GB1598167A

GB1598167A - Method for removal of sulphur dioxide from sulphur dioxide containing gas

Info

Publication number: GB1598167A
Application number: GB2204178A
Authority: GB
Original assignee: Kureha Corp
Current assignee: Kureha Corp
Priority date: 1977-06-06
Filing date: 1978-05-24
Publication date: 1981-09-16
Also published as: FR2393601A1; DE2824247A1; FR2393601B1; DE2824247B2; DE2824247C3

Description

(54) METHOD FOR REMOVAL OF SULPHUR DIOXIDE FROM SULPHUR DIOXIDE CONTAINING GAS (71) We, KUREHA KAGAKU KOGYO KABUSHIKI KAISHA, a company organized under the laws of Japan, of No. 8, Horidome-cho l-chome, Nihonbashi, Chuo-ku, Tokyo, Japan, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a method for the removal of sulphur dioxide from a sulphur dioxide-containing gas e.g. an exhaust gas, by treating the gas with an aqueous solution containing an alkali metal, alkaline earth metal or ammonium salt of organic acid and solid crystalline gypsum and recovering the absorbed sulphur dioxide in the form of gypsum.

U. S. Patent No. 3,928,537 discloses a method for the removal of sulphur dioxide in the form of gypsum from an exhaust gas such as a combustion exhaust gas by the treatment of the exhaust gas with an aqueous solution containing an alkali metal or ammonium salt of organic acid.

According to U. S. Patent No. 3,928,537, the removal of sulphur dioxide is carried out by contacting the gas with an aqueous solution containing an alkali metal or ammonium salt of organic acid to induce absorption of the sulphur dioxide in the form of alkali metal or ammonium sulphite and alkali metal or ammonium sulphate, blowing oxygen or air into the aqueous solution containing the absorbed sulphur dioxide, thereby oxidizing the alkali metal sulphite into the corresponding alkali metal or ammonium sulphate, adding thereto a calcium compound such as calcium carbonate or calcium hydroxide, thereby converting the alkali metal or ammonium sulphate into calcium sulphate (gypsum) and separating the calcium sulphate, and recirculating the solution for contact with the incoming exhaust gas. The reaction mechanism involved in this method is expressed by the following reaction formulas (1) to (4).

2RCOOM+SO2+H20e 2RCOOH+M2SO3, (1) M2SO3+iO2oM2SO4 (2) 2RCOOH+ CaCO3(RCOO)2Ca+COz+ HzO (3) M2SO4+ (RCOO)2CaCaSO4+ 2RCOOM (4) In the preceding formulas, RCOOM represents an alkali metal or ammonium salt of organic acid, RCOO an organic acid group and M an alkali metal or NH4, respectively.

In the treatment of the exhaust gas by the method described above, there is a possibility that the dissolved gypsum present in the solution being circulated through the system will deposit itself on and adhere to the inner wall of the unit (for example, an absorption tower) to induce the phenomenon of "scaling". To overcome the possibility, there has recently been proposed (by Offenlegungsschrift 26 27 705, for example) a method which resorts to use of an aqueous solution containing an alkali metal salt of organic acid and solid crystalline gypsum in place of the aqueous solution containing an alkali salt of organic acid alone. By this method, the exhaust gas is treated with an aqueous solution containing 0.05 to 0.5 mol/liter of an alkali metal salt of organic acid and 0.3 to 10% by weight of solid crystalline gypsum, in the same manner as by the method disclosed in U. S. Patent No. 3,928,537. Although the conversion into gypsum of the sulfur dioxide which has been absorbed in the aqueous solution could be effected by a different procedure that involves first converting the alkali metal sulfite produced in the aqueous solution into calcium sulfite through its reaction with a calcium compound and subsequently oxidizing the calcium sulfite into gypsum, it is effected in this method by the aforementioned procedure that involves first oxidizing the alkali metal sulfite produced in the aqueous solution into a corresponding alkali metal sulfate and thereafter converting this alkali metal sulfate, through its reaction with a calcium compound, into gypsum because the oxidation of alkali metal sulfite proceeds faster than that of calcium sulfite and additionally because this procedure is capable of preventing otherwise possible deposition of scale in the system.

The oxidation of alkali metal sulfite and the reaction for the production of gypsum mentioned above have heretofore been carried out in two separate reaction vessels.

Use of such two reaction vessels of different types has been inevitable because the oxidation of alkali sulfite is a gas-liquid contact reaction and the formation of gypsum is a solid-liquid contact reaction and, thus, they involve different reaction mechanisms. In the gas-liquid contact reaction, for example, the gaseous reactant is divided into very fine bubbles by use of a perforated-plate tower, whereas in the solid-liquid contact reaction, the reactants are stirred in a reaction system provided with agitation blades. The two reactions have thus been effectively carried out.

When the sulphur dioxide which has been absorbed in the aqueous solution is converted into gypsum by the method described above, however, there is inevitably by-produced a dithionate.

Incidentally an alkali metal salt or alkaline earth metal salt of dithionic acid has a high degree of solubility in water. For example, sodium dithionate has a solubility of 32.2% by weight (at 160C) and calcium dithionate a solubility of 28.9viz by weight (at 190C), respectively. The dithionate is by-produced when the sulphite is oxidied into a corresponding sulphate. The dithionate thus by-produced is a substance which is quite resistive to oxidation and exhibits fair stability in the process for the removal of SO2 from the exhaust gas. The by-produced dithionate accumulates gradually in the solution circulating within the reaction system and, consequently, a gradual decline in the concentration of the component which is effective in the removal of sulphur dioxide is experienced.

The present invention is predicated on our observation that, while SO2 absorbed in the aqueous solution is converted into gypsum, the iron ion which is present in a very small amount in the aqueous solution functions catalytically in the formation of dithionate and consequently promotes the production thereof. The presence of the iron ion in the aqueous solution is virtually inevitable because most of the iron ion originates in the iron present (of the order of from 100 to 1000 ppm, for example) in the calcium compound to be used in the formation of gypsum and in the exhaust gas under treatment. We have also ascertained that, during the conversion of the absorbed sulphur dioxide into gypsum the amount of dithionate by-produced in the aqueous solution varies significantly in the region of pH 5 irrespective of the presence of iron ions. Further studies have led us to believe that the by-production of the dithionate can advantageously by inhibited when the conversion into gypsum of the sulphur dioxide absorbed in the aqueous solution is carried out by blowing oxygen, air or some other oxygen-containing gas into the aqueous solution while the pH value of the aqueous solution is maintained in the range of 5 to 9.

According to the present invention we provide a method for the removal of sulphur dioxide from a sulphur dioxide-containing gas e.g. an exhaust gas, which comprises the steps of: a) contacting said gas with an aqueous solution of an alkali metal or alkaline earth metal or ammonium salt of an organic acid (referred to below sometimes as simply "alkali salt") containing solid crystalline gypsum, so as to effect absorption of SO2 into said solution; b) passing oxygen or a free oxygen containing gas mixture through the aqueous solution resulting from step a), in the presence of sufficient calcium compound to maintain the pH of the treated solution in the range of 5 to 9; c) recovering gypsum from the solution; d) returning the solution from step c) to step a).

Examples of suitable alkali metal salts (are, sodium salts and potassium salts), examples of suitable alkaline earth metal salts (are, magnesium salts) and examples of suitable ammonium salts are salts of monobasic acids and dibasic acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, iso valeric acid, malonic acid, fumaric acid and adipic acid, sulpho-substitution products and oxysubstitution products thereof. The concentration of such alkali salt in the aqueous solution with which the gas is brought into contact is not critical.

Although it may be used within the range of its particular solubility in water, it is preferably used in the range of from 0.05 to 0.5 mol/liter, more preferably from 0.05 to 0.3 molniter. In the aqueous solution, the solid crystalline gypsum is preferably contained in a concentration in the range of from 0.3 to 10% by weight. Such solid crystalline gypsum is contained in the aqueous solution in order to preclude deposition of dissolved gypsum, on the inner walls of the process plant, during the SO2 absorption step.

After absorption of the SO2 the absorbed SO2 is converted into gypsum by contacting with oxygen, air or some other free oxygencontaining gas mixture, which may e.g. be blown into the aqueous solution, while the pH value of the aqueous solution is adjusted to or maintained in the range of from 5 to 9 suitably by addition thereto of a calcium compound such as calcium carbonate or calcium hydroxide. The oxygen, air or the oxygen-containing gas mixture and the addition of the calcium compound may be carried out simultaneously or by first adding the calcium compound to adjust the pH value to the range of from 5 to 9 and subsequently effecting the blowing of oxygen, air or the oxygen-containing gas.

The addition of the calcium compound to the aqueous solution may be carried out either continuously or intermittently. When the conversion of the SO2 into gypsum is effected by the latter procedure which involves the oxidation subsequent to the adjustment of the pH value to 5 to 9, the pH value of the aqueous solution upon blowing of oxygen, air or an oxygen-containing gas is preferred to be in the range of from 5.5 to 7 in order to prevent possible occurrence of calcium sulphite scaling within the device.

The pH value at which calcium sulphite will crystallize out upon addition of the calcium compound to the alkali suphite-containing absorption solution (aqueous solution which has absorbed the SO2) varies with the concentration of the alkali salt in the aqueous solution used for the absorption of the SO2, the concentration of the sulphite formed, etc. Under ordinary operating conditions, for example, where 9 to 60 kg of SO2 is absorbed per m3 of the aqueous solution containing 0.1 to 0.2 mol/liter of an alkali salt, substantially no crystallization of calcium sulphite is observed when the calcium compound is added to the absorption solution which has had its pH value lowered below 5 providing the addition is continued for a period long enough for the pH value of the absorption solution to rise beyond the level of 5.5. At the time of the oxidation of the alkali metal, alkaline earth metal or ammonium suphite, therefore, it is preferable from the practical point of view to adjust the pH value of the absoption solution, by addition thereto of the calcium compound, to the highest possible extent which at least exceeds the level of 5.5 and permits substantially no precipitation of calcium sulphite. Since, in most cases, the precipitation of the calcium sulphite tends to occur more readily when the absorption solution has a pH value exceeding 7, the preferred upper limit to which the pH value of the absorption solution is raised by the addition of the calcium compound is 7. The solution which is subjected to the oxidation of the alkali metal, alkaline earth metal or ammonium sulphite contains alkali metal, alkaline earth metal or ammonium sulphate and givers rise to gypsum in the form of precipitate when the calcium compound is added thereto and when the oxidation of the sulphite proceeds.

Nevertheless, since the aqueous solution to be used in the present invention contains solid crystalline gypsum from the beginning, there is a reduced risk of deposition of the precipitated gypsum on the inner wall of the plant.

In the practice of the invention the gypsum which has been formed as described above is recovered by separation from the solution. The solution which remains after the separation of this gypsum is an aqueous solution containing alkali salt and, therefore, can be put to cyclic use for contact with the incoming SO2-containing exhaust gas. If, at this point, this solution does not contain the solid crystalline gypsum at a suitable concentration, further solid crystalline gypsum may be added. At the time the precipitated gypsum is recovered by separation from the aqueous solution, there may be adopted an alternative procedure of separating the precipitated gypsum from a part of the aqueous solution, combining the remaining part of the aqueous solution with the solution which remains after the gypsum separation and putting the combined solution to cyclic use for contact with the incoming SO2-containing exhaust gas.

Now, the present invention will be described more specifically with reference to the accompanying drawing, in which: Fig. 1 represents a process flow diagram illustrating one preferred embodiment of this invention; and Fig. 2 represents another process flow diagram illustrating another preferred embodiment of this invention.

In the flow diagram of Fig. 1, a SO2containing exhaust gas 1 subjected to treatment is fed into an absorption tower 2, brought into contact with an aqueous solution 3 containing an alkali salt of organic acid and solid crystalline gypsum and having a pH value in the range of from 5 to 9, whereby the solution absorbs the SO2.

In consequence of the contact with the exhaust gas, the solution 3 contains the alkali metal alkaline earth metal or ammonium sulfite resulting from the absorption of the SO2, the sulfate occurring as the oxidation product of the aforesaid sulfite, etc. This absorption solution is fed into an oxidation/gypsum production tank 4, in which a calcium compound such as calcium hydroxide or calcium carbonate 5 is continuously or intermittently added downwardly to have the pH value of the absorption solution adjusted to and maintained in the range of from 5 to 9 and air or oxygen is blown upwardly from the bottom of the tank 4 to have the alkali sulfite oxidized and converted into gypsum. In this case, possible occurrence of the dithionate is inhibited because the pH value of the absorption solution is kept above the level of 5 and possible precipitation of calcium sulfite is precluded because the pH value is kept below the level of 9. The gas being introduced for the oxidation and the formation of gypsum can be advantageously broken up into fine bubbles and effectively stirred by means of perforated plates which are adapted to break up the gas into fine bubbles and consequently impart a stirring motion to the formed bubbles or by meanS of stirring blades which are adapted to stir the gas and consequently break up the gas into fine bubbles. Particularly, combined use of these two devices is highly preferable for the purpose of ensuring sufficient breakup of the air into very fine bubbles and effective stirring of the gas in the absorption solution and consequently permitting the relevant reactions to proceed quickly. From a part of the absorption solution which has undergone the reactions in the oxidation/gypsum production tank 4, gypsum 8 owt an amount equivalent to that of the SO2 absorbed in the absorption tower 2 is separated by filtration with a filter 7. The resultant filtrate and the remaining part of the absorption solution emanating from the tank 4 are circulated back to the absorption tower 2 so as to be used as the solution 3 again.

With reference to the flow diagram of Fig. 2, a sulfur dioxide containing exhaust gas 11 subjected to treatment is fed into an absorption tower 12 and brought into contact with an aqueous solution 13 containing 0.05 to 0.5 mollliter of an alkali metal alkaline earth metal or ammonium salt of organic acid and 0.3 to 10 /" by weight solid crystalline gypsum and having a pH value in the range of from 5 to 9, whereby the aqueous solution absorbs the sulfur dioxide. In consequence of the contact with the exhaust gas, the solution now contains the alkali metal alkaline earth metal or ammonium sulfite produced by the absorption of sulfur dioxide, the alkali metal alkaline eath metal or ammonium sulfate occurring as the oxidation product thereof.

etc. This absorption solution is fed into a tank 14, in which calcium hydroxide of calcium carbonate 15 is added thereto to adjust the pH value of the absorption solution to the range of from 5.5 to 7. The absorption solution is then fed into an oxidation tower 16, in which air or oxygen 17 is blown to effect oxidation of the alkali metal alkaline earth metal or ammonium sulfite into the corresponding alkali metal alkaline earth metal or ammonium sulfate.

A part of the formed sulfate forms gypsum.

The absorption solution which has undergone the reactions mentioned above is fed into a gypsum production tank 18, in which calcium hydroxide or calcium carbonate 19 is added so as to have the pH value of the absorption solution adjusted to and maintained in the range of from 5 to 9, with the formation of gypsum brought to completion. A part of the slurry containing the formed gypsum is circulated to the absorption tower 12. From the remaining part of the slurry, a considerable amount of gypsum 21 newly formed in consequence of the absorption of sulfur dioxide is separated by means of a filter 20. The resultant filtrate is circulated back to the absorption tower.

Since the solution intended for the oxidation of the alkali metal alkaline earth metal or ammonium sulfite has its pH value heightened by addition of calcium hydroxide or calcium carbonate and, thereafter, the oxidation itself is effected by having air or oxygen blown into the solution, the concentration of the iron dissolved in the solution is notably lowered and the possible by-production of dithionic acid is virtually inhibited in the course of the oxidation.

According to the present invention, therefore, the process involved is simplified and the cost of desulfurization is lowered as compared with the conventional process and, as demonstrated in the working examples cited herein below, the occurrence of dithionic acid is inhibited to a remarkable extent. By enabling the oxidation of the alkali metal alkaline earth metal or ammonium sulfite and the subsequent conversion into gypsum of the sulfate resulting from the oxidation, i.e. the two reactions which have heretofore been performed in two separate reaction vessels, to be carried out effectively in one and the same reaction tank, this invention, in preferred embodiments, successfully represses the formation of dithionic acid to 1/3 to 1/20 of the conventional level and simplifies the process involved.

Now, the present invention will be described more specifically with reference to working examples described below.

Example 1: A boiler exhaust gas containing 1200 ppm of SO was fed at a flow rate of 100 Nm /hr into contact with 220 litres/hr of an aqueous solution containing 1.2% by weight of sodium acetate, 1.0% by weight of sodium sulfate and 5% by weight of solid crystalline gypsum and having a pH value of 7 to effect absorption of SO2 by the aqueous solution.

After the SO2 absorption, the solution contained 0.8% by weight of sodium acetate, 1.1% by weight of sodium sulfate, 0.2% by weight of sodium sulfite and 5% by weight of solid crystalline gypsum and had a pH value of 4.9. To the solution fed at a rate of 220 litres/hr, 210 g/hr of calcium hydroxide was added to adjust the pH value of the solution to 5.5 and 2 Nm3/hr of air was blown in to effect oxidation of sodium sulfite. After the oxidation, the solution had a pH value of 5.1, contained 3 to 4 ppm of dissolved iron ion and about 0.003 mol of a byproduced dithionate per mol of the sodium sulfite oxidized. The amount of the dithionate thus produced is noted to be about one third of the amount of the dithionate which occurred where the adjustment of pH value was omitted as in Comparitive Example 1 cited below.

Example 2: Under the same conditions as those of Example 1, the exhaust gas was fed into contact with the aqueous solution to effect absorption of the SO2 by the aqueous solution. To the SO,-absorbed aqueous solution fed at a rate of 220 litres/hr, 250 g/hr of calcium hydroxide was added to adjust the pH of the solution to 6.0 and then 2 Nm3/hr of air was blown in to effect oxidation of sodium sulfite. After the oxidation, the solution had a pH value of 5.6, contained 1 to 2 ppm of dissolved iron ion and about 0.0015 mol of a by-produced dithionate per mol of the sodium sulfite oxidized. The amount of the dithionate thus produced is noted to be about one seventh of that of the dithionate which occurred where the adjustment of pH value was omitted as in Comparitive Example I cited below.

To the oxidized solution, 190 g/hr of calcium hydroxide was added to convert the remaining sodium sulfate into gypsum.

From a part (18 liters/hr) of the gypsumcontaining solution, gypsum was separated by filtration. The resultant filtrate and the remaining part of the gypsum-containing solution diluted with added water to 220 litres/hr were recirculated and used for the absorption of SO2. In this manner, the operation was continued for 100 hours.

During this period, no change was observed in the amount of the dithionic acid byproduct in the oxidation of sodium sulfite.

Comparitive Example 1: A boiler exhaust gas containing 1200 ppm of SO2 was fed at a flow rate of 100 Nm /hr into contact with 220 litres/hr of an aqueous solution containing 1.2% by weight of sodium acetate, 1.0% by weight of sodium sulfate and 5% by weight of solid crystalline gypsum and having a pH value of 7, to effect absorption of SO2 by the aqueous solution.

After the absorption of SO2, the solution contained 0.8% by weight of sodium acetate, 1.1% by weight of sodium sulfate, 0.2, by weight of sodium sulfite and 5% by weight of solid crystalline gypsum and had a pH value of 4.9. To the solution fed at a rate of 220 litres/hr, 2 Nm3/hr of air was blown in to effect oxidation of sodium sulfite. After this oxidation, the solution had a pH value of 4.4. This solution contained 13 to 15 ppm of dissolved iron ion and about 0.01 mol of dithionate by-product per mol of the sodium sulfite oxidized.

Example 3: A boiler exhaust gas containing 1200 ppm of SO2 was fed at a flow rate of 100 Nm /hr into contact with an aqueous solution containing 1.2% by weight of sodium acetate, 1.0% by weight of sodium sulfate and 5% by weight of solid crystalline gypsum and having a pH value of 7 to effect absorption of SO2 by the aqueous solution.

After the absorption of SO2, the solution contained 0.8% by weight of sodium acetate, 1.1% by weight of sodium sulfate, 0.2% by weight of sodium sulfite and 5% by weight of solid crystalline gypsum and had a pH value of 4.9.

To this solution fed at a rate of 220 litres/hr, 420 g/hr of calcium hydroxide (purity 95%) was added and, at the same time, 2 m3/litre of air was blown in, to effect oxidation of sodium sulfite and immediate conversion of the oxidation product into gypsum. After the reactions, the solution had a pH value of 7.0. From a part (18 litres/hr) of the gypsum-containing solution, 940 g/hr (purity 98%) of gypsum was separated by filtration. The resultant filtrate (about 17 litres/hr) and the remaining part of the gypsum-containing solution diluted with added water to 220 litres/hr were recirculated to the step of absorption. In this manner, the operation was continued for a period of about 100 hours. In the filtrates resulting from the separation of gypsum which were obtained 24 hours and 72 hours respectively after the start of this continued operation, the concentrations of by-produced dithionic acid were found to be 230 ppm and 690 ppm. These amounts of the dithionic acid were noted to be about one twentieth of the amounts involved where the sodium sulfite formed in consequence of SO2 absorption was first oxidized and the oxidation product was then converted into gypsum as shown in the following comparitive Example 2.

Comparitive Example 2: By following the procedure of Example 3, the same boiler exhaust gas containing 1200 ppm of SO2 was fed at a flow rate of 100 Nm3/hr into contact with an aqueous solution of the same composition as that used in Example 3, to effect absorption of SO2 by the solution.

After the SO2 absorption, the aqueous solution had a pH value of 4.9. To the solution fed at a rate of 220 litres/hr, 2 m3/hr of air was first blown in to effect oxidation of the sodium sulfite present in the solution into sodium sulfate. Consequently, the solution had a pH value of 4.4. To this solution, 420 g/hr of calcium hydroxide was added to induce production of gypsum.

From a part (18 litres/hr) of the gypsumcontaining solution, 940 g/hr of gypsum (purity 98%) was separated. After the manner of Example 3, about 17 litres/hr of the resultant filtrate and the remaining part of the gypsum-containing solution diluted with added water to 220 litres/hr were recirculated to the absorption step to continue the absorption of SO2. In ten hours of this operation, the concentration of dithionate by-product in the solution rose to 870 ppm.

WHAT WE CLAIM IS: 1. A method Tor the removal of sulphur dioxide from a sulphur dioxide-containing gas which comprises the steps of: a) contacting said gas with an aqueous solution of an alkali metal or alkaline earth metal or ammonium salt of an organic acid containing solid crystalline gypsum, so as to effect absorption of SO2 into said solution; b) passing oxygen or a free oxygen containing gas mixture through the aqueous solution resulting from step a), in the presence of sufficient calcium compound to maintain the pH of the treated solution in the range of 5 to 9; c) recovering gypsum from the solution; d) returning the solution from step c) to step a).

2. A method in accordance with Claim 1, wherein the pH of said treated aqueous solution in step b) is maintained in the range of from 5.5 to 7.

3. A method in accordance with Claim 1 or 2, wherein the oxygen, or free oxygencontaining gas mixture is blown into said aqueous solution simultaneously with the addition of the calcium compound.

4. A method in accordance with Claim 1 or 2, wherein the calcium compound is added to said aqueous solution before said solution has the oxygen or gas mixture passed therethrough.

5. A method in accordance with any one of the preceding Claims, wherein the calcium compound is calcium carbonate or calcium hydroxide.

6. A method according to any one of the preceding Claims, wherein the salt of an organic acid is present in the solution in an amount of from 0.05 to 0.5 mol/liter.

7. A method according to Claim 6, wherein the said salt of an organic acid is present in an amount of from 0.05 to 0.3 mol/liter.

8. A method according to any one of the preceding Claims, wherein the solid crystalline gypsum is present in the aqueous solution in an amount of 0.3 to 10% by weight.

9. A method according to any one of the preceding claims, wherein the treated gas in an exhaust gas.

10. A method in accordance with Claim I, substantially as described herein, with reference to Figure I or Figure 2.

11. A method in accordance with Claim 1, and substantially as described with reference to any one of the Examples herein.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. this manner, the operation was continued for a period of about 100 hours. In the filtrates resulting from the separation of gypsum which were obtained 24 hours and 72 hours respectively after the start of this continued operation, the concentrations of by-produced dithionic acid were found to be 230 ppm and 690 ppm. These amounts of the dithionic acid were noted to be about one twentieth of the amounts involved where the sodium sulfite formed in consequence of SO2 absorption was first oxidized and the oxidation product was then converted into gypsum as shown in the following comparitive Example 2. Comparitive Example 2: By following the procedure of Example 3, the same boiler exhaust gas containing 1200 ppm of SO2 was fed at a flow rate of 100 Nm3/hr into contact with an aqueous solution of the same composition as that used in Example 3, to effect absorption of SO2 by the solution. After the SO2 absorption, the aqueous solution had a pH value of 4.9. To the solution fed at a rate of 220 litres/hr, 2 m3/hr of air was first blown in to effect oxidation of the sodium sulfite present in the solution into sodium sulfate. Consequently, the solution had a pH value of 4.4. To this solution, 420 g/hr of calcium hydroxide was added to induce production of gypsum. From a part (18 litres/hr) of the gypsumcontaining solution, 940 g/hr of gypsum (purity 98%) was separated. After the manner of Example 3, about 17 litres/hr of the resultant filtrate and the remaining part of the gypsum-containing solution diluted with added water to 220 litres/hr were recirculated to the absorption step to continue the absorption of SO2. In ten hours of this operation, the concentration of dithionate by-product in the solution rose to 870 ppm. WHAT WE CLAIM IS:

1. A method Tor the removal of sulphur dioxide from a sulphur dioxide-containing gas which comprises the steps of: a) contacting said gas with an aqueous solution of an alkali metal or alkaline earth metal or ammonium salt of an organic acid containing solid crystalline gypsum, so as to effect absorption of SO2 into said solution; b) passing oxygen or a free oxygen containing gas mixture through the aqueous solution resulting from step a), in the presence of sufficient calcium compound to maintain the pH of the treated solution in the range of 5 to 9; c) recovering gypsum from the solution; d) returning the solution from step c) to step a).