A method for preparing hexagonal sheet-like magnesium hydroxide

Abstract

This invention provides a method for preparing hexagonal flake magnesium hydroxide, relating to the field of inorganic flame retardant technology. The invention introduces ultrasonic conditions to assist precipitation during the precipitation reaction stage, combined with controlled ammonia dripping rate, thereby improving the purity of the obtained hexagonal flake magnesium hydroxide. Then, the obtained magnesium hydroxide precursor precipitate is mixed with a surfactant and subjected to staged microwave-assisted hydrothermal treatment. Stepwise heating allows for slow and orderly crystal growth, resulting in hexagonal flake magnesium hydroxide with a regular hexagonal flake structure, uniform dispersion, and small flake size. In the post-treatment stage, a hydrophilic organic solvent replaces water in the crude magnesium hydroxide product, reducing particle agglomeration caused by capillary forces during subsequent drying. The final hexagonal flake magnesium hydroxide has a purity ≥99.0%, regular hexagonal flake morphology, good dispersibility, no obvious agglomeration, and an average flake size in the nanometer range, meeting the requirements for flame-retardant magnesium hydroxide.

Description

Technical Field

[0001] This invention relates to the field of inorganic flame retardant technology, specifically to a method for preparing hexagonal flake magnesium hydroxide. Background Technology

[0002] Magnesium hydroxide, as an additive inorganic flame retardant, undergoes a thermal decomposition reaction at 340℃, releasing water and absorbing a large amount of latent heat, thus exhibiting significant cooling and flame retardant effects. It also provides significant cooling and smoke suppression in the early stages of combustion. Its decomposition product, magnesium oxide, has stable fire resistance and is environmentally friendly. High purity, needle-like or flaky crystal form, and small particle size are the three key indicators determining the performance of flame-retardant magnesium hydroxide.

[0003] Currently, the preparation of high-purity magnesium hydroxide typically uses refined magnesium chloride or magnesium sulfate solutions as raw materials, and involves co-precipitation with the addition of alkaline substances. However, the high cost of pure magnesium salt solutions makes low-cost large-scale production difficult. Salt lakes possess vast reserves of magnesium salts, and using salt lake brine as a raw material for magnesium hydroxide production could significantly reduce costs. However, salt lake brine contains numerous impurity ions such as calcium, boron, and sulfate, which readily enter the magnesium hydroxide precursor as inclusions or co-precipitates during precipitation, resulting in low purity magnesium hydroxide. Furthermore, the complex behavior of impurities in salt lake brine during hydrothermal treatment can affect crystal growth control, making it difficult to achieve regular grain morphology and size in the final product. The resulting magnesium hydroxide is also prone to agglomeration, ultimately resulting in an average particle size in the micrometer range (2~10 μm). Therefore, there is an urgent need for a method that can directly use salt lake brine as a raw material to obtain magnesium hydroxide with high purity, a regular hexagonal plate structure, and nanoscale plate size. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a method for preparing hexagonal sheet magnesium hydroxide, which yields hexagonal sheet magnesium hydroxide with high purity, regular morphology, and nanometer-scale sheet diameter.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing hexagonal sheet-like magnesium hydroxide, comprising the following steps: Ammonia solution is added dropwise to magnesium-containing brine to induce a precipitation reaction, yielding a magnesium hydroxide precursor precipitate. The precipitation reaction is carried out under ultrasonic conditions. The magnesium-containing brine is obtained by clarifying and diluting salt lake brine. The concentration of magnesium ions in the magnesium-containing brine is 0.4–1 mol / L. The rate of ammonia addition is 0.5–5 mL / min. The magnesium hydroxide precursor precipitate and the surfactant were mixed and subjected to microwave-assisted hydrothermal treatment to obtain crude magnesium hydroxide product. The crude magnesium hydroxide product was dehydrated and dried using a hydrophilic organic solvent to obtain hexagonal flake magnesium hydroxide; the surface tension of the hydrophilic organic solvent was 21~25 mN / m. The microwave-assisted hydrothermal treatment includes performing a first microwave-assisted hydrothermal treatment and a second microwave-assisted hydrothermal treatment in sequence; the temperature of the second microwave-assisted hydrothermal treatment is 10~100℃ higher than the temperature of the first microwave-assisted hydrothermal treatment; the ratio of microwave power of the second microwave-assisted hydrothermal treatment to that of the first microwave-assisted hydrothermal treatment is 1.1~1.5:1.

[0006] Preferably, the ultrasonic frequency of the ultrasonic conditions is 20kHz and the ultrasonic power is 30~300W.

[0007] Preferably, the microwave power of the first microwave-assisted hydrothermal treatment is 100~2000W, the temperature is 90~150℃, and the time is 0.5~6h; The microwave power of the second microwave-assisted hydrothermal treatment is 110~3000W, the temperature is 100~250℃, and the time is 0.5~72h.

[0008] Preferably, the surfactant comprises one or more of sodium dodecylbenzenesulfonate, sodium citrate, polyethylene glycol, and polyvinyl alcohol.

[0009] Preferably, the mass ratio of the surfactant to magnesium hydroxide in the magnesium hydroxide precursor precipitate is 0.25~5:100.

[0010] Preferably, the hydrophilic organic solvent includes one or more of acetone, butanone, methyl isobutyl ketone, and isopropanol; The water removal process is performed 3 to 5 times.

[0011] Preferably, the concentration of the ammonia water is 5-28 wt%; The molar ratio of magnesium ions in the magnesium-containing brine to hydroxide ions in the ammonia water is 1:2~4.

[0012] Preferably, the precipitation reaction is carried out at a temperature of 35~95℃.

[0013] Preferably, the precipitation reaction yields a precipitate solution; It also includes aging the precipitate solution to obtain the magnesium hydroxide precursor precipitate solution; The aging time is 0.1 to 4 hours.

[0014] Preferably, the drying includes vacuum drying; The vacuum drying temperature is 60~80℃, and the time is 6~8h.

[0015] This invention provides a method for preparing hexagonal flake magnesium hydroxide, comprising the following steps: adding ammonia water dropwise to magnesium-containing brine to carry out a precipitation reaction, obtaining a magnesium hydroxide precursor precipitate; the precipitation reaction is carried out under ultrasonic conditions; the magnesium-containing brine is obtained by clarifying and diluting salt lake brine; the concentration of magnesium ions in the magnesium-containing brine is 0.4~1 mol / L; the ammonia water is added at a rate of 0.5~5 mL / min; mixing the magnesium hydroxide precursor precipitate with a surfactant and performing microwave-assisted hydrothermal treatment to obtain crude magnesium hydroxide; dehydrating the crude magnesium hydroxide with a hydrophilic organic solvent and then drying it to obtain hexagonal flake magnesium hydroxide; the surface tension of the hydrophilic organic solvent is 21~25 mN / m; the microwave-assisted hydrothermal treatment includes sequentially performing a first microwave-assisted hydrothermal treatment and a second microwave-assisted hydrothermal treatment; the temperature of the second microwave-assisted hydrothermal treatment is 10~100℃ higher than the temperature of the first microwave-assisted hydrothermal treatment; the ratio of microwave power of the second microwave-assisted hydrothermal treatment to that of the first microwave-assisted hydrothermal treatment is 1.1~1.5:1. This invention uses natural salt lake brine as raw material. The magnesium ion concentration in the clarified and diluted magnesium-containing brine is 0.4~1 mol / L. Hexagonal flake magnesium hydroxide is prepared using this brine. Ultrasonic conditions are introduced during the precipitation reaction stage to assist precipitation. Combined with controlled ammonia dripping rate, the supersaturation and nucleation rate during the precipitation process are effectively controlled, avoiding the formation of unstable intermediate phases and co-precipitation of impurities, thus improving the purity of the obtained hexagonal flake magnesium hydroxide. The obtained magnesium hydroxide precursor precipitate is mixed with a surfactant and subjected to staged microwave-assisted hydrothermal treatment. Stepwise heating allows for slow and orderly crystal growth, reducing particle collisions caused by instantaneous high temperatures. Sintering and agglomeration: Magnesium hydroxide undergoes dissolution-recrystallization during hydrothermal treatment. The first microwave-assisted hydrothermal treatment (low-temperature stage) promotes gentle nucleation, forming a large number of fine and uniform crystal nuclei, avoiding the explosive nucleation and abnormal growth caused by the first high-temperature stage. The second microwave-assisted hydrothermal treatment (high-temperature stage) promotes directional growth, with surfactants preferentially adsorbed on specific crystal faces, allowing crystals to grow along the plane and inhibiting vertical growth, resulting in a hexagonal plate structure with clear boundaries and complete edges. At the same time, the addition of surfactants provides steric hindrance or charge repulsion, separating the particles from each other and reducing agglomeration. The resulting hexagonal plate magnesium hydroxide has a regular hexagonal plate structure, is uniformly dispersed, and has a small plate diameter. In the post-processing stage, water in the crude magnesium hydroxide product is replaced by a hydrophilic organic solvent. The good miscibility and low surface tension of the hydrophilic organic solvent with water are used to remove water, which reduces particle agglomeration caused by capillary forces during subsequent drying and maintains the dispersed state of magnesium hydroxide. The final hexagonal flake magnesium hydroxide has a purity of ≥99.0% (Mg(OH)2), regular hexagonal flake morphology, good dispersibility, no obvious agglomeration, and an average flake diameter between 200 and 1000 nm, which can meet the requirements for flame-retardant magnesium hydroxide.Moreover, the preparation method provided by this invention does not introduce complex equipment or high-energy-consuming operations, and the process route is simple, highly controllable, and has good industrialization prospects. Attached Figure Description

[0016] Figure 1 This is a SEM image of the hexagonal sheet magnesium hydroxide prepared in Example 1. Detailed Implementation

[0017] This invention provides a method for preparing hexagonal sheet-like magnesium hydroxide, comprising the following steps: Ammonia solution is added dropwise to magnesium-containing brine to induce a precipitation reaction, yielding a magnesium hydroxide precursor precipitate. The precipitation reaction is carried out under ultrasonic conditions. The magnesium-containing brine is obtained by clarifying and diluting salt lake brine. The concentration of magnesium ions in the magnesium-containing brine is 0.4–1 mol / L. The rate of ammonia addition is 0.5–5 mL / min. The magnesium hydroxide precursor precipitate and the surfactant were mixed and subjected to microwave-assisted hydrothermal treatment to obtain crude magnesium hydroxide product. The crude magnesium hydroxide product was dehydrated and dried using a hydrophilic organic solvent to obtain hexagonal flake magnesium hydroxide; the surface tension of the hydrophilic organic solvent was 21~25 mN / m. The microwave-assisted hydrothermal treatment includes performing a first microwave-assisted hydrothermal treatment and a second microwave-assisted hydrothermal treatment in sequence; the temperature of the second microwave-assisted hydrothermal treatment is 10~100℃ higher than the temperature of the first microwave-assisted hydrothermal treatment; the ratio of microwave power of the second microwave-assisted hydrothermal treatment to that of the first microwave-assisted hydrothermal treatment is 1.1~1.5:1.

[0018] Unless otherwise specified, the materials and equipment used in this invention are all commercially available products in the field.

[0019] In this invention, ammonia water is added dropwise to magnesium-containing brine to carry out a precipitation reaction, thereby obtaining a magnesium hydroxide precursor precipitate.

[0020] In this invention, the magnesium-containing brine is obtained by clarifying and diluting salt lake brine. In this invention, the clarification and dilution may include the following steps: The brine from the salt lake is separated into solid and liquid components to obtain clarified brine. The clarified salt lake brine was diluted with water to obtain magnesium-containing brine.

[0021] In this invention, the solid-liquid separation may include natural sedimentation and / or filtration; the filtration may specifically be performed using gauze. This invention removes large particles of silt and suspended impurities from salt lake brine through solid-liquid separation.

[0022] In this invention, the concentration of magnesium ions in the magnesium-containing brine is 0.4~1 mol / L, specifically 0.5 mol / L, 0.6 mol / L, or 0.8 mol / L. This invention does not have a specific limitation on the amount of water used, as long as the concentration of magnesium ions in the magnesium-containing brine is 0.4~1 mol / L. This invention dilutes the clarified salt lake brine with water, reducing the magnesium ion concentration and facilitating control of the nucleation rate. Slow and uniform nucleation is necessary to obtain nanoscale particles, avoiding the explosive nucleation of particles due to instantaneous mass formation when high-concentration magnesium ions encounter alkaline solutions, preventing particles from growing and sticking together. Furthermore, the diluted system is more fluid, resulting in a more uniform reaction throughout, facilitating the formation of hexagonal, well-dispersed crystals. Simultaneously, high-concentration slurries have higher viscosity, which is detrimental to the transmission of ultrasound during sedimentation and mass transfer in subsequent hydrothermal treatment.

[0023] In this invention, the concentration of the ammonia solution can be 5-28 wt% (the mass percentage of pure NH3 in the ammonia solution), specifically 5 wt%, 10 wt%, 15 wt%, or 20 wt%; the molar ratio of magnesium ions in the magnesium-containing brine to hydroxide ions in the ammonia solution can be 1:2-4, specifically 1:2.5, 1:3, or 1:3.5. In this invention, the ammonia solution is added at a rate of 0.5-5 mL / min, specifically 1 mL / min, 2 mL / min, 3 mL / min, or 4 mL / min.

[0024] In this invention, the precipitation reaction is carried out under ultrasonic conditions; the ultrasonic frequency can be 20 kHz; the ultrasonic power can be 30~300 W, specifically 50 W, 150 W or 250 W. In this invention, the temperature of the precipitation reaction can be 35~95℃, specifically 50℃, 75℃ or 85℃.

[0025] In this invention, the precipitation reaction yields a precipitate solution; it may also include aging the precipitate solution to obtain the magnesium hydroxide precursor precipitate solution. In this invention, the aging time can be 0.1~4 hours, specifically 0.5 hours, 1 hour, 2 hours, or 3 hours. This invention, through aging, initially stabilizes the structure of the magnesium hydroxide precursor.

[0026] After obtaining the magnesium hydroxide precursor precipitate, this invention mixes the magnesium hydroxide precursor precipitate with a surfactant and performs microwave-assisted hydrothermal treatment to obtain crude magnesium hydroxide. Through microwave-assisted hydrothermal treatment, the crystal form of magnesium hydroxide undergoes a transformation, resulting in morphological reshaping and particle size adjustment.

[0027] In this invention, the surfactant may include one or more of sodium dodecylbenzenesulfonate, sodium citrate, polyethylene glycol, and polyvinyl alcohol; the mass ratio of the surfactant to magnesium hydroxide in the magnesium hydroxide precursor precipitate (i.e., the ratio of the mass of the surfactant to the theoretically calculated mass of magnesium hydroxide in the magnesium hydroxide precursor precipitate) may be 0.25~5:100, specifically 1:100, 2:100, 3:100, or 4:100. In one or more embodiments of this invention, using one or more of sodium dodecylbenzenesulfonate, sodium citrate, polyethylene glycol, and polyvinyl alcohol as a surfactant allows for selective adsorption onto specific crystal faces of magnesium hydroxide, thereby inhibiting lateral growth, promoting the growth of the (001) basal plane, and obtaining a hexagonal plate-like morphology.

[0028] In this invention, the microwave-assisted hydrothermal treatment can be carried out in a high-pressure reactor. The microwave-assisted hydrothermal treatment includes sequentially performing a first microwave-assisted hydrothermal treatment and a second microwave-assisted hydrothermal treatment. The temperature of the second microwave-assisted hydrothermal treatment is 10-100°C higher than that of the first microwave-assisted hydrothermal treatment, specifically 10°C, 30°C, or 80°C. By controlling the temperature of the second microwave-assisted hydrothermal treatment to be 10-100°C higher than that of the first microwave-assisted hydrothermal treatment, the large temperature difference between the two stages results in dissolution-recrystallization rates that are not on the same order of magnitude, thus ensuring uniform product particle size.

[0029] In this invention, the ratio of microwave power of the second microwave-assisted hydrothermal treatment to that of the first microwave-assisted hydrothermal treatment is 1.1 to 1.5:1, specifically 1.1:1, 1.2:1, or 1.3:1. By controlling the microwave power ratio of the second microwave-assisted hydrothermal treatment to that of the first microwave-assisted hydrothermal treatment to be 1.1 to 1.5:1, this invention ensures a faster and more stable heating rate during the second microwave-assisted hydrothermal treatment stage.

[0030] In this invention, the microwave power of the first microwave-assisted hydrothermal treatment can be 100~2000W, specifically 200W, 300W, or 400W; the temperature of the first microwave-assisted hydrothermal treatment can be 90~150℃, specifically 95℃, 105℃, 120℃, or 140℃; the time of the first microwave-assisted hydrothermal treatment can be 0.5~6h, specifically 1h, 2h, 3h, or 4h; the microwave power of the second microwave-assisted hydrothermal treatment can be 110~3000W, specifically 500W, 1500W, or 2500W; the temperature of the second microwave-assisted hydrothermal treatment can be 100~250℃, specifically 120℃, 150℃, 200℃, 220℃, or 240℃; and the time of the second microwave-assisted hydrothermal treatment can be 0.5~72h, specifically 8h, 10h, 12h, 16h, or 24h.

[0031] After completing the microwave-assisted hydrothermal treatment, the present invention may further include cooling the hydrothermal liquid obtained by the microwave-assisted hydrothermal treatment to room temperature and then performing solid-liquid separation to obtain a solid phase; washing the solid phase to obtain crude magnesium hydroxide product.

[0032] The present invention does not specifically limit the cooling method; any conventional cooling method in the art can be used, specifically natural cooling. In this invention, the washing may include alternating washing with deionized water and ethanol; the number of alternating washings with deionized water and ethanol may be 3 to 5 times, specifically 4 times.

[0033] After obtaining the crude magnesium hydroxide product, the present invention uses a hydrophilic organic solvent to remove water from the crude magnesium hydroxide product and then dries it to obtain hexagonal flake magnesium hydroxide.

[0034] In this invention, the surface tension of the hydrophilic organic solvent is 21~25 mN / m, specifically 23.7 mN / m, 24.6 mN / m, or 23.6 mN / m (measured according to GB / T 42415-2023); the hydrophilic organic solvent may include one or more of acetone, butanone, methyl isobutyl ketone, and isopropanol. In this invention, the number of water removal processes can be 3~5 times, specifically 4 times; the volume ratio of the hydrophilic organic solvent to the crude magnesium hydroxide product in a single water removal process can be 2~3:1, specifically 2.2:1, 2.5:1, or 2.8:1.

[0035] In this invention, the drying may include vacuum drying; the temperature of the vacuum drying may be 60~80℃, specifically 65℃, 70℃ or 75℃; the time of the vacuum drying may be 6~8h, specifically 6.5h, 7h or 7.5h.

[0036] To further illustrate the present invention, the solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0037] In all embodiments and comparative examples of the present invention, the brine was brine from a salt lake in Qinghai Province, and the concentration of magnesium ions was measured to be 3.2 mol / L. Sodium dodecylbenzenesulfonate (SDBS, purity ≥95%, mixture), sodium citrate (dihydrate, AR, purity ≥99%), polyvinyl alcohol (Mw 13000~23000, purity 87~89%), polyethylene glycol (Mw 4000), acetone and anhydrous ethanol were all purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China).

[0038] The surface tensions of acetone, butanone, isopropanol, and methyl isobutyl ketone were measured at 20°C and 101.325 kPa (according to GB / T 42415-2023).

[0039] Example 1 After filtering the brine from the salt lake through gauze to remove coarse particles, it was diluted with water to obtain a magnesium-containing brine with a magnesium ion concentration of 0.4 mol / L. 50 mL of the obtained magnesium-containing brine was placed in an ultrasonic water bath at a temperature of 75°C, and the ultrasonic treatment was turned on. 25 mL of 5 wt% ammonia water was slowly added dropwise to the magnesium-containing brine at a dropping rate of 1 mL / min to carry out a precipitation reaction. The ultrasonic frequency was 20 kHz and the power was 60 W. After the addition was completed, the ultrasonic conditions were kept unchanged and the mixture was ultrasonically aged for 0.5 h to obtain a magnesium hydroxide precursor precipitate.

[0040] The obtained magnesium hydroxide precursor precipitate was transferred to a hydrothermal reactor, and polyethylene glycol (PEG-4000) with an average molecular weight of 4000 was added as a surfactant. The mass of PEG was 1% of the theoretical mass of magnesium hydroxide in the magnesium hydroxide precursor precipitate. A staged microwave-assisted hydrothermal treatment was performed: the first stage was conducted at a microwave power of 300W and a temperature of 105℃ for 2 hours; the second stage was conducted at a microwave power of 500W and a temperature of 120℃ for 12 hours. After the microwave-assisted hydrothermal treatment, the resulting hydrothermal solution was cooled to room temperature, and solid-liquid separation was performed to obtain a solid phase. The obtained solid phase was washed three times alternately with deionized water and ethanol to obtain crude magnesium hydroxide product.

[0041] The obtained crude magnesium hydroxide product was subjected to three water-replacing reactions with acetone, and then vacuum dried at 60°C for 6 hours to obtain hexagonal flake magnesium hydroxide.

[0042] Figure 1 This is a SEM image of the hexagonal flake magnesium hydroxide prepared in Example 1. Figure 1 It can be seen that hexagonal sheet magnesium hydroxide with an average sheet diameter of 200~500nm was prepared in Example 1.

[0043] Example 2 After filtering the brine from the salt lake through gauze to remove coarse particles, it was diluted with water to obtain a magnesium-containing brine with a magnesium ion concentration of 0.4 mol / L. 50 mL of the obtained magnesium-containing brine was placed in an ultrasonic water bath at a temperature of 75°C, and the ultrasonic treatment was turned on. 25 mL of 5 wt% ammonia water was slowly added dropwise to the magnesium-containing brine at a dropping rate of 1 mL / min to carry out the precipitation reaction. The ultrasonic frequency was 20 kHz and the power was 60 W. After the addition was completed, the ultrasonic conditions were kept unchanged and ultrasonic aging was continued for 1 h to obtain a magnesium hydroxide precursor precipitate.

[0044] The obtained magnesium hydroxide precursor precipitate was transferred to a hydrothermal reactor, and sodium dodecylbenzenesulfonate was added as a surfactant. The mass of sodium dodecylbenzenesulfonate was 2.5% of the theoretical mass of magnesium hydroxide in the magnesium hydroxide precursor precipitate. A staged microwave-assisted hydrothermal treatment was performed: the first stage was conducted at a microwave power of 300 W and a temperature of 95 °C for 3 hours; the second stage was conducted at a microwave power of 500 W and a temperature of 150 °C for 10 hours. After the microwave-assisted hydrothermal treatment, the resulting hydrothermal solution was cooled to room temperature, and solid-liquid separation was performed to obtain a solid phase. The obtained solid phase was washed three times alternately with deionized water and ethanol to obtain crude magnesium hydroxide.

[0045] The obtained crude magnesium hydroxide product was subjected to water replacement with butanone three times, and then vacuum dried at 60°C for 6 hours to obtain hexagonal flake magnesium hydroxide.

[0046] Example 3 After filtering the brine from the salt lake through gauze to remove coarse particles, it was diluted with water to obtain a magnesium-containing brine with a magnesium ion concentration of 0.4 mol / L. 50 mL of the obtained magnesium-containing brine was placed in an ultrasonic water bath at a temperature of 75°C, and the ultrasonic treatment was turned on. 25 mL of 5 wt% ammonia water was slowly added dropwise to the magnesium-containing brine at a dropping rate of 1 mL / min to carry out a precipitation reaction. The ultrasonic frequency was 20 kHz and the power was 60 W. After the addition was completed, the ultrasonic conditions were kept unchanged and the mixture was aged for another 2.5 h to obtain a magnesium hydroxide precursor precipitate.

[0047] The obtained magnesium hydroxide precursor precipitate was transferred to a hydrothermal reactor, and sodium citrate was added as a surfactant. The mass of sodium citrate was 4% of the theoretical mass of magnesium hydroxide in the magnesium hydroxide precursor precipitate. Microwave-assisted hydrothermal treatment was performed in stages: the first stage was conducted at a microwave power of 300 W and a temperature of 140 °C for 1 hour; the second stage was conducted at a microwave power of 500 W and a temperature of 190 °C for 8 hours. After the microwave-assisted hydrothermal treatment, the resulting hydrothermal solution was cooled to room temperature, and solid-liquid separation was performed to obtain a solid phase. The obtained solid phase was washed three times alternately with deionized water and ethanol to obtain crude magnesium hydroxide product.

[0048] The obtained crude magnesium hydroxide product was replaced with isopropanol three times to remove water, and then dried under vacuum at 60°C for 6 hours to obtain hexagonal flake magnesium hydroxide.

[0049] Example 4 After filtering the brine from the salt lake through gauze to remove coarse particles, it was diluted with water to obtain a magnesium-containing brine with a magnesium ion concentration of 0.4 mol / L. 50 mL of the obtained magnesium-containing brine was placed in an ultrasonic water bath at a temperature of 75°C, and the ultrasonic treatment was turned on. 25 mL of 5 wt% ammonia water was slowly added dropwise to the magnesium-containing brine at a dropping rate of 1 mL / min to carry out a precipitation reaction. The ultrasonic frequency was 20 kHz and the power was 60 W. After the addition was completed, the ultrasonic conditions were kept unchanged and ultrasonic aging was continued for 3 hours to obtain a magnesium hydroxide precursor precipitate.

[0050] The obtained magnesium hydroxide precursor precipitate was transferred to a hydrothermal reactor, and polyvinyl alcohol (PVA) was added as a surfactant. The mass of PVA was 0.5% of the theoretical mass of magnesium hydroxide in the magnesium hydroxide precursor precipitate. Microwave-assisted hydrothermal treatment was performed in stages: the first stage was conducted at a microwave power of 300 W and a temperature of 120 °C for 4 hours; the second stage was conducted at a microwave power of 500 W and a temperature of 175 °C for 16 hours. After the microwave-assisted hydrothermal treatment, the resulting hydrothermal solution was cooled to room temperature, and solid-liquid separation was performed to obtain a solid phase. The obtained solid phase was washed three times alternately with deionized water and ethanol to obtain crude magnesium hydroxide.

[0051] The obtained crude magnesium hydroxide product was subjected to water replacement three times with methyl isobutyl ketone, and then vacuum dried at 60°C for 6 hours to obtain hexagonal flake magnesium hydroxide.

[0052] Comparative Example 1 After filtering the brine from the salt lake through gauze to remove coarse particles, it was diluted with water to obtain a magnesium-containing brine with a magnesium ion concentration of 0.4 mol / L. 50 mL of the obtained magnesium-containing brine was placed in an ultrasonic water bath at a temperature of 75°C, and the ultrasonic treatment was turned on. 25 mL of a sodium hydroxide aqueous solution with a concentration of 2.86 mol / L was slowly added dropwise to the magnesium-containing brine at a dropping rate of 1 mL / min to carry out a precipitation reaction. The ultrasonic frequency was 20 kHz and the power was 60 W. After the addition was completed, the ultrasonic conditions were kept unchanged and the mixture was ultrasonically aged for 0.5 h to obtain a magnesium hydroxide precursor precipitate.

[0053] The obtained magnesium hydroxide precursor precipitate was transferred to a hydrothermal reactor, and polyethylene glycol (PEG-4000) with an average molecular weight of 4000 was added as a surfactant. The mass of PEG was 1% of the theoretical mass of magnesium hydroxide in the magnesium hydroxide precursor precipitate. A staged microwave-assisted hydrothermal treatment was performed: the first stage was conducted at a microwave power of 300W and a temperature of 105℃ for 2 hours; the second stage was conducted at a microwave power of 500W and a temperature of 120℃ for 12 hours. After the microwave-assisted hydrothermal treatment, the resulting hydrothermal solution was cooled to room temperature, and solid-liquid separation was performed to obtain a solid phase. The obtained solid phase was washed three times alternately with deionized water and ethanol to obtain crude magnesium hydroxide product.

[0054] The obtained crude magnesium hydroxide product was subjected to three water-replacing reactions with acetone, and then vacuum dried at 60°C for 6 hours to obtain hexagonal flake magnesium hydroxide.

[0055] Comparative Example 2 After filtering the brine from the salt lake through gauze to remove coarse particles, it was diluted with water to obtain a magnesium-containing brine with a magnesium ion concentration of 0.4 mol / L. 50 mL of the obtained magnesium-containing brine was placed in an ultrasonic water bath at a temperature of 75°C, and the ultrasonic treatment was turned on. 25 mL of 5 wt% ammonia water was slowly added dropwise to the magnesium-containing brine at a dropping rate of 1 mL / min to carry out a precipitation reaction. The ultrasonic frequency was 20 kHz and the power was 60 W. After the addition was completed, the ultrasonic conditions were kept unchanged and the mixture was ultrasonically aged for 0.5 h to obtain a magnesium hydroxide precursor precipitate.

[0056] The obtained magnesium hydroxide precursor precipitate was transferred to a hydrothermal reactor, and polyethylene glycol (PEG-4000) with an average molecular weight of 4000 was added as a surfactant. The mass of PEG was 1% of the theoretical mass of magnesium hydroxide in the magnesium hydroxide precursor precipitate. The microwave power was set to 300W and the temperature to 120℃, and microwave-assisted hydrothermal treatment was performed for 14 hours. After the microwave-assisted hydrothermal treatment was completed, the resulting hydrothermal solution was cooled to room temperature and then subjected to solid-liquid separation to obtain a solid phase. The obtained solid phase was washed three times alternately with deionized water and ethanol to obtain crude magnesium hydroxide product.

[0057] The obtained crude magnesium hydroxide product was subjected to three water-replacing reactions with acetone, and then vacuum dried at 60°C for 6 hours to obtain irregularly shaped flake magnesium hydroxide.

[0058] Comparative Example 3 After filtering the brine from the salt lake through gauze to remove coarse particles, it was diluted with water to obtain a magnesium-containing brine with a magnesium ion concentration of 0.4 mol / L. 50 mL of the obtained magnesium-containing brine was placed in an ultrasonic water bath at a temperature of 75°C, and the ultrasonic treatment was turned on. 25 mL of 5 wt% ammonia water was slowly added dropwise to the magnesium-containing brine at a dropping rate of 1 mL / min to carry out a precipitation reaction. The ultrasonic frequency was 20 kHz and the power was 60 W. After the addition was completed, the ultrasonic conditions were kept unchanged and the mixture was ultrasonically aged for 0.5 h to obtain a magnesium hydroxide precursor precipitate.

[0059] The obtained magnesium hydroxide precursor precipitate was transferred to a hydrothermal reactor for staged microwave-assisted hydrothermal treatment: the first stage was performed at a microwave power of 300W and a temperature of 105℃ for 2 hours; the second stage was performed at a microwave power of 500W and a temperature of 120℃ for 12 hours. After the microwave-assisted hydrothermal treatment, the resulting hydrothermal solution was cooled to room temperature and then subjected to solid-liquid separation to obtain a solid phase; the obtained solid phase was washed three times alternately with deionized water and ethanol to obtain crude magnesium hydroxide product.

[0060] The obtained crude magnesium hydroxide product was subjected to three water-replacing reactions with acetone, and then vacuum dried at 60°C for 6 hours to obtain irregularly shaped flake magnesium hydroxide.

[0061] Comparative Example 4 After filtering the brine from the salt lake through gauze to remove coarse particles, it was diluted with water to obtain a magnesium-containing brine with a magnesium ion concentration of 0.4 mol / L. 50 mL of the obtained magnesium-containing brine was placed in an ultrasonic water bath at a temperature of 75°C, and the ultrasonic treatment was turned on. 25 mL of 5 wt% ammonia water was slowly added dropwise to the magnesium-containing brine at a dropping rate of 1 mL / min to carry out a precipitation reaction. The ultrasonic frequency was 20 kHz and the power was 60 W. After the addition was completed, the ultrasonic conditions were kept unchanged and the mixture was ultrasonically aged for 0.5 h to obtain a magnesium hydroxide precursor precipitate.

[0062] The obtained magnesium hydroxide precursor precipitate was transferred to a hydrothermal reactor, and polyethylene glycol (PEG-4000) with an average molecular weight of 4000 was added as a surfactant. The mass of PEG was 1% of the theoretical mass of magnesium hydroxide in the magnesium hydroxide precursor precipitate. A staged microwave-assisted hydrothermal treatment was performed: the first stage was conducted at a microwave power of 300W and a temperature of 105℃ for 2 hours; the second stage was conducted at a microwave power of 500W and a temperature of 120℃ for 12 hours. After the microwave-assisted hydrothermal treatment, the resulting hydrothermal solution was cooled to room temperature, and solid-liquid separation was performed to obtain a solid phase. The obtained solid phase was washed three times alternately with deionized water and ethanol, and then vacuum dried at 60℃ for 6 hours to obtain hexagonal flake magnesium hydroxide.

[0063] Table 1 Technical specifications of the flake magnesium hydroxide prepared in Examples 1-4 and Comparative Examples 1-4

[0064] The technical specifications of the flake magnesium hydroxide prepared in Examples 1-4 and Comparative Examples 1-4 are shown in Table 1. As can be seen from Table 1, the hexagonal flake magnesium hydroxide prepared using the preparation method provided by this invention has a Mg(OH)2 purity of 99.1-99.6%, a hexagonal flake crystal form, good dispersibility, and a minimum average flake diameter as low as 200-500 nm, which is at the nanometer level.

[0065] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing hexagonal sheet-like magnesium hydroxide, characterized in that, Includes the following steps: Ammonia solution is added dropwise to magnesium-containing brine to induce a precipitation reaction, yielding a magnesium hydroxide precursor precipitate. The precipitation reaction is carried out under ultrasonic conditions. The magnesium-containing brine is obtained by clarifying and diluting salt lake brine. The concentration of magnesium ions in the magnesium-containing brine is 0.4–1 mol / L. The rate of ammonia addition is 0.5–5 mL / min. The magnesium hydroxide precursor precipitate and the surfactant were mixed and subjected to microwave-assisted hydrothermal treatment to obtain crude magnesium hydroxide product. The crude magnesium hydroxide product was dehydrated and dried using a hydrophilic organic solvent to obtain hexagonal flake magnesium hydroxide; the surface tension of the hydrophilic organic solvent was 21~25 mN / m. The microwave-assisted hydrothermal treatment includes performing a first microwave-assisted hydrothermal treatment and a second microwave-assisted hydrothermal treatment in sequence; the temperature of the second microwave-assisted hydrothermal treatment is 10~100℃ higher than the temperature of the first microwave-assisted hydrothermal treatment; the ratio of microwave power of the second microwave-assisted hydrothermal treatment to that of the first microwave-assisted hydrothermal treatment is 1.1~1.5:

1.

2. The preparation method according to claim 1, characterized in that, The ultrasonic conditions are defined as follows: ultrasonic frequency of 20kHz and ultrasonic power of 30~300W.

3. The preparation method according to claim 1, characterized in that, The microwave power of the first microwave-assisted hydrothermal treatment is 100~2000W, the temperature is 90~150℃, and the time is 0.5~6h; The microwave power of the second microwave-assisted hydrothermal treatment is 110~3000W, the temperature is 100~250℃, and the time is 0.5~72h.

4. The preparation method according to claim 1, characterized in that, The surfactant includes one or more of sodium dodecylbenzenesulfonate, sodium citrate, polyethylene glycol, and polyvinyl alcohol.

5. The preparation method according to claim 1 or 4, characterized in that, The mass ratio of the surfactant to magnesium hydroxide in the magnesium hydroxide precursor precipitate is 0.25~5:

100.

6. The preparation method according to claim 1, characterized in that, The hydrophilic organic solvent includes one or more of acetone, butanone, methyl isobutyl ketone, and isopropanol; The water removal process is repeated 3 to 5 times. In a single dehydration process, the volume ratio of the hydrophilic organic solvent to the crude magnesium hydroxide product is 2-3:

1.

7. The preparation method according to claim 1, characterized in that, The concentration of the ammonia solution is 5-28 wt%. The molar ratio of magnesium ions in the magnesium-containing brine to hydroxide ions in the ammonia water is 1:2~4.

8. The preparation method according to claim 1 or 7, characterized in that, The precipitation reaction is carried out at a temperature of 35~95℃.

9. The preparation method according to claim 1, characterized in that, The precipitation reaction yields a precipitate solution; It also includes aging the precipitate solution to obtain the magnesium hydroxide precursor precipitate solution; The aging time is 0.1 to 4 hours.

10. The preparation method according to claim 1, characterized in that, The drying includes vacuum drying; The vacuum drying temperature is 60~80℃, and the time is 6~8h.

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