Preparation method of hexagonal flake magnesium hydroxide flame retardant

Abstract

The preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention comprises the following steps: adding a precipitant and a magnesium chloride solution into a vertical ultrasonic tube reactor simultaneously for precipitation reaction to obtain magnesium hydroxide precipitate, and performing hydrothermal reaction with a hydrothermal modifier in a microwave stirring autoclave reactor to obtain the magnesium hydroxide flame retardant; the ultrasonic tube reactor is of a bottom-in and top-out type, and the molar ratio of the precipitant (OH-) to the magnesium chloride solution (Mg2+) is 2:1; the linear velocity at the end of the stirring paddle blade in the microwave stirring autoclave reactor is 0.5-1.5 m / s, and the hydrothermal reaction time is 30-120 min. The hexagonal flake magnesium hydroxide flame retardant prepared by this method has stable quality of hexagonal flake magnesium hydroxide, a narrow particle size distribution, a particle size span of less than 1, a moderate specific surface area, a chloride ion content of less than 100 ppm, and can achieve continuous production with low production cost.

Description

Technical Field

[0001] The invention relates to the technical field of inorganic flame retardants, and in particular to a method for preparing a hexagonal flaky magnesium hydroxide flame retardant. Background Art

[0002] Polymer materials have excellent mechanical properties, processing properties, insulation, corrosion resistance and other advantages. They are widely used in construction, manufacturing, agriculture and other industries, and their demand in national defense science and technology, cutting-edge technology and other fields is also increasing. Although polymer materials have many excellent properties, their easy combustion, release of toxic gases and large amounts of molten droplets during combustion limit their application. How to effectively improve their flame retardancy and minimize the production of toxic and harmful substances during combustion has become the focus of research in the field of flame retardancy of polymer materials in recent years.

[0003] To address the above problems, the traditional method is to add halogen flame retardants to polymer materials to improve their flame retardant effect. Although the amount of halogen flame retardants added is small, such flame retardants produce a large amount of smoke when burned and release toxic gases such as hydrogen halides, which often cause secondary pollution. In recent years, my country has also attached more importance to the application and development of flame retardant technology, and has successively promulgated a series of national mandatory flame retardant regulations to limit the application of non-environmentally friendly flame retardants such as halogen in furniture, electronics, textiles, and automobiles. Therefore, flame retardants that are safe, healthy, and protect the ecological environment are the main research directions at present.

[0004] Magnesium hydroxide is a highly efficient green flame retardant and smoke suppressant. It releases a large amount of crystal water and produces refractory magnesium oxide when it is decomposed by heat, which can reduce the temperature of the heated body and also play a good physical barrier effect on the flame. More importantly, magnesium hydroxide flame retardant does not produce toxic and harmful substances during the flame retardant process, so magnesium hydroxide flame retardant is a highly potential green inorganic flame retardant.

[0005] The preparation methods of magnesium hydroxide are mainly divided into physical method and chemical method. The physical method mainly involves crushing ores such as brucite through a ball mill to obtain powdered magnesium hydroxide. However, the magnesium hydroxide particles obtained by the physical method are usually larger in size and the product purity is low, making it difficult to use directly as a flame retardant. The chemical method generally involves a magnesium source and an alkaline substance through a precipitation reaction to obtain magnesium hydroxide. If it is to be used as a flame retardant, the magnesium hydroxide obtained from the above precipitation reaction needs to be further subjected to a hydrothermal reaction, thereby adjusting the crystal form of the magnesium hydroxide to improve its compatibility with polymer materials.

[0006] For the precipitation reaction in the process of preparing magnesium hydroxide by chemical method, since this reaction is a rapid reaction, the microscopic mixing effect of the magnesium source and the alkaline substance has an important influence on the particle size and particle size distribution of the magnesium hydroxide product. The characteristic time of microscopic mixing in the traditional stirring reactor is longer than the nucleation induction time of magnesium hydroxide precipitation, resulting in the precipitation reaction being carried out under the condition of uneven microscopic mixing, resulting in a wide particle size distribution of magnesium hydroxide particles and poor repeatability between batches, which in turn affects its use effect.

[0007] In recent years, researchers have successively used different process intensification technologies such as impinging stream reactors, high gravity reactors, and microreactors to enhance the microscopic mixing effect in order to prepare high-quality magnesium hydroxide flame retardants. However, the magnesium hydroxide particles prepared by the above technologies are usually prone to agglomeration due to their large surface polarity. On the one hand, the agglomeration of particles affects the uniformity of crystal growth and the control of crystal form in the subsequent hydrothermal process, seriously affecting the subsequent filtration operation, and at the same time, it is easy to cause problems such as fouling and blockage of the reactor and high energy consumption. On the other hand, the agglomeration of particles will cause a large amount of impurities such as chloride ions to remain in the precipitation particles, resulting in the content of impurities such as chlorine element in the magnesium hydroxide flame retardant exceeding the standard, the purity of the magnesium hydroxide crystal decreasing, and being unable to meet the use requirements. If the content of impurities such as chlorine element is to be reduced, it can only be carried out by washing multiple times, which also indirectly reduces the yield of the magnesium hydroxide flame retardant and increases the production cost of the magnesium hydroxide flame retardant. Therefore, new process intensification technologies still need to be developed to achieve the efficient preparation of high-quality magnesium hydroxide.

[0008] For the hydrothermal reaction in the process of preparing magnesium hydroxide by chemical method, the traditional hydrothermal reaction usually places magnesium hydroxide and an alkaline hydrothermal modifier in a traditional stirring reactor for hydrothermal reaction. However, the traditional stirring reactor generally uses conventional heat exchange methods such as heat convection or heat conduction. The above heat exchange methods usually lead to the existence of a temperature gradient in the hydrothermal environment due to uneven heat exchange, which in turn leads to uneven growth of magnesium hydroxide crystals and ultimately affects the crystal form of magnesium hydroxide. At the same time, in order to achieve a better heat exchange effect in the traditional stirring reactor, the rotation speed of the stirring paddle is usually relatively high, which will cause secondary fragmentation of the magnesium hydroxide crystals, resulting in a wider particle size distribution of the magnesium hydroxide crystals. In addition, the above traditional hydrothermal reaction process usually takes a long time. On the one hand, it is not conducive to controlling the aspect ratio of the magnesium hydroxide crystals, increasing the specific surface area of the magnesium hydroxide flame retardant, which is not conducive to the compatibilization and blending of the flame retardant and the polymer material. On the other hand, it increases the preparation cost of the magnesium hydroxide flame retardant and is not conducive to the continuous production of the magnesium hydroxide flame retardant.

[0009] As disclosed in Chinese Patent Document CN201710107494.7, a reaction crystallization device and process for magnesium hydroxide with controllable particle size are provided: Sodium chloride solution is added as the reaction crystallization bottom liquid into three serially connected reaction crystallizers, and stirring is started; Steam is added into the jackets of the reaction crystallizers to make the temperatures of the sodium chloride solutions in the three reaction crystallizers reach the set values of the operating temperatures respectively; Magnesium chloride solution and sodium hydroxide solution are added into the first-stage reaction crystallizer in a set proportion; After a set reaction time, the transfer pump behind the first-stage reaction crystallizer is started to transfer the magnesium hydroxide crystal nucleus-containing suspension generated by the reaction to the second-stage reaction crystallizer, and magnesium chloride solution and sodium hydroxide solution are added into the second-stage reaction crystallizer in a set proportion; After a set reaction time, the transfer pump behind the second-stage reaction crystallizer is started to transfer the magnesium hydroxide suspension generated by the reaction to the third-stage reaction crystallizer, and magnesium chloride solution and sodium hydroxide solution are added into the third-stage reaction crystallizer in a set proportion; After a set reaction time, the transfer pump behind the third-stage reaction crystallizer is started to transfer the magnesium hydroxide suspension generated by the reaction out of the reaction crystallization device, and the reaction crystallization process enters a continuous and stable operation state. The defects of this technology or its deficiencies compared with the present invention are as follows: On the one hand, since the reaction crystallizer in this invention uses a traditional stirred reactor, there are inevitably flow dead zones in the reactor, and the characteristic time of micro-mixing in the reactor is greater than the nucleation induction time of magnesium hydroxide precipitation, resulting in the precipitation reaction being carried out under the condition of uneven micro-mixing, causing a relatively wide particle size distribution of magnesium hydroxide particles and poor repeatability between batches, thereby affecting its use effect. Moreover, using sodium chloride solution as the bottom liquid in the precipitation reaction process will cause a relatively high chloride ion content in the product, which is not conducive to the preparation of green low-chlorine magnesium hydroxide flame retardants. On the other hand, since the reaction crystallizer in this invention uses jacket heating, there are problems of uneven internal temperature in the reaction crystallizer, slow heating speed, and long reaction time.

[0010] Chinese patent document CN201110275320.4 discloses a production method of flame retardant grade magnesium hydroxide: The magnesium salt raw material liquid and the liquid drawn from the reaction kettle are sent into the heat exchanger by a circulation pump. The cooled liquid leaving the heat exchanger enters the gas-liquid mixer. In the mixer and the connecting pipe between the mixer and the reaction kettle, ammonia gas dissolves in the magnesium salt solution and reacts to form magnesium hydroxide. The gas (ammonia gas), liquid (magnesium salt solution) and solid (magnesium hydroxide) three-phase mixed slurry flowing out of the connecting pipe is sprayed into the reaction kettle. In the reaction kettle, ammonia continues to react with the magnesium salt to form magnesium hydroxide. The slurry containing magnesium hydroxide is drawn out from the bottom of the reaction kettle, and after passing through the processes of filtration, washing and drying by a filter press, hexagonal flake magnesium hydroxide products are obtained. The defects of this technology or the deficiencies compared with the present invention: On the one hand, it is impossible to avoid agglomeration between particles in the gas-liquid mixer and the connecting pipe, which will affect the particle size distribution of the product. On the other hand, this invention uses the three-phase mixed slurry to be tangentially sprayed into the reaction kettle from the inner wall of the bottom of the reaction kettle to promote the rotation of the fluid in the reaction kettle to realize the mixing of ammonia gas and magnesium salt solution in the reaction kettle, without the need for mechanical stirring for mixing to generate magnesium hydroxide. However, the three-phase mixed slurry enters the reaction kettle from the bottom of the reaction kettle, and the jacket heating is also at the bottom of the reaction kettle. There is no stirring device at the upper part of the reaction kettle, resulting in insufficient mass transfer and heat transfer at the upper part of the reaction kettle, forming a temperature difference with the bottom of the reaction kettle and affecting the product quality.

[0011] Chinese patent document CN201610886155.9 discloses a device and process for synthesizing ultrafine magnesium hydroxide flame retardant by the high gravity-seeding method: Using magnesium chloride solution and sodium hydroxide solution as raw materials, the preliminarily obtained magnesium hydroxide slurry is used as seeds and added to the raw materials for reaction. The magnesium chloride solution and the sodium hydroxide solution enter the liquid distributor from the liquid inlet of the high gravity reactor at the same time and collide with each other at the nozzle of the inlet pipe to complete the first rapid and uniform mixing and reaction. Subsequently, it enters the inner edge of the packing layer rotating at a certain speed. Under the action of the high gravity field generated by the high-speed rotation, the liquid becomes finer liquid filaments, liquid films or droplets, making the solution in a highly dispersed state. The magnesium chloride solution and the sodium hydroxide solution pass through the packing layer radially from the inside to the outside and complete the second rapid and uniform contact and reaction in the annular packing. The product is discharged from the conical liquid outlet of the high gravity reactor. The obtained magnesium hydroxide slurry provides magnesium hydroxide seeds and enters the rotating packing bed for mixing reaction at the same volume flow rate as the two raw material liquids, and reacts in turn in a cycle. After circulating a certain number of times, the magnesium hydroxide slurry is made into magnesium hydroxide powder with a certain particle size after filtration, washing, drying and grinding. The defects of this technology or the deficiencies compared with the present invention: Although this invention uses a high gravity reactor to strengthen the mixing effect of the raw materials, it is impossible to avoid agglomeration between particles, which affects the particle size distribution of the product, and the agglomerated particles are likely to cause blockage of the reactor.

[0012] Chinese patent document CN201010565634.3 discloses a process for preparing ultra-fine and high-purity magnesium hydroxide flame retardant by high gravity-hydrothermal method, including precipitation reaction: feeding the refined magnesium chloride solution into a high gravity reactor, and then gradually feeding the refined alkali solution into the high gravity reactor within 1 - 3 hours for crystallization precipitation reaction, and the pH of the slurry after the precipitation reaction is ≥ 12.5; heating: transferring the slurry into a hydrothermal reaction kettle, heating for 1 - 4 hours, and the temperature of the slurry rises to 160 - 210 °C, and then holding for 4 - 8 hours; filtering and washing: filtering the material after hydrothermal reaction, and conducting two-step washing with process water for pulping, and the pulping concentration is 8 - 15% by weight of magnesium hydroxide, and finally filtering again to obtain a magnesium hydroxide filter cake; drying: drying the magnesium hydroxide filter cake in two steps, the first step is paddle drying to obtain a wet magnesium hydroxide product with a water content of less than 30%; the second step is carried out in a rotary flash micropowder dryer to obtain a magnesium hydroxide product with a water content of less than 0.2%; modification: dry-modifying the dried magnesium hydroxide powder, adding 100 kg of material each time in a high-speed mixer, heating and adding a modifier for surface treatment. The defects of this technology or the deficiencies compared with the present invention: on the one hand, this invention uses a high gravity reactor for crystallization precipitation reaction, and it is difficult to avoid agglomeration between precipitation particles, thereby affecting the particle size distribution of the product; on the other hand, the conventional heat exchange form adopted by this invention affects the uniformity of the growth of hexagonal plate crystal form due to uneven heat exchange, and the hydrothermal reaction time of this invention is long, affecting production efficiency and increasing product cost.

[0013] Chinese patent document CN202210177029.1 discloses a method for preparing magnesium hydroxide flame retardant using industrial-grade magnesium hydroxide as raw material, including the following steps: (1), mixing the industrial-grade magnesium hydroxide raw material and water and grinding to obtain a magnesium hydroxide slurry; (2), adding water again to the magnesium hydroxide slurry obtained in step (1) to adjust the mass concentration of the magnesium hydroxide slurry to 10% - 50%; (3), subjecting the magnesium hydroxide slurry with adjusted concentration in step (2) to microwave dispersion; (4), subjecting the magnesium hydroxide slurry after microwave dispersion in step (3) to hydrothermal reaction; (5), filtering the product after hydrothermal reaction in step (4) to obtain a filter cake, and the filter cake is magnesium hydroxide that can be used as a flame retardant. The defects of this technology or the deficiencies compared with the present invention: the hydrothermal reaction time of this invention is long, affecting production efficiency and increasing product cost, and except for the product particle size range, no further description is made on other product properties of the flame retardant.

[0014] Therefore, it is of great significance to develop a magnesium hydroxide flame retardant with stable quality, narrow particle size distribution, moderate specific surface area of magnesium hydroxide crystals, low chloride ion content, low production cost, and capable of continuous production. Summary of the Invention

[0015] Therefore, the technical problem to be solved by the present invention is to overcome the defects of poor quality stability, wide particle size distribution, large specific surface area of magnesium hydroxide crystals, high chloride ion content, low production cost and inability to achieve continuous production in the existing magnesium hydroxide flame retardant, so as to provide a preparation method of hexagonal flake magnesium hydroxide flame retardant. The hexagonal flake magnesium hydroxide prepared by this method has stable quality, narrow particle size distribution (particle size span less than 1), moderate specific surface area (about 2-10m 2 / g), low chloride ion content (less than 100 ppm), short hydrothermal reaction time, and can achieve continuous production with low production cost.

[0016] To this end, the present invention provides the following technical solutions:

[0017] A preparation method of hexagonal flake magnesium hydroxide flame retardant, comprising the following steps:

[0018] Precipitation: The precipitant and magnesium chloride solution are respectively and simultaneously added into an ultrasonic tube reactor, and precipitation reaction is carried out in the ultrasonic tube reactor. After the reaction ends, a slurry containing magnesium hydroxide precipitate is obtained;

[0019] Mixing and pulping: The slurry containing magnesium hydroxide precipitate is separated to obtain magnesium hydroxide precipitate; the magnesium hydroxide precipitate is mixed with a hydrothermal modifier to obtain a mixed slurry;

[0020] Hydrothermal treatment: The mixed slurry is added into a microwave stirring autoclave reactor for hydrothermal reaction. After the reaction ends, the obtained slurry is separated and dried to obtain hexagonal flake magnesium hydroxide flame retardant;

[0021] Wherein, based on OH - as the precipitant, based on Mg 2+ as the magnesium chloride solution, the molar ratio of the precipitant to the magnesium chloride solution in the ultrasonic tube reactor is 2:1;

[0022] The ultrasonic tube reactor is vertical, with bottom-in and top-out (the reaction materials enter the reactor from the lower part and are discharged from the upper part of the reactor);

[0023] The linear velocity at the end of the stirring paddle blade in the microwave stirring autoclave reactor is 0.5-1.5 m / s,

[0024] The hydrothermal reaction time is 30-120 min;

[0025] The average particle size of the hexagonal flake magnesium hydroxide flame retardant is 0.5-1.5 μm, the particle size span is less than 1, the specific surface area is 2-10m 2 / g, the magnesium hydroxide content is greater than 99%, and the chloride ion content is less than 100 ppm.

[0026] Aiming at the problems existing in the preparation of magnesium hydroxide flame retardant in the prior art, the present invention provides a preparation method of hexagonal flake magnesium hydroxide flame retardant. By adopting a vertical ultrasonic tube reactor to strengthen the precipitation reaction process, and at the same time controlling the molar ratio of OH - to Mg 2+ in the precipitation reaction process, and then combining with a microwave stirring autoclave reactor with a relatively low stirring blade tip linear velocity (0.5 - 1.5 m / s) to strengthen the hydrothermal modification process of magnesium hydroxide. The whole process can realize continuous reaction, will not cause blockage to the equipment, and the magnesium hydroxide precipitate separated in the mixing and pulping step does not need to be washed and decontaminated, and is directly mixed with the hydrothermal modifier to make pulp, which can greatly shorten the hydrothermal reaction time to 30 - 120 min. Finally, the prepared magnesium hydroxide flame retardant is hexagonal flake with regular morphology, uniform particle size and not easy to agglomerate, with an average particle size of 0.5 - 1.5 μm, a particle size span less than 1, a specific surface area of 2 - 10 m 2 / g, a magnesium hydroxide content greater than 99%, and a chloride ion content less than 100 ppm; thereby improving the compatibility of the magnesium hydroxide flame retardant with the polymer material, reducing the filling amount of the magnesium hydroxide flame retardant, and broadening the application range of the magnesium hydroxide flame retardant in the field of polymer material flame retardancy. If the molar ratio of the precipitant (calculated as OH - ) to the magnesium chloride solution (calculated as Mg 2+ ) in the ultrasonic tube reactor is not within the above limited range, on the one hand, it will cause waste of reaction materials and increase the production cost of the flame retardant, and on the other hand, it will cause the particle size distribution of the precipitate particles to become wider, thereby affecting the particle size distribution of the magnesium hydroxide particles after hydrothermal treatment. If the ultrasonic tube reactor is not vertical or not of the bottom-in and top-out type, it will cause deposition of precipitate particles and blockage of the reactor, which is not conducive to the continuous production of the flame retardant.

[0027] Optionally, in the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention, the ultrasonic tube reactor is a plug flow reactor. Preferably, the aspect ratio of the ultrasonic tube reactor is 10:1 - 30:1. By defining the ultrasonic tube reactor as a plug flow reactor, the reaction residence time can be further accurately controlled, which is beneficial to the control of the particle size of the precipitate particles.

[0028] Optionally, in the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention, the microwave frequency of the ultrasonic tube reactor is 20 kHz - 200 kHz.

[0029] Optionally, in the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention, the precipitant is an alkaline solution containing OH - , and any conventional alkaline solution containing OH - in the industry can be used, such as any one selected from sodium hydroxide, calcium hydroxide, potassium hydroxide, ammonia water, etc.

[0030] Optionally, in the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention, the concentration of the magnesium chloride solution is 1.0 - 5.0 mol / L.

[0031] Optionally, in the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention, the temperature of the precipitation reaction is 10 - 90 °C, and the time is 10 - 60 min.

[0032] Optionally, in the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention, the hydrothermal modifier is sodium hydroxide solution and / or potassium hydroxide solution; preferably, the concentration of OH - in the hydrothermal modifier is 2.0 - 10.0 mol / L.

[0033] Optionally, in the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention, the concentration of magnesium hydroxide in the mixed slurry is 1.0 - 10.0 mol / L.

[0034] Optionally, in the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention, the temperature of the hydrothermal reaction is 120 - 200 °C.

[0035] Optionally, in the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention, the frequency of the microwave stirring autoclave reactor is 915 MHz or 2450 MHz.

[0036] Compared with the prior art, the technical solution provided by the present invention has the following advantages:

[0037] In the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention, the precipitant and the magnesium chloride solution are respectively and simultaneously added into the ultrasonic tube reactor for precipitation reaction, and the OH in the ultrasonic tube reactor is controlled - and Mg 2+The molar concentration ratio is adjusted to generate a slurry containing white magnesium hydroxide precipitate. On the one hand, the magnesium chloride solution and the precipitant are mixed and reacted under an ultrasonic environment. The cavitation effect of ultrasonic waves causes the microbubbles in the solution to continuously oscillate, generate, grow, contract, and break. When the microbubbles break, they release high energy and can produce microjets with a maximum speed of up to 110 m / s, greatly strengthening the microscopic mixing between the reactants. As a result, the characteristic time of microscopic mixing between the reactants under ultrasonic waves is less than the nucleation induction time of magnesium hydroxide precipitation, achieving uniform mixing of the magnesium chloride solution and the precipitant before the precipitation reaction. The strengthening of the above mixing process ensures that the precipitation reaction can strictly carry out the nucleation induction of precipitation particles according to the stoichiometric ratio, and then form uniformly sized crystal nuclei, thereby effectively controlling the particle size and particle size distribution of the magnesium hydroxide solid product obtained from the precipitation reaction, and finally obtaining magnesium hydroxide particles with a narrow particle size distribution. At the same time, the strengthening of microscopic mixing can avoid the use of excessive precipitant, that is, the slurry containing white magnesium hydroxide precipitate does not contain excessive precipitant, so there is no need to wash and remove impurities from the filtered magnesium hydroxide precipitate before proceeding with the subsequent hydrothermal reaction. On the other hand, the cavitation effect of ultrasonic waves can generate a high-temperature and high-pressure environment at local points, accelerating the evaporation of water in the slurry system, effectively reducing the formation of hydrogen bonds, and thus reducing the agglomeration between the generated magnesium hydroxide particles. Moreover, the microjets and shock waves generated by ultrasonic waves have a pulverizing effect on the magnesium hydroxide particle agglomerates. The above process can effectively avoid the residual of a large amount of impurities such as chloride ions in the precipitation particles caused by particle agglomeration, so that the content of impurities such as chlorine element in the magnesium hydroxide flame retardant can meet the use requirements without multiple washings. In addition, the ultrasonic stirring effect of ultrasonic waves can make the magnesium hydroxide solid particles evenly dispersed in the slurry, which is beneficial to the subsequent filtration operation and also beneficial to the control of crystal form in the subsequent hydrothermal process.

[0038] In the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention, after the precipitation reaction is completed, the slurry prepared by mixing the magnesium hydroxide precipitate separated with the hydrothermal modifier is added to a microwave stirring autoclave reactor for hydrothermal reaction, and then a magnesium hydroxide flame retardant with a hexagonal flake crystal form is obtained through the "dissolution-crystallization" process. When performing the hydrothermal reaction in the microwave stirring autoclave reactor, on the one hand, since the microwave reactor is an internal heating mechanism that realizes molecular-level heating by inducing molecular polarization motion, its heating efficiency is significantly higher than the traditional heating methods of heat conduction or heat convection, thereby accelerating the reaction time of the hydrothermal crystallization of magnesium hydroxide particles, greatly reducing the growth time of the hexagonal flake magnesium hydroxide crystals, effectively controlling the aspect ratio of the magnesium hydroxide crystals, reducing the specific surface area of the magnesium hydroxide flame retardant, and ultimately improving the compatibility between the magnesium hydroxide flame retardant and the polymer material. On the other hand, this internal heating mechanism of the microwave reactor can achieve the temperature uniformity during the hydrothermal reaction process, thereby overcoming the phenomenon that the temperature gradient in the hydrothermal environment caused by uneven heat exchange in the traditional heat exchange method leads to uneven crystal growth. At the same time, compared with the higher stirring speed in the traditional stirring reaction kettle, the stirring paddle in the microwave stirring autoclave reactor adopts a lower stirring speed, which can effectively avoid the secondary crushing of the formed magnesium hydroxide crystals, and finally obtain magnesium hydroxide crystals with a narrow particle size distribution.

[0039] Overall, in the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention, the ultrasonic tube reactor can achieve a good microscopic mixing effect between the magnesium chloride solution and the precipitant, thereby obtaining a magnesium hydroxide precipitate that is not easily agglomerated, evenly dispersed, has a narrow and controllable particle size distribution, and has a small impurity content. At the same time, the ultrasonic reactor has the characteristics of low energy consumption and not being easily blocked. The microwave stirring autoclave reactor can greatly reduce the hydrothermal reaction time, is conducive to controlling the specific surface area of the magnesium hydroxide flame retardant, and the uniform hydrothermal temperature helps to eliminate the phenomenon of uneven crystal growth. The magnesium hydroxide flame retardant prepared by the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention has an average particle size of 0.5 - 1.5 μm, a particle size span less than 1, a specific surface area of 2 - 10 m 2 / g, a magnesium hydroxide content greater than 99%, a chloride ion content less than 100 ppm, and has regular morphology, uniform particle size, and is not easily agglomerated. This hexagonal flake magnesium hydroxide flame retardant has good compatibility with polymer materials, can reduce the filling amount of the magnesium hydroxide flame retardant. At the same time, the preparation method provided by the present invention can greatly improve the production efficiency of the hexagonal flake magnesium hydroxide flame retardant, reduce the production cost of the hexagonal flake magnesium hydroxide flame retardant, and thus further broaden the application scope of the magnesium hydroxide flame retardant in the field of polymer material flame retardancy. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a process schematic diagram of the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention;

[0041] Figure 2 It is the histogram of the diameter distribution of magnesium hydroxide precipitate particles obtained from the precipitation reaction in Example 1 of the present invention;

[0042] Figure 3 It is the histogram of the diameter distribution of magnesium hydroxide precipitate particles obtained from the precipitation reaction in Comparative Example 1 of the present invention;

[0043] Figure 4 It is the scanning electron micrograph of the magnesium hydroxide flame retardant prepared in Example 1 of the present invention;

[0044] Figure 5 It is the scanning electron micrograph of the magnesium hydroxide flame retardant prepared in Comparative Example 1 of the present invention;

[0045] Figure 6 It is the scanning electron micrograph of the magnesium hydroxide flame retardant prepared in Comparative Example 2 of the present invention;

[0046] Figure 7 It is the scanning electron micrograph of the magnesium hydroxide flame retardant prepared in Comparative Example 3 of the present invention;

[0047] Figure 8 It is the XRD pattern of the hexagonal flake magnesium hydroxide flame retardant prepared in Example 1 and Examples 3 - 5 of the present invention. Detailed Embodiments

[0048] The present invention will be specifically described below through examples. It is necessary to point out here that the following examples are only used to further illustrate the present invention and cannot be construed as limiting the protection scope of the present invention. Those skilled in the art can make some non - essential improvements and adjustments to the present invention according to the above content of the present invention.

[0049] The following examples are implemented on the premise of the technical solution of the present invention, and the detailed implementation methods and processes are given. However, the protection scope of the present invention is not limited to the following examples. The experimental methods without specific conditions noted in the following examples are usually carried out under conventional conditions.

[0050] Example 1

[0051] This example provides a preparation method of a hexagonal flake magnesium hydroxide flame retardant. The specific process is as Figure 1 shown and includes the following steps:

[0052] Precipitation: An aqueous sodium hydroxide solution with a concentration of 2.0 mol / L and an aqueous magnesium chloride solution with a concentration of 1.0 mol / L were simultaneously added into a vertical ultrasonic tubular plug-flow reactor with an ultrasonic frequency of 20 kHz at a volume flow rate ratio of 1:1. The reaction materials entered from the bottom and exited from the top. The aspect ratio of the reactor was 10:1. The precipitation reaction was carried out at 90 °C for 20 minutes in the ultrasonic vertical tubular reactor, and a slurry containing white magnesium hydroxide precipitate was formed.

[0053] Mixing and pulping: The slurry containing white magnesium hydroxide precipitate was filtered to obtain magnesium hydroxide precipitate without washing. Subsequently, the magnesium hydroxide precipitate was directly stirred and mixed with a sodium hydroxide solution with a concentration of 2.0 mol / L to prepare a mixed slurry with a magnesium hydroxide concentration of 1.0 mol / L.

[0054] Hydrothermal treatment: The above-mentioned mixed slurry was added into a microwave-stirred autoclave reactor. The linear velocity at the end of the stirring paddle blade in the reactor was 1.2 m / s, and the frequency of microwave heating was 915 MHz. After the mixed slurry was subjected to hydrothermal reaction at 140 °C for 120 min, the obtained slurry was filtered and dried to obtain hexagonal flake magnesium hydroxide flame retardant.

[0055] Comparative Example 1

[0056] The preparation method of the magnesium hydroxide flame retardant provided in this comparative example was similar to that in Example 1, except that: the reaction device used in the precipitation step was different. In this comparative example, a traditional stirring reaction kettle was used in the precipitation step, which specifically included the following steps:

[0057] Precipitation: An aqueous sodium hydroxide solution with a concentration of 2.0 mol / L and an aqueous magnesium chloride solution with a concentration of 1.0 mol / L were simultaneously added into a stirring reaction kettle at a volume flow rate ratio of 1:1. The precipitation reaction was carried out at 200 r / min and 90 °C in the stirring reaction kettle for 20 min, and a slurry containing white magnesium hydroxide precipitate was formed.

[0058] Mixing and pulping: The slurry containing white magnesium hydroxide precipitate was filtered to obtain magnesium hydroxide precipitate without washing. Subsequently, the magnesium hydroxide precipitate was directly stirred and mixed with a sodium hydroxide solution with a concentration of 2.0 mol / L to prepare a mixed slurry with a magnesium hydroxide concentration of 1.0 mol / L.

[0059] Hydrothermal treatment: The above-mentioned mixed slurry was added into a microwave-stirred autoclave reactor. The linear velocity at the end of the stirring paddle blade in the reactor was 1.2 m / s, and the frequency of microwave heating was 915 MHz. After the mixed slurry was subjected to hydrothermal reaction at 140 °C for 120 min, the obtained slurry was filtered and dried to obtain magnesium hydroxide flame retardant.

[0060] Comparative Example 2

[0061] The preparation method of the magnesium hydroxide flame retardant provided in this comparative example is similar to that in Example 1, with the only difference being that different reaction apparatuses are used in the hydrothermal step. In this comparative example, a traditional hydrothermal stirring reactor is used in the hydrothermal step, and the specific steps are as follows:

[0062] Precipitation: An aqueous sodium hydroxide solution with a concentration of 2.0 mol / L and an aqueous magnesium chloride solution with a concentration of 1.0 mol / L are simultaneously added into a vertical ultrasonic tubular reactor with a ultrasonic frequency of 20 kHz at a volume flow ratio of 1:1. The reaction materials enter from the bottom and exit from the top, and the aspect ratio of the reactor is 10:1. The aqueous sodium hydroxide solution and the aqueous magnesium chloride solution carry out a precipitation reaction at 90 °C in the ultrasonic vertical tubular reactor for 20 minutes to generate a slurry containing white magnesium hydroxide precipitate.

[0063] Mixing and pulping: The slurry containing the white magnesium hydroxide precipitate is filtered to obtain magnesium hydroxide precipitate without washing, and then the magnesium hydroxide precipitate is directly stirred and mixed with an aqueous sodium hydroxide solution with a concentration of 2.0 mol / L to prepare a mixed slurry with a magnesium hydroxide concentration of 1.0 mol / L.

[0064] Hydrothermal: The above-mentioned mixed slurry is added into a hydrothermal stirring reactor, the linear velocity at the end of the stirring paddle blade in the reactor is 1.2 m / s, and the mixed slurry is subjected to a hydrothermal reaction at 140 °C for 120 min. The obtained slurry is filtered and dried to obtain the magnesium hydroxide flame retardant.

[0065] Comparative Example 3

[0066] The preparation method of the magnesium hydroxide flame retardant provided in this comparative example is similar to that in Example 1, with the only difference being that different reaction apparatuses are used in the precipitation step and the hydrothermal step. In this comparative example, a traditional stirring reactor is used in the precipitation step, and a traditional hydrothermal stirring reactor is used in the hydrothermal step. The specific steps are as follows:

[0067] Precipitation: An aqueous sodium hydroxide solution with a concentration of 2.0 mol / L and an aqueous magnesium chloride solution with a concentration of 1.0 mol / L are simultaneously added into a stirring reactor at a volume flow ratio of 1:1. The aqueous sodium hydroxide solution and the aqueous magnesium chloride solution carry out a precipitation reaction at 300 r / min and 90 °C in the stirring reactor for 20 min to generate a slurry containing white magnesium hydroxide precipitate.

[0068] Mixing and pulping: The slurry containing the white magnesium hydroxide precipitate is filtered to obtain magnesium hydroxide precipitate without washing, and then the magnesium hydroxide precipitate is directly stirred and mixed with an aqueous sodium hydroxide solution with a concentration of 2.0 mol / L to prepare a mixed slurry with a magnesium hydroxide concentration of 1.0 mol / L.

[0069] Hydrothermal method: Add the above-mentioned mixed slurry into a hydrothermal stirring reactor. The linear velocity at the end of the stirring paddle blades in the reactor is 1.2 m / s. After the mixed slurry undergoes a hydrothermal reaction at 140 °C for 120 min, the obtained slurry is filtered and dried to obtain the magnesium hydroxide flame retardant.

[0070] Comparative Example 4

[0071] The preparation method of the magnesium hydroxide flame retardant provided in this comparative example is similar to that of Example 1, except that: the reaction device used in the precipitation step is different. In this comparative example, a traditional stirring reactor is used in the precipitation step, and the precipitation reaction is carried out under an ultrasonic environment, which specifically includes the following steps:

[0072] Precipitation: A sodium hydroxide aqueous solution with a concentration of 2.0 mol / L and a magnesium chloride aqueous solution with a concentration of 1.0 mol / L are simultaneously added into an ultrasonic stirring tank reactor with an ultrasonic frequency of 20 kHz at a volume flow ratio of 1:1. The sodium hydroxide aqueous solution and the magnesium chloride aqueous solution undergo a precipitation reaction at 90 °C in the stirring reactor for 20 min to generate a slurry containing white magnesium hydroxide precipitate.

[0073] Mixing and pulping: The slurry containing white magnesium hydroxide precipitate obtained above is filtered to obtain magnesium hydroxide precipitate, which is not washed and then directly stirred and mixed with a sodium hydroxide solution with a concentration of 2.0 mol / L to prepare a mixed slurry with a magnesium hydroxide concentration of 1.0 mol / L.

[0074] Hydrothermal method: Add the above-mentioned mixed slurry into a microwave stirring tank reactor. The linear velocity at the end of the stirring paddle blades in the reactor is 1.2 m / s, and the frequency of microwave heating is 915 MHz. After the mixed slurry undergoes a hydrothermal reaction at 140 °C for 120 min, the obtained slurry is filtered and dried to obtain the hexagonal flake magnesium hydroxide flame retardant.

[0075] Comparative Example 5

[0076] The preparation method of the magnesium hydroxide flame retardant provided in this comparative example is similar to that of Example 1, except that: when carrying out the hydrothermal reaction in the hydrothermal step, the linear velocity at the end of the stirring paddle blades in the microwave stirring tank reactor is different, which specifically includes the following steps:

[0077] Precipitation: A sodium hydroxide aqueous solution with a concentration of 2.0 mol / L and a magnesium chloride aqueous solution with a concentration of 1.0 mol / L are simultaneously added into a vertical ultrasonic tube plug flow reactor with an ultrasonic frequency of 20 kHz at a volume flow ratio of 1:1. The reaction materials enter from the bottom and exit from the top, and the aspect ratio of the reactor is 10:1. Sodium hydroxide and magnesium chloride undergo a precipitation reaction at 90 °C in the ultrasonic vertical tube reactor for 20 minutes to generate a slurry containing white magnesium hydroxide precipitate.

[0078] Mixed pulping: Filter the slurry containing magnesium hydroxide white precipitate above to obtain magnesium hydroxide precipitate, without washing, and then directly stir and mix the magnesium hydroxide precipitate with sodium hydroxide solution with a concentration of 2.0 mol / L to prepare a mixed slurry with a magnesium hydroxide concentration of 1.0 mol / L.

[0079] Hydrothermal treatment: Add the above-mentioned mixed slurry into a microwave-stirred autoclave reactor. The linear velocity at the end of the stirring paddle blade in the reactor is 2.4 m / s, and the frequency of microwave heating is 915 MHz. After the mixed slurry undergoes hydrothermal reaction at 140 °C for 120 min, the obtained slurry is filtered and dried to obtain hexagonal flake magnesium hydroxide flame retardant.

[0080] Example 2

[0081] This example provides a preparation method of hexagonal flake magnesium hydroxide flame retardant. The specific process is as Figure 1 shown, including the following steps:

[0082] Precipitation: Simultaneously add calcium hydroxide aqueous solution with a concentration of 5.0 mol / L and magnesium chloride aqueous solution with a concentration of 5.0 mol / L into a vertical ultrasonic tubular plug flow reactor with an ultrasonic frequency of 200 kHz at a volume flow ratio of 1:1. The reaction materials enter from the bottom and exit from the top. The aspect ratio of the reactor is 20:1. Calcium hydroxide and magnesium chloride undergo precipitation reaction at 25 °C in the ultrasonic vertical tubular reactor for 10 minutes to generate a slurry containing magnesium hydroxide white precipitate.

[0083] Mixed pulping: Filter the slurry containing magnesium hydroxide white precipitate above to obtain magnesium hydroxide precipitate, without washing, and then directly stir and mix the magnesium hydroxide precipitate with a mixed aqueous solution of sodium hydroxide and potassium hydroxide to prepare a mixed slurry with a magnesium hydroxide concentration of 10 mol / L. Among them, in the mixed aqueous solution of sodium hydroxide and potassium hydroxide, the molar ratio of sodium ions to potassium ions is 1:1, and the total molar concentration of sodium ions and potassium ions is 10 mol / L.

[0084] Hydrothermal treatment: Add the above-mentioned mixed slurry into a microwave-stirred autoclave reactor. The linear velocity at the end of the stirring paddle blade in the reactor is 1.4 m / s, and the frequency of microwave heating is 915 MHz. After the mixed slurry undergoes hydrothermal reaction at 120 °C for 90 min, the obtained slurry is filtered and dried to obtain hexagonal flake magnesium hydroxide flame retardant.

[0085] Example 3

[0086] This example provides a preparation method of hexagonal flake magnesium hydroxide flame retardant. The specific process is as Figure 1 shown, including the following steps:

[0087] Precipitation: An aqueous potassium hydroxide solution with a concentration of 4.0 mol / L and an aqueous magnesium chloride solution with a concentration of 2.0 mol / L were simultaneously added into a vertical ultrasonic tube plug-flow reactor with an ultrasonic frequency of 25 kHz at a volume flow rate ratio of 1:1. The reaction materials entered from the bottom and exited from the top. The aspect ratio of the reactor was 30:1. Potassium hydroxide and magnesium chloride underwent a precipitation reaction at 40 °C in the ultrasonic vertical tube reactor for 30 minutes to form a slurry containing white magnesium hydroxide precipitate.

[0088] Mixing and pulping: The slurry containing white magnesium hydroxide precipitate was filtered to obtain magnesium hydroxide precipitate without washing. Subsequently, the magnesium hydroxide precipitate was directly stirred and mixed with an aqueous potassium hydroxide solution with a concentration of 8.0 mol / L to prepare a mixed slurry with a magnesium hydroxide concentration of 4.0 mol / L.

[0089] Hydrothermal treatment: The above-mentioned mixed slurry was added into a microwave stirring autoclave reactor. The linear velocity at the end of the stirring paddle blade in the reactor was 1.1 m / s, and the frequency of microwave heating was 2450 MHz. After the mixed slurry underwent a hydrothermal reaction at 180 °C for 60 min, the obtained slurry was filtered and dried to obtain hexagonal flake magnesium hydroxide flame retardant.

[0090] Example 4

[0091] This example provides a method for preparing hexagonal flake magnesium hydroxide flame retardant, and the specific process is as Figure 1 shown, including the following steps:

[0092] Precipitation: An aqueous potassium hydroxide solution with a concentration of 6.0 mol / L and an aqueous magnesium chloride solution with a concentration of 3.0 mol / L were simultaneously added into a vertical ultrasonic tube plug-flow reactor with an ultrasonic frequency of 100 kHz at a volume flow rate ratio of 1:1. The reaction materials entered from the bottom and exited from the top. The aspect ratio of the reactor was 25:1. Potassium hydroxide and magnesium chloride underwent a precipitation reaction at 80 °C in the ultrasonic vertical tube reactor for 40 minutes to form a slurry containing white magnesium hydroxide precipitate.

[0093] Mixing and pulping: The slurry containing white magnesium hydroxide precipitate was filtered to obtain magnesium hydroxide precipitate without washing. Subsequently, the magnesium hydroxide precipitate was directly stirred and mixed with an aqueous sodium hydroxide solution with a concentration of 6.0 mol / L to prepare a mixed slurry with a magnesium hydroxide concentration of 2.0 mol / L.

[0094] Hydrothermal treatment: The above-mentioned mixed slurry was added into a microwave stirring autoclave reactor. The linear velocity at the end of the stirring paddle blade in the reactor was 0.9 m / s, and the frequency of microwave heating was 915 MHz. After the mixed slurry underwent a hydrothermal reaction at 160 °C for 100 min, the obtained slurry was filtered and dried to obtain hexagonal flake magnesium hydroxide flame retardant.

[0095] Example 5

[0096] This example provides a method for preparing a hexagonal flake magnesium hydroxide flame retardant. The specific process is as Figure 1 shown and includes the following steps:

[0097] Precipitation: An aqueous sodium hydroxide solution with a concentration of 8.0 mol / L and an aqueous magnesium chloride solution with a concentration of 4.0 mol / L are simultaneously added into a vertical ultrasonic tube plug flow reactor with an ultrasonic frequency of 140 kHz at a volume flow ratio of 1:1. The reaction materials enter from the bottom and exit from the top. The aspect ratio of the reactor is 15:1. Sodium hydroxide and magnesium chloride undergo a precipitation reaction at 10 °C in the ultrasonic vertical tube reactor for 60 minutes to generate a slurry containing white magnesium hydroxide precipitate.

[0098] Mixing and pulping: Filter the slurry containing the white magnesium hydroxide precipitate to obtain magnesium hydroxide precipitate without washing. Subsequently, directly stir and mix the magnesium hydroxide precipitate with an aqueous potassium hydroxide solution with a concentration of 4.0 mol / L to prepare a mixed slurry with a magnesium hydroxide concentration of 6.0 mol / L.

[0099] Hydrothermal treatment: Add the above-mentioned mixed slurry into a microwave stirring autoclave reactor. The linear velocity at the end of the stirring paddle blade in the reactor is 1.3 m / s, and the frequency of microwave heating is 2450 MHz. After the mixed slurry undergoes a hydrothermal reaction at 200 °C for 30 min, the obtained slurry is filtered and dried to obtain the hexagonal flake magnesium hydroxide flame retardant.

[0100] Experimental example

[0101] The magnesium hydroxide flame retardants prepared in each example and comparative example are respectively subjected to performance tests. Among them, the average particle size and particle size span are measured by a laser particle size analyzer, and the specific surface area is measured by a specific surface area analyzer; the magnesium hydroxide content is measured by the EDTA standard solution titration method based on the HG / T 3607-2007 "Industrial Magnesium Hydroxide" standard; the chloride ion content is measured by titration with a mercuric nitrate standard solution based on the GB / T 3051-2000 "General Method for the Determination of Chloride Content in Inorganic Chemical Products - Mercurimetric Method" standard; the hexagonal flake morphology is measured by a scanning electron microscope. The specific test results are shown in Table 1 below.

[0102] Table 1

[0103]

[0104] As can be seen from the data in the above table, the magnesium hydroxide obtained by the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention has stable quality, can form stable hexagonal flakes, has a narrow particle size distribution, a particle size span of less than 1, a moderate specific surface area, a chloride ion content of less than 100 ppm, and can realize continuous production with low production cost. Compared with Example 1, in Comparative Example 1, a traditional stirring reactor was used for precipitation reaction. The particle size span, specific surface area, and chloride ion content of the magnesium hydroxide obtained in Comparative Example 1 were higher than those in Example 1, while the purity of the magnesium hydroxide was lower than that in Example 1.

[0105] The diameter distribution histograms of the magnesium hydroxide precipitate particles filtered in the mixing and pulping steps in Example 1 and Comparative Example 1 are shown in Figure 2 and Figure 3 As shown. From the comparison between Figure 2 and Figure 3 , it can be seen that the diameter distribution of the magnesium hydroxide precipitate particles obtained by the precipitation reaction in the preparation method of the hexagonal flake magnesium hydroxide flame retardant provided by the present invention is narrower, laying a foundation for the small particle size span of the finally obtained magnesium hydroxide flame retardant. While the diameter distribution of the magnesium hydroxide precipitate particles obtained in Comparative Example 1 is wider, which also leads to a wide particle size span of the finally obtained magnesium hydroxide flame retardant in Comparative Example 1. Figure 4 and Figure 5 are the scanning electron microscope images of the magnesium hydroxide flame retardants obtained in Example 1 and Comparative Example 1 respectively. From the comparison between Figure 4 and Figure 5 , it can be seen that the crystal form control of the larger particles in the magnesium hydroxide flame retardant obtained in Comparative Example 1 is not good, and no hexagonal flake morphology is formed. The crystal form control of the magnesium hydroxide flame retardant obtained in Example 1 is very good, and hexagonal flake morphology can be formed regardless of the particle size.

[0106] Compared with Example 1, in Comparative Example 2, hydrothermal reaction was carried out in a traditional hydrothermal stirring reactor. The particle size span, specific surface area, and chloride ion content of the magnesium hydroxide flame retardant obtained in Comparative Example 2 were higher than those in Example 1. Figure 6 are the scanning electron microscope images of the magnesium hydroxide flame retardant obtained in Comparative Example 2 respectively. From the comparison between Figure 4 and Figure 6 , it can be seen that most of the crystals in the magnesium hydroxide flame retardant generated in Comparative Example 2 did not complete the crystal form conversion, the crystal form control was not good, the growth of the crystals in all directions was uneven, and some did not form hexagonal flake morphology.

[0107] Compared with Example 1, in Comparative Example 3, precipitation reaction was carried out in a traditional stirring reactor and hydrothermal reaction was carried out in a traditional hydrothermal stirring reactor. The particle size span, specific surface area, and chloride ion content of the magnesium hydroxide flame retardant obtained in Comparative Example 3 were higher than those in Example 1, while the purity and average particle size of the magnesium hydroxide were lower than those in Example 1. Figure 7SEM image of the magnesium hydroxide flame retardant prepared in Comparative Example 3. From Figure 4 and Figure 7 comparison, it can be seen that the edges of the magnesium hydroxide flame retardant crystals formed in Comparative Example 3 are rough, and almost no particles complete the crystal form conversion. The crystal form control is poor, and no hexagonal plate morphology is formed.

[0108] Compared with Example 1, in Comparative Example 4, the precipitation reaction was carried out in a traditional stirring reactor under ultrasonic conditions. Although there was the strengthening effect of ultrasonic waves, the secondary crushing of the precipitation particles by the stirring paddle ultimately resulted in a larger particle size span and specific surface area of the prepared magnesium hydroxide flame retardant than those in Example 1, while the purity of magnesium hydroxide and the chloride ion content were not much different.

[0109] Compared with Example 1, the tip linear velocity of the impeller used in the microwave stirring autoclave reactor in Comparative Example 5 was significantly higher than that in Example 1. The strong shearing effect caused the secondary crushing of the hydrothermal crystals, and finally the particle size span and specific surface area of the prepared magnesium hydroxide flame retardant were both higher than those in Example 1, while the purity of magnesium hydroxide and the chloride ion content were not much different.

[0110] The XRD patterns of the hexagonal plate-shaped magnesium hydroxide flame retardants prepared in Example 1, Example 3, Example 4 and Example 5 are shown in Figure 8 as follows. Among them, a is Example 5, b is Example 1, c is Example 4, and d is Example 3. From Figure 8 it can be seen that for the magnesium hydroxide flame retardant prepared by the present invention, the peak area of the non-polar plane 001 of the hexagonal plate-shaped crystals is larger than that of the polar plane 101, indicating that the preparation method provided by the present invention can effectively reduce the surface polarity of the magnesium hydroxide flame retardant, thereby facilitating the improvement of the compatibility between the magnesium hydroxide flame retardant and non-polar polymer materials.

[0111] Certainly, the present invention can also have many other embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and deformations according to the present invention, but these corresponding changes and deformations should all fall within the protection scope of the present invention.

Claims

1. A preparation method of a hexagonal flake magnesium hydroxide flame retardant, characterized in that, It includes the following steps: Precipitation: The precipitant and the magnesium chloride solution are simultaneously added into an ultrasonic tubular reactor, and a precipitation reaction is carried out in the ultrasonic tubular reactor. After the reaction ends, a slurry containing magnesium hydroxide precipitate is obtained; Mixing and pulping: The slurry containing magnesium hydroxide precipitate is separated to obtain magnesium hydroxide precipitate; the magnesium hydroxide precipitate is mixed with a hydrothermal modifier to obtain a mixed slurry; Hydrothermal treatment: The mixed slurry is added into a microwave stirring autoclave reactor for hydrothermal reaction. After the reaction ends, the obtained slurry is separated and dried to obtain a hexagonal flake magnesium hydroxide flame retardant; Among them, using OH - as the precipitant, using Mg 2+ as the magnesium chloride solution, the molar ratio of the precipitant to the magnesium chloride solution in the ultrasonic tubular reactor is 2:1; The ultrasonic tubular reactor is vertical, with downward inlet and upward outlet; The linear velocity at the end of the stirring paddle blade in the microwave stirring autoclave reactor is 0.5 - 1.5 m / s, The time of the hydrothermal reaction is 30 - 120 min; The average particle size of the hexagonal flake magnesium hydroxide flame retardant is 0.5 - 1.5 μm, the particle size span is less than 1, the specific surface area is 2 - 10 m 2 / g, the magnesium hydroxide content is greater than 99%, and the chloride ion content is less than 100 ppm.

2. The preparation method of the hexagonal flake magnesium hydroxide flame retardant according to claim 1, characterized in that, The ultrasonic tubular reactor is a plug flow reactor.

3. The preparation method of the hexagonal flake magnesium hydroxide flame retardant according to claim 1, characterized in that, The frequency of the ultrasonic tubular reactor is 20 kHz - 200 kHz.

4. The preparation method of the hexagonal flake magnesium hydroxide flame retardant according to claim 1, characterized in that, The precipitant is an alkaline solution containing OH - .

5. The preparation method of the hexagonal flake magnesium hydroxide flame retardant according to claim 1, characterized in that, The concentration of the magnesium chloride solution is 1.0 - 5.0 mol / L.

6. The preparation method of the hexagonal flake magnesium hydroxide flame retardant according to claim 1, characterized in that, The temperature of the precipitation reaction is 10 - 90 °C, and the time is 10 - 60 min.

7. The preparation method of the hexagonal flake magnesium hydroxide flame retardant according to claim 1, characterized in that, The hydrothermal modifier is sodium hydroxide solution and / or potassium hydroxide solution.

8. The preparation method of the hexagonal flake magnesium hydroxide flame retardant according to claim 1, characterized in that, The concentration of magnesium hydroxide in the mixed slurry is 1.0 - 10.0 mol / L.

9. The preparation method of the hexagonal flake magnesium hydroxide flame retardant according to claim 1, characterized in that, The temperature of the hydrothermal reaction is 120 - 200 °C.

10. The preparation method of the hexagonal flake magnesium hydroxide flame retardant according to claim 1, characterized in that, The microwave frequency of the microwave stirring autoclave reactor is 915 MHz or 2450 MHz.

11. The preparation method of the hexagonal flake magnesium hydroxide flame retardant according to claim 2, wherein the aspect ratio of the ultrasonic tubular reactor is 10:1 - 30:

1.

12. The preparation method of the hexagonal flake magnesium hydroxide flame retardant according to claim 4, wherein the precipitant is sodium hydroxide, calcium hydroxide, potassium hydroxide or ammonia water.

13. The preparation method of the hexagonal flake magnesium hydroxide flame retardant according to claim 7, wherein the concentration of OH - in the hydrothermal modifier is 2.0 to 10.0 mol / L.

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