| dc.description.abstract | Production of misch metal (a mixture of rare earth metals) by electrolysis of molten rare earth chlorides invooves high temperatures and reactive liqueds. Chemical degradation of the sidelining of the cell may terminate the cell life, and improved chemical stability og sidelining of the cell may terminate the cell life, and improved chemical stability of sidelining materials is desired in order to increase the cell life and reduce investment costs. The aim of the present work was to identify potential ceramic sidelining materials and to investigate their chemical stability in laboratory tests reflecting the chemical environments in an electrolysis cell.
Four ceramic materials (A12O3, Y2O3, A1N, BN) were selected as candidates for sidelining materials based on a simple thermodynamic analysis. Dense Y2O3 and A1N were prepared by sintering of fine grained powders, while commercial A!2O3 and BN materials were investigated. The materials were characterized with respect to phase composition, microstructure and oxygen content. The chemical stability of the materials was wxamined in five different laboratory tests which represent various conditions in anelectrolysis cell, defined as:
1) Molten elecrolyte
2) Molten metal
3) Two-phase system consisting of molten electrolyte and molten metal
4) Chlorine gas
5) Chlorine gas and molten electrolyte
Due to the high abundance of cerium in misch metal the metal and elctrolyte used in the tests were molten cerium and a 1:2:2 mole ratio CeC13-NaC1-KC1 melt. The molten elctrolyte and molten metal experiments were performed in molybdenum crucibles in high temperature furnace connected to a glovebox with controlled atmosphere (N2/Ar) with moisture and oxygen contents lower than 1 ppm. The temperature varied from 810 to 1047° C, the time of exposure varied from 1 hour to 28 days, and agitation in the liquids was achieved by rotating the test specimen. The exposure of the materials to chlorine gas was performed in a thermogravimetric balance, while the combination of chlorine gas and molten electrolyte was conducted in a glass carbon crucible in a tight silica vessel. X-ray diffraction analysis, scanning electron microscopy and gravimetry was conducted to evaluate the test specimens after exposure. Chemical analysis of the oxide content in the metal or electrolyte was also conducted in some of the experiments.
Alumina was observed to exhibit the best overall performance in a rare earth electrolysis cell environment. Alumina dissolves slowly in the electrolyte to produce volatile NaA1C14 and KA1C14. The initial dissolution rate at 1022° C was estimated to 4.5 mm/year. Alumina was observed to be attacked in molten cerium, but the extent and character of the reaction is strongly dependent on the convective conditions in the molten metal. When agitation is present a protective layer of CeA1O3 is formed at 810°C, while a protective layer of Ce2O3 is formed at 907° C. The protective layers prevent further attack on alumina. In absence of agitation alumina dissolves rapidly in cerium to form Ce2O3, which precipitates as spherical particles in the molten metal. The reaction between molten alumina and cerium is however reduced when molten electrolyte is present, due to formation of a chloride film on the alumina specimen. The resistence against chlorination of alumina was observed to be high, in line eith thermodynamic calculations.
Yttria was shown to be completely converted to oxy-chlorides according to
Y2O3(s)+3xCeC13(1)=3 CexY1-xOC1 (s) + (3x-1) YC13(1) (0.1)
when exposed to molten electrolyte, and was therefore considered unfit for use in rare earth chloride environments and was not further examined.
A1N exhibits a good chemical stability in the molten electrolyte, and no sighns of chemical reactions were observed neither on the A1N grains nor on the yttria based secondary phase. CeN was observed to be formed when A1N was exposed to molten Ce. However, the extent of the chemical reaction was moderate, and A1N is a potential container materail for cerium metal. the combination of electrolyte and metal was, however, destructive to the part of A1N in contakt with chlorine gas was clearly demonstrated. A1N with yttria based secondary phase (A12Y4O9,A1YO3) disintegrated in chlorine gas, while A1N with calcia based secondary phase was not attacked at the grain boundaries. Only a moderate degree of chlorination of A1N grains was observed, demonstrating the slow kinetics of the energetically favourable reaction.
BN materaials with low oxygen content was observed to be a suited material in conact with molten electrolyte. No signs of chemical attack was observed between BN and the molten electrolyte. Bn with high oxygen content, however, reacts with alkali components of the electrolyte to form a viscous glassy layer on the surface. Considerable molten cerium attack of BN was observed, and the formation of borides changed the rheology of the metal such that electrolysis and metal handling may become difficult. BN is practically inert against chlorination at the temperature considered ( <950° ), in spite of the high thermodynamic driving force for chlorination. | nb_NO |
