Study of Morphology and Magnesium Purity, Formed by Vapor Phase Through Silicothermic Reduction
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INTRODUCTION
THE use of magnesium in different industries, such as aerospace and automobile manufacturing, is significant due to its distinctive characteristics and properties such as light atomic weight and high strength/density ratio.[1,2] Furthermore, pure magnesium is widely used in the production of strategic and important metals such as uranium and titanium.[3] Commercial magnesium was produced through two main approaches, i.e., electrolysis and thermal methods.[4] The thermal methods consist of two techniques of silicothermic and carbothermic.[5–7] Silicothermic reduction requires less capital cost and is easily produced at small scales.[3,7] Compared to other methods, this method is more suitable for the production of high-purity (about 99.95 pct) magnesium.[3,8–11] The silicothermic reduction method extracts magnesium from dolomite, and as a reductant agent, ferrosilicon is used to reduce MgO in dolomite ores.[12–14] This process, the so-called Pidgeon process, is carried out in a reactor with external heating at a temperature of 1150 C to 1300 C.[3,15] This process is represented by Eq. [1]:
AMIR GORJI, MASOUD PANJEPOUR, and MEHDI AHMADIAN are with the Department of Materials Engineering, Isfahan University of Technology, 84156-83111, Isfahan, Iran. Contact email: [email protected] Manuscript submitted August 25, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
2CaOðsÞ þ 2MgOðsÞ þ SiðsÞ ! 2MgðgÞ þ Ca2 SiO4 ðsÞ ½1 This reaction is endothermic and is carried out in steel retorts. The final step is to condense magnesium vapor in a steel condenser to deposit magnesium crystals.[3,16,17] Various parameters, such as temperature, impurities, technical parameters, reductant agent, and reducing agent, are effective in the production of magnesium; furthermore, some have significant effects on the deposit of magnesium vapor. The effect of these parameters on the carbothermic process has been extensively studied. Yang et al.[18] studied the manner of magnesium formation in the carbothermic reduction of dolomite by using a multilevel condenser. The temperature among the levels was different (923 K, 1023 K, 1123 K, and 1223 K). Their results showed that the temperature gradient was effective for the purity and morphology of magnesium in the condenser. In other research, Yang et al.[19] studied the effect of the temperature on the morphology of magnesium nucleation by using the carbothermic process. The results showed that this process was controlled by the temperature and the rate of oxidation reduced when the temperature gradient decreased. Therefore, the quality of magnesium formed in these zones rises significantly at high temperatures. In another study, Yang et al.[20] studied the effect of temperature on the deposit of the impurities of alkaline metals, such as sodium and calcium, in different zones of the condenser by using the carbothermic process. This study revealed that the purity of the magnesium formed in a high-temperature zone was
Table II.
Composition of the Charged Briquettes in the Retort
Composition
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