JP2009066570A - Cement-based muddy water-derived chromium reduction method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for reducing cement-based turbid water-derived chromium to convert hexavalent chromium contained in a cement-based turbid water to trivalent chromium to render the hexavalent chromium harmless, which method uses a reducing agent, which is friendly to people and the environment and can realize recycle of not only waste water but also solid residue. <P>SOLUTION: The method comprises the step of adding an inorganic coagulant and a large amount of calcium sulfite to a hexavalent chromium-containing cement-based turbid water to allow coagulation sedimentation of a suspended component and a reaction for reducing hexavalent chromium in a suspension to trivalent chromium to proceed in a pH range of 5 to 8 and to allow calcium sulfite remaining unreacted to stay as a solid phase component in the liquid (a sedimentation and reduction step) and the step of then performing solid-liquid separation to recover a liquid having a hexavalent chromium content below the level of the wastewater quality standards and the calcium sulfite-containing solid residue (a solid-liquid separation step). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a technology for detoxifying hexavalent chromium contained in cement-based muddy water by reducing it to trivalent. Here, “cement-based muddy water” is water in which fine particulate matter derived from cement-based material is suspended (including cases where it is simply floating), and is generated, for example, by ground improvement using a cement-based solidifying agent. Turbid water, breathing water containing green dams of concrete dams and SS (Suspended Solid), curing water, muddy water containing cement content generated at tunnel construction sites, muddy water containing cement content generated from residual soil treatment facilities, etc. Can be mentioned.

  The cement-based muddy water as described above contains a slight amount of hexavalent chromium resulting from cement (for example, about 0.02 to 1 mg / L in terms of the concentration of hexavalent chromium dissolved in the liquid). Standards related to hexavalent chromium include drainage standards and soil environmental standards. The drainage standards are 0.5 mg / L of hexavalent chromium content (Environment Agency Notification No. 64), and backfilling solid residues such as dehydrated cake The soil environment standard for recycling is that the elution amount of hexavalent chromium satisfies 0.05 mg / L (Environment Agency Notification No. 46). Conventionally, cement-based muddy water generated at a construction site or the like has been subjected to a process of separating it into a liquid and a solid residue in a muddy water treatment facility installed near the site. In muddy water generated in general concrete construction, hexavalent chromium is not a problem because the content of hexavalent chromium in the liquid meets the drainage standard and is twice as high as the soil environmental standard. For this reason, turbid water treatment facilities usually discharge the separated liquid without subjecting the hexavalent chromium to a trivalent reduction. However, when the majority of breathing water occupies, it may exceed the drainage standard, requiring treatment. On the other hand, as for the collected solid residue, the environmental standard of elution amount is as severe as 1/10 of the case of drainage as described above, so it is not easy to clear this, and recycling as embankment etc. It is a difficult situation.

In the treatment of factory wastewater containing chromium, a treatment is performed in the factory to reduce hexavalent chromium to trivalent under low pH (less than 2) using sodium bisulfite (NaHCO ₃ ) or the like as a reducing agent. Yes. In order for sodium bisulfite to act outside the above pH range, excessive addition is required at an equivalent ratio, and solid residue containing harmful substances other than chromium increases, so the pH is adjusted to be low so that the required equivalent is sufficient.

In a muddy water treatment facility installed near a concrete construction site or the like, since it is temporary, a highly harmful one cannot be used, and it is also difficult to perform pretreatment to extremely reduce pH. Patent Documents 1 and 2 show a technique for reducing hexavalent chromium in cement-based wastewater to trivalent by using relatively easy to use general-purpose ferrous sulfate (FeSO ₄ ) as a reducing agent. Yes.

JP 59-199097 A JP 2003-145175 A

  The main purpose of the above-described method of treating industrial wastewater containing chromium using sodium hydrogen sulfite is to remove chromium in the wastewater, so that a recyclable solid residue cannot be obtained. The methods using ferrous sulfate disclosed in Patent Documents 1 and 2 are also intended for wastewater treatment, and are not intended for recycling of solid residues. Moreover, since ferrous sulfate remains in the solid residue, consideration for the environment is necessary. Regarding drainage, it is possible to reduce hexavalent chromium below the drainage standard, and even below the soil environmental standard, but the reducing agent ferrous sulfate is easily dissolved in water. It is difficult to do so, and waste due to excessive addition tends to increase.

  The present invention is a muddy water treatment facility temporarily installed near the site where cement-based muddy water is generated, and is a method suitable for performing treatment for detoxifying hexavalent chromium contained in muddy water into trivalent, It is intended to provide a treatment method that uses an environmentally friendly reducing agent and can recycle the resulting solid residue. Moreover, it aims at providing the processing method which is easy to prevent the excessive addition of a reducing agent regarding the process of waste_water | drain.

In order to achieve the above object, in the present invention, calcium sulfite (CaSO ₃ ) is used as a reducing agent for reducing hexavalent chromium to trivalent. Commonly available calcium sulfite is in the form of half-water CaSO ₃ .0.5H ₂ O.

The invention described in claims 1 to 3 is a process for detoxifying hexavalent chromium from a “liquid” already separated and recovered from cement-based muddy water, and is applied to, for example, step A in FIG. It is something that can be done.
That is, the step of advancing the reaction of reducing hexavalent chromium in the liquid to trivalent by contacting the hexavalent chromium-containing liquid recovered by separating and removing suspended components from the cement-based muddy water with calcium sulfite. This is a reduction treatment method for cement-based muddy water-derived chromium. Calcium sulfite hardly dissolves in water. For this reason, the reduction reaction of hexavalent chromium dissolved in the liquid occurs when calcium sulfite, which is a solid phase, comes into contact with the liquid phase. For example, when calcium sulfite powder is stirred and mixed in a liquid, the particles of calcium sulfite float and disperse in the liquid, so that the reduction reaction proceeds when the particles come into contact with the liquid.

  In order to reduce the amount of excess calcium sulfite added as much as possible and to ensure the detoxification of hexavalent chromium, the ORP (oxidation-reduction potential) after starting the reduction reaction is monitored continuously or periodically. It is effective to allow at least the reduction reaction to proceed until it is confirmed that the ORP has decreased from a fast rate to a slow state, and then stop the reduction reaction. Further, the pH and ORP (oxidation-reduction potential) after the reduction reaction is started are measured at least once, and the presence form of chromium in the solution is changed from hexavalent chromium to trivalent chromium on the pH-Eh (ORP) diagram. It is also effective to control the reaction time so that the reduction reaction proceeds until an ORP value lower than the changing ORP value is reached.

The inventions according to claims 4 and 5 are treatments for detoxifying hexavalent chromium from “solid residue” already separated and recovered from cement-based muddy water, and can be applied to step B in FIG. Is.
In other words, this is a method for reducing cement-based muddy water-derived chromium having a step of mixing a hexavalent chromium-containing solid residue separated and recovered from cement-based muddy water with calcium sulfite. At that time, it is effective to mix the solid residue with calcium sulfite together with the acidifying agent.

The inventions described in claims 6 to 8 are treatments in which a reducing agent is allowed to act in the course of solid-liquid separation of cement-based turbid water, and have a step corresponding to step C in FIG.
That is, by causing an inorganic flocculant and a large amount of calcium sulfite to act on hexavalent chromium-containing cement-based turbid water, the suspension component aggregates and settles and the hexavalent chromium in the liquid is reduced in a pH range of 5-8. Process to proceed (precipitation / reduction process),
Thereafter, a step (solid-liquid separation step) of solid-liquid separation to recover a liquid in which the amount of hexavalent chromium is below the drainage standard and a solid residue,
This is a reduction treatment method for chromium derived from cement-based muddy water.

  In the precipitation / reduction step, unreacted calcium sulfite can be left in the liquid as a solid phase component by adding a large amount of calcium sulfite. In this case, the solid residue containing calcium sulfite is recovered in the solid-liquid separation step. Here, “a large amount of calcium sulfite” means an amount of calcium sulfite that is sufficient to leave unreacted calcium sulfite in the liquid when it is subjected to the solid-liquid separation process.

After the precipitation / reduction step, a step of precipitating trivalent chromium as chromium hydroxide (Cr (OH) ₃ ) in a pH range of 7 to 9 (chromium precipitation step) can be performed. The precipitation / reduction process and the chromium precipitation process may overlap in time. For example, when the pH is maintained between 7 and 8 from the beginning, the trivalent chromium produced by the reduction sequentially changes to chromium hydroxide and precipitates.

  Calcium sulfite used as a reducing agent in the present invention is used as a reducing agent for removing chlorine in tap water and as an antioxidant for wine and the like, and is a highly safe substance for the human body. For this reason, safety improves compared with the process using the conventional reducing agent (Sodium hydrogen sulfite, ferrous sulfate, etc.). In addition, since calcium sulfite is difficult to dissolve in water, when reducing the liquid, soak the calcium sulfite in the net for the time required for the reaction and then pull it up, or pass the liquid through a filter packed with calcium sulfite. The method can be adopted, and waste due to excessive addition of the reducing agent is easily prevented. Moreover, diluting water becomes unnecessary by reducing and removing hexavalent chromium, and the amount of water discharged can be reduced. Furthermore, in the present invention, when the solid residue and calcium sulfite are mixed, the hexavalent chromium remaining without being reduced inside the cement particles of the solid residue is promptly trivalent even when it is eluted in the future. This makes it possible to recycle solid residues derived from cement-based turbid water.

FIG. 1 shows a flow of the process of applying the cement-based muddy water-derived chromium reduction method of the present invention to “liquid” (treated water) generated in a muddy water treatment facility according to the conventional muddy water treatment method or solid residue. An example is shown.
Cement-based muddy water generated from a construction site using cement-based materials is transferred to a muddy water treatment facility provided near the site. In the turbid water treatment facility, after adjusting the pH of the turbid water as necessary, a flocculant is added to promote sedimentation of the suspended components, and then the slurry of the turbid water is separated into solid and liquid so that the treated water and the solid The residue is recovered. In some cases, only pH adjustment may lead to the reservoir.

  Conventionally, in the case of “treated water”, if there is a problem, once it is stored in a reservoir, etc., after taking measures to clear the drainage standard, it is discharged or treated as industrial waste. Was a general processing method. In the case of hexavalent chromium, dilution, reduction, and industrial waste treatment can be considered. However, diluting results in the release of all hexavalent chromium in the solution, which is not preferable for environmental hygiene. It is not a solution. Therefore, the present invention is applied in step A shown in FIG. 1, and the reaction of reducing hexavalent chromium in the solution to trivalent is advanced by bringing calcium sulfite into contact with the treated water.

  Specifically, for example, a method of adding calcium sulfite powder to a hexavalent chromium-containing liquid (including treated water only for pH adjustment) stored in a reservoir and stirring the treated water can be employed. The charged calcium sulfite powder floats in the liquid and a reduction reaction occurs near the particle surface. When calcium sulfite functions as a reducing agent, it becomes sulfuric acid itself and dissolves in the liquid. Therefore, the powdered calcium sulfite powder particles are consumed and become smaller as the reduction reaction proceeds. Regarding the amount of calcium sulfite to be added, as a result of various studies, the addition of the hexavalent chromium concentration that normally satisfies the environmental standards in the amount of calcium sulfite in the range of 2 to 10 equivalents relative to the amount of hexavalent chromium in the liquid. You can find the amount. Moreover, with respect to the liquid that satisfies the drainage standard from the beginning, hexavalent chromium dissolved in the liquid can be reduced almost completely with the addition amount in the above range.

  A method in which a highly permeable bag containing calcium sulfite particles (mesh-shaped bag) is dipped in a hexavalent chromium-containing liquid (treated water) stored in a reservoir by suspending it with a heavy machine, and the reduction reaction proceeds. It is valid. In this case, since the reduction reaction can be stopped by pulling up the bag, for example, an excessive amount of calcium sulfite is immersed in a water-permeable bag, and the concentration of hexavalent chromium in the liquid is determined based on environmental standards due to changes in ORP. It is possible to adopt a method in which the dipping state is continued as it is until it is predicted that the bag has been lowered, and then the bag is pulled up. According to the investigation by the inventors, ORP decreases as the reduction reaction of hexavalent chromium proceeds, but does not decrease uniformly with the amount of reducing agent consumed, but at first, it decreases rapidly. Shows the behavior in which the change of ORP becomes slow. It was confirmed that the reduction reaction had progressed to such an extent that the hexavalent chromium concentration in the liquid was lower than the drainage standard or the soil environmental standard in that the change in ORP became slow. Therefore, when this ORP change behavior is used, it is possible to know the timing suitable for pulling up the water-permeable bag containing calcium sulfite. That is, it is possible to employ a technique in which the ORP of the liquid is continuously or periodically monitored after the reduction reaction starts and the reduction reaction proceeds until the ORP lowering behavior changes from a large state to a slow state. FIG. 3 illustrates a curve showing the change behavior of the ORP. In this example, it is known that the hexavalent chromium concentration becomes lower than the soil environmental standard after passing the point P in FIG.

  In addition, on the theoretical pH-Eh (ORP) diagram based on experiments, the “ORP value at which the form of chromium in the solution changes from hexavalent chromium to trivalent chromium” is based on the initial pH value and ORP value. It is calculated as a function (it is desirable to obtain data that takes into account actual on-site conditions), and after the reduction reaction is started, the ORP of the liquid is monitored, and the measured ORP value is “the presence form of chromium in the solution is hexavalent. It is also effective to adopt a technique in which the reduction reaction proceeds until an ORP value lower than the “ORP value changing from chromium to trivalent chromium”. The measurement of the ORP may be performed at least once at the timing when it is estimated that the timing for stopping the reduction reaction has been reached.

  As another reduction mode, when water in a reservoir is discharged, water containing hexavalent chromium is passed through a filter packed with calcium sulfite to continuously reduce the water to trivalent. May be adopted. In this case, the amount of calcium sulfite to be brought into contact with water is adjusted so that the discharged water has a hexavalent chromium concentration that satisfies environmental standards. In addition, you may use together the method of making such a filter pass water and discharging | emitting, for the safety | security improvement with respect to the water of the reservoir in which the reduction reaction has already advanced as mentioned above.

  On the other hand, the “solid residue” produced in the turbid water treatment facility has a situation where hexavalent chromium remains in the cement particles, and it is difficult to reuse this as soil material such as backfilling. . Therefore, the present invention is applied in step B shown in FIG. 1 to make the solid residue harmless.

  Specifically, solid residue deposited in a storage yard or the like (in the form of precipitated sludge, dehydrated cake, etc.) and powdered calcium sulfite are mixed. It is difficult to immediately reduce the total amount of hexavalent chromium present in the solid residual cement particles even when mixed with a reducing agent. Therefore, the presence of excess reducing agent is essential. Further, since cement particles are fine particles of about 50 μm and an unhydrated component, that is, a strong alkali component remains in the inside thereof, the pH lowering effect by adjusting the pH of the treated water is not so great. For this reason, in order to facilitate the reduction reaction, it is effective to mix with an acidifying agent (neutralizing agent). However, even if they are said to be together, it suffices if they are finally in a mixed state, and either the acidifying agent or the calcium sulfite may be added in either order. When the acidifying agent is not added, the amount of calcium sulfite is preferably 30 equivalents or more with respect to hexavalent chromium in the solid residue. When the pH is adjusted to the acidic range by adding the acidifying agent, It is desirable to be 5 equivalents or more. However, since an effect commensurate with it cannot be obtained even if it is added excessively, it is desirable that the upper limit be about 100 equivalents regardless of whether or not an acidifying agent is added. The mixing of the solid residue and calcium sulfite is effective even if it is simply performed using a heavy machine or the like, but if the mixing is performed, the reduction reaction is more likely to proceed.

The inventors conducted an experiment to examine the elution amount of hexavalent chromium when a green dam of a concrete dam and calcium sulfite (half water) were mixed at various ratios. The experiment is introduced.
[Experimental example]
The mixed powder which mixed the powder of the calcium sulfite which is a reducing agent with various ratios with 500 g of green cutsuri of a concrete dam was obtained (No. 1-4 of Table 1). Further, 500 g of the green cuts were obtained to obtain a mixed powder in which 25 g of a sulfuric acid band (Al ₂ (SO) ₃ ) that is both an aggregating agent and an acidifying agent and calcium sulfite powder were mixed at various ratios (Table 1). No. 11-14). The hexavalent chromium content in the green cuts is approximately 2 mg / kg. Therefore, the mixed powders of No. 1 to 4 and No. 11 to 14 correspond to an addition equivalent ratio of calcium sulfite to hexavalent chromium of about 13.5 times to about 54 times. About each mixed powder, the elution test based on Environment Agency Notification 46 was conducted, and the elution amount of hexavalent chromium was investigated. Moreover, about the liquid after a dissolution test, pH, ORP, and EC (electrical conductivity) were measured. The results are shown in Table 1.

  As can be seen from Table 1, even when the pH is in the alkaline range, mixing of a sufficient amount of calcium sulfite can render the cementitious solid residue (here, green cutsuri) hexavalent chromium harmless. Moreover, when pH is made into an acidic region, hexavalent chromium is rendered harmless with a smaller amount of calcium sulfite mixed.

  As an acidifying agent to be mixed with calcium sulfite, a flocculant conventionally used in turbid water treatment facilities can be used. For example, in addition to the above-mentioned sulfuric acid band, sulfuric acid-based inorganic flocculants such as PAC (polyaluminum chloride) and polyiron (polyferric sulfate) can be used. These also function as acidifying agents.

  In the muddy water treatment process shown in FIG. 2, the present invention can be applied to the place indicated as Step C. This process corresponds to the “precipitation process” and the “solid-liquid separation process” (see FIG. 1) that are generally performed in turbid water treatment facilities. When the present invention is applied, the “precipitation / reduction process” is performed. , A step having a “solid-liquid separation step”. Before the solid-liquid separation step, a “chromium precipitation step” can be performed. The liquid to be treated is cement-based turbid water containing hexavalent chromium, but a liquid whose pH has been lowered to some extent by adjusting pH in advance (for example, pH: about 8 to 11) is a suitable target. This pH adjustment can utilize a known method such as blowing carbon dioxide as a neutralizing agent. Hereinafter, the “precipitation / reduction process”, “chromium precipitation process”, and “solid-liquid separation process” will be described.

[Sedimentation / reduction process]
A reaction to reduce the hexavalent chromium dissolved in the liquid to trivalent by allowing calcium sulfite to act while coagulating and precipitating suspended components by acting a flocculant on hexavalent chromium-containing cement-based turbid water. This is a process to proceed.
As the flocculant, inorganic flocculants such as sulfuric acid band and PAC which are conventionally used in turbid water treatment can be used. Since organic flocculants are expensive, there is little merit in muddy water treatment. The sulfuric inorganic coagulant functions as an acidifying agent. The pH range where the sedimentation action by the flocculant occurs is approximately 5-8. Therefore, while utilizing the acidifying action by the inorganic flocculant itself, the pH is adjusted to 5 to 8, preferably 6 to 7, by adding a pH adjuster as necessary, and the agglomeration / sedimentation reaction proceeds. If the addition amount of the flocculant is adjusted in the range of about 0.1 to 10% by mass ratio with the suspended component (SS) in the liquid, usually good results are obtained.

  On the other hand, hexavalent chromium dissolved in the liquid is reduced to trivalent chromium by adding calcium sulfite as a reducing agent. As for the amount of calcium sulfite added, it is desirable to secure at least an amount sufficient to reduce the total amount of hexavalent chromium dissolved in the liquid to trivalent. If the amount of calcium sulfite is adjusted in the range of about 2 to 5 equivalents relative to hexavalent chromium in the range of pH 5 to 8, hexavalent chromium dissolved in the liquid can be reduced to almost all trivalent amounts.

  However, it is preferable to suppress the elution amount of hexavalent chromium within the environmental standards for the solid residue recovered by solid-liquid separation in the subsequent step. When indicated as step D in FIG. 2, if calcium sulfite is added as shown in Example 2, elution of hexavalent chromium from the solid residue can be sufficiently suppressed. However, if a large amount of calcium sulfite is added in the precipitation / reduction process, the reduction process can be omitted in the subsequent process or the amount of reducing agent added can be greatly reduced, which is more effective. is there. This can be achieved because calcium sulfite has the property of being hardly soluble in water. If it is a water-soluble reducing agent, it moves to the liquid side by solid-liquid separation, so adding a large amount leads to waste of the reducing agent, whereas calcium sulfite is hardly soluble and unreacted as a solid residue. Since it is collected, it is effectively used to make the solid residue recyclable. As a result of various studies, the calcium sulfite remains in the solid residue by adjusting the amount of calcium sulfite added in the range of approximately 20 to 100 equivalents with respect to the amount of hexavalent chromium present in the turbid water that is the liquid to be treated. Can do. In order to obtain a solid residue that can be recycled without adding a reducing agent in the subsequent step, it is desirable to add 50 to 100 equivalents of calcium sulfite with respect to the amount of hexavalent chromium present in the muddy water.

  The order of adding the inorganic flocculant and calcium sulfite is not particularly required. Usually, it may be added at the same time and mixed with the liquid. In addition, since pH may fluctuate in the process of agglomeration / sedimentation reaction and reduction reaction, it is desirable to adjust pH appropriately so as to maintain the above-mentioned appropriate pH range. At that time, the original cement-based turbid water or the dehydrated water generated in Step D of FIG. 2 can be effectively used as the pH raising agent. Further, as the pH lowering agent, treated water that can be recovered at the point indicated as step E in FIG. 2 can be effectively used.

[Chromium precipitation process]
By adding calcium sulfite in the above process, hexavalent chromium dissolved in the liquid is reduced to trivalent, and the liquid recovered by the solid-liquid separation in the subsequent process clears the environmental standard as wastewater It becomes. However, even if water containing chromium, which is a heavy metal, has been rendered harmless by trivalent chromium, it cannot be said that it is environmentally preferable to discharge it as it is. Therefore, in the present invention, a treatment for precipitating trivalent chromium as chromium hydroxide is performed before solid-liquid separation as necessary. This treatment is made possible by adjusting the pH to around 8 (pH: 7 to 9, preferably 7.5 to 8.5), which is the chromium hydroxide production region. As described above, the original cement-based muddy water can be used for pH adjustment. When the agglomeration / sedimentation reaction is almost completed, the pH is adjusted to around 8 and the mixture is stirred for a while, so that the precipitation reaction of chromium hydroxide proceeds. When the precipitation / reduction process is performed at a pH of about 7 to 8, it is considered that a precipitation reaction of chromium hydroxide occurs at that stage, and the precipitation / reduction process and the chromium precipitation process may overlap with each other.

It is considered that the chromium reduction reaction in the precipitation / reduction process and the chromium hydroxide formation reaction in the chromium precipitation process are represented by the following reaction formulas.
2H ₂ CrO ₄ + 3CaSO ₃ .0.5H ₂ O
→ Cr ₂ (SO ₄ ) ₃ + 3Ca (OH) ₂ + 0.5H ₂ O (1)
→ 2Cr (OH) ₃ ↓ + 3CaSO ₄ + 0.5H ₂ O (2)
Here, (1) is a chromium reduction reaction and (2) is a chromium hydroxide formation reaction.

[Solid-liquid separation process]
After the precipitation reaction of the suspended components and the reduction reaction of hexavalent chromium have sufficiently progressed, and the precipitation reaction of chromium hydroxide has also sufficiently progressed as necessary, the liquid (slurry) is subjected to solid-liquid separation. The method of solid-liquid separation may follow a conventional general method. The liquid obtained is detoxified, and most of the trivalent chromium is removed in the product that has undergone the chromium precipitation step, so it is environmentally friendly even if it is discharged as it is. On the other hand, the solid residue recovered in the form containing calcium sulfite satisfies the environmental standard when the elution amount of hexavalent chromium is lower than the environmental standard as it is or supplemented with a slight reducing agent. It is possible. When calcium sulfite is not contained, it can be rendered harmless by the method shown in Example 2.

The figure which showed an example of the flow in the case of applying this invention with respect to the liquid (process water) produced | generated in a muddy water treatment facility according to the conventional muddy water treatment method, or a solid residue. The figure which showed an example of the flow in the case of applying this invention in a muddy water treatment facility. The graph which illustrated the change of ORP accompanying calcium sulfite consumption about hexavalent chromium content cement system muddy water.

Claims (8)

  Derived from cement-based turbid water that causes the hexavalent chromium-containing liquid recovered by separating and removing suspended components from cement-based turbid water to come into contact with calcium sulfite to promote the reaction of reducing hexavalent chromium in the liquid to trivalent Chromium reduction treatment method.   The ORP (oxidation-reduction potential) after starting the reduction reaction is continuously or periodically monitored, and the reduction reaction is allowed to proceed until it is confirmed that the ORP reduction rate has shifted from a fast state to a slow state. The method for reducing cement-based muddy water-derived chromium according to claim 1.   The pH and ORP (redox potential) after starting the reduction reaction are measured at least once, and the form of chromium in the solution changes from hexavalent chromium to trivalent chromium on the pH-Eh (ORP) diagram. The reduction treatment method for cement-based muddy water-derived chromium according to claim 1, wherein the reduction reaction is allowed to proceed until an ORP value lower than the ORP value to be achieved.   A method for reducing chromium derived from cement-based muddy water, comprising a step of mixing a solid residue containing hexavalent chromium separated and recovered from cement-based muddy water with calcium sulfite.   A method for reducing chromium from cement-based muddy water, comprising a step of mixing a solid residue containing hexavalent chromium separated and recovered from cement-based muddy water with calcium sulfite together with an acidifying agent. A step of aggregating and precipitating suspended components and reducing hexavalent chromium in the liquid in a pH range of 5 to 8 by allowing an inorganic flocculant and calcium sulfite to act on hexavalent chromium-containing cement-based muddy water ( Sedimentation / reduction process),
Thereafter, a step (solid-liquid separation step) of solid-liquid separation to recover a liquid in which the amount of hexavalent chromium is below the drainage standard and a solid residue,
A method for reducing a chromium-based cement-based muddy water-containing chromium. By adding an inorganic flocculant and a large amount of calcium sulfite to hexavalent chromium-containing cement-based turbid water, the coagulation sedimentation of suspended components and the reduction reaction of hexavalent chromium in the liquid are advanced within a pH range of 5-8. In addition, a process of leaving unreacted calcium sulfite in the liquid as a solid phase component (precipitation / reduction process),
Thereafter, a step (solid-liquid separation step) of solid-liquid separation to recover a liquid in which the amount of hexavalent chromium is below the drainage standard and a solid residue containing calcium sulfite,
A method for reducing a chromium-based cement-based muddy water-containing chromium.   The reduction treatment of chromium derived from cement-based muddy water according to claim 6 or 7, further comprising a step of precipitating trivalent chromium as chromium hydroxide in a pH range of 7 to 9 (chromium precipitation step) after the precipitation / reduction step. Method.

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