Research On The Preparation Technology Of Magnesium Chloride Hexahydrate And Light Magnesium Oxide From Serpentine

Serpentine minerals are typical Mg-rich layer silicate minerals, and they consist of a silicate tetrahedral sheet and a brucite-like magnesium octahedral sheet. The separation of magnesium and silicon in the mineral was achieved by Titrating after HCl leaching of the mineral. The structure of serpentine before and after acid leaching were characterized by XRD. The acid leaching solution was further purified to obtain high purity magnesium chloride hexahydrate and light magnesia. The economic benefit analysis and environmental analysis were also taken in the paper.The main contents and results of experiments were as follows:(1) Various parameters of acid-leaching serpentine like temperature, time and the dosage of hydrochloric acid have been studied thoroughly and conditions for the maximum recovery of magnesium have been established. It has been found that at 90-100℃with 3-3.5 hours of retention time and using theoretical dosage of hydrochloric acid, it is possible to extract 98% of magnesia from the mineral. The other conditions as follows: stirring speed was maintained over 500 rpm, the mass ratio of liquid to solid equal to 3-4 and the size of particle less than 125μm. The recovery of magnesia was 92% in this step. The structural transformation of serpentine in the acid leaching process was characterized by XRD. According to the results, the structure of serpentine was almost destroyed after acid leaching. The soluble components, like magnesium, existed in the solution. And silicon, which converted into amorphous silicon dioxide (SiO₂·nH₂O), existed in the residues.(2) The process of preparing magnesium chloride hexahydrate from acid leaching solution has been investigated. The precipitant was magnesium hydroxide in the purified step, and the adding sequence was solid firstly, then liquid. The conditions which have great influences on the quality of magnesium chloride hexahydrate were also studied thoroughly, and the optimal conditions were as follows: the cooling was terminated at 20-40℃and the cooling rate was 2-4℃/min, stirring speed was 30-40 rpm, supersaturation of the solution was 40.8-41.00%, the obtained sample was dried at 70-80℃with 1-1.5 hours. The purity of magnesium chloride hexahydrate was over 46% and the quality of the main component accorded with the requirements of national standard, GB 8453-87. The recovery of magnesia was over 74% in this step.(3) The process of preparing light magnesia from acid leaching solution, using ammonia as precipitant, have been investigated. The conditions which have great influences on the quality of light magnesia were studied thoroughly, and the optimal preparation conditions were as follows: the molar ratio between ammonia and NH₄HCO₃ equal to 0.10-0.22, the amounts of ammonia were 1.0-1.1 times of theoretical dosage, the concentration of Mg²⁺ was 25-35 g/L, the reaction temperature and pyrolysis temperature were 50-55℃and 80-90℃, independently, the desiccation conditions was at 130-150℃with 3-5 h, the dried sample was calcined at 800-850℃with 3-3.5 hours. The purity of basic magnesium carbonate was over 42% and the main component meets the requirements of industrial standard for premium, HG/T2959-2000. The recovery of magnesia was over 90%. The purity of light magnesia was over 98% and the main component meet the requirements of industrial standard for premium, HG/T 2573-94.(4) It was also considered that the equipment choice, economic benefit analysis and environmental analysis in the processes of preparing magnesium chloride hexahydrate and light magnesia in this paper.