JPS6140885B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for stabilizing fly ash contained in flue gas discharged from a municipal waste incinerator, and more specifically, the present invention relates to a method for stabilizing fly ash contained in flue gas discharged from a municipal refuse incinerator. Fly ash (dust,
EP ash) is collected, the flue gas from which fly ash has been removed is cooled and washed in a quenching tower using smoke washing water, further washed with alkali in the washing tower, and then released into the atmosphere.
A method of treating the fly ash collected by the cyclone and electrostatic precipitator with mineral acid to selectively elute soluble toxic heavy metals and extracting and separating them from the fly ash, thereby stably processing the ash without causing environmental pollution. It provides: In the flue gas from municipal waste incinerators, municipal waste itself is composed of various miscellaneous materials, whereas incineration ash (clinker ash) is composed of metals whose main component is Ca. There are many harmful heavy metals such as Fe, Pb, Cr, Zn, Cd, Cu, Hg, etc. vaporized as metal chlorides caused by vinyl chloride, etc., and therefore this fly ash (dust,
These harmful heavy metals are also contained in the so-called smoke washing water that collects EP ash. It is said that when one ton of municipal waste is incinerated, 0.3 ton of incinerated ash and 0.01 ton of fly ash are produced, so the disposal of this fly ash has become a problem that cannot be taken lightly. The incineration ash is mainly composed of Ca-Metal, and most of the Ca compounds have high boiling points, are stable, and have low solubility in water, so even if they are disposed of naturally, such as by being used in landfills, the leachate will not cause much of a problem. Nakatsuta. On the other hand, as mentioned above, fly ash often contains harmful heavy metals in the form of soluble salts such as chlorides, and if this is disposed of underground, it will leach out in rainwater, etc., causing river or marine pollution. It is becoming. For this reason, up until now, it has been diluted and dissolved with a large amount of water and then precipitated.
The liquid was discharged to meet environmental standards, and the precipitate was solidified into concrete and discarded to prevent harmful components from leaching out. However, this method is difficult to control because the components in smoke washing wastewater are not constant, and requires a large amount of water. Not only is it difficult to completely prevent leaching from individualized substances, but the treatment is very expensive. There were other drawbacks. In order to eliminate these drawbacks, the inventors of the present invention have repeatedly conducted research on a stable treatment method for fly ash in flue gas from municipal waste incinerators, and found that fly ash is dispersed and mixed in acidic water to form a slurry liquid. By dissolving and removing harmful components while adjusting the pH to a suitable level, the resulting cake is separated from the slurry and then washed with water.
In other words, 10 g of cake with a moisture content of 80% or less was added to 100 ml of water adjusted to pH 5.8 to 6.3, and the superhydrate was shaken for 6 hours, resulting in Cd0.3, Hg0.005, Cr ⁶⁺ 1.5, As1.5, Pb3. 0 each
mg/or less, and furthermore, Cd0.01, Hg0.0005, Cr ⁶⁺ 0.05, As0.05,
The present invention was achieved by discovering that Pb0.1, Mn10, Fe10, T-Cr2 each passed the test. The fly ash is treated with mineral acid, and after the extraction operation and the cake is over-separated, the liquid is combined with the cake washing water and used as the washing water of the quenching tower, which is the smoke washing device (this is called smoke washing water). By using a part of the smoke, it is evaporated and concentrated using the enthalpy of the flue gas, and is processed by ferrite treatment, that is, Japanese Patent Application No. 116653 (1972) filed by the applicant on October 16, 1971 (Japanese Patent Application No. No.) Air is introduced into the heavy metal-containing wastewater containing ferrous ions in accordance with the "method for treating waste liquid from incinerator waste gas washing," which oxidizes it to ferric ions while producing crystals with a spinel-type structure and precipitating it. By applying the so-called ferrite treatment, heavy metals can be separated using a magnetic separator, so that the treatment can be carried out stably. This ferrite sludge, of course, passed the elution standards for industrial waste. The liquid after the ferrite sludge was separated could be used repeatedly as a cleaning liquid for the smoke cleaning device, and also satisfactorily passed the wastewater discharge standards. That is, the present invention provides a method for safely and economically disposing of fly ash (cyclone dust, electrostatic precipitator dust) in flue gas discharged from an incinerator and toxic heavy metals contained in the liquid obtained by washing the flue gas with water. The gist of this project is to provide a method for stably processing fly ash in flue gas from municipal waste incinerators by dispersing and mixing fly ash in mineral acid, maintaining the pH between 4.5 and 5.5, and improving its solubility. a step of selectively eluting and extracting harmful heavy metals from the fly ash; a step of over-separating the extracted fly ash particles from a solution to obtain a cake; and a step of washing the cake with water; It is characterized by comprising the steps of combining the washing water and supplying it as part of the washing water to a quenching tower that cools flue gas, and processing the washing water discharged from the quenching tower into ferrite. This is a method for stabilizing the fly ash. In the mineral acid extraction system used in the present invention, the dust containing the harmful heavy metals is placed in a tank equipped with a stirrer at a ratio of 100 to 400 g per part, and water is added if necessary to adjust the concentration to the above concentration. Add sulfuric acid to adjust the pH so that the alkali equivalent is as small as possible in the neutralization process.
Continue stirring and mixing for up to 6 hours while maintaining the temperature between 4.5 and 5.5. When treated in this manner, harmful heavy metals are dissolved and separated, and almost no Cd is present in the extracted fly ash particles, which pass the industrial waste treatment standards as described above. At this time, the cake is washed with water, or this water is combined with the liquid and used as part of the smoke washing water in a quenching tower, and the enthalpy of the exhaust gas condenses and raises the temperature, so that the ferrite treatment is effectively carried out. Next, the steps of the present invention will be explained with reference to FIG. Flue gas 1 discharged from the incinerator enters a cyclone 2 and an electrostatic precipitator 2', and the fly ash containing harmful heavy metals removed here is sent to a tank 3. Sulfuric acid or hydrochloric acid is added from line 16 into the tank, and water for dissolution is added from line 17 and stirred to disperse and mix the fly ash in the mineral acid. After maintaining the pH in the tank at 4.5 to 5.5 for a maximum of 6 hours, it is sent to filter 4 to separate solid and liquid and obtain a cake.The cake remaining in the filter is then washed with cake washing water from line 18, and this The cake after the acid extraction treatment is referred to as cake A. The liquid from the filter 4 and the washing water after washing the cake are sent to the quenching tower 5.
Used as part of smoke washing water. The smoke washing water discharged from the quenching tower is combined with a part of the liquid (including cake washing water) and sent to the ferrite process 7, where the harmful heavy metals eluted from the fly ash are rendered harmless as ferrite cake B. It is processed. The liquid 8 from which the cake B is separated after the ferrite process is a harmless liquid that causes no problem even if it is discharged as it is outside the system. On the other hand, the flue gas from which fly ash has been removed by the electrostatic precipitator 2' has a temperature of approximately 300°C, and is sent to the quenching tower 5 through line 10, where it is washed with a neutral to slightly acidic state of pH 7 to 6. Wash water from the tower (line 13), liquid from the filter and cake wash water (line 1)
5) and the line 1
Cleans the flue gas from 0 and supplements and cleans the fly ash remaining in the flue gas. Further, in the washing tower 6, it is washed with washing water from line 12 and alkali from line 19, and then discharged into the atmosphere from line 9 at a temperature of about 80°C. The smoke washing water discharged from the quenching tower can be combined with the liquid in the filter 4 and a part of the cake washing water and sent to the next ferrite step 7, but the concentration of the washing water must be below the saturation concentration of salts. Adjust the smoke washing water system to maintain the concentration. As the smoke washing water supplied to the quenching tower 5, a part of the mixture of the liquid from the filter 4 and the cake washing water can be used as described above, and in this way, it is concentrated by the enthalpy of the exhaust gas and the temperature is increased. Therefore, the next ferrite process can be carried out effectively. In addition, in the ferrite process 7, the ferrous ion-containing heavy metal waste liquid is oxidized with air to convert some of it into ferric ions, forming and precipitating crystals with a spinel-type structure. It is more advantageous to introduce so-called spinel by continuously oxidizing it by introducing it in portions. Example 12000 m ³ /Hr of flue gas from a municipal waste incinerator was treated according to the process shown in FIG. 1 above. The fly ash separated from the cyclone and electrostatic precipitator is placed in a tank, and 50% sulfuric acid is added at 1.6 hours per hour, stirring to disperse and mix the fly ash in acidic water. Acid extraction was performed for 5 hours while maintaining the pH at about 5 and stirring.
Here, acid-soluble toxic heavy metals were extracted with acid and dissolved in the liquid. The liquid after the acid extraction was in the form of a slurry, which was sent to the next step, a filter press, where the solid content was separated and washed with water to obtain 9.5 kg of cake per hour. A part of the liquid is combined with washing water and sent to a rectangular quenching tower measuring 1.7 m long x 1.7 m wide x 2.6 m high, and the flue gas (temperature 300 ℃),
After washing the smoke in a quenching tower, it was combined with the filter liquid and the remainder of the cake washing water and sent to the ferrite process. In the ferrite process, ferrous sulfate is added per liquid.
Add 0.3 mol of mixed and then 30% caustic soda per hour
After neutralizing by adding 9.4, air is introduced to make the ferrite sludge into spinel every hour.
24Kg was separated. After quenching, the exhaust gas was further washed with water in a rectangular tubular washing tower with two trays, measuring 1.7 m long x 1.7 m wide x 3.5 m high, and discharged to the outside of the system at 80°C. The results of the example are shown in Table 1 for each item of the fly ash composition before treatment, the cake composition after acid oil extraction, and the industrial waste elution test (Environment Agency Notification No. 13) of the cake after acid extraction and the ferrite cake. Shown below.

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a process diagram showing an example of an embodiment of the present invention.

Claims (1)

[Claims] 1. A method for treating fly ash in flue gas from municipal waste incinerators involves dispersing and mixing fly ash in mineral acid, maintaining the pH between 4.5 and 5.5, and removing soluble harmful heavy metals from the fly ash. A step of selectively eluting and extracting the extracted fly ash particles from a solution, obtaining a cake, and washing the cake with water, and combining the separated liquid with the washing water of the cake. Stabilization of the fly ash characterized by comprising the steps of supplying it as part of the smoke washing water to a quenching tower for cooling flue gas, and treating the smoke washing water discharged from the quenching tower to ferrite. processing method. 2. The method according to claim 1, wherein the mineral acid is sulfuric acid. 3. The method according to claim 1, wherein the mineral acid is hydrochloric acid. 4. The method according to claim 1, wherein the liquid containing the cake washing water is directly used as part of the smoke washing water in the quenching tower, and a part of the liquid is treated to form ferrite. 5. The method according to claim 2 or 3, in which the ferrite treatment step of the smoke washing water involves introducing air in stages to continuously oxidize the water.