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A solvent extraction process for the preparation of ultrahigh purity scandium oxide
hydrometallurgy ELSEVIER
Hydrometallurgy
47 (1997) 47-56
A solvent extraction process for the preparation of ultrahigh purity scandium oxide Pingwei Zhang a,*, Shutong You b, Li Zhang bt Song Feng ‘. Songshou Hou b ”Japan Science and Technology Corporation, Tohoku National Industrial Research Institute, Nigatake 4-2-l. Miyagino-ku, Sendai 983, Japan b Beijing General Research Institute for Non-ferrous Metals, Beijing 100088, Chim Received 28 February
1997; accepted
I May 1997
Abstract The development of a solvent extraction process for the preparation of ultrahigh purity scandium oxide is described. The process consists of two solvent extraction circuits, viz. the removal of zirconium from the starting scandium oxide (purity about 99%) from a 6 M HClO, solution of scandium with 100% tributyl phosphate (TBP) in single stage at an 0:A phase ratio of I : I in the first extraction circuit and the separation from scandium of impurity elements such as calcium, aluminum, manganese, titanium, yttrium and lanthanides, from a 5.8 M HCl solution containing scandium with a 40 ~01% solution of di(l-methyl heptyl) methyl phosphate (P750) in kerosene in three counter-current stages at an 0:A ratio of 1.9: 1 in the second extraction circuit. The scandium in the loaded organic phase is fully stripped in three stages using 1 M HCl at an 0:A ratio of 1:l for the TBP system and 2:1 for the PYzOsy.stem. The scandium is recovered as scandium oxide after precipitation with oxalic acid and calcination at 750-800°C. Experimental data show that the purity of the final scandium oxide product exceeds 99.995% under the selected conditions. The extraction yield of scandium was 99% for the first circuit and 98% for the second. The total recovery of scandium was found to be 93% or higher. 0 1997 Elsevier Science B.V.
1. Introduction It is well known that scandium is a rare and very expensive metal. A number of new applications for scandium have been developed in recent years, including laser crystals
Corresponding
author. Fax: +81-22.2375215;
e-mail: [email protected]
0304-386X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PI/ SO304-386X(97)00033-9
48
P. Zhang et al. / Hydrometallurgy 47 (1997147-56
and coatings, cathode materials for color cathode-ray tubes, advanced ceramics, nuclear materials and catalysts for automobiles [ll. In addition, attention has been paid to the application of scandium in the optical industry [2]. The application of scandium in high temperature superconducting materials is also under investigation [ll. While such applications have heightened the demand for scandium in both quantity and quality, it is difficult to recover scandium because it is associated in trace amounts (from 1O-3 to 10p4%) with natural minerals such as wolframite, titaniferous magnetites, tinstone @no,>, bauxite and uranium ore. Scandium is generally obtained as a by-product in the form of oxide from the processing of these ores [ll. A literature survey revealed that little data is available with respect to the purification of scandium oxide. Herchenroeder et al. [3,4] investigated the process for preparing kilogram quantities of scandium oxide of ultrahigh purity by ion exchange chromatography at 93°C. Guo et al. [5] developed a technique for producing 99.99% pure scandium oxide from starting materials containing about 70% scandium oxide by extraction chromatography with more than 90% recovery. Although the process [5] has a number of advantages, its usefulness as a large-scale production method might be restricted owing to the low scandium concentration in the stock solution used and the long operating period. In previous work [6], the authors have also described an extraction chromatography process using columns containing tributyl phosphate (TBP) supported on polystyrene-divinylbenzene copolymer from hydrochloric acid system by which scandium oxide could be purified from approximately 99% to 99.99% purity with an over 92% recovery. So far, however, no information regarding preparation of ultrahigh purity scandium oxide by solvent extraction has been found. For this reason, the authors investigated in the current work a process for the purification of scandium oxide by means of two-step solvent extraction. Using this process, scandium oxide of ultrahigh purity can be prepared with a recovery greater than 93% from the starting materials containing about 99% scandium oxide.
2. Experimental 2.1. Materials Tributyl phosphate (hereafter abbreviated to TBP), as an extractant for the first extraction circuit, was purified by washing with 10% Na,CO, solution, pre-equilibrated by contacting with an equal volume of 6 M perchloric acid solution. Di(l-methyl heptyl) methyl phosphate (Chinese trade name, P&, as an extractant for the second extraction circuit, was diluted to 40 ~01% with kerosene. Then the organic solution of P,,, was purified by washing with 10% Na,CO, solution and pre-equilibrated with 6 M hydrochloric acid solution. All other chemical reagents used were of guaranteed reagent grade. 2.2. Experimental
procedure
in detail
Scandium oxide of approximately 99% purity which was recovered from the dust produced in chlorinating magnetoilmenite or from the waste fluid in the production of
P. Zhang et al. / Hydrometal1urg.y 47 (1997) 47-56
$3
titanium white was used as the starting material in this work. Concentrated stock solutions of scandium were obtained by dissolving the scandium oxide in 8 M perchloric acid solution. These solutions were diluted with deionized water to the desired extent in order to prepare the aqueous feed solutions for the first extraction circuit. Equal volumes of both the aqueous feed and 100% TBP were mechanically mixed for 30 min (found to be sufficient for reaching equilibrium) in separatory funnels at room temperature (25 f 2°C) unless otherwise specified. After allowing the phases to disengage completely, they were separated and samples were taken for analysis. The organic phase was stripped three times at an organic-aqueous phase ratio of 1: 1 with I M HCI solution. The strip solution was adjusted to pH 2-3 with an aqueous ammonia solution and the scandium in the solution was precipitated at a temperature of around 80°C with a saturated oxalic acid solution as scandium oxalate. The precipitate was filtered and calcined at 750~800°C to scandium oxide. The stripped organic phase was returned to the extraction stage for reuse after regeneration with a 6 M perchloric acid solution. Scandium oxide obtained from the first extraction circuit was again dissolved with a 6 M hydrochloric acid solution so as to prepare the stock solution for the second extraction circuit. The aqueous scandium feed solution was obtained by adjusting the acidity and scandium concentration to the desired level with hydrochloric acid and deionized water. Both the aqueous feed solution of scandium chloride and the organic solution of 40% P 150 in kerosene were mechanically shaken in separatory funnels at room temperature (23 i 2°C). A contact time of 15 min was found in the preliminary experiments to be sufficient for equilibrium to be attained. The scandium extracted to the organic phase was completely stripped by contacting the organic phase with 1 M HCI solution three times at an 0:A ratio of 2:l. (A scrubbing operation can be carried out with 6 M HCI solution before stripping if necessary, depending on the content of impurities in the starting scandium oxide and on the quality requirement for the final product.) The subsequent procedure was the same as that used in the case of the first extraction circuit described above. At ambient temperature no difficulty was encountered in phase disengagement.
2.3. Analysis
and data treatment
The concentrations of scandium and hydrogen ions in aqueous solution were determined gravimetrically with aqueous ammonia precipitation and by titration with standard sodium hydroxide solution using phenolphthalein as an indicator and sodium citrate as a masking agent, respectively. The contents of the impurity elements in scandium oxide were measured by atomic emission spectrometry. The percentage extraction (%) and distribution coefficients (D,) of scandium and zirconium were calculated using the following equations:
% = lOOD/(
D +
RF)
(2)
P. Zhang et al./Hydrometallurgy
50
47 (1997) 47-56
of metal (M) in the equilibrium in which [Mlorg and [Ml,, denote the concentrations organic and aqueous phases, respectively. R is the phase ratio 0:A. The Sc/Zr separation factor ( p> is defined as follows:
(3)
P = b/D,, where D,, and D,, respectively.
represent
the distribution
coefficients
of scandium
and zirconium,
3. Results and discussion 3.1. Removal
of zirconium from scandium
Impurity elements in the initial scandium oxide were mainly calcium, aluminum, titanium, zirconium, manganese, silicon, yttrium and lanthanides. Amongst these impurities, the removal of zirconium from scandium oxide has always been one of the major problems encountered in the development of a process for the preparation of high purity scandium oxide. It is known [7] that the distribution coefficient of scandium between perchloric acid and TBP solutions might reach as high as 30, whereas the extractability of zirconium is much poorer than scandium in this system. Therefore, we selected the HClO,-TBP system for the first extraction circuit to separate scandium from zirconium. 3.1.1. Effect of HCIO, concentration The variations in the extraction of scandium and zirconium with the concentration of perchloric acid were studied using an aqueous solution containing 30 g 1-l of scandium and 0.023 g l- ’ of zirconium with 100% TBP at an 0:A ratio of 1: 1. The experimental data are shown in Fig. 1 and Table 1. As is observed from Fig. 1, scandium extraction increases with increasing acidity and greater than 99% extraction is obtained when the perchloric acid concentration is higher than 6.1 mol l- ’ On the other hand, the acidity has little effect upon extraction of zirconium which only increases from 11 to 16% in the range of acidities examined. Additionally, it is seen from Table 1 that the Sc/Zr
100
rSC
80 9 .g
60
z 40 Iii
3
4
5
6
I
HCIO.concn., molL’
Fig. 1. Effect of HCIO, concentration on extraction l-l, [Zr], =0.023 g I-‘; 0:A = 1:l).
of scandium
and zirconium
with 100% TBP ([SC], = 30 g
P. Zhang et al. / Hydrometallurgy 47 (1997) 47-515 Table
51
I
Effect of acidity on separation
of scandium
HClO, cont. (mot I-’ )
Distribution
7.67 1.66 5.68 6.63 ” The experimental
conditions
from zirconium
in perchloric
coefficients
acid solution by 100% TBP ’ Separation
factor
Dsc
DZ,
P(Ds,/Dzr)
3.4 9.5 23.4 292
0.13 0.19 0.20 0.19
26 so 117 1535
are given in Fig. 1.
separation factor is relatively small when the perchloric acid concentration is less than ca. 4.7 mol I-‘. However, when the acidity is greater than 6.6 M HCIO,, a Sc/Zr separation factor, which exceeds lo3 is observed. Consequently, the increase of HCIO, concentration in the aqueous feed solution is favorable for both the extraction of scandium and the Sc/Zr separation. 3.1.2. Effect of scandium concentration The influence of scandium concentration in the aqueous feed solution was examined by varying its concentration between 10.0 to 50.0 g 1-l. In all of these tests the perchloric acid concentration was 5.7 mol 1-l. The results, given in Fig. 2 and Table 2 show that extraction of scandium is reduced from 99.8 to 82%~ and that of zirconium from 29 to 19%, while the Sc/Zr separation factor decreased from 990 to 20 under the experimental conditions. Such results indicate that scandium concentration in the aqueous feed solution should be maintained at a moderate level to ensure a high extraction of scandium and complete separation of zirconium from scandium. 3.1.3. Scandium stripping Stripping of scandium was examined with 1 M hydrochloric acid solution using cross-current and counter-current operations at the different phase ratio. The experimen-
80
-
20
-
0
1 0
10
20
30
40
50
60
[scli.gL!
Fig. 2. Effect of scandium concentration on extraction of scandium and zirconium with 100% TBP. (The Zr concentration in feed solutions was 0.008, 0.015, 0.023, 0.031 and 0.038 g I-‘, responding to the SC concentration 10, 20, 30, 40 and 50 g I-‘. [HCIO,], = 5.7 mol l-‘, 0:A = 1:I.)
P. Zhang et al. / Hydrometallurgy 47 (1997) 47-56
52 Table 2 Effect of scandium by 100% TBP a
concentration
SC cont. in feed (g
I- ’)
on separation
Distribution
10.0 20.0 30.0 40.0 50.0 a The experimental
conditions
of scandium
from zirconium
coefficients
in 5.7 M perchloric Separation
acid solution
factor
4,
n 2r
P(hc /4,)
406 183 45.2 10.2 4.6
0.4 1 0.34 0.29 0.25 0.23
990 538 IS6 41 20
are shown in Fig. 2.
tal results are given in Table 3. These results clearly indicate that complete stripping scandium from the loaded organic phase can be achieved by contacting it with 1 M hydrochloric acid solution three times at equal phase ratio or using a 3-stage countercurrent operation at an 0:A ratio of 1: 1.5. 3.1.4. Larger-scale tests On the basis of the experimental results discussed above, larger-scale tests were carried out in a 35 1 extraction vessel. The analytical data for the scandium oxide obtained are shown in Table 4, together with the experimental conditions. The content of zirconium in scandium oxide decreases l-2 orders of magnitude from the starting material to the product. Moreover, a product containing less than 0.0005% zirconium was obtained. Such results demonstrate clearly that an effective separation of zirconium from scandium can be achieved using the extraction operation which has been described. In addition, it can be seen from the data in Table 4 that it is possible to obtain not only a zirconium-free scandium oxide product, but also a high extraction yield of scandium ( > 99%) under the given processing conditions. 3.2. Separation
of other impurity elements from scandium
Scandium oxide with free zirconium obtained from the first extraction circuit must be treated further in order to eliminate trace amounts of other impurity elements contained
Table 3 Stripping
of scandium
from the loaded 100% TBP organic phase with I M hydrochloric
acid solution
Sc cont. in organic phase (g 1-l)
Stripping pattern
No. of stages
Phase ratio (0:A)
Percentage stripping (%)
32.2 32.2 34.2
C.C. il CC. Cr.C. h
3 3 3
1:l 1:1.5 1:l
81.4 101.4 100.0
a C.C. represents the counter-current operation. ’ Cr.C. is an abbreviation of cross-current. A 3-stage cross-current phase is contacted with fresh 1 M HCl solution three times.
stripping
means that the loaded organic
P. Zhang et al. / Hydrometallurgy 47 (1997147-X Table 4 Content of zirconium conditions a
in scandium
oxide obtained
Initial feed solutions
[SC](19I_‘)
[HCIO,] (mol I-
30.0
5.6 6. I 6.2 6.2
?I.0
3I.0 31.0 _
’)
after extraction
53
with IGO% TBP under different processing
Phase ratio (0:A)
Content of Zr (wt%) in SczO1 materials
in SC-O, prod;,-&
2:1 I:1 I:1 I:1
0.029 0.043 0.020 0.0085
0.00074 0.00 IO 0.00096 < 0.000.5
” All extraction tests were conducted in single stage. Scandium stripping was aczomplished loaded organic phase with I M HCI solution three times at an 0:A ratio of I: I.
% extraction of scandium 98.4
99.0 > 99 > 99 by contacting
the
in the scandium oxide. In the earlier work [8] we studied the separation of scandium from calcium, titanium, iron and rare earths from hydrochloric acid medium by extraction chromatography using a column containing 40 wt% P350 supported on 60 wt% polystyrene-divinylbenzene copolymer. It can be found from Fig. 3 [8] that calcium. titanium and rare earths are preferentially eluted from the column by 6 M HCl solution, while scandium and iron are retained by the column. Scandium and iron can be eluted when 2.7 M and 0.1 M HCl solutions are used as eluants. respectively. Such results show that P3j0 has strong extraction ability for scandium and iron at high HCI concentration, whereas calcium, titanium and rare earths are hardly extracted by P3?,, under the conditions. Obviously, the separation of scandium from iron can be achieved by selective stripping with different concentration of HCl solutions. The investigation [9] further showed the possibility for separation of scandium from the impurity elements such as silicon, copper, aluminum, manganese and yttrium in addition to calcium. titanium and lanthanides by solvent extraction with Pj5,). Accordingly, the P,,,,--HCI
0
20
40
60 Eluate
80
100
120
volume, ml
Fig. 3. Metal elution curves from a column containing 40 wt% Pj5,] supported on 60 wt% polystyrene-divinylbenzene copolymer with HCI solution. (Amount of metals fed to the column: I5 mg SC, I .5 mg RE “. 2.5 mg Ti, I, 17 mg Ca and I .2 mg Fe; bed dimension: fl I I X 675 mm, elution rate: 0.37 ml cm --’ min ’.I
a Total amount of rare earths
P. Zhang et al. / Hydrometallurgy 47 (1997147-56
54 Table 5 Effect of HCl concentration HCl cont. (mol
on extraction
I ’)
of scandium
Distribution
4.05 4.90 5.65 6.70
coefficient,
with 40% Pas,, in kerosene D,,
% extraction
0.28 0.61 0.88 1.21
The concentration
of scandium
of scandium
21.9 37.9 46.8 54.7
in the feed solution was 20.0 g l-
’ 0:A = 1: 1.
system was chosen for the second extraction circuit so as to isolate these impurities from scandium in this work. Table 5 gives the effect of HCl concentration on extraction of scandium from an aqueous solution containing 20.0 g 1-l of scandium with 40% P3so in kerosene at an 0:A phase ratio of 1: 1. It is clear that the distribution coefficient of scandium increases with increasing the hydrochloric acid concentration. This result is consistent with those stated above. The enhancement of the extraction of scandium with the increase of hydrochloric acid concentration is explained in terms of the extraction reaction as follows [ 101: sc3+ + 3c1-+
3P,,, e ScCl, . 3P3,,
(4)
(where bars denote the species present in the organic phase). It is seen from the above reaction that scandium is extracted into the organic phase in the form of a ScCl, . 3P3,, complex from hydrochloric acid solution in the light of the solvation mechanism. The increase in HCl concentration increases the activity of scandium as a result of the salting-out effect of hydrochloric acid, thereby leading to the increase in scandium extraction. It can be seen from the data in Table 5 that the percentage extraction of scandium is not sufficiently high by one batch operation under the given conditions. Consequently,
[H’] = S.8M HCI O:A= 1.9:1
0
s
10
1s
20
2s
SC in aqueous phase, gL ’
Fig. 4. McCabe-Thiele (Pa,,) in kerosene.
diagram
for extraction
of scandium
by 40% di(l-methyl
heptyl)
methyl phosphate
56
P. Zhang et al. / Hydrometallurgy
47 (I 997) 47-M
4. Conclusions The technical viability of a solvent extraction process for the purification of scandium oxide has been demonstrated on larger-scale tests using 500 g quantities of scandium oxide. The process entails the removal of zirconium from perchloric acid solution by TBP in a first extraction circuit, followed by the removal of other impurity elements such as calcium, titanium, aluminum, yttrium and lanthanides, from hydrochloric acid solution by di(l-methyl heptyl) methyl phosphate in a second extraction circuit. A final product with a purity of 99.995% or better can be obtained from 99% pure scandium oxide using this process. The scandium extraction efficiency is 99% for the first circuit and 98% for the second. The total recovery of scandium is equal to or greater than 93%. A flowsheet diagram of this process is proposed (Fig. 5).
References [I] G. Guo, Y. Chen, Y. Li, J. Metals 40 (1988) 28. [2] Y. Wakui, H. Matsunaga, T.M. Suzuki, Anal. Sci. 5 (1989) 189. [3] L.A. Herchenroeder, H.R. Burkholder, B.J. Beaudry, F.A. Schmidt, J. Less Common Metals 127 (1987) 263. [4] L.A. Herchenroeder, H.R. Burkholder, B.J. Beaudry, F.A. Schmidt, J. Less Common Metals 155 (1989) 37. [5] G. Guo, Y. Chen, Hydrometallurgy 23 (1990) 333. [6] S. You, P. Zhang, L. Zhang, Y. Jiang, S. Hou, Xiyou Jinshu 15 (5) (1991) 330, In Chinese. [7] P.G. Berezhko, Zh. Prikl. Khim. 46 (4) (1973) 757. [8] P. Zhang, Y. Jiang, L. Zhang, S. Hou, Xitu 2 (1990) 12, In Chinese. [9] P. Zhang. L. Zhang, S. You, S. Feng, S. Hou, Xitu 12 (4) (1991) 18, In Chinese. [IO] Y. Zhao, D. Li, Chin. J. Appl. Chem. 7 (2) (1990) 1. In Chinese.
P. Zhang et al. / Hydrometallurgy Table 6 Analysis of the final scandium Impurities ppm
Al 3
Ca 10
47 (1997) 47-56
55
oxide product purified using the present process Si 9
Cu 2
Fe 5
Ti 3
Zr 5
Mn < 0.5
Y
ELI
Yb
the conditions with respect to the phase ratio and number of stages must be optimized in order to raise the extraction yield of scandium. The distribution isotherm of scandium was obtained by contacting the aqueous feed solutions containing various concentrations of scandium from 1.0, 5.0, 10.0 and 20.0 to 30.0 g I-’ and 5.8 M HCl with an organic solution of 40 ~01% P,,, in kerosene at an 0:A phase ratio of 1: 1. The results are presented in Fig. 4. The McCabe-Thiele diagram shown for an 0:A phase ratio of 1.9: 1, indicates that three counter-current stages at 5.8 M HCl are required for essentially complete extraction of scandium. To confirm this, a 3-stage counter-current batch simulation was conducted using an aqueous feed solution containing 20.0 g 1-l of scandium in which zirconium had been removed by the above-described method and the representative raffinate (with 0. 3 g 1-l of scandium) showed an extraction efficiency of > 98%. The loaded organic phase contained 10.37 g 1-I of scandium. The scandium was stripped completely by contacting the organic phase with I M HCl solution three times an 0:A ratio of 2: 1. Typical analyses of the scandium oxide product are given in Table 6. These data show that a final scandium oxide product with a purity of over 99.995% was obtained from 99% pure scandium oxide using the process which has been described.
99% SCANDNM
OXIDE
PRECIPITATION
r-l
CALClNATlON 750.Sco’(
ZRCONNM~FRE SCANDII!M OXIDE
CALClNATlON
SCANDlUM ULTRAHIGH
i OXIDE
OF PURITY
Fig. 5. A flowsheet for the preparation of ultrahigh purity scandium oxide by a solvent extraction process. (The dotted lines indicate that a scrubbing operation can be carried out before stripping, if necessary.) P& = di(lmethyl heptyl) methyl phosphate.
Hydrometallurgy
47 (1997) 47-56
A solvent extraction process for the preparation of ultrahigh purity scandium oxide Pingwei Zhang a,*, Shutong You b, Li Zhang bt Song Feng ‘. Songshou Hou b ”Japan Science and Technology Corporation, Tohoku National Industrial Research Institute, Nigatake 4-2-l. Miyagino-ku, Sendai 983, Japan b Beijing General Research Institute for Non-ferrous Metals, Beijing 100088, Chim Received 28 February
1997; accepted
I May 1997
Abstract The development of a solvent extraction process for the preparation of ultrahigh purity scandium oxide is described. The process consists of two solvent extraction circuits, viz. the removal of zirconium from the starting scandium oxide (purity about 99%) from a 6 M HClO, solution of scandium with 100% tributyl phosphate (TBP) in single stage at an 0:A phase ratio of I : I in the first extraction circuit and the separation from scandium of impurity elements such as calcium, aluminum, manganese, titanium, yttrium and lanthanides, from a 5.8 M HCl solution containing scandium with a 40 ~01% solution of di(l-methyl heptyl) methyl phosphate (P750) in kerosene in three counter-current stages at an 0:A ratio of 1.9: 1 in the second extraction circuit. The scandium in the loaded organic phase is fully stripped in three stages using 1 M HCl at an 0:A ratio of 1:l for the TBP system and 2:1 for the PYzOsy.stem. The scandium is recovered as scandium oxide after precipitation with oxalic acid and calcination at 750-800°C. Experimental data show that the purity of the final scandium oxide product exceeds 99.995% under the selected conditions. The extraction yield of scandium was 99% for the first circuit and 98% for the second. The total recovery of scandium was found to be 93% or higher. 0 1997 Elsevier Science B.V.
1. Introduction It is well known that scandium is a rare and very expensive metal. A number of new applications for scandium have been developed in recent years, including laser crystals
Corresponding
author. Fax: +81-22.2375215;
e-mail: [email protected]
0304-386X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PI/ SO304-386X(97)00033-9
48
P. Zhang et al. / Hydrometallurgy 47 (1997147-56
and coatings, cathode materials for color cathode-ray tubes, advanced ceramics, nuclear materials and catalysts for automobiles [ll. In addition, attention has been paid to the application of scandium in the optical industry [2]. The application of scandium in high temperature superconducting materials is also under investigation [ll. While such applications have heightened the demand for scandium in both quantity and quality, it is difficult to recover scandium because it is associated in trace amounts (from 1O-3 to 10p4%) with natural minerals such as wolframite, titaniferous magnetites, tinstone @no,>, bauxite and uranium ore. Scandium is generally obtained as a by-product in the form of oxide from the processing of these ores [ll. A literature survey revealed that little data is available with respect to the purification of scandium oxide. Herchenroeder et al. [3,4] investigated the process for preparing kilogram quantities of scandium oxide of ultrahigh purity by ion exchange chromatography at 93°C. Guo et al. [5] developed a technique for producing 99.99% pure scandium oxide from starting materials containing about 70% scandium oxide by extraction chromatography with more than 90% recovery. Although the process [5] has a number of advantages, its usefulness as a large-scale production method might be restricted owing to the low scandium concentration in the stock solution used and the long operating period. In previous work [6], the authors have also described an extraction chromatography process using columns containing tributyl phosphate (TBP) supported on polystyrene-divinylbenzene copolymer from hydrochloric acid system by which scandium oxide could be purified from approximately 99% to 99.99% purity with an over 92% recovery. So far, however, no information regarding preparation of ultrahigh purity scandium oxide by solvent extraction has been found. For this reason, the authors investigated in the current work a process for the purification of scandium oxide by means of two-step solvent extraction. Using this process, scandium oxide of ultrahigh purity can be prepared with a recovery greater than 93% from the starting materials containing about 99% scandium oxide.
2. Experimental 2.1. Materials Tributyl phosphate (hereafter abbreviated to TBP), as an extractant for the first extraction circuit, was purified by washing with 10% Na,CO, solution, pre-equilibrated by contacting with an equal volume of 6 M perchloric acid solution. Di(l-methyl heptyl) methyl phosphate (Chinese trade name, P&, as an extractant for the second extraction circuit, was diluted to 40 ~01% with kerosene. Then the organic solution of P,,, was purified by washing with 10% Na,CO, solution and pre-equilibrated with 6 M hydrochloric acid solution. All other chemical reagents used were of guaranteed reagent grade. 2.2. Experimental
procedure
in detail
Scandium oxide of approximately 99% purity which was recovered from the dust produced in chlorinating magnetoilmenite or from the waste fluid in the production of
P. Zhang et al. / Hydrometal1urg.y 47 (1997) 47-56
$3
titanium white was used as the starting material in this work. Concentrated stock solutions of scandium were obtained by dissolving the scandium oxide in 8 M perchloric acid solution. These solutions were diluted with deionized water to the desired extent in order to prepare the aqueous feed solutions for the first extraction circuit. Equal volumes of both the aqueous feed and 100% TBP were mechanically mixed for 30 min (found to be sufficient for reaching equilibrium) in separatory funnels at room temperature (25 f 2°C) unless otherwise specified. After allowing the phases to disengage completely, they were separated and samples were taken for analysis. The organic phase was stripped three times at an organic-aqueous phase ratio of 1: 1 with I M HCI solution. The strip solution was adjusted to pH 2-3 with an aqueous ammonia solution and the scandium in the solution was precipitated at a temperature of around 80°C with a saturated oxalic acid solution as scandium oxalate. The precipitate was filtered and calcined at 750~800°C to scandium oxide. The stripped organic phase was returned to the extraction stage for reuse after regeneration with a 6 M perchloric acid solution. Scandium oxide obtained from the first extraction circuit was again dissolved with a 6 M hydrochloric acid solution so as to prepare the stock solution for the second extraction circuit. The aqueous scandium feed solution was obtained by adjusting the acidity and scandium concentration to the desired level with hydrochloric acid and deionized water. Both the aqueous feed solution of scandium chloride and the organic solution of 40% P 150 in kerosene were mechanically shaken in separatory funnels at room temperature (23 i 2°C). A contact time of 15 min was found in the preliminary experiments to be sufficient for equilibrium to be attained. The scandium extracted to the organic phase was completely stripped by contacting the organic phase with 1 M HCI solution three times at an 0:A ratio of 2:l. (A scrubbing operation can be carried out with 6 M HCI solution before stripping if necessary, depending on the content of impurities in the starting scandium oxide and on the quality requirement for the final product.) The subsequent procedure was the same as that used in the case of the first extraction circuit described above. At ambient temperature no difficulty was encountered in phase disengagement.
2.3. Analysis
and data treatment
The concentrations of scandium and hydrogen ions in aqueous solution were determined gravimetrically with aqueous ammonia precipitation and by titration with standard sodium hydroxide solution using phenolphthalein as an indicator and sodium citrate as a masking agent, respectively. The contents of the impurity elements in scandium oxide were measured by atomic emission spectrometry. The percentage extraction (%) and distribution coefficients (D,) of scandium and zirconium were calculated using the following equations:
% = lOOD/(
D +
RF)
(2)
P. Zhang et al./Hydrometallurgy
50
47 (1997) 47-56
of metal (M) in the equilibrium in which [Mlorg and [Ml,, denote the concentrations organic and aqueous phases, respectively. R is the phase ratio 0:A. The Sc/Zr separation factor ( p> is defined as follows:
(3)
P = b/D,, where D,, and D,, respectively.
represent
the distribution
coefficients
of scandium
and zirconium,
3. Results and discussion 3.1. Removal
of zirconium from scandium
Impurity elements in the initial scandium oxide were mainly calcium, aluminum, titanium, zirconium, manganese, silicon, yttrium and lanthanides. Amongst these impurities, the removal of zirconium from scandium oxide has always been one of the major problems encountered in the development of a process for the preparation of high purity scandium oxide. It is known [7] that the distribution coefficient of scandium between perchloric acid and TBP solutions might reach as high as 30, whereas the extractability of zirconium is much poorer than scandium in this system. Therefore, we selected the HClO,-TBP system for the first extraction circuit to separate scandium from zirconium. 3.1.1. Effect of HCIO, concentration The variations in the extraction of scandium and zirconium with the concentration of perchloric acid were studied using an aqueous solution containing 30 g 1-l of scandium and 0.023 g l- ’ of zirconium with 100% TBP at an 0:A ratio of 1: 1. The experimental data are shown in Fig. 1 and Table 1. As is observed from Fig. 1, scandium extraction increases with increasing acidity and greater than 99% extraction is obtained when the perchloric acid concentration is higher than 6.1 mol l- ’ On the other hand, the acidity has little effect upon extraction of zirconium which only increases from 11 to 16% in the range of acidities examined. Additionally, it is seen from Table 1 that the Sc/Zr
100
rSC
80 9 .g
60
z 40 Iii
3
4
5
6
I
HCIO.concn., molL’
Fig. 1. Effect of HCIO, concentration on extraction l-l, [Zr], =0.023 g I-‘; 0:A = 1:l).
of scandium
and zirconium
with 100% TBP ([SC], = 30 g
P. Zhang et al. / Hydrometallurgy 47 (1997) 47-515 Table
51
I
Effect of acidity on separation
of scandium
HClO, cont. (mot I-’ )
Distribution
7.67 1.66 5.68 6.63 ” The experimental
conditions
from zirconium
in perchloric
coefficients
acid solution by 100% TBP ’ Separation
factor
Dsc
DZ,
P(Ds,/Dzr)
3.4 9.5 23.4 292
0.13 0.19 0.20 0.19
26 so 117 1535
are given in Fig. 1.
separation factor is relatively small when the perchloric acid concentration is less than ca. 4.7 mol I-‘. However, when the acidity is greater than 6.6 M HCIO,, a Sc/Zr separation factor, which exceeds lo3 is observed. Consequently, the increase of HCIO, concentration in the aqueous feed solution is favorable for both the extraction of scandium and the Sc/Zr separation. 3.1.2. Effect of scandium concentration The influence of scandium concentration in the aqueous feed solution was examined by varying its concentration between 10.0 to 50.0 g 1-l. In all of these tests the perchloric acid concentration was 5.7 mol 1-l. The results, given in Fig. 2 and Table 2 show that extraction of scandium is reduced from 99.8 to 82%~ and that of zirconium from 29 to 19%, while the Sc/Zr separation factor decreased from 990 to 20 under the experimental conditions. Such results indicate that scandium concentration in the aqueous feed solution should be maintained at a moderate level to ensure a high extraction of scandium and complete separation of zirconium from scandium. 3.1.3. Scandium stripping Stripping of scandium was examined with 1 M hydrochloric acid solution using cross-current and counter-current operations at the different phase ratio. The experimen-
80
-
20
-
0
1 0
10
20
30
40
50
60
[scli.gL!
Fig. 2. Effect of scandium concentration on extraction of scandium and zirconium with 100% TBP. (The Zr concentration in feed solutions was 0.008, 0.015, 0.023, 0.031 and 0.038 g I-‘, responding to the SC concentration 10, 20, 30, 40 and 50 g I-‘. [HCIO,], = 5.7 mol l-‘, 0:A = 1:I.)
P. Zhang et al. / Hydrometallurgy 47 (1997) 47-56
52 Table 2 Effect of scandium by 100% TBP a
concentration
SC cont. in feed (g
I- ’)
on separation
Distribution
10.0 20.0 30.0 40.0 50.0 a The experimental
conditions
of scandium
from zirconium
coefficients
in 5.7 M perchloric Separation
acid solution
factor
4,
n 2r
P(hc /4,)
406 183 45.2 10.2 4.6
0.4 1 0.34 0.29 0.25 0.23
990 538 IS6 41 20
are shown in Fig. 2.
tal results are given in Table 3. These results clearly indicate that complete stripping scandium from the loaded organic phase can be achieved by contacting it with 1 M hydrochloric acid solution three times at equal phase ratio or using a 3-stage countercurrent operation at an 0:A ratio of 1: 1.5. 3.1.4. Larger-scale tests On the basis of the experimental results discussed above, larger-scale tests were carried out in a 35 1 extraction vessel. The analytical data for the scandium oxide obtained are shown in Table 4, together with the experimental conditions. The content of zirconium in scandium oxide decreases l-2 orders of magnitude from the starting material to the product. Moreover, a product containing less than 0.0005% zirconium was obtained. Such results demonstrate clearly that an effective separation of zirconium from scandium can be achieved using the extraction operation which has been described. In addition, it can be seen from the data in Table 4 that it is possible to obtain not only a zirconium-free scandium oxide product, but also a high extraction yield of scandium ( > 99%) under the given processing conditions. 3.2. Separation
of other impurity elements from scandium
Scandium oxide with free zirconium obtained from the first extraction circuit must be treated further in order to eliminate trace amounts of other impurity elements contained
Table 3 Stripping
of scandium
from the loaded 100% TBP organic phase with I M hydrochloric
acid solution
Sc cont. in organic phase (g 1-l)
Stripping pattern
No. of stages
Phase ratio (0:A)
Percentage stripping (%)
32.2 32.2 34.2
C.C. il CC. Cr.C. h
3 3 3
1:l 1:1.5 1:l
81.4 101.4 100.0
a C.C. represents the counter-current operation. ’ Cr.C. is an abbreviation of cross-current. A 3-stage cross-current phase is contacted with fresh 1 M HCl solution three times.
stripping
means that the loaded organic
P. Zhang et al. / Hydrometallurgy 47 (1997147-X Table 4 Content of zirconium conditions a
in scandium
oxide obtained
Initial feed solutions
[SC](19I_‘)
[HCIO,] (mol I-
30.0
5.6 6. I 6.2 6.2
?I.0
3I.0 31.0 _
’)
after extraction
53
with IGO% TBP under different processing
Phase ratio (0:A)
Content of Zr (wt%) in SczO1 materials
in SC-O, prod;,-&
2:1 I:1 I:1 I:1
0.029 0.043 0.020 0.0085
0.00074 0.00 IO 0.00096 < 0.000.5
” All extraction tests were conducted in single stage. Scandium stripping was aczomplished loaded organic phase with I M HCI solution three times at an 0:A ratio of I: I.
% extraction of scandium 98.4
99.0 > 99 > 99 by contacting
the
in the scandium oxide. In the earlier work [8] we studied the separation of scandium from calcium, titanium, iron and rare earths from hydrochloric acid medium by extraction chromatography using a column containing 40 wt% P350 supported on 60 wt% polystyrene-divinylbenzene copolymer. It can be found from Fig. 3 [8] that calcium. titanium and rare earths are preferentially eluted from the column by 6 M HCl solution, while scandium and iron are retained by the column. Scandium and iron can be eluted when 2.7 M and 0.1 M HCl solutions are used as eluants. respectively. Such results show that P3j0 has strong extraction ability for scandium and iron at high HCI concentration, whereas calcium, titanium and rare earths are hardly extracted by P3?,, under the conditions. Obviously, the separation of scandium from iron can be achieved by selective stripping with different concentration of HCl solutions. The investigation [9] further showed the possibility for separation of scandium from the impurity elements such as silicon, copper, aluminum, manganese and yttrium in addition to calcium. titanium and lanthanides by solvent extraction with Pj5,). Accordingly, the P,,,,--HCI
0
20
40
60 Eluate
80
100
120
volume, ml
Fig. 3. Metal elution curves from a column containing 40 wt% Pj5,] supported on 60 wt% polystyrene-divinylbenzene copolymer with HCI solution. (Amount of metals fed to the column: I5 mg SC, I .5 mg RE “. 2.5 mg Ti, I, 17 mg Ca and I .2 mg Fe; bed dimension: fl I I X 675 mm, elution rate: 0.37 ml cm --’ min ’.I
a Total amount of rare earths
P. Zhang et al. / Hydrometallurgy 47 (1997147-56
54 Table 5 Effect of HCl concentration HCl cont. (mol
on extraction
I ’)
of scandium
Distribution
4.05 4.90 5.65 6.70
coefficient,
with 40% Pas,, in kerosene D,,
% extraction
0.28 0.61 0.88 1.21
The concentration
of scandium
of scandium
21.9 37.9 46.8 54.7
in the feed solution was 20.0 g l-
’ 0:A = 1: 1.
system was chosen for the second extraction circuit so as to isolate these impurities from scandium in this work. Table 5 gives the effect of HCl concentration on extraction of scandium from an aqueous solution containing 20.0 g 1-l of scandium with 40% P3so in kerosene at an 0:A phase ratio of 1: 1. It is clear that the distribution coefficient of scandium increases with increasing the hydrochloric acid concentration. This result is consistent with those stated above. The enhancement of the extraction of scandium with the increase of hydrochloric acid concentration is explained in terms of the extraction reaction as follows [ 101: sc3+ + 3c1-+
3P,,, e ScCl, . 3P3,,
(4)
(where bars denote the species present in the organic phase). It is seen from the above reaction that scandium is extracted into the organic phase in the form of a ScCl, . 3P3,, complex from hydrochloric acid solution in the light of the solvation mechanism. The increase in HCl concentration increases the activity of scandium as a result of the salting-out effect of hydrochloric acid, thereby leading to the increase in scandium extraction. It can be seen from the data in Table 5 that the percentage extraction of scandium is not sufficiently high by one batch operation under the given conditions. Consequently,
[H’] = S.8M HCI O:A= 1.9:1
0
s
10
1s
20
2s
SC in aqueous phase, gL ’
Fig. 4. McCabe-Thiele (Pa,,) in kerosene.
diagram
for extraction
of scandium
by 40% di(l-methyl
heptyl)
methyl phosphate
56
P. Zhang et al. / Hydrometallurgy
47 (I 997) 47-M
4. Conclusions The technical viability of a solvent extraction process for the purification of scandium oxide has been demonstrated on larger-scale tests using 500 g quantities of scandium oxide. The process entails the removal of zirconium from perchloric acid solution by TBP in a first extraction circuit, followed by the removal of other impurity elements such as calcium, titanium, aluminum, yttrium and lanthanides, from hydrochloric acid solution by di(l-methyl heptyl) methyl phosphate in a second extraction circuit. A final product with a purity of 99.995% or better can be obtained from 99% pure scandium oxide using this process. The scandium extraction efficiency is 99% for the first circuit and 98% for the second. The total recovery of scandium is equal to or greater than 93%. A flowsheet diagram of this process is proposed (Fig. 5).
References [I] G. Guo, Y. Chen, Y. Li, J. Metals 40 (1988) 28. [2] Y. Wakui, H. Matsunaga, T.M. Suzuki, Anal. Sci. 5 (1989) 189. [3] L.A. Herchenroeder, H.R. Burkholder, B.J. Beaudry, F.A. Schmidt, J. Less Common Metals 127 (1987) 263. [4] L.A. Herchenroeder, H.R. Burkholder, B.J. Beaudry, F.A. Schmidt, J. Less Common Metals 155 (1989) 37. [5] G. Guo, Y. Chen, Hydrometallurgy 23 (1990) 333. [6] S. You, P. Zhang, L. Zhang, Y. Jiang, S. Hou, Xiyou Jinshu 15 (5) (1991) 330, In Chinese. [7] P.G. Berezhko, Zh. Prikl. Khim. 46 (4) (1973) 757. [8] P. Zhang, Y. Jiang, L. Zhang, S. Hou, Xitu 2 (1990) 12, In Chinese. [9] P. Zhang. L. Zhang, S. You, S. Feng, S. Hou, Xitu 12 (4) (1991) 18, In Chinese. [IO] Y. Zhao, D. Li, Chin. J. Appl. Chem. 7 (2) (1990) 1. In Chinese.
P. Zhang et al. / Hydrometallurgy Table 6 Analysis of the final scandium Impurities ppm
Al 3
Ca 10
47 (1997) 47-56
55
oxide product purified using the present process Si 9
Cu 2
Fe 5
Ti 3
Zr 5
Mn < 0.5
Y
ELI
Yb
the conditions with respect to the phase ratio and number of stages must be optimized in order to raise the extraction yield of scandium. The distribution isotherm of scandium was obtained by contacting the aqueous feed solutions containing various concentrations of scandium from 1.0, 5.0, 10.0 and 20.0 to 30.0 g I-’ and 5.8 M HCl with an organic solution of 40 ~01% P,,, in kerosene at an 0:A phase ratio of 1: 1. The results are presented in Fig. 4. The McCabe-Thiele diagram shown for an 0:A phase ratio of 1.9: 1, indicates that three counter-current stages at 5.8 M HCl are required for essentially complete extraction of scandium. To confirm this, a 3-stage counter-current batch simulation was conducted using an aqueous feed solution containing 20.0 g 1-l of scandium in which zirconium had been removed by the above-described method and the representative raffinate (with 0. 3 g 1-l of scandium) showed an extraction efficiency of > 98%. The loaded organic phase contained 10.37 g 1-I of scandium. The scandium was stripped completely by contacting the organic phase with I M HCl solution three times an 0:A ratio of 2: 1. Typical analyses of the scandium oxide product are given in Table 6. These data show that a final scandium oxide product with a purity of over 99.995% was obtained from 99% pure scandium oxide using the process which has been described.
99% SCANDNM
OXIDE
PRECIPITATION
r-l
CALClNATlON 750.Sco’(
ZRCONNM~FRE SCANDII!M OXIDE
CALClNATlON
SCANDlUM ULTRAHIGH
i OXIDE
OF PURITY
Fig. 5. A flowsheet for the preparation of ultrahigh purity scandium oxide by a solvent extraction process. (The dotted lines indicate that a scrubbing operation can be carried out before stripping, if necessary.) P& = di(lmethyl heptyl) methyl phosphate.