JPH0796133A - Treatment of waste gas containing gaseous chlorine and device therefor - Google Patents

Abstract

PURPOSE:To neutralize a hypochlorite at high pH and to eliminate the need for measuring pH and oxidation-reduction potential by allowing a part of a liq. absorbent contg. a salt formed by the reaction with a waste gas to react with sodium thiosulfate in an alkali side. CONSTITUTION:A waste gas is introduced into an absorption tower 1, and a liq. absorbent is supplied to the tower 1 by an absorbent circulating pump 2 and a contaminated absorbent transport pump 3. An aq. sodium hydroxide soln. kept at about pH10-12 is used as the absorbent, and the waste gas is sufficiently mixed with the absorbent in the tower 1. Sodium chloride formed by the absorption reaction and the absorbent contaminated with sodium hypochlorite are drawn off at all times in a fixed amt. from the bottom of the tower 1 by the pump 3 and sent to a reducing and detoxicating tank 6 together with the sodium thiosulfite which is mixed in a fixed amt. at all times by a reducing agent delivery pump 5. The contaminated absorbent and sodium thiosulfite are sufficiently mixed in the tank still at high pH and detoxicated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to treatment of exhaust gas containing chlorine gas, and more particularly to a treatment method and treatment apparatus for removing pollutant from exhaust gas containing pollutant containing chlorine gas in the form of gas and / or steam. In particular, it can be used for purification of chlorine gas-containing exhaust gas after removing dust in exhaust gas in the production of optical fiber preforms, etc., and is reduced in the final treatment step by the treatment system such as pH adjustment of pollutant absorption liquid. This method is suitable when using a chemical that affects the processing performance.

[0002]

2. Description of the Related Art Conventionally, exhaust gas containing chlorine gas after dust removal
As a processing method of, for example, Japanese Examined Patent Publication No. 3-62442
There is a method described in the report. The outline is as an absorbent
By using an alkaline aqueous solution such as NaOH aqueous solution,
As shown in (1) of chemical formula 1 below, chlorine gas does not react with the absorbing liquid.
To the bottom of the reaction vessel as NaCl and NaOCl.
The absorption liquid contaminated with NaCl and NaOCl
And SiO in exhaust gas ₂And GeO₂Is deposited in the absorbent
To keep the pH of the absorption liquid high so that it does not come out
In addition, while measuring the pH with a pH measuring device, alkaline water
Add the solution. The polluted absorption liquid extracted is the detoxification circulation system.
And added sodium bisulfite as a reducing agent
Then change NaOCl to NaCl as shown in (2) of the following chemical formula 1.
Give back. ClO in liquid^-Ions are shown in (3) of Chemical formula 1 below.
Due to the reversible reaction shown, the existence ratio changes and the pH is high.
The reaction proceeds to the right. Therefore, the neutralization process in the unreduced state
If you do, the pH will decrease and the reaction of (3) will proceed to the left.
For Cl₂Gas will be regenerated. Servant
Therefore, the treatment in the detoxification circulation system is Cl₂To prevent gas re-generation
It is necessary to carry out in an alkaline state. Specifically, detoxification
Salt generated during processing by measuring pH of liquid flowing in the circulation system
Make sure that hydrofluoric acid and sulfuric acid are immediately neutralized, that is,
Control the addition of NaOH so that the pH does not drop below 7
To do. Also, the addition of sodium sulfite flows through the detoxification circulation system.
Continuously measured the redox potential of the liquid to be measured.
Until the free chlorine ion does not exist.
After the reduction process is completed, it is sent to a treatment system such as pH adjustment and is used as wastewater.
After being adjusted to a specified pH, it is drained.

[Chemical 1]

[0003]

According to the prior art,
In the detoxification circulation system of the absorbing solution, the pH and the redox potential of the reducing agent are measured as described above, and sodium bisulfite as a reducing agent is added corresponding to the measured values. The measurement of the redox potential is affected by the salt concentration and pH in the liquid, and the correlation with the chlorite concentration in the liquid changes, so very complicated control is required, and the pH and redox potential are also required. The daily maintenance work to maintain the measurement accuracy of was very important. Further, as described above, the treatment in the detoxification circulation system is C
Although it is carried out in an alkaline state in order to prevent the re-generation of l ₂ gas, sodium sulfite as a reducing agent is not easily oxidized in a state where the pH is high. Therefore, when it is added excessively, it remains and chemicals used in the treatment system such as pH adjustment, for example, Fe (OH) ₃ added for separation / removal of the solid content in the liquid in the solid-liquid separation tank. Is reduced, which hinders the processing performance of the subsequent processing device. Further, when the input amount is insufficient, hypochlorite remains in the liquid and is regenerated as chlorine gas during the neutralization process, so it was very difficult to maintain a proper treatment state. As described above, it has been difficult and time-consuming to maintain a stable reduction detoxification treatment state by the conventional management by liquid quality measurement.

[0004]

As means for solving the above-mentioned problems, the present invention is to pass exhaust gas containing a pollutant containing chlorine gas through a circulation system of an absorption solution consisting of an alkaline aqueous solution to obtain the absorption solution. And a part of the absorption liquid containing the salt generated by the chemical reaction with the exhaust gas, and the extracted liquid is subjected to a chemical detoxification treatment with a reducing agent in the detoxification system, and then after the chemical detoxification treatment. Purify the exhaust gas by introducing the liquid into a treatment system such as pH adjustment to obtain treated wastewater, while replenishing the amount of the absorbing liquid which is reduced by the withdrawal with the unpolluted absorbing liquid in the circulating system of the absorbing liquid. In the treatment method described above, sodium thiosulfate is used as the reducing agent. In the present invention, the sodium thiosulfate is supplied in an amount equal to or more than the amount sufficient to reduce the maximum amount of hypochlorite that can be generated by the reaction of chlorine gas and an alkaline aqueous solution, and the liquid after chemical detoxification treatment is It is particularly desirable to prevent the reduction of the chemicals used in the pH adjusting treatment system by oxidizing excess sodium thiosulfate before introducing it into the pH adjusting treatment system. Further, the treatment apparatus of the present invention is an absorption treatment system having an absorption tower in which an exhaust gas containing a pollutant containing chlorine gas is brought into contact with an absorption liquid composed of an alkaline aqueous solution, and a part of the contaminated absorption liquid is drawn out from the lower part of the absorption tower and reduced. A reduction detoxification tank connected to a pipe for transferring to the detoxification tank and having a supply and mixing means of sodium thiosulfate, and oxidizing residual sodium thiosulfate in the reduction detoxification-treated liquid provided in the downstream of the reduction detoxification tank. Oxidation treatment tank, pH communicating with the oxidation treatment tank
It comprises a processing system for adjustment and the like, and a means for replenishing the absorption processing system with an uncontaminated absorption liquid corresponding to the amount of extraction liquid. In the apparatus of the present invention, the withdrawal of the contaminated absorbent and the replenishment of the uncontaminated absorbent with a metering supply pump are particularly preferred embodiments.

[0005]

The present inventors have found that if sodium thiosulfate is added in the detoxification treatment system, the pH of the extracted liquid is high.
In view of the fact that it becomes possible to neutralize the hypochlorite in the state and that the excessively supplied sodium thiosulfate is subjected to an oxidation treatment by bubbling before being introduced into the pH adjusting system, the present invention has been reached. . When the treatment is carried out by the method of the present invention, the reducing agent is quantitatively supplied in an amount not less than the amount capable of treating the maximum amount of hypochlorite which can theoretically be generated from the amount of raw materials used in the manufacturing process. The situation of shortage is prevented. Moreover, since the pH of the drawing liquid may remain high, it is not necessary to measure the pH and the oxidation-reduction potential as in the case of conventional sodium bisulfite. Therefore, a complicated control system and a pH / oxidation-reduction potential measuring device that requires daily maintenance work are not required, and a stable exclusion processing state can be maintained with little maintenance of the device. Also, sodium thiosulfate can be oxidized even in a high pH state, so if the excess is oxidized by bubbling, etc.
It is possible to prevent the chemicals used in the conditioning system from being reduced and impairing its performance.

[0006]

EXAMPLES The present invention will now be described in detail with reference to examples, but the present invention is not limited thereto. EXAMPLE The present invention will be described in detail with reference to FIG. 1 showing one specific example of the present invention. The exhaust gas containing chlorine gas is introduced into the absorption tower 1, and the absorption liquid is supplied to the absorption tower 1 by the absorption liquid circulation pump 2 and the alkaline aqueous solution supply metering pump 3. Here, an aqueous solution of sodium hydroxide (NaOH) having a pH value of at least 10 to 12 is used as an absorbing liquid for the treatment of removing the pollutants of the chlorine gas tower in the exhaust gas. The exhaust gas to be purified is well mixed with the absorption liquid in the absorption tower 1 by contact with the absorption liquid sprayed by the spray nozzle 15 in the absorption tower 1 and the absorption tower packing 16 moistened with the absorption liquid. As a result, the chemical absorption reaction of the formula (1) shown in Chemical formula 2 occurs.

[Chemical formula 2] Cl ₂ + NaOH → NaCl + NaOCl + H ₂ O ··· (1) As a result of the absorption reaction of (1) above, it is contaminated with NaCl (ordinary salt) and NaClO (sodium hypochlorite). Sodium thiosulfate (Na ₂ S ₂ O _{3) is} used to constantly quantitatively extract the absorbed liquid from the lower part of the absorption tower 1 by the contaminated absorbing liquid transfer pump 3 and constantly quantitatively mix it with the reducing agent supply constant amount pump 5 in the transfer pipe. ) Together with the reduction detoxification tank 6. In the tank 6, a contaminated absorption solution and sodium thiosulfate are thoroughly mixed by a reduction detoxification tank stirrer, and detoxification treatment is performed by the chemical reaction of the formula (4) shown in Chemical formula 3.

Embedded image 4NaClO + Na ₂ S ₂ O ₃ + 2NaOH → 2Na ₂ SO ₄ + 4NaCl + H ₂ O ··· (4) As is clear from the chemical reaction of (3) above, Cl ₂ (chlorine gas) 1 1 mol of NaC generated from that per mol
Since 10 is reduced with 1/4 mol of Na ₂ S ₂ O _3, it is generated by constantly supplying a fixed amount of 1/4 mol or more of sodium thiosulfate, which is the maximum chlorine gas amount that can be loaded in the absorption tower 1. Sodium chlorite is completely detoxified without remaining. Further, when the amount of Cl ₂ generated and the amount used are small, the excess sodium thiosulfate is bubbled with air supplied from the air supply pump 9 in the oxidation treatment tank 8 to carry out the chemical reaction of the formula (5) shown in Chemical formula 4. Oxidation treatment by.

[Chemical formula 4] Na ₂ S ₂ O ₃ + 2O ₂ + 2NaOH → 2Na ₂ SO ₄ + H ₂ O (5) Here, the detoxification-treated contaminated absorbent is in a high pH alkaline state, but Unlike sodium bisulfite that has been conventionally used, sodium sulfate can carry out the above-mentioned oxidation reaction even in an alkaline state. After the above-mentioned detoxification treatment and oxidation treatment of the excess reducing agent are completed, the detoxified absorption liquid drawing pump 11 operated by the water level control by the oxidation treatment tank water level gauge 10 is transferred to a treatment system such as pH adjustment.

As a concrete example, a case will be described in which the exhaust gas from an exhaust gas source having a chlorine gas supply device with a maximum supply capacity of 22.4 liters / min is treated. Chlorine gas is 1
Since it is mol / min, the NaOH required for the treatment has the following chemical formula (1)

[Formula 5] Cl ₂ + NaOH → NaCl + NaOCl + H ₂ O (2) / min from (1). Therefore, when 5% NaOH is supplied by the absorption liquid supply metering pump 4 in FIG. 1, the amount of absorption liquid required is 2 mol / min × 40 (NaOH mass number) /0.05/1.06 (5% NaOH
Specific gravity) = about 1.5 liters / min. Further, the salt generated at this time is 1 mol of NaCl according to the chemical formula (1), and the salt concentration in the absorbing solution is 1 mol / min × 58.5 (NaCl mass number) /1.5 liter / min.
(Liquid volume) = approx. 39 g / liter. Solubility of NaCl is 35.7g (0 ℃) / 100g H ₂ O
Therefore, by drawing out an amount equal to or more than the supply amount of the absorbing liquid as the contaminated absorbing liquid, it is not deposited in the contaminated absorbing liquid.
In this example, when the contaminated absorbent is continuously withdrawn by the contaminated absorbent transfer pump 3 at a rate of 1.6 liters / min, no salt is deposited in the contaminated absorbent, and the amount of absorbed solution decreased by this withdrawal. The absorbent replenishing pump 17 is used to replenish 0.1 liter / min. As a result, the total amount of the absorption liquid supplied to the absorption tower 1 is equal to or more than the amount required for the treatment of all chlorine gas, so that it is possible to always maintain an alkaline state having a high pH. Since NaOCl in the contaminated absorption liquid transferred to the reduction / detoxification tank 6 is 1 mol / min like NaCl, the amount of Na ₂ S ₂ O ₃ necessary for reducing this is represented by the following chemical formula (4).

[Chemical Formula 6] 4NaClO + Na ₂ S ₂ O ₃ + 2NaOH → 2Na ₂ SO ₄ + 4NaCl + H ₂ O ・ ・ (4) gives 1/4 mol / min × 113 (Na ₂ S ₂ O ₃ mass number) = About 28.3 g / mi
n. Further, in consideration of the case where the total amount of Na ₂ S ₂ O _{3 to be} supplied becomes excessive, the oxygen amount required for the oxidation treatment in the oxidation treatment tank 8 is represented by the following chemical formula (5)

Embedded image Since Na ₂ S ₂ O ₃ + 2O ₂ + 2NaOH → 2Na ₂ SO ₄ + H ₂ O (5) is twice the molar amount of Na ₂ S ₂ O ₃ , 28.3 g / min / 113 × 2 × 22.4 liter / mol = 11.2 liter / min. Since the oxygen concentration in the air is about 20%,
The required amount of air supplied by the air supply pump 9 used for bubbling is 11.2 liters / min / 0.2 = 56 liters / min. Therefore, 28.3 g / min of Na ₂ was added to the reduction detoxification tank 6.
By supplying a constant amount of S ₂ O ₃ with the reducing agent supply constant amount pump 5 and supplying air of 56 liters / min or more to the oxidation treatment tank 8 with the air supply pump 9, all of ClO is reduced, and the surplus is increased. All reducing agents can be oxidized.

As described above, when the treatment is carried out by the method and apparatus of the present invention, the reducing agent is supplied by quantitatively supplying the maximum amount of hypochlorite which can be generated or more. , It is possible to sufficiently prevent the situation of insufficient input, and since sodium thiosulfate can be oxidized even in a high pH state, it is possible to oxidize all the excess amount by an oxidation treatment such as bubbling, It is possible to sufficiently prevent the chemicals used in the treatment system such as pH adjustment from being reduced and impairing the detoxification performance. Further, at the same time, since it is possible to operate the device without using a complicated control system and a measurement device for pH and redox potential that require daily maintenance work, the device can be operated with almost no maintenance, It is possible to maintain a stable detoxification treatment state.

[Brief description of drawings]

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

[Explanation of symbols]

1 absorption tower, 2 absorption tower circulation pump, 3 contaminated absorption liquid transfer pump, 4 absorption liquid supply metering pump, 5
Reductant supply metering pump, 6 reduction detoxification tank, 7
Reduction detoxification tank agitator, 8 oxidation treatment tank, 9 air supply pump, 10 oxidation treatment tank water level meter, 11
Detoxified absorption liquid extraction pump, 12 absorption tower blower,
13 NaOH storage tank, 15 Na ₂ S ₂ O ₃ storage tank, 16 absorption tower packing, 17 absorption liquid supply pump, 18 absorption tower circulation tank water level meter.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location // G02B 6/00 356 A 7036-2K

Claims (5)

[Claims] 1. An absorption liquid containing a salt produced by a chemical reaction between an exhaust gas containing a pollutant containing chlorine gas and an absorption liquid made of an alkaline aqueous solution to cause a chemical reaction between the absorption liquid and the exhaust gas. A part of the solution is introduced into the detoxification system and subjected to a chemical detoxification treatment to reduce the salt, and then the solution after the chemical detoxification treatment is introduced into a treatment system such as pH adjustment for treatment. In the treatment method of purifying the exhaust gas by replenishing the amount of the absorption liquid, which is reduced by the extraction in the circulation system of the absorption liquid, with the uncontaminated absorption liquid, the chemical detoxification process is extracted. A method for treating an exhaust gas containing chlorine gas, which comprises reducing the salt by reacting the obtained solution with sodium thiosulfate in an alkaline state. 2. The sodium thiosulfate is supplied in an amount equal to or more than a sufficient amount to reduce the maximum amount of hypochlorite that can be generated by the reaction of chlorine gas and an alkaline aqueous solution, and the liquid after chemical detoxification treatment is used. The method for treating an exhaust gas containing chlorine gas according to claim 1, wherein the sodium thiosulfate remaining in the liquid is oxidized by being brought into contact with the atmosphere and then introduced into a treatment system such as pH adjustment. 3. The processing method according to claim 1, wherein the chlorine gas-containing exhaust gas is exhaust gas from an optical fiber manufacturing process. 4. An absorption treatment system having an absorption tower for contacting an exhaust gas containing a pollutant containing chlorine gas with an absorption solution composed of an alkaline aqueous solution, and extracting a part of the contaminated absorption solution from the lower part of the absorption tower to a reduction detoxification tank. A reduction detoxification tank which is connected to a pipe for transfer and has sodium thiosulfate supply and mixing means, and an oxidation treatment tank which is provided downstream of the reduction detoxification tank and oxidizes the residual sodium thiosulfate in the reduction detoxification-treated liquid An apparatus for treating an exhaust gas containing chlorine gas, comprising: a treatment system communicating with the oxidation treatment tank such as pH adjustment; and a means for replenishing the absorption treatment system with an uncontaminated absorption liquid corresponding to the amount of drawn liquid. 5. The chlorine gas-containing exhaust gas treatment apparatus according to claim 4, wherein the extraction of the contaminated absorption liquid and the replenishment of the uncontaminated absorption liquid are performed by a constant amount supply pump.