Selective Recovery of Zinc over Iron from Spent

Mar 9, 2015 - (EPT) also referred as pseudoemulsion based hollow fiber strip dispersion ..... (1) Cook, S. J.; Perera, J. M.; Stevens, G. W.; Kentish,...
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Selective Recovery of Zinc over Iron from Spent Pickling Wastes by Different Membrane-based Solvent Extraction Process Configurations J. Laso, V. García, E. Bringas, A. M. Urtiaga,* and I. Ortiz Department of Chemical and Biomolecular Engineering, University of Cantabria, Av. de los Castros s/n, 39005 Santander, Spain ABSTRACT: This paper reports the selective separation of zinc over iron from spent pickling wastes or effluents using two different membrane-based solvent extraction process configurations, Nondispersive solvent extraction (NDSX) and emulsion pertraction technology (EPT). The process aims to obtain a highly concentrated zinc solution with a negligible content of iron to allow for Zn electrowinning. The effect of the following process variables on the kinetics and selectivity of zinc separation has been evaluated: (i) process configuration NDSX and EPT, (ii) extractant concentration, tributylphosphate (TBP) in the range of 20−100% v/v and (iii) stripping phase/feed phase volume ratio in the range Vs/Va 0.2−2. The transport of iron, chloride and free acid has been also monitored to gain insight into the separation fundamentals. EPT configuration overcame NDSX in terms of zinc and iron separation kinetics, although separation selectivity (at 30 min) was higher for NDSX configuration, αZn/Fe = 22, compared to EPT process αZn/Fe = 15. The optimum TBP content in the extractant phase was found to be 50% v/v. A further increase did not improve the Zn recovery kinetics and reduced the Zn/Fe selectivity. The increase of the Vs/Va ratio improved the process efficacy in terms of kinetics and zinc recovery.

1. INTRODUCTION Hot-dip galvanizing is the most extensively used surface treatment to protect steel pieces against corrosion. It is reported that steelwork plants in the EU produce 300 000 m3 per year of spent pickling solutions (SPS), which are of environmental concern due to their high content in heavy metals and acids. Typical concentrations of zinc, iron and hydrochloric acid are in the ranges of 20−120, 100−130 and 1−6 mol L−1, respectively.1−4 To move toward an industry with a high level of resource efficiency, the Waste Management Hierarchy must be carefully considered. Resource efficiency by means of recovery practices is a key strategy to reduce the environmental impact of processes that utilize nonrenewable materials and produce hazardous effluents. Further on, process eco-efficiency is enhanced. Galvanizing of steel pieces is high demanding in terms of zinc, which is one of the highest operational costs. The recovery of zinc from SPS decreases the need for virgin zinc metal that has a market value of 1600 € per ton in 2013,5 and reduces the toxicity of the effluents generated in the hot-dip galvanizing process. Regel-Rosocka6 reviewed different separation alternatives to regenerate SPS from steel processing aiming at metal recovery. The studied techniques were ion-exchange resins, crystallization and classical and membrane based solvent extraction. In the past few years, membrane based solvent extraction (MBSX) has drawn the attention of researchers due to the low inventory of extractant, low energy requirement, process selectivity and the possibility of simultaneous extraction and stripping of the target compounds.7,8 The integration of MBSX within the hot-dip galvanizing process considers SPS a source of zinc, as this metal can be selectively transferred to an enriched stream to allow for its further electrowinning recovery. The latter operation is negatively affected by the presence of iron and chloride, © 2015 American Chemical Society

because the oxidation of chloride to chlorine promotes the chemical oxidation of Fe2+ to Fe3+ and ferric iron competes with Zn2+ ions in the cathode.9,10 Further, acidic pH values favor zinc electrodeposition. Consequently, the separation process should be selective toward zinc versus iron and chloride and allow the permeation of protons. In the zinc-based surface treatment sector, conventional solvent extraction and MBSX are being used for the management of passivation baths11−14 and SPS.15−22 Regarding the latter, previous works have focused on the viability of the MBSX process configurations, i.e., nondispersive solvent extraction (NDSX) and emulsion pertraction technology (EPT) also referred as pseudoemulsion based hollow fiber strip dispersion (PEHFSD). These configurations differ in the way of contacting the fluid phases and the number of the membrane contactors involved.8 In most of the reported works, water was used as a stripping agent and tributylphosphate (TBP) was selected as an organic extractant because it selectively removes zinc from iron containing solutions. This is due to the combined effect of the zinc and iron speciation in the SPS and the extraction nature of the TBP, i.e., TBP extracts neutral or anionic compounds and the speciation of zinc and iron in SPS at high chloride concentration and low pH is ZnCl42− and ZnCl3− for zinc, and Fe2+ and a slight ratio of FeCl+ for iron.23,24 The kinetics of zinc removal and recovery from SPS using NDSX and EPT has been well studied from a theoretical and experimental perspective.16,21 However, little attention has been paid to the behavior and transport of other major species Received: Revised: Accepted: Published: 3218

January 8, 2015 March 6, 2015 March 9, 2015 March 9, 2015 DOI: 10.1021/acs.iecr.5b00099 Ind. Eng. Chem. Res. 2015, 54, 3218−3224

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Industrial & Engineering Chemistry Research namely iron, chloride and protons, which are of key importance for the assessment of the viability of zinc recovery. This work reports the performance of the membrane-based configurations, nondispersive solvent extraction and emulsion pertraction technology, for the recovery of zinc over iron from real spent pickling solutions using tributylphosphate as liquid membrane and tap water as stripping agent. The aim is to evaluate and assess the appropriate operation conditions of the process in order to promote the selective separation of zinc from iron containing SPS. Further, the progress of the concentration of iron, chloride and free acid in the feed SPS and in the stripping phase was monitored in order to select suitable conditions for obtaining a stream of enough quality for zinc electrowinning.

Figure 1. Configurations of the membrane-based solvent extraction process for the recovery of zinc from SPS using HFMC: (a) NDSX and (b) EPT. EX, extraction; BEX, back-extraction.

2. MATERIALS AND METHODS 2.1. Materials. Samples of SPS were provided by a local industry and contained mainly zinc, iron, chloride and free acid (Table 1). Further, synthetic solutions were prepared using

fluid into the feed aqueous phases. In the EPT experiments, the pseudoemulsion was prepared using an MRVS-08 stirrer (SBS). The organic and the stripping phases can be easily separated by gravity settling. Further details of the experimental procedure can be found elsewhere.12,14,21 Table 2 lists the experiments conducted for the analysis of the influence of the process variables on the kinetics and on the

Table 1. Composition of the Samples Studied real SPS +

−1

[H ] (mol L ) [Zn2+] (g L−1) [Fe]TOTAL (g L−1) [Fe2+] (g L−1) [Fe3+] (g L−1) [Cl−] (g L−1)

1.1 122 95.6 92.6 3 301

± ± ± ± ± ±

0.04 3 3 2 2 3

synthetic SPS 1 149 121 121 0 354

±5 ±3 ±3

Table 2. Experimental Conditions of the Kinetic Experiments

± 10

organic phase

ZnCl2 (97%, Panreac), FeCl2·4H2O (99%, Sigma-Aldrich) and HCl (37%, Panreac) in order to simulate the composition of SPS in terms of Zn2+, Fe2+, Cl− and H+. The organic liquid membranes consisted of the pure extractant TBP (97%, SigmaAldrich) or of the TBP diluted in the aliphatic solvent Shellsol D70 (Kremer Pigmente). Tap water was utilized as stripping solution in all the experiments. 2.2. Membrane-based Solvent Extraction Experiments. The experiments were performed at laboratory scale using mesoporous hollow fiber membrane contactors (HFMC, Liqui-Cel Extra-Flow 2.5 × 8, Hoechst Celanese). The HFMC presented an effective mass-transfer area of 1.4 m2 and an effective mass-transfer length of 0.15 m, and contained 10 200 polypropylene fibers. The fibers had a nominal porosity of 40%, an average pore size of 0.03 μm and an internal diameter of 240 μm with a wall thickness of 30 μm. Figure 1 shows the experimental setup used for the extraction and back-extraction of zinc from the aqueous systems using (a) NDSX and (b) EPT configurations. In NDSX configuration, the extraction and the back-extraction of the targeted compound were conducted in two different membrane modules and the nondispersive contact occurred between the spent acid and the organic extractant solutions and between the organic and the aqueous stripping solutions, whereas in EPT, the extraction and the backextraction were conducted in a single membrane module and the nondispersive contact occurred between the feed SPS and the pseudoemulsion prepared by dispersing the stripping solution into the organic phase. In both configurations, the temperature of the experiments ranged from 22 to 35 °C due to the heating produced by fluid pumping. The difference in the transmembrane pressure between the aqueous phase and the organic/emulsion phase was maintained constant at approximately 0.15 bar in order to avoid the dispersion of the organic

exp

no. of replicates

1 2d 3 4 5 6d

2 4 2 2 2 4

volume (L)a

configuration

TBP (% v/v)b

Shellsol D70 (% v/v)c

Va

Vo

Vs

NDSX NDSX NDSX EPT EPT EPT

100 50 20 80 80 50

50 80 20 20 50

0.5 0.5 0.5 1 0.5 0.5

1 1 1 0.8 1 1

1 1 1 0.2 1 1

a

Va, feed solution volume; Vo, organic phase volume; Vs, stripping solution volume. bTBP (%v/v): volume percent of TBP (carrier) in the organic phase. cShellsol D70 (%v/v): volume percent of Shellsol D70 (solvent) in the organic phase. dExperiments performed with real and synthetic SPS.

selectivity of the separation process toward zinc over iron. Prior to each experiment, the organic solution was in contact with tap water for 30 min in order to (i) reach the maximum intake of water by fresh TBP and consequently, to minimize the water transport through the membrane and (ii) to wash the loaded organic solution to be reused. During the NDSX and EPT experiments the feed and stripping samples were taken regularly from the corresponding stirred tanks in order to follow the development of zinc, iron, chloride and free acid concentration with time. The concentrations of the different species in the organic phase were calculated according to the mass balances. The following methods were used for the sample quantification: atomic absorption spectrometry (PerkinElmer 3110) for the determination of zinc and total iron, titration with NaOH 0.1 mol L−1 and bromophenol blue solution 0.04% for the determination of H+, UV−vis spectroscopy (SpectroquantPharo 100, λ = 500 nm) for the determination of Fe2+ and Fe3+ 3219

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Industrial & Engineering Chemistry Research by the Standard Method 3500-Fe B and Cl− by the Standard Method 4500-Cl− E.25 When the experiments were repeated the presented data are the mean values of the replications and the related figures incorporate the corresponding error bars.

3. RESULTS AND DISCUSSION 3.1. Effect of Process Configuration. The effect of process configuration, NDSX or EPT, on the selectivity of the liquid membrane toward zinc versus iron separation was evaluated by the comparison of experiments 2 and 6 listed in Table 2. Figure 2 illustrates the development of zinc

Figure 3. Development of the concentration of iron with experimental time in the (a) feed phase and (b) stripping phase using NDSX and EPT configuration, and real and synthetic feed phase; [Fe]o = 97−103 g L−1, [TBP] = 50% (v/v).

total iron is present mainly as iron(II) in the forms of Fe2+ and a minor amount of FeCl+ and both cationic species should be rejected by TBP due to its solvating nature.32 However, TBP may act also as a surfactant and Regel-Rosocka and coworkers29−31 suggested that iron(II) was transported from the aqueous solution to the stripping phase by means of nonselective micromicelles. When the process separation was carried out using EPT, the concentration of iron in SPS decreased by 30%, whereas when using NDSX, it only decreased by 10%. On the other hand, the concentration of iron in the stripping phase when experiments were carried out using EPT configuration reached 4 g L−1, whereas when using NDSX, it was 1.5 g L−1. The higher interfacial area between the dispersed stripping agent into the organic solution exhibited by the EPT configuration compared to NDSX, enhanced the backextraction of iron to the stripping agent and consequently the extraction yield. The difference in the extracted and backextracted mass of iron was assigned to the metal accumulation in the organic phase. The selectivity (α) of the process toward zinc over iron in the stripping solution was calculated in the constant range of the back-extraction fluxes (J) (t < 30 min) in order to further compare the performance of both configurations according to the following equation:

Figure 2. Development of the concentration of zinc with experimental time in the (a) feed phase and (b) stripping phase using NDSX and EPT configuration, and real and synthetic feed phase; [Zn]o = 115− 121 g L−1, [TBP] = 50% (v/v).

concentration in the feed and stripping phases with experimental time. The results point out that the extraction and stripping of zinc took place with higher rate when the EPT configuration was used. The faster extraction and backextraction kinetics are related to the higher interfacial area between the dispersed stripping agent and the continuous organic phase in EPT compared to NDSX.16,17,26 Chemical equilibrium is reached between the zinc species contained in the liquid phases destroying the driving force for zinc transport. The system equilibration was reached after 120 min when the separation was conducted with EPT whereas more than 3 h of experimental run were required to achieve a similar performance when NDSX was employed. When equilibrium conditions were reached, the concentration of zinc in the feed phase had decreased from 120 to 25 g L−1, i.e., 79% of zinc removal, and the final concentration of zinc reached in the stripping solution was 45 g L−1 independently on the process configuration. Figure 3 shows the development of iron concentration in the feed and stripping phases with experimental time. The results indicate that iron is extracted and the rate is affected by process configuration. Several studies have reported extraction of iron(II) from 0.5% to 10% using TBP.1,3,24,27−31 In SPS, the

αZn/Fe|t < 30 =

JZn JFe

t < 30

(1)

The obtained results indicated that NDSX configuration promoted the selectivity of the separation process toward zinc versus iron compared to EPT:αZn/Fe|NDSX = 2.2 × 10−6 kg Zn m−2 s−1/9.9 × 10−8 kg Fe m−2 s−1 = 22 and αZn/Fe|EPT = 1.2 × 10−5 kg Zn m−2 s−1/8.0 × 10−7 kg Fe m−2 s−1 = 15. The flux and selectivity values obtained in this work when using NDSX were of similar level of magnitude than values previously 3220

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Industrial & Engineering Chemistry Research observed in literature.20,33 To the best knowledge of the authors, there is not available data to evaluate the selectivity of the EPT configuration toward zinc over iron. The difference between the selectivity values in EPT and NDSX is attributed to the way in which the process configuration affects the zinc and iron back-extraction fluxes. Due to the smaller driving forces involved in the back-extraction of iron as compared to zinc, the higher interfacial area provided by EPT in comparison to NDSX reported a higher relative iron kinetics increase (JFe,EPT/JFe,NDSX = 8.1) than the observed for zinc ((JZn,EPT/ JZn,NDSX = 5.4) in the constant range of the back-extraction fluxes (t < 30 min). Depending on the galvanic process, SPS may contain different type and concentration of impurities such as metals, ammonia and organic impurities. EPT and NDSX experiments using synthetic samples were performed and compared with experiments 2 and 6 in order to evaluate the effect of the matrix composition on the process selectivity. The experiments were conducted using 50% (v/v) of TBP and 180 min of experimental time. Figures 2 and 3 illustrate the development of the zinc and iron concentration respectively in aqueous and stripping phase when synthetic solutions were used. The results point out that the influence of the SPS complex matrix of in the extraction and back-extraction of zinc and iron was minor for both EPT and NDSX configurations. The lack of iron(III) in synthetic samples and the existence of iron in the obtained stripping phase indicate that Fe2+ is the transported species from the feed phase to the stripping phase. This fact could be supported by the micellar transport theory, in which Fe2+ is transported from the aqueous solution to the stripping phase by means of nonselective micromicelles.29−31 3.2. Effect of Extractant Concentration. The study of the effect of TBP concentration on the extraction of zinc was carried out using the NDSX configuration. This was because of the better selectivity of the process toward zinc when NDSX was used compared to EPT. Three experiments were performed using different concentrations of TBP: 100% (v/ v), 50% (v/v) and 20% (v/v) that correspond to experiments 1−3. Table 2 indicates the experimental conditions. Figure 4 shows the development of the concentration of zinc in the aqueous and stripping phases when different concentrations of TBP were used. The results showed that increasing TBP concentration promoted the extraction kinetics of zinc. The use of 20% (v/v) of TBP implied that the equilibrium conditions were not attained after 240 min of experimental run and led to an extraction of 47% of zinc. On the other hand, the use of 50% (v/v) and 100% (v/v) of TBP implied an extraction of zinc about 80% when equilibrium between the liquid phases was reached at 180 and 120 min, respectively. The difference in the extraction kinetics at different TBP levels was due to the fact that more TBP was available for the generation of the zinc-TBP complex. The removal of zinc from SPS was limited by the chemical equilibrium between the aqueous and organic phases; consequently, increasing TBP concentration impacted on the time needed to reach the equilibrium. Mansur et al.3 reported that the extraction of zinc was 3 times higher when TBP concentration increased from 25% (v/v) to 100% (v/v). In accordance with Figure 4, Regel-Rosocka31 observed that the influence of the concentration of TBP on the extraction of zinc was minor in the range of 80% (v/v) to 100% (v/v). Further, the use of organic solution containing 50% (v/v) of TBP entailed a higher concentration of zinc, 45 g Zn L−1, in the

Figure 4. Development of the concentration of zinc with experimental time in the (a) feed phase and (b) stripping phase using different concentrations of TBP and the NDSX configuration; [Zn]o = 120 g L−1.

stripping phase after 240 min, compared to the use of a 20% (v/v) extractant solution, 35 g Zn L−1. However, the utilization of 100% (v/v) TBP solutions as extractant did not further enhance zinc concentration in spite of the fact that the liquid membrane consisting of 100% (v/v) TBP extracted more zinc than when it was composed of 50% (v/v) TBP. This may be due to the fact that more stripping solution would be needed for back-extracting the accumulated zinc in the organic phase. From the operational process perspective, this need of more volume of stripping could be tackled by the implementation of a semicontinuous system provided with a purge. This system would allow the intake of a stripping agent during the process that will shift the chemical equilibrium between organic/ stripping phases. This fact could allow reaching higher percentages of zinc extraction and back-extraction. Figure 5 shows the development of the concentration of iron in the aqueous and stripping phase using different concentrations of TBP. The amount of total iron in the stripping phase ranged from 1 to 2 g L−1, indicating that TBP concentration affected iron extraction. This may be due to the surfactant properties of TBP; the higher the TBP concentration, the closer to the critical micelle concentration (CMC) and the higher the possibilities of the iron transport by means of nonselective micromicelles. This was also observed by RegelRosocka and co-workers.29−31 On the other hand, the extraction values for iron varied from 10 to 15% and the effect of TBP concentration on iron extraction from SPS could not be accurately identified due to the fact that the amount of iron extracted from SPS was minor compared to the remaining iron in solution. Further, high concentrations of chloride and free acidity in the presence of zinc enhance the formation of acid-complexes with the general formula Hn+[ZnCln+2]−n with n = 0, 1, 2.32 Morris and Short33 proposed that at low acidity the formed zinc/TBP complexes were HZnCl3 and ZnCl2·2TBP, and at acidity above 2.7 mol L−1 the complex H2ZnCl4·2TBP was 3221

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Figure 5. Development of the concentration of iron with experimental time in the (a) feed phase and (b) stripping phase using different concentrations of TBP and the NDSX configuration; [Fe]o = 90−100 g L−1.

Figure 6. Molar ratio of chloride/zinc during the (a) extraction and (b) back-extraction processes using different TBP concentrations as liquid membrane.

detected. Regel-Rosocka et al.24 reported that at low acidity of the feed phase ZnCl2 was the predominant species and at higher acidity HZnCl3 and H2ZnCl4 were found. Several researchers determined that the predominant zinc−chloro complex in the SPS was H 2 ZnCl 4 using MEDUSA software.23,24,27,34−38 The analysis of the molar ratios between the extracted and back-extracted chloride and zinc, and protons and zinc concentrations attained in the present experimental study allows the evaluation of the main zinc chlorocomplexes involved in the separation process. From the results depicted in Figure 6, it is confirmed that all the values of the molar ratio chloride/zinc obtained in the extraction and back-extraction experiments were within the range from 2 to 4, as expected from the general formula of zinc chlorocomplexes. However, most of the data varied in the range from 3 to 2, thus confirming, as expected from the literature, that HZnCl3 and ZnCl2 were the preferable extracted species that react with TBP as follows38 HZnCl3 + 3TBP ⇔ HZnCl3·3TBP

(2)

ZnCl 2 + 2TBP ⇔ ZnCl 2·2TBP

(3)

Figure 7 illustrates that the values of the molar ratio protons/ zinc in the extracted and back-extracted species, HZnCl3·3TBP and ZnCl2·2TBP, were in the range between 0 and 1. These values confirm the extraction mechanisms given by eqs 2 and 3. The presence of H+ in the stripping solution may affect positively the electrodeposition of zinc. Further, the existence of Cl− can have a negative impact on the effectiveness of the process because it is oxidized to chlorine. Therefore, the concentration of the former species should be maximized and minimized, respectively. The concentration of H+ in the stripping solution varied from 0.12 to 0.6 and that of Cl− from 1.1 to 1.8 when the concentration of TBP was modified from 20% (v/v) to 100% (v/v) following a similar trend than

Figure 7. Molar ratio of free acid/zinc during (a) extraction and (b) back-extraction processes using different TBP concentrations as liquid membrane.

that observed in the case of zinc performance. However, these values depend on the zinc concentration achieved in the stripping solution. 3.3. Effect of Phase Volume. EPT was selected to perform the study of the influence of the volume of the liquid phases on the process performance because of the higher rates of zinc and iron extraction when EPT was used compared to NDSX. Table 2 shows the experimental conditions of experiments 4 and 5 3222

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Industrial & Engineering Chemistry Research where the volumes of feed (Va → 1 and 0.5 L), organic (Vo → 0.8 and 1 L) and stripping (Vs → 0.2 and 1 L) phases had been changed. Therefore, the variation of the volume of the organic phase was minor and the volume ratio of the stripping to the feed aqueous phase Vs/Va was varied between 0.2 (experiment 4) and 2 (experiment 5). Figure 8 illustrates the development

4. CONCLUSIONS The separation of zinc from spent pickling solutions (SPS) versus iron is a key strategy to improve the resource recovery efficacy in hot-dip galvanizing processes. Zinc may be further recovered by electrowinning and the viability will depend on the presence of iron, chloride and free acid. This study shows that emulsion pertraction technology (EPT) and nondispersive solvent extraction (NDSX) are effective when removing zinc versus iron from SPS using tributylphosphate (TBP) as an extractant and water as a stripping agent. The kinetics of the extraction and back-extraction of zinc were demonstrated to be promoted by EPT, increasing the TBP concentration in the range between 20% (v/v) and 50% (v/v) and increasing the stripping volume from 0.2 to 1 L. On the other hand, the transport of iron was enhanced by EPT, high TBP concentration and increasing stripping volume. The efficacy of the separation process was established to be mainly limited by the chemical equilibria between the feed/organic and organic/stripping phases. Therefore, this works highlights first the mechanisms and kinetics responsible of the transport of zinc and iron from SPS and second the importance of implementing a semicontinuous system that allows a continuous replacement of stripping in order to further improve the zinc recovery rate. Additionally, the species involved in zinc transfer through the organic liquid membrane were confirmed to be HZnCl3·3TBP and ZnCl 2·2TBP. Further research should be driven by the definition of the maximum acceptable values of pH, chloride and iron concentration for conducting the recovery of zinc by electrodeposition. The results obtained in the present study contribute to the design of the zinc separation process considering this complex scenario.

Figure 8. Development of the concentration of zinc with experimental time in the feed phase and stripping phase using 80% (v/v) of TBP and the EPT configuration.

of the mass of zinc in the feed and stripping phases with experimental time. The comparison of the results obtained in both experiments indicates that increasing the volume of the stripping phase promoted the extraction kinetics of zinc. After 2 h of experimental running, the extraction percentages of zinc were 35% and 81% in experiments 4 and 5, respectively; however, the mass of zinc extracted was around 40 g in both experiments. On the other hand, the concentration of zinc in the stripping phase obtained in experiment 4 was 102 g L−1, whereas in experiment 5, the concentration was 36 g L−1, with the mass of back-extracted zinc being 20 and 36 g, respectively. Therefore, the increase of the volume ratio Vs/Va positively influenced the effectiveness of the back-extraction stage which varied from ≈50% (experiment 4) to ≈90% (experiment 5). The improvement on the stripping performance at high volume ratios minimizes the accumulation of zinc in the organic solution and it is related to the closeness between the extraction and back-extraction rates. The improvement in the back-extraction step promoted the regeneration of TBP. Thus, the amount of free extractant contained in the organic phase is increased. This fact affected positively the extraction step, obtaining faster kinetics of zinc extraction. This was in agreement with results previously obtained by Carrera et al.17 The extraction of iron from SPS was verified by its quantification in the stripping phase in the final conditions. The final concentration of iron in experiment 4 was 2.5 g L−1, whereas in experiment 5, it was 6.6 g L−1. The promotion of iron extraction followed a similar behavior as that observed for zinc, which was related to the better washing of the organic solution at higher water volumes. Consequently, a balance among the phases volume must be considered in order to obtain the maximum extraction of zinc from SPS and the maximum recovery of zinc in the stripping phase with the lowest amount of iron impurities. As was pointed out in Section 3.2, the implementation of a semicontinuous system with continuous replacement of stripping phase seems to be the more convenient process configuration to handle the volume ratios needed to achieve the separation and recovery objectives.

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AUTHOR INFORMATION

Corresponding Author

*Ane Urtiaga. E-mail: [email protected]. Phone: +34 942201587. Fax: +34 942201591. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of the Spanish Ministry of Economy and Competitiveness under the project CTQ2012-31639 (FEDER 2007-2013) and under Juan de la Cierva programme, and of the company is gratefully acknowledged.

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REFERENCES

(1) Cook, S. J.; Perera, J. M.; Stevens, G. W.; Kentish, S. E. The screening of extractants for the separation of Zn(II) from Australian hot-dip galvanizing effluent. Sep. Sci. Technol. 2011, 46, 2066. (2) Csicsovszki, G.; Kékesi, T.; Török, T. I. Selective recovery of Zn and Fe from spent pickling solutions by the combination of anion exchange and membrane electrowinning techniques. Hydrometallurgy 2005, 77, 19. (3) Mansur, M. B.; Ferreira Rocha, S. D.; Silva Magalhaes, F.; dos Santos Benedetto, J. Selective extraction of zinc(II) over iron(II) from spent hydrochloric acid pickling effluents by liquid-liquid extraction. J. Hazard. Mater. 2008, 150, 669. (4) Regel-Rosocka, M.; Wisniewski, M. Selective removal on zinc(II) from spent pickling solutions in the presence of iron ions with phosphonium ionic liquid Cyphos IL 101. Hydrometallurgy 2011, 110, 85. (5) Infomine. Mining intelligence & technology. http://www. infomine.com/ (accessed September 2014).

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DOI: 10.1021/acs.iecr.5b00099 Ind. Eng. Chem. Res. 2015, 54, 3218−3224

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DOI: 10.1021/acs.iecr.5b00099 Ind. Eng. Chem. Res. 2015, 54, 3218−3224