Preparation of magnesium hydroxide

（1） In industry, magnesium hydroxide can be precipitated by reacting seawater with cheap calcium hydroxide solution (lime milk).
In the brine lime process, the lime milk made from the pre purified brine and the lime treated by digestion and slag removal is precipitated in the sedimentation tank. Flocculant is added into the slurry to be fully mixed, and then separated in the sedimentation tank. After filtration, washing, drying and grinding, the finished magnesium hydroxide is prepared. The chemical reaction equation is as follows:
MgCl2+Ca(OH)2→CaCl2+Mg(OH)2↓
Brine ammonia process takes brine which has been purified to remove sulfate, carbon dioxide, a small amount of boron and other impurities as raw materials. Ammonia is used as precipitant to carry out precipitation reaction in the reactor. Before the reaction, a certain amount of crystal seeds are put into the reactor for full stirring. The ratio of brine to ammonia is 1: (0.9-0.93), and the temperature is controlled at 40 ℃. After the end of the reaction, flocculant was added, the precipitate was filtered, washed, dried and crushed to obtain the finished product of magnesium hydroxide. The chemical reaction equation is as follows:
MgCl2+2NH3·H2O→Mg(OH)2↓+2NH4Cl
This test method needs to improve the yield, shorten the washing cycle and improve the production process. Magnesia and carbon dioxide are produced by calcining magnesia ore with anthracite or coke in shaft kiln. Magnesium chloride solution was prepared by the reaction of bitter soil powder with hydrochloric acid of specified concentration after being made into slurry with water. The magnesium chloride solution reacts with a certain concentration of ammonia in the reactor, and the product is washed, settled, filtered and separated, dried and crushed to obtain magnesium hydroxide product. Surface treatment agent can be added for surface treatment as required.
Magnesium hydroxide is a promising inorganic flame retardant filler for polymer matrix composites. Like aluminum hydroxide, magnesium hydroxide flame retardant plays a flame retardant role by chemical decomposition and heat absorption and water release when heated. Therefore, magnesium hydroxide has the advantages of non-toxic, low smoke and magnesium oxide generated after decomposition with stable chemical properties and no secondary pollution. However, compared with halogen-containing organic flame retardants, to achieve a considerable flame retardant effect, the filling amount should be more than 50%. Because magnesium hydroxide is inorganic, the compatibility between the surface and polymer matrix is poor. If the surface modification is not carried out for such a high amount of magnesium hydroxide, the mechanical properties of the composite will decline after it is filled into the polymer material. Therefore, it is necessary to carry out surface modification treatment to improve its compatibility with polymer base materials, so that the mechanical properties of filling materials do not decline, and even improve some mechanical properties of materials.
The results show that the unmodified magnesium hydroxide exists in the form of aggregates in PP. although the powder itself is very fine, there are obvious boundaries and even voids between the particle aggregates and the PP matrix due to the incompatibility between the particle surface and the PP matrix. The results show that the flame retardant of PP does not break due to the formation of a hole in PP due to the fact that the magnesium hydroxide does not break when the magnesium hydroxide particles are filled. The surface modified magnesium hydroxide dispersed uniformly in PP matrix, and the particles were mostly in the form of primary particles or small agglomerated particles.
（2） : a new process for preparing magnesium hydroxide from dolomite: calcined at 950 ℃ for 2.5 hours, the digestion ratio is 1:40, the digestion temperature is 70 ℃, and the digestion time is 50mins; the molar ratio of hydrochloric acid amount to calcium ion in the first acid leaching is 2:1, the molar ratio of sulfuric acid dosage and magnesium ion in secondary acid leaching is 1:1; the pH value of the solution in the precipitation process is 11. Under these conditions, the maximum yield of magnesium hydroxide can be obtained, the total yield is more than 85.20%, and the flake magnesium hydroxide with general purity and dispersion can be obtained. Under the optimal conditions, the extraction rate of magnesium is 90.02% and the product yield is 88.21% in the preparation process of magnesium hydroxide. The selected carbonization temperature is 30 ℃ and the precipitant is ammonia water. The product with good purity and dispersion can be obtained.