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Leaching kinetics of nickel extraction from hazardous waste by sulphuric acid and optimization dissolution conditions
chemical engineering research and design 9 1 ( 2 0 1 3 ) 325–331
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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Leaching kinetics of nickel extraction from hazardous waste by sulphuric acid and optimization dissolution conditions Mahdi Gharabaghi a,∗ , Mehdi Irannajad b , Amir Reza Azadmehr b a b
School of Mining Engineering, College of Engineering, University of Tehran, Tehran, Iran Department of Mining & Metallurgical Engineering, Amirkabir University of Technology, Hafez St, Tehran, Iran
a b s t r a c t In this study, dissolution kinetics of nickel in sulphuric acid was investigated. The effects of various parameters were studied to optimize the dissolution conditions and to determine the kinetics of the leaching. The results indicated that the increase in the leaching time, temperature, sulphuric acid concentration, and decrease in solid–liquid ratio and particle size enhanced the leaching rate of nickel. The results showed that the 96% of nickel content was extracted from the sample of particle size −250 m after 30 min leaching in 8% (v/v) sulphuric acid concentration and solid/liquid of 0.10. The experimental data were best fitted by a diffusion-controlled model indicated that diffusion through the product layer was the rate-controlling step during the dissolution. The activation energy was 9.06 kJ/mol and it was also an indication of the diffusion-controlled process. The following expression based on diffusion-controlled model can be used to describe the dissolution kinetics of nickel: 1–3(1 − ˛) k0 [H2 SO4 ]
1.36
(S/L)
−1.12 −0.22 r0
2/3
+ 2(1 − ˛) = kd t =
exp(9.06/RT)t gypsum was formed as the main leaching product and tended to inhibit the
leaching rate. The leaching mechanism was confirmed by characterizing the original sample and the leach residues using XRD and SEM analysis. © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Hazardous waste; Leaching kinetic; Nickel; Sulphuric acid; Shrinking core model
1.
Introduction
Nickel is an important metal applied in many fields, such as energy materials and functional materials, chemicals and catalysts, cathodes, batteries, and it has several metallurgical benefits such as high melting point, ferromagnetic properties, and ease of electroplating (Anderson, 1996; Mudd, 2010). Nickel can be produced and recycled from both primary and secondary resources such as spent catalysts, waste batteries and tailings (Eramet, 2009; Mudd, 2010; Xu et al., 2005). As the nickel primary resources are getting depleted, the beneficiation of secondary resources has become more important in recent years. These resources are classified as hazardous residues and they cannot be landfilled and they should be collected for recycling. These resources are comprised Ni–Cd batteries, nickel alloys, waste nickel scraps and zinc plant residues. There are many papers reporting extraction of nickel from these wastes using various reagents (Abdel-Aal and Rashad, 2004; Bernardes et al., 2004; Freitas
∗
and de Pietre, 2004; Freitas and Rosalém, 2005; Lee et al., 2010; Lothongkum et al., 2009; Mulak et al., 2005; Nan et al., 2006; Safarzadeh and Moradkhani, 2010; Senanayake et al., 2010; Shen et al., 2008; Tenório and Espinosa, 2002). In addition, the kinetics of nickel leaching from nickel resources has been studied in some recent researches (Abdel-Aal and Rashad, 2004; Mulak et al., 2005). Ni–Cd batteries can be recycled by different methods such as pyrometallurgical, hydrometallurgical or electrochemical recycling processes (Bernardes et al., 2004; Freitas and de Pietre, 2004; Freitas and Rosalém, 2005). There is paucity information about nickel recovery from zinc plant residue. The effects of heat treatment on the nickel extraction from this waste has been investigated (Safarzadeh and Moradkhani, 2010). However, there is not any detail investigation about kinetics of nickel extraction from zinc plant residue. This waste is produced in industrial zinc plants which use hydrometallurgical process for zinc production. Zinc plant residue contains considerable amounts of zinc, cadmium, nickel, copper and other metals and is a hazardous
Corresponding author. Tel.: +98 21 61114556; fax: +98 21 88008838. E-mail addresses: [email protected], [email protected] (M. Gharabaghi). Received 24 March 2012; Received in revised form 17 November 2012; Accepted 27 November 2012 0263-8762/$ – see front matter © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cherd.2012.11.016
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chemical engineering research and design 9 1 ( 2 0 1 3 ) 325–331
100
Dissolved fraction (α)
90 80 70 60 50 40
2%
30
5%
20
8%
10
10 % 12 %
0 0
5
10
15
20
25
30
35
40
45
50
55
Reaction Time (minute)
Fig. 1 – XRD analysis of the head sample.
˘ et al., 1998; waste (Abdel-Aal and Rashad, 2004; Altundogan Gharabaghi et al., 2012; Gouvea and Morais, 2007; Safarzadeh et al., 2009). Based on enviornmental laws assigned to protect the environment from hazardous wastes in the world, it becomes more and more important to satisfy such world demand through recovery of metals of concern from secondary resources such as zinc plant residue, spent magnetic materials, waste alloys (Li et al., 2009; Tan and Shen, 2000). The aim of this work was to investigate the dissolution kinetics of nickel extraction from Iranian zinc plant residue by hydrometallurgical technique using sulphuric acid as lixivant. In each leaching reaction, physicochemical processes control dissolution kinetics. These processes are affected by different parameters, such as acid concentration, solid/liquid ratio, stirring speed, particle size, temperature and etc.
2.
Material and methods
2.1.
Material
Leaching experiments were carried out on the zinc plant residue obtained from Calcimin zinc plant in Zanjan, Iran. The chemical and mineralogical analyses were performed using X-ray diffraction (XRD) spectra and X-ray fluorescence (XRF) and their results are given in Fig. 1 and Table 1 respectively. X-ray diffraction (XRD) analysis showed the zinc-bearing minerals were the major components in the sample, but cadmium, nickel and lead components were also present in the sample. Sulphuric acid (98%) was purchased from Merck (Germany) and used as leaching reagent.
2.2.
Method
The samples were ground and sieved using ASTM standard sieves. After crushing and classification of the samples, the dissolution process was carried out in a 500 ml glass reactor and the slurry was agitated by a glass bladed overhead stirrer. The reaction temperature was maintained constant in water bath heated by electric heater. The leaching experiments were performed using −250 m grain sizes, except those related to the effects of particles size on the reaction kinetics. In the leaching experiments, the influences of acid concentration, particle size, solid–liquid ratio and reaction temperature on the dissolution kinetics were investigated. To prevent liquid loss by evaporation when the system was heated, the reactor was fitted with a reflux condenser.
Fig. 2 – Effects of acid concentration on nickel extraction from waste (25 ◦ C; 500 rpm, S/L = 1/10, particle size: −250 m).
After the reactor containing 100 ml of solution was heated to predetermined constant level, 10 g of sample was added into the solution to commence the leaching experiment while stirring was maintained. The reactor content was filtered as soon as the process finished. Elemental analysis was done using Unicom Atomic Absorption spectrometry (AAS). The calculation of nickel extraction was performed with respect to the amount of nickel in the leach liquor. SEM analysis before and after leaching was examined using a Philips XL30, combined with EDX.
3.
Results and discussion
3.1.
Effect acid concentration
A number of experiments were performed to see the effects of leaching time on the nickel extraction at varying acid concentrations. The effects of acid concentration were studied in the range of 2–12% (v/v) acid concentration at 25 ◦ C using −250 m particle size with a stirring speed of 500 rpm. Depending on the leaching conditions, the results indicated that the dissolution of nickel was strongly dependent on the increase in the acid concentration and leaching rate increased with increasing acid concentration. In the leaching of nickel, the acid concentration could be increased up to a certain limit as shown in Fig. 2. In the low acid concentration, namely 2 and 5% (v/v), there was acid concentration deficiency and only 40.2% and 70.50% of nickel were extracted respectively. The nickel extraction improves from 40% to 96% when the acid concentration increases from 2% to 8%. Increasing nickel extraction as acid concentration increases is due to acid concentration effect on increasing the H+ activity that results further dissolution of nickel containing material. In the nickel extraction from spent nickel–cadmium batteries in sulphuric acid media, the authors have stated that the nickel dissolution rate increased with the increase in acid concentration (Nogueira and Margarido, 2004). The highest nickel extraction was obtained in the experiments carried out for 30 min and after this period, nickel extraction values showed a linear trend with respect to time (Fig. 2). It is obvious from Fig. 2 that any increase in the acid concentration more than 8% in 30 min leaching had no effect on the leaching rate and more than 97% of nickel content was extracted in this condition. In addition, the acid concentration
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chemical engineering research and design 9 1 ( 2 0 1 3 ) 325–331
Table 1 – Chemical analysis of the head sample. Component
ZnO
CdO
NiO
CuO
PbO
SO3
CaO
Fe2 O3
MgO
Al2 O3
LOIa
Amount (%)
38.92
16.56
4.21
1.99
1.38
12.10
2.61
0.44
0.20
0.34
20.54
Loss on ignition (LOI) is the sample weight reduction after being ignited from 550 to 950 ◦ C.
should not be increased more than the required level to avoid the acid attack on other impurities during the process. Similar results for dissolution of spent nickel catalyst in sulphuric acid solution were obtained (Abdel-Aal and Rashad, 2004). The time required for nickel dissolution depends on the particle size, acid concentration and the nature of the adherent materials existing in the samples. The nickel extraction reached to maximum value after 30 min leaching using 8% (v/v) acid concentration. As shown in Fig. 2, any increase in leaching time up to 40 min was immaterial and the reaction reached equilibrium in 30 min. Therefore, 30 min was the optimum leaching time for nickel extraction and extended leaching times (>30 min) had no significant effects on nickel extraction.
3.2.
100 90 80
Dissolved fraction (α)
a
70 60 50 40 20 10 0 0
5
10
15
20
25
30
35
40
45
50
55
Reaction Time (minute) Fig. 4 – Effect of particle sizes on the nickel leaching (8% (v/v) acid, 25 ◦ C; 500 rpm, S/L = 0.10).
Effect solid/liquid ratios 3.3.
The effects of solid/liquid ratio were studied using 8% (v/v) sulphuric acid at 25 ◦ C with −250 m particle size. The results showed that nickel leaching recovery increased with decreasing of S/L ratio, and decreasing the pulp density was beneficial for nickel extraction (Fig. 3). Nickel extraction rose from 52.42% at S/L = 0.30–93% at S/L = 0.10 in 20 min leaching and leaching recovery increased to about 96% in 30 min dissolution at S/L = 0.10. When S/L was less than 0.10, leaching recovery increased slowly. In the other study for zinc extraction from sphalerite concentration, Aydogan et al. has stated that the zinc recovery increased with a decrease in the amount of solid (Aydogan et al., 2005). The results of metals extraction from nickel–cadmium spent batteries showed that the extraction of nickel increased with a decrease in the S/L ratio (Reddy and Priya, 2006). It is possible to improve leaching rate with decreasing pulp solid percent, but it has a limitation because of process economy. Considering the process performance the results indicate S/L = 0.10 as the optimum solid percent.
100 90
Dissolved fraction (α)
-850+425 micron -425+250 micron -250+75 micron -250 micron -75 micron
30
80 70
Effect of particle sizes
In leaching trials, a number of experiments were performed to determine the nickel extraction values at different particle sizes. Within the series of tests, the sulphuric acid concentration and leaching temperature were kept constant at 8% (v/v) and 25 ◦ C, respectively. The solid/liquid ratio was 0.10. These results showed that the nickel extraction was depended to sample particle size (Fig. 4). After 5 min of leaching time, 46.6–60.1% of nickel was extracted and after 30 min it rose to 84 and 98%, for particle size of −850 + 425 and −75 m respectively. Similar trends were observed for various particle sizes. The nickel extraction was achieved with particle sizes −75 m was about 15% higher than that was achieved with particle sizes in the range of −850 + 425 m. In general, the particle size of sample has an important influence on the rate of nickel dissolution. In the nickel extraction, the smaller particle size has the faster extraction rate and nickel extraction from spent catalyst increase with a decrease in the particle size (Mulak et al., 2005). The finer the particle size was, the greater geometric surface area and degree of contact surface between the acid solution and the particles were. In addition with a decrease in the particle size, the surface area per unit weight of sample increases and more nickel compounds liberated from the matrix due to an increase in the degree of size reduction more and more.
60
3.4.
50
Effect of temperature
40 0.05 0.10 0.15 0.20 0.30
30 20 10 0 0
5
10
15
20
25
30
35
40
45
Reaction Time (minute) Fig. 3 – Effect of solid to liquid ratio (S/L) on leaching recovery (8% (v/v) acid, 25 ◦ C; 500 rpm, particle size: −250 m).
50
55
Temperature plays an important role in the dissolution of nickel from zinc plant residue up to 20 min. On the conditions of 8% (v/v) sulphuric acid concentration and S/L = 0.10, the sample of −250 m size was leached at different temperatures (25–75 ◦ C). Data on the leaching rate of nickel are shown in Fig. 5. These results showed that the temperature had appreciable effect on the nickel extraction. The nickel leaching recovery increased from 52.43% at 25 ◦ C to 69.72% at 75 ◦ C. Nickel leaching recovery kept a little invariable between the temperature of
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chemical engineering research and design 9 1 ( 2 0 1 3 ) 325–331
100 90
Dissolved fraction (α)
80 70 60 50
25 C
40
35 C
30
45 C 55 C
20
65 C
10
75 C
0 0
5
10
15
20
25
30
35
40
45
50
Reaction Time (minute)
Fig. 5 – Effect of temperature on the nickel leaching (8% (v/v) acid, 500 rpm, S/L = 0.10). 25 ◦ C and 55 ◦ C. When leaching temperature increased to 75 ◦ C at 15 min dissolution time, leaching recovery of nickel rose to 96%. This increase was due to increase in reaction velocity constant K with the development of reaction temperature. In addition, similar results were obtained in the other investigations (Abdel-Aal and Rashad, 2004; Mulak et al., 2005; Nogueira and Margarido, 2004). As temperature increases, the time required to attain the maximum nickel extraction decreases. At the higher temperatures nickel dissolution increased at a certain leaching time, but by increasing the leaching time, the effects of temperature decreased.
3.5.
Effect of stirring speed
Leaching experiments to determine the effect of the stirring speed were conducted at 25 ◦ C using 8% (v/v) H2 SO4 . Nickel leaching recovery was 25.5% when no stirring was applied at 30 min, while it was 69.9%, 86.6%, and 93.3%, respectively, at 100, 200, 300 rpm stirring speed. The results have shown that leaching rate increased by increasing stirring speed up to 500 rpm and over that amount, the rate of agitation did not influence the nickel extraction (Fig. 6). This was indicating that at 500 rpm stirring speed, there was an adequate suspension of the solid particles in the solution; hence, an intermediate rate of 500 rpm was used in all experiments to assure independence of this variable and the chosen stirring rate removed film diffusion as a control mechanism.
4.
Kinetic analysis
80
For a liquid/solid reaction system, the reaction rate is generally controlled by diffusion through the liquid film, diffusion through the ash/product layer, the chemical reaction at the surface of the solid particles or a mixed of diffusion and chemical reactions (Gharabaghi et al., 2010; Levenspiel, 1998; Zafar, 2008). The shrinking core model considers that the leaching process is controlled by one of these steps. The reaction model between a liquid and a solid may be given as:
70
A(liquid) + B(solid) → Products
100 90 Dissolved fraction (α)
Fig. 7 – Plot of SCM with time at various temperatures (a) diffusion model, (b) chemical reaction model (8% acid concentration, S/L: 0.10, 500 rpm, particle size: −250 m).
60
In order to determine the kinetic parameters and rate controlling step in the nickel leaching in sulphuric acid solutions, the experimental data were analysed based on shrinking core model. The equations of the shrinking core model when either diffusion or the surface chemical reactions are the slowest step can be expressed as follows, respectively (Levenspiel, 1998):
100 rpm 200 rpm 300 rpm 400 rpm 500 rpm 600 rpm
50 40 30 20 10 0 0
5
10
15
20 25 30 35 40 Reaction Time (minute)
45
50
55
Fig. 6 – Effect of stirring speed on nickel extraction (acid concentration 8%(v/v), 25 ◦ C, 30 min, S/L = 0.1, particle size: −250 m).
1–3(1 − ˛)
2/3
[1 − (1 − ˛)
+ 2(1 − ˛) = kd t
1/3
]=
kMB DCA B ar02
t = kr t
(1) (2)
chemical engineering research and design 9 1 ( 2 0 1 3 ) 325–331
329
Fig. 8 – Arrhenius plot for the dissolution process (8% acid concentration, 100 g/L solids, 500 rpm, particle size: −250 m). where ˛ = fraction reacted; k = kinetic constant; MB = molecular weight of solid; CA = acid concentration; a = stoichiometric coefficient of reagent in reaction; r0 = initial radius of particle; t = reaction time; B = density of the solid, D = diffusion coefficient in porous product layer and kd and kr = rate constants, which are calculated from Eqs. (1) to (2), respectively. In order to determine the main controlling step and the effects of temperature on the leaching kinetics, the experimental data in Fig. 5 were analysed based on Eqs. (1) and (2) and shown in Fig. 7aand b respectively. As shown in Fig. 7, there is a good correlation between the experimental data and Eq. (1), indicating that the leaching kinetics can be described by the SCM model with ash layer diffusion control (Aydogan et al., 2006; Habashi, 1999; Jun et al., 2010; Levenspiel, 1998). High value of activation energy indicates that the chemical reaction would be the rate controlling step and low activation energy indicates that the diffusion could be rate-controlling step (Abdel-Aal and Rashad, 2004; Levenspiel, 1998; Souza et al., 2007). Fig. 8 presents the Arrhenius plot is a straight line constructed with the ln k versus 1/T for the k values determined from Eq. (1) and its slope is (−Ea /R). The activation energy of 9.06 kJ/mol was calculated using the Arrhenius equation. This value of activation energy also proves that leaching process is controlled by diffusion as its activation energy is between 4 kJ/mol and 12 kJ/mol (Abdel-Aal, 2000; Aydogan et al., 2006; Habashi, 1999). The rate constants (kd ) for the various acid concentrations were obtained from the slope of straight lines and presented in Fig. 9a. Fig. 9b shows the plot of Log kd versus ln ([H2 SO4 ]) from which the reaction order with respect to the H2 SO4 concentration was determined as 1.36. The results for the effect of liquid to solid ratio on leaching kinetics were applied to the diffusion kinetics models and kd values for each S/L were determined. ln (kd ) versus ln (S/L) was plotted in Fig. 10a and the reaction order with respect to S/L was obtained as −1.12 with a correlation coefficient of 0.9581. From the variation in kinetics model with time at various particle sizes kd values were obtained. From the kd and particle size values, a plot of ln kd versus ln (r0 ) was plotted in Fig. 10b. The order of reaction with respect to particle size was found to be −0.22. As shown in Figs. 7–10, the main resistance to the process is the diffusion through the product layer, so a combination
Fig. 9 – (a) The variation in 1–3(1 − ˛)(2/3) + 2(1 − ˛) with time at different acid concentration (v/v); (b) reaction order with respect to acid concentration (S/L: 0.10, 500 rpm, particle size: −250 m.
Fig. 10 – (a) Determination of reaction order with respect to (S/L), (b) reaction order with respect to particle size (8% acid concentration (v/v), 500 rpm, 25 ◦ C).
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Table 2 – Analysis of the leaching residue after different leaching time. Leaching time (min) Nickel (%) Cd (%) Zn (%) Cu (%) Pb (%) Ca (%)
5
15
30
3.01 9.31 21.36 4.12 7.32 12.35
1.24 3.12 8.73 9.14 15.41 27.72
0.17 1.13 2.11 12.61 18.46 37.62
result showed that by increasing leaching time, Cu, Ca and Pb enriched markedly while Ni content decreased correspondingly. Fig. 11 – XRD pattern of the leaching residue after 30 min.
5.1. of the shrinking core rate expression and the effects of temperature, acid concentration, S/L and particle size on the rate constant gives the following expression for the leaching kinetics of nickel in sulphuric acid: 1 − 3(1 − ˛)
(2/3)
+ 2(1 − ˛)
= kd t = k0 [H2 SO4 ]1.36 (S/L)
5.
−1.12 −0.22 r0
exp
9.06 RT
t
Characterization of the leaching residues
The characterization of the leaching residue included X-ray and SEM/EDX analyses was performed after process. Chemical composition of the residues obtained at 8% (v/v) acid concentration, stirring speed of 500 rpm, solid–liquid ratio of 0.10 and 25 ◦ C after 5, 15 and 30 min leaching are shown in Table 2. The
XRD diffraction patterns after leaching
The leaching residue, obtained at 25 ◦ C after 30 min leaching by 8% (v/v) sulphuric acid was chosen for X-ray and SEM/EDX analyses. As can be seen in Fig. 11, gypsum, PbO2 , Cu2 O, Cu4 O3 and Fe2 O3 composed the residue. It can be concluded that by increasing leaching time, copper, calcium and lead compounds intensity increased and nickel compounds intensity decreased. Increasing percentage of these compounds in the leach residue was due to the sharp decrease in the nickel, cadmium and zinc compounds amounts. The surface morphology of sample after leaching was examined by SEM-equipped with EDX. Fig. 12a shows the effect of sulphuric acid attack and presents some pores that were probably occupied by acid soluble material before leaching process (Vegliò et al., 2001).
Fig. 12 – Particles of the samples after leaching (leaching conditions: 8% acid concentration (v/v), 25 ◦ C, 500 rpm, 100 g/L solids).
chemical engineering research and design 9 1 ( 2 0 1 3 ) 325–331
The study of leach residue in Fig. 12b showed the increase in dissolution of nickel containing phase with increasing leaching time and appearance of significant amount of lead sulphate and gypsum which were indicated by EDX analysis. The SEM image of the leaching residues also showed a progressive reduction in particle diameter Fig. 12c and then an increase in roughness and porosity of the solids. After 30 min, leach residue became quite rich in lead sulphate and gypsum containing cavities, and was spongier in nature. Characterization studies showed that the nickel, zinc and cadmium containing phases progressively disappear while gypsum and lead containing phases predominates at later stages of leaching. Finally, the morphology of leach residue inferred that the sulphuric acid diffused through the pores of the sample and governed the leaching process through a diffusion-controlled mechanism.
6.
Conclusions
The dissolution kinetics of nickel content from zinc plant residue in sulphuric acid was investigated. It was found that the nickel extraction enhanced with the increase in sulphuric concentration, temperature and leaching time. The decrease in the particle size and solid to liquid ratio increased nickel dissolution. The extraction rate increased with increasing stirring speed and it was the highest at 500 rpm. The maximum leaching degree was 97% at 25 ◦ C after 30 min. The shrinking core model was applied to interpret the results and this model with ash layer control fitted the experimental data. The activation energy was 9.06 kJ/mol and it supported that the process was diffusion controlled. The samples before and after leaching were subjected to XRD and SEM-EDS analyses and their results confirmed the kinetics analysis results.
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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Leaching kinetics of nickel extraction from hazardous waste by sulphuric acid and optimization dissolution conditions Mahdi Gharabaghi a,∗ , Mehdi Irannajad b , Amir Reza Azadmehr b a b
School of Mining Engineering, College of Engineering, University of Tehran, Tehran, Iran Department of Mining & Metallurgical Engineering, Amirkabir University of Technology, Hafez St, Tehran, Iran
a b s t r a c t In this study, dissolution kinetics of nickel in sulphuric acid was investigated. The effects of various parameters were studied to optimize the dissolution conditions and to determine the kinetics of the leaching. The results indicated that the increase in the leaching time, temperature, sulphuric acid concentration, and decrease in solid–liquid ratio and particle size enhanced the leaching rate of nickel. The results showed that the 96% of nickel content was extracted from the sample of particle size −250 m after 30 min leaching in 8% (v/v) sulphuric acid concentration and solid/liquid of 0.10. The experimental data were best fitted by a diffusion-controlled model indicated that diffusion through the product layer was the rate-controlling step during the dissolution. The activation energy was 9.06 kJ/mol and it was also an indication of the diffusion-controlled process. The following expression based on diffusion-controlled model can be used to describe the dissolution kinetics of nickel: 1–3(1 − ˛) k0 [H2 SO4 ]
1.36
(S/L)
−1.12 −0.22 r0
2/3
+ 2(1 − ˛) = kd t =
exp(9.06/RT)t gypsum was formed as the main leaching product and tended to inhibit the
leaching rate. The leaching mechanism was confirmed by characterizing the original sample and the leach residues using XRD and SEM analysis. © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Hazardous waste; Leaching kinetic; Nickel; Sulphuric acid; Shrinking core model
1.
Introduction
Nickel is an important metal applied in many fields, such as energy materials and functional materials, chemicals and catalysts, cathodes, batteries, and it has several metallurgical benefits such as high melting point, ferromagnetic properties, and ease of electroplating (Anderson, 1996; Mudd, 2010). Nickel can be produced and recycled from both primary and secondary resources such as spent catalysts, waste batteries and tailings (Eramet, 2009; Mudd, 2010; Xu et al., 2005). As the nickel primary resources are getting depleted, the beneficiation of secondary resources has become more important in recent years. These resources are classified as hazardous residues and they cannot be landfilled and they should be collected for recycling. These resources are comprised Ni–Cd batteries, nickel alloys, waste nickel scraps and zinc plant residues. There are many papers reporting extraction of nickel from these wastes using various reagents (Abdel-Aal and Rashad, 2004; Bernardes et al., 2004; Freitas
∗
and de Pietre, 2004; Freitas and Rosalém, 2005; Lee et al., 2010; Lothongkum et al., 2009; Mulak et al., 2005; Nan et al., 2006; Safarzadeh and Moradkhani, 2010; Senanayake et al., 2010; Shen et al., 2008; Tenório and Espinosa, 2002). In addition, the kinetics of nickel leaching from nickel resources has been studied in some recent researches (Abdel-Aal and Rashad, 2004; Mulak et al., 2005). Ni–Cd batteries can be recycled by different methods such as pyrometallurgical, hydrometallurgical or electrochemical recycling processes (Bernardes et al., 2004; Freitas and de Pietre, 2004; Freitas and Rosalém, 2005). There is paucity information about nickel recovery from zinc plant residue. The effects of heat treatment on the nickel extraction from this waste has been investigated (Safarzadeh and Moradkhani, 2010). However, there is not any detail investigation about kinetics of nickel extraction from zinc plant residue. This waste is produced in industrial zinc plants which use hydrometallurgical process for zinc production. Zinc plant residue contains considerable amounts of zinc, cadmium, nickel, copper and other metals and is a hazardous
Corresponding author. Tel.: +98 21 61114556; fax: +98 21 88008838. E-mail addresses: [email protected], [email protected] (M. Gharabaghi). Received 24 March 2012; Received in revised form 17 November 2012; Accepted 27 November 2012 0263-8762/$ – see front matter © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cherd.2012.11.016
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100
Dissolved fraction (α)
90 80 70 60 50 40
2%
30
5%
20
8%
10
10 % 12 %
0 0
5
10
15
20
25
30
35
40
45
50
55
Reaction Time (minute)
Fig. 1 – XRD analysis of the head sample.
˘ et al., 1998; waste (Abdel-Aal and Rashad, 2004; Altundogan Gharabaghi et al., 2012; Gouvea and Morais, 2007; Safarzadeh et al., 2009). Based on enviornmental laws assigned to protect the environment from hazardous wastes in the world, it becomes more and more important to satisfy such world demand through recovery of metals of concern from secondary resources such as zinc plant residue, spent magnetic materials, waste alloys (Li et al., 2009; Tan and Shen, 2000). The aim of this work was to investigate the dissolution kinetics of nickel extraction from Iranian zinc plant residue by hydrometallurgical technique using sulphuric acid as lixivant. In each leaching reaction, physicochemical processes control dissolution kinetics. These processes are affected by different parameters, such as acid concentration, solid/liquid ratio, stirring speed, particle size, temperature and etc.
2.
Material and methods
2.1.
Material
Leaching experiments were carried out on the zinc plant residue obtained from Calcimin zinc plant in Zanjan, Iran. The chemical and mineralogical analyses were performed using X-ray diffraction (XRD) spectra and X-ray fluorescence (XRF) and their results are given in Fig. 1 and Table 1 respectively. X-ray diffraction (XRD) analysis showed the zinc-bearing minerals were the major components in the sample, but cadmium, nickel and lead components were also present in the sample. Sulphuric acid (98%) was purchased from Merck (Germany) and used as leaching reagent.
2.2.
Method
The samples were ground and sieved using ASTM standard sieves. After crushing and classification of the samples, the dissolution process was carried out in a 500 ml glass reactor and the slurry was agitated by a glass bladed overhead stirrer. The reaction temperature was maintained constant in water bath heated by electric heater. The leaching experiments were performed using −250 m grain sizes, except those related to the effects of particles size on the reaction kinetics. In the leaching experiments, the influences of acid concentration, particle size, solid–liquid ratio and reaction temperature on the dissolution kinetics were investigated. To prevent liquid loss by evaporation when the system was heated, the reactor was fitted with a reflux condenser.
Fig. 2 – Effects of acid concentration on nickel extraction from waste (25 ◦ C; 500 rpm, S/L = 1/10, particle size: −250 m).
After the reactor containing 100 ml of solution was heated to predetermined constant level, 10 g of sample was added into the solution to commence the leaching experiment while stirring was maintained. The reactor content was filtered as soon as the process finished. Elemental analysis was done using Unicom Atomic Absorption spectrometry (AAS). The calculation of nickel extraction was performed with respect to the amount of nickel in the leach liquor. SEM analysis before and after leaching was examined using a Philips XL30, combined with EDX.
3.
Results and discussion
3.1.
Effect acid concentration
A number of experiments were performed to see the effects of leaching time on the nickel extraction at varying acid concentrations. The effects of acid concentration were studied in the range of 2–12% (v/v) acid concentration at 25 ◦ C using −250 m particle size with a stirring speed of 500 rpm. Depending on the leaching conditions, the results indicated that the dissolution of nickel was strongly dependent on the increase in the acid concentration and leaching rate increased with increasing acid concentration. In the leaching of nickel, the acid concentration could be increased up to a certain limit as shown in Fig. 2. In the low acid concentration, namely 2 and 5% (v/v), there was acid concentration deficiency and only 40.2% and 70.50% of nickel were extracted respectively. The nickel extraction improves from 40% to 96% when the acid concentration increases from 2% to 8%. Increasing nickel extraction as acid concentration increases is due to acid concentration effect on increasing the H+ activity that results further dissolution of nickel containing material. In the nickel extraction from spent nickel–cadmium batteries in sulphuric acid media, the authors have stated that the nickel dissolution rate increased with the increase in acid concentration (Nogueira and Margarido, 2004). The highest nickel extraction was obtained in the experiments carried out for 30 min and after this period, nickel extraction values showed a linear trend with respect to time (Fig. 2). It is obvious from Fig. 2 that any increase in the acid concentration more than 8% in 30 min leaching had no effect on the leaching rate and more than 97% of nickel content was extracted in this condition. In addition, the acid concentration
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Table 1 – Chemical analysis of the head sample. Component
ZnO
CdO
NiO
CuO
PbO
SO3
CaO
Fe2 O3
MgO
Al2 O3
LOIa
Amount (%)
38.92
16.56
4.21
1.99
1.38
12.10
2.61
0.44
0.20
0.34
20.54
Loss on ignition (LOI) is the sample weight reduction after being ignited from 550 to 950 ◦ C.
should not be increased more than the required level to avoid the acid attack on other impurities during the process. Similar results for dissolution of spent nickel catalyst in sulphuric acid solution were obtained (Abdel-Aal and Rashad, 2004). The time required for nickel dissolution depends on the particle size, acid concentration and the nature of the adherent materials existing in the samples. The nickel extraction reached to maximum value after 30 min leaching using 8% (v/v) acid concentration. As shown in Fig. 2, any increase in leaching time up to 40 min was immaterial and the reaction reached equilibrium in 30 min. Therefore, 30 min was the optimum leaching time for nickel extraction and extended leaching times (>30 min) had no significant effects on nickel extraction.
3.2.
100 90 80
Dissolved fraction (α)
a
70 60 50 40 20 10 0 0
5
10
15
20
25
30
35
40
45
50
55
Reaction Time (minute) Fig. 4 – Effect of particle sizes on the nickel leaching (8% (v/v) acid, 25 ◦ C; 500 rpm, S/L = 0.10).
Effect solid/liquid ratios 3.3.
The effects of solid/liquid ratio were studied using 8% (v/v) sulphuric acid at 25 ◦ C with −250 m particle size. The results showed that nickel leaching recovery increased with decreasing of S/L ratio, and decreasing the pulp density was beneficial for nickel extraction (Fig. 3). Nickel extraction rose from 52.42% at S/L = 0.30–93% at S/L = 0.10 in 20 min leaching and leaching recovery increased to about 96% in 30 min dissolution at S/L = 0.10. When S/L was less than 0.10, leaching recovery increased slowly. In the other study for zinc extraction from sphalerite concentration, Aydogan et al. has stated that the zinc recovery increased with a decrease in the amount of solid (Aydogan et al., 2005). The results of metals extraction from nickel–cadmium spent batteries showed that the extraction of nickel increased with a decrease in the S/L ratio (Reddy and Priya, 2006). It is possible to improve leaching rate with decreasing pulp solid percent, but it has a limitation because of process economy. Considering the process performance the results indicate S/L = 0.10 as the optimum solid percent.
100 90
Dissolved fraction (α)
-850+425 micron -425+250 micron -250+75 micron -250 micron -75 micron
30
80 70
Effect of particle sizes
In leaching trials, a number of experiments were performed to determine the nickel extraction values at different particle sizes. Within the series of tests, the sulphuric acid concentration and leaching temperature were kept constant at 8% (v/v) and 25 ◦ C, respectively. The solid/liquid ratio was 0.10. These results showed that the nickel extraction was depended to sample particle size (Fig. 4). After 5 min of leaching time, 46.6–60.1% of nickel was extracted and after 30 min it rose to 84 and 98%, for particle size of −850 + 425 and −75 m respectively. Similar trends were observed for various particle sizes. The nickel extraction was achieved with particle sizes −75 m was about 15% higher than that was achieved with particle sizes in the range of −850 + 425 m. In general, the particle size of sample has an important influence on the rate of nickel dissolution. In the nickel extraction, the smaller particle size has the faster extraction rate and nickel extraction from spent catalyst increase with a decrease in the particle size (Mulak et al., 2005). The finer the particle size was, the greater geometric surface area and degree of contact surface between the acid solution and the particles were. In addition with a decrease in the particle size, the surface area per unit weight of sample increases and more nickel compounds liberated from the matrix due to an increase in the degree of size reduction more and more.
60
3.4.
50
Effect of temperature
40 0.05 0.10 0.15 0.20 0.30
30 20 10 0 0
5
10
15
20
25
30
35
40
45
Reaction Time (minute) Fig. 3 – Effect of solid to liquid ratio (S/L) on leaching recovery (8% (v/v) acid, 25 ◦ C; 500 rpm, particle size: −250 m).
50
55
Temperature plays an important role in the dissolution of nickel from zinc plant residue up to 20 min. On the conditions of 8% (v/v) sulphuric acid concentration and S/L = 0.10, the sample of −250 m size was leached at different temperatures (25–75 ◦ C). Data on the leaching rate of nickel are shown in Fig. 5. These results showed that the temperature had appreciable effect on the nickel extraction. The nickel leaching recovery increased from 52.43% at 25 ◦ C to 69.72% at 75 ◦ C. Nickel leaching recovery kept a little invariable between the temperature of
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100 90
Dissolved fraction (α)
80 70 60 50
25 C
40
35 C
30
45 C 55 C
20
65 C
10
75 C
0 0
5
10
15
20
25
30
35
40
45
50
Reaction Time (minute)
Fig. 5 – Effect of temperature on the nickel leaching (8% (v/v) acid, 500 rpm, S/L = 0.10). 25 ◦ C and 55 ◦ C. When leaching temperature increased to 75 ◦ C at 15 min dissolution time, leaching recovery of nickel rose to 96%. This increase was due to increase in reaction velocity constant K with the development of reaction temperature. In addition, similar results were obtained in the other investigations (Abdel-Aal and Rashad, 2004; Mulak et al., 2005; Nogueira and Margarido, 2004). As temperature increases, the time required to attain the maximum nickel extraction decreases. At the higher temperatures nickel dissolution increased at a certain leaching time, but by increasing the leaching time, the effects of temperature decreased.
3.5.
Effect of stirring speed
Leaching experiments to determine the effect of the stirring speed were conducted at 25 ◦ C using 8% (v/v) H2 SO4 . Nickel leaching recovery was 25.5% when no stirring was applied at 30 min, while it was 69.9%, 86.6%, and 93.3%, respectively, at 100, 200, 300 rpm stirring speed. The results have shown that leaching rate increased by increasing stirring speed up to 500 rpm and over that amount, the rate of agitation did not influence the nickel extraction (Fig. 6). This was indicating that at 500 rpm stirring speed, there was an adequate suspension of the solid particles in the solution; hence, an intermediate rate of 500 rpm was used in all experiments to assure independence of this variable and the chosen stirring rate removed film diffusion as a control mechanism.
4.
Kinetic analysis
80
For a liquid/solid reaction system, the reaction rate is generally controlled by diffusion through the liquid film, diffusion through the ash/product layer, the chemical reaction at the surface of the solid particles or a mixed of diffusion and chemical reactions (Gharabaghi et al., 2010; Levenspiel, 1998; Zafar, 2008). The shrinking core model considers that the leaching process is controlled by one of these steps. The reaction model between a liquid and a solid may be given as:
70
A(liquid) + B(solid) → Products
100 90 Dissolved fraction (α)
Fig. 7 – Plot of SCM with time at various temperatures (a) diffusion model, (b) chemical reaction model (8% acid concentration, S/L: 0.10, 500 rpm, particle size: −250 m).
60
In order to determine the kinetic parameters and rate controlling step in the nickel leaching in sulphuric acid solutions, the experimental data were analysed based on shrinking core model. The equations of the shrinking core model when either diffusion or the surface chemical reactions are the slowest step can be expressed as follows, respectively (Levenspiel, 1998):
100 rpm 200 rpm 300 rpm 400 rpm 500 rpm 600 rpm
50 40 30 20 10 0 0
5
10
15
20 25 30 35 40 Reaction Time (minute)
45
50
55
Fig. 6 – Effect of stirring speed on nickel extraction (acid concentration 8%(v/v), 25 ◦ C, 30 min, S/L = 0.1, particle size: −250 m).
1–3(1 − ˛)
2/3
[1 − (1 − ˛)
+ 2(1 − ˛) = kd t
1/3
]=
kMB DCA B ar02
t = kr t
(1) (2)
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Fig. 8 – Arrhenius plot for the dissolution process (8% acid concentration, 100 g/L solids, 500 rpm, particle size: −250 m). where ˛ = fraction reacted; k = kinetic constant; MB = molecular weight of solid; CA = acid concentration; a = stoichiometric coefficient of reagent in reaction; r0 = initial radius of particle; t = reaction time; B = density of the solid, D = diffusion coefficient in porous product layer and kd and kr = rate constants, which are calculated from Eqs. (1) to (2), respectively. In order to determine the main controlling step and the effects of temperature on the leaching kinetics, the experimental data in Fig. 5 were analysed based on Eqs. (1) and (2) and shown in Fig. 7aand b respectively. As shown in Fig. 7, there is a good correlation between the experimental data and Eq. (1), indicating that the leaching kinetics can be described by the SCM model with ash layer diffusion control (Aydogan et al., 2006; Habashi, 1999; Jun et al., 2010; Levenspiel, 1998). High value of activation energy indicates that the chemical reaction would be the rate controlling step and low activation energy indicates that the diffusion could be rate-controlling step (Abdel-Aal and Rashad, 2004; Levenspiel, 1998; Souza et al., 2007). Fig. 8 presents the Arrhenius plot is a straight line constructed with the ln k versus 1/T for the k values determined from Eq. (1) and its slope is (−Ea /R). The activation energy of 9.06 kJ/mol was calculated using the Arrhenius equation. This value of activation energy also proves that leaching process is controlled by diffusion as its activation energy is between 4 kJ/mol and 12 kJ/mol (Abdel-Aal, 2000; Aydogan et al., 2006; Habashi, 1999). The rate constants (kd ) for the various acid concentrations were obtained from the slope of straight lines and presented in Fig. 9a. Fig. 9b shows the plot of Log kd versus ln ([H2 SO4 ]) from which the reaction order with respect to the H2 SO4 concentration was determined as 1.36. The results for the effect of liquid to solid ratio on leaching kinetics were applied to the diffusion kinetics models and kd values for each S/L were determined. ln (kd ) versus ln (S/L) was plotted in Fig. 10a and the reaction order with respect to S/L was obtained as −1.12 with a correlation coefficient of 0.9581. From the variation in kinetics model with time at various particle sizes kd values were obtained. From the kd and particle size values, a plot of ln kd versus ln (r0 ) was plotted in Fig. 10b. The order of reaction with respect to particle size was found to be −0.22. As shown in Figs. 7–10, the main resistance to the process is the diffusion through the product layer, so a combination
Fig. 9 – (a) The variation in 1–3(1 − ˛)(2/3) + 2(1 − ˛) with time at different acid concentration (v/v); (b) reaction order with respect to acid concentration (S/L: 0.10, 500 rpm, particle size: −250 m.
Fig. 10 – (a) Determination of reaction order with respect to (S/L), (b) reaction order with respect to particle size (8% acid concentration (v/v), 500 rpm, 25 ◦ C).
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Table 2 – Analysis of the leaching residue after different leaching time. Leaching time (min) Nickel (%) Cd (%) Zn (%) Cu (%) Pb (%) Ca (%)
5
15
30
3.01 9.31 21.36 4.12 7.32 12.35
1.24 3.12 8.73 9.14 15.41 27.72
0.17 1.13 2.11 12.61 18.46 37.62
result showed that by increasing leaching time, Cu, Ca and Pb enriched markedly while Ni content decreased correspondingly. Fig. 11 – XRD pattern of the leaching residue after 30 min.
5.1. of the shrinking core rate expression and the effects of temperature, acid concentration, S/L and particle size on the rate constant gives the following expression for the leaching kinetics of nickel in sulphuric acid: 1 − 3(1 − ˛)
(2/3)
+ 2(1 − ˛)
= kd t = k0 [H2 SO4 ]1.36 (S/L)
5.
−1.12 −0.22 r0
exp
9.06 RT
t
Characterization of the leaching residues
The characterization of the leaching residue included X-ray and SEM/EDX analyses was performed after process. Chemical composition of the residues obtained at 8% (v/v) acid concentration, stirring speed of 500 rpm, solid–liquid ratio of 0.10 and 25 ◦ C after 5, 15 and 30 min leaching are shown in Table 2. The
XRD diffraction patterns after leaching
The leaching residue, obtained at 25 ◦ C after 30 min leaching by 8% (v/v) sulphuric acid was chosen for X-ray and SEM/EDX analyses. As can be seen in Fig. 11, gypsum, PbO2 , Cu2 O, Cu4 O3 and Fe2 O3 composed the residue. It can be concluded that by increasing leaching time, copper, calcium and lead compounds intensity increased and nickel compounds intensity decreased. Increasing percentage of these compounds in the leach residue was due to the sharp decrease in the nickel, cadmium and zinc compounds amounts. The surface morphology of sample after leaching was examined by SEM-equipped with EDX. Fig. 12a shows the effect of sulphuric acid attack and presents some pores that were probably occupied by acid soluble material before leaching process (Vegliò et al., 2001).
Fig. 12 – Particles of the samples after leaching (leaching conditions: 8% acid concentration (v/v), 25 ◦ C, 500 rpm, 100 g/L solids).
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The study of leach residue in Fig. 12b showed the increase in dissolution of nickel containing phase with increasing leaching time and appearance of significant amount of lead sulphate and gypsum which were indicated by EDX analysis. The SEM image of the leaching residues also showed a progressive reduction in particle diameter Fig. 12c and then an increase in roughness and porosity of the solids. After 30 min, leach residue became quite rich in lead sulphate and gypsum containing cavities, and was spongier in nature. Characterization studies showed that the nickel, zinc and cadmium containing phases progressively disappear while gypsum and lead containing phases predominates at later stages of leaching. Finally, the morphology of leach residue inferred that the sulphuric acid diffused through the pores of the sample and governed the leaching process through a diffusion-controlled mechanism.
6.
Conclusions
The dissolution kinetics of nickel content from zinc plant residue in sulphuric acid was investigated. It was found that the nickel extraction enhanced with the increase in sulphuric concentration, temperature and leaching time. The decrease in the particle size and solid to liquid ratio increased nickel dissolution. The extraction rate increased with increasing stirring speed and it was the highest at 500 rpm. The maximum leaching degree was 97% at 25 ◦ C after 30 min. The shrinking core model was applied to interpret the results and this model with ash layer control fitted the experimental data. The activation energy was 9.06 kJ/mol and it supported that the process was diffusion controlled. The samples before and after leaching were subjected to XRD and SEM-EDS analyses and their results confirmed the kinetics analysis results.
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