CHA.PTER SEVENTEEN -

Solvent Extraction

INTRODUCTION

EXTRACTANTS . . . . . Solubility . . Ethers . . . . . . .

. . . . . . . . Alcohols . . . . . . . Aldehydes

Ketones . . . . . . . . Oximes . . . . . . . .

. . . . . . . Organic acids . . . . . . . . Phenols

Esters . . . . . . . . Amines . . . . . . . . .

. . . . . . Otherextractants THEORY . . . . . . . .

Mechanism . . . . . . . . . . . . . Extraction

Stripping . . . . . . . . . . . Extraction isotherms . . . . THE ORGANIC PHASE

. . . . . . Solventloading . . . Nature of the extracted species . . . Polymerizationofextractant

. . . . . . JRoleofdiluents Effect of extractant concentration .

. . . Extraction by mixed solvents . . . . Third-phase formation

THE AQUEOUS PHASE . . . . . Effect of metal ion concentration . .

. . . . . Effect of foreign ions . Complex formation in the aqueous phase .

332 Principles of extractive metallurgy . . . . . . . . . . . . . . Effect o f p H 369

Effect of ion hydration . . . . . . . . . . . . 370 . . . . . . . . . Extraction of inorganic acids 371

Extraction of water . . . : . . . 373' . . . . . . . . . . ENGINEERTNG ASPECTS 375

. . . . . . . . . . . Percent extraction 375 . . . . . . . . . . . . . . . Phaseratio 376

. . . . . . . . . . . . Separation factor 376 . . . . . . . . . . . Multiple extractions 376

Countercurrent extraction . . . . 378 . . . . . . . . . . . . . Equipment 379 .

. . . . . . . . . . . . . Economic aspects 381 . . . . . . . . . . . . . APPLICATIONS 382

. . . . . . . . . . . . . . Recovery 382 . . . . . . . . . . . . . . . . . Beryllium 382

. . . . . . . . . . . . . . . . . Boron 383

. . . . . . . . . . . . . . Cesium 383 . . . . . . . . . . . . . . . . Copper 383

Gold and silver . . . . . . . . . . . 384 . . . . . . . . . . . . . . Thorium 384

. . . . . . . . . . . . Tungsten 385 . . . . . . . . . . . . . . Uranium 385 . . . . . . . . . . . . . . . Separation : 385

. . . . . . . . . . . . cobalt-nickel 359 . . . . . . . . . . Hafnium-zirconium 389 . . . . . . . . . . . .. Niobium-tantalum 390

.

.. Plutonium-uranium-fission products . . . 390 . . .

. .

. . . . . . . . . . . Scandium-uranium 391

. . . . . . . . . . . Vanadium-uranium 392 . . . . . . . . . . . . . LITERATURE " ' 3 9 2

1NT RODUCTION

SOLVENT EXTRACI ION involves two operations : 1 ) Extraction The metal values in the aqueous phase are extracted by

agitation with a n organic solvent immiscible in that phase . The two phases are then allowed to separate; the aqueous phase is discarded or recycled. and the loaded organic phase saved .

So1;ent estractiorz 333

2) Strippirlg Recovery of the metal values from the loaded organic phase by agitation with a small volume of suitable solution. The stripped solvent is then recycled. In this way a concentrated solution containing the metal values in a relatively pure f o ~ m is obtaiped (Fig. 17-1).

Organic Pure meto l ' Leoch phose Strip solt for reduction

solution (Loaded) solution to metol

(unloaded)

t

Fig.17-1. General scheme in solvent ext ract ion

The earliest record known in which an organic solvent was used to extract metal ions is by PCligot,* who in 1842 observed that uranyl nitrate is

J.

.appreciably soluble in diethyl ether, and used this property for separating uranium from other constituents of pitchblende. Based on this discovery, about one hundred. years later. (from 1942 to 1953), the Mallinckrodt Chemical Works operated a uranium refinery for the U.S. Atomic Energy Commission (the Manhattan Project).

The first large-scale use of solvent extraction in metallurgy was in con- nection with preparing uranium containing < 1 ppm of contaminants for the

t I *

atomic energy program. This requisite purity was obtained by dissolving high-grade ore or concentrate in HNO,, selectively extracting the uranyl nitratc into ether, and stripping the ether with water to give.a conzentrated solution of pure uranyl nitrate. In 1953 the National Lead Company used tributylphosphate as the extractant, and subsequently the Mallinckrodt plant also converted to the use of this reagent.

In 1951 the U.S. Bureau of Mines in cooperation with the U.S. Atomic

* M.E .P i l i go t , "Recherches sur I 'Uranium", A I ~ I ~ . Clrim. PIijs., 5 , 5 4 7 (1842).

Recovery . Extraction

4 .f f P ' Barren Organic Strip Precipitating solution phose solution Ogent (roffinate) recycle recycle

Stripping

334 Prir~ciples 'of e.~tractive metallurgy

Energy Commission started a production-scale operation to separate hafnium from zirconium by solvent extraction. The basic process used was developed by the Oak Ridge National ~ a b o r a t o r ~ .

Systematic searches for other extractants were conducted by a number of laboratories, particularly those of the Dow Chemical Company and the Oak Ridge National Laboratory. These studies led to commercial use of octyl pyrophosphoric acid for recovering by-product uranium from phos- phoric acid in 1955, and to the use of alkylphosphoric acids and aliphatic amines for recovering uranium and vanadium from sulfuric acid solutions, at a number of mills starting in 1956. Also in 1956 a U.S. Bureau of Mines solvent extraction process enabling separation of niobium and tantalum was adapted to commercial use. Solvent extraction has been used since 1959 in processing tungsten ore, and for recovering thorium from waste solutions . of uranium mills. Research is currently under way on the application of the process to the recovery of the less expe~isive metals, such as copper and zinc.

Solvent extraction was successfi~lly demonstrated to be one of the most economical methods for metal recovery. Boron is now recovered from Searles Lake, California, in the form of boric acid of 99.9 "/, purity-a product that sells for only 5.5 cents/lb.

Solvent extraction has been used since the beginning of the twentieth century in the petroleum industry and is widely applied in the purification of organlc chemicals. Therefore, there is extensive literature available on the equipment and chemical engineering aspects which can be readily adopted to metallurgical applications. Also, the application of solvent extraction in analytical chemistry has provided very useful information for the extraction and separation process of potential metallurgical application.

The literature on solvent extraction is very extensive and is increasing at an enormous rate. Reviews appear from time to time. Some of these reviews are in the form of tables listing the extractability of metals from different media by different solvents. In this chapter only systems of metallurgical interest are described, and emphasis is made on organizing the theoretical aspects of such processes in two main sections: the organic phase and the aqueous phase.

So/tient extraction

E X T R A C T A N T S

An ideal extractant should fulfill the following requirements: 1) Selectivity 2).High extraction capacity 3) Easily stripped 4) Separates easily from water, i.e.,

a) Density is appreciably different b) Low viscosity c) High surface tension

5) Safe to handle, i.e., a) Nontoxic bj ~ o n f l a h a b l e c) Nonvolatile

6) Stable during storage or when i n contact with acids or alkalies, i.e., does not hydrolyze during extraction or stripping

7) Cheap In practice, however, it is not possible to find an extractant fulfilling all these requirements, and therefore a compromise is.usually made.

An extractant is seldom used in pure form; it is usually diluted with a cheap organic solvent in order to improve its physical properties, such as its viscosity, density, etc. This organic solvent, called diluent, has no capacity to extract metal ions from solution, i.e., it is inert*. However, as will be seen later, it affects the extracting power of the extractant. The solution of the extractant in the organic solvent composes the organic phase and is often referred to as the solvent. Beside fulfilling most of the requirements of an extractar~t, a diluent should be insoluble in water. Hydrocarbons and sub- stituted hydrocarbons are therefore the most commonly used diluents (Table 17- I).

Solubility

While diluents should be insoluble in water, the solubility of extractants in water may vary greatly. Tributyl phosphate, for example, is practically

* Very few cases are known in which a diluent can extract metal ions from solution- see p. 348.

Tabl

e 17

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81

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Cyc

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270

1.89

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Chl

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9.38

61

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Soluent extraction 337

insoluble while a chelating agent like acetyl acetone is completely soluble. Extraction by chelation is characterized by the fact that- although the extractant (i.e., the chelating agent) may be soluble, yet the metal chelate is insoluble in water but completely soluble in the diluent.

Ethers Simple ethers are light, volatile, and highly flammable liquids having the general formula R.O.Rt. The most commonly used &her is diethyl (Table 17-2), which is simply called ether. Many metal ions are extracted by ether from HF, HCl, and HNO, solutions, but not from H,SO,. The extracting power of ethers decreases with increasing molecular weight.

Table 1722. Physical properties of some ethers. . .

Diethyl B-, B'-dichloro-, ether diethyl ether .

Boiling point, "C 34.6 178.0 Flash point, "C - 45.0 75.6 Solubility in water, % 7.5 1 .O Solubility of water in ether, % 1.3 - Specific gravity 0.717 1.222 Viscosity at 2OoC, centipoise 0.23 2.06

Ether is susceptible to oxidation, forming peroxides which are liable to explode on distillation. It is therefore essential that water be present when ether solutions containing nitrates are being evaporated.

Another commonly used ether is B,Bt-dichlorodiethyl ether, ClCH,CH, OCH,CH,Cl (Table 17-2). This is a colorless liquid, much less volatile than diethyl ether, less soluble in water, and nonflammable at ordinary temper.a ture.

Alcohols Monohydric alcohol., may be primary, secondary, or tertiary:

R R

R-CH20H \

R-COH

R /

R /

Primary Secondary Tertiary 22 Habashi. Metallurgy 11

338 Principles of extractive metallurgy

The lower members are soluble in water, and the solubility decreases with increasing molecular weight. The solubility of n-octyl alcohol (or 1-octanol), CH3 . (CH,), . OH, is 0.03 g in 100 g water. It is used, for example, in separat- ing cobalt from nickel.

Aldehydes A commonly used aldehyde is furaldehyde or furfural,

It is a colorless, pleasant-smelling liquid, of b.p. 162"C, density 1.1598, and solubility in water 9 % by volume. It is produced in large quantities as a by- product of the agricultural waste industry; cheap and nonpoisonuos, it is already used in purifying lubricating oils.

Ketones A commonly used ketone is methyl-isobutyl ketone, known as hexone (Table 17-3).

CH3 I

/ CH3

Hexone

Table 17-3. Physical properties of methyl isobutyl ketone (hexone).

Specific gravity 0.8024 Boiling point, "C 116 Vapor pressure at 20C, mm H g 15.7 Flash point, "C 27 Solubility in water, Val% - 7 Solubility of water in hexone, Val% 2.2 Viscosity at 20C, centipoise 0.585

An important ketonic chelating agent is acetyl acetone, which is a diketone, and its derivative thenoyl trifluoroacetone. Ketones exist in the keto and

Solvent extraction

en01 forms, as shown below

C-OH I I

C=O C=O / /

CH3 CH3 Keto En01

Acetyl acetone 0 0 0 OH

I1 lrll 11 I ~~L!-cH~-c-cF~ S + S / - ~ - ~ ~ = ~ - ~ ~ 3

Keto En01 Thenoyl trifluoroacetone

Oximes Oximes, i.e., compounds containing the group =N-OH, have been used. for many years in analytical chemistry as precipitants for metals. The or- hydroxyoximes are well known as specific for precipitating copper, e.g.,.

Salicylaldoxime 2-Hydroxy-5-dodecylbenzophenone oxime and 5,8-diethyl-7-hydroxy-6-

dodecanone oxime are finding extensive use in extracting copper from leach solutions, under the trade names LIX-63 and LIX-64, introduced to the market by the General Mills Corp.

Dioximes, i.e., compounds containing the (=N-OH), group, e.g., dime-. thy1 glyoxime, have also been known for many years as chelating agents. for metals. Dimethyl glyoxime is specific for nickel:

H3C CH3 \ /

C-C I1 II

N N ,' \

HO OH Dimethyl glyoxime

340 Principles of extractive metallurgy Organic acids

Many organic compounds exhibit acidic properties, but in this section only those which contain the carboxyl group and are of metallurgical interest are , .

1 .

included.

a) Fatty acids, R.COOH The low members are soluble in water and the solubility decreases with increasing molecular weight. Numerous acids have

Table 17-4. Phenols suggested as extractants.

p-Dodecyl phenol

/==\-R R = dodecyl (mixed isomers) \- -/ I R

o-Phenyl phenol OH I

4-Chloro-2-phenyl phenol

4-sec-Butyl-2(&-methyl)- benzyl phenol

\I- v CH,~HCH~CH,

Polyol X I

R - / \ O H \-- /

I

been suggested as extractants, e.g., palmitic, CH,(CH,),,COOH, and stearic, CH,(CH,),,COOH. Others are in use under the trade name Versatic acid. Alpha-substituted acids, e.g., a-bromo lauric (C,,) and a- bromomyristic (C,,) have also been suggested because of their increased strength.

Soluent extraction 341

b) Naphthenic acids are carboylic derivatives of cycloparaffin hydro- carbons, having a structure as shown:

They are derived from crude petroleum oils and have a variable composition. The molecular weight varies from 170 to 330.

Table 17-5. Acidic organic phosphorus compounds.

Group Example Abbreviation

a) Dialkyl phosphoric acid Di (2-ethyhexyl) phosphoric acid D2EHPA 0 R is CH3(CH2), . CH . CH,- i I

RO-P-OH CHz . CH3 I

OR

b) Monoalkyl phosphoric acid Dodecyl phosphoric acid DDPA 0 R is (1,2-methylpropyl-3,Sdimethyl hexyl

1 phosphate) RO-P-OH

c) Alkyl, pyrophosphoric acid Octyl pyro-phosphoric acid 0 0 R is C8H,, I I

RO-P-0-P-OR I I

HO OH

d) Dialkyl phosphinic acid 0 I

R-P-OH I

R

e) Monoalkyl phosphonic acid 0

I R-P-OH

OPPA

Principles of extractive metallurgy Table 17-6. Neutral organic phosphorus compounds.

Group Example Abbreviation

a) Trialkyl phosphate Tributyl phosphate TBP 0 R is C,H,

I RO-P-OR

I OR

b) Trialkyl phosphine oxide 0 I

R-P-R I R

c) Alkyl dialkyl phosphinate 0

I R-P-OR

I R

- d) Dialkyl alkyl phosphonate 0 I

R-P-OR I

OR

Phenols Phenols, in which a hydroxyl group is attached to an aromatic nucleus, are more strongly acidic than alcohols. Table 17-4 shows some phenols of metallurgical interest. Polyol is used on commercial scale for boron extrac- tion.

Esters Esters are formed by the reaction of alcohols with inorganic acids. Those used in extractive metallurgy are derived from phosphoric acid and can be classified into acidic esters (Table 17-5) or neutral esters (Table 17-6). Phosphoric acid is introduced in the form of P205 or POCl,. A mixture of products is generally obtained, and compounds formed from the hydrolysis of the products are common. Moreover, dehydration of the alcohol by P205

Solfient extraction 343 may take place, giving unsaturated hydrocarbons or olefins in the reaction mixture. Thus, an alkyl phosphoric acid such as OPPA was found by Zangen (1960) to be composed of seven components.

The molar ratio of the reactants determines the type of ester. Thus, when an alcohol to P 2 0 5 ratio is 2, an alkyl pyrophosphoric acid is obtained:

0 0 I I

2 ROH + P 2 0 j + RO-P-0-P-OR I I

OH OH

When a ratio of 3: 1 is used, a mixture of mono and dialkyl ortho phosphoric acid is obtained:

3 ROH + P205 + (RO)(H0)2PO + (RO),(HO)PO . When a ratio 6: 1 is used, a trialkyl phosphate is obtained:

6 ROH + P205 + 2 (RO),PO Trialkyl phosphates can also be prepared by the interaction of POClj with alcohols: 3 ROH + POCI, + (RO),PO + 3 HCI A commonly used extractant belonging to this group is tributyl phosphate, physical properties of which are given in ~ a b i e 17-7.

Table 17-7. Physical properties of tributyl phosphate.

Boiling point, "C 177-178 at 27 mmHg Flash point, O C 145 Viscosity at 20C, centipoise 3.41 Solubility in water, Val% 0.6 Solubility of water in TBP, Val% 7 Specific gravity 0.973

To prepare a monoalkyl phosphoric acid, P 2 0 5 is slurried in a diluent such as kerosene, and the alcohol is then added while temperature is kept at about 15C. The reaction product is hydrolyzed at elevated temperature by refluxing with 1N HC1:

0 0 0

Ro-A-0-LOR + H,o -r 2 Ro-A-OH I I I

344 Prirlciples of extractive metallurgy In general the following rules apply:

I 1) solubility in the aqueous phase decreases with increasing chain length. 2) Extracting power increases with increasing chain length, but for

economic reasons chain lengths C,-C,, are used.

3) Low emulsifying tendency is achieved by branching in the chain.

The use of simple amines is limited to those of molecular weight between 250 and 600. Arnines of molecular weight 1owe;'than 250 are appreciably soluble in water, and those of molecular weight higher than 600 are either not easily available or have low solubility in the organic phase. Table 17-8 gives some of the widely used amines as metal extractants. Most of these are commercial products which are mixtures of compounds in isomeric or homologous series. The predominating structure in each mixture is given.

Quaternary ammonium compounds are closely related to amines, but they are significantly different in behavior. While the simple amines are ineffective at high pH, the strong-base quaternary ammonium compounds are effective over a wide pH range.

Solubility of the amines in aqueous and organic solutions, as well as phase-separation properties, depends on the structure of the amine and the nature of the solvent. Water immiscibility decreases with increasing length of the aliphatic chain.

Other extractants

A few of the chelating agents that are used extensively in analytical chemistry and may be of potential metallurgical importance are mentioned below: ,

A colorless solid, m.p. 75"C, slightly soluble in water.

Table 17-8. Typical amines used for extracting metal ions.

Group Example Abbreviation or synonym

Primary Trialkyl methylamine Primene JM RNHz CH3C(CH3)2CHzC(CH3)2CH2C(CH3)2CH2-

Octadecylarnine CH3 (CH2), ,NHz Secondary Dilaurylarnine

Dodecenyltrialkylmethylamine

Arrneen 212

Amine S 24

Amine 9D-178

346 Principles of extractive metallurgy Table 17-8. (cont.)

Abbreviation Group Example or synonynl

CH2CH3 NH I I I

CH3 . (CH 2)3 . CH . (CH 2)2CH I

CH3 . CH2 . CH . (CH2)2 I

Tertiary Tri-11-octylamine (TOA) R R1

R N where R" R /

R = CH3CH2CH2CH2CH2CH2CH2CH2- Tri-iso-octylamine (TIOA)

CH3 CH3 I I

N(CH2CH2CHCH2CHCH3)3 A mixture of tri-n-octylamine and tri-rr-decylamine Alamine 336

Butyldilaurylamine CH3 CH3 I I

CH3CCH2CCH2CH=CHCH2 [ AH3 AH3 Tribenzylamine

348 Principles of extractioe metallurgy Solvent extraction may be physical or chemical in nature. Physical pro-

cesses are very few and in these, Nernst's distribution law is usually obeyed. Examples of such processes are the extraction of arsenic, antimony, ger- manium, and mercury from hydrochloric acid solutions by hydrocarbons or chlorinated hydrocarbons such as benzene, carbon tetrachloride, or chloro- form. Such solvents are usually used as diluents in other extraction systems because of their inertness.

In the majority of cases, however, solvent extraction is a chemical process and Nernst's distribution law is not obeyed. In these processes metal ions, or uncharged species in the aqueous phase enter into interaction with the , extractant, and the reaction product (organometallic complex*) is soluble in the organic phase. Interaction takes place in one of the following ways:

1) Ion-pair transfer In this case electrically neutral molecules interact with the extractant to form an addition compound. The most suitable extractants for such interaction are those having an oxygen atom with a lone pair of electrons, e.g., ethers, alcohols, and the neutral phosphoric acid esters. For example, diethyl ether extracts iron ,from hydrochloric acid solution as follows:

CZH5 C2H5 )O + HFeCI, - )O . HFeCI, CZH5 CZH5

Tributyl phosphate extracts uranium from nitric acid solutions as follows:

where R = C,H,.

2 ) Iorl exchange In this type the metal is transferred from the aqueous phase as a simple ion, and at the same time an ion from the extractant is transferred stoichiometrically to the aqueous phase.

a) Cation exchange The extracted species is a positively charged ion and the extractant is an acid, e.g., monoalkyl phosphoric acid, dialkyl phosphoric acid, alkylpyrophosphoric acid, carboxylic acids, naphthenic acids, etc. For example, the extraction of uranium by a monoalkyl phosphoric

* This'terrn should not be confused with organometallic compounds in which the metal is attached directly to carbon.

acid takes place as follows:

It can.be seen that one uranyl ion is exchanged for two hydrogen idns.

b) Anion exchange The extracted species is a negatively charged ion and the extractant is a base, e.g., an amine,

RNH, + HCI + RNH2CI-

Tt can be seen that an anion A- is stoichiometrically exchanged for another anion.

3) Chelate extraction In this type an electrically neutral metal chelate* is formed which is insoluble in the aqueous phase but readily soluble in the diluent. For example, acetyl acetone in-the en01 form reacts with beryllium ion to give a chelate in which the beryllium replaces .the enolic hydrogen atom and is coordinately bound to the keto oxygen, forming a ring as . follows :

CH3 CH3 CH3 I I I

C-OH C-0 0-C A A \ \\

2 HC + BeZ+ + HC Be CH + 2H' \ \ ;I '.\ ,/

Belonging to this group are the diketones, oximes, oxine, nitroso- naphthols, cupferron, and dithizone.

Stripping In the stripping process, two purposes are achieved:

1) Recovery of the metal values from the organic phase. 2) Regeneration of the extractant for recycle.

When an organic molecule containing both acidic and basic functions combines with a metallic ion, and if in the combination both functional groups are operative, a so-called inner-complex or chelate salt is formed.

350 Principles of extractive melallurgy Stripping can be carried out in one of the following ways:

a) Back-washing the organic phase. . b) Precipitation of metal values directly from the organic phase. c) Selectively, when the organic phase is loaded with more tha ione metal

ion. For example, vanadium and titanium are coextracted with uranium by octyl phosphoric acid. However, separation can be achieved if vanadium is first stripped by 1N HCl, then uranium by 10N HCI, and finally titanium by 5% HF.

The stripping coefficient is defined by

D' = ~oncentr'ation of Metal in Aqueous Phase Concentration of Metal in Organic Phase

The higher the value of D' at equilibrium, the higher is the tendency of the metal ion to be transferred to the aqueous phase. The stripping coefficient is the inverse of the distribution coefficient D.

. Stripping agents. depend on the mature of the extraction mechanism- whether ion-pair transfer or ion exchange. Ethers and neutral organic phosphorus compounds such as TBP are readily stripped by water.

Acidic organophosphorus compounds may be shipped by acids or bases. The acid method employs a concentrated acid to reverse the following reaction :

MZ+ + H2X + M,X + 2 H+

Thus for the stripping of uranium, the following acids are used:

a) 48 % H F In this case uranium is precipitated as UF,. Higher con- centrations of H F result in excessive deterioration of the solvent. Lower concentrations cause losses due to emulsion formation. Contact time should be only 15 sec: longer contact should be avoided.

b) 20 % H F + 20 % H,S04 In this case the rate of deterioration de- creases due to the lower concentration of H F used, and at the same time phase separation is improved by the sulfuric acid added.

c) 10N HC1 This acid is not so effective a stripping agent as H F but due to its lower deteriorating action on the extractant it is much used: Excess acid is recovered by evaporation, and uranium is precipitated by NH,OH.

Soluent extraction 351

Alkalies may be used-to strip extractants of this type by precipitating the metal hydroxide and forming the alkali salt of the organophosphorus acid,e.g.,

M2X + 4NH40H + 2M(OH)2 + (NH4)4X In some cases, e.g., beryllium, the hydroxide is soluble in excess alkali.

Stripping of anlines can be represented as follows: Displacement :

(R,NH)2UO2(SO4)2 (0) + 2 NO, (a) + 2 R3NHNO3 ( 0 ) + UO,Z&) + 2 SO&) Hydrolysis and complexing whereby the free amine is liberated:

(R,NH)2U02(S04)2 + 4 Na2C03 Precipitation:

+ 2 R,N(,, + Na4UO2(CO,),(,, + 2 Na2S04 + H 2 0 + C 0 2 ~(R,NH)ZUO,(SO~);(O) + 5 MgO + 4R3N(o) + MgUzO,(,) + 4 MgSO,(,, + 2 H z 0

Extraction isotherms The plot of the concentration of the metal ion in aqueous phase versus its concentration in the organic phase when the two phases are in equilibrium

Metol concentrotion i n aqueous phase at equilibrium, M i l

Fig. 17-2. Metal extraction from 0.5Msulfate solutions with 0.1 M DZEHP in kerosene [Blake et al. (1 958)l.

352 Principles of.extractiue~metallurgy

is called the extraction isotherm, in analogy to the adsorption isotherm.' Such a plot is very useful, since at each point it directly displays the concen- trations in both phases and consequently the distribution coefficients can be evaluated. With the addition of an operating line based on the volume ratio of the two phases, the extraction isotherm provides a McCabe-Thiele dia- gram for a stagewise analysis of extraction in a countercurrent system, as will be described later under engineering aspects.

The extraction isotherm also shows that with increasing concentration of the metal ion in the aqueous phase, a limiting concentration is reached in the organic phase. There is usually a simple relation between the number of moles of the extractant and that of the metal ion at saturation. This relation determines the stoichiometry of the reaction and the nature of the extracted species in the organic phase. For example, in Fig. 17-2, 0.1 M extractant is saturated with 0.05M UOzZ+, i.e., 0.5 mole metal ion saturates 1 mole of extractant (See also section below on solvent loading).

T H E O R G A N I C PHASE

Solvent loading A certain weight of an extractant has a limited capacity for extracting a cer- tain metal ion from solution. When this limit is reached theextractant is then saturated with the metal ion and is said to have reached its maximum loading. Maximum loading varies widely from one extractant to the other.

The organic phase increases in viscosity with increasing metal ion loading (Fig. 17-3) and sometimes it is necessary to operate at a solvent loading sub- stantially below the maximum loading of the extractant. Saturation con- centration can be determined by cintacting the organic phase several times with fresh aliquots of the.aqueous phase until the amount of metal in the organic phase becomes constant. For example, Table 17-9 shows saturation data for the extraction of thorium from sulfuric acid' solution by di-(2-ethyl hexyl): phosphoric acid at different pH values.

. . .

Nature of extracted species Usually at the saturation concentration the molecular ratio extractantlmetal ion extracted is a whole number, as can be expected from a reaction of the type, . . . .

M"+ + rzHX -+ MX, + nHt

Solvent extraction 353

where n is an integer. However, the simultaneous extraction of other species may cause deviation from awhole number. For example, data in Table 17-9 show that the ratio extractantlmetal ion extracted at saturation is about 3.8 at pH 1.5 and about 3.3 at pH 2-3. This is due to the reactions,

1 ' " " ' ~ 0.4 0.8 1.2 Uronium concentration in TBP, M

Fig. 17-3. Viscosity o f tributyl phosphate as a function o f uranium content [Hesford and McKay (1960)l.

which occur simultaneously and are pH-dependent. According to the first reaction the ratio is 4 and according to the second it is only 2. Therefore, the value of the saturation concentration helps to throw light on the stoichio- metry of the reaction and the nature of the extracted species.

The saturated organic phase so obtained can be studied in a variety of ways. The simplesi way is by heating it at low temperature and under vacuum to evaporate both the diluent and the uncombined extractant. The residue obtained, which. may be a liquid or a crystalline solid, is the extracted spe- cies, i.e., the compound between the metal and the extractant, usually has a definite boiling or melting point. .Table 17-10 shows some examples of compounds isolated during extraction processes. I,t can be seen that some of these compounds contain water of crystallization. 23 Habashi, Metallurgy I1

3 54 Principles of extractive metallurgy

Table 17-9. Saturation of the organic phase in the extraction of thorium. Organic phase: 10 ml of di (2-ethylhexyl) phosphoric acid in kerosene; aqueous phase: 10 ml of 0.1 M

H2S04 containing 2 g/l Th [Tedesco el al. (1966)l.

D2EHPA, Wt. of D2EHPA Wt. of Th Molar pH M in 10 ml, extracted, ratio,

mM mM D2EHPA/Th

1.5 0.407 4.07 1.07 3.80 0.207 2.07 0.54 3.83 0.120 1.20 0.31 3.87

2.0 0.407 4.07 1.20 3.39 0.207 2.07 0.64 3.23 0.120 1.20 0.36 3.33

2.6 0.407 4.07 1.20 . 3.39 0.207 2.07 0.65 '3.18 0.120 1.20 0.37 3.24

3.0 0.407 4.07 1.25 3.25 0.207 2.07 0.59 3.57 0.120 1.20 0.36 3.33

It should be pointed out that when micro amounts are extracted (trace concentrations), the extracted species may be different from those when macro amounts are extracted. Thus Vdovenko et al. (1961) showed that, when trace concentrations of thorium were extracted from H2S04 solutions by primary octylamine, the extracted species was 2 (RNH,),SO,. Th(S04), . When, however, macro amounts (0.0086-0.029M) were extracted by the same solvent, the extracted compound when separated from the organic phase analyzed as (RNH,), Th(S04), .

Infrared and nuclear magnetic resonance measurements are used exten- sively to identify the nature of the extracted species in the organic phase.

Polymerization of extractant Some amines tend to associate in polar hydrocarbon diluents and remain in stable colloidal dispersion (Brown el al., 1957).~~ialk~l~hos~horic acids and the dialkylphosphinic acids are often dimeric'in the organic diluent, pre- sumably by hydrogen bonding (Baes et al., 195:): In the case of the monoalkylphosphoric acids, ROP03H,, and the mono- alkylphosphonic acids, RP03H,, larger polymers'have been found by Koso- lopoff and Powell (1950). Such polymerization directly affects cation ex- change, since the hydrogen atoms are not free to react.

\ Solvenr extraction ' 355

Table 17-10. Examples of extracted species. - - - -

System

Th4+-HzS04-octyl amine

UO:+-~~S0~-d ioc- tylamine

HzPtC16-HzS04-tri- octylamine

UO:+-~~S0,-tri- octylamine

~ 0 : + - ~ ~ 0 ~ - t r i - octylamine

UO:+-HCI-trioctyl- amine

UO:+-HNO~-D~- ethylether

~ r ( ~ ~ ) - ~ ~ l - t r i b u t ~ l phosphate

La(N03),-tributyl phosphate

MO(1V)-HCI-tributyl

- - - -

Extracted species References and remarks

Th(S04)~ . (RNH3)z Vdovenko el al. (1961) FeOH(S04)2 . (R2NHz)z Isolated as orange red

waxy solid, m.p. 204C-Cattrall and West (1966)

UOzS04. 2 (R2NH2)2S04 Zvyagintsev er al. (1964)

PtC16(R3NH)2 Deptula (1966)

U02S04 - 2 (R3NH),S04 Zvyagintsev el al. (1964) U02S04 .'2 (R3NH)$04. 3 H 2 0 Sato (1965) U02S04. 2 (R3NH),S04. nH20 Deptula and Minc (1967) UO2(NO3j3 . R3NH Isolated as yellow

liquid-Vdovenko er al. (1963)

U02(N03)4 . (R3NH)z Sato (1965) UOzC14. (R3NH)z Sato (1 966)

phosphate Niobium fluoride-tributyl

U02(N03)2 . 3 HZO . (CZHS)20 Bachelet and Cheylen (1 947)

U02(N03)2 . 2 HzO . 2 (CzH5),O . UOz(N03)2 . 2 H z O . 4 (CZHS)20 Katzin and Sullivan , -

(1951)

phosphate NbC1,-HCI-tributyl-

phosphate Pt(1V)-HC1-tributyl

phosphate

Tantalum fluoride- tributyl phosphate

HCr03CI. 2 TBP

La(N03)3 . 3 TBP

I M O O ~ C I ~ . 2 TBP

HNbF6 . 3 TBP

HNbOCI4. 3 TBP

HPtCI, - 3 TBP

HPtCI, . 2 TBP H2TaF7 . 2 TBP

Arend and Specker (1964)

Nikolaev er al. (1964)

Arend anh Specker (1964)

Baram er al. (1965)

Startsev and Krylov (1966)

Kulkarni and Sathe (1 966)-

Baram er al. (1965)

356 Principles o f extractive' metallurgy Table 17-10. (cont.)

System Extracted species References and remarks

Uranous nitrate - U(N03), . 2 TBP McKay and Streeton tributyl phosphate (1 965)

Uranyl nitrate - U02(N03)2 ' 2 TBP Solid, m.p.-6C- tributyl phosphate Healy and McKay

(1956) UC14-tri butyl UCI,. 2 TBP Lipovskii and Yakov-

phosphate leva (1964) UCIL. 3 TBP

V(V)-HCI-tributyl HVOICll. 2 TBP Arend and Specker phosphate (1 9 64)

W(V1)-HC1-tributyl WO2CI2 ' 2 TBP Arend and Specker. phosphate (1 964)

/H'.. /"\< / O / O

0 (RO)Z-P\ O> P-(0R)2 , R2-P \ P - - R ~

0 0 \o 0' "."/ \H/

Dialkylphosphoric acid Dialkylphosphinic acid dimer dimer

Role of diluents

The diluents themselves are unable to extract metal ions from the aqueous phase, but they greatly affect the extraction behavior of the solvent (Tables 17-11 and 17-12). The reason is the interaction that may take place between the diluent and the extractant. Further, some diluents favor thepoly- merization of the extractants while others do not. Calorimetric measure- ments usually give a good indication about such interaction.

Afanasev et al. (1966) found that mixing tributyl phosphate with heptane, nonane, and decane was accompanied by absorption of heat, while mixing with' chloroform and carbon tetrachloride was accompanied by liberation of heat. The endothermic effect in the first case is due to the dissociation of TBP dimers in the hydrocarbon diluent, while the exothermic effect in the second case is due to the association of TBP with the chlorinated hydro- carbons to form CHCl, . TBP and CCl, . 2TBP. Such a bond considerfbly reduces the effective concentration of TBP in the organid phase, which is

Solvent extraction

Table 17-11. Effect of diluent on the extraction of uranium from sulfate system by D2EHPA [Blake e! a/. (1958)l.

Dielectric Distribution constant ratio

Kerosene 2.0 135 Carbon tetrachloride - 2.2 17 Benzene 2.3 13 Chloroform 4.8 3 2-Ethylhexanol - 10 0.1

the cause for the low extraction capability of TBP solutions in chloroform and carbon tetrachloride.

Correlation between the effect of a diluent and its dielectric constant is sometimes feasible. In extraction by cation exchange mechanism, the diluents having the least dielectric constant, e.g., kerosene and CCl,, lead to the best extractions (Table 17-11). This is because such diluents do not favor poly-

Table 17-12. Effect of diluents on the Extraction of uranium from sulfate system by different amines [Coleman et al. (1958)l.

Dielectric Distribution ratio

'Onstant Primene JM-T Amine S-24 Tri-n-octylamine,

Kerosene 2 .O 3 110 30 Benzene 2.3 10 20 150 Chloroform 4.8 90 2 5

merization of the extractant in the organic phase through hydrogen bridges. The effect of diluent on amines is complex (Table 17-12), since some amines tend to polymerize, while others do not polymerize in the same organic phase, depending on the structure of the amine.

Effect of extractant concentration

The distribution coefficient increases with increasing concentration of the e-xtractant in the organic phase. When log D is plotted against the logarithm of extractant concentration, a straight line is obtained (Figures 17-4 to 17-6).

Principles of extractive metallurgy

1"- 1.0 10 T B P concentro tion in organic Phase,%

Fig.17-4.lExtraction of uranyl nitrate by TBP in kerosene [McKay (1956)l.

Fig. 17-5. Metal extractions from 0.5M sulfate solutions with DZEHP in kerosene; aqueous pH = 1 plake et nl. (1958)l.

Solvent extraction

This linearity can be interpreted as follows:

103

lo2

10 a -

+ (I rn .-

U 'C 1 L a, 0 U

C 0 .-

- 2 lo-' L +

II 0

ldZ

1ci3

' [MY nX] K = [MY] [XI"

-

D - -

[XI"

Concentrotion of tri-iso-octylomine in organic phose, M j l Fig.17-6. Distribution of Co60 between triisooctylamine chloride and aqueous phase as a function of amlne concentration prooks and Rosen-

baum (1963)l.

I I I I

- -

- -

- -

- -

- -

- -

I I I I 10-3 10-~ lo-' 10

log D = n log [XI + constant

10

This is an equation of a straight line with slope n.

360 Principles 06 extractive metallurgy Extraction by mixed solvents I

Blake et al. (1958) reported that when dialkyl phosphoric acid (RO),PO OH is used in conjunction with certain neutral organophosphorus esters, such as (RO)3P0, (R0)2HP0, (RO)H,PO and R3P0 (R = 11-butyl), the extract- ing power of the mixture exceeds the sum of the extracting powers of the components, as show11 in Fig. 17-7. This phenomena, called synergism, is not confined to organic phosphorus compounds,-but applies to-many types of organic solvents. Thus, Rosenbaum et al. (1958) reported that the combi- nation of alkyl phosphoric acids with alkyl amines alsd showed a synergic effect during the extraction of uranium and vanadium. Irving and Eddington (1959-65) found similar results with mixtures of tributyl phosphate and thenoyltrifluoroacetone (Fig. 17-8). Vdovenko and Krivokhatskii (1960) ob- served the same phenomenon when extracting uranyl nitrate by mixtures of ethers (Fig. 17-9). Weaver (1961) found that a mixture of methyl isobutyl ketone and diisobutyl ketone is a better extractant for niobium from HCI than either of the solvents alone. Newman and Klotz (1961) found that a mixture of TTA and tri-iso-octylamine in benzene extracts thorium from 2N HCI, whereas each of the reagents alone does not. This effect is also dependent on the diluent.

1 '0

I I 0.1 0.2

Concentrotion o f neufrol odditive, M/ I

Fig.17-7. Effect of adding neutral 'organic phosphorus compounds to DZEHPA on the extraction of uranium from H,SO, [Blake er al. (1958)l. (1) tributyl phosphate,. (2) dibutyl butyl .phosphonate, (3) butyl dibutyl

phosphinate, (4) tributyl phosphine oxide.

. Solvent extraction

0 0.5 1.0 Mole fraction of TSP

Fig. 17-8. Extraction of uranium by mixed solvents TTA + TB1' [Irving and Eddington (1960)l.

M ~ x e d extractants

D ibuty l ethe! Dichlorodiethyl ether I I ,

100. 80 60 40 20. 0

Fig.17-9. Extraction of uranyl nitrate by a mixture of dibutyl ether and (l,~'ldichlorodiethyl ether in benzene

362 Principles of extractive metallurgy

In extraction by mixed ketones it can be shown that there is interaction between the ketones themselves, showing positive deviation from Raoults' law. Thus the maximum observed on the vapor pressure curves due to molec- ular interaction coincides with the synergic behavior during extraction with the mixed solvent (Weaver, 1961).

In extraction with a mixture of an acidic (or a chelating) and a nonacidic solvent, the situation is more complicated. Thus Ferraro and Healy (1961) have shown, by means of infrared measurements, that there was no inter- action between thenoyltrifluoroacetone (TTA) and the neutral phosphate ester TBP when mixed together in the absence of metal ions, but that there was a definite interaction in the mixed metal complexes.

If the acidic solvent is denoted by HX, where H is an acidic group, and the neutral solvent by B, then three mechanisms are possible for explaining

, this effect :

1) Addition mechanism The acidic solvent forms uncharged chelate whit the metal ion which is more easily extracted by the neutral solvent than other uncharged complexes of the metal ion. This can be represented as follows, for the case of uranyl ion:

U01+ + 4 HX + U02X2(HX)2 + 2 H+ U02X2(HX)2 + B + U02X2(HX)2 . B

The complex U02X2 . (HX), is more easily extracted by B than the neutral species, UO2(NO,), . The above equation takes in consideration the fact that the solvent HA exists as dimer in the organic phase. This mechanism is supported by Blake et al. (1958), Baes (1963), and Vdovenko and Krovo- khatskii (1960).

2) Substitution mechanism In this mechanism it is assumed that the neu- tral solvent liberates free molecules of the other solvent from the extracted species, thus:

UO$+ + 4 HX ---t U02X2(HX)2 + 2 H+

This liberated solvent now can extract more ions from solution. The mechan- ism is supported by Dryssen (1960), Kennedy (1958), and Deane (1960).

3) Solvation mechanism This mechanism takes into account the water molecules transported to the organic phase during extraction with the acidic solvent. It is assumed that the neutral solvent has the capacity to displace

Solvent extraction 363

this water, thus rendering the extracted species less hydrated, i.e., more readily extracted : I

, [UOlf (HzO),] + 4 HX + UO2(H20), X2(HX), + 2 Hf U02(H20),X2(HX)2 + JJ B + U02X,(HX)2 . JJ B + x Hz0

This mechanism is supported, by Irving (1965) and Healy (1961). The reverse of synergism, i.e., antisynergism, or antagonistic effect, takes

place under certain conditions. According 'to Ishihara and Owada (1966) the distribution ratio of uranium between tributyl phosphate and nitric acid decreased with the addition of benzoic, lauric, oleic, or stearic acids to the organic phase. This was due to a strong hydrogen bond 'formation between

- tributyl phosphate and the carboxylic acid as confirmed by infrared spectro- metry.

Deptula (1967) confirmed the expectation that an antisynergic effect takes place in the extraction of inorganic salts when an amine such as tri-n-octyl- amine is mixed with a phosphoric acid ester such as di-n-butyl phosphoric '

acid.

Third-phase formation /

Some amines, pa~ticularly the tertiary, when in contact with mineral acids form salts which are insoluble in the organic phase, thus a third-phase- liquid or solid-is formed during extraction. Third-phase formation also depends on the diluent. For example, in the extraction of HNO, by methyl dioctylamine, Choi and Tuck (1964) observed that a third phase was formed when the amine was diluted with hexane, pentane, isopentane, and cyclo- hexane, and no third phase was formed when the amine was diluted with benzene, nitrobenzene, CHCI,, and CCI,. The addition of a long chain alco- hol such as n-decanol overcomes this difficulty.

The amount of decanol required to prevent the third-phase formation can be determined in the following way: After equilibrating the organic phase with the aqueous phase and allowing it to settle, the two organic phases are separated and titrated with the alcohol till a single phase is formed. Usually about 3 % alcohol is sufficient to prevent third-phase formation.

T H E A Q U E O U S P H A S E Effect of metal ion concentration For the process

MY(,, + nX,,, z? MY . fix(,,

364 Principles of exlcacliue melallurgy

it was shown that D = K[XIn

where [XI is the concentration of free extractant at equilibrium. But m ] = [S] - [MY . 11x1

where [S] is the total concentration of extractant, and [MY nX] is the con- citntration of the extractant bound to the metal species. This latter concen- tration increases with increasing metal ion concentratioil in the aqueous phase. Therefore [XI and consequently D should decrease ,with increasing metal ion concentration in the aqueous phase. his has been verified experi- mentally (Table 1'7-13).

Table 17-13. Distribution of uranyl nitrate between 2M

HN03 and 40% vol. TBP in kerosene

(Goldschmidt et al., 1956).

Effect of foreign ions In ion-pair transfer, the addition of nonextractable electrolytes to. the aqueous phase greatly enhances the extraction of the metal ion in question. For example, the addition of ammonium, aluminum, or calcium nitrates to nitric acid solutions containing uranium greatly enhances the extraction of uranium by ether, as shown in Fig. 17-10. These electrolytes are called salt- ing-out agents, and they act mainly in suppressing the dissociation of the extractable species.

In extraction by an ion exchange mechanism, the addition of electrolytes has an opposite effect. Foreign ions'in solution will compete with the' ex- changeable ions of the extractant and therefore reduce the distribution ratio. For example, in the extraction of uranium from sulfate medium by tri- n-octylamine, D decreases with increasing concentration.6f foreign anions, as shown in Fig. 17-1 1 (anion exchange). In the extraction of uranium from

Soloenr exrraction

I I I I I 0 2 4 6 8 10

Concenfration of nitrate, M/ I

Fig. 17-10. Extraction of uranyl nitrate by ethyl etiler using various salting- ,

out agents [Furman el al. (1950)l.

0.1 0.2 0.3 toncentrotion ofodded anion, M / I

Fig.17-11. Effect of added anions on uranium extraction from sulfate solution by 0.1M Tri-11-octylamine-Amsco D-95; aqueous phase 1 M SO4,

pH I [Coleman el 01. (1958)l.

sulfate medium by octylphosphoric acid, D decreases with increasing con- centration of foreign cations, e.g., Fe3+ as shown in Fig. 17-12 (cation ex-

, change).

\

Principles of extractive metallurgy

0 1 2 3 4 lnitiol concentrotion o f ~ e ~ + i n aqueousphase,g/l

Fig.17-12. Effect of Fe3+ on the extraction of uranium from sulfate solution by OPA [Ellis et al. (1955)l.

Complex formation in the aqueous phase Highly charged metal ions are usually complexed in the aqueous phase:

Metal ion + Complexing agent + Metal complex If the complex is electrically neutral, then extraction byion-pair transfer is favored. If it is electrically charged (positive or negative), then extraction by ion exchange mechanism (cationic or anionic respectively) is favored. Complexing agents such as EDTA or.ammonium thiocyanate are often

0.1 - 0 5 10 15

HNO, concentration, M - Fig; 17-13. Extraction of uranium from H N O j solution by 19% TBP in

kerosene [Alcock et al. (1958)l.

Solvent extraction 367 .

added to the aqueous phase to facilitate extraction or separation of metal ions. For example, hafnium is extracted selectively by hexone from a hydro- chloric acid solution containing both zirconium and hafnium to which am- monium thiocyanate has been added. On the other hand, the presence of

- P043- ions in the aqueous phase hinders greatly the extraction of uranium by tributyl phosphate, due.to the strong complexing action of phosphate ion. Both processes are ion-pair transfer. In the k s t case, hafnium forms a stable, uncharged complex with ammonium thiocyanate, and is therefore easily ex- '

tracted, while in the second case, phosphate ion forms a charged complex which cannot be extracted by TBP. .

Figure 17-13 shows the effect of HNO, concentration on the extraction of uranium by TBP. It is seen that below 5M HN03, the extraction is favored due to the salting-out effect, but above 5M HNO, the value of D decreases for two reasons:

1) HNO, itself is also extracted, thus competing with metal ion transfer to the organic phase.

2) Great excess of nitrate concentrations promotes the formation of un- extractable anions such as [U02(N03),]-.

- -

0 2 4 6 8 Phosphoric acid Concentration, M/ I

Fig. 17-14. Effect of phosphoric acid concentration on the extraction of uranium by octyl pyrophosphoric acid mabashi (1960)l.

Principles of extractive metallurgy

100 . . , . -

80 . ,

60

LO

20

0.01 0.1 1.0 ' . 10.0 . Concentration of H3PO!, , M / I

Fig. 17-15. Uranium complexes in phosphoric acid solution [Thamer, J. Am. Cltem. Soc., 79, 4298-4305 (1957)l.

Acid .concentration in aqueous phase, M / I

Fig.17-16. Uranium extraction by 0.1M tri-n-octylamine in Amsco D-95 [Coleman (1958)l.

The extraction of uranium from H3P0, solutions by OPPA is shown in Fig.17-14, from which it can be seen that D decreases with increasing H3P04. The reason is that the process involves cation exchange, and, at high H3P04 concentrations, uranium is complexed in the aqueous phase as un- charged species, as shown in Fig. 17-15. These uncharged complexes cannot be extracted by a cationic reagent.

An example of extraction by anion exchange mechanism showing the effect of various acids is given in Fig. 17-16. Numerous data are now available in form of periodic tables showing the extraction of metal ions by different extractants (see, for example, Ishimori et al., 1963, 1964).

Solvent. ex fraction . . . 369. Effect of pH . Extraction by cationic 'solvents is greatly affected by the of the aqueous phase (Fig. 17-17), since H + ion's are taking part in the process:

MZ+ + H;x> MX + 2 H+ Decreasing the hydrogen ion concentration,.i.e., increasing-the pH shifts the equilibrium from left to right thus favoring the extraction.

D H

Fig. 17-17. Metal extractions from 0.5M sulfate solutions with 0.1M DZEHPA in kerosene Blake et al. (1958)l.

Fig.17;18: Effectof the pH on the extraction of uranium (V1)from sulfate solution by tri-n-octylamine [Coleman et al. (195811. .

24 Habashi, Metallurgy I1

370 Principles of: extractive metallurgy /

In many other cases the variation of distribution coefficient with pH may . be . . due . .. to the effect of the latter on the stability of . the. . . complexes in.the aqueous phase,, as. previously mentioned-for example, the extraction. of uranyl ion from sulfate solutions by amines (Fig. 17-18): ,

Effect of ion hydration It is reasonable to expect that strong hydration of an ion reduces its extrac- tability into an organic phase. This has been verified in a number of cases. For example, Fig. 17-19 shows the extraction of the alkali metals with 4-sec- butyl-2(a-methylbenzyl) phenol. The distribution coefficient decreases in the order Na < K < Rb < Cs, which is the reverse order of increasing ion hydration. Sodium ion, being the highest hydrated ion in this series, has the lowest distribution coefficient.

Fig. 17-19. Effect of ion hydration; extraction of the alkali metals by 4-sec-butyl-2'(a-methylbenzyl) phenol [Arnold et 01. (1 965)l.

Also, in the extraction of rare earths by tributyl phosphate and by ketones from nitric acid solution, the distributio? coefficient was found to increase with increasing atomic numbers (Mikhlin and Korpusov, 1967; Panasenko et al., 1966).

+.However, this assumption does not, appear to be valid in all cases. Thus McDowell and Coleman (1966) found that. the distribution coefficient foi the extractibn -of the alkaline. eaith nitrates by di-(2-ethylhexy1)phosphate decreases in the order Ba < Sr < Ca < Be. Beryllium, being the most hy- drated ion in this series, has, nevertheless, the highest distribution coeffi- cient. The reason is that the factors which determine selectivity in extraction are many and they may interact in a very complex way. The characteristics that lead to a strong hydration may also lead to strong coordination with an organic phase extractant, so that the net effect is difficult to predict. Thus for example, Mikhailichenko and Rozen (1967) found that the distribution coefficient in the extraction of the alkali metals by carboxylic acids goes through .a- maximum at potassium.

Extraction of inorganic acids

During the extraction of metal ions by organic solvents, it is found that most inorganic acids are extracted as well. Since metal ion extraction is usu- ally carried out from acid solution, it is important to consider in some detail the extraction of acids. Not only so, but there is much research going on at present to utilize this property to purifying acids by extraction with organic solvents. For example, phosphoric acid obtained from phosphate rock by leaching with H2S04 can be extracted by an organic solvent to get pure H3P04, suitable for purposes other than for making fertilizers.

Sulfuric acid is not extracted by ketones, ethers, or esters, but is extracted by alcohols. Hydrochloric acid is poorly extracted by ethers and only slightly extracted by alcohols, while nitric acid isextracted. Arnines and organic phos- phorus compounds also have the capacity to extract acids.

Table 17-14. Effect of diluents on' the extraction of in- organic acids by 0.1 M Alamine 336 [Agar et al. (1963)l.

% extraction into: Acid

Kerosene Benzene Chloroform

H N 0 3 90.0 . 97.6 99.0 HBr 82.8; 95.2 98.5 HCI 66.0 85.2 84.8 H2S04 51.1 : - - 86.8 . . 92.3 . . . ' .... H3P04 0.7 25.3 , 64.4 ' . '

372 ~rinc ip les qf extractiue metallurgy

, Concentrotion of uranyl nitrate . hexohydrate in organic phase, gl l

Extractionof icidsdepends also on the nature of the.diluent, asshown I in Table 17-14. It'is influenced too by the presence of extractable metilions

in the aqueous phase. Thus the distribution of nitric acid between an aqueous phase containing uranyl: ion and tributyl phosphate decreases with increasing uranyl ion-concentration, which is also extracted by TBP, as sliown in Fig. 17-20. Synergic effect was also observed when extracting an acid by mixed solvents, as shown in Fig. 17-21.

Fig. 17-20. Effect of uranyl ion on the extraction of nitric acid by TBP [Moore (1951)l; 0 2.5 M H N 0 3 ; A 5.0 M H N 0 3 .

. 5 .- 2 U D - . C

.: 0.05 3 9 . -

L -

VI .-

0

: . 1 . Mixed extractants I

I

o " " ~ \ ~ ,

-

0 0

1 1 O

Diethyl ether , . , Acetophenone loo ao s o 40. 20 o

I _ . . . 0 , 50 100 150

Fig. 17-21. Extraction of nitric acid by diethylether-acetophenone mixtures [Vdovenko and Kr@okha!skii.(l960)].

_.

i

Complexes are- also formed between the extractant and the acid in the extraction process,. For example, the extraction of nitric acid by tributyl phosphate can.bk represented by

: . '

. . HNO, ,,,. + TBP,,, =+ HNO, . % : TBP,,, i . . .. K = [HNO, ."BPI(,) ' . .

[HNO,I,a) ' [TBPI(o,

Plotting D. against.[TBP],,, gives a straight line, as shown in Fig. 17-22, thus

Concentration of T8P in organic phase,Oh

'Fig. 17-22. Effect of TBP concentration on the extraction of HN03 from . the aqueous phase [Moore (1951)l; 0 2.5 MHNO,; A 5.0 MHNO,.

- - .. . , .

. .

. .. . .

, .

. .

supporting the above mechanism. Table 17-15 gives selected data on the type of extracted species.

Extraction of water During extraction processes, water is usually coextracted. Figure 17-23 shows the water content of tri-butyl phosphate when contacted with differ- ent mineral acids. Water content in the organic phase is usually determined by titration with the Karl-Fischer reagent.

There appear to be no rules to predict the extractability of water.(Figs. 17-24 and 17-25). While water is coeitracted with uranyl perchlorate by tri- butyl phosphate, this is not the case with uranyl nitrate (Fig. 17-24).

374 Principles of extractive metallurgy Table 17-15. Species extracted by organic phase. .

I

Species System Reference

HCI-TBP HC1. TBP Hesford and McKay (1960) HN03-TBP HNOJ . TBP Hesford and McKay (1960) H2S04-TBP H2S04. TBP Hesford and McKay'(1960) HCI04-TBP HC104. TBP Hesford and McKay (1960) HF-TBP H F . TBP Baram and Laskorin (1964) 0 H3B03-TBP H3BO3 ' 2 TBP Korovin er at. (1966) H3P04-TOA H2P04. TOAH Smirnov and Gordov (1967)

HP04 . (TOAH)2 Smirnov and Gordov (1967) HN03-EHPA H N 0 3 . EHPA Zelikman and Nerezow (1967) HCI-EHPA HCI . 2 EHPA Zelikman and Nerezov (1967) HCI04-EHPA HC104. 2 EHPA Zelikman and Nerezov (1967)

I I I

I I I I I I 0 , 2 4 . 6 8

Acid concentrotion in organic phase,M/I Fig. 17-23. Coextraction of water by TBP from acid medium

[Hesford and McKay (1960)l. .I - .. , .

. .

. . .

" U , .

L L U O ~ [ N O ~ ) Z . . . .

. - .

, .

0 0.2 0.4 0.6 , 0.8 . . Uronium concentrotion in orgonicphase,~/l

Fig. 17-24. Water content of organic phase after extraction of uranyl nitrate and uranyl perchlorate solutions with tributyl phosphate [Hesford '

and McKay (1960)l.

I I I I I I I L 5 .6 7 a 9 10

Initial HNO3 concentration, MI1

30 I . I I I I I I

-

E z -..-

0 E w- 20 - "-3 0

c a

U c 0 F 0 - 0 - C 0, -

5 10 -

Fig. 17-25. Extiaction of water during the equilibration of HNO3 solutions with tri-n-octylamine [Verstegen (1964)).

" . L 0, -

5

E N G I N E E R I N G ASPECTS . .

C

0 No thor~umn~trate 8 Thorum n~trote present

percent extraction

If the original weight of the.solute in the aqueous phase is -w, and after ex- traction it is decreased to w , , then

,

o v - ~ 1 ) l V o D = ~ ~ J L I Va

where Vo is the volume of the ,organic phase and Va is that of the aqueous phase. From this equation it 'f6llows. tliit

376

Therefore

Principles of extractive metallurgy

W - W1 Percent extraction = x 100 W

- - x 1 0 0

D + Val Vo

Phase ratio The volume ratio of the two phase is called the phase ratio. A high aqueous/ organic ratio is sometimes undesirable because it may lead to high solvent losses. On the other hand, a high organic/aqueous ratio requires a large in- ventory of solvent, which may be a financial burden.

While the percent extraction is a function of the phase ratio, the distri- bution coefficient is not.

Separation factor When two metal ions are to be extracted from an aqueous solution. by an organic solvent, the separation factor of the two metal ions is defined by

B = &ID, where Dl and D, are the distribution ratios of the two metals. In order that separation is possible B must not equal 1.

Multiple extractions If Va = volume of aqueous phase containing w grams of the solute, Vo = volume of organic phase, and after the first extraction w, = weight of solute remaining in aqueous phase, then the equilibrium concentration in aqueous phase = IV,/V~, and the equilibrium concentration in organic phase = (w - w1)lVo: Therefore

D = (w - w11lVo w1lV.

Solvent extraction

Dlv, . Vo = V , I ~ - V,IV,

( V, + D Vo) nl, = &\v

r ~ f t e r the second extraction, w, = weight of solute remaining in aqueous phase.; the equilibrium concentration in aqueous phase = ' I ~ J , / v ~ ; and the

. .

equilibrium concentration in organic phase = (lo, - lo,)/ Vo . . , . .

. 8

After n extractions,

where \on = weight remaining in aqueous phase after n extractions, or

where C, = concentration after n extractions, Ci = initial concentration in aqueous phase.

To keep C,, to a minimum, i.e., for extracting the substance from the aqueous I ' phase .practically completely, it is .better to keep Vo small and increase 11, rather than the opposite.

.378 Principles of extractive metallurgy

Countercurrent extraction .. . Complete extraction of a solute in a single stage can be achieved only by using an infinitely large volume of solvent. Multistage extraction is therefore used, since it permits essentially complete recovery using a limited volume of the solvent. In the countercurrent process the aqueous feed and organic phase flow in opposite directions. Thus the fresh extractant contacts nearly barren raffinate, while the nearly saturated extractant contacts fresh aqueous feed.

The aqueous flow volume is designated by A, and the corresponding orga- nic flow volume by 0 (Fig. 17-26). The concentration of the extractable sp& cies are x 'and y in the, aqueous and ogranic phases, respectively. The mass balance over n stages is

I A

Fig. 17-26. Countercurrent extraction.

This is an equatiorof a straight line of slope, AIO. The composition of the organic phase leaving the first stage (y,) is a linear function of the com- position of the aqueous phase leaving the nth stage (x,). The values xo and y,+, , the composition of aqueous.and.organic phases before countercurrent contacts, are constants. This straight line is the operating line in the McCabe- Thiele diagram.

A McCabe-Thiele diagram is useful 'for estimating the number of theo- retical stages required to' obtain specified results in a solvent extraction sys- tem. The operating line is based on mass balance..Hence, the concentration of solute.in the aqueous feed entering any stake, and the organic-phase leav- ing any stage, are coordinates of points on the operating line. since the oper- 'ating line is straight, it is fixed by any two points. Alterliatively, it .can be established by only one point and the r'atibof aqueous to organic feed that determines the' slope of the line. . . - . . .

Soluent extraction. 379

In constructing the diagram, as illustrated in Fig. 17-27, theoretical or ideal stages are "stepped off" by extending a horizontal line from the upper extremity of the operating line to intersect the distribution isotherm, and then a vertical intercept to the operating line, arid so forth, until the other extremity of the operating line is intersected. ~ l t e r n a t i v e l ~ , a vertical inter-; cept can be made initially from the lower extremity of the operating line. Each cycle or "step" is termed an ideal.or theoretical stage. An ideal stage is a contact stage in which equilibrium between the two phases is achieved, and'hence it corresponds to a stage efficiencj of 100%. he intersection of each pair of vertical and horizontal lines with the operating line indicates the solute c0nten.t of the aqueous raffinate leaving an ideal stage and the organic feed entering that stage.

a VI' trotion in aqueous feed, o f a U . -

c trotion in extract, 0 P 0 C .-

C 0 .-

- 0 L -

of aqueous to orgonic flows 2 U

- . . .

0 a

I Ordinate: solute concentration in fresh organic feed ,y ,-,+I , I Equals zero forcompletely str~pped orgonlc

Concentration in oqueous phase Fig. 17-27. McCabe-Thiele diagram.

Equipment Only clear filtered solutions can be extracted by organic solvents, although considerable work has been done on extraction from slurries. Two types of equipment are in common use:

. .

1 ) Mixer settlers These are composed of a mixing chamber where-the . aqueous and organic phases are mixed together by a rotating impeller, and

380. Principles of extracthe metallurgy a settling chamber where the mixed phases are given enough time to sepa- rate (Fig. 17-28). Each such unit composes a stage. The apparatus is very efficient but its principal disadvantage is the large space required per stage.

2) Columrz extractors One of the simplest designs available of this type - is the rotating-disk contactor (Fig. 17-29). It consists of a vertical tower with

annular disks attached to the tower shell, and circular rotor disks attached

Pregnant o q u e x

Fig. 17-28. Mixer settler.

Retarfing shaft + f Orqonic

outlet

& Interface

Organic- inlet Grid

Aqueous outlet

Fig. 17-29. Column extractor.. '

Solvent extractiorz 381 . .

to an axial vertical shaft. Each rotor disk is spaced vertically midway be- twe'en adjacent stator disks. Rotation of ;he central shaft provides controlled dispersion of the-two phases. There no- settling chambers, 'and the two phases drift past one another in countercurrent flow;-separation takes place at the top of the tower.

Another type of c;lumn extractor is the pulse column. In this column the two phases are dispersed in each other by pulsating action through stationary perforated plates extending across the column. The pulsating action is sup- plied by leading a pulse leg from the-bottom of the column to the pulse gene- rator, which may by simply a reciprocating pump with check valves re- moved. The pulsating action forces the two phases through the holes in the plates, forming.bubbles. They rise (or fall) to the next plate, where they coalesce to await the next pulse. If the pulsing is vigorous, the bubbles never coalesce but are repeatedly forced through the holes in the various plates as they work their way up and down the column. The passage of a bubble through a hole in a plate deforms it considerably, and the internal agitation thus produced improves the extraction.

Economic aspects Clemmer el al. (1957) compared the solvent extraction of uranium with ion exchange (Table 17-16) and concluded that the former is more economical than the latter. Most uranium mills built in recent years have chosen the arnine extraction process. A report from South Africa by Carr and Lloyd (1963) was in favor of solvent extraction, and an estimate was given showing that when 1 Ib of uranium is recovered by ion exchange it costs 18.7 cents while by amine extraction it is only 5.2 cents. Agers and House (1965) esti- mate that uranium recovery by amines from a leach solution containing 1 g / l ~ ; 0 , would be about 4 centsllb of U308 including stripping and preci- pitation by ammonia.

Table 17-16. Solvent extraction of uranium versus ion exchange.

Ionexchange ~olven't extraction

Basis of comparison 1 cu ft 1 cu ft 0.1Mextractant Loading capacity 3-5 Ib U 3 0 8 '/4-'/2 Ib U3Os ~roduction capacity,

Ibs U308/day 2-3 4-8 Cost $45 $3-4

382 Principles of extractive metallurgy

Solvent extraction is used extensively in the following fields: - . 1) Recovery of a metal from a leach solution. 2) Separation of two or more closely related metals. 3) Purification, of a leach solution, i.e., removal of an unwanted impurity

such as iron.

Beryllium . . The extraction of beryllium with di-(2-eth~1hexyl)phosphoric acid ihd othe; . ~ . organophosphorus compounds fiom sulfGric acid solutions has been studied by a numberof investigators. A characteristic of the phosphoric acid extrac- tant is its-low beryllium'extraction rate; necessitating long contact times. De Bruin et al. (1962) studied the extraction of anionic beryllium complexes from organic salt solutions (oxalate, salicylate, etc.) with tri-iso-odtylamine. This amine, as wel1.a~ other tertiary, secondary, and quaternary ones, were found to be unable to extract b'eryllium from sulfuric acid leach solutions. Crouse et al. (1965) found, however, that primary' amines, especially 1-(3-ethylpenty1)-4-ethyloctylmine,* can extract beryllium rapidly from sul- . fate solutions.

With both amines and organophosphorus compounds, fluoride ion greatly decreases the extractability of beryllium. Iron is coextracted when present as Fe(II1) ion, but not as Fe(I1). Aluminum, the major impurity in leach solu- tions,.is also coextracted but to a lesser degree. To minimize the contamina- tion of,aluminum, several suggestions have been made:

1) Fluoride ion interferes with beryllium extraction from pure solutipns by primary amines, but its presence in solutions containing aluminum is desirable, since it complexes the aluminum in the aqueous phase and thereby . reduces aluminum competifion,for the amine extractant. To improve the puiity of beryllium, the addition of fluoride in amounts equivalent to the aluminum was therefore suggested by Crouse et al. (1965).

\ 2) Prior to the extraction of beryllium from sulfuric acidleach solutioll by organophosphorus compounds, i t was suggested by ~ r i n s t k d 2nd ~ u r l s (1967) to add ethylene diamine tetraacetic acid to complex all the'impurities.

* Previously identified as amine 21F81, now abbreviated HDA for heptadecylamine.

Solvent extractioti 383

3) In the extraction of beryllium from sulfuric acid solutions by organo- phosphorus compounds, it was suggested by Surls and Grinstead (1967) to strip the organic phase with.9 molar H2S04. Under these conditions, beryl- lium is precipitated as BeS0,-2H20 while aluminum remains in solution.

Boron Borax was formerly recovered from ~ear les Lake, California, by fractional crystallization. Recently, the American Potash and Chemical Corp. devel- oped a process for its recovery using solvent extrac'tion. At present 3000- 4000 gal/min are treated by this method in a full-scale plant.

The water from the lake contains 36% total salts (sodium, potassium, boron, chloride, and sulfate). Borax constitutes 8-10% of.this .total.Extrac- tion is carried out by polyols dissolved in kerosene. Boron is stripped fqom the organic phase by dilute sulfuric acid. The strip solution is passed over activated charcoal to remov,e.traces of entrained organic phase.,On evapora-, tion of the purified strip solution, boric acid crystallizes out and can be sepa- rated by centrifuga.tion and then dried to yield a product containing 99.9 % H~BO,, 0.05 % SO,, and 0.0029 % Na. .The mother liquor from the crystal- lization step is evaporated further to crystallize and recover mixed sulfates of sodium and potassium. , . .

Cesium A process based on the use of a substituted phenol was developed at. Oak Ridge National Laboratory for. the extraction of cesium from p.ollucite ore. The extractant is 4-se~-butyl-2-(~-methylbenzyl)phenol manufactured by the Dow Chemical Company. This extractant is,highly selective for cesium (See- Fig. 17-19). Since the. phenol can be used only in alkaline medium, the pollucite ore is opened by roasting with sodium'carbonate, and it is then leached with water. he process was originally developed for extracting radio-

. . . - . . - .

active cesium from reactor waste solutions.

Copper The recovery of copper from dump leaching solutions containing around 1 g/l Cu is usually achieved by displacing the copper ion by scrap iron-the cementation process. The cost of precipitation by this method is still high despite increased mechanization and better control techniques. Also, the process suffers'from the introduction of ferrous ion in the solutions which are to be recycled in the dumps. Precipitated copper still has to be processed

3 84 Pritlciples oflextractive metallurgy further to yield a marketable product. It is not a particularly attractive mate- rial for handling because of its fineness and the difficulty of washing it free from acid. Solvent extraction with subsequent hydrogen reduction under pressure or electrodeposition of a marketable product offers a convenient method of recovery. There is the added advantage that acid is liberated in the recovery step and can therefore be used as a stripping solution in the extraction process.

The most promising extractant is LIX-64 (see page 339). The process is at present in a commercial scale at Bluebird Ranchers Company in Miami, Arizona, and on pilot stage at Bagdad Copper Corp. and Duval Corp., both in Arizona. Present data available show that the recycle of raffinate contain- ing traces of the organic phase does not have a toxic effect on the bacteria accelerating the leaching process, and that the organic entrainment in the solutions going to electrolysis does not affect electrodeposition if proper handling of solution is taken into consideration.

Gold and Silver Gold and silver leached from their ores by cyanide solution exist as the anions [Au(CN),]- and [Ag(CN),]- respectively. Such anions were found

,

to'be extracted by amines, e.g., tri-octylamine.

Thorium Thorium is recovered on a commercial scale as a by-product of uranium ore processing. The process, based on extraction by an organic phosphorus com- pound, would not be economical for thorium recovery alone in.such a low concentration; but the operation is made feasible by the fact that the ore has ,

Table 17-17. Analysis of Elliot Lake ore and effluent from ion exchange columns after uranium recovery [Vermeulen (1966)l.

Ore, % Effluent, g/l v 0.11 v Th 0.028 Th Rare Earth Ox~des 0.057. Rare Earth Oxides S 3.28 Fe3+ Fe 3.25 Fez+ A1z03 6.2 Ti SiOz 80.3 SO:-

NO3

already been mined and leached by H2S04, and uranium is recovered by ion exchange. The effluent from the ion exchange columns is processed'for tho- rium recovery. Table 17-17 gives an analysis of the ore from Elliot Lake, Canada, and the analysis of the ion exchange effluent. .

Pure thorium is prepared by dissolving the thorium-rich precipitate, ob- tained from monazite sand leach solution, in nitric acid and then extracting t6e thorium with tributyl phosphate. The main impurities are the rare earths; these are left behind, since their distribution coefficient is'very low as com- pared to that of thorium.

T~~ngsletz The recovery of tungsten from wolframite concentrate by extracti,on with

'

ami~ies has recently been applied on a commercial scale. The concentrate is digested with NaOH solution, and filtered to remove the gangue; the leach solution is acidified with H2S04 to theproper pH; then tungsten is extracted with the amine. Stripping is affected by NH4Cl + NH40H solution to yield a solution of ammonium tungstate.

The main impurity in the leach solution is usually molybdenum and .it is coextracted with tungsten by amines and is also co-stripped by ammoniacal ammonium chloride. Removal of most of the molybdenum is affected by evaporating the strip solution to crystallize relatively pure ammonium tung- state.

The application of solvent extraction to the uranium industry is, of course, one of the most important fields in this technology. There is an extensive literature available on both the theoretical and the technical aspects. Table 17-18 summarizes the processes used.

Separation An important application of solvent extraction is its. use in separating a number of metals occurring together. This can usually be achieved by select- ing the proper pH, adding a complexing agent in the aqueous phase, and above all choosing a solvent that shows more selectivity for one metal than for the other. For example, sulfuric acid leach solutions of monazite. sand contain rare earths, thorium, and uranium. On examining the data shown in Table 17-19, it would be seen as possible to devise a method for separation based on extraction by amines. 2 5 Habashi, Metallurgy 11

Principles o f extractive metallurgy Table 17-18. Solvent extraction of uranium.

~ e d i u m Process Remarks

Sulfuric acid Dapex process 3-5% D2EHP in kerosene and about the same concentration TBP (synergic effect). Stripping by 10% Na2C0,. Uranium is recovered from strip solution by precipitation with NaOH-yel- low cake.

DDPA process 0.1 M solution of DDPA in kerosene. Stripping by ION HC1. Strip solution is evaporated to recover HCI, then uranium is precipitated by NH,OH-yel- low cake.

Amex process Secondary 'or tertiary lauryl amines are used as extractants. Stripping by dilute NaCl solu- tion.

Phosphoric acid OPPA in kerosene; stripping by HCI.

Nitric acid TBP'in kerosene; stripping by water. Pure uranyl nitrate solu- tion is obtained, which on eva- poration and decomposition , yields a high-grade UO, (orange cake).

. .

Used in U.S.A. by Climax Ura- ' nium at Grand Junction, Colo- rado, and by Kerr-McGee Ura- nium at Shiprock, New Mexico.

Molybdenunl is coextractedwith uranium but is not 'stripped from the amine during uranium stripping. Sodium carbonate or ammonia strips molybdenum and prevents its buildup in the organic phase.

This method was used in the., 1950's to recover uranium from wet-process phosphoric acid as a by-product of the phosphate fertilizer industry. Still of great potential importance. Phosphate rock contains about l50ppm uranium.

This process is mainly used to get a high-purity uranium suit- able for nuclear reactors: (1) Sodium or ammonium ura-

nate (yellow cake) obtained by processing the strip solu- tions of the above processes are dissolved In HN03, then extracted by TBP.

So1~;ent extraction

Table 17-18, (cont.)

Medium Process ~ e m a r k s '

(2) When uranium is recovered by ion exchange and the resin is eluted by nitrate-nitric acid solution, the eluate is treated with.lime to pH 3.5 to remove sulfate as gypsum and precipitate iron and tho- rium. ~ i a n i u m can then be recovered from the solution by TBP.

Another example of metal separation is given by the Eldorado Mining and Refining Company at Port Hope, Ontario. The process involves the treatment of a concentrate . . . consisting mainly of fine hydroxides of uranium, copper, cobalt, and nickel, andbised on separating the first three metals by. solvent extraction. The solids are leached with dilute H2S0,; uranium is first extracted by a tertiary amine, and then copper by a mixture of LIX 63

Table 17-19. Extraction of thorium, uranium, and rare earths by amines [Crouse and Brown (1959)l.

Thorium Uranium Rare earths

Primary amines Very strong Moderate Moderate

Secondary amines Straight chain or branching far from N-atom Strong Moderate Nil to weak branching near N-atom Weak Strong Nil

Tertiary arnines Nil Strong Nil

1, and DZEHPA. Finally, after pH adjustment, cobalt is extracted, while the nickel remaining in solution is recovered by precipitation (Fig. 17-30). The mixture of the two extractants has the advantage of extracting copper at a low pH at which no precipitation is liable to take place (Fig. 17-31).

Sulfuric ocid,solution containing U,Cu,Co,Ni, Fe,and Al. pH=2.5 . . I

3 % ~sodeconol ~ r o n i u r n extraction. L-+ u traces of Fe Aqueous phase: iu 6.4g, i ----- ------

C O 5.1 Ni 2.8 10010 LIX-63 -4; Copper extraction , 99% Cu recovery 5 Organic phase: 10% D 2 EHPA C U 6.5 g/L CO 0.017 pH adjusted to 4 - - - - - - - - - - - - -

Ni 0.014

10% L I X 63 90% recovery 10% D2 EHPA Cobalt extraction organic phase:

Co 3,58g/1 Ni 0.16

1 To waste

Fig. 17-30. Separation of U, Cu, Co and Ni from H2S04 leach solution [Joe et crl. ( 1 966)].

. . . . . . Equil ibr ium , pH, Fig. 17-31. Separation of copper from cobalt'by extraction with 5% LIX-63 + 10% D2EHPA in flash naphtha1 FN-140 at pH. 2 [,Joe et al. (1966)l.

Numerous schemes have been worked out to separate cobalt from nickel and an extensive literature is available. Figure 17-32 shows the use of octyl alcohol in separating the two metals from HCl solution. Separation can also be achieved by using tertiary amines, e.g., tri-isooctyl amine. This separation is based on the fact that cobalt forms an anionic complex, [COCI,]~-, while nickel does not.

Equilibrium concentration of H ~ I In aqueous phase

Fig. 17-32. Extraction of cobalt and nickel by octyl alcohol [Gindin er a/ . (1960)l.

Hafniuni-zircoriiun~

The advantages of hafnium-free zirconium for nuclear reactors have resulted in the development and application of solvent extraction for the separation of the two metals on a conlmercial scale. Hexone is used to extract hafnium and some zirconium from a hydrochloric acid solution containing both met- als, to which thiocyanic acid has been'added. The organic phase is then stripped with HCI to remove zirconium, and then with H,S04 to remove hafnium. Both metals are next recovered from their respective aqueous solu- tions by precipitation as hydroxides by ammonium hydroxide. The hydrox- ides are filtered, washed, and then ignited to the oxides.

390 Principles of extractive metallurgy

Separation of niobium and tantalum is achieved on a commercial scale by Wah Chang Corp. in Albany, Oregon. Columbite ore containing 58% Nb205 and 17 % Ta205 is digested with 70 % hydrofluoric acid, and the leach solution is then diluted and mixed with H2S04. The solvent used is hexone, and extraction is carried out in pulse-plate columns of all-poly- ethylene construction.

Both metals are extracted simultaneously, leaving the impurities behind. The organic phase is then stripped with demineralized water, whereby all the niobium and only a small amount of tantalum is transferred to the aqueous phase. Removal of this tantalum is achieved by further contacting the strip solution with fresh hexone. The organic phase, containing pure tantalum, is then stripped with water.

Niobium and tantalum hydroxides are precipitated from the respective raffinates by NH40H. Both hydroxides are filtered, washed, and calcined to the respective oxides. Niobium oxide so obtained contains less than 0.03 % Ta,O,, and the tantalum oxide also contains a similar amount of Nb205 (Carlson and Nielson, 1960).

A similar process is also in operation by Fansteel Metallurgical Corp. at Muskogee, Oklahoma (Soisson et al., 1961).

Plutonium-uranium-Jission products

Many methods have been suggisted'for the reprocessing of spent' nuclear fuel element by solvent extraction. Reprocessing has the following purposes:

. .

1) Recovery of plutonium which is produced in small amounts during uranium fission.

2) Removal of fission products which are produced during the fission pro- cess and tend toabsorb the neutrons thus hindering the chain reaction. '

3) Recovery of high-purity uranium suitable for recycle in a nuclear reac- tor.

. . .

Reprocessing by solvent extraction is based on dissolving the spent fuel element in nitric acid to get a solution containing-about 1 part uranium, 0.003 parts plutonium, and 0.01 parts fission products.

In the Purex Process, nitrous acid is added to the feed solution to reduce Pu(V1) to Pu(IV), which then together with UCVI) is extracted by TBP. The

Solvent extraction . - 391

-bulk of the fission products remain in the aqueous phase. The organic phase is then stripped in the following sequence: .

a) By HNO, to remove any fission products extracted. b) By HNO, + Fez+ to remove plutonium. Ferrous ion is added to re-

duce Pu(1V) to Pu(III), which has a very low affinity for the organic phase. c) By water to remove uranium. In the Redox Process, U(V1) and Pu(V1) are extracted by hexone, while

the bulk of the fission products remain in the aqueous phase. The organic phase is then stripped in the following sequence:

a) By aluminum nitrate solution to remove any fission products extracted. b) By HNO, + Fez+ -to remove plutonium. c) By dilute HNO, to remove uranium. In the TTA Process, sodium nitrite is added to the feed solution to reduce

plutonium to Pu(1V) which is then extracted by thenoyltrifluoroacetone in benzene. Zirco~iium is coextracted, but separation is readily achieved by selective stripping. Plutonium is stripped by an acid solution containing fer- rous ion to reduce Pu(1V) to the less extractable Pu(III), while zirconium is stripped by an oxalic acid-HNO, mixture. The raffinate containing uranium and the rare earths is then contacted with a TTA-hexone mixture to extract uranium selectively, leaLing the rare earths behind.

Small amounts of scandium are usually present in uranium ores and are dissolved during acid leaching, yielding up to 0.001 g/l ScZO3. Scandium was found to follow uranium in an organic phase composed of dodecyl phosphoric acid and kerosene. However, it is not stripped with uranium in 10M HCl. After several'contacts of the extractant, scandium builds up into the organic phase and can be stripped with HF. A fluoride precipitate analyzing approximately 10% Sc203 and 20% Tho, is obtained. This is then purified ,by digestion in 15% NaOH to convert it into hydrox- ides, and then digested with hydrochloric acid, the pH being adjusted to 4 .to remove Ti, Zr, Fe, and SiO,. Scandium is recovered from the solution by precipitation with oxalic acid. The oxalate is calcined and the oxide is further purified by dissolution in HC1 and solvent extraction; A description

of scandium recovery as a by-product from uranium leach solutions at Vitro's Salt Lake Plant was published by Lash and Ross (1961):

In Australia uranium is recovered from the leach solution by ion exchange. Scandium, yttrium, thorium, and the rare earths are not sorbed o n the'col- umn and are 'therefore found in the effluent. Canning (1961)-developed a par,ocess using dir(2-ethylhexy1)phosphoric acid to.extract scandium. - .

The association of vanadium with uranium in many uranium ores, and the ease with which a separation by solvent extraction could be accomplished, led to the second major commercial application of solvent extraction, for by-product vanadium recovery. Vanadium is extracted together with ura- nium by DDPA or HDPA. The addition of trialkylamine increases the ex- traction due to synergic effect. Vanadium is stripped first by 2M HCl, and then uranium is stripped by 1OM HCI.

The loaded extractant can also be stripped with Na,CO, solution to give a concentrated solution of uranium and vanadium, from which uranium can be precipitated by NaOH, leaving a solution containing sodium vanadate.