JPH0510993B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a method for treating fluorine in waste water discharged from a desulfurization device for combustion exhaust gas of coal or the like. (Conventional technology) One way to reduce F in wastewater to below the nationwide standard (15 mg/) is to add lime milk or slaked lime powder to the pH level.
7 to 11, and F is calcium fluoride (CaF ₂ )
There is a method of precipitation, ripening, and solid-liquid separation. The outline will be explained with reference to FIG. Dust and ionic oxides are removed from combustion exhaust gas using coal or other fuels by desulfurization equipment such as the lime-gypsum method, and the gas is released as clean gas. ₄ ) and strongly acidic flue gas desulfurization wastewater 21 containing heavy metals, etc. is discharged. The flue gas desulfurization wastewater 21 is led to a rough neutralization step 1, where slaked lime 31 is added to roughly neutralize it, and then to a PH adjustment step 2.
Furthermore, slaked lime 31 is added to adjust the pH to 7 to 11, and F and heavy metals in the wastewater are precipitated as poorly soluble _CaF2 and hydroxides, respectively, but at the same time _SO4 in the wastewater is also precipitated. Gypsum ( _CaSO4・
2H ₂ O). The waste water in which CaF ₂ etc. have been precipitated in the PH adjustment step 2 is then led to a coagulation-sedimentation step 3, where a polymer flocculant 34 is added to turn CaF ₂ and metal hydroxides into coarse flocs, which are then separated by sedimentation. A part of the flocculated and precipitated sludge 23 that has been sedimented and separated is returned to the crude neutralization step 1 to be used as seed crystals for CaF ₂ and CaSO ₄ .2H ₂ O, and the rest is led to the dehydration step 5 where it is dehydrated and then disposed of. The flocculation-sedimentation supernatant water 22 from the flocculation-sedimentation step 3 from which F etc. have been removed is further subjected to post-treatment steps such as filtration, calcium removal with sodium carbonate, and ion exchange.
Water sent to COD removal etc. or directly treated water 2
It is discharged as 4. F-containing wastewater such as flue gas desulfurization wastewater contains many types of ions other than F, and these coexisting ions
They affect treatment performance, but their concentrations vary greatly depending on the history of the wastewater, for example, in the case of wastewater desulfurization wastewater, the type of fuel, the configuration of the exhaust gas treatment system, and its operating conditions. Stable and uniform standard nationwide (15mg/)
It was extremely difficult to do the following. In particular, flue gas desulfurization wastewater is fueled by coal with a low chlorine content, and the molar ratio of aluminum and F in the wastewater is
We encountered a situation in which, even if the PH etc. in the PH adjustment process were set to satisfactory conditions, it did not fall below the nationwide standard (F15 mg/), and as a result of intensive study to solve the problem, we came up with the proposal of the present invention. (Problems to be Solved by the Invention) The present invention solves the drawbacks of conventional methods for treating fluorine in flue gas desulfurization wastewater, and improves the chemical composition of fuel, the amount used,
The stable operating conditions for F treatment can be easily set from values such as the amount of flue gas and the amount of flue gas desulfurization effluent, and the amount of F in the treated water can be reliably reduced to below the nationwide standard. The aim is to provide a basic processing method. (Means for Solving the Problems) The present invention involves adding slaked lime or mineral acid to fluorine-containing wastewater such as flue gas desulfurization wastewater, returning and reacting a part of the coagulated and precipitated sludge under conditions of pH 2 to 4, and then reacting with slaked lime. In the fluorine treatment method, which adjusts the pH to 7 to 9 to precipitate fluorine and separate the generated sludge, the calcium index is higher than the allowable fluorine concentration in treated wastewater. Fluoride in flue gas desulfurization wastewater, characterized by adding chlorine-containing substances such as calcium chloride, aluminum chloride or polyaluminum chloride at any stage before separating the sludge so that the calcium index of the wastewater becomes This is a processing method. (Function) As shown in Fig. 1, the flue gas desulfurization wastewater 21 containing F, SO ₄ , heavy metals, etc. is first led to a rough neutralization step 1, and the pH is reduced to 2 to 4 with slaked lime 31 and mineral acid 32.
The SO ₄ in the wastewater is reacted as shown in the first equation and precipitated as CaSO ₄ .2H ₂ O. In this case, the mineral acid 32 uses HCl, which can be used as a chlorine-containing substance, which will be described later, but other Cl sources, such as CaCl ₂ ,
When using AlCl ₃ , PAC, etc., sulfuric acid (H ₂ SO ₄ ) can be used. SO ^2- ₄ +Ca ²⁺ +2H ₂ OCaSO ₄・2H ₂ O 1st equation At the same time, the metal hydroxides in the coagulated sedimentation sludge 23 returned from the coagulation-sedimentation process 3 are exposed to a low PH atmosphere, resulting in the 2nd equation. As a result, the seed crystal effect of CaSO ₄ 2H 2 O in the flocculation-sedimentation sludge 23, which had been reduced by sludge on the surface with hydroxide in the conventional method, is restored, and the seed crystal effect of CaSO 4 2H ₂ O in the subsequent PH adjustment step 2 and flocculation is restored. It has the effect of reducing the degree of supersaturation of CaSO ₄ .2H ₂ O in the precipitation step 3 and the treated water 24. M(OH) _n +mH ⁺ M ^m+ +mH ₂ O Second Formula Here, M is a metal, and m is the number of charges of M. In addition, PH2 to 4 in the rough neutralization step 1 is contained in wastewater together with F and is difficult to treat, such as borofluoride (e.g.
BF ₄ ^- ) by aluminum (Al) originally present in the wastewater or added externally.
Al-F complexes that can be removed as _CaF2 (e.g.
This is an appropriate range for the reaction shown in the third equation to convert into AlF ₂ ⁺ ), and has the effect of stabilizing the F processing performance. BF ₄ ^- +2Al ³⁺ +3H ₂ O2AlF ₂ ⁺ +H ₃ BO ₃ +3H ⁺
Third Formula Here, in order to complete the reaction of the third formula, the molar ratio of Al and F must be 0.5 (preferably 0.6) or more. Next, lead to PH adjustment step 2, add slaked lime 31 to adjust the PH to 7 to 9 (preferably 7.5 to 8.5),
F and AlF ₂ ⁺ in the wastewater are reacted as shown in the 4th and 5th equations, respectively, to form CaF ₂ which is sparingly soluble in water.
In addition, metals are precipitated as hydroxides etc. that are difficult to tolerate in water by the leftward reaction of the second equation. 2F ^- +Ca ²⁺ CaF ₂ 4th equation AlF ₂ ^- +Ca ²⁺ +3OH ^- CaF ₂ +Al(OH) ₃ 5th equation On the other hand, the precipitation of CaF ₂ is limited by the 6th equation. [Ca ²⁺ ] [F ^- ] ² = K _S1 Equation 6 Here, [ ] is the respective molar concentration (the same applies below), and K _S1 is the concentration-based solubility product of CaF ₂ (M ³ /l ³ )
It is. That is, the processing performance of F is controlled by the concentration of dissolved Ca in the liquid in the PH adjustment step 2 and the coagulation and precipitation step 3, and unless the dissolved Ca concentration is at a certain level or higher, the processing performance of F deteriorates. This dissolved Ca concentration is calculated as the “calcium index”.
ICa (meq/or pm)" is determined by the proposed value. In the liquid of PH adjustment step 2 and coagulation precipitation step 3, metals migrate into the precipitate. It can be ignored, and F is also sufficiently small compared to other ions, so if ignored, the following ion balance holds: [Na ⁺ ] + [K ⁺ ] + 2 {[Ca ²⁺ ] + [Mg ²⁺ ]} = [Cl ^- ]
+2 [SO ^2- ₄ ] Formula 7 ICa (meq/) is defined as follows. ICa = [Cl ^- ] - {[Na ⁺ ] + [K ⁺ ] + 2 [Mg ²⁺ ]} = 2
{[Ca ²⁺ ]−[SO ₄ ^2- ]} The eighth equation [Ca ²⁺ ] is calculated from the eighth equation and the concentration-based solubility product K _S2 (M ² /l ² ) of CaSO ₄ 2H ₂ O. It is given as equation 9. [Ca ²⁺ ] = (ICa + √ ² + 16 _S1 ) / 4 Equation 9 As is clear from the above, ICa is Na in wastewater,
K, Mg, and Cl can be calculated if they are known, but since these four components hardly change their concentration when neutralized with slaked lime, they cannot be calculated by measuring the concentration in the liquid in PH adjustment step 2 or coagulation and precipitation step 3. It can be calculated from the measured values of the flue gas desulfurization waste water 21, and furthermore, it can be predicted from specifications such as the properties and consumption of coal, the amount of exhaust gas, and the amount of waste water. Therefore, it is necessary to understand the ICa of wastewater in advance,
When ICa is negative or less than +30pm, ICa is 30
By adding a chlorine-containing substance 33 such as HCl, CaCl ₂ , AlCl ₃ or PAC to the rough neutralization step 1 or the PH adjustment step 2 so that the concentration of dissolved Ca in the flocculation-precipitation supernatant water 22 is 1400~ 2500
It is possible to maintain stable F treatment performance at around mg/mg/kg. In addition, the chlorine-containing substances added are
For chlorides other than Na, K and Mg, the above
It can be used regardless of HCl, CaCl ₂ , AlCl ₃ or PAC (polyaluminum chloride). The wastewater from the PH adjustment step 2, which contains precipitates such as CaF ₂ , CaSO ₄ .2H ₂ O, and metal hydroxides, is led to the coagulation-sedimentation step 3, where a polymer flocculant 34 is added to turn the precipitates into coarse flocs. Sedimentation and separation. A part of the flocculated and precipitated sludge 23 that has been sedimented and separated is returned to the rough neutralization step 1, and the rest is led to the dewatering step 5 where it is dehydrated and then disposed of. The flocculation and sedimentation supernatant water 22 has F stably reduced to 15 mg/or less, and is either sent to post-treatment process 4, such as decalcification with filtration or sodium carbonate, or COD removal by ion exchange, or is discharged as is as treated water 24. . (Example) Specific examples of implementation related to the present invention are shown below. This example is a coal-fired power generation facility (output 33MW)
Flue gas desulfurization wastewater generated when desulfurizing exhaust gas (exhaust gas amount 170000m ³ N/H) using the coal/gypsum method 21
(Drainage volume: 2 m ³ /h) was treated using the treatment process shown in Figure 1. The results are shown in Figure 3. In Figure 3, measurement point 1 is without HCl addition, and points 2 to 9 are rough neutralization step 1.
and HCl was added as follows.

[Table] In addition, flue gas desulfurization wastewater 2 during the operation period of this example
The average water quality of No. 1 was as follows. PH 1.8 SS 24000mg/F 305mg/Al 345mg/[Al]/[F] 0.8 (mole ratio) ICa -14.8pm Further, the PH in the PH adjustment step 2 was in the range of 8.1 to 8.4. As is clear from this example, in the neutralization treatment of F-containing wastewater such as flue gas desulfurization wastewater with slaked lime,
The following effects can be obtained by adding a chlorine-containing substance. (1) Understand the ICa of wastewater in advance and check that the ICa is +30
By adding chlorine-containing substances so that the chlorine content is pm or higher, treated water that satisfies the national uniform standard F15mg/lower can be obtained in a sufficiently stable manner. (2) You can freely select the ICa that corresponds to the F concentration level of the treated water. In addition, even if there is no actual wastewater, ICa can be predicted from specifications such as coal properties, consumption, exhaust gas volume, and wastewater volume, so it is possible to estimate the amount of chlorine-containing substances added at the design stage of wastewater treatment equipment. It is possible to maintain stable F processing performance at all times.

[Brief explanation of drawings]

Figure 1 shows the flow of the treatment method of the present invention, Figure 2 shows the flow of the conventional treatment method, and Figure 3 shows the results of the example of the present invention. It is a graph showing the relationship with the F concentration in water.

Claims (1)

[Claims] 1. Slaked lime or mineral acid is added to fluorine-containing wastewater such as flue gas desulfurization wastewater, and a part of the coagulated and precipitated sludge is returned and reacted under conditions of pH 2 to 4. Then, slaked lime is added and the pH
In a fluorine treatment method in which the fluoride is adjusted to 7 to 9 to precipitate fluorine and separate the generated sludge, the calcium index of the wastewater is higher than the calcium index corresponding to the allowable fluorine concentration in the treated wastewater. Fluoride in flue gas desulfurization wastewater characterized by adding chlorine-containing substances such as hydrochloric acid, calcium chloride, aluminum chloride or polyaluminum chloride at any stage before separating the sludge so that the calcium index is processing method.