Influence of complexation on the composition of equilibrium phases in

DOI: 10.2478/s11532-006-0060-y Short Communication CEJC 5(1) 2007 13–19

Influence of complexation on the composition of equilibrium phases in the system of Ce2(SO4)3 La2(SO4)3 Maria M. Milanova1∗ , Dimitar S. Todorovsky1, Christo C. Balarew2, Natalia L. Minkova1, Katya I. Ivanova1 1

2

Faculty of Chemistry, University of Sofia, Sofia 1164, Bulgaria

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria

Received 1 August 2006; accepted 23 October 2006 Abstract: The cocrystallization of Ce2 (SO4 )3 and La2 (SO4 )3 is studied in aqueous and H2 SO4 (150 g/dm3 ) solutions at 25 ◦ C and 64 ◦ C. The effect of the formation of inner sphere sulfate complexes of the type LnSO+ 4 in determining the composition of the equilibrium phases is revealed. c Versita Warsaw and Springer-Verlag Berlin Heidelberg. All rights reserved. Keywords: Ce - sulfates, complexes, La-sulfates, solid solutions, solubility

1

Introduction

The solubility of rare earth sulfates is rather important for determination of their behaviour in soils and plants [1, 2], natural waters [3], etc. Knowledge on the composition of the equilibrium phases formed in multicomponent systems, containing rare-earth sulfates is crucial for the application and optimization of sulfuric acid processing of rare earth concentrates [4–6]. The behaviour of the systems and especially the solubility of the different sulfates at ambient temperature (25 ◦ C) and around 64 ◦ C is of primary interest due to the transformation of CaSO4 ·2H2 O into CaSO4 ·0.5H2O at 66 ◦ C. This transformation is very important for the separation of Ca which is the main constituent ∗

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M.M. Milanova et al. / Central European Journal of Chemistry 5(1) 2007 13–19

of the rare earth concentrates produced as a by-product of the H2 SO4 - digestion of apatite [7]. Ce2 (SO4 )3 .xH2 O and La2 (SO4 )3 ·yH2 O are formed at the following temperatures with x = 8, 9 [8] and y = 8, 9 [9, 10] (25 ◦ C); x = 4, 5, 8, 9 [8, 9] and y = 9 [9] (64 ◦ C) on crystallization from aqueous solutions, and x = 4 [9] on crystallization at 64 ◦ C from sulfuric acid solution. No data are available for the crystalohydrate form of La2 (SO4 )3 under the latter conditions. The crystal structure and the solubility of the crystalohydrates under these conditions have been determined [8, 9, 11–14]. The literature data for systems containing salts of two or more rare earth elements are restricted. The solubility of double alkali metal (Na, K) rare earth (La, Ce) sulfates in sulfuric-phosphoric acid solutions at 20 ◦ C is reported in [15]. Data on cocrystallization from water solutions of nitrates, chlorides, bromates, selenates, ethylsulfates and sulfates of La and Nd are published [16–19]. The data on the La-Ce(III) sulfate systems in particular are very limited. Some peculiarities in the solubility of the sulfates at their coexistence in the systems Ln2 (SO4 )3 - H2 O and Ln2 (SO4 )3 -H2 SO4 - H2 O (Ln= La, Ce, Pr, Nd) in the absence or presence of CaSO4 were reported [20]. Some of the factors determining system behaviour such as temperature, H2 SO4 - concentration and nature of the Ln - species existing in the solutions were revealed. The precise interpretation of the results is, however, restricted by the complicated composition of the system. The solid solutions of Ce2 (SO4 )3 -La2(SO4 )3 following their crystalohydrate structure, solubility of the individual sulfates and the consideration of the LnSO+ 4 formation are studied in the present work.

2

Experimental

2.1 Materials The octahydrates of the rare earth sulfates used were obtained from their oxides (having assays >99.9 %) according to the synthetic procedure, presented in [21]. The oxides were dissolved in H2 SO4 (p.a., Merck), the crystals obtained were heated at 450 ◦ C, then dissolved in ice-cold H2 O. After filtration, C2 H5 OH was added to the water solution until a precipitate was formed. After 12 h the precipitate was filtered off and air-dried. The crystallization water content was confirmed by complexometric analysis of the rare earth element in a dried salt.

2.2 Solid solutions preparation In order to obtain the desired solution, the sulfates investigated were dissolved in H2 O or H2 SO4 (150 g/dm3 ) at 1-2 ◦ C. The sulfuric acid concentration is chosen according to the concentration suitable for the processing of the particular rare earth concentrate. Definite volumes of the solutions of both sulfates were mixed and adjusted so that the desired molar ratio of the two lanthanides was obtained. The resulting solution was placed



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in a thermostatic vessel with reflux and magnetically stirred at 25 ◦ C or 64 ◦ C for 168 h. After filtration, both the crystals obtained and the saturated solution were analyzed.

2.3 Analysis The Ce content was determined by cerimetry after its oxidation to Ce(IV). From complexometry the sum of Ce and La was determined and the La-content obtained by difference. Diffractograms of the solid phases were obtained using a powder diffractometer (DRON, former USSR) employing CuKα, 60 kV, 30 mA and a scanning rate of 2 ◦ /min.

3

Results and discussion

The composition of the equilibrium phases obtained with different crystallization conditions is shown on Table 1. The solubilities found for the pure individual compounds are close to those in the literature data. The X-ray diffractometry shows that solid solutions of octa - (at 25 ◦ C) and nonahydrates (at 64 ◦ C) of the two elements, slightly enriched to the less-soluble La2 (SO4 )3 are formed from aqueous solution (in the studied Ce/La mole ratios). The nonahydrate solid solution is based on the La2 (SO4 )3 ·9H2 O crystal structure. A solid solution, crystallizing on the base of the Ce2 (SO4 )3 ·4H2 O, is formed from the H2 SO4 – solution at 64 ◦ C in the studied concentration interval. The formation of solid solutions is to be expected because the respective La- and Ce-sulfates are isomorphous [13]. It is known that at 64 ◦ C, La2 (SO4 )3 forms only the 9-hydrate under these conditions [9]. This fact explains the formation of solid solutions based on the La2 (SO4 )3 ·9H2 O structure, in spite of the pure Ce2 (SO4 )3 crystallizing as the 4-hydrate under the same conditions. The concentrations of both sulfates in the equilibrium liquid phases are proportional to their concentration in the solid phases, remaining (in the studied Ce/La mole ratios) less than the solubility of pure individual sulfates. The only exception is the concentration of Ce2 (SO4 )3 at 64 ◦ C in H2 SO4 -solution which is in equilibrium with a 89 mol % Ce solid solution. Formation of monosulfato-complexes of rare earths has been long established and many investigations have been made on the determination of their stability constants. Previous results are summarized in [22]. A new approach to measure the stability constants with higher precision has been applied [23] in which some of the former published results were confirmed. The formation of inner-sphere CeSO+ 4 in Ce-concentration and acidity intervals similar to the ones used in the present work was shown [22, 24]. In highly concentrated sulfuric acid solutions outer-sphere CeHSO2+ 4 complexes are formed but they dominate at H2 SO4 -concentrations above 1.7 M [25], i.e. above those employed in the present work. The CeSO+ 4 dissociation constant K could be calculated by the equation [24]: lgK = - 3.371 + 6.108 μ1/2 (1 + 1.885 μ1/2 ), where μ is the ionic strength. In Fig. 1, the dependence of the Ce3+ -concentration in the equilibrium solution ([Ce3+ ]l ) vs. its concentration in the solid phase [Ce3+ ]s is compared with the respective values of K. In


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spite of the fact that the equation does not account for the temperature dependence of K (the dissociation is an exothermic process [24]), the symbatic pattern of [Ce3+ ]l and K as functions of [Ce3+ ]s is clearly expressed for both temperatures studied. It can be 3+ concluded that the stability of CeSO+ 4 determines the Ce -concentration in the equilibrium liquid phase. This conclusion is in agreement with the statement made in [24] that the formation of a cerium inner sphere - sulfate complex favours its coprecipitation with collectors in comparison with an aqua-complex. + The analogous LaSO+ 4 is more stable than the CeSO4 species [24]. The formation of both complexes can explain the reduced solubility of both sulfates from the solid solution observed in our experiments (Table 1). The higher stability of LaSO+ 4 may contribute to the observed La enrichment of the solid solutions. With the increasing H2 SO4 - concentration and the respective decrease of [SO2− 4 ] (on − 3+ behalf of [HSO4 ]), the Ce - ions are forced to exist mainly as aqua complexes that suppress their coprecipitation. According to [26] this effect is significant above 0.36 M and may contribute to the [Ce3+ ]l increase above that of the pure Ce2 (SO4 )3 , observed at certain compositions of the solid solution (Table 1). Table 1 Composition of the equilibrium phases. H2 SO4 - concentration (g/dm3 ) in the initial solution

Temperature ◦C

25

0 64

150

∗

64

Equilibrium phases Ce - content Concentration in liquid phase (mol %∗ ) in the (mmol/dm3 ) 3+ solid phase Ce La3+ 34.4 67.4 80.8 100.0 0.0

50 128 205 270 -

48 27 18 75

16.5 21.0 41.6 53.6 68.3 71.4 100.0 0.0

11 20 42 51 56 56 122 -

44 45 22 15 14 13 44

37.0 64.0 89.0 100.0 0.0

60 80 160 123 -

35 23 11 39

Related to the sum Ce + La



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Fig. 1 Ce3+ - concentration in the equilibrium phases at 25 ◦ C (1) and 64 ◦ C (2) and the ◦ values of the CeSO+ 4 - dissociation constant in the equilibrium solutions at 25 C (3) and 64 ◦ C (4).

4

Conclusion

The results reported suggest that the formation of inner sphere sulfate complexes of the type LnSO+ 4 are of primary importance for the determination of the composition of the equilibrium phases even in aqueous solution.


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