Electrorefining Of Aluminum With BMIC-AlCl₃-R Ionic Liquid

Ionic liquids which are liquids at ambient temperature, are completely composed of anions and cations. They have wide electrochemical windows, so many active metals can be eletrodeposited from ionic liquids. Compared with conventional electrodeposition process, the aluminum electrodepsition from ionic liquids can reduce energy consumption and pollution.In this paper, 1-buty-3-methylimidazolium chloride (BMIC) intermediate and BMIC-AlCl₃ ionic liquid were prepared. Aluminum metal was deposited on mild steel in BMIC-AlCl₃ ionic liquid with 1:2 molar ratio of BMIC to AlCl₃ under different conditions.The effect of C₇H₈ and NH₄Cl on electrical conductivities of BMIC-AlCl₃ ionic liquid was investigated. The results showed that the electrical conductivities increased with increasing the concentration C_C7H8 of C₇H₈ when C_C7H8 was less than 2.827mol·L^-1. When C_C7H8 was greater than 2.827mol·L^-1 and the temperature was over 308.15K, the electrical conductivities decreased with increasing C_C7H8 When adding NH₄Cl, the electrical conductivities would become less and less with increasing C_NH4Cl. The conductivities of the ionic liquid containing additives C₇H₈ and NH₄Cl could be described by Arrhenius empirical formulalnκ=ln A-(E_a)╱(RT)whereκis electrical conductivity in S·m^-1, A is pre-exponential factor, and E_a is conductivity activation energy in kJ·mol^-1. In BMIC-AICl₃-C₇H₈ system, both the conductivity activation energy E_a and the pre-exponential factor A increases with increasing C)c₇H₈. In BMIC-AICl₃-NH₄Cl system, however, both E_a and A decreases with increasing C_NH4cl.The deposition potentials after adding C₇H₈ and NH₄Cl shift positively according to the cyclic voltammetry. Moreover, the addition of NH₄Cl made the reaction became more irreversible.At the same scan rate and different temperatures, the peak currents of the linear scanning voltammetry increase and the peak potentials move positively with increasing temperature.At the same temperature and different scan rates, the relationship between I_pc and v^1/2 could be described by Randles-Sevcik formulaI_p＝｛〔0.4463(nF)^3/2 AC₀D_1/2〕/〔(RT)_1/2〕｝v^1/2where S is area of working electrode in cm², C₀ is concentration of active species in mol/cm³, D is diffusion coefficient in cm²/s, R is gas constant, and v is scanning rate in V/s. So it could be deduced that the reaction was controlled by diffusion. After adding NH₄Cl, the relationship between I_pc and v_1/2 could also be described by Randles-Sevcik formula.According to Randles-Sevcik formula, diffusion coefficient D as a function of temperature could be calculated as followslnD＝4836.729/T＝-6.944 3The diffusion activation energy E_D was found to be 40.21 kJ/mol.The cell voltage becomes smaller when NH₄Cl is added in ionic liquid, however, it will become bigger when C₇H₈ is added. The crystal size of the deposition layers becomes smaller when adding NH4CI or C₇H₈. The color of the layer becomes brighter when adding NH₄Cl, but it becomes blacker when adding C₇H₈.At the same current, a thicker aluminum deposition layer could be got at higher temperatures. However, tree crystal phenomena would become more serious when the deposition layer was too thick. At the same temperature, a thicker aluminum deposition layer could be obtained at higher current, but the tree crystal phenomena would become more serious if the current is too large. According to experiments, a better aluminum deposition layer with a thickness of 70μm could be obtained at 373.15K and 110.09A.m^-2. The analysis of the energy disperse X-ray indicated that the aluminum purity of the deposition was over 99.9%.