JP5594123B2 - Alkali etching solution processing apparatus and processing method - Google Patents

Description

  The present invention relates to an alkaline etching solution processing apparatus and processing method, and more particularly to an alkaline etching solution processing apparatus and processing method suitable for forming a textured surface by etching the surface of a crystalline silicon substrate. It is.

  A crystalline silicon substrate used as a substrate for a solar cell is provided with fine pyramidal irregularities on the surface in order to increase the optical path length of incident light and improve power generation efficiency. Such a silicon substrate has, for example, an interfacial activity mainly composed of an alkaline solution such as NaOH or KOH having a concentration of 0.05 to 0.2 mol / L and caprylic acid or lauric acid having a concentration of 0.01 mol / L or more. It can be obtained by etching (texture etching) the surface of the silicon substrate using a mixed solution with an agent as an etchant (Patent Document 1). This etching solution needs to be highly alkaline (high pH) in order to increase the dissolution rate of silicon, and contains organic substances such as caprylic acid and lauric acid in order to make the silicon surface uneven. . As such an organic substance, isopropyl alcohol (IPA) or the like may be used (Patent Document 2).

The alkaline etching solution has a lower pH each time the etching is repeated, and the concentration of SiO ₂ (silica) in the etching solution and the concentration of dopants such as phosphorus (P) and boron (B) doped in silicon. Rises. Also, the concentration of organic additives such as caprylic acid is reduced. This reduces the etching rate and makes it difficult to form fine irregular surfaces. Therefore, it is necessary to replace the etching solution every certain period.

  In the etching solution processing apparatus described in Patent Document 3, a collection tank is provided for separating dopants such as phosphorus and boron dissolved in the etching solution from the etching solution by adsorption, precipitation, or electrical collection. It is reused. However, the replacement frequency cannot be reduced sufficiently, and metal salts and porous materials are brought into contact with the etching solution in the collection tank, so that impurities derived from the metal salts and porous materials are in the etching solution. There is also a risk of contamination.

  In order to solve such a problem, an etching solution containing silica and the like eluted from silicon is extracted from the etching tank and subjected to nanofiltration (NF) membrane separation treatment to remove the silica and the like in the etching solution to the etching tank. A method of circulating was proposed (Patent Document 3). According to the method of Patent Document 3, the etching solution can be used for a longer period of time than before, and the replacement frequency of the etching solution can be reduced.

  In the method of Patent Document 3, an etching solution is extracted from an etching tank, subjected to membrane separation with an NF membrane, circulated permeated water from which silica or the like has been removed is circulated to the etching tank, and a part of the concentrated water is discharged out of the system. A feed-and-bleed method is adopted in which the remaining part is returned to the etching tank. That is, in general, an NF membrane handles a concentrated solution as an application, and therefore, a feed-and-bleed method is employed to increase the amount of water supplied by increasing the amount of concentrated water to prevent concentration polarization.

  However, as a result of the study by the present inventors, it has been found that there is the following problem when an NF membrane separation treatment is performed by a feed-and-bleed method on an alkali etching solution that is a silica-containing alkali solution.

  In other words, an NF membrane with a silica removal rate exceeding 90% has a high removal rate, but it is manufactured densely, so that the amount of permeated water is remarkably small, and in order to obtain a desired amount of permeated water, a large amount of NF membrane is required. Is required. In particular, since the alkali etching solution has a high alkali concentration and high viscosity, the amount of permeated water is small.

On the other hand, when an NF membrane having a silica removal rate of about 50 to 80% is used, it has a structure with a relatively high porosity, although the removal rate is low, the amount of permeated water can be increased. The required number of NF films can be reduced.
Moreover, the amount of permeated water can be further increased by performing a multistage treatment by connecting a plurality of NF membranes having such a low removal rate in series.
That is, for example, as shown in FIG. 2, two stages of NF membrane modules 11 and 12 are provided in series, and the etchant from the etching tank 10 is introduced into the first NF membrane module 11 by the pump P through the relay tank 10A. The NF membrane separation treatment is performed, the permeated water of the first NF membrane module 11 is circulated to the etching tank 10, and the concentrated water is NF membrane separation treated by the second NF membrane module 12. The permeated water recovery rate can be increased by a feed-and-bleed two-stage process in which the permeated water is circulated through the etching tank 10 to discharge a part of the concentrated water out of the system and the remaining part is returned to the relay tank 10A.

However, in a multi-stage NF membrane separation treatment system in which concentrated water is circulated to the supply water side using the feed and bleed method, if an NF membrane with a low silica removal rate is used, the liquid circulated in the system As a result, the NF membrane permeated water returned to the etching tank cannot be obtained with a low silica concentration.
For example, when processing an etching solution having a silica concentration of 2% using an NF membrane having a silica removal rate of 50%, the silica concentration of the NF membrane permeated water is 1%. Such NF membranes are arranged in three stages in series. When connected and treated, the concentrated water in which silica is highly concentrated in the third stage NF membrane is circulated to the feed water side, so the silica concentration of the first stage NF membrane feed water is, for example, 3%, When this NF membrane feed water was subjected to membrane separation treatment with an NF membrane having a silica removal rate of 50%, the silica concentration of the permeated water was 1.5%, and the silica removal rate of the entire system was not 50% but 25% (etching solution) Since the silica concentration of NF membrane permeated water is 1.5% with respect to the silica concentration of 2%, the silica removal rate is (2-1.5) / 2 × 100 = 25%. End up.

Thus, in the NF membrane separation treatment of the etching solution, the feed-and-bleed method for circulating the concentrated water circulates the concentrated water containing high-concentration silica, and as a result, the silica removal efficiency is extremely low.
In addition, not only silica but also impurities in the etching solution such as ions derived from dopants such as heavy metals, organic substances, phosphorus, and boron are concentrated in the concentrated water. There is a problem of accumulation of these impurities in the inside and in the etching bath.

JP 2002-57139 A JP 2006-278409 A International publication WO2010 / 113379 brochure

  The present invention solves the above-described problems. In the multi-stage NF membrane separation treatment of the etchant, the silica removal efficiency is improved, the accumulation of impurities is prevented, and the permeated water having a low silica concentration is returned to the etching tank. It is an object of the present invention to provide an alkali etching solution processing apparatus and method that can be used.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have conducted silica removal performance by increasing the silica concentration in the system by circulating permeate to the membrane feed water side instead of the conventional concentrated water circulation system. It has been found that the problem of decrease in the concentration and accumulation of impurities can be solved, and the construction of a multi-stage NF membrane module for permeate circulation has been constructed to reach the present invention.
That is, the gist of the present invention is as follows.

[1] In an alkaline etching solution processing apparatus in which an alkali etching solution obtained by etching silicon is extracted from an etching tank and subjected to a membrane separation process by a membrane separation means, and the circulation apparatus is circulated to the etching tank, the membrane separation means includes two or more nanometers. The filtration membrane module is connected in series so that the concentrated water of the previous nanofiltration membrane module is introduced as membrane supply water to the second and subsequent nanofiltration membrane modules, and the permeated water of the nanofiltration membrane module A first permeated water circulating means for circulating a part of the permeated water to the etching tank; a second permeated water circulating means for circulating at least a part of the remaining permeated water to the supply water side of the first nanofiltration membrane module; A processing apparatus for an alkaline etching solution, comprising:

[2] In [1], the first permeate circulating means is a means for circulating at least a part of the permeate of the first-stage nanofiltration membrane module to the etching tank, and the second permeate The water circulating means is means for circulating at least a part of permeated water of the second and subsequent nanofiltration membrane modules to the supply water side of the first nanofiltration membrane module. Liquid processing equipment.

[3] In [1] or [2], the water concentration rate of the first-stage nanofiltration membrane module is smaller than the water concentration rate of the second-stage and subsequent nanofiltration membrane modules. Alkali etchant processing equipment.

[4] The alkaline etching solution processing apparatus according to any one of [1] to [3], wherein the nanofiltration membrane module has a silica removal rate of 50 to 80%.

[5] In a method for treating an alkaline etching solution in which an alkaline etching solution obtained by etching silicon is extracted from an etching tank and circulated through a membrane separation treatment, two or more nanofiltration membrane modules are used as the second stage of the alkaline etching solution. Membrane separation is performed by membrane separation means connected in series so that the concentrated water of the previous nanofiltration membrane module is introduced as membrane supply water into each subsequent nanofiltration membrane module, and the permeated water of the nanofiltration membrane module A method for treating an alkaline etching solution, wherein a part of the permeated water is circulated to the etching tank and at least a part of the remaining portion of the permeate is circulated to the supply water side of the first nanofiltration membrane module.

[6] In [5], at least a part of the permeated water of the first nanofiltration membrane module is circulated in the etching tank, and at least one of the permeated water of the second and subsequent nanofiltration membrane modules is circulated. A method of treating an alkaline etching solution, wherein the part is circulated to the supply water side of the first stage nanofiltration membrane module.

[7] In [5] or [6], the water concentration rate of the first-stage nanofiltration membrane module is smaller than the water concentration rate of the second-stage and subsequent nanofiltration membrane modules. Processing method of alkaline etching solution.

[8] The method for treating an alkaline etching solution according to any one of [5] to [7], wherein the nanofiltration membrane module has a silica removal rate of 50 to 80%.

  According to the present invention, the permeated water is circulated not to the NF membrane concentrated water but to the membrane supply water side, so that the silica removal performance decreases due to the increase in the silica concentration in the system due to the concentrated water circulation, and the problem of impurity accumulation. This can be reduced (claims 1 and 5).

  In the present invention, among the NF membrane modules connected in multiple stages, the permeated water of the first stage NF membrane module is circulated to the etching tank and reused as an etching solution, and the permeation of the second and subsequent NF membrane modules. It is preferable to circulate water as membrane feed water to the inlet side of the first stage NF membrane module. In this case, even if the NF membrane has a low silica removal rate, the first stage where silica is not highly concentrated. In the NF membrane module of the eye, permeated water having a low silica concentration can be obtained, and this can be circulated to the etching tank and reused effectively. In the second and subsequent NF membrane modules that further perform membrane separation treatment of the concentrated water in which the silica is concentrated in the first NF membrane module, the silica concentration of the permeated water obtained is that of the first NF membrane module. Since it becomes higher than the silica concentration of the permeated water, it is preferable that this permeated water is not circulated to the etching tank but is circulated to the inlet side of the first stage NF membrane as the membrane supply water. .

  According to the present invention, since the increase in the silica concentration of the circulating fluid in the system due to the conventional concentrated water circulation is prevented, the silica is sufficiently contained even in an NF membrane having a low silica removal rate of about 50 to 80%. The removed permeated water can be obtained, and the amount of permeated water can be increased by using an NF membrane having a low silica removal rate. As a result, the required number of NF membranes can be reduced (claims 3 and 7). .

  In the present invention, the water concentration rate of the first-stage NF membrane module is set smaller than the water concentration rate of the second-stage and subsequent NF membrane modules, and the water concentration of the first-stage NF membrane module is set. By reducing the rate, the concentration polarization on the outlet side of this NF membrane module is kept small, thereby achieving a high silica removal rate, while in the NF membrane modules in the second and subsequent stages, the water concentration rate is increased. It is preferable in terms of improving the recovery rate to reduce the amount of concentrated water that is highly concentrated and discharged out of the system (claims 4 and 8).

  As described above, according to the present invention, among the NF membrane modules connected in two or more stages, the NF membrane module on the front stage side, preferably the NF membrane module on the first stage, is low for circulating to the etching tank. An NF membrane module for obtaining a concentrated silica alkaline solution, and an NF membrane module on the rear stage, preferably the NF membrane module on the second and subsequent stages, for concentrating silica and impurities and discharging them out of the system By using each NF membrane with a low silica removal rate by reducing the required number of NF membranes, the permeated water with a low silica concentration is obtained with a high recovery rate, and this is circulated to the etching tank. Can be reused.

It is a systematic diagram which shows the processing apparatus of the alkaline etching liquid which concerns on embodiment of this invention. It is a systematic diagram which shows the conventional processing apparatus.

  Embodiments of the present invention will be described below in detail with reference to the drawings.

  FIG. 1 is a system diagram showing an alkaline etching solution processing apparatus according to an embodiment of the present invention. In this embodiment, NF membrane modules are connected in series in four stages. Of these, two NF membrane modules 1A and 1B are provided in parallel at the first stage, and one NF membrane module 2, 3, and 4 are connected in series from the second stage.

  Part of the alkaline etching solution in the etching tank 10 is extracted from the pipe 20 to the relay tank 10A, and the etching liquid in the relay tank 10A passes through the pipes 21, 21A, and 21B by the pump P to the first stage. NF membrane modules (sometimes referred to as “first NF membrane modules”) 1A and 1B and subjected to NF membrane separation treatment. The permeated water of the first NF membrane modules 1A, 1B is circulated to the etching tank 10 through the pipes 22A, 22B, 22, and the concentrated water of the first NF membrane modules 1A, 1B is supplied to the second through the pipes 23A, 23B, 23. It is introduced into the NF membrane module at the stage (sometimes referred to as “second NF membrane module”) 2 and subjected to NF membrane separation treatment. The concentrated water of the second NF membrane module 2 is introduced into the third-stage NF membrane module (sometimes referred to as “third NF membrane module”) 3 through the pipe 25 and subjected to NF membrane separation treatment, and this third NF membrane. The concentrated water of the module 3 is further introduced into the fourth-stage NF membrane module (sometimes referred to as “fourth NF membrane module”) 4 from the pipe 27 and subjected to NF membrane separation treatment. The concentrated water of the fourth NF membrane module 4 is discharged out of the system through the pipe 28. On the other hand, the permeated water of the second, third and fourth NF membrane modules 2, 3 and 4 is circulated to the relay tank 10A through the pipes 24, 26 and 29 and the pipe 30, respectively.

  The alkaline etching solution treated in the present invention usually contains 0.05 to 3 mol / L, particularly about 1 to 1.5 mol / L of NaOH and 0.01 mol / L or more, for example, 0.1 to 0. Contains 2 mol / L of organic additive. Examples of the organic additive include isopropyl alcohol as well as surfactants such as caprylic acid and lauric acid.

  A crystalline silicon wafer such as a semiconductor wafer is set in a casing and immersed in the alkaline etching solution in the etching tank 10, and a plurality of silicon wafers are textured simultaneously. By this texture treatment, silicon and dopant are eluted from silicon. The etching solution in the etching tank 10 is usually heated to about 80 to 90 ° C.

  As is well known, the NF membrane has a pore diameter between a UF (ultrafiltration) membrane and an RO (reverse osmosis) membrane and has a charge on the membrane material surface. As the NF film, a film capable of removing at least divalent or higher polyvalent ions is used, and divalent or higher polyvalent ions may be selectively removed.

When an NF film that selectively removes divalent or higher polyvalent ions is used, polyvalent ions such as phosphoric acid are removed from the etching solution. In this case, the NF membrane permeated water becomes alkaline with a pH of about 13 to 14, and contains monovalent silicate ions (for example, HSiO ₃ ⁻ ). Part of silicon is a divalent ion (for example, SiO ₃ ²⁻ ) under highly alkaline conditions of pH 13 or higher. Therefore, by setting the etching solution to pH 13 or more, SiO ₃ ²⁻ and divalent condensed silicate ions are removed from the etching solution.

  In the present invention, it is particularly preferable to use an NF membrane with a low silica removal rate that can increase the amount of permeated water, for example, an NF membrane with a silica removal rate of about 50 to 80%. Unlike the concentrated water circulation system, according to the present invention for circulating permeate, permeate having a sufficiently reduced silica concentration can be obtained and circulated to the etching tank. In addition, since the NF membrane having such a low silica removal rate has a low alkali removal rate, the alkali concentration in the permeated water circulating in the etching tank can be increased, and the alkali recovery and reuse efficiency can be increased.

Further, in the first NF membrane modules 1A and 1B that circulate the permeate to the etching tank, operation with a reduced water concentration rate reduces the concentration polarization on the outlet side of the NF membrane module and allows the transmission of a low silica concentration. On the other hand, it is preferable to obtain water. On the other hand, in the NF membrane module on the rear stage side that circulates the permeated water, it is preferable to increase the water concentration ratio to highly concentrate silica on the concentrated water side.
For this reason, in FIG. 1, the water concentration rate of the first NF membrane module 1A, 1B is 2 times or less, for example, about 1.2 to 1.8 times. The water concentration rate is preferably 2.5 times or more, for example, 2.5 to 5 times.

  In the alkaline etching solution processing apparatus shown in FIG. 1, a solution containing an alkali etching solution from the etching tank and a permeated water circulated from the second to fourth NF membrane modules 2 to 4 is supplied to the membrane. Water is introduced into the first NF membrane modules 1A and 1B. In the first NF membrane modules 1A and 1B, even if the NF membrane has a low silica removal rate, a permeated water having a sufficiently reduced silica concentration is obtained. It can be recycled to the etching tank 10 for reuse. Further, the NF membrane having a low silica removal rate can increase the alkali residual rate in the permeated water and increase the alkali recovery efficiency.

  The concentrated water of the first NF membrane modules 1A and 1B is further sequentially concentrated in the second NF membrane module 2, the third NF membrane module 3 and the fourth NF membrane module 4, and the concentrated water in which silica and other impurities are highly concentrated is out of the system. Since each permeate, that is, permeate having a relatively low concentration of silica and other impurities, is circulated to the relay tank 10A, an increase in the silica concentration of the supply water of the first NF membrane modules 1A and 1B is suppressed. In addition, accumulation of impurities in the system is also prevented.

  The concentrated water of the fourth NF membrane module 4 discharged out of the system is subjected to wastewater treatment, and examples of the treatment method include neutralization, coagulation precipitation, and crystallization. Since the concentrated water of the NF membrane module is strongly alkaline, it is necessary to neutralize it, but according to the present invention, the silicon component and the like contained in the high concentration alkaline liquid is separated by membrane treatment. Since the permeated water is returned to the etching tank, the amount of waste water is reduced compared to the case where the entire amount of the etching liquid in the etching tank is sent to the waste water treatment process when the etching liquid becomes dirty. It is possible to reduce the load of the wastewater treatment process, for example, by reducing the amount of acid required for neutralization.

  Thus, in order to discharge the concentrated water of the NF membrane module in the final stage out of the system and send it to the waste liquid treatment process, so as to keep the liquid level in the etching tank 10 constant during operation, The etching tank 10 is replenished with fresh etching solution. This fresh etching solution is the same as the unused etching solution before starting the etching.

  1 is an example of an embodiment of the present invention, and the present invention is not limited to the illustrated one.

For example, the NF membrane module need only be provided in series in two or more stages, and is not limited to the four-stage configuration as shown in FIG. 1, but may be two stages, three stages, or five stages or more. Further, it is not always necessary to provide two first-stage NF membrane modules in parallel, and one NF membrane module may be provided. Three more NF membrane modules may be provided in parallel for the NF membrane modules in the second and subsequent stages.
However, for the first-stage NF membrane module, it is preferable to provide two or more NF membrane modules in parallel in order to obtain a larger amount of permeated water. In order to carry out, it is preferable to provide one unit in multiple stages.

  Alternatively, the entire amount of permeated water of the first stage NF membrane module is not circulated to the etching tank, but a part is circulated to the relay tank (inlet side of the first stage NF membrane module) as membrane supply water. Well, circulate a part of the NF membrane module in the second and subsequent stages to the etching tank, or discharge a part of the NF membrane module in the second and subsequent stages, for example, a part of the permeated water in the final stage. Also good.

  Although not shown, a UF membrane module is provided in front of the first-stage NF membrane module, such as a polymer in which fine particles contained in an alkali etching solution, a silicon component or the like are polymerized, and other dopants. The polyion complex may be removed by the UF membrane module, thereby reducing the water flow load on the NF membrane module.

  In this case, the UF membrane of the UF membrane module preferably has a pore diameter of 2 to 100 nm and a molecular weight cut-off of about 1000 to 300,000. Further, as a material for the UF membrane, cellulose acetate, polyacrylonitrile, polysulfone, polytetrafluoroethylene, polyethersulfone, polyvinylidene fluoride, and the like are suitable.

  In addition, the piping for circulating the permeated water of the NF membrane module to the etching tank includes alkali concentration measuring means such as Na in the permeated water and concentration measuring means for measuring the concentration of organic additives such as surfactant and IPA, and these Means for adding alkali and organic additives to the permeated water according to the alkali concentration and organic additive concentration in the permeated water measured by the measuring means may be provided.

  In this case, examples of the alkali concentration measurement means include a pH meter, neutralization titration, and ultrasonic waves, and examples of the organic additive concentration measurement means include a TOC meter, IR (infrared), Raman spectroscopy, ultraviolet absorption, and visible light. For example, light absorption. Further, the alkali and organic additives added to the permeated water are the same as the alkali and organic additives in the etching solution in the etching tank.

Further, a liquid temperature adjusting means for adjusting the liquid temperature (T ₁ ) of the supply water of the first stage NF membrane module, and the liquid temperature (T ₂ ) of the permeated water circulated through the etching tank are adjusted. The liquid temperature adjusting means may include at least one liquid temperature adjusting means, and in particular, the liquid temperature adjusting means and the etching tank for adjusting the liquid temperature (T ₁ ) of the first stage NF membrane module supply water And a liquid temperature adjusting means for adjusting the liquid temperature (T ₂ ) of the permeated water circulated in the tank, and heat exchange is performed between the first stage NF membrane module supply water and the permeated water circulated to the etching tank. You may make it perform. When the liquid temperature (T ₁ ) is adjusted by such a liquid temperature adjusting means, the removal rate of divalent silicate ions can be increased by the following effects.

  The removal rate of divalent silicate ions is due to the temperature dependence of the dissociation equilibrium constant of water, and can be explained by equation (1) derived thermodynamically.

In the formula (1), K is an ion product of water, T is an absolute temperature, R is a gas constant, and Δ _r H ^θ is a standard reaction enthalpy. The subscript p on the left side indicates equilibrium under constant pressure.

  As apparent from the above equation (1), the ionic product K of water decreases with increasing temperature, so that the pH of the solution decreases and dissociation hardly occurs. That is, the amount of divalent silicate ions to be removed is reduced. Accordingly, in order to increase the removal efficiency of divalent silicate ions, treatment at a low temperature is preferable. Note that if the liquid temperature is too low, the amount of permeated water of the NF membrane decreases, so the divalent silicate ion and the amount of permeated water are in a trade-off relationship. Therefore, it is preferable to consider the liquid temperature setting and the number of NF films according to the composition of the etching liquid.

Specifically, the liquid temperature (T ₁ ) of the supply water of the first-stage NF membrane module is preferably 10 to 50 ° C., particularly preferably about 15 to 30 ° C. When a liquid having a liquid temperature (T ₁ ) higher than 50 ° C. is passed, the removal rate of divalent silicate ions may be reduced, and the membrane module may be defective, and the liquid having a temperature lower than 10 ° C. If water is passed through, the amount of permeated water of the NF membrane may be reduced.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to the following examples as long as the gist thereof is not exceeded. For convenience of explanation, a comparative example will be described first.

[Comparative Example 1]
As shown in FIG. 2, the etching solution in the etching tank 10 passes through the relay tank 10 </ b> A and is pumped through the NF membrane modules 11 and 12 at a supply water amount of 16 L / min. 12 permeated water is circulated to the etching tank 10, and a part of the concentrated water of the NF membrane module 12 (20% = 3.2 L / min with respect to the supply water of the NF membrane module 11) is discharged out of the system, and the remainder Was circulated to the relay tank 10A. The water quality of the treated etching solution (raw water) is as shown in Table 1. As the NF membranes of the NF membrane modules 11 and 12, 8-inch NF membrane “MPS-34” manufactured by KOCH (SiO ₂ removal rate: 90%, TOC removal rate: 50%) was used.

  The quality of the permeated water (mixed liquid of the permeated water of the NF membrane module 11 and the permeated water of the NF membrane module 12) circulated in the etching tank 10 in the stable operation state two hours after the start of operation and the concentrated water of the NF membrane module 12 The water quality was as shown in Table 1 (a part was circulated as feed water and the remainder was discharged outside the system).

[Example 1]
The same water quality etching solution (raw water) as in Comparative Example 1 was treated with the alkaline etching solution processing apparatus of the present invention shown in FIG.
As the NF membranes of the NF membrane modules 1A, 1B, 2, 3 and 4, 4inch NF membrane “NP-030” (SiO ₂ removal rate: 60%, TOC removal rate: 50%) manufactured by NADIR, respectively, was used. The amount of water (the total amount of water supplied to the NF membrane modules 1A and 1B) is set to 16 L / min as in Comparative Example 1, and the concentrated water from the first NF membrane modules 1A and 1B is supplied to the second NF membrane module 2 to The concentrated water of the 2NF membrane module 2 is supplied to the third NF membrane module 3 in total, the concentrated water of the third NF membrane module is supplied to the fourth NF membrane module 4, and the 4NF membrane module 4 is compared in the amount of concentrated water. The flow rate was adjusted to 3.2 L / min in the same manner as in Example 1, and the concentrated water of the fourth NF membrane module 4 was discharged out of the system. Further, the entire amount of permeated water of the first NF membrane modules 1A and 1B was circulated to the etching tank 10, and the entire amount of permeated water of the second to fourth NF membrane modules 2 to 4 was circulated to the relay tank 10A.
At this time, the water concentration rate of the first NF membrane module 1A, 1B is 1.2 times, the water concentration rate of the second NF membrane module 2 is 2.5 times, the water concentration rate of the third NF membrane module 3 is 2.2 times, The water concentration rate of the fourth NF membrane module 4 was twice.

  The quality of the permeated water (mixed liquid of the permeated water of the first NF membrane modules 1A and 1B) circulated in the etching tank 10 in the stable operation state two hours after the start of operation, the quality of the fourth NF membrane module 4 discharged out of the system Table 1 shows the quality of the concentrated water and the quality of the permeated water (the mixed liquid of the permeated water of the second to fourth NF membrane modules 2 to 4) circulated to the relay tank 10A.

  From Table 1, the following is clear.

In Comparative Example 1 in which the concentrated water is circulated in the system and in Example 1 in which the permeated water is circulated in the system, the silica concentration of the permeated water circulated in the etching tank is substantially the same. Is reduced to about 60%, and the TOC load on the etching tank is reduced. On the other hand, the Na concentration is higher in Example 1, and the alkali recovery and reuse efficiency is higher.
The amount of concentrated water discharged out of the system is the same in Comparative Example 1 and Example 1, but the concentration of Na concentrated water is lower in Example 1, and Example 1 is discharged out of the system. It can be seen that the amount of Na is reduced. That is, Example 1 has a larger amount of alkali recovered and circulated. This is because the NF membrane having a low silica removal rate used in Example 1 has a lower Na removal rate, and therefore, an increase in the Na concentration of concentrated water can be suppressed.
On the other hand, the NF film used in Comparative Example 1 has a higher Na removal rate because the silica removal rate is higher, and thus the Na concentration of the concentrated water is also higher.
About the removal rate of TOC of concentrated water (Organic substance: It is thought that it originates in the coolant which adhered when cutting a silicon wafer, but it is unspecified), both the comparative example 1 and Example 1 were equivalent.

1A, 1B, 2, 3, 4 NF membrane module 10 Etching tank 10A Relay tank 11, 12 NF membrane module

Claims (8)

In the alkaline etching solution processing apparatus, the alkaline etching solution obtained by etching silicon is pulled out of the etching tank and subjected to membrane separation treatment by the membrane separation means, and circulated to the etching tank.
The membrane separation means includes two or more nanofiltration membrane modules connected in series such that the concentrated water of the preceding nanofiltration membrane module is introduced into the second and subsequent nanofiltration membrane modules as membrane supply water, respectively. Become
First permeated water circulating means for circulating a part of the permeated water of the nanofiltration membrane module to the etching tank, and at least a part of the remaining permeated water is circulated to the supply water side of the first-stage nanofiltration membrane module. And a second permeated water circulating means for processing an alkaline etching solution.   2. The first permeated water circulation means according to claim 1, wherein the first permeated water circulation means is a means for circulating at least a part of the permeated water of the first-stage nanofiltration membrane module to the etching tank, and the second permeated water circulation means is The alkaline etching liquid treatment is characterized in that it is means for circulating at least a part of permeated water of the second and subsequent nanofiltration membrane modules to the supply water side of the first nanofiltration membrane module. apparatus.   3. The treatment of an alkaline etching solution according to claim 1 or 2, wherein the water concentration rate of the first-stage nanofiltration membrane module is smaller than the water concentration rate of the second-stage and subsequent nanofiltration membrane modules. apparatus.   4. The alkaline etching solution treatment apparatus according to claim 1, wherein the nanofiltration membrane module has a silica removal rate of 50 to 80%. In the method of treating an alkaline etching solution in which an alkaline etching solution obtained by etching silicon is extracted from an etching tank and circulated through a membrane separation process,
Two or more nanofiltration membrane modules were connected in series so that the concentrated water of the previous nanofiltration membrane module was introduced as membrane supply water into the second and subsequent nanofiltration membrane modules. Membrane separation treatment with membrane separation means,
A part of the permeated water of the nanofiltration membrane module is circulated to the etching tank, and at least a part of the remaining permeated water is circulated to the supply water side of the first nanofiltration membrane module. Processing method of alkaline etching solution.   In claim 5, at least a part of the permeated water of the first nanofiltration membrane module is circulated to the etching tank, and at least a part of the permeated water of the second and subsequent nanofiltration membrane modules A method for treating an alkaline etching solution, comprising circulating to the supply water side of the first nanofiltration membrane module.   In Claim 5 or 6, the water concentration rate of the said 1st stage nanofiltration membrane module is smaller than the water concentration rate of the nanofiltration membrane module of the said 2nd step and subsequent, The process of the alkali etching liquid characterized by the above-mentioned. Method.   The method for treating an alkaline etching solution according to any one of claims 5 to 7, wherein a silica removal rate of the nanofiltration membrane module is 50 to 80%.

Images