Application of extraction chromatography to the preparation of high-purity scandium oxide

HydrometaUurgy, 23 (1990) 333-340

333

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Application of Extraction Chromatography to the Preparation of High-Purity Scandium Oxide GUO GONGYI 1and CHEN YULI 2

1Department of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai (China) 2Department of Chemistry and Chemical Engineering, Shanghai Technology University, Shanghai (China) (Received July 29, 1988; revised and accepted January 27, 1989)

ABSTRACT Guo Gongyi and Chen Yuli, 1990. Application of extraction chromatography to the preparation of high-purity scandium oxide. Hydrometallurgy, 23: 333-340. Extraction chromatography by which 99.99% pure scandium oxide can be prepared from starting material containing about 70% scandium oxide has been developed by taking advantage of some peculiar properties of complexation and extraction chemistry of scandium. The nearly complete separation of scandium from impurities present in starting material is achieved at room temperature at a relatively fast rate through two chromatographic columns containing tributyl phosphate as stationary phase and silanized silica gel or polystyrene-divinylbenzenecopolymer as inert support. On column no. 1, the preliminary separation of scandium from impurities is done by utilizing the different stabilities of metal-citrato complexes, with only a relatively small loss of scandium. On column no. 2, the further separation of scandium from impurities is accomplished by making use of a difference in the extractability of scandium and associated impurities in the tributyl phosphate-perchloric acid system. Scandium is almost completely adsorbed on column no. 2 and can readily be eluted with 1 M hydrochloric acid. The yield of scandium by this extraction chromatography procedure is greater than 90%.

INTRODUCTION

Recently, much interest has been aroused in the study and production of scandium following the discovery of new applications. However, scandium is a truly rare and very expensive metal because of its inherent scarcity and rather difficult metallurgical properties. There are few data reported on the preparation of high-purity scandium oxide. Herchenroeder et al. [ 1 ] have given an abstract of this title, in which they have upgraded scandium oxide of 98% purity to 99.995 + % purity in kilogram quantities by ion exchange chromatography at 93 °C with approximately 90% recovery. 0304-386X/90/$03.50

© 1990 Elsevier Science Publishers B.V.

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Extraction chromatography is particularly useful for separation and purification science because it combines the excellent selectivity and favourable kinetic characteristics of the solvent extraction method with the high efficiency feature of the chromatographic process, and it has been successfully applied to nuclear fuel reprocessing on an industrial scale [ 2 ]. In the last 30 years much effort has been devoted to the application of extraction chromatography to the separation and purification of various metals, but the literature on scandium in this field is limited [3-8]. This paper reports on an extraction chromatography process by which 99.99% pure scandium oxide can be prepared at room temperature from starting material containing about 70% scandium oxide, with greater than 90% recovery. EXPERIMENTAL DETAILS

Preparation of stock solution A crude scandium hydroxide concentrate which was recovered from wolframite residue by solvent extraction was used as starting material. It contained approximately 70% scandium oxide [9]. The stock solution containing scandium was prepared by dissolving the crude scandium hydroxide concentrate in an appropriate amount of analytically pure hydrochloric or nitric acid. Generally, the stock solution containing 2-4 g l-1 scandium oxide was made, then a suitable amount of citric acid was added to the stock solution.

Preparation of chromatographic columns Extraction chromatographic columns containing tributyl phosphate as stationary phase and silanized silica gel or polystyrene-divinylbenzenecopolymer as inert support were prepared by a slurry packing method. The supports were made hydrophobic and coated with tributyl phosphate (hereafter abbreviated to TBP) according to a procedure similar to that described by Horwitz [10]. Column material no. 1 contained 432 mg TBP g-1 of hydrophobic silica gel. Column material no. 2 was 60 wt% undiluted TBP sorbed on 40 wt% polystyrene-divinylbenzene copolymer. These were then packed in two glass columns (no. 1 and no. 2 ) with bed dimensions of 1.5 cm diameter by 45 cm high and 2 cm diameter by 36 cm high, respectively.

Chromatographic separation procedure The purification of scandium was done at room temperature on two chromatographic columns. The stock solution was passed through column no. 1 at a flow rate of 1.7 ml min-1 in order to preliminarily remove some impurities, and then column no. I was consecutively washed with deionized water, hydro-

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chloric acid solution and deionized water to prepare for the next run. The effluent from column no. 1 was adjusted with perchloric acid to a suitable acidity (3-4 M), and then it was passed through column no. 2 at a flow rate of 1.7 ml m i n - 1 so as to further separate scandium from impurities. Scandium adsorbed on column no. 2 was eluted with 1 M hydrochloric acid solution at a flow rate of 1.7 ml min -1. The fractions each of 10 ml were collected and scandium present in each fraction was determined for plotting the scandium elution curve.

Analysis At low scandium concentration, for example, in the effluent from column no. 2, scandium was determined by extraction-spectrophotometry, which is similar to that described by Snell [11], but we used TBP in chloroform to extract scandium from ammonium thiocyanate. At high scandium concentration, for example, in the stock solution or the effluent from column no. 1 and the eluate from column no. 2, scandium was assayed by a new complexometric titration developed by us [ 12 ]. In this complexometry we used diethylenetriamine-penta-acetic acid as a complexing reagent and xylenol orange as an indicator. Impurity elements such as iron, calcium and magnesium in the stock solution and eluate were analyzed by atomic absorption spectrometry using an HITACHI 180-80 Polarized Zeeman Atomic Absorption Spectrophotometer. Yttrium, zirconium and titanium in the stock solution and eluate were determined by inductively coupled plasma-atomic emission spectroscopy using a Jobin Yvon JY38 ICP-AES spectrophotometer. The determination of silicon in the stock solution and eluate was made as quinoline silicomolybdate. The impurity elements in the scandium oxide product were determined by emission spectrometry using type PGS-2 Einstelltabelle Zum Pangitter-Spektrographen or Hilger and Watts Spectrographer. RESULTS AND DISCUSSION

In the extraction chromatography process developed by us, TBP sorbed on the hydrophobic silica gel or polystyrene-divinylbenzene copolymer was used as the stationary phase since TBP shows good selectivity for the extraction of scandium, depending on the extraction medium used and its acidity, and has a favourable back-extraction property. The extraction of scandium from nitric or hydrochloric acid medium into TBP is dependent on the concentration of the acid. If scandium is extracted with TBP from concentrated (6-8 M) hydrochloric or nitric acid aqueous solution, the extraction of scandium is quantitative and the chromatographic distribution ratio of scandium may be as high as 100% [3]. Unfortunately, TBP is also quantitative in extracting scandium from low concentration, e.g., 0.1 M, nitric or hydrochloric acid aqueous solu-

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tion if the aqueous solution is saturated with calcium nitrate or alkali and alkaline earth chlorides [13]. However, these compounds are always present to a greater or lesser degree in the process of extraction of scandium from various scandium-bearing resources. In the following, our results demonstrate that scandium could hardly be extracted by tributyl phosphate from low concentration hydrochloric or nitric aqueous solution containing citric acid so that scandium could be preliminarily separated from some impurities such as iron etc. Subsequently, we converted the extraction medium into a perchloric acid system and utilized the high efficiency feature of the chromatographic process to achieve the nearly complete separation of scandium from impurities.

Yield of scandium Under the conditions specified in the present study, scandium was poorly sorbed on column no. 1 since scandium was not easily extracted by TBP in the presence of citric acid. Citric acid can form a stable complex with scandium ion. The logarithmic stability constant of a scandium citrato complex may be as large as 11.35, and much higher than any of rare earths [ 14 ]. Therefore, the loss of scandium could be as low as 0.83% after the stock solution had passed through column no. 1, as shown in Table 1. Next, perchloric acid was added to the effluent from column no. 1. The distribution ratio of scandium between 5 M perchloric acid and TBP could reach 30 while other metals were less extracted by TBP from aqueous perchloric acid solution [ 15 ]. As shown in Table 1, the break-through loss of scandium through column no. 2 was very small. Scandium adsorbed on column no. 2 could be quantitatively eluted with 1 M hydrochloric acid since scandium is extracted by TBP according to solvation reaction [16 ], since scandium has a low distribution ratio in 1 M HC1-TBP system [17,18] and since scandium ions can form strong chlorocomplexes [19,20]. As shown in Table 1, the percentage elution of scandium approached TABLE 1 Yield of scandium oxide by extraction chromatography Run no.

1 2 3 4 5

Loss through column no. 1

Breakthrough column no. 2

Elution

Yield

(%)

(%)

(%)

(%)

11.2 0.83 3.03 5.45 9.7

0.158 3.88 0.62 0.49 0.52

101.8 98.8 101.2 102.3 102.1

90.3 94.1 97.5 96.3 91.7

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337

25

2O

0

~ 15

b~

c 0

o10 c c o

e

5

c~ Ix1

I i 10

20

E]uate

i |1 i 30

40

volume

50

60

(mL)

Fig. 1. S c a n d i u m elution curve. E l u t i o n of 509 mg Sc203 from c o l u m n no. 2 w i t h I M HC1, elution rate 1.7 ml m i n -1.

100%. Figure 1 shows a typical scandium elution curve with highly concentrated peaks, and no tailing could be observed. It may also be noted from Fig. 1 that 509 mg of scandium oxide could be eluted with only 42 ml of 1 M hydrochloric acid at a flow rate of 1.7 ml min -1. In summary, the recovery of scandium from stock solution by the present method was significantly high. Table 1 presents the results of five extraction chromatographic runs. It can be seen that the yield of scandium is over 90%.

Purification of scandium We took advantage of some peculiar properties of complex chemistry and extraction chemistry of scandium to achieve the separation of scandium from other metals. The scandium ion can form a strong complex with citric acid, whereas impurity elements such as manganese, cobalt, nickel, copper, zinc, cadmium, lead, cerium (III), yttrium and lanthanum ions form only weak citrato complexes, and antimony, iron (III), zirconium, uranium and tungsten ions form weaker citrato complexes [14,21-25 ]. For example, iron (III) is a main associated im-

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purity. The stability constant of the ferric-citrato complex is markedly greater than those of other impurity citrato complexes, but is much less than that of scandium citrato complex. In addition, the distribution ratio of iron(III) is much greater than that of scandium in the low concentration HC1-TBP system [3,17]. The higher stability of the complex of scandium with citric acid avoids scandium being extracted by T B P and hence allows it to pass through column no. 1, whilst impurity metal ions which form relatively weak citrato complexes are retained by column no. 1 to a greater or lesser degree, depending on the stabilities of these complexes and the extractability of the impurity metals with respect to T B P under the conditions given [26]. Therefore, when the stock solution was passed through column no. 1, the good separation of scandium from impurities could be achieved with only a relatively small loss of scandium. After the stock solution had flowed through column no. 1, perchloric acid was added to the effluent, the acidity being then adjusted to 3-4 M perchloric acid. The extractabilities of most metals from perchloric acid solution into T B P are significantly lower than from other mineral acids. The distribution ratio of scandium is about 30, and is therefore higher than that of rare earths by two orders of magnitude, and greater than that of zirconium by three orders of magnitude [15]. Consequently, with the addition of perchloric acid to the effluent, the scandium citrato complex was destroyed and scandium was strongly bound to column no. 2. Other metals not retained by column no. 2 passed through with the mobile phase and appeared in the effluent. As shown in Table 2, the amounts of impurities in the eluate were appreciably lower than those in the stock solution. Scandium in the eluate was precipitated with oxalic acid. Scandium oxalate was then filtered and calcined at 680 °C to scandium oxide product. Table 2 indicates that 99.99% pure scandium oxide can be prepared by the present TABLE2 Removal of impurities by extraction chromatography Impurities (mg 1-1 or mg kg -1) Fe In stock solution In eluate In product (L5) (L3)

Ca 3.82 0.005

<10 11

Si 0.60 0.005

<10 52

Mg 5.0 0.5

<10 16

<0.01 <0.01 <10 <10

Ti

Zr 0.6 0.17

<10 N.D.

Y 0.74 0.075

<10 N.D.

0.011 <0.01 N.D. N.D.

N.D. Not detected. All the a m o u n t s of impurities were expressed in mg 1-1 for solution or in mg k g - ~ for product.

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339

method, and that the impurities in the scandium oxide product, in general, are silicon, iron and calcium.

Performance of column The hydrophobic silica gel has a high capacity for TBP, as described above, but polystyrene-divinylbenzene copolymer has greater stability over a wide pH range of 1-13 and less variation from column to column than silica gel. The column material (polystyrene-divinylbenzene copolymer coated with T B P ) has exhibited reproducible elution characteristics such as reproducible elution peak positions, highly concentrated elution peaks, very little tailing, rapid elution rate etc., and is favourable for reducing silicon contamination in scandium oxide product. The exchange capacity of the column material which has been used for over 1 year is found almost unchanged. In the last run, about 600 mg of scandium oxide could still be adsorbed on the column material with a breakthrough loss of only 0.62% (see run no. 3 in Table 1 ). CONCLUSION

The extraction chromatography of scandium was studied using columns containing T B P supported on hydrophobic silica gel or polystyrene-divinylbenzene copolymer. The purification of scandium was done at room temperature on two chromatographic columns. The stock solution containing 2-4 g l- 1 of scandium oxide and a small amount of citric acid was passed through column no. 1 so that the preliminary separation of scandium from impurities present in the stock solution could be obtained with only a relatively small loss of scandium. Subsequently, the effluent from column no. 1, after being adjusted to 3-4 M perchloric acid, was passed through column no. 2 in order to further separate scandium from impurities. Scandium was eluted with 1 M hydrochloric acid from the second column. The scandium recovery was greater than 90%. Scandium in the eluate was precipitated with oxalic acid as scandium oxalate, which was filtered and calcined to scandium oxide. Scandium oxide of 99.99% purity can be prepared from starting material containing about 70% scandium oxide. It has been demonstrated from these results that it is reasonable and feasible to apply extraction chromatography to the purification of rare, expensive and highly reactive scandium.

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