JPS6317218A - Regeneration of molten salt - Google Patents

Abstract

PURPOSE:To easily regenerate used and degraded molten salt in a short time keeping the molten salt at a high temperature, in the ion exchange treatment of a glass material with molten salt, by contacting the degraded molten salt with an inorganic porous material. CONSTITUTION:A glass rod 1 to be subjected to ion exchange treatment is immersed in a molten salt 2 (e.g. sodium nitrate) maintained at a high temperature. The ions in the glass rod 1 are exchanged with the ion sin the molten salt 2 to form a refractive index distribution gradually varying from the core to the circumference in the cross-section of the glass rod 1. When the molten salt is degraded by the repeated operation, a metallic wire basket 3 containing an inorganic porous material 4 (e.g. activated alumina) is immersed in the degraded molten salt 2 and vertically or horizontally reciprocated. The alkali metal oxide, nitrite ion, etc., produced in the molten salt 2 by decomposition is capture in the porous material 4 by this treatment to enable the regeneration of the molten salt.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a technique for preventing deterioration and regenerating molten salt that is repeatedly used in ion exchange treatment of glass materials and the like.

[Prior art]

The glass material is brought into contact with molten salts such as nitrates and sulfates kept at high temperature, and the ions in the salt are exchanged with the ions in the glass, causing the ions in the salt to diffuse from the surface of the glass material into the interior, thereby modifying the glass. Processing methods for quality are generally known.

For example, a glass rod with a relatively high refractive index composition is brought into contact with a molten salt containing low refractive index ions, and the high refractive index ions in the glass are exchanged with the low refractive index ions in the salt. There are methods to manufacture gradient index lenses in which the refractive index gradually decreases from the axis toward the outer periphery, or chemical strengthening methods that increase the breaking strength of the glass material by creating a compressive strain layer near the surface of the glass material using ion diffusion. be.

As mentioned above, in the process of bringing objects to be treated such as glass into contact with molten salt, the same molten salt is generally used repeatedly while being kept at a high temperature for reasons of economy and productivity. The molten salt gradually decomposes (degrades) at high temperatures, and then K causes problems such as erosion of the glass surface and deterioration of lens performance. Therefore, it is necessary to regenerate the molten salt after an appropriate number of contact treatments. Conventionally, the molten salt at high temperature is dissolved in water at room temperature to precipitate the supersaturated salt (decomposition products are dissolved in a large amount of water). The method used is to centrifuge the precipitate and then re-melt it to reach the desired processing temperature.

[Problem that the invention seeks to solve]

In the conventional salt regeneration method described above, ttoo-soo°C
The molten salt at a high temperature nearby is dissolved in water, cooled to room temperature, and then heated again to the above temperature after regeneration, resulting in a very large loss of thermal energy. The problem is that since it is lost during processing, a large amount of new salt needs to be added, which makes the processing cost very high.

In terms of equipment, it also requires large equipment such as a crystallization furnace, a centrifugal separator, and a Brimelt furnace, and there is also the problem that regeneration takes a long time.

[Means for solving problems]

The present invention aims to solve the above problems and provide an extremely simple and inexpensive molten salt regeneration method,
In the method of the present invention, the molten salt that has been brought into contact with the object to be treated is brought into contact with the inorganic porous material.

The above-mentioned inorganic porous material must be chemically stable to the molten salt used, stable in the high temperature range where the molten salt is held, and do not release impurity ions into the molten salt. It is desirable to use a material that satisfies the following conditions, and according to experimental results, activated alumina (r-alumina) is particularly suitable.

The preferred conditions when using activated alumina for molten salt regeneration are as follows: molten salt temperature 3! 0-t 00''C, and the lower the temperature of the salt, the slower the decomposition of the molten salt itself will be, but on the contrary, the regeneration ability will be better at higher temperatures.

Further, the immersion of activated alumina in the molten salt (2) is preferably about 10 to 30% by weight of the molten salt.

If the amount is small, there will be less salt sucked into the pores when bales of alumina are taken out intermittently, but the regeneration ability will be low.

Also, the immersion time depends on the temperature of the molten salt and the degree of immersion of the activated alumina, so it cannot be stated unconditionally, but in general it is /872
A range of 5 days is desirable. Furthermore, the regeneration ability of alumina gradually decreases as it is used more frequently, so consider economic efficiency! - It is best to replace it with a new one after using it about 10 times.

[For production]

Taking nitrates as an example, the following reactions generally occur as decomposition reactions.

RNO3→R++N○3- (R-alkali) NO3-
→NO2-10/ /202 i'2NO3-→2NO2
'j' + //202↑+Q2-Here, above NO2
- and 02- are analyzed as RuO2°R,20, respectively.

The degree of decomposition can be estimated by analyzing the nitrite ions and active oxygen ions generated by decomposition in this way.

When active oxygen ions 02''' increase in the molten salt, the silica network structure near the glass surface is cut, resulting in clouding of the surface.

As the molten salt decomposes and O2- increases, the erosion of the glass becomes more severe.

On the other hand, when a porous material is brought into contact with a molten salt containing 02- ions and NO2- ions, which are decomposition products, many R+ ions are adsorbed to the active sites on the surface of the porous material, and the 02- ions are absorbed to compensate for the charge. - ions and NO2- ions are bound near R+ ions.

Due to the above-mentioned presumed mechanism, the decomposition products in the molten salt are removed by contact with the inorganic porous material, that is, the molten salt is regenerated, and as shown in the experimental results described below, there is an adverse effect on the glass to be treated due to salt deterioration π. can be eliminated.

According to the method of the present invention, there is no need to cool the molten salt for regeneration treatment, and it is only necessary to immerse the porous material while maintaining the molten salt at the predetermined treatment temperature for the object to be treated, making it extremely economical. It is.

[Embodiments] The present invention will be described in detail below based on embodiments shown in the drawings.

Figure 1 shows the ion exchange treatment process in the production of gradient index lenses, in which a large number of lens material glass rods are immersed in molten salt, for example, sodium nitrate held near soo'c. The ions in the rod l and the ions in the molten salt are exchanged to form a refractive index distribution that gradually changes from the central axis toward the outer periphery within the cross section of the glass rod.

In the above step, while the molten salt 2 is maintained at the above-mentioned constant temperature, the glass pellets of the lens base material are sequentially changed and immersed to perform an ion exchange treatment. With this repeated treatment, the molten salt 2 gradually deteriorates as described above, and if no salt regeneration treatment is performed, the glass rod/...
... may occur, or ion exchange may be interrupted, making it impossible to obtain the desired refractive index distribution.

Therefore, after completing the ion exchange treatment of the glass rod/..., the wire mesh basket 3 containing the inorganic porous material t such as activated alumina is placed in the molten salt λ as shown in Figure 1 (b). Soak in.

If the porous material q is simply immersed and left above the molten salt 2, a wide range of molten salt will come into contact with the porous material q due to the natural convection 3' in the salt solution. In order to enhance the regeneration treatment effect, it is desirable to repeatedly move the basket 3 containing the porous material t in the molten salt 2 in a deep direction or horizontally over a wide range, or to forcibly stir the molten salt.

By bringing the molten salt 2 into contact with the porous material ≠ as described above, the alkali fluoride R20 or sub-Elfm ion NO3'', etc., which are decomposition products in the salt co, are captured in the porous material 1 as described above. After soaking for a certain period of time, the molten salt 2
When taken out, the decomposition products in the molten salt λ are removed, that is, the molten salt λ is regenerated, and the next time the ion exchange of the glass rod 1 is carried out in good condition.

Specific numerical examples are shown below.

Example 1 Two types of molten salts were prepared by melting sodium nitrate IIKf at 520°C, one was kept as it was for 2r days, and the other was prepared using two types of commercially available salts having the specific surface area and pore distribution shown in Table 1. 10077 pieces of each of granular activated alumina A and B were intermittently immersed.

The alumina immersion cycle and immersion time were such that the experiment was carried out for a total of 2 g days by holding the alumina for t days without immersing the alumina, then alternately repeating immersion for 3 days and no immersion for ψ days.

No. 7, which is a molten salt decomposition product before and after one immersion treatment.
Table 2 and FIG. 1 show the results of analyzing the amount of Na2O.

For Na2Ofl, collect 30g of molten salt and add 10OCC of pure water.
was added to form an aqueous solution, colored red with phenolphthalein, and expressed as a titer until it became colorless with //100N HCl. In other words, it can be seen that the greater the titration of //100N HCI, the more Na2OM is contained in the molten salt, and therefore the decomposition is progressing.

Further, Table 3 shows the results of measuring NaNO21 in each molten salt after 2 days by potassium permanganate titration method.

NaNo 2 is one of the decomposition products of the NaNO 3 molten salt, and like Na 2 O, its large amount indicates that the molten salt has been decomposed.

As shown in the example, the blank (alumina immersion No. 2
Compared to the molten salt (without immersion in Table 3), the amounts of Na2O and NaNO3 in the molten salt in which alumina A and B were intermittently immersed are clearly lower, indicating that alumina has the effect of reducing decomposition products. I can see that clearly.

In particular, the amount of Na2O decreased significantly before and after immersion in alumina, indicating that the Na2O component was adsorbed and reduced by alumina, and its ability hardly changed after one use.

Example 2 Next, the following experiment was conducted in order to examine the influence of the decomposition of the molten salt on the glass to be treated and the effect of the activation and regeneration of the molten salt according to the present invention.

Using three types of molten salts after 2 g of treatment in Example 1, 5i02, MgO, P))O, TiO2)N
a2O.

Composed of Li2O! ,/1! A gradient index lens was prepared by subjecting a round glass rod with a diameter of I+1 to ion exchange treatment. The results are shown in Table q.

As can be seen from the results in Table 1, in the case of a blank molten salt which was kept at a high temperature without being immersed in alumina, the glass surface became completely cloudy and the glass became extremely brittle.

Table 1 It shows the percentage of

Furthermore, the lens performance was significantly reduced, with an effective lens radius of 70% compared to the molten salt immersed in alumina.

Example 3 In order to further confirm the molten salt regeneration effect of activated alumina, five types of activated alumina materials with different S-02 contents were prepared, and sodium nitrate soo melted with pso'c was prepared.
The amount of NagO was measured after immersing 30 g of the activated alumina material in each of 5 salt baths of 30 g and holding for 10 days.

And in those salts, 5i02, zno, Li2O
A glass piece having a composition of +N a 20 was introduced and its surface condition was examined after 7 days, and the results shown in Table 5 were obtained.

As a comparative example, after holding the molten salt in which the alumina material was not immersed in ljO''C for 10 days, glass pieces with the same composition as above were introduced and the surface condition was observed 7 days later.Table 5 shows the results. Shown as "blank".

As can be seen from Table 5, when the molten salt has a small titer of t/10ONHc, the erosion of the glass surface is suppressed, that is, the number of decomposition products in the molten salt is reduced, and the contact with activated alumina is It was confirmed that it is effective for playback.

[Effect of four shots]

The present invention has many advantages as listed below.

(1) Molten salt can be recycled while being kept at high temperature.

(2) The regeneration time is short and the molten salt can be used immediately after the regeneration treatment.

(3) There is less loss of thermal energy and salt in the regeneration process.

(4) Regeneration operation is simple and the device can be made smaller.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an embodiment of the present invention, and FIG. 2 is a graph showing an example of the regeneration effect of molten salt by immersion in a porous material. /...Ion exchange treated glass rod 2...
- Molten salt 3... Basket l... Inorganic porous material Figure 1

Claims (2)

[Claims] (1) A method for regenerating molten salt characterized by bringing the molten salt, which has undergone contact treatment with the object to be treated, into contact with an inorganic porous material. (2) A method for regenerating a molten salt according to claim 1, wherein the object to be treated is a glass body, and the molten salt is a salt containing ions that can diffuse into the glass body.