| In China,75% of sponge titanium production capacity is contributed by the companies which buy raw TiCl4 to produce sponge titanium by Kroll method. But the Kroll method needs high equipment investment and consumes a large number of energy in the MgCl2 electrolysis process. The companies sell the molten MgCl2 as a kind of waste at low price after cooling. This causes the waste of resource and energy. A new method named of "situ pyrolysis of molten magnesium chloride" was proposed to solve this problem. The molten MgCl2 produced by vacuum distillation in magnesium thermal reduction process was treated by the direct oxidation thermal decomposition method to produce ultrafine MgO with high purity and chlorine gas. The magnesium produced by thermal reduction of MgO can return to the boiling chlorination of high titanium slag. So, the new cycle of magnesium and chloride in titanium sponge production can be achieved.Based on the thermodynamic analysis of the gas phase reaction between O2 and MgCl2, this paper contains the single factor experiments and orthogonal experiments. The optimal reaction condition was determined and the impact of each factor on the pyrolysis process was studied. The dynamics of the reaction was also explored. XRD, SEM, particle size analysis, and adsorption-desorption analysis were used to characterize the nature of the products. The conclusions are as follows:(1) Thermodynamic calculations were made to study the pyrolysis reaction.The laws derived are as follows:From 750K～1773K, the gibbs free energy change is less than 0. In the vicinity of 1000K, the equilibrium constant Ink:for the reaction reaches the maximum value, which is 3.4. When the temperature is higher than 987K, the endothermic reaction turns to exothermic reaction because of MgCl2 phase transition at 987K. By calculating the saturated vapor pressure of MgCl2, the gas phase reaction happens below its boiling point. The Si-O-C1 system and Mg-O-Cl system predominance area diagrams were calculated at different temperatures by using HSC Chemistry software and the equilibrium composition of different initial ratio between O2 and MgCl2 were also calculated.(2) Oxygen flow rate, oxygen blown time and the reaction temperature were studied in the single factor experiments. The optimal reaction condition is at the temperature of 1200℃, at the oxygen flow rate of 0.25m3/h, and at oxygen blown time of 30min. By operating the improved experiment of the optimal experiment in the single factor experiments, certain magnesia was obtained. The D50 is 18.32 μm and the particles of the magnesia is of different sizes. MgO content of the product is 98.95%. The product quality of the improved experiment is between AR magnesia and GR magnesia and its comprehensive index is better than industrial superior magnesia product.(3) Using the three factors and three levels orthogonal experimental scheme to study the influence of the three influencing factors, the conclusion is:the reaction temperature impacts on the yield of highly significant, oxygen flow rate significantly impacts on the yield of magnesia.(4) The formula of the industrial magnesium chloride is MgCl2 ·O.39H2O by the analysis of TG-DTA and chemical analysis. The industrial magnesium chloride has three kinds of magnesium chloride containing crystal water and they are MgCl2·4H2O, MgCl2·2H2O and MgCl2·H2O. The DTA curves were analyzed by the Freeman-Carroll differential method. When the heating rate is 5℃/min, the apparent activation energy of the chemical reaction equals to 267.18kJ · mol-1 and the reaction order equals to 0.64. When the heating rate is 10℃ /min, the apparent activation energy of the chemical reaction equals to 223.81kJ· mol-1 and the reaction order equals to 0.93. The vapor synthesis mechanism of MgO powder particles was studied. The curve between the activation energy and the gas phase supersaturation degree and the curve between the critical nucleus diameter and the gas phase supersaturation degree were all calculated. |
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