- Email: [email protected]
Magnetic separation of ferrihydrite from wastewater by magnetic seeding and high-gradient magnetic separation
Int. J. Miner. Process. 71 (2003) 45 – 54 www.elsevier.com/locate/ijminpro
Magnetic separation of ferrihydrite from wastewater by magnetic seeding and high-gradient magnetic separation Nuray Karapinar * Technology Department, General Directorate of Mineral Research and Exploration, 06520 Ankara, Turkey Received 22 May 2002; received in revised form 5 March 2003; accepted 6 March 2003
Abstract Ferrihydrite, a member of iron oxides family, has been used as an adsorbent for the removal of heavy metal ions from industrial wastewater. The success of the operation depends mainly on the efficient removal of ferrihydrite from the aqueous phase. Hence, the emphasis of this study was given on the separation of ferrihydrite by high-gradient magnetic separator (HGMS) to overcome solid/liquid separation difficulties of ferrihydrite. This paper clarifies the seeding of ferrihydrite with magnetite mineral and its effects on the operation parameters in HGMS. Magnetic seeding technique was used to make magnetic separation available for the removal of ferrihydrite, and the magnetite mineral was chosen as a seeding material. The method clearly involves the attachment of ferrihydrite to a magnetic seed material and subsequent magnetic separation of ferrihydrite – magnetite coagulates. In seeding process, finely divided magnetite particles were entrapped in ferrihydrite precipitate evolving by increasing pH of solution. In relation with the hydrolysis properties of Fe(III) ion, there is a pH range where the seeding performance is optimal. Iron/magnetite ratio had a marked effect on the separation of seeded ferrihydrite precipitate by HGMS. This ratio determines the operation parameters such as magnetic field strength and flow rate on which the cost and performance of HGMS depend. Test results showed that when used with magnetic seeding technique, HGMS has the potential to overcome separation difficulties associated with adsorption-based treatment techniques with its advantages of high-performance, high-capacity and low-space requirements. D 2003 Elsevier Science B.V. All rights reserved. Keywords: iron oxides; ferrihydrite; magnetic seeding; solid/liquid separation; magnetic separation
* Tel.: +90-312-2873430/2116; fax: +90-312-2875409. E-mail address: [email protected] (N. Karapinar). 0301-7516/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-7516(03)00029-2
46
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
1. Introduction Because of the stringent treatment and disposal regulations, the use of sorption processes for the removal of heavy metals from wastewater has been seen as a promising method. Inorganic sorbents are becoming interesting recently in this regard as being their lower cost when compared to commercial sorbents such as activated carbon and ionexchange resins. The application of several industrial by-products and mineral particles has recently been proposed as alternative sorbents for the removal of toxic ions. Because of the well-known importance of iron oxides in attenuating heavy metal transport, several investigators have suggested iron oxides as appropriate adsorbent. Among the iron oxides, ferrihydrite has been known as a quite efficient sorbent for metal ion removal from industrial wastewater (Schultz et al., 1987; Edwards and Benjamin, 1989), and many laboratory-based adsorption studies have been performed to investigate this phenomenon (e.g., Davis and Leckie, 1978; Benjamin and Bloom, 1981; Dzombak and Morel, 1986). However, solid/liquid separation of ferrihydrite has been accepted as a limitation factor for the wastewater treatment applications because of the difficulty in dewatering of ferrihydrite suspensions (Anderson et al., 1991; Benjamin et al., 1996). Magnetic separation is a method for the separation of particles on the basis of their magnetic properties and has been used in the mineral processing and the recycle industries for many years for concentration and purification requirements. Efficient use of magnetic separation methods in mineral processing has encouraged its use in wastewater treatment. In the 1970s, the introduction of high-gradient magnetic separators (HGMS) developed not only for the recovery of useful materials but also rapid treatment of, wastewater, has made the magnetic separation method applicable to the handling of weekly submicron particles (Svoboda, 1987). As a matter of fact, it has been well known that magnetic separation has been used for the removal of magnetic solid particles in wastewater. Recent progress in this field is the removal of nonmagnetic water pollutants such as virus, algae and dissolved pollutants by magnetic separation that relies on the usage of magnetic seeding technique to enhance the magnetic properties of pollutants to be removed (e.g., Anderson et al., 1983; Anand et al., 1985; Treshima et al., 1986; Van Valsen et al., 1991; De Reuver, 1994; Gillet and Diot, 1999; Franzreb and Ho¨ll, 2000). In spite of the fact that the subject was first studied by De Latour, 1973, and till now a number of reports have been published on the subject (e.g., Moffat et al., 1994; Prenger et al., 1994; Karapinar, 2003), magnetic separation method is a not well-known process in wastewater treatment facilities. In this regard, magnetic separation method may overcome separation difficulties associated with Fe oxides adsorbent by considerably accelerating solid/liquid separation process, and hence increasing the throughput. However, to do this, iron ions can be precipitated a ferrite sludge with magnetic properties (Tamaura et al., 1991; Barrado et al., 1998), or precipitated slurries are seeded with very fine magnetite powder (Treshima et al., 1986; Anderson et al., 1991). When the ferrihydrite is made magnetic, fluid can be treated directly in HGMS without a need for sludge sedimentation. This study is an example of the application of magnetic separation method for the removal of iron oxide adsorbents to overcome solid/liquid separation of it. In this study, it
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
47
was clarified the seeding of ferrihydrite with magnetite mineral and its effects on the operation parameters in HGMS.
2. Experimental 2.1. Materials and method Analytical grade chemicals were used in all experiments. Iron concentration was analyzed by atomic absorption spectrometer. Since being the strongest adsorbent, reasonably inexpensive and insoluble over a wide pH range, freshly precipitated ferrihydrite was chosen as the model adsorbent for the study. A convenient and widespread procedure that was also used in this work is to neutralize ferric solutions with excess of alkali to give a red-brown precipitate that is called ferrihydrite (Schwertmann and Murad, 1983). Because of its large magnetic moment, magnetite mineral (Fe3O4), obtained from Turkish Iron and Steel Works (Divrigi Iron Plant Concentrator, Sivas, Turkey), was used as the seeding material. It was ground in ring mill and sized by coulter counter method. Magnetite, having a mean particle size of 13 Am, was used for seeding purposes. The specific surface area of magnetite seed measured by BET method was approximately 2.0 m2/g. Before using in any experiment, magnetite was washed in diluted HNO3, and after it was rinsed in deionized water, it was dried at 105 jC and demagnetized. A Carpco, WHMS laboratory model HGMS was used in the experiments. The magnetic field in the working gap was generated by a conventional electromagnet and the field gradient was enhanced by a matrix consisting of steel balls having diameter of 4.1 –6.3 mm. The slurry was fed onto the matrix where magnetic particles were captured onto the balls while the nonmagnetic fraction passed through the matrix. The experimental set-up is shown in Fig. 1.
Fig. 1. Experimental set-up (1: mechanic stirrer, 2: peristaltic pump, 3: feed box, 4: coils, 5: current control panel).
48
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
Seeding of ferrihydrite with magnetite mineral was prepared by entrapping of the magnetite in a ferrihydrite precipitate. A certain amount of magnetite was suspended in Fe(NO3)3 solution, and the pH was adjusted to the desired value with NaOH. After 1 h of aging, the suspension was passed through the HGMS at a desired flow rate by using a peristaltic pump. Parameters including amount of iron, magnetite addition and pH of solution were investigated in seeding process. In addition, effects of operation parameters such as flow rate and magnetic field strength in HGMS were investigated on the basis of seeding process conditions. After separation by HGMS, iron and magnetite concentrations were determined after dissolution of iron precipitates in acidified nonmagnetic fraction. After each set, the magnetic field was de-energized and the particles were washed from the matrix.
3. Results and discussion The ferrihydrite was characterized by X-ray diffraction analysis, which showed a spectrum of typical amorphous structure. The specific surface area of ferrihydrite was measured by the single-point BET method (using liquid N2) and found to be 140 m2/g. A pycnometrically measured density of the precipitate was found 3.8 g/cm3. Iron oxides and related amorphous hydrates have been the subject of many studies regarding the preparation conditions and chemical and physical properties (e.g., Van der Giessen, 1966; Towe and Bradley, 1967; Eggleton and Fitzpatrick, 1988). Therefore, it was emphasized to investigate the magnetic seeding and separation of ferrihydrite by magnetic separators rather than its formation and structure. The results of the separation of ferrihydrite seeded with magnetite by HGMS will be discussed in two parts; seeding process and separation of seeded ferrihydrite flocs with magnetite. The effectiveness of the seeding process was evaluated by determining iron concentration of nonmagnetic fraction after HGMS separation. When the magnetite was removed by the separator, the amount of Fe(III) remaining was regarded as the measure of the success of the seeding process of ferrihydrite with magnetite mineral. 3.1. Seeding of ferrihydrite with magnetite To clarify the role of magnetic seeding on the separation of ferrihydrite by HGMS, a part of experiments were done without magnetite addition at different pH values (Fig. 2). Since ferrihydrite without the addition of magnetite particles cannot be separated magnetically, magnetic seeding technique was found to be essential for the separation of ferrihydrite by HGMS. It was observed that any amount of magnetite addition improved the separation efficiency of ferrihydrite by HGMS (Figs. 2 and 3). In seeding process, magnetite seed provides a surface for nucleation of ferrihydrite precipitate. Seeding of ferrihydrite with magnetite particles was achieved by exceeding the solubility limit of the ions in the presence of dispersed magnetite. During the slow neutralization of the mixture, the ferric ions present in solution form a series of hydrolysis product. When the concentration of ferric ions exceeds the solubility limit, a sequence of
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
Fig. 2. Fe(III) removed by HGMS as a function of pH (Fe(III)=0.510 flow rate=0.57 l/min).
2
49
M, magnetic field strength=6900 G,
hydrolytic and condensation reactions occurs, leading to the formation of polynuclear complexes and ultimately to the formation of insoluble hydrous ferric oxide. It is known that these polynuclear complexes, in other words, colloidal hydroxo polymers, have strong affinity for bound surfaces due to OH ions in the ligands (Stumm and Morgan, 1996). Hence, during the precipitation reaction, adsorbed colloidal hydroxo polymer entrapped the magnetite through polymer bridging. Since the iron concentration used was sufficiently high to cause precipitate of metal hydroxide, finely divided magnetite particles were entrapped in the voluminous ferrihydrite precipitate, which is identified as an amorphous iron oxyhydroxide. During the growth of amorphous ferrihydrite precipitate, magnetite particles were entrapped in this structure. The addition of magnetite seed would rather enhance reactions by providing a surface for nucleation during the evaluation of precipitate. Based on this phenomenon, sweep flocculation (or enmeshment) mechanism was apparently responsible for the seeding of ferrihydrite precipitates by magnetite. Fig. 4 shows the concentration of iron species not bound to the magnetite separated by HGMS as a function of pH. It can be concluded that seeding of ferrihydrite with magnetite is maximum in the pH range of 7.0 – 10.0. Out of this pH range, iron concentration of nonmagnetic fraction was increased. Intensifying of magnetic field resulted in no change on removal. The best results were obtained around pH=8.0. pH dependence of the process can be explained by both the hydrolysis properties of ferric ion and surface charge characteristics of precipitate and magnetite mineral. It is known that for pH values near to the point of zero charge (ZPC), the insoluble precipitate forms rapidly; outside of that region, surface charge repulsions retard this rapid precipitation (Stumm and Morgan, 1996). In literature, the ZPC given for ferrihydrite is pH=7.9– 8.2. Therefore, it can be expected that the maximum removal will occur at this pH range. In addition, with the increasing (decreasing) of pH, there will be an increase in the surface charges of both magnetite (in the presence of 210 3 M Fe3+, ZPC value of magnetite is around pH=7.0 (Sun et al., 1998)) mineral and ferrihydrite, which leads to increase electrostatic repulsion force between them.
50
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
3.2. HGMS experiments Magnetic separation of ferrihydrite can be achieved by using magnetic seeding technique. Results showed that the separation efficiency of ferrihydrite by HGMS depends on the ratio of iron concentration to magnetite dosage (Table 1). When the amount of magnetite was small or the amount of Fe(III) is large, that is, the ratio of the amount of nonmagnetic precipitate to that of magnetite was higher, efficiency was decreased. Since the basic property that determines the effectiveness of magnetic separation is the magnetic force exerted on the particles, a possible reason for the reduced effectiveness with higher ratios might be due to the decreasing of the magnetization of precipitate. At higher ratios, concentration of magnetite entrapped in ferrihydrite flocs, consequently magnetization of precipitate, would be decreased, and voluminous fragile flocs would be obtained. According to these findings, improvement of separation efficiency could be achieved by intensifying applied magnetic field, which would result in an increase of magnetic force exerted on the particles. Likewise, when the magnetic field increased from 6900 to 17,700 G, an improvement in magnetic separation of ferrihydrite was obtained at high iron-tomagnetite ratios (Fig. 3). This means that the same performance can be achieved if the iron concentration is decreased at the same magnetite addition. As had been expected, decreased iron concentration to 0.2510 2 M, keeping magnetic addition at 0.5 g/l (Exp. 5), resulted an increase in separation efficiency (Table 1). The effect of magnetic field intensity on the magnetic removal of seeded ferrihydrite was studied by changing the field from 6900 to 17,700 G and keeping the other parameters constant. It was found that there is a very little, if any, effect of magnetic field on the removal (Fig. 4). Therefore, at these conditions, the lowest magnetic field of 6900 G was found to be optimal. However, it was later observed that when the flow rate increased, an increase in applied field is essential to enhance the magnetic force exerted to particle. It was also found that the amount of magnetite addition at a constant iron concentration affects matrix separation capacity (Table 2). With higher ratios of Fe to magnetite obtained by increasing amount of Fe(III), keeping magnetite addition constant, decrease in separation efficiency was more pronounced because of early breakthrough, and some Table 1 Experimental results obtained at the different iron-to-magnetite ratios (magnetic field strength=6900 G, flow rate=0.57 l/min, pH=8.0) Experiment no.
Fe(III)/Magnetite (mmol/g)
Total Fe(III) (M)
1 2 3 4 5 6 7
2.0 2.5 2.5 5.0 5.0 10.0 10.0
0.5010 0.5010 1.0010 0.5010 0.2510 0.5010 1.0010
2 2 2 2 2 2 2
Magnetite (g/l)
Treated volume (l)
Nonmagnetic fraction Fe (ppm)
Magnetite (%)
2.5 2.0 4.0 1.0 0.5 0.5 1.0
1.0 1.0 1.0 1.0 1.0 1.0 0.5 1.0
0.24 0.70 1.08 9.01 0.90 21.43 21.81 35.80
* * * 0.6 * 0.96 0.8 1.27
* There is no considerable amount of magnetite in nonmagnetic fraction.
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
Fig. 3. The effect of magnetite addition on the magnetic separation of ferrihydrite (Fe(III)=0.510 rate=0.57 l/min, pH=8.0).
51
2
M, flow
finely magnetite particles were also obtained in nonmagnetic fraction. However, matrix separation capacity could be increased by adding more magnetite into solution containing same iron concentration. These suggest that the total amount of precipitate captured by a HGMS matrix decreases with the decrease of magnetization of the precipitate. This also means that separation capacity of matrix can be increased by increasing magnetic force exerted on particle achieved by either more magnetic addition or intensifying of magnetic field strength. However, it was observed that more than a certain amount of magnetite causes the plugging of matrix before breakthrough.
Fig. 4. The effect of magnetic field strength on the removal of ferrihydrite as a function of pH (Fe(III)=0.510 magnetite addition=2.0 g/l, flow rate=0.57 l/min).
2
M,
52
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
Table 2 The effect of magnetite addition on the separation capacity of matrix at a 0.510 (magnetic field strength=6900 G, flow rate=0.57 l/min, pH=8.0)
2
M iron concentration
Magnetite (g)
Treated volume (l)
Nonmagnetic fraction Fe (ppm)
Magnetite (%)
2.0
0.255 0.470 0.690 0.875 1.000 0.245 0.465 0.680 0.885 1.000
0.07 0.22 0.22 0.37 0.37 0.31 0.57 1.35 4.31 9.01
* * * * * * * 0.12 0.42 1.27
1.0
* There is no considerable amount of magnetite in nonmagnetic fraction.
According to the results obtained, 2.0 g/l of magnetite dosage was found to be optimal for seeding of 0.510 2 M Fe concentration. In this ratio, heavily impregnated ferrihydrite flocs of reasonable stability were produced. In the case of 2.0 g/l magnetic addition, flow rate was varied from 0.57 to 3.0 l/min for a solution containing 0.510 2 M Fe(III) (Fig. 5). It was found that an increase in flow rate resulted in an increase in iron and magnetite concentration of nonmagnetic fraction. Generally speaking, intensifying of magnetic field permits increased flow rates without sacrifice in performance. It was observed that this is true up to a point. For flow rates higher than 2 l/min, an increase in magnetic field did not improve the magnetic separation efficiency. These data suggest that as water velocity through the canister increases beyond a certain point, the drag force on the trapped magnetite floc particle becomes great enough
Fig. 5. Ferrihydrite removal by HGMS as a function of flow rate (Fe(III)=0.510 pH=8.0).
2
M, magnetite addition=2.0 g/l,
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
53
to shear some of the ferrihydrite precipitates from the magnetite particles, resulting in lower quality product water. It appears that the minimum magnetic field required for maximum removal is just that required to trap and hold the composite magnetite particles against the drag forces exerted by flowing water and the gravitational forces. 4. Conclusion It has been well known that ferrihydrite is a good scavenger for heavy metals and many radio nuclides; its precipitation from waste streams could be used as an effective method of clean up if satisfactory process for the recovery of the ferrihydrite was available. Highgradient magnetic separation could be a solution in this regard for the separation of ferrihydrite. Because of its low magnetic susceptibility, direct application of HGMS for the separation of ferrihydrite is not available. The experiments reported here demonstrate the viability of the magnetic separation of ferrihydrite using magnetic seeding technique. Ferrihydrite was seeded with magnetite mineral and then separated by magnetic separation. The main results are as follows: 1. HGMS treatment with the combination of magnetic seeding technique is available for the solid/liquid separation of ferrihydrite flocs that scavenge the heavy metal ions from solution. 2. Maximum seeding was obtained at a pH range that is the ZPC of ferrihydrite. Sweep flocculation was found to be the main mechanism in seeding process. 3. Iron-to-magnetite ratio that determines the magnetization of floc was found to be main criteria and determines the operation conditions in HGMS separation. At higher ratios, magnetization of precipitate was decreased, which leads to decrease in magnetic separation performance. The time of breakthrough from the HGMS was found to depend on the weight of the chemical precipitates to the magnetite, namely, on the magnetization of the precipitate produced after seeding. Lower ratios of iron to magnetite enhance the matrix separation capacity.
Acknowledgements This work was supported by TUBITAK (The Scientific and Technical Research Council of Turkey, Project No. YDABCßAG-336).
References Anand, P., Etzel, J.E., Friedlaender, F.J., 1985. Heavy metals removal by high gradient magnetic separation. IEEE Transactions on Magnetics mag-21, 2062 – 2064. Anderson, N.J., Bolto, B.A., Blesing, N.V., Kolarik, L.O., 1983. Colour and turbidity removal with reusable magnetite particles: VI. Pilot plant operation. Water Research 17, 1235 – 1243. Anderson, P.R., Chen, W.Y., Roth, K.E., 1991. Evaluation of a magnetite-based adsorption process for the
54
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
recovery of metals from industrial wastewaters. In: Tu¨rkman, A., Uslu, O. (Eds.), New Developments in Industrial Wastewater Treatment. NATO ASI Series, vol. 191. Kluwer Academic Publishers, Netherlands, pp. 183 – 199. Barrado, F., Prieto, F., Vega, M., Fernandez-Polanco, F., 1998. Optimization of the operational variables of a medium scale reactor for metal-containing wastewater purification by ferrite formation. Water Research 32 (10), 3055 – 3061. Benjamin, M.M., Bloom, N.S., 1981. Effects of strong binding of anionic adsorbates on adsorption of trace metals on amorphous iron oxyhdroxide. In: Tewari, P.H. (Ed.), Adsorption from Aqueous Solutions. Plenum, New York, pp. 41 – 60. Benjamin, M.M., Sletten, R.S., Bailey, R.P., Bennett, T., 1996. Sorption and filtration of metals using iron-oxide coated sand. Water Research 30 (11), 2609 – 2620. Davis, J.A., Leckie, J.O., 1978. Surface ionization and complexation at the oxide/water interface: II. Surface properties of amorphous iron oxyhydroxide and adsorption of metal ions. Journal of Colloid and Interface Science 67 (1), 90 – 107. De Latour, C., 1973. Magnetic separation in water pollution control. IEEE Transactions on Magnetics mag-9 (3), 314 – 316. De Reuver, J.L., 1994. Magnetic wastewater treatment in the US chemical industry. Filtration and Separation Journal 31 (6), 605 – 607. Dzombak, D.A., Morel, F.M.M., 1986. Sorption of cadmium on hydrous ferric oxide at high sorbate/sorbent ratios: equilibrium, kinetics and modeling. Journal of Colloid and Interface Science 112 (2), 588 – 598. Edwards, M., Benjamin, M.M., 1989. Regeneration and reuse of iron hydroxide adsorbents in treatment of metal bearing wastes. Journal of Water Pollution Control Federation 61 (4), 481 – 490. Eggleton, R.A., Fitzpatrick, R.W., 1988. New data and a revised structural model for ferrihydrite. Clays and Clay Minerals 36 (2), 111 – 124. Franzreb, M., Ho¨ll, W.H., 2000. Phosphate removal by high-gradient magnetic filtration using permanent magnets. IEEE Transactions on Applied Superconductivity 10 (1), 923 – 926. Gillet, G., Diot, F., 1999. Removal of heavy metal ions by superconducting magnetic separation. Separation Science and Technology 34 (10), 2023 – 2037. Karapinar, N., 2003. Magnetic separation; an alternative method to the treatment of wastewater. The European Journal of Mineral Processing and Environmental Protection (in evaluation, 1 – 15). Moffat, G., Williams, R.A., Webb, C., Stirling, R., 1994. Selective separations in environmental and industrial processes using magnetic carrier technology. Minerals Engineering 7, 1039 – 1056. Prenger, F.C., Stewart, W.F., Hill, D.D., Avens, L.R., Worl, L.A., Schake, A., de Aguero, K.J., Padilla, D.D., Tolt, T.L., 1994. High gradient magnetic separation applied to environmental remediation. Advances in Cryogenic Engineering 39, 485 – 491. Schultz, M.F., Benjamin, M.M., Ferguson, J.F., 1987. Adsorption and desorption of metals on ferrihydrite: reversibility of the reaction and sorption properties of regenerated solid. Environmental Science and Technology 21 (9), 863 – 869. Schwertmann, U., Murad, E., 1983. The effect of pH on the formation of goethite and hematite from ferrihydrite. Clays and Clay Minerals 31, 277 – 284. Stumm, W., Morgan, J.J., 1996. Aquatic Chemistry, Chemical Equilibria and Rates in Natural Water, 3rd ed. John Wiley and Sons, New York. Sun, Z.X., Su, F.W., Forsling, W., Samskog, P.O., 1998. Surface characteristics of magnetite in aqueous suspension. Journal of Colloid and Interface Science 197, 151 – 159. Svoboda, J., 1987. Magnetic Methods for the Treatment of Minerals. Elsevier, Amsterdam, pp. 590 – 605. Tamaura, Y., Katsura, T., Rojarayanont, S., Yoshida, T., Abe, H., 1991. Ferrite process; heavy metal ions treatment system. Water Science and Technology 23, 1893 – 1900. Towe, K.M., Bradley, W., 1967. Mineralogical constitution of colloidal ‘‘Hydrous Ferric Oxides’’. Journal of Colloid and Interface Science 24, 384 – 392. Treshima, Y., Ozaki, H., Sekine, M., 1986. Removal of dissolved heavy metals by chemical coagulation magnetic seeding and high gradient magnetic filtration. Water Research 20 (5), 537 – 545. Van der Giessen, A.A., 1966. The structure of iron (III) oxide-hydrate gels. Journal of Inorganic and Nuclear Chemistry 28, 2155 – 2159.
Magnetic separation of ferrihydrite from wastewater by magnetic seeding and high-gradient magnetic separation Nuray Karapinar * Technology Department, General Directorate of Mineral Research and Exploration, 06520 Ankara, Turkey Received 22 May 2002; received in revised form 5 March 2003; accepted 6 March 2003
Abstract Ferrihydrite, a member of iron oxides family, has been used as an adsorbent for the removal of heavy metal ions from industrial wastewater. The success of the operation depends mainly on the efficient removal of ferrihydrite from the aqueous phase. Hence, the emphasis of this study was given on the separation of ferrihydrite by high-gradient magnetic separator (HGMS) to overcome solid/liquid separation difficulties of ferrihydrite. This paper clarifies the seeding of ferrihydrite with magnetite mineral and its effects on the operation parameters in HGMS. Magnetic seeding technique was used to make magnetic separation available for the removal of ferrihydrite, and the magnetite mineral was chosen as a seeding material. The method clearly involves the attachment of ferrihydrite to a magnetic seed material and subsequent magnetic separation of ferrihydrite – magnetite coagulates. In seeding process, finely divided magnetite particles were entrapped in ferrihydrite precipitate evolving by increasing pH of solution. In relation with the hydrolysis properties of Fe(III) ion, there is a pH range where the seeding performance is optimal. Iron/magnetite ratio had a marked effect on the separation of seeded ferrihydrite precipitate by HGMS. This ratio determines the operation parameters such as magnetic field strength and flow rate on which the cost and performance of HGMS depend. Test results showed that when used with magnetic seeding technique, HGMS has the potential to overcome separation difficulties associated with adsorption-based treatment techniques with its advantages of high-performance, high-capacity and low-space requirements. D 2003 Elsevier Science B.V. All rights reserved. Keywords: iron oxides; ferrihydrite; magnetic seeding; solid/liquid separation; magnetic separation
* Tel.: +90-312-2873430/2116; fax: +90-312-2875409. E-mail address: [email protected] (N. Karapinar). 0301-7516/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-7516(03)00029-2
46
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
1. Introduction Because of the stringent treatment and disposal regulations, the use of sorption processes for the removal of heavy metals from wastewater has been seen as a promising method. Inorganic sorbents are becoming interesting recently in this regard as being their lower cost when compared to commercial sorbents such as activated carbon and ionexchange resins. The application of several industrial by-products and mineral particles has recently been proposed as alternative sorbents for the removal of toxic ions. Because of the well-known importance of iron oxides in attenuating heavy metal transport, several investigators have suggested iron oxides as appropriate adsorbent. Among the iron oxides, ferrihydrite has been known as a quite efficient sorbent for metal ion removal from industrial wastewater (Schultz et al., 1987; Edwards and Benjamin, 1989), and many laboratory-based adsorption studies have been performed to investigate this phenomenon (e.g., Davis and Leckie, 1978; Benjamin and Bloom, 1981; Dzombak and Morel, 1986). However, solid/liquid separation of ferrihydrite has been accepted as a limitation factor for the wastewater treatment applications because of the difficulty in dewatering of ferrihydrite suspensions (Anderson et al., 1991; Benjamin et al., 1996). Magnetic separation is a method for the separation of particles on the basis of their magnetic properties and has been used in the mineral processing and the recycle industries for many years for concentration and purification requirements. Efficient use of magnetic separation methods in mineral processing has encouraged its use in wastewater treatment. In the 1970s, the introduction of high-gradient magnetic separators (HGMS) developed not only for the recovery of useful materials but also rapid treatment of, wastewater, has made the magnetic separation method applicable to the handling of weekly submicron particles (Svoboda, 1987). As a matter of fact, it has been well known that magnetic separation has been used for the removal of magnetic solid particles in wastewater. Recent progress in this field is the removal of nonmagnetic water pollutants such as virus, algae and dissolved pollutants by magnetic separation that relies on the usage of magnetic seeding technique to enhance the magnetic properties of pollutants to be removed (e.g., Anderson et al., 1983; Anand et al., 1985; Treshima et al., 1986; Van Valsen et al., 1991; De Reuver, 1994; Gillet and Diot, 1999; Franzreb and Ho¨ll, 2000). In spite of the fact that the subject was first studied by De Latour, 1973, and till now a number of reports have been published on the subject (e.g., Moffat et al., 1994; Prenger et al., 1994; Karapinar, 2003), magnetic separation method is a not well-known process in wastewater treatment facilities. In this regard, magnetic separation method may overcome separation difficulties associated with Fe oxides adsorbent by considerably accelerating solid/liquid separation process, and hence increasing the throughput. However, to do this, iron ions can be precipitated a ferrite sludge with magnetic properties (Tamaura et al., 1991; Barrado et al., 1998), or precipitated slurries are seeded with very fine magnetite powder (Treshima et al., 1986; Anderson et al., 1991). When the ferrihydrite is made magnetic, fluid can be treated directly in HGMS without a need for sludge sedimentation. This study is an example of the application of magnetic separation method for the removal of iron oxide adsorbents to overcome solid/liquid separation of it. In this study, it
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
47
was clarified the seeding of ferrihydrite with magnetite mineral and its effects on the operation parameters in HGMS.
2. Experimental 2.1. Materials and method Analytical grade chemicals were used in all experiments. Iron concentration was analyzed by atomic absorption spectrometer. Since being the strongest adsorbent, reasonably inexpensive and insoluble over a wide pH range, freshly precipitated ferrihydrite was chosen as the model adsorbent for the study. A convenient and widespread procedure that was also used in this work is to neutralize ferric solutions with excess of alkali to give a red-brown precipitate that is called ferrihydrite (Schwertmann and Murad, 1983). Because of its large magnetic moment, magnetite mineral (Fe3O4), obtained from Turkish Iron and Steel Works (Divrigi Iron Plant Concentrator, Sivas, Turkey), was used as the seeding material. It was ground in ring mill and sized by coulter counter method. Magnetite, having a mean particle size of 13 Am, was used for seeding purposes. The specific surface area of magnetite seed measured by BET method was approximately 2.0 m2/g. Before using in any experiment, magnetite was washed in diluted HNO3, and after it was rinsed in deionized water, it was dried at 105 jC and demagnetized. A Carpco, WHMS laboratory model HGMS was used in the experiments. The magnetic field in the working gap was generated by a conventional electromagnet and the field gradient was enhanced by a matrix consisting of steel balls having diameter of 4.1 –6.3 mm. The slurry was fed onto the matrix where magnetic particles were captured onto the balls while the nonmagnetic fraction passed through the matrix. The experimental set-up is shown in Fig. 1.
Fig. 1. Experimental set-up (1: mechanic stirrer, 2: peristaltic pump, 3: feed box, 4: coils, 5: current control panel).
48
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
Seeding of ferrihydrite with magnetite mineral was prepared by entrapping of the magnetite in a ferrihydrite precipitate. A certain amount of magnetite was suspended in Fe(NO3)3 solution, and the pH was adjusted to the desired value with NaOH. After 1 h of aging, the suspension was passed through the HGMS at a desired flow rate by using a peristaltic pump. Parameters including amount of iron, magnetite addition and pH of solution were investigated in seeding process. In addition, effects of operation parameters such as flow rate and magnetic field strength in HGMS were investigated on the basis of seeding process conditions. After separation by HGMS, iron and magnetite concentrations were determined after dissolution of iron precipitates in acidified nonmagnetic fraction. After each set, the magnetic field was de-energized and the particles were washed from the matrix.
3. Results and discussion The ferrihydrite was characterized by X-ray diffraction analysis, which showed a spectrum of typical amorphous structure. The specific surface area of ferrihydrite was measured by the single-point BET method (using liquid N2) and found to be 140 m2/g. A pycnometrically measured density of the precipitate was found 3.8 g/cm3. Iron oxides and related amorphous hydrates have been the subject of many studies regarding the preparation conditions and chemical and physical properties (e.g., Van der Giessen, 1966; Towe and Bradley, 1967; Eggleton and Fitzpatrick, 1988). Therefore, it was emphasized to investigate the magnetic seeding and separation of ferrihydrite by magnetic separators rather than its formation and structure. The results of the separation of ferrihydrite seeded with magnetite by HGMS will be discussed in two parts; seeding process and separation of seeded ferrihydrite flocs with magnetite. The effectiveness of the seeding process was evaluated by determining iron concentration of nonmagnetic fraction after HGMS separation. When the magnetite was removed by the separator, the amount of Fe(III) remaining was regarded as the measure of the success of the seeding process of ferrihydrite with magnetite mineral. 3.1. Seeding of ferrihydrite with magnetite To clarify the role of magnetic seeding on the separation of ferrihydrite by HGMS, a part of experiments were done without magnetite addition at different pH values (Fig. 2). Since ferrihydrite without the addition of magnetite particles cannot be separated magnetically, magnetic seeding technique was found to be essential for the separation of ferrihydrite by HGMS. It was observed that any amount of magnetite addition improved the separation efficiency of ferrihydrite by HGMS (Figs. 2 and 3). In seeding process, magnetite seed provides a surface for nucleation of ferrihydrite precipitate. Seeding of ferrihydrite with magnetite particles was achieved by exceeding the solubility limit of the ions in the presence of dispersed magnetite. During the slow neutralization of the mixture, the ferric ions present in solution form a series of hydrolysis product. When the concentration of ferric ions exceeds the solubility limit, a sequence of
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
Fig. 2. Fe(III) removed by HGMS as a function of pH (Fe(III)=0.510 flow rate=0.57 l/min).
2
49
M, magnetic field strength=6900 G,
hydrolytic and condensation reactions occurs, leading to the formation of polynuclear complexes and ultimately to the formation of insoluble hydrous ferric oxide. It is known that these polynuclear complexes, in other words, colloidal hydroxo polymers, have strong affinity for bound surfaces due to OH ions in the ligands (Stumm and Morgan, 1996). Hence, during the precipitation reaction, adsorbed colloidal hydroxo polymer entrapped the magnetite through polymer bridging. Since the iron concentration used was sufficiently high to cause precipitate of metal hydroxide, finely divided magnetite particles were entrapped in the voluminous ferrihydrite precipitate, which is identified as an amorphous iron oxyhydroxide. During the growth of amorphous ferrihydrite precipitate, magnetite particles were entrapped in this structure. The addition of magnetite seed would rather enhance reactions by providing a surface for nucleation during the evaluation of precipitate. Based on this phenomenon, sweep flocculation (or enmeshment) mechanism was apparently responsible for the seeding of ferrihydrite precipitates by magnetite. Fig. 4 shows the concentration of iron species not bound to the magnetite separated by HGMS as a function of pH. It can be concluded that seeding of ferrihydrite with magnetite is maximum in the pH range of 7.0 – 10.0. Out of this pH range, iron concentration of nonmagnetic fraction was increased. Intensifying of magnetic field resulted in no change on removal. The best results were obtained around pH=8.0. pH dependence of the process can be explained by both the hydrolysis properties of ferric ion and surface charge characteristics of precipitate and magnetite mineral. It is known that for pH values near to the point of zero charge (ZPC), the insoluble precipitate forms rapidly; outside of that region, surface charge repulsions retard this rapid precipitation (Stumm and Morgan, 1996). In literature, the ZPC given for ferrihydrite is pH=7.9– 8.2. Therefore, it can be expected that the maximum removal will occur at this pH range. In addition, with the increasing (decreasing) of pH, there will be an increase in the surface charges of both magnetite (in the presence of 210 3 M Fe3+, ZPC value of magnetite is around pH=7.0 (Sun et al., 1998)) mineral and ferrihydrite, which leads to increase electrostatic repulsion force between them.
50
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
3.2. HGMS experiments Magnetic separation of ferrihydrite can be achieved by using magnetic seeding technique. Results showed that the separation efficiency of ferrihydrite by HGMS depends on the ratio of iron concentration to magnetite dosage (Table 1). When the amount of magnetite was small or the amount of Fe(III) is large, that is, the ratio of the amount of nonmagnetic precipitate to that of magnetite was higher, efficiency was decreased. Since the basic property that determines the effectiveness of magnetic separation is the magnetic force exerted on the particles, a possible reason for the reduced effectiveness with higher ratios might be due to the decreasing of the magnetization of precipitate. At higher ratios, concentration of magnetite entrapped in ferrihydrite flocs, consequently magnetization of precipitate, would be decreased, and voluminous fragile flocs would be obtained. According to these findings, improvement of separation efficiency could be achieved by intensifying applied magnetic field, which would result in an increase of magnetic force exerted on the particles. Likewise, when the magnetic field increased from 6900 to 17,700 G, an improvement in magnetic separation of ferrihydrite was obtained at high iron-tomagnetite ratios (Fig. 3). This means that the same performance can be achieved if the iron concentration is decreased at the same magnetite addition. As had been expected, decreased iron concentration to 0.2510 2 M, keeping magnetic addition at 0.5 g/l (Exp. 5), resulted an increase in separation efficiency (Table 1). The effect of magnetic field intensity on the magnetic removal of seeded ferrihydrite was studied by changing the field from 6900 to 17,700 G and keeping the other parameters constant. It was found that there is a very little, if any, effect of magnetic field on the removal (Fig. 4). Therefore, at these conditions, the lowest magnetic field of 6900 G was found to be optimal. However, it was later observed that when the flow rate increased, an increase in applied field is essential to enhance the magnetic force exerted to particle. It was also found that the amount of magnetite addition at a constant iron concentration affects matrix separation capacity (Table 2). With higher ratios of Fe to magnetite obtained by increasing amount of Fe(III), keeping magnetite addition constant, decrease in separation efficiency was more pronounced because of early breakthrough, and some Table 1 Experimental results obtained at the different iron-to-magnetite ratios (magnetic field strength=6900 G, flow rate=0.57 l/min, pH=8.0) Experiment no.
Fe(III)/Magnetite (mmol/g)
Total Fe(III) (M)
1 2 3 4 5 6 7
2.0 2.5 2.5 5.0 5.0 10.0 10.0
0.5010 0.5010 1.0010 0.5010 0.2510 0.5010 1.0010
2 2 2 2 2 2 2
Magnetite (g/l)
Treated volume (l)
Nonmagnetic fraction Fe (ppm)
Magnetite (%)
2.5 2.0 4.0 1.0 0.5 0.5 1.0
1.0 1.0 1.0 1.0 1.0 1.0 0.5 1.0
0.24 0.70 1.08 9.01 0.90 21.43 21.81 35.80
* * * 0.6 * 0.96 0.8 1.27
* There is no considerable amount of magnetite in nonmagnetic fraction.
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
Fig. 3. The effect of magnetite addition on the magnetic separation of ferrihydrite (Fe(III)=0.510 rate=0.57 l/min, pH=8.0).
51
2
M, flow
finely magnetite particles were also obtained in nonmagnetic fraction. However, matrix separation capacity could be increased by adding more magnetite into solution containing same iron concentration. These suggest that the total amount of precipitate captured by a HGMS matrix decreases with the decrease of magnetization of the precipitate. This also means that separation capacity of matrix can be increased by increasing magnetic force exerted on particle achieved by either more magnetic addition or intensifying of magnetic field strength. However, it was observed that more than a certain amount of magnetite causes the plugging of matrix before breakthrough.
Fig. 4. The effect of magnetic field strength on the removal of ferrihydrite as a function of pH (Fe(III)=0.510 magnetite addition=2.0 g/l, flow rate=0.57 l/min).
2
M,
52
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
Table 2 The effect of magnetite addition on the separation capacity of matrix at a 0.510 (magnetic field strength=6900 G, flow rate=0.57 l/min, pH=8.0)
2
M iron concentration
Magnetite (g)
Treated volume (l)
Nonmagnetic fraction Fe (ppm)
Magnetite (%)
2.0
0.255 0.470 0.690 0.875 1.000 0.245 0.465 0.680 0.885 1.000
0.07 0.22 0.22 0.37 0.37 0.31 0.57 1.35 4.31 9.01
* * * * * * * 0.12 0.42 1.27
1.0
* There is no considerable amount of magnetite in nonmagnetic fraction.
According to the results obtained, 2.0 g/l of magnetite dosage was found to be optimal for seeding of 0.510 2 M Fe concentration. In this ratio, heavily impregnated ferrihydrite flocs of reasonable stability were produced. In the case of 2.0 g/l magnetic addition, flow rate was varied from 0.57 to 3.0 l/min for a solution containing 0.510 2 M Fe(III) (Fig. 5). It was found that an increase in flow rate resulted in an increase in iron and magnetite concentration of nonmagnetic fraction. Generally speaking, intensifying of magnetic field permits increased flow rates without sacrifice in performance. It was observed that this is true up to a point. For flow rates higher than 2 l/min, an increase in magnetic field did not improve the magnetic separation efficiency. These data suggest that as water velocity through the canister increases beyond a certain point, the drag force on the trapped magnetite floc particle becomes great enough
Fig. 5. Ferrihydrite removal by HGMS as a function of flow rate (Fe(III)=0.510 pH=8.0).
2
M, magnetite addition=2.0 g/l,
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
53
to shear some of the ferrihydrite precipitates from the magnetite particles, resulting in lower quality product water. It appears that the minimum magnetic field required for maximum removal is just that required to trap and hold the composite magnetite particles against the drag forces exerted by flowing water and the gravitational forces. 4. Conclusion It has been well known that ferrihydrite is a good scavenger for heavy metals and many radio nuclides; its precipitation from waste streams could be used as an effective method of clean up if satisfactory process for the recovery of the ferrihydrite was available. Highgradient magnetic separation could be a solution in this regard for the separation of ferrihydrite. Because of its low magnetic susceptibility, direct application of HGMS for the separation of ferrihydrite is not available. The experiments reported here demonstrate the viability of the magnetic separation of ferrihydrite using magnetic seeding technique. Ferrihydrite was seeded with magnetite mineral and then separated by magnetic separation. The main results are as follows: 1. HGMS treatment with the combination of magnetic seeding technique is available for the solid/liquid separation of ferrihydrite flocs that scavenge the heavy metal ions from solution. 2. Maximum seeding was obtained at a pH range that is the ZPC of ferrihydrite. Sweep flocculation was found to be the main mechanism in seeding process. 3. Iron-to-magnetite ratio that determines the magnetization of floc was found to be main criteria and determines the operation conditions in HGMS separation. At higher ratios, magnetization of precipitate was decreased, which leads to decrease in magnetic separation performance. The time of breakthrough from the HGMS was found to depend on the weight of the chemical precipitates to the magnetite, namely, on the magnetization of the precipitate produced after seeding. Lower ratios of iron to magnetite enhance the matrix separation capacity.
Acknowledgements This work was supported by TUBITAK (The Scientific and Technical Research Council of Turkey, Project No. YDABCßAG-336).
References Anand, P., Etzel, J.E., Friedlaender, F.J., 1985. Heavy metals removal by high gradient magnetic separation. IEEE Transactions on Magnetics mag-21, 2062 – 2064. Anderson, N.J., Bolto, B.A., Blesing, N.V., Kolarik, L.O., 1983. Colour and turbidity removal with reusable magnetite particles: VI. Pilot plant operation. Water Research 17, 1235 – 1243. Anderson, P.R., Chen, W.Y., Roth, K.E., 1991. Evaluation of a magnetite-based adsorption process for the
54
N. Karapinar / Int. J. Miner. Process. 71 (2003) 45–54
recovery of metals from industrial wastewaters. In: Tu¨rkman, A., Uslu, O. (Eds.), New Developments in Industrial Wastewater Treatment. NATO ASI Series, vol. 191. Kluwer Academic Publishers, Netherlands, pp. 183 – 199. Barrado, F., Prieto, F., Vega, M., Fernandez-Polanco, F., 1998. Optimization of the operational variables of a medium scale reactor for metal-containing wastewater purification by ferrite formation. Water Research 32 (10), 3055 – 3061. Benjamin, M.M., Bloom, N.S., 1981. Effects of strong binding of anionic adsorbates on adsorption of trace metals on amorphous iron oxyhdroxide. In: Tewari, P.H. (Ed.), Adsorption from Aqueous Solutions. Plenum, New York, pp. 41 – 60. Benjamin, M.M., Sletten, R.S., Bailey, R.P., Bennett, T., 1996. Sorption and filtration of metals using iron-oxide coated sand. Water Research 30 (11), 2609 – 2620. Davis, J.A., Leckie, J.O., 1978. Surface ionization and complexation at the oxide/water interface: II. Surface properties of amorphous iron oxyhydroxide and adsorption of metal ions. Journal of Colloid and Interface Science 67 (1), 90 – 107. De Latour, C., 1973. Magnetic separation in water pollution control. IEEE Transactions on Magnetics mag-9 (3), 314 – 316. De Reuver, J.L., 1994. Magnetic wastewater treatment in the US chemical industry. Filtration and Separation Journal 31 (6), 605 – 607. Dzombak, D.A., Morel, F.M.M., 1986. Sorption of cadmium on hydrous ferric oxide at high sorbate/sorbent ratios: equilibrium, kinetics and modeling. Journal of Colloid and Interface Science 112 (2), 588 – 598. Edwards, M., Benjamin, M.M., 1989. Regeneration and reuse of iron hydroxide adsorbents in treatment of metal bearing wastes. Journal of Water Pollution Control Federation 61 (4), 481 – 490. Eggleton, R.A., Fitzpatrick, R.W., 1988. New data and a revised structural model for ferrihydrite. Clays and Clay Minerals 36 (2), 111 – 124. Franzreb, M., Ho¨ll, W.H., 2000. Phosphate removal by high-gradient magnetic filtration using permanent magnets. IEEE Transactions on Applied Superconductivity 10 (1), 923 – 926. Gillet, G., Diot, F., 1999. Removal of heavy metal ions by superconducting magnetic separation. Separation Science and Technology 34 (10), 2023 – 2037. Karapinar, N., 2003. Magnetic separation; an alternative method to the treatment of wastewater. The European Journal of Mineral Processing and Environmental Protection (in evaluation, 1 – 15). Moffat, G., Williams, R.A., Webb, C., Stirling, R., 1994. Selective separations in environmental and industrial processes using magnetic carrier technology. Minerals Engineering 7, 1039 – 1056. Prenger, F.C., Stewart, W.F., Hill, D.D., Avens, L.R., Worl, L.A., Schake, A., de Aguero, K.J., Padilla, D.D., Tolt, T.L., 1994. High gradient magnetic separation applied to environmental remediation. Advances in Cryogenic Engineering 39, 485 – 491. Schultz, M.F., Benjamin, M.M., Ferguson, J.F., 1987. Adsorption and desorption of metals on ferrihydrite: reversibility of the reaction and sorption properties of regenerated solid. Environmental Science and Technology 21 (9), 863 – 869. Schwertmann, U., Murad, E., 1983. The effect of pH on the formation of goethite and hematite from ferrihydrite. Clays and Clay Minerals 31, 277 – 284. Stumm, W., Morgan, J.J., 1996. Aquatic Chemistry, Chemical Equilibria and Rates in Natural Water, 3rd ed. John Wiley and Sons, New York. Sun, Z.X., Su, F.W., Forsling, W., Samskog, P.O., 1998. Surface characteristics of magnetite in aqueous suspension. Journal of Colloid and Interface Science 197, 151 – 159. Svoboda, J., 1987. Magnetic Methods for the Treatment of Minerals. Elsevier, Amsterdam, pp. 590 – 605. Tamaura, Y., Katsura, T., Rojarayanont, S., Yoshida, T., Abe, H., 1991. Ferrite process; heavy metal ions treatment system. Water Science and Technology 23, 1893 – 1900. Towe, K.M., Bradley, W., 1967. Mineralogical constitution of colloidal ‘‘Hydrous Ferric Oxides’’. Journal of Colloid and Interface Science 24, 384 – 392. Treshima, Y., Ozaki, H., Sekine, M., 1986. Removal of dissolved heavy metals by chemical coagulation magnetic seeding and high gradient magnetic filtration. Water Research 20 (5), 537 – 545. Van der Giessen, A.A., 1966. The structure of iron (III) oxide-hydrate gels. Journal of Inorganic and Nuclear Chemistry 28, 2155 – 2159.