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Solvent binary extraction
journal of MOLECULAR
LIQUIDS
E L S EV I E R Journal of Molecular Liquids 82 (1999) 131-146 S o l v e n t b i n a r y extraction A.I. Kholkin a, V.V. Belova a, G.L. Pashkov b , I.Yu. Fleitlikh b and V.V.Sergeev c ~N.S.Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 31 Leninskii pr., Moscow, 117907, Russia blnstitute of Chemistry and Chemical Technology, Siberian Branch of Russian Academy of Sciences, 42 Marx str., Krasnoyarsk, 660049, Russia c Institute "Hydrotsvetmet", 1 Zelenaya gorka, Novosibirsk, 630036, Russia
1. I N T R O D U C T I O N Processes of the extraction of acids, salts and metal hydroxides by salts of organic acids and organic bases (binary extmctants) relate to binary extraction. A study on these systems allowed to consider them a new class of extraction processes. Data for extraction of some compounds by salts of organic acids and organic bases were first reviewed by Devis and Grinstead. [1], Navtanovich and Heifetz [2], Sato et al. [3]. However, authors studied only several systems and didn't investigate general regularities of binary extraction processes and their practical possibilities. We have shown [4-13] that binary extractant systems are characterized by a number of properties which are similar to those of coordination extraction, and these systems have peculiarities due to properties of anion-exchange and cation-exchange extraetants. On the other hand, binary extractant systems have distinguishing features manifesting in extraction equilibria and suggesting new possibilities of process control. © 1999 Elsevier Science B.V. All rights reserved. 1.1. Theoretical considerations Binary extraction of mineral acids by organic salts of quaternary ammonium bases (QAB) can be described in general form as follows: mH+ta)" + Bm'-(a)+ mR4NAio) ¢:~ (R4N) m B )ot + mHA(o )
(1)
and by salts of amines as follows: + +B(a m )- +mR3NHA(o ) ¢~ (R3NH)mB(o) +mHA(o ) mH(a)
(2)
where HA and A" are organic acid and organic anion, respectively; B m" is anion of mineral acid; "a" and "o" denote aqueous and organic phase, respectively. Comparing this process with acid extraction by neutral extractant, for example, amines: mH~a) + B ~ a ; + m R 3 N ( o ) ¢~ ( R 3 N ) m H m B ( o )
(3)
we see that H m B molecules are distributed into the organic phase in both cases, that is, the acid distribution for two classes of the extraction processes is the same. However, different kinds of chemical reactions are the basis of the extraction processes. In binary extraction it is ion-exchange reaction, and in the extraction by neutral extractant it is an addition reaction. The expressions for the thermodynamic constants of binary extraction (Eq. 4) and extraction by neutral extractant (Eq. 5) can be written as follows: 0167-7322/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0167-7322(99) 00047-1
132
KH B m
.C m C(R4N)m B(o) HA(o) cm "a m R4NA(°) H(a) + .CB (a) m-
Y(R4N)m B(o) "~HA(o ) (4)
Y~R4NA(o) "¥B~f
C
(R3b0m HmB(o ) KH B = m m .a H(a) m+ .CB (a) mCR3N(°)
Y(R3N)m HmB(o ) (5) Y~R3N(o) .YB~a )
From Eqs. (1-5) it follows that anion-exchange extraction can be influenced by change of activity of hydrogen ions, and on the contrary hydrogen ion distribution can be affected by change of anion concentration in the aqueous phase. One can show that under the condition of the constant composition of the organic phase, that is, in the extraction of small amounts of acid, HmB , dependencies of B mdistribution coefficients on aqueous acidity are similar in binary extraction and extraction by neutral extractant. logD B = const- mpH (6) Data for the extraction ofhydrobromic acid dependence on pH of the aqueous phase in solutions of trioctylamine [2], di-2-ethylhexylphosphates of tdoctylamine [2] and tetraoctylammonium [3] are given in Fig. 1. One can see that for both types of the extractants the influence of aqueous acidity on anion extractivity is identical because the dependencies are linear and have the same slope. However, distribution coefficients for binary extractant systems are essentially lower than those for the initial system involving U'i-n-octylamine owing to thermodynamic stability of binary extractants in heterogeneous systems. log Dsr 1.5
log Da~"
1.0
1.5 i
I
1.0 J
0.5
I
I
2
-0.5
-I.0 i -1.5 z
2
\
i
pE
1 0.5 ]
o.o /
' 2
tga=-4 pH:
-0.5 j
3 -1.0
tgtx
1
r
Figure I. HBr extractionby neutraland binary Figure 2. H2SO4 (I),HCI (2) and H4Fc(CN)6 extractants:I- 0.1 M tdoctylarnine;2 - 0.1 M (3) extraction by 0.I M solution of trioctylamine di(2-ethylhexyl)phosphatc,0.3 tetraoctylammonium 4-tert-butylphenolatein M di(2-cthylhexyl)phosphoricacid; 3 - 0.I M toluene in the presence of 4-tert-butylphenol tetraoctylanunonium di(2-ethylhexyl)phos-(0.3 M (I; 2); 0.2 M (3)). Inital phate, 0.3 M di(2-ethylhexyl)phosphodcacid; concentrations, M: I (CI'); 0.01 (SO42-); Diluent - toluene. 0.0025 (Fe(CN)64").
133
Similar data for binary extraction of mineral acids of different basicity by solutions of tetraoctylammoniurn 4-tert-butylphenolate are shown in Fig. 2. The dependencies obtained are also linear with the slopes equal to the charges of anions extracted accordingly to Eq. (6). At stoichiometry of protons and anions of mineral acid in the aqueous (salt-out absence) and organic phases ) and constant activity (mCHA(o) = C(R4N)m B(o) coefficients of components in the organic phase, an isotherm of acid distribution accordingly to Eq. (4) is described by the equation: =l/m + 1. c,m/m + 1 •y + (7) CHmB(o) = l~Hm B -R4NA(o ) "CHmB(a) YR4NA(o ) whereK H B =KH B m m
:
)
(o)
o)
is the concentrationextractionconstant. When mineral acid is extractedby neutralextractant,we have: + + B~a)-mH(a) . . +PL(o) c:~ H rn B. PL(o)
(8)
The analogous expression for an isotherm of acid distribution can be written as: .rm+l .~_++1 (9) CHmB(o) = g.HrnB .CPL(o) -HmB(a ) Eqs. (7) and (9) are similar but have different exponents, (re+l), for factor, CHmB(a) • y + . If binary extraction isotherms are linear (C R4NA(o) = const), in the extraction by neutral extractants the dependencies are curvilinear and depend on the basicity of acids extracted. Fig. 3 shows these differences in the extraction of hydrochloric acid by binary extractant, tetraoctylammonium di(2-ethylhexyl)phosphate (a), and neutral extractant, tri-nbutylphosphate (b). With decreasing CHinain the aqueous phase, Da values decrease in the
CHCl o.10
a
0.05 i
0.00
aoo
134 extraction by neutral extractants (Fig 3, b). Moreover, in binary extraction an exchange reaction is occurred and, as a result, two extracted species are formed in the organic phase and the distribution of these is interdependent. The binary extraction constant can be expressed through constants of simpler processes. In this case, extraetivity of acids (salts and hydroxides in later section) depends on the physical-chemical properties of organic acid systems and systems involving mineral salts of organic bases studied in detail at present. Taking into account compounds, HmB , R4NAand (R4N)mB, which are completely dissociated in the aqueous phase, the binary extraction constant can be expressed as follows:
.KHAm
K(R4N) m B KHmB = m KR4NA
1 Km a
(10)
where K(R4N)m B, KR4NA and KHAare the distribution constants of electrolytes such as (R4N)m B, R4NA and neutral acid, K a is the acidic dissociation constant of the organic acid. For the extraction of acid by neutral extractant one can obtain: KH KH
m
B=
B.pL m .Km a KPL
(11)
where K L and KHmB.pL are the distribution constants of extractant and extracted species respectively, K B is the constant of the extractant basicity. From Eq. (11) it follows that the extraction power of neutral extractants in the extraction of the same acid, HmB, is mainly determined by the extractant basicity (K B), that is, the constant of the process: L + H + ¢:~ LH + (12) Obviously, the extraction of acids by binary extractants depends on the value o f l / K am (Eq. 10) i. e. is determined by the process: A - + H + ¢:~ HA
(13)
Experimetal data [5] on the extraction of mineral acids by salts of tetraoctylammonium and organic acids confirm this conclusion. The data for the binary extraction of hydrochloric acid are given in Fig. 4. As it follows from Fig. 4, the extraction power of the binary extractants increases in the series: dialkyldithiophosphate
135
K~A "KB_oH KHm B = Km
(14)
log Dcl-
log DB-
1.0 0.5
o.o ~6 ~
1.0 1 0.5
'4
,
-0.5
0.0 ,
-I.0 -1.5 -2.0
2
4
I
t
-0.5 4I -1.0 J
Figure 4. HCI extraction by 0.5 M solution of Figure 5. HI (1), I-IBr (2) and HC1 (3) salts of tetraoctylammonium and organic extraction by 0.5 M solution of tetraoctylacids in toluene, ammonium 4-tert-butylphenolate in toluene in 1 - di(2-ethylhexyl)dithiophosphate of QAB, the presence of 1.5 M 4-tert-butylphenol. 0.5 M HA; 2 - di(2-ethylhexyl)phosphate of QAB, 0.5 M HA; 3 - caprylate of QAB, 0.5 M HA; 4 - 4-tert-butylphenolate of QAB, 1.5 M HA. Eq. (14) shows that if using the same binary extractant (values of KHA , K a and K A - O H are the same), the mineral acid extractivity
( K H m B ) would be determined by
the KB-OI-I value. As an example, the data for the extraction of acids by tetraoctylarnmonium 4-tert-butylphenolate are given in Fig. 5. Fig. 5 shows that extractivity decreases in the series HI>HBr>HC1 that corresponds to the known series of extractivity for QAB salts. It should be noted that the distribution coefficients of acids are large in the alkaline region. Binary extractants based on alkylphenolates are very effective and can extract mineral acids from neutral and alkaline solutions, in contrast to known neutral extractants. Binary extraction of metal salts, MmB n, by R4NA binary extractants can be described as follows: mM n+ (a) + nB~a)'-..+ mnR4NA(o ) ¢~ n(R4N)mB(o) + mMAn(o)
(15)
In accordance with the kind of interphase distribution this process is analogous with the extraction of salts by neutral extractants as in the previous case. The binary extraction process differs from the extraction by neutral extractants in the kind of chemical reaction. In the binary extraction the chemical reaction is an ion-exchange, and in the coordination extraction it is an addition reaction. By the kind of the chemical reaction, the binary extraction process is identical to anion and cation- exchange processes. In accordance with Eq. (15), the binary extraction constant can be expressed as:
136 Cn
(R4N)m B(°)
.C m
MAn(°)
KMmB n =C m .C n .C mn n + B (a) mM(a) R4NA(°)
Y~(R4N)mB(°) "~mAn(°) ~M~a; "~B~a; "Y~R4
(16)
NA(o )
According to Eq. (I 6) at constant activity coefficients of components the logarithms of the distribution coefficients of cation and anion are related through a linear dependency with a slope corresponding to the ratio of cation and anion charges: log D Mn+ = const - n log D (17) m BmThe data on the distribution of Ni and Co nitrates in binary extraction by tetraoctylammonium di(2-ethylhexyl)dithiophosphate, which agree with Eq. (17), are given in Fig. 6. For binary extraction the salt-out effect can be used for both aqueous and organic phases, for example, by changing a correlation of extraction products, when organic metal salts or corresponding salts of organic acids are added into the system (see Eq. (16)). This is an important peculiarity of binary extraction. In order to estimate extraction pbwer of binary extractants, the binary extraction constant of salts may be expressed through the constants of simpler processes: mn m n KHA " K M - H " K B - o H (I8) KMmBn Kamn .KAmn_oH where KM.H is the constant of exchange of metal and hydrogen cations in organic acid system. Analysis of Eq. (18) shows that if using the same binary extractant and salts of different metals containing the same anion, the constant of salt extraction is determined by the constant of change, KM-H, that is, the order of extractivity well as the selectivity depend only on the nature of the organic acid. log DM2+
DM 30
0 -1
] /./-
1 /
20 /
3
210g DN~ i 1
Figure 6. Extraction .of nitrates of cobalt (I) and nickel (2)by 0.25 M solution of tetraoctylammonium di(2-ethylhexyl)dithioposphate in toluene. Initial concentrations, M: 0.02 (Ni2+); 0.018 (C02+).
I
2 CNor(,j,
Figure 7. Extraction of nitrates of Cu (1), Zn (2), Ni (3), Co (4) and Ca (5) by 0.2 M solution of tetmoctylammonium di(2ethylhexyl)dithioposphate in toluene. Initial concentrations, M: 0.05 (Cu2+, Ca2+); 0.03 (Zn20; 0.02 (Ni 2*, Co2~.
137
The data on the binary extraction of nitrates of different metals by tetraoctylammonittm di(2-ethylhexyl)dithiophosphate are shown in Fig. 7 [4]. The extractivity of salts decreases in the series Cu(NO3)2 >> Zn(NO3)2 > Ni(NO3)2 > Co(NO3)2 ~> Ca(NO3)2.Thus, the order of the extractivity corresponds to the data on extraction of metals by individual dialkyldithiophosphoric acids, On the other hand, in extraction of salts of the same metal with different anions, the extraetivity is determined by the KB-OH value. It was shown that the increase of nickel distribution coefficients with changing anions in the series NiSO4
(19)
with the equilibrium constant: n
CR4NOH(o ) CMAn(o) KM(OH)n
C n+ "an "Cn M(a) OH?a) R4NA(o)
~R4NOH(o )
"YMAn(o)
(20)
YM~a~ "~R4NA(o)
The KM(OH)nValue depends on the physico-chemical constants for initial extractants according to the equation: n
KHA "KM - H (21) KM(OH)n K n n a "KA-oH from which follows that the extractivity for the same binary extractant is determined by the value of the exchange constant, K M _ H" 2. EXPERIMENTAL AND RESULTS 2.1.Extraction of non-ferrous and associated metals Extraction of copper from leaching solutions of oxidised ores
For the extraction of copper from aqueous solutions we suggested use of a binary extractant such as dialkyldithiophosphate of tetraalkylammonium. It is known that initially dialkyldithiophosphorie acid is a very selective extractant for copper. However, due to the high stability of the salt extracted, copper stripping from the organic phase can't be practically realized. Unlike the cation exchange process, extraction by binary extractant, RnNA, is characterized by simultaneous extraction of copper cation and mineral acid anion into the organic phase [14-16]: 2+) + SO4(a) 2- +2R4NA(o) ¢:~ CuA2(o) +(R4N)2SO4(o) CU(a
(22)
Due to the high stability of CuA2, the extraction of copper sulphate is quantitatively realized from neutral and acidic solutions (upto 5 M H2SO4 ).
138 The presence of sulphate ion in the organic phase and the stability of the binary extractant in a two - phase system create conditions under which copper can be stripped by bonding copper cation in the aqueous phase, for example, using complex-formation with ammonia. Also as in the initial system containing dialkyldithiophosphoric acid, there is an oxidation-reduction equilibrium in the binary extractant system with Cu(II) being reduced by dialkyldithiophosphate ion with the formation of disulphide. Components obtained are labile enough and disulphide is reactive, therefore, Cu(I) is easily oxidized on the addition of disulphide into the system. Significant complications during copper stripping can arise in the case of the high concentration of silicon in the initial solution (above lg/1). In this case, on treating the extract by ammonia solution, gelatinous precipitates are formed at the interface of organic and aqueous phases. Since the decrease of silicon coextraction changes the extraction regime, and its removal from the extract by washing is not possible, combined sorption and extraction technology was suggested for the treatment of solutions with high concentration of silicon. To make efficient use of sorption and extraction method for copper extraction, leaching of copper-containing raw materials using vats is suggested. In this case, the leaching process is combined with the sorption process, that is, copper extraction is proceeded according to the sorption and the filtration free scheme that allows to simplify apparatus and decrease copper level in waste products. Relatively selective aminocarboxylic ampholytes based on polystyrene and divinylbenzene meet most requirements of the sorption and filtration free scheme of copper extraction from pulps. The sorption and filtration free technology in this case has a serious deficiency because it is necessary to use a neutralizer, for example, lime for correcting pH of the aqueous phase of pulp in the sorption process and iron hydrolysis. Copper sorption from acidic solutions (pH=l-l,5) is carded out using sulphocationite. The use of the sulphocationite doesn't allow one to separate Cu and Fe(IU) effectively and to obtain eluates with high copper concentration (eluation is realized by a solution of sulphuric acid). In this case, the separation of copper from iron can be obtained using binary extraction and following the concentration of copper into ammonia stripping solution. From ammoniacal raffinates copper can be obtained as a powder by reduction to metal using sulphur dioxide or hydrogen and also as complex Cu(NH3)4SO4 • H20 by blowing ammonia through raffinates. The purification of electrolytes from copper and iron A study on extraction of non-ferrous metals and iron by binary extractants from sulphate solutions showed the possibility of the effective separation of these metals due to the different thermodynamical stabilities of extracted compounds in the organic phase. The extraction of copper and iron by binary extractants based on QAB and trioctylamine in toluene from sulphate electrolytes of nickel and cobalt has been investigated. The most effective extraction of copper is occurred from the sulphate Nicontaining solutions by alkylcarboxylate and caprylate of triaikylmethylammonium (Table 1). The salts of trialkylmethylammonium and tetraoctylammonium can be used for the extraction of iron from sulphate electrolytes of nickel and, unlike the extraction of copper, the most high values of DFc are observed in the extraction by alkylphosphates (Table 1). The process of the extraction of copper and iron by organic salts of QAB can
139 be expressed in the form of Eqs. 22, 23 (without taking into account additional interactions in the organic phase): 3+ 2Fe(a ) + 3SO2~a) +6 RaNA(o ) =3 (RaN)2SO4(o) + 2FeA3(o) (23) Extraction of copper and iron is obtained due to the high concentration of sulphate ion in the aqueous phase. Stripping of copper and iron is accomplished by water or weak acidic solutions (for the preventation of metal hydrolysis) as the concentration of sulphate ions in the aqueous phase decreases. Indeed, Cu and Fe are effectively stripped by a 0.05 M solution of H2SO4 from the organic phases in the systems involving alkylcarboxylate and caprylate of trialkylmetylammonium in one step and the high values of Dc, and DFc are observed in the extraction by these extractants (Table 1). In comparison with the cation-exchange extraction, the consumption of alkali is eliminated and the consumption of acid is lower. Table 1 Extraction and stripping of Cu and Fe in the system nickel-electrolyte-salts of trialkylmethylammonium (stripping solution - 0.05 M H2SO4). )Hinit. = 0 . 9 0 ; CNi = 120 ~/1; Cc,,.Fc= 1 g/1 D Stripping, % Fc Extractant Cu Fe C©,M Cu Caprylate 71.1 0.53 3.94 0.45 73.9 90.7 1.63 1.0 0.38 97.0 0.426 0.25 32.6 36.4 1.64 0.15 0.002 75.5 Alkylcarboxylate 0,53 82.8 5.23 0.45 59.77 95.0 1.38 12.3 47.9 97.3 0.38 0.212 27.5 16.2 0.25 41.2 0.002 1.31 74.2 0.15 Alkylphosphate 0.45 0.009 73.5 94.4 5.4 0.25 5.90 28.2 0.15 1.22 53.9 .
°
The purificationof solutionsfrom iron
Iron (RI) in chloride solutionsforms stablecomplex anions FcCI4, therefore, saltsof amines and Q A B can bc used for the extractionof iron and the purificationof chloride solutions. The systems containing organic salts (phenolates and alkylcarboxylates) of amines and Q A B are characterized by smaUcr iron (Ill) distributioncoefficientscompared with the chlorides of these extractants[17]. Thc extractionequilibriain binary extractionof complex iron-containingacid from chloride solutionsby saltsof Q A B and trialkylamineare: H(a )+ + FeCl4(a) + R4NA(o ) c~ R4NFeCI4(o) + HA(o ) (24) H~a)_ + FeCl4(a) + R3NHA(o) ¢:> R3NHFeCI4(o) + HA(o)
(25)
In accordance with Eqs. (24, 25), extractionof iron is cnhanced with the increaseof mincral acid concentration in the aqueous phase and also with thc increaseof the
140 concentration of unextracted mineral salts due to the decrease of water activity and the enhancement of activity coefficients of HFeCI4 in the aqueous phase. During the contact of extracts with water, the stripping of iron depends on the stability of binary extractants. Using alkylphenolates of various structure as binary extractants, the most effective stripping of iron takes place in comparison with carboxylates of amine and QAB and the distribution coefficients decrease in the series: 2,6-tert-butylphenolate > 2cumylphenolate > 4-tert-butylphenolate; ~t-monoalkylcarboxylate > caprylate [30]. Such influence of the composition of the binary extractants on the value of DFc is obviously related to the value of acidic dissociation constant of the initial organic acids and also steric hindrances when forming binary extractants for o-substituted alkylphenolates and a-ramified monoalkylcarboxylic acids (ct-MACA). From Eqs. (24, 25) it follows that adding one of the extracted compounds into the organic phase causes a shift of the equilibria to the formation of binary extractant, that is, increase of organic acid concentration improves stripping of iron. One can see from Fig. 8 that the stripping of iron is significantly improved when the concentration of alkylphenole in the organic phase increases.
{ c~,~,), g/l 120
80
40
0
o..,
°'~CHA,M
Figure 8. Iron srripping in the system of 0.3 M solution oftrialkylamine in kerosene as a function of 4-octylphenol concentration in the organic phase. Cv¢o), g/l: 1.0 (1); 1.5 (2); 3.0 (3); 5.0 (4).
Purification of aluminium chloride solutions
One of the main problems of acidic processing of aluminiferrons raw material is the purification of aluminium chloride solutions from iron. The binary extractant systems are characterized by high distribution coefficients of iron (> 340) which slightly decrease for the systems with relatively low stability of these extractaats but in this case DF~ remains sufficiently high for conducting thorough purifications of solutions from iron. Since aluminium-containing chloride solutions atter leaching of aluminiferrons raw material have high temperature (> 90 °C), the extraction can take place using monocarboxylic acids as solvents (for binary extractants [30]). Using the isotherms obtained for iron extraction by 0.3 M solutions of trialkylamine ct-monoearboxylate in ct-MACA at 80 °C and by trialkylamine 4-isooctylphenolate in kerosene at 25 °C two steps of the extraction were found to be quite sufficient for the purification of aluminium chloride solutions from 12 g/l to 0.01 g/l Ire. At low temperatures (20-30 °C), a 0.3-0.5 M solution of triaikylamine p-alkylphenolate in kerosene was used as the
141 extraetant, whereas in the ease of higher temperature (45-80 °C), a 0.3-0.5 M solution of trialkylamine monocarboxylate in earboxylic acid was used. The results obtained in the stripping of iron by HCI solutions (10 g/l) to prevent iron hydrolysis in the aqueous phase at various temperatures for binary extractant systems showed that decrease of temperature leads to significant improvement of stripping. However, for decreasing energy consumption it is essential to conduct stripping at a temperature about 50 °C. The decrease of stability of R3NHA salts and also the use of greater concentration of monoearboxylic acid in the organic phase in comparison with a-rernified acids cause stripping improvement in the systems with monoearboxylic acids of normal structure. Under the optimal conditions iron stripping is occurred in 2-3 steps producing a sufficiently concentrated solution of iron (50-60 g/l). The purification of cobalt-containing solutions The purification of solutions containing cobalt was carried out at room temperature. As the extractant, 0.2 M solution of trialkylamine carboxylate in synthetic fatty acids of C7-C9 fraction was used [30]. The concentration of chloride ion in the aqueous phase was not less than 150 g/1. The character of extraction and stripping isotherms allows to conduct the process in a small number of steps. Iron was extracted practically completely in four steps of the extraction from solution containing, g/l: 120 - Co; 12.9 - Ni; 2.2 - Cu; 5.0 - Fe; 15 HC1. Residual concentration of iron in the raffinate was 0.002-0.05 g/l. Other elements presented in the aqueous solution were practically not extracted. The stripping of iron was conducted by a solution of HC1 (10-15 g/l) in three steps producing sufficiently concentrated stripped solutions of iron (C~eca)=45-50 g/l). 2.2.Extraction of rare metals Extraction of indium In the existing extraction schemes of processing In - containing solutions alkylphosphoric acids and particularly di(2-ethylhexyl)phosphoric acid are used as extractants. Concentrated solutions of HCI (9-11 M) are used for stripping of indium under industrial conditions. The use of binary extractants for indium extraction allows one to eliminate HCI at the stripping stage. The extraction of indium (III) from sulphuric solutions by tetraalkylammonium di(2-ethylhexyl)phosphate has been studied [10]. It was shown that depending on H2SO4 concentration in the aqueous phase a different mechanism of indium extraction is revealed - binary, cation-exchange or anion-exchange mechanisms. The stripping of indium can be proceeded by dilute solutions of H3PO4 or H2SO4 using a binary extractant. The effective binary extractant for indium extraction is also salt of trialkylamine and di(2-ethylhexyl)phosphoric acid. Isocarboxylie or synthetic fatty acids earl be used as solvents. Using binary extractant; the extraction of indium into the organic phase is 98.8-99.8 % and depends on concentrations of copper and iron in solutions. From the extraction isotherms obtained it follows that the theoretical number of extraction steps is equal to 2 for achieving residual concentration of indium in the solution of about 1-2 mg/1. The stripping of indium from the organic phase is proceeded by a mixture of phosphoric and sulphuric acids. Phosphoric acid has the maximum influence on the extent of the stripping. At concentration of 6 M H3PO4 the extent of stripping is 80 % in one step.
142 The full-scale experiments of developed technique were conducted. The initial solutions with the concentration of indium from 0.9 to 1.2 g/l were used. The concentration of indium in the raffinates was 10-12 mg/1 and in the sripped solution it was 28-38 g/1. The extraction of indium into the extract was 98.9-99.4 % and into the strip solution - 95.5 - 98.5 %. Extraction of cadmium
The extraction method is used at Ust-Kamenogorsk lead-zinc combine for the recovery of valuable components from solutions of the sulphatization. The extraction of In, T1, Cd allowed to increase the recovery of these metals from lead dusts. In further processing of solutions in lead production they undergo purification from copper and chloride ions on conducting a labour-consuming operation, namely, filtration and produce cakes difficult for utilization. The extraction method based on the application of trialkylamine monocarboxylate is suggested for a simultaneous extraction of cadmium and chloride ions [18-21 ]. Using this extraetanL cadmium and chloride ions are extracted as follows: 2+ + 4Cl~a) + (R3 NH)2 SO4(0) ¢:> (R3NH) 2 CdCI4(o) + SO2~a) Cd (a)
(26)
(R3NH)eCdCI4(o) +4HA(o ) +4OH~a) <=, 2R3NHA(o) +CdA2(o) +4CI('a) +4H20(a )
(27)
They are separated at the stage of the stripping of chloride ion and cadmium with extractant regeneration: 2+) (28) 2R3NHA(o) + CdA2(o) + 4H~'a)+ SO42~a)<::, (R3NH)2 SO4(o) + 4HA(o) + Cd(a The tests were conducted on the experimental installation of the combine using multistep extractors of the "mixture-sediment" type. A 30 % solution of trialkylamine in kerosene with an addition of ct,ct'-dialkylmonocarboxylic acid and 20 % 2ethylhexanole was used as an extractant. Alcohol was added to improve the phase exfoliation and increase solubility of mineral salts of trioctylamine in the organic phase. As stripping solution the aqueous solution of alkali (200 g/I) was used and the stripping was conducted at an equilibrated pH equal 7.8 - 8.2. During the stripping of chloride ions, the strip solutions were obtained with CI" concentration equal to I00 - 120 g/I and the concentrations of cadmium and zinc equal to 0. I and 0.05 g/I respectively. Thus, thorough separation of cadmium and chloride ions can be reached at the stage of chloride ion stripping. The stripping of. cadmium was conducted by a solution of sulphuric acid (500 g/I) and the residual concentration of H2SO4 in a strip solution was not more than 5 - I0 g/l. A regeneration of the sulphuric salt of tfialkylamine was realized at the stage of cadmium stripping. 2.3, Extraction of noble metals Salts of amines and QAB are understood to be effective extractants for metals of platinum group. The use of amines and QAB in the teehnologycal processes is limited up to now due to difficulties of platinum metal stripping from organic phases. In this connection, systems involving salts of amines, QAB and organic acids as extractants are of great interest [22-29]. In the extraction of H2PdCI4 by salts of dioctylamine, trioctylamine and organic acids, palladium was found to be extracted from 1-3 M HC1 solutions through the anion exchange mechanism. These processes are complicated by interactions of components in organic phase in the presence of organic acids forming compounds (R2NH2)2PdC14 and (R3NH)2PdC14 respectively. A spectrophotometric study showed that in palladium
143 extraction by eaprylate, alkylphenolate, alkylsalicylate of tetraoctylammonium monomeric or dimeric species of Pd (II) are formed in the organic phases depending on the excess of the extractant, aqueous acidity and also the nature of organic acid. When using dioctylamine alkylsalicylate, the extraction of H2PdCI4 decreases in the region of pH>2 according to the rules of mineral acid binary extraction. In the extraction of palladium by alkylphenolate and caprylate of dioctylamine the enhancement of Dpd values in the region of low aqueous acidity is accounted for by the introduction reactions occurring in the organic phase as in the initial system with dioctylamine when compounds (R2NH2)[Pd(R2NH)C13] and Pd(R2NH)2CI2 are formed. In the region of high pH of the aqueous phase, the extraction of palladium by various salts of trioctylarnine including caprylate decreases due to the processes of acid binary extraction and the shift of the extraction equilibrium to the left: 2H~a) + PdC12~a) + 2R3NHA(o) ¢:> (R3NH)2PdCI4(o) + 2HA(o)
(29)
For practical purposes binary extractants based on strong organic acids are of interest, since sripping processes are more effective in these systems. The extraction of palladium chlorocomplexes by salts of tetraoctylammonium and alkylsulphoacids of various structure which are related to the type of strong organic acids (pKa~2) has been studied as a function of HC1 concentration (Fig. 9). The data showed that the prevalent influence on the extractive power of binary extractants causes the structure of corresponding organic acids since they do not significantly differ in the values of acid dissociation constants. An increase of the number of substitutents and the lengthening of the chain of alkyl radicals improve the extractive power of binary extractants, obviously, due to steric hindrances in forming corresponding salts of RaNA. log Dpa log DrL 1,5 1
1.0 0.5
;-log Cn.t,~ -0.5 -
CHCI, M -2,0 J
Figure 9. Tetraoctylammonium salts: 6hexadecyloxynaphthalenesulphonate(1); 4hexadecyloxybenzenesulphonate (2); 2,5di-decyloxybenzenesulphonate (3); 2,3-dihexaoxybenzenesulphonate (4); 5-tetr-butyl-2-hexadecyloxybenzenesulphonate (5).
Figure I0. Platinum (IV) extraction by 0.025 M solutions of dioctylamine caprylatc (I), trioctylaminc caprylatc (2) and 0.004 M tetraoctylammonium caprylate in toluene as a function of aqueous acidity. Ca i,it=2 •10.3 M.
A study of a distributionof H2PtCI6 in the systems with various saltsof dioctylarnine, trioctylamineand tetraoctylammonium showed that in the region from I to 3 M HCI platinum is extracted according to the anion-exchange mechanism and in the region of low aqueous aciditythat is binary extraction mechanism of mineral acids. The extractivepower of binary extractants increases in the series: caprylate of dioctylarninc
144 In platinum extraction by various salts of tetraoctylammonium from solutions with pH=3-7 the following series was obtained: caprylate > p-tert-butylphenolate > alkylsalicylate > alkylphospate > alkylsulphonate of tetraoctylammonium. This series mainly corresponds to the increase of acid dissociation constants of corresponding organic acids with the exception of the alkylphenolate system. The relative decrease in the platinum distribution coefficients during the extraction by alkylphenolate of tetraoctylammonium is obviously accounted for by the formation of stable associated compounds such as R4NCI'nHA. The salts of QAB and strong organic acids (alkylphosphoric acids or alkylsulphoacids) were shown to be of the most interest for practical purposes fi'om the binary extractants used. In the extraction of chlorocomplexes of Pt (IV), Pd (IlI), Ir (IV) by 0.1 M solutions of trialkylbenzylammonium alkylsulphonate in toluene, the distribution coefficients of metals in the region of 0.5-4 M HC1 are varied in the interval of 5-150 while DM for Rh (III) and lr (HI) are not larger than 0.1, which creates possibilities for the platinum metal separation. The calculated coefficients of the platinum metal separation are 13Ptar~Iv)- 20; 13lr(lV)/Ru(Hl) -- 10; 131,qv~ ~ 50; [3VtWd~ 12; 13PtaraV) ~ 20; 13r,vau(nO~ 2"102; 13r~v,h~ 1'103; 15~ar(m)-- 2103. Table 2 Extraction and stripping of platinum (IV) by water in the system with trialkylber~ylammonium alkyls tlphonate in toluene Extraction, % Stripping, % Ce, M CPt(init.), g/1 CHCI,M 52.1 0.10 0.500 80.6 0.1 67.7 97.9 0.5 89.3 -100 1.3 96.9 ~100 2.3 ~I00 ~100 3.3 2.0
0.05 0.10 0.15 0.20 0.25
0.560
91.8 97.9 -100 ~100 ~100
~100 ~100 90.6 72.8 60.4
2.0
0.I0
0.197 0.394 0.780 1,18
~100 95.4 95.3 92.4 88.6
~100 ~100 90.6 72.8 60.4
1.93
Metals of the platinum group are easily stripped from the organic phase by water, which is pratically impossible in the initial systems with QAB mineral salts.The data in Table 2 show that using 0.1 M solutions of trialkylbenzylammonium alkylsulphonate platinum is effectively extracted into the organic p h ~ e with the subsequent stripping of Pt by water. Thus, using platinum extract with 1.6 g/1 concentration after 4 steps of stripping by water (Vo : Va = 1 : 1) a strip solution of platinum with 6.4 g/l concentration was obtained and platinum stripping was 93-100 % for each step (Ce = 0.1 M). The comparison of the binary extractant systems with tributylphosphate showed the advantage of binary extraction for obtaining more concentrated solutions after stripping of platinum metals.
145 CONCLUSION Extraction is widely used due to its advantage in comparison with precipitation and other methods, the potential possibilities of the extraction method are not exhausted. Establishment of basic properties of binary extraction of acids and metal salts, study of peculiarities of compound distribution in these systems and advantages of binary extraction in comparison with extraction processes using initial ion exchange and also neutral extractants, lead to further developments of the extraction method. Since in binary extractant systems the extracted compounds are formed similarly to the systems with initial ion exchange extractants, the use of these properties (for example, selectivity) with combination of new possibilities of distribution process is possible. For example, the increase of ion distribution coefficients results in salting out, and increase of counter ion concentration or on addition of salt containing the same counter ion which can form stable extracted compounds according to anion-exchange or cation-exchange series. Additional possibilities to control binary extraction processes are realized by changing concentration crrrelation of two extracted compounds in the system. Since interphase distributions of cations and anions in the binary extractant systems are interconnected, the use of combination of organic cations and anions with different properties in composition of binary extraetants opens various new possibilities. Thus, in the binary extractant systems the extraction of salts and different compounds that are poorly extracted by neutral extractants (for example, sulphates of non-ferrous metals, salts of alkaline metals, sulphuric acid, hydroxides of metals and so on) is possible. Binary extractants based on alkylphenolates can extract mineral acids from alkaline solutions. On the other hand, the stripping process is essentially facilitated due to formation of stable ionic pairs consisting of an organic cation and an organic anion in a heterogenious system, also during formation of such thermodynamically stable extracted compounds as dialkyldithiophosphate of copper, hexachloroplatinate and tetraehloropalladate of tetraoctylammonium and so on. In comparison with initial ion exchange systems the consumption of alkalies, mineral acids or complexes decreases and in some cases is eliminated. The use of binary extractants causes a decrease of solubility of extractant and extracted compounds in the aqueous phase in comparison with initial systems and also an increase of their solubility in the organic phase. We can assert that development of the binary extraction principle allowed to obtain a large quantity of new extractants with various properties varying their composition in combining various well-known cation exchange and anion exchange extractants including those produced on industrial scale. Extractive properties of binary extractants can be qualitatively predicted on the basis of initial organic acids and organic base salts well studied at present. In this paper we have shown only some developed methods of extraction and separation of metals with help of binary extraction in technological purposes. This principle of the extraction of salts, acids and hydroxides of metals can be used in different "liquid-liquid" systems, for example, in ionic flotation, membrane processes including liquid membranes for ion selective electrodes and also in processes of sorption, flotation of minerals and so on. REFERENCES 1. T.C. Davis and R.R. Grinstead, J. Phys. Chem., 74 (1970) 147. 2. M.L. Navtanovich and V.L. Heifetz, Zh. Obshch. Khim., 45 (1975) 413.
146 3. T. Sato, M. Yamamoto and H. Watanabe, Proc. Int. Solvent Extraction Conf., Belgium, Liege, Vol. 2 (1980). 4. A.I. Kholkin and V.I. Kuzmin, Zhur. neorgan, khimii, 27, (1982)2070. 5. A.I. Kholkin, V.I. Kuzmin and N.V. Protasova, Zhur. neorgan, khimii, 31, (1986) 1245. 6. A.I. Kholkin, V.I. Kuzmin, N.V. Protasova et. al., Proc. Int. Solvent Extraction Conf., Russia, Moscow, Vol. I, (1988) 170. 7. V.I. Kuzmin and A.I. Kholkin, Izv. SO AN SSSR. Ser. khim. nauk, 7, (1989) 3. 8. A.I. Kholkin and V.I. Kuzmin, Isotopenpraxis, 20, (1984) 339. 9. A.M. Rozen and A.I. Kholkin, DAN SSSR. Ser. khim. nauk, 301 (3), (1988) 661. 10. A.I. Kholkin, G.L. Pashkov, I.Yu. Fleitlikh and V.V. Belova, Hydrometallurgy, 36, (1994) 109. I1. G.D. Kholopova, N.V. Protasova, V.I.Kuzmin et. al., Zhur. neorgan, khimii, 28, (1983) 1286. 12. V.I. Kuzmin, N.V. Protasova, A.I. Kholkin et. al., Zhur. neorgan, khimii, 35, (1990) 1886. 13. A.I. Kholkin, V.I. Iqtmnin, G.L. Pashkov et. al., Proc. Int. Solvent Extraction Conf., Russia, Moscow, Vol. 3, (1988) 215. 14. I. Yu. Fleitlikh, K.S. Luboshnikova, A.I. Kholkin et. al., Proc. Int. Solvent Extraction Conf., Elsevier Sci. Publ. (1990) 1187. 15. A.I. Kholkin, V.V. Sergeev, G.L. Pashkov et. al., Zhvetn. met., 1, (1987) 19. 16. G.L. Pashkov, A.M. Kopanev, A.I. Kholkin et. al., Zhvetn. met., 6, (1989) 49. 17. A.I. Kholkin, I.Yu. Fleitlikh, K.S. Luboshnikova et. al., Proc. Int. Solvent Extraction Conf., Russia, Moscow, Vol. 3, (1988) 262. 18. A.I. Kholkin, G.L. Pashkov, V.N. Dokuchaev et.al., Proc. Int. Solvent Extraction Conf., Russia, Moscow, Vol. 4, (1988) 299. 19. G.L. Pashkov, A.I. Kholkin, V.V. Sergeev et. al., Zhvetn. met., 2, (1986) 29. 20. G.L. Pashkov, A.I. Kholkin, V.V. Sergeev et. al., Zhvetn. met., 3, (1986) 36. 21. A.I. Kholkin, I.Yu. Fleitlikh, G.L.Pashkov and V.V.Belova, Proc. Int. Solvent Extraction Conf., London. Elsevier Sci. Publ. (1993) 1679. 22. V.V. Belova, A.I.Kholkin, T.V. Khomchuk, Izv. SO AN SSSR. Ser. khim. nauk, 14 (1990) 134. 23. V.V. Belova, A.I.Kholkin, P. Muhl et. al., Zhur. neorgan, khimii., 36 (1991) 141. 24. V.V. Belova, T.I. Jidkova and A.I. Kholkin, Zhur. neorgan, khimii., 39, (1994) 1851. 25. V.V. Belova, A.I. Kholkin and P. Muhl, Proc. Int. Solvent Extraction Conf., Russia, Moscow, Vol. 3, (1988) 191. 26. V.V. Belova, T.I .Jidkova, A.I. Kholkin, Zhur. neorgan, khimii, 40, (1995) 2066. 27. V.V. Belova, T,I. Jidkova and A.I. Kholkin, Zhur. neorgan, khirnii, 39, (1994) 656. 28. V.V. Belova, A.I. Kholkin, T.I .Jidkova et. al., Zhvetn, met., 7, (1995) 6. 29. V.V. Belova and A.I.Kholkin, Solvent Extraction and Ion Exchange, 16 (1998) 1233. 30. I.Yu. Fleitlikh, A.I. Kholkin, G.L. Pashkov et. al., Zhvem. met., 3 (1995) 22.
LIQUIDS
E L S EV I E R Journal of Molecular Liquids 82 (1999) 131-146 S o l v e n t b i n a r y extraction A.I. Kholkin a, V.V. Belova a, G.L. Pashkov b , I.Yu. Fleitlikh b and V.V.Sergeev c ~N.S.Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 31 Leninskii pr., Moscow, 117907, Russia blnstitute of Chemistry and Chemical Technology, Siberian Branch of Russian Academy of Sciences, 42 Marx str., Krasnoyarsk, 660049, Russia c Institute "Hydrotsvetmet", 1 Zelenaya gorka, Novosibirsk, 630036, Russia
1. I N T R O D U C T I O N Processes of the extraction of acids, salts and metal hydroxides by salts of organic acids and organic bases (binary extmctants) relate to binary extraction. A study on these systems allowed to consider them a new class of extraction processes. Data for extraction of some compounds by salts of organic acids and organic bases were first reviewed by Devis and Grinstead. [1], Navtanovich and Heifetz [2], Sato et al. [3]. However, authors studied only several systems and didn't investigate general regularities of binary extraction processes and their practical possibilities. We have shown [4-13] that binary extractant systems are characterized by a number of properties which are similar to those of coordination extraction, and these systems have peculiarities due to properties of anion-exchange and cation-exchange extraetants. On the other hand, binary extractant systems have distinguishing features manifesting in extraction equilibria and suggesting new possibilities of process control. © 1999 Elsevier Science B.V. All rights reserved. 1.1. Theoretical considerations Binary extraction of mineral acids by organic salts of quaternary ammonium bases (QAB) can be described in general form as follows: mH+ta)" + Bm'-(a)+ mR4NAio) ¢:~ (R4N) m B )ot + mHA(o )
(1)
and by salts of amines as follows: + +B(a m )- +mR3NHA(o ) ¢~ (R3NH)mB(o) +mHA(o ) mH(a)
(2)
where HA and A" are organic acid and organic anion, respectively; B m" is anion of mineral acid; "a" and "o" denote aqueous and organic phase, respectively. Comparing this process with acid extraction by neutral extractant, for example, amines: mH~a) + B ~ a ; + m R 3 N ( o ) ¢~ ( R 3 N ) m H m B ( o )
(3)
we see that H m B molecules are distributed into the organic phase in both cases, that is, the acid distribution for two classes of the extraction processes is the same. However, different kinds of chemical reactions are the basis of the extraction processes. In binary extraction it is ion-exchange reaction, and in the extraction by neutral extractant it is an addition reaction. The expressions for the thermodynamic constants of binary extraction (Eq. 4) and extraction by neutral extractant (Eq. 5) can be written as follows: 0167-7322/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0167-7322(99) 00047-1
132
KH B m
.C m C(R4N)m B(o) HA(o) cm "a m R4NA(°) H(a) + .CB (a) m-
Y(R4N)m B(o) "~HA(o ) (4)
Y~R4NA(o) "¥B~f
C
(R3b0m HmB(o ) KH B = m m .a H(a) m+ .CB (a) mCR3N(°)
Y(R3N)m HmB(o ) (5) Y~R3N(o) .YB~a )
From Eqs. (1-5) it follows that anion-exchange extraction can be influenced by change of activity of hydrogen ions, and on the contrary hydrogen ion distribution can be affected by change of anion concentration in the aqueous phase. One can show that under the condition of the constant composition of the organic phase, that is, in the extraction of small amounts of acid, HmB , dependencies of B mdistribution coefficients on aqueous acidity are similar in binary extraction and extraction by neutral extractant. logD B = const- mpH (6) Data for the extraction ofhydrobromic acid dependence on pH of the aqueous phase in solutions of trioctylamine [2], di-2-ethylhexylphosphates of tdoctylamine [2] and tetraoctylammonium [3] are given in Fig. 1. One can see that for both types of the extractants the influence of aqueous acidity on anion extractivity is identical because the dependencies are linear and have the same slope. However, distribution coefficients for binary extractant systems are essentially lower than those for the initial system involving U'i-n-octylamine owing to thermodynamic stability of binary extractants in heterogeneous systems. log Dsr 1.5
log Da~"
1.0
1.5 i
I
1.0 J
0.5
I
I
2
-0.5
-I.0 i -1.5 z
2
\
i
pE
1 0.5 ]
o.o /
' 2
tga=-4 pH:
-0.5 j
3 -1.0
tgtx
1
r
Figure I. HBr extractionby neutraland binary Figure 2. H2SO4 (I),HCI (2) and H4Fc(CN)6 extractants:I- 0.1 M tdoctylarnine;2 - 0.1 M (3) extraction by 0.I M solution of trioctylamine di(2-ethylhexyl)phosphatc,0.3 tetraoctylammonium 4-tert-butylphenolatein M di(2-cthylhexyl)phosphoricacid; 3 - 0.I M toluene in the presence of 4-tert-butylphenol tetraoctylanunonium di(2-ethylhexyl)phos-(0.3 M (I; 2); 0.2 M (3)). Inital phate, 0.3 M di(2-ethylhexyl)phosphodcacid; concentrations, M: I (CI'); 0.01 (SO42-); Diluent - toluene. 0.0025 (Fe(CN)64").
133
Similar data for binary extraction of mineral acids of different basicity by solutions of tetraoctylammoniurn 4-tert-butylphenolate are shown in Fig. 2. The dependencies obtained are also linear with the slopes equal to the charges of anions extracted accordingly to Eq. (6). At stoichiometry of protons and anions of mineral acid in the aqueous (salt-out absence) and organic phases ) and constant activity (mCHA(o) = C(R4N)m B(o) coefficients of components in the organic phase, an isotherm of acid distribution accordingly to Eq. (4) is described by the equation: =l/m + 1. c,m/m + 1 •y + (7) CHmB(o) = l~Hm B -R4NA(o ) "CHmB(a) YR4NA(o ) whereK H B =KH B m m
:
)
(o)
o)
is the concentrationextractionconstant. When mineral acid is extractedby neutralextractant,we have: + + B~a)-mH(a) . . +PL(o) c:~ H rn B. PL(o)
(8)
The analogous expression for an isotherm of acid distribution can be written as: .rm+l .~_++1 (9) CHmB(o) = g.HrnB .CPL(o) -HmB(a ) Eqs. (7) and (9) are similar but have different exponents, (re+l), for factor, CHmB(a) • y + . If binary extraction isotherms are linear (C R4NA(o) = const), in the extraction by neutral extractants the dependencies are curvilinear and depend on the basicity of acids extracted. Fig. 3 shows these differences in the extraction of hydrochloric acid by binary extractant, tetraoctylammonium di(2-ethylhexyl)phosphate (a), and neutral extractant, tri-nbutylphosphate (b). With decreasing CHinain the aqueous phase, Da values decrease in the
CHCl o.10
a
0.05 i
0.00
aoo
134 extraction by neutral extractants (Fig 3, b). Moreover, in binary extraction an exchange reaction is occurred and, as a result, two extracted species are formed in the organic phase and the distribution of these is interdependent. The binary extraction constant can be expressed through constants of simpler processes. In this case, extraetivity of acids (salts and hydroxides in later section) depends on the physical-chemical properties of organic acid systems and systems involving mineral salts of organic bases studied in detail at present. Taking into account compounds, HmB , R4NAand (R4N)mB, which are completely dissociated in the aqueous phase, the binary extraction constant can be expressed as follows:
.KHAm
K(R4N) m B KHmB = m KR4NA
1 Km a
(10)
where K(R4N)m B, KR4NA and KHAare the distribution constants of electrolytes such as (R4N)m B, R4NA and neutral acid, K a is the acidic dissociation constant of the organic acid. For the extraction of acid by neutral extractant one can obtain: KH KH
m
B=
B.pL m .Km a KPL
(11)
where K L and KHmB.pL are the distribution constants of extractant and extracted species respectively, K B is the constant of the extractant basicity. From Eq. (11) it follows that the extraction power of neutral extractants in the extraction of the same acid, HmB, is mainly determined by the extractant basicity (K B), that is, the constant of the process: L + H + ¢:~ LH + (12) Obviously, the extraction of acids by binary extractants depends on the value o f l / K am (Eq. 10) i. e. is determined by the process: A - + H + ¢:~ HA
(13)
Experimetal data [5] on the extraction of mineral acids by salts of tetraoctylammonium and organic acids confirm this conclusion. The data for the binary extraction of hydrochloric acid are given in Fig. 4. As it follows from Fig. 4, the extraction power of the binary extractants increases in the series: dialkyldithiophosphate
135
K~A "KB_oH KHm B = Km
(14)
log Dcl-
log DB-
1.0 0.5
o.o ~6 ~
1.0 1 0.5
'4
,
-0.5
0.0 ,
-I.0 -1.5 -2.0
2
4
I
t
-0.5 4I -1.0 J
Figure 4. HCI extraction by 0.5 M solution of Figure 5. HI (1), I-IBr (2) and HC1 (3) salts of tetraoctylammonium and organic extraction by 0.5 M solution of tetraoctylacids in toluene, ammonium 4-tert-butylphenolate in toluene in 1 - di(2-ethylhexyl)dithiophosphate of QAB, the presence of 1.5 M 4-tert-butylphenol. 0.5 M HA; 2 - di(2-ethylhexyl)phosphate of QAB, 0.5 M HA; 3 - caprylate of QAB, 0.5 M HA; 4 - 4-tert-butylphenolate of QAB, 1.5 M HA. Eq. (14) shows that if using the same binary extractant (values of KHA , K a and K A - O H are the same), the mineral acid extractivity
( K H m B ) would be determined by
the KB-OI-I value. As an example, the data for the extraction of acids by tetraoctylarnmonium 4-tert-butylphenolate are given in Fig. 5. Fig. 5 shows that extractivity decreases in the series HI>HBr>HC1 that corresponds to the known series of extractivity for QAB salts. It should be noted that the distribution coefficients of acids are large in the alkaline region. Binary extractants based on alkylphenolates are very effective and can extract mineral acids from neutral and alkaline solutions, in contrast to known neutral extractants. Binary extraction of metal salts, MmB n, by R4NA binary extractants can be described as follows: mM n+ (a) + nB~a)'-..+ mnR4NA(o ) ¢~ n(R4N)mB(o) + mMAn(o)
(15)
In accordance with the kind of interphase distribution this process is analogous with the extraction of salts by neutral extractants as in the previous case. The binary extraction process differs from the extraction by neutral extractants in the kind of chemical reaction. In the binary extraction the chemical reaction is an ion-exchange, and in the coordination extraction it is an addition reaction. By the kind of the chemical reaction, the binary extraction process is identical to anion and cation- exchange processes. In accordance with Eq. (15), the binary extraction constant can be expressed as:
136 Cn
(R4N)m B(°)
.C m
MAn(°)
KMmB n =C m .C n .C mn n + B (a) mM(a) R4NA(°)
Y~(R4N)mB(°) "~mAn(°) ~M~a; "~B~a; "Y~R4
(16)
NA(o )
According to Eq. (I 6) at constant activity coefficients of components the logarithms of the distribution coefficients of cation and anion are related through a linear dependency with a slope corresponding to the ratio of cation and anion charges: log D Mn+ = const - n log D (17) m BmThe data on the distribution of Ni and Co nitrates in binary extraction by tetraoctylammonium di(2-ethylhexyl)dithiophosphate, which agree with Eq. (17), are given in Fig. 6. For binary extraction the salt-out effect can be used for both aqueous and organic phases, for example, by changing a correlation of extraction products, when organic metal salts or corresponding salts of organic acids are added into the system (see Eq. (16)). This is an important peculiarity of binary extraction. In order to estimate extraction pbwer of binary extractants, the binary extraction constant of salts may be expressed through the constants of simpler processes: mn m n KHA " K M - H " K B - o H (I8) KMmBn Kamn .KAmn_oH where KM.H is the constant of exchange of metal and hydrogen cations in organic acid system. Analysis of Eq. (18) shows that if using the same binary extractant and salts of different metals containing the same anion, the constant of salt extraction is determined by the constant of change, KM-H, that is, the order of extractivity well as the selectivity depend only on the nature of the organic acid. log DM2+
DM 30
0 -1
] /./-
1 /
20 /
3
210g DN~ i 1
Figure 6. Extraction .of nitrates of cobalt (I) and nickel (2)by 0.25 M solution of tetraoctylammonium di(2-ethylhexyl)dithioposphate in toluene. Initial concentrations, M: 0.02 (Ni2+); 0.018 (C02+).
I
2 CNor(,j,
Figure 7. Extraction of nitrates of Cu (1), Zn (2), Ni (3), Co (4) and Ca (5) by 0.2 M solution of tetmoctylammonium di(2ethylhexyl)dithioposphate in toluene. Initial concentrations, M: 0.05 (Cu2+, Ca2+); 0.03 (Zn20; 0.02 (Ni 2*, Co2~.
137
The data on the binary extraction of nitrates of different metals by tetraoctylammonittm di(2-ethylhexyl)dithiophosphate are shown in Fig. 7 [4]. The extractivity of salts decreases in the series Cu(NO3)2 >> Zn(NO3)2 > Ni(NO3)2 > Co(NO3)2 ~> Ca(NO3)2.Thus, the order of the extractivity corresponds to the data on extraction of metals by individual dialkyldithiophosphoric acids, On the other hand, in extraction of salts of the same metal with different anions, the extraetivity is determined by the KB-OH value. It was shown that the increase of nickel distribution coefficients with changing anions in the series NiSO4
(19)
with the equilibrium constant: n
CR4NOH(o ) CMAn(o) KM(OH)n
C n+ "an "Cn M(a) OH?a) R4NA(o)
~R4NOH(o )
"YMAn(o)
(20)
YM~a~ "~R4NA(o)
The KM(OH)nValue depends on the physico-chemical constants for initial extractants according to the equation: n
KHA "KM - H (21) KM(OH)n K n n a "KA-oH from which follows that the extractivity for the same binary extractant is determined by the value of the exchange constant, K M _ H" 2. EXPERIMENTAL AND RESULTS 2.1.Extraction of non-ferrous and associated metals Extraction of copper from leaching solutions of oxidised ores
For the extraction of copper from aqueous solutions we suggested use of a binary extractant such as dialkyldithiophosphate of tetraalkylammonium. It is known that initially dialkyldithiophosphorie acid is a very selective extractant for copper. However, due to the high stability of the salt extracted, copper stripping from the organic phase can't be practically realized. Unlike the cation exchange process, extraction by binary extractant, RnNA, is characterized by simultaneous extraction of copper cation and mineral acid anion into the organic phase [14-16]: 2+) + SO4(a) 2- +2R4NA(o) ¢:~ CuA2(o) +(R4N)2SO4(o) CU(a
(22)
Due to the high stability of CuA2, the extraction of copper sulphate is quantitatively realized from neutral and acidic solutions (upto 5 M H2SO4 ).
138 The presence of sulphate ion in the organic phase and the stability of the binary extractant in a two - phase system create conditions under which copper can be stripped by bonding copper cation in the aqueous phase, for example, using complex-formation with ammonia. Also as in the initial system containing dialkyldithiophosphoric acid, there is an oxidation-reduction equilibrium in the binary extractant system with Cu(II) being reduced by dialkyldithiophosphate ion with the formation of disulphide. Components obtained are labile enough and disulphide is reactive, therefore, Cu(I) is easily oxidized on the addition of disulphide into the system. Significant complications during copper stripping can arise in the case of the high concentration of silicon in the initial solution (above lg/1). In this case, on treating the extract by ammonia solution, gelatinous precipitates are formed at the interface of organic and aqueous phases. Since the decrease of silicon coextraction changes the extraction regime, and its removal from the extract by washing is not possible, combined sorption and extraction technology was suggested for the treatment of solutions with high concentration of silicon. To make efficient use of sorption and extraction method for copper extraction, leaching of copper-containing raw materials using vats is suggested. In this case, the leaching process is combined with the sorption process, that is, copper extraction is proceeded according to the sorption and the filtration free scheme that allows to simplify apparatus and decrease copper level in waste products. Relatively selective aminocarboxylic ampholytes based on polystyrene and divinylbenzene meet most requirements of the sorption and filtration free scheme of copper extraction from pulps. The sorption and filtration free technology in this case has a serious deficiency because it is necessary to use a neutralizer, for example, lime for correcting pH of the aqueous phase of pulp in the sorption process and iron hydrolysis. Copper sorption from acidic solutions (pH=l-l,5) is carded out using sulphocationite. The use of the sulphocationite doesn't allow one to separate Cu and Fe(IU) effectively and to obtain eluates with high copper concentration (eluation is realized by a solution of sulphuric acid). In this case, the separation of copper from iron can be obtained using binary extraction and following the concentration of copper into ammonia stripping solution. From ammoniacal raffinates copper can be obtained as a powder by reduction to metal using sulphur dioxide or hydrogen and also as complex Cu(NH3)4SO4 • H20 by blowing ammonia through raffinates. The purification of electrolytes from copper and iron A study on extraction of non-ferrous metals and iron by binary extractants from sulphate solutions showed the possibility of the effective separation of these metals due to the different thermodynamical stabilities of extracted compounds in the organic phase. The extraction of copper and iron by binary extractants based on QAB and trioctylamine in toluene from sulphate electrolytes of nickel and cobalt has been investigated. The most effective extraction of copper is occurred from the sulphate Nicontaining solutions by alkylcarboxylate and caprylate of triaikylmethylammonium (Table 1). The salts of trialkylmethylammonium and tetraoctylammonium can be used for the extraction of iron from sulphate electrolytes of nickel and, unlike the extraction of copper, the most high values of DFc are observed in the extraction by alkylphosphates (Table 1). The process of the extraction of copper and iron by organic salts of QAB can
139 be expressed in the form of Eqs. 22, 23 (without taking into account additional interactions in the organic phase): 3+ 2Fe(a ) + 3SO2~a) +6 RaNA(o ) =3 (RaN)2SO4(o) + 2FeA3(o) (23) Extraction of copper and iron is obtained due to the high concentration of sulphate ion in the aqueous phase. Stripping of copper and iron is accomplished by water or weak acidic solutions (for the preventation of metal hydrolysis) as the concentration of sulphate ions in the aqueous phase decreases. Indeed, Cu and Fe are effectively stripped by a 0.05 M solution of H2SO4 from the organic phases in the systems involving alkylcarboxylate and caprylate of trialkylmetylammonium in one step and the high values of Dc, and DFc are observed in the extraction by these extractants (Table 1). In comparison with the cation-exchange extraction, the consumption of alkali is eliminated and the consumption of acid is lower. Table 1 Extraction and stripping of Cu and Fe in the system nickel-electrolyte-salts of trialkylmethylammonium (stripping solution - 0.05 M H2SO4). )Hinit. = 0 . 9 0 ; CNi = 120 ~/1; Cc,,.Fc= 1 g/1 D Stripping, % Fc Extractant Cu Fe C©,M Cu Caprylate 71.1 0.53 3.94 0.45 73.9 90.7 1.63 1.0 0.38 97.0 0.426 0.25 32.6 36.4 1.64 0.15 0.002 75.5 Alkylcarboxylate 0,53 82.8 5.23 0.45 59.77 95.0 1.38 12.3 47.9 97.3 0.38 0.212 27.5 16.2 0.25 41.2 0.002 1.31 74.2 0.15 Alkylphosphate 0.45 0.009 73.5 94.4 5.4 0.25 5.90 28.2 0.15 1.22 53.9 .
°
The purificationof solutionsfrom iron
Iron (RI) in chloride solutionsforms stablecomplex anions FcCI4, therefore, saltsof amines and Q A B can bc used for the extractionof iron and the purificationof chloride solutions. The systems containing organic salts (phenolates and alkylcarboxylates) of amines and Q A B are characterized by smaUcr iron (Ill) distributioncoefficientscompared with the chlorides of these extractants[17]. Thc extractionequilibriain binary extractionof complex iron-containingacid from chloride solutionsby saltsof Q A B and trialkylamineare: H(a )+ + FeCl4(a) + R4NA(o ) c~ R4NFeCI4(o) + HA(o ) (24) H~a)_ + FeCl4(a) + R3NHA(o) ¢:> R3NHFeCI4(o) + HA(o)
(25)
In accordance with Eqs. (24, 25), extractionof iron is cnhanced with the increaseof mincral acid concentration in the aqueous phase and also with thc increaseof the
140 concentration of unextracted mineral salts due to the decrease of water activity and the enhancement of activity coefficients of HFeCI4 in the aqueous phase. During the contact of extracts with water, the stripping of iron depends on the stability of binary extractants. Using alkylphenolates of various structure as binary extractants, the most effective stripping of iron takes place in comparison with carboxylates of amine and QAB and the distribution coefficients decrease in the series: 2,6-tert-butylphenolate > 2cumylphenolate > 4-tert-butylphenolate; ~t-monoalkylcarboxylate > caprylate [30]. Such influence of the composition of the binary extractants on the value of DFc is obviously related to the value of acidic dissociation constant of the initial organic acids and also steric hindrances when forming binary extractants for o-substituted alkylphenolates and a-ramified monoalkylcarboxylic acids (ct-MACA). From Eqs. (24, 25) it follows that adding one of the extracted compounds into the organic phase causes a shift of the equilibria to the formation of binary extractant, that is, increase of organic acid concentration improves stripping of iron. One can see from Fig. 8 that the stripping of iron is significantly improved when the concentration of alkylphenole in the organic phase increases.
{ c~,~,), g/l 120
80
40
0
o..,
°'~CHA,M
Figure 8. Iron srripping in the system of 0.3 M solution oftrialkylamine in kerosene as a function of 4-octylphenol concentration in the organic phase. Cv¢o), g/l: 1.0 (1); 1.5 (2); 3.0 (3); 5.0 (4).
Purification of aluminium chloride solutions
One of the main problems of acidic processing of aluminiferrons raw material is the purification of aluminium chloride solutions from iron. The binary extractant systems are characterized by high distribution coefficients of iron (> 340) which slightly decrease for the systems with relatively low stability of these extractaats but in this case DF~ remains sufficiently high for conducting thorough purifications of solutions from iron. Since aluminium-containing chloride solutions atter leaching of aluminiferrons raw material have high temperature (> 90 °C), the extraction can take place using monocarboxylic acids as solvents (for binary extractants [30]). Using the isotherms obtained for iron extraction by 0.3 M solutions of trialkylamine ct-monoearboxylate in ct-MACA at 80 °C and by trialkylamine 4-isooctylphenolate in kerosene at 25 °C two steps of the extraction were found to be quite sufficient for the purification of aluminium chloride solutions from 12 g/l to 0.01 g/l Ire. At low temperatures (20-30 °C), a 0.3-0.5 M solution of triaikylamine p-alkylphenolate in kerosene was used as the
141 extraetant, whereas in the ease of higher temperature (45-80 °C), a 0.3-0.5 M solution of trialkylamine monocarboxylate in earboxylic acid was used. The results obtained in the stripping of iron by HCI solutions (10 g/l) to prevent iron hydrolysis in the aqueous phase at various temperatures for binary extractant systems showed that decrease of temperature leads to significant improvement of stripping. However, for decreasing energy consumption it is essential to conduct stripping at a temperature about 50 °C. The decrease of stability of R3NHA salts and also the use of greater concentration of monoearboxylic acid in the organic phase in comparison with a-rernified acids cause stripping improvement in the systems with monoearboxylic acids of normal structure. Under the optimal conditions iron stripping is occurred in 2-3 steps producing a sufficiently concentrated solution of iron (50-60 g/l). The purification of cobalt-containing solutions The purification of solutions containing cobalt was carried out at room temperature. As the extractant, 0.2 M solution of trialkylamine carboxylate in synthetic fatty acids of C7-C9 fraction was used [30]. The concentration of chloride ion in the aqueous phase was not less than 150 g/1. The character of extraction and stripping isotherms allows to conduct the process in a small number of steps. Iron was extracted practically completely in four steps of the extraction from solution containing, g/l: 120 - Co; 12.9 - Ni; 2.2 - Cu; 5.0 - Fe; 15 HC1. Residual concentration of iron in the raffinate was 0.002-0.05 g/l. Other elements presented in the aqueous solution were practically not extracted. The stripping of iron was conducted by a solution of HC1 (10-15 g/l) in three steps producing sufficiently concentrated stripped solutions of iron (C~eca)=45-50 g/l). 2.2.Extraction of rare metals Extraction of indium In the existing extraction schemes of processing In - containing solutions alkylphosphoric acids and particularly di(2-ethylhexyl)phosphoric acid are used as extractants. Concentrated solutions of HCI (9-11 M) are used for stripping of indium under industrial conditions. The use of binary extractants for indium extraction allows one to eliminate HCI at the stripping stage. The extraction of indium (III) from sulphuric solutions by tetraalkylammonium di(2-ethylhexyl)phosphate has been studied [10]. It was shown that depending on H2SO4 concentration in the aqueous phase a different mechanism of indium extraction is revealed - binary, cation-exchange or anion-exchange mechanisms. The stripping of indium can be proceeded by dilute solutions of H3PO4 or H2SO4 using a binary extractant. The effective binary extractant for indium extraction is also salt of trialkylamine and di(2-ethylhexyl)phosphoric acid. Isocarboxylie or synthetic fatty acids earl be used as solvents. Using binary extractant; the extraction of indium into the organic phase is 98.8-99.8 % and depends on concentrations of copper and iron in solutions. From the extraction isotherms obtained it follows that the theoretical number of extraction steps is equal to 2 for achieving residual concentration of indium in the solution of about 1-2 mg/1. The stripping of indium from the organic phase is proceeded by a mixture of phosphoric and sulphuric acids. Phosphoric acid has the maximum influence on the extent of the stripping. At concentration of 6 M H3PO4 the extent of stripping is 80 % in one step.
142 The full-scale experiments of developed technique were conducted. The initial solutions with the concentration of indium from 0.9 to 1.2 g/l were used. The concentration of indium in the raffinates was 10-12 mg/1 and in the sripped solution it was 28-38 g/1. The extraction of indium into the extract was 98.9-99.4 % and into the strip solution - 95.5 - 98.5 %. Extraction of cadmium
The extraction method is used at Ust-Kamenogorsk lead-zinc combine for the recovery of valuable components from solutions of the sulphatization. The extraction of In, T1, Cd allowed to increase the recovery of these metals from lead dusts. In further processing of solutions in lead production they undergo purification from copper and chloride ions on conducting a labour-consuming operation, namely, filtration and produce cakes difficult for utilization. The extraction method based on the application of trialkylamine monocarboxylate is suggested for a simultaneous extraction of cadmium and chloride ions [18-21 ]. Using this extraetanL cadmium and chloride ions are extracted as follows: 2+ + 4Cl~a) + (R3 NH)2 SO4(0) ¢:> (R3NH) 2 CdCI4(o) + SO2~a) Cd (a)
(26)
(R3NH)eCdCI4(o) +4HA(o ) +4OH~a) <=, 2R3NHA(o) +CdA2(o) +4CI('a) +4H20(a )
(27)
They are separated at the stage of the stripping of chloride ion and cadmium with extractant regeneration: 2+) (28) 2R3NHA(o) + CdA2(o) + 4H~'a)+ SO42~a)<::, (R3NH)2 SO4(o) + 4HA(o) + Cd(a The tests were conducted on the experimental installation of the combine using multistep extractors of the "mixture-sediment" type. A 30 % solution of trialkylamine in kerosene with an addition of ct,ct'-dialkylmonocarboxylic acid and 20 % 2ethylhexanole was used as an extractant. Alcohol was added to improve the phase exfoliation and increase solubility of mineral salts of trioctylamine in the organic phase. As stripping solution the aqueous solution of alkali (200 g/I) was used and the stripping was conducted at an equilibrated pH equal 7.8 - 8.2. During the stripping of chloride ions, the strip solutions were obtained with CI" concentration equal to I00 - 120 g/I and the concentrations of cadmium and zinc equal to 0. I and 0.05 g/I respectively. Thus, thorough separation of cadmium and chloride ions can be reached at the stage of chloride ion stripping. The stripping of. cadmium was conducted by a solution of sulphuric acid (500 g/I) and the residual concentration of H2SO4 in a strip solution was not more than 5 - I0 g/l. A regeneration of the sulphuric salt of tfialkylamine was realized at the stage of cadmium stripping. 2.3, Extraction of noble metals Salts of amines and QAB are understood to be effective extractants for metals of platinum group. The use of amines and QAB in the teehnologycal processes is limited up to now due to difficulties of platinum metal stripping from organic phases. In this connection, systems involving salts of amines, QAB and organic acids as extractants are of great interest [22-29]. In the extraction of H2PdCI4 by salts of dioctylamine, trioctylamine and organic acids, palladium was found to be extracted from 1-3 M HC1 solutions through the anion exchange mechanism. These processes are complicated by interactions of components in organic phase in the presence of organic acids forming compounds (R2NH2)2PdC14 and (R3NH)2PdC14 respectively. A spectrophotometric study showed that in palladium
143 extraction by eaprylate, alkylphenolate, alkylsalicylate of tetraoctylammonium monomeric or dimeric species of Pd (II) are formed in the organic phases depending on the excess of the extractant, aqueous acidity and also the nature of organic acid. When using dioctylamine alkylsalicylate, the extraction of H2PdCI4 decreases in the region of pH>2 according to the rules of mineral acid binary extraction. In the extraction of palladium by alkylphenolate and caprylate of dioctylamine the enhancement of Dpd values in the region of low aqueous acidity is accounted for by the introduction reactions occurring in the organic phase as in the initial system with dioctylamine when compounds (R2NH2)[Pd(R2NH)C13] and Pd(R2NH)2CI2 are formed. In the region of high pH of the aqueous phase, the extraction of palladium by various salts of trioctylarnine including caprylate decreases due to the processes of acid binary extraction and the shift of the extraction equilibrium to the left: 2H~a) + PdC12~a) + 2R3NHA(o) ¢:> (R3NH)2PdCI4(o) + 2HA(o)
(29)
For practical purposes binary extractants based on strong organic acids are of interest, since sripping processes are more effective in these systems. The extraction of palladium chlorocomplexes by salts of tetraoctylammonium and alkylsulphoacids of various structure which are related to the type of strong organic acids (pKa~2) has been studied as a function of HC1 concentration (Fig. 9). The data showed that the prevalent influence on the extractive power of binary extractants causes the structure of corresponding organic acids since they do not significantly differ in the values of acid dissociation constants. An increase of the number of substitutents and the lengthening of the chain of alkyl radicals improve the extractive power of binary extractants, obviously, due to steric hindrances in forming corresponding salts of RaNA. log Dpa log DrL 1,5 1
1.0 0.5
;-log Cn.t,~ -0.5 -
CHCI, M -2,0 J
Figure 9. Tetraoctylammonium salts: 6hexadecyloxynaphthalenesulphonate(1); 4hexadecyloxybenzenesulphonate (2); 2,5di-decyloxybenzenesulphonate (3); 2,3-dihexaoxybenzenesulphonate (4); 5-tetr-butyl-2-hexadecyloxybenzenesulphonate (5).
Figure I0. Platinum (IV) extraction by 0.025 M solutions of dioctylamine caprylatc (I), trioctylaminc caprylatc (2) and 0.004 M tetraoctylammonium caprylate in toluene as a function of aqueous acidity. Ca i,it=2 •10.3 M.
A study of a distributionof H2PtCI6 in the systems with various saltsof dioctylarnine, trioctylamineand tetraoctylammonium showed that in the region from I to 3 M HCI platinum is extracted according to the anion-exchange mechanism and in the region of low aqueous aciditythat is binary extraction mechanism of mineral acids. The extractivepower of binary extractants increases in the series: caprylate of dioctylarninc
144 In platinum extraction by various salts of tetraoctylammonium from solutions with pH=3-7 the following series was obtained: caprylate > p-tert-butylphenolate > alkylsalicylate > alkylphospate > alkylsulphonate of tetraoctylammonium. This series mainly corresponds to the increase of acid dissociation constants of corresponding organic acids with the exception of the alkylphenolate system. The relative decrease in the platinum distribution coefficients during the extraction by alkylphenolate of tetraoctylammonium is obviously accounted for by the formation of stable associated compounds such as R4NCI'nHA. The salts of QAB and strong organic acids (alkylphosphoric acids or alkylsulphoacids) were shown to be of the most interest for practical purposes fi'om the binary extractants used. In the extraction of chlorocomplexes of Pt (IV), Pd (IlI), Ir (IV) by 0.1 M solutions of trialkylbenzylammonium alkylsulphonate in toluene, the distribution coefficients of metals in the region of 0.5-4 M HC1 are varied in the interval of 5-150 while DM for Rh (III) and lr (HI) are not larger than 0.1, which creates possibilities for the platinum metal separation. The calculated coefficients of the platinum metal separation are 13Ptar~Iv)- 20; 13lr(lV)/Ru(Hl) -- 10; 131,qv~ ~ 50; [3VtWd~ 12; 13PtaraV) ~ 20; 13r,vau(nO~ 2"102; 13r~v,h~ 1'103; 15~ar(m)-- 2103. Table 2 Extraction and stripping of platinum (IV) by water in the system with trialkylber~ylammonium alkyls tlphonate in toluene Extraction, % Stripping, % Ce, M CPt(init.), g/1 CHCI,M 52.1 0.10 0.500 80.6 0.1 67.7 97.9 0.5 89.3 -100 1.3 96.9 ~100 2.3 ~I00 ~100 3.3 2.0
0.05 0.10 0.15 0.20 0.25
0.560
91.8 97.9 -100 ~100 ~100
~100 ~100 90.6 72.8 60.4
2.0
0.I0
0.197 0.394 0.780 1,18
~100 95.4 95.3 92.4 88.6
~100 ~100 90.6 72.8 60.4
1.93
Metals of the platinum group are easily stripped from the organic phase by water, which is pratically impossible in the initial systems with QAB mineral salts.The data in Table 2 show that using 0.1 M solutions of trialkylbenzylammonium alkylsulphonate platinum is effectively extracted into the organic p h ~ e with the subsequent stripping of Pt by water. Thus, using platinum extract with 1.6 g/1 concentration after 4 steps of stripping by water (Vo : Va = 1 : 1) a strip solution of platinum with 6.4 g/l concentration was obtained and platinum stripping was 93-100 % for each step (Ce = 0.1 M). The comparison of the binary extractant systems with tributylphosphate showed the advantage of binary extraction for obtaining more concentrated solutions after stripping of platinum metals.
145 CONCLUSION Extraction is widely used due to its advantage in comparison with precipitation and other methods, the potential possibilities of the extraction method are not exhausted. Establishment of basic properties of binary extraction of acids and metal salts, study of peculiarities of compound distribution in these systems and advantages of binary extraction in comparison with extraction processes using initial ion exchange and also neutral extractants, lead to further developments of the extraction method. Since in binary extractant systems the extracted compounds are formed similarly to the systems with initial ion exchange extractants, the use of these properties (for example, selectivity) with combination of new possibilities of distribution process is possible. For example, the increase of ion distribution coefficients results in salting out, and increase of counter ion concentration or on addition of salt containing the same counter ion which can form stable extracted compounds according to anion-exchange or cation-exchange series. Additional possibilities to control binary extraction processes are realized by changing concentration crrrelation of two extracted compounds in the system. Since interphase distributions of cations and anions in the binary extractant systems are interconnected, the use of combination of organic cations and anions with different properties in composition of binary extraetants opens various new possibilities. Thus, in the binary extractant systems the extraction of salts and different compounds that are poorly extracted by neutral extractants (for example, sulphates of non-ferrous metals, salts of alkaline metals, sulphuric acid, hydroxides of metals and so on) is possible. Binary extractants based on alkylphenolates can extract mineral acids from alkaline solutions. On the other hand, the stripping process is essentially facilitated due to formation of stable ionic pairs consisting of an organic cation and an organic anion in a heterogenious system, also during formation of such thermodynamically stable extracted compounds as dialkyldithiophosphate of copper, hexachloroplatinate and tetraehloropalladate of tetraoctylammonium and so on. In comparison with initial ion exchange systems the consumption of alkalies, mineral acids or complexes decreases and in some cases is eliminated. The use of binary extractants causes a decrease of solubility of extractant and extracted compounds in the aqueous phase in comparison with initial systems and also an increase of their solubility in the organic phase. We can assert that development of the binary extraction principle allowed to obtain a large quantity of new extractants with various properties varying their composition in combining various well-known cation exchange and anion exchange extractants including those produced on industrial scale. Extractive properties of binary extractants can be qualitatively predicted on the basis of initial organic acids and organic base salts well studied at present. In this paper we have shown only some developed methods of extraction and separation of metals with help of binary extraction in technological purposes. This principle of the extraction of salts, acids and hydroxides of metals can be used in different "liquid-liquid" systems, for example, in ionic flotation, membrane processes including liquid membranes for ion selective electrodes and also in processes of sorption, flotation of minerals and so on. REFERENCES 1. T.C. Davis and R.R. Grinstead, J. Phys. Chem., 74 (1970) 147. 2. M.L. Navtanovich and V.L. Heifetz, Zh. Obshch. Khim., 45 (1975) 413.
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