Removal of Low-Content Impurities from Al By Super-Gravity
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hough recycled metals become more ubiquitous under the exhaustion of raw resources, their properties are not always excellent and stable like the new metals. The main reason is because the remained impurities from the environment, which are difficult or costly to be removed entirely, damage the performance of the recycled metals. Some researchers have found that a trace of some elements can decrease metal properties, which means that many recycled metals are used only as low-level products.[1] Aluminum is the second-largest metal material and has one of the highest recovery rates among the metals. A small amount of impurities in its matrix can also worsen some properties of the metal.[1] Besides silicon, iron is the most pervasive impurity in aluminum. Because of its low solid solubility (0.05 wt pct), most formed Fe-rich phases together with other elements are detrimental to the mechanical properties.[2] Among the removal methods employed, the most popular strategy LIXIN ZHAO and MINGYONG WANG, Doctoral Students, are with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, P.R. China 100190 and also with the China Graduate University, Chinese Academy of Sciences, Beijing, P.R. China 100049. ZHANCHENG GUO, Professor, is with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, P.R. China 100190 and also with the Key Laboratory of Ecologic and Recycle Metallurgy, University of Science and Technology Beijing, Beijing, P.R. China 100083. Contact e-mail: [email protected] ZHI WANG, Vice Professor, is with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, P.R. China 100190. Manuscript submitted December 9, 2009. Article published online April 14, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
adopts the precipitation and sedimentation of the melting Al liquid and also requires the addition of other elements called chemical ‘‘neutralizers,’’ such as Mn, Co, etc. However, many additives are harmful to the Al alloy.[3] Zone refining, which is based on the theory of unidirectional solidification, is also an effective method to remove impurities from metal,[4] but it is adapted to the high-purity metals, not to the large-scale product. Although three-layer liquid electrolysis is an effective way to purify aluminum on a large scale, it is costly. So it makes sense to find a new way to remove the low-content impurities from metals rapidly and economically. Macrosegregation caused by gravity, which is called gravity segregation, is ubiquitous in casting metal, and it is harmful to the properties of materials. But sometimes, the phenomenon can be enlarged by super gravity and used for other purposes. J. F. Lo¨ffler et al.[5] have used ultra-high gravity to discover new bulk metallic glasses. When the Mg-Al al
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