Development of aluminum electrorefining in ionic liquids: the effects of experimental conditions on the deposition behavior and microstructure

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Date
2020
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University of Alabama Libraries
Abstract

The electrochemical refining of Al from aluminum alloy scrap (Al2020) on copper substrate cathode at low temperature ionic liquid electrolytes was studied. The main components of ionic liquid electrolytes were the mixture of AlCl3 and 1-butyl-3-methyl imidazolium chloride ([BMIM]Cl), 1-ethyl-3-methyl imidazolium chloride ([EMIM]Cl) or 1-hexyl-3-methyl imidazolium chloride ([HMIM]Cl). The electrodeposition experiments were conducted in a 50-mL glass beaker fitted with Teflon cap. Al2020 aluminum alloy scrap was used as anode material and Cu sheet was used as cathode material. The aluminum alloy scrap (anode material) was cut into thin plate shape and polished mechanically before deposition. To study the effects of various experimental parameters, Al electrodeposition was conducted with temperature ranging from 80 oC to 140 oC, cell voltage ranging from 1.0 V to 1.75 V, stirring rate ranging from 0 to 180 rpm, type of ionic liquids changing from [BMIM]Cl to [EMIM]Cl and [HMIM]Cl, and electrolyte composition ranging from IR=1.0 to 2.4 (IR = AlCl3 : [BMIM]Cl). In addition, the surface roughness of Cu cathode was controlled by polishing through 320, 600, 800, 1200 grits SiC sandpapers and mirror polishing procedure (with 3 μm SiO2 fine colloidal suspension). For all anode materials (Al sheet), they were polished by 320 grits SiC sandpaper. The experiments were performed for 2 h throughout the research and the samples were cleaned by acetone and DI water before characterization. The phase characteristics and crystallinity of Al deposits on Cu cathode sheet were analyzed using X-ray diffraction (XRD). The morphology and chemical composition characterization of Al deposits were carried out using Apreo field-emission scanning electron microscope (Apreo FE-SEM, ThermoFisher Scientific) equipped with an energy dispersive X-ray spectrometer (Bruker XFlash EDS). Al deposits with purity higher than 99.7% were obtained. Among many other merits, this study demonstrates that electrochemical refining of Al from Al2020 alloy scraps using ionic liquid electrolytes is an energy-efficient (current efficiency > 90% and energy consumption < 5 kWh/kg Al) and environmentally friendly method. Furthermore, the microstructure of Al deposits was controlled by the design of experiments. Especially the formation of dendritic structure (a typical structure formed during Al electrodeposition), which can add additional processing cost and has a profound adverse effect on refining of Al, was prevented on smoother cathode surface. The mechanisms of Al electrodeposition and formation of crystal dendrite structure were investigated. The electrodeposition process is primarily controlled by the diffusion of Al2Cl7- and the microstructure formation is concerning the surface energy, local electrolyte concentration, and apparent contact angle of ionic liquids on the substrates.

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Electronic Thesis or Dissertation
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Materials science
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