Selective extraction of uranium from nitric acid leachate of Minjingu rock phosphate

Abstract

A sample of beneficiated rock phosphate from Minjingu Mine and Fertilizer Plant, Tanzania was analysed by energy-dispersive X-ray fluorescence (ED-XRF) spectroscopy and found to contain 15.8 % and 5.30 × 10⁻² % w/w P₂O₅ and U, respectively. The U content of the same sample was estimated to be 3.70 × 10⁻² % w/w when the digested mass was analysed by inductively coupled plasma mass spectrometry (ICP-MS). The rock was leached with three different mineral acids at 65–70 °C. The outlet liquid and solid streams were analysed by ICP-MS and ED-XRF, respectively. Maximum leaching of P₂O₅ and U occurred with 8.9 M HNO₃. Their respective concentrations in the leachate were 6.69 % w/w (99.0 g L⁻¹) and 1.80 × 10⁻² % w/w (0.27 g L⁻¹). The leachate also contained 1.90 × 10⁻² % w/w (0.28 g L⁻¹) rare earth elements (REEs). Selective extraction of U was attempted employing Di-2-ethyl hexyl phosphoric acid (D2EHPA) and Tri-butyl phosphate (TBP) as co-extractants. A 2-stage cross-current solvent extraction (10 g scale; 30 °C) with 1:6 mol ratio of D2EHPA:TBP (0.14 M total extractant concentration; 1:1 w/w organic/aqueous ratio) gave 81–89 % U extraction without co-extraction of REEs. The extraction of U increased to 95.6 % after four stages of solvent extraction but there was a noticeable co-extraction of Y beyond the first two stages. Production of fertilizers from U-free leachate is environmentally safer and the recovered U, in pure form, can be used for fuel production.