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Journal of Minerals & Materials Characterization & Engineering, Vol. 7, No.2, pp 163-173, 2008
jmmce.org Printed in the USA. All rights reserved
Novel Technology for Chlorination of Niobium and Tantalum Oxides and
Their Low-Grade Ore Concentrates
B. A. Shainyan,1 Yu. S. Danilevich,1 Yu. L. Garmazov,2 A. L. Finkelstein,3 T. S. Aisueva,3 V.
1A. E. Favorsky Irkutsk I nstitute of Chemistry of Siberian Branch of Russian Academy of
Sciences, 1 Favorsky Street 664033, Irkutsk, Russia.
2“Tantal” Co. Ltd., Irkutsk, Russia
3A. P. Vinogradov Institute of Geochemistry of Siberian Branch of Russian Academy of Sciences,
4Irkutsk State Technical University, Irkutsk, Russia
A novel energy-economic and environmentally benign technological procedure for
chlorination of niobium and tantalum oxides as well as their low-grade ore concentrates was
elaborated. The process is based on using carbon tetrachloride or silicon tetrachloride as a
chlorinating agent under pressure. It proceeds at moderate temperatures and is free from the
shortcomings of conventional carbochlorination processes such as the use of chlorine gas at
very high temperatures and formation of toxic products and ozone depleting agents (phosgene,
carbon monoxide, chlorohydrocarbons).
Keywords: niobium oxide, tantalum oxide, carbochlorination.
Direct chlorination of refractory metal oxides Nb2O5 and Ta2O5 to their pentachlorides is
thermodynamically unfavorable and takes place only in the presence of a reducing agent binding
the released oxygen. The process is usually carried out in the presence of different forms of
carbon [1-8] or carbon monoxide [9, 10] and is known as carbochlorination process. In all cases
the reaction is carried out at a high temperature, up to 1000oC, therefore, the process is very
energy consuming. Processes using one reagent acting as both the chlorinating and the reducing
agent are also known, for example, carbochlorination of Nb2O5  or Ta2O5  with carbon
tetrachloride vapors at temperatures up to 580oC. Note that chlorination with CCl4 proceeds
under milder conditions than with chlorine, although the temperature is still rather high.
Moreover, the use or formation such toxic and ozone depleting compounds as chlorine, phosgene
and chlorocarbons during the processes makes them dangerous and environmentally harmful.
The autoclave technology for carbochlorination of niobium and tantalum oxides by the use of
carbon tetrachloride as a chlorinating and reducing agent in the original two-autoclave apparatus
164 B. A. Shainyan, et al Vol.7, No.2
was also elaborated . In this paper we describe the results of carbochlorination of niobium
and tantalum oxides and their ore concentrates with carbon tetrachloride as well as the recently
patented [14, 15] novel chlorination system based on silicon tetrachloride, which is free of the
above-mentioned drawbacks and allows to carry out the process under even milder conditions
and make it free from formation of toxic and ozone depleting compounds like phosgene and
chlorocarbons. A comparison is made of the two chlorination systems, CCl4 and SiCl4, including
the thermodynamic analysis of their reactions with the metal oxides.
The experiments were performed on several types of materials. (1) Technical grade metal oxides
Nb2O5 and Ta2O5 prepared by sulfuric acid leaching of niobium- and tantalum-containing ores
and containing about 80–90% of the corresponding metal oxide. (2) Mixed niobium-tantalum ore
concentrate that came from “Zashikhinskoe” deposit (Irkutsk region, Russia) and contained from
2.3 to 16.7% Nb and from 0.1 to 7.2% Ta. (3) Niobium ore concentrate that came from
“Tatarskoe” deposit (Krasnoyarsk region, Russia) and contained 3.7% Nb. Commercial CCl4 and
SiCl4 were used without further purification. Acetonitrile was dried by distillation over P2O5.
The chlorination experiments without mixing were performed in stainless steel bombs of 80–100
mL capacity. The experiments with mixing were performed in the two-autoclave equipment
schematically shown in Fig. 1. It includes a high-pressure autoclave 1 of 150–200 mL capacity,
made of austenitic steel Х18Н10Т, working at up to 100 atm and 500oС. The autoclave is heated
by an electric oven consisting of a tubular heater 2 of 0.5 kilowatt power and heat-insulating
mantle 3. The temperature is regulated by autotransformer 4 and controlled by thermocouple 5
and potentiometer 6. The reaction mixture is stirred by rocking of the working autoclave. The
rocking mechanism consists of electric motor 7, reduction worm-gear 8 and crank-connecting
rod system 9. The rate of rocking is 40 cpm. The products of chlorination were filtered through
pipeline 10 under the pressure developed in the working autoclave 1 into the receiving autoclave
11 whose design and characteristics are similar to those of autoclave 1.
2.3. Chlorination Procedure
2.3.1. Chlorination of niobium oxide with CCl4. The experiments were carried out in a two-
autoclave apparatus. Working autoclave of 200 mL capacity was charged with 25 g of Nb2O5
and 150 mL of CCl4, sealed, heated to 250–290oC and kept for 2 h at this temperature being
stirred by rocking. Then the working autoclave contents was transferred through the filter into
the receiving autoclave, which was cooled and opened. The precipitate formed was filtered off
and thoroughly washed with acetonitrile to give almost colorless niobium oxychloride NbOCl3.
The filtrate (CCl4) was evaporated to give NbCl5.
2.3.2. Chlorination of columbites and pyrochlores with CCl4. The experiments were similar
to those above for chlorination of Nb2O5 with CCl4, the temperature was maintained within 250–
290oC. The results are given in Table 1.
Vol.7, No.2 Novel Technology for Chlorination of Niobium and Tantalum Oxides 165
Fig. 1. Schematic diagram of the experimental two-autoclave unit. (1) working autoclave; (2)
tubular heater; (3) heat-insulating mantle; (4) autotransformer; (5) thermocouple; (6)
potentiometer; (7) electric motor; (8) reduction worm-gear; (9) crank-connecting rod system;
(10) pipeline connecting the working and receiving autoclaves; (11) receiving autoclave.
Table 1. Contents of niobium and tantalum after chlorination of columbites and pyrochlores
Sample Metal Ore
No. 1 Nb 7.74 n/d
Ta 0.78 n/d
No. 2 Nb 16.70 n/d
Ta 7.15 n/d
No. 3 Nb 2.29 n/d
Ta 2.55 0.149
No. 4 Nb 7.83 n/d
Ta n/d n/d
2.3.3. Chlorination of metal oxides with SiCl4. Stainless steel bomb of 80–100 mL capacity
was charged with 10 g of technical grade metal oxide dried at 200оС and poured over with 30–50
mL of SiCl4. The bomb was sealed, placed into an electric tubular oven and kept at 210–280оC
for 2–4 h being periodically shaken. Then the bomb was cooled, opened and the contents
166 B. A. Shainyan, et al Vol.7, No.2
filtered. Metal chlorides are practically insoluble in SiCl4, so their concentration in the filtrate
was negligible. The residue on the filter was treated with dry acetonitrile and filtered from SiO2.
The filtrate was evaporated to give 15.6–17.6 g of crude metal pentachloride as greenish-yellow
residue. The results of chlorination under various conditions and the content of different oxides
before and after chlorination are given in Tables 2–5.
Table 2. Results of chlorination of technical grade Nb2O5 with SiCl4 under various conditions.
Run Temperature, оС Molar ratio Nb2O5:SiCl4 Time, h Yield of Nb chlorides,(%)
1 180 1:3 4 -
2 205 1:5 3 15
3 210 1:3 7 45
4 210 1:10 6 30 (without shaking)
5 210 1:10 5 72 (with shaking)
6 210 1:3 4 75
7 210 1:5 4 90
8 245 1:5 1 73
Table 3. Composition of various products (%) from chlorination of technical grade Nb2O5 (from
After chlorination (run 5 from Table 2)
WO3 6.26 0.111 8.41 2.13
SiO2 0.682 ~90 2.42 0.36
ZrO2 0.348 0.055 n/d 0.207
Fe2O3 7.64 1.96 13.4 n/d
UO2 0.395 0.050 0.922 n/d
SO3 1.95 1.44 0.513 n/d
ZnO 0.196 0.012 0.413 n/d
Cr2O3 n/d 0.047 0.354 n/d
CaO 0.40 0.10 0.33 n/d
NiO 0.072 0.020 0.263 n/d
MnO 0.437 0.158 0.237 n/d
P2O5 1.03 0.723 0.074 n/d
CuO n/d n/d 0.050 n/d
PbO n/d n/d 0.029 n/d
ThO2 n/d 0.011 0.026 n/d
SnO2 2.21 2.21 n/d n/d
As2O3 5.33 0.804 n/d n/d
Al2O3 0.226 0.173 n/d n/d
Vol.7, No.2 Novel Technology for Chlorination of Niobium and Tantalum Oxides 167
Table 4. Results of chlorination of technical grade Ta2O5 with SiCl4 under various conditions.
Run Temperature, оС Molar ratio Ta2O5:SiCl4 Time, h Yield of Ta chlorides,(%)
1 210 1:10 7 23
2 245 1:12.5 5 73
3 245 1:12.5 7 100
4 280 1:10 2 82
5 280 1:18 2 32
6 280 1:18 7 78
7 300 1:12.5 1 69
Table 5. Composition of various products (%) from chlorination of technical grade Ta2O5 (from
After chlorination (run 4 from Table 4)
SiO2 n/d 83.5 1.75 1.98
WO3 1.35 0.126 0.378 0.52
CuO n/d 0.098 0.291 0.287
Fe2O3 0.995 0.442 2.06 n/d
Cr2O3 0.0288 0.135 0.249 n/d
CaO 0.139 n/d 0.179 n/d
NiO n/d 0.030 0.172 n/d
MnO 0.364 0.078 0.073 n/d
P2O5 0.107 0.321 n/d n/d
SO3 0.181 0.207 n/d n/d
Al2O3 0.106 0.1 n/d n/d
As2O3 0.0934 n/d n/d n/d
2.3.4. Chlorination of low-grade niobium ore concentrate with SiCl4. The experiments were
similar to those with CCl4. Working autoclave of 200 mL capacity was charged with 80 g of the
ore concentrate dried at 130оС and poured over with 160 mL of SiCl4. The autoclave was sealed,
placed into an electric tubular oven and kept at 280–300оC for 5 h being stirred by rocking. Then
the working autoclave contents was transferred through the filter into the receiving autoclave,
which was cooled and opened. The contents of the receiving autoclave was filtered, the
precipitate (~4 g) separated and analyzed. The filtrate (SiCl4) was recovered by distillation and
reused. The residue remaining in the working autoclave was also extracted with acetonitrile,
filtered, the filtrate evaporated and the residue analyzed. It consists mainly of ferrum chloride
with only negligible amount of niobium pentachloride. The cake (~60 g) was washed with water,
dried and analyzed. The results are given in Table 6.
168 B. A. Shainyan, et al Vol.7, No.2
Table 6. Contents of various metals after chlorination of low-grade niobium ore concentrate with
Cake after chlorination
at 280oC at 300oC
Nb 3.66 2.06 1.076 (0.47)*
Fe 21.5 1.96 5.74
Sr 1.58 0.22 0.9
Ta 0.088 0.033 0.1 (0.065)*
La 0.087 0.042 0.14
Mn 0.56 0.01 0.07
Ti 0.52 0.1 0.24
Ba 0.11 0.03 n/d
Ce 0.24 0.1 0.24
* Under vigorous stirring, reaction time 30 min.
2.4. Analytical Procedure
Determination of Nb, Ta, and some of the accompanying rock-forming elements was performed
by X-ray fluorescence analysis on a VRA-30 (Carl Zeiss) spectrometer with X-ray tube Ag
anode operated at 50kV and 20 mA, and a S4 Pioneer (Bruker AXS) spectrometer with X-ray
tube Rh anode operated at 50kV and 40 mA. The samples were prepared by two procedures. (1)
For VRA-30: the powdered samples were poured into a cuvette with the bottom shielded by 10
μm polypropylene film and exposed in air. (2) For S4 Pioneer: 300 mg of the sample was mixed
with 2 g of powdered cellulose, pressed pellets and exposed in vacuum. Nb Kα and Ta Lα lines
were used as analytical lines. For calibration, a set of standard reference samples with the content
of the analyzed elements in the range of 0.08-21.5% Nb2O5; 0.147-34% Ta2O5 was used. To take
into account the matrix composition effect, the intensity of the line of incoherently scattered
radiation of the tube anode was measured (AgKα and RhKα Compton line for VRA-30 and S4
Pioneer, respectively). The analytical signal was taken to be equal to the ratio of the analytical
lines to the line of the tube anode incoherent scattering. The calibration curve was determined
using the multiple regression procedure available in the spectrometer software. The standard
deviation from the calibration curve for Nb and Ta is ca. 0.1%. To estimate the content of the
rock-forming elements the procedure available in the S4 Pioneer spectrometer software was
3. RESULTS and DISCUSSION
3.1. Chlorination of Nb2O5 with CCl4
As follows from the experiments with Nb2O5 performed under different conditions, the ratio
between NbOCl3 and NbCl5 is changed in favor of the latter with temperature, so that at ~250oC
NbOCl3 is formed in substantial amounts whereas at temperatures close to or above 300oC NbCl5
is practically the only product of chlorination. In a separate study, we found that the use of a 2:1
mixture of CCl4:CH3CN instead of pure CCl4 (in the same manner as described in Section 2.3)
sharply decreases the amount of phosgene formed, so that it cannot be detected by IR
Vol.7, No.2 Novel Technology for Chlorination of Niobium and Tantalum Oxides 169
3.2. Chlorination of Columbites and Pyrochlores with CCl4
The data of Table 1 show that the average degree of extraction of niobium and tantalum from ore
concentrates is not less than 97%. It clearly demonstrates that the elaborated autoclave
technology is effective for extraction of these metals from their low-grade concentrates. This
allows one to work with gravitation concentrates and avoid operations such as froth flotation and
3.3. Chlorination of Metal Oxides with SiCl4
The results of Nb2O5 chlorination under various conditions are given in Table 2 and the content
of different oxides before and after chlorination in Table 3. The main impurities in the starting
technical grade Nb2O5 were WO3, Fe2O3, As2O3 and SnO2. Sulfur and phosphorus are also
present in amounts of 1-2%. The final product, NbCl5, prepared by sublimation of the crude
product of chlorination, is of >97% purity and contains ~2% of WO3 and traces of SiO2 and
ZrO2. Other elements could not be detected by XRF method. As can be seen from Table 2, after
chlorination the main impurities either remain in the acetonitrile insoluble residue (tin) or are not
sublimed and therefore can be separated from the target product completely (iron) or to a
substantial degree (tungsten). The residue remaining after extraction with acetonitrile represents
sand contaminated by oxides and sulfates of iron, tin and arsenic. Noteworthy is the extraction of
zinc, nickel, chromium and tungsten, whose content in the acetonitrile extract exceeds their
content in the starting material. Of special interest may be the extraction of uranium and thorium,
which are also accumulated in the acetonitrile extract but do not pass into the target product
(NbCl5) after sublimation.
The results of Table 2 (runs 4 and 5) clearly demonstrate the importance of mass transfer in the
process: the yield of niobium chlorides increases more than twice; therefore, further experiments
were performed in the apparatus shown in Fig. 1.
As compared to chlorination with CCl4, the optimal temperature for chlorination with SiCl4 is
notably lower, 210oC (run 7 in Table 2) vs. 250–290oC for experiments in Table 1.
The results of Ta2O5 chlorination under various conditions are given in Table 4 and the content
of different elements after chlorination in Table 5. The final product, TaCl5, prepared by
sublimation of the crude product of chlorination, is also of >97% purity and contains ~2% of
SiO2 and traces of WO3 and CuO. The temperature for successful chlorination of Ta2O5 is
somewhat higher than for Nb2O5 (245oC vs. 210oC).
3.4. Chlorination of Low-Grade Niobium Ore Concentrate with SiCl4
A series of experiments performed on a low-grade niobium concentrate containing ~3.7% of Nb
and only trace amounts of Ta allowed us to determine the optimal parameters of the process. The
optimal ratio of the components, concentrate:SiCl4, is 1:3. The optimal temperature is 300oC,
below 270oC practically no chlorination of the target metal occurs, the yield of NbCl5 is <1%.
The results of chlorination are given in Table 6. The degree of extraction at 280oC is as low as
45%. At 300oC it increases to 71%, and vigorous stirring of the mixture in the autoclave allows
to reach 87% extraction of niobium. The fact that the maximum degree of extraction so far
attained in chlorination with SiCl4 (87%) is still less than with CCl4 (97%) is, most probably, due
to formation of SiO2 for the former chlorinating agent, which is precipitated on the surface of the
ore concentrate and hinders penetration of the liquid chlorinating agent SiCl4 into the pores of
the chlorinated mineral.
170 B. A. Shainyan, et al Vol.7, No.2
3.5. Thermodynamic Analysis
Schematically the chemical process can be expressed by either of the following equations or their
combination (in the presence or absence of crystallization water; M in the equations below
denotes Nb or Ta):
M2O53 SiCl42 MCl5
H2O2 HCl3 SiO2
2 M2O55 SiCl44 MCl55 SiO2
2 MO3 SiCl44 MOCl33 SiO2
An interesting question is why chlorination with SiCl4 occurs as easily as, or even at lower
temperatures than, with CCl4? Equations (4)-(6), which are similar to equations (1)-(3) can be
written for the latter process:
M2O53 CCl42 MCl5
H2O2 HCl3 CO2
2 MO5 CCl44 MCl55 CO
2 53 CCl44 MOCl33 CO2
Calculation of the differences of thermal effects ΔΔH = ΔHo(CCl4) – ΔHo(SiCl4) and of free
energies ΔΔG = ΔGo(CCl4) – ΔGo(SiCl4) for chlorination in CCl4 and in SiCl4 in the gas and the
liquid phase with respect to one molecule of a metal oxide, using the values of ΔHfo(CCl4)g –100.4,
ΔHfo(CCl4)l –132.6, ΔGfo(CCl4)g –58.2, ΔGfo(CCl4)l –62.8, ΔHfo(SiCl4)g –609.6, ΔHfo(SiCl4)l
–640.2, ΔGfo(SiCl4)g –569.9, ΔGfo(SiCl4)l –572.8, ΔHfo(CO2)g –393.3, ΔGfo(CO2)g –394.6,
ΔHfo(SiO2)s –859.4, ΔGfo(SiO2)s –856.7, ΔHfo(COCl2)g –220.9, ΔGfo(COCl2)g –206.7 kJ/mol taken
from  gives the values summarized in Table 7.
Table 7. Thermodynamic parameters (kJ mol-1) for chlorination of metal oxides with CCl4 and
gas liquid gas liquid
eq(4) – eq(1) –129.3–124.5–148.8–143.7
eq(5) – eq(2) –107.8–103.8–124.0–119.8
eq(6) – eq(3) –64.6–62.2–74.4–71.8
eq(7) – eq(2) +323.2+327.2+345.8+350.0
eq(8) – eq(3) +194.0+196.4+207.4+210.0
For simplicity, we neglect here the temperature dependence of thermal effects, since, first, the
temperature is much lower than usual temperatures for conventional carbochlorination, and,
Vol.7, No.2 Novel Technology for Chlorination of Niobium and Tantalum Oxides 171
second, when calculating the ΔΔH and ΔΔG values, the temperature effects are largely
annihilated by subtraction. The values of ΔΔH and ΔΔG are the same for chlorination of both
niobium and tantalum pentoxides since they do not depend on the nature of the chlorinated metal
oxide. Of course, the data in Table 7 can give only an approximate estimation of thermodynamic
parameters of the reactions proceeding at high temperatures and pressures. Nevertheless, because
of large negative or positive values, these data can be considered, at least, qualitatively, to reflect
the behavior of real systems.
The following conclusions can be drawn from analysis of Table 7. First, the values of ΔΔH and
ΔΔG have the same sign and do not differ much in value. Second, the ΔΔG values are more
negative than the corresponding ΔΔH values for the pairs of reactions (4)–(1), (5)–(2), (6)–(3),
and more positive for the pairs of reactions (7)–(2) and (8)–(3). The fact that the ΔΔH and ΔΔG
values in Table 7 are negative for the corresponding pairs of reactions (1)–(6) means that the
process of chlorination should be more exothermic for CCl4 as a chlorination agent, provided that
it is completely transformed into CO2. This seems to be in apparent contradiction with the
experimental observations, which showed that with SiCl4 the chlorination occurs under similar or
even milder conditions. However, the contradiction is removed if we consider that CCl4 is
capable of being also partly dechlorinated to COCl2, whereas no similar process is possible for
SiCl4. Dichlorosilanone Cl2Si=O does not exist as a stable compound but only formed as a
reactive intermediate species by copyrolysis of hexamethyldisiloxane with tetrachlorosilane .
It was also studied theoretically as an elusive intermediate of hydrolysis of SiCl4 . With
CCl4, the reactions of chlorination of metal oxides with only partial dechlorination of CCl4 to
COCl2 can be written as follows:
M2O55 CCl42 MCl55 COCl2
M O3 CCl42 MOCl33 COCl2
The highly positive values of ΔΔH and ΔΔG in Table 7 clearly demonstrate that chlorination with
SiCl4 by reactions (2) and (3) is much more preferable than that with CCl4 by reactions (7) and
(8). Therefore, even if the process of dechlorination of CCl4 to COCl2 comprises a third part of
its dechlorination to CO2, the total process will be less exothermic than that with SiCl4.
Niobium and tantalum pentachlorides after purification by sublimation can be used not only as
starting materials for preparation of pure metals but also as catalysts for various reactions, like
Friedel-Crafts acylation and Sakurai-Hosomi reaction of acetals , ring opening of epoxides
 catalyzed by NbCl5, or preparation of higher esters of acrylic acid  or polymers with
high oxygen permeability  catalyzed by TaCl5.
New processes of conversion of niobium and tantalum oxides into their pentachlorides by the
reaction with silicon tetrachloride under pressure at temperatures of 210–290oC were developed
and a new autoclave technology for extraction of niobium and tantalum from their low-grade ore
concentrates was elaborated. The new technology is based on chlorination of low-grade niobium
and tantalum ore concentrates with carbon tetrachloride or silicon tetrachloride under pressure at
moderate temperatures and represents an alternative to traditional carbochlorination process. It
172 B. A. Shainyan, et al Vol.7, No.2
allows (i) to decrease the temperature of the process from ~1000oC to 250-300oC and, therefore,
minimize energy consumption; (ii) to avoid the use of elemental chlorine; (iii) to remove (with
SiCl4 as a chlorinating agent) the formation of environmentally hazardous exhausts such as toxic
and ozone depleting compounds (phosgene, carbon monoxide, chlorohydrocarbons) unavoidable
in traditional carbochlorination process; (iv) to work with low-grade gravitation concentrates that
decreases the costs for enrichment the ore since it allows to exclude froth flotation and magnetic
separation; and (v) to reuse a substantial part of the chlorinating agents.
The authors are grateful to the Irkutsk Scientific Center for financial support of this work.
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