There are many methods for synthesizing hexagonal magnesium hydroxide. The most commonly used method is precipitation, which is cheap, simple and easy to operate, and easy to control the crystal morphology. In the precipitation method, the main method is salt solution precipitation, usually using strong alkali, such as ammonia or sodium hydroxide. The most widely used magnesium salts are magnesium chloride, magnesium sulfate and magnesium nitrate, and organic magnesium salts such as magnesium acetate are occasionally used.
Magnesium nitrate and ammonia are used to react to generate magnesium hydroxide, and the reaction rate is detected by pH and electrical conductivity. The increase in the solubility of the product ammonium nitrate slows down the nucleation rate. This is because the dissolution of nitrate produces nitrate ions, and anions affect the nucleation rate of precipitation. After the mass fraction of nitrate reaches 15%, the rate no longer changes. Surface modification will not change the morphology and size of the crystal, but will only affect the nucleation rate.
Messi Biology uses sodium hydroxide and magnesium chloride as precipitants to generate magnesium hydroxide. The addition of sodium chloride increases the crystallinity of the crystal, and this method can increase the yield of the product. The particle size of the product flaky magnesium hydroxide is about 6-30um, and the specific surface area is 8.79m2/g.
The concentration of salt and alkali is the key to obtain high-purity magnesium hydroxide. The size of magnesium hydroxide particles mainly depends on the ratio of salt and alkali and the mixed concentration. When MgSO4 is used, the size of the product drops significantly when the concentration is very low and the salt-alkali ratio is very high; when the salt concentration is 0.01 mol/l, all that is obtained is fibrous crystals; above this concentration, fibrous, flaky and even tubular magnesium hydroxide can be obtained.
Messi Biology stated that magnesium hydroxide is prepared by recycling asbestos waste and using 20% sodium hydroxide solution as a precipitant. Although experimental studies have shown that the concentration of magnesium ions is the key to preparing magnesium hydroxide with different morphologies. However, low-concentration alkali solution can also obtain highly dispersed, small-particle magnesium hydroxide products.