Electrorefining Of Aluminum With BMIC-AlCl₃ Ionic Liquids

In this paper , BMIC (1-butyl-3-methylimidazolium chloride) intermediate and BMIC-AlCl₃ ionic liquid were prepared. Metal aluminum was successfully deposited on different substrates such as copper, mild steel and stainless steel in BMIC-AlCl₃ ionic liquids with 1:2 molar ratio of BMIC to AlCl₃ at 100X2.The structures of BMIC intermediate and acidic BMIC-AlCl₃ ionic liquid were characterized by IR, NMR. The result showed that there are a lot of hydrogen bonds in BMIC while the hydrogen bonds are very weak or entirely absent in acidic ionic liquid with 1:2 molar ratio of BMIC to AlCl₃.Electrical conductivities of BMIC-AlCl₃ ionic liquid were measured against temperature and composition. It was demonstrated that the electrical conductivities are increased with increasing temperature, which can bedescribed by Kohlraush empirical formulawhere k is electrical conductivity in S·m～(-1), k₀ is electrical conductivity at room temperature in S·m～(-1), t is temperature in ℃, α and β are temperature coefficients, t₀ is room temperature (25 ℃). An increase in mole fraction of AlCl₃ can enhance the conductivities of the ionic liquid when x_AlCl3 is less than 0.667. However, the effect of AlCl₃ molar fraction on the conductivities is negligible when x_AlCl3 is in between 0.667 and 0.692.The electrode process of aluminum electrodeposition was studied by PC14/300 Potentiostat/Galvanostat/ZRA. Cyclic voltammetry and linear scanning voltammetry showed that the reduction of Al₂Cl₇～- to Al was an irreversible process. There was no intermediate during the reduction, which indicates that the reduction is a process of electron transfer. Therefore, the reaction was three-electron reduction process.The micrographs of aluminum deposits on different substrates were characterized by scanning electron microscopy. It was shown that the thickness of aluminum deposits increases with electrolyzing time. The aluminum deposits on the edge of cathodes are thicker than those in the middle region. The initial deposition of aluminum with thickness 20～40 μm is very compact. However, the tree crystal phenomena will emerge when the thickness of deposition layer exceeds 20-40μm, and will become more obvious with increasing electrolyzing time. The compact deposits on mild steel are thicker than those on copper and stainless steel. The analyses of energy disperse X-ray indicated that the aluminum purity of the deposits is over 99.9%.The addition of NaCl in ionic liquid can change current density and ionic distribution, and has certain effect on the deposits. However, the quality of deposits can not be improved obviously by the addition of NaCl.The deposit is compact at low current density and will become porous at high current density. Besides, the current efficiency is also high at lowcurrent density.The quality of deposits can be improved by choosing substrate, changing cathode current distribution, improving ion transport, controlling current density and adding the appropriate additives.