JP2003185790A - Method and apparatus for recovering nitric acid component from aqueous solution of nitric acid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To recover nitric acid components from an aqueous solution of nitric acid with good energy efficiency by enabling an apparatus to be operated at room temperature under normal pressure, and thus preventing an increase in facility costs. <P>SOLUTION: An aqueous solution 54 of nitric acid is supplied into an anode electrolysis chamber 24 and a cathode electrolysis chamber 26 which are divided by an electrolytic diaphragm 22, and is electrolyzed by feeding electricity from a DC power supply 30 while using an electrode (platinum electrode 28) installed in the anode electrolysis chamber as an anode and using a cathode electrolysis chamber component material (solution retaining container 20 made of glassy carbon) or another electrode installed in the cathode electrolysis chamber as a cathode. A NOx gas generated from the cathode electrolysis chamber is collected by scrubbers 50a, 50b and 50c loaded with distilled water 52 to recover the nitric acid components. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for recovering nitric acid components from a nitric acid aqueous solution without performing a distillation operation. More specifically, it is operated at room temperature and atmospheric pressure by utilizing electrolysis. The present invention relates to a method and an apparatus for recovering nitric acid components from a nitric acid aqueous solution. This technique is not particularly limited, but is effective for the treatment of recovering the nitric acid component from the nitric acid aqueous solution used in the spent nuclear fuel reprocessing step, for example.

[0002]

2. Description of the Related Art Nitric acid has the property of being easily evaporated,
Even if the nitric acid aqueous solution is simply heated, it is impossible to evaporate and concentrate only water. Therefore, in general, distilling is carried out using an evaporative concentrator that combines an evaporator and a rectification tower, nitric acid concentrated to a high concentration is recovered from the evaporator, and steam with a reduced nitric acid concentration is recovered from the top of the rectification tower. ing.

However, there is a problem of corrosion of equipment materials in the distillative concentration of nitric acid. Even with stainless steel that is corrosion resistant at room temperature, grain boundary corrosion of the material becomes a problem at about 120 ° C., which is the boiling point of nitric acid, and it has been observed that the material of the apparatus is thinned.

Stainless steel normally exhibits an excellent corrosion resistance by forming a passive state in a nitric acid solution and forming a chemically stable oxide film on the material surface. But the temperature,
In a strong oxidizing environment in which the effects of nitric acid concentration, coexisting ions, etc. are involved in a complicated manner, the corrosion potential of stainless steel is increased to or near the passivation state, so that intergranular corrosion type local corrosion occurs. In particular, in the case of an acid recovery evaporator, the temperature of the metal material on the surface of the heat transfer surface becomes higher than the temperature of boiling nitric acid, so the corrosive environment becomes more severe.

Therefore, there is a technique of boiling nitric acid under reduced pressure to lower the boiling point of nitric acid and performing distillation operation. Alternatively, an alloy with corrosion resistance to high temperature nitric acid (25C
It is said that the device can be made of r-20Ni-Nb steel, Ti-5% Ta) or the like.

[0006]

However, the boiling treatment under reduced pressure requires a large-scale rectification column due to an increase in the volume of gas, resulting in poor economic efficiency. Further, the equipment using an alloy having corrosion resistance to high-temperature nitric acid has a drawback that it is extremely expensive as compared with the case of using ordinary stainless steel.

An object of the present invention is to provide a method and an apparatus capable of recovering nitric acid components from an aqueous nitric acid solution efficiently in terms of energy without increasing equipment cost because it can be operated at room temperature and atmospheric pressure. .

[0008]

According to the present invention, an aqueous nitric acid solution is supplied to each of an anode electrolysis chamber and a cathode electrolysis chamber divided by an electrolytic diaphragm, and an electrode installed in the anode electrolysis chamber is used as an anode and a cathode electrolysis chamber constituent material. Alternatively, the present invention is a method for recovering nitric acid components from a nitric acid aqueous solution, characterized in that a nitric acid aqueous solution is electrolyzed by passing a direct current through an electrode installed in the cathodic electrolysis chamber as a cathode to recover NOx gas generated from the cathodic electrolytic room.

More specifically, according to the present invention, a nitric acid aqueous solution is supplied to each of an anode electrolysis chamber and a cathode electrolysis chamber which are partitioned by an alumina-based electrolytic diaphragm, and a platinum electrode installed in the anode electrolysis chamber is used as an anode and a cathode electrolyzer. A DC current is made to flow through the glassy carbon constituting the chamber as a cathode to electrolyze the nitric acid aqueous solution, and NOx gas generated from the cathode electrolysis chamber is recovered by a scrubber loaded with distilled water. This is a method for recovering nitric acid components.

The present invention divides an anode electrolysis chamber and a cathodic electrolysis chamber by an electrolysis diaphragm and electrolyzes the nitric acid aqueous solution contained therein, and an NOx gas generated from the cathodic electrolysis chamber of the electrolysis unit. An apparatus for recovering nitric acid components from an aqueous solution of nitric acid, which has a NOx gas recovery section composed of one or more stages of scrubber for recovery.

Here, it is preferable to connect the NOx gas generated from the cathode electrolysis chamber of the electrolysis section and the oxygen gas generated from the anode electrolysis chamber by a gas exhaust pipe so as to supply them to the scrubber. At that time, a heating heater is attached to the NOx gas exhaust pipe from the cathode electrolysis chamber to keep the gas state. Further, it is preferable to connect an air introduction pipe for supplying purge air to the cathode electrolysis chamber and the anode electrolysis chamber of the electrolysis section. The electrolysis section is provided with a bottomed tubular alumina-based electrolytic diaphragm in a glassy carbon solution holding container, and is divided into an inner anode electrolytic chamber and an outer cathode electrolytic chamber by the electrolytic diaphragm. A plate-like platinum electrode is installed, and a direct current power source is energized so that the platinum electrode serves as an anode and the glassy carbon solution holding container serves as a cathode.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, an aqueous nitric acid solution is supplied to each of an anode electrolysis chamber and a cathode electrolysis chamber partitioned by an electrolytic diaphragm, an electrode installed in the anode electrolysis chamber is an anode, and a material constituting the cathode electrolysis chamber is a cathode. As a direct current, a nitric acid aqueous solution is electrolyzed by passing a direct current. Due to the electrolysis of this nitric acid aqueous solution, NOx gas is generated from the cathode electrolysis chamber. On the other hand, oxygen gas is generated from the anode electrolysis chamber by electrolysis of water. These N
By recovering the Ox gas together with the oxygen gas with the scrubber, the nitric acid component can be efficiently recovered. When such treatment is performed, the nitric acid concentration of the nitric acid aqueous solution in the cathode electrolysis chamber decreases, and the amount of nitric acid component in the anode electrolysis chamber does not change, but the water component in the anode electrolysis chamber decreases, so the concentration of nitric acid increases. To do.

The electrode reaction at each electrode is shown below. The first reaction at the cathode is H ⁺ + e ⁻ → (H). Here, (H) indicates a hydrogen atom adsorbed on the cathode. Following this reaction, a reaction of HNO ₃ +2 (H) → HNO ₂ + H ₂ O or 2 (H) → H ₂ occurs.

When HNO ₂ is produced near the cathode,
NOx gas is generated by the following series of reactions. 3HNO ₂ → HNO ₃ + 2NO ↑ + H ₂ O HNO ₂ + HNO ₃ → N ₂ O ₄ + H ₂ O N ₂ O ₄ → 2NO ₂ ↑

The anodic reaction is an oxygen generation reaction as shown below. _{^{4OH - → 2H 2 O + O}} 2 ↑ + 4e -

As shown in the above reaction formula, the main components of the gas generated at the cathode are NO and NO ₂ gas, and these gases are introduced to the NOx gas recovery system and recovered. NO ₂ gas produces nitric acid in the following reaction. 3NO ₂ + H ₂ O → NO + 2HNO ₃

The NO gas generated by electrolysis and the NO gas generated by the above reaction formula pass through the scrubber loaded with distilled water as they are because the solubility of NO gas in water is small. Therefore, it is oxidized by oxygen generated at the anode or by purging air to form NO ₂ . Then, it can be recovered by the scrubber by the reaction shown above.

[0018]

EXAMPLE FIG. 1 is an explanatory view showing an example of a device for recovering nitric acid components from an aqueous nitric acid solution according to the present invention. The present apparatus includes an electrolysis unit 10 and a NOx gas recovery unit 12.

The electrolytic section 10 includes a cylindrical electrolytic alumina membrane 22 with a bottom in a glassy carbon solution holding container 20.
Is installed, and is divided into an inner anode electrolysis chamber 24 and an outer cathode electrolysis chamber 26 by the electrolysis diaphragm 22.
A flag-shaped platinum electrode 28 is installed in the
8 is an anode, the glassy carbon solution holding container 20
The DC power supply 30 is provided so as to energize itself so as to serve as a cathode.

Alumina electrolytic diaphragm 2 having a bottomed cylindrical shape
2 has an outer diameter of 50 mm and a height of 75 mm, and its thickness is 3 mm
Is. This electrolytic diaphragm 22 has a chemical composition of Al ₂ O ₃ +.
It is a porous body of 3Al ₂ O ₃ .2SiO ₂ and has a relatively low electric resistance, but an extremely small amount of solution permeation. By the way, the electric resistance is 5 in 0.1N HCl.
The permeation amount measured by applying a pressure of 9800 Pa to an electrolytic diaphragm having a thickness of about 00 Ω · cm and a thickness of 2 mm is 2.7 cm ³ / cm ² h.

The flag-shaped platinum electrode 28 serving as an anode is formed by welding a platinum support rod having a diameter of 2 mm and a length of 60 mm to a plate-like body (area: 112 cm ^{2 on} both sides) having a length and width of 14 cm × 4 cm and a thickness of 0.1 cm. It is a structure. The solution holding container 20 forming the cathode electrolysis chamber 26 is made of glassy carbon, and the inner surface in contact with the nitric acid aqueous solution also serves as the cathode. Glassy carbon is a black glass-like gas impermeable carbon product, which has characteristics such as high purity and excellent chemical resistance, and is used as a material for crucibles, for example. Is.

As the material of the solution holding container 20, high density graphite can be used for the purpose of holding the nitric acid aqueous solution, but considering that the nitric acid aqueous solution is not contaminated with carbon during electrolysis, the glassy carbon is graphite. It is optimal because it can suppress the diffusion of the fine powder of (3) into the solution. It is possible to use platinum, which is the same material as the anode, as the cathode material, but platinum is expensive, and carbon is used when the nitric acid concentration in the solution is relatively low at 1 to 3 mol / L (liter). It is preferable to use carbon as the cathode constituent material because it is possible to decompose the nitric acid component more efficiently by using the electrode.

The electrochemical measurement and the electrolysis were carried out in a three-electrode system using a reference electrode 32 in addition to the platinum electrode 28 as the anode and the solution holding container 20 made of glassy carbon as the cathode. The reference electrode is an electrode serving as a reference of electric potential, and is used for measuring a target cathode electric potential and regulating the electric potential. In this example, a saturated calomel reference electrode 32 that is most commonly used in an aqueous system experiment is used.

An air introduction pipe 34 from the outside is connected to the anode electrolysis chamber 24 and the cathode electrolysis chamber 26 of the electrolysis section 10. Further, an oxygen gas exhaust pipe 36 is drawn from the anode electrolysis chamber 24, and a NOx gas exhaust pipe 3 is drawn from the cathode electrolysis chamber 26.
8 are extracted, and those gases are combined and sent to the NOx gas recovery unit 12 by the gas discharge pipe 40. In addition,
A heating heater 42 is attached to the NOx gas exhaust pipe 38,
Heat to about 60 to 80 ° C. NOx gas recovery unit 12
Are scrubbers 50a, 50b, which are cascade-connected in three stages.
50c, each scrubber 50a, 50b, 50c
Is loaded with distilled water 52.

60 mL of a nitric acid aqueous solution 54 having a concentration of 6 N was supplied to each of the anode electrolysis chamber 24 and the cathode electrolysis chamber 26 of the electrolysis section 10 constructed as described above, and constant current electrolysis was carried out at an electrolysis current of 3 A to perform anodic electrolysis. The changes in the concentration of the nitric acid aqueous solution in the chamber 24 and the cathode electrolysis chamber 26 were examined. The concentration of the nitric acid aqueous solution was determined by neutralization titration with 1 mol NaOH. Table 1 shows changes in the concentration of the nitric acid aqueous solution in the anode electrolysis chamber and the cathode electrolysis chamber. As the electrolysis progressed, the concentration of nitric acid in the cathode electrolysis chamber decreased due to the decomposition of nitric acid components, and the concentration of nitric acid in the anode electrolysis increased due to the decomposition of water.

[0026]

[Table 1]

Table 2 shows the amount of nitric acid in the nitric acid aqueous solution in the anode electrolysis chamber and the cathode electrolysis chamber before and after the electrolysis, the amount of nitric acid reduced by electrolysis, and the amount of electricity consumed. As can be seen from Table 2, nitric acid in the cathode electrolysis chamber is consumed and the acid concentration is decreasing, and in the anode electrolysis chamber, the nitric acid component is not electrolyzed and the water component is decomposed to increase the acid concentration.

[0028]

[Table 2]

The cathodic reaction is HNO ₃ + 2H ⁺ + 2e ⁻ → HNO ₂ + H ₂ O 3HNO ₂ → HNO ₃ + 2NO ↑ + H ₂ O HNO ₂ + HNO ₃ → N ₂ O ₄ + H ₂ O N ₂ O ₄ → 2NO ₂ ↑ If it occurs in the reaction of 4HNO ₃ + 8H ⁺ + 8e ⁻ → 2NO ↑ + 2NO ₂ ↑ +
It becomes 6H ₂ O. Therefore, theoretically, 0.5 mol of HNO ₃ is decomposed by an electric quantity of 1 F (= 96485 C), and 0.25 mol
Of NO and 0.25 mol of NO ₂ are generated.
In this example, the consumption of nitric acid is 0.211 mol,
The current efficiency is the current efficiency (%) = 0.211 / (42120/9648).
5 × 0.5) × 100 = 96.7%. From this result, it can be seen that the nitric acid component is efficiently electrolyzed even in terms of energy.

N generated in the cathode electrolysis chamber 26 of the electrolysis section 10
Ox gas and oxygen generated in the anode electrolysis chamber 24 are NOx.
The NOx gas is guided to the gas recovery system 12 and scrubber 50
It is recovered by contacting with distilled water 52 in a, 50b and 50c. Air is used for purging the electrolysis generated gas, and promptly guides the electrolysis generated gas to the NOx gas recovery system. Oxygen generated from the anode electrolysis chamber 24 is also introduced into the NOx gas recovery system 12 by the purging air, and the cathode electrolysis chamber 26
It is mixed with NOx gas generated from. Anode electrolysis chamber 24
The oxygen generated in the above step and the oxygen in the purge air are used to oxidize NO, which is difficult to be absorbed by water, into NO ₂ . Therefore, the NOx gas from the cathode electrolysis chamber 26 is collected in the scrubber 50a, 50b, 50c in the NO ₂ state. Each scrubber 50a, 50b, 50c is loaded with 5 L (liter) of distilled water 52, and the amount of change in nitric acid concentration of the scrubber solution as a result of recovering NOx gas by each scrubber 50a, 50b, 50c is shown. It shows in Table 3.

The NOx component in the gas was recovered by the scrubbers 50a, 50b up to the second stage, and the nitric acid component was not recovered by the scrubber 50c in the third stage even after the electrolysis was completed.
As shown in Table 3, NO recovered in all scrubbers
x gas is 0.169 mol (= 0.1
35 + 0.034), which shows that 80% of the nitric acid component reduced from the cathode electrolysis chamber was recovered by the NOx recovery system.

[0032]

[Table 3]

The NOx gas exhaust pipe 38 from the cathode electrolysis chamber 26 is equipped with a heater 42 for heating NOx.
This is because the gas is directed to the NOx gas recovery unit 12 in the gas state. This is because, if not heated, NOx gas condenses in the pipe and returns to nitric acid, and returns to the cathode electrolysis chamber by gravity, which lowers the processing efficiency (current efficiency). However, if the temperature is too high, pipe corrosion may occur, so that the heater temperature is preferably about 60 to 80 ° C. as described above.

[0034]

INDUSTRIAL APPLICABILITY As described above, the present invention supplies an aqueous nitric acid solution to each of the anode electrolysis chamber and the cathode electrolysis chamber divided by the electrolytic diaphragm, and applies a direct current to electrolyze the nitric acid aqueous solution.
Since it is a method and an apparatus for recovering NOx gas generated from the cathode electrolysis chamber, it is possible to efficiently extract nitric acid components from a nitric acid aqueous solution under operating conditions at room temperature and atmospheric pressure with a relatively inexpensive small facility. Can be collected.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an embodiment of a nitric acid component recovery device from a nitric acid aqueous solution according to the present invention.

[Explanation of symbols]

10 Electrolysis Department 12 NOx gas recovery unit 20 Glassy carbon solution holding container 22 Alumina electrolytic diaphragm 24 Anode electrolysis chamber 26 Cathode electrolysis chamber 28 Flag-shaped platinum electrode 30 DC power supply 32 reference electrode 34 Air introduction piping 36 Oxygen gas exhaust pipe 38 NOx gas exhaust pipe 40 gas exhaust pipe 42 Heater 50a, 50b, 50c scrubber 52 distilled water 54 Nitric acid aqueous solution

Claims (6)

[Claims] 1. A nitric acid aqueous solution is supplied to each of an anode electrolysis chamber and a cathode electrolysis chamber partitioned by an electrolysis diaphragm, and an electrode installed in the anode electrolysis chamber is an anode, a cathode electrolysis chamber constituent material or an electrode installed in the cathode electrolysis chamber. A method for recovering nitric acid components from an aqueous nitric acid solution, characterized in that a nitric acid aqueous solution is electrolyzed by flowing a direct current as a cathode to recover NOx gas generated from the cathodic electrolysis chamber. 2. A nitric acid aqueous solution is supplied to each of an anode electrolysis chamber and a cathode electrolysis chamber partitioned by an alumina-based electrolytic diaphragm, a platinum electrode installed in the anode electrolysis chamber serves as an anode, and glassy carbon constituting the cathode electrolysis chamber serves as a cathode. NOx generated from the cathodic electrolysis chamber by electrolysis of nitric acid aqueous solution by passing direct current as
A method for recovering nitric acid components from a nitric acid aqueous solution, characterized in that the gas is recovered by a scrubber loaded with distilled water. 3. An electrolysis section partitioned by an electrolytic diaphragm into an anode electrolysis chamber and a cathode electrolysis chamber, which electrolyzes a nitric acid aqueous solution contained therein, and recovers NOx gas generated from the cathode electrolysis chamber of the electrolysis section. An apparatus for recovering nitric acid components from a nitric acid aqueous solution, which has a NOx gas recovery section composed of one or more stages of scrubbers. 4. NOx generated from the cathode electrolysis chamber of the electrolysis section
The nitric acid according to claim 3, wherein a gas exhaust pipe is connected so that the gas and oxygen gas generated from the anode electrolysis chamber are mixed and supplied to the scrubber, and a NOx gas exhaust pipe from the cathode electrolysis chamber is equipped with a heating heater. Nitric acid component recovery device from aqueous solution. 5. An apparatus for recovering nitric acid components from an aqueous nitric acid solution according to claim 3, wherein an air introduction pipe for supplying purging air is connected to the cathode electrolysis chamber and the anode electrolysis chamber of the electrolysis section. 6. The electrolytic part comprises a bottomed cylindrical alumina-based electrolytic diaphragm placed in a glassy carbon solution holding container,
It is divided into an inner anode electrolysis chamber and an outer cathode electrolysis chamber by the electrolytic diaphragm, and a plate-like platinum electrode is installed in the anode electrolysis chamber, the platinum electrode serves as an anode, and the glassy carbon solution holding container itself serves as a cathode. 6. A device for recovering nitric acid components from an aqueous nitric acid solution according to claim 3, further comprising a direct current power source for energizing the device.