JP2001079565A - Process for removing selenium in wastewater - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an efficient and economical process for removing selenium in wastewater, which is intended for treating wastewater containing hexavalent selenium, such as industrial wastewater, and capable of easily reducing the selenium content in the wastewater to <=0.1 mg/L (effluent standard value) by treatment at room temperature. SOLUTION: This process comprises: introducing wastewater to be treated into a reaction vessel in an air atmosphere under the atmospheric pressure or in a non- oxidizing atmosphere formed by e.g. sealing an upper part of the reaction vessel with gaseous nitrogen or the like; thereafter, adjusting the pH of the wastewater to about 3, adding a reducing agent such as powdery iron to the pH-adjusted wastewater and agitating them together, to reduce the dissolved oxygen content in the wastewater to <=0.5 mg/L; subsequently, adding a divalent iron salt such as FeSO4 to the resulting water, to adjust the pH of the water to 9, to coagulate iron(II) hydroxide and to coprecipitate reduced selenium with the coagulated iron(II) hydroxide; and when a comparatively high content of iron remains in the water thus treated, further increasing the pH of the water to about 9.5, to perform secondary coagulation of iron(II) hydroxide, to reduce the iron content in the water to <=0.1 mg/L and to obtain the objective treated water; wherein the excess powdery iron or iron salt is recovered from the resulting precipitate and reused.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing selenium in wastewater, in particular, hexavalent selenium at room temperature to a wastewater standard value of 0.1 mg / l or less.

[0002]

2. Description of the Related Art As a method for removing selenium in wastewater,
An iron powder replacement treatment method, an adsorption treatment method using iron hydroxide (trivalent), and the like are known. Even in the above-described conventional iron powder replacement treatment method, SeO3 ^2- : selenite ion (hereinafter referred to as 4) is used.
Is almost complete (0.1 mg / selenium).
l) or less, while SeO4
^2- : Regarding the removal of selenate ions (hereinafter referred to as hexavalent selenium), there is a problem that the removal ability is poor.

[0003] Further, the adsorption treatment method for adsorbing and removing selenium from the flocculence of iron hydroxide (trivalent) is a method applicable only to a trace concentration capable of being adsorbed. All have problems in wastewater treatment containing hexavalent selenium (several mg / l to several tens mg / l or more) having a relatively high concentration. Also, for example, see JP-A-6-792.
No. 86 discloses a method in which a large amount of ferrous sulfate corresponding to the amount of selenium is added to wastewater, then a neutralizing agent is added, and selenium is adsorbed and removed by coagulating iron hydroxide (divalent). It has been disclosed. However, this method has a problem that the removal efficiency of hexavalent selenium is low and the cost is high.

[0004] Further, Japanese Patent Application Laid-Open No. Hei 8-132074 discloses that wastewater is treated in the presence of a transition metal compound such as ferrous chloride.
A method for removing selenium as a deposit while maintaining the pH at 9 or more and the temperature at 70 ° C. or more is disclosed. However, in this method, since the hexavalent selenium cannot be efficiently removed unless the liquid temperature is raised to 80 ° C. or more, the cost for raising the temperature is increased, and the temperature of the waste water becomes high. was there.

Japanese Patent Application Laid-Open No. 10-34168 discloses that wastewater is passed through a column filled with iron metal such as iron powder at a pH of 3 or less to reduce selenic acid in the water and to adjust the pH of the effluent from the column. There is disclosed a method in which selenium is adsorbed on floc of iron hydroxide that is agglomerated to remove the selenium by setting it to 7 or more. However, in this method, since an iron metal column is used, it is difficult to directly cope with a high selenium concentration in the wastewater, and a large amount of wastewater becomes a problem when the selenium concentration is low. Further, in consideration of the passing speed of the column, the life of the column, clogging of foreign matter, and the like, a plurality of columns are required, and the related equipment becomes large-scale, so that the cost is large and there is a problem in economy.

On the other hand, as a technique proposed by the present inventors,
There is a method disclosed in Japanese Patent Application Laid-Open No. 8-267076. In this method, ferrous ions are dissolved in wastewater containing selenium, and while maintaining the liquid temperature at 30 ° C. or higher in the atmosphere, an alkaline agent is added to neutralize the solution to a pH of 8 to 10, and the solid-liquid Separate to remove deposits, or add an alkaline agent while keeping the temperature above room temperature, preferably with the air shut off, to adjust the pH to 8
This is a method in which the amount of selenium in the waste water is reduced to 0.1 mg / l or less by neutralizing to 10 to 10 and then performing solid-liquid separation to remove deposits.

[0007]

However, this method has a problem of 6
It was possible to easily remove even valent selenium down to 0.1 mg / l or less, but the temperature was not as high as that of the invention of JP-A-8-132074, but a heating treatment of the liquid was necessary. There were problems with workability and economy.

In view of the above situation, the present invention further develops the technology of the above-mentioned Japanese Patent Application Laid-Open No. 8-267076 and drastically improves its reactivity, thereby easily converting hexavalent selenium into water at room temperature. It is an object of the present invention to provide an efficient and economical method for treating selenium-containing wastewater that can be removed to 0.1 mg / l or less, which is the wastewater standard value in the Pollution Control Law.

[0009]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and found that selenium is neutralized in waste water containing selenium and reduced by iron hydroxide which precipitates. Prior to the treatment, it has been found that the removal of dissolved oxygen present in the wastewater is closely related to the drastic improvement in reactivity. In addition, the present invention can be carried out in an air atmosphere, but preferably carried out in a non-oxidizing atmosphere, which can cope with a higher selenium content.

[0010] The removal of dissolved oxygen includes means for bringing the water to be treated into contact with a metal having a pH of 7 or less, means for adding a dissolved oxygen removing reagent to the water to be treated, means for bubbling the water to be treated with a non-oxidizing gas, Further, a means for reducing the pressure of the water to be treated in a non-oxidizing atmosphere may be mentioned, but other treating means for removing dissolved oxygen in a non-oxidizing atmosphere are naturally included in the present invention. Are used in combination of one or more.

That is, the present invention firstly provides a dissolved oxygen removing step for removing dissolved oxygen contained in a selenium-containing wastewater, and an iron salt adding step for adding a bivalent iron salt to the obtained treated wastewater. And then add an alkaline agent to adjust the pH to 8 to 1.
A selenium co-precipitation step of neutralizing selenium with ferrous hydroxide to zero and a solid-liquid separation step of separating the obtained residue to obtain clean waste water. Selenium removal method 0, secondly, an iron salt addition step of adding a ferrous salt to a selenium-containing wastewater, a dissolved oxygen removal step of removing dissolved oxygen, and A selenium co-precipitation step of neutralizing the pH to 8 to 10 by adding an alkali agent and co-precipitating selenium with ferrous hydroxide, and a solid-liquid separation step of separating the obtained residue to obtain clean waste water Thirdly, a method for removing selenium in wastewater, comprising: a dissolved oxygen removal step of removing dissolved oxygen contained in wastewater containing selenium; Iron salt adding step of adding salt, and then adding alkaline agent to pH
Is neutralized to 8 to 9 and selenium is coprecipitated with ferrous hydroxide, and a selenium coprecipitation step is performed.
Wastewater, comprising: a step of precipitating ferrous hydroxide by neutralizing to 9 to 11 to precipitate ferrous hydroxide; and a solid-liquid separation step of separating the obtained residue to obtain clean wastewater. Fourthly, the method for removing selenium in the wastewater includes the steps of: adding a ferrous salt to a selenium-containing wastewater; adding an iron salt; then, removing a dissolved oxygen; An alkaline agent is added to neutralize the pH to 8 to 9, and selenium is coprecipitated with ferrous hydroxide. A selenium coprecipitation step is performed, followed by adding an alkaline agent to neutralize the pH to 9 to 10. A method for removing selenium in wastewater, comprising a step of precipitating ferrous hydroxide and a step of solid-liquid separation for separating the obtained artifact to obtain clean wastewater, 5. The means for removing dissolved oxygen, wherein the wastewater to be treated is brought into contact with a metal having a pH of 7 or less. The first to fourth, wherein Mukoto
Sixthly, in the sixth aspect, wherein the dissolved oxygen removing step includes means for adding a dissolved oxygen removing agent to the wastewater to be treated. Seventh, the method according to any one of the first to fourth features, wherein the dissolved oxygen removing step includes means for bubbling the treated wastewater with a non-oxidizing gas.
Eighth, the method according to any one of the first to fourth aspects, wherein the dissolved oxygen removing step includes a means for reducing the pressure of the wastewater to be treated. The method according to any one of the first to eighth, characterized in that the method comprises at least means for reducing the pressure of the waste water to be treated in a non-oxidizing atmosphere, The method according to any one of the first to ninth aspects, wherein the selenium co-precipitation step is performed in a non-oxidizing atmosphere. The method according to any one of the third to tenth aspects, wherein the metal is iron powder, and the method according to the fifth aspect, wherein the metal is iron powder. To
The method according to the twelfth aspect, wherein the iron powder is recovered by magnetic separation from the treated wastewater containing a deposit in the iron hydroxide precipitation step, and the iron powder is subjected to the dissolved oxygen removal step again. Fourteenth, after the iron powder is collected by magnetic separation, the wastewater to be treated containing the deposit is subjected to solid-liquid separation, and the resulting deposit is dissolved in an acidic solution having a pH of 4 or less to remove the residue. The method according to the thirteenth aspect, wherein the dissolved substance is separated by filtration, and the obtained filtrate is again subjected to the iron salt addition step as a ferrous iron source. The wastewater to be treated containing the deposit in the process is separated into solid and liquid,
After dissolving the obtained residue in an acidic solution having a pH of 4 or less,
The iron powder in the solution was recovered by magnetic separation and subjected to the dissolved oxygen removing step again, and the solution excluding the iron powder was filtered. The method according to the twelfth aspect, wherein the method is again provided to the iron salt adding step as a source. Sixteenth, the removal of the dissolved oxygen reduces the dissolved oxygen in the water to be treated to 0.5 mg / l or less. 16. A method for removing selenium from wastewater according to any one of the first to fifteenth aspects, wherein

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, dissolved oxygen is basically removed from selenium-containing wastewater, and a divalent iron salt is added to the treated solution to neutralize the solution to a pH of 8 to 10. The selenium is reduced and removed by co-precipitation by adsorbing on coagulated floc of ferrous hydroxide.

By reducing the dissolved oxygen to 0.5 mg / l or less, the ferrous hydroxide can be stabilized, and the selenium in the treated wastewater can be reduced to 0.1 mg / l or less of the wastewater standard value. It becomes possible.

In order to remove dissolved oxygen, (1) means for bringing into contact with a metal such as iron powder in an acidic region having a pH of 7 or less, (2) 2
Means for adding a means or sodium sulfite or the like for removing dissolved oxygen reagent (reducing agent) and neutralizing the dissolved Ataitetsushio, (3) in a non-oxidizing atmosphere, and the in N ₂ gas or the like non-oxidizing gas There are means for bubbling the treated water, and (4) means for subjecting the water to be treated to a reduced pressure in a non-oxidizing atmosphere.

In the non-oxidizing atmosphere, after the water to be treated is put into the reaction tank, a non-oxidizing gas such as N ₂ is caused to flow into the upper space of the liquid tank to replace the air, thereby sealing the water to be treated at the upper part. Obtained by: As the non-oxidizing gas, an inert gas such as argon and helium can be used in addition to the N ₂ gas.
N ₂ gas is cost effective. In the case of the bubbling process using a non-oxidizing gas such as N ₂ gas, by performing the process using a reaction vessel with a lid, the upper space is substantially in a replacement state, that is, a non-oxidizing atmosphere state. There is no need for a top seal.

The effect of the removal of dissolved oxygen on the removal of selenium is remarkable. For example, a reagent was added to pure water, and 300 ml of a solution having a high concentration of hexavalent selenium of 34 mg / l was used. In the case where ferrous hydroxide is aggregated by reacting in the alkaline region at pH = 9 for 30 minutes in the presence of Fe ²⁺ / l in the presence of Fe ²⁺ , the solution which has been subjected to a liquid temperature of 25 ° C. without removing dissolved oxygen is: When the liquid residual selenium concentration was 20 mg / l, when the dissolved oxygen was removed by bubbling N ₂ gas from the bottom of the liquid tank at the same liquid temperature of 25 ° C., the selenium concentration was 1.9 mg / l. Was. In addition, when the liquid temperature was 45 ° C. and the liquid tank was sealed at the top with N ₂ gas, the selenium concentration was 2.0 mg / l (Japanese Patent Application Laid-Open No. H08-208,1992).
-267076 method). That is, in the present invention for removing dissolved oxygen, a sufficient de-selenium effect can be obtained at room temperature.

As described above, the dissolved oxygen in the waste water to be treated and the selenium concentration in the treated waste water are closely related, and the removal of dissolved oxygen is achieved by reducing hexavalent selenium, stabilizing ferrous hydroxide and adsorbing the same. It has a remarkable influence on sex.

Furthermore, compared with wastewater containing less salt, it contains 2.2 mg / l of hexavalent selenium and contains calcium,
According to the investigation of the influence of dissolved oxygen using a high salt concentration wastewater containing a large amount of chlorine and the like, from which hexavalent selenium is difficult to be removed, as a base solution, that is, 900 ml of the base solution was taken in a reaction vessel, and the liquid temperature was set at 25 ° C. N ₂ gas bubbling was continued until dissolved oxygen was reached, and then N ₂ gas was replaced in the upper part of the vessel, and the mixture was stirred and stirred, and the pH was increased in the presence of 500 mg / l of Fe ^{2 +.}
According to the results of a 30-minute agglutination reaction at = 9,
(a) In the case where the treatment for removing dissolved oxygen by bubbling is not performed, the dissolved oxygen is 6.1 to 7.4 mg / l,
While the selenium concentration after the treatment was 0.72 mg / l, the oxygen removal by bubbling as described above was carried out to obtain (b) the dissolved oxygen of 2 mg / l.
1 mg / l, (c) 1 mg / l of dissolved oxygen
The selenium concentration was 0.22 mg / l and (d) the dissolved oxygen was 0.5
mg / l, the selenium concentration was 0.10 mg / l
l, (e) When the dissolved oxygen is 0.1 mg / l or less,
The selenium concentration was reduced to 0.08 mg / l. Therefore, even in the case of high-salt-concentration wastewater from which hexavalent selenium is difficult to remove, the dissolved oxygen of the water to be treated should be reduced to 0 mg / l or less of the treated wastewater at room temperature at room temperature. What is necessary is just to reduce it to 0.5 mg / l or less.

It is preferable that all the steps including the step of removing dissolved oxygen be performed in a non-oxidizing atmosphere. In addition, a ferrous salt can be used as an iron salt for removing dissolved oxygen.
The use of iron (II) salt is 2 times that of coprecipitation treatment of selenium with iron hydroxide.
Becomes a stage use.

In the present invention, since selenium is adsorbed on flocculent floc of ferrous hydroxide, reduction of hexavalent selenium and adsorption of selenium on the floc of floc.
Add the ferrous salt. Therefore, even after removal of selenium at a preferable pH of about 9 for coprecipitation of selenium, iron may remain in an amount slightly exceeding the emission standard of 10 mg / l or less. At this time, in order to remove an excessive amount of residual iron ions, in the present invention, after the coprecipitation reaction of selenium, the pH value can be further increased to cause a new agglomeration reaction of iron hydroxide.

High salt containing 1.1 mg / l of hexavalent selenium
Of pH value, residual selenium and residual iron
According to the investigation of the relationship, ie, the original liquid 900
ml into a reaction vessel,₂Dissolution by gas bubbling
After removal of oxygen, N₂Perform gas replacement
To a non-oxidizing atmosphere, stirring with a stirrer, ₄
 ・ 7H₂Fe by addition of O²⁺500mg / l present
Under pH set to pH = 9 and treated for 30 minutes,
Primary coagulation to co-precipitate selenium,
Without stirring, the pulp-like liquid was pulverized with NaOH under stirring.
H is reset to 9-11 to perform secondary aggregation of iron hydroxide
According to the results of examination of the filtrate, (a)
= 9, the amount of residual selenium was 0.02 mg
/ L, iron content was adjusted to 12 mg / l, (b) pH = 9.5.
, The amount of selenium is 0.04 mg / l and the amount of iron is 0.06 mg / l.
mg / l, (c) When adjusted to pH = 10, the amount of selenium is
0.10mg / l, iron content 0.08mg / l, (d) pH
= 10.5, the amount of selenium is 0.07 mg /
l, iron content was adjusted to 0.06mg / l, (e) pH = 11
, The amount of selenium is 0.09 mg / l and the amount of iron is 0.06 mg / l.
mg / l, and the residual iron content decreases rapidly at pH = 9 or more
did. However, when the pH is 10 or more, selenium in the deposits is
It showed a tendency to dissolve. Therefore, in the previous step, pH = 8
~ 9, preferably about 9, effectively coprecipitated selenium
Thereafter, pH = 9-11, preferably about 10
Readjusted to 10 to perform secondary aggregation of iron hydroxide
Selenium and iron can both be kept below the effluent standard.
it can. At the same time, a small amount of cadmium remaining in the wastewater
Heavy metals such as metals, antimony, mercury, lead, etc.
It can also be held down.

Hereinafter, a specific example of the present invention will be described with reference to the process chart of FIG. Selenium-containing wastewater such as factory wastewater is generally
It is in the state of a room temperature liquid having a pH of 7, and is supplied to the reaction tank as it is. The reaction vessel does not need to be completely sealed, but can be made in a non-oxidizing atmosphere, and is preferably provided with a lid provided with a stirring device together with a gas supply pipe and an inlet for chemicals.

After the selenium-containing waste water, ie, the water to be treated, is supplied to the reaction tank, the space above the reaction tank is filled with N 2 as a non-oxidizing gas.
_{The two} gases are supplied at a constant supply rate to replace the air, and the water to be treated is brought into a sealed state. Next, the water to be treated is kept in an acidic range of about pH = 3 with an acidic agent such as hydrochloric acid, and iron powder is supplied, for example, in an amount of 0.3 to 1.0 g / l, and strongly stirred by a stirrer. A small amount of iron powder melts and reacts with oxygen in the liquid,
Dissolved oxygen can be reduced to 0.1 mg / l or less.

According to a dissolved oxygen meter, 0.5 mg / l is preferable.
Or less than 0.1mg / l dissolved oxygen
After confirming that the
FeCl in water₂ , FeSO₄ Equivalent iron salts²⁺
About 500 mg / l, and stir to dissolve
After that, an alkaline agent such as NaOH is added to adjust the pH to 8-9.
The neutralization reaction is carried out while maintaining this pH.
And divalent iron ions (Fe²⁺) Is Fe (OH)₂Tona
Coagulation (primary coagulation), and selenium in the liquid
(OH)₂ To be adsorbed and co-precipitated. 30 minutes
Selenium is almost precipitated by the degree, but Fe remaining in the liquid
²⁺If the amount of iron exceeds the wastewater standard value, N
The pH is raised to about 9 to 10 with an alkaline agent such as aOH,
Fe (OH) ₂Promotes secondary aggregation. At this time,
Diatomaceous earth is added as a sedimentation filter aid
preferable. The resulting pulp-like treated water containing sediment is settled.
And clean drainage by means such as
Separate the liquid. This makes it easy for selenium to
/ L or less can be obtained. What
Contact, ferrous hydroxide Fe (OH)₂ Before secondary coagulation
As described above, even if the pH is 10 or more, Fe (OH)
₂ Does not increase, while the adsorbed selenium slightly redissolves
Therefore, the pH is kept at 10 or less.

When iron powder is used as a reducing agent to remove dissolved oxygen as described above, an excessive amount of iron powder exceeding several times the required amount is added from the viewpoint of working efficiency. Thereafter, iron powder can be recovered from the deposit-containing treated water by magnetic separation or the like, and can be again subjected to the above-described dissolved oxygen removal treatment. Furthermore, after magnetically separating the iron powder, the treated water containing the residue is separated into solid and liquid, and the resulting residue is dissolved in an acidic solution having a pH of 1 to 4 and preferably 2 to 4 and the insoluble matter is filtered. Separately, the obtained filtered water can be used again as a ferrous iron source and subjected to the above-mentioned ferrous salt addition treatment. If the pH of the acidic liquid is lower than 1, selenium may be slightly dissolved, and if the pH is higher than 4, the dissolution of the iron salt is insufficient.

Further, after the reaction is completed, the treated water containing the deposit is separated into solid and liquid, and the obtained residue is dissolved in an acidic solution having a pH of 4 or less, and the iron powder in the solution is subjected to magnetic separation. The collected solution is again subjected to the above-mentioned dissolved oxygen removal treatment, and the dissolved solution from which the iron powder has been removed is further filtered to remove insolubles. The obtained filtrate is used as a divalent iron source again to remove the above-mentioned divalent iron. It can also be subjected to a salt addition step.

Another embodiment of the present invention will be described with reference to the process chart of FIG. Wastewater containing hexavalent selenium having a pH of 7, such as factory wastewater, is supplied to a reaction tank equipped with a stirrer or the like as the water to be treated as in the case of FIG. 1, and an air supply pipe provided at the bottom of the tank is provided in the water to be treated. Then, by blowing a non-oxidizing gas such as N ₂ gas and performing bubbling, the dissolved oxygen in the waste water to be treated can be reduced to 0.1 mg / l or less in a state where the dissolved oxygen is accompanied by the non-oxidizing gas. In this case, since the non-oxidizing gas fills the upper space of the reaction tank, gas replacement in the upper space is not particularly necessary as in the case of FIG. However, the neutralization step and the selenium coprecipitation step after the removal of dissolved oxygen are preferably performed in a non-oxidizing atmosphere. After bubbling, a small amount of a non-oxidizing gas such as N ₂ is blown into the upper space of the reaction tank. It is more effective to provide a sealing means.

After the end of the dissolved oxygen removal treatment by bubbling, the upper seal is performed with N ₂ gas.
Divalent iron salts such as FeCl ₂ and FeSO ₄ are added to the water to be treated by F
e ^{2 + at} a rate of 500 mg / l,
An alkaline agent such as NaOH is added to adjust the pH to 8-9, preferably 9, and the pH is maintained as it is for about 30 minutes while stirring, so that Fe ²⁺ in the liquid is reduced to Fe (O
H) It becomes ₂ and aggregates, and the reduced selenium is adsorbed and co-precipitated. After about 30 minutes of coprecipitation, NaO
The pH is adjusted to 9 to 10 with an alkali agent such as H, and the remaining F
The secondary aggregation of iron hydroxide is performed for e ²⁺ . During this secondary flocculation, diatomaceous earth is preferably added as a settling filter aid. After finishing the secondary aggregation treatment of iron hydroxide in about 10 minutes,
Treated water containing sediment is subjected to sedimentation or solid-liquid separation by means of filtration, and the sediment is collected. As described above, it is possible to obtain clean wastewater in which selenium and iron are each reduced to below the wastewater standard.

Another embodiment of the present invention will be described with reference to the process chart of FIG. In this example, the aggregation of iron hydroxide is 3
It takes place in stages. Wastewater containing hexavalent selenium having a pH of 7, such as factory wastewater, is supplied to a reaction tank equipped with a stirrer as water to be treated, and is stirred so that air is not involved. This stirring is continued until the end of the reaction. The liquid temperature is preferably 25 to 3
0 ° C. Place the reaction tank under the open air and add F
ESO ₄ · 7H ₂ O, etc. 2 Ataitetsushio 400m as the Fe ²⁺
g / l, and pH is adjusted with an alkaline agent such as NaOH.
Is adjusted to about 8 to 9, preferably about 9, and held for about 15 minutes to produce iron hydroxide (Fe (OH) ₂ ) (primary aggregation), thereby reducing dissolved oxygen. At this time, selenium in the liquid is also coprecipitated to some extent due to the coagulated iron hydroxide.

[0030] Then, pH was adjusted to 7 with hydrochloric acid and the like acidic solution, the pH by the addition of about 400 mg / l again FeSO ₄ · 7H ₂ O, etc. 2 Ataitetsushio as Fe ²⁺ NaOH like alkali agent 8-9 By adjusting to preferably 9 and holding for 15 to 30 minutes, preferably for about 30 minutes, iron hydroxide can be agglomerated (secondary agglutination) and selenium can be adsorbed and co-precipitated. 0.1 mg / l or less.

Next, in order to remove iron ions remaining in the solution, the water to be treated is treated with an alkaline agent such as NaOH to a pH of 9 to 11, preferably pH 1 to 1, without removing iron hydroxide precipitate.
Aggregation of new iron hydroxide is achieved by adjusting the temperature to 0 and holding for about 10 minutes (tertiary aggregation). After a lapse of 10 minutes, diatomaceous earth is added as a flocculant and a sedimentation filter aid, and the flocculate is sufficiently settled. Then, solid-liquid separation is performed to collect a selenium coprecipitate. By the above treatment, it is possible to obtain clean wastewater in which selenium and iron are each reduced to a wastewater standard or less.

[0032]

EXAMPLES Example 1 (Removal of Dissolved Oxygen by Means of Metal Contact) For industrial wastewater containing a large amount of salts such as 1.3 g / l Ca and 40 g / l Cl, a hexavalent selenium concentration of 1. 1m
A treatment test was performed using the g / l solution as a base solution (see FIG. 1).

900 ml of this original solution is placed in a 1 liter beaker, covered, and the upper space is filled with N ₂ gas at 500 ml / min.
, And stirred by a stirrer equipped with two Teflon blades attached to the lid in an upper sealed state to maintain the reaction solution temperature at 25 ° C. Then, the pH is adjusted with hydrochloric acid solution.
Was adjusted to 3, and iron powder was added at a rate of 0.5 g / l to remove dissolved oxygen.

After confirming by a dissolved oxygen meter that the dissolved oxygen in the water to be treated was 0.1 mg / l or less, FeS
500m The O ₄ · _7H 2 O as 2.3 g ^{(Fe 2+}
g / l) was added and stirred for 4 minutes to dissolve. The pH was then adjusted to 9 by adding a NaOH solution,
This pH was maintained for a minute and selenium was co-precipitated with the agglomerated Fe (OH) ₂ . This pulp solution was subjected to suction filtration with a Buchner, and the filtrate was analyzed. As a result, selenium was 0.04 mg / l.

Example 2 (Removal of Dissolved Oxygen by Chemical Addition Means) A 300 ml original solution having a hexavalent selenium concentration of 1.1 mg / l as in Example 1 was placed in a beaker, and N ₂ gas was injected into the upper space thereof. The solution was flowed at a rate of 500 ml / min, and the upper portion was sealed. The original solution was stirred with a stirrer, and the reaction temperature was set to 2 at room temperature.
While maintaining at 5 ° C., the pH was adjusted to 7. Na ₂ SO 3 as a dissolved oxygen removing agent
₃ 2.75 g / 50 ml solution (1 ml) was added (this corresponds to 55 mg of Na ₂ SO ₃ ) to remove dissolved oxygen.

[0036] After the dissolved oxygen was confirmed to have become less 0.1 mg / l by a stirring for 5 minutes the dissolved oxygen meter, as 2 Ataitetsushio FeSO ₄ · 7H ₂ O and 0.77 g (Fe
500 mg / l as ²⁺ was added and stirred for 4 minutes to dissolve. Next, the pH was adjusted to 9 with NaOH, and the reaction was carried out for 30 minutes while maintaining the pH, whereby selenium was adsorbed on the coagulated Fe (OH) ₂ and coprecipitated. The obtained pulp-like liquid was subjected to suction filtration with a Buchner, and the filtrate was analyzed. As a result, the selenium concentration was 0.08 mg / l.

Example 3 (Removal of Dissolved Oxygen by Chemical Addition Means in the Atmospheric Atmosphere) The same factory wastewater as in Example 1 was used, and the concentration of hexavalent selenium was 1.2 mg / l, and the same was repeated four times. A treatment test was performed. At that time, a dissolved oxygen removal treatment was performed in an air atmosphere, and then a selenium coprecipitation treatment was performed. That is,
Each 900 ml of the original solution was placed in each beaker, and each was stirred with a stirrer to such an extent that air was not entrained, and the reaction temperature was maintained at 30 ° C. Then, the original solution was adjusted to pH 7, and then 400 mg / l as Fe ^2+.
It was added considerable FeSO ₄ · 7H ₂ O, then, NaO
H was adjusted to pH 9 and maintained to remove dissolved oxygen to 0.1 mg / l or less. The treatment solution is subsequently adjusted to pH 9 by 15
Hold for minutes.

Thereafter, the water to be treated was adjusted to pH 7 with hydrochloric acid. Further, a FeSO ₄ solution equivalent to 400 mg / l as Fe ²⁺ was added again, and then adjusted to pH 9 with NaOH, and reacted for 15 minutes to produce iron hydroxide Fe (O 2
H) ₂ was used to reduce selenium to cause adsorption and coprecipitation.
The obtained pulp-like liquid was subjected to suction filtration using a Buchner, and the filtrate was analyzed.
g / l, 0.02 mg / l, 0.02 mg / l and 0.01 mg / l.

Example 4 (Removal of Dissolved Oxygen by Chemical Addition Means in Non-Oxidizing Atmosphere) The same factory effluent as in Example 1 was subjected to a liquid having a hexavalent selenium concentration of 1.6 mg / l. A processing test was performed as a base liquid. 900 ml of the original solution was taken in a 1 liter beaker, and N ₂ gas was flowed into the upper space at a rate of 500 ml / min to seal the gas. This stock solution was stirred with a stirrer, and the reaction temperature was maintained at room temperature of 25 ° C.

[0040] Furthermore, after the pH of the Motoeki and 7, the FeSO ₄ · 7H ₂ O as dissolved oxygen removal iron salt 0.4
6 g (corresponding to 100 mg / l as Fe ²⁺ ) was added and stirred for 4 minutes to dissolve. Then, the pH was adjusted to 9 with NaOH, and the pH was maintained at 9 for 3 minutes, whereby the dissolved oxygen was reduced to 0.1 mg / L by the aggregated iron hydroxide.
It was removed to:

Thereafter, the water to be treated was adjusted to pH = 7 with hydrochloric acid. Moreover, again, as the iron salt, FeSO ₄ · 7H ₂
2.30 g of O (equivalent to 500 mg / l as Fe ²⁺ )
Was added and stirred for 4 minutes to dissolve. Then, NaO
The pH was adjusted to 9 with H, and the mixture was reacted for 30 minutes to cause aggregation of iron hydroxide. The obtained pulp-like liquid was subjected to suction filtration with a Buchner, and the filtrate was analyzed.
It was 04 mg / l.

Example 5 (Removal of Dissolved Oxygen by Bubbling Non-Oxidizing Gas) The same factory wastewater as in Example 1 was used, and a liquid having a hexavalent selenium concentration of 0.98 mg / l was used as an original liquid. A treatment test was performed. 900 ml of the original solution was placed in a beaker and stirred with a stirrer. The reaction temperature was normal temperature of 25 ° C. Next, N ₂ gas was blown into the original liquid from the bottom of the beaker, and bubbling was performed at 500 ml / min of N ₂ gas. It was confirmed that the dissolved oxygen in the liquid became 0.1 mg / l or less by bubbling N ₂ gas for 10 minutes.

N₂After gas bubbling, the liquid to be treated
= 7 and adjust the upper space of the beaker container to N₂Gas
I was Then, as a ferrous salt, FeSO₄・ 7
H ₂2.30 g of O (Fe²⁺500 mg / l phase
Was added and stirred for 4 minutes to dissolve. Then N
The pH was adjusted to 9 with aOH, and the mixture was reacted for 30 minutes.
e (OH) ₂Was coagulated, and selenium was coprecipitated by adsorption. Profit
The obtained pulp-like liquid is suction-filtered with a Buchner, and the filtrate is filtered.
Analysis showed that the selenium concentration was 0.05 mg / l.
Was.

Example 6 (Removal of Dissolved Oxygen by Bubbling Means of Non-Oxidizing Gas) For the same factory wastewater as in Example 1, a hexavalent selenium concentration of 1.1 mg / l and mercury of 0.1 mg / l were used. 1, cadmium 1.0 mg / l, arsenic 0.1 mg / l, lead 2.2 mg / l
A treatment test was performed using a liquid to which each heavy metal was added so as to be 1.0 mg / l, zinc 1.0 mg / l, and antimony 0.5 mg / l as a base liquid (see FIG. 2). Take the original solution into a 900 ml beaker, stir with a stirrer,
It was kept at ° C. From the bottom of the _{N 2} gas to the original solution in the beaker blowing at a rate of 500m l / min, it was bubbled process. In the bubbling process for 10 minutes,
It was confirmed that the dissolved oxygen in the liquid was 0.1 mg / l or less.

Next, the solution was adjusted to pH = 7 and beaked.
N in upper space₂Supply gas and N ₂Do the top seal
After that, the reagent FeSO₄・ 7H₂O to 2.
30g (Fe²⁺(Equivalent to 500 mg / l)
And stirred for 4 minutes to dissolve. Then p with NaOH
The reaction was carried out for 30 minutes while adjusting and holding at H = 9, and Fe (OH)
₂Selenium co-precipitation treatment (iron hydroxide primary coagulation treatment)
Was. After the reaction is completed, the treatment is performed without removing the artifact.
The solution was adjusted to pH = 9.5 with NaOH and maintained at 10
Minutes and react with Fe (OH)₂Was subjected to secondary aggregation.
After completion of the reaction, the obtained pulp solution is subjected to suction filtration with a Buchner.
When the filtrate was analyzed, the selenium concentration was 0.0
4mg / l, Fe after treatment also reduced to 0.13mg / l
did. The concentration of heavy metals after the treatment was 0.0005 m of mercury.
g / l, cadmium 0.01 mg / l or less, arsenic 0.0
05 mg / l, lead 0.005 mg / l or less, zinc 0.0
2 mg / l, antimony reduced to 0.008 mg / l,
It was confirmed that selenium was removed from the wastewater at the same time.
Was.

[Comparative Example 1] Two identical treatment tests were performed on the same factory wastewater as in Example 1, using a liquid having a hexavalent selenium concentration of 0.98 mg / l as a base liquid. However,
In each case, the treatment was carried out in the open to the atmosphere without removing the dissolved oxygen. That is, 900 ml of the original solution was placed in each beaker, and the stirrer was stirred to such an extent that air was not entrained, and the reaction temperature was maintained at 25 ° C.

Next, the solution was adjusted to pH = 7 and then divalent.
As an iron salt, the reagent FeSO₄・ 7H ₂2.30 g of O
(Fe²⁺As 500 mg / l) and added 4
Stir for a minute and dissolve. Then, pH = 9 with NaOH.
, And react for 30 minutes.₂Aggregation
Selenium coprecipitation treatment was performed. Pulp-like liquid obtained
Was filtered by suction with a Buchner, and the filtrate was analyzed.
The concentrations of selenium were 0.28 mg / l and
It was 0.34 mg / l.

[0048]

As is clear from the above description, according to the selenium removal method of the present invention for removing dissolved oxygen in water to be treated in advance, high-salt wastewater containing hexavalent selenium such as factory wastewater is treated. Therefore, even at room temperature, hexavalent selenium can be easily reduced to the effluent standard value of 0.1 mg / l or less, and at the same time, the iron content can be sufficiently reduced to the effluent standard value or less. This has the effect that heavy metals can also be removed together. In addition, the workability is good and the water to be treated can be treated at room temperature, so that there is no need for a liquid heating facility, the facility cost is low, and the effect of high economic efficiency can be obtained.

As means for removing dissolved oxygen of the water to be treated, means using iron powder has the effect that iron powder is inexpensive and has good handleability together with the dissolved oxygen removing agent. This has the effect of being efficient. In addition, in the case of removing dissolved oxygen, especially when iron powder is used, it is possible to take a recycle process, thereby achieving an effect of reducing material costs and the like.

[Brief description of the drawings]

FIG. 1 is a process chart showing a specific example of the present invention.

FIG. 2 is a process chart showing another specific example of the present invention.

FIG. 3 is a process chart showing another specific example of the present invention.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Jin Mitsada 1-8-2 Marunouchi, Chiyoda-ku, Tokyo Dowa Mining Co., Ltd. F-term (reference) 4D037 AA11 AB10 AB11 BA04 BA23 BB05 BB07 BB09 CA01 CA02 CA08 CA09 CA14 4D038 AA08 AB27 AB70 AB82 BB03 BB06 BB13 BB15 BB17 BB18

Claims (16)

[Claims] 1. A dissolved oxygen removing step for removing dissolved oxygen contained in a wastewater containing selenium, an iron salt adding step for adding a ferrous salt to the obtained treated wastewater, and then adding an alkali agent. A selenium co-precipitation step of neutralizing the pH to 8 to 10 and co-precipitating selenium with ferrous hydroxide, and a solid-liquid separation step of separating the obtained residue to obtain clean waste water. Characteristic method of removing selenium from wastewater. 2. An iron salt addition step of adding a ferrous salt to waste water containing selenium, a dissolved oxygen removal step of removing dissolved oxygen, and an alkali agent is added to the obtained waste water to be treated to adjust pH. A selenium co-precipitation step of neutralizing selenium with ferrous hydroxide, and a solid-liquid separation step of separating the obtained residue to obtain clean wastewater. To remove selenium from wastewater. 3. A process for removing dissolved oxygen contained in wastewater containing selenium, a process for adding a ferrous salt to the obtained wastewater to be treated, and a process for adding an alkali agent. A selenium co-precipitation step of neutralizing the pH to 8 to 9 and co-precipitating selenium with ferrous hydroxide, and subsequently adding an alkaline agent to neutralize the pH to 9 to 11 and ferrous hydroxide. A precipitation step of iron hydroxide, and a solid-liquid separation step of separating the obtained precipitate to obtain clean wastewater, the method for removing selenium from wastewater. 4. An iron salt addition step of adding a ferrous salt to waste water containing selenium, a dissolved oxygen removal step of removing dissolved oxygen, and an alkaline agent is added to the obtained waste water to be treated to adjust pH. Is neutralized to 8-9 and selenium is coprecipitated with ferrous hydroxide, and then an alkaline agent is added to neutralize the pH to 9-11 to precipitate ferrous hydroxide. A method for removing selenium from wastewater, comprising: a step of precipitating iron hydroxide to be carried out; and a step of separating solid matter from the obtained precipitate to obtain clean wastewater. 5. The removal of selenium in wastewater according to claim 1, wherein the step of removing dissolved oxygen includes means for bringing the wastewater to be treated into contact with a metal having a pH of 7 or less. Method. 6. The method for removing selenium from wastewater according to claim 1, wherein the dissolved oxygen removing step includes a means for adding a dissolved oxygen removing agent to the wastewater to be treated. 7. The method for removing selenium from wastewater according to claim 1, wherein the step of removing dissolved oxygen includes means for bubbling the wastewater to be treated with a non-oxidizing gas. 8. The method according to claim 1, wherein the dissolved oxygen removing step includes a means for reducing the pressure of the waste water to be treated.
The method for removing selenium in wastewater according to any one of the above. 9. The method for removing selenium from waste water according to claim 1, wherein the step of removing dissolved oxygen is performed in a non-oxidizing atmosphere. 10. The method for removing selenium from wastewater according to claim 1, wherein the iron salt adding step and the selenium coprecipitation step are performed in a non-oxidizing atmosphere. 11. The method for removing selenium from waste water according to claim 3, wherein the iron hydroxide precipitation step is performed in a non-oxidizing atmosphere. 12. The method according to claim 5, wherein the metal is iron powder. 13. The method according to claim 11, wherein iron powder is recovered by magnetic separation from the treated wastewater containing deposits from the iron hydroxide precipitation step, and the iron powder is again subjected to the dissolved oxygen removal step. 13. The method for removing selenium from wastewater according to item 12. 14. After collecting the iron powder by magnetic separation, the wastewater to be treated containing the deposit is separated into a solid and a liquid, and the obtained deposit is dissolved and dissolved in an acidic solution having a pH of 4 or less. 14. The method for removing selenium in waste water according to claim 13, wherein the substance is separated by filtration, and the obtained filtrate is used as a ferrous iron source and then subjected to the iron salt addition step again. 15. A wastewater to be treated containing a residue from the iron hydroxide precipitation step is subjected to solid-liquid separation, and the resulting residue is subjected to pH adjustment.
Was dissolved in an acidic solution of 4 or less, and the iron powder in the solution was recovered by magnetic separation, and subjected to the dissolved oxygen removal step again. Further, the solution excluding the iron powder was filtered to remove impurities. 13. The method for removing selenium in waste water according to claim 12, wherein the filtrate separated by filtration is used again as an iron source for the iron salt addition step. 16. The wastewater according to any one of claims 1 to 15, wherein the removal of the dissolved oxygen in the dissolved oxygen removal step is performed until the dissolved oxygen of the wastewater to be treated becomes 0.5 mg / l or less. Method of removing selenium from the inside.