Studies on uranium extraction from phosphoric acid using di-nonyl phenyl phosphoric acid-based synergistic mixtures

Abstract

During acidulation of phosphate rock with sulphuric acid to produce phosphoric acid for fertilizer application, the uranium values contained in the rock also become solubilised. The phosphoric acid produced by the ‘di-hydrate’ process, containing 26–30% P 2O 5, called the ‘weak phosphoric acid’ (WPA), is concentrated to >50% P 2O 5 grade, known as ‘merchant grade acid’ (MGA). The ‘hemi-hydrate’ process directly yields acid of high P 2O 5 content. The uranium concentration varies with acid concentration, provided oxidising conditions are maintained so that selective precipitation of uranium does not take place. Depending on the rock used, the acid produced can contain 0.0050–0.05 g U 3O 8/L. Separation of uranium by solvent extraction employing a novel synergistic combination of organo-phosphorus reagents is reported in this paper. In particular, the solvent mixture of di-nonyl phenyl phosphoric acid (DNPPA) with di-butyl butyl phosphonate (DBBP) in an aliphatic diluent has been found to be a stronger extractant than the commonly used combination of di-(2-ethyl hexyl) phosphoric acid (DEHPA) with tri- n-octyl phosphine oxide (TOPO). The DNPPA–TOPO mixture is an even stronger extractant. Results on extraction from both WPA and MGA are reported. Stripping of uranium from the organic phase was achieved with concentrated phosphoric acid in the presence of Fe +2 reducing agent at elevated temperature. Stripped uranium was subjected to a second cycle of extraction-stripping and recovered as a peroxide precipitate of high purity. Brief results on allied areas of acid pre-treatment and post-treatment are included.