JPH08155461A - Method and apparatus for removing nitric-and/or nitrous-nitrogen - Google Patents

Abstract

PURPOSE: To provide a technical way to conveniently and economically decompose nitric-nitrogen etc., and to solve a problem caused by a conventional electrochemical decomposition technique that nitric-and/or nitrous-nitrogen is reduced and decomposed by a cathode, but since the reaction in an anode, an opposite electrode to the cathode, is a highly electric power-consuming oxygen-producing reaction, the economical decomposition can not be carried out. CONSTITUTION: An object solution for treatment in which nitric-nitrogen is dissolved is supplied to a cathode chamber 4, nitric-nitrogen is converted into gases, and together with hydrogen produced by hydrolysis of water, these gases are circulated to an anode chamber 3 through a circulating line 9 to change an electrolytic reaction in a gas diffusion anode 5 from a highly electric power- consuming oxygen-producing reaction to a low electric power-consuming reaction. Consequently, nitric-nitrogen etc., can be decomposed and removed economically.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for removing nitric and / or nitrite nitrogen components present in a liquid used in the field of wastewater treatment and the like.

[0002]

2. Description of the Related Art In recent years, prevention of pollution of water, air, etc. and improvement of environmental hygiene have emerged as the most important technical problem. Nitrate nitrogen and nitrite nitrogen components contained in tap water and waste water are strictly regulated because they show mutagenicity when taken up in the living body. Biological treatment (for example, “Industrial water”, No. 419, page 37), electrodialysis and the like have been known as methods for decomposing and removing the nitrate nitrogen and the like from water.

In addition to this, attempts to electrochemically decompose and remove it have been reported before, and have been attracting attention as one of the effective removal methods. The nitric acid and nitrite nitrogen are converted into nitrogen and ammonia by electrolytic reduction at the cathode, and NOx such as NO ₂ and N ₂ O are by-produced as an intermediate product.
Decomposition of nitrate nitrogen and the like by this electrolytic reduction has a drawback that the solution resistance is high and that the anodic reaction is an oxygen generation reaction, so that power consumption is large. If the diaphragm is not used, the cathode product is oxidized at the anode and nitrate ions and the like are produced as a by-product.

[0004]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned drawbacks of the conventional nitric acid and nitrite nitrogen decomposing technology, and provides a method and apparatus for decomposing nitric acid and nitrite nitrogen simply and with low power consumption. The purpose is to provide.

The method of the present invention is selected from a gas-liquid permeable porous electrode, a three-dimensional electrode and a fluidized bed electrode as a cathode of an electrolytic cell divided into an anode chamber and a cathode chamber by a diaphragm. In a method for removing nitric acid and / or nitrite nitrogen by reducing nitric acid and / or nitrite nitrogen at the cathode by using an electrode, a gas diffusion electrode is used as an anode, and mainly hydrogen gas is oxidized. A method for removing nitric and / or nitrite nitrogen, which is characterized by carrying out a reaction. The apparatus of the present invention can be applied to this method and can further decompose the generated nitric oxide.

The present invention will be described in detail below. The present invention
Nitrate and nitrite nitrogen, which are difficult to decompose at low cost, are electrochemically decomposed and vaporized as nitrogen, NOx, ammonia, etc. and removed to the outside of the system to decompose and remove the nitrate and nitrite nitrogen. . When the solution containing the nitric and / or nitrite nitrogen is supplied as an electrolytic solution to the cathode chamber of the electrolytic cell, the nitrate ion and the nitrite ion are reductively decomposed according to the following formula.

2NO ₃ ⁻ + 12H ⁺ + 10e ⁻ → N ₂ + 6H ₂ 0 + 2NO ₃ ⁻ 1.246 V 2NO ₃ ⁻ + 10H ⁺ + 8e ⁻ → N ₂ O + 5H ₂ 0 1.116 V NO ₃ ⁻ + 4H ⁺ + 3e ⁻ → NO + 2H ₂ 0 0.937 V NO ₃ ^{- + 2H + + 2e - →} NO 2 - + H 2 0 0.835 V nitrate ion concentration is low 2e ⁺ 2H + → H ₂ (or 2H
_{2 0 + e → H 2 +} 2OH -) ratio of hydrogen generation is increased by.

The produced NOx and nitrogen are partially removed as gas, and the rest is further reduced to produce ammonium ion NH ₄ ⁺ according to the following formula. The ammonium ions are not gasified under neutral to acidic conditions and are dissolved in the solution as ions. NO + 6H ⁺ + 5e ⁻ → NH ₄ ⁺ + H ₂ 0 0.835 V N ₂ O + 10H ⁺ + 8e ⁻ → 2NH ₄ ⁺ + H ₂ 0 0.647 V N ₂ + 8H ⁺ + 6e ⁻ → 2NH ₄ ⁺ 0.275 V As the electrolysis progresses, the catholyte becomes alkaline. When the area is reached, N
The reaction of H ₄ ⁺ → NH ₃ proceeds, NH ₃ is stripped by the bubbling effect of hydrogen gas, and is removed to the gas phase.

The nitrite ion produced by the above equation is further reduced according to the following equation to produce NOx and nitrogen. NO ₂ ⁻ + 2H ⁺ + e ⁻ → NO + H ₂ 0 1.202 V 2NO ₂ ⁻ + 6H ⁺ + 4e ⁻ → N ₂ O + 3H ₂ 0 1.396 V 2NO ₂ ⁻ + 8H ⁺ + 6e ⁻ → N ₂ + 4H ₂ 0 1.520 V (10) Thus the cathode Since NOx, nitrogen, and ammonia produced from nitrate and nitrite nitrogen according to the reaction are easily vaporized, they can be taken out of the system as they are. However, in the present invention, NOx, nitrogen, and hydrogen produced in addition Gas generated in the cathode chamber such as the above may be circulated to the anode chamber.

As a result, the main anodic reaction can be converted from oxygen gas generation by water electrolysis to water generation reaction by hydrogen oxidation, which reduces the cell voltage and allows the reaction to proceed under low overvoltage and low power. You can When the circulating hydrogen from the cathode chamber is not sufficient as the hydrogen supply for the smooth progress of the anode reaction, it is desirable to separately supply hydrogen gas directly to the anode chamber or indirectly through the cathode chamber. When the decomposition of ions such as nitric acid at the cathode is only a part of the whole reaction, most of the by-produced gas is hydrogen and this can be used as the main raw material for the anodic reaction, so the amount of hydrogen supplied separately is Few.

Hydrogen to be separately supplied may be supplied from a commercially available cylinder, or an electrolytic cell for hydrogen gas generation may be installed adjacent to the electrolytic cell to supply the generated hydrogen to the anode chamber or the cathode chamber. Further, a hydrogen generating cathode is installed without a diaphragm in the anode chamber having the gas diffusion electrode, and hydrogen is generated by passing electricity between the porous anode separately installed in the anode chamber and the hydrogen generating cathode through the diaphragm. It is also possible to supply the hydrogen directly to the gas diffusion electrode. Further, when a cation exchange membrane is used as the diaphragm, the target solution having a low electric conductivity can be quickly processed. Further, when a cation exchange membrane is used, a part of the ammonium ions generated in the cathode chamber is diffused in the anode chamber side, reoxidized in the hydrogen gas anode, and decomposed into nitrogen and NOx. At this time, the anode such as NO ₂ ⁻ and NO ₃ ⁻ does not regenerate at the anode at the hydrogen oxidation potential.

In this way, when nitric acid and / or nitrite nitrogen is electrochemically reduced to gases such as NOx and nitrogen and ammonium ions, and at least a part of the generated gas is circulated to the anode chamber, It is possible to convert nitric acid and / or nitrite nitrogen into gas and remove it from the system, and also to convert the anodic reaction from a high power consumption type oxygen generation reaction to a low power consumption type water generation reaction by hydrogen oxidation, which is economical. The denitrification reaction can be promoted. As the cathode used in the present invention, it is preferable to use a porous electrode having gas-liquid permeability, when the conductivity of the liquid to be treated in which the nitrate nitrogen or the like is low or nitric acid or nitrite ion concentration is low. When it is low, use of a three-dimensional electrode (including a gas diffusion electrode) or a fluidized bed electrode is desirable from the viewpoint of efficiency. When forming a porous electrode, a catalyst such as copper and silver, or a platinum group metal such as palladium or platinum is supported on a porous substrate such as carbon or stainless steel. In the case of a three-dimensional electrode or a fluidized bed electrode, it is desirable to use carbon particles or the like. Although carbon materials tend to corrode in concentrated nitric acid, they are cheap and have other performances, so it is desirable to use them while considering consumption.

As the gas diffusion electrode used for the anode, a conventional one may be used. For example, carbon powder to which Teflon (trade name) or carbon fluoride is added to the surface of the carbon paper cloth as a support to improve water repellency. A gas diffusion layer can be formed by forming a gas supply layer made of and a catalyst layer made of carbon powder carrying a catalyst such as platinum, ruthenium or gold on the opposite surface of the support. It is important for the gas diffusion electrode to have some liquid permeability due to the characteristics of the support in order to accelerate the electrolytic reaction.

The diaphragm, preferably a cation exchange membrane, used in close contact with the anode may be a commercially available one.
There are hydrocarbon type and fluororesin type, and the latter is superior in terms of corrosion resistance. When hydrogen is not supplied to the anode and an ion exchange membrane is not used, the reduction product at the cathode reaches the anode and is reoxidized to produce NO ₃ ⁻ or NO ₂ ⁻ , resulting in nitric acid, suboxide Nitrate nitrogen cannot be reduced.
The applied voltage depends on the concentration of the liquid to be treated, but it is desirable to apply it so that a high current density can be obtained in order to efficiently perform stirring due to the lift effect of the generated gas.
On the other hand, it is preferable to have a low current density in terms of power consumption, and therefore, a voltage is generally applied so that the current density is 0.05 to ¹ cm ² . The temperature and pressure of the electrolytic cell are not particularly limited.

In such a method of decomposing nitric acid and / or nitrite nitrogen, although depending on the catalyst, a part of decomposition products are NOx and NH ₃ , and these decomposition products are generated from the liquid to be treated. Even if it can be easily removed, it is not preferable in terms of environmental hygiene to release it as it is into the atmosphere. Therefore, in the device of the present invention, the gas discharged from the anode chamber is converted into nitrogen through a nitric oxide decomposing device filled with a nitric oxide decomposing catalyst, if necessary, and then released into the atmosphere. As the nitric oxide decomposition catalyst, a three-way catalyst containing platinum, which is usually used as a waste gas catalyst for automobiles, is used. You may heat in order to raise the decomposition efficiency by this catalyst. A mist catcher may be installed between the anode chamber and the nitric oxide decomposing device in order to suppress the deterioration of the catalyst. Further, in the apparatus of the present invention, an ammonia removing apparatus based on a stripping method using a catalyst such as palladium or an alkaline aqueous solution and air may be installed before the nitric oxide decomposing catalyst.

FIG. 1 is a schematic vertical sectional view showing an example of the apparatus for decomposing nitrate and nitrite nitrogen according to the present invention. The electrolytic cell body 1 is divided into an anode chamber 3 and a cathode chamber 4 by a cation exchange membrane 2 which is a diaphragm, and a carbon powder and a fluororesin powder carrying a catalyst are provided on the cation exchange membrane 2 side of the anode chamber 3. A gas diffusion electrode 5 made of a mixed sintered body is closely formed, and a sheet-shaped cathode 6 such as carbon cloth is placed in contact with the cation exchange membrane 2 on the side of the cathode chamber 4.

A treatment liquid introduction port 7 and a treatment liquid extraction port 8 are formed in the lower and upper portions of the side wall of the cathode chamber 4, respectively.
One end of a cathode production gas circulation line 9 is connected to the upper surface of the cathode chamber 4, and the line 9 is filled with a mist catcher 10 for dehumidification and a nitric oxide decomposition catalyst.
11 or ammonia decomposition device is installed. One end of a treated gas discharge line 12 is connected to the upper side wall of the anode chamber 3, and the same mist catcher 10 and nitric oxide decomposing device 11 or ammonia decomposing device as described above are installed in the exhaust line 12. Reference numeral 13 is a hydrogen gas generator such as a water electrolysis cell. The generated hydrogen gas is supplied to the cathode chamber 4 of the electrolysis cell main body 1 from the hydrogen gas supply port 14 to increase the hydrogen gas concentration in the cathode generation gas. Reference numeral 15 is a gas supply pipe for supplying gas to the gas diffusion anode 5, and the supply gas is the cathode 6.
The hydrogen gas generated in 1) is supplied through the circulation line 9 or directly from a hydrogen cylinder.

A liquid to be treated in which nitric acid and / or nitrite nitrogen is dissolved is supplied from a liquid to be treated inlet 7 to the electrolytic cell body 1 having such a structure, and hydrogen gas is supplied from a hydrogen gas supply port 14. While energizing between the anode 5 and the cathode 6, and further supplying the gas generated in the cathode chamber 4 to the surface of the anode 5 through the circulation line 9, the nitrate and / or nitrite nitrogen on the surface of the cathode 6 is reduced to the above-mentioned ( Decomposes according to formula 10), nitrogen, NO
x and ammonium ions are generated. Among them, a part of ammonium ions permeates the anode chamber 3 through the cation exchange membrane 2, is oxidized by the hydrogen anode and converted into N ₂ or NOx, and is taken out from the discharge line 10.

Nitrogen and NOx generated in the cathode chamber 4 are supplied to the anode chamber 3 through the circulation line 9 together with hydrogen gas generated by electrolysis of water, and consumed in the anode 5. As a result, the anodic reaction changes from the oxygen generation reaction, which consumes a large amount of power, to the water generation reaction, which is caused by the oxidation of hydrogen, and the power consumption is greatly reduced. Then, the anodic reaction causes not only this water formation reaction but also another oxidation reaction to generate NOx and the like. These anode gases are exhausted from the exhaust line 12, and after deammonification by the ammonia removing device 11 in some cases, dehumidifying by the mist catcher 10 and then decomposing nitrogen oxides by the nitric oxide decomposing device 12, and then the atmosphere. Released inside.

FIG. 2 is a schematic vertical sectional view showing another example of the apparatus for decomposing nitrate and nitrite nitrogen according to the present invention. This example relates to the improvement of the apparatus shown in FIG. 1, and the same members and components are designated by the same reference numerals and the description thereof will be omitted. A circulation line 9'connected to the upper surface of the cathode chamber 3 is bent twice so that its tip reaches the lower surface of the gas diffusion anode 5, and a waste gas outlet 16 is formed in the circulation line 9 '. In the illustrated example, the cathode 6
The hydrogen gas generated in 1 is removed from the waste gas generated from the waste gas outlet 16 when passing through the circulation line 9 ', and is supplied to the anode 5, where the liquid to be treated is treated as in the case of FIG.

[0020]

EXAMPLES Next, examples of the decomposition of nitrate nitrogen etc. of the present invention will be described, but the examples do not limit the present invention.

[Example 1] 100 g / m as a projection surface by electroplating
A stainless fiber sintered body supporting platinum so as to be ² was laminated to form a cathode having a thickness of 5 mm and connected to a glassy carbon power supply plate having a thickness of 3 mm. Nafion 117 (manufactured by DuPont Japan) was used as a cation exchange membrane. Furnace black carbon powder supporting platinum and PTF
E water suspension (30J, manufactured by Mitsui Fluorochemicals Co., Ltd.) was thoroughly mixed in solvent naphtha so that the carbon powder and PTFE were in a weight ratio of 1: 1 and coated on carbon cloth to obtain gas-liquid permeability. A gas diffusion anode was constructed. The electrode area of both electrodes was 15 cm ² .

The above-mentioned anode and the above-mentioned ion exchange membrane were superposed, and the above-mentioned cathode was brought into contact with the opposite surface of the membrane, and these three members were fastened together to be integrated and placed in the electrolytic cell. Anode chamber volume is 10
The volume of the cathode chamber was 0 cm ³ , and the volume of the cathode chamber was 100 cm ³ . In the cathode chamber of this electrolytic cell, 500 ml of a liquid to be treated in which 10 g / l of nitrate ions were dissolved was prepared in a beaker, supplied at a rate of 10 ml / min, and returned to the beaker again. 50% of the theoretical amount of hydrogen gas generated by the pure water electrolysis apparatus was supplied to the lower part of the cathode chamber from the hydrogen gas supply port. A cathode generated gas was circulated on the surface of the gas diffusion anode in the anode chamber through a circulation line connected to the upper part of the cathode chamber.

Further, a discharge line was connected to the side wall of the anode chamber, and a mist catcher and a nitric oxide decomposing device (catalyst: platinum-rhodium-palladium system) were connected to the line. Between both electrodes of the decomposition system such as nitrate nitrogen, 3A
When electrolysis was carried out by applying the current (current density 20 A / dm ² ) of 1., a cell voltage of 1.2 V was obtained at a liquid temperature of 40 ° C. The nitrate ion concentration was 1 g / liter, and the nitrate ion decomposition rate was 90%. Nitrate ions, nitrite ions, ammonium ions at the outlet of the catholyte and cathode chamber,
pH, concentration of ammonia and NOx gas, absorbance method,
It was as shown in Table 1 when measured using a pH meter, a gas detector tube, and gas chromatography. Again 6
The decomposition rate of nitrate ions after the lapse of time was 90%, and the NOx concentration at the outlet of the nitric oxide catalyst was 0.1 ppm or less.

[0023]

[Table 1]

[0024]

[Example 2] The amount of nitrate ions dissolved in the liquid to be treated was 1000.
When the nitrate ions were decomposed under the same conditions as in Example 1 except that the concentration was changed to ppm, a cell voltage of 2.4 V was obtained.
The nitrate ion decomposition efficiency after 6 hours was 85%.

[0025]

[Comparative Example 1] A neutral diaphragm (trade name: Yumicron MF-40, manufactured by Yuasa Ionics) was used in place of the ion-exchange membrane, and an aqueous solution of iridium chloride was applied on a titanium mesh as an anode to prepare by a thermal decomposition method. Electrolysis was performed under the same electrolysis conditions as in Example 1 except that an iridium oxide electrode (20 g / m ² as iridium) was used and hydrogen gas was not supplied to the anode chamber, and a cell voltage of 4.0 V was obtained. The decomposition rate of nitrate ion after 6 hours was 55%.

[0026]

Comparative Example 2 An electrolytic cell similar to that of Example 2 was assembled except that a neutral diaphragm was used instead of the ion exchange membrane, and electrolysis was performed under the same electrolysis conditions. As a result, a cell voltage of about 15 V was obtained for 6 hours. The subsequent decomposition rate of nitrate ions was 80%.

[0027]

The present invention uses an electrode selected from a gas-liquid permeable porous electrode, a three-dimensional electrode and a fluidized bed electrode as a cathode of an electrolytic cell divided into an anode chamber and a cathode chamber by a diaphragm. In the method for removing nitric and / or nitrite nitrogen by reducing and removing volatile and / or nitrite nitrogen at the cathode, a gas diffusion electrode is used as an anode, and an oxidation reaction of hydrogen gas is mainly performed. And a method for removing nitric and / or nitrite nitrogen. Is. According to the present invention, nitric and / or nitrite nitrogen is converted to nitrogen or NO by cathodic reduction.
x is converted into gas such as ammonia and these gases are vaporized and removed from the cathode chamber, or are circulated to the anode chamber along with hydrogen generated by electrolysis of water, and the anode reaction is changed from the high power consumption type oxygen generation reaction. Converts to low power consumption type hydrogen oxidation reaction.

Therefore, the decomposition of nitric and / or nitrite nitrogen, which adversely affects the water quality environment, can be carried out easily and economically. When a cation exchange membrane is used as the diaphragm, the NO produced by the reoxidation of the reduced product produced at the cathode at the anode.
₃ ^- and NO ₂ ^- can be prevented generation of. Further, even when the liquid to be treated has a low electric conductivity, the electrolytic reaction easily proceeds.

The apparatus of the present invention is substantially the same as the apparatus used in the above-described method of the present invention, except that the discharge line for the anode-producing gas is connected to a nitric oxide decomposing device for decomposing nitric oxide in the gas produced at the anode. Is. Nitrate and / or
Alternatively, even after the nitrite nitrogen is decomposed, the gas generated in the anode chamber contains nitric oxide. If this nitric oxide is released as it is, it causes air pollution. Therefore, if the above-mentioned nitric oxide decomposing device used in automobile waste gas is used to decompose the nitric oxide, it is converted into a harmless gas and released into the atmosphere. Will be possible.

[Brief description of drawings]

FIG. 1 is a schematic vertical cross-sectional view showing an example of a nitrate and / or nitrite nitrogen decomposing apparatus of the present invention.

2 is a schematic vertical sectional view showing an example of an improved device of the device of FIG.

[Explanation of symbols]

1 ... Electrolyzer main body 2 ... Cation exchange membrane 3 ...
・ Anode chamber 4 ・ ・ ・ Cathode chamber 5 ・ ・ ・ Gas diffusion electrode 6
・ ・ ・ Cathode 7 ・ ・ ・ Treatment liquid inlet 8 ・ ・ ・ Treatment liquid outlet 9, 9 '・ ・ ・ Circulation line 10 ・ ・ ・ Mist catcher 11 ・ ・ ・ Nitric oxide decomposer 12 ・ ・ ・ Discharge Line 13
..Hydrogen gas generator 14 ... Hydrogen gas supply port 15 ...
・ Gas supply pipe 16 ・ ・ ・ Waste gas outlet

Claims (6)

[Claims] 1. An electrode selected from a gas-liquid permeable porous electrode, a three-dimensional electrode and a fluidized bed electrode is used as a cathode of an electrolytic cell divided into an anode chamber and a cathode chamber by a diaphragm, and a nitric acid and / or In the method for removing nitric acid and / or nitrite nitrogen by reducing and removing nitrite nitrogen at the cathode, a gas diffusion electrode is used as an anode, and an oxidation reaction of hydrogen gas is mainly performed. And / or a method for removing nitrite nitrogen. 2. The method of claim 1, wherein at least a portion of the gas produced at the cathode is fed to the gas diffusion electrode of the anode. 3. The method according to claim 1, wherein a cation exchange membrane is used as the diaphragm. 4. An anode, which is a gas diffusion electrode housed in an anode chamber and a cathode chamber partitioned by an ion exchange membrane, and a cathode selected from a gas-liquid permeable porous electrode, a three-dimensional electrode and a fluidized bed electrode. A liquid to be treated containing nitric and / or nitrite nitrogen, which is installed in the cathode chamber, and an outlet for a gas generated in the cathode chamber, an anode gas inlet, and an anode waste gas outlet. A nitrate and / or nitrite nitrogen removal device comprising. 5. The apparatus according to claim 4, wherein the cathode gas outlet is connected to the anode gas inlet so that at least a part of the gas generated at the cathode is supplied to the anode. 6. The apparatus according to claim 4, wherein an apparatus for decomposing ammonia and nitric oxide is installed at the cathode gas outlet and / or the anode gas outlet.