A Study of the Solubility of Hydrated Orthophosphates of Yttrium, Lanthanum, Cerium, and Neodymium in Sulfuric-Phosphoric Acid Solutions at 20°C

Abstract

The solubility of YPO 4 . 2H 2 O, LaPO 4 . 1.5H 2 O, CePO 4 . 1.5H 2 O, and NdPO 4 . H 2 O 10 360 wt % in sulfuric-phosphoric acid solutions containing 10 360 wt % H 2 SO 4 and 13.8 341.4 H 3 PO 4 was studied.The Khibiny apatite concentrate is a promising source of lanthanides. It is mostly processed by the semihydrate sulfuric acid technology, with the main part of lanthanides concentrated in the phosphosemihydrate [1,2]. It has been shown previously that the main part of lanthanides is present in the phosphosemihydrate in the form of hydrated orthophosphates [3,4]. The content of yttrium, lanthanum, cerium, and neodymium oxides in the sum of lanthanide oxides in the apatite concentrate is about 90 wt % [5]. Therefore, data on the solubility of their hydrated orthophosphates in mineral acids are particularly important for development of processes for acid leaching-out of lanthanides from the phosphosemihydrate. In addition, data on the solubility of YPO 4 . 2H 2 O make it possible to indirectly evaluate the solubility of hydrated orthophosphates of lanthanides of the yttrium group.The solubility of hydrated lanthanum, cerium, and neodymium orthophosphates in nitric acid solutions containing 135 M HNO 3 has been examined [6]. It was found that, for the compounds studied, it increased with the HNO 3 concentration, to reach a value of 70 3100 g l !1 (in terms of oxides) at 20oC. The solubility varied in the order Ce ; Nd > La; however, the solubility of hydrated lanthanum orthophosphate at 100oC markedly exceeded that of hydrated neodymium orthophosphate.The most promising method for recovery of lanthanides from phosphogypsum and phosphosemihydrate formed in sulfuric acid processing of the apatite concentrate into extraction phosphoric acid is their leaching-out with sulfuric acid [7,8]. However, the solubility of hydrated lanthanide orthophosphates in sulfuric acid solutions has been insufficiently studied.It has been found that the solubility of yttrium, lanthanum, and cerium hydrophosphates in sulfuric acid solutions containing 2310 wt % H 2 SO 4 and 2.76 g l !1 H 3 PO 4 increases with the H 2 SO 4 concentration, to reach values of 11.8, 11.1, and 10.7 g l !1 , respectively (in terms of Ln 2 O 3 ). In a H 2 SO 4 solution with a concentration of 2 wt %, their solubility decreased by a factor of 1.8 32.0 as the H 3 PO 4 concentration was raised from 0 to 14 g l !1 . The aim of this study was to determine how the solubility of YPO 4 . 2H 2 O, LaPO 4 . 1.5H 2 O, CePO 4 . 1.5H 2 O, and NdPO 4 . H 2 O at 20oC depends on the concentration of sulfuric acid solutions with c H 2 SO 4 = 103 60 wt % and minor additions of orthophosphoric acid (c H 3 PO 4 = 13.8 3 41.4 g l !1 ). EXPERIMENTAL Reagents of analytically pure grade were used in the study. The hydrated phosphates were synthesized by the procedures described in [9 312]. It was established, using thermogravimetric and X-ray phase analyses, that the phosphates synthesized corresponded to YPO 4 . 2H 2 O and LnPO 4 . 1.5H 2 O (Ln = La, Ce) and NdPO 4 . H 2 O.