Removal of Zinc(II) and Iron(II) from Spent Hydrochloric Acid by Means

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Ind. Eng. Chem. Res. 2005, 44, 1004-1011

Removal of Zinc(II) and Iron(II) from Spent Hydrochloric Acid by Means of Anionic Resins Ireneusz Miesiac Institute of Chemical Technology & Engineering, Poznan University of Technology, 60-965 Poznan, Poland

Anion exchange removed Zn(II) effectively from a spent hydrochloric acid with the practical exchange capacity of 0.62-0.76 mol/L Zn(II). The main species retained was monovalent ZnCl3(80-90%) in the case of strongly basic anion exchangers, whereas in the case of weakly basic anion exchanger, its contribution was only 40%. Regeneration with water gave only dilute (1520 g/L) Zn(II) solution contaminated with Fe(II). Improved regeneration with 6 M HCl and 25% ammonia solution eluted a Zn(II) solution of 42-100 g/L with iron(II) content below 0.1 g/L. Acid retardation provided effective separation of HCl and Fe(II) using the strongly basic Lewatit VP OC 1071. The productivity of the separation column amounted to 50-60 kg/(m3 h) of iron(II) with a reasonable separation factor of Fe(II)/HCl equal to about 10. A new idea of acid retardation mechanism is proposed, in which two Donnan equilibria are considered: normal FeCl2 exclusion and HCl partition due to the membrane potential and salting-out effect. Introduction Pickling of steel goods in hydrochloric acid is the most frequently used technique for surface preparation before hot-dip zinc galvanizing.1 An originally 20% HCl solution becomes inactive when the iron content reaches 80-100 g/L and the HCl concentration decreases to 8-10%. Additionally, the solution also contains 5-20 g/L of zinc(II), which makes it a dangerous waste. The pickling bath used has a high pollution potential, the management of which involves significant economic and environmental costs. For small plants, a common procedure consists of neutralization of spent hydrochloric acid with lime to obtain a solid precipitate of metal hydroxides that require special disposal. The concentration of zinc(II) in the obtained wastewater should not exceed 2 mg/L. Also commonly used is thermal hydrolysis of the spent pickling solution at 800 °C (Ruthner process), providing recovery of the HCl solution and iron(III) oxide. This method has the disadvantages of high energy consumption and technically defined limit of zinc(II) concentration below 0.5 g/L. Many physicochemical methods can be used to separate the species present in the spent hydrochloric acid. Membrane distillation2 enables the recovery of concentrated HCl solution (azeotropic concentration) from the dissolved salts at temperatures below 100 °C, using porous polypropylene membranes. This method still remains at the laboratory scale. Diffusion dialysis enables excellent separation of iron(II) from HCl and Zn(II), but the two latter components permeate together through the membrane. The observed flux values of HCl and Zn(II) are in the range of 20.0 and 0.55 mol/(m2 h), respectively.3 Solvent extraction can be used to remove zinc(II) from aqueous solution.4,5 Our previous works6 show that zinc(II) can be successfully removed from HCl solutions by extraction with tributyl phosphate (TBP), trialkylphosphine oxides, and secondary amines (Amberlite LA-2 and HOE F2562). At the investigated concentra* E-mail: [email protected]. Fax: +4861 6653649.

tions of hydrochloric acid, zinc(II) was present mainly in the form of anionic chloro complexes ZnCl3- and ZnCl42-. The stoichiometry of the zinc chloro complex extraction could then be described by the following reactions for the solvating (S, TBP or Cyanex 923) and basic (B, HOE F2562 or Amberlite LA-2) reagents

2H+w + ZnCl42-w + 2So ) H2ZnCl4‚2So

(1)

ZnCl42-w + 2B‚HClo ) (BH)2ZnCl4o + 2Cl-w (2) The extractants studied were observed to extract Fe(III) efficiently. Their maximum distribution coefficients depend strongly on the actual HCl concentration and are in the range of 102-103 for both Fe(III) and Zn(II). Extraction of iron(II) is usually negligible but can be enhanced under special conditions that promote undesired oxidation of Fe(II) to Fe(III). The said extractants, especially those of basic character, can be considered as liquid anion exchangers. Thus, the solid anion exchangers exhibit similar behavior, i.e., a similarly high level of distribution coefficients for Zn(II) and Fe(III).7 This means that the proper processing of spent pickling acid requires careful elimination of Fe(III) to ensure high selectivity of Zn(II) sorption. On the basis of this principle, an industrial process was proposed by USF Recon.8 The process consists of four steps: loading of a special ion exchanger with spent hydrochloric acid containing 5-10 g/L Zn(II), stripping with a special solution to remove Fe(II) from the bed, regeneration with a special solution to achieve a high concentration of Zn(II) above 60 g/L, and final washing with water. The method continuously decreases the concentration of zinc(II) to 1 g/L or less and produces a concentrated zinc(II) solution to be used as a fluxing bath in the same galvanizing plant. Recently, it has been shown that the removal of Zn(II) and Fe(III) occurred efficiently when caused by the use of the strongly anionic exchangers Lewatit M 504 (gel) and MP 500 (macroporous).9 The total exchange capacity was in the range of 17-31 g/L for Zn(II) and 5-19 g/L for Fe(III). Regeneration of a loaded resin with water

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Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 1005

gave only dilute Zn(II) solution (six bed volumes), except when ammonia was used for regeneration (two bed volumes). Unfortunately, the authors used the same ionexchange method to remove iron(II) after its total oxidation to Fe(III) with hydrogen peroxide. There is no good reason to oxidize all of the Fe(II) present in the spent hydrochloric acid [20 times as much as Zn(II)] to adsorb it as an anionic chloro complex in a separate anion-exchange column. First, strong oxidizing agents can destroy quaternary ammonium functional groups; second, Fe(II) can be removed much more easily using the acid retardation process. Acid retardation systems employ an anion-exchange resin that sorbs strong acids from solution but excludes the metallic salts of those acids (except for salts forming acid-like chloro complexes).10 The process is reversible in that the acid can be readily desorbed from the resin with water. Indeed, the anionic bed works as a chromatographic column and is alternately fed with contaminated acid and water. The free acid thus diffuses into the resin where it is retained, whereas the solution containing the metal salts, e.g., FeCl2, passes through the resin bed and exits the absorber column first. Usually, one bed volume of each contaminated acid and water is used in each cycle. Dissolved iron(II) chloride, free of Zn(II) but with small amounts of HCl, is removed from the unit in a byproduct stream. The acid-enriched fraction is recycled back to the pickle tank. The method was commercialized in 1978 as a RECOFLO Acid Purification System and successfully employed in the purification of sulfuric acid anodizing solutions, mixtures of nitric and hydrofluoric acids, and also hydrochloric acid.11 A similar method has been proposed for industrial use by USF Guetling Ionpure as the KOMParet Retardation System.12 Whereas acid retardation was first observed in the early 1960s, no reasonable explanation for this phenomenon had been found until recently. Hatch and Dillon13 proposed four different models to explain the sorption of acids in the resin: salting-out of acids in the presence of salts repulsed from the resin, hydrogen bonding of protons with the benzene rings of polymer matrix, adsorption of nondissociated acid molecules, and entropic effects associated with the sorption. The idea of nondissociated HCl molecules is proposed as the most probable acid retardation mechanism, but it is poorly understood.14,15 Nevertheless, from a practical point of view, several rules of acid retardation can be derived:14 (i) strongly basic anion exchangers are more suitable than weakly basic ones; (ii) a gellike structure of the resin is better than a macroporous one; (iii) the resin particle size should not be too fine (decreased fraction distance), but it should be uniform; (iv) the resin bed height should not be less than 0.5 m; and (v) flow rates are usually about 2 h-1. The aim of the work was to determine the ionexchange capacity for zinc(II) and especially its recovery in the form of a highly concentrated solution and to estimate the effectiveness of iron(II) removal from spent hydrochloric acid using the acid retardation process. Experimental Section Ion exchange was carried out at ambient temperature (21 ( 1 °C) in a special chromatographic glass column (QVF, Mainz, Germany), 2.6 cm in diameter and 60 cm in height. The total volume of the column was filled with 320 mL of a wet anion exchanger in Cl- form without

Figure 1. Experimental setup.

any voids (Figure 1). The column was fed on top, using a peristaltic pump (Zalimp, Warsaw, Poland) at the flow rate of 15.7 mL/min (3 h-1). After the loading was finished, the column was washed with 3-6 M HCl and then with water or 10-25% ammonia solution at the same flow rate. The resulting effluent was collected in a fraction collector (Laboratorni Pristroje, Prague, Czechoslovakia) from which the samples for the acid, zinc, and iron analyses were taken. Three different types of anion exchangers, supplied kindly by Bayer AG (Leverkusen, Germany), were used as follows: strongly basic Lewatit M500 Mono Plus (gel form, polystyrene matrix, bead size of 0.62 ( 0.05 mm), strongly basic Lewatit VP OC 1071 (gel form, polyacrylamide matrix, polydisperse bead size of 0.4-1.6 mm) and weakly basic Lewatit MP 64 Mono Plus (macroporous, polystyrene, bead size 0.59 ( 0.05 mm). Prior to use, the exchangers were washed with 4% NaOH, then with water and 5% HCl to determine their exchange capacities and to obtain a properly Cl--exchanged form of the anionic resin bed. The obtained exchange capacities were 0.78, 0.81 and 1.05 equiv/L for Lewatit M 500 Mono Plus, VP OC 1071, and MP 64 Mono Plus, respectively. Model solutions containing 3 M HCl (POCh, Gliwice, Poland) and 5.56 g/L Zn(II) were obtained by dissolving anhydrous ZnCl2 (POCh, Gliwice, Poland). The spent hydrochloric acid (Mostostal Siedlce, Siedlce, Poland) contained 2.23 M HCl and 88.5 g/L Fe total, including 1.6 g/L Fe(III) and 4.65 g/L Zn(II). The solution was stored over metallic iron (fine steel wool, ca. 5 g/L) to provide full reduction of Fe(III) (color transition from dark yellow to light green). Higher concentrations of zinc(II) were obtained by dissolving a metallic zinc scrap and adjusting the acid content with a fresh 35% HCl solution. Analytical Procedures. Biamperometric microtitration with 0.05 M K4Fe(CN)6 was used to determine zinc(II) content in the aqueous solution (two Pt wire electrodes polarized at 200 mV, EMU Analyzer, Wroclaw Technical University, Wroclaw, Poland). During the titration, the presence of Fe(II) was masked with 10% NH4F after Fe(II) had been oxidized with 30% H2O2 at pH 3-4, adjusted with a 10% NH3 solution. Spectrophotometric analysis with 1,10-phenanthroline (510 nm) was used to determine the total iron content. Acid content was determined by potentiometric titration with 0.1 M KOH (Titrino 702 SM, Metrohm). Two

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Figure 2. Elution curve of Zn(II) with water [Lewatit M500; model solution, 3 M HCl; c0 ) 5.56 g/L Zn(II)].

Figure 3. Elution curve of Zn(II) with 3 M HCl, water, and 10% ammonia.

equivalent points were obtained: at pH 4.3 for free HCl and at pH 9 for the content of metal chlorides. Results and Discussion Ion Exchange of Zinc(II) Chloro Complexes. The results of loading and regeneration of Lewatit M 500 by means of a model zinc solution in 3 M HCl are presented in Figures 2 and 3. All of the figures are constructed as isoplane diagrams, i.e., the area of each component to be eluted is the same and corresponds to the feed volume. This feature facilitates the calculation of fraction compositions by simple integration of the chosen areas. The processes exhibited a breakthrough at 7.5 bed volumes (BVs), which was related to the actual Zn(II) concentration in the feed (c0 ) 5.56 g/L). The whole loading of the resin bed required an additional 3-5 BV feed solution to achieve the initial Zn(II) concentration. After the loading was finished, it was necessary to remove the remaining feed solution by quick washing. The rinsing with water resulted in direct regeneration of Zn(II). It could be seen that high flows of the regeneration agent diminished the concentration of Zn(II) in the effluent (2.6c0), compared with slow regeneration (3.25c0). From the mass balance, the estimated exchange capacity of the resin bed was equal to 0.77 mol/L Zn(II). This value was very close to the previously determined total exchange capacity (0.78 equiv/L) and indicated that the monovalent chloro complex ZnCl3- was absorbed in the bed. The calculated mean Zn(II) concentration in the effluent (3.7 BV, without tail) was equal to 14.6 g/L. To improve the regeneration of Zn(II), three different media were used to wash the loaded column (Figure 3). First, 3 M HCl (1 BV) was used to exclude the feed solution containing Zn(II); second, water (1.5 BV) was used to separate the acidic effluent from the alkaline one; and finally, 10% ammonia was used to improve

Figure 4. Loading and regeneration of Lewatit M 500 with spent hydrochloric acid [flow rate, 3 h-1; feed concentrations, 1.95 M HCl, 95.4 g/L Fe(II), 27.1 g/L Zn(II)].

Zn(II) regeneration. The use of ammonia effected in a slight increase in the Zn(II) concentration, but the precipitation of zinc hydroxides also occurred during the first 0.5 BV, which caused a significant decrease in the flow in the column. It was possible to obtain a zinc fraction with a narrow volume (2.8 BV) and at the mean concentration of 18 g/L. Figure 4 demonstrates the whole course of loading and regeneration of Lewatit M 500 with the actual spent hydrochloric acid containing 1.95 M HCl, 95.4 g/L Fe(II), and 27.1 g/L Zn(II). A relatively high concentration of Zn(II) in the feed shortened the loading times and did not affect the exchange capacities. This process comprises both ion exchange and acid retardation, but the latter will be discussed later. Even at high Zn(II) concentration in the feed, the full loading required approximately 2 BV (40 min) to achieve the initial Zn(II) concentration in the effluent. This proved the useful effectiveness of a novel technique known as “short bed ion exchange”, which better utilizes the whole resin volume.11 To discharge the feed solution, a volume (0.6 BV) of 6 M HCl solution was used followed by water. The concentration of the HCl solution corresponded to the total chloride ion concentration in the spent hydrochloric acid and should not release the absorbed Zn(II) chloro complexes. The volume of 6 M HCl together with HCl desorbed from the bed by the water rinse gave a large acid fraction (ca. 1.5 BV), which discharged iron(II) from the bed almost completely and could be fed back to the pickling bath. A decrease in the HCl concentration indicated the beginning of Zn(II) regeneration with water. The maximum concentration of Zn(II) did not exceed the feed concentration and dropped slowly as a strong tailing. The amount of Zn(II) remaining in the tail was shown by the peak obtained when 25% ammonia solution was used as a stripping agent (0.3 BV). In this case, the maximum Zn(II) concentration achieved 3.5c0 (95 g/L), and the average fraction concentration (0.5 BV) was equal to 47.7 g/L. Of course, the first contact of ammonia solution with zinc(II) chloride caused a little precipitation, which disappeared quickly in ammonia excess, forming cationic ammonia complexes Zn(NH3)22+. The flow in the column was not disturbed; only small pipe connections could be obstructed at first. From the integration of the whole Zn(II) fraction eluted in the range of 5-6.6 BV, the exchange capacity was calculated as being equal to 0.62 mol/L. Comparing this value with the determined whole exchange capacity (0.78 equiv/L), the distribution of zinc(II) chloro complexes loaded in the resin was estimated as 0.46 mol/L ZnCl3- (80%) and 0.12 mol/L ZnCl42- (20%).


Figure 5. Loading and regeneration of Lewatit VP OC 1071 with spent hydrochloric acid [flow rate, 4 h-1; feed concentrations, 1.95 M HCl, 95.4 g/L Fe(II), 21.9 g/L Zn(II)].

The elution curve of Fe(II) indicated its fully inert character in relation to the resin bed. Washing with 6 M HCl almost completely removed the iron(II) and only small amounts of iron(II) were collected in the Zn(II) fraction. The total Fe(II) concentration found in the Zn(II) effluent was equal to 0.39 g/L, whereas the Zn(II) concentration was 24.7 g/L. The use of ammonia for stripping enabled full elimination of iron(II) from the zinc effluent because traces of iron(II) precipitated on the resin and could be removed by the acid rinse in the next cycle. A great disadvantage of Lewatit M 500 was its strong osmotic contraction on contact with spent hydrochloric acid, causing a 15% decrease in bed volume. This was not found in the case of either of the other anion exchangers of different structure. Figure 5 shows an ion-exchange process effected with the use of the strongly basic resin Lewatit VP OC 1071 specially designed for acid retardation. The sorption of zinc(II) occurred typically without any contraction of bed volume. Washing with 2 M HCl (0.6 BV) displaced iron(II) quickly, but a remaining tail contaminated the zinc(II) fraction. Further washing with water gave a characteristic maximum on HCl concentration curve, which indicated that the excess HCl concentration absorbed into resin particles in comparison with the feed concentration (uphill transport). Zinc(II) desorption began immediately after the acid passed its maximum, quickly attained the maximum concentration of 2c0 (44 g/L) and dropped out monotonically with tailing. The fraction of effluent between 4.6 and 6.3 BV had a mean concentration of 25.4 g/L Zn(II) and 2.2 g/L Fe(II). On the basis of the total zinc(II) amounts eluted in the fraction limit of 4.6-8 BV, the exchange capacity was calculated as 0.74 mol/L Zn(II), whereas the value determined from OH-/Cl- exchange was 0.81 equiv/L. This means that, in this case, the distribution of zinc(II) chloro complexes loaded in the resin bed was shifted almost completely toward monovalent ZnCl3- (ca. 90%). To diminish the retained iron(II) content in the desorbed zinc(II) fraction, the 6 M HCl solution was used as a stripping agent. The column with Lewatit VP OC 1071 was double loaded to show additionally the acid retardation phenomenon on the loaded resin (Figure 6). Rinsing with 6 M HCl solution (0.6 BV) gave an abrupt decrease in Fe(II) concentration toward zero. Zinc(II) desorption occurred similarly to the former case, with the same maximum concentration of about 2c0 (42 g/L). In the fraction limit of 6.7-8.25 BV, the mean concentration of zinc(II) was 30.4 g/L, and that of iron(II) was 0.39 g/L. Thus, the 6 M HCl solution was a suitable agent for stripping the loaded anion-exchange bed prior

Figure 6. Double loading and regeneration of Lewatit VP OC 1071 with spent hydrochloric acid [flow rate, 2 h-1; feed concentrations, 1.95 M HCl, 95.4 g/L Fe(II), 21.9 g/L Zn(II)].

Figure 7. Loading and regeneration of Lewatit MP 64 with spent hydrochloric acid [flow rate, 3 h-1; feed concentrations, 2.23 M HCl, 93.2 g/L Fe(II), 31.8 g/L Zn(II)].

to Zn(II) desorption with water. The process was independent of the flow rate in the column in the range of 2-4 h-1. Figure 7 demonstrates loading and regeneration of the weakly basic Lewatit MP 64 Mono Plus with a macroporous structure. After the bed had been saturated with Zn(II), the rinse with 6 M HCl displaced the remaining iron(II) very effectively and a maximum of acid concentration appeared in the effluent. Desorption of zinc(II) was not as fast as in the case of strongly basic gellike exchangers, and as a result, better separation was observed. Further desorption was gently enhanced by the use of 12% ammonia solution. A narrow fraction of concentrated zinc(II) solution was obtained with the formation of a small amount of precipitate that dissolved quickly in the excess of ammonia. The maximum concentration of Zn(II) exceeded 100 g/L, but the mean concentration of Zn(II) in the fraction limit of 4.9-5.5 BV was equal to 42.4 g/L. The calculated exchange capacity for zinc(II) was 0.66 mol/L, which, compared with the determined model value of 1.05 equiv/L, gave the following distributions of zinc(II) chloro complexes loaded in the resin bed: 40% ZnCl3- and 60% ZnCl42-. Chromatographic Characteristics of IonExchange Columns. Acid retardation is, in principle, a chromatographic process in which the partition isotherm and the shape of the elution curve play an important role. This behavior is better shown when a small feed volume is used for separation. Two separation processes were carried out using a small volume (0.08 BV) of spent hydrochloric acid containing 6.34 g/L Zn(II) and using two resin beds: the strongly basic gel-form Lewatit VP OC 1071 and the weakly basic macroporous Lewatit MP 64 (Figures 8 and 9). The void effluent volume was higher for Lewatit MP 64 (0.5 BV) than for VP OC 1071 (0.33 BV), which confirms the macroporous structure of the former. The

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Figure 8. Chromatographic separation of spent hydrochloric acid on Lewatit VP OC 1071 (flow rate, 3 h-1; feed, 0.08 BV).

Figure 10. Separation of iron(II) and HCl by acid retardation on Lewatit VP OC 1071 [spent hydrochloric acid with a zinc(II) content of 0.1 g/L Zn(II) for a flow rate of 4 h-1]. Table 1. Effectiveness of Iron(II) Removal from Spent Hydrochloric Acid Using Lewatit VP OC 1071a effluent volume (BV) 0.62 0.73 0.79 0.85 0.90 a

Figure 9. Chromatographic separation of spent hydrochloric acid on Lewatit MP 64 (flow rate, 3 h-1; feed, 0.08 BV).

shapes of the HCl peaks were almost the same, although that of Lewatit MP 64 was slightly higher and appeared earlier (relative retention volume of 0.45 BV). The separation of the HCl/Fe(II) peaks related as the difference of retention volumes was thus greater for the gel-form Lewatit VP OC 1071 (0.25 BV) than for the macroporous MP 64 (0.22 BV), i.e., the former is more suitable for acid retardation. The position of the Zn(II) peak was less important for acid retardation, but it indicated the column behavior in the ion-exchange process. The elution of Zn(II) with water began immediately with decreasing concentration of HCl. The increase in Zn(II) concentration was more pronounced for Lewatit VP OC 1071, causing a nonsymmetric peak and indicating a rapid desorption of acidic species such as HZnCl3. Further desorption of Zn(II) was slow with broad tailing, which complicated the whole regeneration of the resin bed. The same tailing was observed in the case of desorption of Fe(II) and was evidently related to the gel form of the resin bed or its polydispersity. Fe(II) tailing was not important for acid retardation but could disturb the regeneration of Zn(II) in the ionexchange process. Acid Retardation. Although acid retardation constitutes the first part of a commonly used ion-exchange process, separate experiments were conducted to avoid any influence on the part of the loading degree of the resin bed. The apparatus and methods used were the same, except for the spent hydrochloric acid, which was purified by ion exchange to keep the content of zinc(II) below 0.1 g/L. The resulting acid solution was fed in the column (1.2 BV, flow rate of 4 h-1) and then washed back with water. The results obtained with Lewatit VP OC 1071 are shown in Figure 10. A void volume equal to 0.3 BV corresponds to the water contained between the resin grains. The increase in the concentration of iron(II) and the shape of the curve depended only on the dynamic flow parameters of the column. On the contrary, the shape of the acid curve was sharp because

contribution Fe(II) HCl 0.25 0.33 0.38 0.44 0.48

0.007 0.037 0.065 0.107 0.146

separation factor, S

productivity [kg of Fe/(m3 h)]

35 9.0 5.9 4.1 3.3

43 59 68 77 85

Flow rate, 4 h-1; Zn(II) content, 0.1 g/L (see Fig. 10).

the acid was strongly absorbed in the resin and its transport path was shorter. The shape of the salt curve was flatter because the salt solution was still diluted with water that was repulsed osmotically from the resin and because of the dispersive flow. The area between the two concentration curves at the beginning of the process (fraction limited by arrows 1 and 2, Figure 10) corresponded to the acid sorption capacity (AC) and can be calculated as follows

AC ) (A1 - A2)c0 where A1 denotes the area of Fe(II) in the chosen fraction, A2 denotes the area of HCl in the chosen fraction, and c0 ) 2.23 mol/L denotes the HCl concentration in the feed. The calculated value of acid sorption capacity was equal to 0.72 mol/L, which was near to the actual anionexchange capacity (0.81 equiv/L). The acid retardation process is not ideal, and only partial separation can be achieved. High productivity of a metal salt is in opposition to the separation factor. Some arbitrarily derived separation parameters are summarized in Table 1. The main parameter is the column productivity or yield, expressed as the amount of iron removed from a unit bed volume per unit time. This parameter can be derived from the relationship

P)

Ac0v F

where P is the column productivity, kg of Fe(II)/[(m3 of resin bed) h]; c0 is the Fe(II) concentration in the feed, kg/m3; v is the flow rate, h-1 {i.e., m3/[(m3 of resin bed) h]}; and F is the effluent volume needed for 1 cycle, BV (typically, F ) 2 BV). The contribution of each component can be calculated by integrating the concentration curve in a chosen fraction boundary. The ratio of these contributions


Figure 11. Separation of iron(II) and HCl by acid retardation on Lewatit VP OC 1071 [spent hydrochloric acid with a zinc(II) content of 4.65 g/L Zn(II) for a flow rate of 4 h-1]. Figure 13. Acid retardation on Lewatit MP 64 [spent hydrochloric acid with a zinc(II) content of 4.65 g/L Zn(II) for a flow rate of 4 h-1; feed 2 denotes the possible beginning of cycle 2 in the absence of zinc(II)]. Table 2. Effectiveness of Iron(II) Removal from Spent Hydrochloric Acid Using Lewatit VP OC 1071a effluent volume (BV)

Figure 12. Separation of iron(II) and HCl by acid retardation on Lewatit VP OC 1071 [spent hydrochloric acid with a zinc(II) content of 0.1 g/L Zn(II) for a flow rate of 2 h-1].

forms the separation factor (S)

S)

A B

where A is the Fe(II) contribution in the chosen fraction and B is the HCl contribution in the chosen fraction. The increase in the fraction volume to be removed caused an increase in the column productivity from 43 to 85 kg of Fe(II)/(m3 h), but simultaneously, the separation factor dropped. It seemed reasonable to select the medium productivity value of about 60 kg of Fe(II)/ (m3 h) with a separation factor of 9.0. Figure 11 demonstrates the acid retardation process when zinc(II) was present in the spent hydrochloric acid. Separation efficiencies for iron(II) and HCl were not altered but a large fraction of regenerated zinc(II) solution appeared. This fraction could contaminate the iron(II) fraction from the next cycle, making the whole process useless. The effect of total loading with Zn(II) upon acid retardation is shown in Figure 6. In the middle region of the process, acid retardation occurred with feed 2, and the calculated HCl sorption capacity was slightly lower, being equal to 0.67 mol/L. This means that the sorption capacity of HCl is not noticeably affected by the anion-exchanger loading with zinc(II) chloro complexes, although from a practical point of view, the two processes of ion exchange and acid retardation should be carried out separately. Figure 12 shows an acid retardation process similar to the process showed in Figure 10 but with one-half the flow rate. The shape of the iron(II) concentration profile was almost the same whereas the initial HCl profile was enlarged. Evidently, the low flow rate accounted for the longitudinal dispersivity of HCl, which decreased the separation efficiency. As a result, the column productivity dropped, in proportion to the

0.51 0.69 0.73 0.78 0.82 0.87 a

contribution Fe(II) HCl 0.18 0.32 0.36 0.40 0.44 0.48

0.012 0.058 0.078 0.100 0.130 0.165



15 5.5 4.6 4.0 3.4 2.9

16 28 32 35 39 42


Table 3. Effectiveness of Iron(II) Removal from Spent Hydrochloric Acid Using Lewatit MP 64a effluent volume (BV) 0.47 0.56 0.62 0.66 0.71 a

contribution Fe(II) HCl 0.15 0.23 0.27 0.32 0.36

0.007 0.041 0.074 0.114 0.157



22 5.6 3.7 2.8 2.3

26 41 48 57 64


decrease in flow rate even by the decreased separation factor (Table 2). The results of using macroporous, weakly basic Lewatit MP 64 for acid retardation are shown in Figure 13. All of the components formed symmetrical peaks without tailing, which was characteristic of the said resin bed. Nevertheless, the distance between HCl and Fe(II) was too small, which resulted in column productivity values about 50% lower than those obtained with Lewatit VP OC 1071 (Table 3). The calculated HCl sorption capacity was much smaller and equal to 0.46 mol/L. This confirmed the poor ability of macroporous anion exchangers for the acid retardation process.14 Mechanism of Acid Retardation. The separation of an acid and its salt by passing them through an anion-exchange bed is based on Donnan equilibrium. The resin bead forms an osmotic cell which has a high concentration of immobile, positively charged functional groups that repulse the same charged ions, i.e., Fe2+, present in the outer solution. Formally, the same mechanism should also exclude HCl molecules, which are fully dissociated in an aqueous solution. It is wellknown that, on the surface that separates two solutions, a membrane potential is formed, i.e., the resin beads are positively charged because of the high concentration of H+ and Fe2+ cations. The membrane charge is

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immediately moved to the hydrated bead core as the result of a diffusionless shift of H+ ions through water hydrogen bonds, i.e., only the electrons are moving.16 In this way, the bead core of an anion-exchange resin is enriched in positively charged, immobile active centers that attract the Cl- anions to attain electroneutrality. Moreover, the hydrochloric acid sorbed in the resin is fully dissociated and can contribute to the Donnan equilibrium. That are two Donnan equilibria in the system: (i) An equilibrium is connected with the concentration of iron(II) and the sum of the chloride ions in the resin bead and in the outer solution, responsible for the repulsion of FeCl2 from the resin

([Fe2+][Cl-sum])out ) ([Fe2+][Cl-sum])in (ii) Another equilibrium exists between the HCl concentrations in the resin beads and in the outer solution, which can also be interpreted as a salting-out effect

([H+][Cl-sum])out ) ([H+][Cl-sum])in where [Cl-sum]out denotes the sum of Cl- concentrations in the outer solution consisting of HCl, FeCl2, and ZnCl2 in the feed solution equal to 5.55 mol/L and [Cl-sum]in denotes the sum of Cl- concentrations in the inner solution of the resin bead coming from the sorbed HCl and ion-exchange capacity. It is hard to find the proper concentration of Cl- ions in the inner solution of the resin bead. The normal exchange capacity of about 1 equiv/L should be increased by factors of 1.33 (33% of bed void volume) and 2 (50% hydration of resin beads), giving a Cl- concentration of about 2.7 equiv/L in the inner solution. This is much lower than the Cl- concentration in the used feed solution, which enables the uphill transport of HCl in the resin bead core. Indeed, as a rule, the concentration of HCl eluted from the resin bed exceeds the feed concentration by a factor of 1.3-1.5. Such behavior has also been observed in the diffusion dialysis process, when the same feed solution was contacted with water through an anion-exchange membrane. After 2 h of the dialysis process, the concentration of HCl in the feed solution decreased to 1.38 mol/L, whereas the concentration in the aqueous strip solution reached 2.4 mol/ L.17 Conclusions Using different anion exchangers, it was possible to remove Zn(II) effectively from spent hydrochloric acid with a practical exchange capacity of 0.62-0.76 mol/L Zn(II). Advantageously, the main species retained on the strongly basic anion-exchange bed was monovalent ZnCl3- (80-90%), whereas in the case of the weakly basic anion exchanger, its contribution was only 40%. The best results were obtained for the strongly basic gel-form Lewatit VP OC 1071 with a polyacrylamide matrix. Another strongly basic anion exchanger (Lewatit M 500, gel form, polystyrene matrix) was not suitable for this purpose because of a significant volume change in the concentrated electrolyte solution (15%). The sensitivity of the osmotic chocks can damage the resin beads and shorten their lifetime. The exchange capacity of Lewatit MP 64, a weakly basic macroporous anion exchanger, was comparable to the best result found for

Lewatit VP OC 1071 and its regeneration properties were even better. The main problem in the case of regeneration of Zn(II) from the resin bed was perhaps the high concentration of the Zn(II) solution, free of iron(II). Normally, the regeneration process occurred with water, but the obtained Zn(II) fraction was dilute (15-20 g/L) and contaminated with Fe(II). Improvement in Zn(II) regeneration was obtained when 6 M HCl was used as a strip solution to wash out the remaining amounts of Fe(II) and then 25% ammonia solution was used to enhance the Zn(II) concentration. This procedure enabled highly concentrated Zn(II) solution in the range of 42-100 g/L with an iron(II) content below 0.1 g/L to be obtained. The separation of HCl and Fe(II) occurred effectively when Lewatit VP OC 1071 was used for acid retardation. Because this process chromatographic in origin, it involved alternately feeding the column with contaminated acid and water, i.e., no zinc(II) should be present in the feed solution because of tailing. With a reasonable separation factor of Fe(II)/HCl equal to about 10, the productivity of the separation column amounted to 5060 kg/(m3 h) of iron(II). The effectiveness of the macroporous Lewatit MP 64 was about 30% lower, which is consistent with literature data. A new idea for acid retardation mechanism was proposed, in which two Donnan equilibria are considered: normal FeCl2 exclusion and new HCl partition due to the membrane potential and salting-out effect. Acknowledgment The work was supported by the Polish State Committee for Scientific Research, KBN Grants 7 T09 B05421 and 32-044/05-DS. Bayer AG, Leverkusen, Germany, is gratefully acknowledged for supplying the Lewatit samples. Literature Cited (1) Winkel, P. Wasser und Abwasser; Eugen G. Leuze Verlag: Saulgau, Germany, 1986; p 117. (2) Tomaszewska, M.; Gryta, M.; Morawski, A. W. The Recovery of Hydrochloric Acid from Industrial Effluents by Membrane Distillation. In Proceedings of Euromembrane, Bath 1995; A. Rowe Ltd.: Chippenham, U.K., 1995; Vol. 1, pp 540-543. (3) Miesia¸ c, I. Utilization methods of spent hydrochloric acid from hot dip zinc galvanizing. Pol. J. Chem Technol. 2003, 5, 3436. (4) Mishonov, I.; Szymanowski, J. Recovery of zinc(II) from primary and secondary chloride solutions by solvent extraction. Pol. J. Chem. Technol. 2002, 4, 187-207. (5) Jha, M. K.; Kumar, V.; Singh, R. J. Solvent Extraction of Zinc from Chloride Solutions. Solvent Extr. Ion Exch. 2002, 20, 389-405. (6) Cierpiszewski, R.; Miesiac, I.; Regel-Rosocka, M.; Sastre, A. M.; Szymanowski, J. Removal of Zinc(II) from Spent Hydrochloric Acid Solution from Hot-Dip Galvanizing Plants. Ind. Chem. Eng. Res. 2002, 41, 598-603. (7) Tremillon, B. Les se´ parations par les re´ sines e´ changeuses d’ions; Gauthier-Villars: Paris, 1965. (8) Das ReZn-Verfahren; USF Recon: Verfahrenstechnik GmbH, D-71108 Waldenbuch, Germany, 1999 (www.usfilter.com). (9) Maranon, E.; Fernandez, Y.; Suarez, F. J.; Alonso, F. J.; Sastre, H. Treatment of Acid Pickling Baths by Means of Anionic Resins. Ind. Eng. Chem. Res. 2000, 39, 3370-3376. (10) Munns, W. K. Iron Removal from Pickle Liquors using Absorption Resin Technology. In Iron Control in Hydrometallurgy; Dutrizac, J. E., Ed., Ellis Horwood Ltd.: Chichester, U.K., 1984; p 26.

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 1011 (11) Brown, C. J. Acid and Metals Recovery by Recoflo Short Bed Ion Exchange. In Separation Processes in Hydrometallurgy; Davies, G. A., Ed., Ellis Horwood Ltd.: Chichester, U.K., 1987; p 36. (12) Komponenten fuer Wassertechnische Systeme, Retardation KOMParet; USF Guetling Ionpure: D-70736 Fellbach, Germany, 1999 (www.usfilter.com). (13) Hatch, M. J.; Dillon, J. A. Acid Retardation. Ind. Eng. Chem. 1963, 2, 253-263. (14) Go¨tzelmann, W.; Hartinger, L.; Gu¨lbas, M. Stofftrennung und Stoffru¨ckgewinnung mit dem Retardation-Verfahren, Teil 1, 2. Metalloberfla¨ che 1987, 41, 208-212; Metalloberfla¨ che 1987, 41, 315-322. (15) Gu¨lbas, M. Regeneration von sauren Prozesslo¨sungen und Endelektrolyten in der Hydrometallurgie durch Retardation. Galvanotechnik 2001, 92, 3386-3397.

(16) Agmon, N. The Grotthuss Mechanism. Chem. Phys. Lett. 1995, 244, 456-462. (17) Miesiac, I, Selectivity of Zinc(II), Iron(II) and HCl Removal from Spent Hydrochloric Acid using Diffusion Dialysis. In Membrany I Procesy Membranowe w Ochronie Srodowiska; Monografie Komitetu Inz˘ ynierii Srodowiska PAN: Gliwice, Poland, 2004; Vol. 22, pp 223-231.

Received for review July 16, 2004 Revised manuscript received November 24, 2004 Accepted November 27, 2004 IE0493762

Removal of Zinc(II) and Iron(II) from Spent Hydrochloric Acid by Means

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