JPS6048220B2 - Method for removing chlorine ions from wet flue gas treatment process - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing chloride ions from a wet flue gas treatment process.

Conventionally, in wet flue gas treatment processes, many methods have been adopted to completely eliminate wastewater from the viewpoint of pollution prevention.

However, this method has the disadvantage that chlorine, which is contained in industrial water, heavy fuel oil, chemicals, etc., is concentrated and accumulated in the circulating absorption liquid due to long-term operation of the device. In this case, when the chlorine ion concentration in the circulating absorption liquid reaches 4,000 to 6,000 ppm, it causes stress corrosion cracking and pitting corrosion of stainless steel, which is the main material of absorption towers, etc., and there is no effective method for removing chlorine ions. It was wanted. Conventionally, in order to avoid problems caused by accumulation of chlorine ions, part or all of the circulating absorption liquid whose chlorine ion concentration exceeds a predetermined concentration has been extracted from the system.

However, this method has the drawback that it is difficult to detoxify the extracted circulating absorption liquid, and discarding it causes secondary pollution.

Moreover, it is necessary to replenish a new amount of absorption liquid corresponding to the amount extracted from the system, and the extracted liquid contains a large amount of dissolved salts, resulting in a large economic disadvantage. Therefore, the present inventors have devised a method to solve the above-mentioned drawbacks and carry out exhaust gas treatment in a closed system as much as possible while maintaining the chlorine ion concentration in the treatment process, especially in the absorption system, below a predetermined level. As a result of intensive research for development, the present invention was arrived at.

The wet method treatment of flue gas containing sulfur dioxide gas is shown in a flow sheet as shown in Figure 1, in which an absorption liquid containing magnesium compounds such as magnesium oxide, magnesium sulfite, and magnesium sulfate is discharged in an absorption system such as absorption tower 1. The sulfur dioxide gas in the flue gas is absorbed by contact with smoke, and the reaction product and unreacted absorbent are separated by a solid-liquid separation system such as a centrifuge 2, and the mother liquor is returned to the absorption system for circulation.

On the other hand, the separated solid content is dried in a drying system such as a dryer 3, and then burned in a calcination system using a calcination machine 4 to recover sulfur dioxide gas and regenerate magnesium compounds, which are then used as reheating absorbents. do. Further, the recovered gas containing sulfur dioxide gas is used as a raw material for simple sulfur or sulfuric acid after being cleaned by a cleaning system such as a cleaning tower 6. Note that the liquid extracted from the washing tower is returned to the circulation absorption system. If we pay attention to the behavior of chlorine ions in the above flue gas treatment process, we can see that these ions not only circulate in the circulation absorption system together with other components, but also partially in the absorption system - solid-liquid separation system -> drying system - calcination system -> It circulates through the cleaning system → absorption system.

The present inventors investigated the latter circulation route and found that solid content (wet cake) with a water content of 2% by weight or more after solid-liquid separation using a centrifuge usually contains 5 to 3% water.
It contains 0% water by weight, and the chlorine ions in this water go through the drying and firing system and are released as hydrogen chloride into the gas generated during firing, and eventually more than 90% of it goes to the cleaning tower. It was confirmed that the substance was trapped in the liquid. Moreover, the amount of chlorine captured is the chlorine ion in the entire system, that is, in the wet flue gas treatment process.
This amount exceeds the amount generated. Therefore, the present invention has been completed based on the discovery that accumulation of chlorine ions in the absorption system can be effectively prevented by discharging a portion of the liquid extracted from the washing tower to the outside of the process. In the present invention, an absorption liquid containing magnesium compounds is circulated and brought into contact with flue gas containing sulfur dioxide gas, and a part of the circulation absorption liquid is separated into solid and liquid, and the resulting solid content is dried and calcined to regenerate the magnesium compounds. In the wet flue gas treatment process in which the gas generated during firing is washed with water, at least a portion of the liquid extracted from the gas cleaning tower generated during firing is used in the wet flue gas treatment process. The water content to be subjected to the regeneration treatment is inversely proportional to the water content of solids of 2% by weight or more, so as to maintain the chloride ion concentration in the absorption system at about 3000 ppm or less by removing it to the outside. The present invention provides a method for removing chlorine ions from a wet flue gas treatment process, which is characterized by adjusting the proportion of liquid extracted from a cleaning tower to be removed outside the wet flue gas treatment process.

Here, the wet flue gas treatment process refers to the entire series of treatment systems including an absorption system, a solid-liquid separation system, a drying system, a firing system, and a cleaning system.

As mentioned above, the liquid extracted from the cleaning tower 6 contains a large amount of chlorine ions, so by removing all or a part of the liquid extracted outside the wet flue gas treatment process, it is possible to remove the liquid from the absorption system. It is possible to prevent the accumulation of chlorine ions and maintain them below a predetermined level.

The proportion of the extracted liquid to be removed outside the wet flue gas treatment process is adjusted in consideration of the relationship between the target chlorine ion concentration level of the absorbing liquid and the moisture content of the wet cake entering the dryer.

For example, if the amount of chlorine ions brought into the wet flue gas treatment process is 0.70 k9/Hr derived from industrial water and 0.18 k9/Hr derived from magnesium compounds, then the water content in the wet cake is 15 If the water content is 2600k9/Hr, if the entire amount of liquid extracted from the cleaning tower is removed outside the wet flue gas treatment process, the chlorine ion concentration in the process, especially in the absorption system, will be as follows: It is maintained at about 340 ppm derived from the formula. 0.70+0.18=2600XX×10-6×-h
-y1+Y2 Here, Y is the amount of liquid extracted from the cleaning tower to the outside of the process.

represents the amount of liquid returned from the washing tower to the absorption tower, Y, = 10
If 0,y2=0 is used as a proxy, then X: 340ppm.Also, if the removal rate of the extracted liquid is about 11%, then Y,! Y.

If -0 tears are substituted into the above formula, it becomes X 3000, and the chlorine ion concentration in the process, especially in the absorption system, becomes about 3000 ppm. Reduce the chloride ion concentration in the absorption system to approximately 3000
By maintaining the concentration below ppm, troubles such as corrosion of the absorption tower etc. can be avoided and stable operation can be performed for a long period of time.

Here, when the chlorine ion concentration exceeds about 3000 ppm,
In particular, if the concentration exceeds 4000 ppm, stress corrosion cracking, pitting corrosion, etc. may occur in absorption towers and the like.

Note that the chloride ion concentration at this time may be measured at any position within the absorption system, but it is usually measured at the outlet of the absorption system, that is, the outlet of absorption tower 1 or its vicinity. be. In the method of the present invention, if the chloride ion concentration in the absorption system is maintained at about 3,000 ppm or less, even if the chlorine ion concentration increases due to various changes in circumstances, it will rarely exceed 14,000 ppm. Therefore, there is no risk of corrosion occurring in the absorption tower. In the present invention, if the water content in the wet cake entering the dryer is 2% by weight or more, the chlorine ion concentration in the absorption system is maintained at 3000 ppm or less by discharging a predetermined amount of the liquid extracted from the washing tower outside the process. I can do that.

The amount of liquid extracted from the cleaning tower to be removed outside the wet flue gas treatment process may be inversely proportional to the water content of the solids to be subjected to regeneration treatment. That is, the higher the water content in the wet cake, the lower the removal rate of the extracted liquid. For this purpose, for example, a method may be adopted in which a portion of the circulating absorption liquid is sent to the dryer bypassing the centrifugal separator. The effluent from the washing tower may be removed continuously or intermittently.

In addition, by temporarily increasing the water content in the wet cake and increasing the chlorine ion concentration in the liquid extracted from the cleaning tower, it is possible to reduce the amount of liquid removed outside the wet flue gas treatment process. can. The liquid extracted from the cleaning tower removed outside the wet flue gas treatment process usually has a pH in the range of 1.5 to 3.5, a slurry concentration in the range of O to 3% by weight, and a total of S.O.

Concentration ranges from 5 to 8% by weight Supernatant COD is 700 to 15
It is in the range of 00 ppm. After neutralizing this by adding an alkaline agent, if necessary, adding a flocculant etc., and separating solid and liquid by methods such as filtration and standing, the suspended solid content of the separated liquid is about 10 to 20 ppm, total SO. The concentration is 2000-3
000 ppm, and COD decreases to about 25 to 30 ppm. Furthermore, this separated liquid is treated with air to remove all SO.
. Since the concentration can be reduced to about 20 to 100 ppm, it can be disposed of without causing secondary pollution. As an alkaline agent for neutralization, as shown in Figure 1, a method can be adopted in which the separated solids are returned to the flue gas treatment process using magnesium oxide slurry during the flue gas treatment process. . Furthermore, when implementing the present invention,
The operating conditions of the entire processing system i, including the temperatures of the drying system and firing system, may be set in the same way as in the normal case.

[Brief explanation of drawings]

FIG. 1 is a flow sheet showing one embodiment of the present invention.

Claims (1)

[Claims] 1. The absorption liquid containing magnesium compounds is circulated and brought into contact with flue gas containing sulfur dioxide gas, and a part of the circulating absorption liquid is separated into solid and liquid, and the resulting solid content is dried and calcined to regenerate and use the magnesium compounds. In addition, in a wet flue gas treatment process in which the gas generated during firing is washed with water, at least a portion of the liquid extracted from the cleaning tower of the gas generated during firing is removed to outside the wet flue gas treatment process. Wet flue gas treatment is carried out so that the water content subjected to the regeneration treatment is inversely proportional to the water content of solids of 2% by weight or more, so as to maintain the chloride ion concentration in the absorption system at about 3000 ppm or less. A method for removing chlorine ions from a wet flue gas treatment process, which comprises adjusting the proportion of liquid extracted from a cleaning tower to be removed outside the process.