JP3049851B2 - Treatment method for wastewater containing selenium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates relates to a method of treating selenium-containing wastewater, SeO ₄ difficult, especially removal by conventional methods
^{The present} invention relates to a method for treating wastewater containing selenium, which is effective for treating wastewater containing ^2- .

[0002]

2. Description of the Related Art Wastewater discharged in the treatment of electrolytic deposits of metals such as copper may contain selenium in a relatively high concentration. Treatment to remove selenium in the wastewater.

Generally, selenium is usually dissolved in the wastewater in the form of SeO ₃ ^2- (tetravalent selenium).
As O ₃ ^2-treatment method, conventionally, for example Japanese Patent Publication 48
There is one disclosed in Japanese Patent No. 30558. This method comprises adding Fe ²⁺ to wastewater containing SeO ₃ ^2-
Then, the addition of Cu ²⁺ in the pH range of 3 to 5 promotes the oxidation and hydrolysis of Fe ²⁺ by SeO ₃ ^2- and further reduces the selenium reduced in the pH range of 5 to 6. ^Is recovered together with Fe ³⁺ hydroxide. Other methods for treating SeO ₃ ^2- include:
A method is known in which one or two of water-soluble ions such as Ag ⁺ , Cu ²⁺ , and Fe ³⁺ or Fe powder are added and fixed as selenide.

However, this wastewater contains SeO.
₄ ^2- when (hexavalent selenium) coexist has low SeO ₄ ^2- in the reducing process by the method is extremely difficult. Therefore, methods as a method for processing these SeO ₄ ^2-, in strongly acidic solution, fixed SeO ₄ ^2-a, as metallic selenium by treatment with a strong reducing agent such as SO ₂ and organic acid salts Is conventionally known.

[0005] On the other hand, selenium is SeO ₃ ^2- and SeO ₄ ^2-
In the reaction of fixing wastewater dissolved in the form of a compound as a compound in a region of room temperature and weakly acidic to slightly acidic, Se is used.
It is necessary to change O ₃ ^2- and SeO ₄ ^2- to compounds having smaller solubility products.

Here, barium and SeO ₃ ^2- or Se
The compound of O ₄ ^2- has a solubility product of 1.0 in BaSeO ₃ .
It is known that both 0 × 10 ⁻⁷ and 3.16 × 10 ^{−8 for} BaSeO ₄ are small. Therefore, a method of applying barium to the wastewater treatment is being studied. In particular, S
In the treatment of eO ₄ ²⁻ , a method of immobilizing it as BaSeO ₄ is appropriate.
₂ are listed.

[0007]

However, among the conventional methods for treating wastewater containing SeO ₄ ^2-, the method using SO ₂ or an organic acid salt is insufficient for the reduction of SeO ₄ ^2-. The treatment efficiency was low, and the recovery of selenium was also low. Moreover, in handling chemicals such as SO ₂ and organic acid salts, sufficient care was required.

Further, in the method of fixing SeO ₄ ^2- as BaSeO ₄ by adding BaCl ₂ , SeO ₄ ^2-
At the same time, SO ₄ ²⁻ dissolved in the wastewater is also fixed as BaSO ₄ , so that BaCl ₂ is
In addition to the necessity of being added in an amount equal to or greater than the integrated content of O ₄ ²⁻ and SO ₄ ²⁻ , that is, in an amount equivalent to 1.0 to 1.2 times the number of moles, BaSeO ₄ Because of the low solubility product of BaCl ₂ , there was a problem that BaCl ₂ had to be added in an excessive amount in order to maintain the selenium concentration in the treated water below the effluent standard value.

It is an object of the present invention to provide a method for treating selenium-containing wastewater which can solve these problems.

[0010]

Hereinafter, a method for treating selenium-containing wastewater according to the present invention will be specifically described. In the method for treating selenium-containing wastewater of the present invention, S O ₃ ²⁻ and SeO ₄ ²⁻ are fixed as compounds having a small solubility product.
In the method of treating eO ₃ ^2- and SeO ₄ ^2- dissolved wastewater,
A metal salt to produce a sparingly soluble salt reacts with the selenium, SO ₄
^{A 2-} free soluble iron salt is added in two steps, and the selenium concentration of the treated water is reduced by maintaining the pH after the reaction within a range of 4.5 ± 0.5. It is characterized by:

In the method for treating selenium-containing wastewater of the present invention, first, as a first step, SeO ₃ ^2- and SeO _{4 are used.}
^In the treated raw water in which ^2- is dissolved, a metal salt such as BaCl ₂ or AgNO ₃ which reacts with selenium to form a sparingly soluble salt is mixed with SeO ₄ ^2- and SO ₄ ^2- dissolved in the treated raw water. Is added in a molar equivalent of 1.0 to 1.2 times the integrated content of, and the pH is maintained in a range of 4.5 ± 0.5 for 10 to 15 minutes. As a result, SeO ₃ dissolved in the treated raw water
^2- , SeO ₄ ^2- and SO ₄ ^2- react with the metal salt,
Each forms a hardly soluble salt having a solubility product of 1.0 × 10 ⁻⁷ or less.

Next, as a second step, a soluble iron salt not containing SO ₄ ²⁻ such as FeCl ₃ is added to the suspension containing the precipitate formed in the first step with the treated raw water. To the integrated content of SeO ₃ ^2- and SeO ₄ ^2- dissolved in water, added in a molar equivalent of 3 times, and further added a neutralizing agent to adjust the pH to a range of 4.5 ± 0.5. For 15-30 minutes. As a result, SeO ₃ ^2- remaining in the suspension
But the solubility product is 2.0 × 10 ^-31, with very poorly soluble is fixed as Fe ₂ (SeO _{3) 3} is a high salt, oxidation and hydrolysis of Fe ²⁺ by SeO ₃ ^2-promoting are, selenium caused by SeO ₃ ^2-of reduction F
Immobilized as adsorbed selenium for e ³⁺ hydrolysis products.

Then, as a third step, a small amount of a polymer flocculant is added to the suspension containing the mixed precipitate formed in the first step and the second step, and the precipitate in the suspension is removed. Agglomeration, sedimentation, sedimentation and sedimentation of sediment, and separation of clear overflow water.

Further, as a fourth step, the precipitate formed as a result of sedimentation and concentration is dehydrated and discharged out of the system as a cake containing a mixed salt of tetravalent selenium, hexavalent selenium and sulfuric acid.

Further, as a fifth step, sulfuric acid or sodium sulfate is added to the overflow water, and excess ions of the metal salt added in the first step are precipitated as sulfate, and then, an alkaline aqueous solution is added and discharged. After maintaining the water pH within the regulation value, a trace amount of suspended substances is separated by filtration in a sand filtration tank to clarify the filtered water, and discharged out of the system as treated water.

As a result, the selenium concentration in the treated water is maintained at or below the effluent standard value, and in particular, SeO ₄ ^2-, which is difficult to remove by the conventional method, can be effectively removed.

Here, FIG. 1 shows a flow sheet diagram of the wastewater treatment method of the present invention. In FIG. 1, the raw water for treatment and the metal salt are mixed and stirred in the first reaction tank 1 in a range of pH = 4.5 ± 0.5 for 10 to 15 minutes. PH with the iron salt and the neutralizing agent
The mixture is mixed and stirred within a range of 4.5 ± 0.5 for 15 to 30 minutes.

The suspension after the reaction treatment is slowly stirred with the flocculant in the flocculation tank 3 and then introduced into the sedimentation tank 4, where it is separated into sediment and overflow water.

The sediment is settled and concentrated in the sedimentation tank 4, continuously extracted from the bottom of the sedimentation tank 4 as a high-concentration suspension, and then dehydrated in the dehydrator 5. Discharged as a cake.

On the other hand, the waste water overflowing from the settling tank 4
After being introduced into the reaction tank 6 and mixed with sulfuric acid or sodium sulfate, the mixture is neutralized with an aqueous alkali solution in the neutralization tank 7, further filtered and separated from a trace amount of suspended solids in the sand filtration tank 8, and clarified. Is discharged out of the system as clean treated water.

[0021]

Next, examples of the present invention will be shown, and effects of the present invention will be described. Under the condition that the treatment according to the present invention is constantly maintained, treated raw water having the same water quality was treated using the following two types of methods, and the results were compared.

(1) Treatment by a conventional method (Comparative Example) Ferrous polysulfate, ferrous sulfate heptahydrate, copper sulfate 5
A total of five drugs, water salt, hydrazine sulfate, and iron powder,
The pH of each reaction solution was measured while adding each amount shown in Table 2 and mixing for 60 minutes. Thereafter, for each reaction solution, the formed precipitate was filtered, and the filtrate was used as treated water, and the selenium concentration thereof (however, the total amount of tetravalent selenium and hexavalent selenium) was measured.

(2) Treatment According to the Method of the Present Invention BaCl ₂ solutions having various concentrations as shown in Table 3 were added to the treated raw water in amounts shown in Table 3 and mixed for 15 minutes. FeCl ₃ which is .385
The solution was added at 2.5 (g / l) or 5.0 (g / l), and the pH of each reaction solution was measured while mixing for further 30 minutes. Thereafter, the precipitate formed was filtered from each reaction solution, and the filtrate was treated water to measure the selenium concentration (however, the total amount of tetravalent selenium and hexavalent selenium).

Table 1 shows the quality of the raw water used in the comparative examples and examples. Table 2 shows the chemicals used in the treatment method (1), the amounts added, and the pH and selenium concentration of the treated water, and the chemicals used in the treatment method (2), the amounts added, and the treated water. p
Table 3 shows the H and selenium concentrations. Here, treated raw water A was used for Examples 1 to 42 of Tables 2 and 3, and treated raw water B was used for Examples 5 to 7 of Table 3.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

Also, here, And

As is apparent from a comparison between Table 2 and Table 3,
In the selenium treatment method of the comparative example, the selenium removal rate was 20%.
When the selenium treatment method of the embodiment is used, the removal rate is as high as 96% to 99% or more. It is possible to reduce the concentration below the emission standard value. In the examples, 79% to 89% of the dissolved selenium in the treated raw water is SeO _4.
Despite being ^2-, it shows high selenium removal rate. Therefore, the use of the present invention enables effective treatment of wastewater particularly containing a large amount of SeO ₄ ^2- .

[0030]

As described above, in the selenium treatment method of the present invention, the selenium removal rate is extremely high.
There is almost no effect on the environment etc. due to the discharge of treated water.
It is possible to remove wastewater containing SeO ₄ ^2- which is difficult to remove by the conventional method with extremely high efficiency. Furthermore,
Since the amount of addition of a metal salt such as BaCl ₂ is smaller than that of the conventional method, the metal salt can be saved, and the medicine can be easily handled.

[Brief description of the drawings]

FIG. 1 is a flow sheet diagram showing the steps of the wastewater treatment method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st reaction tank 2 2nd reaction tank 3 Coagulation tank 4 Sedimentation tank 5 Dehydrator 6 3rd reaction tank 7 Neutralization tank 8 Sand filtration tank

Continuation of the front page (56) References JP-A-2-298989 (JP, A) JP-A-59-164639 (JP, A) JP-A-59-99895 (JP, A) JP-B-45-12003 (JP, A) , B1) (58) Field surveyed (Int. Cl. ⁷ , DB name) C01B 19/00 C02F 1/58

Claims (1)

(57) [Claims] 1. A metal salt which reacts with selenium to form a sparingly soluble salt in treated raw water in which SeO ₃ ^2- and SeO ₄ ^2- are dissolved in an amount equivalent to the total amount of each of the dissolved ions. To form a mixed salt of SeO ₃ ^2- , SeO ₄ ^2- , and SO ₄ ^2- with the metal salt. After the reaction, a soluble iron salt containing no sulfate ion is added, and the pH is adjusted to 4 0.0 to 5.5, and the remaining SeO ₃ ^2- was fixed as Fe ₂ (SeO ₃ ) ₃ and a co-precipitate of the produced iron salt and selenium. A method for treating selenium-containing wastewater, comprising reducing a selenium concentration in treated water by separating a sediment from a liquid phase as a solid phase.