JPH09117771A - Device for treating waste water from stack gas desulfurization process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a scale trouble after an insolubilizing treatment and a solid-liq. separating treatment of waste water from stack gas desulfurization process. SOLUTION: Slaked lime is added to the stack gas desulfurization waste water from a slaked lime storage tank 2, and the mixture is subjected to a settling treatment in a settling tank 3. An electrical conductivity of a separated water is measured with a condactivity meter 4, and a diluting water supply from a diluting water storage tank is controlled based on this measured value. Therefore, a supersaturation of gypsum is eliminated and the scale trouble is prevented by diluting the separated water. The gypsum concn. in the separated water can be estimated from the measured value of the electrical conductivity of a separated water since the degree of the supersaturation of the gypsum in the separated water is interrelated with the electrical conductivity of the separated water, and thereby the amount of diluting water can be minimized.

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 wastewater treatment apparatus, and more specifically, it adds slaked lime or the like to wastewater discharged from a flue gas desulfurization apparatus to insolubilize fluorine ions, heavy metal ions and the like. The present invention relates to a device for solid-liquid separation of insoluble matter to obtain separated water. More specifically, the present invention relates to a treatment device that dilutes the separated water for the purpose of preventing scale failure in the subsequent process.

[0002]

2. Description of the Related Art Combustion exhaust gas using coal or the like as fuel is discharged after dust and sulfur oxides are removed by a flue gas desulfurization apparatus by the limestone gypsum method. In the treatment of combustion exhaust gas in this flue gas desulfurization apparatus, flue gas desulfurization wastewater containing fluorine, sulfate ions, chloride ions, heavy metal ions and the like derived from fuel is discharged. This flue gas desulfurization wastewater is reused after removing the contained fluorine ions and heavy metal ions and then discharging or further performing advanced treatment.

Conventionally, as a method of removing fluorine ions, heavy metal ions and the like in flue gas desulfurization wastewater, lime milk or slaked lime powder is added to the wastewater to adjust the pH to 7 or higher, and the fluoride ions are made into insoluble calcium fluoride. Further, a method is generally used in which heavy metal ions are respectively insolubilized as sparingly soluble hydroxides and the insolubilized products are subjected to solid-liquid separation. As a solid-liquid separation method for the insoluble matter, a sedimentation separation method and a membrane separation method are adopted.

The separated water obtained by solid-liquid separation is discharged as it is, or in order to further reduce fluorine and COD, it is introduced into an ion exchange resin tank and subjected to ion exchange treatment, and then discharged or re-used. Used.

[0005]

In the conventional treatment of flue gas desulfurization wastewater, when the separated water obtained by solid-liquid separation is discharged, a scale mainly composed of gypsum adheres in the discharge pipe, which is remarkable. Had the problem of causing blockages in the piping. Further, when the separated water obtained by solid-liquid separation is introduced into an ion-exchange resin tank for ion-exchange treatment, a scale containing gypsum as a main component is attached to the tank, which causes clogging of the resin layer. There was a problem that became.

An object of the present invention is to solve the above-mentioned conventional problems and prevent scale failure after the flue gas desulfurization wastewater is subjected to insolubilization treatment and solid-liquid separation treatment.

[0007]

A treatment apparatus for flue gas desulfurization wastewater according to the present invention is obtained by an insolubilization treatment means for adding a calcium compound to flue gas desulfurization wastewater to produce an insoluble matter, and the insolubilization treatment means. In an apparatus for treating flue gas desulfurization wastewater having solid-liquid separation means for solid-liquid separation of liquid, electric conductivity measuring means for measuring electric conductivity of separated water obtained by the solid-liquid separation means, and the electric conductivity A diluting water adding means for adding diluting water to the separated water based on the value measured by the degree measuring means is provided.

Separation water obtained by adding a calcium compound to flue gas desulfurization wastewater containing fluoride ions and then subjecting it to solid-liquid separation treatment is a supersaturated solution of gypsum.

Generally, the degree of supersaturation of gypsum is represented by the following equation.

[0010]

(Equation 1)

When the degree of supersaturation greatly exceeds 1.0,
Gypsum deposits as scale. However, it is impossible to measure the degree of supersaturation of gypsum online.

In the case of flue gas desulfurization wastewater, the present inventors
It was found that the electric conductivity in the solid-liquid separation treated water correlates with the gypsum supersaturation degree, and the supersaturation degree can be estimated by the electric conductivity instead of the actual measurement of the gypsum supersaturation degree. The present invention is based on such knowledge. When diluting the separated water from the solid-liquid separation means, the electric conductivity of the separated water is measured, and the amount of dilution water added is controlled based on the measured value. Alternatively, add dilution water while measuring the electric conductivity, and stop when the predetermined electric conductivity is reached. As a result, the degree of gypsum supersaturation can be suppressed to a low level, and scale failure can be reliably prevented. Also, the addition of an excessively large amount of dilution water is eliminated.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a flue gas desulfurization wastewater treatment apparatus according to the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a system diagram showing an embodiment of a flue gas desulfurization wastewater treatment apparatus according to the present invention.

In the apparatus of this embodiment, the flue gas desulfurization wastewater containing fluorine, sulfate ions, heavy metal ions, etc. is introduced into the pH adjusting tank 1 through the pipe 11, and inside the pH adjusting tank 1, the slaked lime storage tank 2 is introduced. Slaked lime is added from the pipe 12 and neutralized to pH 7 or above. As a result, fluorine in the waste water is insolubilized as calcium fluoride (CaF ₂ ), and heavy metal ions are insolubilized as sparingly soluble hydroxide. At the same time, sulfate ions in the wastewater are converted to gypsum (CaSO ₄ ·
2H ₂ O) is deposited.

The insolubilization treatment liquid is introduced into the sedimentation / separation tank 3 through the pipe 13 and is sedimented and separated. The sludge containing the insoluble matter separated in the settling separation tank 3 is extracted from the pipe 14.

The supernatant water of the settling / separating tank 3 is diluted with the diluting water injected from the diluting water storage tank 6 through the pipe 15, and then drained to the outside of the system through the pipe 16. In this embodiment,
The electric conductivity of the supernatant water is measured by the electric conductivity meter 4, and the operation of the dilution water addition pump 5 is controlled based on the measured value. Specifically, the dilution water addition amount is calculated from the measured electric conductivity so that the electric conductivity becomes equal to or less than a predetermined value after addition of the dilution water, and the pump 5 is controlled based on the calculation result. The electrical conductivity after addition of dilution water is usually 4.2 m.
Dilution water is added so as to be S / cm or less.

By controlling the injection amount of the diluting water into the supernatant water based on the electric conductivity in this manner, the gypsum supersaturation degree in the treated water can be reliably suppressed to a predetermined concentration or less, for example 1.0 or less, The scale failure in the treated water discharge pipe 16 and the subsequent stage is reliably prevented.

In FIG. 1, a sedimentation separation tank is used as the solid-liquid separation means, but a membrane separation device or the like may be provided instead of the sedimentation separation tank.

In the apparatus of FIG. 1, after measuring the electric conductivity of the separated water (before adding the diluting water) from the settling separation tank 3, the diluting water is added to the separating water. It is also possible to measure the electric conductivity of the separated water after the addition of and to control the addition amount of the diluting water so that the measured value becomes a predetermined value or less.

The present invention will be described in more detail below with reference to specific examples and comparative examples.

Example 1 The flue gas desulfurization wastewater having the water quality shown in Table 1 was treated by the treatment apparatus of the present invention shown in FIG.

In the pH adjusting tank, slaked lime was added to adjust the liquid pH to 7.0. Other processing conditions were as follows.

Raw water inflow: 180 liters / day pH adjusting tank capacity: 20 liters Sedimentation separation tank capacity: 20 liters

[0025]

[Table 1]

The electric conductivity of the supernatant water of the sedimentation tank was measured by an electric conductivity meter, and the injection amount of dilution water was controlled so that the measured value was 4.2 mS / cm.

After the treatment was continued for 1000 hours, the amount of scale adhered in the treated water discharge pipe 16 (the amount of adhered scale per 1 m ^{2 of} pipe inner wall area) was found to be 0.1 g / m ^3. It was

Comparative Example 1 When the same treatment as in Example 1 was carried out except that the supernatant water was not diluted, the electric conductivity of the treated water was about 6.
It was 0 mS / cm. Further, the amount of scale adhered in the treated water discharge pipe 16 was examined and found to be 200 g / m ³ .

In Example 1 and Comparative Example 1, component analysis of the scale adhering in the pipe was carried out.
More than 9% was gypsum.

From the results of Example 1 and Comparative Example 1, it is clear that the amount of scale adhesion can be significantly reduced by diluting the solid-liquid separated water based on the measured values of electric conductivity.

[0031]

As described in detail above, according to the flue gas desulfurization wastewater treatment apparatus of the present invention, scale failure such as piping in the post-process after the flue gas desulfurization wastewater is insolubilized and solid-liquid separated can be easily and It can be surely prevented. Also, the amount of dilution water added can be minimized.

[Brief description of the drawings]

FIG. 1 is a system diagram showing an embodiment of a flue gas desulfurization wastewater treatment apparatus according to the present invention.

[Explanation of symbols] 1 pH adjusting tank 2 Slaked lime storage tank 3 Sedimentation separation tank 4 Electrical conductivity meter 5 Dilution water addition pump 6 Dilution water storage tank

Claims (1)

[Claims] 1. Flue gas desulfurization having an insolubilization treatment means for generating an insolubilized product by adding a calcium compound to flue gas desulfurization wastewater, and a solid-liquid separation means for solid-liquid separating the liquid obtained by the insolubilization treatment means. In a wastewater treatment apparatus, an electric conductivity measuring means for measuring the electric conductivity of the separated water obtained by the solid-liquid separation means, and a dilution water for the separated water based on the measured value of the electric conductivity measuring means. A device for treating flue gas desulfurization wastewater, which is provided with a means for adding dilution water.