Thermochemical Investigations in the System Cadmium-Praseodymium Relevant for Pyrometallurgical Fuel Reprocessing

Abstract

Vapor pressure measurements, in terms of a (non-)isothermal isopiestic method, were carried out in the system Cd-Pr between 749 K and 1067 K (476°C and 794°C). Thermodynamic activities of cadmium as a function of temperature were obtained directly for the composition ranging from 50.0 to 85.7 at. pct Cd. From these results, partial molar enthalpies of mixing of Cd were derived for the corresponding composition range. The activity values of Cd were converted to an average sample temperature of 823 K (550°C) by applying an integrated form of the Gibbs-Helmholtz equation. These data indicate that Cd 2 Pr and Cd 58 Pr 13 are probably the most stable intermetallic compounds in this system. Using an activity value of Pr from the literature as integration constant, Gibbs-Duhem integration was performed, and integral Gibbs energies are presented at 823 K (550°C), referred to Cd(l) and a-Pr(s). Gibbs energies of formation at the stoichiometric compositions of the phases Cd 6 Pr, Cd 58 Pr 13 , Cd 45 Pr 11 , Cd 3 Pr, and Cd 2 Pr were determined to be about À18.8, À23.5, À24.8, À28.7, and À33.8 kJ g-atom À1 at 823 K (550°C), respectively.

Thermochemical investigations in the system Cd–Gd

“…Besides, no samples were obtained in the phase Cd 3 Gd suggesting that Cd 3 Gd is only slightly more stable than a two-phase mixture of its neighbouring compounds. This phenomenon was actually observed already earlier in the systems Cd–Pr and Cd–Ce where similarly no single-phase samples of the corresponding phases Cd 3 Pr and Cd 3 Ce were formed [8,9] . Nevertheless, it should be mentioned that Cd 3 Gd crystallizes in a different crystal structure compared to Cd 3 Pr and Cd 3 Ce [20] , a fact that makes the comparability regarding stability of these phases more surprising.…”

“…This behaviour is outlined in detail in Ref. [8] . Considering all this, the resulting error in compositions was estimated not to exceed ±0.5 at.%.…”

Enthalpies of formation of Cd–Pr intermetallic compounds and thermodynamic assessment of the Cd–Pr system

“…All isothermal reactions as well as the corresponding reaction temperatures relevant for the present CALPHAD-type optimization are listed in Table 1 . All intermetallic compounds except Cd 6 Pr and Cd 11 Pr show noticeable homogeneity ranges in the order of ~1 at% which were defined by Reichmann et al in their recent phase diagram study [11] in agreement with the results from vapour pressure measurements [10] .…”

“…The corresponding values at 823 K were determined to be −11.2±0.1, −18.7±0.1 and −22.9±0.1 kJ mol(at) −1 . Comparing these values with Gibbs energies of formation given by Reichmann and Ipser [10] a quite good agreement can be observed. In addition, Castrillejo et al listed partial Gibbs energies of Pr in the two-phase fields Cd 58 Pr 13 +Cd 6 Pr, Cd 6 Pr+Cd 11 Pr, and Cd 11 Pr+L, given as −107.6±0.6, −127.4±0.9 and −133.8±1.2 kJ mol(at) −1 for 823 K.…”

“…As far as thermodynamic information is concerned, Reichmann and Ipser [10] determined Cd vapour pressures as a function of composition and temperature, using an isopiestic vapour pressure method. From these data the authors derived activity values of Cd at 823 K. By using an activity value for Pr in the two-phase field Cd 11 Pr+L, taken from Johnson and Yonco [15] , as integration constant, a Gibbs–Duhem integration was performed to calculate Gibbs energies of formation at 823 K. In the study by Johnson and Yonco, the Gibbs energy of formation of the compound Cd 11 Pr had been determined by means of a Gibbs–Duhem integration based on thermodynamic activity values of Pr in Cd 11 Pr from own emf measurements.…”