KR102065713B1 - Method for treating wastewater from flue gas desulfurization using electrolysis device - Google Patents

Abstract

The present invention relates to a treatment method of flue gas desulfurization wastewater using a diaphragm type electrolysis device and, more specifically, to a flue gas desulfurization wastewater treatment method which treats NS-COD by electrolyzing process wastewater and supplying ions necessary for reaction without using harmful chemicals. Preferably, the flue gas desulfurization wastewater treatment method of the present invention includes: a step of removing N-S COD components in flue gas desulfurization wastewater; and a step of removing heavy metal and fluorine. The present invention: adds drain water after completing a treatment process of flue gas desulfurization wastewater to a positive electrode unit formed of a positive electrode chamber and a positive electrode of a diaphragm type electrolysis device; adds tap water to a negative electrode unit formed of a negative chamber and a negative electrode of the diaphragm type electrolysis device; removes N-S COD components by adding hypochlorous acid (HOCI) generated in the positive electrode unit to the N-S COD component removing step; and adds sodium hydroxide (NaOH) generated in the negative electrode unit to the heavy metal and fluorine removing step.

Description

METHODS FOR TREATING WASTEWATER FROM FLUE GAS DESULFURIZATION USING ELECTROLYSIS DEVICE}

The present invention relates to a method for treating flue gas desulfurization wastewater using a diaphragm type electrolysis device, and specifically, to process NS-COD by supplying ions required for the reaction by electrolyzing process wastewater without using harmful chemicals. The present invention relates to a method for treating flue gas desulfurization wastewater.

As the demand for environmental protection increases around the world, the importance of desulfurization wastewater treatment in thermal power plants is emerging. Flue gas emitted from coal or petroleum-fired power plants includes nitrogen oxides and sulfur oxides, and various methods are used to remove them. In general, nitrogen oxides are removed using a selective catalytic reduction device, and sulfur oxides are removed using a wet absorption tower, which is called a wet desulfurization process. In the wet desulfurization process, waste water containing a large amount of inorganic N-S COD component and ammonia nitrogen is discharged.

The chemical oxygen demand (COD) component generated in the desulfurization process is largely composed of NS-based COD component composed of NO ₂ and SO ₂ compounds, COD component by dithio acid ion (S ₂ O ₈ ^-2 ), and limestone as a desulfurization absorber. is classified as COD and COD components with an organic ^{substance-generated} CaCO ₃ it is. Among the COD components contained in the wastewater generated in the desulfurization process, most of the NS-based COD components produced by the NS-based components are used. These are generated by the reaction of NO ₂ and SO ₂ in the exhaust gas of the desulfurization process under acidic conditions and aqueous solutions in the absorption tower.

The removal method of the NS-based COD component is a method of injecting NaNO ₂ at a water temperature of 50 ° C. or higher at pH 2 to remove gaseous N ₂ O or an ionic NO ₃ ⁻ form, and a water temperature of 45 ° C. or higher at pH 4 NaOCl is injected from to remove gaseous NO or ionic NO ₃ ^- form. The above methods all use the oxidizing agents NaNO ₂ and NaOCl, and use a large amount of medicine to adjust to a low pH. In addition, in the case of NaOCl, the concentration is easily reduced according to the ambient temperature, and since humans need to operate it directly, there is a risk of safety accident.

Various documents have been published regarding the method for treating desulfurized wastewater discharged from the desulfurization process.

Korean Laid-Open Patent Publication No. 2006-0026510 describes an apparatus and method for removing nitrogen components such as nitrate and nitrogen at the same time by removing sludge from desulfurized wastewater and directly electrolyzing to remove the bonds of the hardly decomposable NS compounds. .

In addition, in Korean Patent Laid-Open Publication No. 2003-0478206, in order to remove the hard-treatment COD component, after contacting a palladium-based bimetallic catalyst with a large amount of nitrate ions present in the desulfurized wastewater, the active hydrogen ions present in the wastewater The use of nitrates adsorbed on bimetals discloses a process of reducing to nitrite and reacting with a hardly treatable COD component.

In addition, Korean Patent No. 10-1789792 discloses a method for treating desulfurized wastewater by injecting ferric acid salt (IV) metal oxide to treat NS-linked degradable desulfurized wastewater and treating phosphorus and nitrogen through biological treatment using anaerobic microorganisms. Doing. The method has an advantage in that it does not use a flocculant that is consumed separately when treating heavy metals after N-S COD treatment, but has a disadvantage in that iron ions and oxidizing agents, NaOCl and NaOH, are consumed in a large amount.

An object of the present invention is to provide a method for treating N-S COD in flue gas desulfurization waste water without using harmful chemicals.

The above object is the removal of N-S COD component in the flue gas desulfurization waste water; And a method for treating flue gas desulfurization wastewater comprising the step of removing heavy metals and fluorine. Injected into the cathode portion consisting of the cathode and cathode chamber of the electrolysis device, the hypochlorous acid (HOCl) generated in the anode portion is added to the NS COD component removal step to remove the NS COD component, generated from the cathode portion Sodium hydroxide (NaOH) is achieved by the method for treating flue gas desulfurization waste water, characterized in that the step of removing the heavy metal and fluorine.

Preferably, the discharged water after completion of the flue gas desulfurization waste water treatment process is characterized in that it goes through a softening step of removing calcium ions or magnesium ions before being introduced to the anode.

Also preferably, the pH in the N-S-based COD component removal step is 4 to 4.5, the pH in the heavy metal and fluorine removal step is 6.5 to 8.0.

Also preferably, the diaphragm type electrolysis apparatus may include an anode part including an anode and an anode chamber, an anode part provided on both sides of the anode part, and an anode part consisting of a cathode and a cathode chamber, and an ion exchange membrane provided between the anode part and the cathode part. Include.

N-S COD treatment method of the flue gas desulfurization waste water of the present invention can effectively remove the N-S COD components without injecting HCl or NaOCl used in the existing process, it is possible to significantly reduce the injection amount of the neutralizing chemical in the subsequent process. The present invention can significantly reduce the use of strong acids or strong alkaline chemicals, and can reduce the risk of safety accidents.

1 schematically shows a conventional process for treating flue gas desulfurization waste water.
Figure 2 schematically shows a flue gas desulfurization waste water treatment process according to the present invention.
3 is a schematic diagram of a diaphragm type electrolysis tank according to the present invention.
It is a schematic diagram of the negative electrode of a diaphragm type electrolysis tank.
5a and 5b is a graph measuring the HOCl generation concentration and current efficiency of the diaphragm electrolysis tank according to the present invention.
6 is a graph measuring voltage according to whether oxygen (O ₂ ) is injected in a diaphragm type electrolysis tank according to the present invention.

All technical terms used in the present invention, unless defined otherwise, have the following definitions and conform to the meanings commonly understood by those skilled in the art in the relevant field of the present invention. In addition, although a preferred method or sample is described herein, similar or equivalent things are included in the scope of the present invention.

The term "about" means 30, 25, 20, 15, 10, 9, 8, 7, 6, 5, by reference quantity, level, value, number, frequency, percentage, dimension, size, quantity, weight, or length. By amount, level, value, number, frequency, percentage, dimension, size, amount, weight or length, varying by 4, 3, 2 or 1%.

Throughout this specification, the terms “comprises” and “comprising”, unless otherwise indicated in the context, include a given step or component, or group of steps or components, but any other step or component, or It is to be understood that it does not exclude a step or group of components.

The term "N-S-based COD component" as used herein refers to an inorganic or organic compound comprising nitrogen (N) and sulfur (S).

Hereinafter, with reference to the accompanying drawings will be described in detail the present invention.

Figure 1 schematically shows a conventional method for treating flue gas desulfurization waste water.

In general, flue gas desulfurization wastewater is discharged through an N-S-based COD component removal step, heavy metal and fluorine removal step, calcium removal step, residual COD removal and biological denitrification step, and third advanced treatment step.

Waste water discharged from a flue gas desulfurization process is NS COD components consisting of NO ₂ and SO ₂ compound, TN type component ^{_{(NH 4 +, NO 3 -}} ), dt Osan ion ^{_{(S 2 O 8 -2) COD}} , desulfurized sorbent by CaCO ₃ -COD produced by using limestone, COD by organic substances, heavy metals, fluorine, calcium and the like are included. Of these components, the desulfurization drainage COD is mostly NS COD produced by the NS component. These are known to be formed by the reaction of NO ₂ and SO ₂ in the exhaust gas under acidic conditions and aqueous solution in the absorption tower (Khagoeun et al., Korean Journal of Environmental Sciences, Vol. 22, No. 9, 2013, 1073-1078).

At this time, in the step of removing the NS COD component, NaNO ₂ or NaOCl which is an oxidizing agent is added, and hydrochloric acid (HCl) is added to maintain the acidic pH. In addition, a separate reducing agent may be added to remove residual oxidant.

In this step, the N-S-based COD component present in the flue gas desulfurization waste water is removed as in Scheme 1 below.

Scheme 1

^{_{NO (SO 3) 3 -3 +}} HOCl -> NO, NO 3 -

Next, the wastewater discharged from the N-S COD component removal step adds Alum hydrate to remove heavy metals and fluorine, coprecipitates and removes fluoride ions, and adds a chelating agent to form a metal complex to remove heavy metals. The pH of the reactor of this reaction should be maintained at 6.5-8.0, for which NaOH is added.

This is followed by a calcium removal step, residual COD removal and biological denitrification step, and a third advanced treatment step.

Figure 2 schematically shows a flue gas desulfurization waste water treatment process according to the present invention.

Referring to Figure 2, the flue gas desulfurization wastewater treatment process according to the present invention is similar to the conventional process NS COD component removal step 100, heavy metal and fluorine removal step 200, calcium removal step 300, residual COD removal and biological A denitrification step 400 and a third advanced processing step 500. The present invention does not separately add NaNO ₂ or NaOCl as an oxidizing agent in the NS COD component removal step 100, and recycles a part of the discharged water discharged from the flue gas desulfurization waste water treatment process, generated by electrolysis in a diaphragm electrolysis device An oxidant is used and NaOH generated during electrolysis is used for the heavy metal and fluorine removal step 200.

That is, by recycling a part of the discharged water discharged from the flue gas desulfurization waste water treatment process, the discharged water passed through the softening step 600 to remove calcium or magnesium ions contained in the discharged water to the diaphragm type electrolysis device 700 to NS Produces the oxidant required for the COD component removal step. In the softening step (S60) may be used a method such as ion exchange resin tower, alkali precipitation, filtration using a nano-filter.

A schematic diagram of the diaphragm electrolysis device is shown in FIG. 3. Referring to FIG. 3, the diaphragm type electrolysis device 30 includes an anode part composed of an anode 31 and an anode chamber 32, and an anode provided at both sides of the anode part and formed of a cathode 34 and a cathode chamber 35. And an ion exchange membrane 33 provided between the anode and cathode portions. The discharge water of the desulfurization wastewater treatment process which has undergone the softening step (S60) is supplied to the anode chamber 32, and tap water is supplied to the cathode chamber 35.

In the anode chamber 32, chloride gas (Cl ₂ ) is generated as shown in Scheme ₂ below, and Cl ₂ is hydrolyzed by reacting with water as shown in Scheme 3 below to generate hypochlorous acid (HOCl) and hydrochloric acid (HCl). Since the pH in the anode chamber 32 is low, chlorine gas (Cl ₂ ) is present in the anode water, and some HOCl is present in the anode water. Since the pH of the anode water is kept low, the gaseous Cl ₂ is sucked out by the negative pressure condition and mixed with tap water in a separate reservoir (not shown) to exist as HOCl. HOCl thus produced is added to the NS COD component removal step (S10).

Scheme 2

2Cl ^- → Cl ₂ + 2e ^-

Scheme 3

Cl ₂ + H ₂ O → HOCl + HCl

In the separate reservoir, HOCl and OCl ⁻ coexist as in Scheme 4 below. When chlorine gas discharged from the anode is mixed with tap water, chloride gas (Cl ₂ ) is not present due to the pH of the tap water (pH 6-8), but is present as HOCl and OCl ⁻ . When the flow rate of tap water is low, the pH is lowered to increase the HOCl, and when the flow rate of the tap water is increased, the pH is 6-8, so that the HOCl and OCl- coexist.

On the other hand, in the case of the diaphragm type electrolysis device of the present invention, since the pH is very low in the anode chamber, most of them exist in the form of chloride gas (Cl ₂ ).

Scheme 4

HOCl ↔ OCl ^{- +} H +

In the cathode chamber 35, OH ⁻ ions are generated by water decomposition as shown in Scheme 5 below, and react with Na ⁺ having passed through the cation exchange membrane to exist in the form of NaOH in a solution (Scheme 6). The produced NaOH is added to the heavy metal and fluorine removal step 200 to neutralize the pH of the reactor.

Scheme 5

^{_{2H 2 O + 2e - → H}} 2 + 2OH -

Scheme 6

Na ^{+ +} OH ^- → NaOH

According to one embodiment of the invention, it is preferable to inject oxygen gas into the cathode in order to lower the voltage applied to the cathode.

The anode 31 may use iridium oxide (IrOx) or ruthenium oxide (RuOx), which is a platinum group metal, as an electrode for generating chlorine (Cl ₂ ), and detanium (Ta) or titanium (Ti), which is a transition metal, as a binder. It can be prepared in combination with. In the manufacturing method of the positive electrode 31, after sanding the plate, mesh, or perforated titanium plate, one or two or more aqueous solutions of acetone, hydrochloric acid, nitric acid, acetic acid, sulfuric acid, oxalic acid (10-90% concentration) The chemical etching is carried out at 40 to 100 ° C for 5 minutes to 10 hours. After the etching step, after drying until all the water evaporated under the conditions of 50 to 100 ℃, the coating chemical is repeatedly coated several times by brush or dipping or spray or electrostatic coating method and then pyrolyzed at 400 to 600 ℃ conditions to prepare a positive electrode Can be. The coating agent may be prepared by mixing iridium oxide (IrOx) or ruthenium oxide (RuOx) alone or by mixing iridium oxide (IrOx) or ruthenium oxide (RuOx) with a binder.

The cathode 34 is a gas diffusion electrode, and may be manufactured using a catalyst and nickel foam in which 20 to 70 wt% of platinum is supported on a carbon material.

4 is a schematic view of the negative electrode 34 of the diaphragm electrolysis tank according to the present invention. 4, the cathode 34 is divided into a catalytic reaction part and a gas diffusion part, and the gas diffusion part is composed of a power supply part 41 made of stainless steel or titanium plate and a support 42 made of nickel foam. The part consists of a catalyst layer 43 comprising conductive platinum / carbon.

The manufacturing method of the negative electrode 34 is a conductive platinum / carbon (Pt / C) powder and a binder polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE) and the like on the support 42 made of nickel foam The mixed solution may be coated to form a catalyst layer 43, and then hot pressed and manufactured by combining with the power supply unit. Coating on the nickel foam can be carried out with a brush, supported, or the like, preferably spray coating. Hot pressing conditions are suitable for compression from 10 to 300 bar at 100 to 350 ° C.

In the existing NS COD removal process, a large amount of HCl and NaOCl are injected. HCl was injected to maintain the pH of the reactor at 4 ~ 4.5 for effective reaction in the NS COD removal process. However, commercially available NaOCl has a strong alkali pH above 12. Therefore, the conventional drug treatment method has a disadvantage in that the amount of HCl injection increases. In the present invention, by using a diaphragm type electrolysis device in a flue gas desulfurization waste water treatment process, HOCl and HCl generated at the anode part may be introduced into an NS COD removal process, thereby significantly reducing the amount of HOCl and HCl injected in a chemical form. In addition, OH ^- ions required for neutralization in the heavy metal and fluorine removal step can also significantly reduce the amount of NaOH injection in the form of chemicals because NaOH generated from the cathode portion of the diaphragm electrolysis device is introduced in the field.

Hereinafter, the present invention will be described in detail with reference to Examples, but the scope of the present invention is not limited by these Examples.

Experimental Example 1

HOCl generation concentration and current efficiency in diaphragm type electrolysis device using wastewater from flue gas desulfurization were tested. The electrolysis apparatus has the structure of FIG. 3, and the anode is made of RuO ₂ -TiO ₂ prepared using ruthenium oxide and titanium oxide. IrO ₂ -Ta ₂ O ₅ prepared by using an electrode, iridium oxide and tantalum oxide The electrode is a gas diffusion electrode having the structure of FIG. 4. After the water softening step, the wastewater of flue gas desulfurization waste water containing 18,000mg / L of Cl ^- ion was put into the reactor and electrolyzed under current density of 0.1A / cm ² (5A) and flow rate of 4m ³ / hr. The generated Cl ₂ was absorbed into tap water under pH 2 conditions, and the chloride gas (Cl ₂ ) concentration was analyzed by iodine titration, and current efficiency was measured. The results are shown in FIG. 5A (RuO ₂ -TiO ₂ ) and FIG. 5B (IrO ₂ -Ta ₂ O ₅ ). 5A and 5B, the current efficiency was measured to be about 70-80%, and considering some unabsorbed Cl ₂ (existing in the anode chamber), chlorine generation residual efficiency of about 95% or more was measured.

In addition, as a result of measuring the NaOH concentration generated in the cathode, it was measured at 650 mg / L, 700 mg / L, respectively, wherein the pH measured in the cathode water was measured to be about 12 or more.

Experimental Example  2

As in Example 1, the positive electrode and the negative electrode of the electrolytic apparatus were configured, and the influence on the voltage when oxygen (O ₂ ) was injected into the cathode (gas diffusion electrode) or not was measured. Experimental conditions were measured by electrolysis at the current density of 0.1A / cm ² (5A) and flow rate 4m ³ / hr using the discharge water of flue gas desulfurization wastewater containing 8,000mg / L of Cl ^- ions. Indicated. Referring to FIG. 6, when oxygen was added to the cathode, the voltage was measured to be about 2.2V by oxygen reduction reaction, and when oxygen was not added, the voltage was measured to be about 3V. This is because the oxygen reduction reaction is lower than the _{(O 2 + H 2 O +} 4e -, 0.401V - → 4OH) of the water decomposition reaction (0.9V). When oxygen is injected into the cathode, since the voltage of the cathode (gas diffusion electrode) is lowered, the present invention attempts to reduce the power cost. Oxygen injection can be applied even if the cathode is not a gas diffusion electrode, and only the voltage is high due to the water decomposition reaction and has no relation to the HOCl generation efficiency.

Experimental Example 3

NS COD treatment efficiency of the flue gas desulfurization waste water treatment process according to the present invention was measured. CODmn Data on treatment efficiency according to HOCl dose in batch test conditions for 198mg / L. In the test conditions, the water temperature was maintained at 50 ° C. and the stirring condition was 250 rpm at pH 3.5, 4, and 4.5, respectively. HOCl generated by electrolysis in the diaphragm type electrolysis apparatus of the present invention was introduced into each reactor. HOCl is a concentration of about 8,000mg / L HOCl production water produced by maintaining the flow rate of 0.5m ³ / hr under the conditions of Experimental Example 1 concentration of 1,300mg / L, 1,200mg / L, 600mg / L CODmn was analyzed by injecting to and the results are shown in Table 1. In Table 1, pH 4 was analyzed to show the optimum efficiency, and the HOCl / CODmn ratio was found to be about 6. In addition, it was analyzed that the reaction tank stirring time was more than 30 minutes.

Experiment number pH (3.5) pH (4) pH (4.5) T1 (15 minutes stirring time) 28mg / L
(HOCl 1300mg / L @ Cl ₂ ) 24mg / L
(HOCl 1300mg / L @ Cl ₂ ) 43mg / L
(HOCl 1300mg / L @ Cl ₂ ) T2 (30 minutes stirring time) 28mg / L
(HOCl 1300mg / L @ Cl ₂ ) 22mg / L
(HOCl 1300mg / L @ Cl ₂ ) 32mg / L
(HOCl 1300mg / L @ Cl ₂ ) T3 (stirring time 60 minutes) 29mg / L
(HOCl 1300mg / L @ Cl ₂ ) 16 mg / L
(HOCl 1300mg / L @ Cl ₂ ) 31mg / L
(HOCl 1300mg / L @ Cl ₂ ) T4 (15 minutes stirring time) 42mg / L
(HOCl 1,200mg / L @ Cl ₂ ) 28
(HOCl 1,200mg / L @ Cl ₂ ) 53
(HOCl 1,200mg / L @ Cl ₂ ) T5 (30 minutes stirring time) 26mg / L
(HOCl 1,200mg / L @ Cl ₂ ) 21
(HOCl 1,200mg / L @ Cl ₂ ) 32
(HOCl 1,200mg / L @ Cl ₂ ) T6 (stirring time 60 minutes) 24mg / L
(HOCl 1,200mg / L @ Cl ₂ ) 23mg / L
(HOCl 1,200mg / L @ Cl ₂ ) 31mg / L
(HOCl 1,200mg / L @ Cl ₂ ) T7 (15 minutes stirring time) 98mg / L
(HOCl 600mg / L @ Cl ₂ ) 67mg / L
(HOCl 600mg / L @ Cl ₂ ) 112mg / L
(HOCl 600mg / L @ Cl ₂ ) T8 (30 minutes stirring time) 72mg / L
(HOCl 600mg / L @ Cl ₂ ) 42mg / L
(HOCl 600mg / L @ Cl ₂ ) 92mg / L
(HOCl 600mg / L @ Cl ₂ ) T9 (60 minutes stirring time) 60mg / L
(HOCl 600mg / L @ Cl ₂ ) 36mg / L
(HOCl 600mg / L @ Cl ₂ ) 120mg / L
(HOCl 600mg / L @ Cl ₂ )

Having described the specific part of the present invention in detail, it is apparent to those skilled in the art that such a specific technology is only a preferred embodiment, and the scope of the present invention is not limited thereto. Therefore, the substantial scope of the present invention will be defined by the appended claims and equivalents thereof.

Claims (4)

Removing the NS-based COD component in the flue gas desulfurization waste water; And a method for treating flue gas desulfurization wastewater comprising the step of removing heavy metals and fluorine,
The discharged water after the completion of the flue gas desulfurization waste water treatment process is introduced into an anode portion consisting of a cathode and an anode chamber of a diaphragm type electrolysis apparatus, and tap water is introduced into a cathode portion consisting of a cathode and a cathode chamber of the membrane type electrolysis apparatus. Hypochlorous acid (HOCl) generated at the anode part is added to the NS-based COD component removal step, and NS-based COD component is removed, and sodium hydroxide (NaOH) generated at the cathode part is added to the heavy metal and fluorine removal step. ,
After the flue gas desulfurization waste water treatment process is completed, the discharged water is subjected to a softening step of removing calcium ions or magnesium ions by filtration using an ion exchange resin tower, alkali precipitation or nanofilters before being introduced into the anode.
The cathode is a method for treating flue gas desulfurization, characterized in that the catalyst layer containing conductive platinum / carbon is hot-pressed at a condition of 10bar ~ 300bar at 100 ~ 350 ℃ on a support layer made of nickel foam. The method of claim 1, wherein the pH of the N-S-based COD component removal step is 4 to 4.5, and the pH of the heavy metal and fluorine removal step is 6.5 to 8.0. The method of claim 1, wherein the diaphragm electrolysis device comprises an anode part comprising an anode and an anode chamber, an anode part provided at both sides of the anode part, and an anode part consisting of a cathode and a cathode chamber, and an ion exchange membrane provided between the anode part and the cathode part. A method of treating flue gas desulfurization waste, characterized in that it comprises. delete

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