A method for preparing a modified aqueous magnesium hydroxide solution

Abstract

The application belongs to the technical field of inorganic powder material surface modification and water-based dispersion system preparation, and discloses a preparation method of modified magnesium hydroxide water solution, which adopts an in-situ precipitation-surface modification integrated wet process: first, an anionic polymer dispersant is dissolved in deionized water to prepare a 40-70 DEG C bottom solution, then soluble magnesium salt solution and alkaline precipitant solution are added dropwise under high-speed stirring, the pH of the system is controlled to be 9.5-11.0, and after the reaction is completed, the system is preserved and aged, and after solid-liquid separation, washing and purification, the stable dispersion liquid can be obtained through secondary high-speed dispersion. The three steps of powder synthesis, surface modification and slurry preparation are combined in the application, which fundamentally prevents particle hard agglomeration, the obtained dispersion liquid is not obviously layered after being placed at room temperature for 3 months, the median diameter D50 is less than 1 mu m, the absolute value of Zeta potential is greater than 40 mV, and the solid content can be adjusted between 5% and 30%, which is suitable for high-end fields such as environmental protection, medicine and flame-retardant coating.

Description

Technical Field

[0001] This invention belongs to the technical field of inorganic powder material surface modification and aqueous dispersion system preparation, specifically relating to a method for preparing modified magnesium hydroxide aqueous solution. Background Technology

[0002] Magnesium hydroxide, as a green and environmentally friendly inorganic flame retardant and neutralizer, has the advantages of being non-toxic, smoke-suppressing, having a high decomposition temperature, and being widely available as a raw material. Its aqueous solution has been widely used in industrial wastewater treatment, flue gas desulfurization, pharmaceutical acid retardants, flame-retardant coatings, and flame-retardant modification of polymer materials.

[0003] However, magnesium hydroxide crystals are highly polar, with a large specific surface area and high surface energy. Unmodified magnesium hydroxide powder is prone to particle agglomeration and sedimentation in aqueous phases, making it difficult to form a stable aqueous solution. Existing technologies mainly employ the following three preparation methods, all of which have significant drawbacks: Post-modification method: Magnesium hydroxide dry powder is first synthesized by precipitation, and then the dry powder is added to a water or ethanol solution containing a modifier for surface coating. This method has a long process flow, the modifier is difficult to uniformly coat the particle surface, and the hard agglomerates formed during the redispersion of the dry powder cannot be eliminated. The resulting slurry has poor stability and usually requires the addition of a large amount of thickener to delay sedimentation.

[0004] Direct dispersion method: This method uses commercially available magnesium hydroxide powder in conjunction with a dispersant for high-speed mechanical dispersion. The resulting slurry has a large particle size and wide distribution, and is prone to stratification after standing, which cannot meet the requirements of applications with strict dispersibility requirements, such as pharmaceuticals and high-end flame-retardant coatings.

[0005] Traditional precipitation method: Magnesium hydroxide is prepared by direct precipitation reaction of magnesium salts with alkaline solution without the addition of modifiers. The resulting product is in the form of gel-like lumps, which is extremely difficult to filter and wash, and has very poor redispersibility, making it virtually impossible to form a stable aqueous solution.

[0006] Therefore, developing a new method that can generate magnesium hydroxide particles in situ in an aqueous phase and simultaneously complete surface modification to directly obtain a highly dispersed and highly stable aqueous solution of magnesium hydroxide has significant industrial application value. Summary of the Invention

[0007] To overcome the above-mentioned technical problems, the present invention provides a method for preparing a modified aqueous solution of magnesium hydroxide.

[0008] The present invention adopts the following technical solution: A method for preparing a modified magnesium hydroxide aqueous solution includes the following steps: (1) Preparation of base solution: In the reactor, the modifier is dissolved in deionized water to prepare a base solution with a mass concentration of 0.1% to 5%, and heated to 40°C to 70°C; the modifier is an anionic polymeric dispersant, specifically at least one of sodium polyacrylate, sodium hexametaphosphate, and polycarboxylate dispersant.

[0009] (2) Precipitation-modification simultaneous reaction: Under high-speed stirring at 1000-3000 rpm, a soluble magnesium salt aqueous solution with a concentration of 0.5-1.5 mol / L and an alkaline precipitant aqueous solution with a concentration of 1-3 mol / L are simultaneously and slowly added dropwise to the bottom solution of step (1), and the pH value of the system is controlled to be maintained at 9.5-11.0; after the addition is completed, the mixture is kept warm and stirred for 0.5-1 hour for aging.

[0010] (3) Washing and purification: The solution obtained in step (2) is subjected to solid-liquid separation, and the filter cake is repeatedly washed with deionized water until the conductivity of the filtrate drops below the preset value to remove free soluble salt impurities.

[0011] (4) Secondary dispersion: The washed filter cake is added back into fresh deionized water and dispersed at high speed of 1000-5000 rpm for 10-30 minutes using a homogenizer to obtain the modified magnesium hydroxide aqueous solution.

[0012] Further, the amount of the modifier used in step (1) is 0.5% to 5% of the theoretical mass of the generated magnesium hydroxide.

[0013] Furthermore, the deionized water described in step (1) is boiled before use to remove dissolved carbon dioxide and avoid the formation of magnesium carbonate impurities that could affect the purity of the product.

[0014] Furthermore, the soluble magnesium salt mentioned in step (2) is either magnesium chloride or magnesium sulfate.

[0015] Furthermore, the alkaline precipitant mentioned in step (2) is either sodium hydroxide or potassium hydroxide solution.

[0016] Furthermore, after step (4), xanthan gum, a thickener accounting for 0.01% to 0.5% of the total mass of the dispersion, can be added to further improve the long-term stability of the system.

[0017] Compared with the prior art, the beneficial effects of the present invention are: (1) High dispersion stability: This invention adopts an in-situ precipitation-surface modification simultaneous process. At the moment magnesium hydroxide crystal nuclei are generated, the modifier is adsorbed onto the particle surface through electrostatic interaction, forming a dual barrier of steric hindrance and electrostatic repulsion, which fundamentally prevents hard agglomeration between particles. The median diameter D50 of the obtained modified magnesium hydroxide particles is less than 1 μm, exhibiting submicron dispersion, and the absolute value of the Zeta potential is greater than 40 mV. The obtained dispersion is milky white and uniform, and shows no obvious stratification after standing at room temperature for more than 3 months. The volume ratio of the bottom sediment layer does not exceed 5%, and it can be completely redispersed with gentle shaking.

[0018] (2) Simple and efficient process: The three independent steps of "powder synthesis, surface modification and slurry preparation" in the traditional process are combined into one step, which greatly shortens the process flow, reduces energy consumption and production costs, and is suitable for large-scale industrial production.

[0019] (3) Excellent washing performance: After modification, the surface properties of magnesium hydroxide particles change, and they no longer form gel-like precipitates that are difficult to filter. The filter cake is loose and powdery, and the filtration and washing speed is more than 5 times higher than that of the traditional precipitation method, which solves the bottleneck problem of difficult washing in industrial production.

[0020] (4) Adjustable solid content: By adjusting the amount of deionized water added during the secondary dispersion process, an aqueous solution with an adjustable solid content between 5% and 30% can be easily obtained, which can meet the needs of different downstream application scenarios. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the method flow of the present invention. Detailed Implementation

[0022] The present invention will be further described in detail below through specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0023] Example 1 Preparation of the base solution: In a clean 1L glass reactor, add 400mL of deionized water with a conductivity of less than 5μS / cm (pre-boiled for 15 minutes and cooled to room temperature to remove CO2). Add 1.2g of sodium polyacrylate (analytical grade, average molecular weight approximately 4000) and stir at 200 rpm until completely dissolved. Heat the base solution in a water bath and maintain the temperature at 60±1℃.

[0024] Precipitation-modification simultaneous reaction: Increase the stirring speed to 1200 rpm. Measure 200 mL of 1.0 mol / L magnesium chloride hexahydrate solution (MgCl2・6H2O, analytical grade) and 210 mL of 2.0 mol / L sodium hydroxide solution (NaOH, analytical grade). Using a peristaltic pump, slowly add both solutions dropwise to the base solution simultaneously. The MgCl2 solution drop rate is approximately 4 mL / min, and the NaOH solution drop rate is adjusted accordingly to maintain a stable pH of 10.2-10.5 throughout the reaction. The addition is completed in approximately 50 minutes. Afterward, continue stirring at 800 rpm (800 rpm) at 60°C for 60 minutes of aging.

[0025] Washing and purification: The resulting milky white solution was cooled to room temperature and vacuum filtered using a Buchner funnel and slow-speed qualitative filter paper. The filter cake was washed with 100 mL of deionized water each time, for a total of four washes. The conductivity of the filtrate after the fourth wash was measured to be 120 μS / cm, indicating that the free salts had been largely removed. The entire filtration and washing process took only 22 minutes, and the filter cake was a loose powder with no gel clogging.

[0026] Secondary dispersion: Transfer the washed filter cake to a 1L beaker and add deionized water to a total mass of 600g. Disperse the solution using a high-speed homogenizer (model T25digitalULTRA-TURRAX) at 10,000 rpm for 15 minutes to obtain a milky white, homogeneous aqueous solution of modified magnesium hydroxide. The measured solid content was 9.8% (mass percentage).

[0027] Example 2 The operation steps in this embodiment are basically the same as those in Example 1, except that in step (1), 1.2g of sodium polyacrylate is replaced with 0.8g of sodium hexametaphosphate (analytical grade). After washing, the filter cake is also loose and easy to filter, and finally a milky white uniform dispersion is obtained, with a measured solid content of 9.6%.

[0028] Example 3 In this embodiment, after all the steps in Example 1 are completed, 0.6 g of food-grade xanthan gum is added to the obtained modified magnesium hydroxide aqueous solution under slow stirring. Stirring continues for 30 minutes until completely dissolved, resulting in a stable dispersion with slightly increased viscosity.

[0029] Comparative Example 1: Traditional Direct Precipitation Method The operation steps are exactly the same as in Example 1, but the base liquid in step (1) is only 400 mL of deionized water, without adding any modifier.

[0030] Phenomena and Data: The precipitate obtained after the reaction is gel-like. Filter washing is extremely difficult and time-consuming (over 2 hours), and the filter cake becomes lumpy. Attempts to disperse at high speed with added water fail to produce a homogeneous slurry, and a large amount of hard precipitate forms within a short time.

[0031] Comparative Example 2: Post-modification coating method Weigh 30g of commercially available ordinary magnesium hydroxide powder (particle size D50≈8μm) and add it to 400mL of aqueous solution containing 1.2g of sodium polyacrylate. Stir at 60℃ for 2 hours to coat the surface. Then filter, dry, and add water to disperse according to step 4 of Example 1.

[0032] Phenomenon and data: Although a slurry can be formed, the particles are visible to the naked eye, and a clear sedimentation layer appears at the bottom after standing for 1 hour.

[0033] Product performance test results 1. Dispersion stability test Test method: Place 50 mL of sample in a stoppered graduated cylinder, let it stand at room temperature, and observe the stratification or sedimentation. Record the proportion of the bottom sediment layer to the total volume. The results are shown in the table below: 2. Particle size distribution test Testing instrument: Malvern Mastersizer 3000 laser particle size analyzer. Samples were diluted with deionized water before testing, and the results are shown in the table below: 3. Zeta potential test Test instrument: Malvern Zetasizer NanoZS Zeta potential analyzer. Test results: The zeta potential of the product in Example 1 was -42.5mV, and the zeta potential of the product in Comparative Example 2 was -18.7mV.

[0034] Conclusion: The absolute value of the Zeta potential of the product in Example 1 is much greater than 30mV, indicating that there is a very strong electrostatic repulsion between the particles and the system is in a highly stable state. This is attributed to the effective adsorption of the modifier at the moment of particle formation.

[0035] Although embodiments of the present invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and variations can be made to the above embodiments without departing from the principles and spirit of the present invention, the scope of which is defined by the claims and their equivalents.

Claims

1. A method for preparing a modified aqueous solution of magnesium hydroxide, characterized in that, Includes the following steps: (1) Dissolve the modifier in deionized water and heat it to 40℃~70℃ as the base solution; (2) Under high-speed stirring at 1000-3000 rpm, a soluble magnesium salt aqueous solution with a concentration of 0.5-1.5 mol / L and an alkaline precipitant aqueous solution with a concentration of 1-3 mol / L are simultaneously and slowly added dropwise to the bottom liquid, and the pH value of the system is controlled to be maintained at 9.5-11.0; after the addition is completed, the mixture is kept warm and stirred for 0.5-1 hour for aging. (3) Separate the solution obtained in step (2) into solid and liquid, and wash the filter cake repeatedly with deionized water until the conductivity of the filtrate drops below the preset value; (4) Add the washed filter cake back into fresh deionized water and disperse it at high speed of 1000-5000 rpm for 10-30 minutes using a homogenizer to obtain the modified magnesium hydroxide aqueous solution.

2. The method for preparing the modified magnesium hydroxide aqueous solution according to claim 1, characterized in that, The modifier mentioned in step (1) is at least one of sodium polyacrylate, sodium hexametaphosphate, and polycarboxylate dispersant, and its dosage is 0.5% to 5% of the theoretical mass of the generated magnesium hydroxide.

3. The method for preparing the modified magnesium hydroxide aqueous solution according to claim 1, characterized in that, The mass concentration of the base liquid in step (1) is 0.1% to 5%.

4. The method for preparing the modified magnesium hydroxide aqueous solution according to claim 1, characterized in that, The deionized water described in step (1) is boiled before use to remove dissolved carbon dioxide.

5. The method for preparing the modified magnesium hydroxide aqueous solution according to claim 1, characterized in that, The soluble magnesium salt mentioned in step (2) is either magnesium chloride or magnesium sulfate.

6. The method for preparing the modified magnesium hydroxide aqueous solution according to claim 1, characterized in that, The alkaline precipitant mentioned in step (2) is either sodium hydroxide or potassium hydroxide solution.

7. The method for preparing the modified magnesium hydroxide aqueous solution according to claim 1, characterized in that, After step (4), the process also includes adding xanthan gum, a thickener, to the resulting dispersion at a concentration of 0.01% to 0.5% of the total mass of the dispersion.

8. A modified aqueous solution of magnesium hydroxide, characterized in that, Prepared by the method according to any one of claims 1 to 7.

Images