JP2007038196A - Metal etching wastewater treatment method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a treatment method which can improve the treatment properties of metals and hexavalent chromium by a simple operation and prevent the regeneration of hexavalent chromium with time in the treatment of strongly acidic etching wastewater containing small amount of hexavalent chromium as a heavy metal in a large amount of iron, and containing a large amount of hydrogen peroxide. <P>SOLUTION: In the wastewater treatment method, a part of reduction-inhibiting substances is decomposed beforehand by a pretreatment agent, and then a coagulant is added under an alkaline condition. Especially in the treatment of etching wastewater of a chromium-containing iron-based alloy, after decomposing and removing a part of hydrogen peroxide by adding manganese dioxide to the strongly acidic etching wastewater containing a high concentration of hydrogen peroxide, the hydrogen peroxide is decomposed while adding sodium hydroxide, pH is adjusted to an alkaline pH of 9.5 with calcium hydroxide after neutralization to form metal hydroxide mainly comprising iron, and then iron ions and hexavalent chromium in the wastewater are coagulated and removed at the same time by a coagulant having a reduction property. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a wastewater treatment method relating to an etching (etching) waste liquid that is a waste water that retains a metal component and is a strongly acidic liquid mainly containing a small amount of chromium in a large amount of iron. More specifically, wastewater containing a large amount of hydrogen peroxide used to enhance the etching effect and a large amount of heavy metals eluted by the etching process, particularly a small amount of harmful hexavalent chromium, etc. in addition to these, can be easily and efficiently used. The present invention relates to a wastewater treatment method.

Conventionally, techniques related to the treatment of copper etching wastewater have become widespread, and many documents targeting these are known. However, there is no literature regarding the treatment of metal etching wastewater containing chromium.
Etching means that a target metal plate or mold is immersed in a strong acid solution such as nitric acid, hydrochloric acid, hydrofluoric acid or a mixed water thereof, and a part of the metal is eluted (etched) to obtain a predetermined surface shape. This is a technique for imparting properties, and in the case of waste water after metal etching, it contains iron as a main component and a small amount of metal ions derived from alloy components such as chromium.

  Originally, electrochemical methods such as electrolytic deposition and neutralization coagulation precipitation methods using slaked lime have been adopted as methods for treating metal etching wastewater. However, the former has a problem that the apparatus becomes large-scale, and the maintenance and management of the apparatus is very expensive. Further, in the latter treatment method, particularly when etching water is strongly acidic and contains a reduction inhibitor such as hydrogen peroxide, hexavalent chromium exceeding the emission regulation is contained in the treatment water if further containing chromium. There was a problem that was detected.

  In general, hexavalent chromium exists as dichromate ions in the acidic region, and in the alkaline region, it exists as a chromate ion instead of a hydroxide like other heavy metals. Processing is required. That is, as a general treatment method for waste water containing hexavalent chromium, when the mother liquor is not acidic, the solution is first made strongly acidic and then a reducing agent is added to reduce hexavalent chromium to trivalent chromium. Next, it is made alkaline and a chromium hydroxide precipitate is formed. After this, a known method is adopted in which an inorganic flocculant such as polyaluminum chloride is added and then neutralized with slaked lime, and further a polymer flocculant is added to cause flocs, followed by solid-liquid separation (filtration / dehydration). The current situation is that the process becomes extremely complicated and requires a lot of labor and cost.

Furthermore, when hydrogen peroxide is included in the etching wastewater described above, a defoaming agent is used as a countermeasure against the generation of bubbles having poor defoaming properties in the neutralization step with slaked lime, and at the same time a reducing agent such as sodium bisulfite. Has the disadvantage that it must be used in large quantities.
In addition, there are adverse effects such as inhibiting cohesion, and good flocs are not formed, and the frequency of replacement of the filter press cloth is high, which causes processing difficulties and increases in processing costs. In addition, neutralization with slaked lime has the effect of decomposing and removing hydrogen peroxide, but the hexavalent chromium once reduced to trivalent is oxidized by the remaining hydrogen peroxide solution and returned to hexavalent chromium again. There was a problem. On the other hand, the neutralization using sodium hydroxide reduces the generation of sludge, but the cohesiveness of precipitates generated by neutralization is poor, and the sedimentation and filterability are poor because the size of the aggregated sludge is fine. Therefore, a practical solution is desired.

Substances that decompose and remove hydrogen peroxide itself include platinum, palladium gold, silver and other precious metals, manganese oxide, cobalt oxide and other metal oxides, sulfurous acid, sodium bisulfite and other reducing substances, and activated carbon and other inorganic powders. There are many patent documents that use enzymes such as body, catalase, peroxidase (for example, see Patent Document 1), metal salts such as iron, cobalt, rubidium (for example, see Patent Document 2), and manganese dioxide alone. . (For example, see Patent Document 3)
However, when the liquid is in the vicinity of neutrality, the hydrogen peroxide solution is easily decomposed by the above-described substances and generates oxygen, but the solution containing the hydrogen peroxide solution becomes more stable and does not easily decompose. (See Patent Document 1) Further, when the liquid becomes strongly acidic, the hydrogen peroxide solution becomes very stable, and it is almost difficult to decompose with the above-mentioned known substances.
That is, all the matters described in the above-mentioned publicly known documents are techniques for handling the decomposition of hydrogen peroxide water in a pH region of weak acid or higher, and the hydrogen peroxide solution contained in the strong acid as intended by the present inventor. Nothing is suggested about the treatment of hexavalent chromium in metal etching wastewater.

  JP 2003-225678 A   JP 2003-326280 A   JP 2004-042574 A   JP-A-10-314760

  The present invention has been made in view of the above-mentioned problem that has been a problem in the past, and is a strongly acidic etching containing a small amount of hexavalent chromium in a large amount of iron as a heavy metal and a large amount of hydrogen peroxide. It is an object of the present invention to provide a treatment method that can improve the processability of these metals and hexavalent chromium by simple operation and that does not re-express hexavalent chromium over time.

In order to solve the above-mentioned problems, the present inventors diligently studied, and because they are present in a large amount in the wastewater, the cohesiveness is inhibited, and hexavalent chromium once reduced to trivalent chromium is oxidized again to hexavalent chromium. A pretreatment agent for removing hydrogen peroxide water having properties and preventing oxidation was selected.
Furthermore, in order to reduce the generation of sludge, a large amount of precipitates generated by neutralization or alkali range setting and neutralization can be aggregated and coagulated as chromium hydroxide simultaneously with the reduction of hexavalent chromium to trivalent chromium. A simple and reliable method for treating metal etching wastewater was found by using an inorganic powder flocculant having a reducing performance.

That is, if an attempt is made to achieve the object of the present invention by a conventional method, it is generated at the stage of neutralization or alkalinization treatment after initially reducing the hexavalent chromium by introducing a large amount of a reducing agent such as bisulfite solder. In the conventional method using polyaluminum chloride, slaked lime and polymer flocculant for a large amount of precipitates, not only is the process complicated and extremely complicated, but also the frequency of replacement of the filter is high, resulting in high cost. It was.
However, in this respect, in the present invention, hexavalent chromium is reduced to trivalent chromium by adopting a pretreatment agent and using an inorganic powder flocculant having a reducing function, and the reduced trivalent chromium is hydroxylated by an alkali. Since it changes to chromium, it can coagulate simultaneously with the precipitation of other heavy metals, and removal of hexavalent chromium can be achieved simultaneously.
Thus, by adding a pretreatment agent to a strongly acidic metal ion-containing wastewater, it is possible to simultaneously improve the cohesiveness of metal hydroxides mainly composed of ferric hydroxide and prevent the expression of hexavalent chromium. The method has been determined and the present invention has been achieved.

The gist of the present invention is that, in the treatment of strongly acidic metal etching waste water containing a reduction inhibitor, a part of the reduction inhibitor is neutralized after being decomposed by a pretreatment agent, and then coagulated in an alkaline state. In the wastewater treatment method, the metal hydroxide having poor cohesiveness is coagulated by utilizing the remaining pretreatment agent in a series of treatment steps to add residual metal ions.
Here, the reduction inhibitor is usually mainly hydrogen peroxide. The pretreatment agent used in the present invention is desirably a tetravalent or higher valent manganese compound, and preferably 0.01 to 0.3% is added as the pretreatment agent.
In the present invention, it is often applied when chromium or copper is mainly contained as the metal component of the metal etching waste water.

In the present invention, when adjusting the drainage to alkaline, by performing caustic soda up to neutrality and then with slaked lime, it is possible to facilitate the merger with the cost and the prior art.
In the present invention, it is desirable that the flocculant is an inorganic powder flocculant. In particular, the flocculant has a function capable of aggregating a suspension mainly composed of ferric hydroxide, chromium hydroxide or copper hydroxide and a mixture thereof. More preferably, it has.
An inorganic powder flocculant that is suitable when the wastewater contains hexavalent chromium needs to have reducing properties. However, this does not exclude the use of conventional polyaluminum chloride, slaked lime, and polymer flocculants.

As described above, as the hydrogen peroxide decomposition agent, various metal compounds such as iron-based, manganese-based, titanium-based, chromium-based, and vanadium-based materials, metal salts and activated clays, inorganic compounds such as activated clay, activated carbon, sulfite, heavy Inorganic salts such as sulfites and the enzyme catalase are known.
However, they cannot decompose hydrogen peroxide under strong acidity, and the present inventors have intensively studied and discovered that manganese-based compounds are the only effective hydrogen peroxide decomposition performance in strong acidity. .
In particular, compounds with Mn (IV value) or higher, such as manganese dioxide and permanganate, are excellent in the decomposition performance of hydrogen peroxide solution in strong acidity, floc properties during aggregation, and the effect of preventing the expression of hexavalent chromium. I found out. Among them, permanganate (Mn is VII) has high solubility and strong oxidizing power, and the purple coloration of permanganate ions is a practical measure in terms of its own consumption and the decomposability of hydrogen peroxide. Excellent and convenient.

Manganese dioxide used in the present invention is generally used as a catalyst for decomposing hydrogen peroxide to obtain oxygen, and is recovered and reused. However, it is usually neutral to weakly alkaline and is used for catalytic use on the active surface. It has been limited. (Herein, the catalyst refers to a substance that does not alter or dissolve itself and changes only the target)
However, the treatment process of the present invention is extremely characteristic in that it is added and used under strong acidity of pH 1 to 2. In other words, at the beginning of the addition, it acts as a catalyst on the powder solid surface and gradually dissolves to become manganese ions, and once it degrades the hydrogen peroxide solution decomposition action, it strongly develops the catalytic action again in the subsequent neutralization step with caustic soda. Promotes the decomposition of hydrogen peroxide.
It was also discovered that dissolved manganese ions have the effect of preventing flocs with good filterability by preventing mainly iron hydroxides generated in the neutralization step from forming sludge flocs during aggregation. Furthermore, the dissolved manganese ions themselves become hydroxides by alkalinization and are separated and removed from the solution as flocs by agglomeration. Therefore, even if about 0.3% manganese dioxide is added, the manganese drainage regulations are violated. No contamination will occur.

In the present invention, as the pretreatment agent, one having an effect of improving the hydrogen peroxide decomposability and ferric hydroxide cohesiveness is selected and used, and particularly as a compound having hydrogen peroxide decomposing ability under strong acidity. For manganese compounds with higher valence. Examples of the compound include manganese salts such as manganese dioxide and potassium permanganate.
When manganese dioxide is used as a pretreatment agent, the addition amount is preferably 0.005% to 1%. The more hydrogen peroxide decomposes, the more the hydrogen peroxide decomposition ability increases. However, 0.01% to 0.3% is most preferable because sludge increases and manganese ions may remain in the treated water. If it is 0.005% or less, the hydrogen peroxide removal ability is insufficient, which hinders the treatment in the subsequent process.

Manganese dioxide is naturally produced and, as an example of the production method, there is electrolytic manganese obtained by pulverizing manganese carbonate ore, dissolving it in sulfuric acid, and depositing it by electrolysis. Manganese oxide used for the treatment of the metal etching waste water of the present invention is Not only the surface catalytic use but any of them may be used because they are dissolved and act as manganese ions. The particle size is not particularly limited, and is preferably 15 to 400 mesh, and more preferably 30 to 100 mesh from the viewpoints of handleability and dispersibility.
When adding manganese dioxide, it is preferable to add it little by little with stirring in order to level out the instantaneous generation of bubbles. When manganese dioxide is added, fine bubbles (oxygen) having good defoaming properties are generated, so that the bubbles are strong and do not disappear like slaked lime and do not overflow to the water surface. Accordingly, it is not necessary to use an antifoaming agent in the neutralization step.

In the neutralization step, the mixture is neutralized while gradually dropping 1 mol / L or more, preferably 10 mol / L sodium hydroxide under stirring. Initially, ferric hydroxide is generated only in the portion where sodium hydroxide is dripped, and it turns brown, but disappears immediately. However, if the dripping is continued, the liquid gradually turns brown from around pH 4.5, ferric hydroxide is generated, and the whole becomes brown suspended water. It reaches about pH 7 and is switched to addition of slaked lime. Slaked lime may be added as a powder, but is preferably added as a 10 to 20% slurry.
Alkalineization with slaked lime is performed up to pH 9 to pH 10, but at least about pH 9.5 is necessary. If the pH is less than 9, it is difficult to completely remove hexavalent chromium. Further, when the pH is 10 or more, the cohesiveness is deteriorated, and a larger amount of acid is required for neutralizing the treated water.
Further, when fluorine is present in the metal etching waste water, calcium chloride is added for removal. The addition amount is adjusted according to the concentration of fluorine present.

Next, a flocculant is added to remove heavy metal hydroxides and hexavalent chromium mainly from iron in the metal etching waste water. If the inorganic powder flocculant having reducibility is used, hexavalent chromium is reduced to trivalent chromium, resulting in precipitation of chromium hydroxide. To do.
The amount of the flocculant added is preferably from 1,000 mg / l to 5,000 mg / l, more preferably from 2,000 mg / l to 3,000 mg / l. If it is less than 2,000 mg / l, the cohesive strength is insufficient and the hexavalent chromium removability is not sufficient. If it exceeds 5,000 mg / l, the floc becomes too large and the process passability may deteriorate.

Stirring is ideally carried out with a stirrer equipped with an inverter for 1 to 2 minutes and then with weak agitation for 2 to 5 minutes. However, if the initial stirring time is too short, the dispersion and reaction of the flocculant becomes insufficient, and a large floc cannot be formed, resulting in turbid water. If it is too long, the flocs once generated may collapse.
When the stirring is stopped and the mixture is allowed to stand, the floc settles and the supernatant water is separated. The settled floc is filtered and dehydrated with a filtration device. For filtration and dehydration, devices such as a filter press and a belt-type vacuum dehydrator can be used industrially. On the other hand, filtration and dehydration can be easily performed by natural filtration using a nonwoven fabric or cloth having an opening of about 30 microns.
Since the treated water is alkaline with a pH of about 9.3, it must be neutralized with dilute sulfuric acid or the like to have a pH of 5.8 to 8.6 in order to be discharged into the sewer or public water area.

In the present invention, it is not necessary to decompose and remove all of the hydrogen peroxide contained in the pretreatment agent. If all of them are to be decomposed and removed, a large amount of manganese dioxide is required, and further, manganese ions exceeding the regulated concentration value are contained in the treated water. As a countermeasure against this, in the present invention, manganese dioxide is used and removal and reproduction of hexavalent chromium is made possible by using sodium hydroxide which causes precipitation with a low coagulability and a small amount of sludge. The prevention effect was established.
Alkalization after the neutralization treatment can avoid the problem of foaming, so that sludge having good dewaterability and slaked lime which serves the purpose of removing fluorine can be used. Further, when a large amount of fluorine is contained, a calcium fluoride precipitate can be formed by adding calcium chloride, and fluorine can be removed simultaneously by agglomeration.

In the method for treating metal etching wastewater of the present invention, not only decomposition due to normal catalytic effect by prior addition of manganese dioxide but also catalytic effect during neutralization with sodium hydroxide by completely dissolving and changing to manganese ions To effectively decompose hydrogen peroxide.
In addition, ferric hydroxide sludge generated by neutralization or alkalinization is usually fine and sludge, and its filtration and dehydration properties are extremely poor, but due to the effect of manganese ion hydroxide generated by dissolution of manganese dioxide. The aggregating property of the ferric hydroxide aggregating agent is improved, and the effect of improving the dewatering property during sludge filtration is exhibited.

In the present invention, since the process up to neutralization is carried out with sodium hydroxide, the bubbles of oxygen generated by the decomposition of hydrogen peroxide are highly destructive, and the use of a strong antifoaming agent as in the case of using slaked lime is not necessary. Not necessary.
On the other hand, by using a reducing flocculant, hexavalent chromium contained in the waste water is reduced to trivalent chromium, which is aggregated as chromium hydroxide. The agglomeration and hexavalent chromium treatment can be performed by a single agglomeration treatment. Furthermore, the effect of treatment with manganese hydroxide and a reducing flocculant has the effect of suppressing the return phenomenon from trivalent chromium to hexavalent chromium due to the remaining hydrogen peroxide solution.

FIG. 1 schematically shows an embodiment of the present invention, and is a flow sheet of a metal etching wastewater treatment method. That is, the pretreatment agent specified in the present invention is added to raw water containing heavy metal ions, hydrogen peroxide water and the like in a strongly acidic wastewater, and then neutralized and then alkalized. Furthermore, after adding an additive as necessary, flocs are formed with an aggregating agent to perform individual liquid separation.
EXAMPLES Hereinafter, although this invention is demonstrated according to an Example, this invention is not limited only to the Example shown below.

In this example, the effect of adding manganese dioxide to a strongly acidic etching wastewater containing hexavalent chromium is confirmed.
Electrochemical manganese dioxide (purity 90%) (Daiichi Carbon Co., Ltd.) 40 mg is added to 200 cc of strongly acidic (pH 1.4), almost colorless and transparent chromium alloy etching wastewater (obtained from Nippon Etching Co., Ltd.) and stirring is performed for 30 minutes. Stirring was performed. From the raw water, fine bubble gas (oxygen) was vigorously generated immediately after the addition of manganese dioxide, and stopped after about 10 minutes of addition, and all the added manganese dioxide was dissolved and disappeared.
While stirring, a 10 mol / L sodium hydroxide solution was added dropwise to raise the pH. After the addition and stirring of manganese dioxide, the generation of gas once stopped, but the generation started again and continued as sodium hydroxide was added dropwise. From about pH 5, the brown precipitate disappeared, and at pH 7, the whole liquid became a brown suspension.

  Foam generated in the neutralization step with sodium hydroxide has good antifoaming properties, and no antifoaming agent was required. While adding 15% slaked lime slurry to this system, the pH was raised to 9.5 and 0.4 g of calcium chloride was added. Next, 0.5 g of reducing inorganic powder flocculant Super Namit TN315CY-K2 (manufactured by Noatec Co., Ltd.) was added and stirred for 3 minutes to produce large flocs. Upon standing, all flocs settled and separated from the supernatant water. The supernatant and floc were filtered through a nonwoven fabric having an opening of 25 μm. The filtered water was colorless and transparent, had a pH of 9.3, a hexavalent chromium concentration of 0.05 mg / l or less, and a manganese ion of 0.05 mg / l or less.

Further, the dewaterability of the floc separated by filtration was good, and the hexavalent chromium concentration of the squeezed water was 0.05 mg / l or less. The hexavalent chromium concentration after leaving the filtered water in the air for 7 days was 0.05 mg / l or less, and no change with time (oxidation of dissolved trivalent chromium to hexavalent chromium by air and residual hydrogen peroxide) was observed. It was.
As shown in the above examples, the effect of manganese dioxide utilized in the present invention is not only a catalytic effect, but dissolves into manganese ions, and has the effect of promoting decomposition of hydrogen peroxide when sodium hydroxide is added. It is demonstrating.
In addition, manganese ions become manganese hydroxide by neutralization or alkalinization, and this improves the cohesiveness of ferric hydroxide produced by neutralization or alkalinization of iron ions present in large amounts in the raw water, in order to stabilize the floc It can be observed that the sedimentation rate is high and the filtration dewaterability is improved.

Comparative Example 1

In Example 1, when the same procedure as in Example 1 was performed without using manganese dioxide, the floc size was fine and the transparency of the filtrate was poor, and the hexavalent chromium concentration in the filtrate was 0.3 mg / l. Met.
From these results, in the absence of manganese dioxide, the cohesiveness of ferric hydroxide is poor, and the effect of removing hexavalent chromium is not shown at all.

This example confirms the effect of natural manganese dioxide as in Example 1. A colorless and transparent chrome alloy etching wastewater (obtained from Nippon Etching Co., Ltd.) with strong acidity (pH 1.4) 40 cc of natural manganese dioxide (purity 80%) (Daiichi Carbon Co., Ltd.) is added to 200 cc and stirred for 20 minutes. It was. Although fine bubble gas (oxygen) was actively generated from the raw water immediately after the addition of manganese dioxide, it stopped in about 10 minutes, and all the added manganese dioxide dissolved and disappeared.
While stirring, a PH solution was gradually raised while a 10 mol / L sodium hydroxide solution was added dropwise. After the addition and stirring of manganese dioxide, the generation of gas once stopped, but the generation started again as sodium hydroxide was dropped. From about pH 5, the brown precipitate disappeared, and at pH 7, the whole liquid became a brown suspension.

Foam generated during neutralization with sodium hydroxide had good antifoaming properties and did not require an antifoaming agent. The pH was raised to 9.5 while adding 15% slaked lime slurry to the system. Next, 0.4 g of a reducing inorganic powder flocculant Super Namit TN315CY-K2 (manufactured by Noatec Co., Ltd.) was added and stirred for 3 minutes to generate large flocs. Upon standing, all flocs settled and separated from the supernatant water.
The supernatant water was filtered through a nonwoven fabric having an opening of 25 μm. The obtained filtered water was colorless and transparent, had a pH of 9.4, a hexavalent chromium concentration of 0.05 mg / l or less, and a manganese ion concentration of 0.05 mg / l or less.
The filtered and separated floc squeezed water had a hexavalent chromium concentration of 0.05 mg / l or less. The hexavalent chromium concentration after the filtered water was left in the air for 5 days was 0.05 mg / l or less.
As shown from the above Examples, it was confirmed that the use of manganese dioxide utilized in the present invention also showed almost the same effect as Example 1.

Comparative Example 2

In Example 2, when natural manganese dioxide was not used, and the same procedure as in Example 2 was performed using 15% slaked lime slurry instead of sodium hydroxide, large bubbles were generated by the addition of slaked lime slurry. did. Since the bubbles did not disappear easily, the bubbles stayed on the liquid surface as the amount of slaked lime added was increased and overflowed from the beaker, resulting in very poor workability.
Further, the floc size after the addition of the flocculant was small, the transparency of the filtrate was poor, and the hexavalent chromium concentration was 0.5 mg / l. The amount of separated sludge was about 1.3 times that of the sludge of Example 2. The supernatant water was filtered through a nonwoven fabric having an opening of 25 μm. The filtered water was very light brown and transparent, pH 9.0, hexavalent chromium concentration was 0.05 mg / l or less, and manganese ion concentration was 10 mg / l. The hexavalent chromium in the treated water after 7 days was 0.05 mg / l or less.
From these results, when manganese dioxide is not used and neutralization is performed with slaked lime, the generated foam deteriorates workability, the size of floc generated in the aggregation process is small, and the transparency of filtered water is poor. Furthermore, problems such as incomplete removal of hexavalent chromium from the treated water were shown.

This example confirms the effect of manganese dioxide on strongly acidic etching wastewater containing copper and hydrogen peroxide water at a high concentration.
Strongly acidic (pH 0.15) high copper concentration (40 g / L) dark blue copper etching wastewater (obtained from MEC Co., Ltd.) 100 cc, electrolytic manganese dioxide (purity 90%) (Daiichi Carbon Co., Ltd.) ) 30 mg was added and stirred for 20 minutes. Although fine bubble gas (oxygen) was actively generated from the raw water immediately after the addition of manganese dioxide, it stopped in about 10 minutes, and all the added manganese dioxide dissolved and disappeared.
Under stirring, a 10 mol / L sodium hydroxide solution was added dropwise. After the addition and stirring of manganese dioxide, gas generation once stopped, but generation started again as sodium hydroxide was dropped. By adding sodium hydroxide, a brownish brown precipitate was formed while foaming, and it immediately dissolved and became green transparent, and the liquid temperature rose to about 70 ° C.

When the addition was continued, the whole became black-green suspension water. Foam generated in the neutralization step with sodium hydroxide had good antifoaming properties and did not require an antifoaming agent. When the pH reached 6, the pH was adjusted to 8 by switching to the addition of slaked lime.
To this system, 0.3 g of inorganic powder flocculant Super Namit TN315NY (manufactured by Noatec) was added and stirred for 1.5 minutes. Although the generated floc size was small, colorless and transparent treated water was obtained by filtering with a filter cloth having an opening of 25 μm.
The pH of the treated water was 7.3 and the copper concentration was 2.5 mg / l. When the sludge separated by the filter cloth was strongly dehydrated by hand squeezing, the water content of the sludge was 65%.

As shown in the above examples, the effect of manganese dioxide utilized in the present invention is not only a catalytic effect, but also exhibits the effect of promoting decomposition of hydrogen peroxide when sodium hydroxide is added as manganese ions after dissolution. is doing.
Further, manganese ions are converted to manganese hydroxide by neutralization or alkalinization, and this improves the cohesiveness of copper hydroxide generated by neutralization or alkalinization of copper ions present in a large amount in raw water.
As a result, it appears as a phenomenon that the floc is stabilized, the sedimentation property is improved, and the filtration and dewatering property is improved.

Comparative Example 3

In Example 3, when the treatment was performed in the same procedure as Example 4 without adding manganese dioxide, a part of fine floc leaked and the filtered water was colored slightly yellowish green. The pH was 7.7 and the copper concentration was 10 mg / l.
From these results, in the absence of manganese dioxide, the cohesiveness of the copper hydroxide after neutralization is poor and the floc size is small, so that leakage from the filter cloth occurs and the transparency of filtered water is poor and the copper in the treated water is poor. The removal of was insufficient.

  As described above, according to the method of the present invention, the decomposition of hydrogen peroxide which adversely affects the cohesiveness and causes the expression of hexavalent chromium in the treated water can be promoted, and the alkalizing step can be improved. Thus, it is possible to provide a wastewater treatment work process with improved processability without causing re-expression of hexavalent chromium.

The first feature of the present invention is an improvement in processing workability. That is, by adding the manganese dioxide of the present invention at the initial stage of the treatment, it replaces slaked lime that has hydrogen peroxide decomposability but poor defoaming ability in the neutralization step, but the hydrogen peroxide decomposability is small but disappears. It was possible to use sodium hydroxide with excellent foaming properties and no need to use an antifoaming agent. As a result, the amount of generated sludge could be significantly reduced.
In addition, a large amount of iron-based hydroxide precipitates due to neutralization or alkalinization, thereby avoiding a sludge-like and strong floc and difficult to filter. On the other hand, the coexistence of manganese ions as a hydroxide improved the cohesiveness, resulting in a strong floc and improved sedimentation, filtration and dehydration.
Furthermore, the use of inorganic flocculants with reducibility simplifies the work process and improves workability without the need for adding reducing agents in the acidic range and adjusting polymer solutions. , Equipment management work can be reduced.

The second feature of the present invention is to ensure the removal of hexavalent chromium. That is, in the treatment of hexavalent chromium in wastewater, if hydrogen peroxide is present in the wastewater, the conventional method requires a large amount of reducing agent, and hexavalent chromium cannot be removed reliably. In particular, hexavalent chromium was often detected in water produced when sludge was dehydrated with a filter press or the like.
In the present invention, by adding a small amount of manganese dioxide, decomposition and removal of hydrogen peroxide are promoted, and at the same time, manganese ions are converted into hydroxides to coexist with ferric hydroxide and chromium hydroxide precipitates. As a result, re-expression of hexavalent chromium was suppressed. As described in detail above, the method of the present invention improves the processability of the chromium alloy etching wastewater containing hydrogen peroxide, and can stably obtain treated water free of hexavalent chromium. I am convinced that it is industrially significant.

  FIG. 1 is a flow sheet showing a process common as a representative example of the processing of the metal etching waste water of the present invention.

Claims (8)

  In the treatment of strongly acidic metal etching wastewater containing a reduction inhibitor, a part of the reduction inhibitor is neutralized after being decomposed with a pretreatment agent in advance, and then remaining in a series of treatment steps in which an aggregating agent is added in an alkaline state. A wastewater treatment method characterized by coagulating a metal hydroxide having poor cohesiveness by utilizing the pretreatment agent to remove residual metal ions.   The wastewater treatment method according to claim 1, wherein the reduction inhibitor is mainly hydrogen peroxide.   3. The waste water treatment method according to claim 1, wherein the pretreatment agent is at least a tetravalent or higher manganese compound.   4. The waste water treatment method according to claim 1, wherein chromium or copper is mainly contained as a metal component of the metal etching waste water.   The wastewater treatment method according to claim 1, wherein 0.01 to 0.3% of the manganese compound is added as a pretreatment agent.   6. The waste water treatment method according to claim 1, wherein when the waste water is adjusted to be alkaline, the waste water is neutralized with caustic soda and then with slaked lime.   The wastewater treatment method according to claim 1, wherein the flocculant is an inorganic powder flocculant.   The waste water according to any one of claims 1 to 7, wherein the flocculant has a function capable of coagulating a suspension mainly composed of ferric hydroxide, chromium hydroxide or copper hydroxide and a mixture thereof. Processing method.

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