Study On The Dehydration Of Bischofite By Butanol Solvent Complex-Ammonia Chelate Crystallization

Anhydrous magnesium chloride is an important substance that can be used in many industries such as metallurgy, chemistry and medicine. It is very significant to research anhydrous magnesium chloride preparation technology from bischofite. The complex reaction mechanisms between solvent and bischofite are discussed, the rationality of n-butanol as complex dehydration solvent is demonstrated, the solvent complex-ammonia chelate crystallization dehydration technology is studied, and the effects of ultrasound on bischofite dehydration process is also analysed in this dissertation. The main conclusions and findings can be summarized as follows:(1) Anhydrous magnesium chloride can not be obtained while gaseous ammonia is directly introduced into bischofite. On the contrary, the bischofite will acutely hydrolysis, as a result, oxychloride of magnesium, MgOHCl, will appear.(2) The complex substitution reactions between bischofite and some organic solvents may occur. This is the theoretical basis of solvent complex dehydration.(3) Selecting complex dehydration solvent from low-carbon alcohols (LCAs) is logical due to the facts that the solubility of bischofite in LCAs is bigger, and the LCAs are polar ligand solvent which can substitute the coordinated water of bischofite. Being one of LCAs, the n-butanol is selected to be complex hydration solvent in this study due to its higher boiling point than that of water, fewer reaction tendencies with magnesium chloride, and lower toxicity & volatility.(4) During the continuous distillation dehydration of mixed solution containing bischofite and butanol at 101.3KPa, the dehydration percentage of bischofite(DPB) will increase with the amount of butanol adding, so, keeping lower concentration of MgCl₂ is suitable for obtaining higher DPB. The first four coordinated water molecules in bischofite can be removed easilier than the last two, especially the last one that is substituted by butanol molecules with much more difficulty. It is very difficult to remove all coordinated water molecules during this process.(5) Pretreating the bischofite butanol solution with ultrasound is effective on reducing the additive amount of butanol and increasing the DPB. In this work, the optimum pretreatment time is 15 min. After pretreatment with ultrasound, the DPB can reach 80% when the additive amount of butanol is only 3 times of original butanol(OB); in order to reach the same DPB, the additive amount of butanol should be 6 times of OB if having no ultrasound pretreatment. When the additive amount of butanol is 10 times of OB, the DPB of bischofite butanol solution pretreated by ultrasound can reach 90.25%, while the DPB only reach 84.86% if having no ultrasound pretreatment.(6) More than 99.5% coordinated water molecules in bischofite can be removed by the technology combined butanol complex and ammonia chelate crystallization, and the content of MgO in anhydrous magnesium chloride can be lower than 0.5%. The optimum dehydration technology includes two steps: (a) butanol complex dehydration. Primarily, the bischofite and a given amount of butanol solvent (1:11, mol/mol) are mixed. Then the mixed solution is pretreated with ultrosound for 15 min. Subsequently the pretreated solution is heated to boiling (100-120 °C). In order to keep the constant mol ratio of MgCl2:C4H9OH during the boiling(distillation) process, continuously supplying butanol to mixed solution is necessary, the suitable additive amount of butanol is controlled at 10 times of OB. (b) preparation of anhydrous magnesium chloride by ammonia chelate crystallization and de-ammonia. The gaseous ammonia is introduced into the treated mixed solution so as to obtain white crystal, the mol ratio of NH3:MgCl2 should be controlled at 15-20. After the white crystal is dried at 120°C for 6h, it is heated with 6°C/min to 400 °C, and at 400 °C for 60 min.