JPH07136648A - Silica removing device - Google Patents

Abstract

PURPOSE:To reduce the concn. of silica in treated water by providing at least one silica removing unit selected from an ion exchange device, a reverse osmosis membrane device and an electric deionized water making device and a fluoric acid supply device injecting fluoric acid into water to be treated. CONSTITUTION:A silica removing device is equipped with at least one silica removing unit 8 selected from an ion exchange device packed with at least a strong basic anion exchange resin, a reverse osmosis membrane device and an electric deionized water making device packed with at least a strong basic anion exchange resin. Further, a fluoric acid supply device 4 injecting fluoric acid into the water to be treated containing silica supplied to the silica removing unit 8 is provided. Fluoric acid is injected in silica-containing water to be treated to react silica with an F<-> ion to form an SiF6<2-> ion being a strong acid ion of a strong electrolyte and, by this constitution, the silica removing capacity due to the strong basic anion exchange resin or an RO membrane is enhanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silica removing apparatus for producing low-concentration silica-containing water such as boiler feed water, cleaning water for semiconductor production, cleaning water for liquid crystal production, and medical / pharmaceutical water.

[0002]

BACKGROUND OF THE INVENTION Ionic silica in water is generally subjected to an ion exchange method using an anion exchange resin or a reverse osmosis membrane (RO).
Have been removed by the membrane method.

Removal of ionic silica with a strongly basic anion exchange resin is based on the following chemical reaction formula (1).

[0004]

Embedded image R-OH + H ₂ SiO ₃ → R-HSiO ₃ + H ₂ O (1) Further, the silica adsorbed on the ion-exchange resin is, as shown in the following formula (2), Desorbed by alkali.

[0005]

## STR00002 ## R-HSiO ₃ + 2NaOH → R-OH + Na ₂ SiO ₃ + H ₂ O (2) Silica is a weak acid ion, so it is a substance that is difficult to adsorb and is easily desorbed by a strongly basic anion exchange resin. Is.

However, actually, even if the silica is desorbed by the formula (2), the reaction does not completely proceed, and a small amount of silica remains in the resin in the state of R—HSiO ₃ . . Then, when silica is removed by adsorption (that is,
This residual R-HSiO _{3 (} when water to be treated is passed through)
The bond is cleaved by hydrolysis, and the resulting silica is eluted into the treated water. This is the reason why silica is present in the ion exchange treated water.

The above hydrolysis reaction is represented by the equation (3).

[0008]

Embedded image R—HSiO ₃ + H ₂ O → R—OH + H ₂ SiO ₃ ... (3) For example, when the strongly basic anion exchange resin Amberlite (registered trademark) IRA-400 is used, the water temperature is 30 ℃,
Table 1 shows the relationship between the amount of residual silica in the resin and the amount of silica eluted in the treated water under the experimental conditions of water passing SV100.

[0009]

[Table 1] Therefore, in order to minimize the amount of silica eluted, it is necessary to minimize the amount of residual silica in the resin after desorption by using a large amount of resin regenerant, but in order to realize such a thing A large amount of regenerant must be used, which is extremely uneconomical.

Some ion removal methods using RO membranes are very excellent, but the removal rate of silica, which is a weak electrolyte, is lower than that of NaCl, which is a strong electrolyte. Furthermore, if the silica concentration of the water to be treated becomes low, the removal rate of silica tends to decrease. Further, as the RO film is used for a long period of time, the RO film is subject to oxidative deterioration, and along with this, the silica removal rate tends to decrease. For example, 8 μg of treated water at the start of use
There is also an example in which an RO film showing the performance of O ₂ / l and treated water of 1.5 μg SiO ₂ / l showed the performance of treated water of 9 μg SiO ₂ / l and treated water of 8 μg SiO ₂ / l one year later.

Accordingly, there have been various difficulties in producing low-concentration silica-containing water having a low concentration, particularly 1 μg SiO ₂ / l or less, by the conventional method.

As described above, the strongly basic anion exchange resin and R
What makes it difficult to lower the silica to the minimum in the O film is that silica is a weak electrolyte, or in other words, a weak acid ion.

[0013]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a silica removing apparatus which can surely reduce the silica concentration in treated water. Especially.

[0014]

The silica removing device of the present invention for achieving the above object is an ion exchange device having at least a strongly basic anion exchange resin, a reverse osmosis membrane device,
And at least one selected from an electric deionized water producing apparatus filled with at least a strongly basic anion exchange resin
And a hydrofluoric acid supply device for injecting hydrofluoric acid into the water to be treated containing silica to be supplied to the removal unit to remove the silica in the water to be treated to obtain treated water. The ion exchange device is a regenerative mixed bed ion exchange device, a mixed bed cartridge polisher or anion exchange device, the silica concentration in the water to be treated is 1 mgSiO ₂ / l or less, and the silica concentration in the treated water is Is 1 μg SiO ₂ / l or less,
The amount of hydrofluoric acid injected reacts with silica in the water to be treated to produce Si.
It is 1 to 100 times the chemical equivalent required to generate F ₆ ^2-, and it is used as an ultrapure water production apparatus for semiconductor production or liquid crystal production.

[0015]

The present invention applies hydrofluoric acid to water to be treated containing silica.
Inject (HF), silica and F ^- To react the ions
SiF which is a strong acid ion of strong electrolyte by₆ ^2- Io
To produce a strongly basic anion exchange resin or
An attempt to improve the silica removal capability of the RO membrane
Is. SiF₆ ^2- Ions are generated by the following reaction formula

[0016]

Embedded image SiO ₂ + 6HF → H ₂ SiF ₆ + 2H ₂ O ... (4)

[0017]

[Chemical formula 5] H ₂ SiF ₆ → 2H ⁺ + SiF ₆ ² -... (5) Easily S in water at room temperature where HF and SiO ₂ ions coexist.
It is already known that iF ₆ ²⁻ ions are generated.
Further, it is already known that the SiF ₆ ²⁻ ions formed are strong electrolytes and can be easily removed by a weakly basic ion exchange resin.

However, also in this case, the lower limit concentration of silica that can be removed is about 2 ppm as SiO ₂ .

In the present invention, the SiF ₆ ²⁻ ion is removed with a strongly basic anion exchange resin. According to the research by the present inventor, SiF ₆ ²⁻ ions are
Compared to O ₂ ions, it is very easily adsorbed on the strongly basic anion exchange resin, and Si remaining in the resin after regeneration is
It was found that F ₆ is much less likely to be eluted than SiO ₂ .

The reason why the SiF ₆ ²⁻ ion is easily adsorbed on the strongly basic anion exchange resin and is less likely to be eluted is not clarified, but the present inventor suspects that the reason is as follows. There is.

[0021] That is, SiF ₆ ^2-is a divalent ion, and because it is a strong acid ion, the anion-selective ranking strong base anion exchange resin ^{_{SO 4 2-> NO 3 ->}} Cl -> OH ^-> F ^-> HSiO ₃ ^- and, although sO ₄ ^2-is the largest selectivity, selectivity since SiF ₆ ^2-is the same divalent ions are considered to be almost equal to sO ₄ ^2-. Therefore, it is considered that SiF ₆ ²⁻ is an ion that is very easily adsorbed on the strongly basic anion exchange resin and is hardly eluted.

In particular, the elution amount is determined by the following relational expression.

[0023]

Embedded image ^{_{R-SiF 6 + 2OH - →}} R- (OH) assuming the case of treatment with ^{_{2 + SiF 6 2- ···· (6}} ) mixed-bed ion exchanger, eventually present in the water OH ^- ions the amount by dissociation of water, i.e. the following formula (7),

[0024]

[Chemical 7] Determined by

In this case, the residual SiF _{6 in} the resin and Si
The relationship with the elution concentration of F ₆ ^2- is as shown in Table 2. The values in Table 2 are the strongly basic anion exchange resin Amberlite IRA.
-400 and a strongly acidic cation exchange resin IR-124 were used as a mixed bed, and the amount of residual SiF _{6 of} IRA-400 was 20 gS.
The amount of SiF ₆ ^2- eluted when iF ₆ / l-R was adjusted to 30 at the water temperature.
3 × 1 which is the data obtained by measuring at SV50 at ℃
It is an estimated value calculated from 0 ^-2 μg SiF ₆ / l.

[0026]

[Table 2] (However, the values in parentheses indicate the converted values to SiO _2. ) By comparing with the values in Table 1, it can be understood how small the amount of SiF ₆ ^2- eluted in Table 2 is.

In the case of the RO membrane, the same idea is used.
Brush SiO₂ SiF rather than ions₆ ^2-Ion is better
It has the overwhelming advantage in terms of
I can expect to come. This is, of course,
No, but SiF₆ ^2- Ions are strong electrolytes and divalent strong acids
It depends on being an ion.

The present invention will be described in detail below with reference to FIG.

FIG. 1 is a flow chart showing an embodiment of the silica removing apparatus of the present invention. In FIG. 1, 2 is water to be treated containing silica. Silica contains ionic dissolved silica, and the concentration in the water to be treated is not particularly limited, but it is preferably a relatively low concentration, for example, 1 mg SiO ₂ / l or less. If the silica content is high, the hydrofluoric acid (H
Since the amount of F) added increases in proportion to the silica concentration, it may be economically disadvantageous.

Ions other than silica may coexist in the water to be treated.

Hydrofluoric acid is first injected into the water to be treated 2 by the hydrofluoric acid supply device 4 to become hydrofluoric acid-injected water 6. As a result, silica in the water to be treated becomes SiF ₆ ²⁻ ions. The injection amount of hydrofluoric acid is preferably 1 to 1000 chemical equivalents, particularly 1 to 100 chemical equivalents of dissolved silica. Although the reaction proceeds at room temperature, it may be performed in a heated or cooled state.

Next, the hydrofluoric acid-injected water 6 is sent to the silica removing unit 8.

The silica removal unit 8 is, for example, a mixed bed type ion exchange having an anion exchange device filled with a strongly basic anion exchange resin and a regeneration facility filled with a mixed resin of a strongly basic anion exchange resin and a strongly acidic cation exchange resin. apparatus,
Alternatively, there is an ion exchange device such as a mixed-bed cartridge polisher having no regeneration equipment. In any case, it is necessary to include a strongly basic anion exchange resin.

An electric deionized water producing apparatus can also be used as the silica removing unit. In the electric deionized water producing apparatus, basically, a gap formed by an anion exchange membrane and a cation exchange membrane is filled with, for example, a cation exchange resin and an anion exchange resin to form a desalination chamber, The water to be treated is allowed to pass through the room, and a direct current is applied in a direction perpendicular to the flow of the water to be treated through the both ion exchange membranes so that the concentrated water flowing to the outside of both ion exchange membranes is exposed to the treatment. Deionized water is produced by electrically eliminating ions in the treated water.Even if the desalting chamber is filled with an ion exchanger such as an ion exchange resin, no regenerating chemical such as acid or alkali should be used. It is possible to produce deionized water without using it.

As the electric deionized water producing apparatus,
Known (for example, JP-A-4-71624, JP-A-4-16)
No. 6215) can be used as it is.

In this case, the ion exchange resin to be filled must contain at least a strongly basic anion exchange resin.

The anion exchange resin used in the ion exchange apparatus or the electric deionized water producing apparatus of the present invention is not limited to granular ones, and may be fibrous, powdery, etc.
It may have any shape.

As the silica removing unit 8, a reverse osmosis membrane device can be used. Known products can be used as they are.

The fluorine-injected water 6 sent to the removal unit 8 has SiF ₆ ²⁻ ions removed therefrom and treated water 10
SiF ₆ ²⁻ is a divalent strong acid ion as described above, and thus is effectively removed by the silica removing unit 8. Therefore, the silica concentration in the obtained treated water 10 is extremely low, for example, a low concentration of 1 μg SiO ₂ / l or less can be obtained.

As the silica removing unit 8, the above-mentioned ion exchange device, reverse osmosis membrane device, and electric deionized water producing device may be used alone, but two or more of them may be combined appropriately. May be used.

[0041]

The present invention will be described in more detail with reference to the following examples.

Silica in the water to be treated was removed by using the silica removing apparatus having the flow shown in FIG. In FIG. 2, 21 is a treated water tank, 22 is a pump, 23 is a flow meter, and 24 is HF.
A tank, 25 is an HF injection metering pump, 26 is a mixed bed type ion exchange apparatus, 27 is a treated water sampling location, 28 is a reverse osmosis membrane apparatus, and 29 is a treated water sampling location. Either the ion exchange resin device 26 or the reverse osmosis membrane device 28 can be selected by switching the valves 30 and 31.

(Method of Implementation) When the mixed bed type ion exchange device is used as a silica removing unit, water is passed through 4 cycles for each of the case where hydrofluoric acid is not injected and the case where hydrofluoric acid is injected, and data is taken at the 4th cycle. It was The reason for carrying out 4 cycles is to approach the equilibrium state more.

The water exchange cycle of the ion-exchange resin was as follows: water passage-regeneration-water passage-regeneration-water passage-regeneration-water passage 4 cycles (water passage for 7 days, regeneration for 3 hours).

When the reverse osmosis membrane device was used as a silica removing unit, water was passed for 10 hours without injecting hydrofluoric acid, and then water was passed for 10 hours while injecting hydrofluoric acid. Data were obtained at the last 10 hours of each.

(Implementation conditions) Raw water Ultra pure water (18.0 MΩ
-Cm, SiO ₂ 1 μg / l or less) special grade reagent Na ₂ S
A solution obtained by dissolving iO ₃ as SiO ₂ to a concentration of 200 μg / l. Water temperature 25 ° C Silica analysis method After concentration of the sample, measurement by molybdenum blue method Ion exchange resin Strongly acidic cation exchange resin Amberlite IR-124 (H type) 300 ml Regeneration rate 99% Strongly basic anion exchange resin 〃 IRA-400 (OH Type) 300 ml 〃 Ion exchange resin regeneration level IR-124 35% HCl 500 g / lR IR-400 100% NaOH 300 g / lR Ion exchange resin water flow rate SV 50 (30 l / H) Reverse osmosis membrane device NTR-759HR S2B Rotation Rate 90% Permeate amount 30 l / H Operating pressure 15 kg / cm ² Hydrofluoric acid injection amount 800 μg / l (SiO ₂ 200 μg / l is SiF ₆
^-2 times the chemical equivalent of converting to ^-2 ) From the above results, SiO in the water to be treated by injecting hydrofluoric acid
₂ treated silica eluting amount of lack of water mixed bed ion exchanger and a reverse osmosis unit when allowed to SiF ₆ ^2-is evident.

Even when an electric deionized water producing apparatus is used as the silica removing unit and the same ion exchange resin as in the mixed bed type ion exchanging apparatus is filled inside, the same result as in the ion exchanging apparatus is obtained. Was given.

[0048]

EFFECTS OF THE INVENTION The apparatus of the present invention reacts weak acid ion SiO ₂ ions present in water with hydrofluoric acid (HF) to convert it into strong acid ion SiF ₆ ²⁻ ions.
Then, SiF ₆ ²⁻ ions are removed by a strongly basic anion exchange resin or a reverse osmosis membrane device.
The O ₂ ion concentration can be much lower than the treated water obtained by the state of the art.

[Brief description of drawings]

FIG. 1 is a flow chart showing an embodiment of a silica removing apparatus of the present invention.

FIG. 2 is a flow chart showing an embodiment of the present invention.

[Explanation of symbols]

 2 water to be treated 4 hydrofluoric acid supply device 6 hydrofluoric acid injection water 8 silica removal unit 10 treated water

Claims (6)

[Claims] 1. At least one silica selected from an ion exchange device having at least a strongly basic anion exchange resin, a reverse osmosis membrane device, and an electric deionized water producing device filled with at least a strongly basic anion exchange resin. A treatment unit comprising a removal unit and a hydrofluoric acid supply device for injecting hydrofluoric acid into the treatment water containing silica to be supplied to the removal unit to remove the silica in the treatment water to obtain treated water. Equipment for removing silica from water. 2. The silica removing device according to claim 1, wherein the ion exchange device is a regenerative mixed bed type ion exchange device, a mixed bed type cartridge polisher or an anion exchange device. 3. The concentration of silica in the water to be treated is 1 mg SiO 2.
The silica removing apparatus according to claim 1, which has a ratio of ₂ / l or less. 4. The silica concentration in the treated water is 1 μg SiO _2.
The silica removing device according to claim 1, wherein the silica removing amount is not more than 1 / l. 5. The injection amount of hydrofluoric acid is 1 of the chemical equivalent required to react with silica in the water to be treated to produce SiF _6.
The apparatus for removing silica according to claim 1, wherein the silica removal apparatus is 100 times. 6. The silica removing apparatus according to claim 1, which is used as an ultrapure water producing apparatus for semiconductor production or liquid crystal production.