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A selective process for extracting antimony from refractory gold ore
Hydrometallurgy 169 (2017) 571–575
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Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Technical note
A selective process for extracting antimony from refractory gold ore Tianzu Yang a, Shuai Rao a, Weifeng Liu a,b,⁎, Duchao Zhang a, Lin Chen a a b
School of Metallurgy and Environment, Central South University, Changsha 410083, China Henan Yuguang Gold and Lead Group Co., Ltd., Jiyuan 459000, China
a r t i c l e
i n f o
Article history: Received 21 November 2016 Received in revised form 18 February 2017 Accepted 25 March 2017 Available online 28 March 2017 Keywords: Refractory gold ore Sodium sulfide leaching Pressure oxidation Antimony
a b s t r a c t To avoid the adverse effects of antimony on the cyanide leaching process used for the recovery of gold from refractory gold ore, a cleaner production process was proposed that would selectively remove antimony as sodium pyroantimonate. The process included three steps: 1) leaching of antimony from the refractory gold ore with sodium sulfide solution; 2) pressure oxidation of the leaching solution containing sodium thioantimonite, transforming it to sodium pyroantimonate; and 3) concentration/crystallization to recover the sodium thiosulfate byproduct from the oxidized solution. The leaching recoveries of antimony, gold, and arsenic in the first step were 96.64%, 1.44%, and 0.41%, respectively. The precipitation ratio of antimony exceeded 99.80% during the second step. The synthesized sodium pyroantimonate product exhibited a regular tetragonal morphology. The purity of the sodium thiosulfate byproduct obtained during the third step reached 98.0%. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Gold is a strategic precious metal that is widely used in ornamentation, currency reserves, and high-tech industries. According to the United States Geological Survey, globally identified gold reserves amount to 89,000 tons, with approximately one-third of that total contained in refractory ores (George, 2009). With the gradual depletion of high-grade resources, refractory gold ores have attracted increased attention. Refractory gold ores are defined as minerals for which the cyanideleaching recovery of gold is b80%, even after fine grinding. High-arsenic refractory gold ores constitute the largest reserves but are the most difficult from which to extract gold. Gold leaching recoveries can often be as low as 50% (Yang, 2005). These poor results can be attributed to the encapsulation of the fine gold particles—even at micron size—in pyrite or arsenopyrite. Therefore, refractory gold ores must be pretreated prior to the cyanide leaching process to liberate the gold. Currently, there are many methods for the pretreatment of refractory gold ores, including two-stage roasting (Dunn and Chamberlain, 1997), pressure oxidation (Gudyanga et al., 1999), bacterial oxidation (Hol et al., 2011), fixed arsenic roasting (Liu et al., 2000), microwave roasting (Amankwah and Pickles, 2009), chemical oxidation (Saba et al., 2011), and nitric acid catalytic oxidation (Gao et al., 2009). Geochemical studies have shown that gold migrates easily and is concentrated, along with characteristic elements such as arsenic and antimony, during the endogenous mineralization of gold deposits (Nie and Suo, 1997). However, the coexistence of those elements increases the ⁎ Corresponding author at: School of Metallurgy and Environment, Central South University, Changsha 410083, China. E-mail address: [email protected] (W. Liu).
http://dx.doi.org/10.1016/j.hydromet.2017.03.014 0304-386X/© 2017 Elsevier B.V. All rights reserved.
difficulty of extracting gold from refractory gold ores. During the direct cyanide leaching process, the dissolution of stibnite (Sb2S3) not only increases the consumption of OH−, CN−, and O2, but also forms precipitates that coat the surfaces of the gold particles, which leads to low leaching recoveries (Yang, 2005). During the two-stage roasting pretreatment process, it is easy to fuse with each other due to the low melting points of antimony compounds, causing the gold particles to be encapsulated again (Cui et al., 2011). In pressure oxidation pretreatments, precipitates of antimony compounds are produced, which cover the surfaces of the gold particles and hinder further leaching reactions. In the biological oxidation pretreatment process, the antimonycontaining minerals significantly affect the oxidation rates of gold-bearing minerals. Due to the adverse effects of antimony on these pretreatment and direct cyanide leaching processes, it is necessary to remove it from antimonial refractory gold ores before gold extraction (Anderson, 2012). In general, the hydrometallurgical methods for removing antimony can be conducted in either alkaline (sodium sulfide, Na2S) or acidic (hydrochloric acid, HCl) systems. In alkaline sodium sulfide solution, antimony is dissolved in the form of sodium thioantimonite (Na3SbS3). The antimony can be recovered from the leaching solution by neutralization, displacement, electrowinning, air oxidation, and so on. Owing to its excellent selectivity, this method has been widely used for removing antimony from stibnite (Ubaldini et al., 2000), jamesonite (Yang et al., 2005), enargite (Curreli et al., 2009), and refractory antimony ores (Celep et al., 2011). In HCl solution, antimony is dissolved in the form of SbCl3 by adding oxidizing agents such as Cl2, FeCl3, SbCl5, or H2O2 (Zhao, 1987). Methods for the recovery of antimony from the SbCl3 leaching solution include hydrolysis, replacement, distillation, and electrowinning. Although the foregoing methods afford high antimony
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T. Yang et al. / Hydrometallurgy 169 (2017) 571–575
recoveries, they all focus on the unit leaching process or the recovery of antimony from the leaching solution. However, a complete technological process should consider many factors, including the recovery of sulfur, operational environment, and processing costs. In our previous study, low-grade jamesonite concentrate was leached in sodium sulfide solution, affording an antimony recovery exceeding 91.0% (Yang et al., 2005). We have also shown that antimony can be precipitated as sodium antimonite from a solution of sodium thioantimonite by air oxidation (Yang et al., 2002). Based on these findings, a cleaner production process was proposed to selectively remove antimony in the form of sodium pyroantimonate from refractory gold ore. In this work, the complete process is presented in detail so as to provide guidance for the extraction of antimony from antimonial refractory gold ores. 2. Experimental 2.1. Materials The raw material, antimonial refractory gold ore, was provided by the Jinchiling Gold Mine (Zhaojin Mining Industry Co., Ltd., Shandong Province, China). The chemical composition of the ore is given in Table 1. Before the experiments, the ore was dried at 383 K, and then ground and sieved to obtain a particle size below 74 μm. The main phases of the material were analyzed by XRD (Fig. 1). Many mineral components are observed in the refractory gold ore, including quartz (SiO2), muscovite (KAl2Si3AlO10(OH)2), pyrite (FeS2), and arsenopyrite (FeAsS). Antimony is present mainly in the form of stibnite (Sb2S3). All reagents were of analytical grade. The purity of the oxygen used during the pressure oxidation process exceeded 99.9%.
Fig. 1. XRD pattern of the antimonial refractory gold ore.
After completing the leaching process, the leaching solution is transferred into an autoclave to prepare sodium pyroantimonate. The reaction equation is expressed as: 2Na3 SbS3 þ7O2 þ2NaOH þ 5H2 O ¼ 2NaSbðOHÞ6 þ3Na2 S2 O3
The oxidized solution is concentrated and hot filtered to remove the insoluble mixture of sodium sulfate and sodium sulfite. Finally, the sodium thiosulfate byproduct is crystallized from the solution by cooling.
2.2. Process flowsheet
2.4. Analysis and characterization
To avoid the adverse effects of antimony on gold extraction, a clean production process for removing and recovering antimony from refractory gold ore is proposed. The complete process flowsheet is shown in Fig. 2. First, the antimonial refractory gold ore is leached in alkaline sodium sulfide solution. Gold is extracted from the leaching residue by the cyaniding process. The leaching solution is subjected to a pressure oxidation process to prepare sodium pyroantimonate (NaSb(OH)6). Finally, the oxidized solution is concentrated and crystallized to recover sodium thiosulfate (Na2S2O3 ∙5H2O).
Samples with low antimony, arsenic, and gold contents were determined by ICP-AES (IRIS Intrepid II, XRS), and those with high contents of antimony and arsenic were determined by titration with ceric sulfate and potassium bromate, respectively. The gold and silver contents of the solid samples were determined by fire assay. The sulfur content in the solution was analyzed by the iodine-sodium thiosulfate dispersion method. The chemical composition of the solid samples was characterized using X-ray fluorescence (XRF, ZSX Primus II, Rigaku). The phases of the solid sample were identified by X-ray diffraction (XRD) using a Rigaku TTRAX-3 instrument (40 kV, 30 mA, 10°/min). Microstructures were characterized using a Japan Jeol JSM-6360LV instrument equipped with a spectrometer for microanalysis based on an energy dispersive Xray spectroscopy (EDS) system (EDX-GENESIS 60 S, EDAX, USA) with an accelerating voltage of 0.5 kV to 30 kV.
2.3. Experimental procedure The clean production process for extracting antimony from refractory gold ore involves sodium sulfide leaching, pressure oxidation, concentration, and crystallization steps. First, the stibnite in the refractory gold ore reacts with sodium sulfide, and antimony is dissolved into the leaching solution in the form of sodium thioantimonite (Eq. (1)). During the leaching process, a given amount of sodium hydroxide is added into the leaching solution to prevent the hydrolysis of sodium sulfide (Eq. (2)). The main chemical reactions can be described as follows: Sb2 S3 þ3Na2 S ¼ 2Na3 SbS3
ð1Þ
Na2 S þ H2 O ¼ NaOH þ NaHS
ð2Þ
ð3Þ
3. Results and discussion 3.1. Sodium sulfide leaching 3.1.1. Leaching recoveries of antimony, gold, and arsenic Several studies have reported that the cyanide leaching of gold from antimonial refractory gold ore can be enhanced significantly after removing the antimony. Therefore, the selective leaching of antimony before the conventional cyanidation treatment is necessary. Through
Table 1 Chemical composition of the antimonial refractory gold ore. Component
Au/(g/t)
Ag/(g/t)
S
Fe
Sb
As
Cu
SiO2
Al2O3
CaO
K2O
wt%
58.8
42.0
17.68
15.74
6.30
5.50
0.04
36.58
11.94
3.76
2.45
T. Yang et al. / Hydrometallurgy 169 (2017) 571–575
573
Fig. 2. A clean process for extracting antimony from refractory gold ore.
systematic experiments, we established the optimum conditions for antimony removal as follows: sodium sulfide concentration, 50 g/L; sodium hydroxide concentration, 20 g/L; temperature, 323 K; liquid-tosolid ratio, 1.5 L/kg; reaction time, 1.5 h; stirring speed, 120 rpm; and washing water ratio, 1.0. Under these conditions, the antimony recovery reached 96.64%. The yield and antimony content of the leaching residue were 92.0% and 0.23%, respectively. Compared with the leaching of clairite (Celep et al., 2011) and jamesonite (Yang et al., 2005) in the alkaline sodium sulfide system, a higher antimony recovery was obtained for the antimonial refractory gold ore. The main phases of the leaching residue obtained under optimal conditions were analyzed by XRD (Fig. 3). Clearly, the characteristic peaks of the stibnite phase have disappeared completely. The gold and arsenic contents in the leaching solution and residue were analyzed twice. The results, shown in Table 2, demonstrate that minimal amounts of gold and arsenic were dissolved in the leaching solution, suggesting an effective and selective antimony leaching process. 3.1.2. Precipitate from the leaching solution A black precipitate was observed to form in the leaching solution after 48 h. To ascertain its composition and phases, SEM-EDS and XRD
analyses were conducted. The black precipitate consists of many flocculated tiny particles with irregular morphologies (Fig. 4). The EDS pattern indicates that the precipitate contains mainly iron, sodium, sulfur, oxygen, and antimony. Based on Fig. 5, the main phases in the black precipitate can be assigned as Na2FeS2, S, and FeS2. One possible reason for the formation of this precipitate is that a small amount of FeS2 is dissolved into the leaching solution. Then, the following reaction can occur: FeS2 þNa2 S ¼ Na2 FeS2 þ S
ð4Þ
Therefore, before conducting the pressure oxidation process, the leaching solution should be refiltered after 48 h natural settling at room temperature to remove additional impurities. 3.2. Pressure oxidation It is well known that sodium pyroantimonate can be successfully produced from the sodium thioantimonite leaching solution by air oxidation (Yang et al., 2002). However, the reaction rate is relatively slow. To solve this problem, a pressurized oxidation process was proposed to effect the transformation (Zhang et al., 2015). The effects of temperature, oxygen partial pressure, sodium hydroxide concentration, reaction time, and stirring speed on the antimony precipitation ratio were studied in detail. The optimum conditions are as follows: sodium hydroxide concentration, 20 g/L; stirring speed, 800 rpm; oxygen partial pressure, 0.4 MPa; reaction time, 2 h; and reaction temperature, 363 K. Under these conditions, the antimony content in the oxidized solution is reduced to 0.092 g/L, and the antimony precipitation ratio is over 99.80%. Compared with the traditional air oxidation process (Yang et al., 2002), this antimony precipitation process is improved significantly. The reaction time is decreased from 144 to 2 h, and the antimony content in the oxidized solution is lowered from 1.0 to 0.092 g/L. Table 2 Gold and arsenic contents in the leaching solution and residue.
Au As Fig. 3. XRD pattern of the leaching residue obtained under optimal conditions.
No.
Leaching solution
Leaching residue
Concentration
Recovery/%
Content
Recovery/%
1-1 2-1 1-2 2-2
0.40 mg/L 0.56 mg/L 0.15 g/L 0.14 g/L
1.02 1.44 0.41 0.38
64.0 g/t 63.8 g/t 5.96% 5.95%
0.00 0.18 0.31 0.47
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T. Yang et al. / Hydrometallurgy 169 (2017) 571–575
Fig. 4. SEM-EDS pattern of black precipitate formed in the leaching solution.
The phases and morphology of the obtained sodium pyroantimonate product were determined by XRD and SEM-EDS analyses, respectively. As shown in Fig. 6, all characteristic peaks were consistent with the standard diffraction peaks of sodium pyroantimonate (NaSb(OH)6). The chemical composition of the sodium pyroantimonate product was
analyzed (three replicates). The results, shown in Table 3, demonstrate that the sodium pyroantimonate purity is very high. The Sb3+ content, in particular, is b 0.10%, suggesting an adequate oxidation process. The product quality is fully compliant with the first-grade quality requirement of China Non-ferrous Industry Standard YS22-92. SEM micrographs of the sodium pyroantimonate product are shown in Fig. 7. The synthesized material is mainly composed of many agglomerated crystals. Although these crystals exhibit regular tetragonal structures, they are not uniform in terms of particle size, which range from N50 μm for larger crystals to b10 μm for some smaller crystals. Almost all the larger crystals have several adherent smaller crystals. 3.3. Recovery of sodium thiosulfate from the oxidized solution
Fig. 5. XRD pattern of the black precipitate formed in the leaching solution.
After completing the pressure oxidation process, the antimony concentration in the oxidized solution was reduced to 0.092 g/L. However, the oxidized solution still contained many sulfurated compounds, including Na2S2O3, Na2SO4, Na2S, and Na2SO3 at concentrations of 77.50, 7.42, 4.86, and 2.68 g/L, respectively. The main component of the oxidized solution, sodium thiosulfate (Na2S2O3), is worth recovering because of its high economic value. To accomplish this, the oxidized solution is evaporatively concentrated to a slurry with a density of 1.5 g/cm3, and then filtered at 363 K to remove an insoluble mixture of sodium sulfate and sodium sulfite. Finally, sodium thiosulfate is separated by cooling crystallization. The composition of the sodium thiosulfate byproduct was analyzed (two replicates). Table 4 shows that the sodium thiosulfate content exceeds 98.0%. The XRD pattern, shown in Fig. 8, demonstrates an exact match of the standard diffraction peaks for Na2S2O3·5H2O. 4. Conclusion In this study, a complete technological process for extracting antimony from refractory gold ore was proposed, which included leaching, pressure oxidation, concentration, and crystallization steps. The following conclusions may be drawn from this investigation. (1) The leaching experiments demonstrated that antimony could be selectively leached from refractory gold ore. Under optimal conditions, the leaching recoveries of antimony, gold, and arsenic were 96.64%, 1.44%, and 0.41%, respectively.
Table 3 Chemical composition of the sodium pyroantimonate product (wt%).
Fig. 6. XRD pattern of the sodium pyroantimonate product.
No.
Total Sb (Sb2O5)
Na2O
Sb3+
Fe2O3
As2O3
1 2 3
58.98 59.02 59.98
12.55 12.55 12.61
≤0.10 ≤0.10 ≤0.10
0.008 0.007 0.008
0.0006 0.0005 0.0008
T. Yang et al. / Hydrometallurgy 169 (2017) 571–575
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Fig. 7. SEM images of the sodium pyroantimonate product at different magnifications.
References Table 4 Composition of the sodium thiosulfate byproduct (wt%). No.
Na2S2O3·5H2O
Na2S
Fe
Water-insoluble substance
1 2
98.33 98.51
0.003 0.004
0.002 0.001
0.028 0.037
(2) Antimony in the leaching solution could be precipitated in the form of sodium pyroantimonate by pressure oxidation. Under optimal conditions, the antimony precipitation ratio exceeded 99.80%, and the antimony content in the leaching solution was b 0.092 g/L. (3) The sodium thiosulfate byproduct with a purity exceeding 98.3% was recovered from the oxidized solution by evaporation and concentration. Acknowledgments The authors acknowledge the support of the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 51404296) and the China Postdoctoral Science Foundation (Grant No. 2016M602427). We also thank the Jinchiling Gold Mine of the Zhaojin Mining Industry Co., Ltd. for providing the raw material and financial support.
Fig. 8. XRD pattern of the sodium thiosulfate byproduct.
Amankwah, R.K., Pickles, C.A., 2009. Microwave roasting of a carbonaceous sulphidic gold concentrate. Miner. Eng. 22 (13), 1095–1101. Anderson, C.G., 2012. The metallurgy of antimony. Chem. Erde 72, 3–8. Celep, O., Alp, İ., Deveci, H., 2011. Improved gold and silver extraction from a refractory antimony ore by pretreatment with alkaline sulphide leach. Hydrometallurgy 105 (3), 234–239. Cui, R., Yang, H., Fu, Y., Chen, S., Zhang, S., 2011. Biooxidation-cyanidation leaching of gold concentrates with different arsenic types. Trans. Nonferrous Metals Soc. China 21 (3), 694–699 (in Chinese). Curreli, L., Garbarino, C., Ghiani, M., Orrù, G., 2009. Arsenic leaching from a gold bearing enargite flotation concentrate. Hydrometallurgy 96 (3), 258–263. Dunn, J.G., Chamberlain, A.C., 1997. The recovery of gold from refractory arsenopyrite concentrates by pyrolysis-oxidation. Miner. Eng. 10 (9), 919–928. Gao, G., Li, D., Zhou, Y., Sun, X., Sun, W., 2009. Kinetics of high-sulphur and high-arsenic refractory gold concentrate oxidation by dilute nitric acid under mild conditions. Miner. Eng. 22 (22), 111–115. George, M.W., 2009. Mineral resource of the month: gold. Earth 54 (1), 29. Gudyanga, F.P., Mahlangu, T., Roman, R.J., Mungoshi, J., Mbeve, K., 1999. An acidic pressure oxidation pre-treatment of refractory gold concentrates from the KweKwe roasting plant, Zimbabwe. Miner. Eng. 12 (8), 863–875. Hol, A., Weijden, R.D.V.D., Weert, G.V., Kondos, P., Buisman, C.J.N., 2011. The effect of anaerobic processes on the leachability of an arsenopyrite refractory ore. Miner. Eng. 24 (6), 535–540. Liu, J., Chi, R., Xu, Z., Zeng, Z., Liang, J., 2000. Selective arsenic-fixing roast of refractory gold concentrate. Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 31 (6), 1163–1168. Nie, S., Suo, Y., 1997. Leaching of Gold From Refractory Gold Ore. Geology Press, Beijing, pp. 20–48 (in Chinese). Saba, M., Mohammadyousefi, A., Rashchi, F., Moghaddam, J., 2011. Diagnostic pre-treatment procedure for simultaneous cyanide leaching of gold and silver from a refractory gold/silver ore. Miner. Eng. 24 (15), 1703–1709. Ubaldini, S., Vegliò, F., Fornari, P., Abbruzzese, C., 2000. Process flow-sheet for gold and antimony recovery from stibnite. Hydrometallurgy 57 (3), 187–199. Yang, T., 2005. Metallurgy and Product of Precious Metals. Central South University Press, Changsha, pp. 210–248 (in Chinese). Yang, T., Lai, Q., Tang, J., Chu, G., 2002. Precipitation of antimony from the solution of sodium thioantimonite by air oxidation in the presence of catalytic agents. J. Cent. South Univ. 9 (2), 107–111. Yang, T., Jiang, M., Lai, Q., Chen, J., 2005. Sodium sulfide leaching of low-grade jamesonite concentrate in production of sodium pyroantimoniate. J. Cent. South Univ. 12 (3), 290–294. Zhang, D., Xiao, Q., Liu, W., Chen, L., Yang, T., Liu, Y., 2015. Pressure oxidation of sodium thioantimonite solution to prepare sodium pyroantimonate. Hydrometallurgy 151, 91–97. Zhao, T., 1987. Metallurgy of Antimony. Central South University of Technology Press, Changsha, pp. 358–362 (in Chinese).
Contents lists available at ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Technical note
A selective process for extracting antimony from refractory gold ore Tianzu Yang a, Shuai Rao a, Weifeng Liu a,b,⁎, Duchao Zhang a, Lin Chen a a b
School of Metallurgy and Environment, Central South University, Changsha 410083, China Henan Yuguang Gold and Lead Group Co., Ltd., Jiyuan 459000, China
a r t i c l e
i n f o
Article history: Received 21 November 2016 Received in revised form 18 February 2017 Accepted 25 March 2017 Available online 28 March 2017 Keywords: Refractory gold ore Sodium sulfide leaching Pressure oxidation Antimony
a b s t r a c t To avoid the adverse effects of antimony on the cyanide leaching process used for the recovery of gold from refractory gold ore, a cleaner production process was proposed that would selectively remove antimony as sodium pyroantimonate. The process included three steps: 1) leaching of antimony from the refractory gold ore with sodium sulfide solution; 2) pressure oxidation of the leaching solution containing sodium thioantimonite, transforming it to sodium pyroantimonate; and 3) concentration/crystallization to recover the sodium thiosulfate byproduct from the oxidized solution. The leaching recoveries of antimony, gold, and arsenic in the first step were 96.64%, 1.44%, and 0.41%, respectively. The precipitation ratio of antimony exceeded 99.80% during the second step. The synthesized sodium pyroantimonate product exhibited a regular tetragonal morphology. The purity of the sodium thiosulfate byproduct obtained during the third step reached 98.0%. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Gold is a strategic precious metal that is widely used in ornamentation, currency reserves, and high-tech industries. According to the United States Geological Survey, globally identified gold reserves amount to 89,000 tons, with approximately one-third of that total contained in refractory ores (George, 2009). With the gradual depletion of high-grade resources, refractory gold ores have attracted increased attention. Refractory gold ores are defined as minerals for which the cyanideleaching recovery of gold is b80%, even after fine grinding. High-arsenic refractory gold ores constitute the largest reserves but are the most difficult from which to extract gold. Gold leaching recoveries can often be as low as 50% (Yang, 2005). These poor results can be attributed to the encapsulation of the fine gold particles—even at micron size—in pyrite or arsenopyrite. Therefore, refractory gold ores must be pretreated prior to the cyanide leaching process to liberate the gold. Currently, there are many methods for the pretreatment of refractory gold ores, including two-stage roasting (Dunn and Chamberlain, 1997), pressure oxidation (Gudyanga et al., 1999), bacterial oxidation (Hol et al., 2011), fixed arsenic roasting (Liu et al., 2000), microwave roasting (Amankwah and Pickles, 2009), chemical oxidation (Saba et al., 2011), and nitric acid catalytic oxidation (Gao et al., 2009). Geochemical studies have shown that gold migrates easily and is concentrated, along with characteristic elements such as arsenic and antimony, during the endogenous mineralization of gold deposits (Nie and Suo, 1997). However, the coexistence of those elements increases the ⁎ Corresponding author at: School of Metallurgy and Environment, Central South University, Changsha 410083, China. E-mail address: [email protected] (W. Liu).
http://dx.doi.org/10.1016/j.hydromet.2017.03.014 0304-386X/© 2017 Elsevier B.V. All rights reserved.
difficulty of extracting gold from refractory gold ores. During the direct cyanide leaching process, the dissolution of stibnite (Sb2S3) not only increases the consumption of OH−, CN−, and O2, but also forms precipitates that coat the surfaces of the gold particles, which leads to low leaching recoveries (Yang, 2005). During the two-stage roasting pretreatment process, it is easy to fuse with each other due to the low melting points of antimony compounds, causing the gold particles to be encapsulated again (Cui et al., 2011). In pressure oxidation pretreatments, precipitates of antimony compounds are produced, which cover the surfaces of the gold particles and hinder further leaching reactions. In the biological oxidation pretreatment process, the antimonycontaining minerals significantly affect the oxidation rates of gold-bearing minerals. Due to the adverse effects of antimony on these pretreatment and direct cyanide leaching processes, it is necessary to remove it from antimonial refractory gold ores before gold extraction (Anderson, 2012). In general, the hydrometallurgical methods for removing antimony can be conducted in either alkaline (sodium sulfide, Na2S) or acidic (hydrochloric acid, HCl) systems. In alkaline sodium sulfide solution, antimony is dissolved in the form of sodium thioantimonite (Na3SbS3). The antimony can be recovered from the leaching solution by neutralization, displacement, electrowinning, air oxidation, and so on. Owing to its excellent selectivity, this method has been widely used for removing antimony from stibnite (Ubaldini et al., 2000), jamesonite (Yang et al., 2005), enargite (Curreli et al., 2009), and refractory antimony ores (Celep et al., 2011). In HCl solution, antimony is dissolved in the form of SbCl3 by adding oxidizing agents such as Cl2, FeCl3, SbCl5, or H2O2 (Zhao, 1987). Methods for the recovery of antimony from the SbCl3 leaching solution include hydrolysis, replacement, distillation, and electrowinning. Although the foregoing methods afford high antimony
572
T. Yang et al. / Hydrometallurgy 169 (2017) 571–575
recoveries, they all focus on the unit leaching process or the recovery of antimony from the leaching solution. However, a complete technological process should consider many factors, including the recovery of sulfur, operational environment, and processing costs. In our previous study, low-grade jamesonite concentrate was leached in sodium sulfide solution, affording an antimony recovery exceeding 91.0% (Yang et al., 2005). We have also shown that antimony can be precipitated as sodium antimonite from a solution of sodium thioantimonite by air oxidation (Yang et al., 2002). Based on these findings, a cleaner production process was proposed to selectively remove antimony in the form of sodium pyroantimonate from refractory gold ore. In this work, the complete process is presented in detail so as to provide guidance for the extraction of antimony from antimonial refractory gold ores. 2. Experimental 2.1. Materials The raw material, antimonial refractory gold ore, was provided by the Jinchiling Gold Mine (Zhaojin Mining Industry Co., Ltd., Shandong Province, China). The chemical composition of the ore is given in Table 1. Before the experiments, the ore was dried at 383 K, and then ground and sieved to obtain a particle size below 74 μm. The main phases of the material were analyzed by XRD (Fig. 1). Many mineral components are observed in the refractory gold ore, including quartz (SiO2), muscovite (KAl2Si3AlO10(OH)2), pyrite (FeS2), and arsenopyrite (FeAsS). Antimony is present mainly in the form of stibnite (Sb2S3). All reagents were of analytical grade. The purity of the oxygen used during the pressure oxidation process exceeded 99.9%.
Fig. 1. XRD pattern of the antimonial refractory gold ore.
After completing the leaching process, the leaching solution is transferred into an autoclave to prepare sodium pyroantimonate. The reaction equation is expressed as: 2Na3 SbS3 þ7O2 þ2NaOH þ 5H2 O ¼ 2NaSbðOHÞ6 þ3Na2 S2 O3
The oxidized solution is concentrated and hot filtered to remove the insoluble mixture of sodium sulfate and sodium sulfite. Finally, the sodium thiosulfate byproduct is crystallized from the solution by cooling.
2.2. Process flowsheet
2.4. Analysis and characterization
To avoid the adverse effects of antimony on gold extraction, a clean production process for removing and recovering antimony from refractory gold ore is proposed. The complete process flowsheet is shown in Fig. 2. First, the antimonial refractory gold ore is leached in alkaline sodium sulfide solution. Gold is extracted from the leaching residue by the cyaniding process. The leaching solution is subjected to a pressure oxidation process to prepare sodium pyroantimonate (NaSb(OH)6). Finally, the oxidized solution is concentrated and crystallized to recover sodium thiosulfate (Na2S2O3 ∙5H2O).
Samples with low antimony, arsenic, and gold contents were determined by ICP-AES (IRIS Intrepid II, XRS), and those with high contents of antimony and arsenic were determined by titration with ceric sulfate and potassium bromate, respectively. The gold and silver contents of the solid samples were determined by fire assay. The sulfur content in the solution was analyzed by the iodine-sodium thiosulfate dispersion method. The chemical composition of the solid samples was characterized using X-ray fluorescence (XRF, ZSX Primus II, Rigaku). The phases of the solid sample were identified by X-ray diffraction (XRD) using a Rigaku TTRAX-3 instrument (40 kV, 30 mA, 10°/min). Microstructures were characterized using a Japan Jeol JSM-6360LV instrument equipped with a spectrometer for microanalysis based on an energy dispersive Xray spectroscopy (EDS) system (EDX-GENESIS 60 S, EDAX, USA) with an accelerating voltage of 0.5 kV to 30 kV.
2.3. Experimental procedure The clean production process for extracting antimony from refractory gold ore involves sodium sulfide leaching, pressure oxidation, concentration, and crystallization steps. First, the stibnite in the refractory gold ore reacts with sodium sulfide, and antimony is dissolved into the leaching solution in the form of sodium thioantimonite (Eq. (1)). During the leaching process, a given amount of sodium hydroxide is added into the leaching solution to prevent the hydrolysis of sodium sulfide (Eq. (2)). The main chemical reactions can be described as follows: Sb2 S3 þ3Na2 S ¼ 2Na3 SbS3
ð1Þ
Na2 S þ H2 O ¼ NaOH þ NaHS
ð2Þ
ð3Þ
3. Results and discussion 3.1. Sodium sulfide leaching 3.1.1. Leaching recoveries of antimony, gold, and arsenic Several studies have reported that the cyanide leaching of gold from antimonial refractory gold ore can be enhanced significantly after removing the antimony. Therefore, the selective leaching of antimony before the conventional cyanidation treatment is necessary. Through
Table 1 Chemical composition of the antimonial refractory gold ore. Component
Au/(g/t)
Ag/(g/t)
S
Fe
Sb
As
Cu
SiO2
Al2O3
CaO
K2O
wt%
58.8
42.0
17.68
15.74
6.30
5.50
0.04
36.58
11.94
3.76
2.45
T. Yang et al. / Hydrometallurgy 169 (2017) 571–575
573
Fig. 2. A clean process for extracting antimony from refractory gold ore.
systematic experiments, we established the optimum conditions for antimony removal as follows: sodium sulfide concentration, 50 g/L; sodium hydroxide concentration, 20 g/L; temperature, 323 K; liquid-tosolid ratio, 1.5 L/kg; reaction time, 1.5 h; stirring speed, 120 rpm; and washing water ratio, 1.0. Under these conditions, the antimony recovery reached 96.64%. The yield and antimony content of the leaching residue were 92.0% and 0.23%, respectively. Compared with the leaching of clairite (Celep et al., 2011) and jamesonite (Yang et al., 2005) in the alkaline sodium sulfide system, a higher antimony recovery was obtained for the antimonial refractory gold ore. The main phases of the leaching residue obtained under optimal conditions were analyzed by XRD (Fig. 3). Clearly, the characteristic peaks of the stibnite phase have disappeared completely. The gold and arsenic contents in the leaching solution and residue were analyzed twice. The results, shown in Table 2, demonstrate that minimal amounts of gold and arsenic were dissolved in the leaching solution, suggesting an effective and selective antimony leaching process. 3.1.2. Precipitate from the leaching solution A black precipitate was observed to form in the leaching solution after 48 h. To ascertain its composition and phases, SEM-EDS and XRD
analyses were conducted. The black precipitate consists of many flocculated tiny particles with irregular morphologies (Fig. 4). The EDS pattern indicates that the precipitate contains mainly iron, sodium, sulfur, oxygen, and antimony. Based on Fig. 5, the main phases in the black precipitate can be assigned as Na2FeS2, S, and FeS2. One possible reason for the formation of this precipitate is that a small amount of FeS2 is dissolved into the leaching solution. Then, the following reaction can occur: FeS2 þNa2 S ¼ Na2 FeS2 þ S
ð4Þ
Therefore, before conducting the pressure oxidation process, the leaching solution should be refiltered after 48 h natural settling at room temperature to remove additional impurities. 3.2. Pressure oxidation It is well known that sodium pyroantimonate can be successfully produced from the sodium thioantimonite leaching solution by air oxidation (Yang et al., 2002). However, the reaction rate is relatively slow. To solve this problem, a pressurized oxidation process was proposed to effect the transformation (Zhang et al., 2015). The effects of temperature, oxygen partial pressure, sodium hydroxide concentration, reaction time, and stirring speed on the antimony precipitation ratio were studied in detail. The optimum conditions are as follows: sodium hydroxide concentration, 20 g/L; stirring speed, 800 rpm; oxygen partial pressure, 0.4 MPa; reaction time, 2 h; and reaction temperature, 363 K. Under these conditions, the antimony content in the oxidized solution is reduced to 0.092 g/L, and the antimony precipitation ratio is over 99.80%. Compared with the traditional air oxidation process (Yang et al., 2002), this antimony precipitation process is improved significantly. The reaction time is decreased from 144 to 2 h, and the antimony content in the oxidized solution is lowered from 1.0 to 0.092 g/L. Table 2 Gold and arsenic contents in the leaching solution and residue.
Au As Fig. 3. XRD pattern of the leaching residue obtained under optimal conditions.
No.
Leaching solution
Leaching residue
Concentration
Recovery/%
Content
Recovery/%
1-1 2-1 1-2 2-2
0.40 mg/L 0.56 mg/L 0.15 g/L 0.14 g/L
1.02 1.44 0.41 0.38
64.0 g/t 63.8 g/t 5.96% 5.95%
0.00 0.18 0.31 0.47
574
T. Yang et al. / Hydrometallurgy 169 (2017) 571–575
Fig. 4. SEM-EDS pattern of black precipitate formed in the leaching solution.
The phases and morphology of the obtained sodium pyroantimonate product were determined by XRD and SEM-EDS analyses, respectively. As shown in Fig. 6, all characteristic peaks were consistent with the standard diffraction peaks of sodium pyroantimonate (NaSb(OH)6). The chemical composition of the sodium pyroantimonate product was
analyzed (three replicates). The results, shown in Table 3, demonstrate that the sodium pyroantimonate purity is very high. The Sb3+ content, in particular, is b 0.10%, suggesting an adequate oxidation process. The product quality is fully compliant with the first-grade quality requirement of China Non-ferrous Industry Standard YS22-92. SEM micrographs of the sodium pyroantimonate product are shown in Fig. 7. The synthesized material is mainly composed of many agglomerated crystals. Although these crystals exhibit regular tetragonal structures, they are not uniform in terms of particle size, which range from N50 μm for larger crystals to b10 μm for some smaller crystals. Almost all the larger crystals have several adherent smaller crystals. 3.3. Recovery of sodium thiosulfate from the oxidized solution
Fig. 5. XRD pattern of the black precipitate formed in the leaching solution.
After completing the pressure oxidation process, the antimony concentration in the oxidized solution was reduced to 0.092 g/L. However, the oxidized solution still contained many sulfurated compounds, including Na2S2O3, Na2SO4, Na2S, and Na2SO3 at concentrations of 77.50, 7.42, 4.86, and 2.68 g/L, respectively. The main component of the oxidized solution, sodium thiosulfate (Na2S2O3), is worth recovering because of its high economic value. To accomplish this, the oxidized solution is evaporatively concentrated to a slurry with a density of 1.5 g/cm3, and then filtered at 363 K to remove an insoluble mixture of sodium sulfate and sodium sulfite. Finally, sodium thiosulfate is separated by cooling crystallization. The composition of the sodium thiosulfate byproduct was analyzed (two replicates). Table 4 shows that the sodium thiosulfate content exceeds 98.0%. The XRD pattern, shown in Fig. 8, demonstrates an exact match of the standard diffraction peaks for Na2S2O3·5H2O. 4. Conclusion In this study, a complete technological process for extracting antimony from refractory gold ore was proposed, which included leaching, pressure oxidation, concentration, and crystallization steps. The following conclusions may be drawn from this investigation. (1) The leaching experiments demonstrated that antimony could be selectively leached from refractory gold ore. Under optimal conditions, the leaching recoveries of antimony, gold, and arsenic were 96.64%, 1.44%, and 0.41%, respectively.
Table 3 Chemical composition of the sodium pyroantimonate product (wt%).
Fig. 6. XRD pattern of the sodium pyroantimonate product.
No.
Total Sb (Sb2O5)
Na2O
Sb3+
Fe2O3
As2O3
1 2 3
58.98 59.02 59.98
12.55 12.55 12.61
≤0.10 ≤0.10 ≤0.10
0.008 0.007 0.008
0.0006 0.0005 0.0008
T. Yang et al. / Hydrometallurgy 169 (2017) 571–575
575
Fig. 7. SEM images of the sodium pyroantimonate product at different magnifications.
References Table 4 Composition of the sodium thiosulfate byproduct (wt%). No.
Na2S2O3·5H2O
Na2S
Fe
Water-insoluble substance
1 2
98.33 98.51
0.003 0.004
0.002 0.001
0.028 0.037
(2) Antimony in the leaching solution could be precipitated in the form of sodium pyroantimonate by pressure oxidation. Under optimal conditions, the antimony precipitation ratio exceeded 99.80%, and the antimony content in the leaching solution was b 0.092 g/L. (3) The sodium thiosulfate byproduct with a purity exceeding 98.3% was recovered from the oxidized solution by evaporation and concentration. Acknowledgments The authors acknowledge the support of the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 51404296) and the China Postdoctoral Science Foundation (Grant No. 2016M602427). We also thank the Jinchiling Gold Mine of the Zhaojin Mining Industry Co., Ltd. for providing the raw material and financial support.
Fig. 8. XRD pattern of the sodium thiosulfate byproduct.
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