Mechanism. Additives Alcl <sub> 3 </ Sub>-bmic Ionic Liquid Aluminum Electrolytic Refining

Ionic liquids as the electrolytes for active aluminum electrodeposition have been found to be with many advantages and avoid disadvantages involved in these molten salts. In comparison to three-layer electrolytic process, electrorefining aluminum in ionic liquids was carried out at low temperature with low energy consumption and no emission of pollutants. Moreover, the addition of some cosolvents and additives in ionic liquids was observed to produce deposits with fine-grained and good quality. In order to resolve the problems such as easily broken off and dendritic phenomena of aluminum deposits, and higher energy consumption for aluminum deposition in pure ionic liquids. The influence of additives on the specific energy consumption and deposit quality during aluminum electrorefining in acidic aluminum chloride-1-butyl-methylimidazolium chloride (AICI3-BMIC) ionic liquid was investigated. In addition, their mechanism on the deposition process was also discussed.The effects of concentrations of three kinds of additives such as potassium chloride (KC1), choline chloride(ChCl), tetramethyl ammonium chloride (TMAC) on the electrical conductivity of acidic AICI3-BMIC ionic liquid were studied. It was observed that electrical conductivity of AICI3-BMIC ionic liquid increased with increasing the temperature in the absence and presence of these additives. Electrical conductivity of AICI3-BMIC ionic liquid decreased with mole concentrations of KC1, which can be described as linear equation.However, electrical conductivity of AICI3-BMIC ionic liquid was found to increase with the initial addition of ChCl or TMAC, and decrease with further increase in the concentration of ChCl or TMAC. The relationship of the decreased part of electrical conductivity with ChCl or TMAC concentration can be described as linear equation.The electroreduction behavior and nucleation process of aluminum on glassy carbon electrode in AlCl3-BMIC ionic liquid in the absence and presence of these additives were studied by using cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometric current-time transient methods. LSV and CV curves showed that the reduction peak potential positively shifted with a decrease in the peak current in the presence of ChCl. It was found that the initial deposited potential of aluminum negatively moved with little changes in the peak potential and a decrease in the peak current in the presence of KCl. In the case of TMAC the initial deposited potential of aluminum shifted positively, and the peak potential shifted negatively with an increase in the peak current at lower concnetrations. However, the change trend was reversed with further increasing the concentration of TMAC. The chronoamperometric current-time transients showed that the nucleation process of aluminum on glassy carbon electrode was progressive nucleation at low overpotential and instantaneous nucleation at high overpotential without additives. However, the addition of ChCl or KCl or TMAC transfered the progressive nucleation process of aluminum to instantaneous nucleation at low overpotential.The effects of ChCl or KCl or TMAC on technical parameters duiring electrorefining aluminum in acidic AlCl3-BMIC ionic liquid were implemented by constant current electrolysis experiments using two aluminum alloys as the anodes and mild steel as the cathode, respectively. The results showed that the cell voltage decreased significantly, the current efficiency enhanced (nearly 100%), and the specific energy consumption was 0.611 kWh/kg-Al which was only 50% of the pure AlCl3-BMIC ionic liquid and 3.39～4.07% of industrial three-layer electrolyte when cChCl was 0.005 mol/L at 373 K and 50A/m2. However, the current efficiency decreased, the cell voltage and the specific energy consumption increased when cChCL further increased. It was analyzed by XRD that the aluminum deposits were exhibited a strong preferred not (222) but (220) crystallographic orientation in the presence of ChCl. The deposits obtained on mild steel are well-adherent, dense and fine-grained when cChCl was 0.005 mol/L. When cKCL was 0.01 mol/L, the mini energy consumption was 1.526 kWh/kg-Al which was about 93% of the pure A1C13-BMIC ionic liquid and 8.48～10.17% of industrial three-layer electrolyte at 353 K and 100 A/m2. When cKCL was 0.02 mol/L, the (200) reflections were preferred. At other concentrations of KCl, the deposits exhibited a strong preferred (220) crystallographic orientation. However, the surface morphology of the obtained aluminum deposits was little improved in the presence of KCl. The specific energy consumption was 1.363 kWh/kg-Al which was about eighty-three percent of the pure AICI3-BMIC ionic liquid and 7.57-9.09% of industrial three-layer electrolyte when cTMAC was 0.01 mol/L at 353 K and 100 A/m2. However, the cell voltage, the current efficiency and energy consumption were all increased when cTMAC further increased. The energy consumption was 5.977 higher than 1.648 kWh/kg-Al obtained from the pure AICI3-BMIC ionic liquid at 373 K and 50A/m2. When cJMAC was less than 0.02 mol/L, the obtained deposits were exhibited a strong preferred (220) crystallographic orientation and coarse and uneven. Nevertheless, with increasing the concentration of TMAC to 0.5 mol/L, the (200) reflections were preferred, and the deposits were well-adherent, dense and fine-grained.In all cases, the analysis of the energy disperse X-ray indicated that aluminum purity of deposits obtained on mild steel was over 99.9%.