Process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction

Abstract

The application discloses a process for preparing high-purity superfine magnesium hydroxide by low-temperature gradient reaction, belongs to the technical field of magnesium hydroxide preparation, and comprises the following steps: adding a compound impurity removal agent into high-magnesium bitter brine to remove impurities, and rapidly adding part of liquid alkali to form a nucleus at low temperature; in the process of programmed temperature rise, the remaining liquid alkali and a compound crystal form regulating additive are uniformly and dropwisely added; after constant temperature ripening, two-stage filtration purification is conducted, and drying is performed to obtain the product; through the process, the preparation of high-purity, narrow particle size distribution and high-dispersity magnesium hydroxide under low-temperature conditions is realized, the comprehensive energy consumption is reduced, no wastewater is discharged, and the process is green and environment-friendly.

Description

Technical Field

[0001] This application belongs to the field of magnesium hydroxide preparation technology, and in particular relates to a process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction. Background Technology

[0002] Magnesium hydroxide, as a green and environmentally friendly halogen-free flame-retardant filler, has wide applications in wire and cable, rubber and plastics, composite materials and other fields. The preparation of magnesium hydroxide by reacting salt field bittern or underground brine with liquid alkali is an important industrial route. However, existing conventional processes still face technical challenges in obtaining high-purity ultrafine magnesium hydroxide products that can meet the requirements of high-end cable insulation materials (such as purity ≥99.5%, particle size D50 between 1.0-2.0 μm, and excellent dispersibility in organic polymer matrices).

[0003] First, conventional single-stage or isothermal reaction modes struggle to balance raw material conversion rate and particle size distribution control: Currently, most brine-liquid alkali synthesis processes employ single-stage feeding or reactions at a single temperature. In the initial stage of the chemical reaction, the supersaturation is extremely high, resulting in a rapid but uncontrollable nucleation rate, which easily generates a large number of small and unevenly sized primary crystal nuclei. In the later stages of the reaction, the insufficient driving force due to the decrease in alkali concentration leads to incomplete raw material conversion. This uneven nucleation-growth process ultimately results in a wide particle size distribution in the product, with both fine and coarse powders present, directly affecting the powder's filling performance and processing flowability. Patent CN101224901B discloses a continuous preparation method for high-purity magnesium hydroxide, employing a single-stage mixing reaction at isothermal temperature, resulting in a wider product particle size distribution. Secondly, the removal of impurities and the control of product crystal form and dispersibility during the synthesis process are mutually restrictive: Industrial bittern generally contains trace impurities such as calcium, silicon, sulfate, and iron. These impurities are very easy to co-precipitate or surface adsorb during the magnesium hydroxide precipitation process, especially colloidal silicon and calcium ions. They not only reduce the purity of the finished product, but also seriously affect the crystal growth habit and particle surface properties, causing hard agglomeration of particles, which makes the dispersibility of the finished product poor in subsequent applications. The common practice in the industry is to set up a separate high-temperature hydrothermal purification or acid washing process after synthesis. Although this can partially improve the purity, it increases energy consumption and wastewater volume, lengthens the process flow, and goes against the development direction of green chemical industry. Moreover, the high-temperature process can easily cause abnormal crystal growth, destroying the obtained ultrafine particle size. Third, existing technologies mostly focus on optimizing single steps and lack a complete set of processes that couple the entire process from nucleation and growth to in-situ impurity removal and purification. Such fragmented improvements are difficult to generate synergistic effects, and the technology is highly homogenized, making it difficult for the industry to break through the technical barriers of achieving both high purity and ultrafine and narrow particle size distribution in the long term.

[0004] Therefore, developing a green synthesis process that can achieve precise control of nucleation and growth under mild conditions, simultaneous deep impurity removal during synthesis, and produce products with narrow particle size distribution and excellent dispersibility, while generating no additional wastewater, is of great practical significance for breaking through industry technical bottlenecks and meeting the stringent requirements of high-end flame-retardant cable materials for magnesium hydroxide fillers. Summary of the Invention

[0005] The purpose of this application is to provide a process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction, so as to solve the technical problems in the existing magnesium hydroxide production, such as unstable nucleation, incomplete impurity removal, wide particle size distribution, poor dispersibility, and large amount of wastewater generated during the synthesis process, which cannot meet the stringent requirements of high-end flame-retardant cable materials for magnesium hydroxide fillers.

[0006] To achieve the above objectives, the technical solution adopted in this application is: to provide a process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction, specifically including the following steps: (I) Brine pretreatment: Sodium oxalate and sodium fluoride are added to high magnesium bitter brine and stirred to react and obtain brine slurry; (II) Low-temperature gradient reaction S1. Premixing reaction: The brine slurry is heated and stirred, a portion of alkaline solution is added, and after heating, it is filtered to obtain refined magnesium solution. S2, Crystal nucleus growth: Add the refined magnesium liquid and composite additive solution to the reactor and stir, then add the remaining alkaline solution, heat and continue stirring; S3. Constant temperature curing: After the remaining alkali solution is added, the mixture is stirred at a constant temperature to cure the slurry. (III) Post-processing: The matured slurry is filtered, the filter cake is collected, and dried to obtain high-purity ultrafine magnesium hydroxide.

[0007] In one embodiment, The high-magnesium bittern composition mentioned in step (I) includes: Mg 2+ Concentration 60-100 g / L, Ca 2+ Concentration 0.8-1.5 g / L, SO4 2- Concentration 10-35 g / L, Cl - Concentration 150-220 g / L.

[0008] In one embodiment, In step (1), the mass ratio of the high-magnesium bittern to sodium oxalate is 1:0.002-0.003; the mass ratio of the high-magnesium bittern to sodium fluoride is 1:0.0002-0.0003.

[0009] In one embodiment, Step (II) The alkali solution is a 32% NaOH solution. The total mass of the alkali solution added is 102-110% of the theoretical mass of sodium hydroxide required to precipitate all magnesium ions in the high-magnesium bittern into magnesium hydroxide. The alkali solution added in step S1 accounts for 5-8% of the total amount.

[0010] In one embodiment, The composite additive solution is prepared by dissolving tripotassium citrate and hydroxypropyl methylcellulose (HPMC) in pure water and stirring until a transparent viscous solution is obtained. The mass ratio of HPMC to tripotassium citrate is 1:2-5; the amount of the composite additive solution added is 0.3-0.8% of the theoretical mass of magnesium hydroxide produced.

[0011] In one embodiment, In step (ii) S1, the heating temperature is 35-40 ℃, the stirring speed is 350 r / min, and the temperature rise is 60 ℃.

[0012] In one embodiment, In step (ii) S2, the heating temperature is 55-60 ℃ and the stirring speed is 250 r / min.

[0013] In one embodiment, The ripening time in step (II) S3 is 2.5-3.5 h.

[0014] In one embodiment, The filtration in step (iii) is divided into two filtration processes, using a 1 μm filter membrane for thermal filtration.

[0015] In one embodiment, The purity of the high-purity ultrafine magnesium hydroxide is ≥99.5%.

[0016] This application provides a process for preparing high-purity ultrafine magnesium hydroxide using a low-temperature gradient reaction. Sodium oxalate and sodium fluoride are added to high-magnesium bittern for impurity removal. A portion of liquid alkali is rapidly added at low temperature to induce nucleation. The mixture is then heated and filtered through a membrane to simultaneously trap impurities and retain active crystal nuclei. During the programmed temperature increase, the remaining liquid alkali is added dropwise at a uniform rate, along with a crystal form regulating agent composed of tripotassium citrate and hydroxypropyl methylcellulose. After isothermal maturation, the mixture undergoes a two-stage purification process: primary hot filtration and secondary pre-cooled pure water pulping and cold filtration. The final product is then dried. This process offers the following advantages: 1. This process solves the mismatch between nucleation and growth rates from both chemical precipitation kinetics and thermodynamics perspectives by independently controlling the temperature, alkali addition rate, and stirring parameters of the three stages: premixing reaction, crystal nucleus growth, and isothermal ripening. First, in the premixing stage, the system temperature is controlled at a low temperature range of 35-40 °C, and a small portion of liquid alkali is added rapidly in one go, instantly generating extremely high local supersaturation. According to classical nucleation theory, the nucleation rate is proportional to the exponential power of the supersaturation. The low temperature further reduces the thermal motion of solute molecules, inhibiting the dissolution and re-dissolution of crystal nuclei, thereby triggering explosive homogeneous nucleation and forming a large number of nanoscale primary crystal nuclei in a very short time, providing a uniform seed base for subsequent growth. Second, in the crystal nucleus growth stage, the remaining liquid alkali is added dropwise at a uniform rate, while the system temperature is programmed to rise from 40 °C to 55-60 °C. This uniform dropwise addition maintains the Mg content in the system. 2+ and OH - The constant low supersaturation allows solute molecules to deposit orderly on the surface of existing crystal nuclei without secondary nucleation. Programmed temperature control regulates the activation energy of crystal growth by increasing the temperature, promoting directional growth of the crystal along its dominant crystal plane. This coupled control strategy of constant low supersaturation and slow temperature increase effectively avoids the bimodal particle size distribution problem caused by overlapping nucleation and growth in traditional isothermal single-stage reactions. Finally, isothermal ripening utilizes the Ostwald ripening principle, where smaller particles preferentially dissolve due to their higher surface energy, and the solute redeposits on the surface of larger particles, further narrowing the particle size distribution and repairing internal crystal defects. Testing showed that the median particle size of the resulting product is far superior to that of traditional processes. 2. Removal of calcium ions from bittern is a recognized technical challenge in the industry. Traditional processes often use sulfates or carbonates as precipitants. However, the solubility products of calcium sulfate and calcium carbonate are not significantly different from their corresponding magnesium salts, making co-precipitation highly likely in environments with a pH greater than 10, resulting in significant magnesium loss. This process, based on the hard and soft acid-base theory, selects fluoride ions and oxalate ions as a compound impurity remover. 2+ and Ca 2+ Although both are hard acids, F - and C2O4 2+ It is more than OH - A harder alkali, and with Ca 2+ The solubility product of the precipitate formed (CaF2Ksp = 3.9 × 10) -11 CaC₂O₄Ksp = 2.3 × 10⁻⁶ -9 The value is much smaller than that of the corresponding magnesium salt (MgF2Ksp=6.4×10). -9 MgC2O4Ksp=8.5×10 -5This gradient difference in solubility product endows the compound system with excellent calcium-magnesium selectivity, enabling the quantitative removal of calcium ions in the form of CaF2 and CaC2O4 without precipitating magnesium ions. Testing shows that the CaO content in the product can be stably controlled below 0.03%. Simultaneously, the newly generated CaF2 and CaC2O4 precipitates have a large specific surface area and surface activity, serving as highly efficient adsorption carriers to simultaneously remove colloidal silica and trace iron ions from the brine. More importantly, this invention utilizes the significant particle size difference between the impurity-removing precipitate and the active crystal nuclei of magnesium hydroxide at a concentration of 60... Thermal filtration is performed at ℃ using a 0.8μm precision filter membrane. At this temperature, impurity precipitates are fully matured and grown, and the viscosity of the suspension decreases due to the increased temperature, resulting in low filtration resistance. Meanwhile, the nano-sized magnesium hydroxide nuclei, due to their much smaller particle size than the membrane pores, can completely penetrate the membrane pores and enter the next process along with the filtrate. This step simultaneously achieves precise removal of impurities and complete retention of active crystal seeds, avoiding the mechanical entrainment loss of magnesium in traditional separate impurity removal processes. Data from the examples show that the overall magnesium yield is significantly better than the traditional route of first removing impurities and then synthesizing. 3. Magnesium hydroxide belongs to the hexagonal crystal system. Its polar crystal faces exhibit a strong tendency for interlayer stacking due to their high surface energy. This is the chemical reason why the product undergoes hard agglomeration during drying and application. This process introduces a composite crystal form regulating agent, a mixture of tripotassium citrate and hydroxypropyl methylcellulose (HPMC), to synergistically solve the agglomeration problem from two levels: Firstly, the citrate ion, as a polydentate ligand, contains one hydroxyl group and three carboxyl groups in its molecular structure, and its spatial configuration is similar to that of the Mg2+ crystal faces of magnesium hydroxide. 2+ With a well-matched arrangement, it can preferentially anchor to the substrate through chemical adsorption, effectively reducing the surface energy and growth rate of the crystal plane, preventing lamellar stacking, and thus obtaining a well-depolymerized hexagonal lamellar structure; secondly, HPMC is a non-ionic water-soluble cellulose ether, whose long-chain molecules are entangled and coated on the surface of crystal nuclei and primary crystal grains in the slurry system through hydrogen bonds and van der Waals forces, forming a hydration film that provides strong steric hindrance and repulsion potential energy; compared with ionic dispersants, HPMC does not increase the ionic strength of the system, does not change the electrostatic balance of the precipitate, and has better wide pH adaptability; the two work synergistically, tripotassium citrate controls the morphology of primary particles from the perspective of crystal growth habit, while HPMC controls the aggregation of secondary particles from the perspective of colloidal stability, and the product cross-section shows no visible white spots and high dispersibility; 4. Traditional processes for washing and purifying magnesium hydroxide slurry often involve multiple cold water washes, resulting in large water and wastewater volumes. Furthermore, impurities such as colloidal silica are difficult to remove effectively at room temperature. This process utilizes the significant differences in solubility and colloidal stability of impurities at different temperatures. During the first-stage hot filtration, the solubility of byproduct salts such as sodium chloride and sodium sulfate approaches its peak, and most soluble salts are separated along with the high-temperature mother liquor. Simultaneously, due to the high temperature, a small amount of silica colloid remains in a stable sol state, making it difficult to adsorb and deposit on the crystal surface. In the second-stage cold filtration, the filter cake obtained from hot filtration is redispersed in pre-cooled pure water for pulping. Under low-temperature conditions, the trace amounts of soluble salts remaining in the intercrystalline micropores further dissolve under the drive of the concentration gradient. This also weakens the Brownian motion of the dissolved silica colloids, thins the hydration film, reduces colloidal stability, and causes destabilization and precipitation, which adsorbs onto the crystal surface. These destabilized precipitates are effectively retained in subsequent filtrations, significantly reducing impurity content. The low-temperature filtrate from the second-stage cold filtration has extremely low impurity content and can be directly recycled for the next batch of second-stage pulping water, achieving zero wastewater discharge throughout the entire process. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a laser particle size distribution diagram for Example 1; Figure 2 This is a BET multi-point fitting line graph from Example 1; Figure 3 This is a laser particle size distribution diagram for Example 2. Detailed Implementation

[0019] To make the technical problems to be solved, the technical solutions, and the beneficial effects of this application clearer, this application will be further described in detail. It should be understood that the specific embodiments described herein are only for explaining this application and are not intended to limit this application.

[0020] Example 1 (I) Brine pretreatment: The high-magnesium bittern was taken from the salt field. Testing revealed its main components to be: Mg 2+ Concentration 80.5 g / L, Ca 2+ Concentration 1.08 g / L, SO4 2- Concentration 25.2 g / L, Cl - Concentration 182 g / L, trace elements ≤0.5 g / L, density 1.275 kg / L; Measure 100 L of high-magnesium bittern (total mass approximately 127.5 kg) and inject it into a premixing reactor with a jacketed stirrer. Stir at 200 r / min, add 304 g of sodium oxalate and 32 g of sodium fluoride to the brine at room temperature, and continue stirring for 30 min to fully react and obtain brine slurry. At this time, the Ca in the brine 2+ Almost all of it is converted into CaC2O4 and CaF2 precipitates, while at the same time most of the SiO2 colloid and Fe are removed through adsorption and entrapment. 3+ Colloids were used to prepare brine slurry containing fine impurities as precipitates. (II) Low-temperature gradient reaction S1. Premixing reaction: Start the premixing kettle jacket cooling water, control the system temperature at 35 ℃, increase the stirring speed to 350 r / min, and quickly add 4.35 kg (5%) of 32% NaOH liquid alkali to the brine slurry in one go, and control the feeding time within 1 min. A large amount of OH- introduced instantaneously - Local supersaturation caused by low temperature and strong stirring triggers explosive homogeneous nucleation of magnesium hydroxide crystal nuclei, resulting in a mixed slurry containing nanoscale active crystal nuclei and impurity precipitates. Stirring is maintained for 15 minutes to ensure sufficient nucleation. The mixed slurry was heated to 60 °C and fed into a filter equipped with a 0.8 μm precision filter membrane for hot filtration at a pressure of 0.2-0.3 MPa to obtain 115 L of refined magnesium liquid containing crystal nuclei (including 10 L of washing liquid obtained from washing the filter cake with pure water). At this temperature, impurities such as calcium oxalate and calcium fluoride precipitate agglomerates further mature and increase in size, while the stability of active magnesium hydroxide crystal nuclei increases and the viscosity of the suspension decreases. The filtered impurities are completely retained in the filter cake, while the refined slurry containing active magnesium hydroxide crystal nuclei passes through the filter membrane to obtain refined magnesium liquor. The wet filter residue (mainly calcium salts and adsorbed impurities) is treated as solid waste. The amount of 32% NaOH solution added is calculated based on the theoretical mass of sodium hydroxide required to precipitate all magnesium ions in the bittern into magnesium hydroxide: m(Mg) 2+ = 80.5 g / L × 100 L = 8050 g, n = 8050 g / 24.3 g / mol = 331.3 mol, theoretical NaOH required = 331.3 mol × 2 × 40.0 g / mol = 26504 g, theoretical amount of 32% liquid alkali = 26504 g / 32% = 82.83 kg; to ensure complete reaction and maintain alkaline environment, the total amount of liquid alkali used is set to 105% of the theoretical amount, that is: Total amount of liquid caustic soda fed = 82.83 kg × 1.05 = 86.97 kg; S2, crystal nucleus growth: Theoretical yield of magnesium hydroxide = 8050 g × (58.3 g / mol / 24.3 g / mol) = 19320 g. Calculate the total amount of composite additives based on 0.5% of the estimated product mass: 19320 g × 0.5% = 96.6 g. Weigh out tripotassium citrate and HPMC according to a mass ratio of 1:3.33. 74.3 g of tripotassium citrate and 22.3 g of HPMC were dissolved in 500 mL of pure water and stirred until a transparent, viscous solution was obtained to prepare a composite additive solution. The composite additive solution was added to the reactor at once and stirred for 10 min to ensure uniform dispersion. The refined magnesium solution was transferred to the reactor and stirred at 350 r / min. At this time, the temperature inside the reactor was 40 ℃. The remaining 82.62 kg (95%) of liquid alkali was added dropwise at a uniform rate for a total of 3 h. The temperature of the system was programmed to rise from 40 ℃ to 58 ℃ as the alkali was added. After about 2 h of liquid alkali addition, the stirring speed was gradually reduced from 350 r / min to 250 r / min to facilitate uniform grain growth without breakage. S3. Constant temperature curing: After the liquid alkali is added, the system temperature is maintained at 58-60 ℃, the stirring speed is maintained at 250 r / min, and the constant temperature curing is carried out for 3 h to obtain the cured slurry. (III) Post-processing: The matured slurry is hot-filtered through a 1 μm filter membrane with a driving force of 0.3-0.4 MPa. Most of the sodium chloride, soluble sulfate, and residual sodium hydroxide are separated out with the high-temperature mother liquor. 165L of hot mother liquor is separated out. The primary wet filter cake is collected and weighed to obtain 39.8 kg. The primary wet filter cake is put back into the pulping tank, and 39.8 kg of pure water pre-cooled to 6 ℃ is added. The mixture is vigorously stirred and pulped for 30 min. At this point, the trace amounts of soluble salts remaining between the crystals are fully dissolved in the low-temperature water. Simultaneously, the low temperature induces the destabilization and precipitation of trace amounts of silica colloids, which are then adsorbed onto the crystal surface. After slurry preparation, this low-temperature slurry is immediately filtered a second time through a 1 μm filter membrane. The low-temperature conditions reduce the permeability of the colloids, ensuring that impurities are efficiently retained, resulting in a secondary wet filter cake. The secondary wet filter cake is then dried in a 110 ℃ hot air circulating oven for 8 h until the moisture content is <0.3%. After airflow pulverization, 18.96 kg of high-purity ultrafine magnesium hydroxide powder is obtained, with a calculated total magnesium yield of 97.8%. Sodium oxalate oxalate (C2O4) 2- It reacts with calcium ions in the brine to form calcium oxalate precipitate (CaC2O4, Ksp=2.3×10). -9The oxalate ions were removed as a filter cake during the in-situ hot filtration in step one; the excess oxalate ions that did not participate in the reaction existed in the liquid phase as sodium salts and were discharged with the mother liquor from the first-stage hot filtration; the small amount of oxalate ions remaining in the entrainment liquid of the filter cake were mostly removed during the pulping and washing process of the second-stage cold filtration because the solubility of sodium oxalate in water decreases with decreasing temperature (the solubility is about 2.2 g / 100 g water at 0 ℃), and most of them were still dissolved in the low-temperature washing water; the trace oxalate method (diazotization-azo spectrophotometry, GB / T 9730-2007) was used for detection; The fluoride ions (F) of sodium fluoride - It reacts with calcium ions to form calcium fluoride precipitate (CaF2, Ksp=3.9×10). -11 Similarly, fluoride ions are retained during hot filtration; excess fluoride ions exist in the mother liquor as sodium fluoride and are discharged with the first-stage hot filtration; residual fluoride ions in the filter cake are dissolved in the washing water during the second-stage cold filtration and pulping; and are detected by fluoride reagent spectrophotometry (HJ488-2009). Tripotassium citrate is a highly water-soluble organic acid salt. It mainly plays a role in crystal facet selective adsorption in the system, but the adsorption is physical adsorption and the binding force is weak. In the secondary cold filtration stage, strong slurrying with pre-cooled pure water can desorb the citrate ions adsorbed on the magnesium hydroxide surface and dissolve them in the washing water, which is then discharged with the filtrate. HPMC is a non-ionic, water-soluble cellulose ether that dissolves in cold water to form a transparent solution. It acts as a steric hindrance in the system and only physically adheres to the particle surface. During the pulping and washing process of secondary cold filtration, HPMC is redissolved in low-temperature water and removed, with the content being far below the threshold that affects product performance. The core testing indicators for the final product are: Mg(OH)₂ purity of 99.62% and CaO content of 0.0028% as determined by titration, and SO₄²⁻ content. 2- Content 0.04%, Cl - Content 0.07%, C2O4 2- Content 0.003%, F - The residual amount is 0.001%, and the residual amount of citrate is 0.02%; if Figure 1 As shown, the median particle size D50 is 1.309 μm, and the particle size distribution spacing (D90-D10) / D50 is 1.56. Figure 2 As shown, the specific surface area of ​​BET is 6.8 m². 2 / g, whiteness 97.2%.

[0021] Example 2 The difference between this embodiment and Example 1 is that the amount of sodium oxalate added is 255 g, and the mass ratio of sodium fluoride is 25.5 g. The remaining operations are the same, yielding 18.93 g of high-purity ultrafine magnesium hydroxide powder. The calculated overall magnesium yield is 97.6%, and the purity of Mg(OH)₂ measured by titration is 99.59%. Figure 3 As shown, the median particle size D50 is 1.154 μm.

[0022] Example 3 The difference between this embodiment and Example 1 is that the amount of sodium oxalate added is 383 g, the mass ratio of sodium fluoride is 38.25 g, and the rest of the operation is the same. 18.91 g of high-purity ultrafine magnesium hydroxide powder was obtained. The total yield of magnesium was calculated to be 97.5%, the purity of Mg(OH)2 measured by titration was 99.65%, and the median particle size D50 was 1.123 μm.

[0023] Example 4 The difference between this embodiment and Example 1 is that the amount of alkali solution added in the first step accounts for 8% of the total amount, and the amount of alkali solution added in the second step accounts for 92% of the total amount. The rest of the operation is the same, and 18.93 g of high-purity ultrafine magnesium hydroxide powder is obtained. The calculated total yield of magnesium is 97.6%, the purity of Mg(OH)2 measured by titration is 99.60%, and the median particle size D50 is 1.085 μm.

[0024] Example 5 The difference between this embodiment and Example 1 is that the total amount of alkali solution added is 84.49 kg, and the initial addition amount is still 5%, i.e., 4.22 kg. The rest of the operation is the same, and 18.60 g of high-purity ultrafine magnesium hydroxide powder is obtained. The total yield of magnesium is calculated to be 95.9%, the purity of Mg(OH)2 is measured to be 99.62% by titration, and the median particle size D50 is 1.122 μm.

[0025] Example 6 The difference between this embodiment and Example 1 is that the total amount of alkali solution added is 91.11 kg, and the initial addition amount is still 5%, i.e. 4.56 kg. The rest of the operation is the same, and 19.02 g of high-purity ultrafine magnesium hydroxide powder is obtained. The total yield of magnesium is calculated to be 98.1%, the purity of Mg(OH)2 measured by titration is 99.66%, and the median particle size D50 is 1.215 μm.

[0026] Example 7 The difference between this embodiment and Example 1 is that the amount of tripotassium citrate added is 44.6 g, and the rest of the operation is the same. 18.93 g of high-purity ultrafine magnesium hydroxide powder was obtained. The total yield of magnesium was calculated to be 97.6%. The purity of Mg(OH)2 was measured to be 99.51% by titration, and the median particle size D50 was 1.271 μm.

[0027] Example 8 The difference between this embodiment and Example 1 is that the amount of tripotassium citrate added is 111.5 g, and the rest of the operation is the same. 18.91 g of high-purity ultrafine magnesium hydroxide powder was obtained. The total yield of magnesium was calculated to be 97.5%. The purity of Mg(OH)2 measured by titration was 99.68%, and the median particle size D50 was 1.242 μm.

[0028] Example 9 The difference between this embodiment and Example 1 is that the total amount of composite additives used is 57.96 g, the amount of tripotassium citrate added is 44.58 g, and the amount of HPMC added is 13.38 g. The rest of the operation is the same, and 18.95 g of high-purity ultrafine magnesium hydroxide powder is obtained. The calculated total yield of magnesium is 97.7%, the purity of Mg(OH)2 measured by titration is 99.50%, and the median particle size D50 is 1.198 μm.

[0029] Example 10 The difference between this embodiment and Example 1 is that the total amount of composite additives used is 154.56 g, the amount of tripotassium citrate added is 118.89 g, and the amount of HPMC added is 35.67 g. The rest of the operation is the same, and 18.89 g of high-purity ultrafine magnesium hydroxide powder is obtained. The calculated total yield of magnesium is 97.4%, the purity of Mg(OH)2 measured by titration is 99.70%, and the median particle size D50 is 1.078 μm.

[0030] Example 11 The difference between this embodiment and Example 1 is that the heating temperature in step (II) S1 is 40 ℃, the heating temperature in S2 is 60 ℃, and the aging time in S3 is 2.5 h. The remaining operations are the same, and 18.93 g of high-purity ultrafine magnesium hydroxide powder is obtained. The total yield of magnesium is calculated to be 97.6%, the purity of Mg(OH)2 is 99.55% as measured by titration, and the median particle size D50 is 1.214 μm.

[0031] Example 12 The difference between this embodiment and Example 1 is that the heating temperature in step (II) S1 is 38 ℃, the heating temperature in S2 is 55 ℃, and the aging time in S3 is 3.5 h. The other operations are the same, and 18.91 g of high-purity ultrafine magnesium hydroxide powder is obtained. The total yield of magnesium is calculated to be 97.5%, the purity of Mg(OH)2 is 99.63% by titration, and the median particle size D50 is 1.171 μm.

[0032] Comparative Example 1 This comparative example uses a conventional one-time feeding and constant temperature reaction mode, without segmented temperature control, and the rest of the operation is the same as in Example 1; Specific operating steps: Take 100 L of high-magnesium bittern (same as in Example 1), add 304 g of sodium oxalate and 32 g of sodium fluoride, and stir for 30 min; add 86.97 kg of 32% liquid alkali to the bittern in one go, and simultaneously start the jacket heating to maintain the system temperature at a constant 55 ℃, and stir at 350 r / min; after reacting at a constant temperature for 2 h, add the same composite additive solution as in Example 1, and continue stirring for 1 h; use the same hot and cold double-stage filtration, drying, and pulverization as in Example 1; Final product testing indicators: purity 99.45%, D50 3.6 μm, particle size distribution (D90-D10) / D50 2.3, BET specific surface area 11.2 m². 2 / g.

[0033] The median particle size of Comparative Example 1 is much larger than that of Example 1 (1.32 μm), and the diameter spacing (2.3) is much larger than that of Example 1 (1.08), indicating a significantly wider particle size distribution. This demonstrates that the three-stage gradient temperature control process of the present invention plays a decisive role in achieving an ultrafine, narrow particle size distribution, and its effect is significantly better than that of the traditional single-stage isothermal reaction.

[0034] Comparative Example 2 This comparative example uses a route that first adds a purifying agent to the brine, filters out calcium salt precipitates, and then synthesizes magnesium hydroxide. That is, the active seed crystals generated in the purifying stage are not retained. The rest of the operation is the same as in Example 1. Specific operating steps: Take 100 L of high-magnesium bittern (same as in Example 1), add 304 g of sodium oxalate and 32 g of sodium fluoride, and stir for 30 min; filter the above slurry through a 0.8 μm filter membrane, separate and discard the filter cake containing calcium salt precipitate, and obtain clear and refined magnesium liquor; transfer all the refined magnesium liquor into a reaction vessel, heat to 35 ℃, add 4.35 kg of 32% liquid alkali at once, and then add the remaining liquid alkali dropwise, heat at the same gradient, and mature in the same manner as in Example 1; add the same composite auxiliary agent solution, and perform the same hot and cold double-stage filtration, drying, and pulverization; Final product testing indicators: purity 99.48%, D50 1.85 μm, magnesium yield throughout the process: 91.2%.

[0035] The magnesium yield of Comparative Example 2 was 91.2%, which was significantly lower than that of Example 1. The yield loss mainly came from two steps: first, mechanical loss caused by magnesium-containing mother liquor being carried in the calcium salt filter cake during the impurity removal and filtration process; second, the surface of the discarded filter cake adsorbed some magnesium ions, and the lack of seed crystals led to the loss of fine particles through filtration during the subsequent precipitation process. This proves that the coupled process of in-situ impurity removal and seed crystal retention of the present invention can significantly improve the magnesium yield, which is superior to the traditional route of first removing impurities and then synthesizing.

[0036] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0037] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A process for preparing high-purity ultrafine magnesium hydroxide via a low-temperature gradient reaction, characterized in that, Specifically, the following steps are included: (I) Brine pretreatment: Sodium oxalate and sodium fluoride are added to high magnesium bitter brine and stirred to react and obtain brine slurry; (II) Low-temperature gradient reaction S1. Premixing reaction: The brine slurry is heated and stirred, a portion of alkaline solution is added, and after heating, it is filtered to obtain refined magnesium solution. S2, Crystal nucleus growth: Add the refined magnesium liquid and composite additive solution to the reactor and stir, then add the remaining alkaline solution, heat and continue stirring; S3. Constant temperature curing: After the remaining alkali solution is added, the mixture is stirred at a constant temperature to cure the slurry. (III) Post-processing: The matured slurry is filtered, the filter cake is collected, and dried to obtain high-purity ultrafine magnesium hydroxide.

2. The process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction according to claim 1, characterized in that, The high-magnesium bittern composition mentioned in step (I) includes: Mg 2+ Concentration 60-100 g / L, Ca 2+ Concentration 0.8-1.5 g / L, SO4 2- Concentration 10-35 g / L, Cl - Concentration 150-220 g / L.

3. The process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction according to claim 1, characterized in that, In step (1), the mass ratio of the high-magnesium bittern to sodium oxalate is 1:0.002-0.003; the mass ratio of the high-magnesium bittern to sodium fluoride is 1:0.0002-0.0003.

4. The process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction according to claim 1, characterized in that, Step (II) The alkali solution is a 32% NaOH solution. The total mass of the alkali solution added is 102-110% of the theoretical mass of sodium hydroxide required to precipitate all magnesium ions in the high-magnesium bittern into magnesium hydroxide. The alkali solution added in step S1 accounts for 5-8% of the total amount.

5. The process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction according to claim 1, characterized in that, The composite additive solution is prepared by dissolving tripotassium citrate and HPMC in pure water and stirring until a transparent and viscous solution is obtained. The mass ratio of HPMC to tripotassium citrate is 1:2-5; the amount of the composite additive solution added is 0.3-0.8% of the theoretical mass of magnesium hydroxide produced.

6. The process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction according to claim 1, characterized in that, In step (ii) S1, the heating temperature is 35-40 ℃, the stirring speed is 350 r / min, and the temperature rise is 60 ℃.

7. The process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction according to claim 1, characterized in that, In step (ii) S2, the heating temperature is 55-60 ℃ and the stirring speed is 250 r / min.

8. The process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction according to claim 1, characterized in that, The ripening time in step (II) S3 is 2.5-3.5 h.

9. The process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction according to claim 1, characterized in that, The filtration in step (iii) is divided into two filtration processes, using a 1 μm filter membrane for thermal filtration.

10. The process for preparing high-purity ultrafine magnesium hydroxide by low-temperature gradient reaction according to claim 1, characterized in that, The purity of the high-purity ultrafine magnesium hydroxide is ≥99.5%.

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