Distribution of lanthanide and actinide elements between molten lithium halide salts and liquid bismuth solutions

Abstract

Measurements of the equilibrium distribution of several lanthanide and actinide elements between liquid bismuth solutions and a variety of lithium halide salts were made in the temperature range 600–750°C. The salts included molten LiCl, LiBr and several LiFBeF 2ThF 4 solutions. At each temperature, the distribution coefficients (mole fraction in the bismuth phase divided by mole fraction in the salt phase) for most of the solutes obeyed the relationship D M = (N Li n) (K M ∗) , in which n is the oxidation number of solute M n+ in the salt phase and N Li is the mole fraction of lithium in the bismuth phase. Under the experimental conditions employed ( N Li ranging between 10 −5 and 0·3), thorium and protactinium were in the 4+ oxidation state in the salt phase; La, Nd, U, Np, Pu and Cm were tripositive; and Ba and Eu were dipositive. The values of log K M ∗ generally decreased regularly with increasing temperature. The oxidation numbers of Sm, Am and Cf in the salt phase varied between 3+ and 2+, depending on the experimental conditions. Californium existed primarily in the 2+ oxidation state with LiCl as the salt phase, whereas, in fluoride salts, the californium was tripositive. The variation of the distribution coefficients of Sm, Cf and Am with N Li at a given temperature could be expressed as D M = N Li 3/( α+ βN Li), in which α and β are constants.