On the Solubility of Aluminum Carbide and Electrodeposition of Carbon in Cryolitic Melts

Abstract

The solubility of aluminum carbide in cryolitic melts was determined as a function of the following parameters: (i) normalNaF/AlF3 molar ratio false(CRfalse) of normalNaF‐AlF3 melts at 1020°C, (ii) temperature of normalNaF‐AlF3 melts at CR=1.80 , (iii) alumina concentration at CR=1.80 normaland 1020°C , and (iv) CaF2 concentration of normalNaF‐AlF3‐CaF2 melts at CR=1.80 normaland 1020°C . At 1020°C, a maximum concentration of 2.1 weight percent aluminum carbide was found at CR=1.80 . The following model for the aluminum carbide dissolution reaction, based on activity data for normalNaF and AlF3 , was found to fit the experimental solubility dataAl4C3)(normals+5AlF3 )(normaldiss+9normalNaF)(l=3Al3CF83−+9Na+Carbon could be electrodeposited from normalNaF‐AlF3 melts saturated with aluminum carbide by electrochemical oxidation of the dissolved carbide species. Voluminous deposits of amorphous carbon were obtained on several different electrode substrates (graphite, vitreous carbon, iron, tungsten, and platinum).

Electrochemical Behavior of Graphite in KF–AlF[sub 3]-Based Melt with Low Cryolite Ratio

“…Aluminum deposits at a more positive potential on graphite than on platinum. That is probably because the occurrence of a surface chemical reaction between aluminum and graphite in fused salt, which is probably a well-known process, i.e., the formation of aluminum carbide, 8,9,15,16 facilitates the aluminum deposition. The surface chemical reaction may be written as follows…”

“…A thermodynamic calculation shows that the standard decomposition potential of Al 4 C 3 is 0.145 V. Because the aluminum deposition occurs at ca 0.25 V, the anodic wave A 1 at 0.4 can be assigned to the deposition of carbon from aluminum carbide, which agrees with the reports that the carbon deposition occurred at a potential of 150 mV positive to aluminum deposition. 15,16 This anodic reaction can be expressed by Eq. 6 From the analyses mentioned above, it can be concluded that the occurrence of potassium intercalation resulted not only in the deg-radation of cathode carbon but also in the enhancement of the side reaction, which would intensively lower the current efficiency of aluminum production.…”

Wear of Carbon Cathodes in Cryolite-Alumina Melts

“…Wear of carbon cathode blocks used in industrial cells is one if the factors limiting the lifetime of the cathode. Two different mechanisms contribute to the wear; physical wear caused by solid alumina particles and chemical wear caused by the formation and dissolution of aluminium carbide, which is soluble in aluminium and to a larger extent in cryolite [1].…”

“…It has also been demonstrated that the reverse reaction, i.e. the anodic deposition of carbon, may take place at low current densities on, for instance, a graphite substrate [1].…”