JPH0687945B2 - Exhaust gas treatment method - Google Patents

Description

Description: [Object of the invention] Industrial field of application The present invention relates to an exhaust gas treatment method for cleaning and removing reducing substances in exhaust gas with an absorbent containing hydrogen peroxide and ferrous sulfate. is there.

2. Description of the Related Art Generally, exhaust gas discharged from a casting process or the like contains malodorous components such as reducing substances such as phenol and formaldehyde. Therefore, it is necessary to remove such 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 both substances are washed and removed, both substances may be appropriately oxidized and decomposed so that the concentration of both substances in the absorbing 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 oxidative decomposition of both substances in the absorbing solution.

Conventionally, as a method for treating exhaust gas of this type, Japanese Patent Publication No. 51-44898
As disclosed in the publication, the pH value of the absorbing solution is 5
It is adjusted to the following, and the first solution for oxidative decomposition of phenol and formaldehyde is added to this absorbent.
Iron salt and hydrogen peroxide are added so that the concentration of ferrous iron is 1/7 or more of the hydrogen peroxide concentration, and the oxidation-reduction potential is (540-60 × pH value) mV or more before use. There was a way to do it.

Problems to be Solved by the Invention In the exhaust gas treatment, ferrous iron ions are required,
In fact, even in the prior art, ferrous sulfate is added.

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

Therefore, in order to keep the concentration of ferrous iron at an appropriate concentration, ferrous sulfate must be constantly added, and as a result, unnecessary iron must be removed from the system and discarded.

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

After all, in the prior art, "constant supply of ferrous sulfate", "disposal of unnecessary iron content" and "removal of iron sludge accumulated in the system" pose problems in terms of maintenance and economic efficiency.

The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to significantly reduce the supply amount of iron sulfate and the discharge amount of iron sludge, and to reduce the iron content in the system. An object of the present invention is to provide an exhaust gas treatment method capable of preventing the accumulation of sludge, facilitating the maintenance and management of the exhaust gas treatment system, and further improving the economical efficiency.

[Configuration of the Invention] Means for Solving the Problems The exhaust gas treatment method of the present invention is a method of washing and removing a reducing substance in exhaust gas with an absorption liquid containing hydrogen peroxide and ferrous sulfate. Part of the solution is drawn from the absorption system, and the pH and ferric iron concentration of the drawn solution are adjusted to ranges of 1 to 3 (preferably 1 to 2) and 10 mg-mol / mol (preferably 30 or more), respectively, followed by electrolysis. It is characterized in that ferric ions are reduced to ferrous ions in a tank and returned to the absorption system. When the reducing substance is phenol and / or formaldehyde, in the absorption system, , The pH of the absorbing solution, the hydrogen peroxide concentration and the ferrous ion concentration are respectively 1.5 to 5 (preferably 2 to 4),
0.3 mg-mol / min (preferably 0.6) and 0.02 mg-
The absorption liquid is used for cleaning the exhaust gas after being kept in the range of mol / mol or more (preferably 0.04 or more) for 2 minutes or more.

The regulation of the pH and ferric iron concentration of the electrolytic treatment solution should not be so different from the property of the absorbing solution due to the configuration of the exhaust gas treatment system, and the electrolytic treatment should be performed efficiently and iron oxide and iron hydroxide should not be contained. It remains undissolved and prevents it from being deposited as iron sludge in the system.

The reduction treatment amount of the ferrous iron is controlled by the current in the electrolytic cell.

Further, the properties such as pH value, hydrogen peroxide concentration, and ferrous ion concentration of the above-mentioned absorption liquid are the same as those of the first absorption liquid while maintaining the desired absorption performance.
It is a condition necessary to reduce the consumption of iron ions and hydrogen peroxide, and the residence time is the lower limit value required to maintain their concentrations.

Example Hereinafter, an exhaust gas treatment system for carrying out the method of the present invention will be described in detail.

Figure 1 shows the exhaust gas treatment system when the pH and ferric iron concentration of the absorbing solution are set to 2 and 20 mg-mol /, respectively, and 1 is an absorption tower, which is discharged from the casting process. The absorbent 16 is brought into contact with the untreated exhaust gas 8 to allow the absorbent 16 to absorb reducing substances such as phenol and formaldehyde in the exhaust gas.

2 is a circulation tank, which includes water 10, caustic soda 11 and sulfuric acid 12
Is adjusted and supplied, and the liquid amount and pH value of the absorbing liquid 16 are maintained at predetermined values.

Further, hydrogen peroxide 13 and iron sulfate replenishing iron sulfate 14 are also supplied to the circulation tank 2, and the hydrogen peroxide concentration is 0.6 mg-mol /,
In addition, with the ferrous ion concentration set to 0.04 mg-mol /,
The absorbent is allowed to stay for 3 minutes.

Reference numeral 3 is a first settling tank for separating the solid content, which is mainly insoluble and sedimentable, from the absorbent 16.

4 is a pH adjusting tank for precipitating iron in the liquid,
Next, the second settling tank 5 separates the precipitated iron.

Reference numeral 6 denotes a filtration tank, which is adapted to simultaneously perform solid-liquid separation of a solid content mainly composed of floating carbon particles and a solid content such as sand having a sedimentation property.

7 is an electrolytic cell.

Since the treatment system of the present embodiment is configured as described above, first, the untreated exhaust gas 8 is washed with the absorption liquid 16 in the absorption tower 1, and the phenol and formaldehyde therein are absorbed and separated into the absorption liquid 16. And is discharged as treated exhaust gas 9.

The absorption liquid 16 before exhaust gas cleaning is sent from the circulation tank 2 to the absorption tower 1 by a pump (not shown), and after absorbing the phenol and formaldehyde as described above, is returned to the circulation tank 2 as the absorption liquid 17. .

Part of the absorption liquid 16 is used as the first extraction liquid 18 in the first settling tank 3
And the solids mainly composed of sand (sludge 19) are separated. The supernatant liquid 20 is sent to the next pH adjusting tank 4 and becomes caustic soda 15 to become neutral or weakly alkaline, so that almost all of the iron content in the liquid is deposited as ferric hydroxide. Subsequently, iron (iron sludge 22) is settled and separated in the second settling tank 5, and sludge 19 such as sand from which the first settling tank 3 has been separated is added to the supernatant 23, and the next filtration is performed. In the tank 6, a solid content mainly composed of carbon particles and a solid content such as sand are simultaneously subjected to solid-liquid separation, and finally discharged out of the system as sludge 24 and discharged liquid 25.

On the other hand, another part of the absorbing liquid 16 is sent to the electrolytic cell 7 as the second extracting liquid 26, and the required amount of ferric iron ions in the liquid is the first amount.
Reduced to iron ions. The reduction treatment liquid 27 is returned to the circulation tank 2 together with the iron sludge 22 separated in the second settling tank 5.

The operating conditions and performance of the system shown in FIG. 1 are as follows.

(I) Exhaust gas (Ii) Absorption tower Type: Packing tower Packing: Flower type packing (trade name: Terralet) Packing height m: 2 Gas flow rate m / sec: 1.5 Liquid-gas ratio / m ³ N: 2 (iii) Absorbing liquid ( Circulation tank) pH: 2 Hydrogen peroxide concentration mg-mol /: 0.6 Ferrous ion concentration 〃: 0.04 Ferric iron concentration 〃: 20 Circulation tank residence time min: 3 (iv) Electrolyte treatment liquid Figure 2 shows the pH of the absorbing solution and the ferric iron concentration of 3.5.
And 5 mg-mol /, showing another embodiment of the system for carrying out the method of the present invention, which is an absorption tower 1, a circulation tank 2, a first settling tank 3, a pH adjusting tank 4, a second tank. The sedimentation tank 5, the filtration tank 6, and the electrolytic cell 7 have substantially the same functions as those of the embodiment shown in FIG.

Further, in the figure, 28 is an iron dissolution tank, in which sulfuric acid 31 is added to the iron / sand mixed sludge 30 separated in the first settling tank 3 to adjust the pH value to dissolve the iron, Use as a solution.

In the processing system configured as above, the absorption liquid
A part of 16 is sent to the pH adjusting tank 4 as the extraction liquid 29, and is adjusted to neutral or weakly alkaline by the caustic soda 15, so that almost all the dissolved iron components are precipitated.

Then, the iron content along with the settling solids such as sand is collected in the first settling tank 3
The iron / sand mixed sludge 30 was separated in
Sent to. In the iron dissolution tank 28, sulfuric acid 31 is added to the second iron-sand mixing sludge 30 in a state where the iron content in the sludge 30 is dissolved.
It is sent to the settling tank 5.

Therefore, in the second settling tank 5, the sand sludge 33 is settled and separated, and the obtained ferric sulfate solution 34 is sent to the electrolytic tank 7 and reduced to the required amount of ferrous ions. The reduction treatment liquid 27 is returned to the circulation tank 2.

On the other hand, the supernatant 20 of the first settling tank 3 is added with the sand sludge 33 which is the precipitate of the second settling tank 5 and sent to the filtration tank 6, where the solid content such as carbon particles and the sand. Solids such as solids are separated at the same time, and sludge 24 and discharged liquid 25
Is discharged outside the system.

The operating conditions and performance of the system shown in FIG. 2 are as follows.

(I) Exhaust gas (Ii) Absorption tower Type: Packing tower Packing: Flower type packing (trade name: Terralet) Packing height m: 2 Gas flow rate m / sec: 1.5 Liquid-gas ratio / m ³ N: 2 (iii) Absorbing liquid ( Circulation tank) pH: 3.5 Hydrogen peroxide concentration mg-mol /: 0.6 Ferrous ion concentration 〃: 0.04 Ferric iron concentration 〃: 5 Circulation tank residence time min: 3 (iv) Electrolyte treatment liquid [Advantages of the Invention] (1) Since the ferric iron ions in the drawn-out absorption liquid are reduced to the ferrous iron ions in the electrolytic cell and returned to the absorption system, the absorbing action of the reducing substance in the gas is improved. The necessary ferrous iron ions can be regenerated and reused, and in addition to compensating for the small amount of iron discharged out of the system inevitably during waste liquid treatment,
In principle, iron supply is not required.

Therefore, since unnecessary iron does not exist in the system, it is not necessary to forcibly remove and dispose of the iron, and there is no fear of being accumulated in the system as sludge, and maintenance is easy and economical.

(2) Since the pH and ferric iron concentration of the drawn-out absorption liquid entering the electrolytic cell are adjusted in advance to the ranges of 1 to 3 and 10 mg-mol / mol or more, respectively, ferric iron ions are efficiently converted to ferrous iron ions. In addition to being able to reduce the amount of iron, there is no risk that a large amount of iron will remain undissolved in the treatment liquid and accumulate as iron sludge in the system.

(3) When the reducing substance is phenol and / or formaldehyde, the pH of the absorbing solution in the absorption system, the hydrogen peroxide concentration and the ferrous ion concentration are each 1.5 to
Since it was kept at 5, 0.3 mg-mol / min and 0.02 mg-mol / min for 2 minutes or more, not only the absorbing solution can maintain the desired absorption performance, but also ferrous iron And the consumption of hydrogen peroxide can be reduced.

[Brief description of 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 showing another exhaust gas treatment system. 1 ... Absorption tower, 2 ... Circulation tank, 3 ... First settling tank, 4 ...
… PH adjusting tank, 5 …… Second settling tank, 6 …… Filtration tank, 7 ……
Electrolyzer, 8 ... Untreated exhaust gas, 9 ... Treated exhaust gas,
10 …… water, 11 …… caustic soda, 12 …… sulfuric acid, 13 …… hydrogen peroxide, 14 …… iron sulfate, 15 …… caustic soda, 16,17 ……
Absorbing liquid, 18 ...... first drawing liquid, 19 ... sand sludge, 20 ...
… Supernatant, 22 …… iron sludge, 23 …… supernatant, 24 …… sludge, 25 …… effluent, 26 …… second drawing liquid, 27 …… reduction treatment liquid, 28 …… iron dissolution tank , 29 …… Extraction liquid, 30 …… Iron / sand mixed sludge, 31 …… Sulfuric acid, 33 …… Sand sludge, 34
... ferric sulfate solution.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Azuma Higashi Aichi Prefecture, Toyota City, Toyota Town, 1st Toyota Motor Co., Ltd. (72) Inventor Kengo Ando, Aichi Prefecture, Toyota City, 1 Toyota Town, Toyota Motor Co., Ltd.

Claims (3)

[Claims] 1. A method of washing and removing a reducing substance in exhaust gas with an absorbing solution containing hydrogen peroxide and ferrous sulfate,
A part of the absorption liquid is extracted from the absorption system, and the pH and ferric iron concentration of the extraction liquid are adjusted to be 1 to 3 and 10 mg-mol / mol or more, respectively, and then ferric ion is first added to the ferric ion by an electrolytic cell.
An exhaust gas treatment method, which comprises reducing the iron ions and returning them to the absorption system. 2. The exhaust gas treatment method according to claim 1, wherein the reducing substance is phenol and / or formaldehyde. 3. In the above absorption system, the absorption liquid has a pH, a hydrogen peroxide concentration and a ferrous ion concentration of 1.5 to 5, respectively.
2 in the range of 0.3 mg-mol / min and 0.02 mg-mol / min
The exhaust gas treatment method according to claim 2, wherein the absorption liquid is used for cleaning the exhaust gas after being maintained for at least a minute.