Precipitation of Rare Earth Phosphates from Molten Salts: Particle Size Distribution Analysis
Authors: Ivanov Alexander B. , Volkovich Vladimir A. , Maltsev Dmitry S. , Sukhikh Vladislav V. , Griffiths Trevor R. 
Published: February 2020

Phosphate precipitation is one of the possible methods for removing rare earth fission product (FP) elements from the technological melts in pyrochemical processes of spent nuclear fuel (SNF) reprocessing. Chemical and phase composition of the phosphates depends on the cationic composition of the melt and, in some instances, on the precipitant employed. Technologically attractive melts include 3LiCl–2KCl and NaCl–2CsCl eutectic mixtures and NaCl–KCl equimolar mixture. Previous studies showed that the normal rare earth orthophosphates (REEPO4) are formed in the 3LiCl–2KCl based melts. In NaCl–KCl and NaCl–2CsCl melts the composition of the precipitate is influenced by the initial phosphate-to-rare earth element molar ratio, and normal as well as double alkali metal – rare earth phosphates (M3REE(PO4)2 or M3REE2(PO4)3) can be produced. After precipitating rare earth FPs, the precipitate needs to be separated from the melt to allow the electrolyte reuse. Solid phosphates can be removed by filtering or collected employing submergible centrifuges. Particle-size distribution (PSD) of the solid precipitate can greatly affect the efficiency of the collection device. After the phosphates are separated they can be further treated for the final disposal, for example, by incorporating them into a suitable glass matrix (a vitrification process). Here again the PSD of the solid material can be important in understanding the properties of the final waste form. Despite the phosphate precipitation process has been studied by various techniques, there is very little information concerning the particle size of the precipitates formed in the process.

In the present work the effect of the initial phosphate precipitant-to-rare earth mole ratio on the particle size of the rare earth phosphates was investigated. The experiments were performed in NaCl–2CsCl and NaCl–KCl based melts at 650 and 750 oC, respectively. The REEs studied included La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb, where the heavier lanthanides (which are not the fission products) were included for comparison. Phosphate precipitant was added in the amount required for 50 and 100 % precipitation of the REE. In addition, the melts containing the mixture of REEs imitating their content in a typical light water reactor SNF arriving for reprocessing were studied.

Particle size of the precipitated phosphates was determined by laser diffraction method employing ANALYSETTE 22 NanoTec (Fritsch) instrument with a measuring range of 0.02–2100 μm. The measurements were performed both with and without using ultrasound to asses the effect of ultrasonic treatment on the particle deagglomeration.

The obtained results showed that the particle size of the precipitated phosphates typically lies in the range of 0.1–100 μm. Fraction of the particles with the size below 1 μm is relatively small.

The results of the measurements showed that the particles of the phosphates are agglomerated, maxima in the PSD curves shift towards lower values after the ultrasonic treatment of the samples.

Heavier lanthanides tend to form larger particles. For example, maxima in the PSD curves of the phosphates produced at 100 % precipitation in NaCl–2CsCl melts shifted from ca. 6 μm for lanthanum to ca. 20 μm for ytterbium.

The effect of the initial phosphate-to-REE molar ratio in the melt on the particle size is not straightforward. For the lighter lanthanides (from Ce to Sm) PSD curves for the precipitates obtained at 50 % degree of precipitation have two maxima – around 1 and 10–20 μm, and the average particle size is smaller than for the solids obtained at 100 % REE precipitation. For the heavier lanthanides the average size of the particles obtained at 50 % REE precipitation was greater than at complete precipitation.

Cationic composition of the melt does not significantly affect the average size of the particles of REE phosphates, although influences the particle-size distribution. Examples of the results obtained for precipitation of the sum of REE from NaCl–2CsCl and NaCl–KCl based melts are shown in the Figure. Bulk of the particles has the size within 1–100 μm limits.

Figure 1