CA1058830A

CA1058830A - Removal of nitrogen oxides in form of ammonium sulfate from nitrogen oxides-containing gas

Info

Publication number: CA1058830A
Application number: CA218,376A
Authority: CA
Inventors: Tetsuya Watanabe; Koji Konno; Tadashi Nakamura; Shigeru Saitoh
Original assignee: Kureha Corp
Current assignee: Kureha Corp
Priority date: 1974-01-21
Filing date: 1975-01-21
Publication date: 1979-07-24
Also published as: SE406190B; SE7500564L; SE7610594L; SE410303B; NO750154L; NO137679C; NO137679B

Abstract

REMOVAL OF NITROGEN OXIDES IN FORM OF AMMONIUM
SULFATE FROM NITROGEN OXIDES-CONTAINING GAS
ABSTRACT OF THE DISCLOSURE

A method of removing nitrogen oxides from a gas containing nitrogen oxides effectively in the form of ammonium sulfate, wherein a nitrogen oxides-containing gas is brought into contact with an aqueous solution which contains at least a ferrous salt and a sulfurous acid alkali salt to have the nitrogen oxides absorbed in the solution in the form of an alkali salt imidodisulfonic acid, and then the thus formed alkali salt of imidodi-sulfonic acid is converted into ammonium sulfate by hydrolysis at a temperature higher than 100°C after separation and recovery of the salt from the solution.

Description

--` 1058830 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 removing nitogen oxides from the gas efficiently in the form of a solid salt of imidodisulfonic acid.
BACKGROUND OF THE INVENTION:
Examples of gases containing oxides of nitrogen (hereinafter 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 generating plants.
In recent years, it is known that a so-called photo-chemical smog is generated frequently. One of the main causes of such photochemical smog is that a large quantity of NOX is present in the atmosphere. There is, there-fore, 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 reduction of NOX only within narrow limits for both theoretical and economical reasons.

- 2 -1!--` -`` 1058830 . .

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 sulfite; an aqueous solution of potassium permanganate;
an aqueous solution of hypochlorite or chlorite; or an aqueous solution of ferrous salt and sulfurous acid alkali salt (alkali sulfite). The present inventors disclosed in their copending 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 bring-ing 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 i~ a great need for a method which is more efficiently capable of removing NOx from a gas containing NOx by effectively treating the absorption solution.
It is therefore an object of the present invention to provide a method of removing NO efflciently from a gas containin~ 1 . , 1~58830 NOX in the wet pr~cess, by effectively treating an NOX-absorbed solution for the removal therefrom of NOX in the form of ammonium sulfate which is valuable as a fertilizer.
The present inventors have found that when a nitrogen oxides-containing gas is contacted with an aqueous solution which contains a ferrous and a sulfurous acid alkali salt, NOX are absorbed in the solution in the form of an alkali salt of imidodisulfonic acid. They have also found that the imidodisulfonic acid alkali salt can be converted into ammonium sulfate by hydrolysis at a temperature higher than 100C under an acidic range after separation and recovery of.the salt from the absorption solution.
Thus, according to the present invention, there is provided a method of removing nitrogen oxides from a gas containing nitrogen oxides in the form of ammonium sulfate, comprising the ste~s of contacting a nitrogen oxides-containing gas with an aqueous solution which contains at least a ferrous salt and sulfurous acid alkali salt to form therein an alkali salt of imidodisulfonic acid by.absorption of said nitrogen oxides, and converting said alkali salt of imidodisulfonic acid into ammonium sulfate by hydrolysis at a temperature higher than 100C
after separation and recovery of said salt from said solution.
The above and other objects, features and advantages of the invention will become clear from the following description and the claims.

D

DESCRIPTIOW OF THE PREFERRED EMBODIME~TS:
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 con-tains a ferrous salt, an organic acid alkali salt, an organic acid and a sulfurous acid alkali salt, (4) an aqueous solution which contains a ferrous sait 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 salt. Examples of the ferrous salts include 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 tetra-carboxylic 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 . .

L ' ~ ..~
I ` 1058830 ¦ 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 S ¦ these organic acid alkali salts include, for example: monobasic I acids such as acetic acid, propionic acid and butyric acid;
¦ dibasic acids such as malonic acid and succinic acid; polybasic l acids such as ethylenediamine tetracarboxylic acid and nitrilo-¦ tricarboxylic acid. A typical example of ethylenediamine l tetracarboxylic acid is ethylenediamine tetraacetic acid (here-¦ inafter referred to as EDTA) and a typical example of nitrilo-¦ tricarboxylic acid is nitrilo-triacetic acid (hereinafter ¦ referred to as NTA). The carboxylic acids forming ethylenedi-¦ amine tetracarboxylic acids and nitrilo-tricarboxylic acids may ~15 I be, for example, propionic acid, butylic acid or both of these ¦ acids. It should be understood, however, that the carboxylic l acids are not limited only to these acids but other suitable ¦ acids may also be employed. The sulfurous acid aIkali salt is ¦ in the form of M2S03 or MHS03 (wherein, _ represents an alkali l as in the organic acid alkali salt) and includes, for example, ¦ sodium sulfite, potassium sulfite, ammonium sulfite, sodium ¦ bisulfite, potassium bisulfite, ammonium bisulfite and the ¦ like.
~ ¦ It is assumed that, when an N0x-containing gas is ¦ contacted with an aqueous solution which contains at least I . . `' I ` - - 6 -` 1~58~330 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 alkali salt of imidodisulfonic acid 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 rep-resented by NO, and the sulfurous acid alkali salt is represented by sodium sulfite, respectively).
FeSO4 + NO ) Fe(NO)SO4 . . . . . . . . . . (1) Fe(NO)S04 + 2Na253 + 2H20 ~ Fe(OH)3 + Na2S04 ~ NH(S03Na)2 . . . . . . . . . . (2) It will be clear from the foregoing reaction formulae (l) 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 alkali salt of imidodisulfonic acid 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, equal to or greater than the amount of NOX 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 greater 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 alkali salt of imidodisulfonic acid is produced in an increased amount.

D
.. . . . .

When the content of the sulfurous acid alkali salt in the aqueous solution is equal to or greater than 1~ by weight, approximately 9Q% of the NOX which have been absorbed in the solution is converted into alkali salt of imidodi-sulfonic acid. In this manner, when the alkali salt of imidodisulfonic acid is produced, a portion of the NOX
which is absorbed in the aqueous solution forms a nitrilo-trisulfonic acid alkali salt (N(SO3MJ3) and a sulfamic acid alkali salt (NH2SO3M). The thus formed nitrilo-trisulfonic acid alkali salt and sulfamic acid alkali saltare converted easily into ammonium sulfate by hydrolysis when the alkali salt of imidodisulfonic acid is hydrolyzed as will be discussed hereinafter. Therefore, the production of such nitrilo-trisulfonic acid alkali salt and the sulfamic acid alkali salt can impose no adverse ef f ects in the present invention.
It usually takes a relatively lon~ time for the NOX
- in the gas to produce an alkali salt of imidodisulfonic acid after absorption in the aqueous solution. For example, where the NOX are absorbed in the aqueous solution at 55C and the absorption solution, which has absorbed therein NOX, is left standing still at that temperature, it usually takes 3 to 4 hours before a major portion of the absorbed NOX is converted into an alkali salt of imidodisulfonic acid. The reaction time is reduced at a higher temperature level, for example, at 90C, 90~ of the absorbed NOX is converted into an alkali salt of imidodisulfonic acid in 30 minutes. Thus, it is desirable to heat the absorption solution for the purpose of shortening the working time.
The alkali salt of imidodisulfonic acid thus formed in ,; .~
, ...

the absorption solution shows the least solubility when it is in the form of potassium or calcium salt, allowing easy separation from the absorption solution. Therefore, where the aqueous soluti~n to be used for the absorption of NOX contains a potassium salt as the sulfurous acid alkali salt, the alkali salt of imidodisulfonic acid which has been formed in the absorption solution is easily caused to precipitate in the form of potassium salt for separating purposes simply by condensing or cooling the absorption solution. On the other hand, where the potassium salt is not employed as the sulfurous acid alkali salt in the aqueous solution, the alkali salt of imidodisulfonic acid is also easily caused to precipitate in the form of a potassium or calcium salt for separation purposes simply by adding a potassium or calcium compound such as potassium sulfate, potassium chloride, potassium nitrate, calcium hydroxide, calcium carbonate, calcium oxide, calcium chloride or the like to the resultant absorption solution before condensation or cooling the absorption solution. In case a potassium compound is added to the absorption solution for separating the alkali salt of imidodisulfonic acid in the form of potassium salt, it is preferable to employ potassium sulfate as the potassium compound. This is because potassium sulfate has a relatively low solubility as compared with other potassium co~pounds to allow easy recovery and cyclic reuse. In case a calcium compound is added to the absorption solution for separating the alkali salt of imidodisulfonic acid in the form of calcium salt, it is preferable to employ calcium hydroxide as the calcium compound. This is because calcium hydroxide has a proper _ g _ __ . . _ _. .. .... .. ... . .. . ~ . __ ; _ ... _ _ .

solubility as compared with other calcium compounds to allow easly conversion of the alkali salt of imidodi-sulfonic acid to calcium salt. Moreover, potassium sulfate or calcium hydroxide is completely free from accumulation of anions which occurs in the absorption solution when . other potassium or calcium compound such as potassium chloride, potassium nitrate or calcium chloride is used while such anions do not exist originally in the absorption solution. The filtrate obtained after separating the imidodisulfonic acid alkali salt from the absorption solution in the form of potassium or calcium salt may be used again as the aqueous solution for the absorption of NOX. In such a case, there occur no inconveniences even if a portion of the alkali salt of imidodisulfonic acid remains in the filtrate. When the alkali salt of imidodi-sulfonic acid is formed in the absorption solution, 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. Further-more, in a case where the filtrate which has been obtained after separating the alkali salt of imidodisulfonic acid in the form of potassium or calcium salt is reused as an aqueous solution for the absorption of NOX, it is necessary to supplement the ferrous salt for the filtrate in an amount suitable for compensating the loss. However, this is not necessary when a polybasic acid such as EDTA
or NTA is contained in the ~queous 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 lOS8830 easily reduced by the sulfurous acid alkali salt coexisting in the absorption solution, without calling for the need for the removal of the ferric salt nor the supplementation of the ferrous salt. As a portion of the sulfite ions in the NOx-absorbed solution (absorption solution) is consumed by the production of the alkali salt of imidodi-sulfonic acid, it is necessary to supplement the sulfite ions to the filtrate which is obtained after separation of the alkali salt of imidodisulfonic acid from the absorption solution in the form of potassium or calcium salt, before recirculating the filtrate as the aqueous solution for the absorption~of NOX. However, where the gas under treat-ment contains sulfur oxides along with NOX, the sulfur oxides are also absorbed in the absorption solution simultaneously with the NOX to produce sulfite ions in the absorption solution. In such a case, the addition of supplementary sulfite ions to the filtrate is not always necessary.
The alkali salt of imidodisulfonic acid thus formed in the absorption solution is, after separation therefrom, hydrolyzed into ammonium sulfate in an acidic range, preferably at a pH value of below 5Ø In this case, the minimum pH value to be employed may be preferably determined with due consideration to the relationship between the velocity of hydrolyzation for the alkali salt of imidodisulfonic acid and the quantity of an acid to be consumed since a large amount of the acid is required with the lowering of the pH value. The excess lowering of below pH 1.5 cannot be expected ato bring about any effect for promoting hydrolyzation. In this connection, hydrolyzation of the alkali salt of imidodisulfonic acid D

~`1058830 is preferably performed at a pH value of from 5.0 to 1.5, - and at a temperature higher than 100C. As shown by the reaction formulae (3) and (4) given below, it is assumed that the alkali salt of imidodisulfonic acid is firstly converted into a sulfamic acid alkali salt (or sulfamic acid), which is then converted into ammonium sulfate by further hydrolysis.
2NH(S03M)2 + 2H2 t 2NH2SO3M + M2SO4 + H2 4 2 3M 2H20 ~ (NH4)2SO4 + M2SO4 ....................... (4) (where M represents an alkali as mentioned hereinbefore).
The hydrolytic reaction of the alkali salt of imidodi-sulfonic acid which has been formed in the absorption solution does not proceed substantially to the stage as shown by the reaction formula (4) at a temperature below 100C and, as a result, a sulfamic acid alkali salt or sulfamic acid is produced in the solution. On the other hand, when the reaction temperature is higher than 370C
which is almost at the level of the critical temperature of water, the reaction does not occur because water can not exist in the form of liquid. Therefore, in the present invention, it is necessary to maintain the hydrolytic temperature higher than 100C, preferably within a range of from 110 to 200C. In this instance, as mentioned hereinbefore, the absorption solution has formed therein nitrilotrisulfonic acid alkali salt and sulfamic acid alkali salt, which are also converted into ammonium sulfate as shown by the following reaction formula (5) and the afore-mentioned reaction formulae (3) and (4).

3 )3 2H2 ~~~ 2NH(SO3M)2 + M2S4 + H2S4 (5) (where M represents an alkali as mentioned hereinbefore).
The hydrolyze-formed solution thus obtained which contains ammonium sulfate is acidic due to the production ~..,~' lOS8830 o~ sulfuric acid, which, however, may be neutralized with use of ammonia, thereby converting the sulfuric acid in the hydrolyze-formed solution also into ammonium su}fate as shown by the following reaction formula (6).
2H2S4 + 4NH3 --t 2 (N~4)2so4 ~ -----(6) On the other hand, the hydrolyze-formed solution thus obtained, which contains ammonium sulfate, may be alkalized, for example, by addition of an alkaline substance such as sodium hydroxide thereto, in order to generate ammonium gas and recover same.
The thus obtained ammonium sulfate can be advan-tageously used, for example, as a fertilizer, after separation and recovery from the hydrolyze-formed solution.
The filtrate as obtained after recovery of the ammonium sulfate may be used again to form potassium or calcium imidodisulfonate. In this connection, where potassium sulfate is added to the NOx-gas absorbed solution ~absorption solution) in order to produce the alkali salt of imidodisulfonic acid in the form of potassium salt and the thus produced potassium imidodi- sulfonate is hydrolyzed -after separation from the solution, the hydrolyze-formed solution contains therein potassium sulfate which may be separated and recovered from the hydrolyze-formed solution with the ammonium sulfate, so that it becomes possible to add `recircularly the recovered potassium sulfate to the absorption solution. In view of this advantage, it is preferred in the present invention to add potassium su~fate to the absorption solution to produce the alkali salt of imidodisulfonic acid in the form of potassium salt. In connection with the formation of the potassium salt, the NOx-gas absorbed solution should preferably be added with a suitable amownt of potassium sulfate even where , .

~ lOS8830 ,: I
the aqueous solution for the absorption of NOX contains a potassium salt as the sulfurous acid alkali salt. Further, the ammonium sulfate and the potassium sulfate may be separated from the hydrolyze-formed solution individually by precipitating S them one after the other utilizing the difference in solubility.
Alternatively, the ammonium sulfate and the potassium sulfate may be precipitated as a mixture and separated from each other - after recovery from the hydrolyze-formed solution.
It will be appreciated from the foregoing description that, according to the present invention, the NOX which are contained in a gas are efficiently removed therefrom in the form of useful ammonium sulfate. The invention can thus contribute greatly to the treatment of NOx-containing gases, particularly to the treatment of NOx-containing exnaust gases.
The invention will be illustrated more particularly by the following examples, which are given only by way of example and therefore should not be construed as limitative of the invention.
.' , , ,.
EXAMPL~ 1:
FeSO4 ............... 2.0% by weight Na2SO3 .............. 3.2% by weight CH3COONa ........... l0.0% by weight CH3COOH ............. 2.4~ by weight H2O ................ 82.4% by weight ., Il ~058830-300 mi~ of NO gas was contacted at a normal temperature ' with lO0 m~ of an aqueous solution having the above composition absorbing therein 290 m~ of the NO gas. --~ ~he gas absorbed-liquid was heated up to 95C and maintained at that temperature for 30 minutes. As a result, a precipitate of an iron compound appeared in the liquid, and a supernatant liquid was obtained by removing the precipitate.
In order to separate NH(SO3Na)2 which was dissolved in the supernatant liquid, 10 g of KCl was added thereto, followed by cooling to room temperature, to obtain 4.5 g of a precipitate.
i An infrared absorption spectrum analysis revealed that the precipitate separated from the supernatant liquia con-, tained NH(SO3K)2. It was found by an analysis that the nitrogen Il content in the precipitate corresponded to 70% of the absorbed 5 ~ NO gas. A further infrared quantitative analysis revealed that the precipitate contained 47~ by weight of NH(SO3K)2.
Thereafter, the filtrate which was obtained after the !
filtration of the precipitate from the supernatant liquid was I heated and condensed to precipitate a mixture of 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 absorbed NO gas was recovered and fixed.

., I
i; - 15 -`
.,,, , . ~

EXAMPLE 2:
~eSO4 ........ ~.- ----- 2.0~ by weight KHSO3 .................. 3.0% by weight CH3COOK ................ 5.0~ by weight CH3COOH ................ 1.0% by weight H2O - ................... 89% by weight 300 mQ of NO gas was contacted at 55C with 100 m~ 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 liq'uid.
10 g of KCl was added to the separated supernatant liquid, fol-lowed by cooling, to obtain another precipitate.
, As a result of an infrared absorption spectrum analysis it was revealed that the precipit,ate separated from the superna-tant liquid contained NH(SO3K)2. The precipitate had a nitrogen content corresponding to about 70% of the absorbed NO gas.

EXAMPLE 3: , , FeSO4 ................ ~.0%-by weight Na2SO3 ............... 5.0~ by weight CH3COONa ............. 5.0% by weight CH3COOH .............. 3.0% by weight H2O .................. 85.0% by weight ; ~ 1058830 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 KCl was added to the supernatant liquid, followed by cooling, to obtain another precipitate.
As a result of an infrared absorption spectrum analy-sis, it was revealed that the precipitate from the supernatant liquid contained NH(S03K)2 which had a nitrogen content cor-responding to about 90% of the absorbed NO gas.
.. , ' , . .

EXAMPLE 4:
FeS04 ................ 2.0% by weight Na2S03 ................ 2.0% by weight CH3COONa ............. 10.0% by weight CH3COOH .............. 2.4~ by weight EDTA-Na .............. 2.0% by weight ~ethylenediamine sodium tetraacetate) - H20 ................... 81.6~ by weight , ' . .
300 mQ of NO gas was contacted at room temperature with 100 mQ of an aqueous solution having the above composition absorbing therein 280 mQ of the NO gas. The gas absorbed-liquid was heated at 80C for 1 hour and thereafter 8 g of K2S04 dissolv d 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(SO3R)2 and R2SO4. Accord-ing to a quantitative analysis by the infrared spectrum absorp-tion method, NH (S03K)2 component in the precipitate contained apparoximately 50~ of the nitrogen of the absorbed NO gas.
After removal of the precipitate, the remaining liquid was mixed with Na2SO3 in a proportion of about 2~, followed by contact with 300 me of NO gas in the same manner as described hereinbefore and absorbed therein 270 me of the N0 gas.
The gas absorbed-liquid was heated to 80C and then added with

4 g of K2SO4, followed by cooling, to obtain a precipitate of approximately 2.6 g of NH(S03K)2. The precipitate corresponded in nitrogen content to approximately 89% of the NO gas which was absorbed during the second contact absorption.
.. .
EXAMPLE 5:
FeSO4 ................ .4% by weight Na2SO3 ............... .4% by weight NTA-Na ............... .4% by weight (nitrilo sodium triacetate) H20 .................. 88% by weight 300 mQ of N0 gas was contacted at 50C with 100 mQ
of an aqueous solution having the above composition absorbing therein 275 m~ of the NO gas. The gas absorbed-liquid was left ... . . .

standinq for 4 hours at 50C and then 10 g of CH3COOK was dissolved in the liquid, followed by cooling to 10C, to obtain 2.8 g of a precipitate. 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 precipitate was determined to contain 1.7 g of NHtSO3K)2, which corresponded to 58% of the absorbed NO gas. Presumably, the balance of the absorbed NO gas remained, as shown in Example 4 , as a dissolved state of NH(SO3K)2 in the gas absorbed-liquid. It is thus possible to increase the recovery rate of imidodisulfonic acid alkali salt by repeated use of the gas absorbed-liquid.
. ,,.
EXAMPLE 6:
EDTA-Fe chelate compound... 5% by weight Na2SO3 .................... 4% by weight H2O ....................... 91% by weight Acetic acid was added to an aqueous solution having the above composition to adjust the pH value to 5.5. The thus prepared solution was placed in a gas absorption bottle with a blowing pipe. After heating the solution to 60C, a nitrogen gas containing 300 ppm of NO was passed through the bottle at a rate of 100~/h for 10 hours. The gas which was discharged from the bottle had an average NO content of 20 ppm.

i -'i ' . lOS8830 ! The gas absorbed-liquid was heated at 80C for 4 hours, followed , by addition of 4 g of K2SO4. Upon cooling the liquid, 1.4 g of i~ NH~SO3K)2 precipitated. The precipitated NH(S03K)2 corresponded !' in nitrogen content to 54% of the absorbed NO gas. In view of !I the fact that it was possible to precipitate more NH(S03K)2 , by further concentration of the liquid, the balance of the absorbed NO gas was presumably dissolved in the gas absorbed-, liquid in the form of NH(SO3K)2.
l,i , , '.
I' EXAMPLE 7:
I
, FeSO4 ................ 3.0% by weight CH3COONa ............. 10.0~ by weight CH3COOH .............. 4.5% by weight Na2SO3 ............... 5.0% by weight 1 H2O --------------- 77.s% by weight i 8500 mQ of NO gas was absorbed at 50C in 2000 mQ
of an aqueous solution having the above composition. The gas- 1, absorbed solution was heated at 80C for 1 hour. As a result, , a precipitate of iron hydroxide appeared in the solution. The ~ precipitate was filtered out and 180 g of potassium sulfate was l added to the filtrate, followed by cooling down to 30C, to ', precipitate crystals.
The crystals were in the form of a mixture consisting !~ mainly of NH(SO3K)2 and K2SO4 and containing a small amount of ', sodium acetate and NH2SO3K. A quantitative analysis by the ,.
! i .; . . I

.. . .

~ ~058830 ¦ infrared absorption spectrum method revealed that the crystals ¦ contained 44.3 g of NH(S03K)2. After filtering out the crystals, ¦ part of the filtrate was treated to have a pH value of 2.0 and ¦ then heated for 4 hours. Sodium nitrite was added thereto to S ¦ generate nitrogen gas.~ AS a resùlt of calculation based on the amount of nitrogen gas generated from the filtrate, it was ¦ found that 33.4 g of NH(S03K)2 still remained in the filtrate ¦ as a whole. This brought the total amount o~ NHtS03K)2 to 77.7 g , which corresponded in nitrogen content to 95.7% of the absorbed N0 gas.
The above-mentioned crystals were washed with a small amount of cold water to obtain crystals which contained 42.1 g of NH(S03X)2 and 6.8 g of K2S04. The entire amount of the just-mentioned crystals, 0.1 g of concentrated sulfuric acid L5 and 100 m~ of water were put in an autoclave for reaction at 120C for 8 hours underagitating. After the reaction, ammonia was added thereto little by little to adjust the pH value to 6.5, followed by cooling to 20C, to obtain 25.9 g of K2S04 crystals which was 92~ in purity.
~he just-mentioned R2S04 crystals were filtered out and the filtrate was dried under reduced pressure to obtain 32.8 g of crystals which contained 2.1 g of NH2S03K, 19.3 g of ~NH4)2S04 and 8-6 g of K2 4 As a result, it was confirmed that NH(S03K)2 had been !5 hydrolyzed 100~, and NH2S03K, the product of the hydrolysis, ~058830 ., ' . .
was further hydrolyzed 91%. Thus, (N~4)2S04 was produced from NH~S03K)2 at a rate of 96%. That is to say, 91.7% of the absorbed NO gas was recoverea in the form of (NH4~2S04.
; I , ,I E ~ ~LE 8:
Feso4.~ 2~o% by weight .
Na2S03~.......... ~3.2% by weight - C~3COONa........... 10.0~ by weight C~3COOB............ 2.4% by weight ; ~20................ 82.4% by weight 10; 300 mQ of NO gas was contacted at room temperature with 100 m~ of an aqueous solution having the above composition, absoxbing therein 290 mQ of the NO gas. The gas absorbed-liquid was heated at 95C for 30 minutes to form therein a precipitate of an iron compound. After separating the precipitate from the liquid, CaC03 and (CH3C00)2Ca were added to the resultant filt-rate to adjust the pH value thereof to 6.2, converting Na2S04 and Na2S03 which existed therein into CaS04 and CaS03, respectiv _ ly. This is because there occur firstly the precipitates of . ~hese CaS04 and CaS03 in the filtrate in converting imidodi-sulfonic acid alkali salt into the form of calcium salt.
Thereafter, Ca~OH)2 was added little by little to the solution obtained by separating the thus precipitated CaS04 and CaS03 from the filtrate to adjust finally the pH value thereof to 8.0, followed by leaving standstill, to precipitate 3.3 g of crystals.
The crystals werc analyzed after recrystallization fxom water. The analysis revealed that the crystals had the . ' .. ~. ', -.
-22- . .
. ,, ~ , . .
, ...... .. . .

` lOS8~330 chemical composition of NS206NaCa~3EI20. In this connection, 94% of the absor~ed N0 gas was recovered as NS~06NaCa 3H20.
A liquid comprising 2.0g of NS206NaCa 3H20 and 100 m~
. ~f O.lN H2S04 was heated at a temperature of 160C for 4 hours, followed by alkalizing the liquid, to recover 158 m~ of NH3.
. . Accordingly, 0.033 mol/L of (NH4)2S04 was contained in the .. . hydrolyze-formed solution. .

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Claims

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 in the form of ammonium sulfate, comprising the steps of contacting a nitrogen oxides-containing gas with an aqueous solution which contains at least a ferrous salt and sulfurous acid alkali salt to form therein an alkali salt of imidodisulfonic acid by absorption of said nitrogen oxides, and converting said alkali salt of imidodisulfonic acid into ammonium sulfate by hydrolysis at a temperature higher than 100°C
after separation and recovery of said salt from said solution.

2. A method as defined in claim 1, wherein said alkali salt of imidodisulfonic acid is converted into ammonium sulfate by hydrolysis at a temperature higher than 100°C
and the resultant hydrolyze-formed solution which contains said ammonium sulfate is neutralized by means of ammonia.

3. A method as defined in claim 1 or 2, wherein said alkali salt of imidodisulfonic acid is hydrolyzed at a temperature in the range of from 110 to 200°C.

4. A method as defined in claim 1 or 2, wherein said hydrolysis is effected at a pH value in the range of from 5 to 1.5, inclusive.

5. A method as defined in claim 1, wherein a filtrate which is obtained after separating said alkali salt of imidodisulfonic acid from the nitrogen oxides-absorbed solution is recirculated as said aqueous solution for absorption.

6. A method as defined in claim 1, wherein said alkali salt of imidodisulfonic acid is potassium salt formed by adding a potassium compound to the aqueous solution which absorbed therein nitrogen oxides.

7. A method as defined in claim 6, wherein said potassium compound is potassium sulfate.

8. A method as defined in claim 6, wherein said alkali salt of imidodisulfonic acid in the form of potassium salt is formed by adding a filtrate, which is obtained after separating said ammonium sulfate from the hydrolyze-formed solution, to the aqueous solution which absorbed therein nitrogen oxides.

9. A method as defined in claim l, wherein said alkali salt of imidodisulfonic acid is converted into ammonium sulfate after forming said alkali salt of imidodisulfonic acid into the calcium salt by adding a calcium compound to the aqueous solution which absorbed therein nitrogen oxides.

10. A method as defined in claim 9, wherein said calcium compound is calcium hydroxide.

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

12. A method as defined in claim 11, wherein said ethylenediamine tetracarboxylic acid is ethylenediamine tetraacetic acid.

13. A method as defined in claim 11, wherein said nitrilo-tricarboxylic acid is nitrilo-triacetic acid.

14. 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 nitrilo-triacetic acid, at least one alkali salt of said compound, or a mixture thereof.

15. 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.