Thermochimica Acta 443 (2006) 105–108
Solubilities of lanthanum oxide in fluoride melts
Part I. Solubility in M AlF (M = Li, Na, K)
3
6
∗
M. Ambrov a´ , J. Juri sˇ ov a´
Department of Inorganic Technology, Faculty of Chemical and Food Technology, Slovak University of Technology, SK-81237 Bratislava, Slovakia
Received 8 November 2005; received in revised form 9 January 2006; accepted 9 January 2006
Available online 8 February 2006
Abstract
Solubility of lanthanum oxide was measured by thermal analysis. The solubility in alkali cryolites is rather high, because of chemical reactions
between lanthanum oxide and cryolites. In Li
3
AlF
6
–La
2
O
3
, alumina precipitates, in the other systems the mixed oxide LaAlO
3
is formed. In
◦
La
La
©
2
O
O
3
–Li
3
AlF
6
the eutectic point is at 9.5 mol% La
2
O
3
and 755 C. The eutectic points in La
2
O
3
–Na
3
AlF
6
and La
2
O
3
–K
3
AlF
6
are at 11.5 mol%
◦
2
3
, and at 937 and 934 C, respectively.
2006 Elsevier B.V. All rights reserved.
Keywords: Solubility; Lanthanum oxide; Molten cryolite; Molten fluorides; Thermal analysis
1
. Introduction
was kept 1 h at the temperature and regularly stirred by platinum
wire. The sample was slowly cooled after that time. Temperature
was measured with a PtRh10-Pt thermocouple calibrated to the
melting points of NaF, BaCl2, NaCl, KCl, LiF and Na2SO4.
The measured transition temperatures were reproducible within
There is wide interest in reprocessing of spent nuclear fuel.
A promising way of reprocessing is the use of molten salts
and particularly fluoride melts. The advantages are their sol-
vent properties for the dissolution of the fuel; they have a
large electrochemical window and are not sensitive to radiolytic
degradation [1,2]. This work is a part of a project dealing with
electroseparation of the constituents of spent MOX fuel. Lan-
thanum compounds in fluoride melts were chosen as a model
system. This paper deals with solubility and phase diagrams of
lanthanum oxide–alkali cryolite melts. No data on solubility or
phase diagrams of the systems are available.
◦
± 2 C.
The following chemicals were used: LiF, NaF (Lachema, ana-
lytical grade), AlF3, KF, La2O3 (Mikrochem, pure) and P2O5
(Mikrochem, analytical grade). LiF, NaF and La2O3 were dried
◦
at 600 C for 2 h, KF was dried in a vacuum drying oven in the
presence of P2O and AlF3 was purified by sublimation.
5
The samples were analysed by X-ray diffraction (STOE auto-
mated theta/theta diffractometer, Germany, Co K␣1 radiation,
◦
◦
λ = 0.17902 nm) from 5 to 80 2θ. The positions of the basal
reflections were determined by Bede ZDS program.
2
. Experimental
3. Theoretical
The temperatures of phase transitions were determined by
thermal analysis, recording the cooling and heating curves of
mixtures at 2–5 C/min in a resistance furnace with an adjustable
◦
Thermodynamic models of systems containing complex
compounds are usually based on the assumption of partial disso-
ciation of the compounds [3]. However, the structure of cryolite-
based melts is not reliably known [4]. The model presented
below is independent of the structure of the melt. The model is
described in detail by Danielik et al. [5–7]. It assumes the ideal
melt contains only ionic pairs of the basic species. In the case of
the complex compounds in the solid phase, the compounds are
assumed to completely dissociate to the basic species.
cooling rate. A platinum crucible containing 30 g of sample was
placed into the furnace pre-heated to the temperature of fusion of
pure alkali cryolite. (At the compositions where crystallization
of La2O3 was expected, the temperature was higher.) The sample
∗
Corresponding author.
E-mail address: marta.ambrova@stuba.sk (M. Ambrov a´ ).
0
040-6031/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2006.01.007