Experimental Investigation On The Aluminothermic Reduction Of Magnesia Under Vacuum Condition

Magnesium and its alloys offer advantages of low density, high specific strengthand toughness, shielding against electromagnetic radiation and high dampingcharacteristrics. Magnesium and its alloys are considered the lowest constructionmetals. To meet the resources concerns as well as environmental considerations in thecurrent China, it is important to develop a new method to produce primarymagnesium. Recently, a new process was developed to produce primary magnesiumusing dolomite ores as the raw materials. The magnesia and calcia are firstly seperatedfrom the calcined dolomite. Then the magnesia is used to produce primary magnesiumby thermal reduction method and calcia was used to make light weight calciumcarbonate as a coproduct. This work is focused on the production of magnesium bythermal reduction of magnesia using aluminium as a reducing agent.In this dissertation, the thermodynamic analysis for the aluminothermic reductionof magnesia was carried out. The Gibbs free energy change for the possible chemicalreactions was calculated and plotted as a function of temperature and pressure. Basedon the thermodynamic analysis, the reduction temperature and magnesia to Al molarratio for the aluminothermic reduction experiments was designed. The effects of MgOto Al molar ratio, reduction temperature, reduction time and the briquettes preparationpressure on the aluminothermic reduction of magnesia were studied experimentally.The chemical composition, phase constitution and morphology of the condensedmagnesium and the briquettes after thermal reduction were investigated byinductively coupled plasma mass spectroscopy, x-ray diffraction, scanning electronmicroscopy equipped with energy dispersive spectrometry and transmission electronmicroscopy.The aluminothermic reduction of magnesia is a heterogeneous chemical reactioninvolving gas species. The magnesia to Al molar ratio, reduction temperature,reduction time as well as briquettes preparation pressure play important roles in thealuminothermic reduction of magnesia. The reduction ratio of magnesia increaseswith the decrease in MgO to Al molar ratio, the increase in reduction temperature, the prolongation of reaction time and the increase in the briquettes preparation pressure.When the magnesia to Al molar ratio is3:2and the briquettes preparation pressure is200MPa, after5hrs aluminothermic reduction at1200°C, the reduction ratio ofmagnesia reaches up to96%.The reduction of magnesia by aluminum is probably taken place in two stages.At the first stage, the MgO react with aluminium to form magnesium vapor and spinelMgAl2O4. At the second stage, the spinel MgAl2O4reacts with aluminium to formmagnesium vapor and Al2O3. Trace amount of θ-Al2O3was detected in samplesreacted at1200°C and the mechanism for the formation of θ-Al2O3is still underinvestigation.The obtained condensed magnesium is well crystallized and its purity meets therequirement of the China national standard GB/T3499-2003Mg9990.