JPH091142A - Treating device for flue gas desulfurization waste water - Google Patents

Abstract

PURPOSE: To prevent deterioration of a membrane transmitting flow flux due to gypsum in insoluble products and to stably and efficiently treat the waste water for a long period, when fluorine and heavy metal ions in flue gas desulfurization waste water are made insoluble with calcium compounds such as slaked lime and the produced insoluble product is subjected to a solid-liquid separation with a membrane separation. CONSTITUTION: A dispersant and a calcium compound are added to the flue gas desulfurization waste water in a reaction tank 1 and the outflow water from this reaction tank 1 is subjected to a solid-liquid separation with membrane module 5. Growth of gypsum particles depositing from sulfuric acid ions in the flue gas desulfurization waste water is restrained by coexisting the dispersant, and large granulation of the gypsum causing the deterioration of the membrane transmitting flow flux is prevented. Thus, the membrane transmitting flow flux of the membrane module can stably be maintained for a long period.

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 using coal or the like as fuel is released after soot dust and sulfur oxides are removed by a flue gas desulfurization device by 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. Of these, the sedimentation separation method requires a settling tank to settle the insoluble matter, but since the settling tank requires a relatively large installation area, the membrane separation method is adopted in terms of downsizing of the equipment. Preferably.

Japanese Patent Publication No. 55-19676 discloses that
The waste liquid after separating the sulfur oxides in the flue gas as gypsum,
A method is described in which an ion sequestering agent such as EDTA is added to prevent crystallization of metal ions and then a COD component is removed by a membrane separation treatment.

[0005]

When the insoluble matter is subjected to solid-liquid separation by membrane separation after insolubilizing the flue gas desulfurization wastewater,
There is a problem that the membrane permeation flux is likely to decrease and stable and efficient treatment cannot be performed. 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 method described in Japanese Examined Patent Publication No. 55-19676 is a method of removing COD by membrane separation, in which an ion sequestering agent is added to the waste water after the sulfur oxides have been separated as gypsum in advance to remove metal. Ions are complexed and treated to prevent crystallization, and then membrane separation is performed, and insoluble matters of fluorine and heavy metal ions are not removed by membrane separation.

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 subjected to solid-liquid separation 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.

[0008]

A flue gas desulfurization wastewater treatment apparatus according to the present invention comprises an addition means for adding a dispersant and a calcium compound to the flue gas desulfurization wastewater, and a membrane for membrane separation treatment of water from the addition means. And a module.

As a result of repeated investigations by the present inventors, 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, it has been found that when a dispersant coexists during the precipitation and growth of gypsum, the crystal growth of the dispersant is suppressed and the gypsum particles are reduced in particle size.

The present invention has been achieved based on the above findings. By adding a dispersant together with a calcium compound, the growth of gypsum crystals is suppressed and a large particle size that causes a decrease in membrane permeation flux is obtained. It is intended to stabilize the membrane separation process by preventing the generation of gypsum particles.

[0012]

[Function] When a calcium compound is added to the flue gas desulfurization wastewater to insolubilize fluorine and heavy metal ions, coexistence of a dispersant suppresses the growth of gypsum particles precipitated from the sulfate ions in the flue gas desulfurization wastewater. The gypsum is made into small particles by preventing the gypsum from becoming large particles which causes a decrease in the membrane permeation flux.

As a result, the membrane permeation flux of the membrane module can be stably maintained for a long period of time.

As a mechanism of suppressing the growth of gypsum crystals by the dispersant, the growth points of the crystals are blocked by the adsorption of the dispersant to the growth points existing on the surface of the gypsum crystals, and further crystal growth occurs. It is possible that the mechanism is less likely to occur.

[0015]

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 introduced into the reaction tank 1 through the pipe 11, and the slaked lime in the slaked lime storage tank 2 is transferred to the pipe 12.
Further, the pH is adjusted to 7 or more, and the dispersant in the dispersant storage tank 3 is added through the pipe 13. As a result, fluorine is insolubilized as sparingly soluble calcium fluoride (CaF ₂ ), and heavy metal ions are insolubilized as sparingly soluble hydroxides, and at the same time, sulfate ions coexisting in the wastewater are converted to gypsum (CaSO ₄ · 2H ₂ O). In this precipitation, by causing precipitation in the presence of a dispersant, growth of gypsum crystals is suppressed and precipitation of crystals having a large particle diameter of 100 μm or more is prevented.

Therefore, since the deposit particles in the outflow water of the reaction tank 1 flowing into the circulation tank 4 through the pipe 14 are mainly those having a relatively small particle diameter of less than 100 μm, the membrane permeation of the membrane module 5 is performed. A decrease in flux is less likely to occur.

The water flowing out of the reaction tank 1 is the pipe 14 and the circulation tank 4.
And is introduced into the membrane module 5 through the pipe 15, the insoluble matter is captured by the membrane, and the permeated water is discharged from the system as treated water through the pipe 16. On the other hand, the concentrated water is circulated through the pipe 17 to the circulation tank 4, and a part of the concentrated water is discharged from the pipe 18 to the outside of the system.

As the membrane module 5, 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 dispersant, acrylic acid polymers such as sodium polyacrylate, phosphonic acid derivatives such as hydroxyethylidene diphosphonic acid, maleic acid polymers such as maleic acid-isobutylene copolymer, or hexametaline. Polymerized phosphates such as acid salts can be used, and the addition effect can be obtained if the addition amount is 1 mg / l or more with respect to flue gas desulfurization wastewater, but generally 5 to 100 mg / l Is preferably added.

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.

[0025]

[Table 1]

Sodium polyacrylate was used as the dispersant, and the concentration added was 50 mg / l. Also, slaked lime was added so that the pH after addition was 7.0. As the membrane module, polypropylene cross-flow type M
An F 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 procedure of Example 1 was repeated except that the dispersant was not added, and the membrane permeation flux and 30 immediately after the start of the treatment and after 30 days.
The average particle size of the precipitates in the circulation tank after day was examined, and the results are shown in Table 2.
It was shown to.

[0030]

[Table 2]

From Table 2, by adding a dispersant,
It is clear that the crystal growth is suppressed, which prevents the reduction of the membrane permeation flux and keeps the membrane permeation flux high.

[0032]

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 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 Reaction tank 2 Slaked lime storage tank 3 Dispersant storage tank 4 Circulation tank 5 Membrane module

Claims (1)

[Claims] 1. An apparatus for treating flue gas desulfurization wastewater, comprising: an addition means for adding a dispersant and a calcium compound to the flue gas desulfurization wastewater; and a membrane module for membrane-separating the water from the addition means.