Effect of Changes in Structure of Phosphoric Acid

Transcription

1 Effect of Changes in Structure of Phosphoric Acid Esters on Solvent Extraction of Cobalt (II) and Nickel (II)* by Toshio TAKAHASHI 1 and Taichi SATO 2 Complexation behaviors of phosphoric acid esters with cobalt(ii) and nickel (II) in solvent extraction have been assessed to elucidate the relationship between the structure of phosphoric acid esters and the extraction phenomena. By using phosphoric acid esters with aryl and alkyl groups, it is concluded that the structure of the organic groups in the phosphates plays an important role in the formation of the complexes and the extraction behaviors. In the case of di(nonylphenyl)phosphoric acid, the octahedral complexes of cobalt(ii) and nickel (II) form in the extraction, and the compound extracts the metal ions at the lowest ph among the phosphates used. By changing the phenyl group with an alkyl group, the extraction curve shifts to higher ph, and the formation of the tetrahedral complex of cobalt increases in the extraction. Furthermore, the increase of the steric hindrance of the alkyl groups in the phosphates causes the shift of the extraction curves to higher ph. In addition, the selectivity of cobalt over nickel increases with increasing the steric hindrance of the phosphates. KEY WORDS: Solvent Extraction, Cobalt, Nickel, Phosphoric Acid Esters, Structural Effect 1. Introduction During past two decades, solvent extraction process has been applied to refine various metals, and its technique has proved usefulness as the separation process of commercial metal especially in the rare metal processing. Although there are many factors which affect separation ability of metals in the solvent extraction process, extractants play an important role in the separation. For the separation of cobalt and nickel from acid solutions, acid organophosphorus compounds, such as di(2- ethylhexyl)phosphoric acid (DEHPA), 2-ethylhexyl 2- ethylhexyl-phosphonic acid (EHEHPA), and di (2, 4, 4'- trimethylpentyl)phosphinic acid, have been used as one of the most effective extractants in its solvent extraction process1)-4). We have also studied on the composition of divalent cobalt and nickel species extracted from sulfuric acid and hydrochloric acid solutions by DEHPA5)6). Afterwards a comparative study on the extraction of cobalt and nickel was carried out by using three types of alkylphosphorus compounds, i.e., alkyl derivatives of phosphoric acid, phosphonic acid and phosphinic acid7). In addition, some derivatives of those organophosphorus compounds were also used to examine the structural effects of the organophosphorus compounds on the extraction behaviors of cobalt and nickel8)9). The present authors have also previously investigated the effect of changes in *Received March 10, 1995: accepted for publication July 17, Associate Senior Research Manager, Dr. Eng., Lion Corporation, , Hirai, Edogawa-ku, Tokyo 132, Japan 2. Honorary Professor, Dr. Sc., Dr. Eng., Faculty of Engineering, Shizuoka University, Hamamatsu 432, Japan; Visiting Professor, Department of Materials and Metallurgical Engineering, Queen's University, Kingston, Ontario K7L 3N6, Canada For correspondence: , Daita, Setagaya-ku, Tokyo 155, Japan the structure of acid organophosphorus compounds on the extraction of rare earths by use of a systematically varied series of phosphoric acid esters10)-12) In this paper, therefore, the work is extended to obtain further information about the effect of changes in the structure of acid organophosphorus compounds on the extraction behaviors of divalent cobalt and nickel. 2. Experimental 2 1 General Procedure 1H NMR, 13C NMR and 31P NMR spectra were recorded on a JEOL FX90Q spectrometer or a JEOL GSX-400 spectrometer as ca. 10% solutions in CHCl3. Infrared spectra were recorded on a JASCO A202 infrared spectrophotometer as neat samples or nujol mulls. FAB mass spectra were determined on a JEOL DX303 spectrometer by using a 6-8 KeV primary beam and a 3-nitrobenzyl alcohol matrix. Preparative chromatographic columns were packed with SiO2 (Wakogel C200). Thin-layer chromatographic analyses were performed on Kieselgel F254 with a 0.2mm layer thickness. 2 2 Chemicals Phosphoric acid esters were prepared from phosphoryl chloride and two corresponding alcohols according to the reported methods10)13)14). For the resulting phosphates, the structure and purity were identified by NMR, IR, and Mass spectra, TLC and titration with alcoholic KOH. The purities of the phosphates prepared were higher than 98%. DEHPA was obtained from Kanto Chemical Co. Inc., and EHEHPA was purchased from Daihachi Chemical Co. The phosphoric acid esters used are illustrated in Fig. 1 and Table 1. Other reagents were the best grade commercially available and were used

2 Toshio TAKAHASHI and Taichi SATO Fig. 1 Structures of the phosphoric acid esters (* this denotes the compound No. in Table 1). Table 1 Phosphoric acid esters used in the extraction. for the u.v. and visible range. 3. Results without further purification. Solutions of cobalt and nickel were prepared by dissolving their nitrates in nitric acid of required concentration. 2 3 Extraction and analytical procedure Metal extraction equilibria were determined as fol - lows8). Equal volumes (10cm3) of an aqueous metal nitrate (0.04 mol dm-3) in nitric acid solution and a solution of the phosphoric acid ester (0.25 mol dm-3) in kerosene were mixed by stirring. The ph of the solution was adjusted by addition of concentrated ammonia solution, followed by stirring for 10min at 298 and 323K. Preliminary experiments showed that equilibrium between the phases for the extraction of cobalt and nickel is complete in 10 min. After the equilibrium was reached, the metals in the organic phases were stripped with hydrochloric acid at 1 and 2 mol dm-3 for cobalt and nickel, respectively. The concentrations of metals in aqueous solutions were determined by titration with EDTA using Cu-PAN as an indicator. 2 4 Spectrophotometry Structures of the complexes formed in the extraction were assessed by means of spectrophotometry. Absorption spectra were obtained on a Shimadzu UV-160 spectrophotometer, using matched lcm fused silica cells 3 1 Solvent extraction of cobalt and nickel by phosphoric acid esters having aryl and alkyl groups The extraction experiments of cobalt and nickel from the aqueous solution containing nitric acid with phospho - ric acid ester in kerosene were carried out at 298 K and 323 K. Metal distribution equilibria for the extractions of co - balt and nickel by the compounds 1-4 at 298 K and 323 K are shown in Figs. 2 and 3. Table 2 exhibits the coordina - tion number of the complexes formed from the phosphates and the metal ions in the extraction, as tetrahedral or octahedral arrangement5)6). In this case, the tetrahedral or octahedral structure of divalent cobalt and nickel species is determined by the absorption spectral data in aqueous solutions, as indicated in Table 3. Besides the absorption spectra of the organic extracts of cobalt(ii) and nickel(ii) by (4-nonylphenyl)octylphosphoric acid in kerosene at 298 K and 323 K are illustrated ones. in F i g. 4 as the representative As seen in Fig. 2, the data indicate that di(4- nonylphenyl)phosphoric acid (compound 1, signified as 1) extracts cobalt and nickel at lower ph than that in the ex - traction with the other phosphates. In addition, the octahedral complexes of cobalt and nickel are formed with di(4- nonylphenyl)phosphoric acid (1) in the extraction. The ph values at 50% metal extraction (ph1/2) with the compounds 2-4 increase in the order (4-nonylphenyl) octylphosphoric acid (2) < (2,4-dimethylphenyl) (2-ethylhexyl) phosphoric acid (3) <di (2-ethylhexyl)phosphoric acid (4), while the formation of the tetrahedral cobalt complex increases in the same order. That is to say, the extraction of cobalt with the compound 3 forms the mixture of the octahedral and tetrahedral complexes in the organic phase. On the other hand, DEHPA (4) forms the tetrahedral cobalt complex in the extraction5)6) The data also show the temperature effect on the extraction of cobalt and nickel with the phosphates. In the

3 Effect of Changes in Structure of Phosphoric Acid Esters on Solvent Extraction of Cobalt(II) and Nickel(II) Fig.2 Extraction of cobalt and nickel by the compounds 1-4 at 298 K [, cobalt (octahedral) ; œ, cobalt (tetrahedral);, cobalt (mixture of octahedral and tetrahedral);ƒ, nickel (octahedral);*this denotes the compound No. in Table 1]. Fig. 3 Extraction of cobalt and nickel by the compounds 1-4 at 323K [, cobalt (octahedral) ; œ, cobalt (tetrahedral); œ, cobalt (mixture of octahedral and tetrahedral);ƒ, nickel (octahedral); *this denotes the compound No. in Table 1]. Table 3 Absorption spectral data of tetrahedral or octahedral cobalt and nickel species. Fig. 4 Absorption spectra of the organic extracts of cobalt (II) and nickel (II) by (4-nonylphenyl) octylphosphoric acid in kerosene at 298 and 323K (continuous and broken lines represent the extractions at 298 and 323 K, respectively); (a) and (b) denote the species of cobalt (II) and nickel (II) respectively. *) Molar extinction coefficient Table 2 Effect of the phosphate structure on the formation of metal complexes at 298K and 323K. extractions at 323 K, the values of ph1/2 are lower than those of the corresponding extractions at 293K. Furthermore, the formation of the tetrahedral complex of cobalt was enhanced by increasing of temperature. 3 2 Extraction of cobalt and nickel with phosphoric acid esters containing bulky alkyl groups As the results of the extraction at 323 K, percent extractabilities of cobalt and nickel with the compounds 4- a) Extraction temperature 9 are shown in Figs. 5 and 6. The values of ph1/2, defined as the difference in ph1/2 between cobalt and nickel, are

4 Toshio TAKAHASHI and Taichi SATO, cobalt (oc-t ahedral); œ, cobalt (tetrahedral);ƒ, nickel (octahedral); *this denotes the compound No. in Table 1]., cobalt (oc-t ahedral); œ, cobalt (tetrahedral);ƒ, nickel (octahedral); this denotes the * compound No. in Table 11. Table 4 Effect of the phosphate structure on the extraction of cobalt and nickel. shown in Table 4,ƒ ph1/2 of bis [2-(1-methylethyl)-5- methylcyclohexyl] phosphoric acid (10) exhibit the highest selectivity of cobalt over nickel among the phosphates used, and the obtained value is the almost same as that of EHEHPA. From the extraction experiments mentioned above, it is seen that the phosphates with the highly branched alkyl groups form the tetrahedral complexes with cobalt, while they form the octahedral complexes with nickel. 4. Discussion listed in Table 4. The data reveal that cobalt is extracted at lower ph value than nickel in the all extraction system. Since there is little difference in the slope of the extraction curves as illustrated in Figs. 5 and 6, d phl/2 can be applicable to comparison of cobalt - nickel separation of the system. In Fig. 5 and 6, the cobalt - nickel separation, d phi 2, is closely related to the structure of the phosphates. In the case of the 2-ethylhexyl derivatives, the compounds 4-6, the cobalt-nickel separation increase in the order branched alkyl derivative (4)<cyclohexyl derivative (5)< substituted cyclohexyl derivative (6), suggesting that the selectivity of cobalt over nickel increases with increasing the steric hindrance of the organic groups in the phosphate molecules. Branched octadecyl derivatives give the results of the same tendency in the cobalt selectivity. As As indicated in section 3.1, di (4-nonylphenyl) phosphoric acid (1) extracts cobalt ion at lower ph than that in the extraction with DEHPA, and forms the octahedral complex with cobalt in the extraction. By replacing the aryl group with the alkyl group, the extraction curves of the metal ions shift to higher ph. In the case of the cobalt complexes, the complex structure changes from the octahedral to the tetrahedral. These data imply that there is the effect of changes in the structure of phosphoric acid esers on extraction behaviors of the metal ions. In other words, the aryl derivative tends to form the octahedral complex and the introduction of alkyl groups in the phophates causes the formation of the tetrahedral complex, and the extraction curves shift to higher ph in comparison with the aryl derivative. Although it has been well known that the structural effect of extractants arises from the electronic and the steric effects, it seems that the

5 Effect of Changes in Structure of Phosphoric Acid Esters on Solvent Extraction of Cobalt (II) and Nickel (II) Table 5 Effect of the phosphate structure on the formation of the cobalt and nickel complexes. electronic effect is predominant in this case15). In the case of the nickel extraction, the extraction curves of nickel also shift to higher ph by the structural change, but the coordination numbers of the nickel complexes do not change in the series of extractions. As described in section 3.2, phosphoric acid esters having branched alkyl groups form the tetrahedral complexes with cobalt in the extraction. However di(dodecy1)- phosphoric acid (11), a normal alkyl derivative, forms the octahedral complex with cobalt. Further di(3-methylbutyl)phosphoric acid (12), 2-substituted alkyl derivative, gives the mixture of the tetrahedral and octahedral complexes with cobalt. The coordination numbers of the representative phosphate complexes are given in Table 5. From the data, it seems that the steric hindrance of the alkyl groups in the phosphates affects the formation of the cobalt complexes, and the formatiom of the tetrahedral complex is enhanced on the increase of the steric hindrance in the vicinity of the molecule. The following experiment also suggests the importance of the steric hindrance in the complex formation16) In the case of DEHPA (4), addition of ethanol to a kerosene solution of the cobalt complex causes color change of the solution, blue to purple. The result implies that ethanol coordinated the cobalt ion to form the octahedral complex in the solution. The similar change is also observed by addition of 1-butanol to the DEHPA system. The addition of 2-propanol (secondary alcohol) or 2-methyl-2-propanol (tertiary alcohol) does not cause a significant color change. Hence, it is obviously that 2-propanol and 2- methyl-2-propanol do not coordinate to the cobalt complex because of their steric hindrances. In the case of bis [2-(1-methylethyl)-5-methylcyclohexyl]phosphoric acid (10), which has more bulky alkyl groups than those of DEHPA, formation of the octahedral complex with ethanol is less than that of DEHPA. These results imply that the steric hindrance of the ligands play an important role in the formation of the cobalt complexes. The finding that there is no clear correlation between the net charge of the hydrogen in PO (OH) of phosphoric acid esters, which relates to acidity of the phosphates, and the extractability of rare earths reinforces the above notion10) In contrast, all of the phosphoric acid esters used in this study forms the octahedral complexes with nickel in the extraction. As indicated above, the phosphates with the bulky alkyl groups form the tetrahedral complexes with cobalt and the octahedral complexes with nickel. Therefore, the nickel complexes have higher coordination number than that of the corresponding cobalt complexes, and it may be suggested that the steric interaction between the ligands (phosphates) in the nickel complex is stronger than that in the cobalt complex. Namely, the steric effect of the phosphates on the nickel complex should be more influental than that in the cobalt complexes. It is reasonable to assume that the difference in the stability of the cobalt and nickel complexes, the cobalt selectivity over nickel, increases with increasing the steric hindrance of the phosphates. 5. Conclusion The solvent extraction of cobalt (II) and nickel (II) has been carried out by using a systematically varied series of phosphoric acid esters to elucidate the structural effect of the phosphates on the extraction behaviors of the metal ions. In the case of phosphoric acid diaryl ester, the phosphate extracts the both metals at the lowest ph among the phosphates used, and the octahedral complexes of cobalt and nickel are formed in the organic phase. By replacing the aryl group with the alkyl group, the extraction curve shifts to higher ph, and the formation of the tetrahedral complex of cobalt increases in the extraction. The phosphates with dialkyl groups extract the metal ions at higher ph than that with the aryl derivatives. In these cases, the phosphates form the tetrahedral complexes with cobalt and the octahedral complexes with nickel. Further, the increase of the steric hindrance of the alkyl groups in the phosphates causes the increase of the selectivity of cobalt over nickel in the extraction. References 1) Sato, T., Kawamura, M., Nakamura, T. and Ueda, M.: J. Appl. Chem. Biotechnol., Vol. 28, p.85-94, (1978) 2) Sato, T., Sato, K. and Horie, J.: Proc. Symp. Solvent Extr., p.35-40, (1992) 3) Ritcey, G. 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