JPH08267076A - Treatment of selenium-containing waste water - Google Patents

Abstract

PURPOSE: To remove selenium(VI) with the absolute minimum amt. of iron by dissolving iron(II) ion in a waste water contg. selenium, heating the liq., adding an alkaline agent to neutralize the liq. at specified temp. and pH and separating the obtained precipitate from liq. CONSTITUTION: An iron(II) ion is dissolved in a treated liq. 1 as the agent 2 for reducing and fixing selenium(IV) and selenium(VI) ion. The liq. 1 is then heated to >=30 deg.C, and an alkaline neutralizer 3 is added to the liq. 1 in a tank isolated from air or in a nonoxidizing atmosphere to regulate the liq. to pH8 to 10. In this case, a conventional alkaline agent is used as the neutralizer 3. The reacted liq. 1 is separated from solid, and the filtrate is controlled to about pH7 and discharged. A selenium ion reducing tank consisting of a waste water receiving and regulating tank I, an atmosphere-shielding reaction tank II and a treated liq. receiving tank III is used in the treatment of such a waste water contg. selenium.

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 an effluent containing selenium, and more specifically to a selenium value after treatment of 0.
The present invention relates to a method for treating selenium-containing wastewater, which is characterized by removing up to 1 mg / l or less.

[0002]

2. Description of the Related Art Heretofore, as a method for removing selenium from waste water, an iron powder replacement treatment method, an adsorption treatment method using iron hydroxide (trivalent), etc. have been known.

Furthermore, in August 1993, a part of the Enforcement Ordinance of the Water Pollution Control Law was revised, and the drainage standard for selenium was 0.1.
Although it was specified as mg / l or less, there are many industries that are extremely difficult to handle, and the provisional standards up to January 1997 have been applied, and development of highly efficient processing technology is awaited.

In the above-mentioned conventional iron powder replacement treatment method, Se
O ₃ ^2- : selenite ion (hereinafter referred to as tetravalent selenium)
It was possible to remove almost completely (0.1 mg / l or less), but the removal ability of SeO ₄ ²⁻ : selenate ion (hereinafter referred to as hexavalent selenium) was poor. There was a problem.

Further, the adsorption treatment method using iron hydroxide (trivalent) is a method which can be applied only at a trace amount of concentration that can be adsorbed. Therefore, in the above-mentioned conventional methods, the concentration is relatively high. There was a problem with wastewater treatment containing high hexavalent selenium (several mg / l to several tens of mg / l or more).

However, in recent years, the following useful methods have been proposed as a method for removing hexavalent selenium.

Andrew P. Murphy (.Ind. Eng. Chem. Re
1988. 27. p.187-191) report that hexavalent selenium can be reduced to elemental selenium by reacting it with iron hydroxide (divalent) under alkaline conditions.

RH Lien et al. (EPD Congress'
90: The Minerals, Metal & Materials Society, 199
0) conducted a test of the above method on the liquid in a tailing sedimentation tank of an active mine and reported the results. According to this, in order to remove selenium containing 4 ppm up to around 0.1 ppm, at least 200 times the stoichiometric reaction amount (1 equivalent is F
e / Se weight ratio of 4.2 to 6.3) or more is required,
It is poorly reactive, which suggests that it requires a tremendous amount of iron.

[0009]

As described above, although tetravalent selenium can be removed by the conventionally known method, it is difficult to remove hexavalent selenium to 0.1 mg / l or less. It was In addition, iron hydroxide (2
Hexavalent selenium can also remove hexavalent selenium to a low concentration, but the reactivity is poor and the amount of iron required is enormous, which is an unrealistic method.

The present invention dramatically improves the reactivity of the latter reduction treatment method using iron hydroxide (divalent) among these methods, so that the required minimum amount of iron can be efficiently used. 6
The purpose is to enable removal of selenium valence up to 0.1 mg / l or less.

[0011]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to solve the above-mentioned problems. As a result, iron ions (divalent) are added to selenium-containing wastewater under weak acidity, followed by rapid neutralization treatment. In the step of crystallizing iron ions as iron hydroxide (divalent) by subjecting the solution to a reaction and reacting the solution in the air, the phenomenon was found to improve by maintaining the solution temperature at least 30 ° C or higher. .

Further, by carrying out the above-mentioned reaction in an atmosphere of non-oxidizing gas instead of in the atmosphere, the reactivity can be further improved and ensured, and the method of the present invention is provided. I was able to.

That is, according to the present invention, firstly, the first step of dissolving divalent iron ions in the waste water containing selenium, and then the liquid temperature of the first step liquid is heated to 30 ° C. or higher in the atmosphere. A method for treating selenium-containing wastewater, which comprises a second step of solid-liquid separating the precipitate obtained by neutralizing to pH 8 to 10 by adding an alkaline agent while maintaining the second step. ,
The first to dissolve divalent iron ions in wastewater containing selenium
Alkali agent is added to the process and then the first process liquid in a tank in which air is shut off or in a non-oxidizing gas atmosphere.
Second, solid-liquid separation of the precipitate obtained by neutralizing to H8-10
A method for treating selenium-containing wastewater, comprising: a third step, a first step of dissolving divalent iron ions in the selenium-containing wastewater, and a liquid temperature of the first step liquid. In a tank with air shut off or in a non-oxidizing gas atmosphere, while keeping the temperature at room temperature or higher, while adding an alkali agent to neutralize the pH to 8 to 10 The present invention provides a method for treating selenium-containing wastewater, which comprises a second step of separating.

[0014]

The object of the present invention is to remove selenium in effluent, particularly hexavalent selenium, which has been considered to be difficult to remove in the past, up to 0.1 mg / l or less of the standard value of wastewater. It is the establishment of a removal method.

In the method of the present invention, in the first step, first, divalent iron ions to be Fe (OH) ₂ are dissolved in the treatment liquid as a reducing and fixing agent for tetravalent and hexavalent selenium ions.
In this case, the iron ion source is at least one of iron sulfate or iron chloride. At this time, for the purpose of enhancing the dissolution efficiency as ions, one of sulfuric acid or hydrochloric acid as a mineral acid is added to the treatment liquid to adjust the pH. Is preferably adjusted to 6 or less.

The amount of divalent iron ions added for the reaction is about twice the equivalent of the reaction, and the weight ratio (Fe
/ Se) is 8 to 12 times by weight.

Next, in the second step, the treatment liquid after the first step is subjected to a heating treatment and an alkali neutralizing agent is added to adjust the pH to 8 to 10. In this case, as the alkaline neutralizing agent, a general-purpose alkaline agent such as caustic soda or slaked lime is used, and the reason why the pH at the end of neutralization is 8 to 10 is as follows.
This is because all the divalent iron ions in the treatment liquid are crystallized as iron hydroxide, and because the neutralization is for a short time as described below, the range of the neutralization end point is set wide and the treatment operation is easy. It is for the purpose of doing.

Further, the neutralization time (the time from the start of neutralization until the set pH is reached) is the activity F at the initial stage of crystallization under the reaction conditions.
For the purpose of supplying e (OH) ₂ all at once, it is essential to carry out rapid neutralization for a short time. In terms of time, it is within 5 minutes, preferably within 3 minutes from the start of neutralization to the end of neutralization. It is so effective.

The temperature of the treatment liquid which has been further heated,
That is, if the reaction temperature is a reaction in an air atmosphere as shown in FIG. 2, an improvement in reactivity is recognized at 30 ° C. or higher,
It becomes remarkable at 45 ° C or higher.

Further, if the reaction is carried out under the condition that the atmosphere is shut off, the effect is already recognized even at room temperature.
It was confirmed that the temperature was drastically improved at 0 ° C or higher.

This means that, in the present example, the residual hexavalent selenium concentration after treatment at room temperature (25 ° C. ± 3 ° C.) in the atmosphere was about 30 mg / l, whereas in the non-oxidizing gas atmosphere it was 4
The residual hexavalent selenium concentration when heated to 5 ° C for reaction is 1.3 mg / l, which is about 1% compared to the above treatment method.
It becomes / 23, and the remarkableness of the reactivity improvement can be understood.

The reaction time is shown in FIG.
As shown in Table 2 and Table 2, the equilibrium was reached in about 1 hour in the reaction in the air atmosphere, and it was only slightly lower than the value in 30 minutes. Therefore, when the reaction was performed in the air atmosphere, A reaction time of 30 minutes to 1 hour is sufficient.

On the other hand, in the reaction in a nitrogen gas atmosphere with the atmosphere shut off, it is considered that unreacted Fe (OH) ₂ is stable in the reaction system, and the reaction capacity does not deteriorate with time. For example, hexavalent selenium can be reduced to 0.1 mg / l or less in 2 hours at 35 ° C. and 1 hour at 45 ° C. Therefore, when the reaction is performed by a treatment method that shuts off the atmosphere, for example, one hour at 45 ° C. is one guideline.

After the reaction, the treated liquid is subjected to solid-liquid separation using a filter press or the like, the pH of the filtrate is adjusted to around 7 and then discharged. In this case, the precipitation and filtration properties of the reaction product became good from the reaction temperature of 35 ° C., and at 40 ° C. or higher, the use of a flocculant was not necessary at all.

As means for cutting off contact with the atmosphere, any non-oxidizing gas such as an inert gas or a reducing gas can be used in addition to the above-mentioned nitrogen gas.
As other means, a method of forming a layer of a solvent or the like on the surface of the treatment liquid, a method of closing the atmospheric atmosphere by filling the reaction tank with a sealed type, and further, as shown in FIG. The space between the liquid surface and the liquid surface may be narrowed as much as possible, and the above-mentioned gas may be used only when necessary.

Hereinafter, the present invention will be described in detail with reference to Examples, but the scope of the present invention is not limited to these.

[0027]

[Example 1] As a stock solution, total selenium was added at a pH of 3.5
0 mg / l (tetravalent selenium = 60 mg / l, hexavalent selenium = 60 mg /
l) Treatment was performed under the conditions shown below with the contained drainage as the target liquid.

Execution conditions: Treatment liquid amount: 1 liter (magnetic stirrer stirring using a tall beaker) Atmosphere: Two types under atmospheric atmosphere and nitrogen gas reaction temperature: 25, 30, 35, 40, 45, Fifty, five
5,60,70 ° C (use of constant temperature bath) Neutralization time: Within 3 minutes for all tests (Time to reach set pH) Reaction time: 30 minutes for all tests (Retention time after reaching set pH) Reaction pH: All Test pH = within 9.0 to 9.3 Filtration time: Within 7 minutes for all tests (natural filtration with No. C filter paper) Reagents used: sulfuric acid, caustic soda Fe ²⁺ Source: iron sulfate (divalent) Nitrogen flow rate: 800cc / min Take 17 samples of the above stock solution, add ferrous sulfate so that the divalent iron ion concentration becomes 1000 mg / l, and subject them to the test in each atmosphere and temperature to show the selenium grade of the obtained filtrate. 1 and the relationship between the reaction temperature and the concentration of hexavalent selenium remaining in the filtrate are shown in FIG.

[0029]

[Table 1]

From the above results, in the conventional atmosphere,
It can be seen that the reactivity is improved stepwise at 30 ° C. or higher, and further at 45 ° C. or higher in the case of the reaction in the air atmosphere, as compared with the condition of room temperature (25 ° C. and 30 ° C. correspond in this test).

Further, in a test avoiding atmospheric contact by substitution with nitrogen gas, improvement in reactivity was already recognized even at room temperature, and it became dramatic at 30 ° C. or higher, and at 70 ° C., 0.1 mg / s of hexavalent selenium was also contained. It was disassembled to l or less.

[0032]

[Example 2] Exactly the same stock solution and conditions as in Example 1,
The change in reactivity with time was examined. The types of tests are as follows.
It is a kind.

Test type: Reaction time of 30 minutes, 60 minutes at 25 ° C. in air atmosphere,
Investigation of change over time with 3/120 samples. The reaction time is the same as above at 35 ° C. under a nitrogen gas atmosphere. The reaction time is the same as above at 45 ° C. in a nitrogen gas atmosphere.

In this example, after the treatment, the filtration residue was dissolved in pure water to make 1 liter, and the solution was leached and filtered at pH = 4 by adding sulfuric acid, and the divalent iron ion concentration in the obtained filtrate was also examined. It was

Table 2 shows the results, and FIG. 3 shows the relationship between the reaction time and the concentration of hexavalent selenium remaining in the filtrate.

[0036]

[Table 2]

From the above results, the reaction reached equilibrium in about 1 hour in the air atmosphere, but in the system in which the contact with the air atmosphere was cut off, the reaction ability was persistent, and the reaction capacity was 2 hours at 35 ° C. , 45 ° C., all hexavalent selenium (<0.
1 mg / l) was processed.

The deterioration of the reaction ability in the atmosphere is shown in Table 2.
Considering from the unreacted Fe ²⁺ concentration shown in, it is assumed that the Fe (OH) ₂ involved in the reduction reaction reacts with the atmosphere and changes to hematite or magnetite because it is a reaction in the alkaline region. To be done.

In addition, F in the 60-minute reaction of the test type
Considering the amount of e ²⁺ used, the stoichiometric amount of the reaction is 1
Reported value of ndrew P. Murphy (Fe / Se weight ratio 4.2
.About.63), and the intermediate value is 5.23, the 1 × equivalent Fe ²⁺ amount in this example is 120 (mg · Se / l) × 5.25 = 630.
It is mg / l. Therefore, the amount of Fe ²⁺ actually added is 100
Since it is 0 mg / l, the amount of Fe ²⁺ required for complete removal of selenium is considered to be 1000/690 = 1.6 times that of the reaction.
Approximately 1/125 compared to 200 times reported by RH Lien et al.
It will be understood that the effect is enormous.

Furthermore, if the unreacted Fe ²⁺ shown in Table 2 is repeatedly used in the previous step, the required Fe ²⁺ amount can be almost the stoichiometric amount of the reaction, and it is possible to reduce the generation amount of the substance. Also contributes greatly.

[0041]

[Comparative Example 1] With exactly the same stock solution and working conditions as in Example 1,
The treatment was carried out at 25 ° C. in the atmospheric condition of the conventional conditions, and the obtained filtrate was treated again in the same manner.

Table 3 shows the selenium concentrations in the filtrates of the first treatment and the second treatment.

[0043]

[Table 3] As described above, the treatment was repeated twice, but complete removal was impossible. Repeated treatments were required to reduce the wastewater standard to 0.1 mg / l or less.

[0044]

As described above, according to the method of the present invention, hexavalent selenium, which has conventionally been impossible to be removed by performing temperature control in a liquid in which divalent iron ions are dissolved or treatment under an air-shielded condition, is also possible. It is intended to provide an excellent treatment method capable of performing batch treatment like tetravalent selenium.

[Brief description of drawings]

FIG. 1 is a schematic view showing an atmosphere-blocking selenium ion reduction reaction tank in the method of the present invention.

FIG. 2 is a drawing showing the temperature dependence of the reaction in Example 1.

FIG. 3 is a drawing showing the time dependence of the reaction in Example 2.

[Brief description of reference numerals]

I Wastewater receiving adjustment tank II Atmosphere-blocking reaction tank III Post-treatment liquid receiving tank 1 Wastewater 2 Iron salt, acid 3 Alkaline neutralizer (by pH control) 4 Non-oxidizing gas (Use when reducing tank pressure and purging tank) ) 5 Mechanical shutoff type mechanical seal 6 Gas vent valve 7 Liquid seal type discharge pipe 8 Live steam (by temperature control)

Claims (3)

[Claims] 1. A first step of dissolving divalent iron ions in waste water containing selenium, and then adding an alkali agent while maintaining the liquid temperature of the first step liquid at 30 ° C. or higher. A second step of solid-liquid separating the precipitate obtained by neutralizing to pH 8 to 10, and a method for treating selenium-containing wastewater. 2. A first step of dissolving divalent iron ions in waste water containing selenium, and then adding an alkaline agent to the liquid of the first step in a tank in which air is shut off or in a non-oxidizing gas atmosphere. , PH 8 to 1
A second step of solid-liquid separating the precipitate obtained by neutralizing to 0, and a method for treating selenium-containing wastewater. 3. A first step in which divalent iron ions are dissolved in waste water containing selenium, and then the liquid temperature of the first step liquid is kept at room temperature in a tank with air shut off or in a non-oxidizing gas atmosphere. Or a second step of solid-liquid separating the precipitate obtained by neutralizing the pH to 8 to 10 by adding an alkaline agent while keeping the temperature above room temperature, and Processing method.