| Recently, as the high content of lithium oxide and potassium oxide in domestic alumina materials, LiF and KF are enriched in electrolytic cell. Most researches on the lithium salt and potassium are limited to the physicochemical properties. However, there is a shortage of literature about the electrode process and electrochemical mechanism in the melt of cryolite-alumina contained lithium salt and potassium salt. Roles of lithium salt and potassium salt are not clear yet in the cryolite-alumina molten salt. Besides, in order to avoid the pollution concerns arising from using fluoride salt electrolyte system, scientists have been probing a new electrolyte system to replace the fluoride salt system. This paper also explores a new method that the melt NaF-NaCl is used as the main melt to prepare metal aluminum.In this paper, selective and efficient electrochemical measurement methods are applied, like potentiodynamic polarization, recurrent potential pluses, potentiostatic curves, galvanostatic curves and open-circuit chronopotentiometry, to study the influences of LiF and KF on cathodic process in the melt Na3AlF6-Al2O3 at tungsten electrode.In Na3AlFe-Al2O3-LiF melt, the formation of Al-W intermetallic compound and the deposition of metal Al are facilitated with higher LiF concentration. As LiF concentration increases, the dissolution loss of metal aluminum increases and the aluminum activity enhances gradually. Furthermore, the cathodic overvoltage decreases and the conductivity of the melt enhances with the increasing of LiF concentration. When LiF concentration is lower than 3 wt%, and cryolite ratio is less than 2.0, no metal sodium deposites on the tungsten electrode. When LiF concentration remains, the cathodic overvoltage and the dissolution loss of metal aluminum decrease firstly and then increase, and the aluminum activity weakens as cryolite ratio increases. In the melt containing LiF only, neither sodium element nor oxygen element is observed in the metal Al layer, the Al layer purity is not affected in the point of structure under the condition that only the current efficiency is affected.In Na3AlF6-Al2O3-KF melt, as KF concentration increases, it boost that both the cathodic overvoltage and the dissolution loss of metal aluminum increase, and the aluminum activity decreases firstly and then increases. When KF concentration is 3 wt%, the conductivity of the melt reaches to be maximized. The change of cryolite ratio has a significant influence on the dissolution loss of metal aluminum, the thickness of the cathodic product and the aluminum activity. When KF concentration remains, as cryolite ratio increases the cathodic overvoltage and the dissolution loss of metal aluminum decrease firstly and the increase, and the aluminum activity weakens. Furthermore, the conductivity of the melt is also affected as cayolite ratio increases. No metal sodium deposites on the surface of the working electrode when the cryolite ratio is less than 3.0.In Na3AlF6-Al2O3-LiF-KF melt, the tendency of the dissolution loss is consistent with those in the melts of NasAlF6-Al2O3-LiF and Na3AlF6-Al2O3-KF, but the dissolution loss of metal aluminum is much more serious. No metal sodium deposited on the working electrode when both LiF and KF in the molten salt with the cryolite ratio of 1.5 to 3.0. As cryolite ratio increases, the aluminum activity weakens, and the cathodic overvoltage decreases firstly and then increases. The oxygen element is detected in the Al layer in the melts of Na3AlF6-Al2O3-KF and Na3AlF6-Al2O3-LiF-KF with higher cryolite ratio, and the density of the metal Al layer is loose. This explains that under the condition of high cryolite ratio and KF existence in melts, the current efficiency can be influenced during electrolysis aluminum, so can the purity of metal aluminum.This paper also explores the possibility of the electrolysis method to prepare metal aluminum using NaAlO2 and Al2O3 as solute, and discusses the cathodic electrochemical process by means of electrochemical measurement methods. The main results are as follows: When temperature is in 750-850?, the solubility of NaAlO2 and Al2O3 waves slightly as temperature changes, but it grows linearly as temperature rises starting from 850?. Through the electrolysis anlalysis for the systems of NaF-NaCl-NaAlO2 and NaF-NaCl-Al2O3, as well as the XRD analysis for the cathodic metal productions after electrolysis, it concludes that the methods are feasible using NaF-NaCl as solvent to prepare metal aluminum.When studying the cathodic process with electrolysis method to prepare metal aluminum in systems of NaF-NaCl-NaAlO2 and NaF-NaCl-Al2O3, it can be deduced that Al3+ ions is reduced to Al (0) as two ionic structures. In the melt of NaF-NaCl, only metal sodium is reduced on the cathode; when added NaA102 and Al2O3 into the melt, the diffusion and the reduction of Al3+ ions happen. Metal aluminum and metal sodium can be co-deposited on the electrode surface together. |
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