FeCl2-Na2SO3-H2O system as a basis for recovery of iron(II) sulfite from solution

Article abstract of DOI:10.1134/S0036023609100283

The FeCl₂-Na₂SO₃-H₂O system was studied along seven sections with the molar ratios Na₂SO ₃: FeCl₂ = 5: 1, 5: 2, 5: 3, 5: 4, 5: 5, 5: 10, and 5: 15, including six points on each s

Full text of DOI:10.1134/S0036023609100283

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2009, Vol. 54, No. 10, pp. 1674–1677. © Pleiades Publishing, Inc., 2009.
Original Russian Text © D.L. Motov, M.V. Vasekhz, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 10, pp. 1751–1754.
PHYSICOCHEMICAL ANALYSIS
OF INORGANIC SYSTEMS
FeCl2–Na2SO3–H2O System as a Basis for Recovery of Iron(II)
Sulfite from Solution
D. L. Motova and M. V. Vasekhab
a Institute of Rare Element and Mineral Chemistry and Technology, Kola Scientific Center, Russian Academy of Sciences,
ul. Fersmana 14, Apatity, Murmansk oblast, 184200 Russia
b Murmansk State Technical University, Sportivnaya ul. 13, Murmansk, 183010 Russia
Received October 14, 2008
Abstract—The FeCl2–Na2SO3–H2O system was studied along seven sections with the molar ratios Na2SO3 :
FeCl2 = 5 : 1, 5 : 2, 5 : 3, 5 : 4, 5 : 5, 5 : 10, and 5 : 15, including six points on each section where pH ranges
from 1.5 to 4.0 in 0.5 steps. Equal-undersedimentation lines for cations and anions were plotted. The iron(II)
system forms sodium ferrisulfites NaFe5O0.5(SO3)5 · 5H2O, NaHFe5(SO3)6 · 5H2O, Na2Fe5(SO3)6 · 4H2O, and
NaHFe2(SO3)3; the iron(III) system forms Na2Fe6(SO3)7 · 10H2O. The regions where these compounds sediment
were demarcated, and their characterizations were carried out.
DOI: 10.1134/S0036023609100283
THEORETICAL ANALYSIS
these reagents are added in small amounts to reach the
required pH value and are not involved in phase forma-
tion.
The Fe(OH)3(H2SO4)–Na2SO3–H2O system we
studied earlier [1] attracted our attention in the context
of recycling ferrous cake, a byproduct of the hydromet-
allurgy of base metals. In that study we showed that
three Na2SO3 molecules in the said system react with
one Fe(OH)3 molecule; two of these three molecules are
consumed to reduce Fe3+ to Fe2+, and one participates in
the virtually complete sedimentation of iron(II) sulfite
from solution.
It seemed necessary to study the FÂCl2–Na2SO3–H2O
system at room temperate in order to provide a rationale
for the direct recovery of sulfite phases from solutions.
This system has not been yet studied.
The system was studied along seven sections with
molar ratios Na2SO3 : FÂCl2 = 5 : 1, 5 : 2, 5 : 3, 5 : 4, 5 : 5,
5 : 10, and 5 : 15. Six points were taken along each sec-
tion with pHs in the range 1.5–4.0 in 0.5 steps.
Along the sections with molar ratios Na2SO3 : FÂCl2 ≥
5 : 5, only sulfuric acid was added. On the Na2SO3 :
FÂCl2 = 5 : 5 (mol/mol) section, the pH 4.0 point was
without sulfuric acid added. On the Na2SO3 : FÂCl2 <
5 : 5 (mol/mol) sections, aqueous NaOH was added for
points with pH > 3; for those with pH ≤ 3, aqueous
H2SO4 was added. The Na2SO3 : FÂCl2 = 5 : 10
(mol/mol) point for pH 3 was without addition of sulfu-
ric acid or sodium hydroxide.
SUBJECTS AND METHODS
The solutions used contained 0.18 mol/L FÂCl2 (pH
2.6) and 0.18 mol/L Na2SO3 (pH 8.4). The FÂCl2 solu-
tion was added to the Na2SO3 solution, and 1 M sulfuric
acid or 1 M aqueous sodium hydroxide was added to
the mixed solution to reach a certain pH in the range
from 4.0 to 1.5 in 0.5 steps.
The order in which components are added is signif-
icant for the formation of the system. When the Na2SO3
solution is added to the FÂCl2 solution, vigorous SO2
evolution occurs and the system changes its composi-
tion. When the FÂCl2 solution is added to the Na2SO3
solution at pH 1.5, sulfur(IV) oxide evolution is less
than 2%, and at pH 4.0, about 0.1%.
EXPERIMENTAL
Mixtures were stirred and exposed for some time
until sedimentation stopped. The slurry was filtered in
air. The surface of the brown-green sediment turned
gray-brown because of a weak oxidation of Fe2+ to Fe3+
by atmospheric oxygen.
SO23– ion in the filtrate was determined by iodomet-
ric titration, Na2O by atomic emission, and Fe by
atomic absorption on a Kvant AFA instrument.
Solid phases were collected on a filter; washed off
mother solution with water, aqueous ethanol, and etha-
nol; dried in air at 80°ë; and analyzed for the same
components. Water of crystallization was determined
Sulfuric acid and sodium hydroxide are not compo-
nents of the FÂCl2–Na2SO3–(ç+ or éç–)–H2O system;
1674

FeCl2–Na2SO3–H2O SYSTEM AS A BASIS FOR RECOVERY
1675
SO32–
90
by difference and thermogravimetrically on Netzsch
STA 409 PC/PG.
mol %
(a)
Crystal-optical analysis of solid phases was carried
out on a Leica microscope using immersion liquids. X-
ray diffraction patterns were recorded on DRON-2 with
a graphite monochromator and unfiltered Cu radiation.
5
4
70
50
30
10
3
2
IR spectra were recorded as KBr disks on Specord
M-80 in the range 400–3800 cm–1.
50
40
30
RESULTS AND DISCUSSION
25
2+
SO32–
Figure 1 images the system as Fe2+, Na2O, and SO32–
Fe
(b)
equal-sedimentation lines for the component ratios
90
70
50
30
2+
fin
2+
in
2–
3fin
2–
3in
3.0
2.5
2.0
1.5
20
Fe / Fe , Na2Ofin/Na2Oin, and S O
/S O , %
in accordance with [2].
Graphic imaging means that surface projections are
plotted in the planar Cartesian coordinates along sec-
tions as horizontals showing Fe2+, Na2O, and SO32–
undersedimentation relative to their initial percentages.
This is a very illustrative visualization method [3].
15
10
10
Fe
The plots show that the highest and equal values of
sedimentation are achieved over the entire range of pHs
studied: for Fe2+ and SO32– , 98%; and for Na2O, 98.5%.
2+
SO32–
90
(c)
50
At higher Na2SO3 : Fe2+ molar ratios in the initial
70
50
30
10
40
solution, the salting out effect is not manifested; Fe2+
sedimentation decreases to 95%, as the Fe2+ percentage
in the initial solution decreases and Fe2+ undersedimen-
tation relative to this percentage increases. Once the
molar ratio Na2SO3 : Fe2+ is below the equimolar value,
30
2
3
4
5
Fe2+ undersedimentation increases from 2 to 50%, then
decreasing to 25% on account of increasing Fe2+ per-
centage in the initial solution.
6
2+
Fe
1.5
2.0
2.5
pH
3.0
3.5
4.0
The Fe2+, Na2O, and SO23– equal-sedimentation lines
are close to each other only for the equimolar Na2SO3 :
Fe2+ ratio in the initial solution. Once the mole ratio
Fig. 1. FÂCl –Na SO –H O system at 25°ë:
2
2
3
2
exceeds the equimolar value, SO23– undersedimentation
2+
fin
2+
in
(a) Fe / Fe
,
(b)
Na O /Na O ,
and
2
fin
2 in
increases from 2 to 50%; below the equimolar ratio,
undersedimentation increases from 2 to 6%.
2–
3fin
2–
(c) SO
/SO , % equal-sedimentation lines.
3in
Figure 2 displays the compositions of solid phases
and conjugate liquid phases in the FÂCl2–Na2SO3–(ç+
or éç–)–H2O system. When the mole ratio Na2SO3 :
Fe2+ in the initial solution is 0.316 : 1 to 0.613 : 1, the
4.88 : 1
NaHFe2(SO3)3 is formed.
(mol/mol),
sodium
hydroferrisulfite
sediment is phase
A
(sodium oxoferrisulfite
The liquid phases change compositions on going
from the above-equimolar to below-equimolar ratios
NaFe5é0.5(SO3)5 · 5ç2é); when it ranges from 0.613 : 1
to 0.870 : 1, this is phase B (sodium hydroferrisulfite
NaHFe5(SO3)6 · 5ç2é). When the mole ratio Na2SO3 :
Fe2+ is 0.870 : 1 to 1.01 : 1 (in the equimolar region), the
from SO23– : Fe2+ = 14 : 1 to 0.15 : 1 and from Na2O :
Fe 6.7 : 1 to 0.47 : 1.
product is sodium hydroferrisulfite Na2Fe5(SO3)6 ·
4ç2é; in an excess of Na2SO3 up to Na2SO3 : Fe2+
In the equimolar region, the solid and liquid phases
have close compositions.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 10 2009

1676
MOTOV, VASEKHA
Liquid phase
SO23–, mol %
Na O, mol %
2–
3
2
[SO
]
2+
Liquid phase
[Fe ]
[Na O]
90
80
70
60
50
40
30
20
10
90
80
2
2+
[Fe ]
D
C
70
B
Solid
A
60
50
40
30
20
10
phases
2–
[SO
]
3
2+
[Fe ]
Solid
phases
[Na O]
2
D
2+
C
[Fe ]
B
40
A
30
2+
2+
10
20
50
60
70
,%
80
90
Fe
Fe
[Na SO3]
2
Initial
2+
[Fe ] + [Na SO3]
2
+
–
Fig. 2. FÂCl –Na SO –(ç or OH )–H O system at 25°ë. Notations for solid phases and liquid phase: (A) NaFe é (SO ) ·
2
2
3
2
5
0.5
3 5
5ç é, (B) NaHFe (SO ) · 5ç é, (C) Na Fe (SO ) · 4ç é, and (D) NaHFe (SO ) .
2
5
3 6
2
2
5
3 6
2
2
3 3
DTG, %/min
DSC, µV/mg
TG, %
100
452.6°C
351.9°C
0
559.4°C
738.0°C
758.8°C
0.4
95
90
85
80
75
70
65
60
–12.72%
197.8°C
491.9°C
779.2°C
872.2°C
526.4°C
638.8°C
–0.5
–1.0
–1.5
–2.0
0.2
–4.23%
216.3°C
74.3°C
884.4°C
0
–4.20%
655.7°C
–0.2
–0.4
–0.6
–0.8
–5.22%
–3.24%
–5.97%
–4.73%
59.68% (949.2°C)
600 700
123.0°C
100
200
300
400
500
800
900
T, °C
Fig. 3. Thermal curves for the Na Fe (SO ) · 4ç é phase (C).
2
5
3 6
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 10 2009

FeCl2–Na2SO3–H2O SYSTEM AS A BASIS FOR RECOVERY
1677
When the iron-containing component is FeCl3 ç2SO3 evolution; the reaction for phase C characteristic
instead of FeCl2, the molar ratio Na2SO3 : Fe3+ in the ini-
tial solution corresponding to the equimolar region is
3 : 1, because two Na2SO3 molecules are consumed in
of the equimolar ratio SO23– : Fe2+ in the initial solution
is as follows: Na2Fe5(SO3)6 · 4H2O
H2SO3↑ + Na2Fe5O(SO3)5.
3H2O↑ +
Fe3+ reduction and one is consumed in the sedimenta-
tion of the sulfite phase [1]. In this case, sodium fer-
risulfite Na2Fe6(SO3)7 · 10ç2é sediments.
Further heating of the oxo phase brings about stage-
wise oxidation of Fe2+ to Fe3+, elimination of SO2, and
oxidation of sulfur, which is bound with Na+, to SO4 by
the overall reaction
Crystal-optical studies showed that sodium fer-
risulfites are orange-yellow high-refractive grains.
Phases A, B, and D are optically isotropic. Phase C has
weak anisotropy. Refractive indices: for phase A,
1.800 < N < 1,820; for phase B, 1740 < N < 1.770; for
phase C, 1700 < N < 1.720; and for phase D, 1.700 < N <
1.740.
Na2Fe5O(SO3)5 + 1.75O2
Figure 3 displays thermogravimetric curves for the
phase C.
2.5Fe2O3 + 4SO2 + Na2SO4.
X-ray diffraction identifies the final calcination
product as finely disperse hematite Fe2O3 with minor
Na2SO4.
X-ray diffraction patterns show that sodium fer-
risulfites are X-ray amorphous. This likely because the
components rapidly interact during the synthesis to
form many crystallization centers; there are no condi-
tions for crystals to grow, and they remain X-ray amor-
phous.
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
fro Basic Research (project no. 09-08-90701-MOB_ST).
IR spectra of all sodium ferrisulfites in the range
3800–400 cm–1 are identical, containing absorption
bands characteristic of iron sulfite crystal hydrate with
the fundamental absorption band at ~900 cm–1 and,
given that minor iron sulfate appears (likely, due to oxi-
dation), an absorption band at ~1140 cm–1. The IR
absorption bands of water (~3400 and 1630 cm–1) are
well resolved.
REFERENCES
1. D. L. Motov and M. V. Vasekha, Zh. Neorg. Khim. 49
(10), 1742 (2004) [Russ. J. Inorg. Chem. 49 (10), 1617
(2004)].
2. D. L. Motov, Physical Chemistry and Sulfate Technology
of Titanium–Rare Earth Raw Materials (Izd-vo KNTs
RAN, Apatity, 2002), Vol. 2 [in Russian].
Thermogravimetry shows that sodium ferrisulfites
decompose via internal hydrolysis with water and
D. L. Motov and M. V. Vasekha, Dokl. Akad. Nauk 397 (1),
61 (2004).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 10 2009