JPH09117772A - Device for treating waste stack gas desulfurizing water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the lowering of a membrane permeating flux and to minimize the addition of an expensive non-calcium alkali agent by adding a non- calcium alkali agent so that the pH after the agent is added is controlled in a specified range based on the measured result. SOLUTION: The waste stack gas desulfurizing water is introduced into a pH regulating tank 1 from a pipeline 11, and the pH is regulated with slaked lime and sodium hydroxide. When the pH is regulated, the conductivity of the waste water is measured by a conductometer 4 provided to the pipeline 11, and the addition of slaked lime is controlled by a slaked lime injection pump 5 linked with the conductometer so that the total gypsum concn. in the pH-controlled water is not in large excess over the solubility of gypsum. Further, the addition of sodium hydroxide is controlled by a sodium hydroxide injection pump 7 linked with a pH meter 6 furnished in the pH regulating tank 1 so that the insolubilized water is controlled to pH7. Only a small amt. of sodium hydroxide is used in this case, because it is used to supply the deficiency of slaked lime in regulating the pH.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for treating flue gas desulfurization wastewater, and more specifically, it adds calcium compounds such as slaked lime to the wastewater discharged from the flue gas desulfurization apparatus to inactivate fluorine ions, heavy metal ions and the like. The present invention relates to an apparatus for solid-liquid separating the insoluble matter to obtain separated water.

[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 for removing fluorine ions, heavy metal ions and the like in flue gas desulfurization wastewater, an excess amount of a calcium-based neutralizing agent (alkali agent) such as lime milk or slaked lime powder is added to the wastewater to adjust the pH to 7 As described above, a method of insolubilizing fluoride ions as sparingly soluble calcium fluoride and heavy metal ions as sparingly soluble hydroxide, respectively, and solid-liquid separating the insolubilized product from the insolubilized water thus insolubilized is generally used. Target. As the solid-liquid separation means for the insoluble matter, a sedimentation separation method and a membrane separation method are adopted.

[0004]

When the membrane separation method is adopted as a solid-liquid separation means for insoluble matter in the treatment of flue gas desulfurization wastewater, an excessive amount of calcium-based alkaline agent with respect to sulfate ions in the wastewater is used. When the reaction is carried out in step 1, there is a drawback that gypsum with a large particle size of 100 μm or more is deposited on the film surface, and the membrane permeation flux (flux) is reduced early.

In order to prevent the precipitation of gypsum, which is a problem in the membrane separation method, it is considered effective to reduce the amount of the calcium-based alkaline agent added to the waste water. That is, it is presumed that the amount of gypsum deposited can be reduced by using a non-calcium alkaline agent such as sodium hydroxide or magnesium hydroxide in place of the calcium alkaline agent such as slaked lime.

However, since non-calcium alkaline agents such as sodium hydroxide and magnesium hydroxide are more than three times as expensive as slaked lime, it is desirable to minimize the amount used.

The present invention has been made in view of the above-mentioned conventional circumstances. A calcium-based alkaline agent and a non-calcium-based alkaline agent are added to flue gas desulfurization wastewater to form an insoluble matter, and the insoluble matter is separated by a membrane. In the treatment, by reducing the amount of gypsum produced by the calcium-based alkaline agent, it is possible to prevent a decrease in the membrane permeation flux and to minimize the amount of expensive non-calcium-based alkaline agent added. An object of the present invention is to provide a treatment device for smoke desulfurization wastewater.

[0008]

A flue gas desulfurization wastewater treatment apparatus according to the present invention is based on an electric conductivity measuring means for measuring the electric conductivity of the flue gas desulfurization wastewater, and a measurement result of the electric conductivity measuring means. Means for adding a calcium-based alkaline agent to the flue gas desulfurization wastewater, and the pH of the liquid after the addition of the calcium-based alkaline agent
And a means for adding a non-calcium-based alkaline agent so that the amount becomes within a predetermined range.

The inventors of the present invention have examined the decrease in the membrane permeation flux when insoluble matter is subjected to insolubilization treatment and insoluble matter is subjected to membrane separation. As a result, the insoluble matter has a large particle size of 100 μm or more. It was found that the transmembrane flux decreased early when a large amount of gypsum particles were present. Also, if the total concentration of gypsum in the insolubilized water (raw water for membrane separation) is low, the particle size of gypsum particles in the insolubilized material will be small. Conversely, if the gypsum concentration in the insolubilized water exceeds the supersaturated region of gypsum. It was found that plaster of large particle size was deposited.

Furthermore, in the usual case, the concentration of sulfate ion in the flue gas desulfurization wastewater is 3000 to 6000 mg / l (liter).
However, there is a correlation between the sulfate ion concentration in the flue gas desulfurization wastewater and the electrical conductivity of the flue gas desulfurization wastewater.
It was found that the sulfate ion concentration in the flue gas desulfurization wastewater can be grasped from the electric conductivity of the flue gas desulfurization wastewater. That is, as shown in Table 1 below, sulfate ions account for 80% or more of all the ions present in the flue gas desulfurization wastewater, and therefore the electrical conductivity of the flue gas desulfurization wastewater is almost the same. It can be regarded as the electric conductivity due to the sulfate ion. Therefore, the amount of sulfate ion is estimated from the obtained electric conductivity, and the amount of calcium necessary for the reaction is obtained from the solubility product.

In order to prevent a decrease in the permeation flux of the membrane due to the precipitation of gypsum, it is necessary that the total gypsum concentration of the raw water for membrane separation, that is, the insolubilized water does not greatly exceed the solubility of gypsum. Is. Further, as described above, in order to suppress the total gypsum concentration of the insolubilized water, it is necessary to reduce the addition amount of the calcium-based alkaline agent and supplement the alkaline agent necessary for pH adjustment with the non-calcium-based alkaline agent. But,
At this time, it is cost effective to add as much calcium-based alkaline agent as possible within the range where the total gypsum concentration of the insolubilized water does not exceed the solubility of gypsum, and to minimize the amount of non-calcium-based alkaline agent added. Desired from.

In the present invention, the electric conductivity of the flue gas desulfurization wastewater is measured to grasp the sulfate ion concentration in the flue gas desulfurization wastewater, and based on this result, the total gypsum concentration of the insolubilized water determines the solubility of gypsum. The calcium-based alkaline agent is added in such an amount that the amount of the non-calcium-based alkaline agent to be replenished can be minimized for the purpose of adjusting the pH without greatly exceeding it. As a result, it is possible to prevent the deposition of gypsum having a large particle size of 100 μm or more to prevent the decrease of the membrane permeation flux, and to suppress the cost increase due to the combined use of the non-calcium alkaline agent.

[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 in the pH adjusting tank 1, the calcium-based alkaline agent is added. As the slaked lime in the slaked lime storage tank 2 is added from the pipe 12, the sodium hydroxide storage tank 3 is used as a non-calcium alkaline agent as needed.
Sodium hydroxide is added from the pipe 13 to adjust the pH (neutralize).

In adjusting the pH, the electric conductivity of the flue gas desulfurization effluent is measured by the electric conductivity meter 4 provided in the pipe 11, and the pH is adjusted by the slaked lime injection pump 5 which is interlocked with the electric conductivity meter 4. The amount of slaked lime added is controlled so that the total gypsum concentration in the adjusted treated water (insolubilized treated water) does not significantly exceed the solubility of gypsum.

By keeping the total gypsum concentration low in this way, the particle size of the precipitated gypsum particles can be kept small. For example, if the total gypsum concentration in the insolubilized water is less than the solubility, the precipitation hardly occurs, or even if it occurs, the gypsum particles that are precipitated mainly consist of crystals having a small particle size of 30 μm or less. Therefore, there are no precipitate particles flowing into the membrane module 9 via the circulation tank 8 in the latter stage, or the particle size is 30
Since the particles mainly having a small particle diameter of μm or less are prevented, the decrease of the membrane permeation flux is prevented.

Further, a pH meter 6 provided in the pH adjusting tank 1
The amount of sodium hydroxide added is controlled by the sodium hydroxide injection pump 7 that is linked to the so that the pH of the insolubilized water becomes 7 or more. Therefore, a small amount is sufficient and the cost increase due to the addition of sodium hydroxide can be suppressed to a low level.

The total gypsum concentration of the insolubilized water does not exceed the solubility of gypsum (2100 mg / l) and can be any concentration that can prevent a decrease in the membrane permeation flux. It is preferable to control the addition amount of the calcium-based alkaline agent so as to be in the range of 3000 mg / l. If the total gypsum concentration is higher than this range, the effect of preventing the decrease in membrane permeation flux is not sufficient, and conversely, if the total gypsum concentration is low, the amount of non-calcium alkaline agent such as sodium hydroxide supplemented for pH adjustment is low. However, it is not preferable in terms of cost.

By adjusting the pH in the pH adjusting tank 1, fluorine in the waste water is insolubilized as calcium fluoride (CaF ₂ ), and heavy metal ions are insolubilized as hardly soluble hydroxides. At the same time, sulfate ions in the wastewater are converted to gypsum (CaS
O ₄ · 2H ₂ O) is deposited.

The insolubilized treated water is introduced into the membrane module 9 through the pipe 14, the circulation tank 8 and 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 in the circulation tank 8 through the pipe 17, and a part of the concentrated water is discharged out of the system through the pipe 18.

As the membrane module 9, a cross-flow type membrane module provided with a UF (ultrafiltration) membrane or an MF (microfiltration) membrane is suitable.

The apparatus shown in FIG. 1 is an embodiment of the present invention, and the present invention is not limited to the illustrated one as long as it does not exceed the gist of the invention.

For example, pH adjusting tanks may be provided in two stages, and slaked lime and sodium hydroxide may be added to separate pH adjusting tanks.

However, since increasing the number of tanks is disadvantageous in terms of installation area and the like, it is advantageous to add slaked lime and sodium hydroxide to the same pH adjusting tank as shown in FIG. .

In the present invention, slaked lime (Ca (OH) ₂ ) is preferable as the calcium-based alkaline agent. As the non-calcium alkaline agent, magnesium hydroxide or the like can be used in addition to sodium hydroxide.

[0027]

EXAMPLES 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 addition, the water quality shown in Table 1 is an average value, and about sulfate ion,
It varied in the range of 6000 mg / l.

[0029]

[Table 1]

The amount of slaked lime added in the pH adjusting tank was controlled so that the total gypsum concentration produced by the reaction between sulfate ions in the waste water and slaked lime was 4000 mg / l based on the measurement result of the electric conductivity meter. Further, sodium hydroxide was added so that the pH in the pH adjusting tank would be 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 liters / day Membrane area: 0.036m ² Circulating water (concentrated water) amount: 8.4 liters / hr pH adjusting tank, circulation tank residence time: 2.5 hr Treatment The membrane permeation flux at the beginning and 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 conducted except that sodium hydroxide was not added to the pH adjusting tank and the pH was adjusted to 7.0 with only slaked lime. And 30
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.

[0035]

[Table 2]

From Table 2, it is possible to prevent coarse particles from flowing into the membrane module by using sodium hydroxide together in adjusting the pH, thereby preventing a decrease in the membrane permeation flux and preventing the membrane permeation flow. It is clear that the bundles are kept high.

[0037]

As described above in detail, according to the flue gas desulfurization wastewater treatment apparatus of the present invention, a calcium-based alkaline agent is added to the flue gas desulfurization wastewater to insolubilize it, and the insoluble matter is subjected to membrane separation treatment. Therefore, by using a non-calcium alkaline agent in combination, it is possible to prevent a decrease in the membrane permeation flux due to the deposition of gypsum and to perform stable and efficient treatment for a long period of time. Moreover, by controlling the addition amount of the calcium-based alkaline agent based on the measured value of the electrical conductivity of the wastewater, the addition amount of the non-calcium-based alkaline agent can be minimized. By doing so, the increase in cost can be reduced.

[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 Sodium hydroxide storage tank 4 Electric conductivity meter 6 pH meter 8 Circulation tank 9 Membrane module

Claims (1)

[Claims] 1. An electric conductivity measuring means for measuring the electric conductivity of flue gas desulfurization wastewater, and means for adding a calcium-based alkaline agent to the flue gas desulfurization wastewater based on the measurement result of the electric conductivity measuring means. A device for treating flue gas desulfurization wastewater, which comprises a means for adding a non-calcium alkaline agent so that the pH of the liquid after the addition of the calcium alkaline agent falls within a predetermined range.