JPH06304579A - Treatment of hexavalent chromium containing waste liquid - Google Patents

Abstract

PURPOSE:To obtain treated water of high quality with high treatment efficiency by using aq. ferrous salt solution by easily and surely controlling the injection of chemicals when hexavalent chromium contg. waste water is continuously reduced and detoxicated. CONSTITUTION:After acid or alkali is added to hexavalent chromium waste water L to adjust the pH to a prescribed value of 6-12 in a pH conditioning tank 1, the aq. ferrous salt solution is added to the pH-adjusted liquid fed to a reduction treatment tank 2 separated from the pH conditioning tank 1 so that the ORP of the liquid may be kept at +100 to -100mV, and also alkali whose quantity corresponds to the aq. ferrous salt solution is added to the liquid to keep the pH of the liquid in the reduction treatment tank 2 within + or -0.5 from the pH of the liquid adjusted in the pH conditioning tank 1. When the DO of the liquid in the reduction treatment tank 2 is less than 7mg/l, the addition of the aq. ferrous salt solution is immediately stopped. Therefore, since the liquid can be reduced in a pH range of 6-12, the used quantity of acid or alkali is reduced and also the load for providing equipment is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating hexavalent chromium-containing wastewater, in which hexavalent chromium-containing wastewater is continuously reduced and rendered harmless.

[0002]

2. Description of the Related Art Conventionally, as a treatment method for removing hexavalent chromium from wastewater containing hexavalent chromium, a method in which hexavalent chromium is reduced to trivalent chromium and precipitated as an insoluble compound is removed. . Among such conventional treatment methods, the sulfite reduction method is generally used. The reason for this is that it is possible to control the injection of the reducing equivalent of sulfite at pH 2 to 2.5, the amount of sludge produced is small, and the sulfite is available as a liquid and has good operability.

On the other hand, there is also a reduction method using an aqueous solution of ferrous salt such as ferrous sulfate. The reduction method using this ferrous salt aqueous solution has a drawback that a large amount of sludge is generated,
The ferrous salt aqueous solution is inexpensive and has the characteristic that it can be reduced at any pH of the acidic side and the alkaline side. However, the biggest drawback of the reduction method using this ferrous iron salt aqueous solution was the low reliability of the chemical injection control method. That is, if the amount of reducing agent added is not properly maintained, the problem of residual hexavalent chromium and excessive addition of reducing agent will occur. Therefore, the amount of reducing agent added can be detected by detecting the concentration of the remaining hexavalent chromium and excess reducing agent. However, there is almost no detector that can directly measure these concentrations in hexavalent chromium-containing wastewater, and there is a background that an indirect detector is used as an alternative. More specifically, the application of a detector based on the absorptiometric method, which is applied to general chemical analysis of wastewater, was also tried, but it was colored with high sludge concentration like hexavalent chromium-containing wastewater. It was difficult to apply it to wastewater.

For this reason, chemical injection control using an oxidation-reduction potential (hereinafter referred to as ORP) meter has been widely used, but many problems still remain. That is, no clear change in ORP due to the reduction reaction was obtained under any of weakly acidic, neutral and alkaline reducing conditions, and a considerable change in ORP before and after the reduction treatment was obtained only under strongly acidic reducing conditions of pH 1 or less. Therefore, the chemical injection control by the ORP meter has been carried out only under strongly acidic conditions of pH 1 or less. However, under such conditions, a large amount of acid is required for pH adjustment, and ORP indicates the relative electrode potentials of an oxidizing substance and a reducing substance in a solution with respect to the potential of a certain reference electrode. However, it is difficult to obtain an ORP absolute value indicating the reduction treatment of hexavalent chromium to trivalent chromium. Therefore, while reducing the actual wastewater, it is necessary to confirm the ORP at that time, the amount of residual hexavalent chromium in the reduction-treated wastewater and the amount of excess ferrous iron by the chemical analysis method, and set the optimum value. there were. However, since wastewater treatment equipment containing hexavalent chromium often performs integrated treatment of wastewater from various manufacturing processes, sometimes the composition of the wastewater changes greatly depending on the operating conditions of each manufacturing process. In some cases, the hexavalent chromium could not be completely reduced with the set ORP value. As described above, in the reduction method using the aqueous ferrous salt solution, the chemical injection treatment by ORP control has many problems.

In order to solve this problem, the present applicants first added a solution of ferrous salt to wastewater containing hexavalent chromium to reduce hexavalent chromium to trivalent chromium, and then, The pH of the wastewater is adjusted to 4 or more, and a dissolved oxygen (hereinafter referred to as DO) meter is used to add a ferrous salt aqueous solution so that the DO becomes 2 mg / l or less. Proposed (see Japanese Patent Laid-Open No. 3-254889). However, the method of detecting the DO and controlling the addition amount of the ferrous salt aqueous solution has a long response time of about 2 minutes because the chemical reaction between the ferrous ion and the dissolved oxygen is rate-determining, and the DO
When the aqueous solution of ferrous salt is added so that the concentration becomes 2 mg / l or less, it means that the ferrous ion is excessively added, and as a result, the surplus ferrous ion is gradually added to the air or waste water. There was a problem that the wastewater turned red by reacting with oxygen to form ferric ions. That is, the lower limit of measurement of DO is 0 mg
/ L, and when the ferrous iron salt aqueous solution is added so that the DO becomes 2 mg / l or less, the substantial DO control range is 0.
Since it becomes ~ 2 mg / l, there is a problem that it is impossible to judge whether the addition amount of the ferrous iron salt aqueous solution is excessive at the time when DO becomes 0 mg / l.

[0006]

DISCLOSURE OF THE INVENTION The present invention solves the problems of the prior art as described above, and in a method for treating wastewater containing hexavalent chromium using an aqueous ferrous salt solution, chemical injection control can be performed easily and reliably. An object of the present invention is to provide a method for treating hexavalent chromium-containing wastewater, which can obtain treated water of high water quality with high treatment efficiency.

[0007]

[Means for Solving the Problems] When carrying out chemical injection control using an ORP meter in a reduction method using an aqueous ferrous salt solution, the present inventors set the reduction reaction condition of hexavalent chromium-containing wastewater to pH 1 or less. As a result of investigating the necessity of making it strongly acidic and the chemical injection control method using an ORP meter in the neutral and alkaline regions, ORP was found to be used when reducing wastewater containing hexavalent chromium using an aqueous solution of ferrous salt. Is mainly a function of hexavalent chromium and pH, and if the pH is almost constant, ORP
It has been found that can be an index of changes in hexavalent chromium concentration. That is, in the prior art, it is necessary to make the reduction reaction condition strongly acidic with a pH of 1 or less. Since a large amount of hydrogen ions exist in the strongly acidic region, the change in hydrogen ion concentration due to the addition of the ferrous salt aqueous solution can be ignored, This is because only the change in the concentration of hexavalent chromium can be regarded as the change in ORP.
The reason why the chemical injection control method using the ORP meter in the neutral or alkaline region was difficult was that the pH of the ferrous iron salt aqueous solution itself was about 3 and therefore the content of hexavalent chromium in the neutral or alkaline region was included. It was clarified that the pH of the wastewater containing hexavalent chromium shifts to the acidic side when the aqueous solution of ferrous salt is added to the wastewater, which disturbs the ORP. Based on this fact, the inventors of the present invention have a first method as a pH fluctuation suppression measure accompanying addition of an aqueous ferrous salt solution in order to put into practice a chemical injection control method using an ORP meter in a neutral or alkaline range. We adopted a control method that repeats the operation of adding an alkali when the pH of the wastewater containing hexavalent chromium changes due to the addition of an aqueous solution of iron salt, but it is a reliable index for the change in the concentration of hexavalent chromium because of the large fluctuations in pH. No ORP was obtained. Therefore,
By suppressing the pH change of the hexavalent chromium-containing wastewater accompanying the addition of the ferrous salt aqueous solution by simultaneously adding only a preset amount of alkali according to the addition amount of the ferrous salt aqueous solution, Reliable OR as an index of chromium concentration change
P was obtained.

On the other hand, the chemical injection control by the DO meter is not affected by the pH in principle if it is 4 or more of the pH conditions presented by the present applicants, but the pH control method is The effect on unsuitable and large pH fluctuations has not been investigated. Moreover, since the chemical injection control with DO of 2 mg / l or less, which was previously proposed by the present applicants, is not practical as described above, the DO value which can be judged that hexavalent chromium is reduced to the emission standard is investigated again. As a result, DO is 7m
It should be g / l or more, but less than 7 mg / l is 3 mg
It has been found that a range up to / l is preferred. The pH is 4
When the ferrous salt aqueous solution is added to the hexavalent chromium-containing wastewater having the above pH of 12, the pH of the ferrous salt aqueous solution itself is about 3
Since it was acidic, the pH finally reached about 3, and no change in DO was observed during that process. The reason for this is presumed to be that the redox reaction between the reducing agent and the dissolved enzyme occurs at a pH of 4 or higher, and that the pH deviates from this region with the addition of the reducing agent.

The results of the above investigation are shown in FIG. This Figure 2
Is a hexavalent chromium-containing wastewater having a pH of 12 and a hexavalent chromium concentration of 0.5 g / l, and 100 as a pH adjusting agent.
H ₂ SO _{4 at a} concentration of g / l or NaOH at a concentration of 100 g / l
As a ferrous salt aqueous solution and Fe at a concentration of 100 g / l.
It shows changes in ORP (shown by ◯) and DO (shown by ●) with respect to the amount of addition of the ferrous salt aqueous solution when Cl ₂ is used, and (A) shows addition of the ferrous salt aqueous solution for comparison. When the pH of the wastewater containing hexavalent chromium was not adjusted, the pH values after (B) were 12, 10, 8 and
6,4,3,2,1,0.1 and the pH change of the hexavalent chromium-containing wastewater due to the addition of the aqueous ferrous salt solution is suppressed by a pH adjuster to maintain the pH. In (A), the pH decreased to 3 with the addition of the aqueous ferrous salt solution, and the change in ORP also tended to increase, and the ORP and DO values near the theoretical reduction equivalent point relative to the amount of hexavalent chromium present in the experimental conditions were increased. No change was observed. On the other hand, p of (B) to (E)
In H6 ~ 12, clear OR near the theoretical reduction equivalent point
Changes in P and DO were observed. Further, when the pH of (F) to (J) is set to 4 or less, the OR near the theoretical reduction equivalent point is obtained.
The changes in P and DO were unclear. Incidentally, when DO was 6 mg / l under the condition of (D) in which the pH was kept at 8, the residual hexavalent chromium was 0.5 mg / l or less, and the ferrous ion was 1 mg / l or less. An experiment similar to the above was performed using FeSO ₄ as the ferrous salt aqueous solution,
FeCl _{2 at a} concentration of 100 g / l and F at a concentration of 100 g / l
The same result was obtained when eSO ₄ and both were used in an approximately equivalent amount.

As described above, if a pH adjusting agent is added according to the amount of the ferrous salt aqueous solution added to suppress the pH fluctuation, O
It is possible to monitor the reduction reaction of hexavalent chromium by RP,
Further, monitoring by DO is also possible, and since the reduction reaction condition may be in the range of pH 6 to 12, it is not necessary to use a large amount of acid for adjusting the pH to 1 or less in the prior art.
Further, in order to discharge the wastewater after the reduction treatment to the public water area, it is necessary to adjust the pH of the wastewater to 6 to 9 according to the water quality regulation, but an alkali for neutralization treatment such as a liquid having a pH of 1 or less in the prior art is used. It has been clarified that there are advantages such as being unnecessary and not requiring the neutralization tank in the prior art.

From the above investigation results, when reducing the hexavalent chromium-containing wastewater using the ferrous salt aqueous solution, the pH fluctuation of the hexavalent chromium-containing wastewater due to the addition of the ferrous salt aqueous solution was suppressed under the conditions. Then, it was found that the proper addition amount control of the ferrous iron salt aqueous solution can be performed by using the detectors of the ORP meter and the DO meter. Therefore, if both the detectors of the ORP meter and the DO meter are used together. It takes no time to respond even if one of the detectors fails, and the control reliability that exists when only one detector is used when the composition of wastewater containing hexavalent chromium changes significantly We thought that we could alleviate anxiety and proceeded with further study.

A characteristic of ORP meter control is that the chemical potentials of the oxidizing substance and the reducing substance in the solution are detected electrochemically, so that the response is fast, but since the oxidizing substance and the reducing substance cannot be distinguished, they are hexavalent. The ORP in which the potentials of the respective ions are mixed when an oxidizing substance other than chromium and a reducing substance other than ferrous ions are present is shown. Therefore, since the absolute potential corresponding to the end of the hexavalent chromium reduction treatment differs, the ORP at that time while reducing the actual hexavalent chromium-containing wastewater
There is a drawback that it is necessary to confirm the residual hexavalent chromium and excess ferrous iron in the reduction treatment wastewater by a chemical analysis method and set an optimum value. On the other hand, the feature of DO meter control is that it is not affected by coexisting ions because it selectively detects only the dissolved enzyme in the solution, and the measured value is an absolute value, so it is highly reliable. There are drawbacks such as a long response time to obtain a value.

Therefore, the inventors of the present invention aim to make up for the disadvantages of both the ORP meter and the DO meter by using the detectors of the ORP meter and the DO meter together, and to utilize their advantages. To keep both at the proper value,
A control method of adding a ferrous salt aqueous solution (for example, ORP:
-100 mV to +100 mV and DO: 3 to 7 mg /
(range of l) (b) Controlling method by ORP and stopping the addition of ferrous iron salt aqueous solution when DO is out of the set range (c) Controlling chemical by DO and setting by ORP When the three control methods of stopping the addition of the ferrous iron salt aqueous solution when the value is out of the range are examined, the control method of (b) is the most stable control possible and good treatment wastewater can be obtained. Investigated.

When the control system (b) is adopted to reduce the hexavalent chromium-containing wastewater using the ferrous salt aqueous solution, the pH of this system is important as described above.
That is, since the pH of the ferrous salt aqueous solution is about 3 and acidic, the pH of the wastewater during the reduction treatment decreases depending on the amount of the ferrous salt aqueous solution added. To avoid this, simply add an acid or alkali. However, such pH control is not practical because of large pH fluctuations. Therefore, as a measure for suppressing this disturbance, the present inventors investigated the mixing ratio of the two such that the pH of the aqueous ferrous salt solution is apparently neutral or alkaline, and mixing ratio for maintaining the pH. Has been input to the reducing agent addition amount control device, and the amount of alkali addition of the above ratio to the addition amount of the ferrous salt aqueous solution measured by the flow rate adjusting valve is added via the flow rate adjusting valve. The present inventors have completed the present invention by clarifying that treated water of high quality can be obtained with high treatment efficiency. In addition, we also tried to directly change the pH of the ferrous salt aqueous solution by directly adding an alkali to the ferrous salt aqueous solution, but since the precipitation of iron hydroxide occurs and it becomes difficult to transfer the solution, it was implemented. Didn't arrive.

The method for treating hexavalent chromium-containing wastewater according to the present invention will be described in detail below with reference to the drawings. Figure 1
FIG. 3 is a schematic explanatory view of an apparatus example suitable for carrying out the method for treating hexavalent chromium-containing wastewater according to the present invention, in which the hexavalent chromium-containing wastewater L is continuously fed into the pH adjusting tank 1. . The pH adjusting tank 1 has a pH meter 1a (for example, electrochemical measurement
(Manufactured by Co., Ltd., electrode: 6462, converter: HD-36D) and a stirrer 1d are installed and controlled by a pH controller 7 that receives an output from the pH meter 1a, and inside the pH adjusting tank 1. Acid or alkali is added into the pH adjusting tank 1 from the acid supply tank 3 through the flow rate adjusting valve 6a or the alkali supply tank 5 through the flow rate adjusting valve 6b so as to maintain the pH of the wastewater of 6 to 12 .

The hexavalent chromium-containing wastewater L whose pH has been adjusted in this pH adjusting tank 1 is subsequently continuously fed into the reducing treatment tank 2. The reduction tank 2 has a pH meter 2a (for example,
Electrochemical measurement Co., Ltd., electrode: 6462, converter: HD-
36D), ORP meter 2b (for example, Electrochemical Measurement Co., Ltd.)
Made, electrode: 6491, converter: HD-38D), DO meter 2c (for example, manufactured by Electrochemical Measurement Co., Ltd., electrode: 7491,
Converter: OD-36D) and a stirrer 2d are installed and the pH and OR of the hexavalent chromium-containing wastewater L in the reduction treatment tank 2
P and DO are continuously measured and the output is pH / OR
This pH / ORP / D is transmitted to the P / DO control device 8.
The reduction treatment tank is controlled from the reducing agent supply tank 4 via the flow rate adjusting valve 6d so that the ORP of the hexavalent chromium-containing wastewater L in the reduction treatment tank 2 is controlled in the range of +100 to -100 mV by being controlled by the O control device 8. A reducing agent is added to the inside of 2.
At this time, in order to make the pH of the reducing agent apparently neutral or alkaline, at the same time as the addition of the reducing agent, an amount of alkali commensurate with the amount of the ferrous salt aqueous solution previously investigated is adjusted to pH / OR.
It is controlled by the P / DO control device 8 and supplied from the alkali supply tank 5 via the flow rate adjusting valve 6e. Furthermore, the pH of the liquid in the reduction treatment tank 2 is the same as that in the pH adjusting tank 1.
If it is on the acidic side of the pH of the prepared liquid, the pH of the reducing agent
PH / ORP / D more than the amount commensurate with the addition amount of the ferrous salt aqueous solution, which was previously investigated so as to become apparently alkaline.
A liquid in which alkali is added from the above-mentioned alkali supply tank 5 through the flow rate adjusting valve 6e under the control of the O control device 8, and conversely, the pH of the liquid in the reduction treatment tank 2 is adjusted in the pH adjusting tank 1. If it is more alkaline than the pH of
The acid is supplied from the acid supply tank 3 via the flow rate adjusting valve 6c under the control of the pH / ORP / DO control device 8 so that the pH is the same as the pH of the liquid adjusted in the preparation tank 1. Further, even if the ORP is in the range of +100 to -100 mV, when the DO becomes smaller than the lower limit value of 7 mg / l, it is controlled by the pH / ORP / DO control device 8 to immediately adjust the flow rate from the reducing agent supply tank 4. Valve 6d
Is closed and the addition of the ferrous salt solution is stopped, and the addition of an excessive ferrous salt solution can be prevented. By the above operation, the hexavalent chromium-containing wastewater L is p
As hexavalent chromium is reduced while H is prepared and metal ions form hydroxides, the solution is drained through a solid-liquid separator such as a thickener or a centrifuge, and the solid content is appropriately adjusted. It should be processed.

[0017]

Next, the operating conditions will be described. The pH of the wastewater at the time of reduction treatment needs to be 6-12. This is because if the pH of the wastewater is less than 6, the ORP change due to the reduction treatment cannot be obtained even if the pH fluctuation due to the addition of the ferrous salt aqueous solution is suppressed, and if it exceeds 12, the wastewater regulation to the public water area is restricted. This is because it is troublesome to set a value of 6 to 9 and is inferior in economic efficiency. The set pH may be in the above range, but as described above, the ORP is influenced by the pH, and therefore it is preferable to suppress the fluctuation as much as possible. ± 0 at the set value of the range.
If it is adjusted to the range of 5, the ORP fluctuation is small and the ORP control is possible, as well as trivalent chromium, ferrous iron salt aqueous solution,
It was possible to precipitate the aqueous ferric salt solution and other coexisting heavy metal ions as hydroxide simultaneously with the reduction reaction. Further, if a flocculant (polymer) or the like is added to the reduction treatment tank 2, precipitation of this hydroxide is promoted. As described above, by adjusting the pH setting range during the reduction treatment to 6 to 9, the neutralization treatment of the waste water after the reduction treatment in the prior art becomes unnecessary.

The lower limit of the DO setting range is 7 mg / l.
Is. This means that the DO of the wastewater containing hexavalent chromium before the addition of the reducing agent is about 8 to 10 mg / in the factory to which the present inventors belong.
It is because the reduction reaction of hexavalent chromium had already been completed at the time when DO reached 7 mg / l in the investigation by the present inventors. ORP setting range is +100 to -100m
V. This is also the theoretical ORP value of the wastewater in which the hexavalent chromium-containing wastewater is reduced under the condition of about pH 7 and almost no hexavalent chromium remains, and it is 6 in the stainless steel manufacturing plant to which the present inventors belong. It is also the result of an actual survey of waste water containing chromium. However, since this ORP value varies slightly depending on the waste liquid composition, if changes in the waste liquid composition are expected, the residual hexavalent chromium and ferrous iron ion concentrations should be investigated in advance to confirm the optimum values for the above set values. It is preferable to keep.

The type of ferrous salt used in the method of the present invention is not particularly limited, but, for example, ferrous sulfate is the most common, and ferrous chloride, ferrous ammonium sulfate, and ferrous nitrate are the most common. Iron, ferrous hydroxide, etc. can also be used. Further, not only these pure aqueous solutions but also general waste liquids containing these ferrous salts, for example, pickling waste liquids in the iron manufacturing industry, smelting waste water of non-ferrous metals, etc. can be used. As the pH adjusting agent, alkalis such as caustic soda, slaked lime and soda ash, and acids such as sulfuric acid and hydrochloric acid can be used, and as the aggregating agent, various organic polymers can be used.

[0020]

EXAMPLES The method of the present invention will be described more specifically below with reference to experimental examples and comparative examples. For convenience of description, a comparative example will be described first. As the hexavalent chromium-containing wastewater, the wastewater of the steel strip rinsing step on the outlet side of the salt bath in the annealing pickling step of the stainless steel strip (pH: 12, hexavalent chromium concentration 1000 mg /
1) was used.

Comparative Example 1 Waste water of 15 m ³ / H was continuously fed to the apparatus shown in FIG. The volume of the pH adjusting tank 1 and the reducing tank 2 is 5
m ³ and the retention time of the waste water was 20 minutes. p
In the H preparation tank 1, sulfuric acid was added to this wastewater to adjust the pH to 1. Then, the pH-adjusted waste liquid was continuously fed to the reduction treatment tank 2. Here, the ORP of the reduction treatment tank 2 is ORed
The ORP of the reduction treatment tank 2 is 40 while measuring with the P meter 2b.
PH / DO / ORP controller 8 to maintain 0 mV
Was operated to supply an aqueous ferrous sulfate solution having a concentration of 100 g / l as a reducing agent. As a result of continuously treating this reduction treatment for 24 hours, the pH was adjusted to the range of 0.5 to 1.5
P was in the range of +380 to 420 mV. During this time, 1
As a result of performing a chemical analysis by sampling wastewater every hour, the hexavalent chromium concentration was 0.5 mg / l or less, the ferrous ion concentration was 100 ± 50 mg / l, and the wastewater was blue.

Example 1 15 m ³ / H of the waste water was continuously fed to the apparatus shown in FIG. First, the pH of the waste liquid was adjusted to about 7 in the pH adjusting tank 1. Next, the pH-adjusted waste liquid was continuously fed to the reduction treatment tank 2. Here, the control value of the liquid in the reduction treatment tank 2 in the pH / DO / ORP control device 8 is set to 7 for pH, 7 mg / l for DO, and +100 to -100 mV (target value 0 mV) for ORP. did. As the reducing agent, an aqueous solution of ferrous sulfate having a concentration of 100 g / l was used, and caustic soda having a concentration of 100 g / l was simultaneously added to 0.07 mo per 1 mol of the aqueous solution of ferrous sulfate added.
It was added at a ratio of 1 and reduction treatment was carried out while maintaining the pH of the wastewater at about 7. As a result of continuously treating this reduction treatment for 24 hours, pH was in the range of 6.5 to 7.5, and ORP was in the range of +100 to -100 mV. And DO
When the value of DO was less than 7 mg / l, the addition of the ferrous sulfate aqueous solution and caustic soda was stopped immediately, and when the DO exceeded 7 mg / l, the addition of the ferrous sulfate aqueous solution and caustic soda was restarted. During this period, the result of chemical analysis of the wastewater sampled every hour shows that the concentration of hexavalent chromium is 0.5 mg / l or less,
The ferrous ion concentration was 1 mg / l or less, and the water quality was extremely high. The volumes of the pH adjusting tank 1 and the reduction treatment tank were 5 m ³ , and the retention time thereof was 20 minutes.

[0023]

As described above in detail, the method for treating chromium-containing wastewater according to the present invention is designed to control the chemical injection of the ferrous salt solution.
By properly performing the output of DO and ORP in H6 to 12, the reduction process can be easily and reliably controlled for chemical injection. This can be achieved by suppressing the addition of excess reducing agent. Since it is not necessary to reduce the pH excessively during the reduction, it is possible to reduce the amount of acids such as sulfuric acid used and also the amount of alkali such as caustic soda necessary for neutralizing the liquid after the reduction treatment. Can be achieved. Neutralization and coagulation can be performed simultaneously with the reduction, so that the equipment can be reduced. It is possible to obtain hexavalent chromium-containing wastewater easily and surely to obtain treated water of high water quality, and the industrial value thereof is extremely large.

[Brief description of drawings]

FIG. 1 is a schematic system diagram of an example of an apparatus suitable for carrying out the method for treating hexavalent chromium-containing wastewater according to the present invention.

[Fig. 2] 100 g / l as a pH adjusting agent in hexavalent chromium-containing wastewater having a pH of 12 and a hexavalent chromium concentration of 0.5 g / l
Concentration of H ₂ SO ₄ or 100 g / l concentration of NaOH and ORP with respect to the addition amount of the ferrous salt solution when 100 g / l concentration of FeCl ₂ is used as the ferrous salt solution
It shows changes in (denoted by ◯) and DO (denoted by ●).

[Explanation of symbols]

 L 6-valent chromium-containing wastewater 1 pH adjusting tank 1a pH meter 1d Stirrer 2 Reduction tank 2a pH meter 2b ORP meter 2c DO meter 2d Stirrer 3 Acid supply tank 4 Reductant supply tank 5 Alkali supply tank 6a, 6b, 6c , 6d, 6e Flow control valve 7 pH controller 8 pH / ORP / DO controller

Claims (1)

[Claims] 1. When continuously treating the hexavalent chromium-containing wastewater for reduction and detoxification, after adjusting the pH to a predetermined value of 6 to 12 by adding an acid or an alkali to the hexavalent chromium-containing wastewater in a pH adjusting tank. The ORP of the pH-adjusted liquid sent to the reduction treatment tank separated from the pH adjustment tank is +100 to -100 mV.
The pH of the liquid in the reduction treatment tank is adjusted by adding an aqueous solution of ferrous salt so as to maintain the pH of the solution in the reduction treatment tank by adding an amount of alkali corresponding to the aqueous solution of ferrous salt. Hexavalent chromium-containing wastewater characterized by maintaining the pH within ± 0.5 and immediately stopping the addition of the ferrous iron salt aqueous solution when the DO of the liquid in the reduction treatment tank becomes less than 7 mg / l. Processing method.