Pyrohydrolysis of rare-earth trifluorides in moist air

The kinetics of pyrohydrolysis of rare-earth trifluorides, LnF(3) (Ln = Y and La-Lu) have been investigated by means of thermogravimetry. There were two definite breaks on the TG curves that correspond to the conversion of LnF(3) into LnOF and from LnOF into Ln(2)O(3), except for CeF3. The beginning temperature of pyrohydrolysis decreased gradually and the reaction rate increased with increasing the acidity of Ln(3+) due to decrease of the ionic radius. The rate equation of conversion from LnF(3) to LnOF was expressed by [1 - (1 - x)(1/3)] = kt, where x is defined as the reaction ratio, with x = I when LnF(3) is completely transformed into LnOF. This rate equation is derived as the tarnishing reaction (LnF(3) particle is covered with LnOF layer) in which the rate-determining step is the surface reaction. It was assumed that cracks as gas paths were formed in the LnOF layer produced from the surface of sample particles, because of the smaller molar volume of LnOF compared with LnF(3). On the reaction from LnF(3) to LnOF, Ln(4)O(3)F(6) was detected in the XRD pattern. From the results of the kinetic study, the activation energy of pyrohydrolysis of LnF(3) decreased with increasing Ln3+ atomic number by the same reason as the decrease of starting temperature of pyrohydrolysis. (C) 2002 Editions scientitiques et medicales Elsevier SAS. All rights reserved.