Study On Low Temperature Aluminium Electrolysis

Low temperature aluminium electrolysis has been one of active research fields in recent years. Traditional Hall-Heroult electrolysis process for aluminium production usually operates at very high temperature (nowadays about 950℃) and unavoidably shows high energy consumption, complicated operation and pollutant emission. By the introduction of low-melting baths one might expect an increase in current efficiency and lower energy consumption. It is important to reduce liquidus temperature for study on low temperature aluminium electrolysis. But many problems will occur, such as low electrical conductivity, cathode shell, and low alumina solubility and alumina-solution rates when the temperature is too low. Furthermore, over the last decades, many researchers turned their interest in the methods at or near ambient temperatures to reduce pollutant emission and energy consumption. Aluminium electrodeposition in organic solutions was investigated for potential application in aluminium refining and recycling. Development in ionic liquids resulted in another potential approach for aluminium extraction and refining. But the electrolyte properties (e.g. low electrochemical potential windows and low electrical conductivity) limited organic solutions applications. It is difficult to prepare ionic liquids due to the highly exothermic reaction. It limited ionic liquids applications for aluminium extraction and refining.We met the problem by studying fluoride system and chloride system. The purpose of the present work is to reduce liquidus temperature and density, increase electrical conductivity to provide a scientific basis for selecting suitable low temperature aluminium electrolyte composition for fluoride system. At the same time chloride system is investigated near ambient temperatures to obtain a new method for low temperature aluminium electrolysis.For fluoride system, in this study, advanced experiment methods were used. The liquidus temperature, density and electrical conductivity of Na3AlF6-AlF3-Al2O3-CaF2-LiF-NaCl system were measured, but not based on the empirical equations. To a great degree, the accuracy and reliability of the data were better. The liquidus temperature was measured with one Agilent 34401A meter connected to a computer. The method was based on the principle of thermal analysis. The whole process was controlled by computer software. The density measurement method was based on the Archimedes law. RTW-09 melt objects integrated detector made at Northeastern University was used to measure density of the electrolyte. Ac-techniques with a PGSTAT30 and a BOOSTER 20A are used to measure impedance with a sine wave signal with small amplitude in high frequency range. Electrical conductivity is gained by the Continuously Varying Cell Constant (CVCC) technique together with a pyrolytic boron nitride conductance cell. Not only the effects of AIF3, Al2O3, and LiF on liquidus temperature, density and electrical conductivity were discussed but also low concentration NaCl was discussed. For chloride system, the purpose of the present work is to determine the mechanism of the electrodeposition of aluminium from AlCl3-NaCl melts onto W substrate where the methods of cyclic voltammetry, chronopotentiometry and chronoamperometry have been employed. It is necessary to reach a better understanding of electrochemical reactions occurring at very cathodic potentials in the melts. The effects of deposition parameters such as deposition current density on surface morphology and adherence of the deposits were evaluated. We are especially interested in the electrodeposition of aluminium on Al substrates and at relatively high current densities due to their practical importance. Our work aims to explore the possibility of using AlCl3-NaCl melts as potential electrolytes for the electrolytic extraction and recycling of aluminium in aluminium industry.For fluoride system, the liquidus temperature, density and electrical conductivity of Na3AlF6-AlF3-Al2O3-CaF2-LiF-NaCl are discussed. By experiments and discussion, the main contents and results can be summarized as follows:(1) The addition of NaCl and LiF into the electrolyte reduced greatly the liquidus temperature. The Variance analysis showed it is necessary that LiF be partly replaced by NaCl. It not only improves the physicochemical properties of the electrolyte but also decreases cost and increases economic benefits.(2) The result showed increase in NaCl concentration reduced bath density. It is beneficial to industry production. But LiF increased density of the acidic melt. NaCl replace LiF partly in electrolyte system, the density was reduced greatly. (3) The experiments showed that electrical conductivity was increased greatly with NaCl and LiF added. Increasing lwt%LiF resulted in corresponding increase of 0.0276 S/cm for superheat condition 15℃. For NaCl, it was 0.024 S/cm. Electrical conductivity was increased 0.003 S/cm with 1℃increased. In this study, electrical conductivity was lower than that which is predicted by the Wang Model and higher than that which is predicted by the Choudhary Model. Activation energy of conductance was obtained based on the experiment results.(4) The effects of AIF3, Al2O3, LiF, and NaCl on liquidus temperature, density and electrical conductivity were discussed in theory.(5) In experimental range, the regression equations were developed on the basis of experimental data.For Chloride system, the mechanism of the electrodeposition of aluminium from AlCl3-NaCl melts is investigated. By experiments and discussion, the main contents and results can be summarized as follows:(1) The voltammetric studies showed Al (Ⅲ) was reduced in two consecutive steps: 4Al2Cl7-+3e-→Al+7 AICl4-, AlCl4-+3e-→Al+4 Cl-.Certain nucleation overpotential was required during the deposition of aluminium on W electrode.(2) Chronopotentiometry analysis showed that Al (Ⅲ) was reduced in two consecutive steps under certain current density. This is in reasonable agreement with cyclic voltammograms. The electrochemical deposition process of aluminium on tungsten electrode has been investigated and found to proceed by a nucleation and growth mechanism.(3) The current-time characteristics of nucleation on tungsten showed a strong dependency on overpotential. Initially, the current decreases rapidly with the time, then the current began to increase and became flat gradually. These transients showed the typical nucleation characteristics during the deposition of aluminium on tungsten electrode. Chronoamperometric analysis showed that the deposition of aluminium exhibited instantaneous three-dimensional nucleation with hemispherical diffusion-controlled growth of nuclei.(4) By using constant current deposition, the electrodeposits obtained on aluminium substrates between 50 and 100mA/cm2 were quite dense and well adherent to the aluminium substrates. Those obtained at the current density higher than 200mA/cm2 had intergranular crevices growth with relatively poor adherence. Our studies showed that AlCl3-NaCl melt system can be possibly used as potential electrolytes for the electrolytic extraction and recycling of aluminium at less than 100mA/cm2.(5) The electrochemical deposition of aluminium on copper substrates in AlCl3-NaCl melts indicated that the formation of intermetallic compounds was occurred before the metal aluminium was formed. The formation of intermetallic compounds was controlled by the process of which aluminium atoms were diffusing into copper electrode. XRD showed intermetallic compounds were AlCu, Al4Cu9 and Al2Cu. Coating thickness was about 20μm.