Transport numbers in the molten system NaF–KF–AlF3–Al2O3

Transport numbers in the molten system NaF–KF–AlF3 (Al2O3, CaF2) were investigated by the Hittorf method. It was confirmed that in molten cryolite, Na3AlF6, 1,010 °C, the current is transported almost exclusively by the Na+ cations (t(Na+) = 0.99). When AlF3 was added to a Na3AlF6 melt, the transport number of sodium cations decreased to 0.74 at the composition corresponding to NaAlF4. In molten K3AlF6, the transport number of K+ cations equals 0.836 at 1,005 °C. In melts containing both Na+ and K+, the cations contribute to the charge transport approximately in the ratio of the squares of their ionic radii. When 5 mass % of CaF2 was added to the molten NaF–KF–AlF3 system, it remarkably influenced the transport numbers of potassium and fluoride anions.