JPH11239792A - Production of pure water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently remove hydrofluoric acid when a mixed acidic water containing hydrofluoric acid and mineral acids is treated with an electrically deionizing device, by adding an alkali to the mixed acidic water before the treatment until its pH value reaches a range where hydrofluoric acid is dissociated. SOLUTION: An alkaline solution is added to water to be treated, comprising a mixed acidic water of hydrofluoric acid and mineral acids, by a pH controller 1 before supplying the water to an electrically deionizing device (EDI device) 2. The water to be treated, after the pH adjustment, flows into the EDI device 2. The EDI device 2 has a desalination chamber, a concentration chamber, and a pair of electrodes. The water to be treated and concentrated water pass through the desalination and concentration chambers respectively. As the pH of the water to be treated has been adjusted within the range where hydrofluoric acid is dissociated to ions, fluorine ions are remove to the concentrated water which flows through an ion-exchange membrane. Thereby the treated water from which hydrofluoric acid has been removed can be obtained, and concentrated water in which salts have been enriched can be obtained at the same time in the concentration chamber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing pure water for cleaning used, for example, in a semiconductor device production process. And a method for efficiently removing hydrofluoric acid.

[0002]

2. Description of the Related Art In recent years, semiconductor device manufacturing factories have attempted to reduce the amount of wastewater by collecting the wastewater in order to improve the water utilization rate and protect the environment. That is, an ultrapure water production apparatus used in a semiconductor device manufacturing plant generally includes a wastewater recovery system that merges wastewater generated in a semiconductor device production process with raw water of the ultrapure water production apparatus. . The most wastewater generated from this semiconductor device manufacturing process is pH 2 containing mineral acids such as sulfuric acid and hydrofluoric acid.
~ 4 diluted acidic water. Therefore, when this diluted acidic water is combined with raw water as it is and supplied to the ultrapure water production system, hydrofluoric acid and a hard component such as calcium react to generate an insoluble salt, which precipitates at the bottom of the storage tank, and is formed at a later stage. Pumps and reverse osmosis membrane equipment. For this reason, the concentration of hydrofluoric acid in the collected wastewater must be, for example, 0.5 ppm or less.

On the other hand, an ion exchange resin has conventionally been used for producing pure water, and this ion exchange resin usually requires regeneration with a chemical. For this reason, in recent years, the use of an electro-deionized water production apparatus that combines deionization using the ion-exchange resin with electrodialysis to obtain high-purity water without the need for regeneration with a chemical agent has been advanced.

Therefore, a method has been attempted in which the diluted acidic water containing the mineral acid and hydrofluoric acid is treated by an electric deionized water producing apparatus to remove hydrofluoric acid. But in this case,
Although the mineral acid is removed, the hydrofluoric acid leaks without being removed. To solve this, for example,
A method of supplying treated water (for example, collected wastewater) to a reverse osmosis membrane device to remove mineral acids such as sulfuric acid, and then removing hydrofluoric acid by an electric deionized water production device installed at a subsequent stage. Are known.

[0005]

However, this method requires the installation of two devices, thus increasing the capital investment and increasing the running cost.

Accordingly, an object of the present invention is to treat a dilute mixed acidic water containing a mineral acid such as sulfuric acid and a hydrofluoric acid discharged from a semiconductor device manufacturing process with a substantially single electric deionized water manufacturing apparatus. The present invention provides a method for efficiently removing hydrofluoric acid.

[0007]

Under such circumstances, the present inventors have conducted intensive studies. As a result, when the pH of the dilute mixed acidic water containing mineral acid and hydrofluoric acid is, for example, pH 4 or less, the electro-deionized water production is performed. Even if this is processed by the apparatus, the mineral acid which is a strong electrolyte is dissociated, but the hydrofluoric acid which is a weak electrolyte is not dissociated into ions but leaks as it is, so that it becomes a pH region where hydrofluoric acid dissociates. It has been found that if the diluted acidic water is treated by adding an alkali up to this, the mineral acid is neutralized and the hydrofluoric acid can be easily removed, and the present invention has been completed.

That is, the present invention provides a method of treating a mixed acidic water containing hydrofluoric acid and a mineral acid with an electric deionized water producing apparatus, wherein the mixed acidic water is mixed with an alkali until the pH value at which hydrofluoric acid dissociates. It is intended to provide a method for producing pure water in which the treatment is performed after the addition.

According to the method for producing pure water of the present invention, an alkali is added until the pH reaches a pH range in which hydrofluoric acid is dissociated, and then the treatment is carried out in an electric deionized water producing apparatus. At the same time, hydrofluoric acid is dissociated into ions, so that it can be removed by the electric deionized water producing apparatus alone. Therefore, it is not necessary to install a reverse osmosis membrane device in the front part of the electric deionized water production device. Further, in a conventional wastewater recovery system in a semiconductor device manufacturing factory, high-concentration acidic wastewater is discharged, and neutralization by adding an alkali is performed to discharge the acidic wastewater. Since the water is adjusted to pH 6.5 or higher in advance, these wastewaters are neutral or weakly alkaline, and have a pH range that can be released by carbon dioxide that is naturally dissolved in a storage tank or the like. Therefore, there is an advantage that a new neutralization facility is not required.

[0010]

Next, a method for producing pure water according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram illustrating a method for producing pure water according to an embodiment of the present invention. Before being supplied to an electric deionized water producing apparatus (hereinafter also referred to as EDI apparatus) 2, the water to be treated, which is a mixed acidic water of hydrofluoric acid and a mineral acid such as sulfuric acid, is subjected to an alkaline solution by a pH adjusting apparatus 1 in advance. Is added. Thereby, sulfuric acid and the like are neutralized and the pH value becomes 6.5 or more. The treated water whose pH has been adjusted is EDI
It flows into the device 2. The EDI device 2 includes a desalination chamber filled with an ion exchanger such as an ion exchange resin and an ion exchange fiber, a concentration chamber separated from the desalination chamber via an ion exchange membrane, a desalination chamber and a concentration chamber. The chamber has a pair of electrodes for applying a voltage to the chamber. Then, the water to be treated is
In addition, the concentrated water is circulated to the concentration chamber. Since the water to be treated is adjusted to a pH range where hydrofluoric acid is dissociated into ions, fluorine ions are transferred to the concentrated water flowing through the concentrated water via the ion exchange membrane. Thereby, the treated water from which the hydrofluoric acid has been removed can be obtained, and the concentrated water in which the salts have been concentrated can be obtained in the concentration chamber. Therefore, the concentrated water (electrode water) also flows through the electrode chamber that houses the pair of electrodes. Therefore, electrode water is discharged from the electrode chamber.

The water to be treated is not particularly limited as long as it is a mixed acidic solution containing a mineral acid such as sulfuric acid and hydrofluoric acid. Examples of the water to be treated include ammonium hydroxide, sulfuric acid, nitric acid, and the like discharged from a semiconductor device manufacturing process. A diluted acidic wastewater having a pH of 2 to 4 containing hydrofluoric acid or the like can be used.

The pH adjusting device 1 is a device for adding an alkaline solution to the water to be treated, and is composed of, for example, an alkaline solution storage tank and an injection pump. Further, the pH adjusting device 1 is provided with a pH meter for detecting pH before and after the addition of the alkali of the water to be treated, and a regulator for controlling the discharge amount of the infusion pump based on the output of the pH meter. This is preferably a structure that maintains the pH. As the alkali, an alkali metal, a substance which shows alkalinity when dissolved in water such as amines can be arbitrarily selected, and examples thereof include sodium hydroxide and potassium hydroxide. Of these, sodium hydroxide is used for industrial use. It is preferable in that it is inexpensive and easily available. PH at which hydrofluoric acid dissociates by adding alkali
The value is pH 6.5 or more, preferably pH 6.5 to
9.5. If the pH exceeds 9.5, the leakage of alkali becomes large, which is not preferable.

According to the method for producing pure water in the embodiment of the present invention, the pH adjusting device 1 is used to dissociate hydrofluoric acid.
An alkaline solution is added until the H region is reached, and then treated with an electric deionized water production system. At least mineral acid is neutralized and hydrofluoric acid is dissociated into ions. Becomes Therefore, it is not necessary to install a reverse osmosis membrane device in the preceding stage of the EDI device 2.

[0015]

Next, the present invention will be described more specifically with reference to examples. Example 1 A 90-day treatment experiment was performed using an EDI device having the following specifications. The water to be treated before the addition of alkali is recovered wastewater generated in the semiconductor device manufacturing process, and is hydrofluoric acid 15 mg / L,
The water contained 50 mg / L of sulfuric acid, 1 mg / L of nitric acid, and 1 mg / L of ammonia, and had a pH of 3.0 and a conductivity of 500 μS / cm.
As the alkali, sodium hydroxide was used, and this was added to the above-mentioned recovered wastewater to obtain treated water having a pH of 6.5.
The evaluation was performed by measuring the hydrofluoric acid concentration of the treated water after 90 days. As a result, the composition of the treated water was 0.1% hydrofluoric acid.
It was 5 mg / L or less, sulfuric acid 0.1 mg / L or less, nitric acid 0.1 mg / L or less, and ammonia 0.1 mg / L or less, and most of hydrofluoric acid was removed. The conductivity of the treated water is 2μ.
It was S / cm.

(EDI device)-Treated water volume 3.0 m ³ / h, concentrated water volume 0.5 m ³ / h, electrode water volume 0.1 m ³ / h-Applied voltage: 200 V, 3.0 A-Ion exchanger used: cation Exchange resin Amberlite IR120B Anion exchange resin Amberlite IRA400 (all manufactured by Rohm and Haas) Mixing ratio of cation exchange resin and anion exchange resin 1: 2 (capacity ratio) ・ Ion exchange membrane used: cation exchange membrane CMH, anion exchange Membrane AMH (all manufactured by Tokuyama)

Comparative Example 1 The same procedure as in Example 1 was carried out except that the water to be treated was added without alkali. As a result, the concentration of hydrofluoric acid in the treated water was 14 mg / L, and most of the hydrofluoric acid in the water to be treated was leaking.

[0018]

According to the method for producing pure water of the present invention, an alkali is added until the pH reaches a pH range in which hydrofluoric acid is dissociated, and then the resultant is treated in an electric deionized water producing apparatus. At the same time, hydrofluoric acid dissociates into ions, so that it can be removed by the electric deionized water producing apparatus alone. Therefore, it is not necessary to install a reverse osmosis membrane device in the front part of the electric deionized water production device. Further, in a conventional wastewater recovery system in a semiconductor device manufacturing factory, high-concentration acidic wastewater is discharged, and neutralization by adding an alkali is performed to discharge the acidic wastewater. Since the water is adjusted to pH 6.5 or higher in advance, these wastewaters are neutral or weakly alkaline, and have a pH range that can be released by carbon dioxide that is naturally dissolved in a storage tank or the like. Therefore, there is an advantage that a new neutralization facility is not required.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a pure water production method according to an embodiment of the present invention.

[Explanation of symbols]

 1 pH adjustment device 2 Electric deionized water production device

Claims (1)

[Claims] 1. A method of treating a mixed acidic water containing hydrofluoric acid and a mineral acid with an electric deionized water producing apparatus, wherein an alkali is added to the mixed acidic water to a pH value at which hydrofluoric acid dissociates, Thereafter, the treatment is performed.