GB2159508A - Method for regulating concentration of sulfite - Google Patents

Abstract

The present invention is directed to a method for regulating a concentration of a sulfite by oxidizing the sulfite contained in a solution or a suspension, the method being characterised by adjusting a feed rate (via 111 or 112) of an oxidizing agent (via 107) and/or an oxidizing catalyst (via 108) for oxidizing the sulfite in the solution or the suspension in accordance with a signal (110) regarding a deviation of a detection signal of a sulfite concentration in the solution or the suspension from a predetermined sulfite concentration. <IMAGE>

Description

SPECIFICATION Method for regulating concentration of sulfite The present invention relates to a method for controlling a concentration of a sulfite or a hydrogensulfite (hereinafter referred to generically as the sulfite which is inclusive of the -hydrogensulfite therein) such as CaSO#.1/2 H2O, Ca(HSO#)2, H2SOs, Na2SO3, NaHSO3, l'1igSO3, Mg(HSO3)2, K2SO3 or KHSO3, and more particularly, the present invention relates to a method which is remarkably effective to adjust a feed rate of an oxidizing agent or an oxidizing catalyst, for example, when the sulfite contained in an absorbing solution or a suspension (hereinafter reterred to generically as the solution which is inclusive of the suspension therein) with which an exhaust gas containing SO, has been treated is oxidized to a sulfate.

Heretofore, as wet smoke desulfurization methods by which the absorbino solution including SO, in the form of the sulfite is oxidized to the sulfate by blowing air into the absorbing solution, there are methods in Japanese Patent Publication Nos. 12005/1970, 1731811975 and 1247911976 as well as Japanese Patent Provisional Publications Nos. 159897/1975, 8869411978 and 71819/1982, and they are now widely utilized.

Further, techniques of using additives such as catalysts to accelerate the oxidization are also described in the above-mentioned publications.

In these conventional methods, an oxidization rate of the sulfite is first gripped experimentally and a feed rate of an oxidizing agent (in most cases, air is employed as the oxidizing agent) is regulated on the basis of the gripped oxidization rate, and a concentration of the sulfite is controlled by sampling a small amount of the solution and measuring its concentration with the aid of a manual analysis in conformity with JIS K 0102 to check whether or not an amount of the used oxidizing agent is suitable.

Therefore, in the case that an operation of oxidizing the sulfite to the sulfate is continuously carried out in a reaction tank while the concentration of the sulfite is varying with time, the manual analysis is often necessary to regulate the feed rate of the oxidizing agent or the oxidizing catalyst with time, which fact requires much labor and time disadvantageously. Further, when the sulfite present in the absorbing solution is analyzed by a smoke desulfurization method in conformity with JIS K 0102, this analysis will often be hindered by iron ions inconveniently, which fact makes the control of the sulfite concentration difficult.

Now, if it is intended that the sulfite is oxidized to the sulfate on an industrial scale, such an oxidizing operation is to be carried out in a continuous manner. That is to say, the oxidizing agent is successively fed to a reaction tank while a solution containing the sulfite is successively introduced into the reaction tank, and the solution is consecutively taken out of the tank as much as an increased amount. The solution which has been taken out from the tank contains the sulfate as a main component and a small amount of the sulfite together. This is a feature of the continuous operation, and if the sulfate is desired as a product, the control of the sulfite concentration will be an important key.This control is carried out on the basis of data obtained by the above-mentioned manual analysis in conformity with JIS K 0102, but such a control manner takes much labor and time, and it is practically very difficult to properly regulate the feed rate of the oxidizing agent to the reaction tank in response to oxidation load conditions which vary with time. In such situations, a method has heretofore been employed which comprises always feeding an excessive amount of the oxidizing agent to the reaction tank irrespective of increase and decrease of the oxidation load in the reaction tank in order to lower the ratio of the remaining sulfite to the sulfate which is the desired product.

In recent years, the use of the oxidizing agent in the excessive quantity has been considered to be wasteful from the viewpoints of economies of resources and energy, and hence it is strongly required to keep an optimum feed rate of the oxidizing agent or the oxidizing catalyst even when the oxidation load varies abruptly, whereby the concentration of the sulfite can be maintained at a desired level. However, the control by the manual analysis has its limit and thus cannot satisfy the above-mentioned requirements Under such circumstances, the present invention has now been developed, and an object of the present invention is to provide a method for regulating a concentration of the sulfite to a desired level by promptly feeding a proper amount of an oxidizing agent or an oxidizing catalyst without any waste.According to the present invention, a method for regulating a concentration of the sulfite can be provided which adopts a developed detector for successively and instantaneously detecting the concentration of the sulfite on line and which regulates an amount of an oxidizing agent and/or an oxidizing catalyst to be fed to a reaction tank in accordance with a signal regarding a deviation of a detection signal of a sulfite concentration in the solution in the reaction tank from a predetermined sulfite concentration. Further, the regulation of the sulfite concentration can also be accomplished by adjusting the feed rate of the sulfite to the reaction tank instead of the adjustment of the oxidizing agent, or by adjusting feed rates of the oxidizing agent or the oxidizing catalyst and the sulfite together.

The above-mentioned and other objects as well as advantages and features of the present invention will become more apparent from the following detailed description and accompanying drawings, in which: Figure 1 is a flow sheet illustrating an embodiment of the present invention; Figure2 is an explanatory view illustrating a constitution of a sulfite detector used in the present invention; Figure 3 is a diagram illustrating an interrelation between measured values by the sulfite detector shown in Fig 2 and analytical values by a manual analysis; and Figure 4 is a graph showing a transition of a sulfite concentration with respect to an elapsed time, whereby a functional effect of the present invention is elucidated.

Now, the present invention will be described in reference to Fig. 1 about an embodiment in which a method of the present case is applied to a smoke desulfurization apparatus for a wet lime gypsum process.

The present invention has been completed on the basis of a developed manner for continuously and instantaneously detecting a concentration of the sulfite in a solution on line instead of a manual analysis, and hence this detection technique for the sulfite will be first described in detail in reference to Fig. 2.

Fig. 2 is an explanatory view illustrating the constitution of a sulfite detector used in the present invention.

In Fig. 2, reference symbol A represents a sample solution, symbol B is air or nitrogen, C is an acid, for decomposing the sulfite, such as sulfuric acid or hydrochloric acid, D is a waste liquid, E is a drawn gas containing SO2, each of F and G is an exhaust gas and H is a drain. Further, reference numeral 1 represents a constant delivery pump, numeral 2 is a heater, 3 is a temperature controller, 4 is a temperature detector and 5 is a stirring type continuously acid decomposing container which is cut off from the outside air.

Numeral 6 is a stagnant solution, 7 is a stirrer, 8 is a blowing pipe, 9 is a sealant, 10 is a motor, 11 is a flow rate indicator, 12 is a micropump, 13 is a trap, 14 is a pH detector, 15 is a pH controller, 16 is air, 17 is a flow rate controller, 18 is an air pump, 22 is an SO2 analytical instrument, 23 is a sulfite concentration calculator, 24 is a sulfite concentration indicator, 25 is an overflow pipe, 26 is a valve, 27 is a dehumidifier, "p" is a flow rate signal from the flow rate controller 17 and "q" is a flow rate signal from the pump 1.

The sample A containing the sulfite is taken by the constant delivery pump 1 and is delivered to the heater 2 where it is heated so that the stagnant solution 6 in the acid decomposing container 5 may have a predetermined temperature (70or or more), and the sample A is then introduced into the reaction container 5. In this case, a temperature of the stagnant solution 6 is detected by the detector 4, and the heater 2 is controlled by a signal generated from the temperature controller 3 connecting to the detector 4 so that the stagnant solution 6 may have the predetermined temperature (70"C or more).

The stagnant solution 6 in the acid decomposing container 5 is stirred by means of the stirrer 7 so that a solid content in the staqnant solution 6 may not precipitate. A pH of the stagnant solution 6 is detected by the pH detector 14, and in response to results of the pH detection, the micropump 12 is controlled by a signal generated from the pH controller 15 connecting to the pH detector 14, whereby an acid C, for decomposing the sulfite, such as sulfuric acid or hydrochloric acid is introduced into the acid decomposing container 5 so that the stagnant solution may have a predetermined pH (3 or less).

When an acid, for decomposing the sulfite, such as sulfuric acid or hydrochloric acid has been introduced into the stagnant solution 6, the sulfite in the sample A reacts with the acid as shown in the following formulae (1) and (2) to give off SO2 (in the formulae (1) and (2), the sulfite is CaSO2, but the sulfites other than the Ca salt also react likewise).

(In the case that sulfuric acid is added)

(In the case that hydrochloric acid is added)

Produced SO2 is drawn out as follows: A part or all of air (or nitrogen) as a carrier gas adjusted to a predetermined flow rate by the flow rate controller 17 is blown into the stagnant solution 6 through the flow rate indicator 11 and the blowing pipe 8 by operating the distributing valve 26, whereby SO, is drawn out from the stagnant solution 6 as the drawn gas E in which SO2, air (or nitrogen) and water are mixed.

On the other hand, the remaining part of the air (or nitrogen) 16 used in the above-mentioned blowing process is caused to run into the drawn gas E coming from the acid decomposing container 5, and a part of the joined gas is drawn by the air pump 18 and will be used in the SO, analytical instrument 22 and its remaining part is discharged as the exhaust gas F.

Next, the stagnant solution 6 is partially discharged through the overflow pipe 25 as much as an amount of the sample A fed through the constant delivery pump 1 and is delivered to the trap 13. The solution in the trap 13 has a depth enough to overcome an internal pressure in the acid decomposing container 5 and the structure of the trap 13 is designed so as to prevent a solid content in the overflowed solution from precipitating Further, the solution in the trap 13 is discharged as the waste solution D therefrom as much as a volume of the solution delivered thereto.The remaining air 16 (or nitrogen) is caused to run into the drawn gas E consisting essentially of SO2 generated in accordance with the above-mentioned reaction formula (1) or (2), air (nitrogen) and water, and a main part of the joined gas E is then discharged as the exhaust gas F, as mentioned above, but its remaining part is delivered to the dehumidifier 27 where a water content in the gas E is removed therefrom as the drain H. The water-free drawn gas E is then sucked by the air pump 18 and is further delivered to the SO2 analytical instrument 22 where a concentration of SO2 is measured thereby, and it is afterward discharged therefrom as the exhaust gas G. A signal from the SO, analytical instrument 22 is sent to the sulfite concentration calculator 23 to which the flow rate signal "p" of air (or nitrogen) and the flow rate signal "q" of the sample A have already been inputted from the air (or nitrogen) flow rate controller 17 and the constant delivery pump 1, respectively. The calculator 23 carries out the following theoretical calculation, employing these inputted signals in order to seek a sulfite con centration in the-sample A, and the sought value is sent to the sulfite concentration indicator 24.

(Calculation of the sulfite concentration) Sulfite concentration (mol/liter) SO2(%) flow ( flow rate of air (or nitrogen) (Ne/min) 100 - S02(%) (22.4(NUmol) x sample slurry flow rate (e/min) Example for Reference By the use of the apparatus shown in Fig. 2, a concentration of the sulfite CaSO2 was measured. This example will be described with regard to such a case, employing concrete values.

The test apparatus shown in Fig. 2 was used under the following conditions, and measured results are set forth in Fig. 3.

Conc. of CaSO2 in sample slurries : 0.005, 0.1 and 0.2 mol/liter Feed flow rate of sample slurries : 0.12 liter/min Carrier gas : air and nitrogen Carrier gas flow rate : 20 liters/min Predetermined reaction temp. : 70 and 80 C Predetermined pH :3 Blown carrier gas flow rate : 10 liters/min Volume of reaction container : 1.5 liters Fig. 3 is a graph comparatively showing CaSO2 concentrations detected by the present invention and values analysed by a manual analysis, and the abscissa axis in this drawing represents the CaSO2 concentration according to the manual analysis and the ordinate axis therein represents values measured according to the present invention.In Fig. 3, black circles, white circles and triangles represent results at 70'C and 80 C in the presence of air and at 80 C in the presence of nitrogen, respectively. Table 1 given below shows concentrations of SO2 and measured values of CaSO2 concentrations by the present invention and the manual analysis with regard to some samples.

TABLE 1 (carrier gas: air) Case number No. 1 No. 2 No. 3 No. 4 No. 5 Temp. in reaction zone ( C) 70 70 80 80 80 CaSO3 Concentration (mol/liter) Manual analysis 0.044 0.102 0.052 0.103 0.196 Present method 0.042 0.093 0.047 0.098 0.194 SO2 Conc. (vol%) 0.56 1.23 0.63 1.30 2.54 Acid H S S H S In this table, H and S represent hydrochloric acid and sulfuric acid, respectively.

As is definite from Table 1, the present process can successively measure varied concentrations of the sulfite in the sample with about the same accuracy as in the manual analysis and at a high speed.

One example of the sulfite detection process has just been described in detail regarding the case that the sulfite was CaSO2, but similar effective detections have been obtained regarding cases that the sulfites were H2SO3, Na2SO3, NaHSO2, MgSO2, Mg(HSO2)2, K2SO3, KHSO, and Ca(HSO3)2.

Fig. 1 illustrates an embodiment of a method of the present invention. In Fig. 1, an exhaust gas 100 containing SO2 is guided to an absorbing tower 101 where the gas 100 is brought into contact with an absorbing solution to remove SO2 therefrom, and the SO2-free gas is discharged from the tower as a clean gas 102. In the lower portion of the absorbing tower 101, a tank 103 for storing the absorbing solution is provided. The absorbing solution is stirred by a stirrer 104, and it is forwarded to the top of the tower 101 by means of a circulating pump 105 and is sprayed therefrom into the tower in order to be brought into contact with the exhaust gas and to thereby absorb SO2 therefrom.At this time, a sulfite is produced by the absorption of SO, with the absorbing solution, and a part of the produced sulfite is oxidized by oxygen present in the exhaust gas in the gas-liquid contact zone to produce a sulfate. Then, the acidic absorbing solution in which the sulfite and the sulfate are coexistent and which hence has a lower pH drops into the tank 103.The compound CaCO,i3 which is the absorbing agent for SO2 is fed to the tank 103 through a CaCO2 feed line 106 in compliance with an SO, absorption amount in order to neutralize the absorbing solution, and air is fed to the tank 103 through an air nozzle 107, alternatively a solution containing an oxidizing catalyst such as a manganese salt or a cobalt salt is fed thereto through an oxidizing catalyst feeding line 108 in order to oxidize the sulfite in the absorbing solution to the sulfate. Needless to say, air and the oxidizing catalyst may be used together.In the case that an exhaust gas from a boiler is treated, variations in a load of the bciler and an amount of sulfur component will lead to variations in a treating amount of the exhaust gas 100 as well as concentrations of SO2 and 02 and, after all, will bring about a variation in anoxidation amount of the sulfite in the absorbing solution in the tank 103.

That is to say, if the treatment amount of the exhaust gas 100 increases, if the concentration of SO, becomes high, or if the concentration of 02 becomes low, the amount of the air from the air nozzle 107 wili be increased or the concentration of the oxidizing catalyst will be heightened to cope with such variations. On the contrary, if a load amount of the oxidization decreases, the amount of the air will be reduced or the feed of the oxidizing catalyst will be lowered to cope with such a variation. The present invention includes the sulfite detector 109 having the constitution in Fig. 2 to instantaneously detect a concentration of the sulfite in the absorbing solution on line, and a concentration signal of the sulfite is sent to a sulfite controller 110.Then, in the sulfite controller 110, a predetermined sulfite concentration is decided from a relation between an allowable value of the sulfite concentration in a by-product and an allowable value of a sulfite concentration concerning an SO2 absorbing performance, and a signal regarding a deviation of a detected sulfite concentration from the predetermined sulfite concentration is produced. The thus produced deviation signal is then sent to an air flow rate controller 111 and/or an oxidizing catalyst flow rate controller 112.

The air flow rate controller 111 receives signals from an air flowmeter 113 and the sulfite controller 110 in order to control an opening and closing operation of a valve 114 and to thereby adjust an amount of the air which will be blown to the tank 103 through the air nozzle 107, whereby the operation of the present invention can be carried out in a minimum amount of the air which is capable of maintaining the sulfite concentration at a desired level.

On the other hand, the oxidizing catalyst flow rate controller 112 receives signals from a flowmeter 115 for a solution containing an oxidizing catalyst and the sulfite controller 110 in order to control an opening and closing operation of a valve 116 and to thereby adjust a feed rate of the oxidizing catalyst, whereby the operation of the present invention can be carried out in a minimum amount of the catalyst which is capable of retaining the desired sulfite concentration.

The oxidizing catalyst and air may be used alone or in a combination thereof. In the case that the concentration of the sulfite in the absorbing solution is regulated by the use of the oxidizing catalyst alone, the oxidation makes progress depending upon oxygen present in the exhaust gas. Therefore, if the concentration of SO2 is very high and the oxidation load is thus great, the required satisfactory oxidation will not be accomplished only by the oxidizing catalyst sometimes. On the contrary, in the case that the concentration of the sulfite in the absorbing solution is regulated by the blow of air alone, large air bubbles will occur in the tank 103 to lower its oxidizing function at times, and hence a power cost of the air blow will be wasted. In consequence, it is most effective for a great variation of the oxidation load to make use of the oxidizing catalyst and air together.The present invention may suitably select the sole system of the oxidizing catalyst or air which is the oxidizing agent, or the combination system thereof in compliance with the oxidation load amount. When the concentration of the sulfite has been adjusted to a desired level by the oxidation in such a way, the absorbing solution becomes a suspension mainly containing gypsum grains which are the sulfate. The gypsum is then delivered to a centrifugal separator 119 through an absorbing solution drawing line 118 by means of a pump 117 in accordance with quantity balance, and in the centrifugal separator 119, a secondarily produced gypsum 120 is recovered. A filtrate obtained in the centrifugal separator 119 is forwarded to a farther waste liquid treatment process or is recycled through a line 121.

Fig. 1 shows the embodiment where the sulfite concentration regulating method of the present invention is applied to a smoke desulfurization apparatus for a wet lime gypsum method in which CaCO2 is used as the absorbing agent and the gypsum is secondarily produced, but the present invention can be applied also to a soda method in which NaOH or Na2CO2 is used as the absorbing agent and the sulfites of Na2SO3 and NaHSO2 and the sulfate of Na2SO4 are discharged, and a smoke desulfurization method in which a magnesium compound and a potassium compound are used alone or in a combination thereof.

To sum up, any compound which will give off SO2 in reacting with an acid in the sulfite detector shown in Fig. 2 is suitably applicable to the present invention, without limiting to the above-mentioned compounds.

Heretofore, the detection of the sulfite concentration has been carried out by a manual analysis in conformity with JIS K 0102 in which much labor and time are required, and hence it is practically almost impossible to regulate feed rates of air and the oxidizing agent to an optimum level, i.e., to the most economical level in response to oxidation load conditions varied with time. Therefore, in the case of the smoke desulfurization apparatus for the wet lime gypsum method in which the high-purity gypsum is recovered as a by-product, the operating control has been accomplished by excessively using the oxidizing catalyst and air so that the concentration of the sulfite may not be heightened, which fact is not desirable from the viewpoints of economies of energy and resources. In contrast, the present invention can respond instantaneously to noticeably varied oxidation load conditions and can feed optimum amounts of the oxidizing agent (air or the like) and the oxidizing catalyst without any waste in order to regulate the concentration of the sulfite to a desired level, which fact will lead to a specific functional effect of being remarkably economical.

Now, the present invention will be described in detail in accordance with examples.

Example 1 In this example, an apparatus shown in Fig. 1 was employed.

At the outlet of an electrical dust collector, 4,000 m3N/h of an exhaust gas discharged from a coal-fired boiler were sampled and introduced into an absorbing tower 101. The exhaust gas 100 contained 2,000 ppm of SO2, 500 mg/m2N of a dust, 100 ppm of HCI and 30 ppm of HF on the average. As an absorbing agent, a CaCO2 powder prepared by grinding limestone to 325 meshes or less was employed, and this powder was suspended in water and was introduced into a tank 103 through a line 106 in the form of a 2 mol/liter CaCO2 slurry. The tank 103 for storing the absorbing solution had a diameter of 2,000 mm and a solution depth of 2,000 mm, and a level of the solution was controlled so that a volume of the absorbing solution therein might be about 6,000 liters.By means of a circulating pump 105, 60 m3/h of the absorbing solution were sprayed into the absorbing tower 101 from its top, and the exhaust gas was washed with the absorbing solution in the tower which was packed with grids. At this time, Most of SO2, HCI, HF and the dust were caught by the absorbing solution, and a washed and thus cleaned gas 102 contained 50 ppm of SO2, 1 ppm of each HCI and HF, and 60 mg/m3N of the dust. The absorbing solution in which a sulfite was formed by absorbing SO, and which was thus acidified was allowed to stream down to the tank 103, and an alkaline agent CaCO2 was then fed to the tank 103 through a line 106 in order to neutralize the absorbing solution.The absorbing solution was successively sampled 0.12 liter every one minute and was fed to the sulfite detector 109 having the constitution shown in Fig. 2, and a concentration of the sulfite was detected and indicated immediately by the sulfite detector 109. When neither air nor an oxidizing catalyst were fed from an air nozzle 107 and an oxidizing catalyst feeding line 108 respectively, concentration values which were indicated on the sulfite detector 109 varied within the range of 0.3 to 0.6 moljliter in terms of CaSO2, and 0.4 mol/liter on the average. The variation in the CaSO2 concentration was attributable to variations in concentrations of SO2 and oxygen contained in the exhaust gas 100.The absorbing solution containing the sulfite CaSO3 was drawn out by means of a pump 117 as much as an amount of the absorbed 502. Next, the sulfite concentration was set to 0.05 mol/liter in a sulfite controller 110. A deviation signal produced therein was sent to an air flow rate controller 111, and in accordance with the deviation signal, air was blown into the absorbing solution through an air flowmeter 113, a valve 114 and an air nozzle 107. In a steady state, the air flow rate was 80 m3N/h, and the sulfite concentration in the absorbing solution could be maintained at the predetermined level. The predetermined sulfite concentrations were set to 0.1 mol/liter and 0.2 mol/liter in turn, and continuous driving tests were carried out. The detector 109 indicated that the sulfite concentrations in the steady state were 0.047, 0.093 and 0.194 mol/liter.The control accuracy of the sulfite concentration according to the present invention is shown in comparison with data of a manual analysis in Table 1 given above. Further, when the apparatus shown in Fig. 1 had been driven at the predetermined sulfite concentration of 0.0005 moliliter, the steady state was reached at an air flow rate of 250 m3N/h (according to the manual analysis of the absorbing solution, and sulfite concentration was 0.001 mol/liter or less), and a high-purity by-product gypsum 120 could be recovered.

Example 2 In Example 1, the signal from the sulfite controller 110 was sent to the air flow rate controller 111, but in this example, the aforesaid signal was instead forwarded to an oxidizing catalyst flow rate controller 112 and the blow of air to the absorbing solution was stopped. A used exhaust gas 100 contained 1,200 ppm of SO2, and a driving peration was carried out for about 18 hours without adding any oxidizing catalyst. Concentrations of the sulfite in the absorbing solution at this time were indicated by a sulfite detector 109, and these indicated values are exhibited in a zone A in Fig. 4. The concentration of the sulfite in the absorbing solution reached a steady state at about 0.03 mol/liter.Afterward, a 20 wt% aqueous MnSO4 solution which was the oxidizing catalyst began to be fed to the tank through a line 108 and simultaneously a sulfite concentration was set to 0.001 mol/liter in a sulfite controller 110, and then an apparatus was driven. Concentrations of the sulfite in the absorbing solution at this time were recorded, and the results are depicted in a zone B in Fig. 4. In both the zones A and B, the concentrations of the sulfite could be adjusted to the desired and predetermined levels by controlling a feed rate of the aqueous MnSO4 solution. As is definite from the above, the present invention has the effect that the concentration control can be accomplished even within the trace concentration range of the sulfite in contrast with the conventional manual analysis.

Example 3 This example was directed to an embodiment in which air was blown through an air nozzle 107 together with the feed of an aqueous MnSO4 solution. In this example, an amount of SO2 contained in an exhaust gas 100 was 3,000 ppm. Sulfite concentrations were set to 0.03, 0.01 and 0.0005 mol/liter in a sulfite controller 110, and concentration values indicated by a sulfite detector 109 were recorded and shown in zones C, D and E in Fig. 4. In each zone, an Mn concentration in the absorbing solution was interconnected with a blow rate of air through an air nozzle 107, and when the feed rate of the 20 wt% aqueous MnSO4 solution through a line 108 had increased to heighten the concentration of Mn, the blow rate of air correspondingly decreased. In the case that signals produced in a sulfite controller 110 were sent to an air flow rate controller 111 and an oxidizing catalyst flow rate controller 112 simultaneously, it was more preferable to mainly send the signals to the air flow rate controller 111, because a response to the Mn concentration was slow and because if the signals were handled in such a way, control results would be good and the sulfite concentration could easily be regulated. Further, it was confirmed that a power cost necessary for the air blow was remarkably reduced when air was employed together with the oxidizing catalyst.

Claims (5)

1. A method for regulating a concentration of a sulfite by oxidizing said sulfite contained in a solution or suspension, said method being characterized by adjusting a feed rate of an oxidizing agent and/or oxidizing catalyst for oxidizing said sulfite in said solution or suspension in accordance with a signal regarding a deviation of a detection signal of a sulfite concentration in said solution or suspension from a predetermined sulfite concentration.

2. The method for regulating a concentration of a sulfite according to Claim 1 wherein said method for oxidizing said sulfite in said solution or suspension is applicable to a desulfurization treatment of an exhaust gas containing SO2.

3. The method for regulating a concentration of a sulfite according to Claim 1 wherein said sulfite to be treated by said method is at least one selected from the group ccnsisting of H2SO3, Na2SO3, NaHSO3, MgSO3, Mg(HSO3)2, K2SO3, KHSO3, CaSO3#1/2 H20 and Ca(HSO3)2.

4. The method for regulating a concentration of a sulfite according to Claim 1 or 2 wherein a signal regarding a deviation of a detection signal of a sulfite concentration in said solution or suspension from a predetermined sulfite concentration is taken out from a sulfite controller through a sulfite detector, and this deviation signal is sent to an air flow rate controller and/or oxidizing catalyst flow rate controller in order to regulate the concentration of said sulfite.

5. A method for regulating a concentration of a sulphide substantially as herein described with reference to the accompanying drawings.