GB1593745A - Removal of sulphur dioxide from gas mixtures - Google Patents

Description

PATENT SPECIFICATION ( 11) 1 593 745

VI ( 21) Application No 15352/80 ( 22) Filed 16 Aug 1977 ( 19) t ( 62) Divided Out of No 1593742 ( 31) Convention Application No's 714793 ( 32) Filed 16 Aug 1976 -' 804620 8 Jun 1977 in tn ( 33) United States of America (US) -q ( 44) Complete Specification Published 22 Jul 1981 ( 51) INT CL 3 B Ol D 53/34 ( 52) Index at Acceptance C 1 A 5181 518 Y 521 X 521 Y 5221 5222 522 Y 5410 5412 5413 5415 5416 541 Y 544 Y 5450 5451 5471 548 X 5491 5492 5641 5642 5643 5681 5682 SB ( 72) Inventor: HAL BLUFORD HARRISON COOPER ( 54) REMOVAL OF SULFUR DIOXIDE FROM GAS MIXTURES ( 71) I HAL BLUFORD HARRISON COOPER, a citizen of the U S A of 4234 Chevy Chase Drive, Flintridge, California 91011, United States of America, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:-

This invention relates to processes for the removal of sulfur dioxide from gas mixtures, 5 including those containing sulfur dioxide, and nitrogen oxides, regardless of whether removal of such nitrogen oxides from these mixtures is also conducted These processes can achieve higher removal percentages than such widely-adopted processes as the WellmanLord method and lime-limestone scrubbing, and permit flexibility in recovering the sulfur values so removed In particular, recovery of the sulfur in sulfate form through 10 crystallization can be rapid, reliable and high in selectivity Again, the removal and recovery of sulfur values as useful products of commerce through this process can allow an improvement in the economics of the heretofore costly removal of sulfur dioxide pollutants from such gas mixtures as combustion gases.

Where a gas mixture includes both sulfur dioxide and nitrogen oxides, and removal of 15 both is desirable, the new process for removing and recovering sulfur dioxide may be combined into a single step with a process for recovering nitrogen oxides or may be effected sequentially, with the latter offering certain processing advantages over the combined process.

The process of this invention permits the recovery of sulfur dioxide from gas mixture 20 including sulphur dioxide and carbon dioxide regardless of whether nitrogen oxides are present or not and regardless of whether removal of those nitrogen oxides is necessary or desirable Sulfur dioxide removal is effected by:

(a) treating said gas mixture in a zone with an aqueous medium which includes 25 carbonate-bicarbonate of an alkali metal; (b) maintaining said alkali metal carbonate-bicarbonate in stoichiometric excess for reaction with the sulfur dioxide; and (c) forming a product aqueous medium which includes alkali metal sulfite and, optionally alkali metal sulfate 30 Where lower valence nitrogen oxides are present, the process may be conducted in accordance with copending application No 34033/77 (Serial No 1593742) from which the present application is divided so as to oxidize sulfite directly to sulfate with the corresponding alkali metal salts forming in the product aqueous medium together with 35 alkali metal nitrites and nitrates Thereafter, product separation and recovery may proceed as described in Patent Application No 34033/77 (Serial No 1593742) Alternatively, the gas mixtures may be treated countercurrently with a controlled amount of oxidant and excess alkali metal carbonate/bicarbonate in the aqueous medium so that the sulfur dioxide forms alkali metal sulfite salt in the first stage of the gas-liquid contacting The oxidant 40 1 593 745 oxidizes only the lower valence nitrogen oxides passing to the latter stages, and thus does not oxidize the sulfite to sulfate.

The ratio of alkali metal to sulfur dioxide for conversion to sulfite 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 5 sulfate As before, the higher valence nitrogen oxides are trapped in the solution as alkali metal nitrites and nitrates.

The alkali metal is preferably potassium.

Rather than removing sulfur dioxide concomitantly with lower valence nitrogen oxides, however, the process of this invention removes sulfur dioxide separately, and preferably 10 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 medium comprising the carbonate and bicarbonate of an alkali metal where the molar ratio of alkali metal to sulfur dioxide is at least about 2 A product aqueous medium is recovered that includes alkali metal carbonate, alkali metal bicarbonate 15 and alkali metal sulfite formed from the sulfur dioxide Sulfur dioxide removal efficiency of this process can be 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.

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 20 dioxide present from these mixtures As with the processes of copending application No.

34033/77 (Serial No 1593742) for removing lower valence nitrogen oxides from gas mixtures, this process for 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 25 combustion gas from a power plant or other high fuel-consuming plant Air oxidation of sulfite to sulfate, and recovery of sulfate by crystallization may then follow.

The sulfur dioxide removal process of ths invention can not only achieve 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 sulfur 30 dioxide to produce alkali metal sulfite not only ensures 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 raised to a high level by recycling in a 35 manner similar to that described in Patent Application No 34033/77 (Serial No 1593742) for nitrogen oxides removal The sulfur values in the product aqueous medium are preferably recovered as alkali metal 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 40 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 medium by 45 crystallization Crystallization of potassium sulfate from the potassium carbonatecontaining product aqueous medium is particularly efficient Surprisingly, the solubility of potassium sulfate in potassium carbonate solutions is 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 70 C is 19 5 grams/l O O milliliters of water, but 50 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 medium with the sulfate or sulfite salts may be replenished with alkali metal hydroxide or carbonate and fed directly to the recycling product aqueous medium Where the product aqueous medium includes sulfite, 55 nitrite and nitrate salts in solution, these may be separated from one another, as by first 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 medium reduces the solubility of both sulfates and nitrates and aids in the selectivity of the process, thus permitting first sulfates, at around 60 C, and then nitrates to crystallize as the product aqueous medum is cooled The solubility of potassium nitrate in aqueous medium 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 65 3 1 593 745 3 alone at 40 'C.

Where the new sulfur dioxide removal process of this invention is combined with the new process for removing lower valence nitrogen oxides from gas mixtures described and claimed in Patent Application No 34033/77 (Serial No 1593742), even greater advantages accrue from the integration of these two processes into a sequential overall process for 5 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 nitrogen 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 10 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 chloride recovered from the product aqueous medium 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 15 removal, but includes some dinitrogen trioxide, nitrogen dioxide, or both, 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 medium 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 20 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 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 25 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 permits 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 sulfur dioxide in combustion 30 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 process of this invention can meet all these needs.

The process of this invention is illustrated in Figures 1 and 2 of the drawings 35 Figure 1 illustrates an 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 valence nitrogen oxides with hydrogen peroxide in a single scrubber 40 Figure 2 illustrates an 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 formed and removed in the first stage as potassium sulfite in product aqueous medium The lower valence nitrogen oxides in the sulfur dioxide-free gas mixture are removed and recovered in a second stage using the 45 hydrogen peroxide oxidant and aqueous medium containing potassium carbonate/ bicarbonate.

In Figure 1, 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) carbonate and bicarbonate together with hydrogen 50 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 bicarbonate in the alkaline aqueous media react with sulfur dioxide to form potassium bisulfite and potassium sulfite in the aqueous media as follows:

SO 2 + KHCO 3 KHSO 3 + CO 2 KHSO 3 + KHCO 3, K 2503 + CO 2 Lower valence nitric oxide is unaffected and passes upwardly Near the top of the tower, 60 it reacts with hydrogen peroxide to form higher valence nitrogen oxides which pass into solution as potassium nitrite and potassium nitrate.

The product aqueous medium passing from tower 102 via line 105 thus includes potassium nitrate and potassium nitrite, potassium sulfite and potassium sulfate and potassium carbonate and potassium bicarbonate This product aqueous medium passes via 65 4 1 593 745 4 line 105 to decarbonator 106 There, elevated temperature and stripping steam 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 nitrite to nitrate converter 132 in line 118 via line 107 and mixed there with recycling product aqueous media containing potassium nitrite and hydrogen peroxide 5 entering converter 132 from line 118 The carbon dioxide converts carbonate in the product aqueous medium to bicarbonate, lowers the p H below 9, and thus expedites conversion of nitrites to nitrates with hydrogen peroxide in the product aqueous medum Where nitrites are to be recovered, this step is omitted.

Decarbonated product aqueous medium then passes via line 108 to oxidizer 110 to which 10 air or other oxygen-containing gas is added via line 111 The oxygen oxidizes at least a rtion of the potassium sulfite to potassium sulfate under neutral or alkaline conditions, as foollows:

K 2503 + 1/2 02 K 2504 15 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 medium passes from oxidizer 110 via line 112 to hot crystallizer/ 20 separator 113 where potassium 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 medium with hydrogen peroxide, conversion of nitrite to nitrate, and furnishing of makeup potassium are effected 25 Here, product aqueous medium from the potassium sulfate crystallizerseparator 113 passes via line 115 to potassium nitrate crystallizer 116 which operates in the range of about 10 'C.

to about 40 'C Potassium nitrate is removed via line 117 and product aqueous medium containing principally potassium carbonate and lesser amounts of potassium nitrate and nitrite pass through lines 118 and 104 to scrubber 102, making the process cyclic Oxidant 30 hydrogen peroxide for the process 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 130.

Air and hydrogen are fed to plant 119 via lines 120 and 121, respectively Hydrogen may be obtained from an outside source of from electrolytic cell 122 There, potassium hydroxide is made from potassium chloride which enters cell 122 via line 123 At anode 124, 35 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 forms at cathode 127 and passes from cell 122 via line 130 to sulfite oxidizer 110 and recycle line 104 40 Figure 2 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 the removal of sulfur dioxide, and the second for the removal and recovery of lower valence nitrogen oxides Sulfur dioxide is removed by employing aqueous medium comprising alkali metal carbonate and bicarbonate where the 45 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 medium comprising alkali metal carbonate and bicarbonate and hydrogen peroxide.

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 50 particular, this process uses more alkali metal than is required to react with sulfur dioxide so that substantial carbonate and bicarbonate are present in the product aqueous medium.

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, WellmanLord 55 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 than 2 and more preferably between 1 and 2 whereas the new process of this invention employs a ratio of alkali metal to sulfur dioxide of at least 2 and prefererably at least 3 As a result, the process of this invention achieves removal efficiencies of about 60 99 %, whereas the Wellman-Lord process can only reach a practical maximum of about % 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 nitrogen and nitrous oxide (N 20) and are lost as a potential 65 1 593 745 1 593 745 5 source of fixed nitrogen In the process of this invention, the lower valence nitrogen oxides are not destroyed but pass freely to subsequent steps for their removal and recovery.

As shown in Figure 2, a gas mixture including sulfur dioxide, lower valence nitrogen oxides and carbon dioxide, such as in a typical combustion gas, enters scrubber 202 via line 201, and is intimately contacted, cocurrently or countercurrently, with aqueous media 5 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 10 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:

15 2 KOH + CO 2 K 2 C 03 + H 20 K 2 C 03 + CO 2 + H 20 2 KHCO 3 SO 2 + 2 KHCO 3 -> K 2503 + 2 CO 2 + H 20 20 (NO 2 + NO) + 2 K 2 C 03 - 2 KNO 2 + 2 KHCO 3 A substantial portion of the product aqueous media leaving scrubber 202 in line 206 in normally recycled to provide good contact between gas and aqueous phases That portion 25 removed for product recovery is usually decomposed by boiling or by countercurrent steam stripping in decarbonator 211 in which the potassium bicarbonate is converted to carbon dioxide and potassium carbonate The following reaction takes place in zone 211 at boiling temperatures:

30 2 KHCO 3 -> K 2 C 03 + CO 2 + H 20 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 35 potassium carbonate, potassium 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 40 alkaline conditions and may be expedited by operation at elevated pressures and temperatures Any additional base needed to raise p H may be fed to zone 215 at potassium hydroxide through line 271, but this is generally not necessary The reaction taking place in oxidizer 215 is as follows:

45 K 2503 + 1/2 02 K 2504 Potassium sulfate has a significantly lower solubility in water than potassium sulfite and has a low solubility in potassium carbonate solutions Accordingly, potassium sulfate may readily be removed from the product aqueous media by crystallization Normally, the 50 discharge 218 from sulfite oxidizer 215 is a slurry because of substantial water removal and low solubility of potassium sulfate in product aqueous media The slurry in line 218 may then be cooled and potassium sulfate removed in crystallizer-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 55 Product aqueous medium 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 medium is recycled to zone 202 for scrubbing via lines 222, 207 and 208 Some of this may be bled, however, via lines 223 and 224, respectively, to oxidizer 215, to line 258, or both Additional potassium hydroxide is added to the recycling 60 product aqueous medium 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 removed Clean gas emerges via line 205 Unlike the embodiment disclosed in Figure 1, 65 1 593 745 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 removal and recovery of lower valence nitrogen oxides Crystallization and recovery of potassium nitrate is simpler than in Figure 1 because the product aqueous media includes only nitrate, nitrite, carbonate and bicarbonate, but little sulfate Potassium makeup 5 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.

Product aqueous medium comprising unconsumed hydrogen peroxide, potassium nitrate and potassium nitrite, potassium carbonate and potassium bicarbonate, passes from zone 204 via line 251 and is recycled to scrubber 204 via lines 252 and 253, until the concentration 10 of nitrite, nitrate or both reaches a predetermined minimum Thereafter, at least some of the product aqueous medium in line 251 enters the product recovery cycle via line 254.

Preferably, the product aqueous media to be subjected 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 p H to less than 15 about 9, and facilitating oxidation of nitrite to nitrate 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 substantial portion of bicarbonate is converted to carbonate and carbon dioxide by thermal decomposition Carbon dioxide and water pass overhead via line 209 and the 20 carbon dioxide may be recovered and used elsewhere, or may be fed into line 245 to convert potassium carbonate to bicarbonate 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 medium.

Oxidation of nitrite to nitrate proceeds faster when carbonic acid is present and carbonate is 25 absent.

The product aqueous medium leaving the decarbonation tower 257 via line 258 contains primarily potassium nitrate and nitrite and potassium carbonate, and is cooled in crystallizer 260 to around 10 MC Thereupon, potassium nitrate crystallizes and is removed via line 261 following centrifuging Potassium carbonate and potassium nitrite remain in the 30 product aqueous medium which recycles via lines 262 and 253 to zone 204 following addition thereto of oxidant hydrogen peroxide via lines 245 and 253 Some of the recycling aqueous medium 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 35 via line 235 and air, via line 242 Preferably, hydrogen is supplied in whole or in part from 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 electrolytically converted to chlorine at anode 233 and to potassium hydroxide 40 and hydrogen at cathode 234 Chlorine passes from the cell via line 236 and hydrogen via line 235 Cationic membrane 232 is used to produce a chloride-free potassium hydroxide product which passes from cell 231 via line 237.

The process for oxidizing nitrite to nitrate which includes the step of first reducing the p H of the aqueous medium to below 9 forms the subject of patent Application No 15350/80, 45 (Serial No 1593743) which is also divided from application No 34033/77 (Serial No.

1593742).

The following Example illustrates the invention.

Example 1 50

A gas mixture comprising 800 parts per million of sulfur dioxide and 99 92 % nitrogen was passed in countercurrent to an aqueous scrubbing solution including about 20 % potassium carbonate at a rate of about 2 0 foot per second through a 10 foot glass column having a 2 inch diameter and packed with 1/2 inch Pall type rings The p H of the aqueous medium was about 11 2; the 55 reaction temperature about 50 'C Efficiency of sulfur dioxide removal was higher than99 % Removal efficiency was then tested with an aqueous media including about 18 % potassium bicarbonate and about 6 %c potassium carbonate Removal efficiency was again higher than 99 % The p 1-I of the scrubber solution as 9 2, and the temperature was again 500 C 6 By contrast, removal of sulfur dioxide from the same gas stream with an aqueous solution containing 10 % sodium sulfite at 50 'C, in accordance with the WellmanLord 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 p H of the sodium sulfite scrubbing solution was 7 2 at the beginning of the test 65 1 593 745

Claims (1)

1. WHAT WE CLAIM IS:-

   1 A process for removing oxides of sulfur from a gas mixture including carbon dioxide and sulfur dioxide comprising:

   (a) treating said gas mixture in a zone with an aqueous medium which includes carbonate-bicarbonate of an alkali metal; 5 (b) maintaining said alkali metal carbonate-bicarbonate in stoichiometric excess for reaction with the sulfur dioxide; and (c) forming a product aqueous medium which includes alkali metal sulfite and, optionally alkali metal sulfate.

   2 A process according to Claim 1 further comprising recovering at least a portion of 10 said alkali metal sulfite from said product aqueous medium.

   3 A process according to Claim 2, wherein after said recovery, the product aqueous medium is recycled to said treating zone.

   4 A process according to Claim 1 or Claim 2 wherein the said product aqueous medium is recycled to the treating zone until the concentration of alkali metal sulfite in the product 15 aqueous medium reaches a predetermined minimum and the alkali metal sulfite is then recovered.

   A process according to any one of Claims 1 to 4 wherein the alkali metal is potassium.

   6 A process according to any one of Claims 1 to 5 wherein the product aqueous 20 medium is treated to convert said alkali metal bicarbonate therein to carbon dioxide and alkali metal carbonate.

   7 A process according to Claim 6 wherein the carbon dioxide is recovered.

   8 A process according to any one of Claims 1 to 7 wherein the alkali metal sulfite is oxidized to alkali metal sulfate 25 9 A process according to Claim 8 wherein the oxidant is an oxygencontaining gas or hydrogen peroxide.

   A process according to Claim 8 or Claim 9, wherein the oxidation and the conversion of alkali metal bicarbonate to carbon dioxide is carried out simultaneously.

   11 A process according to any one of Claims 1 to 10 wherein alkali metal sulfate is 30 recovered from said product aqueous medium.

   12 A process according to any one of Claims 1 to 11 wherein the alkali metal is provided as an alkali metal hydroxide and is made by electrolysis of an alkali metal chloride.

   13 A process according to Claim 12 wherein the electrolysis of the alkali metal chloride 35 also produces chlorine.

   14 A process according to Claim 12 wherein hydrogen derived from the electrolysis of said alkali metal chloride is used to generate hydrogen peroxide.

   A process according to Claim 14 wherein the hydrogen peroxide is made by the autooxidation of an alkylanthaquinone in organic medium 40 16 A process according to any one of Claims 1 to 15 wherein the gas mixture also includes oxides of nitrogen and after removal of the oxides of sulfur, the oxides of nitrogen are removed by:

   (a) treating said gas mixture in a second zone with an aqueous medium and an oxidant, said aqueous medium including carbonate-bicarbonate of an alkali metal; 45 (b) maintaining said alkali metal carbonate-bicarbonate in stoichiometric excess over that required to convert the nitrogen oxides to alkali metal salts of nitrogen oxides; and (c) forming a product aqueous medium which includes at least one alkali metal salt from the group consisting of alkali metal nitrite and alkali metal nitrate.

   17 A process according to Claim 16 wherein recovering said at least one alkali metal 50 salt from said product aqueous medium.

   18 A process according to Claim 1, conducted substantially as described herein with reference to and as shown in the drawings.

   REDDIE & GROSE, 55 Agents for the Applicants, 16 Theobalds Road, London, WC 1 X 8 PL.

   Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon Surrey 1981.

   Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A IAY, from which copies may be obtained.