The effect of fluoride and silicate ions on the coprecipitation of gadolinium with calcium in phosphoric and sulpho-phosphoric media

Abstract

This work was carried out with the aim to establish the effect of some impurities on the coprecipitation of gadolinium with calcium phosphate and gypsum. The tests were performed using the radioactive tracer technique to monitor the fate of gadolinium in various phosphoric and sulpho-phosphoric media containing fluoride and silicate ions as impurities. In 10⁻² to 10⁻¹ M NH₄H₂PO₄ solution, the Gd(III) at a concentration of 10⁻⁴ M is almost entirely precipitated as amorphous phosphate. However, the presence of H₂SO₄ at C_H2SO4 ≥ 0.1 M, reduces the coprecipitation to less than 8 at. %, on average. The fluoride ions in solution, even at C_HF ≤ 0.1 M, induce a reduction of coprecipitation of 10 to 30% according to HF concentration. In the media containing 10⁻² to 10⁻¹ M NH₄H₂PO₄ and 10⁻¹ M H₂SO₄, the effect of HF is almost negligible in the absence of Si(IV); nevertheless, the presence of fluorosilicate in solution may contribute to the enhancement of the coprecipitation of the REE. The sulphuric acid (0.1 M) in phosphoric media (0.74 ≤ C_H3PO4 ≤ 4.44 M), leads to a significant coprecipitation of the REE (1.3 ± 0.2 ≤ D_Gd ≤ 3.1 ± 0.5), whilst the addition of HF (0.1 M) to these media enhances the solubility of the REE (〈D_Gd〉= 0.06 ± 0.01).

XRD, IR spectroscopy and elemental analyses of the solid phases in conjunction with the variation of the distribution coefficient D indicate that the coprecipitation of the REE is likely controlled by heterovalent substitution of REE in gypsum and its precipitation as phosphate or fluorosilicate.