Solvent Extraction Research and Development, Japan, Vol. 20, 159 – 168 (2013)
Low-Acid Extraction of Tantalum from a Tantalum-Niobium Pulp by MIBK
Xiuli YANG,12 Xiaohui WANG,2* Chang WEI,1 Shili ZHENG2 and Yi ZHANG2
1
Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology,
121 Avenue, Yunnan 650093, China
2
Institute of Process Engineering, Chinese Academy of Sciences, Zhongguancun Avenue,
Beijing 100190, China
(Received March 15, 2012; Accepted April 12, 2012)
The technological parameters and the mechanism of low-acid pulp extraction of
tantalum were investigated. The results show that hydrofluoric acid concentration,
sulfuric acid concentration, organic to pulp phase ratio, the concentration of tantalum
and niobium, contact time and temperature have a significant effect; optimum process
operating parameters were established as follows: hydrofluoric acid concentration 1.6
mol/L, sulfuric acid concentration 0.6 mol/L, organic to pulp phase ratio 3:1, tantalum
concentration 26.61 g/L, niobium concentration 11.64 g/L, contact time 5min and room
temperature. Under these experimental conditions, tantalum extraction can reach more
than 98% and the separation factors of Ta/Nb, Ta/Fe and Ta/Sn were maximal.
According to slope analysis, the equimolar series method and the saturation capacity
method, the extraction product was identified as 3MIBK•H2TaF7.
1. Introduction
Tantalum and niobium are the important elements applied in many high-technology industries such
as superconductors, electronic, energy and aerospace. About 61 % of tantalum is used for capacitors and
about 90 % of niobium in the world is used in the iron and steel industry as additives [1].
The elements tantalum and niobium, because of their similar chemical properties, always occur
together in nature [2]. Extraction as a method for separating and purifying niobium and tantalum, is widely
used in industry [3,4]. A wide range of extractants has been used including: ketones, such as methyl
isobutyl ketone (MIBK) and cyclo-hexanone; alkyl-phosphorus esters, e.g., tri-n-butyl phosphate (TBP)
and trioctyl phosphine oxide (TOPO); N-oxides; sulphoxides; and acidic alkyephosphorus extractants, such
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as di-2-ethylhexyl phosphoric acid (D2EHPA)—which were not very efficient—and long-chain
alkylamines [5-10]. MIBK is the most commonly used extractant worldwide due to its lower density, lower
water solubility, lower cost, higher chemical and operational stability, compared with other extractants [3,
11]. At present MIBK is commonly used in China, at Ningxia Orient Tantalum Industry Co., LTD and
Guangdong Conghua Tantalum &Niobium Smeltery, which are two of the biggest tantalum and niobium
producers in China. Therefore, methyl isobutyl ketone (MIBK) was used as the extractant.
All solvent extraction processes are exclusively operated in the presence of fluoride ions, most
frequently in a mixture with a mineral acid such as sulfuric or hydrochloric acid. As this binary acid system
is normally used for the digestion of tantalum and niobium concentrates, their solvent extraction is often
carried out in the same system. Due to increasingly stringent regulations concerning the protection of
human health and environment, the use of harmful fluorides must be reduced. Therefore, there is an urgent
need to develop a new process for the separation and purification of niobium and tantalum.
Recently, a method for the separation and purification of niobium and tantalum was proposed with
the objective to reduce fluorine pollution [12]. In this method, the pure solution containing only tantalum
and niobium was extracted by MIBK. Based on this method, we proposed a new process to separate and
purify tantalum and niobium in the tantalum-niobium pulp. This new process is based on the difference in
tantalum and niobium extraction performance: niobium complexes in the solution are stronger Lewis acids
than tantalum complexes. Therefore, the extraction of niobium with MIBK requires stronger solution
acidity than that of tantalum. In the traditional process, tantalum and niobium were extracted
simultaneously, followed by the back extraction and separation of tantalum and niobium respectively. In the
new process, first tantalum was extracted, and then niobium was extracted. In comparison with the
traditional process, the hydrofluoric acid concentration used in this process was reduced from 6 mol/L to
1.6 mol/L.
The purpose of the present work is to investigate the extraction behavior of tantalum, niobium, and
other associated mineral impurities such as iron, manganese, aluminum, silicon and tin, and to obtain
optimal oberating parameters.
2. Experimental
2.1 Pulp and reagents
The tantalum and niobium pulps were obtained from the previous low-alkali decomposition process
[13]. In the low-alkali decomposition process the tantalum-niobium ore is decomposed in a low alkali
system under atmospheric pressure. The decomposition product contains some metatantalate and
metaniobate salts, which are insoluble in hydrochloric acid or sulfuric acid, so a transformation agent was
used to solubilise the metatantalate and metaniobate salts transformed in the solution for extraction. The
mass concentrations of major elements in the pulp are shown in Table 1. In the pulp slag rate was 40%,
including tin oxide and aluminosilicate.
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Table 1. The mass concentration of major elements
Elements
Ta
Nb
Fe
Al
Si
Mn
Sn
Concentration (g/L)
26.61
11.64
6.34
1.65
4.06
1.80
7.99
Methyl isobutyl ketone was of analytical grade, which was produced by Beijing Yili Chemical Co. Ltd,
China. Hydrofluoric acid was of analytical grade, which was supplied by Beijing Beihua Fine Chemical Co.
Ltd, China. Distilled water was used in the experiments.
2.2 Pulp extraction procedures
The pulp containing tantalum and niobium was equilibrated with different volumes of methyl isobutyl
ketone by shaking in a separatory funnel for 10 min. The shaking speed was 240 rpm. This period of time
which had been verified in preliminary tests was sufficient to achieve equilibrium.
The pulp after equilibrium was filtered. The concentrations of tantalum and niobium in the filtrate
were measured by ICP-AES (ICP model; Perkins Elmer, OPTIMA 2100DV), after suitable dilution with
distilled water and 12 % (mass fraction) dihydroxysuccinic acid.
3. Results and Discussion
3.1 Effect of hydrofluoric acid concentration on tantalum and niobium extraction
An important factor to be studied is the effect of hydrofluoric acid concentration. The effect of the
hydrofluoric acid concentration on the extraction and separation of tantalum and niobium from the pulp is
shown in Figure 1. It can be seen from Figure 1(a) that the percentage of extracted tantalum and niobium
increased with increasing of hydrofluoric acid concentration. As the concentration of hydrofluoric acid
increased from 1.2mol/L to 1.6mol/L, the degree of extraction of tantalum increased from 86 % to about
98%. Similarly, the degree of extraction of niobium increased from 1% to 6 %. Niobium extraction was far
less than that of tantalum. In the extraction process, the lone pair of electrons on the oxygen atom of MIBK
attracts hydrogen ions, which generates a cation. This cation combines with the tantalum complex anion
electrostatically, which creates the complex H2 TaF7  nMIBK which is hydrophobic and dissolves in methyl
isobutyl ketone [14]. The separation factor is equal to the ratio of the distribution ratio of the two metals,
which expresses the degree of separation of the two metals. Figure 1(b) shows that the separation factor of
Ta/Nb reached a maximum and tantalum and niobium were well separated when the concentration of
hydrofluoric acid was 1.6 mol/L. Meanwhile, the separation factors for Ta/Fe, Ta/Sn also reached a
maximum. Most of the impurities stayed in the pulp, thus achieving the best separation of tantalum and
impurities. Therefore, a hydrofluoric acid concentration of 1.6 mol/L is recommended.
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50
90
40
(a)
80
30
70
20
60
10
50
0
1.2
1.3
1.4
1.5
HF concentration(mol/L)
1.6
2000
Separation factor
Ta
Nb
Extraction rate of niobium(%)
Extraction rate of tantalum(%)
100
Ta/Fe
Ta/Al
Ta/Si
Ta/Mn
Ta/Sn
Ta/Nb
1600
1200
800
(b)
400
0
1.2
1.3
1.4
1.5
HF concentration(mol/L)
1.6
Figure 1. (a) Effect of hydrofluoric acid concentration on the degree of extraction of tantalum and niobium
and (b) Effect of hydrofluoric acid concentration on the separation factor of tantalum and other elements.
Experimental conditions: sulfuric acid 0.4 mol/L, Ta2O5 26.61 g/L, Nb2O5 11.64 g/L, phase ratio
(organic/pulp, volume) 5:1, time 10 min, and room temperature.
3.2 Effect of sulfuric acid concentration on tantalum and niobium extraction
The effect of the sulfuric acid concentration on the extraction and separation of tantalum and niobium
is shown in Figure 2. It can be seen from Figure2(a) that tantalum extraction rate increased and then tended
to level out as the sulfuric acid concentration increased. Niobium extraction increased when the sulfuric
acid concentration increased. Sulfuric acid can combine with water molecules by hydration, so the number
of free water molecules decreases. Therefore, the effective concentration of tantalum and niobium increases
[15]. The effect of sulfuric acid on the extraction of niobium and tantalum with MIBK is a salting out effect
[16]. Figure 2(b) shows that the separation factor of Ta/Nb reached a maximum when the concentration of
sulfuric acid was 0.6mol/L. Tantalum and niobium were well separated. Meanwhile, the separation factors
of Ta/Fe, Ta/Sn also reached a maximum. Most of the impurities stayed in the pulp, thus achieving the best
separation of tantalum and impurities. Therefore, a sulfuric acid concentration of 0.6 mol/L is
recommended.
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90
50
(a)
40
80
30
70
20
60
10
50
0.4
0.5
0.6
0.7
0.8
H2SO4 concentration(mol/L)
0
6000
Separation factor
Ta
Nb
Extraction rate of niobium(%)
Extraction rate of tantalum(%)
100
(b)
4000
2000
Ta/Fe
Ta/Al
Ta/Si
Ta/Mn
Ta/Sn
Ta/Nb
0
0.4
0.5
0.6
0.7
0.8
H2SO4 concentration(mol/L)
Figure 2. (a) Effect of sulfuric acid concentration on the extraction of tantalum and niobium and (b) Effect
of sulfuric acid concentration on the separation factor of tantalum and other elements. Experimental
conditions: hydrofluoric acid concentration 1.6 mol/L, Ta2O5 26.61 g/L, Nb2O5 11.64 g/L, phase ratio
(organic/pulp, volume) 5:1, time 10 min, and room temperature
3.3 Effect of organic to pulp phase ratio on tantalum and niobium extraction
The effect of the phase ratio on the extraction and separation of tantalum and niobium from the pulp is
shown in Figure 3. It can be seen from Figure 3(a) that, when the phase ratio increased, the degree of
tantalum and niobium extraction increased, which can be attributed to the fact that more tantalum and
niobium complexes combined with MIBK. The extraction of tantalum was almost completed at an O/A
ratio of 3:1, but the extraction of niobium was small. Therefore the best separation of tantalum and niobium
was achieved at an O/A ratio of 3:1. Figure 3(b) shows that the separation factor of Ta/Nb reached a
maximum when the phase ratio was 3:1. Meanwhile, the separation factors of Ta/Fe, Ta/Sn also reached a
maximum. Most of the impurities stayed in the pulp, thus achieving the best separation of tantalum and
impurities. Therefore, a phase ratio of 3:1 is recommended.
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40
80
30
70
20
Ta
Nb
60
1
10
2
3
O/A(L/L)
4
5
0
Separation factor
(a)
90
50
2400
50
Extraction rate of niobium(%)
Extraction rate of tantalum(%)
100
Ta/Fe
Ta/Al
Ta/Si
Ta/Mn
Ta/Sn
Ta/Nb
2000
1600
1200
(b)
800
400
0
1
2
3
O/A(L/L)
4
5
Figure 3. (a) Effect of phase ratio on the extraction of tantalum and niobium and (b) Effect of phase ratio on
the separation factor of tantalum and other elements. Experimental conditions: hydrofluoric acid
concentration 1.6mol/L, sulfuric acid 0.6 mol/L, Ta2O5 26.61 g/L, Nb2O5 11.64 g/L, time 10 min, and room
temperature.
3.4 Effect of the concentration of tantalum and niobium on tantalum and niobium extraction
The effect of tantalum and niobium concentration on the extraction and separation of tantalum and
niobium from the pulp is shown in Figure 4. It can be seen from Figure 4(a) that the extraction of tantalum
and niobium remained practically unchanged with increasing concentration of tantalum and niobium from
27 g/L to 50 g/L. Figure 4(b) shows that the separation factor for Ta/Nb decreased when the concentration
of tantalum and niobium increased. However, when the concentration is low, the production capacity will
decline and it is uneconomical. Therefore, a concentration of 38.25 g/L is recommended.
4
98
Ta
Nb
(a)
3
97
2
96
1
95
25
30
35
40
45
50
Ta2O5 and Nb2O5 concentration(mol/L)
0
5000
4000
Separation factor
99
Extraction rate of tantalum(%)
5
Extraction rate of niobium(%)
100
3000
2000
(b)
Ta/Fe
Ta/Al
Ta/Si
Ta/Mn
Ta/Sn
Ta/Nb
1000
0
25
30
35
40
45
50
Ta2O5 and Nb2O5 concentration(mol/L)
Figure 4. (a) Effect of Ta2O5 and Nb2O5 concentration on the extraction rate of tantalum and niobium and (b)
Effect of Ta2O5 and Nb2O5 concentration on the separation factor of tantalum and other elements.
Experimental conditions: hydrofluoric acid concentration 1.6 mol/L, sulfuric acid 0.6 mol/L, phase ratio
(organic/pulp, volume) 3:1, time 10 min, and room temperature.
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3.5 Effect of extraction time on tantalum and niobium extraction
The effect of extraction time on the extraction and separation of tantalum and niobium from the pulp is
shown in Figure 5. It can be seen from Figure 5 that the extraction was very fast. The extraction was
basically complete after only one minute. To ensure that the extraction equilibrium is fully achieved, a
contact time of 5 min is recommended.
8
Ta
Nb
96
6
94
4
92
2
90
0
2
4
6
Time(min)
8
0
10
2.1
Ta
Nb
95
1.8
90
1.5
85
1.2
80
20
30
40
o
Temperature( C)
50
0.9
Extraction rate of niobium(%)
98
100
Extraction rate of tantalum(%)
10
Extraction rate of niobium(%)
Extraction rate of tantalum(%)
100
Figure 5. Effect of time on the extraction of
Figure 6. Effect of temperature on the extraction of
tantalum and niobium. Experimental conditions:
tantalum and niobium. Experimental conditions:
hydrofluoric acid concentration 1.6
mol/L,
hydrofluoric acid concentration 1.6 mol/L, sulfuric
sulfuric acid 0.6 mol/L, Ta2O5 26.61 g/L, Nb2O5
acid 0.6 mol/L, Ta2O5 26.64 g/L, Nb2O5 11.61 g/L,
11.64 g/L, phase ratio (organic/pulp, volume) 3:1,
phase ratio (organic/pulp, volume) 3:1, and time
and room temperature.
10 min.
3.6 Effect of temperature on tantalum and niobium extraction
The effect of the temperature on the extraction and separation of tantalum and niobium from the pulp
is shown in Figure 6. Methyl isobutyl ketone is volatile and toxic, so it is not used at higher temperatures.
Moreover, it can be seen from Figure.6 that the effect of temperature is not significant between 20oC and
50oC. However for MIBK such factors as high volatility and low flash point lead to high reagent losses and
potentially dangerous operating conditions. Therefore, the recommended temperature is room temperature.
4. Mechanism of Tantalum Extraction
In order to be consistent with the above process, the mechanism of tantalum extraction was studied
under the optimum process conditions: tantalum concentration 0.15 mol/L, hydrofluoric acid concentration
1.6 mol/L and sulfuric acid concentration 0.6mol/L. There are mainly H2TaF7 species in the pulp at a
hydrofluoric acid concentration 1.6mol/L [14,16,18]. The predominant areas of TaF72 and TaF6 complex
species in the Ta2O5-HF-H2O system are shown in Figure 7.
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4.1 Slope method
The effect of pH, MIBK concentration, sulfate ion concentration and fluoride ion concentration were
investigated. The plots of log D vs. log[SO42-], log[F-], pH and log[MIBK] are shown in Figure.8, Figure.9,
Figure.10 and Figure.11 respectively. According to Figure 8, Figure 9, Figure 10 and Figure 11, the
experimental points for log D vs. log[SO42-], log[F-], pH and log[MIBK] parameter all fall on a straight line,
respectively, from which the slope can be obtained and are equal to 0.18, 7.06, -2.07 and 2.98 which can be
rounded to be 0, 7, -2 and 3. Figure 8 shows that there is no sulfate ion involved into the reaction. Figure 9,
Figure 10 and Figure 11 show that there are 2 hydrogen ions, 7 fluoride ions and 3 MIBK molecules
involved into the reaction. Therefore, the extraction is considered to be as in Eq. (1).
3MIBK + 2H +TaF72- = H2 TaF7  3MIBK
(1)
2.06
7
2.05
6
2-
5
TaF7
TaF6
3
2.03
2
2.02
1
2.01
0
0
5
R2=0.9762
2.04
4
logD
Tantalum concentration(M)
8
10 15 20 25 30
HF concentration(M)
35
-0.30
40
-0.25
-0.20
log[SO24]
-0.15
-0.10
Figure 7. Predominant areas of TaF72 and
Figure 8. Plot of log D vs. log[SO42-].
TaF6 complex species in the Ta2O5-HF-H2O
Experimental conditions: [F-]=1.6 mol/L,
system (based on [14]).
pH=1
2.0
1.8
R2=0.9880
2.0
R2=0.9817
1.6
logD
logD
1.6
1.4
1.2
1.2
1.0
0.8
0.08
0.12
0.16
log[F-]
0.8
0.20
1.0
1.2
1.4
1.6
pH
Figure 9. Plot of log D vs. log[F-].
Figure 10. Plot of log D vs. pH.
2-
Experimental conditions: [SO4 ]=0.6 mol/L,
Experimental conditions: [SO42-]=0.6 mol/L,
pH=1
[F-]=1.6 mol/L
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4.2 Equimolar series method [16, 19]
The sum of the molar concentration of the extractant in the organic phase and the molar concentration
of tantalum in the aqueous phase is a constant value. When the hydrofluoric acid concentration is 1.6mol/L,
the extraction is carried out for different ratios of the molar concentration of tantalum and the molar
concentration of MIBK. The relationship of the extraction of tantalum and the ratio is shown in Figure 12.
It can be seen from the Figure 12 that the ratio is 0.26 at the maximum extraction of tantalum. From this it
can be concluded that the MIBK: H2TaF7 ratio is 3:1.
40
Extraction of tantalum(mg)
0.0
logD
-0.5
-1.0
-1.5
-2.0
0.0
0.2
0.4
log[MIBK]
0.6
35
30
25
20
15
10
5
0.8
0.1
0.2
0.3
xTa
0.4
0.5
Figure 11. The relationship between log[MIBK]
Figure 12. The relationship of the extraction
and logD.
of tantalum and the Ta MIBK molar ratio.
4.3 Saturated volumetric method
The extraction was carried out at a
hydrofluoric acid concentration of 1.6 mol/L and a tantalum
concentration 0.15mol/L. The concentration of tantalum in the aqueous was analyzed. The content of
tantalum in the organic phase was calculated by the mess balance method. When the organic phase reached
saturation extraction, the molar ratio of MIBK/Ta was 2.98:1. The result is consistent with that of the above
methods.
5. Conclusion
(1) According to a pilot study, low-acid pulp extraction is feasible.
(2) Using this technology, the extraction rate of tantalum can reached to more than 98 % at a
hydrofluoric acid concentration of 1.6 mol/L, a sulfuric acid concentration of 0.6 mol/L, an organic
to pulp phase ratio of 3:1, a tantalum concentration of 26.61 g/L, a niobium concentration of 11.64
g/L, a contract time of 5min and room temperature. Moreover, the separation of tantalum and
niobium was very good under the above conditions.
(3) The slope method, the equimolar series method and the saturated volumetric method were used to
study the mechanism of tantalum extraction. The results of these methods are consistent. The MIBK:
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H2TaF7 ratio is 3:1 in the extracted species.
Acknowledgement
This work was financially supported by Plan 863 Project of China (2009AA06Z103) and National
Science Foundation of China (51004094).
References
1)
“The 2007-2008 market research and investment prospects analysis report of tantalum and niobium
in China”, 2007, Beijing view information consulting Co., Ltd, Beijing.
2)
N.R.Das, S. Lahiri, Analytical Sciences., 8, 317-322 (1992).
3)
A.I. Nikolaev, V.G. Maiorov, Doklady Chemistry, 415, 167-169 (2007).
4)
V.G.Mayorov, A.I.Nikolaev, Hydrometallurgy, 41, 71-78 (1995).
5)
O.M. El Hussaini, N.M. Rice, Hydrometallurgy, 72, 259-267 (2004).
6)
J. Han, Y. Zhou, Rare Metals and Cemented Carbides, 32, 15-20 (2004).
7)
V.G.Maiorov, A.I.Nikolaev, V.K.Kopkov, Russian Journal of Applied Chemistry, 74, 363-367 (2001).
8)
V.G.Mayorov, A.I.Nikolaev, V.G.Mayorov, Inorganic Synthesis and Industrial Inorganic Chemistry,
75, 1389-1393 (2002).
9)
N. Higa, S. Nishihama, K. Yoshizuka, Solvent Extraction Research and Development, Japan, 18,
187-192 (2011).
10) S. Tsukahara, K. Mukai, S. Watanabe, T. Fujiwara, Solvent Extraction Research and Development,
Japan, 18, 149-158 (2011).
11) Z.W. Zhu, C.Y. Cheng, Hydrometallurgy, 107, 1-12 (2011).
12) J. Xu, “Basic research on the process of separating niobium and tantalum by extraction with MIBK
in low concentration hydrofluoric acid system”, 2010, Beijing University of Chemical Technology,
Beijing.
13) X.L Yang, X.L. Wang, C. Wei, Advanced Materials Research, 644-649 (2012).
14) A. Agulyansky, “The Chemical of Tantalum and Niobium Fluoride Compounds”, 2004, Elsevier
Publisher.
15) S.Q. Yu, J.Y. Chen, Acta Metallurgica Sinica, 20, 342-351 (1984).
16) S.N. Bhattacharyya, B. Ganguly, Solvent Extraction and Ion Exchange, 2, 699-740 (1984).
17) Q.W. Guo, Z.X. Wang, “Moden Metallurgy of Tantalum and Niobium”, 2009, Beijing Metallurgical
Industry Press, Beijing.
18) E.W. Baumann, Journal of Inorganic and Nuclear Chemistry, 34, 687-695 (1972).
19) Y.J. Zhang, Z.Y. Bu, X.Y. Wang, Journal of Henan Normal University(Natural Science), 35, 112-114
(2007).
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