Recycling of magnesium alloys: Chemical equilibria between magnesium-lithium-based melts and salt melts
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GENERAL SCOPE OF INVESTIGATION
MAGNESIUM belongs to the light metals. Its density is 1.74 g cm⫺3. There is a strong incentive to use it as a structural material where low weight is required. It is not surprising that the amount of car components made of magnesium is increasing. Unfortunately, primary magnesium is expensive due to the costly electrochemical way of producing it. This situation means that all magnesium parts in cars ought to be recycled. One step in the recycling scheme may be melting in the presence of salt mixtures. The salts are usually chlorides. Still lighter than pure magnesium or its known alloys are the alloys based on the magnesium-lithium (Mg-Li) binary system. Some of the magnesium-lithium alloys that are possibly suitable as structural materials have densities around 1.3 g cm⫺3. This article focuses on the recycling of magnesiumlithium alloys and deals with the chemical equilibria between magnesium-lithium melts and salt mixtures. Melting with salt can be carried out with two alternative aims.
In the present study, the equilibria of the following reactions have been determined experimentally:
(1) No compositional change of the material should occur. That is, the lithium is to be kept in the metal as quantitatively as possible, and side reactions with the salt leading to pickup of other elements must be strictly controlled. Such a process is suitable for clean scraps with known compositions. (2) On the other hand, separation of the lithium from the magnesium may be desired, particularly, if the scrap is not defined or contaminated. In this case, a pure magnesium alloy and a lithium chloride (LiCl)-rich “slag” are obtained, and both products are utilized separately. KLAUS SCHWERDTFEGER, University Professor Emeritus, and ANDRE´ DITZE, Scientist, are with Institut fu¨r Metallurgie, Technische Universita¨t Clausthal, 38678 Clausthal Zellerfeld, Germany. Contact e-mail: [email protected] CHRISTIAN-THALE`S MUTALE, Scientist, formerly with the Institut fu¨r Metallurgie, Technische Universita¨t Clausthal, is with the Department of Materials Science and Metallurgical Engineering, University of Pretoria, Pretoria 0002, South Africa. Manuscript submitted June 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B
MgCl2 ⫹ 2Li ⫽ 2LiCl ⫹ Mg
[1]
CaCl2 ⫹ 2Li ⫽ 2LiCl ⫹ Ca
[2]
BaCl2 ⫹ 2Li ⫽ 2LiCl ⫹ Ba
[3]
Reaction [1] controls the lithium/magnesium distribution between the slag and metal. It may be expected from the negative free-energy change of the reaction that the reaction proceeds much to the right-hand side of Eq. [1]. That is, the high lithium content in the metal can only be maintained in the absence of magnesium chloride (MgCl2) in the salt. But, separation of the lithium from the metal phase should be possible with magnesium chloride, according to Reaction [1]. There is the problem, in a practical process, of good separation of the metal and slag phases, because there is not much density difference between them. Calcium chloride (CaCl2) and barium chloride (BaCl2) were added to make the s
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