Anhydrous magnesium nitrate and preparation method thereof

Abstract

The present invention belongs to the technical field of inorganic material preparation, and in particular, relates to an anhydrous magnesium nitrate and a preparation method thereof. The preparation method provided by the present invention first uses freeze drying to dry magnesium nitrate hexahydrate. Due to the low freeze drying temperature, the freeze drying process has no side reactions, and a dry sample mainly composed of magnesium nitrate dihydrate is formed; at the same time, the dry sample obtained by freeze drying is porous granular and has a large specific surface area, which is conducive to improving the effect of water removal in the next step. Vacuum drying and nitrogen drying are carried out at a relatively low temperature (90-120°C), which minimizes the side reactions and improves the purity of the obtained anhydrous magnesium nitrate; at the same time, vacuum drying is conducive to the dehydration of magnesium nitrate, and nitrogen drying can discharge the trace moisture generated by vacuum drying with nitrogen; the cycle of vacuum drying and nitrogen drying can fully promote the removal of moisture and reduce the water content of the obtained anhydrous magnesium nitrate.

Description

Technical Field

[0001] The invention relates to the technical field of inorganic material preparation, and in particular to anhydrous magnesium nitrate and a preparation method thereof. Background Art

[0002] In the field of molten salt heat transfer and thermal storage, magnesium nitrate-based molten salt has the advantages of low melting point, low cost, and high decomposition temperature, and is a type of molten salt with application prospects. In the field of lithium batteries, adding a certain amount of anhydrous magnesium nitrate to the electrolyte can inhibit the growth of lithium dendrites on the surface of the lithium negative electrode and improve the cycle stability of the lithium battery. Lithium battery electrolytes have high requirements for water content, generally below 20ppm, which puts higher requirements on the water content of magnesium nitrate.

[0003] The stable existence form of magnesium nitrate in the atmospheric environment is magnesium nitrate hexahydrate, and anhydrous magnesium nitrate can be prepared by heating to remove the crystal water. In the process of heating to remove the crystal water, due to thermodynamic limitations, magnesium nitrate inevitably undergoes a hydrolysis reaction to obtain a mixture of magnesium nitrate and basic magnesium nitrate. The basic magnesium nitrate content in the dehydrated product of magnesium nitrate hexahydrate is high, which will cause the decomposition temperature of the prepared magnesium nitrate and magnesium nitrate-based polybasic molten salt to decrease. Therefore, how to reduce the hydrolysis side reaction of magnesium nitrate and improve the purity of anhydrous magnesium nitrate has always been a difficulty in the dehydration technology of magnesium nitrate hexahydrate. Summary of the invention

[0004] In view of this, the object of the present invention is to provide anhydrous magnesium nitrate and a preparation method thereof. The anhydrous magnesium nitrate obtained by the preparation method provided by the present invention has high purity and low water content.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides a method for preparing anhydrous magnesium nitrate, comprising the following steps:

[0007] freeze-drying magnesium nitrate hexahydrate to obtain a dry sample;

[0008] Cyclic vacuum drying and nitrogen drying of the dried sample to obtain anhydrous magnesium nitrate;

[0009] The temperature of the vacuum drying and nitrogen drying is 90-120°C.

[0010] Preferably, the freeze-drying parameters include: freezing temperature of -20 to -60°C for 5 to 24 hours; sublimation vacuum degree of -0.08 to -0.101 MPa for 12 to 24 hours.

[0011] Preferably, the moisture content of the dried sample is 20% to 23%.

[0012] Preferably, the parameters of the vacuum drying include: vacuum degree of -0.095 to -0.1 MPa, and time of 2 to 3 hours.

[0013] Preferably, the parameters of the nitrogen drying include: vacuum degree of -0.05 to -0.06 MPa, nitrogen purity of 99.999%, and time of 2 to 3 hours.

[0014] Preferably, the number of cycles is 6 to 12 times.

[0015] The present invention also provides anhydrous magnesium nitrate obtained by the preparation method described in the above technical scheme, wherein the mass percentage of water in the anhydrous magnesium nitrate is 0.13-0.4%, the mass content of magnesium nitrate is 97.71-97.97%, and the mass content of basic magnesium nitrate is 0.19-0.32%.

[0016] The invention provides a method for preparing anhydrous magnesium nitrate, comprising the following steps: freeze-drying magnesium nitrate hexahydrate to obtain a dried sample; cyclically performing vacuum drying and nitrogen drying on the dried sample to obtain the anhydrous magnesium nitrate; the temperature of the vacuum drying and nitrogen drying is 90-120°C. The invention adopts freeze drying to dry magnesium nitrate hexahydrate. Due to the low freeze drying temperature, there is no hydrolysis side reaction in the freeze drying process, and a dried sample mainly composed of dihydrated magnesium nitrate is formed; at the same time, the dried sample obtained by freeze drying is porous granular and has a large specific surface area, which is conducive to improving the effect of water removal in the next step. Vacuum drying and nitrogen drying are carried out at a relatively low temperature (90-120°C), which minimizes the hydrolysis side reaction and improves the purity of the obtained anhydrous magnesium nitrate; at the same time, vacuum drying is conducive to the dehydration of magnesium nitrate, and nitrogen drying can discharge the trace water generated by vacuum drying with nitrogen; the cycle of vacuum drying and nitrogen drying can fully promote water removal and reduce the water content of the obtained anhydrous magnesium nitrate. The data of the embodiment show that: in the obtained anhydrous magnesium nitrate, the mass percentage of water is 0.13-0.4%, the mass content of magnesium nitrate is 97.71-97.97%, and the mass content of basic magnesium nitrate is 0.19-0.32%.

[0017] The present invention also provides anhydrous magnesium nitrate obtained by the preparation method described in the above technical solution. The anhydrous magnesium nitrate provided by the present invention has high purity and low water content, and can be well applied to the fields of molten salt heat transfer and heat storage and lithium ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the XRD pattern of the dried sample obtained in Comparative Example 2. DETAILED DESCRIPTION

[0019] The present invention provides a method for preparing anhydrous magnesium nitrate, comprising the following steps:

[0020] freeze-drying magnesium nitrate hexahydrate to obtain a dry sample;

[0021] Cyclic vacuum drying and nitrogen drying of the dried sample to obtain anhydrous magnesium nitrate;

[0022] The temperature of the vacuum drying and nitrogen drying is 90-120°C 。

[0023] In the present invention, unless otherwise specified, the raw materials used in the present invention are preferably commercially available products.

[0024] The invention freeze-dries magnesium nitrate hexahydrate to obtain a dry sample.

[0025] In the present invention, the purity of the magnesium nitrate hexahydrate is preferably ≥99%.

[0026] In the present invention, the freeze-drying parameters include: the freezing temperature is preferably -20 to -60°C, more preferably -40 to -50°C; the time is preferably 5 to 24 hours, more preferably 10 to 18 hours; the sublimation vacuum is preferably -0.08 to -0.101MPa, more preferably -0.09 to -0.1MPa; the time is preferably 12 to 24 hours. In the present invention, the moisture content of the dried sample is preferably 20% to 23%; the moisture content of the dried sample is the total moisture content including crystal water.

[0027] In the present invention, the freeze drying is preferably performed in a freeze dryer.

[0028] After obtaining the dried sample, the present invention cyclically performs vacuum drying and nitrogen drying on the dried sample to obtain the anhydrous magnesium nitrate.

[0029] In the present invention, the parameters of the vacuum drying include: the vacuum degree is preferably -0.095 to -0.1 MPa; the temperature is 90 to 120°C, preferably 100 to 110°C; and the time is preferably 2 to 3 hours. In the present invention, the parameters of the nitrogen drying include: the vacuum degree is preferably -0.05 to -0.06 MPa; the temperature is 90 to 120°C, preferably 100 to 110°C; the purity of nitrogen is preferably 99.999%, and the time is preferably 2 to 3 hours. In the present invention, the number of cycles is preferably 6 to 12 times, and more preferably 8 to 10 times.

[0030] In the present invention, the vacuum drying and nitrogen drying are preferably carried out in a vacuum drying oven.

[0031] The present invention also provides anhydrous magnesium nitrate obtained by the preparation method described in the above technical solution. In the present invention, the mass percentage of water in the anhydrous magnesium nitrate is preferably 0.13-0.4%, the mass content of magnesium nitrate is preferably 97.71-97.97%, and the mass content of basic magnesium nitrate is preferably 0.19-0.32%. In the present invention, the anhydrous magnesium nitrate also contains impurities originally present in the raw material hexahydrate magnesium nitrate.

[0032] The anhydrous magnesium nitrate and the preparation method thereof provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0033] Example 1

[0034] A container containing magnesium nitrate hexahydrate (the purity of magnesium nitrate hexahydrate is 99%) was placed in a freeze dryer, frozen at -50°C for 10 hours, and then sublimated and dried at a vacuum degree of -0.1 MPa for 24 hours to obtain a dry sample.

[0035] The dried sample was placed in a vacuum drying oven, heated to 90°C, evacuated to -0.095MPa, and maintained in vacuum for 2 hours, i.e., vacuum drying for 2 hours; then, nitrogen with a purity of 99.999% was filled into the vacuum drying oven to reduce the vacuum degree to -0.05MPa, and maintained for 2 hours, i.e., nitrogen drying for 2 hours; vacuum drying and nitrogen drying were one cycle, and after repeating 10 cycles, the sample was taken out when the temperature reached room temperature, i.e., anhydrous magnesium nitrate.

[0036] It was determined that the mass content of water in the prepared anhydrous magnesium nitrate was 0.4%, the mass content of magnesium nitrate was 97.71%, the mass content of basic magnesium nitrate was 0.19%, and the remainder was other impurities.

[0037] Example 2

[0038] A container containing magnesium nitrate hexahydrate (the purity of magnesium nitrate hexahydrate is 99%) was placed in a freeze dryer, frozen at -50°C for 10 hours, and then sublimated and dried at a vacuum degree of 0.1 MPa for 24 hours to obtain a dry sample.

[0039] The dried sample was placed in a vacuum drying oven, heated to 100°C, evacuated to -0.095MPa, and maintained in vacuum for 2 hours, i.e., vacuum drying for 2 hours; then, nitrogen with a purity of 99.999% was filled into the vacuum drying oven to reduce the vacuum degree to -0.05MPa, and maintained for 2 hours, i.e., nitrogen drying for 2 hours; vacuum drying and nitrogen drying were one cycle, and after repeating 10 cycles, the sample was taken out when the temperature reached room temperature, i.e., anhydrous magnesium nitrate.

[0040] It was determined that the mass percentage of water in the prepared anhydrous magnesium nitrate was 0.16%, the mass percentage of magnesium nitrate was 97.93%, the mass percentage of basic magnesium nitrate was 0.2%, and the remainder was other impurities.

[0041] Example 3

[0042] A container containing magnesium nitrate hexahydrate (magnesium nitrate hexahydrate having a purity of 99%) was placed in a freeze dryer, frozen at -50°C for 10 hours, and then sublimated and dried at a vacuum degree of -0.1 MPa for 24 hours to obtain a dry sample.

[0043] The dried sample was placed in a vacuum drying oven, heated to 120°C, evacuated to -0.095MPa, and maintained in vacuum for 2 hours, i.e., vacuum drying for 2 hours; then, nitrogen with a purity of 99.999% was filled into the vacuum drying oven to reduce the vacuum degree to -0.05MPa, and maintained for 2 hours, i.e., nitrogen drying for 2 hours; vacuum drying and nitrogen drying were one cycle, and after repeating 10 cycles, the sample was taken out when the temperature reached room temperature, i.e., anhydrous magnesium nitrate.

[0044] It was determined that the mass percentage of water in the prepared anhydrous magnesium nitrate was 0.13%, the mass percentage of magnesium nitrate was 97.86%, the mass percentage of basic magnesium nitrate was 0.32%, and the remainder was other impurities.

[0045] Example 4

[0046] A container containing magnesium nitrate hexahydrate (magnesium nitrate hexahydrate having a purity of 99%) was placed in a freeze dryer, frozen at -50°C for 10 hours, and then sublimated and dried at a vacuum degree of -0.1 MPa for 24 hours to obtain a dry sample.

[0047] The dried sample was placed in a vacuum drying oven, heated to 100°C, evacuated to -0.1MPa, and maintained in vacuum for 2 hours, i.e., vacuum drying for 2 hours; then, nitrogen with a purity of 99.999% was filled into the vacuum drying oven to reduce the vacuum degree to -0.05MPa, and maintained for 2 hours, i.e., nitrogen drying for 2 hours; vacuum drying and nitrogen drying were one cycle, and after repeating 10 cycles, the sample was taken out when the temperature reached room temperature, i.e., anhydrous magnesium nitrate.

[0048] It was determined that the mass percentage of water in the prepared anhydrous magnesium nitrate was 0.14%, the mass percentage of magnesium nitrate was 97.97%, the mass percentage of basic magnesium nitrate was 0.22%, and the remainder was other impurities.

[0049] A lithium-ion battery base liquid with a magnesium nitrate content of 1% was prepared, A3 molecular sieve was added to further remove moisture, and then lithium salt and other additives were added to prepare a lithium-ion battery electrolyte. The moisture content of the electrolyte was measured to be 10ppm, which met the use standard.

[0050] Comparative Example 1

[0051] A container containing magnesium nitrate hexahydrate (magnesium nitrate hexahydrate having a purity of 99%) is placed in a vacuum-heatable muffle furnace, the vacuum degree is controlled at -0.05 MPa, and then the temperature is raised to 300°C at a rate of 10°C / min, and heated at 300°C for 0.5h to obtain anhydrous magnesium nitrate.

[0052] After testing, the mass content of basic magnesium nitrate in anhydrous magnesium nitrate is 3.5%, the mass content of magnesium nitrate is 95.1%, and the remainder is other impurities.

[0053] Comparative Example 2

[0054] A container containing magnesium nitrate hexahydrate (magnesium nitrate hexahydrate having a purity of 99%) was placed in a freeze dryer, frozen at -50°C for 10 hours, and then sublimated and dried at a vacuum degree of -0.1 MPa for 24 hours to obtain a dry sample.

[0055] The mass percentage of water in the dried sample was determined to be 20%, the mass percentage of magnesium nitrate was 78.8%, and the remainder was other impurities.

[0056] Figure 1 The XRD pattern of the dried sample obtained in Comparative Example 2 is shown in FIG. Figure 1 It can be seen that the main component of anhydrous magnesium nitrate is magnesium nitrate dihydrate, which means that simple freeze-drying cannot completely remove the crystal water in magnesium nitrate hexahydrate.

[0057] Comparative Example 3

[0058] A container containing magnesium nitrate hexahydrate (purity of magnesium nitrate hexahydrate is 99%) is placed in a vacuum drying oven, evacuated to -0.1 MPa, heated to 120° C., and maintained in vacuum for 12 hours to obtain anhydrous magnesium nitrate.

[0059] It was determined that the mass percentage of water in the prepared anhydrous magnesium nitrate was 5%, the mass percentage of magnesium nitrate was 89.9%, the mass percentage of basic magnesium nitrate was 3.1%, and the remainder was other impurities.

[0060] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing anhydrous magnesium nitrate, characterized in that: The following steps are involved: freeze-drying magnesium nitrate hexahydrate to obtain a dry sample; Cyclic vacuum drying and nitrogen drying of the dried sample to obtain anhydrous magnesium nitrate; The temperature of the vacuum drying and nitrogen drying is 90-120°C; The freeze-drying parameters include: the freezing temperature is -20 to -60°C, and the time is 5 to 24 hours; the sublimation vacuum is -0.08 to -0.101 MPa, and the time is 12 to 24 hours.

2. The preparation method according to claim 1, characterized in that: The moisture content of the dried sample is 20% to 23%.

3. The preparation method according to claim 1, characterized in that: The parameters of the vacuum drying include: vacuum degree of -0.095 to -0.1 MPa, and time of 2 to 3 hours.

4. The preparation method according to claim 1, characterized in that: The parameters of the nitrogen drying include: vacuum degree of -0.05 to -0.06 MPa, nitrogen purity of 99.999%, and time of 2 to 3 hours.

5. The preparation method according to claim 1, 3 or 4, characterized in that: The number of cycles is 6 to 12 times.

6. Anhydrous magnesium nitrate obtained by the preparation method according to any one of claims 1 to 5, characterized in that: In the anhydrous magnesium nitrate, the mass percentage of water is 0.13-0.4%, the mass content of magnesium nitrate is 97.71-97.97%, and the mass content of basic magnesium nitrate is 0.19-0.32%.

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