Method for processing chromium oxide-containing substances in large quantities

Abstract: A method of rapidly reducing chromium oxide-containing slag and the like in large quantities, in a simplified manner and without requiring high temperatures. Chromium oxides are reduced with at least one of elementary sulfur and compounds of sulfur having a valence less than 6. For an aqueous solution of the sulfur component, desirably, its sulfur content is more than 0.03 % by weight. As the sulfur source, preferred is blast furnace slag (e.g., non-aged, gradually-cooled blast furnace slag) that is discharged in large quantities in the iron industry. Cr 6+ in chromium oxides is reduced in one of the following ways: (a) Chromium oxide-containing substances are sprayed with or immersed in blast furnace slag-released water that has been used in cooling blast furnace slag. (b) Chromium oxide-containing substances are mixed with blast furnace slag, and then kept in an air atmosphere. Optionally, the mixture is sprayed with blast furnace slag-released water. (c) Chromium oxide-containing substances are mixed with blast furnace slag, and then steamed. (d) Prior to the reduction treatment, chromium oxide-containing substances are preferably steamed. 100 parts by weight of reduced chromium oxide-containing compounds are mixed with from 0.1 to 90 parts by weight of sulfur-containing slag, and used in various civil engineering works.

Description: FIELD OF THE INVENTION The present invention relates to a method for processing chromium oxide-containing substances, for example, stainless steel slag as discharged in refining stainless steel, chromium slag as discharged in producing chromium compounds such as sodium bichromate, slag of molten wastes, sewage sludge, slag of molten sewage sludge and the like, thereby to prevent the release of Cr 6+ from those substances. BACKGROUND OF THE INVENTION Stainless steel slag as discharged in refining stainless steel, and chromium slag as discharged in producing chromium compounds such as sodium bichromate, contain chromium oxides at a level of a few %. A part of those chromium oxides is often oxidized to Cr 6+ , depending on the working conditions, and Cr 6+ is released from them. Where stainless steel slag and chromium slag are used as materials in roadbeds, materials in temporary works, fillers in civil engineering reclamation works and the like, it is indispensable that Cr 6+ is not released from the slag. Recently, recycling of slag as formed by melting ashes from garbage incinerators, sludge and the like in roadbeds, tiles and the like is now under consideration in the industry. However, the slag formed from some types of ashes from garbage incinerators, sludge and the like will often release Cr 6+ to make it difficult to recycle the slag. Accordingly, for safely recycling such slag in roadbeds, fillers in civil engineering reclamation works and the like, there have been proposed some new techniques of preventing the release of Cr 6+ from the slag. For example, for preventing the release of Cr 6+ from stainless steel slag, there has been disclosed a method of adding aluminum ash and magnesia-based industrial wastes to stainless steel slag (see Japanese Patent Application Laid-Open (JP-A) No. Hei-6-171993). When industrially carrying out the disclosed method, the intended aluminum ash and magnesia-based industrial wastes are previously introduced into a slag-receiving kiln, into which molten stainless steel slag is poured, stirred and mixed by the energy of the falling slag melt. In this method, however, the slag melt is highly viscous, and as to this is different from molten steel, and it is difficult to produce a homogeneously mixed condition by the stirring and mixing means assisted only by the energy of the falling slag melt, resulting in the failure to completely stabilize all chromium oxides in the slag to thereby prevent the release of Cr 6+ from the slag. Another method was proposed, of adding a divalent S compound such as FeS or the like to molten slag resulting from decarburizing and refining stainless steel. The resulting slag is then reduced to recover Cr in the slag. The slag is then stirred with an inert gas stream introduced thereinto, thereby making the slag have an S content of not less than 0.20 % by weight so as not to release Cr 6+ therefrom (see JP-A Hei-8-104553). In this method, an inert gas stream is introduced into the slag melt to homogeneously mix the slag melt with the additive, thereby stabilizing the chromium oxides existing in the slag. This method is effective in preventing the release of Cr 6+ from the slag, but requires the troublesome operation of introducing such an inert gas stream into the reduced slag melt while using the additive for lowering the viscosity of the slag melt. Therefore, the method is not economical. In addition, the method is further troublesome in that, if the additive is added to the slag melt in the refining furnace in order to ensure the homogeneous mixing of the slag melt with the additive, the additive will contaminate the steel melt. On the other hand, for preventing the release of Cr 6+ from chromium slag as discharged in producing chromium compounds, a method has been employed of reductionally roasting the slag to thereby reduce Cr 6+ to Cr 3+ for rendering the slag harmless. Economically, however, this method is troublesome in that it requires a roasting step. Sewage sludge is incinerated to reduce its volume prior to being used for reclamation. To prevent the release of Cr 6+ from the incinerated ashes, a method is used for controlling the air ratio during incineration to be less than 1 (see Journal of the Drainage Works Association of Japan, Vol. 38, No. 378, pp. 29-32, 1994). However, as pointed out in this publication, it is extremely difficult to ensure the optimum operation for a variety of sludges derived from various sites and having different properties, and the same applies also to the incineration of industrial wastes. To reduce Cr 6+ in chromium oxide-containing substances at room temperature, there is generally employed a method using ferrous sulfate as the reducing agent. In this method, however, it is difficult to efficiently lower the Cr 6+ content of the processed substances to an environmentally acceptable level. Where chromium oxide-containing substances are reduced or incinerated according to the methods noted above, and the thus-processed substances are recycled in roadbeds, fillers in civil engineering reclamation works, temporary works and the like, they are checked as to whether or not chromium oxides were fully reduced, prior to being shipped. If the processed substances contain non-reduced chromium oxides, they cannot be recycled in roadbeds, fillers in civil engineering reclamation works, temporary works and the like. From US-A-4,504,321 there is known a method for treating and stabilising chromium ore waste comprising the steps of admixing chromium ore waste mud or sludge, intimately admixing ground granulated blast furnace slag to the above sludge state mixture and placing said sludge state mixture in a quiescent state whereupon a solid mass is obtained by sedimentary separation. From JP-A-53028563 there is known a method comprising reducing hexavalent chromium in chromium-containing substances to trivalent chromium under acidic conditions wherein the reducing agent is the slurry of crystals of calcium sulphite, adding hydroxide so as to convert said trivalent chromium compound to a chromic hydroxide compound in sludge state and mixing said sludge state mixture with cement to make it solid. From US-A-5,304,710 there is known a process in which chromium-containing soil ore waste is treated with a solid in which iron sulphate (FeSO 4 ) is comprised as a reducing agent. OBJECTS OF THE INVENTION An object of the present invention is to solve the problems in the prior art noted above, and to provide an industrial, simple and economical method for processing chromium oxide-containing substances, such as stainless steel slag, chromium slag, slag of molten wastes, sewage sludge, slag of molten sewage sludge, soil contaminated with chromium oxides and the like, thereby to completely prevent the release of Cr 6+ from those chromium oxide-containing substances. SUMMARY OF THE INVENTION Having carefully studied the problems in the prior art noted above, the present inventors have found that, in the prior art in which the defined reduction of chromium is attained in a high-temperature condition for melting, roasting or incineration, the reduction condition is difficult to control and the defined condition is not economical. On the basis of this finding, we have tried methods of reducing Cr 6+ at room temperature or at temperatures near room temperature. Specifically, we have made various studies and experiments for reacting various chromium oxide-containing substances such as those mentioned above with different aqueous solutions containing a variety of reducing agents, thereby to reduce Cr 6+ in those substances. As a result, we have found that, when water that contains sulfur having a valence less than 6, namely water that contains sulfur ions and sulfur as released from the non-aged material of blast furnace slag, is used, then chromium existing in chromium oxide-containing substances can be reduced with high reactivity in an industrial, simple and economical manner. On the basis of this finding, we have achieved the objects of the present invention by a method as defined in claim 1. Preferred embodiments and further improvements of the inventive method are defined in the depending subclaims. In addition, based on this finding, we have further studied and have made the following findings [1] to [3]: [1] Using a sulfur-containing substance listed below as the release of Cr 6+ from chromium oxide-containing substances is extremely effectively prevented. Water that has been used for cooling blast furnace slag ,(hereinafter referred to as blast furnace slag-released water) having a valence less than 6, namely gradually-cooled blast furnace slag spontaneously aged for less than 3 months (hereinafter referred to as non-aged, gradually-cooled blast furnace slag).Generically, this substance set forth above will be hereinafter referred to as sulfur-containing substance. [2] Using the sulfur-containing substance noted above, the release of Cr 6+ from chromium oxide-containing substances is extremely effectively prevented according to any of the following processing methods (a) to (d). (a) They are kept in an aerial atmosphere. (b) Water is sprayed over them. (c) Chromium oxide-containing substances are immersed in sulfur-containing substances. (d) Steam is applied to them. [3] Using the sulfur-containing substance optionally combined with any divalent iron-containing substances (hereinafter referred to as Fe(II)-containing substances), such as ferrous sulfate, produces the following advantages (1) and (2). (1) Even for blast furnace slag-released water having a low concentration of reducing sulfur, of which the concentration may vary in the water, the above mentioned sulfur-containing substance, the compounds of sulfur with a valence less than 6 or the Fe(II)-containing substances may be added thereto or sprayed thereover, and chromium oxide-containing substances to be processed are immersed in the resulting blast furnace slag-released water, whereby Cr 6+ in the chromium oxide-containing substances can be reduced within a short period of time, resulting in that the release of Cr 6+ from the thus-processed chromium oxide-containing substances is completely prevented. Where blast furnace slag-released water is sprayed over chromium oxide-containing substances to be processed therewith, the thus-sprayed substances are preferably kept in an aerial atmosphere to complete the reduction processing. Also preferably, steam is applied to the chromium oxide-containing substances to be processed according to the invention prior to the intended reduction processing of the substances. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a graph showing the relationship between the time for steaming stainless steel slag and the amount of Cr 6+ released from the slag. DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION The invention will be described in detail hereinunder with reference to its preferred embodiments. The first aspect of the invention is a method of reducing chromium oxide-containing substances, such as chromium oxide-containing slag, sewage sludge, contaminated soil and the like, in large quantities and rapidly. 'The method of the invention requires neither particularly high temperatures nor specific large-scale equipment. According to the method of the invention, Cr 6+ in chromium oxide-containing substances is reduced by a reducing sulfur (at least one of elemental sulfur and sulfur ions having a valence of less than 6, e.g. 2 or 4) at atmospheric temperature or at a temperature of up to about 200°C when vapor is optionally used. As the method does not use any slag melt, it does not require particularly high temperatures. In other words, according to the method of the invention, chromium oxide-containing substances can be reduced efficiently at low temperatures. In one preferred embodiment of the method of the invention, chromium oxide-containing substances are kept in contact with an aqueous solution having an overall sulfur content greater than 0.03 % by weight (0.3 g/liter) for sulfur and/or compounds of sulfur having a valence of less than 6 therein. The data are shown in Table 2. In Table 2, the reducibility indicates the decrease (mg) in Cr 6+ per kg of the blast furnace slag-released water used. Table 2 shows that the blast furnace slag-released water also has the ability to reduce Cr 6+ , which is nearly comparable to that of the non-aged, gradually-cooled blast furnace slag in terms of the value per the unit weight. In view of this, we clarified the mechanism of action of those reducing substances. Table 3 shows the concentrations of sulfurs of different types in the gradually-cooled blast furnace slag as used for reducing Cr 6+ , before and after the reducing treatment using it. Comparing the sulfur concentrations before and after the reducing treatment, it is seen that the concentrations of sulfurs except S 2- were lowered after the.reducing treatment. In addition, we measured the concentrations of sulfurs of different types in the reduced aqueous solutions, and found that most sulfurs therein were in the form of SO 4 2- . Accordingly, it is believed that S 0 , S 2 O 3 2- and SO 4 2- in the gradually-cooled blast furnace slag dissolve in water, and that of these, S 0 and S 2 O 3 2- are oxidized to SO 4 2- while reducing Cr 6+ . The reason why the ability to reduce Cr 6+ in the slag powder having a smaller grain size is higher is because the slag powder having a smaller grain size has a larger specific surface area so that the S 0 and S 2 O 3 2- therein are easier to dissolve in water. The present invention has been completed on the basis of the findings noted above. According to the invention, when an aqueous solution containing sulfur and/or compounds of sulfur having a valence less than 6 is contacted with chromium oxide-containing substances, it reduces chromium to stable chromium compounds of Cr(OH) 3 and the like, whereby the release of chromium oxides from the chromium oxide-containing substances is prevented. Even if some chromium ions are released from chromium oxide-containing substances and dissolve to the aqueous solution, they are reduced in the aqueous solution so that the release of Cr 6+ from the chromium oxide-containing substances is prevented. In the preferred embodiment of the first aspect of the invention, the overall sulfur content of the aqueous solution that contains sulfur and/or compounds of sulfur having a valence less than 6 is desirably larger than 0.03 % by weight. This is because an aqueous solution having an overall sulfur content of not larger than 0.03 % by weight for sulfur and/or compounds of sulfur having a valance less than 6 therein takes a few months or longer to reduce chromium oxide-containing substances, and is therefore difficult to use in industrial processes. As sulfur and/or compounds of sulfur having a valence less than 6, and their sources, which are used in the first aspect of the invention, the present invention uses Water for cooling blast furnace slag (blast furnace slag-released water). Type of Slag-released Water Grain Size (mm) Reducibility (mg-Cr 6+ /kg-blast furnace slag- released slag) Blast Furnace Slag-released Water - 1340 Before Reducing Treatment (wt.%) After Reducing Treatment (wt.%) S as S 2- 0.04 0.04 S as S 0 0.22 <0.005 S as S 2 O 3 2- 0.31 0.02 S as SO 4 2- 0.52 0.19 Based on the first aspect of the invention noted above, the present inventors have further studied and obtained the following findings: Using the sulfur-containing substances optionally combined with any of divalent iron-containing substances (Fe(II)-containing substances), such as ferrous sulfate, produces the following advantages (1) and (2). (1) Even for the blast furnace slag-released water having a small concentration of reducing sulfur, of which the concentration may vary in the water, substances that contain sulfur and/or compounds of sulfur having a valence less than 6 may be added thereto, and chromium oxide-containing substances to be processed are immersed in the resulting blast furnace slag-released water, whereby Cr 6+ in the chromium oxide-containing substances can be reduced within a short period of time, resulting in that the release of Cr 6+ from the thus-processed chromium oxide-containing substances is completely prevented. In still another preferred embodiment of the invention, blast furnace slag-released water, which is water as used for cooling blast furnace slag, is sprayed over chromium oxide-containing substances. In still another preferred embodiment thereof, chromium oxide-containing substances are mixed with slag containing sulfur and/or compounds of sulfur having a valence less than 6, and water and/or blast furnace slag-released water, which is water as used for cooling blast furnace slag, is sprayed over the resulting mixture. In this embodiment, non-aged, gradually-cooled blast furnace slag and/or slag as discharged from the pre-treatment of molten pig iron are preferred as the slag containing sulfur and/or compounds of sulfur having a valence less than 6, and more preferred is non-aged, gradually-cooled blast furnace slag. In still another preferred embodiment of the first aspect of the invention, chromium oxide-containing substances are immersed in blast furnace slag-released water, which is water that has been used for cooling blast furnace slag. In still another preferred embodiment thereof, chromium oxide-containing substances are immersed in blast furnace slag-released water to which are added substances containing sulfur and/or compounds of sulfur having a valence less than 6. In those embodiments, preferred are sulfur-containing substances having an overall sulfur content of not less than 10 % by weight for sulfur and/or compounds of sulfur having a valence less than 6 therein, as the substances containing sulfur and/or compounds of sulfur having a valence less than 6. The sulfur-containing substances having an overall sulfur content of not less than 10 % by weight for sulfur and/or compounds of sulfur having a valence less than 6 therein for use in those embodiments are not specifically defined, but preferred are, for example, elemental sulfur; substances containing elemental sulfur such as sinter as recovered from sulfur-containing hot springs (elemental sulfur-containing substances); and compounds of sulfur having a valence less than 6, such as sodium thiosulfate, iron sulfide, hydrogen'sulfide and the like (compounds of sulfur with a valence less than 6). Two or more of these may be used in combination. As the substances containing compounds of sulfur having a valence less than 6, preferred are those that hardly react with oxygen dissolved in water. This is because, if sulfur-containing substances that easily react with oxygen dissolved in water are used, they will react with oxygen in water prior to acting on chromium oxide-containing substances, thereby losing their ability to completely reduce Cr 6+ in the chromium oxide-containing substances. The reducing sulfur concentration in blast furnace slag-released water varies, depending on the season and the weather, and maybe often not more than 0.03 % by weight. If blast furnace slag-released water having such a low reducing sulfur concentration is used, it takes a long time to reduce Cr 6+ in chromium oxide-containing substances. In view of this, the present inventors have further studied and made various experiments in order to find an effective method capable of processing chromium oxide-containing substances within a short period of time even with blast furnace slag-released water having a low reducing sulfur concentration of not more than 0.03 % by weight, thereby completely to prevent the release of Cr 6+ from the thus-processed chromium oxide-containing substances. As a result, we have found that, when substances containing sulfur and/or compounds of sulfur having a valence less than 6 are added to blast furnace slag-released water and chromium oxide-containing substances are immersed in the resulting mixture, then the release of Cr 6+ from the thus-processed chromium oxide-containing substances can be completely prevented, even though the blast furnace slag-released water used has a low reducing sulfur concentration of not more than 0.03 % by weight and even though the processing is effected within a short period of time. Accordingly, even for blast furnace slag-released water having a low reducing sulfur concentration of not more than 0.03 % by weight, if substances containing sulfur and/or compounds of sulfur having a valence less than 6 are added thereto, chromium oxide-containing substances may be immersed in the resulting mixture only within a short period of time thereby completely to prevent the release of Cr 6+ from the thus-processed chromium oxide-containing substances. The reducing sulfur concentration in blast furnace slag-released water as referred to herein is obtained by subtracting the sulfur content of SO 4 2- from the overall sulfur concentration in the water. According to the preferred embodiment noted above, the substances containing sulfur and/or compounds of sulfur having a valence less than 6 added to blast furnace slag-released water shall control the reducing sulfur concentration in the thus-mixed water, with which, therefore, Cr 6+ in the chromium oxide-containing substances processed is rapidly reduced. In this preferred embodiment, the amount of the substances containing sulfur and/or compounds of sulfur having a valence less than 6 to be added to blast furnace slag-released water is preferably so controlled that the reducing sulfur concentration in the blast furnace slag-released water containing those substances is greater than 0.03 % by weight. This is because, if the reducing sulfur concentration in the blast furnace slag-released water used is not more than 0.03 % by weight, it is difficult to completely reduce Cr 6+ in chromium oxide-containing substances with the water within a short period of time. Also preferably, divalent iron-containing substances such as ferrous sulfate, ferrous chloride and the like (Fe(II)-containing substances) may be added to blast furnace slag-released water for use in the invention. Adding divalent iron ions to blast furnace slag-released water ensures more rapid and more complete reduction of Cr 6+ in chromium oxide-containing substances as processed with the divalent iron-containing water. Blast furnace slag-released water: The blast furnace slag-released water for use in the invention is preferably one that has been sprayed over non-aged, gradually-cooled blast furnace slag, or one that has been sprayed over hot slag just having been discharged from a blast furnace. Specifically, as the blast furnace slag-released water for use in the invention is one that has been sprayed over gradually-cooled blast furnace slag that has been spontaneously aged generally for less than 3 months, or one that has been sprayed over hot slag that has just been discharged from a blast furnace. Where the blast furnace slag-released water of that type is used, the overall sulfur content of the water, which results from sulfur and/or compounds of sulfur having a valence less than 6, is preferably greater than 0.03 % by weight. This is because, as mentioned hereinabove, blast furnace slag-released water having an overall sulfur content of not more than 0.03 % by weight for sulfur and/or compounds of sulfur having a valence less than 6 takes a few months or longer to reduce chromium oxide-containing substances, and is therefore difficult to use in industrial processes. (II) Preferred ranges of the amount to be added of slag that contains sulfur and/or compounds of sulfur having a valence less than 6, the amount of blast furnace slag-released water to be sprayed, and the reducing sulfur concentration in blast furnace slag-released water, are as follows: The amount to be added, of the slag that contains sulfur and/or compounds of sulfur having a valence less than 6, the amount of the blast furnace slag-released water to be sprayed, and the reducing sulfur concentration in the blast furnace slag-released water are not specifically defined, but are preferably as follows: [1] In the embodiment of keeping the reduction system in an aerial atmosphere, in the embodiment of spraying water over the reduction system, and in the embodiment of introducing steam into the reduction system: In those embodiments, where the typical blast furnace slag as in Table 3 is used as the slag that contains sulfur and/or compounds of sulfur having a valence less than 6, such as non-aged, gradually-cooled blast furnace slag or the like, it is desirable that the amount of the slag to be added to the reduction system (hereinafter referred to as slag amount), the amount of the blast furnace slag-released water to be sprayed over the system, and the amount to be added thereto of the substances that contain sulfur and/or compounds of sulfur having a valence less than 6 (e.g., elemental sulfur-containing substances, sulfur-containing substances with a valence less than 6) satisfy the following requirements (1) and (2): 10 ≥ A ≥ 0.1 wherein; A = [amount of slag added (wt.pts.)] x 10 + [amount of blast furnace slag-released water sprayed (wt.pts.)] + [amount added of substances containing sulfur and/or compounds of sulfur having a valence less than 6 (wt.pts.)] x 500 In equation (2), parts by weight (wt.pts.) are all relative to one part by weight of the chromium oxide-containing substances being processed, or to one mg/liter of Cr 6+ having been released from the processed chromium oxide-containing substances, and all the amounts in (2) are proportional to the amount of the chromium oxide-containing substances being processed and that of Cr 6+ having been released from the processed chromium oxide-containing substances.If A in (1) is less than 0.1, outside the defined range, the release of Cr 6+ from the chromium oxide-containing substances processed is not inhibited sufficiently. On the other hand, even if A is more than 10, also outside the defined range, the saturated effect of the reducing agents used for preventing the release of Cr 6+ from the processed substances is not enhanced, and the use of such large amounts of the reducing agents is merely wasteful and hence uneconomical.The amount of water to be sprayed over the mixture comprising chromium oxide-containing substances and non-aged, gradually-cooled blast furnace slag is not specifically defined, but is preferably such that the chromium oxide-containing substances in the mixture may be saturated with the water that has been sprayed thereover.This is because, when chromium oxides are reduced with non-aged, gradually-cooled blast furnace slag or the like, the sulfur and the compounds of sulfur having a valence less than 6, which have been released from the slag or the like to dissolve in water, act to reduce Cr 6+ existing in the chromium oxide-containing substances or Cr 6+ that has been released from the substances to dissolve in water. [2] In the embodiment of immersing chromium oxide-containing substances in reducing baths: When chromium oxide-containing substances are immersed blast furnace slag-released water, the reducing sulfur concentration in the water is preferably greater than 0.03 % by weight. This is because, if chromium oxide-containing substances are immersed in blast furnace slag-released water having a reducing sulfur concentration of not more than 0.03 % by weight, Cr 6+ in the substances is difficult to completely reduce within a short period of time. The reducing sulfur concentration in blast furnace slag-released water as referred to herein is obtained by subtracting the sulfur content of SO 4 2- from the overall sulfur concentration in the water. (III) preferred modes of processing chromium oxide-containing substances: In the embodiment of the invention mentioned hereinabove, chromium oxide-containing substances are mixed with slag that contains sulfur and/or compounds of sulfur having a valence less than 6, such as non-aged, gradually-cooled blast furnace slag or the like, and then kept in an air atmosphere. The invention is applicable not only to stainless steel slag, chromium slag, molten slag of industrial waste, sewage sludge, molten slag of sewage sludge and the like slag, but also to any other substances that may release Cr 6+ , thereby preventing the release of Cr 6+ from those substances. Chromium oxide-containing substances to be processed according to the invention may be pre-treated with steam. When slag that releases Cr 6+ in an amount of 10 mg/liter or more in the metal release test as set forth in the Notification No. 46 of the Environment Agency of Japan, or slag having a low degree of porosity is processed according to the method of spraying blast furnace slag-released water over the slag or according to the method of immersing the slag in blast furnace slag-released water, it takes a long time to safely stabilize the slag. On the other hand, when the slag of that type is processed according to the method of applying steam to the slag as mixed with gradually-cooled blast furnace slag, it may be stabilized within a short period of time but its volume increases as this is mixed with gradually-cooled blast furnace slag. Taking those problems into consideration, the present inventors have further studied and made various experiments, and, as a result, have found that, when steam is previously applied to Cr 6+ -containing slag (chromium oxide-containing substances), Cr 6+ in the slag can be rendered easily reducible, and that, when the thus pre-treated, Cr 6+ -containing slag is reduced with blast furnace slag-released water, it can be stabilized within a shorter period of time thereby completely to prevent the release of Cr 6+ from the thus-stabilized slag. In other words, we have found that the pre-treatment with steam shortens the processing time and simplifies the reduction process itself, and that the method comprising 'the pre-treatment with steam is industrially advantageous and economical. Specifically, in those preferred embodiments, steam is previously applied to chromium oxide-containing substances, and then the thus pre-treated substances are reduced in the manner mentioned hereinabove. For example, blast furnace slag-released water is sprayed over the pre-treated, chromium oxide-containing substances; or the pre-treated, chromium oxide-containing substances are immersed in blast furnace slag-released water. These embodiments will be described in more detail hereinafter. The present inventors have studied and made various experiments in order to find out an industrial, simple, economical and effective method capable of processing slag that releases Cr 6+ in an amount of 10 mg/liter or more in the metal release test as set forth in the Notification No. 46 of the Environment Agency of Japan, or slag having a low degree of porosity, within a short period of time without increasing the volume of the processed slag, thereby completely to prevent the release of Cr 6+ from the thus-processed slag. As a result, we have found that, when steam at a high temperature is applied to the slag of that type, Cr 6+ existing in the slag can be made easily releasable or easily reducible. We have further found that, when blast furnace slag-released water is sprayed over the thus pre-treated slag or when the thus pre-treated slag is immersed in blast furnace slag-released water, then the slag can be efficiently and economically processed within a short period of time without increasing the volume of the processed slag, whereby Cr 6+ in the slag is reduced and completely stabilized. Specifically, according to those preferred embodiments of the invention, where steam is previously applied to chromium oxide-containing slag (chromium oxide-containing substances) and then blast furnace slag-released water is sprayed over the thus pre-steamed slag, or where steam is previously applied to chromium oxide-containing slag (chromium oxide-containing substances) and then the thus pre-steamed slag is immersed in blast furnace slag-released water, Cr 6+ in the slag can be reduced within a short period of time without increasing the volume of the processed slag. Accordingly, those embodiments of the invention are extremely economical. Fig. 1 shows the relationship-between the time of steaming stainless steel slag, which is one example of chromium oxide-containing substances, and the amount of Cr 6+ released from the thus-steamed slag. Briefly, steam at 100°C was applied to stainless steel slag, from which Cr 6+ is released, for a predetermined period of time, and thereafter the thus-steamed slag was subjected to the metal release test as set forth in the Notification No. 46 of the Environment Agency of Japan, and the amount of Cr 6+ released from the slag was measured. The data obtained are plotted relative to the steaming time in Fig. 1. The slag used in the test had a grain size of from 13.2 to 26.5 mm, and was not ground prior to the test. As in Fig. 1, the amount of Cr 6+ released from the slag was greater after the steaming time of 24 hours than that before the steaming at 100°C. Since steam does not have the ability to oxidize chromium, the test data obtained herein suggest that the steaming at 100°C makes Cr 6+ in the slag easily releasable, or that is, easily reducible. This is because the capillary action of steam applied to the slag will make Cr 6+ existing in the depth of the slag move to and condense in the surface layer of the slag. Where steam at 100°C was applied to the slag for a period of time longer than 24 hours, the amount of Cr 6+ released from the slag decreased. This is because Cr 6+ that had been made easily releasable from the slag by the initial steaming was dissolved by the successive steaming and removed. Accordingly, Cr 6+ could be completely removed from the slag by continuous steaming for an extremely long period of time, which, however, is not practical. On the basis of those findings noted above, the present inventors have found a preferred embodiment of the invention wherein Cr 6+ -containing slag is previously steamed thereby to make Cr 6+ in the slag easily releasable or easily reducible, and thereafter the thus-steamed slag is then reduced with blast furnace slag-released water, for example, water that has been sprayed over non-aged, gradually-cooled blast furnace slag. The steam to be used in this embodiment is preferably high-temperature steam, but its temperature is not specifically defined. This is because, when steam is applied to Cr 6+ -containing slag or the like in air, its temperature is generally 100°C, but when steam is introduced under high pressure into a closed container containing therein Cr 6+ -containing slag or the like, the boiling point of water will be higher than 100°C as the inside of the container is under increased pressure, resulting in that the temperature of the steam having been introduced into the container increases, depending on the increased pressure. In this embodiment, the steam to be applied to the reduction system is not limited to water vapor only, but may be combined with any other gases, such as air, N 2 and the like. The steaming time is not also specifically defined. Depending on the grain size and the porosity of the slag to be pre-steamed, the steaming time within which Cr 6+ existing in the slag is made releasable or reducible may be determined. In most cases, the steaming time may fall between 1 and 120 hours. The blast furnace slag for use in this embodiment of the invention is preferably non-aged, gradually-cooled blast furnace slag that colors water in the color identification test defined in JIS A5015, Appendix 1. As the blast furnace slag-released water for use therein, in general, preferred is water that has been sprayed over non-aged, gradually-cooled blast furnace slag that has been spontaneously aged for less than 3 months. For example, preferably used is water that has been sprayed over hot slag that has just been discharged from blast furnaces. The. effectiveness of the blast furnace slag-released water to reduce Cr 6+ into Cr 3+ is due to the oxidation of the reducing sulfur of different types (S 2- , S°, S 2 O 3 2- ) that exists in the water. In this embodiment of the invention, the blast furnace slag-released water to be used may be combined with any other reducing agents having the ability to reduce Cr 6+ . For example, ferrous sulfate may be dissolved in the blast furnace slag-released water to be used therein. In this embodiment, the amount of the blast furnace slag-released water to be sprayed over the slag to be reduced, or the time for which the slag to be reduced is immersed in the blast furnace slag-released water, and the ratio of the slag to be reduced to the blast furnace slag-released water in which the slag is immersed are not specifically defined. The preferred values of those parameters may be suitably determined, depending on the grain size, the porosity and the Cr 6+ content of the slag to be reduced. The present invention is especially effectively applied to slag that may release a large amount, for example, 10 mg/liter or more of Cr 6+ , which, however, is not limitative. Needlesstosay, the invention is also effectively applicable to any and every slag that may release a small amount of Cr 6+ . The invention is applicable not only to Cr 6+ -containing stainless steel slag, chromium slag, industrial wastes, slag of molten wastes and the like, but also to any other substances that may release Cr 6+ , thereby preventing the release of Cr 6+ from those substances. Example A1: As chromium oxide-containing substances, herein used were stainless steel slag, chromium slag as discharged during the production of sodium bichromate, and sewage sludge. The stainless steel slag is produced during reduction in the course of refining stainless steel. As sulfur-containing substances, used herein were non-aged, gradually-cooled blast furnace slag, gradually-cooled blast furnace slag spontaneously aged for 6 months, and blast furnace slag-released water. The non-aged, gradually-cooled blast furnace slag is one produced 1 week after cooling and grinding blast furnace slag. The blast furnace slag-released water is water that has been sprayed over hot blast furnace slag. The reducing sulfur concentration in the blast furnace slag-released water used herein was 0.05 % by weight. Table 4 shows the chemical compositions of the samples of non-aged, gradually-cooled blast furnace slag, stainless steel slag, chromium slag and sewage sludge tested herein, and the amount of Cr 6+ released from each sample. In Table 4, the amount of Cr 6+ released from each sample of stainless steel slag, chromium slag and sewage sludge was 1.0 mg/liter, 9.7 mg/liter and 0.80 mg/liter, respectively. Those chromium oxide-containing substances (samples to be reduced) were processed under the different conditions shown in Table 5. The amount of the non-aged, gradually-cooled blast furnace slag added, which is in Table 5, is in terms of % by weight relative to 100 % by weight of each sample to be reduced with it. In Table 5, there are shown the experimental data obtained along with the conditions employed in the experiments that were conducted. The sample to be reduced and the non-aged, gradually-cooled blast furnace slag were mixed uniformly. Each heap of samples was kept in an air atmosphere and weighed about 100 tons and had a height of 2 m. As seen in Table 5, the amount of Cr 6+ released from comparative samples A1 to A4 which had been kept in an air atmosphere or had been steamed at 100°C, without adding non-aged, gradually-cooled blast furnace slag thereto, was not lower than the environmentally acceptable standard value of 0.05 mg/liter. This is because no reducing agent was added to those comparative samples. The amount of Cr 6+ released from comparative sample A5, to which had been added gradually-cooled blast furnace slag and spontaneously aged for 6 months and which had been kept in an air atmosphere, was also no lower than 0.05 mg/liter. As to comparative sample A6, stainless steel slag, which was kept in an air atmosphere while spraying thereover city water of the same weight as the slag, there was found no significant difference in the amount of Cr 6+ released from it before and after the treatment. In contrast to those comparative samples, samples A1 to A16 of the invention were processed as follows: [1] Each sample was mixed with non-aged, gradually-cooled blast furnace slag, and then kept in an air atmosphere (samples A1* to A4*, A10* and A14* ). [2] Each sample was mixed with non-aged, gradually-cooled blast furnace slag, and then kept in an air atmosphere while either one or both of blast furnace slag-released water and ordinary water was sprayed thereover (sample A7, A9, A12 and A16 of the invention). [3] Each sample was immersed in blast furnace-released water (samples A6, A11 and A15 of the invention). [4] Each sample was mixed with non-aged, gradually-cooled blast furnace slag, and high-temperature steam was applied thereto (samples A5* and A13*). As a result, the amount of Cr 6+ released from each sample of the invention thus processed was lower than the environmental standard value of 0.05 mg/liter. As for sample A17, concerning 'stainless steel slag of the invention, over which was sprayed blast furnace slag-released water of the same weight as that of the slag, the amount of Cr 6+ released from the thus-processed slag was lower than the environmentally acceptable standard value. Although the samples, stainless steel slag, chromium slag and sewage sludge, which had been reduced as described herein according to the invention, were kept in an air atmosphere for 1 year, they released no Cr 6+ . Example A3: The same chromium oxide-containing substances as in Example A2 were processed under different immersion conditions shown in Table 8. As sulfur-containing substances, there were used herein blast furnace slag-released water, elementary sulfur, sinter, hydrogen sulfide, and sodium thiosulfate; and as a divalent iron-containing compound, there was used ferrous sulfate. In Table 8, there are shown the experimental data obtained along with the conditions employed. As for test samples A12 to A15 in Table 8, which were immersed in blast furnace slag-released water having a reducing sulfur concentration of 0.03 % by weight for 3 to 5 days, the amount of Cr 6+ released from the processed samples was reduced, but still was not less than 0.05 mg/liter. In contrast to those test samples, samples A27 to A31 of the invention were immersed in blast furnace slag-released water, to which or over which had been added or sprayed any of elemental sulfur, sinter, sodium thiosulfate and hydrogen sulfate (that is, any of elemental sulfur-containing substances or compounds of sulfur having a valence less than 6) thereby to control the reducing sulfur content of the resulting water, for 5 days or less. As a result, the amount of Cr 6+ released from those processed samples of the invention was less than 0.05 mg/liter. As for sample A32 of the invention, it was immersed in blast furnace slag-released water, to which had been added ferrous sulfate, for 3 days. The amount of Cr 6+ released from this sample A32 was also less than 0.05 mg/liter.

Claims: A method for processing chromium oxide-containing substances in large quantities at low temperatures comprising the steps of cooling a blast furnace slag with water for obtaining an aqueous solution released from said slag and containing at least one member selected from a group consisting of sulfur and compounds of sulphur having a valence less than 6, and contacting said chromium oxide-containing substances with said aqueous solution for reducing the chromium oxides in said chromium oxide-containing substances. The method for processing chromium oxide-containing substances in large quantities at low temperatures as claimed in claim 1, wherein said aqueous sulfur-containing solution has a sulfur content of said at least one member of more than 0.03 % by weight. The method for processing chromium oxide-containing substances in large quantities at low temperatures as claimed in claim 1, wherein said chromium oxide-containing substances are sprayed with said aqueous sulfur-containing solution. The method for processing chromium oxide-containing substances in large quantities at low temperatures as claimed in claim 1, wherein said chromium oxide-containing substances are immersed in said aqueous surfur-containing solution. The method for processing chromium oxide-containing substances in large quantities at low temperatures as claimed in claim 4, wherein to said solution are added substances that contain said at least one member.

Inventors: KISHIMOTO YASUO, KUMAGAI MASATO, MATSUNAGA HISAHIRO, SAKURAYA TOSHIKAZU, TOBO HIROYUKI, KISHIMOTO, YASUO, KUMAGAI, MASATO, MATSUNAGA, HISAHIRO, SAKURAYA, TOSHIKAZU, TOBO, HIROYUKI, Kishimoto, Yasuo, c/o Kawasaki Steel Corporation, Kumagai, Masato, c/o Kawasaki Steel Corporation, Matsunaga, Hisahiro, c/o Kawasaki Steel Corp., Sakuraya, Toshikazu, c/o Kawasaki Steel Corp., Tobo, Hiroyuki, c/o Kawasaki Steel Corporation
Applicants: JFE STEEL CORP, JFE STEEL CORPORATION
Application year: 1998
Main classification: C22B7
IPC codes: C22B34, C22B5, C22B7