Oxide Sidewall Materials For Novel Aluminum Reduction Cell

Si3N4bonded SiC materials have been widely used as sidewalls for Hall-Héroultaluminium reduction cell because of its high thermal conductivity, which leads to form afrozen ledge of electrolyte to protect the sidewall materials from the attack of corrosiveelectrolyte. Thus, the service life of the sidewalls can be extended. However, a large heataccounting for approximately35%of the total input energy has to transfer through thesidewalls, which is the reason for the low energy efficiency of40-45%in currentaluminum electrolysis industry. To meet the aim of energy saving in aluminum industry,high insulation layer can be applied outside the sidewalls to avoid the formation of frozenledge, leading to a potentially30-40%energy savings. Such a ledge-free sidewall mayalso increase the productivity of the same dimension cell. Furthermore, if the combinationof inert anodes and ledge-free sidewall were used in new technology, the environmentalimpacts would be reduced significantly. However, in that case the sidewall would beexposed directly to the oxidizing gas, corrosive electrolyte and reducing aluminum fromthe top to bottom causing significantly destruction of Si3N4bonded SiC materials in thethe interface of gas and bath zones. Thus, the Si3N4bonded SiC materials can not meetutilization requirement in ledge-free cell and development of novel sidewall materials isvery important for development of aluminium electrolysis industry.The present work aims to develop oxide sidewall materials that can replace Si3N4bonded SiC materilas for applying in gas and bath zones. Firstly chemical reactionsbetween typical oxides and fluoride are thermodynamically predicted by Factsagesoftware. Then the chemical reactions are verified by experimental method to confirm thechemical stability of the oxides. On the basis of this work, MgO-TiO2-NiFe2O4andMgO-TiO2-SnO2composites are prepared by adding high chemical stable oxides tomagnesia that own good resistance to basic slags and low price. On the other hand,Fe3O4-MgO composites are prepared in air and N2respectively by adding MgO to Fe3O4(spinel structure) powders. The sintering property, phase composite, microstructure and corrosion resistance of the materials prepared are studied and the conclusions can bedrawn as follows:(1) Compared with the oxides of transition elements, the oxides of main groupelements are commonly much easier react with Na3AlF6-AlF3-K3AlF6electrolyte.Thereactions of NiFe2O4, Al2TiO5, Mg2TiO4, MgAl2O4, CaO·6Al2O3with electrolyte becomegradually stronger in order. The results of static corrosion tests indicate that penetrationresistance of sintered specimens to electrolyte could be effectively improved by loweringthe apparent porosity.(2) The addition of TiO2accelerates the densification of the magnesia basedspecimens and improves their corrosion resistance as well. With the increase in the TiO2addition, the amount of Mg2TiO4distributing along the MgO grain increases, the periclasegrain becomes larger, and densification of the specimens improves and reaches itsmaximum in specimens with5wt%TiO2. All the MgO-Mg2TiO4materials preparedexhibit good corrosion resistance to the electrolyte melts due to their high density andMg2TiO4owning good chemical stability depositing along the MgO grain.(3) A reactive sintering process occurs in the MgO-TiO2-NiFe2O4specimens at hightemperatures, which results in the formation of a composite spinel NiχTi1-χFe2χMg2-2χO4inthe matrix and acceleration of densification of the specimens. The crystal structure,morphology and distribution of the composite spinel vary with the amount of the NiFe2O4added. The composite spinel obtains high chemical stability in the electrolyte, and thechemical stability increases as the amount of NiFe2O4added increases. All the specimensprepared show good corrosion resistance to the electrolyte melts due to their high densityand the presence of the composite spinel in the periclase matrix. And the corrosionbehavior and corrosion resistance of the specimens are closely associated with theirmicrostructure and the chemical stability of the composite spinel.(4) In the MgO-TiO2-SnO2specimens, MgO reacts firstly with TiO2and SnO2to formthe Mg2TiO4and Mg2SnO4phases, which in turn form the composite spinelMg2TixSn1-xO4at high temperatures. As the amount of SnO2added is in the range of2wt% to10wt%, the composite spinel distributes homogeneously in the MgO matrix anddensification of the specimens improves with the increase in the SnO2content; whilelarger volume expansion and agglomeration of some composite spinel occur in specimenswith SnO2more than10wt%, leading to the decrease of density for the specimens. Thecomposite spinel has high chemical stability and the corrosion resistance of the specimensis more dependent on the composite spinel content. Therefore, the corrosion resistance ofthe specimens prepared increases progressively with the increase in the SnO2addition.(5) Both sintering atmosphere and MgO content have great influences on the phasecomposite, microstructure and corrosion resistance of Fe3O4-MgO specimens. Forspecimens prepared in air, MgO reacts with Fe2O3that derives from oxidation of Fe3O4toform MgFe2O4spinel. The dense corrosion layers of all specimens are composed of Fe2O3and Al2O3phases. For specimen with20wt%MgO added, a dense and stableMg(AlFe)2O4layer forms between the corrosion and origin layers, which hinders theinfiltration of electrolyte. For Fe3O4-MgO specimens prepared in N2, all specimens arecomposed of FexMg1-xO phase. Comparing with specimens prepared in air, specimensprepared in N2exhibit worse corrosion resistance. Specimens with10wt%and20wt%MgO possess thick and loose corrosion layer, whereas a dense (FeMg)Al2O4layer formsbetween the corrosion and origin layers in specimen with30wt%MgO, leading toimprovement of corrosion resistance of the specimen.(6) The Si3N4bonded SiC sidewall materials exhibit poor resistance to the electrolytemelts in air. Both Si3N4and SiC phases react with the O2and fluorides, which results information of SiF4(g) and destruction of the materials. Compared with the Si3N4bondedSiC materials, the oxide materials prepared exhibit much better corrosion resistance andshow superiority in application as sidewalls in the novel aluminium reduction cell.