Basic Research On Preparation Of Magnesium By Vacuum Carbothermic Reduction From Dolomite

Magnesium and it’s alloys are honoured as the most potential "green engineeringmaterials".With the application of magnesium and it’s alloys extend continually, thedemand for magnesium increase substantially. This greatly promotes the progress intechnology for magnesium manufacturing. At present, the main methods of magnesiumpreparing comprise electrolytic route and vacuum thermalreduction, and both havesome problems such as high cost, big consumption, and serious pollution. In order toimprove the recovery of metallic maguesium, reduce cost, shotren the process flow, andexplore a high efficient, inexpensive, environmental protection way to producemagnesium, this paper put forward a new method of preparing magnesium by vacuumcarbothermic reduction,thermodynamics and kinetics of the new method were studiedsystematically. Metallic magnesium is obtained by vacuum carbothermic reductionexperiment, and the morphology of the production is analyzed.The main contentsstudied and the important conlusion are as follows:（1） Thermodynamic, kinetics analysis and experimental study on direct thermaldecomposition of dolomite and vacuum thermal decomposition of dolomite with carbonwere implemented. The results show, the initial decomposition temperature of vacuumthermal decomposition of calcium carbonate and magnesium carbonate in dolomite withcarbon average decrease about412and85K compared with direct thermaldecomposition of calcium carbonate and magnesium carbonate when the pressure is10¹00Pa.The process of Vacuum decomposition of dolomite mixed with carbonmaterials has advantages compared with the traditional high-temperature calcinationprocess of dolomite.（2） Thermodynamic on vacuum carbothermic reduction producing metallicmagnesium was calculated and analyzed. The results show, the critical reactiontemperature of MgO and CaO is respectively1354,1532K when the system pressure is10Pa; the critical reaction temperature of CaO increase about178K compared withMgO. Magnesia can be reduced when the reduction temperature is from1373K to1523K and the pressure is10Pa, while calcium oxide can not be reduced.Thedecomposition products of dolomite can be used to prepare magnesium.（3） The experiment on vacuum carbothermic reduction producing metallicmagnesium was studied. The results show, adding CaF₂, increasing the reaction temperature and prolonging the reaction time can obviously increase the reaction rate ofreduction and promote the process of the reaction. The reduction rate did not improvesignificantly when the addition of CaF₂was more than5%. When the temperature islower than1473K, extending the reaction time, the reduction rates of increment are verysmall, whereas the reduction rates of f increment are very large. Reaction for60min,the reduction rate can up to83.7%, and the eduction rate did not significant increasewhen prolong the reduction time.（4） It gets metallic magnesium when system pressure is10Pa and reactiontemperature is1473K, and then the product was analyzed. The results show, the shapeof the product in the condensation wall is sheet and pin. The sheet magnesium is finecrystallized, the growth characteristic of which is nuclear growth form, and the Mgcontent of which can up to95%. The production was condensed better.The morphologyof the pin magnesium in the below side of the condensation wall was feathery withloose structure. The magnesium content of the pin production is44.33%, and the surfaceof which is oxidized.（5） The reaction mechanism and kinetics on vacuum carbothermic reductionproducing metallic magnesium was studied. The results show, the activation energiesare451.12⁵28.54kJ·mol^-1under the control of gasification diffusion, the activationenergy is190.28kJ·mol^-1under the control of the boudouard reaction of carbon, theactivation energy is219.71kJ/mol under the control of chemical reaction. The activationenergy under the control of gasification diffusion is the maximum, so the gasificationdiffusion is the rate-determining step of the overall reduction process.