Synthesis and Determination of Thermodynamical Properties of the Compounds of the system Ca-Mg-Bi
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Synthesis and Determination of Thermodynamical Properties of the Compounds of the system Ca-Mg-Bi C. Ramírez1, J. A. Romero1, A. Hernández1, F. Pérez2 ESIQIE-IPN, Z.C. 07738 Tel. 57296000 ext. 55270, México, D.F. 2 University of the State of Hidalgo, Tel. 01-771-717-2000, ext. 2271, México. E-mail: [email protected] 1
ABSTRACT Bismuth is an element obtained as a sub-product in lead production; Mexico occupies the second position in the world in production of this element. Bismuth is used as iron, aluminum and copper alloying, in the pharmaceutical industry, in the cosmetics industry, etc. Bismuth is separated from lead by the Kroll-Betterton Process in which a Ca-Mg alloy is added to the melting lead to form the intermetallic compounds Ca3Bi2, Mg3Bi2 and Ca2MgBi2 which float to the surface of the bath. Unfortunately, there is little thermodynamical information of the compounds of the system Ca-Mg-Bi which can be used to study and optimize the Kroll-Betterton process in a theoretical way. In this work there were synthesized the compounds Ca3Bi2, Mg3Bi2 and CaMg2Bi2 using powders of pure elements (Ca, Mg and Bi) in the required amounts and melted under an inert atmosphere. After synthesis, the samples were characterized by X-Ray Diffraction to ensure the formation of the desire compounds. Later, calorimetric technique was used to determine the thermodynamical properties of the compounds. The results obtained by XRay Diffraction show the formation of Mg3Bi2 and Mg2CaBi2 species; however, there is no crystallographic information of the compound Ca3Bi2. The heating curves obtained by calorimetry show endothermic peaks, due to the presence of phases changes as is indicated in the Ca-Bi, Mg-Bi and Ca-Mg-Bi phases diagrams. Keywords: Bi, Mg, Ca, calorimetry, phase equilibria. INTRODUCTION Bismuth is mainly obtained as a sub-product of the refining of lead. This is because the lead and bismuth are elements with similar properties so that mineral deposits are associated with bismuth and lead. The common method for separating bismuth from lead is called “KrollBetterton Process” which uses calcium and/or magnesium to remove bismuth from lead bath. Kroll-Betterton Process consists in adding a calcium-magnesium alloy to form intermetallic compounds Ca-Mg-Bi of high melting point and lower density than lead. The standard specification of many countries stipulate a maximum of 0.005% bismuth in the metallic lead, nowadays, this is the most widely used process to recover bismuth from lead. Figure 1 shows the phase diagram Ca-Bi[1] where the compound Ca3Bi2 is formed with a composition of 60% at. Ca and 40% at. Bi. Figure 2 shows the phase diagram Mg-Bi[2] with the compound Mg3Bi2 with 60% at. Mg and 40% at Bi, this compound exhibits an allotropic transformation above 700 °C. Figure 3 shows the phase diagram Ca-Mg-Bi[3] where the ternary compound CaMg2Bi2 is formed with a composition of 20% at.Ca, 40% at. Bi and 40% at. Mg.
3
1400
1200°
Temperature, °C
1200
Liquid 1000
842°
786°
800
12
600
400
CaBi3
Ca3Bi2
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