Chem ical R esearch in Ch inese U n iversities V o l. 13 N o. 4 (1997) Separa tion of Samar ium (g ) and Gadol in ium (g ) by A Push-pull L iqu id M em brane System YAN G Guang2hu i 3 (D ep a rtm en t of Chem istry, E d uca tion Colleg e of J ilin P rov ince, Chang chun, 130022) L IM ing2yu, TAN G Q i2hong and GUO Dong2hong (D ep a rtm en t of Chem istry, H enan U n iversity, K a if eng, 475001) W AN G X iang2de and ZHAN G X iu2juan (E nv ironm en ta l S ci. Insṫ, S ou th Ch ina U n iv. of T echnology, Guang z hou, 510641) (R eceived Sep ṫ 3, 1996) Ke yw o rds Sam arium, Gado lin ium, L iqu id m em b rane, Separation In troduction In liqu id m em b rane p rocesses, eu rop ium ( g ) w as successfu lly separated from the concen trate of Sm 3+, Eu 3+, and Gd 3+ [1 ]. T he separation of Tm 3+ and Yb 3+ w as also m ade w ith liqu id m em b rane [2 ]. It has been recogn ized that the separation of Sm 3+ and Gd 3+ is the m o st difficu lt by m ean s of so lven t ex traction p rocesses to separate tw o adjacen t rare earth (R E ) elem en tṡ O u r liqu id m em 2 b rane experim en ts also cam e to the sam e conclu sion. T he m echan ism of en large2 m en t on the difference betw een the tran spo rt rates of Tm 3+ and L u 3+ by a "com 2 p lex2exchange" liqu id m em b rane system [2 ] that is successfu l fo r the separation of Tm 3+ and L u 3+ cou ld no t be u sed effectively fo r the separation of Sm 3+ and Gd 3+. T herefo re, ano ther w ay to en large the difference betw een the tran spo rt rates of Sm 3+ and Gd 3+ in advance by com p lex ing reaction has been developed here, i. e., ethylenediam ine tetra2acetic acid (ED TA ) u sed as com p lex ing agen t is added in feed. T he com p lex con stan t of ED TA w ith individual R E increases w ith the increase of the atom ic num ber of R E in the feed. Bu t the com p lex con2 stan t of trialkylm ethylamm on ium ch lo ride (N 263) w ith each R E, u sed as the m o2 b ile carrier in the o il(m em b rane) phase, decreases w ith the increase of the atom 2 ic num ber of R E. W e call the w ay "pu sh2pu ll" liqu id m em b rane system. Its se2 lectivity is h igher than that of the "com p lex2exchange" liqu id m em b rane system. Experim en tal resu lts also confirm ed th is deduction. 3 To w hom co rrespondence should be addressed.
Exper im en tal T he aqueou s so lu tion s of lan than ide n itrates (Sm and Gd) w ere p repared by disso lving the co rresponding ox ides in n itric acid. T he so lu tion s w ere heated to rem ove w ater and n itric acid, the concen trates of the lan than ide n itrates w ere di2 lu ted w ith w ater, to w h ich amm on ium n itrate w as added to increase the overall n itrate concen tration. T hey w ere u sed as feed (o r ex ternal aqueou s phase). T he o il phase w as p repared by disso lving trialkylm ethylamm on ium ch lo ride (N 263, R 3CH 3N + C l -, R: C 8 C 10) and tribu tyl pho sphate (TBP) in kero sene, respectively. It w as treated w ith amm on ium n itrate so lu tion. So the ch lo rides of quaternary amm on ium bases w ere converted in to n itrates quan titatively by strip2 p ing. T hen the su rfactan t LM A 21 [3 ] w as disso lved in iṫ T he w ater2in2o il em u lsion w as m ade by stirring the in ternal aqueou s phase o r strip liquo r (dilu te amm on ium n itrate) and the o il phase in em u lsion treater. M ak ing the w ater2in2o il em u lsion con tact w ith feed to separate Sm 3+ and Gd 3+ in a separation tank. T he aqueou s so lu tion s con tain ing one R E elem en t and tw o w ere analysed by m ean s of CD 2841 M odel and ICP spectrograph, respectively. Results and D iscuss ion 1 C once ntra tions of Amm onium N itra te in Fe e d a nd Inte rna l A que ous P ha se Amm on ium n itrate u sed as the salting2ou t agen t in feed p lays an im po rtan t ro le in the p rocess of Sm 3+ and Gd 3+ being tran sferred from the feed in to the in2 ternal aqueou s phase. Bo th the tran spo rt rates of Sm 3+ and Gd 3+ w ou ld be very low if the concen tratin of amm on ium n itrate w as less than 2 m o lgl. T herefo re, a m oderate concen tration (4 m o lgl ) of amm on ium n itrate in the feed w as selected in the experim en tṡ In addition, the selection of the in ternal aqueou s phase is also im po rtan t in the liqu id m em b rane p rocess. A cco rding to the theo ry of so lven t ex traction, di2 lu ted n itric acid is the strip liquo r w hen N 263 ex traction system is u sed to sepa2 rate R E. Bu t, if dilu ted n itric acid is also u sed as the in ternal aqueou s phase (the strip liquo r) in liqu id m em b rane p rocesses, the p rob lem s w ou ld be that n itric acid no t on ly co rrodes the equ ipm en t bu t m akes the ph of the feed low er. Be2 cau se the com p lex ing reaction s of ED TA w ith Sm 3+ and Gd 3+ are affected by the ph, ED TA w ou ld no t p lay any ro le if the ph w as no t con tro lled in the feed. So dilu ted amm on ium n itrate w as u sed as the in ternal aqueou s phase in the experi2 m en ts in stead of n itric acid. It w as show n that the strip resu lts of Sm 3+ and Gd 3+ w ith amm on ium n itrate w ere sim ilar to tho se w ith n itric acid w hen their concen2 tration s w ere in the range of 0101 0150 m o lgl. T he tran spo rt rates of Sm 3+ and Gd 3+ decrease w ith the increase of the concen tration of amm on ium n itrate in the in ternal aqueou s phase. In the experim en ts, the concen tration of amm on ium 402
n itrate w as generally selected to be in the range of 0105 0110 m o lgl. It shou ld be po in ted ou t that the concen tration s of amm on ium n itrate in the feed and in ternal aqueou s phase affect on ly the tran spo rt rates of Sm 3+ and Gd 3+. 2 C once ntra tions of TB P a nd N263 T he concen tration s of the co2ex traction m ob ile carriers of N 263 and TBP af2 fect no t on ly the tran spo rt rates bu t also the separation coefficien t of Sm 3+ and Gd 3+. T he difference betw een the tran spo rt rates of Sm 3+ and Gd 3+ increases w ith the increase of the concen tration s of N 263 and TBP. T he selectivity of the liqu id m em b rane system fo r Sm 3+ and Gd 3+ is h igher as the concen tration s of N 263 and TBP are 8%, respectively. If the concen tration s of the carriers are h igher than 8%, som e p rob lem s, fo r exam p le, the b reak ing2dow n of the w ater2 in2o il em u lsion is difficu lt, w ill arise in liqu id m em b rane p rocesses becau se of the em u lsion having a greater visco sity. So the su itab le concen tration s of N 263 and TBP shou ld be 8%. T ab le 1 show s the effect of the concen tration s of N 263 and TBP on the tran spo rt rates of Sm 3+ and Gd 3+. T ab le 2 show s the effect of the co2operation carrier N 263 and TBP on the separation of Sm 3+ and Gd 3+. T ab le 1 T he effect of the concen tration s of N 263 and TBP on the tran spo rt rates of Sm 3+ and Gd 3+ 3 T he concentrations T he carriers: 8% N 263, TBP T he carriers: 8% TBP, N 263 (% ) w (Sm 3+ ) (% ) w (Gd 3+ ) (% ) w (Sm 3+ ) (% ) w (Gd 3+ ) (% ) 2 42. 0 26. 8 28. 9 17. 2 4 58. 8 34. 9 48. 2 27. 9 6 75. 4 45. 0 69. 4 38. 8 8 88. 5 54. 2 87. 6 51. 8 10 90. 1 56. 6 91. 0 54. 1 3 T he internal phase: 0105 mo lgl N H 4NO 3. T he feed: 01025% Sm 3+, 01025% Gd 3+, 4 mo lgl N H 4NO 3. T he o il phase: N 263, TBP, 1% LM A 21, kero sene. R ew = 1 8, R io= 1 1, 6 m in. T ab le 2 T he effect of N 263 and TBP on the separation of Sm 3+ and Gd 3+ 3 T im egm in 2. 0 4. 0 6. 0 8. 0 w (Sm 3+ ) (% ) 41. 4 68. 3 89. 0 93. 2 w (Gd 3+ ) (% ) 24. 0 45. 1 53. 1 61. 0 3 T he o il phase: 8% TBP, 8% N 263, 1% LM A 21, kero sene. T he internal phase: 0105 mo lgl N H 4NO 3. T he feed: 01025% Sm 3+, 01025% Gd 3+, 4 mo lgl N H 4NO 3. R o i= 1 1; R ew = 1 8. 3 C om p le x ing A ge nt ED TA a nd ph in Fe e d A s the com p lex ing agen t ED TA is added in the feed, the ph of the feed af2 fects in ten sely the separation of Sm 3+ and Gd 3+, o r the ph affects on ly the tran s2 po rt rates of Sm 3+ and Gd 3+ becau se the com p lex ing reaction s of ED TA w ith Sm 3+ and Gd 3+ are influenced by the ph of the feed, ED TA cou ld no t p lay any ro le if the ph w ere no t con tro lled w ith in the lim iṫ T he experim en ts show ed that 403
the ph shou ld be w ith in the lim it of 310 315. U nder the condition s, the liqu id m em b rane system has the h ighest selectivity (see T ab le 3). T ab le 3 T he effects of comp lex agen t ED TA and the ph of the feed on the separation of Sm and Gd 3 ph 1. 0 1. 5 2. 0 2. 5 3. 0 3. 5 w (Sm 3+ ) (% ) 86. 4 90. 1 85. 0 81. 5 72. 0 65. 0 w (Gd 3+ ) (% ) 47. 8 55. 0 41. 0 29. 2 15. 8 12. 7 3 T he feed: 213 10-3 mo lgl X, ph. T ransferring tim e: 8 m in. T he o thers just as in T able 2. 4 S e pa ra tion M e cha nism of P ush2pull L iquid M em b ra ne S ys tem A cco rding to the m athem atical m odel of liqu id m em b rane and the tran sfer2 ring m echan ism of R E 3+, the com p lex ing reaction of co2operation carriers w ith R E 3+ on the feed2o il in terface and the decom p lexation of the com p lex com pound on the o il2strip liquo r in terface are described by equation (1). comp lexing R E 3+ (a) + 3NO - 3 (a) + (R 3CH 3N + NO - 3 ) 2 (o) decomp lexation R E (NO 3) 3g 2R 3CH 3N + NO - 3 g TB P (o) (1) w here (a) exp resses the aqueou s phase and (o) the o il phase. T he separation co2 efficien t of Sm and Gd is exp ressed sim p ly by the equation (2) [4 ] in the ab sence of ED TA. ΒSm ggd K ex, Sm gk ex, Gd w here K ex, Sm and K ex, Gd are the equ ilib rium con stan ts of the com p lex ing reaction s of co2operation carrier w ith Sm 3+ and Gd 3+, respectively. Fo r K ex, Sm > K ex, Gd, ΒSm ggd> 1. W hen ED TA w as added in the feed, the com p lex equ ilib rium of ED 2 TA w ith Sm 3+ and Gd 3+ can be described by the fo llow ing equation s (the electric charges of ion s w ere om itted in the fo llow ing eqṡ ). Sm + ED TA Sm g ED TA (3) Gd + ED TA Gdg ED TA (4) Bo th equ ilib rium con stan ts are as fo llow ṡ K Sm = [Sm g ED TA ] [Sm ][ED TA ] (5) K Gd = [Gdg ED TA ] [Sm ][ED TA ] (6) Becau se of reaction s (3) and (4) ex isted in the feed, the distribu tion ratio of Sm 3+ in the o il(m em b rane) phase and feed is discribed by eq. (7). Α Sm = [Sm (NO 3 ) 3g 2N 263g TBP ] (o) [Sm ] (a) + [Sm g ED TA ] (a) T he sub stitu tion of eq. (5) in to eq. (7) gives [Sm (NO 3) 3g 2N 263g TB P ] (o) Α Sm = [Sm ] (a) g {1 + K Sm g [ED TA ] (a) } = ΑSm 1 + K Sm g [ED TA ] (a) w here ΑSm is the distribu tion ratio of Sm in the ab sence of ED TA. O n the accoun t of K Sm [ED TA ] (a) µ 1, eq. (8) is sim p lified as 404 (7) (8)
Fo r the sam e reason, Α Sm ΑSm K Sm g [ED TA ] (a) ΑGd Α Gd K Gdg [ED TA ] (a) In th is m om en t, the separation coefficien t of Sm and Gd is Β Sm ggd Α Sm = ΑSm g K Gd = ΒSm ggdg K Gd (11) Α Gd ΑGd K Sm K Sm Fo r K Gd> K Sm, Β Sm ggd> ΒSm ggd. So, w hen ED TA is added in the feed, the selectivity of the liqu id m em b rane system is raised fu rtheṙ O n the feed2o il in terface, Sm 3+ is first pu lled by carri2 ers from the feed in to the o il(m em b rane) phase and Gd 3+ is com p lexed firstly by ED TA in the feed. T herefo re, the liqu id m em b rane system is called "pu sh2pu ll" liqu id m em b rane system having a h igher selectivity. Re fe re nce s [ 1 ] ZHU J ian2h ua, ZHAN G X iu2juan, T he 1996 International Congress on M em branes and M em brane P rocesses, Yokoham a, Japan, 1996: P282125 [ 2 ] L IU X ing2rong, WAN G X iang2d e, WAN Y in2h ua, et al., T he 1996 International Congress on M em branes and M em brane P rocesses, Yokoham a, Japan, 1996: P282126 [ 3 ] WAN Y in2h ua, ZHAN G X iu2juan, M em branes Science and T echno logy, 12 (4), 17 (1992) [ 4 ] L IU X ing2rong, ZHAN G X iu2juan, R are M etals and H ard A lloy, (3), 1 (1993) (9) (10) 405