Micron-sized hexagonal flaky magnesium hydroxide taking bittern as raw material and preparation method thereof

Abstract

A kind of micron grade hexagonal flaky magnesium hydroxide with bitter brine as raw material and its preparation method, belong to inorganic material preparation technical field, the preparation method of the micron grade hexagonal flaky magnesium hydroxide with bitter brine as raw material includes three steps of preparation of magnesium hydroxide precursor, hydrothermal synthesis, post-processing;The micron grade hexagonal flaky magnesium hydroxide with bitter brine as raw material obtained by the application, the particle size distribution of laser particle size instrument test is D10 at 0.71~0.85 μm, D50 at 1.28~1.47 μm, D90 at 3.26~3.65 μm.

Description

Technical Field

[0001] The invention relates to micron-sized hexagonal flaky magnesium hydroxide using bittern as a raw material and a preparation method thereof, belonging to the technical field of inorganic material preparation. Background Art

[0002] my country's salt lake magnesium reserves total 4.75 billion tons. However, utilization of salt lake brine resources primarily focuses on elements such as lithium, potassium, and boron, while magnesium resources have been wasted and stored for a long time. This has not only led to a resource imbalance but also caused serious "magnesium damage." Magnesium hydroxide is an excellent, environmentally friendly, green flame retardant. Using brine to produce magnesium hydroxide effectively addresses the problem of brine accumulation and enables efficient utilization of magnesium resources. Magnesium hydroxide decomposes at a temperature of approximately 350-400°C. Due to its alkalinity, it can neutralize harmful acidic gases produced during combustion, reducing environmental pollution. It is currently one of the most promising and environmentally friendly inorganic flame retardants. Among them, magnesium hydroxide crystals with a well-dispersed micron-sized hexagonal flake morphology have low surface polarity and surface energy, good compatibility with polymers, and superior flame retardancy, mechanical properties, and mechanical properties, making it one of the most ideal inorganic flame retardants. However, the hexagonal magnesium hydroxide produced by the existing hexagonal magnesium hydroxide preparation process has two prominent problems: wide particle size distribution and poor morphology uniformity. In particular, when bittern is used as a raw material, the content of magnesium ions fluctuates greatly, and the amount of alkali added during the reaction is difficult to accurately control, which causes an unstable precipitation rate of magnesium ions and brings about the problem of difficulty in uniformly controlling the particle size. Therefore, in order to prepare micron-sized hexagonal magnesium hydroxide using bittern as a raw material, it is urgent to develop a preparation method that can better control the particle size distribution and morphology uniformity.

[0003] Chinese patent CN115321565A discloses a method for producing nano-magnesium hydroxide using salt lake brine as raw material, belonging to the fields of waste resource utilization and functional nanomaterials. The method uses salt lake brine as raw material to produce nano-magnesium hydroxide, thereby improving the utilization rate of salt lake brine. The method employs multi-effect evaporation and crystallization to obtain magnesium chloride hexahydrate and magnesium sulfate heptahydrate, respectively, and then prepares ultrapure nano-magnesium hydroxide. The total yield of magnesium in the brine reaches over 90%, thereby improving the utilization rate of magnesium in the salt lake brine. Although the patent achieves nanoscale magnesium hydroxide particle size, the morphology is not hexagonal flakes, which poses significant challenges in uniform dispersion for flame retardant applications.

[0004] It can be seen that the preparation of micron-sized hexagonal flaky magnesium hydroxide using bittern as raw material still has the problems of large particle size fluctuation range and difficulty in controlling the morphology to be uniform hexagonal flakes. Therefore, developing a preparation method of micron-sized hexagonal flaky magnesium hydroxide using bittern as raw material and preparing micron-sized hexagonal flaky magnesium hydroxide with narrow particle size distribution and uniform morphology is of great significance for the efficient utilization of bittern resources. Summary of the Invention

[0005] In view of the shortcomings of the above-mentioned prior art, the present invention provides a micron-sized hexagonal flaky magnesium hydroxide using bittern as raw material and a preparation method thereof, to achieve the following invention objectives: using bittern as raw material, micron-sized hexagonal flaky magnesium hydroxide with narrow particle size distribution and uniform morphology is prepared.

[0006] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0007] Disclosed are micron-sized hexagonal flaky magnesium hydroxide using bittern as a raw material and a preparation method thereof. The micron-sized hexagonal flaky magnesium hydroxide using bittern as a raw material has a particle size distribution as measured by a laser particle size analyzer: D10 is 0.71-0.85 μm, D50 is 1.28-1.47 μm, and D90 is 3.26-3.65 μm.

[0008] The method for preparing micron-sized hexagonal flaky magnesium hydroxide using bittern as raw material comprises three steps: preparing a magnesium hydroxide precursor, hydrothermal synthesis, and post-treatment;

[0009] The following are further improvements to the above technical solution:

[0010] Step 1: Preparation of magnesium hydroxide precursor

[0011] Put bittern into a reactor, add sodium hydroxide solid under uniform stirring, raise the temperature and control the temperature to the reaction temperature, stir and react completely, filter, wash and dry the filtered solid to obtain a magnesium hydroxide precursor;

[0012] The main ion component of the bittern is Mg 2+ Content 90~120g / L, Ca 2+ Content 6~16g / L, Na + Content 1~14g / L, K + Content 0.1~1.5g / L, Li + Content 0.03~0.7g / L, Cl - Content 310~460 g / L;

[0013] The mass ratio of bittern to sodium hydroxide solid is 170-340:26-95;

[0014] The rapid stirring has a stirring rate of 2000 to 3000 rpm;

[0015] The uniform stirring has a stirring rate of 800 to 1500 rpm;

[0016] The reaction temperature is 35-55°C;

[0017] The stirring reaction is complete, and the stirring reaction time is 3 to 7 hours;

[0018] The washing is performed 3 to 6 times with deionized water, and the amount of deionized water used in each washing is equal to the mass of the washed solid;

[0019] The drying step is performed at 60-85° C. for 8-13 hours.

[0020] Step 2: Hydrothermal synthesis

[0021] After the magnesium hydroxide precursor, dispersant, morphology control agent, and composite hydrothermal solvent are mixed and stirred into a uniform slurry, the slurry is added to a hydrothermal reactor with a polytetrafluoroethylene liner, maintained in a sealed state, and heated and maintained at a constant temperature to the hydrothermal reaction temperature under uniform stirring. After the constant temperature hydrothermal reaction is complete, the temperature is cooled to room temperature and discharged to obtain a slurry;

[0022] The dispersant is one of N,N-di(hydroxyethyl)coconut amide, tall oil diethanolamide, and N-(2-hydroxyethyl)dodecylamide;

[0023] The morphology control agent is one of N,N'-bis(2-hydroxyethyl)ethylenediamine and N,N-bis(2-hydroxyethyl)formamide, or a mixture of the two in any mass ratio;

[0024] The composite hydrothermal solvent consists of sulfolane, glycerol and deionized water;

[0025] The mass ratio of sulfolane, glycerol and deionized water is 4-11:8-20:100-250;

[0026] The mass ratio of the magnesium hydroxide precursor, dispersant, morphology control agent and composite hydrothermal solvent is 80-190:2-7:4-9:240-460;

[0027] The uniform stirring has a stirring rate of 450 to 950 rpm;

[0028] After the constant temperature hydrothermal reaction is complete, the reaction time is 7 to 12 hours;

[0029] The hydrothermal reaction temperature is 125-160°C.

[0030] Step 3: Post-processing

[0031] After the slurry is filtered, washed and dried, micron-sized hexagonal flake magnesium hydroxide is obtained;

[0032] The washing is performed 3 to 6 times with deionized water, and the amount of deionized water used in each washing is equal to the mass of the washed solid;

[0033] The drying step is performed at 50-85° C. for 18-26 hours.

[0034] Compared with the prior art, the present application has the following beneficial effects:

[0035] 1、The present application uses N,N-di(hydroxyethyl) coconut amide, tall oil diethanolamide, N-(2-hydroxyethyl) dodecylamide as dispersants, which effectively disperse the growing magnesium hydroxide particles in the hydrothermal synthesis process. The above three substances are all amide substances containing hydroxyl groups, and have good alkali resistance and high temperature resistance, and are more suitable for use in the high temperature alkaline hydrothermal system involved in the present application. The terminal hydroxyl groups and amide groups contained in these substances have strong charge attraction and hydrogen bond adsorption effects with the hydroxyl groups on the surface of magnesium hydroxide particles. These dispersants can well cover the surface of magnesium hydroxide particles, prevent the disorderly coalescence of magnesium hydroxide particles, and promote the magnesium hydroxide particles to exist in the hydrothermal synthesis system in the form of single particles as much as possible. Therefore, the present application can successfully obtain micron-sized hexagonal flake-shaped magnesium hydroxide with narrow particle size distribution and uniform morphology;

[0036] 2、The present application uses N,N'-bis(2-hydroxyethyl)ethylenediamine and N,N-bis(2-hydroxyethyl)formamide as morphology control agents, which effectively control the ordered growth of magnesium hydroxide particles in the hydrothermal synthesis process. The above two substances both contain bis-hydroxyethyl chemical structures, and the bis-hydroxyethyl structure has a very strong chelating adsorption effect on the edges of flake-shaped magnesium hydroxide, thereby promoting the ordered growth of magnesium hydroxide along the edges of the flake, which ultimately promotes the regularity of the hexagonal flake-shaped morphology of magnesium hydroxide particles;

[0037] 3、The present application designs a composite hydrothermal solvent composed of sulfolane, glycerol and deionized water. The polarity of sulfolane and glycerol is much weaker than that of deionized water. Compared with the hydrothermal solvent composed of single deionized water, these organic solvents with weaker polarity than deionized water can more effectively promote the complete dissolution of dispersants and morphology control agents, so that the amide substances containing hydroxyl groups and the substances containing bis-hydroxyethyl chemical structures are more uniformly and stably dissolved in the hydrothermal reaction system, thereby promoting the contact area of the dispersants and the morphology control agents with the magnesium hydroxide particles, so that the surface effect of the above two additives can be maximized, and ultimately promoting the uniformity of the particle size of magnesium hydroxide and the uniformity of its micro-morphology;

[0038] 4、The micron-sized hexagonal flake-shaped magnesium hydroxide obtained by the present application has a particle size distribution of D10 of 0.71-0.85 μm, D50 of 1.28-1.47 μm, and D90 of 3.26-3.65 μm as tested by a laser particle size analyzer. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1This is a scanning electron microscope photograph of the surface of the micron-sized hexagonal flaky magnesium hydroxide obtained in Example 1, magnified 10,000 times;

[0040] Figure 2 This is a scanning electron microscope photograph of the surface of the micron-sized hexagonal flaky magnesium hydroxide obtained in Comparative Example 1, magnified 10,000 times;

[0041] Figure 3 This is a scanning electron microscope photograph of the surface of the micron-sized hexagonal flaky magnesium hydroxide obtained in Comparative Example 2, magnified 10,000 times;

[0042] Figure 4 This is a scanning electron microscope photograph of the surface of the micron-sized hexagonal flaky magnesium hydroxide obtained in Comparative Example 3, magnified 10,000 times. DETAILED DESCRIPTION

[0043] The preferred embodiments of the present invention are described below. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0044] Example 1: A method for preparing micron-sized hexagonal magnesium hydroxide using bittern as raw material

[0045] Step 1: Preparation of magnesium hydroxide precursor

[0046] Put bittern into a reactor, add sodium hydroxide solid under uniform stirring, raise the temperature and control the temperature to the reaction temperature, stir and react completely, filter, wash and dry the filtered solid to obtain a magnesium hydroxide precursor;

[0047] The main ion component of the bittern is Mg 2+ Content 100g / L, Ca 2+ Content 11g / L, Na + Content 9g / L, K + Content 0.9g / L, Li + Content 0.5g / L, Cl - Content 390g / L;

[0048] The mass ratio of bittern to sodium hydroxide solid is 290:65;

[0049] The rapid stirring has a stirring rate of 2700 rpm;

[0050] The uniform stirring has a stirring rate of 1100 rpm;

[0051] The reaction temperature is 40°C;

[0052] The stirring reaction is complete and the stirring reaction time is 6 hours;

[0053] The washing is performed 5 times with deionized water, and the amount of deionized water used in each washing is equal to the mass of the washed solid;

[0054] The drying was carried out at 75° C. for 10 hours.

[0055] Step 2: Hydrothermal synthesis

[0056] After the magnesium hydroxide precursor, dispersant, morphology control agent, and composite hydrothermal solvent are mixed and stirred into a uniform slurry, the slurry is added to a hydrothermal reactor with a polytetrafluoroethylene liner, maintained in a sealed state, and heated and maintained at a constant temperature to the hydrothermal reaction temperature under uniform stirring. After the constant temperature hydrothermal reaction is complete, the temperature is cooled to room temperature and discharged to obtain a slurry;

[0057] The dispersant is N,N-bis(hydroxyethyl)cocamide;

[0058] The morphology control agent is N,N'-bis(2-hydroxyethyl)ethylenediamine;

[0059] The composite hydrothermal solvent consists of sulfolane, glycerol and deionized water;

[0060] The mass ratio of sulfolane, glycerol and deionized water is 8:14:190;

[0061] The mass ratio of the magnesium hydroxide precursor, dispersant, morphology control agent, and composite hydrothermal solvent is 110:5:7:360;

[0062] The uniform stirring has a stirring rate of 650 rpm;

[0063] After the constant temperature hydrothermal reaction is completed, the reaction time is 9 hours;

[0064] The hydrothermal reaction temperature is 140°C.

[0065] Step 3: Post-processing

[0066] After the slurry is filtered, washed and dried, micron-sized hexagonal flake magnesium hydroxide is obtained;

[0067] The washing is performed 5 times with deionized water, and the amount of deionized water used in each washing is equal to the mass of the washed solid;

[0068] The drying was carried out at 65° C. for 22 hours.

[0069] Example 2: A method for preparing micron-sized hexagonal magnesium hydroxide using bittern as raw material

[0070] Step 1: Preparation of magnesium hydroxide precursor

[0071] Put bittern into a reactor, add sodium hydroxide solid under uniform stirring, raise the temperature and control the temperature to the reaction temperature, stir and react completely, filter, wash and dry the filtered solid to obtain a magnesium hydroxide precursor;

[0072] The main ion component of the bittern is Mg 2+ Content 90g / L, Ca 2+ Content 6g / L, Na + Content 1g / L, K + Content 0.1g / L, Li + Content 0.03g / L, Cl - Content 310 g / L;

[0073] The mass ratio of bittern to sodium hydroxide solid is 170:26;

[0074] The rapid stirring has a stirring rate of 2000 rpm;

[0075] The uniform stirring has a stirring rate of 800 rpm;

[0076] The reaction temperature is 35°C;

[0077] The stirring reaction is complete and the stirring reaction time is 3 hours;

[0078] The washing is performed 3 times with deionized water, and the amount of deionized water used in each washing is equal to the mass of the washed solid;

[0079] The drying was carried out at 60° C. for 8 hours.

[0080] Step 2: Hydrothermal synthesis

[0081] After the magnesium hydroxide precursor, dispersant, morphology control agent, and composite hydrothermal solvent are mixed and stirred into a uniform slurry, the slurry is added to a hydrothermal reactor with a polytetrafluoroethylene liner, maintained in a sealed state, and heated and maintained at a constant temperature to the hydrothermal reaction temperature under uniform stirring. After the constant temperature hydrothermal reaction is complete, the temperature is cooled to room temperature and discharged to obtain a slurry;

[0082] The dispersant is tall oil diethanolamide;

[0083] The morphology control agent is N,N-bis(2-hydroxyethyl)formamide;

[0084] The composite hydrothermal solvent consists of sulfolane, glycerol and deionized water;

[0085] The mass ratio of sulfolane, glycerol and deionized water is 4:8:100;

[0086] The mass ratio of the magnesium hydroxide precursor, dispersant, morphology control agent, and composite hydrothermal solvent is 80:2:4:240;

[0087] The uniform stirring has a stirring rate of 450 rpm;

[0088] After the constant temperature hydrothermal reaction is completed, the reaction time is 7 hours;

[0089] The hydrothermal reaction temperature is 125°C.

[0090] Step 3: Post-processing

[0091] After the slurry is filtered, washed and dried, micron-sized hexagonal flake magnesium hydroxide is obtained;

[0092] The washing is performed 3 times with deionized water, and the amount of deionized water used in each washing is equal to the mass of the washed solid;

[0093] The drying was carried out at 50°C for 18 hours.

[0094] Example 3: A method for preparing micron-sized hexagonal magnesium hydroxide using bittern as raw material

[0095] Step 1: Preparation of magnesium hydroxide precursor

[0096] Put bittern into a reactor, add sodium hydroxide solid under uniform stirring, raise the temperature and control the temperature to the reaction temperature, stir and react completely, filter, wash and dry the filtered solid to obtain a magnesium hydroxide precursor;

[0097] The main ion component of the bittern is Mg 2+ Content 120g / L, Ca 2+ Content 16g / L, Na + Content 14g / L, K + Content 1.5g / L, Li + Content 0.7g / L, Cl - Content 460 g / L;

[0098] The mass ratio of bittern to sodium hydroxide solid is 340:95;

[0099] The rapid stirring has a stirring rate of 3000 rpm;

[0100] The uniform stirring has a stirring rate of 1500 rpm;

[0101] The reaction temperature is 55°C;

[0102] The stirring reaction is complete and the stirring reaction time is 7 hours;

[0103] The washing is washing 6 times with deionized water, and the amount of deionized water used in each washing is equal to the mass of the washed solid;

[0104] The drying is drying at 85℃ for 13 hours.

[0105] Step 2, hydrothermal synthesis

[0106] The magnesium hydroxide precursor, dispersant, morphology control agent, and composite hydrothermal solvent are mixed and stirred into a uniform slurry, and then the slurry is added to a hydrothermal reaction kettle with a polytetrafluoroethylene lining, maintained in a sealed state, and uniformly stirred to heat and maintain a constant temperature to the hydrothermal reaction temperature. After the constant temperature hydrothermal reaction is complete, the temperature is lowered to room temperature and the material is discharged, obtaining a slurry;

[0107] The dispersant is N-(2-hydroxyethyl)dodecylamide;

[0108] The morphology control agent is N,N'-bis(2-hydroxyethyl)ethylenediamine;

[0109] The composite hydrothermal solvent is composed of sulfolane, glycerol, and deionized water;

[0110] The mass ratio of sulfolane, glycerol, and deionized water is 11:20:250;

[0111] The mass ratio of the magnesium hydroxide precursor, dispersant, morphology control agent, and composite hydrothermal solvent is 190:7:9:460;

[0112] The uniform stirring is at a stirring rate of 950 revolutions per minute;

[0113] After the constant temperature hydrothermal reaction is complete, the reaction time is 12 hours;

[0114] The hydrothermal reaction temperature is 160℃.

[0115] Step 3, post-treatment

[0116] After the slurry is filtered, washed, and dried, micron-sized hexagonal flaky magnesium hydroxide is obtained;

[0117] The washing is washing 6 times with deionized water, and the amount of deionized water used in each washing is equal to the mass of the washed solid;

[0118] The drying is drying at 85℃ for 26 hours.

[0119] Comparative Example 1: Based on Example 1, in Step 2, hydrothermal synthesis, no dispersant is added, and 5 parts of the dispersant are replaced with an equal amount of composite hydrothermal solvent, and the specific operation is as follows:

[0120] Step 1 is the same as in Example 1;

[0121] Step 2, hydrothermal synthesis

[0122] 5 parts of dispersant were replaced by 5 parts of composite hydrothermal solvent, and other operations were the same as in Example 1;

[0123] The operation of step 3 is the same as that of embodiment 1.

[0124] Comparative Example 2: Based on Example 1, in step 2, hydrothermal synthesis, no morphology control agent was added, and 7 parts of the morphology control agent were replaced by 7 parts of the composite hydrothermal solvent. The specific operation was as follows:

[0125] The operation of step 1 is the same as that of Example 1;

[0126] Step 2: Hydrothermal synthesis

[0127] The 7 parts of morphology control agent were replaced by 7 parts of composite hydrothermal solvent, and the other operations were the same as in Example 1;

[0128] The operation of step 3 is the same as that of embodiment 1.

[0129] Comparative Example 3: Based on Example 1, in step 2, hydrothermal synthesis, no composite hydrothermal solvent was added, and 360 parts of the composite hydrothermal solvent was replaced with 360 parts of deionized water. The specific operation was as follows:

[0130] The operation of step 1 is the same as that of Example 1;

[0131] Step 2: Hydrothermal synthesis

[0132] 360 parts of the composite hydrothermal solvent were replaced with 360 parts of deionized water, and the other operations were the same as in Example 1;

[0133] The operation of step 3 is the same as that of embodiment 1.

[0134] Particle size distribution test:

[0135] The micron-sized hexagonal flaky magnesium hydroxide obtained from Examples 1, 2, 3 and Comparative Examples 1, 2, 3 using bittern as raw material was tested for D10, D50, and D90 using a laser particle size analyzer;

[0136] The test results are shown in Table 1:

[0137] Table 1

[0138]

[0139] It can be seen from the test data in Table 1 that the particle size distribution range of Examples 1-3 is relatively narrow, which indicates that the present invention uses bittern as raw material to prepare micron-sized magnesium hydroxide particles with controllable particle size; Comparative Example 1 does not add a dispersant, and the particle size of Comparative Example 1 is significantly increased, and the range of the particle size distribution is also relatively wide, which shows that the dispersant can effectively inhibit and avoid the agglomeration of the particles to achieve particle size refinement and controllability; Comparative Example 2 does not add a morphology control agent, and the particle size of Comparative Example 2 is significantly increased and the particle size distribution range is significantly widened compared with Example 1, which shows that the morphology control agent has an important regulatory effect on the particle size and particle size range distribution; Comparative Example 3 replaces the composite hydrothermal solvent with deionized water in equal amounts, and the particle size and particle size distribution of Comparative Example 3 are significantly increased compared with Example 1, and the particle size distribution range is also significantly widened, which shows that the composite hydrothermal solvent can also play a role in controlling the particle size and adjusting the particle size distribution.

[0140] Attachment Figure 1 , Attachment Figure 2 , Attachment Figure 3 , Attachment Figure 4 The scanning electron microscope photos of the micron-sized hexagonal flaky magnesium hydroxide obtained in Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3 are magnified 10,000 times, and the attached Figure 1 It can be seen that the morphology of most magnesium hydroxide particles is approximately hexagonal, and the particle size is relatively uniform. There are very few particles with particularly large or small particle sizes. Figure 2 It can be seen that the agglomeration problem of magnesium hydroxide particles is very serious, which shows that in Comparative Example 1 without adding a dispersant, the adsorption and aggregation effect between the magnesium hydroxide particles cannot be suppressed during the hydrothermal synthesis process, resulting in serious agglomeration of the magnesium hydroxide particles, making it difficult to obtain regular hexagonal flake particles; Figure 3 It can be seen that the morphology of magnesium hydroxide particles is very irregular flakes, and does not present hexagonal flakes, which shows that the morphology control agent can effectively control the growth process of magnesium hydroxide particles during the hydrothermal reaction, thereby promoting the regularity and uniformity of their morphology; Figure 4 It can be seen that the magnesium hydroxide is in a very irregular flake shape, and the particle size distribution is wide, with a large number of particles with particularly large or small particle sizes. This shows that the composite hydrothermal solvent has an obvious regulatory effect on the growth of magnesium hydroxide particles during the hydrothermal process, and the composite hydrothermal solvent can promote the uniformity of the particle size and morphology of the magnesium hydroxide particles.

[0141] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing micron-sized hexagonal flaky magnesium hydroxide using bittern as raw material, characterized in that: The method for preparing micron-sized hexagonal flaky magnesium hydroxide using bittern as raw material comprises three steps: preparing a magnesium hydroxide precursor, hydrothermal synthesis, and post-treatment; The magnesium hydroxide precursor is prepared by placing bittern into a reaction kettle, adding sodium hydroxide solid under uniform stirring, heating and controlling the temperature to the reaction temperature, stirring and reacting completely, filtering, and washing and drying the filtered solid to obtain the magnesium hydroxide precursor; The main ion component of the bittern is Mg 2+ Content 90~120g / L, Ca 2+ Content 6~16g / L, Na + Content 1~14g / L, K + Content 0.1~1.5g / L, Li + Content 0.03~0.7g / L, Cl - Content 310~460 g / L; The hydrothermal synthesis comprises mixing a magnesium hydroxide precursor, a dispersant, a morphology control agent, and a composite hydrothermal solvent and stirring them into a uniform slurry, adding the slurry into a hydrothermal reactor lined with polytetrafluoroethylene, maintaining a closed state, heating and maintaining a constant temperature to a hydrothermal reaction temperature under uniform stirring, and cooling to room temperature after the hydrothermal reaction is complete to obtain a slurry; The dispersant is one of N,N-di(hydroxyethyl)coconutamide, tall oil diethanolamide, and N-(2-hydroxyethyl)dodecylamide; The morphology control agent is one of N,N'-bis(2-hydroxyethyl)ethylenediamine and N,N-bis(2-hydroxyethyl)formamide, or a mixture of the two in any mass ratio; The composite hydrothermal solvent consists of sulfolane, glycerol and deionized water.

2. The method for preparing micron-sized hexagonal flaky magnesium hydroxide using bittern as raw material according to claim 1, wherein: The mass ratio of bittern to sodium hydroxide solid is 170-340:26-95.

3. The method for preparing micron-sized hexagonal flaky magnesium hydroxide using bittern as raw material according to claim 1, wherein: The mass ratio of sulfolane, glycerol and deionized water is 4-11:8-20:100-250; The mass ratio of the magnesium hydroxide precursor, the dispersant, the morphology control agent, and the composite hydrothermal solvent is 80-190:2-7:4-9:240-460.

4. The method for preparing micron-sized hexagonal flaky magnesium hydroxide using bittern as raw material according to claim 1, wherein: After the post-treatment, the slurry is filtered, washed and dried to obtain micron-sized hexagonal flaky magnesium hydroxide.

5. The method for preparing micron-sized hexagonal flaky magnesium hydroxide using bittern as raw material according to claim 4, wherein: The washing is performed 3 to 6 times with deionized water, and the amount of deionized water used in each washing is equal to the mass of the washed solid; The drying operation in the post-treatment is drying at 50-85° C. for 18-26 hours.

6. The micron-sized hexagonal flaky magnesium hydroxide prepared by the preparation method according to any one of claims 1 to 5 using bittern as raw material, characterized in that: The particle size distribution of the micron-sized hexagonal flaky magnesium hydroxide prepared from bittern as raw material, as measured by a laser particle size analyzer, is as follows: D10 is 0.71-0.85 μm, D50 is 1.28-1.47 μm, and D90 is 3.26-3.65 μm.

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