Preparation and examination of the properties of complex scandium fluorides

Article abstract of DOI:10.1134/S1070427211080040

Precipitation of scandium with sodium fluoride from an ammonium hydrogen difluoride solution was examined at the molar ratios of Na to Sc within 1-14 and different initial concentrations of scandium in solution. The composition of the resulting precipitate was determined by X-ray phase analysis method. The solubility of the scandium compounds synthesized was examined. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S1070427211080040

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 8, pp. 1319–1323. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © Yu.V. Sokolova, R.N. Cherepanin, 2011, published in Zhurnal Prikladnoi Khimii, 2011, Vol. 84, No. 8, pp. 1254–1258.
INORGANIC SYNTHESIS AND INDUSTRIAL
INORGANIC CHEMISTRY
Preparation and Examination of the Properties
of Complex Scandium Fluorides
Yu. V. Sokolova and R. N. Cherepanin
Moscow Institute of Steel and Alloys, National Research Technological University, Moscow, Russia
Received December 6, 2010
Abstract—Precipitation of scandium with sodium fluoride from an ammonium hydrogen difluoride solution
was examined at the molar ratios of Na to Sc within 1–14 and different initial concentrations of scandium in
solution. The composition of the resulting precipitate was determined by X-ray phase analysis method. The
solubility of the scandium compounds synthesized was examined.
DOI: 10.1134/S1070427211080040
The available published data on the properties of
complex scandium fluorides are mostly concerned with
the solid state. The phase diagrams of the MF–ScF3
(M = Li, Na, K, Rb, Cs, NH4, Ag, Tl) and other
systems are presented in [1].
in those studies do not correspond to the currently
known parameters for KScF4, KSc2F7, K5Sc3F14, and
K3ScF6 (prepared by solid-phase syn-thesis procedure)
[1, 8], i.e., most likely refer to solid solutions. We
studied the interaction of a scandium(III) chloride solu-
tion with crystalline KHF2 and revealed formation of scan-
dium(III) fluoride at n = 2–3, of a mixture of ScF3 with
KSc2F7 at n = 4, of KSc2F7 at n = 5–7, and of K5Sc3F14
at n = 10–12 [9]. A relatively low degree of precipita-
tion of scandium (80%) in preparation of scandium(III)
fluoride, as well as the formation of a KSc2F7 gel,
prevent the use of the above-mentioned system for
quantitative recovery of scandium from solution.
Data concerning the occurrence and properties of
complex scandium fluorides in solution are scarce. In
the early twentieth century Meyer [2] discovered
solubility of scandium(III) fluoride in solutions of
alkali-metal and ammonium fluorides. Sterba–Bohm
[3] described the synthesis of (NH4)3ScF6 and its
decomposition into a number of compounds when
dissolved in water. A more recent study of the
solubility isotherm of (NH4)3ScF6 in an ammonium
fluoride solution at 25°C showed that incongruent
dissolution of the complex salt at the fluoride ion
concentrations under 1.8 M leads to formation of
congruently soluble ammonium tetrafluoroscandate
NH4ScF4 (solubility in water at 25°C 1.29 g l–1, or
9.28 × 10–3 mol l–1). At the fluoride ion concentrations
above 1.8 M scandium occurs in solution as hexa-
fluoroscandate ion [4, 5].
Complex scandium fluorides were prepared by the
reaction between scandium(III) chloride and sodium
fluoride solutions [10]. The conditions for preparation
of scandium fluoride were determined (solid phase had
the composition ScF3·0.16 Н2О), as well as those for
the synthesis of sodium tetrafluoroscandate in the form
of NaScF4·H2O. As to the occurrence of Na3ScF6,
whose preparation was reported in that study, this fact
should not be taken as proved. The maximal F/Sc ratio
obtained for the precipitate was 5.76, and the X-ray
phase analysis data reported in that study are
inconsistent with the currently known characteristics of
Na3ScF6. Consequently, those findings need clarification.
Tananaev and Golubev [6, 7] examined the ScCl3–
KF–H2O system [6, 7] and, based on the optical
density and molar electrical conductivity data for the
solutions characterized by molar ratio of F to Sc
n ranging from 1 to 10, presumed that ScF3 (n = 3) and
K3ScF6 (n = 6) are formed, and at higher n values,
KScF4·0.1H2O and K2.5ScF5.66·H2O (in the presence of
0.2 M KF in solution). The X-ray diffraction data reported
Here, we explored the conditions for the synthesis
of complex scandium fluorides from scandium solution
in ammonium hydrogen difluoride with the use of
sodium fluoride.
1319

1320
SOKOLOVA, CHEREPANIN
S, %
S, %
cSc, g l–1
m
Fig. 1. Degree of precipitation S of Sc(III) from 10%
NH4HF2 solution vs. Sc(III) concentration in the initial
solution. m: (1) 3, (2) 1.5, (3) 1, and (4) 0.5.
Fig. 2. Degree of precipitation S, %, of Sc(III) from 10%
NH4HF2 solution vs. molar ratio of Na to Sc m. сSc, g l–1:
(1) 1.8, (2) 1.5, and (3) 0.67.
EXPERIMENTAL
We examined the degree of precipitation of scan-
dium with crystalline sodium fluoride in relation to the
initial concentration of Sc(III) in solution and to the
molar ratio of Na to Sc m. Figures 1 and 2 show
respectively how the degree of precipitation of
scandium from 10% NH4HF2 solution with sodium
fluoride varies with the Sc(III) concentration in the
initial solution at different molar ratios m and with the
molar ratio m at different Sc(III) concentration in the
initial solution.
The initial solutions of scandium in ammonium
hydrogen difluoride were obtained by the reaction of
0.1–0.2 g of scandium oxide (OS-99.0 grade) with
100 ml of 10% NH4HF2 at 80–90°C. The insoluble
residue was separated from the solution by filtration.
The precipitation of scandium from NH4HF2 solution
was examined with solutions containing 0.6–2 g l–1
scandium as an example.
It is seen that, at Sc(III) concentrations under 0.5 g l–1,
scandium precipitation is incomplete; in the concentra-
tion range 0.5–1.5 g l–1 precipitation requires a
considerable amount of the precipitation agent; and at
the Sc(III) concentration of 1.5 g l–1 and above the
degree of precipitation from the NH4HF2 solution
reaches 96% even at m = 1.5.
The Sc(III) and Na(I) concentrations in the solu-
tions were determined by the inductively coupled
plasma emission spectrometry on a Labtam V-310
Plasma Spectrometer and a Perkin-Elmer AAnalyst
300 atomic absorption spectrometer. The lower
determination limit of the elements was 10–6 %, and
the determination error did not exceed 20%. The F–
and NH4+ concentrations in the solutions were deter-
mined with an ANION-410 pH-meter/ion meter and
The precipitates resulted from introduction into
solution of Sc(III) in 10% NH4HF2 of solid NaF under
stirring and of a NaF solution have different
compositions, specifically, within the m = 1–14 range
they are represented by a mixture of ScF3, Na3ScF6, and
Na(NH4)2ScF6 (Table 1). Figure 3 shows the line
diagram of the powder patterns for the components of
the mixture obtained in experiment no. 4.
ELIS-131
F
and ELIS-121 NH4 ion-selective
electrodes. The X-ray phase analysis was carried out
on a DRON-3 automated X-ray diffractometer with
monochromatized CuKα radiation. The resulting spectra
were processed using a software package for X-ray
analysis of polycrystalline solids [11]. We determined
the phase composition of the samples, as well as the
lattice constants (the latter, with the accuracy Δa/a =
0.0015).
These results suggest proceeding of the following
reactions upon introduction of sodium fluoride into an
NH4HF2 solution:
Ivanov–Emin et al. [5] reported that, in NH4F
solution, Sc(III) occurs in the form of hexafluoro-
scandate ion, which is apparently true for NH4HF2
solution as well. Introduction of sodium fluoride,
either solid or in the form of solution, into an ammo-
nium hydrogen difluoride solution containing the
(NH4)3ScF6 + NaF = Na(NH4)2ScF6 + NH4F,
(NH4)3ScF6 + 3NaF = Na3ScF6 + 3NH4F.
(1)
(2)
A variable number of phases obtained under close
precipitation conditions is evidently associated with
high local supersaturations occurring both during
introduction of solid sodium fluoride and in solution.
hexafluoroscandate ion causes formation of
scandium-containing precipitate.
a
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PREPARATION AND EXAMINATION OF THE PROPERTIES
1321
Table 1. Precipitation of Sc(III) from 10% NH4HF2 solution with sodium fluoride at 20°C. The precipitate composition was
determined by X-ray phase analysis accurately to within 0.1%, except for experiment nos. 1, 3, and 7
Precipitate composition,a %
Initial
Exp. no. concentration of
m
Precipitated Sc, %
Sc, g l–1
ScF3
~20
11.9
~45
7.2
100
Na3ScF6
Na(NH4)2ScF6
NaF
1
2
0.67
1.0
1
~25
20.3
~45
6.2
~55
67.8
~10
86.6
–
–
–
Not determined
1.3
1.5a
1.5
1.5
1.5
3
''
82.1
3
0.67
1.0
–
4
–
Not determined
95.7
5
1.5
–
–
6
1.8
5
8
87
–
Not determined
91.0
7
0.67
1.5
~20
~22
46.0
10.1
28.8
24.4
~58
23.6
84.2
10.8
7.3
–
8
3
30.4
–
99.4
9
1.5
4.5
9
3.9
1.9
28.4
59.6
95.1
10b
11b
1.5
24.5
96.9
1.5
14
6.2
99.4
a
b
Precipitated with 4% solution of sodium fluoride. The rest is sodium fluorosilicate from ammonium hydrogen difluoride and sodium
fluoride.
Table 2. Degree of decomposition of complex scandium fluoride with water at different liquid to solid ratios (T = 20°C)
l/s, ml g–1
Degree of salt decomposition, %
Na/Sc molar ratio in the precipitate
150
250
300
400
500
24.0
68.7
73.5
75.2
89.5
2.7 (mixture of NaScF4 with Na3ScF6)a
1.1 (mixture of NaScF4 with Na3ScF6)
1.0 (NaScF4)
0.9 (mixture of NaScF4 with ScF3)
0.4 (mixture of NaScF4 with ScF3)
a
Given in parentheses is the composition of the compounds, determined by X-ray phase analysis.
At m ≥ 4.5 the precipitate contains sodium fluoride,
which is due to the achievement of the salt solubility
limit.
interaction of a mixture of Na3ScF6 and NaF (it was
obtained by the reaction of a scandium(III) chloride
solution with an overstoichiometric amount of sodium
fluoride, the total composition being 3.2 NaF·ScF3)
with water at different liquid to solid ratios. The time
of establishment of equilibrium was identified from a
constant concentration of the F– ion in solution. The
degree of decomposition of the salt was determined
from the loss in the precipitate weight during the
experiment.
The presence of scandium(III) fluoride in the
mixture may be associated with involvement of the
most stable ScF2+ ion (stability constant 107 [12]) into
interaction by the following scheme:
ScF2+ + HF2– → ScF3 + H+
(3)
or with incongruent dissolution of the resulting
complex salts.
Our results (Table 2) suggest incongruent dissolu-
tion of Na3ScF6 in water with formation of sodium
To determine the conditions conducive to decom-
position of complex fluoride Na3ScF6 we examined the
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1322
SOKOLOVA, CHEREPANIN
(a)
(b)
(c)
2θ, deg
Fig. 3. Line diagrams of the X-ray powder diffraction patterns of (a) Na(NH4)2ScF6 (structural type сF72/5), (b) Na3ScF6
(structural type mP20/13), and (c) ScF3 (structural type hR4/10). (2θ) Bragg’s angle.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 8 2011

PREPARATION AND EXAMINATION OF THE PROPERTIES
1323
tetrafluoroscandate, as well as its decomposition by the
following schemes:
(3) Incongruent solubility in water and dilute solu-
tions of mineral acids with formation of scandium(III)
fluoride was revealed for NaScF4 and (NH4)2NaScF6.
Na3ScF6
NaScF4 + 2NaF,
(4)
NaScF4
ScF3 + 2NaF.
(5)
ACKNOWLEDGMENTS
However, even at large liquid to solid ratios we
obtained a mixture of NaScF4 with ScF3.
The authors are grateful to V.G. Miskar’yants from
GIREDMET Federal State Unitary Enterprise and T.A.
Sviridova from the Center for Composite Materials,
Moscow Institute of Steel and Alloys, National
Research Technological University, for assistance in
conducting the studies.
Treatment of the initial mixture of scandium
fluorides with 1% HCl solution at 70°C at the liquid to
solid ratio of ca. 50 (w/w) for 30 min led to a two-step
formation of scandium(III) fluoride. A similar result
(under identical conditions) was obtained upon
treatment of the salt mixture with 1% nitric acid
solution. Treatment of the precipitates obtained in
experiment nos. 7–9 (Table 1) with dilute solutions of
the same mineral acids also resulted in formation of
scandium(III) fluoride. These findings suggest that
Na3ScF6, NaScF4, and Na(NH4)2ScF6 show incon-
gruent dissolution in an acid medium by schemes (4)–(6):
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Na(NH4)2ScF6
ScF3 + 2NH4F + NaF.
(6)
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fluoride into a solution of ammonium hexafluoro-
scandate in NH4НF2 at molar ratios of F to Sc within
1–14 leads to formation of the precipitate consisting of
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different ratios.
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