CA1058829A - Removal of nitrogen oxides from gas containing same by conversion to harmless nitrogen - Google Patents

Abstract

REMOVAL OF NITROGEN OXIDES FROM
GAS CONTAINING SAME BY CONVERSION
TO HARMLESS NITROGEN

ABSTRACT OF THE DISCLOSURE:

A method of effectively removing nitrogen oxides which are entrained in a gas, wherein a nitrogen oxide-containing gas is brought into contact with an aqueous solution which contains at least ferrous salt and sulfurous acid alkali salt to have the nitrogen oxides absorbed in the solution in the form of imidodi-sulfonic acid alkali salt, hydrolyzing the imidodisulfonic acid alkali salt to obtain a hydrolyze-formed solution, and adding nitrous acid, nitrous acid anhydride, or nitrite to the hydrolyze-formed solution to generate nitrogen.

Description

Field of the Invention:
This invention relates to a method of efficiently removing nitrogen oxides from a gas containing nitrogen oxides, and more particularly to a method of efficiently removing nitrogen oxides from the gas in the form of harmless nitrogen BACKGROUND OF THE INVENTION:
. . .... ___ _ Examples of gases containing oxides of nitrogen (herein-after referred to as NOX) are exhaust gases from combustion apparatuses such as boilers, nitric acid manufacturing plants, various metal treating processes and other nitrogen oxide generat-ing plants.
In recent years, it is known that a so-called photochemical smog is generated frequently. One of the main causes of such photochemical smog is that a large quantit~ of NOX is present in the atmosphere. There is, therefore, a great need to reduce the quantity of NOX contained in such exhaust gases and/or to remove NOX from such exhaust gases.
In combustion apparatuses such as boilers, for example, the NOX content in the exhaust gas has been reduced conventionally by employment of burners and furnaces of improved design.
These methods, however, are not very desirable because they allow the reductlon of NOX only within narrow limits for both the theoretical and economical reasons.
In this connection, it is also well known in the art to employ the so-called wet type processes, for the removal of NOX
contained in an exhaust gas, using an alkaline aqueous solution including an aqueous solution of sodium hydroxide or sodium

- 2 - ~ ~

sulfite; an aqueous solution of potassium permanganatei an aqueous solution of hypochlorite or chlorite; or an aqueous solution of ferrous salt an~ sulfurous acid alkali salt ~alkali sulfite). The present inventors disclosed in their copending ; 5 application Serial No.212,631, filed October 30~ 1974, as a new wet type process, a method for removing nitrogen oxides from a gas containing nitrogen oxides, which is characterized by bringing the nitrogen oxides-containing gas into contact with an aqueous solution containing an organic acid alkali salt and a salt of metal selected from the group consisting of Fe, Co, Ni, Cu and Mn in the presence of sulfurous acid alkali salt. In the above-mentioned wet type processes, as a matter of fact, there has been no established method for effectively treating an absorption solution which has absorbed therein NOX and therefore, there is a strong demand for a method which is more efficiently capable of removing NOX from a gas containing NOX by effectively treating the absorption solution.
An object o the present invention, therefore, is to provide a method of removing NOX from a gas containing NOX in the form of harmless nitrogen, by effectively treating the absorption solution which has absorbed therein NOX in removing NOX from the gas by the wet type process.

The present inventors have found that when a nitrogen oxide-containing gas is brought into contact with an aqueous solution which contains ferrous salt and sulfurous acid alkali salt for the absorption of nitrogen oxides, NOX are absorbed in the form of imidodisulfonic acid alkali salt. They have also found that nitrogèn is generated by adding nitrous acid, nitrous acid anhydride, or nitrite to the hydrolyze-formed solution which is obtained by hydrolyzing the thus produced imidodisulfonic acid alkali salt.
Thus, according to the present invention, there is provided a method of removing nitrogen oxides from a gas containing nitrogen oxides, wherein said gas is brought into contact with an aqueous solution containing at least ferrous salt and sulfurous acid alkali salt to have said nitrogen oxides absorbed in said solution in the form of imidodisulfonic acid alkali salt, hydrolyzing said imidodisulfonic acid alkali salt at a pH below 6.5 to obtain a hydrolyze-formed solution, and adding a nitrogen compound selected from the group consisting of nitrous acid, nitrous acid anhydride, and nitrite to said hydrolyze-formed solution to generate nitrogen.
DESCRIPTION OF THE PREFERRED EMBODIMENTS:
The aqueous solution referred to herein as containing at least ferrous salt and sulfurous acid alkali salt is, for example, (1) an aqueous solution which contains a ferrous salt and a sulfurous acid alkali salt, (2) an aqueous solution which contains a ferrous salt, an organic acid alkali salt and a sulfurous acid alkali salt, (3) an aqueous solution which contains a ferrous salt, an organic acid- alkali salt, an organic acid and a sulfurous i acid alkali salt, ~4) an aqlleous solution which contains a i ferrous salt of organic acid and a sulfurous acid alkali salt, or (5) an aqueous solution which contains a ferrous salt of organic acid, an organic acid alkali salt, an organic acid and $ a sulfurous acid alkali sa~t. Examples of the ferrous salts in-clude inorganic salts such as ferrous sulfate, ferrous nitrate and ferrous chloride, and various water-soluble ferrous salts of organic acids such as acetic acid, propionic acid, butyric acid, malonic acid, succinic acid, ethylenediamine tetracarboxylic acid and nitrilo-tricarboxylic acid. In this connection, when an ; iron salt of ethylenediamine tetracarboxylic acid or nitrilo-tricarboxylic acid is used, the iron salt may not be in the form of ferrous salt but may be a ferric salt.
More particularly, the ferric salt is easily reduced into the form of ferrous salt by a coexisting sulfurous acid alkali salt, forming an aqueous solution which contains substantially a ferrous salt. The organic acid alkali salts are water-soluble salts of organic acids, for example: salts of organic acids with alkali metals such as Li, Na and K; salts of organic acids with alkali earth metals such as Mg and Ca; or ammonium salts of organic acids.
. The organic acids forming these organic acid alkali salts include, for example: monobasic acids such as acetic acid, propionic acid and butyric acid; dibasic acids such as malonic acid and succinic acid; polybasic acids such as ethylenediamine tetracarboxylic acid and nitrilo-tricarboxilic acid. A typical example of ethylene-diamine tetracarboxylic acid is ethylenediamine tetraacetic acid (hereinafter referred to as EDTA) and a typical example of nitrilo-tricarboxylic acid is nitrilo-triacetic acid thereinafter referred .11 .

; to as NT~). The carboxylic acids forming ehtylenediamine tetra-carboxylic acids and nitrilo-tric~rboxylic acids may be, for example, propionic acid, butylic acid or both of these acids.
It should be understood, however, that the carboxylic acids are not limited only to these acids but other suitable acids may also be employed. The sulfurous acid alkali salt is in the form of M2S03 or MHS03 (wherein, M represents an alkali as in the organic acid alkali salt)and includes for example, sodium sulfite, potassium sulfite, ammonium sulfite, sodium bisulfite, potassium bisulfite or ammonium bisulfite, etc.
It is assumed that, when an NOx-containing gas is contacted with an aqueous solution which contains at least ferrous salt and sulfurous acid alkali salt as mentioned hereinabove, the NOx and the ferrous salt form a complex in the aqueous solution and the complex thus produced forms an imidodisulfonic acid alkali salt by reaction with the sulfurous acid alkali salt, according to the following reaction formulae (1) and (2) (where the ferrous salt is represented by ferrous sulfate, NOx is represented by NO, and the sulfurous acid alkali salt is represented by sodium sulfite, respectively).
FeS04 + NO ~-Fe(NO)S04 ........................... (1) Fe(NO)S04 + 2Na2S03 + 2H20 Fe(OH)3 + Na2S04 +
( 3 a)2 -----................................... (2) It will be clear from the foregoing reaction formulae (1) and (2) that the absorption of NOx becomes difficult with an insufficient amount of ferrous salt and that it becomes difficult to satisfactorily produce the imidodisulfonic acid alkali salt with an insufficient amount of sulfurous acid alkali salt.

Therefore, the aqueous solution should contain the ferrous salt and sulfurous acid alkali salt in sufficient amounts. In the present invention, the aqueous solution should contain the ferrous salt in an amount, in terms of moles, e~ual to or greater than the amount of NO~ to be absorbed, preferably in an amount at least 0.02% by weight. Moreover, the aqueous solution to be employed in the present invention should contain the sulfurous acid alkali salt in an amount, in terms of moles, two times great-er than that of the ferrous salt, preferably in an amount at least 0.2% by weight. In this connection, if the amount of sulfurous acid alkali salt in the aqueous solution enhances, the imido-disulfonic acid alkali salt is produced in an increased amount.
When the content of the sulfurous acid alkali salt in the aqueous solution is equal to or greater than 1% by weight, approximately 90% of the N0x which have been absorbed in the solution is convert-ed into imidodisulfonic acid alkali salt. In this manner, when the imidodisulfonic acid alkali acid is produced, a portion of the NOX which is absorbed in the aqueous solution forms a nitrilo-trisulfonic acid alkali salt (N(SO3M)3) and a sulfamic acid alkali salt (NH2S03M). The thus formed nitrilo-trisulfonic acid alkali salt is converted easily into sulfamic acid or its alkali salt by hydrolysis when the imidodisulfonic acid alkali salt is hydrolyzed as will be dlscussed hereinafter. Therefore, the production of such nitrilo-trisulfonic acid alkali salt and the sulfamic acid alkali salt can impose no adverse effects in the present invention.
It usually takes a relatively long time for the N0x in the gas to produce imidodisulfonic acid alkali salt after absorption in the aqueous solution. For example, where the NOX are absorbed ~ I

¦ in the aqueous solution at 55C and the absorption solution, ¦ which has absorbed therein NO , is left standing still at that ¦ temperature, it usually takes 3 to ~ hours before a major portion I of the absorbed NOx is converted into imidodisulfonic acid alkali ¦ salt. The reaction time is reduced at a higher temperature level:
¦ for example, at 90C, 90% of the absorbed NOx is converted into ; ¦ imidodisulfonic acid alkali salt in 30 minutes. Thus, it is desirable to heat the absorption solution for the purpose of ¦ shortening the working time.
¦ When the imidodisulfonic acid alkali salt is produced, precipitation of a ferric salt (Fe~OH)3) takes place in the ¦ absorption solution as shown by reaction formula (2~ above. It is therefore necessary to remove the ferric salt by filtration from the absorption solution. However, this is not necessary when a polybasic acid such as EDTA or NTA is contained in the aqueous solution to be used for the absorption of NOx, since the ferric salt which has been formed in the absorption solution and which has been in the form of a complex based on the polybasic acid is easily reduced by the sulfurous acid alkali salt coexist-ing in the absorption solution. When the gas under treatment contains sulfur oxides along with NOx, the sulfur oxides are also absorbed in the absorption solution simultaneously with the NOX .
The imidodisulfonic acid alkali salt thus formed in the absorption solution, is hydrolyzed into sulfamic acid or its alkal .
salt, as shown by the following reaction formulae (3) and (3)'.
( 3M)2 ~ H20 ~ NH2so3M + MHS03................. . (3) ( 3 )2 i320 ~~~~~ ~U253~ + M2504 - (3)' .1~

(where M represents an alkali as mentioned hereinbefore). In this connection, whether the thus produced sulfamic acid exists in the form of a free acid or in the form of its alkali salt de-pends upon the pH value of the liquid under hydrolysis.
In order to hydrolyze the imidodisulfonic acid alkali salt as shown in the reaction formulae (3) and (3)' given above, the pH value of the liquid under hydrolysis should be below 6.5.
Preferably, the hydrolysis should be carried out at a pH value below 5 and at a temperature of 40 to 100C to ensure a suffici-ently high rate of hydrolysis.
The sulfamic acid and the sulfamic acid alkali salt which are formed in the hydrolyze-formed solution by hydrolysis accord-, ing to the present invention can be converted into nitrogen by adding to the hydrolyze-formed solution a nitrogen compound selected form the group consisting of nitrous acid, nitrous acid anhydride or nitrite, for example, as shown by the following reaction formulae (4) and (5).

2 3 2 N2 + H20 + H2S04 ........................... (4) NH SO M + ~ a(NO2)2 > N2 + H2O ~ 4 (where calcium nitrite represents the nitrite and M stands for an alkali as mentioned hereinbefore). Thus, it becomes possible to remove NOX from a gas containing NOX in the form of harmless nltrogen .
Examples of the nitrites to be added to the hydrolyze-formed solution include nitrous acid alkali salts such as ammonium nitrite, sodium nitrite, potassium nitrite, magnesium nitrite, calcium nitrite and the like. The reaction under formulae 14) -9- .

and (5) may be carried out at a normal temperature when the pEI
value of the solution is below 6. However, it is desirable to effect the reaction under acidic and heated conditions to accele-rate the reaction. The produced nitrogen may be released as it S is, while the liquid whic~h remains after release of nitrogen may be utilized again as an aqueous solution for the absorption of NOX. When a nitrous acid alkali salt is added to the hydrolyze-iormed solution, alkali ions are accumulated in the solution to be recirculated after release of nitrogen. From the standpoint of avoiding such accumulation of alkali ions, it is desirable to employ, as the nitrous acid alkali salt, calcium nitrite which produces hardly soluble calcium sulfate (CaSO4) as shown by the reaction formula (5). In other words, where calcium nitrite is ', used, the accumulation of alkali ions can be suitably prevented as the filtration and separation of calcium sulfate (CaSO4) is easy. On the other hand, where nitrous acid or nitrous acid anhydride is added to the hydrolyze-formed solution, the solution !
after release of nitrogen becomes acidic due to the formation of sulfuric acid, as shown by the formula (4), and may be used for the hydrolysis of imidodisulfonic acid alkali salt as discussed hereinbefore. Thus, the NOX in the gas under treatment are removed therefrom and released in a completely harmless form, that is to say, in the form of nitrogen.
It will be appreciated from the foregoing description that, according to the present invention, the NOX which is contained in a gas is removed therefrom and finally released after conver-sion to harmless nitrogen. Therefore, the present invention is believed to be able to contribute greately to the treatment of i'` ` 11)58829 , N0x-containing gases, particularly, to the treatment of N0x-containing exhaust gases.
,, The invention will be illustrated more particularly by ¦' the following examples, which are given only by way of example 1, and therefore should not be construed ~s limitative o~ the present invention.

Example 1 FeS04....., ................... 2. 0% by weight I Na2S03........................ .. 3.2% by weight i CH3COONa...................... . 10.0% by weight CH3COOH....................... .. 2.4% by weight ~2 ------------- ---------- 8Z-4% by weight 300 mL of N0 gas was contacted at a normal temperature ~ With 100 me of an aqueous solution having the above composition ¦, absorbing therein 290 m~ of the N0 gas.
¦I The gas absorbed-liquid was heated up to 95C and maintaine !1 at that temperature for 30 minutes. As a result, a precipitate ¦ of an iron compound appeared in the liquid.
!1 In order to confirm the formation of NH(S03Na)2 which was 1' dissolved in the liquid obtained by separating the resultant , precipitate, 10 g of XCQ was added thereto, followed by cooling 1~ to room temperature, to obtain 4.5 g of a precipitate.
Il ~An infrared absorption spectrum analysis revealed that the 1, precipitate formed in the liquid contained NH(S03K)2. It was ~1 found by an analysis that the nitrogen content in the precipitate corresponded to 70~ of the absorbed N0 gas. A further infrared ,, quantitative analysis revealed that the precipitate contained 47%
¦I by weight of NH~S03K)2.

.
!
~! .

Il ' Thereafter, the filtrate which was obt~ined after the filtration of the precipitate from the liquid was heated and condensed to precipitate a mixture af- N~SO3K)3 and , NH2~SO3K) which were found, as a result of a quantitative analysis to have nitrogen contents corresponding to about 20~ of absorbed NO. As a result, it was confirmed that about 90~ of used NO gas was converted to the imidodisulfonic acid alkali salt.
,, Example 2 FeSO4 ........................... .2.0% by weight KHSO3 ........................... .3.0% by weight CH3COOK ......................... .5.0% by weight CH3COOH ......................... .1.0% by weight ~2 ............................. 89% by weight 300 mQ of NO gas was contacted at 55C with 100 mQ of an aqueous solution having the above composition absorbing therein 280 mQ of the NO gas. The gas absorbed-liquid was left standing at that temperature for 180 minutes. As a result, a precipitate of an iron compound appeared in the liquid. 10 g of KC~ was added to the separated supernatant liquid, followed by cooling, to obtain another precipitate.
As a result of an infrared absorption spectrum analysis, it was revealed that the precipitate formed in the supernatant liquid contained NH(SO3K)2. The precipitate had a nitrogen content corresponding to about 70% of the absorbed NO gas.

Example 3 . FeSO4 ........................... .2.0~ by weight Na2SO3 .......................... .5.0% by weight 1 1058~3Z9 ^
.
CH3COONa .......... ,.. ,......... 5.0% by weight CH3COOH ....................... . 3.0~ by weight H2O ........................... 85.0% by-weight 300 mQ~of NO gas was contacted at 60C with 100 mQ of an aqueous solution having the above composition absorbing therein 290 mQ of the NO gas. The gas absorbed-liquid was left standing at 60C for 180 minutes. As a result, a precipitate of an iron compound appeared in the liquid. The iron compound was separated and 25 g of KCQ was added to the supernatant liquid, followed by cooling, to obtain another precipitate.
As a result of an infrared absorption spectrum analysis, it was revealed that the precipitate from the supernatant liquid contained NH(SO3K)2 which had a nitrogen content corresponding to about 90% of the absorbed NO gas.

Example 4 FeSO4 .......................... 2.0% by weight CH3COONa ...................... 10.0% by weight CH3COOH ........................ 2.4% by weight Na2SO3 ......................... 2.0% by weight H2O ........................... 83.6% by weight 300 mQ of NO gas was contacted at room temperature with 200 mQ of an aqueous solution having the above composition abs-orbing therein 295 mQ of the NO gas. Sulfuric acid was added to the gas absorbed-liquid to adjust the pH value to 2.0 and the liquid was heated at 80C for 4 hours. Thereafter, 1.0 g of sodium nitrite of about 97% purity was added to the liquid to generate 292 mQ of nitrogen gas, which corresponded to 99% of 1.
~058829 the absorbed NO g~s.

Example 5 FeSO4 ............................ 2.0% by weight CH3COONa ........................ 10.0~ by weight CH3COOH .......................... 2.4% by weight Na2SO3 ........................... 2.0% by weight EDTA ............................. 4.0~ by weight H20 ............................. 79.6% by weight 300 mQ of NO gas was contacted at room temperature with 200 mQ of an aqueous solution having the above composition abs-orbing therein 280 mQ of the NO gas. The gas absorbed-liquid was treated in the same manner as in Example 4 to generate 276 mQ of nitrogen gas, which corresponded to 98.5% of the absorbed NO gas.

Example 6 FeSO4 ............................ 2.0% by weight Na2S03 .......................... 1.5 % by weight NaHS03 .......................... 1.5 % by weight H20 ........................... 95.0 ~ by weight 300 mQ of NO gas was contacted at ~oom temperature with 100 mQ of an aqueous solution having the above composition abs-orbing therein 260 mQ of the N0 gas. The gas absorbed-liquid was heated to produce a precipitate. After filtering the pre-cipitate, sulfuric acid was added to the filtrate to adjust the pH value to 2.0, followed by heating at 80~C for 4 hours.

Thereafter, l.Og of 97% pure sodium nitrite was added to the ~1 10588Z9 filtrate to generate 253 mQ of nitrogen gas, which corresponded to 97.5 % of the absorbed NO gas.
..
Example 7 i FeSO4 ......................... 2.0% by weight S CH3COONa ...................... 10.0% by weight CH3COOH ....................... 2.4% by weight 2S3 ........................ 2.0% by weight EDTA-Na ....................... 4.0% by weight . (ethylenediamine sodium tetraacetate) H20 ........................ ~---- 79.6% by weight 200 mQ of an aqueous solution having the above composition was introduced in a gas scrubbing bottle having an inner diameter of about 5 cm and equipped with a glass filter, and a nitrogen gas containing 300 ppm of NO was passed through the bottle at a rate of 100 Q/h. The NO removal rate after a lapse of 10 hours was 82 %, absorbing 265 mQ of NO in total. The gas absorbed-liquid was treated in the same manner as in Example 4 to generate . 264 mQ of nitrogen gas, which corresponded to about the entire amount of the absorbed NO.

Example 8 . FeSO4 ............................. 3.0% by weight CH3COONa ......................... 10.0% by weight CH3COOH ........................... 4.5~ by weight Na2SO3 ............................ 5.0% by weight H2O ................... ---------- 77.5% by weight . 850 mQ of NO gas was absorbed at 50C in 200 mQ of an .

~ - 15 -.

` 10588Z9 .
aqueous solution having the above composition. The gas absorbed-liquid was heated at 80C for 1 hour to obtain a precipitate of ferrous hydroxide. After separating the precipitate by filtra-tion, 18.0 g of potassium sulfate was added to and dissolved in , the filtrate, followed by cooling to 30C, to obtain crystals ; which mainly consisted of NH(SO3K)2 and K2so4 and contained a small amount of sodium acetate. A quantitative analysis by the infrared spectrum absorption method revealed that the crystals contained 4.3 g of NH(SO3K)2. Part of the filtrate which was obtained after the filtration of the crystals was treated to adjust its pH value to 2.0 and then heated for 4 hours. From the quantity of nitrogen gas which was generated therefrom by reaction with sodium nitrite, it was calculated that the filtrate contained

3.6 g of residual NH(S03K)2. This brought the total quantity of NH(S03K)2 to 7.9 g which corresponded to 97 % of the absorbed NO gas.
The afore-mentioned crystals were washed with a small amount of cold water to obtain 4.1 g of NH(SO3K)2 and 0.7 g of K2S04 in crystalline form. The crystals were suspended in 20 mQ
of water and concentrated sulfuric acid was added to the suspen-sion to adjust its pH to 2Ø After heating the suspension for

4 hours, 1.25 g of calcium nitrite of 91 % purity was added thereto for reaction at 60C for 1 hours. As a result, 432 m~
of nitrogen gas was generated, which corresponded to 97.5 % of the NH(SO3K)2 which participated in the reaction. By an inverse calculation, it was confirmed that 94.6 % of the absorbed NO gas was turned into nitrogen gas.
After separating by filtration the by-product of calcium lOS8829 sulfate, the filtrate was cooled to 20C to obtain crystals containing 1.6 g of K2SO4. The K2SO4 was filtered out and barium chloride was added to the filtrate. By a calculation based on the quantity of barium sulfate which was formed in the filtrate, it was revealed that 1.9 x 10 2 mol of sulfate ions remained in the filtrate.
I .
; Example 9 ; Crystals of 4.2 g of NH(SO3K)2 and 0.9 g of K2SO4 as obtained in Example 8 were suspended in 20 mQ of water. After heating the suspension for 6 hours, it was brought into violent contact with a mixture gas of 230 mQ of nitrogen monoxide and 230 m~ of nitrogen dioxide at 60C. The gas generated in the course of reaction contained 425 mQ of nitroyen gas, which corresponded to 93.7 ~ of NH~SO3K)2 which participated in the reaction. By an inverse calculation, it was determined that 90.8%
of the absorbed nitrogen oxides was turned into nitrogen gas.
Upon cooling the resultant liquid to 20C, there were obtained crystals containing 1.7 g of K2SO4. After filterin~ out the crystals, the filtrate was treated in the same manner as in Example 8. As a result, the filtrate was determined to contain 2.8 x 10 2 mol of sulfate ions.

Example 10 FeSO4 ............................. 2.0% by weight Na2SO3 ............................ 2.0% by weight CH3COONa ......................... 10.0% by weight CH3COOH ........................... 2.4% by weight EDT~-Na ........................... 2.0% by weight thylenediamin~ sodium t traacetate) H2O............................... 81.6~ by weight 300 m of wo gas w~s contacted at room temper~ture with 100 m of an aqueous solution having the above composition ab-sorbing therein 280 m of the NO gas. The gas absorbed-liquid was 1l heated at 80C for 1 hour and thereafter 8 g of K2SO4 was dissol-¦ ed in the liquid, followed by cooling to room temperature, to ¦, obtain 3 g of a precipitate. An infrared spectrum absorption ¦~, analysis and an elementary analysis revealed that the precipitate ¦~, consisted of NH(SO3X)2 and K2SO4. According to a quantitative ¦ analysis by the infrared spectrum absorption methoal NH(SO3K)2 component in the precipitate contained apporoximately 50 % of the nitrogen of the absorbed NO gas. After removal of the precipitate ¦, the xemaining liquid was mixed with Na2SO3 in a proportion of Il about 2 %, followed by contact with 300 m of NO gas in the same I manner as described hereinbefore to absorb therein 270 m of the ¦I NO gas. The gas absorbed-liquid was heated to 80C and then added with 4 g of K2SO4, followed by cooling, to obtain a recipitate of approximately 2.6 g of NH(SO3K)2. The precipitate corresponded in ll nitrogen content to apporoximately 89 % of the NO gas which was ~ I
' absorbed during the second contact absorption.
"
Example 11 FeSO4............................. .4% by weight Na2SO3............................ .4% by weight , N~A-Na............................ .4% by weight ,~ (nitrilo sodium triacetate) H2O .............................. 88% by weight ,1 I

Il - 18 -.j I

~1 `
' lOS8829 300 mQ of NO gas was contacted at 50C with 100 me of an aqueous solution having the above composition absorbing therein 275 m~ of the NQ gas. The gas absorbed-liquid was left standing , for 4 hours at 50C and then 10 g of CH3COOX was dissolved in the I liquid, followed by cooling to 10C, to obtain 2.8 g of a precipi ¦ tate. The precipitate, after centrifugal separation, was studied under the infrared absorption spectrum analysis and found to contain NH(SO3K)2. Under a further quantitative analysis also ¦ employing the infrared absorption spectrum method, the 2.8 g of the pxecipitate was determined to contain 1.7 g of NH(SO3K)2, . which corresponded to 58 % of the absorbed NO gas. Presumably, the balance of the absorbed NO gas remained, as shown in Example 10, as a dissolved state of NH~SO3K)2 in the gas absorbed-liquid.

Example 12 FeSO4 ............................ 2.0% by weight Na2SO3............................ 1.0~ by weight CH3COONa.......................... 15.0% by weight j CH3COOH........................... 0.5% by weight Il EDTA-Na........................... 2.0~ by weight ll H2O....................;.......... 79.5% by weight ¦ NitrOsen gas containing 300 ppm of NO and 750 ppm of SO2 ¦ was blown into 200 m~ of an aqueous solution having the above ¦ composition at a rate of 100 Q/h for 10 hours while maintaining ¦ the temperature of 50C. A gas analysis of the exhaust gas re-¦¦ leased from the a~ueous solution revealed that 88~ of NO and ¦! 95% f S2 were absorbed in the solution.

. 11.................................... . .
1058t3Z9-Air was blown into the thus obtair.ed absorption solution which absorbed therein NO and SO2 to oxidize completely Na2SO3 and FeSO4 which remained in the solution.
I Sulfuric acid was added to the solution to adjust the pH
I value thereof to 2.0, followed by heating the solution at the , temperature of 80c for 3 hours, and then 0.9 g of 96% pure ¦ calcium nitrite was added thereto, to generate 262 me of nitrogen ! I corresponding to 99% of the absorbed NO gas.
¦ Example 13 I FeSO4...... ~....................... 2.0~ by weight Na2SO3............................. 2.0~ by weight CH3COONa........................... 10.0~ by weight CH3COOH............................ 2.4% by weight I EDTA-Na............................ 4.0% by weight H2O................................ 79.6% by weight Nitro~en gas containing 300 ppm of NO was blown into 200 m~
¦ of an a.queous solution having the above composition at a rate of ¦ 100 ~h ~or 10 hours while maintaining the temperature of 50C.
I A gas analysis of the exhaust gas released from the aqueous I solution revealed that 88~ of NO were absorbed in the solution.
Air was blown into the thus obtained absoption solution which absorbed therein NO to oxidize completely Na2SO3 and FeSO4 ¦ which remained in the solution.
I Sulfuric acid was added to the solution to adjust the pH
I value thereof to 2.0, followed by heating the solution at the ¦ temperature o~ 80C for 4 houxs, and then 0.9 g of 96~ pure ' I calcium nitrite was added thereto, to generate 262 mQ of nitrogen ¦ corresponding to 99% of the absorbed NO gas.
Slakea lime (Ca(OH)2) was added, while .preventing the . I violent rise of pH value, little by little to the li~uid which ¦ remained after generation of nitrogen to adjust the pH value thereof to 6.3, to form calcium salfate therein. The filtrate ! obtained after separating the calcium salfate from the liquid : was contacted with So2 to absorb therein llOO mQ of SO2 and reused as the above-mentioned aqueous solution for the absorption of NO to absorb therein 82~ of NO.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of removing nitrogen oxides from a gas containing nitrogen oxides, wherein said gas is brought into contact with an aqueous solution containing at least ferrous salt and sulfurous acid alkali salt to have said nitrogen oxides absorbed in said solution in the form of imidodisulfonic acid alkali salt, hydrolyzing said imidodisulfonic acid alkali salt at a pH below 6.5 to obtain a hydrolyze-formed solution, and adding a nitrogen compound selected from the group consisting of nitrous acid, nitrous acid anhydride, and nitrite to said hydrolyze-formed solution to generate nitrogen.

2. A method as defined in claim 1, wherein said ferrous salt is ferrous sulfate, ferrous nitrate, ferrous chloride, a water-soluble ferrous salt of organic acid, or a mixture thereof.

3. A method as defined in claim 2, wherein said organic acid is ethylenediamine tetracarboxylic acid or nitrilo-tricarboxylic acid.

4. A method as defined in claim 3, wherein said ethylenediamine tetracarboxylic acid is ethylenediamine tetraacetic acid.

5. A method as defined in claim 3, wherein said nitrilo-tricarboxylic acid is nitrilo-triacetic acid.

6. A method as defined in claim 1, wherein said aqueous solution contains at least one compound selected from the group consisting of acetic acid, propionic acid, butyric acid, malonic acid, succinic acid, ethylenediamine-tetraacetic acid and nitrilotriacetic acid, at least one sodium salt, potassium salt magnesium salt calcium salt and ammonium salt of said compound, or a mixture thereof.

7. A method as defined in claim 1, wherein said sulfurous acid alkali salt is sodium sulfite, potassium sulfite, ammonium sulfite, sodium bisulfite, potassium bisulfite, ammonium bisulfite, or a mixture thereof.

8. A method as defined in claim 1, wherein said nitrite is ammonium nitrite, sodium nitrite, potassium nitrite, magnesium nitrite, calcium nitrite, or a mixture thereof.

9. A method as defined in claim 1, wherein said hydrolysis is effected at a pH value below 5 and a temperature within a range of from 40 to 100°C.

10. A method as defined in claim 1, wherein a liquid which remains after release of nitrogen is recirculated as said aqueous solution for absorption.