Solubility of YF3, CeF3, PrF3, NdF 3, and DyF3 in solutions containing sulfuric and phosphoric acids

Article abstract of DOI:10.1134/S0036023607120042

The solubility of YF₃, CeF₃, PrF₃, NdF₃, and DyF₃ in solutions containing 0-4.496% mol/L (0-35 wt %) of H₂SO₄ and 0-27.6 g/L of H ₃PO₄ (0-20 g/L of P_2<

Full text of DOI:10.1134/S0036023607120042

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 12, pp. 1830–1834. © Pleiades Publishing, Inc., 2007.
Original Russian Text © E.P. Lokshin, O.A. Tareeva, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 12, pp. 1946–1950.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Solubility of YF3, CeF3, PrF3, NdF3, and DyF3
in Solutions Containing Sulfuric and Phosphoric Acids
E. P. Lokshin and O. A. Tareeva
Institute of Rare Element and Mineral Chemistry and Technology, Kola Research Center, Russian Academy of Sciences,
ul. Fersmana 14, Apatity, Murmansk oblast, 184200 Russia
Received November 27, 2006
Abstract—The solubility of YF3, CeF3, PrF3, NdF3, and DyF3 in solutions containing 0–4.496% mol/L (0–35 wt %)
of H2SO4 and 0–27.6 g/L of H3PO4 (0–20 g/L of P2O5) at 20°C was determined. Higher solubility in sulfuric
acid solutions compared to that in hydrochloric and phosphoric acid solutions was attributed to the formation
of fluorosulfate complexes M2(SO4)F4 (M =Y, Ce, Pr, Nd, Dy). The effect of minor concentrations of the phos-
phate ions on the solubility of YF3, CeF3, PrF3, NdF3, and DyF3 in sulfuric acid solutions and the effect of flu-
oride ions on the recovery of lanthanides during sulfuric acid leaching from the phosphohemihydrate were dis-
cussed.
DOI: 10.1134/S0036023607120042
The product called phosphohemihydrate, which is
formed upon processing of Khibiny apatite concentrate
into mineral fertilizers, is a promising industry-pro-
duced raw material for lanthanides [1]. Although the
main bulk of lanthanides is contained in phosphohemi-
hydrate as also hydrated orthophosphates [2], phospho-
hemihydrate also includes substantial amounts of
sodium and potassium fluorosilicates, which are hydro-
lyzed to give fluoride ions; this may result in the forma-
tion of poorly soluble lanthanide fluorides. In view of
the development of a sulfuric acid process for the
recovery of lanthanides from phosphohemihydrate [3],
knowledge of the solubility of lanthanide fluorides in
solutions containing sulfuric and phosphoric acids is of
considerable practical interest.
Data on the solubility of lanthanide fluorides in min-
eral acids, in particular, in sulfuric and phosphoric
acids, are scarce. It was found that the solubility of
LaF3 and YbF3 in solutions containing 0–2.355 mol/L
(0–20 wt %) of H2SO4 and 0–27.6 g/L of H3PO4 (0–20 g/L
of P2O5) is much higher than their solubility in monoba-
sic acids, which was attributed to the formation of com-
plex fluorosulfate anions SO4F3– [4].
RESULTS AND DISCUSSION
Dysprosium fluoride (TU 6-09-4677-83), sulfuric
acid (GOST 4204-77), and phosphoric acid (GOST
6552-80) were reagent grade chemicals. Yttrium,
cerium, praseodymium, and neodymium fluorides were
prepared by reacting lanthanide nitrates with hydroflu-
oric acid taken in a 20% excess with respect to the sto-
ichiometry [5]. Gel-like precipitates were separated
from the mother liquor by centrifugation, washed with
distilled water to remove nitrate ions and excess HF,
dried, and triturated in a mortar. Reagent grade chemi-
cals were used for the synthesis. The procedure for
investigation was similar to that reported previously
[4]. The results are presented in Tables 1–5.
The solubilities of lanthanide fluorides in water
were found to be much lower than those reported previ-
ously (calculated for the oxides Ln2O3, g/L) which were
as follows: YF3, 0.0024 [6]; ëÂF3, 0.0051–0.0075;
PrF3, 0.0034–0.005; NdF3, 0.0027–0.0168; and DyF3,
0.0165 [7].
NdF3 is almost insoluble in phosphoric acid solu-
tions, while the other fluorides start to dissolve with an
increase in the H3PO4 concentration. Apparently, this is
due to the fact that neodymium precipitates from dilute
solutions of phosphoric acid as poorly soluble anhy-
drous NdPO4 [8], whereas yttrium, cerium, praseody-
mium, and dysprosium form more soluble hydrated
compounds: YPO4 · 2H2O [9], ëÂêé4 · ç3êé4 · Ûç2é
[10], Prêé4 · 2.5ç2é [11], and Prêé4 · 3ç2é [12]; as
the degree of hydration increases, the solubility
increases.
In phosphoric acid solutions, LaF3 is almost insolu-
ble, whereas YbF3 has a substantial solubility, which
increases with increasing H3PO4 concentration. This
was attributed to the fact that lanthanum precipitates
from dilute solutions of phosphoric acid as LaPO4,
whereas ytterbium apparently forms more soluble
hydrated compounds (YbPO4 · 4H2O or YbPO4 · 2H2O)
in dilute solutions of H3PO4 [4].
This work continues studies on the solubility of lan-
thanide fluorides with the concentration range of sulfu-
ric acid being extended to 4.496 mol/L (35 wt %).
Upon increase in the H2SO4 concentration in the
concentration range studied, the YF3 and ëÂF3 solubil-
1830

SOLUBILITY OF YF3, CeF3, PrF3, NdF3, AND DyF3
1831
Table 1. Solubility of YF3 in H2SO4–H3PO4 solutions at
(20 1)°C
As for LaF3 and YbF3, the solubilities of YF3 in
HNO3 and HClO4 [13] and YF3 and NdF3 in HCl [14]
are lower than those in the sulfuric acid solution with
the same normality.
Content of Y2O3 at P2O5 concentration, g/L
cH SO
,
4
2
mol/L
The dissolution of lanthanide fluorides in sulfuric
acid can be described by the reactions [4]
0
5
12
20
0
0.0002
0.33
0.59
0.79
0.96
1.2
0.0008
0.34
0.61
0.81
1.0
0.0033
0.38
0.63
0.81
1.0
0.0061
0.41
0.66
0.82
1.1
2åF3 + 3H2SO4
2åF3 + 2H2SO4
M2(SO4)3 + 6 HF,
2MSO4F + 4HF
(1)
(2)
0.312
0.635
0.971
1.32
or
2åF3 + H2SO4
M2(SO4)F4 + 2HF,
(3)
with the following equilibrium constants, respectively
[4]:
1.81
1.2
1.2
1.27
1.43
n.d.
66[M2O3]7
44[M2O3]5
2.325
3.004
3.727
4.496
1.35
1.46
1.47
1.46
1.40
n.d.*
n.d.
1.40
n.d.
--------------------------
----------------------------
Kc = K2
2
Kc = K1
,
[H2SO4]3
γ2[H2SO4]2
1
(4)
22[M2O3]3
--------------------------
3 γ[H2SO4]
n.d.
n.d.
and Kc = K
.
3
n.d.
n.d.
n.d.
* Here and in Tables 2–5, n.d. stands for not determined.
In Eqs. (4), [M2O3] and [H2SO4] are, respectively,
the lanthanide oxide and sulfuric acid concentrations in
solution, mol/L; and γ is the activity coefficient of sul-
furic acid.
ities reach a maximum at cH SO = 3.004 mol/L and
2
4
The Kc , Kc , and Kc values calculated from
then remain almost invariable for cH SO = 3.004–
1
2
3
2
4
experimental data are presented in Table 6. In calcula-
tions, the activity coefficients of hydrofluoric acid and
lanthanide cations were assumed to be fixed (they are
taken into account by ä1, ä2, and ä3); the γ values for
sulfuric acid at different concentrations were taken
from [15].
4.496 mol/L. The solubilities of NdF3 and DyF3
increase with an increase in cH SO over the whole
2
4
range studied. In terms of the solubility (mol/L) in solu-
tions of sulfuric and phosphoric acids of the same com-
position, lanthanide fluorides can be arranged in the
following sequence: ëÂF3 < LaF3 < NdF3 < YF3 < PrF3
< DyF3 ꢀ YbF3.
The Kc /ä1, Kc /ä2, and Kc /ä3 values for YF3,
1
2
3
CeF3, PrF3, NdF3, and DyF3 calculated from experi-
Table 2. Solubility of CeF3 in H2SO4–H3PO4 solutions at (20 1)°C
Content of Ce2O3 at P2O5 concentration, g/L
cH SO
,
4
2
mol/L
0
2
5
8
12
16
20
0
0.001
0.0582
0.305
0.381
0.553
0.734
0.820
0.830
0.830
0.820
0.002
0.048
0.29
0.36
0.52
0.66
0.77
n.d.
0.001
0.046
0.31
0.35
0.51
0.70
0.73
n.d.
0.0016
0.032
0.30
0.32
0.49
0.68
0.71
n.d.
0.001
0.041
0.29
0.33
0.49
0.61
0.73
n.d.
0.001
0.040
0.26
0.35
0.49
0.61
0.71
n.d.
0.003
0.040
0.27
0.33
0.48
0.60
0.68
n.d.
0.312
0.635
0.971
1.32
1.81
2.325
3.004
3.727
4.496
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 12 2007

1832
LOKSHIN, TAREEVA
Table 3. Solubility of PrF3 in H2SO4–H3PO4 solutions at
(20 1)°C
Kc /ä3 changes little, which can imply the existence of
complexes M2(SO4)F4 (M =Y, Ce, Pr, Nd, Dy) in sulfu-
3
ric acid solutions.
Content of Pr2O3 at P2O5
concentration, g/L
cH SO
,
4
2
The formation of fluorosulfate ions in concentrated
solutions of sulfuric and hydrofluoric acids was estab-
lished experimentally. In a study of the Ln2O3–TiO2–
H2SO4–HF–H2O systems (Ln = Y, Ce, Nd), the fluoro-
sulfates YSO4F · 2ç2é [16], ëÂSO4F · 2ç2é [17], and
NdSO4F · nç2O [18] crystallized at 50–75°ë (ëÂSO4F ·
2ç2é was not formed at 50°ë) from solutions containing
6–8 wt % HF, 0.15–3.52 wt % Ln2O3, and 44–60 wt %
H2SO4. High concentrations of titanium did not ham-
per the formation of lanthanide fluorosulfates if the
HF : TiO2 molar ratio in the initial solution was not
lower than 4 (mol/mol). For molar ratios HF : TiO2 > 4,
the solid phase consisted of a ëÂSO4F · 2ç2é and ëÂF3
mixture [17]. This attests indirectly to a high stability of
fluorosulfate complexes of lanthanides.
mol/L
0
5
12
20
0
0.00023
0.35
0.84
1.14
1.56
2.29
3.02
3.00
3.08
3.14
0.00035
0.20
0.60
0.94
1.30
2.19
2.71
n.d.
0.0008
0.15
0.54
0.77
1.21
2.00
2.57
n.d.
0.0071
0.14
0.44
0.73
1.13
1.80
2.46
n.d.
0.312
0.635
0.971
1.32
1.81
2.325
3.004
3.727
4.496
A decrease in temperature may result in the forma-
tion of fluorosulfate complexes of more intricate com-
position.
n.d.
n.d.
n.d.
Although a study of LaF3 and YbF3 solubility in
solutions containing 0–2.325 mol/L H2SO4 demon-
strated the formation of MSO4F [4], the composition of
complexes formed at room temperature might be deter-
n.d.
n.d.
n.d.
mental data are summarized in Table 6. It can be seen mined more precisely by studying the solubilities of
that, upon variation of cH SO from 1.32 to 3.727 mol/L, LaF3 and YbF3 over a broader range of cH SO
.
4
2
4
2
Table 4. Solubility of NdF3 in H2SO4–H3PO4 solutions at (20 1)°C
Content of Nd2O3 at P2O5 concentration, g/L
cH SO , mol/L
2
4
0
2
5
8
12
16
20
0
0.001
0.19
0.39
0.52
0.65
0.82
0.95
1.03
1.07
1.11
0.001
0.13
0.37
0.53
0.70
0.84
1.00
n.d.
0.001
0.11
0.35
0.52
0.75
0.88
1.05
n.d.
0.001
0.093
0.36
0.50
0.76
0.95
1.05
n.d.
0.001
0.085
0.37
0.58
0.71
0.90
1.03
n.d.
0.001
0.079
0.37
0.57
0.78
0.98
1.09
n.d.
0.001
0.078
0. 32
0.54
0.70
0.96
1.1
0.312
0.635
0.971
1.320
1.810
2.325
3.004
3.727
4.496
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 12 2007

SOLUBILITY OF YF3, CeF3, PrF3, NdF3, AND DyF3
1833
Table 5. Solubility of DyF3 in H2SO4–H3PO4 solutions at
(20 1)°C
The effects of the phosphoric acid concentrations on
the solubility in sulfuric acid solutions are different.
The solubility of YF3 increases and those of ëÂF3 and
PrF3 decrease with an increase in the H3PO4 concentra-
tion over the whole range of the H2SO4 concentrations
studied. An especially pronounced decrease in the sol-
ubility was found for PrF3. The solubilities of NdF3 and
DyF3 decrease at low H2SO4 concentrations but remain
Content of Dy2O3 at P2O5 concentration, g/L
cH SO
,
4
2
mol/L
0
5
12
20
0
0.0003
0.87
2.01
2.71
3.89
4.45
4.49
4.53
4.59
4.78
0.025
0.76
1.91
2.73
3.62
4.40
4.41
n.d.
0.043
0.80
1.81
2.81
3.60
4.38
4.51
n.d.
0.067
0.77
1.80
2.82
3.50
4.35
4.40
n.d.
0.312
0.635
0.971
1.32
almost invariable or slightly increase at higher cH SO
2
4
concentrations. Generally, the change in the solubility
of lanthanide fluorides in sulfuric and phosphoric acid
solutions attests to an intricate nature of the lanthanide
complexation in these solutions.
1.81
In view of the solubilities of lanthanide fluorides
found experimentally in a previous study [4] and in the
present study, sulfuric acid leaching of praseodymium,
dysprosium, and ytterbium fluorides should proceed
efficiently, while leaching of yttrium, lanthanum,
cerium, and neodymium fluorides from phosphohemi-
2.325
3.004
3.727
4.496
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
Table 6. Dependence of K /K1, K /K2 and K /K3 on H2SO4 concentration
c1
c2
c3
cH SO , mol/L
2
4
Fluoride
0.312
0.635
0.971
1.32
1.81
2.325
3.004
3.727
4.496
K /K1, mol4 L–4
c1
YF3 3.57 × 10–12 4.67 × 10–11 1.43 × 10–10 2.45 × 10–10 4.63 × 10–10 4.45 × 10–10 2.81 × 10–10 1.06 × 10–10 0.37 × 10–10
CeF3 3.37 × 10–18 3.38 × 10–14 6.36 × 10–14 3.78 × 10–13 10.9 × 10–13 9.95 × 10–13 3.96 × 10–13 1.43 × 10–13 0.47 × 10–13
PrF3 3.81 × 10–13 3.92 × 10–11 1.31 × 10–10 5.18 × 10–10 3.02 × 10–9 8.82 × 10–9 3.07 × 10–9 1.33 × 10–9 5.5 × 10–10
NdF3 4.64 × 10–15 1.59 × 10–13 4.70 × 10–13 9.86 × 10–13 19.8 × 10–13 23.4 × 10–13 15.1 × 10–13 7.10 × 10–13 3.31 × 10–13
DyF3 9.48 × 10–11 7.46 × 10–9 2.39 × 10–8 13.2 × 10–8 13.4 × 10–8 6.01 × 10–8 2.33 × 10–8 0.92 × 10–8 0.44 × 10–8
K /K2, mol3 L–3
c2
YF3 5.23 × 10–10 3.53 × 10–9 8.18 × 10–9 12.5 × 10–9 20.6 × 10–9 20.9 × 10–9 15.8 × 10–9 8.28 × 10–9 4.05 × 10–9
CeF3 1.38 × 10–14 2.02 × 10–11 3.31 × 10–11 12.3 × 10–11 27.3 × 10–11 26.7 × 10–11 14.5 × 10–11 7.36 × 10–11 3.50 × 10–11
PrF3 1.06 × 10–10 3.11 × 10–9 7.71 × 10–9 2.14 × 10–8 0.79 × 10–7 1.76 × 10–7 0.87 × 10–7 0.50 × 10–7 0.28 × 10–7
NdF3 4.54 × 10–12 6.09 × 10–11 1.38 × 10–10 2.44 × 10–10 4.19 × 10–10 4.93 × 10–10 3.77 × 10–10 2.31 × 10–10 1.40 × 10–10
DyF3 5.44 × 10–9 1.32 × 10–7 3.17 × 10–7 11.2 × 10–7 11.8 × 10–7 6.94 × 10–7 3.71 × 10–7 2.01 × 10–7 1.24 × 10–7
K /K3, mol2 L–2
c3
YF3 2.19 × 10–7 7.59 × 10–7 1.34 × 10–6 1.82 × 10–6 2.62 × 10–6 2.79 × 10–6 2.52 × 10–6 1.83 × 10–6 1.27 × 10–6
CeF3 3.89 × 10–10 3.41 × 10–8 4.9 × 10–8 1.13 × 10–7 1.95 × 10–7 2.04 × 10–7 1.51 × 10–7 1.07 × 10–7 0.74 × 10–7
PrF3 8.38 × 10–8 7.03 × 10–7 1.29 × 10–6 2.51 × 10–6 5.84 × 10–6 10.0 × 10–6 7.04 × 10–6 5.42 × 10–6 4.09 × 10–6
NdF3 1.27 × 10–8 6.64 × 10–8 1.15 × 10–7 1.72 × 10–7 2.52 × 10–7 2.95 × 10–7 2.69 × 10–7 2.14 × 10–7 1.7 × 10–7
DyF3 8.91 × 10–7 6.68 × 10–6 1.2 × 10–5 2.69 × 10–5 2.97 × 10–5 2.28 × 10–5 1.68 × 10–5 1.24 × 10–5 1.00 × 10–5
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 12 2007

1834
LOKSHIN, TAREEVA
3. E. P. Lokshin, K. G. Ivlev, and O. A. Tareeva, Zh. Prikl.
hydrate may be difficult if the total lanthanide concen-
tration in leaching solutions exceeds 1–1.5 g/L.
Khim. 78 (11), 1796 (2005).
4. E. P. Lokshin, Yu. A. Vershkova, K. G. Ivlev, and
O. A. Tareeva, Zh. Neorg. Khim. 49 (4), 707 (2004)
[Russ. J. Inorg. Chem. 49 (4), 645 (2004)].
CONCLUSIONS
5. V. V. Serebrennikov, The Chemistry of the Rare-Earth
Elements (Tomsk. Univ., Tomsk, 1956), Vol. 1 [in Rus-
sian].
6. V. V. Serebrennikov and O. Vasil’eva, Tr. Tomsk. Univ.,
Ser. Khim., No. 185, 54. (1965).
The solubility of YF3, CeF3, PrF3, NdF3, and DyF3
in solutions containing 0–4.496% mol/L (0–35 wt %)
of H2SO4 and 0–27.6 g/L of H3PO4 at 20°C was deter-
mined. NdF3 is almost insoluble in phosphoric acid
solutions, whereas the other fluorides studied start to
dissolve with an increase in the H3PO4 concentration.
In terms of the solubility in solutions of sulfuric and
phosphoric acids of the same composition, lanthanide
fluorides can be arranged in the following sequence
ëÂF3 < LaF3 < NdF3 < YF3 < PrF3 < DyF3 ꢀ YbF3.
7. P. M. Menon and J. James, J. Chem. Soc., Faraday Trans.
59 (9), 2683 (1989).
8. I. V. Tananaev, E. G. Davitashvili, N. A. Dzhabishvili,
and M. V. Landiya, Izv. Akad. Nauk SSSR, Neorg.
Mater. 9 (12), 2174 (1973).
9. A. I. Ul’yanov and T. I. Kazakova, Izv. Akad. Nauk
SSSR, Ser. Khim., No. 7, 1157 (1963).
10. N. A. Dzhabishvili and L. N. Kargareteli, in Studies in
the Chemistry of Complex and Simple Compounds of
Some Transition and Rare Metals (Metsniereba, Tbilisi,
1978), issue 3, p. 110 [in Russian].
The higher solubility of lanthanide fluorides in solu-
tions of sulfuric acid with respect to their solubility in
monobasic acids is due to the formation of fluorosulfate
complexes. At room temperature, the formation of
M2(SO4)F4 is most probable.
11. I. V. Tananaev and N. A. Dzhabishvili, Izv. Akad. Nauk
Sulfuric acid leaching of praseodymium, dyspro-
sium, and ytterbium fluorides from phosphohemihy-
drate should be efficient, while that of yttrium, lantha-
num, cerium, and neodymium fluorides may be difficult
if the total lanthanide concentration in leaching solu-
tions exceeds 1–1.5 g/L.
SSSR, Neorg. Mater. 5 (8), 1402 (1969).
12. N. A. Dzhabishvili and E. G. Davitashvili, in Studies in
the Chemistry of Complex and Simple Compounds of
Some Transition and Rare Metals (Metsniereba, Tbilisi,
1970), p. 109 [in Russian].
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and V. Karimov, Dokl. Akad. Nauk TadzhikSSR 15 (9),
40 (1972).
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[in Russian].
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research (project no. 06-08-00223-a) and by
the Federal Target Scientific and Technical Program
(2006-RI-112.0/001/268).
16. V. I. Belokoskov, N. I. Zaginaichenko, T. V. Akhmetova,
et al., in Chemistry and Technology of the Natural Min-
eral Resources of the Kola Peninsula (Nauka, St. Peters-
burg, 1992), pp. 94–104 [in Russian].
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et al., in Chemistry and Technology of the Natural Min-
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