JPH069679B2 - Method for removing fluorine in flue gas desulfurization wastewater - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention performs primary treatment of fluorine in wastewater discharged from a flue gas desulfurization apparatus for combustion exhaust gas with slaked lime, and then further The present invention relates to a method for reducing elementary concentration.

(Prior Art) Conventionally, as a method for reducing the concentration of fluorine in flue gas desulfurization wastewater to 15 mg / or less (nationwide uniform regulation), there is a method as shown in FIG.

In FIG. 2, combustion exhaust gas using coal or the like as fuel is
The lime-gypsum desulfurization device removes soot and sulfur oxides and the like and then releases them as clean gas. Flue gas desulfurization wastewater containing F, Cl, SO ₄ , heavy metals, etc. due to the fuel 1 Is discharged. This flue gas desulfurization wastewater 1
Leading to the pH adjusting step a, adding slaked lime 4 to adjust the pH to 7 to 11, and precipitating F and heavy metals in the wastewater 1 as sparingly soluble calcium fluoride (CaF ₂ ) and hydroxide, respectively. due to the SO ₄ in the wastewater 1 gypsum (CaSO ₄ · 2
H ₂ O) also precipitates. At that time, a part of the coagulation sludge 3 containing CaF ₂ , CaSO ₄ .2H ₂ O, etc., which has been separated by sedimentation in the coagulation sedimentation step c, is adjusted to pH.
It is circulated to the adjusting step a to measure the crystal coarsening of CaF ₂ and CaSO ₄ .2H ₂ O, to facilitate the sedimentation separation of the subsequent coagulating sedimentation step c and the dehydration of the dehydration step d, and to reduce the degree of supersaturation of gypsum. It has been reduced and mitigated in scaling. Then, in the aging step b, CaF _{2 was} added to improve the F removal property.
After performing crystallization and crystal growth,
Then, the polymer flocculant 5 is added to coarsely flocculate CaF _2, hydroxide, etc., and then sedimentation is performed. The coagulated sludge 3 that has been separated by settling is guided to a dehydration step d, dehydrated, and then disposed. Then, the wastewater 2a from the flocculation-precipitation step c from which F and the like have been removed does not have the supersaturation of the gypsum still solved and causes an obstacle of gypsum scaling in the next step (for example, excess). Water 6 is added to eliminate supersaturation, or NaOH and Na ₂ CO ₃ are added to make it a high pH (pH 10.0 or higher), and after removing calcium, it is sent to the next step or is directly treated. It is discharged as water 2.

However, the above conventional method has the following drawbacks.

Flue gas desulfurization wastewater 1, (CaF ₂ precipitation inhibition by formation of complex salts, crystallization etc.析速degree drop in) effect on precipitation and aging and separation of CaF ₂ in addition to F contain other kinds of heavy metal impurities, etc. to give However, since the water quality of the treated water fluctuates greatly depending on the fuel conditions, the configuration of the exhaust gas treatment system, the operating conditions thereof, etc., the conventional method has a stable F concentration of treated water of 15 mg /
It was difficult to set below, and it was impossible to set below the standard value of 8 mg / which is seen in some ordinances with strict regulations.

On the other hand, the small clear water of the coagulation-sedimentation step c has a high gypsum supersaturation degree (for example, a supersaturation degree of 1.2 to 2.0), and in order to prevent the gypsum scale from the next step onward, the subsequent preparation step cannot be decided. is there. For example, when decalcifying with sodium carbonate, a large amount of sodium carbonate and NaOH are required to reduce the degree of gypsum supersaturation, so the pH becomes 10 or higher, and the pH is readjusted again to meet the discharge water standard. There must be. Furthermore, the disposal of the large amount of precipitate (calcium carbonate) generated thereby was a problem. Further, in the case of simply diluting to eliminate the degree of supersaturation of gypsum, there is a case in which the amount of dilution water needs to be almost the same as the amount of waste water, and the amount of waste water is significantly increased.

Therefore, in order to solve the above problems, the present inventors calculated the "calcium index ( _Ica )" from the water quality analysis results of the wastewater, and if the value is lower than the appropriate range, during the wastewater or the wastewater treatment process. , I _{ca to} have an appropriate value, hydrochloric acid (HCl),
Calcium chloride (CaCl ₂ ) and aluminum chloride (Al
Cl ₃ ), polyaluminum chloride (PAC) and other chlorine-containing substances are added to stabilize the F in the treated water during the F treatment of wastewater by the lime milk or slaked lime powder neutralization method.
It was proposed that the amount be 5 mg / or less. However, even in this treatment method, it is difficult to reduce F in the treated water to 8 mg / or less.

In addition, the present inventors, after adding and mixing seed crystals consisting of calcium fluoride (CaF ₂ ) and gypsum (CaSO ₄ .2H ₂ O) to the supernatant water of the flocculation-precipitation step, the seed crystals are separated. For example, it has been proposed that F in the treated water can be stably kept at 8 mg / or less and, at the same time, the scaling failure in the subsequent steps is almost eliminated (Japanese Patent Application No. 58-2343). However,
In this way, CaSO ₄ · good CaF ₂ precipitation of a poorly precipitating original
Coprecipitation with 2H ₂ O hinders processability (floating Ca
F ₂ is high, the thickness intended to effectively utilize the seed crystal without causing the F concentration is increased) in the treated water, when increasing the CaF ₂ over a certain ratio, is lost for co沈作There was a tendency to impair the processability. Furthermore, since CaF ₂ is added from outside the system, there is a drawback that the amount of precipitate increases.

(Problems to be Solved by the Invention) The conventional method has a drawback that the national uniform standard of F15 mg / or less cannot be stably obtained. Particularly, the flue gas desulfurization wastewater using coal having a low chlorine content as fuel is Even if the molar ratio of aluminum and F in the wastewater and the pH in the pH adjustment process are set to satisfying conditions, not only does it not fall below 15 mg / F, but 8 mg / F.
/ Faced with a situation where it is impossible to make it below, the present invention has been achieved as a result of intensive studies for solving the problem.

(Means for Solving Problems) The present invention relates to a wastewater treatment method of fixing and removing fluorine from wastewater desulfurization wastewater, wherein slaked lime is added to the wastewater to adjust the pH to
The first step to adjust to 4, pH of 7 ~ by adding slaked lime
In the second step of adjusting to 9, the third step of separating the precipitate produced in the second step, the fourth step of adding a soluble alkali carbonate compound to the supernatant water obtained in the third step, the fourth step It comprises a fifth step of separating the generated precipitate and a sixth step of decomposing the precipitate with a mineral acid acidity, and the gas generated in the sixth step is brought into contact with the mixed liquid of the fourth step, and The decomposition solution obtained in the sixth step is returned to the first step, which relates to a method for removing fluorine in flue gas desulfurization wastewater.

That is, the present invention is characterized by the following points.

(1) Calculate the “calcium index (I _ca )” from the water quality analysis results of the wastewater, and if the value is lower than the appropriate range, make sure that the _Ica will be the appropriate value during the wastewater treatment process or the wastewater treatment process described below. By adding an acid decomposition solution containing calcium chloride (CaCl ₂ ), in the treatment of F in wastewater by the lime milk or slaked lime powder neutralization method, F in treated water can be stabilized to 1
The amount should be 5 mg or less.

(2) By paying attention to the I _ca value as the F treatment stable operation condition, the F treated water quality in the waste water by the lime milk or slaked lime powder neutralization method can be estimated with high reliability from the properties of the desulfurization waste water.

(3) Further, the treated water having F of 15 mg / or less is decalcified with a soluble alkali carbonate at a pH of 8.0 to 8.6 within the standard of discharged water quality to stably stabilize F of 8 mg / or less. Can be Furthermore, not only does the amount of soluble alkali carbonate required for decalcification be small, but there is no need to readjust the pH in order to bring the treated water to a pH of 5.8 to 8.6 within the standard of discharged water quality.

(4) The amount of sludge generated is reduced by using the acid decomposition solution of sludge produced by the calcium removal operation of (3) above in the step of (1) above.

(5) By reusing the gas generated by the acid decomposition of sludge in the above calcium removal operation, the injection amount of the soluble alkali carbonate can be reduced.

(6) The gypsum supersaturation degree of the final treated water is eliminated, and the scaling failure in the subsequent process does not occur.

(Operation) FIG. 1 shows the configuration of the wastewater treatment process according to the present invention.

Flue gas desulfurization wastewater 1 containing F, SO ₄ , heavy metals
Leading to the adjusting step a, slaked lime 4 and acid decomposition liquid 9 described later
The pH is adjusted to 2 to 4 by means of the method, and SO ₄ in the wastewater 1 is reacted as shown in the following formula (1) to precipitate as CaSO ₄ .2H ₂ O.

SO ₄ ²⁻ ＋ Ca ²⁺ ＋ 2H ₂ OCaSO ₄・ 2H ₂ O (1) In this case, the acid decomposition solution 9 contains excess HC as described later.
It contains a large amount of l, Ca ²⁺ , Cl ⁻ and F. at the same time,
By exposing the metal hydroxide in the coagulation sedimentation sludge 3 returned from the coagulation sedimentation step c to a low pH atmosphere,
Re-dissolved according to the formula (2), the surface was contaminated with hydroxide in the conventional method, and CaSO ₄・ 2H ₂ in the reduced coagulation sediment sludge 3 was reduced.
The seed crystal effect of O is recovered, and it has an action of reducing the supersaturation degree of CaSO ₄ .2H ₂ O in the subsequent aging step b, the coagulating sedimentation step c, and the treated water 2a.

M (OH) _m + mH ⁺ H ^{m +} + mH ₂ O (2) where M is a metal and m is the number of charges of M.

Further, pH adjustors step pH2~4 of a is included and refractory properties in the waste water with F reminiscent product (eg BF ₄ ^-)
Al-F which is originally present in wastewater or can be removed as CaF ₂ by externally added aluminum (Al)
It is an appropriate range for the reaction shown in the following formula (3) for converting into a complex (for example, AF ₂ ⁺ ), and has an action of stabilizing the F processing performance.

BF ₄ ⁻ + 2Al ³⁺ + 3H ₂ O 2AlF ₂ ⁺ + H ₃ BO ₃ + 3H ⁺ (3) Here, since the reaction of the above formula (3) is completed, the molar ratio of Al and F is 0.5 (preferably 0. 6) The above is required.

Then, lead to the aging step b, add slaked lime 4a to adjust the pH to 7
To 9 (preferably 7.5 to 8.5), and F and AlF ₂ ⁺ in the waste water are reacted as shown in the following formulas (4) and (5), respectively. As the metal ₂ , the metal is precipitated as a sparingly water-soluble hydroxide or the like by the leftward reaction of the above formula (2).

2F ⁻ + Ca ²⁺ CaF ₂ (4) AlF ₂ ⁺ + Ca ²⁺ + 30H ⁻ CaF ₂ + Al (OH) ₃ (5) On the other hand, precipitation of CaF ₂ is limited by the following formula (6).

[Ca ^2+] [F ^{-] 2} = K _s1 (6) where [] each molar concentration (hereinafter the same), K _s1 is C
It is a concentration-based solubility product (M ³ / l ³ ) of aF ₂ .

That is, the processing performance of F is dominated by the dissolved Ca concentration in the liquid in the aging step b and the coagulation sedimentation step c, and the processing performance of F deteriorates unless the dissolved Ca concentration exceeds a certain level. Then, this dissolved Ca concentration is temporarily calculated as "calcium index I _ca (m
eq / or epm) ”and is uniquely determined by the proposed value. In the liquids of the aging step b and the coagulating sedimentation step c, the metals migrate into the precipitate and F is sufficiently smaller than other ionic species. ) The ion balance shown in the equation is established.

[Na ^+] + [K ^+] + 2 {[Ca ^2+] + [Mg ^2+]} = [Cl ^-] +2
[SO ₄ ^2- ] (7) Further, I _ca (meq /) is defined as shown in the following formula (8).

I _ca = [Cl ⁻ ] − {[Na ⁺ ] + [K ⁺ ] +2 [Mg ²⁺ ]} = 2 {[Ca ²⁺ ] − [SO ₄ ²⁻ ] (8) where [Ca ²⁺ ] Is given by the following formula (9) from the formula (8) and the concentration-based solubility product k _s2 (M ² / l ² ) of CaSO ₄ · 2H ₂ O.

As is clear from the above, the calcium index I _ca can be calculated by knowing Na, K, Mg, and Cl of the wastewater, but these four components hardly change the concentration in the neutralization operation with slaked lime. It is possible to calculate from the measured value of the flue gas desulfurization wastewater 1 without measuring the concentration in the liquid in the aging step b and the coagulation-sedimentation step c, and further, the properties and consumption of coal, the amount of exhaust gas, the amount of wastewater, etc. It can be predicted from the specifications. It was but connexion, previously grasped I _ca wastewater, I _ca negative or +30
When the epm is less than or equal to epm, the dissolved Ca concentration of the coagulation-sedimentation-treated water 2a can be maintained at about 1400 to 2500 mg / by returning the acid decomposition solution 9 to the pH adjusting step a so that I _ca becomes +30 epm or more. It is possible to obtain stable F processing performance.

The wastewater of the aging step b containing precipitates such as CaF ₂ , CaSO ₄ · 2H ₂ O and metal hydroxide was led to the coagulating sedimentation step c, and the polymer coagulant 5 was added to coarsely flocculate the precipitate. After that, sedimentation is performed. A part of the sedimented coagulated sludge 3 is returned to the pH adjusting step a, and the rest is guided to the dehydration step d, dehydrated and then disposed.

The coagulation / precipitation treated water 2a is led to a subsequent reaction step f and mixed with a soluble alkali carbonate 7 and a recovery gas 10 containing carbon dioxide gas (CO ₂ ) generated in an acid decomposition step h described later. As the soluble alkali carbonate 7, sodium carbonate (Na ₂ CO _2
3 ), potassium carbonate (K ₂ CO ₃ ) and the like can be used.

In the reaction step f, calcium carbonate (CaCO ₃ ) is generated by CO ₂ in the dissolved Ca, the soluble alkali carbonate 7 and the recovered gas 10 in the coagulation-treated water 2a according to the reaction represented by the following formula (10).

Ca ²⁺ + CO ₃ ^2- CaCO ₃ (10) At the same time, F in the coagulation-treated water 2a is taken into the generated CaCO ₃ and further decreases. In addition, gypsum is Glauber's salt (N
a ₂ CO ₄ ), and the gypsum supersaturation degree of the coagulation treated water 2a is also eliminated. Further, at this time, if the pH of the coagulation treated water 2a is set to 7 to 9 (preferably 7.5 to 8.5), the pH of the reaction step f reaches equilibrium at about 8.6 at most, No pH control is required. The injection amount of the soluble alkali carbonate 7 is sufficient to eliminate the degree of supersaturation of gypsum.

The wastewater containing CaCO ₃ sludge is then introduced to the subsequent settling separation step g, where the polymer flocculant 5a is added to coarsely flocculate the precipitate, and then the sediment is separated. Sediment separated by sedimentation 8
Is led to the acid decomposition step h and mixed with the mineral acid 11. The mineral acid 11 used here is preferably hydrochloric acid (HCl). In the acid decomposition step h, pH is 4 or less (preferably 2 or less)
The addition amount of the mineral acid 11 is set so that At this time,
CaCO ₃ in the precipitate 8 is decomposed into Ca ²⁺ and CO ₃ ²⁻ by the reaction in the left direction of the above formula (10). Ca ²⁺ remains as CaCl ₂ in the decomposition liquid, but CO ₃ ²⁻ further becomes CO ₂ gas in the low pH atmosphere and is returned to the reaction step f as the recovered gas 10.
Further, the liquid containing CO ²⁺ and F obtained in the acid decomposition step h is returned to the pH adjusting step a as the acid decomposition solution 9.

The treated water 2 which is the supernatant water obtained in the sedimentation separation step g
Has a stable F content of 8 mg / or less, and is sent to a post-treatment step (excess, COD removal by ion exchange, etc.) or discharged as it is.

(Effects of the Invention) The present invention has the following effects.

(1) By grasping the calcium index I _ca of desulfurization wastewater in advance and returning the acid decomposition solution so that the I _ca becomes +30 epm or more, F in the coagulation-treated water is stably stabilized at 15 mg / or less. Can be

(2) I _ca corresponding to the F concentration level of the coagulation-treated water can be selected within an appropriate range, and the calcium removal operation in the subsequent reaction step and sedimentation separation step can be stabilized.

(3) Further, the treated water having F of 15 mg / or less is decalcified with a soluble alkali carbonate at a pH of 8.0 to 8.6 within the standard of discharged water quality to stably stabilize F of 8 mg / or less. Can be Furthermore, not only does the amount of soluble alkali carbonate required for decalcification be small, but there is no need to readjust the pH in order to bring the treated water to pH 8.6 or less within the standard of discharged water quality.

(4) By acid-decomposing the sludge generated by the calcium removal operation, solid substances are reduced and the amount of sludge generated is reduced.

(5) By reusing the gas generated during the acid decomposition of sludge in the above calcium removal operation, the injection amount of the soluble alkali carbonate can be reduced.

(6) The gypsum supersaturation degree of treated water is eliminated, and scaling failure in the subsequent process does not occur.

Examples of the present invention will be shown below.

Example In this example, the flue gas desulfurization wastewater 1 generated when the exhaust gas of the coal brazing power generation facility was desulfurized by the coal-gypsum method was treated in the treatment process shown in FIG. The result is the first
Shown in the table.

The water quality of the flue gas desulfurization wastewater 1 of this example is as follows.

pH 1.8 SS 24000 mg / F 305 mg / Al 345 mg / [Al] / [F] 0.8 (molar ratio) I _ca -14.8 epm In Table 1, test No. 1 is the calcium index I of the pH adjusting step 1. _The amount of the acid decomposition solution 9 was adjusted so that _ca was 30 epm and Test No. 2 was +50 epm.

Comparative Example As a comparative example, the desulfurization wastewater 1 having the same properties as those in the above-mentioned example was used.
The results of the treatment steps shown in FIG. 2 are also shown in Table 1.

[Brief description of drawings]

FIG. 1 is a flow diagram showing the constitution of a treatment process of flue gas desulfurization wastewater according to the present invention, and FIG. 2 is a flow diagram showing the constitution of a conventional treatment process.

Claims (1)

[Claims] 1. A wastewater treatment method for fixing and removing fluorine from flue gas desulfurization wastewater, wherein slaked lime is added to the wastewater to adjust the pH to 2 to 5.
The first step to adjust to 4, pH of 7 ~ by adding slaked lime
In the second step of adjusting to 9, the third step of separating the precipitate generated in the two steps, the fourth step of adding a soluble alkali carbonate compound to the supernatant water obtained in the third step, the fourth step The method comprises a fifth step of separating the formed precipitate and a sixth step of decomposing the precipitate with a mineral acid acidity, and the gas generated in the sixth step is brought into contact with the mixed liquid of the fourth step, and A method for removing fluorine in flue gas desulfurization wastewater, wherein the decomposition liquid obtained in the sixth step is returned to the first step.