JP3572223B2 - Absorbent slurry treatment method and flue gas desulfurization system - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
TECHNICAL FIELD The present invention relates to a method for treating an absorbent slurry and a flue gas desulfurization system, and more particularly, to a wet lime / gypsum method flue gas in which sulfur oxide gas in exhaust gas is absorbed and separated using a limestone or slaked lime absorbent slurry. The present invention relates to a desulfurization system and a method for treating the absorbent slurry.
[0002]
[Prior art]
Combustion exhaust gas using coal as fuel is treated by a desulfurization device using the lime-gypsum method, and wastewater containing persistent COD components (components causing chemical oxygen demand), fluorine compounds, heavy metals, etc. Is discharged. The hardly decomposable COD component contained in the desulfurization wastewater includes an inorganic COD component and an organic COD component. The inorganic COD component is the SO2 absorbed in the absorbing solution in the desulfurizer.₂And NO_xConsists of a nitrogen-sulfur compound (hereinafter also referred to as "NS compound") generated by a part of the reaction, and the organic COD component is mainly used as makeup water in a desulfurization device. It consists of organic components in industrial water.
As a method for treating these COD components, there are a coagulation sedimentation method using a usual coagulant and an activated sludge method using microorganisms. As a method for decomposing the NS compound, a nitrite decomposition method is known, but from the viewpoint of chemical cost and workability, a part of the absorption slurry of the desulfurization device is extracted and solid-liquid separated. There is also known a method of removing the NS compound by adjusting the pH of the separated solution and adding an oxidizing agent.
[0003]
As a method for treating the desulfurization wastewater, in addition to the method for treating the NS compound in the desulfurization wastewater, it is necessary to additionally treat other coexisting components such as organic COD, fluorine, and heavy metals.
Since the organic COD component contained in the desulfurization effluent is hardly decomposable, an activated carbon adsorption method is generally used as a treatment method for the hardly decomposable organic COD component, but the adsorptivity to activated carbon is extremely low. In order to sufficiently adsorb and remove small organic components, it is necessary to enlarge the adsorption equipment.
[0004]
As a method for treating wastewater containing fluorine, a calcium coagulation sedimentation method in which calcium ions are added to fluorine ions and removed as calcium fluoride is generally used. As an improved treatment method, a two-stage coagulation method is used. Precipitation methods are known. In this method, magnesium ion is added to slaked lime by removing a precipitate composed of a heavy metal hydroxide and the like once formed, and further adding an alkaline agent such as sodium hydroxide to make an alkaline region. This is a method in which magnesium oxide (hydroxide) is precipitated and, at the same time, the remaining fluorine ions are coprecipitated and separated.
All of such conventional treatment methods have a problem that the desulfurization wastewater treatment process is large and complicated, and the amount of waste from the desulfurization wastewater treatment process is large.
[0005]
Conventionally, COD treatment, separation by coagulation sedimentation, activated carbon treatment, fluorine treatment, etc. are performed using a part of the filtrate after the gypsum separation as wastewater, and it has been studied only as a method for treating desulfurization wastewater. Complicated workup was required.
That is, conventionally, the absorbent slurry is directly sent to the gypsum separator from the desulfurization apparatus, and the gypsum is removed by the gypsum separator. The filtrate contains components such as heavy metals. After the gypsum separation process, a part of the filtrate is extracted as wastewater in the filtrate treatment process, and is sequentially treated in COD decomposition, coagulation and precipitation by adding a heavy metal chelating agent and an alkali agent, activated carbon adsorption, and fluorine adsorption. It had been. Due to the complicated treatment in these wastewater treatment steps, separation and removal of heavy metals became possible, but the SS component was always generated as sludge in the wastewater treatment step. In addition, a large amount of sludge treatment cost is expensive, which is disadvantageous in terms of cost.
[0006]
Therefore, in a conventional plant, there is a method in which the sludge coming out of the wastewater treatment is again mixed and treated at the preceding stage of the gypsum separation step. That is, in order to reduce the amount of sludge, this is a method of mixing sludge from wastewater treatment into gypsum. In this method, all solid components (such as hydroxides) that have separated from the SS component by coagulation separation or the like are mixed with the absorbent slurry after the desulfurization step and discharged as a gypsum cake.
However, when a method of returning the sludge after coagulation and sedimentation to gypsum before separation by such a method is employed, since the main component of the returned sludge is a hydroxide having poor dehydration properties, the amount of sludge may increase. However, the operation control of the gypsum separator during dehydration is difficult and complicated. That is, since the hydroxide from the desulfurization wastewater treatment process is mixed with the desulfurized gypsum and dehydrated, the operation control of the gypsum separator becomes difficult, and at the same time, the water content and the purity of the obtained gypsum cake are easily affected.
As described above, in the wastewater treatment performed on a part of the filtrate after the separation of the gypsum, the wastewater contains SS components and pollutants such as heavy metals. Was needed.
[0007]
On the other hand, in a wastewater treatment process after the gypsum separation process, a process of adding a heavy metal chelating agent, a coagulation aid, a permanganate, or the like to the filtrate has been conventionally performed. These treatment steps (desulfurization step and wastewater treatment step) have been technically developed as independent ones.
Therefore, it has not been sufficiently studied whether the desulfurization process and the wastewater treatment process can be efficiently combined to promote the treatment of the entire system, simplify the treatment equipment, and reduce sludge. Was.
[0008]
[Problems to be solved by the invention]
In view of the above problems, the present inventors have eagerly developed a flue gas desulfurization system that can improve the treatment efficiency in the desulfurization step and the wastewater treatment step and reduce or reduce the amount of sludge discharged by the treatment. investigated.
As a result, the present inventors have found that in a flue gas desulfurization system, such a problem can be solved by effectively combining and integrating a desulfurization step and a wastewater treatment step.
The present invention has been completed from such a viewpoint, and efficiently and sufficiently removes components such as hardly decomposable COD components, heavy metals, and fluorine from an absorbent slurry of a wet flue gas desulfurization device for coal combustion exhaust gas. At the same time, sludge is prevented from being generated in wastewater treatment, thereby enabling efficient operation of the desulfurization device and facilitation of wastewater treatment.
[0009]
[Means for Solving the Problems]
That is, the present invention provides a method for treating an absorbent slurry that has absorbed sulfur oxide gas in exhaust gas, after a mixing step of adding and mixing a heavy metal chelating agent to the absorbent slurry, a gypsum separation step of separating gypsum. A method for treating an absorbent slurry, comprising: an oxidation step of adding an oxidizing agent to a filtrate from the gypsum separation step; and a membrane separation step of filtering the liquid from the oxidation step with a separation membrane. Is what you do. And, for example, as the absorption liquid slurry, a mode is used in which a part of the liquid that has passed through the gypsum separation step and the oxidation step is returned to the desulfurization step as a concentrated liquid using the slurry that has absorbed the sulfur oxide gas in the desulfurization step.
The treatment method further includes an activated carbon adsorption step of bringing the membrane filtrate having undergone the membrane separation step into contact with activated carbon, and a fluorine adsorption step of bringing the treatment liquid of the activated carbon adsorption step into contact with a fluorine adsorption resin. Can be made. In the mixing step, in some embodiments, a solidified substance such as a permanganate is added in addition to the heavy metal chelating agent.
[0010]
Here, the heavy metal chelating agent is preferably a dithiocarbamic acid group or a thiol group or both, and the treatment solution in the oxidation step is treated in a neutralization step in which a reducing agent is added, and then the membrane separation step is performed. Preferably. As the oxidizing agent, for example, hypochlorite is suitable. Exhaust gas and the like can be used as the reducing agent.
In the treatment method of the present invention, the solid separated in the membrane separation step is returned to the mixing step, mixed with the absorbent slurry, and sent to the gypsum separation step, or the recycled waste liquid in the fluorine adsorption step Is returned to the desulfurization step.
[0011]
Further, the present invention provides a wet lime / gypsum method flue gas desulfurization apparatus for absorbing and separating sulfur oxide gas in exhaust gas using a limestone or slaked lime absorption liquid slurry, wherein a heavy metal chelating agent is added to the absorption liquid slurry. With a mixing tank to mixComprising, after the mixing tank,A gypsum separator for separating gypsum, an oxidation tank for adding hypochlorite to the gypsum-separated filtrate, and a membrane separation tank for filtering the liquid in the oxidation step with a separation membrane, It is intended to provide a flue gas desulfurization device which performs the following. The flue gas desulfurization apparatus further includes an activated carbon adsorption tower for bringing the filtrate in the membrane separation step into contact with activated carbon, and a fluorine adsorption tower for bringing the treatment liquid of the activated carbon adsorption tower into contact with a fluorine adsorption resin. You can also. In the mixing tank, there is also an embodiment in which a solidified substance such as permanganate is added in addition to the heavy metal chelating agent. Further, there is an embodiment in which a neutralization tank for adding a reducing agent to the processing liquid from the oxidation tank is provided between the oxidation tank and the membrane separation tank.
Here, the separation membrane is preferably a tubular microfiltration membrane, a immersion type flat plate microfiltration membrane, or an immersion type hollow fiber microfiltration membrane, and the fluorine adsorption resin is a zirconium supported resin or a cerium supported resin. Is preferred.
In the flue gas desulfurization apparatus of the present invention, an embodiment in which the membrane separation tank and the neutralization tank are installed in an absorption tower of the desulfurization apparatus is also suitably used.
[0012]
The absorption liquid slurry to be treated in the present invention is obtained by absorbing sulfur oxide gas in exhaust gas using limestone or slaked lime, and contains a COD component, heavy metals, fluorine and the like in addition to sulfur oxides.
In the present invention, in the treatment process, by effectively combining and integrating the treatment step and the desulfurization step (desulfurization apparatus), waste containing heavy metals is mixed into desulfurization gypsum, and the waste which is complicated to treat And the amount of final treatment waste discharged as sludge. Further, the treatment process can be simplified as much as possible, and the desulfurization step and the wastewater treatment can be efficiently integrated and performed.
[0013]
The present invention is a desulfurization step in which the entire treatment is promoted and made more efficient by, for example, passing through some of the treatment steps that have been performed in wastewater treatment. Thereafter, the gypsum separation step (solids separation), the oxidation step (COD decomposition step), and the membrane separation step (solids separation step) are performed in this order.
According to the present invention, the desulfurization wastewater treatment process can be simplified, and the amount of precipitated sludge is extremely small as compared with the amount of by-product gypsum, and sludge is hardly generated. Further, since there is no hydroxide generated in the wastewater treatment, there is no need to mix these hydroxides with gypsum, and there is no adverse effect on gypsum moisture content and gypsum purity.
Further, according to the present invention, a processing step can be simplified by adopting a separation step using a separation membrane instead of the conventional coagulation separation step / sand filtration step, etc. Can be avoided.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, specific embodiments of a method for treating an absorbent slurry and a flue gas desulfurization system according to the present invention will be described in detail with reference to the accompanying drawings.
Embodiment (1)
FIG. 1 shows an example of a system capable of implementing the processing method of the present invention.
In FIG. 1, first, wastewater containing heavy metals discharged from the desulfurization device 1 is sent to a mixing step (mixing tank 4). The main component of the absorbent slurry 3a discharged from the desulfurization step (desulfurization device 1) is gypsum, which contains 20 to 30% by weight based on water, and further contains heavy metals as trace components. The ratio of heavy metals varies depending on the constituent components and properties of the fuel, etc., and thus cannot be unconditionally determined.
In the present embodiment, after the desulfurization step, the mixing step of adding a chelating agent, the gypsum separation step (solid content separation), the filtrate treatment step (oxidation step, membrane separation step), the activated carbon adsorption step, the fluorine adsorption step It is performed in order.
[0015]
(1) Mixing process
The mixing step is a step in which a chelating agent for collecting heavy metals, a coagulation aid, and, if necessary, a permanganate are added to the absorption liquid slurry to coagulate and precipitate solids containing heavy metals.
As a chelating agent for collecting heavy metals, a dithiocarbamic acid group (-NH-CS₂Na), a liquid polymer heavy metal collector having a chelate-forming group such as a thiol group (-SNa). The target heavy metal is not particularly limited, but is, for example, a heavy metal such as Cd, Se, and Hg. By adding the chelating agent for collecting heavy metals, microflocs that have collected heavy metals are formed.
The amount of the heavy metal chelating agent added in the mixing step is appropriately determined depending on the amount of the heavy metal in the absorbent and the like, and is usually added to the absorption liquid slurry in an amount of 5 mg / L or more, preferably 10 to 30 mg / L.
[0016]
Coagulation aids are agents that are added as needed to increase the floc of the captured heavy metal chelates or to solidify unreacted heavy metal chelators, such as ferric chloride and sulfuric acid. Iron or the like is used. In addition, since the necessity / unnecessity of the addition of the coagulation aid is determined depending on the components and properties of the fuel, it cannot be unconditionally determined. The amount is 50 to 100 mg / liter, and by this addition, coarse flocs are formed and the separability is improved. The solids in the mixed solution containing these flocs are separated by the gypsum separator 9 and mixed into the gypsum cake.
[0017]
(2) Gypsum separation process
Due to the addition of a chelating agent, a coagulation aid, or the like in the mixing step, heavy metals agglomerated before gypsum removal are mixed into gypsum cake and separated at the same time as gypsum separation. That is, the chelated heavy metals and the like are separated and removed as impurities in the gypsum.
In the present invention, since the heavy metals and the like in the filtrate 11 coming out of the gypsum separation step are removed, even if a part of the filtrate is later drained out, the processing in the subsequent step is easy.
That is, according to the method of the present invention, the heavy metal chelating agent and the like are added to the absorbent slurry withdrawn from the desulfurization device for gypsum separation, all of the filtrate after the gypsum separation is subjected to COD treatment, and a part thereof is separated into a separation membrane. In this embodiment, the treated water is treated as waste water and subjected to post-treatment such as activated carbon treatment. Therefore, in the gypsum separation step, pollutants such as heavy metals are removed, and the membrane separation liquid is extracted separately from the concentrated liquid as wastewater, so that almost no sludge is generated in the post-treatment.
[0018]
Further, among substances contained in the absorption liquid slurry, if an oxidizing agent is added in the oxidation step, it may be converted into a compound that is difficult to recover. In this regard, if a chelating agent or the like is added before gypsum separation to separate heavy metals or the like into gypsum as in the present invention, compounds that are difficult to collect and separate even after COD treatment (oxidation step) Can be prevented beforehand and can be removed by gypsum separation before oxidation.
The gypsum obtained by the gypsum separator contains impurities such as heavy metals that have been solidified, but the purity of the gypsum cake is within a range that does not cause any particular problem.
[0019]
(3) Filtrate treatment process
The filtrate treatment step in the present invention comprises an oxidation step, a membrane separation step, and, if necessary, a neutralization step.
In the present invention, as described above, in the gypsum separation step, heavy metals and the like containing manganese are removed by being mixed into gypsum, and dewatered and filtered. Since the filtrate of the gypsum separator contains hardly decomposable COD, fluorine and the like, the treatment of these compounds is performed on the filtrate 11.
For example, the filtrate treatment step is performed by an apparatus that sequentially passes through an oxidation tank (oxidation reaction), a neutralization tank (neutralization reaction), and a separation membrane.
In the oxidation tank (oxidation reaction) and the neutralization tank (neutralization reaction), first, hypochlorite is added in order to decompose the hardly decomposable COD (oxidation tank). Then, the residual chlorine is reduced with a reducing agent (neutralization tank). In this case, since the reducing agent slightly remains in excess, the reducing agent is oxidized with air to obtain a processing liquid having no problem.
[0020]
Oxidation process / neutralization process
In the oxidation step (COD decomposition step), an oxidizing agent is added to the wastewater to decompose the nitrogen-sulfur compound, which is a COD component in the wastewater, and then adjusted from neutral to weakly alkaline by alkali in the neutralization step. If necessary, a reducing agent is further added to decompose and remove the excess oxidizing agent.
The filtrate 11 from the gypsum separation step is sent to the oxidation step. In the filtrate, SO₂And NO_xAnd an N-S compound (inorganic COD component) having the following composition and a manganese ion mainly produced by the reaction of
Hydroxyamine monosulfonate HONHSO₃ ⁻
Hydroxyamine disulfonate HON (SO₃)₂ ^2-
Hydroxyamine trisulfonate ON (SO₃)₃ ^3-
[0021]
An acid 13a such as hydrochloric acid or sulfuric acid is added to the filtrate to adjust the pH to 4 or less, preferably to about 3 to 4 in order to avoid waste of the amount of chemicals used. Therefore, the amount of the acid added is appropriately determined so as to be within this pH range. Here, as the acid 13a to be added, it is preferable to use hydrochloric acid from the viewpoint of preventing generation of scale.
Thereafter, a predetermined amount of an oxidizing agent such as hypochlorite is added based on the oxidation-reduction potential of the wastewater to decompose the hardly decomposable COD component (NS compound). As the oxidizing agent, hypochlorite, chlorine dioxide solution and the like can be used. Of these, sodium hypochlorite (NaOCl) is preferable from the viewpoint of processability and economy. An example of the reaction formula at this time is shown below.
[0022]
6 ON (SO₃)₃ ^3-+ 18CIO⁻+ 10H₂O
→ 4NO + 2NO₃ ⁻+ 18HSO₄ ⁻+ 18Cl⁻+ 2H⁺+ 3O₂
The amount of the hypochlorite to be added is usually about 2 to 8 mol, preferably about 3 to 5 mol, per mol of the NS compound in terms of mol. The temperature here is preferably 40 ° C. or higher, and the residence time is preferably 2 hours or longer.
In this step, manganese ions (divalent) contained in the filtrate 11 from the gypsum separation step are not oxidized and still remain in a dissolved state.
[0023]
In the neutralization step, the filtrate containing the oxidizing agent added in the oxidation step is first adjusted to pH 7 to 8 with alkali 17a. At that time, the manganese ions (divalent) are oxidized and are precipitated as manganese dioxide (tetravalent).
Further, in order to treat residual chlorine, a reducing agent is added for neutralization. After the NS compound is decomposed, the reducing agent to be added is substantially equivalent to sodium sulfite (Na) based on the remaining amount of the oxidizing agent detected by the oxidation-reduction potential of the wastewater.₂SO₃), Sodium acid sulfite (NaHSO)₃), Sodium thiosulfate (Na₂S₂O₃) Is preferably added to decompose excess oxidizing agent such as sodium hypochlorite.
In addition, as the alkali to be added in the neutralization step, for example, sodium hydroxide or potassium hydroxide can be used.
[0024]
Membrane separation process
With respect to the liquid after the treatment up to the oxidation step and the neutralization step, most of the treatment liquid is returned to the desulfurization step as the concentrated liquid 25 and is circulated and used. That is, at the end of the filtrate treatment step, first, the SS component is removed by membrane separation, and the removed SS component is returned to the desulfurization step again as the concentrated liquid 25. The concentrated liquid returned from the membrane separation tank to the desulfurization device via the concentration pump is about 70% by weight of the filtrate 11 in the gypsum separation step.
On the other hand, the necessary amount of the membrane filtrate 28a separated in the membrane separation step is sent to the subsequent wastewater treatment step. In the wastewater treatment, activated carbon adsorption is performed in the activated carbon adsorption tower 34 in order to treat the organic COD contained in the membrane filtrate 28a, and then the fluorine adsorption treatment with the fluorine adsorption resin is performed in the fluorine adsorption tower 38 to purify the wastewater. Release as water.
[0025]
Examples of the separation membrane 21 include a tubular microfiltration membrane, a submerged flat plate microfiltration membrane, and a submerged hollow fiber microfiltration membrane, and any of them can be used. For example, in the case of a immersion type hollow fiber microfiltration membrane, if the filtrate is allowed to pass through the inside of a hollow tube in the membrane with a slightly reduced pressure, the solid content adheres to the surface, and only water flows inside and is separated. The separation membrane is arranged side by side in the vicinity of the center of the membrane separation tank 20 or slightly above, and if it becomes dirty, it is appropriately cleaned. The separation membrane is always vibrated by the flow of the liquid by the air blown out from the lower diffuser 23. Air volume is usually 0.1 to 0.4m per diffuser³/ H.
The attachment of solids to the membrane surface is prevented by vibration of the membrane due to bubbling. Part of the solid content floats, and the rest precipitates at the bottom of the membrane separation tank 20. This sediment can be returned to the mixing tank 4 before gypsum separation by the sediment pump 26 as needed. This precipitate contains the precipitated manganese dioxide, and is separated into the gypsum simultaneously with the separation of the gypsum.
The reason that the filtrate is separated by the separation membrane is to prevent the solid content from being sent to the wastewater treatment step, and the solid content is removed by solid-liquid separation. In the present invention, since the solid-liquid separation is performed by the separation membrane, there is no need to provide a filtration step (sand filtration or the like) before the activated carbon adsorption tower.
[0026]
(4) Activated carbon adsorption process
The activated carbon adsorption step is a step of adsorbing and removing organic COD components by contacting the activated carbon with the membrane filtrate 28a having undergone the membrane separation step as waste water.
After being stored in the membrane filtrate tank 32, the membrane filtrate 28 a is guided to the activated carbon adsorption tower 34 by the pump 33, passes through the granular activated carbon layer in the activated carbon adsorption tower 34, and is mainly composed of organic water derived from industrial water. Is adsorbed and removed.
The wastewater from which the organic COD component has been adsorbed and removed is led to a fluorine adsorption step for treatment. The activated carbon that has been passed through for a certain period of time and has been clogged with impurities can be removed by backwashing with water.
[0027]
(5) Fluorine adsorption process
The fluorine adsorption step is a step in which the waste water treated in the activated carbon adsorption step is brought into contact with a fluorine adsorption resin to adsorb and remove the remaining fluorine, and then adjusted with an alkali agent.
In the fluorine adsorption step, after adjusting the pH of the wastewater to about 2 to 4 with a mineral acid such as hydrochloric acid, the pH-adjusted wastewater is passed through a fluorine-adsorbing resin layer in a fluorine-absorbing tower, and a small amount of residual water remains in the liquid. To remove fluorine ions. Among the fluorine-adsorbing resins, there are those having various forms as a functional group or a supporting metal, and specific examples thereof include a phosphomethylamino group chelating resin, a zirconium-supporting resin, a cerium-supporting resin, and the like. A mold resin and a cerium-carrying resin are preferably used.
[0028]
Among them, for example, cerium-carrying resin reacts with fluorine ions as follows.
[Adsorption reaction] Ce ... OH⁻  + F- → Ce ... F- + OH⁻
In addition, the adsorbent resin whose fluorine adsorption ability has been reduced by passing water for a certain period of time is regenerated by reacting with an alkali agent such as sodium hydroxide as follows, and then washed with a mineral acid such as hydrochloric acid and water, and regenerated. can do.
[Regeneration reaction] Ce ... F⁻  + NaOH → Ce… OH⁻  + NaF
The regenerated waste liquid 42 discharged at this time can be returned from the regenerated drain tank 43 to the desulfurization apparatus 1. In this case, fluorine ions in the regenerated waste liquid are captured by a large amount of calcium ions in the desulfurization apparatus. Is done. Fluoride ions are fixed as calcium fluoride and the resulting gypsum (CaSO₄). In particular, the regeneration waste liquid 42 contains an excessive amount of NaOH, and has a favorable effect on the desulfurization performance of the desulfurization apparatus.
On the other hand, the fluorinated water 39 from which the fluorine ions have been adsorbed and removed is adjusted to a pH of about 6 to 8 with an alkaline agent such as sodium hydroxide in the treated water tank 40, and is discharged or reused.
[0029]
Embodiment (Part 2)
FIG. 2 shows an embodiment of a system using the processing method according to the present invention.
In the present embodiment, an immobilizing substance such as permanganate 7 is also added to the absorbent slurry in the mixing tank 4. Manganese or the like is usually present in the absorbing liquid. For example, manganese substances cannot be disposed of as they are because they are regulated substances.
Therefore, conventionally, there has been a method in which the pH is raised to 10 or more, the precipitate is formed in the form of a hydroxide, and the precipitate is separated. However, in this method, magnesium contained in the desulfurization wastewater also precipitates as magnesium hydroxide. Therefore, in order to prevent the coprecipitation of magnesium, in the removal of manganese ions, a method of converting manganese into manganese dioxide and then separating and removing the manganese is preferable. For this purpose, permanganate is added.
By adding the permanganate 7 in the mixing tank 4 as described above, the manganese substance in the absorbent slurry is separated and removed from the gypsum in the gypsum separation step.
[0030]
In general, manganese has a lower affinity for a heavy metal chelating agent than other heavy metals, so that other heavy metals are deposited first as solids. Therefore, it may be difficult to remove manganese only with a heavy metal chelating agent. Therefore, for manganese ions (divalent) having a low removal efficiency (solid differentiation ability) by a chelating agent, it is effective to add a specific solidified substance (for example, permanganate or the like). In this case, it is preferable to add the permanganate at about a neutral pH. Examples of the permanganate include potassium permanganate and the like.
When a permanganate is used as described above, its amount ratio is usually 1 to 5 times, preferably 1.5 to 3 times by weight the amount of manganese ions (divalent) in the absorption slurry. Add so that it becomes. Here, the reaction when the permanganate is added is represented by the following reaction formula.
[0031]
2KMnO₄+ 3Mn²⁺+ 4H₂O → 5MnO₂・ 2H₂O + 2K⁺+ 4H⁺
[0032]
Embodiment (Part 3)
In the present embodiment, in a wet lime / gypsum method flue gas desulfurization apparatus that absorbs and separates sulfur oxide gas using an absorption liquid slurry, the above-mentioned membrane separation tank is installed in the absorption tower of the desulfurization apparatus 1. When a neutralization tank for adding a reducing agent to the processing liquid from the oxidation tank is provided between the oxidation tank and the membrane separation tank, the neutralization tank is also used in the desulfurization apparatus 1 in addition to the membrane separation tank. Installed in the absorption tower.
FIG. 3 shows an outline of the apparatus of the present embodiment, and FIG. 4 shows an outline of an apparatus in which an immobilizing substance such as permanganate is further added.
The device of the present embodiment is an integrated device of the filtration process and the desulfurization device 1, and the filtrate treatment process is incorporated in the desulfurization device. Therefore, even if the concentrated liquid 25 obtained by the membrane separation does not return to the desulfurization apparatus 1 through the pump, the concentrated liquid 25 remains in the absorption liquid tank 1a of the desulfurization apparatus as it is, which is efficient.
[0033]
【The invention's effect】
INDUSTRIAL APPLICABILITY In the treatment of an absorbent in a flue gas desulfurization system, the treatment efficiency in the desulfurization step and the wastewater treatment step can be increased, and the amount of sludge discharged by the treatment can be reduced or reduced.
That is, in the present invention, by effectively combining and integrating the desulfurization step (desulfurization apparatus) and the absorption slurry treatment step (gypsum separation), the amount of waste in the entire system can be reduced, and the burden on the post-treatment step is reduced. You.
Further, according to the present invention, components such as persistent COD components, heavy metals, fluorine and the like can be efficiently and sufficiently removed from an absorption slurry of a coal flue gas wet flue gas desulfurization unit, and sludge is treated in wastewater treatment. Is prevented, so that efficient operation of the desulfurization device and simplification of wastewater treatment can be achieved.
Furthermore, since the resin regeneration waste liquid in the fluorine treatment step of the wastewater treatment contains a high pH and an excessive amount of NaOH, by returning it to the desulfurization apparatus, a favorable effect is exerted on the desulfurization performance of the desulfurization apparatus.
[0034]
As described above, according to the present invention, the desulfurization wastewater treatment process can be simplified, and the amount of precipitated sludge is extremely small as compared with the amount of by-product gypsum, and sludge is hardly generated. Further, since there is no hydroxide generated in the wastewater treatment, there is no need to mix these hydroxides with gypsum, and there is no adverse effect on gypsum moisture content and gypsum purity.
Furthermore, by employing a separation step using a separation membrane instead of the conventional coagulation separation step / sand filtration step, etc., the treatment step can be simplified, and the necessity of increasing the size of the equipment and the need for a large-scale treatment facility can be avoided. .
[Brief description of the drawings]
FIG. 1 is a configuration diagram schematically showing an example of a flue gas desulfurization system according to an embodiment (part 1).
FIG. 2 is a configuration diagram schematically showing an example of a flue gas desulfurization system according to an embodiment (No. 2).
FIG. 3 is a configuration diagram schematically showing an example of a flue gas desulfurization system according to an embodiment (part 3).
FIG. 4 is a configuration diagram schematically showing another example of a flue gas desulfurization system according to an embodiment (part 3).
[Explanation of symbols]
1 desulfurization equipment
2 circulation pump
3 Absorbent slurry
4 Mixing tank
5 Heavy metal chelating agents
6 Coagulation aid
7 Permanganate
8 mixture
9 Gypsum separator
10 gypsum (10a gypsum cake)
11 Filtrate
12 oxidation tank
13 Acid
14 hypochlorite
15 Oxidation reaction tank
16 Neutralization tank
17 alkali
18 Reducing agent
19 Neutralization reaction solution
20 Membrane separation tank
21 Separation membrane
22 air
23 diffuser
24 Concentrate pump
25 Concentrate
26 Sediment pump
27 Sediment slurry
28 Membrane filtrate (membrane separation liquid)
29 receiver
30 vacuum pump
31 Exhaust
32 Membrane separation liquid tank
33 activated carbon pump
34 activated carbon adsorption tower
35 Activated carbon treated water
36 activated carbon treatment tank
37 Fluorine adsorption tower pump
38 Fluorine adsorption tower
39 Fluorinated water
40 treated water tank
41 Treated water
42 Recycling waste liquid
43 Recycling waste liquid tank
44 Recycling waste liquid pump

Claims (14)

In a method for treating an absorbent slurry absorbing sulfur oxide gas in exhaust gas, after a mixing step of adding and mixing a heavy metal chelating agent to the absorbent slurry, a gypsum separation step of separating gypsum, 1. A method for treating an absorbent slurry, comprising: an oxidation step of adding an oxidizing agent to the filtrate of step (a), and a membrane separation step of filtering the liquid from the oxidation step with a separation membrane. In a method for treating an absorbent slurry absorbing sulfur oxide gas in exhaust gas, a gypsum for separating gypsum after a mixing step of adding and mixing a heavy metal chelating agent to the absorbent slurry absorbing the sulfur oxide gas in a desulfurization step A separation step, an oxidation step of adding an oxidizing agent to the filtrate from the gypsum separation step, and a membrane separation step of filtering the treatment liquid from the oxidation step with a separation membrane, and the gypsum separation step and the oxidation step A method for treating an absorbent slurry, characterized in that a part of the liquid that has passed through is returned to the desulfurization step as a concentrated liquid. The method further comprises: an activated carbon adsorption step of bringing the membrane filtrate having passed through the membrane separation step into contact with activated carbon; and a fluorine adsorption step of bringing the treatment liquid of the activated carbon adsorption step into contact with a fluorine adsorption resin. Item 3. The method for treating an absorbent slurry according to Item 1 or 2. The method for treating an absorbent slurry according to claim 1 or 2, wherein a heavy metal chelating agent and a solidified substance are added in the mixing step. The method for treating an absorbent slurry according to any one of claims 1 to 4, wherein the heavy metal chelating agent is a dithiocarbamic acid group or a thiol group or both. The method for treating an absorbent slurry according to any one of claims 1 to 4, wherein the oxidizing agent is a hypochlorite. The absorption liquid slurry according to any one of claims 1 to 4, wherein the solid separated in the membrane separation step is returned to the mixing step, mixed with the absorption liquid slurry, and sent to a gypsum separation step. Processing method. The method according to claim 3, wherein the regenerated waste liquid from the fluorine adsorption step is returned to the desulfurization step. A gypsum separator that includes a desulfurization device that absorbs sulfur oxide gas in exhaust gas, and a mixing tank that adds and mixes a heavy metal chelating agent to the absorbent slurry from the desulfurization device, and separates gypsum after the mixing tank. A flue gas desulfurization system comprising: an oxidation tank for adding an oxidizing agent to the gypsum-separated filtrate; and a membrane separation tank for filtering the liquid in the oxidation step with a separation membrane. In addition, claims, characterized by comprising the activated carbon adsorption tower for contacting the filtrate of the membrane separation process in the activated carbon, and fluorine adsorption tower for contacting the treatment solution of the activated carbon adsorption tower a fluorine adsorption resin, the 9. The flue gas desulfurization system according to 9 . 11. The flue gas desulfurization system according to claim 9 , wherein a heavy metal chelating agent and a solidified substance are added to the mixing tank. The flue gas desulfurization system according to claim 9 or 10 , wherein the separation membrane is a tubular microfiltration membrane, a submerged flat plate microfiltration membrane, or a submerged hollow fiber microfiltration membrane. The flue gas desulfurization system according to claim 10 , wherein the fluorine-adsorbing resin is a zirconium-carrying resin or a cerium-carrying resin. The flue gas desulfurization system according to claim 9 or 10 , wherein the membrane separation tank is installed in an absorption tower of a desulfurization device.

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