GB2354516A

GB2354516A - Method for treating a fluorine-containing waste water and treating apparatus

Info

Publication number: GB2354516A
Application number: GB0101249A
Authority: GB
Inventors: Arata Toyoda
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1998-07-17
Filing date: 1999-07-14
Publication date: 2001-03-28
Also published as: KR20010071946A; CN1311759A; WO2000003952A1; GB0101249D0

Abstract

A method for treating a fluorine-containing waste water which comprises fixing a fluoride ion as potassium fluoride in a reacting tank (1), sedimenting it by using aluminum hydroxide as a coagulant aid in a sedimentation tank (3), withdrawing a part of a sediment slurry comprising potassium fluoride and aluminum hydroxide to return it to an aluminum regenerating tank (4), reacting the fluorine adsorbed on aluminum hydroxide particles with a solution containing a high concentration of calcium at a pH of 9 or lower in the aluminum regenerating tank (4), to fix the fluorine as potassium fluoride, and returning it to the reactor (1), thereby conducting circulation in a manner such that aluminum hydroxide, which has adsorptive ability for fluorine, is present at a high concentration in the whole system.

Description

PROCESS AND APPARATUS FOR THE TREATMENT OF FLUORINE- CONTAINING WASTE WATER Technical Field The present invention relates to a process for the treatment of fluorine-containing waste water, particularly a process for the treatment of fluorinecontaining waste water by fixing a large portion of fluorine in the waste water as calcium fluoride and then allowing the remaining portion of fluorine adsorb to aluminum hydroxide, thereby reducing the equipment investment and amount of chemicals required for the treatment and also reducing the amount of sludge generated upon the treatment. The present invention also pertains to an apparatus for the treatment of fluorinecontaining waste water suited for the above-described process.

Background Art While fluorine is a useful substance that is employed in a large quantity in various industrial fields including chemical industry, semiconductor manufacture and the like, it is harmful for the human body and the environment. Fluorine contained in various industrial effluents is regulated to a concentration not higher than 15 mg/l under the Japanese Water Pollution Prevention

Law. In many local governments in Japan, a more stringent, overriding standard is enforced such as limiting the concentration of fluorine in the effluent to 10 mg/l or lower or even to 5 mg/or lower. Among them, the most stringent regulation standard is as low as 0.8 mg/l or less.

In general, fluorine in waste water is conventionally removed by adding, as shown in FIG. 6, a calcium salt to the waste water in primary treating tank 10, thereby forming a sparingly soluble calcium fluoride.

The resulting calcium fluoride particles are very minute and tend to be dispersed in the treated water so that calcium fluoride is coagulated in first coagulation tank 11 by using aluminum hydroxide, which has been formed by dissolving an aluminum salt in an amount about 0.1 time the amount based on the molar concentration of the resulting calcium fluoride in the treated water and neutralizing the same, as a flocculating assistant; and then separated by sedimentation in first sedimentation tank 12. A large portion of fluorine in the waste water can be removed by the above-described procedures, but owing to the interference with the reaction for the formation of calcium fluoride by the impurities contained in the waste water and the solubility of calcium fluoride itself, the fluorine concentration can be usually reduced only to about 20 mg/l at most by this process.

With a view to satisfying the environmental standards, it is the common practice to allow fluorine remaining in the waste water to adsorb to aluminum hydroxide, which has been formed by dissolving a large amount of an aluminum salt in highly advanced treatment tank 13 and neutralizing it, coagulate the resulting aluminum hydroxide in second coagulation tank 14 and carry out sedimentation and separation in second sedimentation tank 15.

The above-described process is however accompanied with the drawback that aluminum hydroxide having calcium fluoride and fluorine adsorbed thereto is generated in a large amount as sludge. In particular, the amount of aluminum hydroxide having fluorine adsorbed thereto, which has been formed as a result of the highly advanced treatment, is markedly large. For example, the amount of calcium fluoride formed upon treating 10 m3 of waste water having a fluorine concentration of 210 mg/l to reduce its concentration to 20 mg/l is about 0.39 kg (about 5 moles), while for the treatment of 10 m3 of waste water having a fluorine concentration of 20 mg/l to reduce its concentration to 5 mg/l, even at least about 2 kg (25.6 moles) of aluminum hydroxide in terms of Al(OH3) is necessary. In practice, aluminum hydroxide in the gel form cannot be dehydrated easily and even if the water content is decreased to 70%, the water-containing weight

is about 5 kg. Such a large amount of aluminum hydroxide is disposed as sludge.

The above-described process is accompanied with another problem that it requires two sedimentation tanks, which need a vast space. For example, it is possible to carry out treatment with only one sedimentation tank, as illustrated in FIG. 7, by adding a calcium salt to fluorine-containing waste water in reaction tank 16, thereby forming calcium fluoride and at the same time dissolving a large amount of an aluminum salt in the waste water, neutralizing it and thereby forming aluminum hydroxide; and utilizing the resulting aluminum hydroxide as a flocculating assistant for sedimentation and separation of calcium fluoride and also for the adsorption treatment of fluorine. In this case, however, since the adsorption site of aluminum hydroxide is occupied by calcium fluoride, a large excess of aluminum as much as tens of times the amount based on the molar concentration of the resulting calcium fluoride is necessary for obtaining sufficient fluorine treating capacity, resulting in an increase in the amount of sludge. This is the reason why the fluorine treatment is generally effected by two stages as described above and requires two sedimentation tanks in spite of their hugeness. When the standard on the concentration of effluents is about 15 mg/l and are not so severe, such a

large amount of aluminum hydroxide is not necessary and in practice, even one-stage treatment can satisfy the standard. Particularly in the case where the impurity content in the waste water is small, it is possible to satisfy the standard by using aluminum hydroxide in an amount a little larger than the amount necessary as a flocculating assistant.

In Japanese Patent Application Laid-Open No.

154767/1994, disclosed is a technique for reducing fluorine in waste water to a sufficiently low concentration by one stage treatment without increasing both the amount of an aluminum salt and the generation amount of sludge. The technique comprises, as illustrated in FIG. 8, adding a calcium salt and an aluminum salt to fluorine-containing waste water in reaction tank 19 to effect neutralization; coagulating the resulting calcium fluoride in coagulation tank 20 with the resulting aluminum hydroxide as a flocculating assistant; causing sedimentation and separation in sedimentation tank 21; and returning a portion of the resulting sediment to the reaction tank, thus circulating the sludge to increase the concentrations of calcium fluoride and aluminum hydroxide, thereby heightening the fluorine treating capacity by the seed crystal effect of the calcium fluoride and co-sedimentation effect of the aluminum hydroxide. Since aluminum hydroxide is used

after circulation and concentration, addition of a large excess of an aluminum salt is not necessary. According to the above-described literature, aluminum is added in an amount of 0.11 to 1.1 times the amount based on the molar concentration of the resulting calcium fluoride, with 0.22 to 0.46 times being more preferred. The abovedescribed technique therefore makes it possible to reduce fluorine in waste water to a sufficiently low concentration by one stage treatment without causing a substantial increase in the amount of sludge.

The above-described technique is however accompanied with the problem that in spite of an improvement in the formation efficiency of calcium fluoride, effects available from aluminum hydroxide used through circulation are not sufficient for the adsorption of fluorine so that even if the fluorine treating capacity is ideal, the final fluorine concentration attained by the technique is limited to about 8 mg/l which corresponds to the solubility of calcium fluoride. This is because when the sludge composed of calcium fluoride and aluminum hydroxide having fluorine adsorbed thereto is returned to the reaction tank, the fluorine adsorbed to aluminum hydroxide is returned to the tank at the same time, which increases not only the concentration of the sludge but also the fluorine concentration in the reaction tank during circulation of the sludge. The

adsorption capacity of aluminum hydroxide is then too low to reduce the fluorine concentration sufficiently.

Described specifically, even if aluminum hydroxide, to which fluorine has already been adsorbed, is returned, the aluminum hydroxide lacks in the capacity of adsorbing more fluorine. Circulation brings about somewhat effects when conducted once or twice as shown in the examples of the above-described literature, but fluorine adsorbing capacity of aluminum hydroxide reaches saturation when circulation is repeated several times or more and after that, aluminum hydroxide does not exhibit any fluorine adsorption effects even when it is circulated.

Accordingly, when continuously supplied fluorinecontaining waste water must be treated to decrease the fluorine concentration to 8 mg/l or less stably, it is necessary to increase the amount of an aluminum salt to be added newly or to additionally conduct highly advanced treatment, which inevitably increases the amount of the sludge. If one-stage treatment is carried out to satisfy the effluent standard of 15 mg/l or so, the amount of aluminum hydroxide cannot be controlled easily and the amount of the aluminum salt to be added must be set much larger than the actually required amount, which has posed a problem of an increase in the amount of sludge. The reason why the amount of aluminum hydroxide is always set greater than the actually required amount is because

owing to a slight difference between the total amount of aluminum hydroxide actually required for lowering of the fluorine concentration in waste water to about 15 mg/l and the amount necessary for coagulation of calcium fluoride, aluminum oxide must always be added in a greater amount with a view to avoiding not only a large influence caused by the fluctuation in the amount of the aluminum salt but also the risk of shortage in the amount of aluminum oxide caused by a temporary increase in the formation amount of calcium fluoride owing to a change in the fluorine concentration in waste water.

Disclosure of Invention An object of the present invention is to provide a process which is capable of overcoming the abovedescribed problems, stably and constantly removing fluorine in continuously supplied high-concentration fluorine-containing waste water down to a sufficiently low concentration by one-stage not-highly advanced treatment, and drastically decreasing both the amount of chemicals necessary for the treatment and the amount of sludge generated by the treatment. Another object of the present invention is to provide a process, which can reduce the generation of sludge to the minimum amount by effectively controlling the optimum chemical amount depending on the target concentration of fluorine by the

treatment.

In a first aspect of the present invention, there is thus provided a process for the treatment of fluorinecontaining waste water which comprises a first step of acting calcium to fluorine-ion-containing waste water and fixing a large portion of fluorine in the waste water as calcium fluoride; a second step of adding to the treated water an aluminum salt in an amount, in terms of aluminum, smaller than calcium fluoride newly generated in the treated water during the first step, causing coagulation and sedimentation of the calcium fluoride with the resulting aluminum hydroxide as a flocculating assistant and forming a sedimentation slurry; and a third step of subjecting the treated water containing said sedimentation slurry to solid-liquid separation and discharging the liquid-phase supernatant and as sludge, the solid-phase sedimentation slurry; wherein calcium is caused to act on a portion of the slurry, which has been taken out from the sedimentation slurry to be discharged as sludge, at a pH not greater than 9, fluorine, which has been adsorbed to aluminum hydroxide contained in the sedimentation slurry, is fixed as calcium fluoride, the resulting sedimentation slurry is returned to said first step and a series of said steps is repeated, whereby the amount of aluminum hydroxide in the system is maintained at an amount at least necessary

for the coagulation of calcium fluoride and an amount contributing to fluorine adsorption is increased.

In a second aspect of the present invention, there is also provided a process for the treatment of fluorinecontaining waste water which comprises a first step of causing calcium to act on fluorine-containing waste water and fixing a large portion of fluorine in the waste water as calcium fluoride; a second step of adding to the treated water an aluminum salt in an amount, in terms of aluminum, smaller than calcium fluoride newly generated during the first step, causing coagulation and sedimentation of the calcium fluoride with the resulting aluminum hydroxide as a flocculating assistant and forming a sedimentation slurry; and a third step of subjecting the treated water containing said sedimentation slurry to solid-liquid separation, discharging the liquid-phase supernatant and as a sludge, the solid-phase sedimentation slurry; wherein calcium is caused to act on a portion of the sedimentation slurry, which has been taken out from the sedimentation slurry to be discharged as the sludge, at pH not greater than 9 and fluorine, which has been adsorbed to aluminum hydroxide contained in the sedimentation slurry, is fixed as calcium fluoride, the resulting sedimentation slurry is returned to said first step and a series of said steps is repeated, whereby the

amount of aluminum hydroxide in the system is maintained at an amount at least necessary for the coagulation of calcium fluoride but an increase in the amount of aluminum hydroxide contributing to fluorine adsorption is controlled by limiting an amount of the aluminum salt to be added.

In a third aspect of the present invention, there is also provided a process for the treatment of fluorinecontaining waste water which comprises a first step of causing calcium to act on fluorine-containing waste water which also contains phosphoric acid under weakly alkaline conditions and fixing a fluorine ion and phosphoric acid as calcium fluoride and calcium phosphate; a second step of adjusting the treated water to weakly acidic or neutral, adding an aluminum salt in an amount, in terms of aluminum, smaller than the total amount of calcium fluoride and calcium phosphate newly generated during the first step, causing coagulation and sedimentation of the calcium fluoride and calcium phosphate with the resulting aluminum hydroxide as a flocculating assistant and forming a sedimentation slurry; and a third step of subjecting the treated water containing said sedimentation slurry to solid-liquid separation, discharging the liquid-phase supernatant and as a sludge, the solid-phase sedimentation slurry; wherein calcium is caused to act on a portion of the

sedimentation slurry, which has been taken out from the sedimentation slurry to be discharged as the sludge, at pH not greater than 9, fluorine, which has been adsorbed to aluminum hydroxide contained in the sedimentation slurry, is fixed as calcium fluoride, the resulting sedimentation slurry is returned to said first step and a series of said steps is repeated.

In the present invention, there is also provided an apparatus for treating fluorine-containing waste water, which comprises a reaction tank for causing calcium to act on fluorine-ion-containing waste water, thereby fixing a large portion of fluorine in the waste water as calcium fluoride, a coagulation tank for causing coagulation and sedimentation of said calcium fluoride with aluminum hydroxide, which has been formed by the addition of an aluminum salt, as a flocculating assistant, and a sedimentation tank for solid-liquid separation of the resulting sedimentation slurry, which apparatus is applied to the above-described first or second process and further comprises an aluminum regeneration tank for adding a calcium salt under the condition of pH 9 or less and fixing fluorine, which has been adsorbed to aluminum hydroxide contained in said sedimentation slurry, as calcium fluoride, thereby regenerating aluminum hydroxide; means for taking out a portion of said sedimentation slurry, which has been

subjected to solid-liquid separation in said sedimentation tank, and returning the same to said aluminum regeneration tank, and means for returning calcium fluoride and aluminum hydroxide regenerated in said regeneration tank to said reaction tank; or an apparatus for treating fluorine-containing waste water which comprises a first reaction tank for causing calcium to act on fluorine-ion-containing waste water which also contains phosphoric acid under weak alkaline conditions, thereby fixing a fluorine ion and phosphoric acid as calcium fluoride and calcium phosphate, a second reaction tank for adjusting the treated water to weak acidic or neutral and adding an aluminum salt to form aluminum hydroxide, a coagulation tank for causing coagulation and sedimentation of said calcium fluoride and calcium phosphate with the resulting aluminum hydroxide as a flocculating assistant, and a sedimentation tank for solid-liquid separation of the resulting sedimentation slurry, which apparatus is applied to the above-described third process and further comprises an aluminum regeneration tank for adding a calcium salt under the condition of pH 9 or less and fixing fluorine, which has been adsorbed to aluminum hydroxide contained in said sedimentation slurry, as calcium fluoride, thereby regenerating aluminum hydroxide, means for taking out a portion of the sedimentation slurry, which has been

subjected to solid-liquid separation in said sedimentation tank, and returning it to said aluminum regeneration tank; and means for returning calcium fluoride and aluminum hydroxide regenerated in said regeneration tank to said reaction tank.

Brief Description Of Drawings FIG. 1 is a block diagram illustrating the system according to the first embodiment of the present invention.

FIG. 2 is a schematic graph of each component existing in the system for illustrating the effect of the present invention.

FIG. 3 is a block diagram illustrating the system according to the second embodiment of the present invention.

FIG. 4 is a block diagram illustrating the system according to the third embodiment of the present invention.

FIG. 5 is a schematic graph of each component existing in the system for illustrating the effect of the fourth embodiment of the present invention.

FIG. 6 is a block diagram illustrating the system according to the general fluorine-containing waste water treatment technique.

FIG. 7 is a block diagram illustrating the general

fluorine-containing waste water treatment technique in the simplified form.

FIG. 8 is a systematic layout of the prior art for overcoming the problems of the general fluorinecontaining waste water treatment technique.

Best Mode for Carrying Out the Invention According to the first treatment process of the present invention, a portion of the sedimentation slurry which has been taken out is always returned to the reaction tank after fluorine adsorbed to aluminum hydroxide contained in the sedimentation slurry is fixed as calcium fluoride by the action of calcium so that aluminum hydroxide circulating at a high concentration in the system has sufficient fluorine adsorption capacity, which makes it possible to decrease the fluorine concentration in the treated water to much lower than the concentration corresponding to the solubility of calcium fluoride. Since in this process, fluorine is fixed mainly by the formation of calcium fluoride, only a necessary amount of an aluminum salt may be added newly to cause coagulation of calcium fluoride appearing newly in the reaction tank, which results in a reduction of the sludge to the minimum amount.

According to the second treating process of the present invention, a range of the amount of the aluminum

hydroxide to be reduced can be widened and the amount of aluminum hydroxide may be controlled within the range so that the control can be carried out very easily and at the same time, a reduction in the amount of aluminum hydroxide leads to a reduction in the amount of the sludge. In addition, even if there is a temporary change in the fluorine concentration in the waste water or somewhat variation in the amount of an aluminum salt added, the slurry to be returned always has an average composition ratio because it is a portion of the slurry having been accumulated in a large amount in the sedimentation tank, and such a factor hardly affects the control of the amount of aluminum hydroxide.

The first treatment process of the present invention will hereinafter be described in detail with reference to attached drawings.

FIG. 1 is a schematic view illustrating one example of the first treatment method of the present invention.

The flow of the treatment system is as follows. In reaction tank 1 maintained at neutral pH, calcium is caused to act on continuously-introduced waste water containing fluorine at a high concentration to fix fluorine ions in the waste water as calcium fluoride and at the same time, an aluminum salt is added and neutralized to form aluminum hydroxide as a flocculating assistant of calcium fluoride. A flocculant is added in

coagulation tank 2 to cause coagulation of solid components, followed by solid-liquid separation of the sediment in sedimentation tank 3. In sedimentation tank 3, a sufficient amount of this sediment is accumulated in advance.

Then, the sediment is partially taken out from the accumulated sediment. A portion of the sediment thus taken out is discharged out of the system as sludge, while the remaining portion is returned to reaction tank 1 through aluminum regeneration tank 4 maintained at pH 3 to 9. In aluminum regeneration tank 4, a calcium salt is charged. The calcium salt flows into reaction tank 1 and acts on fluorine in the waste water to form calcium fluoride. To reaction tank 1, an aluminum salt is added newly. Through repetition of the above-described cycle, the sediment composed of calcium fluoride and aluminum hydroxide is circulated in the system.

In the above process, the amount of calcium salt to be charged in aluminum regeneration tank 4 is set so that the concentration of calcium flowing into reaction tank 1 becomes at least the chemical equivalent, preferably at least twice the chemical equivalent for the formation of calcium fluoride from fluorine in the waste water. The amount of an aluminum salt to be added to reaction tank 1 is the minimum necessary amount which permits aluminum hydroxide formed in the coagulation tank to act as a

flocculating assistant of calcium fluoride and which is, in terms of aluminum, less than the amount of calcium fluoride formed newly by each cycle. Preferably, the aluminum salt is added in an amount within a range of from 1 to 30 mol%, in terms of aluminum, relative to newly formed calcium fluoride. The amount of the sedimentation slurry to be discharged is set so that the amount of calcium fluoride formed newly at each cycle and the amount of calcium fluoride contained in the sediment to be discharged as a sludge become equal each other.

Moreover, the amount of the aluminum salt to be added is set equal to the amount of aluminum hydroxide contained in the sediment to be discharged as sludge, each in terms of aluminum.

The introduction amount of the fluorine-containing waste water and amounts of the sedimentation slurry containing calcium fluoride and aluminum hydroxide to be returned and to be discharged are adjusted to a constant ratio, with the proviso that the introduction amount of the fluorine-containing waste water is greater than the amount of the sedimentation slurry to be returned.

Even after the circulation of the sedimentation slurry is repeated, the solid-liquid interface of sedimentation tank 3 is able to have a constant level by setting the ratio of the introduction amount of the fluorine-containing waste water and amounts of the

sedimentation slurry to be returned and to be discharged, thereby keeping the charge and discharge balance in the system.

As technique for maintaining the charge and discharge balance, the amount of the sedimentation slurry to be discharged may be set after the introduction amount of the fluorine-containing waste water and the amount of the sedimentation slurry to be returned are fixed. When attention is paid to the solid-liquid interface level in sedimentation tank 3, it is possible to adjust the amount of the sedimentation slurry to be discharged so as to keep the solid-liquid interface level falling within a predetermined range upon repetition of the circulation of the sedimentation slurry. More specifically, by operating means for monitoring the solid-liquid interface level, for example, an ordinarily employed level sensor, and a mechanism for controlling the amount of the sedimentation slurry to be taken out so as to give the solid-liquid interface level within a predetermined range, for example, a not illustrated pump or valve, the above-described balance can be maintained.

Although there is no particular limitation imposed on the volume of each tank and it can be adjusted as needed depending on the design, it is possible, for example, to make the volume of aluminum regeneration tank 4 to one-tenth or less of that of reaction tank 1 and

sedimentation tank 3 to at least 5 times the volume of reaction tank 1. Coagulation tank 2 can be adjusted to half of reaction tank 1. The amount of the sediment to be accumulated in advance in sedimentation tank 3 is set at least 10 times the amount of the sediment formed newly at each cycle.

The behavior in the treating system of FIG. 1 will next be described with reference thereto.

In reaction tank 1, a large portion of fluorine ions in the waste water is fixed as calcium fluoride. Since the reaction tank 1 is maintained neutral, the aluminum salt dissolved therein is neutralized and forms aluminum hydroxide. It acts as a flocculating assistant of calcium fluoride and facilitates the coagulation of calcium fluoride particles, which are otherwise readily dispersible in the liquid.

Here, as described above, the aluminum salt is added only in an amount which permits aluminum hydroxide to act as a flocculating assistant of calcium fluoride, so in the case of non-circulated treatment, the fluorine adsorption effects cannot be expected from aluminum hydroxide. Because an adsorption amount of fluorine is small in proportion to a small aluminum amount and effects for reducing a fluorine concentration are hardly brought about. By the non-circulated treatment, the fluorine concentration cannot be made much lower than

about 8 mg/l, which corresponds to the solubility of calcium fluoride, but can be reduced 15 to 20 mg/l at most even under ideal conditions. By the treatment system through the circulation of the sediment as illustrated in FIG. 1, on the other hand, the calcium fluoride and aluminum hydroxide concentrations in reaction tank 1 can be heightened by circulating in the system the sedimentation slurry containing calcium fluoride and aluminum hydroxide. The aluminum hydroxide in the sediment accumulated in sedimentation tank 3 has fluorine adsorbed thereto. The amount of fluorine is not so large as an absolute amount but it almost reaches saturation from the viewpoint of the fluorine adsorption amount per aluminum. The sedimentation slurry contains, in addition to calcium fluoride and aluminum hydroxide, highly concentrated fluorine so that when a calcium salt is added at a high concentration in aluminum regeneration tank 4, fluorine adsorbed to aluminum hydroxide is then fixed as calcium fluoride and aluminum hydroxide fed back to reaction tank 1 is able to contribute to the adsorption of fluorine. As described above, the circulation of the sedimentation slurry increases the concentration of aluminum hydroxide in reaction tank 1.

The amount of fluorine to be treated is fixed so that owing to fluorine adsorption effects of the resulting aluminum hydroxide, the fluorine concentration in the

waste water can be decreased sufficiently.

FIG. 2 is a schematic graph illustrating an increase in the amount of aluminum hydroxide which can contribute to fluorine adsorption in the case of the circulation treatment with the sedimentation slurry containing calcium fluoride and aluminum hydroxide, compared with the non-circulation treatment. If the amounts of newly formed calcium fluoride and newly added aluminum (as an aluminum) and the amounts of calcium fluoride and aluminum (as aluminum hydroxide) contained in the sludge to be discharged are always equal each other at each cycle of the slurry circulation, their ratio is always constant even if the calcium fluoride and aluminum hydroxide concentrations in reaction tank 1 become high.

In addition, repetition of the slurry circulation does not change the solid-liquid interface level of sedimentation tank 3. If the introduction amount of the fluorine-containing waste water and the amounts of the sedimentation slurry to be returned and discharged have a constant ratio, the concentrations of the calcium fluoride and aluminum hydroxide in reaction tank 1 are equilibrated at fixed values and they can be set freely by adjusting each of the above-described amounts.

It is important to adjust the pH of aluminum regeneration tank 4 at 9 or less, because under strongly alkaline pH, aluminum hydroxide dissolves into an

aluminic acid ion and it reacts with a calcium ion, thereby forming stable calcium aluminate, which makes it impossible to regenerate aluminum hydroxide as a fluorine adsorbent. The fluorine adsorbing properties of aluminum hydroxide depend largely on pH so that the pH of reaction tank 1 is preferably maintained within 6 to 7 for effective treatment.

As described above, in this embodiment, it is possible to remove fluorine in the waste water down to a sufficiently low concentration without highly advanced treatment by the minimum system structure which comprises, in addition to a reaction tank, a coagulation tank and a sedimentation tank constituting a system corresponding to primary treatment, out of two-stage treatment for fluorine-containing waste water treatment system, an aluminum regeneration tank which is much smaller than the reaction tank (for example, one-tenth to one-fortieth of the volume of the reaction tank).

It should be noted that the above description may be applied to the second and third treatment processes of the present invention which can be changed as needed depending on the structural requirement.

Example 1 A description will next be made of one example of the first treatment process of the present invention with reference to the treatment flow of FIG. 1.

In this example, the volumes of reaction tank 1, coagulation tank 2, sedimentation tank 3 and aluminum regeneration tank 4 are 30 m3, 15 m3, 300 m3 and 2 m3, respectively. The fluorine-containing waste water to be

treated is introduced into reaction tank 1 at an average fluorine concentration of 200 mg/l, pH 5 and flow rate of I m3/min.

Calcium hydroxide was charged in reaction tank 1 to give the calcium concentration of 500 mg/l and aluminum sulfate was added to give the aluminum concentration of 20 mg/1. In addition, the pH of reaction tank 1 was adjusted with sulfuric acid to constantly fall within a range of 6 to 7. To coagulation tank 2, a polyacrylamide polymer was added as a flocculant to cause coagulation of calcium fluoride and aluminum hydroxide formed in reaction tank 1. The coagulated mixture of calcium fluoride and aluminum hydroxide was sedimented in sedimentation tank 3. The above-described operation was continued to accumulate 100 m3 of the sediment in sedimentation tank 3. The fluorine concentration in the supernatant of sedimentation tank 3 was 18 mg/l.

The sedimentation slurry was then taken out from sedimentation tank 3 at a flow rate of 0.2156 m3/min and a portion of it was fed back to reaction tank 1 through aluminum regeneration tank 4 at a flow rate of 0.2 m3 and the remaining portion was all discharged out of the

system as sludge. The sedimentation slurry had a water content of 97%. Calcium hydroxide was added to aluminum regeneration tank 4 to constantly give the calcium concentration of 3000 mg/l and at the same time, sulfuric acid was added to regulate the pH of aluminum regeneration tank 4 within a range of 8 to 9. To reaction tank 1, aluminum sulfate was added to give the aluminum concentration of 20 mg/l and sulfuric acid or sodium hydroxide was added so that the pH of reaction tank 1 constantly fell within a range of 6 to 7. With the above-described operation as one cycle, the sedimentation slurry was circulated in the system.

By the repetition of the circulation of the sedimentation slurry, the concentrations of calcium fluoride and aluminum hydroxide in reaction tank 1 showed a drastic increase first. The increasing rate gradually slowed down and reached equilibrium. The concentrations of the calcium fluoride and aluminum hydroxide in terms of aluminum at that time were 4535.1 mg/l and 276.7 mg, respectively. The resulting aluminum concentration includes 20 mg/l of aluminum added newly at each cycle and a large portion of it is presumed to be able to contribute to the fluorine adsorption. It was therefore possible to steadily reduce the fluorine concentration in the waste water to 5 mg/l.

The sediment to be discharged out of the system as

sludge theoretically contains calcium fluoride and aluminum hydroxide at a ratio of 4535.1 : 276.7. Judging from that the discharge flow rate of the sediment is 0.0156 m3/min and water content of the sedimentation slurry is 97%, the discharged amounts of calcium fluoride and aluminum hydroxide are 398.2 m3/min and 69.3 m3/min, respectively according to calculation. These amounts are presumed to substantially coincide with the amounts of calcium fluoride and aluminum hydroxide newly formed in reaction tank 1, indicating that the solid-liquid interface level in sedimentation tank 3 did not show a change even after repetition of the treatment cycles.

Example 2 A description will next be made of another example of the first treatment process of the present invention with reference to FIG. 3.

Fig. 3 shows that calcium hydroxide is charged in not only aluminum regeneration tank 4 but also reaction tank 1 as a calcium salt to treat strongly-acidic fluorine-containing waste water. Here, the amount of calcium hydroxide is totally the same as that in Example 1. A portion of calcium hydroxide is added to the reaction tank 1 in an amount permitting the adjustment of reaction tank 1 to neutral (pH 6 to 7) and the remaining portion is all charged in aluminum regeneration tank 4.

For example, when the treatment conditions are similar to

those of Example 1 except that fluorine-containing waste water has a pH of 2.5, calcium hydroxide is added to reaction tank 1 to give a calcium concentration of 210 mg/l and to aluminum regeneration tank 4 to give a calcium concentration of 1740 mg/l.

In this Example, the pH of the strongly-acidic fluorine-containing waste water is adjusted only with calcium hydroxide in reaction tank 1, leading to an advantage that the pH regulation by the addition of sulfuric acid or sodium hydroxide in reaction tank 1 becomes unnecessary. An acidic calcium salt, for example, calcium chloride, can adjust the pH of the weakly-alkaline fluorine-containing waste water.

Example 3 A description will next be made of the example of the third treatment process of the present invention with reference to FIG. 4.

In FIG. 4, calcium is caused to act on the waste water containing both fluorine and phosphoric acid in first reaction tank 5 maintained at pH 8 to 10, whereby calcium fluoride and calcium phosphate are formed to fix fluorine and phosphoric acid in the waste water.

Fluorine remaining in second reaction tank 6 maintained at pH 6 to 7 is subjected to adsorption treatment with aluminum hydroxide. Here, aluminum hydroxide also acts as a flocculating assistant for causing sedimentation and

separation of calcium fluoride and calcium phosphate in sedimentation tank 8. The aluminum salt is added newly in a small amount, as aluminum, only necessary for causing coagulation and sedimentation of calcium fluoride and calcium phosphate, which have been formed newly in reaction tank 1. The other treatment conditions are similar to those of Example 1, but when the phosphoric acid concentration in the waste water is high, the amount of the calcium salt to be added to aluminum regeneration tank 9 is increased correspondingly.

In this Example, two reaction tanks are installed; one is adjusted to have pH suited for the formation of calcium phosphate and the other for the fluorine adsorption treatment by the formation of aluminum hydroxide, which makes it possible to treat the waste water containing both fluorine and phosphoric acid. In addition, this process is accompanied with the advantage that a portion of the resulting calcium phosphate, which is circulated in the system together with calcium fluoride and aluminum hydroxide, can improve the treatment efficiency of fluorine because of its strong fluorine adsorbing action.

Example 4 A description will next be made of the example of the second treatment process of the present invention with reference to the attached drawing.

This example applies to the case where the target fluorine concentration in the waste water is 13 mg/l.

The system structure of the example is similar to that of Example 1 except that the amount of aluminum sulfate to be added to reaction tank 1 is 9 mg/l in terms of aluminum. When treatment is conducted as in Example 1, a markedly large amount of aluminum hydroxide can contribute to the fluorine adsorption in reaction tank 1, which decreases the fluorine concentration in the waste water to 5 mg/l. As illustrated in FIG. 5, the amount of aluminum hydroxide can be drastically reduced if the fluorine concentration in the waste water is reduced to 13 mg/l. By decreasing the amount of aluminum sulfate added to reaction tank 1 from 20 mg/l to 9 mg/l, the fluorine concentration in the waste water can be reduced to 13 mg/l or less.

On the other hand, when the treatment is carried out by the conventional process adopting a non-circulated system, the theoretical amount of aluminum sulfate to be added to the reaction tank is 9 mg/l in terms of aluminum in order to adjust the fluorine concentration in the waste water to 13 mg/l. If the amount of aluminum sulfate temporarily shows a slight decrease and becomes less than 8 mg/l by some reasons, the coagulation property of calcium fluoride shows a rapid deterioration, the treated water becomes turbid and fluorine

concentration exceeds 15 mg/l. When the waste water before treatment is troubled with a large fluctuation in the fluorine concentration and it temporarily exceeds 300 mg/l, the same phenomenon occur. It is therefore necessary to set an amount of aluminum sulfate at 20 mg/l or greater, enough for covering a change in various factors.

This embodiment according to the present invention is accompanied with the advantages that it is free from the problems such as deterioration in the coagulation property of calcium fluoride or fluorine treating property owing to a temporary variation in the fluorine concentration in the waste water before treatment or fluctuations in the amount of an aluminum salt to be added and stable treatment can be carried out with the minimum amount of aluminum hydroxide.

Industrial Applicability As the first advantageous effect, the present invention makes it possible to constantly and stably remove fluorine from waste water, which contains fluorine at a high concentration, into a sufficient low concentration without highly advanced treatment by making use of the conventional equipment and chemicals necessary for primary treatment, whereby equipment including a huge sedimentation tank and chemicals, which have so far been

required for the highly advanced treatment, become unnecessary and at the same time, an enormous amount of sludge by-produced upon the highly advanced treatment does not appear.

This advantage is brought about because an amount of aluminum hydroxide necessary for the coagulation of calcium fluoride in the system is maintained and at the same time, a portion of it contributing to the fluorine adsorption is increased by circulating aluminum hydroxide, which is used in a small amount as a flocculating assistant of calcium fluoride formed in the primary treatment, at a high concentration in the system while regenerating it.

As the second advantageous effect, the amount of a pH regulator to be added to weakly acidic or weakly alkaline fluorine-containing waste water can be optimized, which makes it possible to reduce the using amount of a pH regulator such as sulfuric acid or sodium hydroxide.

This second advantage is brought about, because a portion of strongly alkaline calcium hydroxide or weakly acidic calcium chloride to be added for the formation of calcium fluoride can be used for the neutralization of the acid or base in the reaction tank.

As the third advantageous effect, from the waste water containing both fluorine and phosphoric acid, both

fluorine and phosphoric acid can be removed sufficiently and particularly, fluorine can be removed at a high efficiency, which makes it possible to reduce the amount of aluminum hydroxide also serving as a fluorine adsorbent.

This third advantage is brought about because in addition to calcium fluoride and aluminum hydroxide, calcium phosphate having high fluorine adsorption property is circulated at a high concentration in the system.

As the fourth advantageous effect, in the case where the effluent standard is not so severe as requiring highly advanced treatment, a stable treatment can be attained by the addition of aluminum hydroxide in the minimum amount without causing problems such as deterioration in the coagulation property of calcium fluoride and lowering in the treating efficiency of fluorine owing to a temporary change in the fluorine concentration of the waste water or a variation in the amount of an aluminum salt to be added, which makes it possible to largely reduce the amount of aluminum sulfate, which is otherwise set larger in consideration of the safety, thereby correspondingly reducing the amount of the sludge.

The fourth advantage is brought about because a range of the amount of aluminum hydroxide to be reduced

can be widened sufficiently by returning the aluminumhydroxide-containing slurry to the reaction tank, the amount of aluminum hydroxide can be controlled within this range and the slurry is returned after sufficient amount of it is accumulated in the sedimentation tank so that even if there is a temporary change in the fluorine concentration in the waste water or somewhat variation in the amount of an Al salt, the composition ratio of the slurry stably shows an average value and the amount of aluminum hydroxide can be controlled without being influenced by the above-described factor.

Claims

1. In a process for the treatment of fluorinecontaining waste water which comprises a first step of acting calcium to fluorine-containing waste water and fixing a large portion of fluorine in the waste water as calcium fluoride; a second step of adding to the treated water an aluminum salt in an amount, in terms of aluminum, smaller than calcium fluoride newly generated in the treated water during the first step, causing coagulation and sedimentation of the calcium fluoride with the resulting aluminum hydroxide as a flocculating assistant and forming a sedimentation slurry; and a third step of subjecting the treated water containing said sedimentation slurry to solid-liquid separation and discharging the liquid-phase supernatant and as sludge, the solid-phase sedimentation slurry; the improvement wherein calcium is caused to act on a portion of the slurry, which has been taken out from the sedimentation slurry to be discharged as sludge, at a pH not greater than 9, fluorine, which has been adsorbed to aluminum hydroxide contained in the sedimentation slurry, is fixed as calcium fluoride, the resulting sedimentation slurry is fed back to said first step and a series of said steps is repeated, whereby an amount of aluminum hydroxide in the system is maintained at an amount at least necessary for the coagulation of calcium

fluoride and an amount contributing to fluorine adsorption is increased.

2. A process according to claim 1, wherein a ratio of the introduction amount of the fluorine-containing waste water to be treated to the amount of the sedimentation slurry containing calcium fluoride and aluminum hydroxide to be returned is adjusted to a constant ratio larger than 1.

3. A process according to claim 1, wherein the amount of the sedimentation slurry to be discharged is set so that the amount of calcium fluoride, which has appeared newly in the system by the addition of the calcium salt, is equal to the amount of calcium fluoride contained in the sedimentation slurry to be discharged, and the amount of aluminum to be newly added is equal to the amount of aluminum contained in the sedimentation slurry to be discharged; and the aluminum hydroxide concentration in the system is controlled by setting said amounts, respectively.

4. The process according to claim 1, wherein the amount of the sedimentation slurry to be returned is set to be smaller than the introduction amount of the fluorine-containing waste water to be treated.

5. A process according to claim 1, wherein the adjustment of pH in said first step is affected only by the addition of a calcium salt.

6. In a process for the treatment of fluorinecontaining waste water, which comprises a first step of causing calcium to act on fluorine-containing waste water and fixing a large portion of fluorine in the waste water as calcium fluoride; a second step of adding to the treated water an aluminum salt in an amount, in terms of aluminum, smaller than calcium fluoride newly generated during the first step, causing coagulation and sedimentation of the calcium fluoride with the resulting aluminum hydroxide as a flocculating assistant and forming a sedimentation slurry; and a third step of subjecting the treated water containing said sedimentation slurry to solid-liquid separation, discharging the liquid-phase supernatant and as a sludge, the solid-phase sedimentation slurry; the improvement wherein calcium is caused to act on a portion of the sedimentation slurry, which has been taken out from the sedimentation slurry to be discharged as the sludge, at pH not greater than 9 and fluorine, which has been adsorbed to aluminum hydroxide contained in the sedimentation slurry, is fixed as calcium fluoride, the resulting sedimentation slurry is fed back to said first step and a series of said steps is repeated, whereby the amount of aluminum hydroxide in the system is maintained at an amount at least necessary for the coagulation of calcium fluoride but an increase in

the amount of aluminum hydroxide contributing to fluorine adsorption is controlled by limiting an amount of the aluminum salt to be added.

7. In a process for treating fluorine-containing waste water, which comprises a first step of causing calcium to act on fluorine-containing waste water, which also contains phosphoric acid, under weakly alkaline conditions and fixing a fluorine ion and phosphoric acid as calcium fluoride and calcium phosphate; a second step of adjusting the resulting treated water to weakly acidic or neutral, adding an aluminum salt in an amount, in terms of aluminum, smaller than the total amount of calcium fluoride and calcium phosphate newly generated in the first step, causing coagulation and sedimentation of the calcium fluoride and calcium phosphate with the resulting aluminum hydroxide as a flocculating assistant and forming a sedimentation slurry; and a third step of subjecting the treated water containing said sedimentation slurry to solid-liquid separation, discharging the liquid-phase supernatant and as a sludge, the solid-phase sedimentation slurry; the improvement wherein calcium is caused to act on a portion of the sedimentation slurry, which has been taken out from the sedimentation slurry to be discharged as the sludge, at pH not greater than 9, fluorine, which has been adsorbed to aluminum hydroxide contained in the

sedimentation slurry, is fixed as calcium fluoride, the resulting sedimentation slurry is fed back to said first step and a series of said procedures is repeated.

8. In an apparatus for the treatment of fluorinecontaining waste water, which comprises a reaction tank for causing calcium to act on fluorine-containing waste water and fixing a large portion of fluorine in the treated water as calcium fluoride, a coagulation tank for causing coagulation and sedimentation of said calcium fluoride with aluminum hydroxide, which is formed by the addition of an aluminum salt, as a flocculating assistant, and a sedimentation tank for subjecting the resulting sedimentation slurry to solid-liquid separation; the improvement wherein the treatment process as claimed in any one of claims 1 to 6 is applied to said apparatus, and said apparatus further comprises an aluminum regeneration tank for adding a calcium salt under conditions of pH not greater than 9, fixing fluorine, which has been adsorbed to aluminum hydroxide contained in said sedimentation slurry, as calcium fluoride and regenerating aluminum hydroxide, means for taking out a portion of said sedimentation slurry subjected to solid-liquid separation in said sedimentation tank and returning said portion to said aluminum regeneration tank, and means for returning

aluminum hydroxide and calcium fluoride, which have been regenerated in said regeneration tank, to said reaction tank.

9. An apparatus according to claim 8, wherein said sedimentation tank has means for monitoring a solidliquid interface level, which permits said apparatus to have a system for controlling the amount of said sedimentation slurry to be discharged to keep the solidliquid interface level within a predetermined range.

10. An apparatus according to claim 8, wherein the volume of said regeneration tank is one-tenth or less of that of the reaction tank.

11. In an apparatus for the treatment of fluorinecontaining waste water, which comprises a first reaction tank for causing calcium to act on fluorine-containing waste water which contains phosphoric acid under weakly alkaline conditions and fixing fluorine and phosphoric acid as calcium fluoride and calcium phosphate, a second reaction tank for adjusting the treated water to weakly acidic or neutral and adding an aluminum salt to form aluminum hydroxide, a coagulation tank for causing coagulation and sedimentation of said calcium fluoride and calcium phosphate with the resulting aluminum hydroxide as a flocculating assistant and forming a sedimentation slurry, and a sedimentation tank for subjecting the resulting sedimentation slurry to solid-

liquid separation; the improvement wherein the treatment process as claimed in claim 7 is applied to said apparatus, and said apparatus further comprises an aluminum regeneration tank for adding a calcium salt under conditions of pH not greater than 9, fixing fluorine, which has been adsorbed to aluminum hydroxide contained in said sedimentation slurry, as calcium fluoride and regenerating aluminum hydroxide, means for taking out, under control, a portion of said sedimentation slurry subjected to solid-liquid separation in said sedimentation tank and returning the same to said aluminum regeneration tank, and means for returning aluminum hydroxide and calcium fluoride, which have been regenerated in said regeneration tank, to said first reaction tank.

12. An apparatus according to claim 11, wherein said sedimentation tank has means for monitoring a solidliquid interface level, which permits said apparatus to have a system for controlling the amount of said sedimentation slurry to be discharged in order to keep the solid-liquid interface level within a predetermined range.

13. An apparatus according to claim 11, wherein the volume of said regeneration tank is one-tenth or less of that of the reaction tank.