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Correlation between reducing power and electronic structure of organic reducing agents used in sulfuric acid leaching of polymetallic nodules
Int. J. Miner. Process. 65 (2002) 191 – 202 www.elsevier.com/locate/ijminpro
Correlation between reducing power and electronic structure of organic reducing agents used in sulfuric acid leaching of polymetallic nodules Yahui Zhang a, Qi Liu a,*, Chuanyao Sun b a
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 b Beijing General Research Institute of Mining and Metallurgy, Beijing 100044, China
Received 18 June 2001; received in revised form 13 August 2001; accepted 14 August 2001
Abstract The electronic structures of organic reducing agents used in the leaching of polymetallic nodules and related organic compounds were investigated by means of INDO and CNDO methods of quantum chemistry. It was found that among the electronic structure indices studied, the highest occupied molecular orbital (HOMO) energy of an organic compound correlated best with its reducing power. The HOMO energy of oxalic acid was recommended as the criteria for the selection of effective reducing agents for the acid leaching of polymetallic nodules. D 2002 Elsevier Science B.V. All rights reserved. Keywords: polymetallic ocean nodules; reduction leaching; organic; electronic structure
1. Introduction Polymetallic ocean nodules (also known as manganese nodules), sediments of ferromanganese oxides on ocean floors, contain abundant valuable metals such as copper, nickel, cobalt as well as manganese. They are deemed alternative sources of these metals with the gradual depletion of land-based deposits.
*
Corresponding author. Fax: +1-780-492-2881. E-mail address: [email protected] (Q. Liu).
0301-7516/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 ( 0 1 ) 0 0 0 7 7 - 1
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The processing techniques of the nodules are quite specific. There are usually no separate mineral phases of nickel, copper and cobalt in the nodules. These metals are distributed in the manganese and iron oxide phases as solid solutions (Agarwal et al., 1976; Fuerstenau and Han, 1983). The nodules also contain about 30 –40 wt.% of water due to their fine porous structures. Consequently, the nodules cannot be economically processed using the conventional ore dressing and pyrometallurgical processes. Hydrometallurgy is the process route of choice. Manganese in the nodules is normally present in its tetravalent form, as y-MnO2, birnessite or todorokite, and iron as goethite, maghemite or Mn-hematite (Fuerstenau and Han, 1983). During hydrometallurgical processing, an acid solution or an ammoniacal solution is used to destroy the ferromanganese oxide phases so that the other valuable metals are released. A reducing agent is usually required to transform the tetravalent manganese to its more soluble divalent states in order to leach the nodules at ambient temperature (Fuerstenau and Han, 1983). In fact, most of the studies to date have focused on the selection of an appropriate reducing agent, both inorganic and organic. The inorganic reducing agents studied include sulfur dioxide (Khalafalla and Pahlman, 1981; Kanungo and Das, 1988; Kawahara and Mitsuo, 1993), sulfite (Kanungo and Jena, 1988), sulfides (Kanungo and Jena, 1988; Chen et al., 1992; Belikov, 1993) and hydrogen peroxide (Allen et al., 1991). Organic reducing agents that have been studied include oxalic acid (Zeitlin and Fernando, 1981), glucose (Okuwaki et al., 1977; Das et al., 1986), sucrose (Veglio and Toro, 1994), and alcohols (Jana et al., 1993, 1995; Momade and Momade, 1999). Generally, the consumption of the reducing agents is high and there are no obvious ways to regenerate and recycle them. Besides, the inorganic reducing agents usually result in a leach residue that is difficult to handle, and the organic reducing agents need to be activated at elevated temperatures and even then, have a slow leaching rate. Recently, Sun et al. (1995) and Zhang et al. (2001a,b) reported the use of hydroxybenzene (phenols) and aromatic amines as reducing agents in the sulfuric acid leaching of manganese ores and polymetallic nodules. Comparatively, these reagents do not need to be activated so that the leaching can be carried out at ambient temperatures. The leaching is fast and the residue is easy to handle. The leaching efficiency is comparable to those of the most effective inorganic reducing agents, such as sodium sulfite and hydrogen peroxide. Most importantly, these organic reducing agents can potentially be regenerated through electrochemical methods by taking advantage of the following redox reactions (Fessenden and Fessenden, 1986; Schmid, 1996):
ð1Þ
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ð2Þ
In light of the advantages of the phenol type and aromatic amine type reducing agents, it is felt that a better understanding of the reducing properties of the organic compounds are needed. This is especially so when one realizes that unlike inorganic reducing agents, which can be chosen by the related half-cell reaction potentials, effective organic reducing agents are difficult to select because of the lack of electrochemical data. From a fundamental point of view, the reducing property of organic reducing agents originates from electronic structures of their molecules. It is of theoretical and practical significance to study their electronic structures, find the indices correlated to the reducing property, and build criteria for the selection of new organic reducing agents. In this communication, the electronic structures of a number of organic reducing agents and related organic compounds have been investigated by means of INDO and CNDO methods of quantum chemistry. Based on the results, a criterion has been set up to choose organic compounds as reducing agents in the acid leaching of polymetallic ocean nodules.
2. Experimental The polymetallic ocean nodule samples, collected from the central Pacific Ocean basin, were air-dried, crushed and ground to 77% passing 74 Am. Chemical analysis indicated that the sample contained 21.09% Mn, 0.43% Cu, 0.65% Ni, 0.33% Co, 13.37% Fe, 1.54% TiO2, 3.3% Al2O3, 11.59% SiO2, 2.24% CaO, and 1.97% MgO.
Fig. 1. Effect of substitution on the reactivity of aromatic rings towards electrophiles (re-plotted from McMurray, 1992).
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Leaching experiments were carried out under ambient temperature in a fume hood. A 20-g nodule sample was leached in 80 ml of solution containing 2.36 mol/l sulfuric acid and pre-determined amount of an organic reducing agent. The slurry was agitated in a 250ml beaker with a magnetic stirrer for 20 min. Afterwards, the pulp was filtered, and the
Fig. 2. Structures of studied organic compounds.
Y. Zhang et al. / Int. J. Miner. Process. 65 (2002) 191–202
Fig. 2 (continued ).
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Fig. 2 (continued ).
filter cake (leach residue) was washed, dried and sent for chemical analysis. The sulfuric acid, sodium sulfite and the organic reagents used were of chemical grade and were used directly without further purification. Calculation of the electronic structure indices of the organic compounds was carried out on a Pentium III 450 MHz desktop computer, using a software package Gaussian 98 W
Y. Zhang et al. / Int. J. Miner. Process. 65 (2002) 191–202
197
Fig. 2 (continued ).
and GaussView 2.1. The package was purchased from Gaussain, Pittsburgh, USA. INDO and CNDO methods (Frisch et al., 1998) were used.
3. Results and discussions 3.1. Reducing property of aromatic compounds Lewis acid-base theory has been successfully applied to interpret organic reactions by organic chemists. They use the term ‘‘nucleophile’’ for a Lewis base, which donates electrons, and ‘‘electrophile’’ for a Lewis acid, which accepts electrons. It is known that in redox reactions, electron transfer occurs and the reducing agent can be considered a donor of electrons and the oxidizing agent an acceptor of electrons. Ussanovich (1939) and Gutmann (1978) suggested that there are no clear borderlines between Lewis-type reactions and redox interactions that involve changes in oxidation numbers. The reactivity of benzene derivatives to electrophiles has been well-documented by organic chemists. This can be used as a reference to explore the reducing property of related aromatic compounds. The effect of substitution on the reactivity of aromatic rings towards electrophiles can be sketched as shown in Fig. 1. It can be seen that amino-benzene (aniline), hydroxybenzene (phenol) and methylbenzene (toluene) are more reactive than benzene. On the other hand, – COOH and –NO2 deactivate the aromatic rings. Therefore, the corresponding reducing power of aromatic amines and phenols, as well as their derivatives and related aromatic compounds as shown in Fig. 2, should have the following order: (a) aniline > phenol >toluene> benzene; (b) p-phenylene diamine, o-phenylene diamine, m-phenylene diamine > p-amino
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toluene, o-amino toluene, m-amino toluene>aniline>o-aminobenzoic acid>onitroaniline, p-nitroaniline, m-nitroaniline; (c) 1,2,3-trihydroxybenzene >p-dihydroxybenzene, o-dihydroxybenzene, m-dihydroxybenzene > hydroxybenzene; (d) 3,4,5-trihydroxybenzoic acid >2,4-dihydroxybenzoic acid >o-hydroxybenzoic acid; (e) hydroxybenzene >o-hydroxybenzoic acid; (f) m-dihydroxybenzene >2,4-dihydroxybenzoic acid; (g) 1,2,3-trihydroxybenzene > 3,4,5-trihydroxybenzoic acid. 3.2. Correlation between reducing power and electronic structure indices of aromatic compounds The reducing power of the aromatic compounds is determined by their electronic structures, which can be investigated with the theory of quantum chemistry. First, the semi-empirical INDO method was employed for this purpose. The structural parameters of atomic groups, such as –NH2, –OH, –COOH, –NO2, in the representative compounds adopted in the calculation were from Bowen et al. (1958), and the remainder were set by GaussView 2.1. Electronic structure indices, including the energy of highest occupied molecular orbital (the HOMO), the energy of lowest unoccupied molecular orbital (the LUMO), gross orbital population, atomic Mulliken population, atomic charge, total Mulliken population on benzene ring, were analyzed. It was found that the energy of HOMO correlated best with the reducing power of the studied aromatic compounds. Therefore, only the HOMO and LUMO energies are listed in Table 1. As can be seen, for aromatic amines and phenols, as well as their derivatives and related aromatic compounds, the higher the HOMO energy, the stronger the reducing power, i.e. the HOMO energies follow the same order as the reducing power as shown in the previous section. Therefore, the HOMO energy may be used as an indication of the reducing power of the aromatic compounds. 3.3. Criteria for the selection of reducing agents to be used in the sulfuric acid leaching of polymetallic nodules So far, we have established the correlation between HOMO energy and reducing power for aromatic compounds. In order to find the correlation between the HOMO energy and the reducing power of nonaromatic organic compounds, the electronic structures of other related organic compounds, such as cyclohexane, cyclohexylamine, cyclohexanol, methanol, oxalic acid, and ascorbic acid (vitamin C), were studied using the INDO method. In the meantime, the reducing effects of these aromatic and nonaromatic compounds on the sulfuric acid leaching of polymetallic nodules were tested under ambient temperature. The results are presented in Table 1 and some of the data are plotted in Fig 3. As can be seen from Fig. 3, the HOMO energy of an organic compound (whether aromatic or not) correlates well with the manganese extraction when it was used as reducing agent in the sulfuric acid leaching of polymetallic nodules. Generally, when the HOMO energy of an organic compound is higher than that of oxalic acid, high manganese
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Table 1 The HOMO and LUMO energies of organic compounds calculated with INDO method and the manganese extraction Organic compound Blank (no organic compounds) Sodium sulfite Benzene Methylbenzene (toluene) Amino-benzene (aniline) Hydroxybenzene (phenol) o-Amino toluene p-Amino toluene m-Amino toluene o-Phenylene diamine p-Phenylene diamine m-Phenylene diamine o-Nitroaniline p-Nitroaniline m-nitroaniline o-Aminobenzoic acid o-Dihydroxybenzene p-Dihydroxybenzene m-Dihydroxybenzene 1,2,3-Trihydroxybenzene o-Hydroxybenzoic acid 2,4-Dihydroxybenzoic acid 3,4,5-Trihydroxybenzoic acid 1-Naphthylamine Cyclohexane Cyclohexylamine Cyclohexanol Methanol Oxalic acid Ascorbic acid (vitamin C)
EHOMO (INDO) (a.u.)
0.48766 0.45679 0.40284 0.43533 0.39506 0.38713 0.40069 0.37088 0.36231 0.38970 0.41847 0.42670 0.43227 0.40875 0.41264 0.40540 0.42799 0.41261 0.43955 0.43050 0.42959 0.34813 0.48896 0.45338 0.46077 0.52970 0.43962 0.37745
ELUMO (INDO) (a.u.)
Mn extraction (%)
Dosage of reducing agent (g/g nodule)
0.16627 0.16342 0.16652 0.15915 0.16087 0.16446 0.16085 0.16862 0.16535 0.16928 0.08769 0.08997 0.08715 0.10350 0.15662 0.15184 0.15650 0.15761 0.09678 0.09705 0.08858 0.10019 0.26267 0.25541 0.25025 0.25973 0.09949 0.08656
3.0 98.6 2.8 3.1 99.6 97.2 95.5 96.0 not tested 98.7 96.1 98.5 86.1 not tested 85.1 98.1 99.9 94.7 97.2 99.7 not tested 97.4 99.8 84.1 2.2 2.4 3.3 2.7 60.8 98.8
– 0.30 1.00 1.00 0.20 0.25 0.25 0.25 0.20 0.40 0.20 0.35 0.40 0.25 0.25 0.40 0.25 0.25 0.25 0.20 0.50 1.00 1.00 1.00 1.00 0.40 0.30
1 a.u. (energy) = 27.2116 eV = 2625.5112 kJ/mol.
extraction can be achieved; the leaching efficiency is comparable to that of sodium sulfite, one of the most effective inorganic reducing agents. Otherwise, the manganese extraction is very low, at less than 5%, which is the same as the blank test in which no reducing agent is used. Oxalic acid, whose HOMO energy is 0.43962 a.u. and whose use results in the manganese extraction of about 60%, is located on the separating line between high manganese extraction and low manganese extraction. Actually to all the organic compounds listed in Table 1, the HOMO energy of oxalic acid can be considered the threshold HOMO energy for effective manganese extraction. The analysis of electronic structures of the representative organic compounds with CNDO method shows a similar rule (the same structure parameters as in INDO calculation were used). As illustrated in Fig. 4, those organic compounds that have HOMO energies
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Fig. 3. Correlation between manganese extraction and HOMO energy of organic reducing agent. Data points are taken from Table 1. Manganese extraction was achieved by leaching a 20-g nodule sample in 80 ml of solution containing 2.36 mol/l sulfuric acid in the presence of the organic compound (dosage varied, see Table 1). The HOMO energies of the organic compounds were calculated with INDO method.
above the HOMO energy of oxalic acid can achieve excellent manganese extractions, and those that have HOMO energies below the HOMO energy of oxalic acid have no reducing effect on the acid leaching of polymetallic nodules. As seen from the preceding discussion, the reducing power of an organic compound correlates with its HOMO energy. As an effective organic reducing agent, its HOMO energy should be above a certain threshold to guarantee enough reducing power for the sulfuric acid leaching of polymetallic nodules at ambient temperature. It seems that the HOMO energy of oxalic acid can be taken as the threshold.
Fig. 4. Correlation between manganese extraction and HOMO energy of organic reducing agent. Manganese extraction is taken from Table 1. Manganese extraction was achieved by leaching a 20-g nodule sample in 80 ml of solution containing 2.36 mol/l sulfuric acid in the presence of the organic compound (dosage varied, see Table 1). The HOMO energies of the organic compounds were calculated with CNDO method.
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4. Conclusions INDO and CNDO calculations of the electronic structures of organic compounds indicated that the HOMO energy of an organic compound correlated well with its reducing power. When the HOMO energy was above a threshold value, which in this study was the HOMO energy of oxalic acid, an organic compound may be used as an effective reducing agent for the sulfuric acid leaching of polymetallic nodules. It will be interesting to see if the threshold value of HOMO energy set by oxalic acid in this study is still valid when more organic compounds are examined. Also, it will be significant to find out if such correlation also exists in other reduction leaching systems.
References Agarwal, J.C., Beecher, N., Davis, D.S., Hubred, G.L., Kakaria, V.K., Kust, R.N., 1976. Processing of ocean nodules: a technical and economic review. J. Met. 28, 24 – 31. Allen, J.P., Abercrombie, H.L., Rice, D.A., 1991. Leaching and recovery of metals from cobalt-rich manganese crust. Miner. Metall. Process. 8 (2), 97 – 104. Belikov, V.V., 1993. Russian (Soviet Union) Patent, 1715873. Bowen, H.J.M., Donohue, J., Jenkin, D.G., Kennard, O., Wheatley, P.J., Whiffen, D.H., 1958. Tables of Interatomic Distances and Configuration in Molecules and Ions. The Chemical Society, Special Publication No. 11, Burlington House, London. Chen, H., Fu, C., Zheng, D., 1992. Reduction leaching of manganese nodules by nickel matte in hydrochloric acid solution. Hydrometallurgy 28, 269 – 275. Das, R.P., Anand, S., Das, S.C., Jena, P.K., 1986. Leaching of manganese nodules in ammoniacal medium using glucose as reductant. Hydrometallurgy 16, 335 – 344. Fessenden, R.J., Fessenden, J.S., 1986. Organic Chemistry, 3rd edn. Brooks/Cole Publishing, Monterey, CA, p. 495. Frisch, M.J., Trucks, G.W., Schlegel, H.B., Scuseria, G.E., Robb, M.A., Cheeseman, J.R., Zakrzewski, V.G., Montgomery, J.A., Stratmann Jr., R.E., Burant, J.C., Dapprich, S., Millam, J.M., Daniels, A.D., Kudin, K.N., Strain, M.C., Farkas, O., Tomasi, J., Barone, V., Cossi, M., Cammi, R., Mennucci, B., Pomelli, C., Adamo, C., Clifford, S., Ochterski, J., Petersson, G.A., Ayala, P.Y., Cui, Q., Morokuma, K., Malick, D.K., Rabuck, A.D., Raghavachari, K., Foresman, J.B., Cioslowski, J., Ortiz, J.V., Baboul, A.G., Stefanov, B.B., Liu, G., Liashenko, A., Piskorz, P., Komaromi, I., Gomperts, R., Martin, R.L., Fox, D.J., Keith, T., Al-Laham, M.A., Peng, C.Y., Nanayakkara, A., Challacombe, M., Gill, P.M.W., Johnson, B., Chen, W., Wong, M.W., Andres, J.L., Gonzalez, C., Head-Gordon, M., Replogle, E.S., Pople, J.A., 1998. Gaussian 98, Revision A.9. Gaussian, Pittsburgh, PA. Fuerstenau, D.W., Han, K.N., 1983. Metallurgy and processing of marine manganese nodules. Miner. Process. Technol. Rev. 1, 1 – 83. Gutmann, V., 1978. The Donor – Acceptor Approach to Molecular Interactions. Plenum, New York. Jana, R.K., Singh, D.D.N., Roy, S.K., 1993. Hydrochloric acid leaching of sea nodules with methanol and ethanol addition. Mater. Trans., JIM 34 (7), 593 – 598. Jana, R.K., Singh, D.D.N., Roy, S.K., 1995. Alcohol modified hydrochloric acid leaching of sea nodules. Hydrometallurgy 38 (3), 289 – 298. Kanungo, S.B., Das, R.P., 1988. Extraction of metals from manganese nodules of the Indian Ocean by leaching in aqueous solution of sulfur dioxide. Hydrometallurgy 20, 135 – 146. Kanungo, S.B., Jena, P.K., 1988. Reduction leaching of manganese nodules of Indian Ocean origin in dilute hydrochloric acid. Hydrometallurgy 21, 41 – 58. Kawahara, M., Mitsuo, T., 1993. Selective leaching of manganese nodules using sulfur dioxide as a reductant. In: Heinein, H., Oki, T. (Eds.), Proceedings of the First International Conference on Processing Materials for Properties. The Minerals, Metals & Materials Society, AIME, Warrendale, PA, USA, pp. 471 – 474.
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Khalafalla, S.E., Pahlman, J.E., 1981. Selective extraction of metals from Pacific sea nodules with dissolved sulfur dioxide. J. Met. 33, 37 – 42. McMurray, J., 1992. Organic Chemistry. 3rd edn. Brooks/Cole Publishing, Pacific Grove, CA, p. 573. Momade, F.W.Y., Momade, Zs.G., 1999. Reductive leaching of manganese oxide ore in aqueous methanol – sulfuric acid medium. Hydrometallurgy 51, 103 – 113. Okuwaki, A., Ito, H., Okabe, T., 1977. J. Chem. Soc. Jpn., Chem. Ind. Chem. 10, 1462 – 1468. Schmid, G.H., 1996. Organic Chemistry. Mosby-Year Book, St. Louis, MO, USA, pp. 1015 – 1042. Sun, C., Zhang, Y. and Wang, D., 1995. Chinese Patent, ZL95106747.8. Ussanovich, M., 1939. Zh. Obshch. Khim., USSR 9, 182. Veglio, F., Toro, L., 1994. Reductive leaching of a concentrate manganese dioxide ore in acid solution: stoichiometry and preliminary kinetic analysis. Int. J. Miner. Process. 40, 257 – 272. Zeitlin, H., Fernando, Q., 1981. US Patent, 4259295. Zhang, Y., Liu, Q., Sun, C., 2001a. Sulfuric acid leaching of ocean manganese nodules using phenols as reducing agents. Miner. Eng. 14 (5), 525 – 537. Zhang, Y., Liu, Q., Sun, C., 2001b. Sulfuric acid leaching of ocean manganese nodules using aromatic amines as reducing agents. Miner. Eng. 14 (5), 539 – 542.
Correlation between reducing power and electronic structure of organic reducing agents used in sulfuric acid leaching of polymetallic nodules Yahui Zhang a, Qi Liu a,*, Chuanyao Sun b a
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 b Beijing General Research Institute of Mining and Metallurgy, Beijing 100044, China
Received 18 June 2001; received in revised form 13 August 2001; accepted 14 August 2001
Abstract The electronic structures of organic reducing agents used in the leaching of polymetallic nodules and related organic compounds were investigated by means of INDO and CNDO methods of quantum chemistry. It was found that among the electronic structure indices studied, the highest occupied molecular orbital (HOMO) energy of an organic compound correlated best with its reducing power. The HOMO energy of oxalic acid was recommended as the criteria for the selection of effective reducing agents for the acid leaching of polymetallic nodules. D 2002 Elsevier Science B.V. All rights reserved. Keywords: polymetallic ocean nodules; reduction leaching; organic; electronic structure
1. Introduction Polymetallic ocean nodules (also known as manganese nodules), sediments of ferromanganese oxides on ocean floors, contain abundant valuable metals such as copper, nickel, cobalt as well as manganese. They are deemed alternative sources of these metals with the gradual depletion of land-based deposits.
*
Corresponding author. Fax: +1-780-492-2881. E-mail address: [email protected] (Q. Liu).
0301-7516/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 ( 0 1 ) 0 0 0 7 7 - 1
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The processing techniques of the nodules are quite specific. There are usually no separate mineral phases of nickel, copper and cobalt in the nodules. These metals are distributed in the manganese and iron oxide phases as solid solutions (Agarwal et al., 1976; Fuerstenau and Han, 1983). The nodules also contain about 30 –40 wt.% of water due to their fine porous structures. Consequently, the nodules cannot be economically processed using the conventional ore dressing and pyrometallurgical processes. Hydrometallurgy is the process route of choice. Manganese in the nodules is normally present in its tetravalent form, as y-MnO2, birnessite or todorokite, and iron as goethite, maghemite or Mn-hematite (Fuerstenau and Han, 1983). During hydrometallurgical processing, an acid solution or an ammoniacal solution is used to destroy the ferromanganese oxide phases so that the other valuable metals are released. A reducing agent is usually required to transform the tetravalent manganese to its more soluble divalent states in order to leach the nodules at ambient temperature (Fuerstenau and Han, 1983). In fact, most of the studies to date have focused on the selection of an appropriate reducing agent, both inorganic and organic. The inorganic reducing agents studied include sulfur dioxide (Khalafalla and Pahlman, 1981; Kanungo and Das, 1988; Kawahara and Mitsuo, 1993), sulfite (Kanungo and Jena, 1988), sulfides (Kanungo and Jena, 1988; Chen et al., 1992; Belikov, 1993) and hydrogen peroxide (Allen et al., 1991). Organic reducing agents that have been studied include oxalic acid (Zeitlin and Fernando, 1981), glucose (Okuwaki et al., 1977; Das et al., 1986), sucrose (Veglio and Toro, 1994), and alcohols (Jana et al., 1993, 1995; Momade and Momade, 1999). Generally, the consumption of the reducing agents is high and there are no obvious ways to regenerate and recycle them. Besides, the inorganic reducing agents usually result in a leach residue that is difficult to handle, and the organic reducing agents need to be activated at elevated temperatures and even then, have a slow leaching rate. Recently, Sun et al. (1995) and Zhang et al. (2001a,b) reported the use of hydroxybenzene (phenols) and aromatic amines as reducing agents in the sulfuric acid leaching of manganese ores and polymetallic nodules. Comparatively, these reagents do not need to be activated so that the leaching can be carried out at ambient temperatures. The leaching is fast and the residue is easy to handle. The leaching efficiency is comparable to those of the most effective inorganic reducing agents, such as sodium sulfite and hydrogen peroxide. Most importantly, these organic reducing agents can potentially be regenerated through electrochemical methods by taking advantage of the following redox reactions (Fessenden and Fessenden, 1986; Schmid, 1996):
ð1Þ
Y. Zhang et al. / Int. J. Miner. Process. 65 (2002) 191–202
193
ð2Þ
In light of the advantages of the phenol type and aromatic amine type reducing agents, it is felt that a better understanding of the reducing properties of the organic compounds are needed. This is especially so when one realizes that unlike inorganic reducing agents, which can be chosen by the related half-cell reaction potentials, effective organic reducing agents are difficult to select because of the lack of electrochemical data. From a fundamental point of view, the reducing property of organic reducing agents originates from electronic structures of their molecules. It is of theoretical and practical significance to study their electronic structures, find the indices correlated to the reducing property, and build criteria for the selection of new organic reducing agents. In this communication, the electronic structures of a number of organic reducing agents and related organic compounds have been investigated by means of INDO and CNDO methods of quantum chemistry. Based on the results, a criterion has been set up to choose organic compounds as reducing agents in the acid leaching of polymetallic ocean nodules.
2. Experimental The polymetallic ocean nodule samples, collected from the central Pacific Ocean basin, were air-dried, crushed and ground to 77% passing 74 Am. Chemical analysis indicated that the sample contained 21.09% Mn, 0.43% Cu, 0.65% Ni, 0.33% Co, 13.37% Fe, 1.54% TiO2, 3.3% Al2O3, 11.59% SiO2, 2.24% CaO, and 1.97% MgO.
Fig. 1. Effect of substitution on the reactivity of aromatic rings towards electrophiles (re-plotted from McMurray, 1992).
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Leaching experiments were carried out under ambient temperature in a fume hood. A 20-g nodule sample was leached in 80 ml of solution containing 2.36 mol/l sulfuric acid and pre-determined amount of an organic reducing agent. The slurry was agitated in a 250ml beaker with a magnetic stirrer for 20 min. Afterwards, the pulp was filtered, and the
Fig. 2. Structures of studied organic compounds.
Y. Zhang et al. / Int. J. Miner. Process. 65 (2002) 191–202
Fig. 2 (continued ).
195
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Fig. 2 (continued ).
filter cake (leach residue) was washed, dried and sent for chemical analysis. The sulfuric acid, sodium sulfite and the organic reagents used were of chemical grade and were used directly without further purification. Calculation of the electronic structure indices of the organic compounds was carried out on a Pentium III 450 MHz desktop computer, using a software package Gaussian 98 W
Y. Zhang et al. / Int. J. Miner. Process. 65 (2002) 191–202
197
Fig. 2 (continued ).
and GaussView 2.1. The package was purchased from Gaussain, Pittsburgh, USA. INDO and CNDO methods (Frisch et al., 1998) were used.
3. Results and discussions 3.1. Reducing property of aromatic compounds Lewis acid-base theory has been successfully applied to interpret organic reactions by organic chemists. They use the term ‘‘nucleophile’’ for a Lewis base, which donates electrons, and ‘‘electrophile’’ for a Lewis acid, which accepts electrons. It is known that in redox reactions, electron transfer occurs and the reducing agent can be considered a donor of electrons and the oxidizing agent an acceptor of electrons. Ussanovich (1939) and Gutmann (1978) suggested that there are no clear borderlines between Lewis-type reactions and redox interactions that involve changes in oxidation numbers. The reactivity of benzene derivatives to electrophiles has been well-documented by organic chemists. This can be used as a reference to explore the reducing property of related aromatic compounds. The effect of substitution on the reactivity of aromatic rings towards electrophiles can be sketched as shown in Fig. 1. It can be seen that amino-benzene (aniline), hydroxybenzene (phenol) and methylbenzene (toluene) are more reactive than benzene. On the other hand, – COOH and –NO2 deactivate the aromatic rings. Therefore, the corresponding reducing power of aromatic amines and phenols, as well as their derivatives and related aromatic compounds as shown in Fig. 2, should have the following order: (a) aniline > phenol >toluene> benzene; (b) p-phenylene diamine, o-phenylene diamine, m-phenylene diamine > p-amino
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toluene, o-amino toluene, m-amino toluene>aniline>o-aminobenzoic acid>onitroaniline, p-nitroaniline, m-nitroaniline; (c) 1,2,3-trihydroxybenzene >p-dihydroxybenzene, o-dihydroxybenzene, m-dihydroxybenzene > hydroxybenzene; (d) 3,4,5-trihydroxybenzoic acid >2,4-dihydroxybenzoic acid >o-hydroxybenzoic acid; (e) hydroxybenzene >o-hydroxybenzoic acid; (f) m-dihydroxybenzene >2,4-dihydroxybenzoic acid; (g) 1,2,3-trihydroxybenzene > 3,4,5-trihydroxybenzoic acid. 3.2. Correlation between reducing power and electronic structure indices of aromatic compounds The reducing power of the aromatic compounds is determined by their electronic structures, which can be investigated with the theory of quantum chemistry. First, the semi-empirical INDO method was employed for this purpose. The structural parameters of atomic groups, such as –NH2, –OH, –COOH, –NO2, in the representative compounds adopted in the calculation were from Bowen et al. (1958), and the remainder were set by GaussView 2.1. Electronic structure indices, including the energy of highest occupied molecular orbital (the HOMO), the energy of lowest unoccupied molecular orbital (the LUMO), gross orbital population, atomic Mulliken population, atomic charge, total Mulliken population on benzene ring, were analyzed. It was found that the energy of HOMO correlated best with the reducing power of the studied aromatic compounds. Therefore, only the HOMO and LUMO energies are listed in Table 1. As can be seen, for aromatic amines and phenols, as well as their derivatives and related aromatic compounds, the higher the HOMO energy, the stronger the reducing power, i.e. the HOMO energies follow the same order as the reducing power as shown in the previous section. Therefore, the HOMO energy may be used as an indication of the reducing power of the aromatic compounds. 3.3. Criteria for the selection of reducing agents to be used in the sulfuric acid leaching of polymetallic nodules So far, we have established the correlation between HOMO energy and reducing power for aromatic compounds. In order to find the correlation between the HOMO energy and the reducing power of nonaromatic organic compounds, the electronic structures of other related organic compounds, such as cyclohexane, cyclohexylamine, cyclohexanol, methanol, oxalic acid, and ascorbic acid (vitamin C), were studied using the INDO method. In the meantime, the reducing effects of these aromatic and nonaromatic compounds on the sulfuric acid leaching of polymetallic nodules were tested under ambient temperature. The results are presented in Table 1 and some of the data are plotted in Fig 3. As can be seen from Fig. 3, the HOMO energy of an organic compound (whether aromatic or not) correlates well with the manganese extraction when it was used as reducing agent in the sulfuric acid leaching of polymetallic nodules. Generally, when the HOMO energy of an organic compound is higher than that of oxalic acid, high manganese
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Table 1 The HOMO and LUMO energies of organic compounds calculated with INDO method and the manganese extraction Organic compound Blank (no organic compounds) Sodium sulfite Benzene Methylbenzene (toluene) Amino-benzene (aniline) Hydroxybenzene (phenol) o-Amino toluene p-Amino toluene m-Amino toluene o-Phenylene diamine p-Phenylene diamine m-Phenylene diamine o-Nitroaniline p-Nitroaniline m-nitroaniline o-Aminobenzoic acid o-Dihydroxybenzene p-Dihydroxybenzene m-Dihydroxybenzene 1,2,3-Trihydroxybenzene o-Hydroxybenzoic acid 2,4-Dihydroxybenzoic acid 3,4,5-Trihydroxybenzoic acid 1-Naphthylamine Cyclohexane Cyclohexylamine Cyclohexanol Methanol Oxalic acid Ascorbic acid (vitamin C)
EHOMO (INDO) (a.u.)
0.48766 0.45679 0.40284 0.43533 0.39506 0.38713 0.40069 0.37088 0.36231 0.38970 0.41847 0.42670 0.43227 0.40875 0.41264 0.40540 0.42799 0.41261 0.43955 0.43050 0.42959 0.34813 0.48896 0.45338 0.46077 0.52970 0.43962 0.37745
ELUMO (INDO) (a.u.)
Mn extraction (%)
Dosage of reducing agent (g/g nodule)
0.16627 0.16342 0.16652 0.15915 0.16087 0.16446 0.16085 0.16862 0.16535 0.16928 0.08769 0.08997 0.08715 0.10350 0.15662 0.15184 0.15650 0.15761 0.09678 0.09705 0.08858 0.10019 0.26267 0.25541 0.25025 0.25973 0.09949 0.08656
3.0 98.6 2.8 3.1 99.6 97.2 95.5 96.0 not tested 98.7 96.1 98.5 86.1 not tested 85.1 98.1 99.9 94.7 97.2 99.7 not tested 97.4 99.8 84.1 2.2 2.4 3.3 2.7 60.8 98.8
– 0.30 1.00 1.00 0.20 0.25 0.25 0.25 0.20 0.40 0.20 0.35 0.40 0.25 0.25 0.40 0.25 0.25 0.25 0.20 0.50 1.00 1.00 1.00 1.00 0.40 0.30
1 a.u. (energy) = 27.2116 eV = 2625.5112 kJ/mol.
extraction can be achieved; the leaching efficiency is comparable to that of sodium sulfite, one of the most effective inorganic reducing agents. Otherwise, the manganese extraction is very low, at less than 5%, which is the same as the blank test in which no reducing agent is used. Oxalic acid, whose HOMO energy is 0.43962 a.u. and whose use results in the manganese extraction of about 60%, is located on the separating line between high manganese extraction and low manganese extraction. Actually to all the organic compounds listed in Table 1, the HOMO energy of oxalic acid can be considered the threshold HOMO energy for effective manganese extraction. The analysis of electronic structures of the representative organic compounds with CNDO method shows a similar rule (the same structure parameters as in INDO calculation were used). As illustrated in Fig. 4, those organic compounds that have HOMO energies
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Fig. 3. Correlation between manganese extraction and HOMO energy of organic reducing agent. Data points are taken from Table 1. Manganese extraction was achieved by leaching a 20-g nodule sample in 80 ml of solution containing 2.36 mol/l sulfuric acid in the presence of the organic compound (dosage varied, see Table 1). The HOMO energies of the organic compounds were calculated with INDO method.
above the HOMO energy of oxalic acid can achieve excellent manganese extractions, and those that have HOMO energies below the HOMO energy of oxalic acid have no reducing effect on the acid leaching of polymetallic nodules. As seen from the preceding discussion, the reducing power of an organic compound correlates with its HOMO energy. As an effective organic reducing agent, its HOMO energy should be above a certain threshold to guarantee enough reducing power for the sulfuric acid leaching of polymetallic nodules at ambient temperature. It seems that the HOMO energy of oxalic acid can be taken as the threshold.
Fig. 4. Correlation between manganese extraction and HOMO energy of organic reducing agent. Manganese extraction is taken from Table 1. Manganese extraction was achieved by leaching a 20-g nodule sample in 80 ml of solution containing 2.36 mol/l sulfuric acid in the presence of the organic compound (dosage varied, see Table 1). The HOMO energies of the organic compounds were calculated with CNDO method.
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4. Conclusions INDO and CNDO calculations of the electronic structures of organic compounds indicated that the HOMO energy of an organic compound correlated well with its reducing power. When the HOMO energy was above a threshold value, which in this study was the HOMO energy of oxalic acid, an organic compound may be used as an effective reducing agent for the sulfuric acid leaching of polymetallic nodules. It will be interesting to see if the threshold value of HOMO energy set by oxalic acid in this study is still valid when more organic compounds are examined. Also, it will be significant to find out if such correlation also exists in other reduction leaching systems.
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