United States Patent (19) 11, 3,927,177 Okabe et al. (45) Dec. 16, 1975

54 REMOVAL OF NITROGEN OXDES FROM 2,940,821 7/1960 Carus et al...... 423/599 GASEOUS MIXTURES USENG AOUEOUS 2,940,822 7/1960 Carus et al...... 423/599 ALKALINE OR 3,780, 158 121 1973 Welsh ...... 423/49 PERMANGANATESOLUTIONS 3,780, 159 12/1973 Welsh ...... 423149 (75) Inventors: Taijiro Okabe; Akitsugu Okuwaki, FOREIGN PATENTS OR APPLICATIONS

both of Sendai; Shigetoshi 131,460 1919 United Kingdom...... 423/599 Nakabayashi, Shinminato, all of 219,461 1968 U.S.S.R...... 423/239 Japan 226,568 1968 U.S.S.R...... 423/239 (73) Assignees: Mitsubishi Kinzoku Kogyo Kabushiki Kaisha, Ote; Nippon Primary Examiner-Herbert T. Carter Chemical Industrial Co., Ltd., Assistant Examiner-Eugene T. Wheelock Tokyo, both of Japan Attorney, Agent, or Firm-Wenderoth, Lind & Ponack (22) Filed: Oct. 5, 1973 (21) Appl. No.: 403,974 57) ABSTRACT Nitrogen oxides in a gaseous mixture are removed 30 Foreign Application Priority Data therefrom by a removal method which comprises Oct. 6, 1972 Japan...... 47-100512 causing the gaseous mixture to contact an absorbent comprising a manganate or a or a man (52) U.S. Cl...... 423/235; 423/385; 423/395 ganese-containing substance, which forms a manga 51) Int. Cl’...... C01B 21/00 nate or a permanganate at the time of use, oxidizing 58) Field of Search...... 4231235, 239, 49, 50, 385, oxides formed at the treatment by the re 423/395, 599 duction of the manganate or permanganate with the nitrogen oxides, thereby to generate the oxy-acid 56 References Cited of manganese, and circulating the same. UNITED STATES PATENTS 4 Claims, 1 Drawing Figure 2,793, 112 5/1957 Mancke...... 423150 U.S. Patent Dec. 16, 1975 3,927,177

3,927, 177 1 K.MnO, + 2NO, = 2KNO, MnO, (5) KMnO, + NO =KNO -- MnO + 4 O, (6) REMOVAL OF NITROGEN OXDES FROM KMnO, +3NO + 2KOH = 3KNO -- MnO, + HO (6)" GASEOUS MIXTURES USING AQUEOUS ALKALINE MANGANATE OR PERMANGANATE 5 (In an aqueous solution or in the presence of moisture.) SOLUTIONS In the case where the nitrogen oxide is only NO, BACKGROUND OF THE INVENTION there is no necessity of using the above mentioned This invention relates to a process for economically high-priced salts possessing oxidizing property in addi removing nitrogen oxides in gaseous mixtures through tion to alkalinity for removing only this NO, since NO, the use of (V and VI valence salts of man 10 can be readily reacted and absorbed by contact with an ganese are known and the former may also be called alkaline compound. hypomanganate) or . The expression Accordingly, in the removal of nitrogen oxides, "(per)manganate' used herein means “a salt of oxy which is an object of this invention, on the basis of an acid of manganese'. absorption reaction as described above, the removal of The nitrogen oxides in gaseous mixtures discharged 15 NO only becomes a problem economically and techni from machines and facilities such as internal combus cally. As a method of carrying this out in an effective tion engines, chemical or metallurgical plants, power manner, we have previously conceived the use of alkali generation plants and buildings, gives rise to pollution (per)manganates, of the previous application. such as photochemical smog and is thereby a serious However, as long as the above-mentioned oxy-acid social problem. 20 salts of manganese are dependent on the present pro We have carried out continuous research, from the duction methods, they are of high price. In a method standpoint of eliminating pollution, on treatment of such as that, wherein new lots of these oxy-acid salts of gaseous mixtures containing these nitrogen oxides manganese are used each time in the treatment of gase (which, in actual practice, are principally NO and NO, ous mixtures containing nitrogen oxides, and the oxides and hereinafter referred to collectively as NO). As a 25 of manganese thus formed are discarded, the high cost result, we have discovered that (per)manganates, of the process materials gives rise to an increase in the which have a powerful oxidizing property, are very treatment cost. Furthermore, the discarding of the effective for this purpose as demonstrated in Japanese manganese oxides gives rise to new pollution. These Pat. application No. 17556/1972 (U.S. Pat. Applin. Ser. have been the principal difficulties restricting the use of No. 332,792). 30 (per)manganates. The reactions for absorbing NO and NO, through the Accordingly, it occurred to us that if it were possible use of (per)manganates, which are to be the basis of to oxidize and regenerate at low cost as (per)manga this invention, may be divided into the following three nates the manganese oxide (mostly in the form of man types. ganese dioxide) formed by the reduction of the (per)- 1. NO absorption reaction with a permanganate 35 manganates by the NO in the reactions of Eqs. (1) While this reaction is known with respect to aqueous through (6), it would be possible, by combining this solutions, we have previously disclosed in Japanese regeneration reaction with the above-mentioned NO. Patent Application No. 17556/1972 that an alkali absorption reactions, to provide a process for removing permanganate absorbs NO even in anhydrous NO, which is highly economical. With this possibility solid state. The absorption reaction of NO in the 40 and principle in view, we have carried out various stud case of permanganate, for example, can ies on reactions for regenerating manganese oxides in be represented by the following Eq. (1) or (2), relation to No absorption reactions, and as a result, (2). have arrived at this invention. 45 KMnO, + NO = KNO -- MnO, (I) KMnO, + NO = KNO + MnO, + 4 O, (2) KMnO+ 3NO + 2KOH = 3KNO, + MnO, + HO (2) SUMMARY OF THE INVENTION

(In an aqueous solution or in the presence of moisture.) 50 According to this invention, briefly summarized, 2. NO absorption reaction with a manganate there is provided a process for removal of nitrogen This reaction was made clear for the first time by us oxides in a gaseous mixture containing the same, char as mentioned hereinabove. In the case where acterized by the steps of causing the gaseous mixture potassium manganate (VI) is used as a manga containing a nitrogen oxide to contact an absorbent nate, the reaction can be represented by the 55 comprising a manganate or a permanganate, whereby following Eq. (3), (3)' or (4). the nitrogen oxide is converted into a nitrate or a nitrite KMnO, + 2NO -- O = 2KNOa - MnO, (3) and the manganate or permanganate is converted into (3)' . a manganese oxide, causing the manganese oxide to be (4) oxidized into a manganate or permanganate and circu KMnO, -2NO = 2KNO -- MnO, 60 lating the manganate or permanganate thus regener 3. NO, absorption reaction with a manganate or a ated to the absorption step. permanganate The nature, principle, and utility of the invention, as When potassium manganate (VI), for example, is well as further features thereof, will be apparent from used as a manganate, or potassium permanga the following detailed description beginning with a nate, for example, is used as a permanganate, the 65 consideratien, of general features of the invention and reaction can be represented by the following Eq. concluding with specific examples of practice consti (5) or (6), (6)'. tuting preferred embodiments of the invention. 3 3,927, 177 4 alkaline earth metal salts such as calcium salts, stron BRIEF DESCRIPTION OF THE DRAWING tium salts, and barium salts; Zinc salts and cadmium In the drawing: salts; and ammonium salts. Of these salts, the most FIG. 1 is a schematic flowsheet diagram indicating an representative are alkali metal salts, particularly So example of an absorption-regeneration system accord dium salts and potassium salts. ing to this invention in a state for producing nitrates as The oxides of nitrogen with which this invention is a by-product; and concerned are nitrogen monoxide (NO), dinitrogen FIG. 2 is a similar diagram indicating the system in a trioxide (NO), nitrogen dioxide (NO2), dinitrogen state for system of nitrates. pentoxide (NO), and nitrogen trioxide. Of these ox O ides, NO and NO are the most representative, and the DETAILED DESCRIPTION process of this invention is most effective with respect When the combination of the NO, absorption pro to NO and nitrogen oxide mixtures containing NO. cess step and the step for regeneration of manganese A gaseous mixture containing such oxides of nitrogen oxides is broadly divided, depending on the form of to be disposed of according to this invention has a utilization of the nitrates or nitrites formed as by-pro 5 composition containing nitrogen oxides of from trace ducts in the NO, absorption step, a system for produc to 100 mole percent, preferably from 30 to 50,000 ing nitrates as a by-product as indicated in FIG. 1 and parts per million (ppm) as NO or NO. The composi a nitrate decomposition system as indicated in FIG. 2 tion other than nitrogen oxides, within a gaseous mix result, for example, in the case of salt of an alkali metal. ture of this character, differs with the source of the The invention, however, is not limited to these two gaseous mixture, For example, in the exhaust gas of a systems. gasoline (petrol) engine, there are of the order of from The absorption step and the regeneration step in 100 to 3,000 ppm percent of NO, from 10 to 500 ppm, these nitrate by-product production sytem and nitrate SO, and about 12 mole percent of CO., and CO, hy decomposition system will now be considered in detail. drocarbons and others. In the exhaust gas of a thermal Absorption Step power plant using heavy oil as fuel, there are of the The NO, absorption process step is common to both order of from 100 to 800 ppm of NO, from 30 to 2,000 the nitrate by-production system and the nitrate de ppm of SO, and/or SOs, and about 12 mole percent of composition system of the regeneration step, being in CO and others. In the exhaust gas of chemical plants, accordance with the above Eqs. (1) through (6) in there are of the order of 30 to 50,000 ppm, and HCl, either case. 30 HF and others. According to the results of our studies, the degree of For the absorption step, any reaction process which oxidation of NO due to the (per)manganates, that is, can be adopted for gas-liquid contact reaction (in the whether it stops in the nitrite stage or whether it pro case where the absorbent is a liquid) and gas-solid gresses further to produce nitrates, depends on the contact reaction (in the case where the absorbent is a absorption temperature, the concentration in 35 solid) can be utilized. For example, when a gas-liquid the gas, and further, in the case of an aqueous solution, contact reaction is carried out with a liquid absorbent, on the pH value of the absorbent solution. In the case gas-liquid counterflow or parallel-flow contact can be of an equeous solution, the oxidation progresses to the carried out in a plate column, a bubble capa column, or nitrate indicated in Eq. (1) or (3)' when the absorbent a packed column containing large surface packings. As Solution is neutral or on the acidic side, while a nitrite 40 a result of the reaction, manganese oxides are sepa is formed in accordance with Eq. (2)' of (4) as a first rated out. Accordingly, removal or recovery of these stage when the solution is on the alkaline side, and this oxides must be considered. nitrite is further oxidized into the nitrate, whereby the While any process temperature and pressure may be ultimate result is a reaction according to the Eq. (1) or used, provided that the reaction can be carried out (3)'. 45 satisfactorily, the ordinarily satisfactory levels of tem However, in the case of a dry absorption method perature are from room temperature to 600°C, prefera wherein the absorbent is in a solid state, or even in the bly from 100 to 500C and when the absorbent is in case of an aqueous solution, where absorption takes solid form, and from 0°C to 80°C, preferably 10°C to place in a region of very high content of free alkali, the 70°C when the absorbent is in liquid form; those of rate of oxidation reaction of the nitrite into the nitrate 50 pressure are preferably from 0.95 to 10 atmospheric due to the (per)manganate is considerably slow, and pressure, absolute. The pH value of the absorbent solu the nitrite is included in the absorbent in some cases. In tion is of the order of from 7 to 14. such a case, the nitrite can be readily oxidized to form It is to be understood that the expression “an absor the nitrate by a measure such as electrolytic oxidization bent comprising a manganate or a permanganate' en separately in an aqueous solution state. Therefore, it is 55 compasses the absorbent which yields a manganate or a also possible to utilize this property, to carry out ab permanganate in situ at the time of the absorption step. sorption and removal of NO as the pH value of the Examples of such "precursor' of a manganate or a absorbent solution is held at a high level in the aqueous permanganate include mixtures of MnO, with alkali solution treatment, to cause the absorption reaction to metal oxides with alkali metal hydroxides, and with take place principally according to Eq. (2)' or (4), and 60 salts of nitrogen oxy-acid when the absorbent is in the to reduce the consumption of the (per)manganate per form of a solid. The roasted products of such mixtures unit quantity of the NO in comparison with that in the contain a (per)manganate and are not the precursor case of Eq. (1) or (3)'. but the (per)manganates perse. Examples of (per)manganates suitable for use as the In the case where the absorbent is in the form of an absorbent according to this invention are alkali metal 65 aqueous solution, the solution usually compries (per)- salts such as sodium salts, potassium salts, and rubid manganate in 0.1 to 20% by weight, preferably 1 to 5% ium salts of manganic acid (V), manganic acid (VI), by weight of the solution. The solution can contain a and permanganic acid; copper salts and silver salts; nitrate and/or a nitrite such as an alkali metal nitrate or 3,927, 177 S 6 nitrite which may be a product of the absorption step. are coupled in series or by a single-process regenera In the case where the absorbent in the form of a tion method, wherein these two processes are carried particulate solid, which may be hydrous or anhydrous, out simultaneously. the absorbent can comprise 10 to 100% by weight of More specifically, in the former two-process regener particulate or Solid (per)manganate, the remaining ation method, the first process comprises separating by being inert filler or packing such as Raschig rings or a sedimentation the manganese oxide formed in the ab solid nitrate or nitrite. sorption process, taking this oxide once out of the sys Regeneration Step tem, and oxidizing this oxide with a permanganate The process for regeneration of manganese oxides solution containing free alkali to convert it into a man will be considered separately with respect to the nitrate ganate solution. The second process comprises con by-product producing method and the nitrate decom verting the manganate solution obtained in the first position method. process by electrolytic oxidation, thereby to convert it A. Regeneration process depending on nitrate by into the permanganate. In the case where a manganate product production method is used in the aforementioned absorption step, the oxi The regeneration step of these systems is based on a 15 dation of this into the permanganate is unnecessary. reaction for oxidizing a manganese oxide formed in the The objective process in the case of the latter single NOf absorption step with a permanganate in an aque process regeneration method is limited to a wet absorp ous solution in the presence of an alkali, thereby to tion of NO in which an absorbent in an aqueous solu form a manganate and on a reaction for oxidizing this tion is used. For this purpose there are two methods. manganate by electrolytic oxidation, oxidation with 20 One includes an absorption step and comprises holding chlorine, oxidation with ozone, or some other suitable the concentration, alkalinity, and temperature of the method, as indicated by the following Eqs. (7) and (8). alkali permanganate within suitable ranges within an When is taken as one example of absorption apparatus, oxidizing the manganese oxide the manganese oxide, and caustic potash is specified as formed by the absorption by means of a permanganate the alkali, and electrolytic oxidation is applied, the into a manganate, and, oxidizing this manganate by regeneration reaction is shown by the following formu ordinary electrolytic oxidation thereby to regenerate las: the same into the permangante. The other method MnO, -- 2KMnO -- 4KOH -> 3KMnO, + 2HO (7) comprises taking the absorption solution out of the 2KMnO -- 2HO - 2KMnO, + 2KOH -- H. (8) apparatus, and carrying out concurrently within the In this case, the reactivity of the manganese oxide is 30 regeneration apparatus both of oxidation with a per low for causing the above set forth reactions to take manganate of the manganese oxide into a manganate place, and the higher the alkali concentration and the and electrolytic oxidation of the resulting manganate. reaction temperature are, the more rapid is the Thus, as described above, the manganese oxide progress of the decomposition reaction of the perman formed in the absorption process step can be effec ganate indicated in the following Eq. (9), which is not 35 tively and completely regenerated as (per)manganates. desirable. On one hand, in the case of dry method wherein the 2KMnO, +2KOH - 2K, MnO, + HO +% O, (9) absorbent is used in a solid state for the disposal of the Therefore, the important point of the regeneration NO, the absorbent is treated with water into an aque process of this system is to suppress to a minimum the ous solution of the nitrate. In the case of wet method permanganate decomposition reaction according to wherein the absorbent is used as an aqueous solution, Eq. (9) and to use manganese dioxide of high reactivity the manganese oxide is separated, leaving the aqueous in the oxidation reaction according to Eq. (7). solution containing principally the nitrate. Further We have found that a very convenient feature of the more, in the case where these aqueous solutions con manganese oxide formed when a gaseous mixture con tain a nitrite, they are subjected to a process such as taining nitrogen oxides is treated with an absorbent 45 electrolytic oxidation, thereby to convert all of the comprising a (per)manganate, is its high activity and nitrite into the nitrate, and then can be recovered as high reactivity in the form wherein it is coupled with by-product in a desired form. water, and that, under suitable conditions, it is possible The permanganate to be used in this process step can to cause selectively the reaction of only Eq. (7) to be suitably selected from the permanganates enumer progress, that is, to oxidize selectively manganese diox ated hereinbefore in terms of the absorbent. Preferable ide, thereby to convert it into a manganate. ones are alkali permanganates, particularly sodium As a result, by combining this with the electrolytic permanganate and . Examples oxidation of the manganate of the process presently of alkalis are hydroxides and oxides of alkali metals. practiced as indicated by Eq. (8), it is readily possible While the reaction conditions of the oxidation due to to convert it into the permanganate. Thus, it is possible 55 the permanganate may be freely selected, provided that to regenerate completely and effectively the manga the desired reaction can be carried out, these condi nese oxide formed in the NO absorption reaction as an tions are ordinarily a concentration of the manganese oxyacid salt of manganese. The electrolytic oxidation oxide dispersion of the order of from 0.01 to 10 percent of the manganate represented in Eq. (8) is being prac by weight, a pH value of the order of from 12 to 14, a ticed at present in the process for producing potassium 60 permanganate concentration higher than the stoichio permanganate from manganese ore and does not entail metric quantity, and a reaction temperature of the any problems in production technology. order of from 40 to 200°C. The regeneration by this nitrate by-product produc The oxidation of the manganate formed by oxidation tion system may be carried out by a two-process regen by means of the permanganate can be accomplished by eration method, wherein a process for oxidizing a man 65 any suitable method such as electrolytic oxidation, ganese oxide formed in the absorption process with a chlorine oxidation, or ozone oxidation. While the reac permanganate and another process for electrolytic tion conditions of these oxidation methods may be oxidation of the manganate formed in the first process, selected at will, provided that the desired oxidation can 3,927, 177 7 8 be accomplished, they are ordinarily, for example, in oxide formed in the absorption process as a manganate, electrolytic oxidation a temperature of the order of there is provided a highly economical process for re from 40° to 100°C, an anode current density of the moving NO, which is also extremely useful in the solu order of from 0.001 to 0.02 A/cm, a cathode current tion of pollution problems. density of the order of from 0.1 to 1 a?cn, a terminal 5 In order to indicate still more fully the nature and voltage of the order of from 2 to 3.2 V. In chlorine utility of this invention, the following specific examples oxidation, chlorine gas is blown into the manganate solution at a temperature ordinarily of the order of of practice thereof are set forth, it being understood from 60 to 130°C. that these examples are presented as illustrative only B. Regeneration depending on nitrate decomposition () and that they are not intended to limit the scope of the method invention. The regeneration process depending on this method EXAMPLE in the case of dry method, comprises merely heating of the absorbent. In the case of potassium nitrate, for With the use of a column packed with tellerette pack example, potassium manganate is effectively regener 15 ings manufactured by Nittetu Chemical Engineering ated by a roasting reaction according to the following Ltd., Japan, of an inner diameter of 250 mm and a Eq. (10). packed layer length of 4,000 mm, a 5 percent aqueous MnO -- 2KNO = KMnO, -- N - 20, ( () solution of potassium manganate at 60°C containing 5 In the case of wet treatment, a slurry containing the percent of caustic potash and 10 percent of potassium nitrate and the manganese oxide formed in the step of 20 nitrate, was introduced for irrigation as an absorption absorbing the gas containing NO is first dehydrated agent at a rate of 8 m/m hr, and air containing 1,060 and solidified, and then roasting regeneration is carried ppm of Nor (1,040 ppm of which was NO) was intro out similarly as described above. It is possible to leach duced at a gas velocity of 0.35 m/sec to carry out ab the product of the above-mentioned roasting with a sorption. As a result, the removal of NO in continuous dilute aqueous solution of an alkali, thereby to obtain a absorption was 92 percent. After further continuous manganate solution or, further, to carry out electrolytic operation, and at the stage where a noticeable quantity oxidation of this solution, thereby to obtain the per manganate. of a manganese oxide was formed, this was sampled Thus, by combining a regeneration process as de and analyzed, whereupon its composition was found to scribed above and a NO absorption process depending be as follows: on an absorbent comprising a (per)manganate as de scribed hereinbefore, there is established a process for Moisture content Principal composition absorbing and removing NO in a gaseous mixture in cake of anhydride containing the NO, according to the nitrate decompo HO T.MnO, KO sition method. In the regeneration by the nitrate by 35 60.7% 70.4 2.0% production method, the entire alkali content is replen ished as an alkali hydroxide, and a nitrate is produced as a by-product, while in the regeneration by the nitrate The mother liquor from which the manganese oxide decomposition method, an alkali hydroxide is not re was removed was concentrated, cooled, and the precip plenished, but the nitrate produced as a by-product is 40 itated potassium nitrate was separated. The potassium utilized to regenerate a (per)manganate. By either one nitrate thus produced was mixed with the above-men of these methods the regeneration can be accomplished tioned manganese oxide in a range of KNO/MnO, readily and NO removal can thus be effected at low mole ratio of from 2 to 3, and the resulting mixture was COSt. dried at 10°C. The aforementioned roasting reaction of the manga 45 This mixture was transferred to a steel rotary drum of nese oxide and nitrate (the cations constituting the salt external heating type of 150 mm diameter and 600mm are selected from the aforementioned group with re length, heated to 700°C, and caused to react. Succes spect to the cations constituting the (per)manganate) is sively, damp air, which had been decarbonated and carried out with suitable proportions of the two reac saturated with water at 60°C was introduced, to react tants, for example, with the stoichiometric ratio or with 50 and at the same time the drum was cooled to 350°C, an excess of one reactant, at a temperature of the order whereupon the following results were obtained; of from 300 to 800°C. and ordinarily in air. By carry ing out this roasting reaction in the presence of a hy KNO/MnO, Temp. Time Temp. Time Yield of oxy droxide or oxide of an alkali metal, it is possible to note ratio (C) (min.) (C) . (min.) acid salt of cause this reaction to resemble a reaction included in 55 manganese (%) the presently practiced production of alkali permanga 2 700 60 350 6) 25.4 late. 3 7) () 35 6 37.8 As described above, the manganese oxide formed in 3 700 S} 35) 60 45.1 the absorption process can be regenerated effectively and completed as (per)manganates. Therefore, by cir 60 culating and utilizing the (per)manganates regenerated The fired substance obtained from the above reaction at low cost from manganese oxide by any of these re was mixed into an absorption solution from which po generation processes, a great reduction in material cost tassium nitrate had been separated, and which had can be attained. been heated to 60°C, and the resulting mixture was In accordance with this invention as described above, 65 again, returned to the absorption system, where the by combining a process for absorbing NO in a gaseous absorption test was carried out under the same condi mixture containing the NO by means of (per)manga tions. As a result, the removal of NO in absorption was nates and a process for regeneration of the manganese found to be 92 percent. 3,927, 177 9 10 solution temperature of 0°C to 80°C, said aqueous EXAMPLE 2 absorbent solution consisting essentially of 0.1 to The absorption operation was carried out under the 20% by weight of a manganate or a permanganate same conditions as in Example l. The manganese oxide of sodium or potassium, whereby the nitrogen ox thus formed is separated out and mixed with potassium ides are absorbed into said aqueous absorbent solu permanganate with a KMnO/MnO, mole ratio of from tion and converted into a nitrate or nitrite and l to 4. The resulting mixture was added to a caustic wherein the manganate or permanganate is con potash solution of a concentration of from 30 to 55 verted into a manganese oxide, percent maintained at a temperature of 60°C, and reac 2. separating the manganese oxide from the absor tion was carried with agitation, whereupon the follow 10 bent solution, ing results were obtained: 3. subjecting the manganese oxide to oxidation along with at least a stoichiometric amount of a nitrate by KOH KMnO/MnO, Reaction time Yield of oxy-acid salt roasting in air at a temperature of 300 to 800°C to (%) (mole/ratio) (min.) of manganese (%) convert the manganese oxide into a manganate or 15 3() 3.9 180 97 permanganate, and 4() 3.0 8() 86 4. circulating the manganate or permanganate thus 40 () 8() 3. regenerated to step 1. 55 3.2 18() 92 2. A process for the removal of nitrogen oxides from a gaseous mixture containing such nitrogen oxides The above products of reaction were left standing to which comprises the steps of cause potassium (per)manganate containing some l. contacting a gaseous mixture containing nitrogen manganese oxide to precipitate, and this was mixed oxides with an aqueous absorbent solution at a with a mother liquor from which manganese oxide and, solution temperature of 0°C to 80°C, said aqueous further, potassium nitrate had been separated. Then absorbent solution consisting essentially of 0.1 to the absorption experiment was carried out again, 20% by weight of a manganate or a permanganate whereupon it was found that the removal of No was 90 of sodium or potassium, whereby the nitrogen ox percent. ides are absorbed into said aqueous absorbent solu tion and converted into a nitrate or a nitrite and EXAMPLE 3 wherein the manganate or permanganate is con 30 With the use of a solution containing 2.0 percent of verted into a manganese oxide; potassium permanganate, 2.5 percent of potassium 2. separating the manganese oxide from the absor bent solution; manganate, and 6 percent of caustic potash, a gas ab 3. subjecting the manganese oxide to oxidation sorption operation was carried out under the same wherein the manganese oxide is oxidized with an conditions as in Example l, whereupon a NO absorp 35 alkaline aqueous permanganate solution to pro tion rate of 91 percent was attained. duce a manganate, and The manganese oxide formed in this absorption oper 4. circulating the manganate thus regenerated to step ation was separated and rendered into the permanga 1. nate under the same conditions as in Example 2, and a 3. A process for the removal of nitrogen oxides from solution of the manganese oxy-acid salt was prepared 40 a gaseous mixture containing such nitrogen oxides as follows. which comprises the steps of 1. contacting a gaseous mixture containing nitrogen KMnO, KOH oxides with an aqueous absorbent solution at a 5% 5% solution temperature of 0 to 80°C, said aqueous 45 absorbent solution consisting essentially of 0.1 to 20% by weight of a manganate or a permanganate This solution was transferred to an electrolytic bath of sodium or potassium, whereby the nitrogen ox of a diameter of 800 mm and a depth of 400 mm and ides are absorbed into said aqueous absorbent solu adjusted in temperature to 60°C. Then, electrolysis was tion and converted into a nitrate or a nitrite and carried out for 12 hours with a nickel plate used for the 50 wherein the manganate or permanganate is con anode and an iron bar used for the cathode and with an verted into a manganese oxide; anode current density of 0.008 A/cm, a cathode cur 2, separating the manganese oxide from the absor rent density of 0.4 A/cm, a terminal voltage of 3 V, bent solution; and a current of 100 A. As a result, an electrolytic 3. subjecting the manganese oxide to oxidation oxidation rate of 50 percent was attained. 55 wherein the manganese oxide is oxidized with an With the use of this solution of an alkali (per)manga alkaline aqueous permanganate solution to pro nate containing potassium permanganate, the absorp duce a manganate and the manganate thus pro tion experiment was carried out again, whereupon a duced is then subjected to electrolytic oxidation to NO removal of 91 percent was attained. produce a permanganate; and What we claim is: 60 4. circulating the permanganate thus regenerated to 1. A process for the removal of nitrogen oxides from step 1. a gaseous mixture containing such nitrogen oxides 4. A process for removal of nitrogen oxides as which comprises the steps of claimed in claim 1 in which said nitrate used in step 3 l. contacting a gaseous mixture containing nitrogen is a product of said absorption of step 1. oxide with an aqueous absorbent solution at a 65 ck ki k :k k UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,927, l77 Dated December 16, l975 Inventor(s) Taijiro Okabe ; Akitsugu Okuwaki and Shigetoshi Nakabayashi --- It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below: First page, in the line designated "73" relating to the assignee's name, change "Mitsubishi Kinzoku Kogyo Kabushiki Kaisha" to -- Mitsubishi Kinzoku Kabishiki Kaisha -- . eigned and evealed this thirtieth D 2 y of March 1976 SEAL Attest.

RUTH C. MASON C. MARSHALL D ANN At testing Officer Commissioner of Patents and Trademarks