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Leaching of low grade zinc oxide ores in nitrilotriacetic acid solutions
Hydrometallurgy 161 (2016) 107–111
Contents lists available at ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Technical Note
Leaching of low grade zinc oxide ores in nitrilotriacetic acid solutions Tianzu Yang, Shuai Rao, Duchao Zhang ⁎, Jianfeng Wen, Weifeng Liu, Lin Chen, Xinwang Zhang School of Metallurgy and Environment, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 11 August 2015 Received in revised form 21 January 2016 Accepted 21 January 2016 Available online 22 January 2016 Keywords: Zinc oxide ores Nitrilotriacetic acid Leaching ratio Zinc complexes
a b s t r a c t A new method for the selective extraction of zinc from low grade zinc oxide ore in nitrilotriacetic acid solutions was proposed. A thermodynamic study indicated that the leaching process could be made efficient and selective by controlling the pH of the leaching solution. When low grade zinc oxide ore samples were leached in a 0.25 M nitrilotriacetic acid solution at 40 °C for 2 h with a liquid–solid ratio of 10 mL/g, the leaching ratios of zinc and iron were 91.0% and 4.1%, respectively. Additionally, the nitrilotriacetic acid was regenerated by adjusting the pH of the leaching solution after the leaching process. © 2016 Published by Elsevier B.V.
1. Introduction Currently, the main source of zinc metal is zinc sulphide ore. However, as zinc consumption increases and zinc sulphide ore grades decrease, the supply gap has been a major problem for the zinc industry (Abkhoshk et al., 2014; Ejtemaei et al., 2014). Therefore, the development of new technologies to produce zinc from zinc oxide ore has become an important research field in recent years. Zinc is mainly extracted from zinc oxide deposits using hydrometallurgical methods. Basically, this process contains three steps, leaching, purification and electrolysis (Safari et al., 2009). During the leaching process, the most common leaching agent is sulfuric acid. However, for low grade zinc oxide ore, especially those with high contents of iron, calcium and silicon, excessive acid consumption and complex purification process have caused significant concerns (Xu et al., 2010, 2012). The alkaline leaching process has attracted a great deal of attention due to its higher selectivity. Recently, extensive studies have been conducted on the treatment of zinc oxide ore by alkaline agents such as sodium hydroxide (Chen et al., 2009; Santos et al., 2010) and ammoniacal solutions (Ding et al., 2010; Liu et al., 2012a, 2012b; Yin et al., 2010). During the leaching process, OH− and NH3 serve as ligands by coordinating with Zn2 + to form soluble Zn(OH)2i − i (i = 1 − 4) and (i = 1 − 4) complexes, respectively. However, a relatively Zn(NH3)2+ i high leaching ratio can only be achieved under certain specific conditions, such as a large liquid–solid ratio or a high leaching agent concentration. This can be attributed to the low stability of zinc complexes that cannot completely dissolve insoluble zinc minerals (Yang et al., 2010). ⁎ Corresponding author. E-mail address: [email protected] (D. Zhang).
http://dx.doi.org/10.1016/j.hydromet.2016.01.024 0304-386X/© 2016 Published by Elsevier B.V.
Therefore, it is imperative to identify new ligands (or leaching agents) for the treatment of low grade zinc oxide ore. In this paper, a new ligand, nitrilotriacetic acid (H3NTA), is proposed as a leaching agent for treating low grade zinc oxide ore. Nitrilotriacetic acid is well known as a metal chelate agent that has been successfully employed in the electrogalvanizing industry. With nitrilotriacetic acid as a leaching agent, NTA3− can coordinate with Zn2+ to form soluble Zn(NTA)2i − 3i (i = 1 − 2) complexes. The formation constant of these zinc complexes is relatively high due to the effect of the chelate ring configuration. Therefore, the purpose of this study is to develop a novel hydrometallurgical process for the extraction of Zn from low grade zinc oxide ore that uses nitrilotriacetic acid as the leaching agent.
2. Experimental 2.1. Materials and analysis The low grade zinc oxide ore investigated in the present study was from Lanping town in the Yunnan province of China. Prior to the leaching experiments, all of the samples were washed, dried, ground and sieved until their particle sizes were below 74 μm. The phases of the low grade zinc oxide ore samples were detected using X-ray diffraction analysis (Rigaku TTR-III) on a 2θ scale with Cu Kα radiation (λ = 1.5406 Å, 50 kV and 100 mA) at a scanning rate of 10°/min from 10° to 80°. The XRD pattern is shown in Fig. 1; it indicates that hemimorphite (Zn4Si2O7(OH)2·H2O) and cerussite (PbCO3) comprised the main mineral phase, and quartz (SiO2) and calcite (CaCO3) were identified as the main gangue components. The elemental composition of the samples was characterized using X-ray fluorescence (XRF, Rigaku ZSX Primus II), and the results are listed in Table 1. They show that the
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T. Yang et al. / Hydrometallurgy 161 (2016) 107–111
Fig. 2. The distribution of zinc species in the Zn2+–NTA3− H2O system as the pH varies.
Fig. 1. The XRD pattern of the low grade zinc oxide ore samples.
main components of the low grade zinc oxide ore samples were Zn—10.82%, Fe—7.38%, Pb—7.80%, Ca—21.31% and Si—13.53%. The zinc and iron contents of the leaching residues were determined using potassium dichromate titration and EDTA titration, respectively (Vogel, 2000). Their leaching ratios were calculated using the following equation: ηMe ¼
m1 wt 1 −m2 wt 2 100%; m1 wt 1
ð1Þ
where ηMe is the leaching ratio of a metal, m1 and m2 are the mass of the low grade zinc oxide ore and the leaching residue, respectively, and wt1 and wt2 are the metal contents of the low grade zinc oxide ore and the leaching residue, respectively. 2.2. Leaching experiments The leaching experiments were conducted in a 250 mL three-neck flask immersed in a thermostatically controlled water bath and equipped with a mechanical stirrer, a thermometer and a reflux condenser. A total of 100 mL of the leaching solution was prepared using reagent-grade nitrilotriacetic acid and deionized water. The desired amount of ore was added to the leaching solution when the temperature reached the pre-set value. After the completion of the leaching experiment, the hot slurry was filtered, and the solid residue was dried at 363 K until a constant weight was obtained. The phases and elemental composition of the leaching residue were detected by XRD and XRF, respectively. The pH of the leaching solution was measured using a Mettler MT320-S pH metre with a LE438 electrode.
the equilibrium reactions that occur in an aqueous leaching system and determines the predominant species in the solution under specific conditions. Fig. 2 shows diagrams of the distribution of zinc species in the Zn2+– NTA3− H2O system. It demonstrates that free Zn2+ accounts for most of the total zinc concentration when the pH is less than 1.0, and the Zn(NTA)− complex becomes the predominant species when the pH is between 3.0 and 8.0. It is worth noting that zinc oxide is present when the pH is greater than 9.0. This indicates that zinc ions in solution can be stabilized by NTA3− but can also precipitate as oxides. During the leaching process, the formation of the soluble ZnNTA− complex improves zinc extraction. Therefore, the pH of the leaching solution should be kept below 9.0. Fig. 3 illustrates the distribution of the iron species in the Fe3+– NTA3 − H2O system. Obviously, the FeNTA complex is predominant when the pH is less than 2.0, and iron oxide predominates when the pH is greater than 6.0. Because the dissolution of iron during this leaching process is undesirable, the pH of the leaching solution should be kept above 6.0. Based on this analysis, a leaching solution with a pH between 6.0 and 9.0 should be selected to ensure a selective and efficient leaching process. 3.2. Effects of the nitrilotriacetic acid concentration and the reaction temperature The effect of the nitrilotriacetic acid concentration on zinc and iron extraction was studied. In these experiments, the leaching temperature
3. Results and discussion 3.1. Thermodynamic study of the Zn2+–NTA3− H2O and Fe3+–NTA3− H2O systems To elucidate the predominant species in the leaching system, species distribution diagrams were constructed using Medusa (Puente-Siller et al., 2013). This software is based on an algorithm developed by Eriksson (Eriksson, G., 1979) that minimizes the Gibbs free energy of Table 1 The elemental composition of the low grade zinc oxide ore samples. Component Wt.%
O
Ca
Si
Zn
Pb
Fe
Al
S
Cu
34.24
21.31
13.53
10.82
7.80
7.38
1.66
0.39
0.03
Fig. 3. The distribution of iron species in the Fe3+–NTA3− H2O system as the pH varies.
T. Yang et al. / Hydrometallurgy 161 (2016) 107–111
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Table 2 The elemental composition of the zinc leaching residue. Component Wt.%
Fig. 4. The effect of the nitrilotriacetic acid concentration on zinc and iron extraction and the pH of the leaching solution.
(40 °C), the liquid–solid ratio (10 mL/g) and the leaching time (2 h) were kept constant, as shown in Fig. 4. The results indicate that the nitrilotriacetic acid concentration has a noticeable effect on zinc extraction. When the nitrilotriacetic acid concentration is between 0.1 M and 0.3 M, the amount of zinc extracted improves significantly from 48.3% to 93.2%. However, it should be noted that when the nitrilotriacetic acid concentration is greater than 0.25 M, the zinc extraction improves only slightly, whereas the iron extraction improves significantly. This is largely attributable to the difference in the pH of the leaching solutions. The thermodynamic analysis shows that iron complexes are present when the pH is less than 6.0. When the nitrilotriacetic acid concentration varies from 0.25 M to 0.3 M, the pH of the leaching solution decreases from 6.61 to 5.50. Therefore, the dissolution of iron increases substantially when the nitrilotriacetic acid concentration increases from 0.25 M to 0.3 M. At a nitrilotriacetic concentration of 0.25 M and a liquid–solid ratio of 10 mL/g, the low grade zinc oxide ore samples were leached for 2 h at different temperatures. Data on the zinc and iron extraction and the pH of the leaching solutions are shown in Fig. 5. It can be seen that zinc and iron extraction are nearly independent of the reaction temperature. The result presents a completely different variation tendency than that obtained for sulfuric acid solutions (Souza et al., 2009; Xu et al., 2010, 2012), in which the elevated temperature increases both
Fig. 5. The effect of the reaction temperature on zinc and iron extraction and the pH of the leaching solution.
O
Ca
Si
Zn
Pb
Fe
Al
S
Cu
41.42
16.38
17.70
1.59
4.71
11.66
1.84
0.55
0.03
the reaction rate and the amount of zinc extracted. This is the result of the different leaching mechanisms. During the leaching of zinc oxide ore in a nitrilotriacetic acid solution, the zinc extraction process can be better explained by substituent group effects from the Lewis acid/base theory (Steer and Griffiths, 2013). Therefore, the coordinating capability of a ligand with respect to metal ions is a crucial factor in zinc extraction, whereas the temperature has a negligible effect on the leaching process. The leaching residue, which was obtained from a 0.25 M nitrilotriacetic acid solution with a liquid–solid ratio of 10 mL/g at a leaching temperature of 40 °C after 2 h, was chemically analysed using XRF. The results given in Table 2 show that the leaching residue predominately consisted of calcium, silicon and iron. The leaching process significantly reduced the zinc content. The phases of the leaching residue were detected using XRD. The results given in Fig. 6 show that the main phases of the leaching residue were calcite and silica. All of the characteristic peaks of hemimorphite are absent, which suggests that the hemimorphite was almost entirely leached. 3.3. Regeneration of the leaching agent The thermodynamic analysis suggests that the ZnNTA− complex is predominant in the leaching solution. Because the ZnNTA− complex is very stable in the leaching solution, it is inefficient to produce zinc directly by means of conventional solvent extraction and an electrowinning process. Moreover, the leaching agent used is an unconventional reagent with a competitive price. Therefore, it is necessary to recycle nitrilotriacetic acid before following the conventional technological route. It has been confirmed that free Zn2 + accounts for most of the total zinc concentration when the pH is less than 1.0. Additionally, nitrilotriacetic acid can precipitate due to its low solubility in acidic solutions. Therefore, the leaching agent can be regenerated by adjusting the pH to 1.0. In this study, the pH of the leaching solution was adjusted to 1.0 by adding an appropriate amount of sulfuric acid. The white precipitate obtained from the leaching solution was analysed using XRD and the
Fig. 6. The XRD pattern of the zinc leaching residue.
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A simplified flowchart of the hydrometallurgical processing technique used with low grade zinc oxide ore in this study, which is based on the above-mentioned economic and technical considerations, is presented in Fig. 8. The leaching solution is transformed into a zinc sulphate solution by adjusting its pH; the resulting solution can be used to produce zinc directly using the conventional electrowinning process. The recycled nitrilotriacetic acid is returned for the next leaching process. This provides an alternative method of extracting of zinc from low grade zinc oxide ore. 4. Conclusions On the basis of the results obtained from the process of leaching zinc in a nitrilotriacetic acid solution, the following can be concluded:
Fig. 7. The XRD pattern of the white precipitate obtained from the leaching solution when the pH is 1.0.
results were shown in Fig. 7. The main phases were anglesite (PbSO4), anhydrite (CaSO4) and nitrilotriacetic acid (H3NTA). Then, the nitrilotriacetic acid was further purified in an ammoniacal solution.
1. The thermodynamic study shows that an effective and selective leaching process can be achieved by keeping the pH of the leaching solutions between 6.0 and 9.0. 2. The results of the leaching experiments indicate that zinc and iron extraction is significantly affected by the nitrilotriacetic acid concentration, but is nearly independent of the reaction temperature. When low grade zinc oxide ore samples were leached in 0.25 M nitrilotriacetic acid solutions at 40 °C for 2 h at a liquid–solid ratio of 10 mL/g, the leaching ratios of zinc and iron were 91.0% and 4.1%, respectively. 3. The leaching agent can be regenerated by adjusting the pH of the leaching solution to 1.0.
Fig. 8. A simplified flowchart of the proposed method for the hydrometallurgical processing of low grade zinc oxide ore.
T. Yang et al. / Hydrometallurgy 161 (2016) 107–111
Acknowledgement This study was financially supported by the National Basic Research Programme of China (No. 2014CB643404), the National Natural Science Foundation of China (No. 51504292) and the 2011 Joint Innovation Center — “clean and effective utilization of strategic mineral resources”. References Abkhoshk, E., Jorjani, E., Al-Harahsheh, M.S., Rashchi, F., Naazeri, M., 2014. Review of the hydrometallurgical processing of non-sulfide zinc ores. Hydrometallurgy 149, 153–167. Chen, A., Zhao, Z.W., Jia, X., Long, S., Huo, G., Chen, X., 2009. Alkaline leaching Zn and its concomitant metals from refractory hemimorphite zinc oxide ore. Hydrometallurgy 97 (3–4), 228–232. Ding, Z., Yin, Z., Hu, H., Chen, Q., 2010. Dissolution kinetics of zinc silicate (hemimorphite) in ammoniacal solution. Hydrometallurgy 104 (2), 201–206. Ejtemaei, M., Gharabaghi, M., Irannajad, M., 2014. A review of zinc oxide mineral beneficiation using flotation method. Adv. Colloid Interf. Sci. 206, 68–78. Liu, Z., Liu, Z., Li, Q., Yang, T., Zhang, X., 2012a. Leaching of hemimorphite in (NH4)2SO4– NH3–H2O system and its mechanism. Hydrometallurgy 125–126, 137–143. Liu, Z., Liu, Z., Li, Q., Cao, Z., Yang, T., 2012b. Dissolution behavior of willemite in the (NH4)2SO4–NH3–H2O system. Hydrometallurgy 125–126, 50–54.
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Puente-Siller, D.M., Fuentes-Aceituno, J.C., Nava-Alonso, F., 2013. A kinetic-thermodynamic study of silver leaching in thiosulfate–copper–ammonia–EDTA solutions. Hydrometallurgy 134–135, 124–131. Safari, V., Arzpeyma, G., Rashchi, F., Mostoufi, N., 2009. A shrinking particle-shrinking core model for leaching of a zinc ore containing silica. Int. J. Miner. Process. 93 (1), 79–83. Santos, F.M.F., Penia, P.S., Porcaro, A., Oliviera, V.A., Silva, C.A., Leao, V.A., 2010. The kinetics of zinc silicate leaching in sodium hydroxide. Hydrometallurgy 102 (1–4), 43–49. Souza, A.D., Pina, P.S., Santos, F.M.F., Da Silva, C.A., Leão, V.A., 2009. Effect of iron in zinc silicate concentrate on leaching with sulphuric acid. Hydrometallurgy 95 (3–4), 207–214. Steer, J.M., Griffiths, A.J., 2013. Investigation of carboxylic acids and non-aqueous solvents for the selective leaching of zinc from blast furnace dust slurry. Hydrometallurgy 140, 34–41. Vogel, A.I., 2000. A Text Book of Quantitative Inorganic Analysis. Longmans Green Publishers. Xu, H., Wei, C., Li, C., Fan, G., Deng, Z., Li, M., Li, X., 2010. Sulfuric acid leaching of zinc silicate ore under pressure. Hydrometallurgy 105 (1–2), 186–190. Xu, H., Wei, C., Li, C., Fan, G., Deng, Z., Zhou, X., Qiu, S., 2012. Leaching of a complex sulfidic, silicate-containing zinc ore in sulfuric acid solution under oxygen pressure. Sep. Purif. Technol. 85, 206–212. Yang, T., Dou, A., Lei, C., Ren, J., Liu, Z., 2010. Ligand selection for complex-leaching valuable metals in hydrometallurgy. Trans. Nonferrous Metals Soc. China 20 (6), 1148–1153. Yin, Z., Ding, Z., Hu, H., Liu, K., Chen, Q., 2010. Dissolution of zinc silicate (hemimorphite) with ammonia–ammonium chloride solution. Hydrometallurgy 103 (1–4), 215–220.
Contents lists available at ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Technical Note
Leaching of low grade zinc oxide ores in nitrilotriacetic acid solutions Tianzu Yang, Shuai Rao, Duchao Zhang ⁎, Jianfeng Wen, Weifeng Liu, Lin Chen, Xinwang Zhang School of Metallurgy and Environment, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 11 August 2015 Received in revised form 21 January 2016 Accepted 21 January 2016 Available online 22 January 2016 Keywords: Zinc oxide ores Nitrilotriacetic acid Leaching ratio Zinc complexes
a b s t r a c t A new method for the selective extraction of zinc from low grade zinc oxide ore in nitrilotriacetic acid solutions was proposed. A thermodynamic study indicated that the leaching process could be made efficient and selective by controlling the pH of the leaching solution. When low grade zinc oxide ore samples were leached in a 0.25 M nitrilotriacetic acid solution at 40 °C for 2 h with a liquid–solid ratio of 10 mL/g, the leaching ratios of zinc and iron were 91.0% and 4.1%, respectively. Additionally, the nitrilotriacetic acid was regenerated by adjusting the pH of the leaching solution after the leaching process. © 2016 Published by Elsevier B.V.
1. Introduction Currently, the main source of zinc metal is zinc sulphide ore. However, as zinc consumption increases and zinc sulphide ore grades decrease, the supply gap has been a major problem for the zinc industry (Abkhoshk et al., 2014; Ejtemaei et al., 2014). Therefore, the development of new technologies to produce zinc from zinc oxide ore has become an important research field in recent years. Zinc is mainly extracted from zinc oxide deposits using hydrometallurgical methods. Basically, this process contains three steps, leaching, purification and electrolysis (Safari et al., 2009). During the leaching process, the most common leaching agent is sulfuric acid. However, for low grade zinc oxide ore, especially those with high contents of iron, calcium and silicon, excessive acid consumption and complex purification process have caused significant concerns (Xu et al., 2010, 2012). The alkaline leaching process has attracted a great deal of attention due to its higher selectivity. Recently, extensive studies have been conducted on the treatment of zinc oxide ore by alkaline agents such as sodium hydroxide (Chen et al., 2009; Santos et al., 2010) and ammoniacal solutions (Ding et al., 2010; Liu et al., 2012a, 2012b; Yin et al., 2010). During the leaching process, OH− and NH3 serve as ligands by coordinating with Zn2 + to form soluble Zn(OH)2i − i (i = 1 − 4) and (i = 1 − 4) complexes, respectively. However, a relatively Zn(NH3)2+ i high leaching ratio can only be achieved under certain specific conditions, such as a large liquid–solid ratio or a high leaching agent concentration. This can be attributed to the low stability of zinc complexes that cannot completely dissolve insoluble zinc minerals (Yang et al., 2010). ⁎ Corresponding author. E-mail address: [email protected] (D. Zhang).
http://dx.doi.org/10.1016/j.hydromet.2016.01.024 0304-386X/© 2016 Published by Elsevier B.V.
Therefore, it is imperative to identify new ligands (or leaching agents) for the treatment of low grade zinc oxide ore. In this paper, a new ligand, nitrilotriacetic acid (H3NTA), is proposed as a leaching agent for treating low grade zinc oxide ore. Nitrilotriacetic acid is well known as a metal chelate agent that has been successfully employed in the electrogalvanizing industry. With nitrilotriacetic acid as a leaching agent, NTA3− can coordinate with Zn2+ to form soluble Zn(NTA)2i − 3i (i = 1 − 2) complexes. The formation constant of these zinc complexes is relatively high due to the effect of the chelate ring configuration. Therefore, the purpose of this study is to develop a novel hydrometallurgical process for the extraction of Zn from low grade zinc oxide ore that uses nitrilotriacetic acid as the leaching agent.
2. Experimental 2.1. Materials and analysis The low grade zinc oxide ore investigated in the present study was from Lanping town in the Yunnan province of China. Prior to the leaching experiments, all of the samples were washed, dried, ground and sieved until their particle sizes were below 74 μm. The phases of the low grade zinc oxide ore samples were detected using X-ray diffraction analysis (Rigaku TTR-III) on a 2θ scale with Cu Kα radiation (λ = 1.5406 Å, 50 kV and 100 mA) at a scanning rate of 10°/min from 10° to 80°. The XRD pattern is shown in Fig. 1; it indicates that hemimorphite (Zn4Si2O7(OH)2·H2O) and cerussite (PbCO3) comprised the main mineral phase, and quartz (SiO2) and calcite (CaCO3) were identified as the main gangue components. The elemental composition of the samples was characterized using X-ray fluorescence (XRF, Rigaku ZSX Primus II), and the results are listed in Table 1. They show that the
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T. Yang et al. / Hydrometallurgy 161 (2016) 107–111
Fig. 2. The distribution of zinc species in the Zn2+–NTA3− H2O system as the pH varies.
Fig. 1. The XRD pattern of the low grade zinc oxide ore samples.
main components of the low grade zinc oxide ore samples were Zn—10.82%, Fe—7.38%, Pb—7.80%, Ca—21.31% and Si—13.53%. The zinc and iron contents of the leaching residues were determined using potassium dichromate titration and EDTA titration, respectively (Vogel, 2000). Their leaching ratios were calculated using the following equation: ηMe ¼
m1 wt 1 −m2 wt 2 100%; m1 wt 1
ð1Þ
where ηMe is the leaching ratio of a metal, m1 and m2 are the mass of the low grade zinc oxide ore and the leaching residue, respectively, and wt1 and wt2 are the metal contents of the low grade zinc oxide ore and the leaching residue, respectively. 2.2. Leaching experiments The leaching experiments were conducted in a 250 mL three-neck flask immersed in a thermostatically controlled water bath and equipped with a mechanical stirrer, a thermometer and a reflux condenser. A total of 100 mL of the leaching solution was prepared using reagent-grade nitrilotriacetic acid and deionized water. The desired amount of ore was added to the leaching solution when the temperature reached the pre-set value. After the completion of the leaching experiment, the hot slurry was filtered, and the solid residue was dried at 363 K until a constant weight was obtained. The phases and elemental composition of the leaching residue were detected by XRD and XRF, respectively. The pH of the leaching solution was measured using a Mettler MT320-S pH metre with a LE438 electrode.
the equilibrium reactions that occur in an aqueous leaching system and determines the predominant species in the solution under specific conditions. Fig. 2 shows diagrams of the distribution of zinc species in the Zn2+– NTA3− H2O system. It demonstrates that free Zn2+ accounts for most of the total zinc concentration when the pH is less than 1.0, and the Zn(NTA)− complex becomes the predominant species when the pH is between 3.0 and 8.0. It is worth noting that zinc oxide is present when the pH is greater than 9.0. This indicates that zinc ions in solution can be stabilized by NTA3− but can also precipitate as oxides. During the leaching process, the formation of the soluble ZnNTA− complex improves zinc extraction. Therefore, the pH of the leaching solution should be kept below 9.0. Fig. 3 illustrates the distribution of the iron species in the Fe3+– NTA3 − H2O system. Obviously, the FeNTA complex is predominant when the pH is less than 2.0, and iron oxide predominates when the pH is greater than 6.0. Because the dissolution of iron during this leaching process is undesirable, the pH of the leaching solution should be kept above 6.0. Based on this analysis, a leaching solution with a pH between 6.0 and 9.0 should be selected to ensure a selective and efficient leaching process. 3.2. Effects of the nitrilotriacetic acid concentration and the reaction temperature The effect of the nitrilotriacetic acid concentration on zinc and iron extraction was studied. In these experiments, the leaching temperature
3. Results and discussion 3.1. Thermodynamic study of the Zn2+–NTA3− H2O and Fe3+–NTA3− H2O systems To elucidate the predominant species in the leaching system, species distribution diagrams were constructed using Medusa (Puente-Siller et al., 2013). This software is based on an algorithm developed by Eriksson (Eriksson, G., 1979) that minimizes the Gibbs free energy of Table 1 The elemental composition of the low grade zinc oxide ore samples. Component Wt.%
O
Ca
Si
Zn
Pb
Fe
Al
S
Cu
34.24
21.31
13.53
10.82
7.80
7.38
1.66
0.39
0.03
Fig. 3. The distribution of iron species in the Fe3+–NTA3− H2O system as the pH varies.
T. Yang et al. / Hydrometallurgy 161 (2016) 107–111
109
Table 2 The elemental composition of the zinc leaching residue. Component Wt.%
Fig. 4. The effect of the nitrilotriacetic acid concentration on zinc and iron extraction and the pH of the leaching solution.
(40 °C), the liquid–solid ratio (10 mL/g) and the leaching time (2 h) were kept constant, as shown in Fig. 4. The results indicate that the nitrilotriacetic acid concentration has a noticeable effect on zinc extraction. When the nitrilotriacetic acid concentration is between 0.1 M and 0.3 M, the amount of zinc extracted improves significantly from 48.3% to 93.2%. However, it should be noted that when the nitrilotriacetic acid concentration is greater than 0.25 M, the zinc extraction improves only slightly, whereas the iron extraction improves significantly. This is largely attributable to the difference in the pH of the leaching solutions. The thermodynamic analysis shows that iron complexes are present when the pH is less than 6.0. When the nitrilotriacetic acid concentration varies from 0.25 M to 0.3 M, the pH of the leaching solution decreases from 6.61 to 5.50. Therefore, the dissolution of iron increases substantially when the nitrilotriacetic acid concentration increases from 0.25 M to 0.3 M. At a nitrilotriacetic concentration of 0.25 M and a liquid–solid ratio of 10 mL/g, the low grade zinc oxide ore samples were leached for 2 h at different temperatures. Data on the zinc and iron extraction and the pH of the leaching solutions are shown in Fig. 5. It can be seen that zinc and iron extraction are nearly independent of the reaction temperature. The result presents a completely different variation tendency than that obtained for sulfuric acid solutions (Souza et al., 2009; Xu et al., 2010, 2012), in which the elevated temperature increases both
Fig. 5. The effect of the reaction temperature on zinc and iron extraction and the pH of the leaching solution.
O
Ca
Si
Zn
Pb
Fe
Al
S
Cu
41.42
16.38
17.70
1.59
4.71
11.66
1.84
0.55
0.03
the reaction rate and the amount of zinc extracted. This is the result of the different leaching mechanisms. During the leaching of zinc oxide ore in a nitrilotriacetic acid solution, the zinc extraction process can be better explained by substituent group effects from the Lewis acid/base theory (Steer and Griffiths, 2013). Therefore, the coordinating capability of a ligand with respect to metal ions is a crucial factor in zinc extraction, whereas the temperature has a negligible effect on the leaching process. The leaching residue, which was obtained from a 0.25 M nitrilotriacetic acid solution with a liquid–solid ratio of 10 mL/g at a leaching temperature of 40 °C after 2 h, was chemically analysed using XRF. The results given in Table 2 show that the leaching residue predominately consisted of calcium, silicon and iron. The leaching process significantly reduced the zinc content. The phases of the leaching residue were detected using XRD. The results given in Fig. 6 show that the main phases of the leaching residue were calcite and silica. All of the characteristic peaks of hemimorphite are absent, which suggests that the hemimorphite was almost entirely leached. 3.3. Regeneration of the leaching agent The thermodynamic analysis suggests that the ZnNTA− complex is predominant in the leaching solution. Because the ZnNTA− complex is very stable in the leaching solution, it is inefficient to produce zinc directly by means of conventional solvent extraction and an electrowinning process. Moreover, the leaching agent used is an unconventional reagent with a competitive price. Therefore, it is necessary to recycle nitrilotriacetic acid before following the conventional technological route. It has been confirmed that free Zn2 + accounts for most of the total zinc concentration when the pH is less than 1.0. Additionally, nitrilotriacetic acid can precipitate due to its low solubility in acidic solutions. Therefore, the leaching agent can be regenerated by adjusting the pH to 1.0. In this study, the pH of the leaching solution was adjusted to 1.0 by adding an appropriate amount of sulfuric acid. The white precipitate obtained from the leaching solution was analysed using XRD and the
Fig. 6. The XRD pattern of the zinc leaching residue.
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A simplified flowchart of the hydrometallurgical processing technique used with low grade zinc oxide ore in this study, which is based on the above-mentioned economic and technical considerations, is presented in Fig. 8. The leaching solution is transformed into a zinc sulphate solution by adjusting its pH; the resulting solution can be used to produce zinc directly using the conventional electrowinning process. The recycled nitrilotriacetic acid is returned for the next leaching process. This provides an alternative method of extracting of zinc from low grade zinc oxide ore. 4. Conclusions On the basis of the results obtained from the process of leaching zinc in a nitrilotriacetic acid solution, the following can be concluded:
Fig. 7. The XRD pattern of the white precipitate obtained from the leaching solution when the pH is 1.0.
results were shown in Fig. 7. The main phases were anglesite (PbSO4), anhydrite (CaSO4) and nitrilotriacetic acid (H3NTA). Then, the nitrilotriacetic acid was further purified in an ammoniacal solution.
1. The thermodynamic study shows that an effective and selective leaching process can be achieved by keeping the pH of the leaching solutions between 6.0 and 9.0. 2. The results of the leaching experiments indicate that zinc and iron extraction is significantly affected by the nitrilotriacetic acid concentration, but is nearly independent of the reaction temperature. When low grade zinc oxide ore samples were leached in 0.25 M nitrilotriacetic acid solutions at 40 °C for 2 h at a liquid–solid ratio of 10 mL/g, the leaching ratios of zinc and iron were 91.0% and 4.1%, respectively. 3. The leaching agent can be regenerated by adjusting the pH of the leaching solution to 1.0.
Fig. 8. A simplified flowchart of the proposed method for the hydrometallurgical processing of low grade zinc oxide ore.
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Acknowledgement This study was financially supported by the National Basic Research Programme of China (No. 2014CB643404), the National Natural Science Foundation of China (No. 51504292) and the 2011 Joint Innovation Center — “clean and effective utilization of strategic mineral resources”. References Abkhoshk, E., Jorjani, E., Al-Harahsheh, M.S., Rashchi, F., Naazeri, M., 2014. Review of the hydrometallurgical processing of non-sulfide zinc ores. Hydrometallurgy 149, 153–167. Chen, A., Zhao, Z.W., Jia, X., Long, S., Huo, G., Chen, X., 2009. Alkaline leaching Zn and its concomitant metals from refractory hemimorphite zinc oxide ore. Hydrometallurgy 97 (3–4), 228–232. Ding, Z., Yin, Z., Hu, H., Chen, Q., 2010. Dissolution kinetics of zinc silicate (hemimorphite) in ammoniacal solution. Hydrometallurgy 104 (2), 201–206. Ejtemaei, M., Gharabaghi, M., Irannajad, M., 2014. A review of zinc oxide mineral beneficiation using flotation method. Adv. Colloid Interf. Sci. 206, 68–78. Liu, Z., Liu, Z., Li, Q., Yang, T., Zhang, X., 2012a. Leaching of hemimorphite in (NH4)2SO4– NH3–H2O system and its mechanism. Hydrometallurgy 125–126, 137–143. Liu, Z., Liu, Z., Li, Q., Cao, Z., Yang, T., 2012b. Dissolution behavior of willemite in the (NH4)2SO4–NH3–H2O system. Hydrometallurgy 125–126, 50–54.
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Puente-Siller, D.M., Fuentes-Aceituno, J.C., Nava-Alonso, F., 2013. A kinetic-thermodynamic study of silver leaching in thiosulfate–copper–ammonia–EDTA solutions. Hydrometallurgy 134–135, 124–131. Safari, V., Arzpeyma, G., Rashchi, F., Mostoufi, N., 2009. A shrinking particle-shrinking core model for leaching of a zinc ore containing silica. Int. J. Miner. Process. 93 (1), 79–83. Santos, F.M.F., Penia, P.S., Porcaro, A., Oliviera, V.A., Silva, C.A., Leao, V.A., 2010. The kinetics of zinc silicate leaching in sodium hydroxide. Hydrometallurgy 102 (1–4), 43–49. Souza, A.D., Pina, P.S., Santos, F.M.F., Da Silva, C.A., Leão, V.A., 2009. Effect of iron in zinc silicate concentrate on leaching with sulphuric acid. Hydrometallurgy 95 (3–4), 207–214. Steer, J.M., Griffiths, A.J., 2013. Investigation of carboxylic acids and non-aqueous solvents for the selective leaching of zinc from blast furnace dust slurry. Hydrometallurgy 140, 34–41. Vogel, A.I., 2000. A Text Book of Quantitative Inorganic Analysis. Longmans Green Publishers. Xu, H., Wei, C., Li, C., Fan, G., Deng, Z., Li, M., Li, X., 2010. Sulfuric acid leaching of zinc silicate ore under pressure. Hydrometallurgy 105 (1–2), 186–190. Xu, H., Wei, C., Li, C., Fan, G., Deng, Z., Zhou, X., Qiu, S., 2012. Leaching of a complex sulfidic, silicate-containing zinc ore in sulfuric acid solution under oxygen pressure. Sep. Purif. Technol. 85, 206–212. Yang, T., Dou, A., Lei, C., Ren, J., Liu, Z., 2010. Ligand selection for complex-leaching valuable metals in hydrometallurgy. Trans. Nonferrous Metals Soc. China 20 (6), 1148–1153. Yin, Z., Ding, Z., Hu, H., Liu, K., Chen, Q., 2010. Dissolution of zinc silicate (hemimorphite) with ammonia–ammonium chloride solution. Hydrometallurgy 103 (1–4), 215–220.