Magnetic Properties of Acid Mine Drainage Sludge-Derived Hexagonal Ferrite

MAGNETIC PROPERTIES OF ACID MINE DRAINAGE SLUDGE-DERIVED HEXAGONAL FERRITE

M. LIU* **, A. IIZUKA*** AND E. SHIBATA***

* Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan ** School of Material Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, China *** Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan

SUMMARY: Acid mine drainage sludge was recycled as a source of both calcium and iron to prepare a valuable Ca1-xBaxFe12O19 (x = 0, 0.3, 0.5) hexagonal ferrite by solid-state reaction. The magnetic properties of the resulting samples were studied and are discussed with respect to their phase composition and microstructure. The content of hexagonal ferrite increased with increasing of barium content x when mixtures with a certain molar ratio were calcined at 1100°C for 4 h. The saturation magnetization of samples therefore also increased with increasing x. Energy-dispersive x-ray analysis showed that most calcium in the sludge combined with silicon, rather than entering the hexagonal ferrite structure.

1. INTRODUCTION

Acid mine drainage (AMD), which forms at both active and abandoned mines, is generally characterized as a strong acid containing high concentrations of dissolved metals. It is generated when sulfur-containing mine rock (for example, iron sulfide) is exposed and reacts with air and water to form sulfuric acid and dissolved iron. The acid runoff further dissolves heavy metals, such as copper, lead, mercury, and zinc. AMD can be treated using either active or passive systems. In both types of treatment, acidity and metals are removed from the solutions using (bio)geochemical reactions that lead to the generation of a metal-rich sludge (Macías et al., 2017). Zinck and Griffith (2013) surveyed AMD treatment and sludge management practices at 108 mine sites around the world (66 of them in Canada): on average, sites produced about 9500 tonnes of dry sludge per year, with production ranging from 20 to 135 000 dry tonnes annually. In addition to the problem of land occupation, high disposal costs and uncertainty regarding its long-term chemical stability have led to the need for new technologies for volume reduction and recycling of AMD sludge. There is significant potential to reuse all or part of AMD sludge. To date, only a few investigations focused on the reusability of AMD sludge have been reported, mainly for applications in building materials, pigments, and metal adsorbents, and as a neutralizing material for AMD-generating waste rocks and tailings (Macías et al., 2017). Recycling

Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017 technologies are, however, limited by the hazardous-metal components in these sludges. A product with high chemical stability is therefore desirable; furthermore, a more valuable product is demanded to ensure that treatment is economically feasible. Hexagonal ferrites have been extensively used in permanent magnets, electrical and microwave devices, data storage and recording, plastoferrites, electromagnetic wave absorption, and other applications since they were discovered in the 1950s (Pullar, 2012). The hexagonal ferrites can exist in several structural variants, known as the M, W, Z, and Y structures, in which the metal and oxygen stoichiometries vary. Of these, the most popular is M- type ferrite, which has the general formula MeFe12O19 (Me = Ba, Sr, Pb, or Ca). Whereas the symmetry of spinel ferrite is cubic, M-type ferrite has a major preferred axis called the c-axis. The preferred direction affords good advantages for its use as a permanent magnet (Goldman, 2006). It is very stable and can therefore also be used in heavy-metal stabilization. Reuse of metal-rich AMD sludge in ferrite processing means that the double benefits of both waste recycling and heavy-metal stabilization can be achieved (Liu et al., 2017). Many types of wastes have been used in the preparation of spinel ferrite, including steel picking liquor (Liu et al., 2007), waste batteries (Peng et al., 2008; Hu et al., 2011; Gabal et al., 2013), sludge (Lu et al., 2008; Marcello et al., 2008; Chen et al., 2010; Tu et al., 2013; Zhu et al., 2015), slag (Schwarz et al., 2012), fly ash (Rashad et al., 2005), mill scale (Ahmed et al., 2010), waste catalyst (Hwang, 2006), and gypsum board (Higuchi et al., 2015). For the case of hexagonal ferrite, Pullar and coworkers (2013, 2016) valorized wire-drawing sludge as an iron source to synthesize strontium or barium hexagonal ferrites. To our knowledge, reuse of AMD metal-rich waste as both a calcium and iron source to prepare Ca1-xBaxFe12O19 hexagonal ferrite has not been reported.

2. EXPERIMENTAL

2.1 Materials

AMD sludge was supplied from an abandoned Japanese mine. The sludge was dried at 105°C for 24 h and its elemental composition determined by inductively coupled plasma atomic emission spectroscopy (Spectro Arcos, Spectro, USA) (see Table 1). BaCO3 and Fe2O3 of 99.9% purity (Wako Pure Chemical Industries, Ltd., Japan) were used to adjust the molar ratio of Ca:Ba:Fe.

Table 1. Elemental composition of acid mine drainage sludge (mass %)

Fe Ca Mg Zn Pb Mn As Al Si

36.0 5.20 0.147 0.006 0.021 0.084 0.951 4.39 3.38

2.2 Ca1-xBaxFe12O19 hexagonal ferrite preparation

Mixtures with Ca:Ba:Fe = (1−x):x:12 (molar ratio) were wet ball-milled for 30 min and then dried at 105°C for 24 h. The dried mixtures were crushed and homogenized by mortar grinding, before being calcined at the target temperature for 4 h. The calcined samples were air-cooled

Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017 and ground to powder in a mortar for analysis.

2.3 Characterization

The magnetic properties of the calcined powders were evaluated using a physical property measurement system (PPMS-9T; Quantum Design, Inc., USA) at room temperature with applied fields up to 55 kOe. The crystalline phases were identified by X-ray diffraction analysis (XRD) using a Bruker D2 Phaser with Cu Kα radiation at 30 kV, 10 mA, and using a LYNXEYE detector (Bruker, Germany). The 2θ scan range was from 10° to 80°, with a step size of 0.02° and a scan speed of 0.5 s per step. The morphological study and elemental mapping of calcined powders with carbon coating were performed using an SU6600 field-emission scanning electron microscope (Hitachi, Japan).

3. RESULTS AND DISCUSSION

3.1 Magnetic properties

As shown in Figure 1, the magnetic properties of the calcined powders were measured at room temperature with an applied field up to 55 kOe. In the case of x = 0.0, the M–H curve did not reach saturation. It was evident that there may be a paramagetic component in the calcined powder.

Table 2 summarizes the magnetic parameters of calcined Ca1-xBaxFe12O19 powder. The saturation magnetization (Ms), remanence (Mr), and coercivity (Hc) all increased with increasing barium content x; however, the maximum Ms was 28.3 emu/g, which is lower than that of single- crystal BaFe12O19 (up to 72 emu/g) (Pullar et al., 2016).

Table 2. Magnetic parameters of Ca1-xBaxFe12O19 powder calcined at 1100°C for 4 h

x Ms (emu/g) Mr (emu/g) Hc (Oe)

0.0 _ 0.18 3442

0.3 15.3 7.10 3728

0.5 28.3 13.6 4689

Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017

Figure 1. Hysteresis loops of Ca1-xBaxFe12O19 powders calcined at 1100°C for 4 h

3.2 Structural analysis

Phase identification was carried out by matching the XRD patterns of the calcined powders with those from the standard database of the International Centre for Diffraction Data, PDF 2010. In the case of x = 0.3 and 0.5, the XRD patterns showed the presence of reflection planes (110), (107), and (114), which are characteristic peaks of the family of M-type ferrites

(BaFe12O19, PDF#00-039-1433). Hematite (Fe2O3, PDF#01-084-0311) and esseneite

(CaFe0.6Al1.34Si1.08O6, PDF#01-084-1206) were detected in all the powders. For the x = 0.0 sample, the XRD patterns indicated the existence of trace calcium spinel ferrite (CaFe2O4, PDF#01-076-6952), which contributed to the paramagnetic fraction of the M–H curve.

Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017

Figure 2. X-ray diffraction patterns of Ca1-xBaxFe12O19 powder calcined at 1100°C for 4 h

3.3 Morphological study

A micrograph of a representative calcined Ca0.5Ba0.5Fe12O19 particle is shown in Figure 3. Three phases, including plate-like shaped hexaferrite, bulk hematite, and small irregular impurities, were observed. Elemental mapping of the focused ion beam-milled Ca0.5Ba0.5Fe12O19 particle in Figure 4 clearly showed evidence that most calcium combined with silicon, rather than entering the hexagonal ferrite structure.

Figure 3. Micrograph of Ca0.5Ba0.5Fe12O19 particles calcined at 1100°C for 4 h

Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017

Figure 4. Elemental mapping of focused ion beam-milled Ca0.5Ba0.5Fe12O19 particle calcined at 1100°C for 4 h

4. CONCLUSIONS

M-type hexagonal ferrite can be successfully prepared from AMD sludge with addition of

BaCO3 and Fe2O3 by calcination at 1100°C for 4 h. The magnetic properties of the calcined powders increased with increasing barium addition. The presence of impurities in AMD sludge made it difficult to obtain single-crystal ferrite. Hematite and esseneite existed in all the calcined powders, resulting in negative contributions to their magnetic properties.

AKNOWLEDGEMENTS

This study was supported by the Management Expenses Grants for National Universities Corporations “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan and the Hatakeyama Culture Foundation.

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