Effect of SO2 on chlorination of Bi2O3 + Fe2O3 mixtures

Article abstract of DOI:10.1134/S0036023608090052

The reaction of Bi₂O₃ + Fe₂O₃ mixtures with chlorine and SO₂ at 250-700°C is studied. At 300-500°C, the degree of bismuth chloride sublimation from the oxide mixture increases in the presence of SO

Full text of DOI:10.1134/S0036023608090052

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 9, pp. 1371–1375. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © L.E. Derlyukova, M.V. Vinokurova, T.A. Anufrieva, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 9, pp. 1470–1474.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPUNDS
Effect of SO2 on Chlorination of Bi2O3 + Fe2O3 Mixtures
L. E. Derlyukova, M. V. Vinokurova, and T. A. Anufrieva
Institute for Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia
E-mail: led@icp.ac.ru, alv@icp.ac.ru
Received June 7, 2007
Abstract—The reaction of Bi2O3 + Fe2O3 mixtures with chlorine and SO2 at 250–700°C is studied. At 300–
500°C, the degree of bismuth chloride sublimation from the oxide mixture increases in the presence of SO2.
Chemical sublimation of FeCl3 occurs after BiCl3 is virtually completely recovered from the solid phase.
DOI: 10.1134/S0036023608090052
Preparation of pure metal chlorides is an important
problem of preparative inorganic chemistry. Chemical
sublimation, which is transfer of the substance from a
condensed to gaseous phase in the chloride form as a
result of chemical reaction, is one promising method
for selective separation of solid components [1].
Fe2(SO4)3 is unstable at high temperatures; iron
exists mostly as oxide. The vapor phase contains SO2,
SO3, O2, ël2, and iron chlorides in addition to BiCl3; the
iron chloride proportion is four orders of magnitude
lower than the BiCl3 proportion.
The considerable difference between the partial
pressures of bismuth chloride and iron chloride implies
that selective bismuth chloride sublimation is possible
over a wide range of temperatures (250–1000°ë).
Here, we report the study of reaction of Bi2O3 +
Fe2O3 mixtures with chlorine in the presence of SO2;
this study was undertaken to find optimal parameters
for BiCl3 preparation from iron oxide mixtures. The
importance of preparation of pure BiCl3 arises from its
wide use, in particular, in catalysis and as a precursor
for metallic bismuth [2, 3]. Fe2O3 was shown to
increase the Bi2O3 chlorination rate noticeably [4].
Addition of SO2 to chlorine during chlorination of indi-
vidual oxides increases the formation rates of bismuth
and iron chlorides [5, 6].
EXPERIMENTAL
A flow-through technique [6] was used to study the
reaction of Bi2O3 + Fe2O3 mixtures with chlorine and
sulfur dioxide at 250–500°C. A weighed sample (0.6–
0.9 g) was placed into a heated tubular reactor through
which an inert gas was passed. After a set temperature
was acquired, the inert gas was changed to the reaction
gas. Volatile products were entrapped in cooled traps.
The linear gas flow was 0.02 m/s.
To suggest the composition of reaction products, we
calculated the equilibrium composition of the vapor
and condensed phases in the Bi2O3–Fe2O3–Cl2–SO2 sys-
tem with various component ratios. The basis for calcu-
lations was a variational technique; ASTRA software
was used [7]. Figure 1a demonstrates partial pressure
versus temperature for the vapor components of the
Bi2O3–Fe2O3–3Cl2–3SO2 system; Figure 1b shows the
composition of the condensed phase of this system. The
temperature range 25–1000°ë can be divided into two
smaller ranges: a low-temperature range (up to 600°ë)
and high-temperature range (above 600°ë). In the low-
temperature range, iron in the condensed phase occurs
as sulfate; most bismuth is trichloride and oxychloride.
The overall process can be described as
The laws of chemical sublimation of metal chlorides
at 550–750°ë were studied gravimetrically on a setup
with automated record of weight change [8]. The setup
sensitivity was 0.001 g/mm scale. Sample sizes were
0.180–0.200 g. The overall gas consumption was 150–
200 mL/min at a linear rate of 0.05 m/s. Under these
conditions, the gas delivery rate to the surface did no
control the process. Results were compared with rele-
vant data on individual oxides.
Mechanical mixtures of high-purity grade bismuth
oxide and iron oxide with various component ratios
were used. Table 1 lists the compositions of these mix-
tures. X-ray powder diffraction and standard chemical
analyses were used to characterize the starting samples
and reaction products [9]. X-ray diffraction patterns
were recorded on an ADP-2-01 diffractometer.
Bi2O3 + Fe2O3 + 3Cl2 + 3SO2 = 2BiCl3 + Fe2(SO4)3, (1)
Bi2O3 + Cl2 = BiOCl + 1/2O2.
(2)
A decrease in the chlorine proportion in the system
does not increase the bismuth proportion contained in
the oxychloride form in the condensed phase.
1371

1372
logp [atm]
DERLYUKOVA et al.
∆m/m , %
i
0
(‡)
(‡)
0
–1
–2
–3
–4
–5
20
3
1
10
5
2
0
4
1
20
40
60
80
100
120
2
–10
–20
–30
3
4
6
Weight fraction
(b)
(b)
30
0.5
1
0
–10
–20
–30
–40
–50
–60
–70
–80
10
20
40
50
60
τ, min
0.4
2
1
0.3
0.2
0.1
2
3
3
4
0
200
400
600
800
1000
t, °C
Fig. 1. Panel (a): equilibrium composition of the vapor
phase of the Bi O –Fe O –3Cl –3SO system: (1) Cl ,
Fig. 2. Panel (a): isotherms for the reaction of Bi O +
2
3
2
3
2
3
2
2
2
Fe O mixtures with Cl + SO (1 : 1 mol/mol) below
2
3
2
2
(2) SO , (3) O , (4) SO , (5) BiCl , and (6) FeCl . Panel (b):
2
2
3
3
3
500°ë: (1) 250, (2) 300, (3) 350, and (4) 400°C. Panel (b):
the same above 500°C: (1) 550, (2) 600, (3) 650, and
(4) 700°C.
the same for the condensed phase: (1) BiOCl, (2) Fe (SO ) ,
2
4 3
and (3) Fe O .
2
3
RESULTS AND DISCUSSION
tures their oxides of various compositions during chlo-
rination in the presence of SO2 at 400°ë changes with
time.
The reaction of Bi2O3 + Fe2O3 mixtures with chlo-
rine in the presence of SO2 has a noticeable rate even at
250°ë and is accompanied by weight gain. At higher
temperatures, the initial weight gain is followed by
weight loss (Fig. 2a). The results of chemical and X-ray
powder diffraction analyses imply that the initial
weight gain during chlorination is due to the formation
of bismuth oxychloride BiOCl and iron sulfate
Fe2(SO4)3 in the condensed phase. Subsequent weight
loss is due to transfer of metal chlorides to the gas
phase. Bismuth sulfate is not found in the solid phase,
although its proportion is sufficiently high in the reac-
tion of individual bismuth oxide with chlorine and sul-
fur dioxide [5].
For bismuth oxide, degrees of chloride sublimation
are low; they rise with increasing run duration to reach
12.5% for a contact time of 120 min. Addition of iron
oxide to Bi2O3 considerably increases the degree of bis-
muth chloride sublimation. The degree of BiCl3 chemi-
cal sublimation rises with increasing Fe2O3 proportion
in the starting mixture. For a mixture containing 33.6%
Fe2O3, the degree of bismuth chloride sublimation
reaches 97.0% as soon as after 15 min. A further rise in
Fe2O3 proportion insignificantly influences the degree
of bismuth chloride sublimation. Complete bismuth
chloride sublimation occurs under comparatively mild
Table 1 demonstrates how the degree of chemical
sublimation of bismuth and iron trichlorides from mix- conditions: for example, at 400°ë in 60-min runs for
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 9 2008

EFFECT OF SO2 ON CHLORINATION OF Bi2O3 + Fe2O3 MIXTURES
1373
Table 1. Degree of BiCl3 and FeCl3 sublimation as a func-
tion of mixture composition (T = 400°C)
mixtures containing more than 23% Fe2O3, the bismuth
proportion in the residue is less than 1%.
The iron chloride sublimation depends on the
degree of BiCl3 sublimation. If the degree of bismuth
chloride sublimation does not exceed 60–70%, FeCl3
chemical sublimation virtually does not occur. At high
degrees of bismuth trichloride sublimation, the degree
of iron chloride sublimation from mixtures increases
considerably to approach the respective values for the
individual oxide.
Mixture composition,
Percent chlorination
%
τ, min
Bi2O3
100
Fe2O3
0
Bi
Fe
15
30
60
120
15
30
60
15
30
60
15
30
60
15
30
60
15
30
60
4.0
7.0
–
–
Table 2 displays the composition of chlorination
products obtained from individual oxides and Bi2O3 +
Fe2O3 (1 : 1 wt/wt) mixtures in the presence of SO2.
10.6
12.5
22.0
39.7
67.0
28.8
87.3
99.2
97.0
98.8
99.3
98.1
98.7
99.3
–
–
–
According to previous data, mixtures the bismuth
proportion transferred into sulfate in the reaction of
Bi2O3 with Cl2 + SO2 is about 20% [5]. Bi2(SO4)3 forma-
tion slows down the process. As a result, the degree of
bismuth chloride sublimation at 300–450°ë does not
exceed 50%; the condensed phase contains Bi2(SO4)3,
BiOCl, and 8–15% of unreacted Bi2O3. In the presence
of Fe2O3, the bismuth proportion bound into the trichlo-
ride increases dramatically. In the condensed phase,
bismuth occurs exclusively as oxychloride; bismuth
sulfate does not occur.
The degree of iron oxide conversion in chlorination
of an oxide mixture in the presence of SO2 is higher
than for the individual oxide because of a considerable
increase in the Fe2(SO4)3 proportion. FeCl3 formation is
observed only for high degrees of bismuth chloride
sublimation.
82.8
76.3
67.0
50.0
0
17.4
23.9
33.6
50.0
100
0.3
0.5
2.1
1.3
13.4
15.9
14.9
26.6
26.2
14.6
25.7
25.9
15.0
25.6
24.8
It was demonstrated previously [6] that the propor-
tions of iron in chloride and sulfate are virtually the
same in the reaction of Fe2O3 with a Cl2 + SO2 mixture;
that is, Fe2(SO4)3 is generated by the process described
by the equation
2Fe2O3 + 3Cl2 + 3SO2 = Fe2(SO4)3 + Fe2Cl6.
(3)
Iron sulfate accumulation decelerates chemical sub-
limation of iron trichloride.
–
In chlorination of Bi2O3 + Fe2O3 mixtures in the
presence of SO2, the iron fraction converted into sulfate
is about two times that converted into chloride. The
increased Fe2(SO4)3 proportion in the condensed phase,
the absence of Bi2(SO4)3, and the increased degree of
bismuth chloride sublimation within 250–450°ë can
arise from the fact that bismuth sulfate under the spec-
ified conditions is an intermediate. The increased
degree of bismuth chloride sublimation results from the
occurrence of the exchange reaction
–
The results demonstrate that in cases where reac-
tions rates are high and reaction products are stable in
the reaction medium (iron sulfate), there is a clear cor-
respondence between calculated and experimental data.
Figure 2b displays isotherms for chlorination of
Bi2O3 + Fe2O3 mixtures in the presence of SO2 at 550–
700°ë. An induction period is detected only at 550°ë,
unlike at low temperatures. A sublimate is formed
Bi2(SO4)3 + Fe2Cl6 = 2BiCl3 + Fe2(SO4)3.
(4)
The low thermal stability of Bi2(SO4)3 [10] implies immediately after the gas mixture is admitted to the
that the significance of reaction (4) will decrease at
higher temperatures.
reactor; it contains not only BiCl3 but also FeCl3. Even
at 600°ë, however, the solid residue contains sulfur;
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 9 2008

1374
DERLYUKOVA et al.
Table 2. Composition of the products of reaction in Bi2O3, Fe2O3, and Bi2O3 + Fe2O3 mixtures (1 : 1 wt/wt) with Cl2 + SO2
mixture (1 : 1 mol/mol)
Bi distribution upon chlorination (percent of the starting)
T, °C
τ, min
Bi2O3 + Fe2O3
Bi2O3
BiCl3
BiOCl
BiCl3
BiOCl
Bi2(SO4)3
Bi2O3
300
30
60
120
30
60
30
60
30
60
30
30
27.6
66.7
89.5
77.0
95.9
98.7
99.2
99.0
99.1
99.2
99.3
72.4
33.3
10.5
23.0
4.1
4.4
7.4
14.7
6.5
8.6
7.0
10.6
21.8
34.4
–
62.5
58.1
49.1
62.1
60.3
66.7
62.5
43.9
32.4
–
19.5
20.2
24.1
20.1
21.2
18.9
19.9
19.4
20.5
–
13.5
14.3
12.1
11.3
9.9
350
400
450
1.3
7.4
0.8
7.0
1.0
14.9
12.7
–
0.9
600
650
0.8
0.7
–
–
–
–
Fe distribution upon chlorination (percent of the starting)
Bi2O3 + Fe2O3
Fe2O3
FeCl3
Fe2(SO4)3
Fe2O3
FeCl3
Fe2(SO4)3
Fe2O3
300
350
400
450
600
650
60
60
60
30
60
60
24.1
22.4
24.2
26.5
31.4
48.6
47.2
57.2
57.6
47.6
47.7
2.7
38.7
15.8
18.4
25.9
20.9
49.6
23.7
24.8
24.8
24.6
–
22.8
24.3
22.3
20.3
–
53.5
50.9
52.9
55.2
–
–
–
–
this means that Fe2(SO4)3 is formed. When the reaction Cl2 and SO2 ratio at 600°ë. The degree of BiCl3 chem-
ical sublimation approaches 100% with SO2 concentra-
tion increasing to 60%; a further increase in sulfur diox-
ide proportion brings about a decrease in the degree of
BiCl3 sublimation. For FeCl3 chemical sublimation, this
plot is a curve with a flat maximum, which is character-
istic of conjugate reactions [1]. Similar curves were
obtained previously [1, 6] in the chlorination of individ-
ual oxides (SnO2, Sb2O4, Fe2O3) in the presence of SO2.
This means that the mutual influence of Bi2O3 and
Fe2O3 during chlorination decreases in the high-tem-
time is 90 min, iron is completely transferred into the
sublimate. This demonstrates that Fe2(SO4)3 is an inter-
mediate. At 650°ë, the condensed phase is virtually
free of sulfur (Table 2). The rate of BiCl3 chemical sub-
limation considerably exceeds the FeCl3 formation rate.
The bismuth chloride and iron chloride sublimation
rates are considerably affected by the Cl2 and SO2 ratio
in the vapor phase. Figure 3 displays the degree of
metal chloride chemical sublimation as a function of
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 9 2008

EFFECT OF SO2 ON CHLORINATION OF Bi2O3 + Fe2O3 MIXTURES
1375
perature range is due to the exchange reaction between
FeCl3 and Bi2(SO4)3. Most iron transfers to sulfate. Iron
chloride sublimation occurs only after bismuth is
removed from the solid phase. Thus, selective BiCl3
sublimation in the system in question is possible only
within 300–500°C.
The role of the exchange reaction is reduced with
temperature elevation, because Bi2(SO4)3 degrades
above 405°ë. As a result, the effect of SO2 on bismuth
chloride sublimation is reduced. At the same time, the
degree of FeCl3 chemical sublimation increases; as a
result, bismuth extraction selectivity decreases.
α, %
100
80
60
40
20
0
1
2
REFERENCES
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0
10 20 30 40 50 60 70 80 90 100
c
, %
SO2
2. D. J. Williams and W. T. Pennington, D. VanDerveer,
et al., J. Chem. Crystallogr. 33 (5–6), 465 (2003).
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3. J.-L. Montero, J.-Y. Winum,A. Leydet, et al., Carbohydr.
2
2
Res. 297 (2), 175 (1997).
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showed that iron chloride sublimation within 650–
700°ë obeys the equation
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Zh. Neorg. Khim. 32 (9), 2096 (1987).
Fe2O3 + 3Cl2 + 3SO2 = 2FeCl3 + 3SO3.
(5)
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seev, Computers in Thermodynamic Calculations of
Metallurgical Processes (Nauka, Moscow, 1982) [in
Russian].
CONCLUSIONS
The above results show that several factors affect the
rate of chemical sublimation of bismuth and iron chlo-
rides. The degree of bismuth chloride sublimation from
binary oxide mixtures in the presence of SO2 increases
noticeably only at relatively low temperatures (300–
500°ë), at which the mutual influence of the solid com-
ponents is the strongest. The increase in the rate of the
chemical reaction in the presence of Fe2O3 in this tem-
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