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A modified sulfation-roasting-leaching process for recovering Se, Cu, and Ag from copper anode slimes at a lower temperature
Journal of Environmental Management 235 (2019) 303–309
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Research article
A modified sulfation-roasting-leaching process for recovering Se, Cu, and Ag from copper anode slimes at a lower temperature
T
Misagh Khanlariana, Fereshteh Rashchia,∗, Mojtaba Sabab a b
School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, PO Box 11155/4563, Tehran, Iran Western Australian School of Mines, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Sulfation-roasting Leaching Selenium Silver Copper anode slimes Low temperature
Copper anode slimes are source of valuable elements such as selenium, tellurium, copper, silver, lead, antimony, tin, gold, and platinum group metals. There are vast, diverse methods to recover precious elements from the copper anode slimes. Sulfation roasting is one of the prevalent methods that became widespread because of the simplicity of the technology and available knowledge of the process. High energy consumption and too many stages that produce gaseous and liquid effluents in each stage are the main drawbacks of this approach. In this study, the conventional sulfation roasting approach was environmentally modified to a lower temperature sulfation-roasting-leaching process by reducing the temperature from 600 to 800 °C to ca. 247 °C. Therefore the energy consumption is lowered significantly and the number of stages is shortened to 2 separate stages of sulfation-roasting and leaching as the main stages of the process. In this regard, the gaseous and liquid effluents of the process are reduced and therefore, the environmental drawbacks decreases. The effects of the two parameters of temperature and liquid to solid ratio were investigated using response surface methodology. At the optimum condition, temperature of ca. 247 °C and L/S ratio of 1.95, 99.93% of selenium, 96.53% of copper, and 96.48% of silver were recovered. After separation of selenium, copper and silver were taken apart by solvent extraction using CP-150. Using 10% (V/V) CP-150 in kerosene, a 5 min contact time, pH of 4 and organic to aqueous ratio of 1, 99.87% of copper was extracted.
1. Introduction
low concentration of nitric acid to a sealed sulfuric acid leaching system was examined by Li et al. (2017). In another research, Xing and Lee (2017) utilized a mixture of HCl and oxidizing agents such as H2O2, NaClO, and HNO3 to recover gold and silver. Ranjbar et al. (2014) leached gold using thiourea solution and extracted gold utilizing magnetite nanoparticles (MNPs). Copper and selenium recoveries were studied by Kilic et al. (2013) using two step hydrometallurgical process; adding sulfuric acid to de-copperize the copper anode slimes and then, sodium hydroxide to dissolve selenium. Roasting is one of the most widespread industrial techniques among the copper anode slimes processing procedures. Simplicity in technology and available knowledge of the roasting method spread this method worldwide. Roasting can be used as a pretreatment or the main stage of a hybrid process (Amer, 2002; Hait et al., 2009). Sulfation roasting, oxidizing roasting, and soda roasting are the main roasting techniques. High energy consumption and environmental drawbacks like gaseous and liquid effluents are the main shortcomings of the conventional roasting methods (Cooper, 1990; Hait et al., 2009; Ludvigsson and Larsson, 2003). In recent years, Lu et al. (2015)
Copper anode slimes, the byproduct of the copper electrorefining process often consist of valuable elements such as Se, Te, Cu, Ag, Pb, Sb, Sn, Au and PGMs (Li et al., 2016, 2015; Xiao et al., 2018). There are many diverse methods to recover valuable elements from the copper anode slimes; pyrometallurgical, hydrometallurgical, and hybrid methods. Copper anode slimes are generally processed by hybrid methods that combine pyrometallurgical and hydrometallurgical methods to recover most of the valuable elements of the copper anode slimes. (Chen et al., 2015; Ding et al., 2017; Seisko et al., 2017). Conventional approaches such as soda roasting, selenium leaching, copper leaching, smelting, refining, and silver and gold electrolysis have environmental drawbacks like dust, acid fog, selenium releases and high energy consumption (Ludvigsson and Larsson, 2003). In recent years, many studies have been done to overcome the above-mentioned disadvantages. Xiao et al. (2018) investigated silver recovery from the copper anode slimes by thiosulfate, sulfite and ammonia leaching. Selenium extraction from copper anode slimes by adding a
∗
Corresponding author. E-mail address: [email protected] (F. Rashchi).
https://doi.org/10.1016/j.jenvman.2019.01.079 Received 5 September 2018; Received in revised form 16 December 2018; Accepted 22 January 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.
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decreasing the process temperature from 600 to 800 °C to ca. 247 °C, lowering the energy consumption significantly. The process applied in this study shortens the number of stages in the conventional sulfation roasting process (Fig. 1) to two stages of sulfation-roasting and leaching, decreasing the gaseous and liquid effluents of the process and therefore, reducing the environmental drawbacks. Also, combines the two separate stages of mixing and roasting of the common sulfationroasting-leaching process, and reducing the energy consumption used for each separate stage. In this regard, the effects of temperature and L/ S ratio on the process were investigated and the parameters were optimized by response surface methodology (RSM). Solvent extraction was used to separate copper and silver from the sulfate leachate. The organic extractant, Chemorex CP-150, was used to separate copper and silver from the leachate solution.
performed soda roasting-alkaline leaching-acid leaching process by using sodium carbonate to recover selenium, tellurium, and copper from the copper anode slimes. The optimum roasting temperature was 500 °C (that is relatively high) and the selenium recovery was 94.7%. One of the dominant roasting methods is sulfation roasting. In this process, sulfuric acid is used to sulfatize base metals and oxidize selenium in the presence of air. As a result of SeO2 and water reaction, the selenious acid will be made. The SO2 gas produced in the roasting process reduces the selenious acid to elemental selenium, and the sulfuric acid that was used in the process is restored (Chen et al., 2015; Chiu et al., 1981; Hoffmann, 1989). Selenides are transformed to sulfates in sulfation roasting according to the reaction (Chen et al., 2015; Hyvärinen et al., 1989):
MSe (s) + 4H2 SO4 (l) → MSO4 (s) + SeO2 (g) + 3SO2 (g) + 4H2 O (g) (1)
2. Experimental procedure
where M stands for base metals. A significant sulfation roasting process has been industrialized by Outokumpu Company. In this process, copper anode slimes are treated in an autoclave to leach copper and nickel. The batch from the autoclave is transferred to a flash tank to precipitate selenium and silver by SO2. The solution is filtered and led into the tellurium tank where tellurium is precipitated with copper. The decopperized slimes wet filter cake is roasted at 600 °C (a relatively high temperature) by SO2 and oxygen is introduced into the roaster to recover selenium. The deselenized slimes are separated into two layers of slag and dore metal in the TROF (tilting rotating oxyfuel) converter, with soda and borax as fluxes. Silver is recovered from the dore metal with electrolysis. Gold is obtained by hydrochloric acid leaching of the gold mud of the electrolysis stage (Cooper, 1990; Hait et al., 2009). In another process, provided by Sumitomo Metal Mining Company, first, slimes are roasted in sulfuric acid, and in the next stage, copper is leached in water. The leach filter cake is roasted at 600–800 °C (quite a high temperature) to recover selenium. The roasted slimes are smelted in an electric furnace to produce dore metal. The dore metal is used to cast silver anodes that are used in silver electrolysis to produce silver. Gold is gained by electrorefining the silver anode slimes (Hait et al., 2009). In the Outokumpu (Cooper, 1990; Hait et al., 2009) and Sumitomo techniques (Hait et al., 2009), the sulfation roasting methods to recover valuable elements of copper anode slimes suffer from two main drawbacks of relatively high process temperature and a relatively high number of process stages where gaseous and liquid effluents are produced (Fig. 1). The sulfation-roasting-leaching process is a technique that reduces the conventional sulfation roasting stages. This process was first introduced to extract nickel and cobalt from iron-rich lateritic ores selectively (Borra et al., 2016). In recent years, this method has been applied to selective recovery of rare earths from bauxite residue (Borra et al., 2016; Onghena et al., 2017). The sulfation-roastingleaching process consists of three main stages of mixing the feed material with concentrated sulfuric acid, roasting, and leaching with water. In comparison with the other conventional and novel methods of copper anode slimes recovery, the main benefits of the sulfationroasting-leaching technique are the possibility of acid regeneration and hence, low acid consumption, small volumes of waste water generation, and the close to neutral pH of the leaching residue that makes it manageable with some applications (Borra et al., 2016). Solvent extraction is a dominant method for the selective purification of solutions. There are some commercially developed extractants for copper extraction from the acidic media. Previous studies have revealed the great ability of aldoximes for copper extraction. Chemorex CP-150 consisted of 5-nonylsalicylaldoxime, a branched chain diisobutyrate modifier is a strong copper extractor (Kavousi et al., 2017; Pouramini and Moradi, 2012). In this research, the traditional sulfation roasting technique was modified to a lower temperature sulfation-roasting-leaching method
2.1. Approach overview The experimental procedure for the extraction of valuable elements from the copper anode slimes consist of two main stages: sulfationroasting and water leaching followed by filtration and solvent extraction. Fig. 2 illustrates the process flowsheet. 2.2. Materials and preparations A single batch of copper anode slimes from Sarcheshmeh Copper Company, Iran, was used to perform all the experiments in this research. First, the copper anode slimes were dried in an oven at 100 °C for 24 h. The dried slimes were ground to 38–75 μm and homogenized by a riffle. All of the chemicals used in roasting, leaching, and solvent extraction stages were obtained from Merck Company. Chemorex CP150 organic solvent was used for copper solvent extraction. 2.3. Characterizations The copper anode slimes was characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES, Vista-Pro, Varian Co.), x-ray diffraction (XRD, Philips-3040/60 PW), x-ray fluorescence spectroscopy (XRF, Philips 2000), and energy dispersive x-ray spectroscopy (EDS, Hitachi 8040). Samples were analyzed after each stage by ICP and XRD. 2.4. Design of experiments According to the previous studies, among a large number of parameters that affect the roasting of copper anode slimes, the most significant ones are liquid to solid ratio (L/S) and roasting temperature. Acid concentration and roasting time were considered at constant levels according to the optimum conditions of the previous works (Borra et al., 2016; Dong et al., 2010; Guo et al., 2009; Onghena et al., 2017; Yan et al., 2012; You et al., 2015). The levels of L/S ratio were selected between 1 and 2 mL/g to provide enough acid for sulfation of the slimes (Borra et al., 2016; Guo et al., 2009). Good mixing of the slimes and acid was also provided. For the temperature, the levels were selected between 170 °C and 250 °C due to the preliminary experiments performed. Based on these experiments, below 170 °C, the SeO2 vapor pressure is very low. Thus, the selenium recovery decreases to below 10%. Also, the selenium recovery would be close to 100% at a temperature below 250 °C. As shown in Table 1, sixteen experiments were designed by response surface methodology (RSM) subdivision of Doptimal. To determine the optimum condition of the sulfation-roasting process and in order to maximize the selenium, copper, and silver recovery and also to investigate the possible interactions between temperature and L/S ratio, experimental design and statistical analysis were conducted by Design Expert 7 software (State-Ease Inc., 304
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Fig. 1. Flowsheet for extraction of valuable elements from the copper anode slimes, (a) using autoclave leaching followed by roasting, 600 °C (b) using roasting, 600 °C-800 °C, after Hait et al., 2009. Table 1 The D-optimal experiments design. Run No.
L/S (mL/g)
Temperature (°C)
Se recovery (%)
Cu recovery (%)
Ag recovery (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1.00 1.00 1.38 1.43 1.00 2.00 1.25 1.66 1.50 2.00 1.85 1.00 2.00 1.00 2.00 1.66
170.00 210.00 233.00 205.00 250.00 229.00 187.00 250.00 170.00 170.00 200.00 250.00 229.00 170.00 170.00 250.00
11.37 46.24 90.57 46.93 76.90 84.21 27.49 93.84 36.8 54.40 68.34 81.10 75.77 13.60 24.26 94.61
55.25 62.59 81.56 82.28 71.21 95.24 65.00 87.64 83.85 89.28 89.24 59.31 94.02 57.64 89.15 86.83
55.37 62.10 80.24 82.21 71.37 93.14 67.00 87.36 83.27 91.67 90.37 59.87 94.54 58.84 92.64 87.43
and water, with the purpose of producing selenium. The container was equipped with a three-blade agitator to mix the acid and the slimes. The sand bath in the container provides the required process temperature. The concentrated sulfuric acid and the copper anode slimes were mixed in a 500 mL container until the desired temperature was reached and roasted for 1 h. The selenium recovery was calculated using the following:
Fig. 2. Flowsheet for extraction of valuable elements from the copper anode slimes.
Minneapolis, MN, USA). 2.5. Roasting
Se recovery(%) = The copper anode slimes roasting stage was performed using a roasting setup. Fig. 3 shows the schematic of the setup. The setup components consist of: 1- water reservoir, 2- sand bath, 3condenser, 4- acid resistant pump, 5- slimes and acid container, 6- rotator motor, 7- water pipe. The condenser was filled with a polyvinyl chloride-polyethylene-polypropylene-acrylonitrile butadiene styrene cooling tower packing to increase the interactions between the SeO2 gas
m1 × 100 m0
(2)
where m0 is the weight of selenium in the copper anode slimes and m1 is the weight of produced selenium. 2.6. Leaching The leaching experiments were carried out in a 500 mL three305
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Fig. 5. XRD pattern of the copper anode slimes head sample.
was considered. After separating the organic and aqueous phases, the aqueous phase was analyzed to calculate copper and silver concentration and therefore, the percentage extraction of copper and silver (Eq. (4)). Fig. 3. Schematic of the copper anode slimes roasting setup.
solvent extraction metal recovery (%) =
C1 × 100 C0
(4)
where C0 is the copper/silver concentration in the aqueous phase before solvent extraction and C1 is the copper/silver concentration in the aqueous solution after solvent extraction.
3. Results and discussion 3.1. Characterizations of the head sample Fig. 5 illustrates the XRD pattern of the slimes sample. As it is clear, barite (BaSO4) is the main phase of the copper anode slimes. The majority of copper is present as Cu2Se in the pattern. Silver mainly occurs as Ag2Se, Ag2SeO4, and Ag2O that is confirmed by the fact that copper and silver are commonly associated with selenium in the copper anode slimes. Also, the Ag-Cu selenides and their oxides are the main phases of the copper anode slimes (Chen et al., 2015; Chen and Dutrizac, 1989; Cheng and Hiskey, 1996). Lead and Arsenic appear in complex oxide phases of Pb2(As2O4) and AgPb4(AsO4) (Chen and Dutrizac, 1990). Table 2 shows the XRF analysis of the copper anode slimes head sample. The slimes contain 22.14% Ba, mainly as BaSO4 (according to the XRD pattern (Fig. 5)). Furthermore, 15.21% Se, 7.38% Ag, and 7.34% Cu were the main elements of the slimes. The ICP analysis was conducted to determine the concentration of the elements in the copper anode slimes sample. Table 3 shows that selenium, copper, and silver are the major elements of the copper anode slimes.
Fig. 4. The roasted slimes leaching setup.
necked and round-bottomed thermostatic Pyrex reactor fitted with a reflux condenser placed on a hot plate magnetic stirrer with a temperature control system (Fig. 4) by L/S ratio of 10 and temperature of 90 °C for 1 h. After leaching, the solution was filtered and the separated sulfate leachate was analyzed by ICP to calculate silver and copper recovery.
C Leaching recovery (%) = 1 × 100 C0
Table 2 The XRF analysis of the copper anode slimes head sample.
(3)
where C0 is the copper/silver concentration in the roasted copper anode slimes and C1 is the copper/silver concentration in the leachate. 2.7. Solvent extraction Solvent extraction was performed to extract copper from the leachate at the optimum roasting condition containing 4.46 g/L silver and 4.36 g/L copper. The organic phase consisted of 10% (v/v) Chemorex CP-150 diluted in kerosene. The extraction experiments were conducted at the pH range of 0–4 by using organic to aqueous (O/A) ratio of 1:1 and contact time of 10 min at room temperature (25 ± 2 °C). Each experiment was repeated three times. The average copper extraction 306
Formula
Concentration (%)
Calculated elemental concentration (%)
BaO SeO2 SO3 CuO Ag2O PbO Sb2O3 SiO2 As2O3 TeO2 CaO SrO Cl Al2O3
24.72 21.37 17.57 9.19 7.93 5.34 3.95 2.12 1.54 0.98 0.89 0.86 0.60 0.39
22.14 15.21 7.04 7.34 7.38 4.96 3.29 0.99 1.17 0.78 0.64 0.85 0.60 0.21
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tower as follows (Cooper, 1990; Hait et al., 2009; Hoffmann, 1989):
Table 3 The ICP results of the copper anode slimes. Element
Concentration (%)
Se Ag Cu Pb As Sb Au Bi Zn Ni
13.66 7.07 7.00 4.42 0.42 0.10 0.08 0.08 0.06 0.03
SeO2 (g) + H2 O(l) → H2 SeO3 (l) H2 SeO3 (l) + 2SO2 (g) + H2 O(l) → Se (s) + 2H2 SO4 (l)
The roasting stage was performed with the aim of producing selenium in conjunction with converting water-insoluble compositions of copper and silver, to water-soluble sulfate compositions. During the roasting stage, the SeO2 gas was produced in addition to copper and silver sulfate formation according to the suggested following reactions:
Cu2 Se(s) + 3H2 SO4 (l) + 1.5O2 (g) → 2CuSO4 (s) + SeO2 (g) + SO2 (g)
Ag 2SeO4 (s) + SO2 (g) → Ag2 SO4 (s) + SeO2 (g)
(6) (7)
Copper existed as Cu2Se in the copper anode slimes is likely to be transformed to CuSO4, as shown in Eq. (5). According to the copper anode slimes XRD pattern, silver exists as the three phases of, Ag2Se, Ag2SeO4 and Ag2O. Ag2Se and Ag2SeO4 phases react with sulfuric acid and SO2, respectively, and produce SeO2 and Ag2SO4. The suggested reaction for Ag2O and sulfuric acid to form Ag2SO4 is:
Ag 2O (s) + H2 SO4 (l) → Ag 2 SO4 (s) + H2 O (g)
selenium recovery = +59.13 + 31.93 A+ 12.06B
(11)
copper recovery = +78.77 + 3.61 A+ 15.98B
(12)
silver recovery = +79.21 + 2.81 A+ 16.27B
(13)
where A and B are the coded values of temperature and L/S ratio. The models illustrate that both temperature and L/S ratio have positive effects on the selenium, copper, and silver recoveries. But in the case of selenium, temperature is more effective on the recovery than L/S ratio due to the fact that although L/S ratio can improve SO2 and SeO2 generation, SeO2 vapor pressure will be enhanced at higher temperatures (Behrens et al., 1974; Brebrick, 2000) that is the main factor of selenium recovery according to Eqs. (7) and (8) (Cooper, 1990; Hait et al., 2009; Hoffmann, 1989). While in copper and silver recovery, L/S ratio is more effective in comparison to temperature. The L/S ratio directly intensifies the reactions between Cu2Se, Ag2Se, Ag2O and sulfuric acid (Eqs. (5), (6) and (8)). Therefore the amount of copper and silver sulfate formation will be increased. By increasing the L/S ratio, the amount of SO2 will grow that enhances the Ag2SeO4 and SO2 reaction. Therefore, the silver sulfation reaction intensifies. Hence, due to the above-mentioned reasons, L/S ratio enhances the copper and silver
(5)
Ag 2Se (s) + 2H2 SO4 (l) + O2 (g) → Ag 2SO4 (s) + SeO2 (g) + SO2 (g) + 2H2 O(g)
(10)
First, selenious acid is generated by the reaction of SeO2 and water. In the next stage, selenious acid is reduced to elemental selenium by the SO2 gas produced in the process, and the sulfuric acid that was consumed during the process is regenerated (Borra et al., 2016; Hait et al., 2009). Design of experiments was used to develop an empirical model to obtain optimum conditions for the roasting and leaching process with the goal of maximizing the recovery of selenium, copper, and silver in the range of temperature and L/S ratio investigated. The optimization procedure was based on RSM (see section 2.4). According to the results of the experiments as a function of temperature and L/S ratio, three models are proposed for the recovery of selenium, copper, and silver:
3.2. Statistical analysis and process optimization
+ 3H2 O(g)
(9)
(8)
The produced SeO2 gas is decomposed to selenium in the cooling
Fig. 6. Effect of temperature and L/S ratio on the recovery of selenium (a), copper (b), and silver (c). 307
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recovery. Fig. 6 illustrates the effect of temperature and L/S ratio on the recovery of selenium, copper, and silver. Fig. 6 (a) shows the diagram of selenium recovery. The selenium recovery is clearly enhanced both at a higher temperature and L/S ratio. Temperature has a greater effect than L/S ratio on the selenium recovery. In Fig. 6 (b) and (c) the greater effect of L/S ratio is apparent. Also, the interaction between temperature and L/S ratio on the selenium, copper, and silver recovery is insignificant. The correlation coefficients for the models were computed to be 0.91, 0.91, and 0.93 for the selenium, copper, and silver recoveries, respectively, which give a good agreement between the experimental and the predicted values of the fitted model. The selected predicted optimum condition of the roasting stage followed by the leaching to recover copper and silver was temperature of 247.52 °C and L/S ratio of 1.95 mL/g according to the proposed models (Eqs. (11)–(13)). At this condition, 99.93% of selenium, 96.53% of copper and 96.48% of silver were recovered. The confirmation experiments that were performed at 247 °C and L/S ratio of 1.95 mL/g) repeated three times and results confirmed the above-mentioned condition.
Fig. 8. The XRD pattern of the roasted slimes at the optimum condition (temperature ca. 247 °C of and L/S ratio of 1.95).
Ag2SO4 and CuSO4 peaks approve the sulfation of the slimes. Barium sulfate (BaSO4) is the main phase of the roasted slimes. The XRD pattern also shows that PbSO4 was produced during the roasting.
3.3. Thermodynamic analysis 3.5. Leaching at the optimum condition Fig. 7 shows the diagram of Gibbs free energy (ΔG0) as a function of temperature provided using HSC Chemistry software 5.11 (Outokumpu, Finland) under standard state condition for the main reactions (Eqs. (5)–(10)) occurring during sulfation-roasting with sulfuric acid. The change in the slopes of the reaction lines (except Eq. (7)) at 100 °C is as a result of the change in the state of H2O from liquid to gas. The minus Gibbs free energy for the reactions (5) to (8) confirms that the reactions are spontaneous. The results illustrate that the thermodynamically driven force for the Cu2Se reaction with sulfuric acid (Eq. (5)) is the highest among all reactions shown due to the smaller free energy. The reactions (9) and (10) occur at lower temperatures (below 100 °C) in the cooling tower and result in selenium production. As it can be seen from Fig. 7, at the temperatures below 100 °C, both of the reactions are spontaneous due to the minus Gibbs free energy.
Fig. 9 illustrates the XRD pattern of the leach filter cake produced at the optimum condition (temperature of ca. 247 °C and L/S ratio of 1.95). The main phases of the leach filter cake were low soluble BaSO4 and PbSO4. The leachate was also analyzed by ICP (according to Eq. (3)) and the result shows recovery of 96.53% and 96.48% for copper and silver, respectively. 3.6. Solvent extraction Solvent extraction was performed to separate copper and silver from the leachate obtained at the optimum roasting condition (temperature of ca. 247 °C and L/S ratio of 1.95) containing 4.46 g/L silver and 4.36 g/L copper. Fig. 10 shows the copper extraction as a function of equilibrium pH. Increasing pH enhances the copper extraction. The maximum copper extraction was 99.87% (according to Eq. (4)) at pH = 4 (10% (v/v) CP-150 in kerosene, O/A of 1, and 10 min contact time). The silver extraction was also observed in all the extraction experiments and the results showed that silver was not extracted in the investigated extraction conditions.
3.4. Roasting at the optimum condition At the optimum condition (temperature of ca. 247 °C and L/S ratio of 1.95), the selenium recovery of 99.93% was obtained. Fig. 8 shows the XRD pattern of the roasted slimes at the optimum condition. The
Fig. 7. Diagram of Gibbs free energy (ΔG0) in function of temperature for the main reactions occurring during sulfation-roasting with sulfuric acid. 308
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Ranjbar, R., Naderi, M., Omidvar, H., Amoabediny, G., 2014. Gold recovery from copper anode slime by means of magnetite nanoparticles (MNPs). Hydrometallurgy 143, 54–59. Seisko, S., Aromaa, J., Latostenmaa, P., Forsen, O., Wilson, B., Lundstrom, M., 2017. Pressure leaching of decopperized copper electrorefining anode slimes in strong acid solution. Physicochem. Probl. Miner. Process. 53, 465–474. Xiao, L., Wang, Y.L., Yu, Y., Fu, G.Y., Han, P.W., Sun, Z.H.I., Ye, S.F., 2018. An environmentally friendly process to selectively recover silver from copper anode slime. J. Clean. Prod. 187, 708–716. Xing, W.D., Lee, M.S., 2017. Leaching of gold and silver from anode slime with a mixture of hydrochloric acid and oxidizing agents. Geosyst. Eng. 20, 216–223. Yan, Q., Li, X., Wang, Z., Wu, X., Wang, J., Guo, H., Hu, Q., Peng, W., 2012. Extraction of lithium from lepidolite by sulfation roasting and water leaching. Int. J. Miner. Process. 110, 1–5. You, Z., Li, G., Zhang, Y., Peng, Z., Jiang, T., 2015. Extraction of manganese from iron rich MnO2 ores via selective sulfation roasting with SO2 followed by water leaching. Hydrometallurgy 156, 225–231.
Fig. 9. The XRD pattern of the leach filter cake at the optimum condition (temperature of ca. 247 °C and L/S ratio of 1.95).
Fig. 10. Copper extraction as a function of equilibrium pH.
4. Conclusions In this study, a modified sulfation-roasting-leaching process was performed at a lower temperature than the traditional sulfation roasting technique to recover selenium, copper, and silver from the copper anode slimes. This process consists of two main stages of sulfationroasting and water leaching following by filtration and solvent extraction that even shortens the typical sulfation-roasting-leaching process. Selenium was obtained in the sulfation-roasting stage and copper and silver were recovered from the water leaching of the roasted slimes. At the optimum condition of temperature of ca. 247 °C and L/S ratio of 1.95, the recovery of selenium, copper, and silver were 99.93%, 96.53%, and 96.48%, respectively. The results showed no significant interactions between temperature and L/S ratio on the selenium, copper, and silver recovery. Solvent extraction of the leachate (10% (V/ V) CP-150 in kerosene at the pH of 4, organic to aqueous ratio (O/A) of 1, and 10 min contact time) at the optimum condition resulted in 99.87% copper extraction. The silver extraction was also examined in all the extraction experiments and the results showed that silver was not extracted in the investigated extraction conditions. References Amer, A.M., 2002. Processing of copper anode-slimes for extraction of metal values.
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
A modified sulfation-roasting-leaching process for recovering Se, Cu, and Ag from copper anode slimes at a lower temperature
T
Misagh Khanlariana, Fereshteh Rashchia,∗, Mojtaba Sabab a b
School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, PO Box 11155/4563, Tehran, Iran Western Australian School of Mines, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Sulfation-roasting Leaching Selenium Silver Copper anode slimes Low temperature
Copper anode slimes are source of valuable elements such as selenium, tellurium, copper, silver, lead, antimony, tin, gold, and platinum group metals. There are vast, diverse methods to recover precious elements from the copper anode slimes. Sulfation roasting is one of the prevalent methods that became widespread because of the simplicity of the technology and available knowledge of the process. High energy consumption and too many stages that produce gaseous and liquid effluents in each stage are the main drawbacks of this approach. In this study, the conventional sulfation roasting approach was environmentally modified to a lower temperature sulfation-roasting-leaching process by reducing the temperature from 600 to 800 °C to ca. 247 °C. Therefore the energy consumption is lowered significantly and the number of stages is shortened to 2 separate stages of sulfation-roasting and leaching as the main stages of the process. In this regard, the gaseous and liquid effluents of the process are reduced and therefore, the environmental drawbacks decreases. The effects of the two parameters of temperature and liquid to solid ratio were investigated using response surface methodology. At the optimum condition, temperature of ca. 247 °C and L/S ratio of 1.95, 99.93% of selenium, 96.53% of copper, and 96.48% of silver were recovered. After separation of selenium, copper and silver were taken apart by solvent extraction using CP-150. Using 10% (V/V) CP-150 in kerosene, a 5 min contact time, pH of 4 and organic to aqueous ratio of 1, 99.87% of copper was extracted.
1. Introduction
low concentration of nitric acid to a sealed sulfuric acid leaching system was examined by Li et al. (2017). In another research, Xing and Lee (2017) utilized a mixture of HCl and oxidizing agents such as H2O2, NaClO, and HNO3 to recover gold and silver. Ranjbar et al. (2014) leached gold using thiourea solution and extracted gold utilizing magnetite nanoparticles (MNPs). Copper and selenium recoveries were studied by Kilic et al. (2013) using two step hydrometallurgical process; adding sulfuric acid to de-copperize the copper anode slimes and then, sodium hydroxide to dissolve selenium. Roasting is one of the most widespread industrial techniques among the copper anode slimes processing procedures. Simplicity in technology and available knowledge of the roasting method spread this method worldwide. Roasting can be used as a pretreatment or the main stage of a hybrid process (Amer, 2002; Hait et al., 2009). Sulfation roasting, oxidizing roasting, and soda roasting are the main roasting techniques. High energy consumption and environmental drawbacks like gaseous and liquid effluents are the main shortcomings of the conventional roasting methods (Cooper, 1990; Hait et al., 2009; Ludvigsson and Larsson, 2003). In recent years, Lu et al. (2015)
Copper anode slimes, the byproduct of the copper electrorefining process often consist of valuable elements such as Se, Te, Cu, Ag, Pb, Sb, Sn, Au and PGMs (Li et al., 2016, 2015; Xiao et al., 2018). There are many diverse methods to recover valuable elements from the copper anode slimes; pyrometallurgical, hydrometallurgical, and hybrid methods. Copper anode slimes are generally processed by hybrid methods that combine pyrometallurgical and hydrometallurgical methods to recover most of the valuable elements of the copper anode slimes. (Chen et al., 2015; Ding et al., 2017; Seisko et al., 2017). Conventional approaches such as soda roasting, selenium leaching, copper leaching, smelting, refining, and silver and gold electrolysis have environmental drawbacks like dust, acid fog, selenium releases and high energy consumption (Ludvigsson and Larsson, 2003). In recent years, many studies have been done to overcome the above-mentioned disadvantages. Xiao et al. (2018) investigated silver recovery from the copper anode slimes by thiosulfate, sulfite and ammonia leaching. Selenium extraction from copper anode slimes by adding a
∗
Corresponding author. E-mail address: [email protected] (F. Rashchi).
https://doi.org/10.1016/j.jenvman.2019.01.079 Received 5 September 2018; Received in revised form 16 December 2018; Accepted 22 January 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.
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decreasing the process temperature from 600 to 800 °C to ca. 247 °C, lowering the energy consumption significantly. The process applied in this study shortens the number of stages in the conventional sulfation roasting process (Fig. 1) to two stages of sulfation-roasting and leaching, decreasing the gaseous and liquid effluents of the process and therefore, reducing the environmental drawbacks. Also, combines the two separate stages of mixing and roasting of the common sulfationroasting-leaching process, and reducing the energy consumption used for each separate stage. In this regard, the effects of temperature and L/ S ratio on the process were investigated and the parameters were optimized by response surface methodology (RSM). Solvent extraction was used to separate copper and silver from the sulfate leachate. The organic extractant, Chemorex CP-150, was used to separate copper and silver from the leachate solution.
performed soda roasting-alkaline leaching-acid leaching process by using sodium carbonate to recover selenium, tellurium, and copper from the copper anode slimes. The optimum roasting temperature was 500 °C (that is relatively high) and the selenium recovery was 94.7%. One of the dominant roasting methods is sulfation roasting. In this process, sulfuric acid is used to sulfatize base metals and oxidize selenium in the presence of air. As a result of SeO2 and water reaction, the selenious acid will be made. The SO2 gas produced in the roasting process reduces the selenious acid to elemental selenium, and the sulfuric acid that was used in the process is restored (Chen et al., 2015; Chiu et al., 1981; Hoffmann, 1989). Selenides are transformed to sulfates in sulfation roasting according to the reaction (Chen et al., 2015; Hyvärinen et al., 1989):
MSe (s) + 4H2 SO4 (l) → MSO4 (s) + SeO2 (g) + 3SO2 (g) + 4H2 O (g) (1)
2. Experimental procedure
where M stands for base metals. A significant sulfation roasting process has been industrialized by Outokumpu Company. In this process, copper anode slimes are treated in an autoclave to leach copper and nickel. The batch from the autoclave is transferred to a flash tank to precipitate selenium and silver by SO2. The solution is filtered and led into the tellurium tank where tellurium is precipitated with copper. The decopperized slimes wet filter cake is roasted at 600 °C (a relatively high temperature) by SO2 and oxygen is introduced into the roaster to recover selenium. The deselenized slimes are separated into two layers of slag and dore metal in the TROF (tilting rotating oxyfuel) converter, with soda and borax as fluxes. Silver is recovered from the dore metal with electrolysis. Gold is obtained by hydrochloric acid leaching of the gold mud of the electrolysis stage (Cooper, 1990; Hait et al., 2009). In another process, provided by Sumitomo Metal Mining Company, first, slimes are roasted in sulfuric acid, and in the next stage, copper is leached in water. The leach filter cake is roasted at 600–800 °C (quite a high temperature) to recover selenium. The roasted slimes are smelted in an electric furnace to produce dore metal. The dore metal is used to cast silver anodes that are used in silver electrolysis to produce silver. Gold is gained by electrorefining the silver anode slimes (Hait et al., 2009). In the Outokumpu (Cooper, 1990; Hait et al., 2009) and Sumitomo techniques (Hait et al., 2009), the sulfation roasting methods to recover valuable elements of copper anode slimes suffer from two main drawbacks of relatively high process temperature and a relatively high number of process stages where gaseous and liquid effluents are produced (Fig. 1). The sulfation-roasting-leaching process is a technique that reduces the conventional sulfation roasting stages. This process was first introduced to extract nickel and cobalt from iron-rich lateritic ores selectively (Borra et al., 2016). In recent years, this method has been applied to selective recovery of rare earths from bauxite residue (Borra et al., 2016; Onghena et al., 2017). The sulfation-roastingleaching process consists of three main stages of mixing the feed material with concentrated sulfuric acid, roasting, and leaching with water. In comparison with the other conventional and novel methods of copper anode slimes recovery, the main benefits of the sulfationroasting-leaching technique are the possibility of acid regeneration and hence, low acid consumption, small volumes of waste water generation, and the close to neutral pH of the leaching residue that makes it manageable with some applications (Borra et al., 2016). Solvent extraction is a dominant method for the selective purification of solutions. There are some commercially developed extractants for copper extraction from the acidic media. Previous studies have revealed the great ability of aldoximes for copper extraction. Chemorex CP-150 consisted of 5-nonylsalicylaldoxime, a branched chain diisobutyrate modifier is a strong copper extractor (Kavousi et al., 2017; Pouramini and Moradi, 2012). In this research, the traditional sulfation roasting technique was modified to a lower temperature sulfation-roasting-leaching method
2.1. Approach overview The experimental procedure for the extraction of valuable elements from the copper anode slimes consist of two main stages: sulfationroasting and water leaching followed by filtration and solvent extraction. Fig. 2 illustrates the process flowsheet. 2.2. Materials and preparations A single batch of copper anode slimes from Sarcheshmeh Copper Company, Iran, was used to perform all the experiments in this research. First, the copper anode slimes were dried in an oven at 100 °C for 24 h. The dried slimes were ground to 38–75 μm and homogenized by a riffle. All of the chemicals used in roasting, leaching, and solvent extraction stages were obtained from Merck Company. Chemorex CP150 organic solvent was used for copper solvent extraction. 2.3. Characterizations The copper anode slimes was characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES, Vista-Pro, Varian Co.), x-ray diffraction (XRD, Philips-3040/60 PW), x-ray fluorescence spectroscopy (XRF, Philips 2000), and energy dispersive x-ray spectroscopy (EDS, Hitachi 8040). Samples were analyzed after each stage by ICP and XRD. 2.4. Design of experiments According to the previous studies, among a large number of parameters that affect the roasting of copper anode slimes, the most significant ones are liquid to solid ratio (L/S) and roasting temperature. Acid concentration and roasting time were considered at constant levels according to the optimum conditions of the previous works (Borra et al., 2016; Dong et al., 2010; Guo et al., 2009; Onghena et al., 2017; Yan et al., 2012; You et al., 2015). The levels of L/S ratio were selected between 1 and 2 mL/g to provide enough acid for sulfation of the slimes (Borra et al., 2016; Guo et al., 2009). Good mixing of the slimes and acid was also provided. For the temperature, the levels were selected between 170 °C and 250 °C due to the preliminary experiments performed. Based on these experiments, below 170 °C, the SeO2 vapor pressure is very low. Thus, the selenium recovery decreases to below 10%. Also, the selenium recovery would be close to 100% at a temperature below 250 °C. As shown in Table 1, sixteen experiments were designed by response surface methodology (RSM) subdivision of Doptimal. To determine the optimum condition of the sulfation-roasting process and in order to maximize the selenium, copper, and silver recovery and also to investigate the possible interactions between temperature and L/S ratio, experimental design and statistical analysis were conducted by Design Expert 7 software (State-Ease Inc., 304
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Fig. 1. Flowsheet for extraction of valuable elements from the copper anode slimes, (a) using autoclave leaching followed by roasting, 600 °C (b) using roasting, 600 °C-800 °C, after Hait et al., 2009. Table 1 The D-optimal experiments design. Run No.
L/S (mL/g)
Temperature (°C)
Se recovery (%)
Cu recovery (%)
Ag recovery (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1.00 1.00 1.38 1.43 1.00 2.00 1.25 1.66 1.50 2.00 1.85 1.00 2.00 1.00 2.00 1.66
170.00 210.00 233.00 205.00 250.00 229.00 187.00 250.00 170.00 170.00 200.00 250.00 229.00 170.00 170.00 250.00
11.37 46.24 90.57 46.93 76.90 84.21 27.49 93.84 36.8 54.40 68.34 81.10 75.77 13.60 24.26 94.61
55.25 62.59 81.56 82.28 71.21 95.24 65.00 87.64 83.85 89.28 89.24 59.31 94.02 57.64 89.15 86.83
55.37 62.10 80.24 82.21 71.37 93.14 67.00 87.36 83.27 91.67 90.37 59.87 94.54 58.84 92.64 87.43
and water, with the purpose of producing selenium. The container was equipped with a three-blade agitator to mix the acid and the slimes. The sand bath in the container provides the required process temperature. The concentrated sulfuric acid and the copper anode slimes were mixed in a 500 mL container until the desired temperature was reached and roasted for 1 h. The selenium recovery was calculated using the following:
Fig. 2. Flowsheet for extraction of valuable elements from the copper anode slimes.
Minneapolis, MN, USA). 2.5. Roasting
Se recovery(%) = The copper anode slimes roasting stage was performed using a roasting setup. Fig. 3 shows the schematic of the setup. The setup components consist of: 1- water reservoir, 2- sand bath, 3condenser, 4- acid resistant pump, 5- slimes and acid container, 6- rotator motor, 7- water pipe. The condenser was filled with a polyvinyl chloride-polyethylene-polypropylene-acrylonitrile butadiene styrene cooling tower packing to increase the interactions between the SeO2 gas
m1 × 100 m0
(2)
where m0 is the weight of selenium in the copper anode slimes and m1 is the weight of produced selenium. 2.6. Leaching The leaching experiments were carried out in a 500 mL three305
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Fig. 5. XRD pattern of the copper anode slimes head sample.
was considered. After separating the organic and aqueous phases, the aqueous phase was analyzed to calculate copper and silver concentration and therefore, the percentage extraction of copper and silver (Eq. (4)). Fig. 3. Schematic of the copper anode slimes roasting setup.
solvent extraction metal recovery (%) =
C1 × 100 C0
(4)
where C0 is the copper/silver concentration in the aqueous phase before solvent extraction and C1 is the copper/silver concentration in the aqueous solution after solvent extraction.
3. Results and discussion 3.1. Characterizations of the head sample Fig. 5 illustrates the XRD pattern of the slimes sample. As it is clear, barite (BaSO4) is the main phase of the copper anode slimes. The majority of copper is present as Cu2Se in the pattern. Silver mainly occurs as Ag2Se, Ag2SeO4, and Ag2O that is confirmed by the fact that copper and silver are commonly associated with selenium in the copper anode slimes. Also, the Ag-Cu selenides and their oxides are the main phases of the copper anode slimes (Chen et al., 2015; Chen and Dutrizac, 1989; Cheng and Hiskey, 1996). Lead and Arsenic appear in complex oxide phases of Pb2(As2O4) and AgPb4(AsO4) (Chen and Dutrizac, 1990). Table 2 shows the XRF analysis of the copper anode slimes head sample. The slimes contain 22.14% Ba, mainly as BaSO4 (according to the XRD pattern (Fig. 5)). Furthermore, 15.21% Se, 7.38% Ag, and 7.34% Cu were the main elements of the slimes. The ICP analysis was conducted to determine the concentration of the elements in the copper anode slimes sample. Table 3 shows that selenium, copper, and silver are the major elements of the copper anode slimes.
Fig. 4. The roasted slimes leaching setup.
necked and round-bottomed thermostatic Pyrex reactor fitted with a reflux condenser placed on a hot plate magnetic stirrer with a temperature control system (Fig. 4) by L/S ratio of 10 and temperature of 90 °C for 1 h. After leaching, the solution was filtered and the separated sulfate leachate was analyzed by ICP to calculate silver and copper recovery.
C Leaching recovery (%) = 1 × 100 C0
Table 2 The XRF analysis of the copper anode slimes head sample.
(3)
where C0 is the copper/silver concentration in the roasted copper anode slimes and C1 is the copper/silver concentration in the leachate. 2.7. Solvent extraction Solvent extraction was performed to extract copper from the leachate at the optimum roasting condition containing 4.46 g/L silver and 4.36 g/L copper. The organic phase consisted of 10% (v/v) Chemorex CP-150 diluted in kerosene. The extraction experiments were conducted at the pH range of 0–4 by using organic to aqueous (O/A) ratio of 1:1 and contact time of 10 min at room temperature (25 ± 2 °C). Each experiment was repeated three times. The average copper extraction 306
Formula
Concentration (%)
Calculated elemental concentration (%)
BaO SeO2 SO3 CuO Ag2O PbO Sb2O3 SiO2 As2O3 TeO2 CaO SrO Cl Al2O3
24.72 21.37 17.57 9.19 7.93 5.34 3.95 2.12 1.54 0.98 0.89 0.86 0.60 0.39
22.14 15.21 7.04 7.34 7.38 4.96 3.29 0.99 1.17 0.78 0.64 0.85 0.60 0.21
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tower as follows (Cooper, 1990; Hait et al., 2009; Hoffmann, 1989):
Table 3 The ICP results of the copper anode slimes. Element
Concentration (%)
Se Ag Cu Pb As Sb Au Bi Zn Ni
13.66 7.07 7.00 4.42 0.42 0.10 0.08 0.08 0.06 0.03
SeO2 (g) + H2 O(l) → H2 SeO3 (l) H2 SeO3 (l) + 2SO2 (g) + H2 O(l) → Se (s) + 2H2 SO4 (l)
The roasting stage was performed with the aim of producing selenium in conjunction with converting water-insoluble compositions of copper and silver, to water-soluble sulfate compositions. During the roasting stage, the SeO2 gas was produced in addition to copper and silver sulfate formation according to the suggested following reactions:
Cu2 Se(s) + 3H2 SO4 (l) + 1.5O2 (g) → 2CuSO4 (s) + SeO2 (g) + SO2 (g)
Ag 2SeO4 (s) + SO2 (g) → Ag2 SO4 (s) + SeO2 (g)
(6) (7)
Copper existed as Cu2Se in the copper anode slimes is likely to be transformed to CuSO4, as shown in Eq. (5). According to the copper anode slimes XRD pattern, silver exists as the three phases of, Ag2Se, Ag2SeO4 and Ag2O. Ag2Se and Ag2SeO4 phases react with sulfuric acid and SO2, respectively, and produce SeO2 and Ag2SO4. The suggested reaction for Ag2O and sulfuric acid to form Ag2SO4 is:
Ag 2O (s) + H2 SO4 (l) → Ag 2 SO4 (s) + H2 O (g)
selenium recovery = +59.13 + 31.93 A+ 12.06B
(11)
copper recovery = +78.77 + 3.61 A+ 15.98B
(12)
silver recovery = +79.21 + 2.81 A+ 16.27B
(13)
where A and B are the coded values of temperature and L/S ratio. The models illustrate that both temperature and L/S ratio have positive effects on the selenium, copper, and silver recoveries. But in the case of selenium, temperature is more effective on the recovery than L/S ratio due to the fact that although L/S ratio can improve SO2 and SeO2 generation, SeO2 vapor pressure will be enhanced at higher temperatures (Behrens et al., 1974; Brebrick, 2000) that is the main factor of selenium recovery according to Eqs. (7) and (8) (Cooper, 1990; Hait et al., 2009; Hoffmann, 1989). While in copper and silver recovery, L/S ratio is more effective in comparison to temperature. The L/S ratio directly intensifies the reactions between Cu2Se, Ag2Se, Ag2O and sulfuric acid (Eqs. (5), (6) and (8)). Therefore the amount of copper and silver sulfate formation will be increased. By increasing the L/S ratio, the amount of SO2 will grow that enhances the Ag2SeO4 and SO2 reaction. Therefore, the silver sulfation reaction intensifies. Hence, due to the above-mentioned reasons, L/S ratio enhances the copper and silver
(5)
Ag 2Se (s) + 2H2 SO4 (l) + O2 (g) → Ag 2SO4 (s) + SeO2 (g) + SO2 (g) + 2H2 O(g)
(10)
First, selenious acid is generated by the reaction of SeO2 and water. In the next stage, selenious acid is reduced to elemental selenium by the SO2 gas produced in the process, and the sulfuric acid that was consumed during the process is regenerated (Borra et al., 2016; Hait et al., 2009). Design of experiments was used to develop an empirical model to obtain optimum conditions for the roasting and leaching process with the goal of maximizing the recovery of selenium, copper, and silver in the range of temperature and L/S ratio investigated. The optimization procedure was based on RSM (see section 2.4). According to the results of the experiments as a function of temperature and L/S ratio, three models are proposed for the recovery of selenium, copper, and silver:
3.2. Statistical analysis and process optimization
+ 3H2 O(g)
(9)
(8)
The produced SeO2 gas is decomposed to selenium in the cooling
Fig. 6. Effect of temperature and L/S ratio on the recovery of selenium (a), copper (b), and silver (c). 307
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recovery. Fig. 6 illustrates the effect of temperature and L/S ratio on the recovery of selenium, copper, and silver. Fig. 6 (a) shows the diagram of selenium recovery. The selenium recovery is clearly enhanced both at a higher temperature and L/S ratio. Temperature has a greater effect than L/S ratio on the selenium recovery. In Fig. 6 (b) and (c) the greater effect of L/S ratio is apparent. Also, the interaction between temperature and L/S ratio on the selenium, copper, and silver recovery is insignificant. The correlation coefficients for the models were computed to be 0.91, 0.91, and 0.93 for the selenium, copper, and silver recoveries, respectively, which give a good agreement between the experimental and the predicted values of the fitted model. The selected predicted optimum condition of the roasting stage followed by the leaching to recover copper and silver was temperature of 247.52 °C and L/S ratio of 1.95 mL/g according to the proposed models (Eqs. (11)–(13)). At this condition, 99.93% of selenium, 96.53% of copper and 96.48% of silver were recovered. The confirmation experiments that were performed at 247 °C and L/S ratio of 1.95 mL/g) repeated three times and results confirmed the above-mentioned condition.
Fig. 8. The XRD pattern of the roasted slimes at the optimum condition (temperature ca. 247 °C of and L/S ratio of 1.95).
Ag2SO4 and CuSO4 peaks approve the sulfation of the slimes. Barium sulfate (BaSO4) is the main phase of the roasted slimes. The XRD pattern also shows that PbSO4 was produced during the roasting.
3.3. Thermodynamic analysis 3.5. Leaching at the optimum condition Fig. 7 shows the diagram of Gibbs free energy (ΔG0) as a function of temperature provided using HSC Chemistry software 5.11 (Outokumpu, Finland) under standard state condition for the main reactions (Eqs. (5)–(10)) occurring during sulfation-roasting with sulfuric acid. The change in the slopes of the reaction lines (except Eq. (7)) at 100 °C is as a result of the change in the state of H2O from liquid to gas. The minus Gibbs free energy for the reactions (5) to (8) confirms that the reactions are spontaneous. The results illustrate that the thermodynamically driven force for the Cu2Se reaction with sulfuric acid (Eq. (5)) is the highest among all reactions shown due to the smaller free energy. The reactions (9) and (10) occur at lower temperatures (below 100 °C) in the cooling tower and result in selenium production. As it can be seen from Fig. 7, at the temperatures below 100 °C, both of the reactions are spontaneous due to the minus Gibbs free energy.
Fig. 9 illustrates the XRD pattern of the leach filter cake produced at the optimum condition (temperature of ca. 247 °C and L/S ratio of 1.95). The main phases of the leach filter cake were low soluble BaSO4 and PbSO4. The leachate was also analyzed by ICP (according to Eq. (3)) and the result shows recovery of 96.53% and 96.48% for copper and silver, respectively. 3.6. Solvent extraction Solvent extraction was performed to separate copper and silver from the leachate obtained at the optimum roasting condition (temperature of ca. 247 °C and L/S ratio of 1.95) containing 4.46 g/L silver and 4.36 g/L copper. Fig. 10 shows the copper extraction as a function of equilibrium pH. Increasing pH enhances the copper extraction. The maximum copper extraction was 99.87% (according to Eq. (4)) at pH = 4 (10% (v/v) CP-150 in kerosene, O/A of 1, and 10 min contact time). The silver extraction was also observed in all the extraction experiments and the results showed that silver was not extracted in the investigated extraction conditions.
3.4. Roasting at the optimum condition At the optimum condition (temperature of ca. 247 °C and L/S ratio of 1.95), the selenium recovery of 99.93% was obtained. Fig. 8 shows the XRD pattern of the roasted slimes at the optimum condition. The
Fig. 7. Diagram of Gibbs free energy (ΔG0) in function of temperature for the main reactions occurring during sulfation-roasting with sulfuric acid. 308
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Fig. 9. The XRD pattern of the leach filter cake at the optimum condition (temperature of ca. 247 °C and L/S ratio of 1.95).
Fig. 10. Copper extraction as a function of equilibrium pH.
4. Conclusions In this study, a modified sulfation-roasting-leaching process was performed at a lower temperature than the traditional sulfation roasting technique to recover selenium, copper, and silver from the copper anode slimes. This process consists of two main stages of sulfationroasting and water leaching following by filtration and solvent extraction that even shortens the typical sulfation-roasting-leaching process. Selenium was obtained in the sulfation-roasting stage and copper and silver were recovered from the water leaching of the roasted slimes. At the optimum condition of temperature of ca. 247 °C and L/S ratio of 1.95, the recovery of selenium, copper, and silver were 99.93%, 96.53%, and 96.48%, respectively. The results showed no significant interactions between temperature and L/S ratio on the selenium, copper, and silver recovery. Solvent extraction of the leachate (10% (V/ V) CP-150 in kerosene at the pH of 4, organic to aqueous ratio (O/A) of 1, and 10 min contact time) at the optimum condition resulted in 99.87% copper extraction. The silver extraction was also examined in all the extraction experiments and the results showed that silver was not extracted in the investigated extraction conditions. References Amer, A.M., 2002. Processing of copper anode-slimes for extraction of metal values.
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