Magnesium hydroxide is an important chemical raw material and additive in modern industry. It has the functions of flame retardant, smoke suppression, filling, etc. . It is non-toxic, harmless and non-corrosive, and has great development potential, with high economic and social value, it has become a research hotspot in recent years.
In view of the abundant light-burned powder resources, the market potential of the Magnesium hydroxide and the main problems existing in the production, the refined magnesium sulfate solution was obtained from light-burned powder by acid leaching and impurity removal, the Magnesium hydroxide was synthesized by reacting with ammonia and then modified by hydrothermal method to obtain high purity Magnesium hydroxide.
The optimum conditions of acid leaching of light-burned powder were obtained by experiments: at atmospheric pressure, the concentration of sulfuric acid (mass concentration) was 10.0% , the ratio of sulfuric acid to ammonium sulfate was 7:3, the ratio of magnesium acid to ammonium sulfate was 1.4:1, the reaction temperature was 75 ° C, the reaction time was 40 min, the agitation intensity was 450 r · min -1, and the leaching rate was 96.29% . By X-ray diffraction (XRD) detection, the calcined sample is magnesium sulfate. According to the difference of pH value in hydrolysis of different metal ions, when purifying crude magnesium sulfate solution, the impurities such as iron and aluminum were removed by using ammonia water in constant temperature bath with pH value of 6,60 ° C, the purity of the refined magnesium sulfate solution reached 99.93% after evaporation, crystallization and calcination.
The effects of the concentration of the refined magnesium sulfate solution, the reaction temperature, the amount of ammonia and the stirring speed on the yield of the Magnesium hydroxide were experimentally studied, the optimum conditions for the synthesis of Magnesium hydroxide were determined as follows: magnesium sulfate concentration 1.5 mol · L-1, reaction temperature 50 â ° C, reaction time 50 min, ratio of ammonia to magnesium 3:1, and rotation 450 R · Min-1. The product was a Magnesium hydroxide aggregate with an average particle size of 24.55 μm. The purity of the product was over 99.9% and the yield was 90.16% .
The effects of various types of additives, including anionic, cationic, non-ionic and complex additives, on the Magnesium hydroxide were studied. No change in the crystal form and morphology of the Magnesium hydroxide was observed, however, the change of crystal polarity and crystallinity may be due to the main role of antiions in magnesium sulfate. The effect of hydrothermal modification on the shape and morphology of the Magnesium hydroxide was experimentally investigated. The optimal conditions for hydrothermal modification of the Magnesium hydroxide were determined as follows: hydrothermal time 12 h, hydrothermal temperature 220 ° C, the sodium hydroxide concentration was 6.0 mol · L-1. The obtained samples have small polarity, regular morphology, good dispersion, high crystallinity and good thermal stability. The average apparent particle size is 2.16 μm, the purity is over 99.94% , and the yield is 88.13% .