JP3297062B2 - Method for treating wastewater from flue gas desulfurization equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a flue gas desulfurization method in which sulfur dioxide in exhaust gas is fixed and separated as gypsum. The waste water after removing gypsum is concentrated by an electrodialysis method and an evaporator, and the concentrated waste water is mixed with cement or cement and coal ash. And a method of kneading, solidifying, detoxifying and disposing of the mixture.

[0002]

2. Description of the Related Art As a method for removing sulfurous acid gas from exhaust gas, a method of fixing and separating gypsum using limestone or slaked lime slurry is generally employed, but a large amount of waste liquid remains after gypsum is separated. Although it is circulated for saving the cost of wastewater treatment for the reasons described below, chlorine ions are accumulated in the waste liquid. On the other hand, considering the materials used in flue gas desulfurization equipment, the chlorine ion concentration in
It is necessary that the concentration be below 000 ppm, and waste liquid is extracted up to this value.

[0003] However, this waste liquid contains, in addition to calcium chloride, substances which are subject to pollution control such as magnesium chloride, iron chloride, fluorine and chemical oxygen demanding substances (hereinafter referred to as COD substances). In order to avoid secondary pollution, the following wastewater treatment measures have been required, which has required a large amount of cost.

A conventional wastewater treatment facility is composed of many devices such as an acid decomposition tank for treating COD substances, a solids separation tank for separating solids, and a hydrogen ion concentration adjusting tank for adjusting hydrogen ion concentration. Therefore, there are problems that the equipment cost is high, the equipment area is large, and the operation control method is complicated.

[0005]

SUMMARY OF THE INVENTION The present invention is to solve the above-mentioned disadvantages of the prior art.

[0006]

SUMMARY OF THE INVENTION According to the present invention, gypsum is removed in a flue gas desulfurization method in which an exhaust gas containing a sulfurous acid gas is brought into contact with a slurry of limestone or slaked lime to absorb and remove the sulfurous acid gas and fix and separate it as gypsum. The resulting solution is filtered by a filtration machine, and the filtrate is sent to an electrodialysis tank equipped with a cation exchange membrane and a monovalent anion exchange membrane alternately. A method for treating wastewater from a flue gas desulfurization apparatus, comprising: evaporating and concentrating a concentrated liquid with cement or a mixture of cement and coal ash;

More specifically, the present invention is described in detail. The waste gas generated in a flue gas desulfurization method in which an exhaust gas containing a sulfurous acid gas is brought into contact with a slurry of limestone or slaked lime to absorb and remove the sulfurous acid gas and fixedly separated as gypsum is separated. After removing the solid content concentration of fine gypsum etc. to about 0.2 mg / l by filtration with a filter, dilute sulfuric acid solution is anodized in an electrodialysis tank equipped with a cation exchange membrane and a monovalent anion exchange membrane alternately. Solution and dilute hydrochloric acid solution as catholyte for electrodialysis, while removing sulfate ions (chemical formula: SO ₄ ²⁻ , the same applies hereinafter), concentrating the chloride ion concentration to, for example, about 10%, and attaching the scale using an evaporator. The amount of liquid is reduced by concentrating the chloride ion concentration to, for example, about 25% while preventing this, and this liquid is finally kneaded and solidified with cement or a mixture of cement and coal ash to be disposed of. To.

On the other hand, the dilute water containing sulfate ions generated by the electrodialysis method is circulated to a desulfurizer. Therefore,
The flue gas desulfurization apparatus using the present invention does not generate secondary wastewater.

Various types of filters can be used in the present invention, and it is necessary to remove solids to a solid concentration of about 0.2 mg / l at the request of the electrodialyzer in the next step. For this reason, equipment capable of microfiltration is generally used.

The electrodialysis apparatus used in the present invention comprises a concentrated liquid chamber, a dilute liquid chamber, and a positive and negative electrode plates at both ends of the concentrated liquid chamber and the dilute liquid chamber, which are formed by alternately arranging cation exchange membranes and monovalent anion exchange membranes. The cation exchange membrane selectively permeates cations such as calcium and magnesium, and the monovalent anion exchange membrane selectively transmits monovalent anions such as chlorine, while simultaneously dissolving a large amount of dissolved salts. From the concentrated solution and the diluted solution in which the dissolved salt is diluted.

FIG. 2 is a schematic diagram showing the structure of the electrodialysis apparatus and the movement of ions. As shown in FIG. 2, calcium ions (CaCl ₂ ) and magnesium ions (MgCl ₂ ), which are the main components in the wastewater, are electrically converted by the electric field formed between the positive electrode and the cathode plate. Electrophoresis is performed, during which time it is selectively blocked or permeated by the positive and anion exchange membranes, resulting in a concentrated solution and a dilute solution, which are taken out of the electrodialysis apparatus. For example, cations such as calcium and magnesium migrate toward the cathode plate, pass through the cation exchange membrane, and enter the concentrate chamber.
Further, it is attempted to migrate toward the cathode plate, but is blocked by the monovalent anion exchange membrane and remains in the concentrated solution chamber. On the other hand, chloride ions migrate toward the anode plate, permeate through the monovalent anion exchange membrane, enter the concentrate chamber, and then migrate toward the anode.
It is blocked by the cation exchange membrane and remains in the concentrate chamber, combines with cations migrated from the opposite direction, is carried by the concentrate as the moving medium, and is taken out of the electrodialysis apparatus.

The reason why the monovalent anion selective membrane is used as the anion exchange membrane is to prevent scaling due to gypsum precipitation in the evaporation step for further concentrating the concentrated wastewater from the electrodialysis apparatus. This is because performance degradation, mechanical damage occurs, and long-term stability of the evaporation process is lost.

Although any evaporator can be used in the present invention, it is preferable to use a reduced-pressure evaporator, a thin-film evaporator or the like.

The kneader for kneading the concentrated liquid and cement or cement and coal ash used in the present invention is used for solidifying fly ash of a conventional municipal solid waste incinerator such as a screw feeder type or a vibrating type. It is possible to use an equivalent device.

An example of the mixing ratio of the concentrate and the mixture of the cement or the cement and the coal ash is as follows. When only the cement is used, the concentrate: cement = 2: 1, and when the cement and the coal ash are used, the concentrate is : Cement: Coal ash = 3:
1: 2 is more appropriate in terms of operating and equipment costs,
This mixing ratio varies depending on the required properties such as the priority of the operating cost and the equipment cost, and the hardness of the kneaded material to be produced. Further, the kneaded material from the kneader is optionally shaped and cured (including drying) and then disposed.

[0016]

According to the present invention, a gypsum separation filtrate is separated into a diluent having a low chloride ion concentration and a concentrate having a high chloride ion concentration and a low sulfate ion concentration, and the former is used as water for preparing a desulfurizing agent slurry. By doing so, it is possible to eliminate the adverse effects caused by the accumulation of chlorine ions in the desulfurization system, and to prevent the formation of gypsum scale in the evaporator when the liquid discharged out of the system is concentrated in the evaporator. it can.

[0017]

EXAMPLE In order to demonstrate the effectiveness of the treatment of waste water according to the present invention, an experiment was conducted with an apparatus having the structure shown in FIG. FIG.
In the figure, 1 is a tank for temporarily storing waste water from a flue gas desulfurization unit (not shown), 2 is a precoat filter for removing turbid components in the waste water, 3 is an electrodialysis unit for performing primary concentration of the waste water, and 4 is a primary filter. An evaporator for reconcentrating the concentrated wastewater, a condenser 5 for condensing the vapor generated in the evaporator 4, and a solidifier 6 for kneading the wastewater secondary concentrated in the evaporator 4 with cement or cement and coal ash to solidify. Numeral 7 denotes a powder feeder for mixing cement or cement and coal ash for solidifying waste water and thereafter supplying a fixed amount to the solidifier 6. Reference numeral 8 denotes a pump for uniformly supplying waste water from the tank 1 to the precoat filter 2.

In FIG. 1, the salt concentration from the flue gas desulfurization unit 2
500 kg / hour of [wt% as chlorine] drainage was received in the tank 1 via the line a. Table 1 shows the composition of the main components of the wastewater.

[Table 1]

The drainage of the tank 1 is 500 k
g / h. Diatomaceous earth was used as the filter medium of the precoat filter 2, and the turbid water concentration in the wastewater at the outlet of the precoat filter 2, that is, at the line c, was 0.2 mg / l. The precoat filter 2 was washed for a predetermined time after the operation because the turbid components accumulated over time and the filtration resistance increased.

The waste water from the precoat filter 2 is sent to an electrodialyzer 3 to be separated into a concentrated solution having a high salt concentration (hereinafter, referred to as a primary concentrated solution) and a dilute solution having a low salt concentration. The primary concentrated liquid was discharged to the evaporator 4 via, while the diluent was discharged to the outside of the system. Table 2 shows the specifications and operating conditions of the electrodialysis device 3 at this time.

The salt concentration and the flow rate in the primary concentrate and the diluent in this example are as shown in Table 3. The primary concentrate was 10 wt% as chlorine, and the diluent was 1.13 wt. wt% as chlorine] and the flow rate is 48 kg /
Hour, 452 kg / hour.

[Table 2]

[Table 3]

The primary concentrated liquid from the electrodialyzer 3 is supplied to the evaporator 4 through the line d, and the salt concentration is 25 [w] in the evaporator.
t% as chlorine] (this concentrated solution is hereinafter referred to as a secondary concentrated solution). In this embodiment, a thin-film evaporator was used as the evaporator 4, the inner pressure of the evaporator was controlled at -660 mm (column of mercury), and the temperature of drainage in the can was controlled at 85 ° C.

The steam generated in the evaporator 4 is supplied to the condenser 5 through a line f, where it is cooled and condensed by cooling water supplied from a line g. The cooling water was discharged out of the system via line h, and the condensed water was discharged through line i. 19.2 kg / h of the secondary concentrated liquid from the evaporator 4 was supplied to the solidifier 6 via the line j.

The cement and coal ash required for the solidifying device 6 are sent from the line 1 and the line k, respectively, to the powder supply device 7 in a fixed amount, and after the cement and coal ash are mixed into the powder supply device 7, It was supplied to the solidifier 6 via the metering line m.
The amounts of cement and coal ash supplied from lines l and k were 6.4 kg / hour and 12.8 kg / hour. In the solidifier 6, the secondary concentrated wastewater, cement, and coal ash are kneaded to form a kneaded material. The kneaded material was discharged out of the system via line n.

The cement used in this example was a commercially available ordinary Portland cement, and the coal ash was produced from a currently operating domestic coal-fired power plant (using domestic coal).

The kneaded material was dried with hot air at 107 ° C. for 60 minutes to substantially reduce the contained water content to 30% by weight. As a result of performing a dissolution test according to the method prescribed by the Prime Minister's Office, it was confirmed that the dissolution standard value was satisfied.

The concentration flow rates in each line of this embodiment described above are summarized in Table 3 above.

In this example, continuous operation was performed for 350 hours, operation was possible without any trouble on the apparatus, and the effectiveness of the present invention could be proved.

[0029]

According to the present invention, in a flue gas desulfurization method in which an exhaust gas containing a sulfur dioxide gas is brought into contact with a slurry of limestone powder or slaked lime to absorb and remove the sulfur dioxide gas and fixedly separated as gypsum, the waste water after removing the gypsum is removed. There is no scale adhesion due to the precipitation of gypsum when concentrating, by kneading and solidifying the concentrated liquid with cement or a mixture of cement and coal ash, it has an excellent effect as it is detoxified and no drainage, Furthermore, there are advantages that the number of devices is small, the operation is extremely simple, almost automatic operation is possible, and the frequency of maintenance is much less than that of conventional equipment.

Further, the accumulation of chloride ion concentration in the wastewater from which gypsum has been removed can be reduced, and corrosion of the apparatus can be prevented.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an embodiment of the present invention.

FIG. 2 is an explanatory view of the principle of the electrodialysis apparatus used in the present invention.

Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI C02F 1/04 C02F 9/00 502Z 1/44 11/00 101Z 1/469 B01D 53/34 125R 9/00 502 125E 11/00 101 C02F 1 / 46 103 (72) Inventor Hiroshi Yoshida 1 at 20 Kitakanyama, Odaka-cho, Midori-ku, Nagoya-city, Aichi Prefecture Inside the Electric Power Research Laboratory, Chubu Electric Power Co., Inc. (72) Inventor Mamoru Toriyao, Midori-ku, Nagoya-shi, Aichi Prefecture 20 in Kita-Sekiyama, Takamachi-shi Chubu Electric Power Co., Inc. (72) Inventor Hiroyo Tsutsui 2-5-1 Marunouchi, Chiyoda-ku, Tokyo Mitsubishi Heavy Industries, Ltd. (72) Inventor Shinichiro Kotake Mitsubishi Heavy Industries, Ltd. (5-1) Marunouchi 2-chome, Chiyoda-ku, Tokyo (72) Inventor Naohiko Ugawa 4--22 Kannon Shinmachi, Nishi-ku, Hiroshima-shi, Hiroshima Mitsubishi Heavy Industries, Ltd. Hiroshima Research Laboratory (72) Inventor Susumu Okino Mitsubishi Heavy Industries, Ltd. 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima-shi, Hiroshima Inside the Shima R & D Center (72) Inventor Shizuka Nakamura 4-2-2 Kannon Shinmachi, Nishi-ku, Hiroshima-shi, Hiroshima Mitsubishi Heavy Industries, Ltd.Hiroshima R & D Center (72) Inventor Shiro Fukui 2- 25-14 Kameido, Koto-ku, Tokyo (72) Inventor Norikane Uemachi 2-25-14 Kameido, Koto-ku, Tokyo Asahi Glass Co., Ltd. Chemical Engineering Department (72) Inventor Ryosuke Aoki 10 Goi Kaigan, Ichihara, Chiba Prefecture Asahi Glass Co., Ltd. (56) References JP-A-60-34788 (JP, A) JP-A-3-8411 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01D 53 / 50 B01D 53/77 B01D 61/44 500 B01D 61/44 520 B01D 61/58 C02F 1/04 C02F 1/44 C02F 1/469 C02F 9/00 502 C02F 11/00 101

Claims (1)

(57) [Claims] In a flue gas desulfurization method in which an exhaust gas containing a sulfurous acid gas is brought into contact with a slurry of limestone or slaked lime to absorb and remove the sulfurous acid gas and fixedly separated as gypsum, the liquid after removing the gypsum is filtered by a filter. After filtration, the filtrate is sent to an electrodialysis tank equipped with a cation exchange membrane and a monovalent anion exchange membrane alternately, and the chloride ions in the solution are dialyzed and concentrated. A wastewater from a flue gas desulfurization apparatus, wherein the wastewater is discharged after being kneaded, solidified and rendered harmless with cement or a mixture of cement and coal ash.