Chemical and Electrochemical Processing of Aluminum Dross Using Molten Salts
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TRODUCTION
DROSS is a major by-product of Al smelting or remelting, or molten Al handling.[1,2] The dross from the primary smelters is referred to as primary dross, while that from other processes involving Al is termed secondary dross. Dross is highly heterogeneous in terms of chemical composition and physical features.[2] The primary dross contains high contents of Al2O3 and AlN, certain quantities of metallic Al, and small amounts of cryolite (Na3AlF6), SiO2, Fe2O3, and MgO. The Al industry produces annually around 10 kg of dross per ton of Al produced, depending on the cell technology and processes involved. This amounts to about 250,000 tons of dross per year worldwide. The total Al content of the dross, taking account of metallic Al and Al contained in Al2O3 and AlN, may be as high as 75 wt pct. This represents a significant metal loss in Al production if the value of the dross is not recovered economically. Furthermore, dross contains environmentally undesirable materials, such as AlN or cryolite, and it must be hence treated or disposed of appropriately. A number of the processes based on physical processing of drosses have been investigated extensively.[2–8] These mainly include the following: (1) crushing of dross XIAO Y. YAN, Senior Research Scientist, is with Division of Minerals, The Commonwealth Scientific and Industrial Research Organization, Clayton, Victoria 3169, Australia. Contact e-mail: [email protected] Manuscript submitted May 11, 2006. Article published online April 8, 2008. 348—VOLUME 39B, APRIL 2008
followed by screening of the finer oxides, (2) charging dross into a molten metal pool which has an overlying salt flux layer, (3) flotation/gravity process, and (4) rotary salt furnace (RSF) process. At present, the RSF process is the most widely used commercial process to treat Al dross.[2] It involves crushing the dross to liberate finer oxides and charging the coarser dross containing higher contents of metallic Al to a rotary furnace with a salt flux. The metallic Al is recovered, but a salt cake containing large amounts of Al2O3 and AlN is unavoidably generated. The salt cake is either dumped directly or further processed by water leaching, filtering, and crystallization to recover the residual Al. The former raises environmental concerns because the salt cake contains some toxic species, while the latter is still unable to recover the Al value contained in Al compounds, such as Al2O3 and AlN. All the physical separation processes described previously only allow for the recovery of metallic Al from drosses. It is, therefore, highly desirable to develop a new process where metallic Al and Al contained in the Al compounds in drosses can be recovered in an efficient and economic way in order to maximize the Al recovery. Ueda et al.[9,10] recently investigated a molten salt process to recover Al from A356.0 casting alloy dross. In their process, a floating separation of the Al alloy from the oxides in the dross and subsequent electrolysis of the oxides were carried out in a molten salt bath of 33 mol pc
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