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Preparation and Investigation of Inert Anode for Aluminium Electrolysis

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Tutor: QiuZhuXian ZhangTingAn
School: Northeastern University
Course: Non-ferrous metallurgy
Keywords: aluminium electrolysis,inert anodes,metal anode,cermet anode,oxidation,corrosion
CLC: TF821
Type: PhD thesis
Year:  2008
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Abstract:
There are many disadvantages in traditional Hall-Heroult aluminium-electrolytic cells by using consumable carbon anode, such as high energy consumption, high carbon consumption, high cost and serious environmental pollution. All of these problems can be resolved by using inert anode. So lots of research works have been done to seek appropriate inert anode materials, which can be used in aluminium electrolysis.Fe-Ni metal anode and Fe-Ni-Al2O3 cermet anode were prepared by powder metallurgy method. The effect of forming pressure, sintering temperature, hold time on the porosity and density of inert anode were investingated. The optimal technological paramerter of producing Fe-Ni metal anode are follows, forming pressure 400MPa, sintering temperature 1200¡æ, argon atmosphere protection and holding time 2 hours. And forming pressure 150MPa, sintering temperature 1500¡æ, argon atmosphere protection and holding time 2 hours for Fe-Ni-Al2O3 cermet anode. The conductivity of Fe-Ni-Al2O3 cermet anode shows semiconductor electric and electron conduction behavior at high temperature, the effect of Fe content on anode conductiveity was investingated by percolation theory.The isothermal oxidation kinetics curves of Fe-Ni metal anode were obtain by thermo travimetric analysis. The mictrostructrure and morphology of oxide film were analyzed by X-ray diffraction spectrum and scaning electron microscopy. The isothermal oxidation kinetics curves follow parabolic law, the compact oxide film can protect anode matrix, the component of oxide film are Fe2O3, FeO, NiFe2O4, Fe3O4. Damage mechanism of FeNi oxide film was analyzed, and high temperature oxidation model was built up.Oxidation rate of Fe-Ni-Al2O3 cermet anode increased with temperature increased. The isothermal oxidation kinetics curves follow parabolic law at the initial oxidation 3 hours. During the oxidation progress, the isotherma oxidation kinetics curves follow cubic law and logarithm law at different temperature, which means Fe-Ni-Al2O3 cermet anode is difficult to be oxidized, and the oxide film is compact.Sodium ion and fluoride ion permeated in the matric of AlSi metal anode is the main corrosion in hot corrosion test. Cell voltage of Al-Si metal anode has bigger fluctuation in the electrolysis process. The corrosion layer was loose, that¡¯s Al diffused outward to form oxide film and leave lots of cavities. The corrosion rate can be decreased by improving Al2O3 concentration.Lots of oxygen produced from FeNi metal anode in the electrolysis processs, cell voltage is stable. The surface of anode was analyzed by X-ray diffraction spectrum and scaning electron microscopy, corrosion layer is consisit of Fe2O3, NiO and NiFe2O4, it is compacte and thin, so the electric resistance of FeNi metal anode is small, voltage drop of anode is small.The effect of Al2O3 concentration, current density and cryolite rate on Fe-Ni-Al2O4 cermet anode corrosion behaveior was investigated. High Al2O3 concentration, increasing cryolite rate and decreaseing current density can reducee corrosion rate, the purity reach 97%. Corrosion mechanism was discussed. Corrosion layer is consist of FeAl2O4, NiFe2O4 and Fe2O3, these oxide form net protective film to prevent electrolyte from corroding anode.The effect of Al2O3 concentration and cryolite rate on FeNi base metal anode polarization curve was investigated by electrochemical method. It¡¯s easy to analyze the corrosion performance. Corrosion potential move toward positive direction, and anti-corrosion increased with increasing Al2O3 concentration. Co added in FeNi metal anode can improve anti-corrosion performance than Cu and Al by analyzing polarization curve.The behavior of anodic bubble evolved from carbon and FeNi metal anode was observed by a two compartment see through quartz cell in electrolysis process. The bubble size produced on carbon anode is bigger than FeNi metal anode¡¯s, and it is difficult to desorb from anode surface. The rate of anode reaction increased with increasing current density, and the bubbles production rate increased too.Oxidation peak was observed at 1.2V (Al reference) on carbon in cryolite-alumina molten by testing cyclic boltammetry. The reaction is, 3C+2Al2O3=3CO2+4Al Oxidation peak was observed at 2.3V (Al reference) on FeNi metal anode in cryolite-alumina molten by testing cyclic boltammetry. The reaction is, 2Al2O3=3O2+4Al Anode process of carbon anode and FeNi metal inert anode were simulated to equivalent circuit by testing anode electrochemical impedance spectroscopy, the influence of current density on anode process was investingated.
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