Research On Preparation Of Magnesium By Vacuum Thermal Reduction Of Calcined Dolomite And Magnesite

Magnesium and its alloy have numerous unique properties, such as low density, good vibration damping, superior mechanical properties and recycling property. Magnesium are widely applicable that will be the third engineering material following steel and aluminum. The methods of extract magnesium are two different technologies:molten salt electrolysis and vacuum thermal reduction of MgO. Dolomite and magnesite are rich in natural resources with high grade that preparation of magnesium by vacuum thermal reduction has been the major method. Pidgeon process is a kind of vacuum thermal reduction process as the leading role of magnesium production in China with high energy consumption and high waste emission. Using Al or its alloy as reductant for vacuum thermal reduction of magnesium production can reduce energy consumption, reduction residue and CO2emission obviously. In present paper, the experiment research on vacuum thermal reduction process and analysis of reduction mechanism, which dolomite and magnesite are using as raw materials while the reductant are ferrosilicon alloy, Al-Si-Fe ternary alloy and aluminum powder; the leaching process of alumina from vacuum aluminothermic reduction residue is investigated and make comparison with Pidgeon process.Using experiment to research on calcinations parameters of dolomite, magnesite and huntite which on the basis of phases analysis, chemical composition analysis, differential thermal analysis and thermogravimetric analysis. The effects of calcinations parameters on metallurgy activity include ignition loss, hydration activity degree and loss rate of magnesium mineral under the conditions of mineral particle size1,4,8and12mm, calcinations temperature range from600to1120℃and calcinations time range from30to110min are studied. The optimal calcinations parameters of dolomite, magnesite and huntite with high metallurgy activity of CaOMgO and MgO are determined. The kinetic parameters and mechanism of non-isothermal decomposition process of dolomite is determined by using Coats-Redfern and Doyle mechanism equations. Decomposition process of dolomite is in accordance with Al model of Avrami-Erofeev theory that the nucleation and nucleus growing as the control steps of calcinations. Decomposition kinetic of dolomite is using to predict and determine the results of calcinations. The mechanisms of magnesium production by thermal reduction are analyzed by applying the thermodynamic principle and critical reaction temperatures of reduction reactions with different reductant are calculated. The mechanism of silicothermic reduction process is studied by the view of silicothermic reduction of MgO and silicothermic reduction of calcined dolomite. The result shows that CaO as the role of slagging constituent which reduce critical reaction temperature beyond600K and avoid the loss of MgO. Vacuum condition can decrease critical reaction temperature obviously. The feasibility of silicothermic reduction and effect of facrors on the shape of magnesium crystallization are further determined by calculation of vapor pressure and dew-point of magnesium. The feasibility of vacuum thermal reduction with Al-Si-Fe alloy and aluminum as reductant are confirmed by magnesium vapor pressure and critical reaction temperatures are calculated according to the thermodynamic principle.The experiment include three vacuum thermal reduction process, calcined dolomite as raw material with ferrosilicon alloy as reductant, calcined dolomite as raw material with Al-Si-Fe ternary alloy as reductant, a mixture of calcined dolomite and magneiste or calcined huntite as raw material with aluminum as reductant. The effects of reduction temperature, reduction time, briquetting pressure, added amount of reductant and added amount of villiaumite on reduction rate of magnesium and utilization rate of reductant are investigated. Kinetics of reductions theory at several stages in combination with phase analysis, EDS analysis and SEM analysis for researching on the mechanism and kinetics of reduction process, the control mechanism and kinetics equations of reduction process are obtained.The reduction residue which after vacuum aluminothermic reduction of calcined huntite or a mixture of calcined dolomite and calcined magneiste as raw materials for leaching alumina in alkaline solutions. The leaching mechanism of alumina from CaO·2Al2O3and CaOAl2O3in reduction residue is studied in combination with phases analysis of leaching slag to discuss the function of NaOH and Na2CO3. The effects of leaching temperature, leaching time, liquid-solid ratio, the added amount of NaOH and Na2CO3on leaching rate of alumina from reduction residue under normal pressure, obtain the optimal leaching parameter. The leaching rate of alumina from calcium aluminate is86%and88%of Al element at the optimal leaching parameter. Al(OH)3can form by react between leaching sodium aluminate and CO2, the whiteness degree of product beyond97%. The effect of reduction temperature on phases of reduction residues and leaching alumina are investigated by phase analysis, SEM analysis, EDS analysis of leaching slag and leaching rate of reduction residue under different reduction temperature.Energy consumption cost and production cost of vacuum aluminothermic reducion are compared with current Pidgeon process. Aluminothermic reduction of magnesium make better on exhaust gas emission, waste residue emission and energy consumption:the consumption of energy decrease60%and production cost reduce4000￥t. New technology is a low carbon, little pollution and high efficiency green technology which characteristic of lower production cost, lower pollution emission, higher production efficiency, higher utilization rate of energy and raw materials can be recycled.