JPS5948675B2 - Treatment method for wet flue gas treatment liquid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating waste liquid from wet flue gas treatment, which can reliably precipitate dithionate ions from the flue gas treatment process and convert the precipitate into disposable waste. .

When oil or coal is burned, S is contained in the exhaust gas.
This exhaust gas pollutes the atmosphere because it contains Ox and NOx.

Therefore, oil or coal combustion equipment must be equipped with auxiliary equipment to remove SOx and NOx from the exhaust gas in order to prevent air pollution.

Among these auxiliary equipment, the flue gas treatment equipment uses limestone.
Wet desulfurization equipment using the gypsum method is mainly used.

The waste liquid discharged from wet flue gas treatment equipment contains various components in the flue gas dissolved in it, but the biggest problem in treating this waste liquid is dithionate ion (8206") which is a factor that increases the waste liquid COD. ).

However, this 52o6"- is a very stable substance and cannot be oxidized with strong oxidizing agents such as hydrogen peroxide or ozone.
Simply p
It cannot be removed by normal flocculation and precipitation, which involves raising the H and adding a flocculant.

The conventional technique for removing this dithionate ion is
An extremely large-scale waste liquid treatment method has been adopted in which the waste liquid is concentrated and separated by an ion exchange resin method, an electrodialysis membrane, or a decontamination method such as a diaphragm, and then thermally decomposed.

Furthermore, a processing method that solves the drawbacks of this processing method has been proposed by the present applicant (see Japanese Patent Application No. 53-105956 = Japanese Patent Application Laid-Open No. 55-47181).

The gist of the waste liquid treatment method proposed by the present applicant is that an inorganic flocculant containing aluminum ions is added to the waste liquid obtained by alkali treatment and filtration of the waste liquid containing dithionate ions, so that dithionate ions are removed once (ppm). ) 1, the aluminum ion concentration is 0.5 or more, and an alkaline agent containing calcium ions is added to make the pH 10 or more, and the calcium ion concentration is equal to or more than the equivalent of the dithionate ion concentration. This is a method in which ions are coagulated and precipitated, but the precipitate is very stable in an alkaline region where precipitation can occur.

However, as the pH decreases, it redissolves and dissociates dithionate ions.

For example, about 90% of the amount is dissolved in an aqueous solution with a pH of 7.

Therefore, the above-mentioned precipitate cannot be disposed of as is in the normal living environment.

An object of the present invention is to provide a method for treating waste liquid from wet flue gas treatment, which can reliably precipitate dithionate ions and convert the precipitate into a material that can be disposed of by incinerating the precipitate. be.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a diagram showing a process of treating raw waste liquid from a wet flue gas treatment process, for example, a wet flue gas desulfurization process resulting in a limestone-gypsum process, by the method of the present invention.

The alkali treatment steps 2 and 3 described below perform alkali treatment and filtration of the raw waste liquid.

In step 2, the raw waste liquid from the wet flue gas desulfurization process (step 1) is received in a preliminary coagulation sedimentation tank that also serves as a raw water storage tank.

SS, mainly fly ash, which is contained in the received raw waste liquid at approximately 2%, is naturally settled in the pre-coagulation sedimentation tank, and metal ions such as Fe3+ and Cr3+ contained in the raw waste liquid have a pH of 2. Precipitation is removed (filtration) by maintaining the following acidic raw waste liquid at pH 6 to 8 with an alkaline agent such as calcium hydroxide.

By removing both of these, the load on the coagulation-sedimentation apparatus described below is reduced.

The waste liquid (clarified liquid) obtained in the pre-coagulation sedimentation tank is transferred to step 3.
The liquid is supplied to the coagulation sedimentation device.

In this coagulation-sedimentation apparatus, the pH of the liquid is increased to around 11 using an alkaline agent such as calcium hydroxide.

As a result, most of the metal ions contained in the supplied liquid, which had not yet been precipitated and separated in the pre-coagulation sedimentation tank, can be precipitated and separated in the form of hydroxide in an elevated pH range. I can do it.

At the same time, sulfate ions in the waste liquid that could not be removed in the treatment in step 2 can be precipitated and separated as calcium sulfate as much as possible.

By the precipitation separation in step 3, it is possible to reduce the amount of the inorganic flocculant containing aluminum, which is added in the dithionate ion inorganic flocculant-sedimentation device described later.

That is, this is because sulfate ions, which are a factor that interferes with the inorganic coagulation and precipitation treatment of dithionate ions, are removed.

The waste liquid treated in step 3 is received into the dithionate ion inorganic coagulation and precipitation apparatus of dithionate ion coagulation and precipitation step 4.

Adding an inorganic flocculant containing aluminum ions to the received waste liquid to make the aluminum ion concentration (ppIll) 0.5 or more per dithionate ion concentration (ppm) 1, and adding an alkaline agent containing calcium ions. In addition, the pH of the waste liquid is adjusted to 10 or more, preferably around 12, and the calcium ion concentration is adjusted to be equal to or higher than the dithionate ion concentration (ppIn).

As a result, most of the dithionic hydrochloric acid, the amine nitrogen compound, and a portion of the fluorine could be removed.

The inorganic flocculant containing aluminum ions used for this is an inorganic flocculant whose main component is polyaluminum chloride: PAC! (Product name) (hereinafter referred to as PAO).

), sodium aluminum oxide (AdNa02), aluminum sulfate (A12(804)3), fly ash solution, etc.

In addition, alkaline agents include calcium hydroxide, sodium hydroxide, sodium carbonate, etc., but calcium hydroxide is the main agent, and when the pH is adjusted to pH 11 or higher (optimum 12) with calcium hydroxide, undissolved components remain. In order to prevent this, sodium hydroxide, sodium carbonate, etc. may be used to adjust the pH of a solution containing a predetermined amount of calcium.

PAO is added to the waste liquid and calcium hydroxide is used to reduce the pH of the waste liquid.
When H is around 12, the dithionate ion concentration (p
PAC (ppm as A1203) 0 to pm) 1
．． Requires 5 or more.

In addition, the difference in concentration between calcium ions and dithionate ions required to precipitate dithionate ions is approximately 1.
This means that there is a one-to-one relationship between pH and sodium hydroxide.
This was found from the coagulation and precipitation of dithionate ions caused by the calcium chloride solution prepared in No. 12.

When sodium aluminum oxide or aluminum sulfate is used as an inorganic flocculant containing aluminum ions, and the aluminum ions in the waste liquid are kept at a constant concentration sufficient to precipitate dithionate ions, the removal rate of dithionate ions is In the former case, it reached almost 100%, but in the latter case, it reached only about 50%.

This means that the sulfate ions act as a hindrance to the precipitation of dithionate ions.

Thus, sulfate ions prevent the precipitation of dithionate ions, but if a large amount of an inorganic flocculant containing aluminum ions, such as the above-mentioned PAO, is added, in other words, if the aluminum ion concentration is increased, the coagulation and precipitation of dithionate ions can be prevented. Processing can be carried out.

In the presence of sulfate ions, the required amount of PAO is determined by the dithionate ion concentration (ppIll) and the sulfate ion concentration (ppIll).
ll) is 7 or more.

* The dithionate ion precipitate generated by the coagulation and precipitation in step 4 is taken out from the dithionate ion inorganic coagulation and precipitation equipment, and then put into the incinerator in heat treatment step 5, where it is heat treated at 200 to 300°C. Ru.

Moreover, within this temperature range, exhaust gas can be effectively used as a heating source.

Through this heat treatment, the precipitate is thermally decomposed, and the precipitate thereafter is converted into waste that can be disposed of as is.

Table 1 shows the decomposition rate of 8206'- in the precipitate by thermal decomposition.

The heating conditions in Table 1 are air flow rate: 200 m/min, heating time: 30 minutes, and combustion tube internal volume: 960 frIl.

Moreover, upon thermal decomposition at 300° C., the dithionate ion precipitate does not generate any sulfur dioxide gas.

Due to this heat treatment, a decrease in the weight of the precipitate is also observed, but
This is shown in FIG.

The solid line is a thermogravimetric curve for sodium dithionate trihydrate, and the dotted line is a precipitate of dithionate ions.

Weight loss occurs around 100°C and 270°C for sodium dithionate trihydrate, and around 100°C and 220°C for dithionate ion precipitates.
The weight loss near 00°C is due to volatilization of crystal water and adhering water, and the weight loss near 220°C and 270°C is due to decomposition of dithionate.

In addition, most of the aluminum ions added in the treatment in step 4 are consumed by removing dithionate ions, but the small amount of aluminum ions that remain in the waste liquid reacts with calcium ions in the waste liquid to form calcium aluminate. It has precipitated as.

Therefore, the pH of the effluent from step 4 is changed to the pH of step 6.
Even if it is returned to a neutral region in the H adjustment tank, almost no reprecipitation of aluminum ions occurs.

Therefore, the load on the filtration device in step 7 can be reduced,
A simple sand filtration device can be used as the filtration device.

The COD of the treated water after filtration (step 8) is lQQppm
The concentration has dropped to several ppm and can be released.

In the above example, the coagulated precipitate of dithionate ions was subjected to the treatments of steps 2, 3 and 4 as described above, but the sulfate ion concentration of the raw waste liquid from the wet flue gas desulfurization process was 250.
When the concentration is about 0 pI) In and SS is small, steps 2 and 3 are omitted and the liquid is directly supplied to step 4, where an inorganic flocculant containing aluminum ions and an alkaline agent are added, causing coagulation and precipitation of dithionate ions. You may let them.

In addition, although wet desulfurization waste liquid was given as an example of waste liquid, wet desulfurization waste liquid,
Some desulfurization waste fluids also contain dithionate ions, and Kenpo can be applied to the treatment of these waste fluids.

In summary, according to the present invention, the following excellent effects can be achieved.

■ It is possible to reliably coagulate and precipitate dithionate ions from the waste liquid discharged from the flue gas desulfurization treatment process.

- By thermally decomposing the dithionate ion precipitate, it can be converted into a precipitate that does not contain dithionate ions, and therefore, even if it is disposed of as is, it will not pollute the living environment.

■ It is possible to eliminate the need for complicated equipment such as an ion exchanger, which was conventionally required to remove dithionate ions.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the treatment steps of the present invention, and FIG. 2 is a thermogravimetric curve diagram of a dithionate salt. In the figure, 2 and 3 are alkali treatment steps, 4 is a dithionate ion coagulation and precipitation step, and 5 is a thermal decomposition step.

Claims (1)

[Claims] 1. In a method for treating waste liquid containing dithionate ions from a wet flue gas treatment process, an inorganic flocculant containing aluminum ions is added to the waste liquid to reduce the aluminum ion concentration to 1. 5 or more, and add an alkaline agent containing calcium ions to the waste liquid to make the pH 10 or more, and make the calcium ion concentration equal to or more than the equivalent of the dithionate ion concentration to coagulate and precipitate the dithionate ions, and then the precipitate A method for treating waste liquid from wet flue gas treatment, characterized in that the waste liquid from wet flue gas treatment is separated and thermally decomposed.