Hebei Meixi Biotechnology said that in the preparation of high-purity magnesium hydroxide (99%), under general conditions, it is not easy to prepare high-purity magnesium hydroxide by using lime milk precipitation, and it must go through a more complex purification process including washing and other processes), especially the problem of calcium reduction and boron. This problem is also related to raw materials. Under the same conditions, it is advantageous to use the bitter brine after seawater salt production as raw material to prepare high-purity magnesium hydroxide. When magnesium hydroxide is precipitated by ammonia method, the pH value is controlled to about 11 and the purification process (simpler than the lime milk method) can be easily prepared to high-purity magnesium hydroxide.
In the magnesium hydroxide purification process, the washing step is more likely to produce colloidal phenomenon, the particles become finer, which increases the difficulty of filtration, and it is necessary to try to prevent the occurrence of colloidal phenomenon, and maintain the stability of magnesium hydroxide structure and surface charge change during the washing process. From the comparison of methods, the crystalline magnesium hydroxide prepared by hydrothermal method is easy to prepare high-purity magnesium hydroxide due to less adsorption of impurities. The high-purity magnesium hydroxide prepared by precipitation method is also a medium product for the preparation of other high-purity magnesium series. For example, when the precipitation method is used to prepare high-purity magnesium oxide, the higher the purity of magnesium hydroxide, the more conducive to the preparation of high-purity magnesium oxide.
In general, the main production processes of magnesium hydroxide at present include direct precipitation method, magnesium-containing ore grinding method, magnesium oxide hydration method, etc.
Direct precipitation method.
At present, there are many methods for synthesizing magnesium hydroxide, among which the most used method is precipitation method, which is easy to control crystal morphology due to its low cost and simple operation. In the precipitation method, it is mainly the precipitation of salt solutions, usually using strong bases such as ammonia or sodium hydroxide, the most widely used magnesium salts are magnesium chloride, magnesium sulfate and magnesium nitrate, and the organic magnesium salt magnesium acetate is also occasionally used. The preparation of magnesium hydroxide by ammonia in the direct precipitation method is divided into one-step method and continuous precipitation method, which realizes the recycling of resources, reduces production costs and ensures the quality of products.
Hydrothermal method. The morphology and size of magnesium hydroxide can be effectively controlled by hydrothermal reaction, and the properties of the product mainly depend on the type of precursor magnesium salt, the solvent and the temperature control of the reaction process. The topography of the product depends mainly on the pH of the solution and the temperature of the reaction. By adjusting the size of pH, the morphology of magnesium hydroxide is synthesized in nanofloral, needle-like, sheet-like and spherical shapes. The advantage of hydrothermal synthesis is that magnesium hydroxide with a specific surface area greater than 100 m2 g can be obtained. The disadvantage is that high temperature and high pressure are used in industry, and the cost is higher.
Sonochemical synthesis.
The sonochemical method uses ultrasonic waves with frequencies in the range of 20kHz-10MHz to initiate the formation and collapse of microcells, and generate active sites at high temperature and pressure. Compared to traditional methods, this method occurs under extreme conditions and can greatly increase the rate of the reaction and generate small crystals with more uniform morphology. Sonochemical synthesis is characterized by the fact that different structural types of materials can be produced by changing the reaction medium.
Sol-gel method.
The sol-gel method uses highly reactive metal compounds as precursors, mixes them in liquid phase, hydrolyzes them, and condenses them to generate metal hydroxides. A stable transparent sol is formed, which is slowly polymerized by aging to form a network structure, which loses the solvent during the reaction and forms a gel. The gel is subsequently dried and sintered to prepare nanomaterials. The reaction process of this method is the dispersion and dissolution of precursors, hydrolysis to form monomers, polymerization, and the generation of sols, and the preparation of nano magnesium hydroxide materials after drying and heat treatment.