| KF-AlF3-based system is a promising candidate as low-temperature electrolyte for aluminum electrolysis due to its lower eutectic temperature and higher alumina solubility. The studies on this system are mainly focused on its physical or chemical properties and corrosion effects on inert anodes. Its penetration into cathode block, however, is not yet fully understood at present. Penetration of electrolyte into carbon cathode is important since it affects the life of aluminum reduction cell. Better understanding of the interaction of this system with cathodic carbon block would be of basic importance to predict the performance of carbon cathode when KF-AlF3-based system is used as low-temperature electrolyte.The electrochemical intercalation of potassium into graphite in KF melt was investigated by cyclic voltammetry, potential step, open-circuit potential, and current step. Results indicate that potassium intercalates into graphite during the cathodic polarization. The resultant, a potassium-graphite intercalation compound with high stage number, is quite instable and decomposes very fast. The potassium intercalation results in lattice expansion and structure destruction of graphite. The graphite matrix is seriously eroded after potassium intercalation as many thinner graphite layers and particles exfoliate from the graphite bulk. The graphite matrix completely cracks to fragments when a larger amount of charges are transferred through the cathode. The equation of the current responding to the potential step is deduced. It is found that the current-time transcient deviate from the Cottrellian behavior but can be well fitted with a quasi-reversible equation, indicating that the intercalation of potassium is governed by both the diffusion of potassium ion in graphite bulk and the phase transition kinetics.Cyclic voltammetry, potential step, open-circuit potential, and current step were used to study the cathodic behavior of graphite in KF-AlF3-based melt with low cryolite ratio (CR). The potential of aluminum deposition on graphite electrode is about 0.35 V more positive than that of potassium intercalation in the KF-AlF3-based melt with a cryolite ratio of 1.3 at 750℃. The intercalation of potassium results not only in the graphite erosion but also in the formation of aluminum carbide. The deposition of aluminum on graphite electrode in KF-AlF3-based melt is a mass controlled process. This electrochemical process is limited in the potential region from 0.1 V to 0.03 V due to the absorbance. The critical current density of potassium intercalation is found to be 400mA/cm2. Lowering the melts'CR cause not only the negative shift of the potential but also the decrease of critical current density of potassium intercalation. The formation of Al4C3 occurs more seriously in the melt with lower CR than in the melt with higher CR. The limit current of aluminum deposition increases with the increase of working temperature. The onset potential of potassium intercalation shifts to positive value and the critical current density of potassium intercalation decreases when alumina is added into the KF-AlF3 melt. However, when a relative larger amount of alumina is added into the KF-AlF3-based melt, almost no variety on the cathodic behavior of graphite and diffusion coefficient of AlF4-is found.The galvanostatic electrolyses were carried out in experimental scale using graphite and semographitic carbon block as cathode. Under the same electrolysis conditions, the penetration depth of potassium into graphite is much higher than that of sodium. The main components of penetrated electrolyte within the graphite cathode are KA1F4 and K2.9AlF5.9(H20)0.1. However, KA14F13 also penetrates into graphite cathode when the CR of melt is very low. The electrolyte penetrates the graphite cathode through the pores and cracks. The amount of penetrated electrolyte decreases with increase of the cathode depth. The penetration depth and the amount of penetrated electrolyte increase with increase of the CR of the melt. As the CR of melt increases, the amount of penetrated KAlF4 decreases but that of penetrated K2.9AlF5.9(H20)0.1 increases. When a higher cathodic current density is applied, the penetration depth of potassium and the amount of penetrated K2.9AlF5.9(H20)0.1 increase but the amount of penetrated KAlF4 decreases. The electrolysis temperature has little affects on the penetration depth of potassium. But the amount of penetrated K2.9AlF5.9(H20)0. 1 and KAlF4 increase and decrease with increase of temperature, respectively.The penetration depth of potassium into semigraphitic cathode is lower than that of potassium into graphite cathode but is higher than that of sodium under the same electrolysis conditions. Similar to graphite cathode, the main components of penetrated electrolyte within the semigraphitic cathode are also K2.9AlF5.9(H20)0.1 and KAlF4. The CR and cathodic current density have the same effects on the penetration of electrolyte into semigraphitic cathode as on the penetration into graphite cathode.When KF-AlF3-based melt with low CR is used as low-temperature electrolyte, stronger penetration of melt occurs even if lower cathodic current density and electrolysis temperature are applied. Therefore, both the graphite and the semigraphitic carbon block are not the ideal materials against the penetration of potassium and KF-AlF3-based melt. The inert cathode material, in which TiB2 is the main component in either pure form or composite, should be used to improve the life of aluminum reduction cell when KF-AlF3-based system is used as low-temperature electrolyte.
|
PDF Full Text Request