JPH07108133A - Wet type flue gas desulfurization and device therefor - Google Patents

Abstract

PURPOSE:To prevent the reactivity of limestone generated with the rise in Cl<-1> concentration in a slurry absorbent from lowering and the oxidation performance of sulfite from lowering. CONSTITUTION:In wet type flue gas desulfurization using a slurry absorbent contg. Cl<-1> of high concentration, in order to attain high desulfurizing performance and sulfite oxidizing performance, in a slurry agitation tank 104 of the preceding stage where gypsum is separated from slurry drawn out from an absorber circulating tank 8, soluble sulfate is added to the slurry from a line 103 to decrease the concentration of dissolved calcium salt. In case Cl<-1> and/or Ca<2+> ion concentration attains the high level, causing the oxidation of sulfite in the circulating tank 8 to become incomplate, water solvent is added to the slurry drawn out from the absorber circulating tank 8 to lower the liquid phase soluble salt concentration of the slurry. By making additional oxidation treatment in this state, the same object is also attained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flue gas desulfurization system, and more particularly to the rationalization of the operation of a wet flue gas desulfurization system for recovering gypsum as a by-product by using limestone as a desulfurizing agent.

[0002]

2. Description of the Related Art Environmental problems have been discussed from a global perspective, but the main substances as pollution sources are oxides of nitrogen and sulfur. Excluding the method for removing sulfur oxides, the exhaust gas from a coal or heavy oil fired boiler usually has a concentration of 1
It is common to release a gas containing several tens of ppm of SO ₂ from a chimney by containing sulfur oxide (hereinafter sometimes referred to as SO ₂ ) of 00 ppm and treating it with a desulfurization device. is there. At present, a typical method of a flue gas desulfurization apparatus for a boiler exhaust gas in a thermal power plant uses limestone as an SO ₂ absorbent, and finally gypsum is by-produced and recovered. Among the limestone-gypsum wet flue gas desulfurization methods, a method which is currently becoming the mainstream will be described with reference to FIG.

In FIG. 4, the exhaust gas 1 is guided from the absorption tower inlet duct 2 to the absorption tower 3 to remove the contained SO ₂ . The exhaust gas that has passed through the absorption tower 3 has its entrained mist removed by the demister 4, is heat-exchanged with the exhaust gas guided to the absorption tower 3 by a gas heater (not shown), and is then discharged from the chimney as purified gas. On the other hand, the limestone 5 serving as an absorbent is supplied to the limestone slurry tank 6 and becomes limestone slurry. The amount of limestone slurry supplied by the supply pump 7 is determined by the amount of SO ₂ to be treated. The limestone slurry is
It is extracted from the absorption tower circulation tank 8 below the absorption tower 3 by the absorption tower circulation pump 9, sprayed in the exhaust gas in the absorption tower 3, and then returned to the absorption tower circulation tank 8. Although the limestone slurry at the time of returning to the absorption tower circulation tank 8 contains calcium sulfite generated by absorption of SO ₂ ,
In the system shown in (1), an oxidizing stirrer 11 for dispersing air 10 as fine bubbles in the slurry in the absorption tower circulation tank 8 and, if necessary, a slurry stirrer 12 are installed in the absorption tower circulation tank 8. Calcium sulfite is oxidized to gypsum. As described above, since the sulfite in the slurry that has fallen into the absorption tower circulation tank 8 is rapidly oxidized, most of the solid substance in the slurry in the absorption tower circulation tank 8 is gypsum, and limestone and sulfite are in small amounts. Not included. A predetermined amount of the slurry in the absorption tower circulation tank 8 is sent to the gypsum slurry tank 13. The slurry in the gypsum slurry tank 13 is sent to the gypsum separator 15 by the slurry pump 14,
The powdered gypsum 16 is recovered. The gypsum separation mother liquor 17 generated in the gypsum separator 15 is temporarily stored in a filtered water tank 18 as needed, and a predetermined amount thereof is introduced to a limestone slurry tank 6 by a pump 19 to be used for preparation of limestone slurry. A part of the gypsum-separated mother liquor 17 is sent to the wastewater treatment device 20 as wastewater as needed. In the above method, the dedicated oxidation tower that oxidizes sulfite to sulfate, which was installed by the conventional method, is omitted, and unreacted limestone and sulfuric acid added for the decomposition of sulfite are unnecessary. It has various advantages.

[0004]

By the way, in the absorption part of the conventional wet flue gas desulfurization apparatus described above, hydrogen chloride (HCl) in the exhaust gas is also absorbed and removed, while the gypsum separation mother liquor generated in the gypsum recovery system is absorbed. Since it is circulated in the system, the concentration of chlorine ions (Cl ⁻ ) in the system becomes extremely high unless a predetermined amount of drainage is performed. There is a problem that an increase in chloride concentration will corrode the equipment materials and reduce the purity of gypsum, which is a by-product, but it is necessary to keep the chloride concentration at a low level to replenish the desulfurization equipment. It has the drawback of increasing the amount of water and increasing the size of wastewater treatment facilities. Therefore, when there is a severe restriction on securing water or discarding (discharging) wastewater, the device must be operated with the Cl ⁻ concentration in the desulfurization system being as high as possible. In an extreme case, there is a demand for a system that does not discharge wastewater at all or discharges wastewater in a solution state by drying and solidifying the wastewater. Increasing the concentration of Cl ^-1 in the desulfurizing agent slurry is due to the calcium ion (Ca
²⁺ ) increase. When the concentration of Cl ^-1 and / or Ca ²⁺ increases, the reactivity of limestone decreases, and the oxidation rate of sulfite decreases. The present invention is
It is an object of the present invention to solve the above-mentioned problems of reactivity of limestone and deterioration of sulfite oxidization performance that occur with an increase in Cl ⁻¹ concentration in the absorbent slurries for wet flue gas desulfurization. .

[0005]

The above objects of the present invention can be achieved by the following constitutions. (1) Sulfur oxide in exhaust gas is absorbed using a desulfurizing agent slurry, sulfite obtained by absorbing sulfur oxide in exhaust gas is oxidized into gypsum, and then the gypsum-containing slurry is dehydrated. In a wet flue gas desulfurization method having a function of separating gypsum and a gypsum-separating mother liquor and using the gypsum-separating mother liquor to newly supply a desulfurizing agent slurry, the gypsum-containing slurry is converted into a gypsum-gypsum-separating mother liquor. A method for wet flue gas desulfurization, which comprises performing a reducing operation of the concentration of dissolved calcium salt in the gypsum-containing slurry and an operation of reducing the concentration of dissolved calcium salt in at least the gypsum-containing slurry before the separation into. (2) Sulfur oxide in exhaust gas is absorbed by using a desulfurizing agent slurry, sulfite obtained by absorbing sulfur oxide in exhaust gas is oxidized into gypsum, and then the gypsum-containing slurry is dehydrated. In a wet flue gas desulfurization method having a function of separating gypsum and a gypsum-separating mother liquor and using the gypsum-separating mother liquor to newly supply a desulfurizing agent slurry, the gypsum-containing slurry is converted into a gypsum-gypsum-separating mother liquor. Prior to separation, the gypsum-containing slurry is once subjected to a solid-liquid concentration operation, and at least dissolved calcium salt concentration reduction operation of the solid content concentrated slurry obtained by the operation and oxidation of the solid content concentrated slurry, A wet flue gas desulfurization method for reducing salt concentration. The operation of reducing the concentration of dissolved calcium salt in the wet flue gas desulfurization method of the present invention is magnesium sulfate, sodium sulfate, potassium sulfate, adding a soluble sulfate-containing aqueous solvent other than calcium salts such as ammonium sulfate to the gypsum-containing slurry, aqueous It is carried out by adding a solvent. Further, industrial water or seawater can be used as at least a part of the aqueous solvent.

(3) An absorption tower that absorbs sulfur oxides in exhaust gas using a desulfurizing agent slurry and the sulfite in the sulfite-containing slurry obtained in the absorption tower are oxidized to form gypsum-containing gypsum-containing slurry. A desulfurizing agent slurry to be supplied to an absorption tower is prepared by using a gypsum separator for dehydrating the obtained oxidation tank and the gypsum-containing slurry to separate gypsum and a gypsum separating mother liquor and a gypsum separating mother liquor obtained from the gypsum separating machine In a flue gas desulfurization device equipped with a desulfurizing agent slurry tank, at least between the oxidizing tank and the gypsum separator, the dissolved calcium salt concentration in the gypsum-containing slurry and the oxidation reaction can be reduced. Wet flue gas desulfurization equipment equipped with a slurry stirring tank that performs (4) The oxidation tank of the wet flue gas desulfurization apparatus according to (3) above, wherein the concentration of sulfite or dissolved oxygen in at least one of the inside of the oxidation tank, the outlet thereof, the inside of the slurry stirring tank, and the outlet thereof is measured. A method for operating a wet flue gas desulfurization device, in which the amount of oxidizing air supplied to the slurry inside the slurry stirring tank is determined. (5) Absorption tower that absorbs sulfur oxides in exhaust gas using a desulfurizing agent slurry, and an oxidation tank that oxidizes the sulfite in the sulfite-containing slurry obtained in the absorption tower to form gypsum-containing slurry And a gypsum separator that dehydrates the gypsum-containing slurry to separate it into gypsum and a gypsum separation mother liquor, and a gypsum separation mother liquor obtained from the gypsum separator, using a limestone slurry tank to prepare a limestone slurry to be supplied to an absorption tower. In the flue gas desulfurization equipment provided, a tank for solid-liquid concentration of the gypsum-containing slurry and an operation for reducing the concentration of dissolved calcium salt for the solid-liquid-concentrated gypsum-containing slurry and oxidation between the oxidation tank and the gypsum separator. A wet flue gas desulfurization apparatus provided with a slurry stirring tank for performing at least a reduction operation of dissolved calcium salt concentration in the reaction. (6) The inside of the oxidation tank of the wet flue gas desulfurization apparatus of the above (5), the outlet, the inside of the slurry agitation tank, or at least one of the outlets thereof, the sulfite or dissolved oxygen concentration in the slurry is measured to measure the inside of the oxidation tank. Alternatively, a method for operating a wet flue gas desulfurization device in which the amount of oxidizing air supplied to the slurry in the slurry stirring tank is determined.

[0007]

When the concentration of Ca ²⁺ existing in the liquid phase portion of the slurry in the absorption tower circulation tank increases, the reactivity of limestone and the oxidation performance of sulfite decrease. In the conventional general desulfurization system, the amount of waste water discharged to the outside of the system was adjusted so that the Cl ^-1 concentration in the liquid was maintained at a predetermined level. In the present invention, although the Cl ^-1 concentration is the dominant factor for the corrosion characteristics of equipment materials, it is rather Ca ²⁺ for the above desulfurization equipment characteristics.
It was made clear that it is largely due to the increase in ion concentration. The present invention is based on the basic principle of reducing the concentration of dissolved calcium salt in the absorbent slurry such as limestone used for the reaction with the exhaust gas, and specifically for the slurry extracted from the absorption tower circulation tank. This is accomplished by adding sulfate. By adding soluble sulfates such as magnesium sulfate, sodium sulfate, potassium sulfate and ammonium sulfate to the absorbent slurry, Ca present in high concentration in the slurry in the circulation tank.
²⁺ ions are deposited as gypsum and reduce its concentration.

The gypsum produced by this operation is separated and recovered in the latter stage gypsum separation step together with the gypsum produced in the absorption tower circulation tank, while the filtrate (mother liquor for gypsum separation) is circulated to the limestone slurry adjusting step, but the gypsum separation is performed. The Ca ²⁺ concentration in the mother liquor becomes a low level compared to the time when it was extracted from the absorption tower circulation tank. The basic principle of the present invention is to improve the oxidation characteristic of sulfite by reducing the concentration of dissolved calcium salt in the slurry, and the oxidation rate of sulfite in the absorption tower circulation tank achieves the desired value. If it is not possible, an aqueous solvent is added to the slurry extracted from the absorption tower circulation tank, or an aqueous solvent is added to the slurry in which the solid content is concentrated after the extracted slurry is once subjected to solid-liquid concentration operation. The oxidation treatment can be carried out in the state of adding. The latter method is an effective means when a substance added for the purpose of improving the desulfurization property has an inhibitory effect on sulfite oxidation.

[0009]

The present invention will be described in more detail below with reference to the drawings, but before that, the method of the present invention will be described based on experimental examples. Experimental Example 1 800 ml of pure water was placed in a polyvinyl chloride reaction container having a volume of about 1500 ml, and a predetermined amount of calcium chloride (C
aCl ₂ · 2H ₂ O), calcium sulfite (CaSO ₃ ·
(1/2 H ₂ O) 0.1 mol and gypsum 10 g were added with good stirring, and water was further added to prepare a 1000 ml slurry. The slurry was heated with stirring, and when it was confirmed that the temperature reached 30 ° C., the pH was adjusted to 5 using 1 MH ₂ SO ₄ . Next, while adjusting the pH using 1MCa (OH) ₂ or 1MH ₂ SO ₄ , a predetermined amount of air was supplied from the lower part of the stirring blade to generate fine air bubbles, thereby oxidizing calcium sulfite. Table 1 shows that the oxidation rate was calculated by measuring the total sulfite concentration at every predetermined time, and CaCl ₂ , Cl ⁻¹ and Ca were calculated.
^{It is summarized} as the effect of ²⁺ concentration, but CaCl ₂
As the concentration increases, the rate of sulfite oxidation decreases. Ca
When the concentration of Cl _{2 is} 110 g / l, the oxidation rate is reduced to about 1.7 times that of the case of no addition.

[0010]

[Table 1] The results shown in Table 1 are the basic data of the example. When the Cl ⁻¹ concentration of the absorbent slurry in the absorption tower circulation tank increases, the concentration of the counter ion Ca ²⁺ ion also increases, and sulfite The oxidation rate of is decreased. In such a case, in order to obtain a predetermined oxidation rate, it is necessary to increase the capacity of the absorption tower circulation tank or the supply air amount, but basically, the concentration of the soluble calcium salt present in the absorbent slurry should be controlled. It is efficient to lower it.

Experimental Example 2 Using seawater instead of pure water as the reaction base liquid in Experimental Example 1, an experiment for oxidizing calcium sulfite was conducted in the same manner as in Experimental Example 1. The experimental results are shown in Table 2.

[Table 2]

The seawater used was 18 g / l of Cl ^-1.
Contains ions, but the oxidation rate of sulfite is 1.49 mmo
It is 1 / liter · min, and even if the Cl ⁻¹ ion concentration is the same, the oxidation rate is higher than in the case of using pure water shown in Experimental Example 1-1 of Table 1 above. This is CaSO ₄ 1 / 2H ₂
The rate of oxidation of O is determined by Ca ²⁺ rather than Cl ^-1 ,
It also shows that seawater contains a component that promotes the oxidation of sulfite. The addition of calcium chloride reduces the oxidation rate of sulfite as in the case of using pure water, but the addition of seawater has the higher oxidation rate even with the same CaCl ₂ concentration, and the supply water to the desulfurization equipment is higher. It can be said that it is possible to use seawater as a part of.

Embodiments of the present invention will be described with reference to the drawings based on the facts of the above experimental examples, but the present invention is not limited to those described in each of the following embodiments. First, an embodiment having the configuration of the flue gas desulfurization apparatus shown in FIG. 1 will be described. Figure 1
4, the same members as the respective constituent members of the flue gas desulfurization apparatus shown in FIG. 4 are represented by the same numbers as those in FIG. 4, and the description thereof is omitted. This embodiment is characterized by a method of treating the absorbent slurry withdrawn from the absorption tower circulation tank 8. That is, a predetermined amount of the slurry in the absorption tower circulation tank 8 is extracted by the pump 101.
Thus, the slurry is guided to the slurry stirring tank 104 peculiar to this embodiment. Soluble sulfate in the chemical tank 102 is added to the slurry in the slurry stirring tank 104 via a line 103, and Ca ²⁺ ions in the system are added to gypsum (CaSO ₄ .2H ₂
After being settled as O), the gypsum-concentrated slurry is extracted by the slurry pump 108 and guided to the gypsum separator 15. Subsequent operations are carried out in the same manner as the conventional method shown in FIG.

One of the objects of the present invention is to promote the oxidation reaction of sulfite by lowering the concentration of dissolved calcium salt present in the liquid phase of the absorbent slurry. In the case of this example, the above object is achieved by increasing the concentration of dissolved sulfate in the liquid phase of the absorbent slurry, but in order to obtain the maximum effect when a certain amount of sulfate is added, It is desirable to make the concentration of dissolved sulfate at the sulfate addition site as high as possible. For this purpose, the soluble sulfate salt is not charged into the absorption tower circulation tank 8 and the
The slurry stirring tank 10 different from the absorption tower circulation tank 8
4 to a method for adjusting the dissolved calcium salt concentration in the liquid phase to a predetermined level by adding an aqueous solvent such as industrial water or seawater from line 103. Regarding the addition of seawater, as will be made clear in later examples, the oxidation rate of sulfite does not decrease as long as the Cl ⁻¹ ion concentration in the liquid is kept within a predetermined range.

The method of FIG. 2 is effective when the oxidation of sulfite in the absorption tower circulation tank 8 is insufficient,
The basic method is to oxidize the remaining sulfite with the dissolved calcium salt concentration in the liquid phase being reduced. In the method of FIG. 2, an agitator 105 is installed in the slurry agitating tank 104, and air 106 is provided near the agitating blades of the agitator 105.
Is supplied and the sulfite remaining in the slurry introduced into the slurry stirring tank is oxidized. Even if the Ca ²⁺ ion in the absorbent slurry in the absorption tower circulation tank 8 becomes high in concentration and the oxidation reaction of sulfite becomes difficult to proceed, it exists in the liquid phase of the slurry by the method of this example. By reducing the concentration of dissolved calcium salt, the rate of sulfite oxidation can be increased. A predetermined amount of the slurry in the slurry stirring tank 104 is extracted by the extraction pump 108 and sent to the gypsum separator 15 to collect the powdered gypsum. Gypsum separation mother liquor generated in gypsum separator 15
The treatment of No. 7 is carried out in the same manner as in the conventional method, but the calcium salt concentration in the CaSO ₄ .2H ₂ O separated mother liquor becomes lower than in the conventional method.

In the case of the construction as shown in FIG. 2, the absorption tower circulation tank 8 becomes a part of the multi-stage oxidation reactor for sulfite, and the sulfite is applied only to the slurry in the slurry stirring tank 104 which is the gypsum recovery target. Therefore, the oxidation treatment is thoroughly performed, and extremely good results can be obtained in terms of device efficiency. The present invention is not restricted by the tank capacity, the stirring method, the SO ₂ absorption conditions, and the alkaline calcium compound (carbonate, hydroxide, oxide) that serves as an SO ₂ absorbent, but among these, oxidation With regard to (1), it is easily achieved by supplying fine air into the slurry storage tank, and as a method of generating the fine air bubbles, a method of supplying air in the vicinity of the agitator stirring blades. Figure 2 shows a slurry stirring tank 1
It is the figure which showed the apparatus flow of the method of supplying the air 106 near the stirring blade of the stirrer 105 of No. 04, and reducing the residual sulfite to a trace extent using the fine air bubble generated by this.

The method of the present invention shows excellent desulfurization characteristics even when the absorbent slurry contains a high concentration of dissolved chloride, and is effective in making the desulfurization apparatus compact and reducing the utility. _{2 When} limestone is used as an absorbent and gypsum is recovered, it is sufficient that the gypsum to be finally recovered has a predetermined property, and the slurry in the absorption tower circulation tank 8 contained a small amount of sulfite. However, there is no practical problem. The post-treatment of the absorbent slurry in FIG. 2 may be performed by paying attention to the characteristics of the slurry stirring tank 104.
Therefore, the sulfite content of the slurry may be determined to be equal to or less than the predetermined value. To that end, the sulfite or dissolved oxygen concentration in the slurry discharged from the slurry agitation tank 104 is measured, and when it is determined that a predetermined amount or more of sulfite is contained, the air supply amount is increased,
On the other hand, when the oxidation of the sulfite is sufficiently advanced and thus a predetermined amount or more of dissolved oxygen is present in the slurry, an operation method of reducing the air supply amount is a practical method.

Further, the embodiment of FIG. 3 has an absorption tower circulation tank 8
It shows a flow in the case where the slurry extracted from the slurry is once introduced to the solid-liquid separation device 109, and the concentrated solid content is introduced to the slurry stirring tank 104 to perform the additional oxidation treatment of the remaining sulfite. In this method, the slurry stirring tank 10
In 4, the concentration of dissolved calcium salt in the liquid phase is adjusted to a predetermined level by adding an aqueous solvent consisting of industrial water or seawater from line 103. After the remaining sulfite is oxidized, the slurry is taken out from the pump 108.
Is sent to the gypsum separator 15. The solution portion extracted from the solid-liquid separator 109 is circulated in the limestone slurry tank 6.

In the above embodiment of the present invention, the absorption tower circulation tank 8 of FIG. 1 or the slurry stirring tank 104 of FIGS.
As described in the embodiment of FIG. 2, the oxidation of sulfite carried out in step 1 is carried out by measuring the concentration of sulfite or dissolved oxygen in the slurry inside the absorption tower circulation tank 8 or the slurry stirring tank 104 or at the outlet, and the absorption tower circulation tank. 8 or slurry stirring tank 10
It is desirable to determine the amount of oxidizing air supplied to the inside of No. 4 and adjust it so that the sulfite content of the slurry becomes a predetermined value or less. In the method of each of the embodiments of the present invention described above, sulfite oxidation is carried out in a state in which the concentration of substances that suppress the oxidation of dissolved Ca ²⁺ and other sulfite in the slurry is reduced. .

[0020]

According to the present invention, the absorbent slurry has a high Cl content.
^{Even if the} concentration becomes ^-1 , the desulfurizer can be operated while maintaining the oxidation rate of sulfite and / or the utilization rate of the absorbent at the same level as in the conventional method.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a flow of a wet flue gas desulfurization apparatus in which a soluble calcium salt concentration is reduced by adding a soluble sulfate salt according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a wet flue gas desulfurization apparatus flow of a method of oxidizing residual sulfite with an aqueous solvent added according to an embodiment of the present invention.

FIG. 3 is an illustration of a flow of a wet flue gas desulfurization apparatus of a type in which a solid content is concentrated and then an aqueous solvent is added to oxidize sulfite in a state where the concentration of dissolved calcium salt is low, according to an embodiment of the present invention. It is a figure.

FIG. 4 is an explanatory diagram of a flow of a conventional wet flue gas desulfurization apparatus.

[Explanation of symbols]

1 ... Exhaust gas, 2 ... Absorption tower inlet duct, 3 ... Absorption tower, 4 ...
Demister, 5 ... Limestone, 6 ... Limestone slurry tank, 7 ...
Limestone slurry supply pump, 8 ... Absorption tower circulation tank, 9 ...
Absorption tower circulation pump, 10 ... Air, 11 ... Stirrer for oxidation,
12 ... Slurry agitator, 13 ... Gypsum slurry tank, 15 ...
Gypsum separator, 16 ... Powdered gypsum, 17 ... Gypsum separation mother liquor, 1
8 ... Filtered water tank, 20 ... Wastewater treatment device, 102 ... Chemical tank, 103 ... Chemical or aqueous liquid supply line, 104
... Slurry stirring tank, 105 ... Oxidation stirrer, 106 ... Air, 109 ... Solid-liquid separation device

Claims (12)

[Claims] 1. A desulfurizing agent slurry is used to absorb sulfur oxides in exhaust gas, and the sulfite obtained by absorbing sulfur oxides in exhaust gas is oxidized into gypsum, and then the gypsum-containing slurry is dehydrated. In the wet flue gas desulfurization method having the function of separating the gypsum and the gypsum-separated mother liquor, and using the gypsum-separated mother liquor to newly prepare a desulfurizing agent slurry to be supplied into the exhaust gas, the gypsum-containing slurry is converted into gypsum and gypsum. Before separating into separation mother liquor
A wet flue gas desulfurization method, characterized in that at least the dissolved calcium salt concentration in the gypsum-containing slurry and the oxidation of the gypsum-containing slurry are performed to reduce the dissolved calcium salt concentration in the gypsum-containing slurry. 2. The method for reducing the concentration of dissolved calcium salt is performed by adding a soluble sulfate-containing aqueous solvent other than calcium salts such as magnesium sulfate, sodium sulfate, potassium sulfate and ammonium sulfate to the gypsum-containing slurry. The wet flue gas desulfurization method according to claim 1. 3. The method for reducing the concentration of dissolved calcium salt is performed by adding an aqueous solvent.
The wet flue gas desulfurization method described. 4. The wet flue gas desulfurization method according to claim 3, wherein at least a part of the aqueous solvent is industrial water or seawater. 5. A desulfurizing agent slurry is used to absorb sulfur oxides in exhaust gas, the sulfur oxides obtained by absorbing sulfur oxides in exhaust gas are oxidized into gypsum, and then the gypsum-containing slurry is dehydrated. In the wet flue gas desulfurization method having the function of separating the gypsum and the gypsum-separated mother liquor, and using the gypsum-separated mother liquor to newly prepare a desulfurizing agent slurry to be supplied into the exhaust gas, the gypsum-containing slurry is converted into gypsum and gypsum. Before separating into separation mother liquor
A solid-liquid concentration operation is performed once on the gypsum-containing slurry, and at least the dissolved calcium salt concentration is reduced in the solid content-concentrated slurry obtained by the operation and the solid content-concentrated slurry is oxidized. Wet flue gas desulfurization method characterized by carrying out. 6. The method for reducing the concentration of dissolved calcium salt is performed by adding a soluble sulfate-containing aqueous solvent other than calcium salts such as magnesium sulfate, sodium sulfate, potassium sulfate and ammonium sulfate to the gypsum-containing slurry. The wet flue gas desulfurization method according to claim 5. 7. The wet flue gas desulfurization method according to claim 5, wherein the operation for reducing the concentration of the dissolved calcium salt is performed by adding an aqueous solvent. 8. The wet flue gas desulfurization method according to claim 7, wherein at least a part of the aqueous solvent is industrial water or seawater. 9. An absorption tower that absorbs sulfur oxides in exhaust gas using a desulfurizing agent slurry and the sulfite salt in the sulfite-containing slurry obtained in the absorption tower are oxidized to obtain gypsum-containing slurry. Desulfurization for preparing a desulfurizing agent slurry to be supplied to an absorption tower using a gypsum separator for dehydrating an oxidation tank and the gypsum-containing slurry to separate gypsum and a gypsum separating mother liquor, and a gypsum separating mother liquor obtained from the gypsum separating machine In a flue gas desulfurization device equipped with an agent slurry tank, between the oxidation tank and the gypsum separator, at least the dissolved calcium salt concentration reduction operation for the gypsum-containing slurry and the oxidation reaction should be performed. A wet flue gas desulfurization apparatus, which is provided with a slurry stirring tank for performing. 10. The wet flue gas desulfurization apparatus according to claim 9, wherein the concentration of sulfite or dissolved oxygen in the slurry of at least one of the inside of the oxidation tank, the outlet thereof, the inside of the slurry stirring tank and the outlet thereof is measured to measure the concentration. A method for operating a wet flue gas desulfurization apparatus, characterized in that an amount of air for oxidation supplied to a slurry in an oxidation tank or a slurry stirring tank is determined. 11. An absorption tower that absorbs sulfur oxides in exhaust gas using a desulfurizing agent slurry and the sulfite salt in the sulfite-containing slurry obtained in the absorption tower are oxidized to obtain gypsum-containing slurry. A limestone slurry for preparing a limestone slurry to be supplied to an absorption tower using a gypsum separator for dehydrating an oxidation tank and the gypsum-containing slurry to separate gypsum and a gypsum separating mother liquor, and a gypsum separating mother liquor obtained from the gypsum separating machine In a flue gas desulfurization device equipped with a tank, a tank for solid-liquid concentration of a gypsum-containing slurry and an operation for reducing the concentration of dissolved calcium salt in the solid-liquid concentrated gypsum-containing slurry between an oxidation tank and a gypsum separator And the oxidation reaction,
A wet flue gas desulfurization apparatus comprising a slurry stirring tank for performing at least a reduction operation of dissolved calcium salt concentration. 12. A wet flue gas desulfurization apparatus according to claim 11, wherein the concentration of sulfite or dissolved oxygen in the slurry of at least one of the inside, the outlet and the inside of the slurry stirring tank of the oxidation tank is measured to perform the oxidation. A method for operating a wet flue gas desulfurization apparatus, characterized in that an amount of oxidizing air supplied to a slurry in a tank or a slurry stirring tank is determined.