The preparation of coarse Al-Si alloys by carbon-based electrothermal reduction is a viable means of recycling scrap aluminum on an industrial scale to allow the the high-value utilization of this resource. However, the presence of impurities on the cathode can have significant effects on the product purity. The present study explored the behavior of impurity elements on the cathode and the reasons for the decreased purity of aluminum deposited at the cathode with increases in electrolysis time. X-ray diffraction, scanning electron microscopy, and inductively coupled plasma-atomic emission spectroscopy were used to study the compositions of the molten salt electrolyte and of the cathode product, as well as the phases and element distribution on the working electrode. With increasing cell voltage, the elements added as impurities were found to precipitate in the order of Fe > Al > Si > Mn > Ti > Mg > Ca. Increasing the concentration of these elements in the molten salt electrolyte was determined to increase the cell voltage, with Mg and Ca having the greatest effect.