SOLUBILITY OF GADOLINIUM, NEODYMIUM

AND ITTRIUM OXIDES IN HALIDE MELTS.

 

I.R. Elizarova, E.G. Polyakov, P.T. Stangrit

I.V. Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials, Apatity, Russia

 

The gadolinium, neodymium and ittrium oxides solubility in KCl-KF eutectic mixture was determined voltammetrically (by analogy with work [1]) under isothermic conditions, as well as by the method of isothermal saturation in a wide range of temperatures.

Increasing the content of the above oxides in the melt brought entailed an increase in anodic peak current of oxygen-containing halide complex which was observed on the voltammograms recorded with the scan rate 0.1 V^.s^-1 (the working electrode-glassy carbon, quasi reference electrode -molybdenum.)

The addition of the aforementioned oxides to the melt in a quantity equal to or exceeding the solubility limit resulted in the anodic peak stabilization. The dependencies j^a_p = f([Ln₂O₃]) have a section corresponding to a stable value of j^a_p. The initial rectilinear section (up to the anodic current stabilization) was described by the following equations:

 

Y = 294.020×X + 13.727 for Gd₂O₃                                      (1),

Y = 38.617×X + 7.837               for Y₂O₃                                        (2),

Y = 71.323×X + 19.208 for Nd₂O₃                                      (3),

 

where Y is current density of the anodic peak of the oxygen-containing ion oxidation and X is the oxide concentration in the melt.

By introducing a rare earth metal oxide in the melt in a quantity a fortiori greater than the solubility limit (in our case ~5 w/o) and selecting samples for the Ln₂O₃ content  we constructed the plots of the rare earth metal oxide solubility limit dependence on temperature. The resulting functions are described by equations:

 

Y = 0.0015×X – 0.0794 for Gd₂O₃                                                  (4),

Y = 0.0136×X – 8.4549 for Y₂O₃                                                    (5),

Y = 0.0139×X – 0.2602 for Nd₂O₃                                                  (6),

 

where Y is ultimate concentration Ln₂O₃ in the melt and X is the melt temperature.

Using dependencies (1), (2), (3) and assessing the oxygen content in fluorides of the aforementioned rare earth metals we arrived at the conclusion that the hydrofluoride method of LnF₃ preparation in inert atmosphere is preferable to the following methods: 1) Hydrofluoride method in inert air atmosphere, 2) Hydrofluoride method with a repeat fluorination in air, 3) Hydrofluoride method in inert atmosphere, 4) Deposition from a solution followed by calcination in vacuum.

 

1. Polyakova L.P., Kononova Z.A., Elizarova I.R., Polyakov E.G. Voltammetric determination of the oxide ion in halide melts. // Zhurn. Anal. Khim. (Rus.) -1994, № 11.-p.1228-1232.