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Study on Preparation Al-Mg Alloy by Molten Salt Electrolysis Method from Magnesium Oxide

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Tutor: QiuZhuXian ZhangTingAn
School: Northeastern University
Course: Non-ferrous metallurgy
Keywords: magnesium oxide,molten salt,preparation,electrolysis,alloy
CLC: TF821
Type: PhD thesis
Year:  2008
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Abstract:
Preparation of Al-Mg alloy by molten salt electrolysis method from magnesium oxide has special advantages of environment and economy. At the same time, the method can be further developed to a new electrolytic technology on magnesium industry, and to come forth a new thought for studying on magnesium metallurgy.Preparation of Al-Mg alloy was tried in Na3AlF6-AlF3 electrolyte from magnesium oxide, and the results could not match with satisfaction of experimental goal. Content of magnesium in alloy was very low, the highest was 0.08%(w). The method, which Al-Mg alloy was prepared in Na3AlF6-AlF3 electrolyte from magnesium oxide, was denied on two reduction mechanisms.Content of magnesium in alloy, which its electrolyte was RECl3-KCl-MgCl2, could reach 4.76%(w). However, element of alloy consisted of Rare Earth, and Al-Mg-RE ternary alloy was prepared in this electrolyte. RE element in alloy was proved that it was reduced directly by aluminum by experiment of aluminum reduction. Content of RE in alloy was stable, it was between 0.8-1.2%.Critical current density of electrolyte was studied in MgF2-LiF-KCl electrolyte. The results could show critical current density of electrolyte was affected by content of KCl in electrolyte, and KCl as additive could promote solubility of magnesium oxide in electrolyte. Content of Mg in alloy increased gradually with electrolytic time, the highest was 9.2%(w). Current efficiency increased with electrolytic time, the highest was 79.4%.200A experiment was carried out in laboratory, the results showed that voltage of cell was stable, the range of variation was small, it was about 0.6v, and current efficiency was about 82.6%.Current efficiency was high for BaF2-LiF-MgF2 electrolyte, the highest was 89.4%. Results of research showed that current efficiency was not increased with content of KCl in electrolyte. Compared with no additive of KCl in electrolyte, however, the current efficiency which the electrolyte consisted of amount KCl was higher than that of no KCl in electrolyte. Take 45%BaF2-13%LiF-33%MgF2-9%KCl and 45%BaF2-13%LiF-42%MgF2 for example, the current of former increased about 11% more than that of the latter. The current efficiency was the highest when the electrolytic time was 60 minutes, its value was 87.8%. After that time, the current efficiency decreased slowly, the lowest, however, was higher than 82%. Content of magnesium in alloy increased gradually with electrolytic time. During 3 hours, the content of magnesium reached 18.6%. To guarantee content of magnesium oxide in electrolyte over 2%, the electrolysis could maintain normally with 0.7A/cm2 current density.It was proved by electrochemistry experiment that decomposed voltage of K+, which was the most positive ion in electrolyte, was-2.25V. Mg2+, however, its decomposed voltage relative platinum electrode on tungsten in BaF2-MgF2-LiF-KCl electrolyte was-1.42V. This could show that decomposed voltage of Mg2+was more positive about 830mV than that of other ions in electrolyte. The constant of current and time hardly changed under different current, conclusion could be drawn that process of Mg2+ decomposition was controlled by diffusion of Mg2+ in electrolyte. The rising part of current, which was studied in chronoamperometry experiment, was a good line relationship with time rooting, this could prove that key action of nucleation process was instant nucleation for magnesium ion on tungsten.Density of electrolyte decreased with rising of temperature, common action with increasing content of KCl and decreasing content of MgF2 could lead to reduce density of electrolyte. Liquidus of electrolyte decreased because of common action that content of KCl increased and content of MgF2 decreased in electrolyte. Conductivity of electrolyte was measured by method of continuously vary cell constant, the results showed that conductivity of electrolyte increased with rising of temperature, and the same results with content of KCl increased and content of MgF2 decreased in electrolyte.Process of solubility about magnesium oxide in electrolyte was measured in transparent cell. The results could show the process was four continuous steps. Growth of bubble on anode side was a dynamic continuous process. The big bubbles, which were merged by small bubbles, could overflow electrolyte. On the contrary, bubble at anode bottom was slowly grown up by a small bubble, the big bubble could overflow electrolyte when its diameter was big enough, even the diameter could grow up to 5mm which was the diameter of anode in the experiment. Overflowing speed of bubble on anode side was very fast. Behavior of bubble on anode was in close relationship with cathode. The bubbles grown at bottom of anode were all overflowed on anode side which was near the cathode, and the bubbles grown on side of anode, which was near the cathode, were all smaller than that of opposite side. Diameter of bubble was smaller in low current density than that of high current density.
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