Fundamental studies on electrochemical production of dendrite-free aluminum and titanium-aluminum alloys

Abstract

A novel dendrite-free electrorefining of aluminum scrap was investigated by using AlCl_3 -1-Ethyl-3-methyl-imidazolium chloride (EMIC) ionic liquid electrolyte. Electrodeposition of aluminum were conducted on copper/aluminum cathodes at voltage of 1.5 V, temperatures (50-110ºC), stirring rate (0-120 rpm), molar ratio (MR) of AlCl_3 :EMIC (1.25-2.0) and electrode surface modification (modified/unmodified). The study was focused to investigate the effect of process variables on deposit morphology, cathode current density and their role in production of dendrite-free aluminum. The deposits were characterized using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Modified electrodes and stirring rate (60 rpm) eliminate dendritic deposition by reducing cathode overpotential below critical overpotential (-0.54 V) for dendrite formation. Pure aluminum (>99%) was deposited with current efficiency of 84-99%. Chronoamperometry study was conducted using AlCl_3 -EMIC and AlCl_3 -1-Butyl-3-methyl-imidazolium chloride (BMIC) (MR = 1.65:1) at 90ºC to understand the mechanism of aluminum electrodeposition and find out diffusion parameter of electroactive species Al_2 C_7 ^- . It was concluded that electrodeposition of aluminum is a diffusion controlled instantaneous nucleation process and diffusion coefficient of Al_3 C_7 ^- was found to be 5.2-6.9 × 10-11 m^2 /s and 2.2 × 10-11 m^2 /s for AlCl_3 -EMIC and AlCl_3 -BMIC, respectively. A novel production route of Ti-Al alloys was investigated using AlCl_3 -BMIC-TiCl_4 (MR = 2:1:0.019) and AlCl_3 -BMIC (MR = 2:1) electrolytes at constant voltages of 1.5-3.0 V and temperatures (70-125°C). Ti sheet was used as anode and cathode. Characterization of electrodeposited Ti-Al alloys was carried out using SEM, EDS, XRD and inductively coupled plasma-optical emission spectrometer (ICP-OES). Effect of voltage and temperature on cathode current density, current efficiency, composition and morphology of Ti-Al alloys were determined. The Ti-Al alloys containing about 13-27 atom % Ti were produced using both electrolytes. The current efficiency of AlCl_3 -BMIC was varies between 79-87%. But lower current efficiency (25-38%) was obtained for AlCl_3 -BMIC-TiCl_4 electrolytes due to the formation of TiCl_3 passive layer on the electrodes. To increase the productivity, constant current method (160-210 A/m^2 ) was implemented. This fundamental study on low temperature production of dendrite-free aluminum and Al-Ti alloys is not only efficient but also opens a novel route in aluminum and titanium process metallurgy.