Study Of Crystallization Process Of Magnesium Hydroxide

It is abundant in the natural resource of magnesium chloride at Cha Er Han salt lake in China. In this thesis, magnesium hydroxide with high purity was synthesized via the reaction of NaOH and MgCl2 in order to utilize effectively the magnesium resource at Cha Er Han salt lake. Moreover, the mother liquor after crystallization of magnesium hydroxide could also be used for sodium chloride electrolysis, which was very important for chlor-alkali process.The solubility of MgCl2-NaCl under 25℃,40℃,50℃,60℃was measured to provide the basic information for the salting out of sodium chloride and the preparation of magnesium chloride solution. After salting out, sodium chloride crystal was uniform with the hexahedron morphology, the average particle size was 138.11μm, and the purity of sodium chloride crystal reach 99.80% after washing with the saturated sodium chloride solution.With MSMPR crystallizer, magnesium hydroxide with low-moisture content and high purity was synthesized continuously. The average particle size of the prepared magnesium hydroxide crystal was greater than 31.00μm. The effects of NaOH and MgCl2 concentrations, retention time and impurities on the property of the product were studied in details. It was found that the low supersaturation could be reached with the low NaOH concentration and high retention time. Under the low supersaturation condition, the nuclear rate of crystal was low and the growth rate of crystal was high. Magnesium hydroxide with high-purity and low-moisture content could be obtained when the washing temperature was set 50℃. The morphology of the prepared magnesium hydroxide was spherical assembled by small hexagon crystals.Based on the above-mentioned experimental data, the structure of crystallizer with Mg(OH)2 productivity of 140,000 tons/year was designed by simulation using Fluent6.30 software. In simulation, MRF (multi-reference frame) and standard k-εturbulence model were used to describe the strong swirling flows in the crystallizer. With the design of the propeller position from the crystallizer bottom and the rotational speed, the effects of operating parameters on the flow field distributions of single-phase and two-phase in the crystallizer were investigated.The simulation results for single-phase system showed that:with the increase of propeller height, the kinetic energy at the bottom of crystallizer decreased and the dead zone was expanded, which would affect the velocity distribution in draft tube. When propeller height lowered from H=4.3m to H=3.3m, the mixing was obviously improved. With further lower of propeller height, there was no more improve. In simulation for two-phase system, the Mg(OH)2 crystals with particle size distribution were introduced. The simulated results showed that:the particles entered from top to crystallizer were mainly divided into two parts, one part of particles would entered to the circulation area which finally discharged from the export, the other part of particles would discharged from the export. According to the simulation results, the optimum installed height of propeller was 3.30m.