RQamCLa nature des gaz anodiques est une con&quence directe des processus &kctrolyse des bains cryolithe-alumine. Nourcavons mis au point un dispositif original de captation des gaz in situ & I'iolCtieur de l'anodc (tkctrode creux en gpphitc B paroir minecs sous dtpression intern+ L'analyse par spectromttrie de masse nous permet de euivrc Wolutioo de lo composition des gaz an&qua en fonction de di&ents facteurs expbrimentaux, tels que la den&C de courant, la concentration en aluminc du bain et Ia nature de I'alumine utilii.
L'cau rbiduelk contcnue dam l'alumine de type y dome lieu A la formation de compc&s supcrfickls hydroxy#nts du carbone qui provoquent sa passivation. Le rapport [CO,]/[CO] augmente avec la den&C de courant et diminue lorsque la teneur en aluminc croit.
Une -tation sehhnrtique dsa rtsultats est ptopos& en considtrant quc k produit prim&e de la &action est priacipakment k CO,. Celti-ci se tmnsfornu en CO par la rkction de Boudouard Lc temp, de contact de CO2 BWC k carbnneest It fscteut p&pond&ant quid&ermine le rapport [CO,]/[CO] ainsi quc la consommation en carbone du processus anodique.
Abetract-The composition of anode gas is an important indication for the mechanism of the electrolysis in cryolite-alamina melts. An original technique has been used for the captation of gas in situ (hollow graphite electrode under vacttum ins&). The composition of anode gas was determined by mass spectromctry and shows that the nature of this gas depends on ditTercnt factors such as the crystallographic structure of alumina (a-A1,OI, y-AlaOS, hydrated alumina), the current density, the alumina concentration of the melt, the porosity and the B.E.T. area of carbon anode.
The ratio [CO,]/[CO] increases with incrcasiog current density and dcscrutses with increasing alumina content. The results suggest that~C0, is the primary anode product which reacts partially with the anode following the Boudouard reaction.
The stmctuml water of the aluinina (y-A&O, or hydrated alumina) induas the formation of super6cial GO-B compkwa which lead to the paesivation of the anode.
A schematic representation of these mechanisms is proposed taking into account the kinetics of reaction during BOW gas through the ckctrodc wails. The contract time of gases with the carbon is an important factor in the mechanism of CO built up and the consumption of carbon by the aliodc process.