GB2046231A - Method for removal of sulphur dioxide from exhaust gas - Google Patents

Abstract

The method comprises treating the exhaust gas with an aqueous solution containing an alkali metal or alkaline earth metal or ammonium salt of an organic acid and also solid crystalline gypsum so as to effect absorption of SO2 into solution, whereafter the absorbed sulphur dioxide is recovered from the solution in the form of gypsum by passing oxygen therethrough in the presence of sufficient calcium compound to maintain the pH of the treated solution in the pH range of 5-9. The method inhibits the formation of a dithionate by-product which can cause a reduction in the efficiency of the process.

Description

SPECIFICATION Method for removal of sulphur dioxide from sulphur dioxide containing exhaust gas This invention relates to a method for the removal of sulphur dioxide from a sulphur dioxidecontaining gas e.g. an exhaust gas, by treating the exhaust gas with an aqueous solution containing an alkali 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 salt of organic acid. According to U.S.

Patent No. 3,928,537, the removal of sulphur dioxide in the form of gypsum from the exhaust gas is carried out by contacting the gas with an aqueous solution containing an alkali salt of organic acid to induce absorption of the sulphur dioxide in the form of alkali sulphite and alkali sulphate, blowing oxygen or air into the aqueous solution containing the absorbed sulphur dioxide, thereby oxidizing the alkali sulphite into the corresponding alkali sulphate, adding thereto a calcium compound such as calcium carbonate or calcium hydroxide, thereby converting the alkali 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) through (4).

2RCOOM + SO, + H20 > 2RCOOH + M2SO3 (1) M2503 + +02#M2SO4 (2) 2RCOOH + CaCO3~(RCO0)2Ca + CO2 + H20 (3) M25O4 + (RC0O)2Ca#CaS04 + 2RCOOM (4) In the preceding formulas, RCOOM represents an alkali 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 salt of organic acid and solid crystalline gypsum in place of the aqueous solution containing an alkali salt of organic acid along. By this method, the exhaust gas is treated with an aqueous solution containing 0.05 to 0.5 mol/liter of an alkali 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 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 sulfite produced in the aqueous solution into a corresponding alkali sulfate and thereafter converting this alkali sulfate, through its reaction with a calcium compound, into gypsum because the oxidation of alkali 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 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 of 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 16 C) and calcium dithionate a solubility of 28.9% by weight (at 19 C), respectively. The dithionate is by-produced when the sulphite is oxidized 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 SO, 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 origininates 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 regions 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 be 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 and containing solid crystalline gypsum, so as to effect absorption of SO2 into said solution; b) passing oxygen or an oxygen containing gas mixture through the aqueous solution resulting from step a), in thej 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 salts include alkali metal salts (for example, sodium salts and potassium salts), alkaline earth metal salts (for example, magnesium salts) and ammonium 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, adipic acid and succinic acid, sulpho-substitution products and oxy-substitution products thereof. The concentration of such alkali salt of organic acid in the aqueous solution with which the exhaust 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 mol/Jiter. 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 precluding 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 oxygen-containing 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 of 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 occurence 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 sulphate-containing absorption solution (aqueous solution which has absorbed th SO2) varies with the concentration of the alkali salt of organic acid in the aqueous solution used for the absorption of the SO2, the concentration of the alkali sulphite formed, etc.Under ordinary operation 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 of organic acid, 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, unless 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 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 sulphite contains alkali sulphate and gives rise to gypsum in the form of precipitate when the calcium compound is added thereto and when the oxidation of the alkali 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 the alkali salt of organic acid as represented by the aforementioned formula (4) and, therefore, can be put to cyclic use for contact with the incoming S02-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: Figure 1 represents a process flow diagram illustrating one preferred embodiment of this invention; and Figure 2 represents other process flow diagram illustrating another preferred embodiment of this invention.

In the flow diagram of Fig. 1, a SO2-containing exhaust gas 1 subjected to treatment is fed into an absorption tower 2, brough 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 502. In consequence of the contact with the exhaust gas, the solution 3 contains the alkali sulfite resulting from the absorption of the SO2, the alkali sulfate occurring as the oxidation product of the alkali 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 of 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 ml/liter of an alkali 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 sulfite produced by the absorption of sulfur dioxide, the alkali sulfate occurring as the oxidation product thereof, etc. This absorption solution is fed into a tank 14, in which calcium hydroxide or calcium carbonate 15 is added thereto to have the pH value of the absorption solution adjusted 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 sulfite into the corresponding alkali sulfate. A part of the formed alkali 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 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 sulfite and the subsequent conversion into gypsum of the alkali 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 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 cited herein below. It should be noted, however, that the scope of the present invention is not limited to these examples.

Example 1: A boiler exhaust gas containing 1200 ppm of SO, was fed at a flow rate of 100 Nm3/hr into contact with 220 liters/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 liters/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 by-produced 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 Comparison 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 SO2absorbed aqueous solution fed at a rate of 220 liters/hr, 250 g/hr of calcium hydroxide was added to have the pH value of the solution adjusted 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 Comparison Example 1 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 gypsum-containing solution, gypsum was separated by filteration. The resultant filtrate and the remaining part of the gypsum-containing solution diluted with added water to 220 liters/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 by-produced in the oxidation of sodium sulfite.

Comparison Example 1: A boiler exhaust gas containing 1200 ppm of SO, was fed at a flow rate of 100 Nm3/hr into contact with 220 liters/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 liters/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 a by-produced dithionate per mol of the sodium sulfite oxidized.

Example 3: A boiler exhaust gas containing 1200 ppm of SO, was fed at a flow rate of 100 Nm3/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 liters/hr, 420 g/hr of calcium hydroxide (purity 95 %) was added and, at the same time, 2 m3/liter 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 liters/hr) of the gypsum-containing solution, 940 g/hr (purity 98 %) of gypsum was separated by filtration. The resultant filtrate (about 17 liters/hr) and the remaining part of the gypsum-containing solution diluted with added water to 220 liters/hr were recirculated to the step of absorption. In this manner, the operation was containued 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 SO, absorption was first oxidized and the oxidation product was then converted into gypsum as shown in the following comparison example 2.

Comparison Example 2: By following the procedure of Example 3, the same boiler exhaust gas containing 1200 ppm of SO, 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 SO, absorption, the aqueous solution had a pH value of 4.9. To the solution fed at a rate of 220 liters/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 liters/hr) of the gypsum-containing solution, 940 g/hr of gypsum (purity 98 %) was separated. After the manner of Example 3, about 17 liters/hr of the resultant filtrate and the remaining part of the gypsum-containing solution diluted with added water to 220 liters/hr were recirculated to the step of absorption to continue the absorption of SO2. In ten hours of this operation, the concentration of by-produced dithionate in the solution rose to 870 ppm.

Example 4: A boiler exhaust gas containing 1200 ppm of 802 was fed at a flow rate of 100 Nm3/hr into contact with 220 liters/hr of an aqueous solution containinfg 1.7 % by weight of sodium sulfosuccinate, 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 SO, by the aqueous solution.

After the SO2 absorption, the solution contained 1.1 % by weight of sodium sulfosuccinate, 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 liters/hr, 210 g/hr of calcium hydroxide was added to adjust the pH value of the solution to 3.5 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 by-produced dithionate per mol of the sodium sulfite oxidized.

CLAIMS (24 Nov 1978) 1. 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 and containing solid crystalline gypsum, so as to effect absorption of SO2 into said solution; b) passing oxygen or an 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 oxygen-containing gas

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

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. Example 3: A boiler exhaust gas containing 1200 ppm of SO, was fed at a flow rate of 100 Nm3/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 liters/hr, 420 g/hr of calcium hydroxide (purity 95 %) was added and, at the same time, 2 m3/liter 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 liters/hr) of the gypsum-containing solution, 940 g/hr (purity 98 %) of gypsum was separated by filtration. The resultant filtrate (about 17 liters/hr) and the remaining part of the gypsum-containing solution diluted with added water to 220 liters/hr were recirculated to the step of absorption. In this manner, the operation was containued 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 SO, absorption was first oxidized and the oxidation product was then converted into gypsum as shown in the following comparison example 2. Comparison Example 2: By following the procedure of Example 3, the same boiler exhaust gas containing 1200 ppm of SO, 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 SO, absorption, the aqueous solution had a pH value of 4.9. To the solution fed at a rate of 220 liters/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 liters/hr) of the gypsum-containing solution, 940 g/hr of gypsum (purity 98 %) was separated. After the manner of Example 3, about 17 liters/hr of the resultant filtrate and the remaining part of the gypsum-containing solution diluted with added water to 220 liters/hr were recirculated to the step of absorption to continue the absorption of SO2. In ten hours of this operation, the concentration of by-produced dithionate in the solution rose to 870 ppm. Example 4: A boiler exhaust gas containing 1200 ppm of 802 was fed at a flow rate of 100 Nm3/hr into contact with 220 liters/hr of an aqueous solution containinfg 1.7 % by weight of sodium sulfosuccinate, 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 SO, by the aqueous solution. After the SO2 absorption, the solution contained 1.1 % by weight of sodium sulfosuccinate, 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 liters/hr, 210 g/hr of calcium hydroxide was added to adjust the pH value of the solution to 3.5 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 by-produced dithionate per mol of the sodium sulfite oxidized. CLAIMS (24 Nov 1978)

1. 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 and containing solid crystalline gypsum, so as to effect absorption of SO2 into said solution; b) passing oxygen or an 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 oxygen-containing 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 process 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 process according to Claim 6, wherein the said succinic acid salt is present in an amount of from 0.05 to 0.3 mol/liter.

7. A process 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 process 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 in accordance with Claim 1, substantially as described herein, with reference to Fig. 1 or Fig. 2.

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

AMENDED CLAIMS

1. 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 succinic 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 aqeuous 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.

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