TW201527236A - 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 chemical strengthening of glass

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

A glass substrate that covers a glass or a display such as a digital camera, a mobile phone, or a display device such as a PDA (Personal Digital Assistants) has been subjected to chemical strengthening treatment by ion exchange or the like (hereinafter, sometimes referred to as "chemistry." Tempered glass").

The chemical strengthening treatment by ion exchange is produced on the glass surface by replacing metal ions (for example, Na ions) having a small ionic radius contained in the glass with metal ions (for example, K ions) having a larger ionic radius. The process of compressing the stress layer to enhance the strength of the glass.

In the case where a chemically strengthened glass is produced by ion-exchange of a Na ion in a glass with a K ion in a molten salt in a molten salt containing potassium nitrate (a potassium nitrate molten salt), as the chemical strengthening treatment proceeds, The amount of Na eluted into the molten salt of the glass increases, and the concentration of Na ions in the molten salt becomes high.

Since the surface compressive stress (CS) which is one of the characteristics of chemical strengthening decreases as the Na concentration in the molten salt of potassium nitrate increases, if the CS value of the obtained chemically strengthened glass is lower than the standard value, the The molten salt is used to use a new molten salt.

Molten salts which cannot be obtained by chemical strengthening treatment to obtain the desired CS value are usually pulverized into small pieces after cooling and solidification, and then discarded. However, this treatment method cannot reuse waste melting again. Salt (waste salt), and it is necessary to use a large amount of molten salt or the like.

Therefore, Patent Document 1 discloses a method in which Li or Cs in a glass component is mixed as a impurity into a molten salt as a cause of a decrease in ion exchange capacity of the molten salt, and a salt of a high-temperature molten state is sprayed. The molten salt is dropped on the water in the tank, and the molten salt is dissolved, cooled, and separated in the water to regenerate the molten salt.

[Previous Technical Literature] [Patent Literature]

Patent Document 1: Japanese Patent Laid-Open Publication No. SHO 58-194761

However, in the process of heating the regenerated molten salt to be in a molten state again, there is a case where the container made of stainless steel which holds the molten salt is corroded. When the floating matter is generated in the molten salt due to the corrosion, if the chemical strengthening treatment of the glass is directly performed, there is a concern that the performance of the obtained chemically strengthened glass is affected.

Accordingly, it is an object of the present invention to provide a method for regenerating a molten salt for glass chemical strengthening treatment which has little influence on glass properties.

The inventors of the present invention have diligently studied and found that only the molten salt (waste salt) used for the chemical strengthening treatment is dissolved in an aqueous solution at a temperature not reaching the melting point, and is cooled and dried to be taken out only. The salt having a low Na concentration and the salt having a low Na concentration can be used again as a molten salt for chemical strengthening treatment of glass, thereby completing the present invention.

That is, the present invention relates to the following <1> to <6>.

<1> A method for regenerating a molten salt for chemical strengthening of a glass, which is a method for regenerating a molten salt for chemical strengthening, comprising: dissolving a molten salt after chemical strengthening treatment of glass at a temperature not exceeding a melting point of the molten salt a step in water, a step of cooling the aqueous solution obtained in the above-mentioned dissolving step to obtain a regenerated salt, and regenerating the above by drying The step of the amount of water in the salt is less than 5% by mass.

<2> The method for regenerating a molten salt for chemical strengthening of a glass according to the above <1>, wherein the step of cooling the aqueous solution to obtain a regenerated salt further comprises the step of concentrating the aqueous solution.

The method for regenerating a molten salt for glass chemical strengthening according to the above <1>, wherein the molten salt for glass chemical strengthening contains potassium nitrate.

The method for regenerating a molten salt for chemical strengthening of a glass according to any one of the above aspects, wherein the water content in the regenerated salt is less than 0.2% by mass in the drying step. .

The method for regenerating a molten salt for chemical strengthening of a glass according to any one of the above aspects, wherein the step of cooling the aqueous solution to obtain a regenerated salt is to separate the solid solution after cooling. To regenerate the salt and filtrate, one part of the filtrate is mixed in the solution in the above dissolution step.

(6) The method for regenerating a molten salt for chemical strengthening of a glass according to the above <5>, which is characterized in that the regenerated salt obtained by the solid-liquid separation is washed, and the solid-liquid separation is carried out into a regenerated salt and a filtrate, and the filtrate is filtrated. A part of it is mixed in the solution in the above dissolution step.

According to the method for regenerating a molten salt for chemical strengthening of glass according to the present invention, it is economically useful to reuse a part of the molten salt (waste salt) which has been previously discarded for chemical strengthening treatment for chemical strengthening treatment. Further, since the amount of the molten salt can be reduced, the risk of transportation of the molten salt accompanying the disposal can be reduced, and the load on the environment can be reduced. Further, by making the water content of the obtained regenerated salt 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, and this aspect is achieved. It is also very useful.

Fig. 1 is a view showing the implementation of a method for regenerating a molten salt for chemical strengthening of a glass of the present invention; Flow chart of the form.

Fig. 2 is a graph showing the solubility of a measured value of the solubility of a salt which can be contained in a molten salt after chemical strengthening treatment of glass in 100 g of water.

Fig. 3 is a graph showing the relationship between the number of reuses of the regenerated salt obtained in Example 2 and the recovery ratio of the regenerated salt.

Fig. 4 is a graph showing the relationship between the number of reuses of the regenerated salt obtained in Example 2 and the concentration of the regenerated salt Na.

Fig. 5 is a graph showing the relationship between the number of reuses of the regenerated salt obtained in Example 3 and the recovery ratio of the regenerated salt.

Fig. 6 is a graph showing the relationship between the number of reuses of the regenerated salt obtained in Example 3 and the concentration of the regenerated salt Na.

Fig. 7 is a graph showing the relationship between the number of reuses of the regenerated salt obtained in Example 4 and the recovery salt recovery rate.

Fig. 8 is a graph showing the relationship between the number of reuses of the regenerated salt obtained in Example 4 and the concentration of the regenerated salt Na.

The present invention will be described in detail below, but the present invention is not limited to the embodiments described below, and may be carried out without departing from the scope of the invention.

In the present specification, the meanings of "% by mass" and "% by weight", "ppm by mass" and "ppm by weight" are respectively the same. Further, when it is only referred to as "ppm", it means "weight ppm".

In the present specification, the term "Na concentration" means the concentration in terms of Na.

<Regeneration of molten salt>

The present invention relates to a method for regenerating a molten salt for chemical strengthening, which comprises the steps of: dissolving a molten salt after chemical strengthening treatment of glass in water at a temperature not exceeding the melting point of the molten salt, and cooling the obtained aqueous solution. And the steps of obtaining regenerated salt, And a step of reducing the amount of water in the above-mentioned regenerated salt by less than 5% by mass by drying.

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

The chemical strengthening treatment of the glass is carried out by immersing the glass as a raw material in a molten salt for glass reinforcement (may also be simply referred to as "molten salt"), and ion-exchange of Na in the glass with K in the molten salt. This forms a compressive stress layer as a high density layer on the surface of the glass.

The molten salt of the present invention contains an inorganic potassium salt. The inorganic potassium salt is preferably a melting point of a strain point (usually 500 to 600 ° C) of the glass which is subjected to chemical strengthening. In the present invention, a molten salt containing potassium nitrate (melting point 330 ° C) as a main component is preferred. Potassium nitrate molten salt). As long as potassium nitrate is a main component, it is preferably in a molten state below the strain point of the glass, and is easily handled in the use temperature region. Here, the main component means that the content in the molten salt is 50% by mass or more.

The molten salt preferably further comprises a compound 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. At least one salt of the group, more preferably containing 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 to chemically strengthen the glass, the content of K ₂ CO ₃ in the molten salt is set to 0.1% by mass or more, and the chemical is added. When the strengthening treatment temperature is set to 350 to 500 ° C, the chemical strengthening treatment time is preferably from 1 minute to 10 hours, more preferably from 5 minutes to 8 hours, and still more preferably from 10 minutes to 4 hours.

Further, in the molten salt for chemical strengthening treatment of the present invention, other chemical species may be contained within a range not inhibiting the effects of the present invention, and examples thereof include sodium chloride, potassium chloride, sodium borate, and boric acid. An alkali chloride or an alkali borate such as potassium. These may be added alone or in combination of plural kinds.

The molten salt used for the chemical strengthening treatment of glass can be produced according to a known method, and the molten salt can be used and chemical strengthening treatment of the glass can be carried out by a known method.

The molten salt (waste salt) which cannot be obtained by chemical strengthening treatment to obtain a desired surface compressive stress is cooled or cooled to a temperature which does not reach the melting point of the molten salt, whereby the molten salt is solidified. When a molten salt containing potassium nitrate is used as the molten salt, the waste salt contains potassium nitrate and sodium nitrate. Further, depending on the kind of the salt added to the molten salt, the waste salt may also contain a potassium salt or a sodium salt of the added salt. That is, for example, in the case of adding potassium carbonate (K ₂ CO ₃ ), the waste salt also contains potassium carbonate or sodium carbonate.

The Na concentration in the waste salt is usually from 4,000 to 20,000 ppm by mass.

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

The 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 the inclusion of impurities contained in the aqueous solution, pure water having a conductivity of 10 μS or less is preferable.

The temperature of the water in the case where the waste salt is dissolved may be a temperature which does not reach the melting point of the molten salt, preferably 60 to 120 ° C, and more preferably 80 to 100 ° C in terms of ease of handling. The water temperature can be appropriately adjusted by a known method such as a water bath or an oil bath.

Further, the higher the concentration of the waste salt in the aqueous solution, the better, and more preferably the solubility to the saturated solubility. Further, it is also preferred to dissolve the saturated solubility of the salt to be used as the regenerated salt in the plurality of salts contained in the spent salt, not the saturated solubility of the entire waste salt.

When the waste salt is dissolved, it is preferred to dissolve the waste salt while stirring the water. The stirring speed is usually from 50 to 2,000 rpm, preferably from 100 to 1,000 rpm.

After the waste salt is completely dissolved in water, the aqueous solution is cooled. In the waste salt When the desired salt is dissolved in the saturated solubility, depending on the type of the salt or the ratio contained in the waste salt, there are cases where the other salt is not completely dissolved and remains as a solid in the aqueous solution. In addition, there are cases where foreign matter contained in the waste salt remains. In this case, the undissolved salt or foreign matter is first removed by filtration or the like, and then the filtrate is cooled. The filtration accuracy is preferably 100 μm or less, more preferably 0.2 μm or more and 100 μm or less.

For the cooling, a known method such as natural cooling (cooling), water cooling, or ice cooling can be used. The cooling is preferably carried out until it is cooled to 25 ° C or lower, and more preferably cooled to 20 ° C or lower, and more preferably cooled to 10 ° C or lower in terms of improving the yield.

The precipitate (crystallized) is produced in the cooled aqueous solution by the difference between the solubility of the salt at the temperature at which the waste salt is dissolved and the solubility at the temperature at the time of cooling. When a molten salt containing potassium nitrate is used, potassium nitrate and sodium nitrate are contained in the waste salt and the precipitate. Further, depending on the kind of the salt added to the molten salt, the precipitate may contain a potassium salt or a sodium salt of the added salt.

Fig. 2 is a graph showing the solubility curve (g/water 100 g) of the measured value of the temperature dependence of the solubility of potassium nitrate, sodium nitrate, potassium carbonate and sodium carbonate in water.

Accordingly, the temperature of the aqueous solution is limited to the vicinity of 70 ° C. In the high temperature region, the solubility of potassium nitrate becomes higher than that of sodium nitrate and potassium carbonate, and in the low temperature region, the solubility of potassium nitrate becomes lower than that of sodium nitrate and potassium carbonate.

That is, the difference between the saturated solubility at the dissolution temperature and the saturated solubility at the cooling temperature when the waste salt containing the four salts of potassium nitrate, sodium nitrate, potassium carbonate, and sodium carbonate is dissolved in an aqueous solution having a saturated solubility The salt precipitates as a solid. When the salt to be used as the regenerated salt is used as potassium nitrate, when the difference in the saturated solubility of potassium nitrate is greater than the difference in the saturated solubility of the other salt, the precipitate precipitated by cooling the aqueous solution and the original waste salt The Na concentration in the precipitate becomes lower than the original waste salt compared to the potassium nitrate contained in a high ratio. Therefore, the precipitate can be reused for the molten salt of the glass chemical strengthening treatment, and it can be called "recycled salt".

As described above, in the present invention, the solubility of potassium nitrate and sodium nitrate is reversed in the high temperature region and the low temperature region, and the salt having a low Na concentration can be regenerated from the waste salt having a high Na concentration by recrystallization. When the Na concentration in the regenerated salt is 1000 ppm by mass or less, it can be reused in the chemical strengthening treatment of glass.

Further, in the step of dissolving the waste salt in water and cooling the obtained aqueous solution to obtain a regenerated salt, it is preferred to further concentrate the aqueous solution.

The concentration system increases the salt concentration in the aqueous solution, and a known method such as vacuum concentration (concentration under reduced pressure) or freeze concentration can be used. The salt which was not completely dissolved was precipitated by concentrating the aqueous solution. By combining the cooling step of the aqueous solution with the concentration step, the regenerated salt having a lower Na concentration can be obtained with high efficiency.

Since the obtained regenerated salt is precipitated into the aqueous solution, it is subjected to solid-liquid separation in order to be reused for the chemical strengthening treatment of the glass. For the solid-liquid separation, a known method such as filtration or centrifugation can be used. Since a molten salt (for example, potassium nitrate) remains in the filtrate after the solid-liquid separation, a part of the filtrate can be mixed in the dissolved salt solution and reused. However, when the filtrate is reused and the regeneration is repeated, the Na concentration in the filtrate increases, and as a result, the Na concentration in the regenerated salt also increases. Therefore, it is preferred to partially discard the filtrate to control the Na concentration. The amount of the filtrate to be reused can be determined in consideration of the Na concentration.

In order to increase the purity of the regenerated salt obtained, the regenerated salt may also be washed. The washing can be carried out in pure water having a conductivity of 10 μS or less. Further, the temperature of the washing water is preferably set to 20 ° C or lower. However, if the washing is carried out, the yield of the regenerated salt obtained is lowered. Therefore, the balance between the yield and the purity must be considered in accordance with the purpose, and it is appropriately determined whether or not washing is required.

After washing with the regenerated salt, the washing liquid is further subjected to solid-liquid separation to obtain a regenerated salt. Since a molten salt (for example, potassium nitrate) remains in the filtrate after the solid-liquid separation, a part of the filtrate can be mixed in the dissolved salt solution and reused. However, when the filtrate is reused and the regeneration is repeated, the Na concentration in the filtrate increases, and along with this, The concentration of Na in the raw salt also increases. Therefore, it is preferred to partially discard the filtrate to control the Na concentration. The amount of the filtrate to be reused can be determined in consideration of the Na concentration.

After recovering the regenerated salt by solid-liquid separation, it is preferred to sufficiently dry it 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 mass% of water.

When a molten salt is prepared by heating the regenerated salt for chemical strengthening treatment, a stainless steel (SUS) container is used. However, if the amount of water in the regenerated salt is large, the regenerated salt is heated to form a molten salt. The container is corroded. When the float is generated in the molten salt due to the corrosion, if the chemical strengthening treatment of the glass is directly performed, the performance of the obtained 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 even more preferably less than 0.2% by mass. Further, the amount of water in the regenerated salt can be measured by TGA (thermogravimetric method). The distilled water produced by drying can be reused as a solvent for dissolving the waste salt. By using distilled water again, it can help to reduce the environmental load.

The drying temperature is usually 40 to 300 ° C, more preferably 80 to 200 ° C. The drying time is usually from 1 to 12 hours, more preferably from 1 to 4 hours. Further, it is also possible to perform pressure reduction while heating while drying.

Drying can be carried out by a known method such as heating plate or heating under vacuum.

In order to prevent moisture from being mixed into the regenerated salt after drying, it is preferred to store it in a closed container.

The regenerated salt passing through the drying step can be used as a molten salt for glass chemical strengthening treatment by heating it to a temperature at which chemical strengthening treatment of the glass is performed.

The regenerated salt obtained by the present invention can be reused as a molten salt by the regeneration treatment of the present invention after it is used as a molten salt for chemical strengthening treatment of glass. According to the regeneration method of the present invention, by partially reusing one of the filtrate after solid-liquid separation to dissolve the waste salt, the Na concentration in the regenerated salt can be kept below a specific concentration, and the regenerated salt can be raised. Yield. In the case where the filtrate is not reused, it becomes difficult to increase the yield of the regenerated salt. On the other hand, when the entire filtrate is reused, the Na concentration in the aqueous solution increases, and the Na concentration in the obtained regenerated salt also increases.

In the case where 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. Further, the amount of nitrous acid contained in the new potassium nitrate molten salt which is not chemically strengthened is usually 10 ppm by weight or less. It is considered that the nitric acid in the molten salt is converted into nitrous acid by repeated chemical strengthening, and thus the nitrous acid content is increased. The nitrous acid content in the molten salt can be determined by a naphthylethylenediamine colorimetric method.

In addition, the glass to be subjected to the chemical strengthening treatment in the present invention may contain any composition as long as it contains a component which can be strengthened by molding or chemical strengthening treatment. Specific examples thereof include aluminosilicate glass, soda lime glass, borosilicate glass, lead glass, alkali bismuth glass, and aluminum borosilicate glass.

Among them, since the aluminosilicate glass has a large amount of Na substitution in the glass, the deterioration of the molten salt is severe. Therefore, the effect of the method for regenerating the molten salt of the present invention can be remarkably obtained, which is preferable.

The method for producing the glass to be subjected to the chemical strengthening treatment and the molding method are not particularly limited, and can be produced and molded by a known method. Moreover, the thickness or polishing of the glass for chemical strengthening treatment may be optional.

[Examples]

Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited thereto.

<glass>

In the present embodiment, an aluminosilicate glass having the composition (mol%) shown below 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 the present 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 Co., Ltd.

(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" of the original production facility.

(Measurement of moisture content)

The amount of water contained in the regenerated salt was measured using a heat-drying moisture meter "MS-70" manufactured by A&D.

<Example 1>

The aluminosilicate glass is chemically strengthened in a molten salt composed only of potassium nitrate. The chemical strengthening treatment temperature was set to 450 °C. The concentration of Na 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 the solid waste salt was divided into a size of 30 cm ³ or less, weighed in a 2000 mL beaker, and 800 g of pure water was added. One side was automatically stirred at 200 rpm while being heated to 80 ° C in a water bath to obtain an aqueous solution in which all of the waste salt was dissolved in pure water. It was confirmed that the waste salt was completely dissolved, and the mixture was automatically stirred at 200 to 300 rpm while being ice-cooled to 1 ° C to precipitate a salt (recrystallization).

Then, suction filtration is performed to separate the crystal of the obtained salt from the aqueous solution. The crystals after separation by filtration were collected, and dried on a hot plate set to 80 ° C for 5 hours, whereby 903 g of a regenerated salt was obtained. The amount of water in the obtained regenerated salt was 3% by mass, and the content of nitrous acid was 20 ppm. Regarding the recovery rate, the theoretical yield determined based on the solubility curve was 94%, which was actually 90% with respect to the theoretical yield. Further, the Na concentration in the obtained regenerated salt was 400 ppm by mass.

<Glass chemical strengthening treatment>

The regenerated salt obtained in Example 1 was heated to 450 ° C in a SUS vessel to prepare a molten salt, and the aluminosilicate glass preheated to 200 to 400 ° C was immersed in the molten salt for 2 hours. Chemical strengthening treatment. At this time, it was not visually observed that there was a floating matter due to corrosion of SUS in the molten salt. After the intensive treatment, the glass was washed twice with ion-exchanged water at 20 to 80 ° C, and washed with running water at room temperature with ion-exchanged water. The initial surface compressive stress (initial CS) of the obtained chemically strengthened glass was 844 MPa. Further, the new potassium nitrate which was not subjected to ion exchange treatment was used as a molten salt to chemically strengthen the aluminosilicate glass, and the initial CS at this time was 750 to 900 MPa.

<Example 2>

150 kg of waste salt containing potassium nitrate as a main component and having a Na concentration of 10,000 ppm was placed in a SUS container, and 90.3 kg of pure water was added. It was heated to 90 ° C by an electric heater and dissolved while stirring. After completely dissolving, it was taken out to another SUS container, and cooled to room temperature by cooling to precipitate a salt. Then, centrifugation was carried out to separate the crystal of the obtained salt from the aqueous solution, thereby obtaining a salt and a filtrate having a moisture content of 2% by mass. The obtained salt was washed with pure water and centrifuged again to obtain a salt having a water content of 2% by mass and a filtrate. The obtained salt was dried at 200 ° C for 8 hours to obtain a regenerated salt having a water content of 0.05% by mass, a 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 the filtrate was placed in a SUS container together with 112.5 kg of the waste salt, and 6.5 kg of pure water was added. This was similarly heated to 90 ° C and dissolved while stirring. Thereafter, cooling, centrifugation, washing, and centrifugation are carried out to obtain a regenerated salt and a filtrate. The results of the experiment in which the above operation was repeated were shown in Table 1. Further, the results of the simulation under the same conditions are shown in Table 2. Further, the results are shown in FIGS. 3 and 4 as a graph.

<Chemical strengthening treatment>

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

<Example 3>

In the simulation of Example 2, Table 3 shows the simulation results when the amount of the filtrate was 53.7 kg, and Table 4 shows the simulation results when the amount of the filtrate was 33.7 kg. Moreover, these results are shown in FIG. 5 and FIG.

<Example 4>

In the simulation of Example 2, the results of the simulation when the entire filtrate was discarded were shown in Table 5. Table 6 shows the simulation results in the case where all the filtrates were reused. Moreover, these results are shown in FIG. 7 and FIG.

According to the above results, it is understood that the water content and the Na concentration contained in the regenerated salt obtained by the regeneration method of the present invention are extremely low, and even when reused as a molten salt for glass chemical strengthening treatment, the glass can be imparted and not supplied. The surface compressive stress equivalent to the new molten salt of the ion exchange treatment.

Further, according to the solubility curve shown in Fig. 2, it is suggested that the possibility of the yield of the regenerated salt can be further increased by lowering the precipitation temperature at the time of recrystallization.

Further, it was found that the yield of the regenerated salt can be improved by recycling the filtrate obtained when the solid-liquid separation is carried out. Further, it was found that the Na concentration in the obtained regenerated salt can be controlled by adjusting the amount of the reused filtrate.

The present invention has been described in detail with reference to the preferred embodiments of the invention, and various modifications and changes can be made without departing from the spirit and scope of the invention. The present application is based on Japanese Patent Application No. 2013-247987, filed on Nov. 29, 2013, the content of which is hereby incorporated by reference.

[Industrial availability]

According to the present invention, by regenerating the molten salt used for the chemically strengthened glass, a regenerated salt having the same performance as the new molten salt can be obtained. By this regeneration treatment, the amount of used molten salt (waste salt) can be reduced, environmental load can be reduced, and chemically strengthened glass can be produced at low cost, and high productivity can be achieved.

Claims (6)

A method for regenerating a molten salt for chemical strengthening of a glass, comprising: a step of dissolving a molten salt obtained by chemical strengthening treatment of glass in water at a temperature not exceeding a melting point of the molten salt; and cooling the aqueous solution obtained by the dissolving step The step of obtaining a regenerated salt and the step of drying the water content in the regenerated salt to less than 5% by mass. The method for regenerating a molten salt for chemical strengthening of a glass according to claim 1, wherein the step of cooling the aqueous solution to obtain a regenerated salt further comprises the step of concentrating the aqueous solution. The method for regenerating a molten salt for chemical strengthening of a glass according to claim 1 or 2, wherein the molten salt for chemical strengthening of the glass contains potassium nitrate. The method for regenerating a molten salt for chemical strengthening of a glass according to any one of claims 1 to 3, wherein in the drying step, the amount of water in the regenerated salt is set to less than 0.2% by mass. The method for regenerating a molten salt for chemical strengthening of a glass according to any one of claims 1 to 4, wherein 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 portion of the filtrate is mixed in the solution in the above dissolution step. The method for regenerating a molten salt for chemical strengthening of a glass according to claim 5, wherein the regenerated salt obtained by the solid-liquid separation is washed, and then solid-liquid separation into a regenerated salt and a filtrate, and a part of the filtrate is mixed in the above Dissolve in the solution in the solution.