JPH0687946B2 - Waste liquid treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] Industrial field of application The present invention relates to phenol, formaldehyde,
The present invention relates to a method for treating waste liquid in a system for cleaning and removing sand, carbon particles and the like with an absorption liquid containing hydrogen peroxide and iron sulfate, taking into consideration the conditions of the absorption liquid and the electrolytic treatment liquid.

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 level, ferrous sulfate must be constantly added, and as a result, unnecessary iron is appropriately removed from the system and discarded, so Part of it was withdrawn from the absorption system, neutralized with alkali, and then iron was separated together with sand by filtration, etc. and discharged as sludge outside the system, part of the filtrate was returned to the system, and the other was discharged. .

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" are 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 a waste liquid treatment method capable of preventing sludge accumulation, facilitating maintenance of an exhaust gas treatment system, and further improving economic efficiency.

[Structure of the Invention] Means for Solving Problems The waste liquid treatment method of the present invention is a system for cleaning and removing phenol, formaldehyde, sand, carbon particles and the like in exhaust gas with an absorbing liquid containing hydrogen peroxide and iron sulfate. ,
A step of extracting a part of the absorption liquid from the absorption system, reducing ferric ions of the extraction liquid to ferrous ions by an electrolytic cell and returning the absorption system to the absorption system; Is 1
In the range of 3, the settling and separating of the settling solids containing sand as the main component as sand sludge, the step of settling and separating the iron in the range of pH 7 and above, and the settling solids such as sand and iron are included. The sand sludge generated in the sedimentation separation step is added to a liquid having a pH of 7 or more, and then the sedimentation solids and the floating solids containing carbon particles as the main component are simultaneously separated by filtration to remove the filtrate and the residue. Each of them is characterized in that it is composed of a process of discharging the product out of the system.

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

In addition, the pH is adjusted to the range of 1 to 3 so that the sedimentable solid component containing sand as the main component is sedimented and separated in order to prevent the iron component from becoming undissolved and sedimenting with sand. Is.

In addition, the sedimentation and separation of iron in the pH range of 7 and above is
This is because iron is finally recovered from the waste liquid discharged to the outside of the system and returned to the system, and the waste liquid and sludge discharged to the outside of the system are kept neutral.

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

Figure 1 shows the exhaust gas treatment system when the pH of the absorption liquid and the ferric iron concentration are maintained at 2 and 20 mg-mol / min, respectively, and 1 is an absorption tower, which is discharged from the casting process. The absorbing liquid 16 is brought into contact with the untreated exhaust gas 8 so that the absorbing liquid 16 absorbs 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.

Reference numeral 4 denotes an adjusting tank which is adjusted to be weakly alkaline in order to precipitate the iron content in the liquid, and the subsequent second settling tank 5 separates the precipitated iron content.

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.

Numeral 7 is an electrolyzer, and in order to economically obtain the necessary amount of ferrous iron, the pH of the withdrawn absorbing solution 16 and the ferric iron concentration are adjusted in advance to 1 to 3 and 10 mg-mol / mol or more. It When the absorbing liquid 16 already has these properties, it is directly subjected to electrolytic treatment. If the pH is high, add sulfuric acid. If the ferric iron concentration is low, after adding caustic soda, the iron content is separated by sedimentation and separation, and then the iron content is dissolved with a small amount of sulfuric acid to form a concentrated solution of ferric iron.

Since the treatment system of this embodiment is configured as described above, first, the untreated exhaust gas 8 is absorbed into the absorption liquid 16 in the absorption tower 1.
And the phenol and formaldehyde therein are absorbed and separated by the absorption liquid 16 and discharged as the 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 by adding caustic soda 15, so that almost all of the iron content in the liquid is deposited as iron hydroxide. Subsequently, the iron content (iron sludge 22) is settled and separated in the second settling tank 5, and the alkaline supernatant liquid 23 contains the first
The sludge 19 such as acidic sand separated in the settling tank 3 is added, and in the next filtration tank 6, the solid content mainly composed of carbon particles and the solid content such as sand are simultaneously separated by filtration, and the sludge 24 and the discharged liquid are discharged. It is finally discharged outside the system as 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) Absorption liquid (circulation tank) pH [...]: 2 Hydrogen peroxide concentration [mg-mol /]: 0.6 Ferrous iron ion concentration [mg-mol /]: 0.04 Ferric iron concentration [mg-mol /] ]: 20 Circulation tank residence time [min]: 3 (ii) Electrolyte tank treated liquid flow rate [m ³ / hr]: 1 pH [‥‥]: 2 Ferrous ion concentration [mg-mol /]: 7th 2 Iron concentration [mg-mol /]: 13 (iii) First settling tank treated liquid (supernatant 20) Flow rate [m ³ / hr]: 2 SS [g /]: 0.1 or less (iv) Second settling Tank treated liquid (supernatant 23) Flow rate [m ³ / hr]: 1.5 SS [g /] 0.1 or less (v) Filtration tank treated liquid (effluent 25) Flow rate [m ^{3 /} hr]: 1.5 SS [ g /]: 0.03 or less Fig. 2 shows the pH of the absorbing solution and the ferric iron concentration of 3.5, respectively.
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, It is supposed to be 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, adjusted to be weakly alkaline by the caustic soda 15, and almost all of the dissolved iron content is deposited.

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 alkaline supernatant 20 in the first settling tank 3 is added with the acidic sand sludge 33 which is the precipitate in the second settling tank 5 and sent to the filtration tank 6, where carbon particles and the like are removed. The solid content and the solid content such as sand are simultaneously solid-liquid separated, and discharged as sludge 24 and discharged liquid 25 out of the system.

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

(I) Absorbing liquid (circulation tank) pH [...]: 3.5 Hydrogen peroxide concentration [mg-mol /]: 0.6 Ferrous iron ion concentration [mg-mol /]: 0.04 Ferric iron concentration [mg-mol /] ]: 5 Circulation tank residence time [min]: 3 (ii) Electrolyte tank treated solution (reduction treatment solution 27) Flow rate [m ^{3 /} hr]: 0.5 pH [...]: 2 Ferrous ion concentration [mg- mol /]: 14 Ferric iron concentration [mg-mol /]: 16 (iii) First settling tank treated liquid (supernatant 20) Flow rate [m ³ / hr]: 2.5 SS [g /]: 0.1 or less (Iv) Iron dissolution tank treated liquid flow rate [m ^{3 /} hr]: 0.5 Ferric iron concentration [mg-mol /]: 30 (v) Second settling tank treated liquid (ferric sulfate solution 34) Flow rate [ m ³ / hr]: 0.5 SS [g /]: 0.1 or less (vi) Filter tank treated liquid (effluent 25) Flow rate [m ³ / hr]: 2.5 SS [g /]: 0.03 or less [Effect of invention] (1) Since the ferric ions in the extracted absorption liquid are reduced to ferrous ions in the electrolytic cell and returned to the absorption system, it is necessary for absorbing the reducing substances in the exhaust gas. A first can of iron ions to play for recycling, except to compensate for slight iron which unavoidably is discharged out of the system upon waste 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, there is no fear of being accumulated in the system as sludge, and maintenance is easy and economical.

(2) Part of the absorbing solution was adjusted to a pH range of 1 to 3 and then sedimentable solids containing sand as the main component were separated by sedimentation. Can be prevented from being discharged.

(3) Since the iron content is precipitated and separated in the range of pH 7 or higher, the iron content can be recovered in advance from the waste liquid finally discharged out of the system and returned to the system.

(4) Sedimentary solids such as sand sludge and floating solids containing carbon particles as the main component are separated by filtration at the same time and discharged to the outside of the system. An inexpensive processing device can be obtained.

[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 (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical display location C02F 9/00 Z 7446-4D (72) Inventor Arai Azuma 1 Toyota-cho, Toyota City, Aichi Prefecture Toyota Auto Car Co., Ltd. (72) Inventor Kengo Ando 1 Toyota Town, Toyota City, Aichi Toyota Motor Co., Ltd.

Claims (3)

[Claims] 1. A system for cleaning and removing phenol, formaldehyde, sand, carbon particles and the like in exhaust gas with an absorption liquid containing hydrogen peroxide and iron sulfate, wherein a part of the absorption liquid is extracted from the absorption system, A step of reducing ferric ions to ferrous ions in an electrolytic cell and returning them to the absorption system, and a part of the absorbing solution, which has a sedimentation property mainly composed of sand in a pH range of 1 to 3 It does not contain sedimentable solids such as sand, iron, etc., the process of sedimenting and separating solids as sand sludge, the process of sedimenting and separating iron in the pH range of 7 or more.
The sand sludge generated in the sedimentation separation step is added to a liquid having a pH range of 7 or more, and then the sedimentation solids and the floating solids containing carbon particles as the main component are simultaneously separated by filtration to remove the filtrate and the residue, respectively. A waste liquid treatment method comprising the step of discharging the liquid to the outside of the system. 2. The pH and ferric iron concentration of the absorption liquid are maintained in the ranges of 1.5 to 2.5 and 10 mg-mol / mol, respectively, and a part of the absorption liquid is withdrawn to obtain a solid content containing sand as a main component. After sedimentation and separation, alkali is added to the supernatant to adjust the pH to 7 or above to sediment iron and the iron sludge generated is returned to the absorption system, and the sand sludge generated in the sediment separation is added to the supernatant. The waste liquid treatment method according to claim 1, wherein the waste liquid treatment is performed by filtration. 3. The pH and ferric iron concentration of the absorption liquid are 2.
Keeping in the range of 5 to 5 and 1 mg-mol / or more, and withdrawing a part of the absorbing solution and adding alkali to adjust the pH to 7 or more,
Then, the sedimentable solids such as iron and sand are separated by sedimentation, sulfuric acid is added to the generated sludge to dissolve the iron to perform sedimentation separation, and the supernatant is subjected to electrolytic reduction treatment. The method for treating waste liquid according to item 1 of the above.