Industrially, magnesium hydroxide precipitate is commonly obtained by reacting seawater with an inexpensive calcium hydroxide solution (milk of lime).
Brine-Lime Method: Pre-purified brine and milk of lime (prepared from lime that has undergone slaking and slag removal) are reacted in a precipitation tank. A flocculant is added to the resulting slurry; after thorough mixing, the mixture enters a settling tank for separation. The product then undergoes filtration, washing, drying, and pulverization to yield the final magnesium hydroxide product. The chemical reaction equation is:
MgCl2 + Ca(OH)2 → CaCl2 + Mg(OH)2↓
Brine-Ammonia Method: Brine that has been purified to remove impurities such as sulfates, carbon dioxide, and trace amounts of boron serves as the raw material. Aqueous ammonia acts as the precipitating agent in a reaction vessel. A specific amount of seed crystals is introduced prior to the reaction, accompanied by vigorous stirring. The ratio of brine to aqueous ammonia is maintained at 1:(0.9–0.93), and the temperature is controlled at 40°C. Upon completion of the reaction, a flocculant is added; the precipitate is then filtered, washed, dried, and pulverized to produce the final magnesium hydroxide product. The chemical reaction equation is:
MgCl2 + 2NH3·H2O → Mg(OH)2↓ + 2NH4Cl
This process requires further optimization to improve yield, shorten washing cycles, and refine the production technology. Magnesite-Hydrochloric Acid-Ammonia Method: Magnesite ore is calcined with anthracite or coke in a vertical kiln to produce magnesium oxide and carbon dioxide. The resulting calcined powder is mixed with water to form a slurry and then reacted with hydrochloric acid of a specified concentration to prepare a magnesium chloride solution. This solution reacts with aqueous ammonia of a specific concentration in a reactor. The reaction products undergo washing, sedimentation, filtration/separation, drying, and pulverization to yield the magnesium hydroxide product. Surface treatment agents may be added as required.