Preparation method of magnesium hydroxide

（1） In industry, seawater is often reacted with cheap calcium hydroxide solution (lime milk) to obtain magnesium hydroxide precipitation.
The brine lime method involves the precipitation reaction of pre purified and refined brine and lime milk made from lime treated with digestion and slag removal in a sedimentation tank. Flocculants are added to the obtained slurry, which is thoroughly mixed before entering the sedimentation tank for separation. After filtration, washing, drying, and crushing, magnesium hydroxide finished product is obtained. The chemical reaction equation is:
MgCl2+Ca(OH)2→CaCl2+Mg(OH)2↓
The brine ammonia method uses brine that has been purified to remove impurities such as sulfate, carbon dioxide, and a small amount of boron as the raw material. Ammonia is used as a precipitant for precipitation reaction in a reactor, and a certain amount of crystal seeds are added before the reaction and thoroughly stirred. The ratio of brine to ammonia is 1: (0.9-0.93), and the temperature is controlled at 40 ℃. After the reaction is completed, a flocculant is added, and the precipitate is filtered, washed, dried, and crushed to obtain the finished magnesium hydroxide product. The chemical reaction equation is:
MgCl2+2NH3·H2O→Mg(OH)2↓+2NH4Cl
This experimental method needs to improve yield, shorten washing cycle, improve and perfect production process. Magnesite ore produced by magnesite hydrochloric acid ammonia method is calcined with anthracite or coke in a shaft kiln to produce magnesium oxide and carbon dioxide. Bitter soil powder is mixed into a slurry with water and reacts with a specified concentration of hydrochloric acid to prepare a magnesium chloride solution. The magnesium chloride solution reacts with a certain concentration of ammonia water in a reactor, and the product is washed, settled, filtered, separated, dried, and crushed to obtain magnesium hydroxide product. Surface treatment agents can be added as needed for surface treatment.
Magnesium hydroxide is an inorganic flame retardant filler with good application prospects for polymer based composite materials. Like aluminum hydroxide, magnesium hydroxide flame retardant plays a flame retardant role by absorbing heat and releasing water from chemical decomposition when heated, so it has the advantages of non-toxic, low smoke, stable chemical properties of magnesium oxide generated after decomposition, and no secondary pollution. However, compared to halogenated organic flame retardants, in order to achieve a considerable flame retardant effect, the filling amount generally needs to reach more than 50%. As magnesium hydroxide is a inorganic compound, its surface has poor compatibility with the polymer matrix. If such a high filling amount is not modified on its surface and filled into the polymer material, the mechanical properties of the composite will decline. Therefore, it is necessary to carry out surface modification treatment on it to improve its compatibility with polymer based materials, so that the mechanical properties of the filling material do not decrease, and even improve some of the mechanical properties of the material.
Experiments have shown that unmodified magnesium hydroxide exists in the form of aggregates in PP. Although the powder itself is extremely fine, there is a clear boundary or even a cavity between the particle aggregates and the PP matrix due to the incompatibility between the particle surface and the PP matrix. The void formed after the escape of magnesium hydroxide particles during brittle fracture indicates that the unmodified magnesium hydroxide only plays a role of filling and flame retardancy in PP, and does not form chemical bond with PP. After surface modification, magnesium hydroxide is evenly dispersed in the PP matrix, and the particles are mostly dispersed in the form of primary particles or small agglomerated particles in the material.
（2） A new process for preparing magnesium hydroxide from dolomite. Dolomite is calcined at 950 ℃ for 2.5 hours, with a digestion ratio of 1:40, a digestion temperature of 70 ℃, and a digestion time of 50 minutes; The molar ratio of hydrochloric acid dosage to calcium ion during the first acid leaching is 2:1, and the molar ratio of sulfuric acid dosage to magnesium ion during the second acid leaching is 1:1; The pH value of the precipitation process solution is 11. Under this process condition, the maximum yield of magnesium hydroxide can be obtained, and the total yield is more than 85.20%, and the flaky magnesium hydroxide with ordinary purity and Dispersity can be obtained. Under optimal conditions, the carbonization process achieves a magnesium extraction rate of 90.02% in the calcium magnesium separation process, and a product yield of 88.21% in the magnesium hydroxide preparation process. The selected carbonization temperature is 30 ℃, and the precipitant is ammonia water. This results in a magnesium hydroxide product with good purity, good dispersibility, and flaky shape.