A spectroscopic and electrochemical study of chlorotitanates in molten salt media

Abstract

Ultra-high purity titanium is used as a barrier metal in integrated circuitry. Metallothermic reduction does not produce titanium sufficiently pure for micro-electronics applications so electro refining of the metal in a molten chloride bath at temperatures above 700°C is necessary. The present study focused on the electrorefining of titanium in a bath consisting of the CsCl-NaCl-KCl eutectic as the solvent. Interfacial phenomena (multiple reaction steps and the kinetics associated with each) related to the faradaic process were investigated with electroanalytical techniques. The bulk chemistry of the electrolyte (the structure of the chlorotitanate complexes) that describes the nature of the species present during the electrorefining process was investigated using spectroscopic techniques. Recommendations were made concerning the potential for the various technologies to be used for on-line control to improve operating practices. Electrode kinetics were studied by ac voltammetry. Phase angle information was used to determine the value of the standard rate constant ([alpha) and the transfer coefficient (a) for the reduction couples Ti3+/Ti2+ and Ti2+/Ti0 at a glassy carbon electrode. The reduction from Ti2+ to metal has been identified as the slow step in the electrorefining process. The utility of electrochemical sensing to observe concentration changes has been judged poor. Industrial use of reference electrodes is recommended for controlling the overpotential in the electrorefining process and to improve efficiency. Absorption spectroscopy has established that a temperature sensitive equilibrium between TiC16 3- and TiCl4- exists in the CsCl-NaCl-KCl eutectic. Fiber optic absorption spectroscopy was shown to be capable of detecting additions of Ti2+ to melts containing Ti8+ , as well as sensing Ti3+ concentration fluctuations at a level of ±+5mM. Raman spectroscopy was found to be ill suited for investigating complexation in this system due to the deeply colored nature of the melts.