JP2012125723A - Apparatus for refining waste liquid of nitrohydrofluoric acid and method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus for efficiently removing hydrofluosilicic acid contained in the waste liquid of a nitrohydrofluoric acid without adding an ion that becomes an impurity such as alkali to reduce the amount of hydrofluoric acid and nitric acid to be disposed of, and efficiently recovering the refined liquid of the nitrohydrofluoric acid whose hydrofluosilicic acid concentration is low, and hydrofluoric acid and nitric acid concentrations are high, and a method therefor.SOLUTION: First and second cation exchange membranes C1 and C2 are disposed between a cathode 2 and an anode 3, first and second monovalence selective anion exchange membranes AS1 and AS2 are disposed between these cation exchange membranes, a desalination chamber 4 is formed between these monovalence selective anion exchange membranes, a concentrated liquid chamber 5 is formed between the first cation exchange membrane C1 and the first monovalence selective anion exchange membrane AS1, and a refined liquid chamber 6 is formed between the second monovalence selective anion exchange membrane AS2 and the second cation exchange membrane C2. The waste liquid of the nitrohydrofluoric acid containing the hydrofluosilicic acid is introduced into the desalination chamber 4, an electrolysis solution is transferred from the desalination chamber 4 to the concentrated liquid chamber 5 for electrodialytic treatment, and a refined liquid is recovered from the refined liquid chamber 6.

Description

  The present invention relates to an apparatus and method for purifying hydrofluoric acid waste liquid containing silicic acid, and more particularly to an apparatus and method for removing silicic hydrofluoric acid from hydrofluoric acid waste liquid and recovering purified hydrofluoric acid liquid having a high concentration of hydrofluoric acid.

In a process of surface processing and surface cleaning of a silicon wafer such as in the solar cell manufacturing field, a hydrofluoric acid waste solution containing silicic acid is discharged. For example, in a surface processing process (texture process) of a crystalline silicon solar cell, when a hydrofluoric acid solution, which is a mixed acid of hydrofluoric acid and nitric acid, is used as a processing solution, the surface of the silicon wafer is etched and silicofluoric acid (silicohydrofluoric acid, H ₂ SiF ₆ ) is by-produced. Since the etching ability of the hydrofluoric acid solution decreases with the generation of these by-products, the processing solution is discarded and replaced when the silicic acid concentration becomes high.

  The waste hydrofluoric acid waste solution contains silicic acid and has high concentrations of hydrofluoric acid and nitric acid. Therefore, both nitrogen load and fluorine load are very high, and industrial waste treatment is difficult. In addition, since the concentration of hydrofluoric acid and nitric acid is high, discarding this hydrofluoric acid waste liquid will increase the amount of fluoronitric acid used as the chemical used, and the amount of chemicals purchased will be enormous. It is desired to collect and reuse.

Conventionally, hydrofluoric acid solution is used for metal surface treatment of steel plates and the like, and hydrofluoric acid waste solution containing hydrofluoric acid and nitric acid has been generated, and an electrodialyzer is used to recover the hydrofluoric acid solution from this hydrofluoric acid waste solution. It was. For example, in Patent Document 1 (Japanese Patent Laid-Open No. 9-10557), after deoxidizing by diffusion dialysis, an alkali metal aqueous solution such as KOH is added and neutralized, and then the metal precipitate is filtered, and the filtrate (KF Alternatively, a method for recovering KNO ₃ ) with an electrodialyzer equipped with a cation exchange membrane, an anion exchange membrane and a bipolar membrane is described.

Patent Document 2 (Japanese Patent Laid-Open No. 2009-39672) discloses electrodialysis (desalting in which a plurality of pairs of cation exchange membranes C and monovalent selective cation exchange membranes CS are arranged between the pair of anion exchange membranes A from the anode side. In the deoxidation tank; AC-CS-A), the metal ions (Fe ²⁺ , Cr ²⁺ , Ni ²⁺ ) and the inorganic acid (F ⁻ , NO ₃ ⁻ ) were separated in the desalting chamber (AC). Subsequently, a method is described in which sulfuric acid is added to the cation concentrate, the sulfuric acid-added concentrated waste liquid is placed in (C-CS), and only H ⁺ is passed through the recovery acid chamber (CS-A) to obtain hydrofluoric acid. Yes.

  However, these hydrofluoric acid waste liquids disclosed in Patent Documents 1 and 2 are generated from metal surface treatment, dissolve impurities such as metal oxides and metal salts, and metal such as metal ions, chloride ions and sulfate ions. This method removes these metal salt impurities from fluoric acid nitric acid waste liquid containing salt impurities using a cation or anion permeable membrane, and recovers fluoric nitric acid. Silica hydrofluoric acid generated from surface processing and surface cleaning of silicon wafers is recovered. There is no disclosure about a method for recovering purified hydrofluoric acid solution having a high concentration of hydrofluoric acid by removing silicic acid from the hydrofluoric acid waste solution.

Even if the methods of Patent Documents 1 and 2 are applied to a hydrofluoric acid waste solution containing silicic acid generated from surface processing or surface cleaning of a silicon wafer, it is difficult to separate silicic acid. That is, in Patent Documents 1 and 2, hydrofluoric acid and nitrate ions are separated through a cation or anion exchange membrane. However, electrodialysis using a normal anion exchange membrane removes monovalent ions of hydrofluoric acid and nitric acid to be recovered. Since the desired silicic acid ions (SiF ₆ ²⁻ , HSiF ₆ ⁻ ) exhibit the same behavior, the hydrofluoric acid or nitrate ions cannot be separated from the silicic acid ions. Further, in Patent Documents 1 and 2, metal salts are separated by adding an additive such as an alkali. However, when such an operation is applied to a hydrofluoric acid waste solution containing silicic acid, an additional step is required to remove the alkali and the like. Impurities are added, and there is a problem that the silicon etching process is affected.

Japanese Patent Laid-Open No. 9-10557 JP 2009-39672 A

  In order to solve the conventional problems as described above, an object of the present invention is to efficiently remove silicic hydrofluoric acid contained in a waste hydrofluoric acid solution without adding ions that become impurities such as alkali, and to discard the waste hydrofluoric acid. The present invention proposes a purification apparatus and a method for purifying hydrofluoric acid waste liquid that can efficiently recover a purified hydrofluoric acid liquid with a reduced amount of acid and nitric acid, a low concentration of silicic acid and a high concentration of hydrofluoric acid and nitric acid.

The present invention is the following apparatus and method for purifying hydrofluoric acid waste.
(1) first and second cation exchange membranes disposed between a cathode and an anode;
First and second monovalent selective anion exchange membranes disposed between the first and second cation exchange membranes;
A desalting chamber formed between the first and second monovalent selective anion exchange membranes;
A concentrated liquid chamber formed between the first cation exchange membrane and the first monovalent selective anion exchange membrane;
A purification chamber formed between the second monovalent selective anion exchange membrane and the second cation exchange membrane;
A waste liquid introduction passage for introducing hydrofluoric acid waste liquid containing silicic acid into the desalting chamber;
An electrolyte transfer path for transferring the electrolyte from the desalting chamber to the concentrate chamber;
A concentrate discharge passage for removing silicic acid concentrate from the concentrate chamber;
A purified liquid extraction path for removing the purified liquid from the purified liquid chamber;
A cathode chamber formed between the cathode and the first cation exchange membrane;
An apparatus for purifying hydrofluoric acid waste liquid, comprising an anode chamber formed between an anode and a second cation exchange membrane.
(2) Monovalent selective anion exchange membranes have permselectivity between silicic acid (hexafluorosilicic acid) and monovalent anions.
Silicic acid / monovalent anion <1
The apparatus according to (1), wherein
(3) The apparatus according to (1) or (2) above, comprising a cathode chamber liquid passage for introducing a nitric acid solution into the cathode chamber and an anode chamber liquid passage for introducing the nitric acid solution into the anode chamber.
(4) A hydrofluoric acid waste liquid containing silicic acid is introduced from the waste liquid introduction path of the apparatus according to any one of (1) to (3) above to the desalting chamber,
Transfer the electrolyte from the desalination chamber to the concentrate chamber through the electrolyte transfer path,
Conduct electrodialysis between the cathode and anode,
Take out the hydrofluoric acid concentrate from the concentrate chamber through the concentrate discharge passage,
A method for purifying hydrofluoric acid waste liquid, wherein the purified liquid is taken out from the purified liquid chamber through a purified liquid take-out passage.
(5) The method according to (4) above, wherein nitric acid and / or hydrofluoric acid is added to the purified solution taken out through the purified solution take-out path to prepare a treatment solution.

  In the present invention, the hydrofluoric acid waste liquid to be purified is a hydrofluoric acid waste liquid containing silicic acid. Examples of such a hydrofluoric acid waste liquid include a hydrofluoric acid waste liquid that is discharged when a silicon hydrofluoric acid liquid is used as a processing liquid in a process of surface processing or surface cleaning of a silicon wafer such as in the field of solar cell manufacturing. Specifically, in the surface processing step (texture step) of the crystalline silicon solar cell, a silicon fluoride solution, which is a by-product by etching the surface of the silicon wafer, is obtained by using a hydrofluoric acid solution that is a mixed acid of hydrofluoric acid and nitric acid. An example of the acid-containing hydrofluoric acid waste liquid is given. As the hydrofluoric acid waste liquid to be treated in the present invention, nitric acid is 1 to 68% by weight, preferably 4 to 60% by weight, hydrofluoric acid 0.5 to 30% by weight, preferably 1 to 20% by weight and silicic hydrofluoric acid 0.8%. An aqueous solution containing 5 to 30% by weight, preferably 1 to 10% by weight is suitable.

  In the present invention, silica hydrofluoric acid is removed from such a fluorinated nitric acid waste solution, and a purified fluorinated nitric acid solution having a high concentration of fluorinated nitric acid is recovered. In the process of surface processing and surface cleaning of a silicon wafer, the hydrofluoric acid solution used as the processing liquid is a mixed acid of hydrofluoric acid and nitric acid, and the composition ratio of hydrofluoric acid and nitric acid varies depending on the target processing. The composition ratio of hydrofluoric acid waste liquid produced by these treatments also varies depending on the purpose of the treatment and the object to be treated. The components of the hydrofluoric acid waste liquid are mainly composed of nitric acid, hydrofluoric acid and silicic hydrofluoric acid. Of impurities.

Silicate hydrofluoric acid is generally represented by H ₂ SiF ₆ as silicohydrofluoric acid, but part of it dissociates in an aqueous solution, and the dissociated state is divided depending on pH. In the acid solution, it dissociates into H ₂ SiF ₆ , HSiF ₆ ⁻ , SiF ₆ ^{2− due} to chemical equilibrium, and the existence ratio of chemical species is different. When dialysis is performed using an anion exchange membrane to separate hydrofluoric acid and nitric acid from silicic hydrofluoric acid, HSiF ₆ ⁻ and SiF ₆ ²⁻ pass through along with F ^{− of} hydrofluoric acid and NO ₃ ^{− of} nitric acid. When HSiF ₆ ⁻ and SiF ₆ ²⁻ permeate, H ₂ SiF ₆ is further dissociated so as to maintain chemical equilibrium, so that it is difficult to separate them.

In the present invention, a monovalent selective anion exchange membrane is used to prevent permeation of H ₂ SiF ₆ and SiF ₆ ²⁻ , but silicic acid is dissociated by pH, and the monovalent anion is When permeated, silicic acid is dissociated so as to maintain an equilibrium state, and thus the single monovalent selective dialysis membrane has a low rejection rate of 50% or less. Usually, in such a case, a multi-stage treatment is performed. In the present invention, a pair of cation exchange membranes and a pair of monovalent selective anion exchange membranes are used in combination, so that silicohydrofluoric acid and hydrofluoric acid and The separation performance with nitric acid is improved, and purified hydrofluoric acid solution with low silicic acid concentration and high hydrofluoric acid and nitric acid concentrations is recovered.

  In the apparatus for purifying hydrofluoric acid waste liquid according to the present invention, the first and second cation exchange membranes are disposed between the cathode and the anode, and the first and second monovalent selectivity is provided between the first and second cation exchange membranes. An anion exchange membrane is disposed. A desalting chamber is formed between the first and second monovalent selective anion exchange membranes, a concentrated liquid chamber is formed between the first cation exchange membrane and the first monovalent selective anion exchange membrane, A purified liquid chamber is formed between the two monovalent selective anion exchange membranes and the second cation exchange membrane. The desalting chamber is connected to a waste liquid introduction path for introducing a hydrofluoric acid waste liquid containing silicic acid, the desalting chamber and the concentrated liquid chamber are connected to an electrolytic solution transfer path for transferring the electrolytic solution, and the concentrated liquid chamber is connected to the silicic acid hydrofluoric acid. A concentrate discharge path for taking out the acid concentrate communicates, and a purified liquid take-out path for taking out the purified liquid communicates with the purified liquid chamber. A cathode chamber is formed between the cathode and the first cation exchange membrane, and an anode chamber is formed between the anode and the second cation exchange membrane. It is preferable to communicate with the anode chamber liquid passage to be introduced.

The first and second cation exchange membranes are permeation membranes that selectively permeate cations by electrodialysis and prevent permeation of anions, and cation exchange membranes conventionally used for electrodialysis can be used. . As such a cation exchange membrane, for example, a woven fabric, nonwoven fabric, porous film or the like made of polyolefin resin, polyvinyl chloride, fluorine-based resin or the like is used as a base material, and the base material is filled with an ion exchange resin. What you have. As the ion exchange resin, a cation exchange group is introduced into a hydrocarbon-based or fluorine-based substrate resin. Examples of the cation exchange group include a sulfonic acid group, a carboxylic acid group, and a phosphonic acid group. In general, a sulfonic acid group that is a strongly acidic group is preferable. The cation exchange capacity is 0.1 to 3.0 meq / g, preferably 0.5 to 2.5 meq / g, and the film thickness is 10 to 500 μm, preferably 30 to 300 μm.

  The first and second monovalent selective anion exchange membranes are permeation membranes that selectively permeate monovalent anions by electrodialysis and prevent permeation of cations and divalent or more anions. Conventionally, electrodialysis The monovalent selective anion exchange membrane used in the above can be used. Examples of such a monovalent selective anion exchange membrane include those having a perfluoro cation exchange layer on at least one surface of an anion exchange membrane described in JP-A-8-71387, or other commercially available products. Stuff. The monovalent selective anion exchange membrane is preferably such that the permeation selectivity between silicic acid (hexafluorosilicic acid) and the monovalent anion is silicic acid / monovalent anion <1. The anion exchange capacity is 0.1 to 5 meq / g, preferably 1.5 to 4.5 meq / g, and the film thickness is 10 to 500 μm, preferably 50 to 200 μm.

The size of each permeable membrane is arbitrarily determined depending on the purpose of processing, the scale of the apparatus, etc., but in general, the width is 200 to 500 mm, the length is 300 to 1000 mm, preferably the width is 200 to 300 mm, the length is 300 to 600 mm, and the membrane. The interval is generally 0.2 to 2.0 mm, preferably 0.3 to 0.8 mm, and the current density during dialysis is 0.1 to 10 A / dm ² , preferably 1 to 5 A / dm ² . Is preferred. The apparatus for purifying hydrofluoric acid waste liquid can use one cell stack made of each of the permeable membranes, but a plurality of them can be connected in parallel or in series.

  The method for purifying hydrofluoric acid waste liquid of the present invention is a method for purifying the hydrofluoric acid waste liquid using the above-described apparatus for purifying hydrofluoric acid waste liquid. In this case, hydrofluoric acid waste liquid containing silicic acid is introduced from the waste liquid introduction path into the desalting chamber, and the electrolyte is transferred from the desalination chamber to the concentrated liquid chamber through the electrolyte transfer path and energized between the cathode and the anode. Then, electrodialysis is performed, and the concentrated hydrofluoric acid solution is taken out from the concentrated solution chamber through the concentrated solution discharge passage, and the purified solution is taken out from the purified solution chamber through the purified solution outlet passage. From the desalting chamber, the dissociated monovalent anion permeates through the second monovalent selective anion exchange membrane to the purification liquid chamber, thereby forming a purification liquid. From the concentrate chamber, the dissociated monovalent anion permeates through the first monovalent selective anion exchange membrane to the desalting chamber, thereby suppressing dissociation of silicic acid in the desalting chamber into monovalent ions.

Monovalent anions and hydrogen ions permeate through the membranes on both sides into the purified solution chamber, and a purified solution of hydrofluoric acid is formed. By circulating the purified liquid, concentration polarization can be prevented and the concentration of hydrofluoric acid can be increased. In order to obtain concentrated hydrofluoric acid, it is preferable to circulate the purified solution and concentrate it to a concentration at which F ⁻ and NO ₃ ⁻ do not back-diffuse to the desalting chamber side. The total ion concentration can be about 2 to 3M. However, since hydrofluoric acid is present in HF in a non-dissociated state at a low pH, it can be further concentrated when the nitric acid concentration is high. The purified solution taken out through the purified solution take-out path can be prepared by adding nitric acid and / or hydrofluoric acid to prepare a treatment solution having a predetermined concentration and returning it to the treatment step such as surface treatment and washing.

  It is preferable to introduce a nitric acid solution into the cathode chamber from the cathode chamber liquid passage and to circulate the nitric acid solution from the anode chamber liquid passage into the anode chamber, discharge the generated gas, and stir the solution. Deionized water, nitric acid and / or hydrofluoric acid aqueous solution or the like can be supplied to the purified liquid chamber. In the case of deionized water, it is preferable to move to full-scale operation in a state where conductivity is obtained by diffusion of ions.

In the apparatus and method for purifying a fluorinated nitric acid waste solution according to the present invention, the first and the second are provided between the cathode and the anode.
Between the first and second monovalent selective anion exchange membranes, and between the first and second monovalent selective anion exchange membranes. A desalting chamber is formed, a concentrated chamber is formed between the first cation exchange membrane and the first monovalent selective anion exchange membrane, and the second monovalent selective anion exchange membrane and the second cation exchange are formed. A purification solution chamber was formed between the membranes, and the hydrofluoric acid waste solution containing silicic acid was introduced into the desalting chamber, and the electrolyte was transferred from the desalting chamber to the concentrated solution chamber for electrodialysis. Without adding any ions, the hydrofluoric acid waste liquid efficiently removes hydrofluoric acid and reduces the amount of hydrofluoric acid and nitric acid to be discarded. The silicic hydrofluoric acid concentration is low and the hydrofluoric acid and nitric acid concentrations are high. Purified hydrofluoric acid solution is efficient There is an effect, such as can be recovered to.

It is sectional drawing which shows the refiner | purifier of the hydrofluoric acid waste liquid of embodiment.

  Hereinafter, an embodiment of the present invention will be described with reference to FIG. In FIG. 1, the apparatus 1 for purifying hydrofluoric acid waste liquid has a first cation exchange membrane C 1 and a second cation exchange membrane C 2 disposed between a cathode 2 and an anode 3. A first monovalent selective anion exchange membrane AS1 and a second monovalent selective anion exchange membrane AS2 are arranged between the first cation exchange membrane C1 and the second cation exchange membrane C2. Thus, a desalting chamber 4 is formed between the first monovalent selective anion exchange membrane AS1 and the second monovalent selective anion exchange membrane AS2. A concentrated liquid chamber 5 is formed between the first cation exchange membrane C1 and the first monovalent selective anion exchange membrane AS1, and the second monovalent selective anion exchange membrane AS2 and the second cation exchange membrane AS1. A purified liquid chamber 6 is formed with C2.

  A waste liquid introduction path L1 communicates with the desalting chamber 4 from a waste liquid storage tank 7 to introduce a hydrofluoric acid waste liquid 11 containing silicic acid. An electrolyte solution transfer path L2 communicates between the desalting chamber 4 and the concentrated solution chamber 5, and is configured to transfer the electrolyte solution from the desalting chamber 4 to the concentrated solution chamber 5. A concentrated liquid discharge path L3 communicates with the concentrated liquid chamber 5, and the silicic acid concentrated liquid is taken out of the system. The purified liquid chamber 6 is connected to the purified liquid take-out path L4, and the purified liquid is taken out through the valve V1 and returned to the processing step. The branch path L5 communicates with the purified liquid storage tank 8 from the front side of the valve V1 of the purified liquid extraction path L4, the purified liquid circulation path L6 communicates with the purified liquid chamber 6 from the purified liquid storage tank 8, and the purified liquid in the purified liquid storage tank 8 12 is circulated by a pump P. L <b> 7 is a replenishment water channel and communicates to introduce deionized water into the purified liquid storage tank 8.

  A cathode chamber 9 is formed between the cathode 2 and the first cation exchange membrane C1, and a cathode chamber liquid introduction path L8 for introducing the cathode chamber liquid, a cathode chamber liquid circulation path L9 for circulating the cathode chamber liquid, and hydrogen gas. The discharge path L10 is in communication. An anode chamber 10 is formed between the anode 3 and the second cation exchange membrane C2, and an anode chamber liquid introduction path L11 for introducing the anode chamber liquid and an anode chamber liquid circulation path L12 for circulating the anode chamber liquid. And the oxygen gas discharge path L13 communicate with each other. A nitric acid aqueous solution is used for each of the cathode chamber solution and the anode chamber solution.

  The method for purifying hydrofluoric acid waste liquid by the purification apparatus 1 described above introduces the hydrofluoric acid waste liquid 11 containing silicic acid from the waste liquid storage tank 7 to the desalting chamber 4 through the waste liquid introduction path L1, and from the desalting chamber 4 to the electrolytic solution transfer path. The electrolytic solution is transferred to the concentrate chamber 5 through L2, and electrodialysis is performed by energizing between the cathode 2 and the anode 3. At this time, the silicic acid concentrated liquid is taken out from the concentrated liquid chamber 5 through the concentrated liquid discharge path L3. The purified liquid in the purified liquid chamber 6 is taken out from the purified liquid take-out path L4 through the valve V1. A part of the purified liquid enters and is stored in the purified liquid storage tank 8 from the branch path L5, and is circulated from the purified liquid circulation path L6 to the purified liquid chamber 6 by the pump P. Deionized water is introduced into the purified liquid storage tank 8 as make-up water from the make-up water channel L7.

By electrodialysis, dissociated monovalent anions such as F ⁻ , NO ₃ ⁻ and HSiF ₆ ^{− in} the hydrofluoric acid waste liquid in the desalting chamber 4 permeate into the purified liquid chamber 6 through the second monovalent selective anion exchange membrane AS2. On the other hand, H ⁺ permeates from the anode chamber 10 to the purified solution chamber 6 through the second cation exchange membrane C2. As a result, a purified solution is formed in the purified solution chamber 6 and is taken out from the purified solution take-out path L4. Further, H ⁺ in the desalting chamber 4 permeates through the first monovalent selective anion exchange membrane AS1 to the concentrate chamber 5. The electrolytic solution containing the monovalent ions remaining in the desalting chamber 4 and the divalent or non-permeating non-dissociating or non-dissociating silicic acid is transferred from the desalting chamber 4 to the concentrated solution chamber 5 through the electrolytic solution transfer path L2. The purified solution taken out through the purified solution take-out path L4 is added with nitric acid and / or hydrofluoric acid to prepare a treatment solution having a predetermined concentration, and is returned to the treatment step such as surface treatment and washing.

In the concentrate chamber 5, the dissociated monovalent anion in the electrolytic solution permeates to the desalting chamber 4 through the first monovalent selective anion exchange membrane AS 1, and H ⁺ passes through the first cation exchange membrane C 1 to the cathode chamber 9. To penetrate. As a result, silicic acid in the electrolytic solution is concentrated in the concentrated liquid chamber 5 to generate a silicic acid concentrated liquid, which is taken out of the system from the concentrated liquid discharge path L3.

Silicic acid in the silicic acid waste liquid is dissociated into divalent SiF ₆ ²⁻ and monovalent HSiF ₆ ⁻ in addition to H ₂ SiF ₆ which does not dissociate due to chemical equilibrium. Among these, monovalent HSiF ₆ ⁻ permeates through the first monovalent selective anion exchange membrane AS1 and the second monovalent selective anion exchange membrane AS2 to the desalting chamber 4 and the purified liquid chamber 6, but does not dissociate. ₂ SiF ₆ and divalent SiF ₆ ²⁻ do not pass through and remain as they are. The fact that the ionic composition of silicic acid is in an equilibrium relationship means that if monovalent HSiF ₆ ⁻ is lost due to permeation, it is balanced in the direction in which H ₂ SiF ₆ and divalent SiF ₆ ²⁻ dissociate to compensate for this. Means that the dissociation and permeation of HSiF ₆ ⁻ are repeated. It is presumed that the reason for the low blocking rate of 50% or less when permeating with one monovalent selective dialysis membrane is due to such a cause.

On the other hand, since the monovalent HSiF ₆ ⁻ permeates through the first monovalent selective anion exchange membrane AS1 and the second monovalent selective anion exchange membrane AS2 in the purification apparatus 1 described above, monovalent HSiF ₆ ^- even lost by transmission through the second monovalent selective anion-exchange membrane AS2, HSiF from concentrate chamber 5 through the first monovalent selective anion-exchange membrane AS1 monovalent ₆ ^{Since −} permeates into the desalting chamber 4, there is no significant change in the concentration of monovalent HSiF ₆ ^{— in} the desalting chamber 4. For this reason, dissociation of silicic acid in the desalting chamber 4 into monovalent ions is suppressed, and permeation of silicic acid into the purified liquid chamber 6 is minimized.

As for other ions, when F ⁻ and NO ₃ ^{− in} the hydrofluoric acid waste liquid in the desalting chamber 4 permeate into the purified liquid chamber 6 and the F ⁻ and NO ₃ ⁻ concentration in the electrolytic solution decreases, the conductivity, Although the permeability decreases in terms of dialysis force and the like, since F ⁻ and NO ₃ ⁻ permeate from the concentrate chamber 5 through the first monovalent selective anion exchange membrane AS1 to the desalting chamber 4, There is no significant change in the concentration of monovalent F ⁻ and NO ₃ ^{− in} the desalting chamber 4, the dialysis properties are maintained high, and the separation performance can be maintained high.

In the cathode chamber 9, hydrogen gas is generated by electrolysis of water, and OH ⁻ generated at this time is combined with H ⁺ that permeates the first cation exchange membrane C1 to generate water. A nitric acid solution is introduced into the cathode chamber 9 from the cathode chamber solution introduction path L8 as a cathode chamber solution, and the cathode chamber solution is circulated through the cathode chamber solution circulation path L9 to stir the cathode chamber solution and push away the generated hydrogen gas. To discharge from the hydrogen gas discharge path L10.

In the anode chamber 10, oxygen gas is generated by electrolysis of water, and H ⁺ generated at this time passes through the second cation exchange membrane C2 and passes through the purified liquid chamber 6. The anode chamber 10 has an anode chamber liquid introduction path L1.
1, a nitric acid solution is introduced as an anode chamber solution, and the cathode chamber solution is circulated through the anode chamber solution circulation path L12. The generated oxygen gas is discharged and the solution is stirred. The use of a nitric acid solution as the cathodic chamber solution and the anodic chamber solution is preferable because there is no damage to the electrode and film, contamination, precipitation, etc., but other solutions may be used.

  By introducing deionized water from the replenishing water channel L7 into the purified liquid storage tank 8, deionized water can be supplied to the purified liquid chamber. However, since deionized water is not electrically conductive, it is conductive by ion diffusion. It is preferable to move to full-scale operation in a state where the properties are obtained. At the start, nitric acid or a hydrofluoric acid aqueous solution or the like may be supplied, and deionized water may be supplied when the permeated ions are concentrated.

In the above method, when the hydrofluoric acid waste liquid mixed with silicic acid is passed through the desalting chamber 4, F ⁻ and NO ₃ ⁻ permeate the purified liquid chamber 6 through the second monovalent selective anion exchange membrane AS 2. Part of silicic acid (SiF ₆ ²⁻ ) is blocked. This is because, among silicic acid, H ₂ SiF ₆ , HSiF ₆ ⁻ , SiF ₆ ²⁻ and the chemical equilibrium are different in the existence ratio of chemical species, and only HSiF ₆ ⁻ permeates. Thus, in a normal anion exchange membrane, the product of the molar concentration of each ion and the molar conductivity at infinite dilution is the ion migration speed, and the transmitted ion composition is determined by the speed ratio.

In the case of the present invention, since SiF ₆ ²⁻ hardly permeates the monovalent selective anion exchange membrane, the permeated ion ratio is smaller than the waste liquid supplied by SiF ₆ ²⁻ / (F ⁻ + NO ₃ ⁻ ). Therefore, the electrolytic solution that has passed through the desalting chamber 4 is partially blocked by hydrofluoric acid, and F ⁻ and NO ₃ ⁻ move to the purified liquid chamber 6. The ratio of acid is higher than that of hydrofluoric acid. When the electrolytic solution at the outlet of the desalting chamber 4 is passed through the concentrated solution chamber 5, part of silicic acid is blocked there, and F ⁻ and NO ₃ ⁻ move to the desalting chamber 4. Becomes a solution having a higher ratio of fluorinated nitric acid than silicic acid.

In this way, since the desalination chamber 4 is supplied with a liquid having a lower silicic acid / hydrofluoric acid ratio in the supply waste liquid, F ⁻ , NO ₃ ⁻ moving to the purified liquid chamber 6 through the second monovalent selective anion exchange membrane AS 2. The ratio is also high. As a result, the purified solution obtained from the purified solution chamber 6 has a high concentration of nitric acid and / or hydrofluoric acid and a low concentration of silicic acid, and can be returned to the processing step. On the other hand, the concentrated liquid obtained from the concentrated liquid chamber 5 has a high silicic acid concentration, a low concentration of nitric acid and / or hydrofluoric acid, and a small amount of impurities, which facilitates waste liquid treatment.

  Examples of the present invention and comparative examples will be described below. In each example,% is% by weight.

[Example 1, Comparative Example 1]:
The following simulated acid waste liquid was purified using an electrodialysis apparatus (lab test mini-module) having a structure of a pair of cell stacks (cathode-C1-AS1-AS2-C2-anode) shown in FIG. In Example 1, Astom Neoceptor ACS (trademark) was used as Neoceptor CMX (trademark) manufactured by Astom as the cation exchange membranes C1 and C2, and Monovalent selective anion exchange membranes AS1 and AS2 were used. In Comparative Example 1, in place of the monovalent selective anion exchange membranes AS1 and AS2 of Example 1, a normal anion exchange membrane, Neotomta AMX (trademark) manufactured by Astom Co., was used. Tested.

Example 1, both Comparative Example 1, one side of the membrane area of the cation exchange membrane and one anion exchange membrane purification device 1 ^{5cm × 11cm- = 55cm 2 (0.55dm} 2), current density 2A / ^dm 2, Membrane spacing is 0.6 mm, HNO ₃ concentration: 5%, HF concentration: 2%, H ₂ SiF ₆ concentration; 1 L of simulated acid waste liquid having a composition of 2%, and the water flow rate LV in the desalting chamber 4 is about 0.15cm
/ Sec. And processed for 6 hours. At this time, 500 mL of 1M HNO ₃ aqueous solution was introduced into each of the cathode chamber 9 and the anode chamber 10 and circulated. Further, 100 mL of pure water was introduced into the purification chamber 6 and circulated. The test results of Example 1 and Comparative Example 1 are shown in Table 1.

From Table 1 above, in Example 1 of the present invention, the ratio of HNO ₃ and HF in the recovered purified liquid at the outlet of the purified liquid chamber 6 is higher and the ratio of silicofluoric acid (H ₂ SiF ₆ ) is lower than in Comparative Example 1. Thus, it can be seen that it is possible to recover hydrofluoric acid. Further, in Example 1, the ratio of HNO ₃ and HF in the waste concentrated liquid at the outlet of the concentrated liquid chamber 5 is lower than that in Comparative Example 1, and the ratio of silicic acid (H ₂ SiF ₆ ) is increased, so that the waste liquid can be easily treated. It turns out that it is.

  The above-mentioned recovered purified liquid can be reused by returning to the process after adjusting the concentration with a concentrated solution of hydrofluoric acid. In the silicon etching process, the bath is constructed with a very high concentration of hydrofluoric acid in consideration of the silicon etching rate at the time of the waste liquid. However, by constantly extracting the process liquid and performing the electrodialysis treatment of the present invention, the low concentration of hydrofluoric acid is used. Process control by concentration became possible. In addition, since variations in the concentration of the hydrofluoric acid process liquid are reduced, it is also possible to suppress variations in the etch rate.

  The present invention relates to an apparatus and method for purifying hydrofluoric acid waste liquid containing silicic acid discharged from a step of surface processing and surface cleaning of a silicon wafer such as in the solar cell manufacturing field, in particular, removing silicic acid from the hydrofluoric acid waste liquid, The present invention is applicable to an apparatus and a method for recovering a purified hydrofluoric acid solution having a high concentration of hydrofluoric acid.

1: Purification device for hydrofluoric acid waste liquid, 2: Cathode, 3: Anode, 4: Desalination chamber, 5: Concentrated liquid chamber, 6: Purified liquid chamber, 7: Waste liquid storage tank, 8: Purified liquid storage tank, 9: Cathode chamber 10: Anode chamber, 11: Hydrofluoric acid waste solution, 12: Purified solution, C1: First cation exchange membrane, C2:
Second cation exchange membrane, AS1: First monovalent selective anion exchange membrane, AS2: Second monovalent selective anion exchange membrane, L1: Waste liquid introduction path, L2: Electrolyte transfer path, L3: Concentrated liquid Discharge path, L4: Purified liquid extraction path, L5: Branch path, L6: Purified liquid circulation path, L7: Replenishment water path, L8: Cathode chamber liquid introduction path, L9: Cathode chamber liquid circulation path, L10: Hydrogen gas discharge path, L11: Anode chamber liquid introduction path, L12: Anode chamber liquid circulation path, L13: Oxygen gas discharge path.

Claims (5)

First and second cation exchange membranes disposed between the cathode and the anode;
First and second monovalent selective anion exchange membranes disposed between the first and second cation exchange membranes;
A desalting chamber formed between the first and second monovalent selective anion exchange membranes;
A concentrated liquid chamber formed between the first cation exchange membrane and the first monovalent selective anion exchange membrane;
A purification chamber formed between the second monovalent selective anion exchange membrane and the second cation exchange membrane;
A waste liquid introduction passage for introducing hydrofluoric acid waste liquid containing silicic acid into the desalting chamber;
An electrolyte transfer path for transferring the electrolyte from the desalting chamber to the concentrate chamber;
A concentrate discharge passage for removing silicic acid concentrate from the concentrate chamber;
A purified liquid extraction path for removing the purified liquid from the purified liquid chamber;
A cathode chamber formed between the cathode and the first cation exchange membrane;
An apparatus for purifying hydrofluoric acid waste liquid, comprising an anode chamber formed between an anode and a second cation exchange membrane. Monovalent selective anion exchange membranes have permselectivity between silicic acid (hexafluorosilicic acid) and monovalent anions.
Silicic acid / monovalent anion <1
The apparatus of claim 1.   3. The apparatus according to claim 1, further comprising a cathode chamber liquid path for introducing a nitric acid solution into the cathode chamber and an anode chamber liquid path for introducing the nitric acid solution into the anode chamber. Introducing hydrofluoric acid waste liquid containing silicic acid into the desalting chamber from the waste liquid introduction path of the apparatus according to any one of claims 1 to 3,
Transfer the electrolyte from the desalination chamber to the concentrate chamber through the electrolyte transfer path,
Conduct electrodialysis between the cathode and anode,
Take out the hydrofluoric acid concentrate from the concentrate chamber through the concentrate discharge passage,
A method for purifying hydrofluoric acid waste liquid, wherein the purified liquid is taken out from the purified liquid chamber through a purified liquid take-out passage.   The method according to claim 4, wherein the treatment liquid is prepared by adding nitric acid and / or hydrofluoric acid to the purified liquid taken out through the purified liquid take-out path.

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