Factors affecting crystallization of copper sulfide in fed-batch fluidized bed reactor

Hydrometallurgy 152 (2015) 107–112

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Factors affecting crystallization of copper sulfide in fed-batch fluidized bed reactor Jaeshik Chung a,b,1, Eunhoo Jeong a,c,1, Jae Woo Choi a, Seong Taek Yun c, Sung Kyu Maeng d, Seok Won Hong a,c,⁎ a

Center for Water Resource Cycle Research, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea Department of Environmental Engineering Sciences, University of Florida, P.O. Box 116450, Gainesville, FL 32611-6450, USA Department of Earth and Environmental Sciences, KU-KIST Green School, Korea University, Seoul 136-701, Republic of Korea d Department of Civil and Environmental Engineering, Sejong University, 98 Gunja-Dong, Gwangjin-Gu, Seoul 143-747, Republic of Korea b c

a r t i c l e

i n f o

Article history: Received 9 June 2014 Received in revised form 19 November 2014 Accepted 22 December 2014 Available online 26 December 2014 Keywords: Calcium-coated sand Copper sulfide Crystallization Fed-batch fluidized bed reactor Sodium sulfide pentahydrate

a b s t r a c t This work has assessed factors affecting crystallization of copper sulfide (CuS) under batch conditions and in fed-batch fluidized bed reactor (FBR) using calcium-coated sands as a seed material. Compared with using sodium sulfide nonahydrate as a sulfidation reagent, larger CuS crystals were produced from sodium sulfide pentahydrate, most likely because of the lower surface charge. Due to the partial oxidation of sulfide in the pentahydrate form, the optimum molar ratio of Cu2+ to S2− was found to be 1:2, higher than the theoretical ratio. Crystal growth and aggregation were further increased by using calcium ion as a cross-linker among the CuS fines (b100 nm), resulting in an additional 30% decrease in Cu2+ concentrations from the effluent of fed-batch FBR. The efficacy of CuS crystallization was also remarkably dependent on the operating factors, i.e., the resting height of the seed material and recirculation rate. Under optimal conditions, N 95% of the initial Cu2+ (100 mg Cu2+/L) was successfully transformed to CuS crystals within 120 min. The size and crystallinity of the star-shaped CuS crystals were confirmed by FEG– SEM and XRD analyses, respectively. Principal component analysis indicated that the resting height and the seed type were the primary parameters affecting CuS crystallization in the fed-batch FBR. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Among the common heavy metals, free ionic copper is highly toxic to human and ecosystem health. A direct relation between the concentration of free copper ion and the biological effects has been demonstrated for microalgae and phytoplankton (Sunda and Guillard, 1976; Winner and Gauss, 1986). In 1993, the World Health Organization set a provisional guideline of 2 mg/L for maximum copper content in drinking water. Thus, copper pollution and its removal from water and wastewater have received a great deal of attention during the last several decades. In wastewater, copper is present over a wide concentration range varying from sub-ppb to ppm. For example, copper concentrations in chemical–mechanical polishing wastewater from semiconductor fabrication processing were found to be in the range 45–120 mg/L (Lai and Lin, 2003). Among separation and refinement technologies of metals, crystallization has been technologically advanced from simple ion removal to value adding process, e.g., industrial manufacturing of salt and sugar,

⁎ Corresponding author at: Center for Water Resource Cycle Research, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea. E-mail address: [email protected] (S.W. Hong). 1 These authors contributed equally to this work.

http://dx.doi.org/10.1016/j.hydromet.2014.12.014 0304-386X/© 2014 Elsevier B.V. All rights reserved.

semiconductors, and pharmaceuticals (Demopoulos, 2009). Sulfidation, one of the crystallization reactions, is a well-known process for metal fixation associated with the biogeochemical sulfur cycle (Widdel et al., 1992). Lewis (2010) noted the differences in reaction characteristics of various metals, suggesting the importance of metal sulfidation in hydrometallurgical treatment of ores and effluents. Recently, sulfidation of heavy metals such as copper, zinc, and lead was conducted in fluidized bed reactors (FBRs) (Lee et al., 2004; Lee and Yang, 2005; van Hille et al., 2005; Mokone et al., 2012). Most of these metals were successfully recovered in FBRs under supersaturated conditions. However, Mokone et al. (2012) reported that copper is more difficult to crystallize via sulfidation than other metals (i.e., zinc and lead) because of the small size of CuS particles (ca. 100 nm) and highly charged surfaces (ca. −50 mV). Luther et al. (1996) also pointed out that recovery of CuS is challenging, because copper tends to be reduced under sulfidic conditions, resulting in the formation of bisulfide, sulfide, and polysulfide complexes. In this study, the methods for improving CuS crystallization yields by employing calcium (Ca)-coated sand as a seed material and operating the FBR in a fed-batch mode were employed. Several batch experiments were performed to investigate the effects of the extent of hydration on copper sulfidation using Na2S·5H2O and Na2S·9H2O at various pH values. Other operating parameters of the fed-batch FBR such as Cu2+ to S2− molar ratios, types of seed, seed height, and recirculation rate

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with respect to residual copper concentration in the supernatant were then optimized. Finally, the contribution of each factor to the overall crystallization process using a statistical method was also evaluated. 2. Materials and methods 2.1. Chemicals All the reagents (CuSO4·5H2O, Na2S·5H2O, Na2S·9H2O, and CaCl2) used in this study were analytical grade and used as received from Sigma-Aldrich Korea, Ltd. (Yongin, Korea). All solutions were freshly prepared with deionized (DI) water (N18 MΩ cm; Millipore, Billerica, MA, USA) before each experiment. Due to the hygroscopic nature of the salts, Na2S·5H2O and Na2S·9H2O were used after oven-drying for 30 min at 60 °C. The solution pH was adjusted to the desired value using 0.1 M HCl or 0.1 M NaOH. 2.2. Selection of crystallizing reagent 2.2.1. Speciation of sulfur in two different hydration states Changes in total dissolved sulfide (S2− + HS− + H2S (aq)) and sulfate (SO24 −) concentrations in two different sulfidation reagents, Na 2S·5H2 O and Na 2 S·9H2 O with time over 1 h were measured using a sulfide ion-selective electrode (Orion 94-16; Orion Research, Inc., Cambridge, MA, USA) and an ion chromatography (Dionex, Sunnyvale, CA, USA), respectively. All experiments were conducted in triplicate at room temperature (25 ± 2 °C).

2.4. Optimization of operating conditions for fed-batch fluidized bed reactor 2.4.1. Preparation of seeds Various materials, including fine, coarse, and Ca-coated sands, were introduced as seeds into the fed-batch FBR. Joomoonjin sand, widely used as filtration sand in Korea, was employed as a seed. The sand was separated into two size classes, fine (passing No. 6 sieve) and coarse (retained on No. 6 sieve). The particle size of the fine and coarse sand was less than 1 mm and 1–5 mm, respectively. Prior to use, all sands were washed with 0.1 M HCl to remove impurities from the surface. The Ca-coated sand was prepared by placing the both types of sand in 1 M CaCl2 solution overnight with stirring. After washing thoroughly with DI water, the sands were dried at 60 °C for 12 h and held in a desiccator until use. 2.4.2. Fed-batch fluidized bed reactor operation A fed-batch FBR with an inner diameter and height of 20 and 200 cm, respectively, was constructed using acrylic plastic (Fig. S1). Four different types of seeds, i.e., fine, coarse, and Ca-coated fine and coarse sands, were placed in the FBR at various depths of 10, 20, 30 and 40 cm. In the FBR, the solutions were recirculated from the top to the bottom at various flow rates of 25, 125, 250, and 470 mL/min. Throughout the experiments, 600 mL of 1.5 mM CuSO4 (ca. 100 mg Cu2+/L) and 60 mL of Na2S solutions were fed to the reactor bottom at the beginning of each run, using various molar ratios of Cu2+ to S2− of 1:0.5, 1:1, 1:2, and 1:5. The efficacy of CuS crystallization for both the batch and fed-batch FBR experiments was calculated according to the following equations, modified from Mokone et al. (2012);

2.2.2. Surface charge and size distribution of CuS crystals The surface charge of CuS at various pH values was evaluated by conducting a batch equilibrium experiment: 4 mL of 30 mM Na2S solution was added to 40 mL of 1.5 mM CuSO4 solution and the initial pH (pHi) of the solution was adjusted from 1.0 to 6.0 with 0.1 M HCl or 0.1 M NaOH. The values of pH higher than 6 were not considered because this would result in the formation of insoluble Cu(OH)2 precipitate. The suspensions were sonicated at 20 °C for 5 min and then the zeta potential of each sample in the equilibrated solution was measured using a zeta potential analyzer (Zetasizer Nano ZS90; Malvern Instruments, Malvern, United Kingdom). To determine the effects of Ca2 + and various types of seed on surface charge, samples varying in these respects were prepared using the same procedure.

where Cuin is the initial copper concentration and the concentration in the influent in the batch and fed-batch experiments; Cuout,filtered is the copper concentration in the effluent filtered through 0.45-μm filters; and Cuout,total is the copper concentration in the effluent without filtration.

2.3. Batch kinetics for CuS crystallization

3. Results and discussion

Batch kinetic experiments were carried out to determine the reaction kinetics of CuS crystallization. Forty milliliters of 30 mM Na2S·5H2O solution were added to 400 mL of 1.5 mM CuSO4 solution. The mixture was then agitated at 150 rpm using a horizontal shaker. An aliquot of the sample was taken from the bottle and filtered through a 0.45-μm pore size cellulose acetate membrane filter (Whatman, Germany) prior to conducting ICP-OES (Vista PRO; Varian Inc., Palo Alto, CA, USA) analysis for total copper (i.e., the sum of Cu2+ and CuS presented as fines) and calcium. Another set of batch experiments was performed to verify the effect of Ca on the crystal growth of CuS. These samples were prepared following the same procedures described above with the addition of 1 mL of 1000 mg Ca2+/L CaCl2 solution. The X-ray diffraction (XRD) patterns for the recovered copper sulfide were recorded using a D/max-2500 X-ray diffractometer (Rigaku Corp., Tokyo, Japan) using Cu Kα radiation (λ = 1.5406 Å) operated at 40 kV and 100 mA with a scan speed of 2° min− 1 over 10–90°. The morphologies and compositions of the selected samples were also examined using a field emission gun–scanning electron microscopy (FEG–SEM, Inspect F50; FEI, Eindhoven, The Netherlands) operated at 10–20 kV, equipped with an energy dispersive spectroscopy (EDS) analyzer. All samples were dried in a vacuum oven for 6 h at 60 °C before analysis.

3.1. Changes in speciation of sulfur in water depending on hydration state

Process efficacyð%Þ ¼

Conversion ð%Þ ¼

Cuin −Cuout;filtered  100 Cuin

Cuout;total −Cuout;filtered  100 Cuout;total

ð1Þ

ð2Þ

Sulfur, used as a crystallizing reagent in this study, is an element with variable oxidation states. Since the pKa2 of H2S is fairly high (ca. 13), bisulfide (HS−) and dissolved hydrogen sulfide (H2S (aq)) are the dominant species in water at neutral pH and room temperature. Depending on the hydration state, several types of sodium sulfide are available; pentahydrate (Na2S·5H2O) and nonahydrate (Na2S·9H2O) are the most common. Changes in the speciation and fractionation of sulfur complexes in 0.6 mM Na2S solutions prepared with these two hydration states, Na2S·5H2O and Na2S·9H2O, are shown in Fig. 1. In both cases, the loss of gaseous H2S was negligible (b3%). During exposure to the atmosphere for 120 min, more than 50% of the total dissolved sulfide in the Na2S·5H2O solution was readily oxidized to SO24 − and reached equilibrium within 5 min while only ca. 1.5% of total sulfide in the Na2S·9H2O solution, indicating that chemwas oxidized to SO2− 4 ical stability might be strongly affected by its hydration state. In some cases, the degree of hydration can affect the steric structure, leading to conformational change. As a result, this could affect the chemical stability (Sagarik and Dokmaisrijan, 2005). Thus, the availability of the total sulfide ion should be considered when calculating the required dose of Na2S·5H2O as a sulfidation reagent.

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Fig. 1. Changes in speciation of sulfur from Na2S pentahydrate and Na2S nonahydrate during 120 min exposure to air. Open and closed circles represent concentrations of total dissolved sulfide (S2− + HS− + H2S (aq)) and sulfate (SO2− 4 ), respectively, produced by oxidation of sulfides in aqueous solution.

3.2. Batch experiments for CuS crystallization 3.2.1. Effects of pH on zeta potential and particle size distribution The surface charge of a particle is mostly dependent on the interaction between the surface groups and the surface pH. Nduna et al. (2014) proposed a model for the zeta potential of covellite (CuS) and verified that hydroxyl surface sites did not play an important role in predicting the surface charge behavior under nitrogen conditions. Although metal sulfide has no hydroxyl group, however, the oxidation of sulfide mineral can produce a metal oxide layer under aerobic condition as shown in the following equation (Fullston et al, 1999). It is believed that it provides the surface hydroxyl groups, resulting in pH-dependent zeta potential behavior. MS þ 1=2nO2 þ nH2 O→M1−n S þ nMðOHÞ2

ð3Þ

As shown in Fig. 2a, neither of the CuS zeta potential curves obtained using Na2S·5H2O and Na2S·9H2O as sulfidation reagents were linear with a negative slope. Fairthorne et al. (1997) demonstrated that the shape of zeta potential curve differed significantly between the aged

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(oxidized) and the freshly ground copper sulfide minerals. The zeta potential of CuS produced from Na2S·9H2O was nearly constant at approximately −50 mV over the entire range of pH values, indicating that it is difficult to be developed into larger particles. These results are also consistent with Mokone et al. (2012) who observed that CuS particles with a mean diameter of 10 nm, which originated from using Na2S·9H2O, were stably suspended in water because of their highly charged surfaces. However, when Na2S·5H2O was used as a reagent, the relationship obtained between the zeta potential of CuS particles and pH was rather convex curve (Fig. 2a). The highest charge point of − 8.7 mV was observed at an equilibrium pH of 3.6, while it reduced to between − 20 and − 50 mV at the other equilibrium pH. A similar shape of zeta potential as a function of pH was reported by Fullston et al. (1999) who pointed out that a pronounced hysteresis between the zeta potential acid and base titration curves appeared only under oxidizing conditions ascribed to the dissolution of the minerals at acidic pH values. As shown in Fig. 2b, small particles (ca. 100 nm) were produced using Na2S·9H2O as a sulfidation reagent over the entire pH range studied, presumably resulting from the relatively high negative charge of their surface. However, when Na2S·5H2O was used as the reagent at the pH resulting in the low absolute value of the surface potential, small particles of CuS aggregated to form larger star-shaped crystals (ca. 10 μm) (Fig. 2c). These differences in the properties, e.g., surface charge and shape, of the resulting CuS particles could be resulted from the conformational differences between two hydration states of Na2S used during the sulfidation. Therefore, it is important to consider the effects on the surface charges of the particles induced by sulfidation reagents during crystallization. Defining particles b 100 nm as fines in this study, the percentages of fine particles formed under various conditions (i.e., the initial pH of 1–6 and hydration state of sulfidation reagents) are summarized in Table S1. Within 60 s of sulfidation, the CuS particles produced from Na2S·9H2O over the entire range of pH studied were smaller than those produced from Na2S·5H2O. After 1 d of incubation, the difference in the particle size became more pronounced. Except for the samples with low initial pH (pH 1 and 2), all CuS particles produced from Na2S·5H2O grew larger, up to a mean diameter of 5 μm. However, the mean diameter of those produced from Na2S·9H2O were still less than 800 nm even after 1 d. Based on the crystallization kinetics, the nucleation dominates over the crystal growth at excessively high supersaturation, eventually leading to insufficient development, while the crystal growth is favorable at low supersaturation (Patel and Anderson, 2013). Thus, smaller particle formation from Na2S·9H2O is thought to be favored under high supersaturation, i.e., more available sulfides.

Fig. 2. (a) Zeta potentials measured after sulfidation using Na2S pentahydrate and Na2S nonahydrate at various equilibrium pH values. Field emission gun–scanning electron microscopy (FEG–SEM) images of CuS produced using (b) Na2S·9H2O and (c) Na2S·5H2O as sulfidation reagents. Initial concentration of Cu2+ was 100 mg/L and molar ratio of Cu2+:S2− was 1:2.

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3.2.2. Effect of Ca2+ on crystal growth Several studies have reported on enhancing the crystal growth using various additives such as triethanolamine (TEA) as a linker for the CuS nano-rods (Dhasade et al., 2012). Divalent cations such as Ca2 + and Mg2+ also have been used as cross-linking agents because of their ability to bind to oxygen functional groups on surface (Park et al., 2008). To facilitate the growth of CuS crystals during sulfidation, Ca2+ was introduced as a cross-linking reagent between the oxide layers formed on the surface of CuS under oxidizing conditions as shown in Eq. (3). As early as 60 s after the initiation of sulfidation, there were essentially no fines in the samples; to these samples, 1 mL of 1000 mg Ca2 +/L CaCl2 solution was added, regardless of the initial pH (Table S1). In addition, the FEG–SEM images of the CuS crystals (Fig. S2) revealed that much denser and larger aggregations of crystals formed in the presence of Ca2+ than in its absence. The effect of adding Ca2+ on Cu2+ removal at various pH values is depicted in Fig. S3. As the initial solution pH increased from 1 to 5, the residual copper concentration decreased to a much greater extent with the addition of Ca2 + compared to samples without Ca2+. At an initial pH of 5, only 8% of the copper remained in the sample with Ca2+, whereas ca. 38% of the copper was still present in the sample without Ca2 +. Together with the FEG–SEM results, it can be clearly seen that Ca2+ has a strong influence on the CuS crystal growth, decreasing the copper concentration in the filtered sample. The XRD peaks of the residual black solids confirm that the resulting copper sulfides were mainly in the form of Cu2S and CuS2, both with and without the addition of Ca2+ (Fig. 3).

3.3. Effect of molar ratio of S2− to Cu2+ on nucleus formation in fed-batch fluidized bed reactor Because the only reactive species affecting sulfidation are dissolved sulfides and less than 50% of the total dissolved sulfides from Na2S·5H2O were available for sulfidation (Fig. 1). Hence, it is suggested that Na2S·5H2O be used at a molar ratio of S2−:Cu2+ higher than 2 to supply sufficient dissolved sulfide ions for sulfidation, as predicted by stoichiometry. To determine the optimum molar ratio of S2− to Cu2+, the fed-batch FBR with adding Na2S·5H2O as a reagent was operated for 8 h without adding seeds. Residual copper concentrations in the filtered samples for various molar ratios of Cu2+:S2− of 1:0.5 to 1:5 are shown in Fig. 4. In all cases, the copper removal kinetics was rapid and, equilibrium was attained within the first 10 min. The residual copper concentration decreased abruptly at a molar ratio of 1:2, from 100 to 25 mg Cu2+/L within the first 10 min. However, no further considerable decrease in copper concentration was observed by increasing the dose

Fig. 3. X-ray diffraction (XRD) data for CuS produced with and without adding Ca2+ to solution. Na2S·5H2O was used as sulfidation reagent and molar ratio of Cu2+ to S2− was 1:2.

Cu:S = 1:0.5 Cu:S = 1:1 Cu:S = 1:2 Cu:S = 1:5

1.0 0.8 C/C0

110

0.6 0.4 0.2 0.0

0

100

200

300

400

500

Time (min) Fig. 4. Effect of molar ratio of Cu to S on Cu2+ removal efficacy during operation of fedbatch fluidized bed reactor (FBR). Initial concentration of Cu2+ was 100 mg/L. Recirculation rate was 470 mL/min and operation was conducted without adding seed material.

of Na2S·5H2O to a molar ratio of 1:5, indicating that Cu2+ acted as a limiting reactant rather than S2− at this condition. Thus, a sufficient amount of dissolved sulfide is present at a molar ratio of 1:2, consistent with the batch experiment results in Section 3.1. Conversion (%) of fines to larger crystals at each molar ratio was calculated based on the difference in copper concentration between the unfiltered and filtered samples as expressed in Eq. (2). As a result, conversion (%) increased as the molar ratio of S2− to Cu2+ increased; it was less than 2% at a molar ratio of 1:0.5 (Cu2+:S2−), but ca. 75% of CuS fines successfully formed larger crystals at a molar ratio of 1:5. Because there is an energy barrier for nucleation, a high degree of supersaturation is needed for crystal growth. These results suggest that the degree of supersaturation at lower molar ratios of Cu2+:S2− (b 1:2) is insufficient to develop larger crystals of CuS. 3.4. Effects of seed material and resting height on crystal growth in fedbatch fluidized bed reactor Based on crystallization theory, crystal growth can be accelerated at active growth sites (Patel and Anderson, 2013). Seed materials such as sand can play a vital role in providing active growth sites. Therefore, previous studies have been conducted focusing on providing and maintaining sufficient active growth sites to enhance adhesion of the target materials to the surface of the seeds (Luft and DeTitta, 1999). To promote further uptake of Cu2+ into the seeds, different surface modifications to increase sorption properties and surface roughness have been reported using manganese (Lee et al., 2004) and calcium carbonate (Zhou et al., 1999), respectively. It is expected that Ca coating would give a dual benefit of increasing surface roughness to provide more active sites for crystal growth and providing cross-linking sites for the CuS fines. Various seed materials, including fine, coarse, and Ca-coated fine and coarse sands, were evaluated in terms of residual copper concentrations in the fed-batch FBR. For the same resting height (10 cm, 5% of the total height of the reactor), the coarse sand exhibited higher Cu2+ removal efficacy (up to 70%) than the fine sand (only ca. 30%) after 240 min of reaction time (Fig. 5a). More interestingly, both types of sand coated with Ca exhibited substantial increases in the overall process efficacy by ca. 30%, the Cu2 + concentration approached nearly zero within 120 min when using Cacoated coarse sand as seed. To investigate the differences between the seed types, surface roughness (Ra: center-line mean roughness in μm)

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3.5. Effect of internal recirculation rates on crystal growth in fed-batch fluidized bed reactor In conventional FBR processes, immature fines are susceptible to being washed away during the early stage of operation unless appropriate internal recirculation is provided. Particularly for highly charged small particles such as CuS fines, a high rate of internal recirculation is required for adequate growth and development to larger crystals. Changes in the Cu2+ removal efficacy as a function of the internal recirculation rate are shown in Fig. 6a. The efficacy increased as the recirculation rate increased; the lowest efficacy was observed with a recirculation rate of 25 mL/min and it gradually increased to 55, 67, and ca. 99% at recirculation rates of 125, 250, and 470 mL/min, respectively, after 240-min operation of the fed-batch FBR. This trend can be accounted by increases in the contact opportunities between the CuS fines and the seeds by increasing the internal recirculation rate. Fig. 6b shows the percent conversion at internal recirculation rate of 25 and 470 mL/min. At a recirculation rate of 25 mL/min, the changes in conversion (%) were negligible throughout the 240 min reaction time. With increasing the recirculation rate from 25 to 470 mL/min, the conversion (%) increased from ca. 4 to 68%, respectively. The greater conversion (%) obtained at the higher recirculation rate is an indirect evidence of the presence of CuS suspended in supernatant, not on the seeds. This may be ascribed to the detachment of CuS from the seed surfaces by the increased shear force at the relatively high recirculation rate. These results are consistent with those of Mokone et al. (2012),

Fig. 5. (a) Effect of seed type on Cu2+ removal efficacy during operation of fed-batch fluidized bed reactor (FBR). Initial concentration of Cu2+ was 100 mg/L. Resting height of seeds was 10 cm and recirculation rate was 470 mL/min. (b) Effect of resting height of seeds (Ca-coated coarse sand) on Cu2+ removal efficacy during operation of fed-batch fluidized bed reactor (FBR). Initial concentration of Cu2+ was 100 mg/L and recirculation rate was 470 mL/min.

of 1 μm × 1 μm area at four randomly selected points on each seed was analyzed using contact-mode AFM (Fig. S4). The mean surface roughness was as follows: fine sand (0.0326) b coarse sand (0.0338) b Ca-coated coarse sand (0.0562). Thus, it can be concluded that the overall process efficacy of the fed-batch FBR was affected by the surface roughness of the seeds. The resting height of the seeds is another important factor governing the overall efficacy of the fed-batch FBR. In previous studies, the resting height of the seeds was typically 10–90 cm, equivalent to 5–60% of the total height of the reactor (Lee et al., 2004; Lee and Yang, 2005). Fig. 5b shows changes in the residual copper concentration in the filtered samples as a function of the resting height of the seeds (Ca-coated coarse sand) in the fed-batch FBR. At a constant recirculation rate of 470 mL/min, the process efficacy was significantly correlated with the resting height. However, increased removal efficacy of Cu 2 + was not entirely proportional to the increase in the resting height of the seeds. The lowest efficacy occurred without seeds because of the lack of available active sites for crystal growth. More importantly, when the resting height exceeded a certain critical value, it adversely affected the ability to maintain a properly fluidized bed, resulting in low crystal growth rates. In this study, a resting height of 10 cm (5% of the total height of the reactor) was found to be sufficient for CuS crystal growth at a recirculation rate of 470 mL/min.

Fig. 6. (a) Effect of internal recirculation rate on Cu2+ removal efficacy during operation of fed-batch fluidized bed reactor (FBR). Initial concentration of Cu2+ was 100 mg/L. (b) Percent conversion as a function of time at two different internal recirculation rates, 25 and 470 mL/min. Ca-coated coarse sand was used as seed material and resting height of seeds was 10 cm.

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who reported poor coating of ZnS on silica sands used as seeds at a relatively high recirculation rate (300 mL/min). 3.6. Statistical analysis of factors affecting CuS crystallization Crystallization of CuS is a complex process with various influencing factors, making the process efficacy difficult to predict. The most influential factors were investigated using principal component analysis (PCA), a useful statistical technique for identifying underlying variables and clarifying correlations within a set of observed variables (Kallithraka et al., 2001). PCA was performed using Predictive Analytics Software 18.0 (SPSS Inc., Chicago, IL, USA). The principal component 1 (PC1) reflected the performance of CuS crystallization with highly positive (resting height and seed type) and highly negative (C/C0) loadings (Fig. S5). PC1 explained 33% of the total variance and was the most important PC. PC2 explained 21% of the total variance exhibiting highly positive and negative loadings for recirculation rate and pH, respectively. PC2 reflected the relationship between the recirculation rate and pH that were together inversely proportional to the efficacy of CuS crystallization. Based on the PCA analysis, seed type and resting height were identified as the most influential factors accounting for most of the variance in CuS crystallization in the fed-batch FBR. 4. Conclusion Despite the advantages of sulfidation, CuS is known to be difficult to recover in a FBR because of its small particle size (b 100 nm) and high dispersivity in water resulting from a highly charged surface. However, its overall efficacy could be significantly enhanced by adopting appropriate strategies for crystallization. Na2S·5H2O was found to be an effective sulfidation reagent by decreasing the surface charge of CuS fines. Due to the partial oxidation of sulfide in Na2S·5H2O to sulfate, the optimum molar ratio of Cu2+ to S2− for sulfidation was observed to be 1:2. In addition, Ca2 + coating as a cross-linker on the surface of the seed (coarse sand) was effective for promoting the development of larger and more aggregated CuS crystals in the fed-batch FBR. Operating the FBR in the batch mode minimizes the loss of immature CuS particles in the effluent. PCA indicated that the resting height and the seed type were the most important parameters affecting CuS crystallization in the fed-batch FBR. Acknowledgment This study was supported by the Korea Ministry of Environment as “The Eco-Innovation Program” (No. 2012001340002) and “Global Top Project” (GT-SWS-11-01-006-0), and the National Research Foundation of Korea Grant funded by the Korean Government (MSIP) (2013, University-Institute cooperation program).

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