JP2005177647A - Method and apparatus for removing selenium in wastewater - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a treatment system developing an excellent selenium removing effect even with respect to wastewater having a low concentration of selenium. <P>SOLUTION: This removing apparatus of selenium in wastewater has an electrolytic cell (a) having electrodes wherein a ferrous material is used at least in an anode and applying a voltage of 8-20 V across the electrodes to pass a current through selenium containing wastewater in the electrolytic cell, a means (b) for adding sulfuric acid to the selenium-containing wastewater to adjust the pH of the watewater to 1-2.5, a pH measuring means (c) for measuring the pH of the wastewater, a storage tank (d) for storing the wastewater drawn out of the electrolytic cell after energization, a means (e) for adding alkali to the storage tank to neutralize the wastewater, a pH measuring means (f) of the neutralized wastewater and a means (g) for separating the sediment drawn out of the electrolytic cell and the storage tank to dehydrate the same and is adapted to a selenium removing method not only to adjust the pH of the wastewater to 2.5 or below but also to apply the voltage of 8-20 V across the electrodes to coprecipitate selenium along with an iron hydroxide sediment to preferably reduce the concentration of selenium in the wastewater to 0.01 mg/L or below. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a selenium removal method and a removal system that can effectively remove selenium contained in wastewater and reduce the selenium concentration in the wastewater below an environmental standard.

  Selenium is widely used in glass decolorizers, colorants, pigments, semiconductor materials, battery materials, and the like, and is contained and discharged in factory wastewater. In addition, selenium may be contained in the flue gas desulfurization effluent of a thermal power plant. Since this selenium is a highly toxic environmental pollutant, the amount of selenium in the wastewater is strictly regulated.

Usually, selenium in the waste water is dissolved in the form of selenate ions (SeO ₄ ²⁻ ) and selenite ions (SeO ₃ ²⁻ ). Conventionally, as a method for removing selenium in this wastewater, ferrous salt and copper salt are used in combination, and selenium ion or selenite ion is reduced by iron ion under the catalytic action of copper salt. A method is known in which precipitation is performed and coprecipitation is performed together with the iron hydroxide precipitation that is generated (Patent Document 1). Also known is a method in which selenium-containing wastewater is passed through an iron-based metal packed bed, selenate ions or selenite ions are reduced by iron ions, and this is neutralized to co-precipitate selenium with iron hydroxide. (Patent Document 2, Patent Document 3, Patent Document 4). Furthermore, instead of such a method of passing water through the iron-filled layer, an electrolytic removal method is also known in which selenium-containing wastewater is energized using an iron-based electrode or the like and selenium is co-precipitated together with iron hydroxide precipitation ( Patent Document 5, Non-Patent Document 1).
Japanese Patent Publication No. 48-30558 Japanese Patent No. 3240940 (Japanese Patent Laid-Open No. 10-128343) Japanese Patent Laid-Open No. 11-207364 JP 2003-181467 A JP-A-5-147907 "Resources and Materials" 117, 2001, No.1, pp. 63-71

  Since the method of Patent Document 1 is intended to recover selenium from wastewater, it is effective for wastewater having a relatively high selenium concentration. However, the selenium concentration is relatively low at 15 mg / L or less. In the case of low drainage, there is an aspect that the effect of removing selenium is not sufficient for a large amount of starch. Moreover, since the removal method of patent document 2 and patent document 3 tends to leave a starch in the packed layer of iron-based material, the treatment effect is greatly reduced in a short period of time, and it is troublesome to replace or clean the iron-based material. There is a problem that it takes. In the method of Patent Document 4, with regard to wastewater in which sulfate ions and selenium ions coexist, after sulfate ions are precipitated as sparingly soluble salts, iron powder is added to reduce selenium ions, and composite precipitation of metal selenium or iron selenate is performed. Although it is a method of removing by removing it, it is difficult to proceed with the reduction of selenium when wastewater contains more sulfate ions than selenium, so it is necessary to remove sulfate ions as necessary. The operation is complicated.

  On the other hand, since the electrolytic removal method of Patent Document 5 does not use an iron-based material packed layer, there is an advantage that there is no need to replace or wash it, but the selenium concentration is 15 mg / day as in the previous method. In the case of wastewater having a relatively low selenium concentration of L or less, there is an aspect that the effect of removing selenium is not sufficient. In the method of Non-Patent Document 1, the anode is isolated so as not to elute into the electrolytic solution, carbon particles are used for the cathode, and selenium is deposited on the surface. In this case, selenium on the carbon surface is redissolved in acidic waste water, so it is difficult to reduce the selenium concentration below the environmental standard, for example, 0.01 mg / L or less.

  The selenium concentration in the wastewater determined by the environmental standards tends to become stricter, and there is a need for a treatment method that reliably reduces the selenium concentration to a level lower than the environmental standards even for a relatively low selenium concentration. The present invention solves the above-mentioned problems in the conventional selenium removal method, and provides a treatment method capable of reducing the selenium concentration to an environmental standard or lower even for wastewater having a low selenium concentration and coexisting sulfate ions. .

According to the present invention, the following selenium removal method and removal apparatus are provided.
(1) In a method in which selenium-containing wastewater is energized to co-precipitate selenium together with iron hydroxide precipitates to remove selenium from the wastewater, at least the anode is made of iron-based material and the pH of the wastewater is adjusted to 2.5 or lower. A method for removing selenium in waste water, comprising energizing at a voltage of 8 to 20 V to co-precipitate selenium together with iron hydroxide precipitation.
(2) Adjust the selenium concentration to 15 mg / L or less to pH 1 to 2.5, and energize it at a voltage of 8 to 20 V to coprecipitate selenium with iron hydroxide precipitation, thereby reducing the selenium concentration to 0.01 mg / L. The selenium removal method according to (1), wherein the selenium removal method is reduced to L or less.
(3) (a) An electrolytic cell that has an electrode using an iron-based material at least as an anode and energizes the selenium-containing wastewater in the electrolytic cell at a voltage of 8 to 20 V, and (b) sulfuric acid is added to the selenium-containing wastewater. Means for adjusting the pH to 1 to 2.5, (c) means for measuring the pH of the waste water, (d) a storage tank for collecting waste water extracted from the electrolytic cell after energization, and (e) drainage by adding alkali to the storage tank. An apparatus for removing selenium, comprising: means for neutralization; (f) means for measuring the pH of this waste water; and (g) means for separating and dewatering starch extracted from the electrolytic cell and the storage tank.

[Detailed Description of the Invention]
The method for removing selenium according to the present invention is a method of removing selenium from drainage by co-precipitating selenium together with iron hydroxide precipitation by energizing selenium-containing wastewater, and at least using an iron-based material for the anode, and adjusting the pH of the wastewater to 2. The selenium is co-precipitated with iron hydroxide precipitation by adjusting to 5 or less and energizing at a voltage of 8 to 20V.

  Electricity is supplied to the drainage by adjusting the drainage to pH 2.5 or lower, preferably pH 1 to 2.5, more preferably pH 1.8 to 2.2, and voltage 8-20V, preferably voltage 15-17V. . When the pH of the wastewater is about 4 or more as in the conventional treatment method, it takes time to remove selenium contained in the wastewater, and in particular, selenium is sufficiently removed in wastewater having a low selenium concentration of 15 mg / L or less. difficult. On the other hand, if the pH is lower than 1, it is necessary to use an acid-resistant material for the components of the processing apparatus, which is not practical. Also, if the voltage is lower than 8V, it takes time to remove selenium in the waste water. On the other hand, if the voltage is higher than 20V, a short circuit occurs when a good conductor such as metal scraps enters between the electrodes, and the circuit is likely to be damaged. So it's not practical. The acid added to the waste water is preferably sulfuric acid. When hydrochloric acid is used, a large amount of chlorine gas or the like is generated, and nitric acid is not suitable because nitrogen oxide gas is generated. Also,

The electrode is made of an iron-based material at least for the anode. The cathode is not limited to an iron-based material, and may be a general electrode material. As the iron-based material, various steel materials such as alloy iron and carbon steel can be used in addition to metallic iron. By using a ferrous material anode, when the drainage is energized, divalent iron ions (Fe ²⁺ ) are eluted from the anode into the wastewater, and selenate ions (SeO ₄ ²⁻ ) and selenite in the wastewater. Acid ions (SeO ₃ ²⁻ ) are reduced to precipitate selenium. Moreover, precipitation of iron hydroxide Fe (OH) ₂ is formed by divalent iron ions. Furthermore, since selenium ions in the waste water are attracted around the anode by energization, the selenium concentration becomes relatively high, and selenic acid and selenous acid aggregate at a high concentration. The reduced selenium and agglomerated selenate and selenite can be incorporated into the iron hydroxide precipitate and co-precipitated together and removed from the waste water.

  The above removal method of the present invention is excellent in the effect of removing selenium in waste water, and particularly for waste water having a low selenium concentration of 15 mg / L or less, the selenium concentration can be reduced to 0.01 mg / L or less. It can be applied to strict environmental standards. In the conventional treatment method, it is difficult to further reduce the selenium concentration to 0.01 mg / L or less for the wastewater having such a low selenium concentration. Moreover, the selenium removal method of the present invention can effectively remove selenium from wastewater containing sulfate ions, and the treatment process is simple and excellent in practicality.

  An apparatus configuration example of the removal system of the present invention is shown in FIG. As shown in the figure, the removal system of the present invention includes an electrolytic cell 10 into which selenium-containing wastewater is introduced, a pipe 11 for supplying selenium-containing wastewater to the electrolytic cell 10, and means for adding sulfuric acid to the selenium-containing wastewater (sulfuric acid Supply line) 12 is provided. The electrolytic cell 10 is provided with an electrode 13 using at least an iron-based material for the anode. The cathode is not limited to an iron-based material and may be a general electrode material. The electrolytic cell 10 is provided with a means (pH meter) 14 for measuring the pH of the waste water in the tank and a stirring means 15. The electrolytic cell 10 also serves as a thickener, and is connected to a discharge pipe 20 for extracting starch from the bottom of the cell.

  A storage tank 16 is connected to the electrolytic cell 10 for collecting waste water extracted from the electrolytic cell 10 after energization. The storage tank 16 is provided with means (alkali supply pipe) 17 for neutralizing waste water in the tank by adding alkali, pH measurement means (pH meter) 18 for waste water, and stirring means 19. Further, the storage tank 16 also serves as a thickener, and is connected to a discharge pipe 20 for extracting starch from the tank bottom. Further, the discharge pipe 20 is provided with means 21 for separating and dewatering the extracted starch.

In the above removal system, the selenium-containing wastewater is introduced into the electrolytic cell 10 through the pipe 11. Sulfuric acid is added to the selenium-containing wastewater through the pipe 12 to adjust the pH of the wastewater to 2.5 or less, preferably pH 1 to 2.5, more preferably pH 1.8 to 2.2. After the addition of sulfuric acid, the waste water is stirred by the stirring means 15 to make the pH of the waste water uniform. The pH-adjusted wastewater is energized through the electrode 13 at a voltage of 8 to 20 V, preferably 15 to 17 V. This energization causes divalent iron ions (Fe ²⁺ ) to elute from the anode into the wastewater, reducing the selenium ions (SeO ₄ ^2- ) and selenite ions (SeO ₃ ^2- ) in the waste water. Precipitate. Furthermore, a precipitate of iron hydroxide [Fe (OH) ₂ ] is formed by the divalent iron ions. Moreover, since selenium ions in the waste water are attracted around the anode by energization, the selenium concentration becomes relatively high, and selenic acid and selenous acid aggregate at a high concentration. Reduced selenium and agglomerated selenate and selenite are incorporated into this iron hydroxide precipitate and coprecipitate together. This starch is withdrawn from the bottom of the tank through the discharge line 16.

  The energization time in the electrolytic cell is about 30 to 60 minutes, and the electrolysis may be terminated when the concentration of divalent iron ions generated by electrolysis reaches 1500 mg / L. When the pH of the electrolytic solution becomes high during electrolysis, sulfuric acid is added to adjust the pH of the electrolytic solution to the above range.

  After energization, drainage is extracted from the electrolytic cell 10 and guided to the storage tank 16. Since the waste water extracted from the electrolytic cell 10 is strongly acidic, alkali such as lime or caustic soda is added to this waste water through the pipe line 17 and mixed by the stirring means 19 to neutralize the waste water. Since iron hydroxide ions remain in the wastewater, iron hydroxide starch is produced by neutralizing the wastewater. Neutralize pH to the drainage standard, and discharge wastewater with reduced iron concentration in the drainage system. On the other hand, the starch precipitated on the tank bottom is extracted through the discharge pipe 20.

  The starch extracted through the discharge pipes 16 and 20 of the electrolytic cell 10 and the storage tank 16 is solid-liquid separated by the dehydrating means 21 and dehydrated. Selenium contained in the solid content is reduced and hardly re-eluted, so that it can be treated as general waste.

  Using the removal system shown in FIG. 1, selenium in the waste water was removed according to the treatment conditions shown in Table 1. The results are shown in Table 1. In all the methods (A1, A2, A3) of the present invention, the selenium concentration in the waste water is reduced to 0.01 mg / L or less, but those outside the treatment conditions of the present invention (B1, B2, B3) The removal effect is low.

Device configuration diagram of selenium removal system of the present invention

Explanation of symbols

  10-electrolysis tank, 11-drainage supply line, 12-sulfuric acid supply line, 13-electrode, 14-pH meter, 15-stirring means, 16-storage tank, 17-alkali supply line, 18-pH meter, 19 -Agitation means, 20-discharge line, 21-dewatering means

Claims (3)

  In the method of energizing selenium-containing wastewater and co-precipitating selenium together with iron hydroxide precipitation to remove selenium from the wastewater, at least the anode is made of iron-based material and the pH of the wastewater is adjusted to 2.5 or less, A method for removing selenium in waste water, comprising energizing at a voltage of 20 V to co-precipitate selenium together with iron hydroxide precipitation.   Waste water with a selenium concentration of 15 mg / L or less is adjusted to a pH of 1 to 2.5, and the selenium concentration is reduced to 0.01 mg / L or less by energizing at a voltage of 8 to 20 V to co-precipitate selenium with iron hydroxide precipitation. The selenium removal method according to claim 1, wherein the selenium removal method is reduced. (B) An electrolytic cell having at least an electrode using an iron-based material for the anode and energizing the selenium-containing wastewater in the electrolytic cell at a voltage of 8 to 20 V, and (b) adding sulfuric acid to the selenium-containing wastewater to adjust the pH. Means for adjusting to 1 to 2.5; (c) means for measuring the pH of the wastewater; (d) a storage tank for collecting wastewater extracted from the electrolytic cell after energization; and (e) neutralizing the wastewater by adding alkali to the storage tank. And (f) means for measuring the pH of the waste water, and (g) means for separating and dewatering the starch extracted from the electrolytic cell and the storage tank.

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