A method for preparing HgS crystals by controlling crystal form through wet recrystallization

Abstract

The present invention provides a method for preparing HgS crystals by controlling the crystal form through wet recrystallization, belonging to the technical field of preparing mercury compounds. In the method, mercury sulfide powder is dissolved in an alkali metal sulfide solution, followed by a hydrothermal reaction. After the reaction is completed, the solution is cooled to room temperature. HgS crystals of different crystal forms are prepared by controlling the rate and amount of deionized water added to the reaction solution. The method achieves mild reaction conditions, low cost, and produces millimeter-sized crystals that are easily controllable and have a short crystal growth cycle.

Description

Technical Field

[0001] The invention belongs to the technical field of mercury compound preparation, and particularly relates to a method for preparing HgS crystals by controlling the crystal form through wet recrystallization. Background Art

[0002] Mercury sulfide (HgS) is an important semiconductor material with widespread applications in infrared detection, optoelectronic devices, sensors, and other fields. The two most common crystalline forms of HgS are α-HgS (red hexagonal) and β-HgS (black cubic). These two forms are used in different fields due to their distinct physical properties. For example, the photoelectric properties of α-HgS are suitable for infrared detection, while β-HgS has good conductivity, making it suitable for use in sensors and photoelectric conversion devices. Therefore, the preparation of high-quality HgS crystals is crucial to its performance and application.

[0003] In existing technologies, the synthesis and growth of mercury sulfide crystals are mostly achieved through solid-phase reaction methods and chemical vapor deposition (CVD). However, these methods have numerous limitations: solid-phase reaction methods often suffer from problems such as particle agglomeration and grain inhomogeneity; while CVD methods can produce relatively pure mercury sulfide, they are expensive and require complex equipment, making them unsuitable for large-scale production. Therefore, a method is needed to produce high-purity, large-sized, and uniform mercury sulfide crystals under mild conditions. Summary of the Invention

[0004] To address the above issues, the present invention provides a method for preparing HgS crystals by wet recrystallization with controlled crystal form. This method involves dissolving HgS powder in an alkali metal sulfide solution, performing a hydrothermal reaction, and then controlling the rate at which deionized water is added to control the crystal form of the resulting HgS crystals.

[0005] Specifically, the present invention adopts the following technical solutions to achieve the above objectives:

[0006] A method for preparing HgS crystals by controlling the crystal form through wet recrystallization comprises the following steps:

[0007] S1. Prepare an alkali metal sulfide solution, add HgS powder to the alkali metal sulfide solution, and stir and heat until the HgS is completely dissolved to obtain a saturated HgS solution;

[0008] S2. The HgS saturated solution is placed in a sealed container and heated to perform a hydrothermal reaction; after the reaction is completed, the solution is cooled to room temperature, and deionized water is added dropwise to the reaction solution in batches or all at once, protected from light, and allowed to stand for crystallization; centrifuged, washed, and dried; α-HgS crystals are obtained by adding deionized water dropwise in batches, while β-HgS crystals are obtained by adding deionized water all at once.

[0009] In a preferred embodiment, the alkali metal sulfide in step S1 is sodium sulfide and / or potassium sulfide.

[0010] In a preferred embodiment, the molar ratio of the alkali metal sulfide to the HgS powder in step S1 is 10:3.

[0011] In a preferred embodiment, when preparing the alkali metal sulfide solution in step S1, stirring is performed at 400-500 rpm at 160-180°C.

[0012] In a preferred embodiment, the conditions for the hydrothermal reaction in step S2 are: heating from room temperature to 200-225°C at a rate of 5.5-6.5°C / min, and keeping warm for 1200-1440 min; then cooling to 120-180°C, and keeping warm for 2000-2580 min.

[0013] In a preferred embodiment, step S2 includes the step of adding HgS seed crystals to the HgS saturated solution.

[0014] In a preferred embodiment, the time required for cooling to room temperature in step S2 is 6 to 8 hours.

[0015] In a preferred embodiment, when deionized water is added dropwise in batches in step S2, the ratio of the total volume of the deionized water finally added dropwise to the volume of the reaction solution is 1:12 to 1:2.

[0016] In a preferred embodiment, the deionized water is added dropwise in batches in step S2 as follows: 1 to 2 drops of deionized water are added dropwise every 30 minutes.

[0017] In a preferred embodiment, when deionized water is added at one time in step S2, the ratio of the volume of the added deionized water to the volume of the reaction solution is greater than 1:1.

[0018] In a preferred embodiment, the centrifugal speed in step S2 is 5000-10000 rpm, and the centrifugal time is 1-5 min.

[0019] In a preferred embodiment, the drying temperature in step S2 is 60-90° C., and the drying time is 10-15 hours.

[0020] In a preferred embodiment, the crystals are washed in step S2 in the following manner: the crystals are washed 2 to 4 times with anhydrous ethanol and deionized water, respectively.

[0021] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0022] (1) In the present invention, HgS crystals of different crystal forms are prepared by wet recrystallization. The reaction conditions are mild, the preparation cost is low, the obtained crystals are larger in size (millimeter level), higher in purity, the crystal growth cycle is shorter, and the crystal size is easy to control.

[0023] (2) The millimeter-sized HgS crystals prepared in the present invention can significantly reduce crystal defects and improve the performance and stability of the crystals due to their large size. They are also easy to process and utilize, and can be used in technical fields such as electronics, optics, medicine, and handicraft preparation. For example, they can be used to prepare infrared detectors in the electronics field; they can be used to make optical lenses in optical instruments with special requirements for optical materials in the optical field (such as optical systems with high transmittance or high absorption rate within a specific wavelength range, or for generating lasers with specific wavelengths and characteristics); α-HgS, also known as cinnabar, can be used to make decorations or as a traditional Chinese medicine.

[0024] (3) In the present invention, the conditions for mutual conversion between different HgS crystal forms were studied, and the controllable mutual conversion between the two crystal forms was successfully achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a naked eye appearance picture of the α-HgS crystal prepared in Example 1 of the present invention;

[0026] Figure 2 is the XRD pattern of the α-HgS crystal prepared in Example 1 of the present invention;

[0027] Figure 3 This is a 50-fold magnified appearance image of the α-HgS crystal prepared in Example 1 of the present invention under an optical microscope;

[0028] Figure 4 This is a 100-fold magnified appearance picture of the α-HgS crystal prepared in Example 1 of the present invention under an optical microscope;

[0029] Figure 5 The HAADF-STEM image and EDS image of the α-HgS crystal prepared in Example 1 of the present invention are shown;

[0030] Figure 6 This is the appearance of the HgS crystal prepared in Comparative Example 1, magnified 100 times under an optical microscope;

[0031] Figure 7 The appearance image (Figure (a)) and XRD pattern (Figure (b)) of the β-HgS crystal prepared in Example 6 of the present invention are shown;

[0032] Figure 8 This is an appearance diagram of the bulk α-HgS crystal prepared in Example 5 of the present invention;

[0033] Figure 9 These are the appearance pictures of α-HgS (Figure (a)) and β-HgS (Figure (b)) prepared in Example 7 of the present invention. DETAILED DESCRIPTION

[0034] The following content clearly and completely describes the technical solution of the present application in conjunction with the embodiments so that those skilled in the art can fully understand the present application. Obviously, the embodiments described are only some preferred embodiments of the present application, rather than all embodiments. Any equivalent transformation or substitution made by those of ordinary skill in the art to the following embodiments without creative work falls within the scope of protection of the present application.

[0035] The purity of sodium sulfide nonahydrate used in the following examples is ≥98%.

[0036] Example 1

[0037] A method for preparing α-HgS crystals by wet recrystallization comprises the following steps:

[0038] S1. Clean the beaker and weigh 12 g (0.05 mol) of sodium sulfide nonahydrate (Na2S·9H2O). Add 50 mL of deionized water to prepare a Na2S solution. Stir on a magnetic stirrer at 180°C and 400 rpm until the solution is completely dissolved, resulting in a clear solution. Then, add 3.6 g (0.015 mol) of HgS powder. Continue heating and stirring until the HgS is completely dissolved, resulting in a saturated HgS solution.

[0039] S2. The obtained HgS saturated solution was transferred to the inner lining of a hydrothermal kettle, sealed, and placed in an oven for heating reaction. During the heating process, the temperature was first raised from room temperature to 200°C within 30 minutes (the heating rate was 5.5~5.7°C / min), and kept at 200°C for 1440 minutes. Then the temperature was lowered to 150°C and kept for 2000 minutes, and finally cooled to room temperature within 7 hours. The reaction solution was transferred to a sealed bottle, and 1 to 2 drops of deionized water were slowly added every 30 minutes, and allowed to stand. The final volume ratio of deionized water added to the reaction solution was 1:4. Place in a dark place to allow the solution to slowly crystallize. After the crystallization is completed, centrifuge at 7000rpm for 3 minutes. The solid was washed 3 times with anhydrous ethanol and deionized water respectively, and then dried at 75°C for 12 hours to obtain the following: Figure 1 、 Figure 3 、 Figure 4 The red HgS crystals shown in the figure are ground and then subjected to XRD (X-ray diffraction) testing. The results are as follows: Figure 2 As shown, from Figure 2It can be seen from the above that the obtained product is α-HgS crystal. The α-HgS crystal was analyzed by HAADF-STEM (High Angle Annular Dark Field Scanning Transmission Electron Microscopy) and EDS (Energy Dispersive X-ray Spectroscopy Analysis Technology), and the following results were obtained: Figure 5 The analysis results are shown in Figure 2. Figure 5 The upper left image is the EDS total element distribution diagram of the α-HgS crystal obtained in this example, the HAADF image in the upper right corner is the HAADF-STEM image of the α-HgS crystal, the lower left image is the EDS distribution diagram of the single Hg (mercury) element, and the lower right image is the EDS distribution diagram of the single S (sulfur) element. Figure 5 It can be seen that Hg and S elements are evenly distributed in various parts of the crystal, and no other impurity elements are obviously detected in the test results; at the same time, combined with Figure 2 The XRD test results show that the prepared crystals exhibit excellent crystallinity and uniform element distribution, which proves that the purity of the α-HgS crystals is high.

[0040] Example 2

[0041] The method for preparing α-HgS crystals in this embodiment is basically the same as that in Example 1, except that:

[0042] S2. The resulting HgS saturated solution was transferred to a hydrothermal autoclave, sealed, and placed in an oven for heating and reaction. During the heating process, the temperature was first raised from room temperature to 225°C over 30 minutes (at a rate of approximately 6.5°C / min) and maintained at 225°C for 1200 minutes. The temperature was then lowered to 120°C and maintained for 2200 minutes, before finally cooling to room temperature over 6 hours. The reaction solution was transferred to a sealed bottle and 1-2 drops of deionized water were slowly added every 30 minutes. The solution was allowed to stand until the volume ratio of deionized water to the reaction solution was 1:2. The solution was placed in a dark place to allow the solution to slowly crystallize. After crystallization, the solution was centrifuged at 5000 rpm for 5 minutes. The solid was washed four times with anhydrous ethanol and four times with deionized water, then dried at 60°C for 15 hours to obtain red HgS crystals. The HgS crystals were confirmed to be α-HgS crystals and showed no significant differences from the α-HgS crystals in Example 1.

[0043] Example 3

[0044] The method for preparing α-HgS crystals in this embodiment is basically the same as that in Example 1, except that:

[0045] S1. Clean the beaker and weigh 10 g (0.05 mol) of potassium sulfide pentahydrate (K2S·5H2O). Add 50 mL of deionized water to prepare a K2S solution. Stir on a magnetic stirrer at 160°C and 500 rpm until completely dissolved, resulting in a clear solution. Then, add 3.6 g (0.015 mol) of HgS powder. Continue heating and stirring until the HgS is completely dissolved, resulting in a saturated HgS solution.

[0046] In step S2, red HgS crystals were obtained. The HgS crystals were confirmed to be α-HgS crystals, which were not significantly different from the α-HgS crystals in Example 1.

[0047] Example 4

[0048] A method for preparing α-HgS crystals by wet recrystallization comprises the following steps:

[0049] S1. Clean the beaker and weigh 12 g (0.05 mol) of sodium sulfide nonahydrate (Na2S·9H2O). Add 50 mL of deionized water to prepare a Na2S solution. Stir on a magnetic stirrer at 170°C and 450 rpm until completely dissolved, resulting in a clear solution. Then, add 3.6 g (0.015 mol) of HgS powder. Continue heating and stirring until the HgS is completely dissolved, resulting in a saturated HgS solution.

[0050] S2. The resulting HgS saturated solution was transferred to a hydrothermal kettle, sealed with a liner, and placed in an oven for heating and reaction. During the heating process, the temperature was first raised from room temperature to 210°C within 30 minutes (at a rate of approximately 6.0°C / min) and maintained at 210°C for 1300 minutes. The temperature was then lowered to 180°C and maintained for 2580 minutes, and finally cooled to room temperature within 8 hours. The reaction solution was transferred to a sealed bottle, and 1-2 drops of deionized water were slowly added every 30 minutes. The solution was allowed to stand until the volume ratio of deionized water added to the reaction solution was 1:12. The solution was placed in a dark place to allow the solution to slowly crystallize. After crystallization, the solution was centrifuged at 10,000 rpm for 1 minute. The solid was washed twice with anhydrous ethanol and deionized water, respectively, and then dried at 90°C for 10 hours to obtain red HgS crystals. The HgS crystals were confirmed to be α-HgS crystals and did not show significant differences from the α-HgS crystals in Example 1.

[0051] Example 5

[0052] A method for preparing bulk α-HgS crystals by wet recrystallization comprises the following steps:

[0053] S1. Clean the beaker and weigh 12 g (0.05 mol) of sodium sulfide nonahydrate (Na2S·9H2O). Add 50 mL of deionized water to prepare a Na2S solution. Stir on a magnetic stirrer at 180°C and 400 rpm until the solution is completely dissolved, resulting in a clear solution. Then, add 3.6 g (0.015 mol) of HgS powder. Continue heating and stirring until the HgS is completely dissolved, resulting in a saturated HgS solution.

[0054] S2. The obtained HgS saturated solution was transferred to a hydrothermal kettle, 0.0558 g of HgS seed crystals were added to the solution, and the hydrothermal kettle was sealed. The hydrothermal kettle was placed in an oven for heating reaction. During the heating process, the temperature was first raised from room temperature to 200°C within 30 minutes (the heating rate was 5.5~5.7°C / min), and kept at 200°C for 1440 minutes. Then the temperature was lowered to 150°C and kept for 2000 minutes, and finally cooled to room temperature within 7 hours. The reaction solution was transferred to a sealed bottle, and 1 to 2 drops of deionized water were slowly added every 30 minutes, and the solution was allowed to stand. The final volume ratio of deionized water added to the reaction solution was 1:4. Place in a dark place to allow the solution to slowly crystallize. After the crystallization, centrifuge at 7000 rpm for 3 minutes. The solid was washed 3 times with anhydrous ethanol and deionized water respectively, and then dried at 75°C for 12 hours to obtain the following: Figure 8 The red, blocky α-HgS crystals shown weighed 0.102 g. The size of the α-HgS crystals ranged from 0.8 to 2.3 mm. This indicates that adding HgS seed crystals (either purchased from a commercial source or prepared according to the methods of Examples 1 to 4) to the saturated HgS solution in step S2 significantly increases the size of the α-HgS crystals.

[0055] Example 6

[0056] A method for preparing β-HgS crystals by wet recrystallization comprises the following steps:

[0057] S1. Clean the beaker and weigh 12 g (0.05 mol) of sodium sulfide nonahydrate (Na2S·9H2O). Add 50 mL of deionized water to prepare a Na2S solution. Stir on a magnetic stirrer at 180°C and 400 rpm until the solution is completely dissolved, resulting in a clear solution. Then, add 3.6 g (0.015 mol) of HgS powder. Continue heating and stirring until the HgS is completely dissolved, resulting in a saturated HgS solution.

[0058] S2. Move the obtained HgS saturated solution to the inner lining of a hydrothermal kettle, seal it, and place it in an oven for heating reaction. During the heating process, first raise the temperature from room temperature to 200°C within 30 minutes (the heating rate is 5.5~5.7°C / min), and keep it at 200°C for 1440 minutes. Then cool it to 150°C and keep it for 2000 minutes, and finally cool it to room temperature within 7 hours. Transfer the reaction solution to a sealed bottle, add a large amount of deionized water at one time, let it stand, and the final volume ratio of the added deionized water to the reaction solution is more than 1:1. Place it in a dark place to allow the solution to slowly crystallize. After the crystallization is completed, centrifuge it at 7000rpm for 3 minutes. Wash the solid with anhydrous ethanol and deionized water 3 times each, and then dry it at 75°C for 12 hours to obtain the following: Figure 7 The black HgS crystal shown in Figure (a) is ground and then subjected to XRD testing. The results are as follows Figure 7 As shown in Figure (b), Figure 7 As can be seen from Figure (b), the obtained product is β-HgS crystal.

[0059] Example 7

[0060] This example studies the feasibility of using ultrasound-assisted energy generation to regulate the production of α-HgS and β-HgS. The specific steps are as follows:

[0061] S1. Clean the beaker and weigh 12 g (0.05 mol) of sodium sulfide nonahydrate (Na2S·9H2O). Add 50 mL of deionized water to prepare a Na2S solution. Stir on a magnetic stirrer at 180°C and 400 rpm until the solution is completely dissolved, resulting in a clear solution. Then, add 3.6 g (0.015 mol) of HgS powder. Continue heating and stirring until the HgS is completely dissolved, resulting in a saturated HgS solution.

[0062] S2. Move the obtained HgS saturated solution to the inner lining of the hydrothermal kettle, seal it, and place it in an oven for heating reaction. During the heating process, first raise the temperature from room temperature to 200°C within 30 minutes (the heating rate is 5.5~5.7°C / min), and keep it at 200°C for 1440 minutes. Then cool it to 150°C and keep it for 2000 minutes, and finally cool it to room temperature within 7 hours. Transfer the reaction solution to a beaker, heat it to 60°C, and take two 1mL reaction solutions as sample A and sample B respectively. Add 2~3 drops of deionized water to sample A, and then ultrasonicate sample A at 180W and 40000Hz for 3 minutes. A trace amount of red crystals can be found (such as Figure 9 When 0.5 mL of deionized water was added directly to sample B, the interface of the solution quickly turned black (as shown in Figure (a)). Figure 9Sample B was centrifuged at 4000 rpm for 5 min, and the centrifugation was repeated 3 to 6 times (e.g., 4 times). The resulting crystals were dried at 60°C to obtain black β-HgS crystals.

[0063] As can be seen from the experiment in the present embodiment, the crystal structure of the cubic phase has a lower nucleation barrier due to its higher symmetry, and the cubic structure of β-HgS is formed less energy than the tripartite structure forming α-HgS, and is therefore easier to form. When HgS crystals are required to precipitate out in α phase, it is necessary to provide a certain amount of energy to help overcome the kinetic phase transition barrier of β-HgS to α-HgS during crystallization. Therefore, the regulation of HgS crystalline phase can be achieved by controlling the crystallization speed during crystallization and providing additional energy for the crystallization process.

[0064] Example 8

[0065] In this example, the feasibility of converting α-HgS into β-HgS by heating in the absence of light and in air, and the feasibility of converting β-HgS into α-HgS by heating in the absence of light were studied.

[0066] 8.1 Feasibility of Conversion of α-HgS to β-HgS

[0067] The α-HgS crystal powder prepared in Example 1 was placed in a small crucible, which was then placed in a quartz tube. The quartz tube was then placed in a tube furnace. The tube furnace was first vacuumed to a vacuum degree of 5×10 -5 Pa, then argon was introduced to fill the quartz tube, and then heating was performed. Heating temperature control settings: the temperature was set to 350°C and the heating time was 2 hours. After the heating and holding period, the red crystalline powder in the crucible was observed to turn from red to black. This indicates that heating can transform α-HgS into β-HgS, with a phase transition temperature between 300 and 400°C.

[0068] 8.2 Feasibility of Conversion of β-HgS to α-HgS

[0069] A saturated HgS solution was prepared according to the method in step S1 of Example 6. The resulting saturated HgS solution was transferred to a hydrothermal autoclave, sealed, and placed in an oven for heating. During the heating process, the temperature was first raised from room temperature to 200°C over 30 minutes (at a rate of 5.5-5.7°C / min) and maintained at 200°C for 1440 minutes. The temperature was then lowered to 150°C and maintained for 2000 minutes, before finally cooling to room temperature over 7 hours. 5 mL of the reaction solution was transferred to a sealed bottle, and 3 mL of deionized water was directly added. Black β-HgS crystals quickly precipitated from the solution. The bottle was sealed and placed in the dark. After a period of time (36 hours), the black precipitated crystals were observed to gradually transform into red crystals. This demonstrates that β-HgS can spontaneously transform into α-HgS.

[0070] Comparative Example 1

[0071] A method for preparing HgS crystals by wet recrystallization comprises the following steps:

[0072] P1. Clean the beaker and weigh 12g (0.05mol) of sodium sulfide nonahydrate (Na2S·9H2O). Add 50mL of deionized water to prepare a Na2S solution. Stir on a magnetic stirrer at 180°C and 400r / min until completely dissolved, obtaining a clear solution. Then, add 3.6g (0.015mol) of HgS powder and 0.36g of polyvinylpyrrolidone (PVP, molecular weight 40,000Da). Continue heating and stirring until the HgS is completely dissolved, obtaining a saturated HgS solution.

[0073] P2. Move the obtained HgS saturated solution to the inner lining of a hydrothermal kettle, seal it, and place it in an oven for heating reaction. During the heating process, first raise the temperature from room temperature to 200°C within 30 minutes (the heating rate is 5.5~5.7°C / min), and keep it at 200°C for 1440 minutes. Then cool it to 150°C and keep it for 2000 minutes, and finally cool it to room temperature within 7 hours. Transfer the reaction solution to a sealed bottle, slowly add 1 to 2 drops of deionized water every 30 minutes, let it stand, and the final volume ratio of deionized water added to the reaction solution is 1:4. Place it in a dark place to allow the solution to slowly crystallize. After crystallization, centrifuge it at 7000rpm for 3 minutes. Wash the solid with anhydrous ethanol and deionized water 3 times each, and then dry it at 75°C for 12 hours to obtain the following: Figure 6 The HgS crystals shown are black powders. Observed under an optical microscope, their size is significantly smaller than that of the α-HgS crystals in Example 1, and the crystals appear to be connected in red and black.

[0074] The embodiments described above are merely preferred embodiments of the present application and are not intended to limit the scope of protection of the present application. For any person skilled in the art, the present application may have various modifications and variations. Any simple equivalent changes and modifications made based on the scope of protection of the present application and the contents of the specification should be included in the scope of protection of the present application.

Claims

1. A method for preparing HgS crystals by controlling the crystal form through wet recrystallization, characterized in that: The following steps are involved: S1. Prepare an alkali metal sulfide solution, add HgS powder to the alkali metal sulfide solution, and stir and heat until the HgS is completely dissolved to obtain a saturated HgS solution; the molar ratio of the alkali metal sulfide to the HgS powder is 10:3; S2. The HgS saturated solution is placed in a sealed container, heated, and subjected to a hydrothermal reaction; after the reaction is completed, the solution is cooled to room temperature, and deionized water is added dropwise to the reaction solution in batches or all at once, protected from light, and allowed to stand for crystallization; centrifuged, the crystals are washed, and dried; α-HgS crystals are obtained by adding deionized water dropwise in batches, and β-HgS crystals are obtained by adding deionized water all at once; the conditions for the hydrothermal reaction are: heating from room temperature to 200-225°C at a rate of 5.5-6.5°C / min, and keeping warm for 1200-1440min; then cooling to 120-180°C, and keeping warm for 2000-2580min; when deionized water is added dropwise in batches, the ratio of the total volume of the deionized water finally added to the volume of the reaction solution is 1:12-1:2; when deionized water is added all at once, the ratio of the volume of the deionized water added to the volume of the reaction solution is greater than 1:

1.

2. The method according to claim 1, characterized in that The alkali metal sulfide in step S1 is sodium sulfide and / or potassium sulfide.

3. The method according to claim 1, characterized in that When preparing the alkali metal sulfide solution in step S1, the solution is stirred at 160-180° C. and a speed of 400-500 rpm.

4. The method according to claim 1, wherein Step S2 includes the step of adding HgS seed crystals to the HgS saturated solution.

5. The method according to claim 1, wherein In step S2, deionized water is added dropwise in batches according to the following method: 1 to 2 drops of deionized water are added dropwise every 30 minutes.

6. The method according to claim 1, characterized in that In step S2, the centrifugal speed is 5000-10000 rpm, and the centrifugal time is 1-5 minutes; or / and in step S2, the drying temperature is 60-90° C., and the drying time is 10-15 hours.

7. The method according to claim 1, characterized in that The crystals are washed in step S2 according to the following method: the crystals are washed 2 to 4 times with anhydrous ethanol and deionized water respectively.

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