A t t h e o u t s e t it w a s t h e i n t e n t o f y o u r w r i t e r to r e v i e w all p h a s e s o f t h e s e p a r a t i o n c h e m i s t r y o f t h e l a n t h a n o n s in d e p t h , b u t u p o n d e l v i n g i n t o t h e l i t e r a t u r e it w a s f o u n d t h a t o t h e r s , i n c l u d i n g H u l e t e t al. (1972) a n d W e a v e r (1974), h a d r e c e n t l y r e v i e w e d s o l v e n t e x t r a c t i o n p r o c e s s e s in m u c h g r e a t e r d e t a i l t h a n w o u l d b e f e a s i b l e in t h i s c h a p t e r . A n y a t t e m p t to w a d e t h r o u g h t h e m o r a s s o f s o m e t i m e s f r a g m e n t a r y a n d g e n e r a l l y i n t e r t w i n e d w o r k s i n v o l v i n g s e p a r a t i o n s
SEPARATION CHEMISTRY 105 of lanthanons and actinide elements was judged redundant, if not futile, at this time. Consequently, it was decided to rely principally upon the diligence, expertise, insight and consensus of previous reviewers in discussing this subject.
Excluding consideration of a multitude of representatives of solvent ex- traction classes (water-insoluble alcohols, ethers, acids, esters, ketones and diketones) that have been examined and f o u n d to be inadequate in some respect or another, it is the intent here to f o c u s u p o n only a few representatives of just three classes of extractants: (1) neutral p h o s p h o r o u s agents; (2) monoacidic o r t h o p h o s p h a t e and p h o s p h o n a t e esters; and (3) primary, secondary, tertiary and q u a t e r n a r y ammonium ion species.
Separation of lanthanons by solvent extraction depends upon a preferential distribution of individual lanthanons (either in cationic form or as complex anions) b e t w e e n two immiscible liquid phases that are in contact with each other. One of the liquids is generally an aqueous solution, usually containing a mineral acid or an inorganic salting-out agent, and in some instances an organic acid or anion t h a t acts as a chelating agent. Ideally one hopes to achieve distributions of all c o m p o n e n t s that are i n d e p e n d e n t of their dilution and which can be r e p r e s e n t e d (under a given set of conditions) as distribution coefficient constants ( K ~ = Corg/Caq) which can be used to estimate practical separation factors. T h e separation factor (ratio of distribution coefficients) dictates how many stages will be required to accomplish the attainment of two p r o d u c t s of some specified purity from a binary mixture, or needed to partition a more complex mixture into two less complex mixtures (both of which contain less than specified amounts of c o m p o n e n t s of the other set). U n f o r t u n a t e l y , e x c e p t at very low concentrations, distribution coefficients and, hence, separation factors change markedly with composition and concentration in many systems. Thus, extrapolations (from the tracer-scale) applied to bulk separations are not always valid.
7.1. Neutral pOosphorus agents
Many representatives of t h e trialkyl o r t h o p h o s p h a t e ester, dialkyl alkyl (or aryl) p h o s p h o n a t e ester, alkyl dialkyl (or diaryl)-phosphinate, and trialkyl (or triaryl) phosphine oxide class have been investigated. Judging from the reviews available, none of the more exotic types appears to offer any particular ad- vantage o v e r tributyl o r t h o p h o s p h a t e as a selective e x t r a c t a n t for resolving lanthanide mixtures. Most are either viscous liquids or solids that require a diluent, and for the most part the individual separation factors to be had are unspectacular.
With T B P , from aqueous solutions greater than 8 M in HNO3, extraction into the organic phase increases in the order of increased atomic number, but separation of the Ln's b e y o n d Tb is rather difficult, With T B P , lanthanum, p r a s e o d y m i u m and n e o d y m i u m of rather high purity have been obtained by extracting from 13-14 M HNO3 in as few as 10--14 stages. Similar results have been obtained from nearly saturated rare earth nitrate solutions of low acidity.
106 J.E. POWELL
In 15 M HNO3, KdL, ranges f r o m 0.2 for L a to 450 for Lu. The extraction m e c h a n i s m a p p e a r s to be:
Ln(NO3)3 aq + 3TBP(H20)org,~ [Ln(NO3)a(TBP)3]org + 3H20.
A p p a r e n t l y the extraction into T B P f r o m HC104 and NaC104 solutions is of different c h a r a c t e r than e x t r a c t i o n s f r o m HNO3 and HC1. T h e s e p a r a t i o n f a c t o r s are lower, although extractability is higher, and the lighter e l e m e n t e x t r a c t s m o r e readily than the heavier o n e (Eu/Yb). T h e extraction m e c h a n i s m is p u r p o r t e d to involve [Ln(C 104)3(TB P)6]org.
E x t r a c t i o n of lanthanides f r o m t h i o c y a n a t e solutions by T B P has also been studied, but the highest ( Y b / L a ) separation f a c t o r o b s e r v e d w a s only a b o u t 10.
Thus, there a p p e a r s to be no practical application of a T B P - S C N s y s t e m to separation of lanthanons.
7.2. Monoacidic phosphate and phosphonate esters
The m o s t thoroughly studied and widely applied c o m p o u n d of this class is di-2-ethylhexyl o r t h o p h o s p h o r i c acid ( H D E H P ) . It is readily available and has low a q u e o u s solubility. It is quite viscous, h o w e v e r , p r e s u m a b l y b e c a u s e it exists as a h y d r o g e n - b o n d e d dimeric species, and it m u s t be e m p l o y e d with a diluent.
At low acidities the extraction m e c h a n i s m a p p e a r s to be a c a t i o n - e x c h a n g e process:
3 +
Lnaq + 3(HDEHP)2org ~ Ln[H(DEHPh]3org + 3H,+q
but at high acidities e x t r a c t i o n as a simple trisolvate a p p a r e n t l y occurs in addition to cation-exchange. D a t a on individual separation f a c t o r s o b s e r v e d by Pierce et ai. (1963) are listed in table 22.8.
Kolai'ik et al. (1966) studied e x t r a c t i o n of lanthanons by H D E H P , di-n-amyl p h o s p h o r i c acid ( H D A P ) , diisoamyl p h o s p h o r i c acid ( H D i A P ) , and di-n-octyl p h o s p h o r i c acid ( H D O P ) f r o m HNO3 and HCIO4. All s y s t e m s s h o w e d a 3rd- order d e p e n d e n c e on the extracting agent and a reciprocal 3rd-order d e p e n d e n c e on [H+]. H D E H P , the m o s t b r a n c h e d , w a s the least effective extractant, although the separation f a c t o r s are little affected by chain length and branching.
In the case of isomeric c o m p o u n d s , less sterically hindered di-n-octyl phos- TABLE 22.8
Separation factors for HDEHP-HCI and HDEHP-HCIO4, toluene-water systems at 25°C.
Pair HCI HCI04 Pair HCI HCIO4
La-Ce 2.4 3.0 Gd-Tb 3.2 5.0
Ce-Pr 2.8 2.1 Tb-Dy 2.0 2.1
Pr-Nd 1.7 1.4 Dy-Ho 2.1 1.9
Nd-Pm 2.1 2.2 Ho-Er 2.1 2.3
Pm-Sm 2.4 3.1 Er-Tm 2.5 2.5
Sm-Eu 2.2 1.9 Tm-Yb 1.8 3.1
Eu-Gd 1.6 1.4 Yb-Lu 2.2 1.9
SEPARATION CHEMISTRY 107 phoric acid extracts M 3+ ions 100 times as strongly as does di-2-ethylhexyl phosphoric acid and about 1000 times as strongly as di-2,2-dimethylhexyl phos- phoric acid.
While 3rd-order d e p e n d e n c e of Kd on the e x t r a c t a n t (consistent with forma- tion of Ln(HA2)3 species in the organic phase) is the rule with dialkyl phosphate esters in most solvents, there is considerable evidence that the organic phase species may be LnA(HA2)2 in some solvents and with aromatic diesters of phosphoric acid. By no means can the solvent used for dilution be considered inert. Extractions are definitely lower in those diluents which can interact with the extraction agent.
Replacing an alkoxy group attached to P by an alkyl or aryl group (which converts a phosphoric acid diester into a phosphonic acid m o n o e s t e r ) yields a considerably stronger e x t r a c t a n t for lanthanons, in general, but only slightly increases separation factors for neighboring lanthanons. In toluene, Kd'S with alkyl p h e n y l p h o s p h o n i c acid esters obey the same 3rd-power d e p e n d e n c e on e x t r a c t a n t concentration as with di-2-ethylhexyl phosphoric acid. An increase in t e m p e r a t u r e decreases Kd values as a general rule and has a small effect on separation factors. Separation factors for light Ln's increase, while those for h e a v y Ln's decrease.
A promising application of H D E H P extraction appears to lie in the partition- ing of actinides from lanthanides. T h e T A L S P E A K process (Trivalent Actinide Lanthanide Separation by P h o s p h o r u s Reagent E x t r a c t i o n f r o m Aqueous K o m p l e x e s ) involves extraction of Ln's with H D E H P f r o m an aqueous solution of a c o m p l e x (such as D T P A ) which has a high affinity for and represses extraction of the middle and heavier lanthanons "and actinide elements most of all". The separation f a c t o r b e t w e e n the An's, A m 3+ and C m 3÷, and the Ln's e x c e e d s 100.
H D E H P is also exploited in the commercial production of Eu from bast- naesite concentrates. By a few stages of extraction with H D E H P , f r o m dilute HCI solution, .it is possible to r e m o v e nearly all of the lighter lanthanons, leaving a mixture coritaining several p e r c e n t of Eu, which may then be r e c o v e r e d easily and in high purity by a reduction method.
H D E H P has also been utilized in one f a c e t of a dual extraction process for purifying yttrium.
7.3. Primary, secondary, tertiary and quaternary ammonium ions
In general, extraction of tervalent lanthanons from mineral acid solutions by long-chain amines is not highly favorable unless high concentrations of acids or salts are present in the aqueous phase. Separation factors for adjacent L n ' s are not especially attractive, but the extraction of An's is remarkable. Thus, tertiary amine extraction is the basis for the T R A M E X process for purification of Cm.
In nitrate media light Ln's e x t r a c t more readily than the heavier ones, but in chloride solutions Eu is the most extractable lanthanon.
Q u a t e r n a r y ammonium c o m p o u n d s with high molecular weights behave chemically as strong-base anion exchangers and require lower concentrations of
108 J.E. POWELL
s a l t i n g - o u t a g e n t s . A g a i n s e l e c t i v i t y f o r i n d i v i d u a l l a n t h a n o n s is l o w a n d a p - p a r e n t l y n o n - m o n o t o n i c in S C N . E u 3+ is t h e m o s t e x t r a c t a b l e L n f r o m S C N , b u t e x t r a c t s m u c h l e s s r e a d i l y t h a n A m 3÷ a n d C m 3+. I n n i t r a t e t h e l i g h t e r l a n t h a n o n s a r e m o r e e x t r a c t a b l e t h a n t h e h e a v i e r o n e s .
A p p a r e n t l y t h e f a v o r i t e t e r t i a r y a m i n e s a n d q u a t e r n a r y a l k y l a m m o n i u m s a l t s a r e t h o s e c o n t a i n i n g a m i x t u r e o f e i g h t - a n d t e n - c a r b o n c h a i n s . C o m m e r c i a l m a t e r i a l s in p o p u l a r u s a g e a r e A l a m i n e 336 a n d A l i q u a t 336, a n d t h e i r e q u i v a l e n t s A d o g e n 364 a n d 464.
Acknowledgement
T h i s w o r k w a s s u p p o r t e d b y t h e U S D e p a r t m e n t o f E n e r g y , D i v i s i o n o f B a s i c E n e r g y S c i e n c e s .
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