JPS6339284B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for removing sulfur dioxide from a gas stream, the process being carried out by contacting the gas stream with an aqueous sodium sulfite solution which absorbs the sulfur dioxide and provides a solution rich in sodium bisulfite, the resulting bisulfite The soda solution is treated to desorb the sulfur dioxide and regenerated into an absorption solution for reuse. Sodium sulfate is formed as a by-product in the absorption-degassing solution, which must be removed from the system without undue loss of valuable soda compounds. More particularly, the invention provides a method for preventing the loss of desirable and valuable soda compounds from a circulating sulfur dioxide removal system by removing undesirable sodium sulfate from an absorption-degassing solution as a solid containing large amounts of this salt. Concerning the method. Sulfur dioxide is an air pollutant produced by the oxidation of sulfur and sulfur-containing materials. Sulfur dioxide is present in sufficient quantities as a component of a variety of waste gases, such as smelter gases, chemical plant flue gases, and stack and furnace gases from coal- or oil-fired furnaces such as those used in power plants. ing. The concentration of sulfur dioxide in such gases is generally about, e.g.
0.001 to about 5 mol%, often about 0.5 mol%
Although at low concentrations (less than about 1% by weight), sulfur dioxide emissions are substantial, especially in industrial fields that process large amounts of sulfur-containing materials. For example, in a recent power plant with a capacity of 1350000kW, 1
It burns about 15,000 tons of coal per day.
Although the concentration of sulfur dioxide in the stack gas of such power plants is as low as 0.2 to 0.3 mol%, the total amount of sulfur dioxide produced is
Close to 1000 tons. Similarly, the use of other sulfur-containing fuels produces sufficient amounts of sulfur dioxide. Sulfur dioxide is removed from the sulfur dioxide-containing gas by treatment with an aqueous solution of sodium sulfite. The operating characteristics of an effective and economical system for sulfur dioxide removal are the efficiency of absorbing sulfur dioxide from the sulfur dioxide-containing gas, the efficiency of removing sulfur dioxide from the spent absorption solution, and the purity of the sulfur dioxide product; Furthermore, the loss of metal (soda) value is minimized. A sulfur dioxide-containing gas obtained, for example, by the combustion of sulfur-containing mineral products, such as fuel, yields bisulfite on contact with an aqueous solution of sodium sulfite, thereby reducing the sulfur dioxide content of the gas to approximately 0.2 mol. % sulfur dioxide, the concentration is substantially reduced to, for example, about 0.02 mole % or less. Removal rates of sulfur dioxide from gases often reach approximately 95% or more. The spent absorption solution can be heated to convert its bisulfite to sulfite and sulfur dioxide and produce a liquid or liquid-solid material that can be used as a source of absorption solution. The resulting sulfur dioxide is extracted and either cooled or compressed into a liquid product or sent as a gas to a sulfuric acid or sulfur plant. The regenerated absorption solution can be recycled to the absorption zone. For further information and examples of sulfur dioxide removal systems that can advantageously utilize the techniques disclosed herein, see U.S. Pat.
and No. 3790660. All of these patents are incorporated herein by reference. The sulfur dioxide-containing gas to be treated usually contains materials that facilitate oxidation, such as sulfur trioxide, oxygen, elemental iron, etc., and other materials present, especially when the sulfur dioxide-containing gas originates from the combustion of a fuel. The material also contains oxides of nitrogen. At least some of these materials promote the oxidation of sodium sulfite or bisulfite to sulfate. Sodium sulfate is an irrelevant material for the purpose of sulfur dioxide absorption-degassing treatment since it cannot regenerate sulfite during the degassing operation. Therefore, sulfate will accumulate in the sulfur dioxide absorption-degassing system. A portion of the absorption-degassing solution is withdrawn from the system in order to prevent excessive amounts of the inert sulfate from accumulating in the system. What is withdrawn is a portion of the spent absorption solution or the material obtained during degassing of the sulfur dioxide from the absorption solution. However, these extracted materials contain substantial amounts of sulfite or bisulfite along with their sulfate;
When the withdrawal is discarded, useful soda value is unduly consumed from the system, which must be replaced by addition of a suitable soluble soda compound. removing sulfate from this absorption-degassing system more selectively with respect to sulfite or bisulfite values;
It has then been proposed to return the sulfite or bisulfite to the absorption-degassing system. When the sodium sulfate is simply separated from the sulfite and bisulfite in the spent absorption solution by low temperature crystallization, a sulfate crystallized product containing a small amount of sodium sulfite is obtained. Although this method allows for a more selective separation of sulfate, it requires occasional cooling to lower crystallization temperatures and the need to reheat the crystallization mother liquor back to the absorption-degassing system. This results in unreasonable costs. Furthermore, the obtained crystals are in a hydrated state and require drying to facilitate handling for further processing or sale. Therefore, a large drying step is required to reduce not only the free moisture but also the hydrated moisture. In another method for concentrating sodium sulfate to allow for more economical separation of sodium sulfate from the sulfur dioxide removal system described above, the spent absorption solution discharge stream is contacted with a sulfur dioxide-containing gas. At least a portion of the sodium sulfite is converted to the more soluble bisulfite. This effluent stream is then cooled to the crystallization temperature and the sodium sulfate precipitates more selectively. This method has the disadvantage that it requires two separate treatments of the effluent stream, namely contact with sulfur dioxide and a subsequent crystallization treatment carried out at low temperatures, e.g. approximately 0-10°C, which is significantly more expensive. have. Treatment of the effluent with sulfur dioxide further produces sodium bisulfite, which is contained in the resulting stream and then added to the absorption-degassing system, thereby converting the bisulfite into sulfite. Further heating is required to convert it to salt and sulfur dioxide. Furthermore, in this operation, the sodium sulfate contains sodium sulfate hydrate, which, as mentioned above, requires relatively high drying costs if it is to be further handled or sold. GB 489745 describes a method for removing sulfur dioxide, which relies primarily on the use of ammonium sulphite-absorbing aqueous solutions, but also mentions the possibility of using alkali metal sulphites, And this system is designed to eliminate sulfates. In this method, the spent absorption solution is degassed of sulfur dioxide without salt precipitation. The resulting solution is substantially saturated with sulfate and relatively unsaturated with respect to sulfite and bisulfite. Removal of the water and cooling of the solution, for example from 110°C to 50-70°C, results in sulfate precipitation.
This type of operation is undesirable. This is because the system clearly relies on the liquid from the sulfur dioxide degassing zone being relatively unsaturated with respect to sulfite. This state of unsaturation means that a large amount of absorption solution must be provided to remove a given amount of sulfur dioxide. The present invention provides a highly advantageous method for reducing the amount of sodium sulfate in a sulfite-sodium bisulfite absorption-degassing system for removing sulfur dioxide from a gas stream. In the process of the present invention, the temperature need not be substantially below ambient and the sodium sulfate removed can be essentially non-hydrated or anhydrous. Thus, at least a portion of the aqueous sulfite-sodium bisulfite absorption-degassed solution containing a relatively large amount of sodium bisulfite is treated to remove water, thereby precipitating an amount of sodium sulfate-containing solids from the solution. be able to. The solid described above has a higher sulfate concentration (on a dry basis) than the absorption-degassing solution from which it was produced. Absorption-
Concentration by removal of water from the feed from the degassing system is continued for a sufficient period of time to ensure substantial precipitation of sodium sulfate, but the undissolved solids of the resulting slurry from the feed are approximately 10% by weight. % before it becomes substantially higher than that. The material preferably treated according to the invention is essentially a spent absorption solution. In one embodiment of the method of the invention, the removal of water and precipitation of solids is performed without substantial conversion of bisulfite to sulfite and sulfur dioxide, e.g., less than about 10% by weight of bisulfite. It can be done. The precipitated solids, including the sodium sulfate, can be easily removed from the slurry using conventional liquid-solid separation equipment to yield the sulfate salt, which is at least initially essentially unhydrated. The sulfate salt has a relatively high, preferably at least about 50% by weight, sodium sulfate content on a dry basis. The separated solids contain a large amount of sodium sulfate and a small amount, if any, of sodium sulfite; preferably these amounts are at least about 70% by weight of sodium sulfate and no more than about 30% by weight of sulfite, based on the total amount of the solids. It's soda. The concentration of sodium sulfate in the precipitated solid is often at least twice the sulfate concentration (on a dry basis) in the absorption-degassing solution from which the solid is produced by evaporation of water. In the process of the invention, water is removed to discharge the sodium sulfate and the aqueous absorption precipitates the sulfate.
Based on its total composition, the degassing solution contains a large amount of bisulfite, a small amount of sodium sulfate, and zero to a small amount of dissolved sodium sulfite. Preferably, the solution contains this amount of bisulfite at the beginning of water removal or reaches this concentration in the liquid phase during water removal. The treated liquid exiting the absorption-degassing system contains at least about 25% by weight total salt dissolved, preferably at least about 30% salt, but often this amount does not exceed about 50% by weight. . The sodium sulfate content of this solution usually does not exceed about 10% by weight;
And preferably this amount will often be no more than about 8% by weight. The sodium sulfate content in the solution is generally at least about 1% by weight, and about 3% by weight.
~7% by weight is preferred. The weight ratio of sodium sulfate to sodium sulfite in the liquid that is treated for limited precipitation of solids is usually at least about
0.1:1, preferably at least about 1:1 or so. Generally, the higher the sodium sulfate content in the solution to be treated, the higher the purity of the sodium sulfate obtained by the method of the present invention.
Increasing the amount of inert sodium sulfate material circulating through the absorption-degassing system is a negative factor since less percent active soda is present in a given volume of water in the system. As a result, a larger amount of circulating material is required to achieve a given sulfur dioxide absorption-degassing capacity. In many cases, the absorption-degassing solution to be treated contains about 0.1 to 10% by weight of dissolved sodium sulfite and about 15 to 40% by weight of bisulfite, based on the total ingredients including the sodium sulfate and water. There is. This medium may contain small amounts of other materials dissolved therein, such as sodium thiosulfate. The solution from the absorption-degassing system where the water is removed and the sodium sulfate is precipitated and removed is at least about 2:1, often at least about 3:1 molar at the beginning or during the process. Contains a ratio of sodium bisulfite and sodium sulfite. The amount of dissolved sodium bisulfite in the material to be treated versus the total active soda is "S/C"
It can also be expressed by, and this "S/C" is
Active sulfur per 100 moles of water, e.g. SO ₃ −−
and HSO ₃ − divided by the number of moles of active soda per 100 moles of water. Thus, pure bisulfite solution has an S/C value of 1 and pure sodium sulfite has an S/C value of 0.5. For example, sodium sulfate has no active sulfur or active base. The material from the absorption-degassing system treated according to the present invention preferably has an S/C value of about 0.85 to 0.97. In the present invention, sodium sulfate is used for its aqueous absorption.
The degassed solution, preferably at least a portion of the spent absorption solution, is removed from the system by treating it to evaporate a sufficient amount of water and precipitate a sufficient amount of solids from the solution. However, the evaporation of water is such that the resulting slurry from the absorption-degassing system feed solution contains up to about 10% by weight of crystals, often at least about 1% by weight, and preferably about 5% by weight of crystals.
It is carried out in such a way that it has up to % by weight of crystals. The water content of solutions from absorption-degassing systems undergoing evaporative treatment is often about 78 or more percent by weight.
%, preferably at least 10% by weight, and the slurry is generally sufficiently fluid to be easily pumped. This operation is advantageously carried out at a somewhat elevated temperature sufficient to precipitate essentially non-hydratable crystals without removing excess water. Generally such temperature is at least about 37~
38°C, and a more certain temperature sufficient to produce a non-hydrated product is at least 40°C.
The temperature suitable for desirably evaporating water is approximately 40℃ ~
120°C, preferably about 40-90°C. The choice of temperature will vary depending on the pressure used, and the pressure may be normal, reduced or elevated. Advantageously the pressure is about 7030.7~14061.4Kg/ ^m2 (10~20psi)
Yes, and preferably essential atmospheric pressure is used. The slurry obtained in the water removal operation is solidified.
Liquid separation yields a separate solid phase with a relatively high concentration of sulfate. This separation can be carried out without reducing the temperature of the slurry, which is often about 40-120<0>C, preferably about 40-90<0>C.
The liquid phase after separation, ie its mother liquor, is charged to the absorption-degassing system, preferably to its degassing zone. The separated solids may be dried if necessary and allowed to self-dry by allowing free water to be absorbed as water of hydration. The amount of solids produced in the water evaporation step of the process of the invention and then removed from the absorption-degassing system is sufficient to prevent excessive sodium sulfate buildup in the system. Preferably, the amount of sulfate removed is substantially equal to the amount of sulfate produced in the absorption-degassing system, taking into account the sodium sulfate that is otherwise removed from the system. Furthermore, the amount of solid production required will vary depending on the purity in the precipitated and separated solid phase as well as the total amount of absorption-degassing solution that is subjected to the sulfate removal process. Thus, if the entire absorption-degassed solution is subjected to the sulfate removal treatment, the proportion of solids formed may be lower than if only a portion of the solution is subjected to the sulfate removal treatment. Generally, the purity of the precipitated sulfate increases as the proportion of solids formed in solution decreases. In one embodiment of the invention, substantially all of the absorption-degassing solution is subjected to a water removal treatment and a sulfate precipitation treatment. On the other hand, only a portion of this solution may be so treated, and in such cases about 10 to 90% by weight of the total solution is often treated in this way, and even up to about 75% by weight. , for example about 20
~75% by weight is more often processed. Preferably, this amount is sufficient to produce a slurry from the absorbent-degassed feed with up to about 5% by weight solids precipitated and an adequate discharge of sodium sulfate.
The liquid separated in the sulfate removal operation, the mother liquor, usually contains a high concentration of bisulfite and is sent to a sulfur dioxide degassing stage. Depending on the amount of mother liquor that is recycled, it may be desirable to charge the mother liquor to other parts of the absorption-degassing system. Replenishing soda content in the form of a suitable water-soluble sodium compound such as soda carbonate or caustic soda may be added to the system of the invention to compensate for soda losses. This loss of soda also includes that trapped in the precipitated sodium sulfate solids. Advantageously, this addition is carried out to an absorption solution reduced in soda content, to which makeup water may also be added. In the sulfur dioxide degassing step of the process of the invention, the spent absorption solution is heated to convert sodium bisulfite into sodium sulfite, with the resultant vapor phase containing sulfur dioxide and water being produced as a by-product. The suitable temperature for this operation is about 40-110℃, preferably about 60℃
~95℃. Pressure is approximately 2109.2~14764.4Kg/ ^m2
(3-21psi), preferably about 5624.6-10546.1Kg/
^m2 (8-15 psi). This vapor phase may be processed to recover purer sulfur dioxide and produce sulfur, or may be used, processed, or disposed of in any other manner. Various methods can be used to degas sulfur dioxide, and numerous methods are known in the art. However, this degassing is preferably accompanied by the formation of an undissolved solid or crystalline phase at the same time as the degassing so that degassing can be accomplished with less heating. In such operations, the amount of undissolved solids in the degassing zone is generally at least about 15% by weight of the slurry being decomposed or sulfur dioxide degassed. US Patent No. 3790660
As described in the specification, the amount of such solids is particularly important when passing the slurry through the tubes of an articulating heat exchanger to supply heat to the degassing zone, in order to reduce difficulties arising from fouling of the tubes. When at least approx.
Advantageously, it is 25% by weight. Preferably, the amount of undissolved solids is about 30-50% by weight of the slurry being cracked. Also, if the amount of undissolved solids is high enough, the sodium sulfite content of the slurry is adequate as part of the total slurry that is mixed with water to dissolve the solids, and the resulting The solution can be used as an absorption solution for absorbing sulfur dioxide from the gas being treated in the absorption zone of an absorption-degassing system. Based on the total amount of salts present, this gas absorption solution usually consists of a major portion of sodium sulfite and a minor portion of sodium bisulfite and sodium sulfate.
This gas-absorbing solution often contains about 10 to 35 percent by weight of sodium sulfite, about 3 to 15 percent by weight of sodium bisulfite, and about 1 to 10 percent by weight of sodium sulfate, based on all ingredients, including water. are doing. The invention will be further explained with reference to the drawings. This figure is a schematic flow diagram of a method for using the present invention in an absorption-degassing system that uses sodium sulfite to remove and recover sulfur dioxide from flue gas.
Although equipment such as valves, pumps, heat exchangers, surge tanks, etc. are used in the industrial embodiment of the invention and in absorption-degassing systems, such equipment may be of conventional design and be prepared using well-known methods. It is not shown because it is used in Referring to the drawings, for example, about 0.05 to about 5 mol%
Sulfur dioxide-containing flue gas containing sulfur dioxide enters the absorption tank 10 by line 12 near the bottom of the tank. Water or other aqueous liquid flows with the flue gas and is sent to the bed of a column packed in the lower part of vessel 10 to wash the gas and remove suspended solids such as fly-ash and relatively high water solubility. Components such as sulfur trioxide, hydrogen chloride, etc. are removed from the flue gas. The flue gas flows upwardly through an absorption vessel 10 and liquid-gas contacting means such as sieve trays into a downwardly flowing gas absorption solution which is further supplied to vessel 10 by line 14. This gas absorption solution contains sodium sulfite as the essential sulfur dioxide absorption component. Absorption vessel 10 may use other types of liquid-vapor contact structures such as packing, bubble towers, alternating current rings, and disks. The gas absorbing solution in line 14 is often at a temperature of at least about 30°C, preferably at least about 40°C to about 110°C, and preferably up to about 70°C. The flow rate of the aqueous absorption solution through the absorption zone is such that the sulfur dioxide in the gas to be treated is such that a majority of the sulfur dioxide in the gas, for example about 95% or more, can be removed from the gas by reaction with the gas absorption solution. It can be adjusted according to the concentration and the concentration of sodium sulfite in the solution. The treated gas leaves the absorption vessel 10 via line 13. The used absorption solution is stored in absorption tank 10.
The gas is collected on a gas passage tray 17 inside and removed from the vessel 10 by line 16. The spent absorption solution is transferred by pump 18 and line 54 to heater 52 and then by line 56 to degassing tank 50. Some or all of this spent absorption solution is transferred via line 22 to line 5.
Extracted from 4. The stream in line 22 contains sodium sulfate as well as dissolved sodium sulfite and bisulfite and is sent to an evaporator or dehydrator 24 in accordance with the present invention. The evaporator 24 is heated by a steam coil 26 and the heat removes water from the spent absorption solution and precipitates a solid containing sodium sulfate. Water and sulfur dioxide in the vapor phase in evaporator 24 are removed via line 36. A part of the used absorption medium dehydrated in the evaporator 24 is extracted through a line 40 and sent to a crystal separator 42.
sent to. The crystal separator is selected from conventional processing equipment effective for separating liquids and solids such as filters including rotary filters, centrifuges, clarifiers and other precipitation equipment. The solids rich in sodium sulfate crystals and containing sodium sulfite can be removed by line 44, and the resulting liquid stream is passed through line 46, pump 48, line 4
9, line 54, heater 52 and line 56 to a degassing tank 50 where it is treated to degas the sulfur dioxide and regenerate the absorption solution. The degassing part of this system is described, for example, in US Pat. No. 3,790,660.
It can be operated in the manner shown in that specification, and is shown here as a single stage degassing tank for convenience, although a multistage system of two or more can also be used. The heated solution in line 56 is introduced into degassing tank 50 . The temperature, pressure, and residence time in the degassing tank are selected to provide the desired decomposition of the sodium bisulfite, the desired evaporation of sulfur dioxide and water, and the desired precipitation of the sodium sulfite-containing crystals as described above and in the above-mentioned patents. is maintained. A heat exchanger 5 is used to supply heat to the deaeration tank 50.
The circulating stream is heated in 8. To achieve heating in tank 50, the slurry in this tank is passed through line 60.
and pump 62, line 6
3 through the metal tubes of heat exchanger 58 and return to tank 50 by lines 64 and 56. Steam is introduced into heat exchanger 58 through line 66 as the primary energy source for the degassing zone. Condensate (water) from heat exchanger 58 is removed through line 68. Sulfur dioxide and water vapor from degassing tank 50 are removed by line 74. Deaeration tank 50
A portion of the slurry therein is sent by line 78 to dissolution tank 72 . As water is being removed from the absorption solution during degassing, conditioning water is supplied to tank 72 through line 80, for example from a conditioning tank (not shown). The solution produced in dissolution tank 72 is routed through line 82, pump 84 and line 1.
4 to the absorption tank 10. The conditioning soda ion is an aqueous solution of caustic soda or soda carbonate, which is added to line 82 through line 88. Other methods for precipitating a quantity of solids from an absorption-degassed solution can also be used in carrying out the process of the present invention. For example, this solution is incorporated by reference in U.S. Patent Application No. 647,516 (filed on January 8, 1976, Edgar E. Bailey, Norman
Dehydration can also be carried out during contact with a sulfur dioxide-containing gas charged in a dehydrator as described in E. Nicholson, John Frederick Flintsf and John Scarlet. Next, the present invention will be further explained with reference to Examples.
These examples do not limit the invention. EXAMPLE 1 This example provides the results of a batch evaporation test performed on the synthetic preparation of a spent absorption solution as obtained in the type of absorption operation illustrated. In this test, the spent absorption solution was placed under vacuum in a flask kept in a constant temperature bath. Evaporation was carried out at the temperatures listed in Table 1 and continued until the amounts of solids indicated in the table were formed in the spent absorption solution. The water that evaporated in each test was condensed and collected. The crystals separated from the mother liquor at the temperature of evaporation. The most relevant data obtained in this test are described in Table 1.

[Table] ** Oh my goodness The data in Table 1 shows that as the solids content of the evaporated mixture increases, the sulfate to sulfite ratio in the crystals decreases. Only in Tests 1 and 2, which had 1.1 and 4.8 weight percent solids in the evaporated material, respectively, did the dried crystalline product contain more than about 65 weight percent sulfate. Example 2 Another batch test was carried out in a similar manner, but using spent absorption solution obtained from industrial equipment operation. This device is a device for absorbing sulfur dioxide from gases in a manner of the type shown. In these tests, each spent absorption solution was evaporated with stirring in a stainless steel beaker at about 105° C. and atmospheric pressure until the amounts of solids described in Table 2 were obtained. This solid has a diameter of 15
cm, in a laboratory centrifuge with a height of 5 cm.
Separated at 2800 rpm. The centrifuge basket was covered with 150 mesh 316 stainless steel mesh. The separated crystals and mother liquor were weighed and analyzed.
The crystals formed in almost all cases could be easily centrifuged. The test results are listed in Table 2.

【table】

[Table] 2 = Calculated without mother liquor and assumed to be dry.
The experimental results of Example 1 were confirmed from the results of these large-scale tests. This data clearly showed that the sulfate content of the dry solids was high and that, in general, the sulfate content decreased as the amount of solids produced during evaporation increased. Example 3 In order to investigate the effect of the concentration of sulfate and sulfite in the spent absorption solution on the sulfate content of the separated crystalline product, a spent absorption solution, as obtained in the type of absorption operation illustrated, was prepared. A solution was prepared synthetically and a batch evaporation test was performed on this solution. In this test, the spent absorption solution was placed in a flask and kept in a constant temperature bath of the flask. Evaporation is carried out under vacuum at the temperatures listed in Table 3,
This was continued until the amount of solids indicated in the table was formed in the used spent absorption solution. The water that evaporated in each test was condensed and collected. The crystals were separated from the mother liquor at the temperature of evaporation. Although the concentration of sulfate in this spent absorption solution varied during the test, the percentage of salt in the solution remained relatively constant. The most pertinent data obtained from these tests are described in Table 3.

[Table] * Calculations were made assuming that the mother liquor was not included and that it was dry.
The data in Table 3 shows the effect of sulfate and sulfite concentrations in the spent absorption solution on the sulfate concentration in the separated crystalline product. These data indicate that an approximately 1% higher sulfate concentration in the spent absorption solution (at a constant sulfite concentration) increases the sulfate concentration in the dried crystalline product by approximately 5-10%. ing. Also, if the sulfite concentration in the spent absorption solution decreases by about 6% (at a constant sulfate concentration), the sulfate concentration in the dry crystal product will decrease by about 15-20%.
% increase. Various modifications and equivalents of the method of the invention will be apparent to those skilled in the art and may be made without departing from the spirit or scope of the invention.

[Brief explanation of the drawing]

The figure is a schematic flow diagram of a method of using the present invention in an absorption-degassing scheme using sodium sulfite to remove and recover sulfur dioxide from flue gas. 10... Absorption tank, 24... Evaporator, 42... Crystal separator, 50... Deaeration tank, 52... Heater, 5
8... Heat exchanger, 72... Melting tank.

Claims (1)

[Claims] 1 absorbing sulfur dioxide from a gas into an aqueous absorption solution of sodium sulfite to form the corresponding bisulfite; degassing the resulting bisulfite-containing absorption solution to form sulfur dioxide; and In a process for removing sulfur dioxide from gases which consists in regenerating and recycling the absorption solution and in the presence of sodium sulfate in the absorption-degassing system, a large amount of sodium sulfite, a small amount of sodium sulfite, and an aqueous absorption-degassed solution with at least a portion of the solution to precipitate up to 10% by weight of undissolved solids from said solution. removing sulfate from the absorption-degassing system by forming a slurry, separating a sulfate-containing precipitate and a bisulfite-containing solution from the slurry, and passing the solution through an absorption-degassing cycle; A method for removing sulfur dioxide, characterized by: