Smelting Metallic Magnesium Experimental Study Of Vacuum Metal Thermal Reduction Technology With Magnesite As Raw Material

As a kind of nonferrous metal, magnesium is referred to as the most promising green engineering material. It has such unique properties as light, tough, good heat conductivity, easily processed and recycled and a good recycling characteristics. Magnesium will be the largest engineering materials after iron and steel, aluminum. To extract metallic magnesium, two different technologies are commercially used:Pidgeon method in calcined dolomite and molten-salt electrolysis. An advantage of Pidgeon method is that low investment and simple techniques, so it is widely applied in China, but it still brings energy and resource wasting serious and more environmental pollution, etc. In the South China Liaoning province, there are plenty of magnesite stones and more grade higher. The research on smelting metallic magnesium process use a new method-internal resistance vacuum thermal reduction technology with South China Liaoning province magnesite stones as raw material.By applying the basic principle of thermodynamics, the Gibbs free energy and critical temperature of deoxidization of MgO with ferrosilicon are calculated. Added active CaO during the whole reduction process can obtain Ca2SiO4, make critical temperature decrease from3846K to2820K, avoid the reaction Mg2SiO4product and decrease the loss of magnesium under latm. Vacuum degree has a significant impact on reduction temperature. The calculation results show critical temperature is1358K when the vacuum degree of the reduction system is10.13Pa.The magnesite characteristics and thermal analysis properties are studied. By experiments examine the decomposition rate, burning rate, hydration activity degree and active degree of magnesite calcined process parameters under the condition of magnesite particle size range from0.2mm to8.0mm, calcining temperature range from600℃to800℃and time range from30min to70min, the optimal temperature and time of different size magnesite are determined. Formation mechanism of active magnesium oxide, including three stages of magnesium carbonate absorbing a large number heat, metastable magnesium oxide crystallizing and re-crystallization magnesium oxide sintering, is getting by analysis technology such as XRD and SEM, etc. The kinetic parameters and mechanism functions on magnesite non-isothermal decomposition process is obtained by using Coats-Redfern and Doyle equations. Magnesite decomposition process is in line with Avrami-Erofeev nucleation and nuclear growth as the control of steps Al model and kinetic equitation are expressed as da/dt=1.64×1012/5exp(-225060/RT)(1-α), with an apparent activation energy, E=225.06KJ·mol-1frequency factor, A=1.64×1012s-1. Magnesite thermal decomposition process gives a prediction at different calcination temperature on the decomposition kinetic equaition.The Limestone characteristics and thermal analysis properties are studied. By experiments examining the decomposition rate, burning rate, hydration activity degree and active degree of Limestone calcined process parameters under the condition of Limestone particle size range from0.2mm to8.0mm, calcining temperature range from600℃to800℃and time range from30min to70min, the optimal temperature and time of different size Limestone are determined. Decomposition rate, activity degree process parameters are getting under a condition of additive a small amount of CaF2、NaCl reagent to limestone and the elaborate mechanism of CaF2、NaCl is analyed, too. It derived limestone’ decomposition rate was98.51%, loss rate43.35%, hydration activity30.81%, active degree84.7%and CaF2、 NaCl added3%～5%of total materials when particle size is8.0mm and calcined60min under1100℃. Limestone decomposition process is in line with two-dimensional phase boundary reaction R2model and kinetic equitation is expressed as da/dt=3.34×109/5exp(-222380/RT)(1-α)1/2, with an apparent activation energy, E=222.38KJ· mol-1, frequency factor, A=3.34X109s-1. Lime stone thermal decomposition process gives a prediction at different calcination temperature on the decomposition kinetic equation.Discussion the feasibility on a vacuum process to smelt magnesium with magnesite and limestone as raw material in the paper and investigation factors on the impact of Mg reduction ratio and Si utilization ratio which including reduction temperature and time, FeSi added mount, briquetting pressure, mineralizer added, different reductants, added limestone. The result showed Mg reduction ratio could reach up to94.42%under reduction temperature1473K, vacuum degree10.13Pa, FeSi(Si,75%) as reducing agent, CaO/MgO mole rate1.4,1hour to reduce reaction. Mg product purity is98.36%. Slag mainly include Ca2SiO4和SiO2 which do not detect Mg2SiO4. Reduction process is in line with three-dimensional phase boundary reaction D3model and kinetic equitation are expressed as1-2/3a-(1-a)2/3=2.7×106exp(-313580/RT)t, with an apparent activation energy, E=313.58KJ/mol, frequency factor, A=2.7x106s-1The dolomite characteristics and thermal analysis properties are studied. By experiments examining burning rate, hydration activity degree of dolomite calcined process parameters under the condition of dolomite particle size range from1.0mm to8.0mm, calcining temperature range from700℃to1100℃and time range from50min to80min, the optimal temperature and time of different size dolomite are determined. Smelting metallic magnesium experimental factors on the impact of Mg reduction ratio and Si utilization ratio which including reduction temperature and time, FeSi added to mount, briquetting pressure, mineralizer added, different reductants, added limestone are investigated under vacuum thermal reduction with dolomite as raw material factors. The result showed the Mg reduction rate could reach up to93.70%under reduction temperature1473K, vacuum degree10.13Pa, FeSi(Si,75%) as reducing agent, CaO/MgO mole rate1.4,1hour to reduce reaction.Smelting metallic magnesium experiment is carried out with magnesite and limestone as raw material. It obtained that Mg reduction ratio and Si utilization ratio are more when magnesite is selected as raw material than dolomite as raw material.Some industrialization experiments for the production of magnesium from calcined magnesites and limestones were done on a novel vacuum furnace by using internal heating method. The optimum process parameters are obtained by adjusting the current, voltage and reduction time. The results show that its magnesium yield exceed90%with the working cycle of180min, that its power consumption is below9kWh/kg and thermal efficiency can reach up to50%.The life cycle assessment method was used to investigate the energy consumption and environmental loading of It magnesium production with internal resistance vacuum thermal reduction technology and Pidgeon technology. As a consequence, the consumption of the abundance energy with Pidgeon magnesium production process is1.48×105MJ/t-Mg ingot and greenhouse effect is the biggest one to the environment loading, accounts for the total amount49.65×. The most energy consuming stage is vacuum refining stage and calcination stage, and it occupies80.48%and15.21%for total energy consuming respectively. CO2， SO2, NOX and other exhaust emissions mainly come from coal combustion. Internal resistance vacuum thermal reduction technology use direct-contact heat reaction mass, reduce the heat loss and improve energy efficiency.