Modern techniques for separating the rare-earth elements

JOURNAL OF THE LESS-COMMON METALS

MODERN

TECHNIQUES

FOR SEPARATING

THE

411

RARE-EARTH

ELEMENTS

N. E. TOPP* National Chemical

Laboratory,

Teddington, Middlesex (Great Britain)

(Received

July Sth, 1964)

SInmIARY Fractional crystallisation techniques for purifying the rare-earth elements have been rendered obsolete by the development of chromatographic and counter-current solvent extraction techniques. Two of the older methods retain their importance. These are the sulphate group separation methods, and valency change methods for the isolation of cerium and europium. Four of the newer techniques are discussed. These are ion-exchange chromatography; solvent extraction; amalgam extraction, and ion-exchange membranes. INTRODUCTION

Separation problems

of the

rare-earth

in inorganic

by the fact

that

elements

chemistry.

rare-earth

was for many

The nature

minerals

years

of the problem

normally

contain

one of the

classical

may be stated

the complete

briefly

array

of the

lanthanon elements (atomic numbers 57-71, less promethium). Certain types of mineral contain more of one part of the rare-earth grouping than others, for example, monazite is rich in the light elements from La to Gd, while xenotime contains mainly the heavy elements from Tb to Lu. Minerals rich in the heavy earths frequently contain much yttrium (atomic number 39). In any rare-earth mineral the elements of even atomic number In

aqueous

important

solution, exceptions

and samarium, stability. This

subject

are always more abundant the

lanthanon

elements

to this are cerium,

europium,

tervalent.

The

which also forms the tetravalent

and ytterbium,

was last reviewed

than those of odd atomic number. are normally

which

can

by MARSH in 1947~.

form Since

divalent that

only

ceric ion, ions of low

time,

the new

separation techniques based on chromatography and counter-current extraction have been applied to this problem with notable success. In consequence, the older methods, which were based upon fractional crystallisation techniques, are now mainly of historic interest. Of the older methods, the sulphate separation technique, which gives a group separation, is still important, as also are methods based on the exploitation of valency change. Six methods will be discussed. Certain of these have general application,

while others are of limited

scope.

SULPHATE SEPARATION

Rare-earth sulphates form double sulphates with alkali metal sulphates, which may be salted out by the addition of an excess of alkali sulphate. Double sulphates of the * Present address: Warren Spring Laboratory, ton, Middlesex.

D.S.I.R.

Extraction

of Metals Group, Tedding-

N.

412

E. TOPP

light earths are less soluble than those of the heavy earths. The sulphates also have a negative temperature coefficient of solubility, those of the light earths being less soluble than the heavy earth sulphates at elevated temperatures. Both properties can be exploited for making a rapid light-earth-heavy-earth separation. In the first method, sodium sulphate is stirred into a cold solution of lanthanon sulphates until the double sulphates of the light earths are precipitated. The process may be controlled spectrophotometrically. The filtrate contains most of the heavy earths and yttrium. Alternatively, if rare-earth sulphate solutions are heated, lightearth sulphates are the first to crystallise, and may be separated off. These methods, although rapid, effect only crude separations. EXPLOITATIONOF VALENCYCHANGE Cerium Cerium is unique among the lanthanon elements in forming a stable tetravalent ion in aqueous solution. The stable form of the oxide is also tetravalent. These properties can both be exploited to give a ready separation from the other lanthanides, all of which have a very stable tervalent state. In solution, the tervalent lanthanons are resistant to hydrolysis while the ceric ion hydrolyses readily. The solubility product of the tervalent hydroxides varies from lanthanum (log K.Q = -19.0) to lutetium (log k’s0 = -23.7), while that of ceric hydroxide has been estimated at log Kso = -512~3. If lanthanon solutions are oxidised to the ceric state, for example with chlorine or bromate, basic ceric salts are precipitated4.5. Alternatively, if a mixture of tervalent hydroxides is precipitated, and the product dried, cerium is oxidised to the ceric state and does not dissolve when leached with dilute acids6.7. These methods are valuable, since cerium is a major constituent of the rare earths derived from monazite (usually about 50 i). Neither method gives a pure product, but 997; of the cerium can be eliminated in one operation. Europium This is the least abundant lanthanide element. It has a well-known divalent state, the redox potential of the Eu3+/Eus+ couple being 0.43 V. The europous ion is moderately stable in water, europous sulphate being sparingly soluble and isomorphous with barium sulphate899. Aqueous solutions of lanthanons at pH 3 are treated with zinc, and europium is reduced to the bivalent state. ZEUS++ Zn + zEuz+ + Zn2+ If the reduction is carried out in the presence of sulphate ions, europous sulphate is precipitated. In the treatment of low-grade materials, it is convenient to co-precipitate barium sulphate as a carrier. Recoveries of 80% have been obtained from materials containing only 0.1% EuzO31”. Europium is recovered from the mixed barium europous sulphate precipitate by leaching with an oxidising agent. Final purification is effected by salting out the dichloride with cold concentrated hydrochloric acid, and removing the zinc as a chloride complex on an anion-exchange resing.11. ION-EXCHANGECHROMATOGRAPHY This is now the most efficient

technique

available

for separating

the rare-earth

J. Less-CommonMetals, 7 (1964) 41 r-419

SEPARATING

RARE-EARTH

ELEMENTS

413

elements in a pure state, and is of general application. The development of preparative methods followed the successful application of elution techniques to the separation and identification of fission-product rare earthsr2.13. The affinities of rare-earth cations for ion-exchange materials are similar, and early attempts to obtain separations by passing solutions through columns of cation exchangers met with little success 14915. It was found, however, that the addition of complexing agents (e.g. citric acid) to rare earth solutions improved the separationsl6. Later work has shown that separation requires that the rare-earth cations should have differing association constants for the complexing ligandl7. In the presence of a complexing ligand, the distribution of lanthanon cations between a solution and a cation-exchange resin is the sum of two effects, the affinity of the cations for the resin and complexing agent respectively. Distribution of lanthanon and ammonium cations between the two phases is given by the equation: 3NHa+

+

Ln3+ +

--

Ln3+

+

3NH?+

(I)

and the relative affinity of the competing ions is defined as:

ma+

KvH,+ =

(NH4+)3 (Ln3+)

(2)

(NHa+)3 (Ln3+)

Complexing of metal ions by the reagent (e.g. ethylenediaminetetraacetic is controlled by the reaction: Ln3f

+

Y4-

+

acid, EDTA)

LnY-

(3)

which is defined by the association constant of the cation for the ligand K = c

WY-1

(4)

[Ln3+][Y4-]

Distribution of metal between the two phases is given by the distribution factor KD = MRIMs, where ME and MS refer to resin and solution phases respectively. When anionic complexes are formed, MR = Ln3f and MS e [LnY-1. that :

It follows

(5) where HKa is the product of the acidic dissociation constants of the complexing agent (EDTA). For a pair of rare-earth elements, the resin affinity terms are similar, hence the separation factor KS is simply

that is, the ratio of the association constants of the two ions for the ligand. Equation (6) predicts that the lanthanon element with the highest affinity for the complexing agent will tend to pass into the solution phase, and will therefore elute first from an ion-exchange column. Similarly, the elution of mixtures of tervalent ions will generally follow the sequence of affinity for the complexing agent. However, if metals of J. Less-Common

Metals,

7 (1964) 411-4’9

414

N. E. TOPP

differing valency are present, the expressions for the distribution factors (Ko) will differ in the terms contributed by the affinity of metal ions for the resin. In such cases, the elution sequence of a series of metals will depend on the eluant concentiation. The type of chromatography known as displacement development is applied in preparative work. Usually, two separate cation-exchange columns are employed. The first column is loaded with the mixture of rare-earth elements, while various loadings have been proposed for the second (development) column. After loading, the two columns are coupled together and eluted with a complexing agent solution. H+-form

development

columns

This is restricted to conditions in which the complexing agent is water-soluble. Citric acid was used in the first successful methodia. Coupled ion-exchange columns were eluted with dilute (0.005 M) solutions of tri-ammonium citrate. Anionic complexes are formed on elution:

Lna+ +

3NHa+

Ln3+ +

+

Ln3+ +

7.Cits- +

3NH4+

(7)

Ln(Cit)p

(8)

When the citrate complexes meet the development column, hydrogen ions are displaced from the resin. These break down the complex, and lanthanon cations were exchanged to the resin phase while citric acid formed the eluate. Ln(Cit)z 3- +

6H+

+

Ln3+

+

zHaCit

(9)

If the rare-earth ions have differing affinities for the citrate ligand, separation takes place in the lanthanon band thus : Lnl(Cit)&

+

Ln$+

+

Lng(Cit)+

+

Ln13+

(IO)

This effect is integrated by the movement of the band down the column, the separation factors and column length required to separate a pair of elements being interrelated. The method is effective for separating the light- and heavy-earth groups (La-Sm and Er-Lu). However, the citrate complexes are sparingly soluble in water, and the eluant concentration is near to this limit; the method is thus very tedious. Also, in the early stages of elution, complexes of the type Ln[Ln(Cit)z] are formed; although they supersaturate readily, they are very insoluble in water. The amino-carboxylic acid, j%hydroxyethylethylenediamine triacetic acid (HEEDTA), is water-soluble, and has association constants for the rare-earth elements which make it suitable for separating the La-Sm and Er-Lu groupsig. It may be used in place of citric acid, but when hydrogen ions are displaced from the development column, the acid forms a salt with the resin 19325. The salt of the amino-acid forms a band that moves down the development column in front of the rare-earth band. Although the acid is difficult to precipitate in the presence of electrolytes it tends to precipitate on ion-exchange columns at eluant concentrations above 0.025 M. J. Less-Common

Metals,

7 (1964)

411-419

SEPARATING

RARE-EARTH

ELEMENTS

415

development columns

Metal-loaded

This method

may be used with complexing

acid form. EDTA

is a convenient

reagent

agents

that are insoluble

in the free-

for this purpose as its association

constants

with the rare-earth cations vary in a regular manner with atomic number, there being an increase from lanthanum to lutetium”0. A variety of metals have been proposed”rs”3 for loading the development column, e.g. Fez+, Cu”+, Znz+. Copper appears to be the most commonly used metal. Careful control of the eluant composition is needed, and the tri-ammonium

salt is normally

employed.

This forms acidic complexes

on elution. I>n3+ +

3Cu2+ +

Under

these

conditions,

(NHd)zHY

z(NH&HY

--f 3NHd+

--f 6NHa+

an acidic lanthanon

+

+

Hl

(II)

zHCuo.a(CuY)

(12)

complex

is formed,

while the copper

complex contains some cationic metal. At higher eluant stoichiometry, lanthanon or metal cation-anion salts such as Ln(LnY)3 or Cu(CuY) are formed, which tend to hydrolyse

or precipitate on the columns, while at lower stoichiometry, EDTA acid is precipitated on the lanthanon column 24. This disadvantage restricts the use of the method, and on a large scale a relatively low (0.015 M) eluant concentration is recommended25. If the method

is modified

by making

ammonium

ion to both rare-earth-

lanthanon-

or metal-anionic

by ligand exchange

a controlled

and metal-loaded

complexes

addition

of the non-complexing

columns,

ammonium

salts of the

are formed on elution.

Separation

takes place

on the development

column,

and the difficulties

associated

with

high metal-ion concentrations are overcome; the eluant concentration may be as high as 0.05 M2G. For effecting a rapid preliminary group separation of the rare-earth elements,

Ln3+-NHJ+

successfully

columns

at an eluant

with

concentration

no development

column

have

been

used

of 0.1 MzS.

SOLVEiXT EXTRACTIO?:

This method

may be used for separating

all of the rare-earth

seems to have been used on a macroscopic elements.

Separation

factors

between

elements,

although

it

scale only with the light- and middle-earth

adjacent

tervalent

lanthanons

vary from 1.5 to

2.5, depending upon the system. Solvent-extraction systems require one unit (e.g. mixer-settler) per theoretical stage and do not have the integrating effect of a chromatographic column ; therefore a large number of stages are essential. The separation factors between tetravalent and bivalent lanthanons, and the tervalent ions are much larger, and this has been applied successfully to the purification of cerium and europium. Solvent extractants of current alkylated

phosphoric

interest

are the

tri-alkyl

phosphates

and

the

acids.

Tervalent lanthanons Three systems (T.B.P.)-neutral

have been studied in some detail. These are tri-n-butyl phosphate lanthanon nitrates; T.B.P.-lanthanon nitrate-nitric acid; dialkyl

phosphate-lanthanon

nitrate-nitric

acid.

416

N.

E.

TOPP

Neutral lanthanon nitrates are extracted from aqueous solution by T.B.P. to form tri-solvates of the structure Ln(N0&.3T.B.P. 28. If concentrated lanthanon nitrate solutions are used, or if salting-in agents such as calcium or aluminium nitrates are added to the lanthanon nitrate solution, the organic phase approaches the limiting composition of the tri-solvate. The partition coefficients of the individual lanthanons reach a maximum value at saturation of the organic phase. Under these conditions, these is a steady increase in partition coefficient from lanthanum to holmium, and then a decrease to lutetium 29,aO.The separation factors for the La-Sm elements approach a limiting value of from 1.3 to 2.0, and the separation of these elements is thus feasible if sufficient stages are used. In the presence of nitric acid the system becomes more complex. The solvent forms a compound T.B.P.,HN03 with nitric acid, and, if the nitric acid concentration is greater than 7N, more than the stoichiometric quantity of nitric acid is found in the organic phases’. These compounds compete with lanthanon nitrate for the solvent. The partition of lanthanons between the aqueous and organic phases is thus dependent on the concentrations of the aqueous lanthanon salt and nitric acid. Below acid concentrations of 4N, partition coefficient-atomic number plots showed an inversion at the mid-point of the series. However, at very high acid concentrations (IAN-ISN), a linear relationship was approached with separation factors for adjacent lanthanon elements of 2.0-2.5~~~~~. The position of yttrium in the series depended upon the concentration of nitric acid in the aqueous phase. This method has been used to obtain gadolinium concentrates 34. The initial material was dissolved in 13N nitric acid, and, after counter-current extraction with solvent, the GdaOa content increased from 25% to > 95%. The dialkyl phosphoric acids are normally used in solution in an inert diluent such as kerosine. As a class, these reagents are frequently referred to as liquid cationexchangers,, since metal ions in solution exchange with ions in the organic phase. Partition coefficients of lanthanons vary with the cube of the organic phase phosphate ester concentration, and inversely with the cube of the aqueous phase hydrogen ion concentration. For trace quantities of lanthanons, the variation of partition coefficient with atomic number was linear and the average separation factor between the individual lanthanon elements was 2.535. For separations which require a large loading of the organic phase, such as the separation of large quantities of lanthanons, dialkyl phosphates have the disadvantage of forming hydrogen-bonded dimers in organic solvents; if more than one-sixth of the exchangeable groups are exchanged for multivalent cations, gelation takes place36. Empirical solutions to this difficulty have sometimes been found by adding phase modifiers which are usually long-chain aliphatic alcohols. Extraction of cerium

WARF showed that both nitromethane and T.B.P. were satisfactory for the extraction of the ceric ion from aqueous solution (B-ION nitric acid) which also contained a bromate holding oxidant 37. The ceric ion was readily back-extracted into an aqueous phase containing a reducing agent. An essentially similar method has been used for the preparation of radiochemically pure ceriuma*. Dioctylphosphoric acid has been used also for the purification of cerium3Q. When J. Less-Common Metals, 7 (1964)

411-419

SEPARATING

RARE-EARTH

ELEMENTS

ceric ions were extracted from ION nitric acid solutions thanons, the separation factors from tervalent lanthanons

417 containing tervalent lanwere greater than 106.

The extraction of divalent europium A comparison of the extraction of tervalent lanthanons and alkaline earths by acidic phosphoric and phosphonic esters showed the tervalent elements to transfer more readily to the organic phase. This suggested that a separation of divalent europium from tervalent lanthanons would be feasible. Europium was reduced to the divalent state by divalent chromium, prepared in situ by reducing Cra+ with a zinc amalgam; this procedure also affected the removal of dissolved oxygen from the solution, which was essential with trace quantities of europium. Separation factors of from 105 to 106 were found for Lna+/Euz+ when the organic and aqueous phases were di(z-ethylhexyl) hydrogen phosphate and o.ogM HCl+ O.OIM Crs+ respectively. An aqueous solution of rare earths, containing o.I~~ of EuzOa, was extracted with di(z-ethylhexyl)phosphoric acid to produce a O.IM Lna+ solution in the organic phase. The organic phase was then extracted three times with an aqueous solution which contained 0.5 g/l Zn-Hg and was o.ogM and O.OIM with respect to HCl and Cr2+. The recovery of europium from the aqueous phase was 90%, and separation factors from samarium and gadolinium were both greater than 105 (ref. 40). AMALGAM

EXTRACTION

The extraction of samarium and ytterbium with sodium amalgam from aqueous solutions of lanthanon concentrations was introduced some years ago by MARsH41. Since the original method was difficult to control, it has been re-examined as a batch-extraction technique. Buffered lanthanon solutions were used to control the reaction of sodium with hydrogen ions. Extraction depended on two factors, the samarium or ytterbium content of the concentrate, and the pH of the solution. Ytterbium alone was extracted from heavy-earth concentrates. Samarium was extracted from light-earth concentrates with marginal quantities of light earths, but if europium was present it was extracted at the original Eu : Sm ratio42. Europium and samarium amalgams were stripped quantitatively when run through a column of hot acid (So”C, 4N HCl or ION HOAc). By counter-flowing lanthanon solution and sodium amalgam in one column and treating the lanthanon amalgam similarly with hot acid, in a second column, a continuous extraction process was operated. From 80% to 9096 of the samarium or ytterbium in a lanthanon concentrate was recovered in one operation and the efficiencies of batch and continuous operation were similarda. Seven of the lanthanon elements are extracted from aqueous solutions by alkali metal amalgams, the La-hTd group, and Sm, Eu and Yb. The last three elements appear to be reduced to the divalent state before reduction to the metal. It has been suggested that the extraction process is the sum of two opposed effects, electrochemical and hydrolytic. Samarium, europium, and ytterbium are probably extracted preferentially from lanthanon concentrates for kinetic reasons**. ION-EXCHANGE

Some preliminary

experiments

MEMBRANES

have been described

by BRILQ. The principle

J. Less-Common

of the

Metals, 7 (1964) 411-419

418

N.E.TOPP

method is to apply electromigration to partially complexed rare-earth solutions. A five-compartment cell was described, the electrode compartments of which were separated by alternating cationic and anionic membranes. The rare earth-EDTA solution was fed to the centre compartment of the cell. Current was passed through the electrode compartments which were continually flushed with buffer solutions to prevent excessive pH changes, Cationic lanthanon ions tend to move from the centre compartment to the cathode compartment, and similarly, anionic lanthanon ions move to the anode compartment. It was necessary to add small quantities of buffer solution continually to the intermediate compartments of the cell to stabilise the pH of the solutions; this eliminated the risk of precipitating lanthanon hydroxides or EDTA acid. Experiments were described on the separation of La-Nd and La -Pr-Nd mixtures, partially complexed with EDTA. The separations obtained agreed with the values calculated from the different association constants of these ions for EDTA. A Nd-Th separation was also examined. This was a very effective separation since there is a large difference in the association constants with EDTA (IO?), and the thorium-EDTA complex has no charge. No experiments with multi-compartment cells have been reported. CONCLUSIONS

The lanthanon chemist has a far wider array of separation techniques available than were at his disposal ten years ago. The choice of technique will be decided by a number of factors; for example, will the main product be one lanthanon, or are a number of the elements to be isolated? Other important considerations are the purity of the product, and the scale of operation. The first two techniques described seem unlikely to be supplanted. The classical methods for the removal of cerium is effective and the product is readily purified by solvent extraction, Similarly, the recovery of europium as europous sulphate is efficient, although if feasible, a prior concentration of the element is desirable. Ion-exchange is the best general-purpose method for preparing high-purity lanthanons, given a suitable choice of technique and complexing agent. It is a batch process, but if it is necessary to isolate one of the less abundant elements, the columns can easily be used for storage purposes. It is more difficult to assess the potential of solvent extraction. While the method can be used for separating the light-earth elements a very large number of stages would be needed to approach the purity attained by ion-exchange. These systems also offer scope for making rapid yttrium-heavy-earth separations which have not been exploited. However, until the technical problem of obtaining more theoretical stages per unit volume of plant is solved, the technique seems more useful as an adjunct to ion-exchange for the production of pure lanthanons. Continuous amalgam extraction is probably the best method for preparing pure ytterbium and samarium-europium concentrates, C,oncentrates of these elements are readily obtained by ion-exchange methods. The feasibility of using ion-exchange membranes to separate the lanthanons has been demonstrated. The method is of interest, since it offers the possibility of a continuous process, but more experimental work is needed before a true assessment can be made. J. Less-Conz4nonMetals,

7 (‘964) 411-419

SEPARATING

RARE-EARTH

ELEMENTS

419

ACKNOWLEDGEMENT

The author thanks mission to publish.

the Director

of the National

Chemical

Laboratory

for his per-

REFERENCES 1 J, I<. MARSH, Chem. Rev., 1 (1947) 126. 2 Sfability Constants, Part 2 : Inovganic Ligands, The Chemical Societ)r, London, 1957. 3 T’. M. TARAYAN AND I,.Ai.ELIAZY~N, Izv. Akad. hyaztk .4rm. SSR, Ser. Khim. Nauk, IO (1957) 189. 4 D. 1%‘.PEARCE AXD J. C. BUTLER, Ixorg.Syn., z (1946) 48. 5 R. C. VICKERY, ./.Sot.Chem. Ind. (London), 67 (1948) 333; 69 (1950) 122. 6 A. R. POWELL, Bvit. Patent 5r0, 198 (1938). 7 E. S. PILKIP~GTON AND A. \n;-. WYLIE,,]. Sot.Chem. Ind. (London), 66 (1947) 387. * H. N. McCoy,J. ,4m. Chem. Sot., 58 (1936) 1577. 9 H. N. McCou,J. Am. Chem. SW, 59 (1937) 1131. 10 H. M. MCCOY, J. Am. Chem. Sot., 58 (1936) 2279. 11 K. A. KRAUS AND G. E. MOORE, J. Am. Chem. Sot., 75 (I9j3) 1460. 12 B.H. KETELLE AND G.E.BoYD,,I. Am. Chem.Soc.,6g (1947) 2800. 13 D. H. HARRIS AND E. R. TOMPKINS, J. Am.Chem.Soc., 69 (1947) 2792. 14 R. G. RUSSELL AND D. W. PEARCE, J. Am. Chem. SOL., 65 (1943) 595. 15 B. .4. LISTER AND hi. L. SMITH, J. Chem. Sot., (1948) 1272. 16 D. ERAMETSA, A. SAHAMA AND \l.KANULA, Ann. Acad.Sci. Fennicae, d4j7(1941)5. 17 E. R. TOMPKINS A~TD S. W. MAYER, J. Am.Chem.Soc., 69 (1947) 2959. 18 F. H. SPEDDING, E. I. FULMER, 1. E. POWELL, T. A. BUTLER AND I. S. YAFFE, J. Am. Chem. SOC., 73 (19jI) 4840. 19 F. H. SPEDDING, J. E. POWELL AND E. J. WHEELWRIGHT, J. Am. Chem. Sot., 78 (1956) 34. 9o <;. SCHWARZENBACH, R. GUT .~ND G. ANDREGG, H&J. Chim.Acta,37 (1954)937. F. H. SPEDDING, J. E. POU'ELL AND E. J. WHEELWRIGHT, J. Am. Chem. Sot., 76 (1954) 612. F. H. SPEDDING, J. E. POWELL AND E. J. WHEELWRIGHT, J. Am.Chem. Sot., 76 (1954) 25.57. P. KRUMHOLZ, K. BRIL, S. BRIL et al., Second U.3’. Conf. Atomic Energy, 1958, Ref. 15/P/2491. J. K. MARSH, ,I.Chrm. Ser., 978 (19.57). F. H. SPBDDINC XND J. E. ~‘OWHLL, Chem. I?lzg. I’rogv., .Symp. Ser., jj (24) (1959) 101 t;. H. HOWIE AND N. E. TOPP, unpublished work. N. E. TOPP AND D. 1~. YOUNG, Brit. Patent 880, 561 (1961). E. HESFORD, E. E. JACKSON AI~D H. h. C. MCKAY, f. Inovg. Nucl. Chem., 9 (1959) 279. J. BOCHINSKI,M.SMUTZ AND F.H.SPEDDING, Ind.Eng. Chem.,50 (1958) 1 j7. F. 1’. ROBINSON AND N. E. TOPP, J. Inovg. Nucl. Chem., 26 (1964) 473. I). F. PEPPARD, G. W. MASOIC AND J. L. MAIER, J. Inorg. Fuel. Chem., 3 (1956) 215. 1). SCARGILL, K. ALCOCK, J. M. FLETCHER, E. HESFORD AXD H. A. C. i\lcKAu,J. Inorg. Nucl.

Chem., 4 (19.57) 304. D. I;. PEPPARD, \V. J. DRISCOLL, R. J, SIRONEN AND S. MCCARTHY,

J. Inorg.

Nucl.

Chem.,

4 (‘957) 326. 13. WUVER, F. A. K.~PPELMANN

AND A. c. TOPP, J. Am. Chem. Sot., 75 (x953) 3943. D. I:. PEPPARD, G. W. MASON, J.L.MAIERAND W. J. DRISCOL, J. Inorg. Nucl. Chem., 4 (1957) 334. I). I? I'EPPARD, J. R. FERRARO AND G. W.MASON,J. tnorg. J. c. \\.aRF,J. Am. Ckem. Sot., 71 (1949) 32.j7.

sucl.

Chem.,4

(1957) 371.

t-l. W. KIRBY, Anal. Chem., 29 (1957) Ijgg. 1).1;.PEPPARD, G. W. MASON AND S. W. MOLINE, J. Inorg. Nucl. Chem., 5 (1957) 141. I).F. I'EPPARD, E. P.HORWITZ AND G. W.XASON,J.Inovg.Nucl. Chem.,24 (1962) 429. J. I<. ~T.%RsH,J. Chem. Sot., (1942) 298, _jZj;ibid., (1943) 8, 531. 11. 1'.B.%RR~TT, D. SWEASEY AND N. E. TOPP, J. Inovg. Nucl. Chem., 24 (1962) j7I. fi1.F.BARRETT AND N.E.ToPP, J. Appl.Chem., 13 (1963) 7. \I. I’. BARRETT, D. SWE~SEY AND N. E. TOPP, J. Inorg. Nucl. Chem., 25 (1963) 1273. Ii.BRIL, S. BRIL AND I-'. KRUMHOLZ,,]. Phys,Chem.,63 (1959) 256. J, Less-Common

Metals,

7 (1964)

411-419