Environ. Sci. Technol. 2000, 34, 2576-2581
Magnetic Seed in Ambient Temperature Ferrite Process Applied to Acid Mine Drainage Treatment W. MCKINNON,† J. W. CHOUNG,† Z . X U , * ,† A N D J . A . F I N C H ‡ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 2G6 Canada, and Department of Mining and Metallurgical Engineering, McGill University, Montreal, Quebec, H3A 2B2 Canada
Due to its environmental consequences, acid mine drainage (AMD) has been recognized as a major challenge to the global mining industry. Through the innovative use of magnetic seeds, a versatile ambient temperature ferrite (ATF) process has been developed to treat AMDs containing such nonferrous heavy metal ions as Cu2+, Zn2+, Ni2+, Mn2+, and Al3+. These metal ions proved detrimental to ferrite formation using the existing ATF process, particulary when lime was used as neutralizer. The use of magnetic seeds in the ATF process minimized the interference. The role of the chemical environment in ferrite formation from an AMD with the addition of magnetic seeds was investigated. Compared with the conventional seed processes, controlling the solution chemistry resulted in a reduced amount of seed needed to recover an equal amount of crystalline magnetic precipitates. With a relatively short processing period (less than 2.5 h), up to 100% of the precipitates were magnetically recovered from a simulated AMD. The residual concentrations of major contaminant ions in the treated water were below the corresponding acceptable levels. The XRD pattern showed that the solid products were all of spinel ferrite crystal structure, in contrast to the presence of a substantial amount of noncrystalline phase in the product formed using a conventional seed process.
Introduction Over the last 2 decades, there has been an increasing interest in treating the large volume of industrial effluents to minimize their environmental consequences. The treatment of mine effluents, so-called acid mine drainage (AMD) containing a variety of heavy metallic species, is a typical example. AMD is recognized as a major challenge to the global mining industry due to its persistent generation over long periods of time (centuries), the large volumes involved, and the difficulty in prediction. It has been reported that a typical mine site would treat some 3 million cubic m of AMD annually (1). Left untreated, the AMD will have significant environmental consequences. The objective of the treatment is to reduce the concentration of soluble metallic species below the toxicity level while minimizing production of solid wastes and respecting the inevitable economic constraints. * Corresponding author phone: (403)492-7667; fax: (403)492-2881; e-mail:
[email protected]. † University of Alberta. ‡ McGill University. 2576
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FIGURE 1. Schematic diagrams of (a) HDS and (b) SIROFLOC process. Coprecipitation of metallic species as hydroxides with lime is common practice. This approach is often capable of meeting effluent standards but needs a multistage neutralization/solid-liquid separation process when a variety of heavy metals are present. The process produces a large volume of finely dispersed, low-density precipitates. The nature of the precipitates presents the challenge of effective solid-liquid separation and disposal of inherently unstable material. The development of the high-density sludge (HDS) process targets overcoming these shortcomings. In the HDS process (2), shown schematically in Figure 1a, the externally added or recycled fine solids act as seeds to improve the density of the precipitates and hence solid-liquid separation efficiency. Although the solids generated in this process are more stable and relatively easy to separate from the effluent, they do not have any commercial value and are considered as a solid waste. Producing a byproduct of commercial value from effluent treatment is attractive. In this regard, the ferrite process has been considered as a treatment option for AMD and other industrial effluents. A unique advantage of using the ferrite process is that the magnetic particles formed could be a source material for magnetic fluids, magnetic recording media, magnetic inks, etc. (3). In addition, the magnetic property of the precipitates allows the separation of resultant solids from the effluent to be performed using efficient magnetic filtration methods. However, the conventional ferrite process requires elevated temperatures and/or controlled redox potentials in the presence of catalysts (4). These constraints make the application of the conventional ferrite process to AMD treatment impractical. Although some 10.1021/es9910863 CCC: $19.00
2000 American Chemical Society Published on Web 05/12/2000
attempts have been made to produce magnetic ferrites under ambient conditions, heat treatment of the solid products or prolonged aging over 2 days was found to be necessary (5-8). Recently, the authors have proposed an ambient temperature ferrite (ATF) process (9). In this process, controlling the solution chemistry, mainly the ferric-to-ferrous ratio, allowed magnetic ferrite to form readily under ambient conditions. This novel ATF process has the prime advantages of low energy consumption and short aging time compared to the prior art. However, calcium and other nonferrous heavy metal ions, invariably present in AMD, were found to be detrimental to ferrite formation. The interference by calcium can be minimized by the addition of carbonate compounds, which allow ferric oxyhydroxide intermediates, free of calcium sulfate components, to form (10). The addition of ferric and ferrous ions to the AMD decreased the interference of other nonferrous metal ions significantly. However, the requirement to add a significant amount of iron salts makes this approach unattractive. Alternatively, the complexation of nonferrous metal ions using a selective complexing agent such as diethylene triamine (DETA) was found to be effective in eliminating interference from nickel ions, but only partially effective for zinc, and not effective at all for cupric ions (11). To extend the ATF process to the treatment of industrial effluents containing a variety of “interfering” metal ions at relatively high concentrations, there is a need to develop a versatile ATF process. The use of magnetite particles as seeds for treating wastewater is well documented (12-15). In the conventional seed process, Na2S or CaSx is used to precipitate heavy metal ions as nonmagnetic fine sulfide particles. These particles heterocoagulate with the magnetic seed particles and are removed using magnetic filtration. A variation of the magnetic seed process, called SIROFLOC process shown in Figure 1b, was developed at CSIRO (Australia) to remove suspended solids from industrial effluents. In the presence of added alum or ferric salts as coagulants, increasing effluent pH to 6 caused solid impurities to coagulate and deposit on the magnetic seeds. After exposing the slurry to a magnetic field, the loaded seeds were magnetically flocculated and settled to the bottom of the clarifier. Although magnetic flocculation allowed the solids to be removed, vigorous stirring of resultant sediments was needed to detach the seeds from the solid impurities for recycle. After recovering the seeds by using high gradient magnetic filtration, further removal of deposits by acid washing may be necessary to maintain the magnetic properties of the seeds. As a result, an additional solidliquid separation process is required. These additional requirements cause an increase in operating costs. More importantly, the sludge remains as solid waste and incurs another inevitable environmental consequence. By integrating the seed process with the ATF process, i.e., controlling solution chemistry in the presence of magnetic seeds, all metal ions present in AMD are likely to coprecipitate on magnetic seeds in the form of magnetic ferrite. The interference of nonferrous metal ions with ferrite formation is anticipated to be minimized through their coprecipitation on the surface of the seeds. In this process, a fraction of the resultant magnetic solid products can be recycled as seeds without further treatment, in contrast to the conventional seed process. The remaining solid products, on the other hand, can be a byproduct of AMD treatment. The proof of concept is the focus of this communication. The use of magnetic seed in the ATF process with lime as neutralizer has been investigated systematically. Various nonferrous metal ions, including Cu2+, Zn2+, Ni2+, Mn2+, and Al3+ are considered.
Experimental Section Materials. Analytical grade Fe2(SO4)3‚4H2O, FeSO4‚7H2O, Al2(SO4)3‚18H2O, ZnSO4‚7H2O, NiSO4‚6H2O, CuSO4‚5H2O, MnSO4‚ H2O, and CaO from either Acros or Fisher Scientific were used as received. Lime as neutralizer was used in the powder form. For comparison, a 2 N NaOH stock solution, prepared by dissolving a given amount of analytical grade NaOH (Fisher Scientific) in deionized water, was used as a neutralizer in some tests. In this communication, tests were restricted to sulfate as anions in order to simulate more closely the conditions encountered in practice. Other common anions such as chloride and carbonates were tested in our early work. Compared with sulfate, these anions were found to be less interfering with ferrite formation (10). The synthetic magnetite (98% passing -5 µm) of analytical grade from Aldrich was used as magnetic seeds. Unless otherwise stated, all 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. A given amount of ferric and ferrous sulfates, in some cases along with nonferrous metal sulfates at a desired mole ratio, were dissolved in 500 mL of deionized water. The initial concentration of ferric sulfate was kept at 1 × 10-2 mol/L throughout this study. The pH of the solutions prepared as such was typically around 2.5. The solution was then neutralized by gradual addition of 1.1 g of lime powder or 2 N NaOH stock solutions over a 17 min period, while agitating the solution using a mechanical stirrer at 600 rpm (Caframo Lab. Stirrer supplied by Cole-Parmer, model 440510). The neutralization stopped at pH around 7.8, unless otherwise specified. The precipitates formed during neutralization were aged for 2 h without agitation, in an attempt to form magnetic ferrite precipitates. The magnetic precipitates thus produced were separated from the nonmagnetic precipitates using a hand magnet (0.1 T). Both magnetic and nonmagnetic precipitates were weighed accurately after filtration with a filter circle fixed on a funnel and drying in a vacuum oven at room temperature. The yield, defined as weight fraction of magnetic precipitates, was used to evaluate the process quantitatively. It should be noted that the absolute yield value may vary with the magnetic strength of the hand magnet used in precipitates separation. However, the general trend and the conclusion remain the same regardless of the magnetic strength of the hand magnet used. We found a hand magnet of 0.1 T field strength is convenient and sufficient for the purpose of evaluating ATF process. To determine the role of magnetic seeds, a predetermined amount of magnetite was added to ferric and ferrous salt solutions containing nonferrous metal species of interest. The amount of seeds added was expressed as the weight ratio of the seed to the total precipitates (i.e., both magnetic and nonmagnetic portions formed). After conditioning the suspension for 2 min, followed by the neutralization using the same procedures described above, the magnetic precipitates along with the added seed were separated from the nonmagnetic precipitates using the hand magnet. After subtracting the weight of the seeds from the total magnetic solids (seeds plus magnetic precipitates), the yield of magnetic precipitates was calculated following the same procedures as for the case without seed addition. To test the process, a synthetic AMD was prepared based on the chemical composition of major metallic elements (Table 1) found in the AMD from Noranda’s Geco site (2). Considering that the original Geco effluent contains 5.4 × 10-3 mol/L of Fe3+, 3.1 × 10-3 mol/L ferrous sulfate was added, to adjust Fe3+/Fe2+ molar ratio to 1.75, critical for ferrite formation (9). The required amount of sodium sulfate was added to ensure that the synthetic AMD contains 2500 mg/L sulfate anions as found in the original AMD (2). Since our VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Concentrations of Metallic Elements in the Original AMD from Noranda and in the Treated Effluent of the Simulated AMD elements (ppm) original AMD processed effluent ATF + seed seed limit in drinking water (16)
Fe 300
