The extraction chromatography behavior of zirconium and hafnium is studied using tri-n-octylamine
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| The extraction chromatography behavior of zirconium and hafnium is studied using tri-n-octylamine (TNOA) and tri-caprylyl- monomethyl ammonium chloride (Aliquat 336) as the stationary phase supported on a styrene/divinylbenzene copolymer resin and hydrochloric acid as the mobile phase. The effects of hydrochloric acid concentration, extractant loading on the support, support particle size, column dimensions, and hydrofluoric acid are investigated. Microgram amounts of Zr and Hf are successfully separated on a small size ( ∅5 × 45 mm) column, on which Zr is adsorbed and Hf is completely eluted with 8M HCl. The adsorbed Zr is then eluted with 2M HCl. Aliquat 336 is found to be superior to TNOA for the mutual separation of Zr and Hf. The stability of the extraction chromatography resin is also examined. Introduction It is well known that the mutual separation of zirconium and hafnium is very difficult because of the extreme similarity of these two elements. Both solvent extraction and ion-exchange tech- niques have been effectively applied to this mixture (1–5). However, the solvent extraction technique, which requires multi- stage counter-current mixer settlers using tributyl phosphate (TBP) as extractant, operating at the industrial scale, is not con- venient for analytical separations. Ion-exchange technology is a very effective separation method for similar ions and has been extensively and successfully applied in analytical separations. The selectivity of the ion-exchange process depends on the properties of the ion-exchanger being used and the composition of the aqueous phase. In the case of two ions having the same charge and very similar radii, the selectivity resulting from the properties of the ion-exchanger (such as acidity or basicity and the degree of crossing-linking) is not sufficient to ensure effective separation. In such a case, an appropriate complexing agent has to be added to the aqueous phase; the attained selectivity is then due to either the differences in the stability constants or the different charges or structures of the complexes formed. An alternative is extrac- tion chromatography, which combines the advantages of the high selectivity of solvent extraction and the high efficiency of ion- exchange chromatography. It has been extensively applied to sep- arations in analytical chemistry. Extraction chromatography offers many opportunities for optimizing a separation by adjusting stationary phase parameters (type and size of support particles, type and concentration of the extracting agents used in diluents, or type and degree of the loading of liquid or solid extractants) and mobile phase parameters (flow rate, concentra- tion of partitioning substances, type and concentration of com- plexing or salting out agents). Crawley (6) used TBP on kieselguhr for the separation of zir- conium and hafnium in the HNO 3 –NH
4 NO 3 system, and Ueno and Hoshi (7) reported the same agent absorbed on a celite sup- port for this separation in hydrochloric acid. TBP is a very good extractant for uranium, but the selectivity of TBP for zirconium and haf-nium is not high. The separation factor of Zr–Hf using the TBP–celite–HCl system was approximately 4 (7). Zr and Hf were also separated in the system MIBK–[NH 4 SCN+(NH 4 ) 2 SO 4 ] (8). Although Wang et al. (9) studied the extraction chromatog- raphy behavior of Zr and Hf using sulfoxides as a stationary phase and an NH 4 SCN–HCl solution as a mobile phase, the obtained separation factor of 3.6 was not large, and in addition, the intro- duction of NH 4 SCN salt into the aqueous mobile phase causes complications. As the selectivity of an extraction chromatography column is determined by the nature of the extractant used as the sta- tionary phase, it is important to select a suitable extractant for effective separation. Moore (10) first reported the solvent extraction and separation of Zr and Hf with long-chain aliphatic amines (methyl-
The long-chain aliphatic amine compounds are also known as liquid ion-exchangers and have been widely used in the solvent extraction separation of various metal ions in nuclear fuel pro- cessing and fission product recovery. Cerrai and Testa (11,12) reported detailed investigations on the solvent extraction and separation of Zr and Hf using tri-
1,2
, C. Pin 1
2 1
2 Department of Chemical Engineering, The University of New South Wales, Sydney 2052, Australia Reproduction (photocopying) of editorial content of this journal is prohibited without publisher’s permission. Journal of Chromatographic Science, Vol. 37, May 1999 Downloaded from https://academic.oup.com/chromsci/article-abstract/37/5/171/302600 by guest on 27 February 2018 Journal of Chromatographic Science, Vol. 37, May 1999 172 caprylyl-monomethyl ammonium chloride (Aliquat 336) in a hydrochloric medium, and later separated 9.7 mg of Zr from 0.3 mg of Hf using a TNOA–8M HCl–5% concentrated HNO 3 system on a cellulose powder column (13). However, high con- centrations of HCl pose a problem for the cellulose powder sup- port (14). Das and Chattopadhyay (15) reported briefly on the solvent extraction and chromatographic separation of nio- bium, zirconium, and hafnium with Aliquat 336. These earlier studies suggest that extraction chromatography using TNOA or Aliquat 336 is potentially promising for the mutual separation of Zr and Hf; however, systematic studies of the parameters affecting the separation are lacking. The aim of the present study is to systematically investigate the separation of Zr–Hf by extraction chromatography using TNOA and Aliquat 336. The studied parameters include the loading of the extractant, sup- port particle size, column dimensions, composition of eluents, and stability of the resins in order to explore the potential appli- cation of the method for the separation of Zr–Hf from geolog- ical materials.
TNOA, trioctylmethyl ammonium chloride (marketed as Aliquat 336, molecular weight 404.17), and cyclohexane were obtained from Aldrich Chemical (USA). The ready-to-use Levextrel resins contain TNOA or tricaprylylmethyl ammonium chloride (a mixture of C 8 –C
chains with C 8 predominating; also marketed as Aliquat 336, but referred to as Aliquat 336M here- after in order to distinguish it from the previous Aliquat 336). Levextrel resin is a copolymer of divinylbezene (DVB) with styrene-containing organic extractant, where the organic extrac- tant is added before the polymerisation reaction starts (17,18). The support for the preparation of solvent-impregnated resins (SIR) was Amberchrom CG-300cd, a highly crosslinked styrene/divinylbenzene copolymer, which was obtained from Supelco (USA). The spherical beads of Amberchrom resin offer exceptional mechanical and chemical stability. The physical properties of Levextrel resins and Amberchrom CG-300cd resin are listed in Table I. The weight ratios of TNOA over the support in the Levextrel resins are 1:9, 1:2, and 1:1, and that of Aliquat 336M is 1:2. Zirconium and hafnium standard solutions (Aldrich) were 990 and 1002 µg/mL, respectively. All other chemicals were of analytical grade, and all the chemicals were used as received. Water with 18M Ω •
resistivity that was purified using a Milli-Q (Millipore, Milford, MA) system was used throughout.
Polypropylene columns (inner diameter ∅3.5, 3.8, 5, and 9 mm) with a 30 µm polyethylene frit at the bottom were used for separation work. An inductively coupled plasma atomic emission spectrometry (ICP- AES) instrument (Jobin-Yvon 70 II, Longjumeau, France) was used for the measurement of Zr and Hf concentrations in the solutions collected fractionally from the columns.
The extraction chromatography resin was obtained using two approaches: the solvent impregnating method (direct adsorption of the extractant into macroporous polymeric supports), pro- ducing the SIR resin, and the suspension polymerisation method (polymerisation of styrene and divinyl copolymers in the presence of the extractant, developed by Kroebel and Meyer [17,18]), pro- ducing the Levextrel resin. The SIR resin was prepared by impreg- nating 10 g of Amberchrom CG-300cd resin in a 50–100-mL cyclohexane solution containing 5 g TNOA or Aliquat 336, stir- ring the mixture slightly, and evaporating it at room temperature (20°C) to dryness. This usually takes approximately 5 hours. The Levextrel resin was prepared at the Beijing Institute of Chemical Engineering and Metallurgy, Beijing, China. The Levextrel resin was sieved, and the 75–120 mesh size (125–200 µm) was used in this work. Determination of the distribution and separation coefficients of Zr(IV) and Hf(IV) in an HCl medium using the batch-equilibrium method Resin (0.20 g) was placed into a 10-mL cylindrical vessel, and 5.0-mL portions of aqueous solutions containing 50 µg/mL Zr and Hf in various HCl concentration were added. The vessel was closed and shaken vigorously for 30 min. After filtering, the con- centrations of Zr and Hf in the aqueous phases were measured by ICP-AES. The distribution coefficient (
culated using the calculation D = [(C 0 – C e ) C e ]( V/m) Eq 1
where C 0 and C e are the initial and equilibrium concentrations of Zr and Hf in the aqueous solutions, respectively; V is the volume (mL) of aqueous solution; and m is the amount (g) of resin. The separation factor ( β) is the ratio of distribution coefficients.
Zr β = ——–– Eq 2 D Hf The purification factor (PF) corresponding to the initial solu- tions by the batch-equilibrium method can be calculated using Table I. Physical Properties of Amberchrom and Levextrel Resins Surface Mean Approximate Skeletal Chemical area Porosity pore size Mesh size pore volume density Resin structure (m 2
(%, v) (Å) (µm) (mL/g) (g/mL) CG-300cd
polyaromatic 700
55–75 300
80–160 1.66
1.08 Levextrel polyaromatic 80–345
65 180–800
125–200 NA*
NA* * NA, data not available. Downloaded from https://academic.oup.com/chromsci/article-abstract/37/5/171/302600 by guest
on 27 February 2018 Journal of Chromatographic Science, Vol. 37, May 1999 173 [(
0 –
e )Zr/(
C 0 – C e )Hf] PF Zr = ——————————— Eq 3 (
0 , Zr/
C 0 , Hf) ( C e , Hf/ C e , Zr) PF Hf = ——————— Eq 4 (
0 , Hf/
C 0 , Zr) Determination of exchange capacity A small-size column ( ∅3.5 × 50 mm) and 0.1 g of resin were used for the determination of the exchange capacity of Zr by the breakthrough curve (i.e., the Zr concentration change of the effluent with effluent volume) that was obtained by loading the feed solution until the concentration of Zr in the effluent reached the same concentration of zirconium in the feed solution. Then, the feeding was interrupted, and the column bed was washed with an eluent that displaces the interstitial volume without stripping the Zr retained in the stationary phase. When no Zr was found in this eluate, another eluent (2M HCl) That was able to strip the Zr retained on the column was passed through the column until no Zr was detected in this stripping eluent. The equilibrium exchange capacity (EEC) for Zr was obtained by measuring the concentration of Zr in the collected volume of this stripping eluate:
EEC = ——— Eq 5
where
C is the concentration of Zr in the eluate and V is the volume of eluate. Column separation The resin, suspended in water, was transferred into a chro- matographic column, and the resin bed was pressed slightly with a glass rod to eliminate air bubbles. A 30-µm polyethy- lene frit was placed on top of the column bed to prevent the disturbance of the resin during the elution phase. The column was preconditioned with 8–10 mL of the same con- centration of hydrochloric acid as the first eluant. A solution containing a Zr–Hf mixture to be separated was then loaded into the column. The elution separation was carried out with eluants composed of various concentrations of hydrochloric acid as mobile phases using gravity flow. The concentrations of Zr and Hf in the fractions of the effluents were determined using ICP-AES, and the elution curves were drawn by plot- ting the concentration against the effluent volumes. The purification factor (PF) of the column method is calculated using (
Hf /
Zr )(eluate 1) PF Hf = ——————— Eq 6 (
Hf /
Zr )feed
( C Zr / C Hf )(eluate 2) PF Zr = ——————— Eq 7 (
Zr /
Hf )feed
where eluate 1 is the Hf eluate obtained by passing 8M HCl through the column, and eluate 2 is the Zr eluate obtained by passing 2M HCl through the column.
The distribution coefficient as a function of HCl concentration ranging from 6 to 10M obtained by the batch method is shown in Figure 1. The plots of log D versus log [HCl] give linear relation- ships. It has been assumed that the extraction mechanism of Zr(IV) and Hf(IV) by tertiary amine (TNOA) and quaternary ammonium salt compound may be represented as TNOA: R
N resin
+ HCl aq = R 3 NH + × Cl
– resin
Eq 8 Figure 1. Dependence of log D on log [HCl]. 1:2 Levextrel TNOA resin (A), 1:2 SIR Aliquat 336 resin (B), 1:2 Levextrel Aliquat 336M resin (C). Aqueous phase, 5 mL of 50 µg/mL Zr and Hf in various HCl concentrations; resin phase, 0.20 g.
A B C Downloaded from https://academic.oup.com/chromsci/article-abstract/37/5/171/302600 by guest on 27 February 2018 Journal of Chromatographic Science, Vol. 37, May 1999 174 2R 3 NH + × Cl – resin
+ ZrCl 6 2– aq = (R 3 NH + ) 2 × ZrCl 6 2– resin + 2Cl – aq Eq 9 Aliquat 336: 2R 3
– resin
+ ZrCl 6 2– aq = (R 3 R'N)
+ 2 × ZrCl 6 2– resin + 2Cl – aq Eq 10 where “resin” refers to the resin phase, “aq” is the aqueous phase, R 3
3 R'N
+ Cl – is Aliquat 336, R is the octyl group, and R’ is the methyl group. The distribution coefficients at a given HCl concentration are in the order TNOA > Aliquat 336M > Aliquat 336, showing that the extraction power is in this order. It has been demonstrated that the extraction power of Aliquat 336 is generally lower than that of TNOA and thus of tri- i-octylamine (TIOA) (16). The separation factor as a function of HCl concen- tration is given in Figure 2. The maximum separation is obtain- able at an HCl concentration of 8M, and the separation factor is in the order Aliquat 336M ( β = 27) > Aliquat 336 (β = 23) > TNOA ( β = 12), which is much larger than the literature values obtained by TBP ( β = 4) (7) and sulfoxides (β = 3.6) (9). The Zr and Hf con- centrations in the 8M HCl aqueous solution after the batch equi- librium using Levextrel Aliquat 336M were 7.51 and 42.3 µg/mL, respectively (initial concentration, 50 µg/mL). Therefore, the purification factors obtained by the batch method in 8M HCl were 5.52 for Zr and 5.63 for Hf. It is expected that the separation will be greatly improved by the column method, because numerous equilibria occur in the column during the elution. The column separation will be discussed later. Breakthrough curve and exchange capacity The breakthrough curves for Zr with Aliquat 336M and Aliquat 336 resin are shown in Figure 3. Using Equation 5, the exchange capacities of Levextrel Aliquat 336M resin and Aliquat 336 SIR Figure 2. Separation coefficients as a function of HCl concentration: l, TNOA; n n , Aliquat 336; u, Aliquat 336M. Figure 3. Breakthrough curves of zirconium on the Levextrel Aliquat 336M resin and SIR Aliquat 336 resin columns (0.1 g of resin). l, Levextrel Aliquat 336M resin; l l , SIR Aliquat 336 resin. Figure 4. Separation profiles of Hf and Zr on a ∅5- × 45-mm column (loading, 50 µg Zr and 50 µg Hf): 1:1 Levextrel Aliquat 336M (flow rate, 0.24 mL/min) (A); 1:2 SIR Aliquat 336 (flow rate, 0.24 mL/min) (B); 1:1 Levextrel TNOA (flowrate, 0.2 mL/min) (C).
Downloaded from https://academic.oup.com/chromsci/article-abstract/37/5/171/302600 by guest on 27 February 2018 Journal of Chromatographic Science, Vol. 37, May 1999 175 resin are 24.3 and 30.8 mg Zr/g of resin, respectively. Column separation Figure 4 shows the column separation of Hf from Zr using Aliquat 336, Aliquat 336M, and TNOA as stationary phases and 8M HCl as the mobile phase to elute Hf. The separations of Hf from Zr with Aliquat 336 and Aliquat 336M are excellent using 10 mL of 8M HCl. However, the elution curve with Aliquat 336M seemed to show more tailing than with Aliquat 336, and the elution peak of Hf was very broad (only approximately 50% of the amount of Hf loaded was eluted with 15 mL of 8M HCl from the TNOA resin column). This can be explained by the higher distribution coeffi- cient of the TNOA resin than that of the Aliquat 336 resin (Figure 1). Hence, a larger volume of 8M HCl will be needed to separate Hf from Zr on the TNOA column. The recovery and purification factors in different initial Zr/Hf ratios of the feed solutions obtained by the Aliquat 336M columns are summarized in Table II (10 mL of 8M HCl eluate was collected for recovering Hf, and 10 mL of 2M HCl eluate was collected for Zr). The purification fac- tors are much larger than those obtained by the batch-equilib- rium method. It should be noted that the 5–8 mL of effluent of the condi- tioning phase (10 mL of 8M HCl) became turbid, and white spots were observed in the columns during the course of elu- tion/separation. The turbidity was assumed to be caused by the drainage of some of the extractant from the column. Many more spots appeared in the SIR resin columns than in the Levextrel resin columns. As a result, the flow rate decreased gradually. This could be attributed to the instability of the resin (i.e., the losses of the organic substance from the support). The stability of the SIR and Levextrel resins will be discussed in more detail later.
The effect of extractant loading on the column separation was examined using 1:9, 1:2, and 1:1 Levextrel TNOA resin, and the results are shown in Figures 4 and 5. The separation of Hf from Zr can be effectively performed with 1:2 TNOA (Figure 5B), whereas the extractant loading of 1:9 is not enough to retain Zr on the column (Figure 5A). Using a 1:1 ratio, only approximately 50% Hf loading was eluted with 15 mL of 8M HCl (Figure 4C), reflecting the high affinity of the extractant for Hf and Zr. These results show that 1:2 extractant loading has the optimum column effi- ciency. Grosse-Ruyken and Bosholm (19) reported that the min- imum heights of the equivalent theoretical plate (HETP) values at different di-(2-ethylhexyl) phosphoric acid (HDEHP) loadings on the 0.09-mm diameter silica gel were obtained at the HDEHP/ Figure 5. Effect of the extractant loading on the separation of Hf and Zr: 200 µg Zr and Hf loaded on the ∅9- × 136-mm column containing 2.9 g of 1:9 Levextrel TNOA resin (flow rate, 0.47 mL/min) (A); 50 µg Zr and Hf loaded on the ∅5- × 45-mm column containing 0.40 g of 1:2 Levextrel TNOA resin (flow rate, 0.38 mL/min) (B); 50 µg Zr and Hf loaded on the ∅5- × 45-mm column containing 0.59 g of 1:1 Levextrel TNOA resin (flow rate, 0.2; see Figure 4) (C).
50
50 48.4
0.41 0.56
49.9 96.7
99.9 118
89.2 100
5 102
0.13 0.58
4.77 102
95.4 40.4
164 50
2.5 51.2
0.07 0.38
2.34 102
94.0 36.6
123 Column 2 10
100 9.50
1.18 0.25
94.5 95.0
94.5 80.5
38.3 10
50 9.74
0.49 0.12
48.7 97.4
97.0 99.4
81.2 1 † 40 † 0.93 0.27 0.05
40.8 93.0
102 138
20.4 * Column dimension, 80 mm × 3.8-mm i.d. ; resin type, Levextrel Aliquat 336M; amount, 0.4 g; conditioning, 8 mL 8M HCl. The purification factors by the batch-equilibrium method were 5.52 for Zr and 5.63 for Hf. † The natural Zr/Hf ratio in geological materials is about 40:1. A B Downloaded from https://academic.oup.com/chromsci/article-abstract/37/5/171/302600 by guest on 27 February 2018 Journal of Chromatographic Science, Vol. 37, May 1999 176 support weight ratio of 1:2. Ueno and Hoshi (7) investigated the effect of the TBP-celite weight ratios of 1:1, 1:2, and 1:4 on the separation of Hf from Zr and also found that the 1:2 ratio was the optimum extractant loading for Hf–Zr separation.
Bed length is one of the most important parameters in extrac- tion chromatography; elution volumes, separation time, and the number of theoretical plates increase linearly with bed length, and column resolution increases with its square root. Extraction chromatographic experiments are usually run under gravity flow. When very similar ions are involved, very long lengths of column are usually necessary to obtain a satisfactory resolution. An increase in bed length leads to a proportional increase in pressure needed to force the mobile phase through the column. Most columns used in chromatography for analyt- ical purposes have bed lengths ranging between 50 and 150 mm, mainly determined by the resolution needed and the time that can be tolerated for analysis. The support particle size is of importance to the column performance in both gas and liquid chromatography (LC), because it significantly affects the sur- face area and the HETP. However, the particle size of the support also has a significant influence on the flow resistance. Figure 6 Figure 6. Effect of column bed length and support particle size on flow rate. The column diameter was 6 mm. l, 125–200 µm 1:2 Levextrel TNOA resin; n, 80–160 µm 1:2 SIR TNOA resin.
was 45 mm with SIR Aliquat 336 resin. Figure 8. The elution curves of Hf and Zr with a large-dimension column ( ∅9 × 136 mm): 3.0 g of 1:2 Levextrel TNOA resin (flow rate, 0.59 mL/min) with an elution of 50 mL 8.5M HCl and then 40 mL 4M HNO 3 (A); the same column as in A with an elution of 40 mL 8M HCl and then 30 mL 2M HCl (B); 4.8 g of 1:1 Levextrel TNOA resin (flow rate, 0.75 mL/min) (C); and 3.2 g of SIR TNOA resin (bed lenth, 120 mm; flow rate, 0.22 mL/min) (D). Zr and Hf loadings were 200 µg. Particle sizes were 125–200 µm (A–C) and 80–160 µm (D). The inset shows the Hf elution peak between 1- and 20-mL fractions, drawn to a larger scale.
A B C D Downloaded from https://academic.oup.com/chromsci/article-abstract/37/5/171/302600 by guest on 27 February 2018 Journal of Chromatographic Science, Vol. 37, May 1999 177 shows the effect of column bed length and support particle size on the flow rate. The particle size of the Levextrel resin ranged from 125 to 200-µm diameter, and that of the support for the SIR TNOA resin ranged from 80 to 160-µm diameter; the smaller resin (SIR TNOA) gave a lower flow rate as expected. Figure 7 shows the effect of bed diameter on the flow rate. The flow rate decreased drastically when the bed diameter decreased from 9 to 3.8 mm. Figure 8 shows the separation chromatogram for the larger- dimension column ( ∅9 × 150 mm) using different diameter resins (125–200-µm diameter for 1:1 and 1:2 Levextrel TNOA resins and 80–160 µm diameter for 1:2 SIR TNOA resin). For the diameter of 125–200 µm resins (Figure 8, flow rate = 0.6–0.75 mL/min), in the first effluent fraction of 15 mL of 8M HCl, a por- tion of 1–3% of the Hf amount loaded on the column was detected, whereas in the following fraction of 10 mL, no Hf was detected, indicating that Hf leakage occurred at the beginning of elution. This elution of Hf amounting to 1–3% of the loaded amount was presumably caused by channeling in the column due to the larger size of resin and column diameter and the low dis- tribution coefficient of Hf. The bulk of the Hf was eluted together with the Zr (i.e., no separation occurred) when the mobile phase was changed to 2M HCl. In the case of smaller size resin (80–160 µm diameter) (Figure 8D, flow rate = 0.22 mL/min, much smaller than the previous cases), no obvious peak for Hf in the first frac- tion of 15 mL was observed, indicating that there was no leakage of Hf. Following this, the Hf was eluted separately from Zr with 8M HCl, resulting in excellent separation. It can be concluded that for efficient operation, smaller resins, smaller diameter columns, and slower mobile phase flow rates are preferred. In general, the channeling phenomena led to the loss of resolution and may occur when large-diameter columns and large support particle sizes are used, because in such a case it is difficult to pack the columns uniformly and homogeneously. Therefore, it is good practice to reduce column diameters as much as allowed by experimental limitations. A bed diameter of approximately 5–50 particle diameter (i.e., column internal diameters from 2.5 to 7.0 mm) for 50–150-µm particles is a compromise used frequently (16). Recently, a more detailed investigation on the repro- ducibility of column performance in LC and the role of the packing density was reported by Stanley et al. (20).
Hydrofluoric (HF) acid is commonly used as a complexing agent to stabilize Zr and Hf solutions. In addition, when dealing with geological materials and minerals, HF acid is the most convenient way to create the decomposition. The main purpose of this study was to explore the possibility of the present extraction chromato- graphic system to separate of Zr and Hf from geological materials and minerals. Therefore, the effect of HF acid on the elution behavior of Hf and Zr was investigated and is shown in Figure 9. It can be seen that no separation was achieved when a small amount of HF acid was present in the solution; the elution peaks of Zr and Hf completely overlapped (the amounts eluted in the 8M HCl were 99.8% for Hf and 97.3% for Zr). This is because Hf and Zr form much more stable complexes with the fluoride ion than the chlo- ride ion, and the fluoride complexes are not extractable by the amine compounds. This system was applied to “real world” sam- ples (e.g., geological materials) and found that the successful sep- aration could be obtained in the case of complete removal of fluo- ride ions from the solution. On the other hand, the separation became very complex because of the presence of the major ele- ments Al, Fe, and Ti, which are difficult to separate from Zr and Hf. The details will be published elsewhere (21). Stability and lifetime of resin As mentioned above, instability phenomena were observed with the resin used. The stability characterisation of Levextrel and SIR resins were further examined by measuring the capacity varia- tions of resin during operation. Figure 10 shows the capacity of Levextrel Aliquat 336M resin and SIR Aliquat 336 resin as a func- tion of elution cycle number. In the capacity measurement, no preconditioning was carried out for the first run. It can be seen that the capacity for Zr drops 29% for the Levextrel Aliquat 336M resin and 50% for the SIR Aliquat 336 resin between runs 1 and Figure 9. Effect of hydrofluoric acid on the elution behavior of Hf and Zr. Conditions: column, ∅5 × 45 mm; 1:2 Levextrel TNOA resin; loading, 1 mL of 8M HCl–0.05M HF containing 50 µg Zr and Hf; flow rate, 0.36 mL/min. Figure 10. Exchange capacity for Zr as a function of operation run number. The column was ∅3.5 × 30 mm filled with 0.1 g resin. Each run included 8 mL of 10M HCl for conditioning (except the first run), 40 mL of feed solution in 10M HCl of 100 µg/mL Zr, 10 mL of 2M HCl for eluting Zr from the column, and 10 mL of water for washing the column. l, SIR Aliquat 336 resin (the flowrate decreased from 0.43 mL/min at the beginning to 0.12 mL/min at the end);
l l , Levextrel Aliquat 336M resin (the flowrate decreased from 0.18 mL/min at the beginning to 0.05 mL/min at the end). Downloaded from https://academic.oup.com/chromsci/article-abstract/37/5/171/302600 by guest on 27 February 2018 Journal of Chromatographic Science, Vol. 37, May 1999 178 2, and thereafter there was little change in capacity. One of rea- sons for this difference between runs 1 and 2 could be the pre- conditioning prior to run 2. As has been mentioned previously that at the preconditioning stage, some emulsion formation in the effluent was observed, indicating loss of extractant. The flow rate of the Levextrel Aliquat 336M resin column decreased from 0.43 to 0.12 mL/min, and that of the SIR Aliquat 336 resin column decreased from 0.18 to 0.05 mL/min. The decreases in flow rate and capacity reflect the loss of extractant from the sup- port. The stability of a chromatographic column includes phys- ical stability and chemical stability. Losses of extractant from the column may derive either from dissolution into the eluants or drainage by the eluant of undissolved portions of extractant scarcely retained on the supporting material. They may result in the undesirable presence of extractant in the eluate and also in the variation of the characteristics of the column. When a second, more retained element is involved (in the case of this work, Zr) and loaded at a high ratio, leakages of extractant could also include losses of the extracted compound and give rise to contamination of the eluate and decreases in the purification factor of the first, less retained element or incomplete recovery of the second element in the subsequent steps. In general, a maximum loading of no more than 20% of the column capacity is recommended in extraction chromatography. In the experi- ments shown in Table II, the resin amount in the column was 0.4 g, hence the Zr loading was between 0.003–1%. In addition, 10 mL of conditioning phase (8M HCl) aged the column. Therefore, the instability of the resin has no significant effect on the purification factors when the metals loading is very low (no more than 1% of the maximum capacity) and aging action is taken.
From Figure 10, it can also be seen that the Levextrel resin is more stable than the SIR resin. This may be attributed to the dif- ferent ways the extractant was introduced and held on the sup- port. For the Levextrel resin, the extractant was added before the polymerisation started, and as a result, a large amount of the extractant was adsorbed and fixed in the network or matrix of the porous copolymer. Figure 6A may be further evidence of this, because no adsorption of Zr was observed on the 1:9 Levextrel TNOA resin. For the SIR resin, which was prepared “in house”, a large amount of the extractant was adsorbed onto the surface of the resin by physical adsorption, and some of the extractant entered the cavities of the porous support by capillary forces. Consequently, the Levextrel resin should be more stable than the in-house SIR resin. From Levextrel Aliquat 336M resin (Figure 4A) and SIR Aliquat 336 resin (Figure 4B) and Figure 8, it can be seen that the tailing of Hf and Zr on the Levextrel resin is more serious than on the SIR resin. This verifies the above analysis.
The experimental results showed that TNOA and Aliquat 336 are good extractants for the zirconium–hafnium separation in an extraction chromatography system. Aliquat 336 is more selective than TNOA. The separation factors obtained ranged between 36 and 164 for 1–100 µg of Hf and 2.5–100 µg of Zr. Channeling in the column is more serious with larger column dimensions and support particle sizes. An optimum extractant loading ratio of 1:2 (the weight ratio of extractant over the support) was identified for separation purposes. No separation can be achieved when the feed aqueous solution contains hydrofluoric acid. The stability of the Levextrel resin is better than the solvent impregnated resin pre- pared in house using the conventional method.
This work was carried out at the CNRS, Université Blaise Pascal under the support of the CNRS - K.C. Wong Fellowship (France) to X.J. Yang. The authors would like to thank Dennis Jenke, whose critical comments greatly improved the manuscript.
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