KR20040100082A - Process for removing COD and heavy metals from FGD wastewater using electrolysis - Google Patents

Abstract

PURPOSE: To provide a process for removing COD and heavy metals from flue gas desulfurization (FGD) wastewater, which is capable of reducing steps for the process and maximizing a maintenance efficiency, by using an electrolysis. CONSTITUTION: The process comprises the steps of (a) precipitating floating solid materials in FGD wastewater to remove the floating solid materials therefrom; (b) electrolyzing the floating solid materials removed FGD wastewater to remove refractory COD component, heavy metals, fluorine, calcium and the like therefrom; (c) precipitating a scale component generated by the electrolysis to remove it as a slurry form; and (d) removing the floating solid material remained after the precipitating step.

Description

Process for removing COD and heavy metals from FGD wastewater using electrolysis

The present invention relates to a method for removing COD and heavy metals in desulfurized wastewater using electrolysis. More specifically, by using electrolysis, it is easy to treat COD and heavy metals in the wastewater to be treated, which can significantly reduce the amount of chemicals and is environmentally friendly, and is simpler than conventional processes. The present invention relates to a method for removing COD and heavy metals in desulfurized wastewater by using electrolysis, which has a relatively small required area.

In general, desulfurization wastewater is a wastewater generated during operation of a desulfurization plant of a power plant and contains a large amount of chemical oxygen demand (COD), heavy metals, and the like, and conventional techniques for removing the same include chemicals such as oxidizing agents and flocculants. There is a chemical treatment method used. However, in order to remove impurities in the desulfurized wastewater and to meet the discharge acceptance criteria, suspended solids precipitation, oxidant injection and mixing, inorganic COD oxidation, oxidant reduction, fluorine and heavy metal removal, calcium ion removal, suspended solids removal, organic It had to go through complicated processes such as COD removal, fluorine adsorption removal, and there is a big difference in the proper pH for each process, so that a large amount of caustic soda and hydrochloric acid are used to control this, which requires a high treatment cost.

In particular, the COD component in the desulfurized waste water is composed of a hardly decomposable inorganic COD formed by nitrogen oxides and sulfur oxides generated during combustion of coal and the like. Since the oxidant component has to go through a process of reducing again using a reducing agent, the process is complicated and maintenance costs are high.

Accordingly, the present invention aims to provide an effective treatment of desulfurized wastewater generated in a power plant, and to provide a method for treating desulfurized wastewater including precipitation, electrolysis, and fluorine removal processes.

In the present invention, in the treatment of desulfurized wastewater, the flotation sedimentation tank is complicated to maintain the complex processes such as flotation, COD oxidation, oxidant reduction, heavy metal and fluorine removal, calcium ion removal, suspended solids removal, organic COD removal, and operation cost. The aim is to provide a simple and economical method for treating desulfurized wastewater, which can be replaced by an electrolysis device, a scale settling tank and a float removal process.

1 is an overall schematic diagram of a desulfurization wastewater treatment process using electrolysis according to the present invention.

2 is a graph showing treatment results of COD and heavy metal components by a conventional desulfurization wastewater treatment method.

3 is a graph showing treatment results of COD and heavy metal components by the desulfurization wastewater treatment method of the present invention.

<Description of the symbols for the main parts of the drawings>

2 ---- sedimentation tank 4,6 --- electrolyzer

8 --- Sedimentation tank 10 --- Sand filter

COD and heavy metal removal method in the desulfurized wastewater using the electrolysis of the present invention is a suspended solids precipitation step of removing the suspended solids from the desulfurized wastewater by natural sedimentation method; An electrolysis step of treating and removing heavy metals, fluorine, calcium, etc. in addition to COD components that are hardly decomposable by electrolysis from the desulfurized wastewater from which the suspended solids are precipitated and removed; Removing the scale component produced by the electrolysis into a slurry form by precipitation; And it is characterized in that it comprises a step of further removing the remaining suspended matter not removed in the precipitation step.

Desulfurization waste water treatment method according to the present invention is the chloride ion (Cl ^-) present in the desulfurization waste water a concentration of about 20,000 ppm, electrolysis when OCl ^- in view of the Lim concentration sufficient to produce such oxide components, the conventional drug The oxidation method was replaced by the electrolysis method, and sufficient COD oxidation effect can be obtained without the input of sodium chloride, which is generally required for the electrolysis of waste water.

In addition, the formation of the electrode surface, which should be minimized in the electrolysis process, was promoted by adjusting the flow rate and the inter-pole distance, and then removed during the operation to improve the removal efficiency of heavy metals, fluorine, calcium, etc. in addition to the removal of COD.

When the electrolysis method is applied to the treatment of desulfurized wastewater, inorganic substances such as calcium and magnesium are precipitated on the surface of the electrode in the form of scale, and heavy metals and fluorine ions are formed by precipitation, which is included in the existing desulfurized wastewater treatment process. The effects of COD, heavy metals, calcium ions, fluorine removal process can be achieved at the same time, and can be particularly effective in the removal of hardly decomposable COD components. At this time, the scale component formed on the surface of the electrode is automatically removed by a method of changing or reversing the flow rate inside the electrolytic cell to remove the solids in the scale settling tank at the rear of the electrolysis device.

The present invention will be described in more detail based on the accompanying drawings as follows.

The apparatus for removing COD and heavy metals in the desulfurized wastewater using the electrolysis of the present invention, as illustrated in FIG. 1, is composed of a settling tank 2, an electrolytic tank 4, 6, a settling tank 8, and a sand filter 10. It is.

The desulfurized waste water is removed most of the suspended solids in the settling tank (2), the supernatant is passed to the electrolytic bath (4, 6). At this time, the sedimentation tank (2) is applied to the general natural sedimentation method, if the amount of suspended solids in the nature of the desulfurization waste water to increase the removal performance of the sedimentation tank (2) by injecting flocculant, flocculent aid, chelating agent at the front of the sedimentation tank. Can be.

Electrodes such as SUS316 or Ti are used for the cathodes of the electrolytic cells 4 and 6, and COD treatment efficiency can be improved when using insoluble electrodes (DSA), for example, Ti / RuO ₂ -IrO ₂ electrodes, for the anode. Electrodes such as SUS, Ti, Inconell, Zn, Fe, and Ni, which are not plated, can also be used depending on their properties. The quantity of electrodes used and the specifications of the electrodes can vary depending on the amount of wastewater to be treated.

When COD and heavy metals are removed from the desulfurized wastewater by the electrolysis device, a large amount of scale components are generated on the surface of the electrode. As the distance between poles increases, the formation and removal efficiency of the scale is accelerated. The removal efficiency of COD is hardly affected by the change of the flow rate at the flow rate of 25 cm / sec or less, it is affected by the contact time with the electrode, and the scale component formed on the electrode surface when the flow rate at the electrode surface is 5 cm / sec or more. This tends to be removed by the flow of fluid.

The present invention is a configuration in which at least one electrolytic cell is connected in series, and when the two electrolytic cell configurations are operated in parallel, the flow rate is lowered to be 5 cm / sec or less, for example, to promote COD removal and scale formation, and to promote heavy metals and fluorine. It is possible to increase the removal efficiency of the back. As the formation of scale on the electrode surface progresses and the voltage rises up to a certain level, it is operated in series again to increase the flow rate to 5 cm / sec or more, thereby removing only the scale component formed on the electrode surface while maintaining the COD removal efficiency. Can be.

At this time, the flow rate at which the scale is efficiently removed varies depending on the properties of the desulfurized wastewater, especially the amount of suspended matter and inorganic ions. The scale component formed on the surface of the electrode during the operation of the electrolysis device is slightly reduced compared to the parallel when operating in series, but the formation of the scale is continued and most heavy metals, fluorine, calcium ions and other removal effects are maintained. However, the range of high removal efficiency of the scale formed on the electrode surface is 5 cm / sec or higher and the COD has good removal efficiency at 25 cm / sec or less. Removal efficiency is best in the range of 5 to 25 cm / sec.

At this time, in the case of using an electrode which is not plated with SUS, Ti or the like depending on the properties of the desulfurized wastewater, the scale can be removed by a method of reversing the polarity of the electrodes. If an insoluble electrode (DSA) is used in the electrolysis device, the reversed method is not preferable because the plated film is peeled off when the polarity of the electrode is changed. In addition, in case of desulfurization facilities of power plants installed in Korea, manufacturers and methods are different from each other, and there are differences in the characteristics of desulfurization wastewater due to operating methods and used coal species, limestone, and the like. Differences in operating factors such as electrode materials

On the other hand, the narrower the distance between the electrodes, the lower the power consumption and the formation of scale is facilitated. There may be a slight change depending on the characteristics of the desulfurized wastewater, but in most cases, the distance between the poles should be selected as narrow as possible within the range of 3 to 15 mm.

The width and quantity of the electrodes in the electrolyzer (4,6) are designed according to the amount of wastewater.When the standard of the electrolyzer (4,6) is determined, the size of the electrolyzer (4,6) that meets the capacity to handle the COD concentration is determined. It is desirable to calculate the number and configure the system so that it can be divided into two and connected in series and in parallel. In this case, when the amount of wastewater to be treated is large, the units of the electrolyzers 4 and 6 may be configured in parallel to correspond to the throughput.

In addition, most power plants equipped with desulfurization facilities have a storage tank with sufficient capacity to store wastewater. Therefore, when the storage tank can be used, the discharge amount of the pump is not operated without alternating operation of the electrolysis apparatus configured in series and in parallel. By controlling the flow rate, the operation can be continued by adjusting the flow rate.

The COD removal rate in the desulfurized wastewater during the operation of the electrolysis device may be increased or decreased depending on the operating conditions of the desulfurization facility. On the other hand, if the amount of current applied to the electrolytic cell is adjusted, a desired COD treatment efficiency can be obtained. That is, the amount of current suitable for the treatment of the desulfurized wastewater is 0.5 to 1 kA / m ² , and the amount of ORP (oxidation reduction potential) or oxidizing agent is measured at the rear end of the influent COD and the electrolysis device to measure the amount of oxidant. By automatically adjusting the amount of current applied, the COD removal rate can be kept constant.

The wastewater that has undergone the electrolysis device contains a large amount of scale components formed during the electrolysis process. In the scale component, CaCO ₃ and Mg (OH) ₂ are the main components, and inorganic ions such as Ca and Mg are removed by precipitation, and heavy metals and fluorine ions present in the desulfurized waste water are co-precipitated and removed. This scale component is removed in the form of sludge in the settling tank 8 at the rear of the electrolysis device.

In the sand filter 10 to remove the remaining suspended matter not removed from the settling tank, they are finally discharged. At this time, the sand filter 10 may be replaced by a filter process capable of backwashing according to the site conditions.

In most cases, the concentrations of heavy metals and fluorine ions drop to the discharge limit in the electrolysis process.However, if the concentration of heavy metals and fluorine ions in the desulfurized wastewater is not high enough to treat the discharge limit, the flocculant may be added to the settling tank. By adding heavy metals and fluoride ions can be removed to the desired value. Even in this case, since most heavy metals and fluoride ions are removed during the electrolysis process, only a small amount of chemicals need to be used compared to the conventional process.

Figure 2 of the accompanying drawings shows the results of the treatment of contaminants such as COD and heavy metal components for each process by the conventional desulfurization wastewater treatment process. In FIG. 2, in the X-axis, symbol 1 is a suspended solids precipitation tank, symbol 2 is an oxidant injection and mixing tank, symbol 3 is an inorganic COD oxidation tank, symbol 4 is an oxidant component reducing tank, symbol 5 is a fluorine and heavy metal removal tank, symbol 6 Silver calcium ion removal tank, 7 is a sand filter, 8 is an activated carbon tower, 9 is a fluorine adsorption resin tower.

In the accompanying drawings, Figure 3 is a graph showing the treatment results of COD and heavy metal components of each process by the desulfurization wastewater treatment process of the present invention. In FIG. 3, the sign of the X axis is the same as the process number of FIG. 1. That is, reference numeral 2 denotes a sedimentation tank, numerals 4 and 6 denote electrolyzers, numeral 8 denotes a precipitation tank, and numeral 10 denotes a sand filter.

Table 1 shows the results of elemental analysis of scale precipitates by electrolysis of the present invention.

element Concentration (ppm) MgCaMnFeZnCrPb 257,55229,70346,5354,510940371196

As can be seen from Table 1, elemental analysis results of precipitates formed during electrolysis of desulfurized wastewater show that Ca, Mg and various heavy metals in desulfurized wastewater are effectively removed in the form of precipitation.

Desulfurization wastewater treatment method using the electrolysis of the present invention, in the treatment of desulfurization wastewater, to change the conventional chemical treatment method for the treatment of COD and heavy metals to the electrolysis method, the scale of the electrode surface when applying the electrolysis method By maximizing the formation, it is possible to reduce the chemical usage by continuously removing the scale, and it is possible to replace the fluorine and heavy metal removal process, calcium removal process and organic COD removal process that were separately formed in the existing process with one electrolysis process. .

According to the present invention, by applying the wastewater treatment method including electrolysis to the treatment of desulfurized wastewater, it is possible to reduce as many as nine processes to four processes depending on the situation, thereby maximizing the efficiency of operation and maintenance with economical effects. There is an advantage.

Claims (8)

A suspended solids precipitation step of removing the suspended solids from the desulfurized waste water by natural sedimentation method; An electrolysis step of treating and removing heavy metals, fluorine, calcium, etc. in addition to COD components that are hardly decomposable by electrolysis from the desulfurized wastewater from which the suspended solids are precipitated and removed; Removing the scale component produced by the electrolysis into a slurry form by precipitation; And Removing COD and heavy metals in desulfurized wastewater using electrolysis, characterized in that further comprises removing the remaining suspended matter not removed in the precipitation step. [Claim 2] The method for removing COD and heavy metals in desulfurized wastewater using electrolysis according to claim 1, wherein the flocculant, flocculent aid or chelating agent is added when the amount of the suspended solids is large in the precipitation step of the suspended solids. The method of claim 1, wherein the electrolysis step comprises two or more electrolyzers 4 and 6, and the inflow water is passed through two or more electrolyzers in series and in parallel by changing the flow path. Method for removing COD and heavy metals in desulfurized wastewater using electrolysis. The method according to claim 3, wherein the flow rate of the influent is adjusted to 5 cm / sec to 20 cm / sec when the electrolyzer is operated in series, and the flow rate of the influent is adjusted to 5 cm / sec or less when operated in parallel. Method for removing COD and heavy metals in desulfurized wastewater using electrolysis. The electrode of claim 3 or 4, wherein the anode uses an insoluble electrode such as a Ti / RuO 2 -IrO 2 electrode, the cathode uses stainless steel, and the spacing between the electrodes is narrow within a range of 3 to 15 mm. COD and heavy metal removal method in desulfurized wastewater using electrolysis, characterized in that by selecting the distance. The desulfurization using electrolysis according to claim 5, wherein the positive electrode or the negative electrode is selected from one of stainless steel, Ti, inconel, Fe, Zn, Ni, and carbon electrodes which are not plated. COD and heavy metal removal in waste water. The method of claim 1, wherein in the case of an electrode which is not plated to remove the scale component, COD and heavy metals are removed in the desulfurized wastewater using electrolysis, which is performed by inverting the polarities of the electrodes. Way. The method of claim 1, wherein the amount of current applied in the electrolysis step is 0.5 to 1 kA / m 2.