Application of Phosphate Precipitation for Removing Strontium and Barium from Alkali Chloride Based Melts

Abstract

Alkali chloride based melts can be used as working electrolytes in prospective technologies of pyrochemical reprocessing of spent nuclear fuels. After extracting fissile materials the melts contain ions fission product elements having more negative electrode potentials than uranium and plutonium. These include rare earth, alkaline earth and alkali metals. One of possible methods for removing rare earths from alkali chloride based melts is phosphate precipitation. Alkaline earth metals also form sparingly phosphates and therefore phosphate precipitation can be potentially used for removing strontium and barium from molten electrolytes.The present work was aimed at studying the reaction of solutions of strontium or barium chloride in alkali chloride based melts with sodium orthophosphate. The experiments were conducted in LiCl–KCl and NaCl–KCl–CsCl eutectic based melts at 550 oC, and in NaCl–KCl equimolar mixture at 750 oC.To determine the conditions required for complete removal of strontium and barium, the initial phosphate-to-barium molar ratio in the melt was set at 0.5; 1; 2; 4; 6 and 8. Residual alkaline earth content in the melt was determined by chemical analysis. Precipitated strontium and barium phosphates were subjected to X-ray powder diffraction analysis.Selecting a possible method of separating solid precipitate from molten salt requires information on size of particles forming the solid phase. Particle size was determined by laser diffraction and examples of particle size distribution curves for precipitates formed in LiCl–KCl–BaCl2 melts are shown in Fig. The precipitates consisted of particles ranging from 0.5 to 100 microns. Increasing the initial PO4 3– : Ba2+ molar ratio resulted in increasing particle size but even at the molar ratio of six over 50 % particles were less than 10 μm.Fig. Particle size distribution curves for barium phosphate precipitated from LiCl–KCl eutectic based melts at 550 oC. Initial PO4 3– : Ba2+ mole ratio was 0.5 (line 1); 1.0 (2); 2.0 (3); 4.0 (4) and 6.0 (5). Figure 1