KR101655504B1 - Cold-storage agent - Google Patents

Abstract

The present invention relates to a dry-crosslinked cellulose gel comprising 2 to 4 wt% of a dried crosslinked cellulose gel obtained by irradiating a cellulose-kneaded mixture of pastes obtained by kneading 15 to 25 wt% of cellulose with water under vacuum, will be.

Description

{COLD-STORAGE AGENT}

TECHNICAL FIELD The present invention relates to a shaft coolant, and more particularly to a shaft coolant which is made of only natural materials and is frozen in a short time and exhibits a long cooling effect.

The axial coolant is used for conveying fresh foods and other products while keeping them at a low temperature. In the case of fresh food or other products, the cooling water used for conveying the product may be frozen for 5 to 8 hours / 4 times in accordance with the schedule of delivery (in the case of ordinary use, a plurality of cooling agents are simultaneously frozen. It is frozen in a short time such as 5 hours for the shortest time, 8 hours for the longest time in the freezer), and it is required that the cooling effect is not lowered even during long-time transportation or storage for a long time. There is no satisfying the requirement as the spiruller.

Conventional refrigeration coagulants include aqueous solutions of inorganic salts such as sodium chloride, ammonium chloride and magnesium chloride, or polyhydric alcohols such as ethylene glycol and propylene glycol as a cold agent and use carboxymethylcellulose, polyvinyl alcohol, sodium polyacrylate 0.01 to 10 parts by weight of a hydrophilic polymer such as polyacrylamide is added as a gelling agent per 100 parts by weight of the axial coolant, and as the nucleating agent for freezing the axial coolant, silver iodide, copper sulfide, xanthan gum,? -Phenazine, (Patent Document 1). However, these axial refrigerants have a drawback that the freezing time is long and the cooling effect is lowered in a short time.

In order to solve the above problems, the inventors of the present invention have found that a cold storage-use cold storage agent obtained by mixing 1 to 5% of a gel obtained by cross-linking water-soluble carboxymethyl cellulose in paste form and irradiated with radiation in 1 to 5% Japanese Patent Application Laid-Open Publication No. 2001-34875 proposes a freezing cold storage material in which 1 to 5% of gel is mixed with 5 to 15% of saline and 1 to 20% of propylene glycol is added as a curing agent. This freezing freezing material achieves a freezing point of -27 ° C. However, recently, from the viewpoints of safety and environmental protection, there is a demand for a recyclable product that can be reused as a resource after disposal. The freezing-thawing freezing material proposed by the present inventors is free from short-term freezing and has an excellent long-term cooling effect, and contains propylene glycol as a curing agent and is not a complete natural product. In addition, since the axial coolant described in Patent Document 2 is produced in the air when carboxymethyl cellulose is put into a paste form, bubbles are generated which cause a deviation during irradiation, and a sufficiently uniform gel can not be obtained, It has been confirmed that the freezing and cold-inserting performance is not sufficient because a variation occurs in the absorbency.

Japanese Patent Application Laid-Open No. 11-293234 Japanese Patent Application Laid-Open No. 2007-238735

An object of the present invention is to provide a shaft coolant made of a completely natural material which does not require special chemical treatment in disposal, and which is frozen in a short time and exhibits a long-time cold storage effect.

According to the present invention, there is provided a process for producing a cellulose-based paste, which comprises mixing 2 to 4 wt% of a dry crosslinked cellulose gel obtained by irradiating a paste-like cellulose kneaded product obtained by kneading 15 to 25 wt% of cellulose with water under vacuum, 1 to 21 wt% An aqueous condenser is provided.

The crosslinked cellulose gel used in the present invention is a dried gel having an absorption capacity of 150 to 250 times, preferably 150 to 200 times, obtained by dividing the gel absorbed for 24 hours by the initial weight of the dried crosslinked cellulose gel before absorption desirable. By using the dried crosslinked cellulose gel having such a high absorption capacity, a large amount of water can be retained in the crosslinked cellulose gel. If a large amount of water held in the crosslinked cellulose gel is once frozen, it takes a long time to thaw, so that the cooling time can be lengthened. However, if the absorption magnification is excessively high, a long time is required for freezing and the strength of the gel itself is weakened. Therefore, the above range is preferable.

The cellulose used in the present invention is carboxymethylcellulose sodium, preferably sodium carboxymethylcellulose whose viscosity is not lowered in saline. Particularly preferably, it is a carboxy (meth) acrylate having a viscosity (viscosity by a B-type viscometer) of 2600 mPa.s or more in 1 to 5% saline and a viscosity of 4600 mPa.s or more Methylcellulose sodium. The axial coolant of the present invention has a structure in which a dry cross-linked cellulose gel is present in a saline solution. When the viscosity of the crosslinked cellulose is lowered in the saline solution, the function as a gelling agent can not be exhibited, and the water retention in the crosslinked gel structure is lowered. Therefore, the cellulose used as the raw material preferably has a viscosity within the above range.

The dry cross-linked cellulose gel to be used in the present invention is prepared by adding the above-mentioned cellulose to water in an amount of 15 to 25 wt%, preferably 15 to 20 wt%, and kneading under vacuum to obtain a paste-like cellulose kneaded product , And the electron beam is preferably irradiated at 8 to 16 kGy, more preferably at 9 to 14 kGy.

The water used for preparing the paste-kneaded cellulose mixture preferably contains no salt, and the ion-exchanged water is particularly preferable. It is also necessary to perform kneading under vacuum. By kneading under vacuum, bubbles are not mixed, and the cellulose powder can be sufficiently kneaded in a state in which the cellulose powder is uniformly dispersed in the ion exchange water.

The content of cellulose and the amount of electron beam irradiation in the cellulose kneaded product affect the crosslinking structure formed by electron beam irradiation. Various experiments were carried out in order to achieve the required water absorbency and water retention as the axial coolant. As a result, it was found that the cellulose content and the electron beam irradiation amount in the above range were optimum. If the electron beam irradiation amount is large, the net nose of the crosslinked structure becomes small. Since the moisture is retained in the net nose, the smaller the amount of water retained in one net nose, the shorter the freezing time, and the net is densely formed, so that the defrosting time is lengthened and the cooling time can be lengthened. However, it can not be used if the net becomes so small that it can not keep the water molecule. The inventors of the present invention have found that it is possible to obtain a preferable net structure as the axial coolant of the present invention which can shorten the freezing time and maintain the cooling time longer, The crosslinked cellulose gel obtained under the above-mentioned irradiation conditions has an absorption magnification of 150 to 200 times at the time of drying and is preferable as the axial cooling agent of the present invention.

The starch refrigerant of the present invention is prepared by adding the dried cross-linked cellulose gel to 2 to 4 wt%, preferably 2.5 to 3.5 wt% of saline containing 1 to 21 wt% of salt, Filled into a container such as a container, a film, or a nonwoven fabric.

In the present invention, the saline serves as a cryogen. The salt solution used in the present invention is not a purified salt but a natural salt of salt. Natural rock salt is rich in minerals, unlike tablets (quality standard of salt business center: 99 wt% or more of sodium chloride, 0.02 wt% or less of calcium, 0.02 wt% or less of magnesium and 0.25 wt% or less of potassium) , And serves as an excellent curing agent. It is more preferable that the salt used in the present invention is a salt of NaCl having 99 wt% or more, a total amount of Na ⁺ and Mg ^{2 +} of more than 0 and 0.3 wt% or less, and particularly, a salt formed in Hubei Province, China Do.

The saline concentration differs depending on the temperature range required for the axial coolant. When used as a cold storage substitute for 0 ° C to -17 ° C, the rock salt content is 1 to 17 wt% The content of rock salt is preferably 18 to 21 wt%. In order to lower the solidification point of the axial coolant by 1 ° C, 1 wt% of rock salt may be added.

According to the present invention, an axial coolant that can be frozen in a short time and has a long holding time of the cooling effect is provided. Further, since the axial coolant of the present invention is a completely natural product having biodegradability composed of dry cross-linked cellulose gel, salt, and water, disposal treatment of the axial coolant itself is unnecessary, and even if leakage occurs, Is very easy to use, and is most suitable for storage and transportation of fresh foods or medicines, for which safety is particularly required.

Fig. 1 is a graph showing the results of measurement of freezing time in Example 1 and Comparative Example 1. Fig.
Fig. 2 is a graph showing the measurement results of the cooling time in Example 1 and Comparative Example 1. Fig.
3 is a graph showing the measurement results of freezing time in Example 2 and Comparative Example 2. Fig.
4 is a graph showing the measurement results of the cooling time in Example 2 and Comparative Example 2. Fig.
5 is a graph showing the measurement results of freezing time in Example 3 and Comparative Example 3. Fig.
6 is a graph showing the measurement results of the cooling time in Example 3 and Comparative Example 3. Fig.
7 is a graph showing the measurement results of freezing time in Example 4 and Comparative Example 4. Fig.
8 is a graph showing the measurement results of the cooling time in Example 4 and Comparative Example 4. Fig.

Example

The present invention will be described in detail with reference to Examples and Comparative Examples.

[Production Example 1] <

12 liters of ion-exchanged water was poured into a kneading kiln (content: 60 L) of a vacuum kneading apparatus, 3 kg of powdery carboxymethylcellulose sodium ("Line OZ F350HC-4" manufactured by Nippon Paper Industries Co., Ltd.) was added and water and carboxymethylcellulose Of the total amount. At this time, in order to suppress scattering of sodium carboxymethylcellulose powder, water was added from above the powder while spraying. After the addition of the raw material, the lid of the vacuum kneading apparatus was closed and stirred in a vacuum kneading apparatus for 40 minutes while evacuating, to prepare a cellulose kneaded product.

Subsequently, the cellulose kneaded product was molded under vacuum and irradiated with 14 kGy of electron beam to prepare a crosslinked cellulose gel. The crosslinked cellulosic gel was transferred into a dryer and dried at about 70 ° C.

55 g of rock salt (5% of total amount of 1,100 g) produced in Hubei Province, China was dissolved in ion-exchanged water to prepare 5% saline solution. 27.5 g of dry crosslinked cellulose gel (2.5 wt% of the total amount of 1, 100 g) was added to the 5% saline solution, and the mixture was stirred for 50 minutes and then quenched for 5 minutes and then stirred for 10 minutes. The cold was prepared.

[Comparative Production Example 1]

A shaft cooling agent was prepared in the same manner as in Production Example 1 except that the dry crosslinked cellulose gel described in the Example of Japanese Patent Application Laid-Open No. 2007-238735, which is the crew of the present applicant, was used. That is, 27.5 g of dried crosslinked cellulose gel obtained by irradiating 5 kGy of cobalt 60? Ray onto pasted carboxymethyl cellulose kneaded with an open type kneading kiln and 5% saline were added and stirred to prepare a starch refrigerant . (Total amount 1100 g, gel 2.5% <27.5 g>, salt 5% <55 g>, water 1017.5 cc)

[Example 1]

1100 g of the axial coolant prepared in Production Example 1 was filled in a case of axial coolant (19.5 cm in length x 26 cm in length x 3.5 cm in thickness), and the freezing time and the cooling time were measured.

<Measurement of freezing time>

The axial coolant case was allowed to stand at room temperature and then placed in a freezer free of pan at -35 DEG C for 24 hours to measure the time until it was frozen. The results are shown in Fig.

The initial temperature of the axial coolant was 10.4 캜, but the temperature of the axial coolant rapidly dropped to -10 캜 after about 1 hour and 10 minutes after being put into the freezer, and the freezing was completed at about 2 hours and 20 minutes, 17.6 &lt; 0 &gt; C.

<Measurement of Cooling Time>

The case of the axial cooling agent cooled in a freezer at -35 占 폚 for 24 hours was suspended still in a box of foamed styrol (32 cm in length x 51 cm in height x 15 cm in height) and the box of foamed styrol was allowed to stand at room temperature, The temperature in the box was measured. The results are shown in Fig.

The initial temperature when the condensed refrigerant was put in the box of the foamed styrol was -12.5 DEG C, rose to 0 DEG C after about 8.5 hours, and rose to 3.1 DEG C after about 9.5 hours.

[Comparative Example 1]

The same experiment as in Example 1 was carried out using the axial coolant prepared in Comparative Production Example 1. [ The results are shown in Fig. 1 and Fig.

<Measurement of freezing time>

The initial temperature was 11.8 占 폚, but after about 1 hour and 10 minutes from the freezing, it was frozen at -5 占 폚 for about 4 hours and was -13.9 占 for about 6 hours.

<Measurement of Cooling Time>

The initial temperature was -11.1 deg. C, but it reached 0 deg. C after about 7.5 hours from the introduction into the box of foamed styrol and rose to 10.1 deg. C after about 9.5 hours.

&Lt; Contrast of Example 1 and Comparative Example 1 &gt;

The time required until the freezing is completed is very short, about 2 hours, and the freezing of the instant cooling agent of the present invention reaches to -17.6 캜 by freezing for 6 hours, and freezing is achieved in a very short time. In Comparative Example 1, about 4 hours were required until the freezing was completed, and it reached only -13.9 ° C after 6 hours.

The axial refrigerant of the present invention requires about 8.5 hours to rise to 0 占 폚, and while it stays at 3.1 占 폚 even after 9.5 hours, it is increased to 0 占 폚 in Comparative Example 1 at about 7.5 hours, Reached 10.1 캜. In the case of the co-cooling agent, when the temperature exceeds 10 占 폚, the cold storage property is lost. Therefore, it can be said that Comparative Example 1 can be used as a cooling agent for only about 9 hours. On the other hand, the temperature-rising gradient of the axial refrigerant of the present invention after reaching 3.1 占 폚 is gentler than that of Comparative Example 1, and it can be said that the refrigerant has a long-term cold storage.

[Example 2]

650 g of the axial coolant prepared in Production Example 1 was filled in a case of axial coolant (15 cm in length x 26.5 cm in length x 2 cm in thickness), and the freezing time and the cooling time were measured.

<Measurement of freezing time>

The axial coolant case was allowed to stand at room temperature and then placed in a freezer free of pan at -35 DEG C for 24 hours to measure the time until it was frozen. The results are shown in Fig.

The initial temperature of the axial coolant was 13 ° C, but the temperature of the axial coolant rapidly dropped to about -5 ° C in about 50 minutes after it was put in the freezer. After about 3 hours and 10 minutes, the freezing was completed. .

<Measurement of Cooling Time>

The case of the axial cooling agent cooled in a freezer at -35 占 폚 for 24 hours was suspended still in a box of foamed styrol (32 cm in length x 51 cm in height x 15 cm in height) and the box of foamed styrol was allowed to stand at room temperature, The temperature in the box was measured. The results are shown in Fig.

The initial temperature when the condensed refrigerant was put into the box of the foamed styrol was -17 DEG C, and it rose to 0 DEG C after about 6 hours and rose to 2.9 DEG C after about 6.5 hours.

[Comparative Example 2]

A freezing time and a cooling time were measured in the same manner as in Example 2 except that a commercially available axial cooling agent (gelling agent: polymer, cryogen: propylene glycol) was used. The results are shown in Fig. 3 and Fig.

&Lt; Contrast of Example 2 and Comparative Example 2 &gt;

The time required until the freezing of the present invention is very short, about 3 hours, and the freezing of the present invention reaches -27.1 캜 by freezing for 6 hours, and freezing is achieved in a very short time. Commercially available cooling coagulants require about 4 hours to complete freezing and reach about -25.1 ° C even after about 6 hours.

The axial refrigerant of the present invention requires about 6 hours to rise to 0 占 폚, while it remains at 2.9 占 폚 even after 6.5 hours, while the commercially available axial cooling agent is elevated to 0 占 폚 in about 5 hours, The temperature rapidly increased, and after 6.5 hours, the temperature reached 9.1 ° C. Since the refrigerating apparatus for cold storage substitution will lose the cold storage if it exceeds 10 ° C, the comparative example can be said to be used as the refrigerating apparatus for about 6.5 hours. On the other hand, in the case of the axial refrigerant of the present invention, it can be said that the temperature rising gradient after reaching 2.9 캜 is slower than that of the commercially available axial refrigerant and has a long-time cold keeping performance.

[Production Example 2] &lt;

12 liters of ion-exchanged water was poured into a kneading kiln (an amount of 60 liters) of a vacuum kneading apparatus, 3 kg of powdery carboxymethylcellulose sodium ("Line Oz F350HC-4" manufactured by Nippon Paper Industries Co., Ltd.) was added, and water and carboxymethylcellulose Of the total amount. At this time, in order to suppress scattering of sodium carboxymethylcellulose powder, water was added from above the powder while spraying. After the addition of the raw material, the lid of the vacuum kneading apparatus was closed and stirred in a vacuum kneading apparatus for 40 minutes while evacuating, to prepare a cellulose kneaded product.

Subsequently, the cellulose kneaded product was molded under vacuum and irradiated with 14 kGy of electron beam to prepare a crosslinked cellulose gel. The crosslinked cellulosic gel was transferred into a dryer and dried at about 70 ° C.

A 20% saline solution was prepared by dissolving 130 g of rock salt (20% of the total amount of 650 g) produced in Hubei Province, China, in ion-exchanged water. 19.5 g of dry crosslinked cellulose gel (3 wt% of the total amount of 650 g) was added to the 20% saline solution, and the mixture was stirred for 50 minutes and then quenched for 5 minutes and again stirred for 10 minutes to homogeneously mix. Lt; / RTI &gt;

[Example 3]

650 g of the axial coolant prepared in Production Example 2 was charged into an axial coolant case (15 cm long x 26.5 cm long x 2 cm thick), and the freezing time and the cooling time were measured.

<Measurement of freezing time>

The axial coolant case was allowed to stand at room temperature and then placed in a freezer free of pan at -35 DEG C for 24 hours to measure the time until it was frozen. The results are shown in Fig.

The initial temperature of the axial coolant was 7.9 ° C. However, the temperature of the axial coolant rapidly dropped to -20 ° C. after about 1 hour 30 minutes after being put in the freezer, and the freezing was completed at about 2 hours and 30 minutes. 23.9 &lt; 0 &gt; C.

<Measurement of Cooling Time>

The case of the axial cooling agent cooled in a freezer at -35 占 폚 for 24 hours was suspended still in a box of foamed styrol (32 cm in length x 51 cm in height x 15 cm in height) and the box of foamed styrol was allowed to stand at room temperature, The temperature in the box was measured. The results are shown in Fig.

The initial temperature at the time when the axial refrigerant was put into the box of the foamed styrol was -21.7 캜, and it rose to 0 캜 after about three and a half hours.

[Comparative Example 3]

The freezing time and the cooling time were measured in the same manner as in Example 3, except that a commercially available cooling agent (gelling agent: polymer, curing agent: propylene glycol) was used. The results are shown in Fig. 5 and Fig.

&Lt; Contrast of Example 3 and Comparative Example 3 &gt;

The instant cooling agent of the present invention requires a very short time to complete the freezing to about 2 hours and 30 minutes and reaches -23.9 占 폚 by freezing for 4 hours, thus achieving freezing in a very short time. Commercially available cooling coagulants require about 3 hours to complete freezing and reach about -22.8 ° C even after about 4 hours.

In addition, the axial coolant of the present invention requires about 3 hours and 30 minutes to rise to 0 캜, whereas it increases to 0 캜 at 2 hours and 40 minutes with a commercially available whirling coater, and reaches 8.2 캜 after 4 hours . Since the cold storage refrigerant for freezing substitution exceeds 0 ° C, the cold storage is lost, so that the comparative example can be used for only about 2 hours and 40 minutes. On the other hand, in the case of the axial refrigerant of the present invention, the gradient of the temperature rise until reaching 0 占 폚 is gentle as compared with the commercially available axial refrigerant, so that it can be said that the cold storage is maintained for a long time.

[Production Example 3] &lt;

12 liters of water was poured into a kneading kiln (an amount of 60 L) of a vacuum kneading apparatus, and 3 kg of powdery sodium carboxymethyl cellulose ("Line OZ F350HC-4" manufactured by Nippon Paper Chemicals Co., Ltd.) was added to obtain a total amount of water and carboxymethyl cellulose . At this time, in order to suppress scattering of sodium carboxymethylcellulose powder, water was sprayed from above the powder. After the addition of the raw material, the lid of the vacuum kneading apparatus was closed and stirred in a vacuum kneading apparatus for 40 minutes while evacuating, to prepare a cellulose kneaded product.

Subsequently, the cellulose kneaded product was molded under vacuum and irradiated with 14 kGy of electron beam to prepare a crosslinked cellulose gel. The crosslinked cellulosic gel was transferred into a dryer and dried at about 70 ° C.

A 20% saline solution was prepared by dissolving 100 g of rock salt (20% of the total amount of 500 g) produced in Hubei Province, China, in ion exchange water. 15 g of dry crosslinked cellulose gel (3 wt% of the total amount of 500 g) was added to the 20% saline solution, and the mixture was stirred for 50 minutes and then quenched for 5 minutes. After stirring for 10 minutes, Lt; / RTI &gt;

[Example 4]

500 g of the axial coolant prepared in Production Example 3 was filled in a case of axial coolant (14 cm in length x 20 cm in length x 2.3 cm in thickness), and the freezing time and the cooling time were measured.

<Measurement of freezing time>

The axial coolant case was allowed to stand at room temperature and then placed in a freezer free of pan at -35 DEG C for 24 hours to measure the time until it was frozen. The results are shown in Fig. The initial temperature of the axial coolant was 2.7 캜, but freezing was completed at about 1 hour and 50 minutes after it was put in the freezer, and reached -23.0 캜 after 4 hours.

<Measurement of Cooling Time>

The case of the axial cooling agent cooled in a freezer at -35 占 폚 for 24 hours was suspended still in a box of foamed styrol (32 cm in length x 51 cm in height x 15 cm in height) and the box of foamed styrol was allowed to stand at room temperature, The temperature in the box was measured. The results are shown in Fig.

The initial temperature when the axial coolant was put into the box of the foamed styrol was -23.4 캜, and it remained in the rise to -9.0 캜 even after about 6 hours.

[Comparative Example 4]

A shaft cooling agent was prepared in the same manner as in Production Example 4 except that the dry crosslinked cellulose gel prepared in Comparative Preparation Example 1 was used. That is, 15 g of the dried crosslinked cellulose gel of Comparative Production Example 1 and 20% saline were added and stirred to prepare an axial cooling agent. A freezing time and a cooling time were measured in the same manner as in Example 4 except that the prepared axial coolant was used. The results are shown in Fig. 7 and Fig.

&Lt; Contrast of Example 4 and Comparative Example 4 &gt;

The time required until the freezing is completed is as short as 1 hour 50 minutes and reaches to -23.0 占 폚 by freezing for 4 hours and the freezing is achieved in a very short time. The shaking coolant of Comparative Example 4 requires about 2 hours and 50 minutes until the freezing is completed, and it reaches only -22.9 占 폚 even after about 4 hours.

The axial refrigerant of the present invention was maintained at -9.0 DEG C even after about 6 hours, while it increased to -4.0 DEG C after about 6 hours with the axial cooling agent of Comparative Example 4. [ The temperature-rising gradient of the axial refrigerant of the present invention is gentler than that of the axial refrigerant of Comparative Example 4 up to 6 hours, and thus can be said to have a long-time cold storage capability.

<Summary>

From the above, it can be seen that the axial refrigerant of the present invention has extremely excellent cohesiveness and cold storage as compared with the conventional axial refrigerant. In the above examples and comparative examples, the freezing time and the cooling time for one axial coolant were measured, but normally, a plurality of sheets were frozen at the same time. The simultaneous freezing of a plurality of sheets requires a longer time than that of the above embodiment, but freezing of the shaking coater of the present invention is completed within 5 hours even in the case of simultaneous freezing of four sheets, which is a general use mode.

Industrial availability

INDUSTRIAL APPLICABILITY The axial cooling agent of the present invention is made of only biodegradable natural materials and is very effective for long-distance conveyance and storage of fresh foodstuffs and pharmaceuticals because of freezing in a very short freezing time and long cooling time.

Claims (5)

2 to 4 wt% of a dried crosslinked cellulose gel obtained by irradiating an ultraviolet ray to cellulose-kneaded products obtained by kneading 15 to 25 wt% of sodium carboxymethyl cellulose with water in a vacuum, and 1 to 21 wt% of salt and water Axial coolant. The method according to claim 1,
Wherein the dry cross-linked cellulose gel is a dry gel having an absorption capacity of 150 to 200 times, which is obtained by dividing the gel absorbed for 24 hours by the initial weight before absorption. 3. The method according to claim 1 or 2,
The sodium carboxymethyl cellulose exhibited a viscosity (viscosity by a B-type viscometer) of 2600 mPa s or more in 1 to 5% saline and a viscosity of 4600 mPa s or more (viscosity by a B-type viscometer) in 10% Condensate which is carboxymethylcellulose sodium. 3. The method according to claim 1 or 2,
Wherein the dose of the electron beam is 9 to 14 kGy. 3. The method according to claim 1 or 2,
The rock salt has a NaCl content of 99 wt% or more and contains Na ⁺ and Mg ²⁺ in a total amount of more than 0 and 0.3 wt% or less.

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