JPS621529B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for treating exhaust gas containing fluorine-based components, and is intended to provide a method that can efficiently and inexpensively treat exhaust gas containing fluorine-based components through simple steps. Fluorine-based component-containing exhaust gas is generated from aluminum electrolytic furnaces, ceramic product firing and melting furnaces, fertilizer reaction equipment, stainless steel pickling equipment, etc., and the exhaust gas from these equipment contains HF, _F2, etc. In addition, each equipment releases harmful substances such as SOx and HCl.
It contains CO ₂ and may also contain dust such as silica and iron. Conventional technology for treating such fluorine-based component-containing exhaust gas includes removing fluorine from the exhaust gas by cleaning the exhaust gas with an aqueous NaOH solution, and then adding Ca(OH) ₂ or A method of precipitating and removing CaF ₂ by adding Ca(OH) ₂ + CaCl ₂ (so-called double alkali process), converting exhaust gas to Ca(OH) ₂ + CaCl ₂ solution, Mg
There is a method (mono-alkali process) in which a (OH) ₂ +MgCl ₂ solution or a Ba(OH) ₂ +BaCl ₂ solution is brought into contact with each other (mono-alkali process) (see JP-A-50-113482). However, since the above method uses a double alkali process, the treatment process is complicated, and in order to avoid the complexity of this process, Ca
(OH) ₂ It is possible to use slurry, but
The solubility of Ca(OH) ₂ is low, the loss of Ca(OH) ₂ due to co-precipitation with Ca-based precipitates, and
There are problems in that it is impractical due to loss due to CO ₂ absorption and a decrease in the ability to absorb harmful substances. The above method also includes, as an improvement of the above method,
This was proposed to solve the problems of using Ca(OH) ₂ slurry, and Ca(OH) ₂ in CaCl ₂ solution.
We focused on the fact that the solubility of CaCl ₂ is much greater than the solubility in water.
It is used in a state in which a maximum of 0.76g of Ca(OH) ₂ is dissolved per 100g.
In the figure, CaCl ₂ ; Ca(OH) ₂ in a 30wt% solution;
Exhaust gas is treated with an absorption liquid containing 0.3 to 0.5wt%, but the absorption liquid [Ca(OH) ₂ 0.5wt%]
%] is said to be stable at 8.9, and it is clear that the treatment of exhaust gas is carried out using an absorption liquid in the alkaline region with a pH of 7 or higher, and that fluorine-based components can be removed using an absorbent with a pH of 7 or higher. Since it is carried out using an absorption liquid, it is clear that the absorption and removal reaction of fluorine-based components in this method is Ca(OH) ₂ +2F ^- +2H ⁺ →CaF ₂ +2H ₂ O, and as a side reaction Ca(OH) ) ₂ +CO ₂ →CaCO ₃ +H ₂ O progresses slightly, and it is undeniable that CaCO ₃ is generated to some extent. Therefore, in the above method, Ca(OH) ₂ , which is often used as a fluorine-based component removal agent, is expensive and a portion of it is lost as CaCO ₃ , and furthermore, a highly concentrated CaCl ₂ solution is used. Therefore, in long-term operation, there are disadvantages such as equipment corrosion due to Cl ^- . The present invention was devised after repeated studies in view of the above-mentioned circumstances, and involves defluoridation treatment by bringing exhaust gas containing fluorine-based components into contact with an absorption liquid containing calcium chloride and having a pH of 4.0 to 5.5. , Add a defluoridating agent such as gypsum dihydrate (CaSO ₄ 2H ₂ O), gypsum hemihydrate (CaSO ₄ 1/2H ₂ O) or anhydrite (CaSO ₄ ) to the treated solution after the defluorination treatment. By regenerating the calcium chloride-containing absorption liquid with a pH of 4.0 to 5.5 and recycling the regenerated absorption liquid for the treatment of the fluorine-containing exhaust gas, the above-mentioned fluorine-containing exhaust gas can be removed in a simple process. We succeeded in processing efficiently and inexpensively. That is, to further explain such the present invention, the present inventors previously proposed Patent No. 1046547 (Japanese Patent Publication No. 55-41810) regarding efficient defluorination treatment using calcium carbonate, but in the present invention, , dihydrate gypsum (CaSO ₄ 2H ₂ O), which is much cheaper than calcium carbonate in the prior invention.
In recent years, while the demand for gypsum has been decreasing, by-product gypsum has been increasing due to the spread of desulfurization equipment, etc., and its price is extremely low. Gypsum dihydrate is used as a defluoridant. The fluorine-based component in the fluorine-based component-containing exhaust gas described above reacts with CaCl ₂ in the absorption liquid, and is precipitated as CaF ₂ through a chemical reaction as shown in the following equation (1). CaCl ₂ +2F ^- →CaF ₂ +2Cl ^- ...(1) However, CaSO ₄ 2H ₂ O is added to the treated solution containing Cl ^- after the defluorination treatment, and the chemical reaction as shown in the following equation (2) The reaction regenerates _CaCl2 . CaSO ₄ +2Cl ^- →CaCl ₂ +SO ₄ ^2- ...(2) In addition, the new absorption liquid before the inflow of the flue gas containing fluorine components should be 0.05% or more, preferably 0.1 to 0.5%.
Using a CaCl ₂ aqueous solution with HCl added to adjust the pH to 4.0 to 5.5, preferably 4.5±0.5, the regenerated absorption liquid after the exhaust gas inflow maintains a pH of 4.0 to 5.5, preferably 4.5±0.5. Use a sample that was operated while adding CaSO ₄ .2H ₂ O. The reason why the CaCl ₂ concentration of the new absorption solution was set to 0.05% or more is because if it is less than 0.05%, F in the absorption solution after defluorination treatment will be reduced.
This is because the concentration becomes high, and the reason why it is set to 0.5% or less is because there is no significant change in the defluorination rate even if the content is 0.5% or more, and economic efficiency was taken into consideration. In addition, the pH of the absorption liquid was set to 4.0 or higher because
This is because if it is less than 4.0, the reaction of the above formula (1) will not be carried out efficiently, and the reason why it is set to be 5.5 or less is because if it is 5.5 or more, the reaction of the above formula (2) will not be carried out smoothly. Furthermore, when the PH of the absorption liquid is 4.5±0.5, the fluorine removal rate in the exhaust gas is as high as 98% or more when the F concentration in the gas is about 100 ppm. After that, the F concentration in the exhaust gas becomes 2.0 mg/Nm ³ or less as F. Furthermore, since the PH range (4.0 to 5.5) is an acidic region, most of the components (silica and metals) from the dust in the exhaust gas are dissolved in the absorption liquid, and CaF ₂ is obtained as a precipitate. can be obtained with extremely high purity. The attached drawings show one of the apparatuses for carrying out the present invention.
This is a schematic diagram showing an example, and as shown, 1 is an absorption tower, and in this absorption tower 1, a fluorine-based component-containing exhaust gas 2 is brought into contact with an absorption liquid to remove the fluorine content, and then It is discharged to the outside as treated exhaust gas 3. The treated liquid 4 after the defluorination treatment is led from the lower part of the absorption tower 1 to a supply tank 5 for CaSO ₄ .2H ₂ O, and in this supply tank 5, CaSO ₄ .2H ₂ O 6 and the engineered water 7 are CaCl ₂ is regenerated by being added to the treatment liquid 4, and the resulting CaCl ₂ solution is sent to the upper part of the absorption tower 1 via an absorption liquid circulation pump 8.
It is circulated and supplied as a regenerated absorption liquid. A portion of the CaCl ₂ solution from the pump 8 is led to a thickener 9, and the precipitated concentrate (containing calcium fluoride) from the thickener 9 is led to a solid-liquid separator 10.
In this solid-liquid separator 10, CaF ₂ 11 is recovered and the liquid is returned to the supply tank 5 as furnace liquid together with the supernatant liquid from the thickener 9. By withdrawing a part of the liquid from the extraction port 12, it is possible to prevent the concentration of heavy metals, silica, etc. in the absorption liquid, and to suppress the SO ₄ ^2- concentration in the regenerated liquid, and this extracted liquid is subjected to neutralization treatment. After that, the precipitated product is removed and then discharged as an effluent outside the system. The liquid extracted from the system can generally effectively achieve the above purpose with about 1/8 to 1/10 of the absorption liquid, and the thickener 9 is used to connect the absorption tower 1 and the supply tank 5. It may be installed in between. In addition, as mentioned above, the liquid extracted out of the system from the extraction port 12 can be added with slaked lime or the like to regenerate gypsum, and the gypsum can be recycled. Specific embodiments according to the present invention will be described below. Example 1 SiF ₄ and F ₂ in air as raw gas to be treated
Mix and dilute (SiF ₄ :F ₂ = 1:2 molar ratio),
A fluorine-based component-containing gas with an F concentration of 23 to 27 mg/Nm ³ was synthesized. Using this raw gas at 20 N/min as a test gas, defluorination treatment was performed using the apparatus shown in the attached drawing. As absorption tower 1, a single-stage absorption tower (inner diameter 25.8 mmφ) with a perforated plate (porosity 18%) was used, and the amount of absorbed liquid was 1.7.
, CaCl ₂ concentration in absorption liquid 0.05%, L/G2.5/m ³
Then, fluorine in the gas was removed by supplying CaSO ₄ .2H ₂ O so that the pH of the absorption liquid was maintained at the set value of 3.5 to 6.0. The results are shown in Table 1.

[Table] According to this result, when F23 to 27mg/Nm ³ in the raw gas, CaCl ₂ concentration in the absorption liquid is 0.05%, L/G2.5/m ³
If the pH is 4.0 to 5.5, the defluorination rate is 98% or more, and the F in the treated exhaust gas is 1 mg/ ^Nm3 or less, and the F concentration in the absorbent furnace liquid is 5 to 9 mg/. Ta. In addition, at pH 3.5 and 6.0, F in the absorbent furnace liquid is 15
mg/ or more. In this case, when returning the solid-liquid-separated absorption liquid to the fluorine removal section after supplying the above-mentioned CaSO ₄ 2H ₂ O and removing fluorine in the gas, about one-ninth of the absorption liquid is removed from the system. When extracted and processed, it was possible to appropriately suppress the concentration of sulfate groups in the regenerated solution. Example 2 As a raw gas, SiF ₄ and F ₂ were mixed and diluted with air (SiF ₄ :F ₂ = 1:1 molar ratio) to obtain an F concentration of 98
A gas containing fluorine components of ~101 mg/Nm ³ was synthesized.
Using this raw gas at 20 N/min as a test gas, defluorination treatment was performed using the apparatus shown in the attached drawing. Absorption tower 1
The same one as in Example 1 was used, and CaCl ₂ in the absorption liquid
PH at concentration 0.20%, L/G2.5/ ^m3 , respectively
The results of removing fluorine in the gas by supplying CaSO ₄ 2H ₂ O to maintain the 4.0 to 5.5 are as follows.
As shown in the table.

[Table] That is, according to this result, F98 to 101mg/
When Nm ³ , CaCl ₂ concentration of absorption liquid is 0.20%, L/
If the pH is 4.0 to 5.5 at G2.5/ ^m3 , the defluorination rate is 98% or more, and the F in the treated exhaust gas is 2mg/N.
m ³ or less, and the F concentration in the absorbent furnace liquid is 6 to 8.
It was mg/. Note that equivalent results can be obtained not only with dihydrate gypsum but also with hemihydrate gypsum (CaSO ₄ 1/2H ₂ O) or anhydrite gypsum (CaSO ₄ ) as a defluoridation agent. In addition, in this case, when the solid-liquid separated absorption liquid was returned to the fluorine removal section in the same manner as in Example 1, about one-eighth of the absorption liquid was extracted from the system and treated. As in Example 1, the treatment was possible with the concentration of sulfate roots suppressed. As explained above, in this invention, CaCl ₂ to be defluorinated is regenerated by adding dihydrate gypsum (CaSO ₄ 2H ₂ O), which can be obtained at low cost, and this is used as a (regenerated) absorption liquid. Since the exhaust gas is recycled, it is extremely economical to treat the exhaust gas, and since the pH of the absorption liquid is treated in the acidic range, most of the impurities are dissolved in the absorption liquid, resulting in very high purity precipitate (CaF). ₂ ) is obtained, and furthermore, the CaCl ₂ concentration is low and the treatment can be performed with the concentration of sulfate radicals effectively suppressed in the regenerating solution, so problems such as corrosion of equipment due to Cl etc. can be avoided, resulting in a high industrial effect. This is a great invention.

[Brief explanation of the drawing]

The drawings illustrate embodiments of the invention,
1 is a schematic diagram showing an example of an apparatus implementing the present invention. However, in this drawing, 1 is an absorption tower, 2 is a fluorine-based component-containing gas, 3 is a treated exhaust gas, 4 is a treated liquid, 5 is a supply tank, 6 is dihydrate gypsum, 7 is industrial water, and 8 is an absorption liquid circulation pump. , 9 is a thickener, 10 is a solid-liquid separator, 11 is CaF ₂ , and 12 is a liquid extracted from the system.

Claims (1)

[Claims] 1 Fluorine-containing exhaust gas is brought into contact with an absorption liquid containing calcium chloride with a pH of 4.0 to 5.5 to remove fluoride, and the treated liquid after the defluorination treatment is treated with gypsum dihydrate, gypsum hemihydrate, or gypsum anhydride. Adjust the pH by adding a defluoridating agent such as
The calcium chloride-containing absorption liquid with a concentration of 4.0 to 5.5 is regenerated, the regenerated absorption liquid is recycled for the treatment of the fluorine-containing exhaust gas, and a part of the regenerated absorption liquid is separated into solid-liquid to produce the calcium chloride-containing absorption liquid. A method for treating exhaust gas containing fluorine-based components, characterized in that the returned liquid is returned to a regeneration section, and a part of the returned liquid is extracted as a liquid extracted from the system.