Liquid-liquid extraction of metal salts by amine salts

LIQUID-LIQUID EXTRACTION OF METAL

SAL TS BY AMINE SAL TS

PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE

TECHNISCHE HOGESCHOOL TE DELFT OP GEZAG VAN DE RECTOR MAGNIFICUS DR. R. KRONIG. HOOGLERAAR IN DE AFDELING DER TECHNISCHE NATUURKUNDE. VOOR EEN COMMISSIE UIT DE SENAAT TE. VERDEDIGEN OP WOENSDAG 25 OKTOBER 1961 DES NAMIDDAGS TE 4 UUR

DOOR

KLAAS VAN IPENBURG

CHEMISCH DOCTORANDUS GEBOREN TE SCHOONHOVEN .1 !:

BIBLIOTHEEK

·

D

E

R

:

TECHNISCHE HOGESCHOOL

D'~LFT

List of symbols and abbreviations 3

Chapter I. Introduction 7

Chapter 11. General survey of the SX of metal salts 9

11. A. Classification of the extraction systems 9 11. B. The relations between SX, ion exchange and

adsorption 11

11. C. Synopsis of published theories on SX 11 Chapter 111. Derivation of formulae for D o/a in the case of metal

salt extractions fr om acidic solutions, using amine

salts 19

m.A. D o/a for the case where a number of

restric-tive conditions are fulfilled 20

IIT. B. Associations in the organic phase 25 111. C. Simultaneous extraction of two or morE: metal

salts with the same anion 29

111. D. Extraction of metal salts, if more than one

extractable complex is formed for each metal 30 IIT. E. Formation of non-extra"ctable side complexes

in the aqueous phase 33

m. F. Two or more anions are present, with which

extractable complexes may be formed 34

m. G. Possible further extension of the theory: more than one active solvent; synergistic effect;

in-ertia of the diluent 38

Chapter IV. Application of the formulae for D o/a 40

IV. A. The influence of the constant C .for given

ex-perimental conditions 41

IV. B. The SX of metal salts when association occurs

in the organic phase 46

IV .C. Simultaneous extraction of metal salts and the

effect of solvent saturation 49

IV.D. Comparison of theory and some results from

the literature 52

Chapter V. Solvent extraction experiments 55

V. A. Preliminary investigations 55

V. B. The SX behaviour of ZnC1_2, CuC1₂ and other

metal chlorides with D.E.H.A.-salts 59

V.D. SX of FeCla V.E. SX of ThCl4

V. F. Simultaneous extraction of two metal salts

66 66 67 Chapter VI. The systems D.E.H.A.-xylene and T .I.O.A.-xylene 68 VI.A. D.E.H.A.-xylene and T.I.O.A.-xylene only 68 VI. B. D.E.H.A.-xylene and T .I.O.A.-xylene in

con-tact with aqueous solutions of hydrochloric

acid 81

Chapter VII. Determination of activity coefficients 90 VII. A. Measurement of the water activity [HP]w 90 VII. B. Measurement of' the HCI activity 95 VII. C. The activity coefficients of some metal salts

in 1.00 N HCI solution 100

ChapterVIlI. Comparison of theory and experimental data 101

Tables 111

Appendix I. Analytical procedures 128

Appendix Il. Calculation of phase settling times 136

Appendix III. Calculation of theoretical SX curves 140

Summary 142

Samenvatting 144

D.D.A. = di-n-decyl amine. D.E.H.A. = di-(2-ethylhexyl) amine.

D.E.H.A. -HCI = di-(2-ethylhexyl) ammoniumchloride.

Distribution coefficient (Kv)

=

ratio of the concentrations of a specific solute in the phases.

Distribution ratio (D o/a)

=

stoichiometrie ratio of the concentrations includ-ing all species of the component in the respective phases. Dithizone

=

diphenylthiocarbazone.

E.n.T.A. = ethylenediaminetetraacetic acid.

Extraction percentage (%E) '" the metal salt entracted from the aqueous phase H.D.B.P. H.D.P.A. IX M.I.B.K. oxine P.A.N. SX T.B.P. Thoron T.I.O.A. T. I. O.A. -HCI T.O.A. T.T.A.

in per cent of the amount originally present. = dibutyl phosphate. = heptadecylphosphoric acid. = ion exchange. = methylisobutylketone. = 8 -quinolinol. = 1-(2 -pyridylazo) -2 -naphtol.

= liquid-liquid extraction, or solvent extraction. = tri-n-butyl phosphate.

= 1-(o-arsonophenylazo)-2-naphtol-3, 6-disulphonic acid.

=

tri -(2 -ethylhexyl) amine.

= tri -(2 -ethylhexyl) ammoniumchloride. = tri-n-octyl amine.

= thenoyltrifluoroacetone.

B. Symbols used in the formulae

The formation and dissociation constants mentioned below are all real thermodynamic constants. The distribution ratios give the relationship be-tween concentrations in the organic and the aqueous phases. The numbers in parentheses are thoSe of the relevant equations in this thesis.

a

=

number of molecules of the amine salt, necessary for the formation of one molecule of the extractable complex (III,2).

a*

=

activity. N -

=

p-valent anion.

A'

=

hydrated anion AP- in the aqueous phase (1II,10).

u A

=

degree of dissociation of the associated amine salt in the organic phase.

b

=

number of molecules of the metal salt, necessary for the formation of one molecule of the extractable complex (m,2).

BV

-

=

v-valent anion, forming non-extractable side complexes.

B'

C

=

hydrated anion BV

- in the aqueous phase. The use of B dash indicates, that the corresponding symbol is reHtted to anion B' (m, 84).

= characteristic constant for each combination of metal salt-amine salt (m,34).

C* = rough value for C, calculated without taking into account any activity coefficient (V, 3).

CA = concentration of amine in mole/l (VI, 2). Cx

=

concentration of xylene in mOle/l (VI, 1).

Compl. = the extracted or extractable complex formed by the metal salt MpAm and the amine salt"RpA (m,3).

ComploE = the extractabie complex formed by the metal salt M,Em and the amine salt R,E (m,99).

ComploAE = the extractable complex, formed by the amine salt RpA and the metal salt M,Ern (m,102).

Compl.EA = the extractable complex, formed by the amine salt R,E and the metal salt MpAm (m,103).

(Compl. tot.) = total concentration of a complex, that may exist in monomer form as weU as associated (m,39), (m,113).

d A = path of light in cm through ceU with amine (VI,2). dx = path of light in cm through cell'with xylene (VI,l). DA = distribution ratio for the salt of the amine (m,26). De

=

distribution ratio for the extractable complex (III,28).

D o/a

=

distribution ratio for the metal salt under consideration

(m,

13). IJ

=

number of atoms BV- used in the formation of a non-extractable

side-complex above or below the amount necessary for the formation of a neutral complex (III, 83).

=

increase in the density of aqueous salt solutions per mole/!. of the dis-solved salt (V, 4).

IJE*

=

electromotive force caused by differences in chloride activity (VII, 4). IJN

=

difference between equations (VI, 16) and (VI, 14); (VI,17).

e

=

difference in the density of the aqueous and the organic phase after the SX(V,l).

= number of molecules of hydrate water for the non -extractable side-complex (III, 83).

e' = number of molecules of hydrate water for the anion B V

- (m,84). E'-

=

r-valent anion.

E'

=

hydrated anion Er- in the aqueous phase. The use of E dash indicates, that the corresponding symbol is related to the anion E'.

E*

=

electromotive force in abs. Volts (VII,2).

E~ = standard potentialof the silver-silver chloride electrode in abs. Volts (VII,2).

=

absorbance of amine (VI, 4).

=

apparent absorbance of amine as compared to a blank of xylene (VI,5). = absorbance of xylene.

=

molar absorbance index for amine (VI,2).

=

molar absorbance index for xylene (VI,l).

5 fA

=

activity coefficient of the extracting amine salt (lIl, 24).

fA* = activity coefficient of the associated extracting amine salt (1II,52). fc '" activity coefficient of the extractabie complex (lIl, 15),

fe"

=

activity coefficient of the associated extractable complex (1II,38). f+ = activity coefficient of the hydrated metal ion (lIl, 19).

f- = activity coefficient of the hydrated anion (1II,20). f± = mean activity coefficient (1II,21).

F*

=

coulombs per equivalent (VII,2).

h = height of liquid level in the apparatus, used for equilibration of the phases (App. TI, 5).

IA

=

intensity of light after passing the cell with amine (VI, 2).

Ia

=

original intensity of light (VI, 1).

Ix

=

intensity of light af ter passing the cell with xylene -(VI, 1).

Kl

=

formation constant of the extractable complex in the aqueous phase (lIl, 3).

Kz

=

dissociation constant of the metal salt in water (lIl, 7).

K3

=

dissociation constant of the associated metal salt -amine salt complex in the organic phase (lIl, 37).

K4

=

dissociation constant of the associated amine salt in the organic phase (1II,51).

K5 = dissociation constant of the non-extractable side-complex (1II,86). K6

=

dissociation constant of the amine salt RpA in the aqueous phase (1II,92). K7

=

dissociation constant of the amine salt RrE in the aqueous phase (1II,93). Ka

=

equilibrium constant for the replacement of the acid HpA from its

amine salt by the acid Hr E (ITI, 95). KD' Kb, KB

=

distribution coefficients (11,1,2,5). KF = driving force on a spherical particle (App. TI, 2). Ks

=

frictional resistance on a spherical particle (App,lI, 1). Mm_{+ = m-valent metal ion.}

M'

=

hydrated metal ion Mm_{+ (111,9).}

MA

=

mole fraction of amine in the mixture (VI,l1). lVIx

=

mole fraction of xylene in the mixture (VI,12).

E(M) = total concentration of the met al in the phase under consideration. n

=

number of metals present in the system at the same time.

N = a given property of the solution (= mixture of amine and xylene), e.g. density, absorbance, molar volume, etc. (VI, 6).

NI

=

number of moles HzO in a given amount of solution (e.g. one 1) (VII,5). N

_z

=

number of moles Hel in the same amount of solution as NI (VII,5). N 3

=

number of moles metal salt in the same amount of solution as NI

(V1I,5).

NA

=

the same property as N for the pure amine (VI,7). N x

=

the same peoperty as N for pure xylene (VI, 8). o

=

general indication for the Qrganic phase. p

=

valence of the complexforming anion AP- .

pH

=

negative logarithm of the hydrogen ion concentration,

q

=

number of molecules of hydrate water of the extractable complex (1II,2).

q' = number of molecules of hydrate water of M' (1II,9). q"

=

number of molecules of hydrate water of A' (111,10). Q

=

a constant (combination of several terms) (111,59).

Qs

=

constant necessary to calcufate phase settling times (V, 1). r

=

valenee of the anion E ,-•

rs = radius of the spherical particle (App. 1I, 1). R*

=

the gas constant (V1I,2).

Rl' R2 and R3

=

hydrocarbon chain or H (lII,l).

RpA

=

salt of the amine under consideration and acid HpA (III, 1). R,E

=

as RpA, for the acid H,E (III,91).

(R tot.)o

=

total concentration of the amine in the organic phase before the SX

T*

T

s v

w

x y z ~ ~o (

..

)

[

..

]

experiment (III,23).

=

absolute temperature (Kelvin scale) (VlI,2).

=

phase settling time (V, 1).

=

valence of the anion B v-.

= velocity of a spherical particle (App.lI, 1).

= volume fraction of amine in the mixture (VI,9).

=

volume fraction of xylene in the mixture (VI, 10).

=

general indication for the aqueous (:Ylater) phase.

=

the xth metal salt in a mixture of n metal salts. Used as an subscript it means, that for the symbol the value must be substituted, corre-sponding to the xth metal salt.

= number of complex molecules, associating to a conglomerate (IlI,36).

=

number of molecules of the amine salt, associating to a conglomerate

(III, 50).

=

the hydrated non-extractable side-complex (III, 85).

= viscosity (App. 1I, 1).

=

specific viscosity (ratio of the viscosities of the solution under con-sideration and pure water at the same temperature (App. 1I, A).

=

partial free energy (lI, 3).

=

chemical potential (standard state) (lI, 4).

=

symbol for concentrations.

=

symbol for activities. = symbol for a definition.

= summation operator.

=

summation operator for all the met als M up to and including M .

=

summation operator for all the complexes , formed by one metal.

=

summation operator for all metals and for aU the complexes , formed

byeach metal.

" ", "', etc. = accents, indicating the first, second, third, etc. complex of the same metal salt.

Chapter ·1 INTRODUCTION

Liquid-liquid extraction, defined as the removal of a sub stance from a liquid by a second liquid, not - or at least not completely - miscible with the first one, is frequently used in preparative organic chemistry as weIl as in analytical chemistry(l). It is a long known fact, that it is possible. to extract metal salts from an aqueous solution by organic solvents. In 1842, e. g. , Péligot(2) already applied the extractability of uranyl nitrate by diethyl ether. The high interest in the liquid-liquid extraction (also called solvent-extraction and abbreviated generally as SX) of metal salts ca me in the twentieth century, particularly after the second world war, as the SX proved to be an excellent technique in the field of nuclear energy to heip in the recovery of uranium and thorium from the leach liquors of their ores and in the purification of the fuel of nuclear reactors from fission products. As a consequence of this the amount. of literature, including the many papers of both the Geneva Conferences on the Peaceful Uses of Atomic Energy, held in 1955 and 1958, and the reports made under contract with thè United States Atomic Energy Commission (U.S.A. E. C. ), on this subject, published in the past 15 years, is tremendous.

Indeed the SX enjoys a favored position as compared to the other separa -tion techniques (precipita-tion, ion 3xchange, chromatography, etc.) for in the case of SXwe are only handling liquids. The advantages of the SX are nearly all connected to this fact(3-7): Ease; simplicity; speed, as the time of equili-bration is often very short, enabling a continuous proces with a countercurrent flow of the extractant; applicable both tQ trace and macro levels of metals; no difficulties of inclusions, as in the case of precipitation; little or no dilution of the aqueous phase, which may be of impo"rtance for the recovery of a by-product; and a wide scope of the SX method as a great many organic extrac-tants, diluents and chelating agents are known. Moreover , by a right choice of the conditions in the phase from which the metal salt must be extracted the selectivity for a specific metal salt can often be even further enhanced.

A few drawbacks of the method are: Losses of the (expensive) organic phase during each extraction cycle by dissolution in the aqueous phase, mechanical entrainment, deterioration by agressive chemicals as concentrated HF, HCI, H2S04, HN03, etc., and by radio activity as in the case of re conditioning of reactor fuel.

Most specific for the SX are, however, the costs of dissolution of the orga-nic extractants. Difficulties caused by the formation of stabie emulsions and the formation of (colloidal) precipitates, adhering preferentially to the bound-ary layer between the phases, or the formation of a third phase can mostly be prevented by simple additions, by some alterations to the apparatus, by changing over to another solvent or another volume ratio of the phases, etc. In the case of the SX of metal salts one of the phases is usually an aqueous one, from which the metal salt under consideration is extracted by a suitable

organic solvent, in practice often in several stages (see Treybal(8»). Next the organic phase is sometimes brought into contact with a second aqueous layer, which is, in gene ral , similar to the first one, except that it does not contain contaminating metal salts (impurities), giving in this way a further separation (= sc rubbing) . Finally the metal salt is separated again from the organic phase (= stripping). This may be done by precipitating it as the hydroxyde, or binding it to fluoride, carbonate, etc. Specially uranium salts are of ten stripped by contacting the organic phases with aqueous solutions of Na₂C0₃ or

(NH4hC0s-As organic extractants ethers, ketones (e. g. methylisobutylketone = M. I. B. K.) and esters like ethyl acetate were used most frequently at {irst. Next we had the organic phosphates with long alkylchains, of which T. B. P. (=tri-n-butyl phosphate) is the best known representative. Nowadays the re-search is going in the direction of the phosphinates, phosphine-oxides, etc. Another group of compounds, which received a strongly growing aUention in the past decade, contains one or more nitrogen atoms in the molecule. Parti-cularly the simpie, long-chain aliphatic amines proved to be excellent extrac-tants. A combination of two solvents sometimes leads to an unexpected in-crease in extraction. AUention was drawn to this so-called synergistic effect by Rosenbaum, c. S.(9) and by Blake, Jr. c. sPO).

In order to find useful extractants the old method of trial and error is mostly followed. Coleman c. sP) e. g., synthesized, several hundred organo-nitrogen compounds and tested their ability to extract uranium sulfate. A favorable extraction performance was almost entirely limited to the simple amines, i. e. compounds with a single amine group· and not involving other functional groups or heterocyclic structures. A good example of the trial and error method is also the recent publication of Boyd and Larson(ll) who studied the SX of heptavalent technetium with a large number of members of all the known classes of organic solvents.

The aim of this thesis now is to obtain more insight into the relative im-portance of the many variables governing the SX. So it must be possible via a general theoretical treatment to arrive at:

1. a beUer use of the possibilities of the SX for the recovery of valuable metals from their ore leach liquors and/or separation or purification of metal salts for both technical and analytical purposes;

2. to find facts· or rules, that can serve as a guidance for a more efficient and scientifically -founded choice of optimal extractants to supplement and if possible, to replace the trial. and error method, respectively to indicate the direction to develop new extractants.

As a contribution to this goal we derive in chapterIII, af ter giving a general survey of the SX in chapter II, some formulae for the distribution coefficient D o/a of a metal salt between an acidic aqueous phase and an organic phase containing an amine salt, taking into account a number of possible complica-tions. In chapter IV we consider the applications of these formulae and the results are compared with facts known from literature. In chapter· V up to and including VII the experimental work, necessary for the verification of the theory, is described. The results of theory and experiment are then finally compared in chapter VIII.

Chapter 11

GENERAL SURVEY OF THE SX OF METAL SALTS

IT. A Classification of the extraction systems

In general it is assumed, that formation of uncharged molecules is a pre-requisite for extraction into organic solvents with their normally low dielec-tric constants(e.g. 12,1) Metal salts are often st rong electrolytes with a high solubility in aqueous media (high dielectric constant!). Moreover, water has astrong tendency to solvate the ions. So for neq.rly all metal salt extractions some or all of the water molecules coordinated to the metal ions must be re,.. moved before it is possible to obtain a species that can be extracted into an organic solvent.

Morrison & Freiser(l) classify the extraction systems on the basis of the nature of the extractable species. in addition to the covalent chelate extrac-tion syste'm they distinguish three types of association extraction systems: a) The metal may be incorporated into a very large ion, containing many orga-nic groups, or it may associate with another ion which is of large size; b) The so-called oxonium extraction with esters, ethers and ketones; c) The extrac-tion of metal salts into colloidal aggregates or micelles, formed by the active component in the organic phase.· As possible examples they quote the

extrac-tion of uranium and molybdenum by high molecular weight amines, dissolved in kerosine.

Fletcher (la) combines all extractions with esters, ethers, ketones and also amines to oxonium + aqJ.monium extractions. Chelate extractions are a special case (with extra stability of the complexes by ring formation) of the general reaction of metal ions with weak organic aCids, according to Mn_{+ + n HX t::y} MXn + n H+. Thus dibutyl phosphate e. g. belongs in this classification to the same group as the chelating agents T. T. A. (= thenoyltrifluoroacetone),

oxine, etc., and not to the oxonium extractions like the other esters.

Branica and Bona(14) distinguish for the SX of UOz(NOa)z only two funda-mentally different classes of organic solvents: U02(NOa)2 is not extracted by the first type of solvents (alcohols, ethers, ketones) as long as the concen-tration of the nitrate ions is low.

The second type of solvents (e. g. T. B. P.) are good extractants for uranium salts and do not need the so-called "salting agents". Contrary to the extraction with ethers and ketones we have no co-extraction of water in the case of SX with T. B. P., and the distribution ratios for the SX of UOz (N0:0z with T. B. P. from HCI or HCI04 solutions are muchhigher than thosefor the SX withethers and ketones.

Considering the classifications mentioned above, we see that only the last authors pay attention to the difference in behaviour of ethers and ketones as compared to esters like T. B. P. For the SX of UOz(NOah with ethers, alco-hols and ketones several hydrates were found in the organic phase from the

uranyl nitrate c;lihydrate upwards(15). Moreover, a third nitrate group is easily taken up and the solutions then acquire the characteristic trinitrato-uranyl absorption spectrum. In T. B. P., on the other hand, uranyl nitrate is a neutral, unionized molecule, unhydrated, but solvated by a definite numbet of T. B. P. molecules and unable to 'take up a'third nitrate group(16). General-ly , the number of T. B. P. molecules in complexes with metal salts is onGeneral-ly governed by the valency of the central metal atom, e. g. we have {M(N<l.J)a . 3 T.B.P.}, {M(NOa)4. 2 T.B.P.l and {MO

_z

_(NOa)2. 2 T. B. P.j. The metal atom M is thus always attached to six groups. So in the case 'of T. B. P. sha'rply defined compounds are formed. The same is true for e. g. the com-plex of ZnCl₂ with the HCI salt of methyldioctylamine(17). For ethers and ketones we have a solvation, which only diminishes the hydration. When we have an excess of ether 'or ketone, then the hydration is further reduced, re -sulting in a higher solvate.

In our opinion, taking into account all the facts known so far, the classifi-cation of the SX can be best done in the foÜowing way:

Class I: The extraction of directly as such extractable non-polar inorganic and organic compounds. In as far as this deals with inorganic substances, only compounds as GeCI₄, OsCl4' etc. are involved.

Class 11: The extraction of complexes, which are formed under liberation of H+ ions. An example of this class is the extraction of metals by organic acids. If these complexes, moreover, are stabilized bya ring formation, then we caU them chelates. To this class 11 thus also belong the extractions by chelating agents as dithizone, T. T. A., etc.

Class 111. The extraction of not sharply defined hydrates and solvates, e. g. with alcohols, ethers, ketones, etc.

In the organic phase we find several hydrates and solvates, depending on, the relative concentrations of the compounds involved in the complex for-mation. For a good extraction behaviour we need in general a high concen-tration of a "salting-agent" and a low pH.

Class IV. The extraction of solvate complexes with definite formulae, e. g. with phosphates and amine salts. In addition, higher phosphates and amines often show polymerisation to a high degree in the organic phase(18,19). The compounds of class I dissolve into the organic phase, because they are related to them by their non-polar character ("like extracts like"). The ex-tractions of class 11 are strongly acid dependent, which can advantageously be used for the separation of compounds with different complexing constants. The formation of the extractabie species is often a very slow step for these ex-tractions, 'consequently under circumstances it lasts hours and longer to reach the SX equilibrium. For the classes 111 and IV, where solvations and complex formations with the active solvents play an important rOle, it may be expected that the right choice of the organic phase strongly influences the ex-traction. Gene rally , equilibrium is very fast (from a few seconds to some minutes, see e. g. the related figures in chapterV. The addition to the aqueous phase of an acid, that does not have the same anion and does not form inter-ferlng side-complexes, has only a minor influence on the extraction.

11 II. B The relations between SX) ion exchange and adsorption

Sometimes the relationship between SX and ion exchange is stressed by dividing the organic solvents into cation and anion exchangers(4). As cation exchangers the organophosphates are mentioned, whereas long-chain amines function as anion exchangers. Against this view, which may be very useful in itself, the following objections can be made: The extraction of non-polar substances (our class I) cannot be accounted for; in the case bf ion exchange (further abbreviated as IX) the ratio of the concentrations of the exchanging ions and not the concentrations themselves. determine the final equilibrium(20) . The addition of water, which does not alter this ratio, has no influence. For SX we can th en expect a certain shift (lawof Nernst).

This, of course, is valid only for concentrations which are neither extre-mely high nor low. Further, there are some less important irregularities, connected to the differences in the state of aggregation of the second phase (liquid for SX, solid for IX). .

On the other hand we have certainly some relations between SX, IX and adsorption. In all cases we have two-phase systems, in which one of the phases is usually an aqueous solution (This aqueous phase, however, is not aprere-quisite, as e. g. Larsen and Trovorrow (21) used for their separation of Zr-Hf by SX a combination of acetonitrile -isoamylether as the non -miscible liquids). Further SX, as well as IX and adsorption have as a common goal to pickup one or more components from the aqueous phase for recovery and/or for purification. If an adsorbent is brought into contact with an electrolyte for the very first time, then this electrolyte is adsorbed in the normal manner. Now an electrical double layer is formed and when the adsorbent comes into con-tact with an electrólyte solution for the second time, then we only get an ex-change in the outside of the double layer, in other words, the adsorbent be-haves as an ion exchanging resin.

The SX of classes I, lIl, and IV now can be conceived as the liquid analo-gons of the ordinary. adsorption. Once a metal salt is absorbed in the organic phase the displacement of this salt by a second one is comparable with IX. Class II of the SX immediately greatly resembles the cation exchange in this respect, as metal ions disappear under the formation of the extractabie com-plex and H+ ions are liberated. It is rather striking that, for the extractions of class II and contrary to the extractions of class I, III and IV, the time of equilibration is relatively long.

11. C Synopsis of published theories on SX

In the case of SX we are dealing with two essentially immiscible solvents and one or more solutes distributing between them. As for all phase distribu-tions the phase rule of Gibbs

where Pis the number of phases,

y

the variance or the number of degrees of

freedom, and C the number of components, must be obeyed. At constant tem-perature and pressure and for one solute the rule predicts a variance of unity

as P

=

2 and C

=

3. So if we choose the concentration of the solute in one phase, the solute concentration in the other phase is fixed. Berthelot & Jung-fleisch(22) demonstrated in 1872, that the ratio of the concentrations in the phases was a constant and consequently independent of the total amount of the solute. This ratio is defined as the distribution coefficient

(IT,l)

(A) denotes the corrcentration of the solute A, whereas 0 and w indicate the" organic respectively the aqueous phase. Thts equation for K_D is valid only when the solute has the same molecular weight in both phases and provided that concentrations may be set equal to or at least are proportional to activities. If the molecular weight of the solute is not the same in the aqueQus and the organic phase, as e. g., if we have dimerisation in the organic phase(20,p.98) , then the formula of the sub stance in water can be represented by A and in the organic phase by A_{2 •} According to the equation 2 A~ ~ we expect

(A)o/(A)w2

=

constant (again assuming the proportionality of concentrations and activities). The relation was derived by Nernst(23) in 1891 and hls gene ral formulation of the distribution law is:

K' _ (Af phase 1

" i ) - (A) phase 2 (IT,2)

The extractions of class I (chapter 11 A) are described weU by the formulae of Berthelot & Jungfleisch and Nernst, as the activity coefficients are of ten approximately constant. From a thermodynamic point of view equilibrium is attained at constant temperature and pressure, when the chemical potentials (partial free energies) of the solute in each phase are equal. Thus

or: so

~o = ~w

~oo + RT In

[A]

0

=

~: + RT In

[A]

w

~=

Iq;

= e -(~~ -~~ )/RT

N

(11,3) (IT,4)

(IT,5)

~~ and ~: are the chemical potentials (standard states) of the solute with activity

=

1 for the organic and the aqueous phase and

[A]

denotes the activity of the solute A. At a given constant temperature (~g

-

~:) = constant, if we choose different standard states for each phase and Kj) becomes a constant too. When we decide to use a single standard state for the whole system, then

(~~ -~:)

=

O. Substitution in (IT, 5) results in Kj)

=

1. Defining the activity coefficient f in the usual way by

[A] = f. (A) (IT,6)

and substitution in (IT, 5):

13

From (1I,7) it is apparent, that the activity coefficients fo en fw may strongly influence the ratio (A)o/(A)w . So it is deducted, that the simple rule of Berthe-lot & Jungfleisch (1I, 1) can only hold as long as ~ and fware constants (or at least their ratio must be constant).

Of much greater significanee in practice than the distribution coefficient is the overall or stoichiometrie distribution between the phases of the component of interest, so a more practical quantity to describe the extraction is intro-(jueed, called the distribution ratio Do/a. This is a stoichiometrie ratio, in-cluding all species (converted to gram atoms of the metal) of the same compo-nent in the respective phases.

D / _ Total concentration in the organic phase

o a - Total concentration in the aqueous phase (1I,8) Instead of the distribution ratio the results of experimental work in literature are often given as per cent extracted (%E) from the aqueous layer. This quan-tity is related to D o/a by

(1I,9) where V_{o and} V_{w represent the volume of organic and aqueous phase,} respec-tively. For equal volumes of the phases this simplifies to:

%

E _ 100.D o/a

o - Do/a + 1 (11,10)

From (1I,9) and (IT,10) it can be seen, that D o/a varies from 99 to infinity, for differences in the extraction from 99 to 100%.

The real discrepancy in % E for D o/a = 1000 and e. g. D o/a = 10.000 is very minute as in both cases the extraction is nearly complete. A very high value of D o/a offers the advantage, on the other hand, to enable us to choose a high ratio for V_w /V_o' i. e., we can strongly concentrate a solute from a big volume of an aqueous phase into a little bit of an organic phase, practically without losses of the solute(24).

If the substance A has the same molecular weight in both phases and gives no further complications, then the distribution ratio equals the distribution coefficient:

(IT,l1) The solute A may be involved,however,in reactions in each of the two phases, e. g. associations, (electrolytic) dissociations, interactions with other compo-nents or reagents, etc. For these interactions we can make corrections by

evaluating the effect they have on the concentration (activity) of thè distributing species. In this way the equation for D o/a grows more complicated, but this is not a principal change. Thus. to find the appropriate formula forD o/a for given experimental conditions the influence is taken into account of all sub-stances, present in the system and further possible effects on the activities

14

both in the aqueous and the organic phase. Via measurements of activity co-efficients (or assessing them, e. g. by taking f = 1 in 'very dilute solutions) we can change over to concentrations in one of the usual scales (g/l; "gmol/l; gmol/l000 g of solvent; mole fractions, etc.).

We then have a measure" for the éxtractions to be expected and, in the case of two or more extractabie components (impurities), also for the selectivity. With the aid of the formula derived we have the possibility too to calculate the influence on the extraction of changes in the experimental conditions (see e. g. chapters mand IV).

For extractions of metal salts where formation of the extractable species involves reactions of the metal in the aque.ous phase, equations for D o/a can be de rived , with which the extraction behaviour as a function of the experi-mental parameters is described. Extensive research is done on the extrac~ tions of class II (chelate extractions). A great advantage in the"desçription of

chelate extractions is (Irving~: reagents as weU as metal salt-complexes in

both phases can be considered as simpie, non-associating molecules; solvation is of no importance for the extraction and further the dissolved substances are uncharged, and their concentrations are in general so low, that the be-haviour of their solutions departs litlle from ideality. Kolthoff & SandeU(26) ,

basing themselves on the mass action and the partition laws, found a simple relation between Do/a, the pH and the concentration of the chelating agent.

Their experiments with dithizon and ZnC~ in 0.01 N HCI confirmed their

theory. Irving & Williams(27) elaborated this and they drew atlention to the

high importance of their so-caUed pH! (= pH, where the extraction for a given

concentration of free chelating agent amounts to 50%). They calculated

theo-retical plots for the

%

E as a function of the pH. The form of these plots is

dependent on the valency of the extracting metal and the place is characterized by the pIJl" Also the influence of too short contacting times of the phases (false "equilibria") and the rate of complex formation is treated by them. OostingC28.) deals with the fril.ction non-extracted metal salt for extraction of oxinates in chloroform and he applies his formulae to systems with different metal salts.

The extractions of class m (ethers, ketones, etc.) have been under inves-tigation since the discovery, that FeCIa can be extracted from con~entrated HCI solutions by diethyl ether (Rothe2~" As compared to the extractions of class II the theoretical explanation of the extraction behaviour is complicated by the formation of a number of hyd,rates and solvates, dissociationat low iron

concentrations(30) and association at high concentrations(31,32). McKay

c. SP6,33-38) extensively investigated the extraction of U02(NO~2 by several ethers, alcohols, M.l. B. K. and iso-amylacetate, theoreticaUy as weU as

experimentaUy. The ratio of O/C, the number of oxygen atoms (=0) and the

carbon atoms (=C) in the solvent molecule proved to be very important for the

ethers, as ethers with strongly different molecular weights and the same O/C ratios gave nearly the same extractions(5,34). Even aliphatic and cyclic ethers are then similar in their extraction behaviour(14).

Diamond(19), in order to verify the formulae derived by him, examined the

influence of the metal salt concentration on the partition equilibrium with Mo

(VI) in the range of Hr9 to 10-2 M for extractions by ethers, ketones and one

alcohol from aqueous layers, which contained one or more monobasic acids.

hydration in both phases are taken into account, are only valid in idealized sys-tems, for Diamond does not use activity coefficients. That is why his meas-urements were done .in the range of very low concentrations, where he found that polymer forms of Mo (VI) can exist up to an acid concentration of 6 M. The extractions with phosphates and amines (class IV) are technicaUy as weU as sCientificaUy of prime importance. One of the first phosphates, that gave good extraction results was tri-n-butyl phosphate (Warf4~. Peppard c. s. (41) studied the extraction of the trivalent rare earth metals from HCI or HN03 solutions by T. B. P. For each acid concentration they obtained a constant relationship for the distribution ratios of those lanthanides, which are neighbours in the Periodic system.

McKay c. S.(15.42.16) found that nitrates in T. B. P. only appear as neutral unionized molecules, unhydrated but always solvated by a certain number of molecules of T. B. P. (see chapter 11 A). For the extraction of HNOa in T. B. P. the formation of the complex (HN03 . T. B. P.) .was assuméd. This was con-firmed lately by the work of Collopy & Blum(43). The formation of only neutral molecules in the T. B. P. -ph~se was proved by McKay by measuring the ioni-sation of several metal nitrates. This ioniioni-sation appeared to be always very low and even in the highe st cases it amounted only to a few percent. Hyde(44) mentionsas the mechanism for the extraction of UCl.!(NOa~ in T. B. P., in

accordance with McKay c. s. :

(U02w~+ + 2(NOa,); + 2(T.B.P.~~[U02(N03h· _{2 T.B.P.l o (II,12)}

The equilibrium constant K for this reaction is given by K = 7.70.

In connection with the purification of U from nuclear reactors with T. B. P. , Burger & McClanahan(45) studied the resistance of T. B. P. in several solvents against radio activity. The most important product of the deterioration was

dibutylphosphate (=H. D. B. P.). This H. D~ B. P. gives higher distribution ratios than T. B. P., but the selectivity is less and the stripping is much more diffi-cult. According to Brown c. s.646) the extraction mechanism is:

[U02++l w + 2!(H.D.B.P.hlo~ fU02(D.B.P.~ . 2(H.D.B.P.)lo + 2(H+)w (11,13) Dimerisation of di-alkylphosphoric acids is also found by Baes c. S.(47) from isopiestic measurements, whereas for the distribution ratio D o/a he

ob-tained:

D _{o a""} / (alkylphosphoric acid);

_(H+t

2 (11,14)

(As in these reactions H+ is liberated upon fo.rmation of the extractabie

com-plex these extractions with di -alkylphosphoric acids belong to our class II). If

too little H. D. B. P. is present, then poly-uranyl complexes are formed: n(U0 2

++\/

+ (n+1) [(H.D.B'P')2Io~![U02(D.B'P')2]n' 2(H.D.B.P.)lo +

The experimentally proved strong decrease of D o/a by increasing the HN03 concentration from 0.1 to 3 Min the aqueous pha:se is in qualitative agreement with the last given formula. Increasing the HN03 concentration from 3 to 7 M results in a sudden rise of D o/a for uranium. This is an indication of a change in the reaction mechanism (Brown c. S.46).

The formation of the extractable complex now proceeds as:

(U02 ++)w + 2(NÜ:3 -)w + f (H. D. B. P. )21 0 ~ _{fU0z{N03)2 • 2 (H. D. B. P.) 10} (IT, 16) Organic phosphates, phosphonates, phosphinates and phosphine oxides are compared as to their extraction power for uranium and plutonium(10.48). The substitution of a P-O-C bond by a P-C bond results in a st rong increase in

Do/a, and, e. g., trioctylphosphine oxide extracts uranium with coefficients

which are even 5 orders of magnitude higher, than those shown by T. B. P. The value of D o/a in these groups can be correlated with the electronegati-vity of the P=O bond. As a measure for the electronegatielectronegati-vity Burger(48) took the stretching frequency, that is the frequency of the light, where the absorp-tion reaches its maximum. Burger found the following maxima:

Phosphates Phosphonates Phosphinates Phosphine oxides at ± 7.87 11= 1270 cm-1 ± 8.0511= 1242 cm-1 ± 8.40 11 = 1192 cm-1 ± 8.6411= 1157 cm-1

Introduction of an electronegative group as chlorine increases the frequency.

In good agreement with this fact the extraction is sharply lowered. The

elec-tron accepting properties of the phenyl group lead to the same results, when the oxygen atom exists between the phenyl group and the phosphorus atom. So there seems to be a real correlation between the availabilityof electrons on the oxygen and the extracting power.

SiddaU(49) tested the extractability of U02(N03}z and HNû.J between 00 and

500C with a great many phosphates and phosphonates. From the results he calculated thermodynamic constants for each of the extractants by plotting the constant of complexformation K against the reciprocal value of the absolute temperature. The value for K ,at each temperature is found by dividing the distribution ratio by the square of the nitrate concentration in the aqueous

phase = (N03 -),., and the phosphate concentration in the organic phase:

D o/a K = (Phosph. )02 _•

(N03 -) ; (IT,17)

When the resulting Kis plotted against l/T he does not find a straight line in

all cases. Siddall finally tried to correlate his data with the inducing effects for radical changes in the molecules of the organic solvents, which might be expected via electronegativities.

In 1948 Smith & Page(50) drew attention to the extractions of acids from

17

.principally to separate weak and strong acids. Soon it appeared to be possible to separate metal salts for analytical purposes, e. g. niobium-tantalium(51.52)

cobalt andzinc(53), protactinium-niobium(54) and others(55). Then at the Oak Ridge National Laboratory many hundreds of amines and other organic com-pounds containing nitrogen were screened to see, if they could be used as ex-tractants for uranium and thorium salts(3.56.57).

For the theoretical interpretation of the extractions found experimentally, all authors assume complex formation. The similarity between anion exchange resins and extraction by amine salts is emphasized by many of them(3.19.17) .

According to Coleman et al(3) the extraction of metal ions ~rom sulphuric acid solutions can be described as:

(U~ ++À., + (S04=)...,f=::t (U~S04)w

[(RaNH)2 S04lo+ (U02S0Jw~[(RaNHh· U02(S04hlo

(11,18) (11,19) in which RaN denotes a tertiary amine. Another possibility is anion exchange of a sulphate group with an anionic uranyl sulphate complex, which is already formed in the aqueous phase:

(11,20)

As far as measurable nett results are concerned, these two sets of equations are of course exactly equivalent. According to Coleman et al the real extrac-tion process can follow either the first or the second mechanism, or both. Considering the discussion of the phase in which complexformation is most likely to occur (chapter m), the extraction process along the lines of the first set of equations above is in our opinion the most probable one.

A complication, which must be taken into account for the SX with amine salts is their tendency to associate in the organic phase. So McDowel and Baes J~58) found for di -n-decylamine sulphate, used by them to extract ura-nium, a mean molecular weight of about 30000 for the solution of the amine in xylene. Allen(19) has shown, that the sulphuric acid extraction by tri -n -octyl ·amine (=T.O.A.) in benzene solution follows the law ofmassactionuptoa T. O. A. sulfate concentration of about 0.02 M. Above this concentration he finds an apparent constant amine sulfate activity, which is quantitively treated by him on the basis of partial aggregation of the sulphate species. In the case of di-n-dicylamine (=D.D.A.) the lawof mass action is not followed at all, which is explained by assuming immediate aggregation. Via a hypothesis for the partition of free amine between the solvent and the colloid formed he finds a relation for the reaction of free amine and sulphuric acid, which takes the form of a sOlubility product. Verstegenq;9) also assumes aggregation to col-loids of constant activity, amongst others for his system tri -n -hexylamine-kerosine/water-sulphuric acid-sodium sulphate. In a more recent publication of Allen(60) it is stated that T. O. A. sulfate proved to be monomeric and only di -n -decyl amine sulphate seemed to be associated to such an extent as had been considered reasonable fOT consistency with the equilibration behaviour. The uranyl sulphate complexes of these amine sulphates were all monomeric,

as was confirmed by isopiestic and viscosimetrie measurements. Miss Boirie(61.62) and Bizot and Trémillon(63) raise a protest against the solubili-ty product formula of Allen and showed that their expe riments for the extractions of sulphuric, respectively hydrochloric acid with amines, and of uranium salts by the amine salts formed, agree better with their formulae, derived from the law of mass action, than with the hypothesis of Allen.

KrausC64) gave a review of the state of electrolytes in benzene and he re-ports, that for his quaternary amine chlorides there is al ready a considerable aggregation at a concentration of 1Q-4M. Hughes c. s.(65) extensively

investi-gated the conductivUy of quaternary amine chlorides in benzene and they found that, though the main part of the current is transported by tripel ions at a concentration of ab out 1Q-3M ~NCI, the concentration of the free CI- ions is proportional to the square root of the R4NCI concentration. The latter state-ment is also substantiated by the results of Pearson c. S.(66) for the velocity of the exchange of c:fl6 in Cl complexes of Pt(II) with amine chlorides.

The complexes of amine chlorides wUh the salts of bivalent metals often contain 2 molecules of the amine salt per molecule of metal salt. Schinde-WOlf(17), e. g. , _{proved that for the SX of ZnCl2 (in tracer concentrations) only}

R 2ZnCl4 is present in the organic phase and that the possibility of compounds as RZnCI3, RHZnCl4 and also a high solubility of complexes as Li~nCI4' H 2ZnCI4, etc. can safely be excluded. Also within the experimental error

(± 8%) there was no question of the dissolution of HCI or LiCI in the organic

phase, which contained

2%

of amine hydrochloride, the research having been carried out with Cl36 in concentrations of 0.1 - 8 N HCI, resp. LiCl.

Preparing a great many complexes of cupric chloride with several amine chlorides Whealy c. S.(67) always found complexformulae of the form CuC~.

2 RCL Bizot & Trémillon(63) also mention the formula U02C~. 2 RClforthe complex of U02C~ with the HCI salt of T.O. A.

Finally, Miss Boirie supposes, though on rather weakexperimental mate-rial, the existence of complexes of the form (~S04h. UOil04 for uranyl sulphate with some amine sulphates, in accordance wUh the formula of Brown c. S.(68) for U02S04 wUh triisooctylamine sulphate.

19

Chapter lil

DERIVATION OF FORMULAE FOR D o/a IN THE CASE OF

METAL SALT EXTRACTIONS FROM ACIDIC SOLUTIONS,

USING AMINE SALTS

As there was fairly much known already of the extractions with chelating agents, ethers, ketones and even with the different phosphates (for a review, see Chapter 11 C), when this research was started, we directed our attention mainly to the extractions with amines, as of this class of compounds it was detected scarcely a decade ago that its members could excellently be used as extractants for metal salts. Nowadays the technical extraction of uranium and thorium from their leach liquors by amine salts (=Amex process) is_ already in wide common use. The theoretical interpretation of this form of SX for metal salts received attention only in the past few years and even up to now the interest has been still primarily concerned with uranium and related elements. As the technical utility and even more so the theoretical backgrounds of the SX with amines are lil our opinion not only important for uranium and the ac-tinides, but for a great many metal salts, we have tried to develop a general theory for this form of SX for all metal salts under various conditions.

From a preliminary experimental investigation of the SX of zinc chloride with several amines it was concluded, that extractions with simple aliphatic amines gave the best results. This agrees with the findings of Coleman et al (3) for the SX of uranium salts. For this reason we shall restrict ourselves to the presence of only salts of amines with one N atom per molecule as the ac-tive components in the organic phase. Extraction of metal salts is normally carried out from an acidic aqueous layer and under these conditions the salts of the amines and the acid present are the actual extracting components of the organic phase and not the amines themselves. As is also found e. g. by Bizot and Trémillon(63) it makes no difference as far as the metal salt extraction is concerned whether the amine is saturated with acid before the SX experiment,

or simultaneously with the extraction of the metal salt from an acid solution, provided of course, that the amount of acid in the whole system is enough then. The theory for the SX with amines must make allowance for this forma-tion of the actual active components ..

*

*

An amine itself may function as the active component as was found e. g. by contacting a solution of di-(2-ethyl hexyl)amine (=D. E. H. A.) in xylene with a neutral aqueous solution of cupric chloride. The organic phase became

coloured deeply blue, whereas extraction of CuCl2 from a HCI containing solution yields a yellow-green coloured complex in the aqueous phase (Chap-ter V B). In the first mentioned case cupric hydroxide immediately was precipitated in the aqueous layer. As nearly all extractable metalsare pre-cipitated as hydroxides at a pH of 7 or higher, the extraction of metal salts is normally carried out from an acid aqueous phase to prevent this.

20

The basis of the extraction will be the formation of complexes between metal salts and' amine salts, which then pass into the organic phase. Our as-sumption, that the complex is formed in the aqueous phase or at least on the aqueous side of the boundary layer and then extracted is based on the fact, that reactions and equilibration gene rally will be:much faster in the aqueous phase than in the organic phase. The viscosity is normally lower and the dielectric constant higher for the aqueous layer. Moreover, it is more plausible, that a liftle bft of the polarisable amine should go into the aqueous phase to form there the amine salt and metal salt-amine salt complex, than strongly polar aCids, like Hel, ~S04' _{HN03 and metal salts to pass directly into the}

orga-nic phase. Irving and WilliamsC27) also assume, that their chelates are form-ed in the aqueous phase. As chelating agents are often rather insoluble in water, complex form;l.tion in this case must take place fundamentally on the aqueous side of the boundary layer.

In close agreement with our assumption made above Smith(69) states, that extraction is quickest, where the free chelating agent has appreciable solubi-lity in the aqueous phase, so making the system change from chelation at the

interface to phase transfer of chelate. In accordance with this view the time of equilibration'. varies from 10 minutes to 24 hours or longer for chelate extractions whereas for

s:x

with amines, ethers, ketones, etc., which are all more or less solubleinwater, afew minutesare normallysufficient, as is ver-ified experimentally. A direct proof for the assumption that extractable com-plexes are formed in the aqueous phase is found in the work of Healy and McKay<16). One molecule of U02(N03~ replaces 2 molecules of H20 from an organic phase with T. B. P. If U02(N03)2 as such could pass the boundary layer to replace H20 directly in the organic phase, then it might be expected, that it could pass into other organic solvents as weIl. Experimentally it was found, that uranyl nitrate is completely insoluble in inert diluents such as'

e. g. aliphatic and aromatic hydrocarbons(70), so the complex _U02(N03)2'

2 TBP must have been formed in the aqueous layer prior to extraction. The final equilibrium is of course fully independent of the way in which it is reached. For the reasons outlined above we follow the hypothesis that amines and amine salts can pass the boundary layer, that the metal salt-amine salt-complex is formed in the aqueous phase and that this complex then is extracted into the organic phase.

For the derivation of the formulae the following suppositions are also made in addition to the ones previously mentioned.

1. Only equilibria are discussed.

2. The (thermodynamic) law of mass-action is valid under all circumstances. 3. The phase ratio Vo;Vw = 1 : 1 for reasons of simplicity .

m.

A D a/a for the case where a number of restrictive conditions are ful-filled

To simplify the general case for the time being we assume: a. the amine salt in the organic phase does not associate; b. the extracted complex does not associate;

c. there is only one kind of metal salt in the aqueous phase;

d. the metal salt and the amine salt form only one extractabie complex, that may be hydrated;

e. no formation of non-extractable side-complexes occurs in the aqueous phase;

f. only one kind of anion is present;

g. the diluent in the organic phase behaves fuUy inertly and is insoluble in the aqueous phase;

h. only one kind of active component is present in the organic phase.

In the following sections these restrictions will be dispensed with one by one. The . aqueous and organic phases are indicated by placing a w or an 0 below

the symbol concerned; and [ .... ] and ( .... ) denote respectively activities and concentrations . A full list of symbols and abbreviations is given at the be-ginning of this thesis.

In the general case of an m -valent cation Mm_{+ and a p-valent anion AP- the}

formula of the undissociated salt is MpAm. The formula of the related acid is ~A.

The formation of the amine salt in the aqueous layer proceeds as:

(III,1)

Rl' R₂and R3 are alkyl chains (or H). If we extract with a quaternary amine salt, this mayalso be represented by RpA. In the latter case the nitrogen atom is bound to four alkyl chains.

With (RpA),. an extractable complex may be formed, containing q mole-cules of water:

(III,2)

If we call for brevity:

(III,3) then the formation constant Kl of the complex is:

[Compl.

]w

(1II,4)

The activities of R;' in both phases are equal*, for we assume equili-brium. The same is true for (Compl.). So we find:

* In a state of equilibrium the fuga city of a given substance is the same in every phase, or in every part of a system. Since the activity is defined as the relative fugacity , this activity is the same in every part of the system, when a single standard state for the substance under consideration, e. g. the pure solid state, is chosen for the whole system.

On the other hand, if for the substance in question we had decided to use different standard states in different phases - infinitely diluted solutions are of ten taken in chemical thermodynamics - then the activities in different phases, which are in equilibrium will not be equal, but they will neverthe-less remain proportional to one another as long as equilibrium persists, the factor of proportionality depending upon the choice of standard states. Be-cause the choice of different standard states would unnecessarily complicate the formulae, in this thesis for convenience, a single standard state is as-sumed (compare(71.72) ).

[RpA

10

=

[RpA]w

(1Il,5) and

[Comp1.1o = [Compl.

]w

(1Il,6) Substitution of (lIl, 5) and (lIl, 6) in (lIl, 4) yields:

(1Il,7) The dissociation of ~Am in water gives hydrated ions, according to:

~Am + (pq' + mq") Hp~ p Mm+. q' Hz0 + m AP-. q" H₂0 (1Il,8) The hydrated ions may be called M' and A', thus:

hydrated metalion = Mm+ • q' Hz0 : M' and

hydrated anion

(1Il,9) (1Il,10)

By virtue of supposition 1 reaction (lIl, 8) may be considered as an equili-brium, so we can write for the dissociation constant

K2

of the metal salt in the aqueous phase:

(lIl,l1) In

K2

the variation in [H₂0]w is taken up, as is usually done, because the numbers q' and q1l are often unknown.

Attaching a dissociation constant to any metal salt may seem in disagree-ment with the theory of complete ionisation of st rong electrolytes in dilute solutions. The following remarks may be made on this question, which was reviewed lately by van Ruyven (73) .

1. Even in a dilute solution of a st rong electrolyte we shall have at least a few undissociated molecules on the average. The term molecule is used here in a loose sense and can be taken as meaning: resultantly neutral arrangement of particles (see also Kortüm - BockriS<74) ). So K₂ may have a high, or even a very high value, but it never becomes infinite.

2. The salt MrAn can be a weak as weIl as a st rong electrolyte. Even for st rong electrofytes such as KCI, BaCI_2, K~04 _{and CUS04 the apparent degree} of ionisation in 0.1 N solution lies between 0.45 and 0.86, and st rong acids and bases also have a degree of ionisation considerably lower than unity(75). 3. The dissolution of salts, acids and organic solvent results in the lowering of the dielectric constant of the aqueous phase(76.1) and according to the theory of Bjerrum(77), elaborated by Fuoss and Kraus(78), it may be expect-ed, that the association of ions will be strongly promoted.

Substitution of (TIl, 7) in (TIl, 11) gives:

[Compl.

]0

The distribution ratio D o/a for the metal which can be determined experi-mentally, is an overall or stoichiometric distribution, including all species of the metal in the respective phases.

D

I _

Total concentration in the organic phase

o a - Total concentration in the aqueous phase (m,13) In the case, where only one extractable complex is formed according to equation (m, 2), we get:

r.(M)o = b. P (Compl. )0 (m,14) The activity coefficient of the complex is defined in the usual way by:

[C omp. :;: c. I] f (C omp.I ) , or fc = [Compl. ]

(Compl. ) (m,15) Combination of (m, 14) and (m, 15) gives:

I:(Mk, = b. p. l/fco . [Compl.]o (m,16) The material balance for the aqueous phase results in:

I:(M),. = b. P (Compl.),. + (M·),. + p (MAn),. (m,17) We do not take into account here forms like ~A(~_ll etc. in connection with supposition e (Chapter m A). As the metal concentration I:(M)w after the SX-experiment is as a rule readily accessible or may be found as the differ-ence between the amouilt of metal salt present before the extraction minus the amount taken up by the organic phase, I:(M),. is not further elaborated.

By filling in (m, 16) and (1lI, 12) in.(m, 13) we find:

D o/a

=

E(Mk, /E(M)w

=

b. p. l/_{fc o. [Compl.} ]0. l/E(M)w

=

= Kl/K~. [H20]~. b. p. l/_{fco . l}/I:(M),. . [M· ]~b .

. [A']:,b. [RpA]~

Introducing an activity coefficient as expressed in (nI, 15):

[M·

]w :;:

f+. (M·),. [A']w =: f-. (A')w and Substitution in (1lI, 18): D o/a

=

K/K~. b. p. f±<m+Plb . l/_{fc o. [H}20]~. l/I:(M),. . (A'):,b. (M·)~b. [R~]~ (m,18) (m,19) (m,20) (m,21) (1lI,22)

The total concentration of amine in the organic phase at the beginning of the SX experiment is known and may be represented by (R tot.)o. By bringing the

organic phase into contact with an aqueous solution of the acid BpA the amine

is converted into the amine salt. If the conversion is complete orilearly com-plete, then the concentration before the SX experiment is also equal to (R:tot.)o.

If the amine or acid is so weak, that the amount of free amine remaining is considerable as compared to (R tot.)o, then we have in fact two active solvents (see for this case III GO. )

We assumed, that there were no associations, so the mate rial balance after the SX becomes:

(R tot.)o = P (RpA~ + ap (Compl.)o + p (R~).,. + ap (Compl.)w. (III,23)

If fA is the activity-coefficient of (RpA) , according to

[R~]

=

fA. (R~) then we may change (III, 5) into:

or

fAo· (RpA)o = fAw . (!lpA)w .

(R~)o

I

(RpA).,.

=

DA = fAw/fAo Similar we find from (III,6) and (III, 15):

and fco . (C ompl.)o

=

fc_{w. (C ompl.} ~ (Compl. )o/(Compl.)w == Dc

=

fe,. I

fco

Substitution of (III,26) and (III, 28) in (III,23) gives:

(III,24)

(III, 25) (III, 26)

(III,27) (III,28)

(R tot.)o = P (RpA), + a. p (Compl.)o + P/DA . (HpA)o + a. p/Dc. (Compl.)o (III,29) Taking corresponding terms together:

(R tot.)o = (p + p/DA ). (RpA)o + (a. p + a. piDe). (Compl. ~ (III,30)

Substitution of (III, 14) in (III, 30) yields:

(III,31) and rewriting (111,31):

(III,32) Introducing (111,24) and (III, 32) into (III, 22) gives for D

o

/

a:

25

D o/a = K/K~. b. p. f1m+p)b . fA~/fco' [H20]~ . l/L(M~ •

• vub ( . \pb

~

(R tot.);, - a/b (l+l/Dc) E(M)o

~

a

• (A 'w • M fw • (p / ) .

+ P D_A (III,33)

As Kl and K_{2 are both real thermodynamic equilibrium constants}

andcon-sequently indepent of experimental conditions at a given temperature, they can be combined into one constant C.

Hence:

C ==

Kl

/

K~

and by substitution of (III,34) in (llI, 33):

D o/a = C. b.p. f±(m+p)b . fA~fco' [H b]~. l/E(M)w. ( .)mb ( . \pb'

~

(R tot.)o - a/b (l+l/Dc)E(M},

~

a

.Aw·Mrw· _{(p+p D}/ A) (llI,34) (llI,35) The constant C is dependent only on the selected combination of amine salt and metal salt and may in its turn be used to characterize that combination. The high significance of C for the SX will be outlined more fully in the follow-ing sections and in Chapter N A.

111. B Association in the organic phase

The metal salt-amine salt complex as weIl as the jimine salt itself may as-sociate in the organic phase. Formation of ions (dissociation) in solvents with low dielectric constant is not likely to occur and even in the most favourable case there will be only ari odd percent or so of ions, hence, to prevent unneces-sary complication, no account is given tot this possible effect in the following.

We dispense of restriction b (chapter III A).

If the 'association (conglomeration) of the metal salt-amine salt complex behaves like:

y(Compl. )o~ (Compl.y)o then the dissociation constant K3 is:

[(Compl.

m

K3 = [ _-(Compl)]o fc6. (Compl. _{fcef .(Compl}o}

)b

(III,36)

(llI,37) Here fcef is the activity coefficient of the associated complex, defined as:

fc* (Compl.y) == [Compl.y] (llI,38) The total concentration of the metal M in the organic phase

=

E(M)o' so in the absence of association is

(Comp!. tot. )0=

lA

b. p). L(M)o (ID,39) Complete association would give:

(Comp!. tot. y)o = l/y .1/(b.p). L(M)o (ID,40)

If CIc represents the degree of ionisation, then 1 mole (Compl.yk, yields

upon dissociation CIc. y. mole (Compl. k, and (l-CI

d

mole (Compl-yÀ, will remain

behind.Froml/y.l/(b.p).E(M)o mole (Compl.y)o we get:

CIc. y.l/y. l/(b.p).E(M)o =CIc/(b.p).E(M)o mole (Compl.k, and we retain

(l-CI_c).l/y.l/(b.p).E(M)o mole (Compl.yÀ,. So:

(Compl.)o = CI c' l/(b. p).E (M~

and

(Compl.y)o = (l-CId . l/(b. p) . 1/y. E(M~

Substituting (ID, 41) and (ID, 42) in (ID, 37):

Y/ [CIc. l/(b.p). L(Mk,~y y/( )

K3 _{= fc o} fc~. / / = y. CI c 1-CIc '

(1-a_c).1 (b.p). 1 y.E(M)o . 1/(b. p)y-l . f~ !fc~ . E(Mfo'l and substituting of (ID, 41) in (lIl, 12):

(ID,41)

(ID,42)

(ID,43)

(ID,44)

In accordance with the derivation of (RpA)o, formula (lIl, 32), we assume: (R tot. À, = p(RpAlo +a. p(Compl.)o +a. p. y(Compl.y)o +p(l\Alw +a. p(Compl. )w

(ID,45) Substitution of (ID, 41), (1Il,42) and (lIl, 26) in (ID, 45) then gives:

(R tot. )0 = p(RpAro H . p. CIc' 1/(b. p).L (M)o + a. p. y(1-a_{c)' 1}/(b. p). l/y.

(ID,46)

Gathering together corresponding terms:

27

Hence:

(ID,48) Substitution of (ID, 44), (ID,48) and (ID,34) in (ID, 13) yields as a general formula for association of the complex in the organic phase:

Do/a = C. 1/!l C' b. p. fim+p)b . ~

/fc

_o. [HzO]~

.

(A ,)~b . .

(M·~b

.

~

(R tot')o-a/b (l+ilc/Dc)l:(M)o

la

1: (M)w ~ (p + P/DA) ~ (ID,49)

If the amine salt associates too (dispensing with restriction a, chapterID A, we can write:

(ID,50) The dissociation constant K4 then becomes:

(ID,51)

Similarly defining fA* as in (m,38):

(ID,52) If all !(RpA>Z lo were fully dissociated, then we would find for (RpA)o equation (ID,48).

Complete association would give

!

1 /

~(Rtot.b

- a/b (l+!lc/Dc)·l:(Mb

t

(RpA}z . tot. = 1 z.

~

(p + p/D

A)

~

(ID,53) If the degree of dissociation of the associated (conglomerated) amine salt is represented by !lA, then we find for (R~k

~

(R tot. b -a/b(l+1lc/Dc). l:(Mk,

.

l

(RpA)o

=

!lA' Z,

!

(RA»z, tot.

I

=

!lA' ~ (p + P/DA) ~

(ID,54) and there is left behind

(ID,55) K4 then becomes:

fAz /fA* z /

~

(R tot. k, -a/b (l+a.,,/Dc)l:(M).,

~(Z-l)

(111,56)

K4 = 0 o' Z .!l A (1-!l A) . ( - /D)