JP6149733B2 - Method for regenerating molten salt for glass strengthening and method for producing tempered glass - Google Patents

Description

  The present invention relates to a method for regenerating a used molten salt when the molten salt used for chemically strengthening the glass deteriorates due to use, and further, production of tempered glass using the molten salt regenerated by the method. Regarding the method.

  Glass that has been chemically strengthened by ion exchange or the like (hereinafter also referred to as chemically tempered glass) is used for cover glasses and display glass substrates of display devices such as mobile phones, smartphones, and tablet terminals. Although glass has a high theoretical strength, the strength is greatly reduced by scratching. Chemically tempered glass is ideal for these applications because it has higher mechanical strength than unstrengthened glass and prevents damage to the glass surface.

  Chemical strengthening treatment replaces metal ions with a small ionic radius contained in the surface layer of glass with metal ions with a larger ionic radius, thereby creating a compressive stress layer on the glass surface and improving the mechanical strength of the glass. Ion exchange treatment.

The ionic radius of the alkali metal increases in the order of Li ⁺ <Na ⁺ <K ⁺ <Rb ⁺ <Cs ⁺ . Therefore, for example, when Na ₂ O is contained in the glass composition, the glass is immersed in an inorganic molten salt containing K ions (hereinafter also simply referred to as a molten salt), and Na ions in the glass and K ions in the molten salt are mixed. By performing ion exchange, compressive stress can be generated on the glass surface.

  The strength of chemically tempered glass can be evaluated by several indices, one of which is surface compressive stress (CS, also referred to as CS hereinafter). The CS value of glass is determined by various factors such as the time of immersion in the molten salt, the temperature of the molten salt, the composition of the glass and the composition of the molten salt.

  The CS value of the glass after chemical strengthening using a molten salt is the case where a fresh molten salt that has not been subjected to ion exchange (hereinafter referred to as a new molten salt) and a molten salt that has been subjected to repeated ion exchange (hereinafter referred to as a molten salt). When compared with the case of using a deteriorated molten salt), the CS value generally decreases in the latter case. This is because the K ion concentration of the molten salt is lowered by Na ions and Li ions eluted from the glass by ion exchange.

  In order to suppress the decrease in CS, a method of exchanging part or all of the deteriorated molten salt with a new molten salt is conceivable. However, this method increases the frequency of molten salt exchange, lowers the chemical strengthening treatment efficiency, and requires a purchase cost for new molten salt and a disposal cost for deteriorated molten salt, leading to higher costs.

Therefore, various methods for extending the use period of the molten salt have been considered. For example, Patent Document 1 discloses a method of adding potassium pyrophosphate (K ₄ P ₂ O ₇ ) to a deteriorated molten salt.

Patent Document 2 discloses a method for keeping the molten salt long by adding an additive selected from the group containing potassium orthophosphate (K ₃ PO ₄ ) to the deteriorated molten salt.

Japanese Examined Patent Publication No. 46-38514 JP 2013-67554 A

  It is known that the addition of potassium pyrophosphate described in Patent Document 1 and potassium orthophosphate described in Patent Document 2 to the molten salt has the effect of reducing Na ion concentration in the molten salt and restoring the CS value. Yes. On the other hand, some of these additives remain in the molten salt after addition and adhere to the glass. Patent Document 2 also describes that when it comes into contact with water in a washing step or the like, it dissolves into a strong alkaline solution (pH> 10). Thereby, there exists a possibility of deteriorating the surface roughness of glass.

  Surface roughness is particularly problematic for products that require high form accuracy. Further, each time the addition of an additive containing potassium orthophosphate or potassium pyrophosphate (hereinafter referred to as a sodium precipitant) is repeated, the concentration of the sodium precipitant in terms of phosphorus element mass in the molten salt (hereinafter referred to as the phosphorus concentration). )) Increases, so the effect on the surface roughness of the glass also increases.

  However, at present, there is no known method for reducing the phosphorus concentration derived from the additive in the molten salt. When the phosphorus concentration increases, a part or all of the molten salt is replaced with a new molten salt as usual. There is only. This still reduces the efficiency of the chemical strengthening process.

  Therefore, an object of the present invention is to provide a method for reducing the phosphorus concentration of the molten salt and regenerating the molten salt after adding a sodium precipitant in order to reduce the Na ion concentration in the deteriorated molten salt. Moreover, it aims at providing the manufacturing method of the tempered glass using the molten salt reproduced | regenerated by the said reproduction | regeneration method.

  The present inventors added a sodium precipitant to the deteriorated molten salt, and then added an additive containing at least one Group 2 element nitrate compound (hereinafter referred to as a phosphorus precipitant) to melt the molten salt. It has been found that the Na ion concentration can be recovered to a desired low value while suppressing the phosphorus concentration in the salt, and the present invention has been completed.

That is, the present invention is as follows.
<1> A molten salt regeneration method for removing sodium eluted in a molten salt by chemically strengthening glass using a molten salt containing potassium nitrate,
A sodium precipitation step of adding at least one sodium precipitant selected from the group consisting of potassium orthophosphate or potassium pyrophosphate to the molten salt;
A phosphorus precipitation step of adding a phosphorus precipitant comprising at least one Group 2 element nitrate compound to the molten salt;
A method for regenerating a molten salt comprising:
<2> The method for regenerating a molten salt according to <1>, wherein a phosphorus precipitation step is performed after the sodium precipitation step.
<3> The molten salt regeneration method according to <2>, further including a step of removing the phosphate precipitated in the sodium precipitation step from the molten salt before the phosphorus precipitation step.
<4> The molten salt regeneration method according to <1>, wherein the sodium precipitation step and the phosphorus precipitation step are performed simultaneously.
<5> The method for regenerating a molten salt according to <1> to <4>, wherein the phosphorus precipitant contains magnesium nitrate.
<6> When the phosphorus concentration before adding the phosphorus precipitant is X (mass ppm), the phosphorus precipitant addition amount is Y (mol), and the mass of the molten salt is Z (g), these are the relational expressions. The method for regenerating a molten salt according to <1> to <5>, wherein Y ≧ Z (X−500) / (31 × 10 ⁶ ) is satisfied.
<7> The method for regenerating a molten salt according to <6>, wherein the relational expression Y ≦ ZX / (31 × 10 ⁶ ) is satisfied.
<8> The melting regeneration method according to <1> to <7>, wherein the temperature of the molten salt in the phosphorus precipitation step is 330 to 500 ° C.
<9> A method for producing tempered glass, comprising a step of chemically strengthening glass using the molten salt regenerated by the method according to any one of <1> to <8> above.

  In the molten salt regeneration method according to the present invention, after adding a sodium precipitant for reducing the Na ion concentration, the phosphorus salt is added to the molten salt having a high phosphorus concentration, so that it is low. It is possible to reduce the phosphorus concentration while maintaining the Na ion concentration. Thereby, it is possible to improve the surface quality of the glass and reduce the frequency of the replacement process from the deteriorated molten salt to the new molten salt. Moreover, since the lifetime of molten salt is extended, it leads to cost reduction and improvement of processing efficiency.

FIG. 1 shows the amount of magnesium nitrate added (mol) and the molar mass of precipitated phosphorus when the precipitate is removed after addition of potassium orthophosphate as a sodium precipitant and magnesium nitrate is further added as a phosphorus precipitant. It is a graph which shows the relationship with (mol).

  Hereinafter, the present invention will be described in detail.

  The regeneration method of the present invention can be applied in the following steps including, for example, chemical strengthening treatment. The present invention is not limited to these.

Step 1: Preparation of molten salt Step 2: Chemical strengthening treatment of glass Step 3: Determination of molten salt deterioration Step 4 (sodium precipitation step): Addition of potassium orthophosphate to molten salt Step 5: Precipitated sodium potassium phosphate Separation and removal step 6 (phosphorus precipitation step): Addition of magnesium nitrate to molten salt Step 7: Repeat steps 2 to 6 (step 1)
In step 1, an inorganic salt containing K is put into a container and heated to a temperature equal to or higher than the melting point of the salt to melt it, thereby preparing a molten salt. The inorganic salt is preferably in a molten state below the strain point (usually 500 to 600 ° C.) of the glass to be chemically strengthened, and contains potassium nitrate (melting point 330 ° C.) as a main component in the present invention. 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 a general temperature range when chemical strengthening treatment is performed. Here, the main component means 50% or more by mass ratio.

  The molten salt may contain other inorganic salt besides potassium nitrate. Examples thereof include inorganic salts containing alkali metals such as alkali sulfates such as potassium sulfate, alkali chlorides such as potassium chloride, and alkali carbonates such as potassium carbonate. Among these, a mixed molten salt of potassium nitrate and potassium carbonate is preferable from the viewpoint of cost reduction (for example, reduction in regeneration frequency). In this case, the addition amount of potassium carbonate is preferably 0.01% by mass to 30% by mass and more preferably 0.01% by mass to 13% by mass with respect to potassium nitrate. The upper limit of the amount of potassium carbonate added is 30% by mass. If the amount added is less than this upper limit, the molten salt is easy to handle because there is little potassium carbonate present in the solid phase, and temperature unevenness is unlikely to occur during the ion exchange treatment, so the entire glass is ionized uniformly. It is preferable because it can be exchanged.

Since potassium nitrate has a melting point of 330 ° C. and a boiling point of 500 ° C., the temperature of the molten salt is preferably maintained within the range. In particular, the melting temperature is preferably 350 to 470 ° C. from the viewpoint of the balance between the CS value that can be imparted to the glass and the depth of compressive stress layer (Depth of Layer; hereinafter referred to as DOL) and the strengthening time.
A metal, quartz, ceramics, etc. can be used for the container for melting the inorganic salt. Among these, a metal material is preferable from the viewpoint of durability, and a stainless steel (SUS) material is more preferable from the viewpoint of corrosion resistance.

(Process 2)
In step 2, the glass is preheated, and at the same time, the molten salt prepared in step 1 is adjusted to a predetermined temperature. Next, the preheated glass is immersed in the molten salt for a predetermined time, and then the glass is pulled up from the molten salt and allowed to cool. The composition of the glass will be described later. In addition, you may perform mechanical processing, such as a shape process according to a use, for example, a cutting | disconnection, an end surface process, and a drilling process, before a chemical strengthening process.

  The preheating temperature of the glass depends on the temperature of the molten salt in which the glass is immersed, but is generally preferably 100 ° C. or higher.

  The chemical strengthening temperature is preferably not more than the strain point (usually 500 to 600 ° C.) of the glass to be tempered, and particularly preferably 350 ° C. or more in order to obtain a larger DOL value.

  When the immersion time of the glass in the molten salt is less than 10 minutes, only a small DOL value can be obtained, and a deep flaw that leads to breakage of the glass, that is, a flaw having a depth exceeding the DOL value is likely to occur. Further, when the immersion time exceeds 12 hours, the DOL value becomes too large, so that the internal tensile stress (hereinafter also referred to as CT) also increases, and the impact at the time of breaking increases. When the CT value is large, there is a tendency that the glass breaks into pieces when the glass breaks. Therefore, the immersion time is preferably 10 minutes to 12 hours, more preferably 30 minutes to 10 hours. If it exists in this range, the chemically strengthened glass excellent in the balance of an intensity | strength and a DOL value can be obtained.

(Process 3)
When Step 2 is repeated, Na ions are eluted from the glass in the molten salt, so that the ion exchange capacity of the molten salt decreases as the glass treatment area increases, and the desired CS value cannot be obtained. Therefore, in Step 3, the deterioration state of the molten salt is investigated by measuring the Na ion concentration in the molten salt or the CS value of the glass after chemical strengthening, and the molten salt can be used continuously or In step 4, it is determined whether a process for reducing the Na ion concentration is necessary.
In the present invention, when the CS value obtained with a new molten salt is 100%, a CS value of 95% or more is defined as a desired CS value. Processing to reduce the density is performed.

(Process 4)
In step 4 (hereinafter, also referred to as a sodium precipitation step), a sodium precipitant is added to the molten salt having reduced ion exchange capacity, and the temperature is adjusted to be kept constant and allowed to stand. In order to make the whole uniform or accelerate the reaction, the molten salt after the addition may be mixed with a stirring blade or the like. By this operation, Na ions in the molten salt are ion-exchanged with K ions of the added sodium precipitant, and are precipitated and precipitated as a sodium salt. Therefore, the Na ion concentration in the molten salt decreases and the K ion concentration increases. Thereby, the molten salt which obtains a desired CS value is obtained.
Examples of the sodium precipitant include a combination with at least one selected from the group consisting of potassium orthophosphate and potassium pyrophosphate. The sodium precipitant may be a hydrate or a dehydrated one.

  The optimum addition amount of the sodium precipitant for obtaining a desired CS value varies depending on the Na ion concentration of the molten salt, the composition of the sodium precipitant, and the additive. When the sodium precipitating agent is potassium orthophosphate, the lower limit of the amount added is preferably 1.0-fold molar mass or more (100 mol% or more), 1.5-fold molar mass or more with respect to the Na ion concentration in the molten salt. Is more preferable, and 3.0 times molar mass or more is more preferable. When the sodium precipitating agent is potassium pyrophosphate, the lower limit of the addition amount is preferably 0.75 times or more (75 mol% or more), and 1.13 times or more mol mass with respect to the Na ion concentration in the molten salt. Is more preferable, and 2.25 times the molar mass or more is more preferable. If the lower limit is exceeded, a desired CS value can be obtained. The Na ion concentration in the molten salt can be measured by an atomic absorption analyzer or the like.

  On the other hand, the upper limit of the amount of sodium precipitant added is preferably 10% by mass or less with respect to potassium nitrate. The sodium precipitant has a high melting point (> 1000 ° C.), and the amount dissolved in potassium nitrate is negligible in the temperature range (500 ° C. or lower) used for chemical strengthening. Therefore, if the amount of sodium precipitant added is excessive, it may accumulate as a precipitate on the bottom of the container, making it difficult to handle the molten salt. In particular, when it is added in an amount of more than 10% by mass with respect to potassium nitrate, the ratio of the solid phase of the sodium precipitant becomes 20% by mass or more, and not only the liquid phase volume that can be used for chemical strengthening is greatly reduced, There is a possibility that the sodium precipitant per se precipitates and contacts the glass and induces corrosion of the glass surface. Moreover, as described in Patent Document 2, when the surplus sodium precipitating agent moves to the washing step while adhering to the glass, the sodium precipitating agent is dissolved in water as described above, and a strong alkaline solution (pH> 10). Therefore, there is a possibility that the surface roughness of the glass is deteriorated.

  In addition to the sodium precipitant, other inorganic salts may be added to the deteriorated molten salt. Examples thereof include potassium carbonate and potassium sulfate. Thereby, the Na ion concentration in molten salt can be lowered | hung similarly to the effect of sodium precipitant addition. In addition, the addition amount of other inorganic salts is preferably 30% by mass or less with respect to potassium nitrate. The order of addition is not particularly limited.

  After the addition of the sodium precipitant, it is preferable to stir the molten salt in order to make the whole uniform or to accelerate the reaction. At the time of stirring, the temperature of the molten salt is preferably equal to or higher than the melting point of potassium nitrate, that is, 330 ° C or higher, and more preferably 350 ° C to 500 ° C. The stirring time is preferably 1 minute to 10 hours, and more preferably 10 minutes to 2 hours.

In addition, generally the service life of molten salt can be extended, so that the Na ion density | concentration after finishing the process 4 is low.
The lifetime of the molten salt is evaluated using the Na ion concentration in the molten salt as an index when the CS value is reduced by 10% when the CS value obtained by the chemical strengthening treatment using the molten salt in the initial state is taken as 100%. be able to.
When actually evaluating the molten salt life, for example, in order to create a state in which the Na ion concentration in the molten salt is increased by continuous use, a predetermined amount of Na source such as sodium nitrate is intentionally added. Added. A relational expression between the amount of Na source added and the CS value of the glass obtained after the chemical strengthening treatment is derived, and the amount of Na added when the CS value is reduced by 10% from the initial state is calculated by, for example, linear approximation. It can be used as an index of life.

(Process 5)
In step 5, a precipitate containing a sodium salt precipitated and precipitated by adding a sodium precipitant is separated from the molten salt and then removed.
As a method for removing the precipitate, for example, filtration by a filter or drawing of the precipitate can be considered, but the method is not particularly limited.

  Implementation of step 5 is not essential. However, when the phosphorus precipitant is added in Step 6 to be described later, if Step 5 is not passed, there is a possibility that a part of the sodium salt present as a precipitate is redissolved. That is, it is not preferable because the Na ion concentration is increased again and the phosphorus concentration is not easily lowered. Moreover, since the amount of precipitates increases and accumulates by repeating steps 2 to 6, the precipitates may come into contact with the glass and induce corrosion of the glass. Therefore, it is preferable to go through step 5.

(Step 6)
In step 6 (hereinafter also referred to as phosphorus precipitation step), a phosphorus precipitant containing at least one group 2 element nitrate compound is added to the molten salt having an increased phosphorus concentration, and the temperature is adjusted to be constant. Leave still. In order to make the whole uniform or accelerate the reaction, the molten salt after the addition may be mixed with a stirring blade or the like. By this operation, the orthophosphate ion or pyrophosphate ion in the molten salt is ion-exchanged with the nitrate ion of the added nitrate, and is precipitated and precipitated as a salt having a small solubility product in the molten salt. For this reason, the phosphorus concentration in the molten salt decreases. Thereby, while being able to obtain a desired CS value, it can be again subjected to the chemical strengthening treatment (step 2) as a molten salt that does not deteriorate the surface roughness of the glass.

The phosphorus concentration in the molten salt is preferably 500 mass ppm or less, more preferably 200 mass ppm or less, and even more preferably 100 mass ppm or less. When the phosphorus concentration exceeds 500 ppm by mass, the mechanical strength of the glass is significantly reduced.
The phosphorus concentration in the molten salt can be measured by an inductively coupled plasma (ICP) emission analysis method or the like.

The optimum addition amount of the phosphorus precipitant for obtaining a desired phosphorus concentration varies depending on the phosphorus concentration of the molten salt and the additive, but the lower limit of the addition amount of the phosphorus precipitant for obtaining a phosphorus concentration of C (mass ppm) or less. Is represented by the following (formula 1). [Wherein Y is the addition amount (mol) of the phosphorus precipitant, Z is the total amount (g) of the molten salt, and X is the phosphorus concentration (mass ppm) before the addition of the phosphorus precipitant. ]
Y = Z (X-C) / (31 × 10 ⁶ ) (Formula 1)
If it is more than the said minimum, phosphorus concentration can be adjusted to 500 mass ppm or less which is a desired value.

On the other hand, the upper limit of the addition amount of the phosphorus precipitant is represented by the following (Formula 2).
Y = ZX / (31 × 10 ⁶ ) (Formula 2)
If it is below the said upper limit, the density | concentration of an excess phosphorus precipitant can be suppressed, keeping a phosphorus density | concentration below a desired value.

  Examples of the phosphorus precipitant that is at least one Group 2 element nitrate compound include beryllium nitrate, calcium nitrate, strontium nitrate, barium nitrate, and radium nitrate. The order of addition is not particularly limited.

  After adding the phosphorus precipitant, it is preferable to stir the molten salt in order to make the whole uniform or to promote the reaction. In stirring, the lower limit of the temperature of the molten salt is preferably not less than the melting point of potassium nitrate, that is, 330 ° C. or more, more preferably 350 ° C. or more. Further, the upper limit of the temperature of the molten salt is preferably not more than the boiling point of potassium nitrate, that is, 500 ° C. or less, more preferably 470 ° C. or less, further preferably 400 ° C. or less, and particularly preferably 380 ° C. or less. . The stirring time of the molten salt is preferably 1 minute to 10 hours, and more preferably 10 minutes to 2 hours.

(Step 7)
In step 7, the above steps 2 to 6 are repeated. The molten salt deteriorated by the ion exchange treatment is reduced to a desired value or less by passing through step 4 and adjusted to a desired phosphorus concentration by passing through step 6.

<Glass>
The glass used in the present invention only needs to contain sodium, and glass having various compositions can be used as long as it has a composition that can be strengthened by molding and chemical strengthening treatment. Specific examples include soda lime glass, aluminosilicate glass, borosilicate glass, lead glass, and alkali barium glass.

  The manufacturing method of glass is not specifically limited. A desired glass raw material is charged into a continuous melting furnace, and the glass raw material is heated and melted preferably at 1500 to 1600 ° C., clarified, then supplied to a forming apparatus, and then the molten glass is formed into a plate shape and slowly cooled. Can be manufactured.

  Various methods can be employed for forming the glass. For example, various forming methods such as a down draw method (for example, an overflow down draw method, a slot down method and a redraw method), a float method, a roll-out method, and a press method can be employed.

  The thickness of the glass is not particularly limited, but is usually preferably 5 mm or less and more preferably 3 mm or less in order to effectively perform the chemical strengthening treatment.

Although it does not specifically limit as a composition of the glass for chemical strengthening of this invention, For example, the following glass compositions are mentioned.
(I) SiO ₂ 50-50%, Al ₂ O ₃ 2-25%, Li ₂ O 0-10%, Na ₂ O 0-18%, K ₂ O with the composition expressed in mol% Glass containing 0 to 10% MgO, 0 to 15% MgO, 0 to 5% CaO and 0 to 5% ZrO ₂ (ii) The composition expressed in mol% is SiO ₂ 50 to 74%, Al ₂ O ₃ and 1-10% 6-14% of Na ₂ O, _{K 2} O 3-11% of MgO 2 to 15% of CaO Less than six% and ZrO ₂ to contain 0-5% The total content of SiO ₂ and Al ₂ O ₃ is 75% or less, the total content of Na ₂ O and K ₂ O is 12 to 25%, and the total content of MgO and CaO is 7 to 15%. A certain glass (iii) The composition represented by mol% is 68 to 80% of SiO ₂ and 4 to 1 of Al ₂ O ₃ . 0%, 5-15% of Na ₂ O, the _{K 2} O 0 to 1% a composition displaying at MgO 4-15% and glass containing _{ZrO 2 0~1% (iv) mol} %, SiO ₂ 67 to 75%, the _Al 2 _{O 3} 0 to 4% of Na ₂ O 7 to 15% of _{K 2} O 1 to 9% of MgO having 6 to 14% and ZrO ₂ 0 to 1.5 %, The total content of SiO ₂ and Al ₂ O ₃ is 71 to 75%, the total content of Na ₂ O and K ₂ O is 12 to 20%, and when CaO is contained, the content Is a glass of less than 1%.

  If necessary, the glass may be polished before the chemical strengthening treatment. Examples of the polishing method include a method of polishing with a polishing pad while supplying a polishing slurry. As the polishing slurry, a polishing slurry containing an abrasive and water can be used. As the abrasive, cerium oxide (ceria) or silicon oxide (silica) is preferable.

  When the glass is polished, the polished glass is cleaned with a cleaning liquid. The washing liquid is preferably a neutral detergent and water, and more preferably washed with water after washing with a neutral detergent. A commercially available neutral detergent can be used.

  The glass substrate cleaned by the cleaning step is finally cleaned with a cleaning liquid. Examples of the cleaning liquid include water, ethanol, and isopropanol. Of these, water is preferred.

  After the final cleaning, the glass is dried. The drying conditions may be selected in consideration of the cleaning solution used in the cleaning process, the characteristics of the glass, and the like.

  Examples of the present invention will be specifically described below, but the present invention is not limited to these.

[Examples 1-3: Addition of magnesium nitrate to deteriorated molten salt]
(Test Example 1: Preparation of pseudo-degraded molten salt)
Potassium nitrate 395.7g was added to the SUS cup, and it heated to 430 degreeC with the mantle heater, and prepared the molten salt. To this molten salt, 4.4 g of sodium nitrate was intentionally added in order to artificially create a deteriorated molten salt state (Na ion concentration: 3000 mass ppm) after the glass strengthening treatment.

(Test Example 2: Sodium precipitation step)
16 g (K ₃ PO ₄ 3.0 mass) of potassium orthophosphate trihydrate (K ₃ PO ₄ .3H ₂ O) as a sodium precipitating agent for the molten salt in a deteriorated state that was artificially prepared in Test Example 1. %) Was added. And it stirred for 2 hours using the stirring motor and 4 propeller blades, and left still for 2 hours.
The Na ion concentration of the molten salt after the treatment was measured using an atomic absorption photometer (Z-2310, manufactured by Hitachi High-Tech). Further, the phosphorus concentration of the molten salt was measured by an ICP emission spectrometer (SPS5520 manufactured by Hitachi High-Tech).

(Example 1: Phosphorus precipitation step)
In the test example 2, 0.80 g (Mg (NO ₃ ) ₂ 0.20 mass%) of magnesium nitrate was added as a phosphorus precipitant to the molten salt having a reduced Na ion concentration. And it stirred for 2 hours using the stirring motor and 4 propeller blades, and left still for 2 hours.
The Na ion concentration and phosphorus concentration of the molten salt after the treatment were measured. The results are shown in Table 1.

(Example 2: Phosphorus precipitation step)
The treatment was performed in the same procedure as in Example 1 except that the amount of magnesium nitrate added in Example 1 was changed to 1.60 g (Mg (NO ₃ ) ₂ 0.41% by mass). The Na ion concentration and phosphorus concentration of the molten salt after the treatment were measured. The results are shown in Table 1.

(Example 3: Phosphorus precipitation step)
The treatment was performed in the same procedure as in Example 1 except that the amount of magnesium nitrate added in Example 1 was changed to 2.40 g (Mg (NO ₃ ) ₂ 0.61% by mass). The Na ion concentration and phosphorus concentration of the molten salt after the treatment were measured. The results are shown in Table 1.

  From the above results, the phosphorus concentration was reduced by adding magnesium nitrate to the state in which potassium orthophosphate was added to the deteriorated molten salt (Test Example 2). In particular, in Example 3, the phosphorus concentration was reduced by 47% by mass from the initial state (Test Example 2) by adding magnesium nitrate at least 1.0 times the molar mass of the orthophosphate ion in the molten salt. From this, it can be seen that according to the method of the present invention, the orthophosphate ions of the molten salt precipitate and the phosphorus concentration decreases.

  However, in Examples 1 to 3, it was thought that all the orthophosphate ions could be removed, but actually 50% by mass or more remained. In the sodium precipitation step, potassium orthophosphate added as a potassium salt dissolves in the molten salt, and the orthophosphate ions attract sodium ions in the molten salt to lower the saturation solubility, so that it is considered that the sodium salt precipitates. The Na ion concentration and the phosphorus concentration were increased again because the orthophosphate ions present in excess in the molten salt were precipitated as magnesium phosphate by adding magnesium nitrate, while orthophosphoric acid was precipitated from the sodium salt precipitated in the sodium precipitation step. This is probably because acid ions and Na ions were redissolved.

  Therefore, a step of removing the sodium salt precipitated by the sodium precipitation step from the molten salt was performed before the phosphorus precipitation step.

[Examples 4 to 6: Removal of sodium precipitate and addition of magnesium nitrate to deteriorated molten salt]
(Example 4: Preparation of molten salt in which orthophosphate was precipitated after removal of sodium precipitate)
In Test Example 2, 0.80 g (Mg (NO ₃ ) ₂ 0.23 mass%) of magnesium nitrate was added to the molten salt from which Na precipitate was removed from the molten salt in which the Na ion concentration was reduced from the deteriorated state. . And it stirred for 2 hours using the stirring motor and 4 propeller blades, and left still for 2 hours.
The Na ion concentration and phosphorus concentration of the molten salt after the treatment were measured. The results are shown in Table 2.

(Example 5: Preparation of molten salt in which orthophosphate was precipitated after removal of sodium precipitate)
The treatment was performed in the same procedure as in Example 4 except that the amount of magnesium nitrate added in Example 4 was changed to 1.60 g (Mg (NO ₃ ) ₂ 0.45 mass%). The Na ion concentration and phosphorus concentration of the molten salt after the treatment were measured. The results are shown in Table 2.

(Example 6: Preparation of molten salt in which orthophosphate was precipitated after removal of sodium precipitate)
The treatment was performed in the same procedure as in Example 4 except that the amount of magnesium nitrate added in Example 4 was changed to 2.40 g (Mg (NO ₃ ) ₂ 0.68% by mass). The Na ion concentration and phosphorus concentration of the molten salt after the treatment were measured. The results are shown in Table 2.

  From the above results, it is possible to suppress the rise in Na ion concentration and phosphorus concentration again compared with the examples (Examples 1 to 3) that did not pass through the step of removing the precipitate after separating it from the molten salt. It was. In the state (Examples 5 and 6) in which magnesium nitrate was added in an amount of 1.0 times the molar mass of orthophosphate ions in the molten salt (Examples 5 and 6), the phosphorus concentration was greatly reduced from the initial state (Test Example 2). In No. 6, the phosphorus concentration could be 0 ppm by mass. From this, it can be seen that according to the method of the present invention, the orthophosphate ions of the molten salt are precipitated, and the phosphorus concentration is significantly reduced.

  In Example 5, the Na ion concentration, phosphorus concentration, and Mg ion concentration all showed low values. Thus, according to the present invention, it can be seen that the deteriorated molten salt can be regenerated to a composition close to a new molten salt.

(Relationship between magnesium nitrate addition and phosphorus precipitation)
In addition to the three types of magnesium nitrate addition conditions in Examples 4 to 6, various addition quantity conditions were selected, and the other procedures were performed in the same manner as in Example 4. The phosphorus concentration of each molten salt after the treatment was measured. The relationship between the added amount (mol) of magnesium nitrate obtained and the molar mass (mol) of precipitated phosphorus is shown in FIG.
FIG. 1 shows that phosphorus in the molten salt is precipitated by approximately 1 molar equivalent with respect to the added amount of magnesium nitrate regardless of the amount of added magnesium nitrate. This relationship holds until phosphorus in the molten salt is completely precipitated.

  The method for regenerating a molten salt for strengthening glass and the method for producing a tempered glass of the present invention can be widely used when the molten salt used for chemically strengthening the glass deteriorates due to use.

Claims (10)

A method for regenerating a molten salt that removes sodium eluted in the molten salt by chemically strengthening the glass with a molten salt containing potassium nitrate,
A sodium precipitation step of adding at least one sodium precipitant selected from the group consisting of potassium orthophosphate or potassium pyrophosphate to the molten salt;
A phosphorus precipitation step of adding a phosphorus precipitant comprising at least one Group 2 element nitrate compound to the molten salt;
A method for regenerating a molten salt comprising: The method for regenerating a molten salt according to claim 1, wherein a phosphorus precipitation step is performed after the sodium precipitation step. The method for regenerating a molten salt according to claim 2, further comprising a step of removing the phosphate precipitated by the sodium precipitation step from the molten salt before the phosphorus precipitation step. The method for regenerating a molten salt according to claim 1, wherein the sodium precipitation step and the phosphorus precipitation step are performed simultaneously. The method for regenerating a molten salt according to claim 1, wherein the phosphorus precipitant contains magnesium nitrate. When the phosphorus concentration before adding the phosphorus precipitant is X (mass ppm), the phosphorus precipitant addition amount is Y (mol), and the mass of the molten salt is Z (g), these are the relational expression Y ≧ Z The method for regenerating a molten salt according to claim 1, which satisfies (X-500) / (31 × 10 ⁶ ). The method for regenerating a molten salt according to claim 6, wherein the relational expression Y ≦ ZX / (31 × 10 ⁶ ) is satisfied. The method for regenerating a molten salt according to claim 1, wherein the temperature of the molten salt in the phosphorus precipitation step is 330 to 500 ° C.   The method for regenerating a molten salt according to claim 1, wherein the sodium precipitant is potassium orthophosphate. The manufacturing method of tempered glass including the process of chemically strengthening glass using the molten salt reproduced | regenerated by the method of any one of Claims 1-9.

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