Study On The Preparation Of High Grade Magnesium Hydroxide From Low Grade Magnesite

Contaminated environment and huge waste of resource are caused by lots of magnesite. As an important kind of inorganic protactinides, the magnesium hydroxide has a wide application as flame-retardants, antacids, heavy metal precipitants, desulphurizer, the raw material to produce magnesium oxide and the additive agent for cosmetic and food. In recent years, magnesium hydroxide has attracted more and more attention. Aiming at the existing state of magnesite resource, market potentiality and main problems existing in preparation of Mg(OH)2, the subject has studied systematically preparing process of Mg(OH)2. In this article low grade magnesite from Liaoning province is chosen as raw material. Magnesium sulfate solution is gotten after extraction of magnesium under acid leaching and separation of impurities. And then with the precipitant ammonia, magnesium hydroxide is synthesized by direct precipitation method.Firstly the technics condition of sulfuric acid leaching magnesite was discussed through lots of experiments. The optimum experimental conditions were obtained:the gangue granularity 96μm, the reacting time 4h, the concentration of sulphuric acid (wt%) 40%, the reacting temperature 80℃, the ratio of liquid to solid (4:1) and the whisking speed 400 rpm min-1. The extracted rate of Mg was over 95% under these conditions. The result of XRD indicated that the sample were MgSO4·7H2O and SiO2.Next, row magnesium sulfate solution is purified. The pH values of different hydrates were not same when metal ions are transformed into hydrates and this was used to purify row magnesium sulfate solution. Hydrogen peroxide was used as oxidizer and ammonia was used to adjust pH value to remove ions and aluminum.The thermodynamics study of magnesium hydroxide deposited indicated the reaction of the ammonia-deposited under normal pressure is feasible. In the other hand, the reaction was also belonged to ion-deposited reaction, getting a high velocity, and the technological flow of high grade magnesium hydroxide by ammonia deposition was decided according the principle. Finally, effects of the concentration of magnesium sulfuric, the reacting temperature, the reacting time, the concentration of ammonia, the aging temperature and the aging time on precipitation ratio of Mg2+ was studied by condition experiments. The optimum experimental conditions were obtained:the reacting temperature 50℃, the concentration of Mg2+ 1.5 mol·L-1, the reacting time 45min and the concentration of ammonia 20%, the aging temperature 60℃and the aging time 60min. The sample magnesium hydroxide was purified perfect crystal and had hexagonal crystalline system. The quality of magnesium hydroxide was over 99.6%. The magnesium hydroxide was dispersed well and particles were regular sphericity. The average particle radius is 2μm.