Synthesis Of High Purity Magnesium Hydroxide

Magnesium hydroxide is a very important chemical raw material and an additive agent inmodern industry, it has a function of flame retardant, smoke suppression, filling and otherfunctions, besides it is non-toxic, harmless and non-corrosive. It has great developmentpotentialities and high economic and social value, so that it becomes a hot topic in recent years.Aiming at the existing state of light calcined power, market potentiality and main problemsexisting in productive process of magnesium hydroxide, light calcined power is chosen as rawmaterial in this article. Magnesium sulfate is gotten after extraction of magnesium under acidleaching and separation of impurities and then it reacts with ammonia to synthesis magnesiumhydroxide. Ultimately, high purity magnesium hydroxide is obtained through hydrothermalmodification.The optimum conditions of acid leaching for light calcined power is obtained throughexperiments: under normal pressure, the concentration of sulfuric acid was (wt%)10%, theratio of sulfuric acid to ammonium sulphate at7:3, the ratio of mixed acid to magnesium at1.4:1, the reacting temperature under75℃, the reacting time was40min, the stirring intensityof450r·min-1.The extracted rate of Mg can up to96.29%under these conditions. The result ofX-ray diffraction (XRD) indicates that the calcined sample is magnesium sulfate.According to the fact that different metal ion hydrolyzes at different pH value, whenrefined crude magnesium sulfate with ammonia solution, strictly controlling the pH value ofthe solution at6and maintaining water bath temperature at60℃to remove impurities such asiron and aluminum. Then adjusting pH to about8.5, heating to boiling and then filtered. Afterevaporation, crystallization and calcination, the purity of refined magnesium sulfate can up tomore than99.93%.The effects of the concentration of refined magnesium sulfate, the reacting temperature,the dosage of ammonia and the stirring intensity on the yield of magnesium hydroxide werestudied through experiments. The optimum experimental conditions were obtained: theconcentration of magnesium sulfate was1.5mol·L-1, the reaction temperature under50℃, thereaction time was50min, the ratio of ammonia to magnesium sulfate at3:1and the stirringspeed of450r·min-1. The apparent average particle size of magnesium hydroxide aggregates with card room is about24.55μm, the purity can up to99.9%and the yield of magnesiumhydroxide is90.16%. The effects of different dispersants including anionic, cationic, nonionicand complex-type dispersants on magnesium hydroxide have been studied and the crystal formand appearance of magnesium hydroxide have no change, but the crystalline polarity andcrystallinity have changed, it perhaps indicates that the countra-ion in magnesium sulfate has apredominate effect.The effect of hydrothermal modification on crystal shape and morphology of magnesiumhydroxide synthesized under optimum conditions has been studied through experiments, andthe optimum conditions of hydrothermal modification is obtained: the hydrothermal time of12h, the hydrothermal temperature of220℃, the sodium hydroxide concentration of6.0mol·L-1. Magnesium hydroxide with the characteristic of weak polarity, more regularmorphology, good dispersion, excellent crystallinity and thermal stability is obtained.Apparent average particle size of magnesium hydroxide is2.16μm, the purity of magnesiumhydroxide is over99.94%and the yield is88.13%.