JP2002263662A - Method for treating selenium-containing waste water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for treating a selenium-containing waste water capable of easily achieving high selenium removing ratio while effectively suppressing the generation of sludge even if the selenium-containing waste water contains metal salts. SOLUTION: When a selenium removing treatment in which at least one kind ferrous metal selected from a group consisting of Fe, Mn, Ni and Cu is brought into contact with the waste water containing salts and a dissolved selenium and a separated precipitate is subjected to solid-liquid separation to remove the dissolved selenium is performed, before the selenium removing treatment, a pH value of the waste water is adjusted by adding alkali in the waste water and a pretreatment subjecting the separated precipitate to the solid-liquid separation is performed, and also the selenium removing treatment is performed under the condition whose pH value is not higher than that at the pretreatment.

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 selenium-containing wastewater for efficiently separating and removing dissolved selenium dissolved in wastewater in the form of O ₄ ^2- ) or selenite ion (SeO ₃ ^2- ).

[0002]

2. Description of the Related Art Selenium and selenium compounds are used in glass products, ceramic products, semiconductor materials, solar cells and movie films,
Infrared polarizers, pigments, sensitizers, dehydrogenating agents, foaming agents, etc., are widely used in the production of various industrial products, and from industrial factories that use such selenium and selenium compounds. Inevitably, selenium-containing wastewater containing dissolved selenium is discharged.

[0003] Regarding this selenium-containing wastewater,
The fact that the environmental standard is set to a selenium concentration (as Se) of 0.01 mg / liter or less also triggers various methods for separating and removing dissolved selenium in wastewater. For example, U.S. Pat.
Hexavalent selenium ion [for example, selenate ion (Se
An aqueous solution containing O ₄ ^2- )] and metallic iron are brought into contact with each other at a pH of 6 or less to reduce hexavalent selenium ions to tetravalent selenium ions [for example, selenite ion (SeO ₃ ^2- )] and to reduce metal content. The iron is oxidized and dissolved, the dissolved iron oxide is precipitated in the form of iron hydroxide, and the precipitated iron hydroxide is separated by solid-liquid separation to recover an aqueous solution in which hexavalent and lower selenium ions are reduced. A method has been described, and
No. 2,502 describes a method for reducing and removing selenium concentration in waste liquid by contacting selenium and / or selenium-containing waste liquid with metallic iron in the pH range of 0 to 6 to precipitate selenium on the surface of metallic iron. Have been.

However, in these methods,
In order to reduce the dissolved selenium to a value satisfying the selenium concentration of 0.1 mg / liter or less specified in the wastewater standard, the reaction area where the aqueous solution (waste liquid) is brought into contact with metallic iron has a concentration of several thousand times the selenium concentration. It is necessary to generate a large amount of iron ions, several tens of thousands of times, and as a result, an extremely large amount of iron hydroxide is generated as sludge, and the treatment of this sludge generated in a large amount in the treatment of selenium-containing wastewater is a heavy load. I have.

[0005] The problem of sludge treatment is, for example, that exhaust gas is discharged from a flue gas desulfurization device attached to equipment such as a coal-fired power plant and mainly for removing sulfurous acid gas in flue gas.
-Dissolved selenium and oxidizing substances in a flue gas desulfurization effluent containing oxidizing substances such as sulfur oxides that can be quantified by the -p-phenylenediamine colorimetric method (industrial effluent test method). If this is attempted, the problem becomes even more serious due to the large amount of processing.

[0006] Therefore, the present inventors first contacted wastewater containing dissolved selenium with at least one iron-based metal selected from the group consisting of Fe, Mn, Ni, and Cu. In order to remove the dissolved selenium in the wastewater by solid-liquid separation of the precipitate deposited in the treated water, introduce an inert gas into the reaction area where the wastewater and the iron-based metal come into contact, or remove the dissolved oxygen To control the reaction zone to oxygen depleted conditions compared to air,
It can not only efficiently separate and remove dissolved selenium from selenium-containing wastewater or efficiently separate and remove dissolved selenium and oxidizing substances, but also reduce the amount of sludge generated as much as possible. A method for treating selenium-containing wastewater suitable for treating selenium-containing wastewater was proposed (Japanese Patent Application No. 2000-329,879).

[0007] However, in this method, although the generation of sludge, which imposes a great burden on wastewater treatment, can be reduced as much as possible, selenium-containing wastewater has a certain salt, for example,
When metal salts such as Al, Si, Zn, and Co are included, metal compounds such as metal hydroxides derived from metal salts in wastewater precipitate on the surface of the iron-based metal, and the precipitated metal compounds are dissolved. It has been found that the problem is that the contact between selenium and the iron-based metal is prevented, and as a result, a desired selenium removal rate cannot be achieved. Further, it has been found that this problem occurs when the reaction region is not controlled to the oxygen poor condition, and also when air is not introduced, though the degree thereof is different.

[0008]

Accordingly, the present inventors have studied how to achieve a high selenium removal rate for such selenium-containing wastewater containing salts while suppressing generation of sludge as much as possible. Surprisingly, as a result of an in-depth study, surprisingly, before the de-selenium treatment to remove dissolved selenium by contacting the wastewater with an iron-based metal, the pH of the wastewater was adjusted by adding an alkali to the wastewater, By performing a pretreatment for solid-liquid separation of the deposited precipitate, and by performing the above-mentioned selenium treatment under conditions of the pH value or less at the time of the pretreatment, even if the selenium-containing wastewater contains salts, sludge It has been found that generation can be suppressed to a level that does not cause a problem, and that a high selenium removal rate can be stably achieved, and the present invention has been completed.

Accordingly, an object of the present invention is to provide a selenium-containing wastewater which can easily achieve a high selenium removal rate while suppressing generation of sludge as much as possible, even if the wastewater contains selenium containing salts. It is to provide a processing method.

[0010]

That is, the present invention relates to a method of depositing a wastewater containing salts and dissolved selenium by contacting at least one iron-based metal selected from the group consisting of Fe, Mn, Ni, and Cu. When performing the de-selenium treatment to remove the dissolved selenium in the wastewater by solid-liquid separation of the precipitates, prior to the de-selenium treatment, adjust the pH value of the wastewater by adding alkali to the wastewater, This is a method for treating selenium-containing wastewater, in which a pretreatment for solid-liquid separation of the deposited sediment is performed, and the above-mentioned de-selenium treatment is performed under the condition of the pH value or less during the pretreatment.

Further, the present invention provides a method for adjusting the pH of a pretreated wastewater by adding an acid to the pretreated wastewater before the above-mentioned pretreatment and before the removal of selenium, so that the pH value of the pretreated wastewater at the time of the removal of selenium is 6 or more. This is a method for treating selenium-containing wastewater which is adjusted to a pH value or less during pretreatment.

In the present invention, wastewater containing dissolved selenium to be treated (so-called selenium-containing wastewater)
Dissolved in wastewater discharged in various forms, such as selenate ion (SeO ₄ ^2- ) and selenite ion (SeO ₃ ^2- ), discharged from manufacturing factories of various industrial products using selenium and selenium compounds It means wastewater containing salts of metals such as Al, Si, Zn, and Co together with selenium.If such wastewater contains dissolved selenium and salts, insoluble selenium and selenium compounds in the wastewater May be present, and it can be particularly preferably applied to flue gas desulfurization wastewater containing salts together with dissolved selenium.

In the present invention, prior to contacting the wastewater with an iron-based metal and performing de-selenium treatment, an alkali is added to the wastewater to adjust the pH value of the wastewater to 6 or more, preferably 7 to 7.
It is adjusted to 10, more preferably 7 to 9, and then a pretreatment for solid-liquid separation of the deposited precipitate is performed. P at the time of this preprocessing
When the H value is lower than 6, the removal of salts contained in the wastewater becomes insufficient, and it may not be possible to achieve a desired selenium removal rate during the de-selenium treatment, and further, a desired soap removal rate is achieved. Therefore, the pH value in the selenium treatment reaction zone must be 6 or less, and the consumption of acid increases, which is not economical. Further, even if the pH value at the time of this pretreatment is higher than 10, the selenium removal rate is not so much improved, and the alkali consumption is increased, which is not economical.

Here, the alkali added to the wastewater is not particularly limited as long as the pH value of the wastewater can be adjusted to 6 or more. From the viewpoint of preventing precipitation on the surface of the iron-based metal, sodium is preferably used. Potassium hydroxide,
Magnesium hydroxide and the like, and particularly preferably sodium hydroxide in consideration of cost and the like. Also,
The device for this pretreatment may be any device that can uniformly mix the introduced wastewater and the added alkali, such as a stirring and mixing tank equipped with devices such as a stirrer and a pH meter, and a stirrer. Tic mixers and the like can be exemplified.

The pH of the pretreated wastewater, which is adjusted by this pretreatment and obtained by solid-liquid separation of the deposited precipitate, is subjected to the next de-selenium treatment at a pH of 6 or more, and the pH value at the time of the pretreatment is increased. In order to perform the reaction under the following conditions, if necessary, the pH is further adjusted by adding an acid such as an aqueous solution of hydrochloric acid or an aqueous solution of sulfuric acid. In some cases, the addition of an alkali may be required. The pH adjustment here depends on the type and concentration of the salts contained in the wastewater, but usually the pH during pretreatment is
It is preferable to adjust the pH value to a lower value of about pH 0 to 1 than the value. For example, when the pH value at the time of pretreatment is 7 to 10, the pH value of the pretreated wastewater is preferably adjusted to pH 6 to 9. .

In the present invention, in the de-selenium treatment, at least one ferrous metal selected from the group consisting of Fe, Mn, Ni, and Cu is brought into contact with the pretreated wastewater, and the deposited precipitate is solidified. The separation is performed by liquid separation to remove dissolved selenium in the wastewater. Here, as the iron-based metal used for the de-selenium treatment, iron (Fe) itself, manganese (Mn), nickel (Ni), and iron-based metal consisting of copper (Cu),
Regarding its shape, it can be in efficient contact with the selenium-containing wastewater, and any shape may be used as long as it is easy to handle, for example, spherical, granular, powdery, rod-like, chip-like, rivet-like, Metal molded materials having various shapes such as a columnar shape, a cylindrical shape, and a plate piece shape can be suitably used. The purity is preferably higher, but is not particularly limited.

The de-selenium treatment is carried out at a pH of 6 or more and a pH of the pre-treatment or less, preferably p
H6-9, more preferably 6-7. If the pH value at the time of this de-selenium treatment is lower than pH 6, a large amount of sludge is generated by this de-selenium treatment,
Wastewater treatment is a heavy burden, and conversely, pH during pretreatment
If the value is higher than the value, metal compounds such as metal hydroxides derived from salts in the wastewater are precipitated on the surface of the iron-based metal, and the activity of the iron-based metal is inhibited, so that a sufficient selenium removal rate may not be achieved as a result. Occurs.

In the de-selenium treatment, the pretreated wastewater and the iron-based metal are preferably brought into contact with each other while introducing an oxygen-containing gas such as air into the reaction zone. By the introduction, the elution of the iron-based metal is appropriately promoted to prevent the formation of the passive state, and the dissolved selenium in the pretreated wastewater can be more efficiently removed.

In the present invention, there is no particular limitation on the reactor used for the de-selenium treatment as long as it can bring the pretreated wastewater and the iron-based metal into contact with each other. Fluidized bed reaction tank, fixed bed reaction tank, packed reaction tank, etc. can be mentioned. In the lower part of the tank, a ferrous metal having a small particle diameter flows, and in the upper part of the tank, a clarified state is maintained. A reaction tank of a type in which wastewater flowing by a stirrer is brought into contact with the fixed iron-based metal is suitable. Also, the reaction type is batch type,
Further, it may be a continuous type. Also,

Further, the reaction conditions in the de-selenium treatment, such as the reaction temperature, the contact surface area per unit treatment amount when using an iron-based metal, and the reaction time are not particularly limited. Metal type, shape,
Depending on the size, etc., and also the type of reactor used, etc.,
It can be determined as appropriate. Regarding the reaction temperature, the higher the temperature, the more the selenium removal rate and the reduction in the amount of sludge generated tend to be improved.

In the present invention, p of the treated water collected from the reaction region of the de-selenium treatment and before the solid-liquid separation is performed.
The H value is adjusted to pH 7 or higher, preferably pH 8 to 12, which is higher than the pH value of the reaction region, to further precipitate the iron-based metal-derived precipitate used in the reaction region, thereby further dissolving dissolved selenium into them. You may make it accompany and make it precipitate. When the iron-based metal is, for example, metallic iron,
The formed iron hydroxide forms a precipitate in the form of trivalent iron hydroxide [Fe (OH) ₃ ] or divalent iron hydroxide [Fe (OH) ₂ ],
A precipitate can be deposited more efficiently.

Further, instead of adjusting the pH of the treated water or in combination with the pH adjustment of the treated water, an air backing treatment for introducing air into the treated water may be carried out. Further, the precipitation of precipitates such as iron hydroxide can be promoted, and the removal of selenium can be promoted. Further, for the iron-based metal used in the reaction zone, after contacting with the selenium-containing wastewater, means for removing the deposits (precipitates) attached to the surface of the iron-based metal, for example,
It is preferable to provide means such as stirring, introduction of jet gas, jet water flow, and ultrasonic waves.

The treated water treated by the method of the present invention is discharged in such a manner that precipitates precipitated in the treated water are removed by solid-liquid separation, and then the selenium concentration and the content of oxidizing substances satisfy drainage standards. You.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention shown in the accompanying drawings will be described below more specifically.

In FIG. 1, selenium-containing wastewater containing salts and dissolved selenium is introduced into a stirring and mixing tank 1, and an alkali such as an aqueous sodium hydroxide solution is added thereto. It is adjusted to 7 or more, preferably 7 to 10. This stirring and mixing tank 1 has a pH of 7
The wastewater containing the metal compound such as the metal hydroxide derived from the precipitated salts is transferred to the solid-liquid separator 2, where the metal compound such as the metal hydroxide precipitated by solid-liquid separation is converted into sludge. Removed.

Next, the pH of the pretreated wastewater thus pretreated is adjusted, for example, with an aqueous hydrochloric acid solution or the like in a line mixer 3 so that the pH is adjusted to 6 or more and below the pH value at the time of pretreatment. After that, it is charged into a reaction tank 4 for de-selenium treatment. In the reaction tank 4, an iron-based metal 5 is provided in the reaction area and a stirrer
4a and an air inlet 4b are provided so that the pretreated wastewater can be brought into contact with the ferrous metal 5 while introducing air into the reaction region. Further, when the pH value of the reaction region was greatly changed, the pH was adjusted by adding an alkali.

The treated wastewater extracted from the reaction tank 4 is then transferred to a solid-liquid separator 6, where the treated water from which solid-liquid separation has been performed to remove dissolved selenium and iron hydroxide are separated. Solid-liquid separation into sludge as the main component.

[0028]

Examples 1 to 8 Total selenium concentration (Se) obtained by treating coal-fired exhaust gas with a wet flue gas desulfurization apparatus using limestone as an absorbent
Approximately 0.5 mg / liter of flue gas desulfurization effluent is used as raw water, and selenic acid (H ₂ SeO ₄ ) and selenious acid (H ₂ SeO ₃ ) are added to the raw water to eliminate analytical errors. 0.05 mg / liter of selenium concentration (as Se) derived from tetravalent selenium [selenate ion (SeO ₄ ²⁻ )] and selenium concentration (Se) derived from hexavalent selenium [selenite ion (SeO ₃ ²⁻ )]
A) 0.7 mg / liter sample wastewater was prepared.

2 liters of the sample waste water thus prepared was placed in a 2 liter reaction vessel equipped with a stirrer, a thermometer, a pH meter and the like, and an aqueous sodium hydroxide solution was added with stirring. The pH value was adjusted to the value shown, and a pretreatment was performed to remove the deposited precipitate by filtration to obtain a pretreated wastewater. Metals (Al, Si, Zn and
Se) concentration was determined by atomic absorption spectrometry. Table 1 shows the results
Shown in

Next, a 2 liter reaction vessel equipped with a stirrer, a thermometer, a pH measuring instrument and the like and containing 5 g of metallic iron powder was prepared, and the pretreated wastewater was transferred and charged into the reaction vessel. Under the conditions of the pH value and the reaction temperature of 40 ° C., and contacting the pretreated wastewater and metallic iron powder with stirring while introducing air to perform deselenization, and 3 hours after the start of the deselenization After 5 hours, the selenium concentration was measured, and the selenium removal rate of the treated water relative to the sample wastewater was determined.
Table 1 shows the results. Note that the effect of the pretreatment was also observed when air was not introduced into the reaction area for the selenium removal treatment.

Comparative Examples 1 to 4 Sample wastewater was treated in exactly the same manner as in each of the above Examples, except that the pretreatment and the de-selenium treatment were performed under the conditions shown in Table 1. With respect to the obtained treated water, the concentrations of metals (Al, Si, Zn, and Se) in the pretreated wastewater and the selenium removal rate of the treated water relative to the sample wastewater were determined in exactly the same manner as in the above examples. Table 1 shows the results.

[0032]

[Table 1]

As is clear from the results of each of the examples and comparative examples shown in Table 1, it is understood that a higher selenium removal rate can be achieved in each example than in the comparative example. .

[0034]

According to the present invention, even in the case of selenium-containing wastewater containing salts, a high selenium removal rate can be easily achieved while suppressing the generation of sludge as much as possible.
In particular, it is suitable for treating a large amount of selenium-containing wastewater such as flue gas desulfurization wastewater.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a preferred selenium-containing wastewater treatment step of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Stir-mixing tank, 1a ... Stirrer, 2 ... Solid-liquid separator, 3 ... Line mixer, 4 ... Reaction tank, 4a ... Stirrer, 4b ... Air inlet, 5 ... Iron-based metal, 6 ... Solid-liquid separator.

 ────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kazushige Kawamura 2-1-1, Tsurumichuo, Tsurumi-ku, Yokohama-shi, Kanagawa Prefecture, F-term in Chiyoda Kako Construction Co., Ltd. 4D038 AA08 AB70 AB81 BA04 BB13 BB18 BB20

Claims (5)

[Claims] 1. The wastewater containing salts and dissolved selenium contains Fe, M
n, Ni, and contacted with at least one iron-based metal selected from the group consisting of Cu, when performing a de-selenium treatment to remove the dissolved selenium in the wastewater by separating the precipitated precipitate solid-liquid, Prior to the de-selenium treatment, an alkali is added to the waste water to adjust the pH value of the waste water, and a pre-treatment for solid-liquid separation of the deposited precipitate is performed. A method for treating selenium-containing wastewater, which is performed under the following conditions. 2. The method for treating selenium-containing wastewater according to claim 1, wherein in the selenium-elimination treatment, an oxygen-containing gas is introduced into the reaction region. 3. The method for treating selenium-containing wastewater according to claim 1, wherein the pH value during the pretreatment is 6 to 10 and the pH value during the selenium removal treatment is 6 to 9. 4. The method for treating selenium-containing wastewater according to claim 1, wherein the pH is adjusted in the pretreated wastewater after the pretreatment and before the selenium treatment, or in the selenium treatment reaction zone. . 5. The method for treating selenium-containing wastewater according to claim 1, wherein the wastewater is flue gas desulfurization wastewater discharged from a flue gas desulfurization device.