Purification of Magnesium by Vacuum Distillation
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I. INTRODUCTION
A
number of fields involve high purity magnesium: metallurgy, solid state physics, atomic energy, organic chemistry, biology and so forth. Several years ago we applied a zone melting process to purify magnesium samples previously industrially refined by means of subltmatton.' The metal obtained in this way was useful for numerous studies: preparation efficiency of organomagnesium compounds and stability of these compounds,2,3 formation and migration characteristics of vacancies as analyzed by quenching down to 4 K,4,5 recrystallization after deformation by rolling at several temperatures'v" and preparation of MgxZn1-xTe semiconductor compounda.v'' All these studies showed that even very small impurity contents significantly alter the properties of magnesium. The zone melting method satisfactorily removes a number of impurities, including iron, copper and zinc, but shows virtually no effect concerning manganese elimination. This was not a problem when we could start with an electrolytically produced metal, almost manganese free, but magnesium available in France today is produced by the "Magnetherm" process, applied to dolomite. This process leaves manganese contents of a few ten parts per million, even after industrial sublimation refining. It was therefore necessary to carry out a complementary purification on the metal obtained in this way before using zone melting. Consideration of the equilibrium vapor pressure vs temperature curves drawn for magnesium and its main impurities (Fig. 1),10 as well as some previous works,1l-14 led us to choose vacuum distillation as a purification method. II. EXPERIMENTAL WORK
pO
p1l"MQ 10
In order to study in well defined conditions the difG. REVEL is Maitre de Recherche, J.-L. PASTOL and J.-C. ROUCHAUD are Engineers, and R. FROMAGEAU is Charge de Recherche, Centre d'Etudes de Chimie Metallurgique, 94400 Vitrysur-Seine, France. Manuscript submitted January 5, 1978.
3
10
-
Cd
10
___========No
1
+--------------Mg
Zn
10
_1
10 _2 10
_3
10 _4 10 _5
AI Cu
10 _6
Cr 10 _7 10 _6 10
1. Apparatus
MET ALLURGICAL TRANSACTIONS B
ferent parameters controling the distillation process, we built at first an apparatus capable of distilling 50 g magnesium load. This equipment (Fig. 2) includes a graphite crucible with a diaphragm supporting a stainless steel condenser fitted with inner baffle plates. Heating is achieved by means of a stainless steel electrical resistance surrounding the crucible and kept in position between two pure sintered alumina walls. The inner alumina tube contains only the crucible and is drilled with holes to ensure good thermal exchange; the outer one, thicker walled, contains the heater and the condenser. This design gives good temperature homogeneity from the bottom of the crucible to the diaphragm, and decreasing temperature in the condenser.
Fe Co
_9
10-
10
10- 11 .L...,.----..L.-.L,---r-.L.-+---.L.,r-----r----,-.-------l~ 400 600 800 1000 1200 T (c) Fig. l-Equilibrium vapor pressures of pure elements (po) with respect to th
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