Magnesium Hydroxide Of High Grade And Extra Fine Obtained By Hydration Of Light-burned Magnesia Calcined From Magnesite

Magnesite is a kind of natural MgCO₃ mineral , which is abundant in our country. When magnesite is calcined, magnesium carbonate decomposes to form magnesia and carbon dioxide gas is liberated. Calcining temperature and residence time are the most important parameters which affect the activity of magnesia. When the activity of magnesia decreases, it is hard to react with water and the activity energy of hydration rises. Magnesium hydroxide has some advantages of retarding fire, reducing smoke, safe stuff and cheap price, so it has become an important inorganic flame retardant. Its market prospect is very promising. Presently sea water (brine) reaction with alkali and grinding of brucite are the main methods of producing super fine magnesium hydroxide flame retardant. Magnesia calcined from magnesite as raw material, manufacturing super fine magnesium hydroxide by hydration is a subject which needs in-deep research and has important industrial value. In this dissertation firstly after being ground by ball miller, magnesite powder is purified by a reverse flotation and normal flotation circuit. Mainly silicate minerals are removed in the reverse flotation, while in the normal flotation calcium-containing minerals, residual silicates and other impurities are gotten rid of. Obvious separation efficiency could be obtained using water glass, sodium hexametaphosphate as inhibitor and oleic acid as collector. The results show that in the final product the percentage of silicon and calcium decreases greatly. Especially the impurities of silicate minerals are almost removed completely and the purity of Mg(OH)₂ reachs 99.08%. Secondly effects of calcining temperature and residence time on the activity of MgO are investigated. On the condition that magnesite decomposes completely, the activity of MgO decreases when calcining temperature is too high or residence time is too long. It is noted that when calcining time is too short, magnesium carbonate probably doesn't decompose completely. The activity of MgO is measured by CAA and hydration method, and the product is confirmed as MgO by XRD. It is proved that the optimum calcining condition is 750℃and 1.5 h. Mg(OH)2 is produced by hydration of above mentioned MgO and XRD is used to analyze the product. The hydration curves of 30℃, 50℃, 70℃are gotten by experiments and the reaction rate constant under different temperatures could be calculated. Then according to Arrhenius equation the hydration activity energy of MgO is calculated, about 60.7KJ/mol. The hydration of MgO is controlled by chemical reaction. At last the effects of hydration conditions on the size and morphology of Mg(OH)2 are discussed. The grain size of Mg(OH)2 is characterized by polarizing microscope and Dongtu software, and SEM is used to study the micro size and morphology of product. The optimum condition of producing extra fine Mg(OH)2 by MgO hydration is 70℃, 2h, 0.5mol/L and no-ethanol existing in system. The effects of additives including alkali, magnesium salt and dispersant on the size and morphology of Mg(OH)2 are investigated. It is found that too much alkali in reaction solution is not preferrable for manufacturing super fine Mg(OH)2 and urotropine is the best alkaline additive. Certain amount of Mg2+ is beneficial to produce extra fine Mg(OH)2, but when the concentration of Mg2+ is too high, the size of Mg(OH)2 grain grows evidently. Polyglycol is the best dispersant and under the condition of optimum dosage, the average size of obtained Mg(OH)2 is 0.42μm and the percentage below 1μm is 99.67%. Using scanning electron microscope the size of Mg(OH)2 is 200nm～400nm and the shape is hexagonal plate. This kind of Mg(OH)2 is very fit for advanced flame retardant.