A New Technology Of Magnesium Production By Smelting Reduction Of High Silica Dolomite

China is the largest primary magnesium producer in the world, and the primarymagnesium mainly produced by Pidgeon process. With the growing advancement ofscience and technology and the continuous deterioration of hunan environment, thetechnological backwardness Pidgeon process has became one of national limiteddevelopment projects in China. Combining with rich high-silica dolomite resources inWansheng of Chongqing, this paper developed a new technology of magnesiumproduction by smelting reduction, which has high productivity, low energy consumptionand low pollution. In order to provide references for effective utilization of high-silicadolomite in Chongqing area, some of the basic phenomena and laws of smeltingreduction in silica bath were studed in this paper and the results are as follows:①The chemical composition of Ca, Mg and Si in Wansheng dolomite were21.06%,12.66%and1.6%, respectively, which has the n（CaO）/n（MgO）ratio of1.00. Theimpurity of Si was distributed unevenly with higher chemical composition level indolomite. The analysis of chemical composition indicated that Wansheng dolomite wasa typical high-silica dolomite. Detected by XRD, the mian phase of Wansheng dolomitewas CaMg（CO₃）₂, and the impurity phase was SiO₂. When Wansheng dolomite wasdecomposed by heating, the phase of CaMg（CO₃）₂decomposed into two phases of CaOand MgO. Then, thecombination reaction between CaO and SiO₂was occurred and newimpurity phase of Ca₂SiO₄was formed. The decomposition of Wansheng dolomite wasfrom740℃to880℃end with the weight loss rate of46.4%.②The order of impacts of various process factors on quality of calcined dolomitewas: calcining temperature, dolomite particle seize and calcining time. When thedolomite of particle seize between6and13mm was calcined at1050℃for60min, thecalcined dolomite with high quality was obtained, which the decomposition rate anddegree of activity were98.41%and32.07%, respectively.③CaO and MgO in calcined dolomite are alkaline earth oxides and have veryhigh melting temperatures. In order to get molten slag under the experimentalconditions, it was required to mix appropriate Al₂O₃and SiO₂with calcined dolomite.Through software simulation, thermodynamic analysis and experimental verification,it’s found that the unmelted solid reduced gradually with the addition amount of Al₂O₃and SiO₂increasing and CaO dissolved in the slag before MgO. It’s could got molten slag which contained about55%calcined dolomite and45%slagging agent at1600℃.④The smelting reduction was thermodynamically analyzed. Under the idealstate without considering the activity of the reactants and products in the liquid, thestarting temperature of silicothermic reduction of MgO in molten slag achieved1925℃under atmosphere. When the system pressure reduced to1×10⁴Pa, the reductionreaction could be conducted in1550℃. In actual smelting reduction process, theactivity of substance had important impact on reduction reaction and could continuouschange as reduction proceeding. It’s could use lower system pressure to assure that thelatter stage of smelting reduction could be carried out smoothly.⑤The experiments of silicothermic smelting reduction were carried out undervacuum and the influence of various process factors on reduction extent of MgO in slagwere studied. The chemical composition of slag had important impact on smeltingreduction. Under condition of slag containing more SiO₂, the reduction reaction couldbe restrained because of activity of SiO₂increasing. Reduction rate of MgO couldimprove as the ratio of Al₂O₃/SiO₂increasing. By orthogonal experiment, it’s could gotthat the order of impacts of other various process factors on reduction extent of MgOwas: reaction temperature, the addditon amount of75%ferrosilicon alloy, reaction timeand the addition amount of CaF₂.The experimental results showed that the idealcomposition of slag was55%CaO·MgO-35%Al₂O₃-10%SiO₂and reduction extent ofMgO in this slag was achieved up to97.3%under such optional condition of reactiontemperature of1600℃, n（Si）/n（2MgO）ratio of1.2, reduction time of120min and addingCaF₂of3%, reduction extent of MgO was achieved up to97.3%.⑥The kinetics of silicothermic smelting reduction were studied and the resultsindicated that silicothermic smelting reduction was second order chemical reation andthe reduction extent of MgO inceased by increasing either one of the following factors:the initial mass ratio of Al₂O₃/SiO₂, the addition of CaF₂, the initial molar ratio ofSi/2MgO, and reaction temperature. The overall smelting reduction was controlled bymass transfer in slag with an apparent activation energy586kJ/mol.⑦In silicothermic smelting reduction, the dew point temperature ofmagnesium vapor increased as reaction temperature increasing. When reduction processwas carried at1600℃, the corresponding dew point temperature of magnesium vaporwas870℃, which higher than magnesium melting point（651℃） nearly200℃.If thecoolling temperature of magnesium vapor was controlled at651℃, magnesium vapor of96%converted into liquid, the other vapor could condensated directly to solid by decearing coolling temperature below651℃.⑧A500g-calss and a10kg-class furnace for vacuum smelting reduction weredeveloped. The500g-calss laboratory small furnace had4kW power heated by siliconmolybdenum rod and the furnace temperature was increased to1600℃at full power in1h. The10kg-class furnace which had continuous feeding charge had30kW powerheated by graphite heater and could achieved to the maximum temperature of1800℃.