Precipitation of rare earth phosphates through apatite leach as by-product of nitrophos acid from unconventional phosphate deposit of carbonatite, Malakand, Pakistan

Abstract

The igneous carbonatite apatite ore of Sellai Patti (Malakand, Pakistan) was beneficiated through mineral processing methods (crushing, grinding, gravity separation, and magnetic separation). The concentrate [Ca₅(PO₄)₃, F] (30 % P₂O₅, 0.7 % REEs) was leached with 65 % nitric acid (S/L 0.42, 65 °C, 700 rpm). The leachate (2.78 g/L REEs, 154 g/L P₂O₅) yielded a 92 % recovery of REEs. Precipitation of REEs was performed with 33 % ammonia solution by two different methods. The direct precipitation process (DPP), produced REE precipitates (34 %) at a pH of 0.2–0.5 with a subsequent hot washing. The remaining filtrate, after precipitation, contained REEs at a low concentration of 0.2 g/L, 97 g/L P₂O₅ and calcium up to 102 g/L. In the nitrophos precipitation process (NPP), a pre-concentration of REEs from 2.8 g/L to 4.1 g/L was observed through the removal of dissolved calcium in the form of calcium nitrate tetrahydrate (CNTH) as a result of freezing the leachate at −20 °C. The precipitates (9 % REEs) obtained at pH 1.7 followed by its re-dissolution in 65 % nitric acid (S/L 1:10) and overnight recrystallization with 50 % excess oxalic acid enhanced REEs concentration up to 30 %. The leftover nitrophos acid (NP acid), a precursor of nitrophos fertilizer, contained only <1 mg/L of REEs, 135 g/L of P₂O₅, and up to 40 g/L of calcium. The present research advances a sustainable and efficient methodology for the extraction of REEs through an innovative and significantly effective DPP.