The dependence of the solubility of NaLa(SO 4 ) 2 . H 2 O, NaCe(SO 4 ) 2 . H 2 O, KLa(SO 4 ) 2 . H 2 O, and KCe(SO 4 ) 2 . H 2 O on the concentration of acids in sulfuric, phosphoric, and sulfuric3phosphoric acid solutions containing up to 36 wt % H 2 SO 4 and 33.12 g l !1 H 3 PO 4 was studied at 20oC.The most promising method of the recovery of lanthanides from phosphogypsum and phosphosemihydrate, which are formed in the manufacture of phosphoric acid by the processing of apatite concentrate with sulfuric acid, is the sulfuric acid leaching [1,2]. Because these products contain sodium and potassium hexafluorosilicates, sparingly soluble double sulfates of lanthanides with sodium and (or) potassium can be formed in the leaching solutions. In addition, the sulfuric acid leaching solutions contain residual phosphoric acid from phosphogypsum. NaLa(SO 4 ) 2 . H 2 O and KLa(SO 4 ) 2 . H 2 O are poorly soluble in water [3, 4]. Data on the solubility of double sulfates MLn(SO 4 ) 2 . H 2 O in sulfuric and sulfuric3phosphoric acid solutions are lacking. The concentration of lanthanum and cerium oxides in the sum of lanthanide oxides in the apatite concentrate exceeds 70 wt % [5]. Therefore, data on the solubility of lanthanum and cerium compounds are especially important for understanding processes that occur in sulfuric acid leaching of lanthanides. The goal of this study was to determine the solubility of double sulfates M`M"(SO 4 ) 2 . H 2 O (M`= Na, K; M" = La, Ce) in sulfuric and sul-furic3phosphoric acid solutions of low and medium concentrations. EXPERIMENTALWe used analytically pure grade chemicals. The double sulfates were synthesized by the standard procedure [4, 6], i.e., by evaporation of an aqueous solution containing lanthanide and alkali metal sulfates, with alkali metal sulfates taken in a 20% excess above stoichiometry. The precipitates were filtered off, the residual mother liquor was pressed out by filter centrifugation, excess alkali metal sulfates were removed from the mother liquor by washing with 3% H 2 SO 4 , the residue of the washing solution was pressed out by filter centrifugation, and the precipitate was dried at 1253150oC. The chemical analysis of the products synthesized (Table 1) was consistent (taking into account 3% error of the atomic absorption method used), and the X-ray phase analysis revealed no impurity phases. According to the thermogravimetric (TG) analysis, the products synthesized are monohydrates.The sulfuric3phosphoric acid solutions were prepared by adding small amounts of 85 wt % H 3 PO 4 into sulfuric acid solution of prescribed concentration c H 2 SO 4 ; the corresponding change in c H 2 SO 4 was neglected. The H 3 PO 4 concentration c H 3 PO 4 in the solutions was in the range typical of process solutions obtained from leaching of phosphosemihydrate waste with sulfuric acid.The content of lanthanides in the solutions was determined by oxalate gravimetric method [7] and by atomic-emission spectrometry with an inductively coupled plasma source on a Plasma 400 spectrophoto...

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Leaching-out of lanthanides from waste phosphosemihydrate with 20 330% sulfuric acid solutions was studied.Upon processing of the Khibiny apatite concentrate by the semihydrate technology to obtain wet-process phosphoric acid, the main part of lanthanides is contained in calcium sulfate semihydrate [technical name of the product: phosphosemihydrate (PSH)] [1, 2]. Sulfuric acid leaching can be used for recovery of lanthanides [3,4].In [3,4], the sulfuric acid leaching was performed with low-concentration solutions (c H 2 SO 4 < 10 wt %) at high l : s = (5310) : 1. A high degree of recovery of lanthanides into solution was achieved (up to 84%), but the concentration of lanthanides in solution, c S Ln 2 O 3 , did not exceed 1 g l 31 . All attempts to raise c S Ln 2 O 3 in leaching solutions failed.In the process, it was established that, in leaching of several portions of PSH with the same sulfuric acid solution at l : s = 3 : 1, leaching time of 233 h, and c H 2 SO 4 subsequently raised to 12324 wt %, the concentration of lanthanides in solution increased to 3 g l 31 and more, but the recovery of lanthanides in each stage did not exceed 30 3 40% and decreased as. With a solution with c H 2 SO 4 = 25 wt % and l : s = 5 : 1, the recovery of lanthanides into solution was 30% at c S Ln 2 O 3 = 0.34 g l 31 [5].The experimentally determined concentration of lanthanides in leaching solutions is considerably lower than the solubility of lanthanide sulfates in sulfuric acid solutions. However, the reason for this fact has not been established.The large volumes of the solutions involved and low concentration of lanthanides in them strongly hinder their processing to recover lanthanides and subsequent utilization in manufacture of wet-process phosphoric acid. This makes studies aimed to improve the leaching technology a matter of current interest.The goal of this study was to examine leaching-out of lanthanides from waste PSH with sulfuric acid solutions of medium concentrations (20 332 wt %) andto reveal the factors limiting the accumulation of lanthanides in leaching solutions. EXPERIMENTALExperiments were carried out with waste PSH 1 with a moisture content of 20 wt %. It contained 0.56 wt % 5 Ln 2 O 3 in terms of a dry product.The influence exerted by the conditions of leaching with sulfuric acid solutions of medium concentration (c H 2 SO 4 = 20332 wt %) on the processing efficiency was analyzed. The leaching time was 1 h, because this time is sufficient according to [6]. After the leaching was complete, the solid phase was separated from the solution by filtration. In some of the experiments, the same portion of the sulfuric acid solution was used to successively process two portions of the starting PHS (double-stage leaching). The concentration of the sum of lanthanides c S Ln 2 O 3 and c CaO were determined by the oxalate gravimetric technique, in which the relative determination error was 0.3 rel % and more for the lanthanide concentrations in the range under study [7]. The concentrations of individual oxides were de...

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.

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