Study on the preparation of spinel ferrites with enhanced magnetic properties using limonite laterite ore as raw materials

Accepted Manuscript Study on the preparation of spinel ferrites with enhanced magnetic properties using limonite laterite ore as raw materials Jian-ming Gao, Fangqin Cheng PII: DOI: Reference:

S0304-8853(18)30021-0 https://doi.org/10.1016/j.jmmm.2018.04.010 MAGMA 63856

To appear in:

Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

4 January 2018 4 April 2018 6 April 2018

Please cite this article as: J-m. Gao, F. Cheng, Study on the preparation of spinel ferrites with enhanced magnetic properties using limonite laterite ore as raw materials, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.04.010

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Study on the preparation of spinel ferrites with enhanced magnetic properties using limonite laterite ore as raw materials Jian-ming Gao, Fangqin Cheng Institute of Resources and Environment Engineering, State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, Shanxi Collaborative Innovation Center of High Value-added Utilization of Coal-related Wastes, Shanxi University, Taiyuan 030006, P. R. China

*Corresponding author at: Institute of Resources and Environment Engineering, State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, Shanxi University, Taiyuan 030006, P. R. China. Tel/fax: +86 351 7018813 E-mail address: [email protected] (Jian-ming Gao) 1

Abstract: Preparation of spinel ferrites using limonite laterite ore as raw materials was proposed. The effect of Zn substitution on the structure, magnetic properties of as-prepared spinel ferrites were systematically characterized and discussed. The results show that single-phase spinel ferrite was directly synthesized from the precursor solution with the leaching temperature and leaching acid concentration of 200 C and 2.25 mol·L–1, respectively, and after calcination at 1000 C for 2 h. Moreover, single-phase of spinel ferrites with different Zn substitution contents (x=0.00, 0.05, 0.23, 0.35, 0.47, 0.58) could be also obtained. All the lattice constant (a), average grain sizes (d) and X-ray density (Dx) increase with increasing Zn substitution content. With Zn substitution content increasing from 0.00 to 0.58, the saturation magnetization (Ms) value increased from 34.0 to 65.9 emu·g–1 for x=0.35, and then decreased to 50.7 emu·g–1 while the coercivity (Hc) value decreased from 50 Oe to 14 Oe. This paper provides a pathway for comprehensive utilization of limonite laterite ore and synthesis of spinel ferrites with excellent magnetic performance. Keywords: limonite laterite ore, spinel ferrite, magnetic property, Zn substituted, Raman analysis

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1. Introduction Recently, spinel ferrites with common formula MFe2O4 (M=Ni, Mn, Co, Mg, etc.), have attracted much interest because of their unique structural, electrical, magnetic properties and potential applications in the fields of ferrofluids, catalysts, magnetic high-density storage, high frequency devices, micro-wave absorbers, etc. [1-6] The unit cell of spinel ferrite crystal structure consists of cubic closed-pack arrangement of oxygen ions with 64 tetrahedral (A-site) and 32 octahedral interstitial sites (B-site). Among these sites, 8 of tetrahedral and 16 of octahedral sites are occupied by the divalent or trivalent metal cations. As a result, the large fraction of empty interstitial sites makes a considerable open crystal structure, resulting in migration of cations among interstitial sites [7]. So far, all kinds of spinel ferrites were synthesized from pure chemical reagents by using chemical methods like microemulsion, coprecipitation, hydrothermal, sol-gel auto combustion, solid state reaction, etc. [8-12] As important magnetic materials, the magnetic properties of spinel ferrites depend on preparation method, chemical compositions, distribution of cations among the tetrahedral and octahedral sites, etc. [13] Limonite laterite ores, contain Fe, Ni, Co, Mn, Mg etc., are multi-metal associated and intractable resources. Until now, much effort has been taken on recovery of nickel from limonite laterite ore by high pressure acid leaching method [14]. However, the leaching process needs strict reaction conditions, such as leaching temperature at 250~270 C, and leaching pressure at 4 MPa [15]. Furthermore, there are many other metal ions co-existing in the laterite acid leach solutions, leading to the complex separation and purification processes, which seriously affected the recovery and purity of nickel [16-19]. In addition, many

3

researches have focused on extracting nickel from the laterite ore regardless of the neglected utilization of co-existing valuable metal ions like Fe, Co, Mn, Mg, etc., causing waste of valuable resources. It is important, therefor to develop a relatively simple process to increase nickel recovery, and meanwhile make full use of valuable metals from the limonite laterite ore. Ni, Co, Mn, Mg and Fe are not the main components of limonite laterite ore, but the main metal elements of magnetic spinel ferrites. Furthermore, as mentioned above, lattice defects and gap vacancies in the spinel ferrite structure could provide conditions for synthesis of multiple metals doped ferrites [7]. So it is reasonable to prepare multiple metals doped ferrites using the limonite laterite ore as raw materials. More importantly, it is worth noting that multiple metal ferrites, such as Ni-Mn-Mg ferrite, Ni-Mg-Zn ferrite, etc. synthesized from chemical reagents exhibit better electrical and magnetic performance because of the synergistic effect of metal ions on the properties [20-23]. In our previous work, it is confirmed that the metal ion molar ratios in the precursor play key role in the purity of as-prepared ferrites synthesized from saprolite-limonite laterite blends, saprolite laterite ore or treated saprolite-limonite laterite blends [24-26]. However, synthesis of spinel ferrites using limonite laterite ore as raw materials has not been studied. Furthermore, the magnetic properties and applications of as-prepared samples has not been discussed yet. Thus, how to prepare spinel ferrites with enhanced magnetic properties for great applications from the refractory laterite ore resources become the key problem. In this paper, preparation of multiple metals ferrites using limonite laterite ore as raw materials by the hydrothermal acid leaching-coprecipitation method was proposed. The precursor solutions for synthesis of spinel ferrites were obtained by controlling the critical

4

factors including the leaching temperature and HCl solution concentration. The effects of molar ratio of metal ions on the purity of as-prepared samples were systematically investigated. Then spinel ferrites without or with different Zn substitution contents were synthesized from the precursor solutions by the coprecipitation method. Finally, the structure and magnetic properties of as-prepared spinel ferrites were characterized, especially the effect of Zn substitution content on the structural and magnetic properties were discussed in detail.

2. Experimental 2.1 Materials and reagents The limonite laterite ore, supplied by Beijing Research Institute of Mining and Metallurgy, was firstly dried overnight at 105 C, and then ground to particle size smaller than 150 µm. Analytical reagents concentrated HCl solution and NaOH were purchased from Beijing Chemical Reagent Company and used without any treatment. The chemical compositions of the limonite laterite ore were determined by ICP-AES and the results are given in Table 1. It can be calculated that in the raw limonite laterite ore, the molar ratio of Fe to M (Ni, Co, Mn and Mg) is about 10.4, far away from the stoichiometric ratio 2.0 of spinel ferrites. To synthesize spinel ferrites from limonite laterite ore, the precursor solutions could be obtained by controlling the critical experimental parameters including the leaching temperature and HCl solution concentration. Table 1 The chemical compositions of the limonite laterite ore (wt.%) Constituent

Ni

Co

Mn

Mg

Fe

Cr

Al

Ti

Zn

Si

content

1.02

0.14

0.95

1.04

46.42

1.24

3.57

0.2

0.04

2.03

5

The mineralogical phases of raw limonite laterite ore are shown in Fig. 1(a). The main mineralogical phase is goethite (FeO·OH). However, the nickel-containing phases cannot be found due to the low concentration as listed in Table 1. According to the mineralogical research on the limonite laterite ore [27], the majority of Ni is found in the goethite by substituting Fe element. Fig. 1(b) show the thermogravimetry-differential scanning calorimetry (TG-DSC) plots of the raw limonite laterite ore. The DSC pattern showed two peaks at 87.4 oC and 272.0 oC. The first peak at 85.0 oC can be ascribed to the evaporation of the absorbed water. The second peak at around 272.0 oC may correspond to the endothermic reaction described as reaction (1), and the mass fraction of goethite was calculated as about 80%, suggesting that goethite is the main mineralogical phase as shown in Fig. 1(a). 2FeO·OH→Fe2O3+H2O↑

(1)

Fig. 1 XRD patterns (a) and TG-DSC curves (b) of the limonite laterite ore

2.2. Sample preparation The leaching procedure was performed in a 100 mL Teflon-lined stainless steel autoclave. A certain amount of hydrochloric acid solution (2.0, 2.25 and 2.50 mol·L–1) was added to the Teflon liner. Then the limonite laterite ore (10 g, 150 µm) was also added to the Teflon liner with the liquid to solid ratio of 6:1 mLg–1. The stainless steel autoclave was sealed and put 6

into a drying oven at the certain temperature (125~200 oC) to complete the leaching process. After the leaching process, the solid-liquid separation was conducted in the RJ-TDL-50 A centrifuge with the speed of 4000 r·min–1 for 15 min, and the leach residue was washed with deionized water. The wash water was mixed with the leach solution to obtain the mixed solution as the precursor solutions for spinel ferrites. Then concentrations of Ni2+, Fe3+, Co2+, Mn2+, and Mg2+ etc. in the precursor solutions were determined by ICP-AES. Based on the mole ratio of trivalent Fe ions to divalent metal ions of 2.0, analytical ZnCl2 without or with accurate amount was dissolved in the leach solutions. The sodium hydroxide solution was added dropwise to the solution till the pH value of the solution was adjusted to be about 11.0, and the mixture was placed to react for 15~30 min with magnetic stirring of 300 rpm. After the coprecipitation process, the precipitate was filtered, washed several times with deionized water and dried in an oven at 90 °C for 10 h. Finally, the precipitate was grinded into powder and calcinated at 1000 °C for 2 h, and spinel ferrites were acquired. 2.3 Analysis and characterization Concentrations of Ni2+, Fe3+, Co2+, Mn2+, and Mg2+ etc. in the leach solutions and limonite laterite ore were determined by Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES, America, Varian). X-ray diffraction (XRD) was performed with a Rigaku X-ray diffractometer using Cu Kα radiation (λ=0.15406 nm) at a scanning rate of 0.02 deg·s–1 with a voltage of 40 kV and 40 mA and in the diffraction angle (2θ) 10-90o. The chemical compositions of the laterite ore and as-prepared samples were investigated by X-Ray Fluorescence (XRF-1800, Japan). Room temperature Raman spectra were tested using a Raman spectrometer equipped with an Ar+ laser (532 nm, 10mW) excitation source and a

7

CCD detector. X-ray photoelectron spectroscopy (XPS) studies were performed using an AXIS ULTRA spectrometer with monochromatic Al Ka (1486.6 eV) radiation. The magnetic properties of the samples was tested by Physical Property Measurement System (PPMS, America, 9T (EC-II)) with the applied magnetic field varying from -10 to 10 kOe.

3. Results and discussion 3.1 Precursor solution preparation As analyzed in the Section 2.1, to obtain suitable precursor solutions for spinel ferrites, leaching temperature and acid solution concentration play an important role in the metal leaching efficiencies and molar ratio of Fe3+ ions to M2+ (RFe/M, M represents Ni2+, Co2+, Mn2+ and Mg2+) [20]. Fig.2 and Fig.3 show the effects of acid leaching temperature (from 125 °C to 200 °C) and HCl solution concentration (1.75, 2.0, 2.25, 2.50 and 2.75 molL–1) on the metal leaching efficiency and RFe/M, respectively. As shown in Fig.2, increasing leaching temperature can facilitate the dissolution of Ni, Mn and Mg, simultaneously, promote the hydrolysis reaction of Fe ions in the leach solution. When the leaching temperature was increased from 125 °C to 200 °C, the leaching efficiencies of Ni, Mg and Mn all increased from about 80.0% to 95.0%, however, the Fe leaching efficiency decreased from 35.0% to 20.0%, leading to that the related RFe/M decreased from 4.65 to 1.85. From Fig. 3, it can be clearly observed that increasing the HCl solution concentration all facilitate the dissolution of Ni, Mn, Mg and Fe. As the HCl solution concentration increasing from 1.75 molL–1 to 2.75 molL–1, the leaching efficiencies of Ni, Mg and Mn all increased from 80.0% to 95.0% while the Fe leaching efficiency also increased from 16.0% to 30.0%, resulting in that the RFe/M slightly increased from 1.85 to 2.10, and then sharply changed into 3.75. Considering the

8

stoichiometric ratio for spinel ferrite, the leach liquors obtained with the leaching temperature at 170, 185 and 200 °C, and HCl solution concentration of 2.25 molL–1 were selected as the precursor solutions for preparation of spinel ferrites.

Fig. 2 Effect of hydrothermal acid leaching temperature (125 °C~200 °C) on the metal leaching efficiency and RFe/M. Other experimental conditions: liquid to solid ration 6:1 mL·g–1, HCl solution concentration 2.25 mol·L–1, leaching time 2 h.

Fig. 3 Effect of acid solution concentration (1.75, 2.00, 2.25, 2.50 and 3.0 mol·L–1) on the metal leaching efficiency and and RFe/M. Other experiment conditions: leaching temperature 185 °C, liquid to solid ration 6:1 mL·g–1, leaching time 2 h.

3.2 Spinel ferrite preparation For preparation of single-phase spinel ferrite from the precursor solutions, RFe/M in the

9

leach solutions is an important factor [21]. Fig. 4 shows the XRD patterns of as-prepared samples from the precursor solutions with different RFe/M of 2.6, 2.1 and 1.9. It can be observed that diffraction peaks ((110), (220), (311), (222), (400), (422), (511) and (440), JCPDS card no. 073-1720) of spinel ferrite existed in all of the XRD patterns, and in the meantime, the diffraction peaks of Fe2O3 ((012), (104), (113), (024), (116) and (300), JCPDS card no. 086-0550) were also found in the XRD patterns when the RFe/M are 2.6 and 2.1. Notably, when the RFe/M was controlled at 1.9 (less than 2.0), the diffraction peaks of Fe2O3 disappeared, only single-phase spinel ferrite was obtained. This might be explained that a small amount of trivalent Al ions were remain in the precipitate, and when spinel ferrite generated, the Al ions substituted for trivalent Fe ions in as-prepared ferrite. According to JCPDS 073-1720, the as-prepared sample has a cubic spinel structure whose space group is Fd3m. It can be concluded that single-phase spinel ferrite could be synthesized from the precursor solution with the leaching temperature and leaching acid concentration of 200 C and 2.25 mol·L–1, respectively, and after calcination at 1000 C for 2 h. As discussed in Section 3.1 and listed in Table 2, with the leaching temperature increasing from 125 to 200 C, the RFe/M decreased from 4.6 to 1.9, implying that the obtained precursor solutions with the leaching temperature varying from 125 to 175 C need more divalent metal ions, such as Zn2+ ions. So, it is reasonable to synthesize Zn substituted spinel ferrites with different contents from the precursor solutions. To prepare spinel ferrites with excellent magnetic performance, spinel ferrites with different Zn substitution contents of 0.00, 0.05, 0.23, 0.35, 0.47 and 0.58 were obtained, and the detailed experimental parameters are given in Table 2.

10

Fig. 4 XRD patterns of as-prepared samples synthesized from the precursor solutions with the leaching temperature at 170, 185 and 200 °C, and the acid concentration of 2.25 mol·L–1. Other experimental conditions: calcination temperature at 1000 °C and time for 2 h

Table 2 Experimental parameters for as-prepared Zn substituted spinel ferrites Precursor solution No.

Calcination

Zn content

1.9

1000 °C, 2 h

0.00

185 °C, 2.25 molL–1

2.1

1000 °C, 2 h

0.05

3

170 °C, 2.25 molL–1

2.6

1000 °C, 2 h

0.23

4

155 °C, 2.25 molL–1

3.1

1000 °C, 2 h

0.35

5

140 °C, 2.25 molL–1

3.75

1000 °C, 2 h

0.47

6

125 °C, 2.25 molL–1

4.6

1000 °C, 2 h

0.58

Experimental conditions

RFe/M

1

200 °C, 2.25 molL–1

2

Footnote: RFe/M represents molar ratio of Fe3+ ions to M2+ (Ni2+, Co2+, Mn2+ and Mg2+) in the leach solutions.

3.3 Structure and magnetic properties of as-prepared spinel ferrites Fig. 5 shows the XRD patterns of as-prepared ferrites with varying Zn substitution contents. The diffraction peaks (110), (220), (311), (222), (400), (422), (511) and (440)

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correspond to the standard crystal structure of spinel ferrite (JCPDS card no. 073-1720), which belongs to the space group Fd3m. The absence of additional peaks indicated that single phase of spinel ferrite was obtained. From the shift in 2θ from 35.2o to 35.8o, it can be seen that with the Zn substitution content increasing, the 2θ shift towards a lower angle side, indicating that the lattice constant exhibits a slight increase with increasing Zn content. The lattice constant (a), average grain sizes (d) and X-ray density (Dx) of as-prepared samples were quantitatively calculated according to the following equations (2)-(4) using the prominent (311) XRD peak [28-30], and the results are summarized in Table 3.

a

d

 h2  k 2  l 2 2 sin 

(2)

0.9  cos 

(3)

Dx 

8M Na 3

(4)

Where a, d and Dx presents lattice constant, average grain size and X-ray density, respectively. λ is the wavelength of X-ray radiation, 0.15406 nm; 2θ and β are the position and half-width of the prominent diffraction peak (311); hkl are the corresponding Miller indices; M is the molecular weight for each cell; and N is the Avogadro's constant, 6.02×10²³. From Table 3, it can be found that with the Zn content increasing from 0.00 to 0.58, the lattice constant and average grain size increased from 0.8355 nm to 0.8395 nm, and from 36.8 nm to 40.5 nm, respectively. The lattice expansion can be explained on the basis of replacement of smaller Ni ion (0.069 nm) and Mg ion (0.066 nm) by the lager Zn ion (0.082 nm). It can be also observed that X-ray density of as-prepared samples increased from 4.93 g·cm-3 to 5.19 g·cm-3. The variation trends of characteristic structural parameters with Zn 12

substitution content varying are similar to Mg0.5Zn0.5-xCoxFe2O4 reported in literature [31], however, compared with Zn ion substitution for Co ion in spinel ferrites, the lattice constant of as-prepared Zn substituted Ni-Mn-Mg ferrites become smaller while the X-ray density are increased.

Fig. 5 XRD patterns of as-prepared samples with Zn content x=0.00, 0.05, 0.23, 0.35, 0.47, 0.58, and shift in 2θ from 35.2o to 35.8o

Table 3 The lattice constant (a), average grain sizes (d) and X-ray density (Dx) for as-prepared Zn substituted spinel ferrites Zn content (x)

Formula

a (nm)

d (nm)

Dx (g·cm-3)

0.00

Ni0.22Co0.03Mn0.21Mg0.54Fe2O4

0.8355

36.8

4.93

0.05

Zn0.05Ni0.20Co0.03Mn0.20Mg0.52Fe2O4

0.8369

37.9

4.97

0.23

Zn0.23Ni0.17Co0.02Mn0.16Mg0.42Fe2O4

0.8375

38.6

5.03

0.35

Zn0.35Ni0.14Co0.02Mn0.14Mg0.35Fe2O4

0.8383

39.1

5.08

0.47

Zn0.47Ni0.12Co0.02Mn0.11Mg0.28Fe2O4

0.8391

39.6

5.13

0.58

Zn0.58Ni0.09Co0.01Mn0.09Mg0.23Fe2O4

0.8401

40.5

5.19

13

According to the preferential occupation of cations at tetrahedral (A) and octahedral (B) sites of spinel ferrites, the cation distribution of as-prepared samples can be written as listed in Table 4. Then the radii of the tetrahedral (rA) and octahedral (rB) sites can be calculated using the following equations 5 and 6, and the results are listed in Table 4.

rA  [CZn r (Zn2 )  CMg r (Mg 2 )  CFe r ( Fe3 )]

(5)

1 rB  [CNi r ( Ni 2 )  CCo r (Co2 )  CMn r (Mn 2 )  CFe r ( Fe3 )] 2

(6)

Where CZn, CMg, CFe, CNi, CCo, CMn are the concentration of metal ions on different sites according to the corresponding cation distribution. The ionic radii are taken according to O’Neill [32]. The oxygen positional parameter (u) can be calculated by the equation [33,34]

u  [(rA  rO )

1

1  ] a 3 4

(7)

Where rO represents oxygen parameter (1.38 Å). The bond lengths of A sites (dAO) and B sites (dBO), and the distance between the magnetic ions in A sites (LA) and B sites (LB) were determined using the following equations 8-11 [35-37], and the results are listed in Table 4.

1 d AO  a 3(u  ) 4

d BO  a 3u 2 

11 43 u 4 64

(8) (9)

LA 

3 a 4

(10)

LB 

2 a 4

(11)

From Table 4, it can be clear that the values of rA, rB, u, LA, LB, dAO and dBO are 14

dependent on the Zn substitution content, which is assigned to the substitution process and cation distribution. As Zn content increased, the lattice parameter increases with the faster increasing rate of the ionic radii in tetrahedral sites rA than decreasing rate of rB, resulting in the slight increasing of the oxygen position parameter. Table 4 The ionic radii in tetrahedral sites (rA) and octahedral (rB) sites, the oxygen positional parameter (u), the distance between magnetic ions in tetrahedral (LA) and octahedral sites (LB), average bond lengths in tetrahedral sites (dAO) and octahedral (dBO) sites of as-prepared Zn substituted spinel ferrites (Å) Formula

rA

rB

u

LA

LB

dAO

dBO

0.5390

0.6577

0.3826

3.618

2.954

1.919

2.027

0.5418

0.6568

0.3827

3.624

2.959

1.924

2.030

0.5489

06546

0.3830

3.626

2.961

1.929

2.029

0.5533

0.6532

0.3831

3.630

2.964

1.933

2.030

0.5577

0.6519

0.3833

3.633

2.967

1.937

2.030

0.5631

0.6502

0.3835

3.634

2.970

1.943

2.031

(Mg0.54Fe0.46) [Ni0.22Co0.03Mn0.21Fe1.54]O4 (Zn0.05Mg0.52Fe0.43) [Ni0.20Co0.03Mn0.20Fe1.57]O4 (Zn0.23Mg0.42Fe0.35) [Ni0.17Co0.02Mn0.16Fe1.65]O4 (Zn0.35Mg0.35Fe0.30) [Ni0.14Co0.02Mn0.14Fe1.70]O4 (Zn0.47Mg0.28Fe0.25) [Ni0.12Co0.02Mn0.11Fe1.75]O4 (Zn0.58Mg0.23Fe0.19) [Ni0.09Co0.01Mn0.09Fe1.81]O4 Raman spectroscopy is a useful probe to illustrate the vibrational and structural properties of the materials. Spinel ferrite is known to exhibit cubic spinel structure with unit cell of FCC having eight molecules per unit cell and belonging to Fd3m space group, and give rise to A1g, 2A2u, Eg, 2Eu, F1g, 5F1u, 3T2g and 2F2u vibrational modes. Among these modes, five modes, i.e. A1g, Eg, and 3T2g are Raman active modes [38, 39]. The Raman modes above 600 15

cm–1 correspond to symmetrical stretching of metal-oxygen bonding at tetrahedral sites while the Raman modes below 600 cm–1 correspond to symmetrical and anti-symmetrical bending of metal-oxygen at octahedral sites [40]. Fig. 6 presents the room temperature Raman spectra of all the as-prepared samples with varying Zn content from 0.00 to 0.58 in the range of 100-1000 cm–1. To discover the natural frequency

of

the

Raman

active

modes

of

each

as-prepared

sample,

mixed

Lorentzian-Gaussian line shape was used to fit/deconvolute the Raman spectra. The shift in Raman modes with varying Zn content obtained from Fig.7 is listed in Table 5. It can be observed that the Raman spectra can be deconvoluted into seven components and appear at around 205, 280, 335, 475, 545, 645 and 700 cm–1. The A1g(1) mode and A1g(2) mode appeared around 700 cm–1 and 640 cm–1 are the symmetric stretch of O along the Fe-O bonds and Zn-O bonds at the tetrahedral sites, respectively [13, 41]. The three T2g modes are due to anti-symmetric bending/stretch of ligands. The T2g(3) mode at around 545 cm–1 is an anti-symmetric blending of oxygen with respect to Fe. The T2g(2) mode is anti-symmetric stretch of Fe and O, and gives rise to a peak around 475 cm–1, and the T2g(1) mode is the translational movement of the entire tetrahedral. The Eg mode corresponds to symmetric bending of oxygen ions with respect to Fe. The shift in Raman modes of as-prepared spinel ferrites with varying Zn contents can be ascribed to the incorporation of Zn ions in the ferrites, and the cation redistribution between octahedral and tetrahedral sites. Furthermore, it can be observed that with Zn content increasing, the A1g(1) mode decreases while the A1g(2) mode increases, indicating that when Zn substituted in the as-prepared ferrites, Zn ions occupy the tetrahedral sites, simultaneously, the same amount of Fe ions at the tetrahedral sites transfer to

16

the octahedral sites. Moreover, as Zn ions substitution contents increased, the peak positions of A1g(1) mode and A1g(2) mode shift towards the infrared, confirming the Zn substitution in the as-prepared spinel ferrites.

Fig. 6 Raman spectra of as-prepared samples with Zn content x=0.00, 0.05, 0.23, 0.35, 0.47, 0.58 at room temperature

17

Table 5 Raman mode assignment of as-prepared ferrites with varying Zn content

Samples Raman modes x=0.00

x=0.05

x=0.23

x=0.35

x=0.47

X=0.58

A1g(1)

707

704

705

700

700

697

A1g(2)

647

639

643

646

642

642

Raman

T2g(3)

549

547

544

545

549

541

Shift (cm–1)

T2g(2)

477

478

478

479

478

476

Eg

331

327

331

334

334

335

T2g(1)

212

210

204

209

203

205

To study the chemical elements and oxidation states of as-prepared spinel ferrites, the X-ray photoelectron spectroscopy (XPS) is applied. The collected XPS spectra of sample were analyzed using the CasaXPS software (version2.2.16). All spectra were calibrated using the adventitious C1 speak with a fixed value of 284.8eV. A Shirley-type background subtraction was used to fit the curve using a Gaussian-Lorentzian function. The XPS spectra for as-prepared ferrites with Zn ion content of 0.00, 0.05, 0.35 and 0.58 were collected using high resolution 2p regions of Fe as shown in Fig. 7. Based on the spin orbital splitting, it consists of Fe 2p3/2 and Fe 2p1/2 components, and there is an additional peak existing as satellite peak between Fe 2p3/2 and Fe 2p1/2. The bind energy difference of ~6.0 eV for all the representative spinel ferrites from the satellite peaks to Fe 2p1/2 is identified as Fe3+ state in all the samples [42]. The Fe 2p2/3 region was further deconvoluted for the octahedral versus tetrahedral peaks and analyzed. Deconvolution of the Fe 2p2/3 regions of all the samples shows that there are two nonequivalent bonds at ~710 eV, consistent with Fe3+ ions at octahedral sites (B sites), and at ~712 eV, consistent with Fe3+ ions at tetrahedral sites (A sites) [43, 44]. From Fig.7, it can be clearly observed that with the Zn substitution contents

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increasing from 0.00 to 0.58, the peak area ratio of Fe3+ ions at octahedral sites to Fe3+ ions at tetrahedral sites increased gradually, indicating that a number of Fe3+ ions migrated from the A sites to the B sites as the Zn substitution contents increased. To further confirm the effect of Zn ions substitution content on the cation redistribution at octahedral and tetrahedral sites, the XPS spectra for as-prepared ferrites with Zn ion content of 0.05, 0.35 and 0.58 were also collected using high resolution 2p regions of Zn as shown in Fig. 8. The Zn 2p region spectrum consisting of Zn 2p1/2 and Zn 2p3/2 components could be identified as Zn2+ state in all the samples [45]. Deconvolution of the Zn 2p2/3 regions of all the samples demonstrates that all the Zn2+ ions occupied the tetrahedral sites. With Zn2+ ions substitution contents increasing, the peak area correspondingly increased. The XPS results of Zn and Fe 2p region further confirmed the Raman analysis of all the samples.

Fig. 7 The deconvolution of the Fe 2p3/2 region for as-prepared samples with Zn content x=0.00, 0.05, 0.35, 0.58 19

Fig. 8 The deconvolution of the Zn 2p3/2 region for as-prepared samples with Zn content x=0.05, 0.35, 0.58

Magnetic performance is one of the most important properties for spinel ferrites. The magnetic properties of as-prepared samples with different Zn substitution contents were tested, and Fig.9 shows the room temperature hysteresis loops of all the as-prepared samples. It can be seen that all the spinel ferrites exhibited typical ferrimagnetic behavior at room temperature. The inset in Fig.9 also gives the expanded view of hysteresis loops at low field (-100 Oe~100 Oe), and the variation trend of the saturation magnetization (Ms) and the coercivity (Hc) values with varying Zn substitution contents are plotted in Fig. 10. With Zn substitution contents increasing from 0.00 to 0.58, the Ms values firstly increased from 34.0 emu·g–1 to 65.9 emu·g–1, and then decreased slightly to 50.7 emu·g–1, while the Hc values decreased continuously from 50 to 14 Oe. By comparison with the literatures [46], it is worth noting that the as-prepared spinel ferrite with Zn substitution content of 0.35 exhibited higher 20

Ms value and lower Hc value, which might be due to the synergistic effect of multiple doping of Ni, Co and Mn into the as-prepared ferrites.

Fig. 9 Room temperature hysteresis loops of as-prepared samples with Zn content x=0.00, 0.05, 0.23, 0.35, 0.47, 0.58, and expanded view of hysteresis loops at low field (-100 Oe~100 Oe)

Fig. 10 variation trend of the Ms and Hc values with varying Zn substitution contents

The initial increase of Ms values with increasing Zn substitution contents from 0.00 to 0.35 can be explained by Neel’s two sublattice models [47]. It states that the magnetic moment per formula unit (M) of the lattice is the difference between the magnetic moments of

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B and A sublattices, i.e., M= MB-MA, and the larger M value means higher saturation magnetization of ferrites. As characterized by Raman spectra and XPS analysis, when nonmagnetic Zn ions (0 μB) substituted in the as-prepared ferrites, they prefer to occupy tetrahedral sites (A sites), and the same amount of magnetic Fe ions (5 μB) at tetrahedral sites were forced to occupy octahedral sites (B sites), leading to that the magnetic moment of the B sites increased while the magnetic moment of the A sites decreased, resulting in the increasing of Ms values of as-prepared ferrites. However, when Zn substitution content exceeded 0.35, the Ms values inversely decreased, implying the existence of a non-collinear spin canting structure in the system. The phenomenon was consistent with the famous Yafet-Kittle configuration [48]. Any additional substitution of nonmagnetic Zn ions can cause some magnetic cations (Fe3+, Ni2+) at octahedral sites losing their superexchange interaction partners at tetrahedral sites, which reduce the strength of A-B superexchange interaction. Simultaneously, some magnetic cations at octahedral sites were affected by adjacent magnetic ions (B-B interaction), leading to the antiparallel array of magnetization and decrease of magnetic moment in B sites. Thus, when the Zn substitution contents were 0.47 and 0.58, the Ms values decreased to 57.8 and 50.7 emu·g–1, respectively.

Conclusions In conclusion, high-purity spinel ferrites were directly prepared from limonite laterite ore by a selective leaching-coprecipitation process. The RFe/M played key role in the purity of as-prepared samples. When the leaching temperature and leaching acid concentration were selected as 200 C and 2.25 mol·L–1, single-phase spinel ferrite could be synthesized from the precursor solution with RFe/M of 1.9 after calcination at 1000 C for 2 h. By adjusting the

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experimental conditions of the precursor solution preparation process, spinel ferrites with different Zn substitution contents could also be obtained. XRD and Raman spectra analysis confirmed the formation of single-phase spinel ferrite. Zn substituted in the spinel ferrite could cause the cation redistribution between tetrahedral sites and octahedral sites, and have great effect on the magnetic properties of as-prepared ferrites. Raman spectra and XPS analysis results indicated that when Zn substituted in the as-prepared ferrites, Zn ions prefer to occupy the tetrahedral sites, simultaneously, the same amount of Fe ions at the tetrahedral sites transfer to the octahedral sites. With Zn substitution contents increased from 0.00 to 0.58, the Ms values increased and then decreased while the Hc values decreased all the time. When Zn substitution content was 0.35, the Ms value could reach 65.9 emu·g–1 while the Hc value was 20 Oe. This paper provide a simple way for comprehensive utilization of refractory limonite laterite ore simultaneous preparation of spinel ferrite materials with excellent magnetic properties. Acknowledgments The work was financially supported by the National Key R&D Program of China (No. 2017YFB0603102), the National Natural Science Foundation of China (Nos. 51272025, 50872011 and 51072022), and the National Basic Research Program of China (Nos. 2014CB643401 and 2013AA032003). Conflicts of interest The authors declare that they have no conflict of interest.

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Highlights 1. Synthesis of spinel ferrites from limonite laterite ore was reported. 2. The synergistic effect of multiple doping was propitious for the magnetic properties. 3. The magnetic properties of as-prepared ferrite could be enhanced by Zn substitution.

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