slag to the ladle or with the remaining hot water (residual molten steel) in the ladle, before the electric furnace before oxidation refining or before refining In many cases, the slag is transferred to the ladle, and at the time of or after the transfer of the reducing slag to the slag pot, a reforming treatment for improving the fluidity of the reducing slag may be performed.

  Conventionally, in steel refining, fluorite has been used as a slagging agent in order to lower the melting point of slag, improve the fluidity of slag, and increase the reactivity with molten steel. Was contaminated with fluorine. From the slag mixed with fluorine, fluorine exceeding the environmental standard value of the soil sometimes eluted.

  Therefore, in the past, in order to suppress the elution of fluorine from the slag, a fixing agent such as gypsum, an aluminum-containing material, chromium, and a bulking agent have been used.

Further, as a method for suppressing fluorine eluted from steel slag containing solidified fluorine, it has been disclosed to add 12CaO · 7Al ₂ O ₃ and 3CaO · Al ₂ O ₃ to steel slag containing fluorine (for example, Patent Document 1).

  Further, as a method for preventing the elution of fluorine from slag containing fluorine, it is disclosed to mix aged slag containing fluorine, aged reduced slag containing fluorine, and gypsum (for example, , See Patent Document 2).

In addition, as a method for effectively using the reduced slag, a method is disclosed in which the molten slag is mixed with the oxidized slag, and then the mixed molten slag is granulated and rapidly cooled (see, for example, Patent Document 3). ).
Japanese Patent Laid-Open No. 2003-211118 JP 2004-123476 A JP 2001-262217 A

  However, in the slag fluorine elution suppression as described above, a substance such as slag tempering is added, or a stabilizer is added. It is very expensive. In addition, when changing the composition of the slag, there is a secondary demerit that promotes the wear of refractories and reduces the life of the furnace, and also affects the yield of molten steel in the molten steel production process. was there.

  In the method of the present invention, in the conventional production process, all of the reducing slag (exhaust) and remaining hot water remaining after the steel is discharged from the ladle of the preceding heat to the tundish are mixed with the oxidizing slag produced by the preceding charge. It is based on the fact that elution of fluorine from the electric furnace slag is suppressed by increasing the number of processes to be performed.

  With regard to the suppression of fluorine elution from electric furnace slag, which occurs in steel melting and refining using an electric furnace and ladle refining furnace, the processing costs are small and the quality of steel products is not impaired. By applying the production process mutatis mutandis, new equipment is not required, and a method for suppressing the elution of fluorine from the electric furnace slag generated thereby is provided.

That is, the method for suppressing fluorine elution of the electric furnace slag of the first invention in the present invention for solving the above problems is a method for melting and refining steel in which melting and oxidation refining are performed in an electric furnace and reduction refining is performed in a ladle In
Reductive slag remaining after steel is discharged from the ladle of the preceding heat to the tundish at the preceding charge when transferring the molten steel with a certain amount of oxidizing slag after oxidation refining in the electric furnace to the ladle. The first mixing step of mixing all of the remaining hot water with the oxidizing slag produced by the preceding charge, and the reducing charge from the mixed slag produced by the preceding charge and all of the remaining hot water at the implementation charge. It consists of the 2nd mixing process mixed with the oxidizing slag manufactured by the implementation charge.

According to a second aspect of the present invention, in the first aspect, the main compound form in the oxidizing slag is 2CaO · SiO ₂ , and the main compound form in the reducing slag is 3CaO · Al ₂ O _3. By mixing these, 12CaO · 7Al ₂ O ₃ is produced in the mixed slag.

  According to a third invention of the present invention, in the first or second invention, the reducing slag of the preceding charge which is a mixed source is heated in the initial stage of the reduction refining and the oxidizing slag produced by the preceding charge. Mixed slag is obtained by intermixing, and then reductive refining is performed immediately after the mixed slag is transferred and mixed in a slag pan.

  As described above, in the melting and refining method using an electric furnace and a ladle smelting furnace, by implementing the method of the present invention, while taking advantage of the conventional production process, the cost from the generated electric furnace slag can be reduced without incurring costs such as a stabilizer. Fluorine elution could be suppressed very effectively.

(Function)
(1) On the high-temperature molten steel immediately after being transferred from the electric furnace to the ladle, the reductive slag (exhaust) of the preceding heat and the remaining hot water are mixed with the sufficiently high-temperature oxidizing slag. Even if the temperature of the reducing slag of the preceding heat is lowered, a sufficient reaction temperature is obtained and mixed.

(2) Mixing / stirring takes all the reducing slag (exhaust) and remaining hot water remaining after steel is discharged from the ladle of the preceding heat to the tundish with a certain amount of oxidizing slag after the electric furnace refining. Since it is performed twice, when it is transferred to the pan and when it is stirred and removed after receiving waste, sufficient mixing is performed. Then, 12CaO · 7Al ₂ O ₃ is generated in the mixed slag that has been mixed and stirred twice, and the fluorine in the reduced slag containing a large amount of fluorine is fixed.

(3) That is, since the temperature at the time of mixing of the oxidizing slag and the reducing slag is high, 2CaO · SiO ₂ which is the main compound form in the oxidizing slag and 3CaO which is the main compound form in the reducing slag · Al ₂ O ₃ and then CaO or the like reaction unreacted, the mixing slag, the 12CaO · 7Al ₂ O ₃ is generated.

  (4) In the slag, fluorine exists in a form in which calcium is coordinated. In the fixation of fluorine, the presence of CaO capable of coordinating to non-immobilized fluorine is important.

  (5) Therefore, at the time of mixing slag, the reducing slag of the preceding charge that becomes the mixing source becomes a mixed slag by hot mixing with the oxidizing slag produced by the preceding charging at the initial stage of the reduction refining, and then immediately It is necessary to remove the mixed slag and perform refining from a state in which a certain amount of mixed slag is provided on the surface of the molten metal.

  (6) The reason for this is that if the reducing slag of the preceding charge as the mixing source is not mixed with slag in the early stage of refining refining, the CaO source input during reductive refining takes the molten steel after oxidation refining. When transferred to the ladle, it was used to reform a small amount of oxidizing slag that was transferred along with the molten steel. However, even if slag is mixed, the amount of CaO that can be coordinated to fluorine is not sufficient, and an effective fluorine elution suppression effect cannot be obtained.

(7) In the elution suppressing method of the fluorine, the oxidizing slag mainly comprising calcium silicate, such CaO and Al ₂ O ₃ or 3CaO · Al ₂ O _3, can be a CaO source and Al ₂ O ₃ source material And a fluorine source are mixed in a molten state maintained in a temperature range where calcium aluminate can be generated. However, when the modifier is used, the cost of the modifier and the processing cost such as the energy cost for heating it to the reaction temperature range are newly generated. Therefore, the oxidizing slag and the reducing slag in a sufficiently high temperature state are generated. It is desirable to use and mix.

  (8) Since oxidizing slag is used as a modifier, the cost of the modifier is not incurred at all.

  (9) The work is performed on the hot molten steel immediately after being transferred to the ladle, which is a conventional production process, to the fully heated reducing slag (exhaust) and all remaining hot water. The process cost is almost insignificant with only one additional process of transferring and mixing the slag and discharging the produced mixed slag to the slag pan.

  (10) Conventionally, the remaining hot water (residual molten steel) generated in the ladle is returned to the subsequent heat as much as possible in the operation of the method of the present invention. Costs for processing molten steel are no longer necessary.

  In the present invention, an example in the case of melting and refining case-hardened steel will be shown.

  FIG. 1 is an explanatory view schematically showing a method for suppressing fluorine elution according to an embodiment. Reducing slag (hereinafter referred to as waste slag mixed with slag by pre-charging) The reduced soot is referred to as “exhaust / reduced slag” or “exhaust / reduced slag”). FIG.

  After oxidative refining is completed in a 90-ton electric furnace, molten steel is transferred from the electric furnace to a ladle. At this time, about 1 ton of oxidizing slag is simultaneously transferred. Almost before and after this, the transfer to the tundish is completed in the ladle that precedes two heats. About 4 tons of reducing slag and about 1 ton of residual molten steel remain in the ladle. Bring this over the ladle after steel is discharged from the electric furnace, and take the total amount of about 5 tons into the ladle. Note the tilt transition. After the transfer is completed, the slag mixed by the transfer is scraped into a slag pan. At this time, the slag surface is agitated with a slag drafter (a device that scrapes out the slag), the oxidizing slag and the reducing slag are sufficiently mixed, and then scraped out into the slag pan to form the first mixing step.

  After scraping out the slag, the slagging material is introduced from the state having a slight mixed slag, and the reduction refining is started. The reducing slag generated at the end of the refining and refining from the mixed slag is the reductive slag. In the charge of the waste reduction slag, the reduction smelting is finished, and before and after the steel is tapped into the tundish, the electric furnace oxidation smelting is finished with an implementation charge after about 2 charges.

  Also in the implementation charge, about 1 ton of oxidizing slag is put out to the ladle, and about 4 tons of exhaust charge reduction slag of the preceding charge is transferred and mixed here, and the second mixing step is performed. In the second mixing step, the generated mixed slag having the ability to suppress fluorine elution is discharged into a slag pan, and then slowly cooled in a slag treatment plant and used for roadbed materials.

  With respect to the oxidizing slag, reducing slag, and mixed slag as the mixing source at this time, the composition, the amount of fluorine eluted, and the content were measured. The composition of the slag was determined by the molten glass beet method and in accordance with the provisions of Notification No. 46 of the Environment Agency in 1991. For the fluorine content, the alkali melting-absorptiometry method was used. It measured using K0102 * 34.1.

  Table 1 shows mixed slag using reducing / reducing soot for reducing slag, and the composition (mass%) of the oxidizing slag and reducing slag as the mixing source, fluorine content (%), and fluorine elution amount (mg / L).

  The implementation (1) of the mixed slag is produced by mixing the implementation (1) of the oxidizing slag and the implementation (1) of the reducing slag. Similarly, the implementation (2) of the mixed slag is produced by mixing the implementation (2) of the oxidizing slag and the implementation (2) of the reducing slag. Similarly, the mixed slag implementations (3), (4) and (5) were produced by mixing the same number of oxidizing slag implementations and the same number of reducing slag implementations. It is.

  As shown in Table 1, the fluorine elution amounts of the five types of mixed slags of the implementations (1) to (5) are within the range of 0.4 to 0.8 mg / l, all of 0.8 mg / l or less. It is a numerical value that satisfies the soil environment standard value (0.8 mg / l), and can be applied not only to roadbed materials that are outside the scope of the soil environment standard but also to various civil engineering applications such as a soil conditioner.

(Comparative example)
Next, FIG. 2 is an explanatory view schematically showing a fluorine elution suppression method as a comparative example.

  In other words, reductive slag that has not been mixed with slag in the preceding charge (hereinafter referred to as “new reduced slag” or “newly generated, which has not been subjected to waste after being tapped from the ladle as the mixing source) It is referred to as “reduced soot”.) Is an explanatory view schematically showing a method for suppressing elution of fluorine in which the new reduced slag and the oxidizing slag are mixed in the actual charge.

  First, in the preceding charge, after the steel is discharged from the 90-ton electric furnace to the ladle, reduction refining is performed immediately without receiving reducing slag. The molten steel surface in the initial stage of reduction refining immediately after steelmaking has a small amount of oxidized slag to prevent oxidation of the molten steel, and a reductive slag is generated by adding and heating a new slagging agent here. . A newly generated reducing slag that is not mixed with slag is a new reduced slag. The electric furnace refining is completed in the charge after 2 heats before and after the tundish is discharged in this charge.

  In the implementation charge, the molten steel is discharged from the electric furnace to the ladle with about 1 ton of oxidizing slag as in the embodiment. In this ladle, 1 ton of residual molten steel and 4 ton of new reduced slag after tundish steel are added to the ladle and transferred and mixed. The slag after mixing is stirred with a slag draker, scraped out into a slag pan, and subjected to normal treatment at a slag treatment plant to form a slowly cooled slag.

  Table 2 shows the composition (mass%), fluorine content (%), and fluorine elution amount (mg / mg) of mixed slag and its source, oxidizing slag and reducing slag, using new reduced soot as reducing slag. l) is an example.

  The mixed slag comparison (1) is produced by mixing the oxidizing slag comparison (1) and the reducing slag comparison (1). Similarly, the mixed slag comparison (2) is produced by mixing the oxidizing slag comparison (2) and the reducing slag comparison (2).

  As shown in Table 2, the elution amounts of fluorine in the two types of mixed slags of comparison (1) and comparison (2) using new reduced soot as reducing slag were 2.1 mg / l and 13.0 mg / l. In both cases, the soil environmental standard value was greatly exceeded 0.8 mg / l.

  Next, X-ray diffraction and EPMA mapping analysis were performed on the mixed slag and its oxidizing slag and reducing slag to identify the factors related to the elution behavior of fluorine, and the main mineral layers in each slag And the composition of the fluorine-concentrated site.

As shown in FIG. 3, as a result of investigating the oxidizing slag by X-ray diffraction, strong diffraction patterns of 2CaO.SiO ₂ and FeO were particularly confirmed in the oxidizing slag.

Further, as shown in FIG. 4, as a result of examining the reducing slag by X-ray diffraction, a strong diffraction pattern of 3CaO.Al ₂ O ₃ and CaO could be confirmed particularly in the reducing slag.

Furthermore, as shown in FIG. 5, as a result of examining mixed slag by X-ray diffraction, the mixed slag was confirmed to have peaks of 12CaO · 7Al ₂ O ₃ , 2CaO · SiO ₂ , FeO, and MgO, and in particular, 12CaO · 7Al A strong diffraction pattern of ₂ O ₃ was confirmed. In the mixed slag, 12CaO · 7Al ₂ O ₃ crystals that were not present in either the oxidizing slag or the reducing slag were detected. From this, it was confirmed that in the mixed slag, 12CaO · 7Al ₂ O ₃ crystals were generated by hot mixing, not simply in the physical mixed state of oxidizing slag and reducing slag.

Based on the results of EPMA mapping analysis and X-ray diffraction, the composition in which fluorine is present in each slag was identified by comparing with the identified compound. As a result, the reducing slag was in the 3CaO · Al ₂ O ₃ layer, and the mixed slag was Concentration of fluorine was confirmed in the 12CaO · 7Al ₂ O ₃ layer.

Further, there is a knowledge that fluorine exists in molten slag in a state where Ca is arranged around. From mixing of oxidizing slag and reducing slag, 12CaO · 7Al ₂ O ₃ is produced, and since fluorine is concentrated in this 12CaO · 7Al ₂ O ₃ , from CaO coordinated to fluorine during slag mixing It is thought that 12CaO · 7Al ₂ O ₃ is formed and fluorine is immobilized.

That is, in the process of producing 12CaO · 7Al ₂ O ₃ including the coordination of Ca to fluorine, it is presumed that the factor affecting the fluorine elution suppression behavior is the presence of active CaO capable of giving consideration to fluorine. Is done.

The influence of CaO will be examined from the X-ray diffraction pattern of the exhaust reduction slag, which is the CaO source, and the fluorine content and elution amount of the mixed slag. First, focusing on the X-ray diffraction pattern of the exhaust reduction slag, the ratio of the X-ray peak intensity shown in FIG. 6 is 3CaO.Al ₂ O ₃ : CaO = 4: 6 and the ratio of CaO is large, and FIG. 3CaO.Al ₂ O ₃ : CaO = 9: 1, which is shown, can be roughly divided into those having a small CaO ratio. Here, the ratio of 3CaO.Al ₂ O ₃ : CaO is a ratio of peak intensity in X-ray diffraction, and is different from the ratio of the content of chemical components.

  In Examples, the reducing slag having a high CaO ratio is executions (1), (2), and (5), and the reducing slag having a low CaO ratio is executions (3) and (4). All of the mixed slags that use these as a mixing source exhibit the ability to suppress fluorine elution, but the implementation of reducing slag with a low CaO ratio (3) and (4) is related to the fact that the content of fluorine is smaller than the others. It is presumed that CaO that has a high elution amount and can be coordinated to fluorine affects the fluorine elution suppression behavior of the mixed slag.

Next, paying attention to the production process of reducing slag as a mixing source, the state of slag at the initial stage of refining and refining differs depending on whether the waste is a reductive reduction reed or a new reductive reed. That is, the waste reduction slag is mixed with slag before reductive refining, and reductive refining is started from the mixed slag having 12CaO · 7Al ₂ O _{3 in} which the fluorine is fixed and the main mineral layer. The reductive slag is started from the oxidizing slag having 2CaO · SiO ₂ as the main mineral layer, which was transferred to the ladle along with the steel from the electric furnace.

  Both the waste reduction slag and the new reduction slag are put into the almost same state where the chemical composition and the main mineral layer are indistinguishable at the end of the reduction refining by the introduction of the slagging agent. The ability to suppress fluorine elution of mixed slag produced by mixing with slag is completely different depending on whether the mixing source is exhaust reduction slag or new reduced slag.

  In other words, in the new reduced dredger, the CaO source input during refining and refining is used for reforming the oxidizing slag transferred with molten steel after refining refining, and can be coordinated to fluorine after reductive refining. The amount of CaO is not sufficient, and an effective elution suppression effect cannot be obtained.

Judging from the above comprehensive judgment, 12CaO · 7Al ₂ O ₃ is produced from Ca coordinated to fluorine by mixing of oxidizing slag and reducing slag, and the production process of 12CaO · 7Al ₂ O ₃ taking in this fluorine is mixed. Sufficient suppression of fluorine elution if it affects the slag fluorine elution suppression behavior and if the reducing slag, which is the mixing source, is not the reductive soot that has undergone the first slag mixing process at the initial stage of refining refining It is inferred that the effect cannot be obtained.

  The mixed slag produced by the method of the present invention was able to suppress the elution of fluorine very effectively without taking the cost of a stabilizer and the like while utilizing the conventional production process. Moreover, it can be put to practical use as various civil engineering materials such as soil improvement materials as well as roadbed materials.

It is explanatory drawing which shows typically the elution suppression method of the fluorine which uses a waste reduction cake. It is explanatory drawing which shows typically the elution suppression method of the fluorine which uses a novel reducing soot. It is a peak pattern figure of X-ray diffraction of oxidizing slag. It is a peak pattern figure of X-ray diffraction of reducing slag. It is a peak pattern figure of X-ray diffraction of mixed slag. It is a peak pattern figure of the X-ray diffraction of the high CaO receiving / reducing soot. It is a peak pattern figure of the X-ray diffraction of the low CaO receiving / reducing soot.

Claims (3)

In the melting and refining method of steel, which performs melting and oxidation refining in an electric furnace and reducing refining in a ladle,
Reductive slag remaining after the steel is discharged from the ladle of the preceding heat to the tundish in the preceding charge when transferring the molten steel with a certain amount of oxidizing slag after the smelting of the electric furnace to the ladle. The first mixing step of mixing all of the remaining hot water with the oxidizing slag produced by the preceding charge, and the reducing charge from the mixed slag produced by the preceding charge and all of the remaining hot water at the implementation charge. A method for suppressing fluorine elution of an electric furnace slag, comprising a second mixing step of mixing with oxidizing slag produced by an implementation charge. The major compound form of the oxidizing slag is 2CaO · SiO _2, wherein the main compound form of the reducing slag a 3CaO · Al ₂ O _3, by these mixtures, the 12CaO · 7Al ₂ O ₃ to the mixed slag The method for suppressing fluorine elution of an electric furnace slag according to claim 1, characterized in that it is generated. The reductive slag of the preceding charge which is the mixing source becomes a mixed slag by hot mixing with the oxidizing slag produced by the preceding charge in the early stage of reductive refining, and then this mixed slag is immediately used in a slag pan. The method for suppressing fluorine elution of electric furnace slag according to claim 1 or 2, wherein reductive refining is performed from a state where the mixture is transferred and mixed.

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