A Study On The Dolomite Calcination Process And A New Technology Of Product Magnesium Through Thermal Reduction Process Based On The LCA Theory

This article using life cycle assessment (LCA) evaluated the energy consumption and theimpact on environmental in the entire life cycle of magnesium from ore mining to productionabandonment. Aimed at the abundant dolomite mineral resources in changbai mountainarea，this paper investigated the mineral composition, microstructure, thermal decompositionand the decomposition products. A new process for producing magnesium from dolomite isdesigned based on the existing thermal reduction magnesium method. Combined with lifecycle assessment (LCA), both the energy consumption and environmental impact on theentire life cycle from ore mining to abandonment is reasonably analyzed. These resultsprovide a certain valuably reference for the reasonable exploitation of dolomite, effectivedevelopment, sustainable utilization and magnesium metal smelting industry.Following the principles and framework of life cycle assessment (LCA), this paper putforward a green design decision-making model and an LCA general model for heatingmethod to extract magnesium, based on the valuation of the producing technology, the lifecycle of environmental impact during the process from ore extraction to preparing,producing, using, disposing and reproducing magnesium and magnesium alloy, energyconsumption analysis and economic cost. Introducing the idea of industrial ecology into themanufacture of magnesium and its alloy, this paper proposes the related assessment indicatorsystem, bringing integrated use of natural resources, ecological restoration andreconstruction in industry into LCA theoretical framework. In this system, a virtuous circleof green material-green processing-green recycling is formed; an energy efficient,environmentally friendly and highly efficient magnesium alloy producing system isestablished; the combination of economic and social benefits is well achieved. Based on theexisting basis, we expounded the work centers future of green design and greenmanufacturing from the points of new magnesium alloy materials and process equipment,regeneration theory and technology and green design software design, which will promotethe industrialization of the production of magnesium and magnesium alloy in our country.the dolomite mine from Changbai the mass fraction of C, O, Mg and Ca were14.8%,50.1%,12%and22.5%, respectively, average magnesium content is13.18%, calcium and magnesium ratio of dolomite is essentially1:1, containing a small amount of impuritieswhich is mainly composed of SiO2and Al2O3. which is an ideal material for silicon thermalmethod of metallurgy of magnesium metal. Ore belongs to crystalline dolomite type,Itappears to be crystal rhombohedral, dense-particle structure, parallel, and completerhombohedral. Most of the diamond crystal surface appears to be rock candy structure,visible cleavage crack mutually interlaced structure formation. Owing to lower energy areneeded for priority decomposition along cleavages, the process of calcination are stable atdifferent temperatures. As a result, dolomite from Changbai Mountain area is an idealmaterial for silicon thermal method of metallurgy of magnesium metal.In the dolomite calcination process, with the increase of temperature, the grains of themain product MgO grow rapidly, and finally forming large grains. At the same time, becauseof the thermal motion of molecules, the calcination temperature and the intermolecularkinetic energy increase, which affects the distribution rule of the grains of MgO. When thecalcination temperature is over750℃, the formation of MgO is sensitive to the calcinationtemperature. After calcined for3hours constantly under930℃, the dolomite formed intoaggregate structure, resulting in the surface overburned, thereby reducing the reactingactivity of the transformation from the dolomite into MgO.Through the control of thecalcination temperature and the calcination time during the dolomite calcining experiment,the optimal calcining parameters, which will obtain active MgO and CaCO3, are: calcinationtemperature700℃, constant temperature time3h.Through research on the microstructure of dolomite,combined with calcinedexperimental result at different calcining parameters,show that there is a certain relationshipbetween the microstructure of dolomite and calcined quality,metallurgicalproperties.dolomite minerals which is mainly composed of aphanitic particles calcined atdifferent temperature parameters, calcination are stable at different temperatures mainlybecause of Lower energy are needed for priority decomposition along cleavages.Otherwise,significant crystalline, especially dolomite mineral which is composed by large particles, hasa poor calcined performance, difficult in calcination process comparied with hidden crystalwhichis composed of small particles, the required calcination time is relatively long.Becausein the process of calcination of dense great grain dolomite, the diffusion prcess of CO2iscarried in the interior of crystals slowly, due to the uneven heating of rooms inside thecrystal grains, the different orientation lead to that the process of hermal decomposition goon slowly and significant crystalline dolomite which is composed of large grains is easy to crushing and pulverizing after calcined.Based on the study of the thermal decomposition mechanism of the dolomite inChangbai Mountain area, we found that in the low temperature stage(570～730℃), thereaction equation is CaMg(CO3)2=CaCO3+MgO+CO2and the reaction mechanism isinterface contraction mechanism. The CO32-is the rate-controlling step in the relocationdiffusion. In the intermediate temperature stage (730～750℃), the reaction equation isCaMg(CO3)2=CaCO3+MgO+CO2and CaCO3=CaO+CO2. The two reactions happencollaboratively. Namely, interface contraction mechanism and diffusion control mechanismsexist at the same time. The speed control step of the reaction is determined by Mg2+, O2-,CO32-, Ca2+and CO2. In the high temperature stage (750～930℃), the reaction equation isCaCO3=CaO+CO2. The reaction mechanism is diffusion control mechanism. CO2is therate-controlling step of the diffusion.This article using life cycle assessment, invented a new process using dolomite meltingmagnesium, aiming to overcome the disadvantages existing in magnesium preparation,which are low efficiency, high energy consumption, long production cycle and pollute theenvironment. The method includes crushing dolomite, vacuum light calcined at lowtemperature, recycling of carbon dioxide, cooling and heat recovery, mixing milling, hot mixbriquetting at high temperature, vacuum reduction and magnesium block and slag utilization.Vacuum low temperature light calcination is to calcinate the crushed dolomite lightly for1～3hours under calcination temperature600～700℃and900～1000℃and the pressure10000～60000Pa in the furnace by using continuous vacuum furnace. The MgCO3andCaCO3in dolomite will break down successively. The reaction equation is CaMg(CO3)2=CaCO3+MgO+CO2↑and CaCO3=CaO+CO2↑, obtaining a mixture of CO2, MgO and CaO.The CO2from floated in the furnace by heating dolomite is recycled. After coming out of thecontinuous vacuum furnace,the mixture were directely poured into heat exchanger whichwas filled with non-oxidizing protective gas for cooling. After cooling process, the calcineddolomite,reducing agent and flux were directly put into the mixing machine which wasfilled with non-oxidizing protective gas at room temperature to get mixture. The mixture wasbriqueted after grinding. Then put the Mixture into reduction device to react with thereducing agent (ferrosilicon). After reduction reaction we will get magnesium vapor. Thereaction equation is2(MgO·CaO)+Si(Fe)=2Mg↑+2CaO·SiO2+(Fe), magnesium vapor willbecome collectable metal magnesium after coming into the condenser condensate. The residue in the reduction tank can be made into calcium silicate fertilizer and mold hardenerby grinding.