Method for Desulfurizing Exhaust Gas

(1) Field of the Invention

The present invention relates to a method for desulfurizing an exhaust gas in a wet lime/gypsum process exhaust gas desulfurizing apparatus, and to a method for treating an exhaust gas containing SO.sub.2 and HCl together.

(2) Description of the Prior Art

A wet lime/gypsum process exhaust gas desulfurizing apparatus for removing SO.sub.2 in an exhaust gas therefrom by the use of CaCO.sub.3 as an absorbent is most prevalent for the exhaust gas desulfurization of late, and this type of apparatus is described in detail in many publications. For the purpose of directly recovering gypsum as a by-product from an SO.sub.2 gas absorption tower, a method for blowing air into an absorbing solution is disclosed in Japanese Patent Publication No. 17,318/1975. Here, reference will be made to the conventional method in accordance with FIG. 2.

In FIG. 2, an exhaust gas inlet 2 is provided at an upper portion of a tower body 1, and an exhaust gas outlet 3 is provided at an lower portion thereof. In the tower 1 and under the exhaust gas outlet 3, there is an absorbing solution storage section 5, in which a stirrer 4 and an air introduction pipe 8 are disposed. A spray nozzle 7 for spraying a slurry containing Ca compounds is disposed at an upper portion of the tower 1, and is connected to the absorbing solution storage section 5 via a pipe provided with a circulating pump 6 in the middle thereof.

The exhaust gas containing SO.sub.2 is introduced into the tower 1 through the exhaust gas inlet 2, flows downward in the tower 1, and is discharged through the exhaust gas outlet 3. On the other hand, the slurry jetted through the spray nozzle 7 flows downward through a grid filler 9, while brought into contact with the exhaust gas. Then, in the absorbing storage section 5, the slurry is brought into contact with air fed through the air introduction pipe 8, while stirred by the stirrer 4, and it is delivered to the spray nozzle 7 by means of the circulating pump 6 again.

On the other hand, CaCO.sub.3 which is a kind of absorbent is fed through an absorbent feed pipe 11 in compliance with an amount of absorbed SO.sub.2, and thus the absorbing solution which has become acidic by the absorption of SO.sub.2 is thereby neutralized to calcium sulfite, which is further oxidized to calcium sulfate (gypsum). The thus formed calcium sulfate must be discharged from the system on the basis of material balance. Therefore, a part of the absorbing solution is drawn out from the absorbing solution storage section 5 through an absorbing solution draw pipe 12.

The slurry which has been sprayed through the spray nozzle 7 and which will be brought into contact with the exhaust gas contains calcium sulfate as the main component, and unreacted CaCO.sub.3 and some unoxidized calcium sulfite. By absorbing SO.sub.2, an acidic sulfite is produced in the slurry and the latter falls onto the surface 10 of the solution in the absorbing solution storage section 5.

The part of the absorbing solution drawn out through the absorbing solution draw pipe 12 is delivered to a filter device in order to recover the secondarily produced gypsum, but for the purpose of heightening the purity of the by-product gypsum, it is necessary to lower the concentrations of unreacted CaCO.sub.3 and unoxidized calcium sulfite.

However, in the conventional process, the one tank type absorbing solution storage section 5 is used, and while CaCO.sub.3 which is the alkaline absorbent is fed through the absorbing solution feed pipe 11, the air oxidation is simultaneously carried out. Therefore, such a process has the drawback that the unreacted CaCO.sub.3 and the unoxidized calcium sulfite will remain therein inevitably.

Thus, if it is attempted to decrease a feed of the absorbent CaCO.sub.3 and to thereby diminish an amount of the remaining CaCO.sub.3, a pH of the absorbing solution will lower and an SO.sub.2 absorbing performance will deteriorate inconveniently. Further, in order to reduce this inconvenience as much as possible, it has been tried to increase the volume of the absorbing solution storage section, but such a strategy cannot provide any sufficient effect in spite of a great economical loss.

With regard to oxidation reactions and dissolution reactions of calcium sulfite, intensive researches have been made, and on the basis of the grasp of their characteristics, the present invention has been completed by which the above-mentioned drawbacks of the conventional techniques can be solved.

Moreover, the present invention provides another method by which the above-mentioned disadvantages of the conventional ones can be overcome and by which there can be prevented a phenomenon of impeding the dissolution of CaCO.sub.3 due to CaCl.sub.2 formed by the absorption of HCl.

That is to say, earnest investigations have been performed about the oxidation reaction of calcium sulfite and the dissolution reaction of CaCO.sub.3, and as a result, it has been found that the dissolution reaction of CaCO.sub.3 is slow in the absorbing solution containing the sulfite; that if CaCO.sub.3 is added to the absorbing solution after the perfect oxidation for the formation of the sulfite, the dissolution of CaCO.sub.3 will be accelerated remarkably; and that if CaCl.sub.2 which has been formed by absorbing HCl is converted into a chloride other than calcium chloride, the dissolution of CaCO.sub.3 will be more accelerated, whereby the residual CaCO.sub.3 in the absorbing solution can be decreased outstandingly. In consequence, the present invention has now been achieved .

FIG. 1 shows an exhaust gas desulfurizing apparatus suitable for a practice of the present invention;

FIG. 2 shows a conventional exhaust gas desulfurizing apparatus;

FIG. 3 shows fundamental experimental data to support effects of the present invention;

FIG. 4 is a schematic view illustrating another embodiment of the present invention; and

FIG. 5 is a diagram showing fundamental experimental data to support the effects of the present invention.

The present invention provides a method for desulfurizing an exhaust gas which is characterized by comprising a gas absorbing section in which an absorbing solution absorbs SO.sub.2 in the exhaust gas in order to become an acid solution containing a sulfite; an oxidizing section in which the sulfite is oxidized; a neutralizing section in which the absorbing solution is neutralized by adding an SO.sub.2 absorbent; and a closed circulating circuit for delivering the solution drawn out from the gas absorbing section to the oxidizing section, delivering the solution drawn out from the oxidizing section to the neutralizing section, and delivering the solution drawn out from the neutralizing section to the gas absorbing section.

Further, the present invention provides a method for treating SO.sub.2 and HCl simultaneously which is characterized by comprising a gas absorbing section in which an absorbing solution absorbs SO.sub.2 and HCl to become an acid solution containing a sulfite and a chloride; an oxidizing section in which the sulfite is oxidized by blowing air into the absorbing solution; a neutralizing section in which the absorbing solution is neutralized by feeding at least one of CaCO.sub.3 and Ca(OH).sub.2 thereto which is a neutralizing agent; and a closed circulating circuit for delivering the solution drawn out from the gas absorbing section to the oxidizing section, delivering the solution drawn out from the oxidizing section to the neutralizing section, and delivering the solution drawn out from the neutralizing section to the gas absorbing section; gypsum being recovered as a by-product from the absorbing solution in the closed circulating circuit; a chloride being taken out in the form of an aqueous solution.

In the present invention, it is preferred to employ a means of blowing air into the absorbing solution in the oxidizing section, or into both the absorbing solutions in the oxidizing and neutralizing sections for the sake of the oxidation of the sulfite. In the present invention, as the absorbents for SO.sub.2, there are preferably together used at least one of CaCO.sub.3 and Ca(OH).sub.2 which is a base component, and at least one of a magnesium compound, a sodium compound or a manganese compound which is an additive to improve a reaction percentage of the base component and accelerate the oxidation of the sulfites.

In the present invention, it is preferable to adjust the concentration of CaCO.sub.3 in the absorbing solution in the neutralizing section to less than 2% by weight, and it is also preferable to draw out a part of the absorbing solution from the oxidizing section for the purpose of recovering gypsum which is the by-product.

Now, the present invention will be described in detail in reference to an embodiment shown in FIG. 1.

At the upper portion of a tower 1, an exhaust gas inlet 2 is provided, and at the lower portion thereof, an exhaust gas outlet 3 is provided. Under the exhaust gas outlet 3 and in the tower 1, there is an oxidizing section 20 for oxidizing a sulfite in an absorbing solution, in which a stirrer 4 and an air introduction pipe 8 are disposed.

A neutralizing section 22 which is a liquid chamber separated from the oxidizing section 20 by a partition 21 is disposed in the tower 1, and in the section 22, a stirrer 23 is provided.

In the present invention, an absorbent feed pipe 11 through which CaCO.sub.3 is added to the neutralizing section 22 is disposed therein, and an absorbing solution draw pipe 12 through which the absorbing solution mainly containing gypsum is drawn out is disposed in the oxidizing section 20. According to the above-mentioned constitution, the following characteristic effects of the present invention could be obtained:

(a) Since the CaCO.sub.3 absorbent is not fed to the oxidizing section 20, the acidic sulfite can perfectly be oxidized to a sulfate.

On the contrary, when the oxidization is carried out while the absorbent is being added as in the conventional process, the neutralization rate with alkali CaCO.sub.3 will be faster than the dissolution rate of oxygen, so that neutral calcium sulfite which will be difficult to oxidize will be formed.

Therefore, the conventional process has the drawback that unoxidized calcium sulfite remains.

It is well known that neutral calcium sulfite is hardly oxidized with oxygen. Usually, calcium sulfite would be oxidized with oxygen after its conversion into acidic calcium sulfite (which is also called calcium bisulfite) by adding an acid thereto, which fact indicates that in the conventional technique, the oxidation will be apt to be incomplete.

(b) In the neutralizing section 22, no sulfite is present, i.e., the sulfate into which all the sulfite has been converted is present. Thus, the reaction percentage of CaCO.sub.3 can be improved, and an amount of the remaining unreacted CaCO.sub.3 in the absorbing solution can be diminished remarkably.

CaCO.sub.3 is a compound having a small solubility, but it will dissolve in an acid solution in accordance with the following formula (1):

However, the rate of this dissolution reaction highly varies with whether an anion of the acid compound is a sulfuric ion or a sulfurous ion.

That is to say, the reactions of sulfurous acid are represented by formulae (2) and (3), and those of sulfuric acid are represented by formulae (4) and (5). It can be considered that a difference of whether the product is CaSO.sub.3 of the formula (3) or CaSO.sub.4 of the formula (5) has a great influence on the dissolution reaction of CaCO.sub.3.

FIG. 3 shows experimental data of dissolution rates of CaCO.sub.3 in a sulfurous acid solution and a sulfuric acid solution. The data therein were obtained as follows: A CaCO.sub.3 powder (average grain diameter 15 .mu.m) and water were placed in a beaker (initial concentration of CaCO.sub.3 15 g/l, solution temperature 50.degree. C.), and each acid was added dropwise thereto so that a pH of the reaction solution might be maintained at 5.4 under an automatic adjustment. In this way, the reaction percentage of CaCO.sub.3 was determined repeatedly under identical conditions. By the thus determined average values, the experimental data in FIG. 3 are constituted, and their results indicate that the dissolution rate of CaCO.sub.3 in the presence of sulfurous acid is low.

The experiment by which the results in FIG. 3 were obtained will be described in detail.

In a glass beaker are placed 2 liters of distilled water, and 30 g of CaCO.sub.3 are then added thereto in order to form a CaCO.sub.3 slurry. An SO.sub.2 gas and an O.sub.2 gas are blown thereinto to react all of CaCO.sub.3 therewith, thereby preparing a gypsum slurry, which will be hereinafter referred to as the SO.sub.2 absorbing solution. Gypsum is separated from this solution by the use of a filter, and the consequently obtained filtrate will be used as a regulating solution.

This regulating solution is then placed in a reactor, and limestone (CaCO.sub.3) which has been ground to 325 mesh and under (44 .mu.m or less) is added thereto so that its concentration may be 15 g/l. Stirring is carried out by the use of a stirrer to form a CaCO.sub.3 slurry (average grain diameter of the CaCO.sub.3 powder 15 .mu.m). Since the limestone powder is alkaline, a pH of this CaCO.sub.3 slurry becomes 7 or more.

Next, the acid is added dropwise to the beaker from a burette, and at this time, a pH of the CaCO.sub.3 slurry becomes 4 or less. However, when the addition of the acid is interrupted, the pH changes toward an alkaline side since CaCO.sub.3 begins to be gradually dissolved out therefrom. A settled value of the pH of the slurry in the beaker is decided in proportion of an addition rate of the acid from the burette (i.e., the feed rate of the acid) to the elution rate of CaCO.sub.3.

In short, the dropping addition rate of the acid from the burette can be adjusted so that the pH of the CaCO.sub.3 slurry may be 5.4 (since the elution of CaCO.sub.3 takes a long time, such an operation is possible, but in the instantaneous reaction such as a neutralization reaction between NaOH and H.sub.2 SO.sub.4, it is impossible).

In the experiment, a pH control was carried out by detecting the pH of the slurry by means of a pH meter and automatically operating an acid dropping addition rate regulator via a pH controller.

When the acid is added dropwise thereto, CaCO.sub.3 can be dissolved and a product corresponding to a kind of acid can be obtained. For example, if the acid is sulfuric acid, calcium sulfate (gypsum) will be formed, and if it is sulfurous acid, calcium sulfite will be obtained. Since it is possible to know a concentration of residual CaCO.sub.3 with time by an analysis, a CaCO.sub.3 reaction percentage can be determined from a proportion an amount of remaining CaCO.sub.3 to its initial amount (the reaction solution referred to above means the slurry in the beaker after the acid has begun to be added to CaCO.sub.3).

In the present invention, the neutralization of the acid is carried out with the fed CaCO.sub.3 material in the neutralizing section 22, and thus the dissolving rate of CaCO.sub.3 can be improved and the amount of unreacted CaCO.sub.3 in the absorbing solution can be lowered to almost zero.

In a pilot plant having a scale of 8,000 m.sup.3 N/h for treating an exhaust gas from a coal-fired boiler, an effect of the present invention was established.

The pilot plant had such an SO.sub.2 gas absorbing apparatus as shown in FIG. 1, and the exhaust gas at an exhaust gas inlet 2 had the following properties.

The exhaust gas at the exhaust gas outlet 3 under steady operation conditions had the following properties:

In an oxidizing section 20, a volume of the absorbing solution was 4 m.sup.3, and its pH was 5.0. A CaCO.sub.3 slurry having a concentration of 20% by weight was fed to a neutralizing section 22 through an absorbent feed pipe 11 so that a volume and a pH of the absorbing solution in the neutralizing section 22 might be 4 m.sup.3 and 6.0, respectively.

Air was introduced into the oxidizing section 20 through an air introduction pipe 8 at a flow rate of 400 m.sup.3 N/h. Although not shown in FIG. 1, an air introduction pipe was provided under a stirrer 23 in the neutralizing section 22 in order to introduce air at a flow rate of 200 m.sup.3 N/h, and at this time, a CaCO.sub.3 reaction percentage in the absorbing solution was further improved. However, also when air was not fed additionally, a sufficiently high reaction percentage as high as 95% or more could be obtained. The typical absorbing solution drawn out from the absorbing solution draw pipe 12 had the following properties:

For comparison, when carried out in accordance with the conventional method shown in FIG. 2, a concentration of the remaining sulfite was 0.01 mol/l. Even when a largely excessive amount of the CaCO.sub.3 absorbent was fed to the absorbing solution, its pH of was at most 5.0, and a concentration of the remaining CaCO.sub.3 in the absorbing solution was as high as 2% by weight or more. On the contrary, in the present invention, it was established that the sulfite in the absorbing solution was not detected and the concentration of the remaining CaCO.sub.3 could be maintained at a level of less than 2% by weight. The absorbing solution drawn out was then delivered to a centrifugal separator (not shown in FIG. 1) in order to recover the gypsum by-product, which was high in purity and was industrially valuable as gypsum board.

In the present invention, when as an absorbent for SO.sub.2, there were together used a base component comprising at least one of CaCO.sub.3 and Ca(OH).sub.2 and at least one of a magnesium compound and a sodium compound, the oxidization tended to make progress in a more perfect state and the percentage of the reaction with CaCO.sub.3 and/or Ca(OH).sub.2 which was the base component for SO.sub.2 absorption was also improved. Further, when Mn.sup.2+ was used as an oxidation accelerator, the oxidation of the sulfite was more accelerated, which fact permitted diminishing an amount of blown air and reducing volumes of the oxidizing section and the nuetralizing section.

Furthermore, the present invention is directed to a method for simultaneously treating SO.sub.2 and HCl contained in an exhaust gas to purify the latter, the method being characterized by comprising a gas absorbing section in which an absorbing solution containing a calcium compound absorbs SO.sub.2 and HCl to become an acid solution containing a sulfite and a chloride; an oxidizing section in which the sulfite is oxidized by blowing air into the absorbing solution; a neutralizing section in which the absorbing solution is neutralized by feeding at least one of CaCO.sub.3 and Ca(OH).sub.2 as a neutralizing agent thereto; and a closed circulating circuit for delivering the solution drawn out from the gas absorbing section to the oxidizing section, delivering the solution drawn out from the oxidizing section to the neutralizing section, and delivering the solution drawn out from the neutralizing section to the gas absorbing section; at least one cation selected from the group consisting of Na.sup.+, K.sup.+, Mg.sup.2+ and NH.sub.4.sup.+ being contained in the absorbing solution in an amount of one equivalent or more of a Cl concentration in the absorbing solution; gypsum being recovered as a by-product from the absorbing solution in the closed circulating circuit; the chloride being taken out in the form of an aqueous solution of a chloride other than the calcium compound.

Now, the above-mentioned invention will be described in detail in accordance with an embodiment shown in FIG. 4.

An amount of unreacted CaCO.sub.3 remaining in the absorbing solution can be decreased due to the above functional effects (a) and (b) which have been discussed in the description regarding FIG. 1, and thus the absorbing solution in the oxidizing section 20 contains gypsum grains mainly.

From an absorbing solution delivered to a gypsum separator 14 by means of a pump 13, a by-product gypsum 15 is separated, and the remaining solution is then returned to an oxidizing section 20 via a line 16. On the other hand, most of the oxidized absorbing solution is delivered to a neutralizing section 22 and is neutralized with CaCO.sub.3 fed through an absorbent feed pipe 11, and is then circulated to the absorbing section provided with a spray nozzle 7 and a grid filler 9, by a circulating pump 6.

In the absorbing section, SO.sub.2 and HCl are absorbed by the absorbing solution, and SO.sub.2 is recovered as the by-product gypsum 15 as described above but HCl is dissolved in the absorbing solution in the state of CaCl.sub.2 having a high solubility, when neutralized with CaCO.sub.3. This dissolving chloride is removed in the following manner: A clarified solution collector 17 is used which comprises a tube having an opened bottom end and a closed top end, the closed end of this tube being connected to a pipe for guiding the clarified solution therethrough. The thus constituted collector 17 is dipped in the absorbing solution in which the gypsum grains are mainly suspended, and the aqueous CaCl.sub.2 solution is then sucked up through the collector 17 by means of a pump 18. This solution is delivered to a commercially available drier 19, and is evaporated to dryness thereby in order to recover a solid chloride 24. A heat source for the drier 19 can be preferably supplied by the utilization of the exhaust gas. Although not shown in any drawing, the utilization of the exhaust gas can be preferably made as follows: The aqueous solution containing the chloride is spray-dried into the exhaust gas containing SO.sub.2 and HCl, and the resultant dried solid is then collected by a dry collector. Afterward, the exhaust gas can be utilized in the above-mentioned drier 19 for the wet treatment.

In order to further heighten the effect of the present invention, a magnesium compound, a sodium compound, an ammonium compound, or a potassium compound for decomposing CaCl.sub.2 may be suitably selected and be added through an additive feed pipe 25.

Ca.sup.2+ ions formed in the reactions (6) to (9) are combined with sulfate groups formed by the oxidization of the absorbing solution in the oxidizing section 20, so that a gypsum crystal is produced.

When the dissolved Ca.sup.2+ ions in the absorbing solution diminishes, CaCO.sub.3 having a low solubility instead begins to dissolve acceleratedly, and thus the reaction percentage of CaCO.sub.3 is more improved. FIG. 5 shows experimental data obtained by the same experiment as in the case of FIG. 3.

In this way, when at least one cation selected from the group consisting of Na.sup.+, K.sup.+, Mg.sup.2+ and NH.sub.4.sup.+ is contained in the absorbing solution in an amount of one equivalent or more of a Cl concentration in the absorbing solution, the reaction percentage of CaCO.sub.3 is more enhanced.

Since well soluble in water, the chloride of every cation mentioned above can be partially taken out from the absorbing solution through the clarified solution collector 17 in the form of the aqueous chloride solution by means of the pump 18. The chloride solution thus taken out can be then evaporated to dryness by the spray drier the heat source of which is the exhaust gas, thereby recovering it as a solid chloride.

FIG. 4 exhibits that the aqueous chloride solution is partially taken out from the neutralizing section 22 by the use of the clarified solution collector 17, and the reason why the chloride solution is preferably removed from the section 22 is that the solution to be treated in the drier 19 is not acidic and thus it permits preventing device materials from corroding and restraining an evaporated material from containing an acid gas. However, a position from which the aqueous chloride solution is taken out is not limited to the neutralizing section 22, but it may be partially collected from the oxidizing section or the absorbing section.

In a pilot plant having a scale of 8,000 m.sup.3 N/h for treating an exhaust gas from a coal-fired boiler, an effect of the present invention was established.

The pilot plant had such an SO.sub.2 and HCl absorbing apparatus as shown in FIG. 4, and the exhaust gas at an exhaust gas inlet 2 had the following properties.

The exhaust gas at an exhaust gas outlet 3 under steady operation conditions had the following properties on the average:

A volume of an absorbing solution in an oxidizing section 20 was 4 m.sup.3, and its pH was about 5.0. A volume of the absorbing solution in an neutralizing section 22 was 4 m.sup.3, and a CaCO.sub.3 slurry having a concentration of about 20% by weight was fed to the neutralizing section 22 through an absorbent feed pipe 11 so that a pH of the absorbing solution in the neutralizing section 22 might be about 6.0.

An Na.sub.2 CO.sub.3 solution was added through an additive feed pipe 25 to the neutralizing section 22, and in this case, a feed rate of the Na.sub.2 CO.sub.3 solution was adjusted so that Na.sup.+ might be dissolved therein in an amount of one equivalent or more based on a detected value of the Cl concentration in the absorbing solution.

Air was introduced into the oxidizing section 20 through the air introduction pipe 8 at a flow rate of 400 m.sup.3 N/h, while the absorbing solution was circulated at a flow rate of 120 m.sup.3 /h by means of the circulating pump 6. When a steady operation was maintained, the absorbing solution was drawn out through the absorbing solution draw pipe 12 in compliance with an amount of absorbed SO.sub.2 for the sake of recovering by-product gypsum. The thus drawn absorbing solution had the following properties:

For comparison, when carried out in accordance with the conventional method shown in FIG. 2, a concentration of the remaining sulfite was 0.01 mol/l.

However, heretofore, it has been judged according to a former conception that the remaining amount of 0.01 mol/l is very small. If absorbed SO.sub.2 is not oxidized at all, a concentration of the remaining sulfite in the absorbing solution will be about 1 mol/l, and the remaining amount of 0.01 mol/l corresponds to 99% in terms of an oxidation percentage, and thus such a level is regarded as an almost perfect oxidization from the old thinking. However, in the course of the reseaches regarding the present invention, it has been found that such a small remaining amount of the sulfite inhibits the dissolution of CaCO.sub.3, and the perfect oxidation of the sulfite up to a level of less than 0.001 mol/P has been understood to be a key point to enhance the dissolution of CaCO.sub.3.

In the case that 0.01 mol/l of the sulfite remained, even when the very excessive amount of the CaCO.sub.3 absorbent was fed thereto, a pH of the absorbing solution was at most 5.0, and the concentration of the remaining CaCO.sub.3 in the absorbing solution was as high as 2% by weight or more. On the contrary, in the present invention, it was established that the sulfite in the absorbing solutions in the oxidizing section and the neutralizing section was not detected at less than 0.001 mol/l and the concentration of the remaining CaCO.sub.3 could be maintained at a level of less than 2% by weight.

A by-product gypsum 15 prepared by delivering the absorbing solution drawn out from the oxidizing section 20 to a gypsum separator 14 and carrying out a recovery operation was high in purity and was industrially valuable as gypsum board.

Next, a clarified solution collector 17 was immersed into the absorbing solution in the neutralizing section 22 to suck up a chloride solution therefrom with the aid of a pump 18, which collector 17 was composed of a tube having an inner diameter of 250 mm, a length of 1 m, an opened bottom end and a closed top end. By adjusting an average rise rate of the solution in the collector 17 so as not to exceed a sedimentation rate of the gypsum grains, the chloride solution containing a small amount of the gypsum grains could be taken out.

The aqueous chloride solution thus taken out had the following properties:

The collected solution was condensed by a drier in order to recover an NaCl solid.

The procedure in Example 2 was repeated with the exception that Na.sub.2 CO.sub.3 fed through an additive feed pipe 25 was replaced with Mg(OH).sub.2 and a concentration of Mg.sup.2+ was adjusted so as to be one equivalent or more based on a detected value of a Cl concentration in the absorbing solution.

As in Example 2, a concentration of remaining CaCO.sub.3 in an absorbing solution could be maintained at a level of less than 2% by weight, and SO.sub.2 and HCl could be recovered in the forms of by-product gypsum and MgCl.sub.2, respectively.

Also when (NH.sub.4).sub.2 SO.sub.4 was fed in place of Mg(OH).sub.2, a chloride could be taken out as NH.sub.4 Cl. This makes it clearly definite that the cation to be fed to the absorbing solution may be of any morphology of a hydroxide, a carbonate or a sulfate, so long as it is may be Na.sup.+, K.sup.+, Mg.sup.2+ or NH.sub.4.sup.+ capable of producing the chloride.

Needless to say, when any cation was not fed, the chloride could be taken out in the form of an aqueous CaCl.sub.2 solution, but the dissolution of CaCO.sub.3 deteriorated and thus an amount of remaining CaCO.sub.3 was apt to increase. Nevertheless, in the pilot plant for simultaneously treating SO.sub.2 and HCl, the concentration of remaining CaCO.sub.3 could be maintained at a level of less than 2% by weight, by which it was confirmed that the drawbacks of the conventional process could be overcome sufficiently.

The procedure in Example 2 was repeated with the exception that MnSO.sub.4 was added as an oxidation accelerator to the absorbing solution and a concentration of Mn was adjusted so as to be within the range of 0 to 200 mg/l. When SO.sub.2 was absorbed by the absorbing solution in the absorbing section packed with a grid fillter 9, the oxidation of SO.sub.2 with an oxygen gas contained in an exhaust gas from a coal-fired boiler was accelerated by Mn. In consequence, a concentration of a sulfite in the absorbing solution which would fall onto the surface 10 of the absorbing solution in the oxidizing section 20 diminished, and an amount of air to be blown through an air introduction pipe 8 could be decreased in proportion to a concentration of Mn. In addition thereto, a concentration of the sulfite in the absorbing solution in the oxidizing section could be maintained at a level of less than 0.001 mol/l, and the dissolution of CaCO.sub.3 was good, with the result that a concentration of remaining CaCO.sub.3 could be kept at a level of less than 2% by weight.

As described above in detail, the present invention is concerned with the method comprising the gas absorbing section, the oxidizing section, the neutralizing section and the closed circuit for circulating the absorbing solution through these sections, and thus the present invention can provide the effect of rendering it possible that the unreacted and unoxidized calcium sulfite are scarcely present in the absorbing solution. That is to say, the present invention can provide the effect (oxidation accelerating effect) of preventing the incomplete oxidation phenomenon.

In addition, the exhaust gas containing SO.sub.2 and HCl can be purified with a high efficiency.