JP2000042571A - Fluorine-containing waste water treating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To fluorine make effectively removable with a packed tower filled with granular calcium carbonate by surely adjusting pH of fluorine-containing waste water. SOLUTION: PH in a storage tank 10 is measured with a pH meter 12, and an addition of a pH adjusting agent is controlled and the pH is roughly neutralized. A fluorine concentration in the treated water from the granular calcium carbonate filled tower 22 is measured, then the pH of inflow water into the packed tower filled with granular calcium carbonate 22 is adjusted. By adjusting the pH in two stages, the reaction in the granular calcium carbonate filled tower 22 is always maintained appropriately.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a fluorine-containing wastewater treatment apparatus for removing fluorine as calcium fluoride by passing fluorine-containing wastewater through a column filled with granular calcium carbonate.

[0002]

2. Description of the Related Art In a process of manufacturing electronic components such as a semiconductor integrated circuit and a liquid crystal display, hydrofluoric acid is used as an etching agent, and thus waste water contains fluorine. Although the standard for releasing fluorine is 15 mg / L or less, many municipalities have stricter additional standards, and most of the standards are 1 to 8 mg / L or less.

In the treatment of such fluorine-containing wastewater, a substance that generates calcium ions, such as calcium chloride and slaked lime, is added to the fluorine-containing wastewater, and the fluorine is crystallized as calcium fluoride, followed by solid-liquid separation. The method is fundamental. However, this method has a disadvantage that sludge having a high moisture content is generated.

Therefore, as a method for preventing the generation of such sludge, there has been proposed a method of removing fluorine by flowing fluorine-containing wastewater through a granular calcium carbonate packed tower (Japanese Patent Laid-Open No. Hei 5-201726).

[0005] The method of using the granular calcium carbonate packed tower is not different from the above-mentioned method in that fluorine is removed by forming hardly soluble calcium fluoride. It is characterized in that granular calcium carbonate is directly converted to granular calcium fluoride by passing the contained wastewater. Further, according to this method, the obtained calcium fluoride has a low water content and a high purity, so that it may be used for other uses.

Here, in the process of manufacturing a semiconductor device or the like, an acid such as hydrochloric acid or nitric acid is often used together with fluorine. In this case, the pH of the fluorine-containing wastewater becomes extremely low, and if this flows into the granular calcium carbonate packed tower as it is, the granular calcium carbonate will be dissolved.
Therefore, it is necessary to raise the pH with alkali before flowing into the granular calcium carbonate packed column. On the other hand, if the fluorine-containing wastewater contains ammonia, the pH of the wastewater
And the reaction between the particulate calcium carbonate and fluorine is not sufficiently performed. Therefore, in this case, it is necessary to add an acid to the raw water to lower the pH and increase the concentration of eluted calcium ions.

In Japanese Patent Application Laid-Open No. 7-96258, the concentration of fluorine and the concentration of acid in waste water are measured, the required amount of acid or alkali is calculated, and the obtained amount of acid or alkali (pH adjuster) is added to raw water. Injecting. Also, JP-A-10-217
In Japanese Patent Publication No. 199, the calcium ion concentration of treated water discharged from a granular calcium carbonate packed tower is measured, and the pH is adjusted by injecting a pH adjuster into raw water based on the measurement result.

[0008] By such a conventional method, the pH of the raw water flowing into the granular calcium carbonate packed tower is adjusted so that the fluorine can be effectively removed in the granular calcium carbonate packed tower.

[0009]

However, Japanese Patent Laid-Open No. 7-9 / 1995
The method proposed in Japanese Patent No. 6285 is feedforward control in which the acidity and the fluoride ion concentration of raw water are measured to adjust the amount of a pH adjuster added to raw water. Therefore, unless the measurement of the acidity and the fluoride ion concentration of the raw water is very accurate, the treatment in the granular calcium carbonate packed tower cannot be performed properly. Also, Japanese Patent Application Laid-Open
In the method described in Japanese Patent Application No. 0-57970, when the pH of raw water greatly changes, it may not be able to follow the change.
That is, since the granular calcium carbonate packed tower has a certain residence time, it takes some time before the treated water is affected.

[0010] Therefore, if the pH of the raw water fluctuates rapidly, there is a possibility that sufficient treatment cannot be performed. For example,
If the pH of the raw water suddenly drops and the pH is extremely low, the inflow of the raw water with a low pH will dissolve the calcium carbonate, and when the effect comes out to the treated water, the granular calcium carbonate in the tower will be dissolved. A significant portion may have melted. In addition, if the pH of the raw water suddenly increases, the treatment of fluorine becomes insufficient for a considerable period of time.

The present invention has been made in view of the above problems, and has as its object to provide a fluorine-containing wastewater treatment apparatus using a granular calcium carbonate packed tower capable of effectively treating fluorine-containing wastewater.

[0012]

SUMMARY OF THE INVENTION The present invention provides a fluorine-containing wastewater treatment apparatus for removing fluorine from fluorine-containing wastewater, wherein a pH adjuster is added to the fluorine-containing wastewater to adjust the pH of the fluorine-containing wastewater. pH adjusting means and the first
The flow of the fluorine-containing wastewater whose pH has been adjusted by the pH adjusting means, a granular calcium carbonate packed tower that removes fluorine from the fluorine-containing wastewater, and the pH, the calcium ion concentration or the calcium ion concentration in the treated water discharged from the granular calcium carbonate packed tower Treated water quality detecting means for detecting any one of the fluoride ion concentrations, and based on the detection result of the treated water quality detecting means, flow into the granular calcium carbonate packed tower;
A second pH adjusting means for adjusting the pH of the fluorine-containing waste water flowing into the granular calcium carbonate packed tower by further adding a pH adjuster to the fluorine-containing waste water whose pH has been adjusted by the first pH adjusting means in advance; , Is characterized by having.

As described above, according to the present invention, the first pH
The pH of the fluorine-containing wastewater is adjusted in advance by adjusting means.
Here, hydrofluoric acid is a weak acid and its dissociation constant is 25 ° C.
And 7 × 10 ⁻⁴ . For this reason, the pH in the solution containing only hydrofluoric acid can be approximately determined by the equation (1).

PH = −log [H ⁺ ] = − 0.5 (log 7 × 10 ⁻⁴ + log [HF]) = 1.6-0.5 log [HF] (1) Therefore, in the fluorine-containing wastewater By detecting the fluorine concentration of
The pH corresponding to the fluorine concentration (the pH assuming that all of the fluorine in the wastewater is in the form of hydrofluoric acid (HF)) can be determined, and the first pH adjusting means adjusts the crude pH to this pH value. By adding, acid other than hydrofluoric acid
Alkali can almost certainly be converted to the neutral salt form.

On the other hand, as described above, hydrofluoric acid is a weak acid, and the change in pH due to the change in the concentration of hydrofluoric acid is relatively small. For example, when the fluorine concentration of the wastewater is 190 mg / L to 19
000 mg / L (0.01 M (mol) to 1 M (mol))
And 100-fold change, the pH due to hydrofluoric acid is 2.6-
The pH changes only within the range of 1.6, and the change in pH is small compared to strong acids such as hydrochloric acid and nitric acid. Therefore, large fluctuations in the pH of wastewater are often caused by changes in the concentration of acids and alkalis other than hydrofluoric acid.

Furthermore, hydrofluoric acid is used in a semiconductor manufacturing process.
Used in relatively large quantities. Therefore, the fluorine concentration in the wastewater often does not change much. Therefore, without actually measuring the fluorine concentration of the fluorine-containing wastewater, for example, the pH attributed to fluorine is estimated from the average fluorine concentration of the wastewater,
PH to be adjusted by the pH adjusting means can be determined.

For example, when the fluoride ion concentration is in the order of several percent, the set value of the pH of the waste water to be adjusted is about 1.5 to 2.0, and when the fluoride ion concentration is in the order of several hundred mg / L. In this case, the pH is fixed at about 3.0 to 3.5 to adjust the pH.
Most of acids and alkalis other than hydrofluoric acid can be roughly neutralized.

In this way, the first pH adjusting means roughly neutralizes the wastewater before flowing into the granular calcium carbonate packed column, and converts most of the acids and alkalis other than hydrofluoric acid into neutral salts. Can be changed to

Next, with respect to the fluorine-containing wastewater whose pH has been adjusted by the first pH adjusting means, the pH is adjusted by the second pH adjusting means.

The pH adjustment by the second pH adjusting means is a feedback control based on the detection result of the quality of the treated water in the granular calcium carbonate packed tower. That is, the pH of the treated water from the granular calcium carbonate packed tower, calcium ion, or any one of fluorine compound ions is detected,
Based on this detection result, the pH of the fluorine-containing wastewater flowing into the granular calcium carbonate packed tower, that is, the wastewater that has been roughly neutralized in advance by the first pH adjusting means is further adjusted.

For example, when the concentration of fluorine in the treated water flowing out of the granular calcium carbonate packed column becomes high, the pH of the inflow water into the granular calcium carbonate packed column is lowered to promote the elution of calcium ions and to increase the fluorine concentration. To improve the removal rate. By performing such control, it is possible to control the removal of fluorine in the granular calcium carbonate packed column to a desired one by feedback control.

In particular, the accuracy of the pH adjustment by the second pH adjusting means for controlling the pH of the water flowing into the granular calcium carbonate packed tower by roughly adjusting the pH of the fluorine-containing wastewater by the first pH adjusting means. And the inside of the granular calcium carbonate packed tower can always be controlled to a suitable state.
Therefore, it is necessary to protect the granular calcium carbonate in the granular calcium carbonate packed tower from being exposed to an atmosphere having an extremely low pH or an extremely high pH, and to stably maintain the quality of treated water at a desired level. Can be.

Therefore, according to the present invention, even if the pH of the fluorine-containing wastewater changes greatly, the first change is not affected by this change.
Can be followed in the pH adjusting means of
By performing precise adjustment in the pH adjusting means, reliable fluorine removal can be ensured.

Further, the present invention further comprises a fluorine concentration detecting means for detecting the fluorine concentration of the fluorine-containing waste water subjected to pH adjustment by the first pH adjusting means, and the first pH adjusting means comprises a fluorine concentration detecting means. P based on the detection result of the means
H adjustment is performed.

As described above, by the first pH adjusting means,
PH according to the fluorine concentration of the incoming fluorine-containing wastewater
By performing the adjustment, as described above, it is possible to carry out the crude neutralization for surely converting acids and alkalis other than hydrofluoric acid into a neutral salt form. In particular, reliable neutralization can be performed even if the fluorine concentration in the fluorine-containing wastewater greatly changes. Thereby, the pH adjustment by the second pH adjusting means can be made more precise, and the treatment in the granular calcium carbonate packed tower can be maintained at a favorable level.

[0026]

Embodiments of the present invention (hereinafter referred to as embodiments) will be described below with reference to the drawings.

FIG. 1 shows the overall configuration of the first embodiment of the present invention. In a manufacturing process of a semiconductor device, various chemicals are used for etching and cleaning of a semiconductor, and a large amount of pure water or ultrapure water is used for cleaning. In particular, in a semiconductor manufacturing process, etching using fluorine is often performed, thereby generating fluorine-containing wastewater. Also, nitric acid is often used.
However, the nitric acid wastewater is often intermittently discharged, which greatly changes the pH of the wastewater. Also,
In some processes, ammonia is used, and if ammonia is contained in the wastewater, the pH of the wastewater increases.

In the present embodiment, nitric acid and ammonia are contained in addition to such fluorine (fluoride ion F ⁻ ).
Wastewater whose pH fluctuates greatly is used as raw water. This raw water is
It is introduced into the storage tank 10. This storage tank 10 has a pH meter 12
The pH of the wastewater in the storage tank 10 is measured by the pH meter 12. A pH adjusting agent storage tank 16 is connected to the storage tank 10 via a pH adjusting agent pump 14,
By operating the pH adjusting agent pump 14, the storage tank 1
The pH adjuster is supplied within zero. As a pH adjuster,
An alkali such as sodium hydroxide (NaOH) or an acid such as hydrochloric acid is appropriately selected and used. The measurement result of pH meter 12 is p
The pH control unit 18 controls the operation of the pH adjusting agent pump 14 based on the measurement result of the pH meter 12. Thus, the pH of the wastewater in the storage tank 10 can be adjusted to a desired value.

The storage tank 10 is connected to a granular calcium carbonate packed tower 22 via a raw water pump 20. The granular calcium carbonate packed tower 22 is filled with granular calcium carbonate. Then, the wastewater from the storage tank 10 flows in the granular calcium carbonate packed tower 22 in an upward flow. As a result, the waste water is dispersed in the granular calcium carbonate packed tower 22.
Fluoride in the waste water is replaced by the carbonic acid of the calcium carbonate and removed. Then, the treated water from which fluorine has been removed is discharged from the upper part of the granular calcium carbonate packed tower 22.

A fluorine monitor 24 for detecting the fluorine concentration of the treated water is provided, and constantly detects the fluorine concentration of the treated water. The detected value of the fluorine monitor 24 is supplied to the pH control unit 26. The pH control unit 26 supplies the pH adjusting agent stored in the pH adjusting agent storage tank 28 to the inflow water into the granular calcium carbonate packed tower 22 (the first stage p in the storage tank 10).
PH adjuster pump 30 to be supplied to the H-adjusted wastewater)
Is connected. Then, the pH control unit 26 controls the supply of the pH adjuster according to the measurement result of the fluorine monitor 24. That is, the pH adjuster is supplied to the inflow water into the granular calcium carbonate packed tower 22 so that the fluorine concentration in the treated water becomes a predetermined value.

As described above, in this embodiment, the pH meter 1
2. A first pH adjusting means including a pH adjusting agent pump 14, a pH adjusting agent storage tank 16, and a pH control unit 18, and a second pH including a pH controlling unit 26, a pH adjusting agent storage tank 28, and a pH adjusting agent pump 30. Two independent pH adjustment means
It has adjusting means.

As described above, in the present embodiment, the first-stage pH adjustment is performed by the first pH adjusting means in the storage tank 10, and most of the acids and alkalis other than hydrofluoric acid are changed to the form of a neutral salt. . Accordingly, even in a fluorine-containing wastewater in which nitric acid or ammonia is intermittently mixed and the pH greatly fluctuates, a large fluctuation in the pH of the wastewater flowing into the granular calcium carbonate packed tower 22 can be suppressed.

Next, the second pH adjusting means adjusts the pH of the water flowing into the granular calcium carbonate packed tower 22 according to the fluorine concentration of the treated water from the granular calcium carbonate packed tower 22. In the granular calcium carbonate packed tower 22,
Fluorine is removed by replacing fluoride ions in wastewater with carbonic acid. The lower the pH of calcium carbonate, the easier it is to dissolve, and the reaction with fluoride ions is promoted. Therefore, when the fluoride ion concentration in the treated water becomes higher than a predetermined value, the pH of the wastewater flowing into the granular calcium carbonate packed tower 22 is adjusted so as to lower the pH, whereby the fluorine concentration in the treated water is reduced. Can be maintained at a predetermined value. Therefore, the removal of fluorine in the granular calcium carbonate packed tower 22 can be controlled to a desired one by the second pH adjusting means.

In particular, when the first pH adjusting means is not provided, the pH of the water flowing into the granular calcium carbonate packed tower 22 may be extremely low due to mixing of nitric acid or the like. In this case, a large amount of calcium carbonate is temporarily eluted in the granular calcium carbonate packed tower 22. And since the granular calcium carbonate packed tower 22 has a predetermined residence time, when the effect of the elution of calcium carbonate comes out to the treated water, it will be after much calcium carbonate has already been dissolved. I will. Further, when the pH of the inflow water into the granular calcium carbonate packed tower 22 is high, the fluorine removal reaction becomes insufficient, and the fluorine concentration of the treated water becomes considerably high.

In this embodiment, most of the acids and alkalis other than hydrofluoric acid are converted to neutral salts in the first pH adjusting means. Therefore, the second pH adjustment can be performed with a very small amount of the pH adjuster added. Therefore, this pH adjustment can be made precise, and p
The quality of treated water fluctuates greatly due to excess or deficiency of H adjuster,
It is possible to substantially eliminate that a part of the granular calcium carbonate is dissolved and SS of the treated water is largely increased. That is, by the feedback control, the inside of the granular calcium carbonate packed tower 22 can always be controlled to a suitable state, the granular calcium carbonate in the granular calcium carbonate packed tower 22 is protected, and the quality of the treated water is stably maintained. Can be maintained.

Here, as shown in JP-A-8-217199, the fluoride ion concentration of the treated water depends on the pH and the calcium ion concentration of the treated water. Therefore,
It is also preferable to employ a pH meter or a calcium monitor instead of the fluorine monitor 24, and adjust the pH of the inflow water into the granular calcium carbonate packed tower 22 according to the pH of the treated water and the calcium ion concentration.

The set value for the first stage pH adjustment in the storage tank 10 is set to a relatively high value (pH corresponding to the hydrofluoric acid concentration).
It is also preferable to set the value slightly higher. Thereby, it is possible to prevent the pH of the wastewater flowing into the granular calcium carbonate packed tower 22 from becoming excessively low, and to surely prevent the packed granular calcium carbonate from being dissolved. Also, since the pH in the storage tank 10 becomes relatively high,
The pH meter 12 can be effectively protected from deterioration. In particular, when the wastewater contains a large amount of an alkaline agent, increasing the target pH can also save the pH adjuster.

In the above-described embodiment, the pH adjusting agent storage tanks 16 and 28 are separately provided. However, when the same pH adjusting agent is used in the first and second pH adjusting means, only one is used. Is also good. Further, the first pH adjusting means and the second pH adjusting means are completely different from each other, but they may be combined. For example, it is also preferable that the set target pH is changed in the pH control unit 18 of the first pH adjusting means according to the measurement value from the fluorine monitor 24. As a result, two target pH adjustments can be performed by the one-stage pH adjustment system.

In the first pH adjusting means, it is necessary to follow a large change in drainage, and it is preferable to increase the gain in the adjusting system. p
It is also preferable to use a relatively high concentration of the H regulator. On the other hand, in the second pH adjusting means, it is preferable to perform a precise adjustment by making the system gain relatively small. It is also preferable to use a relatively low concentration of the pH adjuster.

FIG. 2 shows another embodiment of the present invention. In this embodiment, a fluorine monitor (fluorine concentration detecting means) 32 for detecting the fluorine concentration of the fluorine-containing wastewater stored in the storage tank 10 is provided. And p
The H control unit 18 determines, based on the detection value of the fluorine monitor 32,
Calculate the pH attributed to hydrofluoric acid. Then, the pH adjustment is performed by controlling the pH adjusting agent pump 14 with the calculated pH as the target pH. Accordingly, even if the pH of the inflowing fluorine-containing wastewater changes, the acids and alkalis other than fluorine in the inflowing fluorine-containing wastewater can be roughly neutralized to neutral salts. Therefore, the pH of the inflow water into the granular calcium carbonate packed tower 22 at the next stage can be reliably adjusted to the pH attributed to hydrofluoric acid.

[0041]

EXAMPLE A treatment experiment was conducted using fluorine-containing wastewater (semiconductor factory wastewater) having a fluorine concentration of 300 to 900 mg / L, a nitric acid concentration of 30 to 1000 mg / L, and a pH of 1.5 to 3.0. An acrylic column having an inner diameter of 350 mm was packed with 100 kg of granular calcium carbonate having a particle diameter of 0.3 mm to form a granular calcium carbonate packed tower.

Then, as the first-step pH adjustment by the first pH adjusting means, 1N was added to the waste water using an automatic pH adjusting device.
Was added to adjust the pH to 2.8-3.0. The wastewater after pH adjustment is SV-upflowed to the acrylic column
Water was passed under the condition of 2 / h. In this experiment, since the fluctuation of the fluorine concentration was relatively small, the target pH of the neutralization was determined as described above without measuring the fluorine concentration of the wastewater.

As the second stage of pH adjustment by the second pH adjusting means, the concentration of fluorine in the treated water from the upper part of the column was measured, and the fluorine in the treated water from the upper part of the column was measured using another pH adjusting device. A pH adjuster (NaOH) was added to the water flowing into the column so that the concentration was 8 mg / L.
In order to prevent the fixation of the granular calcium carbonate, 6
The treated water was introduced into the column once every 0 minutes, and the granular calcium carbonate was allowed to flow for about 30 seconds. Table 1 shows the results of the continuous water passing treatment for one day.

As a comparative example, the same apparatus as in the above example was used, and only the pH adjustment in the second stage was performed without performing the pH adjustment in the first stage. Other processing conditions were exactly the same as in the example. Table 1 also shows the results of the water passing treatment for one day.

[0045]

[Table 1] As can be seen from Table 1, in the case of the comparative example in which the first-stage pH adjustment was not performed, the stability of the water quality of the treated water was poor, and when the nitric acid concentration was rapidly increased, particulate calcium carbonate was partially eluted,
S increased to a maximum of 500 mg / L or more. As in this example, by roughly neutralizing the pH of the wastewater, the SS of the treated water could be maintained at a low value, and the fluctuation range of the fluorine concentration could be suppressed to a small value.

[0046]

As described above, according to the present invention,
By providing the first and second pH adjusting means, the pH of the wastewater flowing into the granular calcium carbonate packed tower can always be made appropriate, preventing the elution of calcium carbonate,
The quality of the treated water can be stabilized.

[Brief description of the drawings]

FIG. 1 is a diagram showing an overall configuration of an embodiment.

FIG. 2 is a diagram illustrating an overall configuration of another embodiment.

[Explanation of symbols]

10 storage tank, 12 pH meter, 14,30 pH adjuster pump, 16,28 pH adjuster storage tank, 18,26 pH
Control unit, 20 raw water pump, 22 granular calcium carbonate packed tower, 24, 32 fluorine monitor.

Claims (2)

[Claims] 1. A fluorine-containing wastewater treatment device for removing fluorine from fluorine-containing wastewater, comprising: a first pH adjusting means for adding a pH adjuster to the fluorine-containing wastewater to adjust the pH of the fluorine-containing wastewater; A granular calcium carbonate packed tower for circulating the fluorine-containing wastewater whose pH has been adjusted by the pH adjusting means to remove fluorine from the fluorine-containing wastewater; and p in the treated water discharged from the granular calcium carbonate packed tower.
H, a treated water quality detecting means for detecting any one of a calcium ion concentration and a fluoride ion concentration, and based on a detection result of the treated water quality detecting means, the flow into a granular calcium carbonate packed tower, First pH
In the fluorine-containing wastewater whose pH has been adjusted by the adjusting means,
And a second pH adjusting means for adjusting the pH of the fluorine-containing wastewater flowing into the granular calcium carbonate packed tower by adding a pH adjuster. 2. The apparatus according to claim 1, further comprising: a fluorine concentration detecting means for detecting a fluorine concentration of the fluorine-containing wastewater subjected to the pH adjustment by the first pH adjusting means; Is a fluorine-containing wastewater treatment apparatus, wherein the pH is adjusted based on the detection result of the fluorine concentration detecting means.