Interfacial Reactions between Aluminum Alloys and Salt Flux During Melting

Abstract

A ternary mixture of KCl, NaCl and KF may be used as a molten flux cover during melting of aluminum scrap. This practice minimizes oxidation of the liquid metal, and allows the existing oxide layer to be stripped from the scrap more readily. This study was made of the interaction that exists between halide salts and the molten metal during the melting of aluminum beverage containers. The coalescence of metal droplets dispersed in molten salt was found to be influenced by the formation of a high-melting layer that forms at the interface between the molten alloy and the salt. This interfacial layer was detected in the residue that did not dissolve when the solidified salt was water-leached after the melting process. Oxidation of magnesium from the beverage container alloy plays an important role in the formation of this layer. The interfacial layer was identified as KMgF3, in some cases, and as the mineral elpasolite, K2NaAlF6, in other instances. In KMgF3, potassium was partially substituted by sodium to form (K, Na)MgF3; in elpasolite aluminum can be partially substituted by magnesium to form K2Na(Al, Mg)F6. Whether the layer forms as KMgF3 or K2NaAlF6 appears to depend on conditions that exist in the melt at the time of formation-there was no indication of any changes in the structure or chemical composition of the layers that occur as a function of time of contact between metal and salt. The presence of metallic potassium and sodium in the solidified aluminum was detected by SEM micro-analysis of the sample. The origin of these elements in the alloy is apparently the reaction between the molten alloy and the salt mixture that results in the formation of elpasolite and KMgF3. © 1989, The Japan Institute of Metals. All rights reserved.