Preparation Of Al And Alloys In Low Temperature Molten Salt

The method of direct electrochemical reduction solid oxides to prepare solid metals or alloys in molten CaCl₂ base chloride salts is named FFC Cambridge Process, which is simple and low cost, and is a green process using the inert anode.The problems that include oxide required dissolving into molten salt and oxide's low solubility and low dissolved rate in traditional low temperature molten salt can be avoided through FFC method. It is a new method to apply the FFC for preparing metals or alloys in low temperature molten salts. Addition to, CaCl₂-NaCl has the low melt point and it assures the feasibility of application of FFC in low temperature molten salt.In the paper, the FFC Cambridge Process method was used to prepare Al and Al-Mg, DyFe₂ in molten CaCl₂-NaCl. A graphite rod was employed as the anode and Al₂O₃, Al₂O₃-MgO, Dy₂O₃-Fe, Dy₂O₃-Fe₂O₃ were served as cathode respectively. The voltage between the cathode and the anode was 3.1 V. Preparation of Al and Al-Mg, DyFe₂ alloys in low temperature molten salt by FFC was studied and Al, Al-Mg, DyFe₂ alloys were obtained.The electrical conductivity and density of molten salts for Al electrolysis was measured by direct Archimedes method and CVCC technology from 550℃to 800℃respectively. The density of molten CaCl₂-NaCl-Al₂O₃ (w(CaCl₂)= 71%-87% corresponding to NaCl, saturated Al₂O₃) systems was increased with the incressing CaCl₂ content, however, the relation of density and CaCl₂ content was not linear from 550℃to 750℃. The additive Al₂O₃ decreased the electrical conductivity, and increased the activation energy of conductance in CaCl₂-NaCl system. The relation of the electrical conductivity of molten CaCl₂-NaCl-Al₂O₃ (w(CaCl₂)= 71%-82%corresponding to NaCl, saturated Al₂O₃) and temperature was linear from 550℃to 800℃. With the CaCl₂ content incresed, the electrical conductivity of systems was decreased, and the activation energy of conductance was also decreased.The new graphite cavity working electrode is developed and applied. Dependability and validity of the new graphite cavity working electrode was affirmed through testing cycle voltage curves and constant potential electrolysis of the NiO and TiO₂ typical powders. The electrochemical reduction mechanism of solid Al₂O₃ was obtained by the cycle voltage method with the graphite working electrode and the reduction process were known by the constant potential electrolysis at different times. The reduction mechanism of solid Al₂O₃ was one step from Al³⁺→Al. The reduction process was as follows. Firstly, Al₂O₃ was partially electrolyzed to form Al and O^2-, then CaAl₂O₄ and CaAl₄O₇ was formed by the O^2- and Ca²⁺ in the molten salts compound with Al₂O₃, CaAl₂O₄ was electrolyze to form Al and CaAl₄O₇, and CaAl₄O₇ was electrolyzed to form Al and Ca₃Al₂O₆, finally the Ca₃Al₂O₆ was electrolyzed to form Al.To prepare the suitable electrolysis cathode Al₂O₃ pellet, the Al₂O₃ particle size, molding pressure, sinter temperature and sinter time were investigated. The optimal conditions of Al₂O₃ cathode pellet preparation were that the particle size was 38μm, the sinter temperature was 1400℃and the sinter time was 9h. The effect of the different cathode materials, such as the commercial Al₂O₃ powder, the Al₂O₃ sintered pellets, corundum powder, corundum tubes and the sintered Al₂O₃ tubes on the electrolysis were studied by using different conductors including metallic wire, melted Al and Al-Mg alloy. The result showed that the melted Al-Mg alloy as the conductor, the sintered Al₂O₃ rods as cathode, at 800℃and in molten salts with w(CaCl₂)= 82% were optimal electrolysis conditions.The effects of Al₂O₃-MgO mixture's sinter temperature and time on the electrolysis were investigated. The appropriate sintered conditions were at 1300℃and 8h sinter. The electrochemical reduction process of the cathode were as follows. First, Al₂O₃ was electrolyzed to form Al, CaAl₂O₄ and CaAl₄O₇, then MgAl₂O₄ was electrolyzed to form Al and Mg during the Al-Ca compound oxides were electrolyzed, finally the Mg and Al atoms obtained was alloyed.The effects of the temperature and CaCl₂ content in molten salt on electrolysis were studied when Fe-Dy₂O₃ was used as the cathode. The result indicated that high electrolysis temperature and high CaCl₂ content was advantage to prepare pure DyFe₂. The electrochemical reduction process of Fe-Dy₂O₃ was that as follows. First Dy₂O₃ was electrolyzed to form Dy, then Dy atoms diffused into Fe body, the formation sequence of alloy was DyFe₅→DyFe₃→DyFe₂. The effects of the sinter temperature and time of Fe₂O₃-Dy₂O₃ mixture on electrolysis were investigated. The appropriate sinter conditions were at 800℃and 4h sinter. The effect of the molten salt temperature and CaCl₂ content on the electrolysis was studied. The result indicated that high electrolysis temperature and high CaCl₂ content was advantage to prepare pure DyFe₂. Temperature was key factor to form DyFe₂. The electrochemical reduction process of Fe₂O₃-Dy₂O₃ were that first Fe₂O₃ was electrolyzed to form Fe, then DyFeO₃ was electrolyzed to form Fe and Dy₂O₃, and Dy₂O₃ was electrolyzed to form Dy, finally Fe and Dy were alloyed to form DyFe₅→DyFe₂. The anode gases were CO₂ and CO.