Continuous treatment method for nitric acid-containing wastewater and catalyst for nitric acid decomposition

Abstract

To provide a treatment method for the continuous decomposition of nitrate nitrogen, in which, when nitrate nitrogen is decomposed using a catalyst and a reducing agent, ammonia produced by over-reduction and unreacted reducing agent can be decomposed under oxidative decomposition conditions using the same catalyst. [Solution] A method for treating nitric acid-containing wastewater comprising the following steps: A step of adding a reducing agent to a reactor I, which is filled with a reducing catalyst in which metal nanoparticles containing Pd and Cu are supported on a carrier, while flowing in nitric acid-containing wastewater at an adjusted temperature. A step of maintaining the reactor I at a temperature of 5 to 90°C and proceeding with the nitric acid decomposition reaction to obtain a treatment liquid I. A step of introducing oxygen or air bubbles into a reactor II, which is filled with a reducing catalyst in which metal nanoparticles containing Pd and Cu are supported on a carrier, while flowing in the treatment liquid I. A step of obtaining a treatment liquid II by promoting the oxidative decomposition of by-product ammonia by the reducing catalyst and the decomposition of the remaining reducing agent.

Description

[Technical Field]

[0001] This invention relates to a continuous treatment method for nitric acid-containing wastewater and a catalyst for nitric acid decomposition. [Background technology]

[0002] Nitrate nitrogen causes eutrophication of lakes and other bodies of water and harms human health, so it needs to be removed from industrial wastewater and other sources. A known method for removing nitrate nitrogen from wastewater is a chemical treatment method that involves reductive decomposition in the presence of a reducing agent and a catalyst. For example, a catalyst in which metal nanoparticles containing copper and palladium are supported on an inorganic or carbon support is used (see, for example, Patent Document 1). Patent Document 1 discloses the use of a small support to prevent the catalyst from settling in the treated water, and the use of carbon with a large specific surface area as the support. Furthermore, to enable repeated use, the inorganic support (particle size 5-200 nm, specific surface area 10-300 m²) is used. 2 It is known that metal nanoparticles (average primary particle size 1-9 nm) containing copper and palladium are supported on the material (see, for example, Patent Document 2).

[0003] Furthermore, in order to realize a highly active and long-lived catalyst, it is known to support metal particles having planar regions on activated carbon (see, for example, Patent Document 3). In Patent Document 3, Pd-Cu particles are given as an example of metal particles, and phenol resin-based activated carbon is given as an example of a carbon-containing carrier.

[0004] One known method for removing nitrate nitrogen from nitrate-containing wastewater is to apply activated sludge to the wastewater. For example, Patent Document 4 discloses a wastewater treatment method in which activated sludge containing anaerobic microorganisms is used to biologically treat wastewater with a nitrate nitrogen concentration of 5,000 mg / L or more, thereby obtaining treated water with a nitrate nitrogen concentration of 1,000 mg / L or less. However, it is known that in treatment methods that apply activated sludge to nitrate-containing wastewater, it takes several days or more for the nitrate nitrogen concentration in the wastewater to decrease. In addition, other problems have been pointed out, such as the inability to stop the treatment midway because it relies on the action of bacteria in the activated sludge, and the generation of unpleasant odors and disposal problems associated with the activated sludge after it has been applied to wastewater treatment.

[0005] One known method for removing nitrate nitrogen from nitrate-containing wastewater is to utilize electrolysis. For example, Patent Document 6 discloses an invention of a treatment system aimed at efficiently removing nitrogen and phosphorus from wastewater by using oxygen and hydrogen generated by electrolysis of the wastewater to oxidize and reduce nitrogen and phosphorus compounds in the wastewater. However, when using electrolysis to remove nitrate nitrogen from nitrate-containing wastewater, the treatment of wastewater containing high concentrations of nitrate is not easy and takes time, so the energy cost is also a significant issue.

[0006] As a method for removing nitric nitrogen from nitric acid-containing waste liquid, a method of decomposing nitric acid by a reduction method is also known. For example, in Patent Document 7, there is disclosed an invention of a method for treating nitric nitrogen-containing wastewater that can effectively prevent crushing and pulverization of a sponge copper catalyst and efficiently treat nitric nitrogen, and a method for treating nitric nitrogen-containing wastewater using the sponge copper catalyst to perform a reduction treatment on the nitric nitrogen contained in the wastewater. The decomposition treatment of nitric nitrogen by a catalyst is a method in which nitric nitrogen waste liquid, a catalyst, and a reducing agent are mixed, and a reduction reaction is carried out under heating conditions to decompose nitric acid from nitrite to nitrogen, and the nitrogen is discharged into the atmosphere for treatment. Due to such a reaction, no sludge such as microbial decomposition is generated, and it is a relatively clean treatment method. However, nitrite and nitrogen decomposed and generated by the reduction reaction are further reduced, and over-reduction reaction occurs to generate ammonia, which causes problems such as inability to discharge into rivers, etc., and depending on the type of reducing agent used, there is a problem that unreacted reducing agent remains in the waste liquid, increasing the BOD and COD values and making it impossible to discharge into rivers, etc.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Summary of the Invention

Problems to be Solved by the Invention

[0008] The present invention aims to provide a treatment method for continuously decomposing nitrate nitrogen, which allows for the continuous decomposition of high-concentration nitrate nitrogen using a catalyst and a reducing agent. In this method, ammonia produced by over-reduction and unreacted reducing agent can be decomposed under oxidative decomposition conditions using the same catalyst as that used for the decomposition of nitrate nitrogen. Furthermore, the invention aims to provide a treatment method for decomposing nitrate nitrogen that can reduce the treated wastewater to a level suitable for discharge into rivers. Furthermore, the aim is to provide a treatment method that does not require activated sludge, reacts quickly, and can decompose nitrate nitrogen at high concentrations in a short time. Additionally, the aim is to provide a treatment method that can continuously decompose large quantities of nitrate nitrogen over the long term by continuously treating nitrate-containing wastewater. Furthermore, the objective is to provide a reactor packed with a catalyst suitable for a treatment method capable of decomposing nitrate nitrogen, and an apparatus comprising the structure including the reactor. [Means for solving the problem]

[0009] The inventors diligently studied and conducted research to solve the above problems, and have completed the present invention. The present invention is as follows (1) to (7). (1) A method for treating nitric acid-containing waste liquid comprising the following steps: Step 1: Reactor I, packed with a reducing catalyst consisting of metal nanoparticles containing Pd and Cu with an average particle size of 1 to 30 nm supported on a carrier, is subjected to a linear velocity of 1.0 × 10⁻¹⁰ of nitric acid-containing waste liquid (pH 5.0 to 8.0) adjusted to a temperature of 5 to 90°C. -6 ~1.0×10 -2 A step of adding a reducing agent to the reactor I in a molar amount equal to or greater than 1.0 times the amount of nitrate contained in the nitrate-containing waste liquid, while flowing it in at m / second. Step 2: Maintaining the reactor I at a temperature of 5 to 90°C and proceeding with the nitric acid decomposition reaction to obtain a treatment solution I. Step 3: The reducing catalyst, in which the metal nanoparticles having an average particle size of 1 to 30 nm and containing Pd and Cu are supported on a carrier, is filled into reactor II, and the treatment liquid I is passed through it at a linear velocity of 1.0 × 10⁻¹⁰ -6 ~1.0×10 -2A step of introducing oxygen or air bubbles into reactor II while flowing them in at a rate of m / second. Step 4: Adjust the pH of the treatment solution in reactor II to 9.0-12.0, maintain the temperature of reactor II at 20-90°C, and then adjust the pH to 5.0-8.0 to obtain treatment solution II. (2) The method for treating nitric acid-containing waste liquid as described in (1), wherein steps 1 to 4 are carried out continuously. (3) A tank for storing nitrate-containing waste liquid, A reactor I is packed with a reducing catalyst in which metal nanoparticles containing Pd and Cu, with an average particle size of 1 to 30 nm, are supported on a carrier, Reactor II is filled with a reducing catalyst in which metal nanoparticles containing Pd and Cu are supported on a carrier, with an average particle size of 1 to 30 nm, and further comprises a bubble inlet and a bubble generator connected thereto. A nitrate-containing waste liquid treatment apparatus comprising a structure connecting a tank for storing the treatment liquid II obtained from the reactor II, and a reactor II. (4) The nitric acid-containing waste liquid treatment apparatus according to (3), wherein, in the particle size distribution obtained by image analysis of the metal nanoparticles supported on the reducing catalyst packed in reactor I and reactor II, the particle size at which the cumulative frequency reaches 10% is defined as D10, the particle size at which the cumulative frequency reaches 50% is defined as D50, and the particle size at which the cumulative frequency reaches 90% is defined as D90, the condition expressed by the following formula (i). Formula (i): 0.9≦[(D10+D90) / 2] / D50≦2.3 (5) A reactor packed with a reducing catalyst having the following characteristics 1) to 3), wherein metal nanoparticles with an average particle size of 1 to 30 nm are supported on a carrier. 1) The metal nanoparticles contain Pd and Cu, with a mass ratio of Pd / (Pd+Cu)×100 ranging from 2% to 98%, and further contain carbon at a mass ratio of C / (Pd+Cu)×100 ranging from 0.1% to 5%. 2) The content ratio of the metal nanoparticles in the reduction catalyst is 0.1 to 5% by mass. 3) The reduction catalyst has an average particle size of 10 μm to 5 cm and a specific surface area of ​​50 to 3000 m². 2 It must be within the range of / g. (6) A reactor filled with the reducing catalyst described in (5), wherein the reducing catalyst further has the characteristics of 4) below. 4) In the particle size distribution obtained by image analysis of the metal nanoparticles supported on the reduction catalyst, when the particle size is defined as D10 for the cumulative frequency 10%, D50 for the cumulative frequency 50%, and D90 for the cumulative frequency 90%, the following condition expressed by formula (i) is satisfied. Formula (i): 0.9≦[(D10+D90) / 2] / D50≦2.3 (7) A reactor filled with the reducing catalyst described in (5) or (6), wherein the reducing catalyst further has the characteristics of 5) below. 5) In the reduction catalyst, the average value M of the ratio m{m=(s / t)×100} of metal nanoparticles that are separated by 1 nm or more from adjacent metal nanoparticles in all directions is in the range of 50 to 100%. (Here, 10,000 nm in electron microscope image [300,000x magnification]) 2 The number of metal nanoparticles within a 100 nm square area was determined as t, the number of metal nanoparticles in the electron microscope image [300,000x magnification] that are separated from adjacent metal nanoparticles by a minimum distance of 1 nm or more in all directions was determined as s, the percentage of metal nanoparticles in the electron microscope image [300,000x magnification] that are separated from adjacent metal nanoparticles by a minimum distance of 1 nm or more in all directions was determined as m[%], and m was calculated for 50 randomly selected locations in the reduction catalyst, and the average number M[%] was calculated. [Effects of the Invention]

[0010] In the present invention's method for treating nitric acid-containing wastewater, the high concentration of nitrate nitrogen contained in the nitrate-containing wastewater is decomposed using a reducing catalyst and a reducing agent. The ammonia produced by over-reduction during the decomposition process of nitrate nitrogen, along with any unreacted reducing agent used, can be decomposed under oxidative decomposition conditions by the reducing catalyst in the reactor without the need to add any new treatment agents. This method is highly advantageous because it allows for continuous and efficient treatment without the generation of by-products such as sludge, and the treated wastewater can be discharged into rivers or sewers. In addition, a reactor filled with the reducing catalyst of the present invention and an apparatus including the reactor are applicable to the method for treating nitric acid-containing waste liquid of the present invention, and are effective for decomposing nitrous nitrogen, decomposing ammonia generated by over-reduction, and decomposing a reducing agent.

Brief Description of the Drawings

[0011] [Figure 1] It is a schematic side view showing an example of the structure of a nitric acid-containing waste liquid treatment apparatus.

Embodiments for Carrying Out the Invention

[0012] The present invention will be described. The method for treating nitric acid-containing waste liquid of the present invention relates to a treatment method capable of rapidly decomposing / removing nitrous nitrogen from nitric acid-containing waste liquid containing a large amount of nitric acid. Here, the nitric acid concentration of the nitric acid-containing waste liquid is not particularly limited, and for example, a nitric acid concentration of 500,000 ppm level can be mentioned. In addition, examples of the waste liquid include waste liquid from a food manufacturing process, waste liquid from a chemical manufacturing process, waste liquid from a mechanical manufacturing process, etc., but there is no particular limitation as long as it is a liquid waste liquid. The method for treating nitric acid-containing waste liquid of the present invention includes the following steps 1 to 4. Step 1: A nitric acid-containing waste liquid (pH 5.0 or more and 8.0 or less) adjusted to a temperature of 5 to 90°C is fed into a reactor I filled with a reducing catalyst in which metal nanoparticles having an average particle diameter of 1 to 30 nm and containing Pd and Cu are supported on a carrier at a linear velocity of 1.0×10 -6 ~1.0×10 -2 m / sec, and a reducing agent having a molar amount of 1.0 times or more the amount of nitric acid contained in the nitric acid-containing waste liquid is added to the reactor I. Step 2: The reactor I is maintained at a temperature of 5 to 90°C, and a nitric acid decomposition reaction is advanced to obtain a treatment liquid I. Step 3: The treatment liquid I is fed into a reactor II filled with a reducing catalyst in which metal nanoparticles having an average particle diameter of 1 to 30 nm and containing Pd and Cu are supported on a carrier at a linear velocity of 1.0×10 -6 ~1.0×10 -2A step of introducing oxygen or air bubbles into reactor II while flowing them in at a rate of m / second. Step 4: The process of adjusting the pH of the treatment solution in reactor II to 9.0-12.0, maintaining the temperature of reactor II at 20-90°C, and then adjusting the pH to 5.0-8.0 to obtain treatment solution II. The nitric acid-containing waste liquid to be introduced into reactor I can be stored in advance in a sealed storage tank for nitric acid-containing waste liquid (storage tank). A nanobubble generator may be connected to the storage tank to generate nanobubbles inside the storage tank. When introducing the nitric acid-containing waste liquid into the storage tank, the optionally connected nanobubble generator may be activated to generate nanobubbles inside the storage tank. The storage tanks and each reactor can be connected in the inlet line, although this is not particularly limited.

[0013] <Process 1> In step 1, a reducing catalyst consisting of metal nanoparticles (average particle size 1-30 nm) containing Pd and Cu supported on a carrier is poured into reactor I at a linear velocity of 1.0 × 10⁻¹⁰. -6 ~1.0×10 -2 While flowing in at a rate of m / second, a reducing agent in a molar amount equal to or greater than 1.0 times the amount of nitrate contained in the nitrate-containing waste liquid is added to the reactor I. In this specification, unless otherwise specified, "waste liquid" means nitrate-containing waste liquid.

[0014] [Nitrate-containing waste liquid and its nitrate concentration] As mentioned above, there are no particular restrictions on the nitric acid concentration in nitric acid-containing wastewater, but it is preferable that the nitric acid concentration be 500,000 ppm or less. Pretreatment may be performed depending on the composition of the wastewater. Examples of such pretreatment include the removal of turbidity-reducing substances and the removal of COD or BOD components. Examples of turbidity-reducing substances include inorganic oxides and hydroxide gels. Examples of COD components include organic resins, organic solvents, oils and fats, and sugars, while examples of BOD components include organic resins, organic solvents, oils and fats, and sugars. The removal of turbidity-reducing substances can be carried out, for example, by adding cationic or anionic polymers to cause aggregation, followed by centrifugation or filter pressing. The removal of COD or BOD components can be carried out by methods such as microbial decomposition, decomposition with strong oxidizing agents such as hydrogen peroxide or potassium permanganate, or decomposition using ozone or ultraviolet light.

[0015] [Introduction of nitric acid-containing waste liquid into reactor I, which is filled with a reduction catalyst] The pH of the nitric acid-containing waste liquid introduced into reactor I, which is packed with the reduction catalyst, is preferably in the range of pH 5.0 to 8.0. If the pH of the nitric acid-containing waste liquid is below 5.0, the metal nanoparticles contained in the reduction catalyst may dissolve, potentially reducing the catalytic function. If the pH of the nitric acid-containing waste liquid exceeds 8.0, it is likely to reach a high pH of over 13.0 after decomposition, which can make subsequent handling difficult and is therefore undesirable. pH adjusters may be added to the nitric acid-containing waste liquid as needed to adjust its pH. A preferred pH range for the nitric acid-containing waste liquid is 5.5 to 7.5.

[0016] Normally, it is preferable to introduce the nitric acid-containing waste liquid (pH 5.0-8.0) from a sealed storage tank (sealed tank) into reactor I, which is filled with a reduction catalyst, through an introduction line. It is desirable that the sealed storage tank (sealed tank) for the nitric acid-containing waste liquid be a sealed tank equipped with a breather valve that opens when the pressure exceeds 1.05 times atmospheric pressure. Such a sealed tank allows for control of dissolved oxygen and oxygen concentration in both the gas and liquid phases. Therefore, the reduction reaction during nitric acid decomposition proceeds more uniformly, byproduct generation due to side reactions is less likely, and the reaction rate of the nitric acid decomposition reaction is less likely to decrease. The sealed tank may also be equipped with a stirrer and a temperature-adjustable jacket.

[0017] The temperature of the nitric acid-containing waste liquid introduced into reactor I is preferably in the range of 5 to 90°C. This temperature range is desirable because it allows for rapid decomposition of nitric acid. If the temperature of the nitric acid-containing waste liquid is below 5°C, nitric acid can be decomposed, but problems such as low productivity and solidification that prevents decomposition may occur. If the temperature exceeds 90°C, the catalyst tends to deteriorate due to heat, which is undesirable. A temperature range of 10 to 80°C is preferably recommended for the nitric acid-containing waste liquid. Furthermore, it is preferable to adjust the temperature of the nitric acid-containing waste liquid in the sealed tank. Reactor I has a nitric acid waste liquid introduction line and an introduction line for reducing agent at its lower part, and above it is a line for discharging the treated liquid I that has been treated with nitric acid in reactor I, and a breather valve that opens when the pressure exceeds 1.05 times atmospheric pressure. It is desirable that a jacket capable of controlling the temperature inside reactor I with hot or cold water is installed around reactor I. With such a device, the reaction temperature can be controlled to be constant, an efficient nitric acid decomposition reaction can be obtained, and the nitrogen and other substances produced by the decomposition of nitric acid can be released into the atmosphere through a breather valve.

[0018] The linear velocity of the nitric acid-containing waste liquid flowing into reactor I, which is packed with a reduction catalyst, is 1.0 × 10⁻⁶. -6 ~1.0×10 -2 It is desirable that the rate is within the range of m / s. Within this range, the rate of nitrate decomposition tends to improve. Linear velocity is 1.0 × 10⁻⁶ -6 When the linear velocity is less than m / s, the process tends to take too long, resulting in a decrease in resolution efficiency. Also, when the linear velocity is 1.0 × 10⁻⁶ -2 If the linear velocity exceeds m / s, problems such as insufficient decomposition of nitric acid may occur. The linear velocity of the nitric acid-containing waste liquid flowing into reactor I is preferably 3.0 × 10⁻⁶. -6 m / s~1.0×10 -3 A range of m / s is recommended.

[0019] When introducing nitric acid-containing waste liquid (pH 5.0-8.0) into the sealed tank, a nanobubble generator connected to the sealed tank may be driven to generate nitrogen and hydrogen nanobubbles in the waste liquid before introduction. The presence of nanobubbles in the waste liquid tends to accelerate the nitric acid decomposition rate in step 2. The nanobubbles preferably have an average bubble diameter of 50-500 nm. 5 It is preferable that it contains more than 1 / mL. This type of waste liquid contains nanobubbles with an average bubble diameter of 50-500 nm. 5 If the content exceeds 1 / mL, it tends to improve the rate of nitrate decomposition and promote the decomposition of organic impurities. Examples of organic matter that can be broken down by nanobubbles include organic impurities, bacteria, and airborne bacteria. When the average bubble diameter of nanobubbles is less than 50 nm, the improvement in the rate of nitrate decomposition and the decomposition effect of organic impurities tend to be poor. Furthermore, when the average bubble diameter exceeds 500 nm, problems tend to arise such as a short nanobubble lifespan, making it impractical. The lower limit of the number of nanobubbles in the waste liquid is not particularly restricted as long as it does not interfere with the nitric acid decomposition reaction, but 1.0 × 10 5 More preferably 1.0 × 10¹ / mL or higher. 8 A concentration of 1.0 × 10¹ / mL or higher is even more preferable. There is no particular upper limit, but 1.0 × 10¹ / mL is preferable. 11 A concentration of 5.0 × 10 is preferred. 10 More preferably 1.0 × 10¹ / mL. 10 A concentration of 1 / mL is even more preferable. The type of gas contained in the nanobubbles is not particularly limited, but typically nitrogen, hydrogen, and mixtures thereof are used.

[0020] It is desirable to stir the nitric acid-containing waste liquid introduced into reactor I to ensure homogenization. The rotation speed of the stirrer is preferably in the range of 5 to 500 rpm.

[0021] Furthermore, in order to ensure that the nitric acid decomposition reaction proceeds uniformly from step 2 onward, it is preferable to inject an inert gas into the nitric acid-containing waste liquid. The inert gas is injected into reactor I from the introduction line. Once the tank pressure after injection reaches 1.05 times atmospheric pressure and the breather valve opens, the injection of the inert gas may be stopped.

[0022] The type of inert gas is not particularly limited as long as it does not inhibit the reduction reaction, but nitrogen, argon, and hydrogen are preferred. Hydrogen, in particular, also has reducing properties and can be used as a reducing agent. However, since there is a risk of ignition and explosion, for safety reasons it is preferable to mix it with nitrogen or argon to bring it to a concentration outside the explosive limit before use. In this specification, hydrogen gas mixed with nitrogen or argon is also referred to as "diluted hydrogen."

[0023] The gas introduction rate of the inert gas varies depending on the tank size, but it is preferable to use a flow rate of 0.1 to 1 times the tank size. For example, for a 1L tank, a flow rate of 0.1 to 1L / min is preferable. If the flow rate is less than 0.1 times, it will take a long time to remove the oxygen in the tank. If the flow rate is greater than 1 time, the liquid may agitate violently due to the mixing of the gas entering from the introduction line with the liquid, which may hinder the decomposition reaction.

[0024] The gas pressure of the inert gas is not particularly limited as long as the flow rate described above can be obtained, but for example, 0.2 to 1 MPa is preferred.

[0025] The dissolved oxygen content in nitric acid-containing wastewater should preferably be in the range of 0.3 mg / L to 1.0 mg / L. Within this range, the rate of nitric acid decomposition improves, and the decomposition of organic impurities tends to be promoted. When the dissolved oxygen level is less than 0.3 mg / L, the rate of nitrate decomposition decreases, and the decomposition effect of organic impurities tends to be poor. Furthermore, when the dissolved oxygen level exceeds 1.0 mg / L, problems such as a decrease in the rate of nitrate decomposition are more likely to occur. A dissolved oxygen level of 0.3 mg / L to 0.5 mg / L is preferably recommended.

[0026] [Reducing catalyst] In the present invention, the reduction catalyst is a reduction catalyst in which metal nanoparticles (average particle size 1 to 30 nm) are supported on a carrier, and furthermore, a reduction catalyst having the following characteristics can be used. 1) The metal nanoparticles contain Pd and Cu, with a mass ratio of Pd / (Pd+Cu)×100 in the range of 2 to 98, and further contain carbon in a mass ratio of C / (Pd+Cu)×100 of 0.1 to 5%. 2) The content ratio of the metal nanoparticles in the reduction catalyst is 0.1 to 5% by mass. 3) The reduction catalyst has an average particle size of 10 μm to 5 cm and a specific surface area of ​​50 to 3000 m². 2 It must be within the range of / g.

[0027] Details about the reduction catalyst are provided separately later.

[0028] The amount of reducing catalyst used is preferably in the range of 0.01 g / kg to 10 g / kg relative to the mass of the nitric acid-containing waste liquid, in terms of the mass of metal nanoparticles (i.e., the ratio of the mass of metal nanoparticles in the added reducing catalyst to the mass of the nitric acid-containing waste liquid (kg)). If the amount of reducing catalyst used is less than 0.01 g / kg in terms of the mass of metal nanoparticles, the amount of catalyst may be insufficient and the reaction may not proceed. If the amount of reducing catalyst used exceeds 10 g / kg in terms of the mass of metal nanoparticles, the reaction will be too fast, causing a rapid temperature rise and posing a safety problem. A suitable range for the amount of reducing catalyst used is recommended to be 0.1 g / kg to 7 g / kg.

[0029] When packing the reducing catalyst into reactor I, it is preferable to use a packing density in the reaction vessel within the range of 50-90%. If the packing density is less than 50%, the contact efficiency between the nitric acid-containing waste liquid and the catalyst is poor, and efficient nitric acid decomposition may not occur. If the packing density exceeds 90%, the nitric acid decomposition reaction is more likely to occur, but because there are fewer voids, the nitrogen generated by the decomposition can cause a rapid pressure increase, leading to safety problems. A suitable packing density for the reducing catalyst in the reaction vessel is recommended to be in the range of 60-85%. Furthermore, it is preferable that the packing rate of the reduction catalyst in reactor II be the same.

[0030] The temperature of the nitric acid-containing waste liquid after it has been introduced into reactor I, which is packed with the reduction catalyst, is not particularly limited, but a temperature range of 5 to 90°C is preferred, for example.

[0031] The reducing agent added to reactor I in step 1 is used to reduce nitric acid to nitrite, and then to nitrogen.

[0032] The type of reducing agent is not particularly limited as long as it has a reducing effect, but examples include hydrogen, hydrazine, formic acid, citric acid, tannic acid, sodium borohydride, methanol, and ethanol. If the wastewater containing nitric acid is to be discharged into rivers or other bodies of water after treatment, it is preferable that the reducing agent does not contain metals included in wastewater regulations. Wastewater regulations also include BOD and COD regulations, but organic reducing agents and hydrazine are suitable because they undergo oxidative decomposition in treatment step 4 of the present invention.

[0033] The amount of reducing agent to be used should be at least 1.0 times the amount of nitric acid contained in the nitric acid-containing wastewater. A reduction amount of 1.0 times or more is theoretically the minimum amount required for reduction and is therefore preferable. Generally, it is recommended to use an amount between 1.0 and 50 times the amount. Regarding the method of adding the reducing agent, it is preferable to add the reducing agent directly to the waste liquid, to dilute it with pure water or the like before adding it, or to add it all at once or sequentially. It is preferable to add the reducing agent to reactor I, which is filled with the reducing catalyst, through an introduction line.

[0034] <Process 2> In step 2, following step 1, reactor I is kept at a temperature of 5 to 90°C, and the nitric acid decomposition reaction in the nitric acid-containing waste liquid introduced into reactor I is carried out to obtain a treated solution. In this specification, this solution is referred to as "treated solution I". To facilitate the nitric acid decomposition reaction, the temperature of the nitric acid-containing waste liquid after it is introduced into reactor I, which is packed with a reducing catalyst, is preferably in the range of 5 to 90°C. This temperature range is desirable because it allows for rapid decomposition of nitric acid. Although nitric acid can be decomposed even if the temperature of the nitric acid-containing waste liquid introduced into reactor I is below 5°C, this is problematic due to low productivity and the possibility of solidification preventing decomposition. If the temperature exceeds 90°C, the catalyst tends to deteriorate due to heat, which is undesirable. The temperature of the nitric acid-containing waste liquid introduced into reactor I is preferably recommended to be in the range of 10 to 80°C.

[0035] It is preferable to maintain the temperature of reactor I at 5-90°C for 30 minutes to 24 hours. A time of 30 minutes to 24 hours for the nitric acid decomposition reaction in step 2 is preferable because it allows for the completion of nitric acid decomposition. If the nitric acid decomposition reaction takes less than 30 minutes, the reaction time is too short, and the nitric acid may not be decomposed. If it takes longer than 24 hours, the practicality decreases.

[0036] Processed liquid I is obtained through this process 2. A tank for storing the resulting processed liquid I may be provided after reactor I in step 2.

[0037] <Process 3> In step 3, following step 2, the treatment liquid I is introduced into reactor II, which is packed with a reducing catalyst consisting of metal nanoparticles containing Pd and Cu with an average particle size of 1 to 30 nm supported on a carrier, at a linear velocity of 1.0 × 10⁻¹⁰. -6 ~1.0×10 -2 Oxygen or atmospheric bubbles are introduced into reactor II at a rate of m / second.

[0038] The linear velocity when flowing the treatment liquid I into reactor II, which is packed with the reduction catalyst, is 1.0 × 10⁻⁶. -6 ~1.0×10 -2 It is desirable that the rate of oxidation is within the range of m / s. Within this range, oxidative decomposition tends to proceed uniformly. Linear velocity is 1.0 × 10⁻⁶ -6 If the velocity is less than m / s, or if the linear velocity is 1.0 × 10⁻⁶ -2When the rate exceeds m / second, the efficiency of ammonia removal and oxidative decomposition of reducing agents tends to decrease. The linear velocity of the nitric acid-containing waste liquid flowing into the reactor II is preferably 3.0 × 10⁻¹⁰. -6 m / s~1.0×10 -3 A range of m / s is recommended. Furthermore, reactor II is equipped with a line for introducing the treatment liquid I and an introduction line for introducing oxygen and air at its lower part, and at its upper part, a line for discharging the treatment liquid II that has been processed in reactor II, and a breather valve that opens when the pressure exceeds 1.05 times atmospheric pressure. It is also desirable that a jacket be installed around reactor II to control the temperature inside reactor II with hot or cold water. With such equipment, the reaction temperature can be controlled to be constant, an efficient oxidation reaction can be obtained, and nitrogen and other substances produced by the decomposition of ammonia and reducing agents can be released into the atmosphere through the breather valve. It is preferable that the line end of the introduction line for introducing oxygen and air be configured to form bubbles using a sintered filter or the like, as this improves the contact efficiency with the gas and promotes the decomposition reaction.

[0039] In step 3, the gas used for bubbling in reactor II is preferably oxygen and / or air for the purpose of oxidizing the ammonia in the treatment liquid I to nitrogen and decomposing the reducing agent to render it harmless. Bubbling is preferably generated by introducing the gas from the introduction line into reactor II, which is filled with the reduction catalyst. Here, the flow rate of the bubbling gas introduced into reactor II is preferably in the range of 0.1 times / min to 1 time / min of the tank size in order to carry out uniform oxidative decomposition. If the bubbling gas flow rate is less than 0.1 times / min, oxidative decomposition may not occur or ammonia may not be removed, which is undesirable. If the bubbling gas flow rate exceeds 1.0 times / min, the efficiency of ammonia removal and oxidative decomposition of the reducing agent will not increase any further. Preferably, the bubbling gas flow rate is recommended to be in the range of 0.2 times / min to 0.8 times / min.

[0040] The dissolved oxygen content in the treatment liquid I introduced into reactor II should preferably be in the range of 0.3 mg / L to 1.0 mg / L. Within this range, the removal of ammonia and the oxidative decomposition of the reducing agent tend to be promoted. If the dissolved oxygen level is less than 0.3 mg / L, the oxygen level is insufficient, which tends to reduce the oxidative decomposition rate of the reducing agent and result in poor ammonia removal. Conversely, if the dissolved oxygen level exceeds 1.0 mg / L, while there is sufficient oxygen and no problems such as a reduced oxidative decomposition rate of the reducing agent occur, it is undesirable from the standpoint of requiring a large amount of energy to increase the oxygen concentration. A dissolved oxygen level of 0.4 mg / L to 0.8 mg / L is preferably recommended.

[0041] When introducing bubbles and performing ammonia removal and oxidative decomposition of the reducing agent by a reduction catalyst, the temperature of treatment liquid I is preferably in the range of 20 to 90°C. Below 20°C, ammonia tends to remain in the wastewater, and the oxidative decomposition of the reducing agent does not proceed well. Above 90°C, the reduction catalyst may deteriorate, which is undesirable. The temperature of treatment liquid I is preferably recommended to be in the range of 30 to 60°C. Ammonia that vaporizes without oxidative decomposition is preferably diffused into the atmosphere or recovered by installing an absorption tower.

[0042] It is preferable that the bubbles are uniform and small in size, and that the bubbles be generated by passing them through a sintered filter or a ceramic filter with small pores.

[0043] <Step 4> In step 4, following step 3, the treatment solution in reactor II is adjusted to a pH of 9.0 to 12.0, reactor II is kept at a temperature of 20 to 90°C, and the oxidative decomposition of by-product ammonia by the reducing catalyst and the decomposition of the remaining reducing agent are promoted, after which the pH is adjusted to 5.0 to 8.0 to obtain the treatment solution. In this specification, this solution is referred to as "treatment solution II".

[0044] In step 4, it is preferable to add bubbles. In step 4, the ammonia produced by over-reduction and the remaining reducing agent are oxidatively decomposed with a reducing catalyst, and a waste liquid (waste liquid from which nitric acid, ammonia, and reducing agent have been sufficiently removed) from which ammonia and the remaining reducing agent have been removed can be obtained. Furthermore, by adjusting the pH to 5.0 to 8.0, a treated liquid II can be obtained that can be discharged into rivers, for example.

[0045] Preferably, step 4 involves adjusting the temperature of the treatment liquid in reactor II after the processing in step 3 to 20-90°C, adding oxygen or atmospheric bubbles to promote the oxidative decomposition of by-product ammonia and the decomposition of residual reducing agent by the reducing catalyst, and then adjusting the pH (pH 5.0-8.0) to obtain treatment liquid II. Furthermore, the adjustment of the pH of the treatment solution after the decomposition of the residual reducing agent to 5.0-8.0 may be performed in the tank where treatment solution II is stored.

[0046] Treatment solution II (pH 5.0-8.0) can be discharged into sewage systems or the sea. The pH adjusting agent should preferably be one that is not subject to wastewater regulations, and mineral acids such as hydrochloric acid or sulfuric acid are preferred. The temperature for pH adjustment is preferably 10-40°C. The amount of pH adjuster to be added is determined using a pH meter, and the amount of pH adjuster added should be such that the pH is in the range of 5.0 to 8.0.

[0047] When adjusting the pH, stirring is desirable for accurate pH adjustment. A recommended stirring speed is between 5 and 500 rpm.

[0048] In the method for treating nitric acid-containing wastewater of the present invention, it is preferable to treat the nitric acid-containing wastewater continuously by sequentially performing steps 1 to 4 described above. By treating the nitric acid-containing wastewater continuously, a large amount of nitrate nitrogen can be decomposed over a long period of time. Continuous treatment results in high productivity, and when the catalyst reaches the end of its lifespan and needs to be replaced, it can be easily replaced by replacing the entire reaction vessel.

[0049] A preferred embodiment of the present invention's method for treating nitric acid-containing wastewater will be described. [Preferred embodiments of the present invention] The present invention provides a method for treating nitric acid-containing wastewater, which preferably includes the following steps: preparing a sealed storage tank for nitric acid-containing wastewater having a stirring device and a breather valve equipped with a breather valve that opens when the internal pressure is 1.05 times or more atmospheric pressure; carrying out step 1 by adding a reducing agent while flowing the nitric acid-containing wastewater from the sealed tank into reactor I filled with a reducing catalyst; carrying out step 2 by maintaining the reactor I at a temperature of 5 to 90°C and promoting the nitric acid decomposition reaction to obtain treated liquid I; carrying out step 3 by flowing treated liquid I into reactor II filled with a reducing catalyst and introducing oxygen or atmospheric bubbles; adjusting the pH of the treated liquid in reactor II to 9.0 to 12.0; maintaining the reactor II at a temperature of 20 to 90°C to promote the oxidative decomposition of by-product ammonia and the decomposition of residual reducing agent; and then adjusting the pH of the treated liquid to 5.0 to 8.0 to obtain treated liquid II. Furthermore, the nitric acid-containing wastewater treatment apparatus of the present invention preferably has a structure in which a tank for storing nitric acid-containing wastewater is connected to a reactor I, which is filled with a reduction catalyst having an average particle size of 1 to 30 nm and in which metal nanoparticles containing Pd and Cu are supported on a carrier, a reactor II which is filled with a reduction catalyst having an average particle size of 1 to 30 nm and in which metal nanoparticles containing Pd and Cu are supported on a carrier, and a bubble inlet passage and a bubble generator connected thereto, and a tank for storing the treatment liquid II obtained from the reactor II. Furthermore, the reactor used in the treatment of nitric acid-containing wastewater of the present invention is preferably a reactor packed with a reducing catalyst having at least one of the following characteristics 1) to 5), wherein the reducing catalyst is a reducing catalyst in which metal nanoparticles having an average particle size of 1 to 30 nm are supported on a carrier. 1) The metal nanoparticles contain Pd and Cu, with a mass ratio of Pd / (Pd+Cu)×100 ranging from 2% to 98%, and further contain carbon at a mass ratio of C / (Pd+Cu)×100 ranging from 0.1% to 5%. 2) The content ratio of the metal nanoparticles in the reduction catalyst is 0.1 to 5% by mass. 3) The reduction catalyst has an average particle size of 10 μm to 5 cm and a specific surface area of ​​50 to 3000 m². 2 It must be within the range of / g. 4) In the particle size distribution obtained by image analysis of the metal nanoparticles supported on the reduction catalyst, when the particle size is defined as D10 for the cumulative frequency 10%, D50 for the cumulative frequency 50%, and D90 for the cumulative frequency 90%, the following condition expressed by formula (1) is satisfied. Formula (1): 0.9≦[(D10+D90) / 2] / D50≦2.3 5) In the reduction catalyst, the average value M of the ratio m{m=(s / t)×100} of metal nanoparticles that are separated by 1 nm or more from adjacent metal nanoparticles in all directions is in the range of 50 to 100%. (Here, 10,000 nm in electron microscope image [300,000x magnification]) 2 The number of metal nanoparticles within a 100 nm square area was determined as t, the number of metal nanoparticles in the electron microscope image [300,000x magnification] that are separated from adjacent metal nanoparticles by a minimum distance of 1 nm or more in all directions was determined as s, the percentage of metal nanoparticles in the electron microscope image [300,000x magnification] that are separated from adjacent metal nanoparticles by a minimum distance of 1 nm or more in all directions was determined as m[%], and m was calculated for 50 randomly selected locations in the reduction catalyst, and the average number M[%] was calculated.

[0050] [Sealed storage tank for nitric acid-containing waste liquid (storage tank)] The shape of the storage tank is not particularly limited, but a cylindrical shape is preferred. A curved bottom is preferable to a flat bottom. If the bottom is angular, mixing of the liquid may be poor.

[0051] [Agitator, stirring blade shape] While not particularly limited, flat paddle blades, inclined paddle blades, propeller blades, Faudler blades, disc turbine blades, helical ribbon blades, screw blades, etc., are preferred. If the tank size is large, it is preferable to attach multiple stages of blades. If the tank bottom is curved, Faudler blades are preferred. In addition, commercially available agitators such as CONBIJET and PHASEJET manufactured by EKATO can also be suitably used. In particular, combining them with the company's GASJET allows for the efficient generation of a large amount of bubbles, making processes 1 and 2 more efficient.

[0052] [Baffle] It is also preferable to install baffles (obstructing plates) inside the storage tank to generate turbulence during agitation and improve agitation efficiency. The shape of the baffles will vary depending on the shape and size of the tank, but it is preferable to install multiple plate-shaped (plate baffles) or cylindrical (rod baffles). Installing baffles can be expected to make the concentration of the nitric acid waste liquid more uniform. Plate baffles are installed vertically on the inside of the bottom of the storage tank. Rod baffles are installed by suspending rod-shaped members vertically from above the storage tank.

[0053] [Material of the storage tank] The material of the storage tank is not particularly limited as long as it is a low-corrosion material, but stainless steel, plastic, FRP, fluororesin, etc. are preferred. Among stainless steel materials, SUS304 series and the highly corrosive SUS316 series are preferred. SUS304L and 316L are particularly preferred due to their high corrosion resistance. On the other hand, since the pH after the reaction may become highly alkaline, materials that dissolve in alkali, such as glass, are not preferred.

[0054] [Bubbling nozzle] The bubbling nozzle is positioned at the outlet where the gas is discharged in the introduction line. The shape of the bubbling nozzle is not particularly limited as long as it can form bubbles from the gas, but straight or porous shapes are preferred. Porous shapes are especially preferred because they can create smaller bubbles. The size of the holes varies depending on the tank size, but a diameter of 0.1 mm to 1 cm is preferred. Using a nozzle with such a shape allows steps 3 and 4 to be carried out efficiently.

[0055] [Temperature control function] In steps 2 and 4, it is necessary to adjust the reactor temperature, but this is not particularly limited as long as the waste liquid temperature can be raised or lowered. As a heat source, electricity, steam, etc. are preferred, and if cooling is required, the outside of the tank can be cooled by cooling water, antifreeze, etc. When temperature control is performed using steam or water, it is preferable to install a jacket around the reactor through which these substances flow, and the size of the jacket is not particularly limited as long as it can be controlled within the desired range by thermodynamically calculating from the tank size and material to achieve the desired temperature.

[0056] [Reducing agent addition line (line mixing, sequential addition, shower addition)] The reducing agent addition line is not particularly limited, but it is preferable that the reducing agent does not enter locally. The reducing agent is preferably introduced by line mixing while being diluted with water, sequentially, or by shower addition.

[0057] [Breather valve] The breather valve only needs to open when the pressure inside the tank and reactor reaches or exceeds the target pressure (1.05 times atmospheric pressure), and there are no particular restrictions as long as a commercially available breather valve is used.

[0058] [Gas introduction line] The gas introduction line consists of a gas introduction line that can introduce gas into the storage tank and an atmospheric bubble introduction line that can introduce atmospheric bubbles into reactor II. The tip of the gas introduction line is immersed in the liquid stored in the sealed tank (such as nitric acid-containing waste liquid or liquid with other additives), and gas is discharged into the liquid from the tip of the gas introduction line. Gas is present in the upper part of the liquid in the sealed tank, and the gas introduced from the gas introduction line is discharged into this gas-containing area.

[0059] [Nanobubble Generator] It is preferable to pass nitric acid-containing waste liquid through a nanobubble generator before placing it in a storage tank. The nanobubble generator is not particularly limited as long as a commercially available nanobubble generator is used.

[0060] [Nanobubble circulation line] To introduce nanobubbles into nitric acid-containing wastewater in a storage tank, it is preferable to have a circulation line that allows for circulation using a nanobubble generator. Increasing the circulation volume allows for the generation of more nanobubbles in the nitric acid-containing wastewater.

[0061] [pH adjuster addition line] The pH adjusting agent addition lines can be installed at the top and bottom of reactor II. While there are no particular restrictions on the pH adjusting agent addition lines, it is preferable that the pH adjusting agent is not introduced locally. The pH adjusting agent is preferably introduced via line mixing while being diluted with water, added sequentially, or added via a shower.

[0062] [Reducing catalyst] In the nitric acid-containing wastewater treatment method of the present invention described above, it is preferable to use the reduction catalyst of the present invention, which is described below.

[0063] The reduction catalyst of the present invention is a reduction catalyst in which metal nanoparticles (average particle size 1 to 30 nm) are supported on a carrier, and further has the following features 1) to 3). 1) The metal nanoparticles contain Pd and Cu, with a mass ratio of Pd / (Pd+Cu)×100 in the range of 2 to 98, and further contain carbon in a mass ratio of C / (Pd+Cu)×100 of 0.1 to 5%. 2) The content ratio of the metal nanoparticles in the reduction catalyst is 0.1 to 5% by mass. 3) The reduction catalyst has an average particle size of 10 μm to 5 cm and a specific surface area of ​​50 to 3000 m². 2 It must be within the range of / g. The reducing catalyst of the present invention contributes to the decomposition of nitric acid by reduction, the decomposition of ammonia produced by over-reduction, and the decomposition of used reducing agents.

[0064] The metal nanoparticles are preferably Pd-Cu nanoparticles with an average particle diameter of 1 to 30 nm. It is difficult to prepare Pd-Cu nanoparticles with an average particle diameter of less than 1 nm. If the average particle diameter exceeds 30 nm, the decomposition properties of nitric acid decrease. The average particle diameter of the metal nanoparticles was calculated by measuring the particle size using an electron microscope. Specific procedures are described later.

[0065] Furthermore, if the reduction catalyst does not contain Pd-Cu nanoparticles, for example, if it contains only Pd salts, nitrates, or hydroxides, the reduction rate of nitric acid (especially at low temperatures) is slow, which is undesirable.

[0066] The mass ratio of Pd in ​​Pd-Cu nanoparticles (mass ratio of Pd / (Pd+Cu)×100) is preferably in the range of 2 to 98%. If it is less than 2%, the decomposition of nitric acid to nitrite may be reduced. If it exceeds 98%, the decomposition of nitric acid may be reduced. Within the aforementioned mass ratio range, the nitrate decomposition characteristics are excellent, and the amount of ammonia produced by addition and reduction can be kept low. A mass ratio range of 10 to 90% is preferably recommended. In the reduction catalyst of the present invention, the Pd-Cu nanoparticles supported on the support may be Pd and Cu nanoparticles coexisting, or they may be nanoparticles made of a composite of Pd and Cu or an alloy of Pd and Cu. The inclusion of an alloy is preferable as it provides superior nitric acid decomposition characteristics and lower ammonia production. On the other hand, if only Pd nanoparticles are supported on the support alone, nitric acid decomposition may not proceed, and if only Cu nanoparticles are supported on the support alone, nitric acid decomposition proceeds to the stage where nitric acid is decomposed into nitrous acid, but does not proceed to the stage where nitrous acid is decomposed into nitrogen. Furthermore, if both Pd ions and Cu ions are treated on the support, nitric acid decomposition proceeds, but the catalytic activity is low, making it difficult to obtain a practical decomposition rate.

[0067] The dispersant for metal nanoparticles is preferably one containing carbon (C). Among these, hydroxy acids are preferred. In this invention, the amount (mass) of carbon contained in the metal nanoparticles was determined as a ratio (%) to the amount (mass) of the metal nanoparticles, as shown below. Ratio of C component (%) = ([C mass] / [Pd mass + Cu mass]) × 100 The method for analyzing the carbon content in metal nanoparticles is described later. The carbon component is preferably in the range of 0.01 to 15%, and more preferably 0.1 to 5%, relative to the total mass of Pd and Cu contained in the metal nanoparticles. If the ratio of component C is less than 0.01%, the dispersion stability of the metal nanoparticles is low, making them prone to aggregation when supported on a carrier. This can lead to decreased catalytic activity and a shorter lifespan. If it exceeds 15%, the dispersion stability of the nanoparticles is too high, preventing them from adhering to the particles during support, or even if they do, they may act as a protective colloid, causing redispersion and desorption from the carrier when placed back into water. The desired range for the ratio of component C is 0.03 to 12%. Other elements that may be included besides Pd-Cu are preferably those from periods 4 to 15 of the periodic table, and more preferably Fe, Ni, Sn, Mo, and Zn. The presence of such elements may improve decompositionability.

[0068] The reduction catalyst of the present invention preferably further has the following characteristic 4). 4) In the particle size distribution (based on the number of particles) obtained by image analysis of the metal nanoparticles supported on the reduction catalyst, when the particle size is defined as D10 for the cumulative frequency 10%, D50 for the cumulative frequency 50%, and D90 for the cumulative frequency 90%, the following condition expressed by formula (1) must be satisfied. Formula (1): 0.9≦[(D10+D90) / 2] / D50≦2.3 The value of [(D10+D90) / 2] / D50 indicates the sharpness of the particle size distribution. When this value is in the range of 0.9 to 2.3, the particle size distribution of the metal nanoparticles is uniform (high particle size uniformity), which is desirable as it shows good catalytic activity. If the value of [(D10+D90) / 2] / D50 is less than 0.9, there are many small particles, which can cause aggregation or fusion of particles during nitric acid decomposition, reducing catalytic activity, and is therefore undesirable. A value exceeding 2.3 is undesirable because it indicates a large number of large particles and low nitrate decomposition efficiency. A more preferable range for the value of [(D10+D90) / 2] / D50 is recommended to be between 1.0 and 2.2.

[0069] The reduction catalyst of the present invention is preferably further characterized by the following 5). 5) In the reduction catalyst, the average value M of the ratio m{m=(s / t)×100} of metal nanoparticles that are separated by 1 nm or more from adjacent metal nanoparticles in all directions is in the range of 50 to 100%. Here, 10,000 nm in electron microscope image [300,000x magnification] 2The number of metal nanoparticles within the specified range (100 nm square) was t, the number of metal nanoparticles in the electron microscope image [300,000x magnification] that are separated by a minimum distance of 1 nm or more from adjacent metal nanoparticles in all directions was s, the percentage of metal nanoparticles in the electron microscope image [300,000x magnification] that are separated by a minimum distance of 1 nm or more from adjacent metal nanoparticles in all directions was m[%], m was determined for 50 randomly selected locations in the reduction catalyst, and the average number M[%] was calculated. In the reduction catalyst, the average value M of the ratio m{m=(s / t)×100} of metal nanoparticles that are separated by 1 nm or more from adjacent metal nanoparticles in all directions is an indicator that the metal nanoparticles of the reduction catalyst are supported in a monodisperse manner. When the average value M is in the range of 50 to 100%, the frequency of metal nanoparticles aggregating is low, and a catalyst with high nitric acid decomposition properties can be obtained, which is desirable. When the average value M is less than 50%, the frequency of metal nanoparticle aggregation is high, which can lead to problems such as low nitric acid decomposition properties or desorption of metal nanoparticles from the support during use. More preferably, the average value M is recommended to be in the range of 60 to 100%.

[0070] [Catalyst carrier] Next, we will explain catalyst supports. Catalyst supports are used to support metal nanoparticles (including Pd-Cu nanoparticles). Without a support, the metal nanoparticles would aggregate or grow due to the heat generated during the nitric acid decomposition reaction, shortening the catalyst's lifespan. The support material is preferably an organic support such as graphite or activated carbon, and preferably an inorganic support such as titania, zirconia, or alumina. Among these, activated carbon is particularly preferred because it has a large specific surface area and excellent acid and alkali resistance. Activated carbon is a porous material composed of carbon, oxygen, hydrogen, calcium, etc., and is also a porous material that has undergone chemical or physical treatment such as heat. The raw materials for activated carbon can be naturally derived materials such as coconut shells, bamboo, wood, and bagasse, or petroleum-derived materials such as phenolic resin. Among these, phenolic resin is particularly preferred because its raw material purity is high, resulting in high purity activated carbon, and less catalyst poisoning due to impurities is generated during the nitric acid decomposition reaction.

[0071] The average particle size of the support material is 10 μm to 50 mm, with 30 μm to 30 mm being preferred. Within this average particle size range, metal nanoparticles can be monodispersely supported, allowing for the production of a highly active catalyst. If the particle size is less than 10 μm, the surface area is larger, allowing for monodisperse support of metal nanoparticles, but the small particle size of the support material may cause leakage from reactor I. If the particle size exceeds 50 mm, the large particle size of the support material makes it difficult to support the metal nanoparticles, and the porosity when packed into the reactor may be too large, potentially reducing the efficiency of nitric acid decomposition.

[0072] The pore volume of the support is preferably 0.4 to 1.0 mL / g. Within this range, metal nanoparticles can be supported monodispersely, and particle growth among the metal nanoparticles during the nitric acid decomposition reaction is less likely to occur, resulting in a longer catalyst lifetime. If the pore volume is less than 0.4 mL / g, the metal nanoparticles may aggregate and be supported. As a result, during the nitric acid decomposition reaction, the metal nanoparticles may grow together or fall off due to collisions between catalysts, shortening the catalyst lifetime. If the pore volume exceeds 1.0 mL / g, metal nanoparticles can be supported monodispersely, resulting in a catalyst with a longer catalyst lifetime, but because the activated carbon is bulky, it may not be possible to add a sufficient amount of catalyst to the reactor to decompose nitric acid.

[0073] The specific surface area of ​​the carrier is 50-3000 m². 2A value of / g is preferred. Within this range, metal nanoparticles can be supported monodispersely, making it less likely for the metal nanoparticles to grow together during the nitric acid decomposition reaction, thus extending the catalyst lifetime. If the specific surface area is less than 50, the metal nanoparticles may aggregate and be supported. As a result, during the nitric acid decomposition reaction, the metal nanoparticles may grow together or fall off due to collisions between catalysts, which can shorten the catalyst lifetime.

[0074] The short-axis / long-axis ratio of the support is preferably 1.0 to 20. Within this range, metal nanoparticles are easily supported in a monodisperse manner, and the catalyst particles are less likely to break when they collide with each other during the nitric acid dispersion reaction. If the ratio exceeds 20, the support becomes amorphous or angular, making it more prone to breaking when the catalyst particles collide with each other during the nitric acid dispersion reaction.

[0075] The compressive strength of the carrier is 10 gf / mm². 2 It is preferable that the above conditions are met. Within this range, the particles do not break during metal nanoparticle loading or nitric acid decomposition reactions, and no broken catalyst leaks out of the reactor, resulting in a catalyst with a long lifespan.

[0076] Next, the reduction catalyst of the present invention will be described. The reduction catalyst of the present invention has metal nanoparticles supported on the catalyst support. The amount of metal nanoparticles supported is preferably 0.1 to 5%. Less than 0.1% requires the use of a large amount of the reduction catalyst, potentially leading to productivity problems such as the need for larger reaction vessels or extremely slow linear velocity of waste liquid. More than 5% results in poor support and dispersibility, causing the nanoparticles to detach from the support and aggregate. As a result, the activity tends to decrease. Catalyst loading can be done by conventionally known methods, with impregnation methods and vacuum degassing methods being preferred, in which the support is suspended in water, the air in the pores is removed, and the metal nanoparticles are loaded. Such loading allows the metal nanoparticles to be loaded monodisperse not only on the surface of the support but also inside the pores, which may improve catalytic activity and lifespan. Alternatively, the support may be pre-treated with metal ions (Pd ions and Cu ions), and then the metal nanoparticles are reacted with it, allowing for loading by ionic interactions. In this case, the loading force is increased, so the metal nanoparticles can be maintained monodisperse and maintain high activity.

[0077] The average particle size of the reducing catalyst of the present invention is 10 μm to 50 mm, preferably 40 μm to 30 mm. When the average particle size is within this range, steps 1 to 4 can be carried out efficiently. If it is less than 10 μm, the particle size of the support is small and there is a possibility that it will flow out of reactor I. If it exceeds 50 mm, the particle size of the support is large, making it difficult to support the metal nanoparticles, and the porosity when packed into the reactor may be too large, resulting in poor efficiency of nitric acid decomposition.

[0078] The pore volume of the reduction catalyst of the present invention is preferably 0.4 to 1.0 mL / g. Within this range, steps 1 to 4 can be carried out efficiently, and the catalyst life is also extended. If the pore volume is less than 0.4 mL / g, the metal nanoparticles in the catalyst may aggregate and be supported, so during the nitric acid decomposition reaction and oxidative decomposition reaction in steps 3 to 4, the metal nanoparticles may grow together or fall off due to collisions between catalysts, which tends to shorten the catalyst life. If it exceeds 1.0 mL / g, the catalyst becomes bulky, and it may not be possible to add a sufficient amount of catalyst to the reactor to decompose nitric acid.

[0079] The specific surface area of ​​the reduction catalyst of the present invention is 50 to 3000 m². 2 A specific surface area of ​​50 m² is preferred. Within this range, steps 1 to 4 can be carried out efficiently, and the catalyst life is also extended. 2If the value is less than / g, the metal nanoparticles may aggregate and become supported, which can sometimes cause the metal nanoparticles to grow together during the nitric acid decomposition reaction and oxidative decomposition reaction in steps 3 and 4, shortening the catalyst life. The specific surface area of ​​the reduction catalyst of the present invention shall be measured by the BET method, as in the examples described later.

[0080] The short-axis / long-axis ratio of the reduction catalyst of the present invention is preferably 1.0 to 20. Within this range, steps 1 to 4 can be carried out efficiently. If it exceeds 20, the support becomes amorphous or angular, making it prone to cracking when catalyst particles collide with each other during the nitric acid dispersion reaction.

[0081] The compressive strength of the reduction catalyst of the present invention is 10 gf / mm². 2 It is preferable that the values ​​are above this range. Within this range, steps 1 to 4 can be carried out efficiently. [Examples]

[0082] The physical properties of the support (activated carbon), metal nanoparticles, and reduction catalyst in the examples and comparative examples were measured or evaluated using the following methods, and the results are shown in the table.

[0083] (1) Average particle size of metal nanoparticles Using a scanning electron microscope (magnification 200,000x), for a metal nanoparticle dispersion (solid content concentration 0.001% by mass), the maximum diameter of 100 randomly selected metal nanoparticles was measured from images or photographs containing 100 or more metal nanoparticles in the same field of view. The average value of these measurements was then calculated and defined as the average particle diameter [nm] of the metal nanoparticles. In the following examples and comparative examples, a high-resolution field-emission scanning electron microscope S-5500 (manufactured by Hitachi High-Technologies Corporation) was used as the scanning electron microscope (the same scanning electron microscope was used in the various measurements described below). Furthermore, the average particle size of metal nanoparticles supported on the surface of the reducing catalyst can be determined by using the reducing catalyst, and, similar to the case of a metal nanoparticle dispersion, by measuring the maximum diameter of 100 randomly selected metal nanoparticles in an image or photograph containing 100 or more metal nanoparticles in the same field of view, calculating the numerical average of these measurements, and taking this value as the average particle size [nm] of the metal nanoparticles.

[0084] (2) Average particle size of the carrier and reducing catalyst Using a scanning electron microscope (magnification 10x to 200,000x), for each carrier, 100 randomly selected carrier particles were measured in images or photographs containing 100 or more carrier particles within the same field of view. The maximum diameter of each particle was then measured, and the numerical average of these measurements was calculated. This value was defined as the average particle diameter [μm] of the carrier particles. Furthermore, the average particle diameter of the reducing catalyst, which consists of metal nanoparticles supported on carrier particles, was determined by measuring the maximum diameter of 100 randomly selected reducing catalysts in an image or photograph containing 100 or more reducing catalysts within the same field of view, calculating the numerical average of these measurements, and defining this value as the average particle diameter [μm] of the carrier particles.

[0085] (3) Average pore diameter After treating the carrier particles or reduction catalyst in an electric furnace at 300°C for 1 hour, the average pore size was measured using a porosimeter (PM33GT-17, manufactured by Cantachrome) by the mercury intrusion method.

[0086] (4) Pore volume After treating the carrier particles or reduction catalyst in an electric furnace at 300°C for 1 hour, the pore volume was measured using a porosimeter (PM33GT-17, manufactured by Cantachrome) by the mercury intrusion method.

[0087] (5) Specific surface area After treating the carrier particles or reduction catalyst in an electric furnace at 300°C for 1 hour, the specific surface area was measured using the BET method with a specific surface area analyzer (Macsorb HM model-1220, manufactured by Mountec).

[0088] (6) Packing density The carrier particles or reducing catalyst were treated in an electric furnace at 300°C for 1 hour, and then packed into a 100 mL graduated cylinder. The weight at this time was measured, and the packing density (packing weight per 1 mL) was calculated.

[0089] (7) Compressive fracture strength The compressive strength was measured using a micro-compression testing machine (MCT-W500, manufactured by Shimadzu Corporation). Specifically, the sample (carrier particles or reducing catalyst) was compressed to apply a load, and the load at which the sample fractured was measured. This measurement was performed for five samples, and the average value was defined as the compressive fracture strength.

[0090] (8) Composition of metal nanoparticles (Pd-Cu nanoparticles) The Pd-Cu nanoparticle dispersion was analyzed using EXAFS (RIGAKU R-XAM Looper). The results confirmed the presence of a Pd-Cu alloy. Similarly, the reduction catalyst can also be measured in the same manner.

[0091] (9) Percentage of Pd component A Pd-Cu nanoparticle dispersion was dissolved in a 61% by mass nitric acid aqueous solution and diluted with pure water. The masses of Pd and Cu elements in the solution were measured using an ICP inductively coupled plasma emission spectrometer (SPS1200A, Seiko Electronics Co., Ltd.). From the obtained results, the percentage (mass ratio) of the Pd component was calculated as Pd mass / [Pd mass + Cu mass] × 100. Furthermore, to determine the proportion of Pd in ​​a reducing catalyst, the aqueous dispersion of the reducing catalyst can be dissolved in nitric acid to remove the support component, and the measurement can be performed in the same manner as in the case of a Pd-Cu nanoparticle dispersion. Furthermore, the metal nanoparticle content in the Pd-Cu nanoparticle dispersion refers to the sum of the mass percentages of Pd and Cu, which are determined by measuring them using an ICP inductively coupled plasma emission spectrometer as described above.

[0092] (10) Carbon content in metal nanoparticles A Pd-Cu nanoparticle dispersion was dried at 105°C, and tungsten oxide was added as an additive. This was then subjected to high-frequency induction firing to convert the carbon in the Pd-Cu nanoparticles into CO2, and its amount was measured. The amount of carbon [C mass] was quantified from this CO2 amount. From the obtained results, ([C mass] / [Pd mass + Cu mass]) × 100 [%] was calculated to determine the ratio (mass ratio) [%] of the carbon mass [C mass] to the Pd-Cu mass. Note that [Pd mass + Cu mass] is the value obtained in (9) above. The amount of carbon affects the degree of dispersion during loading, and a higher amount has an effect on the degree of dispersion and lifetime. The calculation result ([C mass] / [Pd mass + Cu mass])×100[%] obtained here is, in principle, consistent with the value of ([C mass] / [Pd mass + Cu mass])×100[%] for metal nanoparticles supported on a carrier.

[0093] (11) Amount of Pd-Cu nanoparticles supported on the reduction catalyst The sample (reducing catalyst) was calcined at 600°C and then melted with an alkaline flux. This was dissolved in a 28% by mass hydrochloric acid aqueous solution and diluted with pure water. The content percentage [by mass] of metal elements contained in the reducing catalyst was measured using an ICP inductively coupled plasma emission spectrometer (SPS1200A, Seiko Electronics Industries). The amount of Pd and Cu loaded onto the support was then calculated. If metals other than Pd and Cu were present, the loading amount was calculated without including those metals. The amount of material carried is calculated as [Pd mass + Cu mass] / mass of the carrier × 100 (%).

[0094] (12) Supported state of metal nanoparticles A scanning electron microscope (Hitachi S-5500) was used to photograph the sample (catalyst) at a magnification of 300,000x, and from the resulting photographic projection, the 10,000 nm 2The number of metal nanoparticles (t) within a 100 nm square area and the number of metal nanoparticles (s) that are separated by 1 nm or more from adjacent metal nanoparticles in all directions were measured, and the percentage m {m = (s / t) × 100} of metal nanoparticles separated by 1 nm or more from adjacent metal nanoparticles was calculated. This measurement was performed at 50 locations on the reduction catalyst, and the average value M [%] was calculated.

[0095] (13)[(D10+D90) / 2] / D50 The reduction catalyst [(D10+D90) / 2] / D50 was determined as follows. An electron microscope image (magnification 200,000x, containing 250 or more metal nanoparticles in the same field of view) of a dispersion of metal nanoparticles in a solvent (hereinafter also referred to as "metal nanoparticle dispersion") was taken. For each of the 250 particles randomly selected from the obtained image, the length was measured at the point where the diameter of the primary particle was longest, and a cumulative particle diameter frequency distribution map was created. The particle diameters were defined as follows, with D10 representing the 10% cumulative frequency particle diameter, D50 representing the 50% cumulative frequency particle diameter, and D90 representing the 90% cumulative frequency particle diameter, starting from the smallest particle diameter. Then, using this result, the value of [(D10+D90) / 2] / D50 was calculated. The obtained value is considered to be [(D10+D90) / 2] / D50 in the reducing catalyst.

[0096] (14) Measurement of nitrate decomposition rate, etc. The nitrate ion concentration, nitrite ion concentration, ammonia ion concentration, hydrazine concentration, formic acid concentration, and spheramic acid concentration in the nitric acid-containing waste liquid, treated liquid I obtained in step 2, and treated liquid II obtained in step 4 were measured using the following method.

[0097] Measurement of nitrate ion concentration, etc. The concentrations of nitrate ions and nitrite ions were measured and calculated using spectrophotometric analysis. In this invention, a V-750 spectrophotometer manufactured by JASCO Corporation was used.

[0098] 《Nitric acid decomposition rate》 The conversion rate of nitrate ions [((initial nitrate ion concentration - nitrate ion concentration after reaction) / initial nitrate ion concentration) × 100 (mol%)] was defined as the nitrate decomposition rate.

[0099] Measurement of ammonia ion concentration The amount of NH3 in ammonia was measured using liquid chromatography. This was defined as the amount of NH3 produced. Ammonia in the atmosphere was captured in a TetraBak and measured using an ammonia detection tube.

[0100] Measurement of hydrazine concentration Hydrazine analysis was performed in accordance with JIS B 8224-2005. Specifically, 10 g of sample was taken from the solution after each step, water was added to make 100 ml, and then 2 g of sodium carbonate and 1 ml of starch solution were added and gently stirred to prepare a mixed solution. This mixed solution was titrated with iodine solution (25 mmol / L), and the point where it turned blue was used as the endpoint. The amount of hydrazine was determined from the amount of iodine solution consumed. Measurement of formic acid concentration The formic acid content was measured using liquid chromatography. This was defined as the formic acid content. Measurement of spheramic acid concentration Spheramic acid was analyzed by measuring the amount of spheramic acid using liquid chromatography. This amount was defined as the spheramic acid content.

[0101] [Example 1] (Preparation of Pd-Cu nanoparticle dispersion) 300 g of trisodium citrate aqueous solution (30% by mass) was dissolved with 125 g of ferrous sulfate as a reducing agent. To 350 g of this solution, 40 g of palladium(II) nitrate aqueous solution (20% by mass) was added at room temperature, followed by 10 g of copper(II) nitrate aqueous solution (20% by mass). This solution was thoroughly mixed to prepare a dispersion of Pd-Cu nanoparticles. Then, 100 g of pure water was added to the solid recovered by centrifugation, and further, 100 g of trisodium citrate aqueous solution (30% by mass) was added to remove the reducing agent, and the mixture was stirred for 1 hour to obtain a solution. The solid material recovered from this solution by centrifugation was mixed with 100 g of pure water and stirred. Furthermore, the amphoteric ion exchange resin SMNUPB was added to this dispersion to remove impurities. After separating the ion exchange resin, coarse particles were removed by centrifugation (10000 G - 30 min) to obtain a Pd-Cu nanoparticle dispersion. The metal nanoparticle content (sum of Pd and Cu content) in the obtained Pd-Cu nanoparticle dispersion was 3% by mass. The Pd component ratio in the Pd-Cu nanoparticles contained in this Pd-Cu nanoparticle dispersion was 80% of the sum of the Pd and Cu components (i.e., Pd / (Pd+Cu)×100), and the average particle size of the Pd-Cu nanoparticles was 3 nm. The carbon (C) content of the same Pd-Cu nanoparticles (i.e., C / (Pd+Cu)×100) was 3.0% by mass.

[0102] (Preparation of Pd-Cu nanoparticle-supported catalyst (reducing catalyst-1)) For the catalysis of Pd-Cu nanoparticles, "Taiko A100FB Activated Carbon" (manufactured by Futamura Chemical Co., Ltd.), which is made from carbon derived from phenolic resin, was used as the support. The physical properties of the activated carbon are shown in Table 1. 99 g of this activated carbon was added to 34 g of the Pd-Cu nanoparticle dispersion (metal nanoparticle content: 3% by mass) obtained in the "Preparation of Pd-Cu nanoparticle dispersion" described above, and the mixture was placed in a rotary evaporator and dried under reduced pressure at 50°C. By drying this dried product in a nitrogen atmosphere at a temperature of 105°C for 24 hours, a catalyst (reducing catalyst-1) in which Pd-Cu nanoparticles were supported on activated carbon was obtained. In the Pd-Cu nanoparticles supported on reduction catalyst-1, the Pd component ratio was 80% of the "sum of Pd and Cu components" (i.e., Pd / (Pd+Cu)×100), the average particle size of the same supported Pd-Cu nanoparticles was 3 nm, and the carbon (C) content of the same Pd-Cu nanoparticles (i.e., C / (Pd+Cu)×100) was 3.0 mass%. The physical properties of reduction catalyst-1 are shown in Table 1.

[0103] (Treatment of nitric acid-containing waste liquid) The decomposition characteristics of nitrate nitrogen were evaluated using the reduction catalysts from this example and comparative example, and the flow-type equipment with the structure shown in Figure 1, in the following nitrate nitrogen decomposition process. The treatment conditions are shown in Tables 2 and 5, and the results are shown in Tables 3 and 6.

[0104] The details of the structure of the flow-type equipment used in Example 1 and other examples, as well as comparative examples, are as follows. <Storage tank for nitric acid-containing waste liquid> • Tank volume: 2m³ 3 • Tank shape: cylindrical • Curvature of the tank bottom: Curvature present • Shape of stirring blades: Faudler • Number of stages of stirring blades: 2 • Baffle: Yes • Tank material: SUS316L • Heat source for temperature control: hot water and steam • Gas introduction line: Straight • Waste liquid introduction line: Straight • Presence or absence of breather valve: Yes • Reducing agent addition line: Yes (used as needed; purified water can also be added) • Nanobubble generator: Yes • Nanobubble circulation line: Yes <Reactor I> • Reactor inner diameter 0.5m • Reactor height: 2m • Reactor volume: 0.39 m³ 3 • Temperature-regulating jacket: Yes • Heat source for temperature control: hot water or steam • Presence or absence of breather valve: Yes • Reducing agent addition line: Available. Pure water can also be added. • Waste liquid inlet line: Yes • Outlet line for waste liquid (treatment liquid I): Yes <Reactor II> • Reactor inner diameter 0.5m • Reactor height: 2m • Reactor volume: 0.39 m³ 3 • Temperature-regulating jacket: Yes • Heat source for temperature control: hot water or steam • Presence or absence of breather valve: Yes • pH adjuster addition line: Available. Pure water can also be added. • Atmospheric bubbling line: Yes • Inlet line for waste liquid (treatment liquid I): Yes • Outlet line for waste liquid (treatment liquid II): Yes

[0105] (Preparation of nitric acid-containing waste liquid) Sealed storage tank for nitric acid-containing waste liquid (capacity 2m³) 3 A gas introduction line is provided that allows gas to be introduced into the liquid stored inside, along with an agitator and a jacket with temperature control (hot water / cold water). Furthermore, a breather valve is provided that opens when the internal pressure exceeds 1.05 times atmospheric pressure. 1,000 kg of sodium nitrate solution (nitrate ion concentration 5,000 ppm) is then poured into the sealed tank. Next, 0.1N NaOH was gradually added to the sodium nitrate solution while stirring to adjust the pH of the sodium nitrate solution to 7.0. In this example, this sodium nitrate solution was designated as "nitric acid-containing waste liquid." Subsequently, 50°C hot water was circulated through the jacket to adjust the temperature of the nitric acid-containing waste liquid to 40°C. (Preparation of Reactor I) The prepared reduction catalyst-1 was packed into reactor I (with a temperature-adjustable jacket (hot water / cold water)) to a concentration of 2.0 g / L in terms of metal nanoparticles (total mass of Pd and Cu / (nitric acid-containing waste liquid + reduction catalyst)). Furthermore, reactor I has a waste liquid introduction line (with a metering pump) at the bottom from a sealed storage tank for nitric acid-containing waste liquid, and a reducing agent introduction line (with a metering pump, pure water can also be introduced). At the top, it has a breather valve equipped with a breather valve that opens when the internal pressure exceeds 1.05 times atmospheric pressure, and a discharge line for discharging the treated liquid I to the outside. (Process 1 / Process 2) 50°C hot water is circulated within the jacket of reactor I to adjust the temperature of reactor I to 40°C, and nitric acid-containing waste liquid at 40°C is introduced from the waste liquid introduction line at a linear velocity of 4.72 × 10⁻¹⁰ -5 Hydrazine was simultaneously introduced at a rate of 4.0 kg / h from the reducing agent introduction line at a rate of m / s (reactor residence time of 3 hours) (Step 1). Subsequently, the temperature of reactor I was maintained at 40°C, and the nitric acid decomposition reaction proceeded, obtaining treated liquid I from the discharge line at the top of reaction vessel I (Step 2). (Preparation of Reactor II) For the preparation of reactor II to be used in the next step, the prepared reduction catalyst-1 was packed into reactor II (with a temperature-adjustable jacket (hot water / cold water)) to a concentration of 2.0 g / L in terms of metal nanoparticles (total mass of Pd and Cu / (nitric acid-containing waste liquid + reduction catalyst)), thus forming reactor II. Furthermore, Reactor II has a treatment liquid I introduction line (with a metering pump) and an atmospheric bubble introduction line (with a metering pump, pure water introduction is also possible) at its lower part. At the top of Reactor II, there is a breather valve equipped with a breather valve that opens when the internal pressure exceeds 1.05 times atmospheric pressure, a discharge line for discharging treatment liquid II to the outside, and a sulfuric acid line in between which sulfuric acid is added to adjust the pH. (Process 3 / Process 4) 50°C hot water is circulated within the jacket of reactor II to adjust the temperature of reactor II to 40°C, and treatment liquid I is introduced through the treatment liquid I introduction line connected to the discharge line of reactor I at a linear velocity of 4.72 × 10⁻¹⁰ -5 Ammonia was introduced at a rate of m / second (residence time in the reactor: 3 hours), and at the same time, air was introduced at 50 L / min from the air bubble introduction line (step 3). Furthermore, an aqueous sodium hydroxide solution was introduced from the sodium hydroxide introduction line to adjust the pH to 10.0. Some of the ammonia generated up to step 3 was decomposed, the undecomposed ammonia diffused into the atmosphere, and the remaining hydrazine was also decomposed. Subsequently, the treated waste liquid passed through reactor II and moved to the upper discharge line, where sulfuric acid was added from the sulfuric acid addition line to adjust the pH to 7.0 and obtain treated liquid II (step 4). The detailed conditions and measurement results are shown in Table 2.

[0106] [Example 2] In Example 1, the temperature of the nitric acid-containing waste liquid and the temperature of treatment solution I were controlled to 40°C, but in Example 2, they were adjusted to 8°C. Example 2 was carried out in the same manner as Example 1, except for the following:

[0107] [Example 3] In Example 1, the temperature of the nitric acid-containing waste liquid and the temperature of treatment solution I were controlled to 40°C, but in Example 3, they were adjusted to 87°C. Example 3 was carried out in the same manner as Example 1, except for the differences mentioned above.

[0108] [Example 4] The nanobubble generator was activated. Then, the nitric acid-containing waste liquid was introduced into a sealed storage tank for nitric acid-containing waste liquid through a waste liquid input line connected to the nanobubble generator. A line was installed to directly generate nanobubbles in the liquid inside the sealed tank and connected to the nanobubble generator, generating nanobubbles in a Pd-Cu nanoparticle dispersion. The type of gas used for the nanobubbles was nitrogen, the average bubble diameter was 70 nm, and the nanobubble content in the solution was 240 million nanobubbles / mL. The type of gas used for the nanobubbles, the average bubble diameter, and the nanobubble generation content are shown in Table 2. Example 4 was carried out in the same manner as Example 1, except for the rest.

[0109] [Example 5] In Example 1, the linear velocity of the nitric acid-containing waste liquid introduced into reactors I and II was 4.72 × 10⁻⁶. -5 The test was performed at m / s, but in Example 5, the linear velocity was set to 1.42 × 10⁻¹⁰ -4 Adjusted to m / s. Example 5 was carried out in the same manner as Example 1, except for the differences mentioned above.

[0110] [Example 6] (Preparation of Pd-Cu nanoparticle dispersion) 300 g of trisodium citrate aqueous solution (30% by mass) was dissolved with 125 g of ferrous sulfate as a reducing agent. To 350 g of this solution, 30 g of palladium(II) nitrate aqueous solution (20% by mass) was added at room temperature, followed by 70 g of copper(II) nitrate aqueous solution (20% by mass). This solution was thoroughly mixed to prepare a dispersion of Pd-Cu nanoparticles. Then, 100 g of pure water was added to the solid recovered by centrifugation, and further, 100 g of trisodium citrate aqueous solution (30% by mass) was added to remove the reducing agent, and the mixture was stirred for 1 hour to obtain a solution. The solid material recovered from this solution by centrifugation was mixed with 100 g of pure water and stirred. Furthermore, the amphoteric ion exchange resin SMNUPB was added to this dispersion to remove impurities. After separating the ion exchange resin, coarse particles were removed by centrifugation (10000 G - 30 min) to obtain a Pd-Cu nanoparticle dispersion. The metal nanoparticle content (sum of Pd and Cu content) in the obtained Pd-Cu nanoparticle dispersion was 3% by mass. In this Pd-Cu nanoparticle dispersion, the Pd component ratio of the Pd-Cu nanoparticles was 60% of the "sum of Pd and Cu components" (i.e., Pd / (Pd+Cu)×100), the average particle size of the Pd-Cu nanoparticles was 25 nm, and the carbon (C) content of the same Pd-Cu nanoparticles (i.e., C / (Pd+Cu)×100) was 2.0 mass%.

[0111] (Preparation of Pd-Cu nanoparticle-supported catalyst (reducing catalyst-6)) For the catalysis of Pd-Cu nanoparticles, "Taiko A100FB Activated Carbon" (manufactured by Futamura Chemical Co., Ltd.), which is made from carbon derived from phenolic resin, was used as the support. The physical properties of the activated carbon are shown in Table 1. 99 g of this activated carbon was added to 34 g of the Pd-Cu nanoparticle dispersion (metal nanoparticle content: 3% by mass) obtained in the "Preparation of Pd-Cu nanoparticle dispersion" described above, and the mixture was placed in a rotary evaporator and dried under reduced pressure at 50°C. By drying this dried product in a nitrogen atmosphere at a temperature of 105°C for 24 hours, a catalyst (reducing catalyst-6) in which Pd-Cu was supported on activated carbon was obtained. In the Pd-Cu nanoparticles supported on the reduction catalyst-6, the Pd component ratio was 60% of the "sum of Pd and Cu components" (i.e., Pd / (Pd+Cu)×100), the average particle size of the supported Pd-Cu nanoparticles was 25 nm, and the carbon (C) content of the same Pd-Cu nanoparticles (i.e., C / (Pd+Cu)×100) was 2.0 mass%. Table 1 shows the physical properties of the activated carbon of the reduction catalyst-6.

[0112] (Treatment of nitric acid-containing waste liquid) The treatment of the nitric acid-containing wastewater was carried out in the same manner as in Example 1, except that "Reducing Catalyst-6" was used instead of "Reducing Catalyst-1" which was used in the treatment of the nitric acid-containing wastewater in Example 1.

[0113] [Example 7] (Preparation of Pd-Cu nanoparticle dispersion) 300 g of trisodium citrate aqueous solution (30% by mass) was dissolved with 125 g of ferrous sulfate as a reducing agent. To 350 g of this solution, 40 g of palladium(II) nitrate aqueous solution (20% by mass) was added at room temperature, followed by 10 g of copper(II) nitrate aqueous solution (20% by mass). This solution was thoroughly mixed to prepare a dispersion of Pd-Cu nanoparticles. Subsequently, 100g of pure water was added to the solid recovered by centrifugation, and then 100g of trisodium citrate aqueous solution (30% by mass) was added to remove the reducing agent, and the mixture was stirred for 1 hour to obtain a solution. The solid material recovered from this solution by centrifugation was mixed with 100 g of pure water and stirred. Furthermore, the amphoteric ion exchange resin SMNUPB was added to this dispersion to remove impurities. After separating the ion exchange resin, coarse particles were removed by centrifugation (10000 G - 30 min) to obtain a Pd-Cu nanoparticle dispersion. The metal nanoparticle content (sum of Pd and Cu content) in the obtained Pd-Cu nanoparticle dispersion was 3% by mass. In this Pd-Cu nanoparticle dispersion, the Pd component ratio of the Pd-Cu nanoparticles was 80% of the "sum of Pd and Cu components" (i.e., Pd / (Pd+Cu)×100), the average particle size of the Pd-Cu nanoparticles was 3 nm, and the carbon (C) content of the same Pd-Cu nanoparticles (i.e., C / (Pd+Cu)×100) was 3.0 mass%.

[0114] (Preparation of Pd-Cu nanoparticle-supported catalyst (reducing catalyst-7)) For the catalysis of Pd-Cu nanoparticles, "Taiko A100FB Activated Carbon" (manufactured by Futamura Chemical Co., Ltd.), which is made from carbon derived from phenolic resin, was used as the support. The physical properties of the activated carbon are shown in Table 1. 99 g of this activated carbon was added to 34 g of the Pd-Cu nanoparticle dispersion (Pd-Cu equivalent concentration: 3% by mass) obtained in the "Preparation of Pd-Cu nanoparticle dispersion" described above, and the mixture was placed in a rotary evaporator and dried under reduced pressure at 50°C. By drying this dried product in a nitrogen atmosphere at a temperature of 105°C for 24 hours, a catalyst (reducing catalyst-7) in which Pd-Cu was supported on activated carbon was obtained. In the Pd-Cu nanoparticles supported on the reduction catalyst-7, the Pd component ratio was 80% of the "sum of Pd and Cu components" (i.e., Pd / (Pd+Cu)×100), the average particle size of the supported Pd-Cu nanoparticles was 3 nm, and the carbon (C) content of the same Pd-Cu nanoparticles (i.e., C / (Pd+Cu)×100) was 3.0 mass%. Table 1 shows the physical properties of the activated carbon of the reduction catalyst-7.

[0115] (Treatment of nitric acid-containing waste liquid) Example 7 was carried out in the same manner as Example 1, except that "Reducing Catalyst-7" was used instead of "Reducing Catalyst-1" which was used in the treatment of the nitric acid-containing waste liquid in Example 1.

[0116] [Example 8] In Example 1, the linear velocity of the nitric acid-containing waste liquid introduced into reactors I and II was 4.72 × 10⁻⁶. -5 The test was performed at m / s, but in Example 8, the linear velocity was set to 5.90 × 10⁻¹⁰. -6The speed was adjusted to m / s. In Example 1, 12.1 kg of hydrazine was introduced into reactor I at a speed of 4.0 kg / h as a reducing agent. In Example 8, the process was carried out in the same manner as in Example 1, except that 7.4 kg of formic acid was introduced at a speed of 0.3 kg / h as a reducing agent.

[0117] [Example 9] In Example 1, 12.1 kg of hydrazine was introduced into reactor I at a rate of 4.0 kg / h as a reducing agent. In Example 9, the procedure was the same as in Example 1, except that 15.6 kg of spheramic acid was introduced at a rate of 5.2 kg / h as a reducing agent.

[0118] [Comparative Example 1] In Example 1, the process was carried out up to step 4, but in Comparative Example 1, the process was terminated at the stage where treatment solution I was obtained after step 2.

[0119] [Comparative Example 2] In Example 1, the nitric acid-containing waste liquid and treatment liquid I were treated at a linear velocity of 4.72 × 10⁻¹⁰ -5 The nitrate-containing waste liquid and treatment liquid I were introduced at a linear velocity of 2.80 × 10⁻⁶ m / s. -2 Comparative Example 2 was carried out in the same manner as in Example 1, except that the introduction speed was m / sec.

[0120] [Comparative Example 3] In Example 1, the nitric acid-containing waste liquid and treatment liquid I were treated at a linear velocity of 4.72 × 10⁻¹⁰ -5 The nitrate-containing wastewater and treatment solution I were introduced at a linear velocity of 5.90 × 10⁻¹⁰ m / s. -6 Comparative Example 3 was carried out in the same manner as in Example 1, except that the introduction speed was m / sec.

[0121] [Comparative Example 4] (Preparation of Pd-Cu nanoparticle dispersion) 300 g of trisodium citrate aqueous solution (5% by mass) was dissolved with 125 g of ferrous sulfate as a reducing agent. To 350 g of this solution, 40 g of palladium(II) nitrate aqueous solution (20% by mass) was added at room temperature, followed by 10 g of copper(II) nitrate aqueous solution (20% by mass). This solution was thoroughly mixed to prepare a dispersion of Pd-Cu nanoparticles. Then, 100 g of pure water was added to the solid recovered by centrifugation, and further, 100 g of trisodium citrate aqueous solution (30% by mass) was added to remove the reducing agent, and the mixture was stirred for 1 hour to obtain a solution. The solid material recovered from this solution by centrifugation was mixed with 100 g of pure water. Furthermore, the amphoteric ion exchange resin SMNUPB was added to this dispersion to remove impurities, and the mixture was diluted with pure water. The ion exchange resin was separated to obtain a Pd-Cu nanoparticle dispersion. The metal nanoparticle content (sum of Pd and Cu content) in the obtained Pd-Cu nanoparticle dispersion was 3% by mass. In this Pd-Cu nanoparticle dispersion, the Pd component ratio of the Pd-Cu nanoparticles was 80% of the "sum of Pd and Cu components" (i.e., Pd / (Pd+Cu)×100), the average particle size of the Pd-Cu nanoparticles was 50 nm, and the carbon (C) content of the same Pd-Cu nanoparticles (i.e., C / (Pd+Cu)×100) was 3.0% by mass.

[0122] (Preparation of Pd-Cu nanoparticle-supported catalyst (reducing catalyst - Comparative Example 4)) For the catalysis of Pd-Cu nanoparticles, "Taiko A100FB Activated Carbon" (manufactured by Futamura Chemical Co., Ltd.), which is made from carbon derived from phenolic resin, was used as the support. The physical properties of the activated carbon are shown in Table 1. 99 g of this activated carbon was added to 34 g of the Pd-Cu nanoparticle dispersion (metal nanoparticle content: 3% by mass) obtained in the "Preparation of Pd-Cu nanoparticle dispersion" described above, and the mixture was placed in a rotary evaporator and dried under reduced pressure at 50°C. By drying this dried product in a nitrogen atmosphere at a temperature of 105°C for 24 hours, a catalyst (reducing catalyst - Comparative Example 4) in which Pd-Cu nanoparticles were supported on activated carbon was obtained. In the Pd-Cu nanoparticles supported on the reduction catalyst - Comparative Example 4, the Pd component ratio was 80% of the "sum of Pd and Cu components" (i.e., Pd / (Pd+Cu)×100), the average particle size of the same supported Pd-Cu nanoparticles was 3 nm, and the carbon (C) content of the same Pd-Cu nanoparticles (i.e., C / (Pd+Cu)×100) was 3.0 mass%. Table 4 shows the physical properties of the activated carbon used as a reduction catalyst in Comparative Example 4.

[0123] (Treatment of nitric acid-containing waste liquid) Comparative Example 4 was carried out in the same manner as in Example 1, except that "Reducing Catalyst-1" used in the treatment of nitric acid-containing wastewater in Example 1 was replaced with "Reducing Catalyst-Comparative Example 4", and the jacket temperature of reactor I was set to 5.0°C and the temperature of treatment liquid I was set to 8°C.

[0124] [Comparative Example 5] In Example 1, 12.1 kg of hydrazine was introduced into reactor I at a rate of 4.0 kg / h as a reducing agent. Comparative Example 5 was carried out in the same manner as in Example 1, except that 2.0 kg of hydrazine was introduced into reactor I at a rate of 0.7 kg / h as a reducing agent.

[0125] [Comparative Example 6] Comparative Example 6 was carried out in the same manner as in Example 1, except that the jacket temperatures of reactors I and II were set to -3.0°C and the temperature of treatment liquid I was set to 1°C.

[0126] [Comparative Example 7] In the treatment of nitric acid-containing wastewater in Example 1, the catalyst "Reducing Catalyst - Comparative Example 7," which was treated with Pd ions and Cu ions as described below, was used instead of the Pd-Cu nanoparticle-supported catalyst "Reducing Catalyst-1," but the treatment of nitric acid-containing wastewater was carried out in the same manner as in Example 1.

[0127] (Preparation of a catalyst treated with Pd ions and Cu ions (reducing catalyst - Comparative Example 7)) For catalysis, "Taiko A100FB activated carbon" (manufactured by Futamura Chemical Co., Ltd.) was used as the support, as in Example 1. 99 g of this activated carbon and 10 g of palladium(II) nitrate aqueous solution (20% by mass) were added at room temperature, followed by the addition of 3.8 g of copper(II) nitrate aqueous solution (20% by mass). The mixture was then placed in a rotary evaporator and dried under reduced pressure at 50°C. By drying this dried product in a nitrogen atmosphere at a temperature of 105°C for 24 hours, a reduction catalyst in which Pd ions and Cu ions were supported on activated carbon was obtained - Comparative Example 7.

[0128] [Comparative Example 8] In the treatment of the nitric acid-containing wastewater in Example 1, the Cu nanoparticle-supported catalyst "Reducing Catalyst - Comparative Example 8" (characteristics of which are shown in Table 4) was used instead of the Pd-Cu nanoparticle-supported catalyst "Reducing Catalyst-1" in the same manner as in Example 1.

[0129] (Preparation of Cu nanoparticle dispersion) 300 g of trisodium citrate aqueous solution (30% by mass) was dissolved with 125 g of ferrous sulfate as a reducing agent. 350 g of this solution was mixed with 40 g of copper(II) nitrate aqueous solution (20% by mass). This solution was thoroughly mixed to prepare a dispersion of Cu particles. Then, 100 g of pure water was added to the solid recovered by centrifugation, and further, 100 g of trisodium citrate aqueous solution (30% by mass) was added to remove the reducing agent, and the mixture was stirred for 1 hour to obtain a solution. The solid material recovered from this solution by centrifugation was mixed with 100 g of pure water and stirred. Furthermore, the amphoteric ion exchange resin SMNUPB was added to this dispersion to remove impurities. After separating the ion exchange resin, coarse particles were removed by centrifugation (10000 G - 30 min) to obtain a Cu nanoparticle dispersion. The metal nanoparticle content (total Cu content) in the obtained Cu nanoparticle dispersion was 3% by mass. In this Cu nanoparticle dispersion, the Pd component ratio of the Pd-Cu nanoparticles was 0% relative to the "sum of Pd and Cu components" (i.e., Pd / (Pd+Cu)×100), the average particle size of the Cu nanoparticles was 20 nm, and the carbon (C) content of the Cu nanoparticles (i.e., C / (Pd+Cu)×100) was 2.0 mass%.

[0130] (Preparation of Cu nanoparticle-supported catalyst (reducing catalyst - Comparative Example 8)) A catalyst supported with Cu particles (reducing catalyst - Comparative Example 8) was obtained in the same manner as in Example 1, except that the Cu nanoparticle dispersion of this comparative example was used instead of the Pd nanoparticle dispersion.

[0131] [Comparative Example 9] In the treatment of the nitric acid-containing wastewater in Example 1, the Pd nanoparticle-supported catalyst "Reducing Catalyst-Comparative Example 9" (characteristics of which are shown in Table 4) was used instead of the Pd-Cu nanoparticle-supported catalyst "Reducing Catalyst-1" in the same manner as in Example 1.

[0132] (Preparation of Pd nanoparticle dispersion) 300 g of trisodium citrate aqueous solution (30% by mass) was dissolved with 125 g of ferrous sulfate as a reducing agent. 350 g of this solution was mixed with 40 g of palladium(II) nitrate aqueous solution (20% by mass) at room temperature. This solution was thoroughly mixed to prepare a dispersion of Pd particles. Then, 100 g of pure water was added to the solid recovered by centrifugation, and further, 100 g of trisodium citrate aqueous solution (30% by mass) was added to remove the reducing agent, and the mixture was stirred for 1 hour to obtain a solution. The solid material recovered from this solution by centrifugation was mixed with 100 g of pure water and stirred. Furthermore, the amphoteric ion exchange resin SMNUPB was added to this dispersion to remove impurities. After separating the ion exchange resin, coarse particles were removed by centrifugation (10000 G - 30 min) to obtain a Pd nanoparticle dispersion. The Pd metal equivalent concentration of the metal nanoparticle content (Pd content) in the obtained Pd nanoparticle dispersion was 3% by mass. In this Pd nanoparticle dispersion, the Pd component ratio of the Pd-Cu nanoparticles was 100% of the "sum of Pd and Cu components" (i.e., Pd / (Pd+Cu)×100), the average particle size of the Pd nanoparticles was 2 nm, and the carbon (C) content of the Cu nanoparticles (i.e., C / (Pd+Cu)×100) was 5.5% by mass.

[0133] (Preparation of Pd nanoparticle-supported catalyst (reducing catalyst - Comparative Example 9)) A catalyst supported with Pd particles (reducing catalyst - Comparative Example 9) was obtained in the same manner as in Example 1, except that the Pd nanoparticle dispersion of this comparative example was used instead of the Pd-Cu nanoparticle dispersion.

[0134] [Comparative Example 10] In the sealed storage tank for nitrate-containing waste liquid used in Example 1, denitrifying bacteria (bacteria of the genus Pseudomonas, approximately 1,000,000 cells / L), 10 kg of methanol as nutrients, and 1,000 kg of sodium nitrate solution (nitrate ion concentration 5,000 ppm) were added to the sealed tank. Then, 32°C hot water was circulated through the jacket to adjust the temperature of the sodium nitrate solution to 30°C, and the mixture was stirred at a stirring speed of 60 rpm. Next, nitrogen and air were alternately bubbled for 10 minutes every 3 hours through a gas introduction line into the liquid to decompose the nitrate. During this time, the nitrate concentration, nitrite concentration, and ammonia concentration were measured every 24 hours. At 24 hours, the nitrate concentration was 1,500 ppm, the nitrite concentration was 100 ppm, and the ammonia concentration was 100 ppm. All concentrations reached 0 ppm after 120 hours. After 120 hours, stirring was stopped and the mixture was allowed to stand for 24 hours to check for the presence of sludge, and it was confirmed that sludge had settled.

[0135] [Table 1]

[0136] [Table 2]

[0137] [Table 3]

[0138] [Table 4]

[0139] [Table 5]

[0140] [Table 6]

Claims

1. A method for treating nitric acid-containing waste liquid, comprising the following steps. Step 1: In a reactor I packed with a reducing catalyst consisting of metal nanoparticles containing Pd and Cu with an average particle size of 1 to 30 nm supported on a carrier, nitric acid-containing waste liquid (pH 5.0 to 8.0) adjusted to a temperature of 5 to 90°C is introduced at a linear velocity of 1.0 × 10⁻¹⁰ －６ ~1.0 x 10 －２ A step of adding a reducing agent in a molar amount equal to or greater than 1.0 times the amount of nitrate contained in the nitrate-containing waste liquid to the reactor I while flowing it in at a rate of m / second. Step 2: Maintaining the reactor I at a temperature of 5 to 90°C to carry out the nitric acid decomposition reaction and obtain a treatment liquid I. Step 3: The reducing catalyst, in which the metal nanoparticles having an average particle size of 1 to 30 nm and containing Pd and Cu are supported on a carrier, is filled with the processing liquid I at a linear velocity of 1.0 × 10⁻¹⁰. －６ ~1.0 x 10 －２ A step of introducing oxygen or air bubbles into reactor II while allowing them to flow in at a rate of m / s. Step 4: Adjust the pH of the treatment solution in reactor II to 9.0 to 12.0, maintain the temperature of reactor II at 20 to 90°C, and then adjust the pH to 5.0 to 8.0 to obtain treatment solution II.

2. The method for treating nitric acid-containing waste liquid according to claim 1, wherein steps 1 to 4 are performed continuously.

3. A tank for storing nitric acid-containing waste liquid, A reactor I is filled with a reducing catalyst in which metal nanoparticles containing Pd and Cu, having an average particle size of 1 to 30 nm, are supported on a carrier. Reactor II is filled with a reducing catalyst in which metal nanoparticles containing Pd and Cu, having an average particle size of 1 to 30 nm, are supported on a carrier, and further comprises a bubble inlet and a bubble generator connected thereto. A nitrate-containing waste liquid treatment apparatus comprising a structure connecting a tank for storing the treatment liquid II obtained from the reactor II, and a reactor II.

4. The nitric acid-containing waste liquid treatment apparatus according to claim 3, wherein, in the particle size distribution obtained by image analysis of the metal nanoparticles supported on the reducing catalyst packed in reactor I and reactor II, the particle size at which the cumulative frequency reaches 10% is defined as D10, the particle size at which the cumulative frequency reaches 50% is defined as D50, and the particle size at which the cumulative frequency reaches 90% is defined as D90, the conditions represented by the following formula (1) are satisfied. Formula (1): 0.9≦[(D10+D90) / 2] / D50≦2.3

5. A reactor packed with a reducing catalyst having the following characteristics 1) to 3), wherein metal nanoparticles with an average particle size of 1 to 30 nm are supported on a carrier. 1) The metal nanoparticles contain Pd and Cu, with a mass ratio of Pd / (Pd+Cu)×100 ranging from 2% to 98%, and further contain carbon at a mass ratio of C / (Pd+Cu)×100 ranging from 0.1% to 5%. 2) The content ratio of the metal nanoparticles in the reduction catalyst is 0.1 to 5% by mass. 3) The reduction catalyst has an average particle diameter of 10 μm to 5 cm and a specific surface area of ​​50 to 3000 m². 2 It must be within the range of / g.

6. A reactor filled with the reducing catalyst according to claim 5, wherein the reducing catalyst further has the following characteristic 4). 4) In the particle size distribution obtained by image analysis of the metal nanoparticles supported on the reduction catalyst, when the particle size is defined as D10 for the cumulative frequency 10%, D50 for the cumulative frequency 50%, and D90 for the cumulative frequency 90%, the following condition expressed by formula (1) is satisfied. Formula (1): 0.9≦[(D10+D90) / 2] / D50≦2.3

7. A reactor filled with the reducing catalyst according to claim 5 or claim 6, wherein the reducing catalyst further has the following feature 5). 5) In the reduction catalyst, the average value M of the ratio m {m = (s / t) × 100} of metal nanoparticles that are separated by 1 nm or more from adjacent metal nanoparticles in all directions is in the range of 50 to 100%. (Here, 10,000 nm in the electron microscope image [300,000x magnification]) 2 The number of metal nanoparticles within the specified range (100 nm square) was determined as t, the number of metal nanoparticles in the electron microscope image [300,000x magnification] that are separated from adjacent metal nanoparticles by a minimum distance of 1 nm or more in all directions was determined as s, the percentage of metal nanoparticles in the electron microscope image [300,000x magnification] that are separated from adjacent metal nanoparticles by a minimum distance of 1 nm or more in all directions was determined as m [%], and m was determined for 50 randomly selected locations in the reduction catalyst, and the average number of these locations M [%] was calculated.

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