ON THE CHEMICAL-TRANSPORT OF CR2O3 WITH C L2 AND WITH HGCL2 - EXPERIMENTS AND MODEL-CALCULATIONS

The migration of Cr2O3 in a temperature gradient (1 000-degrees-C --> 900-degrees-C) in the presence of low concentrations of chlorine and w ater (from the wall of silica ampoules) is a result from the endotherm ic reactions (1) Cr2O3,s + H2Og + 3Cl2,g = 2CrO2Cl2,g + 2HClg (2) Cr2O 3,s + 1/2O2,g + 2Cl2,g = 2CrO2Cl2,g With higher concentrations of chlo rine, the transport reaction is (3) Cr2O3,s + 5/2Cl2,g = 3/2CrO2Cl2,g + 1/2CrCl4,g The gas phase of the transport system Cr2O3/Cl2 can be re duced step by step by adding small amounts of chromium, so that CrCl3 and finally also CrCl2 become more important. Further, at a lower rati o n-degrees(Cl)/n-degrees(Cr) three transport reactions have to be tak en into consideration; with the participation of CrOCl2,g (5). (4) Cr2 O3,s + 9/2CrCl4,g = 3/2CrO2Cl2,g + 5CrCl3,g (5) Cr2O3,s + 3CrCl4,g = 3 CrOCl2,g + 2CrCl3,g (6) Cr2O3,s + H2,g + 4 HClg = 2 CrCl2, + 3 H2Og Th e reactions (1), (2) and (6) become possible through the cooperation o f two transport agents at a time. The migration of Cr2O3 with HgCl2 ca n also be described with reactions (1)-(3). The decomposition of HgCl2 produces the small chlorine pressure for the transport reaction. The oxidation potential of the transport agent HgCl2 is too low for the ox idation of Cr(III) to Cr(VI).