JPS6366277B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for purifying wastewater generated when sulfur oxides (hereinafter referred to as SOx) and nitrogen oxides (hereinafter referred to as NOx) in flue gas are removed by wet desulfurization and denitration, and in particular, The present invention relates to a method for decomposing and removing amide and imide nitrogen compounds such as amide sulfuric acid, its salts, and imido disulfate. In order to prevent air pollution caused by SOx and NOx contained in the combustion flue gas of fossil fuels such as coal and oil, various desulfurization and denitrification methods have been proposed, and some of them have been implemented industrially. Although quite excellent methods have been developed for desulfurization, there are still many problems with denitrification that need to be solved. For example, desulfurization and denitrification methods using sulfites are often employed, but the wastewater produced during this process contains imide nitrogen compounds such as imidodisulfate and amide nitrogen compounds such as amide sulfate. These forms of nitrogen compounds are chemically extremely stable and difficult to decompose. In order to decompose nitrogen compounds such as imidodisulfate and amidosulfate, JP-A-51-23470 discloses NO ₂ /NO obtained by oxidizing nitrogen monoxide contained in the exhaust gas of the denitrification process with ozone. A method using a gas mixture with a molar ratio of 1 is disclosed. However, adjusting this molar ratio is extremely complicated. Also, a method has been proposed to decompose these nitrogen compounds into nitrogen gas using nitrous acid or nitrites, but both nitrites and nitrites are designated as deleterious substances, and the lethal dose for humans is 2g. Highly toxic. Therefore, this method poses a major problem in operational safety. Therefore, the present invention overcomes the drawbacks of the prior art and provides flue gas desulfurization and denitrification that can easily decompose imide and amide nitrogen compounds under mild conditions using non-toxic chemicals as purification agents. The purpose is to provide a method for treating wastewater. According to the present invention, this purpose is achieved by using sulfite or bisulfite, which poses no safety problems and is widely used as a reducing agent, in an equivalent amount of 1.0 to 1.0 to This is achieved by adding 5.0 times the amount to decompose the nitrogen compound. The sulfites or bisulfites used may be salts of alkali metals such as sodium, potassium, ammonium salts, and the like. The decomposition reaction is carried out at a pH of 3 or higher, preferably at a pH of 3.0 or higher.
12.0 and is carried out at temperatures between 10 and 100 °C. According to the present invention, since the decomposition reaction is carried out in such a wide PH range and temperature range, there is no need to worry about adjusting the reaction conditions. Next, the present invention will be described in detail based on Examples, but the present invention is not limited thereto. Example 1 As wastewater containing imidodisulfate and amidosulfate, potassium imidodisulfate 425 mg/(nitrogen content 25 mg/) and sulfamic acid 174 mg/
(Nitrogen content 25 mg/) was mixed and adjusted at a ratio of 1:1, and sodium sulfite was added to this wastewater in the equivalent amount shown in Table 1 below based on the amount of potassium imidodisulfate and sulfamic acid. Further, the pH was adjusted to 3 to 12 with sulfuric acid or sodium hydroxide, and the mixture was reacted with stirring using a jar tester at temperatures of 15°C and 90°C for 2 hours to carry out decomposition treatment. Thereafter, the nitrogen concentration of the decomposed water was measured.

[Table] As can be seen from this table, 70 to 80% of potassium imidodisulfate and sulfamic acid were removed in terms of nitrogen concentration at a weakly acidic pH of 3, and 80 to 90% at an alkaline pH. That is, it was confirmed that the reducing action of sodium sulfite decomposes imidodisulfate and amidosulfate. Example 2 The same synthetic wastewater as in Example 1 was purified in the same manner as in Example 1 except that sodium bisulfite was used instead of sodium sulfite. The results and processing conditions are shown in Table 2.

[Table] As shown in Table 2, similar results were obtained for PH3 and PH12 as in Example 1, with 70%
The above nitrogen compounds are decomposed and removed. That is, it was confirmed that imidodisulfate and amidosulfate can be decomposed even when bisulfite is used. In addition, in Tables 1 and 2, the column for nitrogen concentration of treated water shows the lowest and highest values obtained when experiments were conducted several times using the batch method. As mentioned above, according to the method of the present invention, by using sulfite or bisulfite, imidodisulfate and amidosulfate in desulfurization and denitrification wastewater can be decomposed without using chemicals harmful to the human body. There is also the advantage that no complicated adjustment of decomposition conditions is required.

Claims (1)

[Scope of Claims] 1. In order to purify wastewater generated by removing sulfur oxides and nitrogen oxides from flue gas by a wet method, sulfite or bisulfite is added to the wastewater to remove the imide form in the wastewater. and 1.0 to 5.0 times the equivalent amount required for reaction with the amide nitrogen compound.
A method for treating flue gas desulfurization and denitrification wastewater, characterized by decomposing imide and amide nitrogen compounds by reacting at pH 3.0 or higher. 2. The method according to claim 1, wherein the reaction is carried out at a pH of 3.0 to 12.0 and a temperature of 15 to 90°C.