Fluoroacidity evaluation in molten salts

• Published in: Electrochimica acta Vol. 56; no. 14; pp. 5022 - 5027
• Main Authors: Bieber, A.L., Massot, L., Gibilaro, M., Cassayre, L., Chamelot, P., Taxil, P.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 30.05.2011; Elsevier
• Subjects: Chemical engineering; Chemical Sciences; Chemistry; Electrochemistry; Evolution; Exact sciences and technology; Fluorides; Fluoroacidity; Fused salts; General and physical chemistry; High temperature electrochemistry (including molten salts); Media; Melts; Molten fluorides; Monitoring; Silicon; Silicon ions; Stability; Silicon ions; Electrochemistry; Fluoroacidity; Molten fluorides; Free ion; Temperature effect; Molten electrolyte; Sodium Fluorides; Transition metal; Potassium Fluorides; Silicon Fluorides; Operating conditions; Wire electrode; Molten salt; Silver; Binary mixture; Medium effect; Eutectic; Magnesium Fluorides; Calcium Fluorides; Kinetics; Lithium Fluorides
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/j.electacta.2011.03.099

► Si(IV) ions are reduced into Si in a one-step process exchanging 4 electrons in molten fluorides. ► Si(IV) ions stability in molten fluorides depends on the free F− ions. ► Study of SiF4(g) release kinetics allows qualitative fluoroacidity evaluation. ► Several compositions of molten fluoride mixtures have been investigated. The fluoroacidity of several alkaline fluoride media was studied by monitoring the concentration of electroactive species which is decreasing vs. time due to a gas species release, such as silicon fluorides, as indicated by the reaction: SiF4+xx−=SiF4(g)+xF−. This article relates the Si(IV) reaction study to define a relative fluoroacidity scale by studying the silicon ions stability in different melts. Electrochemical techniques allow the measurement of SiF4+xx− concentration evolution and thus the reaction rate constant to be calculated at different temperatures and for several fluoride media. The article shows that the free F− content depends on the fluoride mixture and that the rate values are correlated with the fluoroacidity allowing a qualitative estimation. Then a fluoride solvents fluoroacidity scale was proposed, scaling the different eutectic melts from basic melt to acidic one: NaF–KF<LiF–KF<NaF–MgF2<NaF–CaF2<LiF–NaF<LiF<LiF–CaF2.