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The alkaline leaching of molybdenite flotation tailings associated with galena
Hydrometallurgy 129–130 (2012) 30–34
Contents lists available at SciVerse ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Technical note
The alkaline leaching of molybdenite flotation tailings associated with galena Yan Liu a, b, Yifei Zhang a,⁎, Fangfang Chen a, Yi Zhang a a National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China b Graduate University of Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e
i n f o
Article history: Received 27 April 2012 Received in revised form 9 July 2012 Accepted 28 July 2012 Available online 3 August 2012 Keywords: Molybdenite flotation tailings associated with galena Pre-roasting Alkaline leaching Lead Molybdenum
a b s t r a c t A method for the extraction of both Pb and Mo from molybdenite flotation tailings associated with galena is described. The flotation tailings were pre-roasted and then alkaline leached. The effect of temperature, initial concentration of sodium hydroxide solution, alkali to total calcine mass ratio, leaching time and stirring speed on the leaching of Pb and Mo were systematically investigated by experiment. Optimum conditions were identified as: pre-roasting at 600 °C for 1 h, alkaline leaching for 1 h at 130 °C with an alkali to total calcine mass ratio of 1.2:1, initial NaOH concentration 30%, and a stirring speed of 600 rpm. Under these conditions 98% of the Pb and 98% of the Mo in the flotation tailings were extracted. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum is a strategic metal which has a broad range of applications and uses. Approximately 80% of the world's molybdenum is used in metallurgical applications such as alloy steels, stainless steels and cast irons, and the other 20% is used in chemical applications. The main commercial source of molybdenum is molybdenite (MoS2). Most molybdenite is present as low-grade ores, and is associated with other valuable metals, e.g. Pb, Ni, W, Cu and Re. Generally, molybdenum concentrates are obtained by selective flotation from low-grade ores, and then produce MoO3 or Mo metal through roasting-ammonia leaching process (Zhang and Zhao, 2005). However, the flotation tailings resulting from this process still contain some valuable metals, e.g. Mo, Pb and Cu, so these tailings can be seen as either a resource or an environmental threat with the ceaseless exploitation of resources in the world. Further, the high international Mo price means that it is often economic to re-work the tailings, rather than mine new low-grade ore bodies. A low-grade molybdenum mineral associated with galena was obtained through flotation from these tailings, which cannot be further beneficiated by flotation methods. These tailings contain wulfenite which is insoluble in ammonia and the grade of Mo is also lower; therefore roasting-ammonia leaching process cannot be used to extract Mo from it. Over the past few years work to recover Mo from low-grade and complex ores has proceeded apace (Anand Rao et al., 2001; ⁎ Corresponding author. Tel./fax: +86 10 82544826. E-mail address: [email protected] (Y. Zhang). 0304-386X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2012.07.017
Ebrahimi-Kahrizsangi, et al., 2010; Juneja, et al, 1996; Kar, et al., 2005; Kholmogorov and Kononova, 2005; Kyung, et al., 2006; Nasernejad et al., 1999; Olson and Clark, 2008; Van den Berg, et al., 2002). Sodium hydroxide is also used as lixiviant for the leaching of low-grade Mo minerals (Wang et al., 2009). However, using sodium hydroxide to extract Pb and Mo from molybdenite flotation tailings associated with galena has not been explored. The commercial process for producing metallic lead from galena concentrates involves two stages: feeding to a sintering machine where, by contact with air at an elevated temperature, turning the lead concentrate into lead oxide; heating the lead oxide in a blast furnace with coke to yield metallic lead (Peng, 2003). The main materials of this process are above 55% galena concentrates. If this process is used to treat molybdenite flotation tailings associated with galena, molybdenum in the tailings will go to waste and the metallic lead also contains more impurities. So we cannot choose this process. Hydrometallurgical processes are an alternative approach to obtaining and purifying metallic lead from low-grade galena. Over the past thirty years, extensive work has been carried out on the treatment of galena with hydrochloric acid (Awakura et al., 1980), ferric chloride (Baláz, 1996; Dutrizac, 1986; Dutrizac and Chen, 1990), ferric sulfate (da Silva, 2004; Dutrizac and Chen, 1995), ferric fluorosilicate (Chen and Dreisinger, 1994) and nitrate solutions (Aydoğan et al., 2007; Kholmogorov et al., 2003; Pashkov et al., 2002). However, due to unfavorable economics, as well as the low aqueous solubility of the PbSO4 and PbCl2, none of these methods have been applied commercially. Sodium hydroxide was also used to treat lead minerals, but little progress was made because the reaction rates were slow, and lead compounds rather insoluble in NaOH (Peng, 2003).
Y. Liu et al. / Hydrometallurgy 129–130 (2012) 30–34
Although there are many reports on dealing with molybdenite, galena and wulfenite respectively, there is little information concerning the treatment of low-grade molybdenum mineral associated with galena. Only Shi et al. (2009) reported leaching a high Pb with low Mo ore using sodium hypochlorite. But sodium hypochlorite easily decomposes and is of high cost, in addition, is applied to only treat low S ores. Therefore, new methods need to be studied to efficiently extract Pb and Mo from these tailings. The purpose of this work is to investigate the extraction of both Pb and Mo from molybdenite flotation tailings associated with galena using a pre-roasting and alkaline leaching methodology. 2. Experimental 2.1. Materials The molybdenite flotation tailings associated with galena used in this study were obtained from the molybdenum concentrator plant location within Henan province, China. The molybdenite flotation tailings were enriched by flotation in Pb (to 20–25%) and had a Mo content of up to 2.4%. The X-ray diffraction (XRD) pattern of the flotation tailings is shown in Fig. 1. The chemical composition determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) is given in Table 1. The particle size distribution of the molybdenite flotation tailings associated with galena (measured by laser scattering in a Helos and Rodos granulometer) appears in Table 2. The mean particle diameter was 46.96 μm, which was calculated as the first moment of the volume size distribution function. All the reagents used in chemical analysis, including sodium hydroxide, citric acid and hydrochloric acid, were of analytical grade. And all solutions were made with deionized water. 2.2. Experimental procedure Pre-roasting and leaching studies were carried out on the laboratory scale. Pre-roasting was performed to determine whether new phases were formed and how this influenced leaching efficiency. All experiments were carried out using 80 g samples. The samples were charged into four ceramic crucibles (120 mm × 60 mm) and placed in a pre-heated electric muffle furnace. The temperature range of 550 to 850 °C and a residence time of 1 h were applied in the experiments. The solid mass after roasting is different on different pre-roasting temperature. When the pre-roasting temperature is 600 °C, the solid mass was 99.5% of original tailings mass. After pre-roasting, the calcine was leached using sodium hydroxide solution in a 0.5 L stainless steel autoclave. The temperature of the autoclave was measured with a thermocouple, displayed with a digital multi-meter with a precision of ±0.1 °C, and controlled by the heating furnace. The autoclave was equipped with a pressure gauge
31
Table 1 Chemical composition of the molybdenite flotation tailings associated with galena (A) and the solid residue (B). Component
Pb
Mo
S
Fe
Si
Ca
Cu
Na
(A) wt.% (B) wt.%
24.77 0.87
2.24 0.11
23.58 0.54
12.59 24.21
12.90 17.35
3.45 5.24
0.77 0.27
0.23 1.68
to measure the pressure. The stirring speed was 600 rpm in leaching experiments. Since we found stirring speed have no significant effect on the leaching in pre-experiments, and found that the maximum extraction efficiencies for Pb and Mo occurred at 600 rpm, 600 rpm was selected for use. At the end of each leaching experiment, the slurry was filtered. The resulting filter cake re-suspended with hot water, and the new slurry filtered again. The solid residues were dried to constant weight in an oven at 105 °C. ICP-OES was used to analyze the leach liquor and the solid residues. The mineral phases of the residues were also examined using XRD.
3. Results and discussion 3.1. Comparison of alkaline leaching flotation tailings with and without pre-roasting Comparison of the metal extractions by alkaline leaching with and without the pre-roasting shows that only 25% of the Mo and 11% of the Pb were extracted without pre-roasting, whereas 96% of the Mo and 97% of the Pb were extracted from the pre-roasted flotation tailings. Moreover, these results were obtained using harsher conditions for the non-pre-roasted leach: 180 °C, 20% NaOH, alkali to tailings mass ratio 1.2, stirring speed 600 rpm, duration 3 h. The conditions for the preroasted flotation tailings were 130 °C, 20% NaOH, alkali to total calcine mass ratio 1.2, stirring speed 600 rpm, and duration 1 h. All subsequent experiments were conducted on pre-roasted flotation tailings.
3.2. The mechanism of pre-roasting Within the molybdenite flotation tailings associated with galena, the Pb and Mo mainly occur as sulfides. Whereas the oxides of Pb and Mo leach easily with NaOH, the sulfides leach very slowly until they are converted into the oxides. Therefore, the purpose of pre-roasting is to oxidize the sulfides to either oxides or sulfates. As seen from Fig. 1, the main mineral phases present in the flotation tailings are galena (PbS), quartz (SiO2), pyrite (FeS2), molybdenite (MoS2) and Wulfenite (PbMoO4). In the calcine, also shown in the Fig. 1, the main mineral phases are quartz (SiO2), hematite (Fe2O3), anglesite (PbSO4) and wulfenite (PbMoO4). The main chemical reactions during pre-roasting may be expressed as follows (Peng, 2003; Zhang and Zhao, 2005): 2MoS2 þ 7O2 ⇒ 2MoO3 þ 4SO2 ↑
ð1Þ
PbS þ 2O2 ⇒ PbSO4
ð2Þ
4FeS2 þ 11O2 ⇒ 2Fe2 O3 þ 8SO2 ↑
ð3Þ
2PbS þ 3O2 ⇒2PbO þ 2SO2 ↑
ð4Þ
PbO þ MoO3 ⇒PbMoO4
ð5Þ
Table 2 Particle size distribution of the molybdenite flotation tailings associated with galena.
Fig. 1. XRD patterns of molybdenite flotation tailings associated with galena and calcine: A, calcine; B, molybdenite flotation tailings associated with galena.
Particle size (μm)
−44
+44,−53
+53,−74
+74,−105
vt.%
64.50
5.48
13.22
16.80
32
Y. Liu et al. / Hydrometallurgy 129–130 (2012) 30–34
Fig. 2. Effect of pre-roasting temperature on leaching in 30% NaOH, at 130 °C, alkali to total calcine mass ratio 1.2, 1 h.
Fig. 4. XRD patterns of the leaching residues at 40, 70 and 130 °C.
þ
2−
3.3. Effect of pre-roasting temperature on Pb and Mo extraction
2NaOH þ MoO3 ⇒2Na þ MoO4 þ H2 O
The effect of the temperature of the pre-roasting on the extraction of Pb and Mo was studied from 550 to 850 °C, at a residence time of 1 h. The results are shown in Fig. 2. The extraction of Pb and Mo increased with temperature as long as temperature remained less than 600 °C. However, the extraction of Pb and Mo first decreased, and then increased between temperatures of 600 to 800 °C. Finally and remarkably, the extraction of Pb and Mo decreased above 800 °C. First, with the temperature increased, the percentage of lead in the calcine will decrease due to volatilization. Second, the calcine begins to sinter when the temperature is above 600 °C, but the lead is mainly present as PbSO4 as long as the temperature is below 700 °C. Between 700 and 800 °C, the entire calcine has sintered which will hinder the leaching. And the percentage of PbSO4 in the calcine becomes reduced; the lead is mainly present as PbMoO4 above 800 °C. Therefore, all further experiments were conducted with a pre-roasting temperature of 600 °C.
4NaOH þ PbMoO4 ⇒4Na þ MoO4 þ PbO2 þ 2H2 O
3.4. Alkaline leaching After pre-roasting under optimum conditions, the Mo and Pb minerals formed are easily leached with sodium hydroxide solution in an autoclave. The chemical reactions of the leaching were assumed to occur as: þ
2−
2−
PbSO4 þ 4NaOH⇒4Na þ SO4 þ PbO2 þ 2H2 O
ð6Þ
Fig. 3. Effect of temperature on leaching in 30% NaOH, alkali to total calcine mass ratio 1.2, 1 h.
þ
2−
ð7Þ 2−
ð8Þ
3.4.1. Effect of leaching temperature The effect of temperature on the extraction of Pb and Mo was studied at different temperatures (between 40 and 150 °C). Fig. 3 shows that the extraction percentages of Pb and Mo both increase with temperature. Mo is readily leached at temperatures 40 °C and below, whereas alkaline PbSO4 leaching needs higher temperatures to achieve similar results. When temperature was increased above 90 °C, the increasing trend began to flatten, so that at 130 °C, the extraction percentages of Pb and Mo were both up to 98%. Fig. 4 shows diffraction peaks of the leach residues at 40, 70, and 130 °C obtained under conditions described above. It can be seen that PbO was present in the leach residue. The solubility of lead in sodium hydroxide solution is very low at cooler temperatures (Glasstone, 1921), and so it is easily precipitated as PbO. At higher temperatures (e.g. 70 °C or 130 °C), PbO disappeared, and the diffraction peaks of SiO2 and Fe2O3 were more obvious. 3.4.2. Effect of the alkali to total calcine mass ratio The effect of alkali to total calcine mass ratio on the extraction of Pb and Mo is shown in Fig. 5. Results show that alkali to total calcine mass ratio had an obvious influence on the extraction of Pb, but much less on that of Mo. Higher mass ratios of alkali to total calcine facilitated dissolution of Pb and Mo, and improved surface contact between
Fig. 5. Effect of alkali to total calcine mass ratio on leaching in 30% NaOH, at 130 °C, 1 h.
Y. Liu et al. / Hydrometallurgy 129–130 (2012) 30–34
33
Table 3 Metal content of the leach liquor. Constituent
Pb
Mo
Si
Cu
Al
Concentration (g/l)
85.82
6.63
3.01
2.10
0.27
percentages of Pb and Mo were above 95% after 30 min, and were sufficient to achieve the maximum extraction at 130 °C by about 60 min. At 90 °C, the speed of Pb extraction was slower than that at 130 °C. The reaction needed 60 min to achieve maximum extraction, and the maximum extraction was lower than that at 130 °C. But the maximum extraction of Mo at 90 °C was about the same that at 130 °C.
Fig. 6. Effect of initial NaOH concentration on leaching at 130 °C, alkali to total calcine mass ratio 1.2, 1 h.
alkali and calcine particles. This could have increased the extraction percentages of Pb and Mo observed. But higher ratios increase the amount of NaOH required and reduce the Pb and Mo concentrations in the resultant leach liquor. Therefore, a ratio of 1.2 was employed in order to optimize the leaching process. 3.4.3. Effect of the initial concentration of NaOH solution The effect of the initial concentration of NaOH solution on the extraction of Pb and Mo was studied by varying concentrations of sodium hydroxide over the 10 − 35% range. Fig. 6 shows that the extraction percentage of Pb increased significantly with the initial concentration of sodium hydroxide, increasing when the initial concentration of sodium hydroxide was less than 15%, but increasing more slowly when it was above 15%. However, the extraction percentage of Mo did not change with the initial concentration of sodium hydroxide. The behavior of Pb leaching in these tests was similar to that obtained from oxidized lead ore by Guo (2008). In that work, the initial NaOH concentration also had a pronounced effect on the Pb extraction when it was less than 5 mol/L, and increased more slowly above that point, despite the fact that different minerals and different experimental conditions were used. This may be an effect of the solubility of PbO in sodium hydroxide solutions. 3.4.4. Effect of leaching time The effect of leaching time on the extraction of Pb and Mo is shown in Fig. 7, which shows that the reaction was fast. The extraction
3.4.5. Characterization of the leach residue and leach liquor The chemical compositions and the XRD pattern of the leach residues after leaching (30% NaOH, 130 °C, alkali to total calcine mass ratio 1.2) are shown in Table 1 and Fig. 4 (130 °C). The extraction percentages of Pb and Mo were both 98%, the concentrations of the main metal components of the leach liquor are shown in Table 3. Fig. 4 shows that there were two mineral phases: SiO2 and Fe2O3. Compared with Fig. 1, Fig. 4 shows that PbSO4 and PbMoO4 entered into the leach liquor during the leaching process. The leach liquor can be crystallized to separate Pb and Mo. This will be dealt in a follow-up study. 4. Conclusions Pre-roasting with subsequent NaOH leaching can improve the extraction of Pb and Mo from the molybdenite flotation tailings associated with galena. Temperature, alkali to total calcine mass ratio and initial sodium hydroxide concentration all have strongly influenced the leaching of Pb and Mo from these flotation tailings, while leaching time and stirring speed have less influence. Under the optimum conditions of preroasting at 600 °C for 1 h, and alkaline leaching for 1 h at 130 °C with an alkali to total calcine mass ratio of 1.2:1, NaOH 30%, stirring speed 600 rpm, 98% of the Pb and 98% of the Mo in the molybdenite flotation tailings associated with galena were extracted. Acknowledgment The authors acknowledge the financial support by the National High Technology Research and Development Program of China (863 Program, 2011AA060701), the Knowledge Innovation Project of Chinese Academy of Sciences (KGCX2-YW-321-2), and the National Natural Science Foundation of China (21101159). References
Fig. 7. Effect of leaching time on leaching. (a) NaOH: 30%, temperature: 130 °C, alkali to total calcine mass ratio 1.2; (b) NaOH: 15%, temperature: 90 °C, alkali to total calcine mass ratio 0.6.
Anand Rao, K., Natarajan, R., Padmanabhan, N.P.H., 2001. Studies on recovery of copper, nickel, cobalt and molybdenum values from a bulk sulphide concentrate of an Indian uranium ore. Hydrometallurgy 62, 115–124. Awakura, Y., Kamei, S., Majima, H., 1980. A kinetic study of non-oxidative dissolution of galena in aqueous acid solution. Metall. Trans. B 11 (3), 377–381. Aydoğan, S., Erdemoğlu, M., Uçar, G., et al., 2007. Kinetics of galena dissolution in nitric acid solutions with hydrogen peroxide. Hydrometallurgy 88, 52–57. Baláz, P., 1996. Influence on solid state properties on ferric chloride leaching of mechanically activated galena. Hydrometallurgy 40 (3), 359–368. Chen, A.A., Dreisinger, D.B., 1994. The ferric fluosilicate leaching of lead concentrates: Part I. Kinetic studies. Metall. Mater. Trans. B 25 (4), 473–480. da Silva, G., 2004. Kinetics and mechanism of the bacterial and ferric sulphate oxidation of galena. Hydrometallurgy 75, 99–110. Dutrizac, J.E., 1986. The dissolution of galena in ferric chloride media. Metall. Trans. B 17, 5–17. Dutrizac, J.E., Chen, T.T., 1990. The effect of the elemental sulfur reaction product on the leaching of galena in ferric chloride media. Metall. Trans. B 21 (6), 935–943. Dutrizac, J.E., Chen, T.T., 1995. The leaching of galena in ferric sulfate media. Metall. Mater. Trans. B 26 (2), 219–227. Ebrahimi-Kahrizsangi, R., Abbasi, M.H., Saidi, A., 2010. Molybdenite alkali fusion and leaching: reactions and mechanism. Int. J. Miner. Metall. Mater. 17 (2), 127–131. Glasstone, S., 1921. CXCII.-Physical chemistry of the oxides of lead. Part I. The solubility of lead monoxide. J. Chem. Soc. Trans. 119, 1689–1697.
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Guo, C.X., 2008. Leaching-electrowinning process of extraction of metallic lead from lead bearing wastes and low-grade lead oxide ore. Tongji University, pp. 47–48 (In Chinese). Juneja, J.M., Singh, Sohan, Bose, D.K., 1996. Investigations on the extraction of molybdenum and rhenium values from low grade molybdenite concentrate. Hydrometallurgy 41, 201–209. Kar, B.B., Murthy, B.V.R., Misra, V.N., 2005. Extraction of molybdenum from spent catalyst by salt-roasting. Int. J. Miner. Process. 76, 143–147. Kholmogorov, A.G., Kononova, O.N., 2005. Processing mineral raw materials in Siberia: ores of molybdenum, tungsten, lead and gold. Hydrometallurgy 76, 37–54. Kholmogorov, A.G., Pashkov, G.L., Mikhlina, E.V., et al., 2003. Activation of hydrometallurgical treatment of PbS in nitric solutions. Chem. Sustain. Dev. 11, 879–881. Kyung, Ho Park, Ramachandra Reddy, B., Mohapatra, D., et al., 2006. Hydrometallurgical processing and recovery of molybdenum trioxide from spent catalyst. Int. J. Miner. Process. 80, 261–265. Nasernejad, B., Kaghazchi, T., Edrisi, M., et al., 1999. Bioleaching of molybdenum from low-grade copper ore. Process. Biochem. 35, 437–440.
Olson, G.J., Clark, T.R., 2008. Bioleaching of molybdenite. Hydrometallurgy 93, 10–15. Pashkov, G.L., Mikhlina, E.V., Kholmogorov, A.G., et al., 2002. Effect of potential and ferric ions on lead sulfide dissolution in nitric acid. Hydrometallurgy 63, 171–179. Peng, R.Q., 2003. Metallurgy of lead and zinc. Science Press, Beijing. (In Chinese). Shi, L., Li, J., Xie, J.H., 2009. Research on leaching processes of high-lead and low-grade molybdenum ore in sodium hypochlorite solutions. Min. Metall. Eng. 29, 63–66 (In Chinese). Van den Berg, J.A.M., Yang, Y., Nauta, H.H.K., et al., 2002. Comprehensive processing of low grade sulphidic molybdenum ores. Miner. Eng. 15, 879–883. Wang, M.Y., Wang, X.W., Liu, W.L., 2009. A novel technology of molybdenum extraction from low grade Ni-Mo ore. Hydrometallurgy 97, 126–130. Zhang, Q.X., Zhao, Q.S., 2005. Metallurgy of tungsten and molybdenum. Metallurgy Industry Press, Beijing. (In Chinese).
Contents lists available at SciVerse ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Technical note
The alkaline leaching of molybdenite flotation tailings associated with galena Yan Liu a, b, Yifei Zhang a,⁎, Fangfang Chen a, Yi Zhang a a National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China b Graduate University of Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e
i n f o
Article history: Received 27 April 2012 Received in revised form 9 July 2012 Accepted 28 July 2012 Available online 3 August 2012 Keywords: Molybdenite flotation tailings associated with galena Pre-roasting Alkaline leaching Lead Molybdenum
a b s t r a c t A method for the extraction of both Pb and Mo from molybdenite flotation tailings associated with galena is described. The flotation tailings were pre-roasted and then alkaline leached. The effect of temperature, initial concentration of sodium hydroxide solution, alkali to total calcine mass ratio, leaching time and stirring speed on the leaching of Pb and Mo were systematically investigated by experiment. Optimum conditions were identified as: pre-roasting at 600 °C for 1 h, alkaline leaching for 1 h at 130 °C with an alkali to total calcine mass ratio of 1.2:1, initial NaOH concentration 30%, and a stirring speed of 600 rpm. Under these conditions 98% of the Pb and 98% of the Mo in the flotation tailings were extracted. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum is a strategic metal which has a broad range of applications and uses. Approximately 80% of the world's molybdenum is used in metallurgical applications such as alloy steels, stainless steels and cast irons, and the other 20% is used in chemical applications. The main commercial source of molybdenum is molybdenite (MoS2). Most molybdenite is present as low-grade ores, and is associated with other valuable metals, e.g. Pb, Ni, W, Cu and Re. Generally, molybdenum concentrates are obtained by selective flotation from low-grade ores, and then produce MoO3 or Mo metal through roasting-ammonia leaching process (Zhang and Zhao, 2005). However, the flotation tailings resulting from this process still contain some valuable metals, e.g. Mo, Pb and Cu, so these tailings can be seen as either a resource or an environmental threat with the ceaseless exploitation of resources in the world. Further, the high international Mo price means that it is often economic to re-work the tailings, rather than mine new low-grade ore bodies. A low-grade molybdenum mineral associated with galena was obtained through flotation from these tailings, which cannot be further beneficiated by flotation methods. These tailings contain wulfenite which is insoluble in ammonia and the grade of Mo is also lower; therefore roasting-ammonia leaching process cannot be used to extract Mo from it. Over the past few years work to recover Mo from low-grade and complex ores has proceeded apace (Anand Rao et al., 2001; ⁎ Corresponding author. Tel./fax: +86 10 82544826. E-mail address: [email protected] (Y. Zhang). 0304-386X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2012.07.017
Ebrahimi-Kahrizsangi, et al., 2010; Juneja, et al, 1996; Kar, et al., 2005; Kholmogorov and Kononova, 2005; Kyung, et al., 2006; Nasernejad et al., 1999; Olson and Clark, 2008; Van den Berg, et al., 2002). Sodium hydroxide is also used as lixiviant for the leaching of low-grade Mo minerals (Wang et al., 2009). However, using sodium hydroxide to extract Pb and Mo from molybdenite flotation tailings associated with galena has not been explored. The commercial process for producing metallic lead from galena concentrates involves two stages: feeding to a sintering machine where, by contact with air at an elevated temperature, turning the lead concentrate into lead oxide; heating the lead oxide in a blast furnace with coke to yield metallic lead (Peng, 2003). The main materials of this process are above 55% galena concentrates. If this process is used to treat molybdenite flotation tailings associated with galena, molybdenum in the tailings will go to waste and the metallic lead also contains more impurities. So we cannot choose this process. Hydrometallurgical processes are an alternative approach to obtaining and purifying metallic lead from low-grade galena. Over the past thirty years, extensive work has been carried out on the treatment of galena with hydrochloric acid (Awakura et al., 1980), ferric chloride (Baláz, 1996; Dutrizac, 1986; Dutrizac and Chen, 1990), ferric sulfate (da Silva, 2004; Dutrizac and Chen, 1995), ferric fluorosilicate (Chen and Dreisinger, 1994) and nitrate solutions (Aydoğan et al., 2007; Kholmogorov et al., 2003; Pashkov et al., 2002). However, due to unfavorable economics, as well as the low aqueous solubility of the PbSO4 and PbCl2, none of these methods have been applied commercially. Sodium hydroxide was also used to treat lead minerals, but little progress was made because the reaction rates were slow, and lead compounds rather insoluble in NaOH (Peng, 2003).
Y. Liu et al. / Hydrometallurgy 129–130 (2012) 30–34
Although there are many reports on dealing with molybdenite, galena and wulfenite respectively, there is little information concerning the treatment of low-grade molybdenum mineral associated with galena. Only Shi et al. (2009) reported leaching a high Pb with low Mo ore using sodium hypochlorite. But sodium hypochlorite easily decomposes and is of high cost, in addition, is applied to only treat low S ores. Therefore, new methods need to be studied to efficiently extract Pb and Mo from these tailings. The purpose of this work is to investigate the extraction of both Pb and Mo from molybdenite flotation tailings associated with galena using a pre-roasting and alkaline leaching methodology. 2. Experimental 2.1. Materials The molybdenite flotation tailings associated with galena used in this study were obtained from the molybdenum concentrator plant location within Henan province, China. The molybdenite flotation tailings were enriched by flotation in Pb (to 20–25%) and had a Mo content of up to 2.4%. The X-ray diffraction (XRD) pattern of the flotation tailings is shown in Fig. 1. The chemical composition determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) is given in Table 1. The particle size distribution of the molybdenite flotation tailings associated with galena (measured by laser scattering in a Helos and Rodos granulometer) appears in Table 2. The mean particle diameter was 46.96 μm, which was calculated as the first moment of the volume size distribution function. All the reagents used in chemical analysis, including sodium hydroxide, citric acid and hydrochloric acid, were of analytical grade. And all solutions were made with deionized water. 2.2. Experimental procedure Pre-roasting and leaching studies were carried out on the laboratory scale. Pre-roasting was performed to determine whether new phases were formed and how this influenced leaching efficiency. All experiments were carried out using 80 g samples. The samples were charged into four ceramic crucibles (120 mm × 60 mm) and placed in a pre-heated electric muffle furnace. The temperature range of 550 to 850 °C and a residence time of 1 h were applied in the experiments. The solid mass after roasting is different on different pre-roasting temperature. When the pre-roasting temperature is 600 °C, the solid mass was 99.5% of original tailings mass. After pre-roasting, the calcine was leached using sodium hydroxide solution in a 0.5 L stainless steel autoclave. The temperature of the autoclave was measured with a thermocouple, displayed with a digital multi-meter with a precision of ±0.1 °C, and controlled by the heating furnace. The autoclave was equipped with a pressure gauge
31
Table 1 Chemical composition of the molybdenite flotation tailings associated with galena (A) and the solid residue (B). Component
Pb
Mo
S
Fe
Si
Ca
Cu
Na
(A) wt.% (B) wt.%
24.77 0.87
2.24 0.11
23.58 0.54
12.59 24.21
12.90 17.35
3.45 5.24
0.77 0.27
0.23 1.68
to measure the pressure. The stirring speed was 600 rpm in leaching experiments. Since we found stirring speed have no significant effect on the leaching in pre-experiments, and found that the maximum extraction efficiencies for Pb and Mo occurred at 600 rpm, 600 rpm was selected for use. At the end of each leaching experiment, the slurry was filtered. The resulting filter cake re-suspended with hot water, and the new slurry filtered again. The solid residues were dried to constant weight in an oven at 105 °C. ICP-OES was used to analyze the leach liquor and the solid residues. The mineral phases of the residues were also examined using XRD.
3. Results and discussion 3.1. Comparison of alkaline leaching flotation tailings with and without pre-roasting Comparison of the metal extractions by alkaline leaching with and without the pre-roasting shows that only 25% of the Mo and 11% of the Pb were extracted without pre-roasting, whereas 96% of the Mo and 97% of the Pb were extracted from the pre-roasted flotation tailings. Moreover, these results were obtained using harsher conditions for the non-pre-roasted leach: 180 °C, 20% NaOH, alkali to tailings mass ratio 1.2, stirring speed 600 rpm, duration 3 h. The conditions for the preroasted flotation tailings were 130 °C, 20% NaOH, alkali to total calcine mass ratio 1.2, stirring speed 600 rpm, and duration 1 h. All subsequent experiments were conducted on pre-roasted flotation tailings.
3.2. The mechanism of pre-roasting Within the molybdenite flotation tailings associated with galena, the Pb and Mo mainly occur as sulfides. Whereas the oxides of Pb and Mo leach easily with NaOH, the sulfides leach very slowly until they are converted into the oxides. Therefore, the purpose of pre-roasting is to oxidize the sulfides to either oxides or sulfates. As seen from Fig. 1, the main mineral phases present in the flotation tailings are galena (PbS), quartz (SiO2), pyrite (FeS2), molybdenite (MoS2) and Wulfenite (PbMoO4). In the calcine, also shown in the Fig. 1, the main mineral phases are quartz (SiO2), hematite (Fe2O3), anglesite (PbSO4) and wulfenite (PbMoO4). The main chemical reactions during pre-roasting may be expressed as follows (Peng, 2003; Zhang and Zhao, 2005): 2MoS2 þ 7O2 ⇒ 2MoO3 þ 4SO2 ↑
ð1Þ
PbS þ 2O2 ⇒ PbSO4
ð2Þ
4FeS2 þ 11O2 ⇒ 2Fe2 O3 þ 8SO2 ↑
ð3Þ
2PbS þ 3O2 ⇒2PbO þ 2SO2 ↑
ð4Þ
PbO þ MoO3 ⇒PbMoO4
ð5Þ
Table 2 Particle size distribution of the molybdenite flotation tailings associated with galena.
Fig. 1. XRD patterns of molybdenite flotation tailings associated with galena and calcine: A, calcine; B, molybdenite flotation tailings associated with galena.
Particle size (μm)
−44
+44,−53
+53,−74
+74,−105
vt.%
64.50
5.48
13.22
16.80
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Y. Liu et al. / Hydrometallurgy 129–130 (2012) 30–34
Fig. 2. Effect of pre-roasting temperature on leaching in 30% NaOH, at 130 °C, alkali to total calcine mass ratio 1.2, 1 h.
Fig. 4. XRD patterns of the leaching residues at 40, 70 and 130 °C.
þ
2−
3.3. Effect of pre-roasting temperature on Pb and Mo extraction
2NaOH þ MoO3 ⇒2Na þ MoO4 þ H2 O
The effect of the temperature of the pre-roasting on the extraction of Pb and Mo was studied from 550 to 850 °C, at a residence time of 1 h. The results are shown in Fig. 2. The extraction of Pb and Mo increased with temperature as long as temperature remained less than 600 °C. However, the extraction of Pb and Mo first decreased, and then increased between temperatures of 600 to 800 °C. Finally and remarkably, the extraction of Pb and Mo decreased above 800 °C. First, with the temperature increased, the percentage of lead in the calcine will decrease due to volatilization. Second, the calcine begins to sinter when the temperature is above 600 °C, but the lead is mainly present as PbSO4 as long as the temperature is below 700 °C. Between 700 and 800 °C, the entire calcine has sintered which will hinder the leaching. And the percentage of PbSO4 in the calcine becomes reduced; the lead is mainly present as PbMoO4 above 800 °C. Therefore, all further experiments were conducted with a pre-roasting temperature of 600 °C.
4NaOH þ PbMoO4 ⇒4Na þ MoO4 þ PbO2 þ 2H2 O
3.4. Alkaline leaching After pre-roasting under optimum conditions, the Mo and Pb minerals formed are easily leached with sodium hydroxide solution in an autoclave. The chemical reactions of the leaching were assumed to occur as: þ
2−
2−
PbSO4 þ 4NaOH⇒4Na þ SO4 þ PbO2 þ 2H2 O
ð6Þ
Fig. 3. Effect of temperature on leaching in 30% NaOH, alkali to total calcine mass ratio 1.2, 1 h.
þ
2−
ð7Þ 2−
ð8Þ
3.4.1. Effect of leaching temperature The effect of temperature on the extraction of Pb and Mo was studied at different temperatures (between 40 and 150 °C). Fig. 3 shows that the extraction percentages of Pb and Mo both increase with temperature. Mo is readily leached at temperatures 40 °C and below, whereas alkaline PbSO4 leaching needs higher temperatures to achieve similar results. When temperature was increased above 90 °C, the increasing trend began to flatten, so that at 130 °C, the extraction percentages of Pb and Mo were both up to 98%. Fig. 4 shows diffraction peaks of the leach residues at 40, 70, and 130 °C obtained under conditions described above. It can be seen that PbO was present in the leach residue. The solubility of lead in sodium hydroxide solution is very low at cooler temperatures (Glasstone, 1921), and so it is easily precipitated as PbO. At higher temperatures (e.g. 70 °C or 130 °C), PbO disappeared, and the diffraction peaks of SiO2 and Fe2O3 were more obvious. 3.4.2. Effect of the alkali to total calcine mass ratio The effect of alkali to total calcine mass ratio on the extraction of Pb and Mo is shown in Fig. 5. Results show that alkali to total calcine mass ratio had an obvious influence on the extraction of Pb, but much less on that of Mo. Higher mass ratios of alkali to total calcine facilitated dissolution of Pb and Mo, and improved surface contact between
Fig. 5. Effect of alkali to total calcine mass ratio on leaching in 30% NaOH, at 130 °C, 1 h.
Y. Liu et al. / Hydrometallurgy 129–130 (2012) 30–34
33
Table 3 Metal content of the leach liquor. Constituent
Pb
Mo
Si
Cu
Al
Concentration (g/l)
85.82
6.63
3.01
2.10
0.27
percentages of Pb and Mo were above 95% after 30 min, and were sufficient to achieve the maximum extraction at 130 °C by about 60 min. At 90 °C, the speed of Pb extraction was slower than that at 130 °C. The reaction needed 60 min to achieve maximum extraction, and the maximum extraction was lower than that at 130 °C. But the maximum extraction of Mo at 90 °C was about the same that at 130 °C.
Fig. 6. Effect of initial NaOH concentration on leaching at 130 °C, alkali to total calcine mass ratio 1.2, 1 h.
alkali and calcine particles. This could have increased the extraction percentages of Pb and Mo observed. But higher ratios increase the amount of NaOH required and reduce the Pb and Mo concentrations in the resultant leach liquor. Therefore, a ratio of 1.2 was employed in order to optimize the leaching process. 3.4.3. Effect of the initial concentration of NaOH solution The effect of the initial concentration of NaOH solution on the extraction of Pb and Mo was studied by varying concentrations of sodium hydroxide over the 10 − 35% range. Fig. 6 shows that the extraction percentage of Pb increased significantly with the initial concentration of sodium hydroxide, increasing when the initial concentration of sodium hydroxide was less than 15%, but increasing more slowly when it was above 15%. However, the extraction percentage of Mo did not change with the initial concentration of sodium hydroxide. The behavior of Pb leaching in these tests was similar to that obtained from oxidized lead ore by Guo (2008). In that work, the initial NaOH concentration also had a pronounced effect on the Pb extraction when it was less than 5 mol/L, and increased more slowly above that point, despite the fact that different minerals and different experimental conditions were used. This may be an effect of the solubility of PbO in sodium hydroxide solutions. 3.4.4. Effect of leaching time The effect of leaching time on the extraction of Pb and Mo is shown in Fig. 7, which shows that the reaction was fast. The extraction
3.4.5. Characterization of the leach residue and leach liquor The chemical compositions and the XRD pattern of the leach residues after leaching (30% NaOH, 130 °C, alkali to total calcine mass ratio 1.2) are shown in Table 1 and Fig. 4 (130 °C). The extraction percentages of Pb and Mo were both 98%, the concentrations of the main metal components of the leach liquor are shown in Table 3. Fig. 4 shows that there were two mineral phases: SiO2 and Fe2O3. Compared with Fig. 1, Fig. 4 shows that PbSO4 and PbMoO4 entered into the leach liquor during the leaching process. The leach liquor can be crystallized to separate Pb and Mo. This will be dealt in a follow-up study. 4. Conclusions Pre-roasting with subsequent NaOH leaching can improve the extraction of Pb and Mo from the molybdenite flotation tailings associated with galena. Temperature, alkali to total calcine mass ratio and initial sodium hydroxide concentration all have strongly influenced the leaching of Pb and Mo from these flotation tailings, while leaching time and stirring speed have less influence. Under the optimum conditions of preroasting at 600 °C for 1 h, and alkaline leaching for 1 h at 130 °C with an alkali to total calcine mass ratio of 1.2:1, NaOH 30%, stirring speed 600 rpm, 98% of the Pb and 98% of the Mo in the molybdenite flotation tailings associated with galena were extracted. Acknowledgment The authors acknowledge the financial support by the National High Technology Research and Development Program of China (863 Program, 2011AA060701), the Knowledge Innovation Project of Chinese Academy of Sciences (KGCX2-YW-321-2), and the National Natural Science Foundation of China (21101159). References
Fig. 7. Effect of leaching time on leaching. (a) NaOH: 30%, temperature: 130 °C, alkali to total calcine mass ratio 1.2; (b) NaOH: 15%, temperature: 90 °C, alkali to total calcine mass ratio 0.6.
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