Ind. Eng. Chem. Res. 1999, 38, 4689-4693
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Role of Complexing Agents in Ferrite Formation under Ambient Conditions J. W. Choung and Z. Xu* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6
J. A. Finch Department of Mining and Metallurgical Engineering, McGill University, Montreal, Quebec, Canada H3A 2B2
The effects of nonferrous metal ions in an ambient-temperature ferrite (ATF) process were investigated. Most nonferrous metal ions studied were found to interfere with the formation of magnetic ferrite. The threshold concentration of interference varied in the order of Cu < Ni < Zn < Mn. Complexing Ni2+ and Zn2+ ions using diethylenetriamine (DETA) minimized their interferences. Despite its strong complexing power with cupric ions, DETA was found ineffective in eliminating the interference of copper ions. Ethylenediaminetetraacetic acid (EDTA), a stronger complexing agent but less selective, was found to be unfavorable to the ATF process. Experimental results indicated that, with suitable chemicals, the complexing technique is capable of mitigating metal ion interference in the ATF process and improving the quality of ferrites formed in industrial effluents of complex chemistry. Introduction Ferrite, a stable and nontoxic material, has been widely used in various technical applications including in catalysis, cement products, fabrics, paints, papers, plastics, etc. The magnetic property of ferrites with controlled chemical composition and particle size has made ferrite highly desirable for the production of ferro fluids, information storage media, audio reproduction, color imaging, magnetic inks, magnetic refrigeration, and magnetic sealing, to name a few. The essential requirements of well-controlled uniformity and high purity for these various applications make ferrite production by synthetic routes the preferred choice. Several methods, including coprecipitation, reduction of synthetic iron oxide, and the aniline process, have been developed to produce synthetic ferrite. Among these methods, coprecipitation of spinel ferrites from aqueous solutions has been practiced for the manufacture of high-quality magnetic materials.1,2 The formation reactions of spinel ferrites are generally described by:
xM2+ + (3 - x)Fe2+ + 6(OH)- f MxFe3-x(OH)6 (1) MxFe3-x(OH)6 + 1/2O2 f MxFe3-xO4 + 3H2O (2) where M can be any divalent ion with an unhydrated ion radius between 0.6 and 1 Å.3 When iron is the only metallic constitute, ferrite is known as iron ferrite (or magnetite), characterized by a strong magnetization (e.g., ferromagnetism). If M is exclusively substituted by a diamagnetic (nonferrous) ion, such as Zn2+, Ni2+, or Mn2+, the ferrites formed are paramagnetic with a Neel temperature ranging from 77 to 300 K.4 The particles incorporating partially diamagnetic metal ions will possess a magnetization characteristic between ferromagnetism and paramagnetism, depending not only on the mole ratio of ferrous/nonferrous species but
also on the type of nonferrous metal ions. The authors have demonstrated the formation of magnetic ferrite from ferric and ferrous solutions under ambient conditions.5 The process is referred to as an ambienttemperature ferrite (ATF) process, and the ferric-toferrous mole ratio was identified as the key controlling parameter. There are many economic and environmental incentives to produce magnetic ferrites in the treatment of the large volumes of industrial effluents containing high concentrations of iron (>1000 mg/L). However, when the ATF process is applied in the treatment of industrial effluents, two challenges associated with the complex chemistry of these effluents are anticipated: control of impurities in the ferrite and interference in ferrite formation by nonferrous metallic species.6 The former limits the potential use of the ferrite product while the latter may limit application of the ATF process to industrial effluents. In this paper, a systematic study on the effect of various nonferrous metal ions, including Cu2+, Zn2+, Ni2+, and Mn2+, on ferrite formation in the ATF process is reported. The tolerance level of the process for these ions is determined. The role of complexing agents in moderating the interference and in controlling metal impurities in the resultant ferrite is investigated. Experimental Section Materials. Inorganic sulfate salts, including Fe2(SO4)3‚4H2O, FeSO4‚7H2O, ZnSO4‚7H2O, NiSO4‚6H2O, CuSO4‚5H2O, and MnSO4‚H2O, were from either Acros or Fisher Scientific and used as received. A diethylenetriamine (DETA) solution from Fisher Scientific and ethylenediaminetetraacetic acid (EDTA) from BDH, Inc., were tested as complexing agents for metal ions studied. DETA was used as received, while EDTA was used after dissolving in a 2 N NaOH solution to obtain the concentration of 0.1 mol/L. The solution pH was
10.1021/ie990205k CCC: $18.00 © 1999 American Chemical Society Published on Web 11/09/1999
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adjusted using standard H2SO4 and NaOH solutions from Fisher Scientific. All of the chemicals were of analytical grade. Unless otherwise stated, all of the experiments were carried out at room temperature with deionized water prepared using an Elix-5 followed by purification with a Millipore-UV unit (Millipore, Canada). Procedure. To investigate the effects of nonferrous metal ions in the ATF process, a predetermined amount of pertinent metal sulfates was dissolved in 500 mL of deionized water to obtain a desired mole ratio of various metallic species. The ferric ion concentration in the solution was kept at 2 × 10-2 mol/L throughout. The pH of the resultant solutions was at about 2.5 and was raised to 10.5 within 5 min by adding a 2 N NaOH stock solution. During this neutralization stage, the solution was agitated using a mechanical stirrer at 600 rpm (Caframo Laboratory Stirrer from Cole-Parmer, model 4405-10) and precipitates formed. The agitation was continued for an additional 10 min, and the suspension was then aged for 1 h without agitation, in an attempt to produce magnetic ferrite. The magnetic precipitates produced up to this stage were separated from the nonmagnetic precipitates using a hand magnet (0.1 T). To evaluate the ATF process quantitatively, the weight fraction of magnetic precipitates was calculated after filtering both magnetic and nonmagnetic precipitates with a filter circle fixed on a funnel and drying them in a vacuum oven at room temperature. For a few selected tests, the redox potential was measured using a digital pH/mV meter (Orion model 250A). In DETA ion complexation efficiency tests, a given metal species was dissolved in 500 mL of deionized water at the metal ion concentration of 3 × 10-3 mol/L. A predetermined amount of DETA, based on the [DETA]/ [M2+] mole ratio between 0 and 3, was added drop-bydrop to the solution while maintaining the solution pH at 2.5 by cocurrent addition of sulfuric acid, because the addition of DETA alone would increase the solution pH. The solution was conditioned for 2 min to allow DETA complexation with the metal ions, before neutralization. The complexation efficiency of metal ions with DETA was evaluated by determining the residual metal ions in solution, after precipitating the metal ions as hydroxides by increasing the pH to 10.5. Similar procedures were used to evaluate the complexation efficiency of DETA (or EDTA) in the ATF process. In this case, synthetic iron salt solutions containing a single nonferrous metal species were used, and the magnetic precipitates produced were separated from the nonmagnetic precipitates using the hand magnet. The weight fraction of magnetic precipitates was then determined to examine whether the complexing agent was capable of eliminating the interference with ferrite formation by nonferrous metal species. The solid samples collected were characterized by X-ray diffractometry (XRD, Philips 1710) using Cu KR radiation. Metal ion concentrations in solutions were determined using atomic absorption spectroscopy (AA, Perkin-Elmer 4000). A scanning electron microscope (SEM, Hitachi S-2700, Japan) was used to examine the morphology and size of the magnetic particles. Results and Discussion Ferrite Formation. The precipitates formed by neutralizing acidic solutions containing ferric and divalent nonferrous ions at a mole ratio of 2:1 were found to be nonmagnetic, as judged by the hand magnet. In
Figure 1. X-ray diffraction patterns of precipitates formed at a 2/1 Fe3+/M2+ mole ratio. The vertical lines are from a JCPDS diffraction data card of synthetic ferrite, with the line height representing the relative peak intensity.
contrast, all of the precipitates formed in the ferric and ferrous ion solutions were strongly magnetic under the same precipitation conditions. The XRD pattern in Figure 1 clearly shows that the precipitates formed by coprecipitating ferric with nonferrous divalent ions under ambient conditions were noncrystalline, although the AA analysis of supernatant showed only traces of soluble residual metallic species. It appears that the low-density precipitates formed were a mixture of noncrystalline metal hydroxides, whose exact composition remains to be determined. Because our focus was on the formation of magnetic ferrite, we are not too concerned about the nonmagnetic precipitates. The magnetic precipitates formed from ferric and ferrous ion solutions, on the other hand, were unlikely to be the green rust and/or schwertmannite, which was confirmed by the absence of corresponding XRD patterns. The magnetic precipitates were highly crystalline with a diffraction pattern of spinel structure. This set of experimental results indicates that nonferrous divalent ions are unable to coprecipitate ferric ions as crystalline spinel precipitates under ambient conditions. The implication is that the presence of these divalent ions is likely to interfere with magnetic ferrite formation from industrial effluents. To confirm this, further tests were conducted with solutions containing ferric, ferrous, and divalent nonferrous ions. The concentration of nonferrous ions was given by X defined in terms of a Fe3+/Fe2+/M2+ mole ratio of 2/(1 - X)/X. The solutions with such compositions between trivalent and divalent metal ions satisfy a 2/1 stoichiometric mole ratio of standard spinel ferrite. The results in Figure 2 show that the magnetic fraction of the precipitates decreases with increasing X for all nonferrous metal ions examined. In the case of Cu2+ and Ni2+ ions, all of the precipitates formed were magnetically unrecoverable at an X value of 0.4 or above. The corresponding X value of 0.6 was determined for Zn2+ and Mn2+ ions. These results confirm that nonferrous metal ions interfere with magnetic ferrite formation. The threshold concentration of interference was found to be dependent on the type of metal ions with Cu < Ni < Zn < Mn. It appears that coprecipitation of metal hydroxides is responsible for the interference. Also noted
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Figure 2. Effect of divalent nonferrous metal ions on ferrite formation using the ATF process. X is defined in a mole ratio of Fe3+/Fe2+/M2+ as 2/(1 - X)/X.
Figure 3. Metal ion concentration remaining as soluble metalDETA complexes in solutions at pH ) 10. Initial metal ion concentrations are fixed at 3 × 10-3 mol/L.
is that part of the nonmagnetic precipitates could be converted to weakly magnetic crystalline particles by heating of the suspension or by prolonged aging. This finding suggests that dehydration is the key to phase transition from amorphous hydroxides to crystalline spinel ferrite. It should be noted that the crystalline precipitates formed as such contain divalent nonferrous metals, which is partially responsible for the reduced magnetic strength. Complexation of Metal Ions. Figure 3 shows that even at pH as high as 10 DETA is capable of retaining in solution all of the nonferrous metal ions studied except Mn. At a [DETA]/[M2+] mole ratio of 3, almost all of the metal ions (100%) exist as soluble complexes. Compared with their respective metal hydroxide precipitates, these metal-DETA complexes are more stable, with a complexing strength on the order of Cu > Zn > Ni > Mn. Interestingly, this order agrees with the order of corresponding hydrolysis constants (pKh) of 8.0, 9.0, 9.9, and 10.6 for Cu2+, Zn2+, Ni2+, and Mn2+ ions, respectively.7,8 Also shown in this figure is the marginal complexation of DETA with ferrous ions because the bulk of ferrous ions remained as precipitates over the pH range studied. The preferential complexation of DETA with nonferrous metal ions suggests that the addition of DETA in the ATF process may minimize the interference of nonferrous metal ions in ferrite forma-
Figure 4. Metal ion concentration as soluble metal-DETA complexes in residual solutions after ferrite formation using the ATF process in the presence of DETA.
tion. More importantly, an improved quality of ferrite is anticipated by locking these nonferrous metal species in solution during the ferrite formation. This concept was tested, and the results are shown in Figure 4. It is clear that the interference by Ni2+ ions (X ) 0.2) was minimized by DETA complexation. With a [DETA]/[Ni2+] mole ratio of 3, more than 99% of nickel was kept in solution, which allowed ferrite formation to proceed as if in the absence of nickel. As a result, the magnetic precipitates of the spinel structure formed as such contained minimal nickel ( Zn > Cu (Figure 4). This is in the reverse order of the complexing power of DETA in a simple solution as observed experimentally (Figure 3). There is no direct correlation between the observed stability of the metal complexes in the ATF process, the complexing power, and the corresponding complex stability constants with pK1 values ranging from -16.7, -10.9, to -9.1 for Cu2+, Ni2+, and Zn2+, respectively.9 It is clear that the structure of complexes needs to be considered. For nickel, an octahedral configuration of the Ni-DETA complex, as shown schematically in Figure 5b, satisfies the required sixcoordination number. As a result, all of the amine groups in a DETA molecule, shown in Figure 5a, are occupied, and the nickel ion itself is not accessible by external competitors, such as ferrite. In the case of copper, a planar complex structure, shown in Figure 5c, makes copper accessible outside the plane. The dangling amine groups compete with the bonded amines for copper sites, making the complex labile. These two characteristics of the Cu-DETA complex cause a high probability of Cu being incorporated in precipitates and interfering with ferrite formation. It is clear that, in searching for solutions to the interference of nonferrous divalent metal ions based on the use of complexing agents in the ATF process, not only the complex stability constant but also the structure of specific complexes and possible complexation with iron (i.e., selectivity) need to be considered. In the present case, DETA, a weak complexing agent with iron,10 was found successful to solve interference from
Figure 5. Schematic representation of DETA molecules (a) alone, (b) complexed with a nickel ion, and (c) complexed with a cupric ion in solutions.
nickel ions, but only partially effective to zinc, and not effective at all for cupric ions. It should be noted that the redox potentials varied from +600 to -550 mV during the neutralization. The addition of DETA did not show any significant effect on the redox potential. There was no clear correlation between the ferrite formation and the redox potential over the potential range covered. The authors clearly demonstrated that, using the ATF process in the presence of DETA, high-purity iron ferrite can be produced from an industrial effluent containing a large quantity of ferrous and ferric ions along with substantial amounts of contaminant nickel ions. The nickel ions, remaining in the effluent as nickel-DETA complexes, can be removed or recovered by an ionexchange process or electrowinning while the complex agent itself was shown to decompose under a natural oxidation environment.10 A typical scanning electron micrograph of the product in Figure 6 shows that the morphology of the resultant ferrite particles is not affected by the addition of a complexing agent. Round particles of high purity in a size range of about 300 nm produced in the ATF process are of potential application in magnetic recording media, inorganic pigments, magnetic inks, ferro fluids, etc. To extend the ATF process to industrial effluents containing a variety of “interfering” metal ions, the use of suitable complexing agents
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(4) Unselective complexation of EDTA with all of the metal ions increased soluble free nonferrous metal ions because of its complexation with ferrous ions. These adverse effects by EDTA resulted in little improvement in ferrite formation and impurity level of contaminant ions in the resultant magnetic ferrite. Acknowledgment The authors acknowledge financial support from Natural Sciences and Engineering Research Council of Canada under Strategic Grant NSERC-STP0192953. The support, cash and in-kind, from Falconbridge, Noranda and Inco is also acknowledged. Literature Cited
Figure 6. Scanning electron micrograph of magnetic ferrite obtained in the ATF process in the presence of nickel (X ) 0.2) and DETA at a [DETA]/[Ni2+] mole ratio of 3.
in combination is needed. The experimental results in this paper provide some guidelines for preliminary screening. Conclusions (1) The threshold concentration of nonferrous metal ions interfering with magnetic ferrite formation varied according to the type of metal ions, in the order of Cu < Ni < Zn < Mn. At a solution composition of 2/0.6/0.4 or 2/0.4/0.6 with respect to Fe3+/Fe2+/Cu2+ (Ni2+) or Fe3+/ Fe2+/Zn2+ (Mn2+), all of the precipitates formed were magnetically unrecoverable. (2) DETA formed soluble metal complexes with Cu2+, Zn2+, and Ni2+ but not with Mn2+. The complexing power was in the order of Cu > Zn > Ni > Mn, consistent with the order of the corresponding metal hydrolysis constant. For Ni2+ ions, the use of DETA in the ATF process results in not only the minimized metal ion interference with ferrite formation but also the improved quality of the resultant ferrite. The ATF process with a closely controlled reaction environment is proven capable of producing a high-quality magnetic ferrite of commercial value from industrial effluents. (3) Despite its high complex stability constant, the Cu-DETA complex was found to be unstable in the ATF process and its interference with the ferrite formation cannot be resolved with DETA.
(1) Tsuji, T. Ferrite-Technology Applications and Their Expansion from Electronic to Civil Engineering Fields. Advances in Ceramics; Wang, F. Y., Ed.; American Ceramic Society: Columbus, OH, 1985; Vol. 15, p 573. (2) Sato, T. Formation and Magnetic Properties of Ultrafine Spinel Ferrites. IEEE Trans. Magn. 1970, Mag-6, 4, 795. (3) Smit, J.; Wijin, H. P. J. Ferrites: Physical Properties of Ferrimagnetic Oxides in Relation to their Technical Applications; John Wiley & Sons: New York, 1959; pp 136-142. (4) Sato, T.; Iijima, T.; Seki, M.; Inagaki, N. Magnetic Properties of Ultrafine Ferrite Particles. J. Magn. Magn. Mater. 1987, 65, 252. (5) Wang, W.; Xu, Z.; Finch, J. A. Fundamental Study of an Ambient Temperature Ferrite Process in the Treatment of Acid Mine Drainage. Environ. Sci. Technol. 1996, 30, 2604. (6) Wang, W.; Choung, J. W.; Xu, Z.; Finch, J. A. Challenges for Ambient Temperature Ferrite Process in Acid Mine Drainage Treatment. Processing of Complex Ores: Mineral Processing and the Environment; Finch, J. A., Rao, S. R., Holubec, I., Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, Canada, 1997; p 293. (7) Tamaura, Y. Ni(II)-Bearing Green Rust II and Its Spontaneous Transformation into Ni(II)-Bearing Ferrites. Bull. Chem. Soc. Jpn. 1986, 59, 1829. (8) Stumn, W.; Morgan, J. J. Aquatic Chemistry, Chemical Equilibria and Rates in Natural Waters, 3rd ed.; John Wiley & Sons: New York, 1996. (9) Perrin, D. D. Stability Constants of Metal-Ion Complexes; Pergamon Press: New York, 1979; part B, pp 198 and 774. (10) Rao, S. R.; Xu, Z.; Finch, J. A. Selective Solubilization of Zn(II), Cu(II) and Ni(II) from Fe(III) in Metal Hydroxide Sludges by Diethylene Triamine. In Waste Processing and Recycling in Mineral and Metallurgical Industries II; Rao, S. R., Amaratunga, L. M., Richards, G. G., Kondos, P. D., Eds.; CIM: Montreal, Canada, 1995; p 69.
Received for review March 19, 1999 Revised manuscript received September 1, 1999 Accepted September 29, 1999 IE990205K
