JP2007125510A - Method for treating fluorine-containing water - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain treated water sufficiently removing fluorine by treating fluorine-containing water coexisting with an aluminium salt stably and efficiently by a calcium fluoride method. <P>SOLUTION: A calcium compound is loaded into the fluorine-containing water coexisting with the aluminium salt and also a pH is adjusted at 8 to 10 (first process), then an acid is loaded to adjust the pH at 6 to 6.5 (second process), and next solid-liquid separation is carried out (third process). It is preferable to return sludge obtained in the third process to the first process. At this time, it is preferable to premix the calcium compound to be loaded into raw water with this return sludge, to load this reformed sludge into the raw water and to adjust the pH at 8 to 10. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for stably and efficiently treating fluorine-containing water in which an aluminum salt coexists with a calcium fluoride method.

  Fluorine-containing wastewater discharged from silicon wafer manufacturing process in semiconductor parts manufacturing, pickling wastewater discharged from stainless steel plate manufacturing process, aluminum surface treatment wastewater, hydrofluoric acid manufacturing wastewater, fertilizer manufacturing wastewater, waste incineration wastewater, etc. Needs to be discharged after fluorine removal treatment to meet the drainage standards. Regarding fluorine-containing wastewater, the development of treatment technology that can further reduce the fluorine concentration of treated water is expected as the wastewater standard was strengthened from 15 mg / L to 8 mg / L in 2001. It is rare.

  Conventionally, calcium fluoride method and hydroxide coprecipitation method as advanced treatment are known as treatment methods for fluorine-containing water. After treatment with calcium fluoride method in the first stage, the second stage In addition, a two-stage treatment method in which advanced treatment is performed by a hydroxide coprecipitation method is also known (Non-Patent Document 1). That is, in the calcium fluoride method, the fluorine concentration of the treated water cannot be reduced below the solubility of calcium fluoride, and the waste water standard cannot be satisfied, so such a two-stage treatment is adopted.

  In the calcium fluoride method, after adding a calcium compound to raw water (fluorine-containing water), a neutralizing agent is added to adjust the pH to near neutral, and an insoluble salt of calcium fluoride is generated by the reaction between fluorine and calcium. In this method, a polymer flocculant is added and agglomeration treatment is performed, followed by solid-liquid separation. In this method, slaked lime is generally used as the calcium compound, and inexpensive sulfuric acid is usually used as the neutralizing agent, but hydrochloric acid may also be used.

  The hydroxide coprecipitation method uses a coprecipitation action when aluminum is precipitated as aluminum hydroxide, and is generally used for advanced treatment. In this method, polyaluminum chloride (PAC) or aluminum sulfate (sulfuric acid band) is added to raw water, then neutralized with a neutralizing agent, a polymer flocculant is added for aggregation treatment, and then solid-liquid separation is performed.

In addition, as production pH conditions of calcium fluoride by the calcium fluoride method, Patent Document 1 describes pH 12, and Patent Document 2 describes pH 7, and the pH of calcium fluoride production in the calcium fluoride method is described. The conditions do not necessarily have to be neutral.
Japanese Patent No. 2912237 Japanese Patent No. 3334937 “Pollution Prevention Technology and Regulations” pp. 288-289

  When adopting the above-mentioned two-stage treatment method, if the amount of aluminum salt such as sulfuric acid band added in the second-stage hydroxide coprecipitation method is large, the sludge treatment is caused not only by the chemical cost but also by the amount of generated sludge. Since the cost is high, it is desirable to sufficiently reduce the concentration of treated water fluorine in the first-stage calcium fluoride method and to reduce the amount of aluminum salt added in the second-stage hydroxide coprecipitation method.

  However, since the fluorine concentration in the first-stage treated water is unstable in the conventional two-stage treatment method, the treatment by the second-stage hydroxide coprecipitation method is an aluminum that can cope with the maximum fluctuation value. Since the addition amount of the salt was set, the addition amount of the aluminum salt could not be reduced.

  As a result of examining the cause of the variation in the fluorine concentration of the first-stage treated water, the present inventors have found that the main cause is the aluminum salt in the raw water.

  That is, in the treatment process after the two-stage treatment of fluorine-containing water, a filtration process is usually provided after the precipitation process, but the wastewater discharged by backwashing of the filtration process is sent to the raw water tank. And treated with raw water. The sludge dehydrated filtrate generated here is also fed to the raw water tank and treated together with the raw water. These backwash wastewater and sludge dehydrated filtrate naturally contain an aluminum salt derived from the sulfate band added in this step. And by supplying these to a raw | natural water tank, about several to several dozen mg / L of aluminum salt will coexist in the raw | natural water used for the process by a calcium fluoride method.

  The aluminum salt has an ability to adsorb fluorine, but an aluminum salt mixed at a low concentration of several to several tens mg / L in raw water deteriorates the treatment efficiency in the fluorine treatment by the calcium fluoride method. As a result, the fluorine concentration of the treated water obtained by the treatment by the first-stage calcium fluoride method was unstable.

For _{example, HF200mg / L, H 2 SO} 4 250mg / L, the synthetic wastewater 1 of CaCl ₂ 300mg / L (as Ca), HF50mg / L, H 2 SO 4 250mg / L, ( as Ca) CaCl ₂ 300mg / L As shown in FIG. 2 which shows the result of adjusting the pH to various pH values by adding Ca (OH) ₂ to the synthetic waste water 2 in FIG. The fluorine concentration is not affected by the treatment pH. For this reason, in the conventional calcium fluoride method, treatment in a neutral region of pH 6 to 8 is generally performed in consideration of drainage standards. However, as described in Patent Document 1, the pH is alkaline. May be performed.

On the other hand, synthetic waste water A of HF 50 mg / L, H ₂ SO ₄ 200 mg / L, CaCl ₂ 250 mg / L (as Ca), HF 100 mg / L, H ₂ SO ₄ 200 mg / L, CaCl ₂ 250 mg / L ( 0 to 6000 mg / L of commercially available liquid sulfuric acid band for synthetic waste water B (as Ca) and synthetic waste water C of HF 200 mg / L, H ₂ SO ₄ 200 mg / L, CaCl ₂ 250 mg / L (as Ca) Then, as is clear from FIG. 3 showing the result of adjusting the pH to 6.1 to 6.3 by adding Ca (OH) ₂ , in the raw water containing the aluminum salt, the fluorine treatment characteristics Is significantly worse. However, as the amount of sulfuric acid band added is increased and the sulfuric acid band / fluorine ratio increases, the sulfuric acid band functions as a fluorine adsorbent, and the processing characteristics are restored. Since the liquid sulfuric acid band used here has an Al ₂ O ₃ content of 8% by weight, 8 × ₂ Al / Al ₂ O ₃ = 4.2 mg / L of Al by adding 100 mg / L of the liquid sulfuric acid band. Corresponds to the content.

  Therefore, the present invention solves the problem of instability of processing characteristics when treating fluorine-containing water in which an aluminum salt coexists by the calcium fluoride method, and treats such fluorine-containing water stably and efficiently. An object of the present invention is to provide a method for obtaining treated water from which fluorine has been sufficiently removed.

  In the present invention, as a source of aluminum salt coexisting in fluorine-containing water, in the two-stage treatment method, as described above, the backwash wastewater generated in the filtration step after the two-stage treatment and the generated sludge Examples include dehydrated filtrate. In addition, even in the case of the one-stage calcium fluoride method, wastewater containing aluminum salt generated in other systems (such as dehydrated filtrate generated as a result of sludge dewatering using sulfuric acid bands) is supplied to the raw water tank of the present invention. Fluorine-containing water in which an aluminum salt coexists is a subject of the present invention.

  The method for treating fluorine-containing water of the present invention (Claim 1) is a method for treating fluorine-containing water in which an aluminum salt coexists, wherein a calcium compound is added to the fluorine-containing water and the pH is adjusted to 8-10. Including a first step, a second step of adjusting the pH to 6 to 6.5 by adding an acid to the first step effluent, and a third step of solid-liquid separation of the second step effluent. Features.

  The method for treating fluorine-containing water according to claim 2 is the method according to claim 1, wherein the sludge solid-liquid separated in the third step is returned to the first step, and the calcium compound is mixed with the return sludge. And added to the fluorine-containing water.

  According to the present invention, fluorine-containing water in which an aluminum salt coexists can be stably and stably obtained by adding a calcium compound to raw water and treating it at a pH of 8 to 10 and then returning it to a discharge reference region of a pH of 6 to 6.5. It can be processed efficiently.

  The reaction mechanism by the two-stage neutralization of the present invention is estimated as follows. In the following, x> 3.

That is, when a calcium compound is added to raw water in which fluorine and an aluminum salt coexist, an aluminum fluoride complex (AlF _x ^(3-x) is generated by the reaction between aluminum and fluorine under acidic pH conditions. Cannot be removed as CaF ₂ (the following reaction formula (1)).

Under pH-neutral conditions, calcium fluoride (CaF ₂ ) precipitates react with aluminum fluoride to cause a part of CaF ₂ to colloid and decompose, so that fluorine cannot be sufficiently removed as CaF ₂ (see below). Reaction formula (2)).

On the other hand, if the pH is alkaline, the aluminum fluoride is hydrolyzed to aluminum hydroxide, so that the CaF ₂ dispersing action is lost and fluorine can be removed as CaF ₂ (see below). Reaction formula (3)).
[Acid]
Al ³⁺ + xF ⁻ → AlF _x ^(3-x) (1)
[neutral]
nCaF ₂ (precipitation) + AlF _x ^(3-x) → (CaF ₂ ) _n AlF _x ^(3-x) (dispersion to dissolution) (2)
[alkalinity]
(CaF ₂ ) _n AlF _x ^(3-x) + Ca (OH) ₂ → CaF ₂ (precipitation) + Al (OH) ₃ (precipitation) (3)

When the pH exceeds 10, Al (OH) ₃ becomes Al (OH) ₄ ⁻ and redissolves, and this is adsorbed by CaF ₂ to cause dispersion, so that the fluorine treatment characteristics deteriorate.

  Therefore, in the present invention, in the first step, the calcium compound is added to the raw water and adjusted to pH 8-10.

And the Al (OH) ₃ deposited under the weak alkaline condition of pH 8 to 10 in the first step does not become soluble aluminum fluoride again, even if the pH is adjusted to 6 to 6.5 on the basis of the discharge, and rather precipitated. by Al is (OH) ₃ acts as a flocculant CaF _2, it can be the quality of treated water to facilitate precipitation of CaF ₂ and more made more favorable.

  Therefore, in this invention, it adjusts to pH 6-6.5 in a 2nd process.

  In the present invention, it is further preferable to return the sludge obtained in the third step to the first step. At this time, the calcium compound added to the raw water is mixed with the return sludge in advance (hereinafter, the calcium compound is mixed). The modified sludge is sometimes referred to as “modified sludge.”), It is preferable to add this modified sludge to the raw water to adjust the pH to 8 to 10, and thereby the quality of the treated water by the crystallization effect of the modified sludge. In addition, the amount of sludge generated can be reduced and the dewaterability of the generated sludge can be improved.

  Embodiments of a method for treating fluorine-containing water according to the present invention will be described below in detail with reference to the drawings.

  1 (a) and 1 (b) are system diagrams showing an embodiment of the method for treating fluorine-containing water according to the present invention.

  In FIG. 1 (a), raw water from the raw water tank 1 (fluorine-containing water in which aluminum salt coexists) is introduced into the reaction tank 2, and the calcium compound is added in the reaction tank 2 to adjust the pH to 8-10. Thus, after the insoluble salt of calcium fluoride is generated by the reaction of fluorine and calcium (first step), it is introduced into the pH adjusting tank 3 without solid-liquid separation, and an acid is added to the pH 6 ~ Adjust to 6.5 (second step). The effluent from the pH adjusting tank 3 is introduced into a coagulating tank 4 and a polymer coagulant is added for coagulation treatment, followed by solid-liquid separation in a precipitation tank 5 (third step).

Although there is no restriction | limiting in particular as a calcium compound added to raw | natural water in the reaction tank 2, Usually, slaked lime (Ca (OH) ₂ ) is used. However, it is not limited to slaked lime at all, and calcium chloride, calcium carbonate, or the like may be used. However, when these calcium compounds are used, an alkali for pH adjustment is additionally used. These calcium compounds may be used individually by 1 type, and may use 2 or more types together.

  When the adjusted pH value in the reaction tank 2 is less than 8, sufficient fluorine insolubilization treatment cannot be performed due to the formation of the aluminum fluoride complex as described above, and even if the adjusted pH value exceeds 10, the aluminum hydroxide can be redissolved. The processing characteristics are also deteriorated. The adjusted pH value in the reaction tank 2 is particularly preferably 8.5 to 9.5.

  As the acid added in the pH adjusting tank 3, a mineral acid such as sulfuric acid or hydrochloric acid, preferably sulfuric acid is used. The adjusted pH value in the pH adjusting tank 3 is set to pH 6 to 6.5 suitable for discharge.

  As the polymer flocculant to be adjusted in the flocculation tank 4, one or more of polyacrylamide partial hydrolyzate, sodium polyacrylate, polyvinylamidine and the like can be used. Although it varies depending on the water quality and the polymer flocculant used, it is usually about 0.1 to 5 mg / L.

  For the solid-liquid separation of the aggregating treatment liquid, a membrane separation device or the like can be used in addition to the precipitation tank 5.

  In FIG. 1B, a sludge reforming tank 6 is further provided in the apparatus of FIG. 1A, and a part of the sludge separated into solid and liquid in the sedimentation tank 5 is introduced into the sludge reforming tank 6 as a return sludge. In this sludge reforming tank 6, the calcium compound to be added to the raw water is added to and mixed with the returned sludge to reform the sludge, and this modified sludge is added to the reaction tank 2 to adjust the raw water to a pH of 8 to 10, preferably It is added to 8.5 to 9.5.

  In this way, by returning a part of the separated sludge to the raw water side, the seed crystal effect of the returned sludge can improve the sludge crystallinity, lower the water content, and improve the sedimentation. As shown in FIG. 1B, the return sludge is mixed with a calcium compound such as slaked lime, and the mixture is added to the raw water, thereby further improving the quality of the treated water and reducing the amount of sludge generated.

In addition, if the amount of returned sludge in this case is too small, the above-mentioned effect by performing sludge return cannot be sufficiently obtained, and if it is too large, the load on the settling tank increases, so that the amount of returned sludge is newly generated. It is preferably about 30 to 50 times the amount of sludge. Here, the newly generated sludge is the amount of sludge generated in the reaction tank 2. For example, if the amount of CaF ₂ generated in the reaction tank 2 is 100 mg / L, the SS of the reaction tank 2 is about 3000 to 5000 mg. It is preferable to return the sludge so as to be / L.

  The treated water obtained by the calcium fluoride method treatment by such a two-stage neutralization method is preferably further subjected to a treatment by a hydroxide coprecipitation method and discharged after the fluorine concentration is further reduced.

  Hereinafter, the present invention will be described in more detail with reference to experimental examples, examples and comparative examples.

[Experimental example]
Experimental example 1
First, the influence of pH in the treatment of fluorine-containing water in which an aluminum salt coexists was examined.

The treated water obtained by adding Ca (OH) ₂ to the water quality synthetic waste water I and II shown in Table 1 and adjusting the pH to 5.5 to 10.5, followed by solid-liquid separation. The fluorine concentration of each was examined and the results are shown in Table 2.

  According to Table 2, the effect of pH is specific, and the treatment property of fluorine is poor around pH 7, which is conventionally considered to be appropriate pH, and the optimum pH is narrow, 8 to 10, preferably 8.5 to 9.5. It became clear that it was in range. However, since treated water having a pH of 8.6 or more cannot be discharged as it is, it is necessary to adjust the pH to satisfy the discharge standard.

Therefore, the pH of the synthetic waste water I is adjusted to 6 with H ₂ SO ₄ or NaOH without solid-liquid separation so that the calcium fluoride method optimum pH value of 6 to 6.5, which is a conventional experience value, is obtained. A two-stage neutralization method of 1 to 6.3 was performed.

That is, after the first step of adding Ca (OH) ₂ to the synthetic waste water I to adjust the pH to the pH shown in Table 3, H ₂ SO ₄ or NaOH is added to adjust the pH to 6.1 to 6.3. After performing the two steps, solid-liquid separation was performed, and the fluorine concentration of the obtained treated water is shown in Table 3.

From the above results, even if the treated water treated at the optimum pH of 8 to 10 is neutralized to pH 6.1 to 6.3 as it is, the quality of the treated water is not deteriorated but rather improved, and the two-stage neutralization method according to the present invention is effective. It was confirmed that. That is, as described above, the Al (OH) ₃ deposited under the weak alkaline conditions in the first step does not become soluble aluminum fluoride again even if the pH is adjusted to 6.1 to 6.3 on the basis of the discharge. by precipitated Al (OH) ₃ acts as a flocculant CaF _2, it can be the quality of treated water to facilitate precipitation of CaF ₂ and more made more favorable.

[Examples and Comparative Examples]
In the following examples and comparative examples, the apparatus shown in FIGS. 1A and 1B was used. The capacity of each tank of this apparatus was as follows, and the raw water treatment amount was 2 L / hr.
Raw water tank: 100L
Reaction tank: 0.8L
pH adjustment tank: 0.8L
Coagulation tank: 0.8L
Settling tank: 10L
Sludge reforming tank: 0.4L

Examples 1-5
pH: 2.2, fluorine: 280 mg / L, aluminum ion: 6.5 mg / L, ammoniacal nitrogen: 57 mg / L, sulfate ion: 29 mg / L, nitrate nitrogen: 58 mg / L / L added water is used as raw water, and according to the method shown in FIG. 1A, Ca (OH) ₂ is added to the raw water in the reaction tank 2 to adjust to the pH shown in Table 4, and then in the pH adjusting tank 3 H ₂ SO ₄ was added to adjust the pH to 6.0 to 6.5, and then 2 mg / L of an anionic polymer flocculant (“PA331” manufactured by Kurita Kogyo Co., Ltd.) was added in the agglomeration tank 4 and then precipitated. Solid-liquid separation was performed in the tank 5.

  In this treatment, the fluorine concentration in the effluent of the reaction tank 2 and the fluorine concentration in the effluent of the pH adjustment tank 3 were measured. After removing SS by filtering with 5A, it investigated by analyzing and the result was shown in Table 4.

Examples 6-10
Under the pH conditions of Example 3, 200 to 1000 ml / hr of sludge in the sedimentation tank 5 is fed to the sludge reforming tank 6 so that the sludge return ratio (R) shown in Table 4 is obtained as shown in FIG. Then, the treatment was performed in the same manner except that the modified sludge mixed with Ca (OH) ₂ was added to the reaction tank 2, and the fluorine concentration of each part was similarly examined. The results are shown in Table 4.

  In any case, the generated SS amount in the reaction tank 2 is 600 mg / L, and the sludge return ratio indicates the ratio of the returned sludge amount to the generated SS 600 mg / L. The return ratio (R) in the table is expressed as follows.

Comparative Examples 1-4
In Example 1, the adjusted pH value in the reaction tank 2 was set to the value shown in Table 1, and the pH adjustment tank 3 was treated in the same manner except that the pH adjustment tank 3 was not subjected to pH adjustment, and thus the pH adjustment tank. The effluent concentration of No. 3 was examined, and the results are shown in Table 4.

From Table 4, the following is clear.
In the conventional one-step neutralization treatment at pH 5.8 to 7.5, as shown in the results of Comparative Examples 1 to 4, the treated water fluorine concentration is 30 mg / L or more, whereas Examples 1 to As shown in the results of 5, the treatment at pH 8 to 10, particularly 8.5 to 9.5 can reduce the fluorine concentration to about 12 mg / L, and then to pH 6 to 6.5. As a result, the fluorine concentration of the treated water can be remarkably reduced to about 9 mg / L.

  In addition, as shown in the results of Example 3 and Examples 6 to 10, the treated water fluorine concentration is further improved by returning the sludge.

  In this case, the sludge concentration is 100 g / L, and when the sludge is not returned, the sludge concentration is 20 to 40 g / L. Compared with this, the sludge concentration is much higher, and the amount of sludge generated is greatly reduced. It was confirmed that

  Such a method for treating fluorine-containing water of the present invention includes fluorine-containing wastewater discharged from a silicon wafer manufacturing process in semiconductor component manufacturing, pickling wastewater discharged from a stainless steel plate manufacturing process, aluminum surface treatment wastewater, and hydrofluoric acid manufacturing. It is effective for the treatment of various fluorine-containing water such as wastewater, fertilizer manufacturing wastewater, and waste incineration wastewater. In particular, in the treatment by the two-stage treatment method in which the treatment by the hydroxide co-precipitation method as the advanced treatment is performed after the calcium fluoride method, use of an aluminum salt required for the treatment by the latter-stage hydroxide co-precipitation method The drastic reduction of the amount can reduce the processing cost and the amount of sludge generation, which is extremely advantageous industrially.

It is a systematic diagram which shows embodiment of the processing method of the fluorine-containing water of this invention. It is a graph which shows the relationship between the process pH value of fluorine-containing water which does not contain aluminum salt, and a process water fluorine concentration. It is a graph which shows the relationship between the sulfuric acid band addition amount of the fluorine-containing water which added the sulfuric acid band, and the treated water fluorine concentration.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Raw water tank 2 Reaction tank 3 pH adjustment tank 4 Coagulation tank 5 Precipitation tank 6 Sludge reforming tank

Claims (2)

In a method for treating fluorine-containing water in which an aluminum salt coexists,
A first step of adding a calcium compound to the fluorine-containing water and adjusting the pH to 8 to 10;
A second step of adjusting the pH to 6 to 6.5 by adding an acid to the first step effluent;
And a third step of solid-liquid separation of the second step effluent.   The method of returning the sludge solid-liquid separated in the third step to the first step according to claim 1, wherein the calcium compound is mixed with the return sludge and added to the fluorine-containing water. And a method for treating fluorine-containing water.

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