JP4136016B2 - Exhaust gas purification method - Google Patents

Description

ガスクロマトグラフィから亜酸化窒素が生成しただけであることが判った。
【００３９】
ｂ）反応速度：
周囲温度（２０℃）で反応速度の測定試験を行った。反応速度はｐＨが低くなると速くなる。ｐＨを３．５から１に低下させたときはっきりし、これにより還元速度は２倍になった。ｐＨが１の場合について図１から亜硝酸イオン（又はこの場合は亜硝酸）についての見掛け反応次数は１であり、８秒（洗滌塔での滞留時間）に外挿すると収率は９６％になることが判る。
【００４０】
実施例２〔ＮＨＡによる分子状ヨウ素の還元〕
ａ）反応と生成物：
例えば下記のようないくつかの反応が同時に起るので化学量論量を測定することは本例ではこの前と違ってできなかった
2NH₃OH⁺ ＋2I₂ →N₂O ＋4I^- ＋H₂O ＋6H ⁺
4NH₃OH⁺ ＋3I₂ →N₂＋N₂O ＋5I^- ＋3H₂O＋10 H⁺
これらの反応全体はそれ自体中間工程から成るもので、その中間工程のいくつかは一酸化物ＮＯを生成する。１つの反応が他の反応に対して優位であるかはｐＨ次第であり、この特異点はガスクロマトグラフィの示すところであった。従って、ｐＨ＝１では一酸化窒素が主生成物であるが、プロトキサイド（含有酸素原子数が最小の酸化物）が例えばＮ₂ よりも約３倍も少ない量で発生する。ｐＨ＝２．５ではこの事情は逆転する（一酸化窒素と窒素の７倍量のプロトキサイドが発生する）
【００４１】
ｂ）反応速度論
２０℃で反応速度測定試験を行った。反応速度はｐＨが高くなると速くなる。ｐＨを１から２．５に高くすると反応速度は２０だけ向上する。８秒（洗浄塔の滞留時間）に外挿するとｐＨ２．５で反応は定量的になる。同一ｐＨ範囲でのヨウ素に関する見掛け反応次数は１である。図２はｐＨ＝１のときの速度例である。総ての事例ではＮＨＡはヨウ素と較べてかなり過剰であった。初めのイオン濃度１．２ミリモル／ｌで１２５、１５０、１７５であった。最適ＮＨＡ過剰率は１００に近い。ＮＨＡは化学量論量に較べて大過剰である。初めの亜硝酸イオン濃度が１４、１１、９．４ミリモル／ｌではそれぞれ１００、１２５、１５０であり、最適ＮＨＡ過剰率は５０〜１００である。追加試験を３０°、４０°、５０℃で行った。反応速度は１０℃増加毎に倍になることが巾広く認められた。
【００４２】
実施例３〔吸収溶液中での過酸化水素によるヨウ素イオンの酸化〕
ａ）反応と生成物
ヨウ素イオン酸化反応を下記の標準条件下で検討した、ｐＨ＝１（硝酸添加で調整）、ヨウ化ナトリウム濃度：１ミリモル／ｌ、過酸化水素濃度：０．２モル／ｌ、ＮＨＡ濃度：０．１モル／ｌ。
【００４３】
ｂ）速度についての結果
図３は０．１Ｍ ＮＨＡの存在下はじめの１５分間での時間関数としてのｌｏｇ〔Ｉ^- 〕の変化を示すもので、図４は初めの５時間での試験を示すものである。はじめの２分間で収率は９０％に達し、その後ヨウ素イオン濃度は僅かに増加し、その６時間後には逆に減少し、反応時間が２５時間経てば、収率は少なくとも９９％となる。２０℃での時間関数としてのｌｏｇ〔Ｉ^- 〕の変化はＨ₂ Ｏ₂ による酸化はＮＨＡが存在しないと速くなり、２０分後には収率が９８％になる。その後、ヨウ素イオン濃度は規則的に減少し、５時間後には３．８×１０^-6Ｍとなり、この値は収率９９％に相当する。温度効果は極めて著しく収率９９％を達成するには所要時間は３０℃の６時間から６０℃での４時間に短縮される。これらの温度と時間は硝酸とスルファミン酸混合物にもとずく試薬を用いている既存の回分式方法に完全にみあうもので、これ等の試薬を過酸化水素と代替するだけでよく既存の装置に何等を改造を加える必要はない。
【図面の簡単な説明】
【図１】ｐＨ＝１、２０℃での各種ＮＨＡ過剰率に対する時間（秒）関数としての亜硝酸濃度（モル／ｌ）の常用対数ｌｏｇ〔ＨＮＯ₂ 〕を示すグラフであり、図中の記号は下記ＮＨＡ過剰率で行った測定結果を示すもので、各曲線は線形的に逆累進性である。
◇〔ＮＨＡ〕＝〔ＮＯ₂ ^- 〕×１００
◆〔ＮＨＡ〕＝〔ＮＯ₂ ^- 〕×１２５
×〔ＮＨＡ〕＝〔ＮＯ₂ ^- 〕×１５０
【図２】ｐＨ＝１、２０℃での各種ＮＨＡ過剰率に対する時間（秒）関数としてのヨウ素濃度（モル／ｌ）の常用対数ｌｏｇ〔Ｉ₂ 〕を示すグラフであり、図中の記号は下記ＮＨＡ過剰率で行った測定結果を示すもので、各曲線は線形的に逆累進性である：
◇〔ＮＨＡ〕＝〔Ｉ₂ 〕×１２５
○〔ＮＨＡ〕＝〔Ｉ₂ 〕×１５０
＊〔ＮＨＡ〕＝〔Ｉ₂ 〕×１７５
【図３】ＮＨＡの存在下でのＨ₂ Ｏ₂ による酸化での最初の１５分間での時間（分）関数としてのヨウ素イオン濃度（モル／ｌ）の常用対数ｌｏｇ〔Ｉ^- 〕を示すグラフである。
【図４】ＮＨＡの存在下でのＨ₂ Ｏ₂ による酸での最初の５時間での時間（分）関数としてのヨウ素イオン濃度（モル／ｌ）の常用対数ｌｏｇ〔Ｉ^- 〕を示すグラフであり、２５時間ではヨウ素イオン濃度（図示していない）は１．４２×１０^-6モル／ｌである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for wet-purifying exhaust gas by washing exhaust gas containing nitrogen oxides or nitrite vapor with an aqueous solution containing a nitrite-decomposable ammonium derivative.
[0002]
In addition to nitrous acid vapor, the treated gas contains iodine, in particular iodine 129, and is composed of so-called “dissolved” gas formed during the melting of the reprocessing plant of irradiated nuclear fuel in particular. The method thus comprises a first step of removing nitrous acid vapor from the treated gas and treating iodine and a second step of desorbing iodine from the aqueous solution used using hydrogen peroxide.
[0003]
[Prior art]
Gases obtained from various industrial facilities such as fossil fuel-fired power plants, waste incineration plants, etc., especially exhaust gas containing nitrogen oxides generally called NO _x , and flue gas are various reagents so far Has been processed by a wet method in a cleaning facility using The most frequently used reagent is soda in the form of an aqueous solution. Thus, a large amount of sodium nitrite-containing solution is produced, which in turn requires one or more processing steps before being recycled or released, and is more stringent than ever before to regulate such emissions. Fulfill.
[0004]
Exhaust gas containing NO _x is steam of nitrogen oxides nitrite nuclear industry, in particular a dissolver, for example nuclear fuel dissolution wheels, in particular by contacting the UO ₂ and boiling nitric acid dissolved gas upon dissolving in the following reaction Generate in the form.
UO ₂ + 3HNO ³ → UO ₂ (NO ₃ ) ₂ + 0.5NO ₂ + 0.5NO + 1.5H ₂ O
[0005]
These exhaust gases produced by dissolution of irradiated fuel cannot be released directly into the atmosphere. This is because these exhaust gases contain, in addition to nitrogen oxides NO _x , certain fission products (particularly Kr, Xe, I ₂ (iodine 129) and tritium), volatile fission products (eg ruthenium oxide RuO _4). ), Radioactive aerosol, scavenging gas for the shearing and dissolving means, air lift or bubbling gas used in the dissolving machine, and water vapor due to boiling of the dissolved solution.
[0006]
Accordingly, iodine present in the spent nuclear fuel in the form of iodide, such as cesium iodide, or organic iodide, is released upon dissolution. The amount of release is related to the evaporation rate of the solution, but part of the iodine remains in the solution in the form of non-volatile iodate.
[0007]
It is essential to subject these gases to several treatments after filtration and before being released into the chimney of the reprocessing plant, removing water vapor and regenerating NO, NO ₂ as nitrite vapor in nitric acid. It passes through several devices in series arrangement to be combined: condenser, recombination tower, finishing tower. Recombination of the not perfect, the exhaust gas nitrogen oxide NO _x and other gases, for example Kr, Ze, tritium, air containing iodine is contained mainly.
[0008]
Thus, this gas is further subjected to a wet process consisting essentially of two continuous steps. The first step consists of, for example, continuously absorbing in a 0.4 M soda solution at ambient temperature (20 ° C.) to capture iodine and remove unrecombined nitrogen oxides NO _x. Then, it is considered that the following chemical reaction proceeds.
4NaOH + I ₂ + 2NO → 2NaI + 2NaNO ₂ + 2H ₂ O
(Proceed with a yield of 95%)
2NaOH + 2NO + 1 / 2O ₂ → 2NaNO ₂ + H ₂ O
(Proceed with a yield of 67%)
[0009]
The solution obtained at the end of this process consists essentially of sodium, nitrite and iodide, and this solution used to capture iodine can usually be directly dumped into the ocean if its radioactivity is below the emission standards. .
[0010]
The measurement of the degree of contamination of a sodium solution that has trapped iodine is not performed with the amount of iodine contained in the solution. Because the specific activity of iodine 129 is low, it is porous pollutants, such as ruthenium, that cause the activity to exceed the current standards for direct ocean dumping.
[0011]
Therefore, when the cleaning solution is so contaminated that it cannot be disposed of, the cleaning solution needs to be re-acidified and subjected to a second iodine desorption step in which nitrite is decomposed. It is carried out discontinuously (batch type) at a temperature of 60 ° C. and a contact time of 10 to 17 hours using a reagent consisting of 1.5 M sulfamic acid for nitrate decomposition and 4 M nitric acid for iodine elimination.
[0012]
It is considered that the next chemical reaction proceeds in this process.
NH ₂ HSO ₃ + NaNO ₂ → NaHSO ₄ + N ₂ + H ₂ O
2NH ₂ HSO ₃ + H ₂ O + 2NaI → (NN ₄ ) ₂ SO ₃ + Na ₂ SO ₄ + I ₂ (2)
2NaI + 2NaNO ₂ + 4HNO ₃ → I ₂ + 2NO + 2H ₂ O + 4NaNO ₃ (3)
2NaNO ₂ + HNO ₃ → NO + NO ₂ + 2NaNO ₃ + H ₂ O
The cumulative yield of the above reactions (2) and (3) is 98%.
[0013]
The detached iodine is reconverted to sodium iodide before ocean dumping. The final traces of iodine and volatile organic iodides, such as methyl iodide (which does not react in this process) leaking from this sodium wash, are captured in particular by a catalyst-supported solid material.
[0014]
The final solution obtained in this separation step is an LA-MA solution containing essentially sodium ions and sulfate ions, which is preferably vitrified. However, the presence of sulfate and sodium ions in this solution is particularly harmful to the vitrification process. Sulfate ions have the problem of causing equipment corrosion and are evaded as harmful to vitrification facilities, while the amount of sodium ions taken into the glass is limited.
[0015]
Nitrogen oxides NO _x or industrial waste gas containing nitrous acid vapor, or other may be using the processing method of the exhaust gas, the reagent without the above disadvantages associated with the soda used, i.e., one containing sodium ions, A high salt concentration solution does not form, and one is the presence of a large amount of nitrite in the same washing solution (this makes it impossible to dispose of it, and a removal process involving one or more additional processes is required. There has been a need for gas scrubbing reagents that are free of Accordingly, there is a need for a nitrogen oxide-containing exhaust gas cleaning reagent that can produce an inert product or a product that can be easily removed instead of nitrite and sodium contained in the cleaning solutions used so far. I came.
[0016]
In the case of gases and exhaust gases containing nitrogen oxides and iodine, especially in the nuclear industry, especially so-called dissolved gases, the reagents are sodium ions and sulfate ions for vitrification as a post-treatment in addition to the above-mentioned conditions. Is an organic non-salt reagent that does not contain any impurities, and is at least as efficient as soda in capturing iodine, removing nitrogen oxides and decomposing nitrites to produce inert products, preferably gaseous It is necessary to use a reagent that does not require the addition of a nitrite-resistant reducing reagent in a post-treatment, for example, a separation step. Such a reagent must also be detrimental to the use of stabilizers for iodide in the product effluent.
[0017]
The second step or the separation step is an alternative to the nitric acid-sulfamic acid mixture that has been used so far, and unlike the above mixture, does not particularly generate ions that interfere with the vitrification reaction, such as sulfate ions, There is a need for a reagent that oxidizes iodide to molecular iodine. In addition, such reagents must be able to be used with maximum efficiency in existing equipment and equipment, such as those that provide a reaction rate that matches the residence time in existing equipment, particularly gas-washing towers in continuous operation. Must.
[0018]
[Problems to be solved by the invention]
The object of the present invention viewed from the first angle is to provide a method for purifying or treating a nitrogen oxide-containing exhaust gas by absorption and washing using a reagent that satisfies all the above-mentioned conditions and overcomes the above problems. is there. The object of the present invention from the second angle is to provide a method for treating or purifying exhaust gas containing iodine in addition to nitrogen oxide or nitrous acid vapor, and this exhaust gas is particularly suitable for dissolving irradiated nuclear fuel. This method consists of a dissolved gas to be generated, and this method removes the vapor of nitrous acid and captures iodine, and a first washing and absorption step for capturing iodine, and a second iodine desorption that oxidizes iodine ions to molecular iodine. Consisting of steps, the method meets all the above conditions and criteria and overcomes the problems with prior art methods.
[0019]
[Means for Solving the Problems]
According to the present invention as viewed from the first angle, the above and other objects are obtained by wet purifying exhaust gas by washing exhaust gas containing nitrogen oxide with an aqueous solution, wherein the cleaning water solution is hydroxylammonium nitrate (having 1 to 30 carbon atoms). The alkyl group may be O-substituted and / or N-substituted with one or more alkyl groups. This ammonium derivative has the advantage that it does not contain sulfur, sodium or other elements that are detrimental to the aftertreatment of the cleaning solution. The effluent generated at the end of this washing process has a low salt concentration, and the nitrous acid vapor is decomposed to remove gaseous N ₂ and N ₂ O (which can be easily released and is inert). As it forms, it contains no nitrite. Furthermore, residual excess reagent in the solution is pyrolyzed.
[0020]
From the second angle, the method of the present invention is a method for purifying or treating a gas containing iodine other than nitrogen oxides or exhaust gas. According to this method, the exhaust gas is treated as follows. Hydroxyl ammonium nitrate (which may be O-substituted and / or N-substituted with one or more alkyl groups having 1 to 30 carbon atoms) to capture iodine present in the exhaust gas in the first step In this second step, the aqueous solution of the reagent selected from (1) is removed by removing the nitrogen oxides and decomposing the nitrite. The iodine contained in it is released. These exhaust gases are gases generated by the industry regardless of the type of industry, especially the exhaust gas from the nuclear industry containing iodine in the form of iodine 129, especially the dissolution produced when reprocessing spent nuclear fuel by the dissolution method Gas.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
The ammonium nitrate used in the present invention from the first angle is preferably selected from O-alkyl hydroxyl ammonium nitrate, N-alkyl hydroxyl ammonium nitrate, and hydroxyl ammonium nitrate (NHA), of which NHA is Preferred as a reagent. The reagent is used in an aqueous solution, preferably at a concentration of 0.05 to 0.5 mol / l. A preferred concentration of NHA is, for example, 0.1 mol / l, which is optimal for industrial operation. In addition, it is preferable that the reagent, for example, NHA is present in excess, and it is even better that the amount is larger than the stoichiometric amount (= 1). That is, the concentration (mol / l) ratio between the first reagent and nitrous acid is 5 or more, preferably 25 to 200, more preferably 50 to 100, and ratios of 125, 150 and 175 can also be used.
[0022]
The pH of the cleaning aqueous solution can be adjusted by adding a strong inorganic acid, preferably nitric acid, and is preferably 6 or less, more preferably 3.5 or less, and further preferably 2.5 to 1. Under a certain temperature, the reaction efficiency increases with a decrease in pH. It can be carried out at ambient temperature, generally around 20 ° C., but may be as low as 10 ° C. or as high as 90 ° C., preferably 30 ° -50 ° C., for example 40 ° C. High temperatures are preferred because yields and reaction rates are significantly improved.
[0023]
The cleaning aqueous solution contains the above-mentioned ammonium derivative as a reagent and can be used in any known gas purification and cleaning apparatus. The method is preferably performed continuously. Although the contact time can be easily determined by those skilled in the art, it is usually 6 to 20 seconds in a continuous operation apparatus. The exhaust gas to be treated by the method of the present invention is an exhaust gas industry containing nitrogen oxides or a non-industrial exhaust gas. In particular, exhaust gas from fossil fuel-fired power plants, such as industrial or household waste incineration plants.
[0024]
The first step in the method of the present invention as seen from the second angle essentially corresponds to the method of the present invention as seen from the first angle for exhaust gas containing only nitrogen oxides and is used. The operating conditions are essentially the same as those described above, and the preferred reagent is NHA, but the operating conditions and parameters of the first step of the method of the invention from the second angle are nitrogen oxide and iodine in the exhaust gas. Of course, it can be optimized and adapted in view of the existence of both. Therefore, the temperature is from 10 ° to 90 ° C., and it can be carried out at ambient temperature. However, the higher the temperature, the more advantageous the reaction efficiency. The pH is preferably 6 or less, preferably 3.5 or less, more preferably 2.5 to 1.0.
[0025]
The reagent is used in an aqueous solution, preferably at a concentration of 0.05 to 0.5 mol / l. In particular in the case of NHA, the preferred concentration of this reagent is 0.1 mol / l, which is particularly suitable for industrial operation using existing equipment. In addition, the reagent, for example, NHA is excessive, preferably in excess of iodine, and the concentration ratio (mol / l) of the first reagent to iodine is preferably 5 or more, more preferably 25 to 200, More preferably, it is 50-100, 100 is a preferable ratio, However, 125, 150, 175 can also be used.
[0026]
This step is preferably carried out continuously, and the contact or residence time can be easily determined by a person skilled in the art, but is usually 6 to 20 seconds in continuous operation equipment. It is also possible to determine various parameters, in particular values such as pH, temperature, reagent excess, etc., in order to set a residence time suitable for the equipment used, preferably existing, especially continuously operated equipment. Unlike the soda, the reagent used in the first step, especially NHA, does not contain sodium and does not contain sulfur. Therefore, the discharge cleaning solution has a very low salt concentration and does not cause any problems in post-treatment vitrification. . This reagent effectively captures iodine, removes nitrogen oxides, and decomposes nitrite.
[0027]
Since the cleaning solution obtained at the end of the first step is substantially free of nitrite, it is not necessary to add a nitrite-resistant reagent in the withdrawal step (second step of the method), and thus the specific solution of the present invention. Reagents, N-substituted and / or O-substituted hydroxylammonium nitrate reduce iodine and nitrite, and mainly remove nitrogen N ₂ and nitrous oxide N ₂ O, which are inert gas products that can be easily discharged. Generate. According to the present invention, iodine can be captured and nitrite can be quantitatively (100%) decomposed by using the reagents described above, which represents a significant efficiency improvement over prior art methods using soda.
[0028]
Iodine reduction in the first step is performed at an efficiency of 100%, or depending on the conditions, the decomposition of nitrite becomes a residence time or a limiting function in the first step and is not completely decomposed. Continues, but is completely disassembled at the end of the entire method.
[0029]
The advantages of the reaction and reagent used in the present invention are optimized so that the temperature, pH, and reagent excess conditions become the reaction rate that matches the residence time of the existing equipment, especially the residence time of the dissolved gas absorption treatment equipment of the nuclear fuel reprocessing plant It can be done. Soda can be replaced with the reagent of the present invention without any modification to the apparatus, which is a special advantage of the present invention. For example, in an existing dissolved gas scrubber with a residence time of 8 seconds, iodine is needed at this time, at a pH of 25, a temperature of 20 ° C., and an NHA excess of at least 100 (which corresponds to an NHA concentration of 0.1 mol / l). The reduction yield approaches 100%. In the method using soda, the yield is only 95%. Of course, in other apparatuses, for example, when the temperature rises, the residence time becomes shorter.
[0030]
The reagent used in the second step of the method of the present invention as seen from the second angle is hydrogen peroxide, which oxidizes iodine ions by the following reaction to release iodine.
^{_{H 2 O 2 + 2I - +}} 2H + → I 2 + 2H 2 O
This reagent used in place of the nitric acid and sulfamic acid mixture has the advantage that it does not contain sulfur harmful to vitrification, which is a post-treatment, especially in nuclear fuel reprocessing plants, and does not generate salty emissions. This reagent is also capable of quasi-quantitatively oxidizing iodine ions and detaching iodine, which is an important improvement over the second step of prior art methods using a mixture of nitric acid and sulfamic acid.
[0031]
Hydrogen peroxide is preferably used at a concentration of 0.1 to 0.5 mol / l, and is preferably used in excess as compared with iodine ions. The preferred concentration of hydrogen peroxide is 0.2-0.25M, which is perfectly suitable for industrial operation.
The second step is preferably performed at a pH of 6 or less, more preferably 4 or less, particularly preferably around 1. Generally, it operates at an ambient temperature of about 20 ° C, but low temperatures up to a minimum of 10 ° C, high temperatures up to 30 °, 40 °, 50 °, 60 °, up to 90 ° C are used. When the temperature is increased, the reaction rate is remarkably improved. However, when the residence time is constant, the yield or efficiency is improved. For example, when NHA is used, the reaction rate is considerably faster than when a mixture of nitric acid and sulfamic acid is used, and the efficiency is also improved. That is, the yield for 4 hours at 60 ° C. is 99%.
[0032]
This second step is generally performed discontinuously or batchwise, particularly in the dissolved gas treatment facility of a nuclear fuel reprocessing plant. The residence time is 1 second to 48 hours, preferably 4 to 25 hours, and more preferably 4 to 6 hours.
[0033]
The use of hydrogen peroxide in the present invention is perfectly matched to existing equipment, particularly existing equipment in the dissociation treatment system for dissolved gas cleaning solutions in nuclear fuel reprocessing plants. This is a particular advantage of the present invention because the sulfamic acid nitrate mixture can be replaced by hydrogen peroxide without modification to the equipment, particularly in existing stripped reactors using batch systems, which is a residence time suppression parameter. It means not. In the method of the present invention, the yield after 4 hours at 60 ° C. is 99%, but in the acid-based method of the prior art, the release method continues even after 17 hours, and the yield at this time at 60 ° C. Was found to be only 98%.
[0034]
The solution obtained at the end of the process is preferred for use in a vitrification facility and has a reduced salt concentration and is easy to treat because there is no sulfur present. The method of the present invention can be used to treat any effluent containing nitrogen oxides and, optionally, iodine, regardless of its source, and can be used in existing or new gas treatment facilities.
Other features and advantages of the present invention can be read from the following description, which will be described with reference to the drawings, but the following is not intended to limit the present invention in any way.
[0035]
【Example】
The following examples illustrate the reactions used in the method of the present invention. In the examples, the properties and concentrations of the products in the solution and the gas phase were measured using the following three experimental methods.
· Nitrite ion NO in solution ₂ ^- (or nitrous acid HNO ₂ as a function of pH) the reduction of the iodine discussed quartz cell diode array spectrophotometer.
-The gas was analyzed by gas chromatography.
Oxidation of iodine ions and their release from the first wash solution, two buffer media: sodium nitrate measuring only iodine anions and acetoacetic acid measuring total iodine in the form of iodine ions Followed by specific iodine ion electrode using ascorbic acid mixture.
[0036]
Appropriate molar solutions were prepared from commercially available analytical quality reagents containing hydroxylammonium nitrate (NHA). A molecular iodine solution (maximum concentration 1.2 mmol / l) was obtained by a quantitative Dushman reaction between iodine ion I ⁻ and iodate ion IO ₃ ^{− according} to the following reaction formula.
^{_{IO 3 - + 5I - + 6H}} + → 3I 2 + 3H 2 O
The pH was adjusted with nitric acid. Prior to each test, a fresh solution of NHA, nitrite ions, or iodine was brought to the required temperature in a constant temperature bath and then injected into the spectrophotometer cell using an automatic piget. In gas analysis, gas was bubbled into an NHA solution placed in a sintered plate washing bottle using high purity helium to detect nitrogen. Iodine solution was introduced into the washing bottle little by little for 20 minutes. During the operation, NHA was in a large excess compared with iodine. Gas analysis was performed at regular intervals during and after the iodine injection to release dissolved gas components from the solution. The release of iodine was carried out in a constant temperature controlled glass cell under argon bubbling.
[0037]
Example 1 [Reduction of nitrite ion (or nitrite depending on pH) with NHA]
a) Reactions and products:
The equilibrium reaction stoichiometry was measured at pH 6 or lower by adding a known amount of NHA to excess sodium nitrite in aqueous nitric acid. The nitrite ion (or nitrite) concentration was measured after a sufficient time to complete the reaction at ambient temperature. Table 1 shows that the reaction proceeds in mole to mole because the R ratio (number of moles of nitrite consumed / number of moles of NHA initially present) is very close to 1. Therefore, the reaction formula is as follows.
HNO ₂ + NH ₃ OH ⁺ → N ₂ O (gas) + 2H ₂ O + H ⁺
[0038]
[Table 1]

It was found from gas chromatography that only nitrous oxide was produced.
[0039]
b) Reaction rate:
A test for measuring the reaction rate was carried out at ambient temperature (20 ° C.). The reaction rate increases with decreasing pH. Clearly when the pH was lowered from 3.5 to 1, this doubled the reduction rate. When the pH is 1, the apparent reaction order for nitrite ions (or nitrous acid in this case) is 1 from FIG. 1, and extrapolated to 8 seconds (residence time in the washing tower) yields a yield of 96%. It turns out that it becomes.
[0040]
Example 2 [Reduction of molecular iodine by NHA]
a) Reactions and products:
For example, since several reactions such as the following occur simultaneously, the stoichiometric amount could not be measured in this example unlike before.
^{_{2NH 3 OH + + 2I 2 →}} N 2 O + 4I - + H 2 O + 6H +
^{_{4NH 3 OH + + 3I 2 →}} N 2 + N 2 O + 5I - + 3H 2 O + 10 H +
These reactions as a whole consist of intermediate steps, some of which produce monoxide NO. Whether one reaction is dominant over the other depends on the pH, and this singularity is what gas chromatography shows. Therefore, although nitric oxide is the main product at pH = 1, protoxide (an oxide having the smallest number of oxygen atoms) is generated in an amount about three times smaller than N _2, for example. At pH = 2.5 this situation is reversed (nitrogen monoxide and 7 times as much protocide as nitrogen are generated).
[0041]
b) Reaction kinetics A reaction rate measurement test was conducted at 20 ° C. The reaction rate increases with increasing pH. Increasing the pH from 1 to 2.5 increases the reaction rate by 20. Extrapolating to 8 seconds (wash tower residence time) makes the reaction quantitative at pH 2.5. The apparent reaction order for iodine in the same pH range is 1. FIG. 2 shows an example of the speed when pH = 1. In all cases, NHA was significantly in excess compared to iodine. The initial ion concentration was 125, 150, 175 at 1.2 mmol / l. The optimal NHA excess is close to 100. NHA is in large excess compared to the stoichiometric amount. When the initial nitrite ion concentration is 14, 11, and 9.4 mmol / l, they are 100, 125, and 150, respectively, and the optimum NHA excess is 50 to 100. Additional tests were performed at 30 °, 40 ° and 50 ° C. It was widely observed that the reaction rate doubled for every 10 ° C. increase.
[0042]
Example 3 [Oxidation of iodine ion by hydrogen peroxide in absorbing solution]
a) Reaction and product iodine ion oxidation reaction were examined under the following standard conditions, pH = 1 (adjusted by adding nitric acid), sodium iodide concentration: 1 mmol / l, hydrogen peroxide concentration: 0.2 mol / l, NHA concentration: 0.1 mol / l.
[0043]
b) Results for rate FIG. 3 shows the change in log [I ⁻ ] as a function of time in the first 15 minutes in the presence of 0.1M NHA, and FIG. 4 shows the test in the first 5 hours. Is. In the first 2 minutes, the yield reaches 90%, after which the iodine ion concentration increases slightly, then decreases 6 hours later, and if the reaction time is 25 hours, the yield is at least 99%. The change in log [I ⁻ ] as a function of time at 20 ° C. is accelerated by H ₂ O _{2 in the} absence of NHA, and the yield is 98% after 20 minutes. Thereafter, the iodine ion concentration decreases regularly, and after 5 hours becomes 3.8 × 10 ⁻⁶ M, which corresponds to a yield of 99%. The temperature effect is very significant and the time required to achieve a yield of 99% is reduced from 6 hours at 30 ° C. to 4 hours at 60 ° C. These temperatures and times are perfectly compatible with existing batch methods that use reagents based on a mixture of nitric acid and sulfamic acid, and it is only necessary to replace these reagents with hydrogen peroxide. There is no need to modify anything.
[Brief description of the drawings]
FIG. 1 is a graph showing the common logarithm log [HNO ₂ ] of nitrous acid concentration (mol / l) as a function of time (seconds) for various NHA excess ratios at pH = 1 and 20 ° C. Shows the measurement results of the following NHA excess, each curve is linearly reverse progressive.
◇ [NHA] = [NO ₂ ⁻ ] × 100
◆ [NHA] = [NO ₂ ⁻ ] × 125
× [NHA] = [NO ₂ ⁻ ] × 150
FIG. 2 is a graph showing the common logarithm log [I ₂ ] of iodine concentration (mol / l) as a function of time (seconds) for various NHA excess rates at pH = 1 and 20 ° C. The following shows the results of measurements made with the NHA excess, each curve being linearly reverse progressive:
◇ [NHA] = [I ₂ ] × 125
○ [NHA] = [I ₂ ] × 150
* [NHA] = [I ₂ ] × 175
FIG. 3 is a graph showing the common logarithm log [I ⁻ ] of iodine ion concentration (mol / l) as a function of time (minutes) in the first 15 minutes for oxidation with H ₂ O ₂ in the presence of NHA. It is.
FIG. 4 is a graph showing the common logarithm log [I ⁻ ] of iodine ion concentration (mol / l) as a function of time (minutes) in the first 5 hours with acid with H ₂ O ₂ in the presence of NHA. In 25 hours, the iodine ion concentration (not shown) is 1.42 × 10 ⁻⁶ mol / l.

Claims (19)

  In a method for purifying exhaust gas by washing exhaust gas containing nitrogen oxide using an aqueous solution of the reagent, the reagent is hydroxylammonium nitrate (O-substituted with one or more alkyl groups having 1 to 30 carbon atoms, and / or N -Optionally substituted).   The method according to claim 1, wherein the reagent is selected from O-alkylhydroxylammonium nitrate, N-alkylhydroxylammonium nitrate, hydroxylammonium nitrate (NHA).   The method according to claim 1, wherein the reagent is used at a concentration of 0.05 to 0.5 mol / l.   The method of claim 1, wherein the reagent is present in excess of the stoichiometric amount of the reaction between the reagent and nitrite.   The method of claim 1, wherein the concentration ratio of the initial reagent and nitrite is 5 or more.   6. The method of claim 5, wherein the concentration ratio of the initial reagent to nitrite is 25-200.   The method of claim 1, wherein the concentration ratio of the initial reagent to nitrite is 50-100.   The process according to claim 1, wherein the process is carried out at a pH of 6 or less.   The process according to claim 1, wherein the process is carried out at a temperature between 10 ° and 90 ° C.   The method according to claim 1, wherein the method is continuously performed, and the cleaning liquid and the exhaust gas are contacted for 6 to 20 seconds.   The method according to any one of claims 1 to 10, wherein the exhaust gas also contains molecular iodine. Cleaning the exhaust gas in the first step nitrate hydroxylammonium (carbon atoms O- substituted by one or more alkyl groups of 1 to 30, and / or N- substituted may be) with an aqueous solution of a selected reagent from Then, the iodine present in the exhaust gas is captured, nitrogen oxides are removed, nitrite is decomposed, and iodine contained in the cleaning aqueous solution from the first step in the second step is treated with hydrogen peroxide. The method of claim 11, wherein the method is contacted and released.   The method according to claim 12, wherein the reagent used in the first step is hydroxylammonium nitrate (NHA).   The method according to claim 12, wherein hydrogen peroxide is used in the second step at a concentration of 0.1 to 0.5 mol / l.   The method according to claim 12, wherein the contact is performed at a pH of 6 or less in the second step.   The method according to claim 12, wherein the second step is performed at a temperature of 10 ° to 90 ° C.   The method according to claim 12, wherein the second step is performed in a discontinuous batch mode, and the residence time is 1 second to 48 hours.   The method of claim 12, wherein the solution resulting from the process is then fed to a vitrification facility.   The method of claim 11, wherein the iodine is iodine 129.

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