CA1129628A

CA1129628A - Removal and recovery of nitrogen oxides and sulfur dioxide from gaseous mixtures containing them

Info

Publication number: CA1129628A
Application number: CA284,506A
Authority: CA
Inventors: Hal B.H. Cooper
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-08-16
Filing date: 1977-08-11
Publication date: 1982-08-17
Also published as: AU516135B2; GB1593745A; DE2736768A1; JPS5328558A; GB1593743A; ES461638A1; GB1593742A; AU2794477A; AU6726381A; GB1593744A; ES472199A1

Abstract

ABSTRACT OF THE DISCLOSURE

A cyclic process for removing lower valence nitrogen oxides from gaseous mixtures includes treating the mixtures with an aqueous media including alkali metal carbonate and alkali metal bicarbonate and an oxidant to form higher valence nitrogen oxides and to capture these oxides as alkali metal salts, especially nitrites and nitrates, in a carbonate/
bicarbonate-containing product aqueous media. Highly selective recovery of nitrates in high purity and yield may then follow, as by crystallization, with the carbonate and bicarbonate alkali metal salts strongly increasing the selectivity and yield of nitrates. The product nitrites axe converted to nitrates by oxidation after lowering the product aqueous media pH to below about 9.
A cyclic process for removing sulfur dioxide from gas mixtures includes treating these mixtures with aqueous media including alkali metal carbonate and alkali metal bicarbonate where the ratio of alkali metal to sulfur dioxide is not less than about 2. The sulfur values may be recovered from the resulting carbonate/bicarbonate/sulfite-containing product aqueous media as alkali metal sulfate or sulfite salts which are removed by crystallization from the carbonate-containing product aqueous media. As with the nitrates, the carbonate/
bicarbonate system strongly increases yield of sulfate or sulfite during crystallization.
Where the gas mixtures include both sulfur dioxide and lower valence nitrogen oxides, the processes for removing and recovering lower valence nitrogen oxides and sulfur dioxide may be combined into a single removal/recovery system, or may be advantageously effected in sequence.

Description

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Where a gas mixture includes both sulfur dioxide and lower valence nitrogen oxides, and removal of both is desirable, these new processes for removing and recovering sulfur dioxide and lower valence nitrogen oxides may be combined into a single step, or may be effected sequentially, with the latter offering certain processing advantages over the combined process.
The conversion of lower valence nitrogen oxies to higher valence nitrogen oxide~ to facilitate their removal from a gas mixture is, broadly, not an entirely new concept.
Thus, for example, U.S. Patents 1,420,477; 3,733,3~3; and

3,~27,177 propose oxidizing oxides of nitrogen to remove them more easily from gaseous stream. U.S. Patent 3,873,672 employs a related method for removing sulfur dioxide from gas mixtures containing it. All of these primarily seek simply removal of the pollutants and are costly to operate. None discloses removing pollutant nitrogen oxides or sulur dioxide or both from combustion gases and recovering them as useful products in a practical and economic manner. Combustion gases from the burning of carbonaceous fuels also include substantial quantities of carbon dioxide. These quantities may be in the range of about 5% to ahout 20~ by volume. Although emission of carbon dioxide to the atmosphere is not presently under severe attack as an environmental probl~m, capture and re-covery of carbon dioxide may be highly desirable to provide raw materials for other commercially valuable products. The recovery techniques that form important parts of the processes of this invention permit recovery of large volumes of carbon dioxide in high purity at low cost, thus contributing to the overall efficiency and economy of these processes.
This invention provides processes for treating gas ;

~296~ ~

mixtures including carbon dioxide and at least one lower valence nitrogen oxide for the removal of these lower valence nitrogen oxides comprising treating a gas mixture including these gases with aqueous media including alkali metal carbo-nate, alkali metal bicarbonate and an oxidant to convert the lower valence nitrogen oxides to higher valence nitrogen oxides in which alkali metal carbonate and alkali metal bi-carbonate are present in stoichiometric excess of the amount required to form alkali metal salts of the higher valence nitrogen oxicles, then recovering a product aqueous media from the treating that includes these alkali metal salts. The recovered product aqeuous media is itself a valuable product that may, for example, be used as a liquid fertilizer. Pre-ferably, however, the process also comprises recovering the alkali metal salts from the product aqueous media, and re-cycling the product aqueous media to the treating process.
The gas treating is preferably effected at temperatures in the range of about 50C. to about 60C. where the gas mixture is combustion fuel gas. The oxidant effects conversion from lower valence to higher valence nitrogen. The carbonate/
bicarbonate system immediately forms nitrite and nitrate salts of the higher ~alence nitrogen oxides thus removing them from the gas mixture and facilitating their recovery.
The most common oxide of ni~rogen in combustion gases is the lower valence nitric oxide (NO), which generally exceeds the concentration of nitrogen dioxide by a factor o F
about 10. In these combustion gas mixtures, lower valence oxides of nitrogen may be present in concentrations in the range of about 200 to about 20,000 parts per million. But since these mixtures are evolved at such rapid rates, for example, from about 1,000 cubic feet per minute to about 1,000,000 cubic feet per minute from a 500 megawatt power plant, the quantity emitted to the atmosphere is very large and creates a serious air pollution problem. The processes of this invention are especially applicable in minimizing these emissions, or at least lowering them to, say, about 50 parts per million, without seriously impeding the high flow rate of the gas stream from the power plant to the atmosphere.
These processes employ an acceptor system that ca~tures and holds the higher valence nitrogen oxides formed in this process in solution in non-volatile form, and permits the concentration of these salts ~o rise to a level where recovery is commercially practicable. The acceptor system, including alkali metal carbonate and alkali metal bicarbonate, permits recycling of the product aqueous media while retaini.ng the higher valence nitrogen oxides in solution as nitrite and nitrate salts of alkali metal. And the carbonate/bicarbonate acceptor system, in addition, promotes highly selective re-covery of alkali metal nitrates and nitrites in commercial quantities and crystallization at higher temperatures than would be expected, thus increasing the yield and minimizing the energy needed to evaporate and cool the product aqueous media to effect crystallization of the nitrate and nitrite ` products.
Combustion gas mixtures amenable to the new processes also include carbon dioxide and may include some carbon mono-xide. The carbon dioxide concentration in combustion and similar gases is normally in the range of about 5~ to about 20%, more commonly about 10% to about 16~ by volume. During the treating of gas mixtures in accordance with this in-vention, the carbon dioxide in the gas mixtures is drawn intosolution and forms alkali metal carbonates and bicarbonates -2~31 by reaction with an alkali metal salt such as the hydroxide.
The presence of these alkali metal carbonates and bicarbonates in the aqueous media and their inter-conversion plays an important part in a number of different ways in the practice of this invention, completely aside from their role in removing oxides of nitrogen and sulfur from the gas stream.
For example, the product aqueous media may be treated to recover some or all of this carbon dioxide absorb~d. More-over, the presence of the carbonates in the product aqueous media facilitates the crystallization and recovery of such alkali metal salts as potassium nitrate and potassium nitrite by lowering their solubilities to a major degree. In the produc~ aqueous media, conversion of carbonate to bicarbonate, as by the addition o carbon dioxidle, lowers the pH of the product aqueous media. In turn, that facilitates and expedites the oxidation o~ nitrite to nitrate by the oxidant present in that media. Conversely, conversion of bicarbonate to carbonate by heating permits not only the recovery of relatively pure carbo~ dioxide in large quantitie~, but also facilitates the recovery of the product potassium nitrate by crystallization in higher purity and yield.
The alkali metal carbonate and bicarbonate used in the aqueous treating media may include any of the alkali metals, but potassium i5 pre~erred. Potassium carbonate t in particular, is very soluble in aqueous media, and has the surprising effect of reducing the solubility and raising sub-stantially the temperature at which the nitrate and the nitrite may be crystallized. This means less energy is needed for concentration and refrigeration to cool the product aqueous media to crystallize the nitrate, and a higher potassium nitrate selectivity and yield during crystallization.

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Potassium nitrate is a valuable commercial product and is especially valuable as a fertilizer, both for its fixed nitrogen and its potassium.
The alkali metal is preferably introduced to the treating step of the processes of this invention as alkali metal hydroxide, although it can be supplied as alkali metal carbonate. Preferably, the alkali metal hydroxide is electro-lytically derived from alkali metal halide. Thus, for example, electrolysis of potassium chloride produces potassium hydroxide in aqueous media, as well as hydrogen and chlorine gases. The hydrogen gas may be used to make an oxidant such as hydrogen peroxide, and ~he chlorine gas to make an oxidant such as hypochlorite. Alternatively, chlorine it~elf may be used as the oxidant.
The alkali metal carbonate and bicarbonate used in the treating step of the processes of this invention forms during the treating process by reaction with carbon dioxide from the gas mixture with the alkali metal hydroxide fed ` thereto.
Oxidation of lower valence nitrogen oxides with preferred oxidants produces a product aqueous media including nitrites, nitrates, carbonate and bicarbonates of alkali metal. Because nitrates are more aasily removed from such media than are nitrites, the nitrites are desirably oxidized to nitrates. At the carbon dioxide content normally prevail-ing in combustion gases, e.g., around 14%, both carbonate and bicarbonate are present in the product aqueous media and the pH is likely to be 9 or greater. Surprisingly, lowering the pH to less than about 9 facilitates oxidation of nitrites to nitrates in the product aqueous media. This lowering of the pH may be effected by adding sufficien~ acid to nPutralize the carbonate and convert it to bicarbonate, or, preferably, by adding carbon dioxide to the media.
In these processes, this lowering of pH and con-version of nitrite to nitrate is best effected as the first step in the product recovery so as to utilize any unused oxidant in the product aqueous media remaining from the gas treating and to raise the nitrate concentration prior to its recovery by crystallization. However, this conversion step may be postponed in the recycling of the product aqueous ld media and may even ollow the recovery of nitrate, pro~ided sufficient oxidant is present or is added to the recycling media to efect the oxidation.
Another important step in recovering products from the product aqueous media is decarbonation, where the bi-carbonate formed in the treating step of the processes is converted to carbonate and carbon clioxide by heating the pxoduct a~ueous media and driving off the carbon dioxide.
This heating may be effected by steam stripping or by evaporation. Generally, ~ome evaporation of water is re-quired to facilitate the crystallization of the product.
The carbon dioxide so made is of high purity and may be captured for use in other processes, used for lowering the pH of the product aqueous media to facilitate oxidation of nitrite to nitrate, or both. Although decarbonation pre-ferably follows conversion of nitxite to nitrate, decarbona-tion may follow immediately after the gas treating step of the process. Following the conversion of the bicarbonate to carbonate, the product aqueous media is cooled to recover alkali metal nitrates, preferably by crystallization.
Since the carbonate/bicarbonate containiny product aqueous media employed in the gas sontacting holds alkali .,~, , ' .

3L~l2~316~

metal nitratas and nitrites s~rongly in solution, the product aqueous media may be recycled through gas treating st~ps many times to raise the alkali metal nitrate concentration suffi-ciently high to make product recovery practicable and economic.
Thus, when ~he nitra~e concentration rises into the range of about 2~ to about 45% based on the weight of the product aqueous media, recovery may then be effected efficiently.
Surprisingly, the presence of potassium carbonate in the product aqueous media has a pronounced effect in reducing the solubility of the potassium nitrate and ~hus makes its recovery much easier, and more economical. Not only is the temperature at which crystallization takes place significantly higher than where the potassium carbonate is not present but the yield per pass is also improved.
The oxidants which may be used in these processes are among those known to oxidize lower valence nitrogen oxides, including chlorine, chlorine dioxide, hypochlorite, ozone, and hydrogen peroxide. Of t:hese, hydrogen peroxide is especially preferred bPcause product recovery from the product aqueous media is cleanest and simplest and the hydrogen peroxide may be made at low cost. ~ut halogen-containing oxidants such as chlorine or hypochlorite are more active and oxidize lower valence rlitrogen oxides faster than hydrogen peroxide, and may be preerable where this benefit outweighs the added difficulties that chlorides present in separating and recovering products from the product aqueous media.
Initial oxidation in the vapor phase followed by liquid phase oxidation proceeds faster than oxidation in the liquid phase only. Thus, oxidation may be initiated, and wholly or partly completed, before treatment with oxidant '~ /~. .

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aqueous media begins. Oxidation in the vapor phase is favored by having the oxidant in gaseous form where it can be com-mingled with the gas mixture and directly contact the nitric oxide or nitrogen dioxide. Of the oxidants mentioned above, ozone, chlorin~ and chlorine dioxide are gases at ambient temperatures. Ozone and chlorine dioxide, though effective, are high in cost. Hydrogen peroxide, though a relatively high boiling material, can be flash vaporized into the gas stream to also serve as a vapor phase oxidank. PrPferably, this vapor phase oxidation is effected only for the purpose of oxidizing nitric oxide (NO) to nitrogen dioxide (NO2), with the remaining oxidation being carried out in the aqueous treating media. Where the absorption rate from the gas stream is low, the gas-liquid contacting equipment becomes larger and more contacting stages become necessary to achieve high removal efficiencies. Because of the huge volumes of off-gases that must be treated at power plants and the like, the capital investment can become sizeable. For this reason, addition of initial gas phase oxidation to the process may be desirable.
Where hydrogen peroxide is the oxidant, the peroxide is preferably produced at the site of the treating process by one of the known processes for its production. Incor-porating the peroxide supply process directly into the treating process effects considerable economy because most peroxide producing processes require expensive puri~ication steps, such as extraction and distillation, and movement of large quantities of water. For example, where hydrogen peroxide is made by the air oxidation of 2-alkylanthraquinone, hydrogen peroxide is produced in an anthraquinone-containing organic solvent phase, which is then extracted with water.

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The water layer ~ontaining hydrogen peroxide is then removed and concentrated by distillation. The large amounts of water that must be removed and the considerable decomposition and loss of peroxide that takes place during this recovery is costly. These steps may be eliminated by extracting the hydrogen peroxide directly into the recycling aqueous media used in this process, thus avoiding the investment in recovery equipment and the costly recovery concentration steps. Moreover, the hydrogen gas produced in the electro-lytic formation of potassium hydroxide or other alkali met~l hydroxide may be used to advantage in the anthraquinone re-duction step of this process, thereby lowering substantially the cost of producing this oxidant.
Where chlorine or hypochlorite is the oxidant, chlorine produced in the electroly~;ic formation of alkali metal hydroxide from an alkali metal chloride may be utilized either directly, where chLorine is the oxidant, or indirectly, where hypochlorite .is the oxidant. Thus, chlorine may be converted to hypochlorite by the reaction `~ 20 of alkali metal hydroxide with chlorine in water, and the resulting aqueous hypochlorite may then be fed to the treat-ing process of the invention. The reaction of chlorine, hypochlorite or chlorine dioxide with gas mixtures including lower valence nitrogen oxides and carkon dioxide resul~s in rapid oxidation of nitric oxide to nitrogen dioxide, which then forms nitrite and nitrate in the alkali metal carbonate/
bicarbonate aqueous treating media. Unreacted chlorine or nitrogen dioxide carried over from the primary gas scrubbing may be removed in a cleanup secondary treatment of the gas mixture by scrubbing with an aqueous carbonate/bicarbonate solution. This precludes emission of chlorine to the ~2~336;~
atmosphere with the gas mixtures.
The use of chlorine, chlorine dioxide or hypo-chlorite as the oxidant makes product separation and recovery more difficult than where hydrogen peroxide is the oxidant.
However, alkali metal chlorides and alkali metal nitrates in the product aqueous media may be separated from one another in this process by a combination of steps such as decarbonation and crystallization of potassium chloride followed by isolation and lixiviation of the remaining alkali metal chloride/alkali metal nitrate product. Some o~ the alkali metal chloride so recovered may be recycled to electrolysis for formation of additional oxidant and alkali metal hydroxide. Alternatively, decarbonation and evaporation of the product aqueous media may be followed simply by lixiviation of the solids resulting from the decarbonation/evaporation step.
The processes of this invention also permit the recovery of sulfur dioxide from gas mixtures regardless of wh~ther nitrogen oxides are present or not and regardless of whether removal of those nitrogen oxides is necessary or desira~le. Sulfur dioxide removal may be effected in a number of ways. First, SO2 may be removed from gas mixtures by treating them with an aqueous media including alkali metal carbonate and bicarbonate and oxidant. The ratio of alkali metal to sulfur dioxide (potassium: sulfur dioxide) muæt be at least about 2. Where lower valence nitrogen oxides are present, the process may be conducted so as to oxidize sulfur dioxide to sulfate with the corresponding alkali metal salts forming in the product aqueous media together with alkali metal nitrites and nitrates. Thereafter, product separation and recovery may proceed as described ~ 962~3 ahove. Alternatively, the gas mixtures may be treated countercurrently with a controlled amount of oxidant and excess alkali metal carbonate/bicarbonate in the aqueous media so that the sulfur dioxide forms alkali metal sulfite salt in the first stage of the gas-liquid contacting. The oxidant oxidizes only the lower valence nitrogen oxides passing to the later stages, and thus does not oxidize the sulfite to sulfate. Again, the ratio of alkali metal to sulfur dioxide must be at least about 2. Here, however, the sulfur values may be recovered as an alkali metal sulfite salt such as sodium sulfite, or the sulfite may be oxidized, preferably with oxygen to make alkali metal sulfate. As before, the higher valence nitrogen oxides are trapped in ; the solution as alkali metal nitrites and nitrates.
Rather than removing sulfur dioxide concomitantly with lower valence nitrogen oxides, however, the processes of this invention preferably remove sulfur dioxide separately, and before the lower valence nitrogen oxides are removed.
To do this, the gas mixture, including carbon dioxide and sulfur dioxide, with or without lower valence nitrogen oxides, is treated with aqueous media comprising the carbonate and bicarbonate of an alkali metal where the ratio of the alkali metal to sulfur dioxide is at least about 2. A
product aqueous media is recovered that includes alkali metal carbonate, alkali metal bicarbonate and alkali metal sulfite formed from the sulfur dioxide. Sulfur dioxide removal efficiency of this process is very high, about 98 or 99~, and the process is effective whether or not nitrogen oxides are present and whether or not they are necessarily or desirably removed.

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By contrast, the widely adopted Wellman-Lord and lime-limestone processes for removal of sulfur dioxide from combustion gases are only able to remove about 90~ of the sulfur dioxide present from ~hese mixtures. As with the processes of this invention for removing lower valence nitrogen oxides from gas mixtures, this process or the removal of sulfur dioxide from gas mixtures is particularly useful where the gas stream contains low concentrations of sulfur dioxide and where the gas stream is in large volume and must flow at a high velocity, such as where the gas mixture is a combustion gas from a power plant or other high fuel-consuming plant.
Although this new sulfur dioxide removal process is superior to the Wellman-Lord process, the Wellman-Lord process concept may alternatively be combined with the processes of this invention for removing lower valence nitrogen oxides from gas mixtures including both sulfur dioxide and lower valence nitrogen oxides where there is a direct need for sulfur dioxide as a product in preference to alkali metal sulfate or sulfite. Moreover, ~he Wellman-Lord proce~s and others capturing sulfur dioxide as bisulfite may be combined with the sulfur recovery methods of this invention. Thus, where the product aqueou~ media from gas scrubbing includes such bisulfite, the alkali metal carbonate may be added to the media in an amount sufficient to convert the bisulfite to sulfite. Air oxidation of sulfite to sulfate, and recovery of sulfate by crystallization may then follow.
As in the process for removing and capturing lower valence nitrogen oxides from gas mixtures, the preferred alkali metal is potassium and the preferred source for that ~ ~ f.3~ r~ ~

potassium is electrolytically derived potassium hydroxide made, for example, from the electrolysis of potassium chloride. However, potassium carbonate is also acceptable as a source of potassium.
The sulfur dioxide removal process of this invention not only achieves very high removals of sulfur dioxide, but also can recover large quantities of carbon dioxide at low cost. The employment of an excess of alkali metal beyond that needed for reaction with the sulfux dioxide to produce ; lO alkali metal sulfite not only ensures a high sulfur dioxide removal efficiency, but, depending upon the extent of the excess and the degree of recycling, may also absorb large amounts of carbon dioxide from the gas mixture which can then be recovered in the form of highly pure carbon dioxide upon decarbonation of the alkali metal bicarbonate. The concentration of alkali metal sulfite may be xaised to a high level by recycling in a manner similar to that employed with the nitrogen oxides removal. The sulfur values in the product aqueous media are preferably recovered as alkali matal sulfate, although alkali metal sulfite may be crystallized and recovered as such and may be preferable where the alkali metal is sodium. The sulfate is preferably formed by oxidation with a low-cost oxidant such as oxygen, or an oxygen-containing gas such as air. The by~products of such oxidation are gaseous and consist of evaporated water and air. Much water can be evaporated in this step, as the oxidation is exothermic and the heat of reaction can be used directly. Decarbonation and oxidation may also be effected simultaneously if recovery of carbon dioxide is unnecessary or undesirable. The alkali metal sulfate is recovered from the aqueous media by crystallization. Crystallization of 2~6Z~3 potassium sulfate from the potassium carbonate-containing product aqueous media is particularly efficient. Surprisingly, the solubility of potassium sulfate in potassium carbonate solutions i5 reduced to very low levels, even where the solution is hot. This is one of the important features of the process. For example, the solubility of potassium sulfate in water at 70C. is 19.5 grams/100 milliliters of water, but drops to 0.11 grams/100 milliliters of aqueous solution containing 67 grams of potassium carbonate, a remarkable reduction in solubility.
Alkali metal values removed from the product aqueous media with the sulfate or sulfite salts may be replenished with alkali metal hydroxide or carbonate and fed directly to the recycling product aqueous media. Where the product aqueous media includes sulfite, nitrite and nitrate salts in solution, these may be separated from one another, as by ~irst converting the nitrites to nitrates, then oxidizing the sulfite to sulfate, and finally by separating the sulfate and nitrate by selective crystallization. The presence of alkali metal carbonates in the product aqueous media reduces the solubility of both sulfates and nitrates and aids in the selectivity of the processj thus permitting first sulfates, at around 70C., and then nitrates to crystallize as the product aqueous media is cooled. The solubility of potassium nitrate in aqueous media containing potassium carbonate is reduced to about one tenth of that in water alone. Thus, at 40 C., only 6 grams of potassium nitrate dissolve in 100 milliliters of aqueous solution containing 67 grams potassium carbonate whereas 64 grams of potassium nitrate will dissolve in 100 milliliters of water alone at 40C.

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Where the new sulfur dioxide removal process of this invention is combined with one of the new processes for removing lower valence nitrogen oxides from gas mixtures, even greater advantages accrue from the integration of these two processes into a sequential overall process for removing and preferably recovering both gases from gas mixtures such as combustion gas mixtures. Thus, where the oxidant for oxidation of lower valence nitrogen oxides to higher valence nitxogen oxides is hydrogen peroxide, the electrolytic formation of alkali metal hydroxide also produces hydrogen which may be used to make hydrogen peroxide at the site of the overall process, sharply decreasing the cost of making hydrogen peroxide. Where chlorine or hypochlorite is the oxidant, the same electrolysis process produces chlorine gas, which may be used as the oxidant itself or converted to hypochlorite. Alkali metal c~loride recovered from the product aqueous media may be recycled to electrolysis again thereby decreasing the cost of the overall process.
Where the gas mixture treated includes not only sufficient sulfur dioxide to require its removal, but includes some dinitrogèn trioxide, nitrogen dioxide, or ~oth, the processes of this invention for removing sulfur dioxide also tend to trap these oxides as alkali metal nitrites and nitrates along with the sulfur dioxide as sulfite.
The resulting product aqueous media may then be subjected to the product recovery steps described above to separate the sulfur values from the nitrates and nitrites~ Where removal and recovery of lower valence nitrogen oxides from the same gas stream follows the recovery of sulfur dioxide, the product aqueous media in the sulfur dioxide removal cycle may be bled to the nitrate recovery step of the lower - 16 ~

. , .

valence nitrogen oxide removal process after the sulfur values in the sulfur dioxide removal product aqueous media cycle are removed.
The processes of this invention provide a substantial breakthrough in making the elimination of gaseous pollutants from such gas mixtures as combustion gases both practicable and economic. No practicable process is now known for removing and recovering lower valence nitrogen oxides from such gas mixtures, and none ~ermits the recovery of a saleable product to offset the high operating cost and capital cost of the plant and equipment needed to effect the removal.
The presence of sulur dioxide in combustion gases only compounds the problems that industry faces today and no process exists for achieving the near 100~ removal and recovery of sulfur dioxide from gas mixtures whether or not lower valence nitrogen oxides are present. The processes o~ this invention meet all these needs. Lower valence nitrogen oxides and particularly nitric oxide, which are typically present in quite low concentration in combus ion gas mixtures, cannot be removed by alkaline scrubbing aione.
Nor does oxidation of nitric oxide alone solve the problem.
Combining the use of an alkali metal carbonate/bicarbonate scrubbing media which forms an alkaline acceptor, with an oxidant makes possible the trapping of nitrogen dioxide and other higher valence nitrogen oxides as they form. The resulting product aqueous media is particularly amenable to treatment for removal and recovery of commercially valuable products in commercial quantities. The alkali metal nitrites and nitrates formed in the product aqueous media are strongly held by this acceptor system and may be recycled time and time again through the treating process until the concentration of each rises to a level where its recovery is commercially practicable. Carbonates in the product aqueous me~ia strongly reduce the solubility of the nitrates, and permit them to be crystallized as the temperature of the media is lowered. Nitrites in the produ~t aqueous media are readily converted to nitrates by lowering the pH of the product aqueous media to less than about 9, which expedites the oxidation of nitrite to nitrate. q~he treating step of the process of this invention may be summarized by the ollowing reactions, where the oxidant is hydrogen peroxide and the alkali metal is potassium:

2 KOH + C02 --~ K2C03 + H2 K2C3 + C2 + 2 2 KHCo3 NO Removal and Conversion NO + 1/2 H22 + K2C03 ~ KN2 + KHC03 2 2 2 3 > KN02 + RN03 ~ 2 XHC03 KN02 + H202 ~~ KN03 H2o S2 Removal and Conversion S2 + 2 KHC03 - >K2S03 +2 C02 ~ H20 K2S03 + 1/2 2 ~ 2 4 The processes of this invention are illustrated in Figures 1, 2, 3 and 4. Figure 1 illustrates an embodiment where the gas mixture includes carbon dioxide and lower 6;~

valence nitrogen oxides with or without sulfur dioxide, and gas treatment is made to remove all three simultaneously.
The oxidant is hydrogen peroxide, the alkali metal is potassium, and the oxides of lower valence nitrogen and sulfur, if present, are remo~ed together.
Figure 2 illustrates the embodiment where the gas mixture includes carbon dioxide, sulfur dioxide and lower valence nitrogen oxides, the alkali metal is potassium, the oxidant is hydrogen peroxide, the sulfur dioxide is converted to sulfite by reaction with carbonate and bicarbonate, and the oxides of lower valence nitrogen are thereafter oxidized to higher va:Lence nitrogen oxides with hydrogen peroxide in a single scrubber.
Figure 3 illustrates the embodiment where the gas mixture includes sulfur dioxide, carbon dioxide, and lower valence oxides of nitrogen, the alkali metal is potassium, the oxidant is hydrogen peroxide~ and the sulfur dioxide is ~ormed and removed in ~he first stage as potassium sulfi~e in product aqueous media. The lower valence nitrogen oxidès in the sulfur dioxide-free gas mixture are removed and re-covered in a second stage using the hydxogen peroxide oxidant and aqueous media CGntaining potassium carbonate/bicarbonate.
Figure 4 discloses an embodiment where the gas mixture includes carbon dioxide and lower valence nitrogen oxides, the alkali metal is potassium, the oxidant is chlorine, chlorine dioxide or alkali metal hypochlorite, and the product aqueous media is processed to separate the potassium chloride from the potassium nitrate product.
Referring now to Figure 1, a combustion gas mixture 3~ including carbon dioxide, sulfur dioxide and lower valence nitrogen oxides enters scrubber 2 where the gas is intimately ~.

contacted, either cocurrently or countercurrently, with aqueous media entering scrubber 2 via line 4. Though the drawing shows only one scrubber, there may be several, and each may have multiple stages. The gas mixture exits scrubber 2 via line 5, containing only small amounts of lower valence nitrogen oxides and virtually no sulfur dioxide.
The aqueous media entering scrubber 2 via line 4 includes potassium nitrite and nitrate, carbonate and bicar-bonate, and an oxidant~ hydrogen peroxide. Recycled aqueous media may also enter scrubber 2 via lines 7 and 4. With sulfur dioxicle in the gas mixture entering scrubber 2, the recycled aqueous media also includes some alkali metal sulfate.
The following reactions take place within scrubbar 2:
eactions of C02 2 KOH + C02 ~ K2C3 + H2o K2C3 + C2 H20 ~ 2 KHC03 N0 Con ersion N0 + 1/2 ~22 ~K2C03 ~ KN02 ~ KHC03 22 + 2 K2C3 ~ KN03 + KN0 +

2KHC03 ~ H20 KN02 + H202 ~ KN03 + ~2 S2 Conversion S2 + H202 + 2 K2C03----~K2S~ + 2 KHC03 ~3 Z9~2~3 If the gas mixture includes a large amount of carbon dioxide, the potassium carbonate formed in scrubber 2 changes to bicarbonate in large part. This lowers the pH in scrubber 2 into the range of about 7 to about 10. Where ~he pH is within this range varies with the temperature, carbon dioxide partial pressure in the gas mixture, and the quantity of potassium carbonate and bicarbonate present. Hydrogen per-oxide oxidiæes the oxides of lower valence nitrogen to higher valence nitrogen oxides which then react with the potassium bicarbonate and carbonate in the aqueous media to form potassium nitrate and nitrite. Any sulfur dioxide present oxidizes to potassium sulfate. The scrubbing system operates most economically when all of the oxidant is consumed in the scrubbing step, and that is readily accomplished with counter-current flow of scrubbing solution to the gas mixture.
The product aqueous media from the treating opera-tion, including potassium nitrate, potassium nitrite, potassium sulfate, potassium carbonate and potassium bi-carbonate, leaves scrubber 2 through line 5 and passes to separator 6. From there, a number of process variations may be employed for separating and recovering the products in that media.
Potassium sulfate has a low solubility in water compared to the other products in the product aqueous media, particularly with potassium carbonate present. By contrast~
potassium carbonate and potassium nitrite are highl~ soluble even when cold, compared to the others. By converting most of the bicarbonate to carbonate, the sulfate and nitrate potassium salts may be crystallized from the solution with 3~ no bicarbonate contamination. The carbonate and nitrite remain in solution. The major portion of the product aqueous Z~6Z~3 media is recycled through line 7 to increase the concentration of potassium nitrate in the product aqueous media to a sufficiently high level to make product recovery practicable and economic without excessive evaporation.
Because of its low water solubility and the high sulfur content of many fossil fuels, potassium sulfate tends to crystallize from solution and is removed from separator 6 via line 9 together with the product aqueous media from which potassium nitrate and sulfate products are to be recoveredO Product aqueous media is removed via line 7 from separator 6 and may be recycled to scrubber 2 through line 4.
The product aqueous media passes via line 9 to decarbonator-evaporator 10 where a portion of the bicarbonate i9 normally converted to carbonate with removal and recovery, if desired, of carbon dioxide. Co~version of bicarbonate to carbonate is desirable because potassium bicarhonate tends to crystallize with and contaminate the potassium nitrate, but potassium carbonate will not. The pH of the product aqueous media which includes potassium nitrate, potassium nitrite, potassium bicarbonate and potassium carbonate, together with residual potassium sulfate, may be further adjusted by adding potassium hydroxide through line 11 to prevent crys~allization of potassium bicarbonate.
Distillate water and carbon dioxide leave evaporator-decar-bonator 10 by line 12. The decarbonation and carbon dioxide formation are effected by thermally decomposing bicarbonate by evaporation or steam stripping as follows:

2 KHCO3 > K2CO3 + CO~ + H2O

i,:

1~962~ `

The carbon dioxide removed via line 12 may be recovered as a product or partially used to convert potassium carbonate to bicarbonate to facilitate the oxida~ion of potassium nitrite to nitrate.
Concentrated solution from evaporator-decarbonator 10 passes to crystallizer-separator 13 via line 1~ for recovery of crystalline potassium sulfate via line 15.
Product aqueous media exiting the potassium sulfate crystallizer-separator 13 in line 16 is then deep cooled in crystallizer-separator 17 to recover potassium nitrate which is removed via line 18. Potassium nitrite, which is highly water soluble, remains in solution in the potassium carbonate-containing aqueous media, and recycles in lines 19, 8 and 4 and is ultimately oxidized to potassium nitrate and recovered as such. Impurities accumulated in the product aqueous media may be purged through line 28. This purged material may be used as a fertilizer or fertilizer supplement.
The potassium carbonate/potassium bicarbonate system performs numerous important functions in the practice of this invention. For example, in the trea~ing step, this sys~em insures substantial removal of sulfur dioxide and lower valence nitrogen oxides from the gas mixture~ In product recovery, the system lowers the water solubility of potassium sulfate and potassium nitrate, making recovery of each more selective and higher in yield, thus reducing energy needs for evaporation and refrigeration.
The following data show the unexpectedly strong impact of potassium carbonate in reducing the solubilities of potassium sulfate and potassium nitrate in aqueous media.
Potassium sulfate has a solubility at 70C. of 19,5 grams in 100 milliliters of water, but only 0.11 gram in 100 milliliters . ~

1.~2~6'~

of aqueous solution containing 68 grams of potassium carbonate.
Potassium nitrate has a solubility at 20C. of 33 grams in 100 milliliters of water, but only 2.8 grams in 100 milli-liters of water containing 68 grams of potassium carbonate at 20C. Thus, potassium sulfate can be crystallized with substantially complete removal from aqueous potassium carbonate at temperatures above about 70C., following which the potassium nitrate may be crystallized with increasing recoveries at lower temperatures, ~ay from about lO~C. to about 50C. Separation and recovery of substan~ially pure sulfate and nitrate products is an important and unexpected result of this process, and makes possible the recovery of commercial quantities of these proclucts from gas mixtures containing low concentrations of sulfur dioxide and lower valence nitrogen oxides for the first time.
The product aqueous media in line 19, which contains principally potassium carbonate and lesser amounts of potassium nitrite and nitrate, is recharged with hydrogen peroxiae oxidant. This may be fed via line 21 from an out-side source, or may be produced by autooxidation of certainorganic compounds in on-site hydrogen peroxide plant 22.
In such a plant 22, hydrogen peroxide may be pro duced by the cyclic reduction and oxidation of an alkyl anthraquinone such as 2-ethylanthraquinone in an organic solvent, as follows:

~ 1 ~ 2 ~ ~ i 1~29~8 OH O

~ /2HS ~ ;j,C2H5 Hydrogen peroxide may be extracted from the anthraquinone-containing organic phase directly into the recycling product aqueous media entering plant 22 through line 23 and leaving through line 24, or into a separate water strPam which is then added to the recycling product a~ueous media. The hydrogen entering plant 22 via line 25 may be supplied from electrolytic cell 27 via line 28~
This lowers the cost of producing hydrogen peroxide signi-~icantly because hydrogen is expensive.
Makeup potassium i5 conveniently produced as potassium hydxoxide in electrolytic cell 27. Saturated potassium chloride solution enters cell 27 through line 29, and is electrolytically converted to chlorine at anode 30 and to potassium hydroxide and hydrogen at cathode 31.
Chlorine passes from the cell via line 32 and hydrogen via line 28. A cationic-exchange membrane 33 is used to obtain a potassium chloride-free potassium hydroxide product which passes from cell 27 via line 11. A portion of the potassium hydroxide solution is fed via line 34 to line 8; another portion, to crystallizer 13 via line 11 for adjustment of pH there to a value in the range of about 10 to about 12.
To convert nitrite remaining in the product aqueous 3~ media to nitrate rapidly, carbon dioxide produced in the decarbonation step may be fed via line 12 to line 8.

112962~ ~

There, carbon dioxide converts the carbonate to bicarbonate as follows:

C2 ~ K2C03 ~ 2 ~ 2 KCH03 Optionally, carbonation and oxidation may be expedited by pressurizing carbon dioxide into recycling product aqueous media in line 8. Various mineral acids may alternatively be used to convert carbonate to bicarbonate to expedite the nitrite oxidation but they are costlier than the carbon dioxide made in this process.
Conversion of nitrite to nitrate, and recycle of oxidant-fortified product aqueous media to scrubber 2 via line 4 makes the process fully cyclic.
Figure 2 shows the removal of sulfur dioxide and lower valence nitrogen oxides from a gas mixture containing them and carbon dioxide, where hydrogen peroxide oxidizes only the lower valence nitrogen oxides. Sulfur dioxide is removed rom the gas-liquid contacting in the produc~ aqueous media as alkali metal sulfite. These results are obtained by providing an alkali metal to sulfur dioxide weight ratio of at least about 2, and by limiting the am~unt o oxidant to approximately the amount required for the oxidation of lower valence nitrogen oxides to higher valence nitrogen oxides.
The product aqueous media thus includes (where the alkali metal is potassium) potassium nitrate, potassium nitrite, potassium sulfite, potassium carbonate and potassium bicar-bonate. Countercurrent or staged countercurrent gas-liquid contacting achieves these results. Packed, spray type or other efficient contactvrs may be used. This process embodiment advantageously reduces the quantity of oxidant peroxide needed because sulfite in the product agueous media - 2~ -~1 '29~28 may be oxidized to sulfate with air or other oxygen-containing gas separately rather than with the more expensive oxidant.
In Figure 2, gas mixture 101 including at least one lower valence nitrogen oxide, carbon dioxide and sulfur dioxide, enters near the bottom of tower 102. Alkaline aqueous media containing alkali metal (here potassium) carbo-nate and bicarbonate together with hydrogen peroxide enters near the top of tower 102 via line 104 and passes downwardly and countercurrently to the gas mixture entering in line 101.
Near the bottom of tower 102, potassium carbonate and bi-carbonate in the alkaline aqueous media react with sulfur dioxide to form potassium bisulfite and potassium sulfite in the aqueous media as follows:

S2 + KHC03 3 KHS03 + C02 KHS03 + RHC03 --------~ ~2S03 + C02 Lower valence nitric oxide is unaffected and passes upwardly. Near the top of the tower, it reacts with hydrogen peroxide to form higher valence nitrogen oxides which pass into solution as potassium nitrite and potassium nitrate.
The product aqueous media passing from tower 102 via line 105 thus includes potassium nitrate and potassium nitrite, potassium sulfi~e and potassium sulfate and potassium carbonate and potassium bicarbonate. This product aqueous media passes via line 105 to decarbonator 106. There, elevated temperature and stripping s~eam decompose at least a portion of the bicarbonate, forming carbonate and carbon dioxide which is removed and recovered if desired from line 107~ Alternatively, as shown~ the carbon dioxide may be fed to nitxite to nitrate converter 132 in line 118 via line 107 and mixed there with recycling product aqueous media ~ 27 -6 ". ~ ~

containing potassium nitrite and hydrogen peroxide entering converter 132 from line 118. The carbon dioxide converts carbonate in the product aqueous media to bicarbonate, lowers the pH below 9, and thus expedites conversion of nitrites to nitrates with hydrogen peroxide in the product aqueous media. Where nitrites are to be recovered, this step is omitted.
Decarbonated product aqueous media then passes via line 108 to oxidizer 110 to which air or other oxygen-containing gas is added via line 111. The oxygen oxidizes at least a portion of the potassium sulfite to potassium sulfate under neutral or alkaline conditions, as follows:

K2SO~ ~ 1/2 2 ~ 2 4 Any remaining bicarbonate decomposes to carbonate and carbon dioxide and a substantial amount of water evaporates during this oxidation. Water, air and carbon dioxide pass overhead from oxidizer 110 via line 109.
The product aqueous media passes from oxidizsr 110 via line 112 to hot ~rystallizer~separator 113 where potas-sium sulfate crystallizes and is recovered via line 114.
Potassium nitrate and nitrite remain in solution. Potassium hydroxide may be added to oxidizer 110 via line 140.
Recovery of potassium nitrate, replenishment of product aqueous media with hydrogen peroxide, conversion of nitrite to nitrate, and furnishing of makeup potassium are effected as in Figure lo Here, product aqueous media from the potassium sulfate crystallizer-separator 113 passes via line 115 to potassium nitrate crystallizer 116 which operates in the range of about 10C. to about 40C. Potassium nitrate is removed via line 117 and product aqueous media containing - ~8 -.~ .

1~2962~

principally potassium carbonate and lesser amounts of potas-sium nitrate and nitrite pass through lines 118 and 104 to scrubber 102, making the process cyclic~ Oxidant hydrogen peroxide for the proc~ss is produced in plant 119, and fed to line 118 via line 133. Alternatively, hydrogen peroxide may come from another source to line 118 via line 1300 Air and hydrogen are fed to plant 119 via lines 120 and 121, respectively. Hydrogen may be obtained ~rom an outside source or from electrolytic cell 122. There, potassium hydroxide is made from potassium chloride which enters cell :L22 via line 123. At anode 124, chlorine gas forms and is taken overhead via line 125. Potassium ions pass through cationic permselective membrane 126 to cathode 127, where hydrogen gas forms and passes overhead via line 128. Where hydrogen peroxide is made on site, hydrogen peroxide passes via line 129 and 121 to hydrogen peroxide plant 119. Potassium hydroxide for~s at cathode 127 and passes from cell 122 via line 130 to sulfi~e oxidizer 110 and recycle line 104.
Figure 3 illustrates an embodiment of the process of the invention where a gas mixture including sulfur dioxide, carbon dioxide and lower valence nitrogen oxides are treated in two separate stages, the first for ~he removal of sulfur dioxide, and the second for the removal and recovery of lower valence nitrogen oxides. Sulfur dioxide is removed by employing aqueous media comprising alkali metal carbonate and bicarbonate where ~he ratio of alkali metal to sulfur dioxide is at least about 2. Lower valence oxides of nitrogen are removed from the gas mixture by treating the mixtures with aqueous media comprising alkali metal carbonate and bicarbonate and hydrogen peroxide.

~, ,.

1~29~2~

Removal of sulfur dioxide from gas mixtures and particularly combustion gas mixtures according to this process is strikingly different from any process previously proposed. In particular, this process uses more alkali metal than is required to react with sulfur dioxide so that sub-stantial carbonate and bicarbonate are present in the product aqueous media. Sulfur dioxide is absorbed from the gas mixture and forms sulfite rather than bisulfite as in the Wellman-Lord process. In that process, the scrubber solution is usually slightly acidic in order to recover the sulfur values as sulfur dioxide. To that end, Wellman-Lord decomposes sodium bisulfite thermally to form sulfur dioxide and sodium sulfite which is recycled to gas contact. This process calls for an overall ratio of alkali metal to sulfur dioxide of less tha~ 2 and more preferably bet.ween 1 and 2, whereas the new process of this invention e!mploys a ratio of alkali metal to sulfur dioxide of at least 2 and preferably at least 3. As a result, the process of this invention achieves removal efficiencies of about 99~, whereas the Wellman-Lord process can only reach a practical maximum of about 90%.
Importantly, the process of this invention does not destroy nitrogen oxides present in the gas mixture with the sulfur dioxide. By contrast, in the Wellman-Lord and lime-limestone processes, lower valence nitrogen oxides in the gas mixture are reduced by sulfite-bisulfite solution to n~tro~en and nitrous oxide (N2O) and are lost as a potential source of fixed nitrogen. In the process of this invention, the lower valence nitrogen oxides are not destroyed but pass freely to subse~uent steps for th ir removal and recovery.
Referring now to Figure 3, a gas mixture including sulfur dioxide, lower valence nitrogen oxides and carbon ~ 30 -~129628 dioxide, such as in a typical combus~ion gas, enters scrubber 202 via line 201, and is intimately contacted, cocurrently or countercurrently, with aqueous media entering via line - 208, and including principally alkali metal carbonate and bicarbonate such as potassium carbonate and bicarbonate, together with a lesser amount of potassium sulfite carried from recycle stream 207. The sulfur dioxide is removed from the gas mixture and transfers to the product aqueous media as potassium sulfite. Makeup potassium hydroxide entering via lines 237 and 208 is converted first to potassium carbonate in stream 208 and then to potassium bicarbonate by reaction with the carbon dioxide in the gas mixture.
In turn, the potassium bicarbonate reacts with the more acidic sulfur dioxide to form potassium sulfite. Excess potassium carbonate and bicarbonate effect conversion of potassium bisulfite to sulfite. The reactions taking place in scrubber 202 are as follows:

2 KOH ~ CO~ 3 K2CO3 ~ H2o K2CO3 ~ CO2 + H2o > 2 KHCO3 S2 ~ 2 RHCO - -3 K SO + 2 C02 ~ H2 (NO + NO) * 2 K2CO3 > 2 KN 2 3 A substantial portion of the product aqueous media leaving scrubber 202 in line 206 is normally recycled to provide good contact between gas and aqueous phases. That portion removed for product recovery is usually decomposed by boiling or by countercurrent steam stripping in decar-bonator 211 in which the potassium bicarbonate is convertedto carbon dioxide and potassium carbonate. The following ~ 31 ~

1129~8 reaction takes place in zone 211 at boiling temperatures;

2 KHCO3 ~ K2CO3 + CO2 + H2o The carbon dioxide and steam so formed leave zone 211 via line 213 from which the water may be condensed and the carbon dioxide dried and recovered as a product, or transferred to converter 255 via line 212. The decarbonated product aqueous media comprising potassium carbonate, po~assium sulfite and potassium sulfate in stream 214 passes to sulfite oxidizer/
evaporator 215 where potassium sulfite is oxidized to potassium sulfate. Though the oxidation may be done with hydrogen peroxide, atmospheric oxygen costs less and is preferably used. The oxygen~containing gas is fed to oxidizer 215 via line 217.
Oxidation of sulfite to sulfate by atmospheric oxygen proceeds rapidly under neutral or alkaline conditions and may be expedited by operation at elevated pressures and temperatures. Any additional base needed to raise pH may be fed to zone 215 as potassium hydroxide through line 271, but this is generally not necessary. The reaction taking ~o place in oxidizer 215 is as follows:

K2SO3 ~ 1/2 2 -- ~ K2S4 Potassium sulfate has a significantly lower solu-bility in water than potassium sulfite and has a low sol~ility in potassium carbonate solutions. Accordlngly, potassium sulfate may readily be removed from the product aqueous media by crystallization. Normally, the discharge 218 from sulite oxidizer 215 is a slurry because of sub-stantial water removal and low solubility of potassium sulfate in product aqueous madia. The slurry in line 218 112962~ `

may then be cooled and potassium sulfate removed in crystal-lizer-separator 220. The recovered potassium sulfate is normally centrifuged and removed via line 221 for drying and packaging, and the product aqueous media is removed in line 222.
Produc~ aqueous media in line 222, which is principally potassium carbonate, may also contain a small amount of potassium sulfate, unoxidized potassium sulfite and potassium nitrite. That product aqueous media is re-cycled to zone 202 for scrubbing via lines 222, 207 and 208.
Some of this may be bled, however, via lines 223 and 224, respectivelyl to oxidizer 215, to line 258, or both. Addi-tional potassium hydroxide is added to the recycling product aqueous media via line 237 to compensate for potassium removed in the potassium sulfate product.
The gas stream passing from scrubber 202 via line 203, substantially free of sulfur dioxide, passes to scrubber 204 where lower valence nitrogen oxides and some carbon dioxide are xemoved. Clean gas emerges via line 205. Unlike the embodiment disclosed in Figure 1, however, prior removal of sulfur dioxide reduces substantially the quantity of oxidant required, and precludes formation of more than a small quantity of potassium sulfate during r~moval and recovery of lower valence nitrogen oxides. Crystallization and recovery of potassium nitrate is simpler than in Figure 1 because the product a~ueous media includes only nitrate, nitriter carbonate and bicarbonate, but little sulfateO
Potassium makeup required for producing the potassium nitrate product may be from potassium carbonate via line 224, from potassium hydroxide electrolytically made in cell 231, or both.

1 1 2 9 6 ~ ~ `

Product aqueous media comprising unconsumed hydrogen peroxide, potassium nitrate and po~assium nitrite, potassium carbonate and potassium bicarbonate~ passes ~rom zone 204 via line 251 and is recycled to scrubber 204 via lines 252 and 253, until the concentration of nitrite, nitrate or both reaches a predetermined minimum. Thereafter, at least some of the product aqueous media in line 251 enters the product recovery cycle via line 254. Preferably, the product aqueous media to be subjecte~ to product recovery passes through line 254 to nitrite converter 255. Carbon dioxide from decarbonator 211 passes via line 212 to converter 255 to convert carbonate to bicarbonate, thus lowering the pH to less than about 9, and facilitating oxidation of nitrite to nitra~e by unconsumed oxidant hydrogen peroxide. Product aqueous media, now rich in potassium nitrate and potassium bicarbonate, passes from nitrite converter 255 via line 256 to decarbonator~evaporator 257. There, a substan~ial portion of bicarbonate is converted to carbonate and carbon dioxide by thermal decomposition. Carbon clioxide and water pass overhead via line 209 and the carbon dioxide may be recovered and used elsewhere, or may be fed into line 245 to convert potassium carbonate to hicarbonate in that line and to facilitate oxidation of potassium nitrite there to nitrate.
Pressurizing carbon dioxide aids conversion of carbonate to bicarbonate and facilitates formation of carbonic acid in the product aqueous media. Oxidation of nitrite to nitrate proceeds faster when carbonic acid is present and carbonate is absent.
The product aqueous media leaving the decarbonation tower 257 via line 258 contains primarily potassium nitrate and nitrite and potassium carbonate, and is cooled in 1129~2~

crystallizer 260 to around 10C. Thereupon, potassium nitrate crystallizes and is removed via line 261 following centri-fuging. Potassium carbonate and potassium nitrite remain in the product aqueous media which recycles via lines 262 and 253 to zone 204 following addi~ion thereto of oxidant hydrogen peroxide via lines 245 and 253. Some of the re-cycling aqueous media may be bled into line 258 from line 245 via line 259.
Hydrogen peroxide may be produced on site, as described above, from line 243, or from an outside source via line 244. Where peroxide is made on site, hydrogen is fed to plant 241 via line 235 and air, via line 242. Pr~-ferably, hydrogen is supplied in whole or in part fxom electrolytic cell 231.
Makeup potassium is conveniently produced in electrolytic cell 231 as well. To cell 231, a concentrated solution of potassium chloride is fed via line 238 to the anode compartment where it is elect:rolytically converted to chlorine at anode 233 and to potassium hydroxide and hydrogen at cathode 234. Chlorine passes from the cell via line 236 and hydrogen via line 235. Cationic membrane 232 i5 used to produce a chloride-free potassium hydroxide product which passes from cell 231 via line 237.
Figure 4 illustrates an embodiment of the processes of this invention wherein a gas mixture including carbon dioxide and lower valence nitrogen oxides are treated for removal and recovery of those lower valence nitrogen oxides using gaseous chlorine or alkali metal hypochlorite with an aqueous alkali metal carbonate/bicarbonate acceptor system.
The alkali metal is potassium. Sulfur dioxide, lf present in the gas mixture, may be removed by oxidation with the same 1 1.2~6~

oxidant, or, preferably, before ~reatment with the oxidant as in Figure 3.
Where sulfur dioxide is not present, or if presentt is removed in a prior step, as in Figure 3, the product aqueous media from the treatment with oxidant includes potassium nitrate, potassium nitrite, potassium chloride and potassium carbonate and bicarbonate. Potassium carbonate diminishes the solubility of potassium chloride in aqueous media from, for example, 30% in water at 50C. to around 3%
at the same temperature in a solution containing 50%
potassium carbonate Potassium chloride may therefore be crystallized and removed from the more soluble potassium nitrate and nitrite which remain in the product aqueous media.
When using chlorine or hypochlorite as the oxidant, chlorine carryover may occur at the reduced pH's that arise in the scrubber, and a two-stage scrubbing system is pre-ferred. Hypochlorites are strong oxidizing agents in a pH
range of about 6.5 to about 8 where a substantial amount of hypochlor~us acid is present.
Oxidation and removal of lower valence nitrogen oxides takes place primarily in the ini~ial scrubbing stage.
Unreacted chlorine and nitrogen oxides carried to the second scrubbing stage are removed because of the higher alkalinity and pH there than in the first stage. The same result may be obtained in a single countercurrent scrubbing column but with less flexibility and ease. The reaction of chlorine and hypochlorites with nitric oxide is quite rapid.
Referring now to Figure 4, the gas mixture compris-ing at least one lower valence nitrogen oxide and carbon dioxide enters scrubber 302 via line 301. The gas mixture is intimately contacted by aqueous media including potassium 1129~2~

carbonate and bicarbonate, potassium nitrate and nitrite and the oxidant. Where ~he oxidant is potassium hypochlorite, the oxidant is in solution and is accompanied by potassium chloride. This solution enters scrubber 302 via line 362.
Where chlorine is the oxidant, it may be added to the gas mixture before the mixture enters scrubber 302 or may be fed directly to scrubber 302 through a separate inlet for reaction with potassium carbonate and bicarbonate to form hypochlorite. The scrubbed gas stream passes from scrubber 302 via line 303 to second stage 306 and is treated with aqueous potassium carbonate entering via line 347 to remove the last traces of chlorine. The scrubbed gas exits scrubber 306 via line 307 and passes to the atmosphere. The potassium carbonate conta~ning aqueous solution passes from scrubber 306 via line 304, and is partially recycled to line 347 via line 305. The remainder of the solution passes via line 304 to scrubber 302.
The following reactions take place in scrub~ers 302 and 306:

Zone 306 2 KOH + C02 ______ ~ K2C03 + 2 K2CO3 ~ CO2 H2o ~ 2 KHCO3 Zone 302 Cl + K CO3 ~ KOCl + KCl + CO2 C12 + 2 KHCO3 - > KOC1 + KCl + 2 CO2 ~ H2O

C 2 H20 - HOCl ~ HCl KOCl + H2O - HOC1 + KCl :1 L29628 NO Conversion 2 NO + 3 KOCl (3 KCl) + 2 KHCO3 32 KNO3 ~ 6 KCl -~
2 CO2 ~ ~2 S2 Conversion S2 ~ KOCl ~KCl) ~ 2 KHCO3 2 4 ~ C2 + H20 The oxidant, whether potassium hypochlorite, 10 hypochlorous acid, chlorine, chlorine dioxide or a mixture of two or more of these, oxidizes the lower valence nitrogen oxides to higher valence nitrogen oxides which react with the potassium carbonate and bicarbonate to form non-volatile potassium nitrate and nitrite.
Makeup potassium hydroxide! and oxidant for the system are conveniently produced in electrolytic cell 351.
A saturated potassium chloride solution enters cell 351 via line 319 from dissolver 318. Potassium chloride is fed to dissolver 318 via line 359, and i~ dissolved therein with ~0 water entering via line 350 and with recycled potassium chloride entering via line 317. The potassium chloride is electrolytically converted to chlorine at anode 354 and to potassium hydroxide and hydrogen at cathode 3S3. Chlorine passes frvm the cell via line 357 and hydrogen via line 355.
A cationic-exchange membrane 352 is used ~o obtain potassium - chloride-free potassium hydroxide which passes from cell 351 via line 360.
Where aqueous potassium hypochlorite is the oxidant, potassium hydroxide and chlorine gas are introduced to a 30 potassium hypochlorite reactor 361 through lines 360 and 357 respectively, in the ratio produced in the electrolysis .. . . . .. .. ... .. . .. . ... . .. . . . . . ..... . .. .. . .. ....

1~29628 namely 2:1. The potassium hypochlorite oxidant passes to scrubber 302 via line 362.
Where chlorine is the oxidant, chlorine passes through lines 357 and 358 to line 301, where it commingles with the entering gas mixture before that mixture enters scrubber 302. Potassium hydroxide is, in this embodiment, fed to scrubber 306 via line 347.
The product aqueous media including potassium nitrate, potassium nitrite, potassium carbonate and potassium bicarbonate and potassium chloride leaves scrubber 302 via line 310. Several process variations may be used to separate and recover the products, one of which follows.
The product aqueous media from scrubber 302 passes via line 310 to decarbonator-evaporator-separator 312, in which a substantial portion of the potassium chloride is crystallized. The hot slurry passes to centrifuge 315 via line 313. Alony with the evaporation, bicarbonata in the product aqu~ous media may be at least partially converted to carbonate and carbon dioxide in aecarbonator-evaporator-~0 separator 312, and the relatively pure carbon dioxide may be taken overhead via line 311 and recovered for use in other processes.
Additional potassium carbonate recycles ~o evapo-rator 312 through line 348 to provide a high potassium carbonate concentration therein. This high concentration reduces the soluhility of potassium chloride and a major portion may be crystallized from the aqueous media while retaining the potassium nitrite and nitrate in solution.
Potassium chloride solids centrifuged from solution in centrifuge 315 are discharged via line 317 and recycled to electrolysis via dissolver 318. The aqueous media 1 1~9~

including principally potassium nitrate and carbonate leaves through line 316 and passes to crystallizer 320 where the solution is cooled, generally ~o a temperature of abou~ 30C., to crystallize mixed potassium nitrate-potassium chloride.
The slurry of potassium nitrate-potassium chloride passes from crystallizer 320 via line 321 to contrifuge 322 and the solids, principally potassium nitrate and potassium chloride, are removed through line 3Z3 and fed ~o lixiviator 330. There, the more soluble potassium nitrate is selec-tively dissolved, or lixiviated, from the solids mixture.
The aqueous media comprising principally potassium carbonate passes from centrifuge 322 via line 324 and recycles via lines 347 and 348 to decarbonator-evaporator-crystallizer 312 to promote the crystallization of potassium chloride therein.
In lixiviator 330, the mixed potassium nitrate and chloride solids are contacted intimately, usually in a fluidized bed, with a warm tabout 70 to about 80C.) highly concentrated recycling aqueous potassium carbonate solution. The potassium nitrate tends to dissolve pre-ferentially into the solution but leaving the potassium chloride undissolved. The aqueous potassium carbonate-potassium nitxate solution passes from lixivia~or 330 via line 331 for transfer to the potassium nitrate crystallizer 340. The solution is cooled there to around 20C. and the potassium nitrate crystallizes. The potassium nitrate solids slurry passes via line 341 to centrifuge 342 and is centrifuged. The potassium nitrate solids are removed via line 343 for drying and packaging.
The aqueous potassium carbonate-containing media passes from centrifuge 34Z via line 344 into line 345 for ~ 129628 return to lixiviator 330 to remove additional potassium nitrate~
Undissolved potassium chloride solids in lixiviator 330 are passed via line 332 to centrifuge 333. The aqueous media comprising principally potassium carbonate and potassium nitrate transfers via line 335 to join the main extract stream in line 331. The undissolved potassium chloride solids from centrifuge 333 are recycled to decar-bonator-evaporator-crystallizer 312 via line 334.
The reactions taking place in the elactrolytic cell and the hypochlorite reactor are:

Electrolytic cell 351 2 KCl ~ H O -~ 2 XOH ~ C12~ ~ H2 Hypochlorite reactor 361 2 KOH + C12 - ~ KOCl ~ KCl In the embodiment of Figure 4, any suifur dioxide in the gas mixture is preferably removed inltially as by the process described in reference to s~age 1 of Figure 3.

EXAMP~E I

A gas mixture consisting of 400 parts per million of nitric oxide and 99.96% nitrogen was passed in a s~ream countercurrent to an aqueous scrubbing solution consisting of 20~ potassium carbonate and 1~ hydrogen peroxide (stabilized) at 50C. through a 10 foot glass column having a 2 inch diameter and packed with 1/2 inch Pall-type rings.
The gas stream was fed at a rate corresponding to about 1129~2~ `

0.1 foot per second superficial gas velocity, and the liquid flow rate was jus~ below the flooding point. The concentra~
tion of nitric oxide in the gas stream was reduced to about 30 parts per million which ~orresponds to a removal effi-ciency of about 92%. The scrubbing solution was recircu-lated until the hydrogen peroxide concentration was reduced to approximately 0.1% without any loss in scrubbing efficiency.
The aqueous scrubbing solution was analyzed and the yield of potassium nitrate and po~assium nitrite formed corresponded closely to the amount of nitric oxide removed from the gas mixture. The molar ratio of potassium nitrite to potassium nitrate was about l:l and the usage of hydrogen peroxide was 1.1 times the theoretical requirement.

EXAMPLE II

The same gas mixture treated in Example I was treated in the same equipment and under the same conditions as under Example I, except that a scrubbing solution was used consisting of about 18% potassium bicarbonate, about 6% potassium carbonate and about 1~ hydrogen peroxide in water. The efficiency of nitric oxide removal from this gas stream was 91% and the ratio of potassium nitra~e to potassium nitrite was about 1.2:1. The yield of potassium nitrate and potassium nitrite corresponded closely to the quantity of nitric oxide removed from the stream and the usage of stabilized-hydrogen peroxide was about 1.05 times the theoretical requirement.

EXAMPLE III

The removal of nitric oxide was again tested using the equipment described in Example I but with an aqueous - 42 ~

~129628 scrubbing solution consisting of about 18~ po~assium bicar-bonate, about 6~ potassium carbonate, about 10~ potassium nitrate and about 1% hydrogen peroxide, again at 50C. The nitric oxide removal efficiency from the gas stream remains , at 92~. The treating solu~ion was heated and evaporated after use to convert bicarbonate to carbonate and to adjust the potassium nitrate concentration to approximately 12~.
The solution was cooled to 0C. and nearly 100% pure potassium nitrate product was crystallized from this product aqueous media wi~h little or no potassium carbona~e or potassium nitrate contamination.
EXAMPLE IV
A gas mixture comprising 800 parts per million of sulfur dioxide and 99.92~ nitrogen was tested in the absorp-tion tower described in Example I using a gas velocity of about 2.0 foot per second and an aqueous solution including about 20% potassium carbonate~ The pH of the aqueous media was about 11.2; the reaction temperature, about 50C.
Efficiency of sulfur dioxide removal was higher than 99%.
Removal efficiency was then tested with an aqueous media including about 18~ potassium bicarbona~e and about 6%
potassium carbonate. Removal efficiency was again higher than 99~. The pH of the scrubber solution was 9.2, and the temperature was again 50C.
By contrast, removal of sulfur dioxide from the same gas stream with an aqueous solution containing 10%
sodium sulfite at 50C., in accordance with the Wellman-Lord process, produced a removal efficiency of only about 91~ at peak, which then declined as the concentration of sodium bisulfite began to rise in the recycled aqueous scrubbing media. The pH of the sodium sulfate scrubbing ~ 43 -1~9~

solution was 7.2 at the beginning of the test.

EXAMPLE V
The solubilities in aqueous solution of potassium nitrate, potassium sulfate and potassium chloride are sub~
stantially reduced in the presence of potassium carbonate.
As Table l below shows, an aqueous solution containing 67 grams of potassium carbonate ln 100 milliliters o~ water xeduces the solubility of potassium nitrate to one-tenth that in water without potassium carbonate present and that of potassium sul-fate by greater than one hundredth that in water alone.Potassium chloride solubilities are reduced by one-fourth to one-eighth.

SOLUBILIT~ OF KNO3, K2S04 AND KCl IN AQUEOUS K2C03 SOLUTIONS IN GRAMS PER 100 MIL~ILITERS _ IN AQUEOUS MEDIA INCLUDING
IN WATER ~LONE ~;7 g K CO /100 ml solution _ 2 3 _ _ m p., C: _10 40 70 _ 10 40 70 XNO3 21.5 64.0 138.0 2.80 6.00 13.00 K2S4 9.1 14.5 19.5 0.07 0.09 0.11 KCl 31.0 40.0 48~5 4.00 8.00 12.00 Nitrogen oxides have valences ranging from ~l to ~5, as represented by the following compounds:

~ 44 1~2962~

Valence ~ormula Compound ~1 N2O Nitrous oxide +2 N0 Nitric oxide ~3 N2O3 Dinitrogen trioxide +4 N2,N24 Nitrogen dioxide and Dinitrogen tetroxide ~5 N205 Dinitrogen pentoxide ~3 HNO2 Nitrous acid +5 HNO3 Nitric acid 1~ As used herein, references made to increasing or oxidizing an oxide of lower valence nitrogen to an oxide o~
higher valence nitrogen means oxidizing from a lower vaience state to a higher valence state, e.g., from nitric oxide (NO) to nitrogen dioxide (NO2) and from nitrogen dioxide (N0~) to dinitrogen pentoxide (N205) or nitric acid (HN03).
~ s used herein, alkali metals include potassium, sodium, lithium, rubidium and cesium and alkaline earths include calcium, magnesium, and strontium.

- ~5 -... ... ... .. ... . . .. ...

Claims

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

1. A process for removing oxides of sulfur from a com-bustion gas mixture including carbon dioxide and sulfur dioxide comprising:
a) treating the gas mixture in a zone with an aqueous media which includes the carbonate of an alkali metal wherein the molar ratio of alkali metal, as carbonate-bicarbonate, is greater than 2;
b) converting all of the sulfur dioxide absorbed by the aqueous media to alkali metal sulfite;
c) forming a product aqueous media comprising alkali metal sulfite and alkali metal carbonate-bicarbonate;
d) crystallizing and recovering at least a portion of the alkali metal sulfite from the product aqueous media; and e) recycling the aqueous mother liquor to the zone of said gas treating.

2. The process of claim 1 in which following steps a), b), and c), the process further comprises:
d) treating the product aqueous media to convert at least a portion of said alkali metal sulfite to alkali metal sulfate by oxidation with hydrogen peroxide or an oxygen-containing gas;
e) crystallizing and recovering at least a portion of the alkali metal sulfate from the aqueous media; and f) recycling the aqueous mother liquor to the zone of said gas treating.

3. The process of claim 1 in which following steps a), b), and c), the process further comprises:
d) treating the product aqueous media to decompose least a portion of the alkali metal bicarbonate therein by heating to form alkali metal carbonate and liberate carbon dioxide;
e) crystallizing and recovering at least a portion of said alkali metal sulfite from the aqueous media;
and f) recycling the aqueous mother liquor to the zone of said gas treating.

4. The process of claim 1 in which following steps a), b), and c), the process further comprises:
d) treating the product aqueous media to decompose at least a portion of the alkali metal bicarbonate therein by heating to form alkali metal carbonate and liberate carbon dioxide; then e) further treating to convert at least a portion of said alkali metal sulfite to alkali metal sulfate by oxidation with hydrogen peroxide or an oxygen-containing gas;
f) crystallizing and recovering at least a portion of the alkali metal sulfate from the aqueous media;
and g) recycling the aqueous mother liquor to the zone of said gas treating.

5. The process of claim 1 in which following steps a), b), and c) the process further comprises:
d) crystallizing and recovering alkali metal sulfate therein from the aqueous media;
e) concentrating the aqueous mother liquor;
f) crystallizing and recovering a portion of the alkali metal sulfite from the first aqueous mother liquor; and g) recycling the second aqueous mother liquor to the zone of said gas treating.

6. The process of claim 5 in which following steps a), b), and c) the process further comprises:
d) treating the product aqueous media to decompose at least a portion of the alkali metal bicarbonate therein by heating to form alkali metal carbonate and liberate carbon dioxide; then e) crystallizing and recovering alkali metal sulfate therein from the aqueous media;
f) concentrating the aqueous mother liquor, g) crystallizing and recovering a portion of the alkali metal sulfite from the first aqueous mother liquor; and h) recycling the second aqueous mother liquor to the zone of said gas treating.

7. The process of claim 1, 2 or 3 in which the alkali metal is potassium.

8. The process of claim 4, 5 or 6 in which the alkali metal is potassium.

9. The processes of claims 3, 4 and 6, in which at least a portion of the carbon dioxide liberated is recovered.

10. The processes of claims 2 and 4 including effecting said oxidation and conversion of said alkali metal bicarbonate to carbon dioxide and alkali metal carbonate generally simultaneously.

11. The process of claim 1 wherein the alkali metal of said alkali metal carbonate-bicarbonate is provided as an alkali metal hydroxide and is made by the electrolysis of an alkali metal chloride.

12. The process of claim 2 wherein the alkali metal of said alkali metal carbonate-bicarbonate is provided as an alkali metal hydroxide and is made by the electrolysis of an alkali metal chloride.

13. The process of claim 3 wherein the alkali metal of said alkali metal carbonate-bicarbonate is provided as an alkali metal hydroxide and is made by the electrolysis of an alkali metal chloride.

14. The process of claim 4 wherein the alkali metal of said alkali metal carbonate-bicarbonate is provided as an alkali metal hydroxide and is made by the electrolysis of an alkali metal chloride.

15. The process of claim 5 wherein the alkali metal of said alkali metal carbonate-bicarbonate is provided as an alkali metal hydroxide and is made by the electrolysis of an alkali metal chloride.

16. The process of claim 6 wherein the alkali metal of said alkali metal carbonate-bicarbonate is provided as an alkali metal hydroxide and is made by the electrolysis of an alkali metal chloride.

17. The process of claim 11, 12 ox 13 wherein the electrolysis of said alkali metal chloride also produces chlorine.

18. The process of claim 14, 15 or 16 wherein the electrolysis of said alkali metal chloride also produces chlorine.

19. The process of claim 11, 12 or 13 wherein the electrolysis of said alkali metal chloride also produces hydrogen which is converted to hydrogen peroxide.

20. The process of claim 14, 15 or 16 wherein the electrolysis of said alkali metal chloride also produces hydrogen which is converted to hydrogen peroxide.