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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