Proceedings of the 13 th International Conference of Environmental Science and Technology Athens, Greece, 5-7 September 2013

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1 Proceedings of the 13 th International Conference of Environmental Science and Technology Athens, Greece, 5-7 September 2013 ELECTROCHEMICAL RECOVERY OF THE ZINC PRESENT IN THE SPENT PICKLING BATHS COMING FROM THE HOT DIP GALVANIZING INDUSTRY BY MEANS OF THE USE OF TWO DIFFERENT MEMBRANES J. CARRILLO-ABAD 1, M. GARCÍA-GABALDÓN 1, E. ORTEGA 1, and V. PÉREZ- HERRANZ 1 1 IEC Group, Departamento de Ingeniería Química y Nuclear, Universitat Politècnica de València, Camí de Vera s/n, València, Spain. P.O. Box 22012, E mongarga@iqn.upv.es EXTENDED ABSTRACT Spent pickling baths coming from the hot dip galvanizing process represents an environmental problem due to high ZnCl 2, FeCl 2 and HCl (among other substances) concentrations. This problem, together with the decrease of natural reserves of nonferrous metals, makes zinc electrowinning coming from spent pickling solutions an interesting alternative. Moreover, membranes, or other kind of diaphragms, must be placed between the anode and cathode of the electrochemical reactor to minimize the presence of chlorine gas generated in the anode, which attacks the zinc deposits causing their dissolution. Therefore, the purpose of this work is to compare the behaviour of an Anionic Exchange Membrane (IONICS AR-204) and a Cationic one (NAFION 117). The electrochemical reactor consisted of two connected pyrex glasses of 250 ml each, with the possibility of placing a membrane between them. Two graphite electrodes were used as working and counter electrodes and a standard Ag/AgCl electrode acted as reference one. Both anode and cathode were made of two cylindrical graphite bars with an effective area of cm 2. All the experiences were performed using an Autolab PGSTAT20 potentiostat/galvanostat. Samples were taken from the reactor every 30 min and potential, current, cell voltage and ph were recorded during the electrolysis. The determination of zinc and iron was performed by atomic absorption spectrophotometry (AAS) on a Perkin-Elmer AAnalyst 100 model. For the experiments in the presence of the anion-exchange membrane (AEM), the diluted spent pickling bath was charged in the cathodic chamber while a 0.1M HCl solution was placed in the anodic one. In contrast, in the case of the cation-exchange membrane (CEM) assays, the diluted spent bath was placed in the anodic chamber and an acidic solution was poured in the cathodic one. This acidic solution consisted of 0.1M HCl or 0.1M ZnCl 2 in 0.1M HCl. The CEM allows the passage of zinc and iron cations. When an acidic solution in the absence of zinc is placed in the cathodic chamber, iron codeposits with zinc from the beginning of electrolysis. In contrast, when the acidic solution contains zinc, its presence inhibits iron deposition due to the anomalous codeposition phenomenon. Moreover, the figures of merit (zinc conversion and current efficiency) are higher when the acidic solution initially contains zinc since less amount of energy is employed in the iron deposition process. On the other hand, when the AEM is used, iron deposition starts if its concentration is at least two-fold the zinc concentration value due to the anomalous codeposition phenomenon. The iron codeposition must be taken into account if a separated zinc recovery is required. KEYWORDS: anion-exchange membrane, cation-exchange membrane, electrodeposition, iron, zinc recovery

2 1. INTRODUCTION One of the most widely extended uses of metallic zinc is to protect iron or steel pieces from corrosion by coating them with it (Marder, 2000). To accomplish this objective, the oldest technique used is the hot dip galvanizing, which is based on dipping the pieces into molten zinc. This study is centered in the effluents coming from the pickling step, which is one of the pretreatments previous to the galvanizing process. These effluents contain high concentrations of Zn, Fe and HCl (Kerney, 1994) together with low concentrations of organic compounds, such as hydrogen evolution reaction inhibitors, and other heavy metals. Therefore, spent pickling baths have to be treated before their disposal to accomplish with the environmental restrictions. Many different treatments for purifying spent pickling baths have been suggested due to the complexity of the solution (Regel-Rosocka, 2009) such as liquid-liquid extraction (Regel-Rosocka et al., 2011), anionic resins (Marañón et al., 2000), etc. In this sense, a combination of electrowinning and membrane processes is presented in this paper. A previous electrochemical study of the solution was initially performed (García-Gabaldón et al., 2011) to study the kinetics of the electrochemical processes, and then, an undivided electrochemical batch reactor was used in potentiostatic and galvanostatic mode (Carrillo-Abad et al., 2011; 2012) to determine the viability of zinc recovery from spent pickling baths. During these experiments zinc redissolution was observed at high time values for all the experimental conditions. This process is related to the synergic effect of iron ions and dissolved chlorine gas that attacks zinc deposits causing their oxidation (Caldwell-Ralston, 1921; Thomas et al., 1981). In order to solve the zinc redissolution phenomena, an anionic exchange membrane (AEM) was initially used. Although the results obtained with this membrane were better than those obtained without it, iron began to deposit together with zinc. It is worth to mention that the iron-zinc system deposits following the anomalous codeposition phenomenon (Díaz-Arista et al., 2002; Gómez et al., 1999; Zhang et al., 2001), in which the less noble metal (zinc) deposits preferentially, and iron deposition depends on the zinc-iron ratio, the value of applied current and the ph value. In this sense, a cationic exchange membrane (CEM) was used to prevent iron deposition, and the corresponding figures of merit (fractional conversion and current efficiency) were calculated and were compared with those obtained in the presence of the AEM and with those obtained in the absence of any membrane. 2. METHODOLOGY AND MATERIALS The reactors used in this work were well defined in our previous works (Carrillo-Abad et al., 2011; 2012). The undivided electrochemical reactor consisted of a Pyrex glass of 100 ml with two graphite electrodes acting as working and counter electrodes, and a standard Ag/AgCl saturated KCl electrode acting as reference electrode. The two-compartment electrochemical reactor containing the membrane was made of two Pyrex glass chambers attached by means of four screws on their sides with an anionic or a cationic exchange membrane placed between them. An equal volume (250 cm 3 ) of anolyte and catholyte is poured in their respective chamber after cell assembly. The same Ag/AgCl reference electrode and graphite cathode and anode have been used in this set-up. Both cathode and anode were totally immersed in the solution and they were symmetrically placed with respect to membrane surface. The membranes used as AEM and CEM were, respectively, IONICS AR-204 and NAFION 117. For the two reactors, the anode and cathode were made of two cylindrical graphite bars with an effective area of cm 2.

3 The content of the chambers of the divided reactor was different depending on whether the AEM or the CEM was used. When the AEM was used, the cathodic compartment was filled with the diluted spent pickling bath whereas a 0.1M HCl solution was placed in the anodic chamber. On the other hand, when the CEM was used, the diluted spent pickling bath was placed in the anodic compartment and a synthetic solution composed of 0.1M HCl or 0.1M HCl and 0.1M ZnCl 2 was placed in the cathodic compartment. The synthetic solutions containing ZnCl 2 and/or HCl were made from analytical grade reagents and distilled water. On the other hand, the dilution factors of the spent pickling baths used in this work were 1:50 and 1:10. All experiments were made at room temperature. It is worth to note that temperature was only a control parameter, i.e, temperature mustn t pass 40ºC to avoid a high solution evaporation that would change metal concentration values. Experiments were performed at different applied currents, which ranged from -450 ma to -1A. The equipment used for the electrolysis experiments was an Autolab PGSTAT20 potentiostat/galvanostat. Potential, cell voltage, current, ph and temperature were recorded during the electrowinning. On the other hand, samples were taken from the reactor every 30 minutes and zinc and iron determination was performed by atomic absorption spectrophotometry (AAS) on a Perkin-Elmer model Analyst 100 atomic absorption spectrophotometer using a zinc hollow cathode lamp at nm wavelength, 0.7 nm spectral bandwidth and an operating current of 5 ma. To measure iron concentration, it is used the same equipment changing the Zn hollow lamp for a Fe hollow lamp. The wavelength used is nm, the applied operating current is 5 ma and the spectral bandwidth is 0.2 nm (Carrillo-Abad et al., 2011; 2012). 3. RESULTS AND DISCUSSION 3.1. Analysis of the 1:50 diluted spent pickling bath Figure 1 shows the zinc conversion profiles for both reactors and membranes and for two different values of applied current (-450 and -700 ma). It is worth to note that, when the CEM is used, the cathodic compartment was filled with 0.1M HCl, that is, there is no zinc initially in the cathodic compartment, since iron deposition does not take place in the presence of the CEM when treating the 1:50 diluted spent bath. For all the cases presented in Figure 1, the higher the applied current value, the higher the zinc conversion rate. For both applied currents, CEM presents the lowest zinc conversion values because zinc has to pass through the membrane previously to be deposited. On the other hand, the undivided reactor and the two-compartment one with the AEM present similar zinc conversion values, although those obtained with the AEM are slightly higher especially at high experiment times when zinc redissolution phenomena (Caldwell-Ralston, 1921; Carrillo-Abad et al., 2012) begins to be important in the absence of the membrane. In this sense, the membrane presence acts as a barrier preventing chlorine gas from entering in the cathodic compartment and, therefore, avoiding by this way zinc deposit oxidation. Regarding iron deposition, its conversion profiles for all the conditions under study are presented in Figure 2. Iron deposition presents two different patterns in function of which reactor is used and the kind of membrane placed in the two-compartment reactor. The highest iron conversion values are obtained in the presence of the AEM since this membrane avoids chlorine presence in cathodic compartment which allows iron deposition. If chlorine gas is present together with iron and zinc, is oxidizes Fe 2+ to Fe 3+ and this last causes zinc oxidation and the consequent redissolution, therefore, the deposition of iron is avoided at some point if the chlorine gas is present (Carrillo-Abad et al., 2012). This fact explains the lack of iron deposition in the undivided reactor, which is consistent with the data presented in Figure 2. In the case of the CEM, no iron deposition

4 was observed although Zn/Fe ratio decreases and no chlorine gas is present in the cathodic compartment. This is associated with the fact that the applied current is not only used for ion deposition but also for ion circulation from the anodic to the cathodic compartment. This fact suggests that the real current for ion deposition diminishes, which favors the zinc deposition over the iron one (Gómez et al., 1999) X Zn mA -450mA AEM mA CEM -700mA mA AEM -700mA CEM 0.0 Figure 1. Evolution of zinc conversion vs. time as a function of the applied current in the presence of AEM, CEM and without membrane. 1:50 diluted spent bath X Fe mA -450mA AEM -450mA CEM -700mA -700mA AEM -700mA CEM 0 Figure 2. Evolution of iron conversion vs. time as a function of the applied current in the presence of AEM, CEM and without membrane. 1:50 diluted spent bath. Zinc current efficiency is shown in Figure 3 for all the cases under study. The highest values are obtained in the presence of the AEM since zinc deposition only competes with the other parallel reactions, such as HER process or iron deposition (Carrillo-Abad et al., 2012). On the other hand, in the case of the CEM and for the undivided reactor, zinc current efficiency values are closer but for different reasons. For both cases, parallel processes helps to diminish the zinc current efficiency but they are related to zinc deposits oxidation (Carrillo-Abad et al., 2012) for the undivided reactor, and, to the fact that zinc has to pass through the CEM before being deposited in the case of the twocompartment reactor. This fact implies an energetic cost that diminishes zinc current efficiency. Nevertheless, for high time values, CEM maintain its current efficiency

5 whereas those of the AEM and the undivided reactor decrease as zinc is being depleted from solution. f (t) mA -450mA AEM -450mA CEM -700mA -700mA AEM -700mA CEM Figure 3. Evolution of zinc current efficiency vs. time as a function of the applied current in the presence of AEM, CEM and without membrane. 1:50 diluted spent bath Analysis of the 1:10 diluted spent pickling baths When the 1:10 diluted spent pickling baths were studied in the presence of the CEM, the cathodic compartment was filled with two different solutions: 0.1M HCl or 0.1M ZnCl 2 and 0.1M HCl. This last solution was used to prevent iron deposition, which was observed when treating this concentrated spent bath in the presence of the CEM. The presence of zinc in the cathodic compartment inhibits iron deposition process thanks to its anomalous deposition in the presence of zinc (Gómez et al., 1999; Díaz-Arista et al., 2002). Figure 4 presents the zinc conversion rate for both reactors and membranes when the applied current was -1A, where the presence of zinc in the cathodic chamber of the divided reactor is noted in the legend as CEM C0=0.1M and the absence of this species is referred as CEM C0=0. The highest zinc conversion rate is obtained in the case of the AEM since zinc and iron are together in the cathodic compartment. However, this fact also leads to the undesirable iron codeposition (see Figure 5). On the other hand, these concentrated spent baths present a higher redissolution phenomenon in the undivided reactor, as more chloride gas is formed, which is responsible for the attack of the zinc deposits causing their oxidation. It is also worth to mention that adding zinc to the cathodic compartment improves the zinc conversion rate obtained in the presence of the CEM. The iron conversion rate under the same experimental conditions as those presented in Figure 4 is shown in Figure 5. An increase in the concentration of the spent pickling baths leads to the enhancement of the iron conversion rate in the case of the undivided reactor. In this case, although the chlorine formed produces a great zinc redissolution as observed in Figure 4, it does not avoid the iron codeposition as was observed for the more diluted spent solution (Figure 2). Moreover, in the case of the CEM, when no zinc is initially present in the cathodic compartment, the highest iron conversion values are obtained. This result is related to the anomalous Fe-Zn codeposition as if the zinc/iron ratio is low iron begins to deposit as can also be observed when using the AEM, where iron begins to deposit when zinc conversion is higher than 50%. Finally, if zinc is added to the cathodic compartment of the divided reactor in the presence of the CEM, iron

6 deposition does not take place as a consequence of the anomalous Zn/Fe codeposition phenomenon mentioned above X Zn Undivided reactor AEM 0.1 CEM C0=0 CEM C0=0.1M (ZnCl2+HCl) 0.0 Figure 4. Evolution of zinc conversion vs. time as a function of the applied current in the presence of AEM, CEM and without membrane. 1:10 diluted spent bath. Zinc current efficiency is shown in Figure 6 for all the cases under study. It is worth to note that, as expected, higher current efficiencies are obtained when treating this more concentrated spent baths in relation to the data presented in Figure 3. The presence of the CEM when zinc is added to the cathodic compartment produces high values of f since the iron codeposition, as secondary reaction, is avoided. In the case of the AEM, although iron deposits for time values higher than 200 minutes, current efficiency presents high values since zinc and iron are in the cathodic compartment and the current efficiency is not affected by the mass transfer through the membrane. The lowest values of f are obtained for the undivided reactor due to the zinc redissolution phenomenon that causes zinc oxidation, and for the divided reactor in the presence of the CEM and in the absence of zinc in the cathodic compartment, since part of the current is wasted in the ion transport through the membrane and in the iron deposition Undivided reactor AEM CEM C0=0 CEM C0=0.1M (ZnCl2+HCl) 0.6 X Fe Figure 5. Evolution of iron conversion vs. time as a function of the applied current in the presence of AEM, CEM and without membrane. 1:10 diluted spent bath.

7 Undivided reactor AEM CEM C0=0 CEM C0=0.1M (ZnCl2+HCl) 60 f (t) Figure 6. Evolution of zinc current efficiency vs. time as a function of the applied current in the presence of AEM, CEM and without membrane. 1:10 diluted spent bath. 4. CONCLUSIONS Two kinds of membranes and two kinds of reactors in galvanostatic mode were used for treating two spent pickling baths with dilution factors of 1:50 and 1:10. These solutions mainly contain ZnCl 2 and FeCl 2 in aqueous HCl media. Different figures of merit were calculated in order to compare the behavior of the reactors in the presence of an AEM, a CEM or in the absence of any membrane. In all the cases under study, the use of the membrane prevents the chlorine presence in the cathodic compartment avoiding by this way the redissolution of zinc deposits. Regarding the 1:50 dilution factor, AEM presents the best results in all the figures of merit for both applied currents although it allows iron codeposition. On the other hand, for the 1:10 diluted baths, the results obtained are very different. The undivided reactor presents, the worse results as chlorine attack on zinc deposits become stronger and practically all the zinc deposited is redissolved. In addition, iron codeposites with zinc for all cases under study except for the CEM when zinc is initially added to the cathodic compartment, since this initial zinc together with the zinc that passes through the membrane maintain the Zn/Fe ratio preventing by this way iron codeposition. This addition also improves the current efficiency value which presents values closer to those obtained for AEM. Therefore, the CEM in the presence of initial zinc in the cathodic compartment is selected as the optimal way for zinc recovery although more studies regarding the dilution factor of the spent baths and the applied current have to be done. REFERENCES 1. Marder, A. R. (2000) The metallurgy of zinc-coated steel, Progress in Materials Science, 45, Kerney, U. (1994) Treatment of spent pickling acids from hot dip galvanizing Resources, Conservation and Recycling, 10, Regel-Rosocka, M. (2010) A review on methods of regeneration of spent pickling solutions from steel processing, Journal of Hazardous Materials, 177, Regel-Rosocka, M. and Wisniewski, M. (2011) Selective removal of zinc(ii) from spent pickling solutions in the presence of iron ions with phosphonium ionic liquid Cyphos IL 101, Hydrometallurgy, 110,

8 5. Marañón, E., Fernandez, Y., Suarez, F. J., Alonso, F. J. and Sastre, H. (2000) Treatment of Acid Pickling Baths by Means of Anionic Resins, Industrial & Engineering Chemistry Research, 39, García-Gabaldón, M., Carrillo-Abad, J., Pérez-Herranz, V. and Ortega-Navarro, E. M. (2011) Electrochemical Study of a Simulated Spent Pickling Solution, International Journal of Electrochemical Science, 6, Carrillo-Abad, J., García-Gabaldón, M., Ortega, E. and Pérez-Herranz, V. (2011) Electrochemical recovery of zinc from the spent pickling baths coming from the hot dip galvanizing industry. Potentiostatic operation, Separation and Purification Technology, 81, Carrillo-Abad, J., García-Gabaldón, M., Pérez-Herranz, V. (2012) Electrochemical Recovery of Zinc from the Spent Pickling Solutions Coming from Hot Dip Galvanizing Industries. Galvanostatic Operation, International Journal of Electrochemical Science, 7, Caldwell-Ralston, O. (1921) Electrolytic Deposition and Hydrometallurgy of Zinc, McGraw- Hill Book Company (ed.), University of Wisconsin Madison, Thomas, B. K. and Fray, D. J. (1981) The effect of additives on the morphology of zinc electrodeposited from a zinc chloride electrolyte at high current densities, Journal of Applied Electrochemistry, 11, Díaz-Arista, P., Mattos, O. R., Barcia, O. E. and Fabri Miranda, F. J. (2002) ZnFe anomalous electrodeposition: stationaries and local ph measurements, Electrochimica Acta, 47, Gómez, E., Peláez, E. and Vallés, E. (1999) Electrodeposition of zinc+iron alloys: I. Analysis of the initial stages of the anomalous codeposition, Journal of Electroanalytical Chemistry, 469, Zhang, Z., Leng, W. H., Shao, H. B., Zhang, J. Q., Wang, J. M. and Cao, C. N. (2001) Study on the behavior of Zn-Fe alloy electroplating, Journal of Electroanalytical Chemistry, 516, Carrillo-Abad, J., García-Gabaldón, M., Ortega, E. and Pérez-Herranz, V. (2012) Recovery of zinc from spent pickling solutions using an electrochemical reactor in presence and absence of an anion-exchange membrane: Galvanostatic operation, Separation and Purification Technology, 98,

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