Extraction of Metal Salts by Mixtures of Water-Immiscible Amines and

Aharon M. Eyal, Leni Kogan, and Eyal Bressler. Ind. Eng. Chem. Res. , 1994, 33 (5), pp 1076–1085. DOI: 10.1021/ie00029a002. Publication Date: May 19...
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Ind. Eng. Chem. Res. 1994,33, 1076-1085

1076

Extraction of Metal Salts by Mixtures of Water-Immiscible Amines and Organic Acids (Acid-Base Couple Extractants). 2. Theoretical Treatment and Analysis Aharon M. Eyal' Casali Institute of Applied Chemistry, School of Applied Science and Technology, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel

Leni Kogan Israel Mining Industry, P.O. Box 10140, Haifa 26111, Israel

Eyal Bressler Department of Biological Chemistry, A . Silberman Institute of Life Science, The Hebrew University of Jersualem, 91904 Jerusalem, Israel

A comprehensive discussion is provided for the complex system obtained on equilibrating an aqueous solution of a salt with an acid-base couple (ABC) extractant. These extractants were analyzed as a combination of a liquid cation exchanger and a liquid anion exchanger operating concurrently. The effect of the amine on the organic acid activity and the effect of the acid on the amine activity were compared to those of mineral bases and acids on single active component extractants. The available distribution and spectroscopic data, summarized in the previous article, are discussed. 1. Introduction

A new family of extractants for metal salts was introduced by Grinstead et al. (1969). The extractants denoted here, acid-base couple (ABC) extractants, comprise a mixture of water-immiscible amines and organic acids in a diluent. ABC extractants are complex systems. They are neutral and extract neutral salts rather than just metal cations or their anionic complexes, while their active components are liquid ion exchangers. These active components are well-known and understood as single extractants, but provide novel properties and a variety of new mechanisms and organic-phase species when in a mixture. ABC extractants are less sensitive to acidity in the aqueous phase than liquid cation exchangers and have relatively high efficiency in extraction of alkali and alkaliearth halides. They provide for regeneration through backextraction with water rather than by using acids and bases and formation of salts to be disposed of. Properties of ABC extractants can be adjusted for acid extraction and therefore for the extraction of both acids and their salts. These properties make them highly suitable for applications such as recovery of MgCl2 from concentrated seawater, removal of FeC13 from AIC13 solutions and of CdC12 from ZnSO4, recovery of H2S04 and ZnSO4 from zinc electrowinning bleeds, and other hydrometallurgical and waste treatment processes. The previous article (Eyal et al., 1994)summarized the available distribution data, including effects of extracted salts and extractant characteristics, aqueous- and organicphase concentrations, acidity, and temperature on extraction efficiency and selectivity and on water coextraction. This article analyzes these data and discusses the main mechanisms and the resulting organic-phase species. Being limited to a single family of extracted salts, previous articles usually refer to only a part of the potential mechanisms. Thus, Grinstead et al. (1969; Davis and Grinstead, 19701,Hernandez and Martinez (1981),Hanson et al. (1974,1975),and Kholkin et al. (1982,19861, studying extraction of alkali and alkali-earth chlorides, refer to ABC extractants as salts performing double ion exchange with the extracted salt. However, for many of the ABC compositions studied (particularly for tertiary amine + 0888-5885/94/2633-1076$04.50/0

carboxylic acid) the salt RpNHkpA does not exist in the organic phase as indicated by the IR spectra. Sat0 et al. (1980, 1982), Shmuckler et al. (Hare1 et al., 1987, 1988; Kress et al., 1989, 1990; Cohn et al., 1992),and Gao et al. (1988), studying the extraction of transition metal salts, refer just to anionic complex formation with the anions of the extractant. The following discussion suggests a comprehensive picture through the analysis of the ABC extractants as a combination of a modified liquid anion exchanger and a modified liquid cation exchanger. Each active component is viewed as an independent extractant considering, however, the effect of the other on aqueous-phase acidbase properties and on the species in the organic phase. 2. General 2.1. Acid-Base Properties of Liquid Ion Exchangers and of ABC Extractants. Liquid anion exchangers are water immiscible amine salts binding anionic complexes from the aqueous phase through anion exchange.

0' - m)Rp",pY(o,)

+ [MXjlm-j(aq)=

(RpNH~p)j-m[MXjl(org) + 0' - m)Y(aq)- (1) similarly water-immiscible amines bind acidic molecules of the anionic complex

0'- m)Rp"S-p(org) + Hj-m[MXjl(aq)= (RpNH,p)j-,[MXjl(org) + 0'- m)HY(,,, (2) (For simplicity of presentation RpNHspwill denote waterimmiscible amines including those of different substitutions on the amine (RR'R'"), all the anions including that of the organic acid, A-, will be considered monovalent, and the cations will be Mm+.) Liquid cation exchangers are water-immiscible organic acids or their salts binding cations from the aqueous phase through cation exchange mHA(org)+ Mm+(aq)= MAm(org)+ mH+(aq)

(3)

mM'A(org) + Mm+(aq)= MAmforg)+ mM'+(aq) (4) Acid-base properties of the extractant and of its components are important parameters. Applications of PKa and PKb definitions for organic phases are complex 1994 American Chemical Society

Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994 1077 and do not lend themselves to direct measurement. Grinstead (1966,1968) suggested evaluating the basicity of an amine through the distribution of HC1 between an aqueous 1 M NaCl solution and an organic solution containing the amine. Higher pH values of the aqueous phase at equilibrium indicate higher extraction and hence more basic amines. If the amine/HCl equivalent ratio equals 2/1 the measured pH (denoted pH of half neutralization) resembles the apparent pKa of the amine (B) as

K.

BHCl,,,,

Bor, + H+,q

+ Cl-,,

(1)

y+ is the mean activity coefficient of aqueous HC1. Assuming that the concentration ratio [BI/[BHClI is equal to the ratio of activity coefficients in the organic phase and that [BHClI represents the total of amine hydrochloride in all species, at half neutralization.

pK, = -log [H+l - lOg(y$ (111) Through most of the measured range, the presence of 1 M NaCl does not affect pH measurement markedly and -log(y+)2 equals +0.24. Therefore, pK, = pH 0.24 (Note that in most previous articles pHo.5 was used for pH of half neutralization. However, in order to differentiate from pH of 50% extraction of metal ions, also denoted pH0.6, the term p H h was adopted.) p H b values of four widely used, commercially available amines were determined (0.5 mol/kg in low aromatics kerosene): Primene JMT (a long-chain primary amine, Rohm & Haas), 7.0; Amberlite LA-2 (ALA, a secondary amine, Rohm & Haas), 4.3; Alamine 336 (tricaprylylamine, TCA, Henkel), 3.5; and TEHA (tris(2-ethylhexyl)amine, Fluka), 1.0. These figures and those measured by Grinstead (1966) for many other amines present the general apparent basicity trend: primary amines > secondary amines > tertiary amines and linear chain amines > branched chain amines These trends were explained mainly by the capability of the amine salt (ion pair) formed to stabilize itself in an apolar medium by forming aggregates (Grinstead, 1968). We applied the same approach to evaluating the acidity of water-immiscible acids (HA, liquid cation exchangers, 0.5 mol/kg in low aromatics kerosene) through equilibration with aqueous 0.1 M NaOH + 1.0 M NaCl solutions at an HA/NaOH equivalent ratio of 2/1. pH'h values for lauric acid (LA, Fluka), a-bromolauric acid (ABL, Miles Yeda),bis(2-ethylhexyl)phosphate (DEHPA, Sigma),and dinonylnaphthalenesulfonicacid (DNNSA, King Industries) were 7.7, 5.6, 4.8, and 1.5, respectively. The trend observed is in agreement with expectations according to the water-soluble homologs of these acids. The acidic pH is due to ion exchange between the strong acids in the extractant and the NaCl in the aqueous solution. (5) HA(org)+ NaCl(aq)= N4org) + HC4aq) Applying the p H h method to ABC extractants resulted in most cases in the expected trend, an increase in the overall basicity of the extractant, with increased basicity of the amine, and decreased acidity of the organic acid (Bressler and Eyal, 1993a). p H h was found to be a useful tool for evaluating ABC extractant properties in mineral acid extraction (Eyal et al., 1990) and for adjusting their

+

composition for various feeds. We also noted some correlations between the p H h values of the extractants and the distribution of ZnSO4, CuSO4, and Fez(SO4)a at high aqueous-to organic-phase ratio (Bressler and Eyal, 1993b). The use of pHhn correlations as a basis for the analysis of extraction mechanisms may, however, be misleading. p H h measures apparent basicity. It depends, therefore, on anion properties such as steric hindrance for binding to the amine and for aggregation. Anion charge density is also of importancein determiningthe energy of hydration and solvation of the extracted ion. C1-, the anion of the acid generally used for p H h measurements, differs considerably from the anions of the water-immiscibleacid and from the large anionic complexes suggested for extraction by ABC extractants. The use of pHdextraction correlations as a general basis for extractant design is also questionable. As shown in the following section, both the distribution of the salt and the selectivity of the extraction may depend on aqueous/organicratio. Conclusions based on "limiting conditions" (high aqueous/organic ratio) are not necessarily applicable to industrial conditions. 2.2. Anion Exchange. Amines extract salts composed of transition metal cations and anions capable of forming anionic complexes with them, if these salts are acidic (differently put, if the amines are strong enough bases for M(OH), displacement from MXm).

i

-MXm(aq) + ti - m)H@ m

+ ti - m)RpNH>p(,,) -j-m

(Rp"kp),-m[MXjI(org)

+ ( y ) M ( O H ) m ( a q ) (6)

Extraction according to (6) is limited by the increasing basicity of the medium unless the amine is a much stronger base than M(OH), or unless M(0H)m is removed through precipitation or is neutralized. Neutral or basic salts are not extracted by amines. (Quaternary amines are an exception as they appear in ammonium form R4N+ even at basic conditions. If not indicated otherwise, the term amine will refer in the following to primary, secondary, or tertiary amines. Quaternary amine based extractants are discussed separately in section 2.5.) Amines extract anionic complexes of the cation if an acid of a similar anion (HX) or another acid (HY) is added to the aqueous solution of the neutral salt.

Aqueous-phase acidity decreases on metal extraction

1078 Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994

accordingto eqs 7-11. HX and HY may compete with the metal complex for RpNHg,. The effect of adding a water-immiscible acid, HA, to the extractant is similar to that of adding HX or HY to the aqueous phase, but not identical. Thus, in analogy to eqs 9-11, the species [MXmAj-Jm-j, [MKkAhlmk4, and [MA,+$' may form in the organic phase. In the case of extraction by ABC extractants (from neutral salt solutions), (RpNHkp)j-,[MXj1 does not form (unless Mm+is also transferred into the organic phase (see section 5.4). The reason is that unlike in (8),A- does not distribute into the aqueous phase. Extraction of [MXkAhlm-k-h(where k < m)or [MA,+il" accordingto eqs 10 and 11without subsequent extraction of displaced HX elevates aqueous-phaseacidity. The X/M ratio in the extractant is lower than the equivalent ratio, and acid is required for the stripping of the extracted salt and for the regeneration of the extractant. Reversible extraction (regeneration by back-wash with water) requires participation of equivalent amounts of amine and organic acid according to MXm(aq)+ ti - m)Rp"sp(org)

+ ti - m)HA(org)=

(RpNH,p)j-m[MXmA'-ml(org)

MXm(aq)+ hRp">p(org, (RpNH4-p)k+h-m

(12)

+ hHA(0rg)=

[MXkAh]

X- binding to RpNHkpmay differ considerably from that in the formation of anionic complexes containing X-. f. A- characteristics determine its capability to compete with the metal anionic complexes for RpNHp,. g. HA acidity affects the dependence of A-/HA on aqueous-phase acidity and thereby the availability of Aand HA for the various organic-phase species. h. The presence of HX or HY in the aqueous phase is expected to enhance extraction of anionic metal complexes by ABC extractants, as in the extraction by amines as single extractants, except if the mineral acids compete with the complex on the amine. Other solutes in the aqueousphase may also affect extraction if they are capable of interacting with Mm,X-, or the organic-phase species. 2.3. Cation Exchange. Extraction by ABC extractants of alkali and alkali-earth salts not capable of forming anionic complexes demonstrates the contribution of the cation-exchange mechanism. Water-immiscibleorganic acids, HA, exchange protons for cations from the aqueous phase.

(org) +

(m- k)Rp"kpX(org) (13)

MXm(aq) + (m+ 9RpNH~p(org) + (m+ i)MA(org)= (Rp"kp)i[MAm+il (org) + mRp"kpX(org)

(14)

The degree of extraction through the anion-exchange mechanism as well as the cation/cation and the anion/ anion selectivity is determined by the characteristics of the extracted salt and by extractant properties. a. Extraction according to (12)-(14) depends on the RpNHgp being strong enough to compete with the anions in the system for the proton to form RpNHkp+. HA acidity vs amine basicity determines RpNHtp+ activity and thereby the concentration of its pairs with metal anionic complexes. b. Acid-base properties, charge density, and steric hindrance of Mm+,X-, and A- determine the hydration energy of the first two species in the aqueous phase and the preference of formation of the various anionic complexes: [MXmAj-Jm-', [MXkAhlm-(k+h), [MAm+il', or [MXjIm-j, c. The medium affects the stability of the initial extractants vs that of amine-metal anionic complex ion pairs. In addition apolar diluents are expected to stabilize anionic complexes containing A- better than ones comprising X-. If exactrant loading is not high, solvation by free ABC components may stabilize the ion pairs formed on extraction, acting as a polar modifier of the diluent. In some systems stabilization through aggregation may take place. Amine and HA steric properties might, therefore, be of high importance, as indicated by the results of Sat0 and Yamamoto (1982) for NiClz extraction and those of Fleitlich et al. (1990) for FeCL extraction, where more hindered HA led to better extraction. d. Amine properties determine the selectivity to the various anionic complexes. e. X- properties affect anion/anion selectivity in the formation of RpNHkpX, the metal-free species formed according to eqs 13 and 14. The selectivity sequence in

The equilibrium in eq 3 indicates that the pH of the aqueous phase affects metal-ion extraction by cation exchange. Solvent extraction literature provides much data in the form of pHo.6-the pH at which half of the metal ions are extracted. HA characteristics are of high importance. Weaker acids are more sensitive to aqueousphase acidity. Thus for Fes+, Zn2+, Cd2+, and Cu2+ extraction by DEHPA pHo.6 Values of 1.5,2.1,2.6, and 2.9 were found, respectively, compared to 2.2, 5.5, 5.4, and 3.8, respectively, for extraction by naphthenic acid (Cox and Flett, 1983). PH0.6 is strongly dependent on cation properties. Alkali and alkali-earth extraction is very sensitive to proton existence (PH0.6 for CaZ+ extraction by DEHPA is about 6.5). As a result, commonly used liquid cation exchangers are inefficient in extraction of these cations, and neutral extractants such as crown ethers are preferred. The usual practice in extraction of metal cations by liquid cation exchangers is to shift the equilibrium in eq 3 by neutralizing the released protons through adding a base (B(0H)b) to the aqueous phase.

Metal recovery from the organic phase and extractant regeneration are achieved by back-extraction with an acidic medium.

(Y might be identical to X.) In a process comprising extraction and back-extraction accordingto (15) and (16), a base and an acid are consumed and a salt to be disposed of, BXb, is formed. Consumption of reagents and formation of BXb are avoided in extraction by ABC extractants. The effect of amine presence in the extractant is similar to that of B(OH)baddition, but not identical. The amine salt formed staysin the organic phase rather than in the aqueousphase, as in eq 15.

Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994 1079 basicity. Sensitivity to aqueous-phase acidity, however, exists due to The degree of extraction by ABC extractants acting as cation exchangers and the selectivity of the extraction are determined by A- concentration in the system, by Acharacteristics, by the species competing for it, namely, RpNHpp+,H+, and Mm+,and by HX activity: a. Stronger amines are more efficient in acidity reduction and hence provide for enhanced extraction. b. Weaker HA are more sensitive to acidity and thereby to the amine basicity. Extraction is dependent on R,,NHsp being a stronger base than A-. c. If proton activity is kept low by strongly basic amines the degree of extraction depends on A- efficiency in binding Mm+. Grinstead et al. (1969) claim that anions of weaker acids,showing stronger interactionswith protons, also form stronger interactions with cations. d. RPNHkp+that form strong interactions with A- may compete with Mm+and reduce its extraction. e. The selectivity of extraction depends on the interaction of Mm+with A- and is similar to that of extraction by HA as a single component. f. As in the case of the anion-exchange mechanism, the degree of extraction and its selectivity strongly depend on interactions with the medium, on aggregation, and on complexation with unused extractant components and with water. g. Metal-ion extraction by ABC extractants through the cation-exchange mechanism is dependent on Xcoextraction. The anionlanion selectivity is affected by the anion characteristics determining hydration energy in the aqueous phase, anion-RpNH4+ affinity, and ionpair solvation in the organic phase. It is expected, therefore, to resemble that of mineral acid extraction by amines. 2.4. Mixed Mechanisms. Viewing ABC extractants as a mixture of two single extractants, an anion exchanger (eqs 12-14) and a cation exchanger (eq 17), results in organic-phase species similar to those described in the previous literature. It provides, however, a better understanding of the effects of the active-component characteristics as well as the modification by the other. It also points out an additional mechanism in which the metal cation is extracted in both cation-exchange and anionexchange mechanisms, providing for MAmformation along with species from eqs 12-14. In addition, (RpNH6p)j-m[MXj] may form in extraction from neutral salts according to

2.5. Quaternary Amines Comprising ABC Extractants. Quaternary amine based ABC extractants, RdNA, form organic-phase species similar to those of extraction by RPNHsp + HA. The degree of extraction is, however, determined by the extractant being an ion pair R4N+A-. Hence, extraction in an anion exchange mechanism is not dependent on the low basicity of A- to form the cationic species. Differently put, the extractant acts as if the amine is a much stronger base than A- in the RpNHsp + HA combination. Similarly, the formation of A- in extraction according to cation exchange is not dependent on amine

3. Analysis of Distribution and Spectroscopic Data 3.1. SpeciesFormed in the OrganicPhase. Optional mechanisms for extraction by ABC extractants divide extracted mineral salts into three groups: 3.1.a. Alkali or alkali earth cations hardly form anionic complexes. Salts of these cations, and of others, not capable of forming anionic complexes with the anions present, are extracted by ABC extractants according to the mechanism described in section 2.3, cation exchange modified by coextraction of the released acid. Thus, on extraction of MgC12 by RpNHsp+ HA the organic-phase species formed are CaA2 and RpNHppCl as suggested by Grinstead and others. 3.1.b. Salts of transition metals may be extracted as those in 3.l.a. However, if the properties of the organicphase anion (A-) are suitable, the salts may also form anionic complexes, according to the mechanisms described in section 2.2. Species suggested there, [MX,Aj-,lm-j, [MXkAhlm-(k+h), and [MAm+i]', resemble those suggested by Sat0 et al. (1982): [ZnAal-, [CuAJ-, and [CdC12A212or by Kress et al. (1990) [CuC12A2I2-. As discussed above, properties of the extracted salt components as well as those of the extractant determine the species formed. Complexing properties of the cation should explain why, according to Sato, Zn2+,Mn2+,Co2+,Ni2+,and Cu2+ form [MA&, while Cd2+ extraction by the same extractant forms anionic complexes comprising both A- and C1- as anions. Extractant properties should explain why for quaternary amine carboxylates Sato found [CuA31", while for trioctylamine + 2-ethylhexanoic acid Kress et al. found [CuA2C12l2- (possibly due to steric hindrance in A- in the second case and lower A- activity due to weaker basicity of the amine there). 3.l.c. Salts of transition metals with anions capable of forming anionic complexes with them in the aqueous phase may be extracted as [MXj]"-' in addition to species in 3.1.a and 3.1.b above. Extraction of [MXjIm-j requires acid addition to the aqueous phase as in experiments performed by Belova et al. (1988) with PdCl2 and PtC4 or a mixed mechanism as discussed in section 2.4. In the case of [MXmAj-mlm-jas the sole anion in the organic phase (Kress et al., 1990) or of formation of an aggregate comprising cationic and anionic species in equivalent amounts (Hare1and Schmuckler, 1987and Liu et al., 1990 and their co-workers) there is only a single species in the organic phase. In most cases, however, a variety of species will form comprising RpNHpp+and Mm+. as cations and A-, X-, and various anionic complexes of Mm+as anions. Each speciesmay be solvated or complexed by the amine, the organic acid, or their combination (if extractant is only partially loaded), form aggregates, interact with polar and protic diluents, or bind water molecules (water coextraction). Aggregation and solvation depend on the properties of the system components and on those of the diluent. As in the well-studied case of acid extraction, polar and particularly protic diluents interact with the speciesformed, affecting distribution and selectivity.Thus, Grinstead et al. (1969) explain the enhancing effect of amyl acetate on NaCl extraction by solvation of the NaA formed. The coordination sphere of the sodium is only partly filled by the carboxylate anion, and amyl acetate

1080 Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994 Table 1. Selectivity Sequence in the Extraction of Alkali and Alkali-Earth Chlorides by ABC Extractant# extractant compn aq phase data selectivity sequence ref amine organic acid anion concn (M) remark MTCA EUDd c10-4.5 singlesalt Ca Mg K Na a MTCA/JMT Versatic/DEHPA c1mixture Ca Mg Na b JMT NAf c10-3 mixture Mg Li Na K C secondary 4 Grinetead et al. (1969).* Shibata et al. (1976). Hernandez and Martinez (1981).d Ethylundecanoic acid. e A mixture of C&l and tertiary aliphatic acids. f Naphthenic acid.

contributes to the stabilization through its nonbonding electrons. Chloroform also stabilizes NaA, however, being protic it competes with Na+ on binding to A-. The net enhancing effect on the extraction is limited. In the case of MgC12 extraction the enhancement by chloroform is even smaller as the coordination sphere of Mg is nearly filled by the two carboxylate groups in MgA2. Other species present in the system may also affect it if capable of forming complexes with the metal cation. Effects of such complexations were ignored by investigators adding ammonia or perchloric acid for adjustment of aqueous phase pH or adding TBP to the extractant as emulsion inhibitor. 3.2. Extraction Stoichiometry. Extracted salt to extractant equivalent ratio, extraction stoichiometry, may provide an indication of the extraction mechanism, particularly if saturation is observed in the distribution curve with increasing aqueous-phase concentration. Extraction of alkali-earth chlorides by ABC extractants comparising a strongly basic (or a quaternary) amine reached 1/1 stoichimetry, and in the case of MgC12, saturation is indicated in the distribution curve at this ratio (Grinstead et al., 1969). This stoichiometry agrees with the modified cation-exchange mechanism (eq 17), proposed for 3.1.a salts. Sat0 et al. (1982) use the 2/3 stoichiometry they found for salts relating to group 3.l.c as a support for the mechanism they propose. One should, however, note that equilibrium aqueous phases were dilute and such stoichiometry may result from partial extraction accordingto other mechanisms. Extraction of ZnSO4 by lauric acid (LA) based ABC extractants reaches a salt-to-extractant equivalent ratio of 1/2 in equilibrium with saturated aqueous solutions. Such stoichiometry agrees with the formation of Zn(HA)2A2. Lauric acid is a weak acid, and considerable HA activity is expected even in the presence of a strong amine. However, formation of Zn(HA)2A2,at least in the case of extraction by [MTCA, LA] (extractant composition is presented as in the previous article), should elevate aqueous-phase pH as ZnSO4 extraction should be accompanied by equivalent extraction of H2S04 to maintain the ionic balance. 4R4NA(,,

+ 2ZnS04,,, = Zn(HA)2A2(,rg,+ ~ ( R ~ N ) ~ S O+~ Zn(OH)2(,, (O@ (20)

Similar pH elevation is expected for other mechanisms suggesting HA participation in the cationic complexes (except if the amine is avery weak one). Such pH elevation was not observed for ZnSO4 extraction (Eyal et al., 1994). Extracted salt to extractant equivalent ratios smaller than 1 in extraction of 3.1.b salts from concentrated solutions may thus result from (I) incorporation of HA in a cationic complex bound to A- (elevationof aqueous phase pH); (11)incorporation of RpNHsp in such a complex in analogy to transition metal-ammonia complexes (pH reduction in the aqueous phase); (111) interactions of species containing X-or Mm+with RPNHkpA (unused

extractant molecules); (IV) formation of [MA,+J i- or [MXkAhl”(k+h)with h > m ;or (V)incomplete extraction due to strong interactions between RpNHspand HA that competewith the formationand the binding of components containing X- and Mm+(as in the extraction of Cu, Fe, and Zn sulfates by ABC extractants containing sulfonic acid, which was very low). Extracted salt to extractant equivalent ratios higher than 1were also found. Grinstead suggested the formation of MgACl in equilibrium with highly concentrated aqueous solutions. The above stoichiometric extraction may also be related to micelle formation (although most investigators agree that aggregation numbers are small). For salts of group 3.l.b, high stoichiometries may result from the formation of [MXkAhl”(k+h)where h < m. We have not found such high stoichiometries in extraction of salts from that group. Saturation in the distribution curve at stoichiometries approaching 1/0.5 were, however, found in the extraction of ZnClp, a number of the 3.l.c group. This stoichiometry indicates a strong contribution of the mixed mechanism (section 2.4):

Similarly,high stoichiometries are expected in equilibrium with concentrated solutions of other 3.l.c salts. 3.3. Extraction Selectivity, Amajor part of the data regarding extraction selectivity is based on distribution coefficients (or pHo.6 values) for salts in a single-salt solution. Some data are, however, extracted from multisalt systems and are therefore more reliable, at least as to the degree of selectivity. Salts of group 3.l.a are extracted according to the modified cation-exchange mechanism (eq 17), and selectivity is expected to follow the sequence of extraction by organic acids (liquid cation exchangers) as single extractants. Results by Grinstead (19691, Shibata (1976), Hernandez (1981), and their co-workers confirm this assumption (Table 1). Extraction of 3.l.b salts through the modified cationexchange mechanism follows the selectivity of cation exchange by HA as a single extractant. However, extraction of anionic complexesof Mm+, comprising A- as a ligand, may also contribute. Selectivity for that fraction follows the ability to form the anionic complexes and depends on A- properties. Available selectivity data (Table 2) is not sufficient to determine the contribution of eachmechanism to the extraction. In extraction of group 3.l.c salta, a great contribution of the mixed mechanism was indicated by the stoichiometry (section 3.2) and by the effect of aqueous-phase acidity (see section 3.5). At least two mechanisms determine, therefore, cation/cation selectivity (Table 2). One should note that salts of group 3.l.c may also form A- comprising species in the organic phase. Similar considerations apply to anion/anion selectivity. If cation exchange is the sole mechanism, X- forms ion

Ind. Eng. Chem. Res., Vol. 33, No. 5,1994 1081 Table 2. Selectivity Sequence in the Extraction of Transition Metal Salts by ABC Extractants extractant compn amine BOA 3,4-dimethylaniline MTCA MTCA MTCA/JMT MTCA TCA

organic acid DEHTP carboxylic LA DEHPA LlVABL carboxylic DEHPA

selectivity Sequence

anion Nos-

so4 c1-

Fe Hg

Cu Cu Cd

Zn Zn Cu

sodz

sodz c1c1-

Fe Fe Fe Zn

Cu Zn Cd Fe

Zn Cd Cd Zn

Zn Zn Cu Zn Cu Cu Cu

ref Ni Ni Co

Co Mn Ni

a

Co Mn Mn

b C

d e

Ni Co Co

Co Ni co Ni

Mn Mn

f f

8

h

Kholkin and Kuzmain, 1982. b Kalembkiewicz and Kopacz, 1988. Sat0 et al., 1980,1982. Eyal et al., 1990. e Bresaler et al., 1993b. f Cox and Flett, 1983.8 Sato et al., 1980b. Henkel, 1978. a

pairs with R,NHp,,+ and selectivity is expected to follow that of HX extraction by amines HI > HBr > “ 0 3 > HC1 > HzSO4 (HBr and HN03 may exchange places at lower aqueous-phase concentrations). Results by Grinstead et d. (1969) and by Hernandez and Martinez (1981) for alkali and alkali-earch salts agree with this selectivity sequence. A similar sequence is expected for extraction through anion exchange provided that [MAm+i]” is the main anion, as was found by Kholkin and Kuzmin (1982) for Ni salts. In the case of an important contribution by Mm+complexes comprisingX-, the complexing properties of X- affect selectivity. Hence, for extraction of transition metal salts, the NO3 > C1 preference in binding to R,,NH&,,+ is expected to be balanced by the C1- > Nospreference in anionic complex formation. The high selectivities of extraction by ABC extractants provide for extraction enhancement through the common ion effect as was found for MgClz extraction from concentrated seawater (Grinstead and Davis, 1970;Hanson et al., 1975; Shibata et al., 1976)and for ZnSO4 extraction from zinc electrowinning bleeds (Eyal et al., 1990a). 3.4. Acid Addition to the Aqueous Phase. In most cases of extraction through the anion-exchange mechanism, acid addition to the aqueous phase enhances extraction (see eqs 7-11). Extraction from acidic chloride solution may be viewed as binding of Hj-m[MXj] by the amine. The negative effect of H2SO4 presence on ZnSO4 extraction by 0.5 mol/kg TCA + 1.0 mol/kg DEHPA in kerosene (Eyal et al., 1994) shows the important contribution of the modified cation-exchange mechanism. The contribution of the anion-exchangemechanism in this case is low, as for zinc sulfate, the activity of [MXj]*j is negligible. The formation of Zn complexes comprisingAis hindered by low A- activity in the presence of HzS04 (HA is a much weaker acid than H2S04 and thus 2A- + 2HA + sod2-). The opposite effects of acid addition on the two extraction mechanisms is clearly seen in the results of FeC13 extraction by trialkylamine carboxylates (Fleitlich et al., 1992). Extraction decreases on adding up to 25 g/L HC1 to the aqueous phase and then increases linearly. FeC13 is extracted from neutral solutions mainly according to the cation-exchange mechanism (eq 17)and is negatively affected by acid addition. At higher acidities, however, the main contribution is of the anion-exchange mechanism, which is enhanced by acid addition. 3.5. Effects of Extractant Properties on AqueousPhase Acidity. ABC extractants are viewed as comparising a single active component for many mechanisms proposed. If that was the case, aqueous-phase acidity would not changeduring extraction. However, pH changes in the aqueous phase, as shown in the previous article, and their dependence on extractant properties support the

-

analysis in sections 2.2-2.4 extraction by ABC extractants is better understood as a combination of modified anion exchange and modified cation exchange. 3.5.1. ZnS04Extraction. DEHPA, a relatively strong organic acid, is mostly dissociated at the pH of aqueous ZnSO4 solutions and extracts Zn2+ efficiently. The extraction results in the elevation of aqueous-phaseacidity. Thus, equilibration of [DEHPA, 0.5 mol/kg, kerosene] with ZnSO4 solutions (organic/aqueous w/w ratio of 1/10) increased aqueous-phase acidity by 4 orders of magnitude. Equilibrium pH indicates proton release which is equivalent to Zn extraction. In the case of ABC extractants the amine present binds, partially or completely, the acid released on the cation exchange. JMT, being a strong base (pHh of 7.0), is very efficient in this acid binding. The acidity of a ZnSO4 solution equilibrated with [JMT, DEPHAI is 2 orders of magnitude lower than that of one equilibrated with [DEHPA], while extraction differs only by 1/3 order of magnitude. Quaternary amines in ABC extractants act as strong bases (seesection 2.5). TEHA (tris(2-ethylhexy1)amine), on the other hand, is a weaker base, p H b of 1.0, and thus does not reduce the acidity (Eyal et al., 1994). A similar behavior is shown by lauric acid and ABC extractants containing it. Acidity elevation by LA as a single active component is relatively small due to the low extraction by this extractant. Here the effect of the amine is even more pronounced: [JMT, LA] extracts about 2 orders of magnitude more than the acid by itself but keeps higher pH in the aqueous solution. Tricaprylylamine (TCA,Alamine 336) is a weaker base and thus has a weaker enhancing effect on pH elevation (Eyal et al., 1994). pH values for aqueous solutions in equilibrium with [MTCA, LA] extractants seem too low in consideration with the analysis in 2.5. The reason for that is probably an insufficient removal of HC1 in extractant preparation (see the Experimental Section of the previous article). The small amount of R4NC1+ HA left in it interacts with ZnSO4 according to 2R4NC1(0rg)

+ 2HA(org) + ZnS04(aq)

= (R4N)2S04(org) + ZnAz(org)+ 2HCl(aq) (22)

3.5.2. ZnClz Extraction. On equilibration of dilute ZnClz solutions with [TEHA, DEPHAI (a weak amine and a relatively strong acid) the dominating mechanism is the cation exchange and pH drops to about 1.5, similar to ZnS04 extraction. However, ZnClz belongs to group 3.l.c and acidity elevation combined with a higher ZnCl2 concentration (or in fact HzZnCL concentration) leads to extraction also through anion exchange and thus to pH elevation in concentrated aqueous solutions. Extraction by the quaternary amine containing ABC extractant

1082 Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994

[MTCA, DEHPA] proceeds according to section 2.5 and therefore does not affect the pH of the aqueous phase. The tertiary amine (TCA) and the primary amine (JMT) are of medium basicity and show an intermediate behavior. pH is reduced at low ZnCl2 concentrationsdue to extraction accordingto the cation-exchangemechanism. With acidity elevation and ZnCl2 concentration the anion-exchange mechanism is activated as with TEHA. pH elevation starts,however, at a lower ZnCl2 concentration than for TEHA due to the higher basicity of the amines. At high ZnC12concentrations extraction according to both mechanisms is nearly equal and the effect on aqueous-phase acidity diminishes (Eyal et al., 1994). 3.6. Effect of Extractant Composition on the Degree of the Extraction. ABC extractant’s acid/base properties determine the acidity changes in the aqueous phases (section 3.5) and thereby affect the degree of extraction (section 3.4). Extraction of salts from all three groups should improve with the enhancement of amine basicity. The anion-exchangemechanism resembles amine binding of an acid, the anion of which is the anionic complex of the metal ion. Extraction, therefore, improveswith the elevation of amine basicity. The effect is, however, relatively small for extraction by ABC extractants. That is because, while strong amines are capable of extraction even at low acidity, weak amine containing ABC extractants lead to relatively high acidity in the aqueous phase (see section 3.51, which in turn enhances the extraction. Extraction, according to the cation-exchange mechanism, is more sensitive to the amine basicity. This dependence is particularly strong for weak organic acidcontaining ABC extractants, as shown by Grinstead’s (1969) data for NaCl and MgClz distribution into [amine, ethylundecanoic acid] and by our data for ZnSO4 extraction. Lauric acid (LA) is a weak acid ( p H h of 7.7 (Eyal et al., 1994)). In equilibrium with ZnSO4 solutions, extraction of Zn is too low to be detected. The pH of the aqueous solution reduces to K3.5, a range where LA’S extraction efficiency is very low (pH0.5for Zn extraction by carboxylic acids is about 5.5). TEHA and TCA ( p H h values of 1.0 and 3.5, respectively) are too weak bases for an efficient extraction of the released H2S04. pH stays, therefore, in the range of LA inefficiency. JMT, on the other hand, is a strong base. With p H h = 7.0 it extracts H2S04 even from nearly neutral solutions. pH is maintained at 24.8, high enough to allow Zn extraction by LA. The quaternary amine (MTCA) also acts as a strong base because LA is nearly completely ionized in [MTCA, LA]. The cation-exchange mechanism is expected to be sensitive also to the acidity of the organic acid component of the ABC extractant (HA). Similar to extraction by organic acids as single active components, higher extraction is expected for stronger HA, in agreement with our results for ZnSO4 extraction: [amine, DEHPA] were better extractants than [amine, LA] for all four amines tested. The seemingly contradicting data by Grinstead et al., showing better extraction by ABC extractants comprising weaker HA (EUD > DEHPA > DNNSA), are explained by the amine they chose, MTCA. Being quaternary amine salts these ABC extractants extract MgC12 without affecting aqueous-phase acidity and are therefore not sensitive to the acidity of HA. This interpretation also provides the explanation for Shibata’s results for MgClz extractionshowing[MTCA, VA 9111 > [MTCA,DEHPAl, but [JMT, DEHPAI > [JMT, VA 9111 (Shibata et al., 1976).

On extraction through the anion-exchangemechanism, stronger HA led to higher acidity in the aqueous phase and thereby enhanced the extraction. Toostrong HA may, however, compete with the anionic complex on binding to the amine. This competition explains Belova’s data for HzPtCh and HzPdC4 extraction by [TOA, organic acid] extractants, showing decreasing extraction with increasing acidity of the organic acid in the extractant (caprylic > DEHPA > sulfonic acid) (Belova et al., 1988). This behavior is similar to that of mineral acid extraction by ABC extractants (Eyal and Baniel, 1982; Eyal et al., 1990b,c; Eyal, 1993). The effect of increasingthe amine to organicacid molar ratio is analogous to the effect of increasing the basicity of the amine and vice versa. Hence, higher amine proportion improved CdCl2 extraction (CdC12is a member of the 3.l.c group) (Kholkin, 1988~).Results by Watanabe (1970) and Liu (1990) seem to indicate that extraction by the acid or the amine as single extractants is higher than that by the ABC extractant. Some of our results seem to confirm these findings: distribution curves for ZnS04 extraction by [DEHPAI and [TEHA, DEHPAI are higher through most of the aqueous phase concentration range than those of [MTCA, DEHPAI, [JMT, DEPHAI, and [TCA, DEHPAI. These results are, however, misleading. As shown in section 3.5, extraction affects aqueous-phase acidity and thereby the distribution coefficients. The degree of extraction is therefore affected by the experimental method used. Hence, the distribution coefficient is dependent on the initial concentration and on the phase ratio rather than solely on aqueous phase salt concentration. The effect of the method used and the advantages of the ABC extractants are easily shown by comparing results of two aqueous/organicratios. In limiting conditions (high aqueous to organic phase ratio) [DEHPAI is a markedly more efficientextractant than ABC extractants comprising DEHPA through most of the ZnSO4 concentration range. However, extraction of less than 10% of the Zn from a 1 M solution by [DEHPAI reduces the pH to a range in which further extraction does not take place. (With LA, extraction is limited to less than 1% .) For comparison, contacting an aqueous solution comprisingZnSO4, MgSO4, and MnSO4 (in concentrations of 1.94,0.34, and 0.08 mol/ kg, respectively) with [MTCA, DEHPA, kerosene] in an organic to aqueous w/w ratio of 10/1resulted in extraction of about 80%of the ZnS04. Similarly,nearlyno extraction of alkali-earth chloridestakes place in extraction by liquid cation exchangers, but ABC extractants extracted most of the MgC12 from concentrated brines (Grinstead and Davis, 1970; Hanson et al., 1975). These results show that distribution coefficients are not sufficient for the study of extractant properties. Equilibrium pH (affected also by the degree of extraction and by the organic/aqueous phase ratio) should be followed for a full analysis of extraction efficiency and selectivity. The effect of the amine on the degree of extraction was shown by an experiment in 3- and 4-liquid-phase systems. A beaker was used in which a tube was kept vertically and concentrically so that both its ends were open (Figure 1). An amine or an organicacid in nitrobenzene was introduced first in an amount sufficient to cover the lower part of the inner tube. Then an aqueous solution of CuCl2 was introduced into the inner tube and water was introduced into the shell-side of this tube. The amounts of both aqueous phases were small enough to stay under the opening of the inner tube that avoids direct contact between them. The active component comprising a heavy

Ind. Eng. Chem. Res., Vol. 33, No. 5,1994 1083 -n

I

HA -> CUA, Figure 2. Four-liquid.phaPesystemsforthe extraction of CuCI, by anion exchanger. (A) liquid cation exchanger, (BI aqueous CuCIz solution, (CI aqueous receiver solution. (D)liquid anion exchanger. a liquid cation exchanger and a liquid

extraction by the latter follows CUCI,,,,

+ 2R,N,,,,

+ 2HA,,al =

(R3")2CuC~fiz,,, (23)

+ H2CuC14 2R,N -> (R,NH),CUCl, Figure 1. Three-liquid-phasesystems for the extraction of CuC12 by a liquid anion exchanger or by a liquid cation exchanger. (AI liquid anion or cation exchanger. (B)aqueous CuClz solution. (Cl aqueous receiver solution

organic phase, thus forms a liquid membrane between the aqueous phases. Prolonged experiments show only minimal transport due to the low distribution of Cu into the extractant. Thisexperiment was modified byaddinga fourth phase comprising the other active component (Figure 2). Thus, toasystemcomprising DEHPA in the heavy organicphase, CuCIz aqueous solution in the bore side of the tube, and water in its shell-side, a solution of an amine in kerosene wasadded sothat it covered bothaqueous phases. In this setup the two organic phases serve a liquid membranes between the aqueous phases and vice versa. The Cu" transport rate increased markedly. Thus, transport through the DEHPA-containing phase was enhanced by the presence of the amine, which binds the acid released even thoughthe twoactivecomponentswerenotindirect contact. 3.7. Spectroscopic Data. Kress et al. (1989) studied the extraction of CuCI2 (a 3.l.c Salt, by a weak acid, 2-ethylhexanoic (2 EHA), by trioctylamine (TOA) hydrochloride and by the ABC extractant comprising both [TOA, ZKHA, 0.5 M, toluene]. They claim that the

The Cu2+is extracted in the anion-exchange mechanism as an anionic complex comprising both the mineral and the organic anion as generally presented in eq 12. The contribution of the cation-exchange mechanism is negligahledue tothelowacidityoftheorganicacid in theAHC extractant (as in the case of ZnCI2 extraction by ITCA, LA]). Kresset al. support their mechanism byvisibleand IR spectra. They show that the absorption of CuC12 loaded ABC extractant is shifted from that of Cu2* loaded 2-ethylhexanoic acid (as a single active component in the extractant) 725 and 680 nm, respectively. The latter is ascribed to a dimeric structure with four hidentate carboxylate ligands and two carboxylic acid molecules completing the coordination of the metal to the pseudooctahedralconfiguration,C U Z A ~ H ATheshift )~. to 725 nm in the presence of the amine suggests that the amine replaces the carboxylic acid in the axial position. Thesharp and well-definedabsorption peak of [CuCLIz observed in thespectraofCu**loaded amine hydrochloride (asasingleextractant) ismissing from thespectraofCu2+ loaded ABC extractants. Kress et al. (1989) compare the IR spectra of the unloaded [TOA, 2-ethylhexanoic acid, 0.05 M,CC41 to that of the CuCI2 loaded extractant. Bands at 1420 and 1620 cm-I, added to the latter, were ascribed to the symmetric and antisymmetric carboxylate stretching vibration, respectively. Theabsenceofabsorptionat 2300cm-'isviewed bythe authors as a support to their conclusion that R3NHCI, a product of the cation-exchange mechanism (eq 17),does not exist in the system. They conclude that RlNH* is ion paired withthe largeanion [CuC12AJ-,causinga relatively

1084 Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994

weak H-bond with the anion. As a result the N-H stretching vibration is shifted to higher frequencies where it is obscured by the strong absorbance of the C-H vibration at 2900-3000 cm-l. Liu et al. (1990) studied IR spectra of Fe2(SO& loaded ABC extractants composed of RR’CHNH2 and DEHPA in octane in comparison with IR spectra of Fe3+ loaded single active component systems. Absorption bands at 1034 and 1198 cm-’ appearing in spectra of loaded ABC and loaded DEHPA, were assigned to P-0-C stretching and the PO0 asymmetric frequency, respectively. They indicate that Fe3+ is bonded to the phosphoryl oxygen atom of DEHPA in both systems. Bands a t 617 and 1109 cm-l were ascribed to sulfate groups acting as ligands. This analysisis in agreement with conclusionsof extraction by extractants comprising amines as single active components. Fe(II1) has a higher tendency to form S042containing anionic complexes than Zn2+or Cu2+,and Fezcan therefore be related to group 3.l.c. Sato and Yamamoto (1982) studied the far-IR spectra of quaternary amine carboxylate equilibrated with aqueous solutions of transition metal chlorides (group 3.l.c). They found that the M-C1 absorbance is missing from spectra of organic phases containing Ni2+,Co2+,Zn2+,Mn2+,and Cu2+ and concluded that the C1- in the organic phase is not coordinated with the extracted metal ion. This observation supports their assumption that these metal ions form complexes such as [CuA3]- as generallypresented in eq 14, rather than [CuC12A2J2-suggested by Kress et al. (1990). Spectra of CdC12 loaded ABC extractants, however, do reveal a Cd-Cl vibration at 225 cm-l indicating the formation of [CdC1zA2I2-, according to eq 12. The NMR spectrum of the unloaded extractant [TOA, ZEHA, 0.3 M, benzene] is not a superposition of the individual components and indicates acid-base interactions (Kress et al., 1989). Onlyone resonance peakappears in the range 9-11 ppm, a result of fast proton exchange between the carboxylic and ammonium sites. This peak shift from 11-12 in the unloaded extractant to 9.56 and 9.0 in ZnClz and CaC12 loaded extractants, respectively. According to Hare1 and Schmuckler (1987) the chemical shift indicates an increase in the concentration of the ammonium protons and a decrease in that of the carboxylic proton. Kress et al. (1989) ascribe the shift to a weakening of the H-bond between the anion and R3NH+. 4. Conclusions

The paper analyzed the distribution and spectroscopic data regarding the extraction of salts by the acid-base couple extractants. These data were found to be limited in the range of both the aqueous-phase parameters (characteristics of extracted cations and anions, concentrations, acidities) and the variety of extractants. Essential details were missing, particularly the mutual effects of extraction and aqueous phase acidity changes. A comprehensive analysis was therefore based on viewing the neutral extractant as a combination of a liquid cation exchanger and a liquid anion exchanger operating independently but affecting one another. The effects were found to be similar, but not identical, to those of adding mineral acids and bases to single active component extractants. The derived mechanism divided the extracted salts into three groups according to their ability to form anionic complexes and provided a better understanding of this complex system. The analysis is expected to contribute to both the theoretical study of ABC extractants through further experimentation and to their application in hydrometallurgical and waste treatment processes.

Acknowledgment The authors wish to thank Professor A. Baniel for the fruitful discussion and Mrs. B. Hazan for her technical assistance.

Nomenclature ABC: acid-base couple DEHPA: bis(2-ethylhexyl) phosphate DNNSA: dinonylnaphthalenesulfonic acid 2 EHA 2-ethylhexanoic acid EUD: ethylundecanoic acid JMT: Primene JMT, RR’R”C NH2 LA: lauric acid MTCA: methyltricaprylylamine NA naphthenic acid, a mixture of bicyclic, 5-membered ring, and aliphatic acids TBP: tributylphosphate TCA: tricaprylylamine TEHA tris(2-ethylhexy1)amine VA 911: versatic acid, a mixture of Cg-Cll secondary and tertiary aliphatic acids

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Hanson, C.; Hughes, M. A.; Murthy, S. L. N. Extraction of Magnesium Chloride from Brines Using Mixed Ionic Extractants. J. Znorg. Nucl. Chem. 1975, 37,191-198. Harel, G.; Schmuckler, G. Ion-Pair Extraction of Salts by Mixed Liquid Ion Exchangers. React. Polym. 1987,5, 203-208. Harel, G.;Kress, N.; Schmuckler, G. Extraction of Copper Salts with a Mixed Extractant. Proc. Znt. Solvent Extr. Conf. 1988,I,224225.

Hernandez, R. A.; Martinez, J. M. Extraction of Lithium and Magnesium Salts with Mixed Ionic Extractants. Znd. Eng. Chem. Process Des. Dev. 1981,20, 698-704. Kholkin, A. I.; Kuzmin, V. I. Binary Extraction. Russ. J. Znorg. Chem. 1982,27, 1169-1171. Kholkin, A. I.; Kuzmin, V. I.; Protasova, N. V. The Binary Extraction of Acids. Russ. J. Inorg. Chem. 1986,31, 708-710. Kress, N.; Harel, G.; Shmuckler, G. Extraction of Copper Salts with a Mixed Extractant. Solvent Extr. Zon Exch. 1989, 7,47-56. Kress, N.; Cohen, 0.; Schmuckler, G. Extraction of Copper Salts with a Mixed Extractant (11): Water Uptake and Osmometric Measurements. Solvent Extr. Zon Exch. 1990,8,447-490.

Liu, H.; Yu, S.; Chen, J. Studies on Mechanism of Extraction of Iron (111)from Sulfate Solutions and Stripping with Sulfuric Acid by Ft-IR and Laser Light Scattering Spectrometry. I1 HDEHP and RNH2 as Extractant with Octane as Diluent. Proc. Znt. Solvent Extr. Conf. 1990, A, 853-858. Sato, T.; Yamamoto,M. The Extraction ofBivalent Transition Metals from Aqueous Chloride Solutions by Long-chain Alkyl Quaternary Ammonium Carboxylates. Bull. Chem. SOC. Jpn. 1982,55,9094.

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Shibata, J.; Kawabata, H.; Nishimura, S. Extraction of Magnesium Chloride from Sea Water with Mixed Ionic Extractants. Nippon Kinzoku Gakkaishi 1976,40,412-418. Received for review June 23, 1993 Revised manuscript received December 6, 1993 Accepted January 4,1994. Abstract published in Advance ACS Abstracts, March 15, 1994.