Dialkyl Pyridinedicarboxylates' Extraction Ability toward Copper(II

Modeling of Copper(II) and Zinc(II) Extraction from Chloride Media with KELEX 100. Mariusz B. Bogacki, Svetlana Zhivkova, George Kyuchoukov, and Jan ...
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Ind. Eng. Chem. Res. 1997, 36, 838-845

Dialkyl Pyridinedicarboxylates’ Extraction Ability toward Copper(II) from Chloride Solutions and Its Modification with Alcohols Mariusz B. Bogacki, Artur Jakubiak, Ge´ rard Cote,† and Jan Szymanowski* Institute of Chemical Technology and Engineering, Poznan University of Technology, Pl. Sklodowskiej-Curie 2, 60-965 Poznan, Poland

Dipentyl pyridinedicarboxylates (denoted hereafter as L) with different positions of the ester groups were synthesized and used for copper(II) extraction from chloride solutions containing up to 10 mol‚L-1 Cl-. The effect of decanol addition on copper extraction was studied. A molecular modeling technique was used to estimate the structures of extractants, copper complexes, and associates with alcohol. It was found that the ability of pyridinecarboxylates to extract copper depends on the aqueous phase composition and the position of the ester groups in the pyridine ring. All the investigated compounds except dipentyl pyridine-2,6-dicarboxylate extract copper(II) by formation of CuCl2L2 complexes. Dipentyl pyridine-2,6-dicarboxylate forms another type of complex, probably CuCl2L. However, this compound is not suitable for copper extraction as its copper complex precipitates. Dipentyl pyridine-3,5-dicarboxylate was found to be the most suitable extractant among the various compounds listed. Finally it is shown that the possibilities to modify the extraction ability of pyridinecarboxylates with a hydrophobic alcohol such as decanol are relatively weak. Some enhancement was, however, observed when 20% of decanol was added to the organic phase containing dipentyl pyridine-3,5-dicarboxylate. 1. Introduction The success of the hydrometallurgical technique using extraction of copper with hydroxyoximes and electrowinning in the processing of the oxide ores and various wastes has attracted interest in applying this technology for copper recovery from other raw materials, including sulfidic ores, which constitute the principal raw material for copper production (Szymanowski, 1993). However, the recovery of copper from sulfide ores is more complex than that from oxide ores. Basically, the latter were treated in the past as an off-balance copper resource which provided a strong argument for designing the above-mentioned technology allowing their exploitation. Competition of other hydrometallurgical methods, including those applying cementation or copper electrowinning directly from the solution after leaching with sulfuric acid, was insignificant due to both the low quality of produced copper and the economy of the processes. A totally distinct situation is associated with application of the hydrometallurgical technique for processing sulfide ores for which, almost exclusively, pyrometallurgical techniques have been applied. Neverthless, such techniques constitute a serious hazard for the environment by emitting toxic heavy metals and sulfur dioxide, and under the increasing pressure of environmental concerns and associated legislation, alternative routes to produce copper from sulfidic ores are investigated. It is, however, clear that the developing processes which are based on hydrometallurgy cannot yet compete with the well-established pyrometallurgical processes. It may only be of accessory value, helping to process raw materials unfavorable for pyrometallurgical processes, e.g., complex ores which cannot be concentrated effectively by flotation or specific cuprif* Author to whom correspondence should be addressed. † Laboratoire de Chimie Analytique (Unite ´ associe´e au CNRS No. 437), ESPCI, 10 rue Vauquelin, 75005 Paris, France. S0888-5885(96)00281-3 CCC: $14.00

erous materials and typical sulfide ores occurring in amounts too small for construction of a large, economically profitable pyrometallurgical installation. The first modern attempt was the Arbiter process in which sulfide concentrate was leached with ammonia in the presence of oxygen and copper was extracted from ammonia solutions with hydroxyoximes (Khun et al., 1974). However, after 7 years of operation the installation was closed. The newest effort was done by ICI which developed the CUPREX process. The copper concentrate is leached with ferric chloride, giving a solution containing up to 60 g L-1 of Cu, 150 g L-1 of Fe, and 8 mol‚L-1 Cl-. Copper is then recovered by solvent extraction with pyridine-type extractant ACORGA CLX-50 (Dalton et al., 1983, 1984, 1987, 1988, 1991). The active component of this commercial extractant is an ester of pyridine-3,5-dicarboxylic acid, probably diisodecyl pyridine 3,5-dicarboxylate, and the ICI patents indicate that this type of pyridine compound is particularly suitable for the extraction of copper(II) from chloride systems through a solvation mechanism. ACORGA CLX-50 is capable of transferring large amounts of copper with no need for pH adjustment or control and with very high selectivity over a wide range of metals and metalloids (Dalton et al., 1982; Soldenhoff, 1987). In previous work (Szymanowski et al., 1993; Cote et al., 1994), we have investigated copper(II) extraction by model pyridinemonocarboxylates and compared the results with those obtained for ACORGA CLX-50. In particular, we have modeled the extraction equilibrium at constant water activity and constant total concentration of ionic or molecular species dissolved in the aqueous solution (i.e., by keeping constant, but not necessarily equal, the values of the activity coefficients of all the species involved in the system). The influence of the position of the ester group in the pyridine ring on the extraction ability of pyridinemonocarboxylates was also discussed. Pyridinemonocarboxylates form more stable copper(II) complexes than ACORGA CLX50 and can therefore efficiently extract this metal even © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 839

at low chloride ion concentrations; however, their selectivity over iron is much lower than that of the commercial extractant and copper stripping cannot be achieved as easily as with ACORGA CLX-50. The extraction ability of pyridinecarboxamides was also studied (Borowiak-Resterna, 1994; Borowiak-Resterna and Szymanowski, 1996; Borowiak-Resterna et al., 1993), and their high ability for copper(II) extraction was demonstrated. However, they are even stronger extractants than pyridinemonocarboxylates and extract copper(II) in the region of too low chloride concentrations. The objective of the present work includes an investigation of the ability of various model pyridinedicarboxylates to extract copper(II) from concentrated chloride solutions and a study of the influence of an alcoholtype modifier on extractant properties (formation of extractant-modifier associates) and copper(II) extraction. 2. Experimental Section Materials. Individual dipentyl pyridine-2,3-dicarboxylate (I), dipentyl pyridine-2,4-dicarboxylate (II), dipentyl pyridine-2,5-dicarboxylate (III), dipentyl pyridine-2,6-dicarboxylate (IV), dipentyl pyridine-3,5-dicarboxylate (V), and commercial reagent ACORGA CLX50 (VI) were used. Additionally, our previous results obtained for decyl isonicotinate (VII), decyl nicotinate (VIII), decyl picolinate (IX) (Cote et al., 1994), and diethyl 2,6-dimethyl-4-hexylpyridine-3,5-dicarboxylate (X) (Szymanowski, 1993) are compared with those obtained for diesters. The structures of the studied compounds are as follows:

H11C5OOC

COOC5H11

COOC10H21

N I–V

N VII–IX

H5C2O2C H3C

C6H13 CO2C2H5 N X

CH3

where the positions of the substituents in the pyridine ring are as follows: I, 2,3-; II, 2,4-; III, 2,5-; IV, 2,5-; V, 3,5-; VII, 4-; VIII, 3-; IX, 2-. ACORGA CLX-50, about 1 mol‚L-1 in a high-flashpoint kerosene, was kindly supplied by ZENECA, U.K., and used as delivered. Individual compounds I-V have been synthesized in a two-step process from appropriate pyridinedicarboxylic acids according to the method described previously (Szymanowski et al., 1993b). In the first step the acid reacts with thionyl chloride. In the second, the intermediate acid chloride gives a pyridine diester after reaction with pentanol and neutralization of the resulting hydrochloride with sodium carbonate. The resulting products were purified by vacuum distillation. Characteristics of the synthesized extractants by 1H NMR (CDCl3) are as follows. Dipentyl pyridine-2,3-dicarboxylate (I): δ (ppm) 1.57 (m, 18 H, 2 C4H9), 4.30 (m, 4 H, 2 O-CH2), 7.40 (m, 1 H, 5CH), 8.20 (m, 1 H, 4CH), 8.75 (m, 1 H, 6CH). Dipentyl pyridine-2,4-dicarboxylate (II): δ (ppm) 0.92 (m, 4 H, 2 CH2), 1.42 (m, 8 H, 2 (CH2)2), 1.70 (m, 6 H,

2 CH3), 4.37 (m, 4 H, 2 O-CH2), 8.02 (m, 1 H, 5CH), 8.62 (m, 1 H, 3CH), 8.90 (m, 1 H, 6CH). Dipentyl pyridine-2,5-dicarboxylate (III): δ (ppm) 0.91 (m, 4 H, 2 CH2), 1.41 (m, 8 H, 2 (CH2)2), 1.82 (m, 6 H, 2 CH3), 4.40 (m, 4 H, 2 O-CH2), 8.19 (m, 1 H, 3CH), 8.44 (m, 1 H, 4CH), 9.31 (m, 1 H, 6CH). Dipentyl pyridine-2,6-dicarboxylate (IV): δ (ppm) 0.93 (t, 6 H, 2 CH3), 1.42 (m, 8 H, 2 (CH2)2), 1.83 (q, 4 H, 2 CH2), 4.42 (m, 4 H, 2 O-CH2), 7.40 (m, 1 H, 5CH), 8.00 (m, 1 H, 3CH), 8.50 (m, 1 H, 4CH). Dipentyl pyridine-3,5-dicarboxylate (V): δ (ppm) 1.04 (t, 6 H, 2 CH3), 1.42 (m, 8 H, 2 (CH2)2), 1.81 (q, 4 H, 2 CH2), 4.39 (m, 4 H, 2 O-CH2), 8.83 (m, 1 H, 4CH), 9.51 (m, 2 H, 2CH, 6CH). 1H NMR proves the structures of the obtained compounds. Decanol and toluene were used as a modifier and diluent, respectively. Aqueous solutions of copper(II) were prepared from CuCl2‚2H2O and various amounts of LiCl. Extraction Procedure. Equal volumes of aqueous and organic phases were shaken at room temperature until equilibrium was reached (i.e., 15 min, although equilibrium was obtained within a few minutes). After that, the two phases were allowed to separate and copper(II) was completely stripped from the organic phase by pure water. Copper(II) concentration in the aqueous phase was determined by titration with EDTA with murekside as an indicator. The pH of the aqueous phase was close to 3, and protonation of the extractants did not occur. The initial copper concentration was 0.01 mol‚L-1, while the chloride ion concentration was changed up to 10 mol‚L-1. The extractant concentration in the organic phase was 0.2 mol‚L-1, and the amount of decanol was changed from 0 to 100% v/v. Molecular Modeling of Alcohol-Extractant Association. The 6.00 MOPAC program with PM3 approach (Steward, 1990) and HyperChem Release 3 for Windows (Autodesk, Inc.) with ZINDO 1 have been used. The semiempirical PM3 method is an acceptable quantum chemical method for estimating the energy of small systems possessing intermolecular bonding. It describes the intermolecular hydrogen-bonding geometry of the water dimer and, generally, between neutral molecules. A semiempirical ZINDO 1 method was used for estimating the structures of copper complexes. Molecular modeling was carried out using methanol and dipentyl pyridinedicarboxylates. Methanol was used as the model of an alcohol-type modifier as its low molecular mass increased the rate of computing. Calculations were carried out on an IBM PC computer. The precise mode was used which sets the convergence criteria for GNORM ) 0.01 and SCFRT ) 0.00001. Starting geometries for calculations were obtained with Dreiding models and expressed in internal coordinates. The initial conformation used standard bond distances and bond angles. The angles of hydrogen-bonded ligands were selected to produce tetrahedral association around given oxygen atoms, although these angles were always allowed to optimize. The method was tested for pyridine for which the lengths and angles are given in the literature (Sutton, 1965). The following values were obtained: length (Å, calcd/exptl) N-C, 1.353/1.340; C2-C3, 1.395/1.395, C3C4, 1.393/1.394, C-H, 1.10/1.08; angles (deg, calcd/ exptl): C-N-C, 119.6/116.8; N-C-C, 121.5/123.4; C2C3-H3, 122.7/120.2; C3-C2-H2, 120.3/120.2. The agreement can be considered as satisfactory.

840 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997

For each selected associate structure, i.e., for a given number of associated alcohol, position, and way of hydrogen bonding, the optimized geometry was determined by minimization of the PM3 heats of formation. Stereodrawings of the optimized forms were displayed by the HyperChem Release 3 for Windows program (Autodesk, Inc.). Relative contents of associates having the same number of modifier molecules were estimated from the calculated heats of formation, assuming that the entropy of associate formation depends only upon the number of associated modifier molecules. Thus, the heats of formation were used to estimate the equilibrium contents and component concentrations according to the Brinkley method (Brinkley, 1946, 1947). The parallel formal and independent reactions of isomerization between associates and mass balance equations were considered. 3. Results and Discussion

Figure 1. Distribution of copper(II) chloride complexes: free Cu(II) cation (1 and 1a), CuCl+ (2 and 2a), CuCl2 (3 and 3a), CuCl3(4a), CuCl42- (5a) in the aqueous solutions of chloride ions with water activities aw ) 0.835 and 0.617 (a).

Extraction of Copper(II) with Individual Extractants. The physicochemistry of copper(II) extraction from a chloride solution with esters of pyridinecarboxylic acid is comprehensive and can change depending on the position of the ester group in the pyridine ring. Dalton et al. (1982, 1983) and Soldenhoff (1987) have found that the copper(II) extraction with commercial reagent DS 5443 (predecessor of ACORGA CLX-50) and decyl nicotinate can be described by

extraction of copper should be carried out directly from a leach solution without any adjustment of chloride concentration. Such a liquor obtained by the leaching of copper sulfide concentrates with ferric chloride according to eqs 3 and 4 may contain 10-60 g L-1 of Cu,

Cuw2+ + 2Clw- + 2Lo ) CuCl2L2,o

50-150 g L-1 of Fe, 0.1-1 mol‚L-1 of HCl, and 2-8 mol‚L-1 of total Cl-. The formation of copper chlorocomplexes has been studied by several authors (Bjerrum, 1987, Bjerrum and Skibsted, 1986; Ho¨gfeldt, 1981; Ramette, 1986; Sillen and Martell, 1971). These complexes are poorly stable and their formation constants change in the following ranges depending on the experimental conditions: log β1 ) -0.40 to 1.20, log β2 ) -0.80 to 1.00, log β3 ) -2.80 to 0.80, and log β4 ) -3.00 to 0. As was reported previously (Cote et al., 1994), they strongly depend on water activity (aw), and the following sets of formation constants were obtained: log β1 )0.60 and log β2 ) -0.40 for aw ) 0.835 (β3 and β4 were neglected as very small) and log β1 ) -0.05, log β2 ) -0.80, log β3 ) -1.30 and log β4 ) -2.80 for aw ) 0.617. The considered water activities (0.835 and 0.617) correspond to LiCl concentrations equal to 4 and 6 mol‚L-1, respectively. Thus, they lie in the range of concentration significant for practical purposes. The distributions of copper chloride complexes for these two sets of formation constants are shown in Figure 1. Different concentrations of the copper chlorocomplexes were obtained for both sets of βi constants. However, some similarities can be observed. The content of free Cu(II) cation decreases sharply when the chloride anion concentration rises up to 1 mol‚L-1. Simultaneously, a sharp rise of CuCl+ concentration is observed with a maximum at 1-1.5 mol‚L-1 Cl-. In this range of Cl- concentration, the chloride ions should have a positive mass effect on the extraction of copper(II), as can be expected, for instance, from eq 1. At higher Clconcentration, CuCl2 significantly exists in the solution and anionic copper chlorocomplexes begin to appear as well. The existence of the latter should have, in principle, a negative mass effect on the extraction of copper(II). However, such an effect cannot be observed through the experimental results given in Figure 2,

with

(1)

Kex ) [CuCl2L2]o[Cu2+]w-1[Cl-]w2-[L]o2-

where L represents a molecule of extractant, subscripts w and o refer to the aqueous and organic phases, respectively, and Kex is the extraction constant. The neutral CuCl2 species are thus extracted according to a solvating type reaction. In our previous work in which the copper(II) extraction with decyl pyridinemonocarboxylates was studied, the formation of the complex CuCl2L2 was confirmed (Szymanowski et al., 1993; Cote et al., 1994). An agreement of the model with experimental data was observed when the formation of copper chlorocomplexes was considered

Cuw2+ + iClw- ) CuCli,w2-i with

(2)

βi ) [CuCli2-i]w[Cu2+]w-1[Cl-]w-i, i ) 1, 2, 3, 4

where βi stands for the formation constant of the appropriate chlorocomplex containing i chloroatoms. According to these equations, the copper extraction can be affected by chloride and extractant concentrations. The former seems the most important parameter when the total cycle of extraction and stripping is considered. The extraction is carried out from chloride solutions and is possible above a threshold Cl- concentration. Hence, the stripping can be carried out with water. Thus, the gradient of chloride concentration in the feed and strip is a driving force of the process. It means that the threshold concentration should be enough high to permit an effective driving force. However, it should preferentially be low enough to permit a high concentration of neutral CuCl2 in the aqueous phase. In fact, the

CuFeS2 + 4FeCl3 ) CuCl2 + 4FeCl2 + 2S

(3)

Cu2S + 4FeCl3 ) 2CuCl2 + 4FeCl2 + S

(4)

Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 841

Figure 2. Extraction of copper(II) versus chloride ion concentration with dipentyl pyridine-2,3-dicarboxylate (I), dipentyl pyridine2,4-dicarboxylate (II), dipentyl pyridine-2,5-dicarboxylate (III), dipentyl pyridine-2,6-dicarboxylate (IV), dipentyl pyridine-3,5dicarboxylate (V), commercial reagent ACORGA CLX-50 (VI), decyl isonicotinate (VII), decyl nicotinate (VIII), decyl picolinate (IX), and diethyl 2,6-dimethyl-4-hexylpyridine-3,5-dicarboxylate (X). Reagents concentrations: 0.01 mol‚L-1 Cu(II) and 0.2 mol‚L-1 extractant in toluene. Table 1. Chloride Concentrations for Given Yields of Copper(II) Extraction (Reagent Concentrations: 0.01 mol‚L-1 Cu(II) and 0.2 mol‚L-1 Extractant in Toluene) %E extractant

25

50

75

dipentyl pyridine-2,3-dicarboxylate (I) dipentyl pyridine-2,4-dicarboxylate (II) dipentyl pyridine-2,5-dicarboxylate (III) dipentyl pyridine-2,6-dicarboxylate (IV) dipentyl pyridine-3,5-dicarboxylate (V) ACORGA CLX-50 (VI) decyl isonicotinate (VII) decyl nicotinate (VIII) decyl picolinate (IX)

10.5a 2.9 5.2 3.1 3.8 4.2 0.04 0.5 2.4

11.8a 4.4 7.3 4.5 5.0 5.0 0.4 1.2

12.6a 6.4 10.6a 5.7 6.4 6.2 1.5 4.2

a

Estimated.

where the extraction of copper has been plotted versus chloride ion concentration with decyl pyridinemonocarboxylates, dipentyl pyridinedicarboxylates, diethyl 2,6dimethyl-4-hexylpyridine-3,5-dicarboxylate and commercial reagent ACORGA CLX-50. The obtained results

indicate that the extraction behaviors of the extractants considered depend significantly upon chloride concentration and the number and positions of the ester groups. The extraction rises when the chloride concentration increases. Table 1 gives the values of chloride concentration at which the extraction yield reaches 25, 50, and 75%, i.e., for the values of distribution coefficient 0.33, 1.00, and 3.00, respectively. The apparent extraction equilibrium constant is another parameter which can be used to characterize the extraction ability of the studied extractants (Table 2). The extraction equilibrium constant depends upon extractant structure in agreement with Cl50- and significantly rises with a decrease of water activity and an increase of the total concentration of ionic or molecular substances dissolved in the aqueous phase, as a result of the salting out effect. Decyl pyridinemonocarboxylates are stronger copper extractants than dipentyl pyridinedicarboxylates, and their extraction abilities fall in the order decyl isonicotinate (VII) > decyl nicotinate (VIII) > decyl picolinate (IX). Diethyl 2,6-dimethyl-4-hexylpyridine-3,5-dicarboxylate (X) only slightly extracts copper. The extraction ability of the dipentyl esters decreases in the following order: pyridine-2,6-dicarboxylate (compound IV) ) pyridine-2,4-dicarboxylate (compound II) > pyridine-3,5-dicarboxylate (compound V) . pyridine2,5-dicarboxylate (compound III) > pyridine-2,3-dicarboxylate (compound I). Pyridine-2,3-dicarboxylate (compound I) has a poor affinity for copper(II), and the extraction does not exceed 20%, even at 10 mol‚L-1 Cl-. Pyridine-2,6-dicarboxylate (compound IV) and pyridine-2,4-dicarboxylate (compound II) are the two strongest pyridine dicarboxylate extractants. However, the copper complex of compound IV precipitates as soon as [Cl-] ) 4 mol‚L-1. Thus, the system is not directly suitable for copper(II) extraction. The high affinity of pyridine-2,6-dicarboxylate (compound IV) was not expected due to the potential for significant steric hindrance from the two ester groups near the pyridine nitrogen. Finally, the best compromise is obtained for pyridine-3,5-dicarboxylate (compound V). The extraction of copper is negligible at low chloride ion concentrations (i.e., below 2 mol‚L-1) but sharply rises with the chloride ion concentration above 2 mol‚L-1. Thus, the behavior of compound V is similar to that of ACORGA CLX-50 (compound VI). The observed order of extraction ability of studied mono- and diesters cannot be explained by the order of nitrogen and/or oxygen electronegativity (Table 3). The

Table 2. Values of Kex for Model Pyridinecarboxylates and ACORGA CLX-50 aqueous phase

diluent

aw ) 0.835, σ ) 8 mol‚L-1

toluene

aw ) 0.617, σ ) 12 mol‚L-1

kerosene toluene

kerosene a

extractant

log Kex

decyl isonicotinateb decyl nicotinateb decyl picolinateb dipentyl pyridine-2,3-dicarboxylate dipentyl pyridine-2,4-dicarboxylate dipentyl pyridine-2,5-dicarboxylate dipentyl pyridine-2,6-dicarboxylate dipentyl pyridine-3,5-dicarboxylate ACORGA CLX-50b dipentyl pyridine-2,3-dicarboxylate dipentyl pyridine-2,4-dicarboxylate dipentyl pyridine-2,5-dicarboxylate dipentyl pyridine-2,6-dicarboxylate dipentyl pyridine-3,5-dicarboxylate ACORGA CLX-50b ACORGA CLX-50b

2.43 2.10 1.30 0.36 1.49 0.73 1.44 1.26 0.67 1.26 2.76 2.04 2.82 2.63 2.39 2.80

aw ) water activity. σ - total concentration of ionic or molecular species dissolved in aqueous phase. b From (Cote et al., 1994).

842 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997

Figure 3. Structure of dipentyl pyridine-3,5-dicarboxylate (V) with CuCl2.

Figure 4. Structure of the complex of dipentyl pyridine-3,5dicarboxylate (V) with CuCl2.

Table 3. Electron Charge (in eV) of Nitrogen and Oxygen Atoms in Dipentyl Pyridinedicarboxylates and Decyl Pyridinemonocarboxylates i-C-OR

j-OdCsOR

extractant i,j-

N

CdO

OR

CdO

OR

2,3- (I) 2,4- (II) 2,5- (III) 2,6- (IV) 3,5- (V) 2- (VII) 3- (VIII) 4- (IX)

-0.053 -0.009 -0.049 +0.011 -0.102 -0.035 -0.090 -0.053

-0.364 -0.363 0.361 0.366 0.379 -0.368 -0.380 -0.378

-0.241 -0.254 -0.254 -0.250 -0.266 -0.254 -0.264 -0.266

-0.373 -0.375 -0.379 -0.364 -0.379

-0.267 -0.265 -0.265 -0.251 -0.265

nitrogen electronegativity falls in the order VIII > IX > VII for monoesters, and V . I ) III . II > IV for diesters as a result of different number and positions of the ester groups; even a positive value was estimated for IV. The monoester VIII has a more negative charge on pyridine nitrogen than diesters do, except V. The oxygen of CdO always has a higher electronegativity than the oxygen of the OR group. The differences in the oxygen charge between various esters are small. The considered extractants can be classified into four groups: (1) Extractants having the ester group far away from the pyridine nitrogen, i.e., at positions 3, 4, and/or 5 (compounds V, VI, VII, and VIII) (Figure 3). The access of copper to nitrogen atom in the pyridine ring of these compounds is very easy. The molecules of these extractants cannot form chelates with the metal cation, and the transfer of the latter into the organic phase is possible only via a solvation mechanism. The structure of the complex estimated by the molecular modeling is presented in Figure 4, and characteristic data are given in Table 4. (2) Dipentyl pyridine-2,6-carboxylate (IV) which contains two ester groups in the nitrogen neighborhood (Figure 5). The presented structure suggests a free rotation around the Car-Ccarb bond. In a favorable position the oxygens of the carbonyl groups CdO are, however, on the molecule peripheries, which suggests that the remaining oxygens, i.e., from OR, may act as electron donors in copper complex formation. Analogously to substituted pyridinedicarboxamides (Preez et al., 1980, du Presz and van Brecht, 1989), compound IV may be considered as a tridendate ligand leading to the formation of a relatively stable pentacoordinated complex having two five-membered rings, namely, CuCl2L, which easily precipitates from the extraction

Figure 5. Structure of dipentyl pyridine-2,6-dicarboxylate (IV) with CuCl2. Table 4. Predicted Bond Lengths and Angles for Copper(II) Complexes with Extractants V (CuCl2L2) and IV (CuCl2L) CuCl2L2 Cu-Cl Cu-Cl′ Cu-N Cu-N′

Cl-Cu-Cl′ Cu-N-C Cu-N-C′ N-Cu-N′

CuCl2L Bond Length, Å 2.237 Cu-Cl 2.293 Cu-Cl′ 1.899 Cu-N 1.899 Cu-OR Cu-O′R′ Bond Angles, deg 19.0 Cl-Cu-Cl′ 120.1 Cl-Cu-O 118.5 Cl′-Cu-O′ 158.6 Cl-Cu-O Cl-Cu-O′ O-Cu-Cl′ Cu-N-C

2.931 2.254 2.003 2.189 2.190 172.2 96.5 86.2 96.5 96.4 86.2 109.7

system. However, one must take into account that this is only an assumption and that a direct evidence should be presented. First, a carbonyl oxygen atom of an ester group is a weaker donor than that of a corresponding amide, and, second, other reactions are possible in solution like autoionization in cationic-anionic entities which may occur with Cu(II) in nonaqueous solution. The structure of the probable CuCl2L complex was, however, estimated by molecular modeling (Figure 6). Three different structures were considered, i.e., (i) the one presented in Figure 6 with oxygens of the carbonyl groups in peripheric position, (ii) the opposite structure with the oxygens of the carbonyl groups directed toward

Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 843

Figure 6. Probable structure of the complex of dipentyl pyridine2,6-dicarboxylate (IV) with CuCl2.

pyridine nitrogen, and (iii) the mixed structure with one carbonyl oxygen directed toward pyridine nitrogen and one away. (3) Compound X in which the presence of the methyl groups at positions 2 and 6 essentially disturbs the interaction between pyridine nitrogen and bulky and hydrated copper ion. (4) Extractants having one ester group in the nitrogen neighborhood, i.e., compounds I-III and IX. These compounds probably act as solvating reagents, although the formation of unstable complexes with one fivemembered ring O Cu

C N

cannot be fully excluded. In this case the steric hindrance due to the spacious ester group seems to be also an important parameter which decreases extraction abilities observed for compounds I, III, and IX in comparison to other corresponding analogs of mono- and diesters, respectively. For each of these compounds and

especially for I, the access of the spacious copper ion to pyridine nitrogen can somewhat be restricted. In compound I, the possibility of rotation around the CarCcarb bond is severely limited because of the presence of the second ester substituents at position 3 and repulsion of two negatively charged oxygens from CdO groups. The following distances between the oxygens of two different ester groups were obtained: 1.640, 1.575, and 1.528 Å for 2-CdO and 3-CdO, 2-CdO and 3-OR, and 2-OR and 3-OR, respectively. Extractants II, III, and IX seem to be sterically more favorable than compound I. The bond lengths and angles of copper(II) complexes with extractants IV and V are given in Table 4. The second molecule of V and its appropriate atoms and the second chloride are denoted as “prime”. The CuCl2L2 complex has a typical planar, structure with chloride atoms bent from the planar surface. In the CuCl2L complex assumed to be formed with reagent IV (Figure 6), the complex center deviates from the planar surface of the pyridine ring, with chlorine atoms lying perpendicular to the ring. Association of Dialkyl Pyridinedicarboxylates with Alcohols and Copper Extraction in the Presence of Decanol. The considered extractants contain two ester groups OdCsOR with two different oxygens (CdO and -OR), providing two electron pairs and a pyridine nitrogen carrying one free electron pair able to form hydrogen bonds with alcohol molecules. Thus, theoretically different associates up to those having 9 associated alcohol molecules can be formed. However, such a high degree of association seems to be limited by a steric hindrance. Table 5 shows the relative PM3 heat of formation, ∆Hrel, and dipole moment for various associates of the considered extractants with methanol. The relative heat of formation was calculated according to

∆Hrel ) ∆Hassociate - ∆Hextractant - n∆Halcohol (5) where n is the number of alcohol molecules. The following abbreviations were used: N‚‚‚HOCH3, i-CdO‚‚‚HOCH3, i-RO‚‚‚HOCH3, i-CdO‚‚‚HOCH3,

Table 5. Relative PM3 Heats of Formation (∆Hrel), Lengths and Angles of Hydrogen Bonds, and Dipole Moments (µ) for a 1:1 Methanol-Extractant Associate extractant I

II

III

IV

V

type of hydrogen bonds

-∆Hrel (kJ/mol)

length (Å)

angle (deg)

µ (D)

contents of associates (mol %)

N‚‚‚HOCH3 2OCdO‚‚‚HOCH3 2COO‚‚‚HOCH3 3OCdO‚‚‚HOCH3 3COO‚‚‚HOCH3 N‚‚‚HOCH3 2OCdO‚‚‚HOCH3 2COO‚‚‚HOCH3 4OCdO‚‚‚HOCH3 4COO‚‚‚HOCH3 N‚‚‚HOCH3 2OCdO‚‚‚HOCH3 2COO‚‚‚HOCH3 5OCdO‚‚‚HOCH3 5COO‚‚‚HOCH3 N...HOCH3 2OCdO‚‚‚HOCH3 2COO‚‚‚HOCH3 6OCdO‚‚‚HOCH3 6COO‚‚‚HOCH3 N‚‚‚HOCH3 3OCdO‚‚‚HOCH3 3COO‚‚‚HOCH3 5OCdO‚‚‚HOCH3 5COO‚‚‚HOCH3

19.0 26.4 21.3 26.2 30.4 3.4 11.0 6.4 6.5 6.3 3.4 7.3 5.0 7.7 6.5 5.6 8.0 11.0 6.5 9.2 5.2 10.7 6.7 10.4 8.6

1.862 1.829 1.868 1.830 1.849 1.867 1.823 1.860 1.835 1.878 1.865 1.832 1.846 1.842 1.854 1.899 1.826 1.850 1.825 1.852 1.857 1.833 1.851 1.821 1.873

175.1 165.3 173.9 173.3 173.1 173.5 178.5 175.2 176.4 173.2 173.8 165.1 171.6 168.3 167.8 172.9 172.0 170.9 170.3 177.1 174.5 168.2 167.0 170.2 171.9

3.2 2.9 3.2 2.6 2.2 2.9 2.9 2.5 2.2 4.1 3.5 4.0 1.3 2.4 2.4 3.2 4.2 1.6 3.2 1.5 3.5 2.3 3.4 2.9 3.1

0.7 13.7 1.8 12.9 71.0 3.1 66.2 10.1 10.8 9.9 5.9 28.5 11.2 33.8 20.7 5.5 14.3 49.0 8.0 23.2 4.2 38.0 7.7 33.5 16.5

844 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997

Figure 7. Structure of a 1:1 associate of dipentyl pyridine-3,5dicarboxylate (V) with methanol.

i-CO‚‚‚HOCH3, where i denotes the position of the considered ester group when the two different oxygens atoms (CdO and RO) are distinguished. The obtained results indicate that in each case considered the alcohol forms associates mainly with oxygen atoms of the ester groups. Thus, the modifier molecules do not block the active center (pyridine nitrogen atom) of the extractants. Such a blocking can be only expected when a large excess of alcohol is used, because higher associates can then be formed. However, the differences in relative heats of formation of O‚‚‚HOR and N‚‚‚HOR associates are so significant that such a blocking seems to be rather negligible. Similar effects were reported for decyl pyridinemonocarboxylates considered previously (Bogacki and Szymanowski, 1997). The location of the hydrogen bond in 1:1 associates depends on the position of the ester group, and the following orders of association ability are observed:

for compounds IV and I CO‚‚‚HOCH3 > CdO‚‚‚HOCH3 > N‚‚‚HOCH3 for compounds II, III, and V CdO‚‚‚HOCH3 > CO‚‚‚HOCH3 > N‚‚‚HOCH3 The dominant structure for the 1:1 associate of extractants V is given in Figure 7. The oxygen of the CdO group is more electronegative than the oxygen of OR. Thus, the dominant role of the 1:1 methanol-V associate presented above is obvious. However, the oxygen from OR in IV forms the dominant 1:1 associate. It can be explained by steric effects and the possibility of free rotation discussed previously. Table 5 also gives the estimated approximate contents of 1:1 associates having various locations of hydrogen bonds. Quite similar contents were obtained for appropriate associates with symmetrical esters, i.e., dipentyl pyridine-3,5-dicarboxylate (compound V) and dipentyl pyridine-2,6-dicarboxylate (compound IV). The computing was also carried out for higher associates containing up to 5 alcohol molecules. Such computing indicates that nitrogen blocking depends significantly on the structure of the extractant and the degree of association, i.e., on excess of alcohol. Compounds I and IV are very resistant for the nitrogen blocking because of the steric hindrance. A higher tendency of blocking is observed for reagent V, while the pyridine

Figure 8. Extraction of copper(II) versus decanol content with dipentyl pyridine-3,5-dicarboxylate (V) at [Cl-] ) 4 mol‚L-1 (1) and dipentyl pyridine-2,6-dicarboxylate (IV) at [Cl-] ) 4 mol‚L-1 (2) and [Cl-] ) 8 mol‚L-1 (3).

Figure 9. Extraction of copper(II) versus chloride ion concentration with 0.2 mol‚L-1 dipentyl pyridine-3,5-dicarboxylate (V) in toluene (1) and 0.2 mol‚L-1 dipentyl pyridine-3,5-dicarboxylate in toluene with 20% (v/v) addition of decanol (2).

nitrogens of III and II are strongly blocked in the presence of alcohol excess. The following order for nitrogen blocking is observed for diesters: I < IV < V < II < III. Nevertheless, the above theoretical discussion suggests rather a small effect of alcohols on the ability of extraction of the considered extractants. This conclusion is supported by the experimental data given in Figures 8 and 9. At chloride concentration equal to 4 mol‚L-1, an addition of decanol enhances copper(II) extraction with pyridine-3,5-dicarboxylate (V) and pyridine-2,6-dicarboxylate (IV) up to a content of alcohol 40% and 80% (v/v), respectively (Figure 8). This means that an improvement of the solvating properties of the diluent and/or decanol association with the copper complex has a stronger effect than the alcohol association with the extractant molecules. The latter effect seems to be dominant in the domain of very high alcohol content where the efficiency of copper extraction decreases. However, the positive effect of decanol addition on extraction abilities of IV at chloride concentration equal to 8 mol‚L-1 is not observed. Moreover, the precipitation of the copper complex is not stopped. Thus, it should be concluded that compound IV is not suitable for copper extraction from chloride media. The positive effect of 20% (v/v) alcohol addition upon copper(II) extraction with pyridine-3,5-dicarboxylate (V) is observed. Fortunately, this effect is negligible at

Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 845

chloride concentration below 2 mol‚L-1, and the alcohol addition can be used to improve the copper extraction ability of this extractant by about 20% without disturbing copper stripping with water. 4. Conclusion Dipentyl pyridinedicarboxylates can be used as copper(II) extractants from chloride solutions. Their ability to extract copper depends on the aqueous phase composition and the position of the ester groups in the pyridine ring. All the investigated compounds except dipentyl pyridine-2,6-dicarboxylate extract copper(II) by formation of CuCl2L2 complexes. Dipentyl pyridine-2,6dicarboxylate forms another type of complex, probably CuCl2L. However, the compound is not suitable for copper extraction because of the complex precipitation. Dipentyl pyridine-3,5-dicarboxylate is a suitable extractant. The possibilities to modify the extraction abilities of pyridine carboxylates with an alcohol are relatively weak. Some enhancement is, however, observed when 20% of decanol is added to the organic phase containing dipentyl pyridine-3,5-dicarboxylate. Acknowledgment The work was supported by Polish Research Committee KBN no. 32/932/95. A.J., J.S., and G.C. also are thankful for the French-Polish Scientific and Technological Cooperation Joint Project Grant No. 6619. Literature Cited Bjerrum, J. Determination of Small Stability Constants. A Spectrophotometric Study of Copper(II) Chloride Complexes in Hydrochloric Acid. Acta Chem. Scand. 1987, A41, 328-334. Bjerrum, J.; Skibsted, L. H. Weak Chloro Complex Formation by Copper(II) in Aqueous Chloride Solutions. Inorg. Chem. 1986, 25, 2479-2481. Bogacki, M. B., Szymanowski, J. Association of Alkyl Pyridine Monocarboxylates with Alcohol and their Hydration with Water. Solvent Extr. Ion Exch. 1997, in press. Borowiak-Resterna, A. Extraction of Copper from Acid Chloride Solutions by N-Alkyl- and N,N-Dialkyl-3-pyridinecarboxamides. Solvent Extr. Ion Exch. 1994, 12, 557-569. Borowiak-Resterna, A.; Szymanowski, J.; Cierpiszewski, R.; Prochaska, K.; Banczyk, I. Synthesis and Extraction Properties of N-Alkyl- and N,N-Dialkyl-3-pyridinecarboxamides. In Solvent Extraction in the Process Industries. Proceedings of ISEC’93; Logsdail, D. H., Slater, M. J., Eds.; SCI/Elsevier: London, 1993; pp 578-584. Borowiak-Resterna, A.; Szymanowski, J. Copper(II) Extraction from Chloride Solutions with Model Hydrophobic Pyridinecarboxamides. In Value Adding Through Solvent Extraction. Proceedings of ISEC’96; Shallcross, D. C., Paimin, R., Prvcic, L. M., Eds.; The University of Melbourne: Melbourne, Australie, 1996; pp 569-574. Brinkley, S. R. Note on the Conditions of Equilibrium for Systems of Many Constituents. J. Chem. Phys. 1946, 14, 563-564. Brinkley, S. R. Calculation of the Equilibrium Composition of Systems of Many Constituents. J. Chem. Phys. 1947, 15, 107110. Cote, G.; Jakubiak, A.; Bauer, D.; Szymanowski, J.; Mokili, B.; Poitrenaud, C. Modeling of Extraction Equilibrium for Copper(II) Extraction by Pyridinecarboxylic Acid Esters from Concentrated Chloride Solutions at Constant Activity of Water and Constant Total Concentration of Ionic or Molecular Species Dissolved in Aqueous Solution. Solvent Extr. Ion Exch. 1994, 12, 99-120.

Dalton, R. F.; Price, R.; Quan, P. M.; Steward, D. Extraction of Metal Values. Eur. Pat. 57797, 1982. Dalton, R. F.; Price, R.; Quan, P. M. Novel Solvent Extractants for Chloride Leach Systems. In Proceedings of ISEC’83; American Institute of Chemical Engineers; New York, 1983; pp 189193. Dalton, R. F.; Price, R.; Quan, P. M.; Townson, B. Novel Solvent Extractants for Recovery of Copper from Chloride Leach Solutions Derived from Sulphide Ores. Reagents Miner. Ind., Pap. 1984, 181-188. Dalton, R. F.; Price, R.; Hermana, E.; Hoffmann, B. Cuprex Process. A New Chloride-Based Hydrometallurgical Process for Copper Recovery from Sulphide Ores. Ing. Quim. (Madrid) 1987, 19, 115-120. Dalton, R. F.; Price, R.; Hermana, E.; Hoffmann, B. CuprexsNew Chloride-Based Hydrometallurgy to Recover Copper from Sulphide Ores. Miner. Eng. 1988, 40, 24-28. Dalton, R. F.; Diaz, G.; Hermana, E.; Price, R.; Zunkel, A. D. The Cuprex Metal Extraction Process Pilot Plant Experience and Economics of a Chloride Based Process for the Recovery of Copper from Sulphide Ores. Proc. Copper 91sCobre 91 Int. Synop. 1991, 61-69. du Preez, J. G. H.; van Brecht, B. J. A. M.; van de Water, R. F. Coordination Chemistry of Divalent Cobalt, Copper, and Nickel. Part 7. Penta-coordinated Complexes of Copper(II) with N,N,N′,N′-Tetramethylpyridine-2,6-dicarboxamide. S. Afr. J. Chem. 1980, 33, 9-13. du Preez, J. G. H.; van Brecht, B. J. A. M. The Coordination Chemistry of Divalent Cobalt, Nickel and Copper. Part 8. Selected Complexes Containing N-substituted Pyridine-2,6dicarboxamide Ligands; Crystal Structure of Dibromo(N,N′dimethyl-N,N′-diphenylpyridine-2,6-dicarboxamide)nickel(II). Inorg. Chem. Acta 1989, 162, 49-56. Ho¨gfeldt, E. Stability Constants of Metal-Ion Complexes, Part A: Inorganic Ligands; IUPAC Chemical Data Series No. 21; Pergamon Press: New York, 1981; p 217. Khun, M. C.; Arbiter, N.; Kling, H. Anaconda’s Arbiter Process for Copper, Meeting CIM. CIM Bull. 1974, 67, 62. Ramette, R. W. Copper(II) Complexes with Chloride Ion. Inorg. Chem. 1986, 25, 2481-2482. Sillen, L. G.; Martell, A. E. Stability Constants of Metal-Ion Complexes; Supplement No 1, Special Publication No 25 (Supplement No. 1 to Special Publication No. 17); The Chemical Society: London, 1971; pp 176. Soldenhoff, K. H. Solvent Extraction of Copper(II) from Chloride Solutions by Some Pyridine Carboxylate Esters. Solvent Extr. Ion Exch. 1987, 5, 833-851. Steward, J. J. P. MOPAC version 6.0, QCPE 455, University of Indiana, Bloomington, IN, 1990. Sutton, L. E., Ed. Tables of Interatomic Distances and Configuration in Molecules and Ions: Supplement 1956-1959. Chemical Society(London) Special Publication No. 18; The Chemical Society: London, 1965. Szymanowski, J. Hydroxyoximes and Copper Hydrometallurgy; CRC Press: Boca Raton, FL, 1993. Szymanowski, J. Modifiers in extraction systems. Solvent Extr. Res. Dev. Jpn. 1994, 1, 97-107. Szymanowski, J. Physicochemical Modification of Extractants. Crit. Rev. Anal. Chem. 1995, 25, 143-194. Szymanowski, J.; Jakubiak, A.; Cote, G.; Bauer, D.; Beger, J. Synthesis and Extraction Properties of Pyridinecarboxylic Acid Esters. Solvent Extraction in the Process Industries. Proceedings of ISEC’93; Logsdail, D. H., Slater, M. J., Eds.; SCI/Elsevier: London, 1993; pp 1311-1318.

Received for review May 20, 1996 Revised manuscript received December 3, 1996 Accepted December 14, 1996X IE960281C

X Abstract published in Advance ACS Abstracts, February 1, 1997.