WO2015080095A1 - Method for regenerating molten salt for chemical reinforcement of glass - Google Patents

Abstract

　The present invention relates to a method for regenerating a molten salt for chemical reinforcement, the method for regenerating a molten salt for chemical reinforcement of glass including a step for dissolving a molten salt subsequent to glass chemical reinforcement treatment in water at a temperature less than the melting point of the molten salt, a step for cooling the resultant aqueous solution and obtaining a regenerated salt, and a step for bringing the moisture content in the regenerated salt to less than 5% by mass by drying.

Description

Method for regenerating molten salt for glass chemical strengthening

The present invention relates to a method for regenerating a molten salt used for chemical strengthening treatment, and particularly to a method for regenerating a molten salt containing potassium nitrate.

Cover glass of display devices such as digital cameras, mobile phones, and PDAs (Personal Digital Assistants), and glass substrates of displays are sometimes referred to simply as “chemically tempered glass” glass that has been chemically strengthened by ion exchange or the like. .) Is used.

Chemical strengthening treatment by ion exchange compresses the glass surface by substituting metal ions with a small ionic radius (for example, Na ions) and metal ions with a larger ionic radius (for example, K ions) contained in the glass. This is a process for generating a stress layer and improving the strength of the glass.

When producing chemically strengthened glass by ion exchange of Na ions in glass and K ions in molten salt in molten salt containing potassium nitrate (potassium nitrate molten salt), it is melted from the glass as the chemical strengthening treatment is performed. The amount of Na dissolved in the salt increases, and the Na ion concentration in the molten salt increases.
Since the surface compressive stress (CS), which is one of the characteristics of chemical strengthening, decreases as the Na concentration in the potassium nitrate molten salt increases, the molten salt is discarded when the CS value of the resulting chemically strengthened glass falls below the reference value. However, it is necessary to use a new molten salt.

The molten salt in which the desired CS value cannot be obtained by the chemical strengthening treatment is usually allowed to cool and solidify, and then ground into small blocks and discarded. However, in the said processing method, waste molten salt (waste salt) cannot be used again, but there existed problems, such as having to use a large amount of molten salt.
Therefore, in Patent Document 1, it is assumed that Li or Cs in the glass component is mixed as an impurity in the molten salt, and that the ion exchange capacity of the molten salt is reduced. A method of regenerating a molten salt by allowing it to fall into water as a shower and dissolving, cooling and separating the molten salt in the water is disclosed.

Japanese Unexamined Patent Publication No. 58-194761

However, in the process of heating the regenerated molten salt to a molten state again, the stainless steel container holding the molten salt may corrode. Due to this corrosion, floating substances are generated in the molten salt, and if the glass is chemically strengthened as it is, there is a concern that the performance of the resulting chemically strengthened glass will be affected.
Therefore, an object of the present invention is to provide a method for regenerating a molten salt for glass chemical strengthening treatment that has little influence on glass performance.

As a result of earnest study, the present inventors dissolved the molten salt (waste salt) used in the chemical strengthening treatment in an aqueous solution at a temperature lower than the melting point, cooled and dried, thereby reducing the Na concentration. Only the salt can be taken out, and it has been found that the salt having a low Na concentration can be used again as a molten salt for chemical strengthening treatment of glass, and the present invention has been completed.

That is, the present invention relates to the following <1> to <6>.
<1> A method for regenerating a molten salt for glass chemical strengthening, the step of dissolving the molten salt after glass chemical strengthening treatment in water at a temperature lower than the melting point of the molten salt, and cooling the aqueous solution obtained in the melting step A method for regenerating a molten salt for strengthening glass chemistry, comprising a step of obtaining a regenerated salt and a step of drying to reduce the water content in the regenerated salt to less than 5% by mass.
<2> The method for regenerating a molten salt for strengthening glass chemistry according to <1>, wherein the step of cooling the aqueous solution to obtain a regenerated salt further includes a step of concentrating the aqueous solution.
<3> The method for regenerating a molten salt for glass chemical strengthening according to <1> or <2>, wherein the molten salt for glass chemical strengthening includes potassium nitrate.
<4> The method for regenerating a molten salt for glass chemical strengthening according to any one of <1> to <3>, wherein a moisture content in the regenerated salt is less than 0.2% by mass in the drying step.
<5> In the step of cooling the aqueous solution to obtain a regenerated salt, the cooled solution is solid-liquid separated into a regenerated salt and a filtrate, and a part of the filtrate is mixed with the solution in the dissolution step, <1 The method for regenerating a molten salt for glass chemical strengthening according to any one of> to <4>.
<6> The regenerated salt obtained by the solid-liquid separation is washed, further solid-liquid separated into a regenerated salt and a filtrate, and a part of the filtrate is mixed with the dissolved solution in the dissolving step. A method for regenerating a molten salt for glass chemical strengthening as described.

According to the method for regenerating a molten salt for glass chemical strengthening according to the present invention, a part of the molten salt (waste salt) after the chemical strengthening treatment that has been conventionally discarded can be used again for the chemical strengthening treatment. Useful. In addition, since the amount of molten salt discarded can be reduced, the danger associated with transporting the molten salt to be discarded can be reduced, and the load on the environment can be reduced. Furthermore, by setting the water content of the obtained regenerated salt to less than 5% by mass, the chemically strengthened glass obtained by the chemical strengthening treatment using the regenerated salt exhibits good surface compressive stress and strength. Is very useful.

FIG. 1 is a flowchart showing an embodiment of a method for regenerating a molten salt for glass chemical strengthening according to the present invention. FIG. 2 is a solubility curve showing measured values of the solubility of a salt that can be contained in the molten salt after glass chemical strengthening treatment with respect to 100 g of water. FIG. 3 is a graph showing the relationship between the number of recycles of regenerated salt obtained in Example 2 and the recovered salt recovery rate. FIG. 4 is a graph showing the relationship between the number of recycles of the regenerated salt obtained in Example 2 and the regenerated salt Na concentration. FIG. 5 is a graph showing the relationship between the number of recycles of regenerated salt obtained in Example 3 and the recovered salt recovery rate. FIG. 6 is a graph showing the relationship between the number of recycles of regenerated salt obtained in Example 3 and the regenerated salt Na concentration. FIG. 7 is a graph showing the relationship between the number of recycles of regenerated salt obtained in Example 4 and the recovered salt recovery rate. FIG. 8 is a graph showing the relationship between the number of recycles of regenerated salt obtained in Example 4 and the concentration of regenerated salt Na.

Hereinafter, the present invention will be described in detail, but the present invention is not limited to the following embodiments, and can be arbitrarily modified without departing from the gist of the present invention.
In the present specification, “mass%” and “weight%”, “mass ppm” and “weight ppm” have the same meaning. In addition, when “ppm” is simply described, it indicates “weight ppm”.
In this specification, “Na concentration” means a concentration as Na.

<Regeneration of molten salt>
The present invention is a method for regenerating a molten salt for chemical strengthening, the step of dissolving the molten salt after glass chemical strengthening treatment in water at a temperature below the melting point of the molten salt, cooling the obtained aqueous solution to obtain the regenerated salt. And a step of obtaining a moisture content in the regenerated salt of less than 5% by mass by drying.

FIG. 1 shows an embodiment of a method for regenerating a molten salt for glass chemical strengthening according to the present invention.

The chemical strengthening treatment of glass involves immersing glass as a raw material in a molten salt for glass strengthening (sometimes simply referred to as “molten salt”), and Na in the glass ion exchanges with K in the molten salt. Thus, a compressive stress layer that is a high-density layer is formed on the glass surface.

The molten salt in the present invention contains an inorganic potassium salt. The inorganic potassium salt preferably has a melting point below the strain point (usually 500 to 600 ° C.) of the glass to be chemically strengthened. In the present invention, a molten salt containing potassium nitrate (melting point 330 ° C.) as a main component (potassium nitrate molten) Salt) is preferred. If potassium nitrate is a main component, it is preferable because it is in a molten state below the strain point of glass and is easy to handle in the operating temperature range. Here, the main component means that the content in the molten salt is 50% by mass or more.

The molten salt is further selected from the group consisting of K ₂ CO ₃ , Na ₂ CO ₃ , KHCO ₃ , NaHCO ₃ , K ₃ PO ₄ , Na ₃ PO ₄ , K ₂ SO ₄ , Na ₂ SO ₄ , KOH and NaOH. It is preferable to contain at least one salt, and it is more preferable to contain at least one salt selected from the group consisting of K ₂ CO ₃ , Na ₂ CO ₃ , KHCO ₃ and NaHCO ₃ .

For example, when K ₂ CO ₃ is added to a molten salt containing potassium nitrate as a main component and glass is chemically strengthened, the content of K ₂ CO ₃ in the molten salt is 0.1% by mass or more, and chemical strengthening is performed. When the treatment temperature is 350 to 500 ° C., the chemical strengthening treatment time is preferably 1 minute to 10 hours, more preferably 5 minutes to 8 hours, and even more preferably 10 minutes to 4 hours.

Furthermore, the molten salt used for the chemical strengthening treatment in the present invention may contain other chemical species as long as the effects of the present invention are not impaired, for example, sodium chloride, potassium chloride, sodium borate, boric acid. Examples include alkali chlorides such as potassium and alkali borates. These may be added alone or in combination of two or more.

The molten salt used for the glass chemical strengthening treatment can be produced by a known method, and the glass can be chemically strengthened by a known method using the molten salt.

The molten salt is solidified by cooling or cooling the molten salt (waste salt) in which a desired surface compressive stress cannot be obtained by chemical strengthening treatment to a temperature below the melting point of the molten salt. When a molten salt containing potassium nitrate is used as the molten salt, the waste salt includes potassium nitrate and sodium nitrate. In addition, depending on the type of salt added to the molten salt, the waste salt includes potassium salt and sodium salt of the added salt. That is, for example, when potassium carbonate (K ₂ CO ₃ ) is added, the waste salt includes potassium carbonate and sodium carbonate.
The concentration of Na in the waste salt is generally 4000 to 20000 mass ppm.

In the waste salt, Na is present at a higher concentration than in the molten salt before the chemical strengthening treatment. The solid waste salt having a high Na concentration is taken out and dissolved in water. The waste salt in the solid state is preferably divided as appropriate in order to facilitate dissolution, and is preferably divided into, for example, a size of 1000 cm ³ or less.

Water for dissolving the waste salt is not particularly limited, and pure water, distilled water, or the like can be used. From the viewpoint of preventing an increase in impurities contained in the aqueous solution, pure water having an electric conductivity of 10 μS or less is preferable. .
The temperature of water at the time of dissolving the waste salt may be a temperature below the melting point of the molten salt, preferably 60 to 120 ° C, and more preferably 80 to 100 ° C from the viewpoint of easy handling. The water temperature can be appropriately adjusted by a known method such as a water bath or an oil bath.
In addition, the density | concentration of the waste salt in aqueous solution is so preferable that it is high, and it is more preferable to melt | dissolve to saturation solubility. Moreover, it is also preferable to make it melt | dissolve not to the saturation solubility of the whole waste salt but to the saturation solubility of the desired salt to utilize as a regenerated salt among the plurality of salts contained in the waste salt.

When dissolving the waste salt, it is preferable to dissolve the water while stirring since the solution can be made uniform. The stirring speed is usually 50 to 2000 rpm, preferably 100 to 1000 rpm.

When the waste salt is completely dissolved in the water, cool the aqueous solution. When the desired salt contained in the waste salt is dissolved to saturation solubility, depending on the type of salt and the proportion contained in the waste salt, other salts may not be completely dissolved and remain in the aqueous solution as a solid. . Moreover, the foreign material contained in waste salt may remain. In that case, the filtrate is cooled after removing undissolved salt and foreign matters by filtration or the like. The filtration accuracy is preferably 100 μm or less, more preferably 0.2 μm or more and 100 μm or less.

The cooling can be performed by a known method such as natural cooling (cooling), water cooling, or ice cooling. Cooling is preferably performed to 25 ° C. or lower, more preferably 20 ° C. or lower, and further preferably 10 ° C. or lower, more preferably from the viewpoint of increasing the yield.

Due to the difference between the solubility of the salt at the temperature when the waste salt is dissolved and the solubility of the salt at the temperature at the time of cooling, precipitates are formed in the aqueous solution after cooling (crystallization). In the case of using a molten salt containing potassium nitrate, the waste salt and the precipitate include potassium nitrate and sodium nitrate. Further, depending on the type of salt added to the molten salt, the precipitate contains the potassium salt or sodium salt of the added salt.

FIG. 2 is a solubility curve (g / 100 g of water) of measured values showing the temperature dependence of solubility in water for potassium nitrate, sodium nitrate, potassium carbonate and sodium carbonate.
According to this, when the temperature of the aqueous solution is around 70 ° C., the solubility of potassium nitrate is higher than that of sodium nitrate and potassium carbonate in the high temperature region, and the solubility of potassium nitrate is lower than that of sodium nitrate and potassium carbonate in the low temperature region.
That is, in the case of an aqueous solution in which waste salts containing four kinds of salts of potassium nitrate, sodium nitrate, potassium carbonate and sodium carbonate are dissolved to saturation solubility, the difference between the saturation solubility at the dissolved temperature and the saturation solubility at the cooled temperature Of the salt precipitates as a solid. When the salt to be used as the regenerated salt is potassium nitrate, when the difference in saturation solubility of potassium nitrate is larger than the difference in saturation solubility of other salts, the precipitate deposited by cooling the aqueous solution is not discarded. It contains potassium nitrate at a higher rate than the salt, and the Na concentration in the precipitate is lower than the original waste salt. Therefore, the precipitate can be used again as a molten salt for glass chemical strengthening treatment, and can be called “regenerated salt”.

As described above, in the present invention, by utilizing the fact that the solubility of potassium nitrate and sodium nitrate is reversed between the high temperature region and the low temperature region, recrystallization causes a salt with a low Na concentration from a waste salt with a high Na concentration. Can be played. If the Na concentration in the regenerated salt is 1000 mass ppm or less, it can be reused for the chemical strengthening treatment of glass.

It is also preferable to further concentrate the aqueous solution in the step of dissolving the waste salt in water and then cooling the obtained aqueous solution to obtain a regenerated salt.
Concentration means to increase the salt concentration in the aqueous solution, but a known method such as vacuum concentration (vacuum concentration) or freeze concentration can be used. By concentrating the aqueous solution, a salt that can no longer be dissolved is deposited. By performing a combination of the cooling step and the concentration step of the aqueous solution, a regenerated salt having a lower Na concentration can be obtained with high efficiency.

Since the obtained regenerated salt is precipitated in the aqueous solution, solid-liquid separation is performed in order to use it again for the chemical strengthening treatment of glass. For the solid-liquid separation, known methods such as filtration and centrifugation can be used. Since the molten salt (for example, potassium nitrate) remains in the liquid after solid-liquid separation, a part of the liquid can be mixed and reused in the waste salt solution. However, when recycling is repeated by reusing the liquid smoke, the Na concentration in the liquid smoke increases, and accordingly, the Na concentration in the regenerated salt also increases. Therefore, it is preferable to discard a part of the liquid smoke and control the Na concentration. The amount of the liquid smoke to be reused can be determined in consideration of the Na concentration.

In order to increase the purity of the obtained regenerated salt, the regenerated salt may be washed. The washing can be performed with pure water having an electric conductivity of 10 μS or less. The temperature of the washing water is preferably 20 ° C. or less. However, since the yield of the regenerated salt obtained when washing is reduced, it is necessary to appropriately determine whether or not washing is necessary in consideration of the balance between the yield and purity according to the purpose.

After washing the regenerated salt, the washing liquid is further solid-liquid separated to obtain regenerated salt. Since the molten salt (for example, potassium nitrate) remains in the liquid after solid-liquid separation, a part of the liquid can be mixed and reused in the waste salt solution. However, when recycling is repeated by reusing the liquid smoke, the Na concentration in the liquid smoke increases, and accordingly, the Na concentration in the regenerated salt also increases. Therefore, it is preferable to discard a part of the liquid smoke and control the Na concentration. The amount of the liquid smoke to be reused can be determined in consideration of the Na concentration.

After recovering the regenerated salt by solid-liquid separation, it is preferable to dry well before reuse in the chemical strengthening treatment. By drying, the amount of water in the regenerated salt can be reduced. The regenerated salt before drying contains about 6% by mass of water.
A stainless steel (SUS) container is used when the regenerated salt is heated to be subjected to chemical strengthening treatment to form a molten salt. If the amount of water in the regenerated salt is large, the regenerated salt is heated to form a molten salt. In the process, the SUS container corrodes. Due to the corrosion, suspended matters are generated in the molten salt, and if the glass is subjected to chemical strengthening treatment as it is, the performance of the resulting chemically strengthened glass is affected. Therefore, the smaller the amount of water in the regenerated salt, the better, preferably less than 5% by mass, more preferably less than 2% by mass, still more preferably less than 1% by mass, and particularly preferably less than 0.2% by mass. The water content in the regenerated salt can be measured by TGA (thermogravimetry). Distilled water produced by drying can be reused as a solvent for dissolving the waste salt. By reusing distilled water, it can contribute to the reduction of environmental burden.

The drying temperature may usually be 40 to 300 ° C, more preferably 80 to 200 ° C. The drying time may usually be 1 to 12 hours, and more preferably 1 to 4 hours. Moreover, you may reduce pressure simultaneously with a heating at the time of drying.
For the drying, a known method such as a hot plate or heating vacuum drying can be used.
It is preferable to store the regenerated salt after drying in a sealed container in order to prevent moisture from entering.

The regenerated salt that has undergone the drying step can be used as a molten salt for glass chemical strengthening treatment by heating to a temperature at which the glass is chemically strengthened.

The regenerated salt obtained by the present invention can be reused as a molten salt repeatedly by the regenerating treatment of the present invention after being used as a molten salt for glass chemical strengthening treatment. According to the regeneration method of the present invention, by reusing a part of the liquid smoke after solid-liquid separation to dissolve the waste salt, the yield of the regenerated salt is increased while keeping the Na concentration in the regenerated salt below a predetermined level. Can be raised. If the filtrate is not reused, it is difficult to increase the yield of regenerated salt. On the other hand, when all the filtrates are reused, the Na concentration in the aqueous solution increases, and the Na concentration in the resulting regenerated salt also increases.

When the inorganic potassium of the molten salt is potassium nitrate, the regenerated salt obtained by the present invention contains nitrous acid in the range of 10 to 100 ppm by weight. The amount of nitrous acid contained in a new potassium nitrate molten salt not subjected to chemical strengthening is usually 10 ppm by weight or less. It is thought that the nitric acid content increases because nitric acid in the molten salt changes to nitrous acid by repeated chemical strengthening. The nitrous acid content in the molten salt can be measured by a naphthylethylenediamine colorimetric method.

In addition, as long as the glass used for the chemical strengthening process in this invention should contain sodium and it has a composition which can be strengthened by shaping | molding and a chemical strengthening process, it is possible to use the thing of various compositions. it can. Specific examples include aluminosilicate glass, soda lime glass, borosilicate glass, lead glass, alkali barium glass, and aluminoborosilicate glass.
Among these, aluminosilicate glass has a large amount of Na substitution in the glass, so that the molten salt is severely deteriorated. For this reason, it is preferable because the effect of the method for regenerating a molten salt according to the present invention can be remarkably obtained.

There are no particular limitations on the method of manufacturing and forming the glass subjected to the chemical strengthening treatment, and the glass can be manufactured and formed based on known methods. Moreover, the thickness of the glass used for the chemical strengthening treatment and the presence or absence of polishing are also arbitrary.

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these.
<Glass>
In this example, an aluminosilicate glass having the following composition (mol%) was used. SiO ₂ 64.4%, Al ₂ O ₃ 8.0%, Na ₂ O 12.5%, K ₂ O 4.0%, MgO 10.5%, CaO 0.1%, SrO 0.1%, BaO 0.1%, ZrO ₂ 2.5%

<Evaluation method>
(Measurement of Na concentration)
In this example, the Na concentration in the waste salt and the regenerated salt was identified using an atomic absorption photometer “ZA-3300” manufactured by Hitachi High-Technologies Corporation.
(Measurement of surface compressive stress -CS-)
The surface compressive stress of the aluminosilicate glass after the chemical strengthening treatment was evaluated using a surface stress meter “FSM-6000LE” manufactured by Orihara Seisakusho.
(Measurement of water content)
The amount of water contained in the regenerated salt was quantified using a heat drying moisture meter “MS-70” manufactured by A & D.

<Example 1>
The aluminosilicate glass was chemically strengthened in a molten salt consisting only of potassium nitrate. The chemical strengthening treatment temperature was 450 ° C. The Na concentration in the molten salt (waste salt) after the chemical strengthening treatment was 6000 ppm.
The molten salt after the chemical strengthening treatment was naturally cooled to 25 ° C. to obtain a waste salt.
1000 g of solid waste salt was divided into sizes of 30 cm ³ or less, weighed into a 2000 mL beaker, and 800 g of pure water was added. While this was automatically stirred at 200 rpm, it was heated to 80 ° C. with a water bath to obtain an aqueous solution in which all waste salts were dissolved in pure water. After confirming that the waste salt was completely dissolved, it was ice-cooled to 1 ° C. with automatic stirring at 200 to 300 rpm to precipitate (recrystallize) the salt.
Next, suction filtration was performed to separate the obtained salt crystals from the aqueous solution. The crystals separated by filtration were collected and dried on a hot plate set at 80 ° C. for 5 hours to obtain 903 g of regenerated salt. The obtained regenerated salt had a water content of 3% by mass and a nitrous acid content of 20 ppm. The recovery rate was actually 90% against the theoretical yield of 94% obtained from the solubility curve. Further, the Na concentration in the obtained regenerated salt was 400 mass ppm.

<Glass chemical strengthening treatment>
The regenerated salt obtained in Example 1 was heated to 450 ° C. in a SUS container to form a molten salt, and aluminosilicate glass preheated to 200 to 400 ° C. was immersed therein for 2 hours for chemical strengthening treatment. At this time, no suspended matter due to SUS corrosion was visually confirmed in the molten salt. After the tempering treatment, the glass was washed twice with ion exchange water at 20 to 80 ° C., and washed with running water with ion exchange water at room temperature. The initial surface compressive stress (initial CS) of the obtained chemically strengthened glass was 844 MPa. The initial CS when the aluminosilicate glass is chemically strengthened with new potassium nitrate not subjected to ion exchange treatment as a molten salt is 750 to 900 MPa.

<Example 2>
150 kg of waste salt containing potassium nitrate as a main component and Na concentration of 10,000 ppm was put in a SUS container, and 90.3 kg of pure water was added. This was heated to 90 ° C. with an electric heater and dissolved with stirring. After completely dissolving, it was taken out into another SUS container, cooled to room temperature by allowing to cool, and the salt was precipitated. Next, centrifugation was carried out to separate the obtained salt crystals and the aqueous solution, and a salt and a filtrate having a water content of 2% by mass were obtained. The obtained salt was washed with pure water and centrifuged again to obtain a salt and a filtrate having a water content of 2% by mass. The obtained salt was dried at 200 ° C. for 8 hours to obtain a regenerated salt having a water content of 0.05 mass%, an Na concentration of 70 ppm, and a nitrous acid concentration of 40 ppm. Further, 43.7 kg of the filtrate obtained by centrifugation was discarded, and the remaining 104.8 kg of filtrate was placed in a SUS container together with 112.5 kg of waste salt, and 6.5 kg of pure water was added. This was similarly heated to 90 ° C. and dissolved with stirring. Thereafter, cooling, centrifugation, washing and centrifugation were performed to obtain a regenerated salt and a filtrate. Table 1 shows the experimental results obtained by repeating this. Table 2 shows the result of the simulation performed under the same conditions. Further, the graphs of these results are shown in FIGS.

<Chemical strengthening treatment>
The regenerated salt obtained in Example 2 was heated to 450 ° C. in a SUS container to form a molten salt, and the initial CS of the chemically strengthened glass obtained by performing chemical strengthening treatment in the same manner as in Example 1 was 786 MPa. .

<Example 3>
In the simulation of Example 2, Table 3 shows the simulation result when the amount of discarded liquid is 53.7 kg, and Table 4 shows the simulation result when the amount of discarded waste is 33.7 kg. Moreover, what made these results into a graph is shown in FIG. 5 and FIG.

<Example 4>
In the simulation of Example 2, the simulation results when all of the liquid smoke is discarded are shown in Table 5. Table 6 shows the simulation results when all of the liquid smoke is reused. Moreover, what made these results into a graph is shown in FIG. 7 and FIG.

From the above results, the regenerated salt obtained by the regenerating method according to the present invention has a very low water content and Na concentration, and even when reused as a molten salt for glass chemical strengthening treatment, It was found that a surface compressive stress equivalent to that of a new molten salt not subjected to ion exchange treatment can be applied.
Further, the solubility curve shown in FIG. 2 suggests that the yield of the regenerated salt can be increased by lowering the precipitation temperature during recrystallization.
Furthermore, it was found that the yield of regenerated salt can be increased by reusing the filtrate obtained during the solid-liquid separation. It was also found that the Na concentration in the regenerated salt obtained can be controlled by adjusting the amount of the liquid recycle to be reused.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is based on a Japanese patent application filed on November 29, 2013 (Japanese Patent Application No. 2013-247987), the contents of which are incorporated herein by reference.

According to the present invention, a regenerated salt having a performance equivalent to that of a new molten salt can be obtained by subjecting the used molten salt provided to the chemically strengthened glass to a regeneration treatment. This regeneration process can reduce the amount of used molten salt (waste salt) to be discarded, and can reduce the environmental impact and produce chemically tempered glass at a low cost, realizing high productivity. .

Claims (6)

1. A method for regenerating a molten salt for strengthening glass chemistry,
   A step of dissolving the molten salt after glass chemical strengthening treatment in water at a temperature lower than the melting point of the molten salt, a step of cooling the aqueous solution obtained in the dissolving step to obtain a regenerated salt, and drying in the regenerated salt A method for regenerating a molten salt for strengthening glass chemistry, comprising a step of setting the moisture content of the glass to less than 5% by mass.
2. The method for regenerating a molten salt for glass chemical strengthening according to claim 1, wherein the step of cooling the aqueous solution to obtain a regenerated salt further comprises a step of concentrating the aqueous solution.
3. The method for regenerating a molten salt for glass chemical strengthening according to claim 1 or 2, wherein the molten salt for glass chemical strengthening contains potassium nitrate.
4. The method for regenerating a molten salt for glass chemical strengthening according to any one of claims 1 to 3, wherein the moisture content in the regenerated salt is less than 0.2% by mass in the drying step.
5. In the step of cooling the aqueous solution to obtain a regenerated salt, the cooled solution is subjected to solid-liquid separation into a regenerated salt and a filtrate, and a part of the filtrate is mixed with the solution in the dissolution step. The regeneration method of the molten salt for glass chemical strengthening as described in any one of Claims.
6. The glass chemistry according to claim 5, wherein the regenerated salt obtained by the solid-liquid separation is washed, further solid-liquid separated into a regenerated salt and a filtrate, and a part of the filtrate is mixed with the dissolved solution in the dissolving step. A method for recycling molten salt for strengthening.
   

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