JP2687222B2 - Exhaust gas treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention cleans and removes reducing substances such as phenol and formaldehyde in exhaust gas with an absorbent containing hydrogen peroxide and ferrous ions, The present invention relates to an exhaust gas treatment method that eliminates the need for the iron sludge treatment device.

[Prior Art] Generally, an exhaust gas discharged from a casting process or the like contains a malodorous component such as a reducing substance such as phenol or formaldehyde. Therefore, it is necessary to remove such malodorous components from the exhaust gas before releasing the exhaust gas into the atmosphere.

The reducing substances such as phenol and formaldehyde are water-soluble and have a low equilibrium partial pressure in an aqueous solution. Therefore, when these substances are washed and removed, the substances may be appropriately oxidatively decomposed so that the concentration of these substances in the absorption liquid does not become high.

It is known that the addition of hydrogen peroxide and ferrous ions (usually ferrous sulfate is used) is effective for oxidatively decomposing the substance in the absorbing solution.

As an exhaust gas treatment method of this kind, as disclosed in Japanese Patent Publication No. 51-44898, the absorption liquid is treated with pH ≦ 5 Fe ²⁺ / H ₂ O ₂ ≧ 1/7 redox potential ≧ (540-60 × pH value) mV was used.

Also in this exhaust gas treatment, ferrous iron is necessary, and in fact ferrous sulfate is added.

However, the ferrous ions become ferric ions in the process of oxidative decomposition of phenol or the like and the process of gas-liquid contact.

Therefore, in order to keep the concentration of ferrous iron at a predetermined level, it is necessary to constantly add a large amount of ferrous sulfate, and as a result, unnecessary iron is appropriately removed from the system for secondary treatment. There must be.

Furthermore, looking at the operation results of the practical device to which the above-mentioned conventional technique is applied, unnecessary iron components become sludge and are accumulated in the system.

After all, in the conventional technology, "continuously supplying a large amount of ferrous sulfate", "disposal and secondary treatment of unnecessary iron content" and "removal of iron sludge accumulated in the system" are points of maintenance and economical efficiency. Was a problem.

In order to solve this, the present inventor, as disclosed in JP-A-62-237926, sets the pH, hydrogen peroxide concentration and ferric iron concentration of the absorbing solution to 2 to 4 mg and 0.8 mg to respectively. mol
By maintaining the above / above and above 1 mg-mol / above in the circulation tank for 2 minutes or more, the regeneration of ferrous iron is promoted and the supply amount of ferrous ion is adjusted to the absorption liquid circulation flow rate of 1 m ³ / hr. By reducing the amount of ferrous ions to 30 mg-mol / hr or less per unit time, we have developed a technique to reduce the amount of ferrous ions and consequently reduce the amount of iron sludge and prevent the accumulation of iron sludge in the system.

[Problems to be Solved by the Invention] However, in the above-mentioned conventional method, since reduction of the amount of iron sludge is insufficient, in order to appropriately remove unnecessary iron from the system and to discard it, after the neutralization treatment, the secondary As a treatment, the produced iron sludge (ferric hydroxide) was introduced into a concentrating tank (thickener) 4 as shown in FIG. 2, and the supernatant liquid 18 was discharged to the final treatment plant and the concentrated liquid 19 was solid-liquid. There is a problem that it must be processed by a separating device (filter press) 5 or the like.

The present invention has been made to solve the above-mentioned conventional problems, and an object thereof is to further drastically reduce the supply amount of ferrous iron ions, resulting in a large iron sludge amount. To eliminate the need for secondary treatment equipment (solid-liquid separation equipment, etc.) and prevent the accumulation of iron sludge in the system.
An object of the present invention is to provide an exhaust gas treatment method capable of facilitating maintenance and management of the exhaust gas treatment system and further improving economic efficiency.

[Means and Actions for Solving the Problems] By the way, when phenol and the like are absorbed and removed, the consumption and regeneration of ferrous ions are considered as follows.

That is, ferrous ions become ferric ions in the process of oxidative decomposition of phenol or the like and the process of gas-liquid contact.

On the other hand, ferric ions return to ferrous ions by the following reaction.

C ₆ H ₄ (OH) ₂ + 2Fe ³⁺ → C ₆ H ₄ O ₂ ＋ 2Fe ²⁺ ＋ 2H ⁺ …… (1) Fe ³⁺ ＋ H ₂ O ₂ → Fe ²⁺ ＋ HO ₂ ＋ H ⁺ …… (2) However, C ₆ H ₄ (OH) ₂ represents hydroquinone or catechol, both of which are decomposition products of phenol.

Among the above reactions, the reaction (2) is slow, so it is considered that only the reaction (1) contributes to the regeneration of ferrous ions.

As a result of earnest research, the present inventor has discovered that the reaction of the above (1) can be promoted by setting the state of the absorbing solution as follows.

Furthermore, the concentration of ferrous ion is
It has been found that 0.02 mg-mol / or more is sufficient for oxidative decomposition of phenol and the like, and it has been possible to make the supply amount of ferrous ions much smaller than before.

 As a result, the waste liquid secondary treatment device is no longer required.

That is, in the present invention, the pH of the absorbing solution, the hydrogen peroxide concentration and the ferric iron concentration are respectively set to 2 to 4, 0.5 mg-mol / or more and 0.5 to 0.9 mg-mol / for 2 minutes in the circulation tank. By maintaining the above, it promotes the regeneration of ferrous ions,
The supply amount of ferrous ion is 0.9 per 1 m ³ / hr of absorption liquid circulation flow rate.
The range is to 3.0 mg-mol / hr. As a result, the amount of iron sludge in the waste liquid is greatly reduced, and a secondary treatment device such as a solid-liquid separation device is unnecessary.

[Examples] Hereinafter, an exhaust gas treatment system for carrying out the method of the present invention will be described in detail with reference to the drawings.

Fig. 1 shows the amount of ferrous iron supplied as the absorption liquid circulation flow rate of 1 m ³ /
1 shows an exhaust gas treatment system in the case of setting it in a range of 0.9 to 3.0 mg-mol / hr per hr, 1 is an absorption tower, for example, an untreated exhaust gas 7 discharged from a casting process is treated with an absorption liquid 8 By bringing them into contact with each other, reducing substances such as phenol and formaldehyde in the exhaust gas are absorbed in the absorbing liquid 8.

2 is a circulation tank, water 9, caustic soda 10 and sulfuric acid
11 is supplied while being adjusted, and the amount of absorption liquid and the pH value are maintained at predetermined values.

Further, in the circulation tank 2, hydrogen peroxide 12 and sulfuric acid first
Iron 13 is also supplied.

Since the treatment system of the present embodiment is configured as described above, first, the untreated exhaust gas 7 is washed with the absorption liquid 8 in the absorption tower 1, and the reducing substances such as phenol and formaldehyde therein are removed. It is absorbed and decomposed by the absorbing liquid 8 and discharged as the treated exhaust gas 14.

The absorption liquid 8 before cleaning the exhaust gas is sent from the circulation tank 2 to the absorption tower 1 by a pump (not shown), and after absorbing the reducing substances such as phenol and formaldehyde as described above, the absorption liquid 15 is obtained. It is returned to the circulation tank 2.

Further, a part of the absorption liquid 8 is separated from the absorption system as the extraction liquid 16 and enters the neutralization tank 3, where caustic soda 10 or sulfuric acid is added.
It is neutralized at 11.

The neutralized absorption liquid 17 is directly sent to the terminal treatment plant and treated together with other wastewater.

Examples of operating conditions and performance of the above system are as follows.

Example 1 (i) Exhaust gas pre-treatment and post-treatment flow rate m ³ / hr 90,000 Temperature ℃ 30 Phenol concentration ppm 0.2 0.1 Formaldehyde concentration ppm 0.21〃 Ammonia concentration ppm 5〃 Dust concentration mg / m ³ N 1- (ii) Absorption tower Type: Packing tower Packing: Flower packing (trade name: Terralet) Packing height m: 2.0 Gas flow rate m / sec: 2.5 Liquid gas ratio / m ³ : 2 (iii) Absorbing liquid (circulating tank) pH: 3 Hydrogen peroxide concentration mg-mol /: 1 Ferrous iron ion concentration 〃: 0.02 Ferric iron concentration 〃: 0.5 Circulation tank residence time min: 3 (iv) Ferrous sulfate absorption liquid circulation rate 1m ³ / hr Per mg-mol / hr: 1.4 Total supply amount g-mol / hr: 0.25 (v) Effluent Effluent flow rate m ³ / hr: 0.5 (vi) Effluent SS concentration after neutralization mg /: 82 Example 2 (i) flow amount after the exhaust gas treatment pretreatment m ³ / hr 90,000 temperature ° C. 30 phenol concentration ppm 0.11 0.1 less than the formaldehyde concentration ppm 0.15 〃 ammonia concentration ppm 5 〃 dust concentrated ^{mg / m 3 N 1 - (} ii) the absorption column format: packed column packing: Flower-shaped packing (trade name: Tellerette) fill height m: 2.0 Gas flow rate m / sec: 2.5 solution gas ratio / m ^3: 2 (iii) Absorption liquid (circulation tank) pH: 3 Hydrogen peroxide concentration mg-mol /: 1.8 Ferrous iron ion concentration 〃: 0.04 Ferric iron concentration 〃: 0.7 Circulation tank residence time min: 3 (iv) Sulfuric acid No. 1 Iron absorption liquid circulation rate 1 m ³ / hr mg-mol / hr: 2.0 Total supply amount g-mol / hr: 0.36 (v) Effluent flow rate m ³ / hr: 0.5 (vi) Emission after neutralization Liquid SS concentration mg /: 104 [Effect of the invention] pH, hydrogen peroxide concentration and ferric iron concentration in the absorption liquid
2 to 4, 0.5 mg-mol / min and 0.5 to 0.9 mg, respectively
Since the absorption liquid is used for cleaning the exhaust gas after being kept in the circulation tank in the range of −mol / for 2 minutes or more, as a result, the supply amount of ferrous sulfate can be extremely reduced, and the iron in the waste liquid can be reduced. The amount of sludge can be greatly reduced, and a secondary treatment device such as a solid-liquid separation device can be eliminated.

Further, there is no fear that iron content will be accumulated in the system as sludge, and maintenance is easy and economical.

[Brief description of the drawings]

FIG. 1 is a block diagram of an exhaust gas treatment system for carrying out the method of the present invention, and FIG. 2 is a block diagram of a conventional exhaust gas treatment system. 1 ... Absorption tower, 2 ... Circulation tank, 3 ... Neutralization tank, 4 ... Concentration tank, 5 ... Solid-liquid separation device (filter press), 6 ...
… Hopper, 7… Untreated exhaust gas, 8… Absorption liquid, 9…
… Water, 10 …… Caustic soda, 11 …… Sulfuric acid, 12 …… Hydrogen peroxide, 13 …… Ferrous sulfate, 14 …… Treated exhaust gas, 15 ……
Absorption liquid after absorption treatment, 16 ... Extraction liquid, 17 ... Absorption liquid after neutralization, 18 ... Supernatant liquid, 19 ... Concentrated liquid, 20 ... Residue, 21
... filtrate. Reference numerals 4, 5 and 6 denote secondary processing devices.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Mitsuhiko Hamai 1 Toyota-cho, Toyota-shi, Aichi Toyota Motor Co., Ltd. (56) Reference JP-A-62-237926 (JP, A)

Claims (1)

(57) [Claims] 1. A method of washing and removing a reducing substance containing phenol in exhaust gas with an absorption liquid containing hydrogen peroxide and ferrous ions, the pH, hydrogen peroxide concentration and ferric iron concentration of the absorption liquid being measured. , 2 to 4, 0.5 mg-mol / l or more and 0.5 to 0.9 mg-mol / l in the circulation tank for 2 minutes or more, respectively, and then the absorbent is used for cleaning exhaust gas. Exhaust gas treatment method.