Method for treating hexavalent chromium-containing ash and apparatus for treating hexavalent chromium-containing ash

Abstract

To provide a processing method of hexavalent chromium-containing ash which can suitably suppress elution of hexavalent chromium from hexavalent chromium-containing ash by using a thiosulfuric acid compound as a hexavalent chromium elution inhibitor.SOLUTION: There is provided a processing method of hexavalent chromium-containing ash which adds and kneads a hexavalent chromium elution inhibitor for suppressing elution of hexavalent chromium from hexavalent chromium-containing ash to the hexavalent chromium-containing ash that contains a hexavalent chromium compound, in which the hexavalent chromium elution inhibitor is made of a thiosulfuric acid compound or a liquid solution of the thiosulfuric acid compound, an addition amount of the hexavalent chromium elution inhibitor is the addition amount that becomes equal to or greater than 25 at a molar ratio of a thiosulfate ion constituting the thiosulfuric acid compound to a hexavalent chromium ion constituting the hexavalent chromium compound, and kneading is performed under a non-heating condition.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for treating hexavalent chromium-containing ash and an apparatus for treating hexavalent chromium-containing ash.

Background Art

[0002] Fly ash generated in waste treatment facilities generally contains a large amount of heavy metals such as hexavalent chromium (Cr(VI)). When such heavy metal-containing ash, such as fly ash, is used for landfill disposal or the like, heavy metals may elute from the heavy metal-containing ash, and there are concerns that the environment may be contaminated by the eluted heavy metals. For this reason, when using heavy metal-containing ash for landfill disposal, it is required that the elution amount of heavy metals from the heavy metal-containing ash be small. Specifically, for heavy metal-containing ash used for landfill disposal, it is required to reduce the elution concentration of each heavy metal to a predetermined reference value or less based on the test of Notification No. 13 of the Ministry of the Environment. To achieve this, a treatment for reducing the elution amount of heavy metals from heavy metal-containing ash, that is, a treatment for suppressing the elution of heavy metals from heavy metal-containing ash, is required. Examples of such a treatment include a treatment for suppressing the elution of hexavalent chromium when the heavy metal-containing ash is, for example, hexavalent chromium-containing ash containing a hexavalent chromium compound. Further, in the technical field of suppressing the elution of heavy metals from the heavy metal-containing ash, since the water solubility of the hexavalent chromium compound is relatively high, a treatment for suppressing the elution of hexavalent chromium is more required.

[0003] Examples of such a treatment for suppressing the elution of heavy metals from heavy metal-containing ash include a treatment using the techniques described in Patent Documents 1 to 5.

[0004] Patent Document 1 describes a method for treating dust in a melting treatment facility for incineration ash, which involves adding and kneading water, ferrous chloride, and / or ferrous sulfate to the dust separated by a dust collector in the melting treatment facility for incineration ash. According to Patent Document 1, by adding ferrous chloride and / or ferrous sulfate to the conventionally performed chelate treatment method to add a reduction and non-elution effect, the amount of expensive chelating agent used can be reduced. Also, with the same amount of chelating agent, the elution amount of heavy metals can be reduced to a lower level, and it is disclosed that insolubilization treatment of heavy metals that do not require a chelating agent is also possible.

[0005] Patent Document 2 describes a method for insolubilizing heavy metal-containing ash, which involves adding a solution of a thiosulfate compound to the heavy metal-containing ash and kneading it, and then heat-treating the heavy metal-containing ash. According to Patent Document 2, it is disclosed that the elution of selenium, hexavalent chromium, and mercury contained in the heavy metal-containing ash can be easily and efficiently insolubilized at low cost to prevent their elution.

[0006] Patent Document 3 describes a chemical agent in which a dithiocarbamate-based salt is mixed with a sulfur compound and is composed of one selected from the group such as alkali metals, alkaline earth metals, and ammonium. According to Patent Document 3, it is disclosed that heavy metals and hexavalent chromium Cr(VI) contained in solid waste, etc. can be detoxified (made non-elutable) simultaneously by adding a single compound without the need for a combination of a plurality of compounds or solutions.

[0007] Patent Document 4 describes a method for insolubilizing hexavalent chromium in incineration residues, which involves adding and mixing a heavy metal scavenger having a dithiocarbamate salt as a functional group and a thiosulfate to the incineration residues. According to Patent Document 4, it is disclosed that problems such as re-elution of conventional hexavalent chromium and generation of harmful gases can be solved, and harmful heavy metals such as hexavalent chromium in incineration residues can be reliably immobilized to prevent their elution over a long period.

[0008] Patent Document 5 describes a waste treatment method in which waste is mixed with a waste treatment material containing cements, reducing metals, and sodium thiosulfate, kneaded after adding water as necessary, and cured and solidified. According to Patent Document 5, it is disclosed that the solidified product after reliably solidifying and encapsulating waste containing various harmful heavy metals and the like and stabilizing it is easy to handle, carry, and landfill.

[0009] Patent Document 6 describes a method for reducing the elution of hexavalent chromium in which a treatment object containing hexavalent chromium is mixed with a thiosulfate and at least one compound selected from naphthalene-based polymers, polycarboxylic acid-based polymers, lignin sulfonic acid-based polymers, and melamine-based polymers. According to Patent Document 6, it is disclosed that the elution of hexavalent chromium from objects containing hexavalent chromium such as hydraulic compositions, soil assimilates, and industrial waste can be more efficiently reduced.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0011] In the treatment method described in Patent Document 1, at least one of ferrous chloride and ferrous sulfate is added to incineration ash and kneaded. Although ferrous chloride and ferrous sulfate have the effect of suppressing the elution of hexavalent chromium, they are strongly acidic agents. For this reason, there is a risk of corrosion problems in the liquid contact parts that come into contact with the agent in chemical injection pipes, kneaders, etc. Therefore, in the equipment using this treatment method, modifications such as material changes are required to prevent corrosion.

[0012] In the insolubilization method described in Patent Document 2, a solution of a thiosulfate compound is added to heavy metal-containing ash and kneaded, but the appropriate addition amount of the thiosulfate compound has not been studied. Also, in the insolubilization method described in Patent Document 2, heating is performed, which may increase the running cost. From these facts, the appropriate addition amount of the thiosulfate compound when not heating is unknown in Patent Document 2.

[0013] Although Patent Document 3 exemplifies sodium thiosulfate as a sulfur compound to be added to the agent, the appropriate addition amount when using a thiosulfate compound as a hexavalent chromium elution inhibitor for suppressing the elution of hexavalent chromium from hexavalent chromium-containing ash is not described.

[0014] Although Patent Document 4 describes that the hexavalent chromium insolubilization treatment material to be used contains a thiosulfate, it is also described that the thiosulfate contains a heavy metal collector having dithiocarbamate as a functional group. From this, Patent Document 4 does not describe the appropriate addition amount when using a hexavalent chromium elution inhibitor composed of a thiosulfate compound or a solution of the thiosulfate compound. Also, Patent Document 4 does not describe the addition amount of the hexavalent chromium elution inhibitor in terms of the molar ratio of thiosulfate ions constituting the thiosulfate compound to hexavalent chromium ions constituting the hexavalent chromium compound.

[0015] Although Patent Document 5 describes a waste treatment material containing sodium thiosulfate, the invention described in Patent Document 5 is an invention related to a method for treating waste that solidifies with cement by mixing it with a waste treatment material containing cements. Therefore, Patent Document 5 does not describe an appropriate addition amount when using a hexavalent chromium elution inhibitor composed of a thiosulfate compound or a solution of the thiosulfate compound. Further, Patent Document 5 does not describe the addition amount of the hexavalent chromium elution inhibitor in terms of the molar ratio of thiosulfate ions constituting the thiosulfate compound to hexavalent chromium ions constituting the hexavalent chromium compound.

[0016] Although Patent Document 6 includes a thiosulfate as a compound to be mixed with an object to be treated containing hexavalent chromium, it also includes other compounds such as naphthalene-based polymers in addition to the thiosulfate. Therefore, Patent Document 6 does not describe an appropriate addition amount when using a thiosulfate compound as a hexavalent chromium elution inhibitor for suppressing the elution of hexavalent chromium from hexavalent chromium-containing ash.

[0017] As can be seen from the above prior art, etc., suppressing the elution of hexavalent chromium from hexavalent chromium-containing ash by using a thiosulfate compound in combination with other hexavalent chromium elution inhibitors has been studied. On the other hand, in the treatment of suppressing the elution of hexavalent chromium from the hexavalent chromium-containing ash by adding a hexavalent chromium elution inhibitor to the hexavalent chromium-containing ash, the appropriate addition amount for each type of hexavalent chromium elution inhibitor has not been studied much. The appropriate addition amount when using a thiosulfate compound as the hexavalent chromium elution inhibitor has not been studied either. Since the appropriate addition amount of the thiosulfate compound has not been studied, the appropriate addition amount of the thiosulfate compound according to the content of the hexavalent chromium compound in the hexavalent chromium-containing ash has not been studied either. From the above, using a thiosulfate compound as a hexavalent chromium elution inhibitor to preferably suppress the elution of hexavalent chromium from hexavalent chromium-containing ash has not been sufficiently studied.

[0018] The present invention is an invention made in view of such circumstances, and an object thereof is to provide a method for treating hexavalent chromium-containing ash and an apparatus for treating hexavalent chromium-containing ash, which can suitably suppress the elution of hexavalent chromium from hexavalent chromium-containing ash by using a thiosulfate compound as a hexavalent chromium elution inhibitor.

Means for Solving the Problems

[0019] As a result of various studies, the present inventor has found that the above object is achieved by the following present invention.

[0020] A method for treating hexavalent chromium-containing ash according to one aspect of the present invention is a method for treating hexavalent chromium-containing ash in which a hexavalent chromium elution inhibitor for suppressing the elution of hexavalent chromium from the hexavalent chromium-containing ash is added to the hexavalent chromium-containing ash containing a hexavalent chromium compound and kneaded. The hexavalent chromium elution inhibitor consists of a thiosulfate compound or a solution of the thiosulfate compound, and the addition amount of the hexavalent chromium elution inhibitor is an addition amount such that the molar ratio of thiosulfate ions constituting the thiosulfate compound to hexavalent chromium ions constituting the hexavalent chromium compound is 25 or more, and the kneading is performed under non-heating.

[0021] According to such a configuration, by using a thiosulfate compound as a hexavalent chromium elution inhibitor, the elution of hexavalent chromium from hexavalent chromium-containing ash can be preferably suppressed. This is considered to be due to the following reasons. The addition amount of the hexavalent chromium elution inhibitor is an addition amount suitable for suppressing the elution of hexavalent chromium from the hexavalent chromium-containing ash according to the amount of hexavalent chromium ions constituting the hexavalent chromium compound. That is, the addition amount of the hexavalent chromium elution inhibitor is an addition amount that can preferably suppress the elution of hexavalent chromium from the hexavalent chromium-containing ash according to the content of the hexavalent chromium compound contained in the hexavalent chromium-containing ash. Then, after adding the thiosulfate compound as the hexavalent chromium elution inhibitor to the hexavalent chromium-containing ash in the addition amount and kneading, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be preferably suppressed. Therefore, according to the method for treating the hexavalent chromium-containing ash, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be preferably suppressed by using a thiosulfate compound as a hexavalent chromium elution inhibitor. Furthermore, since the elution of hexavalent chromium from the hexavalent chromium-containing ash is preferably suppressed, even when the kneading is performed under non-heated conditions, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be preferably suppressed. By performing the kneading under non-heated conditions, the method for treating the hexavalent chromium-containing ash can be performed in an energy-saving manner.

[0022] Also, in the method for treating the hexavalent chromium-containing ash, it is preferable that the solution of the thiosulfate compound is an aqueous sodium thiosulfate solution.

[0023] According to such a configuration, by using a thiosulfate compound as a hexavalent chromium elution inhibitor, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be more preferably suppressed.

[0024] Further, in the method for treating the hexavalent chromium-containing ash, the hexavalent chromium-containing ash is obtained from a gasification melting furnace that pyrolyzes the waste to be input and melts and slagifies the ash content in the pyrolysis gas generated by the pyrolysis, or from an incinerator that incinerates the waste to be input. It may be fly ash contained in the exhaust gas discharged from the gasification melting furnace or the incinerator in a waste treatment facility and collected by a dust collector.

[0025] The fly ash is hexavalent chromium-containing ash that contains a relatively large amount of hexavalent chromium compounds. Also, it is required that the elution of hexavalent chromium from the fly ash be suppressed. By treating such fly ash as the hexavalent chromium-containing ash with the method for treating the hexavalent chromium-containing ash, the elution of hexavalent chromium can be preferably suppressed by using a thiosulfate compound as a hexavalent chromium elution inhibitor. This is preferable in that the elution of hexavalent chromium can be preferably suppressed from the fly ash, which has a high requirement for suppressing the elution of hexavalent chromium.

[0026] Further, in the method for treating the hexavalent chromium-containing ash, it is preferable that the kneading be performed at a temperature of 100°C or lower of the hexavalent chromium-containing ash.

[0027] According to such a configuration, after adding a thiosulfate compound as the hexavalent chromium elution inhibitor to the hexavalent chromium-containing ash, the mixture is kneaded under non-heating conditions so that the temperature of the hexavalent chromium-containing ash falls within the above range. By doing so, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be more preferably suppressed by the thiosulfate compound as the hexavalent chromium elution inhibitor.

[0028] Moreover, a treatment apparatus for hexavalent chromium-containing ash according to another aspect of the present invention includes an addition unit that adds a hexavalent chromium elution inhibitor for suppressing elution of hexavalent chromium from the hexavalent chromium-containing ash to the hexavalent chromium-containing ash containing a hexavalent chromium compound, and a kneading unit that kneads the hexavalent chromium-containing ash and the hexavalent chromium elution inhibitor. The hexavalent chromium elution inhibitor is composed of a thiosulfate compound or a solution of the thiosulfate compound, and the addition amount of the hexavalent chromium elution inhibitor is an addition amount such that the molar ratio of thiosulfate ions constituting the thiosulfate compound to hexavalent chromium ions constituting the hexavalent chromium compound is 25 or more. The kneading unit is a treatment apparatus for hexavalent chromium-containing ash that performs the kneading under non-heating conditions.

[0029] According to such a configuration, in the addition section, a hexavalent chromium elution inhibitor that suppresses the elution of hexavalent chromium from the hexavalent chromium-containing ash is added to the hexavalent chromium-containing ash containing a hexavalent chromium compound. At this time, the amount of the hexavalent chromium elution inhibitor added in the addition section is an addition amount corresponding to the amount of hexavalent chromium ions constituting the hexavalent chromium compound as described above. This addition amount is a suitable addition amount for suppressing the elution of hexavalent chromium from the hexavalent chromium-containing ash according to the amount of hexavalent chromium ions constituting the hexavalent chromium compound, that is, it is considered to be an addition amount that can suitably suppress the elution of hexavalent chromium from the hexavalent chromium-containing ash according to the content of the hexavalent chromium compound contained in the hexavalent chromium-containing ash. After adding a thiosulfate compound as the hexavalent chromium elution inhibitor to the hexavalent chromium-containing ash, in the kneading section, the hexavalent chromium-containing ash and the hexavalent chromium elution inhibitor are kneaded without heating. By doing so, a suitable amount of the hexavalent chromium elution inhibitor can be uniformly dispersed in the hexavalent chromium-containing ash, and the elution of hexavalent chromium from the hexavalent chromium-containing ash can be suitably suppressed. Therefore, according to the treatment device for the hexavalent chromium-containing ash, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be suitably suppressed by using a thiosulfate compound as the hexavalent chromium elution inhibitor. Further, since the elution of hexavalent chromium from the hexavalent chromium-containing ash is suitably suppressed, even when the kneading is performed without heating, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be suitably suppressed. By performing the kneading without heating, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be suppressed with energy savings.

[0030] Also, in the treatment device for the hexavalent chromium-containing ash, it is preferable that the solution of the thiosulfate compound is an aqueous sodium thiosulfate solution.

[0031] According to such a configuration, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be more suitably suppressed by using a thiosulfate compound as the hexavalent chromium elution inhibitor.

[0032] In addition, in the hexavalent chromium-containing ash treatment apparatus, the hexavalent chromium-containing ash may be fly ash contained in the exhaust gas discharged from the gasification melting furnace or the incinerator in a waste treatment facility including a gasification melting furnace that thermally decomposes the waste to be input and melts and slagifies the ash content in the pyrolysis gas generated by the thermal decomposition, or an incinerator that incinerates the waste to be input, and collected by a dust collector.

[0033] The fly ash is hexavalent chromium-containing ash containing a relatively large amount of hexavalent chromium compounds. Further, it is required that the elution of hexavalent chromium from the fly ash be suppressed. By treating such fly ash as the hexavalent chromium-containing ash in the hexavalent chromium-containing ash treatment apparatus, the elution of hexavalent chromium can be suitably suppressed by using a thiosulfate compound as a hexavalent chromium elution inhibitor. It is preferable in that the elution of hexavalent chromium can be suitably suppressed from the fly ash with a high demand for suppression of the elution of hexavalent chromium.

[0034] In addition, in the hexavalent chromium-containing ash treatment apparatus, it is preferable that the kneading unit perform the kneading when the temperature of the hexavalent chromium-containing ash is 100°C or lower.

[0035] According to such a configuration, after adding a thiosulfate compound as the hexavalent chromium elution inhibitor to the hexavalent chromium-containing ash in the addition unit, the kneading unit kneads the hexavalent chromium-containing ash without heating so that the temperature of the hexavalent chromium-containing ash falls within the above range. By doing so, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be more suitably suppressed by the thiosulfate compound as the hexavalent chromium elution inhibitor.

Advantages of the Invention

[0036] According to the present invention, it is possible to provide a method for treating hexavalent chromium-containing ash and a hexavalent chromium-containing ash treatment apparatus that can suitably suppress the elution of hexavalent chromium from hexavalent chromium-containing ash by using a thiosulfate compound as a hexavalent chromium elution inhibitor.

Brief Description of the Drawings

[0037]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0038] Hereinafter, embodiments according to the present invention will be described, but the present invention is not limited thereto.

[0039] The method for treating hexavalent chromium-containing ash according to an embodiment of the present invention is a method for treating hexavalent chromium-containing ash in which a hexavalent chromium elution inhibitor for suppressing the elution of hexavalent chromium from hexavalent chromium-containing ash containing a hexavalent chromium compound is added and kneaded. In the method for treating hexavalent chromium-containing ash, the hexavalent chromium elution inhibitor is composed of a thiosulfate compound or a solution of the thiosulfate compound. In the addition, the addition amount of the hexavalent chromium elution inhibitor (hexavalent chromium elution inhibitor composed of a thiosulfate compound or a solution of the thiosulfate compound) is such that the molar ratio (S2O3 6+ ) to hexavalent chromium ions (Cr 2- ) constituting the thiosulfate compound (S2O3 2- / Cr 6+ ) is 25 or more. Such an addition amount is a suitable addition amount for suppressing the elution of hexavalent chromium from the hexavalent chromium-containing ash according to the amount of hexavalent chromium ions (Cr 6+ ) constituting the hexavalent chromium compound, rather than the amount of the hexavalent chromium-containing ash. That is, regardless of the amount of the hexavalent chromium-containing ash, hexavalent chromium ions (Cr 6+) is a suitable addition amount for suppressing the elution of hexavalent chromium from the hexavalent chromium-containing ash according to the amount. Specifically, even if the amount of the hexavalent chromium-containing ash is small, if there are circumstances such as a high concentration of the hexavalent chromium ions in the hexavalent chromium-containing ash, the addition amount of the hexavalent chromium elution inhibitor increases as the amount of the hexavalent chromium ions increases. On the contrary, even if the amount of the hexavalent chromium-containing ash is large, if there are circumstances such as a low concentration of the hexavalent chromium ions in the hexavalent chromium-containing ash, the addition amount of the hexavalent chromium elution inhibitor may be small if the amount of the hexavalent chromium ions is small. From this, after adding a thiosulfate compound as the hexavalent chromium elution inhibitor to the hexavalent chromium-containing ash in the addition amount and kneading, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be preferably suppressed. Therefore, according to the treatment method of the hexavalent chromium-containing ash, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be preferably suppressed by using a thiosulfate compound as the hexavalent chromium elution inhibitor. Specifically, by subjecting the hexavalent chromium-containing ash to the treatment method of the hexavalent chromium-containing ash, regardless of the concentration of the hexavalent chromium compound in the hexavalent chromium-containing ash, the elution concentration of hexavalent chromium ions (Cr 6+ ) based on the test of Notification No. 13 of the Ministry of the Environment can be made 1.5 mg / L or less, which is the national standard (the standard described in Annex 1 of the ministerial ordinance that determines the judgment criteria for industrial waste including metals, etc.). For example, it can be made 1 mg / L or less. Furthermore, since the elution of hexavalent chromium from the hexavalent chromium-containing ash is preferably suppressed, even when the kneading is performed under non-heated conditions, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be preferably suppressed. Therefore, by the treatment method of the hexavalent chromium-containing ash, the kneading is performed under non-heated conditions. By performing the kneading under non-heated conditions, the treatment method of the hexavalent chromium-containing ash can be performed in an energy-saving manner.

[0040] The addition amount of the hexavalent chromium elution inhibitor is the molar ratio (S2O3 6+ ) of the thiosulfate ions (S2O3 2- ) that make up the thiosulfate compound to the hexavalent chromium ions (Cr ) that make up the hexavalent chromium compound.2- / Cr 6+ ) and is 25 or more, preferably 30 or more, and more preferably 30 to 70. If the addition amount is too small, the effect of suppressing the elution of hexavalent chromium from the hexavalent chromium-containing ash tends to be insufficient, and by the addition amount being within the above range, even if the subsequent kneading is carried out without heating, it can be suitably suppressed. On the other hand, if the addition amount is too large, the effect of suppressing the elution of hexavalent chromium from the hexavalent chromium-containing ash will saturate, and there is a tendency for the unnecessary usage amount of the hexavalent chromium elution inhibitor to increase. Further, if the addition amount is too large, there is a risk of generating harmful gases such as sulfur oxide gas.

[0041] In the method for treating hexavalent chromium-containing ash, the kneading performed after the addition is not particularly limited as long as the hexavalent chromium-containing ash and the hexavalent chromium elution inhibitor added to the hexavalent chromium-containing ash are preferably mixed. As described above, the kneading may be performed without heating. Specifically, the temperature of the hexavalent chromium-containing ash during kneading is preferably 100°C or lower, more preferably 10 to 50°C. Even when the temperature of the hexavalent chromium-containing ash during kneading is within the above range, in the method for treating hexavalent chromium-containing ash according to the present embodiment, elution of hexavalent chromium from the hexavalent chromium-containing ash is preferably suppressed, so that elution of hexavalent chromium from the hexavalent chromium-containing ash can be suppressed in an energy-saving manner. The temperature of the hexavalent chromium-containing ash can be measured by a temperature sensor that detects the surface temperature of the hexavalent chromium-containing ash. Examples of the temperature sensor include a temperature sensor capable of detecting temperature non-contact, such as an infrared thermography capable of detecting temperature non-contact or an infrared radiation thermometer capable of detecting temperature non-contact. The conditions during kneading are not particularly limited as long as the hexavalent chromium-containing ash and the hexavalent chromium elution inhibitor are preferably mixed, other than the temperature of the hexavalent chromium-containing ash during kneading. For example, the kneading time is preferably 1 to 10 minutes. Further, the kneading can be performed using a kneading device, and the kneading device is not particularly limited as long as it can knead the hexavalent chromium-containing ash and the hexavalent chromium elution inhibitor. Examples of the kneading device include a known kneader, and specifically, a kneading device provided in a waste treatment facility.

[0042] The hexavalent chromium elution inhibitor consists of a thiosulfate compound or a solution of the thiosulfate compound. The thiosulfate compound is not particularly limited as long as it can suppress the elution of hexavalent chromium from the hexavalent chromium-containing ash by kneading with the hexavalent chromium-containing ash. Further, the hexavalent chromium elution inhibitor may be any component that can suppress the elution of hexavalent chromium from the hexavalent chromium-containing ash and consists of a thiosulfate compound, and as described above, it may be a solution containing a solvent. Examples of the thiosulfate compound include sodium thiosulfate, potassium thiosulfate, magnesium thiosulfate, calcium thiosulfate, and ammonium thiosulfate. The solvent constituting the solution of the thiosulfate compound is not particularly limited as long as it can dissolve the thiosulfate compound and does not inhibit the suppression of hexavalent chromium elution from the hexavalent chromium-containing ash, and examples thereof include water. That is, examples of the solution of the thiosulfate compound include an aqueous solution of the thiosulfate compound.

[0043] The fact that the thiosulfate compound can suppress the elution of hexavalent chromium from the hexavalent chromium-containing ash is considered to be due to, for example, the following reaction. Using a compound containing thiosulfate ion (S2O3 2- ), specifically sodium thiosulfate (Na2S2O3), and using chromate ion (CrO4 2- ) as the hexavalent chromium ion eluted from the hexavalent chromium-containing ash, it will be explained. By generating chromium(III) hydroxide (Cr(OH)3) through the following reaction, the elution of hexavalent chromium from the hexavalent chromium-containing ash can be suppressed.

[0044] Na2S2O3 → 2Na + + S2O3 2- 2S2O3 2- → S4O6 2- + 2e - CrO4 2- + 4H2O + 3e - → Cr(OH)3↓ + 5OH - 2CrO42- + 6S2O3 2- + 8H2O → 6Cr(OH)3+ 3S4O6 2- + 10OH - In the method for treating the hexavalent chromium-containing ash, an inhibitor for suppressing the elution of heavy metals other than hexavalent chromium (a heavy metal elution inhibitor other than the hexavalent chromium elution inhibitor) may be added to the hexavalent chromium-containing ash together with the hexavalent chromium elution inhibitor. As the heavy metal elution inhibitor other than the hexavalent chromium elution inhibitor, as long as the elution of heavy metals (heavy metals other than hexavalent chromium from the hexavalent chromium-containing ash) can be suppressed by kneading with the hexavalent chromium-containing ash, it is not particularly limited. Specifically, as the heavy metal elution inhibitor other than the hexavalent chromium elution inhibitor, an elution inhibitor for Pb that suppresses the elution of lead (Pb) and the like can be mentioned. As the elution inhibitor for Pb, for example, dithiocarbamates such as dipotassium = piperazine-1,4-dicarbodithioate (heavy metal treatment agent TS-275) and the like can be mentioned.

[0045] The hexavalent chromium compound is not particularly limited as long as it is contained in the hexavalent chromium-containing ash and there is concern about the elution of hexavalent chromium from the hexavalent chromium-containing ash. Examples of the hexavalent chromium compound include chromic acid, dichromates, and chromates. Examples of the dichromates include sodium dichromate, potassium dichromate, and ammonium dichromate. Examples of the chromates include sodium chromate, potassium chromate, ammonium chromate, lead chromate, zinc chromate, strontium chromate, and calcium chromate.

[0046] The hexavalent chromium-containing ash is not particularly limited as long as it is ash containing a hexavalent chromium compound. Examples of the hexavalent chromium-containing ash include ash generated from waste treatment facilities. Examples of the ash generated from waste treatment facilities include incineration ash and fly ash such as incineration fly ash and molten fly ash. Further, examples of the fly ash include fly ash generated from waste treatment facilities equipped with incinerators and melting furnaces. More specifically, examples of the fly ash include fly ash generated from waste treatment facilities equipped with fluidized bed gasification incinerators and stoker incinerators. Fly ash generally contains more heavy metals such as hexavalent chromium compounds than incineration ash. Molten fly ash generally contains more heavy metals such as hexavalent chromium compounds than incineration fly ash. The method for treating hexavalent chromium-containing ash according to the present embodiment can preferably suppress the elution of hexavalent chromium from the hexavalent chromium-containing ash. Therefore, in the method for treating hexavalent chromium-containing ash, when using hexavalent chromium-containing ash containing more hexavalent chromium compounds (performing the method for treating hexavalent chromium-containing ash on hexavalent chromium-containing ash containing more hexavalent chromium compounds), the effect of suppressing the elution of hexavalent chromium can be more exerted. Therefore, as the hexavalent chromium-containing ash, fly ash such as incineration fly ash and molten fly ash is preferable, and molten fly ash is more preferable. Examples of the incineration fly ash include fly ash contained in the exhaust gas discharged from the incinerator and collected by a dust collector in a waste treatment facility equipped with an incinerator for incinerating the input waste. Examples of the molten fly ash include fly ash contained in the molten exhaust gas discharged from a gasification melting furnace and collected by a dust collector in a waste treatment facility equipped with a gasification melting furnace that thermally decomposes the input waste and melts and slags the ash content in the pyrolysis gas generated by the thermal decomposition. Among these, fly ash contained in the molten exhaust gas discharged from the gasification melting furnace and collected by a dust collector is preferable. More specifically, examples of this molten fly ash include fly ash generated from the waste treatment facility shown in FIG. 1. The waste is not particularly limited as long as it can be treated in the waste treatment facility, and may be general waste or industrial waste.Examples of the waste include various wastes such as municipal solid waste and sewage sludge.

[0047] As described above, examples of the hexavalent chromium-containing ash include fly ash generated from a waste treatment facility as shown in FIG. 1, for example. This fly ash (hexavalent chromium-containing ash) may be subjected to a treatment method for the hexavalent chromium-containing ash in a waste treatment facility as shown in FIG. 1. That is, the treatment method for the hexavalent chromium-containing ash may be realized by, for example, a treatment device for hexavalent chromium-containing ash provided in a waste treatment device as shown in FIG. 1. The treatment device for hexavalent chromium-containing ash according to the present embodiment includes an addition unit that adds a hexavalent chromium elution inhibitor that suppresses the elution of hexavalent chromium from the hexavalent chromium-containing ash containing a hexavalent chromium compound, and a kneading unit that kneads the hexavalent chromium-containing ash and the hexavalent chromium elution inhibitor. In the treatment device for hexavalent chromium-containing ash, the addition unit adds the hexavalent chromium elution inhibitor (a hexavalent chromium elution inhibitor composed of a thiosulfate compound or a solution of the thiosulfate compound) in an addition amount such that the molar ratio (S2O3 6+ / Cr 2- ) of thiosulfate ions (S2O3 2- ) constituting the thiosulfate compound to hexavalent chromium ions (Cr 6+ ) constituting the hexavalent chromium compound is 25 or more. Thereafter, the kneading unit kneads the hexavalent chromium-containing ash and the hexavalent chromium elution inhibitor without heating. Note that FIG. 1 shows a schematic diagram showing the overall configuration of a waste treatment facility 1 including a treatment device for hexavalent chromium-containing ash according to the present embodiment.

[0048] The waste treatment facility 1 includes a gasification melting furnace 20, a garbage supply unit 10 that supplies garbage as waste to the gasification melting furnace 20, an exhaust gas treatment unit 30 that treats the molten exhaust gas discharged from the gasification melting furnace 20, and a fly ash treatment device 40 that performs detoxification treatment (such as a treatment method for the hexavalent chromium-containing ash) on the fly ash separated from the molten exhaust gas.

[0049] The garbage supply unit 10 includes a garbage pit 11, a garbage conveyor 12, and a dust supply device 13. The garbage pit 11 receives the garbage to be processed carried in from outside the waste treatment facility and temporarily stores it. The garbage conveyor 12 includes a garbage crane 14, and the garbage crane 14 picks up the garbage in the garbage pit 11 and conveys it to the dust supply device 13. The dust supply device 13 includes a garbage input hopper 15, and the garbage input hopper 15 receives the garbage input from the garbage conveyor 12. Then, the dust supply device 13 incorporates a screw conveyor 16 for garbage conveyance and supplies the garbage input into the garbage input hopper 15 to the gasification melting furnace 20.

[0050] The gasification melting furnace 20 includes a gasification furnace 21 and a melting furnace 22. The gasification furnace 21 thermally decomposes the garbage supplied from the dust supply device 13, thereby generating pyrolysis gas. As this gasification furnace 21, a known gasification furnace can be used, for example, a fluidized bed furnace, a kiln furnace, etc. The melting furnace 22 burns the combustible components in the pyrolysis gas at a high temperature and melts the ash in the gas to generate slag. This slag adheres, for example, to the furnace wall of the melting furnace 22. A slag discharge port 23 is provided at the bottom of the melting furnace 22. This slag discharge port 23 is for discharging the slag adhering to the furnace wall and flowing down outside the furnace. Also, in this melting furnace 22, combustion of auxiliary fuel by a burner (not shown) may be performed as necessary for adjusting the temperature inside the furnace.

[0051] The exhaust gas treatment unit 30 includes a waste heat boiler 31, a gas cooling tower 32, a dust collector 34, an induced draft fan 35, and a chimney 36. The waste heat boiler 31 is for recovering heat from the high-temperature molten exhaust gas discharged from the melting furnace 22. Specifically, for example, it generates and discharges steam by utilizing the heat possessed by the molten exhaust gas. The gas cooling tower 32 includes a tower body into which the molten exhaust gas discharged from the waste heat boiler 31 is introduced, a spraying device for spraying cooling water into the tower body, a temperature sensor for detecting the gas temperature at the outlet of the tower body, and a controller for adjusting the cooling water supply flow rate by the spraying device so as to keep the outlet gas temperature detected by this temperature sensor constant. The waste heat boiler 31 and the gas cooling tower 32 lower the temperature of the molten exhaust gas to a temperature suitable for use in the subsequent dust collector 34. The dust collector 34 collects the fly ash by filtering and separating the fly ash in the molten exhaust gas discharged from the gas cooling tower 32. Examples of the dust collector 34 include a device equipped with a bag filter for filtering and separating the fly ash in the molten exhaust gas discharged from the gas cooling tower 32. The fly ash collected by the dust collector 34 corresponds to the hexavalent chromium-containing ash. The exhaust gas from which the fly ash has been removed by this dust collector 34 is discharged from the chimney 36 through the induced draft fan 35.

[0052] The fly ash treatment device 40 is a device for suppressing the elution of heavy metals from the fly ash separated and collected by the dust collector 34 and detoxifying the fly ash. Note that the fly ash treatment device 40 corresponds to the treatment device for the hexavalent chromium-containing ash. The fly ash treatment device 40 includes a fly ash conveying device 41, a fly ash storage tank 42, a fly ash input hopper 43, a kneading device 44, and a heavy metal elution inhibitor adding section 45.

[0053] The fly ash conveying device 41 conveys the fly ash separated and collected by the dust collecting device 34 to the fly ash storage tank 42. The fly ash storage tank 42 receives the fly ash conveyed by the fly ash conveying device 41, stores it temporarily, and then inputs it into the fly ash input hopper 43. The timing of input may be every time a predetermined amount is stored, or after a predetermined time has elapsed. The fly ash input hopper 43 receives the fly ash input from the fly ash storage tank 42 and conveys it to the kneading device 44. The heavy metal elution inhibitor adding section 45 adds a heavy metal elution inhibitor to the fly ash conveyed by the fly ash input hopper 43. Note that the heavy metal elution inhibitor adding section 45 corresponds to the adding section. The kneading device 44 kneads the fly ash and the heavy metal elution inhibitor added to the fly ash by the heavy metal elution inhibitor adding section 45. Note that the kneading device 44 corresponds to the kneading section.

[0054] The heavy metal elution inhibitor adding section 45 includes a heavy metal elution inhibitor storage tank 47 and a pump 48. The heavy metal elution inhibitor storage tank 47 stores a heavy metal elution inhibitor for suppressing the elution of heavy metals from fly ash. The pump 48 adds the heavy metal elution inhibitor stored in the heavy metal elution inhibitor storage tank 47 to the kneading device 44. The heavy metal elution inhibitor includes the hexavalent chromium elution inhibitor. The addition amount of the heavy metal elution inhibitor is appropriately adjusted according to each heavy metal to suppress the elution of that heavy metal. For example, the elution inhibitor for Pb for suppressing the elution of lead (Pb) is adjusted to an addition amount suitable for suppressing the elution of lead. Also, the addition amount of the hexavalent chromium elution inhibitor is adjusted to an addition amount of 25 or more in terms of the molar ratio of the thiosulfate ions constituting the thiosulfate compound to the hexavalent chromium ions constituting the hexavalent chromium compound as described above.

[0055] From the above, the waste treatment facility equipped with the hexavalent chromium-containing ash treatment device according to the present embodiment can turn the fly ash generated from the waste treatment facility into fly ash in which the elution of hexavalent chromium is preferably suppressed.

[0056] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to these examples. EXAMPLES

[0057] 100 g of fly ash (ash containing hexavalent chromium, fly ash generated from a waste treatment facility equipped with a fluidized bed gasification and melting furnace) was placed in a sealable bag. Next, 30 g of water was added to the fly ash placed in the sealable bag (water was added to the sealable bag so that the amount of water was 30% by mass relative to the fly ash). After that, a predetermined amount of 30% by mass of sodium thiosulfate aqueous solution was added to the sealable bag. The amount of the sodium thiosulfate aqueous solution to be added (addition amount) was determined based on the amount of hexavalent chromium ions (Cr 6+ ) to the thiosulfate ion (S2O3 2- ) molar ratio (S2O3 2- / Cr 6+ ) was set to the value shown in Figure 2. Specifically, the following amounts were set.

[0058] First, the content of hexavalent chromium compounds in the fly ash was measured using an energy dispersive X-ray fluorescence analyzer (OURSTEX160 manufactured by Ourstech Co., Ltd.). From the content of hexavalent chromium compounds thus obtained, the molar ratios (S2O3 2- / Cr 6+ The amount of 30 mass% sodium thiosulfate aqueous solution to be added was calculated so that

[0059] After adding the sodium thiosulfate aqueous solution, the mixture was mixed uniformly and kneaded with a kneading machine (Dow Mixer manufactured by Shin-Nichinan Co., Ltd.) The kneaded fly ash was subjected to an elution test based on the Ministry of the Environment Notification No. 13, and the amount of eluted hexavalent chromium [Cr(IV) elution amount (mg / L)] was measured.

[0060] The results are shown in Figure 2. Figure 2 shows the hexavalent chromium ions (Cr 6+ ) to the thiosulfate ion (S2O3 2- ) molar ratio (S2O32- / Cr 6+ ) shows the relationship with the elution amount of hexavalent chromium Cr(IV) from fly ash.

[0061] As can be seen from Figure 2, even when the molar ratio (S2O3 2- / Cr 6+ ) is the same, although there are variations in the elution amount of Cr(IV), when an aqueous solution of sodium thiosulfate, which is a hexavalent chromium elution inhibitor, is added so that the molar ratio (S2O3 2- / Cr 6+ ) is 25 or more, the elution amount of Cr(IV) was 1 mg / L or less. In contrast, when the molar ratio (S2O3 2- / Cr 6+ ) is less than 25, the elution amount of Cr(IV) may exceed 1 mg / L. From this, it can be understood that with the method for treating the hexavalent chromium-containing ash (the method of adding an aqueous solution of sodium thiosulfate, which is a hexavalent chromium elution inhibitor, so that the molar ratio (S2O3 2- / Cr 6+ ) is 25 or more), it is possible to suitably suppress the elution of hexavalent chromium from the hexavalent chromium-containing ash by using a thiosulfate compound as the hexavalent chromium elution inhibitor.

Explanation of symbols

[0062] 1 Waste treatment facility 10 Garbage supply section 11 Garbage pit 12 Garbage conveyor 13 Dust feeding device 14 Garbage crane 15 Garbage input hopper 16 Screw conveyor 20 Gasification melting furnace 21 Gasification furnace 22 Melting furnace 23 Slag discharge port 30 Exhaust gas treatment section 31 Waste heat boiler 32 Gas cooling tower 34 Dust collector 35 Induced draft fan 36 Chimney 40 Fly ash treatment device (treatment device for hexavalent chromium-containing ash) 41 Fly ash conveying device 42 Fly ash storage tank 43 Fly ash input hopper 44 Kneading device (kneading section) 45 Heavy metal dissolution inhibitor addition section (addition section) 47 Heavy metal dissolution inhibitor storage tank 48 Pump

Claims

1. A method for treating hexavalent chromium-containing ash containing a hexavalent chromium compound, comprising adding a hexavalent chromium elution inhibitor for suppressing elution of hexavalent chromium from the hexavalent chromium-containing ash and kneading the mixture, wherein the hexavalent chromium elution inhibitor consists of a thiosulfate compound or a solution of the thiosulfate compound, the addition amount of the hexavalent chromium elution inhibitor is an addition amount such that the molar ratio of thiosulfate ions constituting the thiosulfate compound to hexavalent chromium ions constituting the hexavalent chromium compound is 25 or more, and the kneading is performed without heating. A method for treating hexavalent chromium-containing ash, characterized by the above.

2. The method for treating hexavalent chromium-containing ash according to claim 1, wherein the solution of the thiosulfate compound is an aqueous sodium thiosulfate solution.

3. The hexavalent chromium-containing ash is fly ash contained in exhaust gas discharged from a gasification melting furnace that thermally decomposes waste to be input, melts ash in the pyrolysis gas generated by the thermal decomposition, and forms slag, or an incinerator that incinerates waste to be input, and is collected by a dust collector in a waste treatment facility. The method for treating hexavalent chromium-containing ash according to claim 1.

4. The method for treating hexavalent chromium-containing ash according to any one of claims 1 to 3, wherein the kneading is performed at a temperature of 100 °C or lower of the hexavalent chromium-containing ash.

5. An addition unit for adding a hexavalent chromium elution inhibitor for suppressing elution of hexavalent chromium from hexavalent chromium-containing ash to the hexavalent chromium-containing ash, and a kneading unit for kneading the hexavalent chromium-containing ash and the hexavalent chromium elution inhibitor, wherein the hexavalent chromium elution inhibitor consists of a thiosulfate compound or a solution of the thiosulfate compound, the addition amount of the hexavalent chromium elution inhibitor is an addition amount such that the molar ratio of thiosulfate ions constituting the thiosulfate compound to hexavalent chromium ions constituting the hexavalent chromium compound is 25 or more, and the kneading unit performs the kneading without heating. A device for treating hexavalent chromium-containing ash, characterized by the above.

6. The device for treating hexavalent chromium-containing ash according to claim 5, wherein the solution of the thiosulfate compound is an aqueous sodium thiosulfate solution.

7. The hexavalent chromium-containing ash is fly ash contained in the exhaust gas discharged from the gasification melting furnace or the incinerator in a waste treatment facility equipped with a gasification melting furnace that pyrolyzes the input waste and melts and slags the ash in the pyrolysis gas generated by the pyrolysis, or an incinerator that incinerates the input waste, and is collected by a dust collector. The treatment device for hexavalent chromium-containing ash according to claim 5.

8. The kneading unit performs the kneading at a temperature of 100°C or lower of the hexavalent chromium-containing ash. The treatment device for hexavalent chromium-containing ash according to any one of claims 5 to 7.

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