Anal. Chem. 1986, 58,2222-2226
2222
Dynamic Chromatographic Systems for the Determination of Rare Earths and Thorium in Samples from Uranium Ore Refining Processes D. J. Barkley* and Marcia Blanchette' Mineral Sciences Laboratories, Canada Centre for Mineral and Energy Technology, Ottawa, Ontario, Canada K l A OGl
R. M. Cassidy and Steve Elchuk General Chemistry Branch, Chalk River Nuclear Laboratories, Atomic Energy of Canada Limited, Chalk River, Ontario, Canada KOJ IJO
Dynamic km exchange has been used for the raptd separation (16 mln) and determlnatlon of rare earths and Y In samples from hydrometalturglcal processes used to recover U, Th, Y, and the rare earths from uranium ore. Optlmlzatlon of the effectlve capacHy of the dynamic ion exchanger and the 88lectlvlty of postcolumn reactlon detectlon permitted analysts down to 0.1 pgmL-' of the rare earths and ylttrlum In the presence of U, Th, and a number of other metal Ions. A comparlson with X-ray fluorescence results showed good agreement, and the chromatographic procedure offered conslderable savings In analysts time. Studles with Th( I V ) and U(V1) showed that both metal Ions exhlMted selective sorptlon of thek a-hydroxykobutyriccomplexe8, fanned in M u In the eluent, onto reversed phases. The analytkai results obtained showed that thk dynamic separation process could be used for the determlnatlon of Th, and Its potentlal for the determlnatlon of U was demonstrated.
The Canada Centre for Mineral and Energy Technology is concerned with the development of new and improved refining processes to maximize the recovery of uranium and valuable byproducts, including T h and the rare earths, from low-grade and complex ores. Ion exchange and solvent-extraction processes are used to separate, concentrate, and purify the metals. In a typical process the ore is leached with an acid, and the resulting uranium leach liquid containing U, Th, rare earths, and other metals dissolved from the ore is passed through an ion exchange system to remove U. The uranium-barren solution is used as a feed for a solvent-extraction process to remove Th, and rare earths are then separated from the uranium-thorium-barren solution. In solvent extraction processing, the metal or metals of interest are selectively complex from the feed solution by an organic extractant and are then acid stripped from the organic phase. These procedures have been described in more detail elsewhere (I). To optimize these processes, the determination of rare earths, Y, and Th is required. Analytical methods used previously have included X-ray fluorescence, multichemical separation schemes, and classical ion-exchange techniques. A problem common to these analytical techniques is the separation step, or steps, used to isolate rare earths and T h from extraneous metals. Chemical separations, e.g., fluoride, oxalate, and hydroxide precipitations, are time-consuming, require careful attention, and have the poesibility of rwe earth losses (2). Sufficient sample size is important for chemical separation techniques, and in the development of new refining processes, sample quantity can be limited. Classical ion exPresent address: University of Ottawa, Ottawa, Ontario, Canada. 0003-2700/88/0358-2222$01 SO/O
change separations are also time-consuming and have poor resolution and poor reproducibility. Small-particle styrene-divinylbenzene resins (3, 4 ) and bonded-phase exchangers (5,6) have been used to give improved resolution of the rare earths. However, column stability and resolution were not equivalent to that expected for modern high-performance liquid chromatography (HPLC), and the application of these techniques to samples required preliminary separations to obtain adequate resolution of the rare earths. RRcent studies of dynamic ion exchangers (7) have shown that this technique in combination with postcolumnreaction detection can be used for the sensitive and reproducible determination of rare earths in thorium and uranium fuels (8,9) and for the determination of uranium (8). Dynamic ion exchangers are formed when hydrophobic ions, which are present in the mobile phase, are sorbed onto the hydrophobic surface of a reversed-phase to produce a charged double layer at the surface where ion exchange can occur. These exchangers gave improved column efficiency for metal ions, and greater flexibility with regard to choice of separation conditions. Consequently the application of these dynamic ion exchangers was studied for the determination of rare earths, Y, and T h in samples from different circuits of a refining process for uranium ore. This paper presents the results of this study and compares these results with X-ray fluorescence analyses.
EXPERIMENTAL SECTION Reagents and Materials. Water used for solutions and eluents was freshly prepared distilled water that had been purified in a Milli-Q deionizing unit (Millipore,Bedford, MA). Stock solutions of a-hydroxyisobutyric acid (HIBA) and sodium n-octylsulfonate (CBSOi)were purified by cation ion exchange; the H+ form was used for HIBA and the NH4+form for CsSO,. All other chemicals were reagent grade. The columns used were 4.6 X 150 mm 5-pm Supelcosil LC-18 (Spelco, Bellefonte, PA), 4.6 X 100 mm 3-pm Hypersil C-18 (chromatographic Sciences Co., Montreal, PQ), and 4.6 X 150 mm 5-pm and 10-pm PRP-1 (Hamilton, Reno, NV). Standards were obtained from SPEX Industries (Metuchen, NJ). Apparatus. A variety of HPLC apparatus was used for these studies. It consisted of a Spectra Physics SP-8100 system (Spectra Physics, Santa Clara, CA), a Spectra Physics M8700 pump, Rheodyne (M7125, Berkley, CA) and Valco (Model CV-6UNPa-N60, Valco Instruments, Houston, TX) sample valves, Kratos SF770 and SF773 variable wavelength detectors (Kratos Analytical Instruments, Westwood, NJ), and Spectra Physics SP-4270 computing integrators. The X-ray fluorescence measurements were made with a Philips 1220 X-ray spectrometer. Procedures. For HPLC analysis the 5-pm Cls reversed phase was equilibrated with an aqueous mobile phase containing 2.5 X to 3.0 X rno1.L-l ammonium n-octylsulfonate, 0.05 mo1.L-' HIBA at pH 4.2 (adjusted with ",OH), and 7.5% (v/v) methanol. A 25-pL sample was injected and separated via a gradient from 0.05 to 0.4 mol.mL-' HIBA over 10 min at 1 mL.min-' and then held at 0.4 rnol-L-' for 5 min. The eluted metal ions were monitored at 658 nm after a postcolumn reaction with
Published 1986 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
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Table I. Expected Concentrations of Metala in Samples and Detection Limits for Arsenzao I11
concn of metal
detection limik,b
metal
ion: WgmL-'
WgmL-'
Ni(I1) Zn(I1) Mn(I1) CU(I1) Pb(I1) CO(I1) Fe(II1) Fe(I1) rare earths Y (111) Th
0.64 5.3 3.2 1.78 0.40 0.36 230
2223
.05
.J4
0.14 5-7 40-30
>10 >10 >10 2 2
>10 C
2.5 0.08 0.08 0.2
a Concentrations given for 10-fold dilution of sample. Detection limits are 3X peak-to-peak noise. cElutes at solvent front.
Arsenazo 111 (3,6-bis[(o-arsenophenyl)azo]-4,5-dihydroxy-2,7naphthalenediiaulfonic acid). The postcolumn reagent (1.2 mol-L-' in Arsenazo I11 and 0.5 mo1.L-' in acetic acid) was added to the eluent with a syringe pump (M314, ISCO, Lincoln, NE) via a low dead-volume mixer (6)at 1.2 mL-min-'. Quantitation was done by comparison of sample peak heights with the peak heights for a series of standards. Aqueous samples from the solvent extraction proma were analyzed by direct injection of a 10- to Wfold dilution of the process samples. Aliquots of the organic phase, which consisted of a kerosene solvent and the extractant, were evaporated on a steam bath and treated repeatedly with HN03 and HC104 to oxidize all remaining organic material. The samples were then taken to incipient dryness and dissolved in the eluent. The X-ray fluorescence procedure consisted of chemical separations, preparation of fused glass disks with lithium tetraborate, and a subsequent measurement with a Philips PW 1220 semiautomatic X-ray fluorescence spectrometer. The chemical separations consisted of double precipitations (ammonium hydroxide followed by fluoride) and the fluoride precipitate was used for preparation of the fused-glass disks. Two disks were prepared for each sample aa the calibration technique used was a double-dilution technique with one aliquot being twice the quantity of the other. The spectral corrections and details of the calculations have been published (IO). The reagent Thorin was used for the spectrophotometric determinations of thorium. Samples were first precipitated with NaOH, dissolved in HCl, and then separated on a cation exchange resin. Details of the procedure are given elsewhere (11). RESULTS AND DISCUSSION Detection. In preliminary work to apply HPLC to the separation and determination of rare earths, 4-(2-pyridylazo)resorcinol (PAR) was used as a postcolumn reagent. Satisfactory detection limits were obtained for the rare earths, but it could not be used with direct injection of process samples because of the interference from other metals. Metals present in the samples included Fe(I1) and Fe(III), Mn(II), Co(II), Cu(II), Ni(II), Zn(II), and Pb(I1); the concentrations expected in the samples are given in Table I. Tests with Arsenazo I11 showed that it was more selective for the rare earths than PAR and gave better detection limits (-2 ng, 3X peak-to-peak base line noise); detection limits observed in this work are given in Table I. These data show that the extraneous metals should not interfere and that the detection limits for the metals were sufficient to permit their determination at the required levels. During these studies it was observed that bacteria in the water collected on the column and caused large disruptions in the background (8,9) which could interfere with the detection of small amounts of metal ions. To prevent the buildup of bacteria, particularly during summer months, mobile phases were filtered through a 0.2-wm membrane; the addition of methanol to the mobile phase also helped to reduce this problem.
z
.03
I
P
4 .02
.01
0 ~~
0
4
6 12 RETENTION TIME (min)
16
Flgure 1. Separation of rare earths, yttrium, thorium, and uranium. Experimental conditions: column, 4.6 X 150 mm 5-pm Supeicosil LC-18; gradient separation at 1 mL.min-l from 0.05-0.4 mol-L-' HIBA at pH 4.2 over 10 min and held at 0.4 mobL-' for 5 min; modifiers, 0.03 mol-L-' C,SO,- and 7.5% (v/v) methanol; sample, 25 pL containing 25 ng of each rare earth and 50 ng of Y(III), Th(IV), and U(V1); detection at 658 nm after a postcolumn reaction with Arsenazo 111.
Determination of Rare E a r t h s and Yttrium. The dynamic ion exchange and postcolumn reaction system gave efficient separation and sensitive detection of the rare earths and Y(II1) as shown in Figure 1. The sharp peaks reflect the rapid mass transfer characteristics of the dynamic ion exchanger, and the ability to quickly adjust the effective ion exchange capacity was an important factor in being able to optimize separation from Th(IV) and U(VI). With appropriate adjustments in the concentration of the C8SO; and methanol, which changed the effective ion exchange capacity, and in the pH of the eluent, it was thus possible to optimize the eluant composition to place the Th(1V) and U(V1) peaks between Yb(III)/Tm(III) and Tn(III)/Er(III), rather than in the middle or end of the rare earth separation where the peaks were broad and overlapped rare-earth peaks. The Th(1V) peak was placed near Tm(II1) because the concentration of Tm(II1) in the samples was low and thus of minor importance. Except for the uranium-leach liquor, the concentration of U(V1) was generally low enough that it did not interfere with Er(II1) determinations. Figure 2 shows the chromatogram obtained for a uraniumleach liquor. This chromatogram shows that good peak shape was observed for the rare earths and that this system should also be applicable to the determination of U(V1) in these samples. Although these samples were analyzed by HPLC, no data from X-ray fluorescence measurements were available for comparison. Figure 3 shows the separation of a uranium-barren sample, and as can be seen uranium has been removed by the process. To give perspective to the peak heights in Figure 3, the peak for Gd(II1) was equivalent to 33 ng in the injected 25-pL sample. Ytterbium(1II) and Er(II1) were well-separated from Th(IV), but Tm(II1) was on the side of the Th(1V) peak. There was some band broadening of the Th(IV) peak, which made quantitation of the Th(1V) difficult. The results obtained for this sample are compared with results from XRF analysis in Table 11. The HPLC data were obtained from the average of five duplicate determinations over a 2-week period, and good reproducibility was obtained for all the rare earths. The relative precision (lu) obtained for the determination of Ce, Nd, and Ho in a U-Th-barren sample with five different calibrations over a 2-week period
2224 0 ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
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It
Table 11. Comparison of HPLC and X-ray Results for Uranium-Barren Sample
IY
.05
analysis results HPLC std dev
04
y
z m
rare earth
HPLC," mg-L-'
Lu
