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The leaching kinetics of cadmium from hazardous Cu-Cd zinc plant residues
Waste Management xxx (2017) xxx–xxx
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The leaching kinetics of cadmium from hazardous Cu-Cd zinc plant residues Meng Li a,b,c, Shili Zheng b, Biao Liu b, Hao Du b,d,⇑, David Bruce Dreisinger c,⇑, Leili Tafaghodi c, Yi Zhang b a
School of Metallurgy, Northeastern University, Shenyang 110819, China National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China c Department of Materials Engineering, University of British Columbia, Vancouver V6T1Z4, Canada d University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e
i n f o
Article history: Received 16 November 2016 Revised 17 March 2017 Accepted 24 March 2017 Available online xxxx Keywords: Cu-Cd zinc plant residues (CZPR) Hazardous wastes Sulfuric acid leaching Kinetics Diffusion control
a b s t r a c t A large amount of Cu-Cd zinc plant residues (CZPR) are produced from the hydrometallurgical zinc plant operations. Since these residues contain substantial amount of heavy metals including Cd, Zn and Cu, therefore, they are considered as hazardous wastes. In order to realize decontamination treatment and efficient extraction of the valuable metals from the CZPR, a comprehensive recovery process using sulfuric acid as the leaching reagent and air as the oxidizing reagent has been proposed. The effect of temperature, sulfuric acid concentration, particle size, solid/liquid ratio and stirring speed on the cadmium extraction efficiency was investigated. The leaching kinetics of cadmium was also studied. It was concluded that the cadmium leaching process was controlled by the solid film diffusion process. Moreover, the order of the reaction rate constant versus H2SO4 concentration, particle size, solid/liquid ratio and stirring speed was calculated. The XRD and SEM-EDS analysis results showed that the main phases of the secondary sulfuric acid leaching residues were lead sulfate and calcium sulfate. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Cadmium is an important transition metal that has been widely utilized in pigments, ceramics, chemicals, electroplating, electronic industries and photographic products (Cheremisinoff, 1994; Habashi, 1997). On the other hand, cadmium is a toxic metal that can cause chronic poisoning (Min et al., 2013; Safarzadeh et al., 2007). For instance, the ‘‘Cadmium-contaminated Rice” event happened in Hunan Province in China in recent years was attributed to release of cadmium from industrial wastes of the nonferrous metal production. In this regard, safe handling of industrial discharge containing Cd is critical. In nature, cadmium is a relatively rare element in the earth’s crust and frequently exists as a minor component associated with nonferrous metal ores containing zinc. Therefore, a large amount of cadmium can potentially enter the environment in zinc produc⇑ Corresponding authors at: National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China (H. Du), Department of Materials Engineering, University of British Columbia, Vancouver V6T1Z4, Canada (D.B. Dreisinger). E-mail addresses: [email protected] (H. Du), [email protected] (D.B. Dreisinger).
tion. Analysis of cadmium pollution sources shows that lead and zinc smelting industries are currently the major causes. Zinc is a widely used metal and applied in the field of cosmetics, electronics, construction, automobile corrosion protection and medicines (Li et al., 2016; Xin et al., 2013). The application and demand of zinc have increased significantly recently. The global production of refined zinc metal in 2015 was up to 13.99 million tonnes (2015; Sethurajan et al., 2016). The traditional hydrometallurgical technique featuring oxidative roasting, acid leaching, purification and electrowinning accounts for 80–85% of the world’s zinc production and the balance is recovered by pyrometallurgical processes. A general flow sheet of the zinc hydrometallurgical smelting process is shown in Fig. 1. As can be seen from Fig. 1, there are three steps of the zinc hydrometallurgical process: the leaching of zinc calcines, the purification of the leaching solutions and the electrowinning of ZnSO4 solutions. Further, zinc calcine leaching process includes neutral leaching and hot acid leaching. The aim of the former is to leach most of the ZnO and separate zinc with other impurities, while that of the latter is to dissolve most of the remaining zinc in the zinc calcine (especially ZnFe2O4). This figure also shows the origin of Cu-Cd zinc plant residues (CZPR) generated from the purification of neutral leaching solutions. The CZPR
http://dx.doi.org/10.1016/j.wasman.2017.03.039 0956-053X/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Li, M., et al. The leaching kinetics of cadmium from hazardous Cu-Cd zinc plant residues. Waste Management (2017), http://dx.doi.org/10.1016/j.wasman.2017.03.039
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Zinc calcines
Sulfuric acid
Neutral leaching pH=5.2~5.4
Hot acid leaching
Thickener
Thickener
Neutral supernatant Cd-Cu Cd-Cu residues residues
Waste electrolyte
Neutral leaching residues
Cd &Cu purification Zinc powder and filtration Further purification and electrodeposition
Hot acid leaching solution
Pb-Ag residues
Fe purification and filtration Zinc solution
Fe residues
Zinc metal Fig. 1. Schematic diagram of the hydrometallurgical zinc production.
contains considerable quantity of valuable metals, such as cadmium, zinc and copper, but it is classified as hazardous waste because of containing high concentrations of heavy metals. Due to the different composition of zinc concentrates, some variation in CZPR composition is found. Cadmium pollution caused in the treatment process of CZPR accounts for 70% of the total amount of cadmium pollution (Yang et al., 2015). For every 100 tonnes of electrolytic zinc product, 1.3 tonnes of CZPR is generated (Tan et al., 2008). Therefore, a process for the decontamination treatment for this residue is of great importance for both economic and environmental point of view. In the past decades, the traditional method to recycle cadmium, zinc and copper from CZPR is the hydrometallurgical process. The hydrometallurgical process options are divided into acid method and ammonia method, both of which have their inherent problems. The zinc-ammonia solution produced by the ammonia leaching method cannot be integrated to the existing zinc recovery process; therefore, the ammonia leaching of CZPR has not been widely used (Liu et al., 2010). At present, some large zinc smelter plants in China only partially process CZPR. Cadmium and zinc are recovered first by acid leaching; the leached residues containing copper are sold. However, the copper-containing residues have about 3 wt.% cadmium and 20 wt.% zinc, which will spread the problem of cadmium contamination to the copper recovery process (Shao et al., 2003). Some researchers use low cost manganese dioxide as oxidant in the process for sulfuric acid leaching, and have reported that copper, zinc, cadmium in CZPR will be leached simultaneously (Tan et al., 2008). In addition, to further improve the leaching efficiency of CZPR, researchers have investigated many effective methods such as pressure acid leaching (Ke, 1996) and bioleaching (Cheng et al., 2013) technologies. Pressure acid leaching method operates under high temperature and pressure, which demand high capital and operating cost. Thus, it is not suitable for industrialization. The bioleaching method, on the other hand, is hard to interface with the existing hydrometallurgical zinc processing system. Overall, the existing CZPR treatment technologies have common problems of long process flow, complex operations, low leaching and recovery rate of cadmium, and consequently, high amount of cadmium is released to the environment. Therefore, it is necessary to develop an efficient process for the treatment of CZPR and to investigate the leaching behavior about cadmium, zinc and copper.
In this study, the extraction of cadmium, zinc and copper from a secondary resource, CZPR, by oxidative sulfuric acid leaching was investigated. The preferred process conditions for the maximum recovery of cadmium, zinc and copper and the leaching kinetics of cadmium dissolution were carefully studied. The effects of temperature, sulfuric acid concentration, solid to liquid phase ratio, initial particle size, stirring speed and aeration rate on the leaching efficiency of CZPR were investigated. In addition, the reaction kinetics was fit to different kinetic models and the apparent activation energy. The reaction order was also estimated. 2. Materials and methods 2.1. Cu-Cd zinc plant residues The CZPR used in this research was obtained from Shuikoushan Nonferrous Metals Group Co., Ltd. in Hunan Province, China. After natural drying, the residues were crushed, ground and screened, giving particle size fractions of 830 +425, 425 +250, 250 +150 and 150 +75 mm. The sulfuric acid used in the experiments was of analytical grade and was obtained from Xilong Chemical Group Co., Ltd. Water was purified with Milli-Q (Millipore Corp.) equipment before use. The sample was analyzed by inductively coupled plasma optical emission spectrometry (ICPOES, PerkinElmer Optima 7300V, USA), X-ray diffraction analysis (XRD, PANalytical Empyrean, Netherlands), equipped with a Cu Ka radiation and scanning electron microscopy (SEM, FEI Quanta 250, USA) equipped with energy disperse X-ray spectrometry (EDS, EDAX Genesis, USA). 2.2. Experimental procedure The laboratory-scale experiments were conducted in a 1L fourneck flask in a thermostatically controlled water bath (DF-101S, Bejing Xingde Instrument & Equipment Co., Ltd), equipped with a reflux condenser, mechanical stirring. And the stirrer was protected by Teflon. A volume of 500 mL of solution containing diluted sulfuric acid was added to the flask first. CZPR was added to the four-neck flask when the temperature reached the desired value and the stirrer started. A 1 mL sample of solution was withdrawn at the desired time interval. After the slurry was treated by
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centrifugation, the leachate was analyzed by ICP-OES. All chemical reagents used in this research were of analytical grade. The metal leaching extractions were calculated using Eq. (1):
X¼
V L CL MS W S
ð1Þ
where MS is the mass of CZPR applied in the experiment, g; WS is chemical analysis of the target metal, wt.%; VL is the volume of leaching leachate, mL; CL is the concentration of metals in the leachate, g/mL. Each experiment was repeated three times, and the mean extraction reported to avoid any errors.
Kinetic analysis is of great significance at the industrial level as it is the basis for scale-up and reactor design (Dhawan et al., 2011). The leaching process are generally considered to be controlled by the following steps: (1) by diffusion through the fluid film; (2) by diffusion through the product layer; (3) by the chemical reaction (Safari et al., 2009; Velardo et al., 2002). The step with the slowest speed is the controlling step. The experimental results were analyzed by the shrinking core model in order to find the controlling step. If the leaching process is controlled by diffusion through the solid product layer (Zhang et al., 2016): n
2 b Deff : ½H2 SO4 2 1 X ð1 XÞ2=3 ¼ t ¼ kd t 3 qA r20
ð2Þ
If the leaching process is controlled by the surface chemical reaction, Eq. (2) is replaced by the following equation (Zhang et al., 2016):
2,000
Inteasity (Counts)
b K S ½H2 SO4 n t ¼ kr t qA r0
3 5
1,800
1 4 5
2 5 5 62 5 3 6 3 6 5
1,600 1,400 1,200
6
1,000
ð3Þ
1-Cu 2-CuO 3-ZnO 4-CdO 5-ZnSO4 7H2O
..
1 6
3
6-CuSO4 5H2O
5 6 6
1 6
Ea RT
ð4Þ
4 5
400 200 20
30
40
50
60
3.1. Characterization of the CZPR The residue with the particle size of less than 250 +150 mm was analyzed by XRD, as presented in Fig. 2. The main phases of the CZPR are Cu, CuO, CdO, ZnO, ZnSO47H2O and CuSO45H2O. The content of sulphur was analyzed by X-ray fluorescence spectrometer (XRF, PANalytical B.V. Axios, Netherlands). The other chemical compositions of the CZPR with different particle size diffraction were analyzed by ICP-OES. A CZPR sample of 0.05–0.1 g mixed with 5–10 g sodium fluxing agent containing sodium carbonate and sodium borate (the mass ratio of sodium carbonate to sodium borate is 2:1) was loaded in a 50 mL platinum crucible with a cover and then was calcined in a muffle furnace at 950 °C for 10–30 min. The platinum crucible was then transferred into a 150 mL beaker with a boiling solution (40 mL high pure water and 20 mL concentrated hydrochloric acid) to make sure the solid phase in the platinum crucible was completely dissolved. Then the solution was transferred to a 100 mL volumetric flask and diluted by high pure water if necessary for the following ICP-OES analysis. The results are shown in Table 1. The SEM-EDS analysis of the CZPR was conducted, which was shown in Fig. 3. According to the microphotograph, the particle size of CZPR is not homogeneous but in irregular shapes. Based on the EDS analysis in Fig. 3(a) and (b), the particles contain the elements of cadmium, zinc, calcium and copper.
The redox potential plays a crucial role in the process of leaching metals from residues. The solubilization of metals cannot be solely related to pH-data but must also account for redox potential. Fig. 4 is the Eh-pH diagrams of the Zn-Cd-H2O (a) and Cu-Pb-H2O (b) at the temperature of 25 °C. As can be seen from Fig. 4(a), the stability areas of Zn2+ and Cd2+ are enclosed by the water stability limits, which indicate the feasibility of metals to reacted with acid. Nevertheless, Zn only can be oxidized by the acid when the pH is lower than 6. Similarly, pH of 6.5 (point A) is required for Cd. The areas of solubility regions of Zn2+ and Cd2+ show that zinc
600
10
3. Results and discussion
3.2. Theoretical analysis of the behavior of the phases
2 44
800
0
ln k ¼ ln A
where A is the frequency factor, Ea is the apparent activation energy.
2.3. Kinetic analysis
1 ð1 XÞ1=3 ¼
where X is the fraction reacted; kd is the rate constant for diffusion through the product layer film and t is the leaching time; kr is the rate constant for the chemical reaction; b is the stoichiometric coefficient in the leaching reaction; Deff. is the effective diffusion coefficient; [H2SO4] is the sulfuric acid concentration; n is the order of reaction with respect to H2SO4; Ks is the chemical reaction rate; qA is the molar density and r0 is the initial particle radius. The apparent activation energy can be calculated by the Arrhenius Eq. (4):
70
80
90
2-Theta (degree) Fig. 2. XRD diffraction pattern of the CZPR (250 +150 mm).
Table 1 The chemical compositions of the CZPR (wt.%). Particle size/lm
ZnO
CuO
CdO
SO3
PbO
Al2O3
CaO
SiO2
Fe2O3
+830 830 +425 425 +250 250 +150 150 +75 75
46.83 47.01 47.31 47.42 46.97 47.12
20.1 19.86 19.45 19.95 20.05 19.68
9.32 9.44 9.81 9.48 9.72 9.56
14.71 14.91 14.66 14.78 14.87 14.77
1.92 1.88 1.78 1.81 2.01 1.98
1.43 1.39 1.32 1.37 1.28 1.45
4.38 4.39 4.44 4.36 4.33 4.32
0.43 0.37 0.39 0.41 0.43 0.44
0.88 0.75 0.84 0.42 0.34 0.68
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Fig. 3. The SEM-EDS analysis of the CZPR. (a), (b) SEM of the CZPR, (c) EDS analysis results of position 1 and (d) EDS analysis result of position 2.
2+ 2+ 2+ Fig. 4. The Eh-pH diagrams of the Zn-Cd-H2O (a) and Cu-Pb-H2O (b) at the temperature of 25 °C. (a2+ Cd(aq) = 1, aZn(aq) = 1, aCu(aq) = 1, aPb(aq) = 1, drawn by HSC 6.0 chemistry software, Outokumpu Research, Finland).
Fig. 5. The plots of (a) DrGo T and (b) DrHo T for Eq. (5) (Drawn by HSC 6.0 chemistry software, Outokumpu Research, Finland).
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and cadmium have a greater solubility in the acid solutions, respectively. As can be seen from Fig. 4(b), the stability areas of Cu2+ and Pb2+ are also enclosed by the water stability limits. But higher acidities are needed for the oxidation of copper and lead. As can be seen from Fig. 4(b), Pb only can be oxidized by acid (H+) when the pH is lower than 2 (point B), and Cu cannot be dissolved. In this research, the highest sulfuric acid concentration tested is 25 wt.%, and according to the above analysis Cu cannot be directly dissolved. However, as shown in Fig. 5, Cu can be oxidized in the presence of oxygen and the dissolution reaction is a exothermic
reaction based on the DrHo is negative (Li et al., 2016.). Furthermore, according to the corresponding Eh-pH diagrams (not shown in this article) for the sulfate system, the solubility of metal ions are increased by the presence of SO2 (Safarzadeh et al., 2009). 4 Eq. (5) is the oxidation reaction of copper.
2Cu þ O2 þ 2H2 SO4 ¼ 2CuSO4 þ 2H2 O
ð5Þ 8
The Ksp value of lead(II) sulfate is 1.9 10 , at 25 °C, which indicates the solubility of lead(II) sulfate is very low. As can be seen from Fig. 6(a), Pb(II) ion solubility drops 100,000-fold when the
)
Fig. 6. The plot of concentration of Pb(II) as a function of sulfate radical concentration (a) and the fraction of Pb(II) and PbSO4 as a fraction of sulfate radical concentration (Drawn by Medusa software, Royal Institute of Technology, Sweden).
) Fig. 7. (a) Effect of temperature on cadmium leaching rate from CZPR; (b) 1 (1 X)1/3 versus time at different temperature; (c) 1 2/3X (1 X)2/3 versus time at different temperature and (d) ln kd versus reciprocal temperature.
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concentration of SO2 4 reaches 1 M. Similarly, Fig. 6(b) shows that all Pb(II) transforms to PbSO4 as SO2 4 reaches 1 M. According to this analysis, Pb(II) can be easily separated in the form of PbSO4. 3.3. Cadmium leaching kinetics 3.3.1. Effect of temperature The effect of reaction temperature ranging from 30 to 60 °C was studied, while other parameters were fixed with dilute sulfuric acid concentration of 1.5 M, particle size of 250 +150 mm, solid/liquid ratio of 1:6, stirring speed of 500 rpm and air flow rate of 0. The relationship between cadmium leaching ratio and leaching time at different temperatures is shown in Fig. 7(a), which shows that temperature does not affect the cadmium leaching ratio significantly. Furthermore, the higher temperatures improve the dissolution rate and reactant diffusion slowly. The cadmium leaching extent after 15 min reached 85.4%, 88.3%, 91.3% and 93.6% at the reaction temperature of 30, 40, 50 and 60 °C, respectively. However, the Cd leaching ratio slowly increased after rapidly decomposition in 15 min. Other researchers have found the different conclusion that temperature has a remarkable effect on the cadmium leaching efficiency (Kul and Topkaya, 2008; Safarzadeh et al., 2009; Shahsavari, 2001). Nonetheless, our research showed that the reaction temperature effect decreased by increase of reaction time. Fig. 7(b) and (c) shows the cadmium leaching efficiency of the CZPR at different leaching temperature according to the surface chemical reaction and solid film diffusion control, respectively. The results from Fig. 7(b) and (c) indicate that cadmium leaching
reaction is controlled by solid film diffusion control. The rate constants (kd) are the slopes in Fig. 7(b) and (c). In addition, the Arrhenius plot (ln kd vs 1000/T) was obtained as shown in Fig. 7(d) The calculated activation energy is 10.95 kJ/mol. As the literature suggested, the activation energy of a diffusion controlled process is typically below 12 kJ/mol, while the value for a chemical reaction controlled process in the leaching reaction is usually between 30 and 85 kJ/mol (Momade and Momade, 1999). For the present analysis, it clearly confirms that the leaching process is controlled by diffusion of ions through the product layer. 3.3.2. Effect of dilute sulfuric acid concentration The theoretical consumption of dilute sulfuric acid concentration for the total leaching of CZPR was approximately 1.5 M. Therefore, the effect of dilute acid concentration was studied in the range of 0.8–1.6 M, with other conditions unchanged, including leaching temperature of 30 °C, particle size of 250 +150 mm, solid-to-liquid ratio of 1:6, stirring speed of 500 rpm and air flow rate of 0. As seen in Fig. 8(a), the acid concentration affects the Cd leaching ratio significantly. When the sulfuric acid concentration was 0.8 and 1.1 M, the amount of acid was deficient to completely leach the insoluble cadmium. However, the leaching ratio was not increasing significantly as the sulfuric acid concentration exceeded 1.5 M in 40 min leaching. The results showed that the cadmium leaching extent remained constant after 20 min. The reason is that the leaching product rate increases as the sulfuric acid concentration increases. Furthermore, when the leaching product arrives at the equilibrium value around the CZPR particle, an insoluble product solid film around the unreacted solid particle is
Fig. 8. (a) Effect of dilute sulfuric acid concentration on cadmium leaching ratio from the CZPR; (b) 1 (2/3)X (1 X)2/3 versus time at different dilute sulfuric acid concentration and (c) Variation of ln kd versus ln[H2SO4].
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Fig. 9. (a) Effect of particle size on cadmium leaching efficiency; (b) Plot of 1 (2/3)X (1 X)2/3 versus time at different particle size (c) Plot of kd against 105/d20 and (d) Plot of ln kd against lnd0.
Fig. 10. (a) Effect S/L ratio on cadmium leaching ratio from the CZPR; (b) Plot of 1 (2/3)X (1 X)2/3 versus time at different S/L ratio and (c) Plot of ln kd versus ln(S/L).
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formed. So the leaching process becomes slow (Özmetin et al., 1996). The cadmium leaching extent after 30 min leaching at the sulfuric acid concentration of 0.8 and 1.5 M were 58.1% and 93.6%, respectively. As can be seen from Fig. 8(a), the Cd leaching extent increased with leaching reaction time and with the initial leaching of cadmium very quickly. Approximately 85.4% of cadmium in the residues was extracted at the sulfuric acid concentration of 1.5 M after 15 min and increased to 93.6% after 30 min. The leaching reaction time of 30 min was the optimum leaching time to produce the maximum cadmium extraction. The cadmium leaching ratio data of the CZPR at different sulfuric acid were plotted by the solid film diffusion control as shown in Fig. 8(b). The results from Fig. 8(b) indicate that the data fit the model well. The plot of ln kd versus ln[H2SO4] was presented in Fig. 8(c). The order of reaction with respect to sulfuric acid for the CZPR was 1.74 with a correlation coefficient of 0.99. 3.3.3. Effect of particle size The effect of particle size was examined, while other parameters were fixed with leaching temperature of 30 °C, dilute sulfuric acid concentration of 1.5 M, solid-to-liquid ratio of 1:6, stirring speed of 500 rpm and air flow rate of 0. The relationship of cadmium leaching ratio and leaching time at different particle size was plotted in Fig. 9(a), which shows that the particle size affects the cadmium leaching remarkably. The cadmium leaching extent of 71.6% was obtained using sample size of 830 +425 mm in 15 min, and it improved to 94% using sample size of 150 +75 mm. After 30 min leaching time, 84.6% of cadmium was extracted using sample size of 830 +425 mm.
In this research, the cadmium leaching extent increased a little when the sample size was decreased from 250 +150 to 150 +75 mm, and the leaching ratio keep the level of 94% in 30 min. So 250 +150 mm was the optimum sample size. Fig. 9(b) shows the cadmium leaching ratio data of the CZPR at different particle size analyzed by the model of the diffusion control model. The results from Fig. 9(b) indicate that the data fit the model well. The apparent rate constants (kd) were calculated and plotted in Fig. 9(c) and (d) versus 105/d20 (d0 is the average initial diameter). The direct relationship of kd versus 105/d20 also favors the leaching of Cd, which was controlled by solid film diffusion. The plot of ln kd versus lnd0 is presented in Fig. 9(d). The order of reaction versus particle size for the CZPR was 0.54 with a correlation coefficient of 0.97. 3.3.4. Effect of solid/liquid ratio With other leaching parameters were held at leaching temperature of 30 °C, dilute sulfuric acid of 1.5 M, particle size of 250 +150 mm, stirring speed of 500 rpm and air flow rate of 0, the influence of solid/liquid ratio on Cd leaching ratio was shown in Fig. 10 (a). As seen in Fig. 10(a), the solid-liquid ratio affects the cadmium leaching ratio significantly. The cadmium leaching extent increased from 64.5% at the solid/liquid ratio of 1/3 to 90.2% at the solid/liquid ratio of 1/8 in the leaching time of 15 min. When the solid-liquid ratio was lower than 1/4, cadmium leaching extent increased slowly. Therefore, this value was considered to be the optimum for the extraction of cadmium. However, in other cadmium leaching researches using sulfuric acid for the leaching agent, the optimal solid-liquid ratio values were 5% w/w, 6% (w/v), 1/4 (g/cc) and 1/8 (g/mL), respectively (Gouvea and Morais,
Fig. 11. (a) Effect of stirring speed on Cd leaching ratio from the CZPR; (b) Plot of 1 (2/3)X (1 X)2/3 versus time at stirring speed and (c) Determination of reaction order with respect to stirring speed.
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2007; Kul and Topkaya, 2008; Safarzadeh et al., 2009; Shahsavari, 2001; Shahsavari and Abdollahi, 2005). These may be attributed to the application of the leaching process to different residues. Fig. 10(b) shows the cadmium leaching ratio data of the CZPR at different solid-liquid ratios which were analyzed by the diffusion control model. The results from Fig. 10(b) indicate that the data fit the model well. The plot of ln kd versus ln(S/L) is presented in Fig. 10(c). The order of reaction rate constant versus solid-liquid ratio for the CZPR was be 1.04 with a correlation coefficient of 0.994. 3.3.5. Effect of stirring speed The effect of stirring speed was conducted and other parameters were fixed with leaching temperature of 30 °C, dilute sulfuric acid concentration of 1.5 M, particle size of 250 +150 mm, S/L ratio of 1:6 and air flow rate of 0. The relationship of cadmium leaching extent and reaction time at different particle size was plotted in Fig. 11(a), which indicates that the stirring speed affects the cadmium leaching remarkably. However, the cadmium leaching ratio increased until the stirring speed of 500 rpm in 30 min, where the cadmium leaching extent was 93.6%. Therefore, the optimum stirring speed for the dissolution of cadmium was considered to be 500 rpm. Fig. 11(b) shows the cadmium leaching ratio data of the CZPR at different stirring speed which were analyzed according to the diffusion control model. The results from Fig. 11(b) indicate that the data fit the model well. The plot of ln kd versus ln(Stirring Speed) was presented in Fig. 11(c). The order of reaction rate constant versus stirring speed for the CZPR was found to be 0.34 with a correlation coefficient of 0.99.
9
The cadmium leaching kinetics showed that the higher sulfuric acid concentration, smaller particle size, higher L/S, higher stirring speed and higher leaching temperature contributed to higher cadmium leaching ratio. In addition, the value of apparent activation energy showed that ash diffusion was the controlling process (Aydogan et al., 2006; Habashi, 1999) and this value was in good agreement with the values presented in other researches (Aydogan et al., 2006; Mahlangu et al., 2007; Safarzadeh et al., 2009). 3.4. The behaviors of Zn, Cd, Cu and Pb In this research, the average residue was approximately 4% of the original feed by mass. The leaching behaviors of zinc, cadmium, copper and lead were presented in Fig. 12(a) at proposed conditions. From Fig. 12(a), it can be concluded that zinc can be leached easily and the leaching efficiency can reach 93.5% in 15 min and 99.4% in 40 min. It also can be seen that the leaching efficiency of cadmium was 93.6% in 40 min. However, lead was not leached and was easily separated, which was in concordance with theoretically foreseen. The experiment results of Fig. 12(b) show that the copper leaching ratio can be enhanced effectively by using air as oxidant. These results are consistent with theory. As seen in Fig. 12(b), the copper leaching ratio obtains without aeration is 58.4% in 15 min, while it is 83.6% when the air flow rate of 100 mL/min was applied and this increased to 93% after 40 min. Nevertheless, as
3.3.6. Kinetics model The results of activation energy, sulfuric acid concentration, solid-liquid ratio and particle size of this research showed that the ash layer diffusion was the controlling process. Therefore, the leaching cadmium from the CZPR can be clearly presented by the following equation based on this study:
1:04 2 0:54 S 1 X ð1 XÞ2=3 ¼ kd t ¼ k0 ½H2 SO4 1:74 d0 ðSSÞ0:34 3 L 10:95 t ð6Þ exp RT where X is the fraction reacted; k0 is the correlation coefficient; [H2SO4] is the concentration of dilute sulfuric acid; d0 is the diameter particle size; S/L is the solid/liquid ratio and SS is the stirring speed.
Fig. 13. The XRD analysis of the leaching residue.
Fig. 12. (a) Extraction of zinc, cadmium, copper and lead under the optimum leaching conditions. (Leaching temperature of 30 °C, dilute sulfuric acid concentration of 1.5 M, particle size of 250 +150 mm, solid/liquid ratio of 1:6, stirring speed of 500 rpm and air flow rate of 0.) and (b) The copper leaching ratio at different air flow rate (Leaching temperature of 30 °C, dilute sulfuric acid concentration of 1.5 M, particle size of 250 +150 mm, solid/liquid ratio of 1:6 and stirring speed of 500 rpm.)
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M. Li et al. / Waste Management xxx (2017) xxx–xxx
Fig. 14. The SEM of the leaching residue.
Table 2 The EDS analysis of the leaching residue. No.
1 2
Element (wt.%) O
S
Ca
Pb
Cu
50.99 58.34
18.54 13.87
25.20 22.40
4.81 5.39
0.45 –
the air flow rate increased to 150 mL/min, copper leaching ratio was not significantly increased. Therefore, the required air flow rate for the dissolution of copper was considered to be 100 mL/ min. Moreover, the leaching residue was analyzed by XRD (Fig. 13). And the leaching condition was controlled to be leaching temperature of 30 °C, dilute sulfuric acid concentration of 1.5 M, particle size of 250 +150 mm, solid-to-liquid ratio of 1:6, stirring speed of 500 rpm, air flow rate of 100 mL/min and leaching time of 40 min. It is indicated that PbSO4 and CaSO4 remained in the leaching residue. No cadmium, zinc and copper containing materials were detected in the leaching residue, due to the contents of cadmium, zinc and copper were very low. Therefore, it indicated that nearly all the target elements were leached by sulfuric acid. SEM and EDS analysis as shown in Fig. 14 and Table 2 supported the results. Only insoluble Ca and Pb containing phases were left in the sulfuric acid leaching residue.
4. Conclusions and perspectives The present research showed the Cu-Cd zinc plant residues can be used as a secondary resource for valuable metals of Cd, Zn and Cu extraction. The effect of leaching parameters, including leaching temperature, sulfuric acid concentration, particle size, solid/liquid ratio and stirring speed was assigned to the leaching of cadmium from the CZPR. The recommended leaching conditions were: leaching temperature of 30 °C, dilute sulfuric acid concentration of 1.5 M, particle size of 250 +150 mm, solid-to-liquid ratio of 1:6, stirring speed of 500 rpm, air flow rate of 100 mL/min and leaching time of 40 min. More than 99% Zn, 93% Cd and 93% Cu were leached. The cadmium leaching process was found to be controlled by the shrinking core model and the ash layer diffusion was the controlling process. The apparent activation energy was 10.95 kJ/mol. The order of the reaction with respect to H2SO4 concentration, particle size, solid-liquid ratio and stirring speed were calculated as 1.74, 0.54, 1.04 and 0.34, respectively. The leaching results showed that Pb could be separated from the CZPR. Nevertheless, the secondary leaching residue contains a little amount of insoluble cadmium and copper, which requires a proper separate process design. Therefore, a more in-depth study
about the leaching kinetics of the zinc and copper and the existing form of the insoluble cadmium and copper in the secondary leaching residue need to be conducted. Inasmuch as almost all valuable metals of cadmium, zinc and copper in the CZPR were transferred to the leachate, the overall process about separating copper, zinc and cadmium from the leaching solutions should be considered. One suggestion can be separate copper firstly by the additive of zinc powder. The rest of the solution can be treated by electrowinning or solvent extraction method to separate cadmium and zinc. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51404227, No. 51604254 and No. 91634111), the Science and Technology Service Network Initiative for the Chinese Academy of Sciences (Grant No. KFJSW-STS-148), and the National Basic Research Program of China (973 Program, Grant No. 2013CB632605). The financial support provided by the China Scholarship Council (CSC) for Meng Li (No. 201506080059) to the University of British Columbia (UBC) is also acknowledged. References 2015. International Lead and Zinc Study Group, Lisbon, Portugal. Aydogan, S., Ucar, G., Canbazoglu, M., 2006. Dissolution kinetics of chalcopyrite in acidic potassium dichromate solution. Hydrometallurgy 81, 45–51. Cheng, Y.-X., Wang, Q.-Q., Zhong, Z.-Y., Xu, Y.-Z., 2013. Investigation on Bio-leaching of Cd from Cu /Cd smelting slag. Environ. Chem. 32, 259–265. Cheremisinoff, N.P., 1994. Handbook of Water and Wastewater Treatment Technology. CRC Press. Dhawan, N., Safarzadeh, M.S., Birinci, M., 2011. Kinetics of hydrochloric acid leaching of smithsonite. Russian J. Non-Ferrous Metals 52, 209–216. Gouvea, L.R., Morais, C.A., 2007. Recovery of zinc and cadmium from industrial waste by leaching/cementation. Miner. Eng. 20, 956–958. Habashi, F., 1997. Handbook of Extractive Metallurgy, Wiley-VCH, Heidelberg, Germany, Volume 4. Habashi, F., 1999. Kinetics of Metallurgical Processes. Metallurgie Extractive Quebec, Quebec, Canada. Ke, J.-J., 1996. The progress of pressure leaching in hydrometallurgical process. Hydrometall. China 2, 1–6. Kul, M., Topkaya, Y., 2008. Recovery of germanium and other valuable metals from zinc plant residues. Hydrometallurgy 92, 87–94. Li, M., Zhang, Y., Wang, X.-H., Yang, J.-G., Qiao, S., Zheng, S.-L., Zhang, Y., 2016. Extraction of copper, zinc and cadmium from copper–cadmium-bearing slag by oxidative acid leaching process. Rare Metals, 1–10. http://dx.doi.org/10.1007/ s12598-016-0759-7. Liu, H.-Y., Yan, W.-B., Shi, A.-H., Gao, F., 2010. Research on leaching process of copper, zinc and cadmium from copper-cadmium slag. J. Ji Shou Univ. (Natural Sci. Ed.) 31, 97–100. Mahlangu, T., Gudyanga, F., Simbi, D., 2007. Reductive leaching of stibnite (Sb 2 S 3) flotation concentrates using metallic iron in a hydrochloric acid medium II: kinetics. Hydrometallurgy 88, 132–142. Min, X.-B., Xie, X.-D., Chai, L.-Y., Liang, Y.-J., Mi, L., Yong, K., 2013. Environmental availability and ecological risk assessment of heavy metals in zinc leaching residue. Trans. Nonferrous Metals Soc. China 23, 208–218.
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M. Li et al. / Waste Management xxx (2017) xxx–xxx Momade, F., Momade, Z.G., 1999. A study of the kinetics of reductive leaching of manganese oxide ore in aqueous methanol–sulphuric acid medium. Hydrometallurgy 54, 25–39. Özmetin, C., Kocakerim, M.M., Yapici, S., Yartasi, A., 1996. A semiempirical kinetic model for dissolution of colemanite in aqueous CH3COOH solutions. Ind. Eng. Chem. Res. 35, 2355–2359. Safari, V., Arzpeyma, G., Rashchi, F., Mostoufi, N., 2009. A shrinking particle— shrinking core model for leaching of a zinc ore containing silica. Int. J. Miner. Process. 93, 79–83. Safarzadeh, M.S., Bafghi, M., Moradkhani, D., Ilkhchi, M.O., 2007. A review on hydrometallurgical extraction and recovery of cadmium from various resources. Miner. Eng. 20, 211–220. Safarzadeh, M.S., Moradkhani, D., Ojaghi-Ilkhchi, M., 2009. Kinetics of sulfuric acid leaching of cadmium from Cd–Ni zinc plant residues. J. Hazard. Mater. 163, 880–890. Sethurajan, M., Huguenot, D., Jain, R., Lens, P.N., Horn, H.A., Figueiredo, L.H., van Hullebusch, E.D., 2016. Leaching and selective zinc recovery from acidic leachates of zinc metallurgical leach residues. J. Hazard. Mater. Shahsavari, S., 2001. Leaching of Electrolytic Zinc Filtercake to Extract Nickel and Cadmium. Mining Engineering, Tarbiat Modares, Tehran.
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Shahsavari, S., Abdollahi, M., 2005. The determination of effective parameters on acid leaching of filter cake generated during the electrolytic production of zinc and phase identification of nickel and cadmium. Earth Sci. 57, 126–137. Shao, Q., Du, X., Wang, L., Lan, X.-Z., 2003. Present status of reutilization of coppercadmium slag. Hydrometall. China 22, 66–68. Tan, X.-L., Zhu, G.-C., Zhao, Y.-N., 2008. An investigation on leaching of copper, zinc, and cadmium from copper-cadmium slag. Min. Metall. 17, 28–31. Velardo, A., Giona, M., Adrover, A., Pagnanelli, F., Toro, L., 2002. Two-layer shrinkingcore model: parameter estimation for the reaction order in leaching processes. Chem. Eng. J. 90, 231–240. Xin, W., Srinivasakannan, C., Xin-hui, D., Jin-hui, P., Da-jin, Y., 2013. Leaching kinetics of zinc residues augmented with ultrasound. Sep. Purif. Technol. 115, 66–72. Yang, J.-G., Lei, J., Peng, S.-Y., He, J., Qi, C.-P., Wang, X.-Y., Li, J.-Y., 2015. Extraction of cadmium from leaching solution of Cu-Cd slag through electrically enhanced replacement process. Trans. Nonferrous Metals Soc. China 25, 2268–2274. Zhang, C., Min, X., Zhang, J., Wang, M., Li, Y., Fei, J., 2016. Reductive clean leaching process of cadmium from hydrometallurgical zinc neutral leaching residue using sulfur dioxide. J. Clean. Prod. 113, 910–918.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
The leaching kinetics of cadmium from hazardous Cu-Cd zinc plant residues Meng Li a,b,c, Shili Zheng b, Biao Liu b, Hao Du b,d,⇑, David Bruce Dreisinger c,⇑, Leili Tafaghodi c, Yi Zhang b a
School of Metallurgy, Northeastern University, Shenyang 110819, China National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China c Department of Materials Engineering, University of British Columbia, Vancouver V6T1Z4, Canada d University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e
i n f o
Article history: Received 16 November 2016 Revised 17 March 2017 Accepted 24 March 2017 Available online xxxx Keywords: Cu-Cd zinc plant residues (CZPR) Hazardous wastes Sulfuric acid leaching Kinetics Diffusion control
a b s t r a c t A large amount of Cu-Cd zinc plant residues (CZPR) are produced from the hydrometallurgical zinc plant operations. Since these residues contain substantial amount of heavy metals including Cd, Zn and Cu, therefore, they are considered as hazardous wastes. In order to realize decontamination treatment and efficient extraction of the valuable metals from the CZPR, a comprehensive recovery process using sulfuric acid as the leaching reagent and air as the oxidizing reagent has been proposed. The effect of temperature, sulfuric acid concentration, particle size, solid/liquid ratio and stirring speed on the cadmium extraction efficiency was investigated. The leaching kinetics of cadmium was also studied. It was concluded that the cadmium leaching process was controlled by the solid film diffusion process. Moreover, the order of the reaction rate constant versus H2SO4 concentration, particle size, solid/liquid ratio and stirring speed was calculated. The XRD and SEM-EDS analysis results showed that the main phases of the secondary sulfuric acid leaching residues were lead sulfate and calcium sulfate. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Cadmium is an important transition metal that has been widely utilized in pigments, ceramics, chemicals, electroplating, electronic industries and photographic products (Cheremisinoff, 1994; Habashi, 1997). On the other hand, cadmium is a toxic metal that can cause chronic poisoning (Min et al., 2013; Safarzadeh et al., 2007). For instance, the ‘‘Cadmium-contaminated Rice” event happened in Hunan Province in China in recent years was attributed to release of cadmium from industrial wastes of the nonferrous metal production. In this regard, safe handling of industrial discharge containing Cd is critical. In nature, cadmium is a relatively rare element in the earth’s crust and frequently exists as a minor component associated with nonferrous metal ores containing zinc. Therefore, a large amount of cadmium can potentially enter the environment in zinc produc⇑ Corresponding authors at: National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China (H. Du), Department of Materials Engineering, University of British Columbia, Vancouver V6T1Z4, Canada (D.B. Dreisinger). E-mail addresses: [email protected] (H. Du), [email protected] (D.B. Dreisinger).
tion. Analysis of cadmium pollution sources shows that lead and zinc smelting industries are currently the major causes. Zinc is a widely used metal and applied in the field of cosmetics, electronics, construction, automobile corrosion protection and medicines (Li et al., 2016; Xin et al., 2013). The application and demand of zinc have increased significantly recently. The global production of refined zinc metal in 2015 was up to 13.99 million tonnes (2015; Sethurajan et al., 2016). The traditional hydrometallurgical technique featuring oxidative roasting, acid leaching, purification and electrowinning accounts for 80–85% of the world’s zinc production and the balance is recovered by pyrometallurgical processes. A general flow sheet of the zinc hydrometallurgical smelting process is shown in Fig. 1. As can be seen from Fig. 1, there are three steps of the zinc hydrometallurgical process: the leaching of zinc calcines, the purification of the leaching solutions and the electrowinning of ZnSO4 solutions. Further, zinc calcine leaching process includes neutral leaching and hot acid leaching. The aim of the former is to leach most of the ZnO and separate zinc with other impurities, while that of the latter is to dissolve most of the remaining zinc in the zinc calcine (especially ZnFe2O4). This figure also shows the origin of Cu-Cd zinc plant residues (CZPR) generated from the purification of neutral leaching solutions. The CZPR
http://dx.doi.org/10.1016/j.wasman.2017.03.039 0956-053X/Ó 2017 Elsevier Ltd. All rights reserved.
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Zinc calcines
Sulfuric acid
Neutral leaching pH=5.2~5.4
Hot acid leaching
Thickener
Thickener
Neutral supernatant Cd-Cu Cd-Cu residues residues
Waste electrolyte
Neutral leaching residues
Cd &Cu purification Zinc powder and filtration Further purification and electrodeposition
Hot acid leaching solution
Pb-Ag residues
Fe purification and filtration Zinc solution
Fe residues
Zinc metal Fig. 1. Schematic diagram of the hydrometallurgical zinc production.
contains considerable quantity of valuable metals, such as cadmium, zinc and copper, but it is classified as hazardous waste because of containing high concentrations of heavy metals. Due to the different composition of zinc concentrates, some variation in CZPR composition is found. Cadmium pollution caused in the treatment process of CZPR accounts for 70% of the total amount of cadmium pollution (Yang et al., 2015). For every 100 tonnes of electrolytic zinc product, 1.3 tonnes of CZPR is generated (Tan et al., 2008). Therefore, a process for the decontamination treatment for this residue is of great importance for both economic and environmental point of view. In the past decades, the traditional method to recycle cadmium, zinc and copper from CZPR is the hydrometallurgical process. The hydrometallurgical process options are divided into acid method and ammonia method, both of which have their inherent problems. The zinc-ammonia solution produced by the ammonia leaching method cannot be integrated to the existing zinc recovery process; therefore, the ammonia leaching of CZPR has not been widely used (Liu et al., 2010). At present, some large zinc smelter plants in China only partially process CZPR. Cadmium and zinc are recovered first by acid leaching; the leached residues containing copper are sold. However, the copper-containing residues have about 3 wt.% cadmium and 20 wt.% zinc, which will spread the problem of cadmium contamination to the copper recovery process (Shao et al., 2003). Some researchers use low cost manganese dioxide as oxidant in the process for sulfuric acid leaching, and have reported that copper, zinc, cadmium in CZPR will be leached simultaneously (Tan et al., 2008). In addition, to further improve the leaching efficiency of CZPR, researchers have investigated many effective methods such as pressure acid leaching (Ke, 1996) and bioleaching (Cheng et al., 2013) technologies. Pressure acid leaching method operates under high temperature and pressure, which demand high capital and operating cost. Thus, it is not suitable for industrialization. The bioleaching method, on the other hand, is hard to interface with the existing hydrometallurgical zinc processing system. Overall, the existing CZPR treatment technologies have common problems of long process flow, complex operations, low leaching and recovery rate of cadmium, and consequently, high amount of cadmium is released to the environment. Therefore, it is necessary to develop an efficient process for the treatment of CZPR and to investigate the leaching behavior about cadmium, zinc and copper.
In this study, the extraction of cadmium, zinc and copper from a secondary resource, CZPR, by oxidative sulfuric acid leaching was investigated. The preferred process conditions for the maximum recovery of cadmium, zinc and copper and the leaching kinetics of cadmium dissolution were carefully studied. The effects of temperature, sulfuric acid concentration, solid to liquid phase ratio, initial particle size, stirring speed and aeration rate on the leaching efficiency of CZPR were investigated. In addition, the reaction kinetics was fit to different kinetic models and the apparent activation energy. The reaction order was also estimated. 2. Materials and methods 2.1. Cu-Cd zinc plant residues The CZPR used in this research was obtained from Shuikoushan Nonferrous Metals Group Co., Ltd. in Hunan Province, China. After natural drying, the residues were crushed, ground and screened, giving particle size fractions of 830 +425, 425 +250, 250 +150 and 150 +75 mm. The sulfuric acid used in the experiments was of analytical grade and was obtained from Xilong Chemical Group Co., Ltd. Water was purified with Milli-Q (Millipore Corp.) equipment before use. The sample was analyzed by inductively coupled plasma optical emission spectrometry (ICPOES, PerkinElmer Optima 7300V, USA), X-ray diffraction analysis (XRD, PANalytical Empyrean, Netherlands), equipped with a Cu Ka radiation and scanning electron microscopy (SEM, FEI Quanta 250, USA) equipped with energy disperse X-ray spectrometry (EDS, EDAX Genesis, USA). 2.2. Experimental procedure The laboratory-scale experiments were conducted in a 1L fourneck flask in a thermostatically controlled water bath (DF-101S, Bejing Xingde Instrument & Equipment Co., Ltd), equipped with a reflux condenser, mechanical stirring. And the stirrer was protected by Teflon. A volume of 500 mL of solution containing diluted sulfuric acid was added to the flask first. CZPR was added to the four-neck flask when the temperature reached the desired value and the stirrer started. A 1 mL sample of solution was withdrawn at the desired time interval. After the slurry was treated by
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centrifugation, the leachate was analyzed by ICP-OES. All chemical reagents used in this research were of analytical grade. The metal leaching extractions were calculated using Eq. (1):
X¼
V L CL MS W S
ð1Þ
where MS is the mass of CZPR applied in the experiment, g; WS is chemical analysis of the target metal, wt.%; VL is the volume of leaching leachate, mL; CL is the concentration of metals in the leachate, g/mL. Each experiment was repeated three times, and the mean extraction reported to avoid any errors.
Kinetic analysis is of great significance at the industrial level as it is the basis for scale-up and reactor design (Dhawan et al., 2011). The leaching process are generally considered to be controlled by the following steps: (1) by diffusion through the fluid film; (2) by diffusion through the product layer; (3) by the chemical reaction (Safari et al., 2009; Velardo et al., 2002). The step with the slowest speed is the controlling step. The experimental results were analyzed by the shrinking core model in order to find the controlling step. If the leaching process is controlled by diffusion through the solid product layer (Zhang et al., 2016): n
2 b Deff : ½H2 SO4 2 1 X ð1 XÞ2=3 ¼ t ¼ kd t 3 qA r20
ð2Þ
If the leaching process is controlled by the surface chemical reaction, Eq. (2) is replaced by the following equation (Zhang et al., 2016):
2,000
Inteasity (Counts)
b K S ½H2 SO4 n t ¼ kr t qA r0
3 5
1,800
1 4 5
2 5 5 62 5 3 6 3 6 5
1,600 1,400 1,200
6
1,000
ð3Þ
1-Cu 2-CuO 3-ZnO 4-CdO 5-ZnSO4 7H2O
..
1 6
3
6-CuSO4 5H2O
5 6 6
1 6
Ea RT
ð4Þ
4 5
400 200 20
30
40
50
60
3.1. Characterization of the CZPR The residue with the particle size of less than 250 +150 mm was analyzed by XRD, as presented in Fig. 2. The main phases of the CZPR are Cu, CuO, CdO, ZnO, ZnSO47H2O and CuSO45H2O. The content of sulphur was analyzed by X-ray fluorescence spectrometer (XRF, PANalytical B.V. Axios, Netherlands). The other chemical compositions of the CZPR with different particle size diffraction were analyzed by ICP-OES. A CZPR sample of 0.05–0.1 g mixed with 5–10 g sodium fluxing agent containing sodium carbonate and sodium borate (the mass ratio of sodium carbonate to sodium borate is 2:1) was loaded in a 50 mL platinum crucible with a cover and then was calcined in a muffle furnace at 950 °C for 10–30 min. The platinum crucible was then transferred into a 150 mL beaker with a boiling solution (40 mL high pure water and 20 mL concentrated hydrochloric acid) to make sure the solid phase in the platinum crucible was completely dissolved. Then the solution was transferred to a 100 mL volumetric flask and diluted by high pure water if necessary for the following ICP-OES analysis. The results are shown in Table 1. The SEM-EDS analysis of the CZPR was conducted, which was shown in Fig. 3. According to the microphotograph, the particle size of CZPR is not homogeneous but in irregular shapes. Based on the EDS analysis in Fig. 3(a) and (b), the particles contain the elements of cadmium, zinc, calcium and copper.
The redox potential plays a crucial role in the process of leaching metals from residues. The solubilization of metals cannot be solely related to pH-data but must also account for redox potential. Fig. 4 is the Eh-pH diagrams of the Zn-Cd-H2O (a) and Cu-Pb-H2O (b) at the temperature of 25 °C. As can be seen from Fig. 4(a), the stability areas of Zn2+ and Cd2+ are enclosed by the water stability limits, which indicate the feasibility of metals to reacted with acid. Nevertheless, Zn only can be oxidized by the acid when the pH is lower than 6. Similarly, pH of 6.5 (point A) is required for Cd. The areas of solubility regions of Zn2+ and Cd2+ show that zinc
600
10
3. Results and discussion
3.2. Theoretical analysis of the behavior of the phases
2 44
800
0
ln k ¼ ln A
where A is the frequency factor, Ea is the apparent activation energy.
2.3. Kinetic analysis
1 ð1 XÞ1=3 ¼
where X is the fraction reacted; kd is the rate constant for diffusion through the product layer film and t is the leaching time; kr is the rate constant for the chemical reaction; b is the stoichiometric coefficient in the leaching reaction; Deff. is the effective diffusion coefficient; [H2SO4] is the sulfuric acid concentration; n is the order of reaction with respect to H2SO4; Ks is the chemical reaction rate; qA is the molar density and r0 is the initial particle radius. The apparent activation energy can be calculated by the Arrhenius Eq. (4):
70
80
90
2-Theta (degree) Fig. 2. XRD diffraction pattern of the CZPR (250 +150 mm).
Table 1 The chemical compositions of the CZPR (wt.%). Particle size/lm
ZnO
CuO
CdO
SO3
PbO
Al2O3
CaO
SiO2
Fe2O3
+830 830 +425 425 +250 250 +150 150 +75 75
46.83 47.01 47.31 47.42 46.97 47.12
20.1 19.86 19.45 19.95 20.05 19.68
9.32 9.44 9.81 9.48 9.72 9.56
14.71 14.91 14.66 14.78 14.87 14.77
1.92 1.88 1.78 1.81 2.01 1.98
1.43 1.39 1.32 1.37 1.28 1.45
4.38 4.39 4.44 4.36 4.33 4.32
0.43 0.37 0.39 0.41 0.43 0.44
0.88 0.75 0.84 0.42 0.34 0.68
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Fig. 3. The SEM-EDS analysis of the CZPR. (a), (b) SEM of the CZPR, (c) EDS analysis results of position 1 and (d) EDS analysis result of position 2.
2+ 2+ 2+ Fig. 4. The Eh-pH diagrams of the Zn-Cd-H2O (a) and Cu-Pb-H2O (b) at the temperature of 25 °C. (a2+ Cd(aq) = 1, aZn(aq) = 1, aCu(aq) = 1, aPb(aq) = 1, drawn by HSC 6.0 chemistry software, Outokumpu Research, Finland).
Fig. 5. The plots of (a) DrGo T and (b) DrHo T for Eq. (5) (Drawn by HSC 6.0 chemistry software, Outokumpu Research, Finland).
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and cadmium have a greater solubility in the acid solutions, respectively. As can be seen from Fig. 4(b), the stability areas of Cu2+ and Pb2+ are also enclosed by the water stability limits. But higher acidities are needed for the oxidation of copper and lead. As can be seen from Fig. 4(b), Pb only can be oxidized by acid (H+) when the pH is lower than 2 (point B), and Cu cannot be dissolved. In this research, the highest sulfuric acid concentration tested is 25 wt.%, and according to the above analysis Cu cannot be directly dissolved. However, as shown in Fig. 5, Cu can be oxidized in the presence of oxygen and the dissolution reaction is a exothermic
reaction based on the DrHo is negative (Li et al., 2016.). Furthermore, according to the corresponding Eh-pH diagrams (not shown in this article) for the sulfate system, the solubility of metal ions are increased by the presence of SO2 (Safarzadeh et al., 2009). 4 Eq. (5) is the oxidation reaction of copper.
2Cu þ O2 þ 2H2 SO4 ¼ 2CuSO4 þ 2H2 O
ð5Þ 8
The Ksp value of lead(II) sulfate is 1.9 10 , at 25 °C, which indicates the solubility of lead(II) sulfate is very low. As can be seen from Fig. 6(a), Pb(II) ion solubility drops 100,000-fold when the
)
Fig. 6. The plot of concentration of Pb(II) as a function of sulfate radical concentration (a) and the fraction of Pb(II) and PbSO4 as a fraction of sulfate radical concentration (Drawn by Medusa software, Royal Institute of Technology, Sweden).
) Fig. 7. (a) Effect of temperature on cadmium leaching rate from CZPR; (b) 1 (1 X)1/3 versus time at different temperature; (c) 1 2/3X (1 X)2/3 versus time at different temperature and (d) ln kd versus reciprocal temperature.
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concentration of SO2 4 reaches 1 M. Similarly, Fig. 6(b) shows that all Pb(II) transforms to PbSO4 as SO2 4 reaches 1 M. According to this analysis, Pb(II) can be easily separated in the form of PbSO4. 3.3. Cadmium leaching kinetics 3.3.1. Effect of temperature The effect of reaction temperature ranging from 30 to 60 °C was studied, while other parameters were fixed with dilute sulfuric acid concentration of 1.5 M, particle size of 250 +150 mm, solid/liquid ratio of 1:6, stirring speed of 500 rpm and air flow rate of 0. The relationship between cadmium leaching ratio and leaching time at different temperatures is shown in Fig. 7(a), which shows that temperature does not affect the cadmium leaching ratio significantly. Furthermore, the higher temperatures improve the dissolution rate and reactant diffusion slowly. The cadmium leaching extent after 15 min reached 85.4%, 88.3%, 91.3% and 93.6% at the reaction temperature of 30, 40, 50 and 60 °C, respectively. However, the Cd leaching ratio slowly increased after rapidly decomposition in 15 min. Other researchers have found the different conclusion that temperature has a remarkable effect on the cadmium leaching efficiency (Kul and Topkaya, 2008; Safarzadeh et al., 2009; Shahsavari, 2001). Nonetheless, our research showed that the reaction temperature effect decreased by increase of reaction time. Fig. 7(b) and (c) shows the cadmium leaching efficiency of the CZPR at different leaching temperature according to the surface chemical reaction and solid film diffusion control, respectively. The results from Fig. 7(b) and (c) indicate that cadmium leaching
reaction is controlled by solid film diffusion control. The rate constants (kd) are the slopes in Fig. 7(b) and (c). In addition, the Arrhenius plot (ln kd vs 1000/T) was obtained as shown in Fig. 7(d) The calculated activation energy is 10.95 kJ/mol. As the literature suggested, the activation energy of a diffusion controlled process is typically below 12 kJ/mol, while the value for a chemical reaction controlled process in the leaching reaction is usually between 30 and 85 kJ/mol (Momade and Momade, 1999). For the present analysis, it clearly confirms that the leaching process is controlled by diffusion of ions through the product layer. 3.3.2. Effect of dilute sulfuric acid concentration The theoretical consumption of dilute sulfuric acid concentration for the total leaching of CZPR was approximately 1.5 M. Therefore, the effect of dilute acid concentration was studied in the range of 0.8–1.6 M, with other conditions unchanged, including leaching temperature of 30 °C, particle size of 250 +150 mm, solid-to-liquid ratio of 1:6, stirring speed of 500 rpm and air flow rate of 0. As seen in Fig. 8(a), the acid concentration affects the Cd leaching ratio significantly. When the sulfuric acid concentration was 0.8 and 1.1 M, the amount of acid was deficient to completely leach the insoluble cadmium. However, the leaching ratio was not increasing significantly as the sulfuric acid concentration exceeded 1.5 M in 40 min leaching. The results showed that the cadmium leaching extent remained constant after 20 min. The reason is that the leaching product rate increases as the sulfuric acid concentration increases. Furthermore, when the leaching product arrives at the equilibrium value around the CZPR particle, an insoluble product solid film around the unreacted solid particle is
Fig. 8. (a) Effect of dilute sulfuric acid concentration on cadmium leaching ratio from the CZPR; (b) 1 (2/3)X (1 X)2/3 versus time at different dilute sulfuric acid concentration and (c) Variation of ln kd versus ln[H2SO4].
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Fig. 9. (a) Effect of particle size on cadmium leaching efficiency; (b) Plot of 1 (2/3)X (1 X)2/3 versus time at different particle size (c) Plot of kd against 105/d20 and (d) Plot of ln kd against lnd0.
Fig. 10. (a) Effect S/L ratio on cadmium leaching ratio from the CZPR; (b) Plot of 1 (2/3)X (1 X)2/3 versus time at different S/L ratio and (c) Plot of ln kd versus ln(S/L).
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formed. So the leaching process becomes slow (Özmetin et al., 1996). The cadmium leaching extent after 30 min leaching at the sulfuric acid concentration of 0.8 and 1.5 M were 58.1% and 93.6%, respectively. As can be seen from Fig. 8(a), the Cd leaching extent increased with leaching reaction time and with the initial leaching of cadmium very quickly. Approximately 85.4% of cadmium in the residues was extracted at the sulfuric acid concentration of 1.5 M after 15 min and increased to 93.6% after 30 min. The leaching reaction time of 30 min was the optimum leaching time to produce the maximum cadmium extraction. The cadmium leaching ratio data of the CZPR at different sulfuric acid were plotted by the solid film diffusion control as shown in Fig. 8(b). The results from Fig. 8(b) indicate that the data fit the model well. The plot of ln kd versus ln[H2SO4] was presented in Fig. 8(c). The order of reaction with respect to sulfuric acid for the CZPR was 1.74 with a correlation coefficient of 0.99. 3.3.3. Effect of particle size The effect of particle size was examined, while other parameters were fixed with leaching temperature of 30 °C, dilute sulfuric acid concentration of 1.5 M, solid-to-liquid ratio of 1:6, stirring speed of 500 rpm and air flow rate of 0. The relationship of cadmium leaching ratio and leaching time at different particle size was plotted in Fig. 9(a), which shows that the particle size affects the cadmium leaching remarkably. The cadmium leaching extent of 71.6% was obtained using sample size of 830 +425 mm in 15 min, and it improved to 94% using sample size of 150 +75 mm. After 30 min leaching time, 84.6% of cadmium was extracted using sample size of 830 +425 mm.
In this research, the cadmium leaching extent increased a little when the sample size was decreased from 250 +150 to 150 +75 mm, and the leaching ratio keep the level of 94% in 30 min. So 250 +150 mm was the optimum sample size. Fig. 9(b) shows the cadmium leaching ratio data of the CZPR at different particle size analyzed by the model of the diffusion control model. The results from Fig. 9(b) indicate that the data fit the model well. The apparent rate constants (kd) were calculated and plotted in Fig. 9(c) and (d) versus 105/d20 (d0 is the average initial diameter). The direct relationship of kd versus 105/d20 also favors the leaching of Cd, which was controlled by solid film diffusion. The plot of ln kd versus lnd0 is presented in Fig. 9(d). The order of reaction versus particle size for the CZPR was 0.54 with a correlation coefficient of 0.97. 3.3.4. Effect of solid/liquid ratio With other leaching parameters were held at leaching temperature of 30 °C, dilute sulfuric acid of 1.5 M, particle size of 250 +150 mm, stirring speed of 500 rpm and air flow rate of 0, the influence of solid/liquid ratio on Cd leaching ratio was shown in Fig. 10 (a). As seen in Fig. 10(a), the solid-liquid ratio affects the cadmium leaching ratio significantly. The cadmium leaching extent increased from 64.5% at the solid/liquid ratio of 1/3 to 90.2% at the solid/liquid ratio of 1/8 in the leaching time of 15 min. When the solid-liquid ratio was lower than 1/4, cadmium leaching extent increased slowly. Therefore, this value was considered to be the optimum for the extraction of cadmium. However, in other cadmium leaching researches using sulfuric acid for the leaching agent, the optimal solid-liquid ratio values were 5% w/w, 6% (w/v), 1/4 (g/cc) and 1/8 (g/mL), respectively (Gouvea and Morais,
Fig. 11. (a) Effect of stirring speed on Cd leaching ratio from the CZPR; (b) Plot of 1 (2/3)X (1 X)2/3 versus time at stirring speed and (c) Determination of reaction order with respect to stirring speed.
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2007; Kul and Topkaya, 2008; Safarzadeh et al., 2009; Shahsavari, 2001; Shahsavari and Abdollahi, 2005). These may be attributed to the application of the leaching process to different residues. Fig. 10(b) shows the cadmium leaching ratio data of the CZPR at different solid-liquid ratios which were analyzed by the diffusion control model. The results from Fig. 10(b) indicate that the data fit the model well. The plot of ln kd versus ln(S/L) is presented in Fig. 10(c). The order of reaction rate constant versus solid-liquid ratio for the CZPR was be 1.04 with a correlation coefficient of 0.994. 3.3.5. Effect of stirring speed The effect of stirring speed was conducted and other parameters were fixed with leaching temperature of 30 °C, dilute sulfuric acid concentration of 1.5 M, particle size of 250 +150 mm, S/L ratio of 1:6 and air flow rate of 0. The relationship of cadmium leaching extent and reaction time at different particle size was plotted in Fig. 11(a), which indicates that the stirring speed affects the cadmium leaching remarkably. However, the cadmium leaching ratio increased until the stirring speed of 500 rpm in 30 min, where the cadmium leaching extent was 93.6%. Therefore, the optimum stirring speed for the dissolution of cadmium was considered to be 500 rpm. Fig. 11(b) shows the cadmium leaching ratio data of the CZPR at different stirring speed which were analyzed according to the diffusion control model. The results from Fig. 11(b) indicate that the data fit the model well. The plot of ln kd versus ln(Stirring Speed) was presented in Fig. 11(c). The order of reaction rate constant versus stirring speed for the CZPR was found to be 0.34 with a correlation coefficient of 0.99.
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The cadmium leaching kinetics showed that the higher sulfuric acid concentration, smaller particle size, higher L/S, higher stirring speed and higher leaching temperature contributed to higher cadmium leaching ratio. In addition, the value of apparent activation energy showed that ash diffusion was the controlling process (Aydogan et al., 2006; Habashi, 1999) and this value was in good agreement with the values presented in other researches (Aydogan et al., 2006; Mahlangu et al., 2007; Safarzadeh et al., 2009). 3.4. The behaviors of Zn, Cd, Cu and Pb In this research, the average residue was approximately 4% of the original feed by mass. The leaching behaviors of zinc, cadmium, copper and lead were presented in Fig. 12(a) at proposed conditions. From Fig. 12(a), it can be concluded that zinc can be leached easily and the leaching efficiency can reach 93.5% in 15 min and 99.4% in 40 min. It also can be seen that the leaching efficiency of cadmium was 93.6% in 40 min. However, lead was not leached and was easily separated, which was in concordance with theoretically foreseen. The experiment results of Fig. 12(b) show that the copper leaching ratio can be enhanced effectively by using air as oxidant. These results are consistent with theory. As seen in Fig. 12(b), the copper leaching ratio obtains without aeration is 58.4% in 15 min, while it is 83.6% when the air flow rate of 100 mL/min was applied and this increased to 93% after 40 min. Nevertheless, as
3.3.6. Kinetics model The results of activation energy, sulfuric acid concentration, solid-liquid ratio and particle size of this research showed that the ash layer diffusion was the controlling process. Therefore, the leaching cadmium from the CZPR can be clearly presented by the following equation based on this study:
1:04 2 0:54 S 1 X ð1 XÞ2=3 ¼ kd t ¼ k0 ½H2 SO4 1:74 d0 ðSSÞ0:34 3 L 10:95 t ð6Þ exp RT where X is the fraction reacted; k0 is the correlation coefficient; [H2SO4] is the concentration of dilute sulfuric acid; d0 is the diameter particle size; S/L is the solid/liquid ratio and SS is the stirring speed.
Fig. 13. The XRD analysis of the leaching residue.
Fig. 12. (a) Extraction of zinc, cadmium, copper and lead under the optimum leaching conditions. (Leaching temperature of 30 °C, dilute sulfuric acid concentration of 1.5 M, particle size of 250 +150 mm, solid/liquid ratio of 1:6, stirring speed of 500 rpm and air flow rate of 0.) and (b) The copper leaching ratio at different air flow rate (Leaching temperature of 30 °C, dilute sulfuric acid concentration of 1.5 M, particle size of 250 +150 mm, solid/liquid ratio of 1:6 and stirring speed of 500 rpm.)
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Fig. 14. The SEM of the leaching residue.
Table 2 The EDS analysis of the leaching residue. No.
1 2
Element (wt.%) O
S
Ca
Pb
Cu
50.99 58.34
18.54 13.87
25.20 22.40
4.81 5.39
0.45 –
the air flow rate increased to 150 mL/min, copper leaching ratio was not significantly increased. Therefore, the required air flow rate for the dissolution of copper was considered to be 100 mL/ min. Moreover, the leaching residue was analyzed by XRD (Fig. 13). And the leaching condition was controlled to be leaching temperature of 30 °C, dilute sulfuric acid concentration of 1.5 M, particle size of 250 +150 mm, solid-to-liquid ratio of 1:6, stirring speed of 500 rpm, air flow rate of 100 mL/min and leaching time of 40 min. It is indicated that PbSO4 and CaSO4 remained in the leaching residue. No cadmium, zinc and copper containing materials were detected in the leaching residue, due to the contents of cadmium, zinc and copper were very low. Therefore, it indicated that nearly all the target elements were leached by sulfuric acid. SEM and EDS analysis as shown in Fig. 14 and Table 2 supported the results. Only insoluble Ca and Pb containing phases were left in the sulfuric acid leaching residue.
4. Conclusions and perspectives The present research showed the Cu-Cd zinc plant residues can be used as a secondary resource for valuable metals of Cd, Zn and Cu extraction. The effect of leaching parameters, including leaching temperature, sulfuric acid concentration, particle size, solid/liquid ratio and stirring speed was assigned to the leaching of cadmium from the CZPR. The recommended leaching conditions were: leaching temperature of 30 °C, dilute sulfuric acid concentration of 1.5 M, particle size of 250 +150 mm, solid-to-liquid ratio of 1:6, stirring speed of 500 rpm, air flow rate of 100 mL/min and leaching time of 40 min. More than 99% Zn, 93% Cd and 93% Cu were leached. The cadmium leaching process was found to be controlled by the shrinking core model and the ash layer diffusion was the controlling process. The apparent activation energy was 10.95 kJ/mol. The order of the reaction with respect to H2SO4 concentration, particle size, solid-liquid ratio and stirring speed were calculated as 1.74, 0.54, 1.04 and 0.34, respectively. The leaching results showed that Pb could be separated from the CZPR. Nevertheless, the secondary leaching residue contains a little amount of insoluble cadmium and copper, which requires a proper separate process design. Therefore, a more in-depth study
about the leaching kinetics of the zinc and copper and the existing form of the insoluble cadmium and copper in the secondary leaching residue need to be conducted. Inasmuch as almost all valuable metals of cadmium, zinc and copper in the CZPR were transferred to the leachate, the overall process about separating copper, zinc and cadmium from the leaching solutions should be considered. One suggestion can be separate copper firstly by the additive of zinc powder. The rest of the solution can be treated by electrowinning or solvent extraction method to separate cadmium and zinc. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51404227, No. 51604254 and No. 91634111), the Science and Technology Service Network Initiative for the Chinese Academy of Sciences (Grant No. KFJSW-STS-148), and the National Basic Research Program of China (973 Program, Grant No. 2013CB632605). The financial support provided by the China Scholarship Council (CSC) for Meng Li (No. 201506080059) to the University of British Columbia (UBC) is also acknowledged. References 2015. International Lead and Zinc Study Group, Lisbon, Portugal. Aydogan, S., Ucar, G., Canbazoglu, M., 2006. Dissolution kinetics of chalcopyrite in acidic potassium dichromate solution. Hydrometallurgy 81, 45–51. Cheng, Y.-X., Wang, Q.-Q., Zhong, Z.-Y., Xu, Y.-Z., 2013. Investigation on Bio-leaching of Cd from Cu /Cd smelting slag. Environ. Chem. 32, 259–265. Cheremisinoff, N.P., 1994. Handbook of Water and Wastewater Treatment Technology. CRC Press. Dhawan, N., Safarzadeh, M.S., Birinci, M., 2011. Kinetics of hydrochloric acid leaching of smithsonite. Russian J. Non-Ferrous Metals 52, 209–216. Gouvea, L.R., Morais, C.A., 2007. Recovery of zinc and cadmium from industrial waste by leaching/cementation. Miner. Eng. 20, 956–958. Habashi, F., 1997. Handbook of Extractive Metallurgy, Wiley-VCH, Heidelberg, Germany, Volume 4. Habashi, F., 1999. Kinetics of Metallurgical Processes. Metallurgie Extractive Quebec, Quebec, Canada. Ke, J.-J., 1996. The progress of pressure leaching in hydrometallurgical process. Hydrometall. China 2, 1–6. Kul, M., Topkaya, Y., 2008. Recovery of germanium and other valuable metals from zinc plant residues. Hydrometallurgy 92, 87–94. Li, M., Zhang, Y., Wang, X.-H., Yang, J.-G., Qiao, S., Zheng, S.-L., Zhang, Y., 2016. Extraction of copper, zinc and cadmium from copper–cadmium-bearing slag by oxidative acid leaching process. Rare Metals, 1–10. http://dx.doi.org/10.1007/ s12598-016-0759-7. Liu, H.-Y., Yan, W.-B., Shi, A.-H., Gao, F., 2010. Research on leaching process of copper, zinc and cadmium from copper-cadmium slag. J. Ji Shou Univ. (Natural Sci. Ed.) 31, 97–100. Mahlangu, T., Gudyanga, F., Simbi, D., 2007. Reductive leaching of stibnite (Sb 2 S 3) flotation concentrates using metallic iron in a hydrochloric acid medium II: kinetics. Hydrometallurgy 88, 132–142. Min, X.-B., Xie, X.-D., Chai, L.-Y., Liang, Y.-J., Mi, L., Yong, K., 2013. Environmental availability and ecological risk assessment of heavy metals in zinc leaching residue. Trans. Nonferrous Metals Soc. China 23, 208–218.
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