JPH06246130A - Treatment of incinerator waste gas - Google Patents

Abstract

PURPOSE:To effectively absorb gaseous SO2 by making the difference in the concentration of magnesium hydroxide in a liquid absorbent between a process where hydrogen chloride is absorbed and removed and a process where gaseous SO2 is absorbed and removed. CONSTITUTION:After waste gas from an incinerator 9 is cooled in a quenching tower 1, it is brought into contact with a liquid absorbent contg. magnesium hydroxide in a hydrogen chloride absorber 2 to absorb and remove almost all hydrogen chloride. The absorbent at that time is adjusted so that liquid leaving the absorber 2 may have pH 3-5.5 by adding magnesium hydroxide to the supernatant in a supernatant tank 7 in mid-way of a suction line. Waste gas from which almost all hydrogen chloride has been removed is fed into a gaseous SO2 absorber 3, where when the absorbent adjusted so that liquid leaving the absorber 3 may have pH 5-7 by adding magnesium hydroxide to the supernatant in the supernatant tank 7 is sprayed to the top of the absorber 3, gaseous SO2 is absorbed and removed. Then the liquid is fed to a sedimentation tank 6, where smoke dust is settled.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for treating waste gas from a waste incinerator.

[0002]

2. Description of the Related Art Hydrogen chloride contained in incinerator waste gas generated when incinerating municipal waste or industrial waste,
Cleaning treatment with sulfurous acid gas is required.
Conventionally, sodium hydroxide was used when detoxifying waste gas containing such hydrogen chloride and sulfurous acid gas, but inexpensive magnesium hydroxide is used because sodium hydroxide is expensive. Methods are being considered. However, if magnesium hydroxide is used for cleaning the waste gas containing hydrogen chloride at a high concentration, the sulfurous acid gas cannot be effectively removed. For this reason, a method has been proposed in which magnesium hydroxide and sodium hydroxide are used in combination, firstly hydrogen chloride is mainly removed with magnesium hydroxide, and then sulfurous acid gas is removed with sodium hydroxide (Japanese Patent Laid-Open Publication No. S60-187242). 63-242322).

[0003]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention As described above, the treatment of waste gas containing hydrogen chloride at a high concentration as an absorbent cannot be performed efficiently and economically with magnesium hydroxide alone. The current situation is to use sodium oxide together. However, a waste gas treatment method using only magnesium hydroxide as an absorbent without using expensive sodium hydroxide is desired.

[0004]

Means for Solving the Problems As a result of earnest studies on the treatment of waste gas using magnesium hydroxide as an absorbent, the present inventors have found that a process of absorbing and removing mainly hydrogen chloride and a process of absorbing and removing mainly sulfurous acid gas are performed. Thus, the present invention has been accomplished in which the concentration of magnesium hydroxide in the absorbing solution is varied and the absorbing solutions having different pH are used. That is, according to the present invention, when incinerator waste gas containing soot dust and acidic substances is treated with an absorption liquid containing magnesium hydroxide to absorb and remove hydrogen chloride and sulfur dioxide, the pH of the outlet liquid in the hydrogen chloride absorption tower is 3%. ~
A step of contacting with an absorbing solution whose inlet concentration of magnesium hydroxide is adjusted to 5.5 so as to mainly absorb and remove hydrogen chloride, and then the pH of the outlet solution in the sulfurous acid gas column is 5
A step of contacting with an absorbing solution whose magnesium hydroxide inlet concentration is adjusted to be ~ 7 to absorb and remove mainly sulfurous acid gas, and a step of mixing and leaving both absorbing solutions after gas contact to settle and separate soot dust , And the supernatant liquid from which soot and dust are separated is circulated to each absorption step, and in each step, magnesium hydroxide is added so that the predetermined inlet concentration is obtained and the concentration is adjusted to treat the incinerator waste gas. Is the way.

Hydrogen chloride and sulfurous acid gas contained in the waste gas are absorbed and removed by reacting as shown in the following equation when brought into contact with a liquid containing magnesium hydroxide. Mg (OH) ₂ + 2HCl → MgCl ₂ +
2H ₂ O Mg (OH) ₂ + SO ₂ + 1 / 2O ₂ → M
gSO ₄ + H ₂ O However, the incinerator waste gas such as industrial waste is usually 50 to 800 ppm of sulfurous acid gas, 400 to 4000 pp of hydrogen chloride.
In addition to m, it contains a large amount of dust. When such waste gas is brought into contact with the magnesium hydroxide slurry, most of the hydrogen chloride is absorbed and removed, but the sulfurous acid gas is hardly absorbed. This is because the solubility of magnesium hydroxide is extremely small and the incinerator waste gas contains a large amount of hydrogen chloride as compared with sulfurous acid gas. For this reason,
In the present invention, the step of mainly absorbing hydrogen chloride and the step of mainly absorbing sulfurous acid gas are separated, and the concentration of magnesium hydroxide is changed in each step of the circulating absorption liquid, whereby the absorption of sulfurous acid gas is efficiently absorbed and removed. To do.

The present invention will be described in detail below with reference to FIG. In FIG. 1, 1 is a quenching tower, 2 is a hydrogen chloride absorption tower,
Reference numeral 3 is a sulfur dioxide absorption tower, an absorption liquid discharge pipe is connected to the lower part of each tower, and an absorption liquid introduction pipe is connected to the upper part of the tower. Waste gas from the incinerator 4 is sprayed from the spray nozzle in each tower. It is sequentially contacted with the falling liquid (absorption liquid) and purified, and then discharged as a clean gas through a dust collector. The waste gas from the incinerator 4 is cooled in the quenching tower 1 to 100 ° C. or less, preferably about 80 ° C. At this time, it is preferable to form a wetting wall of a liquid containing magnesium hydroxide in the quenching tower. At this time, a part of hydrogen chloride in the waste gas is absorbed, and this absorption liquid is sent to the mixing tank 5. The waste gas discharged from the quenching tower comes into contact with the absorbing solution containing magnesium hydroxide in the hydrogen chloride absorption tower 2 so that almost all of hydrogen chloride is absorbed and removed. The absorption liquid sprayed to the hydrogen chloride absorption tower 2 is obtained by adding magnesium hydroxide to the supernatant liquid of the supernatant liquid tank 7 in the suction line so that the outlet liquid pH of the tower 2 is 3
Adjust to be ~ 5.5. This concentration adjustment is performed by continuously detecting the pH of the outlet liquid of the tower 2 and adding magnesium hydroxide so that the pH of the outlet liquid of the tower is within the above range while following the fluctuation of the concentration of hydrogen chloride in the waste gas. To do. The magnesium hydroxide concentration of the liquid supplied to the tower 2 varies depending on the concentration of the acid component in the waste gas, the gas / liquid ratio in the absorption tower, etc. is there. The outlet liquid of the tower 2 is also sent to the mixing tank 5.

When the amount of liquid discharged in the quenching tower is large, magnesium hydroxide is replenished to the liquid in the supernatant liquid tank 7 in the same manner as the hydrogen chloride absorption tower supply liquid in order to detect the pH of the liquid discharged. The prepared liquid can also be used as cooling water. The waste gas from which hydrogen chloride has been substantially removed is sent to the sulfurous acid gas absorption tower 3, and here also magnesium hydroxide is added to the supernatant of the supernatant tank 7 in the suction line to adjust the pH of the tower outlet liquid to 5-7. The absorption liquid prepared to have the following concentration is sprayed from the top of the tower,
Sulfurous acid gas is absorbed and removed. The magnesium hydroxide concentration of the absorption liquid supplied to the tower 3 is determined by continuously detecting the pH of the outlet liquid of the tower 3, as in the case of the tower 2. The outlet liquid of the tower 3 is also sent to the mixing tank 5.

In order to sufficiently absorb and remove the sulfurous acid gas in the tower 3, the pH of the outlet liquid of the tower 3 must be 5 to 7, and when the pH of the outlet liquid of the tower 2 exceeds 5.5, it is not possible. The reaction magnesium hydroxide content increases, and the sedimentation loss in the sedimentation tank 6 increases. In the mixing tank 5, the unreacted acid component from each tower and the effluent containing magnesium hydroxide having different pHs are mixed and reacted, and sent to the stationary settling tank 6 where they are allowed to stand to precipitate the soot dust. The supernatant of the stationary settling tank is sent to the supernatant tank 7, and the sediment accumulated at the bottom of the tank is taken out in the form of slurry and sent to the wastewater treatment step for treatment. The mixing tank and the stationary settling tank may have a structure in which one tank is partitioned by a partition plate having a communication hole. In the supernatant liquid tank 7, water extracted in the sedimentation tank and water in an amount corresponding to the amount of evaporated water are replenished, and magnesium hydroxide is circulated in each absorption tower while magnesium hydroxide is added in each suction line.

In the present invention, when the supernatant liquid of the supernatant liquid tank 7 is circulated in the absorption tower as the absorption liquid, the concentration of the liquid supply to the hydrochloric acid absorption tower 2 is adjusted by adjusting the pH of the supernatant liquid and the tower 2. While detecting the pH of the outlet liquid, magnesium hydroxide in an amount such that the pH of the outlet liquid becomes 3 to 5.5 (Mg (OH) ₂ 0.1 to 1.5 g / l) is added. The concentration of the liquid supplied to the sulfur dioxide absorption tower is adjusted while the pH of the sulfur dioxide absorption tower outlet liquid is being detected while the tower outlet liquid pH is 5 to 7, preferably 6 to 6.
The amount of magnesium hydroxide (Mg (OH) ₂ 0.02
˜0.5 g / l) is added to the above supernatant. When the pH of the solution at the outlet of the sulfurous acid gas absorption tower becomes 5 or less, the sulfurous acid gas cannot be sufficiently absorbed and removed. When the pH of the absorbing solution is adjusted as described above by dividing it into a step of mainly absorbing hydrogen chloride and a step of mainly absorbing sulfur dioxide, hydrogen chloride is adjusted to 30 ppm or less even if the absorbent is magnesium hydroxide alone. Sulfurous acid gas can be removed to 5 ppm or less. The method of the present invention is used for incinerator treatment of industrial waste, general waste, and the like.

[0010]

EXAMPLE Waste gas generated by incineration of waste mainly containing vinyl chloride resin was treated using an apparatus as shown in FIG. Waste gas cooled in the quench tower 1 (hydrogen chloride: 2000-
2400 ppm, sulfurous acid gas: 70 to 90 ppm, containing 6 g / Nm ^{3 of} dust) 40000 to 50000 Nm ³ / H were sequentially introduced into the absorption towers 2 and 3. In the hydrochloric acid absorption tower 2, 300m of liquid (pH 9.3, Mg (OH) ₂ 0.06%) in which magnesium hydroxide was added to the supernatant of the sedimentation tank was used.
³ / H sprayed. In the sulfurous acid gas absorption tower 3, a liquid obtained by adding magnesium hydroxide to the supernatant of the sedimentation tank (pH 9.0, Mg
(OH) ₂ 0.005%) was sprayed at 300 m ³ / H. The pH of the sulfur dioxide gas absorption tower outlet liquid was controlled at 6.0 to 6.2.
The pH of the hydrogen chloride absorption tower outlet liquid was 4.2 to 4.5. The hydrogen chloride content in the gas after contact with the absorbing liquid was 20 pp.
m, sulfurous acid gas was 4 ppm.

The outlet liquids of the quenching tower 1, the hydrogen chloride absorption tower 2 and the sulfurous acid gas absorption tower 3 were collected in a mixing tank 5 and reacted with stirring to be sent to a stationary settling tank 6. The residence time in the stationary settling tank 6 was about 15 minutes. This was stored in the supernatant liquid tank 7. The pH of this supernatant was 5.5 to 5.8. Hydrogen chloride absorption tower 2
The concentration of magnesium hydroxide added to the supernatant liquid circulated in the above and that added to the supernatant liquid circulated in the sulfurous acid gas absorption tower 3 was 150 g / l, respectively. The amounts of magnesium hydroxide added to the supernatant were 157 kg / H and 12 kg / H, respectively. The magnesium hydroxide was added in a slurry of about 15% by weight.

[0012]

As described above, the waste gas containing sulfurous acid gas and a large amount of hydrogen chloride is divided into the step of mainly absorbing hydrogen chloride and the step of mainly absorbing sulfurous acid gas, and the liquidity of the absorption liquid is increased. According to the method of the present invention for individually adjusting the above, the waste gas can be sufficiently cleaned by using magnesium hydroxide alone as the absorbent. Further, by doing so, there is little loss of unreacted magnesium hydroxide, and this is an industrially advantageous waste gas treatment method.

[Brief description of drawings]

FIG. 1 is a flow sheet showing the steps of the present invention.

[Explanation of symbols]

 1 Quenching tower 2 Hydrogen chloride absorption tower 3 Sulfurous acid gas absorption tower 4 Incinerator 5 Mixing tank 6 Stationary sedimentation tank 7 Supernatant tank

Claims (1)

[Claims] 1. When the incinerator waste gas containing soot dust and acidic substances is treated with an absorbent containing magnesium hydroxide to absorb and remove hydrogen chloride and sulfur dioxide, the pH of the outlet liquid in the hydrogen chloride absorption tower is 3%. A step of contacting with an absorbing solution whose inlet concentration of magnesium hydroxide is adjusted to be about 5.5 to absorb and remove mainly hydrogen chloride, and then a pH of the mixture after contacting with a sulfurous acid gas absorption tower is 5 to 7 In order to absorb and remove mainly sulfurous acid gas by contacting it with an absorbing solution whose magnesium hydroxide inlet concentration is adjusted, after mixing the two absorbing solutions after gas contact, settling and separating soot and dust, and separating soot and dust The clarified supernatant liquid is circulated in each absorption step, and in each step, magnesium hydroxide is added so that the predetermined inlet concentration is obtained, and the concentration is adjusted. Incinerator waste gas treatment method.