TW201910272A - Fluoride removal method and defluoridation system for flue gas desulfurization wastewater - Google Patents

Abstract

The invention provides a fluoride removal method for flue-gas desulfurization (FGD) wastewater of coal-fired power plants and fluoride removal system thereof. Herein, fluoride in FGD wastewater can be removed by using electrocoagulation process, in which a high frequency pulse power supply combined with alkali addition is used.

Description

   Defluorination method and system for flue gas desulfurization wastewater   

The invention relates to a method and a system for removing fluorine from wastewater, and more particularly to a method and a system for removing fluorine from flue gas desulfurization wastewater from a coal-fired power plant.

It is known that power plants or thermal power plants generate electricity by burning coal. In the process of burning coal, the sulfur contained in coal is released and is present in the flue gas in the form of sulfur oxides. Therefore, in order to reduce the emission of air pollutants, flue gas desulfurization (FGD) has become an important treatment program for gases generated by coal combustion, and this treatment in turn generates a large amount of FGD wastewater. However, the composition of this type of wastewater is complex, the conductivity is high, and even the fluorine exceeds the standard.

For the wastewater generated by FGD, the corresponding treatment method is generally selected according to the characteristics of the FGD wastewater of each plant. At present, the treatment methods commonly used for fluorine-containing wastewater include chemical coagulation sedimentation, adsorption, and electrocoagulation.

Chemical coagulation is a common wastewater treatment method. Its principle for removing fluorine is to add rubber chemicals to react directly or indirectly with pollutants by adding chemical agents under appropriate pH conditions. Removal via precipitation. However, for FGD wastewater with complex ionic components, if the coagulation sedimentation method is used, the addition of a new coagulant will cause the ionic components in the water to become more complex and have a competitive relationship, which will affect the fluoride removal effect; therefore, it will even require multiple dosing Will have a better effect, but this will also generate a large amount of sludge, increasing processing costs. For example, CN 102276034 A uses calcium chloride as the first fluorine removal step, followed by a coagulant (the composition is aluminum chloride, aluminum phosphate, aluminum sulfate, or kaolin) for the second fluorine removal, and finally a flocculant (three Ferric chloride or polyacrylamide) to treat high-concentration fluorine-containing wastewater in power plants. CN 103693770 A uses a multi-stage method to process hexavalent chromium ions, fluoride ions, and COD in FGD wastewater; the treatment effect is achieved by adding calcium hydroxide solution, calcium aluminate powder, and ferric chloride solution.

The adsorption method uses an adsorbent (such as activated aluminum, activated carbon, resin and other materials) to perform adsorption or ion exchange to treat fluorine in industrial wastewater. Generally speaking, different adsorbents have different requirements for the pH value of wastewater. In addition, the excessively high concentration of fluorine-containing wastewater directly into the adsorption system will cause the adsorbent to easily reach saturation, which in turn will reduce the pollutant removal effect. Therefore, if the wastewater is directly treated by adsorption, the cost is usually high. Examples disclosed in the patent literature for treating fluorine-containing wastewater by an adsorption method include, for example, CN 104843818 A, which uses a chelating resin having an aminophosphonic acid compound coated on its surface with chlorine-aluminum ions to adsorb fluorine in the fluorine-containing wastewater And reduce the fluoride ion content of fluorine-containing wastewater to less than 10ppm.

Electrocoagulation (EC) law is an emerging defluorination technology for drinking water and industrial wastewater. Many studies have shown that the use of aluminum (Al) or iron (Fe) as sacrificial electrodes is an effective defluorination method. The conductivity of wastewater can increase the current efficiency and improve the treatment effect by increasing the aluminum or iron ions released by the anodic oxidation reaction. However, the conventional electrocoagulation method has disadvantages such as electrolysis time and large amount of sludge. For example, CN 103086550 B uses a direct current electrolysis method, using a titanium-based coating as the anode and a stainless steel plate as the cathode for electrolytic treatment of FGD wastewater for 60 to 80 minutes, and stirring the FGD wastewater while electrolysis, because the wastewater contains a large amount of chloride ions After electrolysis, oxygen and chlorine gas with strong oxidizing capacity are generated, and then the organic matters in the water are oxidized and decomposed. CN 101817575 B uses electro-flocculation method to treat heavy metals, chlorine, COD, ammonia nitrogen in FGD wastewater; the reactor used is a single-pole variable-frequency pulsed electric flocculation reactor with tubular structure, its pulse frequency is 0.15 ~ 0.2kHz, electrolysis The time is 30 ~ 60 minutes. The anode material is aluminum or iron.

In view of the above-mentioned shortcomings of the current technology for the treatment of FGD wastewater, and the increasingly stricter sewage discharge standards in various countries, a method and system for FGD wastewater treatment that can meet the economic benefits and effectively remove fluorine are urgently needed Therefore, a novel fluorine removal method and a fluorine removal system for FGD wastewater are provided.

An object of the present invention is to provide a method for defluorination of flue gas desulfurization wastewater, including the following steps: (a) electrocoagulation reaction, and (b) stirring and mixing.

Wherein, the step (a) of the electrocoagulation reaction is performed in an electric coagulation device, which is equipped with a power supply that provides a high-frequency pulse current. In step (b), while stirring and mixing, alkali is added to adjust the pH value of the wastewater to pH 5-8.

Optionally, the step (b) of stirring and mixing can be divided into two stages, the first stage is fast mixing, and the second stage is slow mixing.

If necessary, after step (b) is stirred and mixed, precipitation or floating is performed.

Another object of the present invention is to provide a defluorination system for flue gas desulfurization wastewater, which includes the following devices: (a) an electric coagulation device, and (b) a stirring mixing tank.

The electric coagulation device includes one or more pairs of paired anodes and cathodes, a reaction tank, and a power supply providing high-frequency pulse current, wherein the anode and cathode are made of aluminum or iron, and the stirring mixing tank is equipped with Alkali-added components.

Optionally, in addition to the electric coagulation device and the stirring and mixing tank, the fluorine-removing system of the present invention may further include a sedimentation tank or an upper floating tank.

Optionally, the stirring mixing tank is equipped with a stirring speed controller to control the stirring speed.

Optionally, the stirring mixing tank may be two tanks in series, wherein the first tank is a fast mixing tank and the second tank is a slow mixing tank.

In order to make the above-mentioned object, technical features, and advantages of the present invention more comprehensible, the following detailed description will be made with various specific implementation modes.

a‧‧‧step

b, b1, b2‧‧‧step

c‧‧‧step

FIG. 1 is a flowchart of an embodiment of a method for removing fluorine from flue gas desulfurization wastewater according to the present invention.

FIG. 2 is a flowchart of another embodiment of a method for removing fluorine from flue gas desulfurization wastewater according to the present invention.

definition

As used above and throughout the present invention, the following terms should be understood to have the following meanings unless otherwise indicated.

The significance of coagulation is to destroy the stable state of colloidal particles, in order to reduce the mutual exclusion potential between particles and particles, so that the particles can contact each other and agglomerate. After the particles agglomerate, they form a larger rubber plume, which is then removed by precipitation or floating. .

Flocculation, also known as floculation, refers to the stage in which coagulation occurs after particles have aggregated to form a larger gel plume.

Electrocoagulation (EC) means that the aqueous solution to be treated is sent to a reaction tank equipped with a yin and yang electrode, and a voltage is applied to form an electric field effect. In combination with the cations released from the sacrificial electrode, the fine particles of the water body condense Occurs to achieve the effect of separating particles from water.

The following will describe several specific implementation aspects according to the present invention; however, without departing from the spirit of the present invention, the present invention can be practiced in many different forms, and the scope of protection of the present invention should not be interpreted as being limited to what is described in the specification. Presenter. In addition, unless otherwise stated in the text, the terms "a", "the" and similar terms used in this specification (especially in the scope of patent application to be described later) should be understood to include the singular and plural forms.

The method for defluorinating waste water provided by the present invention is an improved electrocoagulation method, which is particularly suitable for treating FGD waste water in power plants. In particular, the method for defluorination of wastewater in the present invention is a method for defluorination by adding high-frequency pulse electric coagulation and adding alkali.

In the method for defluorinating wastewater according to the present invention, the high-frequency pulsed electric coagulation involved is a power supply using high-frequency (preferred range: 0.5 to 60 kHz, optimal range: 3 to 10 kHz) pulses, The electric current belongs to a kind of direct current. The working principle of the system is a high-frequency repetitive pulse process, that is, a high-frequency repetitive "power-off-power-supply" pulse electrolysis process. During pulsed power supply, higher current density can be generated. In the pulse electrolysis, since the energization time is shorter than the total reaction time of the electrolytic treatment, the amount of the metal substance dissolved will be less than that in the direct current electrolysis. Therefore, compared with the direct current electrolysis, the pulse electrolysis greatly reduces the consumption of the electrodes while saving power. In addition, in the high-frequency pulse electric coagulation operation, the pulse signal has the characteristics of periodic commutation, that is, the positive pulse (cathode pulse) is immediately followed by a reverse pulse (anode pulse), so that the electrode plate has both pulse electrolysis, It also has the characteristics of being soluble in both poles, which is more conducive to the flocculation between the released metal ions and colloids, and the polarity of the poles often changes, which also plays a positive role in preventing electrode passivation. Therefore, this method not only has the advantages of fast reaction speed and stable effluent of the traditional electrocoagulation method, but also has the characteristics of short electrolysis time, energy saving, reduced plate consumption, and low sludge production.

Referring to the embodiment shown in FIG. 1, the FGD wastewater sequentially performs the following steps to remove fluorine in the water: electric coagulation, stirring and mixing, and precipitation, wherein each processing step is described in detail below.

As shown in Fig. 1, the fluorine-containing wastewater is first sent to a reaction tank of an electric coagulation device to perform an electric coagulation reaction (step a). The electric coagulation device is provided with a cathode and an anode. Use aluminum or iron. The magnitude of the current generated during the electrocoagulation reaction is determined according to the conductivity of the wastewater. The conductivity of FGD wastewater is generally in the range of 18 to 21 milliSiemens / cm (mS / cm), so the resulting current range is 5 to 10A. The corresponding current density is: 8A / dm ² to 15A / dm ² ; the electrolysis time is 1 to 15 minutes. If both the cathode and the anode are defluorinated with aluminum plates, under the action of electricity, the anode will electrolyze aluminum ions and the cathode will electrolyze water. Depending on the pH value of the wastewater being treated, there will be different forms of aluminum ions in the water. For example, when the pH value is 1 ~ 3, the water is mainly Al ³⁺ ; when the pH value is 4 ~ 5.5, aluminum begins to hydrolyze, and under these conditions, the metal complex (mainly Al (OH) _n . (H ₂ O) _n ^{n +} ), and it is a metal hydroxide (mainly Al (OH) ₃ ) under the condition of high pH (5.5 ~ 10). Therefore, the aluminum species produced under different conditions will react differently with fluoride ions, mainly including the following three reactions: the first is the complexation reaction between aluminum ions and fluoride ions, the main complexes are AlF ²⁺ , AlF ₂ ^+, AlF _3; coprecipitation second reaction, i.e. ^{nAl 3+ + (3n-m)} OH - + mF - → Al n F m (OH) (3n-m); third hydroxide adsorption reaction of aluminum, i.e., _{Al n (OH) 3n + mF} - → Al n F m (OH) (3n-m) + mOH -. All three reactions mentioned above have the effect of removing fluoride ions. If both the cathode and the anode use iron plates to remove fluoride, the mechanism is that under the action of electricity, the Fe ²⁺ produced by the anode reacts with water and oxygen to form Fe (OH) ₃ precipitates, which will adsorb fluoride ions. It _{is: Fe (OH) 3 (s} ) + 3F - → FeF 3 + 3OH - to achieve the removal of fluoride ion.

Referring to Figure 1 again, after the electrocoagulation of the wastewater, the wastewater is then sent to the mixing tank for stirring and mixing (step b). The stirring speed is 20 to 500 rpm and the time is 2 to 30 minutes. The pH value ranges from pH 5 to 8. The OH ^- produced by adding alkali will react with the free Al ³⁺ and aluminum-based metal complexes in the water to further generate an effective fluorine-removing substance; and the wastewater contains a large amount of magnesium ions. Co-precipitation reaction with aluminum ions, hydroxide ions and fluoride ions will further generate effective defluorination substances and improve the defluorination effect. There is no particular limitation on the type of alkali added in this stirring and mixing stage, and the alkali added is preferably a hydroxide of an alkali metal group, for example, sodium hydroxide or potassium hydroxide.

Finally, the wastewater is subjected to a precipitation operation (step c) to achieve the effect of solid-liquid separation. The residence time of the precipitation is between 15 and 60 minutes. In addition, stirring can be accompanied during the precipitation if necessary, wherein the stirring speed is between 5 and 20 rpm. Preferably, the pH value of the wastewater can be adjusted to pH 5-7 before the wastewater is precipitated.

Referring to FIG. 2, according to another embodiment of the present invention, the FGD wastewater is sequentially subjected to the following steps to remove fluorine in the water: electric coagulation, fast mixing, slow mixing, and precipitation. Each of the processing steps is described in detail below. .

As shown in Figure 2, the fluorine-containing wastewater is first sent to the reaction tank of the electric coagulation device for electric coagulation reaction (step a). For the anode and cathode reactions involved in the electric coagulation reaction, please refer to the description above.

Subsequently, the electrocoagulated wastewater is stirred and mixed. As shown in Figure 2, in this embodiment, the stirring and mixing system is performed in two stages, that is, the wastewater after electrocoagulation is first sent to the fast mixing tank for fast mixing (step b1) for 2 to 5 minutes, and stirred The speed is 100 ~ 500rpm. A base such as sodium hydroxide is added simultaneously during the fast mixing stage. Then, the wastewater treated by the fast mixing tank is sent to the slow mixing tank for slow mixing (step b2) for 10 to 30 minutes and the stirring speed is 20 to 100 rpm; the purpose is to aggregate the small rubber feathers under the action of slow mixing. As a result, a larger rubber plume is formed, which is beneficial to the sedimentation of suspended matter.

Finally, the wastewater is subjected to a precipitation operation (step c), so that the solid and liquid are separated. The residence time of the precipitation is between 15 and 60 minutes. In addition, stirring can be accompanied during the precipitation as required, wherein the stirring speed is between 5 and 20 rpm. Preferably, the pH of the wastewater is adjusted to pH 5-7 before the wastewater is precipitated.

Hereinafter, preferred embodiments are proposed to explain the present invention more specifically. However, the following examples are given only to illustrate the present invention, and those skilled in the relevant art will clearly understand that these embodiments do not limit the scope of the accompanying patent application, but can make various changes within the scope and spirit of the present invention. And retouching. Such changes and retouching are of course included in the scope of the accompanying patent application.

Examples

The raw wastewater to be treated used in the examples herein is the wastewater after flue gas desulfurization (FGD) wastewater is treated in the plant to remove some suspended solids (SS).

Example 1

First, the water quality parameters of the wastewater were measured, and the items included pH, conductivity, SS, and fluoride ion concentrations. The results are shown in Table 1. Next, two aluminum plates were used as the cathode and anode, respectively, and the experiment was carried out by using the high-frequency pulse electric coagulation method of the present invention to treat FGD wastewater under different conditions of electrolysis time (5 minutes, 10 minutes, and 15 minutes). 500ml, and then adjusted the pH value of the treated FGD wastewater to neutral, the results are shown in Table 1.2. During the experiment, the pulse frequency of the high-frequency pulse power supply was about 7kHz.

It can be seen from Table 1.2 that a longer period of electrolytic reaction can further reduce the fluoride ion concentration, but it will also cause a large amount of sludge.

Example 2

The high-frequency pulse electric coagulation system was used to treat FGD wastewater. The experimental FGD wastewater had a water volume of about 500ml, the electrolysis time was 5 minutes, and then stirred and mixed while adding different volumes of alkali. The alkali used was: 5% sodium hydroxide. The volume of alkali added are: 0mL, 2mL, 4mL, 8mL, 12mL, 24mL; the experimental results are shown in Table 2.

According to Table 2, it can be found that after the high-frequency pulse electrocoagulation method is added, a small amount of alkali is added, which will have a better defluorination effect.

Example 3

The experiment was carried out by using high-frequency pulse electrocoagulation method combined with alkali addition method to treat FGD wastewater. The amount of water used in the experiment was about 500 ml of FGD wastewater. The experiment was repeated ten times with the same two aluminum electrode plates. The electrolytic reaction time was 5 minutes. Then, fast mixing was performed at 120 rpm and 3 mL of sodium hydroxide was added at a concentration of 5%. The fast mixing time is 2 minutes; and then the slow mixing is performed at a speed of 30 rpm and the slow mixing time is 10 minutes. After the end of each experiment, scrape off the surface of the electrode plate before proceeding to the next experiment. The experimental results are shown in Table 3.

According to Table 3, the change trend of fluoride ion concentration during repeated experiments can be seen. The overall average fluoride removal rate is 70.4%, and the overall average fluorine residual concentration is 35.5mg / L.

Example 4

Compare the effect of high-frequency pulse electric coagulation system with traditional DC electric coagulation system in treating FGD wastewater. Comparative items include electrolysis time, sludge production, and fluoride ion concentration. The amount of FGD wastewater used in the experiment is about 500ml, and the FGD wastewater treated by the two systems with different electrolytic reaction times; both are fast-mixed, while the pH of the wastewater is adjusted to neutral, the speed is 120rpm, and the fast-mixing time is 2 minutes; The slow mixing was performed at a rotation speed of 30 rpm and the slow mixing time was 10 minutes. The results are shown in Table 4. From Table 4, it can be seen that when the raw wastewater is treated with high frequency pulse electric coagulation system and the traditional electric mixing system, the fluoride ion concentration is approximately equal (one is 47.19mg / L, the other is 51.25mg / L). Compared with the traditional electric mixing system, the high-frequency pulse electric coagulation system has the advantages of shorter reaction time and lower sludge production.

Claims (8)

    A method for defluorination of flue gas desulfurization wastewater includes the following steps: (a) electric coagulation reaction, and (b) stirring and mixing, wherein step (a) is performed in an electric coagulation device, the electric coagulation device Equipped with a power supply providing high-frequency pulse current, and adding alkali to adjust the pH of the wastewater to pH 5 ~ 8 while stirring in step (b).         The method of claim 1, wherein the pulse frequency range of the high-frequency pulse current is 0.5 to 60 kHz.         The method as claimed in item 1, wherein a precipitation or floating step is performed after step (b).         The method of claim 1, wherein step (b) is divided into two stages, the first stage is fast mixing, and the second stage is slow mixing.         A defluorination system for flue gas desulfurization wastewater includes the following devices: (a) an electric coagulation device, and (b) a stirred mixing tank, wherein the electric coagulation device includes: one or more sets of paired anodes and cathodes, A reaction tank and a power supply providing high-frequency pulse current, wherein the anode and cathode are made of aluminum or iron, and the stirring mixing tank is equipped with an alkali adding member.         If the system of claim 5, further includes a sedimentation tank or a floating tank.         The system of claim 5, wherein the stirring mixing tank is equipped with a stirring speed controller to control the stirring speed.         The system of claim 5, wherein the agitating mixing tank is two tanks connected in series, the first tank is a fast mixing tank, and the second tank is a slow mixing tank.