Titanium tetrachloride-supercritical carbon dioxide interaction: A solvent extraction and thermodynamic study
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INTRODUCTION
A. Opportunities for Metal Chloride Separations C A R B O C H L O R I N A T I O N of clays I1,2~ or coal ash I31 has been proposed as a method for recovering aluminum from resources available in the United States. Besides bolstering the domestic production of aluminum, electrolysis of aluminum metal from A1C13 consumes less electrical power and capital equipment than present aluminum production, where the metal is electrolyzed from A1203. Carbochlorination yields a mixture containing approximately 10 pct TiCI4 and 90 pct A1C13. The TiCI4 must be removed down to the parts-per-million range to yield acceptable A1C13 for electrolysis. A well-designed purification technique could yield clean TIC14 as a valuable byproduct. The U.S. Bureau of Mines reported that distillation adequately removed all of the common impurities from A1C13 except TiCI4 .tq Although boiling points of TIC14 and A1C13 are quite different, adequate separation by distillation has not been accomplished in practice. Later Bureau studies demonstrated dissolution of several metal chlorides in supercritical CO~, ~4,51with TiCI4 being highly soluble in this solvent. These findings open the possibility of supercritical solvent extraction to separate soluble salts such as TIC14 from less soluble ones such as A1C13. However, a better understanding of the phase behavior of metal chlorides in supercritical CO2 is necessary before practical development of new technology is possible. A study of metal chloride behavior in supercritical solvents has fundamental scientific interest. Transition metal chemistry is strongly influenced by electrons in the d and f shells. Since electrons in these orbitals participate in back bonding, one would expect distinctive solvent-solute
W.K. TOLLEY, Research Scientist, is with the United States Bureau of Mines, Salt Lake City, UT. R.M. IZATI', Professor, Department of Chemistry, and J.L. OSCARSON, Associate Professor, Department of Chemical Engineering, are with Brigham Young University, Provo, UT. Manuscript submitted July 19, 1991. METALLURGICAL TRANSACTIONS B
interactions with transition metal compounds, depending on the electronic configuration of the metal. Supercritical fluids also are of fundamental scientific interest because they exhibit anomalous chemical behavior. I6'7m Solubilities of ionic, metal-containing compounds in supercritical CO2 have received little attention, and the chemistry of the resulting solutions has not been studied. In this article, TiCh is considered to have appreciable ionic character because of the large difference in electronegativities of the constituents. The small size and high formal charge on the titanium also contribute to the ionic character of this compound. Thus, the chemical bonds have a large ionic component. It should not be assumed, however, that the description "ionic" implies ionization of the compounds in the CO2 solutions. Compounds having a strongly polar or ionic nature are poor candidates for extraction with supercritical CO2. t9~ It was expected that stu
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