Article pubs.acs.org/IECR
Silica-Based 2‑Aminomethylpyridine Functionalized Adsorbent for Hydrometallurgical Extraction of Low-Grade Copper Ore Xin Li,*,†,‡ Baohua Li,† Shan Wu,†,‡ Jingbing Li,† Qitai Xu,† Zhiqiang Yang,§ Yuqiang Liu,§ Shaohua Wang,§ and Dongsheng Chen*,† †
Engineering and Technology Research Center for Inorgnic Type Ion-Exchange Resin, Henan University, Kaifeng 475004, People's Republic of China ‡ School of the Chemistry and Environment, Jiaying University, Meizhou 514015, People's Republic of China § Jinchuan Group Limited, Jinchang, 737100, People's Republic of China ABSTRACT: A silica-based 2-aminomethylpyridine functionalized adsorbent was synthesized to extract copper. Characterization of the adsorbent was carried out to determine the surface morphology, specific surface area, pore size, and grafting ratio of 2-aminomethylpyridine groups. The adsorption selectivity of the adsorbent for Cu(II) ions was 1353 times higher than that for Fe(III) ions. The adsorption capacity for Cu(II) ions maintained a stable value of 0.53 mmol/g with and without perturbation of Fe(III) ions. Density functional theory (DFT) calculations revealed a mechanism of selective adsorption of copper, through the formation of 2-aminomethylpyridine-Cu(II) complexes. Semi-industrial scale experiments revealed that Cu(II) ions in the acidic leachate of low-grade copper ore could be efficiently extracted by the adsorbent even in perturbation of substantial amounts of Fe(III) and other metal ions. Based on the adsorbent, a new process for copper hydrometallurgical extraction was developed, in which the adverse environmental impacts in traditional solvent extraction processes could be avoided.
1. INTRODUCTION Copper is a strategic metal ranking the second (after aluminum) among nonferrous metals in amounts of both production and consumption. Low-grade copper oxide and sulfide ores exhibit increasing importance owing to the depleted resources of high-grade copper sulfide.1−3 Compared with pyrometallurgical methods, the hydrometallurgical process is an efficient way in both technology and economy to recover copper from low-grade copper oxide and/or sulfide ores, which involves ore leaching (chemical leaching is usually used for copper oxide ore and bioleaching for copper sulfide ore), copper extraction from aqueous leachate, and electrowinning of copper.3 Hydroxyoximes,4−6 alkyl-8-hydroxyquinoline,7,8 and pyridinecarboxylic acid esters9 are often used to selectively extract Cu(II) ions. The organic extractants and solvents used in the solvent extraction process are typically toxic, flammable, and tend to cause environmental pollution problems. Therefore, there is an urgent need to develop an environmentally friendly process. An alternative to the solvent extraction process is to use a solid-phase polymer-based resin as an extraction medium. However, the leachate of low-grade copper oxide and/or sulfide ores often contain a substantial amount of ferric ions. As a result, the most widely used polymer-based resin for copper extraction (i.e., iminodiacetic acid functionalized cross-linked polystyrene) is easily fouled by Fe(III) ions and loses its ability to extract copper. Theoretically, the ferric ions can be previously removed by chemical precipitation10 or reduced to be ferrous ions by sodium sulfite11 so as not to perturb the extraction of Cu(II) ions. Nevertheless, Cu(II) ions are often coprecipitated in the process of chemical precipitation or reduced to be insoluble copper(I) salts in the process of chemical reduction. In such cases, Cu(II) ions are not extracted © 2012 American Chemical Society
with the iminodiacetic acid functionalized polystyrene resins. Therefore, it is of great importance to fabricate a novel copper selective adsorbent for the hydrometallurgical extraction of lowgrade copper oxide and/or sulfide ores. A few years ago, we noticed that two related N-heterocyclic bidentate chelating ligands (i.e., 2, 2′-bipyridine (Bipy) and 1, 10-phenanthroline (Phen)) can strongly bind Cu(II), Cu(I), or Fe(II) ions. Typical applications of the strong coordination are summarized as follows: Bipy or alkyl substituted Bipy were often introduced into atom transfer radical polymerization(ATRP)12−16 or reverse ATRP17,18 to coordinate the catalyst copper(I/II) halide so as to increase the solubility of copper salts in a hydrophobic polymerization system; Phen-Cu(I/II) complexes were widely used as a catalyst in many organic reactions;19,20 Phen-Fe(II)21 or Bipy-Fe(II)22 complexes were generally used for the photometric determination of Fe(II). On the other hand, Phen and Bipy do not bind Fe(III) ions stably. The decomposition mechanisms of Phen-Fe(III)23,24 and BipyFe(III)24,25 complexes have been proposed to interpret why Phen-Fe(III) and Bipy-Fe(III) complexes are easily decomposed. It is well-known that in aqueous solution the stable ion species are Fe(III) and Cu(II) rather than Fe(II) and Cu(I) ions. Based on these facts, it is envisaged that Bipy or Phen functionalized adsorbent can selectively extract Cu(II) ions without binding Fe(III) ions. As both Bipy and Phen have a large molecular size and are difficult to be grafted, a novel dinitrogen ligand, 2-aminomethylpyridine (AMPy) (see Scheme 1), was designed to Received: Revised: Accepted: Published: 15224
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Scheme 1. Molecular Structure for Bipy, Phen, and AMPy and the Reaction Pathway Yielding SB-AMPy Adsorbent
chelate Cu(II) ions in the present work. A silica-based AMPy functionalized adsorbent (abbreviated as SB-AMPy adsorbent) for selective adsorption of copper was fabricated by grafting 2chloromethylpyridine onto the silica-polyallylamine composites (SPC), as presented in Scheme 1. Characterization of SBAMPy adsorbent was performed. The copper adsorption capacity and selectivity of SB-AMPy adsorbent were investigated under different conditions, and the adsorption mechanism was proposed. Attempts were also made to evaluate its applicability in copper hydrometallurgical extraction by continuous column adsorption-regeneration runs on a semiindustrial scale.
with CPTMOS in toluene bath to produce chloropropylated silica. In this reaction, CPTMOS was hydrolyzed first by the hydrated water molecule monolayer; then, the hydrolyzed products are attached to the silanols on the silica surface through hydrogen bond, followed by Si−O formation with concomitant loss of water. After removing the unreacted CPTMOS, the chloropropylated silica was reacted with polyallylamine aqueous solution to yield SPC. N-Alkylation between chlorine of 2-chloromethylpyridine and primary amino groups on SPC was then carried out to produce the SB-AMPy adsorbent. A typical synthetic procedure was described as follows: 10 parts of 2-chloromethylpyridine, 10 parts of SPC, and 30 parts of methanol were placed into a reactor equipped with a stirrer to start the grafting reaction. The reaction was carried out at 65 °C for 20 h to yield the SB-AMPy adsorbent particles with an amber color. All the synthetic steps for the formation of the SB-AMPy adsorbent are presented in Scheme 1. 2.3. Characterization of SB-AMPy Adsorbent. The surface morphology of SB-AMPy adsorbent was determined with a HITACHI S3500N microscope operating at 20 kV. For sample preparation, the dried adsorbent particles were fixed on a microscope slide and then coated with a 3 nm thin gold layer. The specific surface area and the pore size measurements of the SB-AMPy adsorbent were carried out by a nitrogen adsorption and desorption method using a Micrometics ASAP-
2. EXPERIMENTAL SECTION 2.1. Materials. Raw silica particles were obtained from Qingdao Haiyang Chemicals Co. Ltd., China. γ-Chloropropyl trimethoxysilane (CPTMOS) was purchased from Nanjing Yudeheng Fine Chemicals Co. Ltd., China. Polyallylamine (MW 15 000, 15 wt % aqueous solution) was supplied by Nitto Boseki Co., Tokyo, Japan. 2-Chloromethylpyridine and solvents were provided by Shanghai Tongyuan Chemicals Co., China. Low-grade copper oxide ore was provided by Jinchuan Group Limited, China. All chemicals were used as received. 2.2. Preparation of SB-AMPy Adsorbent. Raw silica was pretreated with an amount of water that leads to approximately one monolayer. The resultant hydrated silica was then treated 15225
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2000 automatic surface area analysis instrument (Micromeritics Instrument, Norcros, U.S.A.). Thermal gravimetric analysis (TGA) was carried out on a Shimadzu DTG-60 instrument with a TA-60WS thermal analyzer operating at a heating rate of 10 °C/min under nitrogen atmosphere to determine the grafting ratio of functional groups. 2.4. Batch and Fixed-Bed Column Adsorption Experiments. Batch adsorption tests were conducted in 250 mL glass flasks. To start the experiments, a desired amount of SB-AMPy adsorbent particles were added to a 100 mL solution containing known metal ions. The flasks were placed in an incubator shaker under 200 rpm, for 4 h at 298 K, and the pH value of the solutions was determined to ensure equilibrium of the adsorption process. Fixed-bed column experiments were performed in a glass column (10 mm diameter and 150 mm length) with a bed volume of 10 mL. A Master Flex L/S 7720060 precision variable speed pump (Cole-Parmer Instrument Company, U.S.A. ) was used to generate a constant flow rate. The concentration of all metal ions was determined by atomic adsorption spectroscopy (Thermal Co., U.S.A.) before and after adsorption. 2.5. Computational Details. All DFT calculations in this work were carried out with a Gaussian 03 program package,26 and the unrestricted Becke3LYP (B3LYP) exchange correlation functional27,28 was used for optimizing geometries of AMPyCu(II) and AMPy-Fe(III) complexes. The 6-31G(d) basis set was used for C, N, H, and O atoms, and the Lanl2DZ29−31 for Cu and Fe atoms. Frequency calculations at the same level of theory have also been performed to identify all stationary points as minima (zero imaginary frequencies).
Table 1. Pore Structural Parameters for the SB-AMPy Adsorbent, SPC, and Raw Silica materials raw silica particles SPC particles SB-AMPy adsorbent
BET surface area (m2/g)
pore vol. (cm3/g)
BJH avg. pore diam. (nm)
339.3
1.2344
14.6
278.3 236.8
0.9502 0.7795
12.0 9.9
Figure 2. BJH differential pore size distribution of raw silica, SPC, and SB-AMPy adsorbent.
has a large surface area (236.8 m2/g) and high pore volume (0.7795 cm3/g), indicating that the textural properties of silica particles can afford adequate space to accommodate these functional groups. TGA was employed to further estimate the relative amount of PAA and AMPy functional groups anchored on the silica matrix. As shown in Figure 3, the weight losses for
3. RESULTS AND DISCUSSION 3.1. Characteristics of SB-AMPy Adsorbent. The SEM micrographs of the SB-AMPy adsorbent are shown in Figure 1.
Figure 1. SEM micrographs showing the surface morphology of the SB-AMPy adsorbent.
The SB-AMPy adsorbent particles have a spherical shape with a rough surface having apparent micropores on it. The large surface area and abundant micropores are favorable for the grafting of AMPy functional groups. The surface area was determined by a Brunauer−Emmett− Teller (BET) method,32 and the pore size distribution was obtained from nitrogen adsorption−desorption measurements according to the Barrett−Joyner−Halenda (BJH) method.33 All pore structural parameters of SB-AMPy adsorbent, SPC, and raw silica particles were compared in Table 1 and Figure 2. Table 1 revealed that there is a significant decrease in surface area, pore volume, and average pore size after loading of polyallylamine and further grafting of methyl-pyridine groups. Furthermore, the pore size distribution shifted to a lower range after each step of modification, as shown in Figure 2. It is also seen that even SB-AMPy adsorbent (the product after grafting of chloropropyl, polyallylamine, and methyl-pyridine groups)
Figure 3. TGA analysis of (a) chloropropylated silica, (b) SPC, and (c) SB-AMPy adsorbent.
chloropropylated silica, SPC, and SB-AMPy adsorbent are ca. 16.0%, 26.0%, and 41.0%, respectively. Clearly, the difference of weight loss between the SPC and chloropropylated silica (26.0−16.0%) represents the amount of grafted PAA, and the difference of weight loss between the SPC and SB-AMPy adsorbent (41.0−26.0%) gives the amount of grafted 2methylenepyridine. As the repeating unit of PAA is −CH2CHCH2NH2− (molar mass 57.1), the grafting density of −CH2CHCH2NH2− groups on SPC was estimated to be 1.75 mmol/g from the amount of grafted PAA. Likewise, the grafting density of 2-methylenepyridine on the resultant SB-AMPy adsorbent was estimated to be 15226
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1.63 mmol/g from the amount of grafted 2-methylenepyridine (molar mass 92.1). Because each 2-methylenepyridine group can just constitute one AMPy functional group, the grafting density of AMPy groups on the exterior and interior surface of SB-AMPy adsorbent is calculated to be 1.63 mmol/g, which is slightly lower than that of −CH2CHCH2NH2− groups. From the grafting density of AMPy groups and chelating mode of AMPy-Cu(II), the theoretical copper adsorption capacity may be estimated. 3.2. Copper Adsorption Performances of SB-AMPy Adsorbent and Selective Mechanisms. Figure 4 depicted
ln qe = ln kF +
1 ln ce n
(1)
Adsorption isotherms of Cu2+ onto SB-AMPy adsorbent at different temperatures are represented by the Freundlich equation (eq 1), where qe is the equilibrium adsorption capacity, ce represents the solute concentration at equilibrium, and kF and n are the Freundlich constants to be determined. The Freundlich model was found to represent all the Cu2+ adsorption isotherms reasonably well, as indicated by its high relative coefficient values (larger than 0.99, as listed in Table 2). Table 2. Freundlich Isotherm Parameters of Cu2+ onto SBAMPy Adsorbent at Different Temperatures T (K)
n
R2
298 318 338
2.3804 2.2178 2.1299
0.9982 0.9972 0.9980
Figure 6 presents the plot of Cu2+ batch adsorption capacity versus contact time for SB-AMPy adsorbent with an initial Cu2+
Figure 4. Effect of pH on the equilibrium adsorption capacity.
the effect of pH on copper adsorption capacity from a batch equilibrium experiment at 298 K, with an initial copper concentration of 1500 mg/L in aqueous solution. At lower pH values, an increase in pH resulted in a rise in copper adsorption capacity. A maximum capacity of 0.70 mmol/g was observed at pH of 4.5−5.0. When pH exceeded 4.5, although a slight increase in copper adsorption capacity was observed, precipitation of Cu(OH)2 may occur. Thus, a pH below 4.5 is recommended for adsorption operation. Figure 5 shows the equilibrium adsorption isotherms of SBAMPy adsorbent at different temperatures with an experimental
Figure 6. Adsorption kinetics curve of SB-AMPy adsorbent at 298 K and pH 4.5 with an initial Cu2+ concentration of 1500 mg/L.
concentration of 1500 mg/L. An equilibrium load capacity of 0.70 mmol/g was observed at ca. 120 min, indicating a sufficiently rapid adsorption kinetics. In general, the adsorption process can be interpreted by a series of steps: (a) mass transfer of solute from the liquid to the particle surface across the boundary layer, (b) diffusion of solute within the pores and deposition on the surface of the particles, and (c) chemical reaction of the solute with the functional groups attached to the matrix.34,35 One of the steps usually exhibits much greater resistance than the other two, and this step is considered as the rate-limiting step of the adsorption process. To determine which step (film diffusion, intraparticle diffusion or chemical reaction) played a dominant role and to obtain the corresponding rate constants, the kinetic data presented in Figure 6 were further processed. The following three models, each assuming a different rate-limiting step in the adsorption process, were used to fit the experimental data:36
Figure 5. Equilibrium adsorption isotherms of Cu2+ onto the SBAMPy adsorbent at different temperatures.
pH of 4.5. The adsorption capacity increased as the adsorption temperature increased. However, the difference of the adsorption capacity determined at 298, 318, and 338 K was not significant; thus, it is feasible to select room temperature for the adsorption operation. 15227
Film diffusion model: ln(1 − F ) = −k f t
(2)
Intraparticle diffusion model: qt = k pt 1/2
(3)
Chemical reaction model: 1 − (1 − F )1/3 = krt
(4)
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where kf, kp, and kr are the rate constants for film diffusion, intraparticle diffusion, and chemical reaction, respectively; and F (qt/qe) is the fractional attainment of the equilibrium. The fitting results with eqs 2−4 are presented in Figure 7. Clearly,
Figure 8. Comparison of adsorption breakthrough curves at different feeding flow rates under the following conditions: 298 K, pH 2.0, and an initial copper concentration of 1500 mg/L.
a higher flow rate will speed up the velocity of treatment, it will also involve two adverse factors: (a) the contact time between SB-AMPy adsorbent and feed solution reduces, thus leading to a lower adsorption capacity; (b) SB-AMPy adsorbent has to be subjected to a higher hydrodynamic pressure which might cause fracture and breakage of adsorbent particles. Figure 9 shows the
Figure 9. Effect of feeding flow rate on the breakthrough adsorption capacity of Cu(II) ions at 298 K and pH 2.0.
breakthrough adsorption capacity drops significantly when the flow rate is above 6 BV/h. Therefore, a flow rate of 6 BV/h is recommended for a column operation. The ratio of the batch equilibrium capacity (0.55 mmol/g) to the breakthrough adsorption capacity (0.53 mmol/g) is ca. 1.04 at pH 2.0. Batch experiments in the perturbation of Fe(III), Al(III), Co(II), Mn(II), and Mg(II) ions were performed to test the adsorption selectivity for Cu(II) ion. The distribution coefficient Kd was determined to quantify the selectivity of the adsorbent, where Kd is defined as the amount of adsorbed metal ion (in micrograms) on 1 g of adsorbent divided by the concentration of metal ion (in micrograms per milliliter) remaining in the treated solution. Table 3 listed the Kd values of six metal ions on SB-AMPy adsorbent obtained at 298 K and the mixed solutions with an initial concentration of 1500 mg/L for all metal ions. At pH 2.0, the Kd value for Cu(II) ion is 1353 times higher than that for Fe(III) ion, and the difference of Kd values between Cu(II) and the other metal ions is more significant. It is obvious that the SB-AMPy adsorbent has an overwhelming adsorption selectivity for Cu(II) ion than for Fe(III), Al(III), Co(II), Mn(II), and Mg(II) ions.
Figure 7. Plots of adsorption kinetics curve at 298 K fitting with (a) film diffusion model, (b) intraparticle diffusion model, and (c) chemical reaction model.
the film diffusion model leads to much better regression coefficients (0.9945) than the other two models, indicating that the adsorption process is limited by film diffusion. Fixed-bed column experiments of SB-AMPy adsorbent were further performed to determine the dynamic adsorption capacity in different feeding flow rates of 2, 4, 6, 8, 10, and 12 Bed Volume/h (BV/h), respectively. Herein, it is defined that a breakthrough occurred when the copper remaining in effluent after adsorption exceeds 1.0 mg/L. The adsorption results at 298 K, pH 2.0, and an initial copper concentration of 1500 mg/L, as shown in Figure 8, revealed that the breakthrough occurred earlier in a higher flow rate. Although 15228
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respectively (see Figure 11A). Obviously, at a pH equal to or lower than 2.0, Fe(III) ions should exist in Fe(H2O)63+, indicated by a positive ΔG for acid dissociation of Fe(H2O)63+, whereas Cu(II) ions still in bare ions, indicated by a negative ΔG for acid dissociation of Cu(H2O)42+. When contacting AMPy ligands, the bare Cu(II) ions will prefer to form AMPyCu(II) square-planar complexes. The negative free energy change (ΔG = −256.1 kcal/mol) value for the formation of AMPy-Cu(II) complexes suggests a strongly spontaneous process. However, Fe(H2O)63+ are not likely to form AMPyFe(III) octahedral complexes under the same condition because the process is thermodynamically unfavorable (ΔG = 10.3 kcal/mol), as shown in Figure 11B. During the formation of AMPy-metal ion complex, the grafted AMPy groups should have enough activity and freedom to coordinate with the metal ion because they are bound to the flexible polyallylamine chain. Due to a high grafting density of AMPy, once an Cu(II) ion associates with an AMPy ligand, it should be rapidly and tightly incorporated into another AMPy ligand to form a stable tetracoordinated, square-planar complex on the adsorbent matrix. However, hexacoordinated octahedral AMPy-Fe(III) complex is difficult to form, since the octahedral space is too crowded to accommodate three AMPy ligands and three corresponding polyallylamine chains. This is another reasonable explanation for the high copper adsorption capacity and selectivity of SB-AMPy adsorbent even at a high concentration of Fe(III) ions. The influence of steric factor on the formation of AMPyCu(II) complexes can not be ignored. An estimated grafting density of 1.63 mmol/g for AMPy groups by TGA suggests a theoretical copper adsorption capacity of 0.81 mmol/g according to the complex ratio of 2:1 between AMPy and Cu(II) ions. However, the experimental copper capacity shown in Figures 4 and 6 is no more than 0.70 mmol/g. This implies that some grafted AMPy groups have not contributed to the effective adsorption. 3.3. Comparison of the Characteristics and Adsorption Performances between SB-AMPy Adsorbent and Amberlite IRC-718. A comparison of the physical and chemical characteristics of SB-AMPy adsorbent and Amberlite IRC-718 (a typical iminodiacetic acid functionalized cross-
Table 3. Distribution Coefficients (Kd) of Six Metals on SBAMPy Adsorbent at 298 K and Different pHa Kd of metal ions (mL/g) pH of solution
Cu(II)
Co(II)
Mn(II)
Mg(II)
Fe(III)
Al(III)
2.0 3.0 4.0
541.3 603.4. 696.8.
0.3 0.47 0.62
0.3 0.41 0.59
0.001 0.001 0.002
0.4 NA NA
0.02 0.05 NA
a Some Kd values are not available (NA) because substantial Fe(III) or Al(III) ions have precipitated at that pH value.
Because detailed structural information of AMPy ligands and metal complex can’t be easily obtained from commonly available spectroscopic techniques, herein DFT calculations were used to better understand the adsorption interaction of SB-AMPy adsorbent and metal ions, especially the formation of AMPy-Cu(II) and AMPy-Fe(III) complexes. Since Fe(III) ions are easily precipitated to Fe(OH)3 precipitates at a pH above 2.0, DFT calculations were carried out at a specified pH of 2.0. The results of DFT calculations, as shown in Figure 10, reveal
Figure 10. Optimized geometries with selected structural parameters (bond lengths in Angstroms) for AMPy-Cu(II) and AMPy-Fe(III) complexes.
that the optimized structure of AMPy-Cu(II) complex should be a tetracoordinated square-planar geometry, whereas AMPyFe(III) complex a hexacoordinated octahedral geometry. Further DFT calculations give the free energy change at pH 2.0 for the hydration process of Cu(II) and Fe(III) ions,
Figure 11. Gibbs free energy and electronic energy change calculated by DFT for the coordinating of Cu(II) and Fe(III). (A) Acid dissociation process of the hydrated Cu(II) and Fe(III) ion. (B) AMPy-Cu(II) and AMPy-Fe(III) complex formation. 15229
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and flow tests. IRC-718, however, suffered from a significant loss in capacity, which decreased to a very low level of 0.11 and 0.05 mmol/g in the batch and flow tests, respectively. In this case, IRC-718 may be regarded as virtually worthless in extracting Cu(II) ions. An interpretation on iron fouling of iminodiacetic acid functionalized resins is given as follows. The electrostatic attraction between −CH2COO− and Cu(II) ions acts as a major driving force for copper extraction. It is known that the electrostatic attraction between −CH2COO− and Fe(III) ions are much stronger than that between −CH2COO− and Cu(II) ions or other metal ions. Accordingly, at a pH lower than 2.0, when Cu(II) ions are coexisting with Fe(III) ions, iminodiacetic acid functionalized resins will always prefer to extract Fe(III) ions rather than Cu(II) ions. Because the bound Fe(III) ions are difficult to be striped, the resins can no longer extract Cu(II) ions or other metal ions. At a pH above 2.0, however, precipitation of Fe(OH)3 is inevitable owing to the very low solubility product of Fe(OH)3. Clearly, both irreversible adsorption of Fe(III) ions and precipitation of Fe(OH)3 within resin particles will cause deterioration in copper adsorption performance. 3.4. Semi-Industrial Scale Column Experiments in Cu Hydrometallurgical Extraction of Low-Grade Copper Oxide Ore. An environmental-friendly process for copper hydrometallurgical extraction was designed based on the SBAMPy adsorbent and then applied to semi-industrial scale experiments, in which the adverse factors appearing in solvent extraction process could be avoided. In the experiments, lowgrade copper oxide ore was used, as its leaching process was simpler and faster than that of copper sulfide ore. The leaching was achieved with sulfuric acid at atmospheric pressure, and then, the pH of leachate was controlled to be 2.0. After careful filtration, the acidic leachate was fed into the adsorption columns at a flow rate of 6 BV/h. The semi-industrial scale column operation (Figure 12) was organized as follows: A set
linked polystyrene resin, which is commercially available from Rohm and Haas) is provided in Table 4. As it can be seen, the Table 4. Comparison of the Characteristics of SB-AMPy Adsorbent and Amberlite IRC-718 matrix functional group physical form particle size (μm) density (g/mL) ionic form max. reversible swelling a
SB-AMPy adsorbent
Amberlite IRC-718
silica−polyallylamine composites 2-aminomethylpyridine amber particles 150−500 0.70 free base free base→Cl−: zero
styrene/divinylbenzene copolymer iminodiacetic acid opaque beige beads 300−1180a 0.68a Na+ a H+→Na+: 15%
Provided by manufacturer.
two adsorption materials are very different in all characteristics. Interestingly, the SB-AMPy adsorbents never swell during repeated adsorption−regeneration cycles, whereas IRC-718 are subjected to a significant reversible swelling with a maximum volume change of 15%. The shrink−swell property of the polymers leads to a short lifetime and requires a dead volume in designing the exchange column, thus limiting their application in packed column systems. In addition, the mechanisms of copper adsorption for the SB-AMPy adsorbent and IRC-718 are also quite different. Cu(II) ions are extracted through forming a four-coordinated complex on SB-AMPy adsorbent. In the case of IRC-718, however, the Cu(II) ions are likely bonded in a tridentate fashion through the amine and both carboxylic acid groups of the iminodiacetic acid ligands, and eventually, each Cu(II) ion may form a six-coordinated complex with two iminodiacetic acid ligands. Batch and flow experiments were also conducted to investigate the difference of the copper adsorption capacity between SB-AMPy and IRC-718. Table 5 revealed that, under Table 5. Copper Adsorption Capacities (mmol/g) of SBAMPy and IRC-718 in Batch and Flow Test with and without Perturbation of Fe(III) Ionsa without perturbation of Fe(III)
with perturbation of Fe(III)
adsorption materials
batch testb
flow testc
batch testb
flow testc
SB-AMPy IRC-718
0.55 0.20
0.53 0.15
0.54 0.11
0.53 0.05
a
The initial concentration of both Cu(II) and Fe(III) are 1500 mg/L. Determined experimentally at 298 K and pH 2.0. cDetermined at at 298 K and pH 2.0 with a flow rate of 6 BV/h. b
Figure 12. Semi-industrial scale columns for Cu hydrometallurgical extraction of low-grade copper ore.
the experimental conditions of 298 K and pH 2.0, the adsorption capacity of Cu(II) ions on SB-AMPy adsorbent maintained a value of above 0.53 mmol/g, which is about 3 times as much as that on IRC-718 in both the batch and flow tests. Apparently, the SB-AMPy adsorbent performed much better than IRC-718 for the copper extraction in such a low pH value. Under the same adsorption temperature and pH, further experiments in the existence of Fe(III) ions were performed to compare the copper adsorption capacity of SB-AMPy and IRC718. The results indicated that with the perturbation of Fe(III) ions, SB-AMPy underwent negligible drop in the capacity, maintaining a stable value of 0.53 mmol/g in both the batch
of three packed columns provide a continuous adsorption− regeneration operation. Two of the columns are connected in series, and the third column serves as a spare. The hydrometallurgical extraction of copper takes place in the two connected columns while the other one is being purged/ regenerated or otherwise for repair or routine maintenance. All treatment streams are introduced in an upflow mode (i.e., the solution was continuously pumped in from the bottom of each column and discharged after treatment from the top of the same column) so that the streams can be in adequate and 15230
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concentration. The decline in copper adsorption selectivity and operating capacity can almost be ignored after 500 continuous adsorption-regeneration runs. More than 96.5% Cu(II) ions in the acidic leachate can be extracted. After extraction of Cu(II) ions, iron and other metal ions leaving in the effluent may be recovered by chemical precipitation or other methods. It was reported37 that high-quality crystalline copper could be obtained in an electrowinning process from an electrolyte containing 17−37g/L of Cu(II) ions, 1−2 g/L of Fe(III) ions, and 152 g/L of H2SO4, with an operating temperature of 30− 50 °C and current density of 300 A/m2. It was further demonstrated that a higher concentration of Cu(II) ions and a lower concentration of Fe(III) ions would be favorable for obtaining high-quality cathode copper. Clearly, the strip solution from the semi-industrial scale column experiments contains adequately pure Cu(II) ions (38.8 g/L of Cu(II) ions and only 0.76 g/L of Fe(III) ions, as shown in Table 6), which is well-suited for the final copper electrowinning. The results suggest a potential application of SB-AMPy adsorbent for copper extraction from low-grade copper ore.
intimate contact with SB-AMPy adsorbent. In the operation, each column is packed with 100 L SB-AMPy adsorbent. Figure 13 shows that Cu(II) ions can be efficiently extracted within about 28 BV per run before a significant breakthrough
Figure 13. Adsorption breakthrough curves of Cu(II) and Fe(III) ions of the acidic leachate in the semi-industrial scale column experiments.
4. CONCLUSIONS
occurred. However, the concentration of Fe(III) ions reach a very high level even in the initial effluent, indicating a poor extraction of Fe(III) ions on SB-AMPy adsorbent. The breakthrough curves of Al(III), Co(II), Mn(II), and Mg(II) ions are not presented in Figure 13 because these ions can hardly be adsorbed by SB-AMPy adsorbent; and their adverse effects on the copper capacity are negligible compared with those of Fe(III) ions. Upon completion of the fixed-bed adsorption, the exhausted adsorbents were regenerated by first rinsing with 5 BV H2SO4 solution of pH 2.0 for a chase of the residual background ions (rinse solution was produced in this process), followed by the stripping step of the bound Cu(II) ions with H2SO4 solution of 150 g/L (strip solution was produced in the step), and then washing the adsorbents to pH 2.0 with water for reuse. It is also seen from Table 6 that the concentration of Cu(II) ions reached a very high level in strip solution (38 824 mg/L) from a low level in feed solution (835 mg/L); meanwhile, the concentration of Fe(III) ions was significantly reduced from 20 372 mg/L to 761 mg/L, and the coexisting Co(II), Mn(II), Mg(II), and Al(III) ions were also reduced to a very low
It is a challenging task to develop an environmental-friendly process for copper extraction from low-grade copper oxide and sulfide ores containing substantial amounts of iron. To address this challenge, a copper selective SB-AMPy adsorbent was designed and fabricated. The characterization of the adsorbent was performed and the adsorption performance for copper was investigated in lab and pilot scales. The results are highlighted as follows. (1) A porous spherical morphology of the adsorbent was observed by SEM. A surface area of 236.8 m2/g and a pore volume of 0.7795 cm3/g were determined by BET method and BJH method, respectively. The grafted AMPy for chelating copper on the exterior and interior surface of SB-AMPy adsorbent were determined to be 1.63 mmol/g by TGA. (2) A maximum capacity of 0.70 mmol/g and 0.53 mmol/g were obtained in a batch test and a flow test, respectively. A stable capacity of 0.53 mmol/g in flow test was observed with and without perturbation of Fe(III) ions, which is much more efficient than that of iminodiacetic acid functionalized cross-linked polystyrene resin. Experiments indicated that adsorption selectivity of SB-AMPy adsorbent for Cu(II) ions is 1353 times higher than for Fe(III) ions. DFT calculations revealed a mechanism of copper selective adsorption through the formation of complexes AMPy-Cu(II) with a complex ratio of 2:1 (AMPy to Cu(II) ion). (3) Based on the SB-AMPy adsorbent, a novel process for hydrometallurgical copper extraction was designed and applied in a semi-industrial scale experiment to process the acidic leachate of low-grade copper oxide ore. More than 96.5% Cu(II) ions in acidic leachate can be extracted into a copper(II) solution of high purity, which contains 38.8 g/L of Cu(II) ions, only 0.76 g/L of Fe(III) ions and very slight amount of other metal ions. In the new process, the adverse environmental impacts appearing in the traditional solvent extraction process could be avoided.
Table 6. Concentration Comparisons of Each Metal Ion in Feed, Effluent, Rinse, and Strip Solution During an Adsorption−Regeneration Run of Semi-Industrial Scale Column Experimentsa concn. metal ions (mg/L)b
solution type
vol. (L)
Cu(II)
Co(II)
Mn(II)
Mg(II)
Al(III)
Fe(III)
feed effluent rinse strip
2800 2800 500 58.5
835 0.3 130 38 824
4.7 4.3 2.1 0.1
54.8 53.7 5.8 1.0
48 100 48 060 195 1.5
40.4 39.5 4.8 0.5
20 372 18 755 8945 761
a
Material balance of each metal ion when breakthrough occurred (28 BV) may be carried out with the following equation: Vf Cf = VeCe + VrCr + VsCs where Vf, Ve, Vr, and Vs represent the volume of feed, effluent, rinse, and strip solution, respectively; Cf, Ce, Cr, and Cs are the concentration of each ion in the solution of feed, effluent, rinse and strip, respectively. bAverage ion concentrations for all types of solution. 15231
dx.doi.org/10.1021/ie301852r | Ind. Eng. Chem. Res. 2012, 51, 15224−15232
Industrial & Engineering Chemistry Research
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Article
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support from Jinchuan Group Limited, China, and the Education Department of Henan Province, China. The authors thank Dr. Yang Weiben from Nanjing Normal University for his help with the measurements of pore structural properties.
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