Preparing Mg-Li And Mg-Sr Alloy By Vacuum Silicothermic Process

In this study, a new method for preparing Mg-Li and Mg-Sr alloys was presentedbased on analysing vacuum silicothermic reduction of MgO,Li2O and SrO. According tothis new method, experimental equipments were designed for verification. A newtechnique, preparing Mg-Li and Mg-Sr alloys directly by vacuum silicothermicreduction, was formed through the theoretic analysis and experimental verification.<vacuum-synchronous hot reduction method for preparing Mg-Li alloys>,<vacuum hotreduction equipments>,and <blowing refining equipments> were applied patents.Furthermore, this method can also be applied to the techniques of preparing other alloysdirectly.The feasibility of silicothermic reduction of MgO, Li2O and SrO was analysed bythermodynamics. The result is Mg, Li and Sr cannot be reduced by Si from MgO,Li2Oand SrO under normal atmosphere, while Mg, Li and Sr can be reduced by Si fromMgO, Li2O,and SrO in vacuum with CaO slagging. According to the functionalrelationship among Gibbs Free Energy Change, temperature, and vapor pressure, therelationships between equilibrium vapor pressure logarithm and temperature on thecritical reaction conditions are confirmed as: lgPMg=6.3-4513.1/T， lgPLi=16.31-18646.9/T，lgPSr=10.53-11786.4/T,respectively.The conditions of vacuum synchronous silicothermic reduction of MgO,Li2Owere studied by thermodynamical analysis. When T>1412K， equilibrium partialpressures of Li> equilibrium partial pressures of Mg> system pressure; when T<1412K,equilibrium partial pressures of Mg> equilibrium partial pressures of Li> systempressure; when T=1412K, equilibrium partial pressures of Mg=equilibrium partialpressures of Li=1259Pa> system pressure. The conditions of vacuum synchronoussilicothermic reduction of MgO,Sr2O are: when T>1719K, equilibrium partial pressuresof Sr> equilibrium partial pressures of Mg> system pressure; when T<1719K,equilibrium partial pressures of Mg> equilibrium partial pressures of Sr> systempressure; when T=1719K, equilibrium partial pressures of Mg=equilibrium partialpressures of Sr=4715Pa> system pressure.The three-phrase equilibrium graphs of Mg, Li and Sr were plotted based on therelational expression between their saturated vapor pressures logarithm andtemperatures. The three-phrase equilibrium points of Mg, Li and Sr are: (923K-358Pa),(454K-1.8×10-8Pa) and (1042K-122Pa),respectively. The vacuumsynchronous condensate temperature was confirmed according to the conditions ofequilibrium partial pressures greater than saturated vapor pressure. When the reductiontemperature is1500K, the vacuum synchronous liquid condensate temperature of Mg-Liand Mg-Sr is T<1023K, the vacuum synchronous solidification condensate temperatureof Mg-Li and Mg-Sr are T<453K and T<924K, respectively.The reactions, happenned in the process of preparing Mg-Li and Mg-Sr byvacuum-synchronous silicothermic reduction, are really solid/solid response. Dynamicsanalysis show that, diffusion mass transfer speed of reaction materials is therate-controlling tache.According to experimental purpose and requirements, a equipment is designed,which can do vacuum melting, disused Mg recovering and vacuum thermal reduction.And for the equipment, the heating temperature is as high as1800℃,vacuum degree canreach10-2Pa,weight of reduzate can reach kilogram levels. There is a suit of metal vaporsolidification cover with condensation recovery reached96.9%. Moreover, a kind ofliquid condensate cover was presented on the basis of solidification cover.Throughdiversion trench of liquid condensate cover, metal products collected from liquefactionare exported for gas-refining.The feasibility of preparing Mg-Li alloys by vacuum silicothermic reduction wasstudied in experiments. After stirring and mixing the distribution raw material, keepingthem compaction in graphite crucible. Reduction was keeped for2hours in1500K and10Pa environment. The result is: residue weight is4474.2g, the content of Mg(wt%) is5.36%, Li(wt%) is0.27%measured by atomic absorption, and the main phase is CaSiO4measured by XRD, in addition, it also contains certain amount of Ca4Si2O7F2. Theobtained products weight is721.7g, the content of Mg(wt%) is83.94%, Li(wt%) is4.49%measured by atomic absorption, and the main phase are Li0.92Mg4.05and Li3Mg17measured by XRD. Growth pattern of Mg-Li products was studied through SEM andEDS. It was found that Mg-Li products grow pinnate, the hairiness is small with lessoxidation at the beginning condensation part, while the hairiness is large with muchoxidation at the last condensation part. The reduction rate and collection rat wascalculated according to Mg\Li weight in raw materials, products and residue. Reductionrate of Mg and Li was75.99%and76.69%, collection rate of Mg and Li was83.51%and84.16%, respectively. The experiments demonstrate it is feasible to prepare Mg-Lialloys with vacuum silicothermic reduction. The feasibility of preparing Mg-Sr alloys by vacuum silicothermic reduction wasstudied in experiments. After stirring and mixing the distribution raw material, keepingthem compaction in graphite crucible. Reduction was keeped for2hours in1500K and10pa environment. The result is: residue weight is4012.0g, the content of Mg(wt%) is5.87%, Sr(wt%) is0.32%measured by atomic absorption, and the main phase is CaSiO4measured by XRD, in addition, it also contains certain amount of Ca14Mg2(SiO4)8andCa13.5Ba0.3Mg1.8. The obtained products weight is692.7g, the content of Mg(wt%) is84.87%, Sr(wt%) is4.27%measured by atomic absorption, and the main phase areMg17Sr2and Mg5.2Sr measured by XRD. Growth pattern of Mg-Sr products was studiedthrough SEM and EDS. It is found that Mg-Sr products grow pinnate, the hairiness issmall with less oxidation at the beginning condensation part, while the hairiness is largewith much oxidation at the last condensation part. The reduction rate and collection ratwas calculated according to Mg\Sr weight in raw materials, products and residue.Reduction rate of Mg and Sr was75.03%and72.80%, collection rate of Mg and Sr was82.09%and81.32%, respectively. The experiments demonstrate it is feasible to prepareMg-Sr alloys with vacuum silicothermic reduction.