- Email: [email protected]
Development of a solvent extraction process for production of nuclear grade dysprosium oxide from a crude concentrate
Desalination 232 (2008) 49–58
Development of a solvent extraction process for production of nuclear grade dysprosium oxide from a crude concentrate D.K. Singh, M.K. Kotekar, H. Singh* Rare Earths Development Section, Materials Group, BARC, Mumbai 400085, India Tel. +91 (22) 25594949; Fax +91 (22) 25505151; email: [email protected]
Received 26 March 2007; accepted revised 12 October 2007
Abstract A solvent extraction process for the production of nuclear grade Dy2O3 for its applications in advance heavy water reactor (AHWR) from a crude concentrate of rare earths containing Y2O3 ~ 67%, Dy2O3 ~ 22%, etc. has been developed and tested by bench-scale counter-current operations. The challenging task of separating Dy2O3 from other rare earths with similar chemical properties has been successfully accomplished by adopting a dual cycle solvent extraction scheme based on an organophosphorus extractant 2-ethylhexylphosphonic acid, mono-2-ethylhexyl ester (EHEHPA). Taking the advantage of the extraction order of rare earths with EHEHPA, in the first cycle heavy rare earths including yttrium fractions are separated in the product strip solution, while dysprosium is concentrated in the raffinate solution along with terbium, gadolinium, etc. In the second cycle dysprosium is purified to the extent of >99.5% with respect to other rare earths from the dysprosium concentrate obtained in the raffinate of the first cycle. Effects of process variables such as aqueous acidity, phase ratio, metal concentration in the aqueous feed, scrubbing and stripping acidity etc on separation of yttrium and other heavy rare earths in the first cycle and upgrading the purity of Dy2O3 in the second cycle have been evaluated. Under optimized conditions of process parameters, continuous operations of mixer settler yielded kilogram quantity of nuclear pure Dy2O3 which exceeds the specifications required. The recovery was found to be >98%. The overall process also produces two concentrates as by-products namely yttrium (>93%, 1st cycle) and terbium (>54%, 2nd cycle) as source materials for further upgradation of these elements. Keywords: Solvent extraction; Rare earths; EHEHPA; Dy2O3; Counter current
*Corresponding author.
Presented at the Symposium on Emerging Trends in Separation Science and Technology — SESTEC 2006 Bhabha Atomic Research Centre (BARC), Trombay, Mumbai, India, 29 September – 1 October 2006 0011-9164/08/$– See front matter © 2007 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2007.10.036
50
D.K. Singh et al. / Desalination 232 (2008) 49–58
1. Introduction India has vast deposits of thorium which are nearly one third of the world reserves but has only limited deposits of uranium [1]. India’s nuclear program envisages the use of thorium nuclear fuel cycle in its third and final phase based on advance heavy water reactor (AHWR) technology [2]. Bhabha Atomic Research Centre, Mumbai proposes to put up a 300 MW demonstration plant which will predominantly use ThO2 as fuel. Dy2O3 admixed and pelletized with Zr2O3 is proposed to be used in the central rod in AHWR for the dual purpose of maintaining negative void coefficient in the reactor and slowing down of the fast neutrons. For such an application the required specifications of Dy2O3 are: lighter rare earths (LRE) <0.1%, Gd2O3 <0.2%, Tb4O7 <1%, Y2O3 ~1.5– 5% and Dy2O3 ~95%. In India, monazite is the main source of LRE and contains ~0.5% Dy2O3 and 4–5% Y2O3 apart from thorium and uranium. Indian Rare Earths Ltd. (IREL), a sister industrial unit of Department of Atomic Energy (DAE), is responsible for the commercial exploitation of monazite mineral for the production of these elements. Digestion of the monazite mineral by caustic soda, followed by selective leaching with hydrochloric acid brings chlorides of rare earths into the solution. The resulting solution is the starting material for separation and purification of rare earths. The individual separation of these elements by methods such as leaching and precipitation is not possible because of their similar chemical behaviour. Generally this multi metal bearing solution is subjected to fractionation into three groups namely: LRE (Ce, La, Pr and Nd), middle rare earths (MRE: Sm, Eu, Gd) and heavy rare earths (HRE: Gd, Tb, Dy, Ho, Er, Y and others) based on solvent extraction technology using EHEHPA prior to their individual separation [3]. The heavy rare earths fraction has the following composition: Y2O3 ~60–67%, Dy2O3 ~20– 25%, Gd2O3 ~4%, Tb4O7 ~4.0%, Ho2O3 ~3%,
Er2O3 ~3.0%, Sm2O3 ~1.06%. This resultant HRE fraction has been taken up for the production of Dy2O3 of required specifications. The solvent extraction processes for the separation and purification of rare earths by liquid cation exchangers such as di(2-ethylhexyl) phosphoric acid (D2EHPA), EHEHPA, Versatic 10 acid and neutral extractant like tri-n-butyl phosphate (TBP) have been developed and implemented by industries [4,5]. However, the details are considered to be a commercial secret. Of late, EHEHPA has been used extensively for the extraction of base metals and rare earths due to its ability to reject calcium and requirement of lower concentration of acid for extraction/back-extraction. Extraction behaviour of rare earths has been investigated and separation factors have been evaluated using PC-88A, which is the trade name of EHEHPA by Daihachi Chemical Industry Co. Ltd, Japan [6,7]. A process for the production of high purity (99.9%) Y2O3 using a dual solvent extraction system comprising of PC-88A and TBP from chloride and thiocyanate medium has been described [8]. Simultaneous purification method for Dy and Tb from a dysprosium concentrate (>50% Dy2O3 and other rare earths) by PC-88A has been reported [9]. The authors reported the production of Dy2O3 of 97% purity with 93% recovery in a 4-exit process. The process is complicated and calls for strict control of process variables. A separation process for Dy and Y from yttrium concentrate involving large number of stages and complicated 4-exit channels using saponified D2EHPA and PC-88A has been described [10]. A process for the purification of Tb from its concentrate (>70% Tb4O7, ~28% Dy2O3, etc.) using EHEHPA has been developed [11]. Though the separation processes have been developed for the purification of Dy2O3 from crude concentrates, they suffer from some practical problems like involvement of a large number of stages, careful monitoring of process variables, etc. Moreover,
D.K. Singh et al. / Desalination 232 (2008) 49–58
the optimized conditions for a particular system cannot be implemented for others as the product specifications change with respect to specific need and also availability as well as the composition of the feed material sources. The information available in the literature provides a guideline to solve the problems. In addition to that, the separation and purification of rare earths either in the organic or aqueous phase from a multi-metal bearing complex solution by liquid cation exchangers of the type EHEHPA depends on many factors such as feed acidity, phase ratio, metal concentration in aqueous solution, etc. Therefore, conditions have to be optimized with respect to these parameters for each single application. In the present work a simple and effective process for the production of nuclear grade Dy2O3 from crude concentrate (HRE fraction) using dual solvent extraction circuit based on EHEHPA has been developed and tested by mixer settler operations. Taking the advantage of extraction order of rare earths (La < Ce < Pr < Nd < Sm < Eu < Gd < Tb < Dy < Ho < Y < Er < Tm < Yb < Lu) with EHEHPA; yttrium and other heavy rare earths are separated in the organic phase in the first cycle leaving Dy, Gd, Tb, Sm, etc. in the raffinate, while Dy in the organic phase of the second cycle leaving LRE, Tb and Gd in the raffinate. Yttrium behaves as a heavy rare earth with EHEHPA. Kilogram quantity of pure Dy2O3 exceeding required specifications has been produced. Process variables such as feed acidity, phase ratio and srcubbing conditions have been optimized. 2. Experimental Rare earths solutions were prepared by dissolving an appropriate quantity of the metal ion trioxides (purity >99.9%) in HCl. After evaporating to dryness, the solutions were made up in distilled water, further diluted solutions were prepared as needed. HRE concentrate containing
51
(Y2O3 ~67%, Dy2O3 ~22%, Gd2O3 ~4%, Tb4O7 ~4.0%, Ho 2O 3 ~2.4%, Er 2O 3 ~3.2%, Sm 2 O 3 ~1.06%) was dissolved in concentrated hydrochloric acid to obtain the starting feed material for solvent extraction test runs. Free acid content in the rare earth chloride solution was determined by precipitating the rare earths with sodium oxalate followed by titration with standard NaOH using phenolphthalein indicator. Total rare earth oxide (TO) concentration was determined by gravimetric method using oxalic acid as precipitant. The concentration of the rare earths was determined by ICP-AES Plasma-400 (inductively coupled plasma-atomic emission spectrometry). Commercially available EHEHPA was used as supplied and was diluted with paraffin (an aliphatic hydrocarbon fraction of refined kerosene with C12–C14 carbon chain of >95% purity) to obtain 1M concentration. The concentration of EHEHPA in the organic phase was determined by titrating with standard NaOH solution using bromocresol purple as indicator. All the other chemicals used were of analytical reagent grade. The extraction experiments were carried out by equilibrating equal volumes of the aqueous and organic phases for five minutes at 30±1°C using glass separating funnels. This contact time was found to be sufficient for attaining the equilibrium. The phases were allowed to settle, centrifuged and the aqueous raffinate removed for assay. The counter-current solvent extraction test runs were performed in a series of separating funnels mounted on a specially designed stand for easy transfer of the phases, each being provided with a motor driven perspex stirrer for mixing. Mixer-settler operations were carried out with 200 ml mixer capacity units made of polypropylene. Batch and continuous counter-current solvent extraction were carried out in glass separating funnels and mixer settlers respectively. The mixing of two phases in mixer compartment was done by mini motors at appropriate rpm. The flowrates of organic and aqueous phases were regulated with metering pumps. The outgoing extracts and
52
D.K. Singh et al. / Desalination 232 (2008) 49–58
raffinates were collected periodically and analysed for total oxide and individual rare earth. Digital pH meter LT-120 ( Elico Pvt. Ltd, India ) was used to measure the pH of the solutions. 3. Results and discussion 3.1. Separation of yttrium and heavy rare earths in the first cycle 3.1.1. Effect of acidity on extraction Experiments to study the effect of HCl concentration on extraction of TO were performed by contacting the feed solution having 25.0 g/L TO with 1.0M EHEHPA at a fixed phase ratio (O/ A) of 2.0. It was observed that the extraction of total oxide decreases with increase in HCl concentration in aqueous feed solution and at 0.3 M HCl, the % extraction of TO was about 50 %. 0.3M acid concentration appeared to be suitable for the preferential extraction of yttrium in counter current test runs (HRE ~67% Y2O3). 3.1.2. Effect of phase ratio Tests for studying the effects of varying phase ratio on percentage extraction of TO from a heavy rare earths concentrate solution containing 22% Dy2O3 and 67% Y2O3 in 0.3 M HCl with 1.0 M EHEHPA were carried out. The percentage extraction was found to increase as follows: 31% at O/A = 0.8, 50% at O/A = 2.0, 55% at O/A = 2.5, 62% at O/A = 3.0, 62.5% at O/A = 3.5 and 63% at O/A = 4.0. The ratio of O/A = 3.5 was found to be optimum for extraction of yttrium. It may be noted that along with Y, some Dy and Tb also get coextracted. The Dy and Tb in the organic phase have to be scrubbed out to maximize the recovery of Dy (in the raffinate) as well as increase the purity of Y in the organic phase. 3.1.3. Effect of acid concentration on scrubbing of dysprosium While extracting Y and other HRE with 1.0 M
EHEHPA, Dy, Tb, Gd etc. also get co-extracted into the organic phase. To obtain a pure yttrium product in the strip solution these rare earths have to be removed. As per the extraction order of rare earths with EHEHPA, if Dy is scrubbed out from the extract, Tb, Gd etc will automatically be separated prior to Dy. An extract comprising of 1.0 M EHEHPA representing the composition with respect to Y and other rare earths was prepared and subjected for scrubbing test at a higher O/A = 6:1 with varying concentration of HCl from 0.5 M to 2.0 M to separate co-extracted Dy. It was observed that with 1.5 M HCl, ~65% of dysprosium and ~30% of yttrium gets scrubbed out in a single contact. With increased concentration of HCl, the increase in scrubbing of Dy as well as Y was observed. At 2.0 M HCl concentration, the separation of Y was > 60%, which is undesirable as it will lead to contamination of the Dy product in raffinate. Hence 1.5 M HCl was selected for futher experiments. 3.1.4. Stripping of yttrium and heavy rare earths D2EHPA requires ~6.0 M HCl to strip yttrium and other heavy rare earths [7]. EHEHPA being less acidic, compared to D2EHPA, it is expected that it requires less acid concentration for the same. In the present work 3.5 M HCl was found to be effective for stripping out Y, Ho, Er, etc. 3.1.5. Counter-current test run After optimizing the process variables including acidity of feed, the phase ratio for extraction, acid concentration for scrubbing and stripping, it was decided to separate yttrium in the product strip solution and obtain a dysprosium-rich product in the raffinate of the first cycle. Accordingly, under optimized process conditions counter-current test runs were performed using separating funnels comprising 8 stages of extraction, 8 stages of scrubbing and 4 stages of stripping with the feed prepared from HRE fraction (25 g TO/L + 0.3 M
D.K. Singh et al. / Desalination 232 (2008) 49–58
HCl) using 1.0 M EHEHPA. The volumes were 55 ml, 14 ml, 8.5 ml, 11 ml for organic, feed, scrub acid (1.5 M HCl) and strip acid (3.5 M HCl) respectively. Outgoing product samples were analyzed for Y2O3 and Dy2O3 content in total oxide at steady state in both phases to know the purity and recovery of Dy2O3. The purity of Dy2O3 in the raffinate was observed to be >97% wrt yttrium with ~90% recovery. To increase the recovery of Dy2O3 in the raffinate without affecting the purity, numbers of stages in extraction (12 stages) as well as in scrubbing (10 stages) cascades were increased, and mixer-settler test runs were carried out as per the conditions shown in Fig. 1. The results obtained with the change in percentage composition are depicted in Table 2. From the results (Table 1) it is evident that in the first cycle the purity of Dy2O3 wrt yttrium > 99% and
53
~94% recovery was achieved. It may also be noted that in earlier report [10] the recovery of Dy2O3 in the raffinate was ~55–60% on total oxide basis with 97% purity as compared to 70% recovery in the present work (Table 1). The recovery of Y2O3 was found to be 99% with 93% purity. The extraction and scrubbing profiles based on the analysis of Dy2O3 and Y2O3 content in both phases at each stage at steady state of the first cycle countercurrent mixer settler test runs are given in Table 2. Further purification of yttrium is difficult in this process because other heavy rare earths such as Er are better extracted than Y due to the low separation factor between them ~1.4 [12]. The LRE including Gd and Tb accompanied Dy in the raffinate, which needs further purification to required specifications in the second cycle.
Fig. 1. Integrated process flow-sheet for the separation and purification of Dy2O3 from heavy rare earths concentrate. Numbers in brackets are relative volumes.
54
D.K. Singh et al. / Desalination 232 (2008) 49–58
Table 1 Compositions of different fractions obtained in the purification process of Dy from HRE concentrate Fractions
TO (g/L)
Feed for the 1st cycle Raffinate 1 Strip liquor 1 Feed for the 2nd cycle Raffinate 2 Strip liquor 2
25 4.5 21 30 7.5 38
Composition (% individual RE oxide/TO) LRE
Gd2O3
Tb4O7
Dy2O3
Y2O3
Ho2O3
Er2O3
0.5 1.0 0.05 1.5 2.4 0.02
4.1 15.7 0.1 15.4 52.4 0.08
4.1 14.5 0.05 13.5 42.6 0.9
21.9 70 1.6 70.0 5.2 >97
67.7 0.15 93.8 0.15 Traces 0.3
2.1 0.4 2.9 0.3 <0.1 0.4
3.2 0.4 4.5 0.25 <0.1 0.4
Table 2 Extraction and scrubbing profile for Y2O3 and Dy2O3 (first cycle: mixer settler test run) Stage No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Aqueous phase (g/L)
Organic phase (g/L)
TO
Dy2O3
Y2O3
TO
4.510 6.560 8.140 9.510 10.500 11.610 13.380 14.950 16.390 18.510 21.160 24.090 40.200 38.930 36.660 34.600 32.610 24.910 24.180 23.960 19.920 11.770
3.060 4.880 6.470 7.500 8.410 8.970 11.040 11.770 12.270 12.140 10.850 11.110 19.360 16.550 13.620 10.540 8.320 4.860 4.040 2.680 1.610 1.483
0.009 0.010 0.021 0.042 0.091 0.179 0.440 1.040 2.020 3.720 6.670 9.860 16.790 18.440 19.670 20.670 21.140 20.100 20.830 19.280 16.320 13.850
0.980 1.730 2.380 2.850 3.380 4.220 4.970 5.650 6.660 7.920 9.320 12.450 12.220 11.810 11.430 11.070 9.680 9.540 9.460 8.730 7.260 5.790
3.2. Purification of dysprosium in the second cycle The total oxide content in the raffinate of the first cycle is 4.5 g/L. The raffinate containing rare
Dy2O3 0.870 1.630 2.120 2.550 2.810 3.800 4.140 4.370 4.300 3.680 3.800 3.610 3.090 2.560 2.000 1.600 0.970 0.830 0.570 0.370 0.190 0.181
Y2O3 0.006 0.007 0.017 0.040 0.082 0.210 0.490 0.960 1.770 3.180 4.700 5.060 5.360 5.580 5.760 5.840 5.650 5.780 5.500 4.960 3.750 2.540
earths was precipitated with oxalic acid followed by calcination to corresponding oxides and subsequent dissolution with HCl to obtain a feed solution of 30 g/L TO for the second cycle opera-
D.K. Singh et al. / Desalination 232 (2008) 49–58
tion. To increase the recovery of Dy in the product strip solution the acid concentration of the feed was fixed at 0.2 M HCl, as lower acidity favours extraction by EHEHPA. 3.2.1. Effect of phase ratio on extraction Data recorded in Table 3 gives the results of the study on the effect of the phase ratio on % extraction of TO (Dy2O3 and Tb4O7) by 1.0 M EHEHPA from a feed of 30 g/L TO (0.2 M HCl). It is evident that % extraction for both elements increased with the increase in O/A, however, the purity of Dy wrt Tb decreased marginally from 87% (O/A = 1) to 84.3% (O/A = 2). Tb is better extracted than Gd and other LRE. To get pure Dy product with a maximum recovery, O/A = 2 was selected. Though under this condition appreciable amount of Tb accompanies Dy in organic phase, its separation is not a major problem and can be achieved by appropriate scrubbing conditions. The separation factor (β) between Dy and Tb was calculated and found to be 2.82, which agrees well with reported value of 2.81 [12]. 3.2.2. Separation of terbium In order to improve the purity of Dy in the organic phase, scrubbing of LRE including Tb is essential. Scrubbing of Tb from the organic phase
55
ensures the removal of Gd and other LRE which are better stripped than Tb as per the extraction order of rare earths with EHEHPA. A synthetic extract analyzing 16.3 g/L TO (90% Dy2O3 and 10% Tb4O7) was prepared and scrubbed with 1.5 M HCl at various phase ratios. Results recorded in Table 4 indicate that % scrubbing of Tb increases with the decrease in O/A. A phase ratio of 6.5 seems to be appropriate for scrubbing of Tb as under this condition 67.3% Tb was scrubbed out along with 42.6% Dy and 9.2 g/L of total oxide remained in the organic phase. As in the first cycle in this case also 3.5 M HCl was found to be suitable for complete stripping of Dy from the scrubbed extract. 3.2.3. Counter-current extraction tests for purification of Dy in the second cycle Continuous counter-current test runs comprising 8 stages of extraction and 16 stages of scrubbing followed by batch stripping of outgoing extract with 3.5 M HCl were carried out under optimized process parameters as per the separation scheme given in Fig. 1. The outgoing samples (raffinate and strip solution) were analyzed at steady state and the results are recorded in Table 1. The purity of Dy2O3 obtained under these conditions was found to be > 99% with respect to other
Table 3 Effect of phase ratio on extraction of Dy in the second cycle
Table 4 Effect of phase ratio on scrubbing of Tb from extract
(O/A)
Phase ratio (O/A)
% extraction TO
1.0 1.5 1.7 2.0
51 57 60 64
Dy2O3 62 70 74 78
Tb4O7 37 45 47 55
Feed: 30 g TO/L (Dy2O3: 66%, Tb4O7: 13.4% and LRE) + 0.2 M HCl Extractant: 1.0 M EHEHPA
5.0 6.5 7.5 10.0
% scrubbing TO 49.2 43.5 38.2 29.2
Dy2O3 45.2 42.6 34.3 25.7
Tb4O7 67.7 62.3 56.7 47.8
Extract: 1.0 M EHEHPA (16.3 g/L TO with 90% Dy2O3 + 10% Tb4O7) Scrub acid: 1.5 M HCl
56
D.K. Singh et al. / Desalination 232 (2008) 49–58
rare earths and with > 98% recovery. The purity of Dy2O3 produced exceeds the technical specifications required for AHWR applications. In addition to this, Tb concentrate with 54% purity with > 99% recovery was also obtained during this purification step as a by-product. 3.3. Performance evaluation test for continuous counter-current set-up Once the system reached equilibrium the periodic samples at regular intervals for outgoing raffinate and strip solutions in the first and the second cycle were withdrawn to evaluate the product purity, recovery and overall material balance (MB) of the system, and also to assess the performance of mixer settler assemblies as some fluctuations in the product purity, recovery and MB were expected to be encountered due to leakages, variations in flowrate, etc. during bench scale
operations. The plot of the total rare earth concentration in the outgoing raffinate as well as in strip solution as a function of time of mixer settler running in h is shown in Fig. 2. In the same plot the data of % recovery of Dy2O3 in the raffinate as well as Y2O3 in strip solution vs. running time of mixer settler in h are also included. It is evident that the concentration of total rare earths in the raffinate and strip solution is almost constant and the average values are ~4.0 and 22.0 g/L respectively with the overall material balance (MB) within an error limit of ±5%. Further the recovery of Dy2O3 in the raffinate and Y2O3 in the strip solution was consistent at ~70% and ~94% respectively. Similar plots for total rare earths concentrations in the raffinate and strip solutions against the contact number for the second cycle of operation are shown in Fig. 3 together with the data of % content of Dy2O3 in the product strip solution.
Fig. 2.Performance evaluation of mixer settler assemblies (first cycle).
D.K. Singh et al. / Desalination 232 (2008) 49–58
57
Fig. 3. Performance evaluation of counter-current assemblies (second cycle).
Here again the MB was found to be within an error limit of ±5%. The purity (> 98%) and recovery (~98%) of Dy2O3 was observed to be consistent throughout the bench scale operations. The % content of Tb4O7, Gd2O3, Y2O3, Sm2O3 was also determined and was found to be in the range of 0.75±0.03, 0.16±0.05, 0.63±0.02 and 0.2±0.05 respectively for these elements. The results indicate the satisfactory performance of the bench scale continuous counter-current operations for both cycles.
quantity of Dy2O3 with > 98% purity has been produced. As by-products, 93% Y2O3 and 54 % Tb4O7 have also been obtained during the dual solvent extraction process, which can be further upgraded to obtain high purity Y and Tb. Acknowledgement The authors are grateful to their colleague Mrs. R. Vijayalakshmi for her valuable suggestions. References
4. Conclusion The feasibility of producing nuclear grade Dy2O3 suitable to be used as central rod material in AHWR from a crude heavy rare earths concentrate employing EHEHPA by a dual solvent extraction scheme has been established. Kilogram
[1] Rea ‘Asu’ (Rare Earths), S. Guhathakurta, translator, H. Singh, ed., Rare Earths Association of India, Mumbai, India, 2000. [2] R.K. Sinha and A. Kakodkar, India’s reactors: past, present and future, Nucl. Eng. Design, 236 (2006) 683–700. [3] N.S. Narayanan, S. Thulsidoss, T.V. Ramachandran,
58
[4] [5] [6] [7] [8]
D.K. Singh et al. / Desalination 232 (2008) 49–58 T.V. Swaminathan and K.R. Prasad, Mater. Sci. Forum, 30 (1988) 46–50. G. Brown and L.G. Sherrington, J. Chem. Tech. Biotechnol., 29 (1979) 193–209. C. Yan, J. Jia, C. Liao, S. Wu and G. Xu, Tsingua Sci. Technol., 11 (2006) 241–247. F. Kubota, M. Goto and F. Nakashio, Solvent. Extr. Ion. Exch., 11 (1993) 437–453. T. Sato, Hydrometallurgy, 22 (1989) 121–140. S.M. Deshpande, R.B. Gajankush, N.V. Thakur and
[9] [10] [11]
[12]
K.S. Koppiker, BARC Research Report No. UED/ 90/5, 1990. S.L. Mishra, H. Singh and C.K. Gupta, Hydrometallurgy, 56 (1989) 33–40. N.V. Thakur and S.L. Mishra, Solvent Extr. Ion Exch., 18 (2000) 853–875. S.L. Mishra, R. Vijayalakshmi, M.K. Kotekar and H. Singh, BARC Research Report No. BARC/2006/ I/005. B.J. Balint, J. Rare Earth, 86 (1991) 386–389.
Development of a solvent extraction process for production of nuclear grade dysprosium oxide from a crude concentrate D.K. Singh, M.K. Kotekar, H. Singh* Rare Earths Development Section, Materials Group, BARC, Mumbai 400085, India Tel. +91 (22) 25594949; Fax +91 (22) 25505151; email: [email protected]
Received 26 March 2007; accepted revised 12 October 2007
Abstract A solvent extraction process for the production of nuclear grade Dy2O3 for its applications in advance heavy water reactor (AHWR) from a crude concentrate of rare earths containing Y2O3 ~ 67%, Dy2O3 ~ 22%, etc. has been developed and tested by bench-scale counter-current operations. The challenging task of separating Dy2O3 from other rare earths with similar chemical properties has been successfully accomplished by adopting a dual cycle solvent extraction scheme based on an organophosphorus extractant 2-ethylhexylphosphonic acid, mono-2-ethylhexyl ester (EHEHPA). Taking the advantage of the extraction order of rare earths with EHEHPA, in the first cycle heavy rare earths including yttrium fractions are separated in the product strip solution, while dysprosium is concentrated in the raffinate solution along with terbium, gadolinium, etc. In the second cycle dysprosium is purified to the extent of >99.5% with respect to other rare earths from the dysprosium concentrate obtained in the raffinate of the first cycle. Effects of process variables such as aqueous acidity, phase ratio, metal concentration in the aqueous feed, scrubbing and stripping acidity etc on separation of yttrium and other heavy rare earths in the first cycle and upgrading the purity of Dy2O3 in the second cycle have been evaluated. Under optimized conditions of process parameters, continuous operations of mixer settler yielded kilogram quantity of nuclear pure Dy2O3 which exceeds the specifications required. The recovery was found to be >98%. The overall process also produces two concentrates as by-products namely yttrium (>93%, 1st cycle) and terbium (>54%, 2nd cycle) as source materials for further upgradation of these elements. Keywords: Solvent extraction; Rare earths; EHEHPA; Dy2O3; Counter current
*Corresponding author.
Presented at the Symposium on Emerging Trends in Separation Science and Technology — SESTEC 2006 Bhabha Atomic Research Centre (BARC), Trombay, Mumbai, India, 29 September – 1 October 2006 0011-9164/08/$– See front matter © 2007 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2007.10.036
50
D.K. Singh et al. / Desalination 232 (2008) 49–58
1. Introduction India has vast deposits of thorium which are nearly one third of the world reserves but has only limited deposits of uranium [1]. India’s nuclear program envisages the use of thorium nuclear fuel cycle in its third and final phase based on advance heavy water reactor (AHWR) technology [2]. Bhabha Atomic Research Centre, Mumbai proposes to put up a 300 MW demonstration plant which will predominantly use ThO2 as fuel. Dy2O3 admixed and pelletized with Zr2O3 is proposed to be used in the central rod in AHWR for the dual purpose of maintaining negative void coefficient in the reactor and slowing down of the fast neutrons. For such an application the required specifications of Dy2O3 are: lighter rare earths (LRE) <0.1%, Gd2O3 <0.2%, Tb4O7 <1%, Y2O3 ~1.5– 5% and Dy2O3 ~95%. In India, monazite is the main source of LRE and contains ~0.5% Dy2O3 and 4–5% Y2O3 apart from thorium and uranium. Indian Rare Earths Ltd. (IREL), a sister industrial unit of Department of Atomic Energy (DAE), is responsible for the commercial exploitation of monazite mineral for the production of these elements. Digestion of the monazite mineral by caustic soda, followed by selective leaching with hydrochloric acid brings chlorides of rare earths into the solution. The resulting solution is the starting material for separation and purification of rare earths. The individual separation of these elements by methods such as leaching and precipitation is not possible because of their similar chemical behaviour. Generally this multi metal bearing solution is subjected to fractionation into three groups namely: LRE (Ce, La, Pr and Nd), middle rare earths (MRE: Sm, Eu, Gd) and heavy rare earths (HRE: Gd, Tb, Dy, Ho, Er, Y and others) based on solvent extraction technology using EHEHPA prior to their individual separation [3]. The heavy rare earths fraction has the following composition: Y2O3 ~60–67%, Dy2O3 ~20– 25%, Gd2O3 ~4%, Tb4O7 ~4.0%, Ho2O3 ~3%,
Er2O3 ~3.0%, Sm2O3 ~1.06%. This resultant HRE fraction has been taken up for the production of Dy2O3 of required specifications. The solvent extraction processes for the separation and purification of rare earths by liquid cation exchangers such as di(2-ethylhexyl) phosphoric acid (D2EHPA), EHEHPA, Versatic 10 acid and neutral extractant like tri-n-butyl phosphate (TBP) have been developed and implemented by industries [4,5]. However, the details are considered to be a commercial secret. Of late, EHEHPA has been used extensively for the extraction of base metals and rare earths due to its ability to reject calcium and requirement of lower concentration of acid for extraction/back-extraction. Extraction behaviour of rare earths has been investigated and separation factors have been evaluated using PC-88A, which is the trade name of EHEHPA by Daihachi Chemical Industry Co. Ltd, Japan [6,7]. A process for the production of high purity (99.9%) Y2O3 using a dual solvent extraction system comprising of PC-88A and TBP from chloride and thiocyanate medium has been described [8]. Simultaneous purification method for Dy and Tb from a dysprosium concentrate (>50% Dy2O3 and other rare earths) by PC-88A has been reported [9]. The authors reported the production of Dy2O3 of 97% purity with 93% recovery in a 4-exit process. The process is complicated and calls for strict control of process variables. A separation process for Dy and Y from yttrium concentrate involving large number of stages and complicated 4-exit channels using saponified D2EHPA and PC-88A has been described [10]. A process for the purification of Tb from its concentrate (>70% Tb4O7, ~28% Dy2O3, etc.) using EHEHPA has been developed [11]. Though the separation processes have been developed for the purification of Dy2O3 from crude concentrates, they suffer from some practical problems like involvement of a large number of stages, careful monitoring of process variables, etc. Moreover,
D.K. Singh et al. / Desalination 232 (2008) 49–58
the optimized conditions for a particular system cannot be implemented for others as the product specifications change with respect to specific need and also availability as well as the composition of the feed material sources. The information available in the literature provides a guideline to solve the problems. In addition to that, the separation and purification of rare earths either in the organic or aqueous phase from a multi-metal bearing complex solution by liquid cation exchangers of the type EHEHPA depends on many factors such as feed acidity, phase ratio, metal concentration in aqueous solution, etc. Therefore, conditions have to be optimized with respect to these parameters for each single application. In the present work a simple and effective process for the production of nuclear grade Dy2O3 from crude concentrate (HRE fraction) using dual solvent extraction circuit based on EHEHPA has been developed and tested by mixer settler operations. Taking the advantage of extraction order of rare earths (La < Ce < Pr < Nd < Sm < Eu < Gd < Tb < Dy < Ho < Y < Er < Tm < Yb < Lu) with EHEHPA; yttrium and other heavy rare earths are separated in the organic phase in the first cycle leaving Dy, Gd, Tb, Sm, etc. in the raffinate, while Dy in the organic phase of the second cycle leaving LRE, Tb and Gd in the raffinate. Yttrium behaves as a heavy rare earth with EHEHPA. Kilogram quantity of pure Dy2O3 exceeding required specifications has been produced. Process variables such as feed acidity, phase ratio and srcubbing conditions have been optimized. 2. Experimental Rare earths solutions were prepared by dissolving an appropriate quantity of the metal ion trioxides (purity >99.9%) in HCl. After evaporating to dryness, the solutions were made up in distilled water, further diluted solutions were prepared as needed. HRE concentrate containing
51
(Y2O3 ~67%, Dy2O3 ~22%, Gd2O3 ~4%, Tb4O7 ~4.0%, Ho 2O 3 ~2.4%, Er 2O 3 ~3.2%, Sm 2 O 3 ~1.06%) was dissolved in concentrated hydrochloric acid to obtain the starting feed material for solvent extraction test runs. Free acid content in the rare earth chloride solution was determined by precipitating the rare earths with sodium oxalate followed by titration with standard NaOH using phenolphthalein indicator. Total rare earth oxide (TO) concentration was determined by gravimetric method using oxalic acid as precipitant. The concentration of the rare earths was determined by ICP-AES Plasma-400 (inductively coupled plasma-atomic emission spectrometry). Commercially available EHEHPA was used as supplied and was diluted with paraffin (an aliphatic hydrocarbon fraction of refined kerosene with C12–C14 carbon chain of >95% purity) to obtain 1M concentration. The concentration of EHEHPA in the organic phase was determined by titrating with standard NaOH solution using bromocresol purple as indicator. All the other chemicals used were of analytical reagent grade. The extraction experiments were carried out by equilibrating equal volumes of the aqueous and organic phases for five minutes at 30±1°C using glass separating funnels. This contact time was found to be sufficient for attaining the equilibrium. The phases were allowed to settle, centrifuged and the aqueous raffinate removed for assay. The counter-current solvent extraction test runs were performed in a series of separating funnels mounted on a specially designed stand for easy transfer of the phases, each being provided with a motor driven perspex stirrer for mixing. Mixer-settler operations were carried out with 200 ml mixer capacity units made of polypropylene. Batch and continuous counter-current solvent extraction were carried out in glass separating funnels and mixer settlers respectively. The mixing of two phases in mixer compartment was done by mini motors at appropriate rpm. The flowrates of organic and aqueous phases were regulated with metering pumps. The outgoing extracts and
52
D.K. Singh et al. / Desalination 232 (2008) 49–58
raffinates were collected periodically and analysed for total oxide and individual rare earth. Digital pH meter LT-120 ( Elico Pvt. Ltd, India ) was used to measure the pH of the solutions. 3. Results and discussion 3.1. Separation of yttrium and heavy rare earths in the first cycle 3.1.1. Effect of acidity on extraction Experiments to study the effect of HCl concentration on extraction of TO were performed by contacting the feed solution having 25.0 g/L TO with 1.0M EHEHPA at a fixed phase ratio (O/ A) of 2.0. It was observed that the extraction of total oxide decreases with increase in HCl concentration in aqueous feed solution and at 0.3 M HCl, the % extraction of TO was about 50 %. 0.3M acid concentration appeared to be suitable for the preferential extraction of yttrium in counter current test runs (HRE ~67% Y2O3). 3.1.2. Effect of phase ratio Tests for studying the effects of varying phase ratio on percentage extraction of TO from a heavy rare earths concentrate solution containing 22% Dy2O3 and 67% Y2O3 in 0.3 M HCl with 1.0 M EHEHPA were carried out. The percentage extraction was found to increase as follows: 31% at O/A = 0.8, 50% at O/A = 2.0, 55% at O/A = 2.5, 62% at O/A = 3.0, 62.5% at O/A = 3.5 and 63% at O/A = 4.0. The ratio of O/A = 3.5 was found to be optimum for extraction of yttrium. It may be noted that along with Y, some Dy and Tb also get coextracted. The Dy and Tb in the organic phase have to be scrubbed out to maximize the recovery of Dy (in the raffinate) as well as increase the purity of Y in the organic phase. 3.1.3. Effect of acid concentration on scrubbing of dysprosium While extracting Y and other HRE with 1.0 M
EHEHPA, Dy, Tb, Gd etc. also get co-extracted into the organic phase. To obtain a pure yttrium product in the strip solution these rare earths have to be removed. As per the extraction order of rare earths with EHEHPA, if Dy is scrubbed out from the extract, Tb, Gd etc will automatically be separated prior to Dy. An extract comprising of 1.0 M EHEHPA representing the composition with respect to Y and other rare earths was prepared and subjected for scrubbing test at a higher O/A = 6:1 with varying concentration of HCl from 0.5 M to 2.0 M to separate co-extracted Dy. It was observed that with 1.5 M HCl, ~65% of dysprosium and ~30% of yttrium gets scrubbed out in a single contact. With increased concentration of HCl, the increase in scrubbing of Dy as well as Y was observed. At 2.0 M HCl concentration, the separation of Y was > 60%, which is undesirable as it will lead to contamination of the Dy product in raffinate. Hence 1.5 M HCl was selected for futher experiments. 3.1.4. Stripping of yttrium and heavy rare earths D2EHPA requires ~6.0 M HCl to strip yttrium and other heavy rare earths [7]. EHEHPA being less acidic, compared to D2EHPA, it is expected that it requires less acid concentration for the same. In the present work 3.5 M HCl was found to be effective for stripping out Y, Ho, Er, etc. 3.1.5. Counter-current test run After optimizing the process variables including acidity of feed, the phase ratio for extraction, acid concentration for scrubbing and stripping, it was decided to separate yttrium in the product strip solution and obtain a dysprosium-rich product in the raffinate of the first cycle. Accordingly, under optimized process conditions counter-current test runs were performed using separating funnels comprising 8 stages of extraction, 8 stages of scrubbing and 4 stages of stripping with the feed prepared from HRE fraction (25 g TO/L + 0.3 M
D.K. Singh et al. / Desalination 232 (2008) 49–58
HCl) using 1.0 M EHEHPA. The volumes were 55 ml, 14 ml, 8.5 ml, 11 ml for organic, feed, scrub acid (1.5 M HCl) and strip acid (3.5 M HCl) respectively. Outgoing product samples were analyzed for Y2O3 and Dy2O3 content in total oxide at steady state in both phases to know the purity and recovery of Dy2O3. The purity of Dy2O3 in the raffinate was observed to be >97% wrt yttrium with ~90% recovery. To increase the recovery of Dy2O3 in the raffinate without affecting the purity, numbers of stages in extraction (12 stages) as well as in scrubbing (10 stages) cascades were increased, and mixer-settler test runs were carried out as per the conditions shown in Fig. 1. The results obtained with the change in percentage composition are depicted in Table 2. From the results (Table 1) it is evident that in the first cycle the purity of Dy2O3 wrt yttrium > 99% and
53
~94% recovery was achieved. It may also be noted that in earlier report [10] the recovery of Dy2O3 in the raffinate was ~55–60% on total oxide basis with 97% purity as compared to 70% recovery in the present work (Table 1). The recovery of Y2O3 was found to be 99% with 93% purity. The extraction and scrubbing profiles based on the analysis of Dy2O3 and Y2O3 content in both phases at each stage at steady state of the first cycle countercurrent mixer settler test runs are given in Table 2. Further purification of yttrium is difficult in this process because other heavy rare earths such as Er are better extracted than Y due to the low separation factor between them ~1.4 [12]. The LRE including Gd and Tb accompanied Dy in the raffinate, which needs further purification to required specifications in the second cycle.
Fig. 1. Integrated process flow-sheet for the separation and purification of Dy2O3 from heavy rare earths concentrate. Numbers in brackets are relative volumes.
54
D.K. Singh et al. / Desalination 232 (2008) 49–58
Table 1 Compositions of different fractions obtained in the purification process of Dy from HRE concentrate Fractions
TO (g/L)
Feed for the 1st cycle Raffinate 1 Strip liquor 1 Feed for the 2nd cycle Raffinate 2 Strip liquor 2
25 4.5 21 30 7.5 38
Composition (% individual RE oxide/TO) LRE
Gd2O3
Tb4O7
Dy2O3
Y2O3
Ho2O3
Er2O3
0.5 1.0 0.05 1.5 2.4 0.02
4.1 15.7 0.1 15.4 52.4 0.08
4.1 14.5 0.05 13.5 42.6 0.9
21.9 70 1.6 70.0 5.2 >97
67.7 0.15 93.8 0.15 Traces 0.3
2.1 0.4 2.9 0.3 <0.1 0.4
3.2 0.4 4.5 0.25 <0.1 0.4
Table 2 Extraction and scrubbing profile for Y2O3 and Dy2O3 (first cycle: mixer settler test run) Stage No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Aqueous phase (g/L)
Organic phase (g/L)
TO
Dy2O3
Y2O3
TO
4.510 6.560 8.140 9.510 10.500 11.610 13.380 14.950 16.390 18.510 21.160 24.090 40.200 38.930 36.660 34.600 32.610 24.910 24.180 23.960 19.920 11.770
3.060 4.880 6.470 7.500 8.410 8.970 11.040 11.770 12.270 12.140 10.850 11.110 19.360 16.550 13.620 10.540 8.320 4.860 4.040 2.680 1.610 1.483
0.009 0.010 0.021 0.042 0.091 0.179 0.440 1.040 2.020 3.720 6.670 9.860 16.790 18.440 19.670 20.670 21.140 20.100 20.830 19.280 16.320 13.850
0.980 1.730 2.380 2.850 3.380 4.220 4.970 5.650 6.660 7.920 9.320 12.450 12.220 11.810 11.430 11.070 9.680 9.540 9.460 8.730 7.260 5.790
3.2. Purification of dysprosium in the second cycle The total oxide content in the raffinate of the first cycle is 4.5 g/L. The raffinate containing rare
Dy2O3 0.870 1.630 2.120 2.550 2.810 3.800 4.140 4.370 4.300 3.680 3.800 3.610 3.090 2.560 2.000 1.600 0.970 0.830 0.570 0.370 0.190 0.181
Y2O3 0.006 0.007 0.017 0.040 0.082 0.210 0.490 0.960 1.770 3.180 4.700 5.060 5.360 5.580 5.760 5.840 5.650 5.780 5.500 4.960 3.750 2.540
earths was precipitated with oxalic acid followed by calcination to corresponding oxides and subsequent dissolution with HCl to obtain a feed solution of 30 g/L TO for the second cycle opera-
D.K. Singh et al. / Desalination 232 (2008) 49–58
tion. To increase the recovery of Dy in the product strip solution the acid concentration of the feed was fixed at 0.2 M HCl, as lower acidity favours extraction by EHEHPA. 3.2.1. Effect of phase ratio on extraction Data recorded in Table 3 gives the results of the study on the effect of the phase ratio on % extraction of TO (Dy2O3 and Tb4O7) by 1.0 M EHEHPA from a feed of 30 g/L TO (0.2 M HCl). It is evident that % extraction for both elements increased with the increase in O/A, however, the purity of Dy wrt Tb decreased marginally from 87% (O/A = 1) to 84.3% (O/A = 2). Tb is better extracted than Gd and other LRE. To get pure Dy product with a maximum recovery, O/A = 2 was selected. Though under this condition appreciable amount of Tb accompanies Dy in organic phase, its separation is not a major problem and can be achieved by appropriate scrubbing conditions. The separation factor (β) between Dy and Tb was calculated and found to be 2.82, which agrees well with reported value of 2.81 [12]. 3.2.2. Separation of terbium In order to improve the purity of Dy in the organic phase, scrubbing of LRE including Tb is essential. Scrubbing of Tb from the organic phase
55
ensures the removal of Gd and other LRE which are better stripped than Tb as per the extraction order of rare earths with EHEHPA. A synthetic extract analyzing 16.3 g/L TO (90% Dy2O3 and 10% Tb4O7) was prepared and scrubbed with 1.5 M HCl at various phase ratios. Results recorded in Table 4 indicate that % scrubbing of Tb increases with the decrease in O/A. A phase ratio of 6.5 seems to be appropriate for scrubbing of Tb as under this condition 67.3% Tb was scrubbed out along with 42.6% Dy and 9.2 g/L of total oxide remained in the organic phase. As in the first cycle in this case also 3.5 M HCl was found to be suitable for complete stripping of Dy from the scrubbed extract. 3.2.3. Counter-current extraction tests for purification of Dy in the second cycle Continuous counter-current test runs comprising 8 stages of extraction and 16 stages of scrubbing followed by batch stripping of outgoing extract with 3.5 M HCl were carried out under optimized process parameters as per the separation scheme given in Fig. 1. The outgoing samples (raffinate and strip solution) were analyzed at steady state and the results are recorded in Table 1. The purity of Dy2O3 obtained under these conditions was found to be > 99% with respect to other
Table 3 Effect of phase ratio on extraction of Dy in the second cycle
Table 4 Effect of phase ratio on scrubbing of Tb from extract
(O/A)
Phase ratio (O/A)
% extraction TO
1.0 1.5 1.7 2.0
51 57 60 64
Dy2O3 62 70 74 78
Tb4O7 37 45 47 55
Feed: 30 g TO/L (Dy2O3: 66%, Tb4O7: 13.4% and LRE) + 0.2 M HCl Extractant: 1.0 M EHEHPA
5.0 6.5 7.5 10.0
% scrubbing TO 49.2 43.5 38.2 29.2
Dy2O3 45.2 42.6 34.3 25.7
Tb4O7 67.7 62.3 56.7 47.8
Extract: 1.0 M EHEHPA (16.3 g/L TO with 90% Dy2O3 + 10% Tb4O7) Scrub acid: 1.5 M HCl
56
D.K. Singh et al. / Desalination 232 (2008) 49–58
rare earths and with > 98% recovery. The purity of Dy2O3 produced exceeds the technical specifications required for AHWR applications. In addition to this, Tb concentrate with 54% purity with > 99% recovery was also obtained during this purification step as a by-product. 3.3. Performance evaluation test for continuous counter-current set-up Once the system reached equilibrium the periodic samples at regular intervals for outgoing raffinate and strip solutions in the first and the second cycle were withdrawn to evaluate the product purity, recovery and overall material balance (MB) of the system, and also to assess the performance of mixer settler assemblies as some fluctuations in the product purity, recovery and MB were expected to be encountered due to leakages, variations in flowrate, etc. during bench scale
operations. The plot of the total rare earth concentration in the outgoing raffinate as well as in strip solution as a function of time of mixer settler running in h is shown in Fig. 2. In the same plot the data of % recovery of Dy2O3 in the raffinate as well as Y2O3 in strip solution vs. running time of mixer settler in h are also included. It is evident that the concentration of total rare earths in the raffinate and strip solution is almost constant and the average values are ~4.0 and 22.0 g/L respectively with the overall material balance (MB) within an error limit of ±5%. Further the recovery of Dy2O3 in the raffinate and Y2O3 in the strip solution was consistent at ~70% and ~94% respectively. Similar plots for total rare earths concentrations in the raffinate and strip solutions against the contact number for the second cycle of operation are shown in Fig. 3 together with the data of % content of Dy2O3 in the product strip solution.
Fig. 2.Performance evaluation of mixer settler assemblies (first cycle).
D.K. Singh et al. / Desalination 232 (2008) 49–58
57
Fig. 3. Performance evaluation of counter-current assemblies (second cycle).
Here again the MB was found to be within an error limit of ±5%. The purity (> 98%) and recovery (~98%) of Dy2O3 was observed to be consistent throughout the bench scale operations. The % content of Tb4O7, Gd2O3, Y2O3, Sm2O3 was also determined and was found to be in the range of 0.75±0.03, 0.16±0.05, 0.63±0.02 and 0.2±0.05 respectively for these elements. The results indicate the satisfactory performance of the bench scale continuous counter-current operations for both cycles.
quantity of Dy2O3 with > 98% purity has been produced. As by-products, 93% Y2O3 and 54 % Tb4O7 have also been obtained during the dual solvent extraction process, which can be further upgraded to obtain high purity Y and Tb. Acknowledgement The authors are grateful to their colleague Mrs. R. Vijayalakshmi for her valuable suggestions. References
4. Conclusion The feasibility of producing nuclear grade Dy2O3 suitable to be used as central rod material in AHWR from a crude heavy rare earths concentrate employing EHEHPA by a dual solvent extraction scheme has been established. Kilogram
[1] Rea ‘Asu’ (Rare Earths), S. Guhathakurta, translator, H. Singh, ed., Rare Earths Association of India, Mumbai, India, 2000. [2] R.K. Sinha and A. Kakodkar, India’s reactors: past, present and future, Nucl. Eng. Design, 236 (2006) 683–700. [3] N.S. Narayanan, S. Thulsidoss, T.V. Ramachandran,
58
[4] [5] [6] [7] [8]
D.K. Singh et al. / Desalination 232 (2008) 49–58 T.V. Swaminathan and K.R. Prasad, Mater. Sci. Forum, 30 (1988) 46–50. G. Brown and L.G. Sherrington, J. Chem. Tech. Biotechnol., 29 (1979) 193–209. C. Yan, J. Jia, C. Liao, S. Wu and G. Xu, Tsingua Sci. Technol., 11 (2006) 241–247. F. Kubota, M. Goto and F. Nakashio, Solvent. Extr. Ion. Exch., 11 (1993) 437–453. T. Sato, Hydrometallurgy, 22 (1989) 121–140. S.M. Deshpande, R.B. Gajankush, N.V. Thakur and
[9] [10] [11]
[12]
K.S. Koppiker, BARC Research Report No. UED/ 90/5, 1990. S.L. Mishra, H. Singh and C.K. Gupta, Hydrometallurgy, 56 (1989) 33–40. N.V. Thakur and S.L. Mishra, Solvent Extr. Ion Exch., 18 (2000) 853–875. S.L. Mishra, R. Vijayalakshmi, M.K. Kotekar and H. Singh, BARC Research Report No. BARC/2006/ I/005. B.J. Balint, J. Rare Earth, 86 (1991) 386–389.