JPH09886A - Treatment device for flue gas desulfurization waste water - Google Patents

Abstract

PURPOSE: To restrain a coarse particle of gypsum from being produced by a method wherein before a calcium compound is added to flue gas desulfurization waste water to insolubilize fluorine and heavy metal ion, a carbonate is added to the waste water to cause calcium carbonate to deposit so that the calcium carbonate is separated and removed and thereafter a calcium compound is added. CONSTITUTION: Flue gas desulfurization waste water containing fluorine, sulfate ions, and noble metal ions is introduced into a carbonate treatment tank 1 via a pipe 11, where a carbonate in a carbonate storage tank 2 is added to the waste water so that calcium ions in the waste water deposit as calcium carbonate. The water in the tank 1 is then introduced into an insolubilizing treatment tank 3 via a pipe 14, where slaked lime is added to the water from a slaked lime storage tank 4 so that pH of the water is adjusted to 7 or more, and the fluorine and noble metal ions are insolubilized to form insoluble calcium fluoride and hydroxide respectively and at the same time sulfate ions deposit as gypsum. The water is then passed through a circulation layer 5 and membrane module 6, whereby the water is purified and discharged.

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 particularly, to a method for treating fluorine or heavy metals contained in wastewater discharged from a flue gas desulfurization apparatus with a calcium compound such as slaked lime. The present invention relates to a device for insolubilizing calcium or hydroxide and performing a membrane separation treatment on the insoluble matter.

[0002]

2. Description of the Related Art Combustion exhaust gas that uses coal as a fuel is discharged after soot and sulfur oxides are removed by a flue gas desulfurization device using the limestone gypsum method. Flue gas desulfurization wastewater containing ions, chloride ions, heavy metal ions, etc. is discharged.

Conventionally, as a method for removing fluorine and heavy metal ions 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 converted into sparingly soluble calcium fluoride. There is a method in which heavy metal ions are made insoluble as insoluble hydroxides and solid-liquid separation is performed. As the solid-liquid separation means, there are a sedimentation separation method and a membrane separation method.

[0004]

When a calcium compound is added to flue gas desulfurization wastewater to insolubilize it, and then the insoluble matter is subjected to solid-liquid separation by membrane separation, the membrane permeation flux is apt to decrease, resulting in stable and efficient treatment. There is a problem that it is not possible to perform specific processing. That is, in the insolubilized product obtained by the insolubilization treatment in which a calcium compound is added to the flue gas desulfurization wastewater, sulfate ions in the flue gas desulfurization wastewater exist as gypsum produced by the reaction with the calcium compound, but this gypsum is a film. This causes a decrease in permeation flux.

The present invention has been made in view of the above conventional circumstances, in which fluorine and heavy metal ions in flue gas desulfurization wastewater are insolubilized with a calcium compound such as slaked lime, and the insolubilized product thus formed is solid-liquid separated by membrane separation. An object of the present invention is to provide a flue gas desulfurization wastewater treatment apparatus which can prevent a decrease in membrane permeation flux due to gypsum in an insoluble matter and can perform stable and efficient treatment for a long time upon separation.

[0006]

A treatment apparatus for flue gas desulfurization wastewater according to the present invention comprises a calcium carbonate separation means for adding a carbonate to the flue gas desulfurization wastewater to separate calcium carbonate deposits.
An insoluble matter depositing means for depositing an insoluble matter by adding a calcium compound to water separated from the calcium carbonate by the calcium carbonate separating means, and a membrane module for subjecting the water obtained by the insoluble matter depositing means to a membrane separation treatment are provided. It is characterized by

As a result of repeated investigations by the inventors of the present invention, the problems of insolubilization of flue gas desulfurization wastewater and reduction of membrane permeation flux due to gypsum in the insoluble matter in the membrane separation treatment of the insoluble matter were obtained.
It has been found that when a large amount of gypsum particles having a large particle size (for example, a particle size of 100 μm or more) is present in the insoluble matter, the membrane permeation flux is significantly reduced.

Further, when a carbonate such as sodium carbonate is added to the flue gas desulfurization wastewater, calcium ions in the wastewater are insolubilized as calcium carbonate, so that the calcium ion concentration of the wastewater is lowered and further the calcium ion concentration is low. It was found that when the wastewater was neutralized with slaked lime, the amount of gypsum deposited was reduced and the particle size of the gypsum deposited was also reduced.

The present invention has been achieved based on the above findings. In advance, by reducing the calcium ion concentration of waste water by carbonate treatment, it is possible to obtain gypsum particles of large particle size that cause a decrease in membrane permeation flux. It is intended to prevent the generation and stabilize the membrane separation process.

[0010]

[Function] Prior to adding a calcium compound to the flue gas desulfurization wastewater to insolubilize fluorine and heavy metal ions, a carbonate is added to the flue gas desulfurization wastewater to precipitate calcium carbonate, and the precipitated calcium carbonate is separated and removed. This reduces the calcium ion concentration in the wastewater. By adding a calcium compound to the wastewater having a reduced calcium ion concentration, it is possible to suppress the generation of coarse particles of gypsum, which causes a decrease in membrane permeation flux, and reduce the amount of gypsum deposited. .

Therefore, the gypsum particles flowing into the membrane module are small and the inflow amount is small, the decrease of the membrane permeation flux of the membrane module is prevented, and the membrane permeation flux can be stably maintained for a long period of time. It will be possible.

[0012]

Embodiments of the present invention will now be described 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 this embodiment, the flue gas desulfurization wastewater containing fluorine, sulfate ions and heavy metal ions is first introduced into the carbonate treatment tank 1 through the pipe 11, and the carbonate in the carbonate storage tank 2 is added through the pipe 12. As a result, calcium ions in the waste water are precipitated as calcium carbonate (CaCO ₃ ).

The carbonate treatment tank 1 has an inverted conical shape in which the bottom portion is reduced in diameter downward, and a withdrawing pipe 13 for the sedimentation precipitate is provided at the lower end thereof. A stirrer 1A is provided in the carbonate treatment tank 1 in order to bring the inside of the tank into a complete mixing condition. However, by stirring the stirrer 1A slowly, a precipitate of calcium carbonate can be obtained. Can be allowed to settle. The sedimented precipitate is discharged to the outside of the system through a pipe 13 for extracting the sedimented sediment at the bottom of the tank.

The outflow water of the carbonate treatment tank 1 is introduced into the insolubilization treatment tank 3 through the pipe 14, the slaked lime in the slaked lime storage tank 4 is added through the pipe 15, and the pH is adjusted to 7 or higher. As a result, fluorine is hardly soluble in calcium fluoride (CaF
₂ ) Also, heavy metal ions are insolubilized as sparingly soluble hydroxides, and at the same time, sulfate ions coexisting in the waste water are precipitated as gypsum (CaSO ₄ · 2H ₂ O). Since the calcium ion concentration of the gypsum is low, the precipitation and growth of gypsum crystals are suppressed, and the particle size is 10
Precipitation of crystals having a large grain size of 0 μm or more is prevented.

Therefore, the amount of gypsum-precipitated particles in the outflow water of the insolubilization treatment tank 3 flowing into the circulation tank 5 from the pipe 16 is small,
Moreover, the gypsum particles that have been deposited are mainly those having a small particle size of, for example, 30 μm or less.
The decrease in the membrane permeation flux is less likely to occur.

The water discharged from the insolubilization treatment tank 3 is introduced into the membrane module 6 through the pipe 16, the circulation tank 5 and the pipe 17,
The insoluble matter is captured by the membrane, and the permeated water is discharged from the system as treated water through the pipe 18. On the other hand, the concentrated water is circulated through the pipe 19 to the circulation tank 5, and a part of the concentrated water is discharged out of the system through the pipe 20.

As the membrane module 6, a cross-flow type membrane module provided with a UF (ultrafiltration) membrane or an MF (microfiltration) membrane is suitable.

In the present invention, as the carbonate, sodium carbonate, ammonium carbonate, barium carbonate, sodium bicarbonate or the like can be used. The concentration of carbonate added is 1000 mg / l for flue gas desulfurization wastewater as carbon dioxide concentration.
As described above, it is usually preferable to set the concentration to 1000 to 2000 mg / l, and it is desirable to reduce the calcium ion concentration in the waste water to 250 mg / l or less by adding this carbonate.

As the calcium compound, the above slaked lime (Ca (OH) ₂ ) is used, and the pH of the waste water after addition is 7.
If necessary, it is added together with another pH adjuster so as to be 0 or more.

The present invention will be described in more detail 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 flue gas desulfurization wastewater treatment apparatus of the present invention shown in FIG.

[0024]

[Table 1]

Sodium carbonate is used as the carbonate,
Addition concentration is 100 as carbon dioxide concentration for flue gas desulfurization wastewater.
The calcium ion concentration was set to 0 mg / l, and the precipitated calcium carbonate was extracted to adjust the calcium ion concentration of the outflow water of the carbonate treatment tank to about 200 mg / l. Also, slaked lime was added so that the pH after addition was 7.0. As the membrane module, a polypropylene cross-flow type MF membrane module was used.

The other processing conditions were as follows.

Treatment conditions Flue gas desulfurization wastewater Treatment water amount: 180 L / day Membrane area: 0.036 m ² Circulating water (concentrated water) amount: 8.4 L / hr Reactor tank, circulation tank residence time: 2.5 hr Initial treatment start and The membrane permeation flux after 30 days is shown in Table 2.
Further, the average particle size of the precipitate in the circulation tank was examined 30 days after the start of the treatment, and the results are shown in Table 2.

Comparative Example 1 The same procedure as in Example 1 was carried out except that the flue gas desulfurization effluent was directly introduced into the insolubilization treatment tank without providing the carbonate treatment tank, and the membrane permeation flux and The average particle size of precipitates in the circulation tank after 30 days was examined, and the results are shown in Table 2.

[0029]

[Table 2]

From Table 2, by adding a carbonate in advance to remove the calcium ions in the waste water as calcium carbonate, the crystal growth is suppressed, whereby the reduction of the membrane permeation flux is prevented and the membrane permeation flux is prevented. It is clear that the flux remains high.

[0031]

As described in detail above, according to the flue gas desulfurization wastewater treatment apparatus of the present invention, the flue gas desulfurization wastewater is insolubilized with the calcium compound and the insoluble matter is subjected to solid-liquid separation by the membrane separation treatment. Thus, it is possible to prevent a decrease in the membrane permeation flux and perform stable and efficient treatment for a long period of time.

[Brief description of 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 Carbonate treatment tank 2 Carbonate storage tank 3 Insolubilization treatment tank 4 Slaked lime storage tank 5 Circulation tank 6 Membrane module

Claims (1)

[Claims] 1. A calcium carbonate separation means for adding a carbonate to flue gas desulfurization effluent to separate calcium carbonate precipitates, and a calcium compound added to the water from which calcium carbonate has been separated by the calcium carbonate separation means to insolubilize it. An apparatus for treating flue gas desulfurization wastewater, comprising an insoluble matter depositing means for depositing a substance and a membrane module for subjecting the water obtained by the insoluble matter depositing means to a membrane separation treatment.